Symposium EM8 : Spin Dynamics in Nonmagnetic Materials and Devices

Many organic semiconductors have weak spin-orbit coupled (SOC) charge carriers and charge transport in these materials takes place through hopping between localized electronic states which are exposed to strong hydrogen induced local and random hyperfine fields. On the first view, these properties seem to render these materials therefore unsuitable for traditional spintronics applications based on spin-injection, spin-transport and spin-manipulation schemes that rely on SOC. However, because of weak SOC, organic semiconductors exhibit very strong spin-controlled magneto-optoelectronic effects^1-3 which are due to spin-selection rules that are most pronounced when spin-to-orbital angular momentum transitions are oppressed. These spin-selection rules can be used for alternative, different approaches to spintronics that are based, for instance, on spin-permutation symmetry states of charge carrier pairs rather than polarization states of charge carriers. In contrast to spin-polarization, electronic transitions controlled by spin-permutation symmetry are not directly dependent on temperature and applied magnetic field strengths but instead, non-equilibrium processes such as electrical or optical charge carrier injection⁴. Weak SOC induced oppression of spin-relaxation can also contribute to long spin-coherence times and thus, allow for electron⁵- or nuclear⁶-spin based memory applications. The implementation of such organic semiconductor-based magnetization-free spintronics requires the understanding of the fundamental physical nature of the underlying spin-dependent processes. In this talk, some of the successes of this approach to spintronics will be discussed, e.g. for the application to highly sensitive, organic thin-film based robust absolute magnetometry but also challenges⁷ that this field still holds. Measurements of spin-exchange, spin-dipolar, SOC as well as hyperfine couplings between charge carriers and their surrounding protons in organic thin films will be presented². Furthermore, exotic non-linear spin-effects such as the spin-Dicke effect within charge carrier pairs (that is illustrated as the appearance of a sudden macroscopic phase coherence between paramagnetic charge carriers under strong AC drive⁸) or the inverse spin-Hall effect⁹ will be discussed.

[1] D. R. McCamey et al., Nature Materials 7, 723 (2008); [2] K. J. van Schooten, et al., Nature Commun. 6:6688, 7688 (2015). [4] W. J. Baker et al., Nature Commun. 3, 898 (2012); [5] W. J. Baker et al., Phys. Rev. Lett. 108, 267601 (2012); [6] H. Malissa, et al., Science 345 1487, (2014); [7] C. Boehme and J. M. Lupton, Nature Nano. 8, 612 (2013); [8] D. P. Waters, et al., Nature Physics 11, (11) 910 (2015); D. Sun et al., Nature Materials 15, 863 (2016).

Techniques based on electron paramagnetic resonance (EPR) spectroscopy can provide valuable insight into excitation transfer pathways in organic semiconductors used as absorber layers in solar cells [1,2]. However, these measurements are usually performed on "model systems", and the conclusions drawn from such experiments may not be valid under true solar cell operating conditions.
Here we report on the development of a setup that allows for simultaneous detection of transient electron paramagnetic resonance as well as transient electrically detected magnetic resonance (trEDMR) signals from fully-processed and encapsulated solar cells. Combining both techniques provides a direct link between photoinduced triplet excitons, charge transfer states and free charge carriers as well as their influence on the photocurrent generated by organic photovoltaic devices.
Our results obtained from solar cells based on poly(3-hexylthiophene) and the fullerene-based electron acceptor PCBM show that the resonant signals observed in low-temperature (T = 80 K) trEDMR spectra can be attributed to positive polarons in the polymer as well as negative polarons in the fullerene phase, indicating that both centers are involved in spin-dependent processes that directly influence the photocurrent [3].
Furthermore, we will discuss how transient EPR measurements on polymers:fullerene blends in combination with charge transfer state and triplet simulations can help to quantify couplings and distances of interacting spins.

References
1. J. Behrends, A. Sperlich, A. Schnegg, T. Biskup, C. Teutloff, K. Lips, V. Dyakonov, and R. Bittl, Phys. Rev. B 2012, 85 , 125206.
2. F. Kraffert, R. Steyrleuthner, S. Albrecht, D. Neher, M. C. Scharber, R. Bittl, and J. Behrends, J. Phys. Chem. C 2014, 118, 28482.
3. F. Kraffert, R. Steyrleuthner, C. Meier, R. Bittl, and J. Behrends, Appl. Phys. Lett. 2015, 107, 043302.

Due to their chemical tunability, low-cost and ease of processing, organic molecules are often used for opto-electronic devices, where the ratio of singlet to triplet excitons can be an important performance parameter. A better understanding of the correlation between the exciton spin states and the polarization of optical transitions will contribute to this field, as well as to the field of spintronics. This theoretical work proposes an experimental technique that can harness this correlation. The Time-Resolved Faraday Rotation technique allows for read-out of triplet-exciton spin dynamics as a function of the delay time between an ultrashort pump and probe pulse. We propose to use a pump pulse to excite to a superposition of triplet sublevels, thereby initiating an oscillation of the expectation value of the total electronic angular momentum as a function of time. To calculate this, we have extended existing theoretical chemistry tools. Experimentally, a suitable measure for such spin precession is the polarization rotation of a linear probe as a function of the delay time between pump and probe.

Due to their exceptionally long spin lifetimes, organic semiconductors could have an important impact on spintronics, where carrier spins play a key role in transmitting, processing and storing information[1]. Recently, the successful conversion of a spin current to a charge current via the inverse spin hall effect (ISHE) has been demonstrated in conjugated polymers[2], potentially enabling fully organic spintronics devices.
However, the importance of SOC in enabling the conversion of spin to charge currents as well as providing a major pathway for spin relaxation[3] clashes with an incomplete understanding of its origin and strength in organic materials. To the best of our knowledge, there is no systematic experimental study of SOC in molecular semiconductors of interest for optoelectronic devices.

In this work, we compare a set of 28 fused-ring molecular semiconductors with systematically varying geometries and atomic substitutions, most of which based on central thiophene or selenophene moieties. Many of the chosen molecules are widely studied and perform exceptionally well in thin film transistors with hole mobilities of 5 - 20 cm² V^-1s^-1 and signs of coherent charge transport[4,5] making them natural candidates for spintronics applications[1].
Because of the difficulty in isolating the contribution of SOC to measurable quantities such as spin lifetimes or inverse spin hall voltages, comprehensive studies of such a scale have previously been challenging. Here, we argue that over a wide range of SOC strengths, the g-factor of an unpaired charge localized on such a molecule can be used as a measure of the effective SOC experienced by a spin. Deviations of the g-factor from its free electron value (Δg) originate from cross-over terms between SOC and the orbital Zeeman energy and can be readily measured by electron spin resonance (ESR). We find that Δg depends linearly on both the spin densities and local SOC strengths at individual atoms in the molecule and is thus suited as an easily accessible indicator for SOC in such molcules.
Aditionally, we can correlate the above g-shifts to spin-lattice relaxation times determined by ESR and demonstrate a change in spin lifetimes over four orders of magnitude, from 200 µs to 0.15 µs. This suggests that Δg can be indeed attributed to an increase in SOC and a corresponding reduction of the spin life time. Even though our simulations and measurements were all performed on molecules in the gas phase and radical cations in solution, respectively, we argue that the gained information will be valuable to understand bulk systems.

[1] G. Szulczewski et al. Nat. Mater. 8 (2009) 693.
[2] K. Ando et al. Nat. Mater. 12 (2013) 622.
[3] Z.G. Yu, Phys. Rev. B 85 (2012) 115201.
[4] V. Podzorov et al. Phys. Rev. Lett. 95 (2005) 226601.
[5] N.A. Minder et al. Adv. Mater. 26 (2013) 1254.

The interaction and dynamic processes of charge pairs in the excited spin states in nonmagnetic organic semiconductors play an important role in determining the performance of organic (opto-)electronic devices; therefore, understanding and engineering these interactions will lead to device advances and improved performance. One way to manipulate the excited spin states and their related dynamic processes in organic semiconductors is by using an external magnetic field. This manipulation technique produces magnetic field effects (MFEs) which are manifested as changes in the optoelectronic and electronic properties of nonmagnetic organic semiconductors at sub-tesla field strength. Many different organic semiconductors and organic-based structures have shown that the magnitude and even the sign of the MFEs can vary by changing the measurement parameters (such as bias voltage) and fabrication conditions (such as film thickness). Such results indicate that there can be multiple physical origins of the MFEs and the final behavior may result from a competition between these different MFE mechanisms.

We present here a simple yet reliable approach to tune the way that MFEs change the electrical resistance of Alq₃(tris-(8-hydroxyquinoline) aluminum)-based systems. Specifically, we modify the interfacial morphology of organic-based devices by adding a fluorinated molecular self-assembled monolayer (heptadecafluoro-1-decanethiol [CF₃(CF₂)₇(CH₂)₂SH]) between the metal electrode and the organic semiconductor. This modification creates an environment for sizable charge accumulation near the interfaces and thus enables changes in the interplay of coexisting MFE mechanisms which we experimentally demonstrate. Our results show that tuning MFE mechanisms by interfacial modification can result in the sign change of MFEs. In addition, by adding a thin TPD (N,N′-Bis(3-methylphenyl)-N,N′-diphenylbenzidine) layer, we harness coexisting MFE mechanisms to create a system whose organic magnetoresistance can be tuned by an external bias voltage.

The spin crossover (SCO) complex [Fe(H₂B(pz)₂)₂(bipy)] has been studied extensively, and has two stable spin states of which can be switched by changing temperature. The spin state can also influenced by other extrinsic stimuli below transition temperature: laser light, electric fields and substrate interactions. Here, we report that while the thin films of [Fe(H₂B(pz)₂)₂(bipy)] grown on SiO₂, NiCoO and La_0.65Sr_0.35MnO₃ favors the low spin state at temperatures even well above the spin crossover transition temperature (which would normally be in the high spin state), [Fe(H₂B(pz)₂)₂(bipy)] can be excited into high spin state with soft X-ray radiation within the time range of minutes. This soft X-ray induced SCO of [Fe(H₂B(pz)₂)₂(bipy)] molecules can be done from 15 K to 345 K. The interface charging, due to the photoemission process, is likely the key to overcoming the potential barrier that locks the spin state of [Fe(H₂B(pz)₂)₂(bipy)] on the SiO₂, NiCoO and La_0.65Sr_0.35MnO₃ surfaces. In the case of La_0.65Sr_0.35MnO₃, the [Fe(H₂B(pz)₂)₂(bipy)] complex thin film can be relaxed back to low spin state, from the excited high spin state, by switching its magnetization of the magnetic La_0.65Sr_0.35MnO₃ substrate at room temperature also. This demonstration of room temperature isothermal switching is an important step towards utilization of the SCO complexes in spintronic and multiferroic molecular devices in a realistic temperature range (i.e. near room temperature).

Silicon Carbide (SiC) has recently garnered attention for its spin-coherent, luminescent defect centers (color centers), occurring in a variety of SiC polytypes. Coupling of these color centers to high quality optical cavities can augment the photonic signature of the defect, allowing for longer-distance, robust information transfer of the color center state. We describe the formation of high quality factor (Q) photonic crystal nanobeam cavities in 4H SiC, whose resonance frequencies have been designed to match either divacancy centers with photoluminescence (PL) emission ranging from 1070-1300 nm or silicon vacancy color centers with PL emission spanning 860-1100 nm. The best coupling conditions require a resonance in frequency between cavity and color center, as well as an overlap between color center position and the spatial extent of the cavity mode.

Color centers were introduced into fabricated 1-dimensional nanobeam photonic crystal cavities, either through ion implantation or electron irradiation. Subsequent thermal annealing resulted in improvements in the cavity Qs, as well as changes in the intensities of the cavity modes. We believe the cavity thus provide a means of sensitively monitoring the spatial diffusion of defects. Ultimately, such behavior may provide insights into approaches for deterministic placement of defects within the cavity.

Tuning of the cavities provided as much as a hundred-fold increase in the intensity of a color center zero phonon line, and lifetime measurements confirm the coupling of the defects to the cavity. The optical data of the tuned cavities-coupled-to-defects will be correlated with corresponding spin lifetimes of the defects. The progress towards strong coupling between cavity and color center, and the understanding of the corresponding spin behavior represent important stepping stones towards the use of such spin centers in quantum information applications.

Efficient interfaces between photons and atoms are crucial for quantum networks and enable nonlinear optical devices operating at the single-photon level. In this talk I will highlight properties of single colour centres at low temperatures and show that single SiV and GeV colour centres in diamond are promising candidates for creating such interfaces. I will also show experiments aiming to create technologies allowing realization of fully integrated, scalable nanophotonic quantum devices.

Long quantum coherence in solid-state systems is the ultimate prerequisite for new technologies based on purely quantum phenomena. In recent years, silicon carbide (SiC) is attracting continuously growing interest for quantum spintronics [1,2] and the longest T₂ reported to date in this system is 1 ms at cryogenic temperature [3]. We observe that T₂ continuously increases up to about 100 ms with the number of decoupling pulses [4]. We estimate the T₂ saturation level to be approximately 0.3 s, which is within the same order of magnitude with the record values for electrons in solid state but achievable without isotope purification of the crystal. Such an exceptional long-lived quantum memory is attained through the suppression of heteronuclear spin cross-talking in a magnetic field above 10 mT, in accord with the theoretical simulations [5].
[1] W. F. Koehl, et al., Nature 479, 84 (2011).
[2] H. Kraus, et al., Nat. Phys. 10, 157 (2014).
[3] D. J. Christle, et al., Nature Mater. 14, 160 (2015).
[4] D. Simin, et al., arXiv:1602.05775 (2016).
[5] Li-Ping Yang, et al., Phys. Rev. B 90, 241203(R) (2014).

Effective fabrication of photonic structures with optically coupled semiconductor spin systems marks an important tool for scalable quantum information processing. To produce aligned nitrogen vacancy centers (NV) with diamond nanostructures, we introduce a lithographic mask with nanoscale implantation apertures for NV creation, together with larger features for producing waveguides and photonic nanocavities. With subsequent nitrogen ion implantation, dry etching and thermal annealing, we obtain self-aligned NV creation at the mode maximum of diamond photonic crystal nanocavities with a single-NV per cavity yield of ∼26%, as well as Purcell induced intensity enhancement of the zero-phonon emission up to five fold. We expect larger enhancement factors by improved fabrication and optimized cavity coupling. Further numerical investigation on the NV formation within nanostructures will also help achieve better alignment between NV and diamond photonic cavities.

The nitrogen vacancy (NV) centers in diamond has emerged as a promising quantum technology in a variety of contexts. Here I discuss our group’s work on diamond NV-based quantum sensing and imaging with a particular emphasis on imaging nanoscale magnetism in condensed matter [1] and biological systems. I also present our work on interfacing NV quantum bits with phonons [2] and photons in a hybrid quantum system, with eventual applications in quantum computing and fundamental studies of macroscopic systems in the quantum regime. These distinct research directions share several similar challenges, the biggest of which is maintaining the NV’s excellent quantum properties while intimately interfacing NV centers with either the system under study or with other quantum elements. I discuss this “quantum interface” problem and present recent work on unraveling the mechanisms of NV decoherence, in particular near interfaces.

[1] “Strain coupling of a mechanical resonator to a single quantum emitter in diamond”,
Kenneth W. Lee, Donghun Lee, Preeti Ovartchaiyapong, Joaquin Minguzzi, Jero R. Maze, Ania C. Bleszynski Jayich, arxiv:1603.07680 (2016).
[2] “Scanned probe imaging of nanoscale magnetism at cryogenic temperatures with a single-spin quantum sensor”, M. Pelliccione, A. Jenkins, P. Ovartchaiyapong, C. Reetz, E. Emmanuelidu, N. Ni, A. C. Bleszynski Jayich, Nature Nanotechnology 10.1038/nnano.2016.68 (2016).

The diamond nitrogen-vacancy (NV) center is a versatile solid-state qubit, exhibiting long spin coherence times and built-in mechanisms for optical initialization and readout at room temperature. However, the standard fluorescence-based spin readout relies on ensemble averaging due to the short timescale (~200 ns) of spin-dependent fluorescence contrast before the qubit state is reset. Typical experiments require ~10⁴ repeats to overcome photon shot noise. At cryogenic temperatures, quantum optics techniques for single-shot readout enable applications involving projective and partial measurements, as well as entanglement-by-measurement of coupled nuclear spins, but these are unavailable at room temperature. For magnetometry, increased readout efficiency translates directly to better sensitivity, higher throughput in scanning based experiments, and lower overall noise.

We present all-optical techniques for improving the signal-to-noise ratio (SNR) of room-temperature NV spin readout and overcoming charge initialization limitations using combined visible and near-infrared illumination. A multiphoton absorption model of the NV center is developed which captures non-monotonic behavior in the steady state charge distribution as a function of near-infrared power. This model describes two distinct power regimes in which mechanisms for all-optical negative charge state (NV^-) initialization exceeding 90% and a new spin-to-charge conversion (SCC) mechanism attributed to ionization of the metastable NV^- singlet manifold dominate the charge dynamics. A new multi-step SCC protocol using this singlet-SCC yields an orders-of-magnitude speedup over traditional photoluminescence readout. A master-equation model of the multi-step SCC quantifies protocol parameters, such as inter-system crossing branching ratios, ionization probability, and initial spin purity. The demonstrated fidelity is limited by laser power, and in the ideal case of 100% ionization and perfect spin initialization, we predict single-shot fidelities approaching 90% and spin projection noise ~1.3x the Heisenberg limit. We discuss a novel post-processing technique utilized in this work that overcomes both poor SCC efficiency and non-ideal charge state readout, which can be used to apply these techniques to nanodiamonds and other nanostructured devices.

*Work supported by a National Science Foundation CAREER Award (ECCS-1553511).

We report on quantitative, high resolution magnetic imaging of antiferromagnetic (AFM) domains using magnetometry based on individual scanning Nitrogen Vacancy (NV) electron spins in diamond[1]. Our results build on the unique performance of our recently developed, all-diamond scanning probes [2,3], which allow for high-performance imaging with an unprecedented combination of nanoscale resolution and sensitivity.
Understanding and controlling magnetism on the nanoscale is a crucial ingredient for designing new types of high-density memory and for implementing new device architectures in spintronics or quantum information processing. Here, we investigate a material system, which is particularly attractive in this regards: The magnetoelectric AFM Cr₂O₃ [4]. This material stands out due to the ability to control its AFM order parameter using combined electric and magnetic fields; a feature which may find applications in purely AFM, magneto-electric random access memory devises. Despite this technological motivation, significant questions remain open, regarding its basic magnetic properties on the nanoscale, such as the occurrence and temperature-dependence of magnetic domains and density of net surface magnetic moments which may occur in Cr₂O₃ due to the crystalline symmetry [5].
Here, we present first nanoscale images of the weak magnetic stray fields emerging from the topmost, magnetically uncompensated surface of the collinear AFM Cr₂O₃. We performed our experiments using a room temperature, scanning NV magnetometer, with the ability to quantitatively image the magnetic field using microwave frequency-locking to the instantaneous NV electron spin resonance [2,6]. The performance of our apparatus allowed us to clearly map stray fields from the Cr₂O₃ surface spins, at a deduced moment density of 2μ_Bohr/nm². As the orientation of the magnetic moment at the top surface is linked to the AFM order parameter, the observed stray field clearly maps the AFM domains of ~200nm average size, which we here studied on the nanoscale the first time. In particular, the robustness with temperature of NV magnetometry allowed us to track the evolution of domain-size across the AFM-paramagnetic phase transition at a Neel temperature ~300 K. As a central result, these studies allowed us to pinpoint the origin of AFM domain formation in Cr₂O₃ to nanoscale variations in Neel temperatures arising from local disorder.
The results presented here are highly relevant for the microscopic understanding of AFM spin ordering in Cr₂O₃ and in general demonstrate the power of NV magnetometry for revealing local properties of magnetically ordered systems which are inaccessible to alternative techniques.
[1] J. Taylor et al., Nature Physics 4, 810
[2] P. Appel et al., Rev. Sci. Instr. 87, 063703
[3] P. Maletinsky et al., Nature Nanotechnology, 7, 320
[4] He et al., Nature Materials 9, 579
[5] K.D. Belaschenko, Phys. Rev. Lett. 105, 147204
[6] R.S. Schoenfeld, Phys. Rev. Lett. 106, 03080

Nitrogen-vacancy (NV) center spins are an attractive candidate for spintronic devices because of their nanoscale size and long coherence times at room temperature. To fully take advantage of their small size, NV centers separated on nanometer length scales must be individually addressable in order to manipulate and read out a particular spin. Sufficiently strong magnetic field gradients, such as those from a magnetic force microscopy (MFM) tip, can satisfy this requirement. An MFM tip, however, requires a complex mechanical nanopositioning system which is inherently slow and non-scalable. Here instead we use a driven ferromagnetic (FM) vortex within a 2-µm- diameter permalloy disk, whose core generates a large local magnetic field and magnetic field gradient. The vortex core can be controllably positioned within the disk on nanosecond time scales by applying modest magnetic fields. We fabricate an array of FM disks atop a gold coplanar waveguide, and then spin-coat diamond nanoparticles, with mean diameter 25 nm and containing zero to several NV centers, onto the FM disks. Measurements were carried out using a combined magneto-optical microscopy/optically detected magnetic resonance technique. The former method tracks the position of the vortex core while the latter is used to initialize and read-out the spin state of the NV centers. Using in-plane magnetic fields, we steer the FM vortex in proximity to the NVs within an individual nanoparticle and map the microwave-driven spin transitions. We find that the FM vortex induces a large spin splitting when brought near the NV spins, due to a field gradient sufficiently large to address spins separated by approximately 10 nm. Moreover, we observe several intriguing phenomena arising from the dynamics of the driven vortex and spins. As the FM vortex move towards NV spins, we find the spin splitting does not change continuously but rather undergoes frequent jumps. These jumps in spin splitting agree well with hysteretic jumps in the vortex position due to pinning of the vortex core at defects in the permalloy. We also find that as the vortex/NV interaction increases, certain spin transitions become broadened by more than an order of magnitude, due to a vortex-induced amplification of the applied microwave field. Finally, we investigate how the coherence of the NV spin state is affected by the proximity to the vortex core and the dynamics of the vortex state. Ramsey measurements reveal that that the effective spin coherence time T₂* ≈ 500 ns is largely independent of the vortex position relative to the NVs. Furthermore, coherence can be preserved even while the vortex is translated during the spin’s free coherent evolution, providing a pathway towards vortex-enabled multi-qubit operations.

We employ diamond-based vectorial magnetometry for imaging the penetration of magnetic field into a type II
superconductor [1]. The technique of diamond magnetometry is based on optical detection of magnetic resonance
(ODMR) of negatively charged nitrogen-vacancy defects inside a single crystal diamond.
Our prototype diamond magnetometer is designed to allow a non scanning magnetic imaging of an electrically
wired sample at sub-Kelvin temperatures through a coherent fiber bundle with 30,000 cores, using a complementary
camera. We use the magnetometer for imaging the penetration of an externally applied magnetic field into a thin
niobium (Nb) film. Magnetometry measurements of the film, whose critical temperature is 9.0K, are performed at a
temperature of 4.3K.
The current distribution in such thin film type-II superconductor under perpendicular magnetic field is theoreti-
cally evaluated by employing the Critical State Model. This model predicts the superconducting shielding currents
distribution, taking into account the penetration of magnetic ux into the superconductor and hysteresis. Comparison
between the experimental findings and theoretical predictions yields a good agreement.
The current experiment has demonstrated the ability of employing diamond magnetometry for low-temperature
study of superconductors. Further improvements in the design of the magnetometer should enable operation at ultra-
low temperatures. Such ability may open the way for a variety of new applications, for example the performance of
a single-shot quantum state readout of a large array of superconducting Josephson qubits.

Efficient, on-demand, and robust single photon emitters (SPE) are of central importance to many areas of quantum information processing. Over the past 10 years, color centers in solids have emerged as excellent SPEs and have also been shown to provide optical access to internal spin states. Color centers in diamond and silicon carbide (SiC) are among the most intensively studied SPEs. Recently other less expensive wide-bandgap materials have become attractive as potential host materials. Calculations have shown that aluminum nitride (AlN) with a bandgap of 6.015 eV can serve as a stable environment for well isolated SPEs. In contrast to diamond or SiC, both with strong covalent bonds, AlN is an ionic crystal with piezoelectric properties that may offer different controlling schemes for quantum spins.

Here we report on room temperature SPEs in thinfilm AlN on a sapphire substrate with an emission spectrum in the visible. The emitter density and photostability can be controlled with annealing treatments. The reported AlN emitters – the first reported to date – add to the promising properties of AlN as an established opto-mechanical system, and can subsequently be used for classical and quantum information processing.

The magnetic and electronic properties of materials often find their origin in basic atomic-scale interactions. Yet, due to the large number of atoms involved, many phenomena can be very difficult to predict: we call these ‘emergent’. The ability to build structures atom-by-atom by means of scanning tunneling microscopy (STM) may provide an excellent platform to explore emergence as a function of system size. For example, by properly tuning the anisotropy of magnetic atoms a thin insulator, we have been able to engineer finite spin chains hosting spin waves (1) as well as the beginnings of a quantum phase transition at a critical magnetic field (2). Unfortunately, the maximum size of such assembled structures is often limited due to e.g. crystal impurity, crystal strain, and general uncontrollability of the STM tip shape, hampering the reliability with which atoms can be manipulated. In this talk, I will demonstrate how atomic assembly can be enhanced dramatically by switching to manipulation of atomic vacancies, rather than adatoms, on a chlorine-terminated copper surface (3). The resulting structures, comprising thousands of vacancies positioned on an exactly defined grid, are found to be stable at temperatures up to 77 K. We use this new technique to construct two-dimensional artificial crystals of various size and atomic spacing, and investigate their collective electronic properties through local tunneling spectroscopy.
1. Imaging of spin waves in atomically designed nano magnets, A. Spinelli et al., Nature Materials 13, 782 (2014).
2. Atomic spin chain realization of a model for quantum criticality, R. Toskovic et al., Nature Physics 12, 656 (2016).
3. A kilobyte rewritable atomic memory, F. Kalff et al., Nature Nanotechnology AOP (2016).

Recent three terminal (3T) measurements in magnet/insulator/semiconductor junctions show that spin-dependent transport through an interfacial defect is vital to understanding spin injection measurements with this 3T technique [1]. Developing a theoretical grasp of such defects is also important for spin pumping and spin thermal transport. To accomplish these goals, we describe the coherent interaction between a defect spin at the interface of a biased magnet and non-magnetic material when the magnet is undergoing ferromagnetic resonance. The magnet allows primarily parallel spins to enter which leads to a dynamic spin accumulation of antiparallel spins on the defect. Local effective fields at the defect site modify the defect spin dynamics and alter the current through the defect. Measurement of this new current-detected spin resonance is a probe to the defect’s local environment and specifically may aide in identifying the type of defect. In the limit of static magnetization, the theory makes precise predictions regarding the current in an oblique field. The theoretical predictions agree very well with recent oblique-field measurements [2]. While the defect is blamed for obfuscating spin injection signals, defects with appropriately positioned energies, can act as intermediaries for spin injection into the semiconductor. Since local effective fields and spin relaxation may operate at the defect site, defects can play a noticeable role in the spin injection by compounding the Hanle effect to produce much narrower line shapes or even inverted line shapes.

[1] H. Inoue, A.G. Swartz, N.J. Harmon, T. Tachikawa, Y. Hikita, M. E. Flatté, H. Y. Hwang, Phys. Rev. X 5, 041023 (2015)
[2] S. He, J.-H. Lee, P. Grünberg, and B. K. Cho, J. Appl. Phys. 119, 113902 (2016)

This material is based on work supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award Number DE-SC0014336.

An atomic scale understanding of the interactions of magnetic atoms with a nonmagnetic host semiconductor is important for future spintronic devices and quantum information processing. Toward this end, scanning tunneling microscopy can be used to probe spin-associated energy levels and study how these states can be tuned by local electric fields from nearby charged defects or the STM tip itself. Atomic manipulation allows us to study exchange coupling between pairs of magnetic impurities. These methods have yielded insight into the properties of a variety of magnetic impurities in GaAs, including Mn, Co, Er, and V, as well as their interactions with charged As vacancies, Ga adatoms, and Zn acceptors. This presentation will focus on tuning Er and Zn dopant properties using local electric fields and laser illumination.
Tunneling spectroscopy of Zn-doped GaAs has revealed a rich spectrum of in-gap states which we have studied as a function of Zn doping level (~10^16-10^19/cm^3), and proximity to the surface and other Zn. Zn acceptors can be grouped into two categories by their response to local electric fields. First, isolated Zn, which are prevalent in low-doped GaAs, exhibit in-gap states which have an apparent energy shift away from the Fermi level which can be understood by considering the STM tip-induced band bending (TIBB). Above-gap illumination reduces TIBB and causes an apparent energy shift towards the Fermi level, which is consistent with the surface photovoltage (SPV) effect. We model these shifts for different tunneling conditions using the SEMITIP Poisson solver by Feenstra. Zn acceptors located near other Zn become more prevalent in higher doped samples, and exhibit in-gap states which do not shift in response to TIBB or SPV.
We have applied these methods in the first STM studies of Er adatoms on GaAs to probe how hybridization with the host influences the Er f-shell. We find that Er has several different adsorption states with different stabilities and in-gap states. Two metastable adsorbate states exhibit broad in-gap peaks in tunneling spectra, but no evidence of the sharp f-shell transitions expected from optical studies. We have characterized the sensitivity of these in-gap states to TIBB/SPV and again find that Er defects fall into in/sensitive categories determined by local environmental factors. In addition to optical illumination at photon energies above the GaAs bandgap, we have used ultrafast illumination in the near IR that is below gap, but resonant with the Er f-shell transitions. Sensitive Er defects exhibit in-gap states that shift in response to both modes of illumination. Because below-gap illumination is not expected to cause SPV, this may indicate an effect of resonant excitation of the Er f-shell.
These studies demonstrate that the interaction of individual impurity atoms with local electric fields and optical illumination is influenced by a variety of local environmental factors.

Ben Warner [a], Fadi El Hallak [a], Nicolae Atodiresei [b], Philipp Seibt [a], Henning Prüser [a], Vasile Caciuc [b], Michael Waters [c], Andrew J. Fisher [a], Stefan Blügel [b], Joris van Slageren [d], and Cyrus F. Hirjibehedin [a]* [a] University College London (UCL), London WC1H 0AH, UK [b] Forschungszentrum Jülich and JARA, D-52425 Jülich, Germany [c] University of Nottingham, Nottingham NG7 2RD, UK [d] University of Stuttgart, 70569 Stuttgart, Germany

Coupling between a magnetic impurity and an external bath can give rise to many-body quantum phenomena, including Kondo and Hund’s impurity states in metals, and Yu-Shiba-Rusinov states in superconductors. While advances have been made in probing the magnetic properties of d-shell impurities on surfaces, the confinement of f orbitals makes them difficult to access directly. Here we show that a 4f driven Kondo resonance can be modulated spatially by asymmetric coupling between a metallic surface and a molecule containing a 4f-like moment [1]. Strong hybridization of dysprosium double-decker phthalocyanine with Cu(001) induces Kondo screening of the central magnetic moment. Misalignment between the symmetry axes of the molecule and the surface induces asymmetry in the molecule’s electronic structure, spatially mediating electronic access to the magnetic moment through the Kondo resonance. This work demonstrates the important role that molecular ligands have in mediating electronic and magnetic coupling and in accessing many-body quantum states.

[1] B. Warner et al., Nature Communications 7, 12785 (2016)

Chiral molecules based spintronics can provide a potential alternative to the semiconductor based spintronic devices.¹ In chiral molecules, the Chiral Induced Spin Selectivity (CISS) effect enables to filter one spin orientation of the electrons transported through the molecules. Hence the effect provides a mean for production of an effective local magnetic field on nanometric scale.² The magnetic field can be monitored applying a Hall setup.³ We found that when electric field is applied on self-assembled monolayer of chiral molecules, spin polarization occurs, due to the charge reorganization. The spin polarization creates a magnetic field that can be sensed by the Hall device. The strength and direction of the magnetic field correlates with the magnitude and sign of the electric field/chirality of molecules.

References
1. Ben Dor, O. ; Yochelis S.; Mathew, S. P.; Naaman, R.; Paltiel, Y., Nature Comm. 2013, 4, 1-6.
2. Ben Dor, O., Morali, N., Yochelis, S., Baczewski, L. T. & Paltiel, Y., Nano Lett. 2014, 14, 6042–6049.
3. Eckshtain-Levi, M.; Capua, E.; Refaely-Abramson, S.; Sarkar, S.; Gavrilov, Y.; Mathew, S. P.; Paltiel, Y.; Levy, Y.; Kronik, L.; Naaman, Nature Comm. 2016, doi: 10.1038/ncomms10744.

Graphene is a potential material for spintronic applications due to the combination of its expected long spin lifetime and high electron mobility. The experimental spin diffusion distances observed in graphene are very long, few micrometers at room temperature [1]. However, previous studies showed that electrical spin injection from ferromagnetic electrodes in direct contact with graphene is not effective. Instead, the use of a thin insulating layer acting as a tunnel barrier (tunneling contact) between the graphene layer and the metal electrodes has proven to be an effective solution [2]. Nevertheless, complete control of tunneling barrier fabrication on graphene sheets is still distant. In this work, we report on the critical steps for graphene spin transport devices fabrication. A detailed investigation of the structural and chemical characteristics of thin evaporated Al₂O₃ tunnel barriers of variable thickness grown onto single-layer graphene sheets is presented. Advanced electron microscopy (HRTEM, STEM) and spectrum-imaging techniques were used to investigate the Co/Al₂O₃/graphene/SiO₂ interfaces. Direct observation of pinhole contacts was achieved using FIB cross-sectional lamellas [3]. The chemical nature of the Al₂O₃ barriers was also analyzed using electron energy loss spectroscopy (EELS). The devices were fabricated using standard lift-off technic and e-beam lithography. The electrical characterization of the device nanocontacts seems to be a resultant of tunneling contact behavior associated to charge trapping and pinhole contacts.

[1] P. Seneor, B. Dlubak, M-B.Martin, A. Anane, H. Jaffres and A. Fert, MRS Bulletin, 37, (2012) 1245.
[2] N. Tombros, C. Jozsa, M. Popinciuc, H.T. Jonkman and B.J. van Wees, Nature,448, (2007) 571.
[3] B. Canto, C. Gouvea, B. Archanjo, J.E. Schmidt and D.L. Baptista, Scientific Reports, 5, (2015) 14332.

Spin-orbit torques arising from current flow in nonmagnetic heavy metals have the potential to enable efficient manipulation of magnetic devices. I will describe recent studies aiming to enhance the strength of these torques via control over materials and interfaces. We have studied how the introduction of both light and heavy impurities into heavy metals affects the strength of spin-orbit torques, finding that significant improvement in both the strength of the torque per unit current and the energy cost for switching can be achieved for heavy metals in which the spin Hall effect is dominated by an intrinsic band structure mechanism. The quality of the interface between the nonmagnetic spin-source material and a ferromagnetic layer is also critical in order to maximize the transmission of spin currents and generate strong torques. We show that thin spacer layers can inhibit intermixing at the interface and reduce the critical currents for magnetic switching by more than a factor of two in some cases. In addition to enhancing the strength of spin-orbit torques, we are also learning to manipulate their direction, by generating spin currents using materials that break inversion symmetry within the sample plane. I will describe measurements in which a thin layer of WTe₂, a low-symmetry transition metal dichalcogenide, produces an out-of-plane anti-damping spin-orbit torque, an orientation that is forbidden by symmetry in more conventional devices.

This work is done in close collaboration with the group of Bob Buhrman at Cornell University.

The spin Hall effect creates a spin current in response to a charge current in a material that has strong spin-orbit coupling. The size of the spin Hall effect in many materials is disputed, requiring independent measurements of the effect. We develop a novel mechanical method to measure the size of the spin Hall effect, relying on the equivalence between spin and angular momentum. The spin current carries angular momentum, so the flow of angular momentum will result in a mechanical torque on the material. We determine the size and geometry of this torque and demonstrate that it can be measured using a nanomechanical device. Our results show that measurement of the spin Hall effect in this manner is possible and also opens possibilities for actuating nanomechanical systems with spin currents.

Current-induced spin polarization is a phenomenon in which carrier spins in semiconductors are oriented when subjected to current flow. However, the mechanism that produces this spin polarization remains an open question. Existing theory predicts that the spin polarization should be proportional to the spin-orbit splitting yet no clear trend has been observed experimentally. We perform experiments on samples designed so that the magnitude and direction of the in-plane current and applied magnetic field can be varied and use optical techniques to measure the electrical spin generation efficiency and spin-orbit splitting [1]. Contrary to expectation, the magnitude of the current-induced spin polarization is shown to be smaller for crystal directions corresponding to larger spin-orbit splitting. Angle-dependent measurements demonstrate that the steady-state in-plane spin polarization is not along the direction of the spin-orbit field, which we attribute to anisotropic spin relaxation. We show that this electrically-generated electron spin polarization can drive dynamic nuclear spin polarization and measure the nuclear spin polarization as a function of laboratory time and applied electric and magnetic field [2]. Furthermore, we use time- and spatially-resolved pump-probe measurements to show that an in-plane electric field can modify the electron g-factor in a semiconductor epilayer [3].
[1] B. M. Norman, C. J. Trowbridge, D. D. Awschalom, and V. Sih, “Current-Induced Spin Polarization in Anisotropic Spin-Orbit Fields,” Phys. Rev. Lett. 112, 056601 (2014).
[2] C. J. Trowbridge, B. M. Norman, Y. K. Kato, D. D. Awschalom, and V. Sih, “Dynamic nuclear polarization from current-induced electron spin polarization,” Phys. Rev. B 90, 085122 (2014).
[3] M. Luengo-Kovac, M. Macmahon, S. Huang, R. S. Goldman, and V. Sih, “g-factor modification in a bulk InGaAs epilayer by an in-plane electric field,” Phys. Rev. B 91, 201110(R) (2015).

Lateral spin valves are now an established tool for investigating spin accumulation in III-V semiconductors. It is apparent, however, that the spin-polarized electrons in these systems cannot be treated as a uniform, homogeneous population. Samples with the longest spin lifetimes at low temperatures are doped only slightly above the metal-insulator transition, and the conduction at lowest temperatures occurs in an impurity band. As a result, spins localized on donors play a particularly important role in governing the spin dynamics in the semiconductor channel. For example, the generation of dynamic nuclear polarization is most efficient near donor sites, and the resulting hyperfine field is spatially inhomogeneous. Electron spin transport therefore occurs through an inhomogeneous landscape of nuclear polarization, and components of the hyperfine field perpendicular to the electron spin lead to an enhancement of spin relaxation.[1] This additional relaxation is observable in high precision Hanle measurements, which are also capable of resolving the exchange field acting on nuclei (the Knight shift) as well as quadrupolar shifts in electrically detected nuclear magnetic resonance.[2] Quadrupolar relaxation is the dominant mechanism limiting the efficiency of dynamic nuclear polarization. As the temperature increases, the hyperfine field and its associate inhomogeneity become less important. Recent progress in spin valve growth and fabrication (particularly the integration of Heusler alloy electrodes) has allowed for precise measurements of spin dynamics at high temperatures, where conventional Dyakonov-Perel relaxation is the dominant process for depolarization of electron spins.

This work was supported by NSF DMR 1104951 and C-SPIN, a SRC STARnet center sponsored by MARCO and DARPA.

[1] N. J. Harmon, T. A. Peterson, C. C. Geppert, S. J. Patel, C. J. Palmstrøm, P. A. Crowell, and M. E. Flatté, Phys. Rev. B 92, 140201(R) (2015).
[2] K. D. Christie, C. C. Geppert, S. J. Patel, Q. O. Hu, C. J. Palmstrøm, and P. A. Crowell, Phys. Rev. B 92, 155204 (2015).

Strong spin-orbital coupling and the inversion asymmetry of transition-metal dichalcogenide (TMD) structures lead to unique spin transport mechanics under an external magnetic field. The presence of spin-orbital fields of opposite directions for electrons in K and K’ Valleys in combination with intervalley scattering results in a nontrivial spin dynamic. This feature can be reflected by measuring nonlocal resistance via electrical Hanle experiments. Optical Hanle measurements revealed 1ns spin life time in monolayer MoS₂; however, no reports have addressed electrical Hanle measurements which could provide more useful information related to additional valley degree of freedom in charge carriers. We first calculated theoretically the Hanle shape in TMDs. Our simulations indicate that, unlike conventional Hanle shape with major peak appeared at zero field, Hanle curve of MoS₂ exhibits a two-peak structure where the maximum occurs at B ≈B_so when the external field compensate the internal spin orbital field of each valley. Also we experimentally measure Hanle curves in 2D MoS₂ films fabricated by pulsed laser deposition. Non-local Hanle measurements under normal field direction were performed at low temperature to find the life-time of injected carriers in the MoS₂ layers. The experimental results match the theoretically-predicted Hanle curve under the regime of slow intervalley scattering. Valley and spin relaxation life time, intervalley scattering rate, spin diffusion length are estimated by fitting the results with our model.

The anomalous Hall (AH) effect is deemed to be a unique transport property of ferromagnetic metals, caused by the concerted action of spin polarization and spin-orbit coupling. Nevertheless, recent experiments have shown that the effect also occurs in a nonmagnetic metal (Pt) in contact with a magnetic insulator [yttrium iron garnet (YIG)], even when precautions are taken to ensure that there is no induced magnetization in the metal. We propose a theory of this effect based on the combined action of spin-dependent scattering from the magnetic interface and the spin-Hall effect in the bulk of the metal. At variance with previous theories, we predict the effect to be of first order in the spin-orbit coupling, just as the conventional anomalous Hall effect—the only difference being the spatial separation of the spin-orbit interaction and the magnetization. For this reason we name this effect the nonlocal anomalous Hall effect and predict that its sign will be determined by the sign of the spin-Hall angle in the metal. The AH conductivity that we calculate from our theory is in order of magnitude agreement with the measured values in Pt/YIG structures.

A recent theory predicts nonlocal magnon drag in a ferromagnetic bilayer system, where magnetic current in one material induces a chemical potential in the neighboring material. [1] This inspires the possibility of inducing magnon drag from a temperature gradient on a ferromagnet into an adjacent paramagnet.

To explore this concept experimentally, we compare the thermopower of Pt films grown on ferrimagnetic yttrium-iron garnet (YIG) to that grown on the paramagnet gadolinium-gallium garnet (GGG). To isolate the hypothetical drag contribution from the magnons in YIG into the adjacent Pt film, we design a “magnon-drag thermocouple”. It consists of a hybrid sample of half GGG/Pt and half GGG/YIG/Pt, where the Pt film is a block U shape with one leg on GGG and the other on YIG. We measure the voltage between the two top ends of the U, while applying a temperature gradient in the direction parallel to the two arms of the U and perpendicularly to the bottom connection. When the gradient is uniform on both sides of the sample, the Pt acts as a differential thermocouple. The effective voltage at the isothermal ends of the Pt arms is ΔV = (α_YIG-α_GGG)ΔT. This provides a direct measure of the difference in thermopower of the two systems, which we attribute to magnon dynamics in YIG and their interactions at the YIG/Pt interface.

We conduct thermopower measurements on this system, investigating the temperature dependence and in-field behavior of the signal. Preliminary results show a clear thermoelectric response of the “magnon thermocouple” that peaks near 340K. At that temperature the signal is unaffected by applied magnetic fields of 1 Tesla either parallel or perpendicularly to the temperature gradient. Low temperature, high field measurements examine the possibility of freezing out magnon dynamics by opening a Zeeman gap in the magnon spectrum of the YIG.

We also report on our results of spin Seebeck (SSE) signals measured on a Pt/NiO/YIG trilayer system. The SSE signal observed on pure Pt/YIG is attenuated by the presence of a NiO intermediate layer, which we vary in thickness from 1 to 10 nm. Yet we observe the spin flux from YIG to go through the NiO into Pt, with a reduction in signal that scales exponentially with thickness of the NiO layer. The attenuation length will be reported as a function of temperature.

Work supported by NSF CEM MRSEC grant DMR-1420451 and ARO MURI grant W911NF-14-1-0016

[1] T. Liu, et al. Phys. Rev. Lett. 116, 237202 (2016)

The emergence of Weyl (massless) fermions in TaAs, due to its electronic band structure, is indicative of a new state of matter in the condensed phase that is of great interest for fundamental physics and possibly new applications. To date, however, the issue of crystal defects and their effect on the electronic energy states and hence the presence of Weyl fermions in TaAs has not been explored. Determining defect structures and assigning physical effects to particular defects is challenging to accomplish experimentally but is an area in which first-principles calculations can offer crucial information. Here we present the results of density functional theory calculations on the intrinsic defect properties of TaAs. We investigate further how the point defects influence the Fermi surface. We demonstrate that there is a thermodynamical driving force to form defects related to different stoichiometries, and determine the phase stabilities as a function of the elemental chemical potentials. Our results show that Weyl fermions can only be observed under specific conditions, and that a small change in the balance of defect concentrations will cause these particles to vanish. We therefore provide key new insights into the physics of Weyl semimetals via their defect chemistry. Our calculations are in excellent agreement with available experiment.

Topological insulators (TIs) have received unparalleled attention in condensed matter physics because of their novel properties such as large spin-orbit interactions and spin momentum locking [1]. Some of TIs have been realized to show higher spin-charge conversion efficiency attributed to large spin-orbit interactions [2]. In this work, we study temperature dependence of spin pumping in Py/(Bi,Sb)2Te3.

We grow ~50-nm thick (Bi1-xSbx)2Te3; x = 0 -1, films (hereafter referred as BST) on a semi-insulating GaAs (111)A substrate by molecular beam epitaxy. We change x by changing flux ratio of Bi and Sb during the growth. The change in x and hence position of Fermi level is reflected in Hall measurements, which show n-type conduction for Bi2Te3 and changes to p-type when x increases above 0.56. The temperature dependence of sheet resistance Rsheet shows increase in Rsheet with decreasing temperature T, most probably due to the freezing of bulk carriers. Rsheet almost saturates or slightly decreases below ~50 K, suggesting that the metallic conduction at low temperatures is dominated by the surface state [3]. We deposit a 20-nm thick permalloy (Py) film on BSTs, and measure ferromagnetic resonance (FMR) spectra as a function of T by sweeping an external in-plane magnetic field. The resonance fields of Py/BSTs are almost the same as that of Py directly deposited on a substrate, while the spectral linewidths are larger than that of Py, indicating the presence of spin pumping in the Py/BST bilayers. The linewidths of Py/BST start to increase rapidly below ~40 K, at which Rsheet starts to saturate, while that of Py around ~40 K decreases with decreasing T without any anomaly. The result indicates that the T- dependent linewidth broadening of Py/BST is related to the surface state of BST.

The work was supported in part by a Grant-in-Aid from MEXT (#26103002).



[1] M. Z. Hasan and C. L. Kane, Rev. Mod. Phys. 82, 3045 (2010).

[2] Jamali et al., Nano Lett. 15, 7126 (2015).

[3] Y. Shiomi et al., Phys. Rev. Lett. 113, 196601 (2014).

In this work, we present an experimental study of thermal spin injection using spin Seebeck tunneling in p-Si from Ni₈₀Fe₂₀ and observation of enhanced negative magnetoresistance in Ni₈₀Fe₂₀ (50 nm)/p-Si (350 nm) bilayer. The resistance measurement shows a reduction of 2.5% for an out-of-plane applied magnetic field of 3T and 0.65% for the transverse in-plane magnetic field. The measurement is carried out using a current density of ~2 x 10² A/cm². The observed change in resistance is higher than the change observed in Ni₈₀Fe₂₀-only thin film. In the Ni₈₀Fe₂₀-only film this behavior is attributed to anisotropic magnetoresistance (AMR). We propose that spin tunneling induced across the Ni₈₀Fe₂₀ and p-Si interface by spin-Seebeck effect (SSE) and spin-Hall effect (SHE) is the underlying cause of the enhanced AMR observed found in the bilayer. . The spin polarization (which is necessary for SHE) of the p-Si layer is confirmed from the pseudo spin-valve behavior observed in the transverse magnetic field dependent measurement of resistance. Magnetic field rotation in direction normal to electric current provide further evidence of both SHE and SSE. We propose that the spin polarization leads to a decrease in hole-phonon scattering due to spin-phonon coupling, resulting in enhanced negative magnetoresistance.

Development of thermoelectric energy harvesting devices has hit an intractable barrier in the strong correlation between the material properties of electrical and thermal conductivity. However, this field can still be further improved by employing devices that depend on a spin-mediated conversion process. When a temperature gradient is applied across a ferromagnetic insulator, a spin current is generated by the Spin Seebeck Effect (SSE), and this current can be converted to a voltage in adjacent conductive films by the Inverse Spin Hall Effect (ISHE). Thus, the thermal energy conversion and charge production steps can be separated into distinct regions of a spin-based thermoelectric device, allowing separate tuning of electrical and thermal conductivities. Here we focus on the second step of this process, ISHE, the strength of which is quantified by a material’s spin hall angle. Platinum has become the standard for ISHE applications due to its relatively high spin hall angle and electrical conductivity. However, we report a better candidate, β-Tantalum, which shows a spin Seebeck voltage approximately 4 times higher than that of Pt at room temperature. Additionally, the sign of the voltage in Ta is opposite to that of Pt, resulting in a voltage with opposite sign under the same conditions. We highlight the importance of this fact in for fabricating spin thermoelectric devices.

The Rashba spin-split effect in non-magnetic semiconductors is due to inversion asymmetry and spin-orbital interaction, which brings about new opportunities for spintronics and other fields of physics and materials science. Typical thermoelectric materials are also narrow-gap semiconductors, converting heat to electricity or vice versa. The connection between spin quantum effect and electrical/thermal transport is studied in the Rashba thermoelectric material BiTeI. Low-dimensional transport phenomena exist in spin-split quantum wells and bulk systems. Because of the unique Fermi topology induced by band shift, the density of states becomes two- and one-dimensional in BiTeI quantum well and bulk material, respectively. The thermopower and electrical properties are thus greatly enhanced, with a factor of up to 2 in electrical term. Besides the charge transport, unconventional phonon transport is also found in bulk BiTeI. The strong dipole field is beneficial to lattice anharmonicity and low thermal conductivity. Through further engineering the carrier scattering and band gap, thermoelectric properties in bulk BiTeI are reasonably enhanced. The unusual electrical and thermal transport may make the Rashba spin-split materials excellent candidates for thermoelectric applications.

References:
1. Wu, L.; Yang, J.; Wang, S.; Wei, P.; Yang, J.; Zhang, W.; Chen, L., Two-dimensional thermoelectrics with Rashba spin-split bands in bulk BiTeI. Phys. Rev. B 2014, 90 (19), 195210.
2. Wu, L.; Yang, J.; Wang, S.; Wei, P.; Yang, J.; Zhang, W.; Chen, L., Thermopower enhancement in quantum wells with the Rashba effect. Appl. Phys. Lett. 2014, 105 (20), 202115.
3. Wu, L.; Yang, J.; Chi, M.; Wang, S.; Wei, P.; Zhang, W.; Chen, L.; Yang, J., Enhanced Thermoelectric Performance in Cu-Intercalated BiTeI by Compensation Weakening Induced Mobility Improvement. Sci. Rep. 2015, 5, 14319.
4. Wu, L.; Yang, J.; Zhang, T.; Wang, S.; Wei, P.; Zhang, W.; Chen, L.; Yang, J., Enhanced thermoelectric performance in the Rashba semiconductor BiTeI through band gap engineering. J. Phys.: Condens. Matter. 2016, 28 (8), 085801.
5. Xi L, Yang J, Wu L, Yang J, Zhang W, Band Engineering and Rational Design of High Performance Thermoelectric Materials from First-Principles, J. Materiomics (2016), doi:10.1016/j.jmat.2016.05.004.
6. Wei, P.; Yang, J.; Guo, L.; Wang, S.; Wu, L.; Xu, X.; Zhao, W.; Zhang, Q.; Zhang, W.; Dresselhaus, M. S., Minimum Thermal Conductivity in Weak Topological Insulators with Bismuth-Based Stack Structure. Adv. Funct. Mater. 2016, DOI: 10.1002/adfm.201600718.

Electron spins in semiconductors holds promise for encoding and manipulating quantum information. The weak coupling of the spin degree of freedom to its environment provides spin qubits exceedingly long storage times. However, while storage is in the spin degrees of freedom, controlling spin qubits often involves manipulating the charge degree of freedom of the participating electrons which is far more susceptible to decoherence due to charge noise. In this talk I will discuss the advantages of operating a two electron singlet-triplet qubit in the rotating frame as a way to mitigate some of the unwanted effects of charge noise. We will demonstrate a new readout scheme for these qubits which is needed for operating in the rotating frame as well as single and two qubit gates with improved performance compared to previously reported results in GaAs spin qubits.

Spin qubits in silicon are excellent candidates for scalable quantum information processing [1] due to their long coherence times and the enormous investment in silicon CMOS technology. While our Australian effort in Si QC has largely focused on spin qubits based upon phosphorus dopant atoms implanted in Si [2,3], we are also exploring spin qubits based on single electrons confined in SiMOS quantum dots [4]. Such qubits can have long spin lifetimes T1 = 2 s, while electric field tuning of the conduction-band valley splitting removes problems due to spin-valley mixing [5]. In isotopically enriched Si-28 these SiMOS qubits have a control fidelity of 99.6% [6], consistent with that required for fault-tolerant QC. By gate-voltage tuning the electron g*-factor, the ESR operation frequency can be Stark shifted by > 10 MHz [6], allowing individual addressability of many qubits. Most recently we have coupled two SiMOS qubits to realize CNOT gates [7] for which over 25 gates can be performed within a two-qubit coherence time of 8 μs. I will conclude by discussing the prospects of scalability of this technology using traditional CMOS manufacturing.

[1] D.D. Awschalom et al., “Quantum Spintronics”, Science 339, 1174 (2013).
[2] J.J. Pla et al., “A single-atom electron spin qubit in silicon”, Nature 489, 541 (2012).
[3] J.T. Muhonen et al., “Storing quantum information for 30 seconds in a nanoelectronic device”, Nature Nanotechnology 9, 986 (2014).
[4] S.J. Angus et al., “Gate-defined quantum dots in intrinsic silicon”, Nano Lett. 7, 2051 (2007).
[5] C.H. Yang et al., “Spin-valley lifetimes in a silicon quantum dot with tunable valley splitting”, Nature Comm. 4, 2069 (2013).
[6] M. Veldhorst et al., “An addressable quantum dot qubit with fault-tolerant control fidelity”, Nature Nanotechnology 9, 981 (2014).
[7] M. Veldhorst et al., “A two-qubit logic gate in silicon”, Nature 526, 410 (2015).


Acknowledgments: We acknowledge support from the Australian Research Council (CE11E0001017), the US Army Research Office (W911NF-13-1-0024) and the NSW Node of the Australian National Fabrication Facility.

Interest in atomically-thin transition metal dichalcogenide (TMD) semiconductors such as MoS₂ has exploded in the last few years, driven by the new physics of coupled spin/valley degrees of freedom and their potential for new spintronic and ‘valleytronic’ devices. Although robust spin and valley degrees of freedom have been inferred from polarized photoluminescence studies of excitons, photoluminescence timescales are necessarily constrained by short-lived (3–30 ps) recombination of excitons. Direct probes of spin & valley dynamics of the resident carriers in electron- (or hole-) doped TMDs, which may persist long after recombination ceases, are still at an early stage.

In this work, we directly measure the coupled spin-valley dynamics of the resident electrons in n-type monolayer MoS₂ using time-resolved Kerr rotation [1], and reveal very long spin lifetimes exceeding 3 ns at 5K -- orders of magnitude longer than typical exciton lifetimes (see Figure). In contrast with conventional III-V or II-VI semiconductors, spin relaxation accelerates rapidly in small transverse magnetic fields B_y. This indicates a novel mechanism of electron spin dephasing in monolayer TMDs that is driven by rapidly-fluctuating internal spin-orbit fields that, in turn, are due to fast electron scattering between the K and K’ conduction bands [1].

Additionally, a small but surprisingly long-lived oscillatory signal is also observed (see Figure), indicating the spin coherence of a small population of localized states [2]. These coherence signals are observed in a variety of samples and are studied as a function of applied field and temperature. Related spin coherence and spin relaxation phenomena have also been observed recently in other monolayer TMDs such as MoSe₂, WS₂, and WSe₂. These studies provide direct insight into the physics underpinning the spin and valley dynamics of electrons in the new monolayer TMD semiconductors.
[1] Luyi Yang, N. Sinitsyn, W. Chen, J. Yuan, J. Zhang, Jun Lou & S. A. Crooker, Nature Physics 11, 830 (2015).
[2] Luyi Yang, W. Chen, K. M. McCreary, Berend T. Jonker, Jun Lou & S. A. Crooker, Nano Letters 15, 8250 (2015).

Collective spin excitations such as spin plasmons and spin waves in quantum wells are potential candidates for information transfer within spintronics e.g. for the implementation of spin-routing devices. However their dynamics and coherence is shaped and affected by the interplay between many-body Coulomb interactions, spin-orbit coupling (SOC) and dissipative effects. There are many possible sources of dissipation: some of them can be controlled, for example by reducing disorder, whereas others are intrinsic and hence unavoidable. One such relaxation mechanism is the spin Coulomb drag (SCD), which occurs when two different spin populations move with different momentum while interacting via Coulomb scattering. The exchange of momentum leads to a drag force between the two spin populations which causes a decay—and eventually a halting—of spin currents. On the other hand, SCD favors coherent transport of spin packets in semiconductors by strongly decreasing the spin-diffusion coefficients with respect to the charge ones, while maintaining the high electron mobility. The standard phenomenological way of introducing the SCD is by considering a homogeneous system in which two populations of spin-up and spin- down electrons move with different velocity, The total SCD dissipation or power loss in an inhomogeneous system can then be obtained using a local approximation. However, such a description becomes questionable when considering nanoscale systems, where interfaces, quantum confinement, or local doping can lead to strong inhomogeneities. Here we discuss a formalism for the SCD valid in the general inhomogeneous case [1], and asses its interplay with SOC and extrinsic dissipation [2]. This nonlocal formulation of SCD is successfully applied to linewidths of intersubband spin plasmons in semiconductor quantum wells, where experiments have shown that the local approximation fails [3]. In view of the immense popularity of local approximations in condensed matter physics and other areas of science, these findings have important general implications for developing new ways for treating dynamical many-body effects in nanoscale systems.

[1] I. D’Amico and C. A. Ullrich, PHYSICAL REVIEW B 88, 155324 (2013)
[2] I. D’Amico and C. A. Ullrich, preprint (2016)
[3] F. Baboux, F. Perez, C. A. Ullrich, I. D’Amico, J. Gomez, and M. Bernard, Phys. Rev. Lett. 109, 166401 (2012).

Spin-orbit interaction can give rise to topologically nontrivial energy bands in solids, but a purely orbital mechanism can also be realized without breaking the spin degeneracies. Circular photogalvanic effect (CPGE) is the generation of photocurrent where the polarity and magnitude of the photocurrent depends on the chirality of the pump light. In this work we studied the surface state properties of Si, a material with inversion symmetry and negligible spin-orbit coupling. CPGE, which is absent in bulk Si is found to arise from interband transitions only at the metal-semiconductor contacts to Si nanowires where inversion symmetry is broken by a Schottky electric field (1). Furthermore, by applying a bias voltage that modulates this field, the sign and magnitude of the CPGE can be controlled. From excitation energy dependent measurements and symmetry considerations, it is argued that the [1-10] surface states due to zig-zag Si chains on the surface that are not aligned with the nanowire growth direction and the Schottky field produce an artificial gyrotropic optical medium that supports CPGE. This work reveals the role of the surface states in the generation of chirality-dependent photocurrents in silicon with a purely orbital-based mechanism, and also opens up new possibilities of engineering new functionalities in Si that can be integrated with conventional electronics.
We will also discuss our work on Bi₂Se₃ nanobelts, a topological insulator to probe its unique surface states. Our work shows that CPGE can be an important tool to probe the spin, angular momentum or valley physics associated with surface states, either in materials with large spin-orbit coupling as in topological insulator Bi₂Se₃, or, engineered artificially in a trivial insulator like Si.

Reference:
1. Dhara, S., et al. (2015). "Voltage-tunable circular photogalvanic effect in silicon nanowires." Science 349(6249): 726-729.

Weyl semimetals exhibit linearly crossing electronic bulk bands, which host quasi-relativistic Weyl fermions. The according band crossing points always come in pairs of opposite chirality, giving the handedness of the Weyl fermions. Weyl semimetals are ideal for probing the quantum mechanical violation of the chiral symmetry, called chiral anomaly, an effect predicted by quantum field theory that has first been observed in high energy particle physics.¹ In solid state matter, chiral anomaly can be demonstrated in Dirac and Weyl metals through negative magnetoresistance (NMR) when electric and magnetic fields are aligned in parallel.² Recently, it has been revealed that NbP is a Weyl semimetal.³

In this study, we perform magnetotransport measurements on a Ga-doped NbP microribbon (49 μm x 2.5 μm x 530 nm) with transport direction along the [001] axis. Our work was motivated by a study of Klotz et al.,⁴ who proposed electron doping of NbP, to move the Fermi level closer than E_F = +5 meV (intrinsic Level) to the Weyl point in order to realize NMR in NbP.

For a magnetic field configuration perpendicular to the electric current direction we observed a quadratic low-field and a linear high-field magnetoresistance (MR). Shubnikov-de-Haas (SdH) oscillations were seen up to 50 K. Our analysis revealed that the Fermi level of the microribbon is located as close as 5 meV below E_F and our longitudinal MR measurements showed an unsaturated NMR up to room temperature which is in the same order of magnitude as the transverse, positive MR. We attribute this observation to chiral anomaly. The NMR follows the expected quadratic dependence at low magnetic fields and reaches the linear quantum limit in the high field regime.

1. Burkov, A. Chiral anomaly without relativity. Science 350, 378–379 (2015).
2. Nielsen, H. B. & Ninomiya, M. The Adler-Bell-Jackiw anomaly and Weyl fermions in a crystal. Phys. Lett. B 130, 389–396 (1983).
3. Xu, D. F. et al. Observation of Fermi Arcs in Non-Centrosymmetric Weyl Semi-Metal Candidate NbP. Chinese Phys. Lett. 32, 107101 (2015).
4. Klotz, J. et al. Quantum oscillations and the Fermi surface topology of the Weyl semimetal NbP. Phys. Rev. B 93, 121105 (2016).