Symposium NM01—Superconductors as Quantum Materials

We report on epitaxial synthesis, excellent p-type transparent conductivity, and two-dimensional superconductivity of the layered lithium niobate [1]. The superconducting films were prepared through the following three steps. Amorphous films of the stoichiometric LiNbO₂ were first deposited on (111) spinel substrates, and then heated to high temperatures for obtaining highly crystalline epitaxial films. A fraction of Li ions were removed from the films by immersing in an iodine solution so that the equivalent amount of hole carriers were introduced. The obtained films showed superconductivity below 4.2 K, and averaged transmittance to the visible light of 77% despite large number of hole carriers. Moreover, we found that the normal-state conductivity and the transmittance simultaneously increased with increasing hole carriers. This behaviour is unusual because higher conductivity usually leads to larger free-carrier absorption. We explained it in terms of strongly correlated electrons at the largely isolated Nb 4d_z2 band. The two-dimensional superconductivity in Li_1−xNbO₂ will be briefly discussed in terms of the observed BKT transition and the electronic phase diagram that was established by electrochemical doping experiments [2].
[1] T. Soma, K. Yoshimatsu, A. Ohtomo, Sci. Adv., 6, eabb8570 (2020).
[2] K. Yoshimatsu, M. Niwa, H. Mashiko, T. Oshima, A. Ohtomo, Sci. Rep., 5, 16325 (2015).

The two-dimensional CuO2 plane plays a fundamental role in the physics of cuprates. Indeed, cuprates (and all other families of high temperature superconductors) adopt a layered atomic structure. In BSCCO, the weak van der Waals interaction between adjacent bismuth-oxygen layers makes the crystal easily cleavable, making atomically-thin BSCCO an ideal system for investigating high temperature superconductivity in the two-dimensional limit. By fabricating samples in an inert atmosphere, we are able to obtain half-unit-cell-thick single crystals (referred to as monolayer) of BSCCO samples and to probe the evolution of superconductivity as the dimensionality is reduced. We probe the electronic structure of monolayer Bi2Sr2CaCu2O8+δ (Bi2212) and Bi2Sr2CuO6+δ (Bi2201) with electronic transport measurements, as well as scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) techniques.

The heavy-fermion superconductor CeCoIn₅ has a pairing symmetry of d_x²_-y² but the pairing mechanism remains unknown. Our planer tunneling spectroscopy measurements on single crystals into the three major crystallographic orientations as a function of temperature down to 20 mK and fields up to 18 T are reproducible, and our diagnostics show transport across the barrier is predominately single-step elastic tunneling. [1] At low temperatures, on the (100) and (001) faces, we find sharp coherence peaks with an estimated gap of 0.65 meV, and on the (110) face, a broad zero-bias peak. Previously published STM work reports observing pair potentials above T_c = 2.3 K [2]; we also find pre-formed pairs up to T_p ~ 5 K. Below T_p, with increasing applied field, the pairing gap evolves smoothly to a field-induced gap, that grows linearly up to 18 T, the highest field measured. At higher temperatures, no field-induced gap is observed, showing that pairing and the field-induced gap exist concomitantly. Kondo scattering plays a role in the pairing mechanism, and omposite pairing remains a possible explanation. [3]


[1] K. Shrestha, S. Zhang, L.H. Greene, J.D. Thompson, Y. Lai, R.E. Baumbach, K. Sasmal, M. B. Maple, and W.K. Park, “Spectroscopic Evidence for the Direct Involvement of Local Moments in the Pairing Process of the Heavy-Fermion Superconductor CeCoIn₅” Submitted.
[2] Ernst et al Phys. Stat, Sol B (2010); Fasano et al Physical B (2018).
[3] Flint and Coleman PRL (2010); Flint, Dzero, Coleman NatPhys (2008)

This work was supported by NSF/DMR-1704712 (FSU), NSF/DMR-1157490 (FSU) NSF/DMR-1644779 and the State of Florida (NHMFL), DOE, Office of BES, and Division of MSE (LANL), and NSF/DMR-1810310 (UCSD).

Using THz electromagnetic cavities to probe and control correlated phases of matter has recently seen a surge in activity, in large part due to remarkable advances in experimental techniques. Some of the most interesting compounds from this point of view are the cuprate high-temperature superconductors and their Mott-insulating parent compounds, which often exhibit long-range antiferromagnetic order. In this work, we present a mechanism by which the electromagnetic field of such a THz cavity can be coherently coupled to the quantum fluctuations of this antiferromagnetic order in a simplified model of an undoped cuprate. In addition to the direct magnetic-dipole interaction between the electromagnetic field and the local moments, we also consider a more elaborate coupling which exploits spin-orbit effects to produce coupling between the antisymmetric spin-exchange interaction and the electric field. Within linear spin-wave theory, we show how these terms can lead to the formation of Neel magnon-polariton resonances, as well as allow for optical parametric driving of spin-waves in the insulating phase. We identify clear experimental signatures to look for and argue these are within experimental reach. Finally, we conclude by speculating on what happens once the system is hole-doped with charge carriers, in light of our outlined coupling scheme.

J.B.C. acknowledges support from the Harvard Quantum Initiative (HQI).

Doping is one of the most relevant paths to tune the functionality of cuprates, it determines carrier density and the overall physical properties of these impressive superconducting materials. We present an oxygen doping study of YBa₂Cu₃O_7-d (YBCO) thin films from underdoped to overdoped state, correlating the measured charge carrier density, n_H, the hole doping, p, and the critical current density, j_c. Our results show a continuous increase of j_c with charge carrier density, reaching 90 MA/cm² at 5 K for p-doping at the Quantum Critical Point (QCP), linked to an increase of the superconducting condensation energy. The ultra-high j_c achieved corresponds to a third of the depairing current, i.e. a value 60% higher than ever reported in YBCO films. The overdoped regime is characterized by a sudden increase of n_H, associated to the reconstruction of the Fermi-surface at the QCP. Pulsed laser and chemical solution deposited films have been studied. Overdoping YBCO, therefore, opens a promising route to extend the current carrying capabilities of REBCO coated conductors for applications.

Motivated by the existence of superconductivity in pyrite CuS₂ [1], we explore the possibility of ionic-liquid-gating-induced superconductivity in the related antiferromagnetic Mott insulator pyrite NiS₂. A clear gating-induced insulator-metal transition is observed, with progressively decreasing low temperature sheet resistance with increasing positive gate bias. We deduce an electrochemical gating mechanism through formation of S vacancies, however, which is entirely non-volatile and irreversible. This is in striking contrast to the completely reversible, volatile electrolyte-gating-induced insulator-metal transition in pyrite FeS₂ [2]. We attribute the difference in behaviors to the much larger S diffusion coefficient in NiS₂ compared to FeS₂ [3,4], analogous to the situation in electrolyte-gated oxides [5].

[1] H. S. Jarrett et al, Phys. Rev. Lett., 21, 617 (1968)
[2] J. Walter, B. Voigt, E. Day-Roberts, K. Heltemes, R. M. Fernandes, T. Birol, and C. Leighton, Sci. Adv. 6, eabb7721 (2020)
[3] C. Clark and F. Friedemann, J. Mag. Mag. Mat. 400, 56 (2016)
[4] E. B. Watson, D. J. Cherniak, and E. A. Frank, Geochim. Cosmochim. Acta, 73, 4792 (2009)
[5] H. Wang, J. Walter, K. Ganguly, B. Yu, G. Yu, Z. Zhang, H. Zhou, H. Fu, M. Greven and C. Leighton, Phys. Rev. Mater. 3, 075001 (2019)

Funding: This work was supported primarily by the National Science Foundation through the University of Minnesota MRSEC under Award Number DMR-2011401.

Achieving higher transition temperature (T_c) is a primary goal in superconductivity research. T_c and doping have been found to have a dome-like universal relation where the peak position is the maximum T_c (T_c-max), which is consistent with previous experimental results in the lower pressure range. By using our newly developed ultra-sensitive magnetization measurement technique under high pressure, we have discovered a universal resurgence of T_c passing the peak predicted by the general T_c-p (doping) or -P (pressure) relation for the BSCCO cuprate high temperature superconductors and have attributed the resurgence to a pressure-induced electronic transition, which is supported qualitatively by our density functional theory calculations. This offers a new path to raise the T_c of the layered cuprate high temperature superconductors to a new height.

We have investigated the bulk superconducting state via dc magnetization measurements by deploying our newly developed HP-DAC/MPMS technique. We discovered a common resurgence of the T_cs of the monolayer Bi₂Sr₂CuO_6+δ (Bi2201) and the bilayer Bi₂Sr₂CaCu₂O_8+δ (Bi2212) to beyond the respective T_c-maxs predicted by the universal relation between T_c and p or P at higher pressures. The T_c of underdoped Bi2201 initially increases from 9.6 K at ambient to a peak at ~ 23 K at ~ 26 GPa and then drops as expected based on the universal T_c-P relation. However, at pressures above ~ 40 GPa, T_c rises rapidly without any sign of saturation up to ~ 30 K at ~ 51 GPa. Similarly, the T_c for the slightly overdoped Bi2212 increases after passing a broad valley at 20-36 GPa and reaches ~ 90 K without any sign of saturation at ~ 56 GPa. We have therefore attributed this T_c-resurgence to a possible pressure-induced electronic transition in the cuprate compounds due to a charge transfer between the Cu 3d_x²-y² and the O 2p bands projected from a hybrid bonding state, leading to an increase in the density of states at the Fermi level, in agreement with our density functional theory calculations. Similar T_c-P behavior has also been reported previously in the trilayer Bi₂Sr₂Ca₂Cu₃O_10+δ (Bi2223). These observations suggest that higher T_cs than those previously reported for the layered cuprate high temperature superconductors can be achieved by breaking away from the universal T_c-P relation through possible electronic transitions induced by the application of higher pressures.

The work at Houston is supported in part by US Air Force Office of Scientific Research Grants FA9550-15-1-0236 and FA9550-20-1-0068, the T. L. L. Temple Foundation, the John J. and Rebecca Moores Endowment, and the State of Texas through the Texas Center for Superconductivity at the University of Houston.

Realizing superconductivity in the vicinity of room temperature in hydrogen-rich materials under pressure is a topic of great current interest. Specifically, high-pressure experiments motivated by density-functional theory and conventional electron-phonon coupling models have uncovered new classes of hydrogen-rich metal hydrides, or superhydrides, with superconducting critical temperatures (Tc) in the vicinity of room-temperature at megabar pressures (i.e., >100 GPa). Original calculations for the rare-earth hydrides predicted that LaH10 and YH10 would form dense hydride clathrate structures exhibiting Tc’s of 257-326 K at pressures of 200-300 GPa.1,2 X-ray diffraction experiments on the La-H system confirmed the formation and stability of the LaH10 structure near the predicted pressures,3 and subsequent electrical conductivity and critical current measurements confirmed the very high-temperature superconductivity of the phase.4,5 Experiments that used ammonia borane (NH3BH3) as the hydrogen source indicated Tc’s beginning at 260 K, including conductivity onsets as high as 290 K that have been confirmed in more recent work.6 It was proposed that the high and variable Tc arises from incorporation of N and/or B in the structure from the ammonia borane starting material.5,6 Moreover, in experiments that confirmed the structure7 and high-temperature superconductivity5,6 of LaH10, Drozdov et al.8 reported a slightly lower maximum Tc of 250 K in experiments conducted without ammonia borane. Using methods developed previously and applied to H3S, B and N doping of the La-based superhydride increases the Tc of the material to room temperature. These techniques were also used to examine the doping of C on the superconductivity of H3S. As found for the La-based superconductor, low-level substitution of C for S can fine-tune the Fermi energy to match the peak in the electronic density-of-states peak, thereby maximizing the electron-phonon coupling and boosting the critical temperature from the original 203 K to 289 K at 260 GPa for 4% doping of C for S in H3S.9 The results provide an explanation for the recent experimental observation of room-temperature superconductivity in a highly compressed C-S-H mixure.10 The above findings open new avenues for creating ‘hot’ hydrogen-rich superconductors with Tc‘s above room temperature.
* Work done in collaboration with M. Ahart, R. Kumar, M. Somayazulu, H. Liu, N. Salke, I. I. Naumov, R. Hoffmann, N. W. Ashcroft, Y. Meng, M. Baldini, Z. M. Geballe, S. W. Tozer, A. D. Grockowiak, E. Zurek, F. Zhang, Y. Ge, R. Dias, Y. Yao, and H. Wang. This research was supported by NSF (DMR-1933622) and DOE (DE-SC0020340 and DE-NA0003975).
1 H. Liu et al., Proc. Natl. Acad. Sci. 114, 6990 (2017).
2 F. Peng et al., Phys. Rev. Lett. 119, 107001 (2017).
3 Z. M. Geballe et al.. Angew. Chem. Inter. Ed. 57, 688 (2018).
4 M. Somayazulu et al., Phys. Rev. Lett. 122, 027001 (2019).
5 R. J. Hemley, M. Ahart, H. Liu, & M. Somayazulu, Road to Room Temperature Superconductivity (Areces, Madrid, Spain, 2018) p. 199.
6 A. D. Grockowiak et al., arXiv: 2006.03004.
7 H. Liu et al., Phys. Rev. B. 98, 100102 (2018).
8 A. P. Drozdov et al., Nature 569, 528 (2019).
9 Y. Ge, F. Zhang, R. Dias, R. J. Hemley & Y. Yao, arXiv: 2011.12891.
10 E. Snider et al., Nature 586, 373 (2020).

Superconducting iron chalcogenides FeSe_xTe_1−x (FST) have attracted a great deal of interest in both fundamental physics and potential applications. While very high upper critical fields make FST an excellent candidate for high-field magnets and energy applications, Majorana zero-modes identified in FST superconductors hold promise for topological quantum computing. In this talk, we discuss multiple topological states and possible broken time reversal symmetry in FST, and their impacts to charge transport, which we studied in both FST single crystals and thin films having a broad range of superconducting transition temperature driven by strain or gating.

Electronic correlation is of fundamental importance to high temperature superconductivity. Iron-based superconductors are believed to possess moderate correlation strength, which combined with their multi-orbital nature makes them a fascinating platform for the emergence of exotic phenomena. Previously, it has been reported that iron-chalcogenide superconductors exhibit strong orbital-dependent correlation effects and that by raising temperature they crossover into an orbital-selective Mott phase. Here, we report spectroscopic evidence of the reorganization of the Fermi surface from FeSe to FeTe as Se is substituted by Te. This evolution is observed to be accompanied by a redistribution of the orbital-dependent spectral weight near the Fermi level together with a divergent behavior of a band renormalization in the d_xy orbital. All of our observations are further supported by our theoretical calculations to be salient spectroscopic signatures of such a non-thermal evolution from a strongly correlated metallic phase towards an orbital-selective Mott phase in FeTe_1-xSe_x as Se concentration is reduced.

Nematicity is a many-body quantum phase that breaks rotational symmetry while preserving translational symmetry. It has been discovered in the phase diagrams of both iron-based and cuprate high-temperature superconductors [1-3]. In the iron-based superconductors, the optimal doping of superconducting phase coincides with the putative nematic quantum critical point, suggesting the possibility of superconducting pairing enhanced by nematic quantum fluctuations [2,4]. However, a comprehensive study of the nematic fluctuations is still absent in the BaFe₂(As_1-xP_x)₂, which exhibits the cleanest signature of quantum criticality among all the iron-based superconductors. In this talk, I will present a detailed study of the nematic fluctuations of BaFe₂(As_1-xP_x)₂ using the elastoresistivity measurement. I will also present the study of strain dependence of superconducting Tc, which shed light on the intricate relations between nematicity and superconductivity.

An (SNS) junction with two superconductors coupled by a normal metal hosts Andreev bound states whose energy spectrum is phase-dependent and exhibits a minigap, resulting in a periodic supercurrent. However, phase-dependent dissipation also appears due to finite-time relaxation of Andreev bound states, providing an ultra-sensitive tool for the carrier dynamics.
In the weakly driven regime, we have measured dissipation in a phase-biased junction built around a bismuth nanowire, a second order topological insulator which was previously shown to host one-dimensional ballistic edge states. We have found absorption peaks at the Andreev level crossings in accordance with predictions of telltale signs of topological junctions [1,2].
I will also present our most recent results on SNS junctions strongly driven out of equilibrium by microwave field where novel physics emerges due to redistribution of quasiparticles, whose understanding is still incomplete. Strong driving power can significantly modify the distribution function from thermal equilibrium, activating additional transitions. We have measured the evolution of both the supercurrent and dissipation, extracted simultaneously from the ac magnetic susceptibility of a phase-biased graphene-superconductor ring driven out of equilibrium by microwave irradiation [3]. Increasing the frequency beyond the system relaxation rate has several measurable consequences: Firstly, the phase dependence of supercurrent is modified. In particular, the Fourier coefficients dependence on power deviates from the Bessel function expected in the adiabatic ac Josephson effect in qualitative agreement with time-dependent Usadel equations. Secondly and more interestingly, the dissipation with irradiation frequency higher than the minigap shows a distinct enhancement around j = 0 where the minigap is the largest and the dissipation is very weak in equilibrium. Using Kubo formalism, we explain this effect by the nonequilibrium distribution function with partial occupation above the minigap permitting transitions between levels on the same side of the minigap (intraband).
More generally, our results demonstrate that phase dependent dissipation measurement allows a much deeper understanding of the relaxation mechanisms than supercurrent in superconducting junctions.

[1] Murani et al, Phys. Rev. Lett. 122, 076802 (2019)
[2] Fu and Kane, Phys. Rev. B 79, 161408(R) (2009)
[2] Dou et al, arXiv:2011.07308

Topological transition transforms common superconductivity into an exotic phase of matter, which holds promise for fault-tolerant quantum computing [1]. A hallmark of this topological transition is the emergence of Majorana bound states (MBS), quasiparticles that nonlocally store a single electron. Their non-Abelian exchange statistics allows for the implementation of quantum gates through braiding operations [1]. While most of the MBS proposals were focused on 1D systems [2], their geometry has inherent difficulties for technological implementation and realizing braiding. MBS detection typically relies on the measurement of zero-bias conductance peak [2,3] that could also arise in topologically-trivial systems. To overcome these challenges, we propose 2D platforms, which could support scalable MBS and their braiding. First we discuss how spin valves, common in spintronics, can be used through their fringing fields [4,5] to proximitize two-dimensional electron gas to implement MBS and their braiding [6,7]. Next we show our measurements of topological superconductivity in epitaxial Al/InAs Josephson junctions [8]. The closing and reopening of the superconducting gap with increasing magnetic field is simultaneously accompanied by the measurement of p-jump in the superconducting phase [8]. Remarkably, this topological transition can be controlled by gate voltage. We propose X-shaped topological junctions [9] in which the observed topological transition can be used to realize scalable MBS and their braiding.
[1] D. Aasen et al., Phys. Rev. X 6, 031016 (2016).
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[3] K. Sengupta, I. Zutic et al., Phys. Rev. B 63, 144531 (2001).
[4] T. Zhou et al., Phys. Rev. B 99, 134505 (2019).
[5] N. Mohanta et al., Phys. Rev. Appl. 12, 034048 (2019).
[6] G. Fatin et al., Phys. Rev. Lett. 117, 077002 (2016).
[7] A. Matos-Abiague et al., Solid State Commun. 262, 1 (2017)
[8] W. Mayer et al., arXiv:1906.01179, preprint.
[9] T. Zhou et al., Phys. Rev. Lett. 124, 137001 (2020)

The proximity effect from superconductors is . Inducing superconductivity on topological insulator surface states through proximity effect is an attractive approach to realize topological superconductivity, and provides great freedom in potential device engineering. Here we aim to look at the coupling between the opposite surfaces of a topological insulator thin film. Using high quality BiSbTe thin films as the tunnel barrier and MgB2 to induce superconductivity, the top and bottom surfaces of the same topological insulator can readily couple between themselves. Other than Josephson coupling between bulk superconductors, one can also identify signatures from the coupling between surface zero-modes, while other modes are largely suppressed due to the opposite spin textures. Interestingly, the edges of the micro sized pillars offer another channel of conductance across the junction. When unfolded, the device is practically equivalent to an in-plane junction with a 1D channel between two topological superconductors and a periodic boundary condition. This circular 1D edge channel can potentially become a host for Majorana-like edge modes on vertical pillar structures.

Advances in nanofabrication has opened new venues for controlling vortex matter, which is responsible for the electro-magnetic response of all applied superconductors. For example, nano-hole and blind-hole structures with innovative patterns have emerged as a versatile platform for controlling and optimizing vortex pinning and dynamics in superconductors for enhanced critical current. Magnetic field pinning of vortices with nanoscale structures has also shown great potential for in-situ manipulation of vortex behaviour. Moreover, magnetic pinning is relatively temperature independent compared with core pinning. I will present recent highlights of our work on tailoring vortex dynamics with magnetic texturing. Namely, we use nano-magnetic patterned structures based on spin-ice rules to explore the effect of pinning, dynamic rectification and geometric frustration in a flux quanta system. In addition, we demonstrate that magnetic pinning can be used to ‘pin’ vortices in the liquid state in high temperature superconductors.
This work was supported by the U.S. Department of Energy, Office of Science, Materials Sciences and Engineering Division.

Superconductivity is a phenomenon that allows for lossless electrical transport and exceptional precision in voltage due to flux quantization, while the quantum Hall effect (QHE) is a paragon of topological protection in electronic states that allows for part-per-billion precision in resistance. Creating seamless heterostructures harboring these two unique electronic phases is highly desirable for the discovery new physics as well as the application of these electronic phases to topological quantum computing. Unfortunately, the quantum Hall state and the superconducting state are typically thought to be incompatible because the same magnetic field that creates the quantum Hall state destroys the superconducting state. In this work, we show that by using growth advances to develop a heterostructure of superconducting NbN and a GaN two-dimensional electron gas (2DEG), the two electronic phases can be seen concurrently in the same structure.

The epitaxial nitride structure begins with a 50 nm NbN layer, which is grown on a 3-inch diameter semi-insulating 6H-SiC wafer by plasma-assisted molecular beam epitaxy (PAMBE). We then use metal organic chemical vapor deposition (MOCVD) to grow a 20 nm AlN nucleation layer, a 1.5 µm GaN buffer layer, and an AlN spacer/AlGaN barrier/GaN cap stack that defines the 2DEG. Gated Hall bar devices were fabricated by using 15 nm of Al₂O₃ as a gate insulator and Ti/Al/Ni/Au annealing for ohmic contacts to the 2DEG. Resistivity and Hall-effect measurements were taken on the gated Hall bars in magnetic fields up to 45 T and at temperatures as low as 350 mK. Over the range of gate voltages used, the 2DEG densities vary from less than 1.1 × 10¹²/cm² to 6.7 × 10¹²/cm², and the 2DEG mobilities vary from 1200 cm²/Vs to 8500 cm²/Vs at 390 mK. The GaN 2DEG was able to enter the integer QHE regime in magnetic fields as low as 15 T. We then made electrical contact to the buried NbN layer to extract the superconducting critical temperature and critical fields via resistance measurements. The T_c is 16.5 K with a sharp transition of ΔT = 0.16 K in width, and the maximum out-of-plane H_c is 17.8 T. Therefore, superconductivity and the integer QHE are concurrent in this structure for T < 1 K and 15 T < µH < 17.8 T. Analysis of these results shows that this window of temperatures and magnetic fields can be significantly expanded, suggesting that the nitride material system could potentially become an industrially viable platform for robust quantum devices based on topologically protected transport.

Quasi-one-dimensional A2Cr3As3 (with A=K, Cs, Rb) is an intriguing new family of superconductors which exhibit many similar features to the cuprate and iron-based unconventional superconductor families. Yet in contrast to these systems, no charge or magnetic ordering has been observed which could provide the electronic correlations presumed necessary for an unconventional superconducting pairing mechanism - an absence which defies predictions of first principles models. We report the results of neutron scattering experiments on polycrystalline K2Cr3As3 (Tc∼7K) which probed the low temperature dynamics near Tc . Neutron diffraction data evidence a strong response of the nuclear lattice to the onset of superconductivity while inelastic scattering reveals short-range magnetic fluctuations.These observations suggest that K2Cr3As3 is in close proximity to a magnetic instability and that the incipient magnetic order both couples strongly to the lattice and competes with superconductivity - in direct analogy with the iron-based superconductors. Further studies of charge doping of KCr3As3 via H intercalation revealed very interesting results. We show that the previously reported ethanol bath deintercalation of K2Cr3As3 to KCr3As3 has a secondary effect whereby H from the bath enters the quasi-one-dimensional Cr6As6 chains. Furthermore, we find that - contrary to previous interpretations - the difference between non-superconducting as-grown KCr3As3 samples and superconducting hydrothermally annealed samples is not a change in crystallinity but due to charge doping with the latter treatment increasing the H concentration in the CrAs tubes effectively electron-doping the 133 compound. These results suggest a new stoichiometry KHxCr3As3, that superconductivity arises from a suppressed magnetic order via a tunable parameter and paves the way for the first charge-doped phase diagram in these materials.

Ever since their discovery, superconductivity in cuprates has motivated the search for materials with analogous electronic or atomic structure. Here we present how soft chemistry approaches can be used to synthesize superconducting infinite layer nickelates from their perovskite precursor phase, using topotactic reactions [1,2]. We will present the synthesis and transport properties of the nickelate superconductors, and our current understanding of aspects of the electronic structure, including the unusual role of rare-earth hybridization [3,4]. A particular focus will be on the observation of a doping-dependent superconducting dome in (Nd,Sr)NiO₂, similar to cuprates [5]. However, while cuprate superconductivity is bounded by an insulator for underdoping and a metal for overdoping, here we observe weakly insulating behavior on either side of the dome. Furthermore, the normal state Hall coefficient is always small and proximate to a continuous zero crossing in doping and in temperature, in contrast to the ~1/x dependence observed for cuprates. This suggests the presence of both electron- and hole-like bands, consistent with band structure calculations.

[1] D. F. Li, K. Lee, B. Y. Wang, M. Osada, S. Crossley, H. R. Lee, Y. Cui, Y. Hikita, and H. Y. Hwang, Nature 572, 624 (2019).
[2] K. Lee, B. H. Goodge, D. F. Li, M. Osada, B. Y. Wang, Y. Cui, L. F. Kourkoutis, and H. Y. Hwang, APL Mater. 8, 041107 (2020).
[3] M. Hepting, D. Li, C. J. Jia, H. Lu, E. Paris, Y. Tseng, X. Feng, M. Osada, E. Been, Y. Hikita, Y.-D. Chuang, Z. Hussain, K. J. Zhou, A. Nag, M. Garcia-Fernandez, M. Rossi, H. Y. Huang, D. J. Huang, Z. X. Shen, T. Schmitt, H. Y. Hwang, B. Moritz, J. Zaanen, T. P. Devereaux, and W. S. Lee, Nat. Mater. 19, 381 (2020).
[4] B. H. Goodge, D. F. Li, M. Osada, B. Y. Wang, K. Lee, G. A. Sawatzky, H. Y. Hwang, and L. F. Kourkoutis, PNAS 118, e2007683118 (2021).
[5] D. F. Li, B. Y. Wang, K. Lee, S. P. Harvey, M. Osada, B. H. Goodge, L. F. Kourkoutis, and H. Y. Hwang, Phys. Rev. Lett. 125, 027001 (2020).

The recent discovery of superconductivity in the infinite-layer nickelates Re_1–xSr_xNiO₂ (Re = Nd, Pr)^1,2 has sparked significant interest in the investigation of the intrinsic superconducting and normal-state properties of this new superconducting system. However, the non-trivial inclusions of defects and structural imperfections in this thermodynamically unstable material^3,4 remain a major materials challenge. The main factors contributing to these imperfections include the unusually high formal nickel valence in the perovskite precursor phase due to the chemical doping of strontium, the unavoidable epitaxial mismatch with the substrate due to the large in-plane lattice change during the topotactic transition, and the decomposition and secondary-phase formation issues arising during the soft-chemistry reduction step.^3-5 Upon careful optimization of the growth and reduction conditions, we have identified the key parameters for achieving higher crystallinity and established a reproducible method to stabilize high-quality Nd_1–xSr_xNiO₂ (001) epitaxial thin films on SrTiO₃ (001) substrate by pulsed-laser deposition and CaH₂-assisted topochemical reduction. In particular, x-ray diffraction and scanning transmission electron microscopy measurements of the newly optimized samples reveal that the Ruddlesden-Popper-type faults prevalent in the previous generation of samples (Ref. 3) are now largely eliminated, indicating major improvements in the crystallinity of this material. The optimization process and the resultant evolution of the structural properties of the nickelate films will be discussed. In addition, the improvement in the crystallinity affects the transport properties and the superconducting dome^5,6 of this material, the details of which will be discussed.

1 Li, D. et al. Superconductivity in an Infinite-Layer Nickelate. Nature 572, 624 (2019).
2 Osada, M. et al. A Superconducting Praseodymium Nickelate with Infinite Layer Structure. Nano Lett. 20, 5735 (2020).
3 Lee, K. et al. Aspects of the Synthesis of Thin Film Superconducting Infinite-Layer Nickelates. APL Mater. 8, 041107 (2020).
4 Wang, B.-X. et al. Synthesis and Characterization of Bulk Nd_1−xSr_xNiO₂ and Nd_1−xSr_xNiO₃. Phys. Rev. Mater. 4, 084409 (2020).
5 Zeng, S. et al. Phase Diagram and Superconducting Dome of Infinite-Layer Nd_1−xSr_xNiO₂ Thin Films. Phys. Rev. Lett. 125, 147003 (2020).
6 Li, D. et al. Superconducting Dome in Nd_1–xSr_xNiO₂ Infinite Layer Films. Phys. Rev. Lett. 125, 027001 (2020).

*Supported by DOE BES MSD (DE-AC02-76SF00515), the Moore Foundation (GBMF9072), and DOD AFOSR (FA 9550-16-1-0305).

The stabilization of superconducting infinite-layer nickelate thin films [1,2] presents a long-awaited platform for experimental comparison to the analogous cuprate superconductors and exploration of the underlying physical mechanisms. Unlike the bulk-synthesized cuprates, however, the thin film geometry of the superconducting nickelates has important implications for both the materials physics and the experimental methods used to probe it. Here, we harness atomic-scale scanning transmission electron microscopy (STEM) and localized electron energy loss spectroscopy (EELS) to explore both the lattice and electronic structure with high real-space resolution across a wide series of nickelate thin films. State-of-the-art electron spectroscopy reveals key electronic differences between the nickelate and cuprate superconductors [3]. Beyond the experimental considerations, the thin film geometry of the nickelate superconductors has also raised speculation about possible phenomena at the nickelate-substrate interface [4-6]. Harnessing the ability of cross-sectional measurements by atomic-resolution STEM and EELS, we discuss the details of both lattice and electronic structure in these materials across the substrate-film interface.

1. Li, D. et al. Superconductivity in an Infinite-Layer Nickelate. Nature 572, 624 (2019).
2. Osada, M. et al. A Superconducting Praseodymium Nickelate with Infinite Layer Structure. Nano Lett. 20, 5735 (2020).
3. Goodge, B. H. et al. Doping evolution of the Mott-Hubbard landscape in infinite-layer nickelates. arXiv:2005.02847 (2020).
4. He, R. et al. Polarity-induced electronic and atomic reconstruction at NdNiO₂/SrTiO₃ interfaces. Physical Review B 102, 035118 (2020).
5. Geisler, B. and Pentcheva, R. Fundamental difference in the electronic reconstruction of infinite-layer versus perovskite neodymium nickelate films on SrTiO3(001). Physical Review B 102, 020502(R) (2020).
6. Bernardini, F. and Cano, R. Stability and electronic properties of LaNiO₂/SrTiO₃ heterostructures. J. Phys. Mater. 3 (2020) 03LT01.

*Supported by DOD AFOSR (FA 9550-16-1-0305), DOE BES MSD (DE-AC02-76SF00515), and the Moore Foundation (GBMF9072).

The discovery of superconductivity in infinite-layer nickelates [1] has the potential to help reveal the origins of high-temperature superconductivity. To do this, we must first develop an understanding of superconductivity in nickelates by measuring their fundamental properties. In this talk, I will present our measurements of the in-plane penetration depth of (Nd,Sr)NiO₂ thin films across the doping-dependent superconducting dome [2]. The penetration depth provides information about the superfluid density and pairing symmetry of the superconducting state, which will help to inform our understanding of the nature of superconductivity in these materials.

[1] Li, D. et al. Superconductivity in an infinite-layer nickelate. Nature 572, 624–627 (2019).
[2] Li, D. et al. Superconducting Dome in Nd_1-xSr_xNiO₂ Infinite Layer Films. Phys. Rev. Lett. 125, 027001 (2020).
Supported by DOE BES MSD (DE-AC02-76SF00515) and the Moore Foundation (GBMF4415)

The recent observation of superconductivity in thin film infinite-layer nickelates [1] offers fresh terrain for investigating superconductivity in layered oxides. A wide range of perspectives [2-4], emphasizing single- or multi-orbital electronic structure, Kondo or Hund’s coupling, and analogies to cuprates, have been theoretically proposed. Clearly, further experimental characterization of the superconducting state is needed to develop a foundational understanding of the nickelates. In this talk, I will discuss using magnetotransport measurements to probe the superconducting anisotropy in Nd_0.775Sr_0.225NiO₂. We find that the upper critical field is surprisingly isotropic at low temperatures, despite the layered crystal structure. We deduce that under magnetic field, superconductivity is strongly Pauli-limited, such that the paramagnetic effect dominates over orbital de-pairing. Underlying this isotropic response is both an enhanced electronic magnetic moment, the measurement of which can serve as a critical test of our interpretation and a substantial anisotropy in the superconducting coherence length, at least four times longer in-plane than out-of-plane. A prominent low-temperature upturn in the upper critical field indicates the likelihood of an unconventional ground state.


[1] Li, D. et al. Superconductivity in an infinite-layer nickelate. Nature 572, 624–627 (2019).
[2] Lee, K.-W. & Pickett, W. E. Infinite-layer LaNiO₂: Ni¹⁺ is not Cu²⁺. Phys. Rev. B 70, 165109 (2004).
[3] Jiang, M., Berciu, M. & Sawatzky, G. A. Critical nature of the Ni spin state in doped NdNiO₂. Phys. Rev. Lett. 124, 207004 (2020).
[4] Botana, A. S. & Norman, M. R. Similarities and differences between LaNiO₂ and CaCuO₂ and implications for superconductivity. Phys. Rev. X 10, 011024 (2020).

*Supported by the Moore Foundation (GBMF9072) and DOD AFOSR (FA 9550-16-1-0305).

In the semimetal WTe₂, low symmetry, band inversion, heavy carrier mass, near-perfect compensation, strong electron-phonon interactions and strong spin-orbit coupling all combine to provide a promising setting for correlated and topological phenonema. Undoped, a monolayer of WTe₂ behaves as a two-dimensional topological insulator with helical conducting edge states. Anisotropic magnetoresistance measurements of the edge conduction reveal the direction of the spin polarization, and surprisingly it is found to be independent of the edge termination. We infer that it is inherited from the bulk bands which are also spin polarized along a special axis, and show that this is consistent with theory. In addition, several lines of evidence indicate that the insulating behavior involves electron-hole pairing, with the possible formation of an excitonic insulator state near the compensated state. Meanwhile, at a surprisingly low level of electrostatic doping the insulating state converts into a metal which superconducts about 0.8 K. It is therefore quite likely that the superconductivity is tilted-Ising like and competes with excitonic ordering. Multi-terminal measurements also suggest that the topological edge states persist while being very weakly coupled to the bulk superconductivity.

Monolayer WTe2, an inversion-symmetric transition metal dichalcogenide, has recently been established as a quantum spin Hall insulator and found superconducting upon gating. It is therefore natural to wonder whether this discovered superconductivity is topological. In this talk, I will first show that gated monolayer WTe2 is a promising candidate for a ``higher-order" topological superconductor, which features Majorana zero modes localized at sample corners. Then I will present a general recipe for realizing superconductors hosting such Majorana corner modes without utilizing proximity effect. Finally, I will discuss systematic searches for new material candidates in databases guided by topological invariants that are applicable to ab initio band structures and can diagnose Majorana boundary types.

Spin-triplet superconductors (SCs) are highly desirable but rarely realized. On the other hand, there are surging interest in the spin-triplet p-wave pairing due to its close relevance to topological superconductivity and noise-resilient quantum computing. The superconducting gaps of triplet and singlet SCs with odd and even parity respectively can be exploited for unequivocal identification. We show that the magnetic flux quantization, one of the defining characteristics of a superconductor (SC), is a decisive phase-sensitive method to distinguish triplet-pairing SCs from the more common spin-singlet SCs. A superconducting ring with triplet pairing may demonstrate half-integer flux quantization (n + ½) Φ₀, where n is an integer, i.e., half flux quanta with half-integer quantum numbers of 1/2, 3/2, 5/2, etc. instead of the exclusive integer quantization nΦ₀ observed in singlet SCs. We have observed half-quantum flux in mesoscopic rings of superconducting β-Bi₂Pd thin films [1]. The result provides conclusive evidence for spin-triplet p-wave pairing in β-Bi₂Pd as a leading candidate of intrinsic topological SCs. We have also observed half-quantum flux in noncentrosymmetric α-BiPd, where an admixture of singlet and triplet pairing is expected from the absence of parity symmetry [2]. Our findings establish a new paradigm for identifying spin-triplet pairing, and usher in new venues for studying topological superconductivity.

[1] Y. Li, X. Xu, M.-H. Lee, M.-W. Chu, C. L. Chien, Science 366, 238-241 (2019).
[2] X. Xu, Y. Li, C. L. Chien, Physical Review Letters 124, 167001 (2020).

In recent years, superconductivity and topological phases (such as quantum spin Hall and anomalous Hall phases) have been discovered in atomically thin two-dimensional materials such as in transition metal dichalcogenides (TMDs) and twisted bilayer graphene. These materials exhibit novel physical properties due to their crystal symmetries, band structures and interaction effects.

In this talk, I would like to discuss:

1. The properties of superconducting 2H-structure TMDs which we called Ising superconductors. In the case of gated 2H-structure MoS2 for example, superconductivity can survive even in the presence of an in-plane magnetic field up to 60 Tesla. We showed that this is due to a special form of spin-orbit coupling (SOC) which we called Ising SOC. We will also discuss how Ising superconductors can be used to realize Majorana fermions which are important for topological quantum computation. [1-4]

3. The current-induced magnetization switching in twisted bilayer graphene. Recently, quantum anomalous Hall effect with spontaneous ferromagnetism was observed in twisted bilayer graphenes (TBG) near 3/4 filling by Goldhaber-Gordon’s group at Stanford [6] and Young’s group at UCSB [7]. Importantly, it was observed that an extremely small current can switch the direction of the magnetization. This offers the prospect of realizing low energy dissipation magnetic memories. However, the mechanism of the current-driven magnetization switching is poorly understood as the charge currents in graphene are generally believed to be non-magnetic. In this talk, the observed current-induced magnetization effect will be explained [8].

1. Science 350, 1353 (2015),
2. Nature Physics 12, 139-143 (2016),
3. Nature Materials 17, 504-508 (2018),
4. Physical Review Research 2, 013026 (2020),
5. A. Sharpe etc. Science 365, 605 (2019),
6. M. Serlin etc. Science 367, 895 (2020),
7. Nature Communications 11, 1650 (2020).

In this talk I will report our recent discoveries of topological superconductivity and Majorana zero mode in iron-based superconductors. We have observed a superconducting topological surface state of Fe(Te, Se) with Tc ~ 14.5K by using low-temperature ARPES [1], and a pristine Majorana zero mode (MZM) inside a vortex core of this material by using low-temperature STM [2]. We have also observed a half-integer level shift of vortex bound states [3] and quantized Majorana conductance [4] in this material, which are hallmarks of MZMs. In addition, we have also found that most of Fe-based superconductors [5], including monolayer Fe(Te, Se)/STO [6], have similar topological electronic structures. One of them, CaKFe₄As₄, an Fe-As bilayer superconductor (Tc ~ 35K), is found to possess MZM and other bound states that can be well reproduced by a simple theoretical model [7]. Our observations offer a new, robust platform for realizing and manipulating MZMs, which can be used for quantum computing at a relatively high temperature.

References
[1] Peng Zhang et al., Science 360, 182 (2018)
[2] Dongfei Wang et al., Science 362, 334 (2018)
[3] Lingyuan Kong et al., Nature Physics 15, 1181 (2019)
[4] Shiyu Zhu et al., Science 367, 189 (2020)
[5] Peng Zhang et al., Nature Physics 15, 41 (2019)
[6] Xun Shi et al., Science Bulletin 62, 503 (2017)
[7] Wenyao Liu et al., Nature Communications 11, 5688 (2020)

Two-dimensional (2D) materials have attracted plenty of interests due to the novel physical properties induced by the reduction of dimension compared to three-demensional (3D) system, and the practical and potential application of revolutionary current technologies as well. It is of great significance to extensively investigate the fundamental physics of the related materials in the two-dimensional limit. The success of 2D materials is based on the development of suitable synthesis methods, mainly including exfoliation, chemical vapour deposition and various solution-phase methods. New and more efficient methods are under developing. Recently, we have successfully intercalated organic ion cetyltrimethyl ammonium (CTA⁺) and tetrabutyl ammonium (TBA⁺) into several layered crystals via electrochemical intercalation method, realizing two-dimensionalization of the related functional layers and observing some intriguing new physical phenomena. We intercalated CTA⁺ and TBA⁺ into the FeSe single crystal by the electrochemical method, achieving high superconducting transition temperature (T_c) to be 45 and 50 K for (CTA)_0.3FeSe and (TBA)_0.3FeSe, respectively. By measuring Knight shift and nuclear spin-lattice relaxation rate, we unambiguously observed a pseudogap behavior below T_p ∼ 60 K in these two kinds of layered FeSe-based high-temperature superconductors, and revealed that the pseudogap behavior is related to strong superconducting fluctuations due to the quasi-2D nature induced by the organic-ion intercalations. With intercalating the same organic ions into an intrinsic semiconductor SnSe₂, superconductivity with T_c to ~6.4 (TBA⁺) and 7.1 K (CTA⁺) was achieved. We also synthesized an organic-ion intercalated transition-metal dichalcogenides (TBA)_0.3VSe₂, where the metallic charge-density-wave (CDW) state with transition temperature (T_CDW) of 110 K in the pristine system was changed to an insulating CDW state with T_CDW of 165 K by the intercalation. With TBA⁺-intercalation, the insulating ferromagnetic van der Waals material Cr₂Ge₂Te₆ with Curie temperature T_curie of 67 K was altered to a metallic ferromagnet with T_curie of 208 K, accompanied by the magnetic easy axis changing from <001> direction to the ab plane. Our work indicated that the 3D-to-2D crossover induced by intercalation of the large organic ion CTA⁺ and TBA⁺ between functional layers plays a key role in the dramatic changes of the physical properties in the several material systems, and demonstrated that the intercalation of organic ions with large size can serves as a convenient and efficient approach to explore and manipulate the versatile electronic and magnetic properties in the layered crystals.
References:
[1] M. Z. Shi, N. Z. Wang, B. Lei, C. Shang, F. B. Meng, L. K. Ma, F. X. Zhang, D. Z. Kuang, and X. H. Chen, Phys. Rev. Mater. 2, 074801 (2018).
[2] M. Z. Shi, N. Z. Wang, B. Lei, J. J. Ying, C. S. Zhu, Z. L. Sun, J. H. Cui, F. B. Meng, C. Shang, L. K. Ma and X. H. Chen, New J. Phys. 20, 123007 (2018).
[3] B.L. Kang, M.Z. Shi, S.J. Li, H.H. Wang, Q. Zhang, D. Zhao, J. Li, D.W. Song, L.X. Zheng, L.P. Nie, T. Wu and X.H. Chen, Phys. Rev. Lett. 125, 097003(2020).
[4] L. K. Ma, M. Z. Shi, B. L. Kang, K. L. Peng, F. B. Meng, C. S. Zhu, J. H. Cui, Z. L. Sun, D. H. Ma, H. H. Wang, B. Lei, T. Wu, X. H. Chen, Phys. Rev. Mater., 4, 124803 (2020).
[5] F. B. Meng, Z. Liu, L. X. Yang, M. Z. Shi, B. H. Ge, H. Zhang, J. J. Ying, Z. F. Wang, Z. Y. Wang, T. Wu and X. H. Chen, Phys. Rev. B 102, 165410 (2020).
[6] N. Z. Wang, H. B. Tang, M. Z. Shi, H. Zhang, W. Z. Zhuo, D. Y. Liu, F. B. Meng, L. K. Ma, J. J. Ying, L. J. Zou, Z. Sun, X. H. Chen, J. Am. Chem. Soc. 141, 17166-17173 (2019).

A Majorana-fermion is an exotic particle that is its own anti-particle and can occur in superconductors, where charge conjugation is realized as a particle-hole symmetry. In recent years, many clever realizations of Majorana-fermions in condensed-matter have been predicted. Many have been verified by exploiting the interplay between superconductivity and band topology in metals and insulators. On the other hand, Weyl-semimetals are three-dimensional materials in which the bulk band-gap closes at an even-number of discrete points (Weyl nodes) in the Brillouin zone. Weyl-summits are characterized by topologically protected surface states consisting of disjoint, open Fermi-arcs, projected from bulk Weyl-nodes to the surface.

Realizations of Majorana-fermions in semimetals remain less explored. We ask, ”under what conditions do superconducting vortices in time-reversal symmetric Weyl-semimetals trap Majorana-fermions on the surface?” If each constant-kz plane, where z is the vortex axis, contains equal numbers of Weyl nodes of each chirality, we predict a generically gapped vortex and derive a topological invariant ν in terms of the Fermi arc structure that signals the presence or absence of surface Majorana fermions. In contrast, if certain constant-kz planes contain a net chirality of Weyl nodes, the vortex is gapless.

We analytically calculate the topological invariant ν within a perturbative scheme and provide numerical support with an orthorhombic lattice model. Using our criteria, we predict phase transitions between trivial, critical and topological vortices by simply tilting the vortex. Finally, we propose Li(Fe0.91Co0.09)As with broken inversion symmetry as a candidate for realizing our proposals.<gdiv></gdiv><gdiv></gdiv><gdiv></gdiv>

It has been established that the superconductivity in a material is dictated by its degrees of freedom of electronic charge, orbital, spin and crystallographic structure, which can be manipulated by the control parameters such as pressure, magnetic field and chemical composition. Pressure is a ‘clean’ way to tune basic electronic and structural properties without changing the basic chemistry, and can help to search for new phenomena and understand the corresponding physics. In this talk, I will present some interesting phenomena obtained by our recent high-pressure studies, including crossover from two-dimensional to three-dimensional superconducting states in bismuth-based cuprate superconductor [1] and discovery of the RSAVS superconductors [2-5].


References:
[1] J. Guo and L. Sun et al, Nature Physics 16,295(2020)
[2] C Huang and L. Sun et al, PRM (R) 4, 071801(2020)
[3] J. Guo and L. Sun et al, Adv. Mater. 31, 1807240 (2019)
[4] L. Sun and J.R. Cava, PRM 3, 090301(2019)
[5] J. Guo and L. Sun et al, PNAS 114, 13144 (2017)

Room-temperature superconductivity has been a century long-held dream of mankind and a focus of intensive research. In an effort to search for room-temperature superconductors, we proposed in 2012 the first-ever clathrate superhydride CaH61 stabilized at an extreme pressure
150 GPa that shows a potential of high temperature superconductivity at Tc about 235 K. In 2017, we extended the superconducting clathrate structures into rare earth (RE, e.g., Sc, Y, La, Ce, Pr., etc) superhydrides at high pressure conditions that commonly have three clathrate
structured stoichiometries of REH6, REH9, and REH10, some of which exhibit extraordinarily high-temperature superconductivity2. Subsequent experiments synthesized the as-predicted clathrate superhydrides YH6, YH9, and LaH10 and measured Tc values at 224, 243, and 260 K, respectively, setting up new Tc records among known superconductors. These discoveries open up the door of achieving superconductors that could work at room temperature (300 K) in superhydrides under high pressures. In the talk, I will give an overview on the current status of research progress on superconductive superhydrides, and then discuss the design principle for achieving room temperature superconductor. Our prediction on a hot superconductor (Tc = ~400 K ) in a clathrate Li2MgH16 superhydride found in a ternary Li-Mg-H system3, and other recent results on clathrate superhydrides together with future research direction along this line will be discussed.
References:
1Wang, et al., PNAS 109, 6463 (2012)
2Peng, et al., PRL 119, 107001 (2017)
3Sun, et al., PRL 123, 097001 (2019)