Symposium CT07—Excited-State Properties of Materials—Theory and Computation

Inspired by optical tweezers, we show that laser pulse with selected frequency can drive an ultrafast diffusionless martensitic phase transition of two-dimensional ferroelastic materials such as SnO and SnSe monolayers, where the unit-cell strain is tweezed as a generalized coordinate that affects the anisotropic dielectric function and electromagnetic energy density. At laser power of 10^10 W/cm2, the transition potential energy barrier vanishes between two 90°-orientation variants of ferroelastic SnO and SnSe monolayer, respectively, so displacive domain switching can occur within picoseconds. The estimated adiabatic thermal limit of energy input in such optomechanical martensitic transition (OMT) is at least 2 orders of magnitude lower than that in Ge-Sb-Te alloy. [Nano Lett. (2018) 7794]

Hydrodynamic electron flow in condensed matters has been one of the most active research areas recently. While progress from both theory and experimental techniques are made, open questions regarding the underlying mechanisms still remain. We utilize ab initio techniques to treat the electron scattering events explicitly, and show in combination with Boltzmann transport equation a more applicable metric of hydrodynamic transport taking into account temperature, channel width, and impurity length, which can be directly verified by various experimental techniques. By investigating different electron scattering mean free paths in PdCoO2, ZrSiS, and TaAs2, we show that that phonon mediated electron-electron interaction could lead to much shorter momentum conserving mean free path (lmc) than momentum relaxing lmr, facilitating hydrodynamic behavior in systems where the direct Coulomb interaction is largely screened.
We further discuss the momentum relaxing lifetimes on their Fermi surfaces, and show that at low temperatures, hole pockets feature much longer lifetimes than electron pockets, providing a window for momentum conserving scattering events to dominate. Importantly, our findings on TaAs2 suggest that linear Dirac-Weyl bands as well as the bands that contribute to phonon-mediated diagrams are critical in search of promising candidates for hydrodynamic flow. Other key ingredients include high mobility carriers, low density of states compensated systems, and large electron-phonon matrix elements. The plethora of low symmetry crystals whose Fermi surfaces are composed of d orbitals from the transition metal and p orbitals from the metalloids provides a much larger pool for further study. This work provides ab initio signatures of material-specifics to explore hydrodynamic electron flow in a much larger family of condensed matter systems, and thus offers insights into study of electron interactions through transport phenomena.

Two-dimensional (2D) materials like the transition metal dichalcogenides (TMDs) and their van der Waals bonded layered structures present interesting opportunities for studying and controlling light-matter interactions and for developing e.g. room temperature excitonic devices. Engineering of the dielectric environment represents a powerful strategy to control the excited states in 2D materials without compromising their structural integrity. We show that the quasiparticle band gap of a 2D semiconductor can be tuned by hundreds of meV by varying the concentration of free carriers in a nearby graphene sheet via electrostatic doping[1]. The recent development of high-κ 2D materials present new opportunities for dielectric engineering. We demonstrate that the exciton Rydberg series of a supported TMD monolayer changes qualitatively when the dielectric screening within the 2D semiconductor becomes dominated by the substrate. In this regime, the distance dependence of the screening is reversed and the effective screening increases with exciton radius, which is opposite to the conventional 2D screening regime. Consequently, higher excitonic states become underbound rather than overbound as compared to the Hydrogenic Rydberg series[2].
Bilayer TMDs present further opportunities for creating novel excitonic states and manipulating their properties. Indeed, interlayer excitons with electrons and holes located in different layers can be realized in structures with Type-II band alignment. I will present a joint theory-experiment collaboration demonstrating electrical control of interlayer excitons in bilayer MoS2 up to room temperature. The large out-of-plane dipole of the interlayer excitons makes then highly sensitive to the perpendicular electric field and leads to giant Stark shifts of up to 60 meV. The work verifies an earlier theoretical prediction about the mixed intra-interlayer nature of the excitons in TMD homobilayers [3].
Finally, I will present some recent results on ab-initio calculations of spontaneous emission rates in van der Waals heterostructures. Specifically, it is shown that highly confined graphene plasmons can lead to extremely high Purcell factors for intersubband transitions in few-layer TMDs sandwiched between graphene and a perfect metal. Among other things, we demonstrate the importance of using ab-initio electronic wave functions (as compared to simpler model wave functions) for obtaining realistic rates.

References:
[1] Electrically Controlled Dielectric Band Gap Engineering in a Two-Dimensional Semiconductor, A. C. Riis-Jensen et al., Phys. Rev. B 101, 121110(R) (2020)
[2] Anomalous Non-Hydrogenic Exciton Series in 2D Materials on High-κ Dielectric Substrates, A. C. Riis-Jensen et al. arXiv:2009.12317
[3] Interlayer Excitons with Large Optical Amplitudes in Layered van der Waals Materials, T. Deilmann and K. S. Thygesen, Nano Lett. 18, 2984 (2018)

Materials with strong nonlinear responses play important roles in advanced optoelectronics and photonics. Large nonlinear responses such as second harmonic generation recently observed in 2D and topological materials spurred tremendous interest, offering great opportunities for developing ultrathin nonlinear optical devices free of phase-matching bottleneck. Here, we report our recent effort on first-principles theory and simulation of nonlinear responses in low-dimensional materials. First, we will present our study of second harmonic generation, shift photocurrent, and circular photocurrent in semiconducting 2D materials and reveal their microscopic origins in terms of interband and intraband contributions. We show that it is possible to realize ferroicity-driven nonlinear photocurrent switching by taking advantage of the inherent coupling between nonlinear susceptibility and underlying symmetry, ferroic order, and light polarization. Second, we will present on theoretical prediction and experimental demonstration of ferroelectric nonlinear anomalous Hall effect in time-reversal invariant few-layer WTe₂ and show that Berry curvature dipole and shift dipole can serve as new order parameters for noncentrosymmetric systems, paving the theoretical foundation for nonlinear quantum electronics such as Berry curvature memory. Finally, we will introduce our recent work on nonlinear photocurrent in PT-symmetry magnetic topological quantum materials. We predict that magnetic shift photocurrent can be magnetically switched between two antiferromagnetic states with time-reversed spin orderings in bilayer antiferromagnetic MnBi2Te4. External electric field can break PT-symmetry and enable normal shift photocurrent that are electrically switchable and tunable down to a few THz regime, suggesting bilayer antiferromagnetic MnBi2Te4 as a tunable platform with rich THz and magneto-optoelectronic applications. Nonlinear optical and photocurrent responses thus provide a powerful alternative for deciphering electronic structures and interactions, particularly fruitful for probing and understanding 2D materials, topological materials, and other quantum materials.

We study the properties of charged interlayer excitons (CIE) in highly excited vdW heterostructures [1] — a compound fermion system with the permanent dipole moment that was observed recently in Transition-Metal-Dichalcogenide bilayers [2]. We predict the existence of new strongly correlated collective CIE states, the long-range ordered phases of the excited bilayer heterostructure — the crystal phase and the Wigner crystal phase. We evaluate the critical temperatures and density for the formation of such many-particle cooperative compound fermion states. We demonstrate that they can be selectively realized with bilayers of properly chosen electron-hole effective mass ratio by just varying their interlayer separation distance. Compound fermion systems featuring permanent electric dipole moments are of both fundamental and practical importance due to their inherently unique many-body correlation effects between electric-dipole and spin degrees of freedom. The spin in such systems could potentially be used for quantum information processing and its correlation with the dipole moment provides an opportunity for spin manipulation through optical means. Fundamental cooperative crystallization phenomena we predict will greatly increase the potential capabilities of such systems to open up new avenues for experimental exploration and novel device technologies with van der Waals heterostructures.

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Funding Acknowledgements:
DOE-DE-SC0007117 (I.V.B.), ARO-W911NF1810433 (O.L.B., R.Y.K.), RFBR-20-02-00410 (Y.E.L.)

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[1] I.V. Bondarev, O.L. Berman, R.Ya. Kezerashvili, and Yu.E. Lozovik, Crystal Phases of Charged Interlayer Excitons in van der Waals Heterostructures, arxiv:2002.09988
[2] L.A.Jauregui, A.Y.Joe, K.Pistunova, D.S.Wild, A.A.High, Y.Zhou, G.Scuri, K.De Greve, A.Sushko, C.-H.Yu, T.Taniguchi, K.Watanabe, D.J.Needleman, M.D.Lukin, H.Park, and P.Kim, Electrical Control of Interlayer Exciton Dynamics in Atomically Thin Heterostructures, Science 366, 870 (2019)

I will explore the properties and performance of novel two-dimensional materials that can be exfoliated from experimentally-known inorganic compounds. I will discuss the cases of novel topological insulators, superconductors, and high-mobility semiconductors. Workd done in collaboration with Davide Campi, Davide Grassano, Antimo Marrazzo, Thibault Sohier, Marco Gibertini, and Giovanni Pizzi.

We discuss first principles, computational methods and strategies to predict light-activated processes in materials for sustainability (e.g. to understand and design photo-electrochemical cells) and for quantum information science (e.g. color centers in semiconductors). In particular we present methods to study materials at finite temperature, by coupling first principles molecular dynamics and many body perturbation theory.

Strong correlations within a symmetry-unbroken ground-state wavefunction can show up in approximate density functional theory as symmetry-broken spin-densities or total densities. They can arise from soft modes of fluctuations (sometimes collective excitations) such as spin-density or charge density waves at non-zero wavevector. Familiar examples are the unobservable but revealing symmetry breaking in stretched H₂ and the observable symmetry breaking in antiferromagnetic solids. The example discussed here is the static charge-density wave/Wigner crystal phase of a low density ( 69) jellium. Time-dependent density functional theory is used to show quantitatively that the static charge density wave is a soft plasmon. More precisely, the frequency of a related density fluctuation drops to zero, as found from the frequency moments of the spectral function. Our calculation is based on a recent constraint-based wavevector- and frequency-dependent jellium exchange-correlation kernel.¹ (Work supported by NSF DMR and DOE BES.)

¹ A. Ruzsinszky, N.K. Nepal, J.M. Pitarke, and J.P. Perdew, Physical Review B 101, 245135 (2020).

Green’s functions are efficient tools to study many-body systems. Because the one-body Green’s function is non-local in space and time, it gives more direct access to phenomena which reflect coupling of different regions of space and to memory effects in time, than approaches based on density functionals. Examples for such couplings are the occurrence of single and multiple plasmons, excitons, or satellite structure in excitation spectra. Still, existing approximations in the framework of many-body perturbation theory are often not sufficient to capture all these phenomena.

In this talk we will discuss extensions of Green’s functions approaches beyond the previous state-of-the-art. This will include the description of coherent inelastic x-ray scattering [1], coupling of electron-hole excitations [2], and the description of charge dynamics. We will conclude by discussing promising routes for the future.

[1] Igor Reshetnyak, Matteo Gatti, Francesco Sottile, and Lucia Reining, Phys. Rev. Research 1, 032010(R) (2019)
[2] Pierluigi Cudazzo and Lucia Reining, Phys. Rev. Research 2, 012032(R) (2020)

Quantum nanostructures, e.g., quantum wires and quantum dots, are needed for applications in quantum information processing devices, such as transistors or qubits. In gapped bilayer graphene (BLG), one can confine charge carriers purely electrostatically, inducing smooth confinement potentials and thereby limiting edge-induced perturbances, while allowing gate-defined control of the confined structure. I will report on a series of works on electrostatically confined nanostructure in gapped BLG. We demonstrated, e.g., how some of BLG's unusual properties, i.e., its states' Berry curvature-induced orbital magnetic moment and the mini valleys and band inversions of its non-parabolic low-energy dispersion, translate into a BLG quantum wire's transport properties and a quantum dot's single- and two-electron states. We investigated both theoretically, and in collaboration with experiments, how to tune these features of BLG nanostructures externally to make them useful in future quantum technology applications.

Owing to their very large geometrical capacitance, ionic liquid gated devices allow unique experiments to be performed. Possibly the best known example is the possibility to induce superconductivity at the surface of different insulators. Ionic liquid gated devices, however, have more to offer. In this talk I will show how ionic liquid gated transistors based on many different 2D semiconductors and van der Waals interfaces can be used to perform precise, quantitative spectroscopic measurements of the energy of band edges. These measurements allow the straightforward determination of the size of the band gap of many semiconducting 2D materials in mono, bi and multilayer form. Thy also allow the determination of the band alignment of two different materials used to form a van der Waals interface, a quantity that is normally difficult to measure reliably by means of other techniques. If times allows, I may also present very recent development on double gated devices, i.e. in devices in which a 2D semiconductor is coupled to two independent ionic gates on opposites sides.

Many fascinating phenomena in nature owe their emergence from the interactions of large number of particles. In this talk, I will discuss some recent progress in understanding and computing excited-state phenomena in materials, especially those that are relevant to energy conversion, transport and storage. Many-electron interactions are dominant in many of these phenomena/properties. Several illustrative examples will be presented, including: strongly bounded correlated multiparticle excitations, such as trions and biexcitons, in quasi 1D and 2D semiconductors; universal slow yet tunable plasmons as well as giant excitonic enhancement of shift currents in 2D materials; and remarkable many-electron features in optical-field-driven, time- and angle-resolved photoemission spectroscopy (ARPES). Ab initio studies of these novel phenomena are made possible because of newly developed first-principles methods based on an interacting many-particle Green’s function approach.

This work was supported by the U. S. Department of Energy and the National Science Foundation. I would like to acknowledge collaborations with members of the Louie group.

Exciton dynamics underlie materials optoelectronic functionality, and it is of great interest to understand how material structure and composition influences the involved transport properties. In this work we study the connection between exciton dispersion and dynamics from a new approach, based on many-body perturbation theory within the GW-BSE approximation. By accounting for the exciton bandstructure, we relate its time-evolution and radiative recombination processes to the underlying material structure. We demonstrate this approach on the pentacene molecular crystal, a well-studied system with nontrivial excitonic processes and unique structural features. We explore exciton dispersion and relate it to the directionality of the light polarization due to crystal packing and to anisotropy in the exciton propagation. We further discuss band to band transitions that dominate the radiative lifetime and thermalization effects. Our results provide an insight into exciton transport properties as they are affected by the structural features of the pentacene crystal.

Many-body perturbation theory (MBPT) is the state of-the-art approach to determine neutral excitations in solids and has been applied with considerable success to determine optical, UV, and x-ray absorption spectra. However, theoretical studies so far have focused on a specific energy region, studying either core or valence excitations. In this talk, we present an all-electron MBPT approach that overcomes these limitations [1]. We show how it can be used to calculate absorption and inelastic scattering spectra, from the optical to the x-ray region. While these spectroscopic techniques probe either the valence or core excitations, the interplay of the two can be revealed by resonant inelastic x-ray scattering (RIXS). We present a novel many-body approach to determine RIXS spectra in solids [2], which makes use of the valence and core excitations determined within our all-electron approach. We demonstrate with the example of the wide-gap oxide Ga₂O₃ [3] how the excitation pathways determine the spectral shape of the emission, and demonstrate the nontrivial role of electron-hole correlation in the RIXS spectra. We also discuss how RIXS can be employed to determine the nature of bound valence excitons in this material.

[1] C. Vorwerk, B. Aurich, C. Cocchi, and C. Draxl, Electronic Structure, 1, 037001 (2019).
[2] C. Vorwerk, F. Sottile, and C. Draxl, Phys. Rev. Research 2, 042003(R) (2020).
[3] C. Vorwerk, D. Nabok, and C. Draxl, in preparation.

I will present our recent study of the quasiparticle excitations and band structures in organized donor-acceptor copolymers. We employ many-body perturbation theory, which accounts for the polarization effects among polymers, to calculate the quasiparticle energies in bulk polymers. Our computed results are in excellent agreements with the photoemission spectra data. We discover two types of states supporting band transport in bulk copolymers: the conjugated bands and impurity states. The non-local exchange interactions are found to enhance the band transport of hole along the polymer axis, but hinder the transport across the chain. The polarization interactions are found to stabilize charge carriers and hinder band transport. Further, we discover that depending on the molecular arrangement, bulk copolymers sustain electron and hole transport in two orthogonal directions; the holes are most efficiently transported along the polymer and the pi-pi stacking directions, while the electrons are transport along the edge-to-edge stacking direction.

First-principles calculations of electron-phonon interactions are becoming an increasingly popular tool for the study of functional materials at finite temperature. As a result, several new techniques have been developed during the past decade to address a broad array of properties and phenomena, ranging from superconductivity to light-matter interactions [1]. Within this context, I will describe some recent developments in calculations of phonon-limited carrier transport and polaron physics. I will outline the Boltzmann transport formalism [2] and its application to the calculation of transport coefficients, and I will illustrate this technique by discussing carrier mobilities in wide-gap semiconductors [3], hybrid organic-inorganic halide perovskites [4] and two-dimensional materials [5]. Then I will describe our work phonon-mediated electron localization, and the formation of polarons in bulk and low-dimensional materials [6,7].

[1] F. Giustino, Rev. Mod. Phys. 89, 015003 (2017).
[2] S. Poncé, W. Li, S. Reichardt, and F. Giustino, Rep. Prog. Phys. 83, 036501 (2020).
[3] S. Poncé, D. Jena, and F. Giustino, Phys. Rev. Lett. 123, 096602 (2019).
[4] S. Poncé, M. Schlipf, F. Giustino, ACS En. Lett. 4, 456 (2019).
[5] W. Li, S. Poncé, F. Giustino, Nano Lett. 19, 1774 (2019).
[6] W. H. Sio, C. Verdi, S. Poncé, and F. Giustino, Phys. Rev. Lett. 122, 246403 (2019).
[7] W. H. Sio, C. Verdi, S. Poncé, and F. Giustino, Phys. Rev. B 99, 235139 (2019).

The ability to synthesize and probe new classes of light-absorbing with tunable structure and composition – such as 2d transition-metal dichalcogenides, organic semiconductors, and halide perovskites – has driven the need for new intuition linking atomic- and molecular-scale morphology to their photophysics, and in particular the nature and fate of excitons. Here, I will discuss the use of ab initio density functional theory and many-body perturbation theory for computing photoactive excited states in complex light-absorbing materials including electron-phonon and exciton-phonon interactions. For transition metal dichalcogenides, we explore how exciton-phonon interactions renormalize defect levels and lead to decoherence channels for excitations associated with deep in-gap states; for organic acene crystals, we develop and use an approach to compute hopping diffusion rates for self-trapped excitons; and, finally, for halide perovskites, we discuss generalization of the BSE framework to include phonon screening via introduction of an exciton-phonon kernel, and demonstrate a significant renormalization of exciton binding energies. We also develop a general Wannier-Mott model for excitons including phonon screening, clarifying its importance in polar semiconductors. In each of the three examples presented, I will emphasize implications for experiments and new principles for the design of these systems materials. This work supported by the Department of Energy and the Department of Defense; computational resources provided by NERSC.

Two dimensional materials (2D) are of interest due to their remarkable physical and chemical properties. Bending is the computationally efficient route to tune fundamental/optical gaps for device functionality [1]. Under mechanical bending, some transition metal dichalcogenide (TMD) monolayer nanoribbons undergo highly nonuniform local strain within the curved layers, much larger than uniaxial strain, making band edge states more tunable in these 2D materials. This helps to remove the strong Fermi-level pinning in the flat states, making the materials usable in contact-engineering. Many-body GW approximations can provide accurate band structures for solid materials. We use GW calculations along with recent meta-GGA density functionals to check the tunability of the band edges of MoS2 nanoribbon with various widths. We are particularly focusing on non-empirical meta-GGAs developed for band-gap prediction [2], as computationally cheaper alternatives to hybrid functionals and GW approximations.
Work is supported by DOE-BES DE-SC0021263.
[1] L. Yu, A. Ruzsinszky, J.P. Perdew, Nano Lett. 16, 2444 (2016)
[2] T. Aschebrock and S. Kümmel, Phys. Rev. Research, 1, 033082 (2019)

Heterostructures formed by vertically stacked layers of transition-metal dichalcogenides (TMDs) exhibit a long-range atomic registry modulation known as a moiré pattern, caused by the slight incommensurability and/or twist angle between the layers. Carriers and excitons in the heterostructure perceive the moiré pattern as a superlattice potential capable of fundamentally altering their energy and optical spectra through miniband formation [1] and localization by confinement [2].

In this talk I shall discuss such effects, paying especial attention to heterobilayers formed by TMDs whose electron or hole band edges are nearly degenerate. This condition promotes resonant interlayer hybridization of both carriers and excitons, leading to the formation of hybridized exciton (hX) states, i.e., coherent superpositions of intra- and interlayer excitons that simultaneously possess a large oscillator strength and electric dipole moment [3]. At small twist angles, umklapp scattering by the moiré superlattice results in hX minibands, recently observed in MoSe₂/WS₂ structures [4]. The optical response of these moiré hX minibads is thus highly tunable by electrical means and twist angle control, providing an engineering pathway for the material’s optoelectronic response. Finally, I will argue that hybridized excitons are ubiquitous in heterostructures of TMDs and their alloys, and discuss their importance for new twistronic materials for optoelectronics.

[1] D.A. Ruiz-Tijerina and V.I. Fal'ko. Phys. Rev. B 99, 125424 (2019)

[2] D.A. Ruiz-Tijerina, I. Soltero and F. Mireles. arXiv:2007.03754 [cond-mat.mes-hall]

[3] L.P. McDonnell, J.J.S. Vinier, D.A. Ruiz-Tijerina et al. arXiv:2010.02112 [cond-mat.mes-hall]

[4] E.M. Alexeev, D.A. Ruiz-Tijerina, M. Danovich et al. Nature 567, 81–86 (2019)

Wide bandgap group II-VI chalcogenides have been used in multiple optical applications. For example, doping ZnSe by first-row transition metal atoms leads to localized states within the bandgap and thus changes to the structural, electronic, and optical properties of the ZnSe host. Here we explore the properties of ZnSe for a series of transition metal dopants using first-principle calculations. We discuss the level of theory applied for calculation of the electronic structures, using a metaGGA exchange-correlation functional with a Hubbard U correction. Our calculated absorption spectra in the infrared and visible spectral ranges demonstrate agreement with experimental measurements. The electronic structures correctly predict the spectroscopic fingerprints, and using Tanabo-Sugano diagrams, the crystal field energy in these materials.

Critical points (CPs) are structures in the dielectric function (DF) which are related to interband transitions and depend on temperature and doping. We analyze CPs in the DF of bulk Ge measured with static and time-resolved spectroscopic ellipsometry using a linear filter technique based on Gaussian kernels recently introduced by Le, Kim, Kim, and Aspnes [1], which combines interpolation, noise reduction, scale change, and differentiation.
Utilizing this linear filter method, we calculate the second derivatives of the complex DF with respect to energy of experimental data taken in the spectral range of the direct band gap E₀ at various temperatures from 10 K to 718 K. The choice of the filter width is crucial to eliminate noise while preserving information at the same time and is defined according to the onset of white noise in the Fourier coefficients determined from a discrete Fourier transform of the data [1].
Applying a Levenberg-Marquardt algorithm, the second derivatives are fitted simultaneously with the imaginary part of the DF to a lineshape based on the Elliott-Tanguy theory [2] considering excitonic effects present at the direct band gap. The Elliott-Tanguy lineshape depends on the excitonic binding energy, the effective masses of the valence and conduction bands, the dipole matrix element, the threshold energy, and a broadening parameter. Effective masses of the conduction band and the heavy (hh) and light hole (lh) valence bands at cryogenic temperatures, as well as the matrix element are taken from literature. For the data set at 10 K, fitting only the band gap energy and the broadening parameters of the hh- and lh-bands provides reasonable fitting results for both the imaginary part and the second derivatives of the DF. For data above 10 K, the temperature dependence of the effective masses and the matrix element are considered. A red shift of the direct band gap energy with increasing temperature is found as well as an increase in broadening which can be fitted using a Bose-Einstein statistical factor taking into account electron-phonon interactions.
The same analysis method is applied to the E₁ and E₁+Δ₁ CPs of Ge dependent on photo-excited charge carrier density and temperature obtained from femtosecond pump-probe ellipsometry measurements [3]. Using a two-dimensional CP lineshape, the amplitude, excitonic phase angle, threshold energy, and broadening parameters are determined as functions of delay time. We find a distinctive change of the various parameters and a relaxation which starts at about 4 ps after the pump pulse. The decrease of the E₁ and E₁+Δ₁ energies suggests a laser heating of about 20 K to 40 K, respectively. In the analysis of these data, we are especially interested in changes to the E₁ and E₁+Δ₁ band gaps due to many-body effects (such as band gap renormalization and band filling), broadenings (due to screening of the electron-phonon interactions), spin-orbit coupling strengths, and excitonic phase angles as a function of the delay time between the pump and probe pulses.

[1] V. L. Le, T. J. Kim, Y. D. Kim, and D. E. Aspnes, J. Vac. Sci. & Technol. B 37, 052903 (2019)
[2] C. Tanguy, Phys. Rev. B 60, 10660 (1999)
[3] S. Espinoza, S. Richter, M. Rebarz, O. Herrfurth, R. Schmidt-Grund, J. Andreasson, and S. Zollner, Appl. Phys. Lett. 115, 052105 (2019)

Single-walled carbon nanotubes (SWNT) and single-layer graphene (SLG) both are allotropes of sp² hybridized carbon with unusual and unique properties owing to their 1D and 2D nature, respectively. In recent years, there has been a push to use hybrids of carbon nanotube and graphene for various applications which take benefit of their unique properties while overcoming the limitations of individual components. A constitutive understanding of the interactions between SWNT and SLG will allow modification and tailoring of the properties of hybrid nanostructures as desired for various applications. This work focuses on the computational study of various interactions between SWNT and SLG in their hybrid nanostructures. In some of the recent experimental works, charge transfer between the constituents (SWNT and SLG), owing to their small work-function difference, is considered to be responsible for changes in their electronic and vibrational properties in the hybrid system. However, other forms of interaction between atoms of SWNT and SLG, such as van der Waal’s (vdW) forces which may lead to localized structural deformations, may also have pronounced effect on measured properties. The interactions between carbon atoms of SWNT and SLG at close proximity may also result in electronic structure of the system that is unique to the hybrid nanostructure and different from either component (SWNT and SLG). These changes affect the overall performance of the devices based on SWNT-SLG hybrids with respect to the pristine constituents. We present the effect of various factors arising due to the interactions between atoms of SWNT and SLG, i.e. the van der Waal’s (vdW) forces, structural deformation and the charge transfer, on the structural, vibrational, electronic, and transport properties of hybrid nanostructures, investigated computationally within the framework of tight-binding density functional theory (TB-DFT). These factors are already known to affect the vibrational properties on SWNT and SLG individually and are also seen to affect the Raman active phonon frequencies of SWNT and SLG within the hybrid system. In this work, we have explored the role of individual factors and interplay between them to estimate their relative contribution to the total changes observed in phonon frequencies in the hybrid systems. From our calculations, it is apparent that the structural deformations and the vdW forces acting on the atoms are the main factors to affect the vibrational properties of components within the hybrid, with structural deformation being the leading factor. We also observe that the charge transfer between SWNT and SLG is not enough at the equilibrium separation of ~3 Å to cause any significant changes in the Raman active phonon mode frequencies. With decreasing separation between SWNT and SLG, the charge transfer increases, however, the resulting vdW forces increase much more rapidly and hence would remain the prime factor to cause changes in vibrational properties. These interactions also affect the electronic structure and electronic transport (calculated within the non-equilibrium Green function (NEGF) formalism) in the hybrid nanostructures. The electronic structure of hybrid nanostructures with various separations between SWNT and SLG and its effect on overall transport behavior is investigated to gain better understanding. We believe that our study would be helpful in comprehending experimentally observed behavior in hybrid nanostructures and may lead to the design of new hybrid systems with desirable properties.

In systems of reduced dimensionality containing metals as constituent building blocks, the pertinent conduction electrons are quantum mechanically confined, and their collective excited states of motion are termed plasmons. Recent research has witnessed intensive efforts on exploiting the rich quantum nature of plasmonic excitations in a wide variety of processes, including photon entanglement, solar energy harvesting, electron dephasing, and catalysis, to name just a few. In this talk, we will briefly review situations where the quantum nature of plasmons is bound to play a vital role. Then we will use a few recent representative systems to demonstrate how quantum plasmonics can be exploited to enhance the performance of low-dimensional heterostructures for optimal functionalities. Our first example is the enhanced energy transfer between plasmons and excitons in the strongly coupled regime known as the plexcitons. Here, we reveal the various spectroscopic signatures of the coupling strength and its tunability in systems of molecular species adsorbed on noble metal nanoparticles. The next example is the demonstration and understanding of drastically enhanced phase coherence of the electron transport in graphene proximity coupled with a plasmonic system. Pushing further on the scope, we show in the third example how plasmons can join force with phonons in enhancing the superconducting transition temperatures of interfacial superconductors and beyond.

Two-dimensional materials are promising candidates for next generation quantum devices. Rational bottom-up design of these functional materials rests on the ability to predict from first principles physical properties that are fundamental to the proposed functionalities. In this talk, we first present a first principles approach to predicting the Landau levels in monolayer H-phase transition metal dichalcogenides, which are 2D valleytronic materials. We obtain Landau levels that are symmetric in the K and K’ valleys, shifted by a valley Zeeman term.[1] By using Hamiltonians with increasing levels of sophistication, we evaluate the effects of many-body interactions on the single-band orbital magnetic moments at the valleys. The resulting Landau levels are valley- and spin-polarized, and are in good agreement with recent experiments.[2] We next address the problem of energy level alignment in mixed-dimensional heterostructures, using state-of-the-art approaches to many-body perturbation theory in the GW approximation. Our GW approach (XAF-GW) [3] can be applied to large interface systems in the presence of non-covalent interfacial hybridization. We show that cobalt phthalocyanine (CoPc) molecules assembled on a 2D vanadium diselenide substrate are promising candidates for spintronics and quantum information applications. The spin on CoPc is not quenched, and spin-dependent tunneling barriers exist due to many-body interactions in the interface-hybridized states.[4] These same many-body interactions result in a shoulder in the unoccupied spectra which is also observed in experiment.

[1] Physical Review Research, 2, 033256 (2020)
[2] Nature Nanotechnology, 12, 144 (2017)
[3] Journal of Chemical Theory and Computation, 15, 3824 (2019)
[4] Journal of Physical Chemistry Letters, 11, 9358 (2020)

Two-dimensional (2D) materials and their interfaces have recently emerged as promising platforms for exotic physical phenomena and outstanding applications. Previous methods of including substrate screening for quasiparticle energies can be only applicable to interfaces of two systems' lattice constants with certain integer proportion, which often requires a few percentage of strain. To solve this problem, we developed an efficient and accurate reciprocal-space interpolation technique for dielectric matrices that made quasiparticle energy calculations possible for arbitrarily mismatched interfaces free of strain [1]. We applied this method to obtain quasiparticle corrections at GW approximation for arbitrary mismatched 2D interfaces, by interfacing hexagonal boron nitride (hBN) with SnS2 and phosphorene at their natural lattice constants.
We then employed this method to study the effect of substrates on optical properties of 2D materials, by solving the Bethe-Salpeter equation. We obtained a perfect agreement with experimental non-rigid 1s and 2s excitonic shifts with increasing layer thickness of WS2, where the 1s peak is nearly unchanged with the number of layers, in a sharp contrast to 2s excitonic peak. Similarly, the 1s peak position of hBN was less sensitive than 2s excitonic peak when varying its substrate materials. We explained this observation in terms of the scaling relation of 1s and 2s exciton binding energy with a 2D hydrogen Wannier exciton model, and the linear scaling between exciton binding energy and quasiparticle band gap due to the environmental screening. At the end, we predicted a longer radiative lifetime of hBN with substrate screening than the one of free-standing hBN. [1] C. Guo et al, arXiv:2007.07982


*This work is supported by the National Science Foundation, under grant number 1760260.

Despite the recognition of two-dimensional (2D) systems as emerging and scalable hostmaterials of single photon emitters or spin qubits, uncontrolled and undetermined chemical nature of thesequantum defects has been a roadblock to further development. Leveraging the design of extrinsic defects cancircumvent these persistent issues and provide an ultimate solution. Here we established a completetheoretical framework to accurately and systematically design new quantum defects in wide-bandgap 2Dsystems. In particular, many-body interactions such as defect-exciton couplings are vital for describing excitedstate properties of defects in ultrathin 2D systems. Meanwhile, nonradiative processes such as phonon-assisted decay and intersystem crossing rates require careful evaluation, which compete together withradiative processes. From a thorough screening of defects based on first-principles calculations, we identifiedthe Ti-vacancy complex as a promising defect in hexagonal boron nitride for spin qubits, with a triplet groundstate, large zero-field splitting, and a prominent intersystem crossing rate highly desirable for spin-stateinitialization and qubit operation.

[1] T. J. Smart, K. Li, J. Xu, and Y. Ping, arXiv:2009.02830 (2020).

Funding Acknowledgement: NSF DMR-1760260, DMR-1956015, DMR-1747426. TJS acknowledges funding provided by LLNL Graduate Research Scholar Program. Part of this work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.

Topological materials having 8-fold electronic degeneracy protection by nonsymmorphic symmetries of a crystal exhibit double Dirac fermions which have been predicted in CuBi₂O_4. Here, we have grown CuBi₂O₄ single crystal using floating zone crystal growth technique. Polycrystalline X-ray diffraction shows that CuBi₂O₄ crystallizes in the space group P4/ncc which consists of planar [CuO₄]⁶⁻ units with Bi³⁺ ions occupying the spaces between units. The magnetization and specific heat indicate a transition to the antiferromagnetic order at TN = 43 K, in agreement with earlier literature reports. In this poster, the detailed magnetoresistance measurements in different crystal orientations will be also presented.

The electronic states for indium-tin-oxide/molybdenum disulfide (In₂Sn₂O₇/MoS₂) crystal interface have been calculated by meaning of density functional theory using CASTEP code with ultrasoft pseudopotentials and revised Perdew-Burke-Ernzerhof (RPBE) functional in General Gradient Approximation (GGA). All molecular models were built using experimental information from atom probe tomography measurements on ITO-MoS₂ thin films as previously reported [1,2]. The experimental APT data indicates no segregation between species; however, some oxygen- molybdenum chemical bonding was detected corresponding to vertical <101>-direction growth of 2H-MoS₂ crystallites onto ITO during RF-sputtering deposits. The density of states indicate a semi- metallic character which are in agreement with Ohmic behavior of resistivity values of ρ ~24-27 (Ω/m) as measured by four-point probe in ITO/MoS₂ thin film samples, as well bending of electronic semiconducting bands of MoS₂ when its on conctact with ITO.

References

[1] Manuel Ramos et al., "Mechanical properties of RF-sputtering MoS₂ thin films", IOP: Surf. Topogr.: Metrol. Prop. 5 (2017) 025003.
[2] Manuel Ramos et al., "Study of indium tin oxide MoS₂ interface by atom probe tomography", MRS Communications, doi:10.1557/mrc.2019.150