Showing papers in "Physical Review B in 2018"

Non-Hermitian robust edge states in one dimension: Anomalous localization and eigenspace condensation at exceptional points

Abstract: Capital to topological insulators, the bulk-boundary correspondence ties a topological invariant computed from the bulk (extended) states with those at the boundary, which are hence robust to disorder. Here we put forward a different ordering unique to non-Hermitian lattices whereby a pristine system becomes devoid of extended states, a property which turns out to be robust to disorder. This is enabled by a peculiar type of non-Hermitian degeneracy where a macroscopic fraction of the states coalesce at a single point with a geometrical multiplicity of 1.

Higher-order topological insulators and superconductors protected by inversion symmetry

Abstract: Here, higher-order topological insulators and superconductors protected by inversion symmetry are investigated. These phases are characterized by gapped bulk and surface with gapless modes confined to hinges or corners of the sample. Such surface states can be understood as topological defects that are globally stabilized by inversion. They can be built using a layer construction that embeds a standard topological insulator/superconductor into a higher dimension by symmetrically adding to it copies of itself. Using this procedure, a complete classification of such states in any dimension is obtained and several examples for possible physical realizations are provided.

Quantum Zeno effect and the many-body entanglement transition

Abstract: We introduce and explore a one-dimensional ``hybrid'' quantum circuit model consisting of both unitary gates and projective measurements. While the unitary gates are drawn from a random distribution and act uniformly in the circuit, the measurements are made at random positions and times throughout the system. By varying the measurement rate we can tune between the volume law entangled phase for the random unitary circuit model (no measurements) and a ``quantum Zeno phase'' where strong measurements suppress the entanglement growth to saturate in an area law. Extensive numerical simulations of the quantum trajectories of the many-particle wave functions (exploiting Clifford circuitry to access systems up to 512 qubits) provide evidence for a stable ``weak measurement phase'' that exhibits volume-law entanglement entropy, with a coefficient decreasing with increasing measurement rate. We also present evidence for a continuous quantum dynamical phase transition between the ``weak measurement phase'' and the ``quantum Zeno phase,'' driven by a competition between the entangling tendencies of unitary evolution and the disentangling tendencies of projective measurements. Detailed steady-state and dynamic critical properties of this quantum entanglement transition are accessed.

Topological phases in the non-Hermitian Su-Schrieffer-Heeger model

Abstract: We address the conditions required for a $\mathbb{Z}$ topological classification in the most general form of the non-Hermitian Su-Schrieffer-Heeger (SSH) model. Any chirally symmetric SSH model will possess a ``conjugated-pseudo-Hermiticity'' which we show is responsible for a quantized ``complex'' Berry phase. Consequently, we provide an example where the complex Berry phase of a band is used as a quantized invariant to predict the existence of gapless edge modes in a non-Hermitian model. The chirally broken, $PT$-symmetric model is studied; we suggest an explanation for why the topological invariant is a global property of the Hamiltonian. A geometrical picture is provided by examining eigenvector evolution on the Bloch sphere. We justify our analysis numerically and discuss relevant applications.

Second-order topological insulators and superconductors with an order-two crystalline symmetry

Abstract: While topological insulators have a gapped bulk band structure, they have gapless surface states. Recently, it was shown that the presence of additional crystalline symmetries can lead to topological phases that combine a gapped bulk spectrum with gapless states on edges or corners of the crystal. Such topological phases have been called ``higher-order topological phases''. This article presents a complete classification of second-order topological insulators and superconductors with mirror, twofold-rotation, or inversion symmetry. The authors show that crystals with a mirror symmetry and a nontrivial bulk band structure either have gapless surfaces or gapless edges. On the other hand, there are crystals with twofold rotation or inversion symmetry with a nontrivial bulk topology but without surface or edge states.

Strain-tunable magnetic anisotropy in monolayer CrCl 3 , CrBr 3 , and CrI 3

Abstract: Recent observation of intrinsic ferromagnetism in two-dimensional (2D) ${\mathrm{CrI}}_{3}$ is associated with the large magnetic anisotropy due to strong spin-orbit coupling of I. Magnetic anisotropy energy (MAE) defines the stability of magnetization in a specific direction with respect to the crystal lattice and is an important parameter for nanoscale applications. In this work we apply the density functional theory to study the strain dependence of MAE in 2D monolayer chromium trihalides $\mathrm{Cr}{X}_{3}$ (with $X$ = Cl, Br, and I). Detailed calculations of their energetics, atomic structures, and electronic structures under the influence of a biaxial strain $\ensuremath{\varepsilon}$ have been carried out. It is found that all three compounds exhibit ferromagnetic ordering at the ground state (with $\ensuremath{\varepsilon}=0$), and upon applying a compressive strain, phase transition to the antiferromagnetic state occurs. Unlike in ${\mathrm{CrCl}}_{3}$ and ${\mathrm{CrBr}}_{3}$, the electronic band gap in ${\mathrm{CrI}}_{3}$ increases when a tensile strain is applied. The MAE also exhibits a strain dependence in the chromium trihalides: it increases when a compressive strain is applied in ${\mathrm{CrI}}_{3}$, while an opposite trend is observed in the other two compounds. In particular, the MAE of ${\mathrm{CrI}}_{3}$ can be increased by 47% with a compressive strain of $\ensuremath{\varepsilon}$ = 5%.

Quantum scarred eigenstates in a Rydberg atom chain: Entanglement, breakdown of thermalization, and stability to perturbations

Abstract: Recent realization of a kinetically constrained chain of Rydberg atoms by Bernien et al., [Nature (London) 551, 579 (2017)] resulted in the observation of unusual revivals in the many-body quantum dynamics. In our previous work [C. J. Turner et al., Nat. Phys. 14, 745 (2018)], such dynamics was attributed to the existence of “quantum scarred” eigenstates in the many-body spectrum of the experimentally realized model. Here, we present a detailed study of the eigenstate properties of the same model. We find that the majority of the eigenstates exhibit anomalous thermalization: the observable expectation values converge to their Gibbs ensemble values, but parametrically slower compared to the predictions of the eigenstate thermalization hypothesis (ETH). Amidst the thermalizing spectrum, we identify nonergodic eigenstates that strongly violate the ETH, whose number grows polynomially with system size. Previously, the same eigenstates were identified via large overlaps with certain product states, and were used to explain the revivals observed in experiment. Here, we find that these eigenstates, in addition to highly atypical expectation values of local observables, also exhibit subthermal entanglement entropy that scales logarithmically with the system size. Moreover, we identify an additional class of quantum scarred eigenstates, and discuss their manifestations in the dynamics starting from initial product states. We use forward scattering approximation to describe the structure and physical properties of quantum scarred eigenstates. Finally, we discuss the stability of quantum scars to various perturbations. We observe that quantum scars remain robust when the introduced perturbation is compatible with the forward scattering approximation. In contrast, the perturbations which most efficiently destroy quantum scars also lead to the restoration of “canonical” thermalization.

Band structure of twisted bilayer graphene: Emergent symmetries, commensurate approximants, and Wannier obstructions

Abstract: Motivated by the exciting discovery of correlated insulating behavior and superconductivity in magic-angle bilayer graphene, the authors discuss here the salient features of the band structure of this system. A remarkable feature is the appearance of isolated, nearly flat bands around the magic angles. The authors discuss the importance of emergent symmetries of this system and clarify some potential confusion regarding commensurate structures of twisted bilayer graphene. Furthermore, they discuss the band topology, which leads to obstructions to constructing well-localized Wannier functions of the nearly flat bands, and give directions towards overcoming the difficulties.

Second-order photonic topological insulator with corner states

Abstract: Higher-order topological insulators (HOTIs) which go beyond the description of conventional bulk-boundary correspondence, broaden the understanding of topological insulating phases. Being mainly focused on electronic materials, HOTIs have not yet been found in photonic crystals. Here, we propose a type of two-dimensional second-order photonic crystals with zero-dimensional corner states and one-dimensional boundary states for optical frequencies. All of these states are topologically nontrivial and can be understood based on the theory of topological polarization. Moreover, by tuning the easily fabricated structure of the photonic crystals, different topological phases can be realized straightforwardly. Our study can be generalized to higher dimensions and provides a platform for higher-order photonic topological insulators and semimetals.

Model for the metal-insulator transition in graphene superlattices and beyond

Abstract: In bilayer graphene with a small twist angle, the moir\'e pattern creates a superlattice with over 10000 atoms per supercell and complex minibands with a small bandwidth. The low-energy narrow bands host correlated insulating and unconventional superconducting phases discovered recently. Here, the authors introduce a two-orbital tight-binding and Hubbard model on a honeycomb lattice to capture the physics of these narrow bands. This model provides a starting point for studying the metal-insulator transition, Landau level degeneracy lifting, and superconductivity in twisted bilayer graphene.

Equivalence of restricted Boltzmann machines and tensor network states

Abstract: The restricted Boltzmann machine is a fundamental building block of deep learning. The authors demonstrate its equivalence with tensor network states with explicit mappings, thus drawing a constructive connection between deep learning and quantum physics. On one side, deep learning approaches can be used to study novel states of matter. In return, investigations of tensor network states and their expressibility can be adapted to guide neural network architecture design.

Entanglement of exact excited states of Affleck-Kennedy-Lieb-Tasaki models: Exact results, many-body scars, and violation of the strong eigenstate thermalization hypothesis

Abstract: We obtain multiple exact results on the entanglement of the exact excited states of nonintegrable models we introduced in Phys. Rev. B 98, 235155 (2018)10.1103/PhysRevB.98.235155. We first discuss a general formalism to analytically compute the entanglement spectra of exact excited states using matrix product states and matrix product operators and illustrate the method by reproducing a general result on single-mode excitations. We then apply this technique to analytically obtain the entanglement spectra of the infinite tower of states of the spin-S AKLT models in the zero and finite energy density limits. We show that in the zero energy density limit, the entanglement spectra of the tower of states are multiple shifted copies of the ground-state entanglement spectrum in the thermodynamic limit. We show that such a resemblance is destroyed at any nonzero energy density. Furthermore, the entanglement entropy S of the states of the tower that are in the bulk of the spectrum is subthermal S∝logL, as opposed to a volume law S∝L, thus indicating a violation of the strong eigenstate thermalization hypothesis (ETH). These states are examples of what are now called many-body scars. Finally, we analytically study the finite-size effects and symmetry-protected degeneracies in the entanglement spectra of the excited states, extending the existing theory.

Nodal surface semimetals: Theory and material realization

Abstract: We theoretically study the three-dimensional topological semimetals with nodal surfaces protected by crystalline symmetries. Different from the well-known nodal-point and nodal-line semimetals, in these materials, the conduction and valence bands cross on closed nodal surfaces in the Brillouin zone. We propose different classes of nodal surfaces, both in the absence and in the presence of spin-orbit coupling (SOC). In the absence of SOC, a class of nodal surfaces can be protected by space-time inversion symmetry and sublattice symmetry and characterized by a ${\mathbb{Z}}_{2}$ index, while another class of nodal surfaces are guaranteed by a combination of nonsymmorphic twofold screw-rotational symmetry and time-reversal symmetry. We show that the inclusion of SOC will destroy the former class of nodal surfaces but may preserve the latter provided that the inversion symmetry is broken. We further generalize the result to magnetically ordered systems and show that protected nodal surfaces can also exist in magnetic materials without and with SOC, given that certain magnetic group symmetry requirements are satisfied. Several concrete nodal-surface material examples are predicted via the first-principles calculations. The possibility of multi-nodal-surface materials is discussed.

Phases of a phenomenological model of twisted bilayer graphene

Abstract: Recent measurements in magic-angle twisted bilayer graphene find a correlated insulator flanked by superconducting domes upon tuning the gate voltage. These results have prompted an analogy with the high-${T}_{c}$ cuprates and the physics of doped Mott insulators. Here, the authors instead propose an analogy with alkali-doped C${}_{60}$ and a violation of Hund's first rule, whereby the resultant attractive interactions, as opposed to a large repulsive Hubbard $U$, are responsible for superconductivity. A two-band phenomenological model for twisted bilayer graphene is studied, which includes nematic orbital ferromagnet and orbital-triplet spin-singlet superconducting broken-symmetry phases.

Towards predictive many-body calculations of phonon-limited carrier mobilities in semiconductors

Abstract: We probe the accuracy limit of ab initio calculations of carrier mobilities in semiconductors, within the framework of the Boltzmann transport equation. By focusing on the paradigmatic case of silicon, we show that fully predictive calculations of electron and hole mobilities require many-body quasiparticle corrections to band structures and electron-phonon matrix elements, the inclusion of spin-orbit coupling, and an extremely fine sampling of inelastic scattering processes in momentum space. By considering all these factors we obtain excellent agreement with experiment, and we identify the band effective masses as the most critical parameters to achieve predictive accuracy. Our findings set a blueprint for future calculations of carrier mobilities, and pave the way to engineering transport properties in semiconductors by design.

Exact excited states of nonintegrable models

Abstract: We discuss a method of numerically identifying exact energy eigenstates for a finite system, whose form can then be obtained analytically. We demonstrate our method by identifying and deriving exact analytic expressions for several excited states, including an infinite tower, of the one-dimensional spin-1 Affleck-Kennedy-Lieb-Tasaki (AKLT) model, a celebrated nonintegrable model. The states thus obtained for the AKLT model can be interpreted as from one to an extensive number of quasiparticles on the ground state or on the highest excited state when written in terms of dimers. Included in these exact states is a tower of states spanning energies from the ground state to the highest excited state. Some of the states of the tower appear to be in the bulk of the energy spectrum, allowing us to make conjectures on the strong eigenstate thermalization hypothesis. We also generalize these exact states including the tower of states to the generalized integer spin AKLT models. Furthermore, we establish a correspondence between some of our states and those of the Majumdar-Ghosh model, yet another nonintegrable model, and extend our construction to the generalized integer spin AKLT models.

Hydrodynamics of operator spreading and quasiparticle diffusion in interacting integrable systems

Abstract: We address the hydrodynamics of operator spreading in interacting integrable lattice models. In these models, operators spread through the ballistic propagation of quasiparticles, with an operator front whose velocity is locally set by the fastest quasiparticle velocity. In interacting integrable systems, this velocity depends on the density of the other quasiparticles, so equilibrium density fluctuations cause the front to follow a biased random walk, and therefore to broaden diffusively. Ballistic front propagation and diffusive front broadening are also generically present in nonintegrable systems in one dimension; thus, although the mechanisms for operator spreading are distinct in the two cases, these coarse-grained measures of the operator front do not distinguish between the two cases. We present an expression for the front-broadening rate; we explicitly derive this for a particular integrable model (the ``Floquet-Fredrickson-Andersen'' model), and argue on kinetic grounds that it should apply generally. Our results elucidate the microscopic mechanism for diffusive corrections to ballistic transport in interacting integrable models.

Strain engineering of the silicon-vacancy center in diamond

Abstract: We control the electronic structure of the silicon-vacancy (SiV) color-center in diamond by changing its static strain environment with a nano-electro-mechanical system This allows deterministic and local tuning of SiV optical and spin transition frequencies over a wide range, an essential step towards multiqubit networks In the process, we infer the strain Hamiltonian of the SiV revealing large strain susceptibilities of order 1 PHz/strain for the electronic orbital states We identify regimes where the spin-orbit interaction results in a large strain susceptibility of order 100 THz/strain for spin transitions, and propose an experiment where the SiV spin is strongly coupled to a nanomechanical resonator

Native point defects and impurities in hexagonal boron nitride

Abstract: Hexagonal boron nitride ($h$-BN) is a technologically important electronic and dielectric material. Recently, research into this material has intensified due to the discovery of bright single-photon-emitting color centers in the form of point defects, with potential applications in advanced quantum technologies. Despite this technological promise, the defect physics of this material is still largely unexplored. Here, the authors have performed detailed first-principles investigations of the defect properties of $h$-BN, finding that the defect physics of this material is dictated by impurities, in particular carbon, oxygen, and hydrogen. They show how the defect properties of this material can be selectively engineered by growth or processing conditions. These insights will be fundamental to controlled generation of single-photon emitters and for general defect engineering of this material.

Theory of optical absorption by interlayer excitons in transition metal dichalcogenide heterobilayers

Abstract: We present a theory of optical absorption by interlayer excitons in a heterobilayer formed from transition metal dichalcogenides. The theory accounts for the presence of small relative rotations that produce a momentum shift between electron and hole bands located in different layers, and a moir\'e pattern in real space. Because of the momentum shift, the optically active interlayer excitons are located at the moir\'e Brillouin zone's corners, instead of at its center, and would have elliptical optical selection rules if the individual layers were translationally invariant. We show that the exciton moir\'e potential energy restores circular optical selection rules by coupling excitons with different center of mass momenta. A variety of interlayer excitons with both senses of circular optical activity, and energies that are tunable by twist angle, are present at each valley. The lowest energy exciton states are generally localized near the exciton potential energy minima. We discuss the possibility of using the moir\'e pattern to achieve scalable two-dimensional arrays of nearly identical quantum dots.

Non-Hermitian perspective of the band structure in heavy-fermion systems

Abstract: We analyze a two-dimensional Kondo lattice model with special emphasis on non-Hermitian properties of the single-particle spectrum, following a recent proposal by Kozii and Fu. Our analysis based on the dynamical mean-field theory elucidates that the single-particle spectral weight shows the exceptional points (EPs). Correspondingly, the spectral weight exhibits the band touching, resulting in a structure similar to the Fermi arc. Furthermore, we find an intriguing phenomenon arising from the periodicity of the lattice. The EPs generated by two distinct Dirac points merge and change into a hybrid point, which vanishes as the exchange interaction is increased. Accordingly, the paramagnetic phase in the low-temperature region shows a significant difference from noninteracting fermions: the imaginary part of the self-energy yields the Fermi loop without any defective points.

Bethe-Boltzmann hydrodynamics and spin transport in the XXZ chain

Abstract: The anomalous nature of spin transport in the XXZ quantum spin chain has been a topic of theoretical interest for some time. Here, the integrability of the underlying dynamics leads to a ballistic component of the spin current, characterized by a spin Drude weight, which measures the degree of divergence of the zero-frequency spin conductivity. However, this quantity had previously proven to be beyond the reach of standard Bethe ansatz techniques. Here, the authors show that a recently developed hydrodynamic formalism for quantum integrable models may be used to compute the spin Drude weight. They also propose a numerical scheme to obtain hydrodynamic predictions for finite-time energy transport. This suggests that the hydrodynamic approach captures completely the ballistic component that dominates transport at long times and distances in the gapless regime of the XXZ model.

Nonlocality of Majorana modes in hybrid nanowires

Abstract: The nonlocal character of Majorana zero modes is a key feature for topological protection of Majorana qubits, and distinguishes separated Majorana bound states from localized Andreev modes. Applying recent theoretical results on the coupling of quantum dots at the wire ends to the zero-energy mode in the wire, the authors provide a first quantitative measure of the nonlocality of Majorana modes in semiconductor-superconductor hybrid nanowires. The spin character of the Majorana mode is revealed by analyzing its coupling to two consecutive quantum dot resonances. Cases of zero modes with moderate and high degrees of nonlocality are compared.

Building blocks of topological quantum chemistry: Elementary band representations

Abstract: The link between chemical orbitals described by local degrees of freedom and band theory, which is defined in momentum space, was proposed by Zak several decades ago for spinless systems with and without time reversal in his theory of ``elementary'' band representations. In a recent paper [Bradlyn et al., Nature (London) 547, 298 (2017)] we introduced the generalization of this theory to the experimentally relevant situation of spin-orbit coupled systems with time-reversal symmetry and proved that all bands that do not transform as band representations are topological. Here we give the full details of this construction. We prove that elementary band representations are either connected as bands in the Brillouin zone and are described by localized Wannier orbitals respecting the symmetries of the lattice (including time reversal when applicable), or, if disconnected, describe topological insulators. We then show how to generate a band representation from a particular Wyckoff position and determine which Wyckoff positions generate elementary band representations for all space groups. This theory applies to spinful and spinless systems, in all dimensions, with and without time reversal. We introduce a homotopic notion of equivalence and show that it results in a finer classification of topological phases than approaches based only on the symmetry of wave functions at special points in the Brillouin zone. Utilizing a mapping of the band connectivity into a graph theory problem, we show in companion papers which Wyckoff positions can generate disconnected elementary band representations, furnishing a natural avenue for a systematic materials search.

Two-terminal charge tunneling: Disentangling Majorana zero modes from partially separated Andreev bound states in semiconductor-superconductor heterostructures

Abstract: We show that a pair of overlapping Majorana bound states (MBSs) forming a partially separated Andreev bound state (ps-ABS) represents a generic low-energy feature in spin-orbit-coupled semiconductor-superconductor (SM-SC) hybrid nanowire in the presence of a Zeeman field. The ps-ABS interpolates continuously between the ``garden variety'' ABS, which consists of two MBSs sitting on top of each other, and the topologically protected Majorana zero modes (MZMs), which are separated by a distance given by the length of the wire. The really problematic ps-ABSs consist of component MBSs separated by a distance of the order of the characteristic Majorana decay length $\ensuremath{\xi}$, and have nearly zero energy in a significant range of control parameters, such as the Zeeman field and chemical potential, within the topologically trivial phase. Despite being topologically trivial, such ps-ABSs can generate signatures identical to MZMs in local charge tunneling experiments. In particular, the height of the zero-bias conductance peak (ZBCP) generated by ps-ABSs has the quantized value $2{e}^{2}/h$, and it can remain unchanged in an extended range of experimental parameters, such as Zeeman field and the tunnel barrier height. We illustrate the formation of such low-energy robust ps-ABSs in two experimentally relevant situations: a hybrid SM-SC system consisting of a proximitized nanowire coupled to a quantum dot and the SM-SC system in the presence of a spatially varying inhomogeneous potential. We then show that, unlike local measurements, a two-terminal experiment involving charge tunneling at both ends of the wire is capable of distinguishing between the generic ps-ABSs and the non-Abelian MZMs. While the MZMs localized at the opposite ends of the wire generate correlated differential conduction spectra, including correlations in energy splittings and critical Zeeman fields associated with the emergence of the ZBCPs, such correlations are absent if the ZBCPs are due to ps-ABSs emerging in the topologically trivial phase. Measuring such correlations is the clearest and most straightforward test of topological MZMs in SM-SC heterostructures that can be done in a currently accessible experimental setup.

Intrinsic and anisotropic Rashba spin splitting in Janus transition-metal dichalcogenide monolayers

Abstract: Transition-metal dichalcogenides (TMDs) monolayers have been considered as important two-dimensional semiconductor materials for the study of fundamental physics in the field of spintronics. However, the out-of-plane mirror symmetry in TMDs may constrain electrons' degrees of freedom and it may limit spin-related applications. Recently, a newly synthesized Janus TMDs MoSSe was found to intrinsically possess both the in-plane inversion and the out-of-plane mirror-symmetry breaking. Here we performed first-principles calculations in order to systematically investigate the electronic band structures of a series of Janus monolayer TMDs with chemical formula $MXY\phantom{\rule{0.16em}{0ex}}(M=\mathrm{Mo},\phantom{\rule{0.16em}{0ex}}\mathrm{W}\phantom{\rule{0.16em}{0ex}}\mathrm{and}\phantom{\rule{0.16em}{0ex}}X,Y=\mathrm{S},\phantom{\rule{0.16em}{0ex}}\mathrm{Se},\phantom{\rule{0.16em}{0ex}}\mathrm{Te})$. It is found that they possess robust electronic properties like their parent phases. We explored also the effect of perpendicular external electric field and in-plane biaxial strain on the Rashba spin splittings. The Zeeman-type spin splitting and valley polarization at $K({K}^{\ensuremath{'}})$ point are well preserved and we observed a Rashba-type spin splitting around the $\mathrm{\ensuremath{\Gamma}}$ point for all the $MXY$ systems. We have also found that these spin splittings can be enhanced by an external electric field collinear with the local electric field derived by the polar bonds and by the compressive strain. The Rashba parameters change linearly with the external electric field, but nonlinearly with the biaxial strain. The compressive strain is found to enhance significantly the anisotropic Rashba spin splitting.

Strong Correlations and d+id Superconductivity in Twisted Bilayer Graphene

Abstract: Recently, the experimental realization of flat bands in twisted bilayer graphene has attracted much attention as in these systems the kinetic energy is almost fully quenched, giving rise to prominent interaction effects. Indeed, experimental indications for correlation-induced insulating as well as superconducting phases show a striking similarity to the physics of unconventional high-${T}_{c}$ superconductors. Here, the authors employ an unbiased functional renormalization group scheme to connect to these findings from a theoretical side. A topological $d$-wave superconducting state as well as insulating behavior are found, depending on the parameters of the model.

Topological surface Fermi arcs in the magnetic Weyl semimetal Co3Sn2S2

Abstract: Very recently, the half-metallic compound Co3Sn2S2 was proposed to be a magnetic Weyl semimetal (WSM) with Weyl points only 60 meV above the Fermi level E-F. Owing to the low charge carrier density and large Berry curvature induced, Co3Sn2S2 possesses both a large anomalous Hall conductivity and a large anomalous Hall angle, which provide strong evidence for the existence of Weyl points in Co3Sn2S2. In this work, we theoretically study the surface topological feature of Co3Sn2S2 and its counterpart Co3Sn2Se2. By cleaving the sample at the weak Sn-S/Se bonds, one can achieve two different surfaces terminated with Sn and S/Se atoms, respectively. The resulting Fermi-arc-related states can range from the energy of the Weyl points to E-F - 0.1 eV in the Sn-terminated surface. Therefore, it should be possible to observe the Fermi arcs in angle-resolved photoemission spectroscopy (ARPES) measurements. Furthermore, in order to simulate quasiparticle interference in scanning tunneling microscopy (STM) measurements, we also calculate the joint density of states for both terminals. This work should be helpful for a comprehensive understanding of the topological properties of these two magnetic WSMs and further ARPES and STM measurements.

Universality of electronic characteristics and photocatalyst applications in the two-dimensional Janus transition metal dichalcogenides

Abstract: Due to mirror symmetry breaking, two-dimensional Janus transition metal dichalcogenides $MXY$ ($M=\text{Mo,W}$; $X,Y=\text{S,Se,Te}$) present charming electronic properties. However, there have not been many related studies as of yet, and the intrinsic physical pictures are unclear. Here, we use first-principles calculations to explore the universality of electronic characteristics and photocatalyst applications of Janus $MXY$, finding that the induced dipole moment, vibrational frequency, Rashba parameters, and direct-indirect band transition of monolayer $MXY$ are deeply associated with the atomic radius and electronegativity differences of chalcogen $X$ and $Y$ elements. The internal electric field renders Janus $MXY$ the ideal photocatalysts. Moreover, the stacking-dependent on/off switch of the dipole moment further confirms that asymmetric Janus $MXY$ serves as a promising candidate for highly efficient photocatalysts within a broad range from infrared, visible, to ultraviolet light.

Hubbard parameters from density-functional perturbation theory

Abstract: We present a transparent and computationally efficient approach for the first-principles calculation of Hubbard parameters from linear-response theory. This approach is based on density-functional perturbation theory and the use of monochromatic perturbations. In addition to delivering much improved efficiency, the present approach makes it straightforward to calculate automatically these Hubbard parameters for any given system, with tight numerical control on convergence and precision. The effectiveness of the method is showcased in three case studies---${\mathrm{Cu}}_{2}\mathrm{O}$, NiO, and ${\mathrm{LiCoO}}_{2}$---and by the direct comparison with finite differences in supercell calculations.