Composite quantum compounds (CQCs) have emerged as a key tool for examining the interactions between two different physical phenomena. Some of these CQCs that have lately received a lot of community attention are topological superconductors and axion insulators, among others. Despite being two distinct quantum phenomena, Rashba spin physics and topological nontriviality can coexist in a CQC platform. In this work, we have performed ab initio calculations to discover materials that inherit both giant Rashba splitting and topological nontrivial states in a single crystalline system, coined as intrinsic CQCs. In this regard, we have investigated two materials, RbSnBi and CsSnBi, that are found to be strong topological insulators along with giant Rashba splitting energy in valence bands (133 and $228\text{meV}$, respectively) with Rashba coefficient ${\alpha }_{\mathrm{R}}$ to be 4.26 and $6.4\text{eV}\AA$, respectively. These values of ${\alpha }_{\mathrm{R}}$ are quite large, and the Rashba coefficient of CsSnBi is the largest, corresponding to valance bands, among previously reported topological materials. Coexisting characteristics of Rashba spin physics and topological nontriviality have the potential to unveil new physical phenomena. For both RbSnBi and CsSnBi, the topological insulating state is found to be associated with coexisting multiple band inversion and multiple Dirac surface states.

A number of compounds in the family of rare-earth half-Heusler alloys have been predicted to be topologically nontrivial semimetals, classified as Weyl, triple-point, and Dirac semimetals based on the multiplicity of degeneracy of the nodal points or crossing of linearly dispersed electronic bands. Here, we present a first-principles theoretical characterization of the electronic topology of the antiferromagnetic half-Heusler alloy DyPdBi. In the antiferromagnetic state preserving $C_{3v}$ symmetry of the crystal, DyPdBi is a triple-point semimetal hosting four triply degenerate nodes along the threefold symmetry axis of the Brillouin zone. In contrast, the antiferromagnetic state of DyPdBi with local magnetic moments on Dy rotated to a direction perpendicular to the $C_3$ axis breaks the threefold rotational symmetry, and hosts four Weyl nodes in its Brillouin zone. Our calculations of the Berry curvature of their electronic states clearly show that the triple-point fermions of DyPdBi exhibit a signature peak in the anomalous Hall conductivity, while the Weyl fermions do not contribute to anomalous Hall conductivity. As these two topologically distinct magnetic states are separated by a small energy difference of $\approx$15 meV, we expect them to be switchable with a magnetic field or spin torque and distinguishable experimentally from anomalous Hall conductance.

The “abnormal” self-reduction of several trivalent lanthanides in borates has been reported for a couple of decades, especially in ${\mathrm{SrB}}_4{\mathrm{O}}_7$, but only a phenomenological explanation has been given. Our calculations of the formation energies of lanthanide-doped and intrinsic defects in ${\mathrm{SrB}}_4{\mathrm{O}}_7$ provide an explanation for the mechanisms of self-reduction phenomena. Furthermore, we also give the trends of the $4f$ and $5d$ host referred binding energies of divalent lanthanides, which well fits and supports the result based on the extrapolation of the experimental data with the semiempirical chemical shift model [P. Dorenbos, Phys. Rev. B 85, 165107 (2012)]. Our results show that the calculations based on the supercell method by employing the projector augmented wave pseudopotentials are capable of interpreting the reduction and charge-transfer phenomena, and can provide a basis for the design of lanthanide-doped optical materials.

Recently, the magnetic domain walls have been experimentally observed in antiferromagnetic topological insulators ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$. Here we study the intrinsic topological zero-line modes (ZLMs) that appear along the domain walls, and find that these ZLMs are layer dependent in ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ multilayers. We reveal the role of the spatial layer degree of freedom and magnetic domain wall configurations in determining the electronic transport properties and the distribution of the ZLMs in antiferromagnetic topological insulator systems. For antiferromagnetic domain wall with out-of-plane magnetization within each domain, we find that ZLMs are only distributed in the odd-number layers equally. For the Néel domain wall, the ZLMs are no longer distributed in the odd-number layers equally due to the mirror symmetry $\left(M_z\right)$ breaking, and can exist in the even-number layers in multilayer systems. Moreover, the ZLMs are mainly distributed in the outermost layers with increasing layer thickness. Our findings lay out a strategy in manipulating ZLMs and can be utilized to distinguish the corresponding magnetic structures.

The advent of two-dimensional (2D) magnetic materials significantly expand the scope of voltage-controlled magnetization-switching scheme as their integration in diverse magnetic tunnel junctions (MTJ) offers a highly attractive perspective for designing future magnetoelectric random access memory (MeRAM). Here, we propose the utilization of 2D ferromagnetic ${\mathrm{Fe}}_2{\mathrm{I}}_2$ to substitute traditional magnetic thin films for assembling energy-efficient MTJs combined with Ir capping layers. We find these multilayers exhibit both giant perpendicular magnetic anisotropy (PMA) and voltage controlled magnetic anisotropy (VCMA) efficiency depending strongly on the Ir thickness and epitaxial strain effect. Furthermore, the spin reorientation has also been achieved with increased Ir thickness, and the magnitude and slope of VCMA behavior are dominated by the biaxial strain. We elucidate that the underlying mechanism is the electric-field-induced modifications of the spin-orbit coupling energies of the spin-polarized $\mathrm{Ir}\text{−}d$ orbitals. These findings not only reveal new approaches controlling 2D magnetism, but also pave the way of an alternative strategy for the design of nonvolatile and ultralow power spintronics and magnetic memory storage devices.

Photoemission from nanostructures under strong fields has attracted significant interests in materials science and optoelectronic applications. Here, we report the extreme nonlinear photoemission in single-walled carbon nanotubes (SWCNTs) and its underlying mechanisms related to the microscopic carrier excitation and the sequential emission of occupied-state electrons. We show that the characteristic photoemission responses, e.g., nonlinear slope, of individual SWCNTs strongly depend on the unique electronic structures near the Fermi level, i.e., localization of the emitting states and their tunneling probabilities. The transition of photoemission mechanisms giving rise to the extreme nonlinear behavior was elucidated using Kohn-Sham potentials and the extended Fowler-Nordheim theory. This paper provides insights into photoemission dynamics of SWCNTs on the ultimate atomic length scale and attosecond time scale, will hopefully help design materials for tunable optoelectronic responses.

In van der Waals (vdW) materials, the electron mean free path (MFP) is largely influenced by the discrete states in the unoccupied band structure. So far, the influence of these states has only been measured in graphene, while all measurements on other vdW materials lack energy resolution. Here, we present reflection and transmission spectra of freestanding, few-layered molybdenum disulfide $\left(\mathrm{Mo}{\mathrm{S}}_2\right)$ samples in the $0–55$ eV electron range. Our measurements reveal states of enhanced electron transmissivity above the vacuum level, that correspond to the (unoccupied) density of states. We also show a full quantum-mechanical calculation that confirms a good understanding of the elastic scattering in $\mathrm{Mo}{\mathrm{S}}_2$. A model is developed to extract the inelastic MFP spectrum, which is a measure of the inelastic scattering cross section. As $\mathrm{Mo}{\mathrm{S}}_2$ is a complicated system of different atomic planes, we expect that our methods generalize well to other van der Waals materials and heterostacks thereof.

Including the phonon-assisted inelastic process in thermoelectric devices makes it possible to enhance the performance of nonequilibrium work extraction. In this work, we demonstrate that inelastic phonon-thermoelectric devices have a fertile functionality diagram, where particle current and phononic heat currents are coupled and fueled by the chemical potential difference. Such devices can simultaneously perform multiple tasks, e.g., heat engines, refrigerators, and heat pumps. Guided by the entropy production, we mainly study the efficiencies and coefficients of performance of multitask quantum thermal machines, where the roles of the inelastic scattering process and multiple biases in multiterminal setups are emphasized. Specifically, in a three-terminal double-quantum-dot setup with a tunable gate, we show that it efficiently performs two useful tasks due to the phonon-assisted inelastic process. For the four-terminal four-quantum-dot thermoelectric setup, we find that additional thermodynamic affinity furnishes the system with both enriched functionality and enhanced efficiency. Our work provides insights into optimizing phonon-thermoelectric devices.

We report ultrafast studies on atomically thin Gallium telluride, a 2D metal monochalcogenide that has appeared to display superior photodetection properties in visible frequencies. Pump photon energy-dependent spectroscopic studies reveal that photoinduced carriers in this direct band-gap material undergo indirect relaxation within $\sim 30$ ps of photoexcitation, which is at least an order slower than that of most 2D materials. Despite the direct band-gap nature, slow and indirect carrier relaxation places this layered material as a prime candidate in the multitude of atomically thin semiconductor-based photodetectors and highlights the potential for prospective optoelectronic applications.

We investigate strain-tuned spin transport and optical conductivity in a monolayer (ML) $n$-type ${\mathrm{MoS}}_2/\mathrm{EuS}$ heterostructure under linearly polarized terahertz radiation using a low-energy effective Hamiltonian, which takes displacement of the Dirac point induced by strain into account. Through modifying the strain modulus, we find an adjustable relationship between the band edge energy and the strain. Thus the threshold and range of allowed angles are strongly dependent on the strength of strain $\epsilon$, whereas the angle $\theta$ along which the strain is applied only acts on the former. Due to spin-flipped scattering generated by Rashba spin-orbit coupling, the in-plane spin polarizations (i.e., $P_X$ and $P_Y$) can be achieved, and their intensity can be enhanced by increasing $\epsilon$. Interestingly, we find that the out-of-plane spin polarization (i.e., $P_Z$) is related to the synergy of $\epsilon$ and the Rashba parameter ${\lambda }_R$. Especially, $P_Z$ can approach $100\%$ when $\epsilon =9\%$ and ${\lambda }_R=2$ meV. Furthermore, the optical transition between the spin-splitting subbands of the conduction band under the strain is studied. It is shown that by varying $\epsilon$ we can control the magnitude of both absorption peaks and absorption valleys in optical conductivity. Moreover, the presence of ${\lambda }_R$ can be used to further enhance the absorption peaks and valleys because ${\lambda }_R$ can tune the energy difference between the spin-splitting subbands within a specific valley and thus affect the optical transition channels. Our findings demonstrate that the strain can influence the tunneling behavior and valley Hall conductivity in a ML $n$-type ${\mathrm{MoS}}_2/\mathrm{EuS}$ heterostructure, and these features are applicable to other two-dimensional ML transition metal dichalcogenides, which is promising for terahertz and valleytronic devices.

The dynamical quantum phase transition (DQPT), which is featured by the nonanalytic behavior of the Loschmidt rate function in the real time evolution, has attracted substantial attention as a valuable theoretical concept for characterizing nonequilibrium states of quantum matter. Although the link between the DQPT and many physical concepts has been established, a thorough understanding of this transition still calls for more studies. In this paper, from the perspective of the quantum steering ellipsoid (QSE), we investigate the DQPTs supported in a one-dimensional spin chain with a deconfined quantum critical point by using the global subspace expansion time-dependent variational principle algorithm. For the quench from the valence-bond-solid phase to the ferromagnetic phase, we find a clear correspondence between the vanishing of the QSE volume and the occurrence of the DQPT. For the quench in the opposite direction, however, the QSE exhibits a rather complicated behavior during the time evolution. We also calculate the quantum entanglement and quantum coherence in the quench processes to unveil the change of the quantum correlations encoded in the QSE picture. These findings could offer a fascinating possibility of revealing DQPTs in a geometrically discernible manner.

We study the interplay between electron-electron interaction and a Jahn-Teller phonon coupling in a two-orbital Hubbard model. We demonstrate that the e-ph interaction coexists with the Mott localization driven by the Hubbard repulsion $U$, but it competes with the Hund's coupling $J$. This interplay leads to two spectacularly different Mott insulators, a standard high-spin Mott insulator with frozen phonons which is stable when the Hund's coupling prevails, and a low-spin Mott-bipolaronic insulator favored by phonons, where the characteristic features of Mott insulators and bipolarons coexist. The two phases are separated by a sharp boundary along which an intriguing intermediate solution emerges as a kind of compromise between the two solutions.

The theoretical scattering cross section of electron energy loss spectroscopy (EELS) is essentially given by $-\text{Im}{\varepsilon }^{-1}(\mathbf{k},\omega )$ with the energy loss $\hslash \omega$ and the momentum transfer $\hslash \mathbf{k}$. The macroscopic dielectric function $\varepsilon (\mathbf{k},\omega )$ can be calculated from first principles using time-dependent density-functional theory. However, experimental EELS measurements have a finite $\mathbf{k}$ resolution or, when operated in spatial resolution mode, yield a $\mathbf{k}$-integrated loss spectrum, which deviates significantly from EEL spectra calculated for specific $\mathbf{k}$ momenta. On the other hand, integrating the theoretical spectra over $\mathbf{k}$ is complicated by the fact that the integrand varies over several (typically six) orders of magnitude around $k=0$. In this article, we present a stable technique for integrating EEL spectra over an adjustable range of momentum transfers. The important region around $k=0$, where the integrand is nearly divergent, is treated partially analytically, allowing an analytic integration of the near divergence. The scheme is applied to three prototypical two-dimensional systems: monolayers of ${\mathrm{MoS}}_2$ (semiconductor), hexagonal BN (insulator), and graphene (semimetal). Here, we are confronted with the added difficulty that the long-range Coulomb interaction leads to a very slow supercell (vacuum size) convergence. We address this difficulty by employing an extrapolation scheme, enabling an efficient reduction of the supercell size and thus a considerable speedup in computation time. The calculated $\mathbf{k}$-integrated spectra are in very favorable agreement with experimental EEL spectra.

Intrinsic inelastic losses in x-ray spectra originate from excitations in an interacting electron system due to a suddenly created core-hole. These losses characterize the features observed in x-ray photoemission spectra (XPS), as well as many-body effects such as satellites and edge-singularities in x-ray absorption spectra (XAS). However, they are usually neglected in practical calculations. As shown by Langreth these losses can be treated within linear response in terms of a cumulant Green's function in momentum space. Here we present a complementary ab initio real-space Green's function generalization of the Langreth cumulant in terms of the dynamically screened core-hole interaction $W_c\left(\omega \right)$ and the independent particle response function. We find that the cumulant kernel $\beta \left(\omega \right)$ is analogous to XAS, but with the transition operator replaced by the core-hole potential with monopole selection rules. The behavior reflects the analytic structure of the loss function, with peaks near the zeros of the dielectric function, consistent with delocalized quasiboson excitations. The approach simplifies when $W_c\left(\omega \right)$ is localized and spherically symmetric. Illustrative results and comparisons are presented for the electron gas, sodium, and some early transition metal compounds.

Walker-Wang models are fixed-point models of topological order in $3+1$ dimensions constructed from a braided fusion category. For a modular input category $\mathcal{M}$, the model itself is invertible and is believed to be in a trivial topological phase, whereas its standard boundary is supposed to represent a $(2+1$)-dimensional chiral phase. In this work we explicitly show triviality of the model by constructing an invertible domain wall to vacuum as well as a disentangling generalized local unitary circuit in the case where $\mathcal{M}$ is a Drinfeld center. Moreover, we show that if we allow for fermionic (auxiliary) degrees of freedom inside the disentangling domain wall or circuit, the model becomes trivial for a larger class of modular fusion categories, namely, those in the Witt classes generated by the Ising unitary modular tensor category. We also discuss general (noninvertible) boundaries of general Walker-Wang models and describe a simple axiomatization of extended topological quantum field theory in terms of tensors.

We adopt the non-Hermitian Hamiltonian formalism to describe Weyl fermions of types III and IV. The spectrum of Hamiltonian has an unusual type of anisotropy. Namely, the hermiticity of Hamiltonian strongly depends on the direction in momentum space: for some directions, the spectrum is real, in contrast for other directions it becomes complex. It is shown that the type III and IV Weyl points are topologically stable and the Chern number is equal to $\pm 1$ despite to the fact that the Hamiltonian is not Hermitian. Furthermore, we calculate the Landau levels and demonstrate that zero Landau level is real, which means that there is a real spectral flow between electronlike and holelike states. Due to the formal analogy with the index theorem, the presence of such a flow as well indicates a nonzero Chern number. In addition, we illustrate that the non-Hermitian Hamiltonian can be regarded as a one-particle problem in the context of topological band theory.

The transverse and longitudinal magnetoresistance (MR) of two samples of the topological chiral semimetal CoSi with different residual resistivity ratios (RRRs) was studied. It is shown that the Kohler rule works for the transverse MR. The Kohler rule is also fulfilled in the case of longitudinal MR at a low reduced magnetic field. A sharp deviation in the longitudinal MR curve for the sample with low RRR from the Kohler prediction at high fields reveals its tendency to change sign at higher magnetic fields. We probably do not see any specific MR features connected to the topological chiral anomaly in the electron spectra of CoSi. This observation implies the need for certain revisions of the theoretical predictions. Shubnikov–de Haas quantum oscillations were observed and analyzed in both perpendicular and parallel configurations of the current and magnetic field in sample CoSi-1 with $\mathrm{RRR}=9.33$ at low temperatures.

Controlling over the electronic structure of two-dimensional (2D) semiconductors is crucial to unlock their full potential in the applications in future nanodevices. Here, we demonstrate a layer-dependent dielectric modulation of electronic structures in multilayer ${\mathrm{WS}}_2/\mathrm{GaN}$ heterostructures. The results investigated by means of in situ scanning tunneling microscopy/spectroscopy and photoluminescence spectroscopy show that the fluctuations of the dielectric environment induced by local corrugation of the GaN surface dramatically tune the band structure of monolayer ${\mathrm{WS}}_2$ with a large downshift of the valence band maximum by about 1.12 eV. Under the shielding effect of the first-layer ${\mathrm{WS}}_2$, the dielectric environment shows a weak modulation effect on the second-layer, and even has negligible influence on the third- and fourth-layer ${\mathrm{WS}}_2$. The relationship between dielectric environment and interfacial distance is further simulated through first-principles calculations. The electrostatic potential energy, differential charge densities, and charge transfer of the ${\mathrm{WS}}_2/\mathrm{GaN}$ heterostructure under different interfacial distances reveal that the differences of charge redistributions between the ${\mathrm{WS}}_2/\mathrm{GaN}$ interface and ${\mathrm{WS}}_2$ interlayer are responsible for the layer-dependent dielectric modulation. This work offers some references for the design and fabrication of novel 2D optoelectronic devices.

Strain engineering is an effective method to extend the properties of two-dimensional (2D) materials. In this paper, we theoretically studied the electron mobility of 2D hexagonal (H) ${\mathrm{MoS}}_2$ and triangular (T) ${\mathrm{PdSe}}_2$ under different strains. We observe contrary trends of the electron transport with strain. Our study shows it inherently comes from the contrary response of chemical bonding to strain. The covalency of both $\mathrm{H}\text{−}\mathrm{Mo}{\mathrm{S}}_2$ and $\mathrm{T}\text{−}\mathrm{PdS}{\mathrm{e}}_2$ decreases with increasing strain due to the elongated bond length. The ionicity of $\mathrm{H}\text{−}\mathrm{Mo}{\mathrm{S}}_2$ is also decreased, resulting in decreased electron-phonon coupling strength. On the contrary, the ionicity of $\mathrm{T}\text{−}\mathrm{PdS}{\mathrm{e}}_2$ is increased, which overwhelms the decrease of the covalency, resulting in enhanced electron-phonon coupling strength. Our work not only uncovers the underlying physics governing the contrary trends of electron mobility with strain in hexagonal and triangular $M{X}_2$, but also offers a paradigm analysis methodology that is applicable to other research areas.