Recent discovery of tellurene, a two-dimensional (2D) allotrope of tellurium (Te), has effectively extended the realm of 2D materials to group VI. In this paper, we have proceeded to determine the electronic and excitonic optical properties of 2D tellurene and selenene, together with stable monolayer ${\mathrm{TeSe}}_2$ in this group, by means of the GW-Bethe-Salpeter equation approach. We find that these materials stabilized in $1\mathrm{T}\text{−}{\mathrm{MoS}}_2$-like structure appear to be indirect semiconductors and possess ultrahigh isotropic carrier mobilities, which outperform their 2D ${\mathrm{MoS}}_2$ counterpart. The exciton is found to be strongly bound and thermally stable at room temperature with a binding energy of 0.37, 0.53, and 0.54 eV, respectively, for the three types of monolayers. Strikingly, we find that the optical gaps of monolayer selenene and ${\mathrm{TeSe}}_2$ lie in the ideal energy window of $1.0–1.5\mathrm{eV}$ corresponding to the maximum photoconversion efficiency. The number of layers in the stack further provides an effective means to tune the absorption edge across a wide energy range. These characteristics combined with the strong optical absorbance in the whole visible-light region make both 2D materials promising in the applications of efficient, ultrathin, and flexible photovoltaic devices with upper bounds to the conversion efficiency that rivals organic and dye-sensitized devices.

Magnetic topological insulators (TIs) are an ideal playground for the study of novel quantum phenomena building on time-reversal symmetry-broken topological surface states. By combining different magnetic TIs in a heterostructure, their magnetic and electronic properties can be precisely tuned. Recently, we have combined high-moment $\mathrm{Dy}:{\mathrm{Bi}}_2{\mathrm{Te}}_3$ with high transition temperature $\mathrm{Cr}:{\mathrm{Sb}}_2{\mathrm{Te}}_3$ in a superlattice, and we found, using x-ray magnetic circular dichroism (XMCD), that long-range magnetic order can be introduced in the $\mathrm{Dy}:{\mathrm{Bi}}_2{\mathrm{Te}}_3$ layers. Accompanying first-principles calculations indicated that the origin of the long-range magnetic order is a strong antiferromagnetic coupling between Dy and Cr magnetic moments at the interface extending over several layers. However, based on XMCD alone, which is either averaging over the entire thin-film stack or is surface-sensitive, this coupling scenario could not be fully confirmed. Here we use polarized neutron reflectometry, which is ideally suited for the detailed study of superlattices, to retrieve the magnetization in a layer- and interface-resolved way. We find that the magnetization is, in contrast to similar recent studies, homogeneous throughout the individual layers, with no apparent interfacial effects. This finding demonstrates that heterostructure engineering is a powerful way of controlling the magnetic properties of entire layers, with the effects of coupling reaching beyond the interface region.

We report that the lattice constant of ${{\mathrm{Dy}}_2\mathrm{Ge}}_{2-x}{\mathrm{Si}}_x{\mathrm{O}}_7$ ($x=0$, 0.02, 0.08, 0.125) can be systematically reduced by substituting the nonmagnetic germanium ion in the cubic pyrochlore oxide with silicon. A multianvil high-pressure synthesis was performed up to 16 GPa and $1100{}^{\circ }\mathrm{C}$ to obtain polycrystalline samples in a solid-state reaction. Measurements of magnetization, ac susceptibility, and heat capacity reveal the typical signatures of a spin-ice phase. From the temperature shift of the peaks, observed in the temperature-dependent heat capacity, we deduce an increase in the strength of the exchange interaction. In conclusion, the reduced lattice constant leads to a changed ratio of the competing exchange and dipolar interaction. This puts the new spin-ice compounds closer towards the phase boundary of a short-range spin-ice arrangement and antiferromagnetic long-range order consistent with an observed reduction in the energy scale of monopole excitations.

We investigate the effect of the degree of metal-oxygen octahedral facesharing on the mechanical and electronic properties of $d^0 {\mathrm{BaTiO}}_3$ and $d^3 {\mathrm{BaMnO}}_3$. We find that increased facesharing softens the elastic constants of both materials due to the increased volume per atom, with polar distortions also contributing to the reduction in the bulk modulus. Owing to orbital filling in the $d$ manifold, we find the electronic band gap of ${\mathrm{BaTiO}}_3$ is relatively unaffected by changes in percent facesharing whereas the band gap of ${\mathrm{BaMnO}}_3$ increases by more than 200% as the percent facesharing increases from 0% (cubic perovskite) to 100% (hexagonal ${\mathrm{BaNiO}}_3$ perovskite). We identify that the trigonal distortions present in the face-connected polymorphs represent useful atomistic structural knobs to tune band structure in hexagonal perovskites. Our results indicate that facesharing hexagonal polymorphs provide an expanded oxides arena with additional structural flexibility beyond the usual fully corner-connected perovskites for property control.

We investigate Josephson flux flow in annular Josephson tunnel junctions (AJTJs) under the application of magnetic fields generating finite-voltage steps in their current-voltage characteristics. Experimental data are presented for confocal AJTJs, which are the natural generalization of the well-studied circular AJTJs for which flux-flow effects have never been reported. Displaced linear slopes, Fiske step staircases, and Eck steps were sequentially recorded at $4.2\text{K}$ with high-quality Nb/Al-AlOx/Nb confocal AJTJs when increasing the strength of a uniform magnetic field applied in the plane of the junction. Their amplitude was found to strongly depend not only on the strength but also on the orientation of the external field. Extensive numerical simulations based on a phenomenological sine-Gordon model developed for confocal AJTJs were carried out to disclose the basic flux-flow mechanism responsible for the appearance of magnetically induced steps and to elucidate the role of several critical parameters, namely, the field orientation, the system loss, and the annulus eccentricity. It was found that in a topologically closed system, such as the AJTJ, where the number of trapped fluxons is conserved and new fluxons can be created only in the form of fluxon-antifluxon pairs, the existence of a steady viscous flow of Josephson vortices only relies on the capability of the fluxons and antifluxons to be generated and to annihilate each other inside the junction. This also implies that flux-flow effects are not observable in circular AJTJs.

In recent years, substantial progress has been made regarding the application of quasiclassical theory on superconducting hybrid structures. This theoretical framework is reliant on a proper set of boundary conditions in order to describe multilayered systems. With the advent of the field of superconducting spintronics, systems that combine heavy metal layers, in which there is large spin-orbit coupling, with ferromagnets have received a great deal of attention, due to their potential for generating long-range triplet superconductivity. In contrast to interfaces of strongly spin-polarized materials, which are well understood, a quasiclassical theory for interfaces in systems in which there is significant spin-orbit coupling does not yet exist. After reviewing the quasiclassical theory for interfaces, we solve this problem here by deriving a set of boundary conditions that take spin-orbit coupling explicitly into account. We then go on to apply these boundary conditions to a superconductor-ferromagnet bilayer and a superconductor-ferromagnet-superconductor Josephson weak link, demonstrating the emergence of long-range triplet superconductivity in these systems.

A variety of exotic non-Fermi liquid (NFL) states have been observed in many condensed matter systems, with different scaling relations between transport coefficients and temperature. The “standard” approach to studying these NFLs is by coupling a Fermi liquid to quantum critical fluctuations, which potentially can drive the system into a NFL. In this work we seek for an alternative understanding of these various NFLs in a unified framework. We first construct two “elementary” randomness-free models with four-fermion interactions only, whose many properties can be analyzed exactly in certain limit just like the Sachdev-Ye-Kitaev model. The most important new feature of our models is that, the fermion scaling dimension in the conformal invariant solution in the infrared limit is tunable by charge density. Then based on these elementary models, we propose two versions of lattice models with four-fermion interactions which give us non-Fermi liquid behaviors with dc resistivity scaling $\varrho \sim T^{\alpha }$ in a finite-temperature window, and $\alpha \in [1,2)$ depends on the fermion density in the model, which is a rather universal feature observed in many experimental systems.

Supervised learning has been successfully used to produce phase diagrams and identify phase boundaries when local order parameters are unavailable. Here, we apply unsupervised learning to this task. By using readily available clustering algorithms, we are able to extract the distinct eigenstate phases of matter within the transverse-field Ising model in the presence of interactions and disorder. We compare our results to those found through supervised learning and observe remarkable agreement. However, as opposed to the supervised procedure, our method requires no strict assumptions concerning the number of phases present, no labeled training data, and no prior knowledge of the phase diagram. We conclude with a discussion of clustering and its limitations.

Outstanding problems in the high-pressure phase diagram of hydrogen have demonstrated the need for more accurate ab initio methods for thermodynamic sampling. One promising method that has been deployed extensively above 100 GPa is coupled electron-ion Monte Carlo (CEIMC), which treats the electronic structure with quantum Monte Carlo (QMC). However, CEIMC predictions of the deuterium principal Hugoniot disagree significantly with experiment, overshooting the experimentally determined peak compression density by $7\%$ and lower temperature gas-gun data by well over $20\%$. By deriving an equation relating the predicted Hugoniot density to underlying equation of state errors, we show that QMC and many-body methods can easily spoil the error cancellation properties inherent in the Rankine-Hugoniot relation, and very likely suffer from error addition. By cross validating QMC based on systematically improvable trial functions against post-Hartree-Fock many-body methods, we find that these methods introduce errors of the right sign and magnitude to account for much of the observed discrepancy between CEIMC and experiment. We stress that this is not just a CEIMC problem, but that thermodynamic sampling based on other many-body methods is likely to experience similar difficulties.

In this study, we calculate the $T=300\mathrm{K}$ scattering and thermoelectric transport properties of rhombohedral GeTe using first-principles modeling. The room-temperature phase of GeTe has a layered structure, with cross-plane and in-plane directions oriented parallel and perpendicular to [111], respectively. Based on rigorous electron-phonon scattering, our transport calculations reveal unusual anisotropic properties; $n$-type GeTe has a cross-plane electrical conductivity that is roughly $3\times$ larger than in plane. $p$-type GeTe, however, displays opposite anisotropy with in-plane conducting roughly $2\times$ more than cross plane, as is expected in quasi-two-dimensional materials. The power factor shows the same anisotropy as the electrical conductivity, since the Seebeck coefficient is relatively isotropic. Interestingly, cross-plane $n$-GeTe shows the largest mobility and power factor approaching $500{\mathrm{cm}}^2/\mathrm{V}$-s and $32\mu \mathrm{W}/\mathrm{cm}\text{−}{\mathrm{K}}^2$, respectively. The thermoelectric figure of merit, $zT$, is enhanced as a result of this unusual anisotropy in $n$-GeTe since the lattice thermal conductivity is minimized along cross plane. This decouples the preferred transport directions of electrons and phonons, leading to a threefold increase in $zT$ along cross plane compared to in plane. The $n$-type anisotropy results from high-velocity electron states formed by Ge $p$ orbitals that span across the interstitial region. This surprising behavior, that would allow the preferential conduction direction to be controlled by doping, could be observed in other quasi-two-dimensional materials and exploited to achieve higher-performance thermoelectrics.

We study the relaxation dynamics of the electron system in graphene flakes under a Landau quantization regime using an approach of time-resolved Raman scattering. The nonresonant character of the experiment allows us to analyze the field dependence of the relaxation rate. Our results clearly evidence a sharp increase in the relaxation rate upon the resonance between the energy of the Landau transition and the G band and shed light on the relaxation mechanism of the Landau-quantized electrons in graphene beyond the previously studied Auger scattering.

We study a possible topological charge pump in a traditional two-parameter pump device, which is based on a one-dimensional (1D) semiconductor nanowire with Rashba spin-orbit interaction. The 1D nanowire has a superlattice structure through periodic folding and the pumping parameters are the two time-dependent Zeeman fields with a phase lag $\phi$ between them. It is shown that the Zeeman field can open an energy gap of the nanowire superlattice and the system enters into a topological state. There are two electron charges pumped out adiabatically in a pumping cycle if the Fermi energy resides in the energy gap and $\phi$ is not close to $n\pi$ ($n$, an integer). This originates from the topologically protected interface state forming between the two pumping sources, which evolves with time, resulting in electron charges from one end transported to the other end of the wire. The quantized current direction can be modulated by some system parameters such as the Fermi energy, pumping phase, and the local potential of the device via gate voltage.

This work unveils how geometric features of two-band non-Hermitian Hamiltonians can classify the topology of their eigenstates and energy manifolds. Our approach generalizes the Bloch sphere visualization of Hermitian systems to a “Bloch torus” picture for non-Hermitian systems, by extending the origin of the Bloch sphere to a singularity ring (SR) in the vector space of the real pseudospin. The SR captures the structure of generic spectral exceptional degeneracies, which emerge only if the real pseudospin vector actually falls on the SR. Applicable to non-Hermitian systems that may or may not have exceptional degeneracies, this SR picture affords convenient visualization of various symmetry constraints and reduces their topological characterization to the classification of simple intersection or winding behavior, as detailed by our explicit study of chiral, sublattice, particle-hole, and conjugated particle-hole symmetries. In 1D, the winding number about the SR corresponds to the band vorticity measurable through the Berry phase. In 2D, more complicated winding behavior leads to a variety of phases that illustrates the richness of the interplay between SR topology and geometry beyond mere Chern number classification. Through a normalization procedure that puts generic two-band non-Hermitian Hamiltonians on equal footing, our SR approach also allows for vivid visualization of the non-Hermitian skin effect.

The largest allowed symmetry in a spin-1 quantum system is an $\mathrm{SU}\left(3\right)$ symmetry rather than the $\mathrm{SO}\left(3\right)$ spin rotation. In this work, we reveal some $\mathrm{SU}\left(2\right)$ symmetries as subgroups of $\mathrm{SU}\left(3\right)$ that, to the best of our knowledge, have not previously been recognized. Then, we construct $\mathrm{SU}\left(2\right)$ symmetric Hamiltonians and explore the ground-state phase diagram in accordance with the $\mathrm{SU}\left(3\right)\supset \mathrm{SU}\left(2\right)\times U\left(1\right)$ symmetry hierarchy. It is natural to treat the eight generators of the $\mathrm{SU}\left(3\right)$ symmetry on an equal footing; this approach is called the eightfold way. We find that the spin spectral functions and spin quadrupole spectral functions share the same structure, provided that the elementary excitations are flavor waves at low energies, which serves as a clue to the eightfold way. An emergent $S=1/2$ quantum spin liquid is proposed to coexist with gapful spin nematic order in one of the ground states. In analogy to quantum chromodynamics, we find the gap relation for hydrodynamic modes in quantum spin-orbital liquid states, which is nothing but the Gell-Mann-Okubo formula.

Despite a long history of research into nonlinear response theory, there has been no systematic investigation into the maximum amount of nonlinear optical response attainable in solid-state materials. In this work, we present an upper bound on the second-order response functions of materials, which controls the shift current response. We show that this bound depends on the band gap, bandwidth, and geometrical properties of the material in question. We find that delocalized systems generally have larger responses than more localized or isolated ones. As a proof of principle, we perform first-principles calculations of the response tensors of a wide variety of materials, finding that the materials in our database do not yet saturate the upper bound. This suggests that new large shift current materials will likely be discovered by future materials research guided by the factors mentioned in this work.

Using linear flavor-wave theory (LFWT) and auxiliary field quantum Monte Carlo (QMC), we investigate the properties of the SU(4) Heisenberg model on the anisotropic square lattice in the fully antisymmetric six-dimensional irreducible representation, a model that describes interacting fermions with four flavors at half-filling. Thanks to the calculations on very large systems, we have been able to convincingly demonstrate that QMC results are consistent with a small but finite antiferromagnetic moment at the isotropic point, in qualitative agreement with LFWT obtained earlier [F. H. Kim et al., Phys. Rev. B 96, 205142 (2017)], and in quantitative agreement with results obtained previously on the Hubbard model [D. Wang et al., Phys. Rev. Lett. 112, 156403 (2014)] after extrapolation to infinite $U/t$. The presence of a long-range antiferromagnetic order has been further confirmed by showing that a phase transition takes place into a valence-bond solid (VBS) phase not too far from the isotropic point when reducing the coupling constant along one direction on the way to decoupled chains.

We study the doping-driven Mott metal-insulator transition for multiorbital Hubbard models with Hund's exchange coupling at finite temperatures. As in the single-orbital Hubbard model, the transition is first order within dynamical mean-field theory, with a coexistence region where two solutions can be stabilized. We find that in the presence of finite Hund's coupling, the insulating phase is connected to a badly metallic phase, which extends to surprisingly large dopings. While fractional power-law behavior of the self-energies on the Matsubara axis is found on both sides of the transition, a regime with frozen local moments develops only on the branch connected to the insulating phase.

We study high-temperature magnetization transport in a many-body spin-1/2 chain with on-site quasiperiodic potential governed by the Fibonacci rule. In the absence of interactions it is known that the system is critical with the transport described by a continuously varying dynamical exponent (from ballistic to localized) as a function of the on-site potential strength. Upon introducing weak interactions, we find that an anomalous noninteracting dynamical exponent becomes diffusive for any potential strength. This is borne out by a boundary-driven Lindblad dynamics as well as unitary dynamics, with agreeing diffusion constants. This must be contrasted to a random potential where transport is subdiffusive at such small interactions. Mean-field treatment of the dynamics for small $U$ always slows down the noninteracting dynamics to subdiffusion, and is therefore unable to describe diffusion in an interacting quasiperiodic system. Finally, briefly exploring larger interactions we find a regime of interaction-induced subdiffusive dynamics, despite the on-site potential itself having no “rare regions.”