A new nonlinear magnetothermal effect, namely unidirectional Seebeck effect (USE), has been recently reported in magnetic/nonmagnetic topological insulator (TI) heterostructure and is ascribed to the asymmetry magnon scattering. Here, we show a new mechanism to generate USE in the Rashba two dimensional electron gas (2DEG), in which the magnetism or magnetic order is completely absent. It's found that the USE has a quantum origin from the spin-momentum locking generated from Rashba spin-orbit coupling. We find that the USE exhibits a sine dependence on the orientation of the magnetic field with respect to the direction of the temperature gradient and is dominant from the states above the Lifshitz point. The USE in the semiconductor Ge(111) subsurface states has been theoretically and systematically investigated.

We introduce a new class of machine learning interatomic potentials—fast general two- and three-body potential (GTTP), which is as fast as conventional empirical potentials and require computational time that remains constant with increasing fitting flexibility. GTTP does not contain any assumptions about the functional form of two- and three-body interactions. These interactions can be modeled arbitrarily accurately, potentially by thousands of parameters not affecting resulting computational cost. Time complexity is O(1) per every considered pair or triple of atoms. The fitting procedure is reduced to simple linear regression on ab initio calculated energies and forces and leads to effective two- and three-body potential, reproducing quantum many-body interactions as accurately as possible. Our potential can be made continuously differentiable any number of times at the expense of increased computational time. We made a number of performance tests on one-, two- and three-component systems. The flexibility of the introduced approach makes the potential transferable in terms of size and type of atomic systems as long as they involve the same atomic species. We show that trained on randomly generated structures with just eight atoms in the unit cell, it significantly outperforms common empirical interatomic potentials in the study of large systems, such as grain boundaries in polycrystalline materials.

Beyond the well-known topological band theory for single-particle systems, it is a great challenge to characterize the topological nature of interacting multiparticle quantum systems. Here, we uncover the relation between topological invariants defined through the twisted boundary condition (TBC) and the center-of-mass (c.m.) momentum state in multiparticle systems. We find that the Berry phase defined through the TBC can be equivalently obtained from the multiparticle Wilson loop formulated by c.m. momentum states. As the Chern number can be written as the winding of the Berry phase, we consequently prove the equivalence of Chern numbers obtained via TBC and c.m. momentum state approaches. As a proof-of-principle example, we study topological properties of the Aubry-André-Harper model. Our numerical results show that the TBC approach and c.m. approach are well consistent with each other for both the many-body case and the few-body case. Our work lays a concrete foundation and provides insights for exploring multiparticle topological states.

Vanadium oxides have been extensively studied as phase-change memory units in artificial synapses for neuromorphic computing due to their metal-insulator transitions (MIT) at or near room temperature. Recently, injection of charge carriers into vanadium oxides, e.g., via optically via a heterostructure, has been proposed as an alternative switching mechanism and also potentially as a means to tune the MIT temperature. In this study, we explore the formation of small polarons in the low temperature (LT) insulating phases for ${\mathrm{V}}_3{\mathrm{O}}_5, {\mathrm{VO}}_2$, and ${\mathrm{V}}_2{\mathrm{O}}_3$, and the barriers to their migration using density functional theory calculations. We find that ${\mathrm{V}}_3{\mathrm{O}}_5$ exhibits very low hole and electron polaron migration barriers ($<100$ meV) compared to ${\mathrm{V}}_2{\mathrm{O}}_3$ and ${\mathrm{VO}}_2$, leading to much higher estimated polaronic conductivity. We also link the relative migration barriers to the amount of distortion that has to travel when the polaron migrate from one site to another. Polarons in ${\mathrm{V}}_3{\mathrm{O}}_5$ also have smaller binding energies to vanadium and oxygen vacancy defects. These results suggest that the triggering of the MIT via injection of charge carriers are due to the formation of small polarons that can migrate rapidly through the crystal.

Materials hosting topologically protected non-Abelian zero modes offer the exciting possibility of storing and manipulating quantum information in a manner that is protected from decoherence at the hardware level. In this paper, we study the possibility of realizing such excitations along line defects in certain fractional quantum Hall states in multivalley systems. Such line defects have been recently observed experimentally between valley polarized Hall states on the surface of Bi(111), and excitations near these defects appear to be gapped (gapless) depending on the presence (absence) of interaction-induced gapping perturbations constrained by momentum selection rules, while the position of defects is determined by strain. In this paper, we use these selection rules to show that a hybrid structure involving a superlattice imposed on such a multivalley quantum Hall surface realizes non-Abelian anyons, which can then be braided by modulating strain locally to move line defects. Specifically, we explore such defects in Abelian fractional quantum Hall states of the form $\nu =2/m$ using a K-matrix approach, and identify relevant gapping perturbations. Charged modes on these line defects remain gapped, while charge neutral valley pseudospin modes may be gapped with the aid of two (mutually orthogonal) superlattices, which pin noncommuting fields. When these superlattices are alternated along the line defect, non-Abelian zero modes result at points where the gapping perturbation changes. Given that these pseudospin modes carry no net physical charge or spin, the setup eschews utilizing superconducting and magnetic elements to engineer gapping perturbations. We provide a scheme to braid these modes using strain modulation, and confirm that the resulting unitaries satisfy a representation of the braid group.

Strong coupling of charge, spin, and lattice in solids brings about emergent elementary excitations with their intertwining and, in one dimension, solitons are known as such. The charge-transferred organic ferroelectric, TTF-CA, has been argued to host charge solitons; however, the existence of the charge solitons remains unverified. Here, we demonstrate that the charge-transport gap in the ionic Mott-Peierls insulating phase of TTF-CA is an order of magnitude smaller than expected from quasiparticle excitations, however, being entirely consistent with the charge soliton excitations. We further suggest that charge and spin solitons move with similar diffusion coefficients in accordance with their coexistence. These results provide a basis for the thermal excitations of the emergent solitons.

We examine the range of validity of the second-order cumulant expansion (CE) for the calculation of spectral functions, quasiparticle properties, and mobility of the Holstein polaron. We devise an efficient numerical implementation that allows us to make comparisons in a broad interval of temperature, electron-phonon coupling, and phonon frequency. For a benchmark, we use the dynamical mean-field theory which gives, as we have recently shown, rather accurate spectral functions in the whole parameter space, even in low dimensions. We find that in one dimension, the CE resolves well both the quasiparticle and the first satellite peak in a regime of intermediate coupling. At high temperatures, the charge mobility assumes a power law $\mu \propto T^{-2}$ in the limit of weak coupling and $\mu \propto T^{-3/2}$ for stronger coupling. We find that, for stronger coupling, the CE gives slightly better results than the self-consistent Migdal approximation (SCMA), while the one-shot Migdal approximation is appropriate only for a very weak electron-phonon interaction. We also analyze the atomic limit and the spectral sum rules. We derive an analytical expression for the moments in CE and find that they are exact up to the fourth order, as opposed to the SCMA where they are exact to the third order. Finally, we analyze the results in higher dimensions.

Real semiconductors usually contain both donor and acceptor impurities. A finite compensation degree leads to the possibility of the single carrier to be bounded by two impurity centers. For such an electron complex (near a simple conduction band), the spectral problem resembles the hydrogen molecule ion problem (up to renormalization of the effective mass and dielectric screening). In $p$-type semiconductors, the spectral problem for a single hole in the field of two attracting centers ($A_2^{-}$ complex) is more complicated due to the complex structure of the valence band. Here such a problem is presented for the case of the hole bounded at two shallow acceptors close to the ${\mathrm{\Gamma }}_8$ valence band edge (${\mathrm{A}}^{\mathrm{III}}{\mathrm{B}}^{\mathrm{V}}$ or group $\mathrm{IV}$ semiconductors). The multicomponent envelope functions are used to develop quantum chemistrylike approach (molecular orbital method). The variational approach is applied to calculate the level structure of the complex. The states of the complex are classified by the total angular momentum projection onto the intercenter axis and by the parity with respect to the intercenter permutation. The ground state has a total angular momentum projection of $\pm 1/2$ and a wave function that is symmetric with respect to the intercenter permutation. The energy levels are found as a function of the intercenter distance. A possible manifestation of $A_2^{-}$ complexes in acceptor-related luminescence is discussed.

The coherent emission of exciton polaritons was proposed as a means of lowering the lasing threshold because it does not require the dissociation of excitons to obtain an electron-hole plasma, as in a classical semiconductor laser based on population inversion. In this work we propose a method to prove clearly the polaritonic nature of lasing by combining experimental measurements with a model accounting for the permittivity change as a function of the carrier density. To do so we use angle resolved photoluminescence to observe the lasing at cryogenic temperature from a polariton mode in a zinc oxide waveguide structure, and to monitor the continuous shift of the polaritonic dispersion towards a photonic dispersion as the optical intensity of the pump is increased (up to 20 times the one at threshold). This shift is reproduced thanks to a model taking into account the reduction of the oscillator strength and the renormalization of the band gap due to the screening of the electrostatic interaction between electrons and holes. Furthermore, the measurement of the carrier lifetime at optical intensities in the order of those at which the polariton lasing occurs enables us to estimate the carrier density, confirming that it is lower than the corresponding Mott density for zinc oxide reported in the literature.

We study the transient dynamics subject to quantum coherence effects of two interacting parallel quantum dots weakly coupled to macroscopic leads. The stationary particle current of this quantum system is sensitive to perturbations much smaller than any other energy scale, specifically compared to the system-lead coupling and the temperature. We show that this is due to the presence of a parity-like symmetry in the dynamics, as a consequence of which two distinct stationary states arise. In the presence of small perturbations breaking this symmetry, the system exhibits metastability with two metastable phases that can be approximated by a combination of states corresponding to stationary states in the unperturbed limit. Furthermore, the long-time dynamics can be described as classical dynamics between those phases, leading to a unique stationary state. In particular, the competition of those two metastable phases explains the sensitive behavior of the stationary current towards small perturbations. We show that this behavior bears the potential of utilizing the parallel dots as a charge sensor, which makes use of quantum coherence effects to achieve a signal to noise ratio that is not limited by the temperature. As a consequence, the parallel dots outperform an analogous single-dot charge sensor for a wide range of temperatures.

The complex structure of the Remeika phases, the intriguing quantum states they display, and their low carrier concentrations are strong motivations to study the nature of their superconducting phases. In this Letter, the microscopic properties of the superconducting phase of single-crystalline ${\mathrm{Lu}}_3{\mathrm{Os}}_4{\mathrm{Ge}}_{13}$ are investigated by muon-spin relaxation and rotation ($\mu \mathrm{SR}$) measurements. The zero-field $\mu \mathrm{SR}$ data reveal the presence of spontaneous static or quasistatic magnetic fields in the superconducting state, breaking time-reversal symmetry; the associated internal magnetic-field scale is found to be exceptionally large ($\simeq$ 0.11 mT). Furthermore, transverse-field $\mu \mathrm{SR}$ measurements in the vortex state of ${\mathrm{Lu}}_3{\mathrm{Os}}_4{\mathrm{Ge}}_{13}$ imply a complex gap function with significantly different strengths on different parts of the Fermi surface. Although our measurements do not completely determine the order parameter, they strongly indicate that electron-electron interactions are essential to stabilizing pairing in the system, thus, demonstrating its unconventional nature.

A DFT+$U$-type corrective functional is derived from first principles to enforce the flat plane condition on localized subspaces, thus dispensing with the need for an ad hoc derivation from the Hubbard model. Small, molecular test systems at the dissociated limit are used to compare the functional form against several previously proposed DFT+$U$-type functionals. The functional derived here yields relative errors below $0.6\%$ in the total energy of the dissociated $s$-block dimers as well as the dissociated ${\mathrm{H}}_5^{+}$ ring system, a challenging test case in which the asymmetric, tilted flat plane condition applies. In comparison, bare PBE and PBE+$U$ (using Dudarev's 1998 Hubbard functional) yields relative energetic errors as high as $8.0\%$ and $20.5\%$, respectively.

We study a parafermion Josephson junction comprising a pair of counterpropagating edge modes of two quantum Hall systems, proximitized by an $s$-wave superconductor. We show that the difference between the lengths (which can be controlled by external gates) of the two counterpropagating chiral edges at the Josephson junction, can act as a source of spontaneous phase bias. For the Laughlin filling fractions, $\nu =1/m, m\in 2\mathbb{Z}+1$, this leads to an electrical control of either Majorana ($m=1$) or parafermion ($m\ne 1$) zero modes.

Within the broad theme of understanding the dynamics of disordered quantum many-body systems, one of the simplest questions one can ask is, given an initial state, how does it evolve in time on the associated Fock-space graph, in terms of the distribution of probabilities thereon? A detailed quantitative description of the temporal evolution of out-of-equilibrium disordered quantum states and probability transport on the Fock space is our central aim here. We investigate it in the context of a disordered quantum spin chain, which hosts a disorder-driven many-body localization transition. Real-time dynamics/probability transport is shown to exhibit a rich phenomenology, which is markedly different between the ergodic and many-body localized phases. The dynamics is, for example, found to be strongly inhomogeneous at intermediate times in both phases, but while it gives way to homogeneity at long times in the ergodic phase, the dynamics remain inhomogeneous and multifractal in nature for arbitrarily long times in the localized phase. Similarly, we show that an appropriately defined dynamical lengthscale on the Fock-space graph is directly related to the local spin autocorrelation, and as such sheds light on the (anomalous) decay of the autocorrelation in the ergodic phase, and lack of it in the localized phase.

The electric-field-induced angular force on the Néel vector in antiferromagnetic (AFM) Weyl semimetals (WSMs) is theoretically investigated. Unlike in the ferromagnetic (FM) counterparts, the magnetic textures in the AFM WSMs, such as the domain walls (DWs) appear to lack the torsion in the magnetization and, thus, unable to benefit from this highly efficient mechanism that originates from the axial magnetic effect. Contrarily, our calculations illustrate that the addition of the Dzyaloshinskii-Morya interaction can introduce a twist in the magnetization around the DW location, giving rise to a nonzero axial magnetic field. This axial magnetic field, when combined with an external electric field, can lead to an imbalance in the fermion density of Weyl cones of opposite chirality and, thus, a spatially localized net electron spin polarization. The resulting effective exchange field can exert an angular force on the AFM textures for spatial movement, which can be significant in certain AFM WSMs even under a moderate external electric field. The dynamics of the DW motion under this emergent angular force is analyzed by considering the balance of energy absorption and dissipation. Our investigation reveals the need to account for the contribution of the exchange dissipation mechanism beyond the typical Gilbert-like (relativistic) term to compensate the unusual superlinear rate of energy absorption by the AFM textures. The obtained DW velocity vs electric-field characteristics show a significant speedup for the Néel DWs in the AFM WSMs over the counterparts in the FM WSMs as well as those in the nontopological magnets. The analysis also elucidates the dependence of the DW motion on the DW chirality in these materials. Our results clearly indicate the significance of the energy-efficient axial magnetic effect in the dynamics of spin textures in AFM WSMs with broken inversion symmetry.

A system of interacting spins that are under the influence of spin-polarized currents can be described using a complex functional, or a non-Hermitian (NH) Hamiltonian. We study the dynamics of two exchange-coupled spins on the Bloch sphere. In the case of currents leading to $\mathcal{PT}$ symmetry, an exceptional point that survives also in the nonlinear system is identified. The nonlinear system is bistable for small currents and it exhibits stable oscillating motion or it can relax to an equilibrium point. The oscillating motion of the two spins is akin to synchronized spin-torque oscillators. For the full nonlinear system, we derive two conserved quantities that furnish a geometric description of the spin trajectories in phase space and indicate stability of the oscillating motion. Our analytical results provide tools for the description of the dynamics of NH systems that are defined on the Bloch sphere.

Pressurized $A{\mathrm{Mn}}_6{\mathrm{Bi}}_5$ $(A=\mathrm{K},\mathrm{Rb},\mathrm{Cs})$ members have recently emerged as a new family of ternary manganese-based superconductors that exhibits a broad superconducting dome with a maximum critical temperature up to $\sim 9.5\mathrm{K}$ adjacent to the long-range antiferromagnetic phase in their $T\text{−}P$ phase diagrams. The analogous compound ${\mathrm{NaMn}}_6{\mathrm{Bi}}_5$, with a smaller $A$-site cation and lower antiferromagnetic transition temperatures $T_1\approx 46\mathrm{K}$ and $T_2\approx 52\mathrm{K}$ is expected to show superconductivity at lower pressures. By performing detailed high-pressure electrical transport measurements on the chemically precompressed ${\mathrm{NaMn}}_6{\mathrm{Bi}}_5$ single crystals, we map out the complex evolution with pressure of its multiple magnetic transitions, which are eventually suppressed by pressures. Contrary to the expectation, the bulk superconductivity is absent near the magnetic quantum critical point, where we observed a weak decrease in resistance below 2.4 K in a narrow pressure range of 12 to 14.4 GPa from the measurements in a diamond anvil cell. The origin of this anomaly remains unclear, and it might be ascribed to the presence of filamentary superconductivity in analogy to other $A{\mathrm{Mn}}_6{\mathrm{Bi}}_5$. These distinct results of ${\mathrm{NaMn}}_6{\mathrm{Bi}}_5$ with respect to other $A{\mathrm{Mn}}_6{\mathrm{Bi}}_5$ members highlight the complexity of magnetism and its interplay with superconductivity in this family of quasi–one-dimensional compounds.

The effect of spin fluctuations on the $\alpha$ (bcc)-$\gamma$ (fcc)-$\delta$ (bcc) structural phase transitions in iron is investigated with a tight-binding (TB) model. The orthogonal $d$-valent TB model is combined with thermodynamic integration, spin-space averaging, and Hamiltonian Monte Carlo to compute the temperature-dependent free-energy difference between bcc and fcc iron. We demonstrate that the TB model captures experimentally observed phonon spectra of bcc iron at elevated temperatures. Our calculations show that spin fluctuations are crucial for both the $\alpha \text{−}\gamma$ and $\gamma \text{−}\delta$ phase transitions but they enter through different mechanisms. Spin fluctuations impact the $\alpha \text{−}\gamma$ phase transition mainly via the magnetic/electronic free-energy difference between bcc and fcc iron. The $\gamma \text{−}\delta$ phase transition, in contrast, is influenced by spin fluctuations only indirectly via the spin-lattice coupling. Combining the two mechanisms, we obtain both the $\alpha \text{−}\gamma$ and $\gamma \text{−}\delta$ phase transitions with our TB model. The calculated transition temperatures are in very good agreement with experimental values.

We reveal a photoinduced control of ferroelectricity in hybrid-improper ferroelectric superlattices, using ab initio theory. Along with a notable photostriction effect, thermalized carriers from photoexcitation affect octahedral tiltings along different directions. A trilinear coupling between antipolar and tilting modes drives corresponding cationic displacements, and this results in a decrease in polarization under light. Together with recent experimental evidence, our study therefore demonstrates that light is a viable route to engineer functionalities in materials.

In the non-Hermitian Landau-Zener models, we investigate the dynamical transition both in parity-time symmetric and symmetry-broken regimes. Taking into account the complex nature of the energy of the non-Hermitian systems, the absolute value of the gap was used to determine the relaxation rate of the system. To show the dynamics of the phase transitions, the relative population is used to estimate the topological defect density in nonequilibrium phase transitions, rather than the excitations in the corresponding Hermitian systems. The result shows that the adiabatic-impulse approximation, which is fundamental to the Kibble-Zurek mechanism, may be adapted to the parity-time symmetric non-Hermitian Landau-Zener models to examine the dynamics near the critical point. The most basic non-Hermitian two-level models with an exact solution exhibiting the Kibble-Zurek mechanism are presented. It would be interesting to extend this scenario to quantum many-body models, such as the quantum phase transition in the Ising model.