Electron-phonon coupling has inspired broad research because of the dominant role in electron and phonon transport properties, whereas how to control the coupling strength through band-structure engineering is a widely debated issue in solid-state physics. Here, we establish a general relationship between valence-band shape and electron-phonon coupling by conducting a comparative study of typical half-Heusler compounds TiCoSb and TiNiSn. The competition between symmetry-allowed $d\text{−}d$ and $p\text{−}d$ orbital couplings eventually determines the valence-band-edge shape. The $d\text{−}d$ coupling between Co/Ni and Ti atoms favors a unique wavelike valence band with multiple nearly equal-energy valleys in TiCoSb and the $p\text{−}d$ coupling between Sb/Sn and Ti atoms prefers a single parabolic valence band in TiNiSn. More importantly, the wavelike band effectively strengthens the electron-phonon coupling by introducing the intervalley scattering that remains stronger than the intravalley scattering. In contrast, the single parabolic band seriously suppresses the scattering process because the intravalley scattering dominates the electron-phonon coupling. The different scattering mechanism results in a series of anomalous electron and phonon transport properties, for example, in sharp contrast to TiNiSn, TiCoSb has a more pronounced decrease in band gap with temperature and a weaker temperature dependence of lattice thermal conductivity. Our findings provide universal insights into the effects of complex band structure on electron-phonon coupling and help to develop different strategies to improve and design better thermoelectric materials.

In this work, coupled field-gauge-invariant self-consistent quantum-kinetic theory in combination with Maxwell equations are established and applied to study ultrafast dynamics of a transient-field driven and photoexcited electron plasma induced by an incident infrared pulse-laser field. Our study reveals that the self-induced plasma-field instability occurs in a quantum-wire system once the transient electric field points to the opposite direction of the pulse-laser-field wave vector. This new effect leads to amplification of an induced plasma field. However, the physical mechanism behind this instability is quite different from earlier reported enhancement of a collective-excitation mode. Here, the latter is enabled by introducing a Doppler-shifted plasmon frequency under a finite drift velocity of electrons, as calculated based on a linear-response theory [e.g., K. Kempa et al., Phys. Rev. B 43, 9273 (1991)]. Dynamically, it is also different from the well-known classical two-stream plasma-fluid instability [e.g., T. M. O'Neil et al., Phys. Fluids 11, 1754 (1968)]. Interestingly, such a transient-field driven instability of a plasma field within a translationally invariant system enables generating a coherent terahertz-radiation wave from a small device with its size comparable to the driving-field wavelength. In current study, the field-gauge invariant semiconductor Bloch equations have been employed [e.g., A. M. Parks et al., Phys. Rev. Lett. 131, 236902 (2023)]. Furthermore, a unique mechanism for amplifying a self-induced plasma field has been identified and analyzed fully based on our self-consistent field-gauge-invariant quantum-kinetic equations for transient-field-driven and laser-excited electron plasma within a quantum-wire system. It is expected that such a unique physical phenomenon is able to generate a tunable high-intensity coherent terahertz-radiation wave.

We employ polarization-resolved Raman spectroscopy and first-principles phonon calculations to study the lattice dynamics and spin-phonon coupling in the kagome spin ice compound HoAgGe. Upon cooling, HoAgGe shows transitions from a nonmagnetic state at 300 K to a partially magnetic ordered phase below $T_2=11.6\mathrm{K}$, eventually reaching a fully magnetic ordered phase below $T_1=7\mathrm{K}$. We detect 8 of 10 Raman-active phonon modes at room temperature, with frequencies consistent with first-principles phonon calculations. We find that the low-energy phonon modes harden upon cooling, soften slightly below $T_2$, and, finally, harden again below $T_1$, suggesting finite spin-phonon coupling in HoAgGe. Furthermore, one high-energy mode at $185{\mathrm{cm}}^{-1}$ with $E^{\prime }$ symmetry exhibits a Fano line shape due to electron-phonon coupling. Our Raman results establish finite spin-phonon coupling and electron-phonon coupling in the magnetic phase of HoAgGe, providing a foundation for future studies of the novel field-induced phases at low temperatures in HoAgGe.

Four-point electrical resistivity measurements were carried out on erbium (Er) metal and dilute magnetic alloy containing 0.5 at.$\%$ Er in superconducting Y for temperatures 3.5–295 K under pressures to 183 GPa in a diamond anvil cell. The magnetic ordering temperature $T_o$ of Er appears to rise rapidly above 80 GPa. Y(Er) alloys display giant superconducting pair breaking $\mathrm{\Delta }{T}_c$ per atomic percent as large as 19 K/at.$\%$ Er. The present results give evidence that, for pressures above 80 GPa, the exchange coupling $J$ between Er ions and conduction electrons becomes negative, thus activating Kondo physics in this highly correlated electron system. We suggest that these anomalously high magnetic ordering temperatures are an unrecognized feature of the Kondo lattice state, providing another perspective for designing magnetic materials with high ordering temperature.

We study the two-dimensional extended Hubbard model on a square lattice and incorporate spin-differentiated nearest neighbor (NN) interactions where the equal-spin ($V_{uu}$) and unequal-spin ($V_{ud}$) terms are independently tuned parameters. We compute single-particle excitations as well as static spin and pairing susceptibilities perturbatively up to fourth order within the thermodynamic limit and at a finite fixed temperature. By explicitly encoding a ferromagneticlike NN interaction ($V_{uu}<V_{ud}$), we induce a competition among the uniform $q=(0,0)$, collinear $q=(\pi ,0)$, and staggered $q=(\pi ,\pi )$ spin excitations. This results in the formation of short-ranged $2\times 2$ ferromagnetic plaquettes arranged in staggered or striped patterns. Kinetic frustration in hopping, both within and between these plaquettes, manifests in single-particle properties, resulting in a reduction of bandwidth and ultimately triggering a Lifshitz transition to quasi-one-dimensional bands. Furthermore, an attractive effective interaction within the localized ferromagnetic plaquette results in the emergence of equal-spin triplet $p$-wave pairing. We demonstrate that sufficiently strong magnetic fluctuations, even at finite length scales, can significantly influence single-particle and pairing properties without breaking translational symmetry. Our approach provides a pathway to realize a variety of rich magnetic phases and Fermi surface reconstruction driven by interactions in the absence of explicit geometric frustration.

We theoretically investigated the influence of Fermi level pinning on contact potential difference (CPD) measurements conducted via Kelvin probe force microscopy (KPFM) on semiconductor surfaces. To systematically modulate the strength of the surface pinning, virtual surface states were introduced within the semiconductor band gap, and the density of states (DOS) was varied. The numerically simulated CPD values varied depending on the DOS of the surface states, reflecting the magnitude of the surface band bending (surface potential). However, we found that under certain conditions, the CPD values obtained by KPFM deviated from the physical quantities directly associated with the surface potential. Our results provide valuable insights for enhancing the accuracy of KPFM data analysis and interpretation.

Small changes in the structure of thin bidimensional transition metal dichalcogenides (TMDs) can radically alter the overall material properties. Here, we investigate the epitaxial self-organized boundary between two tungsten diselenide domains obtained by chemical vapor deposition: the AB ferroelectric bilayer (noncentrosymmetric, $3R$-like) and the conventional AA' bilayer (centrosymmetric, $2H$). Several tools are used to isolate and confirm the different polytype signatures, such as micro-Raman, second harmonic generation (SHG) and photoluminescence. Transmission electron microscopy is found to bring complementary information to Raman and SHG in the polytype determination. Finally, we resolve and index a particular sawtooth structure in the experimental $2H|3R$ boundary wall, whose origin is linked to particular twin boundaries observed in other TMDs.

The fundamental understanding of the driving mechanism of dipolar ordering in complex perovskite oxides (${\mathrm{ABO}}_3$) and its dependence on the degree of doping as well as on the type of dopant is critical for the effective design of novel functional materials with desired properties. To elucidate the temperature-induced transformations in mixed lead-bismuth systems, we have studied two solid solutions with a morphotropic phase boundary (MPB), $(1-x){\mathrm{PbTiO}}_3\text{−}x{\mathrm{BiNi}}_{0.5}{\mathrm{Ti}}_{0.5}{\mathrm{O}}_3$ (PT-$x\mathrm{BNT}$) and $(1-x){\mathrm{PbTiO}}_3\text{−}{\mathrm{xBiMg}}_{0.5}{\mathrm{Ti}}_{0.5}{\mathrm{O}}_3$ (PT-$x\mathrm{BMT}$), by Raman spectroscopy and second-harmonic generation (SHG). The temperature evolution of atomic dynamics in both PT-$x\mathrm{BNT}$ and PT-$x\mathrm{BMT}$ shows that the paraelectric-to-ferroelectric phase transition at the MPB does not involve any dynamical instability, indicating the dominance of order-disorder transformation processes. The temperature dependence of the SHG intensity reveals that the MPB can be identified by the smallest size of the preexisting intrinsic ferroic entities that on cooling merge to form a ferroelectric state with a macroscopic polarization. The difference between Ni- and Mg-containing solid solutions is merely in the correlation length of local structural distortions.

The emergence of nonlinear spin transport phenomena in magnetic heterostructures encourages a careful examination on different origins of the harmonic signals. Here, we report sizable third harmonic nonlinear resistances in permalloy (Py)/Pt bilayers, accounting for approximately 14% of the linear counterparts at low magnetic field of 5 Oe and current density of $7.9\times {10}^6\mathrm{A}\mathrm{c}{\mathrm{m}}^{-2}$. Both the transverse and longitudinal components increase quadratically with the applied current, exhibiting a twofold symmetry on the magnetization direction. By considering the magnetization and resistance modulations due to current-induced spin-orbit torque (SOT) and Joule heating, we performed a comprehensive analysis on spin current-related transport behaviors up to the third-order terms in the bilayer systems. Through a self-consistent fitting process, it is revealed that the low-field signals are dominated by SOT-induced magnetization oscillation, while the high-field ones primarily result from thermally induced resistance changes. We also find that the possible nonlinear spin current generation is negligibly small in the present material system. Our results could contribute to the understanding of nonlinear magnetotransport phenomena in ferromagnetic/heavy-metal bilayers.

We calculate the melting lines of atomic hydrogen and deuterium up to 900 GPa with path-integral Monte Carlo using a machine-learned interatomic potential. We improve upon previous simulations of melting by treating the electrons with reptation quantum Monte Carlo, and by performing solid and liquid simulations using isothermal-isobaric path-integral Monte Carlo. The resulting melting line for atomic hydrogen is higher than previous estimates. There is a small but resolvable decrease in the melting temperature as pressure is increased, which can be attributed to quantum effects.

While recent theoretical studies have positioned noncollinear polar magnets with $C_{nv}$ symmetry as compelling candidates for realizing topological magnetic phases and substantial intrinsic anomalous Hall conductivity, experimental realizations of the same in strongly correlated systems remain rare. Here, we present a large intrinsic anomalous Hall effect and extended topological magnetic ordering in ${\mathrm{Gd}}_3{\mathrm{Ni}}_8{\mathrm{Sn}}_4$ with hexagonal $C_{6v}$ symmetry. Observation of topological Hall response, corroborated by metamagnetic anomalies in isothermal magnetization, peak/hump features in field-evolution of ac susceptibility and longitudinal resistivity suggest the signature of topological magnetic phases. The anomalous Hall effect is quantitatively accounted for by the intrinsic Berry curvature-mediated mechanism. Our results underscore polar magnets as a promising platform to investigate a plethora of emergent electrodynamic responses rooted in the interplay between magnetism and topology.

We investigate the influence of different $3d$ ferromagnetic metals on the fieldlike torque in Ta. We find that the fieldlike torque and dampinglike torque exhibit the same trend: As the orbital-to-spin conversion coefficient of the ferromagnetic metal increases, the orbital torque with a positive sign in Ta also increases, while the spin torque with a negative sign remains nearly constant. Therefore, under the competition between spin torque and orbital torque, both dampinglike torque and fieldlike torque show the same trend. Additionally, we introduce a 1 nm Cu layer at the interface to exclude the interference of spin-orbit torque generated by the spin Rashba-Edelstein effect or the orbital Rashba-Edelstein effect. Due to the significant orbital Hall conductivity in Ta, we identify that an important source of fieldlike torque is the orbital current generated by the orbital Hall effect, which can be tuned through orbital-to-spin conversion.

The interacting resonant level model (IRLM) is the simplest quantum impurity model to display strongly correlated effects in mesoscopic systems, which triggered its extensive theoretical study. However, to date, there have not been any realizations of the model with controllable interaction parameter, and thus the detailed predictions could not be confirmed. Here we use a recently developed approach to Anderson orthogonality catastrophe physics, using a charge detector coupled to a quantum dot (QD) system, to devise a simple experimental system which could display IRLM behavior and detail its predictions. At the same time, the mapping to IRLM allows us to determine the interaction parameter of the charge detector using simple experimental probes.

Quantum phenomena emerging from the interaction of light and matter in low-dimensional systems hold great potential for future quantum technologies. Here using finite-temperature density functional calculations incorporating nonlocal van der Waals interactions and Hubbard corrections, we report simultaneous structural expansion and magnetic collapse in bilayer FeSe induced by photoexcited hot electrons. Our calculations reveal that, while bulk FeSe is nonmagnetic, as observed experimentally, double-layer FeSe exhibits robust staggered antiferromagnetic order at low temperatures with a net site magnetization of $\sim 2.75{\mu }_B$/Fe. Increasing the density of photoexcited electrons systematically enhances the internal electronic entropy, leading to a complete collapse of the antiferromagnetic order accompanied by an abrupt expansion of the interlayer separation. Our findings suggest the structural and magnetic properties of FeSe thin films can be finely tuned via ultrafast laser excitation, offering a pathway to control quantum phases in iron-based compounds through electronic temperature.

Floquet Topological Insulators (FTIs) in two spatial dimensions can exhibit anomalous chiral edge modes despite a fully localized bulk, captured by a new topological invariant other than the Chern number. In this work, we focus on $(2+1)D$ FTIs in symmetry class A with a spacetime symmetry known as $n$-fold time screw symmetry. We show that the bulk invariant can be partially determined by symmetry indicators associated with $n=2,3,4,6$ time screw symmetry, which only depend on the time evolution in high symmetry momenta. We derive a simple formula that relates symmetry indicators to the bulk invariant, and demonstrates its validity in examples of FTI models. Our results can simplify the engineering of FTIs in real materials, and especially shed light on systems driven by circularly polarized light.

We present a comprehensive theoretical framework for calculating the linear and nonlinear optical responses of time-periodic quantum systems. Using density matrix evolution in the Floquet basis and adopting the length gauge, our approach incorporates both interband and intraband contributions of the position operator, enabling detailed insights into photon-assisted transitions and their associated optical phenomena. Notably, we identify a divergent ac response to dc fields in Floquet systems, reminiscent of a Drude peak at finite frequencies. This framework generalizes to optical tensor conductivity calculations at arbitrary perturbation orders and captures various dc photocurrents, including shift current, injection current, and Berry dipole contributions, under specific limits. To demonstrate the versatility of our method, we compute linear and nonlinear optical conductivities for one- and two-dimensional systems, revealing phenomena such as band inversions, $j$-photon-assisted transitions, and high harmonic generation. These results highlight the interplay between periodic driving and nonlinear optical effects, offering different avenues for exploring dynamical topological properties and their applications in ultrafast spintronics, optoelectronics, and strongly correlated systems. Our findings provide a robust platform for analyzing the complex optical behavior of driven quantum systems and guiding experimental investigations in this rapidly evolving field.

We present ab initio theory for electron reflection spectroscopy of few-layer graphene for arbitrary angles of incidence. The inelastic effects are included in a consistent way using the optical potential retrieved from ab initio simulations of electron energy-loss spectra. We demonstrate a significant impact of inelastic effects even for single-layer graphene. Next, we address the ability of the electron reflection spectroscopy to determine specific parameters of graphene including not only the number of layers in the few-layer graphene but also the stacking type in the graphene multilayers, and to resolve moiré patterns in twisted graphene bilayers. We show that the predicted contrast, although significantly reduced by inelastic effects, is sufficient for the experimental detection of all considered parameters. Our findings are corroborated by a fair correspondence of our theoretical predictions with experimental data, both our own and recently published by other authors.