We report a thermal transport study of IrSbSe, which crystallizes in a noncentrosymmetric cubic structure with the $P{2}_{1}3$ space group and shows a narrow-gap semiconducting behavior. The large discrepancy between the activation energy for conductivity [$E_{\rho }=128\left(2\right)$ meV] and for thermopower [$E_S=17.7\left(9\right)$ meV] from 200 to 300 K indicates a polaronic transport mechanism. The electrical resistivity varies as $exp{(T_0/T)}^{1/4}$ and thermopower varies as $T^{1/2}$ at low temperatures, indicating that it evolves into Mott variable-range hopping dominant conduction with decreasing temperature. IrSbSe shows a relatively low value of thermal conductivity ($\sim 1.65$ W/K m) and thermopower of about 0.24 mV/K around 100 K, yet poor electrical conductivity. On the other hand, a high vacancy defect concentration on both Ir and Sb atomic sites of up to 15% suggests a high defect tolerance and points to the possibility of a future improvement of carrier density by chemical substitution or defect optimization.

Two natural bournonite specimens have been characterized by structural, chemical, spectroscopical, magnetic, and thermodynamic analyses. This study confirms a stoichiometric composition of ${\mathrm{PbCuSbS}}_3$ and the crystal to be of an outstanding quality allowing us to consider its properties as being intrinsic for this material. Electronic structure calculations, electrical resistivity measurements, and spectroscopical characterizations reveal ${\mathrm{PbCuSbS}}_3$ to be a direct $n$-type semiconductor with an optical energy gap $E_g^{\mathrm{opt}}$ = 1.69 eV, huge Seebeck coefficient and electrical resistivity (e.g., $\sim -1200 \mu \mathrm{V}{\mathrm{K}}^{-1}$ and $\sim 1000 \mathrm{\Omega }\mathrm{m}$ at room temperature, respectively). The thermal conductivity in ${\mathrm{PbCuSbS}}_3$ is found to be very low [$\kappa \left(T\right)\sim 2–0.5 \mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ in the temperature range 100–600 K] and is dominated by optical phonons ($T>100$ K), which poorly transport heat, strongly scatter the acoustic ones and substantially intensify the phonon-phonon $umklapp$ processes. Additionally, large atomic displacement parameters and the presence of low-energy optical phonons have been identified, evidencing “rattling” effects associated to Cu and Pb atoms in the crystal structure. All these result in high Grüneisen parameters (${\gamma }_{\mathrm{G}}$ = 4.8–3.2) and very short phonon mean free path ($l_{\mathrm{ph}}$ = 11–3 Å for 100–300 K). Thus, the low thermal conductivity in bournonite reflects a combination of different factors leading to a huge phonon anharmonicity.

Transition metal dichalcogenides (TMDCs), belonging to the class of van der Waals materials, are promising materials for optoelectronics and photonics. In particular, their giant optical anisotropy may enable important optical effects when employed in nanostructures with finite thickness. In this paper, we theoretically and numerically study light scattering behavior from anisotropic $\mathrm{Mo}{\mathrm{S}}_2$ nanocylinders, and highlight its distinct features advantageous over the response of conventional silicon particles of the same shape. We establish two remarkable phenomena, appearing in the same $\mathrm{Mo}{\mathrm{S}}_2$ particle with optimized geometry. The first one is a pure magnetic dipole scattering associated with the excitation of the electric-dipole anapole states. Previously reported in core-shell hybrid (metal/dielectric) systems only, it is now demonstrated in an all-dielectric particle. The second phenomenon is the super deflection in the far-field: the maximum scattering may occur over a wide range of directions, including forward-, backward- and side-scattering depending on the mutual orientation of the $\mathrm{Mo}{\mathrm{S}}_2$ nanocylinder and the incident wave. In contrast to the well-known Kerker and anti-Kerker effects, which appear in nanoparticles at different frequencies, the super deflection can be achieved by rotating the particle at a constant frequency of incident light. Our results facilitate the development of functional optical devices incorporating nanostructured anisotropic TMDCs and may encourage further research in meta-optics based on highly anisotropic materials.

Molecules in molecular junctions are subject to current-induced forces that can break chemical bonds, induce reactions, destabilize molecular geometry, and halt the operation of the junction. Theories behind current-driven molecular dynamics simulations rely on a perturbative timescale separation within the system with subsequent use of nonequilibrium Green's functions (NEGF) to compute conservative, nonconservative, and stochastic forces exerted by electrons on nuclear degrees of freedom. We analyze the effectiveness of this approximation, paying particular attention to the phenomenon of negative viscosities. The perturbative approximation is directly compared to the nonequilibrium Ehrenfest approach. We introduce a time-stepping approach to calculate the forces present in the Ehrenfest method via exact integration of the equations of motion for the nonequilibrium Green's functions, which does not necessitate a timescale separation within the system and provides an exact description for the corresponding classical dynamics. We observe that negative viscosities are not artifacts of a perturbative treatment but also emerge in Ehrenfest dynamics. However, the effects of negative viscosity have the possibility of being overwhelmed by the predominantly positive dissipation due to the higher order forces unaccounted for by the perturbative approach. Additionally, we assess the validity of the white-noise approximation for the stochastic forces, finding that it is justifiable in the presence of a clear timescale separation and is more applicable when the current-carrying molecular orbital is moved outside of the voltage window. Finally, we demonstrate the method for molecular junction models consisting of one and two classical degrees of freedom.

We study the rippling instabilities of multilayer graphene and graphite as a function of strain and number of layers using density-functional theory and density-functional perturbation theory calculations. We find that, for multilayer graphene, a small but finite compressive strain is needed to induce a rippling instability, signaled by the onset of imaginary frequencies near the center of the Brillouin zone. For monolayer graphene, this critical strain is zero within numerical uncertainty. Using insights from continuum elasticity theory, we fit the phonon dispersion of the ZA mode near the zone center to a function ${\omega }^2\left(q\right)=C_2{q}^2+C_4{q}^4$ and determine the $C_2$ and $C_4$ coefficients as a function of strain and number of layers. In particular, the $C_2$ coefficient is positive for multilayers in the absence of strain and shows a linear dependence with strain. It assumes negative values for compressive strains beyond a critical value and it is responsible for the appearance of imaginary frequencies in the ZA mode. We discuss these results in light of both continuum and atomistic approaches. Critical parameters vary smoothly with the number of layers from monolayer graphene (two-dimensional) to graphite (three-dimensional).

Half-topological states of matter and two-dimensional (2D) magnetism have gained much attention recently. In this paper, we propose a special topological semimetal phase called a 2D half Chern-Weyl semimetal (HCWS), which is a 2D magnetic Weyl semimetal bound to the half Chern insulator (fully spin-polarized Chern insulator) phase by symmetry, and the two phases can be converted to each other by manipulating the magnetization direction. We provide the symmetry conditions to realize this state in 2D systems with multiple screw axes. Tight-binding models with multiple basis and a predicted 2D material, monolayer TiTe, are shown as the concrete examples for HCWSs. The TiTe monolayer was shown to have a high ferromagnetic Curie temperature ($\approx 966$ K) as well as a Coulomb correlation-enhanced spin-orbit coupling (SOC), and further demonstrates the effect of correlation-enhanced SOC on magnetocrystalline anisotropy energy and energy gap opening. Our work reveals a state with switchable and spin-resolved half-body charge currents as well as half-boundary charge currents, and will provide a platform for novel and high-performance topological spintronics devices.

We report the observation of twofold rotation symmetry in charge-density wave (CDW) state in the newly discovered kagome superconductor ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$. Just below its CDW transition temperature ($T_{\mathrm{CDW}}$), the polarization rotation of the reflected laser beam promptly emerges and increases close to about 1 mrad, and the rotation angle shows twofold rotation symmetry. This observation is very different from many other experimental methods in which nematicity or ${\mathrm{C}}_2$ rotation symmetry appears at low temperatures far below $T_{\mathrm{CDW}}$. There are two components included, where the $d$-wave-like component originates from electronic nematic order and constant term points to the possible existence of time-reversal-symmetry breaking (TRSB), respectively. With femtosecond laser pulse pumping, the likely TRSB and nematicity can be easily suppressed, and the latter is further modulated by coherent phonon. Our investigation provides clear optical evidence for the formation of nematic order with twofold rotation symmetry and possible existence of TRSB just below $T_{\mathrm{CDW}}$ and, moreover, reveals the controlling method with femtosecond laser and coherent phonon.

An Archimedean screw is a classical pump that exploits the equivalence of rotation and translation in helices. Similarly, a spin spiral texture can pump charge and spin by rotating at a frequency $\omega$. In the present paper, we study these pumping phenomena within a microscopic quantum model by both perturbation theory and numerical simulations. Inside the spiral region, the spin polarization and charge current are linear in $\omega$ whereas the spin current is ${\omega }^2$ for small $\omega$. We find that the charge current is related to the mixed momentum-phason Berry phase, which can be viewed as a novel approximate realization of a Thouless pump. It is nearly quantized in spirals with short pitch $\lambda$ but decays with ${\lambda }^{-1}$ for longer pitches, unlike true Thouless pumps or Archimedean screws. Moreover, we study the onset of nonadiabaticity (large $\omega$), the impact of attached nonmagnetic or magnetic contacts, and the real-time evolution of the transport observables. Finally, we analyze the effects of disorders which, surprisingly, might enhance the spin current but suppress the charge current.

We theoretically investigate the topological properties of a dimerized quasi-one-dimensional (1D) lattice comprised of multilegs $\left(L\right)$ as well as multisublattices $\left(R\right)$. The system has main and subsidiary exchange symmetries. In the basis of the latter one, the system can be divided into $L$ 1D subsystems, each of which corresponds to a generalized ${\mathrm{SSH}}_R$ model having $R$ sublattices and on-site potentials. Chiral symmetry is absent in all subsystems except when the axis of the main exchange symmetry coincides on the central chain. We find that the system may host zero- and finite-energy topological edge states. The existence of a zero-energy edge state requires a certain relation between the number of legs and sublattices. As such, different topological phases, protected by subsystem symmetry, including zero-energy edge states in the main gap, no zero-energy edge states, and zero-energy edge states in the bulk states are characterized. Despite the classification symmetry of the system belongs to BDI symmetry class, but each subsystem falls into either AI or BDI symmetry class.

The contribution of free electrons to the dielectric function is largely determined by their plasmonic resonance, a collective density oscillation. We studied this broadband and carrier-density-dependent response in the narrow-gap semiconductor InSb with THz shockwave spectroscopy. A synthesized waveform, with a steep onset followed by a short electric-field plateau, gives access to a multioctave spectrum of frequencies from 100 GHz up to the mid infrared. By measuring the entire spectral characteristics of the plasma, we analyze the dynamics of photogenerated free electrons for a wide range of excitation fluences. Thanks to this analysis, we were able to quantify the coefficients of both electron trapping and the Auger process from cryogenic to room temperature.

We investigate the propagation of surface plasmons in a thin layer of copper surrounded by copper oxide ${\mathrm{Cu}}_2\mathrm{O}$. It is shown that particularly strong excitons in ${\mathrm{Cu}}_2\mathrm{O}$ can have a considerable impact on plasmon propagation, providing many opportunities for plasmon-exciton and plasmon-plasmon interactions. It is demonstrated that by the use of a sufficiently thin metal layer, one can excite the so-called long range plasmons, which can overcome the inherently high ohmic losses of Cu as compared to the usual plasmonic metals such as silver. Analytical results are confirmed by numerical calculations.

Open-shell nanographenes can be covalently bonded and still preserve their local moments, forming interacting spin lattices. In the case of benzenoid nanographenes, the Ovchinnikov-Lieb rules anticipate the spin of the ground state of the superstructure and thereby the sign of the intermolecular exchange. Here we address the underlying microscopic mechanisms for intermolecular exchange in this type of system. We find that, in general, three different mechanisms contribute. First, Hund's ferromagnetic exchange that promotes ferromagnetic interactions of electrons in overlapping orbitals. Second, superexchange driven by intermolecular hybridization, identical to Anderson kinetic exchange, which is a decreasing function of the Hubbard-$U$ energy scale and is always antiferromagnetic. Third, a Coulomb-driven superexchange, that increases as a function of $U$ and involves virtual excitation of excited molecular orbitals that are extended over the entire structure. We find that Coulomb-driven superexchange can be either ferro- or antiferromagnetic, accounting for Ovchinnikov-Lieb rules. We compute these exchange energies for the case of coupled $S=1/2$ phenalenyl triangulenes, using multiconfigurational methods both with Hubbard and extended Hubbard models, thereby addressing the influence of long-range Coulomb interactions on the exchange interactions.

A global quantum quench can be modeled by a quantum circuit with local unitary gates. In general, entanglement grows linearly at a rate given by entanglement velocity. Locality yields a finite light cone, which bounds the velocity. We show that the unitary interactions achieving the maximal rate must remain unitary if we exchange the space and time directions—a property known as dual unitarity. Our results are robust: approximate maximal entanglement velocity also implies approximate dual unitarity. We further show that maximal entanglement velocity is always accompanied by a specific dynamical pattern of entanglement, which yields simpler analyses of several known exactly solvable models.

Non-Hermitian topological corner modes can be formed in exciton-polariton lattices under nonresonant pumping. Signals propagating in the bulk of the system can travel around defects, which is not possible in one-dimensional topological lattices or two-dimensional lattices with Hermitian edge states. In the presence of nonlinear interactions, the system remains topological, as evidenced by the winding of the complex fluctuation spectrum. Finally, as all polariton states are localized, the system offers an opportunity for more accurate measurement of the polariton-polariton interaction strength as one can avoid condensing in a state with large overlap with the pump-induced exciton-reservoir.

Understanding various macroscopic pressure-volume-temperature properties of materials on the atomistic level has always been an ambition for physicists and material scientists. Particularly, some materials such as zirconium tungstate (${\mathrm{ZrW}}_2{\mathrm{O}}_8$), exhibit multiple exotic properties including negative thermal expansion (NTE) and pressure-induced amorphization (PIA). Here, using machine-learning based deep potential, we trace both of the phenomena in ${\mathrm{ZrW}}_2{\mathrm{O}}_8$ back to a common atomistic origin, where the nonbridging O atoms play a critical role. We demonstrate that the nonbridging O atoms confer great flexibility to vibration of polyhedra, and kinetically drive volume shrinking on heating, or NTE. In addition, beyond a certain critical pressure, we find that the migration of nonbridging O atoms leads to additional bond formation that lowers the potential energy, suggesting that the PIA is a potential-driven first-order phase transition. Most importantly, we identify a second critical pressure beyond which the amorphous phase of ${\mathrm{ZrW}}_2{\mathrm{O}}_8$ undergoes a “hidden” phase transition from a reversible phase to an irreversible one.

The atomic-scale structure of glassy ${\mathrm{TeO}}_2$ has been the subject of a longstanding debate. We resort to first-principles molecular dynamics with a careful choice of the exchange and correlation functional to achieve a good agreement with experimental findings for the topology of glassy and molten ${\mathrm{TeO}}_2$. We show that only molecular dynamics at the hybrid functional level of theory is able to reproduce a correct description of the medium-range order in the glass. Based on a Wannier-function decomposition of the electronic structure, we show that the coordination number of tellurium is around 4 and that there is a non-negligible fraction of nonbridging oxygen atoms. An analysis of the net atomic charge distribution shows that an increase of the Te-O-Te bridge asymmetry strongly correlates with the charge on the oxygen atom. Additionally, we find that the oxygen bridge asymmetry increases with temperature, which strengthens the short-range disorder in molten ${\mathrm{TeO}}_2$, and consequently lowers the coordination number of Te. These results provide a revisited picture of the ${\mathrm{TeO}}_2$ network connectivity and its evolution as a function of temperature.

Multireservoir models are widely used for modeling and interpreting ultrafast magnetization dynamics. Here we introduce an alternative formulation to existing three-temperature models for the treatment of spin, electron, and lattice temperatures in magnetization dynamics simulations. In contrast to most existing models of calculations of energy transfer between reservoirs in these types of simulations, the heat distribution of the spin and lattice subsystems is evaluated during the simulation instead of being calculated a priori. The model is applied to investigate the demagnetization and remagnetization of fcc Ni, when subjected to a strong laser pulse. In particular, our model results in a fast interplay between the electron and spin subsystems which reproduces the main features of experimental data for fcc Ni significantly better than most reported three-temperature models. We also show that the way in which the electron, spin, and lattice heat capacities are described can have a significant impact on the simulated ultrafast dynamics. By introducing spin-lattice couplings in the simulation, it is shown that these explicit interactions only have a marginal impact on the magnetization dynamics of fcc Ni, albeit it is more pronounced for higher laser pulse powers.

Spin waves in noncollinear magnets with $C_{nv}$ symmetry are discussed in the context of inelastic neutron scattering in small-angle scattering geometry. In the framework of a minimal model including exchange coupling, Dzyaloshinskii-Moriya interaction (DMI), and single-ion anisotropy, we consider the system properties at moderate external magnetic fields and in the high-field fully polarized phase. In the latter case, the magnon spectrum is gapped and nonreciprocal due to DMI with the minimum perpendicular to the field direction for the in-plane field and is ferromagneticlike for the field along the high-symmetry axis. Inelastic spin-wave small-angle neutron scattering (SWSANS) in the fully polarized phase is considered in four different geometries. It is shown that analysis of the field dependence of the SWSANS cutoff feature in these geometries allows determining all important parameters of the model. The possibility of the proposed method utilization in thin films with interfacial DMI is also discussed.

Stacking two layers of graphene with a relative twist angle gives rise to Moiré patterns, which can strongly modify electronic behavior and may lead to unconventional superconductivity. A synthetic version of twisted bilayers can be engineered with cold atoms in optical lattices. Here, the bilayer structure is mimicked through coupling between atomic sublevels, and the twist is achieved by a spatial modulation of this coupling. In the present paper, we investigate the superconducting behavior of fermionic atoms in such a synthetic twisted bilayer lattice. Attractive interactions between the atoms are treated on the mean-field level, and the superconducting behavior is analyzed via the self-consistently determined pairing gap. A strong enhancement of the pairing gap is found when a quasi-flat band structure occurs at the Fermi surface, reflecting the prominent role played by the twist on the superconductivity. The tunability of interactions allows for the switching of superconducting correlations from intra (synthetic) layer to inter (synthetic) layer. This includes also the intermediate scenario, in which the competition between inter- and intra-layer coupling completely destroys the superconducting behavior, resulting in re-entrant superconductivity upon tuning of the interactions.

Understanding the relation between the structural disorder in the atomic geometry of the recrystallized state of phase-change memory materials and the localized states in the electronic structure is necessary not only for technological advances, but also essential to achieve a fundamental understanding of these materials. In this computational study, hybrid density-functional theory simulations are employed to ascertain the impact of antisite defects on the spatial localization of the electronic states in the bottom of the conduction band in recrystallized models of the prototypical phase-change material ${\mathrm{Ge}}_2{\mathrm{Sb}}_2{\mathrm{Te}}_5$. $\mathrm{Te}\text{−}\mathrm{Te}$ homopolar bonds are the local defective atomic environments mainly responsible for the electron localization of the conduction-band-edge states in the simulated structures, while $\mathrm{Sb}\text{−}\mathrm{Te}$ chains can also induce spatial localization. Unoccupied defect-related electronic states can emerge in the band gap during a crystallization event, while $\mathrm{Sb}\text{−}\mathrm{Sb}$ homopolar bonds have been identified in the defect environment of a deep localized state.