Global symmetries that define the number of low-energy degrees of freedom have profound consequences on universal properties near topological quantum critical points and in other gapless or nearly gapless states of emergent fermions. We take a $Z_2$ global symmetry (such as time-reversal) as an example to study its effect on thermodynamic and transport properties. Although the thermal entropy density of $Z_2$ symmetric systems is simply twice that of their counterparts without any global symmetries or the $Z_1$ class, the temperature dependence of thermal conductivity $\kappa$ is distinctly and drastically different for different symmetries. For systems with dynamic exponent $z=1$, in the $Z_2$ symmetric class, we have $\kappa \propto T^{-(d-1)}$ in the quantum critical regime near weakly interacting fixed points, while for systems with no global symmetries (i.e., the $Z_1$ class), we have $\kappa \propto T^{-(d+3)}$, with $d$ being the spatial dimension. Only near strong-coupling fixed points, both cases with or without $Z_2$ global symmetries follow the same scaling function, $\kappa \propto T^{d-1}$. These distinct scalings of thermal conductivity can also appear in gapless surface Majorana states.

In this work, we present a comparative study of the structural, elastic, electronic, magnetic, and transport properties of Heusler alloys ${\mathrm{Fe}}_2\mathrm{Rh}Z$ and ${\mathrm{Rh}}_2\mathrm{Fe}Z$ with $Z=\mathrm{Al}$, Si, Ga, Ge, In, Sn using the density functional theory. The strongly constrained and appropriately normed functional is considered for the exchange-correlation functional. The newly Heusler T-type pseudocubic structures, ${\mathrm{T}}^p$ and ${\mathrm{T}}^c$, with alternating layers and columns of Fe and Rh along the [001] direction being $\approx 26$ meV/atom lower in energy than the inverse XA structure are predicted as ground states for the ${\mathrm{Fe}}_2\mathrm{Rh}Z$ family. According to the convex hull analysis, all the considered alloys are thermodynamically and mechanically stable or meta-stable. Hybridization schemes and their role in the formation of half-metallicity depending on the weak tetragonal distortion are discussed. XA-${\mathrm{Fe}}_2\mathrm{RhSi}$ is only predicted to be half-metallic in the minority channel with $100\%$ spin polarization and integer magnetic moment of $5{\mu }_B$, while the ground state T structure with layered ordering of Fe and Rh is characterized by the $73\%$ polarization due to a pseudogap around Fermi level. Additionally, a large Seebeck coefficient and figure of merit for the spin-down channel of XA-${\mathrm{Fe}}_2\mathrm{RhSi}$ is predicted.

We developed a theory of collective plasma oscillations in a dc current-biased field effect transistor with an interdigitated dual grating gate and demonstrated a new mechanism of electron plasma instability in this structure. The instability in the plasmonic crystal formed in the transistor channel develops due to conversion of the kinetic energy carried by the drifting plasmons into electromagnetic energy. The conversion happens at the opposite sides of the gate fingers due to the asymmetry produced by the current flow, and occurs through the gate finger fringing capacitances. The key feature of the proposed instability mechanism is the behavior of the plasma frequency peak and its width as functions of the dc current bias. At a certain critical value of the current, the plasma resonant peak with a small instability increment experiencing redshift with increasing current changes to the blue-shifting peak with a large instability increment. This amplified mode switching effect has been recently observed in graphene-interdigitated structures. The obtained theoretical results are in very good qualitative agreement with these experiments and can be used in future designs of the compact sources of THz electromagnetic radiation.

We have observed photocarrier transport behaviors in ${\mathrm{BiFeO}}_3/{\mathrm{La}}_{1-x}{\mathrm{Sr}}_x{\mathrm{MnO}}_3$ (BFO/LSMO) heterostructures by using time-resolved synchrotron x-ray magnetic circular dichroism in reflectivity. The magnetization of LSMO layers was used as a probe of photo-induced carrier dynamics in the photovoltaic BFO layers. During the photo-induced demagnetization process, the decay time of LSMO ($x$=0.2) magnetization strongly depends on the ferroelectric polarization direction of the BFO layer. The variation of decay time should be attributed to the different sign of accumulated photocarriers at the BFO/LSMO interface induced by the photovoltaic effect of the BFO layer. The photocarriers can reach the BFO/LSMO interface and influence the magnetization distribution in the LSMO layers within the timescale of $\sim 100$ ps. Our results provide a novel strategy to investigate carrier dynamics and mechanisms of optical control of magnetization in thin film heterostructures.

Identification of novel multiferroic materials with high-ordering temperatures remains at the forefront of condensed matter physics research. Recently, room-temperature ferroelectricity of structural origin, arising from off-centering displacements of Gd and Cr ions, has been identified in spark-plasma sintered (SPS) ${\mathrm{GdCrO}}_3$ [Suryakanta Mishra et al., Phys. Rev. B 104, L180101 (2021)]. Here, using a similar synthesis protocol (involving SPS), we have been able to engineer room-temperature ferroelectricity (FE) from a similar mechanism in two otherwise nonferroelectric $\mathit{R}{\mathrm{CrO}}_3$ (R= rare-earth) compounds, namely, ${\mathrm{DyCrO}}_3$ (which is reported as a quantum paraelectric) and ${\mathrm{LaCrO}}_3$ (which is already known to be paraelectric). Room-temperature FE in SPS-${\mathrm{LaCrO}}_3$ and SPS-${\mathrm{DyCrO}}_3$ is confirmed through various electrical, calorimetric, and synchrotron-based structural investigations. Out of these two emergent room-temperature FE materials, SPS-${\mathrm{LaCrO}}_3$ also undergoes a high-temperature antiferromagnetic ordering at 290 K, thus coming very close to becoming the first room-temperature multiferroic material in this promising family of $\mathit{R}{\mathrm{CrO}}_3$ compounds.

An atomistic effective Hamiltonian technique is used to investigate the finite-temperature energy storage properties of a ferroelectric nanocomposite consisting of an array of ${\mathrm{BaTiO}}_3$ nanowires embedded in a ${\mathrm{SrTiO}}_3$ matrix, for electric field applied along the long axis of the nanowires. We find that the energy density versus temperature curve adopts a nonlinear, mostly temperature-independent response when the system exhibits phases possessing an out-of-plane polarization and vortices, while the energy density more linearly increases with temperature when the nanocomposite either only possesses vortices (and thus no spontaneous polarization) or is in a paraelectric and paratoroidic phase for its equilibrium state. Ultrahigh-energy density up to $\simeq 141 \mathrm{J}/{\mathrm{cm}}^3$ and an ideal 100% efficiency are also predicted in this nanocomposite. A phenomenological model, involving a coupling between polarization and toroidal moment, is further proposed to interpret these energy density results.

We investigate the influence of three- and four-phonon scattering, perturbative anharmonic phonon renormalization, and off-diagonal terms of coherent phonons on the thermal conductivity of ${\mathrm{CsPbBr}}_3$ phase change perovskite, by using advanced implementations and first-principles simulations. Our study spans a wide temperature range covering the entire structural spectrum. Notably, we demonstrate that the interactions between acoustic and optical phonons result in contrasting trends of phonon frequency shifts for the high-lying optical phonons in orthorhombic and cubic ${\mathrm{CsPbBr}}_3$ as temperature varies. Our findings highlight the significance of wavelike tunneling of coherent phonons in ultralow and glasslike thermal conductivity in halide perovskites.

The optical vortex, an optical beam carrying orbital angular momentum (OAM), has been demonstrated to be promising in manipulating magnetic structure and its dynamics in various systems. In this work, by using numerical and analytical methods, we investigate the optical vortex control of the ultrafast dynamics of skyrmion in antiferromagnets. It is shown that an isolated skyrmion can be generated/erased in an ultrashort time of $\sim \mathrm{ps}$ by beam focusing through the Zeeman effect. Subsequently, the OAM can be transferred to the skyrmion and results in its rotation. Different from the case of ferromagnets, the skyrmion rotation direction in antiferromagnets depends on the light frequency, allowing an easy control of the skyrmion rotation via tuning the frequency of light beam. Furthermore, the skyrmion Hall motion, driven by multipolar spin waves that are excited by optical vortex, is revealed in our calculations, demonstrating the dependence of the Hall angle on the OAM quantum number. This work unveils the interesting optical control of the skyrmion dynamics in antiferromagnets, which is a crucial step towards the development of magneto-optic and spintronic applications.

The motion of a skyrmion driven by propagating spin waves in a thin film track with interfacial perpendicular magnetic anisotropy and Dzyaloshinskii-Moriya interaction was investigated by numerical micromagnetic simulations. At long simulation times, we find that the skyrmion always moves in the direction of the spin-wave propagation. At shorter times, rather than observing skyrmion motion in a single direction, the skyrmion shows transient retrograde motion when the track width is wider than 50 nm (for Gilbert damping $\alpha =0.02$) or $\alpha <0.06$ (for a track width of 100 nm). A simple treatment of spin-wave-driven skyrmion motion based on the Thiele equation predicts that the skyrmion will be pulled back towards the source of the spin waves because of the gyrotropic and dissipative effects. The forward motion away from the spin wave source can be explained by also considering the effect of a repulsive force from the edges of the track when the skyrmion is in close contact with it. We discuss these effects quantitatively in terms of the spin-wave transmission and reflection coefficients of the skyrmion.

A new family of free fermionic quantum spin chains with multispin interactions was recently introduced. Here we show that it is possible to build standard quantum Ising chains—but with inhomogeneous couplings—which have the same spectra as the novel spin chains with multispin interactions. The Ising models are obtained by associating an antisymmetric tridiagonal matrix to the polynomials that characterize the quasienergies of the system via a modified Euclidean algorithm. For the simplest nontrivial case, corresponding to the Fendley model, the phase diagram of the inhomogeneous Ising model is investigated numerically. It is characterized by gapped phases separated by critical lines with order-disorder transitions depending on the parity of the total number of energy density operators in the Hamiltonian.

Topological materials, such as topological semimetals and topological insulators, with robust topological surface states have bright application prospects in electrochemical catalysis. Here, the first-principles calculations indicate the strong topological insulator ${\mathrm{Li}}_2\mathrm{Pt}$ family promotes efficient catalytic response to the hydrogen evolution reaction. For ${\mathrm{Li}}_2\mathrm{Pt}$ and ${\mathrm{Li}}_2\mathrm{Pd}$, the calculated Gibbs free energy $\mathrm{\Delta }{G}_{{\mathrm{H}}^{*}}$ of the bridge site is 0.054 eV and 0.041 eV, while that for the top site is 0.187 eV and 0.641 eV, respectively. The better hydrogen evolution reaction performance of the bridge site can ascribe to H hybridizes with the $d_{xy}+d_{x^2-y^2}$ orbital, which donates the nontrivial topological surface states, while H hybridizes with the $d_{z^2}$ orbital that withholds contribution to topological surface states for the top site. Noticeably, the $\mathrm{\Delta }{G}_{{\mathrm{H}}^{*}}$ of the bridge site for ${\mathrm{Li}}_2\mathrm{Pt}$ (0.054 eV) and ${\mathrm{Li}}_2\mathrm{Pd}$ (0.041 eV) is nearly half of the value of Pt (0.09 eV), indicating an excellent hydrogen evolution reaction activity. This work uncovers the hybridization between adsorbate and topological surface states plays a vital role in enhancing the hydrogen evolution reaction performance and provides a promising route to design topological quantum catalysts.

We study conductance oscillations of antiferromagnetic layer tunnel junctions composed of antiferromagnetic topological insulators such as ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$. In the presence of an in-plane magnetic field, we find that the two terminal differential conductances across the junction oscillates as a function of field strength. Notably, the quantum interference at weak fields for the even-layer case is distinctive from the odd-layer case due to the scattering phase shift $\pi$. Consequently, the differential conductance vanishes (maximized) at integer magnetic flux quanta for even-layer (odd-layer) junctions. The conductance oscillations manifest the layer-dependent quantum interference in which symmetries and scattering phases play essential roles. In numerical calculations, we observe that the quantum interference undergoes an evolution from superconducting quantum interference device-like to Fraunhofer-like oscillations as the junction length increases.

The optical properties of two-dimensional materials are exceptional in several respects. They are highly anisotropic and frequently dominated by excitonic effects. Dipole-allowed second-order nonlinear optical properties require broken inversion symmetry. Hence, several two-dimensional materials show strong in-plane (IP) nonlinearity but negligible out-of-plane (OOP) response due to vertical symmetry. By considering buckled hexagonal monolayers, we analyze the critical role of broken vertical symmetry on their excitonic optical response. Both linear as well as second-order shift current and second-harmonic response are studied. We demonstrate that substantial OOP nonlinear response can be obtained, in particular, through off-diagonal tensor elements coupling IP excitation to OOP response. Our findings are explained by excitonic selection rules for OOP response and the impact of dielectric screening on excitons is elucidated.

The valley-contrasting geometric features of electronic wave functions manifested in Berry curvature and orbital magnetic moment have profound consequences on magnetotransport properties in both three- and two-dimensional systems. Although the importance of employing beyond-relaxation-time-approximation methods and intervalley scatterings in collision integral has been confirmed in three dimensions, they have been widely overlooked in previous studies on two-dimensional multivalley systems. Here, we study the issue of weak-field magnetoresistance in two-dimensional multivalley systems with broken inversion symmetry. We provide an exact solution to the Boltzmann equation and demonstrate that the inclusion of in-scattering terms in the collision integral can change the sign of the magnetoresistance in the high-density regime. With an initial valley polarization, we also predict an orbital magnetic moment-induced intrinsic contribution to Hall conductivity in the time-reversal-broken situation, which is consistently negative and, in contrast to the anomalous Hall term, it does not depend on the polarization sign. Depending on which valley has the excess charge, our calculations show that a completely distinct behavior is exhibited in the magnetoresistance which can be considered as a valley-polarization probe in the experiment.

We study the electronic properties of graphene with triangular defects in the superhoneycomb arrangement by performing first-principles calculations within the framework of density functional theory. First, we investigate the band gaps using spin-unpolarized calculations. Interestingly, we find that the system-size dependence of the band gap value is unique and different from the previously known scaling law, which explains the band gap behaviors of various graphene materials with defects. In particular, the defect size dependence of the band gap value shows a different tendency, and thus, we obtain the universal scaling law with respect to the size of the defect and periodicity. We can estimate the band gap values of larger systems using this law without significant computational cost. In addition, we find that the system usually possesses a direct band gap, indicating that it is a promising material for optoelectronics applications. Further, we perform spin-polarized calculations to study stable magnetic states. We find that the ferromagnetic or the antiferromagnetic states are stable when the defect is large. From the analysis of Néel temperature, we also find that the spin-polarized system at 0 K exhibits spin polarization at room temperature when the distance between the defects is rather short. In addition, the band gap value of the spin-polarized system converges to a nonzero value. These findings indicate that further research on graphene with defects is highly significant and necessary for controlling the electronic properties of graphene-based materials.

We investigate the second-order nonlinear electronic thermal transport induced by the temperature gradient. We develop the quantum kinetic theory framework to describe thermal transport in the presence of a temperature gradient. Using this, we predict an intrinsic scattering time-independent nonlinear thermal current in addition to the known extrinsic nonlinear Drude and Berry curvature dipole contributions. We show that the intrinsic thermal current is determined by the band geometric quantities and is nonzero only in systems in which both the space inversion and time-reversal symmetries are broken. We employ the developed theory to study the thermal response in tilted massive Dirac systems. We show that besides the different scattering time dependencies, the various current contributions have distinct temperature dependencies in the low-temperature limit. Our systematic and comprehensive theory for nonlinear thermal transport paves the way for future theoretical and experimental studies on intrinsic thermal responses.