The development of low contact resistance at metal-semiconductor interfaces in next-generation transistors is being prioritized to improve device performance. By using density functional theory, an intrinsic Ohmic contact between a wide band gap semiconductor MgS and semimetal graphene is predicted herewith theoretically. The zero Schottky barriers in graphene/MgS van der Waals heterostructure (vdWH) can facilitate a high charge injection efficiency, whereas Ohmic contact can be induced in graphene/MgSe under small external perturbation. A comprehensive investigation of the modulation in the electronic contact properties is conducted under the application of vertical compressive strain and a perpendicular electric field to understand their role in the transport mechanism. Under vertical compressive strain, a band gap of ∼0.62 eV in graphene has been opened up. While under perpendicular electric field, the hole carrier concentration in graphene is found to be increased up to $\sim {10}^{14}\mathrm{c}{\mathrm{m}}^{–2}$. Moreover, this work circumvents the prevalent approaches in inducing an Ohmic contact and addresses solutions to very fundamental challenges in pristine graphene, i.e., band gap opening and its tunability, and modulation of carrier concentration. These interesting properties of vdWHs can open up new avenues for constructing these heterojunctions for multifunctional graphene-based field-effect transistors.

Topological metals possess gapless band structures accompanied by nontrivial edge states. Topological metals are created from the topological insulators by adjusting the magnetic flux. Here we propose the time-reversal symmetry protected topological metal without employing the magnetic flux. Topological metallic phase presents although the Chern number vanishes. In the topologically nontrivial phase, three different cases in the metallic phase are distinguished from the band-touching and in-gap features of the topological edge states. These findings shed light on the time-reversal symmetric topological metals and greatly simplify the realization of topological metals.

We find the nonlinear conductance of a dissipative resonant level in the nonequilibrium steady state near its quantum critical point. The system consists of a spin-polarized quantum dot connected to two resistive leads that provide ohmic dissipation. We focus on the crossover from the strong-coupling, non-Fermi-liquid regime to the weak-coupling, Fermi-liquid ground state, a crossover driven by the instability of the quantum critical point to hybridization asymmetry or detuning of the level in the dot. We show that the crossover properties are given by tunneling through an effective single barrier described by the boundary sine-Gordon model. The nonlinear conductance is then obtained from thermodynamic Bethe ansatz results in the literature, which were developed to treat tunneling in a Luttinger liquid. The current-voltage characteristics are thus found for any value of the resistance of the leads. For the special case of lead resistance equal to the quantum resistance, we find mappings onto, first, the two-channel Kondo model and, second, an effectively noninteracting model from which the nonlinear conductance is found analytically. A key feature of the general crossover function is that the nonequilibrium crossover driven by applied bias is different from the crossover driven by temperature—we find that the nonequilibrium crossover is substantially sharper. Finally, we compare to experimental results for both the bias and temperature crossovers: the agreement is excellent.

Local structural correlations in a novel ferroelectric solid solution [$x\mathrm{Bi}\left({\mathrm{Ni}}_{0.5}{\mathrm{Hf}}_{0.5}\right){\mathrm{O}}_3\text{−}(1-x){\mathrm{PbTiO}}_3$ at $x=0.39$] have been drawn out through atomistic modeling against neutron pair distribution functions and complementary Raman scattering measurements. By examining polar displacements of different cations from the refined structural models, we reveal the distributions of cation-specific dipolar fluctuations at the key composition where the maximum piezoelectric response was recorded. It becomes clear that the unusual structural modifications observed after the poling are simply a manifestation driven by the varying extent of the dipolar fluctuations and not a structural transformation caused by adaptive cationic displacements. Moreover, in addition to strong dipolar flexibility under an electric field, both poled and unpoled ceramics exhibit orientation-independent polarized Raman scattering, which is more typical of a dipolar glass with a responsive and polarizable matrix than a polycrystalline material composed of ferroelectric domains. Therefore, compared to other analogous $x\mathrm{Bi}\left(\mathrm{Me}\right){\mathrm{O}}_3\text{−}(1-x){\mathrm{PbTiO}}_3$ systems, the current material is a unique case where chemical substitution has led to a highly polarizable ionic-covalent matrix that seems to be the key ingredient to generate giant piezoelectricity at a composition beyond the morphotropic phase boundary.

We report the results of our experimental studies on the magnetic, transport, and thermoelectric properties of the ferromagnetic metal CoMnSb. A sizable anomalous Hall conductivity ${\sigma }_{yx}$ and transverse thermoelectric conductivity ${\alpha }_{yx}$ are found experimentally and comparable in size to the values estimated from first-principles calculation. Our experiment further reveals that CoMnSb exhibits $-TlogT$ critical behavior in ${\alpha }_{yx}\left(T\right)$, deviating from Fermi liquid behavior ${\alpha }_{yx}\sim T$ over a decade of temperature between 10 and 400 K, similar to ferromagnetic Weyl and nodal-line semimetals. Our theoretical calculation for CoMnSb also predicts the $-TlogT$ behavior when the Fermi energy locates near the Weyl nodes in momentum space.

Using the coherent-potential approximation in heterogeneous-elasticity theory with a log-normal distribution of elastic constants for the description of the Raman spectrum and the temperature dependence of the specific heat, we are able to reconstruct the vibrational density of states and characteristic descriptors of the elastic heterogeneity of a wide range of glassy materials. These descriptors are the nonaffine contribution to the shear modulus, the mean-square fluctuation of the local elasticity, and its correlation length. They enable a physical classification scheme for disorder in modern, industrially relevant glass materials. We apply our procedure to a broad range of real-world glass compositions, including metallic, oxide, chalcogenide, hybrid, and polymer glasses. Universal relationships between the descriptors on the one side, and the height and frequency position of the boson peak, the Poisson ratio and the liquid fragility index on the other side are established.

Spin-orbit torque (SOT) created by a spin current injected into a ferromagnet by an adjacent heavy metal or topological insulator represents an efficient tool for the excitation and manipulation of spin waves. Here we report the micromagnetic simulations describing the influence of SOT on the propagation of spin waves in the $\mathrm{W}/\mathrm{CoFeB}/\mathrm{MgO}$ nanostructure having voltage-controlled magnetic anisotropy (VCMA). The simulations show that two spin waves traveling in the opposite directions can be generated in the center of the $\mathrm{CoFeB}$ waveguide via the modulation of VCMA induced by a microwave voltage locally applied to the $\mathrm{MgO}$ nanolayer. The amplitudes of these waves exponentially decrease with the propagation distance with similar decay lengths of about $2.5\mu \mathrm{m}$. In the presence of a direct electric current injected into the $\mathrm{W}$ film beneath the waveguide center, the decay lengths of two spin waves change in the opposite way owing to different directions of the electric currents flowing in the underlying halves of the $\mathrm{W}$ layer. Remarkably, above the critical current density $J_{\mathrm{W}}\approx 2\times {10}^{10}\mathrm{A}{\mathrm{m}}^{-2}$, SOT provides the amplification of the spin wave propagating in one half of the waveguide and strongly accelerates the attenuation of the wave traveling in the other half. As a result, a long-distance spin-wave propagation takes place in half of the $\mathrm{CoFeB}$ waveguide only. Furthermore, by reversing the polarity of the dc voltage applied to the heavy-metal layer one can change the propagation region and switch the travel direction of the spin wave in the ferromagnetic waveguide. Thus, the $\mathrm{W}/\mathrm{CoFeB}/\mathrm{MgO}$ nanostructure can be employed as an electrically controlled magnonic device converting the electrical input signal into a spin signal, which can be transmitted to one of two outputs of the device.

The dynamics of arbitrary-order quantum correlations in a cavity magnon-polariton system are investigated based on the quantum master equation in the coherent state representation. The phenomena of Rabi-like oscillation and level repulsion of the average cavity-photon number agree remarkably well with existing experimental observations. The competing nature of coherent and incoherent components in these two cases is further revealed by the second-order quantum coherence of the cavity photons and magnons, which can be systematically tuned by the driving microwave and thermal bath. Our results demonstrate the rich higher-order quantum dynamics induced by magnetic light-matter interaction and serve as an indispensable step toward exploring nonclassical states for cavity photons and magnons in quantum cavity magnonics.

Here, we study the effect of the Dzyaloshinskii-Moriya interaction (DMI) on magnetic nuclei expansion in Pt/Co/Ir/Co/Pt perpendicular synthetic ferrimagnets. Magnetization reversal is explored as a response to the DMI, affecting the expansion of the magnetic nuclei. We report a nontrivial inverted dependence of the velocity of the double domain wall on the in-plane (IP) magnetic field. This effect is due to competition of the DMI contributions, resulting in effective fields of different signs in the two Co layers. We report a macroscopic manifestation of the DMI as magnetization reversal accelerated by the applied IP magnetic field. Accelerated magnetic relaxation is provided by the DMI-induced ellipticity of the magnetic nuclei, the spreading of which is accompanied by an enhanced number of nuclei contacts in comparison with round nuclei in the absence of an IP field. The obtained results clarify how the DMI contributes to macroscopic magnetic relaxation in the presence of the interlayer Heisenberg exchange interaction in synthetic ferrimagnets with perpendicular anisotropy.

The quest for Kitaev quantum spin liquids has led to great interest in honeycomb quantum magnets with strong spin-orbit coupling. It has been recently proposed that even Mott insulators with $3d$ transition-metal ions, having nominally weak spin-orbit coupling, can realize such exotic physics. Motivated by this, we study the rhombohedral honeycomb cobaltates ${\mathrm{CoTiO}}_3,$ ${\mathrm{BaCo}}_2{\left({\mathrm{PO}}_4\right)}_2$, and ${\mathrm{BaCo}}_2{\left({\mathrm{AsO}}_4\right)}_2$, using ab initio density-functional theory, which takes into account realistic crystal-field distortions and chemical information, in conjunction with exact diagonalization numerics. We show that these ${\mathrm{Co}}^{2+}$ magnets host $J_{\mathrm{eff}}=1/2$ local moments with highly anisotropic $g$ factors, and we extract their full spin Hamiltonians including longer-range and anisotropic exchange couplings. For ${\mathrm{CoTiO}}_3$, we find a nearest-neighbor easy-plane ferromagnetic $XXZ$ model with additional bond-dependent anisotropies and interlayer exchange, which supports three-dimensional Dirac nodal line magnons. In contrast, for ${\mathrm{BaCo}}_2{\left({\mathrm{PO}}_4\right)}_2$ and ${\mathrm{BaCo}}_2{\left({\mathrm{AsO}}_4\right)}_2$, we find a strongly suppressed interlayer coupling, and significant frustration from additional third-neighbor antiferromagnetic exchange mediated by P or As. Such bond-anisotropic $J_1\text{−}{J}_3$ spin models can support collinear zigzag or coplanar spiral ground states. We discuss their dynamical spin correlations which reveal a gapped Goldstone mode and argue that the effective parameters of the pseudospin-$1/2$ models in these two materials may be strongly renormalized by coupling to a low-energy spin exciton. Our results call for reexamining proposals for realizing Kitaev spin liquids in the honeycomb cobaltates.

We present magnetization measurements on polycrystalline $\beta \text{−}{\mathrm{Li}}_2{\mathrm{IrO}}_3$ under hydrostatic pressures up to 3 GPa and construct the temperature-pressure phase diagram of this material. Our data confirm that magnetic order breaks down in a first-order phase transition at $p_{\mathrm{c}}\approx 1.4$ GPa and additionally reveal a steplike feature—magnetic signature of structural dimerization—that appears at $p_{\mathrm{c}}$ and shifts to higher temperatures upon further compression. Following the structural study by L. S. I. Veiga et al. [Phys. Rev. B 100, 064104 (2019)], we suggest that a partially dimerized phase with a mixture of magnetic and nonmagnetic ${\mathrm{Ir}}^{4+}$ sites develops above $p_{\mathrm{c}}$. This phase is thermodynamically stable between 1.7 and 2.7 GPa according to our ab initio calculations. It confines the magnetic ${\mathrm{Ir}}^{4+}$ sites to weakly coupled tetramers with a singlet ground state and no long-range magnetic order. Our results rule out the formation of a pressure-induced spin-liquid phase in $\beta \text{−}{\mathrm{Li}}_2{\mathrm{IrO}}_3$ and reveal peculiarities of the magnetism collapse transition in a Kitaev material. We also show that a compressive strain imposed by the pressure treatment of $\beta \text{−}{\mathrm{Li}}_2{\mathrm{IrO}}_3$ enhances signatures of the 100 K magnetic anomaly at ambient pressure.

The structural, vibrational and electronic properties of several compositions of amorphous Ge-Se-Te are studied from a combination of x-ray diffraction and density functional-based molecular dynamics. Different structural properties are considered such as structure factors, pair distribution functions, angular distributions, coordination numbers, and neighbor distributions. We compare results with experimental findings and a satisfying agreement is found for the structure functions in real and reciprocal spaces. The short range order is found to be more complex than in related binaries that result in mixed geometries ($\simeq 65\%–75\%$ tetrahedral, and remaining defect octahedral) for a dominant fourfold Ge (80%). The chalcogen atoms are dominantly twofold, the former having furthermore an important fraction of threefold coordinated atoms (30%–40%). The obtained model structures indicate that Ge-Ge, Ge-Se, and Ge-Te bonds dominate with small fractions of Te-Te bonds remaining from the base system GeTe. The investigation of electronic properties indicates that the addition of Se atoms will lead to Te-related bands that are much more localized so that Ge-Te-Se can be regarded as having an increased covalent character with respect to GeTe.

The spin voltages produced by spin accumulation and Hanle spin precession in a lateral spin-transport device with a silicon channel and ferromagnetic tunnel contacts (Fe/MgO) are probed for a wide range of magnetic fields. Signal analysis reveals that for the interpretation of the two-terminal magnetoresistance and the nonlocal spin signals, one needs to consider various superimposed contributions, namely, (i) spin signals arising from spin transport of mobile carriers through the Si channel from one ferromagnetic contact to the other, thus depending on the relative magnetization of the two contacts, (ii) spin signals also arising from the spin accumulation of mobile carriers in the Si channel but generated at each of the ferromagnetic contacts separately, and (iii) spin signals originating from spin accumulation of carriers that are confined at or near the Si/MgO interface of the magnetic tunnel contacts, with rather different spin precession characteristics. Perhaps surprisingly, in the nonlocal spin signal a clear broad Hanle signal from confined electrons is also observed, and argued to be mediated by heat flow from the injector to the nonlocal detector.

Magnetic properties of ${\mathrm{CeZn}}_3{\mathrm{P}}_3$, which has been believed to have the same hexagonal ${\mathrm{ScAl}}_3{\mathrm{C}}_3$-type crystal structure as that of a spin dimer system of ${\mathrm{YbAl}}_3{\mathrm{C}}_3$ at room temperature, have been investigated by magnetization, magnetostriction, specific heat ($C$), and magnetocaloric effect measurements. In this research, we have found that ${\mathrm{CeZn}}_3{\mathrm{P}}_3$ certainly has a slightly deformed crystal structure from the hexagonal one even at room temperature, which is in contrast to the structural phase transition of ${\mathrm{YbAl}}_3{\mathrm{C}}_3$ occuring at around 80 K, although the very slight deformation along the $c$ plane, i.e., slightly deformed triangular lattice, and the formation of the multidomain structure are common to both compounds. ${\mathrm{CeZn}}_3{\mathrm{P}}_3$ is thought to maintain its deformed structure from a high-temperature region above room temperature to an extremely low-temperature region and shows a magnetic order below $T_{\mathrm{N}}=0.8$ K. The analysis of the temperature dependence of the magnetic susceptibility ($\chi$) and $C$ has revealed that a Kramers doublet ground state with an easy-plane type anisotropy on the $c$ plane is well isolated from the excited states by the crystal field splitting energy of more than 300 K. Above $T_{\mathrm{N}}, \chi$ makes a broad peak at around 2 K, which certainly originates from the dimer formation due to the slight deformation of the triangular lattice. On the other hand, below $T_{\mathrm{N}}$, a magnetic phase diagram reminiscent of a magnetic flower blooming on the $c$ plane was observed, which may have a close relation to a quantum effect of a quasi $S=\frac{1}{2}$ spin system and a contribution of the orbital component. With increasing magnetic field, we have found an anomalous magnetic state beyond the usual magnetically ordered state, where $C/T$ is anomalously enhanced. This anomalous magnetic state is similar to that observed in ${\mathrm{YbAl}}_3{\mathrm{C}}_3$ induced by the field, although the magnetic ground state in ${\mathrm{YbAl}}_3{\mathrm{C}}_3$ is a nonmagnetic dimer state different from the normal magnetically ordered state in ${\mathrm{CeZn}}_3{\mathrm{P}}_3$.

A central question in a large class of strongly correlated electron systems, including heavy fermion compounds and iron pnictides, is the identification of different phases and their origins. It has been shown that the antiferromagnetic (AFM) phase in some heavy fermion compounds is induced by Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between localized moments, and that the competition between this interaction and Kondo effect is responsible for quantum criticality. However, conclusive experimental evidence of the RKKY interaction in pnictides is lacking. Here, using high resolution ${}^{23}\mathrm{Na}$ NMR measurements on lightly Cu-doped metallic single crystals of ${\mathrm{NaFe}}_{1-x}{\mathrm{Cu}}_x\mathrm{As}$ $(x\approx 0.01)$ and numerical simulation, we show direct evidence of the RKKY interaction in this pnictide system. Aided by computer simulation, we identify the ${}^{23}\mathrm{Na}$ NMR satellite resonances with the RKKY oscillations of spin polarization at Fe sites. Our ${}^{23}\mathrm{Na}$ spin-lattice and spin-spin relaxation data exhibit a signature of an itinerant and inhomogeneous AFM phase in this system, accompanied by a simulation of Cu-induced perturbation to the ordered moments on the Fe sites. Our NMR results indicate coexistence of local and itinerant magnetism in lightly Cu-doped ${\mathrm{NaFe}}_{1-x}{\mathrm{Cu}}_x\mathrm{As}$.

We theoretically analyze collective modes in unconventional superconductors focusing on the Bardasis-Schrieffer (BS) mode and its contribution to the third-harmonic generation currents. Starting from a model with competing superconducting pairing instabilities we add fluctuations of the fields beyond the saddle point approximation and calculate their response to a driving electric field. To model phase fluctuations appropriately we take into account the effect of the long-range Coulomb interaction. While the phase mode is pushed into a plasmon frequency, as known from the literature, we show that the BS mode remains unaffected. Furthermore, it has a characteristic polarization dependence and, unlike the Higgs mode, generates a current in the perpendicular direction to the applied field. We find that the Bardasis-Schrieffer excitations contribute a sizable signal to the third-harmonic generated current, which is clearly distinguishable from the charge density fluctuations due to Cooper-pair-breaking effects and can be straightforwardly detected in experiment.

We investigate collective spin excitations in graphene with proximity-induced spin-orbit coupling (SOC) of the Rashba and valley-Zeeman types, as it is the case, e.g., for graphene on transition-metal-dichalcogenide substrates. It is shown that, even in the absence of an external magnetic field, such a system supports collective spin modes, which correspond to coupled oscillations of the uniform and valley-staggered magnetizations. These modes can be detected via both zero-field electron spin resonance (ESR) and zero-field electric-dipole spin resonance (EDSR), with EDSR response arising solely due to Rashba SOC. We analyze the effect of electron-electron interaction within the Fermi-liquid kinetic equation and show that the interaction splits both the ESR and EDSR peaks into two. The magnitude of splitting and the relative weights of the resonances can be used to extract the spin-orbit coupling constants and many-body interaction parameters that may not be accessible by other methods.

We examine the thermal properties of the spin-1 Heisenberg antiferromagnet on the honeycomb lattice in the presence of an easy-plane single-ion anisotropy as well as the effects of an additional weak in-plane easy-axis anisotropy. In particular, using large-scale quantum Monte Carlo simulations, we analyze the scaling of the correlation length near the thermal phase transition into the ordered phase. This allows us to quantify the temperature regime above the critical point in which—in spite of the additional in-plane easy-axis anisotropy—characteristic easy-plane physics, such as near a Berezinskii-Kosterlitz-Thouless transition, can still be accessed. Our theoretical analysis is motivated by recent neutron scattering studies of the spin-1 compound $\mathrm{Ba}{\mathrm{Ni}}_2{\mathrm{V}}_2{\mathrm{O}}_8$ in particular, and it addresses basic quantum spin models for generic spin-1 systems with weak anisotropies, which we probe over the full range of experimentally relevant correlation length scales.

Dirac semimetal is an ideal parent state to realize various exotic states of matter, such as quantum spin Hall state, Weyl semimetal phase, and Majorana zero modes. Topological phase transition allows for the switching between these different topological states. Here, in this paper, we exhibit experimentally an effective approach of inducing topological phase transition in ${\mathrm{Cd}}_3{\mathrm{As}}_2$ nanoribbons by applying a bending strain profile onto the sample. The local strain varies linearly from compression to tension through the cross-section of a bent nanoribbon. The strain gradient causes obvious lattice deformation and breaks the $C_4$ rotational symmetry, thus opening an energy gap at the Dirac points and making the bulk gapful. When further increasing the strain strength, the local strain effect dominates over the symmetry-breaking effect, where spatially-varying band shift becomes prominent across the nanoribbon. Our results demonstrate the effect of strain gradient on the evolution of energy band structures, which should be valuable for further study of strain-mediated topological phase transition.

Motivated by recent experiments on ${\mathrm{AV}}_3{\mathrm{Sb}}_5$ (A = K, Rb, Cs), the chiral flux phase has been proposed to explain time-reversal symmetry breaking. To fully understand the physics behind the chiral flux phase, we construct a low-energy effective theory based on the van Hove points around the Fermi surface. The possible symmetry-breaking states and their classifications of the low-energy effective theory are completely studied, especially the flux phases on the kagome lattice. In addition, we discuss the relations between the low-energy symmetry breaking orders, the chiral flux, and charge bond orders. We find all possible 183 flux phases on the kagome lattice within a 2*2 unit cell by brute-force approach and classify them by point-group symmetry. Among the 183 phases, we find 3 classes in a 1*1 unit cell, 8 classes in a 1*2 unit cell, and 18 classes in a 2*2 unit cell, respectively. These results provide a full picture of the time-reversal symmetry breaking in kagome lattices.