In nanocrystalline metallic materials, with the decrease of grain size, the relationship between strength and grain size usually changes from the Hall-Petch (HP) to inverse Hall-Petch (IHP) relationship related to grain boundary (GB) characteristics. A series of nanocrystalline Cu samples with pre-specified grain orientations and different grain sizes were established, in which a part of GBs were replaced with coherent twin boundaries (CTBs). The mechanical behaviors of the samples with and without GB modification under tension were studied using molecular dynamics simulations. It was found that the HP and IHP relationships still work in the nanocrystalline Cu samples with GB modification, but the maximum strength is significantly improved, meanwhile the critical grain size for the transition from HP to IHP is reduced to 7.5 nm. The grain size effects on the flow stress were qualitatively analyzed based on the monitoring microstructure evolutions. The model dominated by GB was combined with that by intracrystalline CTB, which was further extended to quantitatively describe the grain size effect. It showed that the modification of GB could substantially delay the IHP effect, which provides a promising way for the design and optimization of the microstructure of high-performance nanocrystalline materials.

The phase transitions among polymorphic two-dimensional (2D) transition metal dichalcogenides (TMDs) have attracted increasing attention for their potential in enabling distinct functionalities in the same material for making integrated devices. Electron injection to TMDs has been proved to be a feasible way to drive structural phase transition from the semiconducting $H$ phase to the semimetal $dT$ phase. In this paper, based on density functional theory calculations, firstly, we demonstrate that hole injection drives the transition of the $H$ phase more efficiently to the metallic $T$ phase than to the semimetallic $dT$ phase for group-VI-B TMDs (${\mathrm{MoS}}_2, {\mathrm{WS}}_2, {\mathrm{MoSe}}_2$, etc.). The origin can be attributed to the smaller work function (WF) of the $T$ phase than that of the $dT$ phase. Our WF analysis can distinguish the $T$ and $dT$ phases quantitatively, while it is challenging for the commonly used crystal field splitting analysis. In addition, our analysis provides a unified understanding for both hole- and electron-injection induced phase transitions for 2D materials beyond TMDs, such as the synthesized ${\mathrm{MoSi}}_2{\mathrm{N}}_4$ family. Moreover, the hole-driven $T$-phase transition mechanism can explain the recent experiment of ${\mathrm{WS}}_2$ phase transition by hole doping with yttrium (Y) atoms. Using $\frac{1}{3}$ Y-doped ${\mathrm{WS}}_2$ and ${\mathrm{MoSe}}_2$ as examples, we show that the Mo and W valency increases to 5+. These above findings open an avenue to obtain the metallic $T$ phase, which expands the possible stable phases of 2D materials.

Algorithms to simulate ring-exchange models on a square lattice using projected entangled pair states (PEPSs) are developed. We generalize the imaginary time evolution (ITE) method to optimize PEPS wave functions for the models with ring-exchange interactions. We compare the effects of different approximations of the environment. To understand the numerical instability during optimization, we introduce the “singularity” of a PEPS and develop a regulation procedure that can effectively reduce the singularity of a PEPS. We benchmark our method with the toric code model and obtain extremely accurate ground-state energies and topological entanglement entropy. We also benchmark our method with the two-dimensional cyclic ring-exchange model, and find that the ground state has a strong vector chiral order. The algorithms can be a powerful tool to investigate the models with ring interactions. The methods developed in this work, e.g., the regularization process to reduce the singularity, can also be applied to other models.

We demonstrate that the single crystal of ${\mathrm{YCu}}_3$[OH(D)${\mathrm{]}}_{6.5}{\mathrm{Br}}_{2.5}$ (YCOB) is a kagome Heisenberg antiferromagnet without evident orphan spins ($\ll 0.8$%). The site mixing between polar ${\mathrm{OH}}^{-}$ and nonpolar ${\mathrm{Br}}^{-}$ causes local distortions of Cu–O–Cu exchange paths and gives rise to 70(2)% of randomly distributed hexagons of alternate bonds ($\sim J_1-\mathrm{\Delta }J$ and $J_1+\mathrm{\Delta }J$) and the rest of the almost-uniform hexagons ($\sim J_1$) on the kagome lattice. Simulations of the random exchange model with $\mathrm{\Delta }J/J_1$ = 0.7(1) show good agreement with experimental observations, including the weak upturn seen in susceptibility and the slight polarization in magnetization. Despite the average antiferromagnetic coupling of $J_1\sim 60$ K, no conventional freezing is observed down to $T\sim 0.001J_1$, and the raw specific heat exhibits a nearly quadratic temperature dependence below 1 K $\sim 0.02J_1$, phenomenologically consistent with a gapless (spin gap $\le 0.025J_1$) Dirac quantum spin liquid (QSL). Our result sheds light on the theoretical understanding of the randomness-relevant gapless QSL behavior in YCOB, as well as in other relevant materials.

Recent developments in high temperature superconducting materials under high pressure have made numerical evaluation of the superconducting transition temperature ($T_c$) of predicted materials critically important as a means to identification. Existing methods of calculating $T_c$ often do not agree with each other or experiment, often due to the large number of complex factors that contribute to this property; among them is the neglect of dispersion interactions in commonly used density functionals. Here, we evaluate the effect of including dispersion interactions on the predicted superconducting properties of two examples of the covalent superhydride class of very high-$T_c$ superconducting materials. In both cases, dispersion is found to have sizable effects, increasing the electron-phonon coupling as compared to the reference case of elemental niobium. A detailed investigation traces the origin of this effect in a 270 GPa $R3m\mathrm{C}\mathrm{S}{\mathrm{H}}_7$ [Cui et al., Phys. Rev. B 101, 134504 (2020)] structure to structural distortions driven by long-range electron-phonon interactions rather than novel bonding networks.

A mechanism of a superconductivity-mediated interaction of two magnets in a system of coupled superconductor-ferromagnet-superconductor (S-F-S) Josephson junctions (JJs) with spin-orbit interaction is proposed. The predicted indirect magnetic interaction favors the antiparallel orientation of the magnets. Its spatial scale is not restricted by the proximity length scales of the superconductor. Our estimates suggest that the interaction strength is not reduced considerably even at the macroscopic scales of the order of millimeters. At larger distances $l$ between the magnets the coupling constant exhibits the long-range power-law $1/l$ behavior. The mechanism of the interaction is based on two key ingredients: (i) the anomalous ground-state phase shift in the S-F-S JJ provides a magnetoelectric coupling between the condensate phase and the magnetization, and (ii) the interaction is mediated by the condensate phase of the superconducting region connecting both JJs. In addition, we demonstrate high tunability of the total magnetic configuration of the system by the externally controlled superconducting phase between the leads.

We propose a spatially and temporally nonlocal exchange correlation (XC) kernel for the spin-unpolarized fluid phase of ground-state jellium for use in time-dependent density functional and linear response calculations. The kernel is constructed to satisfy known properties of the exact XC kernel to accurately describe the correlation energies of bulk jellium and to satisfy frequency-moment sum rules at a wide range of bulk jellium densities, including those low densities that display strong correlation and symmetry breaking. These effects are easier to understand in the simple jellium model than in real systems. All exact constraints satisfied by the recent MCP07 kernel [A. Ruzsinszky et al., Phys. Rev. B 101, 245135 (2020)] are maintained in the revised MCP07 (rMCP07) kernel, while others are added. The revision $f_{\mathrm{XC}}^{\text{rMCP07}}(q,\omega )$ differs from MCP07 only for nonzero frequencies $\omega$. Only at densities much lower than those of real bulk metals is the frequency dependence of the kernel important for the correlation energy of jellium. As the wave vector $q$ tends to zero, the kernel has a $-4\pi \alpha \left(\omega \right)/q^2$ divergence whose frequency-dependent ultranonlocality coefficient $\alpha \left(\omega \right)$ vanishes in jellium, and is predicted by rMCP07 to be extremely small for the real metals Al and Na.

${\mathrm{Co}}_2$-based Heusler compounds are promising materials for spintronics applications due to their high Curie temperature, large spin polarization, large magnetization density, and exotic transport properties. In the present paper, we report the anomalous Hall effect (AHE) in a polycrystalline ${\mathrm{Co}}_2\mathrm{FeAl}$ Heusler compound using combined experimental and theoretical studies. The Rietveld analysis of high-resolution synchrotron x-ray diffraction data reveals a large degree $(\sim 50\%)$ of antisite disorder between Fe and Al atoms. The analysis of anomalous transport data provides the experimental anomalous Hall conductivity (AHC) about 227 S/cm at 2 K with an intrinsic contribution of 155 S/cm, which has nearly constant variation with temperature. The detailed scaling analysis of anomalous Hall resistivity suggests that the AHE in ${\mathrm{Co}}_2\mathrm{FeAl}$ is governed by the Berry phase driven intrinsic mechanism. Our theoretical calculations reveal that the disorder present in the ${\mathrm{Co}}_2\mathrm{FeAl}$ compound enhances the Berry curvature induced intrinsic AHC.

We perform a theoretical study of the electronic structure and magnetic properties of the prototypical magnetic MAX-phase ${\mathrm{Mn}}_2\mathrm{GaC}$ with the main focus given to the origin of magnetic interactions in this system. Using the density functional $\text{theory}+\text{dynamical}$ mean-field theory ($\text{DFT}+\text{DMFT}$) method, we explore the effects of electron-electron interactions and magnetic correlations on the electronic properties, magnetic state, and spectral weight coherence of paramagnetic and magnetically ordered phases of ${\mathrm{Mn}}_2\mathrm{GaC}$. We also benchmark the DFT-based disordered local moment approach for this system by comparing the obtained electronic and magnetic properties with that of the $\text{DFT}+\text{DMFT}$ method. Our results reveal a complex magnetic behavior characterized by a near degeneracy of the ferro- and antiferromagnetic configurations of ${\mathrm{Mn}}_2\mathrm{GaC}$, implying a high sensitivity of its magnetic state to fine details of the crystal structure and unit-cell volume, consistent with experimental observations. We observe robust local-moment behavior and orbital-selective incoherence of the spectral properties of ${\mathrm{Mn}}_2\mathrm{GaC}$, implying the importance of orbital-dependent localization of the Mn $3d$ states. We find that ${\mathrm{Mn}}_2\mathrm{GaC}$ can be described in terms of local magnetic moments, which may be modeled by DFT with disordered local moments. However, the magnetic properties are dictated by the proximity to the regime of formation of local magnetic moments, in which the localization is in fact driven by Hund's exchange interaction, and not the Coulomb interaction.

The thermoelectric transport properties of a three-dimensional multi-Weyl semimetal in the quantum limit are studied. In the calculation, we consider the different effects of the Gaussian and screened Coulomb electron-impurity scatterings. When the magnetic field is parallel to the electric field or the temperature gradient, the magnetoresistivity is always negative, but the thermopower strongly depends on the scattering potentials and the monopole charge. In the perpendicular field configuration, the situation becomes complicated. For the low electron density case, the increasing magnetic field makes the longitudinal conductivity comparable to the Hall one, which breaks the continuous growth of the Seebeck and Nernst responses, especially for the large monopole charge one. The thermoelectric Hall conductivity still indicates a plateau feature, but its value is multiplied by the monopole charge. Whether the longitudinal thermoelectric conductivity increases with the field depends on the potential type. This may be used to unveil the scattering mechanism in multi-Weyl semimetals.

We study, both experimentally and theoretically, the second-harmonic (SH) generation (SHG) of optical radiation assisted by intense terahertz electromagnetic pulses from the surface of high-resistivity Si(111). The study was performed within the framework of the analysis of the superposition of different SH sources, including the surface-dipole, bulk-quadruple, and electric-field-induced (EFI) bulk-dipole (arising from internal built-in and external terahertz fields) contributions. Azimuthal rotation anisotropy of SHG for an arbitrary mutual polarization of the terahertz, fundamental, and SH optical fields was calculated and experimentally verified. The strongest impact of the terahertz field on SHG was observed in the case where the polarizations of the terahertz and SH fields coincide. In these polarization configurations, the terahertz field added mainly an isotropic term to the SH azimuthal dependences. It has been demonstrated that the application of the terahertz field can be helpful for distinguishing the SHG mechanism. For the sample under study, the contribution to the SH energy from the built-in field-induced source was found to be >4 times greater than the contribution from the surface and quadrupole nonlinear sources. The dominance of the EFISHG allowed us to find the ratio between the components of a silicon third-order nonlinear tensor ${\chi }_{}^{\left(3\right)}(2\omega ;\omega ,\omega ,0)$: ${\chi }_{1122}={\chi }_{1212}\approx 0.51{\chi }_{1111}$ and ${\chi }_{1212}\approx {\chi }_{1221}$ for the fundamental optical wavelength 790 nm.

Realizing tunable and stable magnetic states in two-dimensional (2D) materials has been an attractive issue in current materials and physics fields. To date, however, the reported magnetic 2D materials mostly have low Curie transition temperature. In this paper, we theoretically propose a class of Janus group-III monochalcogenide $\mathrm{O}{M}_2\mathrm{S}(M=\mathrm{Ga},\mathrm{In},\text{and}\mathrm{Tl})$ monolayers. Among them, ${\mathrm{OTl}}_2\mathrm{S}$ possesses the sombrerolike valence band edge (SVBE) and a Van Hove singularity of the density of states, indicating a possible ferromagnetic transition upon hole doping based on the Stoner criteria. In contrast, the SVBE can be formed in ${\mathrm{OGa}}_2\mathrm{S}$ and ${\mathrm{OIn}}_2\mathrm{S}$ via tensile strain. Also, the SVBE is responsible for the intraband Lifshitz transition in these monolayers. Ferromagnetism and half-metallicity can be introduced in Janus $\mathrm{O}{M}_2\mathrm{S}$ monolayers by injecting hole carriers. At the mean field level, the highest Curie transition temperature $T_{\mathrm{C}}$ is estimated to be 627.5 K in freestanding ${\mathrm{OGa}}_2\mathrm{S}$, which is higher than those of GaSe and $M\mathrm{O}(M=\mathrm{Ga}\text{and}\mathrm{In})$ monolayers. The origin of the higher $T_{\mathrm{C}}$ is attributed to the unique sombrerolike band dispersion. We further show that the magnetic properties of ${\mathrm{OGa}}_2\mathrm{S}$ are sensitive to doped hole density and applied strain, implying a potential tunability in experiment. Finally, the feasibility of introducing the magnetic states through $p$-type dopants and monovacancy defect is explored. In this paper, we not only provide a class of nonmagnetic 2D materials for investigating the high-temperature ferromagnetism and half-metallicity but also widen the application potentials of group-III monochalcogenides.

We analyze the charge and thermal transport at a junction of interacting quantum wires close to equilibrium. Within the framework of Tomonaga-Luttinger liquids, we compute the thermal conductance for a wide class of boundary conditions and detail the physical processes leading to the breakdown of the Wiedemann-Franz law at the junction. We show how connecting external reservoirs to the quantum wires affects the conductance tensors close to the various fixed points of the phase diagram of the junction. We therefore distinguish two types of violation of the Wiedemann-Franz law: a “trivial” one, independent of the junction dynamics and arising from the breakdown of the Fermi-liquid picture in the wire, and a junction-related counterpart, arising from multiparticle scattering processes at the junction.

Structural relaxation in slightly misaligned bilayer graphene leads to the formation of domains separated by topological solitons. The influence of such a soliton lattice on phonons is investigated in the framework of the Frenkel-Kontorova model, where a solution is derived in the continuum limit. A nearly free phonon model is developed to explain the characteristic features in the phonon band structure derived from the Frenkel-Kontorova model. Its 2D extension is discussed and applied on the specific case of twisted bilayer graphene. This model explains a number of computational results reported until now and can be used directly to investigate other misaligned two-dimensional networks as found in van der Waals heterostructures.

We theoretically consider two-dimensional moiré transition metal dichalcogenide (TMD) bilayers, which are strongly correlated in the sense that the on-site Coulomb interaction is comparable to or larger than the hopping kinetic energy between the moiré lattice sites. The system accommodates many symmetry-broken ground states both in charge and isospin sectors at various commensurate rational fillings such as 1/2, 1/3, 1/4, 2/3, etc. We investigate two complementary important aspects of the dependence of the symmetry breaking on (1) the range of the electron-electron interaction, which can in principle be experimentally controlled by the nearby gates and the dielectric environment, and (2) temperature, which could thermally suppress the symmetry breaking above a critical temperature. Experimental implications of the theory are discussed.

Green's function in non-Hermitian systems has recently been revealed to be capable of directional amplification in some cases. The exact formulas for end-to-end Green's functions are significantly important for studies of both non-Hermitian systems and their applications. In this work, based on the Widom's formula, we derive exact formulas for the end-to-end Green's functions of single-band systems which depend on the roots of a simple algebraic equation. These exact formulas allow direct and accurate comparisons between theoretical results and experimentally measured quantities. In addition, we verify the prior established integral formula in the bulk region to agree with the result in our framework. We also find that the speed at which the Green's functions in the bulk region approach the prior established integral formula is not slower than an exponential decay as the system size increases. The correspondence between the signal amplification and the non-Hermitian skin effect is confirmed.

The Sb square net in ${\mathrm{BaMnSb}}_2$ is reported to host nearly massless Dirac fermions. At ambient pressure, ${\mathrm{BaMnSb}}_2$ shows strong Shubnikov–de Haas (SdH) oscillations with a $\pi$ Berry phase. In this work, we study ${\mathrm{BaMnSb}}_2$ under different pressures. It is found that the frequency of the SdH oscillation remains almost unchanged, hence the cross-section area of the Fermi surface does not change much with pressure. Notably, there is a sudden change in the phase factor of the oscillations around a critical pressure of $P=$ 2.5 GPa, which suggests a possible topological phase transition in ${\mathrm{BaMnSb}}_2$.

Gain is a blessing and a curse for photonic designs; indeed, gain-based devices may become unprecedentedly efficient but, on the other hand, the local electromagnetic fields can increase exponentially with time and lead the systems to instability. Such stability considerations are presented for a homogenized configuration incorporating tilted active multilayers by determining the poles of its transfer functions across the complex frequency plane and analytically deriving the respective constraints. The reported stable regimes will be crucial in tuning the examined module when incorporated in photonic setups for serving multiple purposes from filtering and imaging to cloaking and lasing.

Band inversion accompanied by the emergence of gapless and helical edge states at the boundary is a hallmark of 2D topological insulators. However, a recent experiment reports the existence of edge states before band inversion in InAs/GaSb quantum wells [F. Nichele et al., New J. Phys. 18, 083005 (2016)]. The underlying physics remains an open problem. Here we provide a possible solution by showing that helical edge states emerge before band inversion as long as the bulk gap falls below a positive threshold determined by the particle-hole asymmetry and interband coupling. In the presence of quantum confinement or local electrostatic potential near the boundary, these edge states may enter the gap of bulk states and be detected as edge conductance in transport measurements. Our work reveals the possible existence of the helical conducting edge states in a large class of narrow-gap semiconductors.

Biased bilayer graphene, with its easily tunable band gap, presents itself as the ideal system to explore the excitonic effect in graphene-based systems. In this paper we study the excitonic optical response of such a system by combining a tight-binding model with the solution of the Bethe-Salpeter equation, the latter being solved in a semianalytical manner, requiring a single numerical quadrature, thus allowing for a transparent calculation. With our approach we start by analytically obtaining the optical selection rules, followed by the computation of the absorption spectrum for the case of a biased bilayer encapsulated in hexagonal boron nitride, a system which has been the subject of a recent experimental study. Excellent agreement is seen when we compare our theoretical prediction with the experimental data.