In this work, single crystals of ${\mathrm{LaMn}}_{0.91}{\mathrm{Sb}}_2$ and ${\mathrm{NdMn}}_{0.88}{\mathrm{Sb}}_2$ with deficiencies of Mn have been successively grown by using the Bi-flux method. They all crystallize in the ${\mathrm{HfCuSi}}_2$-type tetragonal structure with space group $P4$/nmm (No.129). ${\mathrm{LaMn}}_{0.91}{\mathrm{Sb}}_2$ displays signs of successive antiferromagnetic transitions around 133 and 108 K for $H\left|\right|c$. In the case of $H\perp c$, ${\mathrm{LaMn}}_{0.91}{\mathrm{Sb}}_2$ shows signs of weak ferromagnetic transition around 138 K and antiferromagnetic transition at 56 K. However, there is no anomaly observed in the temperature-dependent specific heat curve. For ${\mathrm{NdMn}}_{0.88}{\mathrm{Sb}}_2$ measured with $H\left|\right|c$, the Mn sublattice orders antiferromagnetically below 170 K and the Nd sublattice shows successive antiferromagnetic orders at 34 and 26 K. The antiferromagnetic state of Nd sublattice can be induced to be a ferromagnetic state through a spin-flop transition at 2 K. When $H\perp \mathrm{c}$, the Mn sublattice displays antiferromagnetic orders at 171, 156, and 74 K. The Nd sublattice orders antiferromagnetically below 32 K. Ferromagnetic state for the Nd sublattice is induced by magnetic fields through a spin-flop transition at 2 K. Magnetic phase diagrams for ${\mathrm{NdMn}}_{0.88}{\mathrm{Sb}}_2$ are depicted for cases of $H\left|\right|c$ and $H\perp c$, respectively. ${\mathrm{LaMn}}_{0.91}{\mathrm{Sb}}_2$ and ${\mathrm{NdMn}}_{0.88}{\mathrm{Sb}}_2$ show metallic behavior. Negative magnetoresistance in ${\mathrm{LaMn}}_{0.91}{\mathrm{Sb}}_2$ and ${\mathrm{NdMn}}_{0.88}{\mathrm{Sb}}_2$ are possibly related to canted-antiferromagnetic orders of Mn sublattices.

We consider the interface between a Bardeen-Cooper-Schrieffer superconductor and nonsuperconducting band insulator. We show that under certain conditions, such interfaces can have an elevated superconducting critical temperature, without increasing the strength of the pairing interaction at the interface. We identify the regimes where the interface critical temperature exceeds the critical temperature associated with a superconductor-vacuum interface.

First-principles density functional theory calculations are performed to understand the electronic structure and interaction parameters for recently discovered superconducting kagome metal ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$. A systematic analysis of the tight-binding parameters based on the maximally localized Wannier function method demonstrates that the out-of-plane ${\mathrm{Sb}}^{\mathrm{out}}\text{−}p$ orbital is a key element in complete description of the three Van Hove singularity structures known in this material at the $M$ point near the Fermi level. Further, the correlation strengths are also largely determined by ${\mathrm{Sb}}^{\mathrm{out}}\text{−}p$ states. Based on the constrained random phase approximation, we find that the on-site and intersite interaction parameter are both significantly affected by the screening effect of ${\mathrm{Sb}}^{\mathrm{out}}\text{−}p$ orbitals. As the role of this previously unnoticed orbital state can be tuned or controlled by out-of-plane lattice parameters, we examine the electronic structure and particularly the evolution of Van Hove singularity points as a function of strain and pressure, which can serve as useful knobs to control the material properties.

The $p\text{−}d$ hybridization of transition metal impurities in a semiconductor host is the mechanism that couples valence-band electrons and localized spins. We use scanning tunneling microscopy and spectroscopy combined with density functional theory to probe at the atomic-scale hybridization of Cr single impurities with a GaAs host. Combining spatial density of states mapping and in-gap state spectroscopy of the Cr substituted at the surface of the semiconductor, we give a detailed picture of the spatial extension and the electronic structure of the strongly anisotropic wave function of Cr on GaAs(110). First-principles calculations allow us to identify the electronic character and origin of each state and show that the main resonance peaks and the wave function with “drop-eye” lobes experimentally observed for $3d$ metal impurities in III-V semiconductors are direct local evidences of $p\text{−}d$ hybridization.

In this paper a covalent assembly of two two-dimensional (2D) materials is investigated. It is shown that borophene with periodic perforation (as one of numerous 2D boron nanoallotropes) and graphenylene can form the vertical heterostructure. This borophene-graphenylene bilayer reduces the dynamic instability inherent in certain types of 2D boron. A detailed study of the nature of the interatomic bonds shows that covalent forces and van der Waals forces act alternately between a pair of neighboring boron and carbon atoms. The electronic and mechanical properties of the predicted heterostructure are comprehensively investigated: in the equilibrium condition, under strain, and with passivation by atomic hydrogen. Several calculation schemes are used to increase the accuracy of the electronic structure calculations. It is shown that the tension and the passivation of borophene-graphenylene from the carbon layer by hydrogen can strongly affect the band gap. The mechanical properties of this heterostructure correspond to the properties of one of its materials, graphenylene. The use of this material to stabilize 2D boron nanoallotropes, as well as in flexural nanoelectronics, is promising.

The discovery of ferroelectric behavior in twisted bilayers without inversion symmetry has prompted the consideration of some moiré heterostructures as polar materials. However, misconceptions about the nature and origin of the observed ferroelectricity indicate that a better theoretical understanding of the polar properties of moiré heterostructures is needed. In this paper, it is proposed that all moiré heterostructures in which there is a local breaking of inversion symmetry exhibit an out-of-plane moiré polar domain (MPD) structure, and this is verified using first-principles calculations for several different bilayer systems. In transition-metal dichalcogenide bilayers, a deformation of the charge density on each layer occurs due to the change in stacking arrangements throughout the moiré superlattice, leading to a local out-of-plane dipole moment, with the magnitude and shape of the MPDs being dominated by the chalcogen atoms. While the MPDs in all bilayers considered were found to be sensitive to the moiré period, it is only in the aligned homobilayers that they can be tuned with an out-of-plane electric field. The misconceptions about ferroelectricity in moiré heterostructures are addressed, and it is proposed that the only scenario in which the MPDs can be considered ferroelectric domains is via a global van der Waals sliding in a homobilayer. Finally, a general theoretical discussion of the polar properties of moiré heterostructures without inversion symmetry is provided.

We confirmed the fractional filling of the $4f$ shell of La in ${\mathrm{LaAl}}_2$ by x-ray photoemission spectroscopy (XPS) and ab initio calculations. The La $3d$ core-level XPS spectra were analyzed by use of the Gunnarsson-Schönhammer theoretical model, which allowed us to determine the valency of La smaller than 3 ($v=2.9$). A similar occupation of the La $4f$ shell results from the band structure calculations. In real unstable La atom charge is transferred in the mixing process from its outer $5d6s$ electron shells to the local $4f$ state, according to $4f^{\nu }\left(5d^{1-\nu }6s^2\right)$, where $0<\nu \ll 1$. The calculated density of states fit well the valence band XPS spectra. We have shown that the fractional valence of La plays a key role in electric transport properties of ${\mathrm{LaAl}}_2$ and the fractional filling of the $4f$ shell is really at the root of the resistivity anomalies either at the high or low temperatures.

We report the electronic and local structural investigation of double pervoskites ${\mathrm{Sr}}_{2-x}{\mathrm{La}}_x{\mathrm{CoNbO}}_6$ ($x=0-1$) using x-ray absorption near-edge structure (XANES) and extended x-ray absorption fine structures (EXAFS) at the Nb, Co, and Sr $K$ edges. The ab initio simulations and detailed analysis of the Nb and Co $K$-edge XANES spectra demonstrate that the observed pre-edge features arise from the transition of $1s$ electrons to the $p\text{−}d$ hybridized states. We reveal a $z$-out Jahn-Teller (JT) distortion in the ${\mathrm{CoO}}_6$ octahedra, which decreases monotonically due to an enhancement in the JT inactive ${\mathrm{Co}}^{2+}$ ions with $x$. On the other hand, the $z$-in distortion in ${\mathrm{NbO}}_6$ octahedra remains unaltered up to $x=0.4$ and then decreases with further increase in $x$. This sudden change in the local coordination around Nb atoms is found to be responsible for the evolution of the antiferromagnetic interactions in $x⩾$ 0.6 samples. Also, we establish a correlation between the degree of octahedral distortion and intensity of the white-line feature in the XANES spectra and possible reasons for this are discussed. More interestingly, we observe the signature of ${\mathrm{KN}}_1$ double-electron excitation in the Sr $K$-edge EXAFS spectra for all the samples, which is found to be in good agreement with the $Z+1$ approximation. Further, the Co $L_{2,3}$ edge shows the reduction in the crystal-field strength and hence an increase in the charge-transfer energy (${\mathrm{\Delta }}_{\text{ct}}$) with the La substitution.

Hydrides under ultrahigh pressure, such as ${\mathrm{H}}_3\mathrm{S}$ and ${\mathrm{LaH}}_{10}$, can achieve coveted superconducting critical temperatures via the conventional electron-phonon coupling mechanism. In this work, we report first-principles investigation of high-temperature phonon-mediated superconductivity in a two-dimensional metal hydride, namely, the Janus MoSH monolayer. The MoSH sheet is a recently synthesized intermediate in realizing Janus transition metal dichalcogenides by involving stripping the top-layer S of ${\mathrm{MoS}}_2$ with H atoms [X. Wan et al., ACS Nano 15, 20319 (2021); A. Y. Lu et al., Nat. Nanotechnol. 12, 744 (2017)]. We find coupling of electrons from $\mathrm{Mo}\text{−}d$ orbitals around the Fermi level to ultrahigh frequency H- and S-derived phonons, analogous to superconducting hydrides of ${\mathrm{H}}_3\mathrm{S}$ and ${\mathrm{LaH}}_{10}$. This leads to strong coupling two-gap superconductivity with the calculated critical temperature $T_c$ being about 28.58 K at atmosphere pressure. The presence of soft phonon bands from the Mo in-plane vibrations, in cooperation with the electron susceptibility, accounts for the strong electron-phonon coupling of the MoSH monolayer. By further aligning the Fermi level via doping, the $T_c$ can be boosted to 37.31 K, close to the McMillan limit (39 K). Thus, our work points out a real metal hydride for the realization of two-dimensional high-temperature superconductivity at atmosphere pressure and facilitates further studies on new families of Janus transition-metal sulfhydrates.

Tamm and Shockley states, these two paradigmatic concepts are used to describe surface states not only in electronic systems but also in photonic and phononic crystals. The Re(0001) surface hosts both types of electronic surface states in neighboring but qualitatively different energy gaps. Interestingly, spin-orbit interaction generates a double W-shaped energy vs ${\mathbf{k}}_{\parallel }$ dispersion by mixing both types of states and lifting their spin degeneracy. By combining spin- and angle-resolved photoemission, tight-binding model calculations, as well as density functional theory including the photoemission process, we develop verifiable criteria to distinguish between the two types of surface states and arrive at a consistent picture of the role of spin-orbit interaction in such a scenario.

Unlike isostructural alloys, phase transitions of heterostructural alloys between two different lattice types involve more complexity and are less explored. In this work by analyzing the local environment around each atom, we study the distinct features of phase transitions in three prototypical alloy systems (Cd,Pb)Te, (Cd,Ge)Te and (Zn,Mg)O with different size and chemical mismatches. We show that for the (Zn,Mg)O alloy with small size mismatch and low site preference energy between the Zn and Mg compounds, the phase transition occurs abruptly from the zinc-blende to rock-salt structures, showing the feature of a first-order transition. On the contrary, many distorted local configurations unlike either tetrahedron or octahedron ones in the zinc-blende or rock-salt structures, respectively, exist near the phase transition in (Cd,Ge)Te and (Cd,Pb)Te alloys, which have low formation energies. This is attributed to the large size mismatch between CdTe and PbTe, and stronger preference of Cd occupying the tetrahedral sites, whereas Pb and Ge occupying the octahedral sites in (Cd,Pb)Te and (Cd,Ge)Te alloys. Consequently, the phase transitions in (Cd,Pb)Te and (Cd,Ge)Te alloys happen gradually in a wide composition range, showing the feature of a second-order transition. Our study, therefore, shows that phase transition behaviors of heterostructural alloys can be rather complex, either abrupt or gradual, largely depending on the size mismatch and site preference of the constituents. This provides deep understanding on the phase transition behavior of these types of heterostructural alloys.

We examine the dynamics of an individually driven skyrmion moving through a background lattice of skyrmions coupled to a 2D periodic substrate as we vary the ratio of the number of skyrmions to the number of pinning sites across commensurate and incommensurate conditions. As the skyrmion density increases, the skyrmion Hall angle is nonmonotonic, dropping to low or zero values in commensurate states and rising to an enhanced value in incommensurate states. Under commensuration, the driven skyrmion is channeled by a symmetry direction of the pinning array and exhibits an increased velocity. At fillings for which the skyrmion Hall angle is zero, the velocity has a narrow band noise signature, while for incommensurate fillings, the skyrmion motion is disordered and the velocity noise is broad band. Under commensurate conditions, multi-step depinning transitions appear and the skyrmion Hall angle is zero at low drives but becomes finite at higher drives, while at incommensurate fillings there is only a single depinning transition. As the gyrotropic component of the skyrmion dynamics, called the Magnus force, increases, peaks in the velocity that appear in commensurate regimes cross over to dips, and new types of directional locking effects can arise in which the skyrmion travels along other symmetry directions of the background lattice. At large Magnus forces, and particularly at commensurate fillings, the driven skyrmion can experience a velocity boost in which the skyrmion moves faster than the applied drive due to the alignment of the Magnus-induced velocity with the driving direction. In some cases, an increase of the Magnus force can produce regimes of enhanced pinning when the skyrmion is forced to move along a nonsymmetry direction of the periodic pinning array. This is in contrast to systems with random pinning, where increasing the Magnus force generally reduces the pinning effect. We demonstrate these dynamics for both square and triangular substrates and map out the different regimes as a function of filling fraction, pinning force, and the strength of the Magnus force in a series of dynamic phase diagrams.

In a magnetic multilayer, the spin transfer between localized magnetization dynamics and itinerant conduction spin arises from the interaction between a normal metal and an adjacent ferromagnetic layer. The spin-mixing conductance then governs the spin-transfer torques and spin pumping at the magnetic interface. Theoretical description of spin-mixing conductance at the magnetic interface often employs a single conduction-band model. However, there is orbital hybridization between conduction $s$ electron and localized $d$ electron of the heavy transition metal, in which the single conduction-band model is insufficient to describe the $s-d$ orbital hybridization. In this work, using the generalized Anderson model, we estimate the spin-mixing conductance that arises from the $s-d$ orbital hybridization. We find that the orbital hybridization increases the magnitude of the spin-mixing conductance.

Chainlike structured superconductive materials (such as $A_2{\mathrm{Cr}}_3{\mathrm{As}}_3$, with $A=\mathrm{K},\mathrm{Rb},\mathrm{Cs}$) exhibit the multiband electronic structure of single-particle states, where coexisting quasi-one-dimensional (Q1D) and conventional higher-dimensional energy bands take part in the creation of the aggregate superconducting condensate. When the chemical potential approaches the edge of a Q1D band in a single-band superconductor, the corresponding mean-field critical temperature increases significantly but the superconductivity is quenched by fluctuations. However, recent investigation has revealed that when a Q1D band is coupled to a higher-dimensional one by the interband Cooper-pair transfer, the thermal superconductive fluctuations can be suppressed so that the resulting critical temperature can be close to its mean-field value. In the present work, we calculate the mean-field $T_{c0}$ and fluctuation-shifted $T_c$ critical temperatures for a two-band superconductor where a Q1D band coexists with a higher-dimensional band, and investigate how the thermal fluctuations are sensitive to the system parameters. We find that $T_c$ is close to $T_{c0}$ in a wide range of microscopic parameters, and even the dimensionality of the higher-dimensional band does not play an essential role. Thus, the screening mechanism for suppressing fluctuations via the pair-exchange coupling between the bands is indeed relevant for a large class of Q1D multiband superconducting materials, encouraging further experiments aimed at reaching larger critical temperatures in such multiband superconductors.

Defect-induced magnetism in graphene has been predicted theoretically and observed experimentally. However, there are open questions about the origin of the magnetic behavior when substitutional impurities with $sp$ electrons are considered. The aim of this work is to contribute to the understanding of impurity-induced spin magnetism in doped graphene systems. Thus, the electronic structure and spin magnetic moments for substitutional doped graphene with impurities from groups IIIA (B, Al, and Ga) and VA (N, P, As, Sb, and Bi) of the periodic table were obtained within the framework of density functional theory. The nature of the magnetic ground state was determined from calculations of the total energy as a function of the spin magnetic moment using the fixed spin moment method. We show that the spontaneous magnetization in the studied systems arises from an electronic instability by the presence of a narrow impurity band at the Fermi level. Furthermore, we found that the emergence of spin polarization requires the impurity to introduce an extra electron to the graphene lattice and that the impurity-carbon hybridization is close to the $s{p}^3$ geometry. These features reveal that the charge doping sign and the hybridization degree play a fundamental role in the origin of $sp$ magnetism in substitutional doped graphene.

The spin Hall effect (SHE) and the inverse spin Hall effect (ISHE) have been widely utilized to interconvert the charge and spin currents. In them, the charge current, spin current, and spin polarization are orthogonal to each other, limiting their applications. Based on the general tensor requirement of the spin-charge conversion, additional possibilities can be introduced when the order parameter, the magnetization of a ferromagnet, is involved. We herein report the spin-charge conversion in perpendicular magnetized Co/Pd multilayers with spin pumping measurements. We unambiguously observed the anomalous inverse spin Hall effect (AISHE), where the charge current is collinear with the spin polarization. Both the sign and magnitude of AISHE can be linearly regulated by the out-of-plane magnetization of Co/Pd multilayers. Further, the observed amplitude ratio of AISHE/ISHE in spin pumping measurements is independent of the applied microwave frequency, indicating that it is a material specific parameter.

We uncovered the spin dynamics involved in the birth and growth of giant spin polarons in a magnetic semiconductor. For this purpose, we developed a measurement technique which provides direct access to the spin dynamics, irrespective of phonons and carriers involved in the process. Moreover, we solved the Landau-Lifshitz equation in the specific scenario of spin polarons; this solution fits our data excellently and demonstrates that the spin polaron growth slows down dramatically when the sample is cooled in the paramagnetic phase. Finally, temperature-dependent Monte Carlo simulations were performed; these are in excellent agreement with the observed slowdown, which demonstrates that fluctuations in the Weiss field play a decisive role in spin coherence generation induced by light in magnetic materials.

The classic Bogoliubov theory of weakly interacting Bose gases rests upon the assumption that nearly all the bosons condense into the lowest quantum state at sufficiently low temperatures. Here we develop a generalized version of Bogoliubov theory for the case of a driven-dissipative exciton-polariton condensate with a large incoherent uncondensed component, or excitonic reservoir. We argue that such a reservoir can consist of both excitonic high-momentum polaritons and optically dark superpositions of excitons across different optically active layers, such as multiple quantum wells in a microcavity. In particular, we predict interconversion between the dark and bright (light-coupled) excitonic states that can lead to a dynamical equilibrium between the condensate and reservoir populations. We show that the presence of the reservoir fundamentally modifies both the energy and the amplitudes of the Bogoliubov quasiparticle excitations due to the non-Galilean-invariant nature of polaritons. Our theoretical findings are supported by our experiment, where we directly detect the Bogoliubov excitation branches of an optically trapped polariton condensate in the high-density regime. By analyzing the measured occupations of the excitation branches, we extract the Bogoliubov amplitudes across a range of momenta and show that they agree with our generalized theory.

Transition-metal nitrides (TMNs) are enriched with various exotic phenomena such as superconductivity and quantum magnetism. Vanadium nitride (VN) is a typical superconducting TMN, with its superconductivity sensitively associated with various physical and chemical factors including nitrogen vacancies and lattice constants. Strain tuning is a convenient and effective method to manipulate the superconducting behavior of VN in the thin-film form, providing an additional tuning knob for potential applications in quantum computing and passive microwave devices. Here, we demonstrate the effect of strain tuning on VN films both statically and dynamically. By depositing on various substrates, the static strain states of VN films can be controlled from $-4.04\%$ (compressive) to $+2.0\%$ (tensile), making the superconducting transition temperature $\left(T_c\right)$ tunable by approximately $\pm 10\%$. Applying in situ dynamic compressive strain via a piezoelectric substrate, we are able to achieve control of $T_c$ by a step as fine as 10 mK. Furthermore, our first-principles calculations point out that the strain-tuning effect of $T_c$ originates from a change in the density of states for vanadium's d orbitals.

We derive the nonlinear $\sigma$ model to describe diffusive transport in normal metals and superconductors with intrinsic spin-orbit coupling (SOC). The SOC is described via an SU(2) gauge field, and we expand the model to the fourth order in gradients to find the leading non-Abelian field-strength contribution. This contribution generates the spin-charge coupling that is responsible for the spin-Hall effect. We discuss how its symmetry differs from the leading quasiclassical higher-order gradient terms. We also derive the corresponding Usadel equation describing the diffusive spin-charge dynamics in superconducting systems. As an example, we apply the obtained equations to describe the anomalous supercurrent in dirty Rashba superconductors at arbitrary temperatures.