We propose a static auxiliary field approximation to study the hybridization physics of Kondo systems without the sign problem and use the mutual information to measure the intersite hybridization correlations. Our method takes full account of the spatial fluctuations of the hybridization fields at all orders and overcomes the artificial (first-order) phase transition predicted in the mean-field approximation. When applied to the two-impurity Kondo model, it reveals a logarithmically divergent amplitude mutual information near the so-called “Varma-Jones” fixed point and a large phase mutual information manifesting the development of intersite phase coherence in the Kondo regime, with observable influences on physical properties. These highlight the importance of hybridization fluctuations and confirm the mutual information as a useful tool to explore the hybridization physics in Kondo systems.

We present a thermodynamic model which explains the presence of a negative slope in the melt curve, as observed in systems as diverse as the alkali metals and molecular hydrogen at high pressure. We assume that components of the system can be in one of two well defined states—one associated with low energy, the other with low volume. The model exhibits a number of measurable features which are also observed in these systems and are therefore expected to be associated with all negative Clapeyron-slope systems: first order phase transitions and thermodynamic anomalies along Widom lines. The melt curve maximum is a feature of the model, but appears well below the pressures where the change in state occurs in the solid: the solid-solid transition is related to the melt line minimum. An example of the model fitted to the electride transition in potassium is discussed.

Finite size effects on the ferroic orders in $\mathrm{BiFe}{\mathrm{O}}_3$ are studied by atomic pair distribution function analysis and magnetic measurements. While bulk rhombohedral $\mathrm{BiFe}{\mathrm{O}}_3$ exhibits ferroelectricity and antiferromagnetism with a cycloidal magnetic moment arrangement leading to zero magnetization and weak magnetoelectric coupling, $\mathrm{BiFe}{\mathrm{O}}_3$ nanoparticles with a size smaller than the spin cycloid period of 62 nm preserve their polar rhombohedral structure and develop ferromagnetism, thus exhibiting coexisting polarization and nonzero magnetization that enhances the magnetoelectric coupling. When the nanoparticles become smaller than 17 nm, however, their crystal lattice expands and becomes nonpolar cubic. They also become superparamagnetic and thus simultaneously cease exhibiting both ferroelectricity and ferromagnetism. Our findings shed light on the interaction between the lattice structure and ferroic orders in nanophase perovskites and also provide a rare example of a lattice symmetry breaking phase transition that determines their critical size.

Previous studies on elastic properties of two-dimensional (2D) metals have focused on thin slabs with a clean surface. However, metal surfaces are full of binding sites for adsorption, which makes them interesting materials in catalysis and sensing. In this contribution, we study Young's moduli of ultrathin oxygen-adsorbed 2D Pt slabs with a (111) surface by density functional theory methods. We show that upon oxygen adsorption, 2D Pt slabs experience significant lattice expansion due to adsorbate-induced additional surface stress. Young's moduli of the studied 2D slabs typically decrease when surface oxygen coverage increases. Moreover, thinner slabs in general exhibit higher stiffness. We show that the nonlinear elastic properties of both the core and surface regions of the considered 2D structures underlie the overall effective elastic properties.

We have investigated the dysprosium distribution and its magnetic moment orientation at the region near the surface of ${\mathrm{DyCo}}_{4.4}$ and ${\mathrm{DyCo}}_{4.6}$ ferrimagnetic amorphous films with perpendicular magnetic anisotropy. X-ray magnetic circular dichroism spectroscopy of the films at the Dy $M_{4,5}$ and Co $L_{2,3}$ edges using total electron yield (TEY) detection was performed at 2 K and 300 K temperatures, and at sample orientations ranged from $0^{\circ }$ to ${70}^{\circ }$ with respect to the normal to the sample. The measurements showed an apparent partial decoupling between the cobalt and dysprosium magnetic sublattices. At RT, the magnetic moment per atom of dysprosium was below the minimum value expected if all dysprosium moments were Antiferromagnetic (AF) coupled to cobalt. At 2 K, the cobalt sublattice presented a surprisingly stronger magnetic anisotropy than the dysprosium sublattice. A detailed analysis of the circularly polarized spectra of the Dy $M_5$ edge, based on the deconvolution of the spectra in their related parallel, antiparallel, and transverse to $J_z$ spectral components, demonstrates that the spectra are composed by dysprosium with different magnetic moment distributions. The fit of the Dy $M_5$ spectra using the $J_z$ spectral components evidenced a gradation of dysprosium concentration due to segregation at the region probed by TEY. The topmost layer was magnetically uncoupled from cobalt. At RT, $25\%$ of the dysprosium magnetic moments in the underlayer were found averaged oriented in the same direction of cobalt. The expected weak magnetic coupling of these dysprosium atoms to cobalt should explain the surprisingly lower magnetic anisotropy of the dysprosium sublattice compared to that of cobalt probed by TEY at 2 K.

We present an experimental study of the magnetic structure and dynamics of two frustrated hyperkagome compounds, ${\mathrm{Gd}}_3{\mathrm{Ga}}_5{\mathrm{O}}_{12}$ and ${\mathrm{Gd}}_3{\mathrm{Al}}_5{\mathrm{O}}_{12}$. It has previously been shown that ${\mathrm{Gd}}_3{\mathrm{Ga}}_5{\mathrm{O}}_{12}$ exhibits long-range correlations of multipolar directors that are formed from antiferromagnetic spins on loops of ten ions. Using neutron diffraction and reverse Monte Carlo simulations we prove the existence of similar magnetic correlations in ${\mathrm{Gd}}_3{\mathrm{Al}}_5{\mathrm{O}}_{12}$, showing the ubiquity of these complex structures in frustrated hyperkagome materials. Using inelastic neutron scattering we shed further light on the director state and the associated low-lying magnetic excitations. In addition, we have measured quasielastic dynamics that show evidence of spin diffusion. Finally, we present AC susceptibility measurements on both ${\mathrm{Gd}}_3{\mathrm{Ga}}_5{\mathrm{O}}_{12}$ and ${\mathrm{Gd}}_3{\mathrm{Al}}_5{\mathrm{O}}_{12}$, revealing a large difference in the low-frequency dynamics between the two otherwise similar compounds.

The effect of scattering of conduction electrons by dynamical spin fluctuations on the thermopower in metals near a thermal phase transition into an antiferromagnetic phase is considered. We are interested in a transition at room temperature, as has been studied in a heterostructure involving layers of IrMn. We show that the electrical resistivity exhibits a narrow but low peak at the transition, which may be difficult to detect on top of the main contributions induced by phonons and impurities. By contrast, the thermopower is found to exhibit a prominent peak both as a function of temperature $T$ for fixed layer thickness $t_{\text{AFM}}$ and as a function of $t_{\text{AFM}}$ for fixed $T.$ We conjecture that the transition temperature $T_c$ is a function of both $t_{\text{AFM}}$ and the Fermi energy ${\epsilon }_F$. Both dependencies give rise to a sharp peak of the thermopower as a function of $T$ or $t_{\text{AFM}}$ near the transition. The estimated magnitude and width of the peak for the case of either three- or two-dimensional longitudinal spin fluctuations is in good agreement with experiment.

We report the study of spontaneous magnetization (i.e., spin polarization) for time-reversal-symmetry-breaking (TRS-breaking) superconductors with unitary pairing potentials, in the absence of external magnetic fields or Zeeman fields. Spin-singlet (${\mathrm{\Delta }}_s$) and spin-triplet (${\mathrm{\Delta }}_t$) pairings can coexist in superconductors whose crystal structures lack inversion symmetry. The TRS can be spontaneously broken once a relative phase of $\pm \pi /2$ is developed, forming a TRS-breaking unitary pairing state (${\mathrm{\Delta }}_s\pm i{\mathrm{\Delta }}_t$). We demonstrate that such unitary pairing could give rise to spontaneous spin polarization with the help of spin-orbit coupling. Our result provides an alternative explanation for the TRS breaking, beyond the current understanding of such phenomena in the noncentrosymmetric superconductors. The experimental results of ${\mathrm{Zr}}_3\mathrm{Ir}$ and CaPtAs are also discussed in view of our theory.

It is widely believed that superconducting junctions involving topological insulators and hosting Majorana-like bound states may exhibit an unusual “fractional” ($4\pi$-periodic) ac Josephson effect. Accordingly, “fractional” Shapiro steps on the current-voltage characteristics of such junctions are expected to occur under external microwave radiation. Here, we microscopically evaluate Shapiro steps in topologically trivial highly transparent superconducting weak links. The key features recovered within our analysis—including, e.g., the so-called “missing” Shapiro steps—turn out to be similar to those observed in topological Josephson junctions. Our results demonstrate that caution is needed while interpreting experimental results for superconducting weak links in terms of Majorana physics.

Temperature determination in shock-compressed solids constitutes an important and long-standing scientific need. Since the reduction of Bragg diffracted peaks due to temperature increase, using the Debye-Waller factor, is well established, we examined the use of this approach to determine temperatures in shock-compressed gold and platinum by representing the shocked state as a superposition of density and temperature changes. Comparison of the calculated and measured diffraction peaks did not show good agreement, because x-ray diffraction (XRD) profiles in the shock-compressed state are not governed solely by density and temperature changes. XRD results are also influenced measurably by shock wave induced microstructural changes. Our results demonstrate that contributions from microstructural changes need to be incorporated and modeled in the theoretical analysis to use XRD measurements for reliable temperature determination in shock-compressed solids.

The representative cuprate ${\mathrm{La}}_{2-x}{M}_x{\mathrm{CuO}}_4$ with $M=\mathrm{Sr}$ and $x=1/8$ is studied via first-principles calculations in the high-temperature tetragonal (HTT), low-temperature orthorhombic (LTO), and low-temperature less-orthorhombic (LTLO) structures. By suppressing the magnetism and superconductivity, the LTLO phase, which has rarely been observed in ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$, is found to be the ground state where the structural phase transitions $\mathrm{HTT}\to \mathrm{LTO}\to \mathrm{LTLO}$ can be understood via phonon instability. Although the La-O composition is identified to be responsible for the phonon softening, the superconducting ${\mathrm{CuO}}_2$ layer is dynamically stable. The LTLO phase, which can exhibit an $\sim 20\text{−}\mathrm{meV}$ splitting in the density of states, is proposed to have an intimate relationship with the observed pseudogap and the charge-density wave giving the stripe. We argue that at low temperatures, the superconducting LTO ${\mathrm{La}}_{1.875}{\mathrm{Sr}}_{0.125}{\mathrm{CuO}}_4$ competes with the phonon-preferred LTLO phase by spontaneously forming the Cooper pairs, resulting in suppressing the stripe. Therefore, the revealed LTLO phase is indispensable for understanding ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$.

We study a class of inhomogeneous chiral photonic crystals with topologically nontrivial bands and edge modes using the approach of non-coordinate transformation optics. By transforming the chiral photonic crystals into achiral binary photonic crystals, the closed-form solutions to the photonic bands of the chiral photonic crystals are obtained. We show that the topological edge modes of these chiral photonic crystals can be accurately predicted by the Zak phases developed from the topological band theory for achiral binary photonic crystals.

The realization of a ferromagnetism in bilayer $\mathrm{Cr}{\mathrm{I}}_3$, an antiferromagnetic semiconductor in its ground state, recently has gained extensive attention for both fundamental and applied research. Despite much effort devoted to this topic, so far its nonvolatile ferromagnetism has not been achieved. Here, we report the realization of nonvolatile ferromagnetism in bilayer $\mathrm{Cr}{\mathrm{I}}_3$. Using first-principles calculations, we reveal that when interfacing with monolayer MoSeTe, the magnetic ground state of bilayer $\mathrm{Cr}{\mathrm{I}}_3$ transforms from antiferromagnetism to ferromagnetism. Also, depending on the interfaced surface of monolayer MoSeTe, bilayer $\mathrm{Cr}{\mathrm{I}}_3$ can be ferromagnetic semiconducting or half-metallic. We unveil that the obtained ferromagnetic transformation in bilayer $\mathrm{Cr}{\mathrm{I}}_3$ can be attributed to the competition between orbital-dependent interlayer antiferromagnetic super-super-exchange and ferromagnetic super-super-exchange. Our work not only provides a promising way to engineer nonvolatile ferromagnetism in bilayer $\mathrm{Cr}{\mathrm{I}}_3$, but also renders bilayer $\mathrm{Cr}{\mathrm{I}}_3$ an appealing platform for developing two-dimensional spintronics.

We demonstrate that low-lying triplon excitations in a bilayer Heisenberg antiferromagnet provide a promising avenue to realize magnetic analogs of twisted superfluid and supersolid phases that were recently reported for two-component ultracold atomic condensate in an optical lattice. Using a cluster Gutzwiller mean-field theory, we establish that Dzyaloshinskii-Moriya interactions (DMI), that are common in many quantum magnets, stabilize these phases in a magnetic system, in contrast to the pair hopping process that is necessary for ultracold atoms. The critical value of DMI for transition to the twisted superfluid and twisted supersolid phases depends on the strength of the (frustrated) interlayer interactions that can be tuned by applying external pressure on and / or shearing force between the layers. Furthermore, we show that the strength of DMI can be controllably varied by coupling to tailored circularly polarized light. Our results provide crucial guidance for the experimental search of twisted superfluid and supersolid phases of triplons in real quantum magnets.

We consider a planar SIS-type Josephson junction between diffusive superconductors (S) through an insulating tunnel interface (I). We construct fully self-consistent perturbation theory with respect to the interface conductance. As a result, we find correction to the first Josephson harmonic and calculate the second Josephson harmonic. At arbitrary temperatures, we correct previous results for the nonsinusoidal current-phase relation in Josephson tunnel junctions, which were obtained with the help of conjectured form of solution. Our perturbation theory also describes the difference between the phases of the order parameter and of the anomalous Green functions.

We suggest a generalization of the nonlinear $\sigma$ model for diffusive superconducting systems to account for magnetoelectric effects due to spin-orbit scattering. In the leading orders of spin-orbit strength and gradient expansion, it includes two additional terms responsible for the spin-Hall effect and the spin-current swapping. First, assuming a delta-correlated disorder, we derive the terms from the Keldysh path integral representation of the generating functional. Then we argue phenomenologically that they exhaust all invariants allowed in the effective action to the leading order in the spin-orbit coupling (SOC). Finally, the results are confirmed by a direct derivation of the saddle-point (Usadel) equation from the quantum kinetic equations in the presence of randomly distributed impurities with SOC. At this point, we correct a recent derivation of the Usadel equation that includes magnetoelectric effects and does not resort to the Born approximation.

The origin of the driving force on quantum vortices in superconductors has long been discussed. We investigate the origin of this force using the momentum flux tensor $\mathcal{P}$, Maxwell stress tensor $\mathcal{T}$, and numerical solutions for a flowing rectilinear vortex in the time-dependent Ginzburg-Landau (TDGL) theory for three-dimensional superconductors with finite Ginzburg-Landau parameter $\kappa$ and the Maxwell equations. We calculate the hydrodynamic force ${\mathit{F}}_{\mathrm{hydro}}\left(C\right)$ and magnetic Lorentz force ${\mathit{F}}_{\mathrm{mag}}\left(C\right)$ respectively using the contour integral of $\mathcal{P}$ and $\mathcal{T}$ along a closed path that winds around the vortex line. The calculations show that neither ${\mathit{F}}_{\mathrm{hydro}}\left(C\right)$ nor ${\mathit{F}}_{\mathrm{mag}}\left(C\right)$ reaches the full magnitude of the driving force. However, when the path $C$ is farther than the penetration depth from the vortex line and hence the energy dissipation is negligible on $C$, the sum of the two forces becomes independent of the choice of $C$ and accounts for the full magnitude of the driving force on the vortex. We demonstrate the applicability of this result to a flowing vortex described in the generalized or modified version of the TDGL equation and to a pinned vortex. We then discuss the driving force on the Pearl vortex in two-dimensional superconductors and a curved vortex line in three-dimensional superconductors. We propose an experiment that locally probes the magnetic field with a pinned vortex to verify our results that the contribution of the magnetic pressure (Lorentz force) to the total driving force on the vortex is less than half.

We have used angle-dependent soft x-ray absorption spectroscopy (XAS) at the O $K$ edge and first-principles calculations to investigate the electronic structures of iridate-based superlattices ${\left(\mathrm{SrIr}{\mathrm{O}}_3\right)}_m/\left(\mathrm{SrTi}{\mathrm{O}}_3\right)$ ($m=1$, 2, 3, and ∞). We focus on the pre-edge Ir $5d t_{2g}–\mathrm{O} 2p$ orbital hybridization feature in the XAS spectra. By varying the measurement geometry relative to the incident photon polarization, we are able to extract the dichroic contrast and observe the systematic increase in the anisotropy of Ir $5d$ orbitals as $m$ decreases. First-principles calculations elucidate the orbital anisotropy coming mainly from the enhanced out-of-plane compression of $\mathrm{Ir}{\mathrm{O}}_6$ octahedra in the $\mathrm{SrIr}{\mathrm{O}}_3$ layers that are adjacent to the inserted $\mathrm{SrTi}{\mathrm{O}}_3$ layers. As $m$ decreases, the increased volume fraction of these interfacial $\mathrm{SrIr}{\mathrm{O}}_3$ layers and their contact with the $\mathrm{SrTi}{\mathrm{O}}_3$ layers within the ${\left(\mathrm{SrIr}{\mathrm{O}}_3\right)}_m/\left(\mathrm{SrTi}{\mathrm{O}}_3\right)$ supercell lead to enhanced orbital anisotropy. Furthermore, the tilt and rotation of $\mathrm{Ir}{\mathrm{O}}_6$ octahedra are shown to be essential to understand the subtle orbital anisotropy in these superlattices, and constraining these degrees of freedom will give an incorrect trend. Our results demonstrate that the structural constraint from the inserted $\mathrm{SrTi}{\mathrm{O}}_3$ layers, in addition to other electronic means such as polar interface and charge transfer, can serve as a knob to control the orbital degree of freedom in these iridate-based superlattices.

We propose a semiclassical approach based on the Vlasov equation to describe the time-dependent electronic dynamics in a bulk simple metal under an ultrashort intense laser pulse. We include in the effective potential not only the ionic Coulomb potential and mean-field electronic Coulomb potential from the one-body electron distribution but also the exchange-correlation potential within the local-density approximation (LDA). The initial ground state is obtained by the Thomas-Fermi model. To numerically solve the Vlasov equation, we extend the pseudoparticle method, previously used for nuclei and atomic clusters, to solids, taking the periodic boundary condition into account. We apply the present implementation to a bulk aluminum (fcc) conventional unit cell irradiated with a short laser pulse. The optical conductivity, refractive index, extinction coefficient, and reflectivity as well as energy absorption calculated with the Vlasov-LDA method are in excellent agreement with the results by the time-dependent density-functional theory and experimental references.

The hyperfine interaction is an important probe for understanding the structure and symmetry of defects in a semiconductor. Density functional theory has shown that it can provide useful first-principles predictions for both the hyperfine tensor and the hyperfine constants that arise from it. Recently there has been great interest in using group-IV impurity-vacancy color centers $\mathrm{X}{V}^{-}$ (where X = Si, Ge, Sn, or Pb and $V$ is a carbon vacancy) for important applications in quantum computing and quantum information science. In this paper we have calculated the hyperfine tensors for these $\mathrm{X}{V}^{-}$ color centers using the HSE06 screened Hartree-Fock hybrid exchange-correlation functional with the inclusion of core electron spin polarization. We have compared our results to calculations which only use the PBE exchange-correlation functional without the inclusion of core electron spin polarization and we have found that our results are in very good agreement with available experimental results. Finally, we have theoretically shown that these $\mathrm{X}{V}^{-}$ color centers exhibit a Jahn-Teller distortion which explains the observed anisotropic distribution of the hyperfine constants among the neighboring ${}^{13}\mathrm{C}$ nuclear spins.