Within the framework of a realistic multiband $p-d$ model, we derived an effective Hamiltonian to describe the exchange interaction effects near the spin crossover in magnetic Mott-Hubbard insulators under pressure. It is shown that the single-ion mechanism of spin crossover under the change in the crystal field does not lead to a thermodynamic phase transition; however, at $T=0$ a quantum phase transition appears. It has been found that the cooperativity leads to a modification of the quantum phase transition to a first-order phase transition and the appearance of metastable states of the system. The pressure-temperature phase diagram has been obtained to describe the magnetization and high-spin population near the spin crossover of Mott's insulators with $d^6$ ions.

Random textures of the orbital part of the order parameter of superfluid ${}^3\mathrm{He}$-A in aerogel are analyzed theoretically in the Ginzburg and Landau region both in the presence and in the absence of a global anisotropy. Correlation functions of angles, determining orientation of the order parameter, are found for relative distances which are small in comparison with the characteristic scale of the random texture. Modifications of the Larkin-Imry-Ma state in limiting cases of a relatively strong uniaxial compression and of a uniaxial stretching are analyzed, and characteristic parameters of the emerging states are found.

Femtosecond-laser-induced ultrafast magnetization dynamics have been studied in multisublattice $\mathrm{C}{\mathrm{o}}_x\mathrm{D}{\mathrm{y}}_{1-x}$ alloys. By performing element- and time-resolved x-ray spectroscopy, we distinguish the ultrafast quenching of Co $3d$ and Dy $4f$ magnetic order when the initial temperatures are below ($T=150\mathrm{K}$) or above ($T=270\mathrm{K}$), the temperature of magnetic compensation ($T_{\mathrm{comp}}$). In accordance with former element-resolved investigations and theoretical calculations, we observe different dynamics for Co $3d$ and Dy $4f$ spins. In addition, we observe that for a given laser fluence, the demagnetization amplitudes and demagnetization times are not affected by the existence of a temperature of magnetic compensation. However, our experiment reveals a twofold increase of the ultrafast demagnetization rates for the Dy sublattice at low temperature. In parallel, we measure a constant demagnetization rate of the Co $3d$ sublattice above and below $T_{\mathrm{comp}}$. This intriguing difference between the Dy $4f$ and Co $3d$ sublattices calls for further theoretical and experimental investigations.

We report inelastic neutron scattering measurements of phonon spectra in the parent superconductor iron-tuned chalcogenide $\mathrm{F}{\mathrm{e}}_{1+x}\mathrm{Te}$ for two different $x$ contents $(x\le 0.11)$ using neutron time-of-flight technique. Thermal neutron spectroscopy allowed the collection of the low-temperature Stokes spectra over an extended $Q$ range at 2, 40, and 120 K, hence covering both the magnetic monoclinic and the paramagnetic tetragonal phases, whereas cold neutrons allowed the measurement of high-resolution anti-Stokes spectra at 140, 220, and 300 K, thus covering the tetragonal phase. Our results evidence a spin-phonon coupling behavior towards the observed noticeable temperature-dependent change of the Stokes spectra across the transition temperatures. On the other hand, the anti-Stokes spectra reveal a pronounced hardening of the low-energy, acoustic region of the phonon spectrum upon heating, indicating a strong anharmonicity and a subtle dependence of phonons on structural evolution within the tetragonal phase. Experimental results are accompanied by ab initio calculations of phonon spectra of the tetragonal stoichiometric phase for a comparison with the high-resolution anti-Stokes spectra. Calculations included different density functional methods. Spin polarization and van der Waals interaction were either considered or neglected, individually or concomitantly, in order to study their respective effect on lattice dynamics description. Our results suggest that including van der Waals interaction has only a slight effect on phonon dynamics; however, phonon spectra are better described when spin polarization is included in a cooperative way with van der Waals interactions.

The critical properties of flux-grown single-crystalline quasi-two-dimensional weak itinerant ferromagnet ${\mathrm{Cr}}_{0.62}\mathrm{Te}$ were investigated by bulk dc magnetization around the paramagnetic to ferromagnetic phase transition. Critical exponents $\beta =0.315\left(7\right)$ with a critical temperature $T_c=230.6\left(3\right)$ K and $\gamma =1.81\left(2\right)$ with $T_c=229.1\left(1\right)$ K are obtained by the Kouvel-Fisher method whereas $\delta =6.35\left(4\right)$ is obtained by a critical isotherm analysis at $T_c=230$ K. With these obtained exponents, the magnetization-field-temperature curves collapse into two independent curves following a single scaling equation $M|\frac{T-T_c}{T_c}{|}^{-\beta }=f_{\pm }\left(H\right|\frac{T-T_c}{T_c}{|}^{-\beta \delta })$ around $T_c$, suggesting the reliability of the obtained exponents. Additionally, the determined exponents of ${\mathrm{Cr}}_{0.62}\mathrm{Te}$ exhibit an Ising-like behavior with a change from short-range order to long-range order in the nature of magnetic interaction and with an extension from two to three dimensions on cooling through $T_c$.

The weathervane modes of the classical Heisenberg antiferromagnet on the kagome lattice constitute possibly the earliest and certainly the most celebrated example of a flat band of zero-energy excitations. Such modes arise from the underconstraint that has since become a defining criterion of strong geometrical frustration. We investigate the fate of this flat band when dipolar interactions are added. These change the nearest-neighbor model fundamentally as they remove the Heisenberg spin-rotational symmetry while also introducing a long-range component to the interaction. We explain how the modes continue to remain approximately dispersionless, while being lifted to finite energy as well as being squeezed: they change their ellipticity described by the ratio of the amplitudes of the canonically conjugate variables comprising them. This phenomenon provides interesting connections between concepts such as constraint counting and self-screening underpinning the field of frustrated magnetism. Moreover, this property is found to be remarkably stable to a wide range of additional interactions.

We have performed ${}^{63}\mathrm{Cu}$ nuclear-magnetic-resonance/nuclear-quadrupole-resonance measurements to investigate the magnetic and superconducting (SC) properties on a “superconductivity dominant” ($S$-type) single crystal of ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$. Although the development of antiferromagnetic (AFM) fluctuations down to 1 K indicated that the AFM criticality was close, Korringa behavior was observed below 0.8 K, and no magnetic anomaly was observed above $T_{\mathrm{c}}\sim 0.6$ K. These behaviors were expected in $S$-type ${\mathrm{CeCu}}_2{\mathrm{Si}}_2$. The temperature dependence of the nuclear spin-lattice relaxation rate $1/T_1$ at zero field was almost identical to that in the previous polycrystalline samples down to 130 mK, but the temperature dependence deviated downward below 120 mK. In fact, $1/T_1$ in the SC state could be fitted with the two-gap $s_{\pm }$-wave model rather than the two-gap $s_{++}$-wave model down to 90 mK. Under magnetic fields, the spin susceptibility in both directions clearly decreased below $T_{\mathrm{c}}$, which is indicative of the formation of spin-singlet pairing. The residual part of the spin susceptibility was understood by the field-induced residual density of states evaluated from $1/T_{1}T$, which was ascribed to the effect of the vortex cores. No magnetic anomaly was observed above the upper critical field $H_{c2}$, but the development of AFM fluctuations was observed, indicating that superconductivity was realized in strong AFM fluctuations.

We revisit the interpretation of quasiparticle scattering interference in cuprate high-$T_c$ superconductors. This phenomenon has been very successful in reconstructing the dispersions of $d$-wave Bogoliubov excitations, but the successful identification and interpretation of quasiparticle interference (QPI) in scanning tunneling spectroscopy (STS) experiments rely on theoretical results obtained for the case of isolated impurities. We introduce a highly flexible technique to simulate STS measurements by computing the local density of states using real-space Green's functions defined on two-dimensional lattices with as many as 100 000 sites. We focus on the following question: to what extent can the experimental results be reproduced when various forms of distributed disorder are present? We consider randomly distributed pointlike impurities, smooth “Coulombic” disorder, and disorder arising from random on-site energies and superconducting gaps. We find an apparent paradox: the QPI peaks in the Fourier-transformed local density of states appear to be sharper and better defined in experiment than those seen in our simulations. We arrive at a no-go result for smooth-potential disorder since this does not reproduce the QPI peaks associated with large-momentum scattering. An ensemble of pointlike impurities gets closest to experiment, but this goes hand in hand with impurity cores that are not seen in experiment. We also study the effects of possible measurement artifacts (the “fork mechanism”), which turn out to be of relatively minor consequence. It appears that a more microscopic model of the tunneling process needs to be incorporated in order to account for the sharpness of the experimentally obtained QPI peaks.

We study Andreev reflection and Andreev levels $\varepsilon$ in Zeeman-split superconductor/Rashba wire/Zeeman split superconductor junctions by solving the Bogoliubov–de Gennes equation. We theoretically demonstrate that the Andreev levels $\varepsilon$ can be controlled by tuning either the strength of Rashba spin-orbit interaction or the relative direction of the Rashba spin-orbit interaction and the Zeeman field. In particular, it is found that the magnitude of the band splitting is tunable by the strength of the Rashba spin-orbit interaction and the length of the wire, which can be interpreted by a spin precession in the Rashba wire. We also find that if the Zeeman field in the superconductor has the component parallel to the direction of the junction, the $\varepsilon -\varphi$ curve becomes asymmetric with respect to the superconducting phase difference $\varphi$. Whereas the Andreev reflection processes associated with each pseudospin band are sensitive to the relative orientation of the spin-orbit field and the exchange field, the total electric conductance interestingly remains invariant.

It is shown that the ground states of a system of classical spins on an anisotropic triangular lattice with interactions within an elementary triangular plaquette can be constructed by minimizing the energy of a single plaquette. Even in the case when all three angles between plaquette spins are different, there exist five global ground-state configurations with equal energies. The most complex of these is an incommensurate four-sublattice conical spiral structure. Our results may shed some light on the experimentally observed spin-liquid-like disorder in ${\mathrm{NiGa}}_2{\mathrm{S}}_4$ and ${\mathrm{FeGa}}_2{\mathrm{S}}_4$ where a four-sublattice spin structure was observed.

Recently a new family of Cr-based ${\mathrm{A}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$ ($A=\text{K}$, Rb, Cs) superconductors was reported, which own a rare quasi-one-dimensional (Q1D) crystal structure with infinite ${\left({\mathrm{Cr}}_3{\mathrm{As}}_3\right)}^{2-}$ chains and exhibit intriguing superconducting characteristics possibly derived from spin-triplet electron pairing. The crystal structure of ${\mathrm{A}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$ is actually a slight variation of the hexagonal ${\mathrm{TlFe}}_3{\mathrm{Te}}_3$ prototype, although they have different lattice symmetry. Here we report superconductivity in a 133-type ${\mathrm{KCr}}_3{\mathrm{As}}_3$ compound that belongs to the latter structure. The single crystals of ${\mathrm{KCr}}_3{\mathrm{As}}_3$ were prepared by the deintercalation of K ions from ${\mathrm{K}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$ crystals which were grown from a high-temperature solution growth method, and it owns a centrosymmetric lattice in contrast to the noncentrosymmetric ${\mathrm{K}}_2{\mathrm{Cr}}_3{\mathrm{As}}_3$. After annealing at a moderate temperature, the ${\mathrm{KCr}}_3{\mathrm{As}}_3$ crystals show superconductivity at 5 K revealed by electrical resistivity, magnetic susceptibility, and heat capacity measurements. The discovery of this ${\mathrm{KCr}}_3{\mathrm{As}}_3$ superconductor provides a different structural instance to study the exotic superconductivity in these Q1D Cr-based superconductors.

Controlling and manipulating the topological state represents an important topic in condensed matters for both fundamental researches and applications. In this work, we focus on the evolution of a real-space topological domain structure in hexagonal manganites driven by electric field, using the analytical and numerical calculations based on the Ginzburg-Landau theory. It is revealed that the electric field drives a transition of the topological domain structure from the type-I pattern to the type-II one. In particular, it is identified that a high electric field can enforce the two antiphase-plus-ferroelectric ($\mathrm{AP}+\mathrm{FE}$) domain walls with $\mathrm{\Delta }\mathrm{\Phi }=\pi /3$ to approach each other and to merge into one domain wall with $\mathrm{\Delta }\mathrm{\Phi }=2\pi /3$ eventually if the electric field is sufficiently high, where $\mathrm{\Delta }\mathrm{\Phi }$ is the difference in the trimerization phase between two neighboring domains. Our simulations also reveal that the vortex cores of the topological structure can be disabled at a sufficiently high critical electric field by suppressing the structural trimerization therein, beyond which the vortex core region is replaced by a single ferroelectric domain without structural trimerization ($Q=0$). Our results provide a stimulating reference for understanding the manipulation of real-space topological domain structure in hexagonal manganites.

We develop an approach to apply the Wang-Landau algorithm to multicomponent alloys in a semi-grand-canonical ensemble. Although the Wang-Landau algorithm has great advantages over conventional sampling methods, there are few applications to alloys. This is because calculating compositions in a semi-grand-canonical ensemble via the Wang-Landau algorithm requires a multidimensional density of states in terms of total energy and compositions, and constructing it is difficult from the viewpoints of both implementation and computational cost. In this study, we develop a simple approach to calculate the alloy phase diagram based on the Wang-Landau algorithm, and show that a number of one-dimensional densities of states could lead to compositions in a semi-grand-canonical ensemble as a multidimensional density of states could. Finally, we apply the present method to Cu-Au and Pd-Rh alloys and confirm that the present method successfully describes the phase diagram with high efficiency, validity, and accuracy.

The theory of Bloch electron dynamics for carriers in homogeneous electric and magnetic fields of arbitrary time dependence is developed in the framework of the Liouville equation. The Wigner distribution function (WDF) is determined from the single-particle density matrix in the ballistic regime, i.e., collision effects are excluded. In the theory, the single-particle transport equation is established with the electric field described in the vector potential gauge, and the magnetic field is treated in the symmetric gauge. No specific assumptions are made concerning the form of the initial distribution in momentum or configuration space. The general approach is to employ the accelerated Bloch state representation (ABR) as a basis so that the dependence upon the electric field, including multiband Zener tunneling, is treated exactly. Further, in the formulation of the WDF, we transform to a new set of variables so that the final WDF is gauge invariant and is expressed explicitly in terms of the position, kinetic momentum, and time. The methodology for developing the WDF is illustrated by deriving the exact WDF equation for free electrons in homogeneous electric and magnetic fields resulting in the same form as given by the collisionless Boltzmann transport equation (BTE). The methodology is then extended to the case of electrons described by an effective Hamiltonian corresponding to an arbitrary energy band function; the exact WDF equation results for the effective Hamiltonian case are shown to approximate the free electron results when taken to second order in the magnetic field. As a corollary, in these cases, it is shown that if the WDF is a wave packet, then the time rate of change of the electron quasimomentum is given by the Lorentz force. In treating the problem of Bloch electrons in a periodic potential in the presence of homogeneous electric and magnetic fields, the methodology for deriving the WDF reveals a multiband character due to the inherent nature of the Bloch states. The ${\mathbf{K}}_0$ representation of the Bloch envelope functions is employed to express the multiband WDF in a useful form. In examining the single-band WDF, it is found that the collisionless WDF equation matches the equivalent BTE to first order in the magnetic field. These results are necessarily extended to second order in the magnetic field by employing a unitary transformation that diagonalizes the Hamiltonian using the ABR to second order. The unitary transformation process includes a discussion of the multiband WDF transport analysis and the identification of the combined Zener-magnetic-field induced tunneling.

The role of quenched disorder, a key control to obtain novel phases and functionalities, has not yet been determined in the complex phase diagram of $\mathrm{RNi}{\mathrm{O}}_3$ (R = rare-earth ion) perovskites. Here we present such a study by investigating $\left(\mathrm{L}{\mathrm{a}}_{0.5}\mathrm{E}{\mathrm{u}}_{0.5}\right)\mathrm{Ni}{\mathrm{O}}_3$ (LENO) having large R-site cation disorder. We show that in the presence of quenched disorder, (i) the resistivity drops by a few orders of magnitude across the metal-insulator transition (MIT) but the MIT shows only a subtle decrease, (ii) compressive films are completely metallic while largely tensile films are completely insulating sans a MIT, (iii) orthorhombic distortion promotes sharp MIT, and (iv) a Fermi liquid behavior even as high temperature resistivity exceeds the Mott-Ioffe-Regel limit with a bad metallic state. The low-energy terahertz conductivity dynamics obey Drude and Drude-Smith models for compressive and tensile films, respectively. All these features of disordered LENO, which are not typical of prototype ordered $\mathrm{NdNi}{\mathrm{O}}_3$, reveal an extraordinary sensitivity to slight structural perturbations. This study depicts the ease with which a variety of electronic phases can be tuned in disordered nickelates and emphasize the need to incorporate quenched disorder as a key control in the phase diagram of nickelates.

Topological defects in magnetism have attracted great attention due to fundamental research interests and potential novel spintronics applications. Rich examples of topological defects can be found in nanoscale nonuniform spin textures, such as monopoles, domain walls, vortices, and skyrmions. Recently, skyrmions stabilized by the Dzyaloshinskii-Moriya interaction have been studied extensively. However, the stabilization of antiskyrmions is less straightforward. Here, using numerical simulations we demonstrate that antiskyrmions can be a stable spin configuration in the presence of anisotropic Dzyaloshinskii-Moriya interaction. We find current-driven antiskyrmion motion that has a transverse component, namely, the antiskyrmion Hall effect. The antiskyrmion gyroconstant is opposite to that for skyrmion, which allows the current-driven propagation of coupled skyrmion-antiskyrmion pairs without an apparent skyrmion Hall effect. The antiskyrmion Hall angle strongly depends on the current direction, and a zero antiskyrmion Hall angle can be achieved at a critic current direction. These results open up possibilities to tailor the spin topology in nanoscale magnetism, which may be useful in the emerging field of skyrmionics.

Recent experiments indicate that superconductivity in ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ intercalated with Cu, Nb, or Sr is nematic with rotational symmetry breaking. Motivated by this observation, we present a model study of nematic and chiral superconductivity induced by odd-parity fluctuations. We show that odd-parity fluctuations in the two-component $E_u$ representation of $D_{3d}$ crystal point group can generate attractive interaction in both the even-parity $s$-wave and odd-parity $E_u$ pairing channels, but repulsive interaction in other odd-parity pairing channels. Coulomb repulsion can suppress $s$-wave pairing relative to $E_u$ pairing, and thus the latter can have a higher critical temperature. $E_u$ pairing has two distinct phases: a nematic phase and a chiral phase, both of which can be realized in our model. When $s$-wave and $E_u$ pairings have similar instability temperature, we find an intermediate phase in which both types of pairing coexist.

Time-of-flight momentum microscopy reveals sixfold symmetric sharp features of decreased intensity (dark lines) in constant-energy maps for clean Ir(111) and graphene/Ir(111). The dark lines have been observed for $p$- and $s$-polarized light in the photon-energy range of 20–27 eV and result from scattering of photoelectrons at the surface potential barrier. The phenomenon is strongly related to threshold effects in low-energy electron diffraction. A quantitative analysis of the dark lines' positions shows that the relevant reciprocal-lattice vector corresponds to the lattice of the topmost layer (in our case graphene and Ir, respectively). The dark lines appear in the momentum patterns only in a certain photon-energy range satisfying the additional condition that the electron wavelength matches the lattice periodicity.

We investigate the band structure of the inverse perovskite ${\mathrm{Ca}}_3\mathrm{PbO}$, a candidate three-dimensional (3D) Dirac fermion material, through soft x-ray angle-resolved photoemission spectroscopy. Conelike band dispersions are observed for ${\mathrm{Ca}}_3\mathrm{PbO}$, in close agreement with the predictions of electronic structure calculations. We further demonstrate that chemical substitution of Bi for Pb is effective in tuning the Fermi level of ${\mathrm{Ca}}_3\mathrm{PbO}$ while leaving its electronic structure intact. Our study confirms that the inverse perovskite family provides a promising platform for the exploration of 3D Dirac fermion systems.

Using the density matrix renormalization group technique we study the effect of spin-orbit coupling on a three-orbital Hubbard model in the ${\left(t_{2g}\right)}^4$ sector and in one dimension. Fixing the Hund coupling to a robust value compatible with some multiorbital materials, we present the phase diagram varying the Hubbard $U$ and spin-orbit coupling $\lambda$, at zero temperature. Our results are shown to be qualitatively similar to those recently reported using the dynamical mean-field theory in higher dimensions, providing a robust basis to approximate many-body techniques. Among many results, we observe an interesting transition from an orbital-selective Mott phase to an excitonic insulator with increasing $\lambda$ at intermediate $U$. In the strong $U$ coupling limit, we find a nonmagnetic insulator with an effective angular momentum $\langle {\left({\mathbf{J}}^{\mathrm{eff}}\right)}^2\rangle \ne 0$ near the excitonic phase, smoothly connected to the $\langle {\left({\mathbf{J}}^{\mathrm{eff}}\right)}^2\rangle =0$ regime. We also provide a list of quasi-one-dimensional materials where the physics discussed in this paper could be realized.