The puzzling observation of room temperature ferromagnetism in double perovskites $\left(A_2{BB}^{\prime }{O}_6\right)$, despite having the magnetic lattice of B ions diluted by nonmagnetic $B^{\prime }$ ions, have been examined for ${\mathrm{Sr}}_2{\mathrm{FeReO}}_6$. Ab initio spin spiral electronic structure calculations along various high symmetry directions in reciprocal space are used to determine the exchange interactions entering an extended Heisenberg model, which is then solved classically using Monte Carlo simulations to determine the ferromagnetic transition temperature $T_c$. We find that one must consider on-site Coulomb interactions at the nonmagnetic Re sites $\left(U\right)$ in order to obtain a $T_c$ close to the experimental value. Analysis of the ab initio electronic structure as well as an appropriate model Hamiltonian trace the origin of enhancement in $T_c$ with $U$ to the enhanced exchange splitting that is introduced at these sites. This in turn destabilizes the antiferromagnetic exchange channels, thereby enhancing the $T_c$. The role of occupancy at the nonmagnetic sites is examined by contrasting with the case of ${\mathrm{Sr}}_2{\mathrm{FeMoO}}_6$.

Recently, there has been an increased interest in the application of machine learning (ML) techniques to a variety of problems in condensed-matter physics. In this regard, of particular significance is the characterization of simple and complex phases of matter. Here, we use a ML approach to construct the full phase diagram of a well-known spin model combining ferromagnetic exchange and Dzyaloshinskii-Moriya (DM) interactions where topological phases emerge. At low temperatures, the system is tuned from a spiral phase to a skyrmion crystal by a magnetic field. However, thermal fluctuations induce two types of intermediate phases, bimerons and skyrmion gas, which are not as easily determined as spirals or skyrmion crystals. We resort to large-scale Monte Carlo simulations to obtain low-temperature spin configurations and train a convolutional neural network (CNN), taking only snapshots at specific values of the DM couplings, to classify between the different phases, focusing on the intermediate and intricate topological textures. We then apply the CNN to higher-temperature configurations and to other DM values to construct a detailed magnetic-field–temperature phase diagram, achieving outstanding results. We discuss the importance of including the disordered paramagnetic phases in order to get the phase boundaries, and, finally, we compare our approach with other ML algorithms.

The quantized topological magnetoelectric effect (TME) is the hallmark of the axion insulator but remains unexplored due to its small signal. In axion insulator thin films, TME deviates from the exact quantization due to the finite-size effect. Here we show that such finite-size correction depends on the out-of-plane surface magnetization. This provides a new mechanism for surface magnetization dynamics enabled dynamical magnetoelectric coupling, which further generates a polarization current in the presence of an external magnetic field in the same direction. Such a current measures the finite-size correction to TME, which increases as the film thickness $d$ decreases, in contrast to the TME current, which decreases as $d$ decreases. Remarkably, the current in thin films at magnetic resonance is about several orders of magnitude larger than that of TME, which is absent in trivial insulators and thus could serve as a smoking gun signature for axion insulators. The intimate interplay between surface magnetization dynamics and the magnetoelectric response in the axion insulator phase unveiled here may lead to a new class of electronic and spintronic applications of axion devices.

We present a theory of spin-wave normal modes of a ferromagnetic film with periodically corrugated surfaces in Damon-Eshbach geometry. Two possibly different corrugations on the upper and lower film surface are considered. Both the surface mode and perpendicular standing modes are studied with the partial-wave method, adapted to take into account the modified boundary conditions, within the limit of validity of the Rayleigh's hypothesis. The theory, particularly suited to the case of shallow gratings, shows that the effect of corrugation on the dispersion curves is enhanced on asymmetrical structures, such as films with a sinusoidally corrugated top surface and flat bottom.

A general problem of magnetic sensors is a trade-off between spatial resolution and magnetic-field sensitivity. With decreasing sensor size its resolution is improved but the sensitivity is deteriorated. Obviation of such a trade-off requires development of super-resolution imaging technique not limited by sensor size. Here we present a proof of concept for a super-resolution method of magnetic imaging by a Josephson junction (JJ). It is based on a solution of an inverse problem—reconstruction of a local magnetic-field distribution within a junction from the dependence of the critical current on an external magnetic field, $I_c\left(H\right)$. The method resembles the Fourier-transform holography, with the diffractionlike $I_c\left(H\right)$ pattern serving as a hologram. A simple inverse problem solution, valid for an arbitrary symmetric case, is derived. We verify the method numerically and show that the accuracy of reconstruction does not depend on the junction size and is only limited by the field range of the $I_c\left(H\right)$ pattern. Finally, the method is tested experimentally using planar Nb JJs. Super-resolution reconstruction of stray magnetic fields from an Abrikosov vortex, trapped in the junction electrodes, is demonstrated. Thus our method facilitates both high field sensitivity and high spatial resolution, obviating the trade-off problem of magnetic sensors. We conclude that the holographic magnetic imaging by a planar JJ can be used in scanning probe microscopy.

Recent study of overdoped ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ cuprate superconductor thin films by Božović et al. has revealed several unexpected findings, most notably the violation of the BCS description which was believed to adequately describe overdoped cuprates. In particular, it was found that the superfluid density in ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ films decreases on the overdoped side as a linear function of critical temperature $T_c$, which was taken as evidence for the violation of Homes' law. We show explicitly that the law is indeed violated, and as the main reason for violation we find that the superfluid density in Božović's films is suppressed more strongly than in bulk samples. Based on the existing literature data, we show that the superfluid density in bulk cuprate samples does not decrease with doping, but instead tends to saturate on the overdoped side. The result is also supported by our recent measurement of a heavily overdoped bulk ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ sample. Moreover, this saturation of superfluid density might not be limited to cuprates, as we find evidence for similar behavior in two pnictide superconductor families. We argue that quantum phase fluctuations play an important role in suppressing the superfluid density in thin films.

We study the thermal transport in a two-dimensional system with coexisting superconducting (SC) and nematic orders. We analyze the nature of the coexistence phase in a tight-binding square lattice where the nematic state is modelled as a $d$-wave Pomeranchuk-type instability and the feedback of the symmetry breaking nematic state on the SC order is accounted for by mixing of the $s, d$ paring interaction. The electronic thermal conductivity is computed within the framework of Boltzmann kinetic theory where the impurity scattering collision integral is treated in the Born and unitary limits. We present qualitative, analytical, and numerical results that show that the heat transport properties of SC states emerging from a nematic background are quite distinct and depend on the degree of anisotropy of the SC gap induced by nematicity. We describe the influence of the Fermi surface topology, the van Hove singularities, and the presence or absence of zero-energy excitations in the coexistence phase on the the low-temperature behavior of the thermal conductivity. Our main conclusion is that the interplay of nematic and SC orders has visible signatures in the thermal transport, which can be used to infer SC gap structure in the coexistence phase.

Accurately predicting the formation energy of a compound, which describes its thermodynamic stability, is a key challenge in materials physics. Here, we employ a many-body quantum Monte Carlo (QMC) with single-reference trial functions to compute the formation energy of two electronically disparate compounds, the intermetallic $\mathrm{V}{\mathrm{Pt}}_2$ and the semiconductor CuI, for which standard density functional theory (DFT) predictions using both the Perdew-Burke-Ernzerhof (PBE) and the strongly constrained and appropriately normed (SCAN) density functional approximations deviate markedly from available experimental values. For $\mathrm{V}{\mathrm{Pt}}_2$, we find an agreement between QMC, SCAN, and PBE0 estimates, which therefore remain in disagreement with the much less exothermic experimental value. For CuI, the QMC result agrees with neither SCAN nor PBE pointing towards DFT exchange-correlation biases, likely related to the localized Cu $3d$ electrons. Compared to the behavior of some density functional approximations within DFT, spin-averaged QMC exhibits a smaller but still appreciable deviation when compared to experiment. The QMC result is slightly improved by incorporating spin-orbit corrections for CuI and solid ${\mathrm{I}}_2$, so that experiment and theory are brought into imperfect but reasonable agreement within about 120 meV/atom.

Drag and diffusion of mobile ions in solids are of interest for both purely theoretical and applied scientific communities. This article proposes a theoretical description of ion drag in solids that can be used to estimate ionic conductivities in crystals, and forms a basis for the rational design of solid electrolyte materials. Starting with a general solid-state Hamiltonian, we employ the nonequilibrium path integral formalism to develop a microscopic theory of ionic transport in solids in the presence of thermal fluctuations. As required by the fluctuation-dissipation theorem, we obtain a relation between the variance of the random force and friction. Because of the crystalline nature of the system, however, the two quantities are tensorial. We use the drag tensor to write down the formula for ionic mobility, determined by the potential profile generated by the crystal's ions.

The emergence of a magnetic skyrmion crystal in a nonsymmorphic lattice system with the screw symmetry is numerically investigated. By performing the simulated annealing for a layered spin model with the isotropic exchange interaction and the antisymmetric Dzyaloshinskii-Moriya interaction, and then constructing a low-temperature phase diagram, we reveal that the skyrmion crystal is stabilized in both zero and nonzero fields even without the threefold rotational symmetry in the two-dimensional plane. We show that a competition between the ferromagnetic interlayer exchange interaction and the layer- and momentum-dependent anisotropic interactions is a source of the skyrmion crystals in the presence of the screw axis. Moreover, we find two types of layer-dependent skyrmion crystals, which are characterized as a coexisting state of the skyrmion crystal and the spiral state, in the narrow field region. Our result provides a reference in the further search for skyrmion crystals in nonsymmorphic lattice systems.

Paramagnons are the collective modes that govern the spin response of nearly magnetic conductors. In some cases they mediate electron pairing leading to superconductivity. This scenario may occur in $2H-{\mathrm{NbSe}}_2$ monolayers, that feature spin-valley coupling on account of spin-orbit interactions and their lack of inversion symmetry. Here we explore spin anisotropy of paramagnons both for noncentrosymmetric Kane-Mele-Hubbard models for $2H-{\mathrm{NbSe}}_2$ monolayers described with a DFT-derived tight-binding model. In the infinite wavelength limit we find spatially uniform paramagnons with energies around 1 meV that feature a colossal off-plane uniaxial magnetic anisotropy, with quenched transversal spin response. At finite wave vectors, longitudinal and transverse channels reverse roles: XY fluctuations dominate within a significant portion of the Brillouin zone. Our results show that $2H-{\mathrm{NbSe}}_2$ is close to a Coulomb-driven in-plane (XY) spin density wave instability.

The angular dependence of coercivity in permanent magnets is an unsettled issue in magnetism. We investigate the angular dependences of threshold fields for magnetization reversal in a single-grain and hard-soft-hard magnet model at zero and finite temperatures. We study thermal fluctuation effects on magnetization reversal, which is a stochastic process for barrier crossing at finite temperatures. We show that the mechanism of magnetization reversal in the single grain is different between zero and finite temperatures, although the normalized threshold field at finite temperatures exhibits a similar angular dependence to that of the Stoner-Wohlfarth field. We examine the angular dependences of nucleation and pinning fields for different magnetic parameters in the soft and hard magnet phases. We find that the angular dependences of the nucleation and pinning fields differ from that of the single grain, in which an exchange field resistant to reversal plays an important role, and that the thermal fluctuation reduces the threshold fields, varies the angular dependences, and makes the difference between nucleation and pinning fields smaller. We also analyze the effect of the grain alignment distribution on the threshold fields and find that the distribution changes the angular dependences of the threshold fields in different ways. We discuss the detailed features of these phenomena and the mechanisms behind them.

Vortices in an unconventional superconductor are an important subject for the fundamental study of superconductivity. A spin counterflow half-quantum vortex (HQV) was predicted theoretically for odd-parity, spin-triplet superconductors. Cantilever torque magnetometry measurements revealed previously experimental evidence for HQVs in doubly connected, single-crystal samples of ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ with a mesoscopic size. However, important questions on the HQV, such as its stability, have remained largely unexplored. We report in this paper the detection of distinct features in vortex crossing induced magnetoresistance (MR) oscillations in doubly connected, mesoscopic cylinders of single-crystal ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$, which include a dip and secondary peak in MR, in the presence of a sufficiently large in-plane magnetic field. We argue that these features are due to the formation of spin counterflow HQV in a spin-triplet superconductor, which provides additional evidence for the existence of HQV and insights into the physics of this highly unusual topological object.

We report a high superconducting transition temperature $T_c$ up to 23.6 K, renewing the highest value in transition metals, under high pressure in the elemental metal Ti, one of the top ten most abundant elements in the earth's crust. The $T_c$ increases monotonically from 2.3 K at 40.3 GPa to 23.6 K at 144.9 GPa. With further compression, a robust $T_c$ of $\sim 23$ K is observed between 144.9 and 183 GPa in the $\delta$-Ti phase. The pressure-dependent $T_c$ can be well described by the conventional electron-phonon coupling (EPC) mechanism. Density functional theory calculations show the Fermi nesting and the phonon softening of optical branches at the $\gamma$-Ti to $\delta$-Ti phase transition pressure-enhanced EPC, which results in the high $T_c$. We attribute the robust superconductivity in $\delta \text{−}\mathrm{Ti}$ to the apparent robustness of its strong EPC against lattice compression. These results provide insight into exploring high-$T_c$ elemental metals and Ti-based superconducting alloys.

We report low-temperature (down to $\sim 5$ K) Raman signatures of the recently discovered intercalated honeycomb magnet ${\mathrm{Ag}}_3{\mathrm{LiIr}}_2{\mathrm{O}}_6$, a putative Kitaev quantum spin liquid (QSL) candidate. The Kitaev QSL is predicted to host Majorana fermions as its emergent elementary excitations through a thermal fractionalization of entangled spins $S=1/2$. We observe evidence of this fractionalization in the low-energy magnetic continuum whose temperature evolution harbors signatures of the predicted Fermi statistics obeyed by the itinerant Majorana quasiparticles. The magnetic Raman susceptibility evinces a crossover from the conventional to a Kitaev paramagnetic state below the temperature of $\sim 80$ K. Additionally, the development of the Fano asymmetry in the low-frequency phonon mode and the enhancement of integrated Raman susceptibilities below the crossover temperature signifies prominent coupling between the vibrational and Majorana fermionic excitations.

The graphite $k_z$ dispersion from the region with a size of several micrometers was measured via photoelectron momentum microscopy. The $\pi$-band dispersion over an entire Brillouin zone was investigated. We found that the degeneracy of the $\pi$ bands was eliminated in the ALH reciprocal lattice plane and their symmetries were reduced from $C_{6v}$ to $C_{3v}$. We attributed this phenomenon to the coupling of the graphite surface $\pi$ band of a single-domain threefold structure with the sixfold bulk $k_z$ dispersion which has been neglected to date. We used this symmetry breaking to image the monoatomic step of the graphite surface. Further, the precision of determining $k_z$ dispersion and the effect of surface resonance are discussed.

By combining Wilson's numerical renormalization group (NRG) with a modified Bloch-Redfield approach (BRA), we are able to eliminate the artificial broadening of the Lehmann representation of quantum impurity spectral functions required by the standard NRG algorithm. Our approach is based on the exact reproduction of the continuous coupling function in the original quantum impurity model. It augments each chain site of the Wilson chain by a coupling to an additional reservoir. This open Wilson chain is constructed by a continuous fraction expansion, and the coupling function is treated in second order in the context of the BRA. The eigenvalues of the resulting Bloch-Redfield tensor generate a finite lifetime of the NRG excitations that leads to a natural broadening of the spectral functions. We combine this approach with z-averaging and an analytical exact expression for the correlation part of the self-energy to obtain an accurate representation of the spectral function of the original continuum model in the absence and presence of an external magnetic field.

Magnetic topological insulators provide a prominent material platform for quantum anomalous Hall physics and axion electrodynamics. However, the lack of material realizations with cleanly gapped surfaces hinders technological utilization of these exotic quantum phenomena. Here, using the Zintl concept and the properties of nonsymmorphic space groups, we computationally engineer magnetic topological insulators. Specifically, we explore ${\mathrm{Eu}}_5{M}_2{X}_6$ (M=metal, X=pnictide) Zintl compounds and find that ${\mathrm{Eu}}_5{\mathrm{Ga}}_2{\mathrm{Sb}}_6$, ${\mathrm{Eu}}_5{\mathrm{Tl}}_2{\mathrm{Sb}}_6$, and ${\mathrm{Eu}}_5{\mathrm{In}}_2{\mathrm{Bi}}_6$ form stable structures with nontrivial ${\mathbb{Z}}_2$ indices. We also show that epitaxial and uniaxial strain can be used to control the ${\mathbb{Z}}_2$ index and the bulk energy gap. Finally, we discuss experimental progress towards the synthesis of the proposed candidates and provide insights that can be used in the search for robust magnetic topological insulators in Zintl compounds.