Phase transformations in metallic grain boundaries (GBs) present significant fundamental interest in the context of thermodynamics of low-dimensional physical systems. We report on atomistic computer simulations of the Cu-Ag system that provide direct evidence that GB phase transformations in a single-component GB can continue to exist in a binary alloy. This gives rise to segregation-induced phase transformations with varying chemical composition at a fixed temperature. Furthermore, for such transformations we propose an approach to calculations of free-energy differences between different GB phases by thermodynamic integration along a segregation isotherm. This opens the possibility of developing quantitative thermodynamics of GB phases, their transformations to each other, and critical phenomena in the future.

We show that superconductivity can arise in semiconductors with a band in the shape of a Mexican hat when the chemical potential is tuned close to the band edge, but not intersecting the band, as long as interactions are sufficiently strong. Hence, this is an example where superconductivity can emerge from a band insulator when interactions exceed a threshold. Semiconductors with simple cubic symmetry point groups and with strong spin-orbit coupling provide an example of a system with such band dispersion.

${}^{19}\mathrm{F}$ NMR measurements in optimally electron-doped ${\mathrm{La}}_{1\text{−}y}{\mathrm{Y}}_y{\mathrm{Fe}}_{1\text{−}x}{\mathrm{Mn}}_x{\mathrm{AsO}}_{0.89}{\mathrm{F}}_{0.11}$ superconductors are presented. The effect of Mn doping on the superconducting phase is studied for two series of compounds $(y=0$ and $y=0.2)$ where the chemical pressure is varied by substituting La with Y. In the $y=0.2$ series superconductivity is suppressed for Mn contents an order of magnitude larger than for $y=0$. For both series a peak in the ${}^{19}\mathrm{F}$ NMR nuclear spin-lattice relaxation rate $1/T_1$ emerges upon Mn doping and becomes significantly enhanced on approaching the quantum phase transition between the superconducting and magnetic phases. ${}^{19}\mathrm{F}$ NMR linewidth measurements show that for similar Mn contents magnetic correlations are more pronounced in the $y=0$ series, at variance with what one would expect for $\vec{Q}=(\pi /a,0)$ spin correlations. These observations suggest that Mn doping tends to reduce fluctuations at $\vec{Q}=(\pi /a,0)$ and to enhance other low-frequency modes. The effect of this transfer of spectral weight on the superconducting pairing is discussed along with the charge localization induced by Mn.

The composition-dependent crystal structure, elastic modulus, phase stability, and magnetic property of ${\mathrm{Ni}}_{2-x}{\mathrm{Co}}_x{\mathrm{Mn}}_{1.60}{\mathrm{Sn}}_{0.40}\left(0\le x\le 0.50\right)$ are studied by using first-principles calculations in combination with atomistic spin dynamics method. It is shown that the present lattice parameters and Curie temperature $\left(T_C\right)$ are in agreement with the available experimental data. The martensitic phase transformation (MPT) occurs for $x<0.43$, where the austenite is in the ferromagnetic (FM) state whereas the martensite is in the antiferromagnetic (AFM) one at 0 K. The $x$ dependence of the lattice parameter, elastic modulus, and energy difference between the FM austenite and the AFM martensite well accounts for the decrease of the MPT temperature $\left(T_M\right)$ with the Co addition. With increasing $x$, the increase of the magnetic excitation energy between the paramagnetic and FM austenite of these alloys is in line with the $T_C\sim x$. The Ni $3d$ as well as the Co $3d$ electronic states near the Fermi level are confirmed mainly dominating the phase stability of the studied alloys.

We present an automated scheme to systematically sample energy landscapes of crystalline solids, based on the ideas of metadynamics and evolutionary algorithms. Phase transitions are driven by the evolution of the order parameter (in this case, 6-dimensional order parameters composed of cell vectors components) and aided by atomic displacements corresponding to both zero and nonzero wave vectors, enabling cell size to spontaneously change during simulation. Our technique can be used for efficient prediction of stable crystal structures, and is particularly powerful for mining numerous low-energy configurations and phase transition pathways. By applying this method to boron, we find numerous energetically competitive configurations, based on various packings of ${\mathrm{B}}_{12}$ icosahedra. We also observed a low-energy metastable structure of Si(T32) which is likely to be a product of decompression on Si-II. T32 is calculated to have a quasidirect band gap of 1.28 eV, making it promising for photovoltaic applications.

We introduce a semiclassical limit for many-body localization in the absence of global symmetries. Microscopically, this limit is realized by disordered Floquet circuits composed of Clifford gates. In $d=1$, the resulting dynamics are always many-body localized with a complete set of strictly local integrals of motion. In $d\ge 2$, the system realizes both localized and delocalized phases separated by a continuous transition in which ergodic puddles percolate. We argue that the phases are stable to deformations away from the semiclassical limit and estimate the resulting phase boundary. The Clifford circuit model is a distinct tractable limit from that of free fermions and suggests bounds on the critical exponents for the generic transition.

When wave effects of thermal photons become significant, thermal emitters can exhibit intriguing coherent effects. Here, we show that the superradiant emission, which was originally found in quantum emitters, can be realized in resonant thermal emitters. Similar to the superradiance in quantum emitters, the in-phase oscillation of resonant emitters reduces the lifetime of thermal photons in the emitters. Unlike the atomic superradiance, one remarkable consequence of the thermal superradiance is the anomalous power scaling, where the emission power can scale inversely with the number of thermal emitters. More thermal emitters generate less power due to the coherent interference of thermal photons.

Magnetic as well as calorimetric measurements have been performed on single crystal samples of $\mathrm{P}{\mathrm{r}}_{0.6}\mathrm{S}{\mathrm{r}}_{0.4}\mathrm{Mn}{\mathrm{O}}_3$ and $\mathrm{N}{\mathrm{d}}_{0.6}\mathrm{S}{\mathrm{r}}_{0.4}\mathrm{Mn}{\mathrm{O}}_3$ to develop a complete critical behavior study of the paramagnetic to ferromagnetic transition in both manganites. The critical exponents $\alpha ,\beta ,\gamma ,\mathrm{and}\delta$ have been independently obtained. For $\mathrm{P}{\mathrm{r}}_{0.6}\mathrm{S}{\mathrm{r}}_{0.4}\mathrm{Mn}{\mathrm{O}}_3$, these are $\alpha =0.09$, $\beta =0.312$, $\gamma =1.106$, and $\delta =4.545$, while for $\mathrm{N}{\mathrm{d}}_{0.6}\mathrm{S}{\mathrm{r}}_{0.4}\mathrm{Mn}{\mathrm{O}}_3$ they are $\alpha =0.11$, $\beta =0.308$, $\gamma =1.172$, and $\delta =4.75$. All these values agree with the three-dimensional (3D)-Ising universality class $(\alpha =0.11$, $\beta =0.3265$, $\gamma =1.237$, and $\delta =4.79)$ and are very far away from any other known universality class. This suggests the presence of magnetocrystalline anisotropies in the system that must be taken into account to fully describe the magnetism of these manganites, which deviates from a simple double exchange model.

We present the results obtained by superconducting quantum interference device (SQUID) magnetometry, specific heat, and Mössbauer spectroscopy measurements carried out on ${\mathrm{Np}}_2{\mathrm{Ni}}_{17}$ polycrystalline samples. We show that long-range magnetic order, with a moment ${\mu }_{\left(2b\right)}\sim 2.25$ ${\mu }_B$, occurs below ${\mathrm{T}}_N=17.5$ K on the Np $(2b)$ sites. A nontrivial situation is observed in that the other Np sites $(2d)$ do not take part to the order transition and carry only an induced moment of about 0.2 ${\mu }_B$ below ${\mathrm{T}}_N$. A combined analysis of the different experimental data sets allowed us to determine key parameters associated with the electronic structure of the system. The experimental results are discussed against first-principles electronic structure calculations based on the spin-polarized local spin density approximation plus Hubbard interaction.

The intrinsic spin Hall effect originates from the topology of the Bloch bands in momentum space. The duality between real space and momentum space calls for a spin Hall effect induced from a real space topology in analogy to the topological Hall effect of skyrmions. We theoretically demonstrate the topological spin Hall effect in which a pure transverse spin current is generated from a skyrmion spin texture.

In a bilayer consisting of an insulator and a ferromagnetic metal (FM), interfacial spin-orbit scattering leads to spin mixing of the two conducting channels of the FM, which results in an unconventional anisotropic magnetoresistance (AMR). We theoretically investigate the magnetotransport in such bilayer structures by solving the spinor Boltzmann transport equation with the generalized Fuchs-Sondheimer boundary condition that takes into account the effect of spin-orbit scattering at the interface. We find that the new AMR exhibits a peculiar angular dependence which can serve as a genuine experimental signature. We also determine the dependence of the AMR on film thickness as well as resistivity spin asymmetry of the FM.

A negative oxygen-isotope $({}^{16}\mathrm{O}/{}^{18}\mathrm{O})$ effect (OIE) on the low-temperature tetragonal (LTT) phase-transition temperature $T_{\mathrm{LTT}}$ was observed in ${\mathrm{La}}_{2-x}{\mathrm{Ba}}_x{\mathrm{CuO}}_4\left(x=1/8\right)$ using high-resolution neutron powder diffraction. The corresponding OIE exponent ${\alpha }_{T_{\mathrm{LTT}}}=-0.36\left(5\right)$ has the same sign as ${\alpha }_{T_{\mathrm{so}}}=-0.57\left(6\right)$ found for the spin-stripe order temperature $T_{\mathrm{so}}$. The fact that the LTT transition is accompanied by charge ordering (CO) implies the presence of an OIE also for the CO temperature $T_{\mathrm{co}}$. Furthermore, a temperature-dependent shortening of the $c$ axis with the heavier isotope is observed. These results combined with model calculations demonstrate that anharmonic electron-lattice interactions are essential for all transitions observed in the stripe phase of cuprates.

Recent measurements of the magnetoresistance (MR) of amorphous superconducting thin films in tilted magnetic fields have displayed several surprising experimental details, in particular, a strong dependence of the MR on the field angle at low magnetic fields, which diminishes and then changes sign at large fields. Using a generalized site-bond percolation model, which takes into account both orbital and Zeeman effects of the magnetic field, we show that the resulting MR curves reproduce the main experimental features. Such measurements, accompanied by the corresponding theory, may be crucial in pinpointing the correct theory of the superconductor-insulator transition and of the MR peak in thin disordered films.

The recently observed large Rashba-type spin splitting in the $\mathrm{BiTe}X(X=\mathrm{I},\mathrm{Br},\mathrm{Cl})$ bulk states enables observation of the transition in Fermi surface topology from spindle torus to torus with varying the carrier density and offers an ideal platform for achieving practical spintronic applications and realizing nontrivial phenomena such as topological superconductivity and Majorana fermions. Here we use Shubnikov–de Haas oscillations to investigate the electronic structure of the bulk conduction band of BiTeCl single crystals with different carrier densities. We observe the topological transition of the Fermi surface (FS) from a spindle torus to a torus. The Landau-level fan diagram reveals the expected nontrivial $\pi$ Berry phase for both the inner and outer FSs. Angle-dependent oscillation measurements reveal three-dimensional FS topology when the Fermi level lies in the vicinity of the Dirac point. All the observations are consistent with large Rashba spin-orbit splitting in the bulk conduction band.

The ferromagnetic superconductor UCoGe has been investigated by high field x-ray magnetic circular dichroism (XMCD) at the U $M_{4,5}$ and Co/Ge $K$ edges. The analysis of the branching ratio and XMCD at the U $M_{4,5}$ edges reveals that the U $5f$ electrons count is close to 3. The orbital $(\sim 0.70{\mu }_B)$ and spin $(\sim -0.30{\mu }_B)$ moments of U at 2.1 K and 17 T (H$\parallel$c) have been determined. Their ratio $(\sim -2.3)$ suggests a significant delocalization of the $5f$ electron states. The similar field dependences of the local U/Co and the macroscopic magnetization indicate that the Co moment is induced by the U moment. The XMCD at the Co/Ge $K$ edges reveal the presence of small Co $4p$ and Ge $4p$ orbital moments parallel to the macroscopic magnetization. In addition, the Co $3d$ moment is estimated to be at most of the order of $0.1{\mu }_B$ at 17 T. Our results rule out the possibility of an unusual polarizability of the U and Co moments as well as their antiparallel coupling. We conclude that the magnetism which mediates the superconductivity in UCoGe is driven by U.

A metal-insulator transition (MIT) in ${\mathrm{BiFeO}}_3$ under pressure was investigated by a method combining generalized gradient corrected local density approximation with dynamical mean-field theory (GGA+DMFT). Our paramagnetic calculations are found to be in agreement with the experimental phase diagram: Magnetic and spectral properties of ${\mathrm{BiFeO}}_3$ at ambient and high pressures were calculated for three experimental crystal structures $R3c$, $Pbnm$, and $Pm\overline{3}m$. At ambient pressure in the $R3c$ phase, an insulating gap of 1.2 eV was obtained in good agreement with its experimental value. Both $R3c$ and $Pbnm$ phases have a metal-insulator transition that occurs simultaneously with a high-spin (HS) to low-spin (LS) transition. The critical pressure for the $Pbnm$ phase is 25–33 GPa, which agrees well with the experimental observations. The high-pressure and -temperature $Pm\overline{3}m$ phase exhibits a metallic behavior observed experimentally as well as in our calculations in the whole range of considered pressures and undergoes the LS state at 33 GPa, where a $Pbnm$ to $Pm\overline{3}m$ transition is experimentally observed. The antiferromagnetic GGA+DMFT calculations carried out for the $Pbnm$ structure result in simultaneous MIT and HS-LS transitions at a critical pressure of 43 GPa in agreement with the experimental data.

Fourier transform scanning tunneling spectroscopy (FT-STS) measures the scattering of conduction electrons from impurities and defects, giving information about the electronic structure of both the host material and adsorbed impurities. We interpret such FT-STS measurements in terms of the quasiparticle interference (QPI), here investigating in detail the QPI due to single magnetic impurities adsorbed on a range of representative nonmagnetic host surfaces, and contrasting with the case of a simple scalar impurity or point defect. We demonstrate how the electronic correlations present for magnetic impurities markedly affect the QPI, showing, e.g., a large intensity enhancement due to the Kondo effect, and universality at low temperatures/scanning energies. The commonly used joint density of states interpretation of FT-STS measurements is also considered, and shown to be insufficient in many cases, including that of magnetic impurities.

The all-electron mixed basis $GW$ approach is applied to calculate the electronic structure of rutile ${\mathrm{TiO}}_2$ and ${\mathrm{Ti}}_{0.75}{\mathrm{Nb}}_{0.25}{\mathrm{O}}_2$. The quasiparticle energies are calculated in fine detail, allowing the accurate determination of electronic structures. Although the valence band maximum (VBM) of the pure rutile ${\mathrm{TiO}}_2$ is composed of O $2p$ and Ti $3d$ orbitals, that of ${\mathrm{Ti}}_{0.75}{\mathrm{Nb}}_{0.25}{\mathrm{O}}_2$ is composed of Nb $4d$ orbitals and O $2p$ orbitals together with a few Ti $3d$ orbitals, forming an occupied impurity level, while the conduction band minimum is composed of Nb $4d$ and O $2p$ orbitals. In the calculation of ${\mathrm{Ti}}_{0.75}{\mathrm{Nb}}_{0.25}{\mathrm{O}}_2$, the energy level of the VBM increases, with finite energy gap, in contrast to the local density approximation calculation showing a metallic band structure. The exchange and correlation contributions to the self-energy drastically change in the VBM. The resulting electronic structure is in reasonable agreement with experiments. Based on our results, the $GW$ approximation seems good in describing a not so strongly correlated transition metal oxide system with impurities.

Insulator-to-metal (MI) phase transition in vanadium dioxide $\left(\mathrm{V}{\mathrm{O}}_2\right)$ thin films with controlled lattice distortion was investigated by thermopower measurements. $\mathrm{V}{\mathrm{O}}_2$ epitaxial films with different crystallographic orientations, grown on $\left(0001\right)\alpha \text{−}\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$, $\left(11\overline{2}0\right)\alpha \text{−}\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$, and $\left(001\right)\mathrm{Ti}{\mathrm{O}}_2$ substrates, showed significant decrease of absolute value of Seebeck coefficient $\left(S\right)$ from $\sim 200$ to $23\mu \mathrm{V}{\mathrm{K}}^{-1}$, along with a sharp drop in electrical resistivity $\left(\rho \right)$, due to the transition from an insulator to a metal. The MI transition temperatures observed both in $\rho \left(T_{\rho }\right)$ and $S\left(T_S\right)$ for the $\mathrm{V}{\mathrm{O}}_2$ films systematically decrease with lattice shrinkage in the pseudorutile structure along the $c$ axis, accompanying a broadening of the MI transition temperature width. Moreover, the onset $T_S$, where the insulating phase starts to become metallic, is much lower than the onset $T_{\rho }$. This difference is attributed to the sensitivity of $S$ for the detection of hidden metallic domains in the majority insulating phase, which cannot be detected in $\rho$ measurements. Consequently, $S$ measurements provide a straightforward and excellent approach for a deeper understanding of the MI transition process in $\mathrm{V}{\mathrm{O}}_2$.