Single crystals of solid solutions of ferromagnetic superconductors UCoGe and URhGe were investigated by magnetization and specific-heat measurements. These compounds behave as anisotropic ferromagnets with the easy-magnetization direction along the orthorhombic $c$ axis. The maximum Curie temperature $T_C=20\mathrm{K}$ has been observed in $\mathrm{UC}{\mathrm{o}}_{1-x}\mathrm{R}{\mathrm{h}}_x\mathrm{Ge}$ for $x=0.4$. The main interest of the study was the intriguing behavior in magnetic fields applied along the $b$ axis. The temperature dependence of the $b$-axis magnetic susceptibility exhibits a maximum at a characteristic temperature $T_{\max }$. For low Rh concentrations $\left(x<0.4\right)T_{\max }>T_C$ and the magnetic entropy $S_{\mathrm{mag}}$ at $T_C$ is very low, whereas $T_{\max }=T_C$ and the $S_{\mathrm{mag}}$ at $T_C$ is much higher for $x\ge 0.4$. A metamagneticlike anomaly in magnetization is observed in all compounds, at a critical field which appears on a similar energy scale with the value of the corresponding $T_{\max }$. The character of three paramagnetic regimes which can be distinguished in the T-$x$ and H-$x$ phase diagrams, normal, correlated, and field polarized, is discussed in relation to the field-induced reentrant superconductivity in the parent compounds.

We report the generation of skyrmions under the combined influence of applied perpendicular magnetic fields and electrical currents in Pt/Co/Os/Pt thin films. Under no electrical current, skyrmions are found to nucleate at specific defect sites distributed throughout the film and remain pinned at these sites for small dc electrical currents $\left(<0.05\mathrm{MA}/\mathrm{c}{\mathrm{m}}^2\right)$ . For larger currents $\left(<0.05\mathrm{MA}/\mathrm{c}{\mathrm{m}}^2\right)$, skyrmions can either move or elongate in the direction of the current flow to form stripe domains. We further observed the current-induced generation of skyrmions through two specific processes: (i) stripe domains depinning from defect sites and collapsing into skyrmions, or (ii) skyrmions breaking off from pinned stripes; the rate of these stripe-to-skyrmion creation phenomena is proportional to the applied current density. Additionally, we observe a sudden increase in the skyrmion population immediately after the applied current is removed––a behavior attributed to stripe domains previously elongated by the electrical current collapsing back into skyrmions.

The implementation of ultrasoft pseudopotentials into time-dependent density-functional perturbation theory is detailed for both the Sternheimer approach and the Liouville-Lanczos (LL) method, and equations are presented in the scalar relativistic approximation for periodic solids with finite momentum transfer q. The LL method is applied to calculations of the electron energy loss (EEL) spectrum of face-centered cubic bulk Au both at vanishing and finite q. Our study reveals the richness of the physics underlying the various contributions to the density fluctuation in gold. In particular, our calculations suggest the existence in gold of two quasiseparate $5d$ and $6s$ electron gasses, each one oscillating with its own frequency at, respectively, 5.1 eV and 10.2 eV. We find that the contribution near 2.2 eV comes from $5d\to 6s$ interband transitions modified by the intraband contribution to the real part of the dielectric function, which we call a mixed excitation.

We study the interplay between spin-orbit coupling (SOC) and Coulomb repulsion in a Hubbard model on a decorated honeycomb lattice which leads to a plethora of phases. While a quantum spin Hall insulator is stable at weak Coulomb repulsion and moderate SOC, a semimetallic phase emerges at large SOC in a broad range of Coulomb repulsion. This semimetallic phase has topological properties such as a finite, nonquantized spin Hall conductivity not observed in conventional metals. At large Coulomb repulsion and negligible spin-orbit coupling, electronic correlations stabilize a resonance valence bond (RVB) spin liquid state in contrast to the classical antiferromagnetic state predicted by mean-field theory. Under sufficiently strong SOC, such RVB state is transformed into a magnetic insulator consisting on $S\lesssim 3/2$ localized moments on a honeycomb lattice with antiferromagnetic order and topological features.

We investigate few- and many-body states in half-filled ideal topological insulator flat bands realized by two degenerate Landau levels which experience opposite magnetic fields. This serves as a toy model of flat bands in moiré materials in which valleys have Chern numbers $C=\pm 1$. We argue that although the spontaneously polarized Ising Chern magnet is a natural ground state for repulsive Coulomb interactions, it can be in reasonable energetic competition with correlated Laughlin states of excitons when short-distance corrections to interactions are included. This is because charge neutral excitons in these bands behave effectively as charged particles in ordinary Landau levels. In particular, the Ising Chern magnet is no longer the ground state once the strength of a short-range intravalley repulsion is about $30\%$ larger than the intervalley repulsion. Remarkably, these excitonic Laughlin states feature valley number fractionalization but no charge fractionalization and a quantized charge Hall conductivity identical to the Ising magnet, ${\sigma }_{xy}=\pm e^2/h$, and thus cannot be distinguished from it by ordinary charge transport measurements. The Laughlin state with the highest density of excitons that can be constructed in these bands is an analog of $\nu =1/4$ bosonic Laughlin state and has no valley polarization even though it spontaneously breaks time reversal symmetry.

Negative magnetoresistance is one of the manifestations of the chiral anomaly in Weyl semimetals. The magneto-optical conductivity also shows transitions between Landau levels that are not spaced as in an ordinary electron gas. How are such topological properties modified by interactions and temperature? We answer this question by studying a lattice model of Weyl semimetals with an onsite Hubbard interaction. Such an interacting Weyl semimetal, dubbed as Weyl liquid, may be realized in ${\mathrm{Mn}}_3\mathrm{Sn}$. We solve that model with single-site dynamical mean-field theory. We find that in a Weyl liquid, quasiparticles can be characterized by a quasiparticle spectral weight $Z$, although their lifetime increases much more rapidly as frequency approaches zero than in an ordinary Fermi liquid. The negative magnetoresistance still exists, even though the slope of the linear dependence of the dc conductivity with respect to the magnetic field is decreased by the interaction. At elevated temperatures, a Weyl liquid crosses over to bad-metallic behavior where the Drude peak becomes flat and featureless. We comment on the effect of a Zeeman term.

Electron-electron collisions are known to cause a nonlocal voltage drop in the presence of current flow. The semiphenomenological theory predicts this drop to be opposite to the direction of the current in the ballistic regime. We use a microscopic approach and show that the sign of this drop may be of both signs depending on the temperature and the distance between the source and probe contacts. The change of sign corresponds to the change of the dominant scattering process from head-on collisions to backward scattering of electrons. Our results agree with the experimental data.

Within the effective-mass approximation, we theoretically investigated the electronic and transport properties of 2D semiconductor quantum wires (QWs) with anisotropic effective masses and different orientations with respect to the anisotropic axis. The energy levels in the absence and presence of an external magnetic field are analytically calculated, showing (i) a strong dependence on the spacing of energy levels related to the alignment QW angle and the anisotropy axis, and (ii) for non-null magnetic fields, the quantum Hall edge states are significantly affected by the edge orientation. Moreover, by means of the split-operator technique, we analyzed the time evolution of wave packets in straight and V-shaped anisotropic QWs and compared the transmission probabilities with those of isotropic systems. In the anisotropic case, we found damped oscillations in the average values of velocity in both $x$ and $y$ directions for a symmetric Gaussian wave packet propagating along a straight wide QW, with the oscillation being more evident as the noncollinearity between the group velocity and momentum vectors increases.

X-ray absorption measurements at the U $L_{\mathrm{III}}$, Ru $K$, and Fe $K$ edges are reported for the hidden order (HO) material ${\mathrm{URu}}_{2-x}{\mathrm{Fe}}_x{\mathrm{Si}}_2$ ($x=0$, 0.05, 0.08, 0.10, 0.12, 0.15, and 0.20) as a function of $x$ and temperature $T$. When Fe is substituted for Ru, the local structure about Fe shrinks slightly and the first neighbor Fe-Si bond length decreases by $\approx 0.05\AA$. More importantly excess disorder is observed below 80–100 K (the coherence temperature $T^{*}$) in plots of the Debye-Waller factor ${\sigma }^2$ ($\sigma$ is the width of the pair distribution function); at low $T$ the data deviate from the usual Einstein or correlated-Debye model plots. This excess disorder is most prominent for the Ru-Si bond, and ${\sigma }^2$ actually increases below 80 K. These results suggest a local orthorhombic distortion with $B_{1g}$-like symmetry that develops below 80–100 K. A model that describes these local distortions is presented, and discussed in terms of other measurements that indicate a breaking of fourfold symmetry at low $T$. In addition, the square root of the difference between ${\sigma }^2\left(T\right)$ for the Ru-Si pair and a Debye fit to these data serves as an order parameter for this orthorhombic distortion, in the temperature range below 100 K. This quantity is a length related to $a\text{−}b$, the difference between the $a$ and $b$ lattice constants in the orthorhombic phase, and provides a connection between this distortion and $T^{*}$. X-ray absorption near edge structure (XANES) measurements also show that there are no changes in the edge positions down to 0.1 eV for any edge as a function of $x$, for $T$ in the HO regime.

We study the influence of disorder on the dynamical quantum phase transitions (DQPTs) in the transverse-field anisotropic XY chain after a sudden quench. The formulas to calculate the Loschmidt echo and Fisher zeros of the inhomogeneous XY chain are derived in real space. It is found that new DQPTs appear in the presence of weak disorder for the quenches across the Ising transition and two quantum phase transitions (Ising and anisotropic transitions). As the disorder strength increases, it's demonstrated that the Fisher zeros coalesce to regions on the imaginary axis in the thermodynamic limit. The DQPTs occur at the critical times corresponding to the boundaries of the Fisher zeros regions on the imaginary axis. However, for the quench across the anisotropic transition, the Fisher zeros regions appear even for the very weak disorder. Finally, for the quench within the same phase, the weak disorder is found having no effect on the DQPTs, but the DQPTs will be washed out in the presence of strong disorder.

We identified the spontaneous magnetostriction effects in the chiral magnet ${\mathrm{CrNb}}_3{\mathrm{S}}_6$ with the Dzyaloshinskii-Moriya interaction. The powdered x-ray diffraction patterns indicate a prominent magnetostriction effect at the earth field. The unit-cell volume remains almost constant, as in the so-called Invar effect, below 170 K in the paramagnetic region, where the atomic positions of Nb and S show characteristic changes. Below the magnetic ordering temperature of 127 K, the lattice constants $a$ and $c$ exhibit an opposite temperature dependence. When a finite magnetic field of 1.2 kOe is applied, a magnetostriction appears that counterbalances the spontaneous magnetostriction. The magnetostriction phenomena in ${\mathrm{CrNb}}_3{\mathrm{S}}_6$ were investigated from the viewpoint of the atomic level of the chiral effect.

Using a matrix product state algorithm with infinite boundary conditions, we compute high-resolution dynamic spin and quadrupolar structure factors in the thermodynamic limit to explore the low-energy excitations of isotropic bilinear-biquadratic spin-1 chains. Haldane mapped the spin-1 Heisenberg antiferromagnet to a continuum field theory, the nonlinear sigma model ($\mathrm{NL}\sigma \mathrm{M}$). We find that the $\mathrm{NL}\sigma \mathrm{M}$ fails to capture the influence of the biquadratic term and provides only an unsatisfactory description of the Haldane phase physics. But several features in the Haldane phase can be explained by noninteracting multimagnon states. The physics at the Uimin-Lai-Sutherland point is characterized by multisoliton continua. Moving into the extended critical phase, we find that these excitation continua contract, which we explain using a field-theoretic description. New excitations emerge at higher energies and, in the vicinity of the purely biquadratic point, they show simple cosine dispersions. Using block fidelities, we identify them as elementary one-particle excitations and relate them to the integrable Temperley-Lieb chain.

Fluxonium qubits employ a chain of $N$ Josephson junctions in series. We investigate how the charge-noise induced coherence time $T_2$ of the qubit depends upon $N$, holding fixed the chain's effective capacitance and inductance. The pure dephasing rate decreases with $N$, but the relaxation rate increases. As a result, $T_2$ achieves a maximum. The optimal $N$ can be much smaller than the number typically chosen in experiments, implying that fluxonium qubits can be significantly simplified without compromising performance.

The superconducting $s$-wave state in Weyl semimetals in a strong strain-induced pseudomagnetic field is investigated in a model with local four-fermion interaction. It is found that only the internode pairing is possible in the lowest pseudo-Landau level approximation. Unlike the case of the lowest Landau level in a conventional magnetic field, the corresponding gap equation has only a trivial solution. Nevertheless, superconductivity can be induced via the proximity effect with a usual $s$-wave spin-singlet superconductor. Since a pseudomagnetic field is present necessarily at the surface of a Weyl semimetal, the proximity effect is strongly affected by the pseudomagnetic field. The analysis of such an effect showed that while no gap is opened in the spectrum, the degeneracy of energy levels is lifted. The unique character of the proximity effect in Weyl semimetals can be probed via the density of states, the spectral function, and the tunneling current. The density of states does not vanish at small energies and scales linearly with the pseudomagnetic field strength. This scaling is manifested also in the tunneling current.

Temperature evolutions of the electronic specific heat $C_{\mathrm{e}}\left(T\right)$ [A. Amon et al., Phys. Rev. B 97, 014501 (2018)] and the thermodynamic critical field $B_{\mathrm{c}}\left(T\right)$ [R. Khasanov et al., Phys. Rev. Research 2, 023142 (2020)] of the BeAu superconductor were reconsidered within the framework of the single-gap and two-gap scenarios. The analysis shows that the single-gap approach, by being able to describe satisfactorily the temperature dependence of $C_{\mathrm{e}}\left(T\right)$, fails in the case of $B_{\mathrm{c}}\left(T\right)$. The self-consistent two-gap model, in contrast, is able to describe both thermodynamic quantities by using a similar set of parameters. Our results imply that in some particular cases, such as in the case of BeAu, measurement of one single thermodynamic quantity may not be enough in order to speculate on the nature of the superconducting pairing mechanism.

Quantitative prediction of the gap behavior via first-principles calculations has thus far been restricted to temperature dependence. Here, we present a gap equation developed based on the current-density functional theory for superconductors. This equation is applicable to superconductors immersed in a magnetic field and enables us to quantitatively describe the magnetic-field and temperature dependence of the superconducting gap. We also develop a practical scheme in which the gap equation is solved simultaneously with a proposed relation between the energy gain of the superconducting state at zero magnetic field and that at a finite magnetic field. The presented scheme is applied to aluminum immersed in a magnetic field, and successfully reveals the temperature and magnetic-field dependences of the superconducting gap. The critical magnetic field thus obtained shows good agreement with experimental results.

We present numerical evidence for the existence of an extended valence bond solid (VBS) phase at $T=0$ in the kagome $S=1/2$ Heisenberg antiferromagnet with ferromagnetic further-neighbor interactions. The VBS is located at the boundary between two magnetically ordered regions and extends close to the nearest-neighbor Heisenberg point. It exhibits a diamondlike singlet covering pattern with a 12-site unit cell. Our results suggest the possibility of a direct, possibly continuous, quantum phase transition from the neighboring $\mathbf{q}=0$ coplanar magnetically ordered phase into the VBS phase. Moreover, a second phase which breaks lattice symmetries, and is of likely spin-nematic type, is found close to the transition to the ferromagnetic phase. The results have been obtained using large-scale numerical exact diagonalization. We discuss implications of our results on the nature of the nearest-neighbor Heisenberg antiferromagnet.

The discovery of superconductivity in Sr-doped ${\mathrm{NdNiO}}_2$ is a crucial breakthrough in the long pursuit for nickel oxide materials with electronic and magnetic properties similar to those of the cuprates. ${\mathrm{NdNiO}}_2$ is the infinite-layer member of a family of square-planar nickelates with general chemical formula ${\mathrm{R}}_{n+1}{\mathrm{Ni}}_n{\mathrm{O}}_{2n+2}$ (R = La, Pr, Nd, $n=2,3,...\infty$). In this Rapid Communication, we investigate superconductivity in the trilayer member of this series (${\mathrm{R}}_4{\mathrm{Ni}}_3{\mathrm{O}}_8$) using a combination of first-principles and $t-J$ model calculations. ${\mathrm{R}}_4{\mathrm{Ni}}_3{\mathrm{O}}_8$ compounds resemble cuprates more than ${\mathrm{RNiO}}_2$ materials in that only Ni-$d_{x^2-y^2}$ bands cross the Fermi level, they exhibit a largely reduced charge transfer energy, and as a consequence superexchange interactions are significantly enhanced. We find that the superconducting instability in doped ${\mathrm{R}}_4{\mathrm{Ni}}_3{\mathrm{O}}_8$ compounds is considerably stronger with a maximum gap about four times larger than that in ${\mathrm{Sr}}_{0.2}{\mathrm{Nd}}_{0.8}{\mathrm{NiO}}_2$.

Ferroelectriclike metals are a relatively rare class of materials that have ferroelectriclike distortion and metallic conductivity. ${\mathrm{LiOsO}}_3$ is the first demonstrated and the most investigated ferroelectriclike metal. The presence of free carriers makes them difficult to be studied by traditional ferroelectric techniques. In this paper, using symmetry analysis and first-principles calculations, we demonstrate that the ferroelectriclike transition of ${\mathrm{LiOsO}}_3$ can be probed by a kind of electrical transport method based on nonlinear Hall effect. The Berry curvature dipole exists in the ferroelectriclike phase and it can lead to a measurable nonlinear Hall conductance with a conventional experimental setup. However, the symmetry of the paraelectriclike phase ${\mathrm{LiOsO}}_3$ vanishes the Berry curvature dipole. The Berry curvature dipole shows a strong dependence on the polar displacement, which might be helpful for the detection of polar order. The nonlinear Hall effect provides an effective method for the detection of phase transition in the study of the ferroelectriclike metals and promotes them to be applied in ferroelectriclike electronic devices.