An unbiased one-dimensional weak link between two terminals, subjected to the Rashba spin-orbit interaction caused by an AC electric field which rotates periodically in the plane perpendicular to the link, is shown to inject spin-polarized electrons into the terminals. The injected spin polarization has a DC component along the link and a rotating transverse component in the perpendicular plane. In the low-rotation-frequency regime, these polarization components are proportional to the frequency. The DC component of the polarization vanishes for a linearly polarized electric field.

We develop a numerically exact method for the summation of irreducible Feynman diagrams for fermionic self-energy in the thermodynamic limit. The technique, based on the diagrammatic determinant Monte Carlo and its recent extension to connected diagrams, allows us to reach high $(\sim 10)$ orders of the weak-coupling expansion for the self-energy of the two-dimensional Hubbard model. Access to high orders reveals a nontrivial analytic structure of the self-energy and enables its controlled reconstruction with arbitrary momentum resolution in the nonperturbative regime of essentially strong correlations, which has recently been reached with ultracold atoms in optical lattices.

Spin-dependent photon echoes in combination with pump-probe Kerr rotation are used to study the microscopic electron spin transport in a CdTe/(Cd,Mg)Te quantum well in the hopping regime. We demonstrate that, independent of the particular spin relaxation mechanism, the hopping of resident electrons leads to a shortening of the photon echo decay time, while the transverse spin relaxation time evaluated from pump-probe transients increases due to motional narrowing of spin dynamics in the fluctuating effective magnetic field of the lattice nuclei.

In the context of many-body interacting systems described by a topological Hamiltonian, we investigate the robustness of the Chern number with respect to different sources of error in the self-energy. In particular, we analyze the importance of non-local (momentum dependent) vs. local contributions to the self-energy and show that the local self-energy provides a qualitative description of the topological phase diagrams of many-body interacting systems, whereas the explicit momentum-dependence constitutes a correction to the exact location of the phase transition. For the latter, we propose a statistical analysis, on the basis of which we develop a stochastic upper bound for the uncertainty of the Chern number as a function of the amount of momentum-dependence of the self-energy. We apply this analysis to the Haldane-Hubbard model and discuss the implications of our results for a general class of many-body interacting systems.

Precise heat capacity $C(T,H)$ and electrical resistivity ρ$(T,H)$ down to 0.1 K and magnetic fields up to 10 T on EuPtSi is presented in detail. A detailed $\left(H\text{−}T\right)$ phase diagram constructed, based on the $C\left(H\right)$ and ρ$\left(H\right)$ data, shows the presence of the $A$ phase in EuPtSi, below a paramagnet-to-antiferromagnet/helimagnet transition $T_{\mathrm{N}}\sim 4.1\mathrm{K}$. A clear entropy minimum across the $A$ phase confirms the thermodynamic stability of the phase. Universal scaling performed using the change in magnetic entropy curves evidences the first-order transition below $T_{\mathrm{N}}$ for magnetic fields lower than $H<2.75$ T. The scaling analysis further suggests a field-induced second-order transition from paramagnet to a field-polarized state above $H$ > 3 T, indicating a possible tricritical point at the boundary between a first-order and the field-induced second-order phase transition at $H_{\mathrm{TCP}}\sim 2.75\mathrm{T}$ and $T_{\mathrm{TCP}}\sim 3.2\mathrm{K}$. The critical exponents $\beta \sim 0.5\pm 0.05,\gamma \sim 0.97\pm 0.04$, and $\delta \sim 3.23\pm 0.14$ suggest that the EuPtSi belongs to a mean-field universality class, different from MnSi, which belongs to the universality class of the tricritical mean-field model.

Ferroelectric based piezoceramics exhibiting large electromechanical response are used as sensors, actuators, and transducers in wide-ranging applications spanning sectors like space, defense, medical diagnostics, etc. In general, the large piezoelectric response in ferroelectric solid solutions is associated with a composition driven interferroelectric instability, commonly known as a morphotropic phase boundary (MPB). Here, we show that MPB is not necessarily required to achieve electromechanical response equivalent to, or even more than what can be achieved in MPB based ferroelectric solid solutions. We show this on two ferroelectric solid solution systems, namely, $\left(1–x\right)\mathrm{PbTi}{\mathrm{O}}_3-\left(x\right)\mathrm{Bi}\left(\mathrm{N}{\mathrm{i}}_{1/2}\mathrm{H}{\mathrm{f}}_{1/2}\right){\mathrm{O}}_3$ (PT-BNH) and $\left(\mathrm{Bi},\mathrm{La}\right)\mathrm{Fe}{\mathrm{O}}_3-\mathrm{PbTi}{\mathrm{O}}_3$ (BF-PT:La) which show large piezoelectric response ($d_{33}\sim 450\mathrm{pC}/\mathrm{N}$) and extraordinarily high electrostrain of ∼1.3%, respectively. Although analogous to the conventional MPB systems, the critical compositions of these two alloys mimic a two-phase structural state (cubic + tetragonal), detailed analysis that suggests that it is not so. The cubic phase is rather a manifestation of short correlation length of the tetragonal regions and appears when the system is compositionally driven from a normal ferroelectric state to a relaxor ferroelectric state. This proves that, in contrast to conventional MPB systems, the large electromechanical response of the critical compositions of PT-BNH and BF-PT:La is not due to interferroelectric instability enabled polarization rotation. In the absence of the MPB, the sole contributor to large electromechanical response is a process associated with domain wall motion, large local polarization, and (non-MPB) lattice softening. The generalized ideas derived from our investigation offer scope for expanding the basket of high-performance piezoelectric materials by exploring solid solutions outside of the MPB framework.

In situ high-pressure Brillouin light scattering experiments along loading-unloading paths are used to investigate the compressibility of vitreous silica. Below 9 GPA, the equation of state obtained from the sound velocities corrected for dispersion agrees with volume measurements. Conversely, huge anelastic effects are observed in the range 10–60 GPa, unveiling the reversible transformation from the fourfold-coordinated structure to the sixfold one. The associated density changes correlate with the average Si coordination number. Decompression curves from above 20 GPa reveal abrupt backward coordination changes around 10–15 GPa and significant hysteresis. Contrary to common wisdom, the residual densification of the recovered silica samples can be figured out from changes in elastic properties along pressure cycles, ruling out a plastic description of the latter process.

Germanium sulfide (GeS) and germanium selenide (GeSe) are layered orthorhombic crystals whose structure bears a strong resemblance with that of black phosphorus and, additionally, are expected to exhibit high piezoelectricity in the few layer domain. In this work, we investigate the Raman properties of exfoliated GeS and GeSe and show that their edges exhibit unusual polarized Raman features that were first observed in black phosphorus. The results include the activation in the spectra of otherwise not allowed modes at the edges of the sample, depending on the crystallographic direction of the edge and the polarization configuration used in the measurements. These features are attributed to atomic rearrangements at the crystal terminations, as well as their impact on phonon symmetries, similar to the case of black phosphorus. Our conclusions are further corroborated by using density functional theory and suggest that edge rearrangements, which will have an impact on the mechanical, electronic, and chemical properties of devices, is a general phenomenon of orthorhombic layered structures.

We have used muon-spin rotation, heat capacity, and x-ray diffraction measurements in combination with density functional theory and dipole field calculations to investigate the crystal and magnetic structure of ${\mathrm{FeTi}}_2{\mathrm{O}}_5$. We observe a long-range ordered state below $T_{\mathrm{N}}=41.8\left(5\right)\mathrm{K}$ with indications of significant correlations existing above this temperature. We determine candidate muon stopping sites in this compound, and find that our data are consistent with the spin Jahn-Teller driven antiferromagnetic ground state with $\mathit{k}=(1/2,1/2,0)$ reported for ${\mathrm{CoTi}}_2{\mathrm{O}}_5$ ($T_{\mathrm{N}}=26\mathrm{K}$). By comparing our data with calculated dipolar fields we can restrict the possible moment size and directions of the ${\mathrm{Fe}}^{2+}$ ions.

Layered $\mathrm{V}{\mathrm{I}}_3$ has emerged as a new two-dimensional (2D) van der Waals bonded ferromagnetic semiconductor, which is an important class of materials for spintronics applications. However, the origin of ferromagnetism in $\mathrm{V}{\mathrm{I}}_3$ is still missing. In this work, we systematically investigate the anisotropy of magnetic properties and entropy change in a $\mathrm{V}{\mathrm{I}}_3$ single crystal. Based on the measurement of the angular-dependent magnetization, there is a weak sixfold in-plane symmetry and fourfold out-of-plane symmetry, indicating $\mathrm{V}{\mathrm{I}}_3$ possesses weak in-plane anisotropy and large out-of-plane anisotropy. Meanwhile, anisotropy of magnetic entropy change $\mathrm{\Delta }{S}_M\left(T,H\right)$ is further studied by the isothermal magnetization, showing that the difference of $-\mathrm{\Delta }{S}_M^{\max }$ between the $c$ axis and $ab$ plane reaches a maximum value $\sim 0.95\mathrm{J}\mathrm{k}{\mathrm{g}}^{-1}{\mathrm{K}}^{-1}$ with a magnetic field change of 4.5 T. With the scaling analysis of $\mathrm{\Delta }{S}_M$, the critical exponents can be obtained and indicate a crossover between 2D-Ising and tricritical mean-field magnetic interactions, and the rescaled $\mathrm{\Delta }{S}_M\left(T,H\right)$ curves fall onto a universal curve, demonstrating a second-order type of magnetic transition.

We report results of studying the magnetization, specific heat, and thermal expansion of a single crystal with nominal composition $\left({\mathrm{Mn}}_{1-x}{\mathrm{Fe}}_x\right)\mathrm{Si}$ with $x=0.15$. We found no thermodynamic evidence in favor of a second-order phase transition in this material. The trajectory corresponding to the present composition of (MnFe)Si is a critical one, i.e., approaching quantum critical point at lowering temperature, but some properties may feel the cloud of helical fluctuations bordering the phase transition line.

We use micromagnetic modeling to demonstrate the operation of graded index lenses designed to steer forward-volume magnetostatic spin waves by 90 and 180 degrees. The graded index profiles require the refractive index to diverge in the lens center, which, for spin waves, can be achieved by modulating the saturation magnetization or external magnetic field in a ferromagnetic film by a small amount. We also show how the ${90}^{\circ }$ lens may be used as a beam divider. Finally, we analyze the robustness of the lenses to deviations from their ideal profiles.

We study theoretically the proximity effect in a ferromagnetic semiconductor with the Rashba spin-orbit interaction. The interplay between the exchange potential and the spin-orbit interactions enriches the symmetry variety of Cooper pairs depending on degree of disorder in a ferromagnet. In the ballistic limit, spin-singlet $s$-wave Cooper pairs are the most dominant in the presence of strong spin-orbit interaction because the spin-momentum locking stabilizes a Cooper pair consisting of two electrons of time-reversal partner to each other. We will show that the spin-orbit interactions generate equal-spin-triplet $p$-wave pairs. In the dirty regime, on the other hand, equal-spin-triplet $s$-wave pairs are dominant because random impurity potentials release the locking. The exchange splitting in the conduction band causes the imbalance between two equal-spin pairing components. In a half-metallic ferromagnet, only an equal-spin pairing component survives and carries the spin-polarized supercurrent. We discuss the effects of the spin-orbit interaction on the Josephson current.

The effect of hydrostatic pressure ($P$) on charge density waves (CDWs) in ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{\mathrm{y}}$ has recently been controversial. Using NMR, we find that both the short-range CDW in the normal state and the long-range CDW in high fields are, at most, slightly weakened at $P=1.9$ GPa. This result is in contradiction with x-ray-scattering results finding complete suppression of the CDW at $\approx 1$ GPa and we discuss possible explanations of this discrepancy. Quantitative analysis, however, shows that the NMR data are not inconsistent with a disappearance of the CDW on a larger pressure scale, typically $\approx 10$–20 GPa. We also propose a simple model reconciling transport data with such a hypothesis, provided the pressure-induced change in doping is taken into account. We conclude that it is therefore possible that most of the spectacular increase in $T_c$ upon increasing pressure up to $\approx 15$ GPa arises from a concomitant decrease of CDW strength.

Results are presented for the quench dynamics of a clean and interacting electron system, where the quench involves varying the strength of the attractive interaction along arbitrary quench trajectories. The initial state before the quench is assumed to be a normal electron gas, and the dynamics is studied in a regime where long-range order is absent, but nonequilibrium superconducting fluctuations need to be accounted for. A quantum kinetic equation using a two-particle irreducible formalism is derived. Conservation of energy, particle number, and momentum emerge naturally, with the conserved currents depending on both the electron Green's functions and the Green's functions for the superconducting fluctuations. The quantum kinetic equation is employed to derive a kinetic equation for the current, and the transient optical conductivity relevant to pump-probe spectroscopy is studied. The general structure of the kinetic equation for the current is also justified by a phenomenological approach that assumes model F in the Halperin-Hohenberg classification, corresponding to a nonconserved order parameter coupled to a conserved density. Linear response conductivity and the diffusion coefficients in thermal equilibrium are also derived, and connections with Aslamazov-Larkin fluctuation corrections are highlighted. Results are also presented for the time evolution of the local density of states. It is shown that Andreev scattering processes result in an enhanced density of states at low frequencies. For a quench trajectory corresponding to a sudden quench to the critical point, the density of states is shown to grow in a manner where the time after the quench plays the role of the inverse detuning from the critical point.

${\mathrm{SrTiO}}_3$ is a unique example of a system which exhibits both quantum paraelectricity and superconductivity. Thus, it is expected that the superconducting state is closely related to the intrinsic ferroelectric instability. Indeed, recent experiments suggest existence of a coexistent phase of superconductivity and ferroelectricity in Ca-substituted ${\mathrm{SrTiO}}_3$. In this paper, we propose that ${\mathrm{SrTiO}}_3$ can be a platform of the ferroelectric superconductivity, which is characterized by a ferroelectric transition in the superconducting state. By analyzing a multiorbital model for $t_{2g}$ electrons, we show that the ferroelectric superconductivity is stabilized through two different mechanisms which rely on the presence of the spin-orbit coupling. First, the ferroelectric superconducting state is stabilized in the dilute carrier density regime due to a ferroelectricity-induced Lifshitz transition. Second, it is stabilized under a magnetic field independent of the carrier density. The importance of the multiorbital or multiband nature for the ferroelectric superconductivity is clarified. Then, we predict a topological Weyl superconducting state in the ferroelectric superconducting phase of ${\mathrm{SrTiO}}_3$.

The discovery of superconductivity at 203 K in ${\mathrm{SH}}_3$ is an important step toward higher values of critical temperature $T_c$. Predictions based on state-of-the-art density functional theory for the electronic structure, including one preceding experimental confirmation, showed the mechanism to be the electron-phonon interaction. This was confirmed in optical spectroscopy measurements. In the range of photon energies between $\sim 450$ and 600 meV in ${\mathrm{SH}}_3$, the reflectance in the superconducting state is below that in its normal state. This difference decreases as temperature approaches $T_c$. Decreasing absorption with increasing temperature is opposite to what is expected in ordinary metals. Such an anomalous behavior can be traced back to the energy dependence of the superconducting density of states, which is highly peaked at the energy gap value $\mathrm{\Delta }$ but decays back to the constant normal state value as energy is increased, on a scale of a few $\mathrm{\Delta }$, or by increasing temperature towards $T=T_c$. The process of phonon-assisted optical absorption is encoded with a knowledge of the temperature dependence of $\mathrm{\Delta }$, which is also the order parameter characteristic of the superconducting state. Should the energy of the phonon involved be very large, of order 200 meV or more, this process offers the possibility of observing the closing of the superconducting order parameter with temperature at correspondingly very large energies. The very recent experimental observation of a $T_c\simeq 250$ K in ${\mathrm{LaH}}_{10}$ has further heightened interest in the hydrides. Here, we compare the relevant phonon structure seen in optics with related features in the real and imaginary part of the frequency dependent gap, the quasiparticle density of states, the reflectance, the absorption, and the optical scattering rate. The phonon structures all carry information on the $T_c$ value and the temperature dependence of the order parameter, and can be used to confirm that the mechanism involved in superconductivity is the electron-phonon interaction.

Explicit, analytical forms are obtained for the elastic constants arising from the phase field crystal (PFC) model in which the dependence of the constants on proxies for the temperature and density in the PFC model is clearly exhibited. The expressions indicate that the elastic constants are multivalued in certain temperature ranges corresponding to regions where two or more phases coexist, in agreement with experimental observations. The analytical formulas for the variation of the elastic constants with density agree with those obtained using a computational approach in [Wang, Huang, and Liu, Phys. Rev. B 97, 144112 (2018)] but only in certain regimes. Specifically, if it is assumed that only the bcc state is present, then our formulas agree with the numerical results presented in [Wang, Huang, and Liu, Phys. Rev. B 97, 144112 (2018)]. In general, the bcc state is not the only phase present and, if other phases are taken into account, then the results differ in those regions where alternative states are favored energetically. Similar conclusions hold if one looks at temperature dependence instead of density variation.