Exotic conditions for the existence and evolution of nonlinear spin ensembles (domain walls, spin solitons, skyrmions) in molecular-based magnets are incarnated in the macroscopic response of magnetization corresponding to collective stochastic behavior. The molecular ferrimagnet ${\mathrm{K}}_{0.4}\left[\mathrm{Cr}{\left(\mathrm{CN}\right)}_6\right]\left[\mathrm{Mn}\left(\mathrm{R}/\mathrm{S}\right)-\mathrm{pn}\right]\left(\mathrm{R}/\mathrm{S}\right)-\mathrm{pn}{\mathrm{H}}_{0.6}$ manifests three types of magnetic relaxation: (a) continuous decay of magnetic moment, (b) stepwise relaxation by stochastic magnetization jumps, and (c) a single jump of magnetization in threshold magnetic field. Continuous relaxation at 20–50 K is provided by domain wall movement described in the frames of a strong pinning model, while a low-temperature continuous component of relaxation does not follow this model. Stepwise stochastic relaxation was observed below 8 K in both a sweeping reverse magnetic field and a stationary reverse magnetic field. Statistical treatment of the postponed magnetization jumps revealed a multimodal amplitude distribution of stochastic magnetization jumps corresponding to magnetic moment transitions between few clear distinguishable levels. Spectral density of magnetization jumps in a stationary magnetic field corresponds to white noise, while spectral density in a sweeping magnetic field manifests pink noise $\sim 1/f$ provided by self-organized criticality. Postponed emission of magnetic noise in the ${10}^{-6}-5\times {10}^{-1}\mathrm{Hz}$ frequency range was observed in stationary conditions in contrast to Barkhausen noise.

We use the Boltzmann transport theory in the relaxation time approximation to describe the thermal transport of spin waves in a ferromagnet. By treating spin waves as magnon excitations we are able to compute analytically and numerically the coefficients of the constitutive thermomagnetic transport equations. As a main result, we find that the absolute thermomagnetic power coefficient ${\epsilon }_M$, relating the gradient of the potential of the magnetization current and the gradient of the temperature, in the limit of low temperature and low field, is a constant ${\epsilon }_M=-0.6419k_B/{\mu }_B$. The theory correctly describes the low-temperature and magnetic-field dependencies of spin Seebeck experiments. Furthermore, the theory predicts that in the limit of very low temperatures the spin Peltier coefficient ${\mathrm{\Pi }}_M$, relating the heat and the magnetization currents, tends to a finite value which depends on the amplitude of the magnetic field. This indicates the possibility to exploit the spin Peltier effect as an efficient cooling mechanism in cryogenics.

In this paper, we report experimental evidence of anomalous and planar Righi-Leduc effects on $\mathrm{NiFe}$. The Righi-Leduc effect is the thermal analog of the Hall effect, in which the electric current is replaced by the heat current and the electric field by the temperature gradient. When the material is ferromagnetic, it is well known that there are two other contributions to the Hall voltage which depend on the orientation of the magnetization. These two extra contributions are called the anomalous Hall effect when the magnetization is out of the plane of the sample and the planar Hall effect when the magnetization is in the plane of the sample. In the same way, an anomalous and a planar Righi-Leduc effects are shown to appear when a transverse temperature gradient is generated by a heat current.

The critical behavior of the cubic helimagnet FeGe was obtained from isothermal magnetization data in very close vicinity of the ordering temperature. A thorough and consistent scaling analysis of these data revealed the critical exponents $\beta =0.368,\gamma =1.382$, and $\delta =4.787$. The anomaly in the specific heat associated with the magnetic ordering can be well described by the critical exponent $\alpha =-0.133$. The values of these exponents corroborate that the magnetic phase transition in FeGe belongs to the isotropic 3D-Heisenberg universality class. The specific heat data are well described by ab initio phonon calculations and confirm the localized character of the magnetic moments.

We present a path integral Monte Carlo study of the global superfluid fraction and local superfluid density in cylindrically symmetric reservoirs of liquid ${}^4\mathrm{He}$ separated by nanoaperture arrays. The superfluid response to both translations along the axis of symmetry (longitudinal response) and rotations about the cylinder axis (transverse response) are computed, together with radial and axial density distributions that reveal the microscopic inhomogeneity arising from the combined effects of the confining external potential and the ${}^4\mathrm{He}{-}^4\text{He}$ interatomic potentials. We make a microscopic determination of the length scale of decay of superfluidity at the radial boundaries of the system by analyzing the local superfluid density distribution to extract a displacement length that quantifies the superfluid mass displacement away from the boundary. We find that the longitudinal superfluid response is reduced in reservoirs separated by a septum containing sufficiently small apertures compared to a cylinder with no intervening aperture array, for all temperatures below $T_{\lambda }$. For a single aperture in the septum, a significant drop in the longitudinal superfluid response is seen when the aperture diameter is made smaller than twice the empirical temperature-dependent ${}^4\mathrm{He}$ healing length, consistent with the formation of a weak link between the reservoirs. Increasing the diameter of a single aperture or the number of apertures in the array results in an increase of the superfluid density toward the expected bulk value.

We carried out ${}^{101}\mathrm{Ru}$ nuclear quadrupole resonance (NQR) measurement on superconducting (SC) ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ under zero magnetic field $(H=0)$ and found that the nuclear spin-spin relaxation rate $1/T_2$ is enhanced in the SC state. The $1/T_2$ measurement in the SC state under $H=0$ is effective for detecting slow magnetic fluctuations parallel to the quantized axis of the nuclear spin. Our results indicate that low-energy magnetic fluctuations perpendicular to the ${\mathrm{RuO}}_2$ plane emerge when the superconductivity sets in, which is consistent with the previous ${}^{17}\mathrm{O}\text{-NQR}$ result that the nuclear spin-lattice relaxation rate $1/T_1$ of the in-plane O site exhibits anomalous behavior in the SC state. The enhancement of the magnetic fluctuations in the SC state is unusual and suggests that the fluctuations are related to the unconventional SC pairing. We suggest that this phenomenon is a consequence of the spin degrees of freedom of the spin-triplet pairing.

We study the effects of charge self-consistency within the combination of density functional theory (DFT; wien2k) with dynamical mean field theory (DMFT; w2dynamics) in a basis of maximally localized Wannier orbitals. Using the example of two cuprates, we demonstrate that even if there is only a single Wannier orbital with fixed filling, a noteworthy charge redistribution can occur. This effect stems from a reoccupation of the Wannier orbital in k-space when going from the single, metallic DFT band to the split, insulating Hubbard bands of DMFT. We analyze another charge self-consistency effect beyond moving charge from one site to another: the correlation-enhanced orbital polarization in a freestanding layer of ${\mathrm{SrVO}}_3$.

We show that a series of transition metals with strained body-centered cubic lattice—W, Ta, Nb, and Mo—hosts surface states that are topologically protected by mirror symmetry and, thus, exhibits nonzero topological invariants. These findings extend the class of topologically nontrivial systems by topological crystalline transition metals. The investigation is based on calculations of the electronic structures and of topological invariants. The signatures of a Dirac-type surface state in W(110), e.g., the linear dispersion and the spin texture, are verified. To further support our prediction, we investigate Ta(110) both theoretically and experimentally by spin-resolved inverse photoemission: unoccupied topologically nontrivial surface states are observed.

The neutral excitations in two-dimensional electron systems (2DES) confined at ${\mathrm{Mg}}_x{\mathrm{Zn}}_{1-x}\mathrm{O}/\text{ZnO}$ heterojunctions were studied by a resonant inelastic light scattering (ILS) technique. At zero magnetic field the ILS spectra of intra- and intersubband collective excitations are detected in the wavelength range close to the ZnO direct band gap. The intersubband spectrum is represented by collective modes of charge- and spin-density excitations along with single-particle excitation continuum. The energies of these excitations were characterized on the series of samples and shown to grow sublinearly with the two-dimensional electron density. The intrasubband 2D plasmon was observed at plasma frequency and its square-root momentum dispersion was ascertained. Strong electron-electron correlations are manifested in the energy structure of intersubband excitations. Depolarization and excitonic contributions to the charge- and spin-density excitation energies are extracted in the wide range of 2D-electron densities. The negative excitonic contribution dominates among the many-particle energy terms and results in an unusual hierarchy of 2DES intersubband collective excitation energies.

We show that a local measurement of temperature and voltage for a quantum system in steady state, arbitrarily far from equilibrium, with arbitrary interactions within the system, is unique when it exists. This is interpreted as a consequence of the second law of thermodynamics. We further derive a necessary and sufficient condition for the existence of a solution. In this regard, we find that a positive temperature solution exists whenever there is no net population inversion. However, when there is a net population inversion, we may characterize the system with a unique negative temperature. Voltage and temperature measurements are treated on an equal footing: They are simultaneously measured in a noninvasive manner, via a weakly coupled thermoelectric probe, defined by requiring vanishing charge and heat dissipation into the probe. Our results strongly suggest that a local temperature measurement without a simultaneous local voltage measurement, or vice versa, is a misleading characterization of the state of a nonequilibrium quantum electron system. These results provide a firm mathematical foundation for voltage and temperature measurements far from equilibrium.

A nonperturbative interaction of an electronic system with a laser field can substantially modify its physical properties. In particular, in two-dimensional (2D) materials with a lack of inversion symmetry, the achievement of a regime of strong light-matter coupling allows direct optical tuning of the strength of the Rashba spin-orbit interaction (SOI). Capitalizing on these results, we build a theory of the dynamical conductivity of a 2D electron gas with both Rashba and Dresselhaus SOIs coupled to an off-resonant high-frequency electromagnetic wave. We argue that strong light-matter coupling modifies qualitatively the dispersion of the electrons and can be used as a powerful tool to probe and manipulate the coupling strengths and adjust the frequency range where optical conductivity is essentially nonzero.

We discuss the low-energy physics of the three-orbital Anderson impurity model with the Coulomb interaction term of the Kanamori form which has orbital SO(3) and spin SU(2) symmetry and describes systems with partially occupied $t_{2g}$ shells. We focus on the case with two electrons in the impurity that is relevant to Hund's metals. Using the Schrieffer-Wolff transformation we derive an effective Kondo model with couplings between the bulk and impurity electrons expressed in terms of spin, orbital, and orbital quadrupole operators. The bare spin-spin Kondo interaction is much smaller than the orbit-orbit and spin-orbital couplings or is even ferromagnetic. Furthermore, the perturbative scaling equations indicate faster renormalization of the couplings related to orbital degrees of freedom compared to spin degrees of freedom. Both mechanisms lead to a slow screening of the local spin moment. The model thus behaves similarly to the related quantum impurity problem with a larger SU(3) orbital symmetry (Dworin-Narath interaction) where this was first observed. We find that the two problems actually describe the same low-energy physics since the SU(3) symmetry is dynamically established through the renormalization of the splittings between the orbital and quadrupole coupling constants to zero. The perturbative renormalization group results are corroborated with the numerical-renormalization group (NRG) calculations. The dependence of spin Kondo temperatures and orbital Kondo temperatures as a function of interaction parameters, the hybridization, and the impurity occupancy is calculated and discussed.

We investigate the Peierls-Feynman-Bogoliubov variational principle to map Hubbard models with nonlocal interactions to effective models with only local interactions. We study the renormalization of the local interaction induced by nearest-neighbor interaction and assess the quality of the effective Hubbard models in reproducing observables of the corresponding extended Hubbard models. We compare the renormalization of the local interactions as obtained from numerically exact determinant quantum Monte Carlo to approximate but more generally applicable calculations using dual boson, dynamical mean field theory, and the random phase approximation. These more approximate approaches are crucial for any application with real materials in mind. Furthermore, we use the dual boson method to calculate observables of the extended Hubbard models directly and benchmark these against determinant quantum Monte Carlo simulations of the effective Hubbard model.

Depopulation of the ${\mathrm{Mn}}^{2+}$ state in ZnO:Mn upon illumination, monitored by quenching of the ${\mathrm{Mn}}^{2+}$ EPR signal intensity, was observed at temperatures below 80 K. ${\mathrm{Mn}}^{2+}$ photoquenching is shown to result from the ${\mathrm{Mn}}^{2+}\to {\mathrm{Mn}}^{3+}$ ionization transition, promoting one electron to the conduction band. Temperature dependence of this process indicates the existence of an energy barrier for electron recapture of the order of 1 meV. $\mathrm{GGA}+U$ calculations show that after ionization of ${\mathrm{Mn}}^{2+}$ a moderate breathing lattice relaxation in the 3+ charge state occurs, which increases energies of $d$(Mn) levels. At its equilibrium atomic configuration, ${\mathrm{Mn}}^{3+}$ is metastable since the direct capture of photoelectron is not possible. The metastability is mainly driven by the strong intrashell Coulomb repulsion between $d$(Mn) electrons. Both the estimated barrier for electron capture and the photoionization energy are in good agreement with the experimental values.

Motivated by experimental observation of the Kondo-enhanced Andreev transport [R. S. Deacon et al., Phys. Rev. B 81, 121308(R) (2010)] in a hybrid normal metal–quantum dot–superconductor (N-QD-S) device, we theoretically study the Kondo effect in such a device and clarify the different roles played by the normal and superconducting leads. Due to the Andreev reflection in a QD-S system, a pair of Andreev energy levels form in the superconducting gap, which is able to carry the magnetic moment if the ground state of the QD is a magnetic doublet. In this sense, the Andreev energy levels play a role of effective impurity levels. When the normal lead is coupled to the QD-S system, on the one hand, the Andreev energy levels broaden to form the so-called Andreev bound states (ABSs); on the other hand, it can screen the magnetic moment of the ABSs. By tuning the couplings between the QD and the normal (superconducting) leads, the ABSs can simulate the Kondo, mixed-valence, and even empty orbit regimes of the usual single-impurity Anderson model. The above picture is confirmed by the Green's function calculation of the hybrid N-QD-S Anderson model and is also able to explain qualitatively experimental phenomena observed by Deacon et al. These results can further stimulate related experimental study in the N-QD-S systems.

The layered ternary compound ${\mathrm{TaIrTe}}_4$ has been predicted to be a type-II Weyl semimetal with only four Weyl points just above the Fermi energy. Performing magnetotransport measurements on this material we find that the resistivity does not saturate for fields up to 70 T and follows a $\rho \sim B^{1.5}$ dependence. Angular-dependent de Haas–van Alphen measurements reveal four distinct frequencies. Analyzing these magnetic quantum oscillations by use of density functional theory calculations we establish that in ${\mathrm{TaIrTe}}_4$ the Weyl points are located merely $\sim 40–50$ meV above the chemical potential.

We consider indirect excitons generated at the ring-shaped boundaries between electron- and hole-rich regions in semiconductor quantum wells (QW's). We show theoretically that the in-plane translational motion of the excitons is confined in the radial direction. The confinement potential results from the electrostatic interaction of the exciton dipole moment with the in-plane electric field induced at the boundary by the macroscopic charge separation. Our results directly apply to the external ring and the localized bright spots (LBS) observed in the photoluminescense (PL) pattern of indirect excitons.

We report on the observation of optical activity of quantum wells resulting in the conversion of the light polarization state controlled by the light propagation direction. The polarization conversion is detected in reflection measurements. We show that a pure $s$-polarized light incident on a quantum well is reflected as an elliptically polarized wave. The signal is drastically enhanced in the vicinity of the light-hole exciton resonance. We show that the polarization conversion is caused by the spin-orbit splitting of the light hole states and the birefringence of the studied structure. The bulk inversion asymmetry constant ${\beta }_h\approx 0.14$ eV Å is determined for the ground light hole subband in a 10 nm ZnSe/ZnMgSSe quantum well.