We report magnetotransport measurements on magnetically doped $({\mathrm{Bi},\mathrm{Sb})}_2{\mathrm{Te}}_3$ films grown by molecular beam epitaxy. In Hall bar devices, we observe logarithmic dependence of transport coefficients in temperature and bias voltage which can be understood to arise from electron-electron interaction corrections to the conductivity and self-heating. Submicron scale devices exhibit intriguing quantum oscillations at high magnetic fields with dependence on bias voltage. The observed quantum oscillations can be attributed to bulk and surface transport.

Besides the spin Kondo effect, other degrees of freedom can give rise to the pseudospin Kondo effect. We report a direct observation of the orbital spin Kondo effect in a series-coupled gallium arsenide (GaAs) double quantum dot device where orbital degrees act as pseudospin. Electron occupation in both dots induces a pseudospin Kondo effect. In a region of one net spin impurity, complete spectra with three resonance peaks are observed. Furthermore, we observe a pseudo-Zeeman effect and demonstrate its electrical controllability for the artificial pseudospin in this orbital spin Kondo process via gate voltage control. The fourfold degeneracy point is realized at a specific value supplemented by spin degeneracy, indicating a transition from the SU(2) to the SU(4) Kondo effect.

We study nonequilibrium thermodynamics in a fermionic resonant-level model with arbitrary coupling strength to a fermionic bath, taking the wide-band limit. In contrast to previous theories, we consider a system where both the level energy and the coupling strength depend explicitly on time. We find that, even in this generalized model, consistent thermodynamic laws can be obtained, up to the second order in the drive speed, by splitting the coupling energy symmetrically between system and bath. We define observables for the system energy, work, heat, and entropy, and calculate them using nonequilibrium Green's functions. We find that the observables fulfill the laws of thermodynamics, and connect smoothly to the known equilibrium results.

The rotational state and ortho-para conversion of ${\mathrm{H}}_2$ on a Pd(210) surface is investigated with rotational-state-selective temperature-programmed desorption (RS-TPD) and theoretical calculations. The isotope dependence of TPD shows a higher desorption energy for ${\mathrm{D}}_2$ than that for ${\mathrm{H}}_2$, which is ascribed to the rotational and zero-point vibrational energies. The RS-TPD data show that the desorption energy of ${\mathrm{H}}_2$($J=1$) ($J$: rotational quantum number) is higher than that of ${\mathrm{H}}_2$($J=0$). This is due to the orientationally anisotropic potential confining the adsorbed ${\mathrm{H}}_2$, which is in agreement with theoretical calculations. Furthermore, the ${\mathrm{H}}_2$ desorption intensity ratio in $J=1$ and $J=0$ indicates fast ortho-para conversion in the adsorption state, which we estimate to be of the order of 1 s.

Using the tight-binding approach, we investigate the energy spectrum of square, triangular, and hexagonal ${\mathrm{MoS}}_2$ quantum dots (QDs) in the presence of a perpendicular magnetic field. Novel edge states emerge in ${\mathrm{MoS}}_2$ QDs, which are distributed over the whole edge which we call ring states. The ring states are robust in the presence of spin-orbit coupling (SOC). The corresponding energy levels of the ring states oscillate as a function of the perpendicular magnetic field which are related to Aharonov-Bohm oscillations. Oscillations in the magnetic field dependence of the energy levels and the peaks in the magneto-optical spectrum emerge (disappear) as the ring states are formed (collapsed). The period and the amplitude of the oscillation decrease with the size of the ${\mathrm{MoS}}_2$ QDs.

The number of paramagnetic materials that undergo a structural phase transition is rather small, which can perhaps explain the limited understanding of the band Jahn-Teller mechanism responsible for this effect. Here we present a structural phase transition observed in paramagnetic ${\mathrm{Y}}_2\mathrm{In}$ at temperature $T_0=250\pm 5$ K. Below $T_0$, the high-temperature hexagonal $P{6}_3/mmc$ phase transforms into the low-temperature orthorhombic $Pnma$ phase. This transition is accompanied by an unambiguous thermal hysteresis of about 10 K, observed in both magnetic susceptibility $M/H\left(T\right)$ and resistivity $\rho \left(T\right)$, indicating a first-order transition. Band structure calculations suggest a band Jahn-Teller mechanism, during which the degeneracy of electron bands close to the Fermi energy is broken. We establish that this structural phase transition does not have a magnetic component; however, the possibility of a charge density wave formation has not been eliminated.

This work describes the microstructure and transport properties of ${\mathrm{GeBi}}_{\text{x}}{\mathrm{Te}}_{\text{y}}$ films with emphasis on their out-of-equilibrium behavior. Persistent-photoconductivity (PPC), previously studied in the phase-change compound ${\mathrm{GeSb}}_{\text{x}}{\mathrm{Te}}_{\text{y}}$, is also quite prominent in this system. Much weaker PPC response is observed in the pure GeTe compound and when alloying GeTe with either In or Mn. Films made from these compounds share the same crystallographic structure, the same $p$-type conductivity, a similar compositional disorder extending over mesoscopic scales, and similar mosaic morphology. The enhanced photoconductive response exhibited by the Sb and Bi alloys may therefore be related to their common chemistry. Persistent photoconductivity is observable in ${\mathrm{GeBi}}_{\text{x}}{\mathrm{Te}}_{\text{y}}$ films at the entire range of sheet resistances studied in this work ($\approx {10}^{\text{3}}\mathrm{\Omega }$ to $\approx 55\mathrm{M}\mathrm{\Omega }$). The excess conductance produced by a brief exposure to infrared illumination decays with time as a stretched exponential (Kohlrausch law). Intrinsic electron-glass effects, on the other hand, are observable in thin films of ${\mathrm{GeBi}}_{\text{x}}{\mathrm{Te}}_{\text{y}}$ only for samples that are strongly localized just like it was noted with the seven electron glasses previously studied. These include a memory dip which is the defining attribute of the phenomenon. The memory dip in ${\mathrm{GeBi}}_{\text{x}}{\mathrm{Te}}_{\text{y}}$ is the widest amongst the germanium-telluride alloys studied to date consistent with the high carrier concentration $N\ge {10}^{\text{21}}{\mathrm{cm}}^{-3}$ of this compound. The thermalization process exhibited in either the PPC state or in the electron-glass regime is sluggish but the temporal law of the relaxation from the out-of-equilibrium state is distinctly different. Coexistence of the two phenomena give rise to some nontrivial effects, in particular, the visibility of the memory dip is enhanced in the PPC state. The relation between this effect and the dependence of the memory-effect magnitude on the ratio between the interparticle interaction and quench disorder is discussed.

Three-dimensionally resolved proton momentum distributions and end-to-end distributions have been calculated for hexagonal and cubic water ice. First-principles quantum nuclear wave functions have been used to investigate the impact of vibrational anisotropy, anharmonicity, proton and stacking disorder, temperature, and pressure on these distributions. Moreover, the effects of vibrations on the electronic density in hexagonal ice are shown to lead to a 5% vibrational correction with respect to the static-lattice optical permittivity, and proton disorder is found to be crucial in explaining its experimentally observed temperature dependence.

Single-crystal x-ray diffraction refinements indicate $\mathrm{SrS}{\mathrm{n}}_2\mathrm{F}{\mathrm{e}}_4{\mathrm{O}}_{11}$ crystallizes in the hexagonal $R$-type ferrite structure with noncentrosymmetric space group $P{6}_{3}mc$ and lattice parameters $a=5.9541\left(2\right)\AA , c=13.5761\left(5\right)\AA , Z=2$ ($R\left(F\right)=0.034$). Octahedrally coordinated $2a$ [$M$(1) and $M$(1a)] and $6c$ sites $\left[M\right(2\left)\right]$ have random, mixed occupation by Sn and Fe; whereas the tetrahedrally coordinated $2b$ sites [Fe(3) and Fe(3a)] are exclusively occupied by Fe, whose displacement from the ideal position with trigonal-bipyramidal coordination causes the loss of inversion symmetry. Our dc and ac magnetization data indicate $\mathrm{SrS}{\mathrm{n}}_2\mathrm{F}{\mathrm{e}}_4{\mathrm{O}}_{11}$ single crystals undergo a ferro- or ferri-magnetic transition below a temperature $T_C=630\mathrm{K}$ with very low coercive fields ${\mu }_{\mathrm{o}}{H}_{c\perp }=0.27\mathrm{Oe}$ and ${\mu }_{\mathrm{o}}{H}_{c\parallel }=1.5\mathrm{Oe}$ at 300 K, for applied field perpendicular and parallel to the $\mathit{c}$ axis, respectively. The value for $T_C$ is exceptionally high, and the coercive fields exceptionally low, among the known R-type ferrites. Time-dependent dc magnetization and frequency-dependent ac magnetization data indicate the onset of short-range, spin-glass freezing below $T_f=35.8\mathrm{K}$, which results from crystallographic disorder of magnetic $\mathrm{F}{\mathrm{e}}^{3+}$ and nonmagnetic $\mathrm{S}{\mathrm{n}}^{4+}$ ions on a frustrated Kagome sublattice. Anomalous ac susceptibility and thermomagnetic relaxation behavior in the short-range-ordered state differs from that of conventional spin glasses. Optical measurements in the ultraviolet to visible frequency range in a diffuse reflectance geometry indicate an overall optical band gap of 0.8 eV, consistent with observed semiconducting properties.

We report measurements of elastic moduli of hcp solid ${}^4\mathrm{He}$ down to 15 mK when the samples are rotated unidirectionally. Recent investigations have revealed that the elastic behavior of solid ${}^4\mathrm{He}$ is dominated by gliding of dislocations and pinning of them by ${}^3\mathrm{He}$ impurities, which move in the solidlike Bloch waves (impuritons). Motivated by the recent controversy of torsional oscillator studies, we have performed direct measurements of shear and Young's moduli of annular solid ${}^4\mathrm{He}$ using pairs of quarter-circle-shape piezoelectric transducers (PZTs) while the whole apparatus is rotated with angular velocity $\mathrm{\Omega }$ up to 4 rad/s. We have found that shear modulus $\mu$ is suppressed by rotation below 80 mK, when shear strain applied by PZT exceeds a critical value, above which $\mu$ decreases because the shear strain unbinds dislocations from ${}^3\mathrm{He}$ impurities. The rotation-induced decrement of $\mu$ at $\mathrm{\Omega }=4$ rad/s is about 14.7(12.3)% of the total change of temperature dependent $\mu$ for solid samples of pressure 3.6(5.4) MPa. The decrements indicate that the probability of pinning of ${}^3\mathrm{He}$ on dislocation segment $G$ decreases by several orders of magnitude. We propose that the motion of ${}^3\mathrm{He}$ impuritons under rotation becomes strongly anisotropic by the Coriolis force, resulting a decrease in $G$ for dislocation lines aligning parallel to the rotation axis.

We study numerically and analytically the quench dynamics of isolated many-body quantum systems. Using full random matrices from the Gaussian orthogonal ensemble, we obtain analytical expressions for the evolution of the survival probability, density imbalance, and out-of-time-ordered correlator. They are compared with numerical results for a one-dimensional-disordered model with two-body interactions and shown to bound the decay rate of this realistic system. Power-law decays are seen at intermediate times, and dips below the infinite time averages (correlation holes) occur at long times for all three quantities when the system exhibits level repulsion. The fact that these features are shared by both the random matrix and the realistic disordered model indicates that they are generic to nonintegrable interacting quantum systems out of equilibrium. Assisted by the random matrix analytical results, we propose expressions that describe extremely well the dynamics of the realistic chaotic system at different time scales.

Magnetic permeability, which measures the response of a material to an applied magnetic field, is crucial to the performance of magnetic devices and related technologies. Its dynamic value is usually a complex number with real and imaginary parts that describe, respectively, how much magnetic power can be stored and lost in the material. Control of permeability is therefore closely related to energy redistribution within a magnetic system or energy exchange between magnetic and other degrees of freedom via certain spin-dependent interactions. To avoid a high power consumption, direct manipulation of the permeability with an electric field through magnetoelectric coupling leads to high efficiency and simple operation, but remains a big challenge in both the fundamental physics and material science. Here we report unambiguous evidence of ferroelectric control of dynamic magnetic permeability in a $\mathrm{Co}/\mathrm{Pb}{\left({\mathrm{Mg}}_{1/3}{\mathrm{Nb}}_{2/3}\right)}_{0.7}{\mathrm{Ti}}_{0.3}{\mathrm{O}}_3$ (Co/PMN-PT) heterostructure, in which the ferroelectric PMN-PT acts as an energy source for the ferromagnetic Co film via an interfacial linear magnetoelectric interaction. The electric field tuning of the magnitude and line shape of the permeability offers a highly localized means of controlling magnetization with ultralow power consumption. Additionally, the emergence of negative permeability promises a new way of realizing functional nanoscale metamaterials with adjustable refraction index.

We report on isothermal magnetization, Mössbauer spectroscopy, and magnetostriction as well as temperature-dependent alternating-current (ac) susceptibility, specific heat, and thermal expansion of single crystalline and polycrystalline ${\mathrm{Li}}_2\left({\mathrm{Li}}_{1-x}{\mathrm{Fe}}_x\right)\text{N}$ with $x=0$ and $x\approx 0.30$. Magnetic hysteresis emerges at temperatures below $T\approx 50\mathrm{K}$ with coercivity fields of up to ${\mu }_{0}H=11.6\mathrm{T}$ at $T=2\mathrm{K}$ and magnetic anisotropy energies of 310 K (27 meV). The ac susceptibility is strongly frequency-dependent ($f=10–10000$ Hz) and reveals an effective energy barrier for spin reversal of $\mathrm{\Delta }E\approx 1100\mathrm{K}$ (90 meV). The relaxation times follow Arrhenius behavior for $T>25\mathrm{K}$. For $T<10\mathrm{K}$, however, the relaxation times of $\tau \approx {10}^{10}$ s are only weakly temperature-dependent, indicating the relevance of a quantum tunneling process instead of thermal excitations. The magnetic entropy amounts to more than $25\mathrm{J} {\mathrm{mol}}_{\mathrm{Fe}}^{-1}$ ${\mathrm{K}}^{-1}$, which significantly exceeds $R\ln 2$, the value expected for the entropy of a ground-state doublet. Thermal expansion and magnetostriction indicate a weak magnetoelastic coupling in accordance with slow relaxation of the magnetization. The classification of ${\mathrm{Li}}_2\left({\mathrm{Li}}_{1-x}{\mathrm{Fe}}_x\right)\text{N}$ as ferromagnet is stressed and contrasted with highly anisotropic and slowly relaxing paramagnetic behavior.

The spin-pumping mechanism and associated interfacial Gilbert damping are demonstrated in ion-beam sputtered $\mathrm{C}{\mathrm{o}}_2\mathrm{FeAl}$ (CFA)/Mo bilayer thin films employing ferromagnetic resonance spectroscopy. The dependence of the net spin-current transportation on Mo layer thickness, 0 to 10 nm, and the enhancement of the net effective Gilbert damping are reported. The experimental data have been analyzed using spin-pumping theory in terms of spin current pumped through the ferromagnet/nonmagnetic metal interface to deduce the real spin-mixing conductance and the spin-diffusion length, which are estimated to be $1.56\left(\pm 0.30\right)\times {10}^{19}{\mathrm{m}}^{-2}$ and $2.61\left(\pm 0.15\right)\mathrm{nm}$, respectively. The damping constant is found to be $8.8\left(\pm 0.2\right)\times {10}^{-3}$ in the Mo(3.5 nm)-capped CFA(8 nm) sample corresponding to an $\sim 69\%$ enhancement of the original Gilbert damping $5.2\left(\pm 0.6\right)\times {10}^{-3}$ in the Al-capped CFA thin film. This is further confirmed by inserting the Cu dusting layer which reduces the spin transport across the CFA/Mo interface. The Mo layer thickness-dependent net spin-current density is found to lie in the range of $1-4\mathrm{MA}{\mathrm{m}}^{-2}$, which also provides additional quantitative evidence of spin pumping in this bilayer thin-film system.

We present a combined experimental and theoretical study of the elementary magnetic excitations in ${\mathrm{Ba}}_2{\mathrm{YIrO}}_6$ and ${\mathrm{Sr}}_2{\mathrm{YIrO}}_6$—the two most intensively discussed candidates for a new type of magnetic instability caused by exciton condensation. For both materials, high-resolution resonant inelastic x-ray scattering (RIXS) at the Ir $L_3$ edge reveals sharp excitations around 370 and 650 meV energy loss, which we identify as triplet and quintet spin-orbit excitons. While the momentum-dependent RIXS spectra reveal that both the triplet and the quintet propagate coherently within the nonmagnetic background of the singlet sites, these modes remain fully gapped. The Ir-Ir exchange interactions in both double perovskites are therefore not strong enough to overcome the magnetic gap and, hence, our results exclude an intrinsic magnetic instability due to a condensation of magnetic excitations for both ${\mathrm{Ba}}_2{\mathrm{YIrO}}_6$ and ${\mathrm{Sr}}_2{\mathrm{YIrO}}_6$.

Through 2.6 GeV U irradiations, we have induced bimodal splayed columnar defects in ${\mathrm{Ba}}_{1-x}{\mathrm{K}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ single crystals with splay angles, $\pm 5^{\circ }, \pm {10}^{\circ }, \pm {15}^{\circ }$, and $\pm {20}^{\circ }$. Critical current densities through magnetization measurements were carefully evaluated, where a splay angle of $\pm 5^{\circ }$ brought about the highest $J_{\mathrm{c}}$. Magneto-optical images close to $T_{\mathrm{c}}$ indicate highly anisotropic discontinuity lines in the remnant state, and with anisotropy increasing with greater splay angles. Moreover, amongst those with splayed columnar defects, anomalous nonmonotonic field dependences of $J_{\mathrm{c}}$ and $S$ with an extrema at some fraction of the matching field are observed. We discuss that such $J_{\mathrm{c}}$ enhancement arises from a field-driven coupling transition in which intervortex interactions reorganize the vortex structure to be accommodated into columnar defects, thereby increasing pinning at higher fields.

Developing accurate and computationally efficient methods to calculate the electronic structure and total energy of correlated-electron materials has been a very challenging task in condensed matter physics and materials science. Recently, we have developed a correlation matrix renormalization (CMR) method which does not assume any empirical Coulomb interaction $U$ parameters and does not have double counting problems in the ground-state total energy calculation. The CMR method has been demonstrated to be accurate in describing both the bonding and bond breaking behaviors of molecules. In this study, we extend the CMR method to the treatment of electron correlations in periodic solid systems. Using a linear hydrogen chain as a benchmark system, we show that the results from the CMR method compare very well with those obtained recently by accurate quantum Monte Carlo (QMC) calculations. We also study the equation of states of three-dimensional crystalline phases of atomic hydrogen. We show that the results from the CMR method agree much better with the available QMC data in comparison with those from density functional theory and Hartree-Fock calculations.

The linear response formulation of the Raman susceptibility based on density functional theory (DFT) has been extended to the $\mathrm{DFT}+U$ approach to treat strong correlation effects for localized electrons. The Hubbard model parameters required in the $\mathrm{DFT}+U$ calculation are also determined using the linear response method. The present scheme contains no adjustable parameters and therefore allows an ab initio prediction of the Raman spectra for strongly correlated systems. The method is applied to cathode materials for lithium ion batteries, ${\mathrm{LiCoO}}_2$ and ${\mathrm{LiNi}}_{1/2}{\mathrm{Mn}}_{3/2}{\mathrm{O}}_4$. The Raman spectra for these compounds are remarkably improved by the Hubbard correction with $\mathrm{DFT}+U$, making the spectra in much better agreement than the standard DFT with the experiments. The present approach provides an accurate and efficient prediction of the Raman spectra for correlated systems.

Using optical-pump terahertz-probe spectroscopy, we investigated an ultrafast photocarrier relaxation behavior in a $\mathrm{B}{\mathrm{i}}_{1.5}\mathrm{S}{\mathrm{b}}_{0.5}\mathrm{T}{\mathrm{e}}_{1.7}\mathrm{S}{\mathrm{e}}_{1.3}$ (BSTS) single crystal, which is one of the most bulk-insulating topological insulators. Compared to $n$-type bulk-metallic $\mathrm{B}{\mathrm{i}}_2\mathrm{S}{\mathrm{e}}_3$, we found that BSTS endows distinct behaviors in its photocarrier dynamics; the relaxation time turns out to be an order of magnitude longer, and the transient conductance spectrum exhibits a nonlinear increase as a function of the pumping power. Also, we observed an abrupt reduction of the photocarrier scattering rate in several picoseconds after the initial photoexcitation. We discuss these intriguing experimental observations based on a bulk-to-surface carrier injection assisted by the built-in electric field near the surface and electron-phonon scattering.