We have investigated the electronic excitations in bulk $2H{\text{-MoS}}_2$ using electron energy-loss spectroscopy. The electron energy-loss spectra in the $\mathrm{\Gamma }\text{M}$ and $\mathrm{\Gamma }\text{K}$ directions were measured for various momentum transfer values. The results allow the identification of the ${\text{A}}_1$ and ${\text{B}}_1$ exciton peaks and in particular their energy-momentum dispersion. The dispersions exhibit approximately quadratic upward trends and slight anisotropies in the $\mathrm{\Gamma }\text{M}$ and $\mathrm{\Gamma }\text{K}$ directions. The fitted energy-momentum transfer functions allow the estimation of the effective masses of the excitons which are in close proximity to predicted values.

Semilocal and dispersion-corrected density-functional calculations have been performed to study the crystal structure, equation of state, and electronic structure of metal tellurohalides with chemical formula MeTeI where Me=Bi, Au, or Pd. A comparative investigation of the results of these calculations is conducted which reveals the role of van der Waals attraction. It is shown that the prediction of crystal structure of metal tellurohalides is systematically improved thanks to the inclusion of van der Waals dispersion. It is found for BiTeI and AuTeI that the energy versus volume curve is anomalously flat in the vicinity of equilibrium volume and the calculated equation of state has an excessively steep slope in the low-pressure region; these are also fixed in the dispersion-corrected calculations. Analysis based on the computation of the volume and axial compressibilities shows that predicting the anisotropy of BiTeI via the semilocal calculations yields an unrealistic result, whereas the results of dispersion-corrected calculations agree with the experimental compressibility data. Our calculations render that BiTeI (AuTeI) is a narrow band gap semiconductor with Rashba-type spin splitting at the band edges (with an indirect band gap) while PdTeI is a metal with relatively low density of states at the Fermi level. The band gaps of BiTeI and AuTeI obtained via semilocal (dispersion-corrected) calculations are found to be greater (smaller) than the respective experimental values, which is against (in line with) the expected trend. Similarly, the Rashba parameters of BiTeI are bracketed by the respective values obtained via semilocal and dispersion-corrected calculations, e.g., a larger value for the Rashba parameter ${\alpha }_R$ is obtained in association with the reduction of the band gap caused by modification of the crystal structure owing to van der Waals attraction. Excellent agreement with the experimental Rashba parameters is obtained via interpolation of the calculated (semilocal and dispersion-corrected) values.

The electron-nuclear interaction in optically pumped NMR of semiconductors manifests itself through changes in spectral features (resonance shifts, linewidths, signal amplitudes) and through the magnitude of the nuclear-spin polarization. We show that these spectral features can provide a measure of the parameters that govern the optical pumping process: electron-nuclear cross-relaxation rate, Bohr radius and fractional occupancy of the optically relevant defect (ORD), and electron polarization at the ORD. Applying a model of the spatial and temporal evolution of the nuclear spins under optical pumping to ${}^{31}\mathrm{P}$ in semi-insulating InP we find an ORD Bohr radius of 6 nm, independent of the electron polarization used to fit the data, confirming the ORD is a shallow donor. For an electron polarization of $-0.15$, the ORD fractional occupancy is 0.02, leading to an electron-nuclear cross-relaxation time of 0.20 s and a hyperfine frequency shift of 8.1 kHz for super-bandgap irradiation. Allowing the electron polarization to vary in the model constrained to the hyperfine shift data, we find the fractional occupancy and electron-nuclear cross-relaxation rate to be approximately inversely proportional to the electron polarization. From the long-time evolution of the nuclear polarization we calculate an ORD density of $5\times {10}^{15}{\text{cm}}^{-3}$.

We predict inelastic light scattering spectra from electron collective excitations in a coaxial quantum well embedded in a core-multishell $\mathrm{GaAs}/\mathrm{AlGaAs}$ nanowire. The complex composition, the hexagonal cross section, and the remote doping of typical samples are explicitly included, and the free electron gas is obtained by a density functional theory (DFT) approach. Inelastic light scattering cross sections due to charge and spin collective excitations belonging to quasi-one-dimensional (1D) and quasi-2D states, which coexist in such radial heterostructures, are predicted in the nonresonant approximation from a fully three-dimensional multisubband time-dependent DFT (TDDFT) formalism. We show that collective excitations can be classified in azimuthal, radial, and longitudinal excitations, according to the associated density fluctuations, and we suggest that their character can be exposed by specific spectral dispersion of inelastic light scattering along different planes of the heterostructure.

We develop a Boltzmann-Langevin description of the Coulomb drag effect in clean double-layer systems with large interlayer separation $d$ as compared to the average interelectron distance ${\lambda }_F$. Coulomb drag arises from density fluctuations with spatial scales of order $d$. At low temperatures, their characteristic frequencies exceed the intralayer equilibration rate of the electron liquid, and Coulomb drag may be treated in the collisionless approximation. As temperature is raised, the electron mean free path becomes short due to electron-electron scattering. This leads to local equilibration of electron liquid, and consequently drag is determined by hydrodynamic density modes. Our theory applies to both the collisionless and the hydrodynamic regimes, and it enables us to describe the crossover between them. We find that drag resistivity exhibits a nonmonotonic temperature dependence with multiple crossovers at distinct energy scales. At the lowest temperatures, Coulomb drag is dominated by the particle-hole continuum, whereas at higher temperatures of the collision-dominated regime it is governed by the plasmon modes. We observe that fast intralayer equilibration mediated by electron-electron collisions ultimately renders a stronger drag effect.

We study the entanglement production in a quantum spin Hall ring geometry where electrons of opposite spins are emitted in pairs from a source and collected in two different detectors. Postselection of coincidence detector events gives rise to entanglement in the system, measurable through correlations between the outcomes in the detectors. We have chosen a geometry such that the entanglement depends on the dynamical phases picked up by the edge states as they move around the ring. In turn, the dependence of the phases on gate potential and Rashba interaction allows for a precise electrical control of the entanglement production in the ring.

We study the collective charge excitations (plasmons) in spin-polarized graphene, and derive explicit expressions for their dispersion in the undamped regime. From this, we are able to calculate the critical wave vector beyond which the plasmon enters the electron-hole continuum, its quality factor decreasing sharply. We find that the value of the critical wave vector is strongly spin-polarization dependent, in a way that has no analog in ordinary two-dimensional electron gases. The origin of this effect is in the coupling between the plasmon and the interband electron-hole pairs of the minority spin carriers. We show that the effect is robust with respect to the inclusion of disorder and we suggest that it can be exploited to experimentally determine the spin polarization of graphene.

Manganese-doped germanium films are model systems for the study of nanospinodal decomposition into Mn-rich nanostructures and its influence on the electronic and magnetic properties. Here we study a film of ${\mathrm{Ge}}_{0.9}{\mathrm{Mn}}_{0.1}$ forming self-organized nanocolumns perpendicular to a Ge substrate with high resolution scanning transmission electron microscopy combined with electron energy loss spectroscopy, bulk magnetization, and muon spin rotation and relaxation $\left(\mu \mathrm{SR}\right)$ measurements. The Mn-rich nanocolumns approximately form a triangular lattice with no detectable Mn atoms in the matrix. We find that they consist of cores surrounded by shells. The combined analysis of bulk magnetization and $\mu \mathrm{SR}$ data enables us to characterize the electronic and magnetic properties of both the cores and shells. We argue that the discovered phase separation of the columns between a core and a shell is relevant for other transition-metal-doped semiconductors.

At elevated temperatures, glasses crystallize via thermally activated diffusion. However, metallic glasses can also undergo deformation-induced crystallization at very low temperatures. Here we demonstrate the crystallization of $\mathrm{A}{\mathrm{l}}_{50}\mathrm{F}{\mathrm{e}}_{50}$ metallic glasses under cyclic deformation at 50 K using molecular dynamics simulations and reveal the underlying atomic-scale processes. We demonstrate that stress-driven nonaffine atomic rearrangements, or shear diffusion transformation (SDT) events, lead to successive metabasin-to-metabasin transitions and long-range ordering. We also illustrate that the nucleation and growth of the crystal proceed via collective attachment of ordered clusters, advancing the amorphous/crystal interface in an intermittent manner. The cooperative nature of the steplike crystallization is attributed to the large activation volume of Eshelby transformations which generate as a by-product nonaffine diffusive atomic displacements that accumulate over loading cycles. The dual nature of shear (affine) and diffusion (nonaffine) in low-temperature stress-driven SDT events thus unifies inelasticity with crystallization.

Relations between critical exponents, or scaling laws, at both continuous and discontinuous quantum phase transitions are derived and discussed. In general there are multiple dynamical exponents at these transitions, which complicates the scaling description. Some rigorous inequalities are derived, and the conditions needed for these inequalities to be equalities are discussed. Scaling laws involving the specific-heat exponents that are specific to quantum phase transitions are derived and contrasted with their counterparts at classical phase transitions. We also generalize the ideas of Fisher and Berker and others for applying (finite-size) scaling theory near a classical first-order transition to the quantum case. We then apply and illustrate all of these ideas by using the quantum ferromagnetic phase transition in metals as an explicit example. This transition is known to have multiple dynamical scaling exponents, and in general it is discontinuous in clean systems but continuous in disordered ones. Furthermore, it displays many experimentally relevant crossover phenomena that can be described in terms of fixed points, originally discussed by Hertz, that ultimately become unstable asymptotically close to the transition and give way to the asymptotic fixed points. These fixed points provide a rich environment for illustrating the general scaling concepts and exponent relations. We also discuss the quantum-wing critical point at the tips of the tricritical wings associated with the discontinuous quantum ferromagnetic transition from a scaling point of view.

We numerically investigate the mixed pairing symmetries in mesoscopic superconducting loops in the presence of spin correlations by solving the Bogoliubov-de Gennes equations self-consistently. The spatial variations of the superconducting order parameters and the spontaneous magnetization are determined by the band structure. When the threaded magnetic flux turns on, the charge and spin currents both emerge and depict periodic evolution. In the case of a mesoscopic loop with dominant triplet $p_x\pm i{p}_y$-wave symmetry, a slight change of the chemical potential may lead to novel flux-dependent evolution patterns of the ground-state energy and the magnetization. The spin-polarized currents show pronounced quantum oscillations with fractional periods due to the appearance of energy jumps in flux, accompanied with a steplike feature of the enhanced spin current. Particularly, at some appropriate flux, the peaks of the zero-energy local density of states clearly indicate the occurrence of the odd-frequency pairing. In the case of a superconducting loop with dominant singlet $d_{x^2-y^2}$-wave symmetry, the spatial profiles of the zero-energy local density of states and the magnetization show spin-dependent features on different sample diagonals. Moreover, the evolution of the flux-induced spin current always exhibits an $hc/e$ periodicity.

${\mathrm{LiTaO}}_3$ and ${\mathrm{LiNbO}}_3$ crystals are investigated here in a combined experimental and theoretical study that uses Raman spectroscopy in a complete set of scattering geometries and corresponding density-functional theory calculations to provide microscopic information on their vibrational properties. The Raman scattering efficiency is computed from first principles in order to univocally assign the measured Raman peaks to the calculated eigenvectors. Measured and calculated Raman spectra are shown to be in qualitative agreement and confirm the mode assignment by Margueron et al. [J. Appl. Phys. 111, 104105 (2012)], thus finally settling a long debate. While the two crystals show rather similar vibrational properties overall, the ${\text{E-TO}}_9$ mode is markedly different in the two oxides. The deviations are explained by a different anion-cation bond type in ${\mathrm{LiTaO}}_3$ and ${\mathrm{LiNbO}}_3$ crystals.

We study the equilibrium phase diagram of ultrathin chiral magnets with easy-plane anisotropy $A$. The vast triangular skyrmion lattice phase that is stabilized by an external magnetic field evolves continuously as a function of increasing $A$ into a regime in which nearest-neighbor skyrmions start overlapping with each other. This overlap leads to a continuous reduction of the skyrmion number from its quantized value $Q=1$ and to the emergence of antivortices at the center of the triangles formed by nearest-neighbor skyrmions. The antivortices also carry a small “skyrmion number” $Q_A\ll 1$ that grows as a function of increasing $A$. The system undergoes a first order phase transition into a square vortex-antivortex lattice at a critical value of $A$. Finally, a canted ferromagnetic state becomes stable through another first order transition for a large enough anisotropy $A$. Interestingly enough, this first order transition is accompanied by metastable meron solutions.

We present the results of transverse field (TF) muon-spin rotation (${\mu }^{+}\mathrm{SR})$ measurements on ${\mathrm{Cu}}_2{\mathrm{OSeO}}_3$, which has a skyrmion-lattice (SL) phase. We measure the response of the TF ${\mu }^{+}\mathrm{SR}$ signal in that phase along with the surrounding ones, and suggest how the phases might be distinguished using the results of these measurements. Dipole field simulations support the conclusion that the muon is sensitive to the SL via the TF line shape and, based on this interpretation, our measurements suggest that the SL is quasistatic on a time scale $\tau >100$ ns.

We analyze the coexistence of superfluid and density wave (stripe) order in a quasi-two-dimensional gas of dipolar fermions aligned by an external field. Remarkably, the anisotropic nature of the dipolar interaction allows for such a coexistence in a large region of the zero-temperature phase diagram. In this region, the repulsive part of the interaction drives the stripe formation and the attractive part induces the pairing, resulting in a supersolid with $p$-wave Cooper pairs aligned along the stripes. From a momentum space perspective, the stability of the supersolid phase is due to the fact that the stripe order renders the Fermi surface only partially gapped, leaving gapless regions that are most important for $p$-wave pairing. We briefly discuss how this supersolid phase can be detected in time-of-flight experiments.

Rezende and López Ortiz [Phys. Rev. B 91, 104416 (2015)] published a calculation for the magnon specific heat and thermal conductivity of yttrium iron garnet in the presence of large magnetic fields. The results reproduced our previous temperature-dependent measurements of both quantities in fields up to 70 kOe [Phys. Rev. B 90, 064421 (2014)]. Here, we point out that the experimentally observed magnetic-field dependence of thermal conductivity shows a faster saturation than their model explains. Furthermore, their results do not affect the main conclusion of our paper regarding the magnon thermal mean-free path.

We study spatial correlation effects in multiorbital systems, especially in a paramagnetic metallic state subject to Hund's coupling. We apply a cluster extension of the dynamical mean-field theory (DMFT) to the three-orbital Hubbard model away from half filling, where previous single-site DMFT studies revealed that local correlation effects caused by Hund's coupling bring about unusual strongly correlated metallic behaviors. We find that Hund's coupling significantly affects the nonlocal correlations too; it strongly modulates the electron distribution in the momentum space so as to make a momentum region almost half filled and hence strongly correlated. It leads to an anomalous electronic state distinct both from the Fermi liquid and the Mott insulator. We identify the mechanism of the anomalous state with the intersite ferromagnetic correlations induced by Hund's coupling.

We study the phase diagram of the ionic Hubbard model (IHM) at half filling on a Bethe lattice of infinite connectivity using dynamical mean-field theory (DMFT), with two impurity solvers, namely, iterated perturbation theory (IPT) and continuous time quantum Monte Carlo (CTQMC). The physics of the IHM is governed by the competition between the staggered ionic potential $\mathrm{\Delta }$ and the on-site Hubbard $U$. We find that for a finite $\mathrm{\Delta }$ and at zero temperature, long-range antiferromagnetic (AFM) order sets in beyond a threshold $U=U_{AF}$ via a first-order phase transition. For $U$ smaller than $U_{AF}$ the system is a correlated band insulator. Both methods show a clear evidence for a quantum transition to a half-metal (HM) phase just after the AFM order is turned on, followed by the formation of an AFM insulator on further increasing $U$. We show that the results obtained within both methods have good qualitative and quantitative consistency in the intermediate-to-strong-coupling regime at zero temperature as well as at finite temperature. On increasing the temperature, the AFM order is lost via a first-order phase transition at a transition temperature $T_{AF}(U,\mathrm{\Delta })$ [or, equivalently, on decreasing $U$ below $U_{AF}(T,\mathrm{\Delta })$], within both methods, for weak to intermediate values of $U/t$. In the strongly correlated regime, where the effective low-energy Hamiltonian is the Heisenberg model, IPT is unable to capture the thermal (Neel) transition from the AFM phase to the paramagnetic phase, but the CTQMC does. At a finite temperature $T, \mathrm{DMFT}+\mathrm{CTQMC}$ shows a second phase transition (not seen within $\mathrm{DMFT}+\mathrm{IPT}$) on increasing $U$ beyond $U_{AF}$. At $U_N>U_{AF}$, when the Neel temperature $T_N$ for the effective Heisenberg model becomes lower than $T$, the AFM order is lost via a second-order transition. For $U\gg \mathrm{\Delta }, T_N\sim t^2/U(1-x^2)$, where $x=2\mathrm{\Delta }/U$ and thus $T_N$ increases with increase in $\mathrm{\Delta }/U$. In the three-dimensional parameter space of $(U/t,T/t,\text{and}\mathrm{\Delta }/t)$, as $T$ increases, the surface of first-order transition at $U_{AF}(T,\mathrm{\Delta })$ and that of the second-order transition at $U_N(T,\mathrm{\Delta })$ approach each other, shrinking the range over which the AFM order is stable. There is a line of tricritical points that separates the surfaces of first- and second-order phase transitions.

Ninety-eight one-dimensional channels defined using split gates fabricated on a GaAs/AlGaAs heterostructure are measured during one cooldown at $1.4$ K. The devices are arranged in an array on a single chip and are individually addressed using a multiplexing technique. The anomalous conductance feature known as the “0.7 structure” is studied using statistical techniques. The ensemble of data shows that the 0.7 anomaly becomes more pronounced and occurs at lower values as the curvature of the potential barrier in the transport direction decreases. This corresponds to an increase in the effective length of the device. The 0.7 anomaly is not strongly influenced by other properties of the conductance related to density. The curvature of the potential barrier appears to be the primary factor governing the shape of the 0.7 structure at a given $T$ and $B$.

We present a comparative measurement of the $G$-peak oscillations of phonon frequency, Raman intensity, and linewidth in the magneto-Raman scattering of optical $E_{2g}$ phonons in mechanically exfoliated ABA- and ABC-stacked trilayer graphene (TLG). Whereas in ABA-stacked TLG, we observe magnetophonon oscillations consistent with single-bilayer chiral band doublets, the features are flat for ABC-stacked TLG up to magnetic fields of 9 T. This suppression can be attributed to the enhancement of band chirality that compactifies the spectrum of Landau levels and modifies the magnetophonon resonance properties. The drastically different coupling behavior between the electronic excitations and the $E_{2g}$ phonons in ABA- and ABC-stacked TLG reflects their different electronic band structures and the electronic Landau level transitions and thus can be another way to determine the stacking orders and to probe the stacking-order-dependent electronic structures. In addition, the sensitivity of the magneto-Raman scattering to the particular stacking order in few-layer graphene highlights the important role of interlayer coupling in modifying the optical response properties in van der Waals layered materials.