Ab initio relativistic band structure calculations are performed for the frequency-dependent spin Hall conductivity of two-dimensional atomically thin crystals and one-dimensional nanoribbons. We study the influence of topology, quantization, and topological edge states. As model systems fully halogenated germanene, GeI, and its zigzag nanoribbons are investigated. GeI represents a topological insulator (TI). For comparison, also the TI germanene and the trivial insulator hydrogenated germanene are studied. For the TIs we demonstrate the quantization of the static spin Hall conductivity. It is hardly influenced by temperature and Fermi level shift. Its frequency dependence is governed by the band-structure details. Topological edge states influence the conductivity mainly for vanishing frequencies.

The temperature dependence of microwave-induced resistance oscillations (MIRO), according to the theory, originates from electron-electron scattering. This scattering affects both the quantum lifetime, or the density of states, and the inelastic lifetime, which governs the relaxation of the nonequilibrium distribution function. Here, we report on MIRO in an ultra-high-mobility ($\mu >3\times {10}^7{\mathrm{cm}}^2/\mathrm{V}$ s) two-dimensional electron gas at $T$ between $0.3$ and $1.8$ K. In contrast to theoretical predictions, the quantum lifetime is found to be $T$ independent in the whole temperature range studied. At the same time, the $T$ dependence of the inelastic lifetime is much stronger than can be expected from electron-electron interactions.

We report on a systematic study of the thermodynamic, electronic, and charge transport properties of high-quality single crystals of ${\mathrm{BaNiS}}_2$, the metallic end member of the quasi-two-dimensional ${\mathrm{BaCo}}_{1-x}{\mathrm{Ni}}_x{\mathrm{S}}_2$ system characterized by a metal-insulator transition at $x_{cr}=0.22$. Our analysis of magnetoresistivity and specific heat data consistently suggests a picture of compensated semimetal with two hole bands and one electron band, where electron-phonon scattering dominates charge transport and the minority holes exhibit, below $\sim 100$ K, a very large mobility, ${\mu }_h\sim 15000{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$, which is explained by a Dirac-like band. Evidence of unconventional metallic properties is given by an intriguing crossover of the resistivity from a Bloch-Grüneisen regime to a $\text{linear-}T$ regime occurring at 2 K and by a strong linear term in the paramagnetic susceptibility above 100 K. We discuss the possibility that these anomalies reflect a departure from conventional Fermi-liquid properties in presence of short-range AF fluctuations and of a large Hund coupling.

The heavy fermion compound ${\mathrm{Ce}}_3{\mathrm{Pd}}_{20}{\mathrm{Si}}_6$ displays unconventional quantum criticality as the lower of two consecutive phase transitions is fully suppressed by magnetic field. Here we report on the effects of pressure as an additional tuning parameter. Specific heat and electrical resistivity measurements reveal a converse effect of pressure on the two transitions, leading to the merging of both transitions at 6.2 kbars. The field-induced quantum criticality is robust under pressure tuning. We rationalize our findings within an extended version of the global phase diagram for antiferromagnetic heavy fermion quantum criticality.

We present a generic unraveling scheme for a detailed-balance-preserved quantum master equation applicable for stochastic point processes in mesoscopic transport. It enables us to investigate continuous measurement of a qubit on the level of single quantum trajectories, where essential correlations between the inherent dynamics of the qubit and detector current fluctuations are revealed. Based on this unraveling scheme, feedback control of the charge qubit is implemented to achieve a desired pure state in the presence of the detailed-balance condition. With sufficient feedback strength, coherent oscillations of the measured qubit can be maintained for arbitrary qubit-detector coupling. Competition between the loss and restoration of coherence entailed, respectively, by measurement back action and feedback control is reflected in the noise power spectrum of the detector's output. It is demonstrated unambiguously that the signal-to-noise ratio is significantly enhanced with increasing feedback strength and could even exceed the well-known Korotkov-Averin bound in quantum measurement. The proposed unraveling and feedback scheme offers a transparent and straightforward approach to effectively sustaining ideal coherent oscillations of a charge qubit in the field of quantum computation.

We present a systematic study of the single-particle and collective excitations by femtosecond transient reflectivity measurements in single crystals $\eta \text{−}{\mathrm{Mo}}_4{\mathrm{O}}_{11}$, investigating the dynamics as a function of temperature with two different pump photon energies (3.1 and 1.55 eV). A remarkable slowing down of the relaxation dynamics is observed at the first charge density wave (CDW) transition at $T_{\text{CDW}1}=105$ K associated with hidden one-dimensional Fermi surface (FS) nesting. In contrast, the appearance of the second transition at $T_{\text{CDW}2}$ associated with further CDW ordering is barely perceptible. The coherent response can be described well by the displacive coherent excitation model of Zeiger et al. [Phys. Rev. B 45, 768 (1992)] assuming a coupling of phonons to the photoexcited quasiparticles. The coupling of the collective modes to the electronic order parameter is found to be weak. The exponential relaxation is discussed in terms of single-particle relaxation and an overdamped collective mode.

We present a similarity transformation theory based on a polynomial form of a particle-hole pair excitation operator. In the weakly correlated limit, this polynomial becomes an exponential, leading to coupled cluster doubles. In the opposite strongly correlated limit, the polynomial becomes an extended Bessel expansion and yields the projected BCS wave function. In between, we interpolate using a single parameter. The effective Hamiltonian is non-Hermitian and this polynomial similarity transformation theory follows the philosophy of traditional coupled cluster, left projecting the transformed Hamiltonian onto subspaces of the Hilbert space in which the wave function variance is forced to be zero. Similarly, the interpolation parameter is obtained through minimizing the next residual in the projective hierarchy. We rationalize and demonstrate how and why coupled cluster doubles is ill suited to the strongly correlated limit, whereas the Bessel expansion remains well behaved. The model provides accurate wave functions with energy errors that in its best variant are smaller than 1% across all interaction strengths. The numerical cost is polynomial in system size and the theory can be straightforwardly applied to any realistic Hamiltonian.

We explore two parabolic quantum well (PQW) samples, with and without Bragg mirrors, in order to optimize the building blocks of a bosonic cascade laser. The photoluminescence spectra of a PQW microcavity sample is compared against that of a conventional microcavity with embedded quantum wells (QWs) to demonstrate that the weak coupling lasing in a PQW sample can be achieved. The relaxation dynamics in a conventional QW microcavity and in the PQW microcavity was studied by a nonresonant pump-pump excitation method. Strong difference in the relaxation characteristics between the two samples was found. The semiclassical Boltzmann equations were adapted to reproduce the evolution of excitonic populations within the PQW as a function of the pump power and the output intensity evolution as a function of the pump-pump pulse delay. Fitting the PQW data confirms the anticipated cascade relaxation, paving the way for such a system to produce terahertz radiation.

We report measurements of positronium (Ps) formation following positron irradiation of mesoporous ${\mathrm{SiO}}_2$ films and Ge(100) single crystals at temperatures ranging from 12–700 K. As both of these materials generate Ps atoms via nonthermal processes, they are able to function as positron-positronium converters at cryogenic temperatures. Our data show that such Ps formation is possibly provided the targets are not compromised by adsorption of residual gas. In the case of ${\mathrm{SiO}}_2$ films, we observe a strong reduction in the Ps formation efficiency following irradiation with UV laser light ($\lambda =243.01$ nm) below 250 K, in accordance with previous observations of radiation-induced surface paramagnetic centers. Conversely, Ps emission from Ge is enhanced by irradiation with visible laser light ($\lambda =532$ nm) via a photoemission process that persists at cryogenic temperatures. Both mesoporous ${\mathrm{SiO}}_2$ films and Ge crystals were found to produce Ps efficiently in cryogenic environments. Accordingly, these materials are likely to prove useful in several areas of research, including Ps mediated antihydrogen formation conducted in the cold bore of a superconducting magnet, the production of Rydberg Ps for experiments in which the effects of black-body radiation must be minimized, and the utilization of mesoporous structures that have been modified to produce cold Ps atoms.

Tunneling of quasiparticles between two nearly aligned graphene sheets produces resonant current-voltage characteristics because of the quasiexact conservation of in-plane momentum. We claim that, in this regime, vertical transport in graphene/boron nitride/graphene heterostructures carries precious information on electron-electron interactions and the quasiparticle spectral function of the two-dimensional electron system in graphene. We present extensive microscopic calculations of the tunneling spectra with the inclusion of quasiparticle lifetime effects and elucidate the range of parameters (interlayer bias, temperature, twist angle, and gate voltage) under which electron-electron interaction physics emerges.

We investigate spectral correlations in quasi-one-dimensional Anderson insulators with broken time-reversal symmetry. While energy levels are uncorrelated in the thermodynamic limit of infinite wire length, some correlations remain in finite-size Anderson insulators. Asymptotic behaviors of level-level correlations in these systems are known in the large- and small-frequency limits, corresponding to the regime of classical diffusive dynamics and the deep quantum regime of strong Anderson localization. Employing nonperturbative methods and a mapping to the Coulomb-scattering problem, recently introduced by M. A. Skvortsov and P. M. Ostrovsky [JETP Lett. 85, 72 (2007)], we derive a closed analytical expression for the spectral statistics in the classical-to-quantum region bridging the known asymptotic behaviors. We further discuss how Poisson statistics at large energies develop into Wigner-Dyson statistics as the wire-length decreases.

By applying the alloy design concept that stable intermetallic phases between two immiscible elements can be formed by adding a third element that forms stable compounds with both elements, we have synthesized the first known stable intermetallic compound of Cu, Gd, and Ca, where copper acts as the mediating element between the immiscible Gd and Ca. A compound with the composition $\mathrm{C}{\mathrm{u}}_{84}\mathrm{G}{\mathrm{d}}_9\mathrm{C}{\mathrm{a}}_7$ (equivalent to $\mathrm{C}{\mathrm{u}}_5\mathrm{G}{\mathrm{d}}_{0.54}\mathrm{C}{\mathrm{a}}_{0.42}$) was synthesized by the Czochralski technique in the form of a large single crystal of high structural perfection, and the structural model was determined by x-ray diffraction (XRD). The compound crystallizes in the hexagonal system, space group $P6$/mmm, and the crystal structure is isotypic to the $\mathrm{C}{\mathrm{u}}_{5.44}\mathrm{T}{\mathrm{b}}_{0.78}$. The unit cell contains inherent disorder due to partial occupation of the Cu3 site and the substitutional disorder at the Gd/Ca mixed site located at the vertices of the hexagonal unit cell, where Gd and Ca randomly substitute each other. The random substitution of magnetic Gd by nonmagnetic Ca atoms makes the magnetic Gd lattice disordered, which leads to interesting magnetic ordering at low temperatures that occurs below $T_C=24\mathrm{K}$ in zero and low external magnetic fields. By performing a large set of complementary experiments along two perpendicular crystallographic directions (the [001] hexagonal-axis direction and the [100] hexagonal-plane direction), we show that the zero-field collective magnetic state can be described as a random-anisotropy ferromagnetic state, where random magnetic anisotropies originate from the magnetic dipole interactions between the Gd moments in the magnetically disordered lattice. The random-anisotropy ferromagnetic state in the $\mathrm{C}{\mathrm{u}}_{84}\mathrm{G}{\mathrm{d}}_9\mathrm{C}{\mathrm{a}}_7$ is characterized by randomness and frustration of magnetic interactions, which are the two ingredients that allow classifying this state into the generic class of spin glasses. Our paper opens the possibility to search for new ternary intermetallic phases in the Cu–Gd–Ca phase diagram, including structurally complex phases and quasicrystals.

Raman spectroscopy is used to study magnetic excitations in the quasi-one-dimensional $S=1/2$ quantum spin systems $\text{Cu}\left(\mathrm{Qnx}\right){\left({\mathrm{Cl}}_{1-x}{\mathrm{Br}}_x\right)}_2$. The low energy spectrum is found to be dominated by a two-magnon continuum as expected from the numerical calculations for the Heisenberg spin ladder model. The continuum shifts to higher energies as more Br is introduced. The cutoff of the scattering increases faster than the onset indicating that the increase of the exchange constant along the leg is the main effect on the magnetic properties. The upper and lower continuum thresholds are measured as a function of Br content across the entire range and compared to estimates based on previous bulk studies. We observe small systematic deviations that are discussed.

A neutron powder diffraction (NPD) study of $\mathit{Ln}\mathrm{MnSbO}$ ($Ln=\mathrm{La}$ or Ce) reveals differences between the magnetic ground state of the two compounds due to the strong Ce-Mn coupling compared to La-Mn. The two compounds adopt the P4/nmm space group down to 2 K, and whereas magnetization measurements do not show obvious anomaly at high temperatures, NPD reveals a C-type antiferromagnetic (AFM) order below $T_{\mathrm{N}}=255\mathrm{K}$ for LaMnSbO and 240 K for CeMnSbO. While the magnetic structure of LaMnSbO is preserved to base temperature, a sharp transition at $T_{\mathrm{SR}}=4.5\mathrm{K}$ is observed in CeMnSbO due to a spin-reorientation (SR) transition of the ${\mathrm{Mn}}^{2+}$ magnetic moments from pointing along the $c$ axis to the $ab$ plane. The SR transition in CeMnSbO is accompanied by a simultaneous long-range AFM ordering of the Ce moments, which indicates that the Mn SR transition is driven by the Ce-Mn coupling. The ordered moments are found to be somewhat smaller than those expected for ${\mathrm{Mn}}^{2+}$ ($S=5/2$) in insulators, but large enough to suggest that these compounds belong to the class of local-moment antiferromagnets. The lower $T_{\mathrm{N}}$ found in these two compounds compared to the As-based counterparts ($T_{\mathrm{N}}=317$ for LaMnAsO, $T_{\mathrm{N}}=347\mathrm{K}$ for CeMnAsO) indicates that the Mn-$Pn$ ($Pn=\mathrm{As}$ or Sb) hybridization that mediates the superexchange Mn-$Pn$-Mn coupling is weaker for the Sb-based compounds.

We report muon spin rotation experiments on the magnetic penetration depth $\lambda$ and the temperature dependence of ${\lambda }^{-2}$ in the over-doped Fe-based high-temperature superconductor (Fe-HTS) ${\mathrm{Ba}}_{1-x}{\mathrm{Rb}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ $(x$ = 0.65) studied at ambient and under hydrostatic pressures up to $p=2.3$ GPa. We find that in this system ${\lambda }^{-2}\left(T\right)$ is best described by $d$-wave scenario. This is in contrast to the case of the optimally doped $x=0.35$ system which is known to be a nodeless $s^{+-}$-wave superconductor. This suggests that the doping induces the change of the pairing symmetry from $s^{+-}$ to $d$ wave in ${\mathrm{Ba}}_{1-x}{\mathrm{Rb}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$. In addition, we find that the $d$-wave order parameter is robust against pressure, suggesting that $d$ is the common and dominant pairing symmetry in over-doped ${\mathrm{Ba}}_{1-x}{\mathrm{Rb}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$. Application of pressure of $p=2.3$ GPa causes a decrease of $\lambda$(0) by less than $5\%$, while at optimal doping $x=0.35$ a significant decrease of $\lambda$(0) was reported. The superconducting transition temperature $T_c$ as well as the gap to $T_{\mathrm{c}}$ ratio $2\mathrm{\Delta }/k_{\mathrm{B}}{T}_{\mathrm{c}}$ show only a modest decrease with pressure. By combining the present data with those previously obtained for optimally doped system $x=0.35$ and for the end member $x$ = 1, we conclude that the SC gap symmetry as well as the pressure effects on the SC quantities strongly depend on the Rb doping level. These results are discussed in the light of the putative Lifshitz transition, i.e., a disappearance of the electron pockets in the Fermi surface of ${\mathrm{Ba}}_{1-x}{\mathrm{Rb}}_x{\mathrm{Fe}}_2{\mathrm{As}}_2$ upon hole doping.

An average-atom ($A$-atom) embedded-atom-method potential for random multicomponent alloys at any composition is derived analytically and validated by comparing $A$-atom and true random alloys bulk and defect properties, in model Fe-Ni-Cr systems. The $A$-atom can be mixed with the individual alloying-element potentials, thus enabling computation of defect/defect interactions. Its use provides quantitative insight into the physical role of the fluctuations, and has many applications, such as in atomistic/continuum modeling of random alloys and the development of new potentials with controlled properties.

We investigate energy leakage induced by incomplete measurements of the scattering matrices of complex media. Owing to the limited numerical apertures of an optical system, it is experimentally challenging to access theoretically predicted perfect transmission channels in the diffusive regime. By conducting numerical simulations on scattering matrices, we demonstrate that energy leakage contributed from uncollected transmission in the transmission matrices provides an energy transmission that is more enhanced than that predicted by measurement. On the other hand, energy leakage originating from the uncollected reflection in the partial measurement of a reflection matrix strongly suppresses the energy transmission through a zero-reflection channel, restricting the transmission enhancement to no more than a fivefold enhancement in limited optical systems. Our study provides useful insights into the effective control of energy delivery through scattering media and its ultimate limitation in practical schemes.

The recently discovered dynamical phase transition denotes nonanalytic behavior in the real-time evolution of quantum systems in the thermodynamic limit, and it has been shown to occur in different systems at zero temperature [Heyl et al., Phys. Rev. Lett. 110, 135704 (2013)]. In this paper, we extend the analysis to nonzero temperature by studying a generalized form of the Loschmidt echo, the work distribution function, of a quantum quench in the transverse field Ising model. Although the quantitative behavior at nonzero temperatures still displays features derived from the zero-temperature nonanalyticities, it is shown that in this model dynamical phase transitions do not exist if $T>0$. This is a consequence of the system being initialized in a thermal state. Moreover, we elucidate how the Tasaki-Crooks-Jarzynski relation can be exploited as a symmetry relation for a global quench or to obtain the change of the equilibrium free-energy density.

We report an in situ observation of temperature-dependent phase transition in MnAs thin film by transmission electron microscopy (TEM) techniques. Following the identification of the crystallographic transition, from hexagonal $\alpha$-MnAs to quasihexagonal $\beta$-MnAs, the orbital-to-spin moment ratio is measured and a breaking of the ferromagnetic order locally observed, thanks to the electron magnetic chiral dichroism (EMCD) technique. To achieve quantitative information, applying the sum rules to the dichroic signal of magnetic anisotropic materials is accurately discussed. Finally, the orbital-to-spin moment ratio of $\alpha$-MnAs along the easy, hard, and intermediate magnetic axes is estimated by EMCD and compared to implemented density functional theory (DFT) calculations. The influence of the magnetocrystalline anisotropy is locally demonstrated. This work in particular illustrates the feasibility of the EMCD technique for in situ experiments, and proves its potential to explore the anisotropy of magnetic materials.