We present the Usadel theory describing the superconducting proximity effect in heterostructures with a half-metallic layer. It is shown that the full spin polarization inside the half-metals gives rise to an additional component of the Green's function which results in the giant triplet spin-valve effect in superconductor (S)–ferromagnet (F)–half-metal (HM) trilayers and provides a natural explanation for the ${\phi }_0\text{-junction}$ formation in the S/F/HM/F/S systems. In addition, we consider the exactly solvable model of the S/F/HM trilayers of atomic thickness and demonstrate that it reproduces the main features of the spin-valve effect found within the Usadel approach. Our results are shown to be in qualitative agreement with the recent experimental data on the spin-valve effect in $\mathrm{MoGe}/\mathrm{Ni}/\mathrm{Cu}/{\mathrm{CrO}}_2$ hybrids [Singh et al., Phys. Rev. X 5, 021019 (2015)].

The evolution of the system of fermions with pairing under the action of a pulse (or a quench) of the external potential has been studied. In the dynamical regime the number of fermions oscillates (with beats) as a function of the duration of the pulse (or as a function of time for the quench) about the value determined by the magnitude of the pulse. Oscillations depend on the values of the potential and the pulse. The parameters of those oscillations permit one to determine characteristics of the dynamical quantum phase transitions, such as Fisher's zeros in one-dimensional systems. The effect is absent for zero pairing. In particular, the response of a topological superconductor to the pulse manifests dynamical oscillations, most pronounced for the range of parameters in which the topological superconductivity can occur. The response of Majorana edge states to the pulse is analyzed. The results are generalized for the bosonic system with pairing under the action of the pulse (quench).

We report on the analysis of the root mean square curvature as a function of the numerical resolution for a single reconnection of two quantized vortex rings in superfluid helium. We find a similar scaling relation as reported in the case of decaying thermal counterflow simulations by L. Kondaurova et al. There the scaling was related to the existence of a Kelvin-wave cascade which was suggested to support the L'vov-Nazarenko spectrum. Here we provide an alternative explanation that does not involve the Kelvin-wave cascade but is due to the sharp cusp generated by a reconnection event in a situation where the maximum curvature is limited by the computational resolution. We also suggest a method for identifying the Kelvin spectrum based on the decay of the rms curvature by mutual friction. Our vortex filament simulation calculations show that the spectrum of Kelvin waves after the reconnection is not simply $n\left(k\right)\propto k^{-\eta }$ with constant $\eta$. At large scales the spectrum seems to be close to the Vinen prediction with $\eta =3$ but becomes steeper at smaller scales.

Formed through a closed domain magnetic heterostructure on the surface of a three-dimensional topological insulator, a quantum dot permits a class of quantized interfacial states of topological origin. We find that these states exhibit a remarkable reverse Stark effect in response to an applied electric field. In particular, those topological states whose energies are within the gap exhibit peculiar electrical alignments that are opposite to those associated with the conventional quantum-confined Stark effect in that the positive- (negative-) energy states tend to align with (against) the direction of the field. The phenomenon has unusual implications for the associated optical transitions. Furthermore, the exotic topological states exhibit polarized spin textures that can be effectively controlled electrically or optically, opening an avenue for potential applications in Dirac-material-based spintronics.

We report microscopic studies by muon spin rotation/relaxation as a function of pressure of the ${\text{Ca}}_3{\text{Ir}}_4{\text{Sn}}_{13}$ and ${\text{Sr}}_3{\text{Ir}}_4{\text{Sn}}_{13}$ cubic compounds, which are members of the ${\left({\text{Ca}}_{1-x}{\text{Sr}}_x\right)}_3{\text{Ir}}_4{\text{Sn}}_{13}$ system displaying superconductivity and a structural phase transition associated with the formation of a charge density wave (CDW). We find a strong enhancement of the superfluid density and a dramatic increase of the pairing strength above a pressure of $\approx 1.6$ GPa, giving direct evidence of the presence of a quantum critical point separating a superconducting phase coexisting with CDW from a pure superconducting phase. The superconducting order parameter in both phases has the same $s$-wave symmetry. In spite of the conventional phonon-mediated BCS character of the weakly correlated ${\left({\text{Ca}}_{1-x}{\text{Sr}}_x\right)}_3{\text{Ir}}_4{\text{Sn}}_{13}$ system, the dependence of the effective superfluid density on the critical temperature puts this compound in the “Uemura” plot close to unconventional superconductors. This system exemplifies that conventional BCS superconductors in the presence of competing orders or multiband structure can also display characteristics of unconventional superconductors.

One of the most striking features of novel two-dimensional semiconductors (e.g., transition metal dichalcogenide monolayers or phosphorene) is a strong Coulomb interaction between charge carriers resulting in large excitonic effects. In particular, this leads to the formation of multicarrier bound states upon photoexcitation (e.g., excitons, trions, and biexcitons), which could remain stable at near-room temperatures and contribute significantly to the optical properties of such materials. In the present work we have used the path integral Monte Carlo methodology to numerically study properties of multicarrier bound states in two-dimensional semiconductors. Specifically, we have accurately investigated and tabulated the dependence of single-exciton, trion, and biexciton binding energies on the strength of dielectric screening, including the limiting cases of very strong and very weak screening. The results of this work are potentially useful in the analysis of experimental data and benchmarking of theoretical and computational models.

We show how to achieve optical, spin-selective transitions from the ground state to excited orbital states of group-V donors (P, As, Sb, and Bi) in silicon. We consider two approaches based on either resonant, far-infrared (IR) transitions of the neutral donor or resonant, near-IR excitonic transitions. For far-IR light, we calculate the dipole matrix elements between the valley-orbit and spin-orbit split states for all the group-V donors using effective mass theory. We then calculate the maximum rate and amount of electron-nuclear spin-polarization achievable through optical pumping with circularly polarized light. We find this approach is most promising for Bi donors due to their large spin-orbit and valley-orbit interactions. Using near-IR light, spin-selective excitation is possible for all the donors by driving a two-photon $\mathrm{\Lambda }$ transition from the ground state to higher orbitals with even parity. We show that externally applied electric fields or strain allow similar, spin-selective $\mathrm{\Lambda }$ transition to odd-parity excited states. We anticipate these results will be useful for future spectroscopic investigations of donors, quantum control and state preparation of donor spin qubits, and for developing a coherent interface between donor spin qubits and single photons.

We study the low-frequency admittance of a small metallic island coupled to a gate electrode and to a massive reservoir via a multichannel tunnel junction. The ac current is caused by a slowly oscillating gate voltage. We focus on the regime of inelastic cotunneling in which the dissipation of energy (the real part of the admittance) is determined by two-electron tunneling with creation of electron-hole pairs on the island. We demonstrate that at finite temperatures but low frequencies the energy dissipation is ohmic whereas at zero temperature it is superohmic. We find that (i) the charge relaxation resistance (extracted from the real part of the admittance) is strongly temperature dependent, and (ii) the imaginary and real parts of the admittance do not satisfy the Korringa-Shiba relation. At zero temperature the charge relaxation resistance vanishes in agreement with the recent zero-temperature analysis [M. Filippone and C. Mora, Phys. Rev. B 86, 125311 (2012); P. Dutt, T. L. Schmidt, C. Mora, and K. Le Hur, Phys. Rev. B 87, 155134 (2013)].

By using the variational Monte Carlo technique, we study the spin-$\frac{1}{2}$ XXZ antiferromagnetic model (with easy-plane anisotropy) on the kagome lattice. A class of Gutzwiller projected fermionic states with a spin Jastrow factor is considered to describe either spin liquids [with $U\left(1\right)$ or $Z_2$ symmetry] or magnetically ordered phases [with $\mathbf{q}=(0,0)$ or $\mathbf{q}=(4\pi /3,0)$]. We find that the magnetic states are not stable in the thermodynamic limit. Moreover, there is no energy gain to break the gauge symmetry from $U\left(1\right)$ to $Z_2$ within the spin-liquid states, as previously found in the Heisenberg model. The best variational wave function is therefore the $U\left(1\right)$ Dirac state, supplemented by the spin Jastrow factor. Furthermore, a vanishing $S=2$ spin gap is obtained at the variational level, in the whole regime from the $XY$ to the Heisenberg model.

We report on spin orientation and alignment of ${\mathrm{Mn}}^{2+}$ ions in (Cd,Mn)Te diluted magnetic semiconductor crystals using resonant intracenter excitation with circular- and linear-polarized light. The resulting polarized emission of the magnetic ions is observed at low temperatures when the spin relaxation time of the ${\mathrm{Mn}}^{2+}$ ions is in the order of $1\mathrm{ms}$, which considerably exceeds the photoluminescence decay time of $23\mu \mathrm{s}$. We demonstrate that the experimental data on optical orientation and alignment of ${\mathrm{Mn}}^{2+}$ ions can be explained using a phenomenological model that is based on the approximation of isolated centers.

The ultrafast coupling dynamics of coherent optical phonons and the photoexcited electron-hole plasma in the indirect gap semiconductor GaP are investigated by experiment and theory. For below-gap excitation and probing by 800-nm light, only the bare longitudinal optical (LO) phonons are observed. For above-gap excitation with 400-nm light, the photoexcitation creates a high density, nonequilibrium $e\text{−}h$ plasma, which introduces an additional, faster decaying oscillation due to an LO phonon-plasmon coupled (LOPC) mode. The LOPC mode frequency exhibits very similar behavior for both $n$- and $p$-doped GaP, downshifting from the LO to the transverse optical (TO) phonon frequency limits with increasing photoexcited carrier density. We assign the LOPC mode to the LO phonons coupled with the photoexcited multicomponent plasma. For the 400-nm excitation, the majority of the photoexcited electrons are scattered from the $\mathrm{\Gamma }$ valley into the satellite $X$ valley, while the light and spin-split holes are scattered into the heavy hole band, within 30 fs. The resulting mixed plasma is strongly damped, leading to the LOPC frequency appearing in the reststrahlen gap. Due to the large effective masses of the $X$ electrons and heavy holes, the coupled mode appears most distinctly at carrier densities $\gtrsim 5\times {10}^{18}{\mathrm{cm}}^{-3}$. We perform theoretical calculations of the nuclear motions and the electronic polarizations following an excitation with an ultrashort optical pulse to obtain the transient reflectivity responses of the coupled modes. We find that, while the longitudinal diffusion of photoexcited carriers is insignificant, the lateral inhomogeneity of the photoexcited carriers due to the laser intensity profile should be taken into account to reproduce the major features of the observed coupled mode dynamics.

We have explored, computationally and experimentally, the magnetic properties of (${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x$)${}_2\mathrm{B}$ alloys. Calculations provide a good agreement with experiment in terms of the saturation magnetization and the magnetocrystalline anisotropy energy with some difficulty in describing ${\mathrm{Co}}_2\mathrm{B}$, for which it is found that both full potential effects and electron correlations treated within dynamical mean field theory are of importance for a correct description. The material exhibits a uniaxial magnetic anisotropy for a range of cobalt concentrations between $x=0.1$ and $x=0.5$. A simple model for the temperature dependence of magnetic anisotropy suggests that the complicated nonmonotonic behavior is mainly due to variations in the band structure as the exchange splitting is reduced by temperature. Using density functional theory based calculations we have explored the effect of substitutionally doping the transition metal sublattice by the whole range of $5d$ transition metals and found that doping by Re or W elements should significantly enhance the magnetocrystalline anisotropy energy. Experimentally, W doping did not succeed in enhancing the magnetic anisotropy due to formation of other phases. On the other hand, doping by Ir and Re was successful and resulted in magnetic anisotropies that are in agreement with theoretical predictions. In particular, doping by 2.5 at. % of Re on the Fe/Co site shows a magnetocrystalline anisotropy energy which is increased by 50% compared to its parent (${\mathrm{Fe}}_{0.7}{\mathrm{Co}}_{0.3}$)${}_2\mathrm{B}$ compound, making this system interesting, for example, in the context of permanent magnet replacement materials or in other areas where a large magnetic anisotropy is of importance.

Femtosecond pump-probe spectroscopy of bismuth thin films has revealed strong dependencies of reflectivity and phonon frequency on film thickness in the range of $25-40\mathrm{nm}$. The reflectivity variations are ascribed to distinct electronic structures originating from strongly varying electronic temperatures and proximity of the film thickness to the optical penetration depth of visible light. The phonon frequency is redshifted by an amount that increases with decreasing film thickness under the same excitation fluence, indicating carrier density-dependent bond softening that increases due to suppressed diffusion of carriers away from the photoexcited region in thin films. The results have significant implications for nonthermal melting of bismuth as well as lattice heating due to inelastic electron-phonon scattering.

We study the instabilities towards (exotic) superconductivity of mixtures of $\text{spin-}1/2$ fermions coupled to scalar bosons on a two-dimensional square lattice with the dynamical cluster approximation (DCA) using a numerically exact continuous-time Monte-Carlo solver. The Bogoliubov bosons provide an effective phononic bath for the fermions and induce a nonlocal retarded interaction between the fermions, which can lead to (exotic) superconductivity. Because of the sign problem the biggest clusters we can study are limited to $2\times 2$ in size, but this nevertheless allows us to study the pairing instabilities, and their possible divergence, in the $s-$ and $d\text{-wave}$ channels as well as the competition with antiferromagnetic fluctuations. At fermionic half filling we find that the $d$ wave is stable when the mediated interaction by the bosons is of the same order as the bare fermionic repulsion. Its critical temperature can be made as high as the maximum one for $s$ wave, which opens perspectives for its detection in a cold atom experiment.

In the strictly periodic setting, the electric polarization of inversion-symmetric solids with and without time-reversal symmetry and the isotropic magnetoelectric response function of time-reversal-symmetric insulators are known to be topological invariants displaying an exact ${\mathbb{Z}}_2$ quantization. This quantization is stabilized by the symmetries. In the present work, we investigate the fate of such symmetry-stabilized topological invariants in the presence of a disorder which breaks the symmetries but restores them on average. Using a rigorous analysis, we conclude that the strict quantization still holds in these conditions. Numerical calculations confirm this prediction.

We present a study of electric instability in a two-dimensional electron gas system under high magnetic fields. As the applied dc electric current exceeds a threshold value $I_{th}$, we find that the longitudinal magnetoresistance $R_{xx}$ fluctuates and exhibits negative differential resistivity (NDR). The observed instability occurs only in well-separated low-lying Landau levels (LLs) with a filling factor $\nu \le 2$, and the onset of NDR can be described by the theory of Andreev et al. We find that $I_{th}$ increases with increasing magnetic field $B$ and the lattice temperature $T_L$. In contrast, NDR becomes more pronounced at higher $B$, but gradually diminishes with increasing $T_L$. Data analysis suggests that NDR is actuated by the suppression of $R_{xx}$ with increasing electric field, which can be understood in terms of the capability of the spectral diffusion of electrons and of electron transfer to higher levels via inelastic inter-LLs scattering in the limit of one-occupied LL.

Recent scanning tunneling microscopy experiments reveal niobium diselenide to support domains of striped ($1Q$) charge order side by side with its better-known triangular ($3Q$) phase, suggesting that small variations in local strain may induce a quantum phase transition between the two. We use a theoretical model of the charge order in ${\mathrm{NbSe}}_2$, based on a strong momentum- and orbital-dependent electron-phonon coupling, to study the effect of uniaxial strain. We find that as little as $0.1\%$ anisotropic shift in phonon energies breaks the threefold symmetry in favor of a $1Q$ state, in agreement with the experimental results. The altered symmetries change the transition into the ordered state from weakly first order in the $3Q$ case to second order in the $1Q$ regime. Modeling the pseudogap phase of ${\mathrm{NbSe}}_2$ as the range of temperatures above the onset of long-range order in which phase coherence is destroyed by local phonon fluctuations, we find a shortening of the local ordering wave vector with increasing temperature, complementing recent x-ray diffraction observations within the low-temperature phase.

We present a theory for the recombination of (charged) holons and doublons in one-dimensional organic Mott insulators, which is responsible for the decay of the photoexcited state. Due to the charge-spin separation, the dominant mechanism for recombination at low density of charges involves a multiphonon emission. We show that a reasonable coupling to phonons is sufficient to explain the fast recombination observed by pump-probe experiments in $\mathrm{ET}-{\mathrm{F}}_2\mathrm{TCNQ}$, whereby we can also account for the measured pressure dependence of the recombination rate.

Using first-principles calculations, we predict a Chern insulating phase in thin films of the ferromagnetic semimetal GdN. In contrast to previously proposed Chern insulator candidates, which mostly rely on honeycomb lattices, this system affords a great chance to realize the quantum anomalous Hall effect on a square lattice without either a magnetic substrate or transition metal doping, making synthesis easier. The band inversion between the $5d$ orbitals of Gd and $2p$ orbitals of N is verified by first-principles calculations based on density functional theory, and the band gap can be as large as 100 meV within the GdN trilayer. With a further increase of film thickness, the band gap tends to close and the metallic bulk property becomes obvious.