The intrinsic point defects of $\beta$-${\mathrm{Ga}}_2{\mathrm{O}}_3$ are investigated using density functional theory. We have chosen two different exchange-correlation potentials: the generalized gradient approximation (GGA) and a hybrid potential (HSE06). Defect formation energies were determined taking into account finite-size effects. Schottky, anti-Frenkel, and Frenkel energies have been extracted for $T=0$ K. We calculate formation entropies for an oxygen and a gallium vacancy and determine the Gibbs energy of Schottky disorder. Furthermore, we investigate the defect concentrations as a function of the oxygen partial pressure. The obtained purely intrinsic defect concentrations for charged defects are very small and result in a $p{\mathrm{O}}_2$ dependence of the electron concentration of $\left[{\mathrm{e}}^{\prime }\right]\sim$ $p{\mathrm{O}}_2^{-1/6}$, whereas experimentally $\left[{\mathrm{e}}^{\prime }\right]\sim$ $p{\mathrm{O}}_2^{-1/4}$ is found. So we assume that, experimentally, a small unintentional donor doping is unavoidable. A small extrinsic donor concentration $\left[{\mathrm{D}}^{·}\right]$ $=$ 10${}^{18}$ cm${}^{-3}$ (10 ppm) changes the electron concentration to $\left[{\mathrm{e}}^{\prime }\right]\sim$ $p{\mathrm{O}}_2^{-1/4}$ and gives an activation energy of the conductivity $\sigma$ of 1.7 eV in good agreement to experimental values. So we propose as majority disorder $3\left[{\mathrm{V}}_{\mathrm{Ga}}^{\prime \prime \prime }\right]$ $=$ $\left[{\mathrm{D}}^{·}\right]$ with electrons being minority defects.

We calculate band-to-band radiative transition rate spectra in pure Ge as functions of applied tensile strain, heavy doping, charge injection density, and temperature. Direct and indirect phonon-assisted transitions are considered. Deformation potential theory is adopted to describe the conduction and valence near-gap edges. Biaxial strain has required appropriate treatment of system anisotropy through the evaluation of the mass tensor components for the different bands involved in the studied transitions. Population distribution in the $\Gamma$ and $L$ conduction valleys and in the valence states near $\Gamma$ have been evaluated accordingly considering the degeneracy condition of the sample, induced by high doping and high injection charge density. Also the effect of strain on the dipole matrix elements have been properly taken into account. The energy-resolved near-infrared spontaneous recombination spectra and the TE and TM polarized absorption/gain spectra have been obtained for a broad range of $n$-type doping, strain values, and excitation densities. We conclude that, despite the free carrier losses, net gain values as high as 6000 cm${}^{-1}$, are achievable for doping density and strain values reported in the literature.

The lattice-structure-mismatched CdTe/PbTe heterostructures are emerging materials with unique properties and promising applications in midinfrared optoelectronics and spintronics. High-density two-dimensional electron gas at the metastable twisted interfaces of CdTe/PbTe (111) is investigated experimentally and theoretically. CdTe thin films grown on PbTe(111) using molecular-beam epitaxy are found to have high electron mobilities and carrier concentrations from Hall-effect measurements: $2.02\times {10}^4$ cm${}^2$/V s and $4.5\times {10}^{18}$ cm${}^{-3}$ at 2 K, and $6.70\times {10}^3$ cm${}^2$/V s and $6.0\times {10}^{19}$ cm${}^{-3}$ at 77 K. Our density-functional theory modeling reveals that the epitaxially grown CdTe/PbTe (111) heterostructures form metastable twisted interfaces that exhibit unusual structural and electronic properties: (1) The PbTe epilayer on CdTe(111) substrates is structurally highly distorted from the rocksalt structure within the first ∼4.5 nm from the interface, which is caused by the stereochemical activity of the Pb $s^2$ lone pair. (2) The CdTe eplilayer on the PbTe(111) forms spontaneously a high density two-dimensional electron gas (2DEG) over 10${}^{13}$ cm${}^{-2}$ near the interface, without the need for doping, which explains the experimentally observed high carrier density and mobility. It is a much simpler heterostructure yet able to offer high electron mobility comparable to and one or two order magnitude higher sheet carrier density than the best achieved values for those of Si and II-VI based quantum well structures relying on modulation doping.

We investigate the role that noise plays in the hysteretic dynamics of a suspended nanotube or a graphene sheet subject to an oscillating force. We find that not only the size but also the position of the hysteresis region in these systems can be controlled by noise. We also find that nanoresonators act as noise rectifiers: By increasing the noise in the setup, the resonance width of the characteristic peak in these systems is reduced and, as a result, the quality factor is increased.

We analyze the low-voltage transport response of large molecular wires bridging graphene electrodes, where the molecules are physisorbed onto the graphene sheets by planar anchor groups. In our study, the sheets are pulled away to vary the gap length and the relative atomic positions. The molecular wires are also translated in directions parallel and perpendicular to the sheets. We show that the energy position of the Breit-Wigner molecular resonances is universal for a given molecule, in the sense that it is independent of the details of the graphene edges, gaps lengths, or of the molecule positions. We discuss the need to converge carefully the $k$ sampling to provide reasonable values of the conductance.

The intrinsic spin and valley Hall conductivities of silicene, germanene, and other similar two-dimensional crystals are explored theoretically. Particular attention is given to the effects of the intrinsic spin-orbit coupling, electron doping, and the type of insulating phase of the system (i.e., a topological insulator or a band insulator) which can be tuned by a perpendicular electric field. At finite frequency, the transverse edge to which carriers of a particular spin and valley label flow can be controlled such that an accumulation of a particular combination of spin and valley index can be obtained. The direction of flow is found to be dependent on the type of insulating phase. The magnitude of the Hall conductivity response is enhanced from the DC values at certain incident photon frequencies associated with the onset of interband transitions. Analytic results are presented for both the DC and finite-frequency results.

A single-molecule break junction device serves as a tunable model system for probing the many-body Kondo state. The low-energy properties of this state are commonly described in terms of a Kondo model, where the response of the system to different perturbations is characterized by a single emergent energy scale, $k_{\mathrm{B}}{T}_K$. Comparisons between different experimental systems have shown issues with numerical consistency. With a new constrained analysis examining the dependence of conductance on temperature, bias, and magnetic field simultaneously, we show that these deviations can be resolved by properly accounting for background, non-Kondo contributions to the conductance that are often neglected. We clearly demonstrate the importance of these non-Kondo conduction channels by examining transport in devices with total conductances exceeding the theoretical maximum due to Kondo-assisted tunneling alone.

Spin polarization of a two-dimensional electron gas with Rashba spin-orbit interaction, induced by a thermo-current, is considered theoretically. It is shown that a temperature gradient gives rise to an in-plane spin polarization of the electron gas, which is normal to the temperature gradient. The low-temperature spin polarization changes sign when the Fermi level crosses the bottom edge of the upper electronic subband. We also compare the results with spin polarization induced by an external electric field (current).

The resonant-level model represents a paradigmatic quantum system which serves as a basis for many other quantum impurity models. We provide a comprehensive analysis of the nonequilibrium transport near a quantum phase transition in a spinless dissipative resonant-level model, extending earlier work [Phys. Rev. Lett 102, 216803 (2009)]. A detailed derivation of a rigorous mapping of our system onto an effective Kondo model is presented. A controlled energy-dependent renormalization-group approach is applied to compute the nonequilibrium current in the presence of a finite bias voltage $V$. In the linear-response regime $V\to 0$, the system exhibits as a function of the dissipative strength a localized-delocalized quantum transition of the Kosterlitz-Thouless (KT) type. We address fundamental issues of the nonequilibrium transport near the quantum phase transition: Does the bias voltage play the same role as temperature to smear out the transition? What is the scaling of the nonequilibrium conductance near the transition? At finite temperatures, we show that the conductance follows the equilibrium scaling for $V<T$, while it obeys a distinct nonequilibrium profile for $V>T$. We furthermore provide different signatures of the transition in the finite-frequency current noise and ac conductance via a recently developed functional renormalization group (FRG) approach. The generalization of our analysis to nonequilibrium transport through a resonant level coupled to two chiral Luttinger liquid leads, generated by fractional quantum Hall edge states, is discussed. Our work on the dissipative resonant level has direct relevance to experiments on a quantum dot coupled to a resistive environment, such as H. Mebrahtu et al., [Nature (London) 488, 61 (2012)].

We model the processes induced in a LiF crystal by a swift ion. The impinging ion creates an ionization cylinder of doubly ionized F${}^{+}$ ions, while simultaneously the corresponding electrons are set free. Ions move according to molecular dynamics, while excited electrons are treated by a particle-in-cell scheme. We treat the recombination time of electrons as a free parameter in our model. Electrons are found to partially shield the ions in the track; the shielding is, however, minor (10$\%$ on the average), and occurs mainly in the first 50 fs of the process. We characterize the ion track formed inside the target; it consists of underdense (60$\%$ of the initial density), highly defective material. Within the first 50 fs, a Coulomb explosion is seen in the ion trail. Its most prominent effect is the emission of fast (7–8 eV) Li${}^{+}$ ions from the solid. It is caused by electrical fields at the target surface of the order of ${10}^{10}$ V/m.

We derive effective Hamiltonians for the fractional quantum Hall effect in $n=0$ and $n=1$ Landau levels that account perturbatively for Landau-level mixing by electron-electron interactions. To second order in the ratio of electron-electron interaction to cyclotron energy, Landau-level mixing is accounted for by constructing effective interaction Hamiltonians that include two-body and three-body contributions characterized by Haldane pseudopotentials. Our study builds upon previous treatments, using as a stepping stone the observation that the effective Hamiltonian is fully determined by the few-body problem with $N=2$ and $N=3$ electrons in the partially filled Landau level. For the $n=0$ case we use a first quantization approach to provide a compact and transparent derivation of the effective Hamiltonian which captures a class of virtual processes omitted in earlier derivations of Landau-level-mixing corrected Haldane pseudopotentials.

We report in-plane magnetization measurements on Ni/Nb/Ni/CoO and Co/Nb/Co/CoO spin valve structures with one of the ferromagnetic layers pinned by an antiferromagnetic layer. In samples with Ni below the superconducting transition $T_{\mathrm{c}}$, our results show strong evidence of vortex flipping driven by ferromagnet magnetization. This is a direct consequence of the proximity effect that leads to vortex supercurrent leakage into the ferromagnets. Here, the polarized electron spins are subject to the vortices’ magnetic field, occasioning vortex flipping. Such a novel mechanism has been made possible by fabrication of the ferromagnet/superconductor/ferromagnet/antiferromagnet multilayered spin valves with an S layer thin enough to barely confine vortices inside as well as F layers thin enough to align and control magnetization within the plane. When Co is used, the vortex flipping effect is not observed. This is attributed to the shorter coherence length of Co. Interestingly, a reduction in the pinning field of about 400 Oe is observed instead when the Nb layer is in the superconducting state. This effect cannot be explained in terms of vortex fields. In view of these facts, any explanation must be directly related to the proximity effect and thus a remarkable phenomenon that deserves further investigation.

The implementation of a Lambda scheme in superconducting artificial atoms could allow detection of stimulated Raman adiabatic passage (STIRAP) and other quantum manipulations in the microwave regime. However, symmetries which on one hand protect the system against decoherence yield selection rules which may cancel coupling to the pump external drive. The tradeoff between efficient coupling and decoherence due to broad-band colored noise (BBCN), which is often the main source of decoherence, is addressed in the class of nanodevices based on the Cooper pair box (CPB) design. We study transfer efficiency by STIRAP, showing that substantial efficiency is achieved for off-symmetric bias only in the charge-phase regime. We find a number of results uniquely due to non-Markovianity of BBCN, namely (a) the efficiency for STIRAP depends essentially on noise channels in the trapped subspace; (b) low-frequency fluctuations can be analyzed and represented as fictitious correlated fluctuations of the detunings of the external drives; and (c) a simple figure of merit for design and operating prescriptions allowing the observation of STIRAP is proposed. The emerging physical picture also applies to other classes of coherent nanodevices subject to BBCN.

The classical phase diagram of the Kane-Mele-Heisenberg model is obtained using three complementary methods: Luttinger-Tisza, variational minimization, and the iterative minimization method. Six distinct phases were obtained in the space of the couplings. Three phases are commensurate with long-range ordering: planar Néel states in horizontal plane (phase I), planar states in the plane vertical to the horizontal plane (phase VI), and collinear states normal to the horizontal plane (phase II). However, the other three are infinitely degenerate due to the frustrating competition between the couplings, and they are characterized by a manifold of incommensurate wave vectors. These phases are planar helical states in a horizontal plane (phase III), planar helical states in a vertical plane (phase IV), and non-coplanar states (phase V). Employing the linear spin-wave analysis, it is found that the quantum fluctuations select a set of symmetrically equivalent states in phase III through the quantum order-by-disorder mechanism. Based on some heuristic arguments, it is argued that the same scenario may also occur in the other two frustrated phases VI and V.

Ultrafast laser-induced magnetic switching in rare earth-transition metal ferrimagnetic alloys has recently been reported to occur by ultrafast heating alone. Using atomistic simulations and a ferrimagnetic Landau-Lifshitz-Bloch formalism, we demonstrate that for switching to occur it is necessary that angular momentum is transferred from the longitudinal to transverse magnetization components in the transition metal. This dynamical path leads to the transfer of the angular momentum to the rare earth metal and magnetization switching with subsequent ultrafast precession caused by the intersublattice exchange field on the atomic scale.

On the basis of Eilenberger theory, we study the vortex state when a magnetic field is applied nearly parallel to the $ab$ plane in a chiral $p$-wave superconductor with a large anisotropy ratio of $ab$ and $c$, as in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$. We quantitatively estimate the field dependence of the pair potential, magnetization, and flux-line lattice form factor, and study the transition from the chiral $p_{-}$ state at low fields to the nonchiral $p_y$ state at high fields. Even for exactly parallel fields to the $ab$ plane, transverse fields exist in the chiral state. The chiral-nonchiral transition disappears when the magnetic field orientation is tilted within $1^{\circ }$ from the $ab$ plane. This may be a reason why the experimental detection of this transition is difficult.

The synthesis and crystallography, magnetic susceptibility $\chi$, magnetization $M$, specific heat $C_{\mathrm{p}}$, in-plane electrical resistivity $\rho$, and in-plane magnetic penetration depth measurements are reported for single crystals of $A$Pd${}_2$As${}_2$ ($A$ = Ca, Sr, Ba) versus temperature $T$ and magnetic field $H$. The crystals were grown using PdAs self-flux. CaPd${}_2$As${}_2$ and SrPd${}_2$As${}_2$ crystallize in a collapsed body-centered tetragonal ${\mathrm{ThCr}}_2{\mathrm{Si}}_2$-type structure ($I4/mmm$), whereas BaPd${}_2$As${}_2$ crystallizes in the primitive tetragonal CeMg${}_2$Si${}_2$-type structure ($P4/mmm$), in agreement with literature data. The $\rho \left(T\right)$ data exhibit metallic behavior for all three compounds. Bulk superconductivity is reported for CaPd${}_2$As${}_2$ and SrPd${}_2$As${}_2$ below $T_{\mathrm{c}}=1.27$ and 0.92 K, respectively, whereas only a trace of superconductivity is found in BaPd${}_2$As${}_2$. No other phase transitions were observed. The $\chi \left(T\right)$ and $M\left(H\right)$ data reveal anisotropic diamagnetism in the normal state, with ${\chi }_c>{\chi }_{ab}$ for CaPd${}_2$As${}_2$ and BaPd${}_2$As${}_2$, and ${\chi }_c<{\chi }_{ab}$ for SrPd${}_2$As${}_2$. The normal and superconducting state data indicate that CaPd${}_2$As${}_2$ and SrPd${}_2$As${}_2$ are conventional type-II nodeless $s$-wave electron-phonon superconductors. The electronic superconducting state heat capacity data for CaPd${}_2$As${}_2$, which has an extremely sharp heat capacity jump at $T_{\mathrm{c}}$, are analyzed using our recent elaboration of the $\alpha$-model of the BCS theory of superconductivity, which indicates that the $s$-wave gap in this compound is anisotropic in momentum space.

In low-dimensional electronic materials, the charge or spin ordering can be subtly controlled by specific mode or modes, giving rise to functioning states such as charge- and spin-density waves, Mott insulators, and superconductors. The coupling between the electrons and the atomic lattice can be effectively investigated by ultrafast optical, photoemission, and electron diffraction techniques providing detailed description of microscopic and collective state evolutions in separate electronic and lattice subsystems. However, the electronic and phononic relaxation time scales obtained from these techniques are often distinctly different in low-dimensional electronic materials, even in a system as simple as graphite. Here, we seek to understand their origins from examining the nonequilibrium scenarios considering anisotropic electron-phonon coupling leading to hot phonons, which can be investigated directly from the momentum-dependent scattering changes in the transmission ultrafast electron crystallography. A three-temperature model is constructed to achieve unified understandings combining ultrafast spectroscopy and diffraction results of the nonadiabatic optically driven dynamics in graphite, charge-density waves in CeTe${}_3$, and the Mott insulator VO${}_2$.

We use the operator product expansion to derive exact results for the momentum distribution and the static structure factor at high momentum for a jellium model of electrons in both two and three dimensions. It is shown that independent of the precise state of the Coulomb system and for arbitrary temperatures, the asymptotic behavior is a power law in the momentum, whose strength is determined by the contact value of the pair distribution function $g\left(0\right)$. The power-law tails are quantum effects which vanish in the classical limit $\hslash \to 0$. A leading-order virial expansion shows that the classical and the high-temperature limit do not agree.

We propose and experimentally verify a way to achieve strong nonlinear coupling between the electromagnetic and elastic properties in metamaterials. This coupling is provided through a novel degree of freedom in metamaterial design: the internal rotation within structural elements. Our meta-atoms have high sensitivity to electromagnetic wave power, and the elastic and electromagnetic properties can be independently designed to optimize the response. We demonstrate a rich range of nonlinear phenomena including self-tuning and bistability, and provide a comprehensive experimental demonstration of the predicted effects.