Deep levels in Mg-doped GaN grown by metal organic chemical vapor deposition (MOCVD), undoped GaN grown by MOCVD, and halide vapor phase epitaxy (HVPE)-grown GaN have been studied using deep level transient spectroscopy and minority charge carrier transient spectroscopy on Schottky diodes. One hole trap, labeled HT1, was detected in the Mg-doped sample. It is observed that the hole emission rate of the trap is enhanced by increasing electric field. By fitting four different theoretical models for field-assisted carrier emission processes, the three-dimensional Coulombic Poole-Frenkel (PF) effect, three-dimensional square well PF effect, phonon-assisted tunneling, and one-dimensional Coulombic PF effect including phonon-assisted tunneling, it is found that the one-dimensional Coulombic PF model, including phonon-assisted tunneling, is consistent with the experimental data. Since the trap exhibits the PF effect, we suggest it is acceptorlike. From the theoretical model, the zero field ionization energy of the trap and an estimate of the hole capture cross section have been determined. Depending on whether the charge state is −1 or −2 after hole emission, the zero field activation energy $E_{i0}$ is 0.57 eV or 0.60 eV, respectively, and the hole capture cross section ${\sigma }_p$ is $1.3\times {10}^{-15}\mathrm{c}{\mathrm{m}}^2$ or $1.6\times {10}^{-16}\mathrm{c}{\mathrm{m}}^2$, respectively. Since the level was not observed in undoped GaN, it is suggested that the trap is associated with an Mg related defect.

The ability to transport quantum information across some distance can facilitate the design and operation of a quantum processor. One-dimensional spin chains provide a compact platform to realize scalable spin transport for a solid-state quantum computer. Here, we model odd-sized donor chains in silicon under a range of experimental nonidealities, including variability of donor position within the chain. We show that the tolerance against donor placement inaccuracies is greatly improved by operating the spin chain in a mode where the electrons are confined at the Si-${\mathrm{SiO}}_2$ interface. We then estimate the required time scales and exchange couplings, and the level of noise that can be tolerated to achieve high-fidelity transport. We also propose a protocol to calibrate and initialize the chain, thereby providing a complete guideline for realizing a functional donor chain and utilizing it for spin transport.

We study the polarization dynamics of a spatially expanding polariton condensate under nonresonant linearly polarized optical excitation. The spatially and temporally resolved polariton emission reveals the formation of nontrivial spin textures in the form of a quadruplet polarization pattern both in the linear and circular Stokes parameters, and an octuplet in the diagonal Stokes parameter. The continuous rotation of the polariton pseudospin vector through the condensate due to TE-TM splitting exhibits an ordered pattern of half-skyrmions associated with a half-integer topological number. A theoretical model based on a driven-dissipative Gross-Pitaevskii equation coupled with an exciton reservoir describes the dynamics of the nontrivial spin textures through the optical spin-Hall effect.

We theoretically propose a set of universal quantum gates acting on a hybrid qubit formed by coupling a quantum-dot spin qubit and Majorana fermion qubit. First, we consider a quantum dot that is tunnel coupled to two topological superconductors. The effective spin-Majorana exchange facilitates a hybrid cnot gate for which either qubit can be the control or target. The second setup is a modular scalable network of topological superconductors and quantum dots. As a result of the exchange interaction between adjacent spin qubits, a cnot gate is implemented that acts on neighboring Majorana qubits and eliminates the necessity of interqubit braiding. In both setups, the spin-Majorana exchange interaction allows for a phase gate, acting on either the spin or the Majorana qubit, and for a swap or hybrid swap gate which is sufficient for universal quantum computation without projective measurements.

We examine current fluctuations in tunnel junctions driven by a superposition of a constant and a sinusoidal voltage source. In standard setups, the external voltage is applied to the tunneling element via an impedance providing an electromagnetic environment of the junction. The modes of this environment are excited by the time-dependent voltage and are the source of Johnson-Nyquist noise. We determine the autocorrelation function of the current flowing in the leads of the junction in the weak tunneling limit up to terms of second order in the tunneling Hamiltonian. The driven modes of the electromagnetic environment are treated exactly by means of a unitary transformation introduced recently. Particular emphasis is placed on the spectral function of the current fluctuations. The spectrum is found to comprise three contributions: a term arising from the Johnson-Nyquist noise of the environmental impedance, a part due to the shot noise of the tunneling element, and a third contribution which comes from the cross correlation between fluctuations caused by the electromagnetic environment and fluctuations of the tunneling current. All three parts of the spectral function occur already for devices under dc bias. The spectral function of ac driven tunneling elements can be determined from the result for a dc bias by means of a photoassisted tunneling relation of the Tien-Gordon type. Specific results are given for an Ohmic environment and for a junction driven through a resonator.

We consider the motion of electrons confined to a two-dimensional plane with an externally applied perpendicular inhomogeneous magnetic field, both with and without a Coulomb potential. We find that as long as the magnetic field is slowly decaying, bound states in magnetic quantum dots are indeed possible. Several example cases of such magnetic quantum dots are considered in which one can find the eigenvalues and eigenfunctions in closed form, including two hitherto unknown quasi-exactly-solvable models treated with confluent and biconfluent Heun polynomials. It is shown how a modulation of the strength of the magnetic field can exclude magnetic vortexlike states, rotating with a certain angular momenta and possessing a definite spin orientation, from forming. This indicates one may induce localization-delocalization transitions and suggests a mechanism for spin separation.

We consider spin-dependent scatterers with large scattering cross sections in graphene—a Zeeman-like and an intrinsic spin-orbit coupling impurity—and show that a gated ring around them can be engineered to produce an efficient control of the spin-dependent transport, like current spin polarization and spin Hall angle. Our analysis is based on a spin-dependent partial-waves expansion of the electronic wave functions in the continuum approximation, described by the Dirac equation.

We study how the color and polarization of ultrashort pulses of visible light can be used to control the demagnetization processes of the antiferromagnetic insulator ${\mathrm{Cr}}_2{\mathrm{O}}_3$. We utilize time-resolved second harmonic generation (SHG) to probe how changes in the magnetic and structural state evolve in time. We show that varying the pump photon-energy to excite either localized transitions within the Cr or charge transfer states leads to markedly different dynamics. Through a full polarization analysis of the SHG signal, symmetry considerations, and density functional theory calculations, we show that, in the nonequilibrium state, SHG is sensitive to both lattice displacements and changes to the magnetic order, which allows us to conclude that different excited states couple to phonon modes of different symmetries. Furthermore, the spin-scattering rate depends on the induced distortion, enabling us to control the timescale for the demagnetization process. Our results suggest that selective photoexcitation of antiferromagnetic insulators allows fast and efficient manipulation of their magnetic state.

We report magnetic susceptibility $\chi$, isothermal magnetization $M$, heat capacity $C_{\mathrm{p}}$, and electrical resistivity $\rho$ measurements on undoped ${\mathrm{EuMn}}_2{\mathrm{As}}_2$ and K-doped ${\mathrm{Eu}}_{0.96}{\mathrm{K}}_{0.04}{\mathrm{Mn}}_2{\mathrm{As}}_2$ and ${\mathrm{Eu}}_{0.93}{\mathrm{K}}_{0.07}{\mathrm{Mn}}_2{\mathrm{As}}_2$ single crystals with the trigonal ${\mathrm{CaAl}}_2{\mathrm{Si}}_2$-type structure as a function of temperature $T$ and magnetic field $H$. ${\mathrm{EuMn}}_2{\mathrm{As}}_2$ has an insulating ground state with an activation energy of 52 meV and exhibits antiferromagnetic (AFM) ordering of the ${\mathrm{Eu}}^{+2}$ spins $S=\frac{7}{2}$ at $T_{\mathrm{N}1}=15$ K from $C_{\mathrm{p}}\left(T\right)$ and $\chi \left(T\right)$ data with a likely spin-reorientation transition at $T_{\mathrm{N}2}=5.0$ K. The ${\mathrm{Mn}}^{+2}3d^5$ spins-$\frac{5}{2}$ exhibit AFM ordering at $T_{\mathrm{N}}=142$ K from all three types of measurements. The $M\left(H\right)$ isotherm and $\chi \left(T\right)$ data indicate that the Eu AFM structure is both noncollinear and noncoplanar. The AFM structure of the Mn spins is also unclear. A 4% substitution of K for Eu in ${\mathrm{Eu}}_{0.96}{\mathrm{K}}_{0.04}{\mathrm{Mn}}_2{\mathrm{As}}_2$ is sufficient to induce a metallic ground state. Evidence is found for a difference in the AFM structure of the Eu moments in the metallic crystals from that of undoped ${\mathrm{EuMn}}_2{\mathrm{As}}_2$ versus both $T$ and $H$. For metallic ${\mathrm{Eu}}_{0.96}{\mathrm{K}}_{0.04}{\mathrm{Mn}}_2{\mathrm{As}}_2$ and ${\mathrm{Eu}}_{0.93}{\mathrm{K}}_{0.07}{\mathrm{Mn}}_2{\mathrm{As}}_2$, an anomalous S-shape $T$ dependence of $\rho$ related to the Mn magnetism is found. Upon cooling from 200 K, $\rho$ exhibits a strong negative curvature, reaches maximum positive slope at the Mn $T_{\mathrm{N}}\approx 150$ K, and then continues to decrease but more slowly below $T_{\mathrm{N}}$. This suggests that dynamic short-range AFM order of the Mn spins above the Mn $T_{\mathrm{N}}$ strongly suppresses the resistivity, contrary to the conventional decrease of $\rho$ that is only observed upon cooling below $T_{\mathrm{N}}$ of an antiferromagnet.

Thermal expansion and forced magnetostriction of MnSi were measured as a function of temperature down to 5 K and magnetic field to 3 T. The small length (volume) discontinuity at the magnetic phase transition in MnSi decreases with application of magnetic field to a value $\mathrm{\Delta }L/L\sim {10}^{-7}$, and then suddenly the discontinuity seemingly jumps to zero. Thermal expansivity peaks strongly deteriorate with magnetic fields. No specific features identifying a tricritical point were observed. We propose that the Frenkel concept of heterophase fluctuations may be relevant in the current case. Therefore, we suggest that the magnetic phase transition in MnSi always remains first order at any temperature and magnetic field, but the transition is progressively smoothed by heterophase fluctuations. These results question the applicability of a model of a fluctuation-induced first-order phase transition for MnSi. Probably a model of coupling of an order parameter with other degrees of freedom is more appropriate.

The entanglement properties of quenched quantum systems have been studied for a decade, however, results in dimensions other than $d=1$ are generally lacking. We remedy this by investigating the entanglement properties of bosonic critical systems in $d=3$, both numerically and analytically, comparing the free and the interacting critical quench of an $O\left(N\right)$ model. We find that the evolution of the entanglement entropy for these two systems is nearly identical, as expected from the “quasiparticle” picture. However, the low-lying entanglement spectrum is controlled by the different critical exponent of the two systems, and therefore these exponents may be extracted by purely entanglement-theoretic calculations. We verify this scaling numerically.

To understand the interfacial scattering effect on the anomalous Hall effect (AHE), we prepared multilayers of ${\left(\mathrm{F}{\mathrm{e}}_{\left(36/n\right)\mathrm{nm}}/\mathrm{A}{\mathrm{u}}_{(12/n)\mathrm{nm}}\right)}_n$ using an e-beam evaporator. This structure design allowed us to investigate the effect of interfacial scattering on the AHE, while keeping the samples' thickness and composition unchanged. We measured the (magneto)transport properties of the samples in a wide temperature range (10–310 K) with magnetic fields up to 50 kOe. We found that the scaling between the anomalous Hall resistivity $\left({\rho }_{\mathrm{AHE}}\right)$ and longitudinal resistivity $\left({\rho }_{xx}\right)$ can be roughly described by ${\rho }_{\mathrm{AHE}}\sim {\rho }_{xx}^{\gamma }$ with $\gamma =2.65\pm 0.10$ and 1.90 ± 0.04 for samples from $n=1$ to $n=4$ and samples from $n=4$ to $n=12$, respectively. Our quantitative analysis results showed that the interfacial scattering suppresses the contribution of the intrinsic mechanism and gives rise to a side-jump contribution.

The magnetic and structural properties of cobalt were investigated under high pressure (160 GPa) and low temperature (50 K), by synchrotron K-edge x-ray magnetic circular dichroism and x-ray diffraction. A quasihydrostatic equation of state was measured up to 160 GPa. We found that uniaxial stress plays a role in the hexagonal close packed-face centered cubic (hcp-fcc) structural transition pressure. Also, our data provide the first experimental evidence that changes of the $c/a$ ratio pressure derivative are related to the magnetic behavior. The complete extinction of ferromagnetism is observed above 130 GPa in a mixed hcp-fcc phase with no recovery upon cooling to 50 K, indicating that cobalt at 150 GPa is very likely nonmagnetic, i.e., characterized by zero local spin polarization. Density functional theory calculations point out that the K-edge x-ray magnetic circular dichroism (XMCD) signal is related to the $4p$ orbital moment rather than to the total spin moment and allow us to get a deeper insight into the K-edge XMCD measurements interpretation. The combination of novel theoretical results and experimental outputs provides a detailed scenario of the structural and magnetic properties of cobalt at these extreme conditions answering some previously unsolved issues.

${\mathrm{Re}}_6\mathrm{Hf}$, which crystallizes in $\alpha$-Mn structure(space group $I\overline{4}3m)$ without a spatial inversion center, is a superconductor with a superconducting transition temperature $T_c\approx 6.2\mathrm{K}$. The measurements of magnetic susceptibility $\left(\chi \right)$, resistivity $\left(\rho \right)$, and specific heat capacity $\left(C\right)$ were carried out. Bulk superconductivity is revealed by the jump at $T_c$ of the specific heat with $\mathrm{\Delta }C/{\gamma }_n{T}_c\approx 1.63$, suggesting moderate electron-electron coupling strength in this system. The upper critical field ${\mu }_0{H}_{c2}^{WHH}\left(0\right)$ was estimated to be of 89 kOe, and ${\mu }_0{H}_{c2}^{GL}\left(0\right)=107$ kOe, which is close to the Pauli limiting field. The Ginzburg Landau parameter ${\kappa }_{GL}=50.2$, indicates that ${\mathrm{Re}}_6\mathrm{Hf}$ is a type-II superconductor. The temperature dependence of the electronic specific heat $C_{el}\left(T\right)$ in the superconducting state can be explained by BCS theory. Furthermore, the magnetic-field dependence of $\gamma \left(H\right)$ is found to be linear with respect to $H$. These results imply a dominant $s$-wave superconductivity in ${\mathrm{Re}}_6\mathrm{Hf}$.

We report point-contact spectroscopic studies on high quality single crystalline $\beta \text{−}{\mathrm{PdBi}}_2$ with a superconducting $T_c=5.2\mathrm{K}$. Our measurements show a typical double-peak structure in the voltage-biased junction conductance at low temperatures, probably excluding the existence of topological Andreev bound states. The conductance curves can be well fitted with the standard Blonder-Tinkham-Klapwijk model for $s$-wave superconductors, yielding a superconducting gap ${\mathrm{\Delta }}_0\simeq 0.92\mathrm{meV}$ with $2{\mathrm{\Delta }}_0/k_B{T}_c\sim 4.1$. Our results strongly suggest that $\beta \text{−}{\mathrm{PdBi}}_2$ is a conventional intermediate-coupling $s$-wave superconductor.

It is generally believed that the interedge coupling destroys the quantum spin Hall (QSH) effect along with the gap opening at the Dirac points. Using first-principles calculations, we find that the quantized edge transport persists in the presence of interedge coupling in Ta intercalated epitaxial graphene on SiC(0001), being a QSH insulator with the nontrivial gap of 81 meV. In this case, the band is characterized by two perfect Dirac cones with different Fermi velocities, yet only one maintains the edge state feature. We attribute such an anomalous behavior to the orbital-dependent decay of edge states into the bulk, which allows the interedge coupling between just one pair of edge states rather than two.

We apply a generalized model for the determination and analysis of the dielectric function of optically anisotropic materials with color dispersion to phonon modes and show that it can also be generalized to excitonic polarizabilities and electronic band-band transitions. We take into account that the tensor components of the dielectric function within the Cartesian coordinate system are not independent of each other but are rather projections of the polarization of dipoles oscillating along directions defined by the, non-Cartesian, crystal symmetry and polarizability. The dielectric function is then composed of a series of oscillators pointing in different directions. The application of this model is exemplarily demonstrated for monoclinic ($\beta$-phase) ${\mathrm{Ga}}_2{\mathrm{O}}_3$ bulk single crystals. Using this model we are able to relate electronic transitions observed in the dielectric function to atomic bond directions and orbitals in the real space crystal structure. For a thin film revealing rotational domains we show that the optical biaxiality is reduced to uniaxial optical response.

In the presence of a (time-dependent) macroscopic electric field the electron dynamics of dielectrics cannot be described by the time-dependent density only. We present a real-time formalism that has the density and the macroscopic polarization $\mathcal{P}$ as key quantities. We show that a simple local function of $\mathcal{P}$ already captures long-range correlation in linear and nonlinear optical response functions. Specifically, after detailing the numerical implementation, we examine the optical absorption, the second- and third-harmonic generation of bulk Si, GaAs, AlAs, and CdTe, at different levels of approximation. We highlight links with ultranonlocal exchange-correlation functional approximations proposed within a linear response time-dependent density functional theory framework.

On the basis of many-body ab initio calculations, using the single-shot $G_0{W}_0$ method and Bethe-Salpeter equation, we study phosphorene nanoribbons (PNRs) in the two typical zigzag and armchair directions. The electronic structure, optical absorption, electron-hole (exciton) binding energy, exciton exchange splitting, and exciton wave functions are calculated for different sizes of PNRs. The typically strong splitting between singlet and triplet excitonic states make PNRs favorable systems for optoelectronic applications. Quantum confinement occurs in both kinds of PNRs, and it is stronger in the zPNRs, which behave like quasi-zero-dimensional systems. Scaling laws are investigated for the size-dependent behaviors of PNRs. The first bright excitonic state in PNRs is explored in detail.

Recent experiments on silicon nanostructures have seen breakthroughs toward scalable, long-lived quantum information processing. The valley degree of freedom plays a fundamental role in these devices, and the two lowest-energy electronic states of a silicon quantum dot can form a valley qubit. In this paper, we show that a single-atom high step at the silicon/barrier interface induces a strong interaction of the qubit and in-plane electric fields and that the strength of this interaction can be controlled by varying the relative position of the electron and the step. We analyze the consequences of this enhanced interaction on the dynamics of the qubit. The charge densities of the qubit states are deformed differently by the interface step, allowing nondemolition qubit readout via valley-to-charge conversion. A gate-induced in-plane electric field together with the interface step enables fast control of the valley qubit via electrically driven valley resonance. We calculate single- and two-qubit gate times, as well as relaxation and dephasing times, and present predictions for the parameter range where the gate times can be much shorter than the relaxation time and dephasing is reduced.