As a half-metal is spin polarized at its Fermi level by definition, it was conventionally thought to have little proximity effect to an $s$-wave superconductor. Here we show that with interface spin-orbit coupling $p_x+i{p}_y$ superconductivity without spin degeneracy is induced on the half-metal, and we give an estimate of its bulk energy gap. Therefore, a single-band half-metal can give us a topological superconductor with a single chiral Majorana edge state. Our band calculation shows that two atomic layers of VTe or CrO${}_2$ is a single-band half-metal for a wide range ($\sim$0.1 eV) of Fermi energy and thus is a suitable candidate material.

As low-loss nonlinear elements, Josephson junctions are the building blocks of superconducting qubits. The interaction of the qubit degree of freedom with the quasiparticles tunneling through the junction represents an intrinsic relaxation mechanism. We develop a general theory for the qubit decay rate induced by quasiparticles, and we study its dependence on the magnetic flux used to tune the qubit properties in devices such as the phase and flux qubits, the split transmon, and the fluxonium. Our estimates for the decay rate apply to both thermal equilibrium and nonequilibrium quasiparticles. We propose measuring the rate in a split transmon to obtain information on the possible nonequilibrium quasiparticle distribution. We also derive expressions for the shift in qubit frequency in the presence of quasiparticles.

We present a simple approach for the consideration of bias voltage within the Kohn-Sham formalism of density-functional theory. To be specific, the electronic charging of a metal-insulator-metal capacitor under bias voltage is considered explicitly. This is achieved by separating the Kohn-Sham orbitals around the Fermi level into anode or cathode parts and applying different Fermi levels in the determination of occupation numbers. The formal basis of the present approach is discussed in detail. We test this method against Au-vacuum-Au and Au-MgO-Au capacitors with various dielectric thicknesses. It is shown that the bulk optical and static dielectric constants can be obtained accurately. We also demonstrate that interface effects on capacitance can be investigated straightforwardly. Furthermore, we apply this method to the graphene capacitor and identify the quantum effects in the capacitance, which is well explained by the contribution of the kinetic energy of electrons to the capacitance. The present method can be readily implemented in conventional first-principles codes and provides a unified approach to evaluate capacitance of nanodevices.

We systematically investigate the optical extinction spectra of planar gold split-ring-resonator square arrays operating at $\sim$200-THz frequency versus the lattice constant and versus angle of incidence. We find a strong dependence of the resonance damping on the in-plane wave vector, namely, the resonance damping increases (decreases) versus the in-plane wave vector for small lattice constants (large lattice constants). By comparison with two simple one-dimensional models as well as with more complete numerical calculations, this behavior is interpreted in terms of a long-range retarded interaction among the split-ring resonators. In contrast, the assumptions of only nearest-neighbor interaction and/or of an instantaneous interaction lead to a striking disagreement with the overall experimental facts.

We show that the degenerate region which forms in the vicinity of the Fe/GaAs (001) interface in Schottky tunnel barrier heterostructures plays an important role in spin transport. First, it is a prerequisite for a significant spin current to exist in the absence of an applied bias voltage, which is essential for electrical spin detection. Second, it establishes a well-defined electrochemical potential reservoir which functions as the source of spin currents flowing into the bulk of the semiconductor. A rate-equation analysis shows that the optimal thickness of the highly doped GaAs layer at the interface is in the range of 20 to 25 nm for Si dopings of $5\times {10}^{18}$ cm${}^{-3}$, in reasonable agreement with experiment. Both the sign and magnitude of the spin currents are sensitive to very small changes in interfacial electronic structure, as demonstrated in annealing experiments.

Electron-plasmon coupling in graphene has been shown recently to give rise to a “plasmaron” quasiparticle excitation. The strength of this coupling has been predicted to depend on the effective screening, which in turn is expected to depend on the dielectric environment of the graphene sheet. Here we compare the strength of environmental screening for graphene on four different substrates by evaluating the separation of the plasmaron bands from the hole bands using angle-resolved photoemission spectroscopy. Comparison with $G_{0}W$-random phase approximation predictions are used to determine the effective dielectric constant of the underlying substrate layer. We also show that plasmaron and electronic properties of graphene can be independently manipulated, an important aspect of a possible use in “plasmaronic” devices.

We present the results of a muon-spin relaxation (${\mu }^{\text{+}}$SR) investigation into magnetic ordering in several families of layered quasi-two-dimensional molecular antiferromagnets based on transition-metal ions such as $S=\frac{1}{2}$ Cu${}^{\text{2+}}$ bridged with organic ligands such as pyrazine. In many of these materials magnetic ordering is difficult to detect with conventional magnetic probes. In contrast, ${\mu }^{\text{+}}$SR allows us to identify ordering temperatures and study the critical behavior close to $T_{\mathrm{N}}$. Combining this with measurements of in-plane magnetic exchange $J$ and predictions from quantum Monte Carlo simulations we may assess the degree of isolation of the 2D layers through estimates of the effective inter-layer exchange coupling and in-layer correlation lengths at $T_{\mathrm{N}}$. We also identify the likely metal-ion moment sizes and muon stopping sites in these materials, based on probabilistic analysis of the magnetic structures and of muon-fluorine dipole-dipole coupling in fluorinated materials.

The topological insulating phase results from inversion of the band gap due to spin-orbit coupling at an odd number of time-reversal symmetric points. In Bi${}_2$Se${}_3$, this inversion occurs at the $\Gamma$ point. For bulk Bi${}_2$Se${}_3$, we have analyzed the effect of arbitrary strain on the $\Gamma$ point band gap using density functional theory. By computing the band structure both with and without spin-orbit interactions, we consider the effects of strain on the gap via Coulombic interaction and spin-orbit interaction separately. While compressive strain acts to decrease the Coulombic gap, it also increases the strength of the spin-orbit interaction, increasing the inverted gap. Comparison with Bi${}_2$Te${}_3$ supports the conclusion that effects on both Coulombic and spin-orbit interactions are critical to understanding the behavior of topological insulators under strain, and we propose that the topological insulating phase can be effectively manipulated by inducing strain through chemical substitution.

There are a number of mechanisms that can produce frequency shifts in the NMR spectra of optically pumped semiconductors, including the hyperfine interaction, nuclear dipolar fields, and indirect or $J$ couplings. Using optically pumped Fe-doped InP, we explore how to experimentally distinguish these shift mechanisms from one another, and then exploit the shifts to measure the absolute nuclear polarization. Furthermore, we optically pump, using circularly polarized light, at a much lower field (2.35 T) than previous work, permitting us to explore the field dependence of the nuclear polarization rate, the spin-lattice relaxation time, and the NMR photon energy spectrum. We measure similar polarizations as obtained at higher fields, but with a significantly faster nuclear polarization rate, making operation at lower fields attractive for optically pumped InP as a source of nuclear spin polarization.

The elastic moduli of pure polycrystalline beta plutonium were measured over its full range of existence (417–491 K) using resonant ultrasound spectroscopy. The Debye temperature (138 K), Poisson's ratio (0.28), Gruneisen parameter (2.3), and the zero-temperature atomic volume (21.2 Å${}^3$) were computed from the measurements. Both bulk and shear moduli decrease smoothly on warming with expected discontinuities at the phase boundaries. The shear modulus is surprisingly nearly the same for beta and gamma Pu. The temperature dependence of bulk moduli for beta Pu is, like gamma Pu, unusually small. Poisson's ratio shows very strong differences among alpha, beta, and gamma Pu indicating they are entirely different metals. The zero-temperature elastic moduli were computed for the three phases as well as for gallium-stabilized delta Pu (also measured by us) and compared to calculations.

We demonstrate a mechanism for a dual layer, vertical field-effect transistor, in which nearly depleting one layer will extend its wave function to overlap the other layer and increase tunnel current. We characterize this effect in a specially designed GaAs/AlGaAs device, observing a tunnel current increase of two orders of magnitude at cryogenic temperatures, and we suggest extrapolations of the design to other material systems such as graphene.

Motivated by the occurrence of superconductivity transition in Cu-doped topological insulator Bi${}_2$Se${}_3$, we perform a combined study of low temperature scanning tunneling microscopy/spectroscopy and angle-resolved photoemission spectroscopy experiments and of ab initio density functional theory to clarify the doping nature of Cu atoms in Bi${}_2$Se${}_3$ films. By measuring the structural and electronic properties of the Cu-doped Bi${}_2$Se${}_3$ films at different doping temperatures, we find that Cu atoms behave as donors at intercalated and interstitial sites in Bi${}_2$Se${}_3$ films. Only the interstitial defect density plays an important role in the observation of Landau quantization of the topological surface states in Bi${}_2$Se${}_3$.

Spin-polarized radio frequency (RF) currents and RF-Oersted fields resonantly excite a magnetic vortex core confined in a micron-scale soft magnetic disk. In this study, we measured the rectifying voltage spectra caused by the anisotropic magnetoresistance oscillation due to the gyration of the vortex with different polarity and chirality. The measured spectra are presented such that we can determine the vortex properties and strength of the spin torques and Oersted field accurately and directly through analytical calculation.

We consider a system consisting of a honeycomb lattice of magneto-optical resonators. We show that this system can be very well described by a tight-binding model. Moreover, when truncated, this system supports a one-way edge mode. We discuss the microscopic physics, linking the properties of the one-way edge modes to the corresponding bulk modes.

PbZrO${}_3$/SrRuO${}_3$/SrTiO${}_3$ (100) epitaxial heterostructures with different thickness of the PbZrO${}_3$ (PZO) layer (d${}_{\mathrm{PZO}}$ ∼ 5–160 nm) were fabricated by pulsed laser deposition. The ultrathin PZO films (d${}_{\mathrm{PZO}}$ ≤ 10 nm) were found to possess a rhombohedral structure. On increasing the PZO film thickness, a bulk like orthorhombic phase started forming in the film with d${}_{\mathrm{PZO}}$ ∼ 22 nm and became abundant in the thicker films. Nanobeam electron diffraction and room-temperature micro-Raman measurements revealed that the stabilization of the rhombohedral phase of PZO could be attributed to the epitaxial strain accommodated by the heterostructures. Room-temperature polarization vs electric field measurements performed on different samples showed characteristic double hysteresis loops of antiferroelectric materials accompanied by a small remnant polarization for the thick PZO films (d${}_{\mathrm{PZO}}$ ≥ 50 nm). The remnant polarization increased by reducing the PZO layer thickness, and a ferroelectric like hysteresis loop was observed for the sample with d${}_{\mathrm{PZO}}$ ∼ 22 nm. Local ferroelectric properties measured by piezoresponse force microscopy also exhibited a similar thickness-dependent antiferroelectric-ferroelectric transition. Room-temperature electrical properties observed in the PZO thin films in correlation to their structural characteristics suggested that a ferroelectric rhombohedral phase could be stabilized in thin epitaxial PZO films experiencing large interfacial compressive stress.

The fundamental and optical gaps of relevant molecular systems are of primary importance for organic-based photovoltaics. Unfortunately, whereas optical gaps are accessible with time-dependent density functional theory (DFT), the highest-occupied – lowest-unoccupied eigenvalue gaps resulting from DFT calculations with semi-local or hybrid functionals routinely and severely underestimate the fundamental gaps of gas-phase organic molecules. Here, we show that a range-separated hybrid functional, optimally tuned so as to obey Koopmans’ theorem, provides fundamental gaps that are very close to benchmark results obtained from many-body perturbation theory in the GW approximation. We then show that using this functional does not compromise the possibility of obtaining reliable optical gaps from time-dependent DFT. We therefore suggest optimally tuned range-separated hybrid functionals as a practical and accurate tool for DFT-based predictions of photovoltaically relevant and other molecular systems.

We study the expansion of single-particle and two-particle imaginary-time Matsubara Green’s functions of quantum impurity models in the basis of Legendre orthogonal polynomials. We discuss various applications within the dynamical mean-field theory (DMFT) framework. The method provides a more compact representation of the Green’s functions than standard Matsubara frequencies, and therefore significantly reduces the memory-storage size of these quantities. Moreover, it can be used as an efficient noise filter for various physical quantities within the continuous-time quantum Monte Carlo impurity solvers recently developed for DMFT and its extensions. In particular, we show how to use it for the computation of energies in the context of realistic DMFT calculations in combination with the local density approximation to the density functional theory (LDA+DMFT) and for the calculation of lattice susceptibilities from the local irreducible vertex function.

We study the influence of different edge types on the electronic density of states of graphene nanostructures. To this end we develop an exact expansion for the single-particle Green’s function of ballistic graphene structures in terms of multiple reflections from the system boundary, which allows for a natural treatment of edge effects. We first apply this formalism to calculate the average density of states of graphene billiards. While the leading term in the corresponding Weyl expansion is proportional to the billiard area, we find that the contribution that usually scales with the total length of the system boundary differs significantly from what one finds in semiconductor-based, Schrödinger-type billiards: The latter term vanishes for armchair and infinite-mass edges and is proportional to the zigzag edge length, highlighting the prominent role of zigzag edges in graphene. We then compute analytical expressions for the density of states oscillations and energy levels within a trajectory-based semiclassical approach. We derive a Dirac version of Gutzwiller’s trace formula for classically chaotic graphene billiards and further obtain semiclassical trace formulas for the density of states oscillations in regular graphene cavities. We find that edge-dependent interference of pseudospins in graphene crucially affects the quantum spectrum.

We present an experimental and theoretical study of single semiconductor quantum dots excited by two nondegenerate, resonantly tuned, variably polarized lasers. The first laser is tuned to excitonic resonances. Depending on its polarization it photogenerates a coherent single-exciton state. The second laser is tuned to biexciton resonances. By scanning the energy of the second laser for various polarizations of the two lasers, while monitoring the emission from the biexciton and exciton spectral lines, we map the biexciton photoluminescence excitation spectra. The resonance-rich spectra of the second photon absorption are analyzed and fully understood in terms of a many-carrier theoretical model which takes into account the direct and exchange Coulomb interactions between the quantum confined carriers.

A quantum many-body system that is prepared in the ground state of an integrable Hamiltonian does not directly thermalize after a sudden small parameter quench away from integrability. Rather, it will be trapped in a prethermalized state and can thermalize only at a later stage. We discuss several examples for which this prethermalized state shares some properties with the nonthermal steady state that emerges in the corresponding integrable system. These examples support the notion that nonthermal steady states in integrable systems may be viewed as prethermalized states that never decay further. Furthermore, we show that prethermalization plateaus are under certain conditions correctly predicted by generalized Gibbs ensembles, which are the appropriate extension of standard statistical mechanics in the presence of many constants of motion. This establishes that the relaxation behaviors of integrable and nearly integrable systems are continuously connected and described by the same statistical theory.