We report on ab initio studies of the spin Hall and spin Nernst effect in dilute ternary alloys. Our calculations are performed for a Cu host with different types of substitutional impurities. The obtained numerical results are well approximated by Matthiessen's rule relying on the constituent binary alloys. We show that the spin Nernst effect can be significantly more efficient in a ternary alloy with respect to the related binary alloys. Together with the application of Matthiessen's rule this opens an easy way to design materials for spintronics applications.

By minimizing the difference between the left- and the right-hand sides of the many-body time-dependent Schrödinger equation with the Slater-determinant wave function, we derive a nonadiabatic and self-interaction-free time-dependent single-particle effective potential, which is the generalization to the time-dependent case of the so-called localized Hartree-Fock potential. The new potential can be efficiently used within the framework of the time-dependent density-functional theory as we demonstrate by the evaluation of the wave vector and frequency-dependent exchange kernel $f_x^h(q,\omega )$ of the homogeneous electron gas. This is found to be nonsingular and causal, and it satisfies the positiveness of the dissipation, in contrast to the earlier known kernel from the first-order perturbation theory, which makes our $f_x^h$ promising for applications.

We examine the real-time photoinduced dynamics of a junction composed of the interacting electron systems and propose a path to enhance the solar-cell efficiency using $\mathit{multiplication}$ of electron-hole pairs caused by the electron interaction. The key ingredients for the multiplication are instability and/or metastability of insulator-to-metal transition, and the photoinduced dynamics of the band gap. The band-gap alignment of the junction is also important for efficiency of the carrier generation.

The effect of dipolar and exchange interactions within pairs of paramagnetic electronic states on Pauli-blockade-controlled spin-dependent transport and recombination rates during magnetic resonant spin excitation is studied numerically using the superoperator Liouville-space formalism. The simulations reveal that spin-Rabi nutation induced by magnetic resonance can control transition rates which can be observed experimentally by pulsed electrically (pEDMR) and pulsed optically (pODMR) detected magnetic resonance spectroscopies. When the dipolar coupling exceeds the difference of the pair partners’ Zeeman energies, several nutation frequency components can be observed, the most pronounced at $\sqrt{2}\gamma B_1$ ($\gamma$ is the gyromagnetic ratio, $B_1$ is the excitation field). Exchange coupling does not significantly affect this nutation component; however, it does strongly influence a low-frequency component $<\gamma B_1$. Thus, pEDMR/pODMR allow the simultaneous identification of exchange and dipolar interaction strengths.

We study theoretically the spectrum $\mathbf{F}$(s) of spin-dependent transition rates within dipolar $D$ and exchange $J$ coupled pairs of two spins with $S=\frac{1}{2}$ undergoing Rabi oscillations due to a coherent magnetic-resonant excitation. We show that the Rabi oscillation controlled rates exhibit a spectrum with three frequency components. When exchange $J$ is stronger than the driving field ${\Omega }_R$ (in the frequency units), the frequency components of the Rabi oscillation do not depend on $J$, rather they are determined by the relation between the ${\Omega }_R$ and $D$. We derive analytical expressions for the frequencies and the intensities of all three Rabi oscillation components as functions of ${\Omega }_R/D$ and $\delta /D$, where $\delta$ is detuning of the driving ac field from the Larmor frequency. When ${\Omega }_R\gg D$, the two lower frequencies approach $s={\Omega }_R$, while the upper line approaches $s=2{\Omega }_R$. Disorder of the local Larmor frequencies leads to a Gaussian broadening of the spectral lines. We calculate corresponding widths for different ${\Omega }_R/D$ and $\delta /D$. Unexpectedly, we find that one of the frequency components exhibits an unusual evolution with ${\Omega }_R$: its frequency decreases with ${\Omega }_R$ at ${\Omega }_R<D$. Upon further increase of ${\Omega }_R$, this frequency then passes through a minimum and, eventually, approaches $s={\Omega }_R$. Nonmonotonic behavior of the frequencies is accompanied by nonmonotonic behavior of the respective oscillation intensity.

We present the formation and decay dynamics of a trion (charged exciton) and exciton system in chemically $p$-doped single-walled carbon nanotubes (SWNTs) investigated by time-resolved photoluminescence measurements at 300 K. In (6,5) SWNTs with hole densities in the range of 0.27–1.05 nm${}^{-1}$, a trion is formed from the coalescence of an exciton and a free hole, with a rate of $1.7\times {10}^{13}$ nm s${}^{-1}$. The linear dependence of the trion decay rate on the hole density allows us to find that the trion decay is governed by both a trion-hole Auger recombination process and an internal Auger recombination process. The observed high efficiencies of Auger processes dominate over the radiative recombination rate of trions, which demonstrates dynamic properties of excitonic complexes in the presence of enhanced Coulomb interactions in one-dimensional systems.

We demonstrate a device concept for a lateral spin-transport structure consisting of ferromagnetic (Co${}_2$FeSi) stripes on a nonmagnetic ($n$-GaAs) transport channel. The basic building block of the device consists of a local spin valve which utilizes spin extraction instead of injection at the ferromagnetic contact stripes for its fundamental operation principle. An extended device comprises an array of such spin valves in which the spin polarization in the transport channel results from a cascade of spin extraction events. The achieved functionality can be described in terms of a nonvolatile reconfigurable current divider. We show that for $m$ ferromagnetic contacts, $2^{m-1}$ electrical output levels can occur, where each output level corresponds to a particular magnetization configuration of the entire stripe array.

We present a careful study of the resistive superconducting transition in FeSe single crystals down to $T=40$ mK in continuous magnetic fields up to 30 T applied perpendicular and parallel to the $ab$ plane. In the $\mathbf{H}\parallel \mathbf{c}$ geometry the temperature dependence of the resistive upper critical field $H_{c2}^{*}$, determined as the field at which the in-plane resistivity in the transition region is 90$\%$ of the normal state resistivity is down to temperatures $T/T_c<0.006$, is in close agreement with the Werthamer-Helfand-Hohenberg (WHH) theoretical curve which describes the behavior of the upper critical field in conventional type-II superconductors. In contrast, for the $\mathbf{H}\parallel \mathbf{ab}$ geometry, the data depart from the WHH model with increasing applied magnetic field according to the paramagnetic limitation of superconductivity. An anisotropy parameter $\gamma$ in our FeSe crystals decreases with decreasing temperature and FeSe becomes nearly isotropic when the temperature $T\to 0$.

We report on transport and magnetization measurements of the critical current density $J_c$ in ErNi${}_2$B${}_2$C single crystals that show strongly enhanced vortex pinning at the Néel temperature $T_N$ and low applied fields. The height of the observed $J_c$ peak decreases with increasing magnetic field in clear contrast with that of the peak effect found at the upper critical field. Angular transport measurements of $J_c$ revealed the correlated nature of this pinning enhancement, which we attribute to the formation of antiphase boundaries at $T_N$.

We investigate the effect of surface defects and the related low-coordinated surface atoms on the defect-induced magnetism in MgO nanocrystallites using hybrid density functional theory calculations. It has been demonstrated that when Mg vacancies are introduced at the surface or near surface of cubelike MgO clusters, a magnetic state ($S$ $\ge$ 1) becomes lower in total energy than the nonmagnetic singlet state ($S$ = 0) by several electron volts, resulting in the robust spin-polarized ground state. The total spin $S$ of the clusters in their ground state is equal to the number of the surface Mg vacancies introduced. The resulting spin density is not only located at the surrounding O atoms neighbor to the Mg vacancy site but is also extended to the low-coordinated surface O atoms along the $〈110〉$ direction, forming ferromagneticlike domains. This directional spin delocalization allows a remote (∼1 nm or longer) vacancy-vacancy interaction, eventually leading to a long-range ferromagnetic interaction.

We have investigated electronic structures and phonon spectra of newly discovered isostructural superconductors LaNiBN ($T_c=4.1$ K) and LaPtBN ($T_c=6.7$ K). We have found that their electronic structures are substantially three-dimensional, leading to metallicity both in NiB (PtB) and the intervening LaN layers. Our ab initio phonon calculations show that almost all phonon modes contribute to the electron-phonon coupling (EPC) mechanism, reflecting that both layers are involved in the superconductivity. For LaNiBN, we obtain an EPC strength of $\lambda =0.52$ and a logarithmically averaged characteristic phonon frequency of ${\omega }_{\log }=376$ K, leading to $T_c=3.9$ K. Compared with the Ni $B_{1g}$ mode in LaNiBN, the Pt $B_{1g}$ mode in LaPtBN is reduced by $\sim$70$\%$, leading to a slightly enhanced $\lambda =0.56$ and an $\sim$20$\%$ reduced ${\omega }_{\log }$. The estimated $T_c$ is 5.4 K for LaPtBN, in good agreement with the experiment. We do not find any indication of magnetic instability for either LaNiBN or LaPtBN, which implies that both systems are EPC mediated superconductors. Further, we have found an interesting trend of monotonic increase of $T_c$ with respect to the boron height in the NiB (PtB) layer of both borocarbide and boronitride superconductors, which suggests a possible way to enhance $T_c$ in these systems.

We present a detailed report of microwave irradiation of ultranarrow superconducting nanowires. In our nanofabricated circuits containing a superconducting NbSi nanowire, a dc blockade of current flow was observed at low temperatures below a critical voltage $V_{\mathrm{c}}$, a strong indicator of the existence of quantum phase-slip (QPS) in the nanowire. We describe the results of applying microwaves to these samples, using a range of frequencies and both continuous-wave and pulsed drive, in order to search for dual Shapiro steps which would constitute an unambiguous demonstration of quantum phase-slip. We observed no steps, and our subsequent thermal analysis suggests that the electron temperature in the series CrO resistors was significantly elevated above the substrate temperature, resulting in sufficient Johnson noise to wash out the steps. To understand the system and inform future work, we have constructed a numerical model of the dynamics of the circuit for dc and ac bias (both continuous-wave and pulsed drive signals) in the presence of Johnson noise. Using this model, we outline important design considerations for device and measurement parameters which should be used in any future experiment to enable the observation of dual Shapiro steps at experimentally accessible temperatures and, thus, lead to the development of a QPS-based quantum current standard.

We examine dissipation effects in a multichannel quantum RC circuit, comprising a cavity or single-electron box capacitively coupled to a gate and connected to a reservoir lead via several conducting channels. Depending on the engineering details of the quantum RC circuit, the number of channels contributing to transport varies, as does the form of the interchannel couplings. For low-frequency ac transport, the charge-relaxation resistance ($R_q$) is a nontrivial function of the parameters of the system. However, in the vicinity of the charge-degeneracy points and for weak tunneling, we find as a result of cross-mode mixing or channel asymmetry that $R_q$ becomes universal for a metallic cavity at low temperatures, and equals the unit of quantum resistance. To prove this universality, we map the system to an effective one-channel Kondo model, and construct an analogy with the Coulomb gas. Next, we probe the opposite regime of near-perfect transmission using a bosonization approach. Focusing on the two-channel case, we study the effect of backscattering at the lead-dot interface, more specifically, the role of an asymmetry in the backscattering amplitudes, and make a connection with the weak-tunneling regime near the charge-degeneracy points.

We use entanglement entropy signatures to establish non-Abelian topological order in projected Chern-insulator wave functions. The simplest instance is obtained by Gutzwiller projecting a filled band with Chern number $C=2$, whose wave function may also be viewed as the square of the Slater determinant of a band insulator. We demonstrate that this wave function is captured by the $\text{SU}{\left(2\right)}_2$ Chern-Simons theory coupled to fermions. This is established most persuasively by calculating the modular $\mathcal{S}$-matrix from the candidate ground-state wave functions, following a recent entanglement-entropy-based approach. This directly demonstrates the peculiar non-Abelian braiding statistics of Majorana fermion quasiparticles in this state. We also provide microscopic evidence for the field theoretic generalization, that the $N$th power of a Chern number $C$ Slater determinant realizes the topological order of the $\text{SU}{\left(N\right)}_C$ Chern-Simons theory coupled to fermions, by studying the $\text{SU}{\left(2\right)}_3$ (Read-Rezayi–type state) and the $\text{SU}{\left(3\right)}_2$ wave functions. An advantage of our projected Chern-insulator wave functions is the relative ease with which physical properties, such as entanglement entropy and modular $\mathcal{S}$-matrix, can be numerically calculated using Monte Carlo techniques.

Using low-temperature scanning tunneling microscopy, we demonstrate a one-dimensional system that undergoes a charge-density-wave (CDW) instability on a metallic substrate. For our measurements we utilize a self-assembled monatomic chain of Co atoms aligned by the steps on a vicinal Cu(111) surface. We assign the measured CDW instability to ferromagnetic interactions along the chain. We show that though the linear arrayed dimers are not electronically isolated, they are magnetically independent, and hence can potentially serve as a binary spin-memory system.

Ab initio $GW$ calculations are a standard method for computing the spectroscopic properties of many materials. The most computationally expensive part in conventional implementations of the method is the generation and summation over the large number of empty orbitals required to converge the electron self-energy. We propose a scheme to reduce the summation over empty states by the use of a modified static remainder approximation, which is simple to implement and yields accurate self-energies for both bulk and molecular systems requiring a small fraction of the typical number of empty orbitals.

We study the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction between two contact magnetic impurities placed on bilayer graphene (BLG). We compute the interaction mediated by the carriers of the pristine and biased BLG as well as the conduction electrons of the doped system. The results are obtained from the linear-response expression for the susceptibility written in terms of the integral over lattice Green's functions. For the unbiased system, we obtain some analytical expressions in terms of the Meijer $G$ functions, which consist of the product of two oscillatory terms: one coming from the interference between the two Dirac points and the second coming from the Fermi momentum. In particular, for the undoped BLG, the system exhibits the RKKY interaction commensurate with its bipartite nature as expected from the particle-hole symmetry of the system. Furthermore, we explore a beating pattern of oscillations of the RKKY interaction in a highly doped BLG system within the four-band continuum model. Besides, we discuss the discrepancy between the short-range RKKY interaction calculated from the two-band model and that obtained from the four-band continuum model. The final results for the applied gate voltage are obtained numerically and are fitted with the functional forms based on the results for the unbiased case. In this case, we show that the long-range behavior is scaled with a momentum that depends on Fermi energy and gate voltage, allowing the possibility of tuning of the RKKY interaction by gate voltage.

We present a phase-field crystal model for structural transformations in multicomponent alloys. The presented formalism builds upon the two-point correlation kernel developed in Greenwood et al. for describing structural transformations in pure materials [Greenwood, Provatas, and Rottler, Phys. Rev. Lett. 105, 045702 (2010)]. We introduce an effective two-point correlation function for multicomponent alloys that uses the local species concentrations to interpolate between different crystal structures. A simplified version of the model is derived for the particular case of three-component (ternary) alloys, and its equilibrium properties are demonstrated. Dynamical equations of motion for the density and multiple species concentration fields are derived, and the robustness of the model is illustrated with examples of complex microstructure evolution in dendritic and eutectic solidification and solid-state precipitation.