Structural origins of the properties of rare earth nickelate superlattices

NiO6 octahedral tilts in the LaNiO3/SrTiO3 superlattices are quantified using position averaged convergent beam electron diffraction in scanning transmission electron microscopy. It is shown that maintaining oxygen octahedra connectivity across the interface controls the octahedral tilts in the LaNiO3 layers, their lattice parameters and their transport properties. Unlike films and layers that are connected on one side to the substrate, subsequent LaNiO3 layers in the superlattice exhibit a relaxation of octahedral tilts towards bulk values. This relaxation is facilitated by correlated tilts in SrTiO3 layers and is correlated with the conductivity enhancement of the LaNiO3 layers in the superlattices relative to individual films.Comment: Accepted for publication in Physical Review B (Rapid Communication

Spin injection and electric field effect in degenerate semiconductors

We analyze spin-transport in semiconductors in the regime characterized by $T\stackrel{<}{\sim}T_F$ (intermediate to degenerate), where $T_F$ is the Fermi temperature. Such a regime is of great importance since it includes the lightly doped semiconductor structures used in most experiments; we demonstrate that, at the same time, it corresponds to the regime in which carrier-carrier interactions assume a relevant role. Starting from a general formulation of the drift-diffusion equations, which includes many-body correlation effects, we perform detailed calculations of the spin injection characteristics of various heterostructures, and analyze the combined effects of carrier density variation, applied electric field and Coulomb interaction. We show the existence of a degenerate regime, peculiar to semiconductors, which strongly differs, as spin-transport is concerned, from the degenerate regime of metals.Comment: Version accepted for publication in Phys. Rev.

Holonomic quantum computing in symmetry-protected ground states of spin chains

While solid-state devices offer naturally reliable hardware for modern classical computers, thus far quantum information processors resemble vacuum tube computers in being neither reliable nor scalable. Strongly correlated many body states stabilized in topologically ordered matter offer the possibility of naturally fault tolerant computing, but are both challenging to engineer and coherently control and cannot be easily adapted to different physical platforms. We propose an architecture which achieves some of the robustness properties of topological models but with a drastically simpler construction. Quantum information is stored in the symmetry-protected degenerate ground states of spin-1 chains, while quantum gates are performed by adiabatic non-Abelian holonomies using only single-site fields and nearest-neighbor couplings. Gate operations respect the symmetry, and so inherit some protection from noise and disorder from the symmetry-protected ground states.Comment: 19 pages, 4 figures. v2: published versio

Spin Currents Induced by Nonuniform Rashba-Type Spin-Orbit Field

We study the spin relaxation torque in nonmagnetic or ferromagnetic metals with nonuniform spin-orbit coupling within the Keldysh Green's function formalism. In non-magnet, the relaxation torque is shown to arise when the spin-orbit coupling is not uniform. In the absence of an external field, the spin current induced by the relaxation torque is proportional to the vector chirality of Rashba-type spin-orbit field (RSOF). In the presence of an external field, on the other hand, spin relaxation torque arises from the coupling of the external field and vector chirality of RSOF. Our result indicates that spin-sink or source effects are controlled by designing RSOF in junctions.Comment: 3 figure

Depth-Resolved Composition and Electronic Structure of Buried Layers and Interfaces in a LaNiO3_3/SrTiO3_3 Superlattice from Soft- and Hard- X-ray Standing-Wave Angle-Resolved Photoemission

LaNiO$_3$ (LNO) is an intriguing member of the rare-earth nickelates in exhibiting a metal-insulator transition for a critical film thickness of about 4 unit cells [Son et al., Appl. Phys. Lett. 96, 062114 (2010)]; however, such thin films also show a transition to a metallic state in superlattices with SrTiO$_3$ (STO) [Son et al., Appl. Phys. Lett. 97, 202109 (2010)]. In order to better understand this transition, we have studied a strained LNO/STO superlattice with 10 repeats of [4 unit-cell LNO/3 unit-cell STO] grown on an (LaAlO$_3$)$_{0.3}$(Sr$_2$AlTaO$_6$)$_{0.7}$ substrate using soft x-ray standing-wave-excited angle-resolved photoemission (SWARPES), together with soft- and hard- x-ray photoemission measurements of core levels and densities-of-states valence spectra. The experimental results are compared with state-of-the-art density functional theory (DFT) calculations of band structures and densities of states. Using core-level rocking curves and x-ray optical modeling to assess the position of the standing wave, SWARPES measurements are carried out for various incidence angles and used to determine interface-specific changes in momentum-resolved electronic structure. We further show that the momentum-resolved behavior of the Ni 3d eg and t2g states near the Fermi level, as well as those at the bottom of the valence bands, is very similar to recently published SWARPES results for a related La$_{0.7}$Sr$_{0.3}$MnO$_3$/SrTiO$_3$ superlattice that was studied using the same technique (Gray et al., Europhysics Letters 104, 17004 (2013)), which further validates this experimental approach and our conclusions. Our conclusions are also supported in several ways by comparison to DFT calculations for the parent materials and the superlattice, including layer-resolved density-of-states results

Shot noise in a diffusive F-N-F spin valve

Fluctuations of electric current in a spin valve consisting of a diffusive conductor connected to ferromagnetic leads and operated in the giant magnetoresistance regime are studied. It is shown that a new source of fluctuations due to spin-flip scattering enhances strongly shot noise up to a point where the Fano factor approaches the full Poissonian value.Comment: 5 pages, 3 figure

Pulsational pair-instability supernovae in gravitational-wave and electromagnetic transients

Current observations of binary black-hole ({BBH}) merger events show support for a feature in the primary BH-mass distribution at $\sim\,35\,\mathrm{M}_{\odot}$, previously interpreted as a signature of pulsational pair-instability (PPISN) supernovae. Such supernovae are expected to map a wide range of pre-supernova carbon-oxygen (CO) core masses to a narrow range of BH masses, producing a peak in the BH mass distribution. However, recent numerical simulations place the mass location of this peak above $50\,\mathrm{M}_{\odot}$. Motivated by uncertainties in the progenitor's evolution and explosion mechanism, we explore how modifying the distribution of BH masses resulting from PPISN affects the populations of gravitational-wave (GW) and electromagnetic (EM) transients. To this end, we simulate populations of isolated {BBH} systems and combine them with cosmic star-formation rates. Our results are the first cosmological BBH-merger predictions made using the \textsc{binary\_c} rapid population synthesis framework. We find that our fiducial model does not match the observed GW peak. We can only explain the $35\,\mathrm{M}_{\odot}$ peak with PPISNe by shifting the expected CO core-mass range for PPISN downwards by $\sim{}15\,\mathrm{M}_{\odot}$. Apart from being in tension with state-of-the art stellar models, we also find that this is likely in tension with the observed rate of hydrogen-less super-luminous supernovae. Conversely, shifting the mass range upward, based on recent stellar models, leads to a predicted third peak in the BH mass function at $\sim{}64\,\mathrm{M}_{\odot}$. Thus we conclude that the $\sim{}35\,\mathrm{M}_{\odot}$ feature is unlikely to be related to PPISNe.Comment: Accepted for publication in MNRAS. 19 pages, 8 figures includings appendice

Identification and tunable optical coherent control of transition-metal spins in silicon carbide

Color centers in wide-bandgap semiconductors are attractive systems for quantum technologies since they can combine long-coherent electronic spin and bright optical properties. Several suitable centers have been identified, most famously the nitrogen-vacancy defect in diamond. However, integration in communication technology is hindered by the fact that their optical transitions lie outside telecom wavelength bands. Several transition-metal impurities in silicon carbide do emit at and near telecom wavelengths, but knowledge about their spin and optical properties is incomplete. We present all-optical identification and coherent control of molybdenum-impurity spins in silicon carbide with transitions at near-infrared wavelengths. Our results identify spin $S=1/2$ for both the electronic ground and excited state, with highly anisotropic spin properties that we apply for implementing optical control of ground-state spin coherence. Our results show optical lifetimes of $\sim$60 ns and inhomogeneous spin dephasing times of $\sim$0.3 $\mu$s, establishing relevance for quantum spin-photon interfacing.Comment: Updated version with minor correction, full Supplementary Information include