Quantum transport theory for nanostructures with Rashba spin-orbital interaction

We report on a general theory for analyzing quantum transport through devices in the Metal-QD-Metal configuration where QD is a quantum dot or the device scattering region which contains Rashba spin-orbital and electron-electron interactions. The metal leads may or may not be ferromagnetic, they are assumed to weakly couple to the QD region. Our theory is formulated by second quantizing the Rashba spin-orbital interaction in spectral space (instead of real space), and quantum transport is then analyzed within the Keldysh nonequilibrium Green's function formalism. The Rashba interaction causes two main effects to the Hamiltonian: (i) it gives rise to an extra spin-dependent phase factor in the coupling matrix elements between the leads and the QD; (ii) it gives rise to an inter-level spin-flip term but forbids any intra-level spin-flips. Our formalism provides a starting point for analyzing many quantum transport issues where spin-orbital effects are important. As an example, we investigate transport properties of a Aharnov-Bohm ring in which a QD having Rashba spin-orbital and e-e interactions is located in one arm of the ring. A substantial spin-polarized conductance or current emerges in this device due to a combined effect of a magnetic flux and the Rashba interaction. The direction and strength of the spin-polarization are shown to be controllable by both the magnetic flux and a gate voltage.Comment: 12 pages, 8 figure

Resonant spin Hall conductance in quantum Hall systems lacking bulk and structural inversion symmetry

Following a previous work [Shen, Ma, Xie and Zhang, Phys. Rev. Lett. 92, 256603 (2004)] on the resonant spin Hall effect, we present detailed calculations of the spin Hall conductance in two-dimensional quantum wells in a strong perpendicular magnetic field. The Rashba coupling, generated by spin-orbit interaction in wells lacking bulk inversion symmetry, introduces a degeneracy of Zeeman-split Landau levels at certain magnetic fields. This degeneracy, if occuring at the Fermi energy, will induce a resonance in the spin Hall conductance below a characteristic temperature of order of the Zeeman energy. At very low temperatures, the spin Hall current is highly non-ohmic. The Dresselhaus coupling due to the lack of structure inversion symmetry partially or completely suppresses the spin Hall resonance. The condition for the resonant spin Hall conductance in the presence of both Rashba and Dresselhaus couplings is derived using a perturbation method. In the presence of disorder, we argue that the resonant spin Hall conductance occurs when the two Zeeman split extended states near the Fermi level becomes degenerate due to the Rashba coupling and that the the quantized charge Hall conductance changes by 2e^2/h instead of e^2/h as the magnetic field changes through the resonant field.Comment: 9 pages, 7 figures. This is a sequel to Physical Review Letters 90, 256603 (2004

Constraining the nature of the accreting binary in CXOGBS J174623.5-310550

We report optical and infrared observations of the X-ray source CXOGBS J174623.5-310550. This Galactic object was identified as a potential quiescent low-mass X-ray binary accreting from an M-type donor on the basis of optical spectroscopy and the broad Halpha emission line. The analysis of X-shooter spectroscopy covering 3 consecutive nights supports an M2/3-type spectral classification. Neither radial velocity variations nor rotational broadening is detected in the photospheric lines. No periodic variability is found in I- and r'-band light curves. We derive r' = 20.8, I = 19.2 and Ks = 16.6 for the optical and infrared counterparts with the M-type star contributing 90% to the I-band light. We estimate its distance to be 1.3-1.8 kpc. The lack of radial velocity variations implies that the M-type star is not the donor star in the X-ray binary. This could be an interloper or the outer body in a hierarchical triple. We constrain the accreting binary to be a < 2.2 hr orbital period eclipsing cataclysmic variable or a low-mass X-ray binary lying in the foreground of the Galactic Bulge.Comment: (9 pages, 5 figures, accepted for publication in MNRAS

The Architect Who Lost the Ability to Imagine: The Cerebral Basis of Visual Imagery.

While the loss of mental imagery following brain lesions was first described more than a century ago, the key cerebral areas involved remain elusive. Here we report neuropsychological data from an architect (PL518) who lost his ability for visual imagery following a bilateral posterior cerebral artery (PCA) stroke. We compare his profile to three other patients with bilateral PCA stroke and another architect with a large PCA lesion confined to the right hemisphere. We also compare structural images of their lesions, aiming to delineate cerebral areas selectively lesioned in acquired aphantasia. When comparing the neuropsychological profile and structural magnetic resonance imaging (MRI) for the aphantasic architect PL518 to patients with either a comparable background (an architect) or bilateral PCA lesions, we find: (1) there is a large overlap of cognitive deficits between patients, with the very notable exception of aphantasia which only occurs in PL518, and (2) there is large overlap of the patients' lesions. The only areas of selective lesion in PL518 is a small patch in the left fusiform gyrus as well as part of the right lingual gyrus. We suggest that these areas, and perhaps in particular the region in the left fusiform gyrus, play an important role in the cerebral network involved in visual imagery

Coherent current transport in wide ballistic Josephson junctions

We present an experimental and theoretical investigation of coherent current transport in wide ballistic superconductor-two dimensional electron gas-superconductor junctions. It is found experimentally that upon increasing the junction length, the subharmonic gap structure in the current-voltage characteristics is shifted to lower voltages, and the excess current at voltages much larger than the superconducting gap decreases. Applying a theory of coherent multiple Andreev reflection, we show that these observations can be explained in terms of transport through Andreev resonances.Comment: 4 pages, 4 figure

Zero-field spin splitting in InAs-AlSb quantum wells revisited

We present magnetotransport experiments on high-quality InAs-AlSb quantum wells that show a perfectly clean single-period Shubnikov-de Haas oscillation down to very low magnetic fields. In contrast to theoretical expectations based on an asymmetry induced zero-field spin splitting, no beating effect is observed. The carrier density has been changed by the persistent photo conductivity effect as well as via the application of hydrostatic pressure in order to influence the electric field at the interface of the electron gas. Still no indication of spin splitting at zero magnetic field was observed in spite of highly resolved Shubnikov- de Haas oscillations up to filling factors of 200. This surprising and unexpected result is discussed in view of other recently published data.Comment: 4 pages, 3 figures, submitted to Phys. Rev.

The binary mass ratio in the black hole transient MAXI J1820+070

We present intermediate resolution spectroscopy of the optical counterpart to the black hole X-ray transient MAXI J1820+070 (=ASASSN-18ey) obtained with the OSIRIS spectrograph on the 10.4-m Gran Telescopio Canarias. The observations were performed with the source close to the quiescent state and before the onset of renewed activity in August 2019. We make use of these data and K-type dwarf templates taken with the same instrumental configuration to measure the projected rotational velocity of the donor star. We find $v_{rot} \sin i = 84 \pm 5$ km s$^{-1}$ ($1\!-\!\sigma$), which implies a donor to black-hole mass ratio $q = {M_2}/{M_1} = 0.072 \pm 0.012$ for the case of a tidally locked and Roche-lobe filling donor star. The derived dynamical masses for the stellar components are $M_1 = (5.95 \pm 0.22)\sin ^{-3}i$ $M_\odot$ and $M_2 = (0.43 \pm 0.08) \sin^{-3}i$ $M_\odot$. The use of $q$, combined with estimates of the accretion disk size at the time of the optical spectroscopy, allows us to revise our previous orbital inclination constraints to $66^{\circ} < i < 81^{\circ}$. These values lead to 95% confidence level limits on the masses of $5.73 <M_1(M_\odot) < 8.34$ and $0.28 < M_2(M_\odot) < 0.77$. Adopting instead the $63 \pm 3^{\circ}$ orientation angle of the radio jet as the binary inclination leads to $M_1 = 8.48^{+0.79}_{-0.72} M_\odot$ and $M_2 = 0.61^{+0.13}_{-0.12} M_\odot$ ($1\!-\!\sigma$).Comment: 8 pages, 2 figures. Accepted for publication in ApJ

Effect of Vasopressin on the Hypothalamic-Pituitary-Adrenal Axis in ADPKD Patients during V2 Receptor Antagonism

Background: Patients with autosomal dominant polycystic kidney disease (ADPKD) are treated with a vasopressin V2 receptor antagonist (V2RA) to slow disease progression. This drug increases vasopressin considerably in these patients with already elevated baseline levels. Vasopressin is known to stimulate the hypothalamic-pituitary-adrenal (HPA) axis through V1 and V3 receptor activation. It is unknown whether this increase in vasopressin during V2RA treatment affects glucocorticoid production. Methods: Twenty-seven ADPKD patients were studied on and off treatment with a V2RA and compared to age- and sex-matched healthy controls and IgA nephropathy patients, the latter also matched for kidney function. Vasopressin was measured by its surrogate copeptin. Twenty-four-hour urinary excretions of cortisol, cortisone, tetrahydrocortisone, tetrahydrocortisol, allotetrahydrocortisol, and the total glucocorticoid pool were measured. Results: At baseline, ADPKD patients demonstrated a higher copeptin concentration in comparison with healthy controls, while urinary excretion of cortisol and cortisone was lower (medians of 0.23 vs. 0.34 mu mol/24 h, p = 0.007, and 0.29 vs. 0.53 mu mol/24 h, p <0.001, respectively). There were no differences in cortisol and cortisone excretion compared to IgA nephropathy patients. Cortisol, cortisone, and total glucocorticoid excretions correlated with kidney function (R = 0.37, 0.58, and 0.19, respectively; all p <0.05). Despite that V2RA treatment resulted in a 3-fold increase in copeptin, only cortisone excretion increased (median of 0.44 vs. baseline 0.29 mu mol/24 h, p <0.001), whereas no changes in cortisol or total glucocorticoid excretion were observed. Conclusions: Increased concentration of vasopressin in ADPKD patients at baseline and during V2RA treatment does not result in activation of the HPA axis. The impaired glucocorticoid production in these patients is related to their degree of kidney function impairment

Ballistic spin-polarized transport and Rashba spin precession in semiconductor nanowires

We present numerical calculations of the ballistic spin-transport properties of quasi-one-dimensional wires in the presence of the spin-orbit (Rashba) interaction. A tight-binding analog of the Rashba Hamiltonian which models the Rashba effect is used. By varying the robustness of the Rashba coupling and the width of the wire, weak and strong coupling regimes are identified. Perfect electron spin-modulation is found for the former regime, regardless of the incident Fermi energy and mode number. In the latter however, the spin-conductance has a strong energy dependence due to a nontrivial subband intermixing induced by the strong Rashba coupling. This would imply a strong suppression of the spin-modulation at higher temperatures and source-drain voltages. The results may be of relevance for the implementation of quasi-one-dimensional spin transistor devices.Comment: 19 pages (incl. 9 figures). To be published in PR

Astro 2020 Science White Paper: Time Domain Studies of Neutron Star and Black Hole Populations: X-ray Identification of Compact Object Types

What are the most important conditions and processes governing the growth of stellar-origin compact objects? The identification of compact object type as either black hole (BH) or neutron star (NS) is fundamental to understanding their formation and evolution. To date, time-domain determination of compact object type remains a relatively untapped tool. Measurement of orbital periods, pulsations, and bursts will lead to a revolution in the study of the demographics of NS and BH populations, linking source phenomena to accretion and galaxy parameters (e.g., star formation, metallicity). To perform these measurements over sufficient parameter space, a combination of a wide-field (>5000 deg^2) transient X-ray monitor over a dynamic energy range (~1-100 keV) and an X-ray telescope for deep surveys with <5 arcsec PSF half-energy width (HEW) angular resolution are required. Synergy with multiwavelength data for characterizing the underlying stellar population will transform our understanding of the time domain properties of transient sources, helping to explain details of supernova explosions and gravitational wave event rates.Comment: 9 pages, 2 figures. Submitted to the Astro2020 Decadal Surve