28 research outputs found
Spin imbalance in hybrid superconducting structures with spin-active interfaces
We consider a heterostructure consisting of a normal metal and a
superconductor separated by a spin-active interface. At finite-bias voltages,
spin-filtering and spin-mixing effects at the interface allow for an induced
magnetization (spin imbalance) on the superconducting side of the junction,
which relaxes to zero in the bulk. Such interfaces are also known to host a
pair of in-gap Andreev bound states which were recently observed
experimentally. We show that these states are responsible for the dominant
contribution to the induced spin imbalance close to the interface. Motivated by
recent experiments on spin-charge density separation in superconducting
aluminum wires, we propose an alternative way to observe spin imbalance without
applying an external magnetic field. We also suggest that the peculiar
dependence of the spin imbalance on the applied bias voltage permits an
indirect bound-state spectroscopy.Comment: 7 pages, 4 figure
Shot noise in a harmonically driven ballistic graphene transistor
We study time-dependent electron transport and quantum noise in a ballistic
graphene field effect transistor driven by an ac gate potential. The non-linear
response to the ac signal is computed through Floquet theory for scattering
states and Landauer-B\"uttiker theory for charge current and its fluctuations.
Photon-assisted excitation of a quasibound state in the top-gate barrier leads
to resonances in transmission that strongly influence the noise properties. For
strong doping of graphene under source and drain contacts, when electrons are
transmitted through the channel via evanescent waves, the resonance leads to a
substantial suppression of noise. The Fano factor is then reduced well below
the pseudo-diffusive value, , also for strong ac drive. The good
signal-to-noise ratio (small Fano factor) on resonance suggests that the device
is a good candidate for high-frequency (THz) radiation detection. We show
analytically that Klein tunneling (total suppression of back-reflection)
persists for perpendicular incidence also when the barrier is driven
harmonically. Although the transmission is inelastic and distributed among
sideband energies, a sum rule leads to total suppression of shot noise.Comment: 12 pages, 7 figure
Spectral properties of superconductors with ferromagnetically ordered magnetic impurities
We present a comprehensive theoretical study of thermodynamic properties of
superconductors with a dilute concentration of magnetic impurities, with focus
on how the properties of the superconducting host change if the magnetic
moments of the impurities order ferromagnetically. Scattering off the magnetic
impurities leads to the formation of a band of Yu-Shiba-Rusinov states within
the superconducting energy gap that drastically influences superconductivity.
In the magnetically ordered system, the magnetization displays a sudden drop as
function of impurity density or magnetic moment amplitude. The drop occurs as
the spin-polarized impurity band crosses the Fermi level and is associated with
a quantum phase transition first put forward by Sakurai for the single impurity
case. Taking into account that the background magnetic field created by the
ordered impurity moments enters as a Zeeman shift, we find that the
superconducting phase transition changes from second order to first order for
high enough impurity concentration.Comment: 16 pages, 13 figure
Spin-polarized currents and noise in NS junctions with Yu-Shiba-Rusinov impurities
Conventional superconductors disordered by magnetic impurities demonstrate
physical properties drastically different from their pristine counterparts. In
our previous work [Phys. Rev. B 92, 245430 (2015)] we explored spectral and
thermodynamic properties of such systems for two extreme cases: completely
random and ferromagnetically aligned impurity magnetic moments. Here we
consider transport properties of these systems, and show that they have a
potential to be used in superconducting spintronic devices. Each magnetic
impurity contributes a Yu-Shiba-Rusinov (YSR) bound state to the spectrum,
residing at sub-gap energies. Provided the YSR states form metallic bands, we
demonstrate that the tunneling current carried by these states can be highly
spin-polarized when the impurities are ferromagnetically ordered. The spin
polarization can be switched by simply tuning the bias voltage. Moreover, even
when the impurity spins are completely uncorrelated, one can still achieve
almost 100% spin polarization of the current, if the tunnel interface is
spin-active. We compute electric current and noise, varying parameters of the
interface between tunneling and fully transparent regimes, and analyze the
relative role of single-particle and Andreev reflection processes.Comment: 14 pages, 10 figure
Tunable thermopower in a graphene-based topological insulator
Following the recent proposal by Weeks et al., which suggested that indium
(or thallium) adatoms deposited on the surface of graphene should turn the
latter into a quantum spin Hall (QSH) insulator characterized by a sizeable
gap, we perform a systematic study of the transport properties of this system
as a function of the density of randomly distributed adatoms. While the samples
are, by construction, very disordered, we find that they exhibit an extremely
stable QSH phase with no signature of the spatial inhomogeneities of the adatom
configuration. We find that a simple rescaling of the spin-orbit coupling
parameter allows us to account for the behaviour of the inhomogeneous system
using a homogeneous model. This robustness opens the route to a much easier
experimental realization of this topological insulator. We additionally find
this material to be a very promising candidate for thermopower generation with
a target temperature tunable from 1 to 80K and an efficiency ZT close to 1.Comment: 7 pages, 5 figure
Graphene-based heterojunction between two topological insulators
Quantum Hall (QH) and quantum spin Hall (QSH) phases have very different edge
states and, when going from one phase to the other, the direction of one edge
state must be reversed. We study this phenomena in graphene in presence of a
strong perpendicular magnetic field on top of a spin-orbit (SO) induced QSH
phase. We show that, below the SO gap, the QSH phase is virtually unaffected by
the presence of the magnetic field. Above the SO gap, the QH phase is restored.
An electrostatic gate placed on top of the system allows to create a QSH-QH
junction which is characterized by the existence of a spin-polarized chiral
state, propagating along the topological interface. We find that such a setup
naturally provides an extremely sensitive spin-polarized current switch.Comment: 10 pages, 5 figure
SuperConga: An open-source framework for mesoscopic superconductivity
We present SuperConga, an open-source framework for simulating equilibrium properties of unconventional and ballistic singlet superconductors, confined to two-dimensional (2D) mesoscopic grains in a perpendicular external magnetic field, at arbitrary low temperatures. It aims at being both fast and easy to use, enabling research without access to a computer cluster, and visualization in real-time with OpenGL. The core is written in C++ and CUDA, exploiting the embarrassingly parallel nature of the quasiclassical theory of superconductivity by utilizing the parallel computational power of modern graphics processing units. The framework self-consistently computes both the superconducting order-parameter and the induced vector potential and finds the current density, free energy, induced flux density, local density of states (LDOS), and the magnetic moment. A user-friendly Python frontend is provided, enabling simulation parameters to be defined via intuitive configuration files, or via the command-line interface, without requiring a deep understanding of implementation details. For example, complicated geometries can be created with relative ease. The framework ships with simple tools for analyzing and visualizing the results, including an interactive plotter for spectroscopy. An overview of the theory is presented, as well as examples showcasing the framework\u27s capabilities and ease of use. The framework is free to download from https://gitlab.com/superconga/superconga, which also links to the extensive user manual, containing even more examples, tutorials, and guides. To demonstrate and benchmark SuperConga, we study the magnetostatics, thermodynamics, and spectroscopy of various phenomena. In particular, we study flux quantization in solenoids, vortex physics, surface Andreev bound-states, and a "phase crystal."We compare our numeric results with analytics and present experimental observables, e.g., the magnetic moment and LDOS, measurable with, for example, scanning probes, STM, and magnetometry