18 research outputs found
Resonant spin polarization and spin current in a two-dimensional electron gas
We study the spin polarization and its associated spin-Hall current due to
EDSR in disordered two-dimensional electron systems. We show that the disorder
induced damping of the resonant spin polarization can be strongly reduced by an
optimal field configuration that exploits the interference between Rashba and
Dresselhaus spin-orbit interaction. This leads to a striking enhancement of the
spin susceptibility while the spin-Hall current vanishes at the same time. We
give an interpretation of the spin current in geometrical terms which are
associated with the trajectories the polarization describes in spin space.Comment: (5 pages), updated references, corrected typo
Signatures of topological phase transitions in mesoscopic superconducting rings
We investigate Josephson currents in mesoscopic rings with a weak link which
are in or near a topological superconducting phase. As a paradigmatic example,
we consider the Kitaev model of a spinless p-wave superconductor in one
dimension, emphasizing how this model emerges from more realistic settings
based on semiconductor nanowires. We show that the flux periodicity of the
Josephson current provides signatures of the topological phase transition and
the emergence of Majorana fermions situated on both sides of the weak link even
when fermion parity is not a good quantum number. In large rings, the Majorana
fermions hybridize only across the weak link. In this case, the Josephson
current is h/e periodic in the flux threading the loop when fermion parity is a
good quantum number but reverts to the more conventional h/2e periodicity in
the presence of fermion-parity changing relaxation processes. In mesoscopic
rings, the Majorana fermions also hybridize through their overlap in the
interior of the superconducting ring. We find that in the topological
superconducting phase, this gives rise to an h/e-periodic contribution even
when fermion parity is not conserved and that this contribution exhibits a peak
near the topological phase transition. This signature of the topological phase
transition is robust to the effects of disorder. As a byproduct, we find that
close to the topological phase transition, disorder drives the system deeper
into the topological phase. This is in stark contrast to the known behavior far
from the phase transition, where disorder tends to suppress the topological
phase.Comment: 14 pages, 9 figures, minor changes in the text, published versio
Andreev reflection from non-centrosymmetric superconductors and Majorana bound state generation in half-metallic ferromagnets
We study Andreev reflection at an interface between a half metal and a
superconductor with spin-orbit interaction. While the absence of minority
carriers in the half metal makes singlet Andreev reflection impossible, the
spin-orbit interaction gives rise to triplet Andreev reflection, i.e., the
reflection of a majority electron into a majority hole or vice versa. As an
application of our calculation, we consider a thin half metal film or wire
laterally attached to a superconducting contact. If the half metal is disorder
free, an excitation gap is opened that is proportional to the spin-orbit
interaction strength in the superconductor. For electrons with energy below
this gap a lateral half-metal--superconductor contact becomes a perfect triplet
Andreev reflector. We show that the system supports localized Majorana end
states in this limit.Comment: 14 pages, 3 figure
Mesoscopic fluctuations in the spin-electric susceptibility due to Rashba spin-orbit interaction
We investigate mesoscopic fluctuations in the spin polarization generated by
a static electric field and by Rashba spin-orbit interaction in a disordered 2D
electron gas. In a diagrammatic approach we find that the out-of-plane
polarization -- while being zero for self-averaging systems -- exhibits large
sample-to-sample fluctuations which are shown to be well within experimental
reach. We evaluate the disorder-averaged variance of the susceptibility and
find its dependence on magnetic field, spin-orbit interaction, dephasing, and
chemical potential difference.Comment: 4 pages, 4 figure
Topological superconducting phases in disordered quantum wires with strong spin-orbit coupling
Zeeman fields can drive semiconductor quantum wires with strong spin-orbit
coupling and in proximity to s-wave superconductors into a topological phase
which supports end Majorana fermions and offers an attractive platform for
realizing topological quantum information processing. Here, we investigate how
potential disorder affects the topological phase by a combination of analytical
and numerical approaches. Most prominently, we find that the robustness of the
topological phase against disorder depends sensitively and non-monotonously on
the Zeeman field applied to the wire.Comment: 6 pages, 3 figures; published versio
Dynamic spin-Hall effect and driven spin helix for linear spin-orbit interactions
We derive boundary conditions for the electrically induced spin accumulation
in a finite, disordered 2D semiconductor channel. While for DC electric fields
these boundary conditions select spatially constant spin profiles equivalent to
a vanishing spin-Hall effect, we show that an in-plane ac electric field
results in a non-zero ac spin-Hall effect, i.e., it generates a spatially
non-uniform out-of-plane polarization even for linear intrinsic spin-orbit
interactions. Analyzing different geometries in [001] and [110]-grown quantum
wells, we find that although this out-of-plane polarization is typically
confined to within a few spin-orbit lengths from the channel edges, it is also
possible to generate spatially oscillating spin profiles which extend over the
whole channel. The latter is due to the excitation of a driven spin-helix mode
in the transverse direction of the channel. We show that while finite
frequencies suppress this mode, it can be amplified by a magnetic field tuned
to resonance with the frequency of the electric field. In this case, finite
size effects at equal strengths of Rashba- and Dresselhaus SOI lead to an
enhancement of the magnitude of this helix mode. We comment on the relation
between spin currents and boundary conditions.Comment: 10 pages, 5 figures, added references, corrected typos, extended
section V, VI
Probability distribution of Majorana end-state energies in disordered wires
One-dimensional topological superconductors harbor Majorana bound states at
their ends. For superconducting wires of finite length L, these Majorana states
combine into fermionic excitations with an energy that is
exponentially small in L. Weak disorder leaves the energy splitting
exponentially small, but affects its typical value and causes large
sample-to-sample fluctuations. We show that the probability distribution of
is log normal in the limit of large L, whereas the distribution of
the lowest-lying bulk energy level has an algebraic tail at small
. Our findings have implications for the speed at which a
topological quantum computer can be operated.Comment: 4 pages, 2 figure
Spin Accumulation in Diffusive Conductors with Rashba and Dresselhaus Spin-Orbit Interaction
We calculate the electrically induced spin accumulation in diffusive systems
due to both Rashba (with strength and Dresselhaus (with strength
spin-orbit interaction. Using a diffusion equation approach we find
that magnetoelectric effects disappear and that there is thus no spin
accumulation when both interactions have the same strength, .
In thermodynamically large systems, the finite spin accumulation predicted by
Chaplik, Entin and Magarill, [Physica E {\bf 13}, 744 (2002)] and by Trushin
and Schliemann [Phys. Rev. B {\bf 75}, 155323 (2007)] is recovered an
infinitesimally small distance away from the singular point .
We show however that the singularity is broadened and that the suppression of
spin accumulation becomes physically relevant (i) in finite-sized systems of
size , (ii) in the presence of a cubic Dresselhaus interaction of strength
, or (iii) for finite frequency measurements. We obtain the parametric
range over which the magnetoelectric effect is suppressed in these three
instances as (i) , (ii), and (iii) |\alpha|-|\beta| \lesssiM
\sqrt{\omega/m p_{\rm F}\ell} with the elastic mean free path and
the Fermi momentum. We attribute the absence of spin accumulation
close to to the underlying U (1) symmetry. We illustrate and
confirm our predictions numerically
Electrically controlled spin dynamics in disordered semiconductors
The spin of electrons in a semiconductor environment couples not only to magnetic ļ¬elds, but also to the orbital motion of the electron. As a consequence, transport in semiconductors includes a class of phenomena in which electrically induced charge motion inļ¬uences the electron spin. The intricate interplay of spin and charge makes this type of eļ¬ects a diverse research ļ¬eld of fundamental interest, but is also of practical relevance: Spin-orbit interaction (SOI) provides a mechanism to control the spin with electric ļ¬elds. Being available in tailored materials, that are routinely used in microelectronics, SOI has therefore attracted intense interest for its potential in applications to use the electron spin alternatively to the charge in new types of electronic devices. In this thesis we investigate the interplay of spin and charge transport in disordered electron systems, where random impurities not only determine the electrical resistance but also the spin dynamics through spin-orbit interaction. A focus of this work is electric-dipole-induced spin resonance (EDSR), a versatile scheme of spin control using electric ļ¬elds. Similar to standard paramagnetic resonance where a combination of static and ac magnetic ļ¬elds drive spin rotations, in EDSR ac electric ļ¬elds couple resonantly to the spin. Appropriately chosen pulses of these electric ļ¬elds, which can be generated easier on-chip than ac magnetic ļ¬elds, allow to achieve arbitrary spin rotations. In a diagrammatic analysis we ļ¬nd that the presence of disorder broadens the line-shape of EDSR and determines the maximal achievable polarization. We identify random internal magnetic ļ¬elds as the origin of this line-broadening, which limits the eļ¬ciency of EDSR, and show that these limitations can be overcome in an optimal geometry where the internal ļ¬elds are suppressed by the interference of diļ¬erent spin-orbit mechanisms. This leads to a substantial enhancement of the spin polarization at resonance. We moreover link these ļ¬ndings to spin currents giving rise to the spin-Hall eļ¬ect. We interpret these spin currents in terms of spin polarization components. The behavior of the spin depends sensitively on whether the orbital motion is diļ¬usive or phase coherent. Indeed, qualitatively diļ¬erent ā mesoscopic ā eļ¬ects occur in the spin when electrons ļ¬ow through phase-coherent systems. Whereas in charge transport such mesoscopic eļ¬ects are well known, for the spin they have attracted interest only more recently in the context of spintronics. We ļ¬nd that the spin polarization, that arises due to dc transport and SOI, shows large mesoscopic ļ¬uctuations that exceed the polarization in incoherent samples with self-averaging. Since this average polarization has been successfully measured we expect this mesoscopic ļ¬uctuations to be within experimental reach as well