280 research outputs found
Bends In Nanotubes Allow Electric Spin Control and Coupling
We investigate combined effects of spin-orbit coupling and magnetic field in
carbon nanotubes containing one or more bends along their length. We show how
bends can be used to provide electrical control of confined spins, while spins
confined in straight segments remain insensitive to electric fields. Device
geometries that allow general rotation of single spins are presented and
analyzed. In addition, capacitive coupling along bends provides coherent
spin-spin interaction, including between otherwise disconnected nanotubes,
completing a universal set of one- and two-qubit gates.Comment: 6 pages, 5 figure
Spin Coulomb drag in the two-dimensional electron liquid
We calculate the spin-drag transresistivity
in a two-dimensional electron gas at temperature in the random phase
approximation. In the low-temperature regime we show that, at variance with the
three-dimensional low-temperature result [], the spin transresistivity of a two-dimensional {\it spin unpolarized}
electron gas has the form . In the
spin-polarized case the familiar form is
recovered, but the constant of proportionality diverges logarithmically as
the spin-polarization tends to zero. In the high-temperature regime we obtain
(where
is the effective Rydberg energy) {\it independent} of the density.
Again, this differs from the three-dimensional result, which has a logarithmic
dependence on the density. Two important differences between the spin-drag
transresistivity and the ordinary Coulomb drag transresistivity are pointed
out: (i) The singularity at low temperature is smaller, in the Coulomb
drag case, by a factor where is the Fermi wave vector and
is the separation between the layers. (ii) The collective mode contribution
to the spin-drag transresistivity is negligible at all temperatures. Moreover
the spin drag effect is, for comparable parameters, larger than the ordinary
Coulomb drag effect.Comment: 6 figures; various changes; version accepted for publicatio
Non-linear thermoelectrics of molecular junctions with vibrational coupling
We present a detailed study of the non-linear thermoelectric properties of a
molecular junction, represented by a dissipative Anderson-Holstein model. A
single orbital level with strong Coulomb interaction is coupled to a localized
vibrational mode and we account for both electron and phonon exchange with both
electrodes, investigating how these contribute to the heat and charge
transport. We calculate the efficiency and power output of the device operated
as a heat to electric power converter and identify the optimal operating
conditions, which are found to be qualitatively changed by the presence of the
vibrational mode. Based on this study of a generic model system, we discuss the
desirable properties of molecular junctions for thermoelectric applications.Comment: 8 pages, 5 figure
Image charge effects in single-molecule junctions: Breaking of symmetries and negative differential resistance in a benzene transistor
Both experiments and theoretical studies have demonstrated that the
interaction between the current carrying electrons and the induced polarization
charge in single-molecule junctions leads to a strong renormalization of
molecular charging energies. However, the effect on electronic excitations and
molecular symmetries remain unclear. Using a theoretical framework developed
for semiconductor nanostructure based single-electron transistors (SETs), we
demonstrate that the image charge interaction breaks the molecular symmetries
in a benzene based single-molecule transistor operating in the Coulomb blockade
regime. This results in the appearance of a so-called blocking state, which
gives rise to negative differential resistance (NDR). We show that the
appearance of NDR and its magnitude in the symmetry-broken benzene SET depends
in a complicated way on the interplay between the many-body matrix elements,
the lead tunnel coupling asymmetry, and the bias polarity. In particular, the
current reducing property of the blocking state causing the NDR, is shown to
vanish under strongly asymmetric tunnel couplings, when the molecule is coupled
stronger to the drain electrode. The calculated IV characteristic may serve as
an indicator for image charge broken molecular symmetries in experimental
situations.Comment: Accepted version (Phys. Rev. B), 16 pages, 8 figure
Fidelity and visibility reduction in Majorana qubits by entanglement with environmental modes
We study the dynamics and readout of topological qubits encoded by
zero-energy Majorana bound states in a topological superconductor. We take into
account bosonic modes due to the electromagnetic environment which couple the
Majorana manifold to above-gap continuum quasi-particles. This coupling causes
the degenerate ground state of the topological superconductor to be dressed in
a polaron-like manner by quasi-particle states and bosons, and the system to
become gapless. Topological protection and hence full coherence is only
maintained if the qubit is operated and read out within the low-energy spectrum
of the dressed states. We discuss reduction of fidelity and/or visibility if
this condition is violated by a quantum-dot readout that couples to the bare
(undressed) Majorana modes. For a projective measurement of the bare Majorana
basis, we formulate a Bloch-Redfield approach that is valid for weak
Majorana-environment coupling and takes into account constraints imposed by
fermion-number-parity conservation. Within the Markovian approximation, our
results essentially confirm earlier theories of finite-temperature decoherence
based on Fermi's golden rule. However, the full non-Markovian dynamics reveals,
in addition, the fidelity reduction by a projective measurement. Using a
spinless nanowire model with -wave pairing, we provide quantitative results
characterizing these effects.Comment: 18 pages, 10 figure
Absence of supercurrent sign reversal in a topological junction with a quantum dot
Experimental techniques to verify Majoranas are of current interest. A
prominent test is the effect of Majoranas on the Josephson current between two
wires linked via a normal junction. Here, we study the case of a quantum dot
connecting the two superconductors and the sign of the supercurrent in the
trivial and topological regimes under grand-canonical equilibrium conditions,
explicitly allowing for parity changes due to, e.g., quasi-particle poisoning.
We find that the well-known supercurrent reversal for odd occupancy of the
quantum dot (pi-junction) in the trivial case does not occur in the presence of
Majoranas in the wires. However, we also find this to be a mere consequence of
Majoranas being zero energy states. Therefore, the lack of supercurrent sign
reversal can also be caused by trivial bound states, and is thus not a
discriminating signature of Majoranas.Comment: 6 pages + 1 page appendix + 2 pages bibilography, 4 figure
A number conserving theory for topologically protected degeneracy in one-dimensional fermions
Semiconducting nanowires in proximity to superconductors are among promising
candidates to search for Majorana fermions and topologically protected
degeneracies which may ultimately be used as building blocks for topological
quantum computers. The prediction of neutral Majorana fermions in the
proximity-induced superconducting systems ignores number-conservation and thus
leaves open the conceptual question of how a topological degeneracy that is
robust to all local perturbations arises in a number-conserving system. In this
work, we study how local attractive interactions generate a topological
ground-state near-degeneracy in a quasi one-dimensional superfluid using
bosonization of the fermions. The local attractive interactions opens a
topological quasiparticle gap in the odd channel wires (with more than one
channel) with end Majorana modes associated with a topological near-degeneracy.
We explicitly study the robustness of the topological degeneracy to local
perturbations and find that such local perturbations result in quantum phase
slips which split of the topological degeneracy by an amount that does not
decrease exponentially with the length of the wire, but still decreases rapidly
if the number of channels is large. Therefore a bulk superconductor with a
large number of channels is crucial for true topological degeneracy.Comment: 11 pages, 2 figure
Critical Current 0- Transition in Designed Josephson Quantum Dot Junctions
We report on quantum dot based Josephson junctions designed specifically for
measuring the supercurrent. From high-accuracy fitting of the current-voltage
characteristics we determine the full magnitude of the supercurrent (critical
current). Strong gate modulation of the critical current is observed through
several consecutive Coulomb blockade oscillations. The critical current crosses
zero close to, but not at, resonance due to the so-called 0- transition in
agreement with a simple theoretical model.Comment: 5 pages, 4 figures, (Supplementary information available at
http://www.fys.ku.dk/~hij/public/nl_supp.pdf
Exchange cotunneling through quantum dots with spin-orbit coupling
We investigate the effects of spin-orbit interaction (SOI) on the exchange
cotunneling through a spinful Coulomb blockaded quantum dot. In the case of
zero magnetic field, Kondo effect is shown to take place via a Kramers doublet
and the SOI will merely affect the Kondo temperature. In contrast, we find that
the breaking of time-reversal symmetry in a finite field has a marked influence
on the effective Anderson, and Kondo models for a single level. The nonlinear
conductance can now be asymmetric in bias voltage and may depend strongly on
direction of the magnetic field. A measurement of the angle dependence of
finite-field cotunneling spectroscopy thus provides valuable information about
orbital, and spin degrees of freedom and their mutual coupling.Comment: 5 pages, 2 figure
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