26 research outputs found
Orbital ferromagnetism in interacting few-electron dots with strong spin-orbit coupling
We study the ground state of weakly interacting electrons (with ) in a two-dimensional parabolic quantum dot with strong Rashba spin-orbit
coupling. Using dimensionless parameters for the Coulomb interaction,
, and the Rashba coupling, , the low-energy
physics is characterized by an almost flat single-particle dispersion. From an
analytical approach for and , and from numerical exact
diagonalization and Hartree-Fock calculations, we find a transition from a
conventional unmagnetized ground state (for ) to an orbital
ferromagnet (for ), with a large magnetization and a
circulating charge current. We show that the critical interaction strength,
, vanishes in the limit .Comment: 15 pages, 9 figures; (v2) more discussion added, fig.8 correcte
Non-Fermi liquid manifold in a Majorana device
We propose and study a setup realizing a stable manifold of non-Fermi liquid
states. The device consists of a mesoscopic superconducting island hosting Majorana bound states tunnel-coupled to normal leads, with a Josephson
contact to a bulk superconductor. We find a nontrivial interplay between
multi-channel Kondo and resonant Andreev reflection processes, which results in
the fixed point manifold. The scaling dimension of the leading irrelevant
perturbation changes continuously within the manifold and determines the
power-law scaling of the temperature dependent conductance.Comment: 5 pages, 2 figure
Critical Josephson current through a bistable single-molecule junction
We compute the critical Josephson current through a single-molecule junction.
As a model for a molecule with a bistable conformational degree of freedom, we
study an interacting single-level quantum dot coupled to a two-level system and
weakly connected to two superconducting electrodes. We perform a lowest-order
perturbative calculation of the critical current and show that it can
significantly change due to the two-level system. In particular, the
\pi-junction behavior, generally present for strong interactions, can be
completely suppressed.Comment: 7 pages, 5 figures; v2: minor changes, to be published in Phys. Rev.
Interaction-induced conductance from zero modes in a clean magnetic graphene waveguide
We consider a waveguide formed in a clean graphene monolayer by a spatially
inhomogeneous magnetic field. The single-particle dispersion relation for this
waveguide exhibits a zero-energy Landau-like flat band, while finite-energy
bands have dispersion and correspond, in particular, to snake orbits. For
zero-mode states, all matrix elements of the current operator vanish, and a
finite conductance can only be caused by virtual transitions to finite-energy
bands. We show that Coulomb interactions generate such processes. In stark
contrast to finite-energy bands, the conductance is not quantized and shows a
characteristic dependence on the zero-mode filling. Transport experiments
thereby offer a novel and highly sensitive probe of electron-electron
interactions in clean graphene samples. We argue that this interaction-driven
zero-mode conductor may also appear in other physical settings and is not
captured by the conventional Tomonaga-Luttinger liquid description.Comment: 14 pages, 8 figures, published versio
Adiabatic polaron dynamics and Josephson effect in a superconducting molecular quantum dot
We study the Josephson current through a resonant level coupled to a
vibration mode (local Holstein model) in the adiabatic limit of low oscillator
frequency. A semiclassical theory is then appropriate and allows us to consider
the oscillator dynamics within the Born-Oppenheimer approximation for arbitrary
electron-vibration couplings. The resulting Fokker-Planck equation has been
solved in the most relevant underdamped limit and yields the oscillator
distribution function and the Josephson current. Remarkably, a transition from
single-well to double-well behavior of the effective oscillator potential
surface is possible and can be tuned by variation of the superconducting phase
difference. The Josephson current is shown to be only weakly affected by the
electron-vibration coupling due to strong phonon localization near the bottom
of the potential surface.Comment: 11 pages, 9 figures, final version to appear in Phys. Rev.
Non-equilibrium supercurrent through a quantum dot: current harmonics and proximity effect due to a normal metal lead
We consider a Hamiltonian model for a quantum dot which is placed between two
superconducting leads with a constant bias imposed between these leads. Using
the non-equilibrium Keldysh technique, we focus on the subgap current, where it
is known that multiple Andreev reflections (MAR) are responsible for charge
transfer through the dot. Attention is put on the DC current and on the first
harmonics of the supercurrent. Varying the energy and width of the resonant
level on the dot, we first investigate a cross-over from a quantum dot regime
to a quantum point contact regime when there is zero coupling to the normal
probe. We then study the effect on the supercurrent of the normal probe which
is attached to the dot. This normal probe is understood to lead to dephasing,
or alternatively to induce reverse proximity effect. We describe the full
crossover from zero dephasing to the incoherent case. We also compute the
Josephson current in the presence of the normal lead, and find it in excellent
agreement with the values of the non-equlibrium current extrapolated at zero
voltage
Transport through a molecular quantum dot in the polaron crossover regime
We consider resonant transport through a molecular quantum dot coupled to a
local vibration mode. Applying the non-equilibrium Green function technique in
the polaron representation, we develop a non-perturbative scheme to calculate
the electron spectral function of the molecule in the regime of intermediate
electron-phonon coupling. With increasing tunneling coupling to the leads,
correlations between polaron clouds become more important at relatively high
temperature leading to a strong sharpening of the peak structure in the
spectral function. The detection of such features in the current-voltage
characteristics is briefly discussed
Phonon-mediated negative differential conductance in molecular quantum dots
Transport through a single molecular conductor is considered, showing
negative differential conductance behavior associated with phonon-mediated
electron tunneling processes. This theoretical work is motivated by a recent
experiment by Leroy et al. using a carbon nanotube contacted by an STM tip
[Nature {\bf 432}, 371 (2004)], where negative differential conductance of the
breathing mode phonon side peaks could be observed. A peculiarity of this
system is that the tunneling couplings which inject electrons and those which
collect them on the substrate are highly asymmetrical. A quantum dot model is
used, coupling a single electronic level to a local phonon, forming polaron
levels. A "half-shuttle" mechanism is also introduced. A quantum kinetic
formulation allows to derive rate equations. Assuming asymmetric tunneling
rates, and in the absence of the half-shuttle coupling, negative differential
conductance is obtained for a wide range of parameters. A detailed explanation
of this phenomenon is provided, showing that NDC is maximal for intermediate
electron-phonon coupling. In addition, in absence of a gate, the "floating"
level results in two distinct lengths for the current plateaus, related to the
capacitive couplings at the two junctions. It is shown that the "half-shuttle"
mechanism tends to reinforce the negative differential regions, but it cannot
trigger this behavior on its own
Energy spectrum and broken spin-surface locking in topological insulator quantum dots
We consider the energy spectrum and the spin-parity structure of the
eigenstates for a quantum dot made of a strong topological insulator. Using the
effective low-energy theory in a finite-length cylinder geometry, numerical
calculations show that even at the lowest energy scales, the spin direction in
a topologically protected surface mode is not locked to the surface. We find
"zero-momentum" modes, and subgap states localized near the "caps" of the dot.
Both the energy spectrum and the spin texture of the eigenstates are basically
reproduced from an analytical surface Dirac fermion description. Our results
are compared to microscopic calculations using a tight-binding model for a
strong topological insulator in a finite-length nanowire geometry.Comment: 11 pages, 12 figures, to appear in Physical Review B (2011
Rabi-like oscillations of an anharmonic oscillator: classical versus quantum interpretation
8 pagesInternational audienceWe have observed Rabi-like oscillations in a current-biased dc SQUID presenting enhanced coherence times compared to our previous realization~\cite{Claudon_PRL04}. This Josephson device behaves as an anharmonic oscillator which can be driven into a coherent superposition of quantum states by resonant microwave flux pulses. Increasing the microwave amplitude, we study the evolution of the Rabi frequency from the 2-level regime to the regime of multilevel dynamics. When up to levels are involved, the Rabi frequency is a clear signature of quantum behavior. At higher excitation amplitude, classical and quantum predictions for the Rabi frequency converge. This result is discussed in the light of a calculation of the Wigner function. In particular, our analysis shows that pronounced quantum interferences always appear in the course of the Rabi-like oscillations