97 research outputs found
Quantum wire networks with local Z2 symmetry
For a large class of networks made of connected loops, in the presence of an
external magnetic field of half flux quantum per loop, we show the existence of
a large local symmetry group, generated by simultaneous flips of the electronic
current in all the loops adjacent to a given node. Using an ultra-localized
single particle basis adapted to this local Z_2 symmetry, we show that it is
preserved by a large class of interaction potentials. As a main physical
consequence, the only allowed tunneling processes in such networks are induced
by electron-electron interactions and involve a simultaneous hop of two
electrons. Using a mean-field picture and then a more systematic
renormalization-group treatment, we show that these pair hopping processes do
not generate a superconducting instability, but they destroy the Luttinger
liquid behavior in the links, giving rise at low energy to a strongly
correlated spin-density-wave state.Comment: 16 pages, 9 figures, v.2 section IV D added,accepted for publication
in PR
Breakdown of the Fermi Liquid picture in one dimensional fermion systems: connection with the energy level statistics
Using the adiabatic switching of interactions, we establish a condition for
the existence of electronic quasiparticles in a Luttinger liquid. It involves a
characteristic interaction strength proportional to the inverse square root of
the system length. An investigation of the exact energy level separation
probability distribution shows that this interaction scale also corresponds to
a cross-over from the non interacting behaviour to a rather typical case for
integrable systems, namely an exponential distribution. The level spacing
statistics of a spin , one branch Luttinger model are also analyzed, as
well as the level statistics of a two coupled chain model.Comment: 22 pages, Late
Nonadiabatic Josephson current pumping by microwave irradiation
Irradiating a Josephson junction with microwaves can operate not only on the
amplitude but also on the phase of the Josephson current. This requires
breaking time inversion symmetry, which is achieved by introducing a phase
lapse between the microwave components acting on the two{\dag} sides of the
junction. General symmetry arguments and the solution of a specific single
level quantum dot model show that this induces chirality in the Cooper pair
dynamics, due to the topology of the Andreev bound state wavefunction. Another
essential condition is to break electron-hole symmetry within the junction. A
shift of the current-phase relation is obtained, which is controllable in sign
and amplitude with the microwave phase and an electrostatic gate, thus
producing a "chiral" Josephson transistor. The dot model is solved in the
infinite gap limit by Floquet theory and in the general case with Keldysh
nonequilibrium Green's functions. The chiral current is nonadiabatic: it is
extremal and changes sign close to resonant chiral transitions between the
Andreev bound states.Comment: 13 pages, 7 figures, extended versio
Strong disorder renormalization group on fractal lattices: Heisenberg models and magnetoresistive effects in tight binding models
We use a numerical implementation of the strong disorder renormalization
group (RG) method to study the low-energy fixed points of random Heisenberg and
tight-binding models on different types of fractal lattices. For the Heisenberg
model new types of infinite disorder and strong disorder fixed points are
found. For the tight-binding model we add an orbital magnetic field and use
both diagonal and off-diagonal disorder. For this model besides the gap spectra
we study also the fraction of frozen sites, the correlation function, the
persistent current and the two-terminal current. The lattices with an even
number of sites around each elementary plaquette show a dominant
periodicity. The lattices with an odd number of sites around each elementary
plaquette show a dominant periodicity at vanishing diagonal
disorder, with a positive weak localization-like magnetoconductance at infinite
disorder fixed points. The magnetoconductance with both diagonal and
off-diagonal disorder depends on the symmetry of the distribution of on-site
energies.Comment: 19 pages, 20 figure
Proposal for the observation of nonlocal multipair production: the biSQUID
We propose an all-superconducting three-terminal setup consisting in a carbon
nanotube (or semiconducting nanowire) contacted to three superconducting leads.
The resulting device, referred to as a "biSQUID", is made of four quantum dots
arranged in two loops of different surface area. We show how this biSQUID can
prove a useful tool to probe nonlocal quantum phenomena in an interferometry
setup. We study the measured critical current as a function of the applied
magnetic field, which shows peaks in its Fourier spectrum, providing clear
signatures of multipair Josephson processes. The device does not require any
specific fine-tuning as these features are observed for a wide range of
microscopic parameters -- albeit with a non-trivial dependence. Competing
effects which may play a significant role in actual experimental realizations
are also explored.Comment: 13 pages, 9 figure
Multipair DC-Josephson Resonances in a biased all-superconducting Bijunction
An all-superconducting bijunction consists of a central superconductor
contacted to two lateral superconductors, such that non-local crossed Andreev
reflection is operating. Then new correlated transport channels for the Cooper
pairs appear in addition to those of separated conventional Joseph- son
junctions. We study this system in a configuration where the superconductors
are connected through gate-controllable quantum dots. Multipair phase-coherent
resonances and phase-dependent multiple Andreev reflections are both obtained
when the voltages of the lateral superconductors are commensurate, and they add
to the usual local dissipative transport due to quasiparticles. The two-pair
resonance (quartets) as well as some other higher order multipair resonances
are {\pi}-shifted at low voltage. Dot control can be used to dramatically
enhance the multipair current when the voltages are resonant with the dot
levels.Comment: 6 page
Semiclassical approach to quantum spin ice
We propose a semi-classical description of the low-energy properties of
quantum spin ice in the strong Ising limit. Within the framework of a
semiclassical, perturbative Villain expansion, that can be truncated at
arbitrary order, we give an analytic and quantitative treatment of the
deconfining phase. We find that photon-photon interactions significantly
renormalise the speed of light and split the two transverse photon
polarisations at intermediate wavevectors. We calculate the photon velocity and
the ground state energy to first and second order in perturbation theory,
respectively. The former is in good agreement with recent numerical
simulations
Weak localization in multiterminal networks of diffusive wires
We study the quantum transport through networks of diffusive wires connected
to reservoirs in the Landauer-B\"uttiker formalism. The elements of the
conductance matrix are computed by the diagrammatic method. We recover the
combination of classical resistances and obtain the weak localization
corrections. For arbitrary networks, we show how the cooperon must be properly
weighted over the different wires. Its nonlocality is clearly analyzed. We
predict a new geometrical effect that may change the sign of the weak
localization correction in multiterminal geometries.Comment: 4 pages, LaTeX, 4 figures, 8 eps file
Topological order in the insulating Josephson junction array
We propose a Josephson junction array which can be tuned into an
unconventional insulating state by varying external magnetic field. This
insulating state retains a gap to half vortices; as a consequence, such array
with non-trivial global geometry exhibits a ground state degeneracy. This
degeneracy is protected from the effects of external noise. We compute the gaps
separating higher energy states from the degenerate ground state and we discuss
experiments probing the unusual properties of this insulator.Comment: 4 pages, RevTex 4, 1 EPS figur
Experimental demonstration of Aharonov-Casher interference in a Josephson junction circuit
A neutral quantum particle with magnetic moment encircling a static electric
charge acquires a quantum mechanical phase (Aharonov-Casher effect). In
superconducting electronics the neutral particle becomes a fluxon that moves
around superconducting islands connected by Josephson junctions. The full
understanding of this effect in systems of many junctions is crucial for the
design of novel quantum circuits. Here we present measurements and quantitative
analysis of fluxon interference patterns in a six Josephson junction chain. In
this multi-junction circuit the fluxon can encircle any combination of charges
on five superconducting islands, resulting in a complex pattern. We compare the
experimental results with predictions of a simplified model that treats fluxons
as independent excitations and with the results of the full diagonalization of
the quantum problem. Our results demonstrate the accuracy of the fluxon
interference description and the quantum coherence of these arrays
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