546 research outputs found
Strong coupling of a qubit to shot noise
We perform a nonperturbative analysis of a charge qubit in a double quantum
dot structure coupled to its detector. We show that strong detector-dot
interaction tends to slow down and halt coherent oscillations. The transitions
to a classical and a low-temperature quantum overdamping (Zeno) regime are
studied. In the latter, the physics of the dissipative phase transition
competes with the effective shot noise.Comment: 5 pages, 4 figure
Structure prediction based on ab initio simulated annealing for boron nitride
Possible crystalline modifications of chemical compounds at low temperatures
correspond to local minima of the energy landscape. Determining these minima
via simulated annealing is one method for the prediction of crystal structures,
where the number of atoms per unit cell is the only information used. It is
demonstrated that this method can be applied to covalent systems, at the
example of boron nitride, using ab initio energies in all stages of the
optimization, i.e. both during the global search and the subsequent local
optimization. Ten low lying structure candidates are presented, including both
layered structures and 3d-network structures such as the wurtzite and zinc
blende types, as well as a structure corresponding to the beta-BeO type
Cotunneling at resonance for the single-electron transistor
We study electron transport through a small metallic island in the
perturbative regime. Using a recently developed diagrammatic technique, we
calculate the occupation of the island as well as the conductance through the
transistor in forth order in the tunneling matrix elements, a process referred
to as cotunneling. Our formulation does not require the introduction of a
cut-off. At resonance we find significant modifications of previous theories
and good agreement with recent experiments.Comment: 5 pages, Revtex, 5 eps-figure
Semiconductor saturable absorber mirror structures with low saturation fluence
We present two novel semiconductor saturable absorber mirror (SESAM) designs which can exhibit more than ten times lower saturation fluence than classical SESAM devices. Design considerations and characterization data are presented. These devices are particularly suited for passively mode-locked lasers with ultra-high repetition rate
Revised Phase Diagram of the Gross-Neveu Model
We confirm earlier hints that the conventional phase diagram of the discrete
chiral Gross-Neveu model in the large N limit is deficient at non-zero chemical
potential. We present the corrected phase diagram constructed in mean field
theory. It has three different phases, including a kink-antikink crystal phase.
All transitions are second order. The driving mechanism for the new structure
of baryonic matter in the Gross-Neveu model is an Overhauser type instability
with gap formation at the Fermi surface.Comment: Revtex, 12 pages, 15 figures; v2: Axis labelling in Fig. 9 correcte
Resonant tunneling through ultrasmall quantum dots: zero-bias anomalies, magnetic field dependence, and boson-assisted transport
We study resonant tunneling through a single-level quantum dot in the
presence of strong Coulomb repulsion beyond the perturbative regime. The level
is either spin-degenerate or can be split by a magnetic field. We, furthermore,
discuss the influence of a bosonic environment. Using a real-time diagrammatic
formulation we calculate transition rates, the spectral density and the
nonlinear characteristic. The spectral density shows a multiplet of Kondo
peaks split by the transport voltage and the boson frequencies, and shifted by
the magnetic field. This leads to zero-bias anomalies in the differential
conductance, which agree well with recent experimental results for the electron
transport through single-charge traps. Furthermore, we predict that the sign of
the zero-bias anomaly depends on the level position relative to the Fermi level
of the leads.Comment: 27 pages, latex, 21 figures, submitted to Phys. Rev.
Current and power spectrum in a magnetic tunnel device with an atomic size spacer
Current and its noise in a ferromagnetic double tunnel barrier device with a
small spacer particle were studied in the framework of the sequential tunneling
approach. Analytical formulae were derived for electron tunneling through the
spacer particle containing only a single energy level. It was shown that
Coulomb interactions of electrons with a different spin orientation lead to an
increase of the tunnel magnetoresistance. Interactions can also be responsible
for the negative differential resistance. A current noise study showed, which
relaxation processes can enhance or reduce fluctuations leading either to a
super-Poissonian or a sub-Poissonian shot noise.Comment: 12 pages, 4 figure
Charge Solitons in 1-D Arrays of Serially Coupled Josephson Junctions
We study a 1-D array of Josephson coupled superconducting grains with kinetic
inductance which dominates over the Josephson inductance. In this limit the
dynamics of excess Cooper pairs in the array is described in terms of charge
solitons, created by polarization of the grains. We analyze the dynamics of
these topological excitations, which are dual to the fluxons in a long
Josephson junction, using the continuum sine-Gordon model. We find that their
classical relativistic motion leads to saturation branches in the I-V
characteristic of the array. We then discuss the semi-classical quantization of
the charge soliton, and show that it is consistent with the large kinetic
inductance of the array. We study the dynamics of a quantum charge soliton in a
ring-shaped array biased by an external flux through its center. If the
dephasing length of the quantum charge soliton is larger than the circumference
of the array, quantum phenomena like persistent current and coherent current
oscillations are expected. As the characteristic width of the charge soliton is
of the order of 100 microns, it is a macroscopic quantum object. We discuss the
dephasing mechanisms which can suppress the quantum behaviour of the charge
soliton.Comment: 26 pages, LaTex, 7 Postscript figure
Granulated superconductors:from the nonlinear sigma model to the Bose-Hubbard description
We modify a nonlinear sigma model (NLSM) for the description of a granulated
disordered system in the presence of both the Coulomb repulsion and the Cooper
pairing. We show that under certain controlled approximations this model is
reduced to the Bose-Hubbard (or ``dirty-boson'') model with renormalized
coupling constants. We obtain a more general effective action (which is still
simpler than the full NLSM action) which can be applied in the region of
parameters where the reduction to the Bose-Hubbard model is not justified. This
action may lead to a different picture of the superconductor-insulator
transition in 2D systems.Comment: 4 pages, revtex, no figure
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