1,252 research outputs found
The connection between noise and quantum correlations in a double quantum dot
We investigate the current and noise characteristics of a double quantum dot
system. The strong correlations induced by the Coulomb interaction create
entangled two-electron states and lead to signatures in the transport
properties. We show that the interaction parameter phi, which measures the
admixture of the double-occupancy contribution to the singlet state and thus
the degree of entanglement, can be directly accessed through the Fano factor of
super-Poissonian shot noise.Comment: 5 pages, major revision, to be published in Phys. Rev.
Snake states and their symmetries in graphene
Snake states are open trajectories for charged particles propagating in two
dimensions under the influence of a spatially varying perpendicular magnetic
field. In the quantum limit they are protected edge modes that separate
topologically inequivalent ground states and can also occur when the particle
density rather than the field is made nonuniform. We examine the correspondence
of snake trajectories in single-layer graphene in the quantum limit for two
families of domain walls: (a) a uniform doped carrier density in an
antisymmetric field profile and (b) antisymmetric carrier distribution in a
uniform field. These families support different internal symmetries but the
same pattern of boundary and interface currents. We demonstrate that these
physically different situations are gauge equivalent when rewritten in a Nambu
doubled formulation of the two limiting problems. Using gauge transformations
in particle-hole space to connect these problems, we map the protected
interfacial modes to the Bogoliubov quasiparticles of an interfacial
one-dimensional p-wave paired state. A variational model is introduced to
interpret the interfacial solutions of both domain wall problems
Paramagnetic reentrant effect in high purity mesoscopic AgNb proximity structures
We discuss the magnetic response of clean Ag coated Nb proximity cylinders in
the temperature range 150 \mu K < T < 9 K. In the mesoscopic temperature
regime, the normal metal-superconductor system shows the yet unexplained
paramagnetic reentrant effect, discovered some years ago [P. Visani, A. C.
Mota, and A. Pollini, Phys. Rev. Lett. 65, 1514 (1990)], superimposing on full
Meissner screening. The logarithmic slope of the reentrant paramagnetic
susceptibility chi_para(T) \propto \exp(-L/\xi_N) is limited by the condition
\xi_N=n L, with \xi_N=\hbar v_F/2 \pi k_B T, the thermal coherence length and
n=1,2,4. In wires with perimeters L=72 \mu m and L=130 \mu m, we observe
integer multiples n=1,2,4. At the lowest temperatures, \chi_para compensates
the diamagnetic susceptibility of the \textit{whole} AgNb structure.Comment: 4 pages, 4 figures (color
One-dimensional Josephson arrays as superlattices for single Cooper pairs
We investigate uniform one-dimensional arrays of small Josephson junctions
(, ) with a realistic Coulomb interaction (here is the screening length
in units of the lattice constant of the array). At low energies this system can
be described in terms of interacting Bose particles (extra single Cooper pairs)
on the lattice. With increasing concentration of extra Cooper pairs, a
crossover from the Bose gas phase to the Wigner crystal phase and then to the
superlattice regime occurs. The phase diagram in the superlattice regime
consists of commensurable insulating phases with ( is integer)
separated by superconducting regions where the current is carried by
excitations with {\em fractional} electric charge . The Josephson
current through a ring-shaped array pierced by magnetic flux is calculated for
all of the phases.Comment: 4 pages (LATEX), 2 figure
Quantum Heisenberg Antiferromagnet: Improved Spin-Wave Theories Versus Exact-Diagonalization Data
We reconsider the results cocerning the extreme-quantum
square-lattice Heisenberg antiferromagnet with frustrating diagonal couplings
( model) drawn from a comparison with exact-diagonalization data. A
combined approach using also some intrinsic features of the self-consistent
spin-wave theory leads to the conclusion that the theory strongly overestimates
the stabilizing role of quantum flutcuations in respect to the N\'{e}el phase
in the extreme-quantum case . On the other hand, the analysis implies
that the N\'{e}el phase remains stable at least up to the limit which is pretty larger than some previous estimates. In addition, it is
argued that the spin-wave ansatz predicts the existence of a finite range
( in the linear spin-wave theory) where the Marshall-Peierls
sigh rule survives the frustrations.Comment: 13 pages, LaTex, 7 figures on reques
Photon-Assisted Transport Through Ultrasmall Quantum Dots: Influence of Intradot Transitions
We study transport through one or two ultrasmall quantum dots with discrete
energy levels to which a time-dependent field is applied (e.g., microwaves).
The AC field causes photon-assisted tunneling and also transitions between
discrete energy levels of the dot. We treat the problem by introducing a
generalization of the rotating-wave approximation to arbitrarily many levels.
We calculate the dc-current through one dot and find satisfactory agreement
with recent experiments by Oosterkamp et al. . In addition, we propose a novel
electron pump consisting of two serially coupled single-level quantum dots with
a time-dependent interdot barrier.Comment: 16 pages, Revtex, 10 eps-figure
Site-selective adsorption of naphthalene-tetracarboxylic-dianhydride on Ag(110): First-principles calculations
The mechanism of adsorption of the
1,4,5,8-naphthalene-tetracarboxylic-dianhydride (NTCDA) molecule on the Ag(110)
surface is elucidated on the basis of extensive density functional theory
calculations. This molecule, together with its perylene counterpart, PTCDA, are
archetype organic semiconductors investigated experimentally over the past 20
years. We find that the bonding of the molecule to the substrate is highly
site-selective, being determined by electron transfer to the LUMO of the
molecule and local electrostatic attraction between negatively charged carboxyl
oxygens and positively charged silver atoms in [1-10] atomic rows. The
adsorption energy in the most stable site is 0.9eV. A similar mechanism is
expected to govern the adsorption of PTCDA on Ag(110) as well.Comment: 8 pages, 4 figures, high-quality figures available upon reques
Idiopathic hypercalciuria: Association with isolated hematuria and risk for urolithiasis in children
Idiopathic hypercalciuria: Association with isolated hematuria and risk for urolithiasis in children. A prospective multicenter study was designed to determine the frequency and prognostic importance of hypercalciuria in children with hematuria. Urinary calcium excretion was examined in 215 patients with unexplained isolated hematuria (no proteinuria, urolithiasis, infection or systemic disorder). Hypercalciuria (urinary calcium excretion > 4 mg/kg/day) was identified in 76 patients (35%). Compared to patients with normal urinary calcium excretion, children with hematuria and hypercalciuria were characterized by male preponderance, white race, family history of urolithiasis, gross hematuria and calcium oxalate crystals. Renal biopsies were performed in 10 patients with urinary calcium excretion 0.4 to 2.5 mg/kg/day; three had IgA glomerulonephritis, three had glomerular basement membrane thinning, one had proliferative glomerulonephritis and three were normal. Renal biopsies in three patients with hypercalciuria showed focal segmental glomerulosclerosis, hereditary nephritis or no abnormalities. Oral calcium loading tests showed renal hypercalciuria in 26 patients, absorptive hypercalciuria in 15 patients and were not diagnostic in 35 patients. Serum parathyroid hormone, bicarbonate and phosphorus and urinary cyclic adenosine monophosphate concentrations were similar in the three groups of hypercalciuric patients. Urinary calcium excretion after one week of dietary calcium restriction was higher (5.8 mg/kg/day) in renal hypercalciuria than in other hypercalciuric patients (3.4 mg/kg/day), P < 0.01. One to four years follow-up was available for 184 patients. Eight of 60 hypercalciuric patients developed urolithiasis or renal colic compared to 2 of 124 patients with normal urinary calcium excretion (P < 0.001). Hypercalciuria is commonly associated with isolated hematuria and represents a risk factor for future urolithiasis in children with hematuria. Oral calcium loading tests offer little diagnostic benefit over 24-hour urinary calcium excretion following dietary calcium restriction
Out-of-equilibrium phonons in gated superconducting switches
Recent experiments have suggested that superconductivity in metallic nanowires can be suppressed by the application of modest gate voltages. The source of this gate action has been debated and either attributed to an electric-field effect or to small leakage currents. Here we show that the suppression of superconductivity in titanium nitride nanowires on silicon substrates does not depend on the presence or absence of an electric field at the nanowire, but requires a current of high-energy electrons. The suppression is most efficient when electrons are injected into the nanowire, but similar results are obtained when electrons are passed between two remote electrodes. This is explained by the decay of high-energy electrons into phonons, which propagate through the substrate and affect superconductivity in the nanowire by generating quasiparticles. By studying the switching probability distribution of the nanowire, we also show that high-energy electron emission leads to a much broader phonon energy distribution compared with the case where superconductivity is suppressed by Joule heating near the nanowire
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