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 ($E_J \ll E_C$, $E_C = (2e)^2/2C$) with a realistic Coulomb interaction $U(x) = E_C \lambda \exp( - |x|/\lambda)$ (here $\lambda \gg 1$ 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 $\nu$ 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 $\nu = 1/l$ ($l$ is integer) separated by superconducting regions where the current is carried by excitations with {\em fractional} electric charge $q = \pm 2e/l$. 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

J1−J2J_1-J_2 Quantum Heisenberg Antiferromagnet: Improved Spin-Wave Theories Versus Exact-Diagonalization Data

We reconsider the results cocerning the extreme-quantum $S=1/2$ square-lattice Heisenberg antiferromagnet with frustrating diagonal couplings ($J_1-J_2$ 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 $S=1/2$. On the other hand, the analysis implies that the N\'{e}el phase remains stable at least up to the limit $J_{2}/J_{1} = 0.49$ 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 ($J_{2}/J_{1}<0.323$ 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