452 research outputs found
Recommended from our members
Dissipation and quantum phase transitions of a pair of Josephson junctions
A model system consisting of a mesoscopic superconducting grain coupled by Josephson junctions to two macroscopic superconducting electrodes is studied. We focus on the effects of Ohmic dissipation caused by resistive shunts and superconducting-normal charge relaxation within the grain. As the temperature is lowered, the behavior crosses over from uncoupled Josephson junctions, similar to situations analyzed previously, to strongly interacting junctions. The crossover temperature is related to the energy-level spacing of the grain and is of the order of the inverse escape time from the grain. In the limit of zero temperature, the two-junction system exhibits five distinct quantum phases, including a novel superconducting state with localized Cooper pairs on the grain but phase coherence between the leads due to Cooper pair cotunneling processes. In contrast to a single junction, the transition from the fully superconducting to fully normal phases is found to be controlled by an intermediate-coupling fixed point whose critical exponents vary continuously as the resistances are changed. The model is analyzed via two-component sine-Gordon models and related Coulomb gases that provide effective low-temperature descriptions in both the weak and strong Josephson coupling limits. The complicated phase diagram is consistent with symmetries of the two component sine-Gordon models, which include weak- to strong-coupling duality and permutation triality. Experimental consequences of the results and potential implications for superconductor to normal transitions in thin wires and films are discussed briefly
A Universal Interacting Crossover Regime in Two-Dimensional Quantum Dots
Interacting electrons in quantum dots with large Thouless number in the
three classical random matrix symmetry classes are well-understood. When a
specific type of spin-orbit coupling known to be dominant in two dimensional
semiconductor quantum dots is introduced, we show that a new interacting
quantum critical crossover energy scale emerges and low-energy quasiparticles
generically have a decay width proportional to their energy. The low-energy
physics of this system is an example of a universal interacting crossover
regime.Comment: 4 pages, 1 figur
Chaotic quantum dots with strongly correlated electrons
Quantum dots pose a problem where one must confront three obstacles:
randomness, interactions and finite size. Yet it is this confluence that allows
one to make some theoretical advances by invoking three theoretical tools:
Random Matrix theory (RMT), the Renormalization Group (RG) and the 1/N
expansion. Here the reader is introduced to these techniques and shown how they
may be combined to answer a set of questions pertaining to quantum dotsComment: latex file 16 pages 8 figures, to appear in Reviews of Modern Physic
Electron Pair Resonance in the Coulomb Blockade
We study many-body corrections to the cotunneling current via a localized
state with energy at large bias voltages . We show that the
transfer of {\em electron pairs}, enabled by the Coulomb repulsion in the
localized level, results in ionization resonance peaks in the third derivative
of the current with respect to , centered at . Our
results predict the existence of previously unnoticed structure within
Coulomb-blockade diamonds.Comment: 5 pages, 4 figure
Quantum criticality near the Stoner transition in a two-dot with spin-orbit coupling
We study a system of two tunnel-coupled quantum dots, with the first dot
containing interacting electrons (described by the Universal Hamiltonian) not
subject to spin-orbit coupling, whereas the second contains non-interacting
electrons subject to spin-orbit coupling. We focus on describing the behavior
of the system near the Stoner transition. Close to the critical point quantum
fluctuations become important and the system enters a quantum critical regime.
The large- approximation allows us to calculate physical quantitites
reliably even in this strongly fluctuating regime. In particular, we find a
scaling function to describe the crossover of the quasiparticle decay rate
between the renormalized Fermi liquid regime and the quantum critical regime.Comment: 19 pages, 5 figure
Superconductor-to-Metal Transitions in Dissipative Chains of Mesoscopic Grains and Nanowires
The interplay of quantum fluctuations and dissipation in chains of mesoscopic
superconducting grains is analyzed, and the results are also applied to
nanowires. It is shown that in 1-d arrays of resistively shunted Josephson
junctions, the superconducting-normal charge relaxation within the grains plays
an important role. At zero temperature, two superconducting phases can exist,
depending primarily on the strength of the dissipation. In the fully
superconducting phase (FSC), each grain acts superconducting, and the coupling
to the dissipative conduction is important. In the SC* phase, the dissipation
is irrelevant at long wavelengths. The phase transitions between these two
superconducting phases and the normal metallic phase may be either local or
global, and possess rich and complex critical properties. These are inferred
from both weak and strong coupling renormalization group analyses. At
intermediate temperatures, near either superconductor-to-normal phase
transition, there are regimes of super-metallic behavior, in which the
resistivity first decreases gradually with decreasing temperature before
eventually increasing as temperature is lowered further. The results on chains
of Josephson junctions are extended to continuous superconducting nanowires and
the subtle issue of whether these can exhibit an FSC phase is considered.
Potential relevance to superconductor-metal transitions in other systems is
also discussed.Comment: 42 pages, 14 figure
Electron Correlations in a Quantum Dot with Bychkov-Rashba Coupling
We report on a theoretical approach developed to investigate the influence of
Bychkov-Rashba interaction on a few interacting electrons confined in a quantum
dot. We note that the spin-orbit coupling profoundly influences the energy
spectrum of interacting electrons in a quantum dot. Inter-electron interaction
causes level crossings in the ground state and a jump in magnetization. As the
coupling strength is increased, that jump is shifted to lower magnetic fields.
Low-field magnetization will therefore provide a direct probe of the spin-orbit
coupling strength in a quantum dot
Pseudospin-Resolved Transport Spectroscopy of the Kondo Effect in a Double Quantum Dot
We report measurements of the Kondo effect in a double quantum dot (DQD),
where the orbital states act as pseudospin states whose degeneracy contributes
to Kondo screening. Standard transport spectroscopy as a function of the bias
voltage on both dots shows a zero-bias peak in conductance, analogous to that
observed for spin Kondo in single dots. Breaking the orbital degeneracy splits
the Kondo resonance in the tunneling density of states above and below the
Fermi energy of the leads, with the resonances having different pseudospin
character. Using pseudospin-resolved spectroscopy, we demonstrate the
pseudospin character by observing a Kondo peak at only one sign of the bias
voltage. We show that even when the pseudospin states have very different
tunnel rates to the leads, a Kondo temperature can be consistently defined for
the DQD system.Comment: Text and supplementary information. Text: 4 pages, 5 figures.
Supplementary information: 4 pages, 4 figure
- …