600 research outputs found
Quantum phase-slips in Josephson junction rings
We study quantum phase-slip (QPS) processes in a superconducting ring
containing N Josephson junctions and threaded by an external static magnetic
flux. In a such system, a QPS consists of a quantum tunneling event connecting
two distinct classical states of the phases with different persistent currents
[K. A. Matveev et al., Phys. Rev. Lett. 89, 096802 (2002)]. When the Josephson
coupling energy EJ of the junctions is larger than the charging energy EC =
e2/2C where C is the junction capacitance, the quantum amplitude for the QPS
process is exponentially small in the ratio EJ/EC. At given magnetic flux each
QPS can be described as the tunneling of the phase difference of a single
junction of almost 2pi, accompanied by a small harmonic displacement of the
phase difference of the other N-1 junctions. As a consequence the total QPS
amplitude nu is a global property of the ring. Here we study the dependence of
nu on the ring size N taking into account the effect of a finite capacitance C0
to ground which leads to the appearance of low-frequency dispersive modes.
Josephson and charging effects compete and lead to a nonmonotonic dependence of
the ring critical current on N. For N=infty, the system converges either
towards a superconducting or an insulating state, depending on the ratio
between the charging energy E0 = e2/2C0 and the Josephson coupling energy EJ.Comment: (19 pages, 12 figures) The final version deviated from the original
version. One of the author was removed from the lis
Coulomb Blockade with Dispersive Interfaces
What quantity controls the Coulomb blockade oscillations if the dot--lead
conductance is essentially frequency--dependent ? We argue that it is the ac
dissipative conductance at the frequency given by the effective charging
energy. The latter may be very different from the bare charging energy due to
the interface--induced capacitance (or inductance). These observations are
supported by a number of examples, considered from the weak and strong coupling
(perturbation theory vs. instanton calculus) perspectives.Comment: 4 page
Fractional-flux vortices and spin superfluidity in triplet superconductors
We discuss a novel type of fractional flux vortices along with integer flux
vortices in Kosterlitz-Thouless transitions in a triplet superconductor. We
show that under certain conditions a spin-triplet superconductor should exhibit
a novel state of {\it spin superfluidity} without superconductivity.Comment: Physical Review Lettes, in print. v2: references added, v3:
discussion of several points extended according to referee request. Latest
updates and links to related papers are available at my homepage
http://people.ccmr.cornell.edu/~egor
Charge relaxation resistance in the Coulomb blockade problem
We study the dissipation in a system consisting of a small metallic island
coupled to a gate electrode and to a massive reservoir via single tunneling
junction. The dissipation of energy is caused by a slowly oscillating gate
voltage. We compute it in the regimes of weak and strong Coulomb blockade. We
focus on the regime of not very low temperatures when electron coherence can be
neglected but quantum fluctuations of charge are strong due to Coulomb
interaction. The answers assume a particularly transparent form while expressed
in terms of specially chosen physical observables. We discovered that the
dissipation rate is given by a universal expression in both limiting cases.Comment: 21 pages, 12 figure
Itinerant in-plane magnetic fluctuations and many-body correlations in NaCoO
Based on the {\it ab-initio} band structure for NaCoO we derive the
single-electron energies and the effective tight-binding description for the
bands using projection procedure. Due to the presence of the
next-nearest-neighbor hoppings a local minimum in the electronic dispersion
close to the point of the first Brillouin zone forms. Correspondingly,
in addition to a large Fermi surface an electron pocket close to the
point emerges at high doping concentrations. The latter yields the new
scattering channel resulting in a peak structure of the itinerant magnetic
susceptibility at small momenta. This indicates dominant itinerant in-plane
ferromagnetic fluctuations above certain critical concentration , in
agreement with neutron scattering data. Below the magnetic susceptibility
shows a tendency towards the antiferromagnetic fluctuations. We further analyze
the many-body effects on the electronic and magnetic excitations using various
approximations applicable for different ratio.Comment: 10 page
Vortex ordering in fully-frustrated superconducting systems with dice lattice
The structure and the degenracy of the ground state of a fully-frustrated
XY-model are investigated for the case of a dice lattice geometry.
The results are applicable for the description of Josephson junction arrays
and thin superconducting wire networks in the external magnetic field providing
half-integer number of flux quanta per plaquette. The mechanisms of disordering
of vortex pattern in such systems are briefly discussed.Comment: 10 pages, 3 figure
One- and two-particle correlation functions in the cluster perturbation theory for cuprates
Physics of high- superconducting cuprates is obscured by the effect of
strong electronic correlations. One way to overcome the problem is to seek for
an exact solution at least within the small cluster and expand it to the whole
crystal. Such an approach is in the heart of the cluster perturbation theory
(CPT). Here we develop CPT for the dynamic spin and charge susceptibilities
(spin-CPT and charge-CPT), within which the correlation effects are explicitly
taken into account by the exact diagonalization. We apply spin-CPT and
charge-CPT to the effective two-band Hubbard model for the cuprates obtained
from the three-band Emery model and calculate one- and two-particle correlation
functions, namely, spectral function and spin and charge susceptibilities.
Doping dependence of the spin susceptibility was studied within spin-CPT and
CPT-RPA that is the CPT generalization of the random phase approximation (RPA).
Both methods produce the low energy response at four incommensurate wave
vectors in qualitative agreement to the results of the inelastic neutron
scattering on overdoped cuprates.Comment: 14 pages, 8 figure
The Fermi surface and the role of electronic correlations in SmCeCuO
Using LDA+GTB (local density approximation+generalized tight-binding) hybrid
scheme we investigate the band structure of the electron-doped high-
material SmCeCuO. Parameters of the minimal tight-binding
model for this system (the so-called 3-band Emery model) were obtained within
the NMTO (-th order Muffin-Tin orbital) method. Doping evolution of the
dispersion and Fermi surface in the presence of electronic correlations was
investigated in two regimes of magnetic order: short-range (spin-liquid) and
long-range (antiferromagnetic metal). Each regime is characterized by the
specific topologies of the Fermi surfaces and we discuss their relation to
recent experimental data.Comment: 10 pages, 4 figures, 1 table, Published versio
Quantum instability in a dc-SQUID with strongly asymmetric dynamical parameters
A classical system cannot escape out of a metastable state at zero
temperature. However, a composite system made from both classical and quantum
degrees of freedom may drag itself out of the metastable state by a sequential
process. The sequence starts with the tunneling of the quantum component which
then triggers a distortion of the trapping potential holding the classical
part. Provided this distortion is large enough to turn the metastable state
into an unstable one, the classical component can escape. This process reminds
of the famous baron Muenchhausen who told the story of rescuing himself from
sinking in a swamp by pulling himself up by his own hair--we thus term this
decay the `Muenchhausen effect'. We show that such a composite system can be
conveniently studied and implemented in a dc-SQUID featuring asymmetric
dynamical parameters. We determine the dynamical phase diagram of this system
for various choices of junction parameters and system preparations.Comment: 12 pages, 12 figure
From underdoped to overdoped cuprates: two quantum phase transitions
Several experimental and theoretical studies indicate the existence of a
critical point separating the underdoped and overdoped regions of the high-T_c
cuprates' phase diagram. There are at least two distinct proposals on the
critical concentration and its physical origin. First one is associated with
the pseudogap formation for p<p*, with p~0.2. Another one relies on the Hall
effect measurements and suggests that the critical point and the quantum phase
transition (QPT) take place at optimal doping, p_{opt}~0.16. Here we have
performed a precise density of states calculation and found that there are two
QPTs and the corresponding critical concentrations associated with the change
of the Fermi surface topology upon doping
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