305 research outputs found
Coherent transport in Josephson-Junction rhombi chain with quenched disorder
We consider a chain of Josephson-junction rhombi (proposed originally by
Doucot and Vidal) in quantum regime. In a regular chain with no disorder in the
maximally frustrated case when magnetic flux through each rhombi \Phi_r is
equal to one half of superconductive flux quantum \Phi_0, Josephson current is
due to correlated transport of pairs of Cooper pairs, i.e. charge is quantized
in units of . Sufficiently strong deviation \delta\Phi =|\Phi_r-\Phi_0/2| >
\delta\Phi^c from the maximally frustrated point brings the system back to
usual -quantized supercurrent. For a regular chain \delta\Phi^c was
calculated by us previously. Here we present detailed analysis of the effect of
quenched disorder (random stray charges and random fluxes piercing rhombi) on
the pairing effect.Comment: 21 pages, 5 figure
Non-Abelian symmetries and disorder: a broad non-ergodic regime and anomalous thermalization
Symmetries play a central role in single-particle localization. Recent
research focused on many-body localized (MBL) systems, characterized by new
kind of integrability, and by the area-law entanglement of eigenstates. We
investigate the effect of a non-Abelian symmetry on the dynamical
properties of a disordered Heisenberg chain. While symmetry is
inconsistent with the conventional MBL, a new non-ergodic regime is possible.
In this regime, the eigenstates exhibit faster than area-law, but still a
strongly sub-thermal scaling of entanglement entropy. Using exact
diagonalization, we establish that this non-ergodic regime is indeed realized
in the strongly disordered Heisenberg chains. We use real-space renormalization
group (RSRG) to construct approximate excited eigenstates, and show their
accuracy for systems of size up to . As disorder strength is decreased, a
crossover to the thermalizing phase occurs. To establish the ultimate fate of
the non-ergodic regime in the thermodynamic limit, we develop a novel approach
for describing many-body processes that are usually neglected by RSRG,
accessing systems of size . We characterize the resonances that arise
due to such processes, finding that they involve an ever growing number of
spins as the system size is increased. The probability of finding resonances
grows with the system size. Even at strong disorder, we can identify a large
lengthscale beyond which resonances proliferate. Presumably, this eventually
would drive the system to a thermalizing phase. However, the extremely long
thermalization time scales indicate that a broad non-ergodic regime will be
observable experimentally. Our study demonstrates that symmetries control
dynamical properties of disordered, many-body systems. The approach introduced
here provides a versatile tool for describing a broad range of disordered
many-body systems.Comment: 25 pages, 21 figure
Resistance of two-dimensional superconducting films
We consider the problem of finite resistance R in superconducting films with geometry of a strip of width
W near zero temperature. The resistance is generated by vortex configurations of the phase field. In the first
type of process, quantum phase slip, the vortex world line in 2+1 dimensional space-time is spacelike (i.e., the
superconducting phase winds in time and space). In the second type, vortex tunneling, the world line is timelike
(i.e., the phase winds in the two spatial directions) and connects opposite edges of the film. For moderately
disordered samples, processes of the second type favor a train of vortices, each of which tunnels only across a
fraction of the sample. Optimization with respect to the number of vortices yields a tunneling distance of the
order of the coherence length ξ , and the train of vortices becomes equivalent to a quantum phase slip. Based
on this theory, we find the resistance ln R ∼ − gW/ξ , where g is the dimensionless normal-state conductance.
Incorporation of quantum fluctuations indicates a quantum phase transition to an insulating state for g 1
Exploring lipophilic antioxidants accumulation in field-grown low temperature-stressed Ephedra monosperma
The seasonal patterns of changes in the content of lipophilic antioxidants -carotene (-Car), zeaxanthin (Zx), -tocopherol (-Toc), plastoquinone (PQ)/plastoquinol () were studied in the assimilating shoots of evergreen shrub Ephedra monosperma J.G. Gmel ex C.A. Mey under natural conditions of Central Yakutia. The shortening of the photoperiod and the seasonal decrease in temperature induced a 1.4-fold increase in -Toc content. The fall in the average daily temperature from 0.1 to in October led to a decrease in the content of -Car as a result of the accumulation of rhodoxanthin (Rhd). In this period a sharp increase in the content of Zx retained overnight was also detected. In winter, elevated content of Zx and -Toc persisted. During September, the content of PQH2 increased by 2.5 times and PQ by 1.4 times (compared to July). The beginning of exposure to freezing average daily temperatures from -3 to led to the depletion of the total PQ pool by 18%. However, the content of in the winter months was 1.5 times higher than at the end of July. The results revealed different timing and temperature ranges of variation for individual antioxidants during the development of frost resistance in ephedra
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