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1.
A. I. Belousov S. A. Berzakov Yu. E. Lozovik 《Journal of Experimental and Theoretical Physics》1998,87(2):322-328
The phase diagram of a two-dimensional Josephson array of mesoscopic objects (superconducting granules, superfluid helium
in a porous medium, traps with Bose-condensed atoms, etc.) is examined. Quantum fluctuations in both the modulus and phase
of the superconducting order parameter are taken into account within a lattice boson Hubbard model. Modulating the average
occupation number n
0 of the sites in the system (the “number of Cooper pairs” per granule, the number of atoms in a trap, etc.) leads to changes
in the state of the array, and the character of these changes depends significantly on the region of the phase diagram being
examined. In the region where there are large quantum fluctuations in the phase of the superconducting order parameter, variation
of the chemical potential causes oscillations with alternating superconducting (superfluid) and normal states of the array.
On the other hand, in the region where the bosons interact weakly, the properties of the system depend monotonically on n
0. Lowering the temperature and increasing the particle interaction force lead to a reduction in the width of the region of
variation in n
0 within which the system properties depend weakly on the average occupation number. The phase diagram of the array is obtained
by mapping this quantum system onto a classical two-dimensional XY model with a renormalized Josephson coupling constant and is consistent with our quantum path-integral Monte Carlo calculations.
Zh. éksp. Teor. Fiz. 114, 591–604 (August 1998) 相似文献
2.
A. I. Belousov S. A. Verzakov Yu. E. Lozovik 《Journal of Experimental and Theoretical Physics》1998,86(1):146-155
The boson lattice Hubbard model is used to study the role of quantum fluctuations of the phase and local density of the superfluid
component in establishing a global superconducting state for a system of mesoscopic Josephson junctions or grains. The quantum
Monte Carlo method is used to calculate the density of the superfluid component and fluctuations in the number of particles
at sites of the two-dimensional lattice for various average site occupation numbers n
0 (i.e., number of Cooper pairs per grain). For a system of strongly interacting bosons, the phase boundary of the ordered
superconducting state lies above the corresponding boundary for its quasiclassical limit—the quantum XY-model—and approaches the latter as n
0 increases. When the boson interaction is weak in the boson Hubbard model (i.e., the quantum fluctuations of the phase are
small), the relative fluctuations of the order parameter modulus are significant when n
0<10, while quantum fluctuations in the phase are significant when n
0<8; this determines the region of mesoscopic behavior of the system. Comparison of the results of numerical modeling with
theoretical calculations show that mean-field theory yields a qualitatively correct estimate of the difference between the
phase diagrams of the quantum XY-model and the Hubbard model. For a quantitative estimate of this difference the free energy and thermodynamic averages of
the Hubbard model are expanded in powers of 1/n
0 using the method of functional integration.
Zh. éksp. Teor. Fiz. 113, 261–277 (January 1998) 相似文献
3.
Quantum fluctuations of the phases of the order parameter in two-dimensional arrays of mesoscopic Josephson junctions and
their effect on the destruction of superconductivity in the system are investigated by means of a quantum-cosine model that
is free of the incorrect application of the phase operator. The proposed model employs trigonometric phase operators and makes
it possible to study arrays of small superconducting granules, pores containing superfluid helium, or Josephson junctions
in which the average number of particles n
0 (effective bosons, He atoms, and so on) is small, and the standard approach employing the phase operator and the particle
number operator as conjugate operators is inapplicable. There is a large difference in the phase diagrams between arrays of
macroscopic and mesoscopic objects for n
0<5 and U<J (U is the characteristic interaction energy of the particles per granule and J is the Josephson coupling constant). Re-entrant superconductivity phenomena are discussed.
Pis’ma Zh. éksp. Teor. Fiz. 66, No. 10, 649–654 (25 November 1997) 相似文献
4.
D. M. Kagan M. V. Feigel’man L. B. Ioffe 《Journal of Experimental and Theoretical Physics》1999,89(4):781-787
We show that the ground state of a periodic long-range Josephson array frustrated by a magnetic field is a glass for sufficiently
large Josephson energies despite the absence of quenched disorder. Like superconductors, this glass state has non-zero phase
stiffness and Meissner response; for lower Josephson energies the glass “melts” and the ground state loses its phase stiffness
and becomes insulating. We find the critical scaling behavior near this quantum phase transition: the excitation gap vanishes
as (J−J
c
)2, and the frequency-dependent magnetic susceptibility behaves as
.
Zh. éksp. Teor. Fiz. 116, 1450–1461 (October 1999)
Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor. 相似文献
5.
A. Yu. Aladyshkin A. K. Vorob’ev P. P. Vysheslavtsev E. B. Klyuenkov A. S. Mel’nikov Yu. N. Nozdrin I. D. Tokman 《Journal of Experimental and Theoretical Physics》1999,89(5):940-947
The temperature dependence of the local energy barrier to formation of the mixed state in YBaCuO thin-film superconducting
samples has been determined. The measurement technique is based on use of a small ferromagnetic particle as the magnetic field
source. It is found that the energy barrier to creation of vortices (for the field oriented parallel to the CuO planes) is
anomalously small while the dependence of the corresponding threshold current j
c(T) differs substantially from the temperature dependence of the pair-breaking current. The experimental results are interpreted
in terms of the model of a Josephson medium. The observed temperature dependence of j
c points to a strong suppression of the superconducting order parameter at the intergrain boundaries, which for the most probable
type of boundaries: superconductor-insulator-superconductor, is evidence of anisotropic pairing.
Zh. éksp. Teor. Fiz. 116, 1735–1749 (November 1999) 相似文献
6.
W. Cui Z. R. Xi Y. Pan 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2010,59(3):479-485
We theoretically analyze the entanglement generation and
dynamics by coupled Josephson junction qubits. Considering a
current-biased Josephson junction (CBJJ),
we generate maximally entangled states. In particular, the entanglement
dynamics is considered as a function of the decoherence
parameters, such as the temperature, the ratio
r
≡
wc\omega_c/w0\omega_0 between the reservoir cutoff frequency
wc\omega_c and the system oscillator frequency w0\omega_0,
and the energy levels split of the superconducting circuits in the
non-Markovian master equation. We analyzed the entanglement sudden
death (ESD) and entanglement sudden birth (ESB) by the non-Markovian
master equation. Furthermore, we find that the larger the ratio r
and the thermal energy k
BT
, the shorter the decoherence. In this
superconducting qubit system we find that the entanglement can be
controlled and the ESD time can be prolonged by adjusting the
temperature and the superconducting phases Fk\Phi_k which split the
energy levels. 相似文献
7.
《Superlattices and Microstructures》1999,25(5-6):891-904
Quantum effects in the dynamics of the Josephson phase difference in Josephson junctions with large electron transparency D are studied in the adiabatic regime, when the characteristic charging energyEC of the junction is much smaller than the superconducting energy gap Δ. In isolated junctions, quantum phase fluctuations are large and manifest themselves as Coulomb blockade of Cooper pair tunneling. The amplitude of the Coulomb blockade oscillations is calculated for single-mode junctions with arbitrary D. In particular, it is shown that the chiral anomaly completely suppresses Coulomb blockade in ballistic junctions with D = 1, and the suppression process at D → 1 can be described as the Landau–Zener transition in imaginary time. In the regime when quantum phase fluctuations are small, they lead to quantum decay of supercurrent states due to macroscopic quantum tunneling of phase through the Josephson potential barrier. The decay rate is found in the nearly-ballistic junctions. 相似文献
8.
We study the collective oscillations in two-band superconducting systems with all possible intra-and interband interactions
for arbitrary values of carrier density N
0, including low carrier densities, when μ∼Δ. Allowance for processes of scattering of an interband Cooper pair into an intraband
pair leads to the emergence of a new excitonic mode in addition to the Bogolyubov-Anderson acoustic mode and an excitonic
mode of the Leggett type. The presence of a new order parameter Δ2, in addition to Δ11 and Δ22, and the asymmetric cutoff of integrals lead to the mixing of fluctuations of the phases and amplitudes of the order parameters
of different bands. This mixing and the dependence of the order parameters on N
0 result in a strong dependence of these two excitonic modes on the carrier density N
0.
Zh. éksp. Teor. Fiz. 114, 1047–1066 (September 1998) 相似文献
9.
S.-G. Cheng Y. X. Xing X. C. Xie Q.-F. Sun 《The European Physical Journal B - Condensed Matter and Complex Systems》2009,67(4):551-557
The Josephson current through an Aharonov-Bohm (AB)
interferometer, in which a quantum dot (QD) is situated on one arm
and a magnetic flux Φ threads through the ring, has been
investigated. In the presence of the magnetic flux, the relation
between the Josephson current and the superconductor phase is
complex, and the system can be adjusted to π junction by either
modulating the magnetic flux or the QD’s energy level
εd. Due to the electron-hole symmetry, the Josephson
current I has the property
I(εd,Φ)=I(-εd,Φ+π). The Josephson
current exhibits a jump when a pair of Andreev bound states aligns
with the Fermi energy. The condition for the current jump is given.
Particularly, we find that the position of the current jump and the
position of the maximum value of the critical current Ic are
identical. Due to the interference between the two paths, the
critical current Ic versus the QD’s level εd shows a
typical Fano shape, which is similar to the Fano effect in the
corresponding normal device. However they also show some
differences. For example, the critical current never reaches zero
for any parameters, while the current in the normal device can reach
zero at the destruction point. 相似文献
10.
The effect of quantum fluctuations on a phase transition in a two-dimensional Josephson junction array is studied in terms
of the two-dimensional XY model. A self-consistent harmonic approximation is used to calculate the linear response of the system to a perturbation
by a uniform phase gradient (helicity modulus γ) as a function of dimensionless temperature Θ and of a quantum parameter q appearing due to the finite capacitance of each junction. Calculation of this quantity has permitted us to find the dependence
of the Kosterlitz-Thouless transition temperature on q, which within a broad range of q variation is in agreement with the results of a quantum Monte Carlo simulation.
Fiz. Tverd. Tela (St. Petersburg) 39, 818–822 (May 1997) 相似文献
11.
The current-voltage characteristics of granular YBa2Cu3O6.95 high-temperature superconductor samples have been measured at a temperature of 77.3 K in external transverse magnetic fields
H
ext with a strength of up to H
ext ≈ 500 Oe for low transport current densities (0.1 A/cm2 ≤ j ≤ 0.6 A/cm2). The current-voltage characteristics obtained have been used to construct dependences of the magnetoresistance ρ on the
quantities j (ρ(j)
Hext=const) and H
ext(ρ(H
ext)
j = const). It has been revealed that the current and field dependences of the magnetoresistance exhibit anomalies at H
ext ≥ H
c1g
, where H
c1g
is the lower critical field of superconducting grains. A comparative analysis of the dependences ρ(j)H
ext = const and ρ(H
ext)
j = const has made it possible to develop concepts regarding the influence of the processes of redistribution of the magnetic field
between grain boundaries and superconducting grains on the transport and galvanomagnetic properties of granular high-temperature
superconductors. It has been established that the field dependences of the magnetoresistance exhibit specific features associated
with the beginning of penetration of Josephson vortices into grain boundaries in the magnetic field H
c1J
and with the breaking of a continuous chain of Josephson junctions in the magnetic field H
c2J
. 相似文献
12.
A. F. Andreev 《JETP Letters》1996,64(9):664-669
In NdN and SdS nanostructures, anomalous ground states with a non-integer average number of electrons in the quantum dots
d occur. These states correspond to pair or single-electron superconductivity and are separated from states with a definite
(integer) number of electrons by, as a rule, second-order phase transitions. The characteristic features of the pair and single-electron
superconducting Josephson current in SdS are discussed.
Pis’ma Zh. éksp. Teor. Fiz. 64, No. 9, 618–623 (10 November 1996) 相似文献
13.
The effects of macroscopic ordering in a system of mesoscopic Josephson junctions are investigated by the quantum Monte Carlo
simulation technique (using path integrals). The phase diagram of the system in the T-q plane (q is the dimensionless quantum parameter
, where J is the Josephson coupling constant and C
0 is the self-capacitance of the granules) is investigated in detail. An analysis of the behavior of the relative root-mean-square
phase shifts, as well as the helicity and vorticity moduli, demonstrates the need to employ these two quantities as the parameters
which most completely reflect the character of the topological phase transition in the quantum system under consideration.
Two methods are proposed for calculating the vorticity modulus: 1) a modification of the Gibbs-Bogolyubov variational principle
for calculating the free energy change in response to alteration of the type of boundary conditions; 2) calculation of the
response to the introduction of an infinitesimal magnetic flux at some point in the system. The calculations confirm the absence
of reentrant melting and phase transitions of a non-Kosterlitz-Thouless type in the region of strong quantum phase fluctuations
q>1.
Fiz. Tverd. Tela (St. Petersburg) 39, 1513–1519 (September 1997) 相似文献
14.
15.
S. A. Sergeenkov 《JETP Letters》1998,67(9):680-684
The change of the Josephson supercurrent density j
s
of a weakly connected granular superconductor in response to externally applied arbitrary thermal gradient ∇T (nonlinear Seebeck effect) is considered within a model of 3D Josephson junction arrays. For ∇T>(∇T)c, where (∇T)c is estimated to be of the order of ≃104 K/m for YBCO ceramics with an average grain size d≃10 μ m, the weak-links-dominated thermopower S is predicted to become strongly ∇T-dependent.
Pis’ma Zh. éksp. Teor. Fiz. 67, No. 9, 650–654 (10 May 1998)
Published in English in the original Russian journal. Edited by Steve Torstveit. 相似文献
16.
The nonadiabatic corrections to the self-energy part Σs(q, ω) of the phonon Green’s function are studied for various values of the phonon vectors q resulting from electron-phonon interactions. It is shown that the long-range electron-electron Coulomb interaction has no
direct influence on these effects, aside from a possible renormalization of the corresponding constants. The electronic response
functions and Σs(q, ω) are calculated for arbitrary vectors qand energy ω in the BCS approximation. The results obtained for q=0 agree with previously obtained results. It is shown that for large wave numbers q, vertex corrections are negligible and Σs(q, ω) possesses a logarithmic singularity at ω=2Δ, where Δ is the superconducting gap. It is also shown that in systems with nesting, Σs(Q, ω) (where Q is the nesting vector) possesses a square-root singularity at ω=2Δ, i.e., exactly of the same type as at q=0. The results are used to explain the recently published experimental data on phonon anomalies, observed in nickel borocarbides
in the superconducting state, at large q. It is shown, specifically, that in these systems nesting must be taken into account in order to account for the emergence
of a narrow additional line in the phonon spectral function S(q, ω)≈−π
−1 Im D
s
(q, ω), where D
s
(q, ω) is the phonon Green’s function, at temperatures T<T
c
.
Zh. éksp. Teor. Fiz. 115, 1799–1817 (May 1999) 相似文献
17.
A. V. Pronin B. P. Gorshunov A. A. Volkov H. S. Somal D. van der Marel B. J. Feenstra Y. Jaccard J. -P. Locquet 《JETP Letters》1998,68(5):432-436
Direct measurements of the complex conductivity spectra of thin-film La2−2x
SrxCuO4 are made at frequencies of 5–40 cm−1. Narrow, intense Drude-type excitation is observed in the superconducting phase.
Pis’ma Zh. éksp. Teor. Fiz. 68, No. 5, 406–409 (10 September 1998) 相似文献
18.
M. A. Zelikman 《Technical Physics》1997,42(9):1019-1026
The behavior of planar (laminar) vortices in a three-dimensional, ordered Josephson medium as a function of the parameter
I, which is proportional to the critical junction current and the cell size, is investigated with allowance for pinning due
to the cellular structure of the medium. The minimum possible distances between two isolated vortices are calculated. A system
of vortices formed in a sample in a monotonically increasing external magnetic field is analyzed. The minimum distance from
the outermost vortex to the nearest neighbor is proportional to I
−1.1. For I⩽1.3 each vortex contains a single flux quantum Φ0, and the distance between them does not decrease in closer proximity to the boundary but remains approximately constant,
implying that the magnetic field does not depend on the coordinate in the region penetrated by vortices. These facts contradict
the generally accepted Bean model. The sample magnetization curve has a form typical of type II superconductors. Allowance
for pinning raises the critical field H
c and induces a sudden jump in the curve at H=H
c.
Zh. Tekh. Fiz. 67, 38–46 (September 1997) 相似文献
19.
August Yurgens Dag Winkler Nicolay Zavaritsky Tord Claeson 《Czechoslovak Journal of Physics》1996,46(Z3):1293-1294
We report on thec-axis superconducting energy gap parameter Δ
c
(T) of intrinsic Josephson tunnel junctions inBi
2
Sr
2
CaCu
2
O
8+δ (Bi2212) single crystals. Δ
c
(4.2K)≈10−13 meV, which is approximately a factor of two smaller than reported in the majority of tunneling experiments. Δ
c
(T) deviates strongly from the BCS temperature dependence. These observations may be explained by a multilayer model of Bi2212
which assumes that theBi−O layers are superconducting due to the proximity effects. The Josephson tunneling then takes place between adjacentBi−O layers while there is a strong proximity coupling betweenBi−O andCu−O layers.
The work is supported by Swedish Supercon-ductivity Consortium and NUTEK, and, in part, by Russian Foundation for Basic Research,
grant #95-02-04307 相似文献
20.
A. Smerzi S. Raghavan S. Fantoni S.R. Shenoy 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,24(4):431-435
Oscillations in quantum phase about a mean value of π, observed across micropores connecting two
3
He
-
B
baths, are explained in a Ginzburg-Landau phenomenology. The dynamics arises from the Josephson phase relation,the interbath
continuity equation, and helium boundary conditions. The pores are shown to act as Josephson tunnel junctions, and the dynamic
variables are the inter bath phase difference and fractional difference in superfluid density at micropores. The system maps
onto a non-rigid, momentum-shortened pendulum, with inverted-orientation oscillations about a vertical tilt angle φ = π, and other modes are predicted.
Received 19 March 2001 相似文献