共查询到20条相似文献,搜索用时 46 毫秒
1.
V. L. Gurtovoi S. V. Dubonos A. V. Nikulov N. N. Osipov V. A. Tulin 《Journal of Experimental and Theoretical Physics》2007,105(6):1157-1173
The obtained periodic magnetic-field dependences I c+(Φ/Φ0) and I c?(Φ/Φ0) of the critical current measured in opposite directions on asymmetric superconducting aluminum rings has made it possible to explain previously observed quantum oscillations of dc voltage as a result of alternating current rectification. It was found that a higher rectification efficiency of both single rings and ring systems is caused by hysteresis of the current-voltage characteristics. The asymmetry of current-voltage characteristics providing the rectification effect is due to the relative shifts of the magnetic dependences I c?(Φ/Φ0) = I c+(Φ/Φ0 + Δ?) of the critical current measured in opposite directions. This shift means that the position of I c+(Φ/Φ0) and I c?(Φ/Φ0) minima does not correspond to n + 0.5 magnetic flux Φ quanta, which is in direct contradiction to measured Little-Parks resistance oscillations. Despite this contradiction, the amplitude I c, an(Φ/Φ0) = I c+(Φ/Φ0) ? I c?(Φ/Φ0) of critical current anisotropy oscillations and its variations with temperature correspond to the expected amplitude of persistent current oscillations and its variations with temperature. 相似文献
2.
V. V. Val’kov D. M. Dzebisashvili 《Journal of Experimental and Theoretical Physics》2005,100(3):608-616
The renormalizations of the fermionic spectrum are considered within the framework of the t-J* model taking into account three-center interactions (H(3)) and magnetic fluctuations. Self-consistent spin dynamics equations for strongly correlated fermions with three-center interactions were obtained to calculate quasi-spin correlators. A numerical self-consistent solution to a system of ten equations was obtained to show that, in the nearest-neighbor approximation, simultaneously including H(3) and magnetic fluctuations at n>n1 (n1 ≈ 0.72 for 2t/U = 0.25) caused qualitative changes in the structure of the energy spectrum. A new Van Hove singularity is then induced in the density of states, and an additional maximum appears in the Tc(n) concentration dependence of the temperature of the transition to the superconducting phase with order parameter symmetry of the d x 2?y2 type. 相似文献
3.
A. V. Samokhvalov 《Journal of Experimental and Theoretical Physics》2007,104(3):451-460
Commensurability effects have been theoretically studied in a hybrid system consisting of a Josephson junction located in a nonuniform field induced by an array of magnetic particles. A periodic phase-difference distribution in the junction that is caused by the formation of a regular lattice of Abrikosov vortices generated by the magnetic field of the particles in superconducting electrodes is calculated. The dependence of the critical current through the junction I c on the applied magnetic field H is shown to differ strongly from the conventional Fraunhofer diffraction pattern because of the periodic modulation of the Josephson phase difference created by the vortices. More specifically, the I c(H) pattern contains additional resonance peaks, whose positions and heights depend on the parameters and magnetic state of the particles in the array. These specific features of the I c(H) dependence are observed when the period of the Josephson current modulation by the field of the magnetic particles and the characteristic scale of the change in the phase difference by the applied magnetic field are commensurable. The conditions that determine the positions of the commensurability peaks are obtained, and they are found to agree well with experimental results. 相似文献
4.
A. D. Alekhin O. I. Bilous 《Russian Journal of Physical Chemistry B, Focus on Physics》2015,9(7):1018-1025
This work proposes a mechanism for the physical processes underlying the wide practical application of the unique properties of a substance in a critical state—critical fluid (CF)—in contemporary technologies. According to the fluctuation theory of phase transitions (FTPT), this mechanism may be due to the fluctuation and structural characteristics of a critical fluid, which determine its equilibrium and kinetic properties. Among such characteristics are the system correlation radius Rs, the number of order parameter fluctuations N f ~ R s -3 per mole of critical fluid, and the fluctuation component of the thermodynamic potential F*f = N f k T c/(P c V c) = C 0 R s -3 . These structural characteristics are studied with the use of experimental gravity effect data, such as the altitude and temperature dependencies of the scattered light intensity I(z, t) in a heterogeneous substance (n-pentane) near the critical vaporization temperature. Using these results and the literature data on the formation of Al2O3 nanoparticles with the use of SC-H2O, the propagation velocity of substance molecules v f ≈ 106 cm/s is estimated for the origination and decay of order parameter fluctuations. It has been concluded that just such high propagation velocities of substance molecules most likely cause the unique properties of a critical fluid during their practical application in a number of engineering processes. 相似文献
5.
Near the critical temperature of a superconducting transition, the energy of the threshold perturbation δFthr that transfers a superconducting bridge to a resistive state at a current below the critical current Ic has been determined. It has been shown that δFthr increases with a decrease in the length of a bridge for short bridges with lengths L < ξ (where ξ is the coherence length) and is saturated for long bridges with L ? ξ. At certain geometrical parameters of banks and bridge, the function δFthr(L) at the current I → 0 has a minimum at L ~ (2–3)ξ. These results indicate that the effect of fluctuations on Josephson junctions made in the form of short superconducting bridges is reduced and that the effect of fluctuations on bridges with lengths ~(2–3)ξ is enhanced. 相似文献
6.
S. I. Krasnosvobodtsev A. V. Varlashkin A. I. Golovashkin N. P. Shabanova 《Physics of the Solid State》2005,47(9):1600-1604
The upper critical field H c 2 (H ‖ c) of the two-band superconductor MgB2 is studied as a function of the residual resistivity ρn. It is found that the superconductor follows the standard trend: the slope-dHc2/dT of the temperature dependence of Hc2(T) increases with the number of defects. The upper critical field in the clean limit is found, and direct estimations of the parameters of carriers in the 2D σ band (including the Fermi velocity and the coherence length) are made. The contribution of the electron scattering to the magnitude of Hc2 is determined, and the mean free path of electrons in samples with various defect concentrations is estimated. The density of states of σ electrons at the Fermi level is calculated using the dependence of the slope-dHc2/dT on ρn and a band structure model. It is impossible to estimate this density of states directly, because the upper critical field is determined by the carriers of one band, whereas the resistivity depends on the carriers in both bands. 相似文献
7.
E. Z. Kuchinskii N. A. Kuleeva M. V. Sadovskii 《Journal of Experimental and Theoretical Physics》2017,125(6):1127-1136
We study disorder effects upon the temperature behavior of the upper critical magnetic field in an attractive Hubbard model within the generalized DMFT+Σ approach. We consider the wide range of attraction potentials U—from the weak coupling limit, where superconductivity is described by BCS model, up to the strong coupling limit, where superconducting transition is related to Bose–Einstein condensation (BEC) of compact Cooper pairs, formed at temperatures significantly higher than superconducting transition temperature, as well as the wide range of disorder—from weak to strong, when the system is in the vicinity of Anderson transition. The growth of coupling strength leads to the rapid growth of Hc2(T), especially at low temperatures. In BEC limit and in the region of BCS–BEC crossover Hc2(T), dependence becomes practically linear. Disordering also leads to the general growth of Hc2(T). In BCS limit of weak coupling increasing disorder lead both to the growth of the slope of the upper critical field in the vicinity of the transition point and to the increase of Hc2(T) in the low temperature region. In the limit of strong disorder in the vicinity of the Anderson transition localization corrections lead to the additional growth of Hc2(T) at low temperatures, so that the Hc2(T) dependence becomes concave. In BCS–BEC crossover region and in BEC limit disorder only slightly influences the slope of the upper critical field close to T c . However, in the low temperature region Hc2 (T may significantly grow with disorder in the vicinity of the Anderson transition, where localization corrections notably increase Hc2 (T = 0) also making Hc2(T) dependence concave. 相似文献
8.
V. V. Val’kov A. A. Golovnya 《Bulletin of the Russian Academy of Sciences: Physics》2007,71(5):667-669
The concentration dependence of the transition temperature to the superconducting phase is calculated within the t – t' – t" – J* model, which takes into account tree-site interactions. It is shown that the processes of scattering by spin fluctuations qualitatively change the character of the concentration dependences T c (n). These scattering processes in the normal phase significantly modify the distribution function of Hubbard fermions, determining the pronounced non-Fermi-liquid behavior of the system. 相似文献
9.
To identify the structure of emissive tunnel recombination sites in the emulsion microcrystals of silver bromide AgBr(I) with iodine contaminations and to determine the role of an emulsion medium in their formation, the temperature dependence of the luminescence spectra in the range from 77 to 120 K, the kinetics of the growth of the maximum luminescence intensity value at λ ≈ 560 nm, and the luminescence flash spectrum stimulated by the infrared light are investigated. Two types of the AgBr1 – x(I x ) (x = 0.03) microcrystals—namely, obtained in an aqueous solution and on a gelatin substrate—are used in the studies. It is established that the emissive tunnel recombination sites with a luminescence maximum at λ ≈ 560 nm in AgBr1 – x(I x ) (x = 0.03) are the {(I a - I a - )Ag i + } donor–acceptor complexes with the I a - iodine ions located in neighbor anionic sites of the AgBr(I) crystal lattice, next to which the Ag i + interstitial silver ion is positioned. With an increase in the temperature, the {(I a - I a - )Ag i + } sites undergo structural transformation into the {(I a - I a - )Agin+} sites, where n = 2, 3, …. Moreover, the {(I a - I a - )Ag in + } sites (n = 2) after the capture of an electron and hole also provide the tunnel recombination with a luminescence maximum at λ ≈ 720 nm. The influence of an emulsion medium consists in that gelatin interacts with the surface electron-localization sites, i.e., the interstitial silver ions Ag in + , n = 1, 2, and forms the complexes {Ag in 0 G+} (n = 1, 2) with them. The latter are deeper electron traps with a small capture cross section as compared to the Ag in + sites (n = 1, 2) and that manifest themselves in that the kinetics of the luminescence growth in AgBr(I) to a stationary level at λ ≈ 560 nm is characterized by the presence of “flash firing.” At the same time, the luminescence flash stimulated by IR light, for which the Ag in + (n = 1, 2) electron-localization sites are responsible, is absent. It is supposed that the electrons localized on the {Ag in + G+} complexes (n = 2) retain the capability for emissive tunnel recombination with holes localized on paired iodine sites with a luminescence maximum at λ ≈ 750 nm. 相似文献
10.
This study aims at establishing the interrelation between the current-carrying capacity and peculiarities of magnetoresistance of granular YBa2Cu3O7 ? δ HTSCs (T c = 92.5 K). The transverse magnetoresistance of several batches of YBa2Cu3O7 ? δ HTSC samples with noticeably different values of critical supercurrent density j c is measured in magnetic fields H ext up to H ext max ≈ 500 Oe in a wide range of transport currents (5 mA ≤ I ≤ 1600 mA) at T = 77.4 K. Samples with relatively high values of j c (H ext = 0) ≥ 100 A/cm2 do not exhibit any anomalies in their field dependences. Magnetoresistance jumps δρBG-VG/ρ273K are observed for samples with low values of j c ≥ 20 A/cm2 in fields H BG-VG ≈ 200–260 Oe. The width ΔH BG-VG of the anomalous resistance region increases upon an increase in I. The magnetoresistance jumps decrease with increasing I in increasing field H ext(0 → H ext max ) and increase in decreasing field H ext(H ext max → 0). It is found that these peculiarities of the field dependences of magnetoresistance are associated with a first-order phase transition (in magnetic field) in the vortex structure of YBa2Cu3O7 ? δ HTSCs of the Bragg glass-vortex glass type. 相似文献
11.
V. A. Alekseev 《Journal of Experimental and Theoretical Physics》2001,92(4):608-616
The distribution function W0(n0) of the number n0 of particles in the condensate of an ideal Bose gas confined by a trap is found. It is shown that at the temperature below the critical temperature Tc this function has a Gaussian shape and depends on the trap potential via two parameters only. The center of this function shifts to larger values of n0 with decreasing temperature and its width tends to zero, which corresponds to the suppression of fluctuations. In the narrow vicinity of the critical temperature \(\left| {T - {T_c}} \right| \leqslant {T_c}/\sqrt N \), where N is the number of particles in the trap, the distribution function changes and at the temperature above the critical one it takes the usual form W0(n0) = [1 ? exp(μ)]exp(μn0), where μ is the chemical potential in temperature units. In the limit N→∞, this change occurs at a jump. 相似文献
12.
D. A. Balaev D. M. Gokhfeld A. A. Dubrovskiĭ S. I. Popkov K. A. Shaikhutdinov M. I. Petrov 《Journal of Experimental and Theoretical Physics》2007,105(6):1174-1183
Hysterestic behavior of the magnetoresistance of granular HTSCs and its interaction with the magnetic hysteresis are studied by measuring magnetoresistance R(H) and critical current I c(H) of composites formed by HTSC Y0.75Lu0.25Ba2Cu3O7 and CuO. A network of Josephson junctions is formed in such composites, in which the nonsuperconducting component plays the role of barriers between HTSC grains. Hysteretic dependences R(H) of magnetoresistance are studied in a wide range of transport current density j and are analyzed in the framework of the two-level model of a granular superconductor, in which dissipation takes place in the Josephson medium and the magnetic flux can be pinned both in grains and in the Josephson medium. The interrelation between the hysteresis of critical current I c(H) and the evolution of the hysterestic dependence R(H) of the magnetoresistance upon transport current variation is demonstrated experimentally. The effect of the magnetic past history on the hysteretic behavior of R(H) and the emergence of a segment with a negative magnetoresistance are analyzed. It is shown for the first time that the R(H) dependences are characterized by a parameter that is independent of the transport current, viz., the width of the R(H) hysteresis loop. 相似文献
13.
The mechanism of hole carrier generation is considered in the framework of a model assuming the formation of negative U centers (NUCs) in HTSC materials under doping. The calculated dependences of carrier concentration on the doping level and temperature are in quantitative agreement with experiment. An explanation is proposed for the pseudogap and 60 K phases in YBa2Cu3O6+δ. It is assumed that a pseudogap is of superconducting origin and arises at temperature T* > Tc∞ > Tc in small nonpercolating clusters as a result of strong fluctuations in the occupancy of NUCs (Tc∞ and Tc are the superconducting transition temperatures of an infinitely large and finite NUC clusters, respectively). The T*(δ) and Tc(δ) dependences calculated for YBa2Cu3O6+δ correlate with experimental dependences. In accordance with the model, the region between T*(δ) and Tc(δ) is the range of fluctuations in which finite nonpercolation clusters fluctuate between the superconducting and normal states due to NUC occupancy fluctuations. 相似文献
14.
M. N. Magomedov 《Physics of the Solid State》2003,45(7):1213-1218
A new approach is proposed for calculating the Debye temperature of a nanocrystal in the form of an n-dimensional rectangular parallelepiped with an arbitrary microstructure and the number of atoms N ranging from 2n to infinity. The geometric shape of the system is determined by the lateral-to-basal edge ratio of the parallelepiped. The size dependences of the Debye and melting temperatures for a number of materials are calculated using the derived relationship. The theoretical curves thus obtained agree well with the experimental data. The calculated dependences of the superconducting transition temperature T c on the size d of aluminum, indium, and lead nanocrystals are also in reasonable agreement with the experimental estimates of T c (d). It is demonstrated that, as the nanocrystal size d decreases, the greater the deviation of the nanocrystal shape from an equilibrium shape (in our case, a cube), the higher the temperature of the superconducting transition T c (d). The superconducting transition temperature is calculated as a function of the thickness (diameter) of a plate (rod) with an arbitrary length. It is found that a decrease in the thickness (diameter) of the plate (rod) leads to an increase in the temperature T c (z): the looser the microstructure of the metallic nanocrystal, the higher the temperature T c (z). 相似文献
15.
E. A. Pashitskii V. I. Pentegov 《Journal of Experimental and Theoretical Physics》2017,124(3):433-445
We consider a possible scenario for the evolution of the early cold Universe born from a fairly large quantum fluctuation in a vacuum with a size a 0 ? l P (where l P is the Planck length) and filled with both a nonlinear scalar field φ, whose potential energy density U(φ) determines the vacuum energy density λ, and a nonideal Fermi gas with short-range repulsion between particles, whose equation of state is characterized by the ratio of pressure P(n F ) to energy density ε(n F ) dependent on the number density of fermions n F . As the early Universe expands, the dimensionless quantity ν(n F ) = P(n F )/ε(n F ) decreases with decreasing n F from its maximum value νmax = 1 for n F → ∞ to zero for n F → 0. The interaction of the scalar and gravitational fields, which is characterized by a dimensionless constant ξ, is proportional to the scalar curvature of four-dimensional space R = κ[3P(n F )–ε(n F )–4λ] (where κ is Einstein’s gravitational constant), and contains terms both quadratic and linear in φ. As a result, the expanding early Universe reaches the point of first-order phase transition in a finite time interval at critical values of the scalar curvature R = R c =–μ2/ξ and radius a c ? a 0. Thereafter, the early closed Universe “rolls down” from the flat inflection point of the potential U(φ) to the zero potential minimum in a finite time. The release of the total potential energy of the scalar field in the entire volume of the expanding Universe as it “rolls down” must be accompanied by the production of a large number of massive particles and antiparticles of various kinds, whose annihilation plays the role of the Big Bang. We also discuss the fundamental nature of Newton’ gravitational constant G N . 相似文献
16.
Bernd P. K. Maier 《Zeitschrift für Physik A Hadrons and Nuclei》1965,184(2):153-174
The low energy gamma-rays from neutron-capture in Lu 175 and Lu 176 have been investigated by means of the bent crystal-spectrometer at the DR-3-reactor at Risø. From the transitions in Lu 177 3 rotational bands have been determined. The levels of the (404)K=7/2+ groundstate rotational band are: 121,62 keV (I=9/2), 268,79keV (I=11/2), 440,66 keV (I=13/2), 636,22 keV (I=15/2), 854,34 keV (I=17/2). The level-sequence of the (514)K=9/2?-band is: 150,39 keV (I=9/2), 288,99 keV (I=11/2), 451,49 keV (I=13/2), 637,05 keV (I=15/2) and 844,88 keV (I=17/2). At 457,92 keV is the basis for the (402)K=5/2+-band the higher levels of which are 552,05 keV (I=7/2), 671,89 keV (I=9/2), 816,63 keV (I=11/2), 985,23 keV (I=13/2), 1176,73keV (I=15/2) and probably 1389,5 keV (I=17/2). The energies of the levels apart from the 1389 keV-level have an accuracy of 7×10?5. The energy differences between the 3 bands agree very well with the values expected from the Bohr-Mottelson-formulaE=A·I(I+1)+B·I 2(I+1)2. The calculated branching-ratios within the 3 bands are in fairly good agreement with the experimental values. Theg K -factors have been determined for 2 bands: It was found that for the (514)-bandg K =1,16±0,04 and for the (402)-bandg K =1,33±0,07. 相似文献
17.
W. Biem 《Zeitschrift für Physik A Hadrons and Nuclei》1963,171(1):265-272
Exchange forces modify thef-sum by an additional termCα.C is calculated as a function of the hard core radiusc for several types of potentials and for two densities of nuclear matter (r 0=1·1 and 1·2 f).C is found to depend strongly onc. 相似文献
18.
L. G. Mamsurova K. S. Pigalskiy N. G. Trusevich A. A. Vishnev M. A. Rogova S. Yu. Gavrilkin A. Yu. Tsvetkov 《JETP Letters》2015,102(10):662-667
The magnetization M(H) in the superconducting state, dc magnetic susceptibility χ(T) in the normal state, and specific heat C(T) near the superconducting transition temperature T c have been measured for a series of fine-crystalline YBa2Cu3O y samples having nearly optimum values of y = 6.93 ± 0.3 and T c = (91.5 ± 0.5) K. The samples differ only in the degree of nanoscale structural inhomogeneity. The characteristic parameters of superconductors (the London penetration depth and the Ginzburg–Landau parameter) and the thermodynamic critical field H c are determined by the analysis of the magnetization curves M(H). It is found that the increase in the degree of nanoscale structural inhomogeneity leads to an increase in the characteristic parameters of superconductors and a decrease in H c(T) and the jump of the specific heat ΔC/T c. It is shown that the changes in the physical characteristics are caused by the suppression of the density of states near the Fermi level. The pseudogap is estimated by analyzing χ(T). It is found that the nanoscale structural inhomogeneity significantly enhances and probably even creates the pseudogap regime in the optimally doped high-T c superconductors. 相似文献
19.
The influence of spin fluctuations on the thermodynamic properties of a helical ferromagnet MnSi has been investigated in the framework of the Hubbard model with the electronic spectrum determined from the first-principles LDA + U + SO calculation, which is extended taking into account the Hund coupling and the Dzyaloshinskii–Moriya antisymmetric exchange. It has been shown that the ground state of the magnetic material is characterized by large zero-point fluctuations, which disappear at the temperature T* (<T c is the temperature of the magnetic phase transition). In this case, the entropy abruptly increases, and a lambdashaped anomaly appears in the temperature dependence of the heat capacity at constant volume (C V (T)). In the temperature range T* < T < T c , thermal fluctuations lead to the disappearance of the inhomogeneous magnetization. The competition between the increase in the entropy due to paramagnon excitations and its decrease as a result of the reduction in the amplitude of local magnetic moments, under the conditions of strong Hund exchange, is responsible for in the appearance of a “shoulder” in the dependence C V (T)). 相似文献
20.
The penetration of a magnetic field into superconducting grains and weak links of YBa2Cu3O7?δ ceramic high-temperature superconductors is investigated using measurements of the transverse and longitudinal magnetoresistances at T=77.3 K and 0≤H≤~500 Oe as a function of the transport current in the range ~0.01≤I/I c ≤~0.99. The effects associated with the complete penetration of Josephson vortices into weak links of the high-temperature superconductor in magnetic fields Hc2J, the onset of penetration of Abrikosov vortices into superconducting grains in magnetic fields Hc1A, and the first-order transition from the Bragg glass phase to the vortex glass phase in fields HBG-VG are revealed and interpreted. The I-H phase diagrams YBa2Cu3O7?δ high-temperature superconductors are constructed for I ⊥ H and I ‖ H. 相似文献