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1.
It has been shown that two-and three-center interactions arise in the strong-coupling regime for the 2D Kondo lattice; these interactions both induce and suppress the Cooper instability. It is important that, in contrast to the t-J* model, the three-center interactions promote the Cooper pairing and ensure the appearance of the superconducting phase with a high critical temperature T c. The calculated concentration dependences of T c agree well with the experimental data for the cuprate superconductors.  相似文献   

2.
A microscopic theory of superconductivity in the extended Hubbard model which takes into account the intersite Coulomb repulsion and electron-phonon interaction is developed in the limit of strong correlations. The Dyson equation for normal and pair Green functions expressed in terms of the Hubbard operators is derived. The self-energy is obtained in the noncrossing approximation. In the normal state, antiferromagnetic short-range correlations result in the electronic spectrum with a narrow bandwidth. We calculate superconducting T c by taking into account the pairing mediated by charge and spin fluctuations and phonons. We found the d-wave pairing with high-T c mediated by spin fluctuations induced by the strong kinematic interaction for the Hubbard operators. Contributions to the d-wave pairing coming from the intersite Coulomb repulsion and phonons turned out to be small.  相似文献   

3.
Based on the assumption that the superconducting state belongs to a single irreducible representation of lattice symmetry, we propose that the pairing symmetry in all measured iron-based superconductors is generally consistent with the A 1g s-wave. Robust s-wave pairing throughout the different families of iron-based superconductors at different doping regions signals two fundamental principles behind high-T c superconducting mechanisms: (i) the correspondence principle: the short-range magnetic-exchange interactions and the Fermi surfaces act collaboratively to achieve high-T c superconductivity and determine pairing symmetries; (ii) the magnetic-selection pairing rule: superconductivity is only induced by the magnetic-exchange couplings from the super-exchange mechanism through cation-anion-cation chemical bonding. These principles explain why unconventional high-T c superconductivity appears to be such a rare but robust phenomena, with its strict requirements regarding the electronic environment. The results will help us to identify new electronic structures that can support high-T c superconductivity.  相似文献   

4.
The effect of ion irradiation on the superconducting transition temperatureT c and resistivityρ ab (T) of YBa2Cu3O7-x films with different oxygen content (initial temperatureT c0≈90 K and 60 K) is studied experimentally. The dependenciesT c /T c0 on residual resistivityρ o are obtained in very wide range 0.2<T c /T c0 <1 andρ o μΩ·cm. The critical values ofρ o , corresponding to the vanishing of superconductivity, are found to be an order of magnitude larger then those predicted by theory ford-wave pairing. At 0.5÷0.6<T c /T c0<1 the experimental data are in close agreement with theoretical dependencies, obtained for the anisotropics-wave superconductor within the BCS-framework.  相似文献   

5.
We report on the synthesis and measurements of the temperature dependences of the resistivity ρ, the penetration depth λ, and the upper critical magnetic field Hc2, for polycrystalline samples of dodecaboride ZrB12 and diboride MgB2. We conclude that ZrB12 behaves as a simple metal in the normal state with the usual Bloch-Grüneisen temperature dependence of ρ(T) and with a rather low resistive Debye temperature TR = 280 K (to be compared to TR = 900 K for MgB2). The ρ(T) and λ(T) dependences for these samples reveal a superconducting transition in ZrB12 at Tc = 6.0 K. Although a clear exponential λ(T) dependence in MgB2 thin films and ceramic pellets was observed at low temperatures, this dependence was almost linear for ZrB12 below Tc/2. These features indicate an s-wave pairing state in MgB2, whereas a d-wave pairing state is possible in ZrB12. In disagreement with conventional theories, we found a linear temperature dependence, of Hc2(T) for ZrB12 (Hc2(0) = 0.15 T).  相似文献   

6.
Using the theory of high-temperature superconductivity based on the idea of the fermion-condensation quantum phase transition (FCQPT), we show that neither the d-wave pairing symmetry, the pseudogap phenomenon, nor the presence of the Cu-O2 planes is of decisive importance for the existence of high-T c superconductivity. We analyze recent experimental data on this type of superconductivity in different materials and show that these facts can be understood within the theory of superconductivity based on the FCQPT. The latter can be considered as a universal cause of high-T c superconductivity. The main features of a room-temperature superconductor are discussed.  相似文献   

7.
A theory of the thermodynamic properties of a two-band superconductor with a low carrier density is developed; it is based on a phonon superconductivity mechanism with a strong electron-phonon coupling. This theory can describe the variation of the critical temperature T c, the energy gaps Δ1 and Δ2, and the relative electronic specific heat jump (C S ? C N)/C N at T = T c with the carrier density in the compound MgB2 when substitutional impurities of various valences are introduced into this system. The values of T c, Δ1, and Δ2 are shown to decrease as this compound is doped by electrons and to remain constant (or almost constant) as it is doped by holes. This behavior follows from the mechanism of filling the σ and π energy bands, which overlap at the Fermi surface. The theory agrees qualitatively with experimental data. This agreement is found to be better when intra-and interband electron scattering by an impurity potential is taken into account.  相似文献   

8.
The formation temperature (T*~ 135 K) is determined in the Shubin-Vonsovski approximation for local electron pairs in the CuO2 planes of YBa2Cu3O7 crystal. This estimate is used to obtain the Coulomb pseudopotential µ*≈?0.15. In the presence of strong electron-phonon coupling (λ ~0.5) and electron correlation in the electron pairing, the estimate of critical temperature T c ≈99 K agrees, by the order of magnitude, with its experimental value. The calculated ratio 2Δ/kT c ≈4.13 confirms the presence of strong electron pairing.  相似文献   

9.
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   

10.
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   

11.
Temperature dependences of dielectric permittivity in the improper ferroelastic phase, including the region of the improper ferroelastic phase transition occurring at T=Tc1, were studied in the betaine phosphite-betaine phosphate solid-solution crystals. At a betaine phosphate (BP) concentration of 10%, the phase transition temperature Tc1 was found to shift toward higher temperatures by about 5 K compared to betaine phosphite (BPI) crystals, where Tc1=355 K. The phase transition remains in the vicinity of the tricritical point. As the BP concentration in BPI is increased, the dielectric anomaly at T=Tc1 weakens substantially compared to pure BPI. The nonlinear temperature dependence of reciprocal dielectric permittivity in the improper ferroelastic phase of BPIxBP1?x crystals is described in the concentration region 0.9≤x≤1 in terms of a thermodynamic model taking into account the biquadratic relation of the nonpolar order parameter of the improper ferroelastic phase transition to polarization. The decrease in the ferroelectric phase transition temperature Tc1 (or in the temperature of loss of improper ferroelastic phase stability) with increasing BP concentration in the above limits is due to the decreasing effect of the nonpolar mode on the polar instability, which is accompanied by a weakening of the dielectric anomaly at T=Tc1  相似文献   

12.
The upper critical field H c 2 (Hc) 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   

13.
The temperature dependence of the surface upper critical field,H c3, nearT c is calculated for arbitrary values of the mean free pathl by taking into account the fourthorder term of the generalized Ginzburg-Landau theory. For finitel the boundary condition is modified such that the normal derivative of the energy gap at the surface becomes positive. The slope of the curveH c3/H c2 versust=T/T c att=1 is found to decrease monotoneously from zero to ?1.040 as one goes from the “dirty” to the “clean” limit.  相似文献   

14.
The transition temperatureT c and the critical fieldH c of lead were measured as a function of the concentration of lattice defects. The defects were generated by plastic deformation at liquid Helium temperatures and reduced by annealing. T c is rather insensitive to defects. With increasing residual resistance ratio ρ the transition temperature increases and finally reaches a constant value with onlyΔT c ≈4.5 · 10?3 °K. On the other hand a deformation of the same amount increasesH c more than twice as much as the starting value. Annealing to room-temperature reducesρ, T c andH c to their initial values. During the annealing process,T c shows a distinct maximum and ρ a marked step. Contrary to this behaviourH c decreases linearly during the whole region of annealing. Taking into account the strong influence of ρ uponH c a picture is given about the mechanism of deformation, which allows to understand the results qualitatively. The changes ofT c produced by elastic strain were also measured. The results are in quantitative agreement with those of pressure experiments.  相似文献   

15.
We study the conductivity of two-dimensional interacting electrons on the half-filled Nth Landau level with N?1 in the presence of quenched disorder. The existence of the unidirectional charge-density wave state at temperature T<T c , where T c is the transition temperature, leads to the anisotropic conductivity tensor. We find that the leading anisotropic corrections are proportional to (T c ?T)/T c just below the transition, in accordance with the experimental findings. Above T c , the correlations corresponding to the unidirectional charge-density wave state below T c result in corrections to the conductivity proportional to \(\sqrt {{{T_c } \mathord{\left/ {\vphantom {{T_c } {T - T_c }}} \right. \kern-\nulldelimiterspace} {T - T_c }}} \).  相似文献   

16.
The dependence of the critical temperatureT c upon pressureP is measured in the pressure range up to 160 kbar. The experimental technique developed for very high pressure-low temperature experiments (preceding article) is improved by introducing a double-sample electrical resistance cell. An internal pressure calibration is therefore possible at some well-established room temperature pressure reference points commonly used. Both metals, tetragonal white tin and fcc-lead, show a monotonic decrease ofT c vs.P with upward curvature. The results recommend the use of Pb as a secondary standard for very high pressure experiments at Helium temperatures. In addition, high pressure polymorphic modifications of Sn and Pb are found to show superconductivity withT c =(5.30±0.10) ?K for Sn III atP=113 kbar andT c =(3.55±0.10) ?K for Pb II atP=160 kbar.  相似文献   

17.
The structure of an isolated vortex line, and the lower critical fieldH c 1, is calculated by means of the generalized Ginzburg-Landau (GL) theory for arbitrary values of the GL-parameterk(≧1/√2) and the mean free pathl at temperaturesT in the vicinity ofT c . The free energy functional including the corrections of order [1?(T/T c )] to the GL-functional is derived exactly. The corresponding Euler-Lagrange equations determining the zero-order (GL) contributions and the corrections of order [1?(T/T c )] to the order parameter,f(r), and the superfluid velocity,v(r), have been solved numerically. The shapes of the first-order corrections off(r), v(r), and the magnetic field,h(r) are found to depend markedly, for a given value ofκ, on a second parameter,α=0.882(ξ 0 /l) (whereξ 0 is theBCS-coherence-distance). The deviations from the GL-solutions become largest forh(r) at parameter valuesk≈ 1 andα ≈ 0(the deviation ofh(0) is about 6% atT=0.9T c forκ=1 andα=0). The ratioH c1/H c (where the thermodynamic criticalH c has the BCS-temperature-dependence) is found to increase slightly in the “clean” limit (α=0), and to decrease slightly in the “dirty” limit (α=∞) asT decreases (the variation ofH c 1/H c is always less than 3% for arbitrary values ofκ andα asT decreases fromT c to 0.9T c ).  相似文献   

18.
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   

19.
Thin films of partially deuterated betaine phosphate have been grown by the evaporation on Al2O3(110) and NdGaO3(001) substrates with a preliminarily deposited structure of interdigitated electrodes. The grown films have a polycrystalline block structure with characteristic dimensions of blocks of the order of 0.1–1.5 mm. The degree of deuteration of the films D varies in the range of 20–50%. It has been found that, at the antiferroelectric phase transition temperature Tcafe = 100–114 K, the fabricated structures exhibit an anomaly of the electrical capacitance C, which is not accompanied by a change in the dielectric loss tangent tanδ. The strong-signal dielectric response is characterized by the appearance of a ferroelectric nonlinearity at temperatures T >Tcafe, which is transformed into an antiferroelectric nonlinearity at T <Tcafe. With a further decrease in the temperature, double dielectric hysteresis loops appear in the antiferroelectric phase. The dielectric properties of the films have been described within the framework of the Landau-type thermodynamic model taking into account the biquadratic coupling ξP2η2 between the polar order parameter P and the nonpolar order parameter η with a positive coefficient ξ. The E–T phase diagram has been constructed.  相似文献   

20.
The Efros-Shklovskii (E-S) law for the conductivity of granular metals is interpreted as a result of a variable-range cotunneling process. The cotunneling between distant resonant grains is predominantly elastic at low TT c , while it is inelastic (i.e., accompanied by creation of electron-hole pairs on a string of intermediate non-resonant grains) at TT c . The corresponding E-S temperature T ES , in the latter case, is slightly (logarithmically) T dependent. The magnetoresistance in the two cases is different: it may be relatively strong and negative at T?T c , while, at T > T c , it is suppressed due to inelastic processes, which destroy the interference.  相似文献   

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