首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 46 毫秒
1.
Point-contact spectra of YBa2(Cu1–x Zn x )3O7/Ag are studied at various temperatures. The differential resistance dV/dI of the point contacts shows gap-related structures belowT c which can be attributed to Andreev reflection. Evaluation of many spectra for each sample taken at 4.2 K yields a wide distribution of voltages /e at which these structures occur. The upper limit varies roughly as expected from the depression ofT c by Zn-doping from /e=29 mV (x=0) to 9 mV (x=0.05), while the lower limit decreases much faster and disappears forx=0.05. Hence, the Zn doped samples exhibit a tendency to gapless superconductivity as suggested earlier on the basis of specific-heat measurements.  相似文献   

2.
Using general methods developed in a previous treatment we study correlations in inhomogeneous Ising models on a square lattice. The nearest neighbour couplings can vary both in strength and sign such that the coupling distribution is translationally invariant in horizontal direction. We calculate correlations parallel to the layering in the horizontally layered model with periodv=2. If the model has a finite critical temperature,T c>0, the order parameter in the frustrated case may become discontinuous forT0. Correlations atT=T c decay algebraically with critical exponent =1/4 and exponentially forT>T c. If the critical temperature vanishes,T c=0, we always have exponential decay at finite temperatures, while atT=T c=0 we encounter either long-range order or algebraic decay with critical index =1/2, i.e.T=0 is thus a critical point.Work performed within the research program of the Sonder forschungsbereich 125 Aachen-Jülich-Köln  相似文献   

3.
Al-films, evaporated at room temperature under different oxygen partial pressures, were irradiated with self-ions (500 keV, Al++) at low temperature (<7 K). The observed increase of the resistivity and of the superconducting transition temperatureT c depends strongly on the oxygen contentc 0 present in the layers. A qualitative different behaviour of the above quantities was found for different fluence ranges of the bombarding ions. For high fluences in all cases an oxygen stabilized disorder state was obtained with correspondingT c-increases betweenT c=0.2 K for the purest films (c 00.5 at %) andT c =1.2 K for films with c 0=40 at %. The annealing behaviour of the irradiated films is also dependent on the oxygen contentc 0.  相似文献   

4.
The thermal expansion of squaric acid along the uniqueb-axis was measured between 323 K and 418 K with a resolution of 2·10–7 in strain and 5·10–4 K in temperature using a capacitance dilatometer. The anomaly associated with the antiferro-electric phase transition occuring atT c=373.5 K was observed over the rangeT c–50 K<T<T c+7 K. The regular lattice expansion follows a generalized Grüneisen's law with a Debye-temperature D =660 K and a uniaxial pressure-dependence of D /p b . The critical behaviour close toT c was studied with a sweep rate of 36 mK/h. A hyteresis loop of width 23 mK was observed in the strain curve, indicating a 1st order transition. However, no discontinuity in dilatation was observed, and therefore the transition is close to a multicritical point. The anomalous increase in the expansion coefficient can be described with critical exponents ==0.56 in the interval 0.4 K<|T–T c|<12 K. Closer toT c the divergence is stronger. A generalized Pippard relation between the expansion coefficient and specific heat holds to within 1 K from the transition point. The inconsistencies apparent closer toT c are interpreted as a consequence of a weak 1st order transition.  相似文献   

5.
The critical ordering temperature, T c′, for the antiferromagnetic Heisenberg model is investigated on the basis of the high temperature expansion for the staggered susceptibility. For the simple cubic and body-centred cubic lattices, it appears that

where q is the lattice coordination number, S the spin value and T c the corre-sponding ferromagnetic Curie point. A possible counter argument asserting T c′ ? T c is discussed at some length.  相似文献   

6.
Real glass transitions, where activated processes are included, are analysed near Whitney fold and cusp singularities. In particular the temperature dependence of the viscosity is studied. For sufficiently strongly coupled systems there is close to a fold singularity located atT=T c , a crossover from an algebraic divergence (TT c ) forT>T c , to an Arrhenius dependence exp(E/k B T) forT<T c . If a parameter vector specifying the system also passes close to a cusp singularity, atT 0<T c , there may be a temperature region where the viscosity also shows Fogel-Fulcher like dependence. The results for the viscosity are intimately connected with the dynamics for the so-called -relaxation process, where relaxation occurs via a fractal time process in the delevant time region.  相似文献   

7.
The mechanisms driving the phase separation in high-T c La cuprates is studied by magnetic susceptibility and electrical resistivity measurements. The experiments are performed under different thermal treatment. Three temperature ranges can be resolved characteristic for different physical processes: At 150T170K a space limited diffusion of superparamagnetic clusters (single hole clusters) occurs leading to the formation of a small but rather tight percolative subphase which at highT c values becomes superconducting. Above 180 K the cluster motion becomes unrestricted in space and a more extended subphase is built up, however, with lowerT c values. Above 230 K oxygen diffusion starts leading to an unexpected destruction of the conducting/superconducting phase.  相似文献   

8.
The electronegativity differences, N, between the anions and the cations in suggested resonating elements of some representative high-temperature superconductors with T c - 10 K are evaluated adopting Pauling's scale. The relationship between N and T c was found to separate all the examined high-T csuperconductors onto two curves: One for the cuprate superconductors having two-dimensional layered structures was a straight line, T c=29.8+4.1N; the other correlation curve representing the remaining compound superconductors including the doped 113 perovskite and the perovskite-related 214 structures was at lower T c values but also suggested that increasing electronegativity differences was related to increasing T c.This work was supported by the Director, Office of Energy Research, Office of Basic Energy Sciences, Materials Sciences Division of the U.S. Department of Energy under Contract No. DE-AC03-76SF00098  相似文献   

9.
The phase structure of the (2+1)-dimensional Gross-Neveu model is considered at a nonzero temperatureT and in the presence of an external electromagnetic field. The major results are as follows: 1. AtT=0,H0 (a magnetic field), the original symmetry of the model breaks for anyH values. 2. AtT=0,E0 (an electric field) anE c is shown to exist, such that forEE c the original symmetry is broken, but forE>E c the vacuum of the model is symmetric. 3. AtT0,H0, and at a fixedT value such anH c (T) is shown to exist that forH>H c (T) the chiral invariance of the model is spontancously broken. For any fixedH value there exists such anT c (H) that forT>T c (H) the symmetry restores. The phase portrait of the model is drawn.  相似文献   

10.
We study the block spin transformation for the 2D Ising model at the critical temperatureT c . We consider the model with the constraint that the total spin in each block is zero. An old argument by Cassandro and Gallavotti strongly supports the Gibbsianness of the transformed measure, provided that such model has a critical temperatureT c lower thanT c . After describing a possible rigorous approach to the problem, we present numerical evidence that indeedT c <T c and study the Dobrushin-Shlosman uniqueness condition.  相似文献   

11.
12.
We investigate the electronic structure of the ferromagnetic 4f-metal Gadolinium by use of a many-body evaluation of a generalized model of magnetism, the one-particle part of which is derived from an ASW-LSDA bandstructure calculation. A striking temperature-dependence of the conduction band states is traced back to a 4f-(5d, 6s) interband exchange. The conduction electron polarization (0.63 B atT=0) decreases forTT c very similar to the 4f-magnetization. A red shift of the lower -band edge of about 0.25 eV appears upon cooling fromT=T c toT=0. — The quasiparticle band-structure exibits a remarkable non uniform magnetic behaviour at different positions in the Brillouin zone, and in particular for different subbands. Weakly correlated (s-like) dispersions show a Stoner-likeT-dependence of the exchange splitting. On the other hand, stronger correlated (d-like) dispersions split belowT c into four branches, two for each spin direction. TheirT-dependence mainly concerns the spectral weights of the quasiparticle peaks and not so much the energetic positions. An exchange caused splitting remains even forT<T c .  相似文献   

13.
A bipolaronic model of high-T c superconductivity, that is based upon the idea that a doped oxygenhole reduces the separation between the and levels of the neighbouring Cu-ion and hence causes a Jahn-Teller mixing between the two levels, is suggested. Within the BCS scheme, the bipolaron Hamiltonian is investigated for the superconducting order parameter and the critical temperatureT c as a function of hole-doping. Under suitable approximation for the bipolaronic kinetic energy, the hole-doping dependence ofT c shows a universal behaviour. For temperatures close toT c and forT c 1, obeys the BCS type of relation with pre-factor of (1–T/T c )2 related to the range of superconducting-phase against the doping axis.  相似文献   

14.
We have studied the effect of hydrostatic pressures up to 20 Kbar on the temperature dependence of the resistivity (T) and the effect of quasihydrostatic pressure up to 200 Kbar on the lattice parameters of YBa2Cu3O x for different oxygen concentrations (x=6.95–6.2). Pressure produces a decrease of resistivity in normal state, an increase ofT c , and a suppression of semiconducting-like resistivity (T) at lowx. The dependence of dT c (x)/dP onx is nonmonotonic; the record values of dT c /dP=(1.0±0.1) K/Kbar are observed forx=6.7 and 6.8. The derivative dln/dP atT=293 K differs by the factor 1.8 between superconducting and nonsuperconducting compounds. The compressibility and its pressure derivative alonga, b andc-directions in YBa2Cu3O x have been determined. The most remarkable variation is alongc-direction. A nonmonotonic dependence of dk c (x)/dP onx has been observed.The results are discussed in the context of localized effects in disordered oxygen-deficient YBa2Cu3O x .  相似文献   

15.
Epitaxial YBa2Cu4O7-thin films with thec-axis oriented perpendicular to the film plane were prepared bydc-sputtering from a single stoichiometric target on (100) SrTiO3-substrates. Typical values of the inductively measured superconducting transitions were about 90 K with a width less than 0.5 K. Critical current densities were measured on 5 to 10 m wide strips as function of magnetic field and temperature. The temperature dependences ofj c follow a universal functionj c(B,T)=j c * (T=0,B)·(1–T/T c (B)) with =1.5±0.1. ForB=0 andT=77 K we obtainedj c =4·106 A/cm2. The field dependence of the resistive transitions was measured with the magnetic field parallel to thec-axis. The slope of the upper critical fieldB c2 (T) was determined for different criteria. The carrier concentration evaluated from Hall-effect measurements was found to decrease linearly from one per unit cell at 240 K with decreasing temperature extrapolating nearly through zero forT=0. Highly resolved angular dependent measurements of the critical current density withB perpendicular to the current but tilted from thec-axis show a very strong and sharp enhancement ofj c for the magnetic field parallel to the (CuO2)-layers (Bc). Additionally to this phenomenon, which is caused by an intrinsic pinning mechanism due to the layered structure of high-T c -superconductors the influence of the anisotropy of the upper critical field onj c (B, T, ) is evident nearT c .  相似文献   

16.
We report measurements, between helium and room temperature, of the thermal expansion (T), sound velocitiesv L (T) andv T (v) and specific heatC(T) of YBa2Cu3O7 samples, all cut from the same sintered pellet. A linear term in (T) is resolved at low temperatures. It is compared with a corresponding term in the low temperature specific heat and, using the bulk modulus from our sound propagation measurements, a Grüneisen parameter of usual size (1.2±0.4) is derived-in conformity with the possible existence of non-superconducting carriers well belowT c . Longtime drift effects in the length of the sample between 30 K and 60 K point to the vicinity of a structural instability. The Debye temperature derived fromC(T) amounts to about 450 K atT=T c =91 K and forT0 approaches 350 K, in agreement with the value deduced fromv L (T) andv T (T). The superconducting transition is indicated by anomalies with idealized discontinuities and C. A thermodynamic relation between and C yields the relative pressure derivative ofT c ,t p =T c –1 (T c /p) p0=+(0.7±0.2)·10–7 kbar–1.  相似文献   

17.
With the vibrating reed technique we have measured the critical current densityJ c of a Y 123 200 nm thin film as a function of magnetic field perpendicular to the CuO2 planes (O T<B8 T) and temperature (20KT60 K). At fieldsB<0.1 TJ c is magnetic field independent and decreases at higher fields. A comparison with theory indicates that a crossover from a single pinning to a small bundle collective pinning regime may explain the observed behaviour. According to our estimate the main pinning centers are weak point pins due to oxygen vacancies. From the temperature dependence ofJ c atB0 we obtain a temperature dependence of the thermodynamical critical fieldB c (1–T/T c )2 forT20 K which agrees with the anomalous temperature dependence ofB c2 observed recently in highly anisotropic high temperature superconductors.  相似文献   

18.
The Bose-Einstein condensation of free relativistic particles [=(M 2 c 4 +c 2 p 2 ) 1/2 –Mc 2 ] is studied rigorously. For massless bosons (=cp), the condensation transition of third (second) order occurs in2 (3) dimensions (D). The molar heat capacity follows the T 2 (T 3 ) law below the condensation temperature Tc [k B Tc=(2 2 c 2 n/1.645) 1/2 [( 2 3 c 3 n/1.202) 1/3 ], reaches4.38 (10.8) R at T=Tc, and approaches the high-temperature-limit value2 (3) R with no jump (a jump equal to6.75R) in2 (3)D. For finite-mass (M) bosons, the phase transition occurs only in3D with the condensation temperature Tc always smaller than that of the corresponding nonrelativistic bosons [=(2M) –1 p 2 ]. If the mass M is reduced to zero, the condensation temperature Tc grows monotonically and reaches eventually that of massless relativistic bosons. This mass-dependence of Tc is therefore distinct from the case of nonrelativistic bosons, where Tc grows to infinity as M 0. A brief discussion is given for a possible connection with the normal-to-super transition of the independently moving Cooper pairs (bosons).  相似文献   

19.
The dynamic correlation function for an isotropic ferromagnet forTT c is calculated in first order in =6–d. Applying an exponentiation procedure then leads to results which are relevant for the analysis of neutron scattering experiments in the vicinity of the critical point.Supported by the Fonds zur Förderung der wissenschaftlichen Forschung  相似文献   

20.
A simple model of nonlinear optical systems exhibiting instability—such as in laser action and in bistable absorption—is presented that provides a prototype of nonequilibrium thermodynamics on a statistical basis. The adiabatic reduction of the atomic degrees of freedom in a revised Langevin treatment establishes a fully consistent framework in which the active electromagnetic field mode is in contact with two thermal reservoirs (the cavity and the atoms) as well as being acted upon by an external field. The results are summarized by the first and second laws,dE=W+Q c +Q A anddS Qc/¯Tc+QA/¯TA, with the statistical mechanical representations of the entities therein exhibiting the nature of the mode; i.e., (a) a heat-engine structure operating between two reservoirs of temperatures¯T c > 0 and¯T A < 0 for the laser, and (b) a nonlinear response against external work balanced by a single reservoir (¯ T c = ¯T A ) for the absorptive bistability.Originally presented at the Taniguchi Symposium, October 1979.44  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号