A muon‐spin‐rotation study of the superconducting condensate density in high temperature superconductors

Muon‐spin‐rotation studies on a variety of polycrystalline cuprate high‐T_ c superconductors reveal a generic dependence of the superconducting transition temperature T_ c upon condensate density throughout the entire range of hole doping. Upon full oxygenation the CuO chains in YBa₂Cu₃O_7-\delta become metallic and superconducting and the condensate density is dramatically enhanced. The very rapid suppression of the condensate density n_ s upon Zn substitution in YBa₂(Cu_1-xZn_x)₃O_6+\delta is inconsistent with s‐wave pairing and magnetic scattering but points towards a d‐wave pairing state with non‐magnetic scattering in the unitarity limit. This revised version was published online in August 2006 with corrections to the Cover Date.     

Anomalous peak in the superconducting condensate density of cuprate high- T(c) superconductors at a unique doping state

The doping dependence of the ratio of the superconducting condensate density to the effective mass, n(o)(s)/m(*)(ab), was studied in detail by muon-spin rotation for Y(0.8)Ca(0.2)Ba(2)(Cu(1-z)Zn(z))(3)O(7-delta) and Tl(0.5-y)Pb(0.5+y)Sr(2)Ca(1-x)Y(x)Cu(2)O(7). Our data show that n(o)(s)/m(*)(ab) exhibits a peak at a unique doping state in the overdoped regime. Its position coincides with the critical doping state, where the normal state pseudogap was reported to appear and to deplete the electronic density of states. This finding implies that the pseudogap primarily arises from a change in the electronic ground state rather than from thermal fluctuations.     

Theoretical investigation of the superconducting gaps in the Fe-based superconductors

Based on an antiferromagnetic (AFM) spin fluctuation approximation, we study the superconducting gaps in Fe-based compound using two-band model. We find that our results are consistent with the previous work that concludes sign-reversal extended s-wave pairings between different Fermi surface sheets. The different superconducting gap magnitude around different Fermi surface sheets is probably due to the different density of states on them. This calculation can give insight to the recent angle-resolved photoemission (ARPES) experiments on these materials. To detect the phase variation of the superconducting gap over the Fermi surfaces, we propose a new method for measuring the particular wave vector phonon linewidth. In the case of the sign-reversal superconducting pairing, the linewidth shows continuities compared to the case of no phase variation.     

Plasma excitations in the superconducting state of two-band layered superconductors

We investigate the low-frequency plasma modes polarized in the c-direction in two-band layered superconductors. The dynamic dielectric function (ω,q) is derived at T=0, using the generalized random-phase-approximation which is consistent with the Hartree–Fock single-particle states. It is shown that the dielectric function has two zero-points which correspond to the longitudinal plasma modes in two-band layered superconductors.     

高温超导体中的赝隙与超导位相涨落

高温超导体赝隙态与超导态之间的关系一直是研究的焦点.交流电导和能斯特（Nernst）效应测量相继探测到超导转变温度Tc0以上温区一定范围内存在磁通涡旋激发,利用力矩技术的磁化率测量则探测到超导涨落引起的弱抗磁性.这些发现都支持了高温超导体赝隙相中存在有限的超导序参量振幅和强烈的位相涨落的图像,说明Tc0处的相变是由库珀对之间长程位相关联的消失所驱动的.文章首先简短地介绍高温超导体的电子态相图和赝隙态,以及能斯特效应的原理和测量方法,然后对能斯特效应的测量结果作一评述性介绍,还讨论了相关的理论模型.     

On the possibility of increasing the superconducting gaps in heterogeneous superconductors

A model of the pair scattering of electrons by heterogeneity islands inside a superconducting matrix is presented; states with charge-density waves appear on them. The assumption that such a model can be used in the case of thin heterogeneous films and superconductor boundaries is made.     

高温超导体中的赝隙与超导位相涨落

高温超导体赝隙态与超导态之间的关系一直是研究的焦点.交流电导和能斯特（Nernst）效应测量相继探测到超导转变温度Tc0以上温区一定范围内存在磁通涡旋激发,利用力矩技术的磁化率测量则探测到超导涨落引起的弱抗磁性.这些发现都支持了高温超导体赝隙相中存在有限的超导序参量振幅和强烈的位相涨落的图像,说明Tc0处的相变是由库珀对之间长程位相关联的消失所驱动的.文章首先简短地介绍高温超导体的电子态相图和赝隙态,以及能斯特效应的原理和测量方法,然后对能斯特效应的测量结果作一评述性介绍,还讨论了相关的理论模型.     

On the correlation between order parameter, superconducting volume fraction and critical current density in R: 123 superconductors

Resistivity and low field ac susceptibility measurements are made on R: 123 superconducting samples with different rare earth elements R. The order parameter dimensionality OPD is deduced from resistivity versus temperature plot using the Aslamazov and Larkin expression, while the analysis of the temperature dependence of ac susceptibility is done employing Beans’ critical state model and with the help of the Ravi expression. With increasing R, the critical temperatures T_c are nearly kept constant (∼90 K), while the crossover temperatures T_o are shifted to lower values. Moreover, the superconducting order parameter OPD is shifted toward 2D behavior. On the other hand, the values of superconducting volume fraction f_g decrease with increasing ac field amplitude H_m for all samples and it is higher in Er: 123 sample than in Nd: 123 sample. Although the values of critical current density J_c at the peak temperature T_m are nearly unchanged with increasing R, the values of J_c(T), at T<T_m and T>T_m, are found to be dependent on the chosen R. The correlation between the above calculated parameters against R is also mentioned.     

Mesoscopic samples: the superconducting condensate via the Gross-Pitaevskii scenario

Mesoscopic superconductors are routinely investigated with the Ginzburg-Landau equations, whereby the confinement is taken into account by imposing that the normal component of the superconducting current vanishes through the sample boundary. We argue that this approach gives misleading results when applied to micron- and submicron-sized devices, and we introduce modified Ginzburg-Landau equations that take the confinement potential into account on the level of the free energy functional. For devices much larger than the Ginzburg-Landau coherence length, both approaches agree, but deviations appear for samples of the scale of the coherence length. In the absence of a magnetic field, the modified Ginzburg-Landau equation for the order parameter reduces to the Gross-Pitaevskii equation.     

Features of the magnetoplastic effect in a beryllium condensate

The effect of a weak dc magnetic field on inelastic characteristics of a magnesium-thermal beryllium condensate (99.95 wt % Be) has been studied. It has been shown that the magnetic field differently affects the defect subsystem of beryllium and, depending on sample treatment, leads to its hardening or softening. Based on an analysis of amplitude dependences of the internal friction and elastic moduli, the dislocation velocity after magnetic field exposure has been estimated. It has been shown that the magnetic aftereffect in Be is significantly stronger than the effect in situ.     

NMR studies of the superconducting state of copper oxide superconductors

The authors review and up-date their work on Knight shifts, spin-lattice relaxation, and indirect nuclear spin-spin coupling for YBa₂Cu₃O₇ in the superconducting state. The data are analyzed in particular to show what it may indicate about the orbital and spin pairing of the superconducting state.     

Oxyanion derivatives of cuprate superconductors superconducting 123 phosphates

While YSr₂Cu₃O₇ cannot be prepared under ambient conditions, partial substitution of the phosphate group for copper, as in YSr₂Cu_2.8(PO₄)_0.2O_y, stabilizes this phase in the orthorhombic structure, but the material is not superconducting. Superconductivity in YSr₂Cu_2.8(PO₄)_0.2O_y is obtained by increasing the hole concentration through partial substitution of Y by Ca, as in Y_0.7Ca_0.3Sr₂Cu_2.8(PO₄)_0.2O_y (T_c≈40 K). By incorporating the phosphate group in orthorhombic YBaSrCu₃O ₇, a stable tetragonal derivative of the formula YBaSrCu_2.8(PO₄)_0.2O_y (T_c≈ 47 K) has been prepared; the T_c increases to 70 K by partial substitution of Y by Ca as in Y_0.7Ca_0.3BaSrCu_2.8(PO₄)_0.2O_y.     

Electrical and superconducting properties of BiSrCaCuO ceramics superconductors

Electrical and superconducting properties of BiSrCaCuO superconductor prepared by partial melting method were determined to study its superconducting mechanism. Induced voltage of the BiSrCaCuO superconductor was not so significantly dependent upon on the shape and size of the superconductor. Anti-magnetism was less dependent upon the melting and the annealing temperatures. BiSrCaCuO film prepared at 1158 K shows the maximum magnetic susceptibility in this study. Differential conductance behavior of the superconductor with magnetic flux was not well matched by a conventional flux flow model. It suggests that a superconducting mechanism for type II superconductor should be able to explain the fact that magnetic properties arise from the interaction between the trapped magnetic flux and weak link of the superconducting connectors forms in the superconducting material.     

Thin spectrum states in bulk superconductors and superconducting grains

We show how a local pairing model for superconductivity can be used to describe the symmetry breaking mechanism in exact analogy to the cases of quantum crystals and antiferromagnets. We find that there are low energy states associated with the symmetry breaking process which are not influenced by the Anderson-Higgs mechanism. The presence of these ‘thin spectrum’ states in qubits based on superconducting material leads to a maximum time for which such qubits can remain quantum coherent. We also show how the charging energy of superconducting quantum dots may give the thin spectrum states a finite energy gap, impeding the spontaneous breaking of phase symmetry.     

Coexistence of the superconducting condensate and the quasi-stationary states of hole pairs

The formation of bound states in the case of a direct Coulomb repulsion between two holes whose reciprocal effective mass tensor has principal values of opposite signs is considered as a possible mechanism of high-T _C superconductivity. The study of the specific features of the scattering amplitude shows that, under certain conditions, in addition to the quasi-stationary states, states with a negative attenuation are possible, which corresponds to the tendency toward the formation of a hole pair condensate. The coexistence of the quasi-stationary states and the condensate qualitatively agrees with the phase diagram of p-type doped high-T _C superconducting cuprates.     

The nature of the superconducting state of the high-T c superconductors

At the present time it is possible to get a reasonably good understanding of the nature of the superconducting state in the recently discovered, high transition temperature cuprate superconductors based on experimental results published over the past year or two. These experiments now impose significant constraints on possible theoretical models which can be used to explain the superconductivity. We show how these experiments impose these constraints and discuss those questions which remain unanswered.Invited talk at the International Conference on Macroscopic Quantum Phenomen a, Smolenice Castle, Czechoslovakia, September 18–22, 1989.I am indebted to J. P. Collman, C. Caley, M. Holcomb, and G. Yee for valuable discussion, comments and criticism. I acknowledge financial support for this work from the Department of Energy (grant number DEFG03-86ER45245). I wish also to thank the Czechoslovak Academy of Sciences for the opportunity to attend the International Conference on Macroscopic Quantum Phenomena at Smolenice, Czechoslovakia, September 1989 and support for my stay.     

Fermi arcs in the superconducting clustered state for underdoped cuprate superconductors

The one-particle spectral function of a state formed by superconducting (SC) clusters is studied via Monte Carlo techniques. The clusters have similar SC amplitudes but randomly distributed phases. This state is stabilized by competition with the antiferromagnetism expected to be present in the cuprates and after quenched disorder is introduced. A Fermi surface composed of disconnected segments, i.e., Fermi arcs, is observed between the critical temperature T_(c) and the cluster formation temperature scale T*.