G. G. Sergeeva

On the two types of vortexlike excitations in the pseudogap state of an underdoped high-Tc superconductor

A model of charge transport for U stripes of the pseudogap state of underdoped cuprate high-Tc superconductors is discussed which is topologically consistent with antiferromagnetism. The stripe structure of the CuO2 plane at T<T*(p) suggests that on doping the Cu2+ ions localize the holes, forming planar nonmagnetic Cu3+O42− “molecules” with a nondegenerate A state of the Cu3+ ion. It is shown that in U stripes the vibronic A→E transitions of the Cu3+ ions to the degenerate E state of the Cu2+ ion occur with the release of an energy ΔU, which for T2D≤T≤T*(p) gives rise to a two-dimensional gas of vortices and antivortices “pinned” by complexes Cu↑2+O42−(or Cu↓2+O42−). When the temperature is lowered, T<Tf(p), a transition of the U stripes occurs to a two-dimensional (2D) vortex-metal state with non-Fermi charge carriers (mobile 2D vortices and antivortices) which are consistent with antiferromagnetic order of the CuO2 plane. With the development of 2D superconducting fluctuations at T2D≤T≤T*(p) the pairin...

Jahn-Teller effect in quasi-two-dimensional doped cuprate antiferromagnets and underdoped high-Tc superconductors (review)

The theoretical aspects of research on the role of Jahn–Teller interactions in quasi-two-dimensional cuprate antiferromagnets and high-Tc superconductors (HTSCs) are analyzed. An analysis is made of results that permit one to establish a link between the quasi-two-dimensional character of the properties of underdoped cuprate HTSCs in the normal and superconducting states and the Jahn–Teller (JT) nature of the divalent copper ions. It is shown that the combination of these two features leads to the existence of a pseudogap state. In underdoped cuprate HTSCs with JT lattice distortions the quasi-two-dimensionality gives rise to two-dimensional local and quasilocal states of the charge carriers. This is manifested in substantial temperature dependence of the number of components of the localized and delocalized states of the charge carriers and in repeated dynamical reduction of the dimensionality of underdoped cuprate HTSCs as the temperature is lowered. Such a HTSC, with doping concentrations less than opt...

About the nature of inelastic deformation of HTSC ceramics in region temperature below Tc

Abstract For Y- and Bi-based ceramics in superconducting state the results of the experimental studying the deformation under the loading are presented. The acoustic emission signals, the peculiarities of the intrinsic friction spectrum and ones of the planar contact resistance are studied. These results confirm the full analogy of the inelastic deformation processes at T c for HTSC ceramics and for metals.

On the quantum nature of charge carriers in the pseudogap state of underdoped cuprate high-Tc superconductors

It is conjectured that in underdoped cuprate high-Tc superconductors in the pseudogap state at T⩽T*(psh) the transition of the CuO2 plane to metallic behavior with decreasing temperature is due to quantum diffusion, which was predicted for “defectons” by A. F. Andreev and I. M. Lifshitz in 1969. It is shown that the features of the pseudogap state are determined solely by the properties of the CuO2 plane and the doping concentration psh. When the temperature is decreased to T⩽TAL(psh), the joint magnetic phase diagram of states has a universal line TAL(psh)=815psh of transition of the CuO2 plane to a 2D vortex metal state with mobile charge carriers—2D vortices, which are compatible with antiferromagnetism of the CuO2 plane. A phase diagram of states, with the line TAL(psh)⩽T*(psh), is constructed for the underdoped single crystal YBaCu3Oy; it agrees with the known experimental data for other underdoped cuprate HTSCs.

Investigation of flux-trapping effect in Bi-based ceramics at 65–300 K

The influence of low magnetic fields on the temperature dependence of attenuation decrement is studied. The conservation of this effect after field switching off is discovered.

On the softening effect in superconducting state

It is shown that in superconducting state the increase of creep rate and peculiariries of attenuation decrement have dislocation origin.

A stripe mechanism of electron and hole reflection from the boundary between a normal metal and an underdoped high-Tc superconductor

The processes of reflection of electrons and holes from the boundary between a normal metal and an underdoped high-Tc superconductor (UD HTSC), which were observed by G. Deutcher and co-workers at an N/S point contact, are investigated at T<Tc. It is proposed that, in addition to the known mechanism of reflection on the superconducting potential, developed by Andreev, de Gennes, and Saint-James for a contact between a metal and a low-temperature superconductor, there is a stripe mechanism, which leads to a complete cycle with two reflections of an electron and a hole from the boundary of the normal metal. It is shown that reflections of this kind are due to copper ions in stripes of the CuO2 plane in the point contact: copper ions in the D stripes absorb and emit only electrons into the metal, while those in the U stripes absorb and emit only holes. For T<Tc at a distance from the metal-UD HTSC boundary equal to the coherence length, local fluctuations of the antiferromagnetic (AFM) ordering of the copper...

Dependence of the shear modulus of a Bi ceramic on external influences in the temperature interval 65–130 K

The dynamic shear modulus of a two-phase bismuth ceramic (Bi-2212 and Bi-2223) is measured as a function of temperature, magnetic field, and deformation. The results attest to the multistage penetration of magnetic field into the sample. It is found that the level of the shear modulus is sensitive to the magnetic and defect structure of the ceramic. The conjecture that the observed defects of the shear modulus are due to phase separation in the CuO2 planes of the 2212 phase is discussed.

Peculiarities of vortex state of Bi-2212 single crystals

A phase diagram of the vortex state of Bi-2212 single crystals in a magnetic field parallel to the ĉ-axis proposed under the assumption on the two-stage destruction of long-range vortex correlation in such crystals matches with the experimental results obtained by using different methods.Reference 1Reference 2Reference 3Reference 4Reference 5Reference 6Reference 7 The intersection of the dimensional crossover line Bcr(T) with the line corresponding to violation of long-range vortex correlations in the (ab∧)-plane makes it possible to explain the discontinuity at the critical point of the vortex lattice melting curve Tm(B). Possibilities of experimental investigation of low-temperature condensed state for B>Bcr and the temperature dependence Bcr(T) are considered.

Spontaneous vortex excitations in layered superconductors

The possibilities of using Giaver transformation method for the study of spontaneous vortices and crossover temperature are discussed.