V. M. Dmitriev

Fluctuation conductivity and pseudogap in YBCO high-temperature superconductors (Review)

A considerable part of the theoretical and experimental works reflecting the current status of research on high-temperature superconductivity and the unusual phenomenon of a pseudogap in high-temperature superconductors (HTSCs) is reviewed. The concept of local pairs in systems with low and intermediate charge-carrier density, which can include HTSCs, is examined. The experimental part is primarily based on the study of excess and fluctuation conductivity (FC) in YBa2Cu3O7−y (TBCO) and Y1−xPrxBa2Cu3O7−y (YPrBCO) thin epitaxial films. A new approach to analyzing FC and the pseudogap in such high-temperature systems is proposed and checked experimentally. The approach is based on the idea that excess conductivity σ′(T) forms in HTSCs at temperatures substantially above critical temperature Tc as a result of the formation of pair states in the form of noninteracting strongly bound bosons, demonstrating with decreasing temperature a transition from a regime with localized pairs conforming to the Bose–Einstein...

Resistive studies of the pseudogap in YBCO films with consideration of the transition from BCS to Bose–Einstein condensation

A new approach is proposed for analysis of the excess conductivity σ′(T) arising below a characteristic temperature T*⪢Tc in YBa2Cu3O7−y (YBCO) films with different oxygen concentrations. The approach is based on the idea that σ′(T) is formed as a result of the formation at T⩽T* of local pairs (tightly bound bosons) obeying Bose–Einstein statistics in the temperature interval Tm<T<T*. At temperatures Tcmf<T<Tm the pairs obey BCS theory (Tcmf is the critical temperature separating the phase transition region from the region of critical fluctuations). Thus in Y123 systems a transition from Bose–Einstein condensation to condensation of the BCS type occurs with decreasing temperature. An equation in which the dynamics of formation of the tightly bound bosons is taken into account is proposed which gives a good description of the temperature dependence σ′(T) and in which the parameter Δ*, identified with the pseudogap in high-Tc superconductors, is contained in explicit form. The temperature dependence Δ*(T) i...

Critical currents, phase slip centers, and phase slip lines in superconducting films in the absence of external magnetic field

The temperature dependence of the current–voltage characteristics of high-quality thin films of tin from 7 to 50 μm thick is investigated in the absence of external magnetic field. For the first time on the same samples phase slip centers are observed near Tc, where the films are narrow channels, and phase slip lines are observed at lower temperatures, where the films become wide. The critical current exhibits temperature crossover, which affects its absolute value, but in a certain temperature interval a temperature dependence of the form (1−T/Tc)3/2 is maintained. When the temperature is decreased further, the critical current at which vortices due to the self-field of the current can enter the sample depends linearly on temperature and corresponds to the Aslamazov–Lempitsky theory. The temperature at the start of crossover with decreasing temperature coincides with the temperature at which the film width is equal to four times the penetration depth of a weak magnetic field perpendicular to the film pla...

Fluctuation conductivity and pseudogap in Y1−xPrxBa2Cu3O7−y films

The fluctuation conductivity (FC) and pseudogap (PG) in Y1−xPrxBa2Cu3O7−y with x≈0.1 are investigated. It is shown that the temperature dependence of the FC is the same as in YBa2Cu3O7−y containing defects. At the same time, the temperature dependence of the PG differs from the analogous dependence obtained for YBa2Cu3O7−y films. An increase of the coherence length and decrease of the characteristic temperature T* in Y1−xPrxBa2Cu3O7−y is observed with increasing Pr concentration.

Lattice parameters, magnetic susceptibility and thermal conductivity of ScFe4Al8 and YFe4Al8

Abstract ScFe 4 Al 8 and YFe 4 Al 8 crystallise in a tetragonal structure of the ThMn 12 -type. Although antiferromagnetic ordering in the Fe sublattice exists below about 100 K in these compounds, they can be considered as the reference materials for the RFe 4 Al 8 ternaries where R is a magnetic lanthanide, U or Np. The present magnetic measurements in a low magnetic field of 50 Oe have revealed another pronounced anomaly in the temperature dependence of magnetic susceptibility at temperatures lower than the Neel point. Low temperature (LT) X-ray diffraction (XRD) examination has confirmed the stability of the crystal structure in the whole investigated temperature range. Therefore the lattice distortion as a reason for the LT magnetic anomalies has been excluded. Also heat conductivity measurements have not given any explanation for the LT anomalies. Based on a thermal conductivity investigation it was possible to separate the electron and phonon components from the total thermal conductivity. Thus a deviation from stoichiometry, spin-reorientation transition (SRT) and spin-glass (SG) state are discussed as reasons for the LT anomalies.

Observation of enhancement of superconductivity by an external electromagnetic field and of rf phase-slip lines in wide tin films

The enhancement of superconductivity by an external electromagnetic field and rf phase-slip lines are observed in wide tin films. The temperature, frequency, and power dependences of the critical current are investigated and demonstrate the phenomena observed. It is shown that this class of nonequilibrium effects is common to both narrow and wide films.

Energy gap and upper critical field of the new magnetic superconductor Mo3Sb7 found by the Andreev reflection method

We present the data of the point contact (PC) Andreev-reflection measurements on the new paramagnetic superconductor Mo3Sb7, which were used for finding the energy gap Δ and upper critical field Hc 2 for this compound. The maximum gap value, reduced to the zero temperature via the Bardeen–Cooper–Schrieffer (BCS) theory, turned out to be meV, which is slightly smaller than that expected from the BCS theory, meV. The temperature dependence of the gap obeys the BCS theory approximately. The Hc 2(0) value of about 16.5 kOe was obtained from fitting the experimental data to the conventional H(T) dependence, which is quadratic in temperature. This value is in close agreement with the result from magnetization measurements of 17.2 kOe.

The Volleben effect in magnetic superconductors Dy1–xYxRh4B4 (x = 0.2, 0.3, 0.4, and 0.6)

The Volleben effect (“paramagnetic” Meissner effect) was revealed for the first time while measuring the temperature dependence of magnetic moments of Dy1–xYxRh4B4 (x = 0.2, 0.3, 0.4, 0.6) in magnetic fields 1–20 Oe. The effect decreases with increasing magnetic field strength, and at fields below 1 Oe a diamagnetic signal appears. The ferromagnetism of Dy atoms supposedly plays an essential role in the appearance of paramagnetic signal at temperatures below the superconducting transition temperature.

Advances in the criteria for dividing thin superconducting films into narrow and wide films

The results of experimental investigations of the critical currents and certain nonequilibrium phenomena in thin tin films of different width w are analyzed. Ordinarily, thin superconducting films are divided into two groups: narrow channels w λ⊥. A wide transitional region where the condition w>λ⊥ holds with a large margin and at the same time cannot be explained from the standpoint of the theory of the appearance of a vortex state has been found. This shows that the generally accepted criterion w∼λ⊥ for dividing films into wide and narrow does not work. The transition into a wide-film regime, described by the existing theory of the vortex state, is fully completed only for w∕λ⊥(T)>10–20.

AC Josephson properties of phase slip lines in wide tin films

Current steps in the current–voltage characteristics of wide superconducting Sn films exposed to microwave irradiation were observed in the resistive state with phase slip lines. The behavior of the magnitude of the steps with the applied irradiation power was found to be similar to that for the current steps in narrow superconducting channels with phase slip centers and, to some extent, for the Shapiro steps in Josephson junctions. This provides evidence for the Josephson properties of the phase slip lines in wide superconducting films and supports the assumption about the similarity between the processes of phase slip in wide and narrow films.