A. L. Solovjov

Fluctuation conductivity in YBa2Cu3O7−y films with different oxygen content. II. YBCO films with Tc≈80 K

The fluctuation conductivity in YBa2Cu3O7−y (YBCO) films with T≈80 K is investigated for the first time. Unlike the optimally doped samples, these films exhibit a transition from a Maki–Thompson (MT) mechanism for the scattering of fluctuational pairs to a Lawrence–Doniach (LD) mechanism and, as the temperature approaches Tc, to an Aslamazov–Larkin (AL) mechanism. It is shown that the coherence length ξc(0) along the c axis and the phase relaxation time τφ(100 K) of the fluctuational pairs are determined by the temperature of this second transition. The features observed on both the fluctuation conductivity and resistive behavior are sharply enhanced as Tc approaches 80 K, probably because of a significant increase in the intensity of the magnetic interaction in high-Tc superconductors at these temperatures. In spite of this, for two samples a value τφ(100 K)=(3.35±0.01)×10−13 s is measured, i.e., the same value as for optimally doped YBCO films. It is shown that the dependence of ξc(0) on Tc obeys the st...

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...

Fluctuation conductivity in YBa2Cu3O7−y films with different oxygen content. I. Optimally and lightly doped YBCO films

Fluctuation conductivity in YBa2Cu3O7−y (YBCO) films with different oxygen concentrations is investigated. All of the samples exhibit a clear transition from the Maki–Thompson mechanism of scattering of fluctuational pairs to the Aslamazov–Larkin mechanism as the temperature approaches Tc. The values of the transition temperature are used to determine the coherence length ξc(0) along the c axis and the phase relaxation time τφ(100K) of the fluctuational pairs. Despite the decrease of Tc from 87.4 to 54.2 K with decreasing oxygen content in the samples studied, a value τφ(100K)=(3.35±0.01)×10−13 s is obtained for all the samples. It is shown that the dependence of ξc(0) on Tc conforms to the standard theory of superconductivity. The mechanisms for the scattering of charge carriers and for superconducting pairing in high-Tc superconductors are analyzed.

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.

Fluctuation conductivity in Y–Ba–Cu–O films with artificially produced defects

The fluctuation-induced conductivity (paraconductivity) measured in YBa2Cu3O7−δ (YBCO) films grown on 10° miscut SrTiO3 (001) substrates is analyzed using various theoretical models describing weak fluctuations in high-Tc superconductors and considering both Aslamazov-Larkin and Maki-Thompson fluctuation contributions in the clean limit approach. The analysis reveals a highly anisotropic pair-breaking caused by structural defects produced. This result is in favor of an idea that pseudogap in high-Tc oxydes is mainly governed by the fluctuating pairing.

Fluctuation conductivity and critical currents in YBCO films

A comparative analysis of the results obtained in measurements of the fluctuation conductivity and of the critical current density jc(T) in YBa2Cu3O7−y films containing various numbers of defects is carried out for the first time. It is found that the value and temperature dependence of the fluctuation conductivity are interrelated with the values and temperature dependences of the critical current and resistivity of the samples. It follows from measurements of the fluctuation conductivity that the variation of these temperature dependences is directly related to the variation of the number of defects and, hence, the number of pinning centers in the films studied. It is shown that in films containing practically no defects the jc(T) curve completely follows a model giving jc(T) in high-Tc superconductors with low-angle grain boundaries between crystallization blocks. On the other hand, if the sample has a specific defect structure that is formed under certain conditions in c-oriented epitaxial high-Tc sup...

Hall effect studies in YBCO films

The longitudinal, ρxx(T), transverse, ρxy(T), and Hall, ρH(T) resistivities have been measured for YBa2Cu3Ox (YBCO) films, showing positive resistivity buckling and those with usual linear ρxx(T) dependence. In the former case unexpected peak on ρxy(T) and unusual ρH(T) dependence with double sign change just above transition temperature Tc have been revealed. The data are analyzed using recent theory for the sign of the Hall conductivity in strongly correlated systems.

Possibility of local pair existence in optimally doped SmFeAsO1−x in pseudogap regime

We report the analysis of pseudogap Δ* derived from resistivity experiments in FeAs-based superconductor SmFeAsO0.85, having a critical temperature Tc = 55 K. Rather specific dependence Δ*(T) with two representative temperatures followed by a minimum at about 120 K was observed. Below Ts ≈ 147 K, corresponding to the structural transition in SmFeAsO, Δ*(T) decreases linearly down to the temperature TAFM ≈ 133 K. This last peculiarity can likely be attributed to the antiferromagnetic (AFM) ordering of Fe spins. It is believed that the found behavior can be explained in terms of Machida, Nokura, and Matsubara theory developed for the AFM superconductors.

Electrotransport and Hall effect in Y1−xPrxBa2Cu3O7−y films

The longitudinal and transverse resistivities ρxx(T) and ρxy(T), respectively, and the Hall effect are investigated in Y1−xPrxCu3O7−y films with x≈0.1. Despite the low value of Tc≈78K, the temperature dependence ρxx(T) is linear over a wide temperature range. At the same time, ρxy(T) has a pronounced semiconductor character. The measured Hall coefficient has a value RH≈1.3, which is 3 times less than in a YBa2Cu3O7−y film with a similar Tc. The interaction constant λ≈1.26, on the contrary, is ≈3.5 times greater. It is shown that these and other features revealed can be explained by charge-carrier localization effects in Y1−xPrxCu3O7−y systems.

Electrical and thermal conductivity of the Ti3AlC2 MAX phase at low temperatures

Electrical and thermal conductivities of a sample containing 96% of MAX-phase Ti3AlC2 and 4% of TiC were experimentally studied in the temperature range of 15–300 K. The maximum thermal conductivity is observed at approximately 75 K. As the temperature increases, the fraction of phonon heat transfer decreases from ∼90% at low temperatures to ∼40% at the room temperature.Electrical and thermal conductivities of a sample containing 96% of MAX-phase Ti3AlC2 and 4% of TiC were experimentally studied in the temperature range of 15–300 K. The maximum thermal conductivity is observed at approximately 75 K. As the temperature increases, the fraction of phonon heat transfer decreases from ∼90% at low temperatures to ∼40% at the room temperature.