Transport properties of systems with low charge carrier density and pseudogap state in HTSC materials

Transport properties of systems with low charge carrier density

· For the first time the temperature dependence of the energy gap Δ (T) in high-temperature superconductors YBa2Cu3O7-y [1, 2] and Bi2Sr2CaCu2Ox [3] was measured.

· It is shown that, in contrast to the classical BCS superconductivity, the ratio 2Δ (0) / kBTc ≈ 5 for YBa2Cu3O7-y [1, 2] and 2Δ (0) / kBTc ≥ 6 for Bi2Sr2CaCu2Ox [3], indicating that it is a strong-coupling superconductivity.

· An unusual temperature dependence of the dynamic resistance Rd0 (T) of the Nb- YBa2Cu3O7-y contacts at zero bias was found [2] , indicating that with decreasing temperature quasiparticles in HTSC do not "freeze".

· The effect of depairing of the microwave signal in such contacts was observed, indicating the possible existence of local pairs in high-temperature superconductors up to T * ≈ 240 K [4].

· In contacts of the Nb-Ta2Ba2CaCu2Ox with Tc ≈ 100 K, the stimulation of the excess current by the microwave radiation was observed, which reflects the complex nature of the quasiparticle spectrum and transport properties of new superconductors [5].

1. Verkin B.I., Dmitriev V.M., Zvyagin A.I., Solovjov A.L., Seminozhenko V.P., Christenko E.V., Churilov G.E. and others. Electrical and magnetic properties of superconducting compounds La (Ba, Sr) CuO and YBaCuO // FNT, 13, № 8, 253 (1987).
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4. Dmitriev V.M, Solovjov A.L., Dmitrenko A.I, Ishchenko L.A. Nonlinear properties of YBa2Cu3Ox in the field of microwave radiation in the range 4.2 - 270K // Low Temp. Phys., 15, No. 11, 1223 (1989).
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Excess (fluctuation) conductivity and the pseudogap in the cuprate and iron-based high-temperature superconductors

· A new evidence-based approach to the analysis of fluctuation conductivity (FLC) in the cuprate superconductors was proposed. Within this approach the magnitude and temperature dependence of the FLC in superlattices YBCO-PrBCO [1, 2], in films of YBa2Cu3O7-y with specially embedded defects [3], in well-structured films of YBa2Cu3O7-y with different oxygen content [4]  and in films YxPr(1-x)Ba2Cu3O7-y [5] were investigated for the first time.

· Based on the analysis of fluctuation conductivity (FLC) in various HTS’s systems a model of local pairs was developed to describe the physical nature of the pseudogap (PG) in the high-temperature superconductors [6].

· For the first time the temperature dependence of the excess current (Iexc) in Andreev contacts Ag-Bi2223 with Tc =110 K has been revealed and thoroughly analyzed at T >> Tc [7]. It was shown that the excess current is clearly observed up to Tpair = 180 K. Importantly, in the temperature interval Tc<T<Tc Tpair the temperature dependence Iexc(T) is shown to be linear. Above 180 K Iexc(T) sharply decreases and becomes zero at T= 210 K. According to the theory, the Iexc observation above Tc directly points to the existence of the local pairs with Δ ≠ 0 in high-Tc superconductors. The presence of such local pairs at T>>Tc just produces excess current due to Andreev reflection process in the superconductor

1. Volz W., Razavi F.S., Quirion G., Habermeier H.-U., Solovjov A.L. Magnetoresistance and the effect of superconducting fluctuations in YBaCuO/PrBaCuO superlattices // Phys. Rev. B, 55, № 10, 6631 (1997).
2. Solovjov A.L., Dmitriev V.M., Habermeier H.-U., Trofimov I.E. Analysis of fluctuation conductivity of YBCO-PrBCO superlattices // Phys. Rev. B, 55, № 13, 8551 (1997).
3. Solovjov A.L. Fluctuation conductivity in YBaCuO films with artificially produced defects // Low Temp. Phys., 28, № 11, 1138 (2002).
4. Solovjov A.L, Habermayer H.-U., Haage T. Fluctuation conductivity in YBaCuO films with different oxygen content. I. Optimally doped and slightly doped films // Low Temp. Phys., 28, № 1,  (2002). II. YBCO films with Tc ~ 80 K // Low Temp. Phys., 28, № 2, 99-108 (2002).
5. Solovjov A.L, Dmitriev V.M. Fluctuation conductivity and pseudogap in the films of Y1-xPrx Ba2Cu3O7-y // Low Temp. Phys., 32, № 6, 576-581 (2006).
6. Solovjov A.L, Dmitriev V.M. The study of the pseudogap in YBaCuO films by taking into account the resistive transition from the BCS to Bose-Einstein condensation // Low Temp. Phys., 32, № 2, 139 (2006).
7. Dyachenko A.I., Tarenkov V.Yu., Sidorov S.L., Varyukhin V.N., and Soloviev A.L. An excess current in Bi2223–Ag contacts above the critical temperature // Low Temp. Phys., 39, № 4, 323 (2013).