Experimental parameters:

• Temperature range 2-300 K
• Magnetic field induction – up to 14 T
• Electrical current – up to 250 A
• Measurements of temperature dependences of  electrical resistance for the determination of transition temperature T_c
• Measurements of  current dependences of electrical resistance at T=78 К for a determination of j_c
• R(B) at T=78K for a determination of Вс
• Construction of the triple diagram of critical parameters T_c, j_c and B_c
• Measurements of thermophysical properties of HTSC (heat conduction λ, heat capacity c_p)
• Measurements of thermoelectric power in the temperature range 4-300 K
• Measurements of the Hall effect in magnetic fields up to 14 T

With an increase of irradiation dose up to 5,6х10¹⁶ cm^-2 temperature of superconducting transition (T_c) increases on ~ 10%, after which it decreases down to its initial values. The increase of T_c is concerned with a change of defect structure of crystal lattice and it evidences a non-optimal structure of the material. In the ideal case which should take place for single crystals, the irradiation should lead to the depresses HTSC critical parameters.

Temperature dependences of resistivity Y_0,8Sm_0,2Ba₂Cu₃O₇ ceramics at different irradiation doses ⁶⁰Co: Ф=0; 5,6х10¹⁶; 10,9х10¹⁶ cm^-2. On the insets are: upper part – dose dependence of a temperature of the start of superconducting transition; lower part – that of the end of superconducting transition.

In the range of temperatures before the superconducting transition, the phonon contribution to the heat conduction dominates, and the electron contribution has a very low contribution on the heat conduction, which disappears after the material transition into the superconducting state. The anomaly of phonon contribution, which displays a maximum before the superconducting transition, is explained by a suppression of electron-phonon scattering at a formation  of the Bose-condensate.

Temperature dependence (Т, К) of the  heat conduction λ (W/m.K) of the Y-Sm-Ba-Cu-O system. Marked is its phonon component (curve 1) and electron component  (curve 2).

Three peaks of heat capacity are observed on the sample after synthesis (curve 1), one of which (low-temperature one) agrees with the superconducting transition temperature of 98К. After 3- and 5-fold thermocycling in the range 4-400 К (curves 2 and 3) peaks lower and then they disappear. Their disappearance is explained by the presence of unstable superconducting states.
This example shows the informativity of the heat capacity as a parameter being sensitive to phase states of the material.

Temperature dependence (Т,К) of heat capacity с_р (W/kg.K) of the Y-Sm-Ba-Cu-O.

We have carried out a number of investigations on the Tl-Ba-Ca-Cu-O system, which has several high-temperature phases, one of which (Тс=125 К) is the largest of all known HTSCs.
Thermopower is the most sensitive parameter to the transformation of electronic spectrum among all the kinetic parameters. In this way, the sign and behavior of the thermopower before the superconducting transition has made it possible to determine the hole type of conductance and dominating role of the phonon drag in the charge transfer processes for thallium superconductors.

Temperature dependences of resistivity of 2212 and 2223 samples of the Tl-Ba-Ca-Cu-O system.

Temperature dependences of thermopower of 2212 and 2223 samples of the Tl-Ba-Ca-Cu-O system.

The difference of signs of thermopower and Hall effect is connected with a smaller mobility of holes being compared with that of electrons, though their concentration is smaller. The relationship of concentrations of carriers of both signs remains constant up to the superconducting transition temperature.

Temperature dependence of the Hall coefficient and the Hall concentration of carriers per one Cu atom.

Provided here examples of investigations of HTSC materials show that every measurement method has its own specific characteristics and gives information which could not be obtained by means of other methods. Every one of them complements each other and at complex use they make it possible to obtain complete information on electronic and superconducting properties of HTSC materials.

Investigations of single crystals requires implementation of additional conditions:

• crystal dimensions  are not less than 1х1х6 mm³
• preliminary crystallographic orientation relative to the long axis of the sample
• supporting of a possibility of the sample rotation in magnetic field
• a selection of the method of electrode application, providing their ohmic contact and mechanical strength at the thermocycling.

Shanghai Jiao Tong University in the name of Prof. Xin Yao has provided two HTSC single-crystal samples of  YBa₂Cu₃O₆ system, in frames of implementation of the joint research project “Correlation of crystallization mechanisms of REBCO with their electrical-physical characteristics” which was accepted according to the decision No.9c-14 by the Intergovernmental Belarusian-Chinese commission in the field of science and technologies.
The samples were non-oriented HTSC single crystals with approximate dimensions of 1,5х2х3 mm³. Geometry and dimensions of the samples did not give a possibility to measure their electrical-physical properties. It was only possible to investigate temperature dependences of magnetic moment and magnetic susceptibility in the area of superconducting transition temperatures and in magnetic fields with different orientation relative to crystallographic axes of the samples.

Following investigations of the HTSC single crystals were carried out by researchers of the Cryogenic Research Division:

• X-ray diffraction orientation of the samples with a goal of determination of the main crystallographic axes;
• Measurements of temperature dependences of magnetic moment at some values of magnetic field induction;
• Magnetic field vector was oriented perpendicular and parallel to the crystallographic axis  с in the area of superconducting transition temperature.

Temperature dependence of magnetic moment of YBa₂Cu₃O₆ №1 and №2 with magnetic field induction vector B oriented parallel to the crystallographic axis с.

Averaged characteristic of superconducting transition temperature for the magnetic field induction vector B oriented parallel to the crystallographic axis с.

Temperature dependence of magnetic moment of YBa₂Cu₃O₆ сrystals №1 and №2 with magnetic field induction vector B oriented perpendicular to the crystallographic axis с.

Averaged characteristic of superconducting transition temperature for the magnetic field induction vector B oriented perpendicular to the crystallographic axis с.

The following tendency takes place for two orientations of magnetic field induction vector relative to axis с of the HTSC single crystals:

• With magnetic field increase the temperature of beginning of superconductive transition (T_{c beg}) rises, which is obvious, as magnetic field influence blocks the start of formation of superconductive electronic states. Correspondingly the temperature of end of superconductive transition (T_{c end}) decreases which leads to the growth of superconductive transition width with the magnetic field increase.
• From the point of view of the influence of crystallographic orientation relative to the magnetic field induction vector it follows for the B perpendicular c case a temperature of the complete superconducting transition is considerably larger than that for the B||c case transition width ΔT_c is smaller by a factor of 2.5 - 3. This evidences a dominating role of Cu-O bonds in the ab plane in the superconducting process.