A. I. Golovashkin

Controllable generation of harmonics by means of YBa2Cu3O7 − x superconductors

The nonlinear magnetic properties of YBa2Cu3O7 − x high-temperature superconducting polycrystals are investigated. Using the results of investigations as a basis, a method for controlling the amplitudes of generated magnetization harmonics is worked out and a controllable harmonic generator is devised. The amplitudes of the harmonics are controlled by means of a permanent magnetic field or current. The use of YBa2Cu3O7 − x polycrystals makes it possible to produce a spectrum with offering a large number of higher harmonics. An YBa2Cu3O7 − x-based device has a wide dynamic range; its design is simple and reliable in a wide interval of the amplitudes of an input signal.

Simple high-temperature superconducting yttrium cuprate-based device for measuring weak magnetic fields

The earlier data for the magnetic properties of YBa2Cu3O7 − x polycrystals are used to design a highly sensitive device measuring weak magnetic fields. The sensitivity of the device is close to that of SQUIDs and much higher than the sensitivity of flux-gate meters. At the same time, the device is simpler in design and more reliable in operation than SQUIDs. Unlike SQUIDs, the device makes it possible to directly measure the magnetic field strength, has a wide measuring range, and exhibits a well-defined directional pattern.

Nonlinearity of the current-voltage characteristics for YBa2Cu3O7–x single crystals and the Berezinskii-Kosterlitz-Thouless transition

The nonlinearity of the current-voltage characteristics for YBa2Cu3O7 − x single crystals is investigated in the vicinity of the superconducting transition temperature Tc, and the mechanisms responsible for the nonlinearity of the current-voltage characteristics are considered. The current-voltage characteristics and the amplitudes of the higher voltage harmonics for the YBa2Cu3O7 − x single crystals are directly measured using modulation Fourier analysis. It is shown that the positions of the maxima in the amplitudes of the higher voltage harmonics coincide with the Berezinskii-Kosterlitz-Thouless transition temperature TBKT. This coincidence can provide a means for determining the transition temperature TBKT. A number of parameters characterizing the YBa2Cu3O7 − x single crystals in the superconducting state are estimated using the experimental results and the model of a Berezinskii-Kosterlitz-Thouless transition. The parameters thus estimated are in good agreement with the data available in the literature.

Self-consistent spatially inhomogeneous state in HTSC Ba1-x KxBiO3 single crystals

A spatially inhomogeneous state in HTSC Ba1-xKxBiO3 single crystals (Tc ≈ 30 K, x = 0.4) has been studied at T < Ť ≈ 17 K, where Ť is the temperature of separation into superconducting and dielectric phases. This separation has earlier been detected in Ba1-xKxBiO3 polycrystals. In the single crystals, the main results are obtained from hysteretic magnetization reversal curves, whereas the character of separation in the polycrystals is clearly visible from I–V curves. The results obtained support the theoretical model of HTSC separation proposed in [1].

Analysis of nonlinearity of magnetization of YBa2Cu3O7 − x by the magnetic field modulation method

Temperature dependences of the magnetization harmonics of YBa2Cu3O7 − x monodomain samples are studied experimentally at temperatures of 77–120 K. It is found that nonlinearity of magnetization of YBa2Cu3O7 − x (higher harmonics generation) is observed up to temperatures T = 103–112 K, which are much higher than the superconducting transition temperature of this compound. At the same temperatures, the temperature dependence of resistivity begins to deviate from linearity. The observed singularity of the magnetization of YBa2Cu3O7 − x is associated with the emergence of a pseudogap state in this compound.

Anomalous thermal expansion of high-temperature superconductors at low temperatures

The origins of the anomalous (negative) thermal expansion of high-temperature superconductors (HTSCs) and of the anomalously strong effect of a magnetic field on the thermal expansion coefficient at low temperatures are discussed. A physical model is proposed based on the stabilizing influence of a charge density wave (CDW) arising in the oxygen sublattice in addition to the antiferromagnetic and spin-Peierls ordering in the copper ion sublattice of an HTSC compound. The effects of temperature, magnetic field, and doping on the interaction of such a CDW with the ionic subsystem of an HTSC compound are studied. The conclusions derived from the model are in agreement with experimental data.

Critical magnetic field of the vortex-free state in NbC thin films and prospects for its observation in MgB2

The critical magnetic fields Hc‖ and Hc2 are measured for thin films of the isotropic superconductor NbC. It is revealed that the critical fields exhibit strong anisotropy due to the vortex-free state of the film in a magnetic field aligned parallel to its surface. The Hc‖/Hc2 ratio at 2 K exceeds 6 and increases with increasing temperature. The dependence Hc‖(T) agrees quantitatively with the concepts of microscopic theory on the vortex-free state of a thin film of a clean superconductor in the temperature range below Tc. As the electron mean free path decreases under irradiation of the film with a low dose of He+ ions, the critical field Hc‖ remains unchanged near Tc but increases significantly at lower temperatures. The well-known theoretical models are used to estimate the electronic parameters and thicknesses of MgB2 films for which the specific features associated with the vortex-free state of the two-gap superconductor can manifest themselves in the temperature dependence of the critical magnetic field Hc‖(T).

Observation of latent coherent effects in randomized systems

Coherent interference effects of the following three types are experimentally discovered in disordered (randomized) systems: (i) Josephson behavior of the HTSC polycrystal BaKBiO in the phase-separated state; (ii) oscillations of bismuth film resistance, which are periodic in “direct” magnetic field; and (iii) mesoscopic oscillations of the resistance in the course of film growth. In the first case, the method for detecting the “latent” nonstationary Josephson effect is substantiated by the frequency modulation method for microwave radiation, while in the other two cases, simple models are proposed to explain the nature of coherent oscillations of the resistance. The analogy between the observed oscillations and the Josephson effect in randomized systems is discussed.

Superconducting films with Tc=39 K prepared from stoichiometric MgB2 targets

Oriented films of MgB2 high-Tc superconductor are synthesized by pulsed laser sputtering of stoichiometric MgB2 targets with subsequent annealing of the amorphous Mg-B material deposited onto MgO(111) substrates. The critical temperature of the films depends on the purity of the targets sputtered. The purification of boron powder in a vacuum makes it possible to minimize the content of impurities in the targets and to prepare MgB2 films exhibiting a critical temperature above 39 K and a sharp inductive transition. A high room-temperature to residual resistivity ratio (more than 3) indicates the good quality of the films.

Anomalous low-temperature behavior of the thermal characteristics of MgB2

We have studied the behavior of the thermal expansion coefficient α(T) (in a zero magnetic field and at H≈4 T), the heat capacity C(T), and the thermal conductivity κ(T) of magnesium boride (MgB2) in the vicinity of Tc and at lower temperatures. It was established that MgB2, like oxide-based high-temperature superconductors, exhibits a negative thermal expansion coefficient at low temperatures. The anomaly of α(T) in MgB2 is significantly affected by the magnetic field. It was established that, in addition to the well-known superconducting transition at Tc≈40 K, MgB2 exhibits an anomalous behavior of both heat capacity and thermal conductivity in the region of T≈10–12 K. The anomalies of C(T) and κ(T) take place in the same temperature interval where the thermal expansion coefficient of MgB2 becomes negative. The low-temperature anomalies are related to the presence of a second group of charge carriers in MgB2 and to an increase in the density of the Bose condensate corresponding to these carriers at Tc2≈10–12 K.