L. A. Ponomarenko

Magnetic properties of Cu2V2O7

Abstract Magnetic measurements were performed on Cu2V2O7. Above ∼70 K paramagnetic susceptibility obeys the Curie–Weiss law. Value of effective magnetic moment μeff=1.9μB corresponds to that of Cu2+. The Weiss temperature Θ=−66 K is negative and indicates an antiferromagnetic exchange interaction between Cu2+ ions. At T c =35 K a spontaneous magnetization arises due to the transition from paramagnetic state into either ferrimagnetic or canted antiferromagnetic state.

New 80 K superconductor with structure A14Cu24O41

New 80 K superconductor with structural type A14Cu24O41 was grown and the physical properties of single crystals grown were studied.

Quasi-one-dimensional antiferromagnetic spinel compound LiCuVO4

Abstract An antiferromagnetic transition in the quasi-one-dimensional metaloxide compound LiCuVO4 was found. A broad maximum of the magnetic susceptibility was observed at T M =28 K , while at T N ′=4 K the magnetic susceptibility exhibits a sharp decrease. The Neel temperature T N =3.3 K increases initially with magnetic field, and then decreases. The exchange interaction integral in the chains of edge-sharing CuO6 octahedra and the interchain interaction integral are estimated to be J 1 =22.5 K and J 2 =1.3 K , respectively.

Shielding currents and current-voltage characteristics of Bi2Sr2Ca1Cu2O8+δ single crystal

Abstract The dependence of the irreversible magnetization on the external magnetic field sweep rate, d H e /d t , and its time relaxation are studied in a Bi 2+ x Sr 2+ y Ca 1+ z Cu 2 O 8+δ ( x =0.11, y =0.12, z =-0.25) single crystal. Two characteristic regions are found with a transition temperature T ∗ between 15 and 20 K depending on d H e /d t and relaxation time. Below T ∗ the magnetic field dependence of the shielding currents follows a scaling law J s ( H e , T )/ J s (0, T )= f [ H e / J s (0,T)] with a universal function f independent of temperature and sweep rate of magnetic field. Voltage-current characteristics are described quite well by a power-like law E / E c =( J s / J c ) n with n ∼1/ T . Above T ∗ a deviation from the J s ( H e ) scaling behavior and a transition to much steeper voltage-current characteristics are observed. Possible reasons of these different shielding current behaviours are discussed.

Short-range and long-range magnetic ordering in α-CuV2O6

Abstract Magnetic properties of α-CuV2O6 with linear chains of edge-sharing CuO6 octahedra were studied by magnetic susceptibility and electron spin resonance measurements. The broad maximum of magnetic susceptibility at T M =44 K , corresponding to the intrachain exchange interaction J 1 =34 K , indicates one-dimensional short-range magnetic ordering within the chains. Three-dimensional long-range antiferromagnetic ordering takes place at T N =24 K suggesting a rather big value for the interchain exchange interaction J 2 ∼13 K . The 76 GHz energy gap, found in the antiferromagnetic resonance spectrum, agrees with the spin-flop magnetic field value of about 2.7 T.

Influence of thermal and structural fluctuations on anisotropic resistivity and nonlinear transport properties of ReBa2Cu3O7−δ (Re=Y,Tm) single crystals

Abstract The ϱab and ϱc resistivity components of ReBa2Cu3O7−δ (Re=Y,Tm) single crystals have been studied both above and below Tc. All samples have close ϱab(T) values in normal state and differ mainly by the transition width. We have found that the superconducting transition region isnt described by the thermal fluctuations only and is influenced by structural fluctuations too. For the best sample the anisotropy of the resistivity is ∼40 and shows weak temperature dependence. This behavior is shown to be “intrinsic” property of ReBa2Cu3O7−δ system. For the less perfect samples the resistivity anisotropy is found to be highly temperature dependent which is connected with the layered structural fluctuations. Below Tc the I–V curves show powerlike E∼jn behaviour with sharp n increase between 1 and 3.

Magnetostriction of the spin-Peierls compound CuGeO3

The longitudinal and transverse magnetostriction λ of the spin-Peierls compound CuGeO3 in the b-c plane is measured in magnetic fields up to 20 T both above and below the transition temperature Tsp=14.3K. It is found that for a given crystallographic direction the value of magnetostriction is weakly dependent on the magnetic field direction. In the uniform U phase at T⩾Tsp, λ is negative and approximately equal in the b and c directions, while in the dimerized D phase at Tλc. At low temperatures, λ increases sharply at the magnetic-field-induced transition from the dimerized to the magnetic M phase. The experimental data allow estimation of the stress derivatives of the antiferromagnetic intrachain exchange interaction parameter and of the stress dependence of the critical field of the D-Mphase transition.

Anisotropic conductivity of Bi2Sr2CaCu2O8+δ single crystals near Tc

Abstract The conductivity and the current-voltage characteristics of the Bi2Sr2CaCu2O8+δ single crystals above and below Tc have been studied. The modified Montgomery method is developed for the anisotropic conductivity measurements. In the case of very high anisotropy μ= σ ab / σ c the analytic expression for the conductivity is obtained. Below Tc, the anomalously large increase of anisotropy by several orders of magnitude is observed. This behaviour is connected with transition to the TAFF regime.

Current voltage characteristics of Bi2Sr2Ca1Cu2O8 single crystals - possible plastic deformations of the vortex lattice

Abstract The J-E curves (JEC) for Bi2+xSr2+yCa1+zCu2O8+t single crystals have been extracted from the results of VSM magnetic measurements. The obtained data show the existence of two temperature regions with a transition temperature T∗ between 15 and 20 K being dependent on the voltage criteria. Below T∗, JECs are described quite well by a power-like law E/E c = ( J/ J c ) n with n∝1/T. For high magnetic fields it is found that 1/n ∝B. A model considering the “pancake” vortex-antivortex pair excitation provides good consistency with the experimental data.