G. Kh. Panova

The floating zone growth and superconductive properties of La1.85Sr0.15CuO4 and Nd1.85Ce0.15CuO4 single crystals

Abstract A technology was elaborated to produce large superconducting crystals by use of a non-crucible floating zone melting method with light radiation heating under oxygen atmosphere. Large superconducting single crystals La 2− x Sr x CuO 4 and Nd 2− x Ce x CuO 4 ( x = 0.15) were obtained with diameters and lengths up to 7 mm and 30 mm respectively, and masses exceeding 5 g. Temperatures of the superconducting transition for La- and Nd-based crystals were T c = 39 and 25 K respectively, and the transition width was of the order of 1 K. Superconducting properties were studied by measuring resistance, AC-magnetic susceptibility and specific heat in magnetic fieldsi0 to 8 T. In the temeprature dependence of the specific heat was we observed the jumps corresponding to the superconducting transition. The content of the superconducting phase in the samples is estimated to be about 80–90%.

Experimental study of the magnetic phase transition in the MnSi itinerant helimagnet

Magnetic susceptibility, heat capacity, thermal expansion, and resistivity of a high-quality single crystal of MnSi were carefully studied at ambient pressure. The calculated change in magnetic entropy in the temperature range 0–30 K is less than 0.1R, a low value that emphasizes the itinerant nature of magnetism in MnSi. A linear temperature term dominates the behavior of the thermal expansion coefficient in the range 30–150 K, which correlates to a large enhancement of the linear electronic term in the heat capacity. A surprising similarity between variation of the heat capacity, the thermal expansion coefficient, and the temperature derivative of resistivity through the phase transition in MnSi is observed. Specific forms of the heat capacity, thermal expansion coefficient, and temperature derivative of resistivity at the phase transition to a helical magnetic state near 29 K are interpreted as a combination of sharp first-order features and broad peaks or shallow valleys of yet unknown origin. The appearance of these broad satellites probably hints at a frustrated magnetic state in MnSi slightly above the transition temperature. Present experimental findings bring the current views on the phase diagram of MnSi into question.

Specific heat and electrical resistivity of an icosahedral-structure Zr70Pd30 alloy and of its amorphous and crystalline analogs

Binary icosahedral and crystalline phases of the Zr70Pd30 alloy were obtained in crystallization from the amorphous state during heat treatment. The specific heat and electrical resistivity of the icosahedral, amorphous, and crystalline phases were measured and compared. An increase in the electronic density of states on the Fermi surface, lattice softening, and an increase in the electron-phonon coupling constant were observed to occur with decreasing structural order. Despite the high valence electron density in the icosahedral phase, where the electronic densities of states are twice those in the crystal, the electrical resistivity of the icosahedral phase is ∼50 times as high. Superconductivity was observed for the first time in the icosahedral phase of a binary system of transition metal atoms, Zr70Pd30.

Heat capacity of La1−xSrxMnO3 single crystals in different magnetic states

The heat capacity of three single-crystal samples of La1−xSrxMnO3 (x=0, 0.2, and 0.3) is measured in the temperature range 4–400 K. It is found that the heat capacity undergoes abrupt changes due to the transitions from the antiferromagnetic phase to the paramagnetic phase (x=0) and from the ferromagnetic phase to the paramagnetic phase (x=0.2 and 0.3). The phonon contribution to the heat capacity and the Debye characteristic temperatures for the La0.7Sr0.3MnO3 sample are determined over a wide range of temperatures. The electronic density of states at the Fermi level is evaluated. It is demonstrated that an increase in the strontium concentration x brings about an increase in the electronic density of states at the Fermi level. The contributions of spin waves to the heat capacity and the entropy are estimated under the assumption that the phonon spectrum remains unchanged upon doping with Sr.

Angular dependence of the specific heat of La1.85Sr0.15CuO4 in superconducting mixed state

Abstract The specific heat of single crystals La 1.85 Sr 0.15 CuO 4 has been studied as a function of the relative orientation of the crystal axes and a magnetic field rotating in the Cu–O plane and normal to this plane. Measurements were carried out in the temperature range 2–50 K in magnetic fields up to 8 T and with four directions of the magnetic fields: in the a – b plane (along the [100] and [010] directions) and at angles of 45° and 90° with respect to the a – b plane (along [001] and [103] directions). For all orientations of the magnetic field the specific heat of the mixed state at low temperatures is a nonlinear function of the magnetic field. The dependence of the specific heat on the magnetic field H shows the feature predicted for d -wave pairing: H 1/2 T term. A fourfold symmetry characteristic of the electronic density of states in the crystalline a – b plane and a twofold symmetry in a – c plane was resolved in the magnetic field. The results show unambiguously that the in-plane and out-of-plane electronic density of states in the magnetic field is highly anisotropic and has a minimum when the field is along the a -axis and a maximum when the field makes an angle of 45° with the a and c axes. Using these results, we present an angular mapping of the electronic density of states and the upper critical field H c2 ( T ) estimated from the heat capacity measurements. These results are consistent with d x 2 − y 2 -symmetry of the bulk order parameter.

Atomic dynamics of tin nanoparticles embedded into porous glass

The method of resonant nuclear inelastic absorption of synchrotron radiation has been used to study the phonon spectrum for tin nanoparticles (with a natural isotope mixture) embedded into a porous glassy (silica) matrix with an average pore diameter of 7 nm in comparison to the analogous spectrum of bulk tin enriched with 119Sn isotope. Differences between the spectra have been observed, which are related to both the dimensional effects and specific structural features of the porous glass-tin nanocomposite. Peculiarities in the dynamics of tin atoms embedded into nanopores of glass are interpreted in terms of a qualitative model of the nanocomposite structure.

Surface and volume superconductivity of Pb embedded in nanopores

The heat capacity of lead embedded in glass nanopores (7 nm in diameter) and bulk lead was studied in the temperature range 2–40 K without a magnetic field and in magnetic fields of 1–8 T. The properties of lead nanoparticles and bulk lead were compared. The results obtained allowed us to separate the surface superconductivity from the volume superconductivity. The temperature dependence of the heat capacity of lead nanoparticles was shown to exhibit two superconducting transitions above and below the transition temperature for bulk lead (Tc = 7.2 K), which are associated with the surface and volume superconductivity. The upper critical fields Hc3 for the surface superconductivity and Hc2 for the volume superconductivity were determined. It turned out that these fields for Pb nanoparticles are two orders of magnitude higher than those for bulk lead. The “superconductor-normal metal” phase diagrams were constructed for lead nanoparticles. The study established an increase in the density of low-frequency excitations in Pb nanocrystals as compared to bulk Pb and a difference in the electronic heat capacity of Pb nanoparticles as compared to bulk Pb.

Thermodynamic and kinetic properties of an icosahedral quasicrystalline phase in the Al-Pd-Tc system

The properties of a quasicrystalline phase in the Al-Pd-Tc system are studied for the first time. X-ray investigations demonstrate that the quasicrystalline phase in the Al70Pd21Tc9 alloy has a face-centered icosahedral quasi-lattice with parameter a=6.514 Å. Annealing experiments have revealed that this icosahedral phase is thermodynamically stable. The heat capacity of an Al70Pd21Tc9 sample is measured in the temperature range 3–30 K. The electrical resistivity and magnetic susceptibility are determined in the temperature range 2–300 K. The electrical resistivity is found to be high (600 µΩ cm at room temperature), which is typical of quasicrystals. The temperature coefficient of electrical resistivity is small and positive at temperatures above 50 K and negative at temperatures below 50 K. The magnetic susceptibility has a weakly paramagnetic character. The coefficient of linear contribution to heat capacity (γ=0.24 mJ/(g-atom K2)) and the Debye characteristic temperature (Θ=410 K) are determined. The origin of the specific features in the vibrational spectrum of the quasicrystals is discussed.

Low-temperature vibrational properties of tin nanoparticles in porous glass

The heat capacity has been studied in the temperature range 2.2–40 K and in magnetic fields up to 2 T in tin, which has been embedded in nanometer-size pores in glass having diameter ∼7 nm, in bulk tin and in glass with empty pores. Comparison of the properties of tin nanoparticles and bulk tin has been performed. An increase in the coefficient of electronic heat capacity has been found in nanostructured tin as compared with the bulk tin, and also a considerable deviation of the low-temperature lattice heat capacity from the Debye law in the temperature region T > 3 K has been found. The fact that the density of thermal vibrations in nanocrystalline tin for low energies is higher than in bulk tin has been established using low-temperature heat capacity data.

Resistance and magnetic susceptibility of superconducting lead embedded in nanopores of glass

This paper reports on a study of the resistance and differential magnetic susceptibility χac of lead embedded in nanosized glass pores with a diameter of ∼7 mm, which was performed at temperatures of 6–300 K and magnetic fields of up to 6 T. The field dependence of the resistance R(H) and the temperature dependences of the real, χ″(T), and imaginary, χ″(T), parts of magnetic susceptibility reveal indications of superconducting phase transitions associated with the volume and surface superconductivity of Pb nanopar ticles. The measurements of the field dependence of resistance have been used to set up the Hc-Tc phase diagram and to carry out a comparison with the study of the heat capacity performed on the same samples.