A. A. Shikov

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 lead embedded into nanoporous glass

The thermal atomic vibration spectrum of lead nanostructured in porous glass with an average pore size of 7 nm and the thermal vibration spectrum of conventional bulk lead (taken for comparison) are measured using inelastic neutron scattering. The density of states in the phonon spectrum of lead nanoparticles is found to exceed the density of states in the spectrum of bulk lead at both low (E < 2.5 meV) and high (E > 9.5 meV) energies. These data are used to propose a model for the structure of a porous glass-lead nano-composite.

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.

Atomic dynamics of a d-AlNiFe decagonal quasicrystal

The atomic dynamics of an Al71.3Ni24Fe4.7 decagonal quasicrystal has been investigated using the isotopic contrast method for inelastic neutron scattering. The partial vibrational spectra of the Ni, Fe, and Al atoms and the spectrum of the thermal vibrations of the alloy have been reconstructed directly from the experimental data without any model assumptions. The cutoff energies and the positions of the main features of the spectra have been determined. It has been revealed that the average binding energy of the nickel atoms in the quasicrystal under investigation is lower than that of the iron atoms and the vibrational spectrum of the aluminum atoms is noticeably harder than the spectrum of the pure metal. The results obtained for the d-AlNiFe decagonal quasicrystal have been compared with the previously published data for an i-AlCuFe icosahedral quasicrystal.

Vibrational and electronic characteristics of Zr70Pd30, Zr80Pt20 icosahedral quasicrystals and their amorphous Counterparts

The heat capacity of Zr70Pd30 and Zr80Pt20 icosahedral quasicrystals and their amorphous counterparts is studied in the temperature range 1.5–500 K in order to establish a correlation between the short-range atomic order and the physical properties of these compounds. A comparison of the data made it possible to reveal changes in the vibrational spectra within the low-and high-energy ranges, as well as in the density of states, superconducting characteristics, electron-phonon interaction, and anharmonicity of the lattice thermal vibrations and to calculate the main average frequencies (moments) characterizing the vibrational spectra. The lower superconducting transition temperature Tc of the quasicrystals as compared to that of the amorphous counterparts can be associated with the decrease in the density of states on the Fermi surface, the hardening of the phonon spectrum, and the weakening of the electron-phonon coupling.

Dependence of the specific heat of a La1.85Sr0.15CuO4 superconducting single crystal on the magnetic field direction in the a-b plane

The low-temperature specific heat of a La1.85Sr0.15CuO4 superconducting single crystal was investigated in magnetic fields up to 8 T and with four orientations — in the a-b plane (along the (100) and (110) directions) and at angles of 45° and 90° with respect to the a-b plane (along the (103) and (001) directions). Anisotropy was observed in the field dependence of the specific heat in the a-b plane. The specific heat was found to be minimum with the field oriented in the direction of the a axis and maximum with the field oriented in a direction making an angle of 45° with the a axis. This can be explained by the anisotropy of the energy gap, whose minimum lies along the (110) direction. 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 strength.