Temperature dependence of the specific heat of anisaldazine close to phase transitions

The experimental data for the specific heat C_p is analyzed at various temperatures close to the nematic–isotropic liquid (T_NI = 180.5 °C) and the solid–nematic (T_SN = 168.9 °C) transitions in anisaldazine.

Values of the critical exponent for the specific heat, which describe a λ-transition between nematic and the isotropic liquid, and a jump discontinuity between solid and nematic phases, are deduced for anisaldazine. They are compared with the model predictions.     

Effect of Cu doping in MgB2 superconductor at various processing temperatures

The effects of Cu doping in MgB₂ superconductor has been studied at different processing temperatures. The polycrystalline samples of Mg_1−xCu_xB₂ with x = 0.05 were synthesized through the in-situ solid sate reaction method in argon atmosphere at different temperature range between 800–900 °C. The samples were characterized through X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and low temperature R–T measurement techniques for the phase verification, microstructure and superconducting transition temperature, respectively. The XRD patterns of Mg_1−xCu_xB₂ (x = 0.05) do not exhibit any impurity traces of MgB₄ or MgB₆ and they show the sharp transition in the samples prepared at 850 °C. The onset transition temperature of the prepared samples is around 39 K, which is almost the same as that for the pure MgB₂. This indicates that Cu doping in MgB₂ does not affect the transition temperature. The SEM micrograph of Mg_0.95Cu_0.05B₂ has shown that the sample is dense with grain size smaller than 1 μm.     

27Al NMR study in ZrNiAl

We have studied the microscopic properties of the hexagonal ZrNiAl, a model compound for a wide family of intermetallic compounds crystallizing in this type of structure, by using ²⁷Al NMR spectroscopy. We have investigated the lineshape of static and MAS NMR spectra as a function of magnetic field strength (4.7–9.4 T) and temperature (5–300 K). Our data indicate that the ²⁷Al NMR spectra result from a combined effect of quadrupole and anisotropic shift interactions. The ²⁷Al nuclei are in an environment characterized by the quadrupole coupling constant e²qQ/h of 3.3 MHz, asymmetry parameter η_Q of 0.42, isotropic shift δ_iso of 393 ppm, shift anisotropy δ_anis = δ_zz − (δ_xx + δ_yy)/2 of 150 ppm, and asymmetry factor η_S of 0.5. They are found to be temperature independent. The spin–lattice relaxation rate measured at 7.05 T is proportional to the temperature with T₁T = 135 s K. The mechanisms responsible for observed values of δ_iso, δ_anis, T₁T, and the enhanced Korringa constant are discussed.     

Effect of Yb addition on the superconducting properties of (Bi,Pb)-2212 superconductor

The effects of Yb addition on the phase evolution and superconducting properties of (Bi,Pb)-2212 superconductor prepared by solid state synthesis in the polycrystalline form were studied. Yb content of the samples was varied (x = 0–0.5) on a general stoichiometry of Bi_1.7Pb_0.4Sr_2.0Ca_1.1Cu_2.1Yb_xO_y. Phase analysis by XRD, microstructural examination by SEM, measurements of density and superconducting properties were done to evaluate the relative performance of the samples. A Yb containing secondary phase could be distinguished from XRD analysis from Yb > 0.3 in the stoichiometric level. Microstructural examination showed clear and distinct morphological variation with Yb stoichiometry and a secondary phase with round edged grains was observed in the microstructure of Yb added samples. The critical current density (J_C) and superconducting transition temperature (T_C) of all the Yb added samples were found to be higher than that of the pure sample. A maximum T_C-onset of 94.5 K and a maximum J_C 688 A/cm² has been observed for the sample with Yb = 0.2 in the stoichiometric level. Above this level T_C and J_C began to reduce, may be due to secondary phase formation.     

Study of superconducting state parameters of alkali–alkali binary alloys by a pseudopotential

A detailed study of the superconducting state parameters (SSP) viz. electron–phonon coupling strength λ, Coulomb pseudopotential μ^*, transition temperature T_C, isotope effect exponent and effective interaction strength N_OV of ten alkali–alkali binary alloys i.e. Li_1−xNa_x, Li_1−xK_x, Li_1−xRb_x, Li_1−xCs_x, Na_1−xK_x, Na_1−xRb_x, Na_1−xCs_x, K_1−xRb_x, K_1−xCs_x and Rb_1−xCs_x are made within the framework of the model potential formalism and employing the pseudo-alloy-atom (PAA) model for the first time. We use the Ashcroft’s empty core (EMC) model potential for evaluating the superconducting properties of alkali alloys. Five different forms of local field correction functions viz. Hartree (H), Taylor (T), Ichimaru–Utsumi (IU), Farid et al. (F) and Sarkar et al. (S) are used to incorporate the exchange and correlation effects. A considerable influence of various exchange and correlation functions on λ and μ^* is found from the present study. Reasonable agreement with the theoretical values of the SSP of pure components is found (corresponding to the concentration x = 0 or 1). It is also concluded that nature of the SSP strongly depends on the value of the atomic volume Ω₀ of alkali–alkali binary alloys.     

Nanoparticle in a quantum crystal with a narrow vacancy band

Vacancy-assisted diffusion of a neutral probe nanoparticle with a radius R_p of a few lattice constants in a quantum crystal with a narrow vacancy band is considered. The diffusion coefficient of the probe D_p(T) in such a crystal should fall exponentially near T_melt, and it can go through a maximum at temperatures T_tr, where the transition from thermally activated hopping of localized vacancies to a proper band motion of delocalized vacancions takes place, under the condition that the mean free path of the vacancions l_v(T) at T_tr is less than R_p and increases with lowering the temperature quicker than the inverse value of the relative concentration of vacancies X_v(T). Below T_tr, where l_v is much longer than the probe diameter, the value of D_p should fall proportionally to X_v(T).     

Direct measurement of the magnetocaloric effect in La0.67Ca0.33MnO3

Polycrystalline perovskite La_0.67Ca_0.33MnO₃ was synthesized by a sol–gel method. Its adiabatic temperature change ΔT_ad induced by a magnetic field change was measured directly. At 268 K, near its Curie temperature T_C, ΔT_ad of La_0.67Ca_0.33MnO₃ induced by a magnetic field change of 2.02 T reaches 2.4 K. The latent heat Q and magnetic entropy change −ΔS_M induced by a magnetic field change were calculated from the temperature dependence of ΔT_ad and zero-field heat capacity C_p. The maximum values of Q and −ΔS_M in La_0.67Ca_0.33MnO₃ induced by a magnetic field change of 2.02 T are 1.85 J g⁻¹ and 6.9 J kg⁻¹ K⁻¹, respectively. The former is larger than the phase transition latent heat of heating or cooling, which is about 1.70 J g⁻¹.     

Non-Fermi liquid regimes in U1−xMxPd2Al3(M=Y,Th)

The temperature-composition (T–x) phase diagram and NFL characteristics in the electrical resistivity ρ(T), specific heat C(T), and magnetic susceptibility χ(T) at low temperatures for the systems U_1−xM_xPd₂Al₃ (M=Y,Th) are described. The T–x phase diagram, the NFL characteristics, and the underlying mechanism for the NFL behavior are distinctly different for M=Y³⁺ and Th⁴⁺, apparently reflecting the difference in valence of the M atom substituents, and suggesting that U is tetravalent in these two systems.     

Phase transitions in the superionic protonic conductor CsHSeO4 investigated by Brillouin spectroscopy

A Brillouin investigation in CsHSeO₄ has been performed over the temperature range 20–165 °C which includes two phase transitions, in particular the transition to the superionic phase near T_s = 129 °C. We observed strong discontinuities for elastic constants C₁₁, C₂₂ and C₃₃ at T_s and a broadening of the Brillouin lines above T_s. The results are discussed on the basis of a linear coupling between strains and mobile protons.     

Higher-order elastic constants of La1.84Sr0.16CuO4 high-temperature superconductor

Finite deformation theory is used to obtain the strain energy density of a tetragonal 2–1–4-type single crystal of the high-temperature superconductor La_2−xSr_xCuO₄. The complete set of second and third-order elastic constants of the high-temperature superconductor La_2−xSr_xCuO₄ (x = 0.16) is calculated by taking into account the interactions between nine nearest-neighbour atoms in the lattice and using Mie–Grüneisen interatomic potential. For the sake of comparison we have also computed the values of these constants for x = 0.13–0.20. The values of third-order elastic constants of La_2−xSr_xCuO₄ (x = 0.13–0.20) are negative and their absolute magnitudes are one order higher than those of the second-order elastic constants.     

Influence of fluctuations on thermal conductivity of high-Tc YBa2Cu3O7−δ superconductor

To study a behavior of the thermal conductivity near T_c specific heat and thermal diffusivity of the YBa₂Cu₃O_7−δ high-T_c ceramics were simultaneously measured. Close to T_c = 92.30 K the thermal diffusivity and the thermal conductivity discovered minima and the specific heat – maximum. Quantitative analysis of the influence of thermodynamical fluctuations showed the same power laws with Gaussian exponent equal to 0.5 and existing of crossover from the 3D Gaussian to 3D XY critical behavior in the specific heat and thermal conductivity at the approach to T_c. To explain the minimum in thermal conductivity at T_c we propose a mechanism of scattering of phonons on the superconducting fluctuations.     

A comparative study of donor formation in dysprosium, holmium, and erbium implanted silicon

Formation of donor centers in Czochralski grown silicon doped with dysprosium, holmium, and erbium is discussed. Donor states of three kinds are introduced in the implanted layers after annealing at T=700°C. Shallow donor states with ionization energies between 20 and 40 meV are attributed to oxygen -related thermal donors. Other donor centers in the energy range of E_C−(60…70) meV and E_C−(100…120) meV appear to be dependent on dopants. After a 900°C anneal strong changes in the donor formation are observed only in silicon doped with erbium. Instead of donors at E_C−(118±5) meV, new donor centres at E_C−(145±5) meV are formed. Reportedly, the latter ones are involved in the excitation process of the Er³⁺ ions with a characteristic luminescence line at ≈1.54 μm.     

Synthesis and characterization of the Aurivillius phases Bi2 − xPbxSr1 − xNdxNb2O9

The n = 2 Aurivillius phase Bi_{2 − x}Pb_xSr_{1 − x}Nd₂O₉ was successfully synthesized as a ceramic material over the whole range of simultaneous, charge compensated substitution x = 0–1.0. Structural investigations were performed by Rietveld refinement applying different space groups Fmmm and A2₁am, and additionally by X-ray absorption spectroscopy (EXAFS) on the Nd L_III-edge, confirming the accommodation of Nd on the atomic sites of Sr, which implies the substitution of Bi³⁺ by the isoelectronic Pb²⁺. The ferroelectric transition temperature T_c = 270 °C of the substituted powders with x = 0.4 and 1.0 is distinctly reduced compared to the unsubstituted sample with T_c = 450 °C. In temperature resolved powder X-ray diffraction patterns no structural phase transition could be detected.     

Oxygen nonstoichiometry and phase transitions of the neodymium-rich Nd1+xBa2−xCu3Oz solid solution

On the basis of chemical, thermal analysis and Cu K-edge X-ray absorption measurements, oxygen content in the Nd_1+xBa_2−xCu₃O_z solid solution was determined between 1000°C in air and 400°C in oxygen for x=0.05–0.9 compositions. It has been observed that the oxygen nonstoichiometry Δz of the Nd_1+xBa_2−xCu₃O_7+x/2−Δz solid solution decreases 2–2.5 times for a large substitution (Δz≈0.3–0.33 for x=0.9), despite of the acclaimed higher total oxygen content. The difference in nonstoichiometry is explained by a higher average value of the copper oxidation state (ACV), which is vital for the solid solution with large x even at elevated temperatures (ACV≈2–2.05 for x>0.3 at 1000°C, PO₂=0.21 atm). On the contrary, the ACV after complete oxygenation is almost constant (about 2.25–2.3) for the whole series. The x-dependence of the oxygen content is not monotonous and structural phase transitions can be observed at x=0.3 and x=0.6, as confirmed by the X-ray diffraction and the Raman scattering spectroscopy. The first well-known transition is connected with the oxygen disorder due to the Nd substitution for Ba at random Ba-sites. In the present work, it is proved by the apical oxygen mode broadening in Raman spectra. Ordering of the Nd and Ba atoms with a subsequent orthorhombic distortion of the lattice may occur even at 1000°C in air due to the second transformation at x≈0.6. The invariable orthorhombicity of the Nd-rich solid solution with x>0.6 is not caused by the oxygen absorption as in the x=0.05 case. Existence of high- and low-temperature orthorhombic modifications of this solid solution has been observed for the first time. Finally, a tentative 3D (z–x–T) diagram is suggested for the Nd_1+xBa_2−xCu₃O_z solid solution up to 1000°C in air, including the new x=0.6–0.9 region.     

Magnetic properties of Sm–Co–B-based nanocomposite magnets

The magnetic properties of nanocomposite melt-spun magnets with composition Sm_16−xCo_68+xB₁₆ (x=0–10, 2 at% interval) and Sm₈Co_92−yB_y (y=10–18, 2 at% interval) have been studied systematically. Several ribbons were fabricated with a wheel speed of 50 m/s, followed by annealing in the temperature range of 700–800°C for 2.5–40 min. XRD results and magnetization versus temperature curves showed that almost all of the samples were composed of the tetragonal Sm₂Co₁₄B and rhombohedral SmCo₁₂B₆ phases which are not magnetically hard at room temperature. However, a relatively high coercivity in the range of 3.5–5.5 kOe has been obtained in these samples. The highest coercivity of 5.5 kOe and a very promising β value of −0.28%/°C were obtained in Sm₈Co₇₄B₁₈ ribbons annealed at 750°C for 5 min. The high coercivities are attributed to the small grain size of the 2 : 14 : 1 phase, in which the large surface areas enhance its effective anisotropy, and make it uniaxial type.     

Deposition temperature dependence of the optical properties of a---Ge

The room temperature u.v. reflectance of 1000 Å Germanium films (evaporated onto smooth amorphous substrates) has been measured and observed to vary with substrate deposition temperature T_s. Changes with increasing T_s are ascribed to gradual elimination of highly anisotropic voids for T_s 160°C, and to gradual film crystallization for T_s 200°C.     

Influence of magnetization on lattice constants of 3d transition metal alloys

A simple relation is found in 3d transitional metal alloys between the lattice constant and the magnetization, which can be described with an equation: a(x) = a₀^A· (1 − x) + a₀^B. x + C μ(x). It is proposed that studies of lattice constants at high temperatures may serve as an experimental method to detect the existence of localized moments above T_c. The anomalous thermal expansion of the Invar alloy is explained as a result of the collapse of localized moments above T_c.     

Reduced s-d model with antiferromagnetically coupled impurity spins

An antiferromagnetic equivalent-neighbour Heisenberg interaction H_i between impurity spins is added to the reduced s-d Hamiltonian H_r previously introduced by simplifying the Kondo s-d exchange Hamiltonian H_K. Asymptotic mean-field theory is developed for H_r + H_i, in the presence and absence of external magnetic field, and applied to (La_1−xCe_x)Al₂ alloys. Specific heat c_i(T) and zero-field susceptibility χ_i(0,T) curves for (La_1−xCe_x)Al₂ are depicted. The coupling constants of H_r + H_i and conduction bandwidth are adjusted so that T_c temperatures for x = 0.2, 0.1 are equal to the experimental values. c_i(T) exhibits a jump at T_c and is decreasing for T < T_c. χ_i(0,T) has a first order pole at T_c which corresponds to the maximum of experimental susceptibility and χ_i(0,0) > 0. These results improve those obtained earlier on the grounds of H_r theory.     

Temperature effects in directional elastic peak electron spectroscopy for the Cu(011) face: observation of the surface roughening transition

Polar profiles of directional elastic peak electron spectroscopy (DEPES) and directional Auger electron spectroscopy (DAES) were measured for the Cu(011) face in the vicinity of particular close-packed directions in the sample and for sample temperatures of 400<T<1200 K. For all directions investigated, the ln C versus T curve, where C is the contrast: C = 2(I_max−I_max)/(I_max + I_max) for a particular maximum in the polar profile, is composed of three linear parts. The slope of this particular part is different for the different maxima investigated but the breaks between the linear parts occur always at the temperatures T₁ ≈ 700 K and T₂ ≈ 1000 K. The former break is ascribed to the appearance of anharmonic thermal vibrations while the latter one indicates the surface roughening transition.

DEPES was applied for the first time in the investigation of the surface roughening transition and it was found to be a very convenient and effective method. The surface roughening temperature T_R ≈ 1000 K was found for the Cu(011) face, in an excellent agreement with theoretical predictions.