Magnetic Susceptibility in 2D Superconductor NaxHfNCl System

Abstract

Superconducting critical temperature, T_c and the shielding volume fraction, SVF, of layered nitride superconductor Na_xHfNCl have been studied as a function of x, i.e. Na concentration. Although T_c decreases gradually with increasing x from 20.0 K at x=0.11 to 16.5 K at x=0.85, SVF has a sharp peak around x=1/6, where strong coupling between local ordering of Na atoms and Fermi surface instability can be expected. Structural disorder in the samples above x=0.5, observed by powder neutron diffraction, does not affect the superconductivity appreciably. Electronic specific heat coefficient, γ, is estimated to be about 7.7 mJ/mol/K² by its difference of magnetic susceptibility between HfNCl and Na_0.5HfNCl. The γ value is relatively small compared with the high T_c value, revealing double honeycomb lattice system as new potential higher T_c superconductor series by intercalation.     

Effect of the oxygen pressure during annealing on Tc and lattice parameter c of Bi-2212 single crystals

The influence of oxygen partial pressure (p_o2 = 10⁻⁴–1.5 · 10² atm) during annealing on the T_c and lattice parameter c of Bi-2212 single crystals has been investigated. Antibate correlation between non-monotonous alteration of the c parameter and T_c with the increase in oxygen content has been revealed.     

Dielectric properties of betaine phosphite and deuterated betaine phosphite films

Polycrystalline films of betaine phosphite (BPI) and deuterated BPI have been grown by evaporation on LiNbO₃, α-SiO₂, α-Al₂O₃, and NdGaO₃ substrates. These films consist of large single-crystal blocks in which the polar axis (b) lies in the substrate plane. The results of studying the dielectric properties of the films using interdigital electrodes, X-ray diffraction, and block images in a polarized-light microscope in reflection are reported. The film transition into the ferroelectric state at T = T _c is accompanied by strong anomalies of the capacitance of the film/interdigital structure/substrate structure. The deuteration of BPI films leads to an increase in their temperature T _c: from T _c = 200 K for BPI-based structures to T _c = 280 K for structures with a high degree of deuteration (d ∼ 90%).     

Anomalous Up-and-Down Variation of Positron Lifetime Near Tc in High-temperature Superconductors

The temperature dependence of the positron lifetimes in the polycrystalline high T_c superconducting Y Ba Cu O and Bi Sr Ca Cu O and the non-superconducting Y Ba Cu O have been investigated by using the position lifetime technique. It is found from the experiments that there is an evident temperature dependence for the two kinds of superconducting samples but no temperature dependence for non-superconducting sample. It is important that the anomalous up-and-down variation of positron lifetime near T_c has been observed in the measured superconducting samples. We suggest that this strange temperature dependence of the positron lifetimes near T_C may result from some changes induced by the phase transition.     

Induction periods and mechanism and kinetics of nucleation of barium tungstate crystal growth from sodium tungstate melts in platinum crucibles

Induction periods (t) and mechanism and kinetics of nucleation in barium tungstate crystallization from sodium tungstate melts in platinum crucibles were studied. A theoretical relation has been developed to express the dependence of t on the cooling rates (R_T) and the rate (R_c) of development of excess solute concentration. At any crystallization temperature the average rate-constant (k_n) for heterogeneous nucleation was related to R_c by (k_n/p)^1/(p+1) = 1/(tR_c^γ), where γ is a constant and p is the average number of particles in the critical nuclei. The critical temperature (T), critical supersaturation (S) and γ values were estimated.     

Optical Reflection Spectra of Silicon Clathrate Compounds Ba8AgxSi46−x

Abstract

Optical reflection spectra are measured for Ba₈Ag_xSi_46?x (0 ≤ x ≤ 6) at room temperature. A systematic decrease in the plasmon energy is found with increasing x, indicating that the carrier concentration decreases with increasing x. When x increases, the superconducting transition temperature T_c decreases. An origin of the decrease in T_c is assigned to the decrease in the density of states at the Fermi level due to the decrease in the carrier concentration.     

Effect of Lithium Doping in the Bi-2212 Superconducting System

Samples with the nominal composition Bi₂Sr₂Ca_1–xLi_xCu₂O_y (x = 0.2,0.4, and 0.6) and Bi₂Sr₂CaCu_2−x L1_xO_z (x = 0.4, 0.6, and 1.0) were prepared by the solid state reaction method. The role of L1 at both Ca and Cu sites in the Bi-2212 composition were studied. From the X-ray diffraction data it was found that the L1-doped at the Ca site increases the c-axis and that doped at the Cu site decreases the c-axis. From the D.C. four-probe resistivity data it was found that Li-doped at the Ca site reduces the T_c and the L1-doped at the Cu site gives T_c(0) above the liquid nitrogen temperature. It was observed that the L1 doping reduces the melting point of Bi-2212 composition. The presence of Lithium was confirmed by inductive the coupled plasma method.     

Crystal Growth and Elastic Properties of Hopeite,Zn3(PO4) · 4 H2O

Single crystals of orthorhombic Zn₃(PO₄)₃ · 4 H₂O of optical quality with dimensions up to 12 × 10 × 8 mm were grown from aqueous solutions of ZnSO₄ and NaH₂PO₄ by controlled diffusion of NH₃ into the solution. The elastic constants c_ij and the thermoelastic constants T_ij = dc_ij/dT, T temperature, were determined from ultrasonic resonance frequencies of thick plates in the range between 5 and 40 MHz and between 250 and 320 K. In respect to the longitudinal elastic stiffness hopeite behaves quasi isotropically, however, the elastic shear stiffness shows a large anisotropy of about a factor 2.5. Above 260 K the shear stiffness c₆₆ possesses a quite anomalous temperature dependence (T₆₆ > 0). Further, the linear thermal expansion reveals a strong anisotropy (α₁₁ = −3.3, α₂₂ = 3.4, α₃₃ = 33 · 10⁻⁶/K).     

Anomalous High Pressure Properties in Fullerene Superconductors

Abstract

Correlations among lattice parameter, external pressure, and critical temperature T_c has been surveyed on fullerene superconductors with a wide range of lattice parameters and various valence states. The observed value of dT_c/dP for Na₂Rb_0.5Cs_0.5C₆₀ and Li₃CsC₆₀, having small interfullerene separations, is about a few times lager than that of K₃C₆₀. In contrast, small dT_c/dP values are found in fullerides with expanded unit cells, such as (NH₃)_xNaAA^?C₆₀ (A, A^? = K, Rb and Cs) and A₃Ba₃C₆₀. Interestingly, the chemical and physical pressure effects on T_c are considerably different in these fullerides with large interfullerene spacings. Our results suggest that the pressure dependence is scaled by an interfullerene distance when the interfullerene distances is small, but that this scaling fails when the unit cell is expanded.     

In situ observation of β‐BaB2O4 microcrystal growth in glass matrix

The morphological evolution and growth mechanism of β‐BaB₂O₄ microcrystal in Li₂B₄O₇‐BaB₂O₄ glass (Li₂O‐B₂O₃‐BaO) matrix were investigated by optical in situ observation method. And the crystallization temperature T_c has been examined by differential thermal analysis (DTA). It demonstrates that homogeneous distribution of hexagonal shaped BBO microcrystals with size up to several tens of microns is typical when temperature is much higher than T_c, however, heterogeneous nucleation occurs when annealing temperature is close to T_c. For the latter case, crystal clusters that consist of several microcrystal grains are obvious. When the crystals in one specific cluster grows larger, crystal motion occurs in glass matrix while their orientation and symmetrical shape keep nearly no changes. Additionally, the BBO microcrystal has been determined to grow nearly in linear with time, which suggests a mechanism of interface‐controlled growth. Furthermore, the activation energy of BBO crystal growth in glass matrix is calculated which is around 2.4 eV. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Growth Kinetics of High-Tc and Low-Tc Phases in Bi2-xPbxCa2Sr2Cu3Oy Superconducting Compounds

Superconducting samples of the composition Bi_2-xPb_xCa₂Sr₂Cu₃O_y (x = 0.0, 0.1, 0.2, 0.3, 0.4, and 0.5) are prepared by co-decomposition of metal nitrates. DTA, TGA, density, and porosity studies have been performed on these samples. Characterisation techniques like XRD, SEM, EDXA, d.c. resistance, and a.c. magnetic susceptibility have been employed to study the growth of High-T_c (HTP) and Low-T_c (LTP) phases upon lead substitution for bismuth. Results have revealed the growth of HTP with increase in Pb conc. up to x = 0.3. Further increase in Pb appears to results in the deterioration of HTP resulting in the formation of LTP and Ca₂PbO₄ impurity phase. Transport critical current density (J_c) measurements performed on the samples indicate that Pb doping followed by densification remarkably improves the current carrying capacity of the materials.     

Pressure Effects of Nematic Liquid Crystals

There exists a variety of important new phenomena and plenty of experimental data on the pressure effects of liquid crystals. Yet, no systematic phenomenological or microscopic theory is available. In this paper, the ordinary Landau-deGennes theory is generalized so that the free energy becomes G = G _o(P, T) + a/2 [T - T*(P)]S ² - B(P)/3 S ³ + C(P)/4 S ⁴, T*(P) = T _o + bP - eP ². All the known pressure experiments of nematic PAA (where B and C are independent of P) are explained in one stroke and all parameters are determined. New results including reentrant I phase (I represents isotropic phase), and the independence of T _c—T* and T*—T _c on P, etc., are predicted. Simple methods of experimental confirmation are proposed. Similar discussions on the cases of MBBA, EBBA, etc. are also given. Our theory differs from that of Lin-Keyes-Daniels but agrees better with experiments. Results related to G _O(P, T) will be reported in a separate paper.     

The Order-Disorder Phase Transition in Liquid Crystals as a Function of Molecular Structure. I. The Alkyl Cyanobiphenyls

The pretransitional behaviour of a homologous series of mesomorphic compounds, the alkyl cyanobiphenyls (CNρρC _n H _{2n + 1}) for n = 5–12, has been studied as a function of temperature by light scattering in the isotropic phase. The higher homologues, n = 8–12, exhibit a smectic A phase that becomes increasingly important at the expense of the nematic phase for increasing n and for n = 10 and 12 no nematic phase is observed. We have shown from light scattering and differential scanning calorimetry measurements that the presence of the smectic A phase changes markedly the pretransitional behaviour, and gives rise to two distinct pretransitional regions. The first region is for temperatures greater than ～3°C above the clearing temperature T_c where the systems exhibit a typical pretransitional behaviour adequately described by the phenomenological Landau-de Gennes model of a second order phase transition with an intervering first order transition at T_c . The constants of this model for this region are given and show a distinct odd-even effect. The second region is close to T_c where a strong divergence from the already critical behaviour is observed. This secondary divergence is observed to be a function of the alkyl chain length and its conformation, and is attributed to coupling between smectic A like layers and the orientational order parameter in the isotropic phase.     

Dynamical Properties of a Disordered Crystal: Adamantane—C10D16

Inelastic neutron scattering measurements on C₁₀D₁₆ show the existence of underdamped collective librational modes with temperature-independent frequency widths.

A preliminary dynamical model, with no adjustable parameters, shows that the observed widths are consistent with the existing quasi-static disorder.

Direct measurements of the orientational pair correlation function by means of elastic neutron scattering agree with the prediction of a simple Ising model.

Intensity measurements between room-temperature and the ordering temperature (T _c = 208.6K) indicate that the orientational correlations develop very slowly as T → T _c.     

Thermodynamic analysis and technology of preparation of high-Tc superconducting system YBa2Cu3O7−x

By modeling the equilibrium state of the phase formation reaction of High-T_c superconducting phase 1:2:3 (YBa₂Cu₃O_7−x) and using the results of P-T−x correlation investigation of some authors, the variation of Gibbs free energy (ΔGm) versus temperature has been calculated. On the basis of the variation of ΔGm, the technology procedure of preparation of phase 1:2:3 with desired x has been established to meet requirement of basic investigation and application of High-T_c superconductors YBa₂Cu₃O_7−x.     

Concentration Magnetic Phase Transitions in the (La1−xCaxMnO3 System

Comprehensive investigations into crystal structures, electrical and magnetic properties of the (La_1−xCa_x)MnO₃ (x = 0 ÷ 0.6) solid solutions are performed. Two concentration magnetic phase transitions are found: antiferromagnetic-ferromagnetic with x = 0.14 and ferromagnetic (T_c = 150 K) - ferromagnetic (T_c = 260 K) at f = 0.2. It is shown that the first of them is due to the crystal structure O′ O orthorhombic transition, the second - to the increase of ferromagnetic interaction at the expense of additional interaction through conduction electrons. A diagram of the magnetic states for (La_1−xCa_x)MnO₃ at x = 0 / 0.6 is given.     

Thermoelastic and Piezoelastic Properties of Hexagonal RbMnCl3

Temperature and pressure derivatives of the elastic constants, indices of refraction, dielectric constants, Faraday effect and thermal expansion of RbMnCl₃ near T_c = 278 K confirm the existence of a second-order phase transition. A strongly anomalous behaviour is observed with the elastic shear stiffness c₄₄ which possesses a positive temperature derivative and a negative pressure derivative. These properties and also the pressure derivative of the transition temperature, dT_c/dp = 6.2 K kbar⁻¹ closely resemble the anomalous behaviour of two other ferroelectric species, betaine borate and betaine hydrogen maleinate.     

Optical,X-ray,and calorimetric studies of phase transitions in ND4BeF3

Phase transitions in ND₄BeF₃ crystals at T₁ = 330 and T₂ = 245 K were studied by specific heat (DSC) and X-ray lattice parameters measurements in function of temperature. These ferroelastic transitions are accompanied by anomalous heat absorption, sudden changes in the unit cell parameters and crystal symmetry. Formation of domain structure in particular phases examined under a polarizing microscope is also described. Domain walls of the type (100), (010), and (001) observable at room temperature indicate on the triclinic symmetry of the Phase II. Above T₁ the crystal becomes orthorhombic. First-order phase transition at T₂ causes the symmetry of the crystal in the Phase III to be monoclinic with the c-axis unique. The phase transition diagram is proposed and compared with the phase situation in nondeuterated NH₄BeF₃ crystals.     

Determination of the Effective Vacancy Concentrations and the Vacancy Losses after the Quench of Al–Zn–Mg Alloys

On the basis of

• 1 the experimental observation of dislocation loops by means of TEM,
• 2 the conclusion from this that the concentration of vacancies in the dislocation loops exists at the cost of the vacancy concentration primarily present at T_q, and
• 3 the inversion of the decomposition kinetics of Al–Zn–Mg alloys found out by means of resistivity and microhardness measurements, the effective vacancy concentration c_v.eff being at disposal for the GP-zone formation was found out by means of the relation t_R,max (t_R,max being the time required to reach the maximum of resistivity) and the Arrhenius plot In t_R,max = f(1/T_q) of the isothermal resistivity measurements. From the comparison of c_v.eff with the theoretical V concentration at T_q it is possible to derive statements about the loss of vacancies during and after the quench through loop formation, building-in of vacancies in GP-zones and annealing of vacancies at lattice defects. From this comparison follows:
  • The ideal ZnV concentration at T_q (calculated by means of the Lomer equation) is sufficiently great to account for the effective vacancy concentration.
  • The part of c_MgV in the loss concentration is rising with increasing c_Mg.
  Both these statements support the hypothesis that the initial process of the decomposition in Al–Zn–Mg alloys is caused by the ZnV diffusion.

     

Role of Silver on Superconducting Properties of BPSCCO (2223) System

The role of silver for the superconducting properties of the Bi Pb Sr Ca Cu O (2223) system have been investigated systematically. Samples with various concentrations of Ag were prepared by the matrix method. D.C. resistance results showed that T_c(0) varied between 100–109 K for various compositions of Ag. J_c measured at LNT from I–V data shows an increase with silver addition. J_c decreased from 90 Amp/cm² at zero field to 16 Amp/cm² at 100 Oe. The T_c(0) obtained form the susceptibility data agreed with those obtained from resistance measurements. X-ray diffraction results showed that the dominant phase in BPSCCO is the high-temperature (2223) phase and the same is improved in silver added BPSCCO samples. The scanning electron micrographs indicated that silver is precipated along the grain boundaries, separating the superconducting grains. These results have showed that silver addition does not destroy the superconductivity and at the same time enhances the critical current density.