Features of electrical conductivity in an incommensurate phase of graphite intercalation compound C10HNO3

Electrical conductivity and conduction-electron spin resonance (CESR) have been studied in stage-2 acceptor α-graphite-nitric acid intercalation compound C₁₀HNO₃. It is found that the electrical conductivity σ_c along the c axis in the structurally incommensurate phase of this compound is temperature independent, whereas the electrical conductivity σ_a along carbon layers exhibits “metallic” temperature behavior. Analysis of the temperature dependences of σ_c, σ_a, and the CESR linewidth demonstrates that, in the incommensurate phase of the graphite intercalation compound, the electrical conductivity along the c axis is realized through a nonband mechanism—the transfer of free charge carriers along thin high-conductivity channels shunting the carbon layers adjacent to the intercalate.     

New results on the peierls transition of TTF-TCNQ by electron tunneling

We realized TTF-TCNQ(Al₂O₃)Al junctions with a point contact geometry using the TTF-TCNQ crystal as the point electrode. Junctions have been studied in the two different regimes, T>T_c and T<T_c. For T>T_c the numerically analyzed dynamic conductance shows a parabolic dependence which reflects the metallic regime. For T<T_c a clear deviation from the parabola has been pointed out and a rough estimation of the gap can be deduced from the “conductance well” width.     

Hole mobility in HgI2

Hole mobility along the layers in HgI₂ has been measured for the first time by the nuclear techniques using alpha particles and by cutting the sample perpendicularly to the layers. The value of the hole drift mobility is in agreement with previous results obtained by Photo-Electro-Magnetic (PEM) effect and it is much higher than hole drift mobility along the c-axis.     

Switching between one- and two-dimensional conductance: Coupled chains in the monolayer of Pb on Si(5 5 7)

Four-point conductance measurements of a high temperature annealed monolayer of Pb on Si(5 5 7) are combined with tunneling microscopy (STM) and LEED for structural investigations. We found extremely high surface state conductance which becomes quasi-one-dimensional below a critical temperature of T_c = 78 K. The change from low to extremely high conductance anisotropy is associated with a reversible order-disorder phase transition with a temperature dependence ∝1/T + const. along the chains below the phase transition. Below the phase transition order is found simultaneously in the lateral separation between Pb chains and along the chains. There a 10-fold superperiodicity appears. We suggest that strong two-dimensional coupling, leading to electronic stabilization of terraces with a width of lattice constants, results in perfect nesting normal to the chains at E_F (below T_c), which is the origin of this switching to one-dimensional behavior. Therefore, no metal-insulator transition is expected at low temperature.     

Enhancement of Jc by Hf-doping in the superconductor MgB2: A hyperfine interaction study

Measurements of the critical current density (J_c) by magnetisation and the upper critical field (H_c2) by magnetoresistance have been performed for hafnium-doped MgB₂. There has been a remarkable enhancement of J_c as compared to that by ion irradiation without any appreciable decrease in T_c, which is beneficial from the point of view of applications. The irreversibility line extracted from J_c shows an upward shift. In addition, there has been an increase in the upper critical field which indicates that Hf partially substitutes for Mg. Hyperfine interaction parameters obtained from time differential perturbed angular correlation (TDPAC) measurements revealed the formation of HfB and HfB₂ phases along with the substitution of Hf. A possible explanation is given for the role of these species in the enhancement of J_c in MgB₂ superconductor.     

Anisotropy of the electron momentum distribution in Bi2Sr2CaCu2O8+δ superconductor studied by positron annihilation

The temperature dependent (30-300 K) Doppler broadening of the positron annihilated γ-radiation measurement has been investigated on single crystalline Bi₂Sr₂CaCu₂O_8+δ (Bi-2212) high T_c superconducting samples along two different crystallographic orientations. It has been observed that throughout the temperature range the electron momentum distribution has a larger value along the crystallographic c-axis than in the a-b plane. The temperature dependent Doppler broadened positron annihilation γ-radiation lineshape analysis shows a step like increase of S-parameter at the temperature region 92-116 K.     

Types of defect ordering in undoped and lanthanum-doped Bi2201 single crystals

Undoped and lanthanum-doped Bi2201 single crystals having a perfect average structure have been comparatively studied by x-ray diffraction. The undoped Bi2201 single crystals exhibit very narrow satellite reflections; their half-width is five to six times smaller than that of Bi2212 single crystals grown by the same technique. This narrowness indicates three-dimensional defect ordering in the former crystals. The lanthanumdoped Bi2201 single crystals with x = 0.7 and T _c = 8–10 K exhibit very broad satellite reflections consisting of two systems (modulations) misoriented with respect to each other. The modulation-vector components of these two modulations are found to be q ₁ = 0.237b* + 0.277c* and q ₂ = 0.238b* + 0.037c*. The single crystals having a perfect average structure and a homogeneous average distribution of doping lanthanum consist of 70-to 80-Å-thick layers that alternate along the c axis and have two different types of modulated superlattice. The crystals having a less perfect average structure also consist of alternating layers, but they have different lanthanum concentrations. The low value of T _c in the undoped Bi2201 single crystals (9.5 K) correlates with three-dimensional defect ordering in them, and an increase in T _c to 33 K upon lanthanum doping can be related to a thin-layer structure of these crystals and to partial substitution of lanthanum for the bismuth positions.     

Ferromagnetic and spin-wave resonance in Co/Pd/CoNi multilayer films

It has been found that the magnitude and sign of exchange interaction between Co(5 nm) and CoNi(5 nm) ferromagnetic layers through Pd depend on magnetization orientation of ferromagnetic layers. If magnetization is oriented in a layer plane, exchange interaction can be both ferromagnetic and antiferromagnetic. If magnetization orientation is orthogonal to a layer plane, the exchange constant is always positive at d_Pd<d _c and equals zero at d_Pd>d _c (d _c is the characteristic length).     

Clear experimental evidence for boundary and impurity scattering related crossover field scales in GaAs/AlGaAs split-gate wires

We study the magnetic field dependence of the correlation field ΔB_cand amplitude δgof the conductance fluctuations, observed in the low temperature magnetoresistance of GaAs/AlGaAs split-gate wires. Near zero field, universality of quantum interference is retained and the magnetoresistance shows universal conductance fluctuations. At high magnetic fields, although the discrete Landau level quantization becomes resolved. ΔB_cand δgare found to increase linearly with magnetic field, with a slope which depends upon the nature of electron scatterings in the wire.     

EPR study of the two-dimensional molecular composite [NH3-(CH2)5-NH3]CuCl4: Evidence for both spin diffusion and spin anisotropies

The quasi two-dimensional single crystal [NH₃-(CH₂)₅-NH₃]CuCl₄ has been studied by EPR measurements at X-band, room temperature andT = 100 K in three perpendicular planes (a, b), (a, c), (b, c*). This compound crystallizes in the monoclinic system with four entities per unit cell. Only one EPR exchange-collapsed line was observed for the four magnetically inequivalent copper ions in the lattice at room temperature. A careful analysis of experimental data permits us to obtain the principal values of theg-factor:g _┴=2.052 andg _║=2.267. The EPR linewidth exhibits a minimum along the г п ≈/4 direction of (a, c*) plane attributable to spin diffusion. Along the г п ≈/4 direction of (b, c*) plane the linewidth exhibits a maximum attributable to the two magnetically inequivalent sub-systems located in two adjacent mineral sheets. The simulations of EPR spectra obtained along the bisector of (b, c*) angle at room temperature and atT = 100 K show that exchange interaction between these two dissimilar sub-systems is nearly equal to zero.     

Thermal expansion op NbSe2 and TaS2

The lattice parameters a₀ and c₀ of the hexagonal 2H polytype of NbSe₂; have been measured over the temperature range 156–478 K for a₀, and 138–482 K for c₀. The lattice parameter c₀ of the hexagonal 2H polytype of TaS₂ has been measured between 151 and 472 K. The lattice parameters a₀ and c₀ for the octahedral 1T polytype of TaS₂ been measured between 165 and 488 K. Over these temperature ranges, the following average coefficients of thermal expansion have been measured; 2H-NbSe₂, 6.6 × 10^?6K^?1 along the a-axis, 19.9 × 10^?6 K^?1 along the c-axis; 2H-TaS₂, 15.6 × 10^?6 K^?1 along the c-axis; IT-TaS₂, 12.7 × 10^?6 K^?1 along the c-axis. The parameter c₀ of 1T-TaS₂ undergoes two transitions which may be explained in terms of charge density waves.     

Critical thickness and stress relaxation in YBaCuO (123) strained epitaxial layers and YBaCuO based strained superlattices

An energy model has been used to calculate the critical thickness h _c of YBaCuO thin films and YBaCuO based superlattices within an isotropic or anisotropic approximation. The critical thickness of single layers calculated from the anisotropic model (16 nm) is in good agreement with the previously published experimental values which are spread out from 4 to 20 nm. In the case of superlattices, relaxation appears to be governed by the critical thickness of the elementary sub-layers and is then better evaluated through the calculation performed for YBaCuO single layers. XRD measurements on YBa₂Cu₃O₇/PrBa₂Cu_3?xGa_xO₇ superlattices grown on {100{ SrTiO₃ have evidenced a tetragonal stress in the YBaCuO ab plane which remains expanded when the YBaCuO elementary layer thickness is lower than 4.8 nm (4 YBaCuO cells). However the critical temperature of the shortest period superlattices is only slightly affected by this expanded stress in contrast to the effect of an elastic stress externally applied along the ab plane of YBaCuO thin films.     

A neutron powder diffraction study of the helimagnetic structure of TlCo2Se2−xSx

The magnetic layer structure of TlCo₂Se_2−xS_x has been thoroughly re-investigated with neutron powder diffraction. The cobalt magnetic moments are ferromagnetically arranged within the layers, but the interlayer coupling differs profoundly with varying composition (x): the spins in TlCo₂Se₂ form a helix along the c-axis with a turning-angle of ∼119° at 1.4 K. This kind of helical structure prevails for 0≤x≤1.5 with a gradual decrease of the angle with increasing sulphur content, down to 34°, showing an almost linear relationship with the interlayer distance of Co-Co. For x≥1.75 the interlayer coupling changes to ferromagnetic. Unexpectedly, two helices were found to coexist at x=0.5 and x=1.0. The interaction between adjacent cobalt layers is there characterized by an incommensurate angle (106°, resp., 73°) together with a commensurate angle of 90°. The magnetic structures have been refined as two magnetic phases, each having a characteristic wave vector. A tentative model where the symmetry of the structure and the interlayer distance compete is considered for explaining the simultaneous occurrence of the two kinds of diffraction profile satellites.     

A search for extra lattice distortions associated with the superconductivity and the charge inhomogeneities in the high-Tc La1.85Sr0.15CuO4 system

Precise measurements of the temperature-dependent lattice parameters in a single crystal of La_1.85Sr_0.15CuO₄ are made by X-ray bond method and analyzed comparing the normal thermal expansion term, proportional to T⁴. A spontaneous lattice contraction of about 2.3×10⁻⁵ Å was found in the ab-plane, while along the c-axis a small expansion of about 1.5×10⁻⁵ Å was observed below the superconducting transition temperature of the system, showing some correlation between the structure and the high-T_c superconductivity. A careful search has been made for existence of any extra lattice distortion above the T_c, that might be associated with the stripe inhomogeneities in the title system, however, within the experimental sensitivity we could not find such distortions.     

Einfluß der Bestrahlung mit hochenergetischen Sauerstoffionen auf die kritische Stromdichte supraleitender Nb3Sn-Schichten

Nb₃Sn diffusion layers were irradiated with 24 MeV oxygen ions at fluences from 3.2×10¹³ up to 1.6×10¹⁵ cm^?2. The enhancement of the superconducting critical current density Δj _c has been measured as a function of fluence and of the external magnetic fieldH _a(j _c⊥H _a). The thermal annealing treatment of the defects concerning thej _c and induced by irradiation, has been investigated in the temperature region from 200 to 800 °C. The results are compared with the measurements of irradiation of Nb₃Sn with protons and deuterons. The measured data are discussed in connection with size of defects, cluster distance, fluxline distance and pinning-force.     

Anisotropic thermopower of (TMTSF)2PF6: 1D–2D cross over and SDW ordering

The thermopower of (TMTSF)₂PF₆ has been measured along the a-axis as well as along the b-axis. Marked anisotropy is seen in the whole temperature region studied. Close to 100 K, an anomaly is attributed to cross over from dominating one-dimensionality to dominating two-dimensionality. Precursor effects seen in S_a near T_c are attributed to SDW fluctuations. Well below T_c, both thermopower components exhibit typical semiconducting properties.     

Peculiarities of performance of the spin valve for the superconducting current

The spin valve effect for the superconducting current based on the superconductor/ferromagnet proximity effect has been studied for a CoO _x /Fe1/Cu/Fe2/Cu/Pb multilayer. The magnitude of the effect ΔT _c = T _c ^AP ? T _c ^P , where T _c ^P and T _c ^AP are the superconducting transition temperatures for the parallel (P) and antiparallel (AP) orientation of magnetizations, respectively, has been measured for different thicknesses of the Fe1 layer d _Fe1. The obtained dependence of the effect on d _Fe1 reveals that ΔT _c can be increased in comparison with the case of a half-infinite Fe1 layer considered by the previous theory. A maximum of the spin valve effect occurs at d _Fe1 ～ d _Fe2. At the optimal value of d _Fe1 almost full switching from the normal to the superconducting state when changing the mutual orientation of magnetizations of the iron layers Fe1 and Fe2 from P to AP is demonstrated.     

Heat transport and critical slowing down near the Rayleigh-Bénard instability in cylindrical containers

Measurements of heat transport across layers of liquid He I heated from below show a sharp transition to fluid flow at the gravitationally driven Rayleigh-Bénard instability. The effective thermal conductivity of the convecting fluid is found to be a linear function of the reduced Rayleigh number (R - R_c)/R_c and to have a slope which agrees with calculations. Critical slowing down near R_c is measured and found to be in excellent agreement with theory.     

Ferroelectricity and phase transition from incommensurate phase into commensurate one in (NH4)2ZnCl4

Phase transition has been found in (NH₄)₂ZnCl₄ at T = 266 ± 0.5 K by NQR method. There is a ferroelectric phase below T_c with a space group P2₁cn and with the trebling of the elementary lattice parameter along the axis c. Above the phase transition temperature in the crystal (NH₄)₂ZnCl₄ an incommensurate phase is realized.     

The non-isotropic character of electric and dielectric properties of ammonium zinc chloride crystal

The dielectric constant, ε, and the d.c. conductivity, σ, were measured along the a-, b- and c-axes of (NH₄)₂ZnCl₄ (AZC) crystal in the 300-450 K temperature range. Crystals of AZC grown from aqueous solutions containing excess of ZnCl₂ were used. The value of the dielectric permittivity of AZC is extremely small compared to other ferroelectric crystals. Pronounced broad or step-like peaks at the phase transition temperatures were detected along the a- and b-axes, while ε along the c-axis is temperature independent up to the end of the measuring range. Reciprocal of the dielectric permittivity in the range of the commensurate to incommensurate phase transition obeys a relation similar to the Curie-Weiss law that is valid for second order ferroelectric/paraelectric phase transitions. The constants of the proposed relationship applied to the cooling run are given. The J-E characteristics along the three crystallographic axes were measured in the normal, incommensurate, commensurate and antiferroelectric phases. Hence, the type of conduction mechanism has been estimated. Parameters of Poole-Frenkel and Richardson-Schottky types of conduction mechanism have been determined. The effect of applied electric field on the conductivity measurement was also tested. Conductivity anomalies with different character were observed at the phase transition temperatures. The lnσ−1000/T dependence revealed thermal activation energy of conduction along the a-, b- and c-axes with different values in different phases of AZC.