Evolution of the fermi surface in metastable βL-(BEDT-TTF)2I3

We have been studying the variation of thermodynamic and transport properties as β-(BEDT-TTF)₂I₃ evolves from the ambient-pressure-cooled, structurally metastable low T_c (β_L) phase to the high T_c(β_H) phase. Annealing at ambient pressure for about a hundred hours near 100 K removes the β_L-associated incommensurate superstructure that forms below T₁₇₅ (≈ 175 K at ambient pressure). We find from measurements of the temperature dependent Hall coefficient and heat capacity that long term annealing also suppresses the lower temperature T₂₀ transition (≈ 20 K) that occurs in the β_L phase, leading to bulk β_H phase superconductivity. The results are consistent with the opening of the Fermi surface (FS) at T₂₀ in non-annealed samples but are not consistent with a 50% reduction in FS area for non-annealed β_L phase samples (compared to the area of the Fermi Surface in the β_H phase) or significant localization at T₁₇₅. Finally, we compare these results to our measurements of the temperature dependent Hall coefficient and T_c under pressure.     

The NMR properties of Yb nucleus in AYbI3 (A: K, Rb, Cs) in solid state

The powder and magic-angle spinning (MAS) experiments on ¹⁷¹Yb in AYbI₃ (A: K, Rb, Cs) are reported. The NMR parameters, such as the principal components of the chemical shielding tensor (δ₁₁, δ₂₂, δ₃₃), the spin-lattice relaxation time and the parallel and perpendicular components (T₁, T₁, T₁) are determined in solid state and the transverse relaxation time was obtained from the natural line width. The local symmetry of Yb was discussed according to the spectra.     

High-field magnetization process in Mn1.9Cr0.1Sb

The compound Mn_1.9Cr_0.1Sb shows a first-order transition from the ferrimagnetic (FI) to the antiferromagnetic (AF) state at T₁ = 265 K with decreasing temperature. High magnetic fields (up to 60 T) produce AF-FI transitions below T_t. The temperature dependence of the critical field is nonlinear and shows T² character at low temperatures (T < 150 K), in contrast to the exchange inversion model of Kittel. This behavior is connected with the itinerant magnetism of 3d-electrons of Mn atoms resulting in a large electronic contribution to the change in free energy at the field-induced first-order AF-FI transition. The field-induced phase transition from the AF to the FI state is accompanied by an increase in the electronic specific heat coefficient γ by 13 mJ K⁻² mol⁻¹ as estimated from the magnetization measurements using thermodynamic relation.     

Transport and ultrasonic properties in La0.67Sr0.33Mn0.87Fe0.13O3 perovskite

The longitudinal and transverse ultrasonic sound velocity and attenuation, as well as electric resistance have been carefully measured in single-phase polycrystalline giant magnetoresistance perovskite La_0.67Sr_0.3Mn_0.87Fe_0.13O₃ at a frequency of 10 MHz, from 20 to 300 K. A big electric resistance peak was observed at 85 K (T_C). At the temperature above T_C, the resistivity can be fitted well by Mott’s law ρ=exp (T₀/T)^1/4 and both the longitudinal and transverse sound velocities show a lattice softening, which was accompanied by an attenuation peak. This simultaneous occurrence of electron and lattice softening implies electron–phonon coupling, known to exist for the octahedrally coordinated d⁴ ion, originating in the Jahn–Teller distortion. Below 55 K, pronounced sound-velocity softening for both longitudinal and transverse waves was observed; this may correspond to the formation of a spin-glass state.     

Gd Mössbauer effect and magnetic properties of GdNi2Sb2, GdCu2Sb2 and GdAl2Ga2

We have investigated the magnetic properties and the ¹⁵⁵Gd Mössbauer spectra of the ThCr₂Si₂-type compounds GdNi₂Sb₂, GdCu₂Sb₂ and GdAl₂Ga₂. These three compounds were found to order antiferromagnetically, with T_N=6.5 K, 15.8 K, and 42.4 K respectively. The electric field gradient V₂₂ derived from the quadrupole splitting of the Mössbauer spectra gives rise to a sign change at the end of the T3d series in GdT₂Sb₂, as was observed previously also for the corresponding compounds with Si and Ge. This behaviour was explained in terms of decreasing hybridization between the Gd valence electron states and the d electron states of the T atoms.     

Temperature dependent neutron powder diffraction study of the Laves phase compound TbCo2

The structure, magnetization and magnetostriction of Laves phase compound TbCo₂ are investigated by temperature dependent high resolution neutron powder diffraction. The compound crystallizes in the cubic Laves phase C15 structure above its Curie temperature T_C and exhibits a rhombohedral distortion (space group ) below T_C. By an appropriate extrapolation of the temperature factor of Co atom above T_C, the Rietveld refinement of the neutron powder diffraction data of the rhombohedral structure converges satisfactorily and reveals that the moments of Co1(3b) and Co2(9e) are almost equal. Tb moment follows well the Brillouin function. The total magnetic moment of TbCo₂ is about 5.8μ_B/f.u., the anisotropic magnetostriction constant λ₁₁₁ is about 4.6 × 10⁻³ and the volume magnetostriction ω_s is about 8.7 × 10⁻³ at 14 K.     

Einkristall-Röntgenstrukturanalyse und magnetische eigenschaften von Ba3YRu2O9, Ba3GdRu2O9 und Ba3YbRu2O9

Single crystals of (I) Ba₃YRu₂O₉, (II) Ba₃GdRu₂O₉ and (III) Ba₃YbRu₂O₉ were obtained by high temperature solid state reactions and examined by X-ray analysis. They crystallize with hexagonal symmetry, space group D_{6h⁴-P63/mmc}, with (I) A = 5.987 Å, C = 14.464 Å; (II) A = 5.895 Å, C = 14.592 Å; (III) A = 5.858 Å, C = 14.432 Å; Z = 2. The crystal structures belong to the 6L perovskites. The magnetic behaviour was investigated between 77 and 650 K using a Faraday balance. The magnetic susceptibility of (I) shows the characteristic behaviour of an Ru₂O₉ face-shared double octahedron, with a Neél temperature T_N = 220 K. The compounds (II) and (III) comply with the Curie-Weiss law, indicating the oxidation state Ru⁴⁺.

Zusammenfassung

(I) Ba₃YRu₂O₉, (II) Ba₃GdRu₂O₉ und (III) Ba₃YbRu₂O₉ wurden mit Hilfe von Hochtemperatur-Festoffreaktionen einkristallin dargestellt und röntgenographisch untersucht. Sie kristallisieren hexagonal, in der Raumgruppe D_{6h⁴ - P63/mmc}, mit (I) A = 5,987 Å, C = 14,464 Å; (II) A = 5,895 Å, C = 14,592 Å; (III) A = 5,858 Å, C = 14,432 Å; Z = 2. Die Kristallstrukturen gehören zur Klasse der 6L-Perowskite. Ihr magnetisches Verhalten wurde mit der Faradaymethode zwischen 77 und 650 K untersucht. Die magnetische Suszeptibilität von (I) zeigt das typische Verhalten von Substanzen mit isolierten Ru₂O₉-Doppeloktaedern mit einer Neél-Temperatur von T_N = 220 K. Die Verbindungen (II) und (III) erfüllen das Curie-Weiss-Gesetz und weisen auf Ru⁴⁺ hin.     

H-NMR-NMR study of magnetic anomaly in (BEDT-TTF)3CuBr4

The H-NMR¹-NMR measurements were carried out on the organic conductor, (BEDT-TTF)₃CuBr₄, where strong coupling between π and 3d electrons are expected. The H¹ spin-lattice relaxation rate, T₁⁻¹, was large about 20 sec⁻¹ and almost temperature independent above 60 K indicating the effect of localized magnetic moments. Anomalies at the structural transition (60 K) was small. An antiferromagnetic transition was confirmed: The magnitude of the moment at Cu site was estimated as 0.6-0.7 μ_B. The moment at the Cu site was found to survive in the metallic state under pressure.     

Magnetic structures of Zr6CoAs2-type Ho6−xErxMnBi2 solid solutions

Investigations were made by neutron diffraction on Zr₆CoAs₂-type (space group no. 189) Ho_6−xEr_xMnBi₂ solid solutions. The ferromagnetic ordering temperature decreases from Ho₆MnBi₂ (T_C = 200(6) K) to Er₆MnBi₂ (T_C = 100(4) K), whereas temperatures of ferrimagnetic (or antiferrimagnetic) ordering (T_Ferri and T_AFerri) are found to have non-monotonic dependences on the content of Er: T_Ferri = 58(4) K for Ho₆MnBi₂, T_Ferri = 162(4) K for Ho_4.5Er_1.5MnBi₂, T_Ferri = 150(4) K for Ho₃Er₃MnBi₂, T_AFerri = 78(4) K for Ho_1.5Er_4.5MnBi₂ and T_AFerri = 52(4) K for Er₆MnBi₂.

In these compounds, no local moment was detected on the manganese ion site, except for Ho_1.5Er_4.5MnBi₂ and Er₆MnBi₂ compounds. The manganese magnetic moments (μ_Mn) is 1.5μ_B and these are antiferromagnetically coupled with that of rare earth moments.     

Thermochemistry of GaBO3 and phase equilibria in the Ga2O3–B2O3 system

Employing a Tian-Calvet-type calorimeter operating in the scanning mode at temperatures from 1120 to 1220 K, the enthalpy change, Δ_dH, associated with the decomposition of GaBO₃ (=1/2β-Ga₂O₃+1/2B₂O₃(liq.)) and the corresponding decomposition temperature, T_d, were determined: Δ_dH=30.34±0.6 kJ/mol, T_d=1190±5 K. Using the transposed-temperature-drop method the thermal enthalpy, H(T)−H(295 K), of GaBO₃ was measured as a function of temperature, T, in the region from 760 to 1610 K; the results obtained are

[H(T)−H(295 K)]/(J/mol)=104.8·(T/K)−31 300 (760 K<T<1190 K),

[H(T)−H(295 K)]/(J/mol)=138.8·(T/K)−41 480 (1190 K<T<1590 K).

On the basis of the experimental results, the enthalpy and entropy of formation, Δ_fH and Δ_fS, respectively, of GaBO₃ from the component oxides were derived:

Δ_fH=−30.34 kJ/mol,Δ_fS=−25.50 J/(K·mol) at 1190 K,

Δ_fH=−10.55 kJ/mol,Δ_fS=−5.48 J/(K·mol) at 298 K.

The enthalpy versus temperature curve shows, apart from a step associated with the decomposition of GaBO₃, a further step at 1593 K which is attributed to a monotectic equilibrium.     

Single crystal study on UNi0.5Sb2

Single crystals of UNi_1−xSb₂ have been grown from an Sb-rich melt and studied by means of X-ray diffraction, magnetic and electrical transport measurements. Crystal structure refinements indicated significant deficiency on the transition metal sites in the tetragonal HfCuSi₂-type unit cell, yielding the actual composition UNi_0.5Sb₂. The single crystals studied order antiferromagnetically below T_N=161 K and exhibit another phase transition at T_t=60 K, presumably caused by a spin-reorientation. No crystal structure distortion could be detected at 10 K. Above T_N the electrical resistivity is dominated by a Kondo effect, whereas at lower temperatures it shows a behavior characteristic of antiferromagnets. The overall magnetic and electrical transport properties of UNi_0.5Sb₂ are highly anisotropic both in the ordered and paramagnetic states.     

Magnetic ordering in ternary RMn2Ge2 compounds (R = Tb, Ho, Er, Tm, Lu) from neutron diffraction study

The compounds RMn₂Ge₂ (R = Tb, Ho, Er, Tm, Lu) have been investigated by neutron diffraction. TbMn₂Ge₂ is a collinear ferrimagnet with the Mn and Tb moment aligned along the c axis (μ_TB = 8.81(59) μ_B: μ_Mn = 2.21(44) μ_B). HoMn₂Ge₂ exhibits incommensurale ordering below 2.1 K characterized by two wavevectors at 1.3 K: q₁ = (0.1543(4), 0.1543(4), 0) and q₂ = (0.210(1), 0.007(1), 0). The Mn sublattice remains antiferromagnetic down to 1.3 K (μ_Mn = 2.38(6) μ_B). The Er moments order ferromagnetically below 5.5 K in ErMn₂Ge₂ (μ_Mn = 6.81(31) μ_B). The moments are perpendicular to the c axis. The Mn sublattice remains antiferromagnetic down to 1.8 K (μ_Mn = 2.34(18) μ_B). The magnetic structure of TmMn₂Ge₂ is characterized by the propagation vector (0.0.1/2). the Tm moments lying in the basal plane. The ordering of the Tm moments yields a canting of the Mn moments (τ = 21(3)°); μ_Tm = 6.63(18) μ_B; μ_Mn = 2.28(27) μ_B). The antiferromagnetic structure of LuMn₂Ge₂ has been determined (μ_Mn = 2.32(14) μ_B). The evolution of the magnetic properties of the heavy rare earth compounds RMn₂Ge₂ is discussed.     

On the synthesis and characterization of a new manganite type CMR material: La0.7Hg0.3MnO3

A new manganite type CMR material, La_0.7Hg_0.3MnO₃ has been successfully synthesized and has been found to exhibit magnetoresistance (≈9%) at low fields (≈1.5 kG). The synthesis has been carried out through a solid state reaction route consisting of the formation of La_0.7MnO₃ followed by diffusion of Hg leading to La_0.7Hg_0.3MnO₃. The as grown samples are polycrystalline and correspond to an orthorhombic unit cell with the lattice parameters; a=5.5183 Å, b=5.6383 Å and c=7.5368 Å. The typical grain size as revealed by scanning electron microscopy is in the range of 0.5–2 μm. The ρ–T behaviour shows a peak at T_IM=227 K. The ρ–T behaviour above this temperature corresponds to that of an insulator and below this to that of a metal. The ρ–T behaviour remains unaltered when a magnetic field (H_dc=1.5 kG) is applied. However, with this magnetic field a drop in the resistivity is observed up to 77 K. At room temperature the magnetoresistance ratio (MRR) is too small but it steadily increases as the temperature is decreased. Thus, MRRs at 227.13 and 77 K are 3.41 and 9.05%, respectively, in an applied field of H_dc=1.5 kG. At a given temperature the variation in MRR with field H_dc is rapid at lower field values (H_dc<1.2 kG) and scales linearly for higher field values (H_dc>1.2 kG). It may be mentioned that the present work on the synthesis and magnetoresistance behaviour of La_0.7Hg_0.3MnO₃, is the first of its type.     

Electronic state of κ-(BMDT-TTF)2Au(CN)2 studied by NMR and thermoelectric measurements

The electronic state of κ-(BMDT-TTF)₂Au(CN)₂ was investigated by measurements of H-NMR¹-NMR and thermoelectric power. In H-NMR,¹-NMR, the profile of the nuclear relaxation curve changed drastically at 75 K from exponential time-dependence to non-single exponential one reflecting inhomogeneous relaxation. This change evidences a transition in the electronic state at this temperature. However, the longest T₁ below the transition temperature behaves like a Korringa law, suggesting that the low-temperature state is another metallic state. The thermoelectric power shows a strong anomaly around 75 K, being consistent with the NMR results. Above that, the temperature dependence is similar to that found in the κ phase of BEDT-TTF compounds, suggesting a similar Fermi-surface topology. These results suggest that the transition at 75 K is not a conventional metal-to-insulator transition but a metal-to-metal transition.     

Influence of HDDR treatment on magnetic properties of TbFe2- and DyFe2-based intermetallics

Magnetic properties of TbFe₂, DyFe₂, Tb(Fe_0.8M_0.2)₂ and Dy(Fe_0.8M_0.2)₂ with M=Co, Al, Si, Ga alloys affected by the Hydrogenation-Decomposition-Desorption-Recombination processing have been studied. After hydrogen treatment the coercive force H_c grows sharply, so HDDR-powders can be used as isotropic permanent magnets with the energy product up to 26 MG Oe at T=77 K. In Dy(Fe_0.8Al_0.2)₂ at T<10 K the stepwise magnetic reversal has been observed. The HDDR-treatment increases the critical field of magnetic reversal from 11 up to 18.4 kOe at 4.2 K. It is shown that the effect of the stepwise magnetic reversal is caused by a heat release in a sample during an avalanche motion of narrow domain walls.     

Effect of anion ordering in the superconducting phase of (TMTSF)2ClO4

The anion-ordering effect for the superconducting phase of (TMTSF)₂ClO₄ was investigated by both measurements of static magnetization and resistance for various cooling rate through the ClO₄⁻ anion ordering temperature (24K). With increasing cooling rate, the temperature of superconducting transition, T_c, determined through the measurements of Meissner effect continuously decreases. The resistance along the a-axis monotonically decreases as deceasing temperature below the cooling rate of 0.2K/sec and the residual resistance caused by imperfectness of anion ordering increases as increasing cooling rate. The decrease of T_c is attributed to the pair breaking effect of nonmagnetic scattering due to the suppression of anion ordering.     

Structure of Y3TaNi6+xAl26: a filled-up substitution variant of the BaHg11 type

The crystal structure of Y₃TaNi_6+xAl₂₆ (refined composition Y₄TaNi_6+[7]Al_20+[6]) was determined by single-crystal X-ray diffraction (λ(Mo K) −0.71073 A. μ −17.827 mm¹, F(000) = 700, T = 293 K, wR = 0.015 for [8] unique reflections). This new quaternary aluminide crystallizes with a cubic structure. Pearson code cP49-12.85, (221) Pm-3m-ji'gdba, a = 8.3600(1) Å. V = 584.28(2) Å, Z = 1, M₁ = 1510.25, D_x = 4.292 mg mm¹. The structure of Y_xTaNi_6+xAl₂₆ is filled-up substitution variant of the BaHg₁₁ structure type with one additional atom site, partly occupied (around 15%) by Ni atoms, located at the centre of a cube formed by Al atoms. Distinct atom coordinates were refined for Ni and Al atoms on a site for which mixed occupation (approximately 50% Ni/50% Al) was found. The Ta atoms centre regular Al atom cuboctahedra, and the Y atoms 20-vertex polyhedra, formed by Al and Ni atoms, similar to those observed in CeMn₄Al₈ and YbFe₂Al₁₀.     

Freezeout of the electrical resistivity of (BEDT-TTF)2l3 below 20 K

At temperatures above 20K, the resistivity of ET is given by: ρab ρ0 + BT². Below 20K, we found that the resisitivity falls significantly below ρ₀. Resistance ratios ρ(300K)/ρ(2K) exceed several thousand and sometimes even exceed 10⁴. The freezeout at temperatures just below the phonon frequency is a strong indication that the resisitivity above 20K is due to scattering by phonons. The T² law is due to a very strong momentum-dependence of the velocity v(k), with a sharp peak at the Fermi level. This peak causes the residual resisitivity due to scattering by defects to become temperature-dependent and to diminish greatly at very low temperatures. These results resolve the mystery of the highly-anomalous behavior of the resistivity of organic metals, suggested by Heeger and Garito in 1973. We present an ab-initio theory accounting for the sharp velocity peak. The theory considers a low-density electron-gas imbedded in a highly-dispersive medium, ε₀/ε∞ 20. This theory applies also to high-T_c cuprates, as well as to some other systems possessing a high dispersion.     

Structure and anelasticity of Fe3Ge alloy

The structure and anelastic properties of Fe-27 at.%Ge alloy are studied. Long-term annealing of the as-cast alloy at 1273 K leads to homogenising and several transformations take place below 873 K. These low temperature transitions are studied by several methods: X-ray diffraction, calorimetry, vibrating-sample magnetometry and internal friction, and are related to magnetic transitions in the different phases. A high stability of the hexagonal (D0₁₉) phase at room temperature is recorded. The hexagonal β (B8₁) phase is also detected in the alloy at room temperature, while the presence of the ′ and phases is doubtful. A broad internal friction relaxation peak with the relaxation strength of Δ = 0.0036, the activation energy of about 1.78 eV and the preexponential relaxation time of τ₀ = 2 × 10⁻¹⁷ s was discovered and classified as the Zener peak in both the and β phases.     

Crystallographic and magnetic properties of HfFe6Ge6-type REFe6Sn4Ge2 compounds (RE = Y, Gd–Er)

The REFe₆Sn₄Ge₂ (RE = Y, Gd–Er) compounds have been synthesized and studied by powder X-ray diffraction and magnetisation measurements. These compounds crystallize in the hexagonal HfFe₆Ge₆ structure although the parent ternary compounds REFe₆X₆ (X = Ge, Sn) display more complicated orthorhombic crystal structure. This evolution is discussed and interpreted on the basis of the relaxation of some RE–X contacts in the quaternary compounds. The iron sublattice order antiferromagnetically above room temperature (554 ≤ T_N ≤ 560 K) while the paramagnetic RE compounds display a second transition at low temperature (7.3 ≤ T_t ≤ 42.7 K). The magnetisation versus field curves display a metamagnetic behaviour at 4.2 K. The corresponding value of the magnetisation suggests a non-collinear ordering of the RE sublattice.