-NMRC-NMR study on κ-(BEDT-TTF)2X

The electronic state of the κ phase family of BEDT-TTF compounds, κ-(BEDT-TTF)₂X [X = Cu(NCS)₂, Cu[N(CN)₂]Br and Cu[N(CN)₂]Cl], has been investigated by -NMR¹³C-NMR as well as H-NMR.¹-NMR. The ¹³C isotope was substituted selectively into the central double bonded carbon sites of BEDT-TTF. The ¹³C nuclear spin-lattice relaxation rate and line spectra have been measured for the Cu(NCS)₂ and Cu[N(CN)₂]Br salts. The Knight shift evaluated from the the width of the spectra of the powdered samples is temperature-insensitive and scaled to the uniform susceptibility. However, the relaxation rate, T₁⁻¹, does not obey the Korringa law but exhibits anomalous temperature dependence with a peak formation around 50 K. The absolute value of T₁⁻¹ is in excess of the Korringa relation. These behaviors are considered to come from strong aniferromagnetic spin fluctuation with finite wave vector, which is suppressed below 50 K. On the other hand, the H¹ NMR and magnetization measurements gave an unambiguous evidence for antiferromagnetic transition with spin canting at 25 K in κ-(BEDT-TTF)₂Cu[N(CN)₂]Cl. The κ phase family, situated in the vicinity of metal-insulator and magnetic transitions, is characterized by strong electron correlations.     

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.     

Low-frequency Raman spectra in κ-(BEDT-TTF)2Cu(NCS)2 and κ-(BEDT-TTF)2Cu[N(CN)2]Br

The ordering of the terminal ethylene groups of the BEDT-TTF molecule, i.e., the staggered or eclipsed conformation, in κ-(BEDT-TTF)₂Cu(NCS)₂ and κ-(BEDT-TTF)₂Cu[N(CN)₂]Br was studied between 18 and 295 K by Raman scattering. The low-frequency spectra of these compounds are similar to each other. Broad peaks at about 55 cm⁻¹ extremely broadened and their intensities became weak with decreasing temperature. The anomalous behaviour was interpreted in terms of critical dynamics of the pseudospin-phonon coupled system, where the spin states represent the conformations of the terminal ethylene groups. It was found that the ordered state is formed around the superconducting critical temperature.     

New organic superconductor, λ-(BETS)2GaCl4 and metal-insulator transition of BETS conductor with magnetic anions (BETS = bis(ethlenedithio)tetraselenafulvalene)

Crystal structutres and electronic properties of BETS compounds with tetrahedral anions, MX₄ (BETS = bis(ethylenedithio)tetraselenafulvalene; M = Ga, Fe, In; X = Cl, Br) were examined. The salts can be classified into two types (κ, λ). All the compounds with so-called κ-type molecular arrangements retained metallic behavior down to 4 K. λ-(BETS)₂GaCl₄ demonstrated a a superconducting transition around 8 K. Whereas λ-(BETS)₂FeCl₄ with the isomorphic λ-type structure exhibited a sharp metal-insulator transition at 8 K. Low-temperature X-ray studies showed that the structural difference between λ-GaCl₄ and λ-FeCl₄ salts is very small. The structure analysis of κ-(BETS)₂FeCl₄ at 18 K indicated that the FeCl₄ anion has approximately C_3v symmetry. The ESR measurements revealed the high-spin state of Fe³⁺ in λ-FeCl₄ salt and and low-spin state in κ-FeCl₄ salt. ESR intensities of κ- and λ-FeCl₄ salts suggested antiferromagnetic interaction between Fe³⁺ ions.     

Structural and physical properties of θ -(BEDT-TTF)2Cu2(CN)[N(CN)2]2

The title compound was obtained during the crystal growth of two κ-type superconductors, κ-(BED-TTF)₂Cu(CN)[N(CN)₂] and κ′ -(BEDT-TTF)₂ Cu₂(CN)₃. This compound is a magnetic semiconductor, whose spin susceptibility is expressed by quadratic-layer Heisenberg antiferromagnet model (QLAF) between 32.6K and 220K. At 220K, a semiconductor-semiconductor transition into a high conductivity phase was observed by transport measurements.     

Anisotropic superconductivity in κ-(BEDT-TTF)2Cu(NCS)2

To determine the symmetry of the superconducting energy gap, we measured the ac-susceptibility of κ-(BEDT-TTF)₂Cu(NCS)₂. With shielding geomagnetic field, we confirmed the power law temperature dependence of magnetic-field penetration depth. This result indicates an anisotropic superconductivity of gapless nature in this system. The penetration depth extrapolated to 0K for parallel field was found to be 0.22mm of macroscopic size.     

Studies of the microwave resistivity of κ-(BEDT-TTF)2CU[N(CN)2]Cl

We have performed temperature dependent measurements of the surface impedance of κ-(BEDT-TTF)₂Cu[N(CN)₂]Cl in both directions, parallel and perpendicular to the highly conducting planes. In both orientations the microwave resistivity basically follows the dc behavior for T > 15 K: the crystal undergoes a broad metal-insulator transition at around 50 K. While there are no indications of superconductivity at ambient pressure found in the dc resistiviy and by ac susceptibility, the microwave resistivity clearly drops at 13 K and again at 5 K, indicating two supercondcuting phases. When pressure is applied by embedding it in grease, only the well known superconducting phase transition at T_c= 13 K remains.     

Spectroscopy, magnetism and non-radiative processes in heteronuclear Cu:Pr squarate; [Pr2Cu(C4O4)4(H2O)16]·2H2O

In the present work, the spectroscopic and magnetic properties of heteronuclear Cu:Pr squarate are reported. Single crystals of [Pr₂Cu(C₄O₄)₄(H₂O)₁₆]·2H₂O were obtained by reaction of squaric acid, praseodymium chloride and copper chloride in water solution according to the procedure described earlier. The crystals of title compound are isomorphic with [La₂Cu(C₄O₄)₄(H₂O)₁₆]·2H₂O crystal, where squarate anions participate as bridging ligands between metal ions.

The UV region of absorption spectra of the title compound is dominated by C–T band of Cu(II), f–d transition of Pr(III) and internal π–π*(A_1g→E_u) and π–π*(A_1g→E_g) ligand transitions. In visible and IR regions, t_2g–e_g of copper Cu(II) as well as ³H₄→³P_J, ¹D₂, ¹G₄, ³F_J, ³H₆ Pr(III) transitions at 293 and 4 K were recorded. At low temperature splitting given by Jahn–Teller effect can be observed. Significant anisotropy of d–d transitions intensities confirms well the Jahn–Teller effect, too. Unexpectedly high intensity of ³H₄→¹G₄ transition is probably due to the intensity borrowing from the Cu (II) d–d transition.

The ³P₀ and ¹D₂ emission of Pr(III) in the [Pr₂Cu(C₄O₄)₄(H₂O)₁₆]·2H₂O crystals is quenched even at 77 K. Whereas emission of appropriate polynuclear europium squarate was detected. The pathways of excited state quenching by e_g levels of Cu(II), multhiphonon relaxation and concentration quenching can be considered in the system under studies. Magnetic susceptibility measurements were carried out in 300–1.7 K temperature range and are discussed in relation to the structure.

Effect of the polymeric structure on spectroscopic behaviour is presented. Selectivity of polymeric europium squarate in vitro test for different tumor cells is shown.     

Observation of the coherence peak of H-NMR-NMR relaxation rate in the superconducting state of (MDT-TTF)2Aul2

We have investigated the superconductivity in the organic conductor, (MDT-TTF)₂Aul₂ (T_c = 4.1 K) from the temperature dependence of H¹ nuclear relaxation rate, 1/T₁, measured by the field cycling method. The nuclear relaxation above 3 K is well characterized by a single relaxation time for nearly two decades. In the normal state, the standard Korringa relation holds. 1/T₁ just below T_c showed a clear enhancement, followed by a falling-oil at lower temperatures. This behavior is just as predicted in the BCS theory (coherence peak) and implies that this material is a conventional superconductor with an isotropic gap. The decay of the nuclear magnetization below 3 K appeared non-sinle exponential. This is explained as the effect of trapped vortices.     

Revisiting the superconducting phase diagram of(TMTSF)2ClO4

We present results of a search for evidence of reentrance at high magnetic field of the superconducting state in the quasi-one dimensional organic superconductor (TMTSF)₂ClO₄, for a specific orientation of the field at low temperature. This search was motivated by the theoretical work of Lebed' and of Dupuis and Montambaux, and in part by that of Te anovic and coworkers, who predict novel magnetic field-temperature phase diagrams in quasi-1-D and quasi-2-D superconductors in a strong magnetic field. In the quasi-1-D case, a field directed both normal to the chains and along the layers is proposed to induce exotic oscillations of the critical temperature at high field, while for the 2-D case, quantum effects associated with Landau quantization lead to similar predictions. We have investigated the superconducting transition in a quasi-1-D candidate material, (TMTSF)₂ClO₄, to 0.06 K with field oriented along the b′-axis (± 0.02°). From mostly constant field temperature sweeps, we find positive curvature in H_c2(T) below about T_c/2. In addition, a resistivity decrease is observed to persist to 7 T, the highest field employed, with a slightly positive slope ∂H/∂T above 5T. The possibility of these features being representative of high field superconductivity is discussed.     

Thermoelectric properties of A-site doped perovskites (Sr0.6Ba0.4)1−xMxPbO3 (M = La, K)

La- and K-doped perovskite-type ceramics, (Sr_0.6Ba_0.4)_1−xLa_xPbO₃ with x = 0.0−0.1 and (Sr_0.6Ba_0.4)_1−xK_xPbO₃ with x = 0.00−0.15, were prepared to modify thermoelectric properties of semi-metallic Sr_0.6Ba_0.4PbO₃ via the doping of electrons and holes, respectively. The electrical conductivity σ and Seebeck coefficient S for the ceramics were measured at temperatures of 373–1073 K in air. With the La doping, electron carriers were successively doped and the material changed from a semi-metal for the undoped Sr_0.6Ba_0.4PbO₃ to a metal for the (Sr_0.6Ba_0.4)_0.9La_0.1PbO₃. With the K doping, the thermoelectric properties were essentially unchanged probably due to the carrier compensation effect by the generation of oxygen deficiencies. The thermoelectric power factor S²σ was maximized to a value of 3.1 × 10⁻⁴ Wm⁻¹ K⁻² at 773 K for the undoped Sr_0.6Ba_0.4PbO₃ ceramic.     

Optical study of praseodymium dicarboxylate [Pr(H2O)]2[O2C(CH2)3CO2]3.4H2O

Praseodymium dicarboxylate, [Pr(H₂O)]₂[O₂C(CH₂)₃CO₂]₃.4H₂O]–glutarate, Pr[glut], is synthesized by hydrothermal techniques. The title compound crystallizes in the monoclinic space group C2/c (No. 15). The rare earth cation is coordinated by nine oxygen atoms, eight oxygen atoms from the carboxylate groups and one from the water molecule. The local symmetry of Pr site is low, C_s. The absorption spectra of Pr[glut] are recorded from the visible to the far IR domain at 300, 77 and 9 K. Under various Ar⁺ laser excitations no emission is detected from ³P₀ and ¹D₂ excited levels of Pr³⁺ ion. In the low temperature absorption spectra only one electronic line is recorded for ³H₄→³P₀ transition. It confirms a unique local environment for the rare earth ion in Pr[glut]. The utility of the ‘barycenter curves’ in the attribution of electronic lines is demonstrated. Energy level scheme of 36 Stark components is deduced from the absorption spectra. The parametric calculation was performed on the whole 4f² (Pr³⁺) configuration with the starting set of crystal field parameters obtained previously for the Eu³⁺ ion in the isostructural compound. Eight free ion and nine phenomenological crystal field parameters in C_2v symmetry reproduce quite well several electronic levels of Pr³⁺ ion experimentally observed in Pr[glut]. A good r.m.s. standard deviation of 14.8 cm⁻¹ is obtained.     

STM spectroscopy in superconducting phase of (BEDT-TTF)2Cu(NCS)2

The superconducting phase of (BEDT-TTF)₂Cu(NCS)₂ was investigated with use of the scanning tunneling microscope (STM).The tunneling conductance was measured at the b-c surface of a single crystal. The differential conductance shows a clear superconducting gap structure below the transition temperature and its value is reduced to almost zero around zero bias voltage at 1.9 K. The conductance curve is not well fitted to the BCS one but suggests the gap anisotropy. However, the gap with line nodes, expected from the simple d-wave symmetry, is excluded. Possible models for the superconducting gap are as follows; (1) the gap with point nodes or (2) the anisotropic gap with a minimum value Δ_min1 mev. It is understood that the superconducting gap is highly anisotropic in k-space.     

Static magnetization study of κ-(BEDT-TTF)2Cu[N(CN)2]Cl under high pressure: Part 2. Low magnetic fields

Low-field measurements have revealed the presence of paramagnetic contribution to the field-cooled magnetization of single crystal of title salt at pressures as high as 1 kbar and 1.5 kbar. This magnetization appears if field is oriented normal to the conducting ac plane, at temperature just below the onset of bulk SC transition, and saturates at fields of a few Oe. Depending on thermal history, it can even dominate over the diamagnetic Meissner signal. This behavior is similar to recently discovered (due to study of HTSC oxides) intensively studied but still remaining not understood Wohlleben effect (or paramagnetic Meissner effect). For the κ-(BEDT-TTF)₂Cu[N(CN)₂]Cl we propose that the coexisting magnetic phase might be a cause for the phenomenon.     

Thermal expansion studies on Th(MoO4)2, Na2Th(MoO4)3 and Na4Th(MoO4)4

Thermal expansion behavior of Th(MoO₄)₂, Na₂Th(MoO₄)₃ and Na₄Th(MoO₄)₄ was studied under vacuum in the temperature range of 298–1123 K by high temperature X-ray diffractometer. Th(MoO₄)₂ was synthesized by reacting ThO₂ with 2 mol of MoO₃, at 1073 K in air and Na₂Th(MoO₄)₃ and Na₄Th(MoO₄)₄ were prepared by reacting Th(MoO₄)₂ with 1 and 2 mol of Na₂MoO₄, respectively at 873 K in air. The XRD data of Th(MoO₄)₂ was indexed on orthorhombic system where as XRD data of Na₂Th(MoO₄)₃ and Na₄Th(MoO₄)₄ were indexed on tetragonal system. The lattice parameters and cell volume of all the three compounds, fit into polynomial expression with respect to temperature, showed positive thermal expansion (PTE) up to 1123 K. The average value of thermal expansion coefficients for Th(MoO₄)₂, Na₂Th(MoO₄)₃ and Na₄Th(MoO₄)₄ were determined from the high temperature data.     

AC ionic conductivity investigations on the CsK(SO4)·Te(OH)6 material

Single crystals of CsK(SO₄)·Te(OH)₆ are obtained by slow evaporation at room temperature. Differential scanning calorimetry (DSC) measurements have been carried out in the temperature range 350–650 K. It reveals three anomalies in the studied temperature range. Complex impedance measurements are performed on this material as a function of both temperature and frequency. The electric conduction and dielectric relaxation have been studied. The temperature dependence of DC conductivity indicates that the sample became an ionic conductor at high temperature. The frequency dependence of conductivity follows the Jonscher’s dynamic law. The high level of conductivity at high temperature is explained by the dynamical disordering of protons between the neighboring SO₄ groups. This behavior indicates the presence of superprotonic phase transition in CsK(SO₄)·Te(OH)₆.     

β-phase transformation and thermoelectric power in FeSi2 and Fe2Si5 based alloys containing small amounts of Cu

The effects of Cu addition on the β phase formation rate and the thermoelectric power in various FeSi₂ and Fe₂Si₅ based alloys was examined. The peritectoid reaction (a+→β) in FeSi₂ alloys was initially enhanced by the addition of Cu but it became slower for longer annealing times. The retained metallic ε was harmful for the thermoelectric power. The inherent thermoelectric properties of (FeSi₂)_99−XMn₁Cu_X (X=0–1.O at.%), (FeSi₂)_99−X Co₁Cu_X (X=0–1.0 at.%) alloys were attained after the elimination of ε. In the case of eutectoid reaction (→β+Si). Differential thermal analysis, X-ray diffraction and microscopic observation clearly confirmed that the eutectoid reaction rate was drastically enhanced by the addition of a small amount of Cu and its rate decreased with decreasing Cu content. The rate also depends on the annealing temperature and reached a maximum at about 1073 K for most alloys. The addition of only 0.1 at.% Cu was still very effective even in Mn or Co doped alloys. The thermoelectric power of these alloys increased very quickly with annealing time. Their final values decreased with Cu content and saturated at 0.2 at.% Cu. The value of the 0.1 at.% Cu added alloy was higher than that of both the conventional p- and a-type FeSi₂ based alloys. These results suggest that the Fe₂Si₅ alloys with a small amount of Cu may be attractive as new thermoelectric materials.     

Fabrication and electrical and thermal properties of Ti2InC, Hf2InC and (Ti,Hf)2InC

In this paper we report on the characterization of predominantly single phase, fully dense Ti₂InC (Ti_1.96InC_1.15), Hf₂InC (Hf_1.94InC_1.26) and (Ti,Hf)₂InC ((Ti_0.47,Hf_0.56)₂InC_1.26) samples produced by reactive hot isostatic pressing of the elemental powders. The a and c lattice parameters in nm, were, respectively: 0.3134; 1.4077 for Ti₂InC; 0.322, 1.443 for (Ti,Hf)₂InC; and 0.331 and 1.472 for Hf₂InC. The heat capacities, thermal expansion coefficients, thermal and electrical conductivities were measured as a function of temperature. These ternaries are good electrical conductors with a resistivity that increases linearly with increasing temperatures. At 0.28 μΩ m, the room temperature resistivity of (Ti,Hf)₂InC is higher than the end members (0.2 μΩ m), indicating a solid solution scattering effect. In the 300 to 1273 K temperature range the thermal expansion coefficients are: 7.6×10⁻⁶ K⁻¹ for Hf₂InC, 9.5×10⁻⁶ K⁻¹ for Ti₂InC, and 8.6×10⁻⁶ K⁻¹ for (Ti,Hf)₂InC. They are all good conductors of heat (20 to 26 W/m K) with the electronic component of conductivity dominating at all temperatures. Extended exposure of Ti₂InC to vacuum (10⁻⁴ atm) at 800 °C, results in the selective sublimation of In, and the conversion of Ti₂InC to TiC_x.     

Thermal expansion in single crystals of κ-(BEDT-TTF)2Cu(NCS)2 in magnetic fields up to 6 Tesla

The linear thermal expansion coefficients _i (i = a^*,b,c) of single crystals of the organic superconductor κ-(BEDT-TTF)₂Cu(NCS)₂ (T_c = 9.15K) have been measured in the temperature range from 2 to 300K, by means of capacitive dilatometry. At T_c, we resolved second order jumps Δ_i, which disappear in magnetic fields above H_c2 From the initial slopes dH_c2/dT parallel and perpendicular to the superconducting layers the Ginzburg-Landau coherence lengths ξ(O) = 67 å and ξ(O) = 7.3 å were determined. Via Ehrenfest's relation it is possible to deduce the uniaxial stress dependence of T_c which was found to be negative in the a^*-direction ( ∂ T_c/ ∂ σ_a* = −3.20 K/kbar), positive in the c-direction ( ∂ T_c/ ∂ σ_c = 1.46 K/kbar) and zero in the b-direction.     

Magnetic field dependence of the cyclotron effective mass in the organic superconductor κ-(BEDT-TTF)2I3

κ-(BEDT-TTF)₂I₃ is an electronically extreme two-dimensional (2d) organic metal with a superconducting transition at around 4K. In magnetic fields above 12T the effective mass, as obtained from the temperature dependence of the amplitudes of Shubnikov-de Haas (SdH) oscillations, is magnetic field dependent as long as the magnetic field is arranged perpendicular to the conducting planes. In contrast to this by turning the magnetic field 27° or even only by 9° the observed effective mass (as obtained from SdH- and de Haas-van Alphen (dHvA)-measurements) is field independent. We suppose that the occurrence of anyons at temperatures below 1K and in fields above 12T is the reason for the observed field dependence of the effective mass.