V. N. Zverev

Effect of electrocrystallization medium on quality, structural features, and conducting properties of single crystals of the (BEDT-TTF)4AI[FeIII(C2O4)3]·G family

A modified procedure for the electrocrystallization of organic conductors with paramagnetic anions of the (BEDT-TTF)4AI[MIII(C2O4)3]·G family has been proposed. It is found that single crystals of different phases of the family can be prepared if the electrocrystallization medium is represented by the mixture of 1,2,4-trichlorobenzene (or 1,3-dibromobenzene), 96% ethanol and different solvents (G), only the latter being included into the composition of the resulting salts as neutral guest molecules (G = benzonitrile, fluorobenzene, chlorobenzene, 1,2-dichlorobenzene, bromobenzene, nitrobenzene). Using this approach, a number of known and new BEDT-TTF salts with the tris(oxalato)ferrate anion have been synthesized. Among them, there are superconducting crystals of monoclinic β′′-series with different guest solvents (G) and their mixtures. For the first time, crystals of a triclinic phase (G = 1,2-dibromobenzene), with alternating α- and ‘pseudo-κ’ BEDT-TTF layers and metallic behaviour down to 1.5 K, were obtained. Additionally, monoclinic crystals having another stoichiometry and α-type donor packing were prepared.

Experimental studies of Coulomb drag between ballistic quantum wires

The Coulomb drag between two spatially separated one-dimensional (1D) electron systems in lithographically fabricated 2??m long quantum wires is studied experimentally. The drag voltage VD shows peaks as a function of a gate voltage which shifts the position of the Fermi level relative to the 1D subbands. The maximum in VD and the drag resistance RD occurs when the 1D subbands of the wires are aligned and the Fermi wave vector is small. The drag resistance is found to decrease exponentially with interwire separation. In the temperature region 0.2?K?T?1?K, RD decreases with increasing temperature in a power-law fashion RDTx with x ranging from -0.6 to -0.77 depending on the gate voltage. We interpret our data in terms of the Tomonaga-Luttinger liquid theory.

Coexistence of two donor packing motifs in the stable molecular metal α-‘pseudo-κ’-(BEDT-TTF)4(H3O)[Fe(C2O4)3]·C6H4Br2

The crystal and electronic structure of a new radical cation salt α-‘pseudo-κ’-(BEDT-TTF)4H3O[Fe(C2O4)3]·C6H4Br2 have been studied. The new triclinic crystals contain two conducting organic layers which are characterized by different BEDT-TTF packing motifs: a ‘pseudo-κ’-layer which is composed of charged dimers and neutral monomers of BEDT-TTF orthogonal to each other and an α-layer which consists of inclined, uniformly charged BEDT-TTF stacks. According to electronic band structure calculations, the ‘pseudo-κ’ layer has a large gap between the HOMO bands at the Fermi level and should be associated with an activated conductivity. In contrast, the α-layer is a strongly two-dimensional electronic system with uniform intermolecular interactions. The absence of any nesting in the Fermi surface of the α-layer suggests that this salt should be a stable metal down to low temperatures. Metallic properties have been observed in the crystals in the 300–0.4 K temperature range. Besides, well pronounced Shubnikov–de Haas oscillations of the magnetoresistance have been revealed at B > 8 T. The salt investigated is a new phase in the (BEDT-TTF)4AI[MIII(C2O4)3]G family of organic molecular conductors with paramagnetic anions and different guest solvent molecules G in the anion layer. Structural features of the new α-‘pseudo-κ’-crystals and other known phases of the family (β″, ‘pseudo-κ’ and α-β″) have been compared.

Structure and magnetotransport properties of the new quasi-two-dimensional molecular metal β″-(BEDT-TTF)4H3O[Fe(C2O4)3] · C6H4Cl2

The β″-(BEDT-TTF)4AI[MIII(C2O4)3] · G(AI=NH4+, H3O+, K+, Rb+; MIII=Fe, Cr; G = “guest” solvent molecule) family of layered molecular conductors with magnetic metal oxalate anions exhibits a pronounced dependence of the conducting properties on the type of neutral solvent molecules introduced into the complex anion layer. A new organic dichlorobenzene (C6H4Cl2)-containing conductor of this family, namely, β″-(BEDT-TTF)4H3O[Fe(C2O4)3] · C6H4Cl2, is synthesized. The structure of the synthesized single crystals studied by X-ray diffraction is characterized by the following parameters: a = 10.421(1) Å, b= 19.991(2) Å, c= 35.441(3) Å, β = 92.87(1)°, V= 7374(1) Å3, space groupC2/c, and Z = 4. In the temperature range 0.5&2-300 K, the conductivity of the crystals is metallic without changing into a superconducting state. The magnetotransport properties of the crystals are examined in magnetic fields up to 17 T at T = 0.5 K. In fields higher than 10 T, Shubnikov-de Haas oscillations are detected, and the Fourier spectrum of these oscillations contains two frequencies with maximum amplitudes of about 80 and 375 T. The experimental results are compared with the related data obtained for other phases of this family. The possible structural mechanisms of the effect of a guest solvent molecule on the transport properties of the β″-(BEDT-TTF)4AI[MIII(C2O4)3] · G crystals are analyzed.

Structural phase transition in the β′′-(BEDT-TTF)4H3O[Fe(C2O4)3]·G crystals (where G is a guest solvent molecule)

A structural phase transition from monoclinic C2/c to triclinic P symmetry has been found by X-ray diffraction in a number of single crystals of the known family of organic metals and superconductors β′′-(BEDT-TTF)4H3O[Fe(C2O4)3]·G where G stands for halogenated benzene derivatives and their mixtures with benzonitrile. The transition occurs upon lowering the temperature at 180–230 K. Comparison of the crystal and electronic structure of the monoclinic and triclinic phases reveals details of the structural transformations in the (PhCl + PhCN)-containing superconducting β′′-crystal, as an example. It is shown that the transition concerns mainly the anion layer and has a weak influence on the structure of the BEDT-TTF layer and, consequently, on the conducting properties of the single crystals.

Structure and magnetotransport properties of the new quasi-two-dimensional molecular metal {beta} Double-Prime -(BEDT-TTF){sub 4}H{sub 3}O[Fe(C{sub 2}O{sub 4}){sub 3}] {center_dot} C{sub 6}H{sub 4}Cl{sub 2}

The β″-(BEDT-TTF)4AI[MIII(C2O4)3] · G(AI=NH4+, H3O+, K+, Rb+; MIII=Fe, Cr; G = “guest” solvent molecule) family of layered molecular conductors with magnetic metal oxalate anions exhibits a pronounced dependence of the conducting properties on the type of neutral solvent molecules introduced into the complex anion layer. A new organic dichlorobenzene (C6H4Cl2)-containing conductor of this family, namely, β″-(BEDT-TTF)4H3O[Fe(C2O4)3] · C6H4Cl2, is synthesized. The structure of the synthesized single crystals studied by X-ray diffraction is characterized by the following parameters: a = 10.421(1) Å, b= 19.991(2) Å, c= 35.441(3) Å, β = 92.87(1)°, V= 7374(1) Å3, space groupC2/c, and Z = 4. In the temperature range 0.5&2-300 K, the conductivity of the crystals is metallic without changing into a superconducting state. The magnetotransport properties of the crystals are examined in magnetic fields up to 17 T at T = 0.5 K. In fields higher than 10 T, Shubnikov-de Haas oscillations are detected, and the Fourier spectrum of these oscillations contains two frequencies with maximum amplitudes of about 80 and 375 T. The experimental results are compared with the related data obtained for other phases of this family. The possible structural mechanisms of the effect of a guest solvent molecule on the transport properties of the β″-(BEDT-TTF)4AI[MIII(C2O4)3] · G crystals are analyzed.

Coulomb drag between ballistic one-dimensional electron systems

The presence of pronounced electronic correlations in one-dimensional systems strongly enhances Coulomb coupling and is expected to result in distinctive features in the Coulomb drag between them that are absent in the drag between two-dimensional systems. In this review, we review recent Fermi and Luttinger liquid theories of Coulomb drag between ballistic one-dimensional electron systems, also known as quantum wires, in the absence of inter-wire tunnelling, to focus on these features and give a brief summary of the experimental work reported so far on one-dimensional drag. Both the Fermi liquid (FL) and the Luttinger liquid (LL) theory predict a maximum drag resistance RD when the one-dimensional subbands of the two quantum wires are aligned and the Fermi wave vector kF is small, and also an exponential decay of RD with increasing inter-wire separation, both features confirmed by experimental observations. A crucial difference between the two theoretical models emerges in the temperature dependence of the drag effect. Although the FL theory predicts a linear temperature dependence, the LL theory promises a rich and varied dependence on temperature depending on the relative magnitudes of the energy and length scales of the systems. At very low temperatures, the drag resistance may diverge due to the formation of locked charge density waves. At higher temperatures, it should show a power-law dependence on temperature, RD Tx, experimentally confirmed in a narrow temperature range, where x is determined by the Luttinger liquid parameters. The spin degree of freedom plays an important role in the LL theory in predicting the features of the drag effect and is crucial for the interpretation of experimental results. Substantial experimental and theoretical work remains to be done for a comprehensive understanding of one-dimensional Coulomb drag.

On the synthesis and the electric and magnetic properties of superconducting barium–niobium–oxide compounds

Abstract In the system Ba–Nb–O–N a superconducting ceramic material with T c =22 K was synthesized. The temperature dependencies of magnetic susceptibility and resistivity of these ceramics with different oxygen concentrations was studied. A large increase of T c was observed, from 12 K for samples prepared in O 2 up to 23 K prepared in air due to the variation of oxygen composition and Y doping. The XRD structure and Auger electron spectroscopy composition investigations indicate the formation of oxinitrides BaNbO x N y with a cubic structure having a lattice parameter a =4.32 A, which is apparently responsible for the highest T c observed. The incorporation of nitrogen into the crystal lattice results in an increase of T c and stabilization of the superconducting structure.

The Conducting Spin-Crossover Compound Combining Fe(II) Cation Complex with TCNQ in a Fractional Reduction State

The radical anion salt [Fe{HC(pz)3}2](TCNQ)3 demonstrates conductivity and spin-crossover (SCO) transition associated with Fe(II) complex cation subsystem. It was synthesized and structurally characterized at temperatures 100, 300, 400, and 450 K. The compound demonstrates unusual for 7,7,8,8,-tetracyanoquinodimethane (TCNQ)-based salts quasi-two-dimensional conductivity. Pronounced changes of the in-plane direct-current resistivity and intensity of the electron paramagnetic resonance (EPR) signal, originated from TCNQ subsystem, precede the SCO transition at the midpoint T* = 445 K. The boltzmannian growth of the total magnetic response and structural changes in the vicinity of T* uniquely show that half [Fe{HC(pz)3}2] cations exist in high-spin state. Robust broadening of the EPR signal triggered by the SCO transition is interpreted in terms of cross relaxation between the TCNQ and Fe(II) spin subsystems.

The first molecular superconductor based on BEDT-TTF radical cation salt with paramagnetic tris(oxalato)ruthenate anion

The first molecular superconductor based on BEDT-TTF radical cation salt with the paramagnetic tris(oxalato)ruthenate anion, β′′-(BEDT-TTF)4Kx(H3O)1−x[RuIII(ox)3]C6H5Br (x ∼ 0.8), is synthesized and its crystal and electronic structure as well as transport and magnetotransport properties are studied. The single crystals have monoclinic C2/c symmetry and β′′-packing of the conducting BEDT-TTF layer. The samples were found to be superconductors with Tc = 2.8–6.3 K (depending on the sample). Well pronounced Shubnikov–de Haas oscillations are observed in the field range 7–17 T. The Fourier spectrum of the oscillations mainly consists of two frequencies, which correspond to the Fermi surface cross sections 6.3% and 0.85% of the Brillouin zone. The first one with good accuracy corresponds to the calculated cross section 7.5%, the origin of the second one is discussed. Semiconducting orthorhombic phase ‘pseudo-κ’-(BEDT-TTF)4Kx(H3O)1−x[RuIII(ox)3]C6H5CN is also synthesized and studied.