Eva Rose

Comprehensive transport study of anisotropy and ordering phenomena in quasi-one-dimensional(TMTTF)2Xsalts (X=PF6,AsF6,SbF6,BF4,ClO4,ReO4)

The temperature dependent dc resistivity of the quasi-one-dimensional organic salts (TMTTF)2X ( X = PF6, AsF6, SbF6; BF4, ClO4, ReO4) has been measured in all three crystal directions in order to investigate anisotropy, localization effects, charge and anion ordering phenomena at low temperatures. For all compounds and directions we extract the transport mechanisms in different regimes. The data are thoroughly analyzed, related to structural properties, and extensively discussed in view of previous studies and latest theories. It becomes apparent that the anions have a severe influence on the physical properties of the TMTTF salts.

Electronic properties of Fabre charge-transfer salts under various temperature and pressure conditions

Using density functional theory, we determine parameters of tight-binding Hamiltonians for a variety of Fabre charge transfer salts, focusing, in particular, on the effects of temperature and pressure. Besides relying on previously published crystal structures, we experimentally determine two new sets of structures: (TMTTF)2SbF6 at different temperatures and (TMTTF)2PF6 under various hydrostatic pressures. We find that a few trends in the electronic behavior can be connected to the complex phase diagram shown by these materials. Decreasing temperature and increasing pressure cause the systems to become more two dimensional. We analyze the importance of correlations by considering an extended Hubbard model parameterized using Wannier orbital overlaps and show that while charge order is strongly activated by the intersite Coulomb interaction, the magnetic order is only weakly enhanced. Both orders are suppressed when the effective pressure is increased.

Pressure-dependent structural and electronic properties of quasi-one-dimensional (TMTTF)2PF6.

We have performed detailed x-ray investigations of the quasi-one-dimensional organic conductor (TMTTF)(2)PF(6) at room temperature and hydrostatic pressures up to 27 kbar. Based on the pressure-dependent crystal structure, the electronic band structure was calculated by density functional theory (DFT). Our systematic study provides important information on the coupling among the organic molecules but also to the anions. We discuss the consequences for the electronic properties and compare them with optical investigations under pressure. The increasing plasma frequency observed perpendicular to the stacks corresponds to a widening of the bands for the b-direction. Around 20 kbar a dimensional crossover occurs from a one-dimensional Mott insulator to a two-dimensional metal.

Pressure dependence of the metal-insulator transition in κ-(BEDT-TTF)2Hg(SCN)2Cl: optical and transport studies

The two-dimensional organic conductor κ-(BEDT-TTF)2-Hg(SCN)2Cl exhibits a pronounced metal-insulator transition at [Formula: see text] K. From the splitting of the molecular vibrations, the phase transition can be unambiguously assigned to charge-ordering with [Formula: see text]. We have investigated the pressure evolution of this behavior by temperature-dependent electrical transport measurements and optical investigations applying hydrostatic pressure up to 12 kbar. The data reveal a mean-field like down-shift of [Formula: see text] with a critical pressure of [Formula: see text] kbar and a metallic state above the suppression of the charge-ordered state; no traces of superconductivity could be identified down to T  =  1.5 K. As the charge order [Formula: see text] sets in abruptly with decreasing temperature, its size remains unaffected by pressure. However, the fraction of charge imbalanced molecules decreases until it is completely absent above 1.6 kbar.

Synthesis and Characterization of Superconducting Ca1−xNaxFFeAs

A representative of the fluoride-containing iron pnictide high-temperature superconductors, namely CaFFeAs, was doped with sodium up to the composition Ca0.86Na0.14FFeAs for the first time. Single crystals with an edge length in the range of 0.1 – 2.0 mm were obtained via solid-state and flux syntheses, respectively. The composition of the crystals was verified by means of single crystal X-ray diffractometry and energy dispersive X-ray spectroscopy (EDX). Measurements of the electrical resistivity, as well as the magnetization on a crystal of Ca0.89Na0.11FFeAs both show a transition to the superconducting state on cooling to 34.5 K. Investigations of the upper critical fields reveal an anisotropy ratio of about five. The lattice parameters and molar volumes increase with rising sodium content. This effect is clearly observable for the c-axis and the volume, whereas the increase of the a-axis is rather minor.

The metal-insulator transition in the organic conductor β″-(BEDT-TTF)2Hg(SCN)2Cl

We explore the nature of the metal-insulator transition in the two-dimensional organic compound β″-(BEDT-TTF)2Hg(SCN)2Cl by x-ray, electrical transport, ESR, Raman, and infrared investigations. Magnetic and vibrational spectroscopy concurrently reveal a gradual dimerization along the stacking direction (a-b), setting in already at the crossover temperature of 150 K from the metallic to the insulating state. A spin gap of Δσ=47 meV is extracted. From the activated resistivity behavior below T = 55 K, a charge gap of Δρ=60 meV is derived. At TCO = 72 K, the C=C vibrational modes reveal the development of a charge-ordered state with a charge disproportionation of 2δρ=0.34e. In addition to a slight structural dimerization, charge-order causes stripes most likely perpendicular to the stacks.

Superconductivity at Tc = 36.5 K in Na-Substituted SrFe2As2 Single Crystals

We report superconductivity, which is achieved in Sr1-xNaxFe2As2 at Tc = 36.5 K for x = 0.47. Single crystals of Na-substituted SrFe2As2 up to 2 × 2 mm2 were grown from NaF flux. The superconducting transition temperature with a narrow width less than 2 K, confirmed by both magnetic and resistivity measurements, indicates the high quality of our samples. The upper critical fields (0) = 87 T and (0) = 217.5 T are extremely high. The anisotropy ratio γ ~ 2.5 is close to that of other hole-or electron-doped 122 phases but lower than γ ~ 4.3 – 5 of phases from the 1111 series and much lower than γ ~ 7 – 10 of superconducting cuprates. Magnetic relaxation measurements, which are consistent with the collective pinning model, reveal that the flux-creep rate in Na substituted 122 SrFe2As2 is smaller than in 1111 type iron pnictides.