John Anton Fendrich

Superconductivity up to 11.1 K in three solvated salts composed of [Ag (CF3)4]- and the organic electron-donor molecule bis (ethylenedithio) tetrathiafulvalene (ET)

Abstract Electrocrystallization of the electron-donor molecule BEDT-TTF [bis (ethylenedithio) tetrathiafulvalene] with the square planar organometallic anion [Ag(CF 3 ) 4 ] − in TCE (1,1,2-trichloroethane) results in the formation of at least four crystalline phases. A plate-like phase, κ L -(ET) 2 Ag(CF 3 ) 4 · TCE, is isostructural to κ L -(ET) 2 Cu(CF 3 ) 4 · TCE T c = 4.0 K with a slightly expanded unit cell, but a lower critical temperature of 2.6 K. Two superconducting needle phases of (ET) 2 Ag(CF 3 ) 4 · TCE have been observed, with critical temperatures of 9.4 and 11.1 K. The critical temperature of this latter phase is within 0.5 K of the current record for ambient pressure organic superconductors, 11.6 K in κ-(ET) 2 Cu[N(CN) 2 ]Br. A fourth phase, (ET) 2 Ag(CF 3 ) 4 , crystallizes simultaneously with the plates and needles, contains no solvent molecules, and does not superconduct down to 0.7 K.

Time-dependent Ginzburg-Landau simulations of vortex guidance by twin boundaries

Abstract The driven motion of vortices in the presence of a planar defect is simulated by means of the time-dependent Ginzburg-Landau equations. Guided motion of the vortices, both internal and external to the twin boundary, is found to occur over a range of driving forces. Experimental transport data on a single crystal of YBa 2 Cu 3 O 7 are consistent with guided motion.

The first organic cation-radical salt superconductor (Tc= 4 K) with an organometallic anion: superconductivity, synthesis and structure of κL-(BEDT-TTF)2Cu(CF3)4·TCE

Superconductivity at ambient pressure with onset Tc= 4.00 ± 0.05 K is reported in a unique organic cation-radical salt κL-(ET)2Cu(CF3)4·TCE, [ET = bis(ethylenedithio)tetrathiafulvalene, TCE = 1,1,2-trichloroethane] with the Cu(CF3)4– species being the first Cu3+ and F containing organometallic anion in an organic superconductor.

A new 9 K superconducting organic salt composed of the bis (ethylenedithio) tetrathiafulvalene (ET) electron-donor molecule and the tetrakis (trifluoromethyl) cuprate (III) anion, [Cu(CF3)4]-

Abstract We report the discovery of a second, higher- T c superconducting organic charge-transfer salt derived from the electron-donor molecule BEDT-TTF (or ET), the novel organometallic anion [Cu(CF 3 ) 4 ] − , and the neutral solvent molecule 1, 1, 2-trichloroethane (TCE). We have very recently reported that this charge-transfer system yields a new superconducting ϰ phase salt, ϰ L - (ET) 2 Cu(CF 3 ) 4 ·TCE, with inductive onset T crmc =4.0 K at ambient pressure. This ϰ phase salt (ϰ denotes a particular packing arrangement of the ET organic donor molecules) is electrocrystallized in the habit of hexagonal plates. Crystals possessing a needle-like habit electrocrystallize simultaneously with these plates, and we find these needles to be a distinctly different superconducting phase with diamagnetic onset T c =9.2±0.1 K at ambient pressure. On the basis of our experiments, we denote this new superconducting phase as ϰ H −( ET ) 2 Cu ( CF 3 ) 4 ·( TCE ) x , X

Synthesis and characterization of two new organic superconductors, κL- and κH-[bis(ethylenedisulfanyl)tetrathiafulvalene]2Au(CF3)4·(1,1,2-trichloroethane)via microelectrocrystallization

A novel microelectrocrystallization (MEC) procedure (ten-fold decrease in halogenated solvent use, micromolar quantities of electron donor molecule and supporting electrolyte, and Hg free electrodes), necessitated by the very small available quantities of the novel organometallic Au(CF3)4– anion, has been developed to prepare two distinct phases of κ-(ET)2Au(CF3)4·(TCE)[ET = bis(ethylenedisulfanyl)tetrathiafulvalene, TCE = 1,1,2-trichloroethane] which exhibit ambient pressure superconductivity with onset temperatures of 2.1 and 10.5 K.

Dynamic instabilities in the vortex lattice of YBa2Cu3O7−δ

Abstract A dynamic instability in the AC response of the vortex solid phase just below the melting/freezing line is reported. The instability takes the form of a voltage oscillation whose frequency is much lower than the driving frequency. The dependence of the voltage oscillation on experimental parameters is described.

Static and dynamic vortex transitions in clean YBa2Cu3O7

The evidence establishing first order melting of the vortex lattice in clean YBa2Cu3O7 is reviewed. Dynamic transitions in the moving vortex system are demonstrated experimentally through resistivity and magnetization measurements.

Magneto-optical study of twin boundary pinning in YBa2Cu3O7-δ

Abstract Using a high-resolution magneto-optical technique we studied the flux penetration and pinning in YBa 2 Cu 3 O 7-δ crystals in the presence of isolated twin boundaries. The most important factor determining the behavior of the twin boundaries is the direction of flux motion with respect to the boundary. Up to temperatures close to T c twin boundaries are effective barriers for flux motion transverse to the boundaries. For flux motion parallel to the boundary the characteristic “flame”-shaped flux patterns indicative of weak link behavior (i. e. low J c ) of the boundary are observed.

χ″ peaks, flux pinning, and the irreversibility line in high-temperature superconductors

Abstract Physical processes that produce various peaks in the adsorption component χ″ of the AC susceptibility of high-temperature superconductors are described. It is shown that for good YBa 2 Cu 3 O 7− x crystals twinning planes are the main source of flux pinning and the primary determinant of the reported irreversibility line (IL) position in the H–T plane. In other situations, particularly ones involving weaker pinning, the apparent IL is found to lie much lower in the H–T plane. Ambiguous results from experiments designed to produce pinning sites by irradiation are discussed in the context to this new observation.

Induced microwave absorption by magnetic modulation in untwinned and twinned YbaCuO crystals and its comparison with resistivity measurements.

We studied the magnetic field modulated microwave absorption across T{sub c} for an untwinned and for a twinned YBa{sub 2}Cu{sub 3}O{sub 7-{delta}} crystal (T{sub c}{approx_equal}93 K) which were cleaved from the same high quality single crystal. The absorption signals were analyzed using the Tinkham model. A good fit to the model was obtained for the untwinned crystal for Bc and for B>0.2 T, while for the twinned crystal we observed pronounced deviations from the model which we attribute to the presence of twin boundary pinning. The angular dependence of the activation energy of vortex motion in the untwinned crystal shows a sharp increase when the direction of magnetic field is almost parallel to the a-b plane, which is suggestive of a lock-in transition. Although field-dependent resistivity measurements on untwinned crystals clearly demonstrate a vortex melting transition, microwave studies do not reveal any signature of it. This agrees with results of other groups showing that high frequency measurements in YbaCuO{sub 7-{delta}} single crystals, unlike Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub x} single crystals, are rather insensitive to the vortex melting transition.