Joachim Wosnitza

Calorimetric Evidence for a Fulde-Ferrell-Larkin-Ovchinnikov Superconducting State in the Layered Organic Superconductor κ − ( BEDT − TTF ) 2 Cu ( NCS ) 2

The specific heat of the layered organic superconductor kappa-(BEDT-TTF)(2)Cu(NCS)(2), where BEDT-TTF is bisethylenedithio-tetrathiafulvalene, has been studied in magnetic fields up to 28 T applied perpendicular and parallel to the superconducting layers. In parallel fields above 21 T, the superconducting transition becomes first order, which signals that the Pauli-limiting field is reached. Instead of saturating at this field value, the upper-critical-field increases sharply and a second first-order transition line appears within the superconducting phase. Our results give strong evidence that the phase, which separates the homogeneous superconducting state from the normal state is a realization of a Fulde-Ferrell-Larkin-Ovchinnikov state.

Stacking-order dependent transport properties of trilayer graphene

We report markedly different transport properties of ABA- and ABC-stacked trilayer graphenes. Our experiments in double-gated trilayer devices provide evidence that a perpendicular electric field opens an energy gap in the ABC trilayer, while it causes the increase of a band overlap in the ABA trilayer. In a perpendicular magnetic field, the ABA trilayer develops quantum Hall plateaus at filling factors of \nu = 2, 4, 6... with a step of \Delta \nu = 2, whereas the inversion symmetric ABC trilayer exhibits plateaus at \nu = 6 and 10 with 4-fold spin and valley degeneracy.

Magnetic torque evidence for the Fulde-Ferrell-Larkin-Ovchinnikov state in the layered organic superconductor κ − ( BEDT − TTF ) 2 Cu ( NCS ) 2

We present magnetic-torque measurements of the organic superconductor k-(BEDT-TTF)2Cu(NCS)2 for in-plane magnetic fields up to 32 T. In this layered two-dimensional compound the superconductivity can persist even in fields above the Pauli limit of about 21 T. There, a pronounced upturn of the upper-critical-field line occurs and the superconducting phase-transition line splits and forms an additional high magnetic field phase. k-(BEDT-TTF)2Cu(NCS)2 is a spin-singlet superconductor, therefore, such a superconducting high-field phase beyond the Pauli limit can originate only from Cooper pairing with finite center-of-mass momentum. The measurements are discussed in connection with a Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, in accordance with earlier specific-heat observations. The torque experiments allow us to investigate the high magnetic-field phase diagram and the FFLO state of k-(BEDT-TTF)2Cu(NCS)2 in great detail.

An insight into what superconducts in polycrystalline boron-doped diamonds based on investigations of microstructure

The discovery of superconductivity in polycrystalline boron-doped diamond (BDD) synthesized under high pressure and high temperatures [Ekimov, et al. (2004) Nature 428:542–545] has raised a number of questions on the origin of the superconducting state. It was suggested that the heavy boron doping of diamond eventually leads to superconductivity. To justify such statements more detailed information on the microstructure of the composite materials and on the exact boron content in the diamond grains is needed. For that we used high-resolution transmission electron microscopy and electron energy loss spectroscopy. For the studied superconducting BDD samples synthesized at high pressures and high temperatures the diamond grain sizes are ≈1–2 μm with a boron content between 0.2 (2) and 0.5 (1) at %. The grains are separated by 10- to 20-nm-thick layers and triangular-shaped pockets of predominantly (at least 95 at %) amorphous boron. These results render superconductivity caused by the heavy boron doping in diamond highly unlikely.

A Star-Shaped Heteronuclear CrIIIMnII3 Species and Its Precise Electronic and Magnetic Structure: Spin Frustration Studied by X-Ray Spectroscopic, Magnetic, and Theoretical Methods

Molecular magnets incorporate transition-metal ions with organic groups providing a bridge to mediate magnetic exchange interactions between the ions. Among them are star-shaped molecules in which antiferromagnetic couplings between the central and peripheral atoms are predominantly present. Those configurations lead to an appreciable spin moment in the nonfrustrated ground state. In spite of its topologically simple magnetic structure, the [Cr(III)Mn(II)(3) (PyA)(6)Cl(3)] (CrMn(3)) molecule, in which PyA represents the monoanion of syn-pyridine-2-aldoxime, exhibits nontrivial magnetic properties, which emerge from the combined action of single-ion anisotropy and frustration. In the present work, we elucidate the underlying electronic and magnetic properties of the heteronuclear, spin-frustrated CrMn(3) molecule by applying X-ray magnetic circular dichroism (XMCD), as well as magnetization measurements in high magnetic fields, density functional theory, and ligand-field multiplet calculations. Quantum-model calculations based on a Heisenberg Hamiltonian augmented with local anisotropic terms enable us not only to improve the accuracy of the exchange interactions but also to determine the dominant local anisotropies. A discussion of the various spin Hamiltonian parameters not only leads to a validation of our element selective transition metal L edge XMCD spin moments at a magnetic field of 5 T and a temperature of 5 K but also allows us to monitor an interesting effect of anisotropy and frustration of the manganese and chromium ions.

Spin-orbit interaction in chiral carbon nanotubes probed in pulsed magnetic fields

The magnetoconductance of an open carbon nanotube (CNT)-quantum wire was measured in pulsed magnetic fields. At low temperatures, we find a peculiar split magnetoconductance peak close to the chargeneutrality point. Our analysis of the data reveals that this splitting is intimately connected to the spin-orbit interaction and the tube chirality. Band-structure calculations suggest that the current in the peak regions is highly spin polarized, which calls for application in future CNT-based spintronic devices.

Nanostructured thin manganite films in megagauss magnetic field

We report on the use of the colossal magnetoresistance (CMR) effect in manganites for the measurement of pulsed magnetic fields up to the megagauss limit. To increase the application range in a magnetic field, we fabricated nanostructured La-Sr-Mn-O films consisting of nanocrystallites cummulated into clusters separated by highly amorphous inter-cluster boundaries. We demonstrate that the CMR effect does not saturate in these films at 77 K up to 91.4 T. Moreover, the magnetoresistance behavior at 290 K shows that nanostructured manganite films are promising candidates for the development of magnetic field scalar sensors operating in wide field and temperature ranges.

Implementation of specific-heat and NMR experiments in the 1500?ms long-pulse magnet at the Hochfeld-Magnetlabor Dresden

We report on the design and successful testing (up to 55 T) of a 60 T long-pulse (LP) magnet with a pulse duration of 1500?ms at the Hochfeld-Magnetlabor Dresden. This magnet has the capability of producing magnetic fields with time and homogeneity characteristics suitable to host specific-heat and nuclear magnetic resonance (NMR) experiments. We describe the development of a specific-heat probe for this magnet, which uses the quasi-adiabatic heat-pulse method and enables magnetocaloric-effect measurements. Furthermore, we report the implementation of a NMR setup, consisting of home-built probe and spectrometer. Both experimental devices were tested successfully in the LP magnet up to 31 and 44 T, respectively.

On-chip superconductivity via gallium overdoping of silicon

We report on superconducting properties of gallium-enriched silicon layers in commercial (100) oriented silicon wafers. Ion implantation and subsequent rapid thermal annealing have been applied for realizing gallium precipitation beneath a silicon-dioxide cover layer. Depending on the preparation parameters, we observe a sharp drop to zero resistance at 7 K. The critical-field anisotropy proofs the thin-film character of superconductivity. In addition, out-of-plane critical fields of above 9 T and critical current densities exceeding 2 kA/cm2 promote these structures to be possible playgrounds for future microelectronic technology.

Large carbon-isotope shift of TC in boron-doped diamond

We report the carbon-isotope effect for boron-doped diamond (BDD). Resistive as well as specific-heat measurements reveal a 0.2K shift in the superconducting transition temperature Tc between BDDs containing C13 and C12. This is more than two times larger than could be expected from the mass difference by the use of the simple BCS formula in case the phenomenon is related to electron-phonon mediated superconductivity in BDD.