Taro Yakabe

Magnetic-field-induced superconductivity in a two-dimensional organic conductor.

The application of a sufficiently strong magnetic field to a superconductor will, in general, destroy the superconducting state. Two mechanisms are responsible for this. The first is the Zeeman effect, which breaks apart the paired electrons if they are in a spin-singlet (but not a spin-triplet) state. The second is the so-called ‘orbital’ effect, whereby the vortices penetrate into the superconductors and the energy gain due to the formation of the paired electrons is lost. For the case of layered, two-dimensional superconductors, such as the high-Tc copper oxides, the orbital effect is reduced when the applied magnetic field is parallel to the conducting layers. Here we report resistance and magnetic-torque experiments on single crystals of the quasi-two-dimensional organic conductor λ-(BETS)2FeCl4, where BETS is bis(ethylenedithio)tetraselenafulvalene. We find that for magnetic fields applied exactly parallel to the conducting layers of the crystals, superconductivity is induced for fields above 17 T at a temperature of 0.1 K. The resulting phase diagram indicates that the transition temperature increases with magnetic field, that is, the superconducting state is further stabilized with magnetic field.

Global Phase Diagram of the Magnetic Field-Induced Organic Superconductors λ-(BETS)2FexGa1-xCl4

Organic alloys λ-(BETS) 2 Fe x Ga 1- x Cl 4 show superconductivity only under very high magnetic fields parallel to the conducting layer for x ≥0.47. As x decreases, the field induced superconducti...

Fermi surface and resistance anomalies in ET-TCNQ

Recently, a new organic conductor ET-TCNQ, which exhibits two resistance anomalies at T 1 = 80 K and T 2 = 20 K, was synthesized. We present a systematic study of transport properties of ET-TCNQ under magnetic fields. Shubnikov-de Haas (SdH) oscillations are clearly observed and we find five distinct small Fermi surfaces (FSs). The cross-sectional areas of FS are estimated to be from I %(α) to 3.5 %(e) of the first Brillouin zone. We detect Lebed resonance and Danner-Chaikin oscillation in measurements of angular-dependent magnetoresistance oscillation (AMRO), suggesting the existence of the quasi-one dimensional FS related to ET molecules. The origins of the two resistance anomalies are discussed in terms of nesting-driven density-wave transition.

Shubnikov–de Haas Effect and Angular-dependent Magnetoresistance in New Layered Organic Conductors ET3Cl(DFBIB) and ET3Br(pBIB)

This paper reports the measurements of Shubnikov–de Haas (SdH) and angular-dependent magnetoresistance oscillations (AMROs) for new layered organic conductors ET 3 Cl(DFBIB) and ET 3 Br( p BIB), wh...

Metal Atomic Chains on the Si(100) Surface

The selection of a single-domain Si(100)2×1 surface enables us to make an indium atomic chain over 70 nm in length. Such self-assembled chains can be extended by atomic manipulation using a scanning tunneling microscope tip, as briefly demonstrated in this work on In/Si(100). The advantage of a single-domain Si(100) surface over a double-domain one for the growth of long chains is rationalized, and the mechanism behind the atomic manipulation is noted.

Fermi surface in new layered organic conductors (BEDT-TTF)3Br(pBIB) and (BEDT-TTF)3Cl(DFBIB)

Abstract We report the measurements of Shubnikov–de Haas (SdH) oscillation and angular dependent magnetoresistance oscillation (ADMRO) in a new layered organic conductor (BEDT-TTF)3Br(pBIB), where pBIB stands for p-bis(iodoethynyl)benzene. When magnetic fields are applied perpendicular to the layers, two distinct SdH oscillations are clearly observed, which arise from the two extremal cross-sectional areas of the slightly warped cylindrical Fermi surface (FS). The cross-sectional areas correspond to 50.9 and 51.6% of the first Brillouin zone. From the analysis of the ADMRO, the area of FS is estimated to be 52.5% of the first Brillouin zone. The ADMRO data also shows that the anisotropy of the FS is enhanced by change of Br(pBIB) for Cl(DFBIB), where DFBIB stands for 1,4-difluoro-2,5-bis(iodoethynyl)benzene. These results are consistent with the band calculation.

Atomic scale Pb chains on Si(100)

The growth of Pb on Si(100) 2×1 is found by scanning tunneling microscopy to form one-dimensional ad-dimer chains at a coverage far below a monolayer, analogous to the behavior of group-III elements (Ga, Al, In) on the same surface but with a buckled dimer configuration. The asymmetric dimerized structure is retained for c(8×4) and c(4×4) reconstructions of higher coverage. Tunneling spectra for the Pb dimers exhibit a surface-state band gap of ∼1.2 eV and thus suggest a nonmetallic property for the Pb chains, in agreement with the dimerization and buckling feature of Pb atoms. The parallel ad-dimer adsorption structure also suggests an approach to obtaining long atomic lines via the selection of the single domain Si(100) as substrates.

Observation of negative differential resistance on AgSi(100) using STM

Abstract We investigated the initial stage of Ag adsorption on the Si(100) surface at low temperature. From STM images at − 1 and + 1 V, occupied and unoccupied orbitals of neighboring two Ag sites are uncoupled and coupled, respectively. We observed another adsorption structure at low temperature in comparison with the room temperature. We also measured the I-V characteristics over Ag sites which indicate two distinct peaks of negative differential resistance in the empty state. The mechanism behind this phenomenon is attributed to the localized surface states of Ag within the Si bulk band gap.

Superconductivity in organic alloys λ-(BETS)2FexGa1−xCl4

We have performed resistance measurements in a wide field region for various λ-(BETS) s Fe x E 1-x Cl 4 and obtained the global magnetic phase diagram. For x>0.45, λ-(BETS) 2 Fe X Ga 1-4 Cl 4 shows superconductivity only under high magnetic fields parallel to the conducting layer. As x decreases, the field induced superconducting phase shifts towards lower fields and a striking field-induced insulator to superconductor transition is observed below 4 T for x=0.45. The overall features of the global phase diagram are well understood in terms of Jaccarino-Peter compensation mechanism.

Novel electronic properties under magnetic fields in organic conductors λ-(BETS)2FexGa1-xCl4

Abstract Electronic states have been investigated under high magnetic fields for quasi-two-dimensional organic conductors λ-(BETS) 2 Fe x Ga 1− x Cl 4 ( x =1.0, 0.47, 0.45, 0; BETS: bis(ethylenedithio)tetraselenafulvalene). Under magnetic field parallel to the c -axis, various electronic states appear, depending on x , field, and temperature. The global phase diagram for this alloy is presented and the evidence of the strong internal field due to the Fe moments is shown.