Yutaka Nishio

A new type oscillatory phenomenon in the magnetotransport of θ-(BEDT―TTF)2I3

Abstract A new type oscillatory magnetotransport phenomenon has been observed in θ-type crystals of (BEDT-TTF) 2 I 3 at temperatures below 6 K and in the magnetic field above 3 T. The oscillation appears when the magnetic field of a fixed strength is rotated from the direction normal to the conductive two dimensional plane to a direction parallel to the plane. The period of the oscillation is described by an equation tan ( θ min )= sN ( s =0.39, N =0, 1, 2, 3,…), where θ min is the angle giving the position of the trough of the oscillation. The amplitude of the oscillation is primarily determined by the magnetic field component normal to the conductive plane. It is related to the temperature through the change in the resistivity ⪢ 0 .

Electronic Phases in an Organic Conductor α-(BEDT-TTF)2I3: Ultra Narrow Gap Semiconductor, Superconductor, Metal, and Charge-Ordered Insulator

We review the transport phenomena in an organic conductor α-(BEDT-TTF) 2 I 3 . It exhibits various types of transport depending on the circumstance in which it is placed. Under the ambient pressure, it is a charge-ordered insulator below 135 K. When high hydrostatic pressures are applied, it changes to a new type of narrow gap (or zero gap) semiconductor. The conductivity of this system is nearly constant between 300 and 1.5 K. In the same region, however, both the carrier (hole) density and the mobility change by about six orders of magnitude, in a manner so that the effects just cancel out giving rise to the temperature independent conductivity. The temperature ( T ) dependence of the carrier density n obey n ∝ T 2 below 50 K. When it is compressed along the crystallographic a -axis, it changes from the charge ordered insulator to a narrow gap semiconductor. At the boundary between these phases, there appears a superconducting phase. On the other hand, when compressed in the b -axis, the system changes ...

Transport Property of an Organic Conductor α-(BEDT-TTF) 2I 3 under High Pressure - Discovery of a Novel Type of Conductor -

We have found quite a new type of transport phenomenon in an organic crystal α-(BEDT-TTF) 2 I 3 under high pressure. Essentially, it is a semimetal or a narrow gap semiconductor. But, the transport property is peculiar. The conductivity of this the carrier (hole) density and mobility change by a about 6 orders of magnitude. They change in a manner so that the effects just cancel out giving rise to the temperature independent conductivity. At low temperatures, the system is in a state with high mobility (3 ×10 5 cm 2 /V·sec) and low carrier density (5 ×10 15 cm -3 ). This state has been found to be very sensitive to magnetic field. We propose a mechanism for the extremely strong temperature dependence of the carrier density. It is based on the band structure and takes the thermal effect into consideration.

Magnetotransport Phenomena of α-Type (BEDT-TTF) 2I 3 under High Pressures

The magnetotransport phenomenon is investigated in α-type crystals of (BEDT-TTF) 2 I 3 which are metallized by applying quasi-hydrostatic pressures. At liquid helium temperatures, a fairly large magnetoresistance which rises in very low field and saturates above 0.5 T is observed. The effect of the magnetic field of 1.2 T is found to be recognizable at temperatures above 50 K. Our interpretation of the phenomenon is that the metal-insulator transition which has been suppressed by the pressure arises again, aided by the magnetic field.

Effects of Uniaxial Strain on Transport Properties of Organic Conductor α-(BEDT-TTF)2I3 and Discovery of Superconductivity

We demonstrate that an organic conductor α-(BEDT-TTF) 2 I 3 exhibits various types of transport phenomena when compressed along some crystal axes. When the sample is strained along the a -axis, the low-temperature electronic state changes from a charge-ordered insulating state under small strains to a narrow gap semiconductor state under large strains. In between these two states, there appears a superconducting state with a T c of about 7 K. When compressed along the b -axis, on the other hand, this material behaves as a typical quasi-two-dimensional metal with large Fermi surfaces.

Transport properties of ((CH3)4N) (Ni(dmit)2)2: A new organic superconductor

Abstract Transport properties of ((CH 3 ) 4 N) (Ni(dmit) 2 ) 2 crystals are studied in the temperature region from 300 to 1.5 K under hydrostatic pressure up to 15 kbar. A resistivity jump at around 100 K and a sharp rise of resistivity below 20 K are characteristic of this crystal. Hydrostatic pressure reduces both anomalies. In the sample where the low temperature anomaly was suppressed by high pressure, we have observed an onset of a superconducting transition under 3.2 kbar at 3.0 K. The transition temperature becomes high with increasing pressure. At 7 kbar, we find T c = 5.0 K .

Effect of the zero-mode landau level on interlayer magnetoresistance in multilayer massless Dirac fermion systems.

We report on the experimental results of interlayer magnetoresistance in the multilayer massless Dirac fermion system alpha-(BEDT-TTF)2I3 under hydrostatic pressure and its interpretation. We succeeded in detecting the zero-mode Landau level (n=0 Landau level) that is expected to appear at the contact points of Dirac cones in the magnetic field normal to the two-dimensional plane. The characteristic feature of zero-mode Landau carriers including the Zeeman effect is clearly seen in the interlayer magnetoresistance.

New organic superconductors K- and θ-(BEDT-TTF)2I3: Transport property

Abstract Transport properties of newly synthesized organic conductors K - and θ-(BEDT-TTF)2I3 are investigated. We find that these materials are quasi-two dimensional metals. Both materials exhibit ambient pressure superconductivity with Tc around 3.6K. The upper critical magnetic field is determined as a function of temperature and the coherence length is estimated. We find that the coherence lengths for k -type in the two dimensional plane and normal to it are about 360A and 13.5A, respectively.

A supramolecular superconductor θ-(DIETS)2[Au(CN)4]

A supramolecular conductor θ-(DIETS)2[Au(CN)4] [DIETS = diiodo(ethylenedithio)diselenadithiafulvalene] has been revealed to be a new superconductor with Tc = 8.6 K (onset, 10 kbar) under uniaxial strain parallel to the crystallographic c-axis.

Hydrogen-Bond-Dynamics-Based Switching of Conductivity and Magnetism: A Phase Transition Caused by Deuterium and Electron Transfer in a Hydrogen-Bonded Purely Organic Conductor Crystal

A hydrogen bond (H-bond) is one of the most fundamental and important noncovalent interactions in chemistry, biology, physics, and all other molecular sciences. Especially, the dynamics of a proton or a hydrogen atom in the H-bond has attracted increasing attention, because it plays a crucial role in (bio)chemical reactions and some physical properties, such as dielectricity and proton conductivity. Here we report unprecedented H-bond-dynamics-based switching of electrical conductivity and magnetism in a H-bonded purely organic conductor crystal, κ-D3(Cat-EDT-TTF)2 (abbreviated as κ-D). This novel crystal κ-D, a deuterated analogue of κ-H3(Cat-EDT-TTF)2 (abbreviated as κ-H), is composed only of a H-bonded molecular unit, in which two crystallographically equivalent catechol-fused ethylenedithiotetrathiafulvalene (Cat-EDT-TTF) skeletons with a +0.5 charge are linked by a symmetric anionic [O···D···O](-1)-type strong H-bond. Although the deuterated and parent hydrogen systems, κ-D and κ-H, are isostructural paramagnetic semiconductors with a dimer-Mott-type electronic structure at room temperature (space group: C2/c), only κ-D undergoes a phase transition at 185 K, to change to a nonmagnetic insulator with a charge-ordered electronic structure (space group: P1). The X-ray crystal structure analysis demonstrates that this dramatic switching of the electronic structure and physical properties originates from deuterium transfer or displacement within the H-bond accompanied by electron transfer between the Cat-EDT-TTF π-systems, proving that the H-bonded deuterium dynamics and the conducting TTF π-electron are cooperatively coupled. Furthermore, the reason why this unique phase transition occurs only in κ-D is qualitatively discussed in terms of the H/D isotope effect on the H-bond geometry and potential energy curve.