Y.J. Jo

The novel role of anion ordering in angle dependent magnetotransport of one-dimensional organic conductors

Abstract We have studied the angle dependent magnetoresistance of the various Bechgaard salts when the magnetic field rotates in the b ’ c ∗ plane. Lebed-type resonance is clearly observed for all the compounds both in R xx and in R zz . In all cases, conductivity is enhanced at angles proposed by Lebed. (TMTSF) 2 ClO 4 shows almost the same behavior as (TMTSF) 2 PF 6 in spite of the presence of anion ordering. Only considered is doubling of unit cell along the intermediate conducting direction ( b ). Conductivity enhancement is very much pronounced in R zz than in R xx . The resonance effect is particularly well established in R zz of (TMTSF) 2 ReO 4 where more than seventh resonance can be easily observed. The angles at which resonance takes place can be fitted with the general formula, but only with p / q =odd integers. Also found are additional fast oscillatory parts superposed on the Lebed oscillations.

Thermoelectric power of anisotropic electron system in quasi-one-dimensional organic conductors

We investigate thermoelectric power of Bechgaard salts. Metallic state can clearly be divided into two regions where the linearly temperature dependent thermoelectric power along the most conducting direction has distinct slopes. Thermoelectric power along the second and the least conducting direction has a broad maximum between 50 and 150K depending on the direction or compounds, which is contradictory to the old investigation in early eighties. Our results suggest that the high temperature metallic state is highly one dimensional with b- and c- axis transport being only diffusive. We also suggest that thermoelectric power measurement could be used to estimate the dimerization gap which resides on the most of the quasi-one-dimensional electron system.

Quantum Oscillation Signatures of Pressure-induced Topological Phase Transition in BiTeI

We report the pressure-induced topological quantum phase transition of BiTeI single crystals using Shubnikov-de Haas oscillations of bulk Fermi surfaces. The sizes of the inner and the outer FSs of the Rashba-split bands exhibit opposite pressure dependence up to P = 3.35 GPa, indicating pressure-tunable Rashba effect. Above a critical pressure P ~ 2 GPa, the Shubnikov-de Haas frequency for the inner Fermi surface increases unusually with pressure, and the Shubnikov-de Haas oscillations for the outer Fermi surface shows an abrupt phase shift. In comparison with band structure calculations, we find that these unusual behaviors originate from the Fermi surface shape change due to pressure-induced band inversion. These results clearly demonstrate that the topological quantum phase transition is intimately tied to the shape of bulk Fermi surfaces enclosing the time-reversal invariant momenta with band inversion.

NMR studies of the exotic members of the Bechgaard salts NO3 and FSO3 salts

1H, 19 F and Se NMR measurements were performed to investigate the properties of Bechgaard salts, (TMTSF) 2 X, with noncentrosymmetric anions, X=NO 3 and FSO 3 . Single crystal Se NMR signal for NO 3 salt detects the spin density wave transition. Preliminary analyses of the angular dependence of 1 H NMR spectra were also carried out. Spin density wave in the NO 3 salt was confirmed to have an incommensurate wave number and relatively small arnplitude. 19 F and 77 Se NMR signals in the FSO 3 salt could detect drastic anomaly at the anion ordering temperature. The insulating state was confirmed to be nonmagnetic with a large spin gap of 420 K.

Resonant angular magnetoresistance in the ac-plane of the Bechgaard salts: comparative studies

Lebed resonance phenomenon which manifests sharp resistance minima at angles where the magnetic field imposes electrons a periodic motion on the Fermi surface, is readily observed in most of Bechgaard salts such as (TMTSF)2ClO4, (TMTSF)2PF6, (TMTSF)2ReO4. On the other hand, another type of oscillatory behavior, observed when the magnetic field rotates from the most conducting direction towards the least conducting direction, in the acplane, is found to vary much from one compound to another, having been clearly observed only in (TMTSF)2ClO4. We have performed a systematic study of both the bc- and ac-rotation measurements and report that the ac-plane resonances are also omnipresent and universal in all the three compounds. Difference among compounds, notably the strength of the coherence peak may be used to account for the evolution of coherence from one compound to another.

New look on dimensionality of the Bechgaard salts

We present our comparative study of Bechgaard salts which have played a key role to develop the physics of quasi-one-dimensional electron systems. Angular magnetoresistance as well as magnetic oscillations has been successfully interpreted in terms of the quasi-one-dimensional Fermi surface. However, our recent investigation of the Bechgaard salt revealed that the physics of the Bechgaard salts can be extended to the two dimensional electron systems.

Genuine two-dimensional electrons in Bechgaard salts

Abstract We will present our comparative study of Bechgaard salts which have played a key role to develop the physics of quasi-one-dimensional electron systems. Our recent investigations revealed that a genuine two-dimensional electron system can also be realized in these materials. When the angular magnetoresistance of (TMTSF)2FSO3 under 6.2 kbar showed oscillatory behavior, the peak positions could not be explained with Lebed resonance model but with Yamaji resonance model.