Quasi Two-dimensional Organic Conductor Research Articles

Angle dependent magnetothermopower of α-(ET)2KHg(SCN)4

The magnetic field and angle dependences of the thermopower and Nernst effect for the quasi-two dimensional (q2D) organic conductor α-(ET)2KHg(SCN)4 are measured at temperatures below (4 K) and above (9 K) the transition temperature in fields of up to 31 T. In addition, a theoretical model involving a magnetic breakdown effect between the q1D and q2D bands is proposed in order to simulate the data. Analysis of the background components of the thermopower and Nernst effect imply that at low temperatures, in the CDW state, the properties of α-(ET)2KHg(SCN)4 are determined mostly by the orbits on new, open Fermi sheets. The quantum oscillations observed in both thermoelectric effects at fields above 8 T are caused only by the α orbit.

13C-NMR study of charge order state of the organic conductor [formula omitted

It is said that the quasi-two dimensional organic conductor ( BEDT-TTF ) 2 X forming two columns structure like α or θ salts, shows charge order (CO) state at low temperature. To confirm the existence of the CO state and the electron magnetism in α ′ phase whose symmetry is different from that of α and θ phases, we investigated 13C-NMR on α ′ ‐ ( BEDT-TTF ) 2 IBr 2 , in which one-sided central carbon on BEDT-TTF molecules is substituted with 13C. As a result, the change of spectrum was observed at 200 K which corresponds to CO transition and the angular dependence of spectrum at 15 K indicated that the charge ordering pattern was intra-column separating type. The spectrum in paramagnetic state can be examined by not only the difference of the charge density on on-site molecule but also the contribution from off-site molecules. This result suggested that the hyperfine coupling tensor changes after CO transition in which the charge separates extremely.

Non-Linear Conduction in the Density Wave State of Quasi-Two Dimensional Organic Conductor α-(BEDT-TTF)2KHg(SCN)4

Current–voltage characteristics are measured in the quasi-two dimensional organic conductor α-(BEDT-TTF) 2 KHg(SCN) 4 at temperatures down to 0.5 K and in the magnetic field up to 25 T. The non-linear conduction with a threshold electric field is found in the density wave state. The features of threshold electric field obtained in the low magnetic field region are explained by the unconventional charge density wave model. In the high magnetic field region, where the Shubnikov–de Haas oscillations appear, the current–voltage characteristics reveal that the density wave state synchronizes with the filling of the electron on the Landau level and continues even above a kink field 23 T.

Charge disproportionation in the metallic state of α -(BEDT-TTF)2I3

13C-NMR measurements have been carried out on the quasi two-dimensional organic conductor, α-(BEDT-TTF) 2 I 3 . We measured the angular dependence of 13 C-NMR spectrum at several temperatures above metal-insulator transition temperature, 135 K (=T MI, ). We found that charge (spin) disproportionation already exists in the metallic state and gradually develops as temperature approaches to T MI .

Pressure effect on β′-Pd(dmit)2 salts

Large enhancement of antiferromagnetic (AF) transition temperature T N by pressure was observed in a quasi two-dimensional organic conductor, β'-(Me 4 P)[Pd(dmit) 2 ] 2 . Similar results are obtained in Et2Me2P salt at 5 kbar. It is indicated that pressure tends to reduce magnetic frustrations leading to the stabilization on AF state. A single crystal of β'-(Et2Me2P)[Pd(dmit) 2 ] 2 was also examined at ambient pressure. A clear evidence of AF transition was obtained at around 16 K by 13 C NMR relaxation rate 1/T 1 . This is consistent with ESR study, but different from our polycrystalline experiments.

Imperfect nesting and transport properties in unconventional density waves

We consider the effect of imperfect nesting in quasi-one dimensional unconventional density waves. The phase diagram is very close to those in a conventional DW. The linear and non-linear aspects of the electric conductivity are discussed. At T=0 the frequency dependent electric conductivity develops a small dip at low frequencies. The threshold electric field depends strongly on the imperfect nesting parameter, allowing us to describe the measured threshold electric field in the low temperature phase of the quasi-two dimensional organic conductor, alpha-(BEDT-TTF)_2KHg(SCN)_4 very well.

THE STUDY OF THE MAGNETIC BREAKDOWN EFFECT AS A FUNCTION OF ANGLE IN THE ORGANIC CONDUCTOR K- (BEDT-TTF)2Cu(NCS)2 IN HIGH MAGNETIC FIELDS

The magnetic breakdown effect can be seen by the growth of new frequencies in the quantum oscillations in clean metals as a function of magnetic field. We have studied the variation of the amplitudes in the quantum oscillations in the resistance (the Shubnikov-de Haas effect) as a function of angle in the quasi-two dimensional-organic conductor κ-(BEDT-TTF)2Cu(NCS)2. The measurements were made by means of a radio frequency (rf) tank circuit (~ 50 MHz) at very high magnetic fields(50T-60T) and low temperature(500 mK). The geometry of the rf excitation we used excited in-plane currents, and therefore we measured the in-plane resistivity. In contrast to conventional transport measurements that measure the inter-plane resistivity, the in-plane resistivity is dominated by the magnetic breakdown frequencies. As a result we measured much higher breakdown frequency amplitudes than conventional transport experiments. As is expected, the angular dependence of the Shubnikov-de Haas frequencies have a 1/cosθ behavior. This is due to the change of the cross sectional area of the tubular Fermi surface as the angle with respect to the magnetic field is changed. The amplitude of the oscillations changes due to the spin splitting factor which takes into account the ratio between the spin splitting and the energy spacing of the Landau levels which also has 1/cosθ behavior. We show that our data agree with the semi-classical theory (Lifshitz-Kosevich formula).

Charge ordering in α-(BEDT-TTF) 2I 3

13C-NMR measurements were performed to the quasi two-dimensional organic conductor α-(BEDT-TTF) 2 I 3 , which exhibits metal-insulator transition at 135 K (=T MI ) at ambient pressure. The relaxation rate T 1 -1 has two components below T MI indicating two nonequivalent sites. T-dependence of the lineshape clearly indicates that there exist two differently charged BEDT-TTF molecules below T MI .

Phase transition in magnetic field parallel to the conducting plane for λ-(BETS) 2FeCl 4

Magneto-resistance and magnetic torque measurements were made for a quasi two-dimensional organic conductor λ-(BETS) 2 FeCl 4 . Under magnetic fields along the c-axis (parallel to the conducting plane), the resistance decreases above 15 T and then becomes too small to detect above 17 T at 0.02 K. The behaviour is associated with change in the magnetic torque. The results show that a phase transition to a very high conductive state is present around 16 T.

Charge disproportionation in the organic conductor, α-(BEDT-TTF) 2I 3

We have carried out 13C-NMR measurements on the quasi two-dimensional organic conductor α-(BEDT-TTF) 2I 3, which exhibits metal–insulator transition at 135 K at ambient pressure. The relaxation rate T 1 −1 has two components below the transition. Temperature dependence of the lineshape clearly indicates that there exist two differently charged BEDT-TTF molecules below the transition temperature.

Angular dependence of the high field phase transition in α-(BEDT-TTF) 2TlHg(SCN) 4

We report results of a systematic study of the anomalous low temperature state (LTS) angular magnetoresistance (MR) in the quasi-two dimensional organic conductor α-(BEDT-TTF) 2TlHg(SCN) 4 in fields up to 30 tesla. The high field suppression of the LTS is found to be independent of angle, while the MR maximum, H max increases with angle in conjuction with the well studied angular dependent magnetoresistance oscillations (ADMRO). The different magnetic breakdown probabilities above and below H max are found to be the source of a novel high field change in the ADMRO shape.

Uniaxial stress study of the magnetotransport properties of the quasi-two dimensional organic conductor α-(BEDT-TTF) 2KHg(SCN) 4

Measurements of the low temperature magnetoresistance of α-(BEDT-TTF) 2KHg(SCN) 4 under uniaxial stress perpendicular to the BEDT-TTF molecular layers indicate that the density wave ground state is suppressed by 1.5 Kbar. The fundamental Shubnikov-de Haas frequency decreases at an approximate rate of -20 tesla/Kbar and a slow, stress dependent oscillation appears for stresses above 0.9 Kbar. These enhanced stress effects are discussed in light of the high sensitivity of the Fermi surface to small changes in the crystal structure.

Open and closed Fermi surface contributions to the anomalous angular magnetoresistance of α-(BEDT-TTF) 2RbHg(SCN) 4

Anomalous angular magnetoresistance (AMR) in the quasi-two dimensional organic conductor α-(BEDT-TTF) 2RbHg(SCN) 4 is reported. The AMR appears as oscillations with sharp minima below the anitiferromagnetic ordering temperature. The period of these oscillations is anisotropic with respect to the plane of rotation cutting through the conducting layers. Above the ordering temperature, the nature of the AMR changes fundamentally. We propose a model for the AMR that incorporates both open and closed Fermi surfaces, and discuss how temperature and field dependent behaviors of the individual FS contribute to the conductivity.