A. Kawamoto

Nesting instability in a family of α-(BEDT-TTF)2MHg(SCN)4 [M = K, Rb, NH4] studied by 13C-NMR

Abstract We have investigated low-temperature electronic states of the organic conductors, α-(BEDT-TTF) 2 MHg(SCN) 4 [M = K, Rb and NH 4 ] in terms of 13 C-NMR and dc susceptibility measurements. For all the compounds, the overall temperature dependence of 1/( T 1 T ) scales to that of χ s 2 except below the nesting instability. For M = Rb and K salts, the 1/( T 1 T ) decreased below the transition temperatures without line-broadening or relaxation enhancement. The spectra changed from complicated lines to an apparently simple Pake doublet below the nesting temperatures. All of the NMR data are successfully explained by reduction of the electronic density-of states below the transition but do not have any sign of magnetic order.

NMR study of (BEDT-TTF)(TCNQ) at selective nuclear sites

Abstract The charge transfer complex, (BEDT-TTF)(TCNQ), is a hybrid system which consists of one-dimensional (1D) columns and two-dimensional (2D) BEDT-TTF layers and is known to be a paramagnetic insulator below room temperature. In order to examine the electronic structures of 1D and 2D layers separately, we prepared the complex between TCNQ and 13C,deuterium-substituted BEDT-TTF. We report the site-selective NMR study on this sample and discuss the electronic structure of the 1D and 2D electronic subsystems.

Resistive transition in an extremely two-dimensional superconductor, α-(BEDT-TTF)2NH4Hg(SCN)4

Abstract In order to investigate the superconductivity in α-(BEDT-TTF) 2 NH 4 Hg(SCN) 4 , resistivity was measured as a function of temperature, magnetic field and exciting current. In addition, simultaneous measurements of I–V characteristics and ac susceptibility were carried out. The results indicate highly two-dimensional character of the superconductivity.

Specific heat study of α-(BEDT-TTF)2MHg(SCN)4

Abstract The low-temperature specific heat of α-phase of (BEDT-TTF) 2 MHg(SCN) 4 (M=K, Rb, NH 4 ), where BEDT-TTF is bis- (etylenedithio) tetrathiafulvalene, is studied. The electronic specific heat coefficient (γ) estimated from the data between 1K and 2.3K amounts to 5.3, 10.3, 28.8mJ/mol K 2 for M=K, Rb and NH 4 , respectively. The K-salt does not have distinct anomally around 8K within the precision of our experiments, implying that the transition occurs with a gradual change of electronic structure.

13C-NMR study on β-(BEDT-TTF)2I3

Abstract The electronic state of the κ phase family of BEDT-TTF compounds, κ-(BEDT-TTF) 2 X [XCu(NCS) 2 , Cu[N(CN) 2 ]Br and Cu[N(CN) 2 ]Cl], has been investigated by 13 C-NMR as well as 1 H-NMR. The 13 C isotope was substituted selectively into the central double bonded carbon sites of BEDT-TTF. The 13 C nuclear spin-lattice relaxation rate and line spectra have been measured for the Cu(NCS) 2 and Cu[N(CN) 2 ]Br salts. The Knight shift evaluated from the the width of the spectra of the powdered samples is temperature-insensitive and scaled to the uniform susceptibility. However, the relaxation rate, T 1 −1 , does not obey the Korringa law but exhibits anomalous temperature dependence with a peak formation around 50 K. The absolute value of T 1 −1 is in excess of the Korringa relation. These behaviors are considered to come from strong aniferromagnetic spin fluctuation with finite wave vector, which is suppressed below 50 K. On the other hand, the 1 H NMR and magnetization measurements gave an unambiguous evidence for antiferromagnetic transition with spin canting at 25 K in κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]Cl. The κ phase family, situated in the vicinity of metal-insulator and magnetic transitions, is characterized by strong electron correlations.

13C-NMR study on α-(BEDT-TTF)2MHg(SCN)4

Abstract The low temperature electronic states of the organic conductors, α-(BEDT-TTF) 2 MHg(SCN) 4 [MK, Rb], have been studied by 13 C-NMR with the use of 13 C labeled samples. We observed anomalies in the nuclear spin-lattice relaxatin rate, T 1 , and shift of the NMR line below 8 K for the K salt and below 12 K for the Rb salt. These indicate some phase transition. However, absence of critical fluctuation near the transition temperature and absence of splitting or broadening of the NMR line is against the static magnetic ordering as found in usual SDW systems such as the TMTSF compounds. The present results imply that the electronic density of states decreases to a half below the transition.

Metal-insulator transition in partially deuterated κ-(BEDT-TTF)2Cu[N(CN)2]Br

Abstract The κ-(BEDT-TTF) 2 Cu[N(CN) 2 ]Br salts were synthesized from non-, partially, and fully deuterated BEDT-TTF molecules. The electrical resistivities of this family have been studied. The resistive state is progressively changed from metallic to insulating behaviors by the partial deuterations. The kink of the resistive curve with hysteresis was observed around 78K irrespectively of the degree of the deuteration.

13C and 1H NMR study of vortex dynamics in a layered organic superconductor at selective nuclear sites

Abstract The quasi-two-dimensional superconductor, κ-(BEDT-TTF) 2 Cu(NCS) 2 , under a perpendicular field was investigated by NMR relaxation rate measurements at the three different nuclear sites. The vortex dynamics contribution to the relaxation rate was separated from the quasiparticle contribution and found to show a peak formation around 2 K with a site-dependent peak height. The analysis supports that the peak behavior does not come from the simple crossover but from some qualitative change in the vortex dynamics.

Two types of charge ordering in DCNQI-metal complexes studied by 13C-NMR

Abstract The π electron charge ordering in (DI-DCNQI) 2 Ag with a 1/4-filled l-d band and the d electron charge ordering in deuterated (DMe-DCNQI) 2 Cu with the π -d hybridization have been characterized by 13 C NMR. The profile of the ordering is quite different between the two systems, reflecting the different driving force. The intersite Coulomb interaction between the π-electrons is considered to be crucial in the former case while the perfect charge ordering found in the latter salt seems to invoke cooperative mechanism including the π electron-lattice (Peierls) instability, dry level-lattice instability and strong correlation in the Cu site.

13C-NMR Study of Mott Insulating Phase in κ-(BEDT-TTF)2X Family

Abstract The κ-(BEDT-TTF)2X family are quite attractive in the study of metal-insulator transitions and competition between superconductivity and magnetic ordering. We report 13C-NMR study of X= Cu[N(CN)2]Cl salt and discuss on the electronic structure, compared with the results of superconducting salts. In addition, we found that the deuterated Cu[N(CN)2]Br salt, when cooled rapidly, showes AF ordering like the Cu[N(CN)2]Cl salt with only minor fraction of superconducting phase. We also report 13C-NMR study of this salt.