Spin dynamics near the metal-insulator transition in (TMTSF)2ReO4

We have studied the metal-insulator transition in the organic superconductor (TMTSF)2ReO4, which is accompanied by anion ordering, by means of electron paramagnetic resonance measurements. As a result, a peculiar spin dynamics, indicative of the opening of the spin gap around the Tc, associated with an antiferromagnetic transition was revealed

Magnetic-field-induced phase transitions in the quasi-one-dimensional organic conductor HMTSF-TCNQ

Motivated by an interest to see if the field-induced (FI) phase in the charge-density wave (CDW) system is similar to the field-induced-SDW (FISDW) in (TMTSF)(2)X, (TMTSF: tetramethyltetraselenafulvalene), we examined the magnetic-field-induced phases in a quasi-one-dimensional (Q1D) organic conductor HMTSF-TCNQ (hexamethylene- tetraselenafulvalene- tetracyanoquinodimethane) under a pressure of 1.1 GPa, where the CDW occurring at 30K is suppressed. The work was carried out by measurements of angular-dependent magnetoresistance oscillations and exploratory work on the Hall effect. It turned out that the FI-phase, most likely a FICDW for B > 0.1 T, accompany a quantum Hall effect, and the FI-phase transitions are controlled by the field component along the least conducting axis. Above 10 T, the lowest Landau level of the small 2D Fermi pocket (due to incomplete nesting of Fermi surface) exceeds the Fermi level, reaching the quantum limit. Although there are many differences between the CDW (HMTSF-TCNQ) and SDW ((TMTSF) 2X) systems, a similar scenario for field-induced phases seems to hold. (C) 2014 AIP Publishing LLC

Reconstruction of Fermi surface of (TMTSF)2NO3

Magnetoresistance was investigated for the spin-density-wave state of the organic conductor (TMTSF)2NO3 at ambient pressure and in a tilted magnetic field. The Shubnikov de Haas oscillations were used to calculate the cross-sectional area and to reconstruct the quasi-two-dimensional Fermi surface. We found that the warping of Fermi surface along the c* axis is not asymmetric but depends on the azimuthal angle. Furthermore, there was an evidence that the warping is not regular but twisted along the c* axis, that might explain absence of Yamaji-type resonance.Ministere de l'Education Nationale;Universite de Montpellier II;Conseil Regional du Languedoc-Roussillon;District de Montpellie

H-T phase diagram of λ-(BETS)2FeCl4 under pressure

Interlayer magnetoresistance measurements under the pressure of 6 kbar have been performed for the magnetic field induced organic superconductor λ-(BETS)2FeCl4 to investigate the role of pressure and magnetic field. No evidence of the metal-insulator phase transition is observed down to 1.5 K and a broad superconducting transition occurs. A series of magnetic transition from antiferromagnetic to paramagnetic phase are observed with increasing magnetic field. © EDP Sciences.Region Pays de la Loire;Ministere Delegue a la Recherche et aux Hautes Technologies;CNRS-Chemistry and Physics Departments;Universite d'Angers;Delegation General a l'Armemen

NMR evidences of the coupling between conduction electrons and molecular degrees of freedom in the exotic member of the Bechgaard salt (TMTSF)2FSO3

We performed 77Se- and 19F-NMR measurements on single crystals of (TMTSF)2FSO3 to characterize the electronic structures of different phases in the temperature-pressure phase diagram, determined by precise transport measurements [Jo et al., Phys. Rev. B 67, 014516 (2003)]. We claim that such varieties of electronic states in the refined phase diagram are caused by strong couplings of the conduction electrons with FSO3 anions, especially with the permanent electric dipoles on the anions. We suggest that as temperature decreases, the FSO3 anions form orientational ordering through two steps; first, only the tetrahedrons form an orientational order leaving the orientations of the electronic dipoles in random (transition I); then the dipoles form a perfect orientational order at a lower temperature (transition II). In the intermediate temperature range between transitions I and II, we found an appreciable enhancement of homogeneous and inhomogeneous widths of the 77Se-NMR spectrum. From the analysis of the angular dependence of the linewidth, we attributed these anomalies to the intramolecular charge disproportionation or imbalance and its slow dynamics caused by the coupling with the permanent electric dipole of the anion. Results of 19F-NMR relaxation and lineshape measurements support this picture very well. Electronic structures at higher pressures up to 1.25 GPa are discussed on the basis of the results of the 77Se- and 19F-NMR measurements. ©2016 The Physical Society of Japan

EPR Observation of the dimensional crossover in (TMTSF)2ClO4

High temperature electron spin dynamics in the organic superconductor (TMTSF)2ClO4 (tetramethyltetraselenafulvalenium-perchlorate) was studied by means of EPR measurements. Distinct temperature dependencies were observed from the line components attributed to the highly conductive stacking direction and the transverse plane, respectively. In particular, while the stacking direction component shows a metallic behavior in the whole temperature range investigated, the other component shows an apparent dimensional crossover at around 160 K. © 1998 Elsevier Science Ltd. All rights reserved

Observation of non-trivial quantum hall effect

We report that the quantum Hall effect can be anisotropic in a two-dimensional conducting plane of the organic superconductor (TMTSF) 2ClCO 4 under certain conditions. When the FISDW was depinned with high current and repinned, the hysteresis around N = 3 quantum Hall state disappears anisotropically in ρ xy and ρ yx. It is the first result which shows the anisotropic quantum Hall effect

Possible quantum Hall effect in a magnetic-field-induced phase transition in the quasi-one-dimensional CDW organic conductor, HMTSF-TCNQ

We have studied the angular dependence of magnetoresistance and Hall effect of the CDW organic conductor, HMTSF-TCNQ in order to see whether a magnetic-field-induced phase exists in the charge density wave (CDW) system, similarly to the magnetic-field-induced SDW phases in (TMTSF)(2)X. The anomaly in magnetoresistance was observed only around the pressure where the CDW is almost suppressed, i.e. around 0.8-1.1 GPa, but neither at low pressures (0 and 0.5 GPa) nor at high pressure above 2 GPa. This behavior is quite similar to that of (TMTSF)(2)X. At 1.1 GPa anomalies were found at fields of 0.2 T and 10 T. We speculate that at 1.1 GPa the field-induced phase is located between 0.2 T and 10 T, where 1D Fermi surface sheet and 20 Fermi-surface pocket are present. The R-xy shows plateau structure and R-xx was very small in the same region, suggestive of quantum Hall effect. (C) 2014 Elsevier B.V. All rights reserved

Fabrication of (TMTSF)2PF6 thin crystals in a confined electrode

In this study of crystal engineering of an organic superconductor, the single crystals of (TMTSF)2PF6 were fabricated in the form of thin crystals by the electrocrystallization method with a confined electrode by applying a low current (10 nA). Almost all of the flattened needle-shape thin crystals grown in the specially designed confined electrode had an enhanced anisotropic morphology with a thickness-to-width (t / w) ratio of about 6%, being about 19 μm thick, which is an order of magnitude smaller than that of the representative bulk crystals (t / w ≈ 57 %) grown in a free environment. The X-ray crystal structure analysis of the (TMTSF)2PF6 thin crystals revealed that they have the same crystal structure and packing mode as the bulk ones. The ν4 and ν3 mode frequencies in the micro-Raman spectra corresponding to the C{double bond, long}C in-phase and out-of-phase stretching vibrations were 1457 cm-1 and 1595 cm-1, respectively. These frequencies are in between those of the neutral TMTSF and TMTSF+ cation. The thin crystals also show the same SDW transition at 12.2 K as do the bulk ones. It is concluded that the fabricated thin crystals retain the properties of bulk crystals. © 2007 Elsevier B.V. All rights reserved