X. Chen

New clues in the mystery of the ground state of the (BEDT-TTF)2MHg(SCN)4 family of organic conductors

Abstract Since the discovery of a kink in the magnetoresistance of (BEDT-TTF)2KHg(SCN)4 by the Tokyo group [Osada et al., Phys. Rev. B41 (1990) 5428] several years ago, this family (where K may be replaced by NH4, Tl, or Rb) has become one of the most exciting areas of organic conductor research. At the heart of the matter is the interplay of the open and closed orbit sections of the Fermi surface, in which subtle details of topology apparently play major roles in the low temperature state (LTS). Recent experiments involving angular dependent magnetoresistance, and determinations of magnetic breakdown orbits provide clues to the mechanisms which act in the LTS. We present a model which addresses the anomalous magnetoresistance phenomena in the LTS. The model involves the connection via magnetic breakdown of otherwise disconnected closed hole and open electron bands.

Specific heat measurements of pressure-induced reentrant superconductivity in Eu0.9Ho0.1Mo6S8

Specific heat measurements of Eu0.9Ho0.1Mo6S8 were performed using an ac calorimetry technique under the extreme conditions of low temperature, high magnetic field, and hydrostatic pressure. The title compound is a Chevrel phase material, which is paramagnetic at ambient pressure, becomes superconducting for pressures above 7 kbar, and has a reentrant upper critical field [dHc2(T)/dT≥0, as T→0 K]. Evidence that this material also has a high magnetic field‐induced superconducting phase at low temperatures (H≥5 T, T≤1 K) was previously reported. The present ac calorimetry study carried out for 0.15≤T≤6 K, 0≤H≤20 T, and P≂8 kbar, shows a sharp superconducting transition for T≤1 K in the specific heat, which broadens at higher fields and temperatures. The results are analyzed to obtain a reentrant phase diagram in agreement with previous transport work, but direct evidence for the high field‐induced phase was not found. The mechanism for the field and temperature dependence of the superconductivity of Eu0.9Ho...

Magneto-oscillatory behavior and magnetism in quasi-two-dimensional organic conductors

Abstract We have studied the magneto-oscillatory behavior of quasi-two-dimensional (quasi-2D) organic conductors of the type (BEDT-TTF)2XHg(SCN)4, where X = K, Rb, NH4, in order to probe the underlying magnetic behavior of these materials. The K and Rb compounds are metallic down to 50 mK, but are not superconducting. They both display a dual series of magneto-oscillations, a magnetic ordering at low temperatures, below about 10 K, and a sharp drop in the magnetoresistance associated with the destruction of this phase with the application of high magnetic fields at low temperatures. In contrast, the NH4 material is a superconductor at low temperatures (below 1K) up to a pressure of at least 4 kbar. It also displays a complex background magnetoresistance upon which is superposed a single oscillation series. In all three organic materials, there is a strong interaction between the underlying magnetic phases (which influences the open orbits) and the magneto-oscillations which result from the quasi-2D Fermi surface behavior.

Fermi surface and magnetic properties of low-dimensional organic conductors

Abstract We report systematic high-magnetic-field and low-temperature measurements on several members of the α-(BEDT- TTF)2MHg(SCN)4 family of low-dimensional organic conductors. For M = K, transport, high-pressure and magnetic studies show the presence of a magnetic phase which is removed either with high magnetic fields or with high pressure. For M = Rb, transport measurements indicate the existence of a magnetic phase similar to the case for M = K, but with a higher transition temperature and critical field. For both M = K and Rb the split Landau level phenomenon is observed. We also report the pressure dependence of the Fermi surface for M = NH4 and K.

Pressure effects on the low temperature state of the α-(BEDT-TTF)2MHg(SCN)4 organic conductor family

Abstract Systematic magnetotransport measurements as a function of hydrostatic pressure have been made on four members of the α-(BEDT-TTF) 2 MHg(SCN) 4 organic conductor family (M = K, Tl, Rb, NH 4 ). Applied pressure above about 6 Kbar removes the density wave state for M = K, Tl, and Rb. For M = NH 4 the superconducting state is removed with pressure as dT c dP = − 0.25 K/Kbar . In all cases the Shubnikov de Haas oscillation frequency increases with pressure, including the β orbit (which involves the entire Brillouin zone), and new orbits involving very small fractions of the Fermi surface are formed.

Magnetic ordering and Fermi surface effects in the low-dimensional organic conductor α-(BEDT-TTF)2RbHg(SCN)4

Members of the family of low-dimensional organic conductors α-(BEDT-TTF) 2 MHg(SCN) 4 , with M=K, Rb, Tl, or NH 4 , have a Fermi surface (FS) consisting of both quasi-two-dimensional (Q2D) closed hole orbits and quasi-one-dimensional (Q1D) open orbits. The charge transfer salts with M=K, Rb, and Tl undergo an antiferro-magnetic ordering transition at about 10 K, which results in a density wave ground state. We have performed magnetoresistance (MR) measurements on α-(BEDT-TTF) 2 RbHg(SCN) 4 in order to understand details of the FS, and the nature of the interaction of the FS with the magnetic ordering in this material

Magnetic phase diagram for the quasi-two-dimensional organic conductor α-(BEDT-TTF)2RbHg(SCN)4

We have studied the magnetoresistance (MR) of the low-dimensional organic charge transfer salt α-(BEDT-TTF)2RbHg(SCN)4 at low temperatures and at high dc magnetic fields. Electrical conduction occurs via both the quasi-two-dimensional closed hole orbit portion of the Fermi surface (FS), as well as the quasi-one-dimensional open orbit FS. An antiferromagnetic ordering transition occurs in this material below 10.5 K, which results in a density wave ground state. The consequences of this ordering can be seen in the MR background, which measures the quasi-1D FS. This ordering can be broken with the application of magnetic fields in excess of 32 T for T < 4 K, or with applied pressures of about 8 kbar.

Magnetoresistance and SdH Oscillation of α-(BEDT-TTF)2RbHg(SCN)4 at low temperatures and high pressures

Abstract We report the first systematic measurement of magnetoresistance (MR) and Shubnikov-de Haas Oscillation (SdHO) in the organic conductor α-(BEDT-TTF)2RbHg(SCN)4, at low temperatures T = 150 mK to 3 K, high magnetic fields H=0 to 20 T, and pressures P=0 kbar to 12 kbar, where BEDT-TTF (abbreviated ET and hereafter) represents bis(ethylenedithiolo)tetrathiafalvalene. SdHO frequency F=637.6 T at 0 kbar and increases with pressure (dF/dP = 11.74 T/kbar). Splitting in SdHO is observed at low pressures and the effect is enhanced by increasing pressure until it disappears at above 4 kbar. No superconductivity is found. The results are compared to an early study on a related material α-(BEDT-TTF)2KHg(SCN)4. The phenomena are discussed in the regime of available mechanisms.

superconductivity and localipation in the density pave state of the α-(BEDT-TTF)2MHg(SCN)4 organic conductor family

Abstract pe have measured the low temperature transport and magnetic behavior in α-(BEDTTTF) 2 RbHg(SCN) 4 [alpha-bis(ethylenedithio) tetrathiafulvalene-rubidium-mercurp-thiocpanate]. Below 12 K this material is believed to go into a spin density pave state. At lower temperatures, below 1 K, a further transition to a lower resistance state occurs, which may be a tendency for superconductivity. We have determined the corresponding T - H phase diagram for the new state, and report shielding experiments which show less than 0.1% diamagnetic effect consistent pith filamentary superconductivity. pe also consider the effect from the point of view of a peak anti-localization mechanism.