E. Steep

Magnetic field-temperature phase diagram of the organic conductor α-(BEDT-TTF)2KHg(SCN)4

We present systematic magnetic torque studies of the “magnetic field-temperature” phase diagram of the layered organic conductor α-(BEDT-TTF)2KHg(SCN)4 at fields nearly perpendicular and nearly parallel to the highly conducting plane. The shape of the phase diagram is compared to that predicted for a charge-density-wave system in a broad field range.

High-field studies of the H-T phase diagram of α-(BEDT-TTF)2KHg(SCN)4

Abstract Detailed comparative studies of magnetoresistance and torque of α-(BEDT-TTF) 2 KHg(SCN) 4 as a function of temperature and magnetic field in fields up to 28 T have been carried out. At high fields a phase boundary is directly shown to exist between the high- and low-temperature regions. At fields 16 ⩽ H ⩽ 21 T two maxima have been observed in the R ( T ) curves revealing likely two successive phase transitions. The field is shown to suppress the transition into the magnetic groundstate, at least in the low-field region.

B–T–P phase diagram of α-(BEDT-TTF)2KHg(SCN)4

We present detailed studies of the B-T-P phase diagram of α-(BEDT-TTF) 2 KHg(SCN) 4 by means of magnetic torque and magnetoresistance measurements. The phase diagram for magnetic field oriented almost perpendicular to the highly conducting layers closely resembles that expected for a charge-density-wave (CDW) system with an imperfectly nested Fermi surface. The influence of pressure can be readily understood as deteriorating the nesting conditions and thus enhancing the role of the orbital effect of magnetic field. At field strongly tilted from the normal to the layers, both the torque and magnetoresistance display a number of irreversible features that may be associated with a series of first-order phase transitions. This behaviour is not understood within the existing CDW theories.

High field magnetization measurements on α-(BEDTTTF)2KHg(SCN)4

We report torque measurements of the steady and oscillatory magnetization of α-(BEDTue5f8TTF)2KHg(SCN)4, where BEDTue5f8TTF is bis(ethylenedithio)tetrathiafulvalene, at temperatures down to 0.4 K in magnetic fields around at the magnetic phase transition at 24 T, the so called kink transition. The effective cyclotron mass changes only slightly at the kink transition being m∗=mme=1.77 (me is the free electron mass) above 24 T and m∗–1.7 at 20 T for a magnetic field perpendicular to the highly conducting ac plane (Θ=0°). For Θ=60.2° we find m∗=3.55 (B – 18 T). In the low field state the amplitude of the oscillatory magnetization (μ0Mosc=7.5 × 10−7 T) is small; this corresponds with a high Dingle temperature (3–4 K). At 24 T a strong increase in the oscillation amplitude appears accompanied by a decrease in the Dingle temperature to TD ⩽ 2 K. The steady torque shows a quadratic increase with field at low B-values and tends to saturate near the phase transition for small angles Θ. For tilt angles Θ⩾60° the typical increase of the de Haas-van Alphen amplitude at the kink transition is no longer observed, but several anomalies occur in the steady torque above 15 T; even at 27 T α-(BEDTue5f8TTF)2KHg(SCN)4 is in a complex magnetic groundstate.

Investigations of Fermi surfaces and cyclotron effective masses of organic superconductors by Shubnikov-de Haas (SdH)- and de Haas-van Alphen (dHvA)-measurements

Abstract The Fermi surfaces (FS) and effective masses of the organic superconductors (BEDO-TTF) 2 ReO 4 (H 2 O) ( 1 ) and κ-(BEDT-TTF) 2 I 3 ( 2 ) were investigated by SdH- and dHvA-measurements in magnetic fields up to 27T in the temperature range from 0.5K to 4.2K. In 1 two small closed pockets (0.7% and 1.5% of the first Brillouin zone [FBZ] are observed corresponding very well with two cross-sectional areas of the FS obtained for a hole and an electron pocket from tight binding calculations. In contrast to 1 in 2 two relatively large closed sections (13% and 85% of the FBZ) of the FS are observed, again confirming the tight binding calculations. For 2 in magnetic fields above 12T the effective mass for the larger orbit, as obtained from the temperature dependence of the SdH-oscillation amplitudes, is magnetic field dependent as long as the field is arranged perpendicular to the conducting planes. In contrast, from dHvA-measurements — which were performed by turning the magnetic field by 27° with respect to the SdH-experiments — the observed effective mass is field independent (m ∗ = 3.9m O ). We suppose that the occurrence of particles with fractional statistics at temperatures below 1K and in fields above 12T might be the reason for the observed field dependence of the effective mass in the SdH investigations.

Fermi surface studies of the borocarbide superconductor YNi2B2C

The Fermi surface of the nonmagnetic borocarbide superconductor YNi2B2C was studied by the de Haas-van Alphen (dHvA) effect. Five branches of dH-vA frequencies are observed in the normal state for B ∥ c and the angular dependence was investigated with magnetic fields in the (110) plane. Together with the effective cyclotron masses, mc, the result is discussed in the context of electronic band-structure calculations.

dHvA studies of NbSe2 using the torque method

Abstract We present torque measurements on 2H-NbSe2 in strong magnetic fields up to 27 T and at temperatures between 0.4 K and 4.2 K. The angular dependence of the de Haas-van Alphen (dHvA) frequency and of the effective mass has been studied for angles between 5° and 89° yielding a larger lateral extent and a smaller thickness of the well-known pancake Fermi surface compared to previous estimations. Two so-called spin zeros were detected at the angles Θ=43° and Θ=14.5°. Assuming the same angular dependence for the effective mass as for the dHvA frequency we give an estimation of the effective g-factor for the case that g is isotropic to g=1.2 ± 0.1. Measurements below the critical field Bc2 show first of all a stronger damping of the oscillations compared to the normal state and, depending on the field sweep rate, a slight frequency shift.

Forbidden orbits in the magnetic breakdown regime of κ-(BEDT-TTF)2Cu(NCS)2

Abstract We report on de Haas–van Alphen measurements on the quasi two-dimensional organic superconductor κ -(BEDT-TTF) 2 Cu(NCS) 2 in magnetic fields up to 27xa0T. Above 16xa0T where magnetic breakdown occurs we observe besides F α (closed orbit) and F β (breakdown orbit) also the difference frequency F β − α which cannot be explained by the semiclassical theory of magnetic breakdown. We will discuss the experimental results, also the observation of unusual effective electron masses, in the context of chemical potential oscillations.

Anomalous damping effects of magneto-quantum oscillations in the extremely 2D electronic system κ-(BEDT-TTF)2I3

Abstract The electronic system of the organic superconductor κ-(BEDT-TTF) 2 I 3 (BEDT-TTF = bis(ethylenedithiolo)tetrathiafulvalene) is identified as extremely two-dimensional (2D). The topology of the Fermi surface (FS) was investigated by means of Shubnikov-de Haas (SdH) as well as de Haas-van Alphen (dHvA) experiments focusing on the verification of the 2D character of the system. This two-dimensionality specially takes effect as soon as the magnetic field is oriented exactly perpendicular to the conducting planes (i.e. Θ = 0°). Under such conditions strong anomalous damping effects in the field and temperature dependence of quantum oscillation amplitudes are observed. These anomalous damping effects are discussed in terms of the occurrence of quasi-particle excitations with fractional statistics (QPFS) which may only occur in extremely 2D systems at high magnetic fields and low temperatures (i.e., only when ħω C ≫ k B T ). Taking up these requirements, the aim of this work is to quantify the extreme two-dimensionality of the electronic system of κ-(BEDT-TTF) 2 I 3 and to show that the observed damping effects in fact are determined by the ratio ħω C / k B T . These facts may support the interpretation of the observed damping effects of quantum oscillation amplitudes at high magnetic fields, low temperatures and Θ = 0° as generated by the possible occurrence of such QPFS.

Correlation between a new frequency (F0 = 13.2 t) and anomalous damping effects of magneto-quantum oscillations in the 2D organic superconductor κ-(BEDT-TTF)2I3

Abstract A new magneto-quantum oscillation (QO) frequency F0=13.2 T is observed in the normal conducting state of the strong two-dimensional (2D) organic superconductor κ-(BEDT-TTF)2I3. Such a frequency is neither predicted by band-structure calculations nor yet observed in Shubnikov–de Haas experiments on any κ-structured material of the BEDT-TTF family. An oscillatory structure with the same frequency is observed in the anomalous damping of the QO amplitudes of the known frequencies F2=570 T and F3=3883 T. These damping effects are restricted to the magnetic field orientation perpendicular to the conducting (b,c)-planes (i.e., Θ=0°), high fields B>12 T and low temperatures T