Quantum spin dynamics

Abstract:

We describe a range of techniques developed by the Oxford group for use in conjunction with the Millimetre-wave Vector Network Analyser in measurements of magnetic resonance and high-frequency conductivity, at extremely low temperatures and high magnetic fields. Included are a variety of resonant cavity techniques. The cylindrical geometry is used to produce high-Q tuneable cavities, ideally suited to measurements of the frequency and temperature dependence of, for example, cyclotron resonance of carriers in GaAs-(Ga,Al)As heterojunctions. A family of rectangular cavities has been designed specifically for measurements of the angle-dependent high-frequency conductivity of organic molecular metals; these systems allow us either to rotate the whole cavity (containing a sample) in the external magnetic field, thus measuring the dependence of a particular component of the conductivity tensor on magnetic field orientation, or to rotate the sample within the cavity, thus measuring different components of the magneto-conductivity. We also describe a non-resonant measurement using a pressure cell with optical access permitting experiments at up to 1.8 GPa. Examples of data obtained from each technique are included.

Abstract:

We have used a novel millimetre-wave magneto-optical technique to study the angle-dependence of the high-frequency conductivity of the molecular superconductor kappa-(BEDT-TTF)(2)Cu(NCS)(2). The data, strongly suggest that the superconducting gap has nodes directed along the b and c directions of the crystal, in agreement with recent theoretical predictions. This supports the idea that the superconductivity in kappa-(BEDT-TTF)(2)Cu(NCS)(2) is d-wave in nature, and is mediated by spin fluctuations.

Abstract:

We experimentally demonstrate a mechanism for a new kind of magnetic resonance, the Fermi-surface traversal resonance (FTR). This is caused by the periodic traversal of carriers across quasi-one-dimensional (Q1D) sections of Fermi surface (FS) in an external magnetic field. Owing to the warping of the Q1D Fermi sheets, the real space velocities of the carriers oscillate as they cross the FS, generating resonances in the high frequency conductivity of the material. The results contain information about the FS, including the direction and harmonic content of the warping components. Using a rotating resonant-cavity system, FTRs have been observed in alpha-(BEDT-TTF)(2)KHg(SCN)(4). (C) 1998 Elsevier Science B.V. All rights reserved.

Abstract:

The FELIX free-electron laser is continuously tuneable over the wavelength range 5-100 mu m and can provide great power and the possibility of time-resolved experiments in the ns to mu s range. We describe a pulsed magnet specifically designed for semiconductor experiments using FELIX, which provides fields of up to 45 T, with a pulse-shape tailored to the macropulses of the FELIX source. The magnet consists of a conventional outer coil providing fields of up to 30 T with a pulse length of several ms, plus a single-layer inner coil with a rise-time of similar to 10 mu s, matched to duration of the FELIX macropulses. The magnet is to be extended to fields of up to 60 T. (C) 1998 Elsevier Science B.V. All rights reserved.