Quantum spin dynamics

Abstract:

We describe a quantum-mechanical model of Fermi-surface traversal resonance (FTR), a magneto-optical resonance that occurs in quasi-one-dimensional metals. We show that the predictions of this model are in quantitative agreement with earlier semiclassical models of FTR. The agreement between the two approaches, whose starting assumptions are very different, demonstrates that it is a fundamental property of quasi-one-dimensional systems. © 1999 The American Physical Society.

Abstract:

Fermi-surface traversal resonance (FTR) is caused by the periodic motion of carriers in a magnetic field across open sections of Fermi surface (FS). Owing to the warping of the FS, the real space velocities of the carriers oscillate, generating resonances in the high frequency conductivity which may be described by a semiclassical model. A rectangular resonant cavity, oscillating at 70 GHz, which can rotate in the external magnetic field, has been used to confirm the existence of the effect in the organic metal alpha-(BEDT-TTF)(2)KHg(SCN)(4). The data contain a great deal of information about the FS, including the direction and anharmonicity of warping components. A quantum mechanical model is presented which predicts all of the features of FTR appearing in the semiclassical model. This confirms that FTR is a fundamental property of few-dimensional systems, existing under a very wide range of conditions.

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.