Quantum spin dynamics

Abstract:

We present the results of a study of the millimetre-wave response of the organic metal alpha-(BEDT-TTF)(2)NH4Hg(SCN)(4), between 500 mK and 4.2 K and in magnetic fields of up to 17 T. Two relatively broad features are interpreted as a two dimensional (2D) cyclotron resonance (CR) with a CR mass m(2D)(CR) = 1.35 +/- 0.1 m(e), and a 1D CR with CR mass m(1D)(CR) = 0.85 +/- 0.05 m(e). Conduction electron spin resonance and magnetic quantum oscillations are also observed. Analysis of the quantum oscillations yields an effective mass of m(RF)* = 2.7 +/- 0.2 m(e).

Abstract:

Electrons in cyclotron orbits around closed pockets in the Fermi surfaces of organic metals possess an oscillating real-space velocity. Higher harmonics of this real-space velocity lead to predicted cyclotron resonances additional to those normally expected. We calculate these resonances semiclassically using the Boltzmann transport equation. These higher harmonics are expected to occur remarkably often, and we show that they are found even in a very simple tight-binding model. A similar effect occurs in quasi-one-dimensional Fermi surfaces which are highly corrugated and in this case the oscillating part of the real-space velocity as it traverses the Fermi surface can couple directly to the microwave frequency. © 1997 The American Physical Society.

Abstract:

In undoped pure single crystals of the mixed valence compound SmB6 anomalous ESR absorption is observed in the frequency range v=40-120 GHz at temperatures of 1.8-4.2 K. The ESR for the case of the coherent ground state consists of two components corresponding to g-factors g1=1.907±0.003 and g2=1.890±0.003. The amplitude of both ESR lines strongly depends on temperature in the temperature range studied: the amplitude of the first line with g=gt increases and the amplitude of the second line decreases with temperature. A model based on consideration of intrinsic defects in the SmB6 crystalline lattice, with a density ∼1015-1016 cm-3, is suggested as an explanation for the anomalous ESR-behavior. In the frequency range v>70 GHz at T=4.2 K, in addition to the main ESR lines, a new magnetic resonance with a hysteretic field dependence is discovered. © 1996 American Institute of Physics.