Spin dynamics in the quasi-one-dimensional S = 1/2 Heisenberg antiferromagnet copper benzoate

The one-dimensional {dollar}S=1/2{dollar} Heisenberg antiferromagnet (HAFM) is one of the oldest many-body problems in condensed matter physics, dating back to the Bethe Ansatz solution of 1931. Experimental interest in this problem gained considerable momentum with the discovery of materials which exhibit quasi-one-dimensional magnetic behavior. Copper benzoate, {dollar}rm Cu(Csb6Hsb5COO)sb2cdot3Hsb2O,{dollar} is a quasi-one-dimensional {dollar}S=1/2{dollar} antiferromagnet with near-Heisenberg interactions. In copper benzoate, a unique set of experimental requirements are met which make it a good choice for the study of spin dynamics in the {dollar}S=1/2{dollar} HAFM in a magnetic field. The magnetic coupling strength J is small enough to permit significant magnetization in an applied magnetic field, yet large enough that the low energy part of the excitation spectrum may be explored by inelastic neutron scattering.; I report on our experiments with copper benzoate using the techniques of AC magnetic susceptibility, heat capacity, and inelastic neutron scattering to probe the spin dynamics of this material. We determine the exchange constant {dollar}J=1.57{dollar} meV and explore low field and low temperature properties by AC susceptibility measurements. The dynamic correlation function {dollar}{lcub}cal S{rcub}(tilde q,omega){dollar} is measured with inelastic neutron scattering in zero magnetic field, where we find that copper benzoate exhibits only 1D spin correlations for {dollar}hbaromega>0.4{dollar} meV at T = 1.8 K. Further inelastic scattering measurements explore the temperature dependence of the generalized susceptibility {dollar}chisp{lcub}primeprime{rcub}(q,omega){dollar} and find good agreement with scaling predictions derived from field theory predictions.; Our central result is that neutron scattering from copper benzoate provides the first direct experimental evidence for field-dependent incommensurate low energy modes in a one-dimensional spin S= 1/2 antiferromagnet. Soft modes occur for wavevectors {dollar}tilde q=pipmdeltatilde q(H){dollar} where {dollar}deltatilde q(H)approx2pi M(H)/gmusb{lcub}B{rcub}{dollar} as predicted by Bethe ansatz and spinon descriptions of the S = 1/2 chain. A result not predicted by these theories is a field-induced energy gap {dollar}Delta(H)propto Hsp{lcub}alpha{rcub},{dollar} where {dollar}alpha=0.65(3){dollar} as determined from specific heat measurements.