Spin-Peierls and Antiferromagnetic Phases in Cu{1-x}Zn{x}GeO{3}: A Neutron Scattering Study

Comprehensive neutron scattering studies were carried out on a series of high-quality single crystals of Cu_{1-x}Zn_xGeO_3. The Zn concentration, x, was determined for each sample using Electron Probe Micro-Analysis. The measured Zn concentrations were found to be 40-80% lower than the nominal values. Nevertheless the measured concentrations cover a wide range which enables a systematic study of the effects due to Zn-doping. We have confirmed the coexistence of spin-Peierls (SP) and antiferromagnetic (AF) orderings at low temperatures and the measured phase diagram is presented. Most surprisingly, long-range AF ordering occurs even in the lowest available Zn concentration, x=0.42%, which places important constraints on theoretical models of the AF-SP coexistence. Magnetic excitations are also examined in detail. The AF excitations are sharp at low energies and show no considerable broadening as x increases indicating that the AF ordering remains long ranged for x up to 4.7%. On the other hand, the SP phase exhibits increasing disorder as x increases, as shown from the broadening of the SP excitations as well as the dimer reflection peaks.Comment: 17 preprint style pages, 9 postscript files included. Submitted to Phys. Rev. B. Also available from http://insti.physics.sunysb.edu/~mmartin/pubs.htm

Aging and fluctuation-dissipation ratio in a nonequilibrium qq-state lattice model

A generalized version of the nonequilibrium linear Glauber model with $q$ states in $d$ dimensions is introduced and analyzed. The model is fully symmetric, its dynamics being invariant under all permutations of the $q$ states. Exact expressions for the two-time autocorrelation and response functions on a $d$-dimensional lattice are obtained. In the stationary regime, the fluctuation-dissipation theorem holds, while in the transient the aging is observed with the fluctuation-dissipation ratio leading to the value predicted for the linear Glauber model

Replica-symmetric solutions of a dilute Ising ferromagnet in a random field

We use the replica method in order to obtain an expression for the variational free energy of an Ising ferromagnet on a Viana-Bray lattice in the presence of random external fields. Introducing a global order parameter, in the replica-symmetric context, the problem is reduced to the analysis of the solutions of a nonlinear integral equation. At zero temperature, and under some restrictions on the form of the random fields, we are able to perform a detailed analysis of stability of the replica-symmetric solutions. In contrast to the behaviour of the Sherrington-Kirkpatrick model for a spin glass in a uniform field, the paramagnetic solution is fully stable in a sufficiently large random field

High-field Electron Spin Resonance of Cu_{1-x}Zn_{x}GeO_{3}

High-Field Electron Spin Resonance measurements were made on powder samples of Cu_{1-x}Zn_{x}GeO_{3} (x=0.00, 0.01, 0.02, 0.03 and 0.05) at different frequencies (95, 110, 190, 220, 330 and 440 GHz) at low temperatures. The spectra of the doped samples show resonances whose positions are dependent on Zn concentration, frequency and temperature. The analysis of intensity variation of these lines with temperature allows us to identify them as originating in transitions within states situated inside the Spin Peierls gap. A qualitative explanation of the details of the spectra is possible if we assume that these states in the gap are associated with "loose" spins created near the Zn impurities, as recently theoreticaly predicted. A new phenomenon of quenching of the ESR signal across the Dimerized to Incommensurate phase-boundary is observed.Comment: 4 pages, 5 ps figures in the text, submitted to Phys. Rev. Let