Anomalous dynamical line shapes in a quantum magnet at finite temperature

Abstract

The effect of thermal fluctuations on the dynamics of a gapped quantum magnet is studied using inelastic neutron scattering on copper nitrate, a model material for the spin-1/2, one-dimensional (1D) bond alternating Heisenberg chain. A large, highly deuterated, single-crystal sample of copper nitrate is produced using a solution growth method and measurements are made using the high-resolution backscattering spectrometer OSIRIS at the ISIS Facility. Theoretical calculations and numerical analysis are combined to interpret the physical origin of the thermal effects observed in the magnetic spectra. The primary observations are (1) a thermally induced central peak due to intraband scattering, which is similar to Villain scattering familiar from soliton systems in 1D, and (2) the one-magnon quasiparticle pole is seen to develop with temperature into an asymmetric continuum of scattering. We relate this asymmetric line broadening to a thermal strongly correlated state caused by hard-core constraints and quasiparticle interactions. These findings are a counter example to recent assertions of the universality of line broadening in 1D systems and are applicable to a broad range of quantum systems.

Time-dependent correlations in quantum magnets at finite temperature

“…Note that the signal obtains an asymmetric tail towards higher energies at T = 2 K. This phenomenon was already reported in Ref. [9] for copper nitrate and in Ref. [10] for BaCu 2 V 2 O 8 using conventional INS.…”

“…The dimers are coupled by a weaker interaction J . For copper nitrate the coupling constants are J = 0.443 meV and J = 0.101 meV [9]. Hence the alternation ratio α = J /J ≈ 0.227 is rather small and the ground state of the chain is close to the product state with singlets on each dimer, r |s r = r (| ↑↓ r − | ↓↑ r )/ √ 2.…”

“…However, recent investigations of one-dimensional [9,10] and three-dimensional [11] materials of coupled spin dimers * benedikt.fauseweh@tu-dortmund.de † felix.groitl@psi.ch ‡ goetz.uhrig@tu-dortmund.de using inelastic neutron scattering (INS) show that the lineshape in the frequency domain develops an asymmetric tail. It has been argued that such tails may be found in a broad range of quantum systems.…”

“…Using conventional INS it has been shown that copper nitrate [Cu(NO 3 ) 2 × 2.5 D 2 O], a model material for a onedimensional alternating Heisenberg chain (AHC), displays a temperature dependent asymmetric lineshape in the frequency domain [9,23,24]. The material was recently investigated in the time domain using the high resolution neutron resonance spin-echo triple-axis spectroscopy (NRSE-TAS) [25].…”

“…In a previous study of the material [25], the experimental results in the time domain were analyzed by fitting it to a phenomenological formula [9]. This formula models an asymmetric lineshape in the frequency domain and was used to compare NRSE-TAS and ToF data to demonstrate the capabilities of NRSE-TAS.…”