Wouter Montfrooij

Magnetic Spin Ladder (C5H12N)2CuBr4: High Field Magnetization and Scaling Near Quantum Criticality

The magnetization, M(H< or =30 T,0.7< or =T< or =300 K), of (C5H12N)2CuBr4 has been used to identify this system as an S = 1/2 Heisenberg two-leg ladder in the strong-coupling limit, J( perpendicular) = 13.3 K and J( parallel) = 3.8 K, with H(c1) = 6.6 T and H(c2) = 14.6 T. An inflection point in M(H,T = 0.7 K) at half saturation, M(s)/2, is described by an effective XXZ chain. The data exhibit universal scaling behavior in the vicinity of H(c1) and H(c2), indicating that the system is near a quantum critical point.

Zener double exchange from local valence fluctuations in magnetite.

Magnetite (Fe3O4) is a mixed valent system where electronic conductivity occurs on the B site (octahedral) iron sublattice of the spinel structure. Below T(V)=123 K, a metal-insulator transition occurs which is argued to arise from the charge ordering of 2+ and 3+ iron valences on the B sites (Verwey transition). Inelastic neutron scattering measurements show that optical spin waves propagating on the B site sublattice (approximately 80 meV) are shifted upwards in energy above T_{V} due to the occurrence of B-B ferromagnetic double exchange in the mixed valent phase. The double exchange interaction affects only spin waves of Delta(5) symmetry, not all modes, indicating that valence fluctuations are slow and the double exchange is constrained by short-range electron correlations above T(V).

Cage diffusion in liquid mercury

We present inelastic neutron scattering measurements on liquid mercury at room temperature for wave numbers q in the range 0.3 <q<7.0 A(-1). We find that the energy half width of the incoherent part of the dynamic structure factor S(q,E) is determined by a self-diffusion process. The half width of the coherent part of S(q,E) shows the characteristic behavior expected for a cage diffusion process. We also show that the response function at small wave numbers exhibits a quasielastic mode with a time scale characteristic of cage diffusion, however, its intensity is larger by an order of magnitude than what would be expected for cage diffusion. We speculate on a scenario in which the intensity of the cage diffusion mode at small wave numbers is amplified through a valence fluctuation mechanism.

Angular and time resolution of neutron time of flight spectrometers

Abstract We show that the angular resolution Δφ and time resolution Δt of neutron time-of-flight spectrometers can be determined accurately by measuring the scattering spectrum of superfluid 4He at low temperature. The method is used to determine Δφ and Δt of the IRIS spectrometer at ISIS (UK), yielding the values (FWHM) Δφ=0.118 rad (6.8°) and Δt=212 μs, and to reinterpret the results of Blagoveshchenskii et al., showing that their measured linewidths for superfluid 4He can be ascribed solely to the effect of the angular resolution of their time-of-flight spectrometer.

Investigation of the presence of charge order in magnetite by measurement of the spin wave spectrum

Inelastic neutron scattering results on magnetite (Fe{sub 3}O{sub 4}) show a large splitting in the acoustic spin wave branch, producing a 7 meV gap midway to the Brillouin zone boundary at q = (0,0,1/2) and {h_bar}{omega} = 43 meV. The splitting occurs below the Verwey transition temperature, where a metal-insulator transition occurs simultaneously with a structural transformation, supposedly caused by the charge ordering on the iron sublattice. The wavevector (0,0,1/2) corresponds to the superlattice peak in the low symmetry structure. The dependence of the magnetic superexchange on changes in the crystal structure and ionic configurations that occur below the Verwey transition affect the spin wave dispersion. To better understand the origin of the observed splitting, several Heisenberg models intended to reproduce the pair-wise variation of the magnetic superexchange arising from both small crystalline distortions and charge ordering were studied. None of the models studied predicts the observed splitting, whose origin may arise from charge-density wave formation or magnetoelastic coupling.

Non-Fermi-liquid behavior in Ce(Ru1-xFex)2Ge2: cause and effect

We present inelastic neutron scattering measurements on the intermetallic compounds

Influence of the Verwey transition on the spin-wave dispersion of magnetite

\mathrm{Ce}{({\mathrm{Ru}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x})}_{2}{\mathrm{Ge}}_{2}

Cluster formation in quantum critical systems

(

The role of grain boundaries in determining the transport properties of magnetite

x=0.65

Direct determination of the helium interatomic core potential by neutron scattering

, 0.76, and 0.87). These compounds represent samples in a magnetically ordered phase, at a quantum critical point, and in the heavy-fermion phase, respectively. We show that at high temperatures the three compositions have the identical response of a local moment system. However, at low temperatures the spin fluctuations in the critical composition are given by non-Fermi-liquid dynamics, while the spin fluctuations in the heavy-fermion system show a simple exponential decay in time. In both compositions, the lifetime of the fluctuations is determined solely by the distance to the quantum critical point. We discuss the implications of these observations regarding the possible origins of non-Fermi-liquid behavior in this system.