arXiv subjects

W. Montfrooij

Publications and source records attributed to W. Montfrooij.

7 recordsLinked to original sources

arXiv2024

A comparison of the hidden order transition in URu$_2$Si$_2$ to the $λ$-transition in $^4$He

The low-temperature states of ambient URu$_2$Si$_2$ and superfluid $^4$He are both characterized by momentum-dependent energy gaps between the ground and excited states. This behavior weakly persists even above the transition temperatures but becomes over-damped (ungapped) because of the number of excitations present at elevated temperature. We show that akin to the normal fluid to superfluid transition in $^4$He, the hidden-order (HO) transition in URu$_2$Si$_2$ can be understood by a change of the ungapped excitations to the gapped, elementary excitations (EE) of the unknown ordered state. These under-damped EEs reflect the basic character and order parameters of the different phase transitions. This view accounts for the full amount of entropy released in these transitions, the jumps in the resistivity and thermal conductivity directly below the transition, as well as the reduction of the Fermi surface. We argue that the behavior in the HO phase is that of a gas of weakly interacting excitations from charge density wave or crystal field states in a similar manner to that of the phonon-roton excitations of the superfluid $^4$He phase. We discuss the influence of applying pressure and magnetic fields within this scenario and the role of the small moment antiferromagnetic clustering in the hidden order phase.

cond-mat.str-el↗

arXiv2007

Zener double exchange from local valence fluctuations in magnetite

Magnetite (Fe$_{3}$O$_{4}$) is a mixed valent system where electronic conductivity occurs on the B-site (octahedral) iron sublattice of the spinel structure. Below $T_{V}=122$ 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 ($\sim$80 meV) are shifted upwards in energy above $T_{V}$ due to the occurrence of B-B ferromagnetic double exchange in the mixed valent metallic phase. The double exchange interaction affects only spin waves of $Δ_{5}$ symmetry, not all modes, indicating that valence fluctuations are slow and the double exchange is constrained by electron correlations above $T_{V}$.

cond-mat.str-el↗

arXiv2006

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

Inelastic neutron scattering results on magnetite (Fe3O4) 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 E = 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, we have constructed a series of Heisenberg models intended to reproduce the pairwise variation of the magnetic superexchange arising from both small crystalline distortions and charge ordering. We find that none of the models studied predicts the observed splitting, whose origin may arise from charge-density wave formation or magnetoelastic coupling.

cond-mat.str-el↗

arXiv2005

Non-Fermi-liquid behavior in Ce(Ru$_{1-x}$Fe$_x$)$_2$Ge$_2$: cause and effect

We present inelastic neutron scattering measurements on the intermetallic compounds Ce(Ru$_{1-x}$Fe$_x$)$_2$Ge$_2$ ($x$=0.65, 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.

cond-mat.str-el↗

arXiv2004

The Role of Grain Boundaries in Determining the Transport Properties in Magnetite

We present magnetoresistance and Hall-effect measurements on single crystal magnetite Fe3O4 close to the Verwey transition Tv = 123.8 K. We show that the formation of grain boundaries accompanying the reduction in crystal symmetry plays a significant role in the electron scattering mechanism, and that grain boundaries can account for the apparent change in sign of the charge carriers below Tv.

cond-mat.str-el↗

arXiv2003

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 Å$^{-1}$. We find that the energy halfwidth of the incoherent part of the dynamic structure factor $S(q,E)$ is determinded by a self-diffusion process. The halfwidth 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 quasi-elastic 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.

cond-mat.dis-nn↗

arXiv2002

non-Local Quantum Criticality in Ce(Ru_{1-x} Fe_x)_2 Ge_2 (x = x_c = 0.76)

We present the results of inelastic neutron scattering measurements, performed near the antiferromagnetic quantum critical point in Ce(Ru_{0.24} Fe_{0.76})_2 Ge_2. Both local and long range fluctuations of the local moments are observed, but due to the Kondo effect only the latter are critical. We propose a phenomenological expression which fits the energy $E$, temperature T and wave vector q dependences of the generalized susceptibility chi(q,E,T), describing the non-Fermi liquid E/T scaling found at every wave vector.

cond-mat.str-el↗