arXiv subjects

Peter Fouquet

Publications and source records attributed to Peter Fouquet.

12 recordsLinked to original sources

arXiv2026

Dynamical magnetism in the disordered cubic lattice material $\gamma$-${\rm Ba}_{3}{\rm CoNb}_{2}{\rm O}_{9}$

$\gamma$-${\rm Ba}_{3}{\rm CoNb}_{2}{\rm O}_{9}$ realizes a disordered simple-cubic spin-$1/2$ lattice in which Co$^{2+}$ ions randomly occupy one third of the sites, placing the system close to the site-percolation threshold for magnetic order. Specific-heat, susceptibility, neutron spin-echo, and muon spin-rotation measurements reveal a broad thermodynamic crossover, short-range magnetic correlations, and persistent fast spin dynamics down to at least 0.1~K, with no evidence for static order or conventional spin-glass freezing. Monte Carlo simulations yield a broad distribution of orphan spins, finite clusters, and an infinite network. The calculated orphan-spin fraction ($\approx 8.8\%$) agrees well with the weakly correlated spin fraction inferred from magnetization ($\approx 8.2\%$). Exact diagonalization of a diluted $S = 1/2$ Heisenberg model captures the broad magnetic specific-heat anomaly and supports the coexistence of weakly and strongly correlated spin environments. These results support a picture in which spin-$1/2$ quantum fluctuations, together with dilution and proximity to the percolation threshold, can support a disorder-driven dynamical state with short-range correlations in three dimensions, distinct from both classical spin glasses and geometrically frustrated quantum spin liquids.

cond-mat.str-el↗

arXiv2022

Structure and dynamics investigations of a partially hydrogenated graphene/Ni(111) surface

Using helium-3 atom scattering, we have studied the adsorption kinetics, the structure and the diffusional dynamics of atomic hydrogen on the surface of a graphene monolayer on Ni(111). Diffraction measurements reveal a 4$^\circ$ rotated rectangular hydrogen overstructure. Hydrogen adsorption and desorption exhibit activation barriers of $E_a=(89\pm7)\,$meV and $E_d=(1.8\pm0.2)\,$eV, respectively. Helium-3 spin-echo measurements showed no decay of the spatial correlation function (or intermediate scattering function) within the time range of the spectrometer. Hence, we are able to set lower limits for a possible hydrogen surface diffusion rate.

cond-mat.mes-hall↗

arXiv2022

Structural Dynamics of Polymer:Non-Fullerene Organic Solar Cell Blends: A Neutron Spectroscopy Perspective

Organic solar cells (OSCs) based on ADA-type (acceptor-donor-acceptor) non-fullerene acceptors (NFAs) exhibit improved power conversion efficiency (PCE) compared to the conventional fullerene-based analogues. The optoelectronic properties of OSC active layer blends are correlated to their underlying structural dynamics and therefore influence the device performance. Using synergistically different neutron spectroscopy techniques, we studied the dynamics of binary and ternary blends made of the NFAs O-IDTBR and O-IDFBR and the regioregular donor polymer P3HT. Deuteration was considered for a contrast variation purpose. In addition to shedding light on the miscibilty and alloying characters of the blends, a main outcome of this work is the evidenced similar dynamical response of the blend components. This finding is in contrast with our previous neutron spectroscopy and molecular dynamics studies of the fullerene-based blend P3HT:PCBM, where we highlighted distinct behaviors of P3HT and PCBM in terms of the vitrification/frustration of P3HT and the plasticization of PCBM by P3HT upon blending. Alike P3HT vitrification is not presently observed. The absence or the weak vitrification evidenced here is in line with recent reports and is likely related to the improved PCE exhibited by the ADA-type NFA-based OSCs.

cond-mat.mtrl-sci↗

arXiv2021

Motion of water monomers reveals a kinetic barrier to ice nucleation on graphene

The interfacial behaviour of water remains a central question to fields as diverse as protein folding, friction and ice formation[1,2]. While the structural and dynamical properties of water at interfaces differ strongly from those in the bulk, major gaps in our knowledge at the molecular level still prevent us from understanding these ubiquitous chemical processes. Information concerning the microscopic motion of water comes mostly from computational simulation[3,4] but the dynamics of molecules, on the atomic scale, is largely unexplored by experiment. Here we present experimental results combined with ab initio calculations to provide a detailed insight into the behaviour of water monomers on a graphene surface. We show that motion occurs by activated hopping on the graphene lattice. The dynamics of water diffusion displays remarkably strong signatures of cooperative behaviour due to repulsive forces between the monomers. The repulsive forces enhance the monomer lifetime ($t_m \approx 3$ s at $T_S = 125$ K) in a $\textit{free-gas}$ phase that precedes the nucleation of ice islands and, in turn, provides the opportunity for our experiments to be performed. Our results give a unique molecular perspective of barriers to ice nucleation on material surfaces, providing new routes to understand and potentially control the more general process of ice formation.

cond-mat.mtrl-sci↗

arXiv2020

Evolution of Helimagnetic Correlations when approaching the Quantum Critical Point of Mn$_{1-x}$Fe$_x$Si

We present a comprehensive investigation of the evolution of helimagnetic correlations in Mn$_{1-x}$Fe$_x$Si with increasing doping. By combining polarised neutron scattering and high resolution Neutron Spin Echo spectroscopy we investigate three samples with $x$=0.09, 0.11 and 0.14, i.e. with compositions on both sides of the concentration $x^* \sim 0.11$ where the helimagnetic Bragg peaks disappear and between $x^*$ and the quantum critical concentration $x_C \sim 0.17$, where $T_C$ vanishes. We find that the abrupt disappearance of the long range helical periodicity at $x^*$, does not affect the precursor fluctuating correlations. These build up with decreasing temperature in a similar way as for the parent compound MnSi. Also the dynamics bears strong similarities to MnSi. The analysis of our results indicates that frustration, possibly due to achiral RKKY interactions, increases with increasing Fe doping. We argue that this effect explains both the expansion of the precursor phase with increasing $x$ and the abrupt disappearance of long range helimagnetic periodicity at $x^*$.

cond-mat.str-el↗

arXiv2019

Mapping Microstructural Dynamics up to the Nanosecond of the Conjugated Polymer P3HT in the Solid State

We present a detailed study of the structure-dynamics relationship of regio-regular and regio-random PEHT using different neutron scattering techniques. Deuteration is employed to modulate the coherent and incoherent cross-sections, allowing particularly to access both self-motions and collective dynamics of the materials. The measurements are underpinned by extensive quantitative calculations using classical MD, as well as first principles quantum chemistry. MD reproduced well the main structural features and slow motions, and shed light on differences in collective dynamics between Q-values linked with the $π-π$ stacking and the lamellar stacking, with the crystalline phase being the most impacted. On the other hand MD led to a limited description of molecular vibrations. In this context, first principles molecular calculations described well the high-energy vibrational features ( $>$ 900 cm$^{-1}$ ), while periodic calculations allowed to better describe the low- and mid-energy vibrational ranges ( 200-900 cm$^{-1}$ ). The mid-energy range is predominantly associated with both intra-molecular and inter-molecular mode coupling, which encloses information about both the polymer conformation and the polymer packing at short range. One of the outcomes of this study is the validation of the common assumption made that RRa-P3HT is a good approximation for the amorphous phase of RR-P3HT at the macroscopic level. The present work helps to clarify unambiguously the latter point which has been largely overlooked in the literature. We highlight the importance to complement optical spectroscopy techniques with inelastic neutron scattering. The latter offering the advantage of being insensitive to the delocalized $π$-electron system, and thus enabling to infer relevant quantities like conjugation lengths, for instance, impacting properties of conjugated polymer.

cond-mat.mtrl-sci↗

arXiv2018

Dipolar spin ice states with fast monopole hopping rate in CdEr$_2$X$_4$ (X = Se, S)

Excitations in a spin ice behave as magnetic monopoles, and their population and mobility control the dynamics of a spin ice at low temperature. CdEr$_2$Se$_4$ is reported to have the Pauling entropy characteristic of a spin ice, but its dynamics are three-orders of magnitude faster than the canonical spin ice Dy$_2$Ti$_2$O$_7$. In this letter we use diffuse neutron scattering to show that both CdEr$_2$Se$_4$ and CdEr$_2$S$_4$ support a dipolar spin ice state -- the host phase for a Coulomb gas of emergent magnetic monopoles. These Coulomb gases have similar parameters to that in Dy$_2$Ti$_2$O$_7$, i.e. dilute and uncorrelated, so cannot provide three-orders faster dynamics through a larger monopole population alone. We investigate the monopole dynamics using ac susceptometry and neutron spin echo spectroscopy, and verify the crystal electric field Hamiltonian of the Er$^{3+}$ ions using inelastic neutron scattering. A quantitative calculation of the monopole hopping rate using our Coulomb gas and crystal electric field parameters shows that the fast dynamics in CdEr$_2$X$_4$ (X = Se, S) are primarily due to much faster monopole hopping. Our work suggests that CdEr$_2$X$_4$ offer the possibility to study alternative spin ice ground states and dynamics, with equilibration possible at much lower temperatures than the rare earth pyrochlore examples.

cond-mat.str-el↗

arXiv2017

Ultrafast Molecular Transport on Carbon Surfaces: The Diffusion of Ammonia on Graphite

We present a combined experimental and theoretical study of the self-diffusion of ammonia on exfoliated graphite. Using neutron time-of-flight spectroscopy we are able to resolve the ultrafast diffusion process of adsorbed ammonia, NH$_3$, on graphite. Together with van der Waals corrected density functional theory calculations we show that the diffusion of NH$_3$ follows a hopping motion on a weakly corrugated potential energy surface with an activation energy of about 4 meV which is particularly low for this type of diffusive motion. The hopping motion includes further a significant number of long jumps and the diffusion constant of ammonia adsorbed on graphite is determined with $D=3.9 \cdot 10^{-8}~\mbox{m}^2 /\mbox{s}$ at 94 K.

cond-mat.mtrl-sci↗

arXiv2016

Observation of low frequency spin dynamics in INVAR Fe$_{65}$Ni$_{35}$

We present ferromagnetic neutron spin echo measurements and muon spin relaxation ($μ$SR) measurements of the spin dynamics in the ordered phase of the metallic alloy INVAR Fe$_{65}$Ni$_{35}$. Both techniques display evidence of slow spin dynamics in the ordered phase which could be associated with the INVAR effect. Zero field (ZF) and longitudinal field $μ$SR measurements suggest the presence of two separate timescales of slow spin-dynamics in the ordered phase; a fast component corresponding to that observed in our neutron-spin-echo measurements and a second slow component which may be related to the "cluster" spin-fluctuations or to the dynamics of disordered local moments or both as suggested by previous neutron scattering measurements and recent theoretical studies.

cond-mat.str-el↗

arXiv2008

Hidden magnetic frustration by quantum relaxation in anisotropic Nd-langasite

The static and dynamic magnetic properties of the Nd$_3$Ga$_5$SiO$_{14}$ compound, which appears as the first materialization of a rare-earth kagome-type lattice, were re-examined, owing to contradictory results in the previous studies. Neutron scattering, magnetization and specific heat measurements were performed and analyzed, in particular by fully taking account of the crystal electric field effects on the Nd$^{3+}$ ions. One of the novel findings is that the peculiar temperature independent spin dynamics observed below 10 K expresses single-ion quantum processes. This would short-circuit the frustration induced cooperative dynamics, which would emerge only at very low temperature.

cond-mat.str-el↗

arXiv2007

Critical Scattering and Dynamical Scaling in an Heisenberg Ferromagnet Neutron Spin Echo versus Renormalization Group Theory

High resolution Neutron Spin Echo (NSE) spectroscopy was used to investigate the dynamics of an 3D Heisenberg ferromagnet in the exchange-controlled regime over a broad range of temperatures and momentum transfer. These results allow for the first time an extensive comparison between the experimental dynamical critical behavior and the predictions of the Renormalization Group (RG) theory. The agreement is exhaustive and surprising as the RG theory accounts not only for the critical relaxation but also for the shape crossover towards an exponential diffusive relaxation when moving from the critical to the hydrodynamic regime above $T_C$.

cond-mat.stat-mech↗

arXiv2002

The van der Waals Potential between Metastable Atoms and Solid Surfaces: Novel Diffraction Experiments versus Theory

Highly polarizable metastable He* ($\mathrm{2^3S}$) and Ne* ($\mathrm{2^3P}$) atoms have been diffracted from a 100 nm period silicon nitride transmission grating and the van der Waals coefficients $C_3$ for the interaction of the excited atoms with the silicon nitride surface have been determined from the diffraction intensities out to the 10th order. The results agree with calculations based on the non-retarded Lifshitz formula.

quant-ph↗