arXiv subjects

Wouter Montfrooij

Publications and source records attributed to Wouter Montfrooij.

At least 19 recordsLinked to original sources

arXiv2026

Breakdown of the periodic potential ansatz in correlated electron systems

Our electronic structure theory for crystalline solids is commonly built on the periodic potential assumption $V(\mathbf r)=V(\mathbf r+\mathbf R)$ for every lattice translation $\mathbf R$, enabling Bloch eigenstates, crystal momentum as a good quantum number, and the standard quasiparticle-based description of the behavior of metals. Because the zero-point motion of the ions, however, in correlated electron systems the electronic environment experienced by an itinerant electron is neither static nor self-averaging at the single-particle level, even in perfectly stoichiometric crystals, leading to a distribution of local Kondo scales that spans two orders of magnitude in temperature. We discuss, through a comparison between uniform scenarios and one that breaks with perfect lattice translational symmetry, how incorporating this distribution yields a unified description for all heavy-fermion systems at the quantum critical point.

cond-mat.str-el↗

arXiv2023

A Quantitative Solution to the Kondo Lattice Problem

Metallic Kondo Lattice systems that have been prepared to exhibit a competition between ordering of magnetic moments and shielding of those moments by the conduction electrons down to absolute zero display unusual low-temperature responses. Here we show that the dominant response of such systems is caused by two quantum effects: zero-point motion of the ions, and the size of the system restricting the allowed wavelengths of fluctuations. This zero-point motion of the ions induces a broad distribution in Kondo shielding temperatures that renders the assumption of a uniform heavy-fermion ground state untenable. However, letting go of this assumption and instead incorporating these two quantum effects leads to percolation physics that quantitatively captures the non-Fermi liquid response in both stoichiometric and doped quantum critical compounds, allowing for a unified description of all quantum critical systems.

cond-mat.str-el↗

arXiv2023

Role of the single-particle dynamics in the transverse current autocorrelation function of a liquid metal

A very recent simulation study of the transverse current autocorrelation of the Lennard-Jones fluid revealed, as expected, that this function can be perfectly described within the exponential expansion theory. However, above a certain wavevector $Q$, not only transverse collective excitations are found to propagate in the fluid, but a second oscillatory component of unclear origin (thereby called X) must be considered to properly account for the time behavior of the correlation. Here we present an extended investigation of the transverse current autocorrelation of liquid Au as obtained by ab initio molecular dynamics in the very wide range 5.7 nm$^{-1}$ $\le Q \le$ 32.8 nm$^{-1}$ in order to follow the behavior of the X component, if present, also at large $Q$ values. By combining the study of the transverse current autocorrelation with the analogous analysis of its self part, we show that the second oscillatory component originates from the longitudinal dynamics and appears in the same form as a collective excitation is represented in the single-particle behavior. Therefore, the signature of the longitudinal processes (sound waves) in the transverse current autocorrelation is not due to often conjectured couplings of longitudinal and trasverse modes, but descends from the self part of the function, which contains the traces of all processes acting in the fluid as the density of states, that is the spectrum of the velocity autocorrelation function, does.

cond-mat.stat-mech↗

arXiv2022

Onset of two collective excitations in the transverse dynamics of a simple fluid

A thorough analysis of the transverse current autocorrelation function obtained by molecular dynamics simulations of a dense Lennard-Jones fluid reveals that even such a simple system is characterized by a varied dynamical behavior with changing length scale. By using the exponential expansion theory, we provide a full account of the time correlation at wavevectors $Q$ between the upper boundary of the hydrodynamic region and $Q_p/2$, with $Q_p$ the position of the main peak of the static structure factor. In the $Q$ range studied we identify and accurately locate the wavevector at which shear wave propagation starts to take place, and show clearly how this phenomenon may be represented by a damped harmonic oscillator changing, in a continuous way, from an overdamped to an underdamped condition. The decomposition into exponential modes allows one to convincingly establish not only the crossover related to the onset of transverse waves but, surprisingly, also the existence of a second pair of modes equivalent to another oscillator that undergoes, at higher $Q$ values, a similarly smooth over- to underdamped transition.

cond-mat.stat-mech↗

arXiv2021

Modeling neutron and X-ray scattering by liquids

We review exact formalisms for describing the dynamics of liquids in terms of static parameters. We discuss how these formalisms are prone to suffer from imposing restrictions that appear to adhere to common sense but which are overly restrictive, resulting in a flawed description of the dynamics of liquids. We detail a fail-safe way for modeling the scattering data of liquids that is free from any unwarranted restriction and that models the scattering using the fewest possible number of free parameters. We also list some common habits in analyzing data and how these habits do not do justice to the accuracy of the results obtained in scattering experiments, and how these habits may stand in the way of rejecting some models used in describing the dynamics of liquids.

cond-mat.stat-mech↗

arXiv2021

Evidence for magnetic clusters in stoichiometric quantum critical CeRu$_2$Si$_2$

Systems that have been prepared to undergo a second-order phase transition at zero Kelvin, the so-called quantum critical systems, appear to fall into two categories: (chemically) heavily-doped systems where the unusual properties can be related to a disorder-induced distribution of Kondo shielding temperatures, and (almost) stoichiometric systems where the departures from Fermi-liquid theory have been attributed to intrinsic instabilities. Here we show that this distinction is not as clear cut and that magnetic clusters associated with a distribution of Kondo shielding temperatures are also present in CeRu$_2$Si$_2$, a system close to a quantum critical point. By revisiting published data on this system and comparing them to the results for heavily-doped quantum critical Ce(Ru$_{0.755}$Fe$_{0.245}$)$_2$Ge$_2$, we show that clusters exist in both systems at low temperatures, and that the moments of the Ce-ions within these clusters have all lined up with their neighbors. This implies that the dominant physics that drives heavily-doped systems, namely spontaneous formation of magnetic clusters, should also play a leading role in the response of homogenous systems. This represents a notable departure of how the physics that governs quantum critical points is treated in the literature.

cond-mat.str-el↗

arXiv2021

The effects of disorder on Harris-criterion violating percolation

We present the results of computer simulations on a class of percolative systems, called protected percolation, that violates the Harris criterion. The Harris criterion states whether the critical behavior at a phase transition from a disordered state to an ordered state will be altered by impurities. We have incorporated impurities into our simulations to test whether the critical exponents for protected percolation are altered by impurities. We find that the critical exponents for three-dimensional protected percolation simulations indeed change with impurities in the form of missing sites and immortal sites. On the other hand, the critical exponents for both standard percolation and protected percolation in two dimensions are stable against impurities.

cond-mat.stat-mech↗

arXiv2020

Protected percolation: a new universality class pertaining to heavily-doped quantum critical systems

We present the results of computer simulations on a class of percolative systems that forms a new universality class. We show the results for the critical exponents for this new class, inferred from simulations of two- and three-dimensional lattices consisting of up to one billion lattice sites. These new percolative systems differ from standard percolative systems in that once a cluster breaks off the lattice spanning cluster, its sites become protected and cannot be removed. This situation closely mimics the situation in heavily-doped quantum critical systems where isolated magnetic clusters are protected from (further) Kondo screening. Our results indicate that protected percolation violates the Harris criterion, which yields a natural explanation as to why universal exponents for quantum phase transitions have been elusive.

cond-mat.str-el↗

arXiv2018

Quantum critical behavior in Ce(Fe$_{0.76}$Ru$_{0.24}$)$_2$Ge$_2$: the full story

Systems with embedded magnetic ions that exhibit a competition between magnetic order and disorder down to absolute zero can display unusual low temperature behaviors of the resistivity, susceptibility, and specific heat. Moreover, the dynamic response of such a system can display hyperscaling behavior in which the relaxation back to equilibrium when an amount of energy E is given to the system at temperature T only depends on the ratio E/T. Ce(Fe$_{0.755}$Ru$_{0.245}$)$_2$Ge$_2$ is a system that displays these behaviors. We show that these complex behaviors are rooted in a fragmentation of the magnetic lattice upon cooling caused by a distribution of local Kondo screening temperatures, and that the hyperscaling behavior can be attributed to the flipping of the total magnetic moment of magnetic clusters that spontaneously form and order upon cooling. We present our arguments based on the review of two-decades worth of neutron scattering and transport data on this system, augmented with new polarized neutron scattering experiments.

cond-mat.str-el↗

arXiv2016

Identification of the low-energy excitations in a quantum critical system

We have identified low-energy magnetic excitations in a doped quantum critical system by means of polarized neutron scattering experiments. The presence of these excitations could explain why Ce(Fe$_{0.76}$Ru$_{0.24}$)$_2$Ge$_2$ displays dynamical scaling in the absence of local critical behavior or long-range spin-density wave criticality. The low-energy excitations are associated with the reorientations of the superspins of fully ordered, isolated magnetic clusters that form spontaneously upon lowering the temperature. The system houses both frozen clusters and dynamic clusters, as predicted by Hoyos and Vojta [Phys. Rev. B 74, 140401 (R) (2006)].

cond-mat.str-el↗

arXiv2016

Hard-sphere behavior in the dynamics of all monoatomic liquids at the de Gennes minimum

We show that the position of the de Gennes minimum in scattering spectra, where the dynamics of liquids shows down, is given by a hard-sphere expression for a range of mono-atomic liquids that crystallize in a close packed structure. This expression relates the position of the minimum to the number density of the liquid, without any adjustable or unknown parameters. We argue that this implies that a liquid can be viewed as a close packed structure of the cages that represent the confinement of atoms by their neighbors. We further show that some metals deviate from this expression, namely those metals that crystallize in a structure that is not close packed. Our expression should prove very useful in identifying what liquids to study in inelastic scattering experiments given that deviations from normal fluid behavior can already be predicted based on the peak position of the static structure factor.

cond-mat.soft↗

arXiv2016

On the extraction of paramagnon excitations from resonant inelastic X-ray scattering experiments

Resonant X-ray scattering experiments on high-temperature superconductors and related cuprates have revealed the presence of intense paramagnon scattering at high excitation energies, of the order of several hundred meV. The excitation energies appear to show very similar behavior across all compounds, ranging from magnetically ordered, via superconductors, to heavy fermion systems. However, we argue that this apparent behavior has been inferred from the data through model fitting which implicitly imposes such similarities. Using model fitting that is free from such restrictions, we show that the paramagnons are not nearly as well-defined as has been asserted previously, and that some paramagnons might not represent propagating excitations at all. Our work indicates that the data published previously in the literature will need to be re-analyzed with proper models.

cond-mat.supr-con↗

arXiv2012

Modified percolation theory and its relevance to quantum critical phenomena

We present the results of a percolation-like model that has been restricted compared to standard percolation models in the sense that we do not allow finite sized clusters to break up once they have formed. We calculate the critical exponents for this model and derive relationships between these exponents and those of standard percolation models. We argue that this restricted model represents a new universality class that is directly relevant to the critical physics as observed in quantum critical systems, and we describe under what conditions our percolation results can be applied to the observed temperature and field dependencies of the specific heat and susceptibility in such systems.

cond-mat.stat-mech↗

arXiv2011

Cluster formation in quantum critical systems

The presence of magnetic clusters has been verified in both antiferromagnetic and ferromagnetic quantum critical systems. We review some of the strongest evidence for strongly doped quantum critical systems (Ce(Ru$_{0.24}$Fe$_{0.76}$)$_2$Ge$_2$) and we discuss the implications for the response of the system when cluster formation is combined with finite size effects. In particular, we discuss the change of universality class that is observed close to the order-disorder transition. We detail the conditions under which clustering effects will play a significant role also in the response of stoichiometric systems and their experimental signature.

cond-mat.str-el↗

arXiv2010

Comment on "Evidence for a non Fermi liquid phase in Ge-substituted YbRh$_2$Si$_2$"

In a recent paper, Custers {\it et al.} \cite{custers} argue for the existence of a new metallic quantum critical phase at 0 K in the Ge-doped heavy-fermion system YbRh$_2$Si$_2$ in the presence of magnetic frustration. In here we discuss the consequences of this identification for the (more standard) field induced quantum critical phase.

cond-mat.str-el↗

arXiv2009

Magnetic excitations in the spinel compound Li$_x$[Mn$_{1.96}$Li$_{0.04}$]O$_4$ (x= 0.2, 0.6, 0.8, 1.0): how a classical system can mimic quantum critical scaling

We present neutron scattering results on the magnetic excitations in the spinel compounds Li$_x$[Mn$_{1.96}$Li$_{0.04}$]O$_4$ (x= 0.2, 0.6, 0.8, 1.0). We show that the dominant excitations below T ~ 70 K are determined by clusters of Mn^4+ ions, and that these excitations mimic the E/T-scaling found in quantum critical systems that also harbor magnetic clusters, such as CeRu$_{0.5}$Fe$_{1.5}$Ge$_2$. We argue that our results for this classical spinel compound show that the unusual response at low temperatures as observed in quantum critical systems is (at least) partially the result of the fragmentation of the magnetic lattice into smaller units. This fragmentation in quantum critical systems is the direct and unavoidable result of intrinsic disorder.

cond-mat.str-el↗

arXiv2008

Teaching superfluidity at the introductory level

Standard introductory modern physics textbooks do not exactly dwell on superfluidity in 4He. Typically, Bose-Einstein condensation (BEC) is mentioned in the context of an ideal Bose gas, followed by the statement that BEC happens in 4He and that the ground state of 4He exhibits many interesting properties such as having zero viscosity. Not only does this approach not explain in any way why 4He becomes a superfluid, it denies students the opportunity to learn about the far reaching consequences of energy gaps as they develop in both superfluids and superconductors. We revisit superfluid 4He by starting with Feynman's explanation of superfluidity based on Bose statistics as opposed to BEC, and we present exercises for the students that allow them to arrive at a very accurate estimate of the superfluid transition temperature and of the energy gap separating the ground state from the first excited state. This paper represents a self-contained account of superfluidity, which can be covered in one or two lessons in class.

cond-mat.supr-con↗

arXiv2006

The ground state of a quantum critical system

The competition between the tendency of magnetic moments to order at low temperatures, and the tendency of conduction electrons to shield these moments, can result in a phase transition that takes place at zero Kelvin, the quantum critical point (QCP). So far, the ground state of these types of systems has remained unresolved. We present neutron scattering experiments that show that the ground state of a sample representative of a class of QCP-systems is determined by the residual interactions between the conduction electrons, resulting in a state with incommensurate intermediate-range order. However, long-range order is thwarted by quantum fluctuations that locally destroy magnetic moments, leaving the system with too few moments to achieve long-range order.

cond-mat.str-el↗