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F. Demmel

Publications and source records attributed to F. Demmel.

At least 19 recordsLinked to original sources
arXiv2025

Zeeman split Kramers doublets in spin-supersolid candidate Na$_{2}$BaCo(PO$_{4}$)$_{2}$

Na$_{2}$BaCo(PO$_{4}$)$_{2}$ is a triangular antiferromagnet that displays highly efficient adiabatic demagnetization cooling (J. Xiang $\textit{et al.}$ Nature ${\bf{625}}$, 270 (2024)) near a quantum critical point at $\mu_{0}H_{c}\sim 1.6$ T, separating a low-field magnetically disordered from a high-field fully polarized ferromagnetic phase. We apply high resolution backscattering neutron spectroscopy in an applied field to study the magnetic excitations near $\mu_{0}H_{c}$. At large fields we observe ferromagnetic fluctuations that gradually transition to being overdamped in energy below $\mu_{0}H_{c}$ where the magnetism is spatially disordered. We parameterize the excitations in the high field polarized phase in terms of coupled Zeeman split Kramers doublets originating from the presence of spin-orbit coupling. On reducing the field, the splitting between the Kramers doublets is reduced and if done adiabatically, provides a mechanism for reducing temperature. On lowering the applied field through the $\mu_{0}H_{c}$ the excitations characterize a textured phase that we suggest is inefficient for cooling. Low temperature disordered frustrated magnets built on Kramers doublets with nearby quantum critical points provide a route for efficient magnetocalorics.

cond-mat.str-el↗
arXiv2022

Quantum critical spin-liquid-like behavior in S = 1/2 quasikagome lattice CeRh1-xPdxSn investigated using muon spin relaxation and neutron scattering

We present the results of muon spin relaxation ($\mu$SR) and neutron scattering on the Ce-based quasikagome lattice CeRh$_{1-x}$Pd$_{x}$Sn ($x=0.1$ to 0.75). Our ZF-$\mu$SR results reveal the absence of static long-range magnetic order down to 0.05~K in $x = 0.1$ single crystals. The weak temperature-dependent plateaus of the dynamic spin fluctuations below 0.2~K in ZF-$\mu$SR together with its longitudinal-field (LF) dependence between 0 and 3~kG indicate the presence of dynamic spin fluctuations persisting even at $T$ = 0.05~K without static magnetic order. On the other hand, $C_{\text{4f}}$/$T$ increases as --log $T$ on cooling below 0.9~K, passes through a broad maximum at 0.13~K and slightly decreases on further cooling. The ac-susceptibility also exhibits a frequency independent broad peak at 0.16~K, which is prominent with an applied field $H$ along $c$-direction. We, therefore, argue that such a behavior for $x=0.1$ (namely, a plateau in spin relaxation rate ($\lambda$) below 0.2~K and a linear $T$ dependence in $C_{\text{4f}}$ below 0.13~K) can be attributed to a metallic spin-liquid (SL) ground state near the quantum critical point in the frustrated Kondo lattice. The LF-$\mu$SR study suggests that the out of kagome plane spin fluctuations are responsible for the SL behavior. Low energy inelastic neutron scattering (INS) of $x$ = 0.1 reveals gapless magnetic excitations, which are also supported by the behavior of $C_{\text{4f}}$ proportional to $T^{1.1}$ down to 0.06~K.

cond-mat.str-el↗
arXiv2020

Non-linear soliton confinement in weakly coupled antiferromagnetic spin chains

We analyze the low-energy dynamics of quasi one dimensional, large-$S$ quantum antiferromagnets with easy-axis anisotropy, using a semi-classical non-linear sigma model. The saddle point approximation leads to a sine Gordon equation which supports soliton solutions. These correspond to the movement of spatially extended domain walls. Long-range magnetic order is a consequence of a weak inter-chain coupling. Below the ordering temperature, the coupling to nearby chains leads to an energy cost associated with the separation of two domain walls. From the kink-antikink two-soliton solution, we compute the effective confinement potential. At distances large compared to the size of the solitons the potential is linear, as expected for point-like domain walls. At small distances the gradual annihilation of the solitons weakens the effective attraction and renders the potential quadratic. From numerically solving the effective one dimensional Schr\"oedinger equation with this non-linear confinement potential we compute the soliton bound state spectrum. We apply the theory to CaFe$_{2}$O$_{4}$, an anisotropic $S=5/2$ magnet based upon antiferromagnetic zig-zag chains. Using inelastic neutron scattering, we are able to resolve seven discrete energy levels for spectra recorded slightly below the N\'eel temperature $T_\textrm{N}\approx 200$~K. These modes are well described by our non-linear confinement model in the regime of large spatially extended solitons.

cond-mat.str-el↗
arXiv2019

Spinon Confinement and a Sharp Longitudinal Mode in Yb$_2$Pt$_2$Pb in Magnetic Fields

The fundamental excitations in an antiferromagnetic chain of spins-1/2 are spinons, de-confined fractional quasiparticles that when combined in pairs, form a triplet excitation continuum. In an Ising-like spin chain the continuum is gapped and the ground state is N{\'e}el ordered. Here, we report high resolution neutron scattering experiments, which reveal how a magnetic field closes this gap and drives the spin chains in \YPP\ to a critical, disordered Luttinger-liquid state. In \YPP\ the effective spins-1/2 describe the dynamics of large, Ising-like Yb magnetic moments, ensuring that the measured excitations are exclusively longitudinal, which we find to be well described by time-dependent density matrix renormalization group calculations. The inter-chain coupling leads to the confinement of spinons, a condensed matter analog of quark confinement in quantum chromodynamics. Insensitive to transverse fluctuations, our measurements show how a gapless, dispersive longitudinal mode arises from confinement and evolves with magnetic order.

cond-mat.str-el↗
arXiv2018

From Ising resonant fluctuations to static uniaxial order in antiferromagnetic and weakly superconducting CeCo(In$_{1-x}$Hg$_{x}$)$_{5}$ ($x$=0.01)

CeCo(In$_{0.990}$Hg$_{0.010}$)$_{5}$ is a charge doped variant of the $d$-wave CoCoIn$_{5}$ superconductor with coexistent antiferromagnetic and superconducting transitions occurring at T$_{N}$= 3.4 K and T$_{c}$=1.4 K, respectively. We use neutron diffraction and spectroscopy to show that the magnetic resonant fluctuations present in the parent superconducting phase are replaced by collinear $c$-axis magnetic order with three-dimensional Ising critical fluctuations. No low energy transverse spin fluctuations are observable in this doping-induced antiferromagnetic phase and the dynamic resonant spectral weight predominately shifts to the elastic channel. Static ($\tau$ $>$ 0.2 ns) collinear Ising order is proximate to superconductivity in CeCoIn$_{5}$ and is stabilized through hole doping with Hg.

cond-mat.supr-con↗
arXiv2017

Orphan spins and bound in-gap states in the $S={5\over2}$ antiferromagnet CaFe$_{2}$O$_{4}$

CaFe$_{2}$O$_{4}$ is an anisotropic $S={5\over 2}$ antiferromagnet with two competing $A$ ($\uparrow \uparrow \downarrow \downarrow$) and $B$ ($\uparrow \downarrow \uparrow \downarrow$) magnetic order parameters separated by static antiphase boundaries at low temperatures. Neutron diffraction and bulk susceptibility measurements, show that the spins near these boundaries are weakly correlated and a carry an uncompensated ferromagnetic moment that can be tuned with a magnetic field. Spectroscopic measurements find these spins are bound with excitation energies less than the bulk magnetic spin-waves and resemble the spectra from isolated spin-clusters. Localized bound orphaned spins separate the two competing magnetic order parameters in CaFe$_{2}$O$_{4}$.

cond-mat.mtrl-sci↗
arXiv2017

Short-range dynamics in the solid and liquid phases

The existence of the phonon-roton minimum has been widely observed for both the solid and liquid phases but so far there is no sufficient theoretical explanation of its origin. In this paper we use a range of techniques to study the dynamics and short-range order for a range of simple materials in their crystalline, amorphous and liquid phases. We perform inelastic neutron scattering (INS) experiments of polycrystalline and liquid barium to study the high-frequency dynamics and understand the mechanisms underlying the atomic motion. Moreover we perform INS simulations for crystals and supercooled liquids, compare the collective excitation spectra and identify similarities. We perform molecular dynamics (MD) simulations for the same materials and present results of population and bond angle distribution showing a short-range order dependence of the different phases, expanding the current knowledge in literature. Finally, we support our findings with a theoretical explanation of the origin of the phonon-roton minima which is observed in both solids and liquids. We study this as a classical phenomenon and we base our explanation on short range interatomic interactions.

cond-mat.other↗
arXiv2016

Solitary magnons in the $S={5\over2}$ antiferromagnet CaFe$_{2}$O$_{4}$

CaFe$_{2}$O$_{4}$ is a $S={5\over 2}$ anisotropic antiferromagnet based upon zig-zag chains having two competing magnetic structures, denoted as the A ($\uparrow \uparrow \downarrow \downarrow$) and B ($\uparrow \downarrow \uparrow \downarrow$) phases, which differ by the $c$-axis stacking of ferromagnetic stripes. We apply neutron scattering to demonstrate that the competing A and B phase order parameters results in magnetic antiphase boundaries along $c$ which freeze on the timescale of $\sim$ 1 ns at the onset of magnetic order at 200 K. Using high resolution neutron spectroscopy, we find quantized spin wave levels and measure 9 such excitations localized in regions $\sim$ 1-2 $c$-axis lattice constants in size. We discuss these in the context of solitary magnons predicted to exist in anisotropic systems. The magnetic anisotropy affords both competing A+B orders as well as localization of spin excitations in a classical magnet.

cond-mat.str-el↗
arXiv2016

Magnetic transitions in the topological magnon insulator Cu(1,3-bdc)

Topological magnon insulators are a new class of magnetic materials that possess topologically nontrivial magnon bands. As a result, magnons in these materials display properties analogous to those of electrons in topological insulators. Here, we present magnetization, specific heat, and neutron scattering measurements of the ferromagnetic kagome magnet Cu(1,3-bdc). Our measurements provide a detailed description of the magnetic structure and interactions in this material, and confirm that it is an ideal prototype for topological magnon physics in a system with a simple spin Hamiltonian.

cond-mat.str-el↗
arXiv2015

Finite-temperature scaling of spin correlations in a partially magnetized Heisenberg $S=1/2$ chain

Inelastic neutron scattering is employed to study transverse spin correlations of a Heisenberg $S=1/2$ chain compound in a magnetic field of 7.5 T. The target compound is the antiferromagnetic Heisenberg $S=1/2$ chain material 2(1,4-Dioxane)$\cdot$2(H$_2$O)$\cdot$CuCl$_2$, or CuDCl for short. The validity and the limitations of the scaling relation for the transverse dynamic structure factor are tested, discussed and compared to the Tomonaga-Luttinger spin liquid theory and to Bethe-ansatz results for the Heisenberg model.

cond-mat.str-el↗
arXiv2014

The effect of substitution of Mn for Ni on the magnetic and transport properties of CeNi0.8-xMnxBi2

We report the results of Mn substitution for Ni in CeNi0.8Bi2, (i.e. CeNi0.8-xMnxBi2). All the samples have an antiferromagnetic ordered state below TN = 5.0 K due to localized 4f-magnetic moment on the Ce ions. Besides this antiferromagnetic ordering caused by Ce, the magnetic and transport properties are abruptly changed with increasing Mn contents at the boundary composition of x = 0.4. The magnetic state is changed into a ferromagnetic state around 200 K for x > 0.4, where the electrical resistivity is strongly suppressed to become simple metallic. These results of ferromagnetism and metallicity can be explained by the double-exchange mechanism. The mixed valence states of Ni and Mn ions are confirmed by X-ray photoelectron spectroscopy (XPS). For x <= 0.4, the initial Ni3+ state gradually changes to the Ni2+ state with increasing x up to 0.4. On further increase of x > 0.4, the Ni2+ state is substituted for the Mn2+ state, which gradually changes to the final Mn3+ state. We also present an inelastic neutron scattering (INS) measurements on CeNi0.8Bi2 (i.e. x=0) between 1.2 and 12 K. The high energy INS study reveals the presence of two well defined crystal electric field (CEF) excitations near 9 meV and 19 meV at 1.2 K and 6 K, while the low energy INS study reveals the presence of quasi-elastic scattering above 4 K. We will discuss our INS results of CeNi0.8Bi2 based on the crystal electric field model.

cond-mat.str-el↗
arXiv2013

Competing 4f-electron dynamics in Ce(Ru1-xFex)2Al10 (x=0 to 1): magnetic ordering emerging from the Kondo semiconducting state

We have carried out muon spin relaxation (muSR), neutron diffraction and inelastic neutron scattering (INS) investigations on polycrystalline samples of Ce(Ru1-xFex)2Al10 (x=0, 0.3, 0.5, 0.8 and 1) to investigate the nature of the ground state (magnetic ordered versus paramagnetic) and the origin of the spin gap formation as evident from the bulk measurements in the end members. Our zero-field muSR spectra clearly reveal coherent two-frequency oscillations at low temperature in x=0, 0.3 and 0.5 samples, which confirms the long-range magnetic ordering of the Ce-moment with TN=27, 26 and 21 K respectively. On the other hand the muSR spectra of x=0.8 and x=1 down to 1.4 K and 0.045 K, respectively exhibit a temperature independent Kubo-Toyabe term confirming a paramagnetic ground state. The long-range magnetic ordering in x=0.5 below 21 K has been confirmed through the neutron diffraction study. INS measurements of x=0 clearly reveal the presence of a sharp inelastic excitation near 8 meV between 5 K and 26 K, due to an opening of a gap in the spin excitation spectrum, which transforms into a broad response at and above 30 K. Interestingly, at 4.5 K the spin gap excitation broadens in x=0.3 and exhibits two clear peaks at 8.4(3) and 12.0(5) meV in x=0.5. In the x=0.8 sample, which remains paramagnetic down to 1.2 K, there is a clear signature of a spin gap of 10-12 meV at 7 K, with a strong Q-dependent intensity. Evidence of a spin gap of 12.5(5) meV has also been found in x=1. The observation of a spin gap in the paramagnetic samples (x=0.8 and 1) is an interesting finding in this study and it challenges our understanding of the origin of the semiconducting gap in CeT2Al10 (T=Ru and Os) compounds in terms of hybridization gap opening only a small part of the Fermi surface, gapped spin waves, or a spin-dimer gap.

cond-mat.str-el↗
arXiv2012

From incommensurate correlations to mesoscopic spin resonance in YbRh2Si2

Spin fluctuations are reported near the magnetic field driven quantum critical point in YbRh2Si2. On cooling, ferromagnetic fluctuations evolve into incommensurate correlations located at q0=+/- (delta,delta) with delta=0.14 +/- 0.04 r.l.u. At low temperatures, an in plane magnetic field induces a sharp intra doublet resonant excitation at an energy E0=g muB mu0 H with g=3.8 +/- 0.2. The intensity is localized at the zone center indicating precession of spin density extending xi=6 +/- 2 A beyond the 4f site.

cond-mat.str-el↗
arXiv2012

Magnetic field splitting of the spin-resonance in CeCoIn5

Neutron scattering in strong magnetic fields is used to show the spin-resonance in superconducting CeCoIn5 (Tc=2.3 K) is a doublet. The underdamped resonance (\hbar \Gamma=0.069 \pm 0.019 meV) Zeeman splits into two modes at E_{\pm}=\hbar \Omega_{0}\pm g\mu_{B} \mu_{0}H with g=0.96 \pm 0.05. A linear extrapolation of the lower peak reaches zero energy at 11.2 \pm 0.5 T, near the critical field for the incommensurate "Q-phase" indicating that the Q-phase is a bose condensate of spin excitons.

cond-mat.supr-con↗
arXiv2011

Dynamical Response of Single Bi-layer Spin Model : A Theoretical Analysis

The spin dynamics of a single bi-layer ferromagnetic model, as proposed for a manganite system $La_{1.2}Sr_{1.8}Mn_{2}O_{7}$ as an approximate minimal model, is studied both below and above the bulk magnetic transition temperature using the semi-classical Monte Carlo-molecular dynamics technique. The quantities studied are:- (i) the static spin configurations, (ii) the spin auto-correlation function (SAF) $C(t)$ and (iii) the dynamical structure function (DSF) $S(\bf q, ω)$. The major aim is to probe the nature of collective modes and in particular to ascertain whether the propagating modes can exist above the ordering temperature. We find that the typical spin configurations contain a high degree of mis-aligned spins, particularly in the high temperature phase. Nevertheless, in the low temperature phase a long range ferromagnetic ordering can be seen. The shape of the curves corresponding to $S(\bf q, ω)$ vs. $ω$ in the constant $\bf q$-scan are found in general to be quite complex viz. containing peaks, cusps and broad plateau. Moreover, in a large regime of $(\bf q, ω)$ space, $S(\bf q ,ω)$ is found to be negative, signalling a total breakdown of the semi-classical approximation in the presence of the enormous thermodynamic and quantum fluctuations. Considering the physically allowed regime only, we can theoretically extract an effective $ω$ vs. $q$ dispersion curve for the collective excitations, which exhibits a slope opposite to that expected for a full three-dimensional system and disagrees with the results of inelastic neutron scattering experiments in the nature of the dispersion curve along the $<100>$ direction in the ordered phase. The theoretical results also exhibit the existence of propagating modes even in the high temperature (disordered) phase from our calculations based on a minimal single bi-layer ferromagnetic model.

cond-mat.str-el↗
arXiv2010

Level crossings and zero-field splitting in the \{Cr$_8$\}-cubane spin-cluster by inelastic neutron scattering and magnetization studies

Inelastic neutron scattering in variable magnetic field and high-field magnetization measurements, at the milikelvin temperature range, were performed to gain insight into the low-energy magnetic excitation spectrum and the field-induced level crossings in the molecular spin cluster \{Cr$_8$\}-cubane. These complementary techniques provide consistent estimates of the lowest level-crossing field. The overall features of the experimental data are explained using an isotropic Heisenberg model, based on three distinct exchange interactions linking the eight Cr$^{\text{III}}$ paramagnetic centers (spins $s = 3/2$), that is supplemented with a relatively large molecular magnetic anisotropy term for the lowest S=1 multiplet. It is noted that the existence of the anisotropy is clearly evident from the magnetic field dependence of the excitations in the INS measurements, while the magnetization measurements are not sensitive to its effects.

cond-mat.mes-hall↗
arXiv2009

Anomalous spin-waves and the commensurate-incommensurate magnetic phase transition in LiNiPO4

Detailed spin-wave spectra of magneto-electric LiNiPO4 have been measured by neutron scattering at low temperatures in the commensurate (C) antiferromagnetic (AF) phase with ordering temperature 20.8 K. An anomalous low-energy mode is observed at the modulation vector of the incommensurate (IC) AF phase appearing above the 20.8 K. A linear spin-wave model based on Heisenberg exchange couplings and single ion anisotropies accounts for all the observed spin-wave dispersions and intensities. Along the b axis an unusually strong next-nearest-neighbor AF coupling competes with the dominant nearest-neighbor AF exchange interaction and causes the IC structure.

cond-mat.mtrl-sci↗