arXiv subjects

D. L. Abernathy

Publications and source records attributed to D. L. Abernathy.

At least 37 records · Page 2Linked to original sources

arXiv2019

Experimental measurements of the temperature-dependent Van Hove function in a $\text{Zr}_{80} \text{Pt}_{20}$ liquid

Even though the viscosity is one of the most fundamental properties of liquids, the connection with the atomic structure of the liquid has proven elusive. By combining inelastic neutron scattering with the electrostatic levitation technique the time-dependent pair-distribution function (i.e. the Van Hove function) has been determined for liquid Zr80Pt20. We show that the decay-time of the first peak of the Van Hove function is directly related to the Maxwell relaxation time of the liquid, which is proportional to the shear viscosity. This result demonstrates that the local dynamics for increasing or decreasing the coordination number of local clusters by one determines the viscosity at high temperature, supporting earlier predictions from molecular dynamics simulations.

cond-mat.mtrl-sci↗

arXiv2019

Coexistence of ferromagnetic and stripe antiferromagnetic spin fluctuations in SrCo$_2$As$_2$

We use inelastic neutron scattering to study energy and wave vector dependence of spin fluctuations in SrCo$_2$As$_2$, derived from SrFe$_{2-x}$Co$_x$As$_2$ iron pnictide superconductors. Our data reveals the coexistence of antiferromagnetic (AF) and ferromagnetic (FM) spin fluctuations at wave vectors $\textbf{Q}_{\rm AF}$=(1,0) and $\textbf{Q}_{\rm FM}$=(0,0)/(2,0), respectively. By comparing neutron scattering results with those of dynamic mean field theory calculation and angle-resolved photoemission spectroscopy experiments, we conclude that both AF and FM spin fluctuations in SrCo$_2$As$_2$ are closely associated with a flat band of the $e_g$ orbitals near the Fermi level, different from the $t_{2g}$ orbitals in superconducting SrFe$_{2-x}$Co$_x$As$_2$. Therefore, Co-substitution in SrFe$_{2-x}$Co$_x$As$_2$ induces a $t_{2g}$ to $e_g$ orbital switching, and is responsible for FM spin fluctuations detrimental to the singlet pairing superconductivity.

cond-mat.str-el↗

arXiv2019

Long range antiferromagnetic order in a rocksalt high entropy oxide

We report for the first time the magnetic structure of the high entropy oxide $(Mg_{0.2}Co_{0.2}Ni_{0.2}Cu_{0.2}Zn_{0.2})O$ using neutron powder diffraction. This material exhibits a sluggish magnetic transition but possesses a long-range ordered antiferromagnetic ground state, as revealed by DC and AC magnetic susceptibility, elastic and inelastic neutron scattering measurements. The magnetic propagation wavevector is k=(1/2, 1/2, 1/2) based on the cubic structure Fm-3m, and the magnetic structure consists of ferromagnetic sheets in the (111) planes with spins antiparallel between two neighboring planes. Inelastic neutron scattering reveals strong magnetic excitations at 100 K that survive up to room temperature. This work demonstrates that entropy-stabilized oxides represent a unique platform to study long range magnetic order with extreme chemical disorder.

cond-mat.mtrl-sci↗

arXiv2018

Discovery of coexisting Dirac and triply degenerate magnons in a three-dimensional antiferromagnet

Topological magnons are emergent quantum spin excitations featured by magnon bands crossing linearly at the points dubbed nodes, analogous to fermions in topological electronic systems. Experimental realization of topological magnons in three dimensions has not been reported so far. Here, by measuring spin excitations (magnons) of a three-dimensional antiferromagnet Cu$_{3}$TeO$_{6}$ with inelastic neutron scattering, we provide direct spectroscopic evidence for the coexistence of symmetry-protected Dirac and triply degenerate nodes, the latter involving three-component magnons beyond the Dirac-Weyl framework. Our theoretical calculations show that the observed topological magnon band structure can be well described by the linear-spin-wave theory based on a Hamiltonian dominated by the nearest-neighbour exchange interaction $J_1$. As such, we showcase Cu$_{3}$TeO$_{6}$ as an example system where Dirac and triply degenerate magnonic nodal excitations coexist, demonstrate an exotic topological state of matter, and provide a fresh ground to explore the topological properties in quantum materials.

cond-mat.str-el↗

arXiv2018

A nuclear quantum effect with pure anharmonicity and the anomalous thermal expansion of silicon

Despite the widespread use of silicon in modern technology, its peculiar thermal expansion is not well-understood. Adapting harmonic phonons to the specific volume at temperature, the quasiharmonic approximation, has become accepted for simulating the thermal expansion, but has given ambiguous interpretations for microscopic mechanisms. To test atomistic mechanisms, we performed inelastic neutron scattering experiments from 100-1500K on a single-crystal of silicon to measure the changes in phonon frequencies. Our state-of-the-art ab initio calculations, which fully account for phonon anharmonicity and nuclear quantum effects, reproduced the measured shifts of individual phonons with temperature, whereas quasiharmonic shifts were mostly of the wrong sign. Surprisingly, the accepted quasiharmonic model was found to predict the thermal expansion owing to a fortuitous cancellation of contributions from individual phonons.

cond-mat.mtrl-sci↗

arXiv2017

Muon spin relaxation and inelastic neutron scattering investigations of all-in/all-out antiferromagnet Nd2Hf2O7

Nd2Hf2O7, belonging to the family of geometrically frustrated cubic rare earth pyrochlore oxides, was recently identified to order antiferromagnetically below T_N = 0.55 K with an all-in/all-out arrangement of Nd3+ moments, however with a much reduced ordered state moment. Herein we investigate the spin dynamics and crystal field states of Nd2Hf2O7 using muon spin relaxation (muSR) and inelastic neutron scattering (INS) measurements. Our muSR study confirms the long range magnetic ordering and shows evidence for coexisting persistent dynamic spin fluctuations deep inside the ordered state down to 42 mK. The INS data show the crytal electric field (CEF) excitations due to the transitions both within the ground state multiplet and to the first excited state multiplet. The INS data are analyzed by a model based on CEF and crystal field states are determined. Strong Ising-type anisotropy is inferred from the ground state wavefunction. The CEF parameters indicate the CEF-split Kramers doublet ground state of Nd3+ to be consistent with the dipolar-octupolar character.

cond-mat.str-el↗

arXiv2017

Stabilization of Polar Nano Regions in Pb-free ferroelectrics

Formation of polar nano regions through solid-solution additions are known to enhance significantly the functional properties of ferroelectric materials. Despite considerable progress in characterizing the microscopic behavior of polar nano regions, understanding their real-space atomic structure and dynamics of formation remains a considerable challenge. Here, using the method of dynamic pair distribution function, we provide direct insights into the role of solid-solution additions towards the stabilization of polar nano regions in the Pb-free ferroelectric of Ba(Zr,Ti)O3. It is shown that for an optimum level of substitution of Ti by larger Zr ions, the dynamics of atomic displacements for ferroelectric polarization are slowed sufficiently, which leads to increased local correlation among dipoles below THz frequencies. The dynamic pair distribution function technique demonstrates unique capability to obtain insights into locally correlated atomic dynamics in disordered materials, including new Pb-free ferroelectrics, which is necessary to understand and control their functional properties.

cond-mat.mtrl-sci↗

arXiv2017

Pseudo-Goldstone magnons in the frustrated S=3/2 Heisenberg helimagnet ZnCr2Se4 with a pyrochlore magnetic sublattice

Low-energy spin excitations in any long-range ordered magnetic system in the absence of magnetocrystalline anisotropy are gapless Goldstone modes emanating from the ordering wave vectors. In helimagnets, these modes hybridize into the so-called helimagnon excitations. Here we employ neutron spectroscopy supported by theoretical calculations to investigate the magnetic excitation spectrum of the isotropic Heisenberg helimagnet ZnCr2Se4 with a cubic spinel structure, in which spin-3/2 magnetic Cr3+ ions are arranged in a geometrically frustrated pyrochlore sublattice. Apart from the conventional Goldstone mode emanating from the (0 0 q) ordering vector, low-energy magnetic excitations in the single-domain proper-screw spiral phase show soft helimagnon modes with a small energy gap of ~0.17 meV, emerging from two orthogonal wave vectors (q 0 0) and (0 q 0) where no magnetic Bragg peaks are present. We term them pseudo-Goldstone magnons, as they appear gapless within linear spin-wave theory and only acquire a finite gap due to higher-order quantum-fluctuation corrections. Our results are likely universal for a broad class of symmetric helimagnets, opening up a new way of studying weak magnon-magnon interactions with accessible spectroscopic methods.

cond-mat.str-el↗

arXiv2017

Effective One-Dimensional Coupling in the Highly-Frustrated Square-Lattice Itinerant Magnet CaCo$_{\mathrm{2}-y}$As$_{2}$

Inelastic neutron scattering measurements on the itinerant antiferromagnet (AFM) CaCo$_{\mathrm{2}-y}$As$_{2}$ at a temperature of 8 K reveal two orthogonal planes of scattering perpendicular to the Co square lattice in reciprocal space, demonstrating the presence of effective one-dimensional spin interactions. These results are shown to arise from near-perfect bond frustration within the $J_1$-$J_2$ Heisenberg model on a square lattice with ferromagnetic $J_1$, and hence indicate that the extensive previous experimental and theoretical study of the $J_1$-$J_2$ Heisenberg model on local-moment square spin lattices should be expanded to include itinerant spin systems.

cond-mat.supr-con↗

arXiv2017

Higgs mode and its decay in a two dimensional antiferromagnet

Condensed-matter analogs of the Higgs boson in particle physics allow insights into its behavior in different symmetries and dimensionalities. Evidence for the Higgs mode has been reported in a number of different settings, including ultracold atomic gases, disordered superconductors, and dimerized quantum magnets. However, decay processes of the Higgs mode (which are eminently important in particle physics) have not yet been studied in condensed matter due to the lack of a suitable material system coupled to a direct experimental probe. A quantitative understanding of these processes is particularly important for low-dimensional systems where the Higgs mode decays rapidly and has remained elusive to most experimental probes. Here, we discover and study the Higgs mode in a two-dimensional antiferromagnet using spin-polarized inelastic neutron scattering. Our spin-wave spectra of Ca$_2$RuO$_4$ directly reveal a well-defined, dispersive Higgs mode, which quickly decays into transverse Goldstone modes at the antiferromagnetic ordering wavevector. Through a complete mapping of the transverse modes in the reciprocal space, we uniquely specify the minimal model Hamiltonian and describe the decay process. We thus establish a novel condensed matter platform for research on the dynamics of the Higgs mode.

cond-mat.str-el↗

arXiv2017

Robust antiferromagnetic spin waves across the metal-insulator transition in hole-doped BaMn$_{2}$As$_{2}$

BaMn$_{2}$As$_{2}$ is an antiferromagnetic insulator where a metal-insulator transition occurs with hole doping via the substitution of Ba with K. The metal-insulator transition causes only a small suppression of the Néel temperature ($T_\mathrm{N}$) and the ordered moment, suggesting that doped holes interact weakly with the Mn spin system. Powder inelastic neutron scattering measurements were performed on three different powder samples of Ba$_{1-x}$K$_{x}$Mn$_{2}$As$_{2}$ with $x=$0, 0.125 and 0.25 to study the effect of hole doping and metallization on the spin dynamics of these compounds. We compare the neutron intensities to a linear spin wave theory approximation to the $J_{1}-J_{2}-J_{c}$ Heisenberg model. Hole doping is found to introduce only minor modifications to the exchange energies and spin gap. The changes observed in the exchange constants are consistent with the small drop of $T_\mathrm{N}$ with doping.

cond-mat.str-el↗

arXiv2016

Hourglass dispersion and resonance of magnetic excitations in the superconducting state of the single-layer cuprate HgBa2CuO4+δ near optimal doping

We use neutron scattering to study magnetic excitations near the antiferromagnetic wave vector in the underdoped single-layer cuprate HgBa2CuO4+δ (superconducting transition temperature Tc ~ 88 K, pseudogap temperature T* ~ 220 K). The response is distinctly enhanced below T* and exhibits a Y-shaped dispersion in the pseudogap state, whereas the superconducting state features an X-shaped (hourglass) dispersion and a further resonance-like enhancement. A large spin gap of about 40 meV is observed in both states. This phenomenology is reminiscent of that exhibited by bilayer cuprates. The resonance spectral weight, irrespective of doping and compound, scales linearly with the putative binding energy of a spin-exciton described by an itinerant-spin formalism.

cond-mat.supr-con↗

arXiv2016

Spin excitations used to probe the nature of the exchange coupling in the magnetically ordered ground state of Pr$_{0.5}$Ca$_{0.5}$MnO$_{3}$

We have used time-of-flight inelastic neutron scattering to measure the spin wave spectrum of the canonical half-doped manganite Pr$_{0.5}$Ca$_{0.5}$MnO$_{3}$, in its magnetic and orbitally ordered phase. The data, which cover multiple Brillouin zones and the entire energy range of the excitations, are compared with several different models that are all consistent with the CE-type magnetic order, but arise through different exchange coupling schemes. The Goodenough model, i.e. an ordered state comprising strong nearest neighbor ferromagnetic interactions along zig-zag chains with antiferromagnetic inter-chain coupling, provides the best description of the data, provided that further neighbor interactions along the chains are included. We are able to rule out a coupling scheme involving formation of strongly bound ferromagnetic dimers, i.e. Zener polarons, on the basis of gross features of the observed spin wave spectrum. A model with weaker dimerization reproduces the observed dispersion but can be ruled out on the basis of discrepancies between the calculated and observed structure factors at certain positions in reciprocal space. Adding further neighbor interactions results in almost no dimerization, i.e. recovery of the Goodenough model. These results are consistent with theoretical analysis of the degenerate double exchange model for half-doping, and provide a recipe for how to interpret future measurements away from half-doping, where degenerate double exchange models predict more complex ground states.

cond-mat.str-el↗

arXiv2016

Momentum and Energy Dependent Resolution Function of the ARCS Neutron Chopper Spectrometer at High Momentum Transfer: Comparing Simulation and Experimen

Inelastic neutron scattering at high momentum transfers (i.e. $Q\ge20$ Å) or DINS provides direct observation of the momentum distribution of light atoms, making it a powerful probe for studying single-particle motions in liquids and solids. The quantitative analysis of DINS data requires an accurate knowledge of the instrument resolution function $R_{i}({Q},E)$ at each $Q$ and energy transfer $E$, where the label $i$ indicates whether the resolution was experimentally observed $i={obs}$ or simulated $i=sim$. Here, we describe two independent methods for determining the total resolution function $R_{i}({Q},E)$ of the ARCS neutron instrument at the Spallation Neutron Source, Oak Ridge National Laboratory. The first method uses experimental data from an archetypical system (liquid $^4$He) studied with DINS, which are then numerically deconvoluted using its previously determined intrinsic scattering function to yield $R_{obs}({Q},E)$. The second approach uses accurate Monte Carlo simulations of the ARCS spectrometer, which account for all instrument contributions, coupled to a representative scattering kernel to reproduce the experimentally observed response $S({Q},E)$. Using a delta function as scattering kernel, the simulation yields a resolution function $R_{sim}({Q},E)$ with comparable lineshape and features as $R_{obs}({Q},E)$, but somewhat narrower due to the ideal nature of the model. Using each of these two $R_{i}({Q},E)$ separately, we extract characteristic parameters of liquid $^4$He such as the intrinsic linewidth $α_2$ (which sets the atomic kinetic energy $\langle K\rangle\simα_2$) in the normal liquid and the Bose-Einstein condensate parameter $n_0$ in the superfluid phase. The extracted $α_2$ values agree well with previous measurements, independently of which $R_i(Q,y)$ is used to analyze the data.

physics.ins-det↗

arXiv2016

Orbital selective spin excitations and their impact on superconductivity of LiFe1-xCoxAs

We use neutron scattering to study spin excitations in single crystals of LiFe$_{0.88}$Co$_{0.12}$As, which is located near the boundary of the superconducting phase of LiFe$_{1-x}$Co$_{x}$As and exhibits non-Fermi-liquid behavior indicative of a quantum critical point. By comparing spin excitations of LiFe$_{0.88}$Co$_{0.12}$As with a combined density functional theory (DFT) and dynamical mean field theory (DMFT) calculation, we conclude that wave-vector correlated low energy spin excitations are mostly from the $d_{xy}$ orbitals, while high-energy spin excitations arise from the $d_{yz}$ and $d_{xz}$ orbitals. Unlike most iron pnictides, the strong orbital selective spin excitations in LiFeAs family cannot be described by anisotropic Heisenberg Hamiltonian. While the evolution of low-energy spin excitations of LiFe$_{1-x}$Co$_x$As are consistent with electron-hole Fermi surface nesting condition for the $d_{xy}$ orbital, the reduced superconductivity in LiFe$_{0.88}$Co$_{0.12}$As suggests that Fermi surface nesting conditions for the $d_{yz}$ and $d_{xz}$ orbitals are also important for superconductivity in iron pnictides.

cond-mat.supr-con↗

arXiv2016

Electron doping evolution of the magnetic excitations in NaFe$_{1-x}$Co$_x$As

We use time-of-flight (ToF) inelastic neutron scattering (INS) spectroscopy to investigate the doping dependence of magnetic excitations across the phase diagram of NaFe$_{1-x}$Co$_x$As with $x=0, 0.0175, 0.0215, 0.05,$ and $0.11$. The effect of electron-doping by partially substituting Fe by Co is to form resonances that couple with superconductivity, broaden and suppress low energy ($E\le 80$ meV) spin excitations compared with spin waves in undoped NaFeAs. However, high energy ($E> 80$ meV) spin excitations are weakly Co-doping dependent. Integration of the local spin dynamic susceptibility $χ^{\prime\prime}(ω)$ of NaFe$_{1-x}$Co$_x$As reveals a total fluctuating moment of 3.6 $μ_B^2$/Fe and a small but systematic reduction with electron doping. The presence of a large spin gap in the Co-overdoped nonsuperconducting NaFe$_{0.89}$Co$_{0.11}$As suggests that Fermi surface nesting is responsible for low-energy spin excitations. These results parallel Ni-doping evolution of spin excitations in BaFe$_{2-x}$Ni$_x$As$_2$, confirming the notion that low-energy spin excitations coupling with itinerant electrons are important for superconductivity, while weakly doping dependent high-energy spin excitations result from localized moments.

cond-mat.supr-con↗

arXiv2016

Comments on 'Phonon and magnetic structure in δ-plutonium from density-functional theory' by P. Söderlind et al

In their recent paper [Söderlind, P. et al., Sci. Rep. 5, 15958 (2015)], Söderlind et al. discuss two subjects from a theoretical point of view: the phonon spectra and the possible magnetic structure of δ-plutonium (Pu). Here, we comment on the second subject. Söderlind et al. compare the Pu magnetic form factor F(Q) calculated via density functional theory (DFT) with measurements of F(Q) by neutron spectroscopy [Janoschek, M. et al., Sci. Adv. 1, e1500188 (2015)]. In particular, this comparison does not consider a number of experimental facts established in the neutron spectroscopy study.

cond-mat.str-el↗

arXiv2016

Commensurate antiferromagnetic excitations as a signature of the pseudogap in the tetragonal high-Tc cuprate HgBa$_2$CuO$_{4+δ}$

Antiferromagnetic correlations have been argued to be the cause of the d-wave superconductivity and the pseudogap phenomena exhibited by the cuprates. Although the antiferromagnetic response in the pseudogap state has been reported for a number of compounds, there exists no information for structurally simple HgBa$_2$CuO$_{4+δ}$. Here we report neutron scattering results for HgBa$_2$CuO$_{4+δ}$ (superconducting transition temperature T$_c$ $\sim$ 71 K, pseudogap temperature T* $\sim$ 305 K) that demonstrate the absence of the two most prominent features of the magnetic excitation spectrum of the cuprates: the X-shaped 'hourglass' response and the resonance mode in the superconducting state. Instead, the response is Y-shaped, gapped, and significantly enhanced below T*, and hence a prominent signature of the pseudogap state.

cond-mat.supr-con↗