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Tennant, David Alan

Publications and source records attributed to Tennant, David Alan.

10 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Direct Observation and Analysis of Low-Energy Magnons with Raman Spectroscopy in Atomically Thin NiPS 3

van der Waals (vdW) magnets have rapidly emerged as a fertile playground for fundamental physics and exciting applications. Despite the impressive developments over the past few years, technical limitations pose a severe challenge to many other potential breakthroughs. High on the list is the lack of suitable experimental tools for studying spin dynamics on atomically thin samples. Here, Raman scattering techniques are employed to directly observe the low-lying magnon (~1 meV) even in bilayer NiPS 3 . Further, the advantage is that it offers excellent energy resolutions far better on low-energy sides than most inelastic neutron spectrometers can offer. More importantly, with appropriate theoretical analysis, the polarization dependence of the Raman scattering by those low-lying magnons also provides otherwise hidden information on the dominant spin-exchange scattering paths for different magnons. By comparing with high-resolution inelastic neutron scattering data, these low-energy Raman modes are confirmed to be indeed of magnon origin. Because of the different scattering mechanisms involved in inelastic neutron and Raman scattering, this information is fundamental in pinning down the final spin Hamiltonian. This work demonstrates the capability of Raman spectroscopy to probe the genuine two-dimensional spin dynamics in atomically thin vdW magnets, which can provide insights that are obscured in bulk spin dynamics.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Dynamics of nonequilibrium magnons in gapped Heisenberg antiferromagnets

Nonequilibrium dynamics in spin systems is a topic currently under intense investigation as it provides fundamental insights into thermalization, universality, and exotic transport phenomena. While most of the studies have been focused on ideal closed quantum many-body systems such as ultracold atomic quantum gases and one-dimensional spin chains, driven-dissipative Bose gases in steady states away from equilibrium in classical systems also lead to intriguing nonequilibrium physics. Here, in this work, we theoretically investigate out-of-equilibrium dynamics of magnons in a gapped Heisenberg quantum antiferromagnet based on Boltzmann transport theory. We show that, by treating scattering terms beyond the relaxation-time approximation in the Boltzmann transport equation, energy and particle number conservation mandate that nonequilibrium magnons cannot relax to equilibrium, but decay to other nonequilibrium stationary states. The only decay channel for these stationary states back to equilibrium is through the nonconserving interactions (i.e., changing particle number and/or energy within the magnon system) such as boundary or magnon-phonon scattering. At low temperatures, these nonconserving interactions are much slower processes than intrinsic magnon-magnon interaction in a gapped spin system. Using magnon-phonon interaction as a quintessential type of nonconserving interaction, we then propose that nonequilibrium steady states of magnons can be maintained and tailored using periodic driving at frequencies faster than relaxation due to phonon interactions. These findings reveal a class of classical material systems that are suitable platforms to study nonequilibrium statistical physics and macroscopic phenomena such as classical Bose-Einstein condensation of quasiparticles and magnon supercurrents that are relevant for spintronic applications.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Topological Singularity Induced Chiral Kohn Anomaly in a Weyl Semimetal

The electron-phonon interaction (EPI) is instrumental in a wide variety of phenomena in solid-state physics, such as electrical resistivity in metals, carrier mobility, optical transition, and polaron effects in semiconductors, lifetime of hot carriers, transition temperature in BCS superconductors, and even spin relaxation in diamond nitrogen-vacancy centers for quantum information processing. However, due to the weak EPI strength, most phenomena have focused on electronic properties rather than on phonon properties. One prominent exception is the Kohn anomaly, where phonon softening can emerge when the phonon wave vector nests the Fermi surface of metals. In this paper, we report a new class of Kohn anomaly in a topological Weyl semimetal (WSM), predicted by field-theoretical calculations, and experimentally observed through inelastic x-ray and neutron scattering on WSM tantalum phosphide. Compared to the conventional Kohn anomaly, the Fermi surface in a WSM exhibits multiple topological singularities of Weyl nodes, leading to a distinct nesting condition with chiral selection, a power-law divergence, and non-negligible dynamical effects. Our work brings the concept of the Kohn anomaly into WSMs and sheds light on elucidating the EPI mechanism in emergent topological materials.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Studies of Spinons, Majoranas, and Monopoles in Spin Liquid and Quantum Critical Magnets with Neutrons

Quantum and frustrated materials have eminent potential for the future with the promise of transformative properties in energy storage, conversion, and transmission as well as for new types of computing. Many of the new materials being explored are introduced to have highly unusual ground states that show quantum critical behavior or exotic disorder such as spin liquids. Intense interest is focused on the unusual types of fractionalized quasiparticles that these phases can support such as spin-1/2 spinons, Majorana fermions, and magnetic monopoles. Neutron scattering provides a remarkable tool for probing quantum and frustrated states of real magnetic system by measuring the structure and dynamics of atomic spins on the important energy scales involved. Here I review the signatures of these unusual quasiparticles and the evidence for them starting with quasi-1D magnets covering fractionalization of magnons into spinons, confinement effects, and three classes of quantum critical behavior. In 2D I report on the status of quantum spin liquids in triangular and Kagome magnets before considering the current evidence for exotic Majorana fermions in materials that approximate the Kitaev model. Following this I outline the current status of research into magnetic monopoles in 3D spin ice and the potential for emergent electrodynamics in quantum spin ice. Finally, I consider where new developments in materials, experiment, and theory and modeling could lead to new discoveries.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Electrochemical properties of the interaction between cytochrome c and a hematite nanowire array electrode

We investigate the interaction of horse heart cytochrome c (cyt c ) with hematite nanowire array electrodes by cyclic voltammetry to study the electron transfer between redox active proteins and mineral surfaces. Using this model system, we quantify electron transfer rates between cyt c and hematite under varying electric potential and pH conditions. The results are consistent with two cyt c conformations adsorbed at the hematite surface: the native and a partially unfolded form. The partially unfolded protein maintained redox activity, but at a lower redox potential than the native protein. Adsorption of cyt c allowed direct electron transfer between cyt c and hematite, with an interfacial electron transfer rate, k ° ET , of 0.4 s -1 for the native form and 0.55 s -1 for the partially unfolded protein at pH 7.07. At pH 4.66, protein adsorption decreased compared to neutral pH and the fraction of partially unfolded protein increased. Additionally, the diffusion controlled electron transfer rate between hematite and the electron shuttling compound anthraquinone-2,6-disulfonate (AQDS) was determined to be k ° ET = 8.0·10 -3 cm·s -1 at pH 7.07. In conclusion, modulation of electron transfer rates as a result of conformational changes by redox active proteins has broad implications for describing chemical transformations at biological-mineral interfaces.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

Classical and quantum spin dynamics of the honeycomb Γ model

Quantum-to-classical crossover is a fundamental question in dynamics of quantum many-body systems. In frustrated magnets, for example, it is highly nontrivial to describe the crossover from the classical spin liquid with a macroscopically degenerate ground-state manifold to the quantum spin liquid phase with fractionalized excitations. This is an important issue, as we often encounter the demand for a sharp distinction between the classical and quantum spin liquid behaviors in real materials. Here we take the example of the classical spin liquid in a frustrated magnet with novel bond-dependent interactions to investigate the classical dynamics, and critically compare it with quantum dynamics in the same system. In particular, we focus on signatures in the dynamical spin structure factor. Combining Landau-Lifshitz dynamics simulations and the analytical Martin-Siggia-Rose approach, we show that the low-energy spectra are described by relaxational dynamics and highly constrained by the zero mode structure of the underlying degenerate classical manifold. Further, the higher energy spectra can be explained by precessional dynamics. Surprisingly, many of these features can also be seen in the dynamical structure factor in the quantum model studied by finite-temperature exact diagonalization. Here, we discuss the implications of these results and their connection to recent experiments on frustrated magnets with strong spin-orbit coupling.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Excitations in the field-induced quantum spin liquid state of α-RuCl 3

The celebrated Kitaev quantum spin liquid (QSL) is the paradigmatic example of a topological magnet with emergent excitations in the form of Majorana Fermions and gauge fluxes. Upon breaking of time-reversal symmetry, for example in an external magnetic field, these fractionalized quasiparticles acquire non-Abelian exchange statistics, an important ingredient for topologically protected quantum computing. Consequently, there has been enormous interest in exploring possible material realizations of Kitaev physics and several candidate materials have been put forward, recently including α-RuCl 3 . In the absence of a magnetic field this material orders at a finite temperature and exhibits low-energy spin wave excitations. However, at moderate energies, the spectrum is unconventional and the response shows evidence for fractional excitations. Here in this paper, we use time-of-flight inelastic neutron scattering to show that the application of a sufficiently large magnetic field in the honeycomb plane suppresses the magnetic order and the spin waves, leaving a gapped continuum spectrum of magnetic excitations. Our comparisons of the scattering to the available calculations for a Kitaev QSL show that they are consistent with the magnetic field induced QSL phase.

36 MATERIALS SCIENCE↗
DOE OSTI2017

A magnetic topological semimetal Sr 1-y Mn 1-z Sb2 (y, z < 0.10)

Weyl (WSMs) evolve from Dirac semimetals in the presence of broken time-reversal symmetry (TRS) or space-inversion symmetry. The WSM phases in TaAs-class materials and photonic crystals are due to the loss of space-inversion symmetry. For TRS-breaking WSMs, despite numerous theoretical and experimental efforts, few examples have been reported. Here in this paper, we report a new type of magnetic semimetal Sr 1-y Mn 1-z Sb 2 (y, z < 0.1) with nearly massless relativistic fermion behaviour (m* = 0.04 - 0.05m 0 , where m 0 is the free-electron mass). This material exhibits a ferromagnetic order for 304 K < T < 565 K, but a canted antiferromagnetic order with a ferromagnetic component for T < 304 K. The combination of relativistic fermion behaviour and ferromagnetism in Sr 1-y Mn 1-z Sb2 offers a rare opportunity to investigate the interplay between relativistic fermions and spontaneous TRS breaking.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Technical Design Report, Second Target Station

The Second Target Station (STS) is a proposed upgrade for SNS. It includes a doubling of the accelerator power and an additional instrument hall. The new instrument hall will receive a 467 kW 10 Hz beam. The parameters and preliminary design aspects of the STS are presented for the accelerator, target systems, instrument hall, instruments and civil construction aspects.

43 PARTICLE ACCELERATORS↗