arXiv subjects

Jason N. Hancock

Publications and source records attributed to Jason N. Hancock.

14 recordsLinked to original sources

arXiv2024

Preparation of Epitaxial Scandium Trifluoride Thin Films using Pulsed Laser Deposition

Bulk Scandium trifluoride ($\mathrm{ScF_3}$) is known for a pronounced negative thermal expansion (NTE) over a wide range of temperature, from $10~\mathrm{K}~\text{to}~ 1100~\mathrm{K}$. The structure of $\mathrm{ScF_3}$ can be described as an $\mathrm{ABX_3}$ perovskite with an empty A-site and a space group of Pm-3m. Growing thin films of $\mathrm{ScF_3}$ allows for tuning the lattice constant, the thermal expansion, and the construction of devices based upon differential thermal expansion. We have investigated the growth of $\mathrm{ScF_3}$ films on oxide and fluoride substrates using pulsed laser deposition (PLD) This letter describes the successful growth recipe for producing high quality epitaxial $\mathrm{ScF_3}$ thin films on positive thermal expansion (PTE) lithium fluoride ($\mathrm{LiF}$) substrates, at substrate temperature, $350^{\circ}\mathrm{C}$ with a laser repetition rate of $1~\mathrm{Hz}$, with an energy per pulse of $600~\mathrm{mJ}$, under a vacuum of $1.5\times 10^{-6}~ \mathrm{torr}$, for a growth time of $6$ hours. However, even for films with excellent epitaxy and sharp peaks along the principal axes, diffraction peaks from certain crystallographic directions are extremely broad, with the example of ($104$) reflections, in this work. We attribute this broadening to disorder in the $\mathrm{F_6}$ octahedral rotations that occur as an attempt to accommodate the large temperature-induced lattice mismatch that results in cooling from the growth temperature for this system of a NTE film mated to a PTE substrate.

cond-mat.mtrl-sci↗

arXiv2023

Mott insulating negative thermal expansion perovskite TiF3

We characterize perovskite TiF_3, a material which displays significant negative thermal expansion at elevated temperatures above its cubic-to-rhombohedral structural phase transition at 330 K. We find the optical response favors an insulating state in both structural phases, which we show can be produced in density functional theory calculations only through the introduction of an on-site Coulomb repulsion. Analysis of the magnetic susceptibility data gives a S=1/2 local moment per Ti+3 ion and an antiferromagnetic exchange coupling. Together, these results show that TiF_3 is a strongly correlated electron system, a fact which constrains possible mechanisms of strong negative thermal expansion in the Sc_1-xTi_xF3 system. We consider the relative strength of the Jahn-Teller and electric dipole interactions in driving the structural transition.

cond-mat.str-el↗

arXiv2020

An apparatus for nondestructive and rapid comparison of mask approaches in defense against infected respiratory aerosols

At the front lines of the world's response to the COVID-19 pandemic are hero-clinicians facing a lack of critical supplies including protective medical grade breathing masks and filtering materials. At the same time, the general public is now being advised to wear masks to help stop the spread. As a result, in the absence of centrally coordinated production and distribution efforts, supply chains for masks, respirators, and materials for advanced filtration technology are immensely burdened. Here we describe experimental efforts to nondestructively quantify three vital characteristics of mask approaches: breathability, material filtration effectiveness, and sensitivity to fit. We focus on protection against water aerosols $>$0.3$μ$m using off-the-shelf particulate, flow, and pressure sensors, permitting rapid comparative evaluation of these three properties. We present and discuss both the pressure drop and the particle transmission as a function of flow to permit comparison of relative protection for a set of proposed filter and mask designs. The design considerations of the testing apparatus can be reproduced by university laboratories and medical facilities and used for rapid local quality control of respirator masks which are of uncertified origin, monitoring the long-term effects of various disinfection schemes, and evaluating improvised products not designed or marketed for filtration.

physics.ins-det↗

arXiv2018

Negative thermal expansion near the precipice of structural stability in open perovskites

Negative thermal expansion (NTE) describes the anomalous propensity of materials to shrink when heated. Since its discovery, the NTE effect has been found in a wide variety of materials with an array of magnetic, electronic and structural properties. In some cases, the NTE originates from phase competition arising from the electronic or magnetic degrees of freedom but we here focus on a particular class of NTE which originates from intrinsic dynamical origins related to the lattice degrees of freedom, a property we term \textit{structural} negative thermal expansion (SNTE). Here we review some select cases of NTE which strictly arise from anharmonic phonon dynamics, with a focus on open perovskite lattices. We find that NTE is often present close in proximity to competing structural phases, with structural phase transition lines terminating near $T$=0 K yielding the most superlative displays of the SNTE effect. We further provide a theoretical model to make precise the proposed relationship among the signature behavior of SNTE, the proximity of these systems to structural quantum phase transitions and the effects of phase fluctuations near these unique regions of the structural phase diagram. The effects of compositional disorder on NTE and structural phase stability in perovskites are discussed.

cond-mat.mtrl-sci↗

arXiv2017

Large isotropic negative thermal expansion above a structural quantum phase transition

Perovskite structured materials contain myriad tunable ordered phases of electronic and magnetic origin with proven technological importance and strong promise for a variety of energy solutions. An always-contributing influence beneath these cooperative and competing interactions is the lattice, whose physics may be obscured in complex perovskites by the many coupled degrees of freedom which makes these systems interesting. Here we report signatures of an approach to a quantum phase transition very near the ground state of the nonmagnetic, ionic insulating, simple cubic perovskite material ScF3 and show that its physical properties are strongly effected as much as 100 K above the putative transition. Spatial and temporal correlations in the high-symmetry cubic phase determined using energy- and momentum-resolved inelastic X-ray scattering as well as X-ray diffraction reveal that soft mode, central peak and thermal expansion phenomena are all strongly influenced by the transition.

cond-mat.str-el↗

arXiv2017

Kondo lattice excitation observed using resonant inelastic X-ray scattering at the Yb $M_5$ edge

We present a study of the resonant inelastic scattering response of \ybin\ excited at the tender Yb $M_5$ X-ray edge. In the high-temperature, paramagnetic phase, we observe a multiplet structure which can be understood at an ionic level. Upon cooling through the valence transition at $T_v\sim$ 40$K$, we observe a strong renormalization of the low-energy spectra, indicating a sensitivity to the formation of an intermediate valence phase at low temperatures. Similar spectrum renormalization has been observed in the optical conductivity, which suggests that the low-energy electronic structure possesses both mixed conduction and localized character.

cond-mat.str-el↗

arXiv2017

Negative thermal expansion near two structural quantum phase transitions

Recent experimental work has revealed that the unusually strong, isotropic structural negative thermal expansion in cubic perovskite ionic insulator ScF3 occurs in excited states above a ground state tuned very near a structural quantum phase transition, posing a question of fundamental interest as to whether this special circumstance is related to the anomalous behavior. To test this hypothesis, we report an elastic and inelastic X-ray scattering study of a second system Hg2I2 also tuned near a structural quantum phase transition while retaining stoichiometric composition and high crystallinity. We find similar behavior and significant negative thermal expansion below 100K for dimensions along the body-centered-tetragonal c axis, bolstering the connection between negative thermal expansion and zero temperature structural transitions. We identify the common traits between these systems and propose a set of materials design principles that can guide discovery of new materials exhibiting negative thermal expansion.

cond-mat.mtrl-sci↗

arXiv2014

Magnetoplasmon resonances in polycrystalline bismuth as seen via terahertz spectroscopy

We report the magnetic field-dependent far-infrared reflectivity of polycrystalline bismuth. We observe four distinct absorptions that we attribute to magnetoplasmon resonances, which are collective modes of an electron-hole liquid in magnetic field and become optical and acoustic resonances of the electron-hole system in the small-field limit. The acoustic mode is expected only when the masses of distinct components are very different, which is the case in bismuth. In a polycrystal, where the translational symmetry is broken, a big shift of spectral weight to acoustic plasmon is possible. This enables us to detect an associated plasma edge. Although the polycrystal sample has grains of randomly distributed orientations, our reflectivity results can be explained by invoking only two, clearly distinct, series of resonances. In the limit of zero field, the optical modes of these two series converge onto plasma frequencies measured in monocrystal along the main optical axes.

cond-mat.str-el↗

arXiv2011

Surface state charge dynamics of a high-mobility three dimensional topological insulator

We present a magneto-optical study of the three-dimensional topological insulator, strained HgTe using a technique which capitalizes on advantages of time-domain spectroscopy to amplify the signal from the surface states. This measurement delivers valuable and precise information regarding the surface state dispersion within <1 meV of the Fermi level. The technique is highly suitable for the pursuit of the topological magnetoelectric effect and axion electrodynamics.

cond-mat.mes-hall↗

arXiv2006

Spectroscopy in the Presence of Geometrical Constraints: A Torsional Pendulum

We demonstrate that an effect other than anharmonicity can severely distort the spectroscopic signatures of quantum mechanical systems. This is done through an analytic calculation of the spectroscopic response of a simple system, a charged torsional pendulum. One may look for these effects in the optical data of real systems when for example a significant rocking component of rigid polyhedra plays a significant role in the lattice dynamics.

physics.ed-ph↗

arXiv2004

The Kondo Dynamics of YbIn(1-x)AgxCu4

We present an infrared/optical study of the dynamics of the strongly correlated electron system YbIn(1-x)AgxCu4 as a function of doping and temperature for x ranging from 0 to 1, and T between 20 and 300 K. This study reveals information about the unusual phase transition as well as the phases themselves. Scaling relations emerge from the data and are investigated in detail using a periodic Anderson model based calculation. We also provide a picture in which to view both the low and high-energy x-dependent features of the infrared data, including identification of high energy, temperature dependent features.

cond-mat.str-el↗

arXiv2004

Unusual Low-Energy Phonon Dynamics in the Negative Thermal Expansion Compound ZrW2O8

An infrared study of the phonon spectra of ZrW2O8 as a function of temperature which includes the low energy (2-10 meV) region relevant to negative thermal expansion is reported and discussed in the context of specific heat and neutron density of states results. The prevalence of infrared active phonons at low energy and their observed temperature dependence are highly unusual and indicative of exotic low-energy lattice dynamics. Eigenvector calculations indicate a mixing of librational and translational motion within each low-frequency IR mode. The role of the underconstrained structure in establishing the nature of these modes, and the relationship between the IR spectra and the large negative thermal expansion in ZrW2O8 are discussed.

cond-mat.mtrl-sci↗

arXiv2004

Kondo Scaling in the Optical Response of YbIn_(1-x)Ag_xCu_4

Theoretical work on Kondo systems predicts universality in the scaling of observable quantities with the Kondo temperature, T_K. Here we report infrared-frequency optical response measurements of the correlated system \ybinag. We observe that x-dependent variations in the frequency and strength of a low energy excitation are related to the x-dependent Kondo temperature. Comparison of the inferred trends with existing theory and a model calculation provides a framework in which to view these experimental results as scaling phenomena arising from local-moment/conduction electron hybridization.

cond-mat.str-el↗

arXiv2000

Optical study of the electronic phase transition of strongly correlated YbInCu_4

Infrared, visible and near-UV reflectivity measurements are used to obtain conductivity as a function of temperature and frequency in YbInCu_4, which exhibits an isostructural phase-transition into a mixed-valent phase below T_v=42 K. In addition to a gradual loss of spectral weight with decreasing temperature extending up to 1.5 eV, a sharp resonance appears at 0.25 eV in the mixed-valent phase. This feature can be described in terms of excitations into the Kondo (Abrikosov-Suhl) resonance, and, like the sudden reduction of resistivity, provides a direct reflection of the onset of coherence in this strongly correlated electron system.

cond-mat.str-el↗