Frances HELLMAN

In this talk I report on the disappearance of a magnetic helical phase as it approaches a paramagnetic phase in thin film amorphous Fe51Ge49 (a-FeGe). a-FeGe hosts a randomized Dzyaloshinskii-Moriya interaction that competes with the exchange interaction to stabilize magnetic spin helices. By combining transmission resonant soft X-ray scattering and micromagnetic simulations, the phase transition was determined to be defect mediated with an accompanied reorientation of the helical propagation direction.

One of the leading candidates for next-generation magnetic memory applications is the class of devices employing Spin-Orbit Torque (SOT) where a spin generating layer exerts a switching torque on a magnetic layer. The observation of large anomalous hall effect in amorphous FeSi (a-Fe-Si) and large spin torque efficiency generated interest in amorphous 3d metals alloyed with semiconductors like silicon and germanium. We sputtered various compositions of amorphous CoGe (a-Co-Ge) with Ni 81 Fe 19 and measured spin torque efficiencies with spin-torque ferromagnetic resonance and second harmonic voltage measurements. We also explored the possibility of enhancing the spin torque efficiency by introducing various thin oxide layers between a-Co-Ge and Ni 81 Fe 19.

In this work we report low temperature mechanical loss, specific heat and structural characterization of amorphous silica (a-SiO 2) films grown by UHV electron-beam evaporation at substrate temperatures from 50 to 900 C. The films’ density increases with increasing growth temperature. Through positron annihilation studies and shear modulus measurements, we also observe an increase in the average nanovoid size with growth temperature while the network becomes more compact and rigid. Mechanical loss and specific heat below 10 K decrease with increasing growth temperature; both properties are associated with tunneling states whose density decreases with increasing growth temperature, correlated with the structural changes discussed above. However, the density of tunneling states lies within the universal glassy range for all growth temperatures, unlike what was seen in a-Si, and is larger than that …

Y62. 00009: Studying the Temperature Dependence of the Transient Reflectance of FeRh Thin Films on the Subpicosecond Timescale*

B32. 00008: Effect of fragility on the stability and thermodynamic properties of amorphous Te-Ge films*

The ability to observe astronomical events through the detection of gravitational waves relies on the quality of multilayer coatings used on the optical mirrors of interferometers. Amorphous Ta 2 O 5 (including TiO 2: Ta 2 O 5) currently limits detector sensitivity due to high mechanical loss. In this paper, mechanical loss measured at both cryogenic and room temperatures of amorphous Ta 2 O 5 films grown by magnetron sputtering and annealed in air at 500∘ C is shown to decrease for elevated growth temperature. Films grown at 310∘ C and annealed yield a mechanical loss of 3.1× 10− 4 at room temperature, the lowest value reported for pure amorphous Ta 2 O 5 grown by magnetron sputtering to date, and comparable to the lowest values obtained for films grown by ion beam sputtering. Additionally, the refractive index n increases 6% for elevated growth temperature, which could lead to improved sensitivity of …

The second Gravitational-Wave Transient Catalog, GWTC-2, reported on 39 compact binary coalescences observed by the Advanced LIGO and Advanced Virgo detectors between 1 April 2019 15∶ 00 UTC and 1 October 2019 15∶ 00 UTC. Here, we present GWTC-2.1, which reports on a deeper list of candidate events observed over the same period. We analyze the final version of the strain data over this period with improved calibration and better subtraction of excess noise, which has been publicly released. We employ three matched-filter search pipelines for candidate identification, and estimate the probability of astrophysical origin for each candidate event. While GWTC-2 used a false alarm rate threshold of 2 per year, we include in GWTC-2.1, 1201 candidates that pass a false alarm rate threshold of 2 per day. We calculate the source properties of a subset of 44 high-significance candidates that have a …

APS -APS March Meeting 2024 - Event - A brief history of women in APS American Physical Society American Physical Society Sites|APS|Journals|Physics APS Meetings Home|Help|Contact APS Meetings Bulletin of the American Physical Society Bulletin Home My Scheduler Epitome Author Index Session Index Invited Speakers Chair Index Search Abstract Search Affiliation Using My Scheduler APS March Meeting 2024 Monday–Friday, March 4–8, 2024; Minneapolis & Virtual Session Q56: The American Physical Society at 125 3:00 PM–5:24 PM, Wednesday, March 6, 2024 Room: 205AB Sponsoring Unit: FHP Chair: Joseph Martin, Durham University; Alberto Martinez, University of Texas at Austin Abstract: Q56.00004 : A brief history of women in APS 4:48 PM–5:24 PM Abstract Presenter: Frances Hellman (University of California, Berkeley) Author: Frances Hellman (University of California, Berkeley) TBD Follow …

Strong disorder has a crucial effect on the electronic structure in quantum materials by increasing localization, interactions, and modifying the density of states. Bi x TeI films grown at room temperature and 230 K exhibit dramatic magnetotransport effects due to disorder, localization, and electron correlation effects, including a metal-insulator transition at a composition that depends on growth temperature. The increased disorder caused by growth at 230 K causes the conductivity to decrease by several orders of magnitude for several compositions of Bi x TeI. The transition from metal to insulator with decreasing composition x is accompanied by a decrease in the dephasing length, which leads to the disappearance of the weak-antilocalization effect. Electron-electron interactions cause low temperature conductivity corrections on the metallic side and Efros-Shklovskii variable range hopping on the insulating side …

N23. 00003: Ergodic dynamics near the order-disorder transition of spin-helices in amorphous Fe-Ge thin-film*

A system that possesses translational symmetry but breaks orientational symmetry is known as a nematic phase. While there are many examples of nematic phases in a wide range of contexts, such as in liquid crystals, complex oxides, and superconductors, of particular interest is the magnetic analogue, where the spin, charge, and orbital degrees of freedom of the electron are intertwined. The difficulty of spin nematics is the unambiguous realization and characterization of the phase. Here we present an entirely new type of magnetic nematic phase, which replaces the basis of individual spins with magnetic helices. The helical basis allows for the direct measurement of the order parameters with soft X-ray scattering and a thorough characterization of the nematic phase and its thermodynamic transitions. We discover two distinct nematic phases with unique spatio-temporal correlation signatures. Using coherent X-ray methods, we find that near the phase boundary between the two nematic phases, fluctuations coexist on the timescale of both seconds and sub-nanoseconds. Additionally, we have determined that the fluctuations occur simultaneously with a reorientation of the magnetic helices, indicating that there is spontaneous symmetry breaking and new degrees of freedom become available. Our results provide a novel framework for characterizing exotic phases and the phenomena presented can be mapped onto a broad class of physical systems.

Thermoelastic loss is an important energy dissipation mechanisms in resonant systems. A careful analysis of the thermoelastic loss is critical to the design of low-noise devices for high-precision applications, such as the mirrors used for gravitational-wave (GW) detectors. In this paper, we present analytical solutions to the thermoelastic loss due to thermoelasticity between different materials that are in contact. We find expressions for the thermoelastic loss of multimaterial coatings of finite substrates, and analyze its dependencies on material properties, mirror design and operating experimental conditions. Our results show that lower operating mirror temperature, thinner layers and higher number of interfaces in the coating, and the choice of the first layer of the coating that minimizes the thermal expansion mismatch with the substrate are strategies that reduce the thermoelastic loss and, therefore, diminish the thermal …

Amorphous materials, even of the same composition, can have highly variable material properties due to the local structure. This local structure also leads to the so-called''energy landscape''of a glass. An ultrastable glass is any that lies lower in this energy landscape than a traditional quenched glass. It has been hypothesized that fragility is a measure of surface to bulk diffusivity, and is therefore a controlling factor in allowing for vapor-deposited glasses to be grown as ultrastable films. By using ultrafast differential scanning calorimetry, we measure the glass transition temperature and thermodynamic properties of chalcogenide as-deposited films (including Ge-Te and Si-Te) to explore the dependence of ultrastability on fragility.

Magnetic thin-films provide a unique system to study the evolution of exotic spin-textures which are fundamentally intriguing and potentially promising for spintronics application. A key question to address is how symmetric and anti-symmetric exchange coupling between atomic spins modulate the nature of fluctuations as the system approaches phase transition. In amorphous Fe-Ge thin films by controlling the Fe concentration one can tune the magnitude of the symmetric Heisenberg exchange and randomness of anti-symmetric Dzyaloshinskii-Moriya interaction (DMI) respectively. In this work we performed soft x-ray photon correlation spectroscopy (XPCS) measurements to probe the nanoscale magnetic fluctuations of stripe domains in amorphous Fe x Ge 1-x thin films for x= 0.51, 0.52 and 0.53. The fluctuations were found to be length-scale specific heterogeneous; starting over a small fraction of the domains well …

While topological materials are not restricted to crystals, there is no efficient method to diagnose topology in noncrystalline solids such as amorphous materials. Here we introduce the structural spillage, a new indicator that predicts the unknown topological phase of a noncrystalline solid, which is compatible with first-principles calculations. We illustrate its potential with tight-binding and first-principles calculations of amorphous bismuth, predicting a bilayer to be a new topologically nontrivial material. Our work opens up the efficient prediction of noncrystalline solids via first-principles and high-throughput searches.

The discovery of a gravitational wave (GW) signal from a binary neutron star (BNS) merger (Abbott et al. 2017a) and the kilonova emission from its remnant (Coulter et al. 2017; Abbott et al. 2017b) provided the first GW standard siren measurement of the cosmic expansion history (Abbott et al. 2017c). As pointed out by Schutz (1986), the GW signal from a compact binary coalescence directly measures the luminosity distance to the source without any additional distance calibrator, earning these sources the name “standard sirens”(Holz & Hughes 2005). Measuring the cosmic expansion as a function of the cosmological redshift is one of the key avenues with which to explore the constituents of the universe, along with the other canonical probes such as the cosmic microwave background (CMB; Spergel et al. 2003, 2007; Komatsu et al. 2011; Ade et al. 2014, 2016; Aghanim et al. 2020), baryon acoustic oscillations …

The anomalous properties of glasses at low temperature and their seeming universality have been a challenge since first observed 50 years ago. The standard tunneling model describes these anomalous properties through the interaction of elastic and electric fields with tunneling two-level systems. The realization of glasses without glassy properties added more questions than answers: What glasses can be made without two-level systems, and why some and not others? Can we connect the ability to make some two-level free amorphous materials to their structure? The reduction of two-level systems at low temperature has been suggested to be related to ultrastability and perhaps ideality — glasses that show enhanced kinetic and thermodynamic stabilities and low enthalpy and entropy. In this chapter, we present experimental results and discuss the properties that may lead to the formation of two-level systems …

Amorphous silicon (a-Si) is an attractive candidate for photovoltaics, photonics, and dielectric mirror coatings for gravitational-wave detectors. Its use however is often limited by optical absorption due to its low band gap. Due to its disordered nature, carrier recombination and photon absorption are enabled by defects in the bonding network, which poses challenges to fabricating device-quality a-Si. Hydrogenation has been shown to be effective in improving the electrical performance of a-Si. In spite of many advances made, the mechanism by which introduction of hydrogen to a-Si suppresses recombination and light absorption remains elusive. It is widely accepted that the improvement is related to hydrogen passivation of dangling bonds. However, here we report that despite only a tiny amount of hydrogen in our hydrogenated a-Si films and almost no change in dangling bond density after hydrogenation, the …

Magnetic spin textures are a non-trivial distribution of magnetization vectors in a sample which are promising for future low power computing in part due to their topological protection. A popular material that hosts these spin textures is FeGe which shows a wide range of various spin textures at relatively high temperatures. In this talk, I will focus on the amorphous counterpart of this material and investigate the phase transition pathway between a disordered helical phase and a paramagnetic phase. As a result of the unique energy landscape provided by the amorphous nature of our sample, we find that the helical propagation direction reorients from being purely in-plane to canted out-of-plane and is mediated by the production of topological defects. By using coherent soft X-ray scattering, we were able to investigate the magnetic fluctuations as this helix reorients.

Amorphous thin films of TbCo (a-Tb-Co) grown by magnetron co-sputtering exhibit changes in magnetic anisotropy with varying growth and annealing temperatures. The magnetic anisotropy constant increases with increasing growth temperature, which is reduced or vanishes upon annealing at temperatures above the growth temperature. The proposed explanation for this growth-induced anisotropy in high orbital moment Tb-based transition metal alloys such as a-Tb-Co is an amorphous phase texturing with preferential in-plane and out-of-plane local bonding configurations for the rare-earth and transition metal atoms. Scanning nanodiffraction performed in a transmission electron microscope (TEM) is applied to a-TbCo films deposited over a range of temperatures to measure relative changes in medium-range ordering (MRO). These measurements reveal an increase in MRO with higher growth temperatures and a decrease in MRO with higher annealing temperatures. The trend in MRO indicates a relationship between the magnetic anisotropy and local atomic ordering. Tilting select films between 0 and 40 in the TEM measures variations in the local atomic structure a function of orientation within the films. The findings support claims that preferential ordering along the growth direction results from temperature-mediated adatom configurations during deposition, and that oriented MRO correlates with the larger anisotropy constants.