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It remains highly controversial whether long-distance spin transport or only Joule heating without spin transport exists in the disordered magnetic amorphous ${\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$ (a-YIG). Here, we carefully check and analyze the origin of the observed electrical signals in the Py/a-YIG/Pt-based nonlocal spin-transport experiments with both horizontal and vertical geometries, based on the ferromagnetic resonance (FMR)-induced spin pumping and inverse spin Hall effect. In all lateral nonlocal devices with a distance from 2.0 \ensuremath{\mu}m to 200 nm and vertical structures with a $\ensuremath{\ge}\text{40-nm-thick}$ a-YIG spacer layer, the spin-current-induced direct voltage peak signal is absent under FMR conditions, indicating that a-YIG does not possess long-distance spin transport. Furthermore, we demonstrate that a weak spin-torque FMR voltage signal, which gradually reduces to zero with increasing a-YIG thickness in the vertical geometry, is attributed to the leaked spin rectification voltage generated in the Py layer due to the electric leakage between the top Pt layer and the bottom Py layer through the pinhole defects in the thin middle a-YIG layer. Additionally, we find an improvement of the interfacial spin-mixing conductance due to spin accumulation in the interface of a-YIG/Py, which can significantly enhance spin-orbit torque efficiency in the ferromagnet/heavy metal systems with a disordered magnetic a-YIG buffer layer.

Boron carbide (${\mathrm{B}}_{4}\mathrm{C}$) is superhard, but its engineering applications are limited by the abnormal brittle failure arising from amorphous shear band formation. Mitigating the local amorphization is essential to improve the ductility of ${\mathrm{B}}_{4}\mathrm{C}$. Here, we carried out ab initio molecular dynamics (AIMD) simulations to examine the response of ${\mathrm{B}}_{4}\mathrm{C}$ to shear along two plausible slip systems $(001)/[100]$ and $(111)/[11\overline{2}]$ at finite temperature. We found that the icosahedra of ${\mathrm{B}}_{4}\mathrm{C}$ gradually deconstruct at finite temperature, resulting in local amorphization, thereby giving rise to a lower stress barrier than that of density functional theory simulations at zero temperature. The deconstruction of the icosahedral clusters arises from the interaction with the neighboring chains. The failure mechanism at the finite temperature suggested that the local amorphization can be suppressed by altering the structure of the 12-atom icosahedron and the 3-atom chain of ${\mathrm{B}}_{4}\mathrm{C}$. To demonstrate this, we replaced the ${\mathrm{B}}_{11}{\mathrm{C}}^{\mathrm{P}}$ icosahedron in ${\mathrm{B}}_{4}\mathrm{C}$ with ${\mathrm{B}}_{12}$ icosahedron to form boron-rich boron carbide (${\mathrm{B}}_{13}{\mathrm{C}}_{2}$) and then performed the same shear deformations. We found that local amorphization significantly decreases, which results from the modified icosahedral interaction. We also altered the 3-atom C-B-C chain to the 2-atom P-P chain and found that the accumulated shear stress can be released through icosahedral slipping, which is achieved by breaking the chains. The icosahedral slipping then prevents the destruction of icosahedra under shear deformations, expected to improve the ductility. Our results demonstrate two design strategies toward improved ductility of ${\mathrm{B}}_{4}\mathrm{C}$: (1) boron enrichment can alleviate amorphous shear band formation in boron carbide, and (2) altering the 3-atom chain to a 2-atom chain and meanwhile decreasing the strength of the chain to make it less stable than the icosahedron cage may lead to an icosahedral-slipping-dominated mechanism, thereby avoiding icosahedral fracture.

We experimentally observed an ultrahigh blocking temperature, above 600 K, in an exchange-biased Fe/FeO bilayer after field annealing at 773 K. This is far above the 198 K N\'eel temperature of bulk FeO. By comparing with an Fe/FeO/Fe trilayer and studying the Fe thickness dependence, we find that the magnetic proximity effect, due to coupling between the Fe layer and the ultrathin amorphous FeO layer, is responsible for the high blocking temperature. The realization of high-temperature exchange bias using an ultrathin antiferromagnet paves the way to further miniaturize magnetic devices for high-information-density applications.

The existence of photonic band gap (PBG) in amorphous networks has been associated with structural short-range order and internal similarity. In this study we present amorphous networks that possess low degrees of orderness and similarity, yet still exhibit decent PBG forming ability. The robustness of PBG is found to be related to less-deviated ring distributions. We develop regional ring disorder to quantify variations of rings, revealing the complementary roles of regional ring uniformity and local similarity in PBG formation. The evolution of PBG with respect to structural deformation is also presented to elucidate the effect of regional characters on PBG formation. Our study brings new insights towards the understanding of PBG formation in amorphous structures, paving the way for novel design and manipulation of amorphous PBG materials.

Optimizing materials' properties and functions by controlling defects in the crystalline phase has been a cornerstone of materials science and condensed matter physics. However, this paradigm has yet to be established in the broadly defined amorphous materials, which implies the identification of very subtle structural features in an otherwise uniformly disordered medium. Here we propose and define a new integrated glassy defect (IGD), based on machine learning strategy informed by atomistic physics, and also by an extremely wide configurational, thermodynamic, and dynamic variables space of the disordered state. The IGD simultaneously includes positional topology and vibrational features, as well as the local morphology of the potential energy landscape. This unprecedented combination gives rise to a much more comprehensive and more effective definition of the ``glassy defect,'' much beyond the conventional, purely structural input. IGD can be used not only as an efficient predictor of athermal plasticity but is also transferable to detect both short-time vibrational anomalies (the boson peak), and long-time relaxation and diffusion dynamics in glasses. The integrated strategy is instrumental to build the long-sought structure-property relationship in complex media.

In this work, we study the magnetoelastic behavior of ${\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Ga}}_{x}$ ($0.11lxl0.19$) thin films, grown on glass and oxidized Si(100) amorphous substrates, that present an isotropic crystalline texture in the film plane. The magnetoelastic coupling coefficients are obtained through the cantilever deflection technique. We find that the magnetoelastic response is larger for samples grown on glass with respect to those grown on Si(100), and such a response increases for larger Ga concentrations for both substrates used. Furthermore, the increasing substrate temperature during growth does not appear to have a significant effect on magnetoelastic behavior of samples grown on Si(100). From a model that takes into account the elastic grain interaction for isotropic systems, we are able to describe the experimentally observed behavior. We find that the magnetoelastic response of the samples grown on glass are well described by the Voigt model, while the samples on Si(100) present an intermediate response between the Voigt and Reuss models.

Longitudinal sound velocities (and elastic moduli) were determined in soda lime glass (SLG) at peak shock stresses ranging between 40 and 90 GPa. Laser interferometry was used to obtain particle velocity histories and sound velocities by impacting SLG samples on lithium fluoride (LiF) optical windows. In all experiments, the SLG response consists of a sharp jump to a constant state followed by a release wave. The measured longitudinal sound velocities and moduli showed a marked decrease between 52 and 58 GPa, providing experimental evidence for the transformation from an amorphous solid to a liquid in shock-compressed SLG. The stress threshold (\ensuremath{\sim}55 GPa) for melting in SLG is considerably lower than the threshold reported in shock-compressed fused silica (\ensuremath{\sim}72 GPa), showing the effect of network-modifying cations on the onset of melting. The relative values of sound velocities, shock velocities, and the Hugoniot slopes---between 58 and 90 GPa---are fully consistent with the thermodynamic response of a shock-compressed liquid. Using the experimental results, the Gr\"uneisen parameter (\ensuremath{\Gamma}) values were determined for liquid SLG to 90 GPa and then used to provide the Mie-Gr\"uneisen equation of state for liquid SLG.

Physics letters / B 103(17), 174515 (2021). doi:10.1103/PhysRevB.103.174515, The application of pressure has been speculated to boost the search for high-temperature superconductors, especially in superhydrides. However, the applied pressure as high as hundreds of GPa needed to create superconductivity in those materials limits their technological application. Finding a route to achieve the high-temperature superconductivity at near-ambient conditions is attractive. By choosing a phase-change alloy Ge$_2$Sb$_2$Te$_5$, we study the phase evolution of this material with pressure from the trigonal phase through the amorphous to the body-centered cubic one by the measurements of x-ray diffraction, Raman scattering, resistivity, and Hall coefficient. Superconductivity is observed to take place in the last two phases and can maintain at nearly ambient pressure in the decompression run. Pressure-induced disorder is found to be the key for holding superconductivity in the compressed phase-change alloy., Published by North-Holland Publ., Amsterdam

Kinetics in amorphous $\mathrm{Ti}{\mathrm{O}}_{2}$ driven by ion irradiation are discussed in the framework computer simulations. Thermal spike like regions along the path of incident ions are identified and their effects on the glassy system are described. Benefiting from the complementary approaches of purely ballistic TRIM (transport in matter) simulations and molecular dynamics simulations, a distinction between characteristic scales for short-range diffusive relocations ($\ensuremath{\lesssim}4.5\phantom{\rule{0.28em}{0ex}}\text{\AA{}}$) and long-range ballistic relocations is achieved. It is found that diffusion dynamics within thermally activated cascade zones sets in at the critical temperature ${T}_{c}$, identified as the critical temperature of the mode coupling theory, while the mean relocation limit for thermal diffusion in the glassy system is attributed to an overlap between the energy regime of purely ballistic and thermally activated relocation processes.

Structural models of the prototypal phase change material Ge-Sb-Te are generated from first-principles molecular dynamics simulations with a particular attention to the ${\mathrm{Ge}}_{x}{\mathrm{Sb}}_{x}{\mathrm{Te}}_{100\ensuremath{-}2x}$ join. Constraint counting algorithms permit us to analyze the obtained amorphous networks within the framework of topological constraint (rigidity) theory. A flexible-to-rigid transition is found at $x\ensuremath{\simeq}8.5$% which satisfies the Maxwell isostatic criterion and is characterized by an important topological disorder with large amounts of mixed geometries and miscoordinations. The angular constraint count furthermore leads to the identification of tetrahedral Ge sites whose population decreases with growing Ge/Sb content and is about 55% for ${\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{5}$. Sb sites change from a dominant pyramidal geometry typical of Group V chalcogenides to a defect octahedral one which is reminiscent of the crystalline polymorph of ${\mathrm{Ge}}_{2}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{5}$.

Tunneling two-level systems (TLSs), generic to amorphous solids, dictate the low-temperature properties of amorphous solids and dominate noise and decoherence in quantum nano-devices. The properties of the TLSs are generally described by the phenomenological standard tunneling model. Yet, significant deviations from the predictions of this model found experimentally suggest the need for a more precise model in describing TLSs. Here we show that the temperature dependence of the sound velocity, dielectric constant, specific heat, and thermal conductivity, can be explained using an energy-dependent TLS density of states reduced at low energies due to TLS-TLS interactions. This reduction is determined by the ratio between the strengths of the TLS-TLS interactions and the random potential, which is enhanced in systems with dominant electric dipolar interactions., 12 pages, 7 figures

The recently found crystalline silicon-amorphous boron ($c$-Si/$a$-B) heterojunction has been successfully applied in the detection of short-wave UV photons. These detectors play a decisive role in the progress of nanoelectronics fabrication. The $c$-Si/$a$-B heterojunction could not be explained using the existing `instrumentarium' in semiconductor physics. We investigated the $c$-Si/$a$-B interfaces using ab initio molecular dynamics simulations. The simulations reveal atomic ordering of the $a$-B atoms adjacent to both the Si{0 0 1} and Si{1 1 1} substrates. Charge transfer occurs from the interfacial Si to B, thereby forming ${\mathrm{Si}}^{+}/{\mathrm{B}}^{\ensuremath{-}}$ charge barriers, which induce an electric field in the nearby regions. The obtained information here is helpful in furthering our understanding of the physics behind the $c$-Si/$a$-B junctions, as well as driving the development of a new `instrumentatrium' in solid state physics.

We study the magnetic coupling in a thin film bilayer exchange-spring magnet composed of an amorphous ${\mathrm{Tb}}_{10}{\mathrm{Co}}_{90}$ layer with a large perpendicular anisotropy and an amorphous ${\mathrm{Co}}_{85}{({\mathrm{Al}}_{70}{\mathrm{Zr}}_{30})}_{15}$ layer with uniaxial in-plane anisotropy. The CoAlZr is directly exchange coupled to the TbCo with an interface region of at least 7.5 nm within which the CoAlZr magnetization is perpendicular to the plane and switches with the underlying TbCo. The influence of the coupling extends up to 30 nm into the CoAlZr resulting in an effective tilted anisotropy. The coercivity of the CoAlZr is greatly enhanced due to the coupling and is tuneable over a large range by varying its thickness.

Utilizing the synchrotron x-ray powder diffraction ${\mathrm{As}}_{2}{\mathrm{Se}}_{3}$ has been investigated under high pressure in a diamond anvil cell with hydrogen and neon applied as the pressure media. For both systems the amorphous samples were compressed to ca. 2--3 GPa and heated in situ by a laser, which led to their crystallization in the $R$-3 $m, {\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$-type phase. During further compression this phase transforms to the $C2/m, \ensuremath{\beta}\text{\ensuremath{-}}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}$-type structure, and they coexist within the pressure range of 21.5--33.0 GPa. The latter phase is observed up to the highest achieved pressure of 52.0 GPa, revealing no signs of reaction with the hydrogen pressure-transmitting medium. The pressure evolution of the volume of the reported phases has been described by the third-order Birch-Murnaghan equation of state. The relative stability of the experimentally detected crystalline phases is confirmed by the density functional theory calculations. The pressure-driven evolution of the As--Se distances and coordination number has been discussed in comparison to the recent findings concerning the amorphous $a\text{\ensuremath{-}}{\mathrm{As}}_{2}{\mathrm{Se}}_{3}$ phase.

Prediction of the stable crystal structure for multinary (ternary or higher) compounds with unexplored compositions demands fast and accurate evaluation of free energies in exploring the vast configurational space. The machine-learning potential such as the neural network potential (NNP) is poised to meet this requirement but a dearth of information on the crystal structure poses a challenge in choosing training sets. Herein we propose constructing the training set from densityfunctional-theory (DFT) based dynamical trajectories of liquid and quenched amorphous phases, which does not require any preceding information on material structures except for the chemical composition. To demonstrate suitability of the trained NNP in the crystal structure prediction, we compare NNP and DFT energies for Ba2AgSi3, Mg2SiO4, LiAlCl4, and InTe2O5F over experimental phases as well as low-energy crystal structures that are generated theoretically. For every material, we find strong correlations between DFT and NNP energies, ensuring that the NNPs can properly rank energies among low-energy crystalline structures. We also find that the evolutionary search using the NNPs can identify low-energy metastable phases more efficiently than the DFTbased approach. By proposing a way to developing reliable machine-learning potentials for the crystal structure prediction, this work will pave the way to identifying unexplored multinary phases efficiently., 8 pages, 5 figures

We report spatially resolved measurements of free carrier collective excitations in a doped semiconductor. Using $\ensuremath{\sim}10$ meV resolution electron energy loss spectroscopy (EELS) in an electron microscope with $\AA{}$ spatial resolution, we identify both surface and bulk carrier plasmons at infrared energies in a freestanding film of indium tin oxide (ITO). The interference patterns of long wavelength propagating surface carrier plasmons are revealed using spatially resolved EELS, from which we extract a dispersion relation. We further show that the energies of these plasmons vary near the surfaces and grain boundaries of the film due to band bending. Modeling based on dielectric theory agrees very well with experimental results. Finally, carrier plasmons in amorphous and crystalline ITO films are compared. These results should also be helpful for understanding the free carrier plasmons in other doped semiconductors in nanoscale volumes.

We use time-resolved x-ray resonant magnetic scattering (TR-XRMS) at the Co ${M}_{2,3}$ and Tb ${O}_{1}$ edges to study ultrafast demagnetization in an amorphous ${\mathrm{Co}}_{88}{\mathrm{Tb}}_{12}$ alloy with stripe domains. Combining the femtosecond temporal with nanometer spatial resolution of our experiment, we demonstrate that the equilibrium spin texture of the thin film remains unaltered by the optical pump pulse on ultrashort timescales ($l1\phantom{\rule{0.28em}{0ex}}\mathrm{ps}$). However, after $\ensuremath{\simeq}4$ ps, we observe the onset of a significant domain wall broadening, which we attribute to a reduction of the uniaxial magnetic anisotropy of the system, due to energy transfer to the lattice. Static temperature-dependent magnetometry measurements combined with analytical modeling of the magnetic structure of the thin film corroborate this interpretation.

We report on pressure tuning of superconductivity in a strongly coupled superconductor ${\mathrm{Mo}}_{8}{\mathrm{Ga}}_{41}$ with a critical temperature ${T}_{c}\ensuremath{\sim}9.8\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Resistance measurements show that ${T}_{c}$ is suppressed to 3.6 K at 19 GPa followed by a nonmonotonic variation at higher pressures. Synchrotron x-ray diffraction experiments reveal an amorphization taking place at the critical pressure ${P}_{c}\ensuremath{\sim}19\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$. As comparing with $\mathrm{Mg}{\mathrm{B}}_{2}$, the ${\mathrm{Mo}}_{8}{\mathrm{Ga}}_{41}$ shows a relative low decreasing rate of ${T}_{c}$, which might be attributed to its unique vertex-sharing type of cluster architecture. Survival of superconductivity across the crystalline to amorphous transition indicates that, although long-range structural order of endohedral Ga clusters is destroyed, short-range-ordered intracluster structure still exists and allows for local Cooper pairing. Due to presence of disorders in the amorphous phase, establishment of Cooper pair coherence is shifted to lower temperatures, which causes unusual transport behaviors near ${T}_{c}$, e.g., the anomalous resistance peak and dip. The amorphous superconducting phase is able to be retained to ambient pressure and therefore provides an ideal platform to study disorder effects on superconductivity.

Thin films of amorphous (a)-TiO2 are ubiquitous as photocatalysts, protective coatings, photoanodes and in memory application, where they are exposed to excess electrons and holes. We investigate trapping of excess electrons and holes in the bulk of pure amorphous titanium dioxide, a-TiO2, using hybrid density functional theory (h-DFT) calculations. Fifty 270-atom a-TiO2 structures were produced using classical molecular dynamics and their geometries fully optimised using h-DFT simulations. They have the density, distribution of atomic coordination numbers and radial pair-distribution functions in agreement with experiment. The calculated average a-TiO2 band gap is 3.25 eV with no states splitting into the band gap. Trapping of excess electrons and holes in a-TiO2 is predicted at precursor sites, such as elongated Ti-O bonds. Single electron and hole polarons have average trapping energies (ET) of -0.4 eV and -0.8 eV, respectively. We also identify several types of electron and hole bipolaron states and discuss their stability. These results can be used for understanding the mechanisms of photo-catalysis and improving the performance of electronic devices employing a-TiO2 films., 12 pages, 10 figures. This article has been submitted to Physical Review B

Low frequency quasi-localized modes of amorphous glasses appear to exhibit universal density of states, depending on the frequencies as $D(\omega) \sim \omega^4$. To date various models of glass formers with short range binary interaction, and network glasses with both binary and ternary interactions, were shown to conform with this law. In this paper we examine granular amorphous solids with long-range electrostatic interactions, and find that they exhibit the same law. To rationalize this wide universality class we return to a model proposed by Gurevich, Parshin and Schober (GPS) and analyze its predictions for interaction laws with varying spatial decay, exploring this wider than expected universality class. Numerical and analytic results are provided for both the actual system with long range interaction and for the GPS model., Comment: 10 Pages, 7 Figures, Physical Review B

The structure of amorphous ${\mathrm{Cu}}_{2}{\mathrm{GeTe}}_{3}$ is investigated by a combination of anomalous x-ray scattering and extended x-ray absorption fine-structure experiments. The experimental data are analyzed with reverse Monte Carlo modeling, and they are interpreted in terms of short-range-order parameters as well as by using ring statistics and persistent homology to study the intermediate-range order. Based on this information, the structural relationship of the amorphous phase to the corresponding crystal is discussed. It is found that the amorphous network can be rationalized by small atomic displacements of the crystal structure, directed toward the intrinsic void regions. This structural similarity establishes the possibility of a fast phase-change process. On the other hand, the atomic rearrangements also lead to the formation of new chemical bonds and to distortions on the intermediate-range-order level. These are realized by a collapse and contraction of the strict hexagonal ring arrangements of the crystal and by the formation of small, triangular rings as well as Cu cluster configurations. These structural features allow for a new understanding of the phase-change property contrast of this material, especially concerning the density change and the optical contrast.

Multilayers composed of aluminum (Al) and platinum (Pt) exhibit a nonmonotonic trend in thermal resistance with bilayer thickness as measured by time domain thermoreflectance. The thermal resistance initially increases with reduced bilayer thickness only to reach a maximum and then decrease with further shrinking of the multilayer period. These observations are attributed to the evolving impact of an intermixed amorphous complexion approximately 10 nm in thickness, which forms at each boundary between Al- and Pt-rich layers. Scanning transmission electron microscopy combined with energy dispersive x-ray spectroscopy find that the elemental composition of the complexion varies based on bilayer periodicity as does the fraction of the multilayer composed of this interlayer. These variations in complexion mitigate boundary scattering within the multilayers as shown by electronic transport calculations employing density-functional theory and nonequilibrium Green's functions on amorphous structures obtained via finite temperature molecular dynamics. The lessening of boundary scattering reduces the total resistance to thermal transport leading to the observed nonmonotonic trend thereby highlighting the central role of complexion on thermal transport within reactive metal multilayers.

Structural transformations at high pressure in NaAlGe3O8 glass were investigated by means of x-ray absorption spectroscopy at the Ge K edge in combination with a diamond anvil cell. The obtained results provide a detailed picture of the local structural behavior of Ge in a chemically complex glass under compression. First and second shell bond distances (RGe-O and RGe...Ge) were extracted assuming contributions of two scattering paths (Ge-O and Ge…Ge). We observed a significant extension of the Ge-O distance from 1.73 to 1.82 A between 3 and ∼26GPa, accompanied by an increase of the fitted number of nearest neighbors from ∼4 to ∼6. These observations can be attributed to the change from tetrahedral to octahedral Ge coordination. Second shell bond distances Ge…Ge are also consistent with this structural transformation. Between 34 and 131 GPa, the evolution of the fitted Ge-O distance implies a gradual volume reduction of the Ge octahedra. At the highest probed pressure of 131 GPa a Ge-O distance of 1.73 A was found, which is similar to the one obtained at ambient conditions for Ge in fourfold coordination. The compressibility of the Ge-O octahedron in NaAlGe3O8 beyond 34 GPa is considerably higher than the one reported for amorphous GeO2 from x-ray diffraction analysis but it is similar to the one reported for the Ge octahedron in crystalline rutile-type GeO2. We attribute the high compressibility of the Ge-O bond in NaAlGe3O8 glass to the presence of Al and Na that increase the system's complexity and therefore its degrees of freedom. Beyond 110 GPa the data on NaAlGe3O8 glass indicate the onset of polyhedral distortion. The performed study provides insights into the structural changes of complex and polymerized germanate glasses or melts at extreme pressure conditions.

Angular resolved scanning transmission electron microscopy is an important tool for investigating the properties of materials. However, several recent studies have observed appreciable discrepancies in the angular scattering distribution between experiment and theory. In this paper we discuss a general approach to low-loss inelastic scattering which, when incorporated in the simulations, resolves this problem and also closely reproduces experimental data taken over an extended angular range. We also explore the role of ionic bonding, temperature factors, amorphous layers on the surfaces of the specimen, and static displacements of atoms on the angular scattering distribution. The incorporation of low-loss inelastic scattering in simulations will improve the quantitative usefulness of techniques such as low-angle annular dark-field imaging and position-averaged convergent beam electron diffraction, especially for thicker specimens.

The wide application of amorphous materials in solar cells, memristors, and optical sensors has stimulated interest in understanding heat conduction in amorphous systems owing to their thermal management issues. Thermal transport in amorphous materials fundamentally differs from their crystalline counterparts due to the lack of long-range order. Despite great progress in understanding the thermal transport in crystalline materials over the past few decades from both first-principles computations and thermal transport characterizations, details of heat conduction in amorphous systems remain largely unknown. Here, we quantify different types of heat carriers in amorphous silicon using mean free path spectroscopy, with characteristic sizes down to 50 nm. We show that despite its disordered nature, more than half of thermal conductivity is contributed by propagating vibrational waves, which have mean free paths mostly above 100 nm. This provides direct evidence supporting the diversity of heat carriers in amorphous systems; some modes transport heat as propagating waves, while others do not. Our results suggest mean free path spectroscopy is a versatile tool for understanding thermal transport in disordered systems.

In this study, we used the thermally driven spin current to investigate the spin frustrations and spin fluctuations in spin-glass (SG) $\mathrm{C}{\mathrm{u}}_{1\ensuremath{-}x}\mathrm{M}{\mathrm{n}}_{x}$ alloys. Tuning the $\mathrm{C}{\mathrm{u}}_{1\ensuremath{-}x}\mathrm{M}{\mathrm{n}}_{x}$ composition results in a transition of the alloys from the SG state to the antiferromagnetic state; these states have different spin-freezing temperatures $({T}_{f})$. Most spins randomly freeze at temperatures lower than the ${T}_{f}$ of the alloy. For each alloy composition, we obtained a temperature-dependent inverse spin Hall voltage with a peak at ${T}_{p}$. Crucially, ${T}_{p}$ had nearly identical composition dependence as that of ${T}_{f},$, with ${T}_{p}$ being nine times larger than ${T}_{f}$. Similar behavior was captured using the SG insulator, amorphous ${\mathrm{Y}}_{3}\mathrm{F}{\mathrm{e}}_{5}{\mathrm{O}}_{12}$. These results indicated that the maximum spin fluctuation in both conducting and insulating SGs occurred at temperatures considerably higher than the ${T}_{f}$ of each. In addition, we demonstrated the importance of the effective number of valence electrons in tailoring the spin Hall angle in binary alloys.

CoFeB alloys are highly relevant materials for spintronic applications. In this work, the crystallization of CoFeB alloys triggered by thermal annealing was investigated by x-ray diffraction techniques and scanning electron microscopy, as well as spectroscopic ellipsometry and magneto-optical Kerr effect spectroscopy for annealing temperatures ranging from 300 to ${600}^{\ensuremath{\circ}}\mathrm{C}$. The transformation of \ensuremath{\sim}100-nm-thick ${\mathrm{Co}}_{x}{\mathrm{Fe}}_{(80\ensuremath{-}x)}{\mathrm{B}}_{20}$ films from amorphous to polycrystalline was revealed by the sharpening of spectral features observed in optical and magneto-optical dielectric functions spectra. The influence of B on the dielectric function was assessed both experimentally and by optical modeling. By analyzing the Drude component of the optical dielectric function, a consistent trend between the charge-carrier scattering time/resistivity and the annealing temperature was observed, in agreement with the electrical investigations by means of the four-point-probe method.

Dynamic heterogeneity is an intrinsic characteristic of amorphous materials that is closely related to the mysterious glass transition. However, there is seldom an intuitive physical parameter characterizing the degree of dynamic heterogeneity and linking it quantitatively to the dynamic arrest phenomenon at the glass transition. Here, we propose a general theoretical protocol to explain the glass transition via a statistical parameter quantifying the dynamic heterogeneity of glass-forming systems. The parameter can be calculated using the concept of the Shannon information entropy associated with the variation in the activation barriers to local structural excitations on the underlying potential energy landscape, which can be explored extensively using the recently developed activation-relaxation technique in inherent structures spanning a wide range of configurational space. The concept is demonstrated successfully in a model of a prototypical glass-forming system ${\mathrm{Cu}}_{50}{\mathrm{Zr}}_{50}$. The Shannon entropy and statistical variation in the activation barriers are found to change dramatically at the glass-to-liquid transition and, therefore, can be treated as a novel signature of the glass transition, beyond the conventional thermodynamic indicators, such as the volume, potential energy, enthalpy, and heat capacity. The temperature-dependent Shannon entropy coincides with the evolution of the experimentally available stretching exponent during the glass-to-liquid transition and provides an intuitive explanation for the obscure decrease in dynamic heterogeneity from a metastable glass to an equilibrium liquid. Finally, possible relationships among structures, thermodynamics, and dynamics are discussed in terms of quantitative correlations among the structural Shannon entropy, excess total entropy, and dynamic Shannon entropy, respectively.

We investigate a symmetry protected $Z_2$ topological electron glass -- a glassy equivalent of the $Z_2$ topological band insulator in crystalline systems -- and uncover associated quantum phase transitions in this three dimensional amorphous network of atoms. Through explicit numerical calculations of the Witten effect, we show that the $Z_2$ glass is characterized by an anomalous electromagnetic response -- dyons with 1/2 electronic charge. We further study, using a variety of numerical diagnostics including such electromagnetic responses, the phase transitions of the $Z_2$ glass into a metallic and/or a trivial insulating phase. We find that the phase transitions here are governed by subtle features of mobility edges and "spectral inversion" which are possibly unique to structurally amorphous systems. Our results provide a concrete setting to understand the general underpinnings of such phases -- where strong structural disorder interplays with symmetry-protected topological order., 8 pages, 9 figures

The formation of novel two-dimensional electron gas (2DEG) with high mobility in metal/amorphous interfaces has motivated an ongoing debate regarding the formation and novel characteristics of these 2DEGs. Here we report an optical study, based on infrared spectroscopic ellipsometry, of nonmagnetic metal and amorphous semiconducting oxide (Cu/Bi$_2$O$_3$) interfaces that confirms the formation of a 2DEG with spin orbit coupling (SOC). The 2DEG optical response was simulated with a uniaxial diagonal dielectric tensor within a sub-nanometer thin layer, where its $x$ and $z$ components lineshapes resolved in both free-electron and peak-like contributions, resulted very similar to theoretical predictions [M. Xie et al., Phys. Rev. B $\bf{89}$, 245417 (2014)] of a two dimensional electron gas confined in the normal direction of a perovskite interface. In particular, the small but finite conducting character of the $z$ component provides a unambiguous signature of the presence of the 2DEG in the Cu/Bi$_2$O$_3$ system. Although the original constituent materials do not possess spin-orbit coupling (SOC), the resulting interfacial hybridization of such states induce electronic asymmetric wave functions. This work demonstrates the detection of 2DEG in amorphous crystals allowing to study its challenging interfacial phenomena such as SOC and interface-bulk coupling, overcoming an experimental impediment which has hold back for decades important advancements for the understanding of 2DEGs in amorphous materials., 10 pages main text, under review at Phys. Rev. B

It is well known that twinning deformation plays an important role in the plastic deformation of hexagonal close-packed (HCP) metals because of the insufficient independent slip systems in HCP metals. However, recent works show twinning deformation does not happen in compression in the $c$ axis of the magnesium (Mg) single crystal, and it is not clear that how it deforms with limited dislocation glide in absence of twinning deformation. In the present work, molecular dynamics simulations with well validated modified embedded-atom method potential for Mg are performed to reveal the microscopic deformation behaviors of Mg single crystal compressed in the $c$ axis. We focus on the microstructures and the evolution during both the deformation and the relaxation of the Mg single crystal. Twinning deformation is not observed in the simulations as expected, and dislocations and the amorphous regions are found to be dominant under different loading conditions. The amorphous regions are dominant in the deformation under ultrahigh strain rate, and it results in more homogenous deformation and lower flow stress than dislocation glide. The dislocation glide is dominant under lower strain rate, and the insufficient dislocations may lead to quite inhomogeneous plastic deformation. Furthermore, it is revealed that the amorphous regions consist of irregularly distributed atoms with higher per-atom potential than those in the crystalline state. The amorphous regions thus annihilate rapidly once the high-rate deformation is stopped. The annihilated amorphous regions turn into crystalline ones with dense dislocation networks. The annihilation rate of the amorphous regions decreases with increasing pressure, while it hardly depends on temperature.

Germanium oxide is a prototype network-forming oxide with pressure-induced structural changes similar to those found in crystals and amorphous silicate oxides at high pressure. Studying density and coordination changes in amorphous $\mathrm{Ge}{\mathrm{O}}_{2}$ allows for insight into structural changes in silicate oxides at very high pressure, with implications for the properties of planetary magmas. Here, we report the density of germanium oxide glass up to 133 GPa using the x-ray absorption technique, with very good agreement with previous experimental data at pressure below 40 GPa and recent calculation up to 140 GPa. Our data highlight four distinct compressibility domains, corresponding to changes of the local structure of $\mathrm{Ge}{\mathrm{O}}_{2}$. Above 80 GPa, our density data show a compressibility and bulk modulus similar to the counterpart crystal phase, and we propose that a compact distorted sixfold coordination, similar to the structural motif of the pyritelike crystalline $\mathrm{Ge}{\mathrm{O}}_{2}$ polymorph, is likely to be stable in that pressure range. Our density data point to a smooth continuous evolution of the average coordination for pressure above 20 GPa with persistent sixfold coordination, without sharp density or density slope discontinuities. These observations are in very good agreement with theoretical calculations and spectroscopic measurements, and our results indicate that glasses and melts may behave similarly to their high-pressure solid counterparts with comparable densities, compressibility, and possibly average coordination.

Laser pulse induced terahertz (THz) emission from Ta/${({\mathrm{Co}}_{x}{\mathrm{Fe}}_{1\ensuremath{-}x})}_{80}{\mathrm{B}}_{20}$/MgO thin films, with varying compositions $x$ and annealing temperatures, is investigated. With increasing annealing temperature, the THz emission intensity exhibits significant dependence on $x$, with maxima at $x$ of 0.1--0.3. The $x$ dependence of the THz emission originated from the composition dependence of the spin currents induced in ${\mathrm{Co}}_{x}{\mathrm{Fe}}_{1\ensuremath{-}x}$ formed by the crystallization of amorphous CoFeB after the annealing. The origin of the laser-induced spin current is qualitatively discussed in terms of a ballistic transport of hot electrons and a spin-dependent Seebeck effect with different compositions.

It is generally believed that the lattice thermal conductivity in crystalline solids is mainly carried by phonons, whereas diffusons are regarded as the most important vibrational entity in amorphous solids. Contrary to this belief, here we show that the behavior of lattice thermal conductivity (LTC) of simple crystalline $\mathrm{Tl}X\mathrm{T}{\mathrm{e}}_{2}(X=\mathrm{Ga},\mathrm{In})$ compounds cannot be characterized either by phonons or diffusons alone. The calculated LTC based on first-principles density functional theory and the semiclassical Boltzmann transport equation (BTE) is only half of the experimental value. We show that the relatively weak bonding between Tl atoms and rigid $X\mathrm{T}{\mathrm{e}}_{2}$ chains gives rise to strong vibrational anharmonicity, which suppresses the mean free path of a large portion of phonon modes to even below the Ioffe-Regel limit and thus invalidates the precondition of the semiclassical BTE. We further show that the ultralow LTC of $\mathrm{Tl}X\mathrm{T}{\mathrm{e}}_{2}(X=\mathrm{Ga},\mathrm{In})$ can be rationalized by using the two-channel transport model. This finding would enhance the understanding of the underlying heat transport behavior in such materials.

Magnetism in palladium has been the subject of much work and speculation. Bulk crystalline palladium is paramagnetic with a high magnetic susceptibility. Palladium under pressure and palladium nanoclusters have generated interest to scrutinize its magnetic properties. Here we report another possibility: Palladium may become an itinerant ferromagnet in the amorphous bulk phase at atmospheric pressure. Atomic palladium is a d$^{10}$ element, whereas bulk crystalline Pd is a d$^{10-x}$(sp)$^{x}$ material; this, together with the possible presence of 'unsaturated bonds' in amorphous materials, may explain the remnant magnetism reported herein. This work presents and discusses magnetic effects in bulk amorphous palladium., Comment: 6 Pages, 4 Figures, 1 Table

Pressure-induced glass-glass transition (GGT) has been reported in a few rare-earth based and main-group metallic glasses (MGs), a process during which the compressibility decreases with the transformation from a low-density to a high-density amorphous state. Herein, we report an unexpected GGT behavior in Pd-based MGs under pressure, which is characterized by an anomalous increase in the compressibility with the pressure. State-of-the-art high-energy synchrotron technique, coupled with theoretical simulations, reveals that this unique GGT behavior is primarily caused by the change in bonding characteristics, i.e., some covalentlike atomic bonds in the as-prepared state change to metallic ones under high pressure, which also leads to the increase in the compressibility. Our current findings shed light on the understanding of the nature of polyamorphic transitions in MGs.

Nanoporous materials are of broad interest for various applications, in particular advanced thermoelectric materials. The introduction of nanoscale porosity, even at modest levels, has been known to drastically reduce a materials thermal conductivity, in some cases even below its amorphous limit, thereby significantly increasing its thermoelectric figure of merit, ZT. The details of the important attributes that drive these large reductions, however, are not yet clear. In this work, we employ large-scale equilibrium molecular dynamics to perform an exhaustive atomistic-scale investigation of the effect of porosity on thermal transport in nanoporous bulk silicon. Thermal transport is computed for over 50 different geometries, spanning a large number of geometrical degrees of freedom, such as cylindrical pores and voids, different porosities, diameters, neck sizes, pore/void numbers, and surface-to-volume ratios, placed in ordered fashion, or fully disordered. We thus quantify and compare the most important parameters that determine the thermal conductivity reductions in nanoporous materials. Ultimately, we find that, even at the nanoscale, the effect of merely reducing the line-of-sight of phonons, i.e. the clear pathways that phonons can utilize during transport, plays the most crucial role in reducing the thermal conductivity in nanoporous materials, beyond other metrics such as porosity and surface/boundary scattering., Comment: 33 pages, 9 figures

While it is known that the charge-carrier mobility in amorphous metal oxide semiconductor thin film transistors (TFT) deviates from Arrhenius temperature dependence, we found that the Hall mobility measured in amorphous In-Ga-Zn-O (a-IGZO) follows an Arrhenius relation surprisingly well. We explain these observations by the effect of strong vertical electric field created by the gate voltage, which facilitates direct tunneling of trapped carriers into the conductive band and leads to virtually temperature independent mobility. We present a generalized Arrhenius model based on the effective temperature concept. We show that our model allows quantitative description of the temperature dependence of the mobility in a-IGZO TFTs over a broad temperature range.

Polycrystalline tellurium becomes amorphous after irradiation with strong femtosecond pulses. The amorphization is sensitive to the initial temperature but not very sensitive to the temporal profile of the optical excitation. Above the amorphization threshold, single-shot transient reflectivity traces show clear coherent phonon oscillations within 1 ps. These results suggest that amorphization is due to thermal melting rather than nonthermal melting or switching for pump fluences up to the ablation threshold.

Silica glass is an optically homogeneous material with many practical applications. Unlike crystalline materials, it remains optically homogeneous during transformations. However, high-pressure in situ small-angle x-ray-scattering measurements indicate an increase in the scattering intensity during the transformation in short-range order between fourfold-coordinated and sixfold-coordinated amorphous polymorphs, providing strong evidence for heterogeneity. Detailed analyses suggest that silica glass consists of optically invisible subnanometer-scale domains for the two amorphous polymorphs and boundary region between them. The boundary region, which likely has an intermediate structure, is comparable in size to the domains, and works as a buffer to prevent the glass from cracking during the transformation. The transformation is more drastic and the domains are more distinct upon decompression than compression.

The vitrification of single-element metallic glass remains a challenging research topic and attracts a great deal of attention. It is reported that the propensity for formation of the amorphous phase increases as the size of the system decreases in experiments. However, the thermodynamic contributions to this phenomenon are still elusive. Here, we study the size effects on the thermodynamic properties of the Cu nanoparticles in both crystalline and amorphous phases using the Frenkel-Ladd methods. Smaller particles show stronger size dependences on these quantities. From the perspective of free-energy difference between the amorphous phase and the crystalline phase, the thermodynamic driving force for vitrification of single-element metallic liquids catches up with that for crystallization dramatically, given the particle diameter smaller than a critical value (i.e., $\ensuremath{\sim}4$ nm for Cu), which provides a thermodynamic interpretation for the size dependence of the glass-forming ability.

We investigate the spin Hall effect of W-Hf thin films, which exhibit a phase transition from a segregated phase mixture to an amorphous alloy below 70% W. The spin Hall angle was determined with a planar harmonic Hall voltage technique. Due to the accompanying jump in resistivity, the spin Hall angle shows a pronounced maximum at the composition of the phase transition. The spin Hall conductivity does, however, reduce from W to Hf with a weak discontinouity across the phase transition. The maximum spin Hall angle of $\theta_\mathrm{SH} = -0.25$ is obtained for amorphous W$_{0.7}$Hf$_{0.3}$. A detailed comparison with spin Hall conductivities calculated from first principles for hcp, fcc, and bcc solid solutions provides valuable insight into the alloying physics of this binary system., Comment: 6 pages, 4 figures

Spin superfluidity, i.e., coherent spin transport mediated by topologically stable textures, is limited by parasitic anisotropies rooted in relativistic interactions and spatial inhomogeneities. Since structural disorder in amorphous magnets can average out the effect of these undesired couplings, we propose this class of materials as platforms for superfluid spin transport. We establish nonlinear equations describing the hydrodynamics of spin in insulating amorphous magnets, where the currents are defined in terms of coherent rotations of a noncollinear texture. Our theory includes dissipation and nonequilibrium torques at the interface with metallic reservoirs. This framework allows us to determine different regimes of coherent dynamics and their salient features in nonlocal magneto-transport measurements. Our work paves the way for future studies on macroscopic spin dynamics in materials with frustrated interactions., 9 pages, 5 figures; to appear in Phys. Rev. B

This work presents both the parametrization procedure of a self-consistent charge density functional based tight-binding scheme (SCC-DFTB) for hafnium oxide and the structural as well as the electronic results of the simulations of amorphous hafnium oxide $a\text{-HfO}{}_{2}$ using the data resulting during the parametrization. While aiming at describing amorphous hafnium oxide, the created parameters could be successfully compared to ab initio DFT data for hafnium and hafnium oxide crystalline structures. Furthermore the atomic structure and electronic properties obtained in the DFTB simulations of $a\text{-HfO}{}_{2}$ compare well to other theoretical studies using classical many-body as well as quantum-mechanical approaches.

The recently advocated power-law scaling for fractal packing in amorphous metals is examined in a metallic glass under hydrostatic pressure. We show that the scaling relation ${v}_{a}\ensuremath{\sim}{q}_{1}^{\ensuremath{-}\ensuremath{\zeta}}$ between the sample molar volume ${v}_{a}$ and the first peak position ${q}_{1}$ in the scattering function exhibits a varying exponent \ensuremath{\zeta} from 2.77 to 3.72 in different stages of compression, rather than a constant as the universal fractal dimensionality. Fractal packing of short- and medium-range icosahedral clusters is found to exist but undetectable with high angle scattering. Therefore, the substructure of the metallic glass does not contribute to the power-law exponent. Moreover, we show that the space filling contributed from different alloy components with varying atomic sizes is overlooked in the scaling relation that gives rise to the so-called fractal dimension. The amorphous packing of metallic glasses is actually compact with dimension 3.

Weakly disordered superconducting films can be driven into an anomalous low temperature resistive state upon applying a magnetic field. Recent experiments on weakly disordered amorphous InO$_x$ have established that both the Hall resistivity and the frequency of a cyclotron-like resonance in the anomalous metal are highly suppressed relative to the values expected for a conventional metal. We show that both of these observations can be understood from the flux flow dynamics of vortices in a superconductor with significant vortex pinning. Results for flux flow transport are obtained using a systematic hydrodynamic expansion, controlled by the diluteness of mobile vortices at low temperatures. Hydrodynamic transport coefficients are related to microscopics through Kubo formulae for the longitudinal and Hall vortex conductivities, as well as a `vorto-electric' conductivity.

The transmission and reflection processes of THz phonons at solid interfaces are of fundamental interest and of importance to thermal conduction in nanocrystalline solids. The processes are challenging to investigate, however, because typical experiments and many computational approaches do not provide transmission coefficients resolved by phonon mode. Here, we examine the modal transmission and reflection processes of THz phonons across an amorphous Si region connected to two crystalline Si leads, a model interface for those that occur in nanocrystalline solids, using mode-resolved atomistic Green's functions. We find that the interface acts as a low-pass filter, reflecting modes of frequency greater than around 3 THz while transmitting those below this frequency, in agreement with a recent experimental report [C. Hua et al., Phys. Rev. B 95, 205423 (2017)]. Further, we find that these low frequency modes travel nearly unimpeded through the interface, maintaining their wave vectors on each side of the interface. Our work shows that even completely disordered regions may not be effective at reflecting THz phonons, with implications for efforts to alter thermal conductivity in nanocrystalline solids.

Scattering of phonons at boundaries of a crystal (grains, surfaces, or solid/solid interfaces) is characterized by the phonon wavelength, the angle of incidence, and the interface roughness, as historically evaluated using a specularity parameter p formulated by Ziman [J. M. Ziman, Electrons and Phonons (Clarendon Press, Oxford, 1960)]. This parameter was initially defined to determine the probability of a phonon specularly reflecting or diffusely scattering from the rough surface of a material. The validity of Ziman's theory as extended to solid/solid interfaces has not been previously validated. To better understand the interfacial scattering of phonons and to test the validity of Ziman's theory, we precisely measured the in-plane thermal conductivity of a series of Si films in silicon-on-insulator (SOI) wafers by time-domain thermoreflectance (TDTR) for a Si film thickness range of 1 - 10 {\mu}m and a temperature range of 100 - 300 K. The Si/SiO2 interface roughness was determined to be 0.11+/-0.04 nm using transmission electron microscopy (TEM). Furthermore, we compared our in-plane thermal conductivity measurements to theoretical calculations that combine first-principles phonon transport with Ziman's theory. Calculations using Ziman's specularity parameter significantly overestimate values from the TDTR measurements. We attribute this discrepancy to phonon transmission through the solid/solid interface into the substrate, which is not accounted for by Ziman's theory for surfaces. We derive a simple expression for the specularity parameter at solid/amorphous interfaces and achieve good agreement between calculations and measurement values., Comment: 4 figures, submitted to PRB

In the quest for materials with emergent or improved properties, an effective route is to create artificial superstructures. Novel properties emerge from the coupling between the phases, but the strength of this coupling depends on the quality of the interfaces. Atomic control of crystalline interfaces is notoriously complicated and to elude that obstacle, we suggest here an all-amorphous design. Starting from a model amorphous iron alloy, we locally tune the magnetic behavior by creating boron-doped regions by means of ion implantation through a lithographic mask. This process preserves the amorphous environment, creating a non-topographic magnetic superstructure with smooth interfaces and no structural discontinuities. The absence of inhomogeneities acting as pinning centers for the magnetization reversal is demonstrated by the formation of magnetic vortexes for ferromagnetic disks as large as 20 \textmu{}m in diameter embedded within a paramagnetic matrix. Rigid exchange coupling between two amorphous ferromagnetic phases in a microstructured sample is evidenced by an investigation involving first-order reversal curves. The sample consists of a soft matrix with embedded elements constituting a hard phase where the anisotropy originates from an elongated shape of the elements. We provide an intuitive explanation for the micrometer-range exchange coupling mechanism and discuss how to tailor the properties of all-amorphous superstructures.