Session Y7: Glassy Metallic, Semiconductor, Oxide and Chalcogenide Systems

Ion irradiation induced suppression of shear banding in amorphous ZrCuAl nanowires under simulated compression

Metallic glasses (MG) are amorphous metallic solids which exhibit extraordinary mechanical properties including large elastic strain limit and high tensile strength; at the same time, MGs are generally brittle due to catastrophic failure of shear banding. Controversy remains, however, as to whether shear banding is size-dependent in MG nanopillar samples prepared using Focused Ion Beam (FIB) technique. Here we modeled Zr50Cu40Al10 MG nanopillars under irradiation via molecular dynamic simulations (MD) and found a transition in deformation mode from dominant shear-banding to homogeneous shear-flow. Our results call for careful consideration of irradiation damage as a result of FIB sample-preparation-technique. In addition, we also show the amorphization of icosahedral structures as a result of irradiation, which might be responsible for the homogeneous deformation mode of irradiated MG nanowires upon compression tests.   

Two Level Systems in Amorphous Silicon

The specific heat of e-beam evaporated a-Si thin films prepared at various growth temperatures was measured from 2-300K. Below 20K, films with low density have a specific heat in excess of the predicted Debye value while higher density films do not. The excess heat capacity is typical of a glass with a linear contribution that is characteristic of two-level systems (TLS) and a T$^{3}$ contribution that is in excess of the Debye specific heat calculated from the measured sound velocity. The excess specific heat is independent of the elastic properties of the materials as determined by shear modulus and sound velocity measurements but depends on film density. The density dependence suggests that the low energy excitations form in voids or low density regions and are not intrinsic to the amorphous silicon network. A correlation is found between the density of TLS and the excess T$^{3}$ specific heat suggesting that they have a common origin. Comparisons will be made between the specific heat and internal friction.   

First principles calculations of the Urbach tail in the optical absorption of silica glass

We present density-functional theory calculations of the optical absorption spectra of silica glass for temperatures up to 2400\,K. The calculated spectra exhibit exponential tails near the fundamental absorption edge that follow the Urbach rule, in good agreement with experiments. We also discuss the accuracy of our results by comparing to hybrid e xchange correlation functionals. By deriving a simple relationship between the exponential tails of the absorption co efficient and the electronic density-of-states, we establish a direct link between the photoemission and the absorpti on spectra near the absorption edge. This relationship is subsequently employed to determine the lower bound to the U rbach frequency regime. Most interestingly, in this frequency interval, the optical absorption is Poisson distributed with very large statistical fluctuations. Finally, We determine the upper bound to the Urbach frequency regime by id entifying the frequency at which transition to Poisson distribution takes place.   

Substrate- and interface-mediated photocrystallization in a-Se films and multi-layers

Photocrystallization in a-Se films and layered a-Se structures is studied by Raman scattering as a function of temperature for photon energies near or slightly below the band gap. The samples are $\sim $16.5 $\mu $m thick films of a-Se grown i) directly on glass, ii) on indium tin oxide (ITO) coated glass, iii) on glass that is spin coated with 800nm polymide, and iv) on a Capton sheet. A low As-concentration ($<$ 0.2 {\%}) is present in several of the a-Se films. We compare the results on these samples to prior findings on a-Se HARP targets, and on a polymer-encapsulated a-Se film [1]. We observe strong evidence that the interface between the a-Se film and the underlying substrate and/or multi-layers plays an important role in the onset time and growth rate of photocrystallized Se domains. In some samples a discontinuous increase in the onset time with increasing temperature occurs near the glass transition ($\sim $310K), and there is a surprising ``dead zone'' of no crystallization in this region. Other samples merely show a minimum in the onset time at similar temperatures, but no discontinuity and no region where crystallization is absent. Soft intermediate layers appear to increase stability against crystallization in an overlying a-Se film. The competing effects of substrate shear strain and thermal driving forces on the photocrystallization process are considered to account for these findings. \\[4pt] [1] R.E. Tallman et. al. J. Non-crystalline Sols. \textbf{354,} 4577-81 (2008)   

Lattice irreversibility and enhanced fragility under fatigue in amorphous solids

The enhanced brittleness observed in glassy ZrCuAl metallic alloys under mechanical cyclic fatigue loading is linked to a local atomic structure reorganization and suppression of atomic fluctuations. From the analysis of neutron scattering and the pair density function technique, an atomic cluster restructuring is observed that intensifies with increasing the compression cycles ex-situ. This is accompanied by an attenuation of the extended phonon-like lattice dynamics, beyond the region in the momentum space where the Boson peak appears, observed by inelastic neutron scattering. Together both effects most likely render the glass more fragile. These findings provide a direct link between the plasticity and the internal structure of metallic glasses under fatigue.   

Probing the liquid behavior in La-based metallic glasses using NMR spectroscopy

The nature of liquid structure and its temperature and/or pressure dependent behavior is currently an active area of scientific investigation. Temperature dependent $^{27}$Al nuclear magnetic resonance (NMR) experiments were carried out above the liquid temperature in La-based metallic glasses. The strong coexistence of two liquid states was observed in addition to nonlinear liquid behavior. NMR results also provide thermodynamic insight for the structural changes observed.   

Chemical ordering in Cu-Zr and Cu-Hf liquids and glasses

Evidence for chemical ordering in Cu-Zr and Cu-Hf metallic liquids and glasses is presented. High-energy X-ray diffraction data has been taking using the Beamline Electrostatic (BESL) technique and stationary scattering methods up to a high momentum transfer. From this data, we obtained the total pair-correlation functions. By examining the structural evolution in the equilibrium and undercooled liquid, and the glassy state, we find that ordering occurs, increasing the Cu-Zr (Cu-Hf) and Zr-Zr (Hf-Hf) correlations over a wide composition and temperature range. This ordering is evident by a judicious choice of scattering lengths and compositions. Growth is observed at two distinct r-space coordinates in the pair-correlation function, unique to these two metallic systems. These results will be discussed in the context of previous experiments which indicate that this ordering is chemical and topological in nature, likely due to the development of Cu-centered icosahedral clusters.   

Molecular structure of Si$_x$Ge$_x$Te$_{100-2x}$ glasses

Bulk glasses of the titled ternary have been synthesized in the 8\% $<$ x $<$ 16\% range by melt quenching. Glass transition temperatures, T$_g$(x), increase linearly from 140$^{\circ}$C at x = 8\% to 200$^{\circ}$C at x = 12\%, then decrease steadily to 160$^{\circ}$C thereafter. The non-reversing enthalpy of relaxation at T$_g$ shows a broad minimum near 9\% but a maximum near 12\%. Glasses appear to be fully polymerized at x $<$ 12\%, but segregate as x $>$ 12\%. The broad minimum near 9\%, most likely, represents the opening of an Intermediate Phase. Raman scattering, excited using 785 nm radiation of a glass at x = 10\%, shows two modes, a broad one near 160 cm$^{-1}$ and a narrow one near 127 cm$^{-1}$. A mode near 157 cm$^{-1}$ has been previously\footnote{M.Brodsky, Phys. Stat. Solidi(b) 52, 609 (1972)} identified with polymeric Te$_n$ chains of a-Te. Tentatively, we assign the 127 cm$^{-1}$ mode with face-sharing\footnote{M. Malyj et al. Phys. Rev. B 31, 3672 (1985)} GeTe$_4$, and a mode near 170 cm$^{-1}$ with edge-sharing SiTe$_4$ tetrahedra in these glasses. The nature of glass structure evolution with composition will be elucidated.   

The sharpness of thermally reversing windows as a measure of glass network homogeneity

Reversibility windows(RWs) have been observed\footnote{Boolchand et al., Phil. Mag 85, 3823 (2005)} in Chalcogenides, modified- and unmodified-oxides, and solid electrolytes. These are identified with isostaticaly rigid networks formed in narrow compositional windows between flexible and stressed-rigid elastic phases. Until recently, we found RWs in oxides and solid electrolytes to be sharper than in chalcogenides. Recently we introduced\footnote{Bhosle et al., Sol. St. Comm. 151, 1851 (2011)} a Raman profiling method to track homogeneity of melts during synthesis, and found that the kinetics of homogenization of chalcogenide melts are slow. The enthalpy of relaxation at T$_g$ measured in binary Ge$_x$Se$_{100-x}$ glasses show the RW to be intrinsically square-well like with sharp edges in homogeneous samples ($\Delta$x $\sim$ 0), and becomes trapezoidal ($\Delta$x = 1.5\%), then triangular ($\Delta$x = 3\%) and eventually disappears as glass heterogeneity increases. The heterogeneity deduced from Raman profiling experiments provides the Ge-stoichiometry variation ($\Delta$x) across a batch composition, which can be used to predict the observed RWs.   

Boson mode, Medium Range Structure and Intermediate Phase (IP) in (Na$_{2}$O)$_{x}$(B$_{2}$O$_{3})_{1-x}$ glasses

Raman scattering of titled glasses are examined using a T64000 Dispersive system. Scattering strengths of the Boson mode (40 cm$^{-1}$, 70 cm$^{-1})$ and the Boroxyl ring (BR) mode (808 cm$^{-1})$ are found to decrease with increasing x at the same rate in the 0 $<$ x $<$ 20{\%} soda range. Apparently, the 2D character of BRs embedded in a 3D network gives rise to the Boson mode.\footnote{M. Flores-Ruiz and G. Naumis, PRB, 2011. \textbf{83}: p. 184204} The triad of modes (705, 740, 770 cm$^{-1})$ near the 808 cm$^{-1}$ mode are found to display a maximum in scattering strength near x = 37{\%} (705 cm$^{-1})$, 33{\%} (740 cm$^{-1})$ and 25{\%} (770 cm$^{-1})$, suggesting that these are also ring modes of Na-tripentaborate (STPB), Na-diborate (SDB) and Na-triborate (STB) super-structures. Variations in Raman scattering strengths also suggest that STB percolate near x = 20{\%}, the \textit{stress} transition, while the STPB and SDTB percolate near x = 40{\%}, the \textit{rigidity} transition. These transitions were inferred from m-DSC experiments that show an intermediate phase in the 20{\%} $<$ x $<$ 40{\%} range in dry and homogeneous glasses.   

Boson mode, Intermediate Phase, and glass molecular structure of heavy metal Oxides

We have synthesized bulk glasses of (B$_{2}$O$_{3})_{5}$(TeO2)$_{95-x}$(V$_{2}$O$_{5})_{x}$ ternary and examined their thermal and optical properties as a function of composition ``x.'' The enthalpy of relaxation at T$_{g}$ shows a global minimum in the Vanadia concentration of 24{\%} $<$ x $<$ 26.5{\%}, which we identify\footnote{P. Boolchand et al. Phil. Mag. \textbf{85}, 3823 (2005)} as the intermediate phase (IP) with compositions at x $<$ 24{\%} to be in stressed-rigid and those at x $>$ 26.5{\%} in the flexible phase. Raman scattering reveals a rich lineshape including a Boson mode, whose scattering strength steadily decreases with increasing x, possibly due to bifurcation of weak (Te-O$_{axial})$ and strong (Te-O$_{equatorial})$ springs characteristic of TeO$_{2}$ building blocks.\footnote{S. Sakida, J. Phys.: Condens. Matter \textbf{12} (2000)} Vanadia alloying brings in isostatic building blocks, pyramidal V(O$_{1/2})_{3}$ and quasi-tetrahedral O= V(O$_{1/2})_{3 }$as suggested by present Raman scattering and recent $^{51}$V NMR data.\footnote{Ibid.} We describe the structure evolution of these glasses in terms of the Te-centered and V-centered local structures.   

Identification of strong and weak interacting two-level systems in KBr:CN

Tunneling two-level systems (TLSs) are believed to be the source of phenomena such as the universal low temperature properties in disordered and amorphous solids, and $1/f$ noise. The existence of these phenomena in a large variety of dissimilar physical systems testifies for the universal nature of the TLSs, which however, is not yet known. Following a recent suggestion that attributes the low temperature TLSs to inversion pairs [M. Schechter and P.C.E. Stamp, arXiv:0910.1283] we calculate explicitly the TLS-phonon coupling of inversion symmetric and asymmetric TLSs in a given disordered crystal. Our work (a) estimates parameters that support the theory in M. Schechter and P.C.E. Stamp, arXiv:0910.1283 in its general form, and (b) positively identifies the relevant TLSs in a given system. Consequences to the understanding of the microscopic structure of amorphous solids will be discussed. A. Gaita-Arino and M. Schechter, Phys.~Rev.~Lett.~{\bf 107}, 105504 (2011)   

Coulomb Glass: a Mean Field Study

We study the Coulomb glass model of disordered localized electrons with long-range Coulomb interaction, which describes systems such as disordered insulators, granular metals, amorphous semiconductors, or doped crystalline semiconductors. Long ago Efros and Shklovskii showed that the long-range repulsion induces a soft Coulomb gap in the single particle density of states at low temperatures. Recent works suggested that this gap is associated to a transition to a glass phase, similar to the Almeida-Thouless transition in spin glasses. In this work, we use a mean field approach to characterize several physical properties of the Coulomb glass. In particular, following a seminal work of Bray and Moore, we show that the Edward-Anderson parameter $q_{EA}$ and the spin glass susceptibility $\chi_{SG}$ are directly related to spectrum distribution of the Hessian matrix around free energy minima. Using this result, we show that no glass transition is associated to the gap formation.   

Non-linear dielectric response of glass formers under oscillating temperature

Exploiting a simple model we investigate linear and non-linear dielectric responses of glass formers under oscillating temperature. We demonstrate that three characteristic temperatures, Vogel-Fulcher, glass transition and cross-over temperatures, can be determined from the analysis of the dielectric response. We first show that the real part of the linear susceptibility at the static limit becomes a cusp below the cross-over temperature and its curvature changes at the glass transition temperature. We also analyze the non-linear susceptibility under oscillating temperature which contains information of the response of the system, in particular of the free energy landscape, against the temperature modulation. We find that the real part of the non-linear susceptibility shows anomalies at the characteristic temperatures similar to the linear susceptibility.