Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session J29: Glassy & Amorphous Systems, including Quasicrystals |
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Sponsoring Units: DCMP Room: 603 |
Tuesday, March 4, 2014 2:30PM - 2:42PM |
J29.00001: Medium-range order in Al$_{90}$Sm$_{10}$ liquids revealed by a pre-peak in the structure factor Feng Zhang, Yang Sun, Xiao-Wei Fang, Matthew Kramer, Mikhail Mendelev, Ryan Ott, Cai-Zhuang Wang, Kai-Ming Ho Aluminum (Al) alloyed with about 10 at.{\%} rare-earth metals such as samarium (Sm) can display promising mechanical properties upon rapid quenching from the liquid state. Knowledge about the structure of the liquid phase is an important starting point for understanding the profound phase selection during the rapid solidification process, which plays a key role in materials performance. We have performed ab-initio molecular dynamics (AIMD) calculations on the liquid Al$_{90}$Sm$_{10}$ system with 500 atoms per unit cell at T $=$ 1300 K, which is well above the melting temperature of the system. The AIMD simulations show that the liquid system develops a medium-range order by segregating into nanometer-sized regions with different atomic compositions. This segregation ultimately gives rise to a distinct pre-peak in the structure factor observed both in experiments and in AIMD calculations. Our results are in good agreement with experimental measurements using three-dimensional atom probe on the amorphous Al$_{90}$Sm$_{10}$ structure rapidly quenched from the liquid state. [Preview Abstract] |
Tuesday, March 4, 2014 2:42PM - 2:54PM |
J29.00002: Atomic structural evolution in metallic liquids and glasses: A measure of fragility Nicholas Mauro, Matthew Blodgett, Mark Johnson, Adam Vogt, Kenneth Kelton The glass forming ability (GFA) of metallic alloys is widely varied. Bulk metallic glasses (BMGs) have been identified in a number of alloy systems but far more compositions can be vitrified only when their liquids are rapidly quenched. Understanding the structural evolution of metallic liquids as they are supercooled and quenched into glasses is critically important, not only for providing insight into the nature of the glass transition, but also for understanding technical aspects of glass formation and the thermal stability of the glassy solid. In this talk, we discuss the results of viscosity and high energy X-ray diffraction studies on a range of transition metal-based liquids and glasses. The temperature dependence of the X-ray structure factor has been measured in the glass by means of stationary diffraction and in the equilibrium and supercooled liquid state using Beamline Electrostatic Levitation. As will be shown, the temperature dependence of the structure factor above the glass transition shows anomalous acceleration. The degree of this acceleration has a strong correlation with liquid fragility as measured from non-contact viscosity data. These results suggest a \textit{structural fragility} metric distinguishing good glass formers from poor ones. [Preview Abstract] |
Tuesday, March 4, 2014 2:54PM - 3:06PM |
J29.00003: Two-level tunneling systems in amorphous alumina Irina V. Lebedeva, Alejandro P. Paz, Ilya V. Tokatly, Angel Rubio The decades of research on thermal properties of amorphous solids at temperatures below 1 K suggest that their anomalous behaviour can be related to quantum mechanical tunneling of atoms between two nearly equivalent states that can be described as a two-level system (TLS) [1]. This theory is also supported by recent studies on microwave spectroscopy of superconducting qubits [1]. However, the microscopic nature of the TLS remains unknown. To identify structural motifs for TLSs in amorphous alumina we have performed extensive classical molecular dynamics simulations. Several bistable motifs with only one or two atoms jumping by considerable distance $\sim$ 0.5 {\AA} were found at T=25 K. Accounting for the surrounding environment relaxation was shown to be important up to distances $\sim$ 7 {\AA}. The energy asymmetry and barrier for the detected motifs lied in the ranges 0.5 - 2 meV and 4 - 15 meV, respectively, while their density was about 1 motif per 10 000 atoms. Tuning of motif asymmetry by strain was demonstrated with the coupling coefficient below 1 eV. The tunnel splitting for the symmetrized motifs was estimated on the order of 0.1 meV. The discovered motifs are in good agreement with the available experimental data. \\[4pt] [1] G. J. Grabovskij et al. Science 338, 232 (2012) [Preview Abstract] |
Tuesday, March 4, 2014 3:06PM - 3:18PM |
J29.00004: ABSTRACT WITHDRAWN |
Tuesday, March 4, 2014 3:18PM - 3:30PM |
J29.00005: Fragility, network adaptation, rigidity- and stress- transitions in homogenized binary Ge$_{x}$S$_{100-x}$ glasses Shibalik Chakraborty, Punit Boolchand Binary Ge$_{x}$S$_{100-x}$ glasses reveal elastic and chemical phase transitions driven by network topology. With increasing Ge content x, well defined rigidity (x$_{c}$(1)$=$19.3{\%}) and stress(x$_{c}$(2)$=$24.85{\%}) transitions and associated optical elasticity power-laws are observed in Raman scattering. Calorimetric measurements reveal a square-well like minimum with window walls that coincide with the two elastic phase transitions. Molar volumes show a trapezoidal-like minimum with edges that nearly coincide with the reversibility window. These results are signatures of the isostatically rigid nature of the elastic phase formed between the rigidity and stress transitions. Complex C$_{p}$ measurements show melt fragility index, m(x) to also show a global minimum in the reversibility window, underscoring that \textit{melt dynamics encode the elastic behavior of the glass} formed at T$_{g}$. The strong nature of melts formed in the IP has an important practical consequence; they lead to slow homogenization of non-stoichiometric batch compositions reacted at high temperatures. Homogenization of chalcogenides melts/glasses over a scale of a few microns is a \textit{pre-requisite} to observe the intrinsic physical properties of these materials. [Preview Abstract] |
Tuesday, March 4, 2014 3:30PM - 3:42PM |
J29.00006: Consequences of the superstrong nature of chalcogenide glass-forming liquids at select compositions Kapila Gunasekera, Siddhesh Bhosle, Punit Boolchand, Matthieu Micoulaut Growth of homogeneous melts of stoichiometric compositions of chalcogenides is facilitated by underlying crystalline phases. Such is not the case for non-stoichiometric melt compositions in which, for example, variation of fragility (m) from complex specific heat measurements show global minimum [1] at an extremely low value (m$=$14.8(0.5)) in the 21.5{\%} \textit{\textless }x \textit{\textless }23{\%} range of Ge in homogenized Ge$_{x}$Se$_{100-x} $melts. This has unwittingly led to variability of results in physical properties of \textit{non-stoichiometric} melts/glasses due to their heterogeneity. By directly mapping melt stoichiometry variation along a quartz tube as a function of reaction time of starting materials at a fixed temperature T\textit{\textgreater }T$_{g}$ over days, we have observed a slowdown [1] of melt-homogenization by the super-strong melt compositions, 21.5{\%} \textless x \textit{\textless }23{\%}. This range, furthermore, appears to be correlated to the one observed between the ?exible and stressed rigid phase in network glasses.\\[4pt] [1] K.Gunasekera et al. , J.Chem Phys. 139, 164511 (2013). [Preview Abstract] |
Tuesday, March 4, 2014 3:42PM - 3:54PM |
J29.00007: Fragility and super-strong character of non-stoichiometric chalcogenides: implications on melt homogenization Sriram Ravindren, Kapila Gunasekera, Punit Boolchand, Matthieu Micoulaut The kinetics of homogenization of binary As$_{x}$Se$_{100-x}$ melts in the As concentration range 0\%$<$x$<$50\% are followed using Raman profiling, and show that 2 gm sized melts in the range 20\%$<$x$<$30\% take nearly two weeks to homogenize when the starting materials are reacted at 700$^{\circ}$C. The enthalpy of relaxation at T$_{g}$ - $\Delta$ H$_{nr}$(x) - shows a minimum in 27\%$<$x$<$37\% in aged samples. In such homogeneous glasses, molar volumes vary non-monotonically with composition and the fragility index $\textit{m}$ displays a broad global minimum in 20\%$<$x$<$40\% where $\textit{m}$$<$20. The super-strong nature of melt compositions in 20\%$<$x$<$30\% hinders melt diffusion at high temperatures, leading to the observed slow kinetics of melt homogenization. In comparing these results with earlier reports, there is evidence that fragility decreases as melts are homogenized. Furthermore, a clear scaling of $\textit{m}$ vs. T$_{g}$ is observed with a negative slope for Flexible glasses and a positive slope for Rigid and Stressed-rigid ones. The absence of a melting endotherm in non-stoichiometric As-Se compositions is reported. Fragilities of the Ge-As-Se are reported and a correlation observed with fragilities of As-Se and Ge-Se. [Preview Abstract] |
Tuesday, March 4, 2014 3:54PM - 4:06PM |
J29.00008: Modeling amorphous thin films: Kinetically limited minimization Pawel Zawadzki, John Perkins, Stephan Lany Amorphous materials become increasingly attractive components of thin film devices such as thin film displays or solar cells. They are typically prepared using physical vapor deposition (PVD) techniques at temperatures well below the melting point of deposited material ($< 0.2T_m$). Computational models of amorphous structures, however, are almost elusively constructed from a high temperature equilibrated crystal melt using simulated annealing (SA) protocol. While such procedure imitates the quench form melt preparation of bulk glasses, its applicability to modeling low temperature synthesized amorphous thin films is unclear. To account for low T growth conditions we propose a new method. The method, kinetically limited minimization (KLM), starts from a randomly initialized structure and minimizes the total energy in a number of local structural perturbation-relaxation events. We compare KLM and SA with quench rates ranging from 64 K/ps to 2500 K/ps using two prototypical ionic and covalent materials: In2O3 and Si, respectively. While both methods provide qualitatively similar structures, we find that, compared to KLM, slow quench SA provides stronger medium range order in a-In2O3 and fast quench SA provides weaker short range order in a-Si. [Preview Abstract] |
Tuesday, March 4, 2014 4:06PM - 4:18PM |
J29.00009: Optical spectroscopy of RE-Cd (RE = Gd, Y) quasicrystals and approximants N.M.R. Armstrong, Kim Mortimer, M. Rahimi Movassagh, T. Timusk, T. Kong, S.L. Bud'ko, P.C. Canfield To date, the optical conductivity of icosahedral quasicrystals, and their approximants, either have lacked an intraband Drude peak altogether or have shown an optical conductivity that, at best, can be described as an unresolved Drude peak with significant broadening, due to intense scattering, which is difficult to separate from interband transitions. We have measured the optical conductivity of the new family of RE-Cd (RE $=$Gd, Y) icosahedral quasicrystals and their approximant and found that the approximants show a well-defined peak at low frequency that cannot be fit with standard Drude theory. We will discuss our findings in terms of Mayou's generalized Drude theory of anomalously diffusing electrons. [Preview Abstract] |
Tuesday, March 4, 2014 4:18PM - 4:30PM |
J29.00010: Physical properties of i-R-Cd quasicrystals(R = Y, Gd-Tm) Tai Kong, Sergey L. Bud'ko, Anton Jesche, Alan I. Goldman, Andreas Kreyssig, Kevin W. Dennis, Mehmet Ramazanoglu, Paul C. Canfield, John McArthur Detailed characterization of recently discovered i-R-Cd (R = Y, Gd-Tm) binary quasicrystals by means of room-temperature powder x-ray diffraction, dc and ac magnetization, resistivity and specific heat measurements will be presented. i-Y-Cd is weakly diamagnetic. The dc magnetization of i-R-Cd (R = Gd, Ho-Tm) shows typical spin-glass type splitting between field-cooled (FC) and zero-field-cooled (ZFC) data. i-Tb-Cd and i-Dy-Cd do not show a clear cusp in their ZFC dc magnetization. ac magnetization measured on i-Gd-Cd indicates a clear frequency-dependence and the third-order non-linear magnetization, $\chi_{3}$, is consistent with a spin-glass transition. The resistivity for i-R-Cd is of order 100 $\mu \Omega$ cm and weakly temperature-dependent. No feature that can be associated with long-range magnetic order was observed in any of the measurements. Characteristic freezing temperatures for i-R-Cd (R = Gd-Tm) deviate from ideal de Gennes scaling. [Preview Abstract] |
Tuesday, March 4, 2014 4:30PM - 4:42PM |
J29.00011: Complex antiferromagnetic order in the Cd$_6R$ approximants to the i-$R$-Cd quasicrystals A. Kreyssig, G. Beutier, J.-U. Hoffmann, T. Kong, M.G. Kim, G.S. Tucker, B.G. Ueland, T. Hiroto, D. Liu, T. Yamada, M. de Boissieu, R. Tamura, S.L. Bud'ko, P.C. Canfield, A.I. Goldman The observation of antiferromagnetic order in the Cd$_6R$ ($R$ = rare earths) approximants [1-2] to the recently discovered related i-$R$-Cd quasicrystals [3] provides new and exciting opportunities to unravel the nature of magnetism in these materials. We present single-crystal studies employing x-ray and neutron scattering that revealed complex antiferromagnetism in the Cd$_6R$ approximants. Resolution-limited magnetic Bragg peaks have been observed at lattice points forbidden by the center-symmetry and at incommensurate positions demonstrating long-range antiferromagnetic correlations between the $R$ moments. The work at the Ames Laboratory was supported by US DOE, Office of Basic Energy Sciences, DMSE, contract DE-AC02-07CH11358. Work at the Tokyo University of Science was supported by KAKENHI (Grant No. 20045017). \\[4pt] [1] M. G. Kim et al., Phys. Rev. \textbf{B 85} (2012) 134442. \\[0pt] [2] A. Kreyssig et al., Phil. Mag. Lett. \textbf{93} (2013) 512. \\[0pt] [3] A. I. Goldman et al., Nature Mater. \textbf{12} (2013) 714. [Preview Abstract] |
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