arXiv subjects

K. F. Kelton

Publications and source records attributed to K. F. Kelton.

27 records · Page 2Linked to original sources

arXiv2016

A locally preferred structure characterises all dynamical regimes of a supercooled liquid

Recent experimental results suggest that metallic liquids universally exhibit a high-temperature dynamical crossover, which is correlated with the glass transition temperature ($T_{g}$). We demonstrate, using molecular dynamics results for Cu64Zr36, that this temperature, $T_{A} \approx 2 \times T_{g}$, is linked with cooperative atomic rearrangements that produce domains of connected icosahedra. Supercooling to a new characteristic temperature, $T_{D}$, is shown to produce higher order cooperative rearrangements amongst connected icosahedra, leading to large-scale domain fluctuations and the onset of glassy dynamics. These extensive domains then abruptly stabilize above $T_{g}$ and eventually percolate before the glass is formed. All characteristic temperatures ($T_{A}$, $T_{D}$ and $T_{g}$) are thus connected by successive manifestations of the structural cooperativity that begins at $T_{A}$.

cond-mat.dis-nn↗

arXiv2015

Liquid-to-liquid phase transition underlying the structural crossover in a supercooled metallic liquid

The existence of a 'crossover region' in glass-forming liquids has long been considered as a general phenomenon that is as important as the glass transition. One potential origin for the crossover behavior is a liquid-to-liquid phase transition (LLPT). Although a LLPT is thought to exist in all forms of liquids, structural evidence for this, particularly in supercooled liquids, is scarce, elusive, and in many cases controversial. A key challenge to the search for a LLPT in a supercooled liquid is the interference of crystallization during cooling. Crystallization induces major structural changes, which can overwhelm and therefore mask the more subtle changes associated with a LLPT. Here, we report the results of an in-situ containerless synchrotron study of a metallic-glass-forming liquid (Zr57Nb5Al10Cu15.4Ni12.6) that show distinct changes in the liquid structure at ~1000 K, a temperature well below the melting temperature of the liquid and 150 K above the crystallization temperature. The structural transition is characterized by growing short- and extended-range order below the transition temperature, and is accompanied by a concurrent change in density. These data provide strong evidence for a LLPT in the supercooled metallic liquid, particularly in the light of a recent computer simulation study. That a LLPT is found in a metallic liquid supports the increasingly widely held view that a LLPT may be a common feature of a variety of liquids.

cond-mat.mtrl-sci↗

arXiv2015

Thermalization and possible quantum relaxation times in "classical" fluids: theory and experiment

Quantum effects in material systems are often pronounced at low energies and become insignificant at high temperatures. We find that, perhaps counterintuitively, certain quantum effects may follow the opposite route and become sharp when extrapolated to high temperature within a "classical" liquid phase. In the current work, we suggest basic quantum bounds on relaxation (and thermalization) times, examine kinetic theory by taking into account such possible fundamental quantum time scales, find new general equalities connecting semi-classical dynamics and thermodynamics to Planck's constant, and compute current correlation functions. Our analysis suggests that, on average, the extrapolated high temperature dynamical viscosity of general liquids may tend to a value set by the product of the particle number density ${\sf n}$ and Planck's constant $h$. We compare this theoretical result with experimental measurements of an ensemble of 23 metallic fluids where this seems to indeed be the case. The extrapolated high temperature viscosity of each of these liquids $η$ divided (for each respective fluid by its value of ${\sf n} h$) veers towards a Gaussian with an ensemble average value that is close to unity up to an error of size $0.6 \%$. Inspired by the Eigenstate Thermalization Hypothesis, we suggest a relation between the lowest equilibration temperature to the melting or liquidus temperature and discuss a possible corollary concerning the absence of finite temperature "ideal glass" transitions. We suggest a general quantum mechanical derivation for the viscosity of glasses at general temperatures. We invoke similar ideas to discuss other transport properties and demonstrate how simple behaviors including resistivity saturation and linear $T$ resistivity may appear very naturally. Our approach suggests that minimal time lags may be present in fluid dynamics.

cond-mat.stat-mech↗

arXiv2014

Unexpected Universality in the Viscosity of Metallic Liquids

The range of the magnitude of the liquid viscosity as a function of the temperature (T) is one of the most impressive of any physical property, changing by approximately 17 orders of magnitude from its extrapolated value at infinite temperature to that at the glass transition. We present experimental measurements of containerlessly processed metallic liquids that reveal that the ratio of the viscosity to its extrapolated infinite temperature value follows a universal function of Tcoop/T. The temperature Tcoop corresponds to the onset of cooperative motion and is strongly correlated with the glass transition temperature. On average the extrapolated infinite temperature viscosity is found to be nh, where h is Planck's constant and n is the particle number density. A surprising universality in the viscosity of metallic liquids and its relation to the glass transition is demonstrated.

cond-mat.soft↗

arXiv2013

Connectivity of the Icosahedral Network and a Dramatically Growing Static Length Scale in Cu-Zr Binary Metallic Glasses

We report on and characterize, via molecular dynamics (MD) studies, the evolution of the structure of Cu50Zr50 and Cu64Zr36 metallic glasses (MGs) as temperature is varied. Interestingly, a percolating icosahedral network appears in the Cu64Zr36 system as it is supercooled. This leads us to introduce a static length scale, which grows dramatically as this three dimensional system approaches the glass transition. Amidst interpenetrating connections, non-interpenetrating connections between icosahedra are shown to become prevalent upon supercooling and to greatly enhance the connectivity of the MG's icosahedral network. Additionally, we characterize the chemical compositions of the icosahedral networks and their components. These findings demonstrate the importance of non-interpenetrating connections for facilitating extensive structural networks in Cu-Zr MGs, which in turn drive dynamical slowing in these materials.

cond-mat.mtrl-sci↗

arXiv2011

Detecting hidden spatial and spatio-temporal structures in glasses and complex physical systems by multiresolution network clustering

We elaborate on a general method that we recently introduced for characterizing the "natural" structures in complex physical systems via a multiscale network based approach for the data mining of such structures. The approach is based on "community detection" wherein interacting particles are partitioned into "an ideal gas" of optimally decoupled groups of particles. Specifically, we construct a set of network representations ("replicas") of the physical system based on interatomic potentials and apply a multiscale clustering ("multiresolution community detection") analysis using information-based correlations among the replicas. Replicas may be (i) different representations of an identical static system or (ii) embody dynamics by when considering replicas to be time separated snapshots of the system (with a tunable time separation) or (iii) encode general correlations when different replicas correspond to different representations of the entire history of the system as it evolves in space-time. We apply our method to computer simulations of a binary Kob-Andersen Lennard-Jones system, a ternary model system, and to atomic coordinates in a ZrPt system as gleaned by reverse Monte Carlo analysis of experimentally determined structure factors. We identify the dominant structures (disjoint or overlapping) and general length scales by analyzing extrema of the information theory measures. We speculate on possible links between (i) physical transitions or crossovers and (ii) changes in structures found by this method as well as phase transitions associated with the computational complexity of the community detection problem. We briefly also consider continuum approaches and discuss the shear penetration depth in elastic media; this length scale increases as the system becomes increasingly rigid.

cond-mat.mtrl-sci↗

arXiv2010

Detection of hidden structures on all scales in amorphous materials and complex physical systems: basic notions and applications to networks, lattice systems, and glasses

Recent decades have seen the discovery of numerous complex materials. At the root of the complexity underlying many of these materials lies a large number of possible contending atomic- and larger-scale configurations and the intricate correlations between their constituents. For a detailed understanding, there is a need for tools that enable the detection of pertinent structures on all spatial and temporal scales. Towards this end, we suggest a new method by invoking ideas from network analysis and information theory. Our method efficiently identifies basic unit cells and topological defects in systems with low disorder and may analyze general amorphous structures to identify candidate natural structures where a clear definition of order is lacking. This general unbiased detection of physical structure does not require a guess as to which of the system properties should be deemed as important and may constitute a natural point of departure for further analysis. The method applies to both static and dynamic systems.

cond-mat.mtrl-sci↗

arXiv2002

Icosahedral Ti-Zr-Ni: A groundstate quasicrystal?

The first complete ab initio zero-temperature ternary phase diagram is constructed from the calculated energies of the elemental, binary and ternary Ti-Zr-Ni phases; for this, the icosahedral i-TiZrNi quasicrystal phase is approximated by periodic structures of up to 123 atoms/unit cell, based on a decorated-tiling model. The approximant structures containing the 45-atom Bergman cluster were nearly degenerate, and stable against the competing binary phases. It is speculated that i-TiZrNi may be a ground state quasicrystal, as it is experimentally the low-temperature phase for its composition.

cond-mat.mtrl-sci↗

arXiv2002

Structure of the icosahedral Ti-Zr-Ni quasicrystal

The atomic structure of the icosahedral Ti-Zr-Ni quasicrystal is determined by invoking similarities to periodic crystalline phases, diffraction data and the results from ab initio calculations. The structure is modeled by decorations of the canonical cell tiling geometry. The initial decoration model is based on the structure of the Frank-Kasper phase W-TiZrNi, the 1/1 approximant structure of the quasicrystal. The decoration model is optimized using a new method of structural analysis combining a least-squares refinement of diffraction data with results from ab initio calculations. The resulting structural model of icosahedral Ti-Zr-Ni is interpreted as a simple decoration rule and structural details are discussed.

cond-mat.mtrl-sci↗