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An observable for vacancy characterization and diffusion in crystals
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10.1063/1.4796322
/content/aip/journal/jcp/138/14/10.1063/1.4796322
http://aip.metastore.ingenta.com/content/aip/journal/jcp/138/14/10.1063/1.4796322
View: Figures

Figures

Image of FIG. 1.
FIG. 1.

Series of atomic configurations along the local migration path in a 2D trigonal lattice of WCA particles superimposed to the corresponding V(x; R) potential (see Sec. IV A for all the details). V(x; R) is reported in Lennard-Jones units (these units are used throughout the text) and represented according to the colormap reported in the bar at the bottom of the figure. The simulation box is also reported in the figure (periodic boundary conditions are used in all calculations discussed in the text). The (ideal) local migration path is the (linear) path followed by a vacancy migrating from a lattice site to one of its nearest neighbor sites (indicated by the white arrow in the top panel). Panel (a) corresponds to the configuration in which the vacancy is located on a lattice site. In the next two panels, (b) and (c), the atomic configuration and the potential V(x; R) at intermediate states between the initial and the mid-way states are shown. In these states, the potential V(x; R) already presents the characteristic “double-well” shape. In panel C is reported the mid-way configuration and the corresponding potential V(x; R). In this state, the two wells of V(x; R) are symmetric.

Image of FIG. 2.
FIG. 2.

V(x; R) (left) and ρ(x|R) (right) for four atomic configurations along the ideal local vacancy migration path in a system of WCA particles.

Image of FIG. 3.
FIG. 3.

V(x; R) (left) and ρ(x|R) (right) for a system of hard disks in the equilibrium and mid-way configurations along the ideal local migration path. The blue color in the left-hand panel denotes V(x; R) = 0; the red V(x; R) = ∞.

Image of FIG. 4.
FIG. 4.

and as a function of a at the configurations of panels (a) (●), (b) (■), (c) (◆), and (d) (x) of Fig. 1 . a is reported on a logarithmic scale. Vertical lines are inserted to facilitate the comparison of values of and at a given value of a. Zone 1, 2, and 3 denote the three regions of a in which and have different behaviors (see text).

Image of FIG. 5.
FIG. 5.

V(x; R) (left) and ρ(x|R) (right) at few atomic configurations selected along the migration event observed during an unbiased simulation at T = 2.5. The arrows in the left column point to representative narrow minima of the potential V(x; R) not associated to the vacancy (see text).

Image of FIG. 6.
FIG. 6.

The two components, and , of the first moment (top) and the trace of the second moment (bottom) of the density ρ(x|R) as measured along a high temperature (T = 2.5) simulation. The arrow in the bottom panel indicates a failed migration event.

Image of FIG. 7.
FIG. 7.

Potential (left) and density ρ(x|R) (right) computed at configurations taken along the avoided migration event around timestep 5600 of the high temperature MD simulation. The white cross indicates the position of . When the vacancy is located at a crystal site (top and bottom panels), is approximately at the maximum of ρ(x|R) and the vacancy minimum of the potential . During the attempted migration event (central panels), the main peak of the density is shifted towards the bottom-left with respect to , while a long tail is formed at its top-right.

Image of FIG. 8.
FIG. 8.

Comparison of the first moment and the trace of the second moment of the density ρ(x|R) as obtained from (unbiased) MC at T = 0.5 (left) and TAMC at the same T and at T* = 8.5 (right). The first component of ( ) is denoted by a continuous black line, while the second component ( ) by a dotted red line. The dashed ellipses indicate events that are discussed in the text. The inset shows a zoom of the along the TAMC simulation in the interval in which the system was in a reoriented crystalline state (see text). The red line in the inset corresponds to the bottom of the curve in the simulation time complementary to the inset interval.

Image of FIG. 9.
FIG. 9.

Trajectories along a local (a) and non-local (b) migration event. The particles are colored in red at the beginning of the trajectory and, passing by green, become blue at the end. The boundary of the simulation box is also reported.

Image of FIG. 10.
FIG. 10.

Atomic configurations corresponding to the misoriented crystal. The stress due to the wrong orientation is accommodated by the formation of a wide vacancy defect. During the interval ∼(2.3–5) × 105 TAMC steps the misoriented crystal maintained the same geometry (trigonal) but its orientation and the position of the vacancy changed several times. The misorientation of the crystal is emphasized by reporting on the plot the original lattice vectors a and b, and the lattice vectors of the reoriented crystal, a′ and b′, and a″ and b″.

Image of FIG. 11.
FIG. 11.

3D representation of the classical (a) and quantum (b) densities in the finite temperature system at the same configuration of the third panel of Fig. 5 . The classical density has been computed at = 0.1, the lowest with associated negligible density values at crystal minima. The scales of the panels (a) and (b) are set such that the densities’ maxima have a comparable magnitude.

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/content/aip/journal/jcp/138/14/10.1063/1.4796322
2013-04-11
2014-04-17
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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: An observable for vacancy characterization and diffusion in crystals
http://aip.metastore.ingenta.com/content/aip/journal/jcp/138/14/10.1063/1.4796322
10.1063/1.4796322
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