Large kinetic asymmetry in the metal-insulator transition nucleated at localized and extended defects

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Large kinetic asymmetry in the metal-insulator transition nucleated at localized and extended defects

W. Fan, J. Cao, J. Seidel, Y. Gu, J. W. Yim, C. Barrett, K. M. Yu, J. Ji, R. Ramesh, L. Q. Chen, and J. Wu

Phys. Rev. B 83, 235102 – Published 2 June 2011

Abstract

Superheating and supercooling effects are characteristic kinetic processes in first-order phase transitions, and asymmetry between them is widely observed. In materials where electronic and structural degrees of freedom are coupled, a wide, asymmetric hysteresis may occur in the transition between electronic phases. Structural defects are known to seed heterogeneous nucleation of the phase transition, hence reduce the degree of superheating and supercooling. Here we show that in the metal-insulator transition of single-crystal VO $_{2}$ , a large kinetic asymmetry arises from the distinct spatial extension and distribution of two basic types of crystal defects: point defects and twin walls. Nanometer-thick twin walls are constantly consumed but regenerated during the transition to the metal phase, serving as dynamical heterogeneous nucleation seeds and eliminating superheating. On the other hand, the transition back to the insulator phase relies on nucleation at point defects because twinning is structurally forbidden in the metal phase, leading to a large supercooling. By controlling the formation, location, and extinction of these defects, the kinetics of the phase transition might be externally modulated, offering possible routes toward unique memory and logic device technologies.

Received 22 March 2011

DOI:https://doi.org/10.1103/PhysRevB.83.235102

Authors & Affiliations

W. Fan^1,2, J. Cao^1,3, J. Seidel³, Y. Gu⁴, J. W. Yim^1,3, C. Barrett^1,3, K. M. Yu³, J. Ji², R. Ramesh^1,3, L. Q. Chen⁴, and J. Wu^1,3,*

¹Department of Materials Science and Engineering, University of California, Berkeley, Berkeley, California 94720, USA
²Department of Thermal Science and Energy Engineering, University of Science and Technology of China, Hefei, China
³Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA
⁴Department of Materials Science and Engineering, Pennsylvania State University, University Park, Pennsylvania 16802, USA

^*wuj@berkeley.edu

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Vol. 83, Iss. 23 — 15 June 2011

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Images

Figure 1
Kinetic asymmetry in the metal-insulator transition of VO $_{2}$ . (a) Resistance of a clamped device and a freestanding device measured at a temperature changing rate of 2 °C/min. The resistance of the freestanding device is divided by 10 to add a vertical offset for clarity. (b) Distinct distribution of resistance steps on the resistance curve measured from a clamped device. The inset shows a closeup view of the boxed area on the clamped device curve in (a) where ministeps can be seen.Reuse & Permissions
Figure 2
(a) Optical images of an initially $M_{1}$ -twinned VO $_{2}$ MB recorded during heating and cooling. $M$ and $R$ phases have bright and dark reflections, respectively. The left end of the MB is freestanding. Note the gradual (irregular) domain formation during heating vs abrupt (regular) domain formation during cooling. During cooling the $M$ phase emerges abruptly within <1 °C between 75 and 74 °C, and is “born periodic.” (b) Optically determined transition temperatures of freestanding VO $_{2}$ as a function of 3-MeV $α$ -particle irradiation dose. Different symbols represent different samples. The curves are a guide to the eye. Inset: Optical image of a freestanding VO $_{2}$ MB before and after the MIT. (c) Temperature-dependent four-probe resistance of a clamped device before and after two doses of $α$ -particle irradiation. The superheating is not affected, yet the supercooling is clearly reduced by the irradiation.Reuse & Permissions
Figure 3
The twin wall structure. (a) A schematic illustrating the orientation of 180° and 90° $M_{1}$ twin walls and of 180° $M_{2}$ twin wall along a MB. (b) Crystal structure of the 180° twin wall (green dashed line) between two variants of the $M_{1}$ phase viewed along the $\pm [011]_{M_{1}}$ direction, and of the $R$ phase viewed along the same direction ( $[1 \bar{1} 0]_{R}$ ). This is the side-view direction of the MB, while the MB length is along the horizontal $c_{R}$ (or $a_{M_{1}}$ ) direction. A small angle of 0.23° exists between the $a_{M_{1}}$ axes of the two $M_{1}$ variants. (c) Crystal structure of the 180° twin wall (green dashed line) between two variants of the $M_{2}$ phase viewed along the $\pm b_{M_{2}}$ direction, and of the $R$ phase viewed along the same direction (i.e., $c_{R}$ ). This is the direction along the MB length. In (b) and (c), small (red) circles are O atoms, and large (blue and green) circles are V atoms.Reuse & Permissions
Figure 4
The twin walls as dynamical nucleation sites for the MIT. (a) An optical image (top) and polarized image (middle) of a clamped VO $_{2}$ MB showing twinned $M_{1}$ domains with twin walls (indicated by arrows) perpendicular to the MB axis, and the nucleation of $R$ domains at some of the twin walls upon heating (bottom). (b) SEM images showing twinned $M_{2}$ domains among $R$ domains in a clamped VO $_{2}$ beam at 70 °C with $M_{2}$ twin walls parallel to the MB axis. (c) Tapping-mode AFM topography images showing $M_{1 α}$ / $M_{1 β}$ twin walls at room temperature. At higher temperatures $R$ domains nucleate at some of the $M_{1}$ twin walls. When the $R$ domains are sufficiently large, they induce a twinned $M_{2}$ phase in the neighborhood whose twin walls in turn mediate the growth of the $R$ phase. Some of the $M_{1}$ twin walls are highlighted by vertical dashed lines in the top image, and some of the $M_{2}$ twin walls are highlighted by horizontal dashed lines in the bottom image. The $c_{R}$ axis is horizontal. Note that the $R$ phase has a larger height than both $M_{1}$ and $M_{2}$ phases.Reuse & Permissions
Figure 5
Phase field modeled domains of bottom constrained VO $_{2}$ . (a) (001) $_{R}$ twins of the $M_{1}$ phase at 25 °C. (b) Domain structures after relaxation at 79 °C. The $M_{1}$ domain edges make an angle of ∼65° with the $c_{R}$ direction. The yellow and green colors represent two variants of the $M_{1}$ phase. The semitransparent part represents the $R$ phase.Reuse & Permissions

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