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Quantum and Spontaneous Breakage of Translational Symmetry due to Light Excitation in Photo-Induced Structural Phase Transitions

100%
Nasu K.
Acta Physica Polonica A
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2012
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vol. 121
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issue 2
347-349
EN
Real time quantum dynamics of the spontaneous translational symmetry breakage due to light excitation in the early stage of photo-induced structural phase transitions is reviewed under the guide of the Toyozawa theory, which is in exact compliance with the conservation law of the total momentum. At the Franck-Condon state, an electronic excitation just created by a visible photon is in a plane wave state, extended all over the crystal. While, after the lattice relaxation having been completed, it is localized as a new excitation. So, is there the shrinkage of the excitation wave function? No! The wave function never shrinks, but only the spatial (or inter lattice-site) quantum coherence (interference) of the excitation disappears, as the lattice relaxation proceeds. This is the breakage of translational symmetry.
2
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Photoinduced sp^3 Nanosize Domain with Frozen Shear Displacement in Graphite

63%
Nishioka K., Nasu K.
Acta Physica Polonica A
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2012
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vol. 121
|
issue 2
350-354
EN
A novel sp^3-bonded nanosize domain, known as a diaphite which is an intermediate state between a graphite and a diamond, is generated by the irradiation of visible laser pulse onto a graphite crystal. The sp^3 structure is well stabilized by shear displacement between neighboring graphite layers. We theoretically study the interlayer sp^3 bond formation with frozen shear displacement in a graphite crystal, using a classical molecular dynamics and a semi-empirical Brenner potential. We show that a pulse excitation under the fluctuation of shearing motion of carbons in an initial state can generate interlayer sp^3 bonds which freeze the shear, though no frozen shear appears if there is no fluctuation initially. Moreover, we investigate a pulse excitation under the coherent shearing motion and consequently obtain that the sp^3-bonded domain with frozen shear is efficiently formed. We conclude that the initial shear is important for the photoinduced sp^3 nanosize domain formation.
3
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Domain h Type Collective Dimerization of Graphite and Possible sp^2 rightarrow sp^3 Transition Induced by Inter h Layer Charge Transfer Excitations in the Visible Region

45%
Radosinski L., Ostasiewicz K., Luty T., Radosz A., Nasu K.
Acta Physica Polonica A
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2010
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vol. 118
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issue 3
500-506
EN
We theoretically study possible domain-type collective dimerizations of graphite induced by inter-layer charge transfer excitations in the visible region. Using the semiempirical Brenner theory, we have calculated the adiabatic energy along the path that starts from original two distant graphite layers, but finally reaches the dimerized domain which consists of about 100 carbons with inter-layer σ-bonds. The energy barrier between this new domain and the starting graphite is shown to be of the order of 1 eV, being easily overcome by applying a few visible photons. We have also shown the optimal path of transformation via step by step increase of the domain size.
4
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Photoinduced Conversion of Hybridization in Graphite

39%
Radosinski L., Luty T., Nasu K., Ohnishi H., Nishioka K., Radosz A., Wójt D.
Acta Physica Polonica A
|
2012
|
vol. 121
|
issue 2
359-368
EN
Recent experiments indicate that a photostimulated graphite with a femtosecond laser pulse results in the formation of a stable domain with sp^3 like interlayer bonds. By means of the energy barrier minimization and molecular dynamics using the empirical Brenner potential we study a geometrical structure of the new phase. We clarify proliferation of the initial domain and prove that the overall process is a multiphoton one. Furthermore, we present a model describing the initial transformation - an interlayer charge transfer resulting in the localization of an exciton-like state. The local density approximation electronic structure analysis reveals that the electronic state of the new phase is an insulator immersed in semimetal. We study by means of the long-range carbon bond order potential the effect of the existence of the new phase on the surrounding graphite and propose a new mid step structure on the path of a photoinduced graphite-diamond conversion.
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