Based on the Holstein model Hamiltonian of one-dimensional molecular crystals, by making use of the expansion approach of the correlated squeezed-coherent states of phonon instead of the two-phonon coherent state expa...Based on the Holstein model Hamiltonian of one-dimensional molecular crystals, by making use of the expansion approach of the correlated squeezed-coherent states of phonon instead of the two-phonon coherent state expansion scheme, the properties of the ground state and the anomalous quantum fluctuations are investigated in a strongly coupled electron-phonon system with special consideration of the electron-two-phonon interaction. The effective renormalization （ai） of the displacement of the squeezed phonons with the effect of the squeezed-coherent states of phonon and both the electron-displaced pbonon and the polaron-squeezed phonon correlations have been combined to obtain the anomalous quantum fluctuations for the corrections of the coherent state. Due to these non-adiabatic correlations, the effective displacement parameter ai is larger than the ordinary parameter ai （0） In comparison with the electron-one-phonon interaction （g） corrected as oig, we have found the electron-two-phonon interaction （gl） corrected as ai2gi is enhanced significantly. For this reason, the ground state energy （E（2）） contributed by the electron-two-phonon interaction is more negative than the single-phonon case （E01）） and the soliton solution is more stable. At the same time, the effects of the electron-two-phonon interaction greatly increase the polaron energy and the quantum fluctuations. Furthermore, in a deeper level, we have considered the effect of the polaron-squeezed phonon correlation （f-correlation）. Since this correlation parameter f 〉 1, this effect will strengthen the electron-one and two-phonon interactions by fai9 and f2ai2g1, respectively. The final results show that the ground state energy and the polaron energy will appear more negative further and the quantum fluctuations will gain further improvement.展开更多
The structural and elastic properties of the recently-discovered wⅡ- and δ-Si3N4 are investigated through the plane-wave pseudo-potential method within ultrasoft pseudopotentials.The elastic constants show that wⅡ-...The structural and elastic properties of the recently-discovered wⅡ- and δ-Si3N4 are investigated through the plane-wave pseudo-potential method within ultrasoft pseudopotentials.The elastic constants show that wⅡ- and δ-Si3N4 are mechanically stable in the pressure ranges of 0-50 GPa and 40-50 GPa,respectively.The α→wⅡ phase transition can be observed at 18.6 GPa and 300 K.The β→δ phase transformation occurs at pressures of 29.6,32.1,35.9,39.6,41.8,and 44.1 GPa when the temperatures are100,200,300,400,500,and 600 K,respectively.The results show that the interactions among the N-2s,Si-3s,3p bands（lower valence band） and the Si-3p,N-2p bands（upper valence band） play an important role in the stabilities of the wⅡ and S phases.Moreover,several thermodynamic parameters（thermal expansion,free energy,bulk modulus and heat capacity） of δ-Si3N4 are also obtained.Some interesting features are found in these properties.δ-Si3N4 is predicted to be a negative thermal expansion material.The adiabatic bulk modulus decreases with applied pressure,but a majority of materials show the opposite trend.Further experimental investigations with higher precisions may be required to determine the fundamental properties of wⅡ- andδ-Si3N4.展开更多
目前,我国正处于国家公园体制稳步推进,自然保护地体系转型重构的关键时期,明确国家公园的发展思路与理念是我们亟待解决的关键问题。National Parks:The American Experience一书较为全面地探讨了美国国家公园体系的发展历程及发展理...目前,我国正处于国家公园体制稳步推进,自然保护地体系转型重构的关键时期,明确国家公园的发展思路与理念是我们亟待解决的关键问题。National Parks:The American Experience一书较为全面地探讨了美国国家公园体系的发展历程及发展理念变迁,讲述了联邦政府、铁路公司和环保组织等利益团体围绕"国家公园该如何发展"这条主线展开的一系列博弈过程,涵盖了重要的历史事件、关键的推动性人物及相关开创性法案等内容。书中对推动美国国家公园发展的思潮和理念做了总结回顾,尤其是国家公园应该"保护什么"以及"如何保护"的思考,对完善我国国家公园体制、重构自然保护地体系具有一定的借鉴和启示意义。展开更多
基金Project supported by the National Natural Science Foundation of China (Grant No. 10574163)
文摘Based on the Holstein model Hamiltonian of one-dimensional molecular crystals, by making use of the expansion approach of the correlated squeezed-coherent states of phonon instead of the two-phonon coherent state expansion scheme, the properties of the ground state and the anomalous quantum fluctuations are investigated in a strongly coupled electron-phonon system with special consideration of the electron-two-phonon interaction. The effective renormalization （ai） of the displacement of the squeezed phonons with the effect of the squeezed-coherent states of phonon and both the electron-displaced pbonon and the polaron-squeezed phonon correlations have been combined to obtain the anomalous quantum fluctuations for the corrections of the coherent state. Due to these non-adiabatic correlations, the effective displacement parameter ai is larger than the ordinary parameter ai （0） In comparison with the electron-one-phonon interaction （g） corrected as oig, we have found the electron-two-phonon interaction （gl） corrected as ai2gi is enhanced significantly. For this reason, the ground state energy （E（2）） contributed by the electron-two-phonon interaction is more negative than the single-phonon case （E01）） and the soliton solution is more stable. At the same time, the effects of the electron-two-phonon interaction greatly increase the polaron energy and the quantum fluctuations. Furthermore, in a deeper level, we have considered the effect of the polaron-squeezed phonon correlation （f-correlation）. Since this correlation parameter f 〉 1, this effect will strengthen the electron-one and two-phonon interactions by fai9 and f2ai2g1, respectively. The final results show that the ground state energy and the polaron energy will appear more negative further and the quantum fluctuations will gain further improvement.
基金Funded by National Natural Science Foundation of China(Nos.61475132,61501392,11475143,11304141)the National Training Programs of Innovation and Entrepreneurship for Undergraduates(No.201510477001)
文摘The structural and elastic properties of the recently-discovered wⅡ- and δ-Si3N4 are investigated through the plane-wave pseudo-potential method within ultrasoft pseudopotentials.The elastic constants show that wⅡ- and δ-Si3N4 are mechanically stable in the pressure ranges of 0-50 GPa and 40-50 GPa,respectively.The α→wⅡ phase transition can be observed at 18.6 GPa and 300 K.The β→δ phase transformation occurs at pressures of 29.6,32.1,35.9,39.6,41.8,and 44.1 GPa when the temperatures are100,200,300,400,500,and 600 K,respectively.The results show that the interactions among the N-2s,Si-3s,3p bands（lower valence band） and the Si-3p,N-2p bands（upper valence band） play an important role in the stabilities of the wⅡ and S phases.Moreover,several thermodynamic parameters（thermal expansion,free energy,bulk modulus and heat capacity） of δ-Si3N4 are also obtained.Some interesting features are found in these properties.δ-Si3N4 is predicted to be a negative thermal expansion material.The adiabatic bulk modulus decreases with applied pressure,but a majority of materials show the opposite trend.Further experimental investigations with higher precisions may be required to determine the fundamental properties of wⅡ- andδ-Si3N4.
文摘目前,我国正处于国家公园体制稳步推进,自然保护地体系转型重构的关键时期,明确国家公园的发展思路与理念是我们亟待解决的关键问题。National Parks:The American Experience一书较为全面地探讨了美国国家公园体系的发展历程及发展理念变迁,讲述了联邦政府、铁路公司和环保组织等利益团体围绕"国家公园该如何发展"这条主线展开的一系列博弈过程,涵盖了重要的历史事件、关键的推动性人物及相关开创性法案等内容。书中对推动美国国家公园发展的思潮和理念做了总结回顾,尤其是国家公园应该"保护什么"以及"如何保护"的思考,对完善我国国家公园体制、重构自然保护地体系具有一定的借鉴和启示意义。
