- Transients in disordered solids
Electron transients induced by femtosecond laser pulses in semiconductors, in particular semiconductor alloys, were studied in the self-consistent Born approximation. The tool was an analytical and direct numerical solution of the Dyson equations for non-equilibrium Green's functions. The particle correlation function carrying the statistical information about the transient response was found to consist of a coherent transient and an incoherent back-scattered component. The particle number was conserved only if the field induced coherence was fully incorporated. The statistical correlation of disorder in the valence and conduction bands givers rise to kinematical correlation of electrons and holes manifested in effects similar to true excitons and well known for linear response. The onset and the ripe stage of the pulse excitation clearly demonstrated that the effective kinematic correlation in non-linear transients can be charaterized as a "final state interaction", an e-h interaction in the outgoing channel.
A. Kalvová and B. Velický: Photoexcited Transients in Disordered Semiconductors: Quantum Coherence at Very Short to Intermediate Times, Phys. Rev. B 65 155329 (2002)
A. Kalvová and B. Velický: Photoexcited Transients in Disordered Semiconductors: Kinematically Induced Electron-Hole Correlation: in book Progress in Nonequilibrium Green’s Function II, eds. M. Bonitz, D. Semkat, World Scientific, London, (2003), p. 322-329
A. Kalvová and B. Velický: Disorder Induced e-h Correlation in Photoexcited Transients in Semiconductors: in book Nonequilibrium Physics at Short Time Scales, Formation of Correlations, ed. K. Morawetz, Springer, Berlin, (2004), p. 273-293
22nd General Conference of the CMD EPS, 25-28 August 2008 „La Sapienza“ University, Roma, Italy
poster: A NGF solver for fast transients in semiconductors PosterRoma.pdf
- Fast transients in mesoscopic systems
A transient response of electrons in a mesoscopic system depends on the joint effect of the initial state with correlations and of the driving external disturbances abruptly setting on at the initial time. If the initial state results from the previous history of the system, its role can be transformed onto the coherence effects between the transient and its antecedent, as given by the blocks of the self-energy off-diagonal in time. The early period of the transient requires the full NGF for its description. After the disturbances cease acting and the initial correlations evanesce, the process enters the non-equilibrium quasi-particle mode permitting a reduced description by a generalized master equation. As an example, we consider a molecular bridge between two leads. The transient is induced by rapid changes in the coupling between the leads and the bridge. Without interactions the model is soluble to the end.
B. Velický, A. Kalvová and V. Špička: Single molecule bridge as a testing ground for using NGF outside of steady current regime, Physica E 42, 539 (2010) DOI:10.1016/j.physe.2009.08.006
V. Špička, A. Kalvová and B. Velický : Dynamics of mesoscopic systems: Non-equilibrium Green's functions approach Physica E 42, 525 (2010)
A. Kalvová, V. Špička and B. Velický: Fast Transients in Mesoscopic Systems , Non-equilibrium Statistical Physics Today, AIP Conf. Proc. 1332, 223-225 (2011)
V. Špička, A. Kalvová, and B. Velický: Fast Dynamics of Molecular Bridges , Physica Scripta T151 (2012),014037(17pp)
A. Kalvová, V. Špička and B. Velický: Fast Transient Current Response to Switching Events in Short Chains of Molecular Islands , Superconductivity and Novel Magnetism 26: 773-777 (2013), DOI: 10.1007/s10948-012-2081-4; The final publication is available at link.springer.com.
A. Kalvová, V. Špička and B. Velický: Transient magnetic tunneling mediated by a molecular bridge in the junction region , EPJ Web of Conferences 75, 02004 (2014), DOI: 10.1051/epjconf/20147502004
A. Kalvová, V. Špička and B. Velický: Transient Magnetic Tunneling Mediated by a Molecular Bridge, Superconductivity and Novel Magnetism 28:1087-1091 (2015), DOI: 10.1007/s10948-014-2702-1; The final publication is available at link.springer.com.
A. Kalvová, B. Velický and V. Špička : Transient Magnetic Tunneling Currents Through a Molecular Bridge: Limits to Reduction of Nonequilibrium Green's Functions to a Generalized Master Equation, Superconductivity and Novel Magnetism (2015), DOI: 10.1007/s10948-016-3680-2
Progress in Nonequilibrium Green’s Functions IV, Glasgow 17 – 21 August, 2009
11th Granada Seminar on Computational and Statistical Physics (Foundations of Nonequilibrium Statistical Physics), La Herradura 13-17 September, 2010, Spain
3rd International Conference on Superconductivity and Magnetism, Istanbul 29 April – 4 May 2012, Turkey
seminář MFF UK, katedra elektronových struktur, 17.2. 2012
seminář teor.odd. FZU AVČR, 19.6. 2012
JEMS 2013 Joint European Magnetic Symposia, August 25-30, 2013, Rhodes, Greece
4th International Conference on Superconductivity and Magnetism, Antalja 27 April – 2 May 2014, Turkey
5th International Conference on Superconductivity and Magnetism, Fethiye 24 April – 30 April 2016, Turkey
- Time dynamics of many-electron systems
These studies were extended to a general investigation of time dynamics of many-electron systems with correlations on the time scale from the fast initial transients up to the kinetic stage of evolution. Several results were established developing the general Non-Equilibrium Green's Function (NGF) theory without the need for resorting to specific models. A Reconstruction Theorem was proved answering to the Bogolyubov Postulate concerning the possibility of inverse recovery of the non-equilibrium correlation functions from the one-particle quantum distribution function. The NGF theory was in this way paralleled to the Time Dependent Density Functional Theory. The corresponding reconstruction equations represent a renormalized version of the approximate factorizations known as the family of causal ansatzes generalizing the Kadanoff-Baym ansatz for NGF. A new Time Partitioning Method was developed and applied to the Dyson equation for the particle correlation function to take account of the weak time non-locality of the action of the correlated initial conditions and their decay.
V. Špička, B. Velický and A. Kalvová: Long and Short Time Quantum Dynamics: I. Between Green’s Functions and Transport Equations : Physica E 29, (2005) p. 154-174
V. Špička, B. Velický and A. Kalvová: Long and Short Time Quantum Dynamics: II Kinetic Regime . Physica E 29, (2005) p. 175-195
V. Špička, B. Velický and A. Kalvová: Long and Short Time Quantum Dynamics: III Transients Physica E 29, (2005) p.196-212
B. Velický, A. Kalvová and V. Špička: Between Green’s Functions and Transport Equations: Reconstruction Theorems and the Role of Initial Conditions, Journal of Physics: Conference Series Progress in Nonequilibrium Green’s Functions III, Vol. 35, (2006), p. 1-16
B. Velický, A. Kalvová and V. Špička: Correlated Initial Condition for an Embedded Process by Time Partitioning , Phys. Rev. B 81, 235116 (2010) DOI: 10.1103/PhysRevB.81.235116
B. Velický, A. Kalvová and V. Špička: Ward identity for nonequilibrium Fermi systems, Phys. Rev. B 77, 041201 (2008)
V. Špička, B. Velický and A. Kalvová : Electron systems out of equilibrium: Nonequilibrium Green's function approach International Journal of Modern Physics B, Vol. 28, No. 23 (2014) 1430013-1,DOI:10.1142/S0217979214300138
- Non-equilibrium quasiparticles
The approximate factorization of propagators known as the Semi-Group Rule also permits a renormalization, which serves to guarantee validity of the non-equilibrium generalization of the Ward Identity leading to the particle number conservation. The quasi-particle form of the semi-group rule leads to an operational definition of non-equilibrium quasi-particles: the quasi-particle regime is achieved, if the propagator possesses a time-local optical potential. This can be determined from a non-linear integral Phase Equation obtained as a condition for self-consistent cancellation of the vertex in the renormalized semi-group rule.
B. Velický, A. Kalvová and V. Špička: Quasiparticle States of Electron Systems out of Equilibrium , Phys. Rev. B 75, 195125 (2007)
LECTURES (in pdf format):
December 2004, Seminar at the Theor. Dpt., Prague: Disorder induced e-h correlations in photo-transients in semiconductors
May 2005, Interdisciplinary Seminar, University Plzeň: Slow and stop light after five years
LectureStopLight.pdf (in Czech language)
ZpomalenéSvětlo.doc (sborník z XII semináře o filosofických otázkách matematiky a fyziky)
November 2005, Topical Problems in Theor. Stat. Phys., Chemnitz University: Correlated initial condition for restart process
December 2007, Topical Problems in Theor. Stat. Phys., Chemnitz University: Non-equilibrium Quasiparticles
December 01-06, 2008, Workshop on Nonequilibrium Nanostructures NONNA08: Non-equilibrium Ward Identity
Faculty of Science, Palacký University, Olomouc
Introduction into the Physics of the Condensed Systems