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“Two-band superconductors : extended Ginzburg-Landau formalism by a systematic expansion in small deviation from the critical temperature”. Vagov A, Shanenko AA, Milošević, MV, Axt VM, Peeters FM, Physical review : B : condensed matter and materials physics 86, 144514 (2012). http://doi.org/10.1103/PhysRevB.86.144514
Abstract: We derive the extended Ginzburg-Landau (GL) formalism for a clean s-wave two-band superconductor by employing a systematic expansion of the free-energy functional and the corresponding matrix gap equation in powers of the small deviation from the critical temperature tau = 1 – T/T-c. The two lowest orders of this expansion produce the equation for T-c and the standard GL theory. It is shown that in agreement with previous studies, this two-band GL theory maps onto the single-band GL model and thus fails to describe the difference in the spatial profiles of the two-band condensates. We prove that this difference appears already in the leading correction to the standard GL theory, which constitutes the extended GL formalism. We derive linear differential equations that determine the leading corrections to the band order parameters and magnetic field, discuss the validity of these equations, and consider examples of an important interplay between the band condensates. Finally, we present numerical results for the thermodynamic critical magnetic field and temperature-dependent band gaps for recent materials of interest, which are in very good agreement with those obtained from the full BCS approach in a wide temperature range. To this end, we emphasize the advantages of our extended GL theory in comparison with the often used two-component GL-like model based on an unreconstructed two-band generalization of the Gor'kov derivation.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 3.836
Times cited: 44
DOI: 10.1103/PhysRevB.86.144514
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“Atypical BCS-BEC crossover induced by quantum-size effects”. Shanenko AA, Croitoru MD, Vagov AV, Axt VM, Perali A, Peeters FM, Physical review : A : atomic, molecular and optical physics 86, 033612 (2012). http://doi.org/10.1103/PhysRevA.86.033612
Abstract: Quantum-size oscillations of the basic physical characteristics of a confined fermionic condensate are a well-known phenomenon. Its conventional understanding is based on the single-particle physics, whereby the oscillations follow variations in the single-particle density of states driven by the size quantization. Here we present a study of a cigar-shaped ultracold superfluid Fermi gas, which demonstrates an important many-body aspect of the quantum-size coherent effects, overlooked previously. The many-body physics is revealed here in the atypical crossover from the Bardeen-Cooper-Schrieffer (BCS) superfluid to the Bose-Einstein condensate (BEC) induced by the size quantization of the particle motion. The single-particle energy spectrum for the transverse dimensions is tightly bound, whereas for the longitudinal direction it resembles a quasi-free dispersion. This results in the formation of a series of single-particle subbands (shells) so that the aggregate fermionic condensate becomes a coherent mixture of subband condensates. Each time when the lower edge of a subband crosses the chemical potential, the BCS-BEC crossover is approached in this subband, and the aggregate condensate contains both BCS and BEC-like components.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 2.925
Times cited: 34
DOI: 10.1103/PhysRevA.86.033612
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“Phonon-induced pure dephasing in exciton-biexciton quantum dot systems driven by ultrafast laser pulse sequences”. Axt VM, Kuhn T, Vagov A, Peeters FM, Physical review : B : condensed matter and materials physics 72, 125309 (2005). http://doi.org/10.1103/PhysRevB.72.125309
Abstract: A semiconductor quantum dot model accounting for single exciton as well as biexciton states coupled to phonons and laser light is investigated in the limit of strong electronic confinement. For an arbitrary sequence of excitations with ultrafast pulses analytical solutions are obtained for all density-matrix elements. The results are nonperturbative with respect to both the carrier-phonon and the carrier-light coupling. Numerical results for a single pulse excitation are presented illustrating spectral features of our solution as well as pulse area and temperature dependences.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 3.836
Times cited: 48
DOI: 10.1103/PhysRevB.72.125309
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“The Cooper problem in nanoscale : enhancement of the coupling due to confinement”. Croitoru MD, Vagov A, Shanenko AA, Axt VM, Superconductor science and technology 25, 124001 (2012). http://doi.org/10.1088/0953-2048/25/12/124001
Abstract: In 1956 Cooper demonstrated (1956 Phys. Rev. 104 1189) that, no matter how weak the attraction is, two electrons in three-dimensional (3D) space just above the Fermi sea could be bound. In this work we investigate the influence of confinement on the binding energy of a Cooper pair. We show that confinement-induced modification of the Fermi sea results in a significant increase of the binding energy, when the bottom of an energy subband is very close to the Fermi surface.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 2.878
Times cited: 9
DOI: 10.1088/0953-2048/25/12/124001
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“Ginzburg-Landau theory for multiband superconductors : microscopic derivation”. Orlova NV, Shanenko AA, Milošević, MV, Peeters FM, Vagov AV, Axt VM, Physical review : B : condensed matter and materials physics 87, 134510 (2013). http://doi.org/10.1103/PhysRevB.87.134510
Abstract: A procedure to derive the Ginzburg-Landau (GL) theory from the multiband BCS Hamiltonian is developed in a general case with an arbitrary number of bands and arbitrary interaction matrix. It combines the standard Gor'kov truncation and a subsequent reconstruction in order to match accuracies of the obtained terms. This reconstruction recovers the phenomenological GL theory as obtained from the Landau model of phase transitions but offers explicit microscopic expressions for the relevant parameters. Detailed calculations are presented for a three-band system treated as a prototype multiband superconductor. It is demonstrated that the symmetry in the coupling matrix may lead to the chiral ground state with the phase frustration, typical for systems with broken time-reversal symmetry. DOI: 10.1103/PhysRevB.87.134510
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 3.836
Times cited: 57
DOI: 10.1103/PhysRevB.87.134510
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“Nanofilms as quantum-engineered multiband superconductors : the Ginzburg-Landau theory”. Shanenko AA, Orlova NV, Vagov A, Milošević, MV, Axt VM, Peeters FM, Europhysics letters 102, 27003 (2013). http://doi.org/10.1209/0295-5075/102/27003
Abstract: Recently fabricated single-crystalline atomically flat metallic nanofilms are in fact quantum-engineered multiband superconductors. Here the multiband structure is dictated by the nanofilm thickness through the size quantization of the electron motion perpendicular to the nanofilm. This opens the unique possibility to explore superconductivity in well-controlled multi-band systems. However, a serious obstacle is the absence of a convenient and manageable theoretical tool to access new physical phenomena in such quasi-two-dimensional systems, including interplay of quantum confinement and fluctuations. Here we cover this gap and construct the appropriate multiband Ginzburg-Landau functional for nano-thin superconductors. Copyright (C) EPLA, 2013
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 1.957
Times cited: 8
DOI: 10.1209/0295-5075/102/27003
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“Coherent dynamics of confinement-induced multiband superconductors”. Croitoru MD, Zachmann M, Vagov A, Axt VM, Shanenko AA, Kettmann P, Papenkort T, Kuhn T, Physica: C : superconductivity 503, 183 (2014). http://doi.org/10.1016/j.physc.2014.04.014
Abstract: We study the coherent dynamics of pairing in a nanoscale superconductor, that is intrinsically multiband, after an external perturbation in the non-adiabatic regime. The description of the dynamics of the pairing order is within the density-matrix approach based on the BCS model and the Bogoliubov-de Gennes equations. We find that for certain resonant wire widths the superconducting order parameter exhibits two oscillatory frequencies which are determined by the long-time asymptotic values of the subgaps. This in turn leads to a pronounced beating phenomenon. (C) 2014 Elsevier B.V. All rights reserved.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 1.404
Times cited: 1
DOI: 10.1016/j.physc.2014.04.014
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“Competition between pure dephasing and photon losses in the dynamics of a dot-cavity system”. Vagov A, Glaessl M, Croitoru MD, Axt VM, Kuhn T, Physical review : B : condensed matter and materials physics 90, 075309 (2014). http://doi.org/10.1103/PhysRevB.90.075309
Abstract: We demonstrate that in quantum-dot cavity systems, the interplay between acoustic phonons and photon losses introduces novel features and characteristic dependencies in the system dynamics. In particular, the combined action of both dephasing mechanisms strongly affects the transition from the weak-to the strong-coupling regime as well as the shape of the spectral triplet that represents the quantum-dot occupation in Fourier space. The width of the central peak in the triplet is expected to decrease with rising temperature, while the widths and heights of the side peaks depend nonmonotonically on the dot-cavity coupling.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 3.836
Times cited: 11
DOI: 10.1103/PhysRevB.90.075309
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“Phonon limited superconducting correlations in metallic nanograins”. Croitoru MD, Shanenko AA, Vagov A, Milošević, MV, Axt VM, Peeters FM, Scientific reports 5, 16515 (2015). http://doi.org/10.1038/srep16515
Abstract: Conventional superconductivity is inevitably suppressed in ultra-small metallic grains for characteristic sizes smaller than the Anderson limit. Experiments have shown that above the Anderson limit the critical temperature may be either enhanced or reduced when decreasing the particle size, depending on the superconducting material. In addition, there is experimental evidence that whether an enhancement or a reduction is found depends on the strength of the electronphonon interaction in the bulk. We reveal how the strength of the e-ph interaction interplays with the quantum-size effect and theoretically obtain the critical temperature of the superconducting nanograins in excellent agreement with experimental data. We demonstrate that strong e-ph scattering smears the peak structure in the electronic density-of-states of a metallic grain and enhances the electron mass, and thereby limits the highest T-c achievable by quantum confinement.
Keywords: A1 Journal article; Engineering sciences. Technology; Condensed Matter Theory (CMT)
Impact Factor: 4.259
Times cited: 9
DOI: 10.1038/srep16515
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“Influence of disorder on superconducting correlations in nanoparticles”. Croitoru MD, Shanenko AA, Vagov A, Vasenko AS, Milošević, MV, Axt VM, Peeters FM, Journal of superconductivity and novel magnetism 29, 605 (2016). http://doi.org/10.1007/s10948-015-3319-8
Abstract: We investigate how the interplay of quantum confinement and level broadening caused by disorder affects superconducting correlations in ultra-small metallic grains. We use the electron-phonon interaction-induced electron mass renormalization and the reduced static-path approximation of the BCS formalism to calculate the critical temperature as a function of the grain size. We show how the strong electron-impurity scattering additionally smears the peak structure in the electronic density of states of a metallic grain and imposes additional limits on the critical temperature under strong quantum confinement.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 1.18
Times cited: 7
DOI: 10.1007/s10948-015-3319-8
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“Quench dynamics of an ultracold Fermi gas in the BCS regime : spectral properties and confinement-induced breakdown of the Higgs mode”. Hannibal S, Kettmann P, Croitoru MD, Vagov A, Axt VM, Kuhn T, Physical review : A : atomic, molecular and optical physics 91, 043630 (2015). http://doi.org/10.1103/PhysRevA.91.043630
Abstract: The Higgs amplitude mode of the order parameter of an ultracold confined Fermi gas in the BCS regime after a quench of the coupling constant is analyzed theoretically. A characteristic feature is a damped oscillation which at a certain transition time changes into a rather irregular dynamics. We compare the numerical solution of the full set of nonlinear equations of motion for the normal and anomalous Bogoliubov quasiparticle excitations with a linearized approximation. In doing so the transition time as well as the difference between resonant systems, i.e., systems where the Fermi energy is close to a sub-band minimum, and off-resonant systems can be well understood and traced back to the system and geometry parameters.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 2.925
Times cited: 10
DOI: 10.1103/PhysRevA.91.043630
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“Superconductivity between standard types: Multiband versus single-band materials”. Vagov A, Shanenko AA, Milošević, MV, Axt VM, Vinokur VM, Aguiar JA, Peeters FM, Physical review B 93, 174503 (2016). http://doi.org/10.1103/PhysRevB.93.174503
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 3.836
Times cited: 37
DOI: 10.1103/PhysRevB.93.174503
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“High pulse area undamping of Rabi oscillations in quantum dots coupled to phonons”. Vagov A, Croitoru MD, Axt VM, Kuhn T, Peeters FM, Physica status solidi B –, Basic solid state physics 243, 2233 (2006). http://doi.org/10.1002/pssb.200668029
Keywords: A1 Journal article; Condensed Matter Theory (CMT); Electron microscopy for materials research (EMAT)
Impact Factor: 1.674
Times cited: 16
DOI: 10.1002/pssb.200668029
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“Coherent nonlinear optical response of excitons and biexcitons in quantum dots coupled to phonons”. Krugel A, Axt VM, Kuhn T, Vagov A, Peeters FM, Physica status solidi B –, basic solid state physics 243, 2241 (2006). http://doi.org/10.1002/pssb.200668034
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 1.674
Times cited: 1
DOI: 10.1002/pssb.200668034
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“Nonmonotonic field dependence of damping and reappearance of Rabi oscillations in quantum dots”. Vagov A, Croitoru MD, Axt VM, Kuhn T, Peeters FM, Physical review letters 98, 1 (2007). http://doi.org/10.1103/PhysRevLett.98.227403
Keywords: A1 Journal article; Condensed Matter Theory (CMT); Electron microscopy for materials research (EMAT)
Impact Factor: 8.462
Times cited: 91
DOI: 10.1103/PhysRevLett.98.227403
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“Parity-fluctuation induced enlargement of the ratio \DeltaE/kBTc in metallic grains”. Croitoru MD, Shanenko AA, Peeters FM, Axt VM, Physical review : B : condensed matter and materials physics 84, 214518 (2011). http://doi.org/10.1103/PhysRevB.84.214518
Abstract: We investigate how the interplay of quantum confinement and particle number-parity fluctuations affects superconducting correlations in ultra-small metallic grains. Using the number-parity projected BCS formalism we calculate the critical temperature and the excitation gap as a function of the grain size for grains with even and odd number of confined carriers. We show that the experimentally observed anomalous increase of the coupling ratio ΔE/kBTc with decreasing superconducting grain size can be attributed to an enhancement of the number-parity fluctuations in ultra-small grains.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 3.836
Times cited: 14
DOI: 10.1103/PhysRevB.84.214518
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“Real time path integrals in studies of quantum dots dynamics: non-monotonous decay rate and reappearance of rabi rotations”. Vagov A, Croitoru MD, Axt VM, Kuhn T, Peeters F, Path Integrals : New Trends and Perspectives, Proceedings , 57 (2008). http://doi.org/10.1142/9789812837271_0007
Abstract: The dynamics of strongly confined laser driven semiconductor quantum dots coupled to phonons is studied theoretically by calculating the time evolution of the reduced density matrix using the path integral method. We explore the cases of long pulses, strong dot-phonon and dot-laser coupling and high temperatures, which up to now have been inaccessible. We find that the decay rate of the Rabi oscillations is a non-monotonic function of the laser field leading to the decay and reappearance of the Rabi oscillations in the field dependence of the dot exciton population.
Keywords: P1 Proceeding; Condensed Matter Theory (CMT)
DOI: 10.1142/9789812837271_0007
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“Extended Ginzburg-Landau formalism : systematic expansion in small deviation from the critical temperature”. Vagov AV, Shanenko AA, Milošević, MV, Axt VM, Peeters FM, Physical review : B : condensed matter and materials physics 85, 014502 (2012). http://doi.org/10.1103/PhysRevB.85.014502
Abstract: Based on the Gor'kov formalism for a clean s-wave superconductor, we develop an extended version of the single-band Ginzburg-Landau (GL) theory by means of a systematic expansion in the deviation from the critical temperature T(c), i.e., tau = 1 – T/T(c). We calculate different contributions to the order parameter and the magnetic field: the leading contributions (proportional to tau(1/2) in the order parameter and. t in the magnetic field) are controlled by the standard GL theory, while the next-to-leading terms (proportional to tau(3/2) in the gap and proportional to tau(2) in the magnetic field) constitute the extended GL (EGL) approach. We derive the free-energy functional for the extended formalism and the corresponding expression for the current density. To illustrate the usefulness of our formalism, we calculate, in a semianalytical form, the temperature-dependent correction to the GL parameter at which the surface energy becomes zero, and analytically, the temperature dependence of the thermodynamic critical field. We demonstrate that the EGL formalism is not just a mathematical extension to the theory: variations of both the gap and the thermodynamic critical field with temperature calculated within the EGL theory are found in very good agreement with the full BCS results down to low temperatures, which dramatically improves the applicability of the formalism compared to its standard predecessor.
Keywords: A1 Journal article; Condensed Matter Theory (CMT)
Impact Factor: 3.836
Times cited: 36
DOI: 10.1103/PhysRevB.85.014502
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