68 research outputs found
Discrete Time Crystals with Absolute Stability
We show that interacting bosons on a ring which are driven periodically by a
rotating potential can support discrete time crystals whose absolute stability
can be proven. The absolute stability is demonstrated by an exact mapping of
discrete time crystal states to low-lying eigenstates of a time-independent
model that reveals spontaneous breaking of space translation symmetry. The
mapping ensures that there are no residual time-dependent terms that could lead
to heating of the system and destruction of discrete time crystals. We also
analyze periodically kicked bosons where the mapping is approximate only and
cannot guarantee the absolute stability of discrete time crystals. Besides
illustrating potential sources of instability, the kicked bosons model
demonstrates a rich field for investigating the interplay between different
time and space symmetry breaking, as well as the stability of time crystal
behavior in contact with a thermal reservoir.Comment: Version accepted for publication in Physical Review B as a Lette
Crystallographic structure of ultrathin Fe films on Cu(100)
We report bcc-like crystal structures in 2-4 ML Fe films grown on fcc Cu(100)
using scanning tunneling microscopy. The local bcc structure provides a
straightforward explanation for their frequently reported outstanding magnetic
properties, i.e., ferromagnetic ordering in all layers with a Curie temperature
above 300 K. The non-pseudomorphic structure, which becomes pseudomorphic above
4 ML film thickness is unexpected in terms of conventional rules of thin film
growth and stresses the importance of finite thickness effects in ferromagnetic
ultrathin films.Comment: 4 pages, 3 figures, RevTeX/LaTeX2.0
Symphony on strong field approximation
This paper has been prepared by the Symphony collaboration (University of Warsaw, Uniwersytet Jagiellonski, DESY/CNR and ICFO) on the occasion of the 25th anniversary of the 'simple man's models' which underlie most of the phenomena that occur when intense ultrashort laser pulses interact with matter. The phenomena in question include high-harmonic generation (HHG), above-threshold ionization (ATI), and non-sequential multielectron ionization (NSMI). 'Simple man's models' provide both an intuitive basis for understanding the numerical solutions of the time-dependent Schrodinger equation and the motivation for the powerful analytic approximations generally known as the strong field approximation (SFA). In this paper we first review the SFA in the form developed by us in the last 25 years. In this approach the SFA is a method to solve the TDSE, in which the non-perturbative interactions are described by including continuum-continuum interactions in a systematic perturbation-like theory. In this review we focus on recent applications of the SFA to HHG, ATI and NSMI from multi-electron atoms and from multi-atom molecules. The main novel part of the presented theory concerns generalizations of the SFA to: (i) time-dependent treatment of two-electron atoms, allowing for studies of an interplay between electron impact ionization and resonant excitation with subsequent ionization; (ii) time-dependent treatment in the single active electron approximation of 'large' molecules and targets which are themselves undergoing dynamics during the HHG or ATI processes. In particular, we formulate the general expressions for the case of arbitrary molecules, combining input from quantum chemistry and quantum dynamics. We formulate also theory of time-dependent separable molecular potentials to model analytically the dynamics of realistic electronic wave packets for molecules in strong laser fields. We dedicate this work to the memory of Bertrand Carre, who passed away in March 2018 at the age of 60
Surface structure of epitaxial Gd(0001) films on W(110) studied by quantitative LEED analysis
The surface structure of thick (400 Å) Gd(0001) films, epitaxially grown on W(110), is investigated by low-energy electron-diffraction (LEED) IV measurements in combination with dynamical LEED calculations. A first-layer contraction of 2.4% and a second-layer spacing expansion of 1% is found. These findings are in good agreement with literature values determined for the (0001) surface of bulk Gd crystals. No significant difference in the LEED IV data is found between films grown at room temperature and films grown at elevated temperatures.published_or_final_versio
Determination of the map of efficiency of the J-PET detector with the GATE package
A novel PET detector consisting of strips of polymer scintillators is being
developed by the J-PET Collaboration. The map of efficiency and the map of
geometrical acceptance of the 2-strip J-PET scanner are presented. Map of
efficiency was determined using the Monte Carlo simulation software GATE based
on GEANT4. Both maps were compared using method based on the chi2 test.Comment: 14 pages, 9 figures, proceeding from conference Symposium on Positron
Emission Tomography: http://koza.if.uj.edu.pl/pet-symposium-2013
Electron correlation effects and magnetic ordering at the Gd(0001) surface
Effects of electron correlation on the electronic structure and magnetic
properties of the Gd(0001) surface are investigated using of the full-potential
linearized augmented plane wave implementation of correlated band theory
("LDA+U"). The use of LDA+U instead of LDA (local density approximation) total
energy calculations produces the correct ferromagnetic ground state for both
bulk Gd and the Gd surface. Surface strain relaxation leads to an 90 %
enhancement of the interlayer surface-to-bulk effective exchange coupling.
Application of a Landau-Ginzburg type theory yields a 30 % enhancement of the
Curie temperature at the surface, in very good agreement with the experiment.Comment: revised version: minor typos correcte
From bi-layer to tri-layer Fe nanoislands on Cu3Au(001)
Self assembly on suitably chosen substrates is a well exploited root to
control the structure and morphology, hence magnetization, of metal films. In
particular, the Cu3Au(001) surface has been recently singled out as a good
template to grow high spin Fe phases, due to the close matching between the
Cu3Au lattice constant (3.75 Angstrom) and the equilibrium lattice constant for
fcc ferromagnetic Fe (3.65 Angstrom). Growth proceeds almost layer by layer at
room temperature, with a small amount of Au segregation in the early stage of
deposition. Islands of 1-2 nm lateral size and double layer height are formed
when 1 monolayer of Fe is deposited on Cu3Au(001) at low temperature. We used
the PhotoElectron Diffraction technique to investigate the atomic structure and
chemical composition of these nanoislands just after the deposition at 140 K
and after annealing at 400 K. We show that only bi-layer islands are formed at
low temperature, without any surface segregation. After annealing, the Fe atoms
are re-aggregated to form mainly tri-layer islands. Surface segregation is
shown to be inhibited also after the annealing process. The implications for
the film magnetic properties and the growth model are discussed.Comment: Revtex, 5 pages with 4 eps figure
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