| 1. |
- Almqvist, Tim, et al.
(författare)
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Domain formation and self-sustained oscillations in quantum cascade lasers
- 2019
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Ingår i: European Physical Journal B. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 92:4
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Tidskriftsartikel (refereegranskat)abstract
- Abstract: We study oscillations in quantum cascade lasers due to traveling electric field domains, which are observed both in simulations and experiments. These oscillations occur in a range of negative differential resistance and we clarify the condition determining whether the boundary between domains of different electric field can become stationary. Graphical abstract: [Figure not available: see fulltext.].
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| 2. |
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| 3. |
- Aurell, Erik, et al.
(författare)
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Dynamics and performance of susceptibility propagation on synthetic data
- 2010
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Ingår i: European Physical Journal B. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 77:4, s. 587-595
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Tidskriftsartikel (refereegranskat)abstract
- We study the performance and convergence properties of the susceptibility propagation (SusP) algorithm for solving the Inverse Ising problem. We first study how the temperature parameter (T) in a Sherrington-Kirkpatrick model generating the data influences the performance and convergence of the algorithm. We find that at the high temperature regime (T > 4), the algorithm performs well and its quality is only limited by the quality of the supplied data. In the low temperature regime (T < 4), we find that the algorithm typically does not converge, yielding diverging values for the couplings. However, we show that by stopping the algorithm at the right time before divergence becomes serious, good reconstruction can be achieved down to T a parts per thousand 2. We then show that dense connectivity, loopiness of the connectivity, and high absolute magnetization all have deteriorating effects on the performance of the algorithm. When absolute magnetization is high, we show that other methods can be work better than SusP. Finally, we show that for neural data with high absolute magnetization, SusP performs less well than TAP inversion.
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| 4. |
- Baldissera, Gustavo, et al.
(författare)
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Vacancy induced magnetism in WO3
- 2013
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Ingår i: European Physical Journal B. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 86:6, s. 273-
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Tidskriftsartikel (refereegranskat)abstract
- The possibility to obtain ferromagnetic (FM) phase from native defects in WO3 is investigated by theoretically analyzing six different crystalline structures. The local magnetic moment from vacancies is calculated using the projector augmented wave method in combination with the local spin density approximation including a Coulomb correction (LSDA + U) of the W d-states. We find that tungsten vacancies V-W can induce a magnetic phase of similar to 3.5 mu(B)/V-W with a local magnetic moment on the oxygen atoms of at most similar to 1 mu(B)/V-W, whereas corresponding oxygen vacancies V-O have no impact on the magnetic coupling. Intriguingly, although the six crystalline structures have very comparable bonds, the magnetic moment generated by the cation vacancies is different, showing higher local magnetic moments for WO3 structures with low crystalline symmetry. The results indicate that WO3:V-W cannot induce a hole-mediated FM phase, and instead V-W in WO3 induces a local magnetic moment on the unpaired states at surrounding O atoms.
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| 5. |
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| 6. |
- Barra, Adriano, et al.
(författare)
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Mean field spin glasses treated with PDE techniques
- 2013
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Ingår i: European Physical Journal B. - : Springer Berlin/Heidelberg. - 1434-6028 .- 1434-6036. ; 86:7
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Tidskriftsartikel (refereegranskat)abstract
- Following an original idea of F. Guerra, in this notes we analyze the Sherrington-Kirkpatrick model from different perspectives, all sharing the underlying approach which consists in linking the resolution of the sta- tistical mechanics of the model (e.g. solving for the free energy) to well-known partial differential equation (PDE) problems (in suitable spaces). The plan is then to solve the related PDE using techniques involved in their native field and lastly bringing back the solution in the proper statistical mechanics framework. Within this strand, after a streamlined test-case on the Curie-Weiss model to highlight the methods more than the physics behind, we solve the SK both at the replica symmetric and at the 1-RSB level, obtaining the correct expression for the free energy via an analogy to a Fourier equation and for the self-consistencies with an analogy to a Burger equation, whose shock wave develops exactly at critical noise level (triggering the phase transition).Our approach, beyond acting as a new alternative method (with respect to the standard routes) for tackling the complexity of spin glasses, links symmetries in PDE theory with constraints in statistical mechanics and, as a novel result from the theoretical physics perspective, we obtain a new class of polynomial identities (namely of Aizenman-Contucci type, but merged within the Guerra’s broken replica measures), whose in- terest lies in understanding, via the recent Panchenko breakthroughs, how to force the overlap organization to the ultrametric tree predicted by Parisi.
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| 7. |
- Borysov, Stanislav, et al.
(författare)
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U.S. stock market interaction network as learned by the Boltzmann machine
- 2015
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Ingår i: European Physical Journal B. - : Springer Berlin/Heidelberg. - 1434-6028 .- 1434-6036. ; 88:12, s. 1-14
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Tidskriftsartikel (refereegranskat)abstract
- We study historical dynamics of joint equilibrium distribution of stock returns in the U.S. stock market using the Boltzmann distribution model being parametrized by external fields and pairwise couplings. Within Boltzmann learning framework for statistical inference, we analyze historical behavior of the parameters inferred using exact and approximate learning algorithms. Since the model and inference methods require use of binary variables, effect of this mapping of continuous returns to the discrete domain is studied. The presented results show that binarization preserves the correlation structure of the market. Properties of distributions of external fields and couplings as well as the market interaction network and industry sector clustering structure are studied for different historical dates and moving window sizes. We demonstrate that the observed positive heavy tail in distribution of couplings is related to the sparse clustering structure of the market. We also show that discrepancies between the model’s parameters might be used as a precursor of financial instabilities.
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| 8. |
- Boström, Mathias, et al.
(författare)
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Casimir attractive-repulsive transition in MEMS
- 2012
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Ingår i: European Physical Journal B. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 85:11, s. 377-
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Tidskriftsartikel (refereegranskat)abstract
- Unwanted stiction in micro-and nanomechanical (NEMS/MEMS) systems due to dispersion (van der Waals, or Casimir) forces is a significant hurdle in the fabrication of systems with moving parts on these length scales. Introducing a suitably dielectric liquid in the interspace between bodies has previously been demonstrated to render dispersion forces repulsive, or even to switch sign as a function of separation. Making use of recently available permittivity data calculated by us we show that such a remarkable nonmonotonic Casimir force, changing from attractive to repulsive as separation increases, can in fact be observed in systems where constituent materials are in standard NEMS/MEMS use requiring no special or exotic materials. No such nonmonotonic behaviour has been measured to date. We calculate the force between a silica sphere and a flat surface of either zinc oxide or hafnia, two materials which are among the most prominent for practical microelectrical and microoptical devices. Our results explicate the need for highly accurate permittivity functions of the materials involved for frequencies from optical to far-infrared frequencies. A careful analysis of the Casimir interaction is presented, and we show how the change in the sign of the interaction can be understood as a result of multiple crossings of the dielectric functions of the three media involved in a given set-up.
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| 9. |
- Boström, Mathias, et al.
(författare)
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Casimir force between atomically thin gold films
- 2013
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Ingår i: European Physical Journal B. - : Springer Science and Business Media LLC. - 1434-6028 .- 1434-6036. ; 86:2, s. 43-
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Tidskriftsartikel (refereegranskat)abstract
- We have used density functional theory to calculate the anisotropic dielectric functions for ultrathin gold sheets (composed of 1, 3, 6, and 15 atomic layers). Such films are important components in nano-electromechanical systems. When using correct dielectric functions rather than bulk gold dielectric functions we predict an enhanced attractive Casimir-Lifshitz force (at most around 20%) between two atomically thin gold sheets. For thicker sheets the dielectric properties and the corresponding Casimir forces approach those of gold half-spaces. The magnitude of the corrections that we predict should, within the today's level of accuracy in Casimir force measurements, be clearly detectable.
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| 10. |
- Catalano, Francesco, et al.
(författare)
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Equation of motion truncation scheme based on partial orthogonalization
- 2021
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Ingår i: European Physical Journal B. - : Springer Nature. - 1434-6028 .- 1434-6036. ; 94
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Tidskriftsartikel (refereegranskat)abstract
- We introduce a general scheme to consistently truncate equations of motion for Green's functions. Our scheme is guaranteed to generate physical Green's functions with real excitation energies and positive spectral weights. There are free parameters in our scheme akin to mean field parameters that may be determined to get as good an approximation to the physics as possible. As a test case we apply our scheme to a two-pole approximation for the 2D Hubbard model. At half-filling we find an insulating solution with several interesting properties: it has low expectation value of the energy and it gives upper and lower Hubbard bands with the full non-interacting bandwidth in the large U limit. Away from half-filling, in particular in the intermediate interaction regime, our scheme allows for several different phases with different number of Fermi surfaces and topologies.Graphic abstract
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