Quantum Mechanics
http://hdl.handle.net/20.500.11824/17
Sun, 17 Feb 2019 08:17:50 GMT2019-02-17T08:17:50ZIsotropic Bipolaron-Fermion-Exchange Theory and Unconventional Pairing in Cuprate Superconductors
http://hdl.handle.net/20.500.11824/930
Isotropic Bipolaron-Fermion-Exchange Theory and Unconventional Pairing in Cuprate Superconductors
Bru J.B.; de Siqueira Pedra W.; Delgado de Pasquale A.
The discovery of high-temperature superconductors in 1986 represented a major experimental breakthrough (Nobel Prize 1987), but their theoretical explanation is still a subject of much debate. These materials have many exotic properties, such as d- and p-wave pairing and density waves. The appearance of unconventional pairing is examined from a microscopic model, taking into account important properties of hole-doped copper oxides. Weconsider an exchange interaction between fermions and dominantly inter-site bipolarons to be the mechanism which leads to the pairing. We connect its momentum dependency to the well-established fermion-phonon anomalies in cuprate superconductors. Since charge carriers in these materials are strongly correlated, we add a screened Coulomb repulsion to this exchange term. We avoid any ad hoc assumptions like anisotropy, but rather provide a microscopic explanation of unconventional pairing for coupling strengths that are in accordance with experimental facts. One important outcome is a mathematically rigorous elucidation of the role of Coulomb repulsion in unconventional pairing, which is shown to be concomitant with a strong depletion of superconducting pairs. Our theory, applied to the special case
of LaSr 214, predicts at optimal doping (i) a coherence length of 21A, which is the same as that obtained from the Ginzburg-Landau critical magnetic field measured for this material, and (ii) d-wave pair formation in the pseudogap regime, i.e., at temperatures much higher thanthe superconducting transition temperature. We think that the understanding of pairing symmetry and the pseudogap phase are central issues in the theoretical comprehension of high-temperature superconductivity, with
possible technological applications like s-, d-, and p-wave Josephson junctions used nowadays in quantum computers.
Mon, 10 Dec 2018 00:00:00 GMThttp://hdl.handle.net/20.500.11824/9302018-12-10T00:00:00ZAccuracy of Classical Conductivity Theory at Atomic Scales for Free Fermions in Disordered Media
http://hdl.handle.net/20.500.11824/929
Accuracy of Classical Conductivity Theory at Atomic Scales for Free Fermions in Disordered Media
Aza N.J.B.; Bru J.B.; de Siqueira Pedra W.; Ratsimanetrimanana A.
The growing need for smaller electronic components has recently sparked the interest in the breakdown of the classical conductivity theory near the atomic scale, at which quantum effects should dominate. In 2012, experimental measurements of electric resistance of nanowires in Si doped with phosphorus atoms demonstrate that quantum effects on charge transport
almost disappear for nanowires of lengths larger than a few nanometers, even at very low temperature (4.2K). We mathematically prove, for non-interacting lattice fermions with disorder, that quantum uncertainty of microscopic electric current density around their (classical) macroscopic values is suppressed, exponentially fast with respect to the volume of the region of the lattice where an external electric field is applied. This is in accordance with the above experimental observation. Disorder is modeled
by a random external potential along with random, complex-valued, hopping amplitudes. The celebrated tight-binding Anderson model is one particular example of the general case considered here. Our mathematical analysis is based on Combes-Thomas estimates, the Akcoglu-Krengel ergodic theorem, and the large deviation formalism, in particular the Gärtner-Ellis theorem.
Tue, 22 Jan 2019 00:00:00 GMThttp://hdl.handle.net/20.500.11824/9292019-01-22T00:00:00ZDecay of Complex-time Determinantal and Pfaffian\ Correlation Functionals in Lattices
http://hdl.handle.net/20.500.11824/928
Decay of Complex-time Determinantal and Pfaffian\ Correlation Functionals in Lattices
Aza N.J.B.; Bru J.B.; de Siqueira Pedra W.
We supplement the determinantal and Pfaffian bounds of Sims and Warzel (Commun Math Phys 347:903--931, 2016) for many-body localization of quasi-free fermions, by considering the high dimensional case and complex-time correlations. Our proof uses the analyticity of correlation functions via the Hadamard three-line theorem. We show that the dynamical localization for the one-particle system yields the dynamical localization for the many-point fermionic correlation functions, with respect to the Hausdorff distance in the determinantal case. In Sims and Warzel (2016), a stronger notion of decay for many-particle configurations was used but only at dimension one and for real times. Considering determinantal and Pfaffian correlation functionals for complex times is important in the study of weakly interacting fermions.
Wed, 24 Jan 2018 00:00:00 GMThttp://hdl.handle.net/20.500.11824/9282018-01-24T00:00:00ZThe Discreteness-driven Relaxation of Collisionless Gravitating Systems: Entropy Evolution in External Potentials, N-dependence, and the Role of Chaos
http://hdl.handle.net/20.500.11824/917
The Discreteness-driven Relaxation of Collisionless Gravitating Systems: Entropy Evolution in External Potentials, N-dependence, and the Role of Chaos
Beraldo e Silva L.; de Siqueira Pedra W.; Valluri M.; Sodré L.; Bru J.-B.
We investigate the old problem of the fast relaxation of collisionless N-body systems that are collapsing or perturbed,
emphasizing the importance of (noncollisional) discreteness effects. We integrate orbit ensembles in fixed potentials,
estimating the entropy to analyze the time evolution of the distribution function. These estimates capture the correct
physical behavior expected from the second law of thermodynamics, without any spurious entropy production. For
self-consistent (i.e., stationary) samples, the entropy is conserved, while for non-self-consistent samples, it increases
within a few dynamical times, stabilizing at a maximum (even in integrable potentials). Our results make transparent
that the main ingredient for this fast collisionless relaxation is the discreteness (finite N) of gravitational systems in
any potential. Additionally, in nonintegrable potentials, the presence of chaotic orbits accelerates the entropy
production. Contrary to the traditional violent relaxation scenario, our results indicate that a time-dependent potential
is not necessary for this relaxation. For the first time, in connection with the Nyquist–Shannon theorem, we derive the
typical timescale T tcr » 0.1N 1 6 for this discreteness-driven relaxation, with slightly weaker N-dependencies for
nonintegrable potentials with substantial fractions of chaotic orbits. This timescale is much smaller than the
collisional relaxation time even for small-N systems such as open clusters and represents an upper limit for the
relaxation time of real N-body collisionless systems. Additionally, our results reinforce the conclusion of Beraldo e
Silva et al. that the Vlasov equation does not provide an adequate kinetic description of the fast relaxation of
collapsing collisionless N-body systems.
Thu, 10 Jan 2019 00:00:00 GMThttp://hdl.handle.net/20.500.11824/9172019-01-10T00:00:00Z