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A tale of three models - How annealing can give rise to localization, liquids phases, and topological order.
(CFP Room (326), from 2019-01-30 14:00 to 2019-01-30 15:00)
Annealed, as opposed to quenched, degrees of freedom are allowed to choose their equilibrium state rather than remaining frozen in a predetermined configuration. Models of annealed classical degrees of freedom in contact with quantum mechanical variables can emerge in the presence of quasi-conserved quantities or as effective descriptions of collective excitations, valid away from the zero temperature limit. In contrast to their classical counterparts, these models can be efficiently simulated by classical Monte Carlo algorithms. The Falicov-Kibble (FK) model is the simplest of this kind. It has been widely studied and used as testing grounds to dynamical mean field theory methods. In this talk, I will first show that the phase diagram of the FK model still held some surprises, including an example of a disordered-free localized phase. Second, I will show that introducing frustration, by going to the triangular lattice, an FK-like model can support rather exotic liquid phases. Thirdly, I will show how FK interactions affect the topological properties of the Haldane model. References: [1] Interaction-tuned Anderson versus Mott localization A. E. Antipov, Y. Javanmard, P. Ribeiro, S. Kirchner Phys. Rev. Lett. 117, 146601 (2016) [2] Classical and quantum liquids induced by quantum fluctuations. M. M. Oliveira, P. Ribeiro, S. Kirchner. arXiv:1810.10582 (2018) [3] Temperature-driven gapless topological insulator. M. Gonçalves, P. Ribeiro, R. Mondaini, E. V. Castro. arXiv:1808.00978 (2018)
Many-body localization and Thermalization in isolated quantum systems
(, from 2019-01-14 22:32 to 2019-01-14 22:32)
The fundamental question of how an isolated interacting quantum system, subjected to only unitary time-evolution, loses information about its initial preparations has been the focus of a variety of studies [1, 2]. More recently, however, another angle of this problem has also been investigated: When they are influenced by quenched disorder, information of the initial conditions can be preserved for arbitrarily long times, whose potential application to quantum memories is immediate. This phenomenon is dubbed many-body localization and can be seen as the generalization of the fundamental problem of the Anderson localization when its constituents are interacting. In this talk, I will present an overall picture of this interplay of the many-body localization [3, 4] and thermalization [5, 6], describing the conditions they are manifest. Importantly, it has been the focus of not only numerical studies but also of experimental ones, via the emulation in optical lattices trapping cold atoms [7, 8]. I will also glance on some recent generalizations investigated by our group showing that many-body localization may also be manifest in systems that are translationally invariant, i.e., even in the absence of quenched disorder [9], and the possible investigation of many-body mobility edges. [10] [1] M. Srednicki, Phys. Rev. E 50, 888 (1994) [2] M. Rigol, V. Dunjko, and M. Olshanii, Nature 452, 854-858 (2008) [3] R. Mondaini, M. Rigol, Phys. Rev. A 92, 041601(R) (2015) [4] C. Cheng, R Mondaini, Phys. Rev. A 94 (5), 053610 (2016) [5] R. Mondaini, K. R. Fratus, M. Srednicki, M. Rigol, Phys. Rev. E 93 (3), 032104 (2016) [6] R. Mondaini and M. Rigol, Phys. Rev. E 96, 012157 (2017) [7] M. Schreiber, S. S. Hodgman, P. Bordia, H. P. Luschen, M. H. Fischer, R. Vosk, E. Altman, U. Schneider, I. Bloch, Science 349, 842 (2015) [8] J.-Y. Choi, S. Hild, J. Zeiher, P. Schauß, A. Rubio-Abadal, T. Yefsah, V.Khemani, D. A. Huse, I. Bloch, and C. Gross, Science 352, 1547 (2015) [9] R. Mondaini and Z. Cai, Phys. Rev. B 96, 035153 (2017) [10] Xing Bo Wei, Chen Cheng, Gao Xianlong, Rubem Mondaini, arXiv:1810.08209
On the energy transport in a non-integrable Ising chain
(CFP Room (326), from 2018-11-07 13:00 to 2018-11-07 14:00)
Typicality arguments have been first proposed as a well-defined framework to study quantum thermodynamics, from a more fundamental point of view. The key idea of typicality is to describe thermal ensembles from a pure state formalism. The applicability of typicality extends beyond equilibrium quantum physics and possibly to other areas of physics. I will present a study of the energy transport in a non-integrable spin-1/2 Ising chain using typicality arguments, which amounts to characterizing the quench dynamics after a local perturbation. W​​​e obtained numerical results regarding transport coefficients both at finite and infinite temperature, which question the understanding in non-integrable models. Second, I will introduce Matrix Product States (MPS) and explain the connection with typicality arguments. We obtained several results regarding typicality of MPS and showed their limitations to study quench dynamics.
Can Teleparallel gravity (TEGR) really be the Translations gauge theory?
(CFP Room (326), from 2018-10-17 13:30 to 2018-10-17 14:30)
I will report on my work reexamining the translation gauge aspect of Teleparallel Equivalent to GR. TEGR is presented as the gravity theory equivalent to GR that is also the gauge theory of translations. We addressed that claim and discovered that it is not tenable given that the principal fiber bundle of translation cannot be built in general and that the need for general frames entails the use of the Lorentz group. We found the solution in a Weitzenbock-Cartan-Poincaré construction that is not yet clearly a gauge theory. This talk is funded by Fundação para a Ciência e a Tecnologia (FCT) through the project UID/FIS/04650/2013.
Planar four-point functions of protected operators in N=4 SYM
(CFP Room (326), from 2018-10-15 13:30 to 2018-10-15 14:30)
We study four-point functions of protected operators in planar N=4 SYM up to the five-loop order. We use a light-cone OPE analysis to constrain the integrands and then fix the integrated correlators with input from integrability. The OPE data we extract allows us to determine the triple wrapping correction in the Hexagon approach to three-point functions, which contributes earlier than expected.
Dark side of the seesaw
(Room 326 (CFP room), from 2018-10-03 14:30 to 2018-10-03 15:30)
I present a model where there is a connection between two apparently uncorrelated sectors, namely neutrino and dark matter. In the model, a scalar field acts as mediator between these two sectors, its vacuum expectation value generating the mass of the dark matter and also taking part in generating neutrino mass. A Z4 symmetry is used, broken by the scalar field vacuum to a remnant Z2 responsible for dark matter stability. The observed light neutrino masses and relic abundance constraint on the dark matter combined lead to predictions of the heavy seesaw scale. This framework to connect dark matter and neutrino sector introduced here is a generic one and can be applied to other possible neutrino mass generation mechanism and different dark matter candidate(s). This talk is funded by Fundação para a Ciência e a Tecnologia (FCT) through the project UID/FIS/04650/2013.
Berry phases and topological physics in one-dimensional systems
(CFP Room - 326, from 2018-07-26 14:00 to 2018-07-26 15:00)
Topological systems are one of the most active research areas in condensed matter physics. The topological characterization of a condensed matter system relies on mathematical constructs such as the Berry phase, the winding number, or the Chern number. In the first part of the talk, I will explain how the Berry phase can be understood as the first cumulant of a series. We calculate higher order cumulants and reconstruct the underlying distribution of the polarization for the Rice-Mele model. Our approach allows the visualization of a topological transition, how a system goes between phases with different quantization. In the second part of the talk, I will go through constructing one-dimensional analogs of the Haldane and Kane-Mele models. In the former, the overall winding number does not indicate topological behavior, but the model falls into two independent Creutz models with opposite windings, and the topological transition occurs within each one separately. The latter ladder model also consists of two Creutz models, one for each spin-channel and falls in the CII symmetry class. In its analysis, the thermodynamic derivation of the generalization of the Streda-Widom formula to the quantum spin Hall effect turned out to be useful.
Extreme Holography
(CFP Room - 326, from 2018-07-25 14:00 to 2018-07-25 15:00)
A massive experimental effort will be devoted in the coming years to the physics of QCD at high energy density and/or high baryon density. Understanding this physics, especially out of equilibrium, is an important theoretical challenge. I will discuss how holography can help us address this challenge. Topics covered will include the far-from-equilibrium dynamics near the QCD critical point and colour superconducting phases. No prior knowledge of string theory required.
Nonlinear conductivities of crystals in the independent electron approximation: Results from a velocity gauge analysis
(CFP Room - 326, from 2018-07-11 13:30 to 2018-07-11 14:30)
Some formal developments concerning the calculation of nonlinear conductivities of crystals are presented. Even under the simplification of the long wavelength limit and the independent electron approximation, there are subtleties in this calculations, namely on how to deal with particular choices of gauge that define the perturbation. Here, the difficulties inherent to perturbative calculations in the velocity gauge are addressed. In particular, it is shown how, contrary to the common belief in the literature, calculations can be done to any order and for any finite band model in this gauge. The procedure and advantages of the velocity gauge in such calculations are described.
The Analytic Functional Bootstrap I: 1D CFTs and 2D S-Matrices
(CFP Room - 326, from 2018-06-29 14:00 to 2018-06-29 15:00)
We study a general class of functionals providing an analytic handle on the conformal bootstrap equations in one dimension. We explicitly identify the extremal functionals, corresponding to theories saturating conformal bootstrap bounds, in two regimes. The first corresponds to functionals that annihilate the generalized free fermion spectrum. In this case, we analytically find both OPE and gap maximization functionals proving the extremality of the generalized free fermion solution to crossing. Secondly, we consider a scaling limit where all conformal dimensions become large, equivalent to the large AdS radius limit of gapped theories in AdS2. In this regime we demonstrate analytically that optimal bounds on OPE coefficients lead to extremal solutions to crossing arising from integrable field theories placed in large AdS2