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Observing a Global Delocalization Transition in the de Moura-Lyra Model
(CFP room (326), from 2019-05-31 14:30 to 2019-05-31 15:30)
The possibility of having a delocalisation transition in the 1D de Moura-Lyra class of models (having a power-spectrum $q^{-\alpha}$) has been the object of a long standing discussion in the literature. In this seminar, we report the first numerical evidences that such a transition happens at $\alpha=1$, where the localisation length (measured from the scaling of the conductance) is shown to diverge as $(1-\alpha)^{-1}$. The persistent finite-size scaling of the data is shown to be caused by a very slow convergence of the nearest-neighbor correlator to its infinite-size limit, and controlled by the choice of a proper scaling parameter. Our results for these models are consistent with a localisation of eigenstates that is driven by a persistent small-scale noise, which vanishes as $\alpha\to1^{-}$. This interpretation in confirmed by analytical perturbative calculations which are built on previous work. Finally, the nature of the delocalisation transition is discussed and the conclusions are illustrated by numerical work done in the $\alpha>1$ regime.
Colliders and conformal interfaces
(CFP room (326), from 2019-05-17 14:00 to 2019-05-17 15:00)
We set up a scattering experiment of matter against an impurity which separates two generic one-dimensional critical quantum systems. We compute the flux of reflected and transmitted energy, thus defining a precise measure of the transparency of the interface between the related two-dimensional conformal field theories. If the largest symmetry algebra is Virasoro, we find that the reflection and transmission coefficients are independent of the details of the initial state, and are fixed in terms of the central charges and of the two-point function of the displacement operator. The situation is more elaborate when extended symmetries are present. Positivity of the total energy flux at infinity imposes bounds on the coefficient of the two-point function of the displacement operator, which controls the free-energy cost of a small deformation of the interface.
Holographic modeling of neutron stars
(CFP room (326), from 2019-05-15 14:00 to 2019-05-15 15:00)
Gauge/gravity duality can be used to study QCD at high densities and low temperatures where many other theoretical tools do not work. In particular one may constrain the equation of state of QCD matter in neutron star cores, which currently has large theoretical uncertainties. I will review recent progress on this topic. Bottom-up models which are carefully fitted to available data suggest that the equation of state is stiff (i.e., the speed of sound is high) in the nuclear matter phase and soft (i.e, the speed of sound is low) in the quark matter phase. The two phases are separated by a strong first order phase transition, which makes quark matter cores of neutron stars unstable. This picture agrees with current experimental constraints from LIGO/Virgo and from Shapiro delay measurements. This Journal Club was supported by the project UID/FIS/04650.
Positive helicity Einstein-Yang-Mills Amplitudes from the Double Copy
(CFP room (326), from 2019-05-14 14:00 to 2019-05-14 15:00)
All positive helicity four-point gluon-graviton amplitudes in Einstein-Yang-Mills theory coupled to a dilaton and axion field are computed at the leading one-loop order using the double copy method. Using the double copy method the gravity integrand is obtained from integrands of a gauge theory. The resulting purely rational expressions take very compact forms. The previously seen vanishing of the single-graviton-three-gluon amplitude at leading order in $\kappa$ is seen to be lifted at order $\kappa^{3}$.
An introduction to (improved) holographic QCD
(CFP room (326), from 2019-05-13 14:00 to 2019-05-13 15:00)
I start with a generic review of gauge/gravity duality and discuss how it can be used to obtain information for QCD in the strongly coupled regime. I give a brief discussion of various top-down and bottom-up approaches. I go on presenting in detail a specific class of models, that is improved holographic QCD (IHQCD) and V-QCD. IHQCD and V-QCD are bottom-up models for Yang-Mills theory in the 't Hooft limit and for QCD in the Veneziano limit, respectively. I demonstrate that these models compare well with available data for QCD. This Journal Club is supported by the project UID/FIS/04650.
Scattering States in AdS/CFT
(CFP Room (326 DFA), from 2019-03-22 13:30 to 2019-03-22 14:30)
We show that suitably regulated multi-trace primary states in large N CFTs behave like `in' and `out' scattering states in the flat-space limit of AdS. Their transition matrix elements approach the exact scattering amplitudes for the bulk theory, providing a natural CFT definition of the flat space S-Matrix. We study corrections resulting from the AdS curvature and particle propagation far from the center of AdS, and show that AdS simply provides an IR regulator that disappears in the flat space limit.
Entanglement and formation of Black Holes
(CFP room (326), from 2019-02-27 14:00 to 2019-02-27 15:00)
 
"Holography, higher spin and gravity"
(CFP Room (326), from 2019-02-25 13:30 to 2019-02-25 14:30)
 
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