Solving fermionic many-body problems by summing Feynman diagrams
le vendredi 31 mars 2017 à 11h00

Séminaire théorie

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : It is commonly believed that in quantum Monte Carlo (QMC) approaches to fermionic many-body problems, the infamous sign problem generically implies prohibitively large computational times in the thermodynamic limit. I will point out that for convergent (or subject to resummation)
Feynman diagrammatic series evaluated with the Monte Carlo algorithm of [Rossi, arXiv:1612.05184], the computational time increases only polynomially with the inverse error on thermodynamic-limit quantities. I will discuss the computational complexity problem for different QMC approaches: conventional techniques (auxiliary-field, path-integral and diffusion QMC) and diagrammatic Monte Carlo approaches. I will also report on recent progress of Diagrammatic Monte Carlo simulation of the resonant Fermi gas and the homogeneous electron gas.

Energy transport and topological aspects of collective plasmons in chains of metallic nanoparticles
le jeudi 6 avril 2017 à 13h30

Séminaire interne LPMMC

Personne à contacter : Vincent Rossetto ()

Lieu : Salle de lecture 2, maison des Magistères

Résumé : One of the primary goals of plasmonics is to confine light at subwavelength scales. This aim is motivated by the desire to both transport and manipulate light over macroscopic distances. While metallic nanostructures have been proposed and widely studied to achieve such "plasmonic circuits", both radiative and nonradiative losses inherent to metals are rather significant. Hence, the possible applications for energy and information transport at the nanoscale are seemingly limited. Understanding the different damping mechanisms in radiatively-coupled metallic nanostructures is thus of paramount importance to the field of plasmonics, from both a fundamental point of view and in order to increase the efficiency of signal transmission.
We investigate the collective plasmonic modes in chains of spherical metallic nanoparticles that are coupled by near-field interactions. These dipolar interactions between the nanoparticles gives rise to collective plasmons, which are extended over the whole plasmonic lattice. We study both a simple chain composed of regularly-spaced nanoparticles, which displays phenomena fundamental to all one-dimensional nanoparticle arrays, and a bipartite chain, which exhibits nontrivial topological features.
We obtain the size- and momentum-dependent nonradiative Landau damping and radiative decay rates, which determine the excitation propagation along the regular chain [1]. We find that the behavior of the radiative decay rate as a function of the plasmon wavelength leads to a transition from an exponential decay of the collective excitation for short distances to an algebraic decay for large distances. Importantly, we show that the exponential decay is of a purely nonradiative origin. These findings constitutes an important step in the quest for the optimal conditions for plasmonic propagation in nanoparticle chains.
We also study a bipartite chain constituted by metallic nanoparticle dimers [2]. We find an effective Dirac Hamiltonian describing the collective plasmons, and show that the corresponding spinor eigenstates represent Dirac-like massive bosonic excitations. We show that the system is governed by a topologically nontrivial Zak phase, which predicts the manifestation of edge states in the chain. When two bipartite chains with different topological phases are connected, we find the appearance of a bosonic version of a Jackiw-Rebbi midgap state. We investigate losses of the collective plasmonic excitations in the bipartite chain, and comment on the challenges for experimental realization of the topological effects found theoretically.

[1] A. Brandstetter-Kunc, G. Weick, C. A. Downing, D. Weinmann, R. A. Jalabert, Nonradiative limitations to plasmon propagation in chains of metallic nanoparticles, Phys. Rev. B94, 205432 (2016)

[2] C. A. Downing and G. Weick, Topological collective plasmons in bipartite chains of metallic nanoparticles, Phys. Rev. B95, 125426 (2017)

Gergely Zarand (Budapest University of Technology and Economics)

Designed Quantum Criticality
le vendredi 7 avril 2017 à 11h00

Colloque CPTGA

Personne à contacter :

Lieu : Amphithéâtre, maison des Magistères

Résumé : Recent years witnessed an astonishing increase of control over quantum systems. Today it is possible to design and construct quantum systems with unprecedented control, realize novel quantum states, and study their dynamical and out of equilibrium behavior using nanotechnological and optical tools. In my talk, I will demonstrate the power of this approach on a few specific examples of artificially designed quantum-critical systems, mostly from the nanophysics perspective. I will review how one can gain insight to certain paradigmatic quantum phase transitions by combining artificial atoms with resistive elements, carbon nanotubes, or topological edge states, and how a combination of detailed field theoretical modeling and accurate experiments may lead to a complete characterization. I will also discuss future perspectives such as, e.g., interfacing nanotechnological circuits with cold atomic or cavity QED elements.
References:
* R.M. Potok, I.G. Rau, H. Shtrikman, Y. Oreg, David Goldhaber-Gordon, Nature 446, 167-171 (2007)
* G. Zaránd, C.H. Chung, P. Simon, M. Volta, Physical Review Letters 97, 166802 (2006)
* H.T. Mebrahtu, I.V. Borzenets, H. Zheng, Y.V. Bomze, A.I. Smirnov, S. Florens, H.U. Baranger, and G. Finkelstein, Nature Physics 9, 732 (2013)
* J. Simon, W.S. Bakr, R. Ma, M.E. Tai, P.M. Preiss, and M. Greiner, Nature 472, 307–312 (2011)
* B. Dóra, F. Pollmann, J. Fortágh, G. Zaránd, Physical Review Letters 111, 046402 (2013)
* Xibo Zhang, Chen-Lung Hung, Shih-Kuang Tung, Cheng Chin, Science 335, 1070 (2012)
* A. J. Keller, L. Peeters, C. P. Moca, I. Weymann, D. Mahalu, V. Umansky, G. Zaránd, David Goldhaber-Gordon, Nature 526, 237 (2015)