Seminars

Seminars in Condensed Matter Theory

Organisers: Charles Creffield and Fernando Sols

This is a series of seminars dealing with topics related to condensed matter physics. We interpret condensed matter quite broadly, including mesoscopic physics, atomic and molecular physics as well as more traditional topics. Our standard seminar time is Wednesday at 12 o' clock, and they are held in the seminar room of the Departamento de Fisica de Materiales in the Universidad Complutense (sala 124, on the second floor)

Academic year 2018 - 2019

20 February Diego Porras, Institute of Fundamental Physics, CSIC

"Topological Amplification in Photonic Lattices"

Driven-dissipative lattices are quantum models where dissipation and/or decoherence are added to the unitary quantum dynamics of tight-binding models. Those models are implemented, for example, in photonic setups such as superconducting circuits or coupled photonic cavities. The same theoretical paradigm can be used to describe vibronic lattices in trapped ions or nano-mechanical systems. The presence of dissipation and gain/loss mechanisms make the description of driven-dissipative lattices very different form their unitary counterparts. For example, it is a priori not trivial at all how to extend the theory of topological phases and topological insulators to this dissipative scenario. In my talk I will introduce a theoretical formalism that allows us to classify topological phases of driven-dissipative lattices by a formal mapping between dissipative lattices and effective chiral Hamiltonians. Our theory reveals the existence of topologically non-trivial dissipative phases in which photonic lattices act as directional amplifiers. This surprising connection will allow us to use Topological Band Theory to predict the performance of quantum amplifiers and sensors based on the symmetries of the underlying photonic lattice.

D. Porras & Samuel Fernández-Lorenzo, arXiv:1812.01348

13 February Toni Ramsak, University of Ljubljana

"Stability analysis of non-adiabatic qubit manipulations"

A promising method of qubit malipulation in quantum information processing applications is the manipulation where the Rashba effect in non-adiabatic systems induces quantum phases, including the spin rotation. By the virtue of exact unitary transformations [1] we recently proved that the ratio of the non-adiabatic Anandan phase and the adiabatic Wilczek-Zee counterpart can be tuned to any real number [2].

Stability properties of qubit transformations and the corresponding fidelity can also be studied exactly and as an example we will present results for spin-orbit dynamics influenced by the Ornstein-Uhlenbeck coloured noise of driving fields [3]. We will demonstrate also how these non-adiabatic systems can be coupled to thermal baths. In particular, by the known unitary transformation [1] the system can be expressed in the Floquet basis which enables an exact derivation of dissipators and the Lindblad equation. Some typical solutions of the corresponding Lindblad equation will be presented [4].

[1] T. Cadez, J. H. Jefferson, and A. Ramsak, Phys. Rev. Lett. 112, 150402 (2014).

[2] A. Ramsak, T. Cadez, A. Kregar, and L. Ulcakar, Eur. Phys. J. ST 227, 353 (2018).

[3] L. Ulcakar and A. Ramsak, New J. Phys. 19, 093015 (2017); L. Ulcakar and A. Ramsak, Int. J. Mod. Phys. B 32, 1840028 (2018).}

[4] B. Donvil, L. Ulcakar, T. Rejec, and A. Ramsak, in preparation.

16 January Juan Carlos Cuevas, Universidad Autonoma de Madrid

"Super-Planckian radiative heat transfer"

Understanding heat exchange via thermal radiation is key for many areas of science and engineering [1]. Our knowledge about the thermal radiation is still largely based on Planck’s law for black bodies. In particular, Planck’s law establishes an upper limit for the thermal energy that can be transferred between two objects via radiation. However, this fundamental law of physics has known limitations and, in principle, it is only valid when all the dimensions involved in the problem are larger than the so-called thermal wavelength (l_Th), which is around 10 microns at room temperature. In this talk, I will present an overview of our efforts devoted to explore the limits of Planck’s law in two situations in which it is no longer valid. First, I will discuss the radiative heat transfer between two objects in cases in which they are separated by a distance smaller than l_Th and the thermal exchange is dominated by evanescent waves [2-4]. Then, I will discuss the radiative heat transfer between objects with some of their dimensions being smaller than l_Th. In particular, I will show that in this case it is possible to overcome the blackbody limit by orders of magnitude even in the far-field regime [5], i.e., when they are separated by macroscopic distances. I will illustrate this phenomenon in the case of micron-sized dielectric devices [5,6] and 2D materials such as graphene [7].

[1] J.C. Cuevas and F.J. García-Vidal, Radiative Heat Transfer, ACS Photonics 5, 3896 (2018).

[2] B. Song, Y. Ganjeh, S. Sadat, D. Thompson, A. Fiorino, V. Fernández-Hurtado, J. Feist, F.J. Garcia-Vidal, J.C. Cuevas, P. Reddy, E. Meyhofer, Nature Nanotechnol. 10, 253 (2015).

[3] K. Kim, B. Song, V. Fernández-Hurtado, W. Lee, W. Jeong, L. Cui, D. Thompson, J. Feist, M.T.H. Reid, F.J. García-Vidal, J.C. Cuevas, E. Meyhofer, P. Reddy, Nature 528, 387 (2015).

[4] L. Cui, W. Jeong, V. Fernández-Hurtado, J. Feist, F.J. García-Vidal, J.C. Cuevas, E. Meyhofer, P. Reddy, Nature Commun. 8, 14479 (2017).

[5] V. Fernández-Hurtado, A.I. Fernández-Domínguez, J. Feist, F.J. García-Vidal, J.C. Cuevas, Phys. Rev. B 97, 045408 (2018).

[6] D. Thompson et al., Nature 561, 216 (2018).

[7] V. Fernández-Hurtado, A.I. Fernández-Domínguez, J. Feist, F.J. García-Vidal, J.C. Cuevas, ACS Photonics 5, 3082 (2018).

5 December Miguel A. F. Sanjuán, Universidad Rey Juan Carlos

"Nonlinear Dynamics, Chaos and Complex Systems: A historical perspective"

When we talk about dynamics, we do not only understand the motion of celestial bodies and solid mechanical systems, but any changes with respect to time of one or more variables. From that point of view, we can find dynamics everywhere, in any field of science. Thus, now we have a more general vision, including stock market movements and economic variables, concentration changes in chemical reactions, changes in physiological, biological and medical variables, action potentials of neurons, etc … providing a more interdisciplinary perspective.

The various interactions between the constituent parts of a physical system and their feedback mechanisms, are a source of nonlinearity and complexity, which added to the sensitivity dependence to initial conditions which is a hallmark of chaotic behavior, constitutes a change of perspective in dynamical systems with important consequences for the understanding of science.

I will give a historical perspective of Nonlinear Dynamics, Chaos Theory and Complex Systems, including some of the different sources that have contributed to the construction of the discipline as we know it today. Among them, the three-body problem in celestial mechanics, turbulence in fluid dynamics, irreversibility and fundamentals of statistical physics and the logistic map and population dynamics in biology. Many schools of mathematics and physics have played an essential role in the historical development of the subject, including the French, Russian, Japanese and American school. The knowledge of this historical perspective allows us to understand the breadth of the discipline itself and the multiple interdisciplinary applications to various fields of science.

Academic year 2017 - 2018

4 December David Pérez García, Universidad Complutense

"Undecidability in physics... and its consequences"

The pioneering work of Goedel and Turing in the 30s showed that there exist problems in mathematics and computer science that cannot be solved. They are called undecidable. Since then, several problems in physics have been shown to be undecidable too. In this talk I will show that many interesting properties of a quantum many body system are indeed undecidable. This negative result has, however, a positive side. It predicts the existence of a new effect that we name "size-driven quantum phase transition”. I will present this effect, its characteristic features, as well as our recent ideas to try to observe it.

The results presented in the talk have been done in collaboration with J. Bausch, T. Cubitt, B. Doucot, S. Iblisdir, A. Lucia and M.M. Wolf.

17 November Jesús Pérez Ríos, School of Natural Sciences and Technology, Universidad del Turabo, USA

"Ultracold Rydberg atoms in high density media: chemistry and many-body physics"

A single Rydberg atom in a Bose-Einstein condensate (BEC), where thousands of neutral atoms are within the Rydberg orbit, experiences a fast decay depending on its principal quantum number n, in comparison with the natural decay rate of Rydberg atom. The physics behind this phenomenon has remained unexplored until the present work, where it is shown that the decay mechanism of a Rydberg atom in a density medium is due to both reactive and non-reactive ultracold chemical processes: l-mixing collisions and chemi-ionization. These chemical reactions have been studied by means of a new theoretical framework including explicitly the role of the Rydberg electron on the dynamics of the Rydberg-neutral energy landscape, as well as the short-range Rydberg core-neutral potential energy curves coming from quantum chemistry calculations. On the other hand, several Rydberg atoms in a BEC may form ultra-long Rydberg molecules through light-assisted chemical reactions, where a photon provides the necessary energy for binding the Rydberg atom to a neutral one, leading to the formation of ultralong Rydberg molecules. The production and control of these homonuclear molecules with giant dipole moments may be used to design novel many-body Hamiltonians as well as to explore intriguing anisotropic blockade effects leading to novel quantum phase transitions.

Previous years: 2017-2016, 2015-2014, 2014-2013, 2013-2012, 2012-2011, 2011-2010, 2010-2009, 2009-2008

If you have any further suggestions for speakers, please email me at: charles.creffield AT gmail.com