Seminars and Events

Seminars and Events

Theory and Simulation of Materials

Coordinators: Álvaro Gómez León, Tobías Stauber

28 February 2019, 12:00 h. Sala de Seminarios, 182

Quantum dissipation in the context of the Maxwell-Chern-Simons theory

Alejandro Valido
Instituto de Física Fundamental (CSIC)

In the last decade the quantum physics of two-dimensional systems have attracted a renewed interest, for which the Maxwell-Chern-Simons electrodynamics has provided useful insight about a great diversity of phenomena connected to condensed matter systems or quantum information theory (for instance, the statistics transmutation). The present talk is devoted to examine the Maxwell-Chem-Simons theory from the perspective of the quantum open-system theory. Concretely, we shall address the dissipative dynamics of the Brownian motion of planar harmonic oscillators minimally interacting with a Maxwell-Chern-Simons electromagnetic field acting as a heat bath. Unlike conventional Brownian motion, the Chern-Simons action endows the system particles with a magneticlike flux giving rise to an ordinary Hall response and flux noise. In particular, we shall show that the Chern-Simons dissipative effects represent a second-order correction to the well-known damped harmonic oscillator in the Markovian limit.

21 February 2019, 12:00 h. Sala de Seminarios, 182

Are colourful photons introvert or extrovert?

Elena del Valle
Universidad Autónoma de Madrid

Measuring and understanding correlations between photons, emitted by a given quantum system, with simultaneous resolution in both frequency and time, is not a trivial task, since these are Heisenberg-conjugate variables. I will present, from the theory point of view, how to obtain such correlations and clarify all their enigmas so they can be interpreted in a meaningful and useful way, unveiling hidden relationships between the emitted photons.

14 February 2019, 12:00 h. Sala de Seminarios, 182

Ultrastrong coupling with propagating photons

Juan Jose García Ripoll
Instituto de Fisica Fundamental (CSIC)

In the ultrastrong coupling regime, light and matter interact at a rate that approaches the speed with which the matter and light evolve. In this challenging regime, light entangles and dresses the states of matter, complicating a traditional quantum optics description based on the Jaynes-Cummings model, rotating-wave approximation and similar techniques. In this talk I will summarize our work on the USC regime with propagating light, introducing the underlying spin-boson model, the numerical and analytical methods we use [1] and a experimental collaboration to reproduce this regime with superconducting circuits [2].

[1] Shi, T., Chang, Y. and García-Ripoll, J.J., 2018. Ultrastrong coupling few-photon scattering theory. Physical review letters, 120(15), p.153602.

[2] Forn-Díaz, P., García-Ripoll, J.J., Peropadre, B., Orgiazzi, J.L., Yurtalan, M.A., Belyansky, R., Wilson, C.M. and Lupascu, A., 2017. Ultrastrong coupling of a single artificial atom to an electromagnetic continuum in the nonperturbative regime. Nature Physics, 13(1), p.39.

07 February 2019, 12:00 h. Sala de Seminarios, 182

An electron-phonon mechanism for electron pairing and high-Tc superconductivity?

John Sous
University of British Columbia

It is widely accepted that strong particle-phonon coupling generally makes the resulting quasiparticles (polarons and bipolarons) heavy, with masses that increase with the coupling strength. This is the characteristic behaviour in the Holstein and Fr{"o}hlich models. Here, I study the one-dimensional Peierls/Su-Schrieffer-Heeger(SSH) model necessary for describing coupling of hopping to breathing-mode distortions in certain oxides, polyenes and other quantum systems. I show that the Peierls interactions bind electron pairs. The paired quasiparticles, known as bipolarons, are much lighter than the Holstein and Fr{"ohlich} counterparts and are much more stable to strong Coulomb repulsion. I explain these effects as a result of an unusual phonon-induced kinetic interaction (not density-density interaction) that forces electron pairs to move coherently together. These light pairs could undergo Bose-Einstein condensation at high temperatures, opening a door to phonon-mediated high-Tc superconductivity.

31 January 2019, 12:00 h. Sala de Seminarios, 182

Relativistic motion of an Airy wavepacket in a lattice - quantum mechanics at high speed

Charles Creffield
Dpto de Física de Materiales, Universidad Complutense de Madrid

What happens when particles move at high speeds, comparable to the speed of light? Classically the result is well-known; Newtonian mechanics evolves into special relativity. We can also ask the same question for a quantum mechanical system - will a quantum wavepacket pass into the relativistic regime as its speed increases?

The Airy wavepacket is a particular solution of the Schrödinger equation that appears to undergo a constant acceleration. It should thus eventually become relativistic when its velocity becomes similar to the speed of light. We can study this conveniently by confining it to move in a lattice instead of free space. The lattice provides a natural "speed limit" given by its maximum group velocity, which can be many orders of magnitude lower than the true speed of light. In this talk I will show that an Airy wavepacket moving in a lattice is indeed described by relativistic equations, which, rather unexpectedly, arise from evolution under the standard non-relativistic Schrödinger equation [1].

A natural system to study this effect is in gases of cold atoms held in optical lattice potentials. I will show how these are thus excellent candidates for studying quantum systems in extreme relativistic conditions in the laboratory, and how Floquet engineering techniques can be used to control their properties.

1. C.E. Creffield, Phys. Rev. A 98, 063609 (2018).

24 January 2019, 12:00 h. Sala de Seminarios, 182

Topological Wilson-Hubbard matter: a bridge between condensed matter and high-energy physics

Alejandro Bermúdez
Dpto de Física Teórica, Universidad Complutense de Madrid

In this talk, I will describe our recent efforts to understand correlation effects in fermionic symmetry-protected topological (SPT) phases of matter by exploring a family of Hubbard ladders that can be understood as discretized versions of the Gross-Neveu model in relativistic QFTs. I will discuss how the combination of condensed-matter tools (i.e. quantum impurity models and DMRG) and high-energy physics techniques (large-N and Wilsonian RG) offer a neat understanding of strongly-correlated SPT phases and possible quantum phase transitions to other Landau-ordered phases. Finally, I will discuss a possible scheme for a cold-atom implementation of such Hubbard ladders.

17 January 2019, 12:00 h. Sala de Seminarios, 182

Magnetocrystalline anisotropy: from the isolated ion to the bulk

Maria Blanco-Rey
Departamento de Fisica de Materiales UPV/EHU and Donostia International Physics

The magnetic anisotropy determines the magnetization of a system at different orientations of the applied filed. This process is often dominated by the so-called magnetocrystalline contribution, which originates from the spin-orbit interaction (SOI). I will review some theoretical aspects of the magnetocrystalline anisotropy (MCA). First, I will talk about effective models for single-ions (spin Hamiltonians and multiplets). In the opposite limit of extended systems, the SOI effects on the bandstructure drive the MCA. I will show that, under suitable assumptions, a second-order perturbative treatment of the SOI can efficiently account for the MCA behaviour. As illustrative examples, we will see results on rare-earth alloys, metal-organic coordination networks, and Fe-based alloys.

15 January 2019, 12:00 h. Sala de Seminarios, 182

Quantum transport properties of an exciton insulator/superconductor hybrid junction

Dario Bercioux

We present a theoretical study of electronic transport in a hybrid junction consisting of an excitonic insulator sandwiched between a normal and a superconducting electrode. The normal region is described as a two-band semimetal and the superconducting lead as a two-band superconductor. In the excitonic insulator region, the coupling between carriers in the two bands leads to an excitonic condensate and a gap Gamma in the quasiparticle spectrum. We identify four different scattering processes at both interfaces. Two types of normal reflection, intra- and inter-band [1]; and two different Andreev reflections, one retro-reflective within the same band and one specular-reflective between the two bands [2,3]. We calculate the differential conductance of the structure and show the existence of a minimum at voltages of the order of the excitonic gap. Our findings are useful towards the detection of the excitonic condensate and provide a plausible explanation of recent transport experiments on HgTe quantum wells and InAs/GaSb bilayer systems [3-5].

[1] M. Rontani and L. J. Sham, Phys. Rev. Lett. 94 186404 (2005).
[2] D.B., T. M. Klapwijk and F. S. Bergeret, Phys. Rev. Lett. 119 067001 (2017).
[3] D.B, B. Bujnowski and F. S. Bergeret, arXiv:1806.03991, Advanced Quantum Technology, in press (2019).
[3] A. Kononov et al., Phys. Rev. B 93 041303 (2016).
[4] A. Kononov et al., Phys. Rev. B 96, 245304 (2017).
[5] Yu et al., New J. Phys. 20 053063(2018).

10 January 2019, 12:00 h. Sala de Seminarios, 182

AdS/CMT: From Black Holes to Strange Metals

Daniel Areán
Instituto de Física Teórica (UAM-CSIC)

After a basic introduction to Gauge/Gravity duality I will show it at work by reviewing some of its applications to Condensed Matter physics like the realization of a strongly coupled superfluid or the more ambitious attempt at describing quantum critical phases relevant for the physics of strange metals.


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