Seminars and Events

Seminars and Events

Theory and Simulation of Materials

Coordinators: Álvaro Gómez León, Tobías Stauber


06 May 2019, 12:00 h. Sala de Seminarios, 182

Internal screening and dielectric engineering in magic-angle twisted bilayer graphene

J. M. Pizarro
University of Bremen

Stacking two graphene layers at a twist angle θ on top of each other leads to twisted bilayer graphene (tBLG) featuring a moiré pattern with an intricate emergent electronic structure. For small twist angles θ<20 the resulting superlattices host several thousand atoms per unit cell. In this situation, the electronic bands around the charge neutrality point (CNP) become very flat, facilitating strong correlation effects. Recent experiments reported the emergence of possibly unconventional superconducting and insulating states in magic-angle tBLG (MA-tBLG) at different levels of doping. The insulating states occur for commensurate fillings at both electron and hole dopings, signaling a possible Mott-Hubbard origin. Around these insulating states, superconductivity emerges, resembling the phase diagram of high-Tc cuprates. While the impact of external parameters such as doping or magnetic field can be conveniently modified, the all-surface nature of the quasi-2D electron gas combined with its intricate internal properties pose a challenging task to characterize the nature of the different insulating and superconducting states found in experiments. We analyze the interplay of internal screening and dielectric environment on the intrinsic electronic interaction profile of MA-tBLG. We find that interlayer coupling generically enhances the internal screening. The influence of the dielectric environment on the effective interaction strength depends decisively on the electronic state of MA-tBLG. Thus, we propose the experimental tailoring of the dielectric environment, e.g. by varying the capping layer composition and thickness, as a promising pursuit to provide further evidence for resolving the hidden nature of the quantum many-body states in MA-tBLG.



25 April 2019, 12:00 h. Sala de Seminarios, 182

Thermoelectric graphene nanostructures

Francisco Domínguez-Adame
Universidad Complutense de Madrid

Thermoelectric materials offer the possibility to harness dissipated energy and make devices less energy-demanding. Heat-to-electricity conversion requires materials with a strongly suppressed thermal conductivity but still high electronic conduction. Graphene nanostructures can meet nicely these two requirements because enhanced phonon scattering at the bends and defects reduces the lattice thermal conductivity while electric conductivity is not severly deteriorated, leading to an overall remarkable thermoelectric efficiency. Therefore, they are regarded as a promising route to achieving valuable thermoelectric materials at the nanoscale. In this talk, I will present an overview of key experimental and theoretical results concerning the thermoelectric properties of graphene nanostructures. The focus of this review is put on the physical mechanisms by which the efficiency can be improved. Phonon scattering and enhancement of the power factor by quantum effects will be thoroughly discussed.



11 April 2019, 12:00 h. Salón de Actos

The Interacting Semiconductor Hole Gas

John Schliemann
Institute for Theoretical Physics, University of Regensburg

I will discuss the homogeneous interacting hole gas in p-doped bulk III-V
semiconductors. The structure of the valence band is modeled by Luttinger’s
Hamiltonian in the spherical approximation, giving rise to heavy and light
hole dispersion branches, and the Coulomb repulsion is taken into account
via a self-consistent Hartree-Fock treatment. As a nontrivial feature of the
model, the self-consistent solutions of the Hartree-Fock equations can be
found in an almost purely analytical fashion, which is not the case for
other types of effective spin-orbit coupling terms. In particular, the
Coulomb interaction renormalizes the Fermi wave numbers for heavy and light
holes. As a consequence, the ground state energy found in the self-consistent
Hartree-Fock approach and the result from lowest-order perturbation theory
do not agree.

Moreover, I will report on a recent study of the dielectric function of the
homogeneous hole gas in p-doped zinc-blende III-V bulk semiconductors within
random phase approximation. In the static limit we find a beating of
Friedel oscillations between the two Fermi momenta for heavy and light holes,
while at large frequencies dramatic corrections to the plasmon dispersion
occur.


References:

Theoretical study of interacting hole gas in p-doped bulk III-V semiconductors
John Schliemann, Phys. Rev. B 74, 045214 (2006).

Dielectric function of the semiconductor hole gas
John Schliemann, Europhys. Lett. 91, 67004 (2010).

Dielectric function of the semiconductor hole liquid: Full frequency and wave vector dependence
John Schliemann, Phys. Rev. B 84, 155201 (2011).



04 April 2019, 12:00 h. Salón de Actos

Topological waves in continuous media

Pierre Delplace
École Normale Supérieure de Lyon

Unidirectionnal boundary modes are the hallmark of Chern insulators. Such topological states have been engineered in various platforms, from condensed matter to artificial crystals e.g. in photonics, acoustics or cold atoms physics. Remarkably, such chiral modes also exist in continuous media encountered in nature. This is the case of oceanic and atmospheric equatorial waves that only propagate their energy eastward. This remarkable property, that triggers the El niño southern oscillations and impacts the climate over the globe, has a topological interpretation analogous to those of Chern insulators [1]. Similar topological arguments also allow the prediction of new kinds of waves in strongly stratified fluids that might be observed e.g. in stars [2].
In the presence of a solid boundary, Kelvin already pointed out the existence of one-way directional waves propagating along the coasts of lakes. In strong contrast with crystals, the existence of these chiral modes in continuous media depends on the boundary conditions: they are thus not topologically protected as we would naively expect by analogy with the celebrated bulk-boundary correspondence in condensed matter [3].
[1] Topological origin of equatorial waves. P. Delplace, B. Marston and A. Venaille, Science 358, 1075 (2017).
[2] Topological transition in stratified atmospheres. M. Perrot, P. Delplace and A. Venaille, arXiv:1810.03328 (2018).
[3] Anomalous bulk-edge correspondence in continuous media. C. Tauber, P. Delplace and A. Venaille, arXiv:1902.10050 (2019)



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

Some questions about twisted bilayer graphene at magic twist angle

José González
Instituto de la Estructura de la Materia (IEM-CSIC)

We will review basic electronic properties of twisted bilayer graphene in order to address some of the puzzling questions (old and recent) posed by such a carbon material. These include the intriguing nature of the flat bands that develop at the so called magic twist angles, and which lack a proper physical understanding. More recent questions have arisen with regard to the superconductivity and the adjacent Mott-insulating phase found at the first magic angle. The central problem has become to discern the interaction mechanism responsible for those phases, and which should also account for the deviations from Fermi liquid behavior observed in the metallic state.



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

Spin-orbit coupling studies of materials: some examples

Roberto Robles
Centro de Física de Materiales CFM/MPC (CSIC-UPV/EHU)

Spin-orbit coupling (SOC) plays a central role in the properties of many technologically and fundamentally relevant materials. One of the most commonly studied consequences of SOC is the preferential orientation of the magnetic moments along certain directions of the lattice: the magnetic anisotropy energy (MAE). We have studied MAE in low-dimensional systems consisting of heterogeneous transition metal chains supported on a Cu2N/Cu(100) surface [1,2].

MAE is also relevant in the study of materials for magnetic data storage. Here we show the reduction of the coercivity in nanoporous Cu-Ni films just by the action of an electric field. By performing ab-initio calculations we offer some insight into the mechanisms behind the observed behavior [3].

Another interesting topic is the enhancement of spin-orbit interactions in materials with small intrinsic SOC, like graphene. We have studied the spin properties of Graphene/Bi2Se3 heterostructures, in particular the nontrivial spin-texture induced in the graphene states, and their possible implications on the spin transport properties of the resulting system [4].

Finally, I will comment on recent advances of the treatment of SOC in the ab-initio DFT code Siesta.

[1] D-J. Choi et al. Physical Review B 94, 085406 (2016)
[2] D-J. Choi et al. Nano Letters 17, 6203 (2017)
[3] A. Quintana et al. Advanced Functional Materials 27, 1701904 (2017)
[4] K. Song et al. Nano Letters 18, 2033 (2018)



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.



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

Topological Amplification in Photonic Lattices

Diego Porras
Instituto de Física Fundamental (CSIC)

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.

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



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
DIPC

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].

References
[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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