Seminars and Events

Seminars and Events

Theory and Simulation of Materials

Coordinators: Álvaro Gómez León, Tobías Stauber


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

Coherent population trapping by dark states formation in carbon nanotubes

Milena Grifoni
Regensburg University

Illumination of atoms by resonant lasers can pump electrons into a coherent superposition of hyperfine levels which can no longer absorb the light. Such superposition is known as dark state, because fluorescent light emission is then suppressed. Here we report an all-electric analogue of this destructive interference effect in a carbon nanotube quantum dot. The dark states are a coherent superposition of valley (angular momentum) states which are decoupled from either the drain or the source leads. Their emergence is visible in asymmetric current-voltage characteristics, with missing current steps and current suppression which depend on the polarity of the applied source-drain bias. Our results demonstrate coherent-population trapping by all-electric means in an artificial atom.



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

Unconventional quantum optics in topological waveguide QED

Miguel Bello
ICMM

Abstract: In the race to find the next big technological application of quantum systems, quantum emitters coupled to photonic crystals seems a promising platform with many potential applications. Specifically, they can be used to simulate spin Hamiltonians whose interactions can be tailored by tuning the properties of the photonic crystals or those of the emitters. In this talk I will review a recent work [1] where we explore the physics emerging when quantum emitters interact with a topological 1D photonic crystal, namely a photonic analogue of the SSH model. We will see that the topology of the crystal reflects itself in the effective dipole-dipole interactions produced between the emitters and also the single-photon scattering properties.

[1] M. Bello, G. Platero, J. I. Cirac, and A. González-Tudela,
"Unconventional quantum optics in topological waveguide QED". Sci.
Adv. 5, eaaw0297 (2019).



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

Drift instabilities in graphene as a source of THz radiation

Hugo Terças
Instituto Superior Técnico, University of Lisbon

Electrically injected terahertz (THz) radiation sources are extremely appealing given their versatility and miniaturization potential, opening the venue for integrated-circuit THz technology. In this work, we show that coherent THz frequency combs in the range 0.5 THz<ω/2π<10 THz can be generated making use of graphene plasmonics. Our setup consists of a graphene field-effect transistor with asymmetric boundary conditions, with the radiation originating from a plasmonic instability that can be controlled by direct current injection. To continue the debate, we discuss some of the controversy still present around the various hydrodynamical models for graphene and present a strategy to mitigate it based on microscopic wave kinetics.



09 October 2019, 12:00 h. Sala de Seminarios, 182

Long-range chirality dimer order in the spin S = 1/2 Heisenberg chain with alternating Dzyaloshinskii-Moriya interaction

George I. Japaridze
Ilia State University

The ground state phase diagram of a spin S=1/2 XXZ Heisenberg chain with spatially modulated Dzyaloshinskii-Moriya (DM) interaction parametrized by D0, D1 is studied using the continuum-limit bosonization approach and extensive density matrix renormalization group computations. It is shown that at finite values of the parameters, depending on the value of an effective anisotropy parameter, the ground state phase diagram of the model contains four phases: the ferromagnetic phase, the gapless Luttinger-liquid (LL) phase,the gapped composite (C1) phase characterized by coexistence of the long-range-ordered (LRO) dimerization pattern with the LRO alternating spin current (chirality) pattern, and the gapped composite (C2) phase characterized in addition to the coexisting spin dimerization and alternating chirality patterns, by the presence of LRO antiferromagnetic order. The transition from the LL to the C1 phase belongs to the Berezinskii-Kosterlitz-Thouless universality class, while the transition from C1 to C2 phase is of the Ising type. Magnetic phase diagram of a spin S = 1=2 antiferromagnetic two-leg ladder with period two lattice units modulated DM interaction along the legs will is also discussed.



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

Electron-Phonon Interaction in Nanoelectronic Circuits:
toward the control of single phonons

Stefan Ludwig
Paul Drude Institute for Solid-State Electronics, Berlin, Germany

Electron-phonon interaction strongly influences the dynamics of mesoscopic quantum circuits; in nanostructures it can be further enhanced by size effects. This leads to detector backaction [1] and dephasing of solid state qubits [2]. Can we take a more optimistic perspective and aim at a full control of phonons? Are coherent electron-phonon-hybrid nanosystems interesting for quantum technology applications? In this talk I will present a series of experiments performed in semiconductor based mesoscopic samples ranging from phonon-mediated energy transfer in a two-dimensional electron-system to coherent electron-phonon interaction in lateral coupled quantum dot systems [1-4]. Finally, I will provide an outlook on how we plan to achieve strong coupling between confined electrons and surface phonons stored in cavities.

[1] Phonon-mediated vs. Coulombic Back-Action in Quantum Dot circuits; D. Harbusch et al.: Phys. Rev. Lett. 104, 196801 (2010).
[2] Characterization of Qubit Dephasing by Landau-Zener-Stückelberg-Majorana Interferometry; F. Forster et al.: Phys. Rev. Lett. 112, 116803 (2014).
[3] Phonon-mediated non-equilibrium interaction between nanoscale devices; G.J. Schinner et al.: Phys. Rev. Lett. 102, 186801 (2009).
[4] Quantum interference and phonon-mediated back-action in lateral quantum-dot circuits; G. Granger et al.: Nature Phys. 8, 522–527 (2012).



19 September 2019, 12:00 h. Sala de Seminarios, 182

Transport phenomena in hybrid superconductor devices

Robert HusseinEve
University of Konstanz

Cooper-pairs are a source of correlated electrons. Their nonlocal breaking can be used in entanglement generation, thermoelectric transport, and spin-current-control. On the basis of a double-quantum-dot-superconductor three-terminal system, we demonstrate that Cooper-pair splitting induces a thermo-current in the superconducting lead without any transfer of charge between the two normal metal leads [1]. Conversely, we show that a nonlocal heat exchange between the normal leads is mediated by nonlocal Andreev reflection. Coupling each quantum dot to an electromagnetic or mechanical resonator, realizes a device which can simultaneously cool down both resonators into their ground state [2]. We show that the process of cross-Andreev reflection can additionally be employed to coherently transfer single photons, and therewith heat, between both the oscillators. Furthermore, we investigate the Josephson current through a suspended carbon nanotube quantum dot, which acts likewise as a mechanical resonator. We discuss that a finite resonator coupling can induce phase transitions between groundstates of different total spin. A footprint also manifests in the critical current. Finally, we study spin-dependent quasiparticle and Cooper-pair transport through an island interfaced with two superconductors and two ferromagnets [3]. We demonstrate that unequal spin-mixing conductances for the two interfaces with the ferromagnets result in equal-spin triplet correlations on the island, detectable via a net charge current between the two magnets. The setup enables the generation, control, and detection of typically elusive equal-spin triplet Cooper-pairs.
[1] R. Hussein et al, PRB 99, 075429 (2019).
[2] M. Mantovani et al, arXiv:1907.04308 (2019).
[3] A. Rezaei et al, arXiv:1908.09610 (2019)



12 September 2019, 12:00 h. Sala de Seminarios, 182

Quantum sensing with NV centers. Limits of spectral resolution

Javier Prior
Departamento de Física Aplicada, Universidad Politécnica de Cartagena

In Quantum Sensing (QS) exploiting the quantum properties of entanglement and coherence, quantities such as charge, magnetic and electric fields, pressure, and temperature can be measured with a sensitivity which significantly exceeds any classical methodologies. New concepts of QS currently being developed could provide groundbreaking measurement techniques. It is believed that the field of QS will transform science and technology in the next decade. Among the most prominent devices, NV centers in diamonds represent one of the most promising platforms for sensing or metrology and they are proposed as a new generation for Nuclear Magnetic Resonance (NMR) and Magnetic Resonance Imaging (MRI).

The limits of frequency resolution in nano-NMR experiments have been discussed extensively in recent years. It is believed that there is a crucial difference between the ability to resolve a few frequencies and the precision of estimating a single one. Whereas the efficiency of single frequency estimation gradually increases with the square root of the number of measurements, the ability to resolve two frequencies is limited by the specific timescale of the signal and cannot be compensated for by extra measurements. In this talk, we show theoretically and demonstrate experimentally that the relationship between these quantities is more subtle and both are only limited by the Cramér-Rao bound of a single frequency estimation [1].

[1] Amit Rotem, Tuvia Gefen, Santiago Oviedo-Casado, Javier Prior, Simon Schmitt, Yoram Burak, Liam McGuiness, Fedor Jelezko, and Alex Retzker Phys. Rev. Lett. 122, 060503 (2019).



11 July 2019, 12:00 h. Sala de Seminarios, 182

Exotic stationary states in ac-driven open quantum systems: Breakdown of the Floquet-Gibbs distribution

Georg Engelhardt
Beijing Computational Science Research Center

Periodic driving is a flexible tool for quantum control of various systems. While the dynamics of isolated quantum system is well understood and used in experiments, the behavior of open ac-driven quantum system has gained less attention. Under special conditions of the ac drive, Floquet-Gibbs states, generalization of Gibbs states to ac-driven systems [1], arise.
Here, we investigate possible exotic stationary states in ac-driven open quantum systems which appear after the breakdown of the Floquet-Gibbs states. We use exact and approximate methods to study different regimes of driving field and system-environment coupling.
Besides others, we show that the probability distribution of the stationary state of a dissipative ac-driven two-level system can exhibit jump discontinuities. These discontinuities can be observed as jumps in the emission. Overall, the stationary states depends sensitively on the system-environment coupling operator. The jumps are the consequence of discontinuities in the transition rates, which we calculate numerically and analytically based on the secular Floquet-Redfield formalism [2].

[1] T. Shirai, J. Thingna, T. Mori, S. Denisov, P. Hänggi, and S.
Miyashita, New J. of Phys. 18 (2016) 053008.
[2] G. Engelhardt, G. Platero, J. Cao, arXiv:1903.00190



27 June 2019, 12:00 h. Sala de seminarios, 182

Revealing the fine structure of Andreev bound states in hybrid nanowires

Alfredo Levy Yeyati
Universidad Autónoma de Madrid

In atomic physics the fine structure refers to the splitting of the spectral lines due to relativistic effects, leading to the spin-orbit interaction. Analogously, the Andreev bound states in a weak link between two superconducting leads connected through a semiconducting nanowire are expected to exhibit a splitting due to the Rashba spin-orbit interaction in the wire.

In a recent work [1] we provide evidence of this effect through microwave spectroscopic measurements. We have developed a simple model which allows us to fit the transition lines in terms of a few parameters, like the effective junction length and its transparency. A basic ingredient in the model is to account for the nanowire multichannel structure [2]. Our theoretical analysis allows to get an estimate on the size of the spin-orbit interaction in the nanowire, which is found to be in agreement with values reported in the literature. It also allows to describe with high accuracy the effect on the Andreev levels of an applied Zeeman field along an arbitrary orientation.

I shall discuss several open issues connected to understanding the excitation and detection mechanisms of the Andreev transitions in a circuit QED geometry like the one used in the experiments. This knowledge constitutes a basic requirement towards the development of an Andreev spin qubit based on this setup.

[1] L. Tosi, C. Metzger, M.?F. Goffman, C. Urbina, H. Pothier, Sunghun Park, A. Levy Yeyati, J. Nygård, and P. Krogstrup, Phys. Rev. X 9, 011010 (2019) [2] S. Park and A. Levy Yeyati, Phys. Rev. B 96, 125416 (2017)



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

Phonons or phasons? The influence of soliton fluctuations on the electronics of Moiré-patterned materials.

Hector Ochoa
Columbia University

Sliding and twisting the layers of a Van der Waals material give rise to superstructures with emergent functionalities like, for example, the plethora of strongly correlated phenomena (superconductivity included) observed in twisted bilayer graphene. One of the most recent development on this front the observation of a remarkably large, linear in temperature (T) resistivity in the normal state, which some theories have attributed to electron-phonon scattering. In this talk, I will argue that the long-wavelength dynamics of these generically incommensurate structures are dominated by new collective modes, phasons. These modes correspond to coherent superpositions of optical phonons describing the sliding motion of stacking solitons separating regions of partial commensuration. I will illustrate this physics with recent data acquired in strained MoSe2 multilayers [1], where scanning tunnel microscopy revealed the formation of sharp stacking textures with distinct imprints in tunnel spectroscopy. In twisted bilayer graphene, transverse phason modes dominate the interaction of electrons with the lattice [2]. This coupling lifts the layer degeneracy of the reconstructed Dirac cones, which could explain the observed 4-fold Landau level degeneracy in magneto-transport. Electron-phason scattering gives rise to a linear-T contribution to the resistivity that increases with decreasing twist angle. This contribution, however, seems to be insufficient to explain the rapid increase of the resistivity close to the magic angle condition.
[1] D. Edelberg et al., in preparation.
[2] H. Ochoa, arXiv:1905.10850.



13 June 2019, 12:00 h. Salón de Actos

Non-equilibrium almost-stationary states and linear response for gapped quantum systems

Stefan Teufel
University of Tübingen

I will first review the problem of justifying linear response theory for gapped extended interacting quantum systems and briefly discuss existing mathematical results. Then I will present a recent approach to the rigorous justification of linear (and higher order) response based on non-equilibrium almost-stationary states and an adiabatic theorem for time-dependent perturbations that close the spectral gap. My talk is based on [arXiv:1708.03581, CMP Online First] and an ongoing joint project with Giovanna Marcelli.



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

Designing spin-spin interactions within the ultrastrong light- matter coupling

David Zueco
Universidad de Zaragoza

Photon mediated spin-spin interactions is a standard and interesting way of engineering spin models that can be used to design quantum simulators, to perform quantum gates or to generate spin squeezed states. Photons can live on a cavity and the underlying light-matter model is given by the Dicke Hamiltonian. They also can form a continuum. In this case they are described via spin(s)-boson like models. If the light-matter coupling is sufficiently big (as compared to the bare frequencies of the spins) these spins-bosons mixtures enter in a nonperturbative regime. Novel transitions, non-trivial ground state or renormalization of the spin parameters are the most important effects occurring in this nonperturbative regime. As a consequence, these phenomena determine the effective spin-spin models that can be obtained.

In this talk, I discuss two effective spin-spin models obtained in this ultrastrong coupling regime. In a first example, I consider an ensemble of N spins coupled to a single mode cavity. I show that both the ultrastrong and an explicit parity symmetry breaking (via an external bias) allows to engineer two-axis twisting interactions generating spin squeezing that scales in an optimal way (∼1/N). Then, I consider spins ultrastrongly coupled to a finite-band set of photons. In the ground state of the model, the spins interact via virtual photons. This interaction is of the Ising type, it decays exponentially with the spin- spin distance and it is ferromagnetic. Finally, the photonic edge causes the appearance of spin-boson bound states: localized excitations in the vicinity of the spins. I show that, mediated by these bound states, effective tight-binding effective Hamiltonians can be built and transport like properties as perfect state transfer between distant spins are explained.



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

Resonant magneto-optically active structures

Antonio García Martín
Instituto de Micro y Nanotecnología (IMN-CSIC)

Abstract: The control of light propagation in the visible and near-infrared domain using resonant systems such as plasmonic excitations or optical nanoantennas has been a matter of intense research during the last decades. The possibility to create and manipulate nanostructured materials encouraged the exploration of new strategies to control the electromagnetic properties without the need to modify the physical structure, i.e. by means of an external agent. A possible approach is combining magnetic responsive materials (magneto-opticaly active) and resonant materials (e.g. metals exhibiting plasmonic modes), where it is feasible to control the optical properties with magnetic fields in connection to the excitation of plasmonic resonances [1] (magnetoplasmonics).

These nanostructures can involve localized resonances or nano-antenna modes, or extended resonances such as surface plasmon polaritons SPP in thin continuous or perforated films. Here I will review the fundamental aspects behind magneto-optically active resonant nanostructures and then show that they can be employed in a wide variety of systems and ranges of the electromagnetic spectrum.

I will specifically show that these structures:
(i) can be used to modulate the propagation wavevector of SPPs [2], which allows the development of label free sensors with enhanced capabilities [3-5]
(ii) give rise to enhanced values of the magneto-optical response in isolated or interacting entities as well as perforated films, either metallic or dielectric, but always in connection with a strong localization of the electromagnetic field [6-8]
(iii) can be used to actively control thermal emission and the radiative heat transfer between objects in the near and far field [9-11]

References
[1] G. Armelles, et al., Adv. Opt. Mat. 1, 10 (2013)
[2] V.V. Temnov et al., Nat. Photon. 4, 107 (2010)
[3] B. Sepúlveda, A. Calle, L.M. Lechuga, G. Armelles, Opt. Lett. 31, 1085 (2006)
[4] M.G. Manera, et al., Biosens. Bioelectron. 58, 114 (2014)
[5] B. Caballero, A. García-Martín, and J. C. Cuevas, ACS Photonics 3, 203 (2016)
[6] N. de Sousa et al., Phys. Rev. B 89, 205419 (2014)
[7] N. de Sousa et al., Sci. Rep. 6, 30803 (2016)
[8] M. Rollinger et al., Nano Lett. 16, 2432-2438 (2016)
[9] E. Moncada-Villa, et al., Phys. Rev. B 92, 125418 (2015).
[10] R. M. Abraham Ekeroth, et al., Phys. Rev. B 95, 235428 (2017).
[11] R. M. Abraham Ekeroth, et al., ACS Photonics 5, 705 (2018).



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

Protected cat states in a driven superfluid boson gas

Fernando Sols
Dpto de Física de Materiales, Universidad Complutense de Madrid

We investigate the behavior of a one-dimensional Bose-Hubbard gas in both a ring and a hard-wall box, whose kinetic energy is made to oscillate with zero time-average, which suppresses first-order particle hopping while allowing higher-order processes [1]. At a critical value of the driving amplitude, the system passes from a Mott insulator to a superfluid formed by two quasi-condensates with opposite nonzero momenta. The superfluid system in the ring has similarities to the Richardson model, but with a peculiar type of pairing and an attractive interaction in momentum space. This analogy permits an understanding of some key features of the interacting boson problem. The ground state is a macroscopic quantum superposition, or cat state, of two superfluid states which collectively occupy opposite momentum eigenstates. Interactions give rise to a reduction (or modified depletion) cloud that is common to both macroscopically distinct states. Symmetry arguments permit a precise identification of the two orthonormal macroscopic many-body branches which combine to yield the ground state. In the ring, the system is sensitive to variations of the effective flux but in such a way that the macroscopic superposition is preserved. The shared nature of the reduction cloud provides some protection against collapse of the cat state due to particle losses. For the hard-wall case, the macroscopic quantum superposition is preserved because the system cannot collapse into a nonzero current state [2].

[1] G. Pieplow, F. Sols, C. E. Creffield, New J. Phys. 20, 073045 (2018).
[2] G. Pieplow, C. E. Creffield, F. Sols, in preparation.



09 May 2019, 12:00 h. Salón de Actos

Quantum spin liquids in honeycomb materials

Jaime Merino
Dpto. Fisica Teorica de la Materia Condensada, IFIMAC

Quantum spin liquids are magnetically disordered states with no broken symmetries even at zero temperature. In contrast to trivial quantum paramagnets they are highly entangled states with long range topological order and fractional excitations. In spite of decades of intense research an unambiguous signature of a quantum spin liquid remains elusive.

In the present talk I will discuss possible ways of achieving quantum spin liquid behavior in honeycomb lattices motivated by the organometallic materials, Mo3S7(dmit)3, and iridates. The honeycomb layers of Mo3S7(dmit)3 can be modeled by a Heisenberg model with frustrated exchange couplings. We have found a gapped quantum spin liquid of the Resonance Valence Bond type with fractional spinon excitations driven by magnetic frustration on honeycomb lattices. In contrast quantum compass interactions relevant to the Iridates can also induce a quantum spin liquid which, in contrast to the RVB, is gapless with Majorana excitations.



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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