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Ricardo Lobo

ESPCI Paris – PSL University – CNRS – Sorbonne Université, France

Correlations and dispersive Dirac physics in the quantum material family BaCoS2-BaNiS2 – Reverse band-structure engineering of the optical conductivity

When: 12:00-13:00 CET, May 30th (Thursday), 2024

Where: Seminar Room (182), ICMM-CSIC, Campus de Cantoblanco, Madrid

BaCoS2 and BaNiS2 are the end members of a solid solution that shows a vast array of quantum properties. The Co material is close to a strongly correlated insulator with an antiferromagnetic transition, as well as a structural phase transition, around room temperature. At 28% Ni doping this material undergoes an electronic metal-insulator phase transition to a Drude metal. The metallic state persists all the way to the pure Ni compound. At this point, in addition to a Drude peak, we observe a strong contribution from bands with linear dispersion at the Fermi level, which give origin to dispersive Dirac nodal lines. We measured the optical conductivity of these materials and combined them with ab-initio calculations to reverse engineer the role of each band in the physical response of these materials. We explained uncommon features in their optical response such as a linear dispersion of the optical conductivity [1] and the existence of an isosbestic line separating a spectral-weight transfer across Dirac nodal states [2].

[1] D. Santos-Cottin, Y. Klein, P. Werner, T. Miyake, L. de’Medici, A. Gauzzi, R.P.S.M. Lobo, and M. Casula, Linear behavior of the optical conductivity and incoherent charge transport in BaCoS2, Phys. Rev. Materials 2, 105001 (2018). arXiv: 1712.01539.

[2] D. Santos-Cottin, M. Casula, L. de’Medici, F. Le Mardelé, J. Wyzula, M. Orlita, Y. Klein, A. Gauzzi, A. Akrap, and R.P.S.M. Lobo, Optical conductivity signatures of open Dirac nodal lines, Phys. Rev. B 104, L201115 (2021). arXiv: 2104.05521.

Nicolas Leconte

University of Seoul, Korea

Christopher W. Wächtler 

Department of Physics

University of California at Berkeley

Dissipation as versatile resource for collective quantum dynamics

When: 12:00-13:00 CET, April 16th (Tuesday), 2024

Where: Salon de Actos, ICMM-CSIC, Campus de Cantoblanco, Madrid

The widespread belief is that quantum systems need to be protected from the environment as well as possible for quantum technology to fulfill its promise of revolutionizing computing, communication, and sensing. However, the advent of the Noisy Intermediate-Scale Quantum (NISQ) era forces us to investigate dissipation and decoherence and to find ways to utilize them effectively. This talk aims to provide examples where interactions with the environment play a pivotal role in generating and detecting collective quantum phenomena, inspiring new perspectives on harnessing environment interactions for advancing quantum technologies. Firstly, we will delve into the emerging field of topological quantum synchronization, a novel form of synchronization where topology and dissipation intertwine to protect synchronized dynamics against perturbation. Next, we will explore how carefully tailored interactions with the environment induce energy migration within small quantum spin networks characterized by a superradiant speed-up, demonstrating the potential for utilizing dissipation as a resource rather than a hindrance. Finally, if time permits, I will introduce a novel methodology for probing quantum criticality in non-equilibrium systems. This method circumvents the shortcomings of standard perturbative expansions, enabling a comprehensive and thermodynamically consistent understanding of critical phenomena in systems coupled to non-Markovian reservoirs.

Georg Engelhardt

Southern University of Science and Technology in the Shenzhen Institute of Quantum Science and Engineering

Photon-Resolved Floquet Theory and its application to quantum sensing

When: 12:00-13:00 CET, April 11th (Thursday), 2024

Where: Seminar Room (182), ICMM-CSIC, Campus de Cantoblanco, Madrid

Quantum sensing uses quantum properties of matter to enhance the sensitivity in precision measurements. Besides others, it already finds important applications in atomic clocks, for medical purposes, and in the search for dark matter. Many currently employed quantum sensing protocols exhibit a simple setup, in which a laser probes the optical properties of an ensemble of atoms, molecules, or other quantum emitters, which are subject to the external stimulus to be measured. An important figure of merit to predict the sensitivity is the signal-to-noise ratio.  While it is easy to theoretically calculate the signal, the accurate prediction of the noise is challenging.

Motivated by this, we have developed the Photon-Resolved Floquet Theory (PRFT), which besides predicting the state of a driven quantum system (e.g., the atom or molecule), can also predict the number of photons exchanged with the coherent driving field [1,2]. To this end, the PRFT introduces counting fields into the semiclassical equations of motions, that track the photons in the driving field.  Interestingly, the PRFT predicts light-matter entanglement in the Floquet-state basis. This effect can be employed to devise a measurement-based quantum communication protocol, which has favorable scaling properties over long distances.  We apply the PRFT to spectroscopy, where it can predict the Fisher information of coherent spectroscopic signals [3]. The PRFT thus opens new paths to design and optimize quantum sensors based on AMO systems, which might assist in the discovery of new physics.

[1] G. Engelhardt, S. Choudhury, and W. V. Liu, Phys. Rev. Research 6, 013116 (2024)

[2] G. Engelhardt, JY. Luo, V. M. Bastidas, and G. Platero, arXiv: 2311.01509

[3] G. Engelhardt et al., in preparation

Paloma A. Huidobro 

Departamento de Física Teórica de la Materia Condensada and Condensed Matter Physics Center (IFIMAC), Universidad Autónoma de Madrid

Light and photons in time varying media

When: 12:00-13:00 CET, March 21st (Thursday), 2024

Where: Salon de Actos, ICMM-CSIC, Campus de Cantoblanco, Madrid

In this talk I will first introduce the basic concepts of wave interactions in time varying media. I will discuss how temporal modulations of the optical parameters offer new pathways in light control, as energy is not necessarily conserved in these time-dependent systems, and non-reciprocal effects can be realized [1]. I will concentrate on a class of space-time modulations where parameters are modulated in a travelling-wave form, such that there is an apparent motion of the optical properties, and discuss light propagation in these systems: from non-reciprocal effects even in the quasistatic limit, to synthetic motion and the link to the Fresnel drag effect of light in moving media, through an unconventional linear gain mechanism.

Next, I will discuss photon pair creation and squeezing in a photonic time crystal, the temporal counterpart of a conventional photonic crystal. I will show how modulating the refractive index in time causes dynamical Casimir processes, in which pairs of photons can be created. However, when there are multiple time interfaces, these photon pair creation phenomena depend dramatically on the photon’s wavelength, as well as on the contrast between the refractive indices, the duration of each temporal period and the number of periods that form the photonic time crystal [2].

 References:

[1] Galiffi, E., Tirole, R., Yin, S., Lia, H., Vezzoli, S., Huidobro, P.A., Silveirinha, M.G., Sapienza, R., Alu, A., and Pendry, J.B. “Photonics of time varying media” Advanced Photonics, 4(1), 014002 (2022)

[2] Echave, J., García-Vidal, F.J., &  Huidobro, P.A., “Photon squeezing in time varying media”, to be submitted (2024)

Belén Valenzuela

ICMM-CSIC, Madrid, Spain

A toy Landau model for illustrating learning and unlearning of nociplastic pain

When: 12:00-13:00 CET, March 3rd, 2024

Where: Salon de Actos, ICMM-CSIC, Campus de Cantoblanco, Madrid

Chronic pain is increasing at an alarming rate specially among young people and kids. This disturbing situation has increased the research interest in gaining a deeper understanding of chronic pain. Physiologically, nociplastic pain has been defined as a significant component of chronic pain not linked to tisular damage but to a nocive plasticity pattern of the nervous and immune system. From phenomenological cognitive sciences, there is compelling evidence that the consolidation of nociplastic pain is a complex, nonconscious learned process of threat perception that gives rise to maladaptive loops. Embodied neurobiological pain education is emerging as a promising approach to reduce the perception of threat which leads to a decrease of symptom intensity and frequency, improved functionality and eventual symptom alleviation. However, this approach is not well known among clinicians and society at large, creating a communication problem that unfortunately perpetuates the suffering of the patients. We propose a toy Landau model to describe the learning and unlearning process of nociplastic pain, aiming to clarify this complex situation and facilitate communication across different sectors of society. Nociplastic pain corresponds to a first-order transition, with attention more likely in the alert-protection state than in the trust-explore state. Two appealing results of the model are that the perception of the critical context depends on personal history regarding the symptom and that maladaptive loops are formed when there is alarming learned historical information about the symptom, along with confused and contradictory expert information, as seen in nocebo messages. Learning and unlearning in the model correspond to a change in control parameters that can weight more the alert-protection state, the trust-explore state, the uncertain state or the neutral state. This description clarifies why neurobiological education is the ground therapy from which others must be built to embody the accesible, clear, and trustworthy information. The model could be used to address other mind-body syndromes.

YouTube link: https://www.youtube.com/watch?v=i7mHqDFsc_4

Dario Bahamon

Mackenzie Presbyterian University, São Paulo, Brazil

Quantum transport in twisted bilayer graphene

When: 12:00-13:00 CET, February 29th, 2024

Where: Salon de Actos, ICMM-CSIC, Campus de Cantoblanco, Madrid

While the van der Waals interaction in layered crystals is weak, there is hybridization that gives rise to a superlattice potential as a function of the rotation in twisted heterostructures. The long-range modulation of this potential results in both commensurate and incommensurate structures, involving tens of thousands of atoms in a single moiré supercell. This poses significant challenges to conventional methods for studying the electronic properties of these materials. In this talk, I will present new approaches to accessing the quantum transport properties of twisted heterostructures, specifically applied to twisted bilayer graphene.

Nicolas Leconte

University of Seoul, Korea

Open-orbit induced extreme magnetoresistance and fractional Chern insulator in graphene/h-BN
superlattices at low fields

When: 12:00-13:00 CET, June 20th (Tuesday), 2024

Where: Seminar Room, ICMM-CSIC, Campus de Cantoblanco, Madrid

In this talk, I will discuss our recent work where we report on the hitherto unexplored low magnetic field regime in ultrahigh-quality graphene/h-BN superlattices where intrinsic band structure features perturb the single-band Hofstadter butterfly picture. For magnetic fields as low as 0.1 − 1 Tesla, we uncover extreme magnetoresistance patterns, attributed to the formation of open orbits at Lifshitz transitions, providing a truly 2D experimental realization of the commonly discussed 2D theoretical open-orbit models, and illustrated through a real-space analysis of the current densities. We further observe trigonal warping-induced Landau level splitting near the secondary Dirac point in monolayer graphene superlattice, where enhanced many-body effects induce a fractional Chern insulator, a primer at such low magnetic fields. Finally, a transition from linear to parabolic Landau level dispersion is uncovered, a phenomenon we link to miniband overlap through a semiclassical analysis. Our combined theoretical-experimental study provides insights into the intricacies of the low-field fractal spectrum ahead of the quantum Hall regime.

Antonio Prados

Departamento de Física Atómica, Molecular y Nuclear
Universidad de Sevilla, Spain

Swift state-to-state transformations in stochastic systems

When: 12:00-13:00 CET, July 17th (Monday), 2023

Where: Seminar Room, ICMM-CSIC, Campus de Cantoblanco, Madrid

In the last years, there has been a great interest in the possibility of accelerating the connection between given initial and final states. The field was started with the idea of “shortcuts to adiabaticity” (STA) in the quantum framework, in which the general goal was to obtain the adiabatic evolution but in a finite time—by driving the system with an additional term in the Hamiltonian. Several techniques have been proposed to achieve this goal: inverse engineering, counter-diabatic driving, fast-forward, to name a few [1]. Both the idea of accelerating the connection between states and the techniques employed have later been transposed to stochastic systems, mainly trying to find shortcuts for the relaxation between equilibrium states. Since “adiabatic” has the meaning of zero-heat in thermodynamics, the term “swift state-to-state transformations” (SST) has been proposed to encompass all the protocols that aim at accelerating the connection in stochastic systems [2]. This is done by tailoring the time evolution of physical properties that control the time evolution of the system of interest, e.g. the stiffness of a harmonic trap or the temperature of the thermal bath—i.e. the control functions. Once the feasibility of connecting in a finite time is shown, there appears the problem of optimising it in a certain sense: connection time, dissipation, or other figures of merit. This makes it necessary, in general, to resort to optimal control theory to find the optimal time protocol for the control functions. The above general ideas will be illustrated with several examples of stochastic systems.

[1] D. Guéry-Odelin, A. Ruschhaupt, A. Kiely, E. Torrontegui, S. Martínez-Garaot, and J. G. Muga. Rev. Mod. Phys. 91, 045001 (2019).
[2] D. Guéry-Odelin, C. Jarzynski, C. A. Plata, A. Prados, and E. Trizac, Driving rapidly while remaining in control: classical shortcuts from Hamiltonian to stochastic dynamics, Rep. Prog. Phys. 86, 035902 (2023).

Mikhail M. Otrokov,

Centro de Física de Materiales (CFM-MPC), Centro Mixto CSIC-UPV/EHU, San Sebastián, Spain

IKERBASQUE, Basque Foundation for Science, Bilbao, Spain

Combining magnetism and topology: from magnetic doping to novel interfaces and intrinsic magnetic topological insulators

When: 12:00-13:00 CET, June 15th (Thursday), 2023

Where: Seminar Room, ICMM-CSIC, Campus de Cantoblanco, Madrid

In this talk, I will overview the developments in the field of magnetic topological insulators (MTIs) that led to the discovery of the intrinsic MTIs of the MnBi₂Te₄ family that attracts a great deal of attention nowadays. First, to describe the context in which materials such as MnBi₂Te₄ appeared in the research arena, I will discuss the magnetic doping and magnetic proximity effect approaches of introducing magnetism into a TI. Then, the two types of novel and promising interfaces involving MnBi₂Te₄ compounds will be discussed, as they are expected to show certain advantages over the latter two approaches. Next, the discovery of intrinsic MTIs of the MnBi₂Te₄ family will be overviewed. Finally, concerning current challenges of this field, we will consider in detail the issue of the Dirac point gap in the MnBi₂Te₄topological surface state that has caused a lot of controversy recently.