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.

Prof. S.-R. Eric Yang,

Korea University, Dept. of Physics, 02855, Seoul, South Korea

Topologically ordered zigzag graphene nanoribbon

When: 12:00-13:00 CET, June 1st (Thursday), 2023

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

Interacting disordered graphene zigzag ribbons are a new topologically ordered Mott–Anderson insulator displaying e^–/2 fractional charges, spin-charge separation, and two degenerate ground states^1,2. The disorder is a singular perturbation that couples electrons on opposing zigzag edges, resulting in instantons. This effect converts zigzag ribbons from a symmetry protected phase to a topologically ordered phase and generates e^–/2 fractional charges on the opposite zigzag edges. These fractional charges are protected by an exponentially decaying soft gap. Furthermore, an interacting disordered zigzag nanoribbon has a finite topological entanglement entropy and its entanglement spectrum resembles the corresponding edge spectrum of the system. Doped ribbons display the following effects³, which can be experimentally tested: (1) In the low doping case and weak disorder regime, the soft gap in the tunneling density of states of the undoped case is replaced by a sharp peak at the midgap energy with two accompanying peaks. The e^–/2 fractional charges that reside on the boundary of the zigzag edges are responsible for the midgap peak. Localization effects play an important role in the quantization of these fractional charges. (2) The midgap peak disappears as the doping concentration increases. The presence of e^–/2 fractional charges will be strongly supported by the detection of these peaks. Doped zigzag ribbons may also exhibit unusual transport, magnetic, and inter-edge tunneling properties.

1. S.-R. Eric Yang, Topologically Ordered Zigzag Nanoribbon: e/2 Fractionally Charged Anyons and Spin-Charge Separation. (World Scientific, Singapore 2023).

2. S.-R. Eric Yang, Min-Chul Cha, Hye Jeong Lee, and Young Heon Kim, Phys. Rev. Research 2, 033109 (2020).

3. Young Heon Kim, Hye Jeong Lee, Hyun-Yong Lee, and S.-R. Eric Yang, Sci. Rep. 12, 14551 (2022).

Prof. Giorgos Katsaros,

Institute of Science and Technology Austria (ISTA), Austria

Spin qubits and hybrid devices in planar Ge

When: 12:00-13:00 CET, May 26th (Friday), 2023

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

Ge, the material that was first utilized for the development of transistors at Bell Labs in 1947, has recently attracted significant attention for its potential in the field of quantum information. Particularly, the focus has shifted towards hole gases in Ge/SiGe heterostructures due to their combination of high mobilities, strong spin-orbit interaction, and electrically adjustable g-factors. These attributes make Ge quantum wells not only a promising candidate for spin qubits but also for the creation of hybrid superconductor-semiconductor devices. In this presentation, I will present our latest findings on planar Germanium, emphasizing its potential for co-integration of semiconductor with superconducting technology.

Dr. Rafael Sanchez ,

Department of Theory of Condensed Matter Physics, Universidad Autonoma de Madrid, Spain

Single-electron (or photon) heat currents and how to control them

When: 12:30-13:30 CET, May 11th (Thursday), 2023

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

The flow and dissipation of heat is unavoidable in the operation of any circuit. Exploiting the properties of nanoscale conductors, one should be able to define devices able to control it on-chip, such as thermal rectifiers, transistors or circulators. These typically rely on strong nonlinearities and far from equilibrium configurations. In this talk I will discuss how these effects appear in minimal systems with a few number of levels (such as quantum dots [1,2] or qubits [3]) are coupled to two or more reservoirs, close to the linear response regime.

[1] R. Sánchez, H. Thierschmann and L. W. Molenkamp, Phys. Rev. B 95, 241401 (2017).
[2] A. Marcos-Vicioso et al., Phys. Rev. B 98, 035414 (2018).
[3] D. Goury and R. Sánchez, Appl. Phys. Lett. 115, 092601 (2019).
[4] I. Díaz and R. Sánchez, New J. Phys. 23, 125006 (2021).

YouTube link: https://www.youtube.com/watch?v=rrvH95h6s3E

Prof. Yigal Meir,

Department of Physics, Ben Gurion University, Israel

Measuring Entropy of Exotic Particles

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

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

In recent years many candidate setups have been proposed to support exotic quasi-particles, such as Majorana fermions (MFs), which may be relevant for quantum computing, but whether these particles have been observed experimentally is currently a topic of a vivid debate. Entropy measurements can unambiguously separate these quasi-particles from other, simpler excitations. The entropy of a MFs is, for example, log2/2 (in units of the Boltzman constant), a fractional value that cannot be attributed to a localized excitation. However, standard entropy measurements applicable to bulk systems cannot be utilized in measuring the additional entropy of a mesoscopic device, which may be due to less than a single electron in the device. In this talk I will describe recent theoretical and experimental progress in performing such measurements, either using thermopower and/or using the Maxwell relations [1,2]. Particular examples will be single and double quantum dots in the Coulomb blockade regime. Lastly I will show how the formalism has been generalized to deduce the entropy from conductance measurements, and, applying it to a setup where two and three-channel Kondo physics have been observed, yields the fractional entropy of a single MF and a single Fibonacci anyon [3]. Lastly I will discuss the backaction of the measurement and discuss the possibility of measuring entanglement entropy [4].

[1]   Direct entropy measurement in a mesoscopic quantum system, N. Hartman, et al., Nature Physics 14, 1083 (2018).
[2]   How to measure the entropy of a mesoscopic system via thermoelectric transport, Y. Kleeorin et al., Nature Comm. 10 , 5801 (2019)
[3]   Fractional Entropy of Multichannel Kondo Systems from Conductance-Charge Relations, C. Han et al., Phys. Rev. Lett. 128, 146803 (2022).
[4]   Realistic protocol to measure entanglement at finite temperatures, C. Han, Y. Meir and E. Sela, Phys. Rev. Lett., in press.

Julien Varignon,

Laboratoire CRISMAT, CNRS UMR 6508, ENSICAEN, Normandie Université, France

First-principles studies of oxide superconductors

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

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

Along with the famous cuprates, ABiO3 (A=Ba, Sr) and BaSbO3 oxide perovskites as well as nickel oxides, either as an infinite layer RNiO2 or Reduced Ruddlesden-Popper phase Rn+1NinO2(n+1) (R=La, Pr or Nd), belong to the few oxide systems exhibiting superconductivity once appropriately doped [Phys. Rev. B 37, 3745 (1988), Nature 390, 148 (1997), Nature 572, 624 (2019), Nat. Mater. 21, 160 (2021)]. They are thus alternative platforms for understanding the formation of bound electrons at the core of superconductivity. However, these systems look rather different in their pristine form (i) Ni+ cations exhibit a magnetic moment while Bi4+ or Sb4+cations do not, (ii) nickelates are prone to correlation effects while the two other materials do not, (iii) RNiO2 compounds are metallic while BaSbO3 and ABiO3 are insulators and (iv) nickelates and antimonates present a Mott-like regime while ABiO3 materials possess a charge-transfer like behavior. Although different at first glance, we reveal on the basis of Density Functional Theory (DFT) calculations, involving all relevant degrees of freedom and an exchange-correlation functional sufficiently amending self-interaction errors, that all these materials are prone to exhibit charge orderings (CO) accompanied by bond disproportionation and insulating phases. Once doping drive the materials in a metallic regime at the vicinity of a CO phase, the vibration associated with the bond disproportionation is sufficient to explain the formation of Cooper pairs and to reproduce the evolution of the critical temperature versus doping content observed experimentally for the different compounds. Finally, strong hybridizations between the O-p states and the relevant cation states as those appearing in bismuthates are shown to be a determining factor behind high temperature superconductivity in oxides.

YouTube link: https://www.youtube.com/watch?v=Azs6rw0OlzE

Dr. Sol Carretero Palacios,

Universidad Autónoma de Madrid

Quantum trapping modelling based on the Casimir-Lifshitz force

When: 12:00-13:00 CET, March 16th (Thursday), 2023

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

The Casimir-Lifshitz force originates from the quantum vacuum fluctuations of the electromagnetic field. This force is especially intense between interacting objects at nanoscale distances, and it can be attractive or repulsive depending on the optical properties of the materials involved (amongst other parameters). This fundamental phenomenon is at the heart of the malfunctioning of nano- and micro-electromechanical devices (NEMS and MEMS) that integrate many of the gadgets we use in our daily lives. Absolute control over these forces would make it possible to suppress adhesion and friction in these NEMs and MEMs.

During this talk, I will show the possibility of controlling the Casimir-Lifshitz force by tuning the optical properties of the interacting objects. Specifically, I will present diverse examples of quantum levitation of self-standing thin films comprising multilayer structures or films with spatial inhomogeneities, based on the Casimir-Lifshitz force.

YouTube link: https://www.youtube.com/watch?v=iA0VfSgdGbw