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

Prof. Manuel Nieto-Vesperinas, Instituto de Ciencia de Materiales de Madrid

New scenery of electrodynamics forces, including a view into nanophotonics

When: 12:00-13:00 CET, March 2nd (Thursday), 2023

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

I shall demonstrate that the theory of electrodynamical forces that makes use of Maxwell’s stress tensor, only describes half the physics of these phenomena. The other half, that I shall uncover, and whose law we have formulated, is governed by the imaginary part of a complex stress tensor of which Maxwell’s is only its real part.

This complex stress tensor law constitutes a new paradigm of the mechanical efficiency of light on matter, and completes the landscape of electromagnetic forces in photonics and electrodynamics. It  widens our understanding in the design of both illumination and matter, in optical manipulation and propulsión by light in e.g solar sails.

In tis context, one may look at the energy conservation law of Electromagnetism: The Poynting theorem:  Energy transport is determined by two quantities of the momentum of light: one is real, well-known and currently observed; the other is imaginary, also known,
and alternating. However, the latter hinders the former; it is a workhorse of engineering in the design of transmission lines and antennas, and it is known as reactive power, which impairs the system performance by dissipation of feeding power.

I shall show that the complex momentum conservation law, which in a dielectric is relevant to the Abrahme-Minkowski debate, conveys a quantity which we coined as the “reactive strength of canonical momentum”, whose built-up hinders the efficiency of all currently
observed (time-averaged) electrodynamical forces, and  I shall illustrate it with its consequences in the optical force on nanoparticles and nanoantennas: beads employed in optical tweezers in Biology, high index-resonators which are the basis of a wide range of
metasurfaces in Mie-resonant photonics or Mie-tronics, and plasmonic nanoparticles.

References:

M. Nieto-Vesperinas and X. Xu, Light: Sci. & Appl. 11:297 (2022).
J.Zeng and J. Wang, Light: Sci. & Appl.  News&Views  12:20 (2023)
M. Nieto-Vesperinas and X. Xu, Phys. Rev. Res. 3, 043080 (2021).

Akashdeep Kamra, Universidad Autonoma de Madrid

The magnon-cooparon quasiparticle

Generating and moving unconventional spinal Cooper pairs using magnons

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

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

A superconductor is formed when pairs of electrons undergo condensation into a macroscopically coherent state with a common order parameter. Its supporting dissipationless flow of charge currents underlies the central role that superconductors are playing in various emerging quantum technologies. The widely used conventional superconductors are constituted by spin-singlet Cooper pairs. These are formed by electrons bearing opposite spin, and therefore bear no net spin. For various phenomena, such as dissipationless magnetic memories and Majorana excitations, it is desirable to have equal-spin triplet Cooper pairs formed by electrons with the same spin. Engineering and controlling such exotic Cooper pairs has thus been a main goal and challenge for the scientific community [1].

In this talk, we will discuss some recent theoretical efforts in achieving the generation and control of such spin-triplet Cooper pairs using magnetic insulators and their spin excitations – magnons. We will show that if the superconductivity is mediated by antiferromagnetic magnons, instead of phonons, the resulting superconducting state is unconventional and can achieve a relatively high critical temperature [2,3]. This can be accomplished in bilayers comprising an antiferromagnetic insulator (AFI) interfaced to a normal metal. Furthermore, unconventional Néel spin-triplet Cooper pairs with pairing amplitude oscillating at atomic length scales have recently been predicted to emerge in AFI/superconductor bilayers [4]. Finally, we will discuss the emergence of a novel quasiparticle – magnon-cooparon – in a conventional superconductor interfaced with a ferromagnetic insulator [5]. The spatially inhomogeneous exchange field generated by a magnon in the ferromagnet induces spin-triplet Cooper pairs in the adjacent superconductor which act to screen the magnon spin. This quasiparticle, reminiscent of the polaron excitation, allows driving the spin-triplet Cooper pairs in a desired direction employing mature techniques from the field of magnonics. The magnon-cooparon also enables a powerful magnonic directional coupler, a key element in magnon-based logic and computing paradigms.

References:

[1] Matthias Eschrig. Spin-polarized supercurrents for spintronics: a review of current progress. Rep. Prog. Phys. 78, 104501 (2015).
[2] A. Kamra, A. Rezaei, and W. Belzig. Spin splitting induced in a superconductor by an
antiferromagnetic insulator. Phys. Rev. Lett. 121, 247702 (2018).
[3] E. Erlandsen, A. Kamra, A. Brataas, and A. Sudbø. Enhancement of superconductivity
mediated by antiferromagnetic squeezed magnons. Phys. Rev. B 100, 100503(R) (2019).
[4] G. A. Bobkov, I. V. Bobkova, A. M. Bobkov, and A. Kamra. Néel proximity effect at
antiferromagnet/superconductor interfaces. Phys. Rev. B 106, 144512 (2022).                              [5] I. V. Bobkova, A. M. Bobkov, A. Kamra, and W. Belzig. Magnon-cooparons in magnet-
superconductor hybrids. Communications Materials 3, 95 (2022).

David Martinez-Martin, University of Sidney

Life and cell’s mass dynamics

When: 12:00-13:00 CET, January 19th (Thursday), 2023

Where: Main Hall, ICMM-CSIC, Campus de Cantoblanco, Madrid

Living cells sense and exchange biological, chemical, and mechanical information, as well as nutrients, water and waste products with their surroundings. These processes involve changes of a cell’s volume and mass[1] and are tightly linked to fundamental processes such as metabolism, proliferation[2], gene expression[3] and cell death. Yet it remains challenging to characterise the dynamics and regulation of a cell’s mass and volume in real time and with high accuracy, hampering our understanding of cell physiology. Moreover, dysregulation of cell mass is a critical underlying force in the development and progression of many disorders[4] such as cancer, diabetes type 2, obesity, cardiovascular disease and ageing. Therefore understanding how cells regulate their mass has enormous potential to transform the way we diagnose, monitor and treat disease[5].

I will introduce a new technology (picobalance) that we have developed, which is based on an optomechanical microresonator[6]. It measures the mass of single or multiple cells in culture conditions over days at millisecond time resolution reaching subpicogram mass sensitivity. Besides, this technology allows measuring  cells’ rheological properties[7]. I will present some of the results we have discovered using this technology in both mammalian cells[6] and yeast[1], and which challenge models in biology that have been central for decades[1, 5, 6]. 

References

1. Cuny, A.P., et al., High-resolution mass measurements of single budding yeast reveal linear growth segments. Nat Commun, 2022. 13(1): p. 3483.

2. Lang, F., et al., Functional significance of cell volume regulatory mechanisms. Physiological Reviews, 1998. 78(1): p. 247-306.

3. Haussinger, D., The role of cellular hydration in the regulation of cell function. Biochemical Journal, 1996. 313: p. 697-710.

4. Lloyd, A.C., The Regulation of Cell Size. Cell, 2013. 154(6): p. 1194-1205.

5. Martinez-Martin, D., Dynamics of cell mass and size control in multicellular systems and the human body. Journal of Biological Research-Thessaloniki, 2022. 29.

6. Martinez-Martin, D., et al., Inertial picobalance reveals fast mass fluctuations in mammalian cells. Nature, 2017. 550(7677): p. 500-505.

7. Flaschner, G., et al., Rheology of rounded mammalian cells over continuous high-frequencies. Nat Commun, 2021. 12(1): p. 2922.