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.

Herbert Fertig, Indiana University

Quantum Geometric Dipole in Collective Excitations

When: 12:00-13:00 CET, December 19th (Monday), 2022

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

In recent years it has become increasingly appreciated that electrons in solids possess quantum geometric structure that impact the electronic properties of the system. Typically, this takes the form of a Berry curvature which contributes to the electron velocity in its response to external fields. In this talk we discuss quantum geometric properties of collective modes of electronic materials, focusing on those that can be described as two-body excitations. We show that generally such excitations possess their own type of geometric measure, closely related to an electric dipole moment, which we call the quantum geometric dipole (QGD). We will focus on two examples of this: excitons in semiconducting systems, and plasmons in two-dimensional metals. We show that for excitons, a non-zero QGD appears when there is no effective Lorentz invariance in the system, even at long wavelengths, and that its presence leads to a perpendicular exciton drift in an electric field. For the case of plasmons, we consider the impact of the QGD on scattering from a circularly symmetric potential, showing that the QGD necessarily gives rise to non-reciprocal behavior. In general the presence of a non-vanishing QGD impacts the dynamics of these collective modes, and we discuss some implications for experiment.

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

Maryam Khosravian, Aalto University

Impurity-induced excitations in a topological two-dimensional ferromagnet/superconductor van der Waals moiré heterostructure

When: 12:00-13:00 CET, November 24th (Thursday), 2022

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

The emergence of a topological superconducting state in van der Waals heterostructures provides a new platform for exploring novel strategies to control topological superconductors. In particular, impurities in van der Waals heterostructures, generically featuring a moiré pattern, can potentially lead to the unique interplay between atomic and moiré length scales, a feature absent in generic topological superconductors. Here we address the impact of nonmagnetic impurities on a topological moiré superconductor, both in the weak and strong regime, considering both periodic arrays and single impurities in otherwise pristine infinite moiré systems. We demonstrate a fine interplay between impurity-induced modes and the moiré length, leading to radically different spectral and topological properties depending on the relative impurity location and moiré lengths. Our results highlight the key role of impurities in van der Waals heterostructures featuring moiré patterns, revealing the key interplay between length and energy scales in artificial moiré systems.

[1] Phys. Rev. Materials 6, 094010(2022).

José C. Abadillo-Uriel, CEA Grenoble

Manipulating and extending the coherence of hole spins

When: 12:00-13:00 CET, November 16th (Wednesday), 2022

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

Hole spin qubits in semiconductor quantum dots afford the advantage of efficient electrical control. This control is enabled by the strong spin-orbit interaction (SOI) in the  valence band of semiconductors, which couples the spin to the real-space motion of the hole in the applied electric fields. In this talk, I will present our recent theoretical and experimental progress on hole spin qubits. We show that the intrinsic SOI of the semiconductor valence band offers unique mechanisms to manipulate the hole spins [1]. While this electrical susceptibility couples the hole spin to charge noise, I will show that hole qubits can be engineered to minimize decoherence at sweet spots [2, 3]. Finally, I will cover how the SOI allows the coupling of the hole spin to cavity photons [3, 4], going well beyond what has been achieved with electron spins and paving the way toward a long-range photon-mediated two-qubit gate.

YouTube link: https://www.youtube.com/watch?v=bnOnMz8Ek5U

[1] B. Martínez, JC Abadillo-Uriel, et al., arXiv:2209.10231
[2] N. Piot et al., Nat. Nano. 17, 1072–1077 (2022)
[3] Michal, JC Abadillo-Uriel, et al., arXiv:2204.00404
[4] C. Yu et al., arXiv:2206.14082

Jens Paaske, Niels Bohr Institute

Microwave response of superconducting sub-gap states

When: 11:00-12:00 CET, November 10th (Thursday), 2022

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

Josephson junctions spanning a Coulomb-blockaded quantum dot host subgap states with a characteristic dispersion with phase difference and gate voltage, which to a large extent determine the microwave response of the junction. In this lecture, I will present our calculations of this linear microwave response, with special emphasis on spinful (odd occupied) quantum dots giving rise to Yu-Shiba-Rusinov bound states and the accompanying interaction driven quantum phase transition from π- to 0-junction behavior. I shall also discuss the intricate dc current response of a Josephson junction based on a double quantum dot with two phase shifted microwave tones on the individual gate voltages. This is shown to lead to a tunable phi_0 junction and to allow for supercurrent rectification.

YouTube link: https://www.youtube.com/watch?v=gFviD0Ktypc

[1] Cecilie Hermansen, Alfredo Levy Yeyati, JP, Phys. Rev. B 105, 054503 (2022)
[2] Carlos Ortega-Taberner, Antti-Pekka Jauho, JP; arXiv:2207.06152