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