Sensing strongly correlated electrons in 2D materials
le mardi 21 février 2023 à 14:00
Séminaire nano-électronique quantique
Personne à contacter : Jeremie Viennot ()
Salle Rémy Lemaire K223, Institut Néel et via https://univ-grenoble-alpes-fr.zoom.us/j/91808901596?pwd=UWZ2cml2N1VBOEZBenk0d3RJek9rdz09
Résumé : When the strength of Coulomb interaction between itinerant electrons in a two-dimensional system becomes significantly larger than the kinetic energy, the electrons start to develop strong correlations. A paradigm phase that is expected to emerge in this regime is an electronic Wigner crystal, in which the electrons spontaneously form a periodic lattice mimicking that of atoms in the real crystals. However, in order for this electronic crystallization to occur, the actual ratio of the above energy scales must exceed 30, which turns out to be notoriously difficult to fulfill in conventional semiconductors (e.g., GaAs).
Recently, atomically-thin transition metal dichalcogenides (TMDs) have emerged as a highly-tunable experimental platform that unlocks the access to uncharted territories of strongly correlated electron physics. This is due to reduced dielectric screening and large carrier effective masses, which endow TMD monolayers with excellent optical properties and give rise to strong inter-electron interactions enabling to reach Coulomb-to-kinetic energy ratios being more than an order of magnitude larger than that for the GaAs at comparable electron densities.
In this talk, I will review our recent optical investigations of landmark correlated phases in charge-controlled TMD-based van der Waals heterostructures. In particular, I will present our novel spectroscopic technique enabling us to detect the Wigner crystal in a TMD monolayer through the periodic potential it generates for the excitons (1). In the presence of this potential, the excitons Bragg scatter off the Wigner crystal, which gives rise to the emergence of a Bragg-umklapp resonance in the reflectance spectrum that heralds the presence of an electronic lattice.
Our observation of this resonance provides the first unequivocal evidence for the formation of the Wigner crystal that has been thus far probed only by indirect methods in two-dimensional systems. In the second part of the talk, I will also show how the Rydberg excitons in a TMD monolayer can be exploited to optically probe the formation of correlated electronic phases in an adjacent graphene layer (2), which is otherwise optically inaccessible owing to the lack of a robust energy gap. I will demonstrate that this approach allows for sensing fractional quantum Hall effect in graphene with a similar sensitivity to that of state-of-the-art transport tools.
References:
(1) T. Smolenski, P. E. Dolgirev, C. Kuhlenkamp, A. Popert, Y. Shimazaki, P. Back, X. Lu, M. Kroner, K. Watanabe, T. Taniguchi, I. Esterlis, E. Demler, and A. Imamoglu, Nature 595, 53-57 (2021).
(2) A. Popert, Y. Shimazaki, M. Kroner, K. Watanabe, T. Taniguchi, A. Imamoglu, and T. Smolenski, Nano Letters 22, 7636 (2022)
Bloch oscillations in the magnetoconductance of twisted bilayer graphene
le mercredi 22 février 2023 à 11:00
Séminaire LPMMC
Personne à contacter : Vincent Rossetto ()
G 421
Résumé : We identify a mapping between two-dimensional (2D) electron transport in
a minimally twisted graphene bilayer and a 1D quantum walk, where one
spatial dimension plays the role of time. In this mapping a magnetic
field B perpendicular to the bilayer maps onto an electric field. Bloch
oscillations due to the periodic motion in a 1D Bloch band can then be
observed in purely DC transport as magnetoconductance oscillations with
periodicity set by the Bloch frequency.
Ref : Phys. Rev. B 105, L241408 (2022)
Can deep sub-wavelength cavities induce Amperean superconductivity in a 2D material?
le vendredi 24 février 2023 à 11:00
Séminaire théorie
Personne à contacter : Serge Florens ()
G 421
Résumé : Amperean superconductivity is an exotic phenomenon stemming from attractive effective electron-electron interactions (EEEIs) mediated by a transverse gauge field. Originally introduced in the context of quantum spin liquids and high-Tc superconductors, Amperean superconductivity has been recently proposed to occur at temperatures on the order of 1-20 K in two-dimensional, parabolic-band, electron gases embedded inside deep sub-wavelength optical cavities.
I will first generalize the microscopic theory of cavity-induced Amperean superconductivity to the case of graphene and then argue that this superconducting state cannot be achieved in the deep sub-wavelength regime. In the latter regime, indeed, a cavity induces only EEEIs between density fluctuations rather than the current-current interactions which are responsible for Amperean pairing.
Analytical description of the superfluid stiffness in strongly disordered superconductors
le mercredi 15 mars 2023 à 11:00
Séminaire interne LPMMC
Personne à contacter : Denis Basko ()
G 421
Résumé : The problem of the temperature dependence of the superfluid stiffness Θ in strongly disordered pseudo-gapped superconductors is relevant for the search of superconducting superinductors – the devices much wanted for several fields of modern quantum technology. The key theoretical challenge is twofold: 1) one has to consistently describe the substantially inhomogeneous superconducting state, known to exist in such superconductors [1,2] even moderately far from the superconductor-insulator transition, and 2) after that one has to calculate the macroscopic electromagnetic response of the resulting disorder media, a challenging problem in its own right. In this talk, we address both of these problems and derive an analytical equation that describes the temperature variation of the superfluid stiffness. At low temperatures, the temperature variation of the superfluid stiffness is close to a power law, Θ(T = 0) - Θ(T) ∝ Tb, where the exponent b ~ 2-3 is disorder-dependent. This result is in qualitative agreement with the experimental results obtained recently by Thibault Charpentier from the Néel Institute.
[1] B. Sacépé, T. Dubouchet, C. Chapelier, M. Sanquer, M. Ovadia, D. Shahar, M.V. Feigel'man and L.B. Ioffe, "Localization of preformed Cooper pairs in disordered superconductors", Nature Physics 7, 3 (2011), pp. 239–244.
[2] A.V. Khvalyuk and M.V. Feigel'man, "Distribution of the order parameter in strongly disordered superconductors: An analytic theory", Phys. Rev. B 104 (2021), pp. 224505.
