Quantum phase transitions: microscopic scale and Planckian time
le mardi 07 mai 2024 à 14:00
Séminaire nano-électronique quantique
Personne à contacter : Jeremie Viennot ()
Lieu : Salle Rémy Lemaire K223, Institut Néel
Résumé : For more than thirty years, experimental analysis of quantum phase transitions (QPTs) has been
largely focused on finding critical exponents and universality classes of studied systems. This
approach emphasizes scale-invariance of QPTs and ignores the fact that system response also
depends on two non-universal length scales: microscopic “seeding” scale of the correlation
length and the dephasing length. Correcting this deficiency, we have developed a
phenomenological model of QPTs based on conjecture that the dephasing length is set by a
distance travelled by a system-specific semi-classical elementary excitation over the Planckian
time, and that the scaling function assumes the generic exponential form predicted
by the scaling theory of localization (the figure shows some examples). Using this model, we
have quantitatively explained QPTs in eighteen systems including: magnetic-field-driven QPT
in superconducting films, nanowires, La1.92Sr0.08CuO4 and Josephson junction chains; QPT in
Ising and Heisenberg spin chains, the Mott transition in 2d cold atomic gases and moiré
superlattices; and QPT between the states of quantum Hall and other topological insulators. The
model illuminates the universal microscopic nature of many-body gapless state of matter
emerging near QPTs. Surprisingly, the only system deviating from the trend is doped Si : P,
where metal-insulator transition is explained by the non-interaction version of the model. We
anticipate that shifting emphasis from critical exponents to the microscopic parameters of a
phase transition will be a fruitful approach for many systems beyond equilibrium condensed
matter physics.
Ref. :
[1] A. Rogachev, Microscopic scale of quantum phase transition: from doped semiconductors to
spin chains, cold gases and moiré superlattices, arXiv:2309.00749.
[2] A. Rogachev and K. Davenport, Microscopic scale of pair-breaking quantum phase
transitions in superconducting films, nanowires and La1.92Sr0.08CuO4, arXiv:2309.00747.
[3] A. Rogachev, Quantum phase transitions in quantum Hall and other topological systems: role
of the Planckian time, arXiv:2309.00747.
Liens :
Stephan Roche ( ICREA Research Professor at Institut Català de Nanociència i Nanotecnologia (ICN2))
Ten Years of 2D Materials based Spintronics Research: Highlights and Future
le mardi 14 mai 2024 à 14:00
Séminaire QuantAlps
Personne à contacter : Michele Filippone ()
Lieu : Bât. GreEn-Er (Salle à confirmer)
Résumé :
In this talk, I will review more than 10 years of research activities of concerning the exploration of the potential of two-dimensional materials and van der Waals heterostructures for spintronic applications. Starting from the first demonstration of long spin diffusion length in graphene to most recent breakthroughs of active spin devices, we will discuss the progress achieved within the Spintronics workpackage of the Graphene Flagship (from 2013-2023), highlighting the main milestones of the field and the current perspective and challenges concerning the use of 2D materials for spintronic applications as recently presented by a consortium of leading groups in the field (H. Yang et al. Nature 606 (7915), 663-673 (2022)).
I acknowledge the European Union Seventh Framework Program under Grant Agreement No. 881603 Graphene Flagship.
Theory of free fermion dynamics - from monitored to post selected evolution
le vendredi 17 mai 2024 à 11:00
Séminaire théorie
Personne à contacter : Serge Florens ()
Lieu : G421
Résumé : Monitored quantum systems undergo Measurement-induced Phase Transitions (MiPTs) stemming from the interplay between measurements and unitary dynamics. When the detector readout is post- selected to match a given value, the dynamics is generated by a Non-Hermitian Hamiltonian with MiPTs characterized by different universal features. Here, we derive a partial post-selected stochastic Schrodinger equation based on a microscopic description of continuous weak measurement. This formalism connects the monitored and post-selected dynamics to a broader family of stochastic evolution. We apply the formalism to a chain of free fermions subject to partial post-selected monitoring of local fermion parities. Within a 2-replica approach, we obtained an effective bosonized Hamiltonian in the strong post-selected limit. Using a renormalization group analysis, we find that the universality of the non-Hermitian MiPT is stable against a finite (weak) amount of stochasticity. We further show that the passage to the monitored universality occurs abruptly at finite partial post-selection, which we confirm from the numerical finite size scaling of the MiPT. Our approach establishes a way to study MiPTs for arbitrary subsets of quantum trajectories and provides a potential route to tackle the experimental post-selected problem.
