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Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

Mon, 10 Apr 2023

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1.Identifying dissipative phase transitions from entropy and conductance

Authors:Zhanyu Ma, Cheolhee Han, Yigal Meir, Eran Sela

Abstract: Dissipative phase transitions (DPT) occur when a small quantum system interacts with a bath of harmonic oscillators. At equilibrium, DPTs are accompanied by an entropy change, signaling the loss of coherence. Despite extensive efforts, equilibrium DPTs have yet to be observed. Here, we demonstrate that ongoing experiments on double quantum dots that measure entropy using a nearby quantum point contact (QPC) realize the celebrated spin-boson model and allow to measure the entropy change of its DPT. We find a Kosterlitz-Thouless flow diagram, leading to a universal jump in the spin-bath interaction, reflected in a discontinuity in the zero temperature QPC conductance.

2.Chiral chains with two valleys and disorder of finite correlation length

Authors:Jean-Baptiste Touchais, Pascal Simon, Andrej Mesaros

Abstract: In one-dimensional disordered systems with a chiral symmetry it is well-known that electrons at energy $E = 0$ avoid localization and simultaneously exhibit a diverging density of states (DOS). For $N$ coupled chains with zero-correlation-length disorder, the diverging DOS remains for odd $N$, but a vanishing DOS is found for even $N$. We use a thin spinless graphene nanotube with disordered Semenoff mass and disordered Haldane coupling to construct $N = 2$ chiral chain models which at low energy have two linear band crossings at different momenta $\pm K$ (two valleys) and disorder with an arbitrary correlation length $\xi$ in units of lattice constant $a$. We find that the finite momentum $\pm K$ forces the disorder in one valley to depend on the disorder in the other valley, thus departing from known analytical results which assume having $N$ independent disorders (whatever their spatial correlation lengths). Our main numerical results show that for this inter-dependent mass disorder the DOS is also suppressed in the limit of strongly coupled valleys (lattice-white noise limit, $\xi/a = 0$) and exhibits a non-trivial crossover as the valleys decouple ($\xi/a\gtrsim 5$) into the DOS shapes of the $N = 1$ continuum model with finite correlation length $\xi$. We also show that changing the intra-unit-cell geometry of the disordered Haldane coupling can tune the amount of inter-valley scattering yet at lowest energies it produces the decoupled-valley behavior ($N = 1$) all the way down to lattice white noise.

3.Effect of Inversion Asymmetry on Bilayer Graphene's Superconducting and Exciton Condensates

Authors:Xiang Hu, Enrico Rossi, Yafis Barlas

Abstract: Inversion asymmetry in bilayer graphene can be tuned by the displacement field. As a result, the band dispersion in biased bilayer graphene acquires flat band regions near the Dirac points along with a non-trivial band geometry. We analyze the effect of inversion symmetry on the critical temperature and superfluid stiffness of the superconducting state of AB-stacked graphene bilayer and on the exciton condensate in double layers formed by two AB-stacked graphene bilayers. The geometric superfluid stiffness in bilayer graphene superconductors is found to be negligible due to the small superconducting gap. Furthermore, we show that the geometric superfluid stiffness is maximized for a constant order parameter. Therefore, it can be neglected in biased bilayer graphene superconductors with any pairing symmetry. However, the displacement field enhances the geometric superfluid stiffness in exciton condensates. It is most prominent at low densities and high displacement fields. A consequence of the geometric superfluid stiffness is a modest enhancement of the Berezinskii-Kosterlitz-Thouless transition temperature in bilayer graphene's exciton condensate.