arXiv subjects

Andrea Blason

Publications and source records attributed to Andrea Blason.

5 recordsLinked to original sources

arXiv2024

Exciton condensation driven by bound states of Green's functions zeros

The interaction driven transition between quantum spin-Hall and Mott insulators in the Bernevig, Hughes and Zhang model is studied by dynamical cluster approximation, and found to be accompanied by the emergence of Green's function zeros already in the quantum spin-Hall regime. The non-trivial interplay between Green's function poles and zeros leads to an exotic quantum spin-Hall insulator exhibiting two chiral branches of edge Green's function poles and one of zeros. When symmetry breaking is allowed, a non-topological excitonic insulator is found to intrude between quantum spin-Hall and Mott insulators. We find evidence that excitons in the Mott insulator, which become soft at the transition to the excitonic insulator, are actually bound states between valence and conduction bands of Green's function zeros, rather than between lower and upper Hubbard bands.

cond-mat.str-el↗

arXiv2024

Luttinger surface dominance and Fermi liquid behaviour of topological Kondo insulators SmB$_6$ and YbB$_{12}$

Defying the traditional classification into metals and insulators, several materials simultaneously display metallic thermal properties and insulating electric behaviour, as if they hosted quasiparticles carrying entropy but not charge. Among them, some materials also possess quantum oscillations in magnetic fields as if they had well-defined Fermi surfaces despite the insulating gap. This remarkable dichotomy has been observed in the topological Kondo insulators SmB$_6$ and YbB$_{12}$. Prompted by the peculiar mixed-valence nature of these compounds, involving $f$ and $d$ electrons of the lanthanide, we propose an explanation of their intriguing properties drawing inspiration from the physics of the pseudogap phase in underdoped cuprates. We argue that the $f$ and $d$ subsystems, when considered separately, act, respectively, as electron- and hole-doped Mott insulators, featuring Fermi pockets coexisting with Luttinger surfaces responsible for the pseudogap. When the two are coupled to each other a hybridisation gap opens up, and the whole turns into a topological insulator endowed with genuine chiral edge states. However, the Luttinger surfaces persist and support neutral quasiparticles. This scenario, supported by numerical simulations within the dynamical cluster approximation, effectively resolves the paradoxical phenomenology of SmB$_6$ and YbB$_{12}$.

cond-mat.str-el↗

arXiv2023

Unified role of Green's function poles and zeros in topological insulators

Green's function zeros, which can emerge only if correlation is strong, have been for long overlooked and believed to be devoid of any physical meaning, unlike Green's function poles. Here, we prove that Green's function zeros instead contribute on the same footing as poles to determine the topological character of an insulator. The key to the proof, worked out explicitly in 2D but easily extendable in 3D, is to express the topological invariant in terms of a quasiparticle thermal Green's function matrix $G_*(iε,\mathbf{k})= 1/\big(iε-H_*(ε,\mathbf{k})\big)$, with hermitian $H_*(ε,\mathbf{k})$, by filtering out the positive definite quasiparticle residue. In that way, the topological invariant is easily found to reduce to the TKNN formula for quasiparticles described by the non-interacting Hamiltonian $H_*(0,\mathbf{k})$. Since the poles of the quasiparticle Green's function $G_*(ε,\mathbf{k})$ on the real frequency axis correspond to poles and zeros of the physical-particle Green's function $G(ε,\mathbf{k})$, both of them equally determine the topological character of an insulator.

cond-mat.mes-hall↗

arXiv2022

Local Kekulé distortion turns twisted bilayer graphene into topological Mott insulators and superconductors

Magic-angle twisted bilayer graphene displays at different fillings of the four flat bands lying around the charge neutrality point a wealth of notable phases that include magnetic Chern insulators, whose magnetization is mostly of orbital nature, and contiguous superconducting domes. Such rich phase diagram is here explained through the positive interplay of Coulomb repulsion and the electron coupling to a twofold optical mode that corresponds to Kekulè distortions localized into the small AA stacked regions of the moiré supercells. A static distortion stabilizes, at any integer filling of the flat bands, valence-bond insulators that carry finite Chern number away from charge neutrality. Similarly, a dynamic distortion that resonates between the two lattice vibrations leads to resonating-valence-bond topological insulators with built-in chiral d-wave pairs that have finite Chern number equal to the angular momentum, and thus are prone to turn superconducting upon doping away from integer filling.

cond-mat.str-el↗

arXiv2019

Quantum model for Impulsive Stimulated Raman Scattering

The interaction between ultrashort light pulses and non-absorbing materials is dominated by Impulsive Stimulated Raman Scattering (ISRS). The description of ISRS in the context of pump\&probe experiments is based on effective classical models describing the interaction between the phonon and pulsed electromagnetic fields. Here we report a theoretical description of ISRS where we do not make any semi-classical approximation and we treat both photonic and phononic degrees of freedom at the quantum level. The results of the quantum model are compared with semiclassical results and validated by means of spectrally resolved pump\&probe measurements on $α$-quartz.

quant-ph↗