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Register a new userQuantum metric and effective mass of a two-body bound state in a flat band
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We consider two-body bound states in a flat band of a multiband system. The existence of pair dispersion predicts the possibility of breaking the degeneracy of the band and creating order, such as superconductivity. Within a separable interaction potential approximation, we find that finiteness of the effective mass of a bound pair is determined by a band structure invariant, which in the uniform case becomes the quantum metric. The results offer a simple foundation to understand and predict flat band superconductivity. We propose an experiment to test the interaction-induced pair motion.
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We report on the engineering of a non-dispersive (flat) energy band in a geometrically frustrated lattice of micro-pillar optical cavities. By taking advantage of the non-hermitian nature of our system, we achieve bosonic condensation of exciton-polaritons into the flat band. Due to the infinite effective mass in such band, the condensate is highly sensitive to disorder and fragments into localized modes reflecting the elementary eigenstates produced by geometric frustration. This realization offers a novel approach to studying coherent phases of light and matter under the controlled interplay of frustration, interactions and dissipation.
We study the dynamics of the Bogoliubov wave packet in superconductors and calculate the supercurrent carried by the wave packet. We discover an anomalous contribution to the supercurrent, related to the quantum metric of the Bloch wave function. This anomalous contribution is most important for flat or quasiflat bands, as exemplified by the attractive Hubbard models on the Creutz ladder and sawtooth lattice. Our theoretical framework is general and can be used to study a wide variety of phenomena, such as spin transport and exciton transport.
Recent ab initio lattice studies have found that the interactions between alpha particles (4He nuclei) are sensitive to seemingly minor details of the nucleon-nucleon force such as interaction locality. In order to uncover the essential physics of this puzzling phenomenon without unnecessary complications, we study a simple model involving two-component fermions in one spatial dimension. We probe the interaction between two bound dimers for several different particle-particle interactions and measure an effective potential between the dimers using external point potentials which act as numerical tweezers. We find that the strength and range of the local part of the particle-particle interactions play a dominant role in shaping the interactions between the dimers and can even determine the overall sign of the effective potential.
The recent realization of twisted, two-dimensional, bilayers exhibiting strongly correlated stateshas created a platform in which the relation between the properties of the electronic bands and the nature of the correlated states can be studied in unprecedented ways. The reason is thatthese systems allow an unprecedented control of the electronic bands properties, for exampleby varying the relativetwist angle between the layers forming the system. In particular, in twisted bilayers the low energy bands canbe tuned to be very flat and with a nontrivial quantum metric. This allows the quantitativeand experimental exploration of the relation between the metric of Bloch quantum statesand the properties of correlated states. In this work we first review the general connectionbetween quantum metric and the properties of correlated states that break a continuous symmetry.We then discuss the specific case when the correlated state is a superfluid and show how the quantum metric is related to the superfluid stiffness.To exemplify such relation we show results for the case of superconductivityin magic angle twisted bilayer graphene.We conclude by discussing possible research directions to further elucidatethe connection between quantum metric and correlated states properties.
Superconductivity is a quantum phenomena arising, in its simplest form, from pairing of fermions with opposite spin into a state with zero net momentum. Whether superconductivity can occur in fermionic systems with unequal number of two species distinguished by spin, atomic hyperfine states, flavor, presents an important open question in condensed matter, cold atoms, and quantum chromodynamics, physics. In the former case the imbalance between spin-up and spin-down electrons forming the Cooper pairs is indyced by the magnetic field. Nearly fifty years ago Fulde, Ferrell, Larkin and Ovchinnikov (FFLO) proposed that such imbalanced system can lead to exotic superconductivity in which pairs acquire finite momentum. The finite pair momentum leads to spatially inhomogeneous state consisting of of a periodic alternation of normal and superconducting regions. Here, we report nuclear magnetic resonance (NMR) measurements providing microscopic evidence for the existence of this new superconducting state through the observation of spin-polarized quasiparticles forming so-called Andreev bound states.
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