Abstract
The electron-electron Coulomb interaction in Dirac-Weyl semimetals harbours a novel paradigm of correlation effects that hybridizes diverse realms of solid-state physics with their relativistic counterpart. Driving spontaneous mass acquisition, the excitonic condensate of strongly-interacting massless Dirac fermions is one such example whose exact nature remains debated. Here, by focussing on the two-dimensional tilted Dirac cones in the organic salt $\alpha$-(BEDT-TTF)$_2$I$_3$, we show that the excitonic instability is controlled by a small chemicalpotential shift and an in-plane magnetic field. In combined analyses based on renormalization-group approaches and ladder approximation, we demonstrate that the nuclear relaxation rate is an excellent probe of excitonic-spin fluctuations in an extended parameter region. Comparative nuclear magnetic resonance (NMR) experiments show good agreements with this result, jointly revealing the importance of intervalley nesting between field-induced, spin-split Fermi pockets of opposite charge polarities. Our work provides an accurate framework to search for excitonic instability of strongly-interacting massless fermions.
Highlights
The notable electronic properties of graphene as well as topological semimetals and insulators have been attracting increasing attention because of their exotic topological nature, and due to their unusual effects induced by the electron-electron Coulomb interaction [1,2,3,4,5,6,7]
Chiral excitonic instability and its impact on the spinlattice relaxation rate in pressurized α-(BEDT-TTF)2I3 with small charge off-neutrality (μ = 0) at in-plane magnetic field H is determined by two logarithmically reshaped massless Dirac cones for even-parity, spin-triplet, and intervalley pairings. (Notice that this is a natural extension of Ref. [11], performed at μ = 0 and small H, to a more general case.) At a weak-coupling mean-field-level treatment of gap equation, we find that the contribution from the two cones to intervalley excitonic response strongly depends on μ and H
We have investigated excitonic instability of the continuum model for the pressurized organic conductor α-(BEDT-TTF)2I3, hosting two massless charge-neutral cones with a tilted dispersion relation
Summary
The notable electronic properties of graphene as well as topological semimetals and insulators have been attracting increasing attention because of their exotic topological nature, and due to their unusual effects induced by the electron-electron Coulomb interaction [1,2,3,4,5,6,7]. The quasi-2D electron system in the organic conductor α-(BEDT-TTF)2I3 [where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene] provides a good example of this, where extensive studies have revealed the presence of a pair of 2D tilted Dirac cones under hydrostatic pressure [10,11,45,46,47,48,49,50,51,52,53] that are charge neutral by 3/4 filling of the electronic band [54,55,56,57,58,59,60,61] In this system, the cones are isotropic but canted towards each valley [Fig. 1(a)], which in an in-plane H lifts the spin degeneracy and generates elliptic Fermi pockets for the electron and hole bands near the crossing points (at ±k0), where the electron pocket is relatively shifted from the hole pocket in opposite directions at the two valleys [Fig. 1(b)]. Details of calculations and supportive experiments are given in the Supplemental Material [67]
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