Tilted-Cone-induced easy-plane pseudo-spin ferromagnet and Kosterlitz-Thouless transition in massless Dirac fermions

TL;DR

This paper investigates the quantum Hall ferromagnetism in massless Dirac fermions with tilted cones, revealing an easy-plane pseudospin ferromagnetism and a Kosterlitz-Thouless transition, with implications for experimental resistivity data.

Contribution

It demonstrates how tilted Dirac cones induce easy-plane pseudospin ferromagnetism and a KT transition in massless Dirac fermions, extending understanding of quantum Hall states in such systems.

Findings

Tilted cones lead to non-zero inter-valley scattering.

Easy-plane pseudospin ferromagnetism emerges due to tilt.

Kosterlitz-Thouless transition occurs at low temperatures.

Abstract

The possible quantum Hall ferromagnet at a filling factor $\nu =0$ is investigated for the zero-energy (N=0) Landau level of the two dimensional massless Dirac fermions in $\alpha$-(BEDT-TTF)$_2$I$_3$ under pressure with tilted cones and a twofold valley degeneracy resulting from time-reversal symmetry. In the case of the Dirac cones without tilting, the long-range Coulomb interaction in the N=0 Landau level exhibits the SU(2) valley-pseudo-spin symmetry even to the order $O(a/l_{\rm H})$, in contrast to $N \ne 0$ Landau levels, where $a$ and $l_{\rm H}$ represent the lattice constant and the magnetic length, respectively. Such a characteristic comes from a fact that zero-energy states in a particular valley are restricted to only one of the spinor components, whereas the other spinor component is necessarily zero. In the case of the tilted Dirac cones as found in $\alpha$-(BEDT-TTF)$_2$I$_3$, one obtains a non-zero value of the second component and then the ackscattering processes between valleys becomes non-zero. It is shown that this fact can lead to easy-plane pseudospin ferromagnetism (XY-type). In this case, the phase fluctuations of the order parameters can be described by the XY model leading to Kosterlitz-Thouless transition at lower temperature. In view of these theoretical results, experimental findings in resistivity of $\alpha$-(BEDT-TTF)$_2$I$_3$ are discussed.