arXiv subjects

Ankur Das

Publications and source records attributed to Ankur Das.

23 records · Page 2Linked to original sources

arXiv2021

Coexistence of Canted Antiferromagnetism and Bond-order in $ν=0$ Graphene

Motivated by experimental studies of graphene in the quantum Hall regime, we revisit the phase diagram of a single sheet of graphene at charge neutrality. Because of spin and valley degeneracies, interactions play a crucial role in determining the nature of ground state. We show that, generically, in the regime of interest there is a region of coexistence between magnetic and bond orders in the phase diagram. We demonstrate this result both in continuum and lattice models, and argue that the coexistence phase naturally provides an explanation for unreconciled experimental observations on the quantum Hall effect in graphene.

cond-mat.mes-hall↗

arXiv2020

On the topological character of three-dimensional Nexus triple point degeneracies

Recently a generic class of three-dimensional band structures was identified that host two-fold line degeneracies meeting at three-fold or triple point degeneracies, which resist the usual topological characterization of isolated point degeneracies as in Dirac/Weyl semimetals. For these so-called ``Nexus" fermions which lie beyond Dirac/Weyl fermions, we lay out several concepts to characterize the wavefunction geometry and spell out its topology. Our approach is based on an understanding of the analyticity properties of Nexus wavefunctions building on a two-dimensional analogue studied recently by us. We use this to write down a homological classification of various Nexus triple point degeneracies in three dimensions.

cond-mat.mes-hall↗

arXiv2020

SU(3) fermions in a three-band Graphene-like model

Two-dimensional Graphene is fascinating because of its unique electronic properties. From a fundamental perspective, one among them is the geometric phase structure near the Dirac points in the Brillouin zone, owing to the SU(2) nature of the Dirac cone wave functions. We ask if there are geometric phase structures in two dimensions which go beyond that of a Dirac cone. Here we write down a family of three-band continuum models of non-interacting fermions which have more intricate geometric phase structures. This is connected to the SU(3) nature of the wavefunctions near three-fold degeneracies. We also give a tight-binding free fermion model on a two-dimensional Graphene-like lattice where the three-fold degeneracies are realized at fine-tuned points. Away from them, we obtain new "three-band" Dirac cone structures with associated non-standard Landau level quantization, whose organization is strongly affected by the non-SU(2) or beyond-Dirac geometric phase structure of the fine-tuned points.

cond-mat.str-el↗

arXiv2019

Stability of zero energy Dirac touchings in the honeycomb Hofstadter problem

We study the band structure of electrons hopping on a honeycomb lattice with 1/q (q integer) flux quanta through each elementary hexagon. In the nearest neighbor hopping model the two bands that eventually form the n = 0 Landau level have 2q zero energy Dirac touchings. In this work, we study the conditions needed for these Dirac points and their stability to various perturbations. We prove that these touchings and their locations are guaranteed by a combination of an anti-unitary particle-hole symmetry and the lattice symmetries of the honeycomb structure. We also study the stability of the Dirac touchings to one-body perturbations that explicitly lower the symmetry.

cond-mat.mes-hall↗

arXiv2018

Phase Transition In $SU(N)\times U(1)$ Gauge Theory With Many Fundamental Bosons

Here we study the Renormalization group flow of $SU(N)\times U(1)$ gauge theory with $M$-fundamental bosons in $4-ε$ dimension by calculating the beta functions. We found a new stable fixed point in the zero mass plane for $M>M_\text{crit}$ by expanding upto $O(ε)$. This indicates a second order phase transition. We also calculated the critical exponents in both $ε$ expansion and also in the large-$M$ expansion.

cond-mat.stat-mech↗