Fractional Solitons in Excitonic Josephson Junctions

Abstract

The Josephson effect is especially appealing to physicists because it reveals macroscopically the quantum order and phase. In excitonic bilayers the effect is even subtler due to the counterflow of supercurrent as well as the tunneling between layers (interlayer tunneling). Here we study, in a quantum Hall bilayer, the excitonic Josephson junction: a conjunct of two exciton condensates with a relative phase ϕ0 applied. The system is mapped into a pseudospin ferromagnet then described numerically by the Landau-Lifshitz-Gilbert equation. In the presence of interlayer tunneling, we identify a family of fractional sine-Gordon solitons which resemble the static fractional Josephson vortices in the extended superconducting Josephson junctions. Each fractional soliton carries a topological charge Q that is not necessarily a half/full integer but can vary continuously. The calculated current-phase relation (CPR) shows that solitons with Q = ϕ0/2π is the lowest energy state starting from zero ϕ0 – until ϕ0 > π – then the alternative group of solitons with Q = ϕ0/2π − 1 takes place and switches the polarity of CPR.

Single interface effects dominate in exciton-condensate/normal-barrier/exciton-condensate (EC/N/EC) structures of long-barrier

“…the crossover happens at L∼3λ. We believe that the finding summarized here on the EC/N/EC structures of long barrier, together with our previous discussion [52] on the same structure of short-barrier can shed light on the realization of excitonic-based structures and fractional solitons-which might lead to new types of quantum qubits.…”

“…When the enclosing normal-barrier is short, this EC/N/EC structure resembles a superconducting Josephson junction [47,[49][50][51] but with coherent interlayer tunneling [3,[6][7][8][9][10] ; such structure is also referred to as the excitonic Josephson junction (EJJ). In our previous work of EJJ [52], we found exotic fractional soliton, an object that carries a topological charge of Q≡f 0 /2π, which resembles fractional vortices [53][54][55][56] in 0-κ superconducting Josephson junction [57,58]. This type of topological objects is a potential candidate for fluxor phase-base qubits.…”

“…We use the developed lattice model [52,[65][66][67] and tailor it to describe specifically our EC/N/EC structures. Consider that the electrons can only be in either top or bottom layer, the wave function of the eigenstate can be generally expressed as:…”

Pump-driven normal-to-excitonic insulator transition: Josephson oscillations and signatures of BEC-BCS crossover in time-resolved ARPES

“…Josephson oscillations between two superconducting electrodes separated by a thin insulator arise by applying a DC voltage to the equilibrium junction, thus effectively introducing a difference in the electrochemical potentials. This mechanism has recently been investigated by replacing the superconductors with EI electrodes [52][53][54][55]. The NEQ-EI oscillations in Eq.…”

Spatially indirect exciton condensate phases in double bilayer graphene

“…They are counterflow superfluids and exhibit, among other properties, spontaneous phase coherence between electrically isolated subsystems. These condensates can be manipulated by external electrical contacts [14][15][16][17] via an excitonic generalization of Andreev scattering. Excitonic superflow is manifested most explicitly in a variety of transport experiments, 18,19 in which the electrically isolated two-dimensional electron systems are contacted independently.…”