A type I defect and new integrable boundary conditions for the coupled nonlinear Schr\"odinger equation

TL;DR

This paper introduces a type I defect condition and new integrable boundary conditions for the coupled nonlinear Schrödinger equation, demonstrating their integrability through conserved quantities, classical r-matrix, and Sklyanin's approach.

Contribution

It presents a novel type I defect condition via a Bäcklund transformation and new integrable boundary conditions involving time derivatives, expanding the integrability framework for coupled NLS.

Findings

Existence of an infinite set of conserved quantities for the defect system

Construction of new boundary conditions with non-constant K matrices

Proof of integrability using classical r-matrix and Sklyanin's approach

Abstract

We study two integrable systems associated with the coupled NLS equation: the integrable defect system and the integrable boundary systems. Regarding the first one, we present a type I defect condition, which is described by a B\"{a}cklund transformation frozen at the defect location. For the resulting defect system, we prove its integrability both by showing the existence of an infinite set of conserved quantities and by implementing the classical $r$-matrix method. Regarding the second one, we present some new integrable boundary conditions for the coupled NLS equation by imposing suitable reductions on the defect conditions. Our new boundary conditions, unlike the usual boundary conditions (such as the Robin boundary), involve time derivatives of the coupled NLS fields and are characterised by non constant $K(\lambda)$ matrices. We prove the integrability of our new boundary conditions by using Sklyanin's approach.