arXiv subjects

Yun Yong Terh

Publications and source records attributed to Yun Yong Terh.

3 recordsLinked to original sources

arXiv2024

Topological disclination states and charge fractionalization in a non-Hermitian lattice

We show that a non-Hermitian lattice with a disclination can host topological disclination states that are induced by on-site gain and loss. The disclination states are inherently non-Hermitian as they do not exist in the limit of zero gain/loss. They arise from charge fractionalization in the non-Hermitian lattice, which we establish using non-Hermitian Wilson loops calculated with biorthogonal products. The model can be realized using an array of optical resonators, with the emergence of the topological disclination states manifesting as an abrupt shift in emission intensity and frequency upon tuning the gain/loss level.

cond-mat.mes-hall↗

arXiv2024

Dirac mass induced by optical gain and loss

Mass is commonly regarded as an intrinsic property of matter, but modern physics reveals particle masses to have complex origins, such as the Higgs mechanism in high-energy physics. In crystal lattices such as graphene, relativistic Dirac particles can exist as low-energy quasiparticles with masses imparted by lattice symmetry-breaking perturbations. These mass-generating mechanisms all assume Hermiticity, or the conservation of energy in detail. Using a photonic synthetic lattice, we show experimentally that Dirac masses can be generated via non-Hermitian perturbations based on optical gain and loss. We then explore how the space-time engineering of the gain/loss-induced Dirac mass affects the quasiparticles. As we show, the quasiparticles undergo Klein tunnelling at spatial boundaries, but a local breaking of a non-Hermitian symmetry can produce a novel flux nonconservation effect at the domain walls. At a temporal boundary that abruptly flips the sign of the Dirac mass, we observe a variant of the time reflection phenomenon: in the nonrelativistic limit, the Dirac quasiparticle reverses its velocity, while in the relativistic limit the original velocity is retained.

cond-mat.mes-hall↗

arXiv2023

Scattering Dynamics and Boundary States of a Non-Hermitian Dirac Equation

We study a non-Hermitian variant of the (2+1)-dimensional Dirac wave equation, which hosts a real energy spectrum with pairwise-orthogonal eigenstates. In the spatially uniform case, the Hamiltonian's non-Hermitian symmetries allow its eigenstates to be mapped to a pair of Hermitian Dirac subsystems. When a wave is transmitted across an interface between two spatially uniform domains with different model parameters, an anomalous form of Klein tunneling can occur, whereby reflection is suppressed while the transmitted flux is substantially higher or lower than the incident flux. The interface can even function as a simultaneous laser and coherent perfect absorber. Remarkably, the violation of flux conservation occurs entirely at the interface, as no wave amplification or damping takes place in the bulk. Moreover, at energies within the Dirac mass gaps, the interface can support exponentially localized boundary states with real energies. These features of the continuum model can also be reproduced in non-Hermitian lattice models.

cond-mat.mes-hall↗