Unveiling the Phase Diagram and Nonlinear Optical Responses of a Twisted Kitaev Chain

TL;DR

This paper proposes a twisted Kitaev model for CoNb$_2$O$_6$, demonstrating how terahertz 2D coherent spectroscopy can detect weak Kitaev interactions through characteristic nonlinear optical signals related to spin excitations.

Contribution

It introduces a calibrated twisted Kitaev model for CoNb$_2$O$_6$ and shows how 2DCS can identify fractionalized excitations in quantum magnets.

Findings

Revealed discrete peaks in 2DCS signals from spin excitations.

Linked 2DCS signals to two-spinon and four-spinon processes.

Demonstrated detection of weak Kitaev interactions in quantum materials.

Abstract

Detecting Kitaev interactions in real materials remains challenge, as conventional experimental techniques often have difficulty distinguishing fractionalized excitations from other normal contributions. Terahertz two-dimensional coherent spectroscopy (2DCS) offers a novel approach for probing many-body phenomena, such as exotic excitations in quantum magnets. Motivated by recent experiments on CoNb$_2$O$_6$ and the development of the terahertz spectroscopy in Kitaev quantum spin liquid, we proposed a twisted Kitaev model for CoNb$_2$O$_6$ and determined the precise twist angle according to experimental specific-heat phase diagram. With this calibrated model, we found that non-rephasing diagonal and rephasing anti-diagonal signals appear in the 2DCS nonlinear response. The $x$ and $y$ components of the spin superexchange interactions split the rephasing signals into a grid of discrete peaks. We further demonstrate that the diagonal and the discrete rephasing signals primarily originate from two-spinon and four-spinon excitation processes based on numerical projection method. These findings indicate that even weak Kitaev interactions in quantum materials can be effectively detected via two-dimensional coherent spectroscopy .