Electric field driven flat bands: Enhanced magnetoelectric and electrocaloric effects in frustrated quantum magnets

TL;DR

This paper demonstrates that applying an electric field to a frustrated quantum spin chain can induce flat-band phenomena, leading to enhanced magnetoelectric and electrocaloric effects, thus broadening the scope of flat-band physics in materials.

Contribution

It introduces a method to induce flat-band conditions via electric fields in quantum magnets, enabling control over magnetization and polarization effects without fine-tuning exchange interactions.

Findings

Electric field induces flat-band scenarios in quantum spin chains.

System exhibits a magnetization jump driven by electric field.

Enhanced magnetocaloric and electrocaloric effects observed.

Abstract

The $J_1$-$J_2$ quantum spin sawtooth chain is a paradigmatic one-dimensional frustrated quantum spin system exhibiting unconventional ground-state and finite-temperature properties. In particular, it exhibits a flat energy band of one-magnon excitations accompanied by an enhanced magnetocaloric effect for two singular ratios of the basal interactions $J_1$ and the zigzag interactions $J_2$. In our paper, we demonstrate that one can drive the spin system into a flat-band scenario by applying an appropriate electric field, thus overcoming the restriction of fine-tuned exchange couplings $J_1$ and $J_2$ and allowing one to tune more materials towards flat-band physics, that is to show a macroscopic magnetization jump when crossing the magnetic saturation field, a residual entropy at zero temperature as well as an enhanced magnetocaloric effect. While the magnetic field acts on the spin system via the ordinary Zeeman term, the coupling of an applied electric field with the spins is given by the sophisticated Katsura-Nagaosa-Balatsky (KNB) mechanism, where the electric field effectively acts as a Dzyaloshinskii-Moriya spin-spin interaction. The resulting novel features are corresponding reciprocal effects: We find a magnetization jump driven by the electric field as well as a jump of the electric polarization driven by the magnetic field, i.e.\ the system exhibits an extraordinarily strong magnetoelectric effect. Moreover, in analogy to the enhanced magnetocaloric effect the system shows an enhanced electrocaloric effect.