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

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 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.

Automated summary

The J(1)-J(2) quantum spin sawtooth chain is a one-dimensional frustrated quantum spin system that exhibits unconventional ground-state and finite-temperature properties. By applying an appropriate electric field, the system can be driven into a flat-band scenario, overcoming the restriction of fine-tuned exchange couplings and allowing for enhanced magnetocaloric effects. This interaction between electric field and spins via the Katsura-Nagaosa-Balatsky mechanism results in strong magnetoelectric effects and enhanced electrocaloric effects.