Flat band driven competing charge and spin instabilities in the altermagnet CrSb

TL;DR

This study reveals how flat electronic bands in CrSb lead to competing charge and spin orders, causing significant spin-phonon coupling and complex phase behavior near the magnetic transition, highlighting the material's potential for exploring intertwined quantum phases.

Contribution

It demonstrates the role of flat bands in mediating strong spin-phonon coupling and charge-spin competition in CrSb, a simple altermagnetic material, through combined experimental and first-principles analysis.

Findings

Charge-order fluctuations persist above TN and collapse below it.

Pronounced Kohn-like phonon anomaly appears at TN.

Largest spin-phonon coupling (~6 meV) reported to date.

Abstract

The confinement of electronic wavefunctions in momentum space can give rise to flat electronic bands, where the quenching of kinetic energy enhances the density of states and amplifies interaction effects. Such conditions are fertile ground for emergent quantum phases, as spin, charge and lattice degrees of freedom become strongly entangled. In these regimes, subtle competitions between intertwined order parameters often dictate the macroscopic ground state, producing complex and sometimes unexpected collective behavior. Here we show that the altermagnet CrSb provides a realization of this scenario, and uncover short-range charge-order fluctuations at the M point of the Brillouin zone, q*=(1/2 0), persisting above the Neel temperature (TN). Remarkably, these fluctuations collapse upon entering the magnetically ordered phase, revealing a direct and robust competition between charge and spin order. At TN, the phonon dispersion at q* develops a pronounced Kohn-like anomaly, signaling strong electron-phonon coupling in the vicinity of the magnetic transition. Below TN, exchange striction dramatically renormalizes the associated soft phonon mode by approximately ~6 meV, the largest spin-phonon coupling ever reported. First-principles calculations attribute this behavior to a strong coupling between nearly dispersionless electronic states and a phonon branch that appears unstable at the harmonic level only when no magnetic order is considered, revealing the large sensitivity of the lattice to magnetic symmetry breaking. The competition between charge and spin order parameters, amplified by flat-band physics, drives the observed phonon anomaly and its abrupt reconstruction at TN. With its chemically simple structure and symmetry-protected altermagnetic state, CrSb emerges as a model platform to explore how flat electronic bands mediate giant spin-phonon coupling and competing broken symmetries.