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arXiv · 2509.23221

Evidence of electronic instability driven structural distortion in the nodal line semimetal CoSn$_2$

Suman Nandi·Bishal Maity·Shovan Dan·Khadiza Ali·Bikash Patra·Anshuman Mondal·Gaston Garbarino·Pierre Rodière·Sitaram Ramakrishnan·Bahadur Singh·Arumugam Thamizhavel
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Abstract

Understanding the mechanisms that drive spontaneous rotational symmetry breaking in correlated electron systems is a central challenge in condensed matter physics. Although such symmetry breaking phases have been studied in low-dimensional and strongly correlated materials, its emergence in structurally simpler compounds remains less explored. Here, we investigate non-magnetic CoSn$_2$ that is a centrosymmetric intermetallic compound crystallizing in a tetragonal structure at ambient conditions, and discover an electronically driven symmetry breaking instability. Electrical resistivity reveals a distinct change in the slope below 25 K, deviating from the expected Bloch-Gr\"uneisen behavior. This anomaly is attributed towards a structural change as at 22 K single crystal X-ray diffraction using synchrotron radiation uncovers weak superlattice reflections that leads to a doubling of $\textbf{a}$ and $\textbf{c}$, resulting in a 4-fold superstructure. The symmetry of the lattice reduces from tetragonal to acentric monoclinic but without any discernible monoclinic distortion down to 10 K. This structural transition is accompanied by a twofold symmetry in angular magnetoresistance, contrasting the fourfold symmetry observed at higher temperatures. First-principles calculations show no phonon softening but reveal enhanced electronic susceptibility, suggesting an electronic instability. Polarization-dependent ARPES measurements further identify a strong orbital anisotropy dominated by the in-plane Co-$d_{xy}$ states. Collectively, our results point to an electronic instability driven structural distortion in CoSn$_2$, offering a rare platform to study symmetry breaking in a non-magnetic metallic system.

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Suman Nandi, Bishal Maity, Shovan Dan, Khadiza Ali, Bikash Patra, Anshuman Mondal, Gaston Garbarino, Pierre Rodière, Sitaram Ramakrishnan, Bahadur Singh, Arumugam Thamizhavel. 2025-09-27. Evidence of electronic instability driven structural distortion in the nodal line semimetal CoSn$_2$. https://arxiv.org/abs/2509.23221

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