Symmetry Breaking and Ascending in the Magnetic Kagome Metal FeGe

Open Access

Symmetry Breaking and Ascending in the Magnetic Kagome Metal FeGe

Shangfei Wu, Mason L. Klemm, Jay Shah, Ethan T. Ritz, Chunruo Duan, Xiaokun Teng, Bin Gao, Feng Ye, Masaaki Matsuda, Fankang Li, Xianghan Xu, Ming Yi, Turan Birol, Pengcheng Dai, and Girsh Blumberg

Phys. Rev. X 14, 011043 – Published 8 March 2024

Abstract

Spontaneous symmetry breaking—the phenomenon in which an infinitesimal perturbation can cause the system to break the underlying symmetry—is a cornerstone concept in the understanding of interacting solid-state systems. In a typical series of temperature-driven phase transitions, higher-temperature phases are more symmetric due to the stabilizing effect of entropy that becomes dominant as the temperature is increased. However, the opposite is rare but possible when there are multiple degrees of freedom in the system. Here, we present such an example of a symmetry-ascending phenomenon upon cooling in a magnetic kagome metal FeGe by utilizing neutron Larmor diffraction and Raman spectroscopy. FeGe has a kagome lattice structure with simple A-type antiferromagnetic order below Néel temperature $T_{N} \approx 400 K$ and a charge density wave (CDW) transition at $T_{CDW} \approx 110 K$ , followed by a spin-canting transition at around 60 K. In the paramagnetic state at 460 K, we confirm that the crystal structure is indeed a hexagonal kagome lattice. On cooling to around $T_{N}$ , the crystal structure changes from hexagonal to monoclinic with in-plane lattice distortions on the order of $10^{- 4}$ and the associated splitting of the double-degenerate phonon mode of the pristine kagome lattice. Upon further cooling to $T_{CDW}$ , the kagome lattice shows a small negative thermal expansion, and the crystal structure gradually becomes more symmetric upon further cooling. A tendency of increasing the crystalline symmetry upon cooling is unusual; it originates from an extremely weak structural instability that coexists and competes with the CDW and magnetic orders. These observations are against the expectations for a simple model with a single order parameter and hence can only be explained by a Landau free energy expansion that takes into account multiple lattice, charge, and spin degrees of freedom. Thus, the determination of the crystalline lattice symmetry as well as the unusual spin-lattice coupling is a first step towards understanding the rich electronic and magnetic properties of the system, and it sheds new light on intertwined orders where the lattice degree of freedom is no longer dominant.

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Received 15 September 2023
Accepted 24 January 2024

DOI:https://doi.org/10.1103/PhysRevX.14.011043

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Charge density waves Phase transitions by order

Strongly correlated systems

Neutron scattering Raman spectroscopy

Condensed Matter, Materials & Applied Physics

Authors & Affiliations

Shangfei Wu ^1,*,†, Mason L. Klemm^2,*, Jay Shah³, Ethan T. Ritz³, Chunruo Duan², Xiaokun Teng², Bin Gao², Feng Ye⁴, Masaaki Matsuda ⁴, Fankang Li ⁵, Xianghan Xu¹, Ming Yi ², Turan Birol ^3,‡, Pengcheng Dai ^2,§, and Girsh Blumberg ^1,6,∥

¹Department of Physics and Astronomy, Rutgers University, Piscataway, New Jersey 08854, USA
²Department of Physics and Astronomy, Rice University, Houston, Texas 77005, USA
³Department of Chemical Engineering and Materials Science, University of Minnesota, Minnesota 55455, USA
⁴Neutron Scattering Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
⁵Neutron Technologies Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
⁶National Institute of Chemical Physics and Biophysics, 12618 Tallinn, Estonia

^*These authors contributed equally to this work.
^†wusf@baqis.ac.cn Present address: Beijing Academy of Quantum Information Sciences, Beijing 100193, China.
^‡tbirol@umn.edu
^§pdai@rice.edu
^∥girsh@physics.rutgers.edu

Popular Summary

Symmetry breaking occurs when a solid changes from one crystalline phase to another at its phase-transition temperature. In second-order phase transitions, the crystal structure changes continuously from a highly symmetrical phase to a less symmetrical one. Generally, the more symmetrical phase corresponds to higher temperatures and the less symmetrical phase to lower temperatures. The opposite scenario, in which the material becomes more symmetrical upon cooling, is unusual but possible in complex matter where multiple electronic, magnetic, and lattice degrees of freedom interact with each other. Here, we present an example of such a symmetry-ascending phenomenon in a magnetic kagome metal, FeGe.

In the paramagnetic state, we confirm that the crystal structure of FeGe is indeed a hexagonal kagome lattice. On cooling to around the antiferromagnetic phase-transition temperature, the crystal structure changes from hexagonal to monoclinic with in-plane lattice distortions. Upon further cooling to the charge-density-wave transition temperature, the kagome lattice shows a small negative thermal expansion, and the crystal structure becomes gradually more symmetric due to the competition between lattice distortion, charge density wave, and magnetic orders.

The charge-density-wave order in FeGe can be tuned by a simple annealing process, where the correlation length of the charge density wave can change from 0 to 100%. Thus, it is interesting and an open question to see how the annealing process influences the interplay between the charge, magnetic, and lattice degrees of freedom in FeGe.

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