Time-reversal symmetry broken by charge order in <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi mathvariant="normal">CsV</mml:mi><mml:mn>3</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">Sb</mml:mi><mml:mn>5</mml:mn></mml:msub></mml:math>
R. Khasanov, Debarchan Das, Ritu Gupta, C. Mielke, Matthias Elender, Qiangwei Yin, Zhijun Tu, Chunsheng Gong, Hechang Lei, Ethan T. Ritz, Rafael M. Fernandes, Turan Birol, Zurab Guguchia, H. Luetkens
Abstract
The recently discovered vanadium-based kagome metals $A{\mathrm{V}}_{3}{\mathrm{Sb}}_{5}$ ($A$ = K, Rb, Cs) exhibit superconductivity at low temperatures and charge density wave (CDW) order at high temperatures. A prominent feature of the charge ordered state in this family is that it breaks time-reversal symmetry (TRSB), which is connected to the underlying topological nature of the band structure. In this work, a powerful combination of zero-field and high-field muon-spin rotation/relaxation is used to study the signatures of TRSB of the charge order in ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$, as well as its anisotropic character. By tracking the temperature evolution of the in-plane and out-of-plane components of the muon-spin polarization, an enhancement of the internal field width sensed by the muon-spin ensemble was observed below ${T}_{\mathrm{TRSB}}={T}_{\mathrm{CDW}}\ensuremath{\simeq}95$ K. Additional increase of the internal field width, accompanied by a change of the local field direction at the muon site from the $ab$ plane to the $c$ axis, was detected below ${T}^{*}\ensuremath{\simeq}30$ K. Remarkably, this two-step feature becomes well pronounced when a magnetic field of 8 T is applied along the crystallographic $c\phantom{\rule{4.pt}{0ex}}\mathrm{axis}$, thus indicating a field-induced enhancement of the electronic response at the CDW transition. These results point to a TRSB in ${\mathrm{CsV}}_{3}{\mathrm{Sb}}_{5}$ by charge order with an onset of $\ensuremath{\simeq}95$ K, followed by an enhanced electronic response below $\ensuremath{\simeq}30$ K. The observed two-step transition is discussed within the framework of different charge-order instabilities, which, in accordance with density functional theory calculations, are nearly degenerate in energy.