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arXiv · 2610.08934

Remote magnon sensing detects the universal phase stiffness jump in Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$

Yue Sun·Mai Nguyen·Naiyuan James Zhang·Dingbin Huang·Shuai Yuan·Aljoscha Söll·Arun Ramanathan·Takashi Taniguchi·Kenji Watanabe·Xavier Roy·Zdeněk Sofer·Shigeyuki Ishida·Hiroshi Eisaki·Makoto Hashimoto·Zhi-Xun Shen·Joseph Orenstein·Xiaodong Xu
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Abstract

Phase stiffness governs macroscopic quantum coherence in superfluids and superconductors, providing a sensitive probe of condensate depletion, fluctuations, and phase transitions. However quantitative and spatially resolved measurements remain challenging. Here we report a contactless, local, all-optical measurement of superconducting phase stiffness using propagating antiferromagnetic magnons as remote sensors. In CrSBr, strong exciton-magnon coupling enables optical excitation and detection of magnon wavepackets and precise determination of their group velocity. When CrSBr is interfaced with underdoped Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$, we find that the magnon group velocity is substantially enhanced below the superconducting transition and quantitatively encodes the phase stiffness through Meissner screening currents. Using this capability, we identify a parallel-field-induced Berezinskii-Kosterlitz-Thouless transition through direct observation of the universal phase-stiffness jump, revealing a field-controlled route to expose intrinsic two-dimensional phase-ordering physics in a bulk cuprate. Our work establishes propagating magnons as local probes of superconducting electrodynamics and phase-coherent phenomena in low-dimensional quantum materials.

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Yue Sun, Mai Nguyen, Naiyuan James Zhang, Dingbin Huang, Shuai Yuan, Aljoscha Söll, Arun Ramanathan, Takashi Taniguchi, Kenji Watanabe, Xavier Roy, Zdeněk Sofer, Shigeyuki Ishida, Hiroshi Eisaki, Makoto Hashimoto, Zhi-Xun Shen, Joseph Orenstein, Xiaodong Xu. 2026-10-06. Remote magnon sensing detects the universal phase stiffness jump in Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$. https://arxiv.org/abs/2610.08934

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