Hiroto Takahashi
Date: Wednesday, October 15th, 2025
Location: Elings Hall, room 1601
Time: 12:30 pm - 1:30 pm
Spin Noise Spectroscopy of Herbertsmithite ZnCu3(OH)6Cl2: Spinon Mediation of Witness-Spin Dynamics
Abstract: An emerging concept for identifying various types of spin liquid states, which are often altogether designated by the absence of order, is through their characteristic spin noise. This spin noise approach has been successfully demonstrated in a classical spin ice material Dy2Ti2O7 and a spiral spin liquid material Ca10Cr7O28. Here we extend the technique to herbertsmithite ZnCu3(OH)6Cl2, an iconic quantum spin liquid candidate with S = 1/2 Cu2+ kagome layers that are separated by nonmagnetic Zn2+ layers. Some S = 1/2 Cu2+ randomly substitute Zn2+ to contribute weakly interacting quasi-free spins that are usually regarded as “impurities”. We reconceptualize them as “witness-spins” through which one can probe a conjectured quantum spin liquid state on the kagome layer. By developing a millikelvin SQUID-based spin noise spectrometer with fT/√Hz- sensitivity and μs-time resolution, we detect magnetization noise M(t,T) of the witness-spins in ZnCu3(OH)6Cl2 which fluctuates intensively at pT-order. A scale-invariant noise power spectral density S_M (ω,T)∝〖1/ω〗^α(T) and noise variance σ_M^2 (T) indicate a sharp transition at T^* 260 mK. We introduce a model of spinon-mediated witness-spin interactions which, despite having only one free parameter, is consistent with the full range of experimental observations.
Bio: Hiroto Takahashi is currently a Postdoctoral Research Assistant at the University of Oxford, working with Prof. J. C. Séamus Davis. He received his BSc in Physics in 2020 from the University of Tokyo, during which he completed a research internship with Prof. Hidenori Takagi and Prof. Bernhard Keimer at the Max Planck Institute for Solid State Research. There, he explored spin-orbital physics in ruthenates using resonant inelastic x-ray scattering. Following his undergraduate studies, he joined the University of Oxford and Max Planck Graduate Center for Quantum Materials, earning a DPhil in Condensed Matter Physics in 2025. During his doctoral research, he investigated the spontaneously generated spin noise dynamics in quantum spin liquid materials, designing and launching a millikelvin ultra-low noise SQUID spin noise spectrometer laboratory with Prof. J. C. Séamus Davis.
