Directly Probing Spin Fluctuations in Quantum Spin Liquids with X-Ray Free Electron Lasers

Tommaso Pardini | 18-LW-022


Quantum spin liquids (QSLs) represent a class of quantum magnets for which the ground state is understood to be a non-trivial superposition of many states, connected by local spin fluctuations. This complex entanglement, which is the defining feature of a QSL phase, does not couple directly to experimental probes, making the detection of candidate QSL materials difficult.

Our research advanced the understanding of the effect of impurities on the theoretical stability of QSL phases in a special class of materials described by the Transverse Field Ising Model on the pyrochlore lattice. This topic is of great relevance to the advanced materials community, as any experiment must address the ever-present deviations of each sample from its ideal atomic composition and crystalline structure. Our initial question–whether it is possible to directly couple to the quantum spin fluctuations typical of QSLs by combining standard spectroscopic techniques with the high temporal resolution of x-ray free electron lasers (XFELs)–was not answered in our study. This argument had relied on the observation that quantum spin fluctuations belong to the same femtosecond time scale typical of XFEL pulses.

Impact on Mission

Our findings and the experimental designs and numerical codes developed to conduct our research contribute to Lawrence Livermore National Laboratory's core competency in advanced materials and the Laboratory's expertise in highly entangled quantum systems. We built a strong collaborative relationship with the Physics departments at UC Davis and Georgetown University with the goal of advancing new ideas in the field of magnetism.

Publications, Presentations, Etc.

Pardini, A., et al. 2019. "Local Entanglement and Confinement Transitions in Random Transverse-Field Ising Model on the Pyrochlore Lattice" Phys. Rev. B 100, 144437 (2019). LLNL-JRNL-778510.