近日，意大利佛罗伦萨大学的G. Modugno&A.Smerzi及其研究团队取得一项新进展。经过不懈努力，他们利用约瑟夫森效应测量超固体的超流体分数。相关研究成果已于2024年5月8日在国际权威学术期刊《自然》上发表。

该研究团队演示了一种基于约瑟夫森效应的测量超流体分数的创新方法，约瑟夫森效应是一种普遍存在的现象，与两种超流体或超导体之间存在物理屏障有关，由于空间调制，这种现象也可能出现在超固体中。研究人员证明了超固体的单个单元能够维持约瑟夫森振荡，并且进一步展示了如何从电流相动力学直接推导出超流体分数。他们对冷原子偶极超固体的深入研究揭示了一个显著小于1的超流体分数，这一发现使得研究之前未知的现象，如部分量子化漩涡和超流，成为可能。这项研究结果开辟了一个新的研究方向，可能统一所有超固体系统的描述。

据悉，一类具有空间周期调制超流体密度的新型超流体和超导体正在兴起。这可能与物质的超固相有关，其中规范对称性和平移对称性的自发破缺导致了空间调制的宏观波函数。然而，人们只有在某些情况下才认识到这种关系，并且需要一个通用的性质来量化超固体和普通物质之间的差异，例如超流体分数，它测量由空间调制引起的超流体刚度减少的程度。超流体分数是很久以前提出的，但尚未经过实验评估。

附：英文原文

Title: Measurement of the superfluid fraction of a supersolid by Josephson effect

Author: Biagioni, G., Antolini, N., Donelli, B., Pezz, L., Smerzi, A., Fattori, M., Fioretti, A., Gabbanini, C., Inguscio, M., Tanzi, L., Modugno, G.

Issue&Volume: 2024-05-08

Abstract: A new class of superfluids and superconductors with spatially periodic modulation of the superfluid density is arising. It might be related to the supersolid phase of matter, in which the spontaneous breaking of gauge and translational symmetries leads to a spatially modulated macroscopic wavefunction. This relation was recognized only in some cases and there is the need for a universal property quantifying the differences between supersolids and ordinary matter, such as the superfluid fraction, which measures the reduction in superfluid stiffness resulting from the spatial modulation. The superfluid fraction was introduced long ago, but it has not yet been assessed experimentally. Here we demonstrate an innovative method to measure the superfluid fraction based on the Josephson effect, a ubiquitous phenomenon associated with the presence of a physical barrier between two superfluids or superconductors, which might also be expected for supersolids, owing to the spatial modulation. We demonstrate that individual cells of a supersolid can sustain Josephson oscillations and we show that, from the current–phase dynamics, we can derive directly the superfluid fraction. Our study of a cold-atom dipolar supersolid reveals a relatively large sub-unity superfluid fraction that makes realistic the study of previously unknown phenomena such as partially quantized vortices and supercurrents. Our results open a new direction of research that may unify the description of all supersolid-like systems.

DOI: 10.1038/s41586-024-07361-9

Source: https://www.nature.com/articles/s41586-024-07361-9