Flux Line Lattice Melting and the Formation of a Coherent Quasiparticle Bloch State in the Ultraclean URu$_2$Si$_2$ Superconductor

TL;DR

This study reveals a flux line lattice melting transition in ultraclean URu$_2$Si$_2$ superconductor, driven by large thermal fluctuations due to heavy carriers, leading to a flux liquid phase and potential quasiparticle Bloch state formation.

Contribution

It demonstrates the occurrence of flux line lattice melting and quasiparticle Bloch state formation in an ultraclean heavy-fermion superconductor, highlighting the role of thermal fluctuations.

Findings

Flux line lattice melts below the upper critical field.

Large thermal fluctuations cause a flux liquid phase.

Quasiparticle mean free path increases below melting transition.

Abstract

We find that in ultraclean heavy-fermion superconductor URu$_2$Si$_2$ ($T_{c0}=1.45$ K) a distinct flux line lattice melting transition with outstanding characters occurs well below the mean-field upper critical fields. We show that a very small number of carriers with heavy mass in this system results in exceptionally large thermal fluctuations even at subkelvin temperatures, which are witnessed by a sizable region of the flux line liquid phase. The uniqueness is further highlighted by an enhancement of the quasiparticle mean free path below the melting transition, implying a possible formation of a quasiparticle Bloch state in the periodic flux line lattice.