New magnetic-field-induced macroscopic quantum phenomenon in a superconductor with gap nodes

TL;DR

This paper reports a novel magnetic-field-induced macroscopic quantum phenomenon in high-$T_c$ superconductors with gap nodes, involving quantization of superfluid density due to confined density waves, distinct from known quantum effects.

Contribution

It introduces a new macroscopic quantum state characterized by a non-integer quantum number, arising from confined density waves in superconductors with gap nodes, explaining recent experimental anomalies.

Findings

Observation of magnetic-field-induced transition to a state without gap nodes.

Identification of superfluid density quantization linked to confined density waves.

Explanation of unexplained experimental reports in cuprates using the new quantum state.

Abstract

High-$T_c$ superconductivity is unconventional because the gap is not isotropic as in simple metals but has $d_{x^2-y^2}$ symmetry with lines of nodes. In a fascinating thermal transport experiment on a high-$T_c$ superconductor, Krishana et al \cite{Krishana} have reported mysterious magnetic field induced first order transitions from a superconducting state with gap nodes to a state without gap nodes. We show here that this is an experimental manifestation of a novel {\it macroscopic} quantum phenomenon induced by the magnetic field, qualitatively different from the usual quantum Hall effects. It corresponds to the {\it quantization of the superfluid density} in a superconductor with gap nodes due to the generation of Confined Field Induced Density Waves (CFIDW) in the node regions of the Fermi surface. The Landau numbers $L$ are not sufficient to index these macroscopic quantum states and the addition of a new quantum number $\zeta$ is necessary. Distinct qualitative implications of this non-integer $|L,\zeta>$ quantization are also evident in a number of recent unexplained experimental reports in the cuprates.