Resistive Transition and Upper Critical Field in Underdoped YBa_2Cu_3O_{6+x} Single Crystals

TL;DR

This study investigates the resistive transition and upper critical field in underdoped YBa_2Cu_3O_{6+x} single crystals, revealing unique magnetic field responses and universal behavior consistent with boson superconductivity models.

Contribution

It provides new insights into the magnetic field dependence of the resistive transition and critical fields in underdoped cuprates, highlighting a universal behavior described by a boson superconductivity model.

Findings

Transition width increases insignificantly with magnetic field

B_{c2}(T) curves do not saturate and have positive second derivatives

Universal scaling of B_{c2}(T) across different crystal states

Abstract

A superconducting transition in the temperature dependence of the ab-plane resistivity of underdoped YBa_2Cu_3O_{6+x} crystals in the range T_c<30 K has been investigated. Unlike the case of samples with the optimal level of doping, the transition width increased insignificantly with magnetic field, and in the range T_c<13 K it decreased with increasing magnetic field. The transition point T_c(B) was determined by analyzing the fluctuation conductivity. The curves of B_{c2}(T) measured in the region T/T_c>0.1 did not show a tendency to saturation and had a positive second derivative everywhere, including the immediate neighborhood of T_c. The only difference among the curves of B_{c2}(T) for different crystal states is the scales of T and B, so they can be described in terms of a universal function, which fairly closely follows Alexandrov's model of boson superconductivity.