Evolution of the electronic excitation spectrum with strongly diminishing hole-density in superconducting Bi_{2}Sr_{2}CaCu_{2}O_{8+\delta}

TL;DR

This study uses tunnelling spectroscopy to analyze how the electronic excitation spectrum in superconducting Bi2212 evolves with decreasing hole density, revealing a doping-dependent energy gap and scattering rate without invoking additional ordered states.

Contribution

It introduces a parameterization of the excitation spectrum with an anisotropic energy gap and a doping-dependent scattering rate, successfully fitting spectra across the phase diagram.

Findings

The energy gap /Delta_{1} increases as hole density decreases.

The effective scattering rate /Gamma_{2}^{*} rises significantly in the underdoped regime.

Two diverging energy scales, /Delta_{1} and /Delta_{0}, are identified.

Abstract

A complete knowledge of its excitation spectrum could greatly benefit efforts to understand the unusual form of superconductivity occurring in the lightly hole-doped copper-oxides. Here we use tunnelling spectroscopy to measure the T\to 0 spectrum of electronic excitations N(E) over a wide range of hole-density p in superconducting Bi_{2}Sr_{2}CaCu_{2}O_{8+/delta}. We introduce a parameterization for N(E) based upon an anisotropic energy-gap /Delta (\vec k)=/Delta_{1}(Cos(k_{x})-Cos(k_{y}))/2 plus an effective scattering rate which varies linearly with energy /Gamma_{2}(E) . We demonstrate that this form of N(E) allows successful fitting of differential tunnelling conductance spectra throughout much of the Bi_{2}Sr_{2}CaCu_{2}O_{8+/delta} phase diagram. The resulting average /Delta_{1} values rise with falling p along the familiar trajectory of excitations to the 'pseudogap' energy, while the key scattering rate /Gamma_{2}^{*}=/Gamma_{2}(E=/Delta_{1}) increases from below ~1meV to a value approaching 25meV as the system is underdoped from p~16% to p<10%. Thus, a single, particle-hole symmetric, anisotropic energy-gap, in combination with a strongly energy and doping dependent effective scattering rate, can describe the spectra without recourse to another ordered state. Nevertheless we also observe two distinct and diverging energy scales in the system: the energy-gap maximum /Delta_{1} and a lower energy scale /Delta_{0} separating the spatially homogeneous and heterogeneous electronic structures.