Artist’s impression of a high critical temperature superconductor immersed in a magnetic field. The magnetic field generates whirls of current called vortices. These allow to better perceive an ordered electronic structure that coexists with the superconducting state. Credit: © UNIGE – Xavier Ravinet
New research shows electronic densities measured in these superconductors are a combination of 2 separate effects. As a result, a new model suggests the existence of 2 coexisting states, rather than competing ones as was postulated for the past 30 years. A small revolution in the world of superconductivity.
A superconducting material is a material that, below a certain temperature, loses all electrical resistance. When immersed in a magnetic field, high-temperature superconductors (high-Tc) allow this field to penetrate as filamentary regions ie vortices, in which the material is no longer superconducting. Each vortex is a whirl of electronic currents generating their own magnetic field and in which electronic structure is different from the rest of the material.
Some theoretical models describe high-Tc superconductors as a competition between 2 fundamental states, each developing its own spectral signature. The 1st is characterized by an ordered spatial arrangement of electrons. The 2nd, corresponding to the superconducting phase, is characterized by electrons in pairs.
Subgap states are observed in zero field….the average of 2,704 spectra recorded over a 120 × 120 nm2 area in zero field. (a) Average dI/dV conductance of a 120 × 120 nm2 area at T=0.4 K in zero field. The junction resistance was 1.2 GΩ and the regulation voltage 300 mV. Five electron-hole symmetric spectral features and the background are indicated. (b) Spatial map of the superconducting gap ΔSC over the same area. (c) Histogram of the gap values shown in b. The colouring of bars corresponds to the colour scale shown in b.
“However, by measuring the density of electronic states with local tunneling spectroscopy, we discovered that the spectra that were attributed solely to the core of a vortex, where the material is not in the superconducting state, are also present elsewhere, that is to say in areas where the superconducting state exists. This implies that these spectroscopic signatures do not originate in the vortex cores and cannot be in competition with the superconducting state,” explains Prof. Christoph Renner, UNIGE. “This study therefore questions the view that these two states are in competition… Instead, they turn out to be two coexisting states that together contribute to the measured spectra,” professor Renner says. Indeed, physicists from UNIGE have shown, using theoretical simulation tools, that the experimental spectra can be reproduced perfectly by considering the superposition of the spectroscopic signature of a superconductor and this other electronic signature, brought to light through this new research.
This discovery is a breakthrough towards understanding the nature of the high temperature superconducting state. It also sheds new light on the electronic nature of the vortex cores, which potentially has an impact on their dynamics. Mastery of this dynamics, and particularly anchoring of vortices that depend on their electronic nature, is critical for many apps, eg high field electromagnets.
http://dqmp.unige.ch/super-conductivity-seen-in-a-new-light/
http://www.nature.com/ncomms/2016/160331/ncomms11139/full/ncomms11139.html
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