The nature of superconductivity, a phase in which electron form pairs and condense into a special zero-resistance state, is not understood in SrTiO3. PQI researcher Guanglei Cheng’s work, which appeared in Nature , provides a surprising new insight into this unconventional superconducting system, thereby revealing a novel phase in which electrons remain paired far outside of the superconducting regime.
Superconductivity can persist to very low carrier densities in STO , placing it outside conventional BCS theory and prompting early speculation about the possibility of real-space electron pairs existing outside of the superconducting state . However, no evidence for such pairs was observed until Cheng et al’s experiments utilizing a quantum dot geometry (left panel). In this geometry, individual electrons tunnel onto and off of a conducting island, permitting single-electron spectroscopy of the electronic states on the dot and their evolution as a function of external parameters such as magnetic field and temperature.
Surprisingly, at a magnetic field around 2 Tesla (but sometimes as large as 7 Tesla!), the tunneling conductance peaks (the bright green and yellow features, right panel) start to bifurcate. As the magnetic field increases further, the split peaks shift with typical Zeeman energy. This curious behavior suggests that below ~2 Tesla, each conductance peak actually corresponds to the tunneling of not one, but two electrons. These electron pairs are extraordinarily strong, persisting to magnetic fields an order of magnitude larger than the critical field for superconductivity, 0.2 Tesla. Therefore, this constitutes clear evidence for electron pairs persisting in a non-condensed, non-superconducting phase.
- Cheng, G. et al. "Electron Pairing without Superconductivity," Nature 521, 196-199 (2015).
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- Eagles, D. M. "Possible Pairing without Superconductivity at Low Carrier Concentrations in Bulk and Thin-Film Superconducting Semiconductors," Phys. Rev. 186, 456–463 (1969).