Seminar

In situ doping control of the surface of high-temperature superconductors

Prof. Andrea Damascelli
Department of Physics and Astronomy
University of British Columbia
Vancouver, Canada
Tuesday, November 24, 2009
Higgins 235, 2 pm

Central to the understanding of high-temperature superconductivity is the evolution of the electronic structure as doping alters the density of charge carriers in the CuO2 planes. Superconductivity emerges along the path from a normal metal on the overdoped side to an antiferromagnetic insulator on the underdoped side. This path also exhibits a severe disruption of the overdoped normal metal’s Fermi surface [1,2]. Angle-resolved photoemission spectroscopy (ARPES) on the surfaces of easily cleaved materials such as Bi2Sr2CaCu2O8+ shows that in zero magnetic field the Fermi surface breaks up into disconnected arcs [3]. However, in high magnetic field, quantum oscillations at low temperatures in YBa2Cu3O6.5 indicate the existence of small Fermi surface pockets [4]. Reconciling these two phenomena through ARPES studies of YBa2Cu3O7− (YBCO) has been hampered by the surface sensitivity of the technique. Here, we show that this difficulty stems from the polarity and resulting self-doping of the YBCO surface. Through in situ deposition of potassium atoms on cleaved YBCO, we can continuously control the surface doping and follow the evolution of the Fermi surface from the overdoped to the underdoped regime; the results differ markedly from the interpretation of the high-field measurements in terms of electron and hole pockets on the same material [5]. The present approach opens the door to systematic studies of high-temperature superconductors, such as creating new electron-doped superconductors from insulating parent compounds.

Figure 1: Schematic phase diagram of YBCO. The hole doping p per planar copper (p=0 for the 1/2-filled Mott insulator with 1 hole per Cu atom), and the corresponding oxygen content (7−), are indicated on the bottom and top axes. The ARPES Fermi surface for under- and overdoped YBCO is also shown. (Click on an image to see a larger version).

1. N.E. Hussey et al., Nature 425, 814 (2003).
2. M. Platé et al., Phys. Rev. Lett. 95, 077001 (2005).
3. M.R. Norman et al., Nature 392, 157 (1998).
4. N. Doiron-Leyraud et al., Nature 447, 565 (2007).
5. M.A. Hossain et al., Nature Physics 4, 527 (2008).