Fermi Surface Evolution in Na x CoO 2 Hong Ding, Boston College, DMR-0353108 (SRC DMR-0084402)

1. Fermi Surface Evolution in NaxCoO2Hong Ding, Boston College, DMR-0353108(SRC DMR-0084402) The recent discovery of superconductivity in NaxCoO2.yH2O (Tc ~ 5 K) has generated great interests.We report systematic angle-resolved photoemission studies on NaxCoO2 single crystals for a wide range of Na concentrations. As shown in Fig. 1, we observe a large Fermi surface (FS) centered at the G point, which satisfies Luttinger theorem. However, the small FS pockets predicted by band theory near the K points are not observed. Instead, “sinking islands” with the binding energy of 100 – 200 meV are observed. The disappearance of the small FS pockets are explained well by our calculation that considers large electron correlations, as shown in Fig. 2. In addition, at x = 1/3 where superconductivity occurs with proper water intercalation, we found that the large Fermi surface coincides with the new zone boundary of a commensurate charge ordering, suggesting that the charge fluctuations may play an important role in the superconductivity of this material. H.-B. Yang et al., accepted by Phys. Rev. Lett. S. Zhou et al., Phys. Rev. Lett. 94, 206401 (2005) Fig. 1 Fig. 2

2. Fermi Surface Evolution in NaxCoO2Hong Ding, Boston College, DMR-0353108 (SRC DMR-0084402) Education One undergraduate student (P.M.Bishay), two graduate students (H.-B. Yang, Z.-H. Pan), and one postdoc (A.K.P. Sekharan) from my group have contributed to the experimental work. Two graduate students (S. Zhou, M. Guo) from Z. Wang’s group have contributed to the theoretical work. H.-B. Yang has received his Ph.D. in August 2005 and is becoming a postdoc at Brookhaven National Laboratory. This work is his Ph.D. thesis project. Social Impact The knowledge of band structure and Fermi surface topology is important for understanding unconventional superconductors, such as high temperature superconductors, which have great application potential, including non-dissipating energy transport, high speed computing, and new medical devices.