Fermi Surface Evolution in Na x CoO 2

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.
Wangs 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.