High-resolution spectroscopy of extremely metal-poor stars from
  SDSS/SEGUE. III. Unevolved Stars with $[\mathrm{Fe/H}]\lesssim -3.5$
  We present elemental abundances for eight unevolved extremely metal-poor
stars with $T_{\rm eff}>5500\,\mathrm{K}$, among which seven have
$[\mathrm{Fe/H}]<-3.5$. The sample is selected from the Sloan Digital Sky
Survey / Sloan Extension for Galactic Understanding and Exploration
(SDSS/SEGUE), and our previous high-resolution spectroscopic follow-up with the
Subaru Telescope (Aoki et al.). Several methods to derive stellar parameters
are compared, and no significant offset in the derived parameters is found in
most cases. From an abundance analysis relative to the standard extremely
metal-poor star G 64-12, an average Li abundance for stars with
$[\mathrm{Fe/H}]<-3.5$ is $A(\mathrm{Li}) =1.90$, with a standard deviation of
$\sigma =0.10$ dex. This result confirms that lower Li abundances are found at
lower metallicity, as suggested by previous studies, and demonstrates that the
star-to-star scatter is small. The small observed scatter could be a strong
constraint on Li-depletion mechanisms proposed for explaining the low Li
abundance at lower metallicity. Our analysis for other elements obtained the
following results: i) A statistically significant scatter in $[\mathrm{X/Fe}]$
for Na, Mg, Cr, Ti, Sr, and Ba, and an apparent bimodality in
$[\mathrm{Na/Fe}]$ with a separation of $\sim 0.8\, \mathrm{dex}$, ii) an
absence of a sharp drop in the metallicity distribution, and iii) the existence
of a CEMP-$s$ star at $[\mathrm{Fe/H}]\simeq -3.6$ and possibly at
$[\mathrm{Fe/H}]\simeq-4.0$, which may provide a constraint on the mixing
efficiency of unevolved stars during their main-sequence phase.
