1,157 research outputs found
STS Observations of Landau Levels at Graphite Surfaces
Scanning tunneling spectroscopy measurements were made on surfaces of two
different kinds of graphite samples, Kish graphite and highly oriented
pyrolytic graphite (HOPG), at very low temperatures and in high magnetic
fields. We observed a series of peaks in the tunnel spectra, which grow with
increasing field, both at positive and negative bias voltages. These are
associated with Landau quantization of the quasi two-dimensional electrons and
holes in graphite in magnetic fields perpendicular to the basal plane. Almost
field independent Landau levels fixed near the Fermi energy, which are
characteristic of the graphite crystalline structure, were directly observed
for the first time. Calculations of the local density of states at the graphite
surfaces allow us to identify Kish graphite as bulk graphite and HOPG as
graphite with finite thickness effectively
Scanning tunneling microscopy and spectroscopy of the electronic local density of states of graphite surfaces near monoatomic step edges
We measured the electronic local density of states (LDOS) of graphite
surfaces near monoatomic step edges, which consist of either the zigzag or
armchair edge, with the scanning tunneling microscopy (STM) and spectroscopy
(STS) techniques. The STM data reveal that the and honeycomb superstructures coexist over a length scale of 3-4 nm
from both the edges. By comparing with density-functional derived nonorthogonal
tight-binding calculations, we show that the coexistence is due to a slight
admixing of the two types of edges at the graphite surfaces. In the STS
measurements, a clear peak in the LDOS at negative bias voltages from -100 to
-20 mV was observed near the zigzag edges, while such a peak was not observed
near the armchair edges. We concluded that this peak corresponds to the
graphite "edge state" theoretically predicted by Fujita \textit{et al.} [J.
Phys. Soc. Jpn. {\bf 65}, 1920 (1996)] with a tight-binding model for graphene
ribbons. The existence of the edge state only at the zigzag type edge was also
confirmed by our first-principles calculations with different edge
terminations.Comment: 20 pages, 11 figure
Possibility of valence-fluctuation mediated superconductivity in Cd-doped CeIrIn probed by In-NQR
We report on a pressure-induced evolution of exotic superconductivity and
spin correlations in CeIr(InCd) by means of
In-Nuclear-Quadrupole-Resonance (NQR) studies. Measurements of an NQR spectrum
and nuclear-spin-lattice-relaxation rate have revealed that
antiferromagnetism induced by the Cd-doping emerges locally around Cd dopants,
but superconductivity is suddenly induced at = 0.7 and 0.9 K at 2.34 and
2.75 GPa, respectively. The unique superconducting characteristics with a large
fraction of the residual density of state at the Fermi level that increases
with differ from those for anisotropic superconductivity mediated by
antiferromagnetic correlations. By incorporating the pressure dependence of the
NQR frequency pointing to the valence change of Ce, we suggest that
unconventional superconductivity in the CeIr(InCd) system may
be mediated by valence fluctuations.Comment: Accepted for publication in Physical Review Letter
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