NMR-Experimente zur elektronischen Struktur der Si(111)(7x7)-Oberfläche
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Philipps-Universität Marburg
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Abstract
Despite of being one of the most intensively
studied semiconductor
surfaces the electronic structure of the
Si(111)-(7x7) surface is
still under discussion particularly
in the vicinity of the Fermi
energy.
The density of states
(DOS) at the Fermi level can be derived from
temperature
dependent measurements of the 8Li nuclear spin relaxation
rate. The measurements were carried out at lithium coverages in
the
10^(-3) ML range. In order to enhance the energy
resolution of states
close to E_F, which is proportional to
k_B T, I modified the cooling
of the surface to operate with
liquid Helium instead of liquid
Nitrogen. In order to be able
to perform experiments down to 50K
Sample temperature. A fit
of a schematic model DOS to the data
resulted in a band at the
Fermi energy as narrow as 5~meV. This band
is probably
generated by many body effects among the surface
electrons,
which is a hint that the Si(111)-(7x7) surface stands at
the
verge of a Hubbard metal insulator transition which is driven
by
electron correlations. This is the first indication of this
transition
at the Si(111)-(7x7) surface.
From the weak
dependence of the relaxation rate from the external
magnetic
field together with its constancy in between 48 K and 400 K I
estimated a lower limit of 0.5 eV for the activation energy of
diffusion.
Slightly shifted absorption lines were found in
beta-NMR-resonance
absorption experiments for positive and
negative polarisation of the
8Li ensemble respectively. From
this shift the sign of the electric
field gradient (EFG) of Li
adsorbed at Si(111)-(7x7) was deduced. The
EFG was found to be
positive,indicating that Li is adsorbed not on top
but within
the rather open (7x7) reconstruction. Due to the width of the
lines of 12 Mhz the the calulation of the EFG from the
lineshift underestimates the actual value,
Vzz>2,44*10^15 V/cm^2.Despite of being one of the most intensively
studied semiconductor
surfaces the electronic structure of the
Si(111)-(7x7) surface is
still under discussion particularly
in the vicinity of the Fermi
energy.
The density of states
(DOS) at the Fermi level can be derived from
temperature
dependent measurements of the 8Li nuclear spin relaxation
rate. The measurements were carried out at lithium coverages in
the
10^(-3) ML range. In order to enhance the energy
resolution of states
close to E_F, which is proportional to
k_B T, I modified the cooling
of the surface to operate with
liquid Helium instead of liquid
Nitrogen. In order to be able
to perform experiments down to 50K
Sample temperature. A fit
of a schematic model DOS to the data
resulted in a band at the
Fermi energy as narrow as 5~meV. This band
is probably
generated by many body effects among the surface
electrons,
which is a hint that the Si(111)-(7x7) surface stands at
the
verge of a Hubbard metal insulator transition which is driven
by
electron correlations. This is the first indication of this
transition
at the Si(111)-(7x7) surface.
From the weak
dependence of the relaxation rate from the external
magnetic
field together with its constancy in between 48 K and 400 K I
estimated a lower limit of 0.5 eV for the activation energy of
diffusion.
Slightly shifted absorption lines were found in
beta-NMR-resonance
absorption experiments for positive and
negative polarisation of the
8Li ensemble respectively. From
this shift the sign of the electric
field gradient (EFG) of Li
adsorbed at Si(111)-(7x7) was deduced. The
EFG was found to be
positive,indicating that Li is adsorbed not on top
but within
the rather open (7x7) reconstruction. Due to the width of the
lines of 12 Mhz the the calulation of the EFG from the
lineshift underestimates the actual value, Vzz>2,44*10^15
V/cm^2.
Review
Metadata
Contributors
Supervisor:
Dates
Created: 2003Issued: 2003-12-23Updated: 2011-08-10
Faculty
Fachbereich Physik
Publisher
Philipps-Universität Marburg
Language
ger
Data types
DoctoralThesis
Keywords
Korrelationen , Vielteilcheneffektecorrelationselektrischer FeldgradientHubbarddensity of states, electric field gradientFermi energySi(7x7)
DFG-subjects
ZustandsdichteOberflächeBeta-NMR-Spektroskopie , Hubbard-ModellSiliciumFermi-Energie
DDC-Numbers
530
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Schillinger, Richard (128640189): NMR-Experimente zur elektronischen Struktur der Si(111)(7x7)-Oberfläche. : Philipps-Universität Marburg 2003-12-23. DOI: https://doi.org/10.17192/z2003.0777.
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