UltraLow Temperature Scanning Tunneling Microscopy

1
Ultra-Low Temperature Scanning Tunneling
Microscopy
M.D. Upward, J.W. Janssen, L. Gurevich and L.P.
Kouwenhoven
Department of Applied Physics, Delft University
of Technology, P.O. Box 5046, 2600 GA Delft,
The Netherlands.
We have developed a home built ultra-low
temperature STM capable of continuous operation
at temperatures of lt100mK. The STM is attached to
a modified Oxford Instruments Kelvinox 100
dilution refrigerator. The system contains a 14T
superconducting magnet that allows magnetic
fields to be applied perpendicular to the sample.
The entire system is suspended by elastic ropes
to provide very effective vibration isolation.
Extensive testing of the system has been
conducted recently and it was found to be very
stable. We have studied the superconductor NbSe2
and have obtained images with atomic resolution
and spectroscopy with a resolution of 20mV. We
measured differential conductance spectra using a
lock-in amplifier with a 20mVpp oscillation. This
technique was used to image and perform
spectroscopy measurements on the Abrikosov flux
vortex lattice formed with magnetic fields of
0.1-2T. We are able to measure the density of
states inside and outside the vortices with a
resolution equal to that of Hess et al. (Phys.
Rev. Lett. 64, 2711 (1990)).
Schematic of the STM Head
Kelvinox 100 fridge insert
Differential Conductance Spectra
Abrikosov flux lattice
NbSe2 is a layered material that cleaves easily
to produce flat inert surfaces. At temperatures
lt7.2K NbSe2 is a type II superconductor with an
energy gap of 1.1meV. When a magnetic field is
applied it penetrates the sample through flux
vortices, which interact to arrange in a
triangular lattice. At the centre of each vortex
is a region of normal material that allows the
field to penetrate. The lattice can be imaged
either with a voltage less than the energy gap
or, as here, by recording the differential
conductance at a voltage slightly larger than the
gap energy (1.3mV). In such images the vortices
appear as darker regions. Spectroscopy
measurements are made using a lock-in amplifier
with a 20mV oscillation. This measures the
differential conductance vs. voltage, which is
proportional to the local density of states
(LDOS). At zero field or far away from a vortex
the LDOS is BCS-like, however the properties of
the material give a distribution of gap sizes.
This produces a more gradual slope at the gap
edges. Since the vortex core is normal metal
one might expect a flat DOS. However the
measurements show a pronounced peak at EF. This
is due to coupling between the normal metal and
the superconductor, which results in
quasiparticle (QP) bound states.
dI/dV
super- conducting region
3mV
-3mV
0
centre of a vortex
dI/dV
Vbias
-2mV
2mV
0
Imaging the Density of States of a Vortex at 0
Volts
Evolution of the Spectra Across a Single Vortex
-2mV
Voltage
120x120 nm, 0.1 T
160x160 nm, 0.4 T
-2mV
A
B
Distance
The wavefunctions of the QP bound states are
peaked near the centre of the vortex. This
produces a peak in the LDOS near the centre of
the vortex. Spatial maps of the conductance at 0V
have been measured. This is done by stopping the
STM feedback loop and measuring the differential
conductance for different biases. The feedback is
then restarted and the tip moved to the next
pixel location. All the points for the same bias
are then collated to produce images such as those
shown above.
It is very clear that the QP bound states are not
symmetric around the vortex, there is a clear
6-fold star pattern. It has been shown by Hayashi
et al. (Phys. Rev. Lett. 77, 4074 (1996)) that
while the vortex lattice can explain some of the
observed features only the anisotropic energy gap
can explain all of the features.
The image above is a graphical representation of
differential conductance measurements taken along
a line passing through the centre of single
vortex. The vertical scale is the bias voltage,
and the horizontal scale is the lateral
displacement, as shown in the schematic diagram.
The colours represent the magnitude of the
conductance (red high, bluelow). It is possible
to see the evolution from a BCS gap on the left
and right to a conductance peak in the centre.
For a review of all of the earlier work performed
by H.F. Hess and co-workers see Physica B 169,
422 (1991).