Selected Applications
Cryogenic Probe Stations
Differential Conductance Measurements on Pd Break-
Junctions
In this application, an attocube 4-point probe station is used to perform
tunneling spectroscopy measurements in metallic nanogap devices at
cryogenic temperatures. The samples consist of palladium nanowire con-
striction patterns on a Si/SiO 2 substrate, fabricated in arrays containing
between 60 and 100 devices. A single nanowire is contacted by two probes
and a thermally-assisted electromigration process is employed to create
a sub-nanometer tunneling gap (a). Differential conductance across the
gap is measured as a function of source-drain bias. A third probe is used to
contact the conducting substrate, which is used as a back gate.
a
Nanopositioners independently move each DC probe along the x, y, and z
directions, enabling electrical contact to each device in succession. The
contacting process is monitored by an inspection optics that provides
good optical resolution to securely contact the pads of about 50 - 100 µm
in size.
b
c
Tunneling gaps formed with a break junction technique may be used to
study a variety of complex physical phenomena, but the resulting de-
tailed geometric configuration of the electrodes will be different for each
device. Through the analysis of many samples one can extract generic
features which may then be anticipated with a certain frequency of occur-
rence. Transport measurements in Pd break junctions reveal anomalous
features in the spectrum of dI/dV vs. V SD at low temperatures (b). The
sharp, bias-symmetric features undergo a rapid decrease in both ampli-
tude and voltage position as temperature is increased, as demonstrated
in the colormap (c). The associated energy of these features evolves with
temperature in accord with a mean-field transition. As discussed in [1],
the origin of these differential conductance features is attributed to
inelastic processes involving a paramagnetic to ferromagnetic phase tran-
sition occurring at the site of the break junction tips. The accessibility of
this experiment has been greatly increased by attocube’s probe station
and has demonstrated how nanoscale confinement of seemingly simple
materials can lead to phenomena not observable in bulk samples and mac-
roscale thin films.
[1] G . D . Scott, J . J . Palacios, and D . Natelson, ACS Nano 4, 2831-2837 (2010)
attoMICROSCOPY
Sophisticated Tools for Science
PAGE 189