Jeehoon Kim, Alex Frenzel
Photo of force microscope.
We have constructed a custom variable temperature force microscope with the following features:
- Vibration isolation: floating room, floating table, internal suspension from damped springs.
- Temperature: 2K up to 340K
- Magnetic field: 5 Tesla vertical
- Scan Range: 35μm scan at room temperature;
several mm coarse (x,y,z) motion of sample with respect to tip
- Cantilever position detection:
fiber optic interferometer with λ=1550nm diode laser;
position noise better than 2×10-3 Å/√Hz
- Geometry: allows either horizontal or vertical cantilever orientation
Photo of force microscope setup, including vibration isolation.
NSF PHY 01-17795
Metal-Insulator Transition in VO2
Geometry of the atomic force microscope setup used to measure the conductivity of VO2
as a function of voltage applied to the sample.
We are currently investigating the electric field triggered insulator-to-metal transition in vanadium dioxide (VO2
) using conducting atomic force microscopy (CAFM). We have
demonstrated that a biased CAFM tip can be used to simultaneously change and measure the local conducting state of the VO2
sample at room temperature. Additionally, we are able
to visualize the transition in individual VO2
grains by collecting spatially-resolved IV characteristics. The goals of this research are to understand the physics behind the
electric field triggered MIT and to provide detailed spatial information about the transition, which will be necessary for incorporation of VO2
into practical devices.
Hysteresis loop in the conductivity of VO2
as a function of applied bias, in conducting-AFM geometry.
Shriram Ramanathan, Harvard University
Vortex Depinning in Iron Pnictide Superconductor NdO1-xFxFeAs
Iron pnictide superconductors, discovered in February 2008, are a new class of high Tc superconductors that have drawn much attention, both for their role as a foil to better
understand the cuprates, and for their possibly superior application potential. Key to the latter will be the understanding of vortex pinning in these materials. The two main families of
iron pnictide superconductors, "1111" and "122", have different vortex pinning properties. We use our magnetic force microscope to focus on the "1111" family of materials, especially
NdO1-xFxFeAs, which shows Tc>50K, among the highest of all the iron-pnictides. The "1111" system shows a large electrical anisotropy compared with the
"122" system. This may allow exotic vortex phases such as pancake vortices and Josephson vortices. We are exploring the nature of vortex pinning by using a magnetic-tipped
cantilever to manipulate vortices as a function of applied magnetic field and temperature.
Photo of single crystal NdO1-x
FeAs sample, grown by the group of Prof. Paul Canfield, Ames Lab, Iowa.
We use the MFM to image and manipulate a vortex in NdO1-x
FeAs at 5 K with a magnetic field of 1 Gauss. (A)
Image at a "safe" height where
tip-sample interactions are too weak to depin vortices. (B)
Lower the tip to increase the interaction and drag a vortex. (C)
Re-image at a "safe" height to demonstrate the
Paul Canfield, Ames Lab, Iowa