Current research projects

 

MRFM experiments in Condensed Matter at millikelvin temperatures

We explore the possibilities of using Magnetic Resonance Force Microscopy (MRFM), also called MRI-AFM, as a tool for experiments involving quantum matter like topological insulators (TI). Recent observations with ARPES and STS on TI have shown the presence of unique surface states; however these techniques have the disadvantage that they are limited to measuring the extremal surface. Conventional NMR techniques have the disadvantage that they are equally sensitive to the bulk as to the surface. MRI-AFM is able to combine microwave techniques of NMR with the surface sensitivity of scanning probes to perform NMR experiments locally, enabling the probing of the electronic susceptibility of the topological surface states. If successful, this technique could also be used to reveal the pairing mechanism in unconventional superconductors like the LAO/STO hetero-interface which is difficult to measure with conventional NMR.

NMR

The Lead Zeppelin

Magnetic Resonance Force Microscopy (MRFM) is a nanoscale 3D imaging technique based on the magnetic coupling between spins in a sample and a magnetic tip on a mechanical resonator. High sensitivity MRFM requires the lowest possible thermo-mechanical force noise, at sub-attonewton level, which can be achieved by using ultrasoft resonators at very low temperatures.

In traditional MRFM experiments the force sensor is an ultrasoft silicon cantilever with a small magnet attached to its end, whose motion is detected by means of a Superconducting QUantum Interference Device (SQUID); this scheme has been proven to be fully compatible with operation at milliKelvin temperatures. As an alternative, we are now developing a magnetically levitated small superconducting particle – or ‘‘Lead Zeppelin’’ – as our mechanical resonator. This could overcome the imperfections and noise of the silicon cantilever to such an extent, as to obtain a Q-factor of more than ~106, while having full control over the spring constant.

Going to ever higher sensitivities, we expect our resonator to become susceptible to the state of a single spin, e.g. an NV-center in diamond. This would allow us to study the quantum mechanical behaviour of a heavy mechanical resonator: by controlling the NV-center with a laser, we can have full control over an entangled NV-center – massive magnetic dipole system. Measuring very accurately the decoherence time of the heavy dipole, we expect to see a deviation from the environmental decoherence times. One of the motivations for this experiment is the breaking of unitarity in Quantum Mechanics for heavy objects.

 LeadZeppelin website combined

 

Breaking of Unitarity in Quantum Mechanics for Heavy Objects

Magnetic Resonance Force Microscopy (MRFM) is a nanoscale 3D imaging technique based on the interaction between spins in a sample and a magnetic tip on a mechanical resonator. High sensitivity MRFM requires the lowest possible thermo-mechanical force noise, at sub-attonewton level, which can be achieved by using ultrasoft silicon cantilevers at millikelvin temperatures. Because of this very low temperature the cantilever motion is detected by means of a superconducting pick-up loop coupled to a SQUID.

Active feedback cooling can cool the resonator further untill only a few phonons are left. Meanwhile the force sensitivity is low enough to measure single NV centers. The NV center under study can be optically pumped into its ground state which can be orientated orthogonal to the sensitive direction of the cantilever. This would create an interesting quantum system of a NV center and a macroscopic resonator. We are interested in the quantum state collapse time behaviour of this cantilever.

 

3D imaging of Biological Structures

In progress