The Degen Lab

Magnetic sensing with single spins in diamond

Diamond crystals host a number of fluorescent defects. One of these defects, the so-called nitrogen-vacancy (NV) color center, has a single electronic spin associated with it. This spin can be controlled individually based on optically detected magnetic resonance, can be implemented in nanodiamonds less than ten nanometers in size, and can thus be used as sensitive magnetic detector with nanometer spatial resolution. This "scanning diamond magnetometry" was proposed only very recently and is a genuine idea from our lab. See highlight in Nature Nanotechnology, June 2008 (link, pdf).

In our lab, we intend to integrate high-quality NV centers into scanning probes, so that we can apply them for directly mapping electron and nuclear spins on surfaces with nanometer resolution. Combination with magnetic resonance spectroscopy may then permit full chemical analysis of materials and study of surface dynamics.

On this chart you see how the sensitivity of diamond sensors compares to other nanoscale magnetic sensors.

• Papers: C. L. Degen, "Scanning magnetic field microscope with a diamond single-spin sensor", Appl. Phys. Lett. 92, 243111 (2008). (link)
  See also: J. R. Maze et al., Nature 455, 644 (2008) (link) and G. Balasubramanian et al., Nature 455, 648 (2008) (link).

Nano-MRI of 1-100 nm sized biological objects

One of our main goals is the application of nanoscale spin detection to the 3D imaging of important biological structures by MRI. The main obstacles to these experiments are sensitivity and spatial resolution. Nanomechanical force sensors have, in the latest demonstration experiments, pushed the resolution of MRI to about 5 nm and enabled three-dimensional imaging of individual virus particles (a roughly 1000x improvement compared to conventional MRI techniques).

Our objective is to make these methods more widely applicable and to apply them to 1-100 nm-sized biological objects. Many of these objects are of extraordinary relevance to biology, including, for example, single virus particles, functional cellular units (like the ribosome), or Amyloid fibrils (implicated in Alzheimer’s disease). Our aim is to use Nano-MRI for resolving their superstructure and eventually locating specific functional units using isotopic labeling techniques. Another strategy is the combination with atomistic data from X-ray analysis (that is generally incomplete for large molecules), with the aim of reconstructing full atomic structures.

• Papers: C. L. Degen, M. Poggio, H. J. Mamin, C. T. Rettner, and D. Rugar, "Nanoscale magnetic resonance imaging", PNAS 106, 1313 (2009). (link)
• See also: The New York Times, January 13, 2008, on page D3 (link to the online version), and the News & Views article by Chris Hammel: P. C. Hammel, Nature 458, 844 (2009). (link) 

Nanomechanics and ultrasensitive force detection

Sensitive detection of magnetic forces requires mechanical structures that can measure very small forces. Some of the best sensitivities are achieved with micromechanical cantilevers, which reach detection limits in the range of Attonewtons (1e-18 N). Making even more sensitive mechanical structures is important not only for nanoscale spin detection, but also for mass sensing or testing of fundamental laws in, e.g., gravity.

Our lab explores limitations to these detectors. This includes studies of the various dissipation mechanisms. We are also interested in resonators made from new materials. These might not only offer better force sensitivity, but by studying their behavior we also learn more about the material’s fundamental properties.

Chemical surface identification with nanometer resolution

Local chemical characterization of nanostructured surfaces, especially when working  with organic materials like self-assembled monolayers or thin polymer films, is currently very hard or not possible. Due to the chemical specificity of magnetic resonance, it may be possible to apply spectroscopy techniques (like NMR) for exactly that purpose. Our lab’s objective is to adopt such techniques so as to simultaneously image a surface's chemical composition (using spectroscopy), spin density (using Nano-MRI) and topography (using the sensor in force microscopy mode) with nanometer resolution. This application is also very interesting for semiconductor devices as well as for a variety of nanostructured surfaces in material and energy science, like catalysts.

Spin physics in small structures

The physics of microscopic spin ensembles can be distinctly different from that of macroscopic ensembles. For example, in volumes of nuclear spins smaller than about < (100 nm)3, random spin flips generate a fluctuating polarization that exceeds the typical thermal (or Boltzmann) polarization. These spin fluctuations are a major source of dephasing in solid-state quantum systems, and their control is an important prerequisite for nanometer-scale magnetic resonance imaging (MRI) and spectroscopy. In our lab we focus on the fundamental physics that goes into these experiments on small numbers of spins. We also try to understand “back-action” effects from the detector on dynamics of spins as they become strongly coupled to, for example, a nanomechanical resonator.

• Papers: C. L. Degen, M. Poggio, H. J. Mamin, and D. Rugar, Nuclear spin relaxation induced by a mechanical resonator", Phys. Rev. Lett. 100, 137601 (2008). (link)