Nitrogen-vacancy (NV) defect centers in diamond have recently exploded onto the scientific research scene. NV centers are extremely stable and have unique optical properties that enable a wide range of applications. In the field of quantum information science, NV centers may act as single photon sources for quantum computing applications. NV centers have also been demonstrated as quantum assisted sensing devices to resolve nanoscale variations in magnetic fields, electric fields, strain, temperature, and pressure. In the biological realm, NV centers have proven to be excellent biomarkers with unlimited photostability and low cytotoxicity.
These fields of research were sparked by the first electron paramagnetic resonance (EPR) measurements of a single NV defect, performed in 1997. The optically active version of the NV center involves an overall negative charge state with six electrons. Two electrons are donated by the nitrogen atom, three from the surrounding dangling carbon bonds of the vacancy, and one more electron is donated from the diamond lattice. The luminescence of the NV center is strongly coupled to its spin state and can therefore act as a quantum based sensor.
Montana Instruments has developed a cryogenic platform to meet the demanding needs of the NV center research community and help alleviate the barrier to entry for doing NV center research. Below you will find common experimental challenges faced by the research community and how Montana Instruments has helped researchers overcome these barriers.
- Diamond NV centers have low coherent evolution times at room temperature which can make specific spin state readout difficult without the use of complicated spectroscopic techniques.
- Coherent evolution time of the spin state is directly proportional to the sensitivity of pulsed detection techniques. Therefore, the sensitivity of low temperature experiments is much greater compared to those performed at room temperature!
- Temperature fluctuations can alter the response of the NV center and cause distortion of the experimental results
- Interesting material transitions often occur at low temperature. Examples include magnetic transitions and formation of magnetic textures such as vortices in high temperature superconductors, ferromagnetism, and skyrmion phase formation in helimagnets.
- Low vibrations and low temperature are required for maximum resolution of scanning probe experiments. Minimal vibrational disturbance of the experimental setup is also required for nanodiamond dispersal on a sample for NV center assisted mapping of material properties.
Keys for Optimizing an NV Center Experiment
An ultra-stable sample space enables the experimenter to carefully control critical environmental variables that can affect the fidelity of experimental results. A versatile and reconfigurable sample space provides experimental flexibility to adjust the optical setup as the laboratories needs and experimental requirements grow in complexity.
|Focus Area||Why It's Important||The Cryostation Difference|
|Low Temperatures (<4K)||Cryogenic temperatures maximize the readout sensitivity for specific spin states of the NV center.||Cryogenic temperatures <4K are accessed seamlessly. Dial in your experimental temperature using the automated software and your sample will be there shortly.|
|Temperature Stability||Temperature stability is required to maintain a constant signal response from the NV center.||The temperature stability at a specific set point can exceed 10mK.|
|Low Vibrations||Low vibrations provide a stable experimental platform with minimal disturbance to the local sample region and NV center being investigated for maximum resolving power.||-Minimal system vibrations (< 5nm; with options for < 1nm available) provide an ultra-stable cryogenic platform where the NV center can be used as a quantum sensor to map a sample with nanometer resolution. -Minimal sample drift when cooling from 300K to 4K allows you to easily track a single NV center feature across a wide range of temperatures.|
-Minimal sample drift when cooling from 300K to 4K allows you to easily track a single NV center feature across a wide range of temperatures.
|Sample & Optical Access||The sample needs to be optically accessible. A high NA objective lens for collecting fluorescence signal emanating from the NV center is required for single NV center experiments. Alternatively, a longer working distance, larger field of view experimental setup requires a large acceptance angle in the cryostat viewport for collecting array data on a CCD detector.||The Cryostation provides easy access to the sample space with unparalleled optical access options for low working distances (1-2mm) and high numerical aperture (NA) with the patented Cryo-Optic option. This helps optimize signal sensitivity and experimental resolution.|
|XYZ Positioning||Optionally, an XYZ platform may be required to perform scanning measurements to map the property of interest across a macroscopic range, or position a cantilever tip with a NV center under the focal spot of the objective lens.||Piezoelectric positioners can be added to your sample platform. Closed-loop stages enable scanning of the sample for mapping the property of interest across a large area.|
|Usability||Minimal disruptions to the experimental environment allow long collection times for low signal experiments.||No liquid helium refills are required! An experiment can be run for days or weeks at a time without ever stopping to refill the system from a helium Dewar. Helium transfers disrupt the local sample environment through mechanical vibrations.|
related techniques & CONFIGURATIONS
The Cryostation Base Platforms offer multiple solutions for Diamond NV Center research. A robust family of configurable options and accessories can be combined to meet the needs of various experimental techniques, including.
|Experimental Technique||Recommended Configuration||Research Spotlight|
|Confocal Microscopy||4106-A: CFM for Large Field of View|
|Scanning Confocal Microscopy||4106-B: Low Working Distance CFM|
4106-C: Scanning CFM
|Scanning Probe Microscopy||HILA Workstation||Jayich Lab|
Experimental Configurations for Cryogenic (4K) NV Center Research
Data Collection for an Ensemble of NV Centers
A magnetic sample is placed on a bulk diamond substrate. The diamond substrate has a thin surface layer that is rich with NV defects. The thin layer of uniform density NV defects can be formed using ion-implantation methods or by incorporating a nitrogen rich surface layer during the synthesis of a diamond substrate. An excitation laser, typically 532nm is used to promote the NV centers to the excited state and the fluorescence signal is collected between 630-800nm. A microwave signal is simultaneously used to excite and probe the spin state (ESR). The fluorescence signal is detected by a CCD camera that collects the data in a 2D array and can be mapped back to the original sample. The ensemble technique allows large area collection by using a long working distance for a larger field of view compared to single NV center measurements.
Single NV Center Research: Nanodiamond Dispersal on a Sample
Single NV centers in nanodiamonds can be studied in a confocal microscopy setup. The experimental setup includes a cryogenically compatible XYZ piezo stage stack. The Z motion allows the focal plane to be adjusted, and the XY motion enables scanning over the sample area (limited to roughly 5mm x 5mm at cryogenic temperatures). The patented Cryo-Optic design from Montana Instruments enables a low working distance and high NA objective lens to be used while maintaining the sample at 4K to provide the required collection efficiency and spot size for focusing on a single NV center. The excitation wavelength used is 532nm, and the fluorescence signal is collected between 630-800nm. A variable microwave signal is used to excite the spin state of the diamond NV center, and specific spin state of the NV center corresponds to a peak shift in the detected fluorescence signal. In order to study a sample, nanodiamonds (typically 20-30nm in diameter) are dispersed uniformly across a sample surface of interest. The XYZ piezo stack is then used to scan the sample and focus on individual NV centers. A single NV center can then be used to monitor the property of a material across a broad temperature range.
The Tokura Lab successfully implemented this technique to study the helimagnetic transition in an FeGe sample. The details of the experiment can be read here: Using NV-Center Optically Detected Magnetic Resonance (ODMR) as a Probe for Local Magnetic Dynamics in Transition Metals
Scanning Probe Quantum Sensor (ex. Scanning Magnetometry)
A NV center is located at the end of a cantilever tip and the tip is scanned in close proximity across the surface of a sample. In this scanning probe microscopy setup, a confocal light microscopy setup focuses the excitation light on the tip of the cantilever that hosts the NV center. The sample is scanned using cryogenically compatible piezo stages. The NV center can either be placed at the tip of the cantilever by grafting a nanodiamond crystal, or the tip can be manufactured using nanoimprint lithography combined with O2 etching of a bulk diamond substrate and subsequently performing 14N implantation to generate the NV center defect at the cantilever tip. The cantilever tip is aligned to the plane of the sample surface using a three axis tilting stage. The excitation laser is focused and located on the tip of the cantilever by adjusting the XYZ position of a final set of piezo stages anchored to the cold plate of the cryostat. The technique can be performed at cryogenic temperatures in a low vibration cryostat such as the Montana Instruments Nanoscale Workstation NW2.
The Jayich Lab successfully implemented this technique to image magnetic vortices in the iron pnictide superconductor BaFe2(As0.7P0.3)2 with a critical temperature of 30K. The technique creates great promise for studying solid state systems that exhibit exotic behavior at cryogenic temperatures. Additional details about the experiment can be read here: Scanned probe imaging of nanoscale magnetism at cryogenic temperatures with a single-spin quantum sensor
- Shields, B. J., Unterreithmeier, Q. P., de Leon, N. P., Park, H. & Lukin, M. D. Efficient readout of a single spin state in diamond via spin-to-charge conversion. Physical Review Letters 114, (2015).
- Dussaux, A. et al. Local dynamics of topological magnetic defects in the itinerant helimagnet FeGe. Nature Communications 7, 12430 (2016).
- Hong, S. et al. Nanoscale magnetometry with NV centers in diamond. MRS Bulletin 38, 155–161 (2013).
- Pelliccione, M. et al. Scanned probe imaging of nanoscale magnetism at cryogenic temperatures with a single-spin quantum sensor. arXiv:1510.02780 [cond-mat] (2015).