Author: Zhewen Xu

Minneapolis — Conference

In March, I had the opportunity to travel to Minneapolis, USA, to attend the APS March Meeting 2024. The APS March Meeting is renowned for bringing together experts in condensed matter physics and quantum physics from around the world. It was also my first time participating in such a prestigious international conference during my PhD studies. 

At the conference, I presented a talk titled “Nuclear Magnetic Resonance with a Single NV Scanning Probe.” The focus of my presentation was on showcasing the innovative potential of using a scanning NV (nitrogen-vacancy) probe to perform nanoscale NMR (Nuclear Magnetic Resonance) measurements. This technology represents a significant advancement, as it enables high-resolution imaging at the nanoscale, which can be applied across a wide variety of scientific domains.

Figure 1. ESR 13 Zhewen XU at 2024 APS March Meeting

After the conference, my colleague Chaoxin and I embarked on a memorable trip along the East Coast of the United States. Our journey began in Boston, where we explored the historic Freedom Trail. Figure 2 captures the start of our adventure on this iconic trail, which is rich with American history and landmarks.

Figure 2. The start of the Freedom Trail in Boston
Figure 3. Ismael and I on the top of Rockefeller Center

From Boston, we headed to Washington DC and then to the New York City. One of the highlights of our visit was going to the top of Rockefeller Tower. The view from the observation deck was breathtaking, offering a perfect view of the sprawling city below. During our time at Rockefeller Center, I have met Ismael (ESR 10). Afterwards,  we had our pleasant dinner at a shrimp restaurant (from the movie “Forrest Gump”) near the Times Square.

Quantum sensing magnetometer and its spatial resolution

In recent years, the field of quantum sensing[1] has witnessed a revolutionary advancement with the emergence of nitrogen vacancy (NV)[2] centers as versatile and highly sensitive quantum probes. NV centers, found in diamond crystals, exhibit unique quantum properties that make them ideal candidates for a wide range of sensing applications.

The NV center consists of a nitrogen atom adjacent to a vacancy in the diamond lattice, along with two neighboring carbon atoms. It can be interrogated using optical techniques. When illuminated with green light, NV centers absorb photons and enter an excited state. Subsequently, they relax back to their ground state, emitting red fluorescence (c.f., Figure 1, box 1). The intensity and polarization of this fluorescence depend on the spin state of the NV center, which can be manipulated and read out using microwave and optical techniques. By precisely measuring changes in fluorescence, NV centers can be employed to sense and characterize various physical phenomena. On the other hand, as a single Q-bit, the electronic ground state of the NV center consists of a triplet spin configuration, i.e., m_s = 0, ±1. When no external magnetic field is applied, |-1> and |+1> energy levels are degenerated. However, when an external magnetic field is introduced, the energy levels experience a Zeeman splitting due to the interaction between the electron’s magnetic moment and the magnetic field. According to the Zeeman effect, the energy levels shift in energy, with the amount of splitting depending on the strength and direction of the magnetic field (c.f. Figure 1, box 2). By precisely measuring the energy splitting and phase differences between spin states, NV centers can be employed as ultra-sensitive sensors in diverse fields such as magnetometry, bio-imaging, quantum information processing.

Figure 1. NV center energy diagram and Zeeman splitting

Although NV center offers unparalleled sensitivity in real space imaging, one of its limitations is the requirement for close proximity to the sample surface. This constraint poses challenges for imaging finer structures that require a lower stand-off distance. The spatial resolution of Scanning NV magnetometry (SNVM) depends on the sensor-to-sample distance dNV , as opposed to optical microscopy, whose resolvability is constrained by the diffraction limit, and other quantum sensing technique such as scanning superconducting quantum interference devices microscopy (or scanning SQUID microscopy) , whose resolvability is limited by the sensor size. Figure 2 shows the change of the z-component of magnetic stray field originating from two dipoles[3] separated by distance δd measured by NV at different dNV .

Figure 2. The SNVM measured z-component magnetic stray field above two dipoles separated by distance δd is modified by the sensor to sample distance dNV. At dNV = δd (light green curve), the FWHM of individual dipole equals to the separation between two maxims. Rainbow color sequence represents the different dNV , from 0.4 δd to 1.6 δd at each 0.2 δd interval. Stray field curves have been vertically offset for clarity and have been multiplied by a given factor to compensate for the weaker signal due to the large dNV . 

It is evident that the peaks from two separated magnetic dipoles can be well resolved at small sensor-sample separation (dNV in the range of 0.4 to 0.8 of the dipoles separation δd). When dNV approximates δd, the distance between the two peaks equals the full width at half maximum (FWHM) of the stray field distribution curves. It makes distinguishing between two individual dipoles difficult. Beyond this point, the two magnetic dipoles can not be resolvable any longer. Analogous to the Rayleigh criterion, the spatial resolution in SNVM is defined as the smallest distance between NV and sources to be resolved.

Figure 3. Images of a room temperature multiferroics bismuth ferrite (BiFeO3) by scanning NV magnetometry at different NV-to-sample distance. With decreasing standoff distance dNV, the spin cycloid (‘zig-zag’ shape patterns) can be well resolved.

Figure 3 illustrates the improved spatial resolution of SNVM through imaging the widely studied room temperature multiferroic material bismuth ferrite (BiFeO3)[4-6], known for its non-collinear antiferromagnetic spin cycloid that has garnered significant research interest in recent years. By shifting the NV center 21 nm closer to the surface of the BiFeO3 sample, the intricate ‘zig-zag’ pattern of the spin cycloid becomes clearly discernible.

In conclusion, the improved spatial resolution of scanning NV magnetometry represents a significant technological advancement with far-reaching implications across scientific disciplines. Through innovative techniques and methodologies, we have pushed the boundaries of spatial resolution, enabling nanoscale imaging of magnetic fields with unparalleled precision. As this field continues to evolve, scanning NV magnetometry promises to revolutionize our understanding of magnetism, quantum phenomena, and biological systems, paving the way for transformative discoveries and technological innovations.

[1] Degen, Christian L., Friedemann Reinhard, and Paola Cappellaro. “Quantum sensing.” Reviews of modern physics89.3 (2017): 035002.

[2] Degen, Christian L. “Scanning magnetic field microscope with a diamond single-spin sensor.” Applied Physics Letters 92.24 (2008).

[3] Lima, Eduardo A., and Benjamin P. Weiss. “Obtaining vector magnetic field maps from single‐component measurements of geological samples.” Journal of Geophysical Research: Solid Earth 114.B6 (2009).

[4] Gross, Isabell, et al. “Real-space imaging of non-collinear antiferromagnetic order with a single-spin magnetometer.” Nature 549.7671 (2017): 252-256.

[5] Haykal, A., et al. “Antiferromagnetic textures in BiFeO3 controlled by strain and electric field.” Nature communications11.1 (2020): 1704.

[6] Chauleau, J-Y., et al. “Electric and antiferromagnetic chiral textures at multiferroic domain walls.” Nature materials 19.4 (2020): 386-390.

Hamburg — Secondment

I had a wonderful stay in Hamburg for the secondment in the middle of the year (from April to July). Thank Kirsten (SPEAR second supervisor), Vishesh (ESR 11) and Arturo (ESR 12) for the kind host. I enjoyed the lab work and discussions a lot with the group members. It was also very lucky that the period of time has actually the best weather of the year in Hamburg, with sunshine and breeze almost very single day.

The institution is located in the very center of the city, opposite with a prison, which looks quite similar as the one in TV series. Besides neighboring with the prison, in the vicinity of the institution there is also a Japanese garden. Jogging around the lovely garden after doing the daily lab work was perhaps one of the nicest things to do while in Hamburg secondment.

Japanese garden nearby

During the stay in Hamburg, I gave a presentation in the topic of ‘the spatial resolution of the scanning nitrogen vacancy magnetometer (SNVM) and nano-meter scale nuclear magnetic resonance (NMR) via nitrogen vacancy (NV) center in diamond’ at Prof. Wiesendanger’s group seminar. The knowledge exchange between SNVM and STM makes me realize the similarities between the two techniques are pretty intriguing. More and more researchers in the STM community are currently employing STM to study electron paramagnetic resonance (EPR) at the atomic scale.

Group activity
Escape room

I also had lots of fun in Hamburg, participating in the escape room activity with the group (thank Vishesh for the organization), attending Arturo’s birthday party, and viewing the movie Oppenheimer with the group members. Farewell to people who graduated (Julia) with PhD degree, and (Roberto) left for professorship in Netherlands, is also a nice part of memory in Hamburg.

Looking forward to visiting Hamburg again next year and working together with the Lab013 fellows.

Aachen — Farewell

Right before the 2nd ESR training event in IMEC, Leuven, I went back to Aachen, where I had spent 3 unforgettable years, to participate in the long overdue master’s graduation ceremony. The festival was a one-day event, filled with a variety of activities including exhibitions of academic work, live performances, and group photos of graduates. The highlight of the festival was the graduation ceremony, where the student received their degree certificates with pride and joy. I was so excited and so lucky to additionally be awarded the ‘Springorium Commemorative Medal’ for my Master’s degree with distinction in materials science. The city mayor and the university president also participated in the ceremony and delivered great speeches. A variety of cultural and entertainment shows were also performed by the students.

2022 RWTH Graduation Ceremony [Image is taken from]

Aachen is a city overflowing with culture, history, and innovations. It is also a very tolerant city, where people speaking different languages and having different backgrounds gather together here from all over. I am so grateful that I have lived here for years, have studied with plenty of smart brains, and have made lots of friends with whom I share considerably unforgettable memories.

Springorium Commemorative Medal
Aachen’s autumn

Zurich — Inception

It has been several months since I settled down in Zurich. I really enjoy the new start as a PhD student here at QZabre and Degen’s Group. People here are very nice and helpful, and willing to help me out when I get problems. Now I am occupied with the first project in my PhD life, i.e., to improve the spatial resolution of quantum sensor.

Quantum Sensing Magnetometer, QZabre AG

During the spare time, I’d like to explore the nature here in this country. Hiking around and breathing the fresh air, can always recharge me with maximum of energy so that I am able to dedicate myself back to the scientific work. 

É Mé (on the top of Rigi)

Sometimes hiking brings me unexpected rewards. One huge stone engraved with Chinese characters ‘É Méi (峨眉, one of the most well-known mountain in China)’ on the top of Mountain Rigi is one of these unexpected rewards. You cannot imagine how excited and touched I was, when I saw thing possessing the same root and culture with me myself in a foreign country, especially as a ‘wanderer’ who has been away from home for more than two years.

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