In the 2023 edition of IMPACT Research at Brown magazine, the University highlights quantum science research conducted by faculties from multiple departments including Physics. IMPACT, an annual publication from the Office of the Vice President for Research at Brown University, illuminates the trailblazing work of researchers who are pushing boundaries in their respective fields. This sixth issue of IMPACT continues its tradition of spotlighting influential research contributing to global advancement.
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Brown researchers use tiny magnetic swirls to generate true random numbers
Professor Gang Xiao’s Lab has developed a technique that can potentially generate millions of random digits per second by harnessing the behavior of skyrmions — tiny magnetic anomalies that arise in certain two-dimensional materials. Generating true random numbers is difficult, but they are extremely useful in cryptography and probabilistic computing. Read more.
CMOS transistors functioning at cryogenic temperatures point to potential capacitorless memory
Source: AIP Publishing Scilight, By Adam Liebendorfer
Cryogenic use of complementary metal oxide semiconductor (CMOS) transistors has garnered interest for providing control and readout circuitry in future quantum sensing and information technologies. At cryogenic temperatures, however, CMOS devices can deviate from standard transistor behavior.
Working in a joint Brown University-NIST collaboration on cryogenic magnetic sensor arrays, Zaslavsky et al. have observed and proposed a new application for the bistability that occurs in bulk CMOS transistors. Due to impact ionization changing the transistor body, the drain current exhibits sharp current jumps and stable hysteretic loops as a function of gate voltage, a finding that can be potentially used as a memory element.
“It should be noted that our memory only works at cryogenic temperatures, but we found that the retention time in our devices is surprisingly long, making the memory effectively nonvolatile on the time scales relevant to quantum sensing and computation,” said author Alexander Zaslavsky.
The approach was demonstrated for both n-type and p-type commercial-foundry 180 nanometer-process CMOS transistors when operated at voltages exceeding 1.3 to 1.5 V at cryogenic temperatures.
As various quantum computation schemes based on solid-state qubits advance towards practical implementation, Zaslavsky hopes the group’s work further stokes interest in filling future needs for compact capacitorless cryogenic memory fully compatible with the standard CMOS process.
The group looks to further characterize the performance parameters of cryogenic silicon transistors for low-temperature applications, such as their work on high-resolution imaging of magnetic fields.
Source: “Impact ionization-induced bistability in CMOS transistors at cryogenic temperatures for capacitorless memory applications,” by A. Zaslavsky, C. A. Richter, P. R. Shrestha, B. D. Hoskins, S. T. Le, A. Madhavan, and J. J. McClelland, Applied Physics Letters (2021). The article can be accessed at https://doi.org/10.1063/5.0060343.
Professors Mitrovic and Marston develop new technique to investigate electron spins in topological insulators
Working together, Professors Mitrovic and Marston and their graduate student have developed a technique called resistively detected nuclear magnetic resonance (RDNMR) that can be used to investigate the behavior of electron spins in topological insulators (TIs) with strong spin-orbit coupling. In these TIs, electron spins are locked to their momentum, making direct manipulation with microwaves impossible. However, RDNMR can be employed to probe the helical nature of surface conducting states in TIs. In this technique, a radio frequency (RF) field is applied to reorient nuclear spins, which then interact with electronic spins through the hyperfine interaction. This modulation of the electron spin affects the conductance, leading to changes at nuclear resonance frequencies. The direction of the applied magnetic field with respect to the helicity of the electrons influences the conductivity and provides insight into the nature of the conductive edge or surface states. Furthermore, their findings suggest that RDNMR can not only probe TI states but also enable the coherent manipulation of topologically protected states using RF control of nuclear spins, which could be valuable for future device applications.
This work has been published in Physical Review B. The article can be found here.
Researchers develop ultra-sensitive device for detecting magnetic fields
The new magnetic sensor is inexpensive to make, works on minimal power and is 20 times more sensitive than many traditional sensors.
A team of Brown University physicists has developed a new type of compact, ultra-sensitive magnetometer. The new device could be useful in a variety of applications involving weak magnetic fields, the researchers say.
“Nearly everything around us generates a magnetic field — from our electronic devices to our beating hearts — and we can use those fields to gain information about all these systems,” said Gang Xiao, chair of the Brown Department of Physics and senior author of a paper describing the new device. “We have uncovered a class of sensors that are ultra-sensitive, but are also small, inexpensive to make and don’t use much power. We think there could be many potential applications for these new sensors.”
The new device is detailed in a paper published in Applied Physics Letters. Brown graduate student Yiou Zhang and postdoctoral researcher Kang Wang were the lead authors of the research.
A traditional way of sensing magnetic fields is through what’s known as the Hall effect. When a conducting material carrying current comes into contact with a magnetic field, the electrons in that current are deflected in a direction perpendicular to their flow. That creates a small perpendicular voltage, which can be used by Hall sensors to detect the presence of magnetic fields.
The new device makes use of a cousin to the Hall effect — known as the anomalous Hall effect (AHE) — which arises in ferromagnetic materials. While the Hall effect arises due to the charge of electrons, the AHE arises from electron spin, the tiny magnetic moment of each electron. The effect causes electrons with different spins to disperse in different directions, which gives rise to a small but detectable voltage.
The new device uses an ultra-thin ferromagnetic film made of cobalt, iron and boron atoms. The spins of the electrons prefer to be aligned in the plane of the film, a property called in-plane anisotropy. After the film is treated in a high-temperature furnace and under a strong magnetic field, the spins of the electrons develop a tendency to be oriented perpendicular to the film with what’s known as perpendicular anisotropy. When these two anisotropies have equal strength, electron spins can easily reorient themselves if the material comes into contact with an external magnetic field. That reorientation of electron spins is detectable through AHE voltage.
It doesn’t take a strong magnetic field to flip the spins in the film, which makes the device quite sensitive. In fact, it’s up to 20 times more sensitive than traditional Hall effect sensors, the researchers say.
Key to making the device work is the thickness of the cobalt-iron-boron film. A film that’s too thick requires stronger magnetic fields to reorient electron spins, which decreases sensitivity. If the film is too thin, electron spins could reorient on their own, which would cause the sensor to fail. The researchers found that the sweet spot for thickness was 0.9 nanometers, a thickness of about four or five atoms.
The researchers believe the device could have widespread applications. One example that could be helpful to medical doctors is in magnetic immunoassay, a technique that uses magnetism to look for pathogens in fluid samples.
“Because the device is very small, we can put thousands or even millions of sensors on one chip,” Zhang said. “That chip could test for many different things at one time in a single sample. That would make testing easier and less expensive.”
Another application could be as part of an ongoing project in Xiao’s lab supported by the National Science Foundation. Xiao and his colleagues are developing a magnetic camera that can make high-definition images of magnetic fields produced by quantum materials. Such a detailed magnetic profile would help researchers better understand the properties of these materials.
“Just like a regular camera, we want our magnetic camera to have as many pixels as possible,” Xiao said. “Each magnetic pixel in our camera is an individual magnetic sensor. The sensors need to be small and they can’t consume too much power, so this new sensor could be useful in our camera.”
The research was supported by the National Science Foundation (OMA-1936221).
$2 million grant will support development of ‘magnetic camera’
A team of Brown University researchers will use a $2 million grant from the National Science Foundation to build a quantum mechanical magnetic camera, which will take snapshots of weak magnetic fields emanating from quantum materials. The camera will help researchers to understand the exotic materials that may one day be used in quantum computers and other quantum devices.
“Just as the camera on your phone has an array of photosensors that register light and create an image, our device will use magnetic sensors that can ‘see’ magnetic fields and make images or movies of magnetic patterns,” said Gang Xiao, chair of the physics department at Brown and principal investigator on the new grant. “We can learn a lot about quantum materials by observing in great detail the magnetic fields they produce, and that’s what this device will let us do.”
Quantum technologies make use of the often-peculiar behavior of individual subatomic particles. Harnessing that behavior could create computers than can perform calculations far beyond the reach of even the fastest of today’s supercomputers, sensors far more powerful than those used currently and potentially unbreakable encryption modes. Making these quantum tools work depends on a deeper understanding of how particles in quantum systems interact. Magnetic fields offer a window into those interactions, and the magnetic camera could potentially reveal the intricacies of those fields.
The challenge is making the device sensitive enough to register the ultra-weak magnetic signals generated by many quantum materials. To do that, the researchers will have to improve magnetic tunnel junctions (MTJs), tiny quantum mechanical sensors currently used to read information from computer hard disks. Xiao, who has studied MTJs and related nanoscale magnetic phenomena for years, will lead the team in investigating new materials for assembling MTJs and work with electronics experts to build specialized circuitry around them.
Joining Xiao on the team are three experts in quantum materials and phenomena: Vesna Mitrovic, Brad Marston and Kemp Plumb from Brown’s physics faculty. They’ll work with Professor of Engineering Alexander Zaslavsky and Senior Research Engineer William Patterson, both microelectronics experts.
The grant also includes funding for student entrepreneurship training, with an eye toward marketing the technology once it’s completed.
“The use for us is in exploring quantum materials, but if we’re able to scale this up, it could be useful for industry as well,” Xiao said. “A large enough camera could be useful in quality control for magnets used in a range of electronic devices. Similar devices could also be used in medical diagnostics to sense tiny shifts in magnetic fields generated by the heart or nervous system.”
Work on the project is scheduled to begin in January 2020.
Original article: https://www.brown.edu/news/2019-11-06/qmmc