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Author: Geraint Lewis

A Graphene Superconductor That Plays More Than One Tune

Researchers at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have developed a graphene device that’s thinner than a human hair but has a depth of special traits. It easily switches from a superconducting material that conducts electricity without losing any energy, to an insulator that resists the flow of electric current, and back again to a superconductor — all with a simple flip of a switch.

“Usually, when someone wants to study how electrons interact with each other in a superconducting quantum phase versus an insulating phase, they would need to look at different materials. With our system, you can study both the superconductivity phase and the insulating phase in one place,”

said Guorui Chen, the study’s lead author and a postdoctoral researcher in the lab of Feng Wang, who led the study. Wang, a faculty scientist in Berkeley Lab’s Materials Sciences Division, is also a UC Berkeley physics professor.

Trilayer Graphene Superlattice

The graphene device is composed of three atomically thin (2-D) layers of graphene. When sandwiched between 2-D layers of boron nitride, it forms a repeating pattern called a moiré superlattice.

The material could help other scientists understand the complicated mechanics behind a phenomenon known as high-temperature superconductivity, where a material can conduct electricity without resistance at temperatures higher than expected, though still hundreds of degrees below freezing.

In a previous study, the researchers reported observing the properties of a Mott insulator in a device made of trilayer graphene. A Mott insulator is a class of material that somehow stops conducting electricity at hundreds of degrees below freezing despite classical theory predicting electrical conductivity.

But it has long been believed that a Mott insulator can become superconductive by adding more electrons or positive charges to make it superconductive, Chen explained.

Superconductivity

For the past 10 years, researchers have been studying ways to combine different 2-D materials, often starting with graphene — a material known for its ability to efficiently conduct heat and electricity. Out of this body of work, other researchers had discovered that moiré superlattices formed with graphene exhibit exotic physics such as superconductivity when the layers are aligned at just the right angle.

“So for this study we asked ourselves, ‘If our trilayer graphene system is a Mott insulator, could it also be a superconductor?'”

said Chen.

Working with David Goldhaber-Gordon of Stanford University and the Stanford Institute for Materials and Energy Sciences at SLAC National Accelerator Laboratory, and Yuanbo Zhang of Fudan University, the researchers used a dilution refrigerator, which can reach intensely cold temperatures of 40 millikelvins — or nearly minus 460 degrees Fahrenheit — to cool the graphene/boron nitride device down to a temperature at which the researchers expected superconductivity to appear near the Mott insulator phase, said Chen.

Mott Insulator

Once the device reached a temperature of 4 kelvins (minus 452 degrees Fahrenheit), the researchers applied a range of electrical voltages to the tiny top and bottom gates of the device.

As they expected, when they applied a high vertical electrical field to both the top and bottom gates, an electron filled each cell of the graphene/boron nitride device. This caused the electrons to stabilize and stay in place, and this “localization” of electrons turned the device into a Mott insulator.

The graphene/boron nitride moiré superlattice
The graphene/boron nitride moiré superlattice material is composed of three atomically thin (2D) layers of graphene (gray) sandwiched between 2D layers of boron nitride (red and blue) to form a repeating pattern called a moiré superlattice. Superconductivity is indicated by the light-green circles.
Credit: Guorui Chen et al./Berkeley Lab

Then, they applied an even higher electrical voltage to the gates. To their delight, a second reading indicated that the electrons were no longer stable. Instead, they were shuttling about, moving from cell to cell, and conducting electricity without loss or resistance.

In other words, the device had switched from the Mott insulator phase to the superconductor phase.

Chen explained that the boron nitride moiré superlattice somehow increases the electron-electron interactions that take place when an electrical voltage is applied to the device, an effect that switches on its superconducting phase. It’s also reversible — when a lower electrical voltage is applied to the gates, the device switches back to an insulating state.

A Tiny Playground

The multitasking device offers scientists a tiny, versatile playground for studying the exquisite interplay between atoms and electrons in exotic new superconducting materials with potential use in quantum computers — computers that store and manipulate information in qubits, which are typically subatomic particles such as electrons or photons — as well as new Mott insulator materials that could one day make tiny 2-D Mott transistors for microelectronics a reality.

“This result was very exciting for us. We never imagined that the graphene/boron nitride device would do so well,” Chen said. “You can study almost everything with it, from single particles to superconductivity. It’s the best system I know of for studying new kinds of physics,”

Chen said.

This study was supported by the Center for Novel Pathways to Quantum Coherence in Materials (NPQC), an Energy Frontier Research Center led by Berkeley Lab and funded by the DOE Office of Science. NPQC brings together researchers at Berkeley Lab, Argonne National Laboratory, Columbia University, and UC Santa Barbara to study how quantum coherence underlies unexpected phenomena in new materials such as trilayer graphene, with an eye toward future uses in quantum information science and technology.

Original Study: Signatures of tunable superconductivity in a trilayer graphene moiré superlattice

Top Image: Guorui Chen/Berkeley Lab

Programmed Nanorobots Seek And Destroy Cancer Tumors

Nanorobots have been successfully programmed to shrink tumors by cutting off their blood supply. This major advancement came from Arizona State University (ASU) scientists, in collaboration with researchers from the National Center for Nanoscience and Technology (NCNST), of the Chinese Academy of Sciences.

The demonstration of the technology, the first-of-its-kind study in mammals, used breast cancer, melanoma, ovarian and lung cancer mouse models.

“We have developed the first fully autonomous, DNA robotic system for a very precise drug design and targeted cancer therapy. Moreover, this technology is a strategy that can be used for many types of cancer, since all solid tumor-feeding blood vessels are essentially the same,”

said Hao Yan, director of the ASU Biodesign Institute’s Center for Molecular Design and Biomimetics and the Milton Glick Professor in the School of Molecular Sciences.

DNA Origami

Yan is an expert in the field of DNA origami, which in the past two decades, has developed atomic-scale manufacturing to build more and more complex structures.

The bricks to build their structures come from DNA, which can self-fold into all sorts of shapes and sizes  – all at a scale one thousand times smaller than the width of a human hair – in the hopes of one day revolutionizing computing, electronics and medicine.

That one day may be coming a bit faster than anticipated.

Nanomedicine is a new branch of medicine that seeks to combine the promise of nanotechnology to open up entirely new avenues for treatments, such as making minuscule, molecule-sized nanoparticles to diagnose and treat difficult diseases, especially cancer.

Nanoscale robots
Image courtesy of ASU Biodesign Institute

Until now, the challenge to advancing nanomedicine has been difficult because scientists wanted to design, build and carefully control nanorobots to actively seek and destroy cancerous tumors, while not harming any healthy cells.

The international team of researchers overcame this problem by using a seemingly simple strategy to very selectively seek and starve out a tumor.

Drug Nanocarriers

The work was initiated about 5 years ago. The NCNST researchers first wanted to specifically cut-off of tumor blood supply by inducing blood coagulation with high therapeutic efficacy and safety profiles in multiple solid tumors using DNA-based nanocarriers.

Prof. Hao Yan’s expertise has upgraded the nanomedicine design to be a fully programmable robotic system, able to perform its mission entirely on its own.

“These nanorobots can be programmed to transport molecular payloads and cause on-site tumor blood supply blockages, which can lead to tissue death and shrink the tumor,”

said Baoquan Ding, a professor at the NCNST, located in Beijing.

To perform their study, the scientists took advantage of a well-known mouse tumor model, where human cancer cells are injected into a mouse to induce aggressive tumor growth.

Once the tumor was growing, the nanorobots were deployed to come to the rescue.

Each nanorobot is made from a flat, rectangular DNA origami sheet, 90 nanometers by 60 nanometers in size. A key blood-clotting enzyme, called thrombin, is attached to the surface.

Thrombin

Thrombin can block tumor blood flow by clotting the blood within the vessels that feed tumor growth, causing a sort of tumor mini-heart attack, and leading to tumor tissue death.

First, an average of four thrombin molecules was attached to a flat DNA scaffold. Next, the flat sheet was folded in on itself like a sheet of paper into a circle to make a hollow tube.

They were injected with an IV into a mouse, then traveled throughout the bloodstream, homing in on the tumors.

The key to programming a nanorobot that only attacks a cancer cell was to include a special payload on its surface, called a DNA aptamer. The DNA aptamer could specifically target a protein, called nucleolin, that is made in high amounts only on the surface of tumor endothelial cells, and not found on the surface of healthy cells.

Once bound to the tumor blood vessel surface, the nanorobot was programmed, like the notorious Trojan horse, to deliver its unsuspecting drug cargo in the very heart of the tumor, exposing an enzyme called thrombin that is key to blood clotting.

The nanorobots worked fast, congregating in large numbers to quickly surround the tumor just hours after injection.

Safe and Effective

First and foremost, the team showed that the nanorobots were safe and effective in shrinking tumors.

“The nanorobot proved to be safe and immunologically inert for use in normal mice and, also in Bama miniature pigs, showing no detectable changes in normal blood coagulation or cell morphology,”

said Yuliang Zhao, also a professor at NCNST and lead scientist of the international collaborative team.

Most importantly, there was no evidence of the nanorobots spreading into the brain where it could cause unwanted side effects, such as a stroke.

“The nanorobots are decidedly safe in the normal tissues of mice and large animals,”

said Guangjun Nie, another professor at the NCNST and a key member of the collaborative team.

The treatment blocked tumor blood supply and generated tumor tissue damage within 24 hours while having no effect on healthy tissues. After attacking tumors, most of the nanorobots were cleared and degraded from the body after 24 hours.

by two days, there was evidence of advanced thrombosis, and 3 days, thrombi in all tumor vessels were observed.

thrombin molecule
Image courtesy of ASU Biodesign Institute

The key is to trigger thrombin only when it is inside tumor blood vessels. Also, in the melanoma mouse model, 3 out of 8 mice receiving the nanorobot therapy showed complete regression of the tumors. The median survival time more than doubled, extending from 20.5 to 45 days.

They also tried their system in a test of a primary mouse lung cancer model, which mimics the human clinical course of lung cancer patients. They showed shrinkage of tumor tissues after a 2-week treatment.

Important Milestone

For Yan, the important study milestone represents the end of the beginning for nanomedicine.

“The thrombin delivery DNA nanorobot constitutes a major advance in the application of DNA nanotechnology for cancer therapy,” said Yan. “In a melanoma mouse model, the nanorobot not only affected the primary tumor but also prevented the formation of metastasis, showing promising therapeutic potential.”

Yan and his collaborators are now actively pursuing clinical partners to further develop this technology.

“I think we are much closer to real, practical medical applications of the technology,” said Yan. “Combinations of different rationally designed nanorobots carrying various agents may help to accomplish the ultimate goal of cancer research: the eradication of solid tumors and vascularized metastases. Furthermore, the current strategy may be developed as a drug delivery platform for the treatment of other diseases by modification of the geometry of the nanostructures, the targeting groups and the loaded cargoes.”

The work was supported by grants from National Basic Research Plan of China, the National Natural Science Foundation of China, the National Distinguished Young Scientists program, Innovation Research Group of National Natural Science Foundation, Beijing Municipal Science & Technology Commission, CAS Interdisciplinary Innovation Team, Key Research Program of Frontier Sciences, CAS, and US National Institute of Health Director’s Transformative Research Award.

Original Study: A DNA nanorobot functions as a cancer therapeutic in response to a molecular trigger in vivo

Top Image: Jason Drees, Arizona State University

Testing Hyper-complex Quantum Theories With An Exotic Metamaterial

Physicists have looked for deviations from standard quantum mechanics, testing whether quantum mechanics requires a more complex set of mathematical rules.

A research team led by Philip Walther at the University of Vienna designed a photonic experiment using exotic metamaterials, which were fabricated at the University of California Berkeley. Their experiment supports standard quantum mechanics and allows the scientists to place bounds on alternative quantum theories.

The results could help to guide theoretical work in a search for a more general version of quantum mechanics.

Quantum mechanics is based on a set of mathematical rules, describing how the quantum world works. These rules predict, for example, how electrons orbit a nucleus in an atom, and how an atom can absorb photons, particles of light.

The standard rules of quantum mechanics work extremely well, but, given that there are still open questions regarding the interpretation of quantum mechanics, scientists are not sure whether the current rules are the final story.

This has motivated some scientists to develop alternative versions of the mathematical rules, which are able to properly explain the results of past experiments, but provide new insight into the underlying structure of quantum mechanics. Some of these alternative mathematical rules even predict new effects, which require new experimental tests.

Mathematical Rules In Daily Life

In everyday life, if we walk all the way around a park we end up back at the same place regardless of whether we choose to walk clockwise or counter-clockwise. Physicists would say that these two actions commute.

Not every action needs to commute, though.

If, on our walk around the park, we walk clockwise, and first find money lying on the ground and then encounter an ice cream man, we will exit the park feeling refreshed. However, if we instead travel counter-clockwise, we will see the ice cream man before finding the money needed to buy the ice cream. In that case, we may exit park feeling disappointed.

In order to determine which actions commute or do not commute physicists provide a mathematical description of the physical world.

In standard quantum mechanics, these mathematical rules use complex numbers. However, recently an alternative version of quantum mechanics was proposed which uses more complex, so-called “hyper-complex” numbers. These are a generalization of complex numbers.

With the new rules, physicists can replicate most of the predictions of standard quantum mechanics. However, hyper-complex rules predict that some operations that commute in standard quantum mechanics do not actually commute in the real world.

Hyper-complex Numbers

The research team has now tested for deviations from standard quantum mechanics predicted by the alternative hyper-complex quantum theory. In their experiment the scientists replaced the park with an interferometer, a device which allows a single photon to travel two paths at the same time.

They replaced the money and ice cream with a normal optical material and a specially designed metamaterial. The normal optical material slightly slowed down light as it passed through, whereas the metamaterial slightly sped the light up.

The rules of standard quantum mechanics dictate that light behaves the same no matter whether it first passes through a normal material and then through a metamaterial or vice versa. In other words, the action of the two materials on the light commutes.

In hyper-complex quantum mechanics, however, that might not be the case. From the behavior of the measured photons the physicists verified that hyper-complex rules were not needed to describe the experiment.

“We were able to place very precise bounds on the need for hyper-complex numbers to describe our experiment,” says Lorenzo Procopio, a lead author of the study.

However, the authors say that it is always very difficult to unambiguously rule something out. Lee Rozema, another author of the paper, says

“we still are very interested in performing experiments under different conditions and with even higher precision, to gather more evidence supporting standard quantum mechanics.”

This work has placed tight limits on the need for a hyper-complex quantum theory, but there are many other alternatives which need to be tested, and the newly-developed tools provide the perfect avenue for this.

Funding for the work was provided by the European Commission, the Austrian Science Fund, the Vienna Science and Technology Fund, and the United States Air Force Office of Scientific Research.

Original Paper: Single-photon test of hyper-complex quantum theories using a metamaterial
Image: MSc. Jonas Schmöle, Faculty of Physics, University of Vienna

Meter-scale Optical Coherence Tomography Depth Barrier Broken

The first ever optical coherence tomography (OCT) images of cubic meter volumes have been produced, through an industry-academic collaboration. The advance could open up many new uses for OCT in industry, manufacturing and medicine.

It also marks a milestone toward developing a high-speed, low-cost optical coherence tomography system on a single integrated circuit chip. James G. Fujimoto of the Massachusetts Institute of Technology (MIT), said:

“Our study demonstrates world-record results in cubic meter volume imaging, with at least an order of magnitude larger depth range and volume compared to previous demonstrations of three-dimensional OCT. These results provide a proof-of-principle demonstration for using OCT in this new regime.”

1.5-meter Area 3D OCT

Optical coherence tomography, first invented by Fujimoto’s group and collaborators in the 1990s, is currently the gold standard of care in ophthalmology and is increasingly used in cardiology and gastroenterology. Although OCT provides useful 3-D images with micron-scale resolution, it has been limited to imaging depths of just millimeters to a few centimeters.

The researchers report achieving high speed, 3-D OCT imaging with 15-micron resolution over a 1.5-meter area.

They demonstrated the new OCT approach by imaging a mannequin, (see top photo) a bicycle and models of a human brain and skull. They also conducted measurements of objects ranging in scale from meters to microns.

In addition to the advantages of high speeds and fine resolution, OCT enables imaging, profiling and distance measurement at multiple depths simultaneously while rejecting stray light.

The new technique could be particularly useful for industrial and manufacturing settings, where it could potentially be used to monitor processes, take technical measurements and nondestructively evaluate materials.

Macro-scale OCT could also enhance medical imaging, for example, by providing three-dimensional measurements in laparoscopy or mapping structures such as the upper airway.

Vertical Cavity Surface-emitting Laser

The light source behind meter-range OCT is a tunable vertical cavity surface-emitting laser (VCSEL) developed by Thorlabs Inc. and Praevium Research. It uses a MEMS device to rapidly change, or sweep, the laser’s wavelength over time to perform what is called swept-source OCT.

“Research by our group at MIT and our collaborators at Praevium Research and Thorlabs indicated that the coherence length of the VCSEL source was orders of magnitude longer than other swept laser technologies suitable for OCT, which suggested the possibility of long-range OCT imaging,”

said Ben Potsaid of MIT and Thorlabs Inc., coauthor of the paper.

The MIT researchers have experimented with the VCSEL light source for several years, but light detection and data acquisition remained a challenge. These hurdles were overcome by advanced optical components designed for telecommunications applications.

In this study, the researchers used a new silicon photonics coherent optical receiver developed by Acacia Communications that replaced several bulky OCT components with integrated optics on a tiny, low-cost, single-chip photonic integrated circuit (PIC).

Photonic Integrated Circuit

Importantly, the PIC receiver supports the very high electrical frequencies and wide range of optical wavelengths required for swept-source OCT while also enabling what is known as quadrature detection, which doubles the OCT imaging range for a given data acquisition speed.

“The development of OCT in the early 1990s greatly benefited from components and methods used in fiber optical communications,” said Fujimoto. “And still, 25 years later, advances in the optical communications industry continue to greatly benefit OCT.”

In the paper, the researchers showed that meter-range OCT can obtain a strong signal from surfaces of varying geometry and materials. Their tests also indicated the technique’s performance has not reached the fundamental limits for the VCSEL laser source or PIC receiver.

The researchers are working to develop and utilize even more low-cost, high-speed components with the goal of speeding up the data acquisition and processing steps. This could eventually allow real-time OCT imaging using customized integrated circuit chips.

“As PIC technology continues to advance, one can realistically expect full OCT systems on a single chip within the next five years, dramatically lowering the size and cost,” said Chris Doerr of Acacia Communications, coauthor of the paper. “This would allow more people all over the world to benefit from OCT and open up new applications.”

Original Paper: Cubic meter volume optical coherence tomography

Image: James G. Fujomoto, MIT

Early Universe Cosmic Microwave Background 3D Printed

A 3D printed map of the oldest light in the universe has been created by researchers at Imperial College London, and you can download the files and your print your own baby universe.

The cosmic microwave background is a glow that the universe has in the microwave range that maps the oldest light in the universe. It was imprinted when the universe first became transparent, instead of an opaque fog of plasma and radiation.

The cosmic microwave background (CMB) formed when the universe was only 380,000 years old – very early on in its now 13.8 billion-year history.

Cosmic Microwave Background Map

The Planck satellite is making ever-more detailed maps of the CMB, which tells astronomers more about the early universe and the formation of structures within it, such as galaxies. However, more detailed maps are increasingly difficult to view and explore.

To solve this difficulty, Dr Dave Clements from the Department of Physics at Imperial, and two final-year undergraduate students in Physics, have created the plans for 3D printing the CMB.

Dr Clements explains:

“Presenting the CMB in a truly 3D form, that can be held in the hand and felt rather than viewed, has many potential benefits for teaching and outreach work, and is especially relevant for those with a visual disability.

Differences in the temperature of the CMB relate to different densities, and it is these that spawned the formation of structure in the universe, including galaxies, galaxy clusters and superclusters.

Representing these differences as bumps and dips on a spherical surface allows anyone to appreciate the structure of the early universe. For example, the famous ‘CMB cold spot’, an unusually low temperature region in the CMB, can be felt as a small but isolated depression.”

The 3D Cosmic Microwave Background Map can be printed from a range of 3D printers, and two files types have been created by the team:

  • an STL file used for printing the monochrome version of the project
  • a VRML file used for printing the hollow coloured version of the project that includes the temperature differences represented as colors in addition to bumps and dips

STL stands for “stereolithography”, which is a 3D rendering that contains only a single color. This is the file format most people would use with desktop 3D printers. VRML (“vermal”, .WRL file extension) stands for “Virtual Reality Modeling Language”. VRML is a newer digital 3D file type that also includes color, so it can be used on desktop 3D printers with more than one extruder (i.e. two more nozzles that each can print with a different color plastic), or with full-color binder jetting technology.

You can download the files at this link.

Dr Dave Clements’ latest book is Infrared Astronomy – Seeing the Heat: from William Herschel to the Herschel Space Observatory

Original Study: Cosmic sculpture: a new way to visualise the cosmic microwave background

Image: courtesy of Imperial College London