Physicists at the University of Basel have developed a minuscule instrument able to detect extremely faint magnetic fields. At the heart of the superconducting quantum interference device are two atomically thin layers of graphene, which the researchers combined with boron nitride. Instruments like this one have applications in areas such as medicine, besides being used to research new materials.
To measure very small magnetic fields, researchers often use superconducting quantum interference devices, or SQUIDs. In medicine, their uses include monitoring brain or heart activity, for example, while in the earth sciences researchers use SQUIDs to characterize the composition of rocks or detect groundwater flows. The devices also have a broad range of uses in other applied fields and basic research.
The team led by Professor Christian Schönenberger of the University of Basel's Department of Physics and the Swiss Nanoscience Institute has now succeeded in creating one of the smallest SQUIDs ever built. The researchers described their achievement in the scientific journal Nano Letters.
A superconducting ring with weak links
A typical SQUID consists of a superconducting ring interrupted at two points by an extremely thin film with normal conducting or insulating properties. These points, known as weak links, must be so thin that the electron pairs responsible for superconductivity are able to tunnel through them. Researchers recently also began using nanomaterials such as nanotubes, nanowires or graphene to fashion the weak links connecting the two superconductors.
As a result of their configuration, SQUIDs have a critical current threshold above which the resistance-free superconductor becomes a conductor with ordinary resistance. This critical threshold is determined by the magnetic flux passing through the ring. By measuring this critical current precisely, the researchers can draw conclusions about the strength of the magnetic field.
SQUIDs with six layers
"Our novel SQUID consists of a complex, six-layer stack of individual two-dimensional materials," explains lead author David Indolese. Inside it are two graphene monolayers separated by a very thin layer of insulating boron nitride. "If two superconducting contacts are connected to this sandwich, it behaves like a SQUID - meaning it can be used to detect extremely weak magnetic fields."
In this setup, the graphene layers are the weak links, although in contrast to a regular SQUID they are not positioned next to each other, but one on top of the other, aligned horizontally. "As a result, our SQUID has a very small surface area, limited only by the constraints of nanofabrication technology," explains Dr. Paritosh Karnatak from Schönenberger's team.
The tiny device for measuring magnetic fields is only around 10 nanometers high - roughly a thousandth of the thickness of a human hair. The instrument can trigger supercurrents that flow in minuscule spaces. Moreover, its sensitivity can be adjusted by changing the distance between the graphene layers. With the help of electrical fields, the researchers are also able to increase the signal strength, further enhancing the measurement accuracy.
Analyzing topological insulators
The Basel research team's primary goal in developing the novel SQUIDs was to analyze the edge currents of topological insulators. Topological insulators are currently a focus of countless research groups all over the world. On the inside, they behave like insulators, while on the outside - or along the edges - they conduct current almost losslessly, making them possible candidates for a broad range of applications in the field of electronics.
"With the new SQUID, we can determine whether these lossless supercurrents are due to a material's topological properties, and thereby tell them apart from non-topological materials. This is very important for the study of topological insulators," remarked Schönenberger of the project. In future, SQUIDs could also be used as low-noise amplifiers for high-frequency electrical signals, or for instance to detect local brainwaves (magnetoencephalography), as their compact design means a large number of the devices can be connected in series.
By layering different two-dimensional materials, physicists at the University of Basel have created a novel structure with the ability to absorb almost all light of a selected wavelength. The achievement relies on a double layer of molybdenum disulfide. The new structure’s particular properties make it a candidate for applications in optical components or as a source of individual photons, which play a key role in quantum research. The results were published in the scientific journal Nature Nanotechnology.
Novel two-dimensional materials are currently a hot research topic around the world. Of special interest are van der Waals heterostructures, which are made up of individual layers of different materials held together by van der Waals forces. The interactions between the different layers can give the resulting material entirely new properties.
Double layer unlocks crucial properties There are already van der Waals heterostructures that absorb up to 100 percent of light. Single-layers of molybdenum disulfide offer absorption capacities in this range. When light is absorbed, an electron vacates its original position in the valence band, leaving behind a positively charged hole. The electron moves to a higher energy level, known as the conduction band, where it can move freely.
The resulting hole and the electron are attracted to each other in accordance with Coulomb’s law, giving rise to bound electron-hole pairs that remain stable at room temperature. However, with single-layer molybdenum disulfide there is no way to control which light wavelengths are absorbed. “It is only when a second layer of molybdenum disulfide is added that we get tunability, an essential property for application purposes,” explains Professor Richard Warburton of the University of Basel’s Department of Physics and Swiss Nanoscience Institute.
Absorption and tunability Working in close collaboration with researchers in France, Warburton and his team have succeeded in creating such a structure. The physicists used a double layer of molybdenum disulfide sandwiched between an insulator and the electrical conductor graphene on each side.
“If we apply a voltage to the outer graphene layers, this generates an electric field that affects the absorption properties of the two molybdenum disulfide layers,” explains Nadine Leisgang, a doctoral student in Warburton’s team and lead author of the study. “By adjusting the voltage applied, we can select the wavelengths at which the electron-hole pairs are formed in these layers.”
“This research could pave the way for a new approach to developing optoelectronic devices such as modulators,” adds Richard Warburton. Modulators are used to selectively change a signal’s amplitude. Another potential application is generating individual photons, with important implications for quantum technology.
Graphene consists of a single layer of carbon atoms arranged in a honeycomb structure. The material is of interest not only in basic research but also for various applications given to its unique properties, which include excellent electrical conductivity as well as astonishing strength and rigidity. Research teams around the world are working to further expand these characteristics by substituting carbon atoms in the crystal lattice with atoms of different elements. Moreover, the electric and magnetic properties can also be modified by the formation of pores in the lattice.
Now, a team of researchers led by the physicist Professor Ernst Meyer of the University of Basel and the chemist Dr. Shi-Xia Liu from the University of Bern have succeeded in producing the first graphene ribbons whose crystal lattice contains both periodic pores and a regular pattern of nitrogen atoms. The structure of this new material resembles a ladder, with each rung containing two atoms of nitrogen.
In order to synthesize these porous, nitrogen-containing graphene ribbons, the researchers heated the individual building blocks step by step on a silver surface in a vacuum. The ribbons are formed at temperatures up to 220°C. Atomic force microscopy allowed the researchers not only to monitor the individual steps in the synthesis, but also to confirm the perfect ladder structure - and stability - of the molecule.
Using scanning tunneling microscopy, the scientists from the Department of Physics and the Swiss Nanoscience Institute (SNI) at the University of Basel also demonstrated that these new graphene ribbons were no longer electrical conductors, like pure graphene, but actually behaved as semiconductors. Colleagues from the Universities of Bern and Warwick confirmed these findings by performing theoretical calculations of the electronic properties. "The semiconducting properties are essential for the potential applications in electronics, as their conductivity can be adjusted specifically," says Dr. Rémy Pawlak, first author of the study.
From the literature, it is known that a high concentration of nitrogen atoms in the crystal lattice causes graphene ribbons to magnetize when subjected to a magnetic field. "We expect these porous, nitrogen-doped graphene ribbons to display extraordinary magnetic properties," says Ernst Meyer. "In the future, the ribbons could therefore be of interest for applications in quantum computing."
Silicene consists of a single layer of silicon atoms. In contrast to the ultra-flat material graphene, which is made of carbon, silicene shows surface irregularities that influence its electronic properties. Now, physicists from the University of Basel have been able to precisely determine this corrugated structure. As they report in the journal PNAS, their method is also suitable for analyzing other two-dimensional materials.
Since the experimental production of graphene, two-dimensional materials have been at the heart of materials research. Similar to carbon, a single layer of honeycombed atoms can be made from silicon. This material, known as silicene, has an atomic roughness, in contrast to graphene, since some atoms are at a higher level than others.
Silicene not completely flat
Now, the research team, led by Professor Ernst Meyer of the Department of Physics and the Swiss Nanoscience Institute of the University of Basel, has succeeded in quantitatively representing these tiny height differences and detecting the different arrangement of atoms moving in a range of less than one angstrom – that is, less than a 10-millionth of a millimeter.
“We use low-temperature atomic force microscopy with a carbon monoxide tip,” explains Dr. Rémy Pawlak, who played a leading role in the experiments. Force spectroscopy allows the quantitative determination of forces between the sample and the tip. Thus, the height in relation to the surface can be detected and individual atoms can be chemically identified. The measurements show excellent agreement with simulations carried out by partners at the Instituto de Ciencia de Materiales de Madrid (ICMM).
Different electronic properties
This unevenness, known as buckling, influences the electronic properties of the material. Unlike graphene, which is known to be an excellent conductor, on a silver surface silicene behaves more like a semiconductor. “In silicene, the perfect honeycomb structure is disrupted. This is not necessarily a disadvantage, as it could lead to the emergence of interesting quantum phenomena, such as the quantum spin hall effect,” says Meyer.
The method developed by the researchers in Basel offers new insights into the world of two-dimensional materials and the relationship between structure and electronic properties.