Research Highlights

Nanostructuring on length scales corresponding to phonon mean free paths provides control over heat flow in semiconductors and makes it possible, in principle, to engineer their thermal properties. However, this is currently not feasible because there is no general description for heat flow in 3D nanostructured semiconductors. In recent research, STROBE scientists used short wavelength extreme ultraviolet beams to study heat transport in a silicon metalattice with deep nanoscale features. They observed dramatically reduced thermal conductivity relative to bulk—about x50 times less than current model predictions. To explain this, they developed a new predictive theory that incorporates the idea that heat-carrying lattice vibrations can behave like a fluid—spreading out instead of just moving ballistically in straight lines. Moreover, this new theory of heat transport can be used to predict and engineer phonon transport in many other 3D nanosystems including nanowires and nanomeshes, that are of great interest for next-generation energy-efficient devices.

Though microscopes have been in use for centuries, there is still much that we cannot see at the smallest length scales. Current microscopies range from the simple optical microscopes used in high school science classes, to x-ray microscopes that can image through visibly-opaque objects, to electron microscopes that use electrons instead of light to capture images of vaccines and viruses. However, there is a great need to see beyond the static structure of an object—to be able capture how a nano- or biosystem functions in real time, or to visualize magnetic fields on nanometer scales. A team of researchers from the STROBE Center have been working together to overcome these challenges. STROBE is an NSF Science and Technology Center that is building the microscopes of tomorrow. A large multidisciplinary team from the Miao and Osher groups from UCLA, the Kapteyn-Murnane group at CU Boulder, Ezio Iacocca from CU Colorado Springs, David Shapiro and collaborators at Lawrence Berkley National Laboratory, and the Badding and Crespi groups from Pennsylvania State University. They developed and implemented a new method to use x-ray beams to capture the 3D magnetic texture in a material with very high 10-nanometer spatial resolution for the first time (published in Nature Nanotechnology, see reference below).

Hedgehogs and Anti-Hedgehogs

The team investigated a nanostructured magnetic sample, consisting of tiny spheres of nickel, only ~30nm across, connected together by slender few-nm “necks” of nickel, that together form a structure called a magnetic metalattice. This complex nanostructured magnet is expected to produce swirling magnetic fields with topological spin textures that are far more complex than in a uniform magnet. These are called 3D topological magnetic monopoles – or hedgehogs, due to their spiny shape in magnetic rotation – if the magnetic field points outward. Conversely, they can be thought of anti-hedgehogs if the magnetic field points inward.  However, until recently, there was no experimental method to measure the 3D spin texture at the deep nanoscale. Using advanced algorithms to recover the image, and a microscope at the x-ray synchrotron light source at the Lawrence Berkley National Laboratory, the researchers overcame these challenges.

Imaging spin textures is extremely important, as it can help physicists to better understand magnetism at a fundamental level, and to design more energy-efficient data storage, memory, and nanodevices.  Using electron microscopy, one can capture beautiful 2D images of a static spin-texture, but it is challenging to capture a full 3D image. In the past, other scientists were able to capture a 3D image at a spatial resolution of about 100 nanometers, but they had to make assumptions about the sample to extract the 3D image. With this new technique, researchers do not have to make any assumptions.

Armed with this new visualization technique, the team of researchers is excited to study spin textures further. STROBE is developing tabletop setups and helping with national facilities that can capture the static and dynamic spin texture in materials. All algorithms developed for this data analysis will be open-sourced soon. In this experiment, as with others, they found that collaboration is key for moving scientific progress forward.

Over the last decade, course-based undergraduate research experiences (CUREs) have been recognized as a way to improve undergraduate science, technology, engineering, and mathematics education by engaging students in authentic research practices. One of these authentic practices is participating in teamwork and collaboration, which is increasingly considered to be an important component of undergraduate research experiences and laboratory classes. For example, the American Association of Physics Teachers Recommendations for the Undergraduate Physics Laboratory Curriculum suggest that one of the goals for students in physics labs should be to develop “interpersonal communication skills” through “teamwork and collaboration.” Teamwork can have tremendous benefits for students, including increased motivation, creativity, and reflection; however, it can also pose an array of new social and environmental challenges, such as differing styles of communication, levels of commitment, and understanding of concepts. It can also be difficult for lab course instructors to evaluate and assess. In this work, we study student teamwork in a large-enrollment physics CURE. The CURE was specifically designed to emphasize teamwork as a scientific practice. We use the two sources of data, the adaptive instrument for regulation of emotions questionnaire and students’ written memos to future researchers, to measure the students’ teamwork goals, challenges, self, co-, and socially shared regulations, and perceived goal attainment. We find that students overwhelmingly achieved their teamwork goals by overcoming obstacles using primarily socially shared regulatory strategies, and that the vast majority of students felt teamwork was an essential part of their research experience. We discuss implications for the design of future CUREs and lab courses and for lab instructors desiring to assess teamwork in their own courses.

Electromigration is one of the most critical problems limiting the future scaling of integrated circuits (ICs).  As more transistors are packed into ever smaller volumes, the total length of interconnecting wire is increasing as the wire simultaneously becomes narrower and more vulnerable to failure. Surprisingly, while chip fabs are presently churning out microprocessors at the “5-nm process node” with correspondingly tiny interconnects, before recent work by a STROBE team no measurements of electromigration pressures had been reported at length scales smaller than 10 μm.

Using electron-beam lithography, the  team fabricates tiny aluminum nanowires on electron-transparent silicon nitride membranes. They then apply an electrical current density of 10⁸ A/cm²,  which is enough to drive electromigration, and map the nanowire’s density with parts-per-thousand precision using electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope (STEM).  Two separable effects appear.  First, thermal expansion decreases the wire’s density as Joule heating raises its temperature. Using aluminum’s known coefficient of thermal expansion, one can convert the density map into a temperature map with the same spatial resolution.  Second, via the electron “wind force” the electrical current simultaneously compresses one end of the wire while it tensions the other. Using aluminum’s known bulk modulus,  one can convert the density map into a pressure map.  The temperature effect is independent of the direction of the current, while the pressure effect changes sign with the current’s direction, which allows the two effects to be separated.  Thus the thermodynamic state variables T and P can be mapped with high spatial resolution everywhere inside the wire. The ability to simultaneously see the prompt thermal and electron-wind-force effects in nanoscale interconnects opens entirely new avenues for studying electromigration, a key technological problem in the microprocessors powering modern computing devices.

Liquids and solids are two fundamental states of matter. Although the structure of crystalline solids has long been solved by crystallography, our understanding of the 3D atomic structure of liquids and amorphous materials remained speculative due to the lack of direct experimental determination. Now, a collaborative team from UCLA, Lawrence Berkeley National Lab and Brown University has advanced atomic electron tomography to determine for the first time the 3D atomic positions in monatomic amorphous materials, including a Ta thin film and two Pd nanoparticles. Despite different chemical composition and synthesis methods, they observed that pentagonal bipyramids are the most abundant atomic motifs in these amorphous materials. Contrary to traditional understanding, most pentagonal bipyramids do not assemble icosahedra, but are closely connected to form networks extending to medium-range scales. Molecular dynamics simulations further revealed that pentagonal bipyramid networks are prevalent in monatomic metallic liquids, which rapidly grow in size and form more icosahedra during the quench from the liquid to the glass state. These results expand our fundamental understanding of the atomic structure of amorphous solids and will encourage future studies on amorphous-crystalline phase and glass transitions in non-crystalline materials with three-dimensional atomic resolution.

A team of physicists and engineers at CU Boulder have solved the mystery behind a perplexing phenomenon in the nano realm: why some ultra-small heat sources cool down faster if you pack them closer together. The research began with an unexplained observation: a team led by Margaret Murnane and Henry Kapteyn at JILA were experimenting with metallic nanolines on a silicon substrate that when heated with a laser, something strange occurred. Nanoscale heat sources do not usually dissipate heat efficiently. But if you pack them close together, they cool down much more quickly.

Now, the researchers know why it happens. The team joined forces with a group of theorists led by Mahmoud Hussein in Aerospace Engineering Sciences to use computer-based simulations to track the passage of heat from their nano-sized bars. The simulations were so detailed that they could follow the behavior of each and every atom in the model—millions of them in all. They discovered that when they placed the heat sources close together, the vibrations of energy (called phonons) they produced bounced off each other more efficiently when other heat sources were nearby, scattering heat away and cooling the bars down.

The group’s results highlight a major challenge in designing the next generation of tiny devices, such as microprocessors or quantum computer chips. When you shrink down to very small scales, heat does not always behave the way you think it should.