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Kin Fai Mak Group Trap Elusive Electrons

Now, a Cornell-led collaboration has developed a way to stack two-dimensional semiconductors and trap electrons in a repeating pattern that forms a specific and long-hypothesized crystal.

The team’s paper, “Correlated Insulating States at Fractional Fillings of Moiré Superlattices,” published Nov. 11 in Nature. The paper’s lead author is postdoctoral researcher Yang Xu.

The project grew out of the shared lab of Kin Fai Mak, associate professor of physics in the College of Arts and Sciences, and Jie Shan, professor of applied and engineering physics in the College of Engineering, the paper’s co-senior authors. Both researchers are members of the Kavli Institute at Cornell for Nanoscale Science; they came to Cornell through the provost’s Nanoscale Science and Microsystems Engineering (NEXT Nano) initiative.

Read More at Cornell Chronicle

Brad Ramshaw research team identify new type of superconductor

Until now, the history of superconducting materials has been a tale of two types: s-wave and d-wave.

Now, Cornell researchers – led by Brad Ramshaw, the Dick & Dale Reis Johnson Assistant Professor in the College of Arts and Sciences – have discovered a possible third type: g-wave.

Their paper, “Thermodynamic Evidence for a Two-Component Superconducting Order Parameter in Sr2RuO4,” published Sept. 21 in Nature Physics. The lead author is doctoral student Sayak Ghosh, M.S. ’19.

Electrons in superconductors move together in what are known as Cooper pairs. This “pairing” endows superconductors with their most famous property – no electrical resistance – because, in order to generate resistance, the Cooper pairs have to be broken apart, and this takes energy.

In s-wave superconductors – generally conventional materials, such as lead, tin and mercury – the Cooper pairs are made of one electron pointing up and one pointing down, both moving head-on toward each other, with no net angular momentum. In recent decades, a new class of exotic materials has exhibited what’s called d-wave superconductivity, whereby the Cooper pairs have two quanta of angular momentum.

Read more about the discovery on the Cornell Chronicle.

Natasha Holmes Helps Physics Teachers Transition to Online Teaching

APS Physics Editorial honors Natasha Homes amongst others for helping Physics Teachers making the transition to teaching online during the COVID pandemic.

The rapid switch to online teaching earlier this year was a huge and stressful adjustment, but physics teachers also learned valuable new tools and practices.

Group Dynamics Matter

My team develops introductory labs for undergraduates. In the shift to remote instruction, I found that the biggest loss was in the interactions with and between students. As we prepare for continued online instruction in the Fall, we are therefore deliberately structuring group interactions.

For example, we used to randomly assign students to groups of two to three. We’d then have students change groups a couple of times each semester. This year, we’ll instead survey students about their group-work preferences, asking whether they prefer to take a particular role, have a team leader, tinker with equipment before deciding what to do, and so on. These responses will help us form the groups. A couple of weeks into the semester, students will be asked to evaluate their group-work experiences, themselves, and their team. We will intervene if there are any issues. Each group will document how its members are participating in lab activities, so that they can consider whether their group work is fair and equitable. Finally, we will create ways for the students to check in with other groups and discuss their work.

These structures would have seemed burdensome before the pandemic. Now, they seem necessary to support students’ sense of community and sense of belonging. It’s also become clear that this kind of structure is important no matter how we teach, so we’ll continue to use it when we return to in-person instruction.

Natasha Holmes is the Ann S. Bowers Assistant Professor of Physics at Cornell University.

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Katja Nowack's Group Enhances Magnetic Imaging Scanning Probes

Cornell researchers used an ultrathin graphene “sandwich” to create a tiny magnetic field sensor that can operate over a greater temperature range than previous sensors, while also detecting miniscule changes in magnetic fields that might otherwise get lost within a larger magnetic background.

The group’s paper, “Magnetic Field Detection Limits for Ultraclean Graphene Hall Sensors,” published Aug. 20 in Nature Communications.

The team was led by Katja Nowack, assistant professor of physics in the College of Arts and Sciences and the paper’s senior author.

Nowack’s lab specializes in using scanning probes to conduct magnetic imaging. One of their go-to probes is the superconducting quantum interference device, or SQUID, which works well at low temperatures and in small magnetic fields.

“We wanted to expand the range of parameters that we can explore by using this other type of sensor, which is the Hall-effect sensor,” said doctoral student Brian Schaefer, the paper’s lead author. “It can work at any temperature, and we’ve shown it can work up to high magnetic fields as well. Hall sensors have been used at high magnetic fields before, but they’re usually not able to detect small magnetic field changes on top of that magnetic field.”

Read More Cornell Chronicle

Brad Ramshaw Named CIFAR Azrieli Global Scholar

Brad Ramshaw, the Dick & Dale Reis Johnson Assistant Professor of Physics in the College of Arts and Sciences, have been named CIFAR Azrieli Global Scholars.

CIFAR’s global scholars program supports outstanding early-career researchers through mentorship, an international network and professional skills development. The scholars also receive $100,000 CAD in unrestricted research support for two years.

McMahon and Ramshaw are among 13 recipients who were selected from 184 applications from 31 countries. The 2020-22 scholars represent academic institutions in Canada, France, Germany, Ireland, the United Kingdom and the United States. Cornell is the only university with more than one scholar in this year’s cohort.

In addition to funding emerging projects in interdisciplinary theme areas such as Life and Health, and Information and Matter, the program also helps the researchers connect with their peers.

Read More Cornell Chronicle

Eun-Ah Kim's Group Create First Model of Planckian Behaviour Down to Absolute Zero

A Cornell-led collaboration has used state-of-the-art computational tools to model the chaotic behavior of Planckian, or “strange,” metals. This behavior has long intrigued physicists, but they have not been able to simulate it down to the lowest possible temperature until now.

The team’s paper, “Linear Resistivity and Sachdev-Ye-Kitaev (SYK) Spin Liquid Behaviour in a Quantum Critical Metal with Spin-1/2 Fermions,” published July 22 in the Proceedings of the National Academy of Sciences. The study’s lead author is doctoral student Peter Cha.

Leading the collaboration is Eun-Ah Kim, professor of physics in the College of Arts and Sciences, who is interested in the social phenomena of electrons and how they interact as a society, with all the complications that entails.

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“Just as we have social distancing recommendations at the order of our governor, electrons have social distancing recommendations at the order of Mother Nature,” Kim said. “But exactly how this social distancing order resulted in this particular, maximally chaotic behavior has been a mystery. How do you go from the mandate of, okay, you’re all repelling each other, to this particular form of chaotic, incongruent behavior? It suggests there is something in this very confusing state that is a seed for a very organized state.”

Read More Cornell Chronicle