The 91探花 is proud to announce that 56 faculty and researchers who completed their work while at 91探花have been named on the list from Clarivate.

The annual list identifies researchers who demonstrated significant influence in their chosen field or fields through the publication of multiple highly cited papers during the last decade. Their names are drawn from the publications that rank in the top 1% by citations for field and publication year in the .

Highly Cited Researchers demonstrate significant and broad influence in their fields of research. The total list includes 7,131 awards from more than 1,300 institutions in 60 countries and regions. This small fraction of the global researcher population contributes disproportionately to extending the frontiers of knowledge and contributing to innovations that make the world healthier, more sustainable and which drive societal impact, according to Clarivate.

The that determines the 鈥渨ho鈥檚 who鈥� of influential researchers is drawn from data and analysis performed by bibliometric experts and data scientists at the Institute for Scientific Information at Clarivate.

The list below includes faculty and researchers whose primary affiliation is with the UW, Fred Hutch Cancer Center, and the Institute for Health Metrics and Evaluation.

Please note: Some of the people on the list are no longer with the 91探花and their current affiliation is noted. This list reflects initial data from Clarivate and may be updated.

Ivan Anishchenko (Vilya)

David Baker

William A. Banks

Gregory N. Bratman

Steven L. Brunton

Guozhong Cao

Ting Cao

Lauren Carter (Gates Medical Research Institute)

Helen Chu

David H. Cobden

Katharine H. D. Crawford

Riza M. Daza

Frank DiMaio

Kristie L. Ebi

Evan E. Eichler

Emmanuela Gakidou

David Ginger

Raphael Gottardo (CHUV)

Alexander L. Greninger

Simon I. Hay

Andrew Hill (Infinimmune)

Eric Huang

Michael C. Jensen (BrainChild)

Neil P.聽 King

C. Dirk Keene

J. Nathan Kutz

Eric H. Larson

Aaron Lyon

Michael J. MacCoss

Brendan MacLean

C. M. Marcus

Julian D. Marshall

Ali Mokdad

Thomas J. Montine (Stanford)

Mohsen Naghavi

Marian L. Neuhouser

Julian D. Olden

Robert W. Palmatier

David Pigott

Hannah A. Pliner (Bristol Myers Squibb)

Ganesh Raghu

Stanley Riddell

Andrea Schietinger (Memorial Sloan Kettering Cancer Center)

Jay Shendure

M. Alejandra Tortorici

Troy R. Torgerson (Allen Institute)

Cole Trapnell

Katherine R. Tuttle

David Veesler

Theo Vos

Alexandra C. Walls (BioNTech SE)

Bryan J. Weiner

Di Xiao

Jie Xiao

Xiaodong Xu

Jihui Yang

The 91探花 is proud to announce that more than 40 faculty and researchers who completed their work while at 91探花have been named on the annual list from Clarivate.

The annual list identifies researchers who demonstrated significant influence in their chosen field or fields through the publication of multiple highly cited papers during the last decade. Their names are drawn from the publications that rank in the top 1% by citations for field and publication year in the Web of Science citation index.

The list of faculty and researchers whose primary affiliation is with the 91探花or with the Institute for Health Metrics and Evaluation who were acknowledged for their work includes:

David Baker

William A. Banks

Gregory N. Bratman

Steven L. Brunton

Guozhong Cao

William A. Catterall

Helen Chu

David H. Cobden

Katharine H.D. Crawford

Riza M. Daza

Frank DiMaio

Evan E. Eichler

Michael Gale Jr.

Raphael Gottardo

Allison J. Greaney

Alexander L. Greninger

Simon I. Hay

Celestia S. Higano

Neil P. King

James B. Leverenz

Charles M. Marcus

Philip Mease

Ali Mokdad

Thomas J. Montine*

Christopher J. L. Murray

Mohsen Naghavi

William S. Noble

Young-Jun Park

David M. Pigott

Stanley Riddell

Andrea Schietinger **

Jay Shendure

M. Alejandra Tortorici

Troy R. Torgerson***

Cole Trapnell

David Veesler

Theo Vos

Alexandra C. Walls****

Bryan J. Weiner

Spencer A. Wood

Sanfeng Wu

Di Xiao

Xiaodong Xu

The that determines the 鈥渨ho鈥檚 who鈥� of influential researchers draws on the data and analysis performed by bibliometric experts and data scientists at the Institute for Scientific Information at Clarivate. It also uses the tallies to identify the countries and research institutions where these scientific elite are based.

The full 2023 Highly Cited Researchers list and executive summary can be found online .

* now is at Stanford University.

** now is at Memorial Sloan Kettering Cancer Center.

*** now is at the Allen Institute.

**** now is at BoiNTech SE.

now is at Princeton University.

Quantum computing could revolutionize our world. For specific and crucial tasks, it promises to be exponentially faster than the zero-or-one binary technology that underlies today鈥檚 machines, from supercomputers in laboratories to smartphones in our pockets. But developing quantum computers hinges on building a stable network of qubits 鈥� or quantum bits 鈥� to store information, access it and perform computations.

Yet the qubit platforms unveiled to date have a common problem: They tend to be delicate and vulnerable to outside disturbances. Even a stray photon can cause trouble. Developing fault-tolerant qubits 鈥� which would be immune to external perturbations 鈥� could be the ultimate solution to this challenge.

A team led by scientists and engineers at the 91探花 has announced a significant advancement in this quest. In a pair of papers published and , they report that, in experiments with flakes of semiconductor materials 鈥� each only a single layer of atoms thick 鈥� they detected signatures of 鈥渇ractional quantum anomalous Hall鈥� (FQAH) states. The team鈥檚 discoveries mark a first and promising step in constructing a type of fault-tolerant qubit because FQAH states can host anyons 鈥� strange 鈥渜uasiparticles鈥� that have only a fraction of an electron鈥檚 charge. Some types of anyons can be used to make what are called 鈥渢opologically protected鈥� qubits, which are stable against any small, local disturbances.

鈥淭his really establishes a new paradigm for studying quantum physics with fractional excitations in the future,鈥� said , the lead researcher behind these discoveries, who is also the Boeing Distinguished Professor of Physics and a professor of materials science and engineering at the UW.

FQAH states are related to the , an exotic phase of matter that exists in two-dimensional systems. In these states, electrical conductivity is constrained to precise fractions of a constant known as the conductance quantum. But fractional quantum Hall systems typically require massive magnetic fields to keep them stable, making them impractical for applications in quantum computing. The FQAH state has no such requirement 鈥� it is stable even 鈥渁t zero magnetic field,鈥� according to the team.

Hosting such an exotic phase of matter required the researchers to build an artificial lattice with exotic properties. They stacked two atomically thin flakes of the semiconductor material molybdenum ditelluride (MoTe₂) at small, mutual 鈥渢wist鈥� angles relative to one another. This configuration formed a synthetic 鈥渉oneycomb lattice鈥� for electrons. When researchers cooled the stacked slices to a few degrees above absolute zero, an intrinsic magnetism arose in the system. The intrinsic magnetism takes the place of the strong magnetic field typically required for the fractional quantum Hall state. Using lasers as probes, the researchers detected signatures of the FQAH effect, a major step forward in unlocking the power of anyons for quantum computing.

The team 鈥� which also includes scientists at the University of Hong Kong, the National Institute for Materials Science in Japan, Boston College and the Massachusetts Institute of Technology 鈥� envisions their system as a powerful platform to develop a deeper understanding of anyons, which have very different properties from everyday particles like electrons. Anyons are quasiparticles 鈥� or particle-like 鈥渆xcitations鈥� 鈥� that can act as fractions of an electron. In future work with their experimental system, the researchers hope to discover an even more exotic version of this type of quasiparticle: 鈥渘on-Abelian鈥� anyons, which could be used as topological qubits. Wrapping 鈥� or 鈥渂raiding鈥� 鈥� the non-Abelian anyons around each other In this quantum state, information is essentially 鈥渟pread out鈥� over the entire system and resistant to local disturbances 鈥� forming the basis of topological qubits and a major advancement over the capabilities of current quantum computers.

鈥淭his type of topological qubit would be fundamentally different from those that can be created now,鈥� said 91探花physics doctoral student Eric Anderson, who is lead author of the Science paper and co-lead author of the Nature paper. 鈥淭he strange behavior of non-Abelian anyons would make them much more robust as a quantum computing platform.鈥�

Three key properties, all of which existed simultaneously in the researchers鈥� experimental setup, allowed FQAH states to emerge:

• Magnetism: Though MoTe₂ is not a magnetic material, when they loaded the system with positive charges, a 鈥渟pontaneous spin order鈥� 鈥� a form of magnetism called ferromagnetism 鈥� emerged.
• Topology: Electrical charges within their system have 鈥渢wisted bands,鈥� similar to a M枚bius strip, which helps make the system topological.
• Interactions: The charges within their experimental system interact strongly enough to stabilize the FQAH state.

The team hopes that, using their approach, non-Abelian anyons await for discovery.

鈥淭he observed signatures of the fractional quantum anomalous Hall effect are inspiring,鈥� said 91探花physics doctoral student , co-lead author on the Nature paper and co-author of the Science paper. 鈥淭he fruitful quantum states in the system can be a laboratory-on-a-chip for discovering new physics in two dimensions, and also new devices for quantum applications.鈥�

鈥淥ur work provides clear evidence of the long-sought FQAH states,鈥� said Xu, who is also a member of the Molecular Engineering and Sciences Institute, the Institute for Nano-Engineered Systems and the Clean Energy Institute, all at UW. 鈥淲e are currently working on electrical transport measurements, which could provide direct and unambiguous evidence of fractional excitations at zero magnetic field.鈥�

The team believes that, with their approach, investigating and manipulating these unusual FQAH states can become commonplace 鈥� accelerating the quantum computing journey.

Additional co-authors on the papers are William Holtzmann and Yinong Zhang in the 91探花Department of Physics; Di Xiao, Chong Wang, Xiaowei Zhang, Xiaoyu Liu and Ting Cao in the 91探花Department of Materials Science & Engineering; Feng-Ren Fan and Wang Yao at the University of Hong Kong and the Joint Institute of Theoretical and Computational Physics at Hong Kong; Takashi Taniguchi and Kenji Watanabe from the National Institute of Materials Science in Japan; Ying Ran of Boston College; and Liang Fu at MIT. The research was funded by the U.S. Department of Energy, the Air Force Office of Scientific Research, the National Science Foundation, the Research Grants Council of Hong Kong, the Croucher Foundation, the Tencent Foundation, the Japan Society for the Promotion of Science and the 91探花.

For more information, contact Xu at xuxd@uw.edu, Anderson at eca55@uw.edu and Cai at caidish@uw.edu.

The 91探花 is proud to announce that 47 faculty and researchers who completed their work while at 91探花have been named on the annual list from Clarivate.

The highly anticipated annual list identifies researchers who demonstrated significant influence in their chosen field or fields through the publication of multiple highly cited papers during the last decade. Their names are drawn from the publications that rank in the top 1% by citations for field and publication year in the Web of Science citation index.

The list of faculty and researchers who were acknowledged for their work while at 91探花includes:

• David Baker
• Frank DiMaio
• William Sheffler
• Dr. Jay Shendure
• Cole Trapnell
• David Veesler
• Alexandra C. Walls*
• Philip Mease
• Dr. Christopher J. L. Murray
• Dr. Ganesh Raghu
• Dr. Stanley Riddell
• Alejandra Tortorici
• Dr. William A. Banks
• Gregory N. Bratman
• Steven L. Brunton
• Guozhong Cao
• William A. Catterall
• David H. Cobden
• Riza M. Daza
• Dr. E. Patchen Dellinger
• Dr. Janet A. Englund
• E. Erskine
• Michael Gale Jr.
• Raphael Gottardo
• Celestia S. Higano
• Neil P. King
• Ali Mokdad
• William S. Noble
• Julian D. Olden
• L. Patrick
• David L. Smith
• Dr. Piper Meigs Treuting
• Spencer A. Wood
• Jesse R. Zaneveld
• Ning Zheng
• Dr. Hans D. Ochs
• Simon I. Hay
• Evan E. Eichler
• Deborah A. Nickerson**
• John A. Stamatoyannopoulos***
• Dr. Thomas J. Montine****
• Di Xiao
• Xiaodong Xu
• Bryan J. Weiner
• Mohsen Naghavi
• Theo Vos
• David M. Pigott

The that determines the 鈥渨ho鈥檚 who鈥� of influential researchers draws on the data and analysis performed by bibliometric experts and data scientists at the Institute for Scientific Information at Clarivate. It also uses the tallies to identify the countries and research institutions where these scientific elite are based. This year Clarivate partnered with Retraction Watch and extended the qualitative analysis of the Highly Cited Researchers list, addressing increasing concerns over potential misconduct.

The full 2022 Highly Cited Researchers list and executive summary can be found online .

* now is at BioNTech SE.

** on Dec. 24, 2021.

*** now is at Altius.

**** now is at Stanford University.

Researchers have discovered that light 鈥斅爄n the form of a laser 鈥斅燾an trigger a form of magnetism in a normally nonmagnetic material. This magnetism centers on the behavior of electrons. These subatomic particles have an electronic property called 鈥渟pin,鈥� which has a potential application in quantum computing. The researchers found that electrons within the material became oriented in the same direction when illuminated by photons from a laser.

The experiment, led by scientists at the 91探花, the University of Hong Kong and the Pacific Northwest National Laboratory, was April 20 in Nature.

By controlling and aligning electron spins at this level of detail and accuracy, this platform could have applications in the field of quantum simulation, according to co-senior author , a Boeing Distinguished Professor at the 91探花in the Department of Physics and the Department of Materials Science and Engineering, and scientist at the Pacific Northwest National Laboratory.

鈥淚n this system, we can use photons essentially to control the 鈥榞round state鈥� properties 鈥斅爏uch as magnetism 鈥斅爋f charges trapped within the semiconductor material,鈥� said Xu, who is also a faculty researcher with the UW鈥檚聽, the , and the . 鈥淭his is a necessary level of control for developing certain types of 鈥斅爋r 鈥榪uantum bits鈥� 鈥斅爁or and other applications.鈥�

Xu, whose research team spearheaded the experiments, led the study with co-senior author Wang Yao, professor of physics at the University of Hong Kong, whose team worked on the theory underpinning the results. Other 91探花faculty members involved in this study are co-authors , a 91探花professor of physics and of materials science and engineering who also holds a joint appointment at the Pacific Northwest National Laboratory, and , a 91探花professor of chemistry, director of the , and faculty member in the Clean Energy Institute and the Molecular Engineering & Sciences Institute.

The team worked with ultrathin sheets 鈥斅爀ach just three layers of atoms thick 鈥斅爋f tungsten diselenide and tungsten disulfide. Both are semiconductor materials, so named because electrons move through them at a rate between that of a fully conducting metal and an insulator, with potential uses in photonics and solar cells. Researchers stacked the two sheets to form a 鈥渕oir茅 superlattice,鈥� a stacked structure made up of repeating units.

Stacked sheets like these are powerful platforms for quantum physics and materials research because the superlattice structure can hold excitons in place. Excitons are bound pairs of 鈥渆xcited鈥� electrons and their associated positive charges, and scientists can measure how their properties and behavior change in different superlattice configurations.

The researchers were studying the exciton properties within the material when they made the surprising discovery that light triggers a key magnetic property within the normally nonmagnetic material. Photons provided by the laser 鈥渆xcited鈥� excitons within the laser beam鈥檚 path, and these excitons induced a type of long-range correlation among other electrons, with their spins all orienting in the same direction.

鈥淚t鈥檚 as if the excitons within the superlattice had started to 鈥榯alk鈥� to spatially separated electrons,鈥� said Xu. 鈥淭hen, via excitons, the electrons established exchange interactions, forming what鈥檚 known as an 鈥榦rdered state鈥� with aligned spins.鈥�

The spin alignment that the researchers witnessed within the superlattice is a characteristic of ferromagnetism, the form of magnetism intrinsic to materials like iron. It is normally absent from tungsten diselenide and tungsten disulfide. Each repeating unit within the moir茅 superlattice is essentially acting like a to 鈥渢rap鈥� an electron spin, said Xu. Trapped electron spins that can 鈥渢alk鈥� to each other, as these can, have been suggested as the basis for a type of qubit, the basic unit for quantum computers that could harness the unique properties of quantum mechanics for computation.

In a separate published Nov. 25 in Science, Xu and his collaborators found new magnetic properties in moir茅 superlattices formed by ultrathin sheets of chromium triiodide. Unlike the tungsten diselenide and tungsten disulfide, chromium triiodide harbors intrinsic magnetic properties, even as a single atomic sheet. Stacked chromium triiodide layers formed alternating magnetic domains: one that is ferromagnetic 鈥斅爓ith spins all aligned in the same direction 鈥斅燼nd another that is 鈥渁ntiferromagnetic,鈥� where spins point in opposite directions between adjacent layers of the superlattice and essentially 鈥渃ancel each other out,鈥� according to Xu. That discovery also illuminates relationships between a material鈥檚 structure and its magnetism that could propel future advances in computing, data storage and other fields.

鈥淚t shows you the magnetic 鈥榮urprises鈥� that can be hiding within moir茅 superlattices formed by 2D quantum materials,鈥� said Xu. 鈥淵ou can never be sure what you鈥檒l find unless you look.鈥�

First author of the Nature paper is Xi Wang, a 91探花postdoctoral researcher in physics and chemistry. Other co-authors are Chengxin Xiao at the University of Hong Kong; 91探花physics doctoral students Heonjoon Park and Jiayi Zhu; Chong Wang, a 91探花researcher in materials science and engineering; Takashi Taniguchi and Kenji Watanabe at the National Institute for Materials Science in Japan; and Jiaqiang Yan at the Oak Ridge National Laboratory. The research was funded by the U.S. Department of Energy; the U.S. Army Research Office; the U.S. National Science Foundation; the Croucher Foundation; the University Grant Committee/Research Grants Council of Hong Kong Special Administrative Region; the Japanese Ministry of Education, Culture, Sports, Science and Technology; the Japan Society for the Promotion of Science; the Japan Science and Technology Agency; the state of Washington; and the UW.

For more information, contact Xu at xuxd@uw.edu.

Scientists can have ambitious goals: curing disease, exploring distant worlds, clean-energy revolutions. In physics and materials research, some of these ambitious goals are to make ordinary-sounding objects with extraordinary properties: wires that can transport power without any energy loss, or quantum computers that can perform complex calculations that today鈥檚 computers cannot achieve. And the emerging workbenches for the experiments that gradually move us toward these goals are 2D materials 鈥� sheets of material that are a single layer of atoms thick.

In a published Sept. 14 in the journal Nature Physics, a team led by the 91探花 reports that carefully constructed stacks of graphene 鈥� a 2D form of carbon 鈥� can exhibit highly correlated electron properties. The team also found evidence that this type of collective behavior likely relates to the emergence of exotic magnetic states.

鈥淲e鈥檝e created an experimental setup that allows us to manipulate electrons in the graphene layers in a number of exciting new ways,鈥� said co-senior author , a 91探花assistant professor of physics and of materials science and engineering, as well as a faculty researcher at the UW聽.

Yankowitz led the team with co-senior author , a 91探花professor of physics and of materials science and engineering. Xu is also a faculty researcher with the 91探花, the 91探花 and the Clean Energy Institute.

Since 2D materials are one layer of atoms thick, bonds between atoms only form in two dimensions and particles like electrons can only move like pieces on a board game: side-to-side, front-to-back or diagonally, but not up or down. These restrictions can imbue 2D materials with properties that their 3D counterparts lack, and scientists have been probing 2D sheets of different materials to characterize and understand these potentially useful qualities.

But over the past decade, scientists like Yankowitz have also started layering 2D materials 鈥� like a stack of pancakes 鈥� and have discovered that, if stacked and rotated in a particular configuration and exposed to extremely low temperatures, these layers can exhibit exotic and unexpected properties.

The 91探花team worked with building blocks of bilayer graphene: two sheets of graphene naturally layered together. They stacked one bilayer on top of another 鈥� for a total of four graphene layers 鈥� and twisted them so that the layout of carbon atoms between the two bilayers were slightly out of alignment. Past research has shown that introducing these small twist angles between single layers or bilayers of graphene can have big consequences for the behavior of their electrons. With specific configurations of the electric field and charge distribution across the stacked bilayers, electrons display highly correlated behaviors. In other words, they all start doing the same thing 鈥� or displaying the same properties 鈥� at the same time.

鈥淚n these instances, it no longer makes sense to describe what an individual electron is doing, but what all electrons are doing at once,鈥� said Yankowitz.

鈥淚t鈥檚 like having a room full of people in which a change in any one person鈥檚 behavior will cause everyone else to react similarly,鈥� said lead author , a 91探花doctoral student in physics and a former Clean Energy Institute fellow.

Quantum mechanics underlies these correlated properties, and since the stacked graphene bilayers have a density of more than 10¹², or one trillion, electrons per square centimeter, a lot of electrons are behaving collectively.

The team sought to unravel some of the mysteries of the correlated states in their experimental setup. At temperatures of just a few degrees above absolute zero, the team discovered that they could 鈥渢une鈥� the system into a type of correlated insulating state 鈥� where it would conduct no electrical charge. Near these insulating states, the team found pockets of highly conducting states with features resembling superconductivity.

Though other teams have recently reported these states, the origins of these features remained a mystery. But the 91探花team鈥檚 work has found evidence for a possible explanation. They found that these states appeared to be driven by a quantum mechanical property of electrons called 鈥渟pin鈥� 鈥� a type of angular momentum. In regions near the correlated insulating states, they found evidence that all the electron spins spontaneously align. This may indicate that, near the regions showing correlated insulating states, a form of is emerging 鈥� not superconductivity. But additional experiments would need to verify this.

These discoveries are the latest example of the many surprises that are in store when conducting experiments with 2D materials.

鈥淢uch of what we鈥檙e doing in this line of research is to try to create, understand and control emerging electronic states, which can be either correlated or topological, or possess both properties,鈥� said Xu. 鈥淭here could be a lot we can do with these states down the road 鈥� a form of quantum computing, a new energy-harvesting device, or some new types of sensors, for example 鈥� and frankly we won鈥檛 know until we try.鈥�

In the meantime, expect stacks, bilayers and twist angles to keep making waves.

Co-authors are 91探花researchers Yuhao Li and Yang Liu; 91探花physics doctoral student and Clean Energy Institute fellow Jiaqi Cai; and K. Watanabe and T. Taniguchi with the National Institute for Materials Science in Japan. The research was funded by the 91探花Molecular Engineering Materials Center, a National Science Foundation Materials Research Science and Engineering Center; the China Scholarship Council; the Ministry of Education, Culture, Sports, Science and Technology of Japan; and the Japan Science and Technology Agency.

###

For more information, contact Xu at xuxd@uw.edu and Yankowitz at myank@uw.edu.

Scientists have visualized the electronic structure in a microelectronic device for the first time, opening up opportunities for finely tuned, high-performance electronic devices.

Physicists from the 91探花 and the University of Warwick developed a technique to measure the energy and momentum of electrons in operating microelectronic devices made of atomically thin 鈥� so-called 2D 鈥� materials.

Using this information, the researchers created visual representations of the electrical and optical properties of the materials to guide engineers in maximizing 2D materials’ potential in electronic components.

The , published July 17 in the journal , could also pave the way for the types of 2D semiconductors that are likely to play a role in the next generation of electronics, in applications such as photovoltaics, mobile devices and quantum computers.

The electronic structure of a material describes how electrons behave within that material, and therefore the nature of the current flowing through it. That behavior varies depending upon the voltage 鈥� the amount of “pressure” on its electrons 鈥� applied to the material, and so changes to the electronic structure with voltage determine the efficiency of microelectronic circuits.

These changes in electronic structure in operating devices are what underpin all of modern electronics. But until now there has been no way to directly see these changes to help us understand how they affect the behavior of electrons.

Their technique used angle-resolved photoemission spectroscopy, or ARPES, to “excite” electrons in the chosen material.

“It used to be that the only way to learn about what the electrons are doing in an operating semiconductor device was to compare its current-voltage characteristics with complicated models,” said co-corresponding author , a 91探花 professor of physics and faculty member in both the 聽and the . “Now, thanks to recent advances, which allow the ARPES technique to be applied to tiny spots, combined with the advent of two-dimensional materials where the electronic action can be right on the very surface, we can directly measure the electronic spectrum in detail and see how it changes in real time.”

By applying this technique, scientists will have the information they need to develop “fine-tuned” electronic components that work more efficiently and operate at high performance with lower power consumption. It will also help in the development of 2D semiconductors that are seen as potential components for the next generation of electronics, with applications in flexible electronics, photovoltaics and spintronics. Unlike today’s 3D semiconductors, 2D semiconductors consist of just a few layers of atoms.

“How the electronic structure changes with voltage is what determines how a transistor in your computer or television works,” said co-corresponding author , an associate professor of physics at the University of Warwick. “For the first time we are directly visualising those changes. Not being able to see how [structure] changes with voltages was a big missing link. This work is at the fundamental level and is a big step in understanding materials and the science behind them.”

By focusing a beam of ultraviolet or x-ray light on atoms in a localized area, the excited electrons are knocked out of their atoms. Scientists can then measure the energy and direction of travel of the electrons, which 鈥� thanks to laws for the conservation of energy and momentum 鈥� allows them to work out the energy and momentum they had within the material. That determines the electronic structure of the material, which can then be compared against theoretical predictions based on state-of-the-art electronic structure calculations performed by co-author ‘s group at the University of Warwick.

“This powerful spectroscopy technique will open new opportunities to study fundamental phenomena, such as visualization of electrically tunable topological phase transition and聽 doping effects on correlated electronic phases, which are otherwise challenging,” said co-corresponding author , a 91探花 professor of both physics and materials science and engineering, as well as faculty member in the .

The team first tested the technique using graphene before applying it to 2D transition metal dichalcogenide, or TMD, semiconductors.

“The new insight into the materials has helped us to understand the band gaps of these semiconductors, which is the most important parameter that affects their behavior, from what wavelength of light they emit, to how they switch current in a transistor,” said Wilson.

The measurements were taken at the at the , in collaboration with co-author .

“This changes the game,” said Cobden.

Lead author is Paul Nguyen, a 91探花 doctoral student in the Department of Physics. Co-authors are Nathan Wilson and Joshua Kahn, both 91探花 doctoral students in the Department of Physics; Natalie Teutsch and Xue Xia with the University of Warwick Department of Physics; Viktor Kandyba with the Elettra – Sincrotrone Trieste; and Gabriel Constantinescu with the University of Cambridge. The research was funded by the U.S. Department of Energy, the U.K. Engineering and Physical Sciences Research Council, the National Science Foundation, the University of Warwick, the Winton Programme for the Physics of Sustainability, the Cambridge Trust European Scholarship and the University of Cambridge.

###

Adapted from by Peter Thorley at the University of Warwick.

Future technologies based on the principles of quantum mechanics could revolutionize information technology. But to realize the devices of tomorrow, today’s physicists must develop precise and reliable platforms to trap and manipulate quantum-mechanical particles.

In a published Feb. 25 in the journal , a team of physicists from the 91探花, the University of Hong Kong, the Oak Ridge National Laboratory and the University of Tennessee reports that they have developed a new system to trap individual . These are bound pairs of electrons and their associated positive charges, known as holes, which can be produced when semiconductors absorb light. Excitons are promising candidates for developing new quantum technologies that could revolutionize the computation and communications fields.

The team, led by , the UW’s Boeing Distinguished Professor of both physics and materials science and engineering, worked with two single-layered 2D semiconductors, and , which have similar honeycomb-like arrangements of atoms in a single plane. When the researchers placed these 2D materials together, a small twist between the two layers created a “superlattice” structure known as a 鈥� a periodic geometric pattern when viewed from above. The researchers found that, at temperatures just a few degrees above absolute zero, this moir茅 pattern created a nanoscale-level textured landscape, similar to the dimples on the surface of a golf ball, which can trap excitons in place like eggs in an egg carton. Their system could form the basis of a novel experimental platform for monitoring excitons with precision and potentially developing new quantum technologies, said Xu, who is also a faculty researcher with the UW’s 聽and the .

Excitons are exciting candidates for communication and computer technologies because they interact with 鈥� single packets, or quanta, of light 鈥� in ways that change both exciton and photon properties. An exciton can be produced when a semiconductor absorbs a photon. The exciton also can later transform back into a photon. But when an exciton is first produced, it can inherit some specific properties from the individual photon, such as spin. These properties can then be manipulated by researchers, such as changing the spin direction with a magnetic field. When the exciton again becomes a photon, the photon retains information about how the exciton properties changed over its short life 鈥� typically, about a hundred nanoseconds for these excitons 鈥� in the semiconductor.

In order to utilize individual excitons’ “information-recording” properties in any technological application, researchers need a system to trap single excitons. The moir茅 pattern achieves this requirement. Without it, the tiny excitons, which are thought to be less than 2 nanometers in diameter, could diffuse anywhere in the sample 鈥� making it impossible to track individual excitons and the information they possess. While scientists had previously developed complex and sensitive approaches to trap several excitons close to one another, the moir茅 pattern developed by the UW-led team is essentially a naturally formed 2D array that can trap hundreds of excitons, if not more, with each acting as a , a first in quantum physics.

A unique and groundbreaking feature of this system is that the properties of these traps, and thus the excitons, can be controlled by a twist. When the researchers changed the rotation angle between the two different 2D semiconductors, they observed different optical properties in excitons. For example, excitons in samples with twist angles of zero and 60 degrees displayed strikingly different magnetic moments, as well as different helicities of polarized light emission. After examining multiple samples, the researchers were able to identify these twist angle variations as 鈥渇ingerprints鈥� of excitons trapped in a moir茅 pattern.

In the future, the researchers hope to systematically study the effects of small twist angle variations, which can finely tune the spacing between the exciton traps 鈥� the egg carton dimples. Scientists could set the moir茅 pattern wavelength large enough to probe excitons in isolation or small enough that excitons are placed closely together and could 鈥渢alk鈥� to one another. This first-of-its-kind level of precision may let scientists probe the quantum-mechanical properties of excitons as they interact, which could foster the development of groundbreaking technologies, said Xu.

“In principle, these moir茅 potentials could function as arrays of homogenous quantum dots,” said Xu. “This artificial quantum platform is a very exciting system for exerting precision control over excitons 鈥� with engineered interaction effects and possible properties, which could lead to new types of devices based on the new physics.”

“The future is very rosy,” Xu added.

Co-lead authors on the paper are Pasqual Rivera and Kyle Seyler, who conducted this research as 91探花doctoral students in physics and are now postdoctoral researchers at the 91探花and the California Institute of Technology, respectively. Co-corresponding author is Wang Yao at the University of Hong Kong. Co-authors are Hongyi Yu at the University of Hong Kong; Nathan Wilson and Essance Ray at the UW; and David Mandrus and Jiaqiang Yan at the Oak Ridge National Laboratory and the University of Tennessee. The research was funded by the U.S. Department of Energy, the CEI, the Croucher Foundation and the Research Grants Council of Hong Kong.

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For more information, contact Xu at xuxd@uw.edu or 206-543-8444.

Grant numbers: DE-SC0018171, HKU17302617

In published online May 3 in the journal , the researchers report that they used stacks of ultrathin materials to exert unprecedented control over the flow of electrons based on the direction of their spins 鈥� where the electron “spins” are analogous to tiny, subatomic magnets. The materials that they used include sheets of chromium tri-iodide (CrI₃), a material described in 2017 as . Four sheets 鈥� each only atoms thick 鈥� created the thinnest system yet that can block electrons based on their spins while exerting more than 10 times stronger control than other methods.

“Our work reveals the possibility to push information storage based on magnetic technologies to the atomically thin limit,” said co-lead author , a 91探花doctoral student in physics.

In , published April 23 in , the team found ways to electrically control the magnetic properties of this atomically thin magnet.

“With the explosive growth of information, the challenge is how to increase the density of data storage while reducing operation energy,” said corresponding author , a 91探花professor of physics and of materials science and engineering, and faculty researcher at the 91探花. “The combination of both works points to the possibility of engineering atomically thin magnetic memory devices with energy consumption orders of magnitude smaller than what is currently achievable.”

The new Science paper also looks at how this material could allow for a new type of memory storage that exploits the electron spins in each individual sheet.

The researchers sandwiched two layers of CrI₃ between conducting sheets of graphene. They showed that, depending on how the spins are aligned between each of the CrI颅颅₃ sheets, the electrons can either flow unimpeded between the two graphene sheets or were largely blocked from flowing. These two different configurations could act as the bits 鈥� the zeroes and ones of binary code in everyday computing 鈥� to encode information.

“The functional units of this type of memory are magnetic tunnel junctions, or MTJ, which are magnetic ‘gates’ that can suppress or let through electrical current depending on how the spins align in the junction,” said co-lead author , a 91探花postdoctoral researcher in physics. “Such a gate is central to realizing this type of small-scale data storage.”

With up to four layers of CrI₃, the team discovered the potential for “multi-bit” information storage. In two layers of CrI₃, the spins between each layer are either aligned in the same direction or opposite directions, leading to two different rates that the electrons can flow through the magnetic gate. But with three and four layers, there are more combinations for spins between each layer, leading to multiple, distinct rates at which the electrons can flow through the magnetic material from one graphene sheet to the other.

“Instead of your computer having just two choices to store a piece of data in, it can have a choice A, B, C, even D and beyond,” said co-author Bevin Huang, a 91探花doctoral student in physics. “So not only would storage devices using CrI₃ junctions be more efficient, but they would intrinsically store more data.”

The researchers鈥� materials and approach represent a significant improvement over existing techniques under similar operating conditions using magnesium oxide, which is thicker, less effective at blocking electrons and lacks the option for multi-bit information storage.

“Although our current device requires modest magnetic fields and is only functional at low temperature, infeasible for use in current technologies, the device concept and operational principle are novel and groundbreaking,” said Xu. “We hope that with developed electrical control of magnetism and some ingenuity, these tunnel junctions can operate with reduced or even without the need for a magnetic field at high temperature, which could be a game changer for new memory technology.”

Additional co-authors are Nathan Wilson, Kyle Seyler, Lin Zhu and David Cobden at the UW; co-corresponding author Wang Yao and Matisse Wei-Yuan Tu at the University of Hong Kong; co-corresponding author Di Xiao and Xiao-Ou Zhang at Carnegie Mellon University; Takashi Taniguchi and Kenji Watanabe at the National Institute for Materials Science in Tsukuba, Japan; and Michael McGuire at the Oak Ridge National Laboratory in Tennessee. The major funder of the research were the U.S. Department of Energy. Part of this work was performed at the at the Clean Energy Institute.

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For more information, contact Xu at xuxd@uw.ed or Song at tcsong@uw.edu.

Grant numbers: DE-SC0018171, DE-SC0012509, DMR-1708419, MRSEC-1719797

Magnetic materials form the basis of technologies that play increasingly pivotal roles in our lives today, including sensing and hard-disk data storage. But as our innovative dreams conjure wishes for ever-smaller and faster devices, researchers are seeking new magnetic materials that are more compact, more efficient and can be controlled using precise, reliable methods.

A team led by the 91探花 and the Massachusetts Institute of Technology has for the first time discovered magnetism in the 2-D world of monolayers, or materials that are formed by a single atomic layer. The findings, June 8 in the journal , demonstrate that magnetic properties can exist even in the 2-D realm 鈥� opening a world of potential applications.

“What we have discovered here is an isolated 2-D material with intrinsic magnetism, and the magnetism in the system is highly robust,” said , a 91探花professor of physics and of materials science and engineering, and member of the UW’s . “We envision that new information technologies may emerge based on these new 2-D magnets.”

Xu and MIT physics professor led the international team of scientists who proved that the material 鈥� chromium triiodide, or CrI₃ 鈥� has magnetic properties in its monolayer form.

Other groups, including co-author at the Oak Ridge National Laboratory, had previously shown that CrI₃ 鈥� in its multilayered, 3-D, bulk crystal form 鈥� is ferromagnetic. In ferromagnetic materials, the “spins” of constituent electrons, analogous to tiny, subatomic magnets, align in the same direction even without an external magnetic field.

But no 3-D magnetic substance had previously retained its magnetic properties when thinned down to a single atomic sheet. In fact, monolayer materials can demonstrate unique properties not seen in their multilayered, 3-D forms.

“You simply cannot accurately predict what the electric, magnetic, physical or chemical properties of a 2-D monolayer crystal will be based on the behavior of its 3-D bulk counterpart,” said co-lead author and 91探花doctoral student .

Atoms within monolayer materials are considered “functionally” two-dimensional because the electrons can only travel within the atomic sheet, like pieces on a chessboard.

To discover the properties of CrI₃ in its 2-D form, the team used Scotch tape to shave a monolayer of CrI₃ off the larger, 3-D crystal form.

“Using Scotch tape to exfoliate a monolayer from its 3-D bulk crystal is surprisingly effective,” said co-lead author and 91探花doctoral student Genevieve Clark. “This simple, low-cost technique was first used to obtain , the 2-D form of graphite, and has been used successfully since then with other materials.”

In ferromagnetic materials, the aligned spins of electrons leave a telltale signature when a beam of polarized light is reflected off the material’s surface. The researchers detected this signature in CrI₃ using a special type of microscopy. It is the first definitive sign of intrinsic ferromagnetism in an isolated monolayer.

Surprisingly, in CrI₃ flakes that are two layers thick, the optical signature disappeared. This indicates that the electron spins are oppositely aligned to one another, a term known as anti-ferromagnetic ordering. Ferromagnetism returned in three-layer CrI₃. The scientists will need to conduct further studies to understand why CrI₃ displayed these remarkable layer-dependent magnetic phases. But to Xu, these are just some of the truly unique properties revealed by combining monolayers.

“2-D monolayers alone offer exciting opportunities to study the drastic and precise electrical control of magnetic properties, which has been a challenge to realize using their 3-D bulk crystals,” said Xu. “But an even greater opportunity can arise when you stack monolayers with different physical properties together. There, you can get even more exotic phenomena not seen in the monolayer alone or in the 3-D bulk crystal.”

Much of Xu’s research centers on creating heterostructures, which are stacks of two different ultrathin materials. At the interface between the two materials, his team searches for new physical phenomena or new functions to allow potential applications in computing and information technologies.

In a related advance, Xu’s research group, 91探花electrical engineering and physics professor 聽and a team of colleagues published a May 31 in showing that an ultrathin form of CrI₃, when stacked with a monolayer of tungsten diselenide, creates a ultraclean “heterostructure” interface with unique and unexpected photonic and magnetic properties.

“Heterostructures hold the greatest promise of realizing new applications in computing, database storage, communications and other applications we cannot even fathom yet,” said Xu.

Xu and his team would next like to investigate the magnetic properties unique to 2-D magnets and heterostructures that contain a CrI₃ monolayer or bilayer.

The third co-lead author on the Nature paper is MIT researcher . Other co-authors are Dahlia Klein at MIT; Ran Cheng and Di Xiao at Carnegie Mellon University; Kyle Seyler, Ding Zhong, Emma Schmidgall and at the UW; and at the University of Hong Kong. Seyler, Zhong and Xiayu Linpeng, who are all 91探花doctoral students, are co-lead authors on the Science Advances paper.

The 91探花researchers for the Nature publication were funded by the Department of Energy and a 91探花 Innovation Award.

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For more information, contact Xu at xuxd@uw.edu or 206-543-8444.

Grant numbers for 91探花researchers for the Nature publication: DE-SC0008145, DE-SC0012509.