SLAC/Stanford team discovers new way of switching exotic properties on and off in topological material

Credit: Edbert Sie/Stanford University; Ella Maru Studio

SLAC/Stanford researchers have switched a material in and out of a topological state with novel electronic properties. The scientists controlled the switch with an invisible form of light, called terahertz radiation, which made layers of the material swing back and forth.

https://www.newswise.com/doescience/?article_id=706074&returnurl=aHR0cHM6Ly93d3cubmV3c3dpc2UuY29tL2FydGljbGVzL2xpc3Q=

A weird feature of certain exotic materials allows electrons to travel from one surface of the material to another as if there were nothing in between. 

Now, researchers have shown that they can switch this feature on and off by toggling a material in and out of a stable topological state with pulses of light. The method could provide a new way of manipulating materials that could be used in future quantum computers and devices that carry electric current with no loss.

Topological materials are particularly interesting for these applications because their electronic states are extraordinarily resistant to external perturbations, such as heating, mechanical pressure and material defects. But to make use of these materials, scientists also need ways to fine-tune their properties.

“Here, we’ve found an ultrafast and energy-efficient means of using light as an external perturbation to drive a material in and out of its stable topological state,” said Aaron Lindenberg, the study’s principal investigator and an associate professor at the Department of Energy’s SLAC National Accelerator Laboratory and Stanford University.

The SLAC/Stanford team published their results in Nature.

Controlling topology with light

In mathematics, topology describes how a geometric object can transform into various shapes without losing certain properties. For example, a sphere can morph into a flat disk but not into a doughnut, because that would require poking a hole in it.

In materials, the concept of topology is more abstract, but it similarly leads to extraordinary robustness: Materials in a topological state maintain their exotic properties, such as the ability to conduct electricity with very little loss, under external perturbation. 

“These materials offer an exciting platform for understanding new concepts in materials physics, and we’ve been actively learning new ways of utilizing their unique potential,” said Edbert Sie, a fellow at the Geballe Laboratory for Advanced Materials at Stanford working with Lindenberg and one of the new study’s lead authors. Research on topological materials has been honored with the 2016 Nobel Prize in Physics and a 2019 Breakthrough Prize.

Although topological materials are known for their stability, certain perturbations can also drive them out of their stable state. “In our own work, we’re looking for ways to use light and strain to manipulate topological materials and create new material states that could be useful for future applications,” Sie said.

This study focused on a topological material called tungsten ditelluride, which is made of stacked two-dimensional layers. Scientists have already proposed that when the material is in its topological state, the particular arrangement of atoms in those layers can generate what are called Weyl nodes that exhibit unique electronic properties such as zero-resistance conductivity. These points can be thought of as wormhole-like features that tunnel electrons between opposite surfaces of the material.

Sie and his colleagues set out to tweak the material’s properties with pulses of terahertz radiation, an invisible form of light whose wavelengths lie between infrared and microwave radiation. What they found took them by surprise: With the light, they were able to rapidly switch the material between its topological state and a non-topological state, effectively switching the zero-resistance state off and back on again.     

“It’s the first time anyone has seen this switching behavior,” said Clara Nyby, a graduate student on Lindenberg’s team and another lead author of the study. “Using terahertz radiation was the key here because its energy can efficiently drive this motion.”

Ultrafast ‘electron camera’ reveals material switch

To find out what exactly happened in the material, the researchers used SLAC’s instrument for ultrafast electron diffraction (UED) – a high-speed “electron camera” – to take rapid snapshots of the material’s atomic structure immediately after it was hit by a terahertz pulse.

They discovered that the pulses shifted neighboring atomic layers in opposite directions, distorting the material’s atomic structure. The structure began to oscillate, with layers swinging back and forth around their original positions (see animation above). Swinging in one direction, the material lost its topological property. Swinging in the other direction, the property reappeared and became more stable.

“There are many atomic motions that can potentially occur in the material,” said co-author Xijie Wang, head of SLAC’s UED team. “The combination of terahertz pulses and UED, used here for the first time, made this experiment possible. It allowed us to quickly identify this particular oscillatory motion.”

Co-author Das Pemmaraju, an associate staff scientist at SLAC, said, “The UED data were also the basis for calculations of the material’s electronic structure and its response to terahertz radiation. Our results demonstrate that the radiation drives the material out of its topological state and then back into it.”

It remains to be seen how this switching mechanism, for which the team has obtained a provisional patent, can actually be used. “It’s early in the game,” Sie said. “But the fact that we can manipulate topological materials in a rather simple manner using light and strain is of great potential.”

Next, the scientists want to apply their method to more materials and investigate how these structural modifications change their electronic properties, further exploring the world of topological materials science.

Parts of this work were done by researchers with the Stanford Institute for Materials and Energy Sciences (SIMES) and the Stanford PULSE Institute, which are jointly operated by SLAC and Stanford. Other institutions involved in the study were Columbia University and Florida State University. The work was primarily supported by the Department of Energy’s Office of Science.             

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SLAC is a multi-program laboratory exploring frontier questions in photon science, astrophysics, particle physics and accelerator research. Located in Menlo Park, Calif., SLAC is operated by Stanford University for the U.S. Department of Energy's Office of Science.

SLAC National Accelerator Laboratory is supported by the Office of Science of the U.S. Department of Energy. The Office of Science is the single largest supporter of basic research in the physical sciences in the United States, and is working to address some of the most pressing challenges of our time. For more information, please visit science.energy.gov.

Presentation & Abstract-SWITCHING OF 2D TOPOLOGICAL MATERIALS VIA SINGLE TERAHERTZ LIGHT PULSES

Abstract:  Tungsten ditelluride is a layered transition-metal dichalcogenide that crystalizes in a distorted hexagonal net with an orthorhombic unit cell (Td phase). The lack of inversion symmetry in this phase leads to a predicted new topological semimetal. Here, we use a single THz pulse to trigger a structural phase transition from the orthorhombic structure to a different metastable phase of this material in a reversible manner, and probe the switching using an ultrafast electron diffraction technique. Related studies in the alloy MoWTe2 as well as ultrafast measurements of the atomic-scale response in the just below switching threshold regime will also be presented. These findings serve as the first direct evidence of a THz field induced structural transition in a two-dimensional material, and offer a new promising way to optically control the topological properties of solids.

Location: 

3 Le Conte Hall

Speaker: 

Aaron M. Lindenberg

Affiliation: 

Stanford University

Research Area: 

Condensed Matter Physics And Materials Science

SLAC Researchers Explore Terahertz Realm

SLAC's Alan Fisher leads a talk, "Presentation and Future Concepts for Intense Terahertz from SLAC Accelerators," during the Sept. 5-6 "Frontiers of THz Science"... (Photo by Matt Beardsley)

https://news.slac.stanford.edu/features/researchers-explore-terahertz-realm

September 18, 2012

by Glenn Roberts Jr.

From detecting concealed weapons and other security threats to manipulating and studying molecules and nanomaterials, potential applications for terahertz (THz) radiation are varied and growing, noted scientists who participated in this month's "Frontiers of THz Science" workshop at SLAC.

The international workshop, held Sept. 5 and 6, drew 130 attendees from as far away as China and Germany for discussions about the latest techniques to generate and use THz radiation, which sits in a largely untapped band of the electromagnetic spectrum between far-infrared and microwave radiation.

In final sessions at the conference, scientists considered the most promising areas for discovery and the types of terahertz research that could best benefit from use of SLAC facilities and expertise.

There was general consensus that experiments using powerful sources of X-ray radiation, including SLAC’s Stanford Synchrotron Radiation Lightsource and Linac Coherent Light Source, could provide insights into how terahertz radiation can drive and control biomechanical, chemical and electrical processes, for example.

Mark Sherwin, a physicist at the University of California, Santa Barbara, who served as a co-chairman for a 2004 conference titled "Opportunities in THz science," said terahertz research since that time has become "a very broad worldwide community." 

Sherwin said SLAC, with its powerful X-ray facilities, is well-positioned to assist this community in exploring some of the previously unreachable areas of terahertz science. 

Because terahertz research is still a fledgling field, "There's a very problem-rich environment" to delve into, Sherwin told SLAC organizers of the 2012 workshop.

Andrea Markelz, a physicist at New York's University at Buffalo, suggested that using terahertz radiation to introduce structural changes, such as controlling a biomolecular function to study its various states, is one promising area of research. 

In a summary presentation, Aaron Lindenberg of SLAC and Stanford concluded that experiments can use terahertz radiation to study a wide range of condensed-matter physics and materials-science problems, from phase transitions to high efficiency, non-contact measurements of electrical properties of nanomaterials. 

And SLAC’s Kelly Gaffney said experiments using terahertz radiation as a "pump" to excite changes in sample materials, followed by X-rays to probe those changes, are perhaps the "most important opportunity to pursue." 

Also, he said, "If terahertz is an important spectral range to measure the properties of materials, it should be an important range to manipulate those properties and how they change over time."

Conference organizers, including SLAC's Chief Scientist Z-X Shen, Norbert Holtkamp, director of the Accelerator Directorate, and Jo Stöhr, LCLS director, will prepare a report based on the discussions and conclusions at the workshop. The report will chart progress in terahertz research since the 2004 conference and plot a course for the future of terahertz science.

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Shen will give a summary of this workshop at the LCLS/SSRL Annual Users' Meeting on Oct. 5. The deadline for registration and poster abstracts for LCLS/SSRL 2012 has been extended until Sept. 27.

New life for old electrons in biological imaging, sensing technologies

In this illustration of a terahertz pulse, light emerges from small sheet of metallic foil. (Image by Greg Stewart) http://www.nanowerk.com/news/newsid=23334.php (Nanowerk News) Using leftover high-speed electrons from SLAC's Linac Coherent Light Source, researchers have successfully generated intense pulses of light in a largely untapped part of the electromagnetic spectrum – the so-called terahertz gap.

Terahertz pulses – T-rays, for short – get their name from their frequency, which is 1 to 10 trillion cycles per second. Falling between visible frequencies and microwaves on the electromagnetic spectrum, terahertz waves are being used in new sensing technologies and biological imaging techniques that cause minimal radiation damage to samples.

Other potential uses include chemical and biological materials identification, photonic devices, microelectronics characterization and biomedical imaging. Until now, however, the quest to develop many of these applications has been frustrated by an inability to produce terahertz light that's bright enough.

A team led by SLAC National Accelerator Laboratory's Aaron Lindenberg reported success in a paper published Oct. 7 in Applied Physics Letters ("Single-cycle terahertz pulses with >0.2 V/Â field amplitudes via coherent transition radiation") We've generated terahertz light pulses with field strengths comparable to the forces that hold atoms together in materials or exist within nanoscale devices," said Lindenberg, a member of the Stanford PULSE Institute for Ultrafast Energy Science, and SIMES, the Stanford Institute for Materials and Energy Science. "It's a big leap forward in terms of the magnitude of the electromagnetic fields that we're generating." The work was a collaborative effort between investigators in photon science and the LCLS at SLAC. The LCLS generates intense X-rays for research by accelerating electrons and sending them wiggling through magnets. Once their work is done, the still-speedy electrons are shunted off to a particle dump. But Lindenberg and team thought of a clever way to reuse these electrons: They sent the electrons, which are gathered into breathtakingly short bunches only 100 millionths of a billionth of a second long, through a thin metallic foil. The electrons in the metal responded by radiating in phase with each other, emitting short, coherent bursts of terahertz light. Because these LCLS bursts are so short – comparable to the duration of a single cycle of the terahertz light wave, from peak to peak or trough to trough – they might be used to investigate and even control the functional properties of materials, said Lindenberg, an assistant professor of materials science and engineering, and of photon science, at Stanford. Beyond their potential for sensing and imaging, terahertz pulses create electric and magnetic fields that are capable of pushing atoms in specific directions, he said. These fields could be used, for example, to push electrons or ions within a solar cell or electrochemical device in order to study the first stages of processes within them. "In this paper, we've demonstrated the source and observed some first exciting examples of how these intense fields interact with materials. The next step is to apply this," Lindenberg said. "We're also interested in using these kinds of fields to understand how fast ferroelectric materials can switch their polarity, which is important for developing materials for next-generation information storage technologies, and understanding their photovoltaic response. You can use these kinds of single-cycle fields to study these intrinsic properties in ways never before possible." Using both the high-intensity terahertz fields and the LCLS X-rays simultaneously, "you could additionally probe with the X-rays to visualize what the atoms are doing," Lindenberg said. Other groups at SLAC already have expressed interest in using these new fields to manipulate surface chemistry. They could be used to snip a bond that binds atoms to the surface of a material, either to eject it from the sample or to drive it along the surface and control chemistry in defined ways. "Imagine you could take two atoms on a surface, push them together, and cause them to form a bond on an extremely short time scale," he said. "What would happen? It depends on the system, but no one really knows because no one has ever been able to do these kinds of things before." Source: By Janet Rae-Dupree, SLAC National Accelerator Laboratory