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.

Watching a Quantum Material Lose Its Stripes

Berkeley Lab study uses terahertz laser pulses to reveal ultrafast coupling of atomic-scale patterns

Sarah Yang

http://newscenter.lbl.gov/2017/11/29/watching-quantum-material-lose-stripes/

Stripes can be found everywhere, from zebras roaming in the wild to the latest fashion statement. In the world of microscopic physics, periodic stripe patterns can be formed by electrons within so-called quantum materials.

Illustration of an ultrashort laser light striking a lanthanum strontium nickel oxide crystal, triggering the melting of atomic-scale stripes. The charges (yellow) quickly become mobile while the crystal distortions react only with delay, exposing the underlying interactions. (Credit: Robert Kaindl/Berkeley Lab)

Scientists at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have now disentangled the intriguing dynamics of how such atomic-scale stripes melt and form, providing fundamental insights that could be useful in the development of novel energy materials.

In strongly correlated quantum materials, interactions between the electrons reign supreme. The complex coupling of these electrons with each other – and with electron spins and crystal vibrations – results in exotic phases such as charge ordering or high-temperature superconductivity.

“A key goal of condensed matter physics is to understand the forces responsible for complex phases and the transitions between them,” said Robert Kaindl, a principal investigator and staff scientist at Berkeley Lab’s Materials Sciences Division. “But in the microscopic world, interactions are often extremely fast. If we just slowly heat or cool a material to change its phase, we can miss out on the underlying action.”

Kaindl and his colleagues have been using ultrafast laser pulses to tease apart the microscopic dynamics of correlated quantum materials to access the interactions among the electrons and with the crystal’s atomic lattice in the time domain.

For this study, the researchers worked with lanthanum nickelate, a quantum material and model stripe compound. In particular, the researchers investigated the electronic charges that form the stripe pattern and how they couple to the crystal lattice.

How charges interact with the crystal is a key ingredient to stripe physics, the researchers said.

“The crystal lattice strongly distorts around the charge stripes,” said Giacomo Coslovich, who did the work while he was a postdoctoral researcher at Berkeley Lab. “This change of the crystal symmetry results in new lattice vibrations, which we can in turn detect with light at terahertz frequencies.”

Robert Kaindl (left) and Giacomo Coslovich next to the experimental setup that generates ultrashort light pulses in the near-infrared and terahertz spectral range. (Credit: Lingkun Zeng/Berkeley Lab)

Kaindl and Coslovich are corresponding authors of a paper reporting these results in Science Advances.

In their experiments, the material is optically excited by a near-infrared laser pulse with a duration of 50 femtoseconds, and probed with a terahertz pulse with variable time delay. A femtosecond is one millionth of one billionth of a second.

The researchers found unexpected dynamics when using the laser to disrupt the microscopic order.

“The interesting thing is that while the laser immediately excited the electrons, the vibrational distortions in the crystal initially remained frozen,” said Coslovich, who is now associate staff scientist at SLAC National Accelerator Laboratory. “The stripe-phase vibrations disappeared only after several hundred to a few thousand femtoseconds. We also concluded that the speed depends on the direction of the interactions.”

The interpretation of the experiments was supported by simulations of the phonon dispersion by Alexander Kemper of North Carolina State University.

The results provide important insight into the interactions, or “glue,” that couple electrons to lattice vibrations in the lanthanum nickelate. However, their broader relevance stems from recent observations of charge order in high-temperature superconductors – materials where electrical currents can flow without resistance at temperatures above the boiling point of liquid nitrogen. While the mechanism remains puzzling, recent studies demonstrated the ability to induce superconductivity by suppressing stripes with short light pulses.

“Fluctuating stripes are thought to occur in unconventional superconductors. Our study puts a speed limit on how fast such patterns can change,” said Kaindl. “It highlights the importance of considering both the spatial and temporal structure of the glue.”

This work was supported by the DOE Office of Science. The material’s equilibrium optical properties were characterized using Berkeley Lab’s Advanced Light Source, a DOE Office of Science User Facility.

SLAC Invention Could Lead to Novel Terahertz Light Sources That Help Us See the World with Different Eyes

A new device could open new avenues for the generation of high-frequency radiation with applications in science, radar, communications, security and medical imaging.

By Manuel Gnida

https://www6.slac.stanford.edu/news/2017-10-04-slac-invention-could-lead-novel-terahertz-light-sources-help-us-see-world-different

October 4, 2017

Ever since the discovery of X-rays in 1895, their ability to reveal things hidden to the human eye has created endless opportunities. But X-rays by far aren’t the only option to see the world with different eyes. Researchers hope to make better use of a different form of light, called terahertz radiation, which has broad applications in science, radar, security, medicine and communications. 

“Terahertz technology has a lot of prospect for innovation,” says Filippos Toufexis, a graduate student at the Department of Energy’s SLAC National Accelerator Laboratory and Stanford University. “Yet, it has been exploited only very little due to a lack of suitable radiation sources, particularly for applications that require them to have a small footprint and high output power.”

In a recent paper, published in Applied Physics Letters, Toufexis and his colleagues described an approach that could aid the development of future terahertz sources.

gyrocon-images-2-cropped.jpg

Mechanical design of a novel compact source of millimeter waves, developed by SLAC’s Technology Innovation Directorate. At the core of the source is an electron beam that traverses two specifically shaped metal cavities. (Andrew Haase/SLAC National Accelerator Laboratory)

Challenging Terahertz Technology

With wavelengths ranging from one-tenth of a millimeter to a few millimeters, terahertz light is several hundred times less energetic than visible light and occupies a middle ground between infrared radiation and microwaves. It could be used in radars to detect small objects, such as space debris. Or it could be used in navigation systems, security scanners and devices that search for explosives and drugs. In medicine, it has been used as an alternative to X-rays in some areas, such as 3-D imaging of teeth. Terahertz radiation could also be used in communications, where it would enable high data transfer rates.

However, there are only few options for terahertz sources, which are often not very practical. Some require large superconducting magnets or use giant particle accelerators, others produce radiation that is not powerful enough.

The SLAC team opted for another idea for the production of high-power radiation with shorter wavelengths: They used an electron beam that interacts with a specifically shaped cavity – a hollow metal structure – in a microwave vacuum tube. As the electron beam passes through the cavity, it excites electromagnetic radiation, or light, of a particular wavelength. Since the wavelength of the excited radiation scales with the cavity dimension, the output radiation can be shifted to shorter wavelengths by making smaller cavities.

However, making these cavities very small has several unwanted side effects, including a drop in output power and issues with heating and manufacturing.

“Our design eliminates these issues because it uses a cavity that is very large compared to the wavelength it produces and because it has a wide opening at one end,” Toufexis says. “It also doesn’t use magnets and is relatively small.”

gyrocon-images-6.jpg

At left: Cross section of the output cavity of a new source of millimeter waves at SLAC, showing the electric field and the cavity’s particular geometry. At right: Photo of the cavity’s top before assembly with the spherical shell seen at left. (Filippos Toufexis/John Van Pelt/SLAC National Accelerator Laboratory)

‘Whispering Gallery’ for Light

In fact, the new device has two different cavities.

The first cavity uses microwaves to deflect an electron beam coming from an “electron gun,” forcing it onto a corkscrew-like path into the second cavity – the one with the wide opening. As the result, the deflected beam appears to be rotating along that opening.

A new source of millimeter wave radiation, developed at SLAC, uses two metallic cavities – one operating at a low frequency to deflect an electron beam and another to extract power from it at a higher frequency. This simulation shows the electron beam flight path through the cavities. (Filippos Toufexis/SLAC National Accelerator Laboratory)

The electron beam excites a rotating electromagnetic field in the second cavity, which generates the output radiation. Since beam and field move synchronously along the rim, the field can continuously draw energy from the beam.

The traveling field is known as a “whispering gallery” mode because it has an analogy in acoustics. The effect occurs, for instance, in St. Paul’s Cathedral in London, where whispers – soundwaves – travel around a gallery beneath the cathedral’s dome and can be clearly heard anywhere on the gallery.

A new source of millimeter wave radiation, developed at SLAC, uses two metallic cavities – one operating at a low frequency to deflect an electron beam and another to extract power from it at a higher frequency. This simulation shows the rotating electric field at the output cavity when excited by a rotating electron beam. (Filippos Toufexis/SLAC National Accelerator Laboratory)

Due to its particular shape, the second cavity produces radiation with a wavelength five times shorter than the microwave radiation that goes into the first cavity, shifting the output wavelength toward the terahertz region.

From Proof of Principle to Application

In experiments at SLAC’s Klystron Test Lab, the device, for which the researchers obtained a provisional patent, produced stable radiation with a wavelength of about 5 millimeters and an output power of 50 watts. Calculations show that the source even has the potential to produce radiation that is 1,000 times more powerful.

However, more development work is needed to push the technology into the terahertz region with wavelengths of about a millimeter, says SLAC accelerator scientist Sami Tantawi, who had the idea for the new design, which borrows elements from an established method for the generation of radiation with longer wavelengths.

“Our proof-of-principle experiment shows that we can, in principle, use vacuum electronics to produce radiation with much shorter wavelengths,” he says. “But it also showed many difficulties in controlling the manufacturing process of our apparatus and achieving the desired outcome. We’re working diligently on developing better simulation codes that will allow us to address these issues, improve our design and advance the technology.”

The mechanical designs for the radiation source were done by Dymenso, a San Francisco-based company. Funding for this work came from the DOE Office of Science, the National Science Foundation and SLAC’s Laboratory Directed Research and Development (LDRD) Program.    

test stand.jpg

Tests of the novel millimeter wave source at SLAC. (Filippos Toufexis/SLAC National Accelerator Laboratory)

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Citation: F. Toufexis et al., Applied Physics Letters, 30 June 2017 (10.1063/1.4990970)

For questions or comments, contact the SLAC Office of Communications at communications@slac.stanford.edu.

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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.

Synopsis: Terahertz-Driven Chemistry

Terahertz pulses drive certain reactions on a metal surface by selectively exciting some of the adsorbed molecules.

H. Ogasawara/SLAC and APS/Joan Tycko

http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.115.036103

Certain chemical reactions occur at a higher rate when the reactants are stuck to a two-dimensional surface. But sometimes the desired reaction competes with other processes. To control the surface chemistry, Jerry LaRue, from the SLAC National Accelerator Laboratory in California, and his colleagues demonstrate a new technique in which pulses of light in the terahertz frequency range selectively drive an oxidation reaction on a metal surface. The pulses effectively gives a kick to adsorbed oxygen atoms, so that they move along the surface and interact with other molecules more often.

The surface reaction studied by LaRue et al. is the conversion of carbon monoxide (CO) and oxygen into carbon dioxide (CO2). This oxidation process is commonly performed in a car’s catalytic converter, where metals serve as the surface catalyst. The researchers looked specifically at ruthenium—a relatively inexpensive catalyst material but one in which CO oxidation must compete with CO desorbing from the surface.

Previous work showed that optical pulses could increase CO oxidation on ruthenium, but the pulses also heat the surface, causing increased desorption. In their experiments, LaRue and his colleagues utilized terahertz pulses produced at SLAC’s Linac Coherent Light Source. The team showed that the electric field from the pulses causes electrons to oscillate near the surface, and this electron motion weakens the bonds that hold oxygen atoms to the metal. The partly released oxygen atoms have extra energy for moving along the surface and colliding with COmolecules forming CO2. The terahertz pulses do not increase the surface temperature, so the CO desorption rate remains unchanged. The researchers found that terahertz stimulation led to as much as a third of adsorbed COconverting to CO2.

This research is published in Physical Review Letters.

–Michael Schirber

Researchers try out terahertz electrical switch using magnetite material

                                         An optical laser pulse (red streak from upper right) shatters 
                                         the ordered electronic structure (blue) in an insulating sample of magnetite, 
                                         switching the material to electrically conducting (red) in one trillionth of a                                
                                         second.(Greg Stewart/SLAC)
http://www.eeherald.com/section/news/onws2013080409.html
Today's widely used and popular nanoscale switch, the silicon MOSFET transistor, what we find in Ultra integrated VLSI chips can switch at speeds of 10s gigahertz, speed can be pushed further by using high electron mobility compound semiconductor material. But the researchers U.S. Department of Energy's (DOE) SLAC National Accelerator Laboratory go for a big jump in switching speeds by able to use magnetic material called magnetite as an electric switch at the frequencies in terahertz range, thousand times higher than today's switching in any of the semiconductor material.

Scientists have used SLAC's Linac Coherent Light Source (LCLS) X-ray laser to switch from on to off in 1 trillionth of a second in samples of magnetite.

"This breakthrough research reveals for the first time the 'speed limit' for electrical switching in this material," said Roopali Kukreja, a materials science researcher at SLAC and Stanford University who is a lead author of the study.

Scientists could able to see how the electronic structure of magnetite sample rearranged into conducting and nonconducting regions, which formed in just hundreds of quadrillionths of second. They could see how a conducting and nonconducting states coexist in the material to create electrical pathways in high-speed chips which can be used to process high definition video thousand times faster than the present VLSI chips.

If this research becomes successful it paves way to a new generation of chips which can be configured using light or any such emission.

The process used is somewhat similar to present chipmaking, the laser light was flashed on the sample material resulting in fragmentation of material's electronic structure at an atomic scale. After beaming laser, scientist hit the material with ultrabright, ultrashort X-ray pulse to study the timing and details of changes in the sample excited by the initial laser strike. It's not clear whether they used masks for the material to undergo electronic structural change.

By slightly adjusting the interval of the X-ray pulses, they precisely measured how long it took the material to shift from a non-conducting to an electrically conducting state, and observed the structural changes during this switch, as said in the release.

The release states "Scientists had worked for decades to resolve this electrical structure at the atomic level, and just last year another research team had identified its building blocks as "trimerons" – formed by three iron atoms that lock in the charges. That finding provided key insights in interpreting results from the LCLS experiment."

The magnetite had to be cooled to minus 190 degrees Celsius to lock its electrical charges in place, so the next step is to study more complex materials and room-temperature applications, Kukreja said.

Future experiments will aim to identify exotic compounds and test new techniques to induce the switching and tap into other properties that are superior to modern-day silicon transistors. The researchers have already conducted follow-up studies focusing on a hybrid material that exhibits similar ultrafast switching properties at near room temperature, which makes it a better candidate for commercial use than magnetite, explained in the release.

Hermann Dürr, the principal investigator of the LCLS experiment and senior staff scientist for the Stanford Institute for Materials and Energy Sciences (SIMES), said there is a major global effort underway to go beyond modern semiconductor transistors using new materials to satisfy demands for smaller and faster computers, and LCLS has the unique ability to home in on processes that occur at the scale of atoms in trillionths and quadrillionths of a second.

This research tells, for faster switching it's not just semiconductor material, but also any other materials can be explored for terahertz switches, which are necessary for future computing comparable to human brain.

The institutes collaborating on this project includes Helmholtz-Zentrum Berlin for Materials and Energy; Hamburg University/Center for Free Electron Laser Science (CFEL); University of Amsterdam; the T-REX laboratory at the ELETTRA-Sincrotrone Trieste and University of Trieste; Cologne, Potsdam Regensburg and Purdue universities; the Advanced Light Source at Lawrence Berkeley National Laboratory; and SwissFEL.

Research at Stanford University was supported through SIMES and LCLS by the DOE Office of Science. Portions of this research were carried out on the Soft X-ray (SXR) instrument at the LCLS, a user facility operated by Stanford University for the DOE. SXR is funded by a consortium including LCLS, Stanford, Berkeley Lab, CFEL, University of Hamburg and several other research organizations in Europe.

In its insulating state, the magnetite sample has electrical charges locked into structures known as "trimerons" that are composed of three iron atoms (a). An optical laser pulse was used to fracture trimerons (b), creating strands of electrical conductivity (red) surrounding islands of non-conducting trimeron structures (c). (S. de Jong et al./Nature Materials)