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Magnetism drives metals to insulators in new experiment: Study provides new tools to probe novel spintronic devices

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Home > Press > Magnetism drives metals to insulators in new experiment: Study provides new tools to probe novel spintronic devices

An illustration of two domains (blue and orange) divided by a domain wall (white area) in a material. The magnetic order is designated with organized arrows (electron spins) while the colors represent two different domains (but the same magnetic order). In the material pictured here, the domain walls are conductive and the domains are insulating. CREDIT
Yejun Fang
An illustration of two domains (blue and orange) divided by a domain wall (white area) in a material. The magnetic order is designated with organized arrows (electron spins) while the colors represent two different domains (but the same magnetic order). In the material pictured here, the domain walls are conductive and the domains are insulating. CREDIT
Yejun Fang

Abstract:
Like all metals, silver, copper, and gold are conductors. Electrons flow across them, carrying heat and electricity. While gold is a good conductor under any conditions, some materials have the property of behaving like metal conductors only if temperatures are high enough; at low temperatures, they act like insulators and do not do a good job of carrying electricity. In other words, these unusual materials go from acting like a chunk of gold to acting like a piece of wood as temperatures are lowered. Physicists have developed theories to explain this so-called metal-insulator transition, but the mechanisms behind the transitions are not always clear.

Magnetism drives metals to insulators in new experiment: Study provides new tools to probe novel spintronic devices


Pasadena. CA | Posted on June 4th, 2021

“In some cases, it is not easy to predict whether a material is a metal or an insulator,” explains Caltech visiting associate Yejun Feng of the Okinawa Institute for Science and Technology Graduate University. “Metals are always good conductors no matter what, but some other so-called apparent metals are insulators for reasons that are not well understood.” Feng has puzzled over this question for at least five years; others on his team, such as collaborator David Mandrus at the University of Tennessee, have thought about the problem for more than two decades.

Now, a new study from Feng and colleagues, published in Nature Communications, offers the cleanest experimental proof yet of a metal-insulator transition theory proposed 70 years ago by physicist John Slater. According to that theory, magnetism, which results when the so-called “spins” of electrons in a material are organized in an orderly fashion, can solely drive the metal-insulator transition; in other previous experiments, changes in the lattice structure of a material or electron interactions based on their charges have been deemed responsible.

“This is a problem that goes back to a theory introduced in 1951, but until now it has been very hard to find an experimental system that actually demonstrates the spin-spin interactions as the driving force because of confounding factors,” explains co-author Thomas Rosenbaum, a professor of physics at Caltech who is also the Institute’s president and the Sonja and William Davidow Presidential Chair.

“Slater proposed that, as the temperature is lowered, an ordered magnetic state would prevent electrons from flowing through the material,” Rosenbaum explains. “Although his idea is theoretically sound, it turns out that for the vast majority of materials, the way that electrons interact with each other electronically has a much stronger effect than the magnetic interactions, which made the task of proving the Slater mechanism challenging.”

The research will help answer fundamental questions about how different materials behave, and may also have applications in technology, for example in the field of spintronics, in which the spins of electrons would form the basis of electrical devices instead of the electron charges as is routine now. “Fundamental questions about metal and insulators will be relevant in the upcoming technological revolution,” says Feng.

Interacting Neighbors

Typically, when something is a good conductor, such as a metal, the electrons can zip around largely unimpeded. Conversely, with insulators, the electrons get stuck and cannot travel freely. The situation is comparable to communities of people, explains Feng. If you think of materials as communities and electrons as members of the households, then “insulators are communities with people who don’t want their neighbors to visit because it makes them feel uncomfortable.” Conductive metals, however, represent “close-knit communities, like in a college dorm, where neighbors visit each other freely and frequently,” he says.

Likewise, Feng uses this metaphor to explain what happens when some metals become insulators as temperatures drop. “It’s like winter time, in that people–or the electrons–stay home and don’t go out and interact.”

In the 1940s, physicist Sir Nevill Francis Mott figured out how some metals can become insulators. His theory, which garnered the 1977 Nobel Prize in Physics, described how “certain metals can become insulators when the electronic density decreases by separating the atoms from each other in some convenient way,” according to the Nobel Prize press release. In this case, the repulsion between the electrons is behind the transition.

In 1951, Slater proposed an alternate mechanism based on spin-spin interactions, but this idea has been hard to prove experimentally because the other processes of the metal-insulator transition, including those proposed by Mott, can swamp the Slater mechanism, making it hard to isolate.

Challenges of Real Materials

In the new study, the researchers were able at last to experimentally demonstrate the Slater mechanism using a compound that has been studied since 1974, called pyrochlore oxide or Cd2Os2O7. This compound is not affected by other metal-insulator transition mechanisms. However, within this material, the Slater mechanism is overshadowed by an unforeseen experimental challenge, namely the presence of “domain walls” that divide the material into sections.

“The domain walls are like the highways or bigger roads between communities,” says Feng. In pyrochlore oxide, the domain walls are conductive, even though the bulk of the material is insulating. Although the domain walls started out as an experimental challenge, they turned out to be essential to the team’s development of a new measurement procedure and technique to prove the Slater mechanism.

“Previous efforts to prove the Slater metal-insulator transition theory did not account for the fact that the domain walls were masking the magnetism-driven effects,” says Yishu Wang (PhD ’18), a co-author at the Johns Hopkins University who has continuously worked on this study since her graduate work at Caltech. “By isolating the domain walls from the bulk of the insulating materials, we were able to develop a more complete understanding of the Slater mechanism.” Wang had previously worked with Patrick Lee, a visiting professor at Caltech from MIT, to lay the basic understanding of conductive domain walls using symmetry arguments, which describe how and if electrons in materials respond to changes in the direction of a magnetic field.

“By challenging the conventional assumptions about how electrical conductivity measurements are made in magnetic materials through fundamental symmetry arguments, we have developed new tools to probe spintronic devices, many of which depend on transport across domain walls,” says Rosenbaum.

“We developed a methodology to set apart the domain-wall influence, and only then could the Slater mechanism be revealed,” says Feng. “It’s a bit like discovering a diamond in the rough.”

###

The paper, titled, “A continuous metal-insulator transition driven by spin correlations,” was funded by the Okinawa Institute, with subsidy funding from the Cabinet Office, Government of Japan; the National Science Foundation; the Air Force Office of Scientific Research; and the U.S. Department of Energy. Other authors include Daniel M. Silevitch of Caltech and Scott E. Cooper of the Okinawa Institute of Science and Technology. Mandrus is also affiliated with the Oak Ridge National Laboratory.

####

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@caltech

Copyright © California Institute of Technology

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Nano Technology

New family of atomic-thin electride materials discovered

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Home > Press > New family of atomic-thin electride materials discovered

Yellow isosurfaces on left panel indicate electrons localized in-between the C3 trimers. Ionized structure on the right has no trapped electrons, and some of the M atoms have been largely displaced. This displacement of the M atoms again significantly stabilizes the ionized structure. CREDIT
Soungmin Bae and Hannes Raebiger
Yellow isosurfaces on left panel indicate electrons localized in-between the C3 trimers. Ionized structure on the right has no trapped electrons, and some of the M atoms have been largely displaced. This displacement of the M atoms again significantly stabilizes the ionized structure. CREDIT
Soungmin Bae and Hannes Raebiger

Abstract:
An exploratory investigation into the behavior of materials with desirable electric properties resulted in the discovery of a structural phase of two-dimensional (2D) materials. The new family of materials are electrides, wherein electrons occupy a space usually reserved for atoms or ions instead of orbiting the nucleus of an atom or ion. The stable, low-energy, tunable materials could have potential applications in nanotechnologies.

New family of atomic-thin electride materials discovered


Yokohama, Japan | Posted on June 11th, 2021

The international research team, led by Hannes Raebiger, associate professor in the Department of Physics at Yokohama National University in Japan, published their results on June 10th as frontispiece in Advanced Functional Materials.

Initially, the team set out to better understand the fundamental properties of a 2D system known as Sc2CO2. Containing two atoms of metallic scandium, one atom of carbon and two atoms of oxygens, the system belongs to a family of chemical compounds collectively referred to as MXenes. They are typically composed of a carbon or nitrogen layer one atom thick sandwiched between metal layers, dotted with oxygen or fluorine atoms.

The researchers were particularly interested in MXene Sc2CO2 due to the predictions that, when structured into a hexagonal phase, the system would have desired electrical properties.

“Despite these fascinating predictions of hexagonal phases of Sc2CO2, we are not aware of its successful fabrication as of yet,” said Soungmin Bae, first author and researcher in the Department of Physics at Yokohama National University. “Analyzing its fundamental properties, we discovered a completely new structural phase.”

The new structural phase results in new electride materials. The atomic-thin 2D structural phase is described as tiled shapes forming the central carbon plane. The previously predicted shape was a hexagon, with a carbon atom at every vertex and one in the middle. The new materials have a rhombus-like shape, with electrons at the vertices and a carbon trimer — three carbon atoms in a row — in the middle.

“Carbon is one of the most common materials on our planet, and quite important for living beings, but it is hardly ever found as trimers,” Raebiger said. “The closest place where carbon trimers are typically found is interstellar space.”

The overall shape is less symmetric than the previously described hexagonal structure, but it is more symmetric with regard to the central plane. This structure offers unique characteristics due to the appearance of the new family of electrides, according to Raebiger.

“Electrides contain electrons as a structural unit and often are extremely good electrical conductors,” Raebiger said. “The present family of electrides are insulators, and while most insulators can be made conductive by adding or removing electrons, these materials simply become more insulating.”

MXenes are particularly attractive as a material, because they can be reconfigured with other metallic elements to offer a cornucopia of properties, including tunable conductivity, various forms of magnetism, and/or accelerate chemical reactions as catalysts. On top of this, they are ultra-thin sheets only a few atoms thick, that is, 2D materials. The newly discovered electrides have electrons in lattice voids between atoms and ions, which can be readily emitted into surrounding space, such as the electron sources for large particle accelerators, as well as be borrowed to catalyze a specifically desired chemical reaction.

“We made this discovery because we wanted to understand how these materials work better,” Bae said. “If you encounter something you don’t understand, dig deeper.”

Co-authors include William Espinosa-García and Gustavo M. Dalpian, Centro de Ciências Naturais e Humanas, Universidade Federal do ABC, Brazil; Yoon-Gu Kang and Myung Joon Han, Department of Physics, Korea Advanced Institute of Science and Technology; Juho Lee and Yong-Hoon Kim, Department of Electrical Engineering, Korea Advanced Institute of Science and Technology; Noriyuki Egawa, Kazuaki Kuwahata and Kaoru Ohno, Department of Physics at Yokohama National University; and Mohammad Khazaei and Hideo Hosono, Materials Research Center for Element Strategy, Tokyo Institute of Technology. Espinosa-García is also affiliated with Grupo de investigación en Modelamienot y Simulación Computacional, Facultad de Ingenierías, Universidad de San Buenaventura-Medellín.

The Iwaki Scholarship Foundation; São Paulo Research Foundation; Korea’s National Research Foundation, Ministry of Science and ICT and Ministry of Education; KAIST (formerly the Korea Advanced Institute of Science and Technology); and Samsung Research Funding & Incubation Center of Samsung Electronics funded this work.

####

About Yokohama National University
Yokohama National University (YNU or Yokokoku) is a Japanese national university founded in 1949. YNU provides students with a practical education utilizing the wide expertise of its faculty and facilitates engagement with the global community. YNU’s strength in the academic research of practical application sciences leads to high-impact publications and contributes to international scientific research and the global society.

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Akiko Tsumura

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Molecular coating enhances organic solar cells

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Home > Press > Molecular coating enhances organic solar cells

The team fabricated an organic solar cell that, unlike conventional solar cells, can be easily recycled following the simple steps shown above. Adapted from Lin et al. (2021)
The team fabricated an organic solar cell that, unlike conventional solar cells, can be easily recycled following the simple steps shown above. Adapted from Lin et al. (2021)

Abstract:
An electrode coating just one molecule thick can significantly enhance the performance of an organic photovoltaic cell, KAUST researchers have found. The coating outperforms the leading material currently used for this task and may pave the way for improvements in other devices that rely on organic molecules, such as light-emitting diodes and photodetectors.

Molecular coating enhances organic solar cells


Thuwal, Saudi Arabia | Posted on June 11th, 2021

Unlike the most common photovoltaic cells that use crystalline silicon to harvest light, organic photovoltaic cells (OPVs) rely on a light-absorbing layer of carbon-based molecules. Although OPVs cannot yet rival the performance of silicon cells, they could be easier and cheaper to manufacture at a very large scale using printing techniques.

When light enters a photovoltaic cell, its energy frees a negative electron and leaves behind a positive gap, known as a hole. Different materials then gather the electrons and holes and guide them to different electrodes to generate an electrical current. In OPVs, a material called PEDOT:PSS is widely used to ease the transfer of generated holes into an electrode; however, PEDOT:PSS is expensive, acidic and can degrade the cell’s performance over time.

The KAUST team has now developed a better alternative to PEDOT:PSS. They use a much thinner coating of a hole-transporting molecule called Br-2PACz, which binds to an indium tin oxide (ITO) electrode to form a single-molecule layer. The organic cell using Br-2PACz achieved a power conversion efficiency of 18.4 percent, whereas an equivalent cell using PEDOT:PSS reached only 17.5 percent.

“We were very surprised indeed by the performance enhancement,” says Yuanbao Lin, Ph.D. student and member of the team. “We believe Br-2PACz has the potential to replace PEDOT:PSS due to its low cost and high performance.”

Br-2PACz increased the cell’s efficiency in several ways. Compared with its rival, it caused less electrical resistance, improved hole transport and allowed more light to shine through to the absorbing layer. Br-2PACz also improved the structure of the light-absorbing layer itself, an effect that may be related to the coating process.

The coating could even improve the recyclability of the solar cell. The researchers found that the ITO electrode could be removed from the cell, stripped of its coating and then reused as if it was new. In contrast, PEDOT:PSS roughens the surface of the ITO so that it performs poorly if reused in another cell. “We anticipate this will have a dramatic impact on both the economics of OPVs and the environment,” says Thomas Anthopoulos, who led the research.

####

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Copyright © King Abdullah University of Science and Technology

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Researchers turned transparent calcite into artificial gold

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Home > Press > Researchers turned transparent calcite into artificial gold

Figure shows 3D reconstruction of the golden vaterite and the laser-induced heating of the spherulites. CREDIT
Tel Aviv University
Figure shows 3D reconstruction of the golden vaterite and the laser-induced heating of the spherulites. CREDIT
Tel Aviv University

Abstract:
Breakthrough in metamaterials: for the first time in the world, researchers at Tel Aviv University developed an innovative nanotechnology that transforms a transparent calcite nanoparticle into a sparkling gold-like particle. In other words, they turned the transparent particle into a particle that is visible despite its very small dimensions. According to the researchers the new material can serve as a platform for innovative cancer treatments.

Researchers turned transparent calcite into artificial gold


Tel Aviv, Israel | Posted on June 11th, 2021

In a new paper published in Advanced Materials, an international team of scientists, coordinated by Dr. Roman Noskov and Dr. Pavel Ginzburg from the Iby and Aladar Fleischman Faculty of Engineering at Tel Aviv University, Prof. Dmitry Gorin from the Center for Photonics and Quantum Materials at the Skolkovo Institute of Science and Technology (Skoltech) and Dr. Evgeny Shirshin from M.V. Lomonosov Moscow State University, has introduced the concept of biofriendly delivery of optical resonances via a mesoscopic metamaterial, a material with properties that are not found in nature. This approach opens promising prospects for multifunctionality in biomedical systems, allowing the use of a single designer-made nanoparticle for sensing, photothermal therapy, photoacoustic tomography, bioimaging, and targeted drug delivery.

“This concept is the result of cross-disciplinary thinking at the interface between the physics of metamaterials and bioorganic chemistry, aiming to meet the needs of nanomedicine. We were able to create a mesoscopic submicron metamaterial from biocompatible components that demonstrates strong Mie resonances covering the near-infrared spectral window in which biological tissues are transparent,” says Dr. Roman Noskov.

The nanostructures capable of nanoscale light localization as well as performing several functions are highly desirable in a plethora of biomedical applications. However, biocompatibility is typically a problem, as engineering of optical properties often calls for using toxic compounds and chemicals. The researchers have resolved this issue by employing gold nanoseeds and porous vaterite (calcium carbonate) spherulites, currently considered promising drug-delivery vehicles. This approach involves controllable infusion of gold nanoseeds into a vaterite scaffold resulting in a mesoscopic metamaterial – golden vaterite – whose resonance properties can be widely tuned by changing the quantity of gold inside the vaterite. Additionally, high payload capacity of vaterite spherulites allows simultaneous loading of both drugs and fluorescent tags. To exemplify the performance of their system, the researchers demonstrated efficient laser heating of golden vaterite at red and near?infrared wavelengths, highly desirable in photothermal therapy, and photoacoustic tomography.

Prof. Pavel Ginzburg summarizes: “This novel platform enables the accommodation of multiple functionalities – as simple add-ons that can be introduced almost on demand. Alongside optical imaging and thermotherapy, MRI visibility, functional biomedical materials and many other modalities can be introduced within a miniature nano-scale particle. I believe that our collaborative efforts will lead to in-vivo demonstrations, which will pave the way for a new biomedical technology.”

####

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New family of atomic-thin electride materials discovered

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Home > Press > New family of atomic-thin electride materials discovered

Yellow isosurfaces on left panel indicate electrons localized in-between the C3 trimers. Ionized structure on the right has no trapped electrons, and some of the M atoms have been largely displaced. This displacement of the M atoms again significantly stabilizes the ionized structure. CREDIT
Soungmin Bae and Hannes Raebiger
Yellow isosurfaces on left panel indicate electrons localized in-between the C3 trimers. Ionized structure on the right has no trapped electrons, and some of the M atoms have been largely displaced. This displacement of the M atoms again significantly stabilizes the ionized structure. CREDIT
Soungmin Bae and Hannes Raebiger

Abstract:
An exploratory investigation into the behavior of materials with desirable electric properties resulted in the discovery of a structural phase of two-dimensional (2D) materials. The new family of materials are electrides, wherein electrons occupy a space usually reserved for atoms or ions instead of orbiting the nucleus of an atom or ion. The stable, low-energy, tunable materials could have potential applications in nanotechnologies.

New family of atomic-thin electride materials discovered


Yokohama, Japan | Posted on June 11th, 2021

The international research team, led by Hannes Raebiger, associate professor in the Department of Physics at Yokohama National University in Japan, published their results on June 10th as frontispiece in Advanced Functional Materials.

Initially, the team set out to better understand the fundamental properties of a 2D system known as Sc2CO2. Containing two atoms of metallic scandium, one atom of carbon and two atoms of oxygens, the system belongs to a family of chemical compounds collectively referred to as MXenes. They are typically composed of a carbon or nitrogen layer one atom thick sandwiched between metal layers, dotted with oxygen or fluorine atoms.

The researchers were particularly interested in MXene Sc2CO2 due to the predictions that, when structured into a hexagonal phase, the system would have desired electrical properties.

“Despite these fascinating predictions of hexagonal phases of Sc2CO2, we are not aware of its successful fabrication as of yet,” said Soungmin Bae, first author and researcher in the Department of Physics at Yokohama National University. “Analyzing its fundamental properties, we discovered a completely new structural phase.”

The new structural phase results in new electride materials. The atomic-thin 2D structural phase is described as tiled shapes forming the central carbon plane. The previously predicted shape was a hexagon, with a carbon atom at every vertex and one in the middle. The new materials have a rhombus-like shape, with electrons at the vertices and a carbon trimer — three carbon atoms in a row — in the middle.

“Carbon is one of the most common materials on our planet, and quite important for living beings, but it is hardly ever found as trimers,” Raebiger said. “The closest place where carbon trimers are typically found is interstellar space.”

The overall shape is less symmetric than the previously described hexagonal structure, but it is more symmetric with regard to the central plane. This structure offers unique characteristics due to the appearance of the new family of electrides, according to Raebiger.

“Electrides contain electrons as a structural unit and often are extremely good electrical conductors,” Raebiger said. “The present family of electrides are insulators, and while most insulators can be made conductive by adding or removing electrons, these materials simply become more insulating.”

MXenes are particularly attractive as a material, because they can be reconfigured with other metallic elements to offer a cornucopia of properties, including tunable conductivity, various forms of magnetism, and/or accelerate chemical reactions as catalysts. On top of this, they are ultra-thin sheets only a few atoms thick, that is, 2D materials. The newly discovered electrides have electrons in lattice voids between atoms and ions, which can be readily emitted into surrounding space, such as the electron sources for large particle accelerators, as well as be borrowed to catalyze a specifically desired chemical reaction.

“We made this discovery because we wanted to understand how these materials work better,” Bae said. “If you encounter something you don’t understand, dig deeper.”

Co-authors include William Espinosa-García and Gustavo M. Dalpian, Centro de Ciências Naturais e Humanas, Universidade Federal do ABC, Brazil; Yoon-Gu Kang and Myung Joon Han, Department of Physics, Korea Advanced Institute of Science and Technology; Juho Lee and Yong-Hoon Kim, Department of Electrical Engineering, Korea Advanced Institute of Science and Technology; Noriyuki Egawa, Kazuaki Kuwahata and Kaoru Ohno, Department of Physics at Yokohama National University; and Mohammad Khazaei and Hideo Hosono, Materials Research Center for Element Strategy, Tokyo Institute of Technology. Espinosa-García is also affiliated with Grupo de investigación en Modelamienot y Simulación Computacional, Facultad de Ingenierías, Universidad de San Buenaventura-Medellín.

The Iwaki Scholarship Foundation; São Paulo Research Foundation; Korea’s National Research Foundation, Ministry of Science and ICT and Ministry of Education; KAIST (formerly the Korea Advanced Institute of Science and Technology); and Samsung Research Funding & Incubation Center of Samsung Electronics funded this work.

####

About Yokohama National University
Yokohama National University (YNU or Yokokoku) is a Japanese national university founded in 1949. YNU provides students with a practical education utilizing the wide expertise of its faculty and facilitates engagement with the global community. YNU’s strength in the academic research of practical application sciences leads to high-impact publications and contributes to international scientific research and the global society.

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Akiko Tsumura

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