Jan 07, 2021 (Nanowerk News) All photo-electronic devices work on the basis that the materials inside them absorb, transmit and reflect light. Understanding the photo properties of a specific material at the atomic level not only helps to decide what material to choose for a given application but also opens up ways to control such properties on demand. In a new collaborative work (Nature Physics, “Vibrational coherent control of localized d–d electronic excitation”), researchers from Italy, Germany and the United States show how ‘kicking’ the atoms in a CuGeO3 crystal with an infrared laser pulse can not only make the material transparent but that the transparency can then be controlled on an ultrafast femtosecond scale. This result paves the way for the further application of the atomic kicking scheme to enhance other phenomena such as, for example, superconductivity. The work has now been published in Nature Physics. Artistic impression of vibrationally induced transparency in CuGeO3. (Image: University of Trieste / INSRL) The design of complex materials with new functionalities is often a result of the interplay between different components of the matter, such as electrons and crystal vibrations – the so-called phonons. The coupling between these matter components can be of an incoherent or coherent nature. While the former is usually the result of the nuclear fluctuations induced by the temperature, the latter is achieved when the crystal vibrations and the electronic excitations propagate in the material with the same frequency and at constant phase difference. Here, the researchers use resonant vibrational excitation to coherently control the crystal field surrounding the Cu2+ ions in a CuGeO3 crystal. This material is ideal for two main reasons: the phonons can be kicked selectively via mid-infrared laser pumping and the three characteristic d–d electronic transitions at high energy (around 1.7eV) are isolated from other spectral features that could interfere with the electron-phonon coupling. In particular, the resonant excitation of IR-active phonon modes, which are non-linearly coupled to Raman active phonon modes, results in a coherent vibrational motion of the apical oxygen that dynamically controls the energy and oscillator strength of the orbital transition between different crystal levels on Cu2+ ions. By controlling the parameter of the phonon pumping schemes it is then possible to achieve a transparency in the energy window of the d-d electronic transitions. “It is fascinating to see how distinct matter excitations that belong to completely different energy regions can coherently interact and affect the macroscopic properties of a crystal,” says Simone Latini, a post-doc and former Humboldt fellow at the MPSD. “We are currently investigating if a similar phenomenon can be observed elsewhere and we have a hint that it could be present in two-dimensional materials such as WS2.” “This study shows how far we have gone experimentally in terms of controlling matter with ultrashort light pulses,” says Alexandre Marciniak, the author of this work together with Stefano Marcantoni of the University of Trieste. “It is indeed remarkable how we can unveil the intimate microscopic relationships between excitations in a material and how this understanding can be utilized to fabricate functional devices that can become transparent on demand.” The project, financially supported mainly by the European Research Council (project INCEPT), was carried out at the Q4Q lab led by Daniele Fausti of the University of Trieste at Elettra Sincrotrone Trieste. The theoretical model was developed in the group of Fabio Benatti at the University of Trieste, in collaboration with researchers in Ángel Rubio’s group at the MPSD and Jeroen van den Brink at the IFW / the Institute for Theoretical Physics in Dresden. MPSD Theory director Ángel Rubio concludes: “This work opens up new avenues to control and design phenomena in correlated and topological materials.”

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Jan 06, 2021 (Nanowerk News) Using copper foil, glass containers and a conventional household microwave oven, University of Wyoming researchers have demonstrated that pulverized coal powder can be converted into higher-value nano-graphite. The discovery is another step forward in the effort to find alternative uses for Wyoming’s Powder River Basin coal, at a time when demand for coal to generate electricity is declining due to concerns about climate change. In a paper published in the journal Nano-Structures & Nano-Objects (“Converting raw coal powder into polycrystalline nano-graphite by metal-assisted microwave treatment”), the UW researchers report that they created an environment in a microwave oven to successfully convert raw coal powder into nano-graphite, which is used as a lubricant and in items ranging from fire extinguishers to lithium ion batteries. This “one-step method with metal-assisted microwave treatment” is a new approach that could represent a simple and relatively inexpensive coal-conversion technology. In a microwave oven, sparks are generated inside a glass vial containing coal powder and copper foil as part of an experiment by University of Wyoming researchers. They successfully converted the coal powder to nano-graphite, demonstrating a novel and inexpensive coal-conversion technology. (Image: Chris Masi) “This method provides a new route to convert abundant carbon sources to high-value materials with ecological and economic benefits,” wrote the research team, led by Associate Professor TeYu Chien, in UW’s Department of Physics and Astronomy. While previous research has shown that microwaves can be used to reduce the moisture content of coal and remove sulfur and other minerals, most such methods require specific chemical pretreatment of the coal. In their experiment, the UW researchers simply ground raw Powder River Basin coal into powder. That powder was then placed on copper foil and sealed in glass containers with a gas mixture of argon and hydrogen, before being placed in a microwave oven. A conventional microwave oven was chosen because of convenience and because it provided the desired levels of radiation. “By cutting the copper foil into a fork shape, the sparks were induced by the microwave radiation, generating an extremely high temperature of more than 1,800 degrees Fahrenheit within a few seconds,” says Masi, lead author of the paper. “This is why you shouldn’t place a metal fork inside a microwave oven.” The sparks caused by the microwaves generated the high temperatures necessary to transform the coal powder into polycrystalline graphite, with the copper foil and hydrogen gas also contributing to the process. While the experiment included microwave durations ranging from 3 to 45 minutes, the optimal duration was found to be 15 minutes. The researchers say this new method of coal conversion could be refined and performed at a larger scale to yield both a higher quality and quantity of nano-graphite materials. “Finite graphite reserves and environmental concerns for the graphite extraction procedures make this method of converting coal to graphite a great alternative source of graphite production,” the scientists wrote.

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