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Perovskites could be platforms for exciton condensates – Materials – Physics World

Appearance of exciton vortices in hybrid halides suggests a possible route to Bose–Einstein condensates of electron–hole pairs at liquid nitrogen temperatures

The post Perovskites could be platforms for exciton condensates appeared first on Physics World.

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emerging exciton vortices
Laser pumping applied on a perovskite monolayer and the emerging exciton vortices. Courtesy: D Zhang

Organic–inorganic perovskite materials are usually studied in the context of making solar cells and other photovoltaic devices. Now researchers from the Institute of Semiconductors at the Chinese Academy of Sciences in Beijing have shown that these hybrid halide materials could also be ideal platforms for realizing Bose–Einstein condensates of excitons (electron–hole pairs). Such condensates, which appear as vortex patterns, could be produced at liquid nitrogen temperatures – positively balmy, by the standard of the field – thanks to the long lifetimes of the excitons in the materials and their huge binding energies.

Bose–Einstein condensation (BEC) occurs when all the bosonic atoms or particles in a gas collapse into the same quantum ground state and can therefore be described by the same wavefunction. Such collapses are triggered by cooling the gas until the de Broglie wavelength of its constituent atoms or particles is comparable to the distance between them. Once in this state, the atoms or particles behave as a superfluid, flowing without friction.

Exciton BECs

Researchers made the first BEC in 1995 from rubidium atoms. Since then, condensates have been observed in various other types of particles, including polaritons, photons and magnons as well as other species of atoms and molecules. In all cases, however, the phenomenon has only appeared at ultralow temperatures of no more than a few Kelvin above absolute zero.

To make BECs easier to study – and perhaps also to put their amazing properties to practical use – researchers have long sought to increase the temperature at which they form. One way of doing this might be to make a BEC using excitons, which are bosons composed of the bound states of two fermions: a negatively charged electron and a positively charged hole, or electron vacancy. These fermions are bound together via weak Coulomb interactions that cause them to form dipoles. Since these bound states are much lighter than atoms, they can be packed together with higher density – meaning that they ought to Bose condense at much higher temperatures.

That, at least, is the theory. Unfortunately, previous attempts to make such excitonic BECs – for example in semiconductor wells and graphene – have succeeded only at disappointingly low temperatures of around 1 K, due to the small exciton binding energy in these material systems.

Calculating the exciton binding energy

In their new work, researchers led by Kai Chang studied a 2D hybrid perovskite with the chemical formula (PEA)2PbI4. Perovskites in general are promising thin-film solar-cell materials thanks to the fact that they can absorb light over a broad range of solar spectrum wavelengths. Electrons and holes have a long lifetime in these materials too (that is, they can diffuse through the material over long lengths) and this is the property that Chang and colleagues focused on.

The team’s chosen perovskite has a stable layered structure comprising layers of [PbI6]4− octahedra and long-chain organic molecules with the formula C6H5C2H4NH3+ (abbreviated PEA+). The inorganic PbI4 layers are sandwiched between two organic layers and have effective potential barriers with energies of 8.1 eV. These barriers make (PEA)2PbI4 behave like stacked quantum wells confined by “hard-wall” energy potentials, the researchers explain.

Using first-principles calculations and a theoretical framework known as the Keldysh model, the researchers calculated a binding energy as high as 238.5 meV for the excitons in monolayer (PEA)2PbI4, a value that agrees with that obtained in laboratory experiments. “The Keldysh model is a standard treatment for describing 2D excitons with ‘unscreened’ Coulomb interactions and the large exciton binding energy we calculated means that the critical temperature of the exciton BEC could approach the liquid nitrogen regime (77 K),” team member Dong Zhang tells Physics World.

Vortex patterns

The researchers studied their flakes of (PEA)2PbI4 further by applying an electric field perpendicular to them. From this, they found that the applied field slightly changed the material’s binding energy, while also causing all the electron–hole dipoles to line up in the same direction. In this situation, the interaction between the dipoles becomes repulsive.

When the researchers then “pumped” the flakes of the (PEA)2PbI4 using pulses from a laser, they found that the repulsive dipole–dipole interaction created by the perpendicular electric field can drive the laterally confined excitons into various vortex patterns. The time it takes for these vortices to evolve is on a par with the lifetime of the exciton itself and the result is a stable pattern with a certain number of vortices rotating at the centre.

Members of the team, who report their work in Chinese Physics Letters, say they now plan to study exciton BEC in few-layered hybrid perovskites as opposed to just monolayer ones. “We will also be looking at how to manipulate exciton vortices and make vortex-based information-storage porotype devices,” Zhang says.

Source: https://physicsworld.com/a/perovskites-could-be-platforms-for-exciton-condensates/

Nano Materials

Inducing transparency by kicking the atoms

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.

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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 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.”

Source: https://feeds.feedblitz.com/~/641157294/0/nanowerk/agwb~Inducing-transparency-by-kicking-the-atoms.php

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Nano Materials

Researchers turn coal powder into graphite in microwave oven

Using copper foil, glass containers and a conventional household microwave oven, researchers have demonstrated that pulverized coal powder can be converted into higher-value nano-graphite.

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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. vial with sparks in a microwave 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.

Source: https://feeds.feedblitz.com/~/641131698/0/nanowerk/agwb~Researchers-turn-coal-powder-into-graphite-in-microwave-oven.php

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Nano Materials

A better pen-and-ink system for drawing flexible circuits

Scientists have developed inexpensive conductive inks for clog-free ballpoint pens that can allow users to ‘write’ circuits almost anywhere — even on human skin.

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Source: https://feeds.feedblitz.com/~/641131206/0/nanowerk/agwb~A-better-penandink-system-for-drawing-flexible-circuits.php

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