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Quantum materials: Where many paths meet

  • By Leena Aggarwal
  • 27 October 2017

In Nature, there exist materials with exotic properties that cannot be understood in the framework of classical theories. Such properties, however, are beautifully described by more sophisticated theoretical tools involving quantum mechanics.  Such materials are now known as the “quantum materials”. The range of exotic properties exhibited by the quantum materials is extremely broad and includes superconductivity, superfluidity, ferromagnetism, quantum hall effect, spin-liquidity, topological insulation, to name a few.

Superconductors, discovered by Kammerlingh Onnes, 1911, were first to emerge as quantum materials. In normal metals, the resistance arises due to inelastic scattering between the charge carriers (electrons) and defects in the periodic crystal lattice. The defects or scattering centres can be any distortion to the periodicity of the lattice like those due to presence of impurity or the thermal vibration of the lattice points. In superconductors, surprisingly, the resistance becomes zero despite the presence of a large number of impurities and at high temperatures where the lattice points can undergo vigorous thermal vibration. The question that how the charge carriers remained insensitive to such strong scattering centres could not be answered within any classical picture. A microscopic understanding of superconductivity was first provided by Bardeen, Cooper and Schrieffer (BCS) in 1951, only after substantial development of quantum mechanics and quantum field theories – the theories where quantum mechanics is combined with Einstein’s theory of relativity.

New Technique for Measuring the Layer-Resolved Charge Density

  • By Burcu Ozden
  • 25 October 2017

Recently Benjamin M. Hunt and his colleagues developed a new technique for measuring the layer-resolved charge density, from which they can map layer polarization of the valley or spin quantum numbers in bilayer graphine and other two dimensional materials. In this study, they demonstrated direct measurement of valley and orbital levels in bilayer graphite. They have detected that the four valley and orbital components have different weights on the two layers of the bilayer. By using Hunt’s technique one can probe layer, valley, and spin polarization quantitatively in other atomic layered materials, including twisted bilayer graphene and both homobilayer and heterobilayer of transition metal dichalcogenide

 

The new era of Polariton condensates

  • By Leena Aggarwal
  • 17 October 2017

In a recently published paper in Physics Today, David Snoke, a professor of physics and astronomy at the University of Pittsburgh and Jonathan Keeling who is a reader in theoretical condensed-matter physics at the University of St. Andrews in Scotland, have shown the superfluidity of light where photon treats as a gas of interacting bosonic atoms. They have demonstrated that how to engineer a Bose–Einstein condensation from light.

 

Trapping of Polariton in Microcavities

  • By Leena Aggarwal
  • 9 October 2017

Bose-Einstein Condensation (BEC) is a process which occurs at low temperature when an ensemble of bosons cools down and enters a single quantum state. In most of the experiments, this condensation process includes the atomic gases. But in solid-state systems, which has a long history of generating new types of particles and quasiparticles, BEC can occur in quasiparticles e.g. fermion-like excitations of Bose- condensed Cooper pairs in a superconductor.

One class of such quasiparticles is polaritons, which form from electronic excitations coupled to photons in a microcavity. Polaritons are not fundamentally different in character from elementary particles; they are just highly renormalized to have different properties. In particular, polaritons can be viewed as photons having an effective mass and much stronger interactions than photons in a vacuum.

“A carefully engineered coupling between light and matter could pave the way to a room-temperature Bose–Einstein condensate.”

 

Giannis Mpourmpakis and His Colleagues Revealed the Mystery Behind Formation of Metal Nano-particles at Specific Sizes

  • By Burcu Ozden
  • 30 August 2017

Nature Communications, co-authored by Giannis Mpourmpakis, and PhD candidate Michael Taylor, offers a possible way to unravel these mysteries, with the help of computer simulations. “In applying our new theory, we aim to accelerate discovery and application,” said Mpourmpakis. From molecular carriers for targeted drug delivery to systems for energy generation and storage to solar cells, this research could help.

Computational Study of Ni-Catalyzed C−H Functionalization: Factors That Control the Competition of Oxidative Addition and Radical Pathways

  • By Aude Marjolin
  • 2 August 2017

To guide the development of a more diverse set of Ni-catalyzed C−H bond functionalizations, a thorough understanding of the mechanisms, reactivity, and selectivity of these reactions is required. Peng Liu and his student Humair Omer have undertaken a computational study of the functionalization of the C−H bonds in molecules that contain the N,N-bidentate directing group with Ni catalyst and various coupling  partners, e.g. phenyl iodide (Ph−I), which has been published in the July 26, 2017 issue of the Journal of the American Chemical Society.

Uncovering the Connection Between Negative Stiffness and Magnetic Domain Walls

  • By Aude Marjolin
  • 19 July 2017

Nature doesn't like having interfaces—this is why bubbles like to be round, and the surface of a pond settles to flat as long as it's not disturbed. These trends minimize the total amount of interface (or surface) that is present. As an exception to this behavior, certain materials are known to have a property, called negative stiffness, where the interface prefers to become distorted, or wavy, even without any external stimulation. Interfaces with negative stiffness have been considered in crystals before, but the characteristic has now also been found in modern magnetism.

Kevin Chen's Nature Physics Article Highlighted in News and Views Article

  • By Aude Marjolin
  • 2 June 2017

Kevin Chen's article Experimental observation of optical Weyl points and Fermi arc-like surface states, published in Nature Physics, was the subject of a "News and Views" article entitled Topological Photonics: Come to Light. The physics idea leading to this paper originated from Penn State collaborator Mikael Rechtsman.

Topological states of matter can exhibit a range of unique quantum phenomena that are of interest to various fields of classical physics, such as acoustics, mechanics or photonics. Although a number of these topological states have been successfully emulated in photonics, their application has been restricted to certain frequencies. Most topological properties have been demonstrated in two-dimensional (2D) systems; however, a variety of new topological properties have been predicted for three-dimensional (3D) systems. The study published in Nature Physics marks an important step by emulating Weyl points, which constitute the simplest possible topologically nontrivial band structure, in three dimensions.

Hot Electrons at the Interface between Silver Nanoparticles and a Graphite Substrate

  • By Aude Marjolin
  • 8 May 2017

The interface between nano-sized precious metal clusters such as Silver (Ag) and a semiconductor such as graphite (Gr) is called a heterojunction (Ag/Gr). Heterojunctions have great promise in enhance solar energy conversion due to their unique and enhanced optical, electronic, and chemical properties. When excited with laser pulses, electrons in the system acquire a mean energy higher than its thermal equilibrium value and are referred to as "hot electrons". In fact, graphitic materials are model systems for the study of hot electron dynamics. An ineffective screening within the layers of graphite allows the hot electrons to reach temperatures comparable to that in the solar photosphere!

In a study supported by the Center for Chemistry at the Space-Time Limit recently published in the Journal of the American Chemical Society, Hrvoje Petek and his group modified the Gr surface with Ag nanoclusters (NC)s to investigate how the excitation of the plasmonic resonance of the Ag/Gr heterojunction affects the generation, spatial distributions, and relaxation processes of hot electrons. Plasmonic resonance is a prominent feature of precious-metal nanoparticles; it is a sharp and intense absorption band in the visible range that arise from a collective resonant oscillation of the free electrons of the conduction band of the metal called plasmon.

Graphane as an Efficient and Water-Free Hydrogen Fuel Cell Membrane

  • By Aude Marjolin
  • 8 May 2017

Hydrogen powered fuel cell cars, developed by almost every major car manufacturer, are ideal zero-emissions vehicles because they produce only water as exhaust. However, their reliability is limited because the fuel cell relies upon a membrane that only functions in when enough water is present, limiting the vehicle’s operating conditions. 

Karl Johnson and his group have found that the unusual properties of graphane – a two-dimensional polymer of carbon and hydrogen – could form a type of anhydrous “bucket brigade” that transports protons without the need for water, potentially leading to the development of more efficient hydrogen fuel cells for vehicles and other energy systems. Graduate research assistant Abhishek Bagusetty is the lead author on their paper “Facile Anhydrous Proton Transport on Hydroxyl Functionalized Graphane”, recently published in Physical Review Letters. Computational modeling techniques coupled with the high performance computational infrastructure at the University’s Center for Research Computing enabled them to design this potentially groundbreaking material. 

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