Quantum Fluctuation Theory of Pseudogap Physics Presented

by Tommy on 30/09/2014


Pseudogap phenomena in ultracold atomic Fermi gases, Qijin Chen and Jibiao Wang, Phys. 9(5), 539-570 (2014)

The pairing and superfluid phenomena in a two-component ultracold atomic Fermi gas is an analogue of Cooper pairing and superconductivity in an electron system, in particular, the high Tc superconductors. Owing to the various tunable parameters that have been made accessible experimentally in recent years, atomic Fermi gases can be explored as a prototype or quantum simulator of superconductors. It is hoped that, utilizing such an analogy, the study of atomic Fermi gases may shed light to the mysteries of high Tc superconductivity. One obstacle to the ultimate understanding of high Tc superconductivity, from day one of its discovery, is the anomalous yet widespread pseudogap phenomena, for which a consensus is yet to be reached within the physics community, after over 27 years of intensive research efforts. In this article, we shall review the progress in the study of pseudogap phenomena in atomic Fermi gases in terms of both theoretical understanding and experimental observations. We show that there is strong, unambiguous evidence for the existence of a pseudogap in strongly interacting Fermi gases. In this context, we shall present a pairing fluctuation theory of the pseudogap physics and show that it is indeed a strong candidate theory for high Tc superconductivity.

This stunning review lays it all out in black and white for anybody who is still interested.

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Orbitronics Proposed for 1T-TaS2 Tantalum Disulfide

by Tommy on 26/09/2014


Orbital textures and charge density waves: towards orbitronics in transition metal dichalcogenides, T. Ritschel, J. Trinckauf, K. Koepernik, B. Büchner, M. v. Zimmermann, H. Berger, Y. I. Joe, P. Abbamonte and J. Geck

Low-dimensional electron systems, as realized naturally in graphene or created artificially at the interfaces of heterostructures, exhibit a variety of fascinating quantum phenomena with great prospects for future applications. Once electrons are confined to low dimensions, they also tend to spontaneously break the symmetry of the underlying nuclear lattice by forming so-called density waves; a state of matter that currently attracts enormous attention because of its relation to various unconventional electronic properties. In this study we reveal a remarkable and surprising feature of charge density waves (CDWs), namely their intimate relation to orbital order. For the prototypical material 1T-TaS2 we not only show that the CDW within the two-dimensional TaS2-layers involves previously unidentified orbital textures of great complexity. We also demonstrate that two metastable stackings of the orbitally ordered layers allow to manipulate salient features of the electronic structure. Indeed, these orbital effects enable to switch the properties of 1T-TaS2 nanostructures from metallic to semiconducting with technologically pertinent gaps of the order of 200 meV. This new type of orbitronics is especially relevant for the ongoing development of novel, miniaturized and ultra-fast devices based on layered transition metal dichalcogenides.

So now in addition to spintronics and magnonics, we also have orbitronics!

Shocking I tell you, shocking!

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Bismuth Iodide and Topological Insulators are in the News

by Tommy on 25/09/2014


Epitaxial growth of large-gap quantum spin Hall insulator on semiconductor surface, Miao Zhou, Wenmei Ming, Zheng Liu, Zhengfei Wang, Ping Li, and Feng Liu, Proceedings of the National Academies of Sciences of the United States of America, PNAS, 22 September 2014

DOI: 10.1073/pnas.1409701111

Quantum phase of matter is of great scientific and technological interest. The quantum spin Hall (QSH) insulator is a newly discovered two-dimensional material that exhibits topological edge state residing inside bulk energy gap, so that its edge is metallic with quantized conductance and its bulk is insulating. For its potential applications in spintronics and quantum computing, a large energy gap is desirable, e.g., for room-temperature application. So far, large-gap QSH insulators have been predicted only in freestanding films. Here we demonstrate the formation of a large-gap QSH state on a semiconductor substrate through epitaxial growth of heavy metal atoms on halogenated Si surface. Our findings not only reveal a new formation mechanism of large-gap QSH insulator, but may also pave the way for its experimental realization.

Formation of topological quantum phase on a conventional semiconductor surface is of both scientific and technological interest. Here, we demonstrate epitaxial growth of 2D topological insulator, i.e., quantum spin Hall state, on Si(111) surface with a large energy gap, based on first-principles calculations. We show that the Si(111) surface functionalized with one-third monolayer of halogen atoms [Si(111)-3√×3√-X (X = Cl, Br, I)] exhibiting a trigonal superstructure provides an ideal template for epitaxial growth of heavy metals, such as Bi, which self-assemble into a hexagonal lattice with high kinetic and thermodynamic stability. Most remarkably, the Bi overlayer is atomically bonded to but electronically decoupled from the underlying Si substrate, exhibiting isolated quantum spin Hall state with an energy gap as large as ∼0.8 eV. This surprising phenomenon originates from an intriguing substrate-orbital-filtering effect, which critically selects the orbital composition around the Fermi level, leading to different topological phases. In particular, the substrate-orbital-filtering effect converts the otherwise topologically trivial freestanding Bi lattice into a nontrivial phase; and the reverse is true for Au lattice. The underlying physical mechanism is generally applicable, opening a new and exciting avenue for exploration of large-gap topological surface/interface states.

Supplemental Supporting Information (PDF)

See also : http://arxiv.org/abs/1401.3392

Large-gap quantum spin Hall state on a semiconductor surface: The orbital filtering by substrate, Miao Zhou, Wenmei Ming, Zheng Liu, Zhengfei Wang, Yugui Yao, Feng Liu

For potential applications in spintronics and quantum computing, it is desirable to place a quantum spin Hall insulator [i.e., a 2D topological insulator (TI)] on a substrate while maintaining a large energy gap. Here, we demonstrate an approach to create the large gap 2D TI state on a semiconductor surface, based on extensive first principles calculations. We show that when Bi, Pb and Au atoms are deposited on a patterned H-Si(111) surface into a hexagonal lattice, both the Bi@H-Si(111) and Pb@H-Si(111) surfaces exhibit a 2D TI state with a large gap of {greater than} 0.5 eV while the Au@H-Si(111) surface is a trivial insulator. These interesting results are found to originate from the fact that the H-Si(111) surface acts as an atomic orbital filter to critically select the orbital composition around the Fermi level, resulting in different topological phases. In particular, the substrate orbital filtering effect converts the otherwise topologically trivial freestanding Bi and Pb lattices into a nontrivial phase; while the reverse is true for the Au lattice. The physical mechanism underlying this approach is generally applicable, opening up a new and exciting avenue for future design and fabrication of large-gap topological surface/interface states.

So, after almost exactly 20 years, it begins. Unfortunately, this story seems to have taken on a life of its own, reminiscent of the quantum computer in the Forbin Project. The way it sounds Google will have Colossus running on the barge next week, and we’ll have thinking and talking animatronic toys by Christmas. Such is life in the new university press release era of science.

On the Nature of Bismuth (I) Iodide in the Solid State, T. L. Elifritz, Spec. Sci. Tech., 17, 85, 1994

I doubt anyone remembers the 1995 Laboratory of the Year issue of R&D Magazine. Old news.

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High Temperature Superconductivity in the Cuprates

by Tommy on 18/09/2014
High Temperature Superconductivity in the Cuprates

High Temperature Superconductivity in the Cuprates


High Temperature Superconductivity in the Cuprates, B. Keimer, S. A. Kivelson, M. R. Norman, S. Uchida and J. Zaanen

The discovery of high temperature superconductivity in the cuprates in 1986 triggered a spectacular outpouring of creative and innovative scientific inquiry. Much has been learned over the ensuing 28 years about the novel forms of quantum matter that are exhibited in this strongly correlated electron system. This progress has been made possible by improvements in sample quality, coupled with the development and refinement of advanced experimental techniques. In part, avenues of inquiry have been motivated by theoretical developments, and in part new theoretical frameworks have been conceived to account for unanticipated experimental observations. An overall qualitative understanding of the nature of the superconducting state itself has been achieved, while profound unresolved issues have come into increasingly sharp focus concerning the astonishing complexity of the phase diagram, the unprecedented prominence of various forms of collective fluctuations, and the simplicity and insensitivity to material details of the “normal” state at elevated temperatures. New conceptual approaches, drawing from string theory, quantum information theory, and various numerically implemented approximate approaches to problems of strong correlations are being explored as ways to come to grips with this rich tableaux of interrelated phenomena.

This review should get you started and up to speed on recent developments.

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The Delta 9 Reusable Launch Vehicle

by Tommy on 27/08/2014


I have unretired briefly to do this for the United States Air Force (USAF).

The Delta 9 reusable launch vehicle concept consists essentially of a SpaceX Falcon 9R clone, implemented in hydrogen fuel, using recently developed and tested Blue Origin BE-3 engines.

The intent of such a program is to produce separate propulsion, airframe and operations sectors for the emerging commercial space flight industry in the same way our current airline industries are structured. This specific vehicle is designed in such a way as to motivate our legacy aerospace companies to begin participating substantively and competitively in this new commercial spaceflight industry, within their respective market niches and areas of expertise, while driving the industry forward with substantial Air Force funding, in much the same way that NASA has done with the COTS program, but focused more on specific Department of Defense space launch needs, with near term operational time frames (2018). The most obvious existing tank suitable for modification into the required launch vehicle would be the Boeing Delta IV Medium, but alternative airframe providers and technologies would not be ruled out.

So yeah, I’m a rocket scientist and a space architect. Now all I need is a rocket.


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