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  • richardmitnick 6:09 pm on March 4, 2019 Permalink | Reply
    Tags: “What is interesting” says first author Tatsuya Kaneko a postdoctoral researcher at the RIKEN Cluster for Pioneering Research “is that our calculations showed that this takes place based on the , But for the present study the researchers used non-equilibrium dynamics to analyze the effect of pulses of light on a Mott insulator and found that the effect would in fact happen in the real world, Mott insulators, , Scientists from the RIKEN Cluster for Pioneering Research have shown that pulses of light could be used to turn these materials beyond simple conductors to superconductors—materials that conduct ele, superconductors, Thirty years ago the mathematical physicist Chen-Ning Yang originally proposed the idea of eta-pairing but because it was a purely mathematical concept it was understood as a virtual phenomenon that w, This process would happen through an unconventional type of superconductivity known as “eta pairing”, Under normal electron band theory they ought to conduct electricity but they do not due to interactions among their electrons, What remains is to perform actual experiments with Mott insulators to verify that this process actually takes place   

    From RIKEN: “Light pulses provide a new route to enhance superconductivity” 

    RIKEN bloc

    From RIKEN

    March 4, 2019

    Chief Scientist
    Seiji Yunoki
    Computational Condensed Matter Physics Laboratory
    Chief Scientist Laboratories

    Jens Wilkinson
    RIKEN International Affairs Division
    Tel: +81-(0)48-462-1225 / Fax: +81-(0)48-463-3687
    Email: pr@riken.jp

    1
    Schematic of eta-pairing

    Materials known as Mott insulators are odd things. Under normal electron band theory they ought to conduct electricity, but they do not, due to interactions among their electrons. But now, scientists from the RIKEN Cluster for Pioneering Research have shown that pulses of light could be used to turn these materials beyond simple conductors to superconductors—materials that conduct electricity without energy loss. This process would happen through an unconventional type of superconductivity known as “eta pairing.”

    Using numerical simulations, the researchers found that this unconventional type of conductivity, which is believed to take place under non-equilibrium conditions in strongly correlated materials such as high-Tc cuprates and iron-pnictides, arises due to a phenomenon known as eta pairing. This is different form the superconductivity observed in the same strongly correlated materials under equilibrium conditions, and is thought to involve repulsive interactions between certain electrons within the structure. It is also different from traditional superconductivity, where the phenomenon arises due to interactions between electrons and vibrations of the crystal structure, inducing mutual interactions between electrons through vibrations and thus overcoming the repulsion between the electrons.

    Thirty years ago, the mathematical physicist Chen-Ning Yang originally proposed the idea of eta-pairing, but because it was a purely mathematical concept, it was understood as a virtual phenomenon that would not take place in the real world. But for the present study, the researchers used non-equilibrium dynamics to analyze the effect of pulses of light on a Mott insulator, and found that the effect would in fact happen in the real world. “What is interesting,” says first author Tatsuya Kaneko, a postdoctoral researcher at the RIKEN Cluster for Pioneering Research, “is that our calculations showed that this takes place based on the beautiful mathematical structure that Yang and his followers formulated so many years ago.”

    According to Seiji Yunoki, who led the research team, “This work provides new insights not only into the phenomenon of non-equilibrium dynamics, but also could lead to the development of new high-temperature superconductors, which could be useful in applications. What remains is to perform actual experiments with Mott insulators to verify that this process actually takes place.”

    The research was published in Physical Review Letters.

    See the full article here .


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    RIKEN is Japan’s largest comprehensive research institution renowned for high-quality research in a diverse range of scientific disciplines. Founded in 1917 as a private research foundation in Tokyo, RIKEN has grown rapidly in size and scope, today encompassing a network of world-class research centers and institutes across Japan.

     
  • richardmitnick 11:04 am on January 2, 2019 Permalink | Reply
    Tags: , , , , Physicists record “lifetime” of graphene qubits, , , superconductors   

    From MIT News: “Physicists record ‘lifetime’ of graphene qubits” 

    MIT News
    MIT Widget

    From MIT News

    December 31, 2018
    Rob Matheson

    1
    Researchers from MIT and elsewhere have recorded the “temporal coherence” of a graphene qubit — how long it maintains a special state that lets it represent two logical states simultaneously — marking a critical step forward for practical quantum computing. Stock image

    First measurement of its kind could provide stepping stone to practical quantum computing.

    Researchers from MIT and elsewhere have recorded, for the first time, the “temporal coherence” of a graphene qubit — meaning how long it can maintain a special state that allows it to represent two logical states simultaneously. The demonstration, which used a new kind of graphene-based qubit, represents a critical step forward for practical quantum computing, the researchers say.

    Superconducting quantum bits (simply, qubits) are artificial atoms that use various methods to produce bits of quantum information, the fundamental component of quantum computers. Similar to traditional binary circuits in computers, qubits can maintain one of two states corresponding to the classic binary bits, a 0 or 1. But these qubits can also be a superposition of both states simultaneously, which could allow quantum computers to solve complex problems that are practically impossible for traditional computers.

    The amount of time that these qubits stay in this superposition state is referred to as their “coherence time.” The longer the coherence time, the greater the ability for the qubit to compute complex problems.

    Recently, researchers have been incorporating graphene-based materials into superconducting quantum computing devices, which promise faster, more efficient computing, among other perks. Until now, however, there’s been no recorded coherence for these advanced qubits, so there’s no knowing if they’re feasible for practical quantum computing.

    In a paper published today in Nature Nanotechnology, the researchers demonstrate, for the first time, a coherent qubit made from graphene and exotic materials. These materials enable the qubit to change states through voltage, much like transistors in today’s traditional computer chips — and unlike most other types of superconducting qubits. Moreover, the researchers put a number to that coherence, clocking it at 55 nanoseconds, before the qubit returns to its ground state.

    The work combined expertise from co-authors William D. Oliver, a physics professor of the practice and Lincoln Laboratory Fellow whose work focuses on quantum computing systems, and Pablo Jarillo-Herrero, the Cecil and Ida Green Professor of Physics at MIT who researches innovations in graphene.

    “Our motivation is to use the unique properties of graphene to improve the performance of superconducting qubits,” says first author Joel I-Jan Wang, a postdoc in Oliver’s group in the Research Laboratory of Electronics (RLE) at MIT. “In this work, we show for the first time that a superconducting qubit made from graphene is temporally quantum coherent, a key requisite for building more sophisticated quantum circuits. Ours is the first device to show a measurable coherence time — a primary metric of a qubit — that’s long enough for humans to control.”

    There are 14 other co-authors, including Daniel Rodan-Legrain, a graduate student in Jarillo-Herrero’s group who contributed equally to the work with Wang; MIT researchers from RLE, the Department of Physics, the Department of Electrical Engineering and Computer Science, and Lincoln Laboratory; and researchers from the Laboratory of Irradiated Solids at the École Polytechnique and the Advanced Materials Laboratory of the National Institute for Materials Science.

    A pristine graphene sandwich

    Superconducting qubits rely on a structure known as a “Josephson junction,” where an insulator (usually an oxide) is sandwiched between two superconducting materials (usually aluminum). In traditional tunable qubit designs, a current loop creates a small magnetic field that causes electrons to hop back and forth between the superconducting materials, causing the qubit to switch states.

    But this flowing current consumes a lot of energy and causes other issues. Recently, a few research groups have replaced the insulator with graphene, an atom-thick layer of carbon that’s inexpensive to mass produce and has unique properties that might enable faster, more efficient computation.

    To fabricate their qubit, the researchers turned to a class of materials, called van der Waals materials — atomic-thin materials that can be stacked like Legos on top of one another, with little to no resistance or damage. These materials can be stacked in specific ways to create various electronic systems. Despite their near-flawless surface quality, only a few research groups have ever applied van der Waals materials to quantum circuits, and none have previously been shown to exhibit temporal coherence.

    For their Josephson junction, the researchers sandwiched a sheet of graphene in between the two layers of a van der Waals insulator called hexagonal boron nitride (hBN). Importantly, graphene takes on the superconductivity of the superconducting materials it touches. The selected van der Waals materials can be made to usher electrons around using voltage, instead of the traditional current-based magnetic field. Therefore, so can the graphene — and so can the entire qubit.

    When voltage gets applied to the qubit, electrons bounce back and forth between two superconducting leads connected by graphene, changing the qubit from ground (0) to excited or superposition state (1). The bottom hBN layer serves as a substrate to host the graphene. The top hBN layer encapsulates the graphene, protecting it from any contamination. Because the materials are so pristine, the traveling electrons never interact with defects. This represents the ideal “ballistic transport” for qubits, where a majority of electrons move from one superconducting lead to another without scattering with impurities, making a quick, precise change of states.

    How voltage helps

    The work can help tackle the qubit “scaling problem,” Wang says. Currently, only about 1,000 qubits can fit on a single chip. Having qubits controlled by voltage will be especially important as millions of qubits start being crammed on a single chip. “Without voltage control, you’ll also need thousands or millions of current loops too, and that takes up a lot of space and leads to energy dissipation,” he says.

    Additionally, voltage control means greater efficiency and a more localized, precise targeting of individual qubits on a chip, without “cross talk.” That happens when a little bit of the magnetic field created by the current interferes with a qubit it’s not targeting, causing computation problems.

    For now, the researchers’ qubit has a brief lifetime. For reference, conventional superconducting qubits that hold promise for practical application have documented coherence times of a few tens of microseconds, a few hundred times greater than the researchers’ qubit.

    But the researchers are already addressing several issues that cause this short lifetime, most of which require structural modifications. They’re also using their new coherence-probing method to further investigate how electrons move ballistically around the qubits, with aims of extending the coherence of qubits in general.

    See the full article here .


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  • richardmitnick 3:09 pm on January 16, 2014 Permalink | Reply
    Tags: , , , , superconductors   

    From Berkeley Lab: “Natural 3D Counterpart to Graphene Discovered” 


    Berkeley Lab

    Researchers at Berkeley Lab’s Advanced Light Source Find New Form of Quantum Matter

    January 16, 2014
    Lynn Yarris (510) 486-5375 lcyarris@lbl.gov

    The discovery of what is essentially a 3D version of graphene – the 2D sheets of carbon through which electrons race at many times the speed at which they move through silicon – promises exciting new things to come for the high-tech industry, including much faster transistors and far more compact hard drives. A collaboration of researchers at the U.S Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) has discovered that sodium bismuthate can exist as a form of quantum matter called a three-dimensional topological Dirac semi-metal (3DTDS). This is the first experimental confirmation of 3D Dirac fermions in the interior or bulk of a material, a novel state that was only recently proposed by theorists.

    graph
    A topological Dirac semi-metal state is realized at the critical point in the phase transition from a normal insulator to a topological insulator. The + and – signs denote the even and odd parity of the energy bands.

    “A 3DTDS is a natural three-dimensional counterpart to graphene with similar or even better electron mobility and velocity,” says Yulin Chen, a physicist with Berkeley Lab’s Advanced Light Source (ALS) when he initiated the study that led to this discovery, and now with the University of Oxford. “Because of its 3D Dirac fermions in the bulk, a 3DTDS also features intriguing non-saturating linear magnetoresistance that can be orders of magnitude higher than the materials now used in hard drives, and it opens the door to more efficient optical sensors.”

    Chen is the corresponding author of a paper in Science reporting the discovery. The paper is titled Discovery of a Three-dimensional Topological Dirac Semimetal, Na3Bi. Co-authors were Zhongkai Liu, Bo Zhou, Yi Zhang, Zhijun Wang, Hongming Weng, Dharmalingam Prabhakaran, Sung-Kwan Mo, Zhi-Xun Shen, Zhong Fang, Xi Dai and Zahid Hussain.

    See the full article and all of the excitment here.

    A U.S. Department of Energy National Laboratory Operated by the University of California

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