From: Burt Kessler (bcomet@postoffice.pacbell.net)
Date: Fri Aug 16 2002 - 12:34:45 PDT
Date: Fri, 16 Aug 2002 12:34:45 -0700 From: Burt Kessler <bcomet@postoffice.pacbell.net> Subject: superconductor theory Message-id: <a05100306b98304217c42@[64.164.11.120]>
Burt
>Contact: Paul Preuss, (510) 486-6249, paul_preuss@lbl.gov
>
>(For an illustrated html version of this press release, with
>links, see
>http://www.lbl.gov/Science-Articles/Archive/MSD-superconductor-Cohen-Louie.html)
>
>A MOST UNUSUAL SUPERCONDUCTOR AND HOW IT WORKS
>First-principles calculation explains the strange behavior
>of magnesium diboride
>
>BERKELEY, CA -- Magnesium diboride (MgB2) becomes
>superconducting at 39 degrees Kelvin, one of the highest
>known transition temperatures (Tc) of any superconductor.
>What's more, its puzzling characteristics include more than
>one superconducting energy gap, a state of affairs
>anticipated in theory but never before seen experimentally.
>
>Now theorists at Lawrence Berkeley National Laboratory and
>the University of California at Berkeley, led by Marvin
>Cohen and Steven Louie of Berkeley Lab's Materials Sciences
>Division, both professors of physics at UC Berkeley, have
>calculated the properties of this unique superconductor from
>first principles, revealing the secrets of its anomalous
>behavior. Collaborators in the project included postdoctoral
>fellow Hyoungjoon Choi, graduate student David Roundy, and
>visitor Hong Sun.
>
>In the August 15 issue of Nature, the theorists report that
>MgB2's odd features arise from two separate populations of
>electrons -- nicknamed "red" and "blue" -- that form
>different kinds of bonds among the material's atoms. As well
>as explaining conflicting observations, their calculations
>led to predictions subsequently born out by experiment.
>Further, they suggest the possibility of creating radically
>new materials with analogous electronic structure.
>
>Bottles of powdered MgB2 have been sitting on the chemical
>laboratory shelf since the 1950s, but not until January of
>2001 did Japanese researchers announce their discovery that
>it was a relatively high-temperature superconductor. Like
>high-Tc superconductors made of cuprate ceramics, MgB2 is a
>layered material; while undoped cuprates are insulators at
>ordinary temperatures, however, MgB2 is always a metal.
>
>"Structurally, magnesium diboride is almost as simple as
>pencil lead, graphite," says Louie. "It consists of
>hexagonal honey-combed planes of boron atoms separated by
>planes of magnesium atoms, with the magnesiums centered
>above and below the boron hexagons."
>
>This remarkably simple atomic structure would eventually
>prove the key to understanding MgB2. But in the hundreds of
>papers produced in the first rush to examine the new
>superconductor, experimenters using different techniques
>found many different, unusual, and sometimes conflicting
>properties.
>
>"It was like the blind men looking at the elephant," Cohen
>remarks. "Everybody who looked at MgB2 saw a different
>picture. Some said the superconducting energy gap was this,
>others said it was that; still others found anomalies in
>measurements of specific heat."
>
>It quickly became apparent that theories developed to
>explain superconductivity in the layered, high-Tc cuprates
>would not be helpful in understanding MgB2. Instead, Louie
>and Cohen and their colleagues used the well-established
>Bardeen-Cooper-Schrieffer (BCS) theory to examine the
>fundamental properties of MgB2, an effort made possible by a
>technique Choi developed to solve the BCS equations for
>materials with complex electronic structure.
>
>"When we looked at the elephant," says Cohen, "we saw that
>almost everybody had been right!" The many different
>pictures were in fact consistent.
>
>In BCS theory, electrons overcome their mutual repulsion to
>form pairs that can move through the material without
>resistance. Vital to pair formation are the quantized
>vibrations of the crystal lattice, known as phonons.
>
>"Electrons pair by exchanging a phonon. If you think of a
>lattice of positive ions, you can picture them 'pulling' the
>electrons together into pairs, as vibration moves them
>toward passing electrons," says Cohen.
>
>What was puzzling was that, in BCS theory, the coupling to
>the lattice required to form an electron pair should be
>equivalent to the coupling of a single electron emitting and
>reabsorbing a phonon, giving rise to an enhanced electron
>mass. But in MgB2 these two values were apparently different
>-- a clue that more than one kind of electron might be
>involved in pairing. So the theorists began with basic
>considerations of MgB2's elemental constituents and layered
>structure.
>
>"To understand the importance of crystal structure to MgB2's
>electronic states, compare it to graphite," Louie suggests.
>In the hexagonal planes of graphite, each carbon atom, which
>has four valence electrons, is bonded to three others,
>occupying all available planar bonding states, the sigma
>bonds; its remaining electron moves in orbitals above and
>below the plane, forming pi bonds.
>
>MgB2, like graphite, has strong sigma bonds in the planes
>and weak pi bonds between them, but since boron atoms have
>fewer electrons than carbon atoms, not all the sigma bonds
>in the boron planes are occupied. And because not all the
>sigma bonds are filled, lattice vibration in the boron
>planes has a much stronger effect, resulting in the
>formation of strong electron pairs confined to the planes.
>
>"Partially occupied sigma bonds driving superconductivity in
>a layered structure is one of the new concepts that appeared
>from the theoretical studies; generally speaking, nature
>does not like unoccupied sigma bond states," says Louie.
>"Our other major finding is that not all the boron electrons
>are needed in strong pair formation to achieve high Tc. In
>addition to the strongly bonded sigma pairs, the boron
>electrons involved in pi bonds form much weaker pairs."
>
>Stated differently, electrons on different parts of the
>Fermi surface form pairs with different binding energies.
>The theorists' graph of MgB2's extraordinary Fermi surface
>-- a way of visualizing the highest-energy states its
>electrons can occupy -- clearly shows the two populations of
>electrons and the different energies needed to break their
>superconducting pairs -- a graph that incidentally gives
>rise to the nicknames "red" and "blue" electrons.
>
>Four distinctive kinds of sheets make up the Fermi surface.
>Two form nested cylinders: these map differently oriented
>sigma bonds and are colored orange and red to indicate the
>large amount of energy needed to break these superconducting
>pairs -- a large superconducting "gap," ranging from 6.4 to
>7.2 thousandths of an electron volt (meV) at 4 degrees
>Kelvin.
>
>Two other sheets of the Fermi surface form "webbed tunnels"
>and represent the pi-bonded electrons; they are colored
>green and blue to indicate the low energy (1.2 to 3.7 meV)
>required to break superconducting pairs of these electrons
>at 4 degrees K, constituting a separate superconducting gap.
>
>The two kinds of electron pairs are coupled, and as
>temperature increases the superconducting gaps for "red" and
>"blue" pairs rapidly converge, until at about 39 degrees K
>both vanish. Above this temperature, all pairs are broken
>and the material does not superconduct.
>
>The detailed theoretical calculations of the superconducting
>gaps and their temperature dependence for the "red" and
>"blue" electrons made it possible to interpret the
>experimental measurements, including those from scanning
>tunneling microscopy, optical studies, electron
>photoemission, and neutron analyses, and from heat capacity
>and infrared studies -- each a different way of "seeing the
>elephant."
>
>Cohen and Louie and their colleagues performed their
>first-principles calculations on supercomputers at the
>Department of Energy's National Energy Research Scientific
>Computing Center (NERSC) based at Berkeley Lab and first
>shared them with the condensed-matter community last fall.
>Experimentalists have since confirmed many of the explicit
>predictions of their model. Among these was the existence of
>two superconducting gaps in MgB2, never before seen in any
>material.
>
>Yet BCS theory contemplated the possibility of materials
>with multiple superconducting energy gaps early on, and the
>discovery of MgB2 raises the possibility that others could
>be made. Louie and Cohen have long studied the electronic
>properties of unusual materials incorporating boron, carbon,
>and nitrogen. MgB2 offers a new model for layered materials
>capable of high-temperature superconductivity.
>
>"The origin of the anomalous superconducting properties of
>MgB2," by Hyoung Joon Choi, David Roundy, Hong Sun, Marvin
>L. Cohen, and Steven G. Louie, appears in the 15 August 2002
>issue of Nature.
>
>The Berkeley Lab is a U.S. Department of Energy national
>laboratory located in Berkeley, California. It conducts
>unclassified scientific research and is managed by the
>University of California. Visit our website at
>http://www.lbl.gov.
-- A truly happy person is one who can enjoy the scenery on a detour.
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