[Paleopsych] New Scientist: Solids that can pass through solids

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Solids that can pass through solids

      * 08 October 2005
      * Catherine Zandonella

    IS it possible to walk through walls? Can solid objects really pass
    through each other? Moses Chan thinks they can, and he says he has the
    proof. Chan and his colleagues at Pennsylvania State University have
    created the world's first "supersolids", bizarre crystals that slide
    through each other like ghosts. It is a finding that promises to
    revolutionise the way we think about matter. "It really changes one's
    concept of solids," says Jason Ho, a solid-state theorist at Ohio
    State University in Columbus.

    The idea that one solid object can flow through another contradicts
    all our everyday experiences: no one has ever seen a teacup dissolve
    through a saucer. And when you prop up the bar on Friday nights there
    is no danger of you slowly melting into the surface and falling out
    the other side.

    Solids get their reassuring rigidness from the orderly way their atoms
    are arranged. Unlike liquids and gases, the atoms in a crystal are
    fixed, a bit like the squares on a chequerboard. While they may quiver
    a little due to their thermal energy, this normally isn't enough to
    dislodge the atoms and cause them to flow over each other.

    However, physicists have long suspected that this rule could be
    broken. In 1969, Russian theorists Alexander Andreev and Ilya Lifshitz
    were studying the properties of solids and found that there was indeed
    a way for them to flow. In certain crystals, the bonds between atoms
    are so weak that you can squeeze the solids like a sponge. Such weak
    bonds give a crystal another property. Close to absolute zero its
    atoms have barely enough thermal energy to shiver. But for some
    crystals, even this tiny movement is thought to be enough for an atom
    to break free. This leaves the crystal latticework full of gaps called
    "zero-point vacancies" and these vacancies are mobile, even at
    absolute zero.

    It might sound odd that a gap in a crystal can have any physical
    properties. But physicists think of these vacancies as having energy
    and mass just as atoms do. And they can even move around the crystal.
    Andreev and Lifshitz predicted even more bizarre behaviour: the
    vacancies can move through the solid in synchrony as if one solid were
    flowing through another.

    This happens because of the peculiar nature of quantum mechanics.
    Close to absolute zero, quantum theory says that groups of atoms can
    lose their individual identities and start behaving like a single,
    giant atom. Instead of dancing around randomly in a gas or liquid,
    these atoms can condense into a single quantum state and start moving
    in perfect lock-step. This happens because their quantum wave
    function, the region of space in which a particle is found, spreads
    out and grows larger than the distance between the atoms.

    The details of the process depend on a basic property of particles
    called spin. Atoms divide into two families called bosons and fermions
    depending on the nature of their spin. Quantum mechanics dictates that
    no two identical fermions can share the same quantum state, so they
    cannot move as a single entity. But there is no such restriction on
    bosons. They can crowd together in the same energy state when the wave
    functions begin to overlap. It is at this point, they start behaving
    like one massive atom, dubbed a Bose-Einstein condensate. In principle
    at least, this means that solids made of bosons can flow through each

    In many solids this cannot happen because the atoms are locked in
    their individual fixed positions like people sitting in their assigned
    seats in a theatre. But Andreev and Lifshitz recognised that atoms
    have more freedom in crystals that have weak bonds. And they realised
    that the same thing should apply to vacancies, so they would have the
    freedom to move around too and condense into one, giant vacancy. Like
    a ghost walking through a wall, they predicted that this "supersolid"
    made of vacancies would pass eerily though the rest of the crystal.

    Andreev and Lifshitz were thinking of solid helium-4 when they did
    their calculations because helium-4 atoms are bosons and the bonds
    between them are so weak. Helium is also the second lightest element
    in the periodic table, so its quantum behaviour should become more
    apparent when it cools than with heavier elements. Supersolids should
    therefore show up in helium and hydrogen first.

    But they could form in any solid under the right conditions. "If this
    is what quantum mechanics tells us about the ground state of solid
    hydrogen and solid helium, it is true of every solid in the universe,"
    says Philip Anderson of Princeton University, an expert on the quantum
    theory of solids and winner of the 1977 Nobel prize for physics.

    Since Andreev and Lifshitz made their prediction, however, many
    researchers have looked in vain for supersolids. Yet there are some
    tantalising hints that supersolids do indeed exist. In the late 1990s,
    John Goodkind of the University of California in San Diego and his
    colleagues saw something strange in experiments with crystals made
    from helium's isotopes helium-3 and helium-4.

    Creating helium crystals is not easy. You need more than just freezing
    temperatures to transform liquid helium-4 into a solid: you also have
    to crush it to at least 25 times normal atmospheric pressure.

    Goodkind's team was bombarding a solid helium-4 crystal with
    ultrasound. As they cooled the crystal close to absolute zero, the
    researchers noticed the ultrasound waves speeding up. This could have
    been down to the formation of a supersolid. When sound travels through
    a solid, it causes the atoms to vibrate. If part of the crystal had
    turned into a supersolid, it would have decoupled from the rest of the
    crystal and allowed the sound waves to travel faster. But the team
    needed to do more experiments to be sure.

    Enter Chan and his graduate student Eunseong Kim. Inspired by
    Goodkind's results, they decided to take a look for themselves in
    2001. They began by putting some solid helium-4 in a bucket, hanging
    it from a string, and spinning it first clockwise then counter
    clockwise at 1000 times a second while freezing it. This experiment
    would enable them to see what happens to solid helium as it oscillates
    rapidly at very low temperatures.

    In reality the device is more complicated, surrounded by tubes and
    wires that deliver helium and record measurements. The "bucket" is
    actually smaller than a sewing thimble and rotates on a shaft rather
    than a string. To compress the helium into a crystal Chan and Kim
    ratcheted up the pressure to 60 times atmospheric pressure.

    Success finally came last year. As they cooled the crystal down below
    2 K, the experimenters carefully monitored the rate at which the
    thimble oscillated. The frequency of this oscillation is governed by
    the shaft's stiffness and the thimble's inertia, which is determined
    by the mass of helium inside.

Supersolid debut

    At about 0.2 K, the thimble began to oscillate faster, as if some of
    the helium had escaped from the bucket. Except that no escape was
    possible since Chan and Kim had already checked for leaks in the
    system. They concluded that roughly 1 per cent of the solid had
    stopped moving with the rest of the helium crystal and was standing
    stock still. The rest of the crystal appeared to be passing through

    It was an electrifying moment, but Chan and Kim had to make sure that
    what they were seeing wasn't the result of a flaw in their set-up. So
    they replaced helium-4 with helium-3, whose atoms are fermions and
    should not form a supersolid. If they saw no effect with helium-3,
    they could be confident that the original result was real.

    Sure enough, there was no sign of supersolid flow with helium-3. But
    still the researchers weren't completely sure of what they saw. They
    published the results in Nature (vol 427, p 225) under the rather
    hesitant title "Probable observations of a supersolid helium phase".

    Chan's fellow physicists were intrigued. "It is hard to believe that a
    solid can behave that way," says Wayne Saslow, a condensed matter
    physicist at Texas A&M University in College Station. "If it is right,
    it means that this particular solid is like a localised fluid rather
    than a solid."

    While their fellow physicists scratched their heads, Kim and Chan did
    more experiments to check their results. They published their later
    findings in Science (vol 305, p 1941) but this time they dropped
    "probable" from the title.

    The most logical explanation for his results, thinks Chan, is that 1
    per cent of the atoms or vacancies condensed into a single unit that
    then played by the rules of quantum mechanics while the rest of it
    continued to live in the classical world.

    "You think of solids as very reliable and boring, with all the atoms
    fixed in their expected positions," says Chan. "We are saying no,
    there is actually flow within a solid."

    Yet Chan's results are proving controversial. While everyone agrees
    that the experiment was carried out carefully, other groups of
    researchers are still trying to replicate it. A Japanese group led by
    Masaru Suzuki of the University of Electro-Communications in Tokyo and
    Keiya Shirahama at Keio University in Yokohama, has recently conducted
    a similar experiment to Goodkind's, using sound waves to study solid
    helium-4. So far they have found no evidence of supersolid-like

    Meanwhile theorists are arguing about what Chan has really seen.
    Nikolai Prokof'ev, a theorist at the University of Massachusetts in
    Amherst and his colleague Boris Svistunov think what Chan spotted were
    myriad microscopic crystals slipping around in a sea of liquid helium,
    rather than a supersolid passing through a single crystal. "Think of
    chunks of packed ice with water in between," says Prokof'ev. Yet Chan
    argues that such behaviour would have shown up in his experiments.

    Theorists are also going back to basics and re-examining Andreev and
    Lifshitz's theory. Some, like David Ceperley, a theorist at the
    University of Illinois in Urbana-Champaign, thinks vacancies cannot be
    present at the high pressures in Chan's experiment. He argues that
    rising pressure would squeeze the vacancies out of the crystal. He
    expects them to migrate to the end of the crystal and disappear like
    bubbles escaping from a fizzy drink.

    Others, including Tony Leggett, also at Illinois and winner of the
    2003 physics Nobel for his work on superfluid helium, argue that
    vacancies might not even be needed for supersolid behaviour. Leggett
    says it might be down to atoms switching places like in a game of
    musical chairs. Chan is convinced vacancies and other types of crystal
    defects are involved, but he is leaving it to the theorists to work
    out the details. "The explanation is probably a bit more subtle than
    what was proposed by Andreev and Lifshitz," he says.

    But many researchers simply want to skip the controversy and go
    straight to the question of how the stuff would act if it really was a
    supersolid. One reason they are so keen is that if Chan is correct,
    that means that quantum mechanics can operate on large scales in all
    three states of matter - solids, liquids, and gases. "Quantum
    mechanics is supposedly the theory that explains nature, yet we don't
    see it existing in our environment," says Seamus Davis, a physicist at
    Cornell University in Ithaca, New York. "We know of liquids and gases
    that can be macroscopic quantum systems. It would be very exciting if
    we now have an example in a solid."

Towards absolute zero

    It is too early to say what supersolids could be used for. But the
    very fact that Chan may have uncovered an entirely new state of matter
    is reason enough to get excited. So could supersolids turn up in any
    other elements? In March, Chan and his students Anthony Clark and Xi
    Lin reported similar behaviour in solid hydrogen at even lower
    temperatures. Hydrogen molecules are bosons, so the result ties in
    nicely with their previous experiments. And Chan is confident that
    other elements could follow.

    Researchers have already coaxed over half a dozen different gases into
    Bose-Einstein condensates using special traps that employ magnetic
    fields and lasers to hold and cool the atoms. And it is possible to
    make superfluids from fermions, such as helium-3. Here the atoms first
    pair up to form bosons before crowding into the same quantum state. So
    who knows what elements might follow next.

    But don't get too excited just yet: other supersolids may be hard to
    detect. As you move down the periodic table, the atoms act less like
    quantum particles because they are more massive and bond more strongly
    to neighbouring atoms. So the supersolid effect is likely to be
    smaller. To see it, you would need extremely low temperatures, perhaps
    as low as a few thousandths or a few millionths of a degree above
    absolute zero. Yet it is still possible. "It is just that the
    supersolid fraction will be smaller and we will have a difficult time
    measuring it," says Anderson.

    So it is unlikely that the supersolid phase will help you walk through
    walls, at least not yet. But studying supersolid behaviour could usher
    in a new way of thinking about solids, says Anderson. The textbook
    picture of a solid as a chequerboard array of atoms with one atom per
    space could be replaced by something a whole lot weirder. "It really
    shakes your confidence in what a solid is," says Anderson.

Go with the flow

    Helium is strange stuff by anyone's standards. At temperatures where
    most other elements just freeze into lumps, liquid helium can flow
    straight up and over the side of a cup on its own. Stir some chilled
    helium in a coffee cup and it will continue to swirl around
    effortlessly forever; it is not slowed by viscosity the way ordinary
    liquids are. Such renegade liquids are called superfluids. Both of
    helium's isotopes, helium-3 and helium-4, morph into superfluids when
    the thermometer dips within a few degrees of absolute zero.

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