Experiments may soon confirm a computer simulation of hydrogen, at high
pressures, turning into a superconducting superfluid. Besides having
the ability to conduct electricity without loss, this new form of
hydrogen would also be a liquid that would flow without friction.

Phys. Rev. Lett. 95, 135301 (2005) Super tangles. According to computer simulations, under the right conditions liquid hydrogen is a superconducting superfluid, where combined vortex lines from electrons (blue) and protons (red) form a rigid lattice (top). At higher temperatures the vortex lines remain in pairs, but they now move freely through the superfluid (bottom). (See animations below.)

Under extremes of pressure and temperature, hydrogen will likely
become a superconductor, conducting electricity without resistance. But
it may simultaneously become a liquid that flows without friction–a
superfluid. This unprecedented combination state, and other related
ones, are predicted by computer simulations reported in the June Physical Review B and the 23 September PRL. Whether these unusual states actually occur remains contentious, but experimental tests may soon settle the debate.

Under sufficient pressure–millions of atmospheres, such as occurs
in Jupiter’s interior–hydrogen will turn into a liquid metal. At
temperatures within a few degrees of absolute zero, it may become a
superconductor, in which a current of electrons can flow without
resistance. At these temperatures a superconducting state of protons
may also emerge [1].
Remarkably, the electron and proton states might cooperate to form a
superfluid, in which electrons and protons would move in concert,
allowing friction-free motion of mass with no net motion of charge.

Asle Sudbø of the Norwegian University of Science and Technology in
Trondheim, in collaboration with two Cornell University physicists,
recently predicted some properties of the superconducting superfluid
state and two other states–one superconducting but not superfluid, the
other superfluid but not superconducting [2].
In their latest two papers, Sudbø and his colleagues describe
simulations that explore in detail the interactions of electrons and
protons in liquid hydrogen. All three states appear in the simulations,
with transitions between them controlled by temperature and an
externally applied magnetic field.

A magnetic field can force many superconductors into a sort of Swiss
cheese state, where the field only penetrates through tubes of
non-superconducting material called vortex lines, around which
electrons circulate. At low temperatures, in the superconducting
superfluid phase of liquid hydrogen, vortex lines of both electron and
proton superconductors physically coincide and form a fixed lattice.

As you heat the hydrogen, two things can happen, depending on the
strength of the magnetic field, the researchers say. In a modest field,
the proton vortex lines begin to detach from the electron vortex lines
as the hydrogen warms. This separation disrupts the coherent motion of
the two components and destroys superfluidity, while superconductivity
persists. In their simulations, the team observed the transition to
this "electronic superconductor" state.

If the field is fixed at a higher strength as the temperature
climbs, the two types of vortex lines cling together, but the composite
vortices break out of their rigid lattice positions and move freely,
creating what researchers call a vortex liquid. This phase is not a
superconductor because an electric current would force the magnetic
flux tubes to move, and thus would expend energy. But because the
electron and proton charges remain coordinated everywhere,
superfluidity is preserved. Sudbø and his colleagues also saw this
phase in their simulations.

"The [superconducting superfluid] state would be interesting," says
David Ceperly of the University of Illinois at Urbana-Champaign, but he
is skeptical that it can actually occur. The crucial question is
whether hydrogen freezes into a solid or remains liquid at low
temperatures, and his own computer calculations suggest that it will
freeze first. Others, however, disagree [3].

Fortunately, Ceperly and Sudbø agree, experiments may resolve the question before too long [4].
New laboratory methods to make flawless diamonds should soon allow the
construction of compression devices that do not fracture at the
pressures needed to create metallic superfluid hydrogen. "Only
experiments will tell whether or not this state exists," Sudbø says.

–David Lindley
David Lindley is a freelance writer in Arlington, Virginia, and author of Degrees Kelvin: A Tale of Genius, Invention, and Tragedy (Joseph Henry Press, March 2004).

References:

[1] K. Moulopoulos and N.W. Ashcroft, "Generalized Coulomb Pairing in the Condensed State," Phys. Rev. Lett. 66, 2915 (1991).
[2] E. Babaev, A. Sudbø, and N.W. Ashcroft, "A Superconductor to Superfluid Phase Transition in Liquid Metallic Hydrogen," Nature (London) 431, 666 (2004).
[3] S. A. Bonev, E. Schwegler, T. Ogitsu, and G. Galli, "A Quantum
Fluid of Metallic Hydrogen Suggested by First-Principles Calculations,"
Nature (London) 431, 669 (2004).
[4] E. Babaev, A. Sudbø, and N.W. Ashcroft, "Observability of a Projected New State of Matter: A Metallic Superfluid," Phys. Rev. Lett. 95, 105301 (2005).

Videos

Dancing Vortices

A
computer-generated animation shows electronic (blue) and protonic (red)
vortex lines created by a magnetic field passing through liquid
hydrogen at low temperature and under extreme pressure. At first the
system is a superconducting superfluid. As the temperature climbs, the
composite vortex lines begin to move off of their lattice sites and
eventually separate when the system becomes a normal liquid.