2ε4f + U + J
2ε4f + U – J
L = 0, S = 0
L = 1, S = 1
A flipping cell
Upon reflection. Image showing how circularly polarized light interacts with superconducting Cooper pairs bound by
Hund’s rule coupling. Two electrons that move into the 5f shell of the same uranium atom have an interaction energy
U + J if they form a singlet spin = 0 state, and U – J if they form a triplet spin = 1 state. Here, U represents the average
Coulomb interaction between two electrons occupying the same 5f shell, and J is the Hund’s rule exchange interaction.
U is much more strongly screened than J. For U < J, an attractive interaction results between two conduction electrons
hybridizing with the same uranium atom, but only if their spins are parallel. The antiparallel spin state would result
in a repulsive interaction with this mechanism. Because of the angular momentum of the pair, left-handed and right-handed photons do not have the same absorption coefficient, and, because linear polarized light is a superposition of
left- and right-handed light, the handedness of the absorption causes a finite rotation of the polarization angle.
magnetism was thought to be incompatible
with the superconducting state. This view,
however, has changed with the advent of the
high-temperature superconducting cuprates
and, more recently, by the iron pnictides
and chalcogenides, both of which are fundamentally magnetic. The possibility of mixed
singlet-triplet character has been proposed,
because the low symmetry of the underlying crystal lattice allows for such a mixture
to exist. If the interactions present that give
rise to this type of pairing can be identified,
finding the correct balance between these interaction channels may be sufficient to form
an exotic superconductor with a desired set
Recently, superconducting two-dimen-
sional electron gases have been generated by
molecular beam epitaxy methods to physi-
cally grow dissimilar materials in layered
structures, creating entirely new properties
at the interfaces. Controlling the nature
of the pairing interaction for triplet super-
conductors could lead to ways of creating
new triplet superconductors based on the
interface properties. The road of materi-
als engineering combining ferromagnetism
and superconductivity in a single material
has been paved by the discovery of mag-
netic field–induced superconductivity in the
chalcogenides (11), and the demonstration
of spin triplet supercurrent through the
half-metallic ferromagnet CrO2 (12). Because
of the nature of the pairing type, p-wave
superconductors are more compatible with
ferromagnetism than conventional s-wave
superconductors. Further improvement of
these materials may be possible if they
can be based on triplet (instead of singlet)
In the experiments reported by Schemm
et al., the sensitivity to time-reversal symmetry breaking relies on the chiral nature of
the photon. Left-handed and right-handed
photons will have different absorption coefficients if the state of matter of the material to which they couple has itself a certain
handedness (see the figure). However, as
the energy of the two states is independent
of the handedness, textures (in 3He, for example, textures are observed in the form
of lines, planes, and points) involving these
two degenerate states can in principle exist.
And because each circularly polarized photon transfers a quantum of angular momentum, one may speculate on future spintronic
devices in which the spin state of the pair-condensate is manipulated by the transfer of
photon angular momentum. ■
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3. C.M.Varma, Comm. Solid State Phys. 11, 221(1985).
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5. P. W.Anderson, Phys. Rev. B 30,1549(1984).
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Most bacteria surround themselves with atough, highlyinterconnected polymer called peptidoglycan. To create this wall, bacteria synthesize precursors in the cytoplasm, attach them to a lipid carrier (lipid II),
transport (“flip”) this compound across the
cytoplasmic membrane, and then polymerize
and cross-link the monomers to the existing
cell wall (1). The enzymes responsible for the
cytoplasmic and extracytoplasmic steps are
well known. However, hydrophilic precursors must cross the hydrophobic membrane,
and the protein that conducts this critical
transfer has stubbornly resisted identification. On page 220 of this issue, Sham et al.
(2) present evidence that the MurJ protein
of Escherichia coli performs this “flippase”
function in vivo, thus solving a major puzzle
in the basic pathway that synthesizes the
bacterial cell wall. The new data are somewhat disconcerting, though, because in vitro
experiments identified another candidate,
Fts W, as the relevant flippase, with MurJ being inactive (3). We are left with the tantalizing question of how these results are to be
reconciled and which protein fills this fundamental biochemical role.
At issue is how hydrophilic compounds
traverse a highly hydrophobic and nearly impenetrable lipid bilayer. Numerous receptors,
transporters, pores, pumps, and transducers
help transform lipid bilayers into the semipermeable membranes required for cellular
life, but it is unclear how several important
carbohydrates cross bacterial and eukaryotic
membranes (1, 4). There are a limited number of ways to export polysaccharides, so
studying the mechanisms used by E. coli has
the potential to shed light on other transport
systems (5). The issue has practical import,
as well, since the cell wall allows bacteria to
exist in the osmotically inhospitable environments that cover most of Earth and oceans,
and removing the wall or inhibiting its synthesis is one of the most frequently used
means of controlling bacterial infection.
By Kevin D. Young
How bacteria “flip”
across the cytoplasmic