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If there were a Hall of Fame for materials, manganites would be among its
members. Some manganites, compounds of manganese oxides, are renowned for
colossal magnetoresistance. Manganites are also promising candidates for
spintronics applications.
What’s not particularly unusual about manganites, however, is that they have
stripes, regions where the material’s electrical charges gather and
concentrate. Other so-called correlated-electron materials also have stripes,
including many high-temperature superconductors having the same crystal
structure: arrangements of layers of atoms named for the mineral perovskite.
Now a team of researchers from the University
of Colorado at Boulder, the U.S. Department of Energy’s
Lawrence Berkeley National Laboratory (Berkeley Lab), and Argonne National
Laboratory have used the technique of angle-resolved photoelectron spectroscopy
(ARPES), at beamline 12.0.1 at Berkeley Lab’s Advanced Light Source, to
demonstrate a startling new feature of one kind of lanthanum strontium manganese
oxide.
This “2D bilayer manganite” can change its stripes from fluctuating to
static and back. As a result, at the right temperature it switches from a
metallic state, a good conductor of electricity, to an insulator—a colossal
change in conductivity. The researchers report their results in Proceedings
of the National Academy of Sciences (PNAS).
New stripes versus old
“Self-organization of charges into static stripes isn’t new, but as far as we
know this is the first good insight into what happens to the electronic
properties of a material when stripes ‘fluctuate’—in other words, break their
perfect order—and fracture to pieces,” says Alexei Fedorov, staff scientist for
ALS beamline 12.0.1 and a coauthor of the PNAS
paper. “It establishes the existence of a distinct new phase of the material,
which we call fluctuating bi-stripes.”
Unlike the stripes in some high-temperature superconductors, in which the
electrons move freely, the electrons in the manganite bi-stripes are frozen in place.
Electrons not trapped inside a stripe are free to move, but when the stripes
are lined up side by side, all the way across the crystal, free electrons are
stymied at every turn.
The bilayer compound used in the study has the formula La2?2xSr1+2xMn2O7.
The x’s in the formula indicate the degree of positive doping, the introduction
of additional positive charge carriers or holes (in fact, the absence of
electrons), which markedly affect how the compound behaves.
Previous studies of lanthanum strontium manganese oxides focused on low to
medium doping levels, where the sample behaves either as a ferromagnetic metal
or an anti-ferromagnetic insulator. The new study was conducted at higher
doping over a range of temperatures, where earlier X-ray scattering experiments
had observed static stripes.
“Static bi-stripes have a wider spacing and only occur at higher
temperatures,” says Fedorov. “It sounds counterintuitive, but below the
critical temperature the bi-stripes ‘melt,’ and instead of forming long
parallel bands they become disordered.” The disordered stripes exist as broken
fragments, allowing free electrons to find a path among them.
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Detecting disordered or “fluctuating stripes” is tricky. ARPES can detect
their presence, but X?ray scattering, which reconstructs an image from X-rays
diffracted by atoms of specific elements in the sample, doesn’t see them
individually; rather it responds to their periodicity. Since there’s no
periodicity in the “melted” stripes of highly doped La2-2xSr1+2xMn2O7,
the scattering signal quickly disappears below the temperature at
which the static bi-stripes disappear.
How to catch stripes in the act
ARPES can see both kinds of stripes because it draws a spectrum of electronic
states directly, when a bright beam of soft X?rays from ALS beamline 12.0.1
falls on the sample and excites the electrons into the vacuum. These electrons
are caught by a detector that measures their kinetic energy and direction. From
this data the electronic structures in the sample can be identified.
The remarkable ability to turn the conductivity of properly doped bilayer
manganite on and off just by adjusting its temperature a few degrees holds
obvious promise. The bi?stripes act like electronic valves and could be used to
tune various materials—perhaps even high-temperature superconductors—by
altering the material’s stripe structure.
“We’re after smart materials,” says Fedorov. “We need them for all kinds of
reasons—for example, for electronic devices so energy efficient that they can
run on a lighter batteries or fewer solar cells. In this area, applications are
already getting ahead of what we comprehend—for example, colossal
magnetoresistance or high-temperature superconductivity. To make real progress,
it’s important that we understand the physics of highly correlated materials.”
The physical picture of bilayer manganite the researchers have come up with
shows how electrons can hop from manganese atom to manganese atom. Some of
these atoms lack three electrons (Mn3+) and others lack four (Mn4+),
and at low temperature the free electrons alternate from one to the next as
they find paths through the disordered landscape. But at high temperature the
stripes line up—and the gaps between them are wider, accommodating an extra row
of Mn4+ atoms, as a result of the extra hole doping. Electrons can’t
negotiate these irregularly spaced “stepping stones.”
