A manganite changes its stripes

July 15, 2011
At the right temperature, a special kind of manganite can change its stripes from a disordered state to an ordered static state, switching from a good conductor of electricity to an insulator -- a colossal change in conductivity. Credit: Lawrence Berkeley National Laboratory

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 – the ability to suddenly boost resistance to electrical conductivity by orders of magnitude when a magnetic field is applied – and manganites are also promising candidates for spintronics applications – devices that can manipulate electrons according to their quantum spin as well their charge.

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 "two-dimensional 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 co-author of the PNAS paper. "It establishes the existence of a distinct new phase of the material, which we call fluctuating bi-stripes."

With ARPES data from the Advanced Light Source's beamline 12.0.1, researchers charted electron energy and momentum to plot the band structure of highly doped lanthanum strontium manganese oxide. At 170 degrees Kelvin (154 F) the material switches from a metal to an insulator. Below that temperature, electrons can move freely among disordered insulating fragments. Above that temperature the patches align into coherent stripes. An extra row of highly ionized manganese atoms between the stripes acts as a barrier to electron mobility. Credit: Lawrence Berkeley National Laboratory

Unlike the stripes in some high-temperature superconductors, in which the 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 focused on low to medium doping levels, where the sample behaves either as a ferromagnetic metal or an anti-ferromagnetic insulator. (Ferromagnetism is the kind of magnetism displayed by iron, which can be magnetized by a magnetic field applied from outside and can retain that magnetism.) 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.

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 vaccum. 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, 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."

For the success of the team's work, Fedorov credits a long history of manganite research by the University of Colorado physicists, Zhe Sun and Quinn Wang, led by Daniel Dessau; the high-quality manganite samples made by Jennifer Hong Zheng and group leader John Mitchell from Argonne; and finally the characteristics of the 12.0.1 beamline, which was built for ARPES. He says, "The beamline was recently upgraded for greater photon flux, improved resolution, and excellent sample positioning, and" – because experiments that rely on temperature change need a good vacuum – "the stable vacuum environment, which made this study possible."

Explore further: Looking for 'Stripes' in High-Tc superconductors

More information: PNAS June 29, 2011 , doi: 10.1073/pnas.1018604108

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1 / 5 (1) Jul 15, 2011
The self-organization into stripes is probably an instance of the Brazil Nut Effect, also known as the Paranuss Problem. (When you Google the phrase, it's better to use Brazil Nut Problem, which pops up literature about the theoretical underpinnings and the difficulty of making precise predictions.)

When a jar of nuts or balls of different sizes is shaken sufficiently, the objects will self-organize into discrete layers. Each layer will contain only (or mostly) objects of a particular size--and none of the "other" objects in the jar.

In this manganite case, the bands appear at and above the transition temperatures. The higher temperature is equivalent to a sufficient shaking of a jar. Also in the case of this manganite, the separation will result in bands due to lattice constraints.

Here, the key difference between the "objects" may simply be that some "objects" are minus three electrons, while other "objects" are minus four electrons.

1 / 5 (1) Jul 15, 2011
Pretty quiet around here. So let me throw out a little more bait for the many skeptics who rise up against posters like me. I say that the Brazil Nut Effect, and the self-organization in manganite as described in this article, and the self-organization of the many rings of Saturn are all governed by the same principle.

Fire away.
1 / 5 (2) Jul 16, 2011
Not bad, not bad at all. I can see how your theory could work. Here's a theory of mine that is unrelated, but I believe to be more audacious. I remember a television manufacturer proposing a 3D TV by creating a multiple file of clear parallel screens -2 Dimensional screens that played only the images within their assigned ranges , but added together, would smoothly create the effect of three dimensions. Imagine, and by that I ask for suspension of disbelief for a moment , that our universe is composed of many(infinite) 2 dimensional screens, parallel and infinitesimally apart from each other, that we perceive as three dimensions- like looking through the filed screens of the aforementioned TV. The Casimir effect, or something akin to it, is at play between these closely spaced dimensions, creating a vast network of, and as of yet unexplained, cosmic energy sources.
not rated yet Jul 16, 2011
The self-organization into stripes is probably an instance of the Brazil Nut Effect, also known as the Paranuss Problem.
It's interesting idea, but A) The temperature corresponds the intensity of shaking in this model. The more intensive shaking will only make the Brazil Nut Effect established faster. Your model has no explanation, why this effect is reversible with temperature. B) The direction of layers in Brazil Nut Effect depends on gravity, which wasn't observed in real samples of manganite.
not rated yet Jul 16, 2011
My explanation can be only rough one and it's based on the analogy with metallic glasses. These materials are amorphous, i.e with regular layers because the size of atoms differs only slightly each other, which prohibits the formation of regular crystal lattice. This picture illustrates an example of metallic glass.


The heating of manganite could make atomic bonds expanded in such way, their mutual length will become more similar and as the result the more regular structure becomes preferred. A similar transition is responsible for martensite-austenite phase transition inside of superellastic metal alloys, which are exhibiting shape memory.
1 / 5 (1) Jul 16, 2011
Thanks for your interest and your comments. I am working on another project now but plan to respond with a post later today.
1 / 5 (1) Jul 16, 2011
Let's start with the order that appears in the manganite (stripes) at and above a certain temperature. I say that order results from a forced separation, not from an attractive force. The difference between a minus three object and a minus four object becomes significant--the driving force--at and above a critical temperature.

The same effect is now at work with the euro. Tempers are flaring and pressure is increasing. As it is with phase transitions in physics, so it is with unions of people and countries. A union that is stable under what we consider to be normal conditions can be split asunder at higher temperatures and greater pressures.

Or think of the gaits of a horse. Here, all four objects--the hooves--are identical. Even so, the horse prefers different configurations (gaits) at different speeds (frequencies, which are akin to temperature variations).

The Brazil Nut Effect is a forced separation caused by varied oscillation rates due to varied nut size, shape.
1 / 5 (1) Jul 16, 2011
The article says that the conditions are "counterintuitive," because the order emerges at higher temps, and "melts" at lower temps. Why counterintuitive? Because order in physics generally increases at lower temperatures. While usually true, order can also emerge from forced separation, which is often the culprit when we encounter new or more order at higher temps or pressures. Plasma is an easy example. See also the germanium high pressure result on phys org a few months ago.

Here, there is a useful result stemming from the change in order, which is the point of the article. That's good. I suspect that similar surprising results will emerge--at high temps or pressures--with other materials that have small variations in their constituent objects, as forced separation creates new and surprising order. The consequences of any new order are hard to predict, but some consequences will be useful and will add to the tool kit that material scientists have made available.
1 / 5 (1) Jul 16, 2011
"The Brazil Nut Effect is a forced separation caused by varied oscillation rates due to varied nut size, shape."
That's an excellent response to Callipo's astute critique, but I didn't take the Brazil nut analogy literally or that you meant that gravity was the only operational force at work here. For that matter, the rings of Saturn may have been arranged by other forces besides gravity. By creating materials with a similar structure but act in the reverse of this manganite, room-temperature superconductors could become a reality.
not rated yet Jul 16, 2011
creating materials with a similar structure but act in the reverse of this manganite, room-temperature superconductors could become a reality
Such superconductors exist already and they're prepared just with using of metallic glass trick, which I described above - i.e. with crystallization of copper oxide (forming superconductive stripes) with mixture of metallic oxides, forming the insulating layers between stripes.


There is no need of some structural transition with temperature for superconductors, on the contrary. The stripes should form continuous phase through whole crystal and every structural change would destroy their superconductivity.
not rated yet Jul 16, 2011
Like I've said, the Brazil Nut Effect doesn't explain, why the manganite structural change is reversible with temperature. The layers of Brazil Nut wouldn't mix again, when the frequency of vibrations is decreased (which would correspond the temperature decreasing in your analogy). In addition, the atoms aren't behaving like freely movable nuts within crystal lattice - they're connected mutually with directional forces together.
1 / 5 (1) Jul 16, 2011
I stand corrected, Callippo, on a few counts. I think I meant, though, that only the structure that allows for the superconductivity in the manganite be imitated rather than its
temperature transition properties. I'm also thinking in terms of electric power line loss, so the material you mentioned I do remember reading about, but with the advent of cheap graphene, isn't that more likely a candidate to replace the grid's antiquated and expensive cable?
1 / 5 (1) Jul 16, 2011
with the advent of cheap graphene, isn't that more likely a candidate to replace the grid's antiquated and expensive cable?
I don't think so, because the cold fusion produces copper waste - we would get a lotta copper for production of conductors. And you cannot replace the superconductors with copper or graphene in applications, when the true superconductivity is required.
1 / 5 (1) Jul 16, 2011
A PhysOrg article I overlooked from July 14th. The country is too broke to implement this anyway. Someday, maybe.

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