Physicists find surprising distortions in high-temperature superconductors

August 7, 2018 by Mike Williams, Rice University
Rice University researchers used experiments and simulations to discovery small distortions in the lattice of an iron pnictide that becomes superconductive at ultracold temperatures. They suspect these distortions introduce pockets of superconductivity in the material above temperatures at which it becomes entirely superconductive. Credit: Weiyi Wang/Rice University

There's a literal disturbance in the force that alters what physicists have long thought of as a characteristic of superconductivity, according to Rice University scientists.

Rice physicists Pengcheng Dai and Andriy Nevidomskyy and their colleagues used simulations and neutron scattering experiments that show the atomic structure of materials to reveal tiny distortions of the crystal lattice in a so-called iron pnictide compound of sodium, iron, nickel and arsenic.

These local distortions were observed among the otherwise symmetrical atomic order in the material at ultracold temperatures near the point of optimal . They indicate researchers may have some wiggle room as they work to increase the temperature at which iron pnictides become superconductors.

The discovery reported this week in Nature Communications is the result of nearly two years of work by the Rice team and collaborators in the U.S., Germany and China.

Dai and Nevidomskyy, both members of the Rice Center for Quantum Materials (RCQM), are interested in the fundamental processes that give rise to novel collective phenomena like superconductivity, which allows materials to transmit electrical current with no resistance.

Scientists originally found superconductivity at ultracold temperatures that let atoms cooperate in ways that aren't possible at room temperature. Even known "high-temperature" superconductors top out at 134 Kelvin at ambient pressure, equivalent to minus 218 degrees Fahrenheit.

So if there's any hope for widespread practical use of superconductivity, scientists have to find loopholes in the basic physics of how atoms and their constituents behave under a variety of conditions.

That is what the Rice researchers have done with the iron pnictide, an "unconventional superconductor" of sodium, iron and arsenic, especially when doped with nickel.

To make any material superconductive, it must be cooled. That sends it through three transitions: First, a structural phase transition that changes the lattice; second, a magnetic transition that appears to turn paramagnetic materials to antiferromagnets in which the atoms' spins align in alternate directions; and third, the transition to superconductivity. Sometimes the first and second phases are nearly simultaneous, depending on the material.

In most unconventional superconductors, each stage is critical to the next as electrons in the system begin to bind together in Cooper pairs, reaching peak correlation at a quantum critical point, the point at which magnetic order is suppressed and superconductivity appears.

Rice University physicists find surprising distortions in high-temperature superconductors
These single crystals of nickel-doped compounds of sodium, iron and arsenic are like those used by Rice University researchers in experiments to determine the material's superconductive properties at ultracold temperatures. They used simulations and precise neutron scattering experiments to show the presence of tiny lattice distortions near the optimal superconductivity of an iron pnictide compound. Credit: Rice University

But in the pnictide superconductor, the researchers found the first transition is a little fuzzy, as some of the lattice took on a property known as a nematic phase. Nematic is drawn from the Greek word for "thread-like" and is akin to the physics of liquid crystals that align in reaction to an outside force.

The key to the material's superconductivity seems to lie within a subtle property that is unique to iron pnictides: a structural transition in its , the ordered arrangement of its atoms, from tetragonal to orthorhombic. In a tetragonal crystal, the atoms are arranged like cubes that have been stretched in one direction. An orthorhombic structure is shaped like a brick.

Sodium-iron-arsenic pnictide crystals are known to be tetragonal until cooled to a transition temperature that forces the lattice to become orthorhombic, a step toward superconductivity that appears at lower temperatures. But the Rice researchers were surprised to see anomalous orthorhombic regions well above that structural transition temperature. This occurred in samples that were minimally doped with nickel and persisted when the materials were over-doped, they reported.

"In the tetragonal phase, the (square) A and B directions of the lattice are absolutely equal," said Dai, who carried out neutron scattering experiments to characterize the material at Oak Ridge National Laboratory, the National Institute of Standards and Technology Center for Neutron Research and the Research Neutron Source at the Heinz Maier-Leibnitz Center.

"When you cool it down, it initially becomes orthorhombic, meaning the lattice spontaneously collapses in one axis, and yet there's still no magnetic order. We found that by very precisely measuring this lattice parameter and its distortion, we were able to tell how the lattice changes as a function of temperature in the paramagnetic tetragonal regime."

They were surprised to see pockets of a superconducting skewing the towards the orthorhombic form even above the first transition.

"The whole paper suggests there are local distortions that appear at a temperature at which the system, in principle, should be tetragonal," Dai said. "These local distortions not only change as a function of temperature but actually 'know' about superconductivity. Then, their temperature dependence changes at optimum superconductivity, which suggests the system has a nematic quantum critical point, when local nematic phases are suppressed.

"Basically, it tells you this nematic order is competing with superconductivity itself," he said. "But then it suggests the nematic fluctuation may also help superconductivity, because it changes dependence around optimum doping."

Being able to manipulate that point of optimum doping may give researchers better ability to design with novel and predictable properties.

"The electronic nematic fluctuations grow very large in the vicinity of the , and they get pinned by local crystal imperfections and impurities, manifesting themselves in the local distortions that we measure," said Nevidomskyy, who led the theoretical side of the investigation. "The most intriguing aspect is that superconductivity is strongest when this happens, suggesting that these nematic fluctuations are instrumental in its formation."

Explore further: Scientists uncover the microscopic origin of a magnetic phase in iron-based superconductors

More information: Weiyi Wang et al, Local orthorhombic lattice distortions in the paramagnetic tetragonal phase of superconducting NaFe1−xNixAs, Nature Communications (2018). DOI: 10.1038/s41467-018-05529-2

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Whart1984
Aug 07, 2018
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Hyperfuzzy
not rated yet Aug 12, 2018
can't figure it out. the stable atomic elements move as a collective, i.e. the atom moves at one energy state. The collective fields produced re feedback for similar phenomena, yeah, yeah, all charge respond, see the near field, or better the point of 'action' for each charge. Since the molecular motion is random; first: Can it be abolished, i.e. a superimposed field. Might want to make the thermaL EFFECTS nonrandom. Can the motion be controlled?
Hyperfuzzy
not rated yet Aug 12, 2018
Else know what is required. It is all a simple ply. Don't think most humans can figure it out. Think you need a Necessary and Sufficient Theory before you begin and a computer program to define events you seek. Anyway, I'll play nice. Only charge exists. Within the space you are concerned, count the sources. Maybe begin Atomically. Construct requirements based only upon these. No made up $hit, all definable! Can we define the Total spectrum instead of temperature, N counts of motion, RMS value, how does that help? Juz says som'n going on! So let's sort 'em. How clever are ya?
Hyperfuzzy
not rated yet Aug 12, 2018
For goodness sake, if you cannot design for results; then you have no idea what you're doing. Review what you Know!
Hyperfuzzy
not rated yet Aug 12, 2018
If you are using QM; you are 'EasterEgging' for surprises.
Hyperfuzzy
not rated yet Aug 12, 2018
Notice the space about each charge center, all we know is that this is the center of an E field & is unique it moves according to Coulomb. About the charge the field is updated; think of it as the equal potential spheres or hypothetical shells update at the speed of light. The center has existed since ? Therefore it's DC field is felt everywhere according to Coulomb. Since a Neutron is really a combo electron proton at a Point, adjust for others; there are always N pairs to each atom. Orbiters exist without ions from 1 to ? Solvable as 2N Eq's and 2N unknowns. Make your unknown an r , v, a or whatever, ..
Hyperfuzzy
not rated yet Aug 12, 2018
It all is a superposition of all potentials, continuously.
Hyperfuzzy
not rated yet Aug 12, 2018
Can we play with real Physics instead of QM? Each potential has a source. So be more specific.
Whart1984
Aug 12, 2018
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Whart1984
Aug 12, 2018
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Whart1984
Aug 12, 2018
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Whart1984
Aug 12, 2018
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Whart1984
Aug 12, 2018
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Whart1984
Aug 13, 2018
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Whart1984
Aug 14, 2018
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Whart1984
Aug 14, 2018
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Whart1984
Aug 14, 2018
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Whart1984
Aug 14, 2018
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Hyperfuzzy
not rated yet Aug 27, 2018
The high temperature superconductors work in the following way: they contain long lines of positively charged atoms (so called charge stripes) which attract electrons like the hens to the line of feeders. The electrons are mutually repulsive particles, so that they hate each other - but they're all attracted to positively charged atoms and they concentrate there. Their repulsive forces overlap and compensate to a high degree there, so that the electrons will start to propagate freely. The linear character of charge stripes also contributes to their mutual entanglement (shielding effect): the electrons move fast about axis of stripe but very slowly in perpendicular direction, so that the boson condensate gets formed there.


Let logic, knowledge of charge, composition of object under investigation. if you can't explain it geometrically or temporally, simple from the number of +/- free centers, potential spheres update, cumulative response per center! If 1 then ...

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