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
1 / 5 (1) Aug 07, 2018
these distortions introduce pockets of superconductivity in the material above temperatures at which it becomes entirely superconductive
This is so-called pseudogap phase: small islands of superconductive material, which gradually merge together during decreasing temperature.
that superconductivity is strongest when this happens, suggesting that these nematic fluctuations are instrumental in its formation
It's very trivial artifact - but BCS theory doesn't predict it (it occurs only within HT superconductors) - so that its understanding is some 35 years delayed.
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
not rated yet Aug 12, 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.

Whart1984
not rated yet Aug 12, 2018
As one may expect, this effect establishes gradually: some places along hole stripes serve as a flux pinning centers and the electron condense along them in droplets like the beads on rosary. This is just the pseudogap phase, because individual droplets are already well superconductive - but because they're not still interconnected, no macroscopic current can still flow along them.

Phase diagram of pseudogap

What can we understand from this diagram? First of all, the pseudogap forms only when the density of charge stripes remains low (low density of holes), because as one can imagine, when the density of stripes is high, then the electrons will form a continuous layer around them and they already have no place for formation of islands.
Whart1984
not rated yet Aug 12, 2018
Furthermore, we can see from diagram, that the superconductivity temperature goes through optimum of doping (hole density), because when the holes get too sparse, then there is not enough of free electrons for their full surrounding with electrons and merging into a continuous condensate phase: we get lotta pseudogap phase with high pressure of electrons within its condensate droplets - but no bulk superconductivity. Such an effect will survive high magnetic fields thought, so it was just the regime studied in the above articles.

When the concentration of holes gets too high, then the electrons will start to repel mutually rather than to get attracted to holes and they will form a continuous phase - but the level of mutual squeezing of electrons will remain low, so that no quantum condensation can occur there. We get less or more uniform strange metal with high amount of Dirac electrons, but no superconductivity.
Whart1984
not rated yet Aug 12, 2018
This explanation may look complex for someone, but it's actually based just on balance of attractive and repulsive forces between fixed and movable electrons. The main secret here is, with compare to atoms the electrons have no solid surfaces, so that when electrons move at large distances from each other, they can move freely. When we compress them, then they get locked into fixed places within so-called Wigner lattice. When we compress them even more, then this lattice would dissolve and the electrons will start move freely again - and this is just the moment when the superconductivity may apply.
This explanation actually applies to low-temperature superconductivity too, because BCS theory doesn't actually explain, why some elements can get superconductive whereas others not and why the good superconductors are so brittle. The answer is, the squeezing of electrons poses strong force to lattice which must counteract their repulsive forces, so that strength of lattice decreases.
Whart1984
not rated yet Aug 12, 2018
In the tetragonal phase, the directions of the lattice are absolutely equal,..When you cool it down, it initially becomes orthorhombic, meaning the lattice spontaneously collapses in one axis... They were surprised to see pockets of a superconducting nematic phase skewing the lattice towards the orthorhombic form even above the first transition. This occurred in samples that were minimally doped with nickel ...
It's easily understandable, because the Ni4+ atoms play a role of holes in similar way like Cu3+ atoms within cuprates. When the lattice collapses, some places become superconductive, but because electrons exhibit strong repulsive pressure to lattice just around superconducting centers the orthorhombic lattice expands back into tetragonal phase again. Many simulations don't account to flexibility of lattice. This is the problem with overdoping that the repulsive electrons may overcome the attractive forces of lattice, which expands and the superconductivity gets lost.
Whart1984
not rated yet Aug 13, 2018
Magic Twist in Stacked Graphene Reveals Potentially Powerful Superconducting Behavior Actually there is already a few of room superconductivity observations connected with graphite (1, 2) for complete lack of replications in this matter. Are we waiting for dying out supporters of BCS theory - or what?
Whart1984
not rated yet Aug 14, 2018
Researchers Find Evidence of Ambient Temperature Superconductivity (Tc=236K) in Au-Ag Nanostructures This finding has
already been challenged though - in postmodern science challenges always advance attempts for replications - often by many decades of years. First room superconductivity finding is 45 years (Grigorov 1984) and so far it hasn't been officially replicated. The doubting study points to an identical pattern of noise for two presumably independent measurements of the magnetic susceptibility as a function of temperature.
Whart1984
not rated yet Aug 14, 2018
Researchers Find Evidence of Ambient Temperature Superconductivity (Tc=236K) in Au-Ag Nanostructures This finding has
already been challenged though - in postmodern science challenges always advance attempts for replications - often by many decades of years. First room superconductivity finding is 45 years (Grigorov 1984) and so far it hasn't been officially replicated. The doubting study points to an identical pattern of noise for two presumably independent measurements of the magnetic susceptibility as a function of temperature.
Whart1984
not rated yet Aug 14, 2018
Researchers Find Evidence of Ambient Temperature Superconductivity (Tc=236K) in Au-Ag Nanostructures This finding has
already been challenged though - in postmodern science challenges always advance attempts for replications - often by many decades of years. First room superconductivity finding is 45 years (Grigorov 1984) and so far it hasn't been officially replicated. The doubting study points to an identical pattern of noise for two presumably independent measurements of the magnetic susceptibility as a function of temperature.
Whart1984
not rated yet Aug 14, 2018
My apology for multiplicated posts.. :-(

Actually none of these announcements of room temperature superconductivity (1, 2, 3, 4, 5, 6, 7,...) were even attempted to replicate - which speaks for something... :-\

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