Mathis on Graphene? Any hints?

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Re: Mathis on Graphene? Any hints?

Post by Cr6 on Sat Mar 10, 2018 2:18 am

This came out recently...might need to review Miles' paper one more time to see if the charge field aligns differently with a "twist":
(more at link....)

Give double-layer graphene a twist and it superconducts
A ‘magic angle’ lets electrons flow freely

By Emily Conover
4:10pm, March 8, 2018

DOUBLE UP  A device made of two layers of graphene (illustrated) can conduct electricity without resistance when one layer is rotated relative to the other.


LOS ANGELES — Give a graphene layer cake a twist and it superconducts — electrons flow freely through it without resistance. Made up of two layers of graphene, a form of carbon arranged in single-atom-thick sheets, the structure’s weird behavior suggests it may provide a fruitful playground for testing how certain unusual types of superconductors work, physicist Pablo Jarillo-Herrero of MIT reported March 7 at a meeting of the American Physical Society.

The discovery, also detailed in two papers published online in Nature on March 5, could aid the search for a superconductor that functions at room temperature, instead of the chilly conditions required by all known superconductors. If found, such a substance could replace standard conductors in various electronics, promising massive energy savings.

Layered graphene’s superconductivity occurs when the second layer of graphene is twisted relative to the first, at a “magic angle” of about 1.1 degrees, and when cooled below 1.7 kelvins (about –271° Celsius). Surprisingly, Jarillo-Herrero and colleagues report, the same material can also be nudged into becoming an insulator — in which electrons are stuck in place — by using an electric field to remove electrons from the material. That close relationship with an insulator is a characteristic shared by certain types of high-temperature superconductors, which function at significantly warmer temperatures than other superconductors, although they still require cooling.

APS March Meeting 2018
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session K35: 2D Materials - Superconductivity and Charge Density Waves I
8:00 AM–11:00 AM, Wednesday, March 7, 2018
LACC Room: 409B
Sponsoring Unit: DMP
Chair: Daniel Rhodes, Columbia Univ
Abstract: K35.00007 : Topology, correlations, and superconductivity in 2D
9:12 AM–9:48 AM
Abstract Presenter: Pablo Jarillo-Herrero (Physics, MIT)

In this talk I will review our recent quantum electronic transport experiments in a variety of 2D materials and van der Waals heterostructures, where we show interplay between topology, strong electron-electron correlations and electrically tunable superconductivity. In some of these materials, different phases can be achieved simply by tuning the electric field applied to the material.


(more at link....)

Some high-temperature superconductors might not be so odd after all
Finding hidden swirls of electric current shows that the material’s behavior matches standard theory

By Emily Conover
7:00am, December 8, 2017
illustration of a superconductor

VORTEX FOUND Newly observed swirls of electric current in a high-temperature superconductor (shown in an artist’s conception) may indicate that the unusual material fits within the standard theoretical picture.

Magazine issue: Vol. 193, No. 1, January 20, 2018, p. 11

A misfit gang of superconducting materials may be losing their outsider status.

Certain copper-based compounds superconduct, or transmit electricity without resistance, at unusually high temperatures. It was thought that the standard theory of superconductivity, known as Bardeen-Cooper-Schrieffer theory, couldn’t explain these oddballs. But new evidence suggests that the standard theory applies despite the materials’ quirks, researchers report in the Dec. 8 Physical Review Letters.

All known superconductors must be chilled to work. Most must be cooled to temperatures that hover above absolute zero (–273.15° Celsius). But some copper-based superconductors work at temperatures above the boiling point of liquid nitrogen (around –196° C). Finding a superconductor that functions at even higher temperatures — above room temperature — could provide massive energy savings and new technologies (SN: 12/26/15, p. 25). So scientists are intent upon understanding the physics behind known high-temperature superconductors.

When placed in a magnetic field, many superconductors display swirling vortices of electric current — a hallmark of the standard superconductivity theory. But for the copper-based superconductors, known as cuprates, scientists couldn’t find whirls that matched the theory’s predictions, suggesting that a different theory was needed to explain how the materials superconduct. “This was one of the remaining mysteries,” says physicist Christoph Renner of the University of Geneva. Now, Renner and colleagues have found vortices that agree with the theory in a high-temperature copper-based superconductor, studying a compound of yttrium, barium, copper and oxygen.

Vortices in superconductors can be probed with a scanning tunneling microscope. As the microscope tip moves over a vortex, the instrument records a change in the electrical current. Renner and colleagues realized that, in their copper compound, there were two contributions to the current that the probe was measuring, one from superconducting electrons and one from nonsuperconducting ones. The nonsuperconducting contribution was present across the entire surface of the material and masked the signature of the vortices.

Subtracting the nonsuperconducting portion revealed the vortices, which behaved in agreement with the standard superconductivity theory. “That, I think, is quite astonishing; it's quite a feat,” says Mikael Fogelström of Chalmers University of Technology in Gothenburg, Sweden, who was not involved with the research.


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Re: Mathis on Graphene? Any hints?

Post by Cr6 on Sat Mar 10, 2018 3:11 am

It is curious that the bigger "lattice" type molecules often display "superconducting" properties at very cold temps. I wonder if at cold temps/high pressure they just become charge-field tamped down "blocks" and certain charge field streams then just penetrate across these non-cycling or reduced cycling atom-blocks (charge field reduced cold atoms). Basically, "recycling" per Mathis stops and then flow occurs around the "semi-frozen" atoms? In many of these cases, the charge cycling balance is off.

Atomic Number: 63

240c. Period 6 Why Isn't Hafnium a Noble Gas?
We have more evidence of my diagrams from Europium, whose density goes way down compared to Samarium. That is because Samarium has gone all blue in the inner levels, with two protons on both sides of each inner hole. That only gives us four parcels of charge through a hole that can take five, but remember, this 5-stack contains a single proton, and that proton is not part of an alpha. This means that there is charge leakage around that inner proton (in the sandwich), so the 5-stack can't really channel 5 proton's worth of charge. The inner proton channels, but it doesn't spin up the charge a fifth amount. So the charge strength of the 5-stack stays at 4. Therefore, Europium is actually at its inner limit. It can't put any more protons in the inner levels. So it switches to a different plan, one more like we saw with Dysprosium:

We can see why that is considerably less dense, since it has more mass out in the 4th level and less on the axis. We can also see that Europium now has enough protons to work with that it can bump up all the numbers in the 4th level by one. In this way, it avoids having the same number at each pole. Instead of 1 North and two South, it has 2 north and 3 South. That solves the problem of equal charge.

This means that once again Europium isn't doing what it is doing to find a +3 oxidation number. That is just a side-effect of a deeper mechanics.

Notes on Dysprosium as SC:

Journal of Inorganic and Organometallic Polymers and Materials

September 2012, Volume 22, Issue 5, pp 1081–1086 | Cite as
Growth of the Dysprosium–Barium–Copper Oxide Superconductor Nanoclusters in Biopolymer Gels

Clusters of DyBa2Cu3O7−y high TC type II nanosuperconductor were prepared by sol–gel method in the presence of biopolymer chitosan. At the first step, the precursor and biopolymer were aggregated into amorphous matrix and then hydrogels were formed by thermogelling. Nucleation and growth of discrete nanoparticles is controlled by the biopolymer gel owing to retention of the fibrous nature of the chitosan at high temperatures up to 500 °C. After heating to 900 °C and complete decomposition of BaCO3, nanoparticles of DyBa2Cu3O7−y superconductor with diameter of 10–20 nm in the form of nanoclusters are prepared. Critical temperature (Tc) of the nanoparticles was found to be above 83 K. Characterizations of specimens were performed using scanning electron microscopy and transmission electron microscopy, supported by other techniques including XRD diffraction, energy dispersive X-ray, FT-IR spectrum and magnetic susceptibility measurements.

July 2016, Volume 29, Issue 7, pp 1787–1791 | Cite as
The Effect of Dy Doping on the Magnetic Behavior of YBCO Superconductors


The effect of dysprosium (Dy) doping on yttrium barium copper oxide (YBCO) prepared by conventional solid-state reaction method has been investigated by means of XRD, AC susceptibility, and DC magnetization measurements. AC susceptibility measurements for sintered YBCO pellets have been performed as a function of temperatures at constant frequency and AC field amplitude in the absence of a DC bias field. DC magnetization measurements were done at 5, 20, and 77 K upon zero field cooling (ZFC) process. The magnetization measurements showed a paramagnetic behavior existing at high magnetic fields. The magnetic field dependence of critical current density of the samples has been estimated from DC magnetization data. The partial Dy substitution for Y on YBCO superconductors improves the bulk critical current density at high magnetic fields and at high-temperature regions (higher than 20 K).

Posted: May 16, 2009
Europium found to be a superconductor
(Nanowerk News) Of the 92 naturally occurring elements, add another to the list of those that are superconductors.

James S. Schilling, Ph.D., professor of physics in Arts & Sciences at Washington University in St. Louis, and Mathew Debessai, Ph.D., — his doctoral student at the time — discovered that europium becomes superconducting at 1.8 K (-456 °F) and 80 GPa (790,000 atmospheres) of pressure, making it the 53rd known elemental superconductor and the 23rd at high pressure.

Debessai, who received his doctorate in physics at Washington University's Commencement May 15, 2009, is now a postdoctoral research associate at Washington State University.

"It has been seven years since someone discovered a new elemental superconductor," Schilling said. "It gets harder and harder because there are fewer elements left in the periodic table."

This discovery adds data to help improve scientists' theoretical understanding of superconductivity, which could lead to the design of room-temperature superconductors that could be used for efficient energy transport and storage.

The results are published in the May 15, 2009, issue of Physical Review Letters in an article titled "Pressure-induced Superconducting State of Europium Metal at Low Temperatures."

Schilling's research is supported by a four-year $500,000 grant from the National Science Foundation,Division of Materials Research.

Europium belongs to a group of elements called the rare earth elements. These elements are magnetic; therefore, they are not superconductors.

"Superconductivity and magnetism hate each other. To get superconductivity, you have to kill the magnetism," Schilling explained.

Of the rare earths, europium is most likely to lose its magnetism under high pressures due to its electronic structure. In an elemental solid almost all rare earths are trivalent, which means that each atom releases three electrons to conduct electricity.

"However, when europium atoms condense to form a solid, only two electrons per atom are released and europium remains magnetic. Applying sufficient pressure squeezes a third electron out and europium metal becomes trivalent. Trivalent europium is nonmagnetic, thus opening the possibility for it to become superconducting under the right conditions," Schilling said.

Schilling uses a diamond anvil cell to generate such high pressures on a sample. A circular metal gasket separates two opposing 0.17-carat diamond anvils with faces (culets) 0.18 mm in diameter. The sample is placed in a small hole in the gasket, flanked by the faces of the diamond anvils.
Pressure is applied to the sample space by inflating a doughnut-like bellow with helium gas. Much like a woman in stilettos exerts more pressure on the ground than an elephant does because the woman's force is spread over a smaller area, a small amount of helium gas pressure (60 atmospheres) creates a large force (1.5 tons) on the tiny sample space, thus generating extremely high pressures on the sample.

Unique electrical, magnetic properties

Superconducting materials have unique electrical and magnetic properties. They have no electrical resistance, so current will flow through them forever, and they are diamagnetic, meaning that a magnet held above them will levitate.

These properties can be exploited to create powerful magnets for medical imaging, make power lines that transport electricity efficiently or make efficient power generators.

However, there are no known materials that are superconductors at room temperature and pressure. All known superconducting materials have to be cooled to extreme temperatures and/or compressed at high pressure.

"At ambient pressure, the highest temperature at which a material becomes superconducting is 134 K (-218 °F). This material is complex because it is a mixture of five different elements. We do not understand why it is such a good superconductor," Schilling said.

Scientists do not have enough theoretical understanding to be able to design a combination of elements that will be superconductors at room temperature and pressure. Schilling's result provides more data to help refine current theoretical models of superconductivity.

"Theoretically, the elemental solids are relatively easy to understand because they only contain one kind of atom," Schilling said. "By applying pressure, however, we can bring the elemental solids into new regimes, where theory has difficulty understanding things.

"When we understand the element's behavior in these new regimes, we might be able to duplicate it by combining the elements into different compounds that superconduct at higher temperatures."
Schilling will present his findings at the 22nd biennial International Conference on High Pressure Science and Technology in July 2009 in Tokyo, Japan.


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Re: Mathis on Graphene? Any hints?

Post by Cr6 on Sat Mar 10, 2018 3:37 am

28 August 1993
Technology: US Navy superconductor sails past Britain


The US Navy is pioneering the use of high-temperature superconductors
in working machinery with a newly delivered sonar system. Now British scientists
are urging the government to fund more research into superconductivity.

Meanwhile the American Superconductor Corporation of Westboro, Massachusetts,
and the US Naval Undersea Warfare Center, based in Connecticut, have finished
a prototype sonar system incorporating bismuth strontium copper oxide, which
is superconducting at 73 K (-200 °C). It was delivered two years ahead
of schedule.

Nick Kerley of Oxford Instruments in Eynsham says, ‘It is a very good
demonstrator for high-temperature superconductors. The message (is) that
real practical devices are beginning to appear on the horizon – it is no
longer a dream.’ He was among the group who visited the US, and says Britain
must now decide on a device which is within its capabilities, and start
making a prototype by the end of the year. ‘I’d like to think the DTI would
be influenced and call for demonstration proposals,’ he says. Tim Button
of ICI Superconductors in Billingham, Cleveland, thinks the DTI may act
more quickly now, but he says: ‘I always felt it was urgent anyway. The
science isn’t any better in the US but the transfer into usable technology
is strides ahead of what we could do.’

The American instrument is an acoustic transducer, which converts electrical
energy into sound waves. It is used in sensitive sonar systems probing shallow
seas, such as the Persian Gulf. The company says this is the first time
high-temperature superconductors, which lose all their electrical resistance
at temperatures below about 113 K, have been combined with the ordinary
room-temperature electronics that control them. The transducer, which cost
$800 000, is similar to a hi-fi speaker: it has a coil made of the superconductor
and generates a magnetic field when an alternating electric current passes
through it. Inside the coil is a rod of terbium dysprosium, which is a magneto-strictive
material – its length changes with the magnetic field. The rod is connected
to pistons immersed in the sea, whose movements generate sound waves. The
US Navy has tested it successfully in a 30-metre-deep test lake.

The superconductor is refrigerated at between 50 and 70 K, which is
also the optimum operating temperature for the terbium rod. ‘The marriage
of the high-temperature superconductor and the terbium dysprosium gives
rise to low frequencies and high powers not available before
,’ says Greg
Yurek, the company president. This means the system can detect quiet submarines.
A coil made from an ordinary conductor such as copper would overheat from
the electric current, while one made of a low-temperature superconductor,
which is only superconducting up to about 10 K, would be too expensive to


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Re: Mathis on Graphene? Any hints?

Post by Cr6 on Sat Mar 10, 2018 3:41 am

Terbium–neodymium co-doping in Bi sites on the BPSCCO bismuth cuprate superconductor

Morsy M A Sekkina, Hosny A El-Daly and Khaled M Elsabawy1

Published 19 November 2003 • 2004 IOP Publishing Ltd
Superconductor Science and Technology, Volume 17, Number 1


The parent BiPbSr2Ca2Cu3O10 (BPSCCO) and samples of the general formula Bi1-(x+y)NdxTbyPbSr2Ca2Cu3O10, where x = y = 0.05, 0.1 and 0.2, were prepared using the conventional high-temperature solid-state reaction technique. The superconducting measurements proved that the best value of Tc, 108 K, is for the sample with x = y = 0.1 mol% while the lowest value of Tc, 101 K, is for the sample with maximum dopant concentration x = y = 0.2 mol%. The evaluated crystalline lattice structure of the prepared samples mainly belongs to the superconductive tetragonal phase (2223) besides the secondary (2212) phase. Also, thermogravimetric and differential thermal analyses were studied on the green mixture showing an endothermic peak at 790 °C corresponding to the superconductive phase formation. The microstructures of the prepared samples were investigated by scanning electron microscopy and energy dispersive x-ray analysis.


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Re: Mathis on Graphene? Any hints?

Post by Cr6 on Sat Mar 10, 2018 4:00 am

The Ytterby Elements

About a half hours drive out of Stockholm, Sweden, there’s a tiny nondescript town. During the 18th – 20th centuries there was slightly more life here, with a mine operating out of the town.

Ytterby mine, roughly 1910. Courtesy of Tekniska museet on Flickr

One day in 1787, Carl Arrhenius, an army lieutenant, discovered a strange, unusually heavy black ore in the mine. Over the next 100 years, many different elements where discovered from this one ore, later named gadolinite.

These elements are all “rare earth” elements, which, despite the name, are not particularly rare. Their rarity comes more from the difficulty in separating them from each other. It took many scientists many years of research and arguing to discover them all.

Now the little town of Ytterby is not the first place you’d pick to hold a scientific record. Yet it does, as it’s the origin of the names of not one, but four elements. Of the elements in the original gadolinite ore, yttrium, terbium, erbium, and ytterbium all ended up named after the town.


The first element in Ytterby’s story is yttrium. A Finnish chemist, Johan Gadolin, received a sample of the mysterious black ore from Arrhenius. He isolated an unknown element – which later become known as yttrium.

Yttrium’s claim to fame is a discovery made two centuries later in 1987. Yttrium barium copper oxide, a material containing yttrium, is a superconductor at “high” temperatures. Superconductors are strong magnets at low temperatures, and are used in places like in MRIs in hospitals. It is worth noting that to scientists researching superconductors, -180ºC is a high temperature, although to anyone else on the planet it seems freezing. This is because to reach these temperatures, it needs liquid helium cooling. As liquid helium is both expensive and in short supply, hopefully a material like this will soon replace conventional superconductors in MRIs and scientific instruments.

A superconductor becomes magnetic at low temperatures and floats above another magnet. Courtesy of Trevor Prentice on Flickr


Further along the table lives terbium. Another Swedish man called Carl, this time Carl Gustaf Mosander, found two more elements in the black gadolinite. A thoroughly unimaginative guy, he named all three after the mine where they were first found – naming Gadolin’s element yttrium, and the other two terbium and erbium.

Terbium is another rare-earth, and is often used in TV screens to make yellow and green phosphors. It’s also mixed with blue and red phosphors to make white light in LEDs.

Terbium sulfate glowing green under UV light. Terbium green is often used in TV phosphors. Credit Chemical Elements, A Virtual Museum

Terbium is versatile, and can also be used in single molecules that act as tiny bar magnets. Usually magnets need hundreds and thousands of individual atoms or molecules. One of these terbium molecules can instead act as a magnet on its own.

The reason why this is important is due to the device you’re reading this on. The storage in computers and phones is made up of tiny little magnetic areas coded with data. These magnetic bits have gotten smaller and smaller as well as faster with time, as computers have gone from filling an entire room to fitting in your pocket. We are now reaching the limit of how small we can make these magnets, but imagine if single molecules could be used instead! With terbium, the dream of minuscule computers may come true.


Now erbium is just confusing! The name, as you may have noticed, is very close to that of terbium. This led to some mishaps when the two where first discovered.

Out of the black gadolinite came different coloured element oxides. The original “erbium” was the yellowish coloured stuff, while “terbium” the bright pink stuff. These two were simple enough to separate from the white yttrium. However, when they were discovered people weren’t convinced that they were two separate elements, and by the time they sorted it out, the original names were switched!

These days, erbium refers to the element whose salts are a beautiful rose-pink. If you buy a pair of rose-coloured glasses, chances are there’s erbium!

Small amounts of erbium are also used with yttrium in Er:YAG lasers. They’re useful because the light from these lasers doesn’t travel through the human body. They are therefore useful for dermatology and dentistry, where only the skin or surface of a tooth needs to be treated.

A rare earth (neodymium and yttrium Nd:YAG) laser. Similar lasers are made with erbium or ytterbium and yttrium. Courtesy of Ben Williams on Flickr


Finally, we have ytterbium. Ytterbium was discovered a lot later than the other three elements, from a sample of erbium by a Swiss chemist Jean Charles Galissard de Marignac. Just like Mosander, he had little imagination, and decided again to name the element after the town.

Ytterbium is also used like erbium in yttrium-based Yb:YAG lasers. Its most interesting use though is in atomic clocks.

Atomic clocks use a vibrating atom to keep time. The ytterbium clock is even more accurate than the caesium atomic clock currently used to define the second. This improved accuracy means that super super fast things in earth sciences and astronomy can be measured.

The Ytterby elements in the lab. Author’s own

Countless other applications of these Ytterby elements exist, and they’re the focus of a lot of research. Any dedicated rare-earth chemist will eventually make the trip to Ytterby to visit the birthplace of these versatile elements.

However, the story doesn’t end with these four. From gadolinite and the mine in Ytterby, 6 other rare-earth elements were discovered:

Scandium: named after Scandinavia

Gadolinium: named after Johan Gadolin

Dysprosium: from the Greek “hard to find”

Holmium: named after Stockholm county, where Ytterby is found

Thulium: after “Thulia”, a Greek name for Scandinavia

Lutetium: from the Latin name for Paris (where the mineral sample was analysed)


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Re: Mathis on Graphene? Any hints?

Post by Cr6 on Sat Mar 10, 2018 4:11 am

Polonium's Superconductivity

Quasi-two-dimensional superconductivity from dimerization of atomically ordered AuTe2Se4/3 cubes
J. G. Guo ORCID:, X. Chen ORCID:,2, X. Y. Jia3, Q. H. Zhang1, N. Liu1,2, H. C. Lei4, S. Y. Li3,5, L. Gu1,6,7, S. F. Jin1,6 & X. L. Chen1,6,7

Nature Communicationsvolume 8, Article number: 871 (2017)
Download Citation


The emergent phenomena such as superconductivity and topological phase transitions can be observed in strict two-dimensional (2D) crystalline matters. Artificial interfaces and one atomic thickness layers are typical 2D materials of this kind. Although having 2D characters, most bulky layered compounds, however, do not possess these striking properties. Here, we report quasi-2D superconductivity in bulky AuTe2Se4/3, where the reduction in dimensionality is achieved through inducing the elongated covalent Te–Te bonds. The atomic-resolution images reveal that the Au, Te, and Se are atomically ordered in a cube, among which are Te–Te bonds of 3.18 and 3.28 Å. The superconductivity at 2.85 K is discovered, which is unraveled to be the quasi-2D nature owing to the Berezinsky–Kosterlitz–Thouless topological transition. The nesting of nearly parallel Fermi sheets could give rise to strong electron–phonon coupling. It is proposed that further depleting the thickness could result in more topologically-related phenomena.


The dimensional reduction or degeneracy usually induces the significant change of electronic structure and unexpected properties. The monolayer, interface and a few layers of bulky compounds are typical resultant forms of low dimensionality. The two dimensional (2D) material, for instance, graphene, is found to have a linear energy dispersion near Fermi energy (EF) and possess a number of novel properties1,2,3. Monolayer MoS2 exhibits a direct energy gap of 1.8 eV4 and pronounced photoluminescence5, in contrast to trivial photo-response in bulky MoS2 with an indirect band-gap.

2D superconductivity (SC), a property closely related to dimensional reduction, has been observed in a variety of crystalline materials like ZrNCl6, NbSe27, and MoS28 recently through the electric-double layer transistor (EDLT)9 technique. Many emerged properties, i.e., the well-defined superconducting dome, metallic ground state and high upper critical field6, 8, significantly differ from those of intercalated counterparts. Besides, the lack of in-plane inversion symmetry in the outmost layer of MoS2/NbSe2 with strong Ising spin-orbital coupling induces a valley polarization7, 8. In the scenario of low-dimensional interface, the unexpected 2D SC10, 11, the remarkable domed-shaped superconducting critical temperature (Tc)12, pseudo-gap state13, and quantum criticality14 have been demonstrated in La(Al,Ti)O3/SrTiO3(001) film. Very recently, the interface between Bi2Te3 and FeTe thin films displayed 2D SC evidenced by Berezinsky–Kosterlitz–Thouless (BKT) transition at 10.1 K15. The tentative explanations are related to the strong Rashba-type spin–orbit interactions in the 2D limit.

At the moment, the way to fabricating low-dimensional materials generally involves molecule beam epitaxy and exfoliation from the layered compounds. The top-down reduction processes usually are sophisticated and time consuming for realizing scalable and controllable crystalline samples. There are other chemical routes to tuning dimensionality by means of either changing the size of intercalated cations or incorporating additional anions. It is reported that increasing the size of alkaline-earth metals Ae (Ae=Mg, Ca, and Ba) between [NiGe] ribbons can reduce three dimensional (3D) structure to quasi-1 dimensional one16. In addition, the ternary CaNiGe can be converted into ZrCuSiAs-type CaNiGeH by forming additional Ca–H bonds, which exhibits different properties owing to the emergence of 2D electronic states17. The metastable Au1−x Te x (0.6 < x < 0.85) show an α-type polonium structure18, 19, in which the Au and Te disorderly locate at the eight corners of a simple cubic unit cell. The Tc fluctuates in the range of 1.5–3.0 K, but the mechanism of SC has been barely understood20. Besides, the equilibrium phase AuTe2, known as calaverite, is a non-superconducting compound, in which distorted AuTe6 octahedra are connected by Te–Te dimers21.

Through incorporating more electronegative Se anions, we fabricate a new layered compound AuTe2Se4/3 by conventional high temperature solid-state reaction. In a basic cube subunit, the Se anions attract electrons from Te and lead to the ordered arrangement of Au, Te and Se atoms. The cubes stack into strip through Te–Te dimers at 3.18 Å and 3.28 Å along the a- and b-axis, respectively, which composes 2D layers due to the existence of weak Te–Te interaction (~4 Å) along the c-axis. Electrical and magnetic measurements demonstrate that the SC occurs at 2.85 K. Furthermore, this SC exhibits 2D nature evidenced by the BKT transition in the thin crystals. The observed results are interpreted according to the crystallographic and electronic structure in reduced-dimensionality.


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Re: Mathis on Graphene? Any hints?

Post by Cr6 on Sat Mar 10, 2018 4:12 am

Hydrogen‐rich superconductors at high pressures
Advanced Review
Hui Wang, Xue Li, Guoying Gao, Yinwei Li, Yanming Ma
Published Online: Sep 05 2017
DOI: 10.1002/wcms.1330


The hydrogen‐rich superconductors stabilized at high‐pressure conditions have been the subject of topic interests. There is an essential hope that hydrogen‐rich superconductors are promising candidates of room‐temperature superconductors. Recent advances in first‐principles crystal structure prediction techniques have opened up the possibility of reliable prediction of superconductive structures, and subsequent superconductivity calculations based on phonon‐mediated superconducting mechanism revealed a general appearance of high temperature superconductivity in pressurized hydrides. Theory‐orientated experiments at high pressure discovered a number of hydrogen‐rich superconductors, among which sulfur hydrides exhibit a remarkably high superconducting critical temperature reaching 203 K. In this review, we discuss the emerging research activities towards hydrogen‐rich superconductors at high pressures and outlook the future direction in the field. WIREs Comput Mol Sci 2018, 8:e1330. doi: 10.1002/wcms.1330

This article is categorized under:

   Structure and Mechanism > Computational Materials Science

The crystal structure of (a) the Pm‐3n phase of YH3, (b) the Im‐3m phase of H3S, (c) the R‐3m phase of H3S, (d) the P63/mmm phase of TeH4, (e) the Cmmm phase of RbH3, (f) the Cc phase of H5Cl, (g) the Im‐3m phase of CaH6, and (h) the I4/mmm phase of CaH4. Hydrogen and the heavier element are shown by the small and large balls, respectively.
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(a) Calculated PHDOS (top panel) and the Eliashberg phonon spectral function α2F(ω)/ω, electron–phonon integral λ(ω) (lower panel) of SiH4(H2)2 at 250 GPa. (b) Top panel is the collected estimated Tc values of SiH4(H2)2, GeH4(H2)2, and AlH3H2 at 250 GPa, and the lower panel describes the contributions of the low‐frequency vibrations from heavy atoms (green bars), the intermediate‐frequency intermolecular vibrations (blue bars), and the high‐frequency phonons from the H2 vibrons (yellow bars) to the total λ.
[ Normal View | Magnified View ]
(a) Calculated PHDOS (top panel) and the Eliashberg phonon spectral function α2F(ω)/ω, electron–phonon integral λ(ω) (lower panel) of AsH8 at 350 GPa. (b) Top panel is the collected estimated Tc values of H4I, PoH4, LiH6, PbH8, AsH8, and MgH12 at selected pressures, and the lower panel describes the contributions of the low‐frequency vibrations from metal atoms (green bars), the intermediate‐frequency intermolecular vibrations (blue bars), and the high‐frequency phonons from the H2 vibrons (yellow bars) to the total λ.
[ Normal View | Magnified View ]
(a) The crystal structure and electron localization function of the Im‐3m phase of CaH6. (b) Phonon dispersion curves of Im‐3m‐CaH6 (left panel). Olive circles indicate the phonon line width with a radius proportional to the strength. Calculated Eliashberg phonon spectral function α2F(ω) and electron–phonon integral λ(ω) (right panel). Band structures of (c) CaH6 and (d) Ca0H6 (Im‐3m‐CaH6 with Ca removed), respectively.
[ Normal View | Magnified View ]
(a) The crystal structure and electron localization function (ELF) of the Im‐3m phase of H3S. (b) Calculated Eliashberg phonon spectral function α2F(ω) and electron–phonon integral λ(ω) (top panel) and the PHDOS (lower panel) of Im‐3m‐H3S at 200 GPa. (c) The crystal structure of the Pm‐3n phase of GaH3. (d) Calculated Eliashberg phonon spectral function α2F(ω)/ω and electron–phonon integral λ(ω) (top panel) and the PHDOS (lower panel) of Pm‐3n‐GaH3 at 160 GPa.
[ Normal View | Magnified View ]


Ionic hydrides

Ionic or saline hydrides are composed of hydride bound to an electropositive metal, generally an alkali metal or alkaline earth metal. The divalent lanthanides such as europium and ytterbium form compounds similar to those of heavier alkali metal. In these materials the hydride is viewed as a pseudohalide. Saline hydrides are insoluble in conventional solvents, reflecting their non-molecular structures. Ionic hydrides are used as bases and, occasionally, as reducing reagents in organic synthesis.[6]

C6H5C(O)CH3 + KH → C6H5C(O)CH2K + H2

Typical solvents for such reactions are ethers. Water and other protic solvents cannot serve as a medium for ionic hydrides because the hydride ion is a stronger base than hydroxide and most hydroxyl anions. Hydrogen gas is liberated in a typical acid-base reaction.

NaH + H2O → H2 (g) + NaOH ΔH = −83.6 kJ/mol, ΔG = −109.0 kJ/mol

Often alkali metal hydrides react with metal halides. Lithium aluminium hydride (often abbreviated as LAH) arises from reactions of lithium hydride with aluminium chloride.

4 LiH + AlCl3 → LiAlH4 + 3 LiCl

From Wikipedia, the free encyclopedia

The pseudohalogens are polyatomic analogues of halogens, whose chemistry, resembling that of the true halogens, allows them to substitute for halogens in several classes of chemical compounds[1]. Pseudohalogens occur in pseudohalogen molecules, inorganic molecules of the general forms Ps–Ps or Ps–X (where Ps is a pseudohalogen group), such as cyanogen; pseudohalide anions, such as cyanide ion; inorganic acids, such as hydrogen cyanide; as ligands in coordination complexes, such as ferricyanide; and as functional groups in organic molecules, such as the nitrile group. Well-known pseudohalogen functional groups include cyanide, cyanate, thiocyanate, and azide.


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Re: Mathis on Graphene? Any hints?

Post by Cr6 on Sat Mar 10, 2018 4:34 am

WHAT'S HOT IN... PHYSICS , May/Jun 2009
The Good Samarium Hots Up Superconductivity
by Simon Mitton

The Physics Hot Papers in this period show that superconductivity research is now hotting up, thanks to the unexpected discovery of a new class of iron-based superconductors. Papers #2, #3, #7 and #9 capture the tremendous interest stimulated by the recent discovery of superconductivity at Tc = 26 K in Hideo Hosonothe iron-based oxypnictide La(O1-xFx)FeAs (Y. Kamihari, et al., J. Am. Chem. Soc., 130 [11]: 3296-7, 2008; currently #1 in the Chemistry Top Ten). That research, by Hideo Hosono and colleagues of the Tokyo Institute of Technology, put high-temperature superconductivity back on the agenda with a bang.

Paper #2 describes an experiment designed by Xian Hui Chen, and conducted together with colleagues at the University of Science and Technology, Hefei, China. They followed up on the Japanese discovery paper by looking at superconductivity in a related compound, SmFeAsO1-x Fx, in which samarium is substituted for lanthanum. They aimed to see how high they could push Tc in Jonathan Baggera layered rare-earth superconductor. In doing so they broke the record for a non-copper-oxide superconductor, by reaching Tc = 43 K, comfortably above the previous record of 39 K for magnesium diboride.

The Sm-doped material is intriguing: according to Chen, it has Tc above that suggested by standard BCS theory, which argues for the oxypnictides being unconventional superconductors. Furthermore, the jump in Tc from 26 K to 43 K just by substituting Sm for La immediately suggested that further research would produce higher Tc in layered oxypnictides doped with F.

That’s where #3 takes us: in it Zhi-An Ren and colleagues from Beijing, China, report Tc = 55 K in the same F-doped compound. In fact, related experiments by this group, in which they also substituted Ce, Pr, and Nd, have shown that FeAs superconductors constitute a new family with Tc > 50 K. The high-citation rate of #3 is partly driven by the comprehensive information it gives on fabrication. The materials are grown using a high-pressure technique similar to that used for turning graphite to diamond.

Zhi-An Ren’s collaboration is also responsible for #7, in which they point out that the compounds have a simple structure of alternating FeAs and ReO layers (where Re is a rare earth). Instead of doping with F to achieve superconductivity, they created vacancies of oxygen atoms in the lattice. That move creates more electron carriers, which should be a more efficient approach to the realization of superconductivity. And indeed, tuning the O content leads to the occurrence of superconductivity in a way that resembles the situation in cuprates. That’s encouraging because the parallels between the two compounds suggest that the arsenides with O vacancies rather than F doping could be the more competitive choice for higher Tc.

Newcomer #9 is a paper that neatly illustrates how research on F-doped arsenides may contribute to fundamental physics. The experiments described in this paper show how F doping suppresses spin-density-wave (SDW) instabilities and leads to superconductivity. SDW is a low-energy ordered state that occurs at low temperatures. SDW inhibits the onset of superconductivity.

Superconductivity is one of the most dramatic phenomena in condensed matter physics. Part of the motivation for the groups in China and Japan is the ultimate goal: the realization of the phenomenon at room temperature. There are plenty of physicists who will state informally that room temperature operation is about as likely as cold fusion, or hot fusion. But fast progress has energized research. In 2008 there were at least seven international symposia devoted to Fe-based superconductors, and those events have no doubt propelled the citation rates. For researchers it’s a matter of striking while the iron is hot!

Dr. Simon Mitton is a Fellow of St. Edmund’s College, Cambridge, U.K.


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Re: Mathis on Graphene? Any hints?

Post by Cr6 on Sat Mar 10, 2018 4:42 am

Princeton’s Robert Cava: From Superconductivity to Topological Insulators
Scientist Interview: March 2011

SW: What do you consider the critical questions in high-Tc that still have to be answered?

Well, why do these things superconduct at all, and what’s going on in these materials at a very, very local scale? The most interesting physics is probably occurring at a scale of tens of angstroms. You can see wonderfully complicated things going on at the nanometer-length scale with different kinds of experimental probes. It’s not just a uniform sea of electrons like we learned a piece of copper is. It’s a very inhomogeneous distribution of electrons doing all kinds of crazy stuff. The more people look, the more complicated it gets.

SW: Are any researchers seriously looking for new high-Tc superconductors anymore, or is that part also done?

A small number of people are, and every once in a while a big surprise appears—somebody finds a new superconductor that nobody expected. In 2008, for example, a Japanese group found a new superconductor, a combination of iron, arsenic, lanthanum, oxygen, and fluorine that was superconducting at 26 Kelvin. What made it so interesting is that the superconductor seems to arise from the iron and arsenic, and the iron should typically give you a magnet.

Until the high-temperature superconductors came along, people thought magnetism and superconductivity were incompatible, whereas in many cases they’re probably just two sides of the same coin. You can change a magnet into a superconductor and a superconductor into a magnet by changing some chemical parameter.

So in 2008, a group of Japanese researchers discovered that iron and arsenic are the basis of a new class of superconductors whose superconductivity and magnetism seem to be related (Y. Kamihara, et al., J. Am. Chem. Soc., 130[11]: 3296-7, 2008; see also ).

After that big discovery, another group discovered that the temperature of the superconductor could go up to 50 or 60 Kelvin with the right combination of elements. That makes these the second-highest-temperature superconductors known, and with a whole new element involved—not copper anymore, but iron.

Of course, thousands of people also jumped onto this new one really fast. The interesting difference between now and 1986 is that back then you had to hear about the discovery through word of mouth. Somebody talked to somebody who talked to somebody on the other side of the world. Occasionally, a fax of a preprint appeared. Information didn’t travel very fast.

SW: Let’s begin at the beginning. How far have high-temperature superconductors come in the quarter-century since they were discovered, and what are the key research areas still being studied?

I’d say there are a couple of things that are still going on. First, there’s no universally accepted theory yet about why they work. We know a lot about them, and we have much of the phenomenology worked out, but we still have no theory about what makes them superconducting that the community as a whole accepts. That’s a remarkable situation, if you ask me. It goes to show how complicated physics can be sometimes.

There are so many interesting phenomena that occur in conjunction with the superconductivity that the whole package has not really been put together yet in a way that satisfies everybody, at least not like the BCS theory explains basic superconductivity. There’s nothing yet established that will go into the textbooks as explaining it. There’s a lot of action on the theoretical side, and it’s very sophisticated, but nobody has explained it all.


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