Tag Archives: #superconductivity

Superconductivity in Cuprates: ‘From Maximal To Minimal Dissipation’ – A New Model? (Physics)

Researchers from the University of Bristol’s School of Physics used some of Europe’s strongest continuous magnetic fields to uncover evidence of exotic charge carriers in the metallic state of copper-oxide high-temperature superconductors.

Their results have been published this week in Nature. In a related publication in SciPost Physics last week, the team postulated that it is these exotic charge carriers that form the superconducting pairs, in marked contrast with expectations from conventional theory.

Conventional superconductivity

Superconductivity is a fascinating phenomenon in which, below a so-called critical temperature, a material loses all its resistance to electrical currents. In certain materials, at low temperatures, all electrons are entangled in a single, macroscopic quantum state, meaning that they no longer behave as individual particles but as a collective – resulting in superconductivity. The general theory for this collective electron behaviour has been known for a long time, but one family of materials, the cuprates, refuses to conform to the paradigm. They also possess the highest ambient-pressure superconducting transition temperatures known to exist. It was long thought that for these materials the mechanism that ‘glues together’ the electrons must be special, but recently the attention has shifted and now physicists investigate the non-superconducting states of cuprates, hoping to find clues to the origin of high-temperature superconductivity and its distinction from normal superconductors.

High-temperature superconductivity

Most superconductors, when heated to exceed their critical temperature, change into ‘ordinary’ metals. The quantum entanglement that causes the collective behaviour of the electrons fades away, and the electrons start to behave like an ordinary ‘gas’ of charged particles.

Cuprates are special, however. Firstly, as mentioned above, because their critical temperature is considerably higher than that of other superconductors. Secondly, they have very special measurable properties even in their ‘metallic phase’. In 2009, physicist Prof Nigel Hussey and collaborators observed experimentally that the electrons in these materials form a new type of structure, different from that in ordinary metals, thereby establishing a new paradigm that scientists now call the ‘strange metal’. Specifically, the resistivity at low temperatures was found to be proportional to temperature, not at a singular point in the temperature versus doping phase diagram (as expected for a metal close to a magnetic quantum critical point) but over an extended range of doping. This extended criticality became a defining feature of the ‘strange metal’ phase from which superconductivity emerges in the cuprates.

Magnetoresistance in a strange metal

In the first of these new reports, EPSRC Doctoral Prize Fellow Jakes Ayres and PhD student Maarten Berben (based at HFML-FELIX in Nijmegen, the Netherlands) studied the magnetoresistance – the change in resistivity in a magnetic field – and discovered something unexpected. In contrast to the response of usual metals, the magnetoresistance was found to follow a peculiar response in which magnetic field and temperature appear in quadrature. Such behaviour had only been observed previously at a singular quantum critical point, but here, as with the zero-field resistivity, the quadrature form of the magnetoresistance was observed over an extended range of doping. Moreover, the strength of the magnetoresistance was found to be two orders of magnitude larger than expected from conventional orbital motion and insensitive to the level of disorder in the material as well as to the direction of the magnetic field relative to the electrical current. These features in the data, coupled with the quadrature scaling, implied that the origin of this unusual magnetoresistance was not the coherent orbital motion of conventional metallic carriers, but rather a non-orbital, incoherent motion from a different type of carrier whose energy was being dissipated at the maximal rate allowed by quantum mechanics.

From maximal to minimal dissipation

Prof Hussey said: “Taking into account earlier Hall effect measurements, we had compelling evidence for two distinct carrier types in cuprates – one conventional, the other ‘strange’. The key question then was which type was responsible for high-temperature superconductivity? Our team led by Matija Čulo and Caitlin Duffy then compared the evolution of the density of conventional carriers in the normal state and the pair density in the superconducting state and came to a fascinating conclusion; that the superconducting state in cuprates is in fact composed of those exotic carriers that undergo such maximal dissipation in the metallic state. This is a far cry from the original theory of superconductivity and suggests that an entirely new paradigm is needed, one in which the strange metal takes centre stage.” 

Reference Paper: Ayres, J., Berben, M., Čulo, M. et al. Incoherent transport across the strange-metal regime of overdoped cuprates. Nature 595, 661–666 (2021). https://doi.org/10.1038/s41586-021-03622-z

Provided by University of Bristol

New Mechanism of Superconductivity Discovered in Graphene (Physics)

Placing a 2D Bose-Einstein condensate in the vicinity of a graphene layer confers superconductivity to the material

Superconductivity is a physical phenomenon where the electrical resistance of a material drops to zero under a certain critical temperature. Bardeen-Cooper-Schrieffer (BCS) theory is a well-established explanation that describes superconductivity in most materials. It states that Cooper pairs of electrons are formed in the lattice under sufficiently low temperature and that BCS superconductivity arises from their condensation. While graphene itself is an excellent conductor of electricity, it does not exhibit BCS superconductivity due to the suppression of electron-phonon interactions. This is also the reason that most ‘good’ conductors such as gold and copper are ‘bad’ superconductors.

Researchers at the Center for Theoretical Physics of Complex Systems (PCS), within the Institute for Basic Science (IBS, South Korea) have reported on a novel alternative mechanism to achieve superconductivity in graphene. They achieved this feat by proposing a hybrid system consisting of graphene and 2D Bose-Einstein condensate (BEC). This research is published in the journal 2D Materials.

(a) Temperature dependence of the superconducting gap for bogolon-mediated process with temperature correction (dashed) and without temperature correction (solid). (b) The critical temperature of the superconductivity transition as a function of condensate density for bogolon-mediated interaction with (red dashed) and without (black solid) the temperature correction. The blue dash-dotted line shows the BKT transition temperature as a function of the condensate density. © Institute for Basic Science

Along with superconductivity, BEC is another phenomenon that arises at low temperatures. It is the fifth state of matter first predicted by Einstein in 1924. The formation of BEC occurs when low-energy atoms clump together and enter the same energy state, and it is an area that is widely studied in condensed matter physics. A hybrid Bose-Fermi system essentially represents a layer of electrons interacting with a layer of bosons, such as indirect excitons, exciton-polaritons, etc. The interaction between Bose and Fermi particles leads to various novel fascinating phenomena, which piques interests from both the fundamental and application-oriented perspectives.

In this work, the researchers report a new mechanism of superconductivity in graphene, which arises due to interactions between electrons and “bogolons”, rather than phonons as in typical BCS systems. Bogolons, or Bogoliubov quasiparticles, are excitation within BEC which has some characteristics of a particle. In certain ranges of parameters, this mechanism permits the critical temperature for superconductivity up to 70 Kelvin within graphene. The researchers also developed a new microscopic BCS theory which focuses specifically on the novel hybrid graphene-based system. Their proposed model also predicts that superconducting properties can be enhanced with temperature, resulting in the non-monotonous temperature dependence of the superconducting gap.

Furthermore, the research showed that the Dirac dispersion of graphene is preserved in this bogolon-mediated scheme. This indicates that this superconducting mechanism involves electrons with relativistic dispersion — a phenomenon that is not so well-explored in condensed matter physics.

“This work sheds light on an alternative way to achieve high-temperature superconductivity. Meanwhile, by controlling the properties of a condensate, we can tune the superconductivity of graphene. This suggests another channel to control the superconductor devices in the future.”, explains Ivan Savenko, the leader of the Light-Matter Interaction in Nanostructures (LUMIN) team at the PCS IBS.

Featured image: A hybrid system consisting of an electron gas in graphene (top layer) separated from a two-dimensional Bose-Einstein condensate, represented by indirect excitons (blue and red layers). The electrons in the graphene and the excitons are coupled by the Coulomb force. © Institute for Basic Science

Provided by Institute for Basic Science

Two-dome Superconductivity in Kagome Superconductor CsV3Sb5 Discovered under High Pressure (Physics)

Recently, a research team led by Prof. YANG Zhaorong from the Hefei Institutes of Physical Science (HFIPS) of the Chinese Academy of Sciences (CAS), in collaboration with researchers from the Anhui University and other institutions, discovered pressure-induced two-dome superconductivity in the quasi-two-dimensional topological kagome superconductor CsV3Sb5. This work was published in Physical Review B and selected as Editors’ Suggestion.

Owing to its unique geometry, kagome lattice intrinsically hosts electronic flat bands (strong correlations), Dirac band crossings like in graphene, and Van hove singularities, enabling realizations of quantum diversities. Recently, the kagome superconductors AV3Sb(A=K, Rb, Cs) have attracted much research interests due to discoveries of superconductivity, chiral charge effect, giant anomalous Hall effect, non-trivial topological electronic bands etc.

Pressure, as one of the three fundamental thermodynamic parameters, is known as a clean and powerful means to directly manipulate the lattice and further tune the electronic states. Naturally, one may ask how these phenomena interact with each other and what kind of exotic states may emerge for the systems under pressure.

In this research, the team chose CsV3Sb5 as an example since it has the highest superconducting transition temperature of about 5.0 K amongst the systems at ambient pressure. They used the so-called diamond anvil cell to generate high pressures up to 47.9 GPa. They found that the transition temperature first increases and then decreases rapidly under pressure, which is undetectable in the intermediate pressure range of 5-16 GPa.

Unexpectedly, superconductivity reemerges above 16 GPa, with transition temperature first increasing slightly and then almost leveling off.

Therefore, a two-dome superconducting phase diagram was revealed for CsV3Sb5 under high pressure. In terms of the high-pressure synchrotron x-ray diffraction measurements, they didn’t find any structural transition but an anomaly in ratio of the lattice parameters just around the same critical pressure, indicating a Fermi surface reconstruction via Lifshitz transition that might be responsible for the reemergence of superconductivity.

Many experiments suggests that the ambient-pressure superconductivity in CsV3Sb5 should be unconventional. In this sense, this work evidences a two-dome superconductivity in the first V-based unconventional superconductor.

In addition to previous reports in tremendous unconventional superconductors like Cu-based, Fe-based and heavy-fermion systems, two-dome superconductivity seems to be a common feature for these systems under external parameters, which may provide an important clue for understanding mechanisms of the unconventional superconductivity.

This work was supported financially by the National Key Research and Development Program of China, the National Natural Science Foundation of China, the High Magnetic Field Laboratory of Anhui, as well as the Youth Innovation Promotion Association of CAS.

Featured image: Temperature-pressure phase diagram of kagome superconductor CsV3Sb5 (Image by CHEN Xuliang)

Reference: Zhuyi Zhang, Zheng Chen, Ying Zhou, Yifang Yuan, Shuyang Wang, Jing Wang, Haiyang Yang, Chao An, Lili Zhang, Xiangde Zhu, Yonghui Zhou, Xuliang Chen, Jianhui Zhou, and Zhaorong Yang, “Pressure-induced reemergence of superconductivity in the topological kagome metal CsV3Sb5”, Phys. Rev. B 103, 224513 – Published 9 June 2021. DOI: https://doi.org/10.1103/PhysRevB.103.224513

Provided by Chinese Academy of Sciences

Researchers Discover Pressure-induced Superconductivity in Photovoltaic Semiconductor ZnSiP2 (Physics)

Very recently, pressure-induced superconductivity has been unveiled in photovoltaic semiconductor ZnSiP2, according to a study conducted by a team from the High Magnetic Field Laboratory of the Hefei Institutes of Physical Science and the Zhengzhou University.

As a promising candidate for transitional tandem solar cells, ZnSiP2 can be further used as a high-performance anode material for the next-generation Li-ion batteries, especially after disorder is introduced into the cation sublattice.

“Pressure is an effective and clean way to tune the lattice constant and electronic state, thus varying the fundamental physical properties of materials.” said YANG Zhaorong, who led the team, “that’s why we built and used the ultra-high-pressure physical property measurement system in our laboratory.”

In this research, they systematically investigated the pressure effect on the structural, optical, and electronic properties of the chalcopyrite semiconductor ZnSiP2 through various experimental measurements. “It exhibited an unusual V-shaped superconducting phase diagram.” said ZHOU Yonghui, who joined the research.

During compression, the photoluminescence (PL) peak energy exhibited a plateau between 1.4 and 8.7 GPa, which was accompanied by a piezochromic transition and correlated with the progressive development of cation disorder. Upon further compression across a phase transition from tetragonal to cubic rock-salt structure, superconductivity with a critical temperature 8.2K emerged immediately. Temperature decreased in the range of 24.6–37.1GPa but inversely increased at higher pressures.

These findings present vivid structure-property relationships, which not only offer important clues to optimize the optical and electronic properties, but also provide a new way to use compression to switch between different functionalities.

This work was supported by the National Key Research and Development Program of China, the National Natural Science Foundation of China, the Youth Innovation Promotion Association CAS and so on.

Featured image: Phase diagram of ZnSiP2 at high pressures (Image by HFIPS)

Reference: Yuan, Y., Zhu, X., Zhou, Y. et al. Pressure-engineered optical properties and emergent superconductivity in chalcopyrite semiconductor ZnSiP2. NPG Asia Mater 13, 15 (2021). https://www.nature.com/articles/s41427-021-00285-0 https://doi.org/10.1038/s41427-021-00285-0

Provided by Chinese Academy of Sciences

Researchers Create ‘Beautiful Marriage’ of Quantum Enemies (Quantum)

Cornell scientists have identified a new contender when it comes to quantum materials for computing and low-temperature electronics.

Using nitride-based materials, the researchers created a material structure that simultaneously exhibits superconductivity – in which electrical resistance vanishes completely – and the quantum Hall effect, which produces resistance with extreme precision when a magnetic field is applied.

“This is a beautiful marriage of the two things we know, at the microscale, that give electrons the most startling quantum properties,” said Debdeep Jena, the David E. Burr Professor of Engineering in the School of Electrical and Computer Engineering and Department of Materials Science and Engineering. Jena led the research, published Feb. 19 in Science Advances, with doctoral student Phillip Dang and research associate Guru Khalsa, the paper’s senior authors.

The two physical properties are rarely seen simultaneously because magnetism is like kryptonite for superconducting materials, according to Jena.

“Magnetic fields destroy superconductivity, but the quantum Hall effect only shows up in semiconductors at large magnetic fields, so you’re having to play with these two extremes,” Jena said. “Researchers in the past few years have been trying to identify materials which show both properties with mixed success.”

The research is the latest validation from the Jena-Xing Lab that nitride materials may have more to offer science than previously thought. Nitrides have traditionally been used for manufacturing LEDs and transistors for products like smartphones and home lighting, giving them a reputation as an industrial class of materials that has been overlooked for quantum computation and cryogenic electronics.

“The material itself is not as perfect as silicon, meaning it has a lot more defects,” said co-author Huili Grace Xing, the William L. Quackenbush Professor of Electrical and Computer Engineering and of Materials Science and Engineering. “But because of its robustness, this material has thrown pleasant surprises to the research community more than once despite its extremely large irregularities in structure. There may be a path forward for us to truly integrate different modalities of quantum computing – computation, memory, communication.”

Such integration could help to condense the size of quantum computers and other next-generation electronics, just as classical computers have shrunk from warehouse to pocket size.

“We’re wondering what this sort of material platform can enable because we see that it’s checking off a lot of boxes,” said Jena, who added that new physical phenomena and technological applications could emerge with further research. “It has a superconductor, a semiconductor, a filter material – it has all kinds of other components, but we haven’t put them all together. We’ve just discovered they can coexist.”

For this research, the Cornell team began engineering epitaxial nitride heterostructures – atomically thin layers of gallium nitride and niobium nitride – and searching for conditions in which magnetic fields and temperatures in the layers would retain their respective quantum Hall and superconducting properties.

They eventually discovered a small window in which the properties were observed simultaneously, thanks to advances in the quality of the materials and structures produced in close collaboration with colleagues at the Naval Research Laboratory.

“The quality of the niobium-nitride superconductor was improved enough that it can survive higher magnetic fields, and simultaneously we had to improve the quality of the gallium-nitride semiconductor enough that it could exhibit the quantum Hall effect at lower magnetic fields,” Dang said. “And that’s what will really allow for potential new physics to be seen at low temperature.”

Potential applications for the material structure include more efficient electronics, such as data centers cooled to extremely low temperatures to eliminate heat waste. And the structure is the first to lay the groundwork for the use of nitride semiconductors and superconductors in topological quantum computing, in which the movement of electrons must be resilient to the material defects typically seen in nitrides.

“What we’ve shown is that the ingredients you need to make this topological phase can be in the same structure,” Khalsa said, “and I think the flexibility of the nitrides really opens up new possibilities and ways to explore topological states of matter.”

Other co-authors of the paper include David Muller, the Samuel B. Eckert Professor of Engineering in the School of Applied and Engineering Physics; and researchers from the U.S. Naval Research Laboratory, the National High Magnetic Field Laboratory, and semiconductor company Qorvo.

The research was funded by the Office of Naval Research and the National Science Foundation.

Featured image: Doctoral students Phillip Dang (left) and Reet Chaudhuri at the National High Magnetic Field Laboratory, where measurements were made on a material structure that concurrently has superconductivity and the quantum Hall effect. © Jena-Xing Lab/Provided

Reference: Phillip Dang, Guru Khalsa, Celesta S. Chang et al., “An all-epitaxial nitride heterostructure with concurrent quantum Hall effect and superconductivity”, Science Advances  19 Feb 2021: Vol. 7, no. 8, eabf1388 DOI: 10.1126/sciadv.abf1388

Provided by Cornell University

Physicists Create Tunable Superconductivity in Twisted Graphene “Nanosandwich” (Physics)

Structure may reveal conditions needed for high-temperature superconductivity.

When two sheets of graphene are stacked atop each other at just the right angle, the layered structure morphs into an unconventional superconductor, allowing electric currents to pass through without resistance or wasted energy.

This “magic-angle” transformation in bilayer graphene was observed for the first time in 2018 in the group of Pablo Jarillo-Herrero, the Cecil and Ida Green Professor of Physics at MIT. Since then, scientists have searched for other materials that can be similarly twisted into superconductivity, in the emerging field of “twistronics.” For the most part, no other twisted material has exhibited superconductivity other than the original twisted bilayer graphene, until now.

In a paper appearing today in Nature, Jarillo-Herrero and his group report observing superconductivity in a sandwich of three graphene sheets, the middle layer of which is twisted at a new angle with respect to the outer layers. This new trilayer configuration exhibits superconductivity that is more robust than its bilayer counterpart.

The researchers can also tune the structure’s superconductivity by applying and varying the strength of an external electric field. By tuning the trilayer structure, the researchers were able to produce ultra-strongly coupled superconductivity, an exotic type of electrical behavior that has rarely been seen in any other material.

“It wasn’t clear if magic-angle bilayer graphene was an exceptional thing, but now we know it’s not alone; it has a cousin in the trilayer case,” Jarillo-Herrero says. “The discovery of this hypertunable superconductor extends the twistronics field into entirely new directions, with potential applications in quantum information and sensing technologies.”

His co-authors are lead author Jeong Min Park and Yuan Cao at MIT, and Kenji Watanabe and Takashi Taniguchi of the National Institute of Materials Science in Japan.

A new super family

Shortly after Jarillo-Herrero and his colleagues discovered that superconductivity could be generated in twisted bilayer graphene, theorists proposed that the same phenomenon might be seen in three or more layers of graphene.

A sheet of graphene is an atom-thin layer of graphite, made entirely of carbon atoms arranged in a honeycomb lattice, like the thinnest, sturdiest chicken wire. The theorists proposed that if three sheets of graphene were stacked like a sandwich, with the middle layer rotated by 1.56 degrees with respect to the outer layers, the twisted configuration would create a kind of symmetry that would encourage electrons in the material to pair up and flow without resistance — the hallmark of superconductivity.

“We thought, why not, let’s give it a try and test this idea,” Jarillo-Herrero says.

Park and Cao engineered trilayer graphene structures by carefully slicing a single gossamer sheet of graphene into three sections and stacking each section on top of each other at the precise angles predicted by the theorists.

They made several trilayer structures, each measuring a few micrometers across (about 1/100 the diameter of a human hair), and three atoms tall.

“Our structure is a nanosandwich,” Jarillo-Herrero says.

The team then attached electrodes to either end of the structures, and ran an electric current through while measuring the amount of energy lost or dissipated in the material.

“We saw no energy dissipated, meaning it was a superconductor,” Jarillo-Herrero says. “We have to give credit to the theorists — they got the angle right.”

He adds that the exact cause of the structure’s superconductivity — whether due to its symmetry, as the theorists proposed, or not — remains to be seen, and is something that the researchers plan to test in future experiments.

“For the moment we have a correlation, not a causation,” he says. “Now at least we have a path to possibly explore a large family of new superconductors based on this symmetry idea.”

“The biggest bang”

In exploring their new trilayer structure, the team found they could control its superconductivity in two ways. With their previous bilayer design, the researchers could tune its superconductivity by applying an external gate voltage to change the number of electrons flowing through the material. As they dialed the gate voltage up and down, they measured the critical temperature at which the material stopped dissipating energy and became superconductive. In this way, the team was able to tune bilayer graphene’s superconductivity on and off, similar to a transistor.

The team used the same method to tune trilayer graphene. They also discovered a second way to control the material’s superconductivity that has not been possible in bilayer graphene and other twisted structures. By using an additional electrode, the researchers could apply an electric field to change the distribution of electrons between the structure’s three layers, without changing the structure’s overall electron density.

“These two independent knobs now give us a lot of information about the conditions where superconductivity appears, which can provide insight into the key physics critical to the formation of such an unusual superconducting state,” Park says.

Using both methods to tune the trilayer structure, the team observed superconductivity under a range of conditions, including at a relatively high critical temperature of 3 kelvins, even when the material had a low density of electrons. In comparison, aluminum, which is being explored as a superconductor for quantum computing, has a much higher density of electrons and only becomes superconductive at about 1 kelvin.

“We found magic-angle trilayer graphene can be the strongest coupled superconductor, meaning it superconducts at a relatively high temperature, given how few electrons it can have,” Jarillo-Herrero says. “It gives the biggest bang for your buck.”

“The work is a meaningful step up in structural complexity of a twistronic system that can be faithfully reproduced in several samples,” says David Goldhaber-Gordon, a professor of physics at Stanford University who was not involved in the study. “That structural complexity is not just pursued for its own sake but rather aims to make the effect of electronic interactions tunable. Applications of such sophisticated multilayer structures will likely be in quantum information science where the exquisite control of electronic structure will be important. ”

The researchers plan to fabricate twisted graphene structures with more than three layers to see whether such configurations, with higher electron densities, can exhibit superconductivity at higher temperatures, even approaching room temperature.

“Our main goal is to figure out the fundamental nature of what underlies strongly coupled superconductivity,” Park says. “Trilayer graphene is not only the strongest-coupled superconductor ever found, but also the most tunable. With that tunability we can really explore superconductivity, everywhere in the phase space.”

This research was supported, in part, by the Department of Energy, the National Science Foundation, the Gordon and Betty Moore Foundation, and the Ramon Areces Foundation.

Featured image: This artist’s rendition shows magic-angle twisted trilayer graphene, composed of three honeycomb lattices. The tightly bound electrons (yellow spheres connected by blue halos) indicate the new structure’s strongly coupled superconducting state. Credits: Image: Ella Maru Studio

Reference: Park, J.M., Cao, Y., Watanabe, K. et al. Tunable strongly coupled superconductivity in magic-angle twisted trilayer graphene. Nature (2021). https://doi.org/10.1038/s41586-021-03192-0

Provided by MIT

New Way to Control Electrical Charge in 2D Materials: Put a Flake On It (Physics)

Physicists at Washington University in St. Louis have discovered how to locally add electrical charge to an atomically thin graphene device by layering flakes of another thin material, alpha-RuCl3, on top of it.

(Image: Shutterstock)

A paper published in the journal Nano Letters describes the charge transfer process in detail. Gaining control of the flow of electrical current through atomically thin materials is important to potential future applications in photovoltaics or computing.

“In my field, where we study van der Waals heterostructures made by custom-stacking atomically thin materials together, we typically control charge by applying electric fields to the devices,” said Erik Henriksen, assistant professor of physics in Arts & Sciences and corresponding author of the new study, along with Ken Burch at Boston College. “But here it now appears we can just add layers of RuCl3. It soaks up a fixed amount of electrons, allowing us to make ‘permanent’ charge transfers that don’t require the external electric field.”

A layered device transfers electric charge. (Image courtesy Nano Letters)

Jesse Balgley, a graduate student in Henriksen’s laboratory at Washington University, is second author of the study. Li Yang, professor of physics, and his graduate student Xiaobo Lu, also both at Washington University, helped with computational work and calculations, and are also co-authors.

Balgley © wustl

Physicists who study condensed matter are intrigued by alpha-RuCl3 because they would like to exploit certain of its antiferromagnetic properties for quantum spin liquids.

In this new study, the scientists report that alpha-RuCl3 is able to transfer charge to several different types of materials — not just graphene, Henriksen’s personal favorite.

They also found that they only needed to place a single layer of alpha-RuCl3 on top of their devices to create and transfer charge. The process still works, even if the scientists slip a thin sheet of an electrically insulating material between the RuCl3 and the graphene.

“We can control how much charge flows in by varying the thickness of the insulator,” Henriksen said. “Also, we are able to physically and spatially separate the source of charge from where it goes — this is called modulation doping.”

Adding charge to a quantum spin liquid is one mechanism thought to underlie the physics of high-temperature superconductivity.

“Anytime you do this, it could get exciting,” Henriksen said. “And usually you have to add atoms to bulk materials, which causes lots of disorder. But here, the charge flows right in, no need to change the chemical structure, so it’s a ‘clean’ way to add charge.”

Read more in Nano Letters: Modulation Doping via a Two-Dimensional Atomic Crystalline Acceptor

Provided by Washington University in Saint Louis

Transition Metal ‘Cocktail’ Helps Make Brand New Superconductors (Material Science)

Concept of high entropy alloys provides a discovery platform for new superconductors.

Researchers from Tokyo Metropolitan University mixed and designed a new, high entropy alloy (HEA) superconductor, using extensive data on simple superconducting substances with a specific crystal structure. HEAs are known to preserve superconducting characteristics up to extremely high pressures. The new superconductor, Co0.2Ni0.1Cu0.1Rh0.3Ir0.3Zr2, has a superconducting transition at 8K, a relatively high temperature for an HEA. The team’s approach may be applied to discovering new superconducting materials with specific desirable properties.

Schematic of the CuAl2-type crystal structure of the newly created superconducting Co0.2Ni0.1Cu0.1Rh0.3Ir0.3Zr2 compound, with an HEA-type Tr site. © Tokyo Metropolitan University

It’s been over a hundred years since the discovery of superconductivity, where certain materials were found to suddenly show minimal resistance to electrical currents below a transition temperature. As we explore ways to eliminate power waste, a way to dramatically reduce losses in power transmission is a fascinating prospect. But the widespread use of superconductivity is held back by the demands of existing superconductors, particularly the low temperatures required. Scientists need a way to discover new superconducting materials without brute-force trial and error, and tune key properties.

A team led by Associate Professor Yoshikazu Mizuguchi at Tokyo Metropolitan University have been pioneering a “discovery platform” that has already led to the design of many new superconducting substances. Their method is based on high entropy alloys (HEAs), where certain sites in simple crystal structures can be occupied by five or more elements. After being applied to heat resistant materials and medical devices, certain HEAs were found to have superconducting properties with some exceptional characteristics, particularly a retention of zero resistivity under extreme pressures. The team surveys material databases and cutting-edge research and finds a range of superconducting materials with a common crystal structure but different elements on specific sites. They then mix and engineer a structure that contains many of those elements; throughout the crystal, those “HEA sites” are occupied by one of the elements mixed (see Figure 1). They have already succeeded in creating high entropy variants of layered bismuth-sulfide superconductors and telluride compounds with a sodium chloride crystal structure.

(a) Temperature dependence of resistivity of the new CuAl2-type Co0.2Ni0.1Cu0.1Rh0.3Ir0.3Zr2 in magnetic fields of different strengths. (b) Temperature dependence of electronic specific heat Cel/T. © Tokyo Metropolitan University

In their latest work, they focused on the copper aluminide (CuAl2) structure. Compounds combining a transition metal element (Tr) and zirconium (Zr) into TrZr2 with this structure are known to be superconducting, where Tr could be Sc, Fe, Co, Ni, Cu, Ga, Rh, Pd, Ta, or Ir. The team combined a “cocktail” of these elements using arc melting to create a new HEA-type compound, Co0.2Ni0.1Cu0.1Rh0.3Ir0.3Zr2, which showed superconducting properties. They looked at both resistivity and electronic specific heat, the amount of energy used by the electrons in the material to raise the temperature, and identified a transition temperature of 8.0K. Not only is this relatively high for an HEA-type superconductor, they confirmed that the material had the hallmarks of “bulk” superconductivity.

The most exciting aspect of this is the vast range of other transition metals and ratios that can be tried and tuned to aim for higher transition temperatures and other desirable properties, all without changing the underlying crystal structure. The team hopes their success will lead to more discoveries of new HEA-type superconductors in the near future.

This work was supported by a JSPS KAKENHI Grant (Grant Number: 18KK0076) and a grant under the Advanced Research Program of the Human Resources Funds of Tokyo [Grant Number: H31-1].

Reference: Yoshikazu Mizuguchi, Md. Riad Kasem & Tatsuma D. Matsuda (2021) Superconductivity in CuAl2-type Co0.2Ni0.1Cu0.1Rh0.3Ir0.3Zr2 with a high-entropy-alloy transition metal site, Materials Research Letters, 9:3, 141-147, DOI: 10.1080/21663831.2020.1860147 https://www.tandfonline.com/doi/full/10.1080/21663831.2020.1860147

Provided by Tokyo Metropolitan University

Scientists Discover a New Complex Europium Hydride (Chemistry)

A team of researchers from Russia, the United States, and China led by Skoltech Professor Artem R. Oganov has discovered an unexpected very complex europium hydride, Eu8H46. The paper detailing the discovery has been published in The Journal of Physical Chemistry Letters.

The novel strongly correlated Europium superhydride ©Dmitrii V. Semenok et al/The Journal of Physical Chemistry Letters

Superhydrides of rare-earth metals are interesting compounds that form under pressure: some exhibit high-temperature superconductivity that scientists have been chasing for over 100 years, and some possess magnetic properties. Although devoid of superconductivity, europium hydrides are very interesting in view of chemical anomalies that make europium different from other rare-earth atoms.

Armed with the efficient and reliable USPEX crystal structure prediction tool developed by Oganov and his students, the team predicted the structure of the remarkably complex compound, Eu8H46, which helped understand and explain experimental data.

“I am pleasantly surprised that USPEX has easily predicted a highly complex structure of 54 atoms, which is quite a lot. Curiously enough, our colleagues obtained this hydride in an experiment earlier but got the structure and composition wrong, assuming it was EuH5. Now we know that the compound is much trickier,” Oganov comments.

“Such unusual compounds can be predicted in theory and proved by experiment, but there is no simple rule for identifying probable chemical compositions of stable compounds without performing arduous calculations,” says Dmitrii Semenok, the first author of the paper and a PhD student at Skoltech.

Artem R. Oganov is a Professor at Skoltech, a member of Academia Europaea, a Fellow of the Royal Society of Chemistry, a Fellow of the American Physical Society, a Fellow of the Mineralogical Society of America, and a Professor of the Russian Academy of Sciences.

References: Dmitrii V. Semenok, Di Zhou, Alexander G. Kvashnin, Xiaoli Huang, Michele Galasso, Ivan A. Kruglov, Anna G. Ivanova, Alexander G. Gavriliuk, Wuhao Chen, Nikolay V. Tkachenko, Alexander I. Boldyrev, Ivan Troyan, Artem R. Oganov, and Tian Cui, “Novel Strongly Correlated Europium Superhydride”, J. Phys. Chem. Lett. 2021, 12, XXX, 32–40, 2020. https://pubs.acs.org/doi/10.1021/acs.jpclett.0c03331

Provided by SKOLTECH