UChicago physicists first to see behavior of quantum materials in curved space

Harnessing the shared wave nature of light and matter, researchers at the University of Chicago led by Neubauer Family Assistant Professor of Physics Jonathan Simon have used light to explore some of the most intriguing questions in the quantum mechanics of materials. The topic encompasses complex and non-intuitive phenomena that are often difficult to explain in non-technical language, but which carry important implications to specialists in the field.

In work published online June 6, 2016, in the journal Nature, Simon's group presents new experimental observations of a quantum Hall material near a singularity of curvature in space.

Quantum effects give rise to some of the most useful and promising properties of materials: they define standard units of measurement, give rise to superconductivity, and describe quantum computers. The quantum hall materials are one prominent example in which electrons are trapped in non-conducting circular orbits except at the edges of the material. There, electrons exhibit quantized resistance-free electrical conduction that is immune to disorder such as material impurities or surface defects.

Furthermore, electrons in quantum Hall materials do not transmit sound waves but instead have particle-like excitations, some of which are unlike any other particles ever discovered. Some of these materials also exhibit simultaneous quantum entanglement between millions of electrons, meaning that the electrons are so interconnected, the state of one instantly influences the state of all others. This combination of properties makes quantum Hall materials a promising platform for future quantum computation.

Researchers worldwide have spent the past 35 years delving into the mysteries of quantum Hall materials, but always in the same fundamental way. They use superconducting magnets to make very powerful magnetic fields and refrigerators to cool electronic samples to thousandths of a degree above absolute zero.

Trapping light...

In a new approach, Simon and his team demonstrated the creation of a quantum Hall material made up of light. "Using really good mirrors that are pointed at each other, we can trap light for a long time while it bounces back and forth many thousands of times between the mirrors," explained graduate student Nathan Schine.

In the UChicago experiment, photons travel back and forth between mirrors, while their side-to-side motion mimics the behavior of massive particles like electrons. To emulate a strong magnetic field, the researchers created a non-planar arrangement of four mirrors that makes the light twist as it completes a round trip. The twisting motion causes the photons to move like charged particles in a magnetic field, even though there is no actual magnet present.

"We make the photons spin, which leads to a force that has the same effect as a magnetic field," explained Schine. While the light is trapped, it behaves like the electrons in a quantum Hall material.

First, Simon's group demonstrated that they had a quantum Hall material of light. To do so, they shined infrared laser light at the mirrors. By varying the laser's frequency, Simon's team could map out precisely at which frequencies the laser was transmitted through the mirrors. These transmission frequencies, along with camera images of the transmitted light, gave a telltale signature of a quantum Hall state.

Next, the researchers took advantage of the precise control that advanced optical systems provide to place the photons in curved space, which has not been possible so far with electrons. In particular, they made the photons behave as if they resided on the surface of a cone.

...near a singularity

"We created a cone for light much like you might do by cutting a wedge of paper and taping the edges together," said postdoctoral fellow Ariel Sommer, also a co-author of the paper. "In this case, we imposed a three-fold symmetry on our light, which essentially divides the plane into three wedges and forces the light to repeat itself on each wedge."

The tip of a cone has infinite curvature--the singularity--so the researchers were able to study the effect of strong spatial curvature in a quantum Hall material. They observed that photons accumulated at the cone tip, confirming a previously untested theory of the quantum Hall effect in curved space.

Despite 20 years of interest, this is the first time an experiment has observed the behavior of quantum materials in curved space. "We are beginning to make our photons interact with each other," said Schine. "This opens up many possibilities, such as making crystalline or exotic quantum liquid states of light. We can then see how they respond to spatial curvature."

The researchers say this could be useful for characterizing a certain type of quantum computer that is built of quantum Hall materials.

"While quantum Hall materials were discovered in the eighties, they continue to reveal their fascinating secrets to this day," said Simon. "The final frontier is exploring the interplay of these beautiful materials with the curvature of space. That is what we've begun to explore with our photons."

Reference:

Synthetic Landau levels for photons
Nature (2016) doi:10.1038/nature17943

University of Chicago

Protected Majorana states for quantum information

A nanowire device similar to those used in the study. The semiconducting nanowire (green), one-thousandth the width of a human hair, is coated with a superconductor (light blue) and electrically contacted with gold leads and electrostatic gates (yellow).(Credit: Shivendra Upadhyay / Sven Albrecht)

Normal computers are limited in their ability to solve certain classes of problems. The limitation lies in the fact that the operation of a conventional computers is based on classical states, or bits, the fundamental unit of information that is either 0 or 1.

In a quantum computer, data is stored in quantum bits, or qubits. According to the laws of quantum mechanics, a qubit can be in a superposition of states --- a 0 and 1 at the same time. By taking advantage of this and other properties of quantum physics, a quantum computer made of interconnected qubits should be able to tackle certain problems much more efficiently than would be possible on a classical computer.

There are many different physical systems that could in principle be used as quantum bits. The problem is that most quantum systems lose coherence very quickly—the qubit becomes a regular bit once measured. This is why researchers are still searching for the best implementation of quantum hardware. Enter the Majorana zero mode, a delocalized state in a superconductor that resists decoherence by sharing quantum information between separated locations. In a Majorana mode, the information is stored in such a way that a disturbance of either location leaves the quantum information intact.

“We are investigating a new kind of particle, called a Majorana zero mode, which can provide a basis for quantum information that is protected against measurement by a special and who knows, perhaps unique property of these particles. Majorana particles don’t exist as particles on their own, but they can be created using a combination of materials involving superconductors and semiconductors. What we find is that, first of all, the Majorana modes are present, verifying previous experiments, but more importantly that they are protected, just as theory predicts,” says Villum Kann Rasmussen Professor Charles Marcus, Director of the Center for Quantum Devices (QDev) and Station Q Copenhagen, at the Niels Bohr Institute, University of Copenhagen.

Nanowires for quantum technology

The Center for Quantum Devices is a leading research center in quantum information technology – with activities in theory, experiment, and materials research.

Semiconductor nanowires around 10 micrometers long and around 0.1 micrometers in diameter, coated with superconducting aluminum were used to form isolated islands of various lengths. By applying a strong magnetic field along the axis of the wire, and cooling the wires to below a tenth of a kelvin, a new kind of superconducting state, called a topological superconductor, was formed.

Quantum states are protected  

In 2012, physicists at Delft University in the Netherlands found the first signatures of Majorana zero modes in a similar system, with further evidence revealed in subsequent experiments around the world. Now, researchers at the Center for Quantum Devices have demonstrated critical predictions regarding their behavior, namely that their quantum states are protected in a fundamentally different manner from conventional quantum states.

The experiments were carried out by PhD Candidate Sven Albrecht and postdoc Andrew Higginbotham, now at the University of Colorado/NIST, USA, using new superconductor-semiconductor hybrid nanowires developed by Assistant Professor Peter Krogstrup in collaboration with Marcus and Professor Jesper Nygard.

“The protection is related to the exotic property of the Majorana mode that it simultaneously exists on both ends of the nanowire, but not in the middle. To destroy its quantum state, you have to act on both ends at the same time, which is unlikely”, says Sven Albrecht.

Albrecht explains that it was a challenging effort to demonstrate the protection experimentally. The researchers had to repeat their experiment many times with nanowires of different lengths in order to show that the protection improved with wire length.

“Exponential protection is an important check as we continue our basic exploration, and ultimately application, of topological states of matter.  Two things have pushed the field forward—from the first Majorana sightings at Delft to the present results—the first is strong interaction between theory and experiment. The second is remarkable materials development in Copenhagen, an effort that predates our Center. Without these new materials, the field was rather stuck. That’s behind us now.” says Charles Marcus.

Niels Bohr Institute - University of Copenhagen

A Proposed Superconductivity Theory Receives Exclusive Experimental Confirmation

Superconductivity – a quantum phenomenon in which metals below a certain temperature develop flow of current with no loss or resistance – is one of the most exciting problems in physics, which has resulted in investments worldwide of enormous brain power and resources since its discovery a little over a century ago. Many prominent theorists, Nobel laureates among them, have proposed theories for new classes of superconducting materials discovered several decades later, followed by teams of experimentalists working furiously to provide solid evidence for these theories.  More than 100,000 research papers have been published on the new materials.

One such theory began with a proposal in 1989 by Chandra Varma while at Bell Laboratories, NJ, and now a distinguished professor of physics and astronomy at the University of California, Riverside. At UC Riverside, he further developed the theory and proposed experiments to confirm or refute it.  That theory has now been experimentally proven to be a consistent theory by physicists in China and Korea.

The experimental results, published in Science Advances today (March 4), now allow for a clear discrimination of theories of high-temperature superconductivity, favoring one and ruling others out.  The research paper is titled “Quantitative determination of pairing interactions for high-temperature superconductivity incuprates.”

“At the core of most models for the high-temperature superconductivity in cuprates lies the idea of the electron-electron pairing,” said Lev P. Gor’kov, a theoretical physicist at Florida State University who is renowned for making the most important formal advance in the superconductivity field in 1958, while at the Soviet Academy of Sciences. “The paper by Prof. Chandra Varma and his colleagues from China and Korea is the daring and successful attempt to extract the relevant electron-electron interactions directly from experiment. Their elegant approach opens new prospects also for studying the superconductivity mechanisms in other systems with strongly correlated electrons.”

A boon to technology

Superconductors are used in magnetic-imaging devices in hospitals. They are used, too, for special electrical switches.  The electromagnets used in the Large Hadron Collider at CERN use superconducting wire.  Large-scale use of superconductivity, however, is not feasible presently because of cost.  If superconductors could be made cheaply and at ordinary temperatures, they would find wide use in power transmission, energy storage and magnetic levitation.

First discovered in the element mercury in 1911, superconductivity is said to occur when electrical resistance in a solid vanishes when that solid is cooled below a characteristic temperature, called the transition temperature, which varies from material to material. Transition temperatures tend to be close to 0 K or -273 C.  At even slightly higher temperatures, the materials tend to lose their superconducting properties; indeed, at room temperature most superconductors are very poor conductors.  In 1987, some high-temperature superconductors, called cuprates, were discovered by physicists Georg Bednorz and Alexander Müller, so named because they all contain copper and oxygen.  These new materials have properties which have raised profound new questions.  Why these high-temperature superconductors perform as they do has remained unknown.

A brief history lesson

The superconductivity problem was considered solved by a theory proposed in 1957: the BCS theory of superconductivity. This comprehensive theory, developed by physicists John Bardeen, Leon Cooper and John Schrieffer (the first letter of their last names gave the theory its name), explained the behavior of superconducting materials as resulting from electrons forming pairs, with each pair being strongly correlated with other pairs, allowing them all to function coherently as a single entity.  Concepts in the BCS theory and its elaborations have influenced all branches of physics, ranging from elementary particle physics to cosmology.

“But in the cuprates, some of the founding concepts of the physics of interacting particles, such as the quasi-particle concept, were found to be invalid,” Varma said. “The physical properties of superconductors  above the superconducting transition temperature were more remarkable than the superconductivity itself. Subsequently, almost all the leading theoretical physicists in the world proposed different directions of ideas and calculations to explain these properties as well as superconductivity.  But very few predictions stemming from these ideas were verified, and specific experiments were not in accord with them.”

A quasi-particle is a packet of energy and momentum that can, in some respects, be regarded as a particle. It is a physical concept, which allows detailed calculation of properties of matter.

In 1989, while at Bell Laboratories, Varma and some collaborators proposed that the breakdown of the quasi-particle concept occurs due to a simple form of quantum-critical fluctuations – fluctuations which are quantum in nature and occur when symmetry of matter breaks down, such as at the phase transition critical point near absolute zero of temperature.

In physics, symmetry is said to occur when some change in orientation or movement by any amount leaves the physical situation unchanged (empty space, for example, has symmetry because it is everywhere the same). Relativity, quantum theory, crystallography and spectroscopy involve notions of symmetry.

“It was at this time that we introduced the concept of marginal Fermi-liquids or marginal quasi-particles through which various properties of superconductivity were explained,” Varma said. “We also provided some definitive predictions, which could only be tested in 2000 by a new technique called Angle Resolved Photoemissions or ARPES.”

Varma explained that in 1989 there was also no evidence that the same quantum-critical fluctuations promoted the superconductivity transition.

“There was no theory for the cause of such quantum-critical fluctuations or for the symmetry which must change near absolute zero to realize them,” he said.

In 1997, Varma proposed transitions to a new class of symmetries, in which the direction of time was picked by the direction of currents. These currents, he suggested, begin to spontaneously flow in each microscopic cell of the cuprates.  Since 2004, a group of French scientists at Saclay has been reporting evidence of such symmetries in every high-temperature superconducting compound it could investigate with neutron scattering.  Several other kinds of experiments by other research groups are in accord also.

Varma cautioned that some unresolved issues persist. His group is proposing experiments to address them.

In 2003, the year Varma moved to UC Riverside, he formulated a theory for how quantum fluctuations coupled to electrons give rise to the observed symmetry in superconductivity.

“This was a completely new kind of coupling,” he said. “It had very remarkable and unusual predictions for experiments designed to decipher such a coupling.”

ARPES to the rescue

In 2010, Varma became aware of high-quality laser-based ARPES in a laboratory at the Institute of Physics in the Chinese Academy of Sciences, Beijing, China. A collaboration with physicist Xingjiang Zhou at the institute ensued, with numerical analysis of the data being done by Han-Yong Choi, a physicist at SungKyunKwan University, Korea, who, in the past, worked with Varma at UCR.

Zhou’s team made several improvements in the ARPES technique, which ensured that the quality of data was high and reproducible enough to have full confidence.

“The data obtained and the analysis we describe in our paper are conclusive on the most important issues relevant to superconductivity,” Varma said. “Our conclusions – namely, that the quantum fluctuations promoting superconductivity are the same as those that lead to the marginal Fermi-liquid and they are consistently of the form predicted, being stretched exponentially in time in a scale-invariant way relative to stretching in space – also have no theoretical approximations.  They are as precise as the quality of the data allows.  They also unambiguously address the question of symmetry of superconductivity.  Further, they rule out many of the alternative ideas that have been proposed on this problem in the last thirty years since the original discovery.  Our observations of the breakdown of time-reversal symmetry and of the fluctuations that follow complete major aspects of our understanding of these problems.”

Varma, Zhou and Choi were joined in the research by Jin Mo Bok (first author of the paper) and Jong Ju Bae at SungKyunKwan University, Korea; and Wentai Zhang, Junfeng He, Yuxiao Zhang and Li Yu at the Institute of Physics, Chinese Academy of Sciences, Beijing, China. Varma was partially supported by a grant from the National Science Foundation.

About Varma

After he received his doctoral degree in physics from the University of Minnesota, Varma joined Bell Labs in 1969, one of the most coveted positions at the time for young physicists anywhere in the world. The following year, he became a permanent member of the laboratory.  He was the head of the theoretical physics department at Bell Labs from 1983 to 1987, and was awarded the Distinguished Member of Research in 1988.  He has served as a visiting professor at the University of Chicago, Stanford University, MIT, the College de France in Paris, France, and at CNRS, France; and a senior visiting fellow at Cavendish Lab at Cambridge University.  In 2000, he was selected to the Lorentz Visiting Chair at Leiden University, the Netherlands.  In 2009, he held a Miller Professorship at UC Berkeley.

He is a fellow of the American Physical Society and of the American Association for the Advancement of Science. A member of the World Academy of Sciences, he is the recipient of the Alexander Humboldt Prize and the Bardeen Prize for theoretical advances in superconductivity.

Varma has published nearly 200 scientific papers, which have in all about 18,000 citations. He has made seminal contributions to the theory of glasses, to Kondo and mixed valence and heavy-fermion phenomena, novel forms of superconductivity, charge density waves, co-existing magnetic and superconducting states, the Higgs boson in superconductors, quantum criticality, singular Fermi-liquids and associated superconductivity.

UC Riverside

Chiral Magnetic Effect Generates Quantum Current

Separating left- and right-handed particles in a semi-metallic material produces anomalously high conductivity

 

Scientists at the U.S Department of Energy's (DOE) Brookhaven National Laboratory and Stony Brook University have discovered a new way to generate very low-resistance electric current in a new class of materials. The discovery, which relies on the separation of right- and left-"handed" particles, points to a range of potential applications in energy, quantum computing, and medical imaging, and possibly even a new mechanism for inducing superconductivity—the ability of some materials to carry current with no energy loss. 

The material the scientists worked with, zirconium pentatelluride, has a surprising trait: When placed in parallel electric and magnetic fields, it responds with an imbalance in the number of right- and left-handed particles—a chiral imbalance. That imbalance pushes oppositely charged particles in opposite directions to create a powerful electric current. 

This "chiral magnetic effect" had long been predicted theoretically, but never observed definitively in a materials science laboratory at the time this work was done. 

"The resistance of this material drops as the magnetic field strength increases, which could open up a completely different route toward achieving something like superconductivity." — Brookhaven Lab/Stony Brook University nuclear physics theorist Dmitri Kharzeev

In fact, when physicists in Brookhaven's Condensed Matter Physics & Materials Science Department (CMP&MS) first measured the significant drop in electrical resistance, and the accompanying dramatic increase in conductivity, they were quite surprised. "We didn't know this large magnitude of 'negative magnetoresistance' was possible," said Qiang Li, a physicist and head of the advanced energy materials group in the department and a co-author on a paper describing these results just published in the journal Nature Physics. But after teaming up with Dmitri Kharzeev, the head of the RIKEN-BNL theory group at Brookhaven and a professor at Stony Brook, the scientists had an explanation. 

Kharzeev had explored similar behavior of subatomic particles in the magnetic fields created in collisions at the Lab's Relativistic Heavy Ion Collider (RHIC), a DOE Office of Science User Facility where nuclear physicists explore the fundamental building blocks of matter. He suggested that in both the RHIC collisions and zirconium pentatelluride, the separation of charges could be triggered by a chiral imbalance.

To test the idea, they compared their measurements with the mathematical predictions of how powerful the increase in conductivity should be with increasing magnetic field strength. 

"We looked at the data and we said, 'Gee, that's it!' We tested six different samples and confirmed that no matter how you do it, it's there as long as the magnetic field is parallel to the electrical current. That's the smoking gun," Li said. 

Going Chiral

 

Right- or left-handed chirality is determined by whether a particle's spin is aligned with or against its direction of motion. In order for chirality to be definitively established, particles have to behave as if they are nearly massless and able to move as such in all three spatial directions. 

While free-flowing nearly massless particles are commonly found in the quark-gluon plasma created at RHIC, this was not expected to occur in condensed matter. However, in some recently discovered materials, including "Dirac semimetals"—named for the physicist who wrote the equations to describe fast-moving electrons—nearly massless "quasiparticle" versions of electrons (and positively charged "holes") propagate through the crystal in this free manner. 

Some aspects of this phenomenon, namely the linear dependence of the particles' energy on their momentum, can be directly measured and visualized using angle-resolved photoemission spectroscopy (ARPES).

"On first sight, zirconium pentatelluride did not even look like a 3D material," said Brookhaven physicist Tonica Valla, who performed the measurements with collaborators at the Advanced Light Source (ALS) at Lawrence Berkeley National Laboratory and at Brookhaven's National Synchrotron Light Source (NSLS)—two additional DOE Office of Science User Facilities. "It is layered, similar to graphite, so a quasi-2D electronic structure would be more expected. However, as soon as we did the first ARPES measurements, it was clear that the material is a 3D Dirac semimetal."

These results agreed nicely with the ones on conductivity and explained why the chiral magnetic effect was observed in this material.

In the absence of magnetic and electric fields, zirconium pentatelluride has an even split of right- and left- handed quasiparticles. But adding parallel magnetic and electric fields introduces a chiral preference: The magnetic field aligns the spins of the positive and negative particles in opposite directions, and the electric field starts the oppositely charged particles moving—positive particles move with the electric field, negative ones against it. If the two fields are pointing in the same direction, this creates a preference for positive and negative particles that are each moving in a direction aligned with their spin orientation—right-handed chiral particles—but with positive and negative particles moving away from one another. (If the magnetic field orientation is flipped relative to the electric field, the preference would be for left-handed particles, but still with opposite charges separating.) 

"This chiral imbalance gives a big boost to the separation of the oppositely charged particles, which can be connected through an external circuit," Kharzeev said. And once the chiral state is set it's hard to alter, "so very little energy is lost in this chiral current." 

Potential applications

 

The dramatic conductivity and low electrical resistance of Dirac semimetals may be key to potential applications, including "quantum electricity generators" and quantum computing, Li said.

"In a classic generator, the current increases linearly with increasing magnetic field strength, which needs to be changing dynamically. In these materials, current increases much more dramatically in a static magnetic field. You could pull current out of the 'sea' of available quasiparticles continuously. It's a pure quantum behavior," Li said.

Separating the two chiral states could also give a new way of encoding information—analogous to the zeros and ones of computing. And because the chiral state is very stable compared with other electrical states, it's much less prone to interference from external influences, including defects in the material. It could therefore be a more reliable material for quantum computing, Li said.

Kharzeev has some other ideas: "The resistance of this material drops as the magnetic field strength increases, which could open up a completely different route toward achieving something like superconductivity—zero resistance," he said. Right now the materials show at least some reduction in resistance at temperatures as high as 100 Kelvin—in the realm of the best high-temperature superconductors. But there are many different types of Dirac semimetals to experiment with to explore the possibility of higher temperatures or even more dramatic effects. Such low-resistance materials could help overcome a major limit in the speed of microprocessors by reducing the dissipation of current, Kharzeev added. 

"In zirconium pentatelluride and other materials that have since been discovered to have the chiral magnetic effect, an external magnetic field is required to start reducing resistivity," Valla said.

"However, we envision that in some magnetic materials, the electrical current could flow with little or no resistance in a direction parallel with the material's internal magnetic field. That would eliminate the need for external magnetic fields and would offer another avenue for dissipationless transport of electrical current."

Kharzeev and Li are also interested in exploring unusual optical properties in chiral materials. "These materials possess collective excitations in the terahertz frequency range, which could be important for wireless communications and also in imaging techniques that could improve the diagnosis of cancer," Kharzeev said.

Getting back to his nuclear physics roots, Kharzeev added, "The existence of massless quasiparticles that strongly interact makes this material quite similar to the quark-gluon plasma created in collisions at RHIC, where nearly massless quarks strongly interact through the exchange of gluons. So this makes Dirac semimetals an interesting arena for testing some of the ideas proposed in nuclear physics."

"This research illustrates a deep connection between two seemingly unrelated fields, and required contributions from an interdisciplinary team of condensed matter and nuclear physicists," said James Misewich, the Associate Laboratory Director for Energy Science at Brookhaven Lab and a professor of physics at Stony Brook University, who played the central role of introducing the members of this research team to one another. "We're fortunate to have scientists with expertise in these fields here at Brookhaven and nearby Stony Brook University, and the kind of collaborative spirit to make such a project come to fruition," he said.

Brookhaven National Laboratory 

New properties of superconductivity discovered

Superconductivity could have implications for creating technologies like ultra-efficient power grids and magnetically levitating vehicles.

Physicists at the University of Waterloo have led an international team that has come closer to understanding the mystery of how superconductivity, an exotic state that allows electricity to be conducted with practically zero resistance, occurs in certain materials.

Physicists all over the world are on a quest to understand the secrets of superconductivity because of the exciting technological possibilities that could be realized if they could make it happen at closer to room temperatures. In conventional superconductivity, materials that are cooled to nearly absolute zero ( −273.15 Celsius) exhibit the fantastic property of electrons pairing up and being able to conduct electricity with practically zero resistance. If superconductivity worked at higher temperatures, it could have implications for creating technologies such as ultra-efficient power grids, supercomputers and magnetically levitating vehicles.

New superconductivity findings published in journalScience

 

The new findings from an international collaboration, led by Waterloo physicists David Hawthorn, Canada Research Chair Michel Gingras, doctoral student Andrew Achkar and post-doctoral student Zhihao Hao, present direct experimental evidence of what is known aselectronic nematicity - when electron clouds snap into an aligned and directional order - in a particular type of high-temperature superconductor.

The results, published in the prestigious journalScience, may eventually lead to a theory explaining why superconductivity occurs at higher temperatures in certain materials.

“In this study, we identify some unexpected alignment of the electrons – a finding that is likely generic to the high-temperature superconductors and in time may turn out be a key ingredient of the problem,” says Hawthorn, a professor in Waterloo’s Department of Physics and Astronomy.

The findings show evidence of electronic nematicity as a universal feature in cuprate high-temperature superconductors. Cuprates are copper-oxide ceramics composed of two-dimensional layers or planes of copper and oxygen atoms separated by other atoms. They are known as the best of the high-temperature superconductors. In the 1980s, materials that exhibit superconductivity under somewhat warmer conditions (but still -135 Celsius, so far from room temperature) were discovered. But how superconductivity initiates in these high-temperature superconductors has been challenging to predict, let alone explain.

“It has become apparent in the past few years that the electrons involved in superconductivity can form patterns, stripes or checkerboards, and exhibit different symmetries – aligning preferentially along one direction,” says Hawthorn. “These patterns and symmetries have important consequences for superconductivity – they can compete, coexist or possibly even enhance superconductivity.”

Scientists use soft x-ray scattering in superconductivity research

 

The scientists used a novel technique called soft x-ray scattering at the Canadian Light Source synchrotron in Saskatoon to probe electron scattering in specific layers in the cuprate crystalline structure. Specifically, they looked at the individual cuprate (CuO2) planes where electronic nematicity takes place, versus the crystalline distortions in between the CuO2 planes.

Electronic nematicity happens when the electron orbitals align themselves like a series of rods.

The term nematicity commonly refers to when liquid crystals spontaneously align under an electric field in liquid crystal displays. In this case, the electron orbitals enter the nematic state as the temperature drops below a critical point.

Cuprates can made to be superconducting by adding elements that will remove electrons from the material, a process known as “doping.”

A material can be optimally doped to achieve superconductivity at the highest and most accessible temperature, but in studying how superconductivity happens, physicists often work with material that is “underdoped,” which means the level of doping is less than the level that maximizes the superconducting temperature.

Results from this study show electronic nematicity likely occur in all underdoped cuprates.

Physicists also want to understand the relation of nematicity to a phenomenon known as charge density wave fluctuations. Normally, the electrons are in a nice, uniform distribution, but charge-ordering can cause the electrons to bunch up, like ripples on a pond. This sets up a competition, whereby the material is fluctuating between the superconducting and non-superconducting states until the temperature cools enough for the superconductivity to win.

Future work will tackle how electrons can be tuned for superconductivity

 

Although there is not yet an agreed upon explanation for why electronic nematicity occurs, it may ultimately present another knob to tune in the quest to achieve the ultimate goal of a room temperature superconductor.

“Future work will tackle how electronic nematicity can be tuned, possibly to advantage, by modifying the crystalline structure,” says Hawthorn.

Hawthorn and Gingras are both Fellows of the Canadian Institute For Advanced Research. Gingras holds the Canada Research Chair in Condensed Matter Theory and Statistical Mechanics and spent time at the Perimeter Institute for Theoretical Physics as a visiting researcher while this work was being carried out.

Other Canadian collaborators include the Canadian Light Source and H. Zhang and Y.-J. Kim from the University of Toronto.

University of Waterloo