Potassium-ion batteries (PIBs) have been considered as promising alternatives to lithium-ion batteries due to the rich natural abundance of potassium (K) and similar redox potential with Li+/Li.
|However, due to the large K ion radius and slow reaction dynamics, the previously reported PIB anode materials (carbon-based materials, alloy-based anodes such as tin and antimony, metal oxides, etc.) suffer from a low capacity and fast capacity decay.|
|In order to achieve a high capacity and excellent cycle stability for K storage process, rational design of the electrode materials and proper selection of the electrolytes should be considered simultaneously.|
|Recently, two research teams led by Prof. Chunsheng Wang and Prof. Michael R. Zachariah from the University of Maryland, College Park, have designed and fabricated a novel antimony (Sb) carbon composite PIB anode via a facile and scalable electrospray-assisted strategy and found that this anode delivered super high specific capacities as well as cycling stability in a highly concentrated electrolyte (4M KTFSI/EC+DEC).|
|This work has been published in Energy and Environmental Science (“Super Stable Antimony-carbon composite anodes for potassium-ion batteries”).
|Figure 1. Schematic illustration of electrospray-assisted strategy for fabricating antimony @carbon sphere network electrode materials. (© Royal Society of Chemistry)|
|We have successfully fabricated a novel antimony carbon composite with small Sb nanoparticles uniformly confined in the carbon sphere network (Sb@CSN) via a facile and scalable electrospray-assisted strategy.|
|Such a unique nanostructure can effectively mitigate the deleteriously mechanical damage from large volume changes and provide a highly conductive framework for fast electron transport during alloy/de-alloy cycling process.|
|Alongside the novel structural design of the anode material, formation of a robust solid-electrolyte-interphase (SEI) on the anode is crucially important to achieve its long-term cycling stability.|
|The formation of a robust SEI on the anode material is determined by both the surface chemistries of active electrode materials as well as electrolyte compositions such as salt anion types and concentrations.|
|Therefore, designing a proper electrolyte is extremely important for the anode to achieve a high cycling stability.|
|In our study, we have for the first time developed a stable and safe electrolyte of highly concentrated 4M KTFSI/EC+DEC for PIBs to promote the formation of a stable and robust KF-rich SEI layer on an Sb@CSN anode, which guarantees stable electrochemical alloy/de-alloy reaction dynamics during long-time cycling process.|
|Figure 2. Cycling performance of antimony carbon sphere network electrode materials at 200mA/g current density in the highly concentrated electrolyte (4M KTFSI/EC+DEC). (© Royal Society of Chemistry)|
|In the optimized 4M KTFSI/EC+DEC electrolyte, the Sb@CSN composite delivers excellent reversible capacity of 551 mAh/g at 100 mA/g over 100 cycles with a capacity decay of 0.06% per cycle from the 10st to 100th cycling and 504 mAh/g even at 200 mA/g after 220 cycling. This demonstrates the best electrochemical performances with the highest capacity and longest cycle life when compared with all K-ion batteries anodes reported to date.|
|The electrochemical reaction mechanism was further revealed by density functional theory (DTF) calculation to support such excellent Potassium-storage properties.|
|Figure 3. Capacity comparison of Sb@CSN anode with previous reported anodes in potassium ion batteries. (© Royal Society of Chemistry)|
|In conclusion, these outstanding performances should be attributed to the novel nanostructure of Sb nanoparticles uniformly encapsulated into conductive carbon network and the formation of a more stable and robust KF-rich SEI layer on Sb@CSN in the optimized 4M KTFSI electrolyte.|
|These encouraging results will significantly promote the deep understanding of the fundamental electrochemistry in Potassium-ion batteries as well as rational development of efficient electrolyte systems for next generation high-performance Potassium-ion batteries.|
|Yong Yang, Research Associate, Prof. Zachariah Research Group, Department of Chemical and Environmental Engineering, University of California, Riverside|
Microelectronic devices – from pacemakers to cellphones – have long shaped the course of human health and telecommunications. But, scientists have struggled to navigate the technology gap between microelectronics and the biological world.
For example, today’s consumers cannot tap into their smartphones to uncover information about an infection or illness affecting their body, nor can they use their phones to signal a device to administer an antibiotic or drug.
One of the primary reasons for this disconnect between the body and everyday technology is that microelectronic devices process information using materials such as silicon, gold, or chemicals, and an energy source that provides electrons; but, free electrons do not exist in biology. As such, scientists encounter a major roadblock in their efforts to bridge the gap between biological systems and microelectronics.
But, engineers at the University of Maryland’s A. James Clark School of Engineering, along with researchers from the University of Nebraska-Lincoln and the U.S. Army Research Laboratory, may have found a loophole.
In biological systems, there exists a small class of molecules capable of shuttling electrons. These molecules, known as “redox” molecules, can transport electrons to any location. But, redox molecules must first undergo a series of chemical reactions – oxidation or reduction reactions – to transport electrons to the intended target.
By engineering cells with synthetic biology components, the research team has experimentally demonstrated a proof-of-concept device enabling robust and reliable information exchanges between electrical and biological (molecular) domains.
“Devices that freely exchange information between the electronic and biological worlds would represent a completely new societal paradigm,” said bioengineering professor William E. Bentley, director of the UMD Robert E. Fischell Institute for Biomedical Devices. “It has only been about 60 years since the implantable pacemaker and defibrillator proved what devices could achieve by electronically stimulating ion currents. Imagine what we could do by transferring all the knowledge contained in our molecular space, by tapping into and controlling molecules such as glucose, hormones, DNA, proteins, or polysaccharides in addition to ions.”
Building on their progress, the research team is now working to develop a novel biological memory device that can be written to and read from via either biological and/or electronic means. Such a device would function like a thumb drive or SD card, using molecular signals to store key information, and would require almost no energy. Inside the body, these devices would serve the same purpose – except, instead of merely storing data, they could be used to control biological behaviors.
“For years, microelectronic circuits have had limited capabilities in maximizing their computing and storage capacities, mainly due to the physical constraints that the building-block inorganic materials – such as silicon – imposed upon them,” said UMD professor Reza Ghodssi, who specializes in electrical and computer engineering. “By exploring and utilizing the world of biology through an integrated and robust interface technology with semiconductor processing, we expect to address those limitations by allowing our researchers and students to design and develop first-of-kind innovative and powerful bioelectronic devices and systems.”
The collaborative research team will work to integrate subsystems and create biohybrid circuits to develop an electronically controlled device for the body that interprets molecular information, computes desired outcomes, and electronically actuates cells to signal and control biological populations.
The hope is that such a system could seek out and destroy a bacterial pathogen by recognizing its secreted signaling molecules and synthesizing a pathogen-specific toxin. In this way, the group will, for the first time, explore electronic control of complex biological behaviors.
This year, the group was awarded a $1.5 million National Science Foundation grant through the Semiconductor Synthetic Biology for Information Processing and Storage technologies (SemiSynBio) program. Their earlier related work was published in Nature Communications.
Read about related microbiology research at Maryland.
This electron microscope image shows a hybrid nanoparticle consisting of a nanodiamond (roughly 50 nanometers wide) covered in smaller silver nanoparticles that enhance the diamond’s optical properties.Credit: Min Ouyang
Researchers develop a new method for pairing nanoscale diamonds with other nanomaterials
Nanomaterials have the potential to improve many next-generation technologies. They promise to speed up computer chips, increase the resolution of medical imaging devices and make electronics more energy efficient. But imbuing nanomaterials with the right properties can be time consuming and costly. A new, quick and inexpensive method for constructing diamond-based hybrid nanomaterials could soon launch the field forward.
University of Maryland researchers developed a method to build diamond-based hybrid nanoparticles in large quantities from the ground up, thereby circumventing many of the problems with current methods. The technique is described in the June 8, 2016 issue of the journal Nature Communications.
The process begins with tiny, nanoscale diamonds that contain a specific type of impurity: a single nitrogen atom where a carbon atom should be, with an empty space right next to it, resulting from a second missing carbon atom. This “nitrogen vacancy” impurity gives each diamond special optical and electromagnetic properties.
By attaching other materials to the diamond grains, such as metal particles or semiconducting materials known as “quantum dots,” the researchers can create a variety of customizable hybrid nanoparticles, including nanoscale semiconductors and magnets with precisely tailored properties.
“If you pair one of these diamonds with silver or gold nanoparticles, the metal can enhance the nanodiamond’s optical properties. If you couple the nanodiamond to a semiconducting quantum dot, the hybrid particle can transfer energy more efficiently,” said Min Ouyang, an associate professor of physics at UMD and senior author on the study.
Evidence also suggests that a single nitrogen vacancy exhibits quantum physical properties and could behave as a quantum bit, or qubit, at room temperature, according to Ouyang. Qubits are the functional units of as-yet-elusive quantum computing technology, which may one day revolutionize the way humans store and process information. Nearly all qubits studied to date require ultra-cold temperatures to function properly.
A qubit that works at room temperature would represent a significant step forward, facilitating the integration of quantum circuits into industrial, commercial and consumer-level electronics. The new diamond-hybrid nanomaterials described in Nature Communications hold significant promise for enhancing the performance of nitrogen vacancies when used as qubits, Ouyang noted.
While such applications hold promise for the future, Ouyang and colleagues’ main breakthrough is their method for constructing the hybrid nanoparticles. Although other researchers have paired nanodiamonds with complementary nanoparticles, such efforts relied on relatively imprecise methods, such as manually installing the diamonds and particles next to each other onto a larger surface one by one. These methods are costly, time consuming and introduce a host of complications, the researchers say.
“Our key innovation is that we can now reliably and efficiently produce these freestanding hybrid particles in large numbers,” explained Ouyang, who also has appointments in the UMD Center for Nanophysics and Advanced Materials and the Maryland NanoCenter, with an affiliate professorship in the UMD Department of Materials Science and Engineering.
The method developed by Ouyang and his colleagues, UMD physics research associate Jianxiao Gong and physics graduate student Nathaniel Steinsultz, also enables precise control of the particles’ properties, such as the composition and total number of non-diamond particles. The hybrid nanoparticles could speed the design of room-temperature qubits for quantum computers, brighter dyes for biomedical imaging, and highly sensitive magnetic and temperature sensors, to name a few examples.
“Hybrid materials often have unique properties that arise from interactions between the different components of the hybrid. This is particularly true in nanostructured materials where strong quantum mechanical interactions can occur,” said Matthew Doty, an associate professor of materials science and engineering at the University of Delaware who was not involved with the study. “The UMD team’s new method creates a unique opportunity for bulk production of tailored hybrid materials. I expect that this advance will enable a number of new approaches for sensing and diagnostic technologies.”
The special properties of the nanodiamonds are determined by their nitrogen-vacancies, which cause defects in the diamond’s crystal structure. Pure diamonds consist of an orderly lattice of carbon atoms and are completely transparent. However, pure diamonds are quite rare in natural diamond deposits; most have defects resulting from non-carbon impurities such as nitrogen, boron and phosphorus. Such defects create the subtle and desirable color variations seen in gemstone diamonds.
The nanoscale diamonds used in the study were created artificially, and have at least one nitrogen vacancy. This impurity results in an altered bond structure in the otherwise orderly carbon lattice. The altered bond is the source of the optical, electromagnetic and quantum physical properties that make the diamonds useful when paired with other nanomaterials.
Although the current study describes diamonds with nitrogen substitutions, Ouyang points out that the technique can be extended to other diamond impurities as well, each of which could open up new possibilities.
“A major strength of our technique is that it is broadly useful and can be applied to a variety of diamond types and paired with a variety of other nanomaterials,” Ouyang explained. “It can also be scaled up fairly easily. We are interested in studying the basic physics further, but also moving toward specific applications. The potential for room-temperature quantum entanglement is particularly exciting and important.”
- Jianxiao Gong, Nat Steinsultz, Min Ouyang. Nanodiamond-based nanostructures for coupling nitrogen-vacancy centres to metal nanoparticles and semiconductor quantum dots. Nature Communications, 2016; 7: 11820 DOI: 10.1038/ncomms11820
Extremely small batteries built inside nanopores show that properly scaled structures can use the full theoretical capacity of the charge storage material. The batteries are part of assessing the basics of ion and electron transport in nanostructures for energy storage. These nanobatteries delivered their stored energy efficiently at high power (fast charge and discharge) and for extended cycling.
Precise structures can be constructed to assess the fundamentals of ion and electron transport in nanostructures for energy storage and to test the limits of three-dimensional nanobattery technologies.
Nanostructured batteries, when properly designed and built, offer promise for delivering their energy at much higher power and longer life than conventional technology. To retain high energy density, nanostructures (such as nanowires) must be arranged as dense “nanostructure forests,” producing three-dimensional nanogeometries in which ions and electrons can rapidly move. Researchers have built arrays of nanobatteries inside billions of ordered, identical nanopores in an alumina template to determine how well ions and electrons can do their job in such ultrasmall environments.
The nanobatteries were fabricated by atomic layer deposition to make oxide nanotubes for ion storage inside metal nanotubes for electron transport, all inside each end of the nanopores. The tiny nanobatteries work extremely well: they can transfer half their energy in just a 30 second charge or discharge time, and they lose only a few percent of their energy storage capacity after 1000 cycles. Researchers attribute this performance to rational design and well-controlled fabrication of nanotubular electrodes to accommodate ion motion in and out and close contact between the thin nested tubes to ensure fast transport for both ions and electrons.
This work was performed at the University of Maryland and was supported by the Nanostructures for Electrical Energy Storage (NEES) Center, an Energy Frontier Research Center funded by the DOE Office of Science, Office of Basic Energy Sciences.
- Chanyuan Liu, Eleanor I. Gillette, Xinyi Chen, Alexander J. Pearse, Alexander C. Kozen, Marshall A. Schroeder, Keith E. Gregorczyk, Sang Bok Lee, Gary W. Rubloff. An all-in-one nanopore battery array. Nature Nanotechnology, 2014; 9 (12): 1031 DOI: 10.1038/nnano.2014.247
- Paul V. Braun, Ralph G. Nuzzo. Batteries: Knowing when small is better. Nature Nanotechnology, 2014; 9 (12): 962 DOI: 10.1038/nnano.2014.263
Scientists are using biology to improve the properties of lithium ion batteries. Researchers at the University of Maryland, Baltimore County (UMBC) have isolated a peptide, a type of biological molecule, which binds strongly to lithium manganese nickel oxide (LMNO), a material that can be used to make the cathode in high performance batteries. The peptide can latch onto nanosized particles of LMNO and connect them to conductive components of a battery electrode, improving the potential power and stability of the electrode.
The researchers will present their results at the 59th annual meeting of the Biophysical Society, held Feb. 7-11 in Baltimore, Maryland.
“Biology provides several tools for us to solve important problems,” said Evgenia Barannikova, a graduate student at UMBC. Barannikova works in the lab of Mark Allen and studies how biological molecules in general can improve the properties of inorganic materials in batteries. “By mimicking biological processes we can find the better solution,” she said.
One of the problems currently facing battery researchers is the difficulty of working with nanoscale materials, which due to their extra tiny size can be hard to control and hold in place. The frustrations of working with nanosized materials are worth overcoming, however. Nanostructured electrodes in Li-ion batteries have several advantages over bulk material electrodes, including shorter distances for charge-carrying particles to travel and a high surface area that provides more active sites for electrochemical reactions to occur — all of which translates to batteries that are lighter and longer-lasting.
To take on the challenge of manufacturing on the nanoscale Barannikova and her colleagues have turned for help to biological molecules called peptides. Themselves made up from strings of molecules known as amino acids, peptides are naturally occurring and bind to many different types of organic and inorganic materials, depending on their sequence of the amino acids. They play many roles in the human body, from signaling in the brain to regulating blood sugar, and some drugs, like insulin, are made up of peptides.
One of the inspirations for her research, Barannikova said, was the way that organisms such as mollusks use peptides to control the growth of their shells. They demonstrate remarkable control in order to build intricate nano- and macrostructures from inorganic materials like calcium carbonate, she said.
The researchers borrowed the general approach of the mollusks, but had to employ some lab-bench wizardry to find the appropriate peptide. No snail, after all, makes its shell from lithium manganese nickel oxide.
Barannikova and her colleagues used a procedure called “Phage Display” to screen more than one billion possible peptides in search of one that would stick strongly to lithium manganese nickel oxide. The “peptide library” through which the researchers searched is commercially produced by a laboratory supply company, and contains a vast number of randomly combined amino acid sequences incorporated into a protein made by a virus called the M13 bacteriophage.
The researchers isolated a peptide that binds to lithium manganese nickel oxide by combining the library with a sample of the metal oxide and then repeatedly washing away the peptides that didn’t stick to it. The researchers then combined the newly-discovered peptide with a previously isolated peptide that binds to carbon nanotubes. Carbon nanotubes can serve as conductive nanowires in Li-ion electrodes.
The resulting peptide could then form a bridge, binding to both the lithium manganese nickel oxide nanoparticles and the carbon nanotubes and keeping them close to each other so that they can maintain a connection through multiple charging cycles. By helping to maintain a highly organized architecture at the nanoscale, the researchers expect that their peptides will improve the power and cycling stability of future Li-ion batteries, allowing them to be smaller and maintain longer lifetimes.
The team is currently testing how well the new cathodes perform. Going forward, Barannikova plans to make an anode with similar techniques and to integrate the two components. “I hope to demonstrate an entire biotemplated battery in my Ph.D. thesis,” she said.
The University of Maryland (UMD) and the U.S. Department of Commerce’s National Institute of Standards and Technology (NIST) announced today the creation of the Joint Center for Quantum Information and Computer Science (QuICS), with the support and participation of the Research Directorate of the National Security Agency/Central Security Service (NSA/CSS). Scientists at the center will conduct basic research to understand how quantum systems can be effectively used to store, transport and process information.
This new center complements the fundamental quantum research performed at the work of the Joint Quantum Institute (JQI), which was established in 2006 by UMD, NIST and the NSA. Focusing on one of JQI’s original objectives to fully understand quantum information, QuICS will bring together computer scientists—who have expertise in algorithm and computational complexity theory and computer architecture—with quantum information scientists and communications scientists.
“This new endeavor builds on an already successful and fruitful collaboration at JQI,” said Acting Under Secretary of Commerce for Standards and Technology and Acting Director of NIST Willie May. “The new center will be a venue for groundbreaking basic research that will help to build our capacity for quantum research and train the next generation of researchers.”
UMD and NIST have a shared history of collaboration and cooperation in education, research and public service. They have long cooperated in building collaborative research consortia and programs that have resulted in extensive personal, professional and institutional relationships.
“By deepening our partnership with NIST, we now have all the ingredients in place to make major advances in quantum science,” said UMD President Wallace Loh. “This superb, world-class quantum program will team some of the best minds in physics, computer science and engineering to overcome the limitations of current computing systems.”
Dianne O’Leary, Distinguished University Professor Emerita in computer science at UMD, and Jacob Taylor, a NIST physicist and JQI Fellow, will serve as co-directors of the new center. Like the JQI, QuICS will be located on the UMD campus in College Park, Md.
The capabilities of today’s embedded and high-performance computer architectures have limited advances in critical areas, such as modeling the physical world, improving sensors and securing communications. Quantum computing could enable us to break through some of these barriers.
QuICS’ objectives will be to:
- Develop a world-class research center that will build the scientific foundation for quantum information science to enable understanding of the relationships between information theory, computational complexity theory and nature, as well as the advances in computer science necessary to support potential quantum computing and communication devices and systems;
- Maintain and enhance the nation’s leading role in quantum information science by expanding an already-powerful collaboration between UMD, NIST and NSA/CSS; and
- Establish a unique, interdisciplinary center for the interchange of ideas among computer scientists, physicists and quantum information researchers.
Some of the topics QuICS researchers will initially examine include understanding how quantum mechanics informs computation and communication theories, determining what insights computer science can shed on quantum computing, investigating the consequences of quantum information theory for fundamental physics, and developing practical applications for theoretical advances in quantum computation and communication.
QuICS is expected to train scientists for future industrial and academic opportunities and provide U.S. industry with cutting-edge research results. By combining the strengths of UMD and NIST, QuICS will become an international center for excellence in quantum computer and information science.
QuICS will be the newest of 16 centers and labs within the University of Maryland Institute for Advanced Computer Studies (UMIACS). The center will bring together researchers from UMIACS; the UMD Departments of Physics and Computer Science; and the UMD Applied Mathematics & Statistics, and Scientific Computation program with NIST’s Information Technology and Physical Measurement laboratories.
About the University of Maryland
The University of Maryland is home to three quantum science research centers: the Joint Center for Quantum Information and Computer Science, the Joint Quantum Institute, and the Quantum Engineering Center. UMD has nation-leading computer science, physics and math departments, with particular strengths in the areas relevant to quantum science research.
In the 2015 Best Graduate Schools ranking by U.S. News & World Report, UMD’s Department of Physics ranked 14th, the Department of Computer Science ranked 15th, and Department of Mathematics ranked 17th. The atomic/molecular/optical physics specialty ranked 6th, the quantum physics specialty ranked 8th, and the applied math specialty ranked 10th. Visit UMD’s website to learn more.
As a non-regulatory agency of the U.S. Department of Commerce, NIST promotes U.S. innovation and industrial competitiveness by advancing measurement science, standards and technology in ways that enhance economic security and improve our quality of life. Visit NIST’s website for more information.
Seeking a robust home for qubits | October 8th, 2012
If quantum computers are ever going to perform all those expected feats of code-breaking and number crunching, then their component qubits—tiny ephemeral quantum cells held in a superposition of internal states—will have to be protected from intervention by the outside world. In other words, decoherence, the loss of the qubits’ quantum integrity, has to be postponed. Now theoretical physicists at the Joint Quantum Institute (JQI) and the University of Maryland have done an important step forward to understand qubits in a real-world setup. In a new study they show, for the first time, that qubits can successfully exist in a so called topological superconductor material even in the presence of impurities in the material and strong interactions among participating electrons.
To see how qubits can enter into their special coherence-protection program, courtesy of “Majorana particles,” an exotic form of excitation, some groundwork has to be laid.
Credit: Emily Edwards
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Most designs for qubits involve materials where quantum effects are important. In one such material, superconductors (SC), electrons pair up and can then enter into a large ensemble, a supercurrent, which flows through the material without suffering energy loss. Another material is a sandwich of semiconductors which support the quantum Hall effect (QHE). Here, very low temperatures and a powerful external magnetic field force electrons in a thin boundary layer to execute tiny cyclone motions (not exactly, but ok—also isn’t a cyclone a storm?). At the edge of these layers, the electrons, unable to trace out a complete circular path, will creep along the edge, where they constitute a net electrical current.
One of the most interesting and useful facts about these electrons at the edge is that they move in one direction. They cannot scatter backwards no matter how many impurities (which in ordinary conductors can lead to energy dissipation) may be in the material. If, furthermore, the electrons can be oriented according to their spin—their intrinsic angular momentum—then we get what is called the quantum spin Hall effect (QSH). In this case all electrons with spin up will circulate around the material (at the edge) in one direction, while electrons with spin down will circulate around in the opposite direction.
The QHE state is depicted in figure 1.
Credit: Emily Edwards
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In some materials the underlying magnetism of the nuclei in the atoms making of the material is so strong than no external magnet is needed to create the Hall effects. Mercury-cadmium-telluride compounds are examples of materials called topological insulators. Insulators (not sure how this sentence was supposed to start, but grammatically is currently confusing) because even as electrons move around the edge of the material with very little loss of energy, the interior of these 3-dimensional structures is an insulator; no current flows. The “topological” is a bit harder to explain. Partly the flow of current on the outside bespeaks of geometry: the electrons flow only at the edge and are unable (owing to quantum interactions) from scattering backwards if they meet an impediment.
But topology in this case has more to do with the way in which the motion of the electrons in these materials are described in terms of “dispersion relations.” Just as waves of white light will be dispersed into a spectrum of colors when the waves strike the oblique side of a prism, so electron waves (electrons considered as quantum waves) will be “dispersed,” in the sense that electrons with the same energy might have different momenta, depending on how the electrons move through the material in question.
The idea of electron dispersal is often depicted in the form of an energy-level diagram. In insulators (the left panel of Figure 2) electrons remain in a valence band; they don’t have enough energy to visit the conduction band of energies; hence the electrons do not move; the material is an insulator against electricity. In a conductor (middle part) the conduction and valence bands overlap. In the QHE (right panel) electrons in the interior of the material also do not move along; the bulk of the material is an insulator. But for electrons at the edge there is a chance for movement into the conduction band.
Now for the topology: just as a coffee cup is equivalent to a donut topologically—either can be transformed into the other by stretching but not by any tearing—so here the valence band can be transformed into a conduction band (at least for edge states) no matter what impurities might be present in the underlying material. In other words, the “topological” nature of the material offers some protection for the flow of electrons against the otherwise-dissipating effects of impurities.
The marvelous properties of superconductors and topological materials can be combined. If a one-dimensional topological specimen—a nanowire made from indium and arsenic—is draped across a superconductor (niobium, say) then the superconductivity can extend into the wire (proximity effect). And in this conjunction of materials, still another hotly-pursued effect can come into play.
The orange balls (the letter gamma) mark the two places, at either end of the nanowire, where the Majorana excitations appear. The wire sits atop the superconductor. (Credit: Alejandro Lobos)
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One last concept is needed here—Majorana particles—named for the Italian physicist Ettore Majorana, who predicted in 1937 the existence of a class of particle that would serve as its own antiparticle. Probably this object would not exist usefully in the form of a single real particle but would, rather, appear in a material as a quasiparticle, an ensemble excitation of many electrons.
Some scientists believe that qubits made from Majorana pulses excited in topological materials (and benefitting from the same sort of topological protection that benefits, say, electrons in QHE materials) would be much more immune from decoherence than other qubits based on conventional particles.
Specifically Sankar Das Sarma and his colleagues at the University of Maryland (JQI and the Condensed Matter Theory Center) predicted that Majorana particles would appear in topological quantum nanowires. In fact part of the Majorana excitation would appear at both ends of the wire. These predictions were borne out. It is precisely the separation of these two parts (each of which constitutes a sort of “half electron”) that confers some of the anticipated coherence-protection: a qubit made of that Majorana excitation would not be disrupted by merely a local irregularity in the wire.
A recent experiment in Holland provides preliminary evidence for exactly this occurrence (***).
Robust Qubits Amid Disorder
Here is a picture of the prospective experimental setup in which Majorana particles could be made in a hybrid superconductor-nanowire material. In (a) the topological semiconductor bridges a gap between two parts of a superconductor. The letter phi represents an external magnetic field which can tailor conditions in the semiconductor. In (b) the immediate nanowire (gray rod) environment is shown. L is the size of the gap between the superconductor halves while L1 is the distance over which the underlying superconductivity will persist within the overlying semiconductor. (c) shows how the superconductor (SC)-semiconductor (SM) sandwich can be further tuned by a nearby electrical circuit. (Credit: Lutchyn, Sau and Das Sarma)
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One of the authors of the new study, Alejandro Lobos, said that the earlier Maryland prediction, useful as it was, was still somewhat idealistic in that it didn’t fully grapple with the presence of impurities, a fact of life which all engineers of actual computers must confront. This is what the new paper, which appears in the journal Physical Review Letters, addresses.
The problem of impurities or defects (which flowing electrons encounter as a form of disorder) is especially important for components which are two or even one dimensional in nature. The same is true for the repulsive force among electrons. “In 3-dimensional materials,” said Lobos, “electrons (and their screening clouds of surrounding holes) can avoid each other thanks to the availability of space. They can just go around each other. In 1-D materials, this is not possible, since electrons cannot pass each other. In 1D, if one electron wants to move, it has to move all the other electrons! This ensures that excitations in a 1D metal are necessarily collective, as opposed to the single-particle excitations existing in a 3D metal.
So, in summary, the new Maryland work shows that disorder and electron interactions, two things that normally work to disrupt superconductivity, can be overcome with careful engineering of the material. “A number of important theoretical studies before ours have focused on the destabilizing effects of either disorder or interaction on topological superconductors,” said Lobos. “These studies showed the extent to which a topological superconductor could survive under these effects separately. But to make contact with real materials, disorder and interactions have to be considered on equal footing and simultaneously, a particular requirement imposed by the one-dimensional geometry of the system. It was then an important question to determine if it was possible to stabilize a topological superconductor under their simultaneous presence. The good news is that the answer is yes: despite their detrimental effect, there is still a sizable range of parameters where topological superconductors hosting Majorana excitations can exist. That’s the main result of our study, which will be useful to understand and characterize topological superconductors in more realistic situations.”
(*) The Joint Quantum Institute is operated jointly by the National Institute of Standards and Technology in Gaithersburg, MD and the University of Maryland in College Park.
(**) “Interplay of disorder and interaction in Majorana quantum wires,” Alejandro M. Lobos, Roman M. Lutchyn, and S. Das Sarma, Physical Review Letters, 5 October 2012
(***) See earlier Majorana JQI press release and several pertinent research papers
Alejandro M. Lobos, (301)405-0603, email@example.com