Category: Physics

by University of Cambridge

Researchers have found a way to control the interaction of light and quantum ‘spin’ in organic semiconductors, that works even at room temperature.

Spin is the term for the intrinsic angular momentum of electrons, which is referred to as up or down. Using the up/down spin states of electrons instead of the 0 and 1 in conventional computer logic could transform the way in which computers process information. And sensors based on quantum principles could vastly improve our abilities to measure and study the world around us.

An international team of researchers, led by the University of Cambridge, has found a way to use particles of light as a ‘switch’ that can connect and control the spin of electrons, making them behave like tiny magnets that could be used for quantum applications.

The researchers designed modular molecular units connected by tiny ‘bridges.’ Shining a light on these bridges allowed electrons on opposite ends of the structure to connect to each other by aligning their spin states. Even after the bridge was removed, the electrons stayed connected through their aligned spins.

This level of control over quantum properties can normally only be achieved at ultra-low temperatures. However, the Cambridge-led team has been able to control the quantum behavior of these materials at room temperature, which opens up a new world of potential quantum applications by reliably coupling spins to photons. The results are reported in the journal Nature.

Almost all types of quantum technology—based on the strange behavior of particles at the subatomic level—involve spin. As they move, electrons usually form stable pairs, with one electron spin up and one spin down. However, it is possible to make molecules with unpaired electrons, called radicals. Most radicals are very reactive, but with careful design of the molecule, they can be made chemically stable.

“These unpaired spins change the rules for what happens when a photon is absorbed and electrons are moved up to a higher energy level,” said first author Sebastian Gorgon, from Cambridge’s Cavendish Laboratory. “We’ve been working with systems where there is one net spin, which makes them good for light emission and making LEDs.”

Gorgon is a member of Professor Sir Richard Friend’s research group, where they have been studying radicals in organic semiconductors for light generation, and identified a stable and bright family of materials a few years ago. These materials can beat the best conventional OLEDs for red light generation.

“Using tricks developed by different fields was important,” said Dr. Emrys Evans from Swansea University, who co-led the research. “The team has significant expertise from a number of areas in physics and chemistry, such as the spin properties of electrons and how to make organic semiconductors work in LEDs. This was critical for knowing how to prepare and study these molecules in the solid state, enabling our demonstration of quantum effects at room temperature.”

Organic semiconductors are the current state-of-the-art for lighting and commercial displays, and they could be a more sustainable alternative to silicon for solar cells. However, they have not yet been widely studied for quantum applications, such as quantum computing or quantum sensing.

“We’ve now taken the next big step and linked the optical and magnetic properties of radicals in an organic semiconductor,” said Gorgon. “These new materials hold great promise for completely new applications, since we’ve been able to remove the need for ultra-cold temperatures.”

“Knowing what electron spins are doing, let alone controlling them, is not straightforward, especially at room temperature,” said Friend, who co-led the research. “But if we can control the spins, we can build some interesting and useful quantum objects.”

The researchers designed a new family of materials by first determining how they wanted the electron spins to behave. Using this bottom-up approach, they were able to control the properties of the end material by using a building block method and changing the ‘bridges’ between different modules of the molecule. These bridges were made of anthracene, a type of hydrocarbon.

For their ‘mix-and-match’ molecules, the researchers attached a bright light-emitting radical to an anthracene molecule.

After a photon of light is absorbed by the radical, the excitation spreads out onto the neighboring anthracene, causing three electrons to start spinning in the same way. When a further radical group is attached to the other side of the anthracene molecules, its electron is also coupled, bringing four electrons to spin in the same direction.

“In this example, we can switch on interaction between two electrons on opposite ends of the molecule by aligning electron spins on the bridge absorbing a photon of light,” said Gorgon. “After relaxing back, the distant electrons remember they were together even after the bridge is gone.

“In these materials we’ve designed, absorbing a photon is like turning a switch on. The fact that we can start to control these quantum objects by reliably coupling spins at room temperature could open up far more flexibility in the world of quantum technologies. There’s a huge potential here to go in lots of new directions.”

“People have spent years trying to get spins to reliably talk to each other, but by starting instead with what we want the spins to do and then the chemists can design a molecule around that, we’ve been able to get the spins to align,” said Friend. “It’s like we’ve hit the Goldilocks zone where we can tune the spin coupling between the building blocks of extended molecules.”

The advance was made possible through a large international collaboration—the materials were made in China, experiments were done in Cambridge, Oxford and Germany, and theory work was conducted in Belgium and Spain.

More information: Emrys Evans, Reversible spin-optical interface in luminescent organic radicals, Nature (2023). DOI: 10.1038/s41586-023-06222-1. www.nature.com/articles/s41586-023-06222-1

Journal information: Nature 

Provided by University of Cambridge 

by Raphael Rosen, Princeton University

Scientists have found a mathematical shortcut that could help harness fusion energy, a potential source of clean electricity that could mitigate floods, heat waves, and other rising effects of climate change. The method allows researchers to more easily predict how well a stellarator—a twisty device designed to reproduce the fusion energy that powers the sun and stars—can retain the heat crucial to fusion reactions.

The technique measures how well a stellarator’s magnetic field can hold on to the fastest-moving atomic nuclei in the plasma, boosting the overall heat and aiding the fusion reactions. But how can scientists find a shape that holds in as much of the heat as possible?

Finding magnetic cages that hold heat

“We can’t simulate the motions of all the individual particles in all the possible magnetic fields—that would require almost infinite computing power,” said Alexandra LeViness, a graduate student in plasma physics at the U.S. Department of Energy’s (DOE) Princeton Plasma Physics Laboratory (PPPL). “Instead, we have to use a shortcut,” said LeViness, lead author of the paper reporting the results in the journal Nuclear Fusion.

“This research shows that we can find the best magnetic field shape for confining heat by calculating something easier—how far the fast particles drift away from the curved magnetic field surfaces in the center of the plasma,” LeViness said. “This behavior is described by a number known as gamma C, which we discovered consistently corresponds with plasma confinement.”

In effect, the shortcut advances future stellarator research, LeViness said, “because the more fast-moving particles that stay in the center of the plasma, the hotter the fuel and the more efficient the stellarator will be.”

The history and future of fusion

Fusion releases vast amounts of energy by combining light elements in the form of plasma—the hot, charged state of matter composed of free electrons and atomic nuclei that makes up 99% of the visible universe. Scientists around the world are seeking to harness fusion reactions to create a virtually inexhaustible supply of safe and clean power to generate electricity.

PPPL has more than a half-century of experience developing the theoretical scientific knowledge and advanced engineering to enable fusion to power the U.S. and the world. At the same time, the Laboratory has long advanced the basic scientific understanding of the plasma universe from laboratory to astrophysical scales.

Stellarators, developed by PPPL founder Lyman Spitzer in the 1950s, operate without the risk of damaging disruptions that doughnut-shaped fusion devices called tokamaks face. But stellarators have long been unable to hold in the heat as well as tokamaks, which have similar magnetic fields.

“But using techniques like the one LeViness studied, we have been able to find magnetic configurations for stellarators that contain heat as well as tokamaks can,” said Elizabeth Paul, an assistant professor of applied physics at Columbia University. “It’s more challenging for stellarators, but LeViness has helped show that it’s possible,” said Paul, a former presidential fellow at Princeton University.

More information: Alexandra LeViness et al, Energetic particle optimization of quasi-axisymmetric stellarator equilibria, Nuclear Fusion (2022). DOI: 10.1088/1741-4326/aca4e3

Provided by Princeton University 

by Trinity College Dublin

Trinity’s quantum physicists in collaboration with IBM Dublin have successfully simulated super diffusion in a system of interacting quantum particles on a quantum computer.

This is the first step in doing highly challenging quantum transport calculations on quantum hardware and, as the hardware improves over time, such work promises to shed new light in condensed matter physics and materials science.

The work is one of the first outputs of the TCD-IBM predoctoral scholarship programwhich was recently established where IBM hires Ph.D. students as employees while being co-supervised at Trinity. The paper was published recently in npj Quantum Information.

The early stage quantum computer used in this study consists of 27 superconducting qubits (qubits are the building blocks of quantum logic) and is physically located in IBM’s lab in Yorktown Heights in New York and programmed remotely from Dublin.

Quantum computing is currently one of the most exciting technologies and is expected to be edging closer towards commercial applications in the next decade. Commercial applications aside there are fascinating fundamental questions which quantum computers can help with. The team at Trinity and IBM Dublin tackled one such question concerning quantum simulation.

Explaining the significance of the work and the idea of quantum simulation in general, Trinity’s Professor John Goold, director of the newly established Trinity Quantum Alliance, who led the research, says, “Generally speaking the problem of simulating the dynamics of a complex quantum system with many interacting constituents is a formidable challenge for conventional computers.”

“Consider the 27 qubits on this particular device. In quantum mechanics the state of such a system is described mathematically by an object called a wave function. In order to use a standard computer to describe this object you require a huge number of coefficients to be stored in memory and the demands scale exponentially with the number of qubits; roughly 134 million coefficients, in the case of this simulation.”

“As you grow the system to say 300 qubits you would need more coefficients than there are atoms in the observable universe to describe such a system and no classical computer will be able to exactly capture the system’s state. In other words we hit a wall when simulating quantum systems,” Goold said.

“The idea of using quantum systems to simulate quantum dynamics goes back to the American Nobel prize winning Physicist Richard Feynman who proposed that quantum systems are best simulated using quantum systems. The reason is simple—you naturally exploit the fact that the quantum computer is described by a wave function thus circumventing the need for exponential classical resources for storage of the state.”

So what exactly did the team simulate? Prof. Goold continues, “Some of the simplest non-trivial quantum systems are spin chains. These are systems of little connected magnets called spins, which mimic more complex materials and are used to understand magnetism. We were interested in a model called the Heisenberg chain and we were particularly interested in the long-time behavior of how spin excitations are transported across the system. In this long-time limit, quantum many-body systems enter a hydrodynamic regime and transport is described by equations that describe classical fluids.

“We were interested in a particular regime where something called super-diffusion occurs due to the underlying physics being governed by something called the Kardar-Parisi-Zhang equation. This is an equation which typically describes the stochastic growth of a surface or interface like how the height of snow grows during a snowstorm, how the stain of a coffee cup on cloth grows with time, or how a fluff fire grows. The propagation is known to give super diffusive transport.”

“This is transport which becomes faster as you increase the system size. It is amazing that the same equations that govern these phenomena crop up in quantum dynamics and we were able to use the quantum computer to verify that. This was the main achievement of the work.”

IBM-Trinity predoctoral scholar Nathan Keenan, who programmed the device as part of the project tells us of some of the challenges to program quantum computers.

“The biggest problem with programming quantum computers, is performing useful calculations in the presence of noise,” he said. “The operations performed at the chip-level are imperfect, and the computer is very sensitive to disturbances from its laboratory environment. As a result, you want to minimize the runtime of a useful program, as this will shorten the time in which these errors and disturbances can occur and affect your result.”

Juan Bernabé-Moreno, Director of IBM Research UK & Ireland, said, “IBM has a long history of advancing quantum computing technology, not only by bringing decades of research but also by providing the largest and most extensive commercial quantum program and ecosystem. Our collaboration with Trinity College Dublin, through the MSc for Quantum Science and Technology and Ph.D. program, exemplifies this and I am delighted that it is already delivering promising results.”

More information: Nathan Keenan et al, Evidence of Kardar-Parisi-Zhang scaling on a digital quantum simulator, npj Quantum Information (2023). DOI: 10.1038/s41534-023-00742-4

Provided by Trinity College Dublin 

by RIKEN

A mysterious magnetic effect that causes the path that electrons take through a material to bend—called the anomalous Hall effect—has been elucidated in a new mathematical analysis by two RIKEN physicists. Their work has been published in the journal Physical Review B.

First discovered nearly a century and a half ago by American physicist Edwin Hall, the conventional Hall effect is a well-understood electrical and magnetic phenomenon. When just an electric field is applied to a conducting material, the electrons will move in a straight line that is parallel to that field. But when a magnetic field is added too, it causes the electrons’ path to curve.

The anomalous Hall effect is a related phenomenon that happens in some magnetic materials. In this case, no external magnetic field needs to be applied since the material supplies the magnetic field.

But the cause of the anomalous Hall effect seems to vary between materials. “The difficulty is that there are many possible mechanisms but no unifying explanation,” says Hiroki Isobe of the RIKEN Center for Emergent Matter Science, who co-authored the analysis with RIKEN colleague Naoto Nagaosa. “This makes it very complicated, even for specialists.”

More information: Hiroki Isobe et al, Anomalous Hall effect from a non-Hermitian viewpoint, Physical Review B (2023). DOI: 10.1103/PhysRevB.107.L201116

Journal information: Physical Review B 

Provided by RIKEN 

by Chinese Academy of Sciences

For years, capturing detailed three-dimensional images of complex biological specimens has posed a major challenge due to their intricate composition and the multiple scattering of light. A game-changing moment has arrived, as scientists from the MIT Laser Biomedical Research Center and the Chinese University of Hong Kong have introduced an innovative method called speckle diffraction tomography (SDT).

In a remarkable feat, the research team harnessed the power of SDT to produce high-resolution images of thick biological samples, achieving an impressive lateral resolution of approximately 500 nanometers and an axial resolution of 1 micrometer using a reflection-based full-field optical setup. The significance of this achievement lies in the capability of SDT to reveal even the tiniest height variations on tissue surfaces, down to the nanometer scale.

The SDT method leverages dynamic speckle-field interferometry and a low-coherence light source to mitigate the unwanted multiple-scattering and out-of-focus signals, offering remarkable optical sectioning in full-field imaging. Combined with an advanced inverse scattering model that takes specimen-induced distortions into consideration, this unique spatiotemporal gating mechanism facilitates mapping of refractive index distributions in multi-layered tissue specimens.

The power of SDT is highlighted through simulations showcasing its performance, including spatial frequency coverage and resolution at various depths. Moreover, the researchers have developed a 3D deconvolution algorithm that improves spatial resolution by almost 30%, elevating the precision of the technique even further.

In essence, SDT now makes it possible to capture detailed label-free images of thick biological specimens, boasting an unparalleled lateral resolution of approximately 500 nanometers and an axial resolution of around 1 micrometer, all within a reflection-based full-field optical setup. This revolutionary achievement sets the stage for unprecedented 3D label-free in vivo imaging applications that were once deemed unfeasible.

The capabilities of SDT were put to the test in imaging red blood cells and quantifying their membrane fluctuations behind a challenging turbid medium, spanning 2.8 scattering mean-free-paths. SDT’s high-resolution and full-field quantitative imaging capabilities were pivotal in enabling this breakthrough.

Additionally, the researchers accomplished volumetric imaging of the cornea within an ex vivo rat eye, shedding light on its optical properties. Notably, SDT unveiled nanoscale topographic features of Dua’s and Descemet’s membranes, an uncharted territory until now.

The results of this research are published in Light: Science & Applications.

More information: Sungsam Kang et al, Mapping nanoscale topographic features in thick tissues with speckle diffraction tomography, Light: Science & Applications (2023). DOI: 10.1038/s41377-023-01240-0

Journal information: Light: Science & Applications 

Provided by Chinese Academy of Sciences 

by University of Innsbruck

Researchers from Austria and the U.S. have designed a new type of quantum computer that uses fermionic atoms to simulate complex physical systems. The processor uses programmable neutral atom arrays and is capable of simulating fermionic models in a hardware-efficient manner using fermionic gates.

The team led by Peter Zoller demonstrated how the new quantum processor can efficiently simulate fermionic models from quantum chemistry and particle physics. The paper is published in the journal Proceedings of the National Academy of Sciences.

Fermionic atoms are atoms that obey the Pauli exclusion principle, which means that no two of them can occupy the same quantum state simultaneously. This makes them ideal for simulating systems where fermionic statistics play a crucial role, such as molecules, superconductors and quark-gluon plasmas.

“In qubit-based quantum computers extra resources need to be dedicated to simulate these properties, usually in the form of additional qubits or longer quantum circuits,” explains Daniel Gonzalez Cuadra from the research group led by Zoller at the Institute for Quantum Optics and Quantum Information (IQOQI) of the Austrian Academy of Sciences (ÖAW) and the Department of Theoretical Physics at the University of Innsbruck, Austria.

Quantum information in fermionic particles

A fermionic quantum processor is composed of a fermionic register and a set of fermionic quantum gates. “The register consists on a set of fermionic modes, which can be either empty or occupied by a single fermion, and these two states form the local unit of quantum information,” says Gonzalez Cuadra. “The state of the system we want to simulate, such as a molecule composed of many electrons, will be in general a superposition of many occupation patterns, which can be directly encoded into this register.”

This information is then processed using a fermionic quantum circuit, designed to simulate for example the time evolution of a molecule. Any such circuit can be decomposed into a sequence of just two types of fermionic gates, a tunneling and an interaction gate.

The researchers propose to trap fermionic atoms in an array of optical tweezers, which are highly focused laser beams that can hold and move atoms with high precision. “The required set of fermionic quantum gates can be natively implemented in this platform: tunneling gates can be obtained by controlling the tunneling of an atom between two optical tweezers, while interaction gates are implemented by first exciting the atoms to Rydberg states, carrying a strong dipole moment,” says Gonzalez Cuadra.

Quantum chemistry to particle physics

Fermionic quantum processing is particularly useful to simulate the properties of systems composed of many interacting fermions, such as electrons in a molecule or in a material, or quarks inside a proton, and has therefore applications in many fields, ranging from quantum chemistry to particle physics. The researchers demonstrate how their fermionic quantum processor can efficiently simulate fermionic models from quantum chemistry and lattice gauge theory, which are two important fields of physics that are hard to solve with classical computers.

“By using fermions to encode and process quantum information, some properties of the simulated system are intrinsically guaranteed at the hardware level, which would require additional resources in a standard qubit-based quantum computer,” says Gonzalez Cuadra.

“I am very excited about the future of the field, and I would like to keep contributing to it by identifying the most promising applications for fermionic quantum processing, and by designing tailored algorithms that can run in near-term devices.”

More information: D. González-Cuadra et al, Fermionic quantum processing with programmable neutral atom arrays, Proceedings of the National Academy of Sciences (2023). DOI: 10.1073/pnas.2304294120

Journal information: Proceedings of the National Academy of Sciences 

Provided by University of Innsbruck 

by Monash University

“Trimming” the edge-states of a topological insulator yields a new class of material featuring unconventional “two-way” edge transport, as reported in a new theoretical study from Monash University, Australia, published in Materials Today Physics.

The new material, a topological crystalline insulator (TCI) forms a promising addition to the family of topological materials and significantly broadens the scope of materials with topologically nontrivial properties.

Its distinctive reliance on symmetry also paves the way for novel techniques to manipulate edge transport, offering potential applications in future transistor devices. For example, “switching” the TCI via an electric field that breaks the symmetry supporting the nontrivial band topology, thus suppressing the edge current.

This discovery significantly advances our fundamental understanding of how spin currents travel in topological materials, providing valuable insights into the behavior of these intriguing systems.

Challenging the common definition of topological insulators

Let’s begin by quoting the elegant definition of topological insulators according to the vision of FLEET: “Topological insulators conduct electricity only along their edges, and strictly in one direction. This one-way path conducts electricity without loss of energy due to resistance.”

However this new theoretical study, conducted by the computational group at Monash University, challenges that standard topological-physics view by uncovering a new type of edge transport, which prompts reconsideration of the phrase “strictly in one direction.”

Modifying this phrase is not a simple task. The topological material is akin to a large tree rooted in the solid soil of “bulk-edge correspondence,” meaning that the intrinsic properties of the bulk will dictate the nature of the edge current.

Just as a tree requires pruning to maintain its shape and health, the edge states of a topological material also need to be tailored to adapt towards various applications in electronics and spintronics.

The research team successfully achieved the objective of extracting a new type of edge spin current in a 2D topological material, planar bismuthine, by proposing a novel method to manipulate edge states through bulk-edge interactions, similar to the pruning work done in gardening routines.

Unconventional spin texture hidden in the symmetry-protected topology

The newly discovered material, named a topological crystalline insulator (TCI), stands as a promising addition to the family of topological materials, operating on the principle that conducting edge currents remain resilient as long as specific crystalline symmetries exist within the bulk.

The discovery of TCI significantly broadens the scope of materials with topologically nontrivial properties. Its distinctive reliance on symmetry also paves the way for novel techniques to manipulate edge transport, offering potential applications in transistor devices.

For instance, by subjecting TCI to a strong electric field, the edge current can be suppressed when the symmetry supporting the nontrivial band topology is broken. Once the field is removed, the conducting edge currents promptly return, showcasing TCI’s advantageous on-demand switch property, ideal for integration into transistor devices.

Beyond offering an alternative form of topological protection, the exciting potential of TCI goes further. The research team has uncovered an unconventional type of spin transport hidden within the edge of two-dimensional TCI bismuthene, a phenomenon previously overlooked in prior reports.

“While the common belief is that TCI exhibits the same edge transport mode observed in topological insulators, where each stream of spin current (spin-up or spin-down) strictly travels in one direction, our findings reveal that TCI planar bismuthene hosts a new type of spin transport protected by mirror symmetry,” explains lead author Dr. Yuefeng Yin, a research fellow at Monash.

“In this mode, the spin current is no longer confined to fixed directions along the edge.”

This new-found spin transport mode unlocks innovative design concepts for topological devices, enabling support for “both pure charge current without net spin transport, and pure spin currents without net charge transport”—a possibility not comprehensible in conventional understanding of topological materials.

“This discovery opens up a new path toward achieving FLEET’s goal of creating low-energy-consuming electronic devices,” adds corresponding author Prof. Nikhil Medhekar, also affiliated with Monash.

“While identical spin-polarized streams traveling in opposing directions may not seem immediately useful, they offer new opportunities for spin manipulation that are otherwise inaccessible in other topological materials.”

The research team anticipates that this computational breakthrough will inspire further follow-up studies, both experimental and theoretical, to fully harness the potential of this novel edge transport in electronic and spintronic applications.

Extracting the spin current with bulk-edge interactions

Following the discovery of an unconventional spin texture in 2D TCI planar bismuthene, the research team’s objective is to extract the exotic spin currents from the entangled edge bands by utilizing bulk-edge interactions.

The term “bulk-edge interactions” refers to employing various tuning strategies, such as applying external electric fields and substrate potentials, to selectively adjust the alignment between the bulk and edge bands while preserving the bulk band topology.

“By carefully choosing the tuning factors, we can isolate specific branches of edge states from the original entangled configuration,” explains Dr. Yin.

“This is crucial for further investigating the unconventional spin texture we have identified. Another advantage of this approach is that we can retain the protection offered by the intact bulk-edge correspondence.”

Through the use of a large external electric field and weak substrate potential, the research team can isolate the unconventional spin texture within the edge, effectively concealing the conventional spin transport components in the bulk.

Moreover, these bulk-edge interactions allow for the existence of conducting edge channels even under the influence of a large external electric field, in contrast to the common understanding that applying an electric field opens a band gap in the edge region.

The research team has also demonstrated the ability to revert the edge region back to a fully conventional spin transport setup, akin to what is observed in topological insulators, by applying substrate potentials to selective orbitals.

Prof. Medhekar remarks “This is a truly remarkable finding. Not only have we uncovered a new type of edge spin texture in topological materials, but we have also demonstrated an effective way to manipulate and preserve it while maintaining the rigorous bulk-edge topology.”

The research team anticipates that these innovative “topological gardening techniques” can be extended to other topological systems, offering efficient and flexible means to manipulate edge currents.

More information: Yuefeng Yin et al, Extracting unconventional spin texture in two dimensional topological crystalline insulator bismuthene via tuning bulk-edge interactions, Materials Today Physics (2023). DOI: 10.1016/j.mtphys.2023.101168

Qile Li et al, Localized Wannier function based tight-binding models for two-dimensional allotropes of bismuth, New Journal of Physics (2021). DOI: 10.1088/1367-2630/ac04c9

Journal information: New Journal of Physics 

Provided by Monash University 

by University of Science and Technology of China

A team led by Prof. He Junfeng from University of Science and Technology of China (USTC) of the Chinese Academy of Sciences (CAS), in collaboration with Academician Gao Hongjun’s team from CAS and other domestic and international research teams, has discovered the van Hove singularity (VHS) at Fermi level in kagome superconductors and revealed its relationship with superconductivity. Their work was published in Nature Communications on June 28.

VHS is a saddle point connecting electron-like and hole-like energy bands that can generate divergent electron density of states. On the one hand, the luge electron density of states near VHS can cause strong instability in the electronic structure. On the other hand, hole-like and electron-like conduction can coexist at VHS, giving rise to unconventional electronic pairing.

However, in reality, VHS tends to deviate from the Fermi level of the material, resulting in very little effect on the low-energy states of the material. Therefore, it is important to find the suitable kagome material to explore the effect of VHS on superconductivity.

The researchers investigated Ta-doped CsV₃Sb₅ samples, and the use of Ta atoms instead of V atoms can increase the superconducting transition temperature from 2.5 K in CsV₃Sb₅ to 5.5 K in CsV_3-xTa_xSb₅ (x~0.4). Angle-resolved photoemission spectroscopy was used to investigate the electronic structures of both CsV₃Sb₅ and Ta-doped CsV_3-xTa_xSb₅ samples.

The results showed that the VHS in CsV₃Sb₅ lies below the Fermi level before entering the superconducting state due to the reconfiguration of energy bands by the electron density wave and contributes almost nothing to superconductivity, whereas in CsV_3-xTa_xSb₅, the VHS is located exactly at the Fermi level, in agreement with first-principles calculations.

Further experiments demonstrated that there is a strong correlation between the superconducting transition temperature and the energy position of the VHS relative to the Fermi energy level, revealing the feasibility of VHS-enhanced superconductivity in kagome superconductors.

In addition, the researchers found that the superconducting state in CsV_3-xTa_xSb₅ has significantly different characteristics from the superconducting state in CsV₃Sb₅ through scanning tunneling microscopy experiments, indicating the possibility of unconventional pairing superconductivity in the van Hove scenario.

More information: Yang Luo et al, A unique van Hove singularity in kagome superconductor CsV3-xTaxSb5 with enhanced superconductivity, Nature Communications (2023). DOI: 10.1038/s41467-023-39500-7

Journal information: Nature Communications 

Provided by University of Science and Technology of China

by University of Science and Technology of China

A team led by Prof. He Junfeng from University of Science and Technology of China (USTC) of the Chinese Academy of Sciences (CAS), together with domestic and international collaborators, discovered that the energy level of the van Hove singularity (VHS) in the novel Ti-based kagome metal CsTi₃Bi₅ can be tuned without lattice structural instability. Their work was published in Physical Review Letters on July 12 as the cover article.

In kagome metal, the electronic instabilities provided by VHS usually coexist with lattice structural instabilities, making it difficult to distinguish the effect of different instabilities on charge density waves (CDW).

To understand the effect of different instabilities on CDW, the team conducted research on the novel Ti-based kagome metal CsTi₃Bi₅. CsTi₃Bi₅ has the same lattice structure as AV₃Sb₅ (A=K, Rb, Cs) but does not have charge density wave states.

The team first studied the electronic structure of CsTi₃Bi₅ using high-resolution angle-resolved photoemission spectroscopy and found it in good agreement with the results of first-principles calculations. The energy position of the VHS in pristine CsTi₃Bi₅ was well above the Fermi level and cannot bring about electronic instability. First-principles calculations and low-temperature X-ray diffraction measurements showed no lattice instability in CsTi₃Bi₅.

Researchers went on to find that electrons can be introduced by Cs surface doping, enabling the modulation of the VHS in CsTi₃Bi₅ in a wide energy range. When the VHS approaches Fermi level, it can generate electronic instabilities. At the same time, first-principles calculations showed no lattice structural instability in CsTi₃Bi₅ after electron doping. In this way, the team realized the decoupling of the two instabilities. CsTi₃Bi₅ can be a unique platform where the electronic instabilities can be modulated solely without being affected by the structural instabilities.

The researchers also found that even if the VHS is tuned to introduce electronic instability near the Fermi energy level, it still can’t generate the energy gap in CDW in CsTi₃Bi₅. Thus, the electronic instability itself can’t generate charge density waves in CsTi₃Bi₅.

First-principles calculations further showed that during the evolution from CsV₃Sb₅ to CsTi₃Bi₅, the appearance of CDW directly corresponds to the change in the total energy of the system. CDW phase transition occurs only when the corresponding crystal structure has the lowest total energy. Therefore, lattice structural instability plays an important role in CDW phase transition in kagome metals.

This work is instructive for the further understanding of the effects of electronic instability and lattice structural instability on CDW in kagome metals.

More information: Bo Liu et al, Tunable Van Hove Singularity without Structural Instability in Kagome Metal CsTi3Bi5, Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.131.026701

Journal information: Physical Review Letters 

Provided by University of Science and Technology of China

by Alison Hatt, Lawrence Berkeley National Laboratory

The prospect of a quantum internet, connecting quantum computers and capable of highly secure data transmission, is enticing, but making it poses a formidable challenge. Transporting quantum information requires working with individual photons rather than the light sources used in conventional fiber optic networks.

To produce and manipulate individual photons, scientists are turning to quantum light emitters, also known as color centers. These atomic-scale defects in semiconductor materials can emit single photons of fixed wavelength or color and allow photons to interact with electron spin properties in controlled ways.

A team of researchers has recently demonstrated a more effective technique for creating quantum emitters using pulsed ion beams, deepening our understanding of how quantum emitters are formed. The work was led by Department of Energy Lawrence Berkeley National Laboratory (Berkeley Lab) researchers Thomas Schenkel, Liang Tan, and Boubacar Kanté who is also an associate professor of electrical engineering and computer sciences at the University of California, Berkeley.

The results appeared in Physical Review Applied and are part of a larger effort by the team to identify the best quantum defect emitters for processing and transporting quantum information and to produce them with precision.

“The color centers we’re making are candidates for becoming the backbone of a quantum internet and a key resource for scalable quantum information processing,” said Schenkel, a senior scientist in Berkeley Lab’s Accelerator Technology & Applied Physics (ATAP) Division. “They could support linking quantum-computing nodes for scalable quantum computing.”

In this work, the team targeted the fabrication of a specific type of color center in silicon comprising two substitutional carbon atoms and a slightly dislodged silicon atom. The conventional method of producing the defects is to hit the silicon with a continuous beam of high-energy ions; however, the researchers discovered that a pulsed ion beam is significantly more efficient, producing many more of the desired color centers.

“We were surprised to find these defects can be more easily generated with pulsed ion beams,” said Wei Liu, a postdoctoral scholar in ATAP and first author of the publication. “Right now, industry and academia mainly use continuous beams, but we’ve demonstrated a more efficient approach.”

The researchers believe that the transient excitations created by the pulsed beam, where the temperature and system energetics change rapidly, are key to the more efficient color center formation, which they established through an earlier study using pulsed ion beams from a laser-driven accelerator published in Communications Materials.

The team characterized the color centers at cryogenic temperatures using highly sensitive near-infrared detectors to probe their optical signals. They found that the intensity of the ion beam used to create the color centers changed the optical properties of the photons they emitted.

Large-scale computer simulations on the Perlmutter system at the National Energy Research Scientific Computing Center (NERSC) provided further insight into the discovery, revealing that the wavelength of emitted photons is sensitive to strain in the crystal lattice.

“First-principles electronic structure calculations have become the go-to method for understanding defect properties,” added Vsevolod Ivanov, a postdoctoral scholar at the Molecular Foundry and co-first author of the publication. “We have reached the point where we can predict how a defect behaves, even in complex environments.”

The findings also suggest a new application for quantum emitter color centers as sensors for radiation.

“It opens new directions,” said Tan, a staff scientist at Berkeley Lab’s Molecular Foundry. “We can form this color center by just hitting silicon with a proton. We could potentially use that as a dark-matter or neutrino detector with directionality because we see these different strain fields depending on which way the radiation came.”

With this deeper understanding of quantum emitter formation and properties, the team continues to expand its exploration of color centers. Ongoing work includes generating a database of color centers predicted to exist in silicon, using computer simulations to identify those best suited to quantum computing and networking applications, and refining fabrication techniques to gain deterministic control over creating individual color centers.

“We’re working towards a new paradigm of qubits by design,” said Kanté. “Can we reliably make a given color center that operates in the telecom band, has sufficient brightness, isn’t too hard to make, has a memory, etcetera? We’re engaged in that quest and have demonstrated some exciting progress.”

“The new pathways to forming color centers using intense beams uncovered in this work are an exciting application of high energy density conditions and plasma science to improving technologies for quantum information science,” said ATAP Division Director Cameron Geddes.

More information: Wei Liu et al, Quantum Emitter Formation Dynamics and Probing of Radiation-Induced Atomic Disorder in Silicon, Physical Review Applied (2023). DOI: 10.1103/PhysRevApplied.20.014058

Provided by Lawrence Berkeley National Laboratory