Category: Physics

Quark gluon plasma (QGP) is an exciting state of matter that scientists create in a laboratory by colliding two heavy nuclei. These collisions produce a QGP fireball. The fireball expands and cools following the laws of hydrodynamics, which govern how fluids behave in various conditions. Eventually, subatomic particles (protons, pions, and other hadrons, or particles made up of two or more quarks) emerge and are observed and counted by detectors surrounding the collision.

Fluctuations in the number of these particles from collision to collision carry important information about the QGP. However, extracting this information from what scientists can observe is a difficult task. An approach called the maximum entropy principle provides a crucial connection between these experimental observations and the hydrodynamics of the QGP fireball.

The approach is described in the journal Physical Review Letters.

As a QGP fireball expands and cools, it eventually becomes too diluted to be described by hydrodynamics. At this stage the QGP has “hadronized.” This means its energy and other quantum properties are carried by hadrons. These are subatomic particles such as protons, neutrons, and pions that are made up of quarks. The hadrons “freeze out”—they freeze information about the final hydrodynamic state of the QGP fireball, allowing the particles streaming from the collision to carry this information to the detectors in an experiment.

The research provides a tool for using simulations to compute observable fluctuations in the QGP. This has allowed the researchers, from the University of Illinois, Chicago, to use freeze-out to identify hints of a critical point between a QGP fireball and a gaseous hadronized state. This critical point is one of scientists’ unresolved questions about quantum chromodynamics, the theory of the strong gluon-driven interactions between quarks.

Fluctuations in the QGP carry information about the region of the QCD phase diagram where the collisions “freeze out.” This makes connecting fluctuations in hydrodynamics to fluctuations of the observed hadrons a crucial step in translating experimental measurements into the map of the QCD phase diagram. Large event-by-event fluctuations are telltale experimental signatures of the critical point.

Data from the Run-I Beam Energy Scan (BES) program at the Relativistic Heavy-Ion Collider (RHIC) hint at the presence of the critical point. To follow this hint, the researchers proposed a novel and universal approach to converting hydrodynamic fluctuations into fluctuations of hadron multiplicities.

The approach elegantly overcomes challenges faced by previous attempts to solve this problem. Crucially, the new approach based on the maximum-entropy principle preserves all the information about the fluctuations of conserved quantities described by hydrodynamics. The novel freeze-out procedure will find applications in the theoretical calculations of event-by-event fluctuations and correlations observed in experiments such as the Beam Energy Scan program at RHIC aimed at mapping the QCD phase diagram.

More information: Maneesha Sushama Pradeep et al, Maximum Entropy Freeze-Out of Hydrodynamic Fluctuations, Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.130.162301

Journal information: Physical Review Letters 

Provided by US Department of Energy 

by Chinese Academy of Sciences

Near-infrared (NIR)-emitting phosphor-converted light-emitting diodes (pc-LEDs) have attracted attention in emerging technology fields such as night-vision and bio-imaging. Currently, the development of NIR-emitting pc-LEDs has encountered a bottleneck due to the lack of blue-light excitable high-performance NIR-emitting phosphor materials.

Although Cr³⁺-activated phosphors stand out among numerous NIR-emitting phosphors and has recently made progress in realizing tunable broadband emission, major issues still remain their unsatisfactory luminescence efficiency and poor thermal stability.

A₃B₂C₃O₁₂-typed garnets are considered as promising host materials that can address these issues, because their compact coordinated environment and tunable structures may provide diverse luminescence properties including the required ones.

Unfortunately, there is a trade-off between the emission wavelength and efficiency as well as thermal stability. That is, the highly efficient and thermally stable NIR luminescence generally accompanied with short emission wavelength (< 750 nm). Challenge still lies in improving luminescence efficiency and thermal stability while maintaining original longer wavelength for Cr³⁺-doped NIR-emitting garnet phosphors.

In a new paper published in Light: Science & Applications, a team of scientists, led by Professor Jun Lin from Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, and Professor Guogang Li from China University of Geosciences have reported a “kill two birds with one stone” strategy to simultaneously improve luminescence efficiency and thermal stability of the NIR-emitting Ca₃Y_2-2x(ZnZr)_xGe₃O₁₂:Cr garnet system by chemical unit co-substitution, accompanied with only slight emission shift.

The developed NIR-emitting phosphors show potential applications in information encryption, bio-tissue imaging, and night-vision. This work provides a new insight for developing high-performance NIR-emitting phosphor materials.

To achieve high luminescence efficiency and thermal stability for Cr³⁺-doped garnet phosphors, two dominant factors should be considered. One is luminescence “killer” Cr⁴⁺ that shows intensive absorption in NIR region. Another crucial factor is structural rigidity.

In this work, the authors chose Ca₃Y₂Ge₃O₁₂ with a typical garnet structure as an initial host for chromium doping. Through a cation co-substitution of [Zn²⁺–Zr⁴⁺] for [Y³⁺–Y³⁺], a series of Ca₃Y_2-2x(ZnZr)_xGe₃O₁₂:Cr NIR-emitting phosphors were synthesized using a traditional high-temperature solid-state method. The underlying luminescence optimization mechanism of this garnet system were investigated.

The structural analysis and density functional theory (DFT) calculations indicate that chromium ions are very likely to enter the Ge⁴⁺ sites of Ca₃Y₂Ge₃O₁₂ in the form of tetravalent Cr⁴⁺ ions. The coexistence of Cr³⁺ and Cr⁴⁺ is demonstrated to be responsible for the low quantum efficiency of Ca₃Y₂Ge₃O₁₂:Cr. The designed co-substitution of smaller [Zn²⁺–Zr⁴⁺] for [Y³⁺–Y³⁺] plays an expected role as reductant, which promotes the transformation from Cr⁴⁺ luminescence killers to beneficial Cr³⁺ emission centers.

This result is also demonstrated by the diffuse reflectance spectra and Cr K-edge X-ray absorption near-edge structure spectra. The valence reduction is related to the successful reconstruction of the octahedral sites for Cr³⁺ ions. Moreover, the introduction of [Zn²⁺–Zr⁴⁺] unit also contributes to a rigid crystal structure.

These two aspects together achieve the simultaneous high internal quantum efficiency of 96% and excellent thermal stability of 89% at 423 K, which almost surpasses all of the reported Cr³⁺-doped garnet phosphors within similar emission region (770–820 nm). This proves the feasibility of the designed co-substitution in optimizing luminescence properties for Cr³⁺-doped garnet phosphors.

Furthermore, benefiting from the reconstructed rigid covalent structure, the acid resistance of the phosphor is also greatly improved. Inspired by this, information encryption with “burning after reading” is achieved. Finally, the fabricated NIR-emitting pc-LED shows promising applications in bio-tissue imaging and night-vision.

This work provides a new perspective of luminescence optimization by chemical unit co-substitution, and the revealed universal mechanism could motivate further exploration of high-performance Cr³⁺-doped NIR-emitting phosphor materials.

More information: Dongjie Liu et al, Valence conversion and site reconstruction in near-infrared-emitting chromium-activated garnet for simultaneous enhancement of quantum efficiency and thermal stability, Light: Science & Applications (2023). DOI: 10.1038/s41377-023-01283-3

Journal information: Light: Science & Applications 

Provided by Chinese Academy of Sciences 

by Chinese Academy of Sciences

Exciton polaritons, hybrid quasiparticles caused by the strong exciton-photon coupling, constitute a unique prototype for studying many-body physics and quantum photonic phenomena traditionally in cryogenic conditions.

Atomically thin transition-metal dichalcogenides (TMDs), as exceptional semiconductors with room-temperature operations, have received much attention due to their fascinating valleytronics features and strong exciton resonance. Nevertheless, in TMDs microcavities, the overall nonlinear interaction strength of polaritons can be insignificant compared to that of other wide-bandgap semiconductors.

Considerable effort has been devoted to improving the nonlinear interactions, for instance, by resorting to 2s states, trion, and moiré or dipolar excitons. However, these excitons quickly dissipate at elevated temperatures and then destroy the strong coupling condition. Thus, achieving an appropriate combination of strong nonlinearity together with the thermal stability of the TMDs polaritons is highly sought after for realistic polariton-based integrated devices.

In a recent paper published in Light: Science & Applications, a team of scientists, led by Professor Qihua Xiong from the State Key Laboratory of Low-Dimensional Quantum Physics, Department of Physics, Tsinghua University, China, Beijing Academy of Quantum Information Sciences, China, and co-workers have presented fully deterministic potential wells via the lithographic mesas to trap polaritons through the photonic component in a monolayer WS₂ microcavity.

Experimentally, their mesa cavities show the discretization of photoluminescence dispersions and spatially-confined patterns, unambiguously demonstrating the deterministic on-site trapping effect. More interestingly, they have systematically studied the polariton nonlinearity under such cavities by non-resonant power-dependent measurements and found that the polariton-exciton interaction dominates the observed spectral shift, which can be increased by about six times through improving spatial confinement at room temperature.

Meanwhile, the coherence of trapped polaritons is significantly improved due to the spectral narrowing and tailored in a picosecond range.

Therefore, these results prove a convenient method based on the programmed micro-nano fabrication to achieve controllable nonlinearity and coherence of polaritons in TMD at room temperatures, opening new avenues for future polariton-based integrated devices, such as polariton modulators, polariton quantum sources, and quantum neural networks.

The scientists summarize the innovation and significance of their work:

“Employing the artificial mesa cavities to manipulate interacting exciton polaritons has three prominent advantages. First, the approach allows to operate at ambient conditions, which is highly sought after for realistic polariton-based integrated devices. Second, the mesa cavities will confine polaritons through their photonic part instead of the excitonic part, which is more practical considering the extremely small Bohr radius and a sub-micrometer transport length of excitons.”

“Last, the utilization of mesa cavities enables us to realize fully deterministic potential wells rather than random traps probably induced by strain or air gaps involved in sample preparation.”

“This work indicates the feasibility of controlling polariton properties in TMDs microcavities by engineering artificial potential wells and establishes the foundation for simulating the polariton Hamiltonian with further complex potential landscapes and realizing integrated polaritonic devices at room temperature,” the scientists say.

More information: Yuan Luo et al, Manipulating nonlinear exciton polaritons in an atomically-thin semiconductor with artificial potential landscapes, Light: Science & Applications (2023). DOI: 10.1038/s41377-023-01268-2

Journal information: Light: Science & Applications 

Provided by Chinese Academy of Sciences 

by Carmen Rotte, Max Planck Society

Electron microscopes provide unique vistas of nanoscale structures, but their resolution is limited by the mutual repulsion of electrons. Researchers in Göttingen have now succeeded in precisely measuring the influence of these interactions. They discovered an “energetic fingerprint” in which the distribution of the electrons’ velocities is characteristic of their respective numbers. This finding has enabled the team to develop a method that could increase the performance of established electron microscopes and open up a new interface between electron microscopy and quantum technology.

Our understanding of nanoscale phenomena largely rests on the performance of modern microscopy. For example, transmission electron microscopes routinely achieve atomic resolution nowadays. In these microscopes, electrons are sent through an object under investigation to obtain an image—in some analogy to a light microscope. Thereby, electron microscopes can visualize molecular structures, the atomic ordering in solids, and the shape of nanoparticles.

However, the contrast and resolution of electron microscopes is limited, among other things, by interactions between electrons: when two electrons come close to each other, they mutually repel due to the Coulomb force. This limits the maximum usable brightness of an electron beam. Researchers led by Claus Ropers, director at the Max Planck Institute (MPI) for Multidisciplinary Sciences, have now resolved and analyzed the repulsion between individual electrons in the microscope for the first time. Using the new insights, they developed methods that make use of this interparticle repulsion.

Counted electrons

“Electrons in a beam are randomly distributed. Therefore, one cannot control the inaccuracies introduced by Coulomb forces,” says Rudolf Haindl, first author of the study recently published in Nature Physics.

But when the physicists use a laser to generate electrons in the form of ultrashort pulses, they also create packets with exactly two, three, or four electrons. These electrons are closely confined in space and time such that they interact with each other. With the help of a spectrometer and an event-based detector, the energy exchange between electrons in a pulse becomes visible.

“Depending on how many electrons are in a pulse, the electrons repel each other to different degrees—this allowed us to determine an energetic fingerprint for the number of electrons in a pulse,” Haindl points out.

New possibilities

Based on their findings, the team developed new schemes to use the multi-electron states in electron microscopes. “We have worked out a procedure that will enable us to generate electron pulses with a fixed number of electrons in the future. This can significantly increase the performance of electron microscopes in basic research and technology applications, for example in semiconductor manufacturing,” explains Armin Feist, co-author and physicist in Ropers’ team.

Max Planck Director Ropers adds, “In addition to the implications for electron microscopy and lithography, we believe that the electrons are also quantum mechanically ‘entangled,’ tied to each other in a specific quantum way, which opens up a new interface between electron microscopy and quantum technology.”

More information: Rudolf Haindl et al, Coulomb-correlated electron number states in a transmission electron microscope beam, Nature Physics (2023). DOI: 10.1038/s41567-023-02067-7

Journal information: Nature Physics 

Provided by Max Planck Society 

by The Ohio State University

In a new breakthrough, researchers have used a novel technique to confirm a previously undetected physics phenomenon that could be used to improve data storage in the next generation of computer devices.

Spintronic memories, like those used in some high-tech computers and satellites, use magnetic states generated by an electron’s intrinsic angular momentum to store and read information. Depending on its physical motion, an electron’s spin produces a magnetic current. Known as the “spin Hall effect,” this has key applications for magnetic materials across many different fields, ranging from low power electronics to fundamental quantum mechanics.

More recently, scientists have found that electrons are also capable of generating electricity through a second kind of movement: orbital angular momentum, similar to how Earth revolves around the sun. This is known as the “orbital Hall effect,” said Roland Kawakami, co-author of the study and a professor in physics at The Ohio State University.

Theorists predicted that by using light transition metals—materials that have weak spin Hall currents—magnetic currents generated by the orbital Hall effect would be easier to spot flowing alongside them. Until now, directly detecting such a thing has been a challenge, but the study, led by Igor Lyalin, a graduate student in physics, and published today in the journal Physical Review Letters, showed a method to observe the effect.

“Over the decades, there’s been a continuous discovery of various Hall effects,”‘ said Kawakami. “But the idea of these orbital currents is really a brand new one. The difficulty is that they are mixed with spin currents in typical heavy metals and it’s difficult to tell them apart.”

Instead, Kawakami’s team demonstrated the orbital Hall effect by reflecting polarized light, in this case, a laser, onto various thin films of the light metal chromium to probe the metal’s atoms for a potential build-up of orbital angular momentum. After nearly a year of painstaking measurements, researchers were able to detect a clear magneto-optical signal which showed that electrons gathered at one end of the film exhibited strong orbital Hall effect characteristics.

This successful detection could have huge consequences for future spintronics applications, said Kawakami.

“The concept of spintronics has been around for about 25 years or so, and while it’s been really good for various memory applications, now people are trying to go further,” he said. “Now, one of the field’s biggest goals is to reduce the amount of energy consumed because that’s the limiting factor for jacking up performance.”

Lowering the total amount of energy needed for future magnetic materials to operate well could potentially enable lower power consumption, higher speeds and higher reliability, as well as help to extend the technology’s lifespan. Utilizing orbital currents instead of spin currents could possibly save both time and money in the long term, said Kawakami.

Noting that this research opens up a way to learn more about how these strange physics phenomena arise in other kinds of metals, the researchers say they want to continue delving into the complex connection between spin Hall effects and orbital Hall effects.

More information: Igor Lyalin et al, Magneto-Optical Detection of the Orbital Hall Effect in Chromium, Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.131.156702

Journal information: Physical Review Letters 

Provided by The Ohio State University 

by Bob Yirka , Phys.org

A team of atmospheric scientists, chemists and infectious disease specialists at the Max Planck Institute for Chemistry, working with colleagues from the Max Planck Institute for Dynamical Systems, the University of Denver, Georg August University and St. Petersburg State University, has embarked on an effort to collate publicly available information on droplet properties, such as the way they are distributed by size, their composition, and the ways they are emitted, as a means of helping to develop mitigation strategies for fighting infectious agents.

In their paper published in the journal Reviews of Modern Physics, the group describes their collating process and why they believe it could help fight non-contact infectious diseases.

In the early days of the pandemic, as people around the world locked themselves inside their residences, scientists, including those not in the medical field, looked for ways to help. One such pair of researchers, Christopher Pöhlker, an atmospheric scientist, and his wife, Mira, a cloud scientist, began to wonder about the nature of droplet size—something related to both their fields of work.

After getting online and doing some searching, they found little research had been done regarding respiratory droplet size as it relates to airborne disease transmission. That led them to begin a research effort of their own that involved gathering known information and collating it in a way that might prove useful to traditional medical researchers. To that end, they joined up with specialists in other fields to form a team with the goal of parameterizing droplets involved in respiratory infections such as COVID-19.

The team first searched for available information regarding infectious droplet size. They then embarked on a mission to create a parameterization scheme that would collate the data. To that end, they created a classification system based on what they describe as modes, where different modes are based on the size of droplets created in various parts of the body.

They ultimately defined five types in all, each described by its size (from less than 0.2 µm to 130 µm), rather than a name. Each was also classified by the location where it was created: in the lungs, the mouth, tongue or lips and the larynx–trachea. The researchers also left space for data that correlates droplet size with infection potential—specifics that are still not known.

The team concluded by suggesting that human studies will have to be conducted to fully complete their collating process, which they suggest should, when complete, provide medical researchers with a valuable resource as they continue to look for ways to develop anti-transmission measures to combat infectious diseases.

More information: Mira L. Pöhlker et al, Respiratory aerosols and droplets in the transmission of infectious diseases, Reviews of Modern Physics (2023). DOI: 10.1103/RevModPhys.95.045001

Journal information: Reviews of Modern Physics 

© 2023 Science X Network

by Ingrid Fadelli , Phys.org

Superconductivity is the ability of some materials to conduct a direct electrical current (DC) with almost no resistance. This property is highly sought after and favorable for various technological applications, as it could boost the performance of different electronic and energy devices.

In recent years, condensed-matter physicists and material scientists have been trying to identify strategies to enhance the superconductivity of specific materials. This includes the material K₃C₆₀, an organic superconductor that has been found to enter a phase characterized by zero resistance when mid-infrared optical pulses are applied to it.

Researchers at the Max Planck Institute for the Structure and Dynamics of Matter, Università degli Studi di Parma and University of Oxford have now identified a strategy to enhance the light-induced superconductivity of K₃C₆₀. This strategy, outlined in Nature Physics, has so far yielded very promising results, increasing the photo-susceptibility of this superconducting material by two orders of magnitude.

“We have been exploring for approximately a decade the possibility of using light to enhance superconductivity starting form an equilibrium state at base temperature above Tc,” Andrea Cavalleri, one of the researchers who carried out the study, told Phys.org. “We have shown that this works in some cuprates, in certain charge transfer salts and in K₃C₆₀.”

“In this paper, we have explored the mechanism underlying the K₃C₆₀ optically induced superconductivity using a special optical source that is much more tunable than that used previously, reaching 10 THz frequency.”

Cavalleri and his research team have been exploring the superconductivity of K₃C₆₀ for a few years now. In their previous experiments, they were able to realize the superconducting phase of this material with excitation photon energies ranging between 80 and 165 meV (20–40THz).

In their new study, they set out to explore excitation in the material at lower energies between 24 and 80 meV (6–20 THz), using a strategy that was previously inaccessible to them. The researchers achieved this using a terahertz source that generates narrow-bandwidth pulses by combining the near-infrared signal beams of two distinct phase-locked optical parametric amplitudes.

“The underlying physics is not yet clear, but the experiment targets selected molecular vibrations that are driven directly to large amplifies at their resonance frequency,” Cavalleri said. “The driven vibrations appear to couple with the electronic states and enhance the pairing and coherence that give rise to superconductivity. The present paper shows that this effect works especially well at 10 THz, where a certain molecular vibration is found.”

The recent work by Cavalleri and his collaborators sheds some new light on the possible mechanisms underpinning photo-induced superconductivity in K₃C₆₀ and potentially other superconductors. In addition, it introduces a strategy that could help to prolong photo-induced superconductivity for longer periods of time, which could have interesting implications for the development of light-driven quantum technologies.

“We realized a 10 ns long lived superconducting state at room temperature,” Cavalleri added. “In principle, this could be used for future quantum devices powered by light. We want to study the properties of this transient state, especially magnetic properties and we are going to try to compare the properties of the photo-induced phase to those of equilibrium SC.”

More information: E. Rowe et al, Resonant enhancement of photo-induced superconductivity in K3C60, Nature Physics (2023). DOI: 10.1038/s41567-023-02235-9.

Journal information: Nature Physics 

by Tohoku University

A collaborative group of researchers has manipulated the behavior of light as if it were under the influence of gravity. The findings, which were published in the journal Physical Review A on September 28, 2023, have far-reaching implications for the world of optics and materials science, and bear significance for the development of 6G communications.

Albert Einstein’s theory of relativity has long established that the trajectory of electromagnetic waves—including light and terahertz electromagnetic waves—can be deflected by gravitational fields.

Scientists have recently theoretically predicted that replicating the effects of gravity—i.e., pseudogravity—is possible by deforming crystals in the lower normalized energy (or frequency) region.

“We set out to explore whether lattice distortion in photonic crystals can produce pseudogravity effects,” said Professor Kyoko Kitamura from Tohoku University’s Graduate School of Engineering.

Photonic crystals possess unique properties that enable scientists to manipulate and control the behavior of light, serving as “traffic controllers” for light within crystals. They are constructed by periodically arranging two or more different materials with varying abilities to interact with and slow down light in a regular, repeating pattern. Furthermore, pseudogravity effects due to adiabatic changes have been observed in photonic crystals.

Kitamura and her colleagues modified photonic crystals by introducing lattice distortion; gradual deformation of the regular spacing of elements, which disrupted the grid-like pattern of protonic crystals. This manipulated the photonic band structure of the crystals, resulting in a curved beam trajectory in-medium—just like a light-ray passing by a massive celestial body such as a black hole.

Specifically, they employed a silicon distorted photonic crystal with a primal lattice constant of 200 micrometers and terahertz waves. Experiments successfully demonstrated the deflection of these waves.

“Much like gravity bends the trajectory of objects, we came up with a means to bend light within certain materials,” adds Kitamura.

“Such in-plane beam steering within the terahertz range could be harnessed in 6G communication. Academically, the findings show that photonic crystals could harness gravitational effects, opening new pathways within the field of graviton physics,” said Associate Professor Masayuki Fujita from Osaka University.

More information: Kanji Nanjyo et al, Deflection of electromagnetic waves by pseudogravity in distorted photonic crystals, Physical Review A (2023). DOI: 10.1103/PhysRevA.108.033522

Journal information: Physical Review A 

Provided by Tohoku University 

by Bob Yirka , Phys.org

A team of physicists at the Technical University of Denmark has found the reason a spinning magnet can cause a secondary magnet to levitate without the need for stabilization. In their paper published in the journal Physical Review Applied, the group describes experiments they conducted to learn more about the phenomenon and what they learned from them.

Prior research and anecdotal evidence have shown that if two magnets with north poles facing one another are brought close together, they will repel one another. Such force has been used for applications such as levitating trains. But these applications must account for the inherent instability that arises when magnets repel each other.

More recently, scientists have found that if one of the magnets is spun at high speed, a second magnet can be repelled without the need for stabilizing—it remains levitated even when the first magnet is moved around. In this new effort, the researchers have uncovered the reason for such behavior.

To learn more about the phenomenon, the research team paired several different types of magnets and spun them at different speeds while recording the action with high-speed cameras and motion tracking software. In studying the resulting imagery, the team was able to uncover the reason for the behavior.

The researchers found that the secondary magnet (which they call a floater) rotated in sync with the rotor magnet—they spun at the same speed. They also found that the axis of the rotor magnet spun with a slight tilt—a situation that would destabilize the two magnets if they were not spinning. To better understand what was happening, the researchers created a simulation that allowed them to more easily manipulate the two magnets and their behavior.

They found that the magnetic field of the rotor magnet exerted some amount of torque on the floater resulting in the two magnets rotating in sync due to a gyroscopic effect. But the floater resisted, if only slightly, which accounted for the parallel configuration that developed. They also found that there was a very small amount of misalignment of the polar axis of the rotor magnet relative to its magnetic field—the resulting attractive and repulsive forces balanced each other out, allowing the floater to be held in a steady position during levitation.

More information: Joachim Marco Hermansen et al, Magnetic levitation by rotation, Physical Review Applied (2023). DOI: 10.1103/PhysRevApplied.20.044036. On arXiv: doi.org/10.48550/arXiv.2305.00812

Journal information: Physical Review Applied  , arXiv 

© 2023 Science X Network

by ARC Centre of Excellence in Future Low-Energy Electronics Technologies

Researchers from Monash University have unlocked fresh insights into the behavior of quantum impurities within materials.

The new, international theoretical study introduces a novel approach known as the “quantum virial expansion,” offering a powerful tool to uncover the complex quantum interactions in two-dimensional semiconductors.

This breakthrough holds potential to reshape our understanding of complex quantum systems and unlock exciting future applications utilizing novel 2D materials.

The research is published in the journal Physical Review Letters

Unveiling quantum impurities

The study of “quantum impurities” has far-reaching applications across physics in systems as diverse as electrons in a crystal lattice to protons in neutron stars. These impurities can collectively form new quasiparticles with modified properties, essentially behaving as free particles.

Although a straight-forward many-body problem to state, quantum impurity problems are difficult to solve.

“The challenge lies in accurately describing the modified properties of the new quasiparticles,” says Dr. Brendan Mulkerin, who led the collaboration with researchers in Spain.

The study offers a novel perspective on impurities in 2D materials known as exciton-polarons, bound electron-hole pairs immersed in a fermionic medium.

A new path: Quantum virial expansion

The Monash University team introduced the “quantum virial expansion” (QVE), a powerful method that has long been indispensable in ultracold quantum gases.

In this case, the integration of QVE into the study of quantum impurities meant that only the interactions between pairs of quantum particles needed to be taken into account (rather than large numbers of them), with the resulting, solvable, model shedding new light on the interplay between impurities and their surroundings in 2D semiconductors.

The new approach is remarkably effective at “high” temperatures (e.g., in a semiconductor anything above a few degrees Kelvin) and low doping, where the electrons’ thermal wavelength is smaller than their interparticle spacing, leading to a “perturbatively” exact theory (referring to a quantum system being perturbed from a simple, solvable limit).

“One of the most intriguing aspects of this research is its potential to unify different theoretical models, with the ongoing debate surrounding the appropriate model for explaining the optical response of 2D semiconductors being resolved through the quantum virial expansion,” says corresponding author Jesper Levinsen (also at Monash).

Opening doors to the future

The quantum virial expansion is expected to have a broad impact, extending its applications to various systems beyond 2D semiconductors.

“Understanding quantum impurity physics will continue to reveal insights and unlock novel properties and new possibilities for understanding, harnessing, and controlling quantum interactions,” says corresponding author Prof Meera Parish (Monash).

More information: Brendan C. Mulkerin et al, Exact Quantum Virial Expansion for the Optical Response of Doped Two-Dimensional Semiconductors, Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.131.106901

Journal information: Physical Review Letters 

Provided by ARC Centre of Excellence in Future Low-Energy Electronics Technologies