Category: Physics

From fundamental physics of light-matter interaction to fabrication of targeted optical properties in highly complex optical engineering, the femtosecond (fs) laser plays a crucial role in laser manufacturing. Ultrashort light pulses can precisely deposit light energy in a given transparent material volume by controllable focusing conditions. Nonlinear absorption of high-density photon energy leads to the creation of free electrons plasma within a few fs, before the electron-phonon energy transfer to the lattice.

At a low repetition rate, the glass network heating is decoupled from light exposure and the plasma itself, enabling localized modifications or even breakdown without surrounding damage. Such nonlinear processes contribute to multiple types of modifications according to laser parameters. The fs laser-induced modifications have found many applications in most branches of nonlinear science, ranging from plasma physics and nano-photonics to material science, bio-photonics and quantum information science.

In a new paper published in Light: Science & Applications, a team of scientists led by Professor Matthieu Lancry from the University Paris Saclay has developed a new technique to tailor chiral optical properties. Their novel procedure implements fs laser 3D direct writing on glasses.

Recent progress highlights that an fs laser beam (axially symmetric intensity distribution, linearly polarized with orthogonal incidence) can create optical chirality inside an achiral material through a direct laser writing. This concept shed light on a new approach for tailoring chiral optical properties in 3D, providing a wider landscape of laser manufacturing. A prerequisite to exploiting such new potentials is to elucidate of how one can manipulate these chiral optical properties.

In this work, they suggested that the breaking of symmetry arises from the combined action of a stress field and a form birefringence due to nanograting formation with the non-parallel non-orthogonal neutral axis. This extrinsic chiral assembly is the source of the observed optical chirality, namely optical rotation and circular dichroism.

The researchers attempted to “disassemble” the dependence of the form birefringence and stress contributions concerning the laser polarization direction. In a simple view, the slow/fast axis of the form birefringence is controlled by the laser polarization direction and the related amplitude by the laser fluence.

Whereas the orientation of stress-induced birefringence is mostly dictated by the scanning geometry. A two-layer model is then developed based on Mueller formalism. It quantitatively explains the creation of both linear and circular anisotropic optical properties.

Finally, the researchers exploited their model to engineer chiral optical properties following two different designs: multilayer “nanograting-based waveplates” and an ultralow loss design based on “stress-engineered waveplates.” The results provide clear evidence that the origin of fs laser-induced circular properties includes two contributions and propose a strategy for tailoring chiral optical properties in any glasses by fs laser direct writing.

More information: Jiafeng Lu et al, Tailoring chiral optical properties by femtosecond laser direct writing in silica, Light: Science & Applications (2023). DOI: 10.1038/s41377-023-01080-y

Journal information: Light: Science & Applications 

Provided by Chinese Academy of Sciences 

Warming a crystal of the mineral fresnoite, Oak Ridge National Laboratory scientists discovered that excitations called phasons carried heat three times farther and faster than phonons, the excitations that usually carry heat through a material.

“Neutrons were ideal for exploring these sources of heat transport because they interact with both phasons and phonons,” said Michael Manley, who led the study with Raphael Hermann.

In most crystals, atomic vibrations propagate excited waves through the lattice as phonons. However, in certain crystals, atomic rearrangements also propagate excited waves as phasons. Because phasons can move faster than sound, physicists anticipated they would excel at moving heat.

The team mapped paths of phasons and phonons and characterized their vibrations at ORNL’s Spallation Neutron Source and measured the transport of heat through the lattice in a Materials Science and Technology Division laboratory.

“We observed phasons carrying heat through the crystal by improving the experimental resolution, like going from the Hubble to the James Webb Space Telescope,” Hermann said, referring to iconic telescopes launched three decades apart.

The results may help theorists improve accuracy for heat transport simulations of energy materials.

The study is published in the journal Physical Review Letters.

More information: M. E. Manley et al, Phason-Dominated Thermal Transport in Fresnoite, Physical Review Letters (2022). DOI: 10.1103/PhysRevLett.129.255901

Journal information: Physical Review Letters 

Provided by Oak Ridge National Laboratory 

In 2018, a team of physicists at Purdue University invented a device which experimentally showed quasiparticles interfering for the first time in the fractional quantum Hall effect at filling factor v=1/3. Further development of these heterostructures has allowed the Manfra Group to expand their research to experiments that explore counterflowing charged edge modes at the 2/3 fractional quantum Hall state.

They have recently published their findings, “Half-Integer Conductance Plateau at the ν = 2/3 Fractional Quantum Hall State in a Quantum Point Contact,” in Physical Review Letters. This novel work has been selected as the Editor’s Suggestion for the forthcoming issue.

The team is led by Dr. Michael J. Manfra, Bill and Dee O’Brien Distinguished Professor of Physics and Astronomy, Professor of Electrical and Computer Engineering, Professor of Materials Engineering, and Scientific Director of the Microsoft Quantum Lab West Lafayette. The lead author of the publication is Dr. James Nakamura, senior research scientist. Dr. Geoffrey Gardner and graduate student Shuang Liang were also co-authors of this publication making valuable contributions to heterostructure growth.

In the experiment, the team produced a semiconductor material which contains a sheet of two-dimensional electrons. On top of this semiconductor, they built a quantum point contact which consists of metal gates with a very narrow 300 nanometer gap. They used the quantum point contact to direct the conducting edge states through the narrow gap.

In this configuration, demonstrated by the graphic above, they were able to measure an electrical conductance equal to half the fundamental value of e²/h. This experimental result is consistent with longstanding theoretical predictions for the edge states of the ν = 2/3 fractional quantum Hall state.

“We have a semiconductor structure that contains electrons arranged in a plane, called a two-dimensional electron system. When you cool the electrons down to low temperature and put them in a strong magnetic field, they form special states of matter called quantum Hall states,” explains Nakamura.

“At a certain value of the magnetic field, the quantum Hall state is called the ν = 2/3 fractional quantum Hall state. At all quantum Hall states, electrical current is carried by edge states that flow around the edge of the sample, and they are chiral, meaning each edge state only flows in one direction (clockwise or counterclockwise). The ν = 2/3 state is predicted by theoretical physicists to have the special property that the there are two edge states which flow in the opposite direction to each other, one clockwise and the other counterclockwise.”

“This is different from most quantum Hall states, where all the edge states flow in the same direction. We used a device with metal gates called a quantum point contact to control the edge states, and our measurements of the edge states in the quantum point contacts confirm the counterflowing edge states in our device.”

“The quantum point contact brings the edge states on opposite edges of the sample close together. We measured a value of electrical conductance across the device equal to half of the value e²/h, where e is the electron’s charge and h is Planck’s constant. This value of the conductance is strong experimental evidence that our system has the edge structure with two counterflowing edge states.”

This research is part of an ongoing quest to understand and manipulate fractionally charged anyons in fractional quantum Hall regime, a rich testbed for exploring the impact of topology in condensed matter physics which may possibly be used to create qubits.

More information: J. Nakamura et al, Half-Integer Conductance Plateau at the ν=2/3 Fractional Quantum Hall State in a Quantum Point Contact, Physical Review Letters (2023). DOI: 10.1103/PhysRevLett.130.076205

Journal information: Physical Review Letters 

Provided by Purdue University 

Burning hydrocarbon fuels produces nano-sized soot particles and Polycyclic Aromatic Hydrocarbons (PAH)—harmful emissions that impact our environment. The carbon-made particles make up 70% of our interstellar space, and black carbon particles from flames are exciting nanomaterials for electronic devices and sustainable energy applications—making their study important.

The lifetimes of soot and PAHs are extremely short (sub nanoseconds to hundreds of nanoseconds) in turbulent flames. Therefore, it requires ultrafast imaging approaches to resolve combustion both in space (2D/3D) and time.

The current state-of-the-art planar imaging systems are limited to just a few million frames per second. To extract 2D maps of flame species, they also require multiple consecutive laser pulses, causing undesired thermal issues. Furthermore, the traditional pump-probe ultrafast imaging methods can only capture the processes which are “repeatable” because several images of the same process are taken at different time instances to extract a complete picture of spatiotemporal dynamics.

Therefore, researchers in the field of combustion science have long awaited a tool that can overcome the limitations of the current systems.

In a new paper published in Light: Science & Applications, a team of scientists, led by Dr. Yogeshwar Nath Mishra, Dr. Peng Wang and Professor Lihong V. Wang, from California Institute of Technology and their collaborators from University of Gothenburg in Sweden, and Friedrich-Alexander University Erlangen in Germany, have developed the world’s fastest planar imaging camera: laser-sheet compressed ultrafast photography (LS-CUP).

Using LS-CUP, they have captured the entire movies of laser-flame dynamics at a record imaging speed of 12.5 billion frame per second (Gfps), which is at least three orders of magnitude higher than the current state-of-the-art systems. Using only one single laser pulse, LS-CUP enabled wide-field real-time imaging of laser-induced fluorescence from PAHs, elastic light scattering and laser-induced incandescence from soot particles.

Dr. Yogeshwar Nath Mishra, one of the leading authors of this paper said, “Laser sheet imaging is one of the most popular techniques for characterizing flows and combustion in two-dimension (2D) because it preferably resolves a plane in both time and space. Using LS-CUP, we can perform many exciting studies and ‘film’ fast chemical reactions and non-repeatable flame-laser interactions using a single laser pulse in real-time beyond the MHz imaging range.”

“We can combine it with pre-existing planar imaging methods for combustion research. Further, we can apply LS-CUP for real-time observation of hydrogen combustion, plasma-assisted combustion, and metal powder combustion—some of the recent hot topics in the field. Temperature is a crucial property in many thermodynamic systems, and to the best of our knowledge, we have reported its fastest wide-field measurements.”

Dr. Peng Wang, the other major contributor to this work, said, “LS-CUP is perfect: it is single-shot, only needs a single laser pulse, has a wide field-of-view, and can be easily adapted to observe all kinds of laser-induced signals over the entire lifetime of soot particles. We extracted critical parameters from the fast dynamics, such as fluorescence lifetimes of PAH molecules, soot nanoparticle sizes and cluster sizes, particle temperature, etc.”

“LS-CUP, in general, allows us to study extremely fast phenomena from a completely new and unique perspective. Reaching far beyond combustion research, the applications of our technique are extremely broad in physics, chemistry, biology and medicine, energy, and environmental research. The capability of capturing ultrafast phenomena represents an important metric of our human’s technology development, which is also driven by the curiosity and needs of scientists and engineers across various fields.”

More information: Yogeshwar Nath Mishra et al, Single-pulse real-time billion-frames-per-second planar imaging of ultrafast nanoparticle-laser dynamics and temperature in flames, Light: Science & Applications (2023). DOI: 10.1038/s41377-023-01095-5

Journal information: Light: Science & Applications 

Provided by Chinese Academy of Sciences 

Photonic integrated circuits, or in short, PICs, utilize particles of light, better known as photons, as opposed to electrons that run in electronic integrated circuits. The main difference between the two: A photonic integrated circuit provides functions for information signals imposed on optical wavelengths typically in the near infrared spectrum.

“Actually, these PICs with many integrated photonic components are able to generate, route, process and detect light on a single chip,” says Dr. Georgy Astakhov, Head of Quantum Technologies at HZDR’s Institute of Ion Beam Physics and Materials Research. “This modality is poised to play a key role in upcoming future technology, such as quantum computing. And PICs will lead the way.”

Before, quantum photonics experiments were notorious for the massive use of “bulk optics” distributed across the optical table and occupying the entire lab. Now, photonic chips are radically changing this landscape. Miniaturization, stability and suitability for mass production might turn them into the workhorse of modern-day quantum photonics.

From random to control mode

Monolithic integration of single-photon sources in a controllable way would give a resource-efficient route to implement millions of photonic qubits in PICs. To run quantum computation protocols, these photons must be indistinguishable. With this, industrial-scale photonic quantum processor production would become feasible.

However, the currently established fabrication method stands in the way of the compatibility of this promising concept with today’s semiconductor technology.

In a first attempt reported about two years ago, the researchers were already able to generate single photons on a silicon wafer, but only in a random and non-scalable way. Since then, they have come far. “Now, we show how focused ion beams from liquid metal alloy ion sources are used to place single-photon emitters at desired positions on the wafer while obtaining a high creation yield and high spectral quality,” says Dr. Nico Klingner, physicist.

Furthermore, the scientists at HZDR subjected the same single-photon emitters to a rigorous material testing program: After several cooling-down and warming-up cycles, they did not observe any degradation of their optical properties. These findings meet the preconditions required for mass production later on.

To translate this achievement into a widespread technology, and allow for wafer-scale engineering of individual photon emitters on the atomic scale compatible with established foundry manufacturing, the team implemented broad-beam implantation in a commercial implanter through a lithographically defined mask.

“This work really allowed us to take advantage of the state-of-the-art silicon processing cleanroom and electron beam lithography machines at the Nano Fabrication facility Rossendorf,” explains Dr. Ciarán Fowley, Cleanroom group leader and Head of Nanofabrication and Analysis.

Using both methods, the team can create dozens of telecom single-photon emitters at predefined locations with a spatial accuracy of about 50 nm. They emit in the strategically important telecommunication O-band and exhibit stable operation over days under continuous-wave excitation.

The scientists are convinced that the realization of controllable fabrication of single-photon emitters in silicon makes them a highly promising candidate for photonic quantum technologies, with a fabrication pathway compatible with very large-scale integration. These single-photon emitters are now technologically ready for production in semiconductor fabs and incorporation into the existing telecommunication infrastructure.

The findings are published in the journal Nature Communications.

More information: Michael Hollenbach et al, Wafer-scale nanofabrication of telecom single-photon emitters in silicon, Nature Communications (2022). DOI: 10.1038/s41467-022-35051-5

Journal information: Nature Communications 

Provided by Helmholtz Association of German Research Centres 

Metasurfaces are highly versatile for manipulating the amplitude, phase, or polarization of light. During the last decade, metasurfaces have been proposed for a vast range of applications—from imaging and holography to the generation of complex light field patterns. Yet, most optical metasurfaces developed to date are isolated optical elements that work only with external light sources.

Despite their versatility for manipulating a light field spatially, most metasurfaces have only a fixed, time-invariant response and a limited ability to control the temporal shape of a light field. To overcome such limitations, researchers are looking into ways to use nonlinear metasurfaces for spatiotemporal light field modulation. However, most materials for constructing metasurfaces have a relatively limited nonlinear optical response on their own.

One solution to the limited nonlinearity of metasurface materials is near-field coupling to a medium with extremely large optical nonlinearity. Epsilon-near-zero (ENZ) materials, an emerging class of materials with vanishing permittivity, have drawn much attention in recent years. For instance, indium tin oxide (ITO), a conductive metal oxide widely used as transparent electrodes in solar cells and consumer electronics, typically has permittivity beyond zero in the near-infrared regime.

An ENZ material, with its linear refractive index approaching zero, is endowed with an extremely large nonlinear refractive index and nonlinear absorption coefficient.

As reported in Advanced Photonics, researchers from Tsinghua University and the Chinese Academy of Sciences recently generated laser pulses with tailored spatiotemporal profiles by directly incorporating an ENZ material coupled to a metasurface in a fiber laser cavity.

The researchers used the geometric phase of a metasurface made of spatially inhomogeneous anisotropic metallic nano-antennas to tailor the transverse mode of the output laser beam. The giant nonlinear saturable absorption of the ENZ-coupled system allows pulsed laser generation via a Q-switching process. To provide a prototype, the researchers realized a microsecond pulsed vortex laser with varying topological charges.

This work provides a new route to construct a laser with a tailored spatiotemporal mode profile in a compact form. For further system miniaturization, the metasurface may be integrated on the fiber-end face.

According to corresponding author Yuanmu Yang, professor at the Tsinghua University State Key Laboratory of Precision Measurement Technology and Instruments, “We hope that our work may further exploration of metasurface versatility for spatial light field manipulation, with its giant and tailorable nonlinearity for generating laser beams with arbitrary spatial and temporal profiles.”

Yang notes that this innovative method may pave the way for the next generation of miniaturized pulsed laser sources, which could be used in various applications, such as light trapping, high-density optical storage, superresolution imaging, and 3D laser lithography.

More information: Wenhe Jia et al, Intracavity spatiotemporal metasurfaces, Advanced Photonics (2023). DOI: 10.1117/1.AP.5.2.026002

Provided by SPIE 

It’s not easy to make sense of quantum-scale motion, but a new mathematical theory developed by scientists at Rice University and Oxford University could help—and may provide insight into improving a variety of computing, electrochemical and biological systems.

The theory developed by Rice theorist Peter Wolynes and Oxford theoretical chemist David Logan gives a simple prediction for the threshold at which large quantum systems switch from orderly motion like a clock to random, erratic motion like asteroids moving around in the early solar system. Using a computational analysis of a photosynthesis model, collaborators at the University of Illinois Urbana-Champaign showed that the theory can predict the nature of the motions in a chlorophyll molecule when it absorbs energy from sunlight.

The theory applies to any sufficiently complex quantum system and may give insights into building better quantum computers. It could also, for instance, help design features of next-generation solar cells or perhaps make batteries last longer.

The study is published this week in the Proceedings of the National Academy of Sciences.

Nothing is ever completely still on the molecular level, especially when quantum physics plays a role. A water droplet gleaming on a leaf may look motionless, but inside, over a sextillion molecules are vibrating nonstop. Hydrogen and oxygen atoms and the subatomic particles within them—the nuclei and electrons—constantly move and interact.

“In thinking about the motions of individual molecules at quantum scale, there is often this comparison to the way we think of the solar system,” Wolynes said. “You learn that there are eight planets in our solar system, each one with a well-defined orbit. But in fact, the orbits interact with each other. Nevertheless, the orbits are very predictable. You can go to a planetarium, and they’ll show you what the sky looked like 2,000 years ago. A lot of the motions of the atoms in molecules are exactly that regular or clocklike.”

When Wolynes and Logan first posed the question of predicting the regularity or randomness of quantum motion, they tested their math against observations of vibrational motions in individual molecules.

“You only have to know two things about a molecule to be able to analyze its quantum motion patterns,” Wolynes said. “First, you need to know the vibrational frequencies of its particles, that’s to say the frequencies at which the vibrations occur which are like the orbits, and, second, how these vibrations nonlinearly interact with each other. These anharmonic interactions depend mostly on the mass of atoms. For organic molecules, you can predict how strongly those vibrational orbits would interact with one other.”

Things are more complicated when the molecules also dramatically change structure, for instance as a result of a chemical reaction.

“As soon as we start looking at molecules that chemically react or rearrange their structure, we know that there’s at least some element of unpredictability or randomness in the process because, even in classical terms, the reaction either happens, or it doesn’t happen,” Wolynes said. “When we try to understand how chemical changes occur, there’s this question: Is the overall motion more clocklike or is it more irregular?”

Aside from their nonstop vibrations, which happen without light, electrons can have quantum-level interactions that sometimes lead to a more dramatic turn.

“Because they’re very light, electrons normally move thousands of times faster than the centers of the atoms, the nuclei,” he said. “So though they are constantly moving, the electrons’ orbits smoothly adjust to what the nuclei do. But every now and again, the nuclei come to a place where the electronic energies will almost be equal whether the excitation is on one molecule or on the other. That’s what’s called a surface crossing. At that point, the excitation has a chance to jump from one electronic level to another.”

Predicting at which point the transfer of energy that takes place during photosynthesis turns from orderly motion to randomness or dissipation would take a significant amount of time and effort by direct computation.

“It is very nice that we have a very simple formula that determines when this happens,” said Martin Gruebele, a chemist at the University of Illinois Urbana-Champaign and co-author on the study who is a part of the joint Rice-Illinois Center for Adapting Flaws into Features (CAFF). “That’s something we just didn’t have before and figuring it out required very lengthy calculations.”

The Logan-Wolynes theory opens up a wide array of scientific inquiry ranging from the theoretical exploration of the fundamentals of quantum mechanics to practical applications.

“The Logan-Wolynes theory did pretty well in terms of telling you at roughly what energy input you’d get a change in quantum-system behavior,” Wolynes said.

“But one of the interesting things that the large-scale computations of (co-author Chenghao) Zhang and Gruebele found is that there are these exceptions that stand out from all the possible orbiting patterns you might have. Occasionally there’s a few stragglers where simple motions persist for long times and don’t seem to get randomized. One of the questions we’re going to pursue in the future is how much that persistent regularity is actually influencing processes like photosynthesis.”

“Another direction that is being pursued at Rice where this theory can help is the problem of making a quantum computer that behaves as much as possible in a clocklike way,” he said.

“You don’t want your computers to be randomly changing information. The larger and more sophisticated you make a computer, the likelier it is that you’ll run into some kind of randomization effects.”

Gruebele and collaborators at Illinois also plan to use these ideas in other scientific contexts. “One of our goals, for instance, is to design better human-built light-harvesting molecules that might consist of carbon dots that can transfer the energy to their periphery where it can be harvested,” Gruebele said.

More information: Zhang, Chenghao et al, Surface crossing and energy flow in many-dimensional quantum systems, Proceedings of the National Academy of Sciences (2023). DOI: 10.1073/pnas.2221690120

Journal information: Proceedings of the National Academy of Sciences 

Provided by Rice University 

Google scientists said Wednesday they have passed a major milestone in their quest to develop effective quantum computing, with a new study showing they reduced the rate of errors—long an obstacle for the much-hyped technology.

Quantum computing has been touted as a revolutionary advance that uses our growing scientific understanding of the subatomic world to create a machine with powers far beyond those of today’s conventional computers.

However the technology remains largely theoretical, with many thorny problems still standing in the way—including stubbornly high error rates.

In new research published in the journal Nature, the Google Quantum AI lab described a system that can significantly decrease the error rate.

That could give the US tech giant a step up on its rivals such as IBM, which is also working on superconducting quantum processors.

While traditional computers process information in bits that can be represented by 0 or 1, quantum computers use qubits, which can be a combination of both at the same time.

This property, known as superposition, means that a quantum computer can crunch an enormous number of potential outcomes simultaneously.

The computers harness some of the most mind-boggling aspects of quantum mechanics, including a phenomenon known as “entanglement”—in which two members of a pair of bits can exist in a single state, even if far apart.

‘Magic’

But a problem called decoherence can cause the qubits to lose their information when they leave their quantum state and come into contact with the outside world.

This fragility causes high error rates, which also increase with the number of qubits, frustrating scientists wanting to ramp up their experiments.

However Google’s team said it had demonstrated for the first time in practice that a system using error-correcting code can detect and fix errors without affecting the information.

The system was first theorized in the 1990s, however previous attempts had just thrown up more errors, not less, said Google’s Hartmut Neven, a co-author of the study.

“But if all components of your system have sufficiently low error rates, then the magic of quantum error correction kicks in,” Neven told a press conference.

Julian Kelly, another study co-author, hailed the development as “a key scientific milestone”, saying that “quantum error correction is the single most important technology for the future of quantum computing“.

Neven said the result was still “not good enough, we need to get to an absolute low error rate”.

He added that “there are more steps to come” to achieve the dream of a useable quantum computer.

Google claimed in 2019 it had passed a milestone known as “quantum supremacy”, when the tech giant said its Sycamore machine executed a calculation in 200 seconds that would have taken a conventional supercomputer 10,000 years to complete.

However the achievement has since been disputed, with Chinese researchers saying last year that a supercomputer could have beaten Sycamore’s time.

More information: Suppressing quantum errors by scaling a surface code logical qubit, Nature (2023). DOI: 10.1038/s41586-022-05434-1

Towards quantum computers that are robust to errors, Nature (2023). DOI: 10.1038/d41586-022-04532-4

Journal information: Nature 

© 2023 AFP

One of the striking aspects of the quantum world is that a particle, say, an electron, is also a wave, meaning that it exists in many places at the same time. In a new study, reported today in Nature, researchers from the Weizmann Institute of Science make use of this property to develop a new type of tool—the quantum twisting microscope (QTM)—that can create novel quantum materials while simultaneously gazing into the most fundamental quantum nature of their electrons.

The study’s findings may be used to create electronic materials with unprecedented functionalities.

The QTM involves the “twisting,” or rotating, of two atomically-thin layers of material with respect to one another. In recent years, such twisting has become a major source of discoveries. It began with the discovery that placing two layers of graphene, one-atom-thick crystalline sheets of carbon, one atop the other with a slight relative twist angle, leads to a “sandwich” with unexpected new properties.

The twist angle turned out to be the most critical parameter for controlling the behavior of electrons: Changing it by merely one-tenth of a degree could transform the material from an exotic superconductor into an unconventional insulator. But critical as it is, this parameter is also the hardest to control in experiments. By and large, twisting two layers to a new angle requires building a new “sandwich” from scratch, a process that is very long and tedious.

“Our original motivation was to solve this problem by building a machine that could continuously twist any two materials with respect to one another, readily producing an infinite range of novel materials,” says team leader Prof. Shahal Ilani of Weizmann’s Condensed Matter Physics Department. “However, while building this machine, we discovered that it can also be turned into a very powerful microscope, capable of seeing quantum electronic waves in ways that were unimaginable before.”

Creating a quantum picture

Pictures have long played a central role in scientific discovery. Light microscopes and telescopes routinely provide images that allow scientists to gain a deeper understanding of biological and astrophysical systems. Taking pictures of electrons inside materials, on the other hand, has for many years been notoriously hard, owing to the small dimensions involved.

This was transformed some 40 years ago with the invention of the scanning tunneling microscope, which earned its developers the 1986 Nobel Prize in Physics. This microscope uses an atomically sharp needle to scan the surface of a material, measuring the electric current and gradually building an image of the distribution of electrons in the sample.

“Many different scanning probes have been developed since this invention, each measuring a different electronic property, but all of them measure these properties at one location at a time. So, they mostly see electrons as particles, and can only indirectly learn about their wave nature,” explains Prof. Ady Stern from the Weizmann Institute, who took part in the study along with three other theoretical physicists from the same department: Profs. Binghai Yan, Yuval Oreg and Erez Berg.

“As it turned out, the tool that we have built can visualize the quantum electronic waves directly, giving us a way to unravel the quantum dances they perform inside the material,” Stern says.

Spotting an electron in several places at once

“The trick for seeing quantum waves is to spot the same electron in different locations at the same time,” says Alon Inbar, a lead author on the paper. “The measurement is conceptually similar to the famous two-slit experiment, which was used a century ago to prove for the first time that electrons in quantum mechanics have a wave nature,” adds Dr. John Birkbeck, another lead author. “The only difference is that we perform such an experiment at the tip of our scanning microscope.”

To achieve this, the researchers replaced the atomically sharp tip of the scanning tunneling microscope with a tip that contains a flat layer of a quantum material, such as a single layer of graphene. When this layer is brought into contact with the surface of the sample of interest, it forms a two-dimensional interface across which electrons can tunnel at many different locations.

Quantum mechanically, they tunnel in all locations simultaneously, and the tunneling events at different locations interfere with each other. This interference allows an electron to tunnel only if its wave functions on both sides of the interface match exactly. “To see a quantum electron, we have to be gentle,” says Ilani. “If we don’t ask it the rude question ‘Where are you?’ but instead provide it with multiple routes to cross into our detector without us knowing where it actually crossed, we allow it to preserve its fragile wave-like nature.”

Twist and tunnel

Generally, the electronic waves in the tip and the sample propagate in different directions and therefore do not match. The QTM uses its twisting capability to find the angle at which matching occurs: By continuously twisting the tip with respect to the sample, the tool causes their corresponding wave functions to also twist with respect to one another. Once these wave functions match on both sides of the interface, tunneling can occur.

The twisting therefore allows the QTM to map how the electronic wave function depends on momentum, similarly to the way lateral translations of the tip enable the mapping of its dependence on position.

Merely knowing at which angles electrons cross the interface supplies the researchers with a great deal of information about the probed material. In this manner they can learn about the collective organization of electrons within the sample, their speed, energy distribution, patterns of interference and even the interactions of different waves with one another.

A new twist on quantum materials

“Our microscope will give scientists a new kind of ‘lens’ for observing and measuring the properties of quantum materials,” says Jiewen Xiao, another lead author.

The Weizmann team has already applied their microscope to studying the properties of several key quantum materials at room temperature and is now gearing up toward doing new experiments at temperatures of a few kelvins, where some of the most exciting quantum mechanical effects are known to take place.

Peering so deeply into the quantum world can help reveal fundamental truths about nature. In the future, it might also have a tremendous effect on emerging technologies. The QTM will provide researchers with access to an unprecedented spectrum of new quantum interfaces, as well as new “eyes” for discovering quantum phenomena within them.

More information: A. Inbar et al, The quantum twisting microscope, Nature (2023). DOI: 10.1038/s41586-022-05685-y

Journal information: Nature 

Provided by Weizmann Institute of Science 

Researchers at Harvard University, the Lawrence Berkeley National Laboratory, Arizona State University, and other institutes in the United States have recently observed an antiferromagnetic metal phase in electron-doped NdNiO₃ a material known to be a non-collinear antiferromagnet (i.e., exhibiting an onset of antiferromagnetic ordering that is concomitant with a transition into an insulating state).

“Previous works on the rare-earth nickelates (RNiO₃) have found them to host a rather exotic phase of magnetism known as a ‘noncollinear antiferromagnet,'” Qi Song, Spencer Doyle, Luca Moreschini and Julia A. Mundy, Four of the researchers who carried out the study, told Phys.org.

“This type of magnet has unique potential applications in the field of spintronics, yet rare-earth nickelates famously change spontaneously from being metallic to insulating at the exact same temperature that this noncollinear antiferromagnet phase turns on. We wanted to see if we could somehow modify one of these materials in a way so that it remained metallic, but still had this interesting magnetic phase.”

Ensuring that rare-earth nickelates remain metallic at low temperatures, where their antiferromagnetic phase appears, would ultimately enable their use for the development of spintronic devices. In their experiments, Song, Doyle, Moreschini, Mundy and their colleagues tried to achieve this using the rare-earth nickelate NdNiO₃.

To prompt the material to retain its antiferromagnet metal phase without eliciting its transition into an insulator, they used electron doping, a technique for changing the number of electrons in materials. Essentially, they grew a series of NdNiO₃ samples in which they added varying amounts of cerium atoms in the place of neodymium atoms, to add more electrons to the system.

“Once we had these samples, we collected electrical transport measurements, where we applied a small current through each sample and measured the resistance,” Doyle said. “By performing this measurement as we changed the temperature of the sample, we were able to deduce whether or not the sample was metallic or insulating in the magnetic phase.”

Ultimately, to demonstrate the potential of the electron-doped antiferromagnetic metal sample they realized for the development of spintronic devices, the team also collected a further measurement, known as the “zero field planar Hall effect.” Simply put, this measurement can be used to verify whether a material can “remember” whether a magnetic field had been applied to it or not, even after this field was turned off.

These tests yielded very promising results, as the electron-doped samples produced by Doyle and his colleagues demonstrated this “memory effect.” Remarkably, the effect observed in these materials was very strong compared to those typically observed in antiferromagnets.

“We created a new phase in the family of nickelates, which was not seen before. The key property of these materials is that they have a metal-insulator transition, which comes together with a magnetic one, as it often happens,” Moreschini explained. “Above the transition, you have weak or no magnetic order, and below you do have one, in this case, antiferromagnetism. In other previous studies, this transition had been suppressed in some conditions, but for the first time we succeeded in decoupling them: the magnetic transition is still there, but the metal-insulator transition is gone.”

The electron doping-based strategy proposed by this team of researchers allowed them to elicit an antiferromagnetic metal phase in a rare-earth nickelate. Below the temperature at which the material transitions to an antiferromagnetic phase, it is still a metal, even if a less performing metal. Overall, their electron doped-samples are thus metals with an antiferromagnetic order, a phase that had not been observed before in nickelates.

In the future, the recent study by Song, Doyle, Moreschini, Mundy and their colleagues could open new and exciting possibilities for the development of spintronic devices based on rare-earth nickelates. In their next work, the team plans to explore strategies that would allow them to further increase the temperature at which the metal-metal transition of these rare-earth nickelates takes place.

“The ultimate goal of course is to push it all the way to room temperature, because that is where you want your devices to work. At the moment honestly speaking, the nickelates are still far from there, but in a broader perspective what we learned here from this new phase can hopefully guide the engineering of new phases in other families of oxides or other materials in general where these transitions happen a little closer to room temperature, and tweaking a bit the electronic correlations you can give them that last push to have them at room temperature,” say the researchers.

More information: Qi Song et al, Antiferromagnetic metal phase in an electron-doped rare-earth nickelate, Nature Physics (2023). DOI: 10.1038/s41567-022-01907-2

Journal information: Nature Physics 

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