Category Archives: PHYSICS

Researchers Develop Sealed X-Ray Microchannel Plate Imager with Higher Sensitivity and Stability (Physics)

The gated X-ray framing camera (XFC) is an indispensable piece of two-dimensional diagnostic equipment used to measure implosion symmetry, hydrodynamic instability, and plasma uniformity. Therefore, this device plays an irreplaceable role in the research of laser inertial confinement fusion (ICF) and high energy density physics.

The open structure and outgassing materials of the micro-channel plate (MCP) imager equipped in current XFCs may lead to the contamination of the MCP and the Au photocathode, resulting in further reduction of the gain of the framing camera as storage time prolongs. Besides, Au has a lower quantum efficiency as an X-ray photocathode, which has been found to reduce the camera detection quantum efficiency and increase the image noise.

In a new study, a team led by Prof. Dr. YANG Yang from Xi’an Institute of Optics and Precision Mechanics (XIOPM) of the Chinese Academy of Sciences proposed a hermetically sealed MCP imager with a CsI photocathode. Compared with traditional MCP imagers, this imager solves the problems of poor stability and the low quantum efficiency brought by the open structure and the Au photocathodes.

According to the results of the experiments, with two microstrip photocathodes of 100 nm Au and 100 nm CsI, respectively, the proposed imager has a measured exposure time of 65 ps when applied with a 200 ps width gate pulse, and the image intensity of the CsI microstrip photocathode is 3.4 times that of the Au microstrip photocathode when illuminated by a non-monochromatic portable X-ray source with high energy photons.

Furthermore, the results showed that the sealed structure effectively improves the performance stability of the MCP imager. After being stored in the laboratory air for 1000 hours, the sealed MCP imager recorded a 17% drop from the initial value in its static response intensity. As for the open-structured MCP imager, the static response intensity was reduced by more than half when the exposure time accumulated to more than 24 hours.

The proposed hermetically sealed X-ray MCP imager with a CsI microstrip photocathode improves both sensitivity and stability. With its enhanced detection efficiency and potential to reduce crosstalk and artifacts, the sealed MCP imager is expected to achieve quantitative measurements with higher precision.

Featured image: Schematic structure of the sealed MCP imager. © Authors


Provided by Chinese Academy of Sciences

Exotic Property Of ‘Ambidextrous’ Crystals Points To A New Magnetic Phenomena (Physics)

Researchers from Skoltech, KTH Royal Institute of Technology, and Uppsala University have predicted the existence of antichiral ferromagnetism, a nontrivial property of some magnetic crystals that opens the door to a variety of new magnetic phenomena. The paper was published in the journal Physical Review B.

Chirality, or handedness, is an extremely important fundamental property of objects in many fields of physics, mathematics, chemistry and biology; a chiral object cannot be superimposed on its mirror image in any way. The simplest chiral objects are human hands, hence the term itself. The opposite of chiral is achiral: a circle or a square are simple achiral objects.

Chirality can be applied to much more complex entities; for instance, competing internal interactions in a magnetic system can lead to the appearance of periodic magnetic textures in the structure that differ from their mirror images – this is called chiral ferromagnetic ordering. Chiral crystals are widely considered promising candidates for magnetic data storage and processing device realization as information can be encoded via their nontrivial magnetic textures.

Anastasia Pervishko, research scientist at the Skoltech Center for Computational and Data-Intensive Science and Engineering (CDISE), and her colleagues used symmetry-based analysis and numerical computations to predict the existence of antichiral ferromagnetism – a kind of ferromagnetic ordering when both types of chirality (handedness) exist simultaneously and alternate in space.

“In contrast to chiral and achiral textures, we predict a fundamentally different magnetic ordering in tetrahedral ferromagnets. We use the term ‘antichirality’ to highlight the spontaneous modulation of magnetization direction with a spatial chirality alternation between right- and left-handedness induced by crystal symmetry,” Pervishko says.

She explains that the chirality in this periodic magnetic texture alternates in space while the average torsion invariant is zero. “One can picture it as a magnetic modulation where some part is characterized by right-handedness and the other by left-handedness; that differs drastically from conventional chiral textures where handedness is preserved,” Pervishko adds.

The team showed that antichiral ferromagnetism can be observed in a class of crystals in which many minerals are formed naturally. To do this, they studied magnetic ordering in the structure with tetrahedral crystal symmetry and used micromagnetic analysis to derive this new antichiral ordering.

”Thanks to this unconventional ground state, the proposed magnetic ordering might result in a rich family of magnetic phenomena including unique magnetic domains and skyrmions that are fundamentally different from chiral textures. This finding triggers further theoretical and experimental investigation in this type of magnetic materials,” Anastasia Pervishko concludes.

The work was done with the financial support provided by Russian Foundation for Basic Research Project No. 19-32-60020 and the Russian Federation President Scholarship.

Featured image: Examples of periodic magnetic textures and their geometric reflections. (a) Cycloid, also known as Néel-type walls. (b) Helix, also known as Bloch-type walls. (c) Antichiral modulation found in the present work. Credit: DOI: 10.1103/PhysRevB.104.L020406


Reference: Filipp N. Rybakov et al, Antichiral ferromagnetism, Physical Review B (2021). DOI: 10.1103/PhysRevB.104.L020406


Provided by Skolkovo Institute of Science and Technology

Why Strong Interaction at Large Distances And Weaker At Short? (Particle Physics / Quantum)

Recently, Subhash Kak proposed that space is e-dimensional, rather than the commonly supposed 3-dimensions, where, e is Eulers number that is about 2.71828, and he used this to provide a resolution to the problem of Hubble tension, which is the disagreement between values of the rate at which the universe is expanding obtained using two different methods. Now, Kak has provided further support to this theory by a model that may explain why quarks, constituents of matter, don’t interact with each other when they are up close, but are bound strongly when pulled apart.

“Noninteger dimensionality leads to the anomalous situation of strong interaction at large distances and much weaker interaction at short distances, which is a characteristic of asymptotic freedom,” writes Subhash Kak, Professor in the School of Electrical and Computer Engineering at Oklahoma State University.

His new paper shows that, as the dimensionality falls below the value of critical dimension (dcrit), which lies between 2 and 3, there arises strange behavior where increasing energy reduces the strength of interaction between the particles. 

And when the value is 2 or below, the potential becomes constant (independent of separation) and force between objects or particles completely disappears. This new version of asymptotic freedom, which arises from the squeezing the dimensionality of space, could be of use in studying the anomalous mechanical properties of metamaterials.

“Future investigation will reveal whether this phenomenon based on dimensionality of space has any connection with other models of asymptotic freedom,” he concludes.

Featured image: Potential with respect to dimensionality; blue line p3,B(d), and red line p3,C(d) © S. Kak


Reference: Kak, S. Asymptotic freedom and noninteger dimensionality. Sci Rep 11, 3406 (2021). https://doi.org/10.1038/s41598-021-83002-9


Note for editors of other websites: To reuse this article fully or partially kindly give credit either to our author/editor S. Aman or provide a link of our article


Reviewed by S. Kak

Researchers Observed an Unexpected Response To Electron Injection In The Mott Insulator (Physics)

Unexpected peaks in a spectrum upset conventional models of an exotic quantum material

In a finding that will give theorists plenty to ponder, an all-RIKEN team has observed an unexpected response in an exotic material known as a Mott insulator when they injected electrons into it1. This observation promises to give physicists new insights into such materials, which are closely related to high-temperature superconductors.

Neither a chunk of silicon nor a Mott insulator conduct electricity—but for very different reasons. In silicon, electrons are tightly bound to atoms and require a lot of energy to become mobile conduction electrons. In contrast, in a Mott insulator, electrons may not be strongly bound to the atoms, but their movement is instead curbed by their mutual repulsion.

The Mott state’s emergence from interactions between electrons leads to unusual properties. “A small excess or deficit of electrons in a Mott insulator can lead to high-temperature superconductivity, which could be of enormous practical value in the future,” says Christopher Butler of the RIKEN Center for Emergent Matter Science (CEMS). “In Mott-insulating tantalum disulphide, electrons are localized not at each atom, but instead on the crests of a pre-existing ‘charge density wave’. Because the charge density wave is rather delicate, the Mott state can easily be tweaked.”

But to harness the potential of this Mott-insulating state and the charge density wave that hosts it, scientists need to better understand the physics connecting them.

Now, Butler and three colleagues, all at CEMS, have added excess electrons to a Mott insulator using the tip of a scanning tunneling microscope (Fig. 1) and observed a surprising response—tunneling spectra showed a sharp feature, a distinct state that set off vibrations in the ionic lattice.

The conventional theoretical model for Mott insulators predicts that the spectrum should be smooth and non-descript. “It was most surprising that we saw such sharp features in our tunneling spectroscopy measurements,” says Butler. “They may indicate that something is going on that is outside the bounds of the usual theory.”

Butler notes that some theoretical calculations do predict sharp features similar to those his team saw, but they involve particle-like entities known as quasiparticles, which are controversial since they are not thought to exist in true Mott insulators. “There are competing explanations for the observation that are less controversial,” says Butler. “But if it eventually turns out that the calculation results indicating the existence of quasiparticles are right, it might shake up the theoretical understanding of Mott insulators.”

Figure 1: RIKEN physicists used the tip of a scanning tunneling microscope (gray inverted pyramid) to inject single electrons (gold sphere) into the surface of a Mott insulator). © RIKEN Center for Emergent Matter Science


Reference

  • 1.Butler, C.  J., Yoshida, M., Hanaguri, T. & Iwasa, Y. Doublonlike excitations and their phononic coupling in a Mott charge-density-wave system. Physical Review X 11 011059 (2021). doi: 10.1103/PhysRevX.11.011059

Provided by RIKEN

Physicists Show That a Quantum Particle Made of Light and Matter Can Be Dragged by a Current of Electrons (Physics)

A pair of studies in Nature show that a quasiparticle, known as a plasmon polariton, can be pulled with and against a flow of electrons, a finding that could lead to more efficient ways of manipulating light at the nanoscale.

Light was thought to move at a fixed rate until 1851, when a French physicist—the first to accurately clock the speed of light—showed it could also be slowed or accelerated simply by shining a light beam with or against the flow of moving water. Decades later, Einstein seized on Hippolyte Fizeau’s landmark water-pipe experiments in developing his theory of relativity.

Now, new research in Nature shows that a quasiparticle made of waves of photons and electrons—a plasmon polariton—has a similar ability to change speeds when immersed in an electrical current flowing through a sheet of graphene. But there’s a hitch: the polaritons appear to more easily shift gears in one direction—to a slightly slower speed—when traveling against the flow of electrons.

The finding is a big deal for plasmonics, a field with a rock-star name dedicated to finding new and efficient ways of controlling light down at the nearly invisible scale of individual atoms—for optical computing, nanolasers, and other applications, including imprinting patterns into semiconductors. Polaritons have two perks. Their relatively slow speed compared to photons makes them a good proxy for manipulating light. Polariton waves are also minuscule; dozens can squeeze into the wavelength of one photon.

Dmitri Basov, a physics professor at Columbia, has devoted most of his lab to studying their behavior. “Polaritons possess the best virtues of electrons and photons,” he said. “They’re compact but still quantum, which means they can be manipulated on ultra-fast time scales.”   

Illustration of polariton waves interacting with drifting electrons in a sheet of graphene.
In this illustration, a set of polariton waves (at left), interact with drifting electrons in a sheet of graphene. The warped fabric of space-time (upper left) represents the related concept of relativity. Credit: Yinan Dong, Denis Bandurin, and Ella Maru Studio

In the recent Nature study, Basov and his colleagues recreated Fizeau’s experiments on a speck of graphene made up of a single layer of carbon atoms. Hooking up the graphene to a battery, they created an electrical current reminiscent of Fizeau’s water streaming through a pipe. But instead of shining light on the moving water and measuring its speed in both directions, as Fizeau did, they generated an electromagnetic wave with a compressed wavelength—a polariton—by focusing infrared light on a gold nub in the graphene. The activated stream of polaritons look like light but are physically more compact due to their short wavelengths.

The researchers clocked the polaritons’ speed in both directions. When they traveled with the flow of the electrical current, they maintained their original speed. But when launched against the current, they slowed by a few percentage points.

An Unexpected Result

“We were surprised when we saw it,” said study co-author Denis Bandurin, a physics researcher at MIT. “First, the device was still alive, despite the heavy current we passed through it—it hadn’t blown up. Then we noticed the one-way effect, which was different from Fizeau’s original experiments.”

The researchers repeated the experiments over and over, led by the study’s first-author, Yinan Dong, a Columbia graduate student. Finally, it dawned on them. “Graphene is a material that turns electrons into relativistic particles,” Dong said. “We needed to account for their spectrum.”

A group at Berkeley Lab found a similar result, published in the same issue of Nature. Beyond reproducing the Fizeau effect in graphene, both studies have practical applications. Most natural systems are symmetric, but here, researchers found an intriguing exception. Basov said he hopes to slow down, and ultimately, cut off the flow of polaritons in one direction. It’s not an easy task, but it could hold big rewards.

“Engineering a system with a one-way flow of light is very difficult to achieve,” said Milan Delor, a physical chemist working on light-matter interactions at Columbia who was not involved in the research. “As soon as you can control the speed and direction of polaritons, you can transmit information in nanoscale circuits on ultrafast timescales. It’s one of the ingredients currently missing in photon-based circuits.”

An invitation to a lecture series at Columbia in 1906, which helped to popularize the work of physicist Hippolyte Fizeau and his discovery that light can move at different speeds.
A lecture series at Columbia in 1906 helped to popularize the work of physicist Hippolyte Fizeau, which included his discovery that light can move at different speeds. © Columbia University

Optical isolators are currently used to limit the bounce-back of light in everything from lasers to the fiber optic cables in broadband. But they’re bulky and incompatible with modern nanocircuits, making polaritons, with their potential to be shut off in one direction, so appealing.

Plasmonics researchers are also excited about the detailed images to come out of the experiments. They show that polaritons can serve as nanoscale probes, they said, triggering and recording electron-electron interactions in a system. This information provides clues about how graphene and other quantum materials will behave in the real world.

“The images are effectively a read-out of the material properties of graphene,” Delor said.

‘The Enablers of Nanoptics’

“I like to call polaritons the enablers of nanoptics,” says James Schuck, a mechanical engineer and plasmonics researcher at Columbia Engineering who was not involved in the work. “They’re useful for probing all sorts of materials at the nanoscale.”

Most of the experiments were done during the pandemic; the researchers wore masks and gloves and disinfected the lab after each visit. “There was no slow-down for quantum physics,” says Basov, with a laugh, evoking Fizeau.

The French physicist’s name was later inscribed on the Eiffel Tower; not for the effect that bears his name, but for precisely measuring the speed of light. Fizeau’s work was popularized in a lecture series at Columbia in 1906, as Basov likes to remind students. Fizeau was also an early photographic experimenter. Some of his ghostly daguerreotype views of the rooftops of Paris are held by The Metropolitan Museum of Art, not far from the Columbia campus.

Featured image: Columbia University graduate students Lin Xiong (left) and Yinan Dong image polaritons using a cryogenic microscope. (Credit: Yinan Dong)


Reference: Dong, Y., Xiong, L., Phinney, I.Y. et al. Fizeau drag in graphene plasmonics. Nature 594, 513–516 (2021). https://doi.org/10.1038/s41586-021-03640-x


Provided by Columbia University

High-precision Frequency Measurement (Physics)

Many scientific experiments require highly precise time measurements with the help of a clearly defined frequency. Now, a new approach allows the direct comparison of frequency measurements in the lab with the atomic clock in Bern, Switzerland.

For many scientific experiments, today’s researchers require a precise reference frequency that allows them to calibrate the time measurements made by their equipment. Such experiments include spectroscopy investigations – in which chemical reactions between molecules are examined in real time – and physical studies on natural constants.

Access to exactly this kind of highly precise reference frequency could soon become standard for Swiss research institutions. In a joint project funded as part of the Swiss National Science Foundation’s Sinergia programme, researchers at ETH Zurich, the University of Basel, the Swiss Federal Institute of Metrology (Metas) – Switzerland’s «guardian of measurement units» – and the Switch Foundation, which operates Switzerland’s academic data network, have demonstrated that such a precision reference signal can be sent via conventional telecommunications infrastructure.

«Initial results show that this permits chemical spectroscopy analyses that are 100 times more accurate than before», reports Stefan Willitsch, Professor of Physical Chemistry at the University of Basel and coordinator of the project. «With this precision, the laws of nature are verified by spectroscopic measurements on molecules with unprecedented accuracy,» adds Frédéric Merkt, Professor of Physical Chemistry at ETH Zurich.

Continuous correction

Specifically, the project established a trial network that connects the METAS site in Wabern near Bern with the University of Basel and ETH Zurich. A clever process synchronises the output signal with the Metas atomic clock. This signal is transmitted via the fibre-optic network operated by Switch – which manages IT network infrastructures for Swiss universities – to Basel and Zurich, where researchers can use it to calibrate their measuring devices.

«To ensure that the signal reaches the researchers with the desired level of precision, transmission must be continuously adjusted. Even the slightest variation in the length of the fibre-optic cable – caused by vibrations or temperature changes – affect the frequency», explains Jacques Morel, Head of the Photonics, Time and Frequency Laboratory at Metas. Therefore the signal is bounced back from Basel and Zurich to Bern, where the output signal is corrected as required.

High quality, lower costs

«In Switzerland, we’re only now beginning to establish this kind of network,» says Jérôme Faist, Professor at the Institute for Quantum Electronics at ETH Zurich, who contributed his expertise in laser technology to the project. «Other countries like Italy, Germany and France are already a step ahead in this area.»

In these countries, the reference frequencies have, up to now, been transmitted in one of two ways – each with its own specific drawbacks. Either the signal is sent via a dedicated cable, which produces an optimum physical result but is expensive, or the signal is transmitted via the telecommunications provider’s existing infrastructure. While this is much cheaper, it is technically inferior because the reference signal for measuring time is transmitted within the C band, in other words at a similar base frequency to data traffic. Not only does this leave the reference signal open to potential disruption by the rest of the data traffic, it blocks a channel that would normally be used for data transmission, which in turn complicates operation.

«We’ve now developed a third option,» explains Fabian Mauchle, project manager at Switch: «For reasons of cost, we use the existing Switch network. But instead of transmitting the reference signal within the physically optimum C band – which is largely taken up by data traffic – we use the L band, which is still mostly uncongested and has a different base frequency.» The results now show that the L band is also a viable option for transmitting reference signals at excellent quality without encountering disruption from data traffic. This did, however, require Switch to make certain modifications to its network infrastructure.

International networking

The next step will be to further expand the network to include other Swiss institutions such as Cern in Geneva, EPFL or the University of Neuchâtel. There are also plans to take the network to an international level. The goal is to establish a transnational network capable of comparing signals from various atomic clocks.

This would pave the way for an even more precise time measurement for defining the second as an SI unit. To ensure consistent time measurement worldwide, atomic clocks are currently compared with satellite signals in the gigahertz range. Synchronising atomic clocks using optical signals in the terahertz range would allow measurements of the second up to 18 decimal places instead of the «mere» 16 decimal places previously achieved. But the only way this can work is if the signals used to compare these optical clocks are transmitted as light via fibre optics.

Interesting for other disciplines

Faist also points out that it’s not just chemists and physicists who could benefit from the new network. It could provide geoscientists with new insights, too. Geoscientists might not require highly precise time signals for their experiments, but since even the tiniest disruption will affect the signal frequency, they could use the approach to detect subsurface vibrations that are too subtle for today’s measuring devices to register.

Featured image: Close-up of the optical components used to stabilise the light of the infrared laser for the precise reference frequency. (Image: Metas)


Reference

Husmann D et.al.: SI-traceable frequency dissemination at 1572.06 nm in a stabilized fiber network with ring topology. Vol. 29, No. 16 /2 August 2021. doi: 10.1364/OE.427921


Provided by University of Basel

Ultrafast X-ray Provides New Look At Plasma Discharge Breakdown in Water (Physics)

Occurring faster than the speed of sound, the mystery behind the breakdown of plasma discharges in water is one step closer to being understood as researchers pursue applying new diagnostic processes using state-of-the-art X-ray imaging to the challenging subject.

These diagnostic processes open the door to a better understanding of plasma physics, which could lead to advances in green energy production through methods including fusion, hydrocarbon reforming and hydrogen generation.

Dr. David Staack and Christopher Campbell in the J. Mike Walker ’66 Department of Mechanical Engineering at Texas A&M University are part of the team pioneering this approach to assessing plasma processes. Partners on the project include diagnostics experts from Los Alamos National Laboratories and using the facilities at the Argonne National Laboratory Advanced Photon Source (APS).

The team is working with LTEOIL on patented research into the use of multiphase plasma in carbon-free fuel reforming. The research is supported by the dynamic materials properties campaign (C2) and the advanced diagnostics campaign (C3) at Los Alamos National Laboratories through the Thermonuclear Plasma Physics group (P4) principal investigator, Zhehui (Jeph) Wang.

The research, which was recently published in Physical Review Research, is producing the first-known ultrafast X-ray images of pulsed plasma initiation processes in water. Staack, associate professor and Sallie and Don Davis ’61 Career Development Professor, said these new images provide valuable insight into how plasma behaves in liquid.

“Our lab is working with industry sponsors on patented research into the use of multiphase plasma in carbon-free fuel reforming,” Staack said. “By understanding this plasma physics, we are able to efficiently convert tar and recycled plastics into hydrogen and fuels for automobiles without any greenhouse gas emissions. In the future, these investigations may lead to improvements in inertial confinement fusion energy sources.”

Inertial confinement fusion — in which high temperature, high energy density plasmas are generated — is a specific focus of the project. To better understand the plasma physics involved in this type of fusion, Staack said the team is developing short timescale, high-speed imaging and diagnostic techniques utilizing a simple, low-cost plasma discharge system.

Additionally, they are seeking to better understand the phenomena that occur when plasma is discharged in liquid, causing a rapid release of energy resulting in low-density microfractures in the water that move at over 20 times the speed of sound.

High-speed X-ray image of plasma discharge.
Even using state-of-the-art X-ray imaging, the plasma discharge occurs so quickly that researchers were only able to record one frame per event. | Image: Courtesy of Dr. David Staack

Campbell, a graduate research assistant and Ph.D. candidate, said the team hopes their discoveries can prove to be a valuable contribution to the collective knowledge of their field as researchers seek to develop robust predictive models for how plasma will react in liquid.

“Our goal is to experimentally probe the regions and timescales of interest surrounding this plasma using ultrafast X-ray and visible imaging techniques, thereby contributing new data to the ongoing literature discussion in this area,” said Campbell. “With a complete conceptual model, we could more efficiently learn how to apply these plasmas in new ways and also improve existing applications.”

Although they have made progress, Campbell said current methods are not yet sophisticated enough to collect multiple images of a single plasma event in such a short amount of time — less than 100 nanoseconds.

“Even with the state-of-the-art techniques and fast framerates available at the Advanced Photon Source, we have only been able to image a single frame during the entire event of interest — by the next video frame, most of the fastest plasma processes have concluded,” Campbell said. “This work highlights several resourceful techniques we have developed to make the most of what few images we are able to take of these fastest processes.”

The team is currently working to measure the pressures induced by the rapid phenomena and preparing for a second round of measurements at APS to investigate interacting discharges, discharges in different fluids and processes that may limit confinement of higher energy discharges. They look forward to the opportunity of using even higher-framerate X-ray imaging methods ranging up to 6.7 million frames per second, compared to 271 thousand frames per second in this study.

Featured image: Christopher Campbell and Dr. Xin Tang work to record plasma discharge in the Argonne National Laboratory Advanced Photon Source. | Image: Courtesy of Dr. David Staack


Reference: Christopher Campbell, Xin Tang, Yancey Sechrest, Kamel Fezzaa, Zhehui Wang, David Staack. Ultrafast x-ray imaging of pulsed plasmas in water. Physical Review Research, 2021; 3 (2) DOI: 10.1103/PhysRevResearch.3.L022021


Provided by TEES Texas A&M University

New Solid-state Thermal Diode Developed with Better Rectification Performance (Physics)

The effective control of heat transfer is of great significance to improve energy efficiency. Thermal diode is one of the key elements for heat flow controlling. Similar to the current rectification effect of electronic diodes, heat flow is easily directed through one direction in a thermal diode, while obstructed in the opposite direction. Sizable heat rectification can be obtained using a junction of two solid materials with opposite trends in thermal conductivity as a function of temperature. This type of thermal diodes is attractive due to their scalability and analogy to electrical diode design.

A team led by Prof. TONG Peng from the Hefei Institutes of Physical Science (HFIPS) of the Chinese Academy of Sciences (CAS) had reported that they’ve found sulfides Ni1-xFexS, a series of materials that may unlock new ways to better thermal rectification.

Recently, the same team announced that they constructed a novel thermal diode with a combined material of Ni0.85Fe0.15S and alumina, which displayed excellent performance over solid-state thermal diodes ever reported. Their up-to-date result was published on Journal Physical Review Applied.

In their previous study, they discovered the abrupt jump of thermal conductivity in the vicinity of the first-order phase transition (FOPT) in Ni1-xFexS. The change of thermal conductivity reaches as large as 200%, which suggests the sulfides are promising materials for designing solid-state thermal diodes.

On this base, they constructed a thermal diode with Ni0.85Fe0.15S (bonded by 10wt.%Ag) and Al2O3 as two segments. The thermal diode exhibits excellent thermal rectification performance. When the cold end of the thermal diode is set at 250 K, at a temperature bias of 97 K, the maximum thermal rectification coefficient γmax reaches 1.51.

The Ni0.85Fe0.15S/Al2O3 thermal diode shows advantages over other solid-state thermal diodes ever reported. Namely, its γmax is the largest among the reported values, meanwhile the requested temperature bias for driving γmax is at least 100 K less than that of reported thermal diodes having comparable γmax values. 

The outstanding thermal rectification effect of the current thermal diode may have potential applications in thermal management systems, for example, caloric refrigeration and energy conversion.

Moreover, on the base of systematical experimental and theoretical analysis, the team clarified how the thermal rectification factor is affected by the cold terminal temperature, the length ratio of Ni0.85Fe0.15S and Al2O3 segments, and the sharpness of the FOPT of Ni0.85Fe0.15S.

These new results provide guides for designing new solid-state thermal diodes in the future.

This work was supported by the National Natural Science Foundation of China, the Key Research Program of Frontier Sciences and the Users with Excellence Program of Hefei Science Center of CAS.

Featured image: (a) and (b) represent for the schematic geometry of the thermal diode consisting of Ni0.85Fe0.15S and Al2O3 for forward and reverse direction. (c) Thermal rectification factor (γ) as a function of temperature bias (ΔT) along with those reported. (Image by ZHANG Xuekai)  


Reference: Xuekai Zhang, Peng Tong, Jianchao Lin, Kun Tao, Xuelian Wang, Lulu Xie, Wenhai Song, and Yuping Sun, “Large Thermal Rectification in a Solid-State Thermal Diode Constructed of Iron-Doped Nickel Sulfide and Alumina”, Phys. Rev. Applied 16, 014031 – Published 13 July 2021. DOI: https://doi.org/10.1103/PhysRevApplied.16.014031


Provided by Chinese Academy of Sciences

How Secondary Electron Emission Yield Properties Affect Atomic Layer Deposition-coated Microchannel Plate?(Physics)

The microchannel plate-photomultiplier tube (MCP-PMT), a kind of compact high-sensitive photo device consisting of photocathode, MCPs, anode and tube shell, has attracted much attention in the field of modern high-energy physics detection.

However, the damage caused by the ions feedback in the channel to the photocathode limits the lifetime of the MCP-PMT. To solve the problem, the atomic layer deposition (ALD) technique is used to deposit the novel separate resistive and emissive layers onto the inner surface of the channel.

A research team led by Prof. Dr. TIAN Jinshou from Xi’an Institute of Optics and Precision Mechanics (XIOPM) of the Chinese Academy of Sciences (CAS) revealed how the secondary electron emission (SEE) yield properties of the emissive materials affect the behavior of the ALD-coated MCP.

Their findings were published in Nuclear Instruments and Methods in Physics Research.

Fig. 1 Schematic structure and principle of the MCP (Image by XIOPM)   

The researchers investigated the situation about three important secondary electron emissions – the backscattered, rediffused and true SEE, respectively.

They made all settings the same as the conventional MCP, and then changed the key parameters with other parameters unchanged to derive the main characteristics of the ALD-coated MCP, gain and timing performance, and dependency, etc.

The simulation results indicated the opportunities for improving the gain of the ALD-coated MCP by improving the SEE yields corresponding to the incident energies of 0eV-100eV.

“We found that the backscattered and rediffused electrons have strong effects on the gain and timing performance of the MCP. Although the higher the SEE yield the higher the MCP gain, the extremely high SEE yield will make the MCP saturated prematurely and degrade the time resolution,” said Dr. TIAN.

This work may provide inspiration to the study and selection of material of emissive layer and provide references for developing the next generation ALD-coated MCP-PMT.

Featured image: Cross section of the 3D model of the MCP single channel (Image by XIOPM)


Reference: Lehui Guo, Liwei Xin, Lili Li, Yongsheng Gou, Xiaofeng Sai, Shaohui Li, Hulin Liu, Xiangyan Xu, Baiyu Liu, Guilong Gao, Kai He, Mingrui Zhang, Youshan Qu, Yanhua Xue, Xing Wang, Ping Chen, Jinshou Tian, Effects of secondary electron emission yield properties on gain and timing performance of ALD-coated MCP, Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, Volume 1005, 2021, 165369, ISSN 0168-9002, https://doi.org/10.1016/j.nima.2021.165369. (https://www.sciencedirect.com/science/article/pii/S0168900221003533)


Provided by Chinese Academy of Sciences