In the video below, Porter shows how the precise movements of the robotic grippers can be used to fold the tiny creases on a very basic paper airplane, which is about the size of a penny. Porter controls the robotic grippers with joystick-like hand controllers while looking at a 3D image on a viewfinder. The system translates the surgeon’s movements into more precise micro-movements while reducing any shaking. © 2010 PhysOrg.com Dr. James Porter folds a paper airplane using the da Vinci surgical robot. Currently, 1,000 of the $1.3-million da Vinci robots are being used worldwide to perform surgeries. Among the advantages of the system are that many procedures that traditionally require large incisions can now be made minimally invasive, and many patients have shorter recovery times. This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only. Explore further Citation: Da Vinci surgical robot makes a tiny paper airplane (2011, April 5) retrieved 18 August 2019 from https://phys.org/news/2011-04-da-vinci-surgical-robot-tiny.html (PhysOrg.com) — The da Vinci surgical robot may be best known for performing prostate, gynecological, and heart valve surgeries. But in its spare moments, as Dr. James Porter of the Swedish Medical Center in Seattle has recently demonstrated, the da Vinci robot can also make and fly paper airplanes. This image taken from the video below shows da Vinci’s robotic grippers making a paper airplane. Image credit: Swedish Medical Center. More information: via: IEEE Spectrum Robots help surgeons transcend human limits
Image: Wikipedia This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only. Simple technique results in surprising repellency results Explore further (PhysOrg.com) — Anyone who has ever swum around near the bottom of a swimming pool, or flippered along an ocean floor for any length of time without benefit of an air supply knows that there is a decision making process going on from the moment the dive begins: when to surface? In people, the process clearly involves some calculating. The deeper a person dives, the more time must be allotted to reach the surface. A miscalculation can result in panic, or worse tragedy. But then, people aren’t exactly at home in the deep water; but penguins are. So, how do they figure out when it’s time to surface? Surely they’re not thinking it over the whole time, that would take away from focusing on the reason for the dive. Finding and eating fish. That’s what Dr Kozue Shiomi and his colleagues from the University of Tokyo wanted to know, so they set about studying emperor penguins to find out. As it turns out, as they explain in their paper published in The Journal of Experimental Biology, it’s not so much about timing as it is about energy used in flapping their wings underwater to chase after prey.Dr. Shiomi and his team studied the penguins in their two major diving environments: in open water, and when diving from and returning to a hole in the ice. In both cases, the birds were timed to see how long their foraging expeditions under the water lasted.When fishing in open water, the ten free-rangers studied, over the course of 15,978 dives stayed under for an average of 5.7 minutes. When fishing from a hole in the ice however, the three birds under study dived 495 times but stayed under much longer, which led the researchers to believe that the penguins’ decision to end their time under water wasn’t about how long they’d been under at all. This led them to consider the possibility that it was based on energy expended instead, which is how they came to start counting how many times the penguins flapped their wings to propel themselves while chasing after fish.Turns out regardless of whether the penguins are fishing in open water, or through a hole in the ice, they flap on average 237 times before surfacing. Thus, it seems rather clear that they are basing their time spent under water on energy spent flapping, rather than on some predetermined time span; though, how they count and keep track, is still anyone’s guess. © 2011 PhysOrg.com More information: Point of no return in diving emperor penguins: is the timing of the decision to return limited by the number of strokes? J Exp Biol 215, 135-140. January 1, 2012. doi: 10.1242/jeb.064568AbstractAt some point in a dive, breath-hold divers must decide to return to the surface to breathe. The issue of when to end a dive has been discussed intensively in terms of foraging ecology and behavioral physiology, using dive duration as a temporal parameter. Inevitably, however, a time lag exists between the decision of animals to start returning to the surface and the end of the dive, especially in deep dives. In the present study, we examined the decision time in emperor penguins under two different conditions: during foraging trips at sea and during dives at an artificial isolated dive hole. It was found that there was an upper limit for the decision-to-return time irrespective of dive depth in birds diving at sea. However, in a large proportion of dives at the isolated dive hole, the decision-to-return time exceeded the upper limit at sea. This difference between the decision times in dives at sea versus the isolated dive hole was accounted for by a difference in stroke rate. The stroke rates were much lower in dives at the isolated hole and were inversely correlated with the upper limit of decision times in individual birds. Unlike the decision time to start returning, the cumulative number of strokes at the decision time fell within a similar range in the two experiments. This finding suggests that the number of strokes, but not elapsed time, constrained the decision of emperor penguins to return to the surface. While the decision to return and to end a dive may be determined by a variety of ecological, behavioral and physiological factors, the upper limit to that decision time may be related to cumulative muscle workload. Citation: Researchers find clue to explain how penguins know when to surface (2011, December 9) retrieved 18 August 2019 from https://phys.org/news/2011-12-clue-penguins-surface.html
Explore further (Phys.org)—A team of researchers working at the University of Hawaii using data from the Kepler space telescope, has found that the oscillations made by a star conform closely to the golden mean—further study showed that it also behaves in a fractal pattern. In their paper published in the journal Physical Review Letters, the team describes their analysis of data from the pulsating star KIC 5520878, captured over a period of several years, and why what they found is cause for excitement. This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only. Credit: J. Lindner et al., Phys. Rev. Lett. (2015) Market crashes are anomalous features in the financial data fractal landscape © 2015 Phys.org Journal information: arXiv More information: Strange Nonchaotic Stars, Phys. Rev. Lett. 114, 054101 – Published 3 February 2015. dx.doi.org/10.1103/PhysRevLett.114.054101 . On Arxiv: arxiv.org/abs/1501.01747ABSTRACTThe unprecedented light curves of the Kepler space telescope document how the brightness of some stars pulsates at primary and secondary frequencies whose ratios are near the golden mean, the most irrational number. A nonlinear dynamical system driven by an irrational ratio of frequencies generically exhibits a strange but nonchaotic attractor. For Kepler’s “golden” stars, we present evidence of the first observation of strange nonchaotic dynamics in nature outside the laboratory. This discovery could aid the classification and detailed modeling of variable stars. , Physical Review Letters Citation: Researchers find evidence of fractal behavior in pulsating stars (2015, February 4) retrieved 18 August 2019 from https://phys.org/news/2015-02-evidence-fractal-behavior-pulsating-stars.html In studying the Kepler data, the team was able to track the pulses that emanated from the star over a period of four years—taken at 30 minute intervals. They found that two of star KIC 5520878’s pulsating frequencies occurred at 4.05 and a 6.41 hour cycles—which the team noted had a ratio of 1.58, which is close to 1.618, aka the Golden Ratio—famously found in nature and sometimes artistic renderings. Intrigued, they looked deeper and found that the frequencies conformed to fractal patterns—separating the oscillations into their constituent parts revealed additional weaker frequencies, similar to the way, the team points out, that images of shorelines display craggy lines regardless of how close or far away they are viewed from. Counting bumps on converted plots which had heights greater than a certain threshold revealed a power law dependence—one of the accepted signs of fractal behavior. The difference here of course is that with traditional fractal systems, the behavior is seen visually—here it was seen as factor of time. The group suggests that their finding appears to be the first example of a “strange nonchaotic attractor” which is a system that displays a fractal structure but does not have the sensitivity to beginning conditions of other known chaotic systems, such as the weather.The team looked at five other pulsating stars to see if they could spot a pattern and found mixed results, three of them had near golden ratios and fractal patterns, while two others had neither. What is still unclear at this point is whether the behavior of the stars that do show fractal structure is something that happens for a reason, which could perhaps offer new clues about stellar physics, or if the ratios found by the team are merely coincidence.
Journal information: Nature Nanotechnology (Phys.org)—A team of researchers with members from the Netherlands, Australia, and the U.K. has developed a new way to build an extremely sensitive magnetic sensor. As they describe in their paper published in the journal Nature Nanotechnology, their sensors are based on sensing with a single electron spin using real-time adaptive measurements. The work by the team marks the development of the first quantum sensor to be based on the spin of a single electron, which in this case, was trapped in a diamond nitrogen-vacancy center. It is so sensitive that it is able to measure the strength of a magnetic field to the very limits of that described by quantum physics.The problem with attempting to use the spin of an electron as a sensor, of course, is that it must be measured, which causes the quantum state to be affected. To get around this problem the researchers used an atomic sized defect in diamond kept in an extremely cold environment—the spin in its defect (nitrogen-vacancy) is not very sensitive to environmental noise because it has no net nuclear spin. The sensor works by taking multiple measurements as the electron is exposed to the magnetic field, on the spin defect, using optimal settings based on prior measurements and then adjusting those that come after using Bayesian statistics—it is based on Zeeman interactions, the researches explain—which is what happens when an electron moves into an magnetic field. The actual measurements are taken by subjecting the spin to microwave radiation, then exciting it with a laser and then measuring the fluorescent signals that are produced. The data is then processed (on an off-the-shelf microprocessor they programmed for their purposes) and the results are used to set the settings for the next measurement, and so on.The result is a sensor that is 100 times more precise than previous sensors, though the team acknowledges that to make it useful, they will have to find a way to make it usable at room temperature. If they can do that, the sensor could conceivably be used to image the makeup of individual molecules, or perhaps as a method for storing qubits in a quantum computer. Explore further More information: C. Bonato et al. Optimized quantum sensing with a single electron spin using real-time adaptive measurements, Nature Nanotechnology (2015). DOI: 10.1038/nnano.2015.261AbstractQuantum sensors based on single solid-state spins promise a unique combination of sensitivity and spatial resolution. The key challenge in sensing is to achieve minimum estimation uncertainty within a given time and with high dynamic range. Adaptive strategies have been proposed to achieve optimal performance, but their implementation in solid-state systems has been hindered by the demanding experimental requirements. Here, we realize adaptive d.c. sensing by combining single-shot readout of an electron spin in diamond with fast feedback. By adapting the spin readout basis in real time based on previous outcomes, we demonstrate a sensitivity in Ramsey interferometry surpassing the standard measurement limit. Furthermore, we find by simulations and experiments that adaptive protocols offer a distinctive advantage over the best known non-adaptive protocols when overhead and limited estimation time are taken into account. Using an optimized adaptive protocol we achieve a magnetic field sensitivity of 6.1 ± 1.7 nT Hz−1/2 over a wide range of 1.78 mT. These results open up a new class of experiments for solid-state sensors in which real-time knowledge of the measurement history is exploited to obtain optimal performance. Spin lifetime of electrons in graphene increased by magnetic fields © 2015 Phys.org Experiment apparatus. Credit: (c) 2015 Nature Nanotechnology (2015) doi:10.1038/nnano.2015.261 Citation: Researchers build quantum sensors based on single solid-state spins (2015, December 2) retrieved 18 August 2019 from https://phys.org/news/2015-12-quantum-sensors-based-solid-state.html This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only.
This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only. An international team of researchers has experimentally observed Bethe strings for the first time. In their paper published in the journal Nature, the team describes their experiments and what they observed, and offer possible implications of their work. Citation: Bethe strings experimentally observed for the first time (2018, February 8) retrieved 18 August 2019 from https://phys.org/news/2018-02-bethe-experimentally.html Explore further Researchers observe dynamical quantum phase transitions in an interacting many-body system Journal information: Nature More information: Zhe Wang et al. Experimental observation of Bethe strings, Nature (2018). DOI: 10.1038/nature25466AbstractAlmost a century ago, string states—complex bound states of magnetic excitations—were predicted to exist in one-dimensional quantum magnets. However, despite many theoretical studies, the experimental realization and identification of string states in a condensed-matter system have yet to be achieved. Here we use high-resolution terahertz spectroscopy to resolve string states in the antiferromagnetic Heisenberg–Ising chain SrCo2V2O8 in strong longitudinal magnetic fields. In the field-induced quantum-critical regime, we identify strings and fractional magnetic excitations that are accurately described by the Bethe ansatz. Close to quantum criticality, the string excitations govern the quantum spin dynamics, whereas the fractional excitations, which are dominant at low energies, reflect the antiferromagnetic quantum fluctuations. Today, Bethe’s result is important not only in the field of quantum magnetism but also more broadly, including in the study of cold atoms and in string theory; hence, we anticipate that our work will shed light on the study of complex many-body systems in general. © 2018 Phys.org Bethe strings were first proposed over a century ago by German physicist Hans Bethe, who based them on theories by Werner Heisenberg, and are defined as a type of collective behavior in electrons—a behavior that can travel between particles. The research team with this new effort refers to them as “complex bound states of magnetic excitations.” Some have described them as being similar to a line of people at a soccer game performing the “wave”—in this analogy, their up-and-down arm movements represent the up or down spin states of electrons. Bethe suggested their existence back in the 1930s, and others have refined the theory, but only recently has highly sophisticated technology emerged to test their existence.As a means of observing them experimentally, the researchers first synthesized SrCo2V2O8 crystals—an antiferromagnetic Heisenberg-Ising chain. They then used high-resolution terahertz spectroscopy to resolve the spin states. Doing so allowed for calculating the string suggestions using the Bethe approach, which showed that the spectroscopy had served as an indirect means of observation of Bethe strings.The team describes their work as a step forward in research into spin dynamics as they pertain to quantum magnetism. They suggest further that their findings could also have an impact on research in other areas such as cold quantum gases.On the other hand, since eventual observation of Bethe strings was expected at some point, the achievement by the research team is not likely to change much of anything in the physics world except for the possibility of using them in some way, such as manipulating them to allow for data storage. Less practically, it is also possible that gaining a better understanding of Bethe strings through observation might lead to a breakthrough in string theory. Quantum spin chain in SrCo2V2O8, psinon–(anti)psinon pairs and strings. Credit: Nature (2018). DOI: 10.1038/nature25466
In the present work, Andersen et al. used electrically gated graphene devices fabricated on diamond and silicon/silicon dioxide substrates, encapsulated in hexagonal boron nitride (hBN) at cryogenic temperatures (T= 10 to 80 K) to conduct the proposed experiments. The experimental setup provided low-bias transport properties for the ultraclean graphene system with a mobility ranging from 20 to 40 m2/V.s at a carrier density (2 x 1012 cm-2), corresponding to nearly ballistic transport. Due to high mobility, carriers could be accelerated by an electric field to high drift velocities to observe nonlinear current response, while a disordered device contrastingly showed linear ohmic behavior. To study the nonequilibrium behavior, first, Andersen et al. measured the global noise in the source-drain current with a spectrum analyzer, while varying the applied bias power (P). The results indicated a new source of noise in graphene devices with low disorder, encapsulated in hBN. To gain insight to the observed anomaly, the scientists performed spatially resolved noise measurements by constructing graphene devices on diamond substrates with shallow nitrogen-vacancy color-center impurities of 40 to 60 nm in depth. They measured the atom-like spin qubits using confocal microscopy and probed the nanoscale current noise by measuring the resulting magnetic fields. Andersen et al. probed the spatial dependence of the anomalous noise by optically observing single NV centers along the device to measure their spin relaxation rate . The noise exhibited clear symmetry with the direction of the current, an unexpected outcome since global noise and transport properties are independent of the direction of the current. Then using the device gate, Andersen et al. demonstrated that the local noise signal depended on the flow direction of momentum and not charge. The scientists also showed that the noise was small at the carrier entry point but grew exponentially as the carrier flowed across the 17-µm long device. Journal information: Science Understanding nonequilibrium phenomena to effectively control it is an outstanding challenge in science and engineering. In a recent study, Trond. I. Andersen and colleagues at the departments of physics, chemistry, materials science and engineering in the USA, Japan and Canada used electricity to drive ultraclean graphene devices out-of-equilibrium and observe the manifested instability as enhanced current fluctuations and suppressed conductivity at microwave frequencies. Nonequilibrium dynamics in graphene, probed both globally and locally. (A) Device schematic: hBN (hexagonal boron nitride) -encapsulated graphene device on diamond substrate containing NV (Nitrogen-Vacany) centers for nanomagnetometry. (Inset) The optical image of clean hBN-encapsulated device A1 (6 μm x5.4 μm) (B) Condition for Cerenkov emission of phonons: when vD>vs, stimulated phonon (ph) emission dominates over absorption (right). (C) Two-probe resistance versus carrier density of device A1 (T = 10 K). (D) Current density as a function of applied electric field (T = 80 K) in clean device A1 (blue) and disordered device B1 (7 μm by 18 μm, black). The gray dashed line indicates where vD=vs for the longitudinal acoustic mode. (E) Global electronic noise PSD (averaged over 100 to 300 MHz) as a function of bias power in devices A1 (blue) and B1 (black). Blue curve satisfies vD>vs for P > 0.12 μW/μm2. (F) Local magnetic noise (measured by NV nanomagnetometry) versus applied bias power in clean device C1 on diamond substrate. Error bars represent 95% confidence intervals. Credit: Science, doi: 10.1126/science.aaw2104 Using the experimental setup, they found that direct current at high drift velocities generated a large increase in the noise at gigahertz frequencies and the noise grew exponentially in the direction of the current. Andersen and co-workers credited the observed emission mechanism, to the amplification of acoustic phonons by the Cerenkov effect (a characteristic blue glow resulting from charged particles passing through an insulator at a speed greater than the speed of light in that medium) and have now published the results on Science. The scientists spatially mapped the nonequilibrium current fluctuations using nanoscale magnetic field sensors to reveal that they grew exponentially along the direction of carrier flow. Andersen et al. credited the observed dependence of the phenomenon on density and temperature, to electron-phonon Cerenkov instability at supersonic drift velocities. Supersonic drift velocities occurred when the population of certain phonons increased with time due to forced Cerenkov emission, when the drift velocity of electron conduction was greater than the velocity of sound (VD>VS) in the medium. The experimental results can offer the opportunity to generate tunable terahertz frequencies and construct active phononic devices on two-dimensional materials. Nonequilibrium phenomena driven in electronic and optical systems display rich dynamics, which can be harnessed for applications as Gunn diodes and lasers. Two-dimensional materials such as graphene, are an increasingly popular new platform to explore such phenomena. For instance, modern ultraclean graphene devices demonstrate high mobilities and can be driven to high electronic velocities with predicted instabilities to include hydrodynamic instabilities in electronic fluids and Dyakonov-Shur instabilities where the driven electrons can amplify plasmons. Since the Cerenkov amplification is sensitive to the phonon lifetime, the scientists expected the effects to intensify at lower temperatures due to slower anharmonic decay. However, as Andersen et al. reduced the temperature from 300 to 10 K, they observed a strong increase in noise – in clear contrast to the decreasing thermal noise observed at low drives (vD≲vs), suggesting that the amplification process was limited by scattering with thermally occupied modes. In this way, Andersen et al. extensively detailed how nonequilibrium dynamics stemming from electron-phonon instability could be demonstrated in a 2D material. In the experiments, the driven electron-phonon system showed rich nonequilibrium dynamics that merit further investigations using new techniques to directly characterize the phonon spectrum and gain further insights. Previous theoretical studies had predicted amplified phonons in graphene with frequencies as high as 10 THz, substantially higher than those in several other materials. The experimental system can offer pure electrical generation and phonon amplification in a single micrometer-scale device with wide frequency tunability. Andersen et al. envision applications that will explore coupling to a mechanical cavity to develop a phonon laser, and outcoupling of the amplified sound waves to far-field terahertz radiation for medical imaging and security screening imaging (due to the degree of imaging transparency offered), wireless communications, quality control and process monitoring in manufacturing applications. The results by Andersen et al. represent a promising step towards the development of new-generation active phononic and photonic devices for multidisciplinary applications in future work. The study of electronic properties of graphene under extreme nonequilibrium conditions therefore provides a productive testbed to assess and monitor exotic transport phenomena. In addition to the use of high-frequency signal generation, Andersen et al. investigated the underlying non-equilibrium dynamics during electron transport in ultraclean graphene devices containing an extremely high electron drift velocity. Understanding nonequilibrium dynamics is vital for many technical applications of graphene; including high frequency transistors, ultrafast incandescent light sources and flexible transport interconnects. However, it is difficult to realize the electronic stabilities in practice, due to increased phonon scattering at high drift velocities. Dependence on bath temperature and charge density. (A) Global noise PSD as a function of bath temperature at constant drift velocities and n = 2 × 10^12 cm−2. (B) Calculated peak phonon emission frequency, which can be tuned via the graphene carrier density (blue: Te = 0 K; red: Te = 320 K). (C) Normalized global current noise as a function of carrier density for different device lengths (j = 0.6 mA/μm). Solid curves show predicted total phonon emission. (D) The charge density at which the noise peaks (npeak) for a wider variety of samples than in (C), with fit (blue). Error bars represent sampling spacing of carrier densities. Credit: Science, doi: 10.1126/science.aaw2104. © 2019 Science X Network The scientists consistently explained all observations using the electro-phonon Cerenkov instability. As a key insight of the study, Andersen et al. showed that when the electronic drift velocity exceeded the speed of sound (supersonic drift velocity), the forward-moving acoustic phonons experienced a faster rate of simulated emission than absorption. Pristine graphene also exhibited long acoustic phonon lifetimes; therefore, an emitted phonon could stimulate the emission of exponential growth in the setup. When they modelled these effects mathematically, the results agreed well with experimental outcomes, while the anomalous noise further increased with increasing device length. The model predicted that the observed electron-phonon instability would give rise to a conductivity spectrum. The scientists continued to explore the nonequilibrium dynamics using models of the electron-phonon system. Amplifier for terahertz lattice vibrations in a semiconductor crystal TOP: Measurement circuit. Circuit diagram for the measurement of noise (red box) and AC differential conductivity (yellow box). LEFT: Device fabrication on diamond substrate. (A) Device schematic: Monolayer graphene (grey chain) was graphite contacted and encapsulated with hexagonal boron nitride (hBN). Few-layer graphene (FLG) was used as topgate. (B-H) Micrographs of device fabrication, with 40 µm scalebar in (B)-(G) and 500 µm in (H). (B) Exfoliated graphene. White dashed line indicates monolayer region. (C) Complete stack on diamond substrate with shallow implanted (40 − 60 nm deep) NV centers. (D) Initial contacts and wire for delivering reference noise (left-most electrode). (E) Device after etch to define geometry. (F) Edge contacts constructed through etching and subsequent thermal evaporation. (G) Device with etch mask for disconnecting topgate from edge contacts. Note that ripples visible in the image are entirely contained in the top gate graphene and are not expected to affect the transport properties of the channel graphene, due to the thick (∼ 90 nm) hBN dielectric. (H) Entire (2×2 mm2 ) single crystal diamond, with wire bonded device. RIGHT: Device fabrication on Si/SiO2 substrate. (A) Device schematic: Monolayer graphene (grey chain) was encapsulated with hexagonal boron nitride (hBN). Silicon substrate was used as a global backgate. (B)-(F) Micrographs of device fabrication, with 20 µm scalebar. (B) Exfoliated graphene. (C) Complete stack on substrate. (D) Initial contacts. (E) Edge contacts constructed through etching and subsequent thermal evaporation. (F) Device after geometry-defining etch. Credit: Science, doi: 10.1126/science.aaw2104 More information: 1. Electron-phonon instability in graphene revealed by global and local noise probes DOI: 10.1126/science.aaw2104 , https://science.sciencemag.org/content/364/6436/154 Trond I. Andersen et al. 12 April 2019, Science.2. Cerenkov emission of terahertz acoustic-phonons from graphene aip.scitation.org/doi/abs/10.1063/1.4808392 C.X. Zhao et al. 2013, Applied Physics Letters.3. Nanoscale magnetic sensing with an individual electronic spin in diamond www.nature.com/articles/nature07279 J.R. Maze et al. 2008, Nature. 4. Acoustic amplification in semiconductors and metals www.tandfonline.com/doi/abs/10 … 7?journalCode=tphm19 A.B. Pippard, 1962, The Philosophical Magazine: A Journey of Theoretical Experimental and Applied Physics. Spatially resolved local noise measurements with NV magnetometry. (A) Fluorescence image of NV centers underneath device C2, with false-colored contacts and borders added. (B) NV spin relaxation from polarized to thermal state (dashed line), when current densities j = 0 mA/μm (dark blue) and j = −0.19 mA/μm (light blue) are passed through the device. Solid lines are fits. ms, spin quantum number. (C) Local magnetic noise near drain contact as a function of graphene current density (device C1) in electron (e)– and hole (h)–doped regime (blue and red, respectively). (D) Spatial map of the local magnetic noise (device C2) at j = 0.18 mA/μm and n = 0.92 × 1012 cm−2. The spatial profile is consistent with the exponential growth of phonons due to Cerenkov amplification (cartoon, top). Dashed black curve shows the theoretically predicted excess phonon population (offset to account for background noise). a.u., arbitrary units. (E) The growth direction is reversed by changing the current direction (left) or the charge carrier sign (right). Error bars represent 95% confidence intervals. Credit: Science, doi: 10.1126/science.aaw2104 Explore further In principle, while phonon scattering loss is typically irreversible, long-lived phonons can act as a dominant source of instability within the experimental setup. When the electronic drift velocity (VD) exceeds the velocity of sound (VS), phonon emission becomes greater than phonon absorption, resulting in an exponential growth of the phonon population, known as phonon Cerenkov amplification. The phenomenon was long explored in theory as a technique to produce high-frequency acoustic waves, with accompanying experimental evidence in bulk systems and semiconductor superlattices obtained using acoustic and optical measurements thereafter. Slow dynamics in global electronic measurements. (A) Global noise spectra at n = 2 × 1012 cm−2. Colored curves: clean device A2 (9.5 μm by 11 μm) at bias ranging from 0 to 0.8 V (bottom to top). Black curve: disordered device B1 at maximum power applied to device A2 (scaled 7×). (B) Ac differential conductivity spectra (excitation: −20 dBm) (19) with biases 0 to 0.8 V [top to bottom, colors same as in (A)]. The real (Re) component is suppressed at low frequencies. Gray curve: imaginary (Im) component at 0.8 V. Black curves are fits. (C and D) Features in noise and conductivity spectra shift to higher frequencies in a shorter (6-μm) device (device A1) under similar electric field as maximum in (A) and (B). (E and F) Extracted traversal time from (B) and (D) as a function of drift velocity and device length. Dashed curves correspond to speed of sound in graphene [light gray, transverse acoustic (TA); dark gray, longitudinal acoustic (LA)]. (G) Cartoon of important rates in the driven electron-phonon system. During Cerenkov amplification, the correlation time observed in electronic measurements is limited by the phonon traversal time, tT=L/vs. Credit: Science, doi: 10.1126/science.aaw2104 Citation: Electron-phonon instability in graphene revealed by global and local noise probes (2019, April 23) retrieved 18 August 2019 from https://phys.org/news/2019-04-electron-phonon-instability-graphene-revealed-global.html This document is subject to copyright. Apart from any fair dealing for the purpose of private study or research, no part may be reproduced without the written permission. The content is provided for information purposes only.
Darjeeling: Twenty-two police personnel, who were returning home to North Dinajpur from Kalimpong where they were deployed for chief minister Mamata Banerjee’s visit, were injured when their van overturned on Friday. Police sources said the van overturned in a bend at 3rd Mile, about 9km from Kalimpong town around 2pm. “The van hit the guard rails on the road and overturned. Had it jumped the guard walls, it would have tumbled down the hill,” said a police source.Thirty-six police personnel were travelling in the van.”Investigations are on to ascertain the cause of the accident,” said a police source.
“She thinks of him and so she dresses in black, and though he’ll never come back, she’s dressed in black Oh dear, what can I do? Baby’s in black and I’m feeling
Beijing Olympics champion Abhinav Bindra produced some excellent shooting to clinch the gold while London Games bronze medallist Gagan Narang failed to finish on the podium in the men’s 10m Air Rifle event of the Asian Air Gun Championships here on Sunday.32-year-old Bindra, who won gold in 2008 Olympics in 10m Air Rifle event, shot 208.3 to bag the top prize ahead of Kazakhstan’s world number eight Yurkov Yuriy (206.6) and Korea’s Yu Jaechul (185.3) on the first day of competition at the Dr Karni Singh Shooting Range. Also Read – A league of his own!Narang, who won a bronze in the same event in 2012 London Olympics, finished fourth with a score of 164.5 while another Indian, Chain Singh was two places below at seventh after notching up 122.7.Narang started on a good note, shooting 10.6 and 10.6, but fell behind in the following attempts while Chain Singh was out after a shoot-off with Korea’s Kim
As many as 94 per cent of Indian netizens surveyed by a telecom service provider feel that internet has improved their lives while 40 per cent think spreading rumours was the most annoying internet habit among Indian netizens.Inviting people to play online games, sharing inappropriate content, sympathy-seeking posts on Facebook and trolling (offensive posts to elicit angry responses) are the other major bad habits, the study by Telenor company said. According to the study, 33 per cent Indians hate excessive selfie-takers against the regional average of 21 per cent.The survey revealed that 65 per cent of Indian netizens are internet addicts. In an effort to learn more about their customers, Telenor conducted the internet behavioural survey across India, Thailand, Singapore and Malaysia.From profanity-tolerance levels to selfie-approval ratings, respondents across the region replied about what they love and loathe most about the net.“This survey gives us a very stimulating way to look at our customers are and their online preferences. As online access increases in the country, it is great to see that 94 per cent of the Indians surveyed say that internet has improved their lives, the highest percentage among the surveyed nations,” said Sharad Mehrotra, CEO, Telenor India.