Laser pulses help scientists tease apart complex electron interactions

Laser pulses help scientists tease apart complex electron interactions

Microscopic image of one of the bismuth strontium calcium copper oxide samples the scientists studied using a new high-speed imaging technique. Color changes show changes in sample height and curvature to dramatically reveal the layered structure and flatness of the material. Credit: Brookhaven National Laboratory

Scientists studying high temperature superconductors-materials that carry electric current with no energy loss when cooled below a certain temperature-have been searching for ways to study in detail the electron interactions thought to drive this promising property. One big challenge is disentangling the many different types of interactions-for example, separating the effects of electrons interacting with one another from those caused by their interactions with the atoms of the material.

Now a group of scientists including physicists at the U.S. Department of Energy’s Brookhaven National Laboratory has demonstrated a new laser-driven “stop-action” technique for studying complex  under dynamic conditions. As described in a paper just published in Nature Communications, they use one very fast, intense “pump” laser to give  a blast of energy, and a second “probe” laser to measure the electrons’ energy level and direction of movement as they relax back to their normal state.

“By varying the time between the ‘pump’ and ‘probe’ laser pulses we can build up a stroboscopic record of what happens-a movie of what this material looks like from rest through the violent interaction to how it settles back down,” said Brookhaven physicist Jonathan Rameau, one of the lead authors on the paper. “It’s like dropping a bowling ball in a bucket of water to cause a big disruption, and then taking pictures at various times afterward,” he explained.

The technique, known as time-resolved, angle-resolved photoelectron spectroscopy (tr-ARPES), combined with complex theoretical simulations and analysis, allowed the team to tease out the sequence and energy “signatures” of different types of electron interactions. They were able to pick out distinct signals of interactions among  (which happen quickly but don’t dissipate much energy), as well as later-stage random interactions between electrons and the atoms that make up the crystal lattice (which generate friction and lead to gradual energy loss in the form of heat).

But they also discovered another, unexpected signal-which they say represents a distinct form of extremely efficient  at a particular energy level and timescale between the other two.

“We see a very strong and peculiar interaction between the excited electrons and the lattice where the electrons are losing most of their energy very rapidly in a coherent, non-random way,” Rameau said. At this special energy level, he explained, the electrons appear to be interacting with lattice atoms all vibrating at a particular frequency-like a tuning fork emitting a single note. When all of the electrons that have the energy required for this unique interaction have given up most of their energy, they start to cool down more slowly by hitting atoms more randomly without striking the “resonant” frequency, he said.

Laser pulses help scientists tease apart complex electron interactions
Brookhaven Lab physicists Peter Johnson (rear) and Jonathan Rameau. Credit: Brookhaven National Laboratory

The frequency of the special lattice interaction “note” is particularly noteworthy, the scientists say, because its energy level corresponds with a “kink” in the energy signature of the same material in its superconducting state, which was first identified by Brookhaven scientists using a static form of ARPES. Following that discovery, many scientists suggested that the kink might have something to do with the material’s ability to become a superconductor, because it is not readily observed above the superconducting temperature.

But the new time-resolved experiments, which were done on the material well above its superconducting temperature, were able to tease out the subtle signal. These new findings indicate that this special condition exists even when the material is not a superconductor.

“We know now that this interaction doesn’t just switch on when the material becomes a superconductor; it’s actually always there,” Rameau said.

The scientists still believe there is something special about the energy level of the unique tuning-fork-like interaction. Other intriguing phenomena have been observed at this same , which Rameau says has been studied in excruciating detail.

It’s possible, he says, that the one-note lattice interaction plays a role in superconductivity, but requires some still-to-be-determined additional factor to turn the superconductivity on.

“There is clearly something special about this one note,” Rameau said.

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Hubble ‘cranes’ in for a closer look at a galaxy

Hubble 'cranes' in for a closer look at a galaxy

IC 5201 sits over 40 million light-years away from us. As with two thirds of all the spirals we see in the universe — including the Milky Way, the galaxy has a bar of stars slicing through its center. Credit: ESA/Hubble & NASA

In 1900, astronomer Joseph Lunt made a discovery: Peering through a telescope at Cape Town Observatory, the British-South African scientist spotted this beautiful sight in the southern constellation of Grus (The Crane): a barred spiral galaxy now named IC 5201.

Over a century later, the galaxy is still of interest to astronomers. For this image, the NASA/ESA Hubble Space Telescope used its Advanced Camera for Surveys (ACS) to produce a beautiful and intricate image of the galaxy. Hubble’s ACS can resolve individual stars within other galaxies, making it an invaluable tool to explore how various populations of stars sprang to life, evolved, and died throughout the cosmos.

IC 5201 sits over 40 million light-years away from us. As with two thirds of all the spirals we see in the Universe—including the Milky Way—the galaxy has a bar of stars slicing through its center.

Verlinde’s new theory of gravity passes first test

Verlindes new theory of gravity passes first test

The gravity of galaxies bends space, such that the light traveling through this space is bent. This bending of light allows astronomers to measure the distribution of gravity around galaxies, even up to distances a hundred times larger than the galaxy itself. Credit: APS/Alan Stonebraker; galaxy images from STScI/AURA, NASA, ESA, and the Hubble Heritage Team

A team led by astronomer Margot Brouwer (Leiden Observatory, The Netherlands) has tested the new theory of theoretical physicist Erik Verlinde (University of Amsterdam) for the first time through the lensing effect of gravity. Brouwer and her team measured the distribution of gravity around more than 33,000 galaxies to put Verlinde’s prediction to the test. She concludes that Verlinde’s theory agrees well with the measured gravity distribution. The results have been accepted for publication in the British journal Monthly Notices of the Royal Astronomical Society.

The gravity of galaxies bends space, such that the light traveling through this space is bent, as through a lens. Background galaxies that are situated far behind a foreground galaxy (the lens), thereby seem slightly distorted. This effect can be measured in order to determine the distribution of gravity around a foreground-galaxy. Astronomers have measured, however, that at distances up to a hundred times the radius of the galaxy, the force of gravity is much stronger than Einstein’s  of gravity predicts. The existing theory only works when invisible particles, the so-called dark matter, are added.

Verlinde now claims that he not only explains the mechanism behind gravity with his alternative to Einstein’s theory, but also the origin of the mysterious extra gravity, which astronomers currently attribute to dark matter. Verlinde’s new theory predicts how much gravity there must be, based only on the mass of the .

Brouwer calculated Verlinde’s prediction for the gravity of 33,613 galaxies, based only on their visible mass. She compared this prediction to the distribution of gravity measured by gravitational lensing, in order to test Verlinde’s theory. Her conclusion is that his prediction agrees well with the observed  distribution, but she emphasizes that dark matter could also explain the extra gravitational force. However, the mass of the dark matter is a free parameter, which must be adjusted to the observation. Verlinde’s theory provides a direct , without free parameters.

The new theory is currently only applicable to isolated, spherical and static systems, while the universe is dynamic and complex. Many observations cannot yet be explained by the new theory, so  is still in the race. Brouwer: “The question now is how the theory develops, and how it can be further tested. But the result of this first test definitely looks interesting.”

Astronomers discover new gas giant exoplanet

Astronomers discover new gas giant alien world

Observed data for event OGLE-2014-BLG-0676/MOA-2014-BLG-175 from the MOA (gray), OGLE (red) and Wise (green) microlensing survey groups along with data from the RoboNET/LCOGT (cyan) and MiNDSTEp (magenta) groups. Also shown is the best-fitting binary lens model light-curve (black line). The epoch when the OGLE collaboration issued an alert for this event is indicated with a black arrow. Data with extremely large errors are omitted. Credit: Nicolas Rattenbury et al., 2016.

Using the gravitational microlensing method, an international team of astronomers has recently detected a new gas giant exoplanet three times more massive than Jupiter. The newly discovered planet received designation OGLE-2014-BLG-0676Lb and is an important addition to the short list of extrasolar worlds detected by the microlensing technique. The discovery was described in a paper published Dec. 12 on arXiv.org.

Unlike other methods of detecting exoplanets, microlensing is most sensitive when it comes to searching for exoworlds that orbit around one to 10 AU away from their host stars. These planets are of special interest for astronomers studying planetary formation theories due to proximity to their parent stars, within the so-called “snow line.” Just beyond this line, the most active planet formation occurs; therefore, understanding the distribution of exoplanets in this region could offer important clues to how planets form.

So far, 47 planets have been discovered by microlensing. Currently, several ground-based observation programs routinely monitor dense stellar fields to search for microlensing events. When a new event is discovered, an alert to the broader scientific community is issued in order to allow follow-up observations. Astronomers are particularly interested in events showing evidence for perturbations that could be due to the presence of a planet, or which are predicted to have a high sensitivity to such perturbations.

OGLE-2014-BLG-0676, discovered in April 2014 by a Polish astronomical project called the Optical Gravitational Lensing Experiment (OGLE), is one of those interesting microlensing events. Recently, a collaboration of researchers consisting of the OGLE group, the Microlensing Observations in Astrophysics (MOA), the Wise Observatory Group and the Microlensing Network for the Detection of Small Terrestrial Exoplanets (MiNDSTEp), has detected an anomalous signal in this event consistent with a planetary lens system.

“The source star passed through the central caustic, with the second caustic crossing being well recorded by the MOA microlensing survey collaboration. Observations at epochs between the unrecorded first caustic crossing and the second caustic crossing were made by the OGLE, Wise and MOA collaborations. (…) All analyses of the light curve data favor a lens system comprising a planetary mass orbiting a host star,” the paper reads.

According to the research, the newly discovered planet has a mass of about 3.1 Jupiter masses and orbits its parent star at a deprojected orbital separation of about 4.4 AU. The host star is approximately 38 percent less massive than our sun and was classified as a K-dwarf. The distance to the  is about 7,200 light years.

Moreover, the team revealed some information about the source star. They revealed that is rather faint and very red, noting that there is a possibility that the source may be blended with a nearby red star, causing an incorrect identification of the source star type.

In conclusion, the scientists emphasize the importance of their discovery, noting that OGLE-2014-BLG-0676Lb could serve as a test bed for planet formation scenarios. “Planet OGLE-2014-BLG-0676Lb can be added to the growing list of planets discovered by microlensing against which planetary formation theories can be tested,” the researchers wrote in the paper.

Researchers report possible solution to a long-standing solar mystery

Researchers report possible solution to a long-standing solar mystery

An image of the sun taken with The Helioseismic and Magnetic Imager (HMI) on the Solar Dynamics Observatory spacecraft. HMI is an instrument designed to study oscillations and the magnetic field at the solar surface, or photosphere. HMI observes the full solar disk with a resolution of 1 arcsecond. Credit: NASA

Astronomers from the University of Hawaii Institute for Astronomy (IfA), Brazil, and Stanford University may have solved a long-standing solar mystery.

Two decades ago, scientists discovered that the outer five percent of the sun spins more slowly than the rest of its interior. Now, in a new study, to be published in the journal Physical Review Letters, IfA Maui scientists Ian Cunnyngham, Jeff Kuhn, and Isabelle Scholl, together with Marcelo Emilio (Brazil) and Rock Bush (Stanford), describe the physical mechanism responsible for slowing the sun’s outer layers.

Team leader Jeff Kuhn said “The sun won’t stop spinning anytime soon, but we’ve discovered that the same solar radiation that heats the Earth is ‘braking’ the sun because of Einstein’s Special Relativity, causing it to gradually slow down, starting from its surface.”

The sun rotates on its axis at an average rate of about once per month but that rotation isn’t like, for example, the solid Earth or a spinning disk because the rate varies with solar latitude and distance from the center of the sun.

The team used several years of data from the Helioseismic and Magnetic Imager on NASA’s Solar Dynamics Observatory satellite to measure a sharp down-turn in the sun’s  in its very outer 150km. Kuhn said, “This is a gentle torque that is slowing it down, but over the sun’s 5 billion year lifetime it has had a very noticeable influence on its outer 35,000km.” Their paper describes how this photon-braking effect should be at work in most stars.

This change in rotation at the sun’s surface affects the large-scale  and researchers are now trying to understand how the solar magnetism that extends out into the corona and finally into the Earth’s environment will be affected by this braking.

Gravity sensors might offer earlier warning of earthquakes

earthquake

Ruins from the 1906 San Francisco earthquake, remembered as one of the worst natural disasters in United States history. Credit: Public Domain

A team of researchers from France, the U.S. and Italy has found evidence from the Tohoku-Oki earthquake that sensors that measure changes in gravity might offer a way to warn people of impending disaster faster than traditional methods. In their paper published in the journal Nature Communications, the group describes how they analyzed data from gravity sensors near the epicenter of the Tohoku-Oki quake back in 2011 and found that it was possible to isolate gravitational changes due to the earthquake from the noise of other events.

Current earthquake warning systems rely on a network of seismic —they listen for P-waves below the ground which are generated by an earthquake and send a signal to an alarm if they are heard. Such a system offers those in the vicinity of a quake from a few seconds to perhaps a minute to take safety measures. In this new effort, the researches wondered if it might be possible to detect subtle changes in  near the epicenter of a quake to offer those in harm’s way a little more time to prepare for it—because  waves travel at the speed of light.

Prior research has shown that there are subtle changes in gravitational pull around the epicenter of a quake, due to changes in the density of the rock in the area. But until now, it was not clear if such changes could be picked out from all the other background noise. To find out, the researchers pulled data from gravimeter sensors located approximately 500 kilometers from the epicenter of the Tohoku-Oki quake and compared what they found in the record with data from five  in the same area. They noted also that it took 65 seconds for the P-waves to reach the seismic stations. To find out if the quake data would stand out amongst the noise of other natural events (such as the changing tides) the team looked at measurements taken over the 60 days prior to the quake and then at the data from the day before, the day of, and the day after the quake. In looking at the data, the researchers found that they were able to “see” a small blip—one that stood out enough to confirm a quake had occurred.

More research will have to be done before it can be proven that a network of gravity sensors would truly offer people more time to prepare for a  (depending on how close they are to the ), but the results from this initial study seem promising.

Scientists discover a nearby superearth

Scientists from the IAC discover a nearby 'superearth'

Artistic design of the “superearth” GJ 536 b and the star GJ 536. Credit: Gabriel Pérez, SMM (IAC).

Ph.D. student Alejandro Suárez Mascareño, of the Instituto de Astrofísica de Canarias (IAC) and the University of La Laguna (ULL), and his thesis director, Rafael Rebolo and Jonay Isaí González Hernández, have discovered a “superearth”-type planet, GJ 536 b, whose mass is around 5.4 Earth masses, in orbit around a nearby very bright star. The study has been accepted for publication in the journal Astronomy & Astrophysics; researchers from several countries are involved.

This exoplanet orbiting the star GJ 536 is not within the star’s habitable zone, but its short orbital period of 8.7 days and the luminosity of its star, a red dwarf which is quite cool and near to our sun, make it an attractive candidate for investigating its atmospheric composition. During this research, a cycle of magnetic activity similar to that of the sun has been observed, but with a shorter period, three years.

“So far, the only planet we have found is GJ 536 b, but we are continuing to monitor the star to see if we can find other companions,” says Alejandro Suárez Mascareño, who is the first author on the article. “Rocky planets are usually found in groups,” he explains, “especially around  of this type, and we are pretty sure that we can find other low-mass planets in orbits further from the star, with periods from 100 days up to a few years. We are preparing a programme of monitoring for transits of this new exoplanet to determine its radius and mean density.”

“This rocky exoplanet is orbiting a star much smaller and cooler than the sun,” says Jonay Isaí González. “But it is sufficiently nearby and bright. It is also observable from both the northern and southern hemispheres, which is very interesting for future high-stability spectrographs, and in particular, for the possible detection of another rocky planet in the habitability zone of the star.”

“To detect the planet”, says Rafael Rebolo, “we had to measure the velocity of the star with an accuracy of the order of a metre per second. With the construction of the new instrument ESPRESSO, co-directed by the IAC, we will improve this accuracy by a factor of 10, and will be able to extend our search to  with conditions very similar to Earth, around this and many other nearby stars.”

The planet has been detected in a joint effort between the IAC and the Geneva Observatory, using the HARPS (High Accuracy Radial velocity Planet Seeker) spectrograph on the 3.6M ESO Telescope at La Silla (Chile) and HARPS North, on the Telescopio Nacional Galileo (TNG) at the Roque de los Muchachos Observatory, Garafia (La Palma).