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Category: physics – Page 243

A technique to over-dope graphene beyond the van Hove singularity

For over a decade, theoretical physicists have predicted that the van Hove singularity of graphene could be associated with different exotic phases of matter, the most notable of which is chiral superconductivity.

A van Hove is essentially a non-smooth point in the density of states (DOS) of a crystalline solid. When reaches or is close to this specific energy level, a flat band develops in its electronic structure that can occupy an exceptionally large number of electrons. This leads to strong many-body interactions that promote or enable the existence of exotic states of matter.

So far, the exact degree to which the available energy levels of graphene need to be filled with electrons (i.e., “doped”) in order for individual phases to stabilize has been very difficult to determine using model calculations. Identifying or designing techniques that can be used to dope graphene to or beyond the van Hove singularity could ultimately lead to interesting observations related to exotic phases of matter, which could in turn pave the way towards the development of new graphene-based technology.

Orbits of ancient stars prompt rethink on Milky Way evolution

Theories on how the Milky Way formed are set to be rewritten following discoveries about the behavior of some of its oldest stars.

An investigation into the orbits of the Galaxy’s metal-poor stars—assumed to be among the most ancient in existence—has found that some of them travel in previously unpredicted patterns.

“Metal-poor stars—containing less than one-thousandth the amount of iron found in the Sun—are some of the rarest objects in the galaxy,” said Professor Gary Da Costa from Australia’s ARC Center of Excellence in All Sky Astrophysics in 3 Dimensions (ASTRO 3D) and the Australian National University.

Unexpectedly, The Universe Is Getting Hotter And Hotter as It Expands

So the Universe is getting hotter? 😃

For almost a century, astronomers have understood that the Universe is in a state of expansion. Since the 1990s, they have come to understand that as of 4 billion years ago, the rate of expansion has been speeding up.

As this progresses, and the galaxy clusters and filaments of the Universe move farther apart, scientists theorize that the mean temperature of the Universe will gradually decline.

But according to new research led by the Center for Cosmology and AstroParticle Physics (CCAPP) at Ohio State University, it appears that the Universe is actually getting hotter as time goes on.

Understanding astrophysics with laser-accelerated protons

Bringing huge amounts of protons up to speed in the shortest distance in fractions of a second—that’s what laser acceleration technology, greatly improved in recent years, can do. An international research team from the GSI Helmholtzzentrum für Schwerionenforschung and the Helmholtz Institute Jena, a branch of GSI, in collaboration with the Lawrence Livermore National Laboratory, U.S., has succeeded in using protons accelerated with the GSI high-power laser PHELIX to split other nuclei and to analyze them. The results have now been published in the journal Nature Scientific Reports and could provide new insights into astrophysical processes.

For less than one picosecond (one trillionth of a second), the PHELIX laser shines its extremely intense light pulse onto a very thin gold foil. This is enough to eject about one trillion hydrogen nuclei (protons), which are only slightly attached to the gold, from the back-surface of the foil, and accelerate them to high energies. “Such a large number of protons in such a short period of time cannot be achieved with standard acceleration techniques,” explains Pascal Boller, who is researching laser acceleration in the GSI research department Plasma Physics/PHELIX as part of his graduate studies. “With this technology, completely new research areas can be opened that were previously inaccessible.”

These include the generation of nuclear fission reactions. For this purpose, the researchers let the freshly generated fast protons impinge on uranium material samples. Uranium was chosen as a case study material because of its large reaction cross-section and the availability of published data for benchmarking purposes. The samples have to be close to the production to guarantee a maximum yield of reactions. The protons generated by the PHELIX laser are fast enough to induce the fission of uranium nuclei into smaller fission products, which remain then to be identified and measured. However, the laser impact has unwanted side effects: It generates a strong electromagnetic pulse and a gammy-ray flash that interfere with the sensitive measuring instruments used for this detection.

No losses: Scientists stuff graphene with light

O,.o.

Physicists from MIPT and Vladimir State University, Russia, have converted light energy into surface waves on graphene with nearly 90% efficiency. They relied on a laser-like energy conversion scheme and collective resonances. The paper was published in Laser & Photonics Reviews.

Manipulating light at the nanoscale is a task crucial for being able to create ultracompact devices for optical conversion and storage. To localize light on such a small scale, researchers convert optical radiation into so-called plasmon-polaritons. These SPPs are oscillations propagating along the interface between two materials with drastically different refractive indices—specifically, a metal and a dielectric or air. Depending on the materials chosen, the degree of surface wave localization varies. It is the strongest for light localized on a material only one atomic layer thick, because such 2-D materials have high refractive indices.

The existing schemes for converting light to SPPs on 2-D surfaces have an efficiency of no more than 10%. It is possible to improve that figure by using intermediary signal converters—nano-objects of various chemical compositions and geometries.

Does all the gold in the universe come from stars?

In a remote galaxy, two neutron stars circled one another in a ballet of ultimate destruction and inevitable creation. Both objects were the remnants of massive stars, probably from a binary system, that had become supernovae long before. Each was incredibly massive, with neutrons so closely packed that their cores became diamond. The dance, alas, could not go on forever and the stars collided, releasing unimaginable energy and sending gravitational waves speeding through the fabric of space-time.

In 2017, 1.3 billion years later, astronomers detected those waves with the Laser Interferometer Gravitational-wave Observatory. Albert Einstein’s prediction that the universe should be filled with such faint ripples caused by gravity from massive objects included sources such as neutron star mergers. Yet finding a disturbance in the fabric of space-time from this kind of event had proven elusive until then. When news of the detection of gravitational waves broke, the media wanted to know what else happens when neutron stars collide. Astronomers explained that, beyond the destruction of the stars and the ripples in space, such events also create all the heavy elements we know in the blink of an eye. But what did the media key into? That gold comes from outer space.

Researchers make most precise measurements of deuterium fusing with a proton to form helium-3

A large team of researchers affiliated with a host of institutions in Italy, the U.K and Hungary has carried out the most precise measurements yet of deuterium fusing with a proton to form helium-3. In their paper published in the journal Nature, the group describes their effort and how they believe it will contribute to better understanding the events that transpired during the first few minutes after the Big Bang.

Astrophysics theory suggests that the creation of deuterium was one of the first things that happened after the Big Bang. Therefore, it plays an important role in Big Bang nucleosynthesis—the reactions that happened afterward that led to the production of several of the light elements. Theorists have developed equations that show the likely series of events that occurred, but to date, it has been difficult to prove them correct without physical evidence. In this new effort, the researchers working at the Laboratory for Underground Nuclear Astrophysics in Italy have carried out experiments to simulate those first few minutes, hoping to confirm the theories.

The work was conducted deep under the thick rock cover of the Gran Sasso mountain to prevent interference from —it involved firing a beam of protons at a deuterium target—deuterium being a form of hydrogen with just one and one neutron—and then measuring the rate of fusion. But because the rate of fusion is so low, the bombardment had to be carried out many times—the team carried out their work nearly every weekend for three years.

Black hole or no black hole: On the outcome of neutron star collisions

A new study lead by GSI scientists and international colleagues investigates black-hole formation in neutron star mergers. Computer simulations show that the properties of dense nuclear matter play a crucial role, which directly links the astrophysical merger event to heavy-ion collision experiments at GSI and FAIR. These properties will be studied more precisely at the future FAIR facility. The results have now been published in Physical Review Letters. With the award of the 2020 Nobel Prize in Physics for the theoretical description of black holes and for the discovery of a supermassive object at the center of our galaxy, the topic currently also receives a lot of attention.

But under which conditions does a black hole actually form? This is the central question of a study lead by the GSI Helmholtzzentrum für Schwerionenforschung in Darmstadt within an international collaboration. Using , the scientists focus on a particular process to form namely the merging of two stars.

Neutron stars consists of highly compressed dense matter. The mass of one and a half is squeezed to the size of just a few kilometers. This corresponds to similar or even higher densities than in the inner of atomic nuclei. If two neutron stars merge, the matter is additionally compressed during the collision. This brings the merger remnant on the brink to collapse to a black hole. Black holes are the most compact objects in the universe, even light cannot escape, so these objects cannot be observed directly.