Two electrons and two holes, created by light quanta, held together by a chessboard-like background.
TU Wien
In physics, there are very different types of particles: Elementary particles are the fundamental building blocks of matter. Other particles, such as atoms, are bound states consisting of several smaller constituents. And then there are so-called “quasi-particles”—excitations in a system that consists of many particles, which in many ways behave just like a particle themselves.
Such a quasiparticle has now been discovered in computer simulations at TU Wien (Vienna) and named pi-ton. It consists of two electrons and two holes. The new particle is presented in the journal Physical Review Letters, the article also describes how the pi-ton can be detected experimentally.
A hole is almost a particle
“The simplest quasi-particle is a hole”, explains professor Karsten Held from the Institute for Solid State Physics at Vienna University of Technology (TU Wien). “Let us imagine, for example, that many atoms are arranged in a regular pattern in a crystal and that there is a moving electron at each atom. Only at one particular atom the electron is missing—this is called a hole.” Now an electron can move up from the neighboring atom. The original hole is closed, a new hole opens.
Instead of describing the motion of constantly moving electrons, it is easier to study the motion of the hole. If the electrons move to the right, the hole moves to the left—and this movement follows certain physical rules, just like the movement of an ordinary particle. However, unlike an electron, which can also be observed outside the crystal, the hole only exists in conjunction with the other particles. In this case we speak of a “quasi-particle.”
“However, the dividing line between particles and quasi-particles is not as clear as one might think,” says Held. “Strictly speaking, even ordinary particles can only be understood in the context of their environment. Even in a vacuum, particle-hole excitations occur constantly, albeit for a very short time. Without them, the mass of an electron for example would be completely different. In this sense, even in experiments with ordinary electrons, what we see is really a quasi-particle electron.”
More complicated bonds
But there are also more complex quasi-particles: The exciton, for example, which plays an important role in semiconductor physics. It is a bound state consisting of an electron and a hole, which is created by light. The electron is negatively charged, the hole is the absence of a negative charge – and thus positively charged. Both attract each other and can form a bond.
“We actually wanted to investigate such excitons,” report Dr. Anna Kauch and Dr. Petra Pudleiner, the first authors of the paper. “We developed computer simulations to calculate quantum physical effects in solids.” But soon Kauch, Pudleiner, and their colleague Katharina Astleithner realized that they had come across something totally different in their calculations—a completely new type of quasi-particle. It consists of two electrons and two holes that couple to the outside world via photons.
The team gave this previously unknown object the name “pi-ton.”
“The name pi-ton comes from the fact that the two electrons and two holes are held together by charge density fluctuations or spin fluctuations that always reverse their character by 180 degrees from one lattice point of the crystal to the next—i.e. by an angle of pi, measured in radians,” explains Kauch.
“This constant change from plus to minus can perhaps be imagined like a change from black to white on a chessboard,” says Pudleiner. The pi-ton is created spontaneously by absorbing a photon. When it disappears, a photon is emitted again.
The particle that came out of the computer
So far, the pi-ton has been discovered and verified by computer simulations. For the research team, there is no doubt about the existence of the pi-ton: “We have now investigated the phenomenon of the pi-ton using various models—it shows up again and again. Therefore, it should definitely be detectable in a variety of different materials, ” Held says. “Some experimental data obtained with the material samarium titanate already seem to point to the pi-ton. Additional experiments with photons and neutrons should soon provide clarity.”
Even though we are constantly surrounded by countless quasiparticles—the discovery of a new quasiparticle species is something very special. Besides the exciton, there is now also the pi-ton. In any case, this contributes to a better understanding of the coupling between light and solids, a topic that plays an important role not only in basic research but also in many technical applications—from semiconductor technology to photovoltaics.
– This press release was originally published on the TU Wien website
NAIROBI, Kenya (AP) — The body of Ugandan Olympic athlete Rebecca Cheptegei — who died after being set on fire by her partner in Kenya — was received Friday by family and anti-femicide crusaders, ahead of her burial a day later.
Cheptegei’s family met with dozens of activists Friday who had marched to the Moi Teaching and Referral Hospital’s morgue in the western city of Eldoret while chanting anti-femicide slogans.
She is the fourth female athlete to have been killed by her partner in Kenya in yet another case of gender-based violence in recent years.
Viola Cheptoo, the founder of Tirop Angels – an organization that was formed in honor of athlete Agnes Tirop, who was stabbed to death in 2021, said stakeholders need to ensure this is the last death of an athlete due to gender-based violence.
“We are here to say that enough is enough, we are tired of burying our sisters due to GBV,” she said.
It was a somber mood at the morgue as athletes and family members viewed Cheptegei’s body which sustained 80% of burns after she was doused with gasoline by her partner Dickson Ndiema. Ndiema sustained 30% burns on his body and later succumbed.
Ndiema and Cheptegei were said to have quarreled over a piece of land that the athlete bought in Kenya, according to a report filed by the local chief.
Cheptegei competed in the women’s marathon at the Paris Olympics less than a month before the attack. She finished in 44th place.
Cheptegei’s father, Joseph, said that the body will make a brief stop at their home in the Endebess area before proceeding to Bukwo in eastern Uganda for a night vigil and burial on Saturday.
“We are in the final part of giving my daughter the last respect,” a visibly distraught Joseph said.
He told reporters last week that Ndiema was stalking and threatening Cheptegei and the family had informed police.
Four in 10 women or an estimated 41% of dating or married Kenyan women have experienced physical or sexual violence perpetrated by their current or most recent partner, according to the Kenya Demographic and Health Survey 2022.
TEL AVIV, Israel (AP) — A rare Bronze-Era jar accidentally smashed by a 4-year-old visiting a museum was back on display Wednesday after restoration experts were able to carefully piece the artifact back together.
Last month, a family from northern Israel was visiting the museum when their youngest son tipped over the jar, which smashed into pieces.
Alex Geller, the boy’s father, said his son — the youngest of three — is exceptionally curious, and that the moment he heard the crash, “please let that not be my child” was the first thought that raced through his head.
The jar has been on display at the Hecht Museum in Haifa for 35 years. It was one of the only containers of its size and from that period still complete when it was discovered.
The Bronze Age jar is one of many artifacts exhibited out in the open, part of the Hecht Museum’s vision of letting visitors explore history without glass barriers, said Inbal Rivlin, the director of the museum, which is associated with Haifa University in northern Israel.
It was likely used to hold wine or oil, and dates back to between 2200 and 1500 B.C.
Rivlin and the museum decided to turn the moment, which captured international attention, into a teaching moment, inviting the Geller family back for a special visit and hands-on activity to illustrate the restoration process.
Rivlin added that the incident provided a welcome distraction from the ongoing war in Gaza. “Well, he’s just a kid. So I think that somehow it touches the heart of the people in Israel and around the world,“ said Rivlin.
Roee Shafir, a restoration expert at the museum, said the repairs would be fairly simple, as the pieces were from a single, complete jar. Archaeologists often face the more daunting task of sifting through piles of shards from multiple objects and trying to piece them together.
Experts used 3D technology, hi-resolution videos, and special glue to painstakingly reconstruct the large jar.
Less than two weeks after it broke, the jar went back on display at the museum. The gluing process left small hairline cracks, and a few pieces are missing, but the jar’s impressive size remains.
The only noticeable difference in the exhibit was a new sign reading “please don’t touch.”
VICTORIA – The British Columbia government is partnering with a bear welfare group to reduce the number of bears being euthanized in the province.
Nicholas Scapillati, executive director of Grizzly Bear Foundation, said Monday that it comes after months-long discussions with the province on how to protect bears, with the goal to give the animals a “better and second chance at life in the wild.”
Scapillati said what’s exciting about the project is that the government is open to working with outside experts and the public.
“So, they’ll be working through Indigenous knowledge and scientific understanding, bringing in the latest techniques and training expertise from leading experts,” he said in an interview.
B.C. government data show conservation officers destroyed 603 black bears and 23 grizzly bears in 2023, while 154 black bears were killed by officers in the first six months of this year.
Scapillati said the group will publish a report with recommendations by next spring, while an independent oversight committee will be set up to review all bear encounters with conservation officers to provide advice to the government.
Environment Minister George Heyman said in a statement that they are looking for new ways to ensure conservation officers “have the trust of the communities they serve,” and the panel will make recommendations to enhance officer training and improve policies.
Lesley Fox, with the wildlife protection group The Fur-Bearers, said they’ve been calling for such a committee for decades.
“This move demonstrates the government is listening,” said Fox. “I suspect, because of the impending election, their listening skills are potentially a little sharper than they normally are.”
Fox said the partnership came from “a place of long frustration” as provincial conservation officers kill more than 500 black bears every year on average, and the public is “no longer tolerating this kind of approach.”
“I think that the conservation officer service and the B.C. government are aware they need to change, and certainly the public has been asking for it,” said Fox.
Fox said there’s a lot of optimism about the new partnership, but, as with any government, there will likely be a lot of red tape to get through.
“I think speed is going to be important, whether or not the committee has the ability to make change and make change relatively quickly without having to study an issue to death, ” said Fox.
This report by The Canadian Press was first published Sept. 9, 2024.
