An international research team involving a researcher at Tsukuba University uses attosecond laser pulses to study electronic excitations in a dielectric crystal, which may open the way for novel computer logical elements and optical devices

The international research team has analyzed the ultrafast electronic response inside magnesium fluoride (MgF₂) crystals caused by very short laser bursts. They found that the electron behavior at attosecond timescales was similar to that seen in both atoms and solids. This work may lead to better control of the electronic activity in previously overlooked materials.

In the past decade, light-induced ultrafast electron dynamics in solids in the femtosecond (10^-15 second) and the attosecond (10^-18 second) regimes have been intensively studied towards the realization of future ultrafast electronics, optics, and optoelectronics. However, the fundamental understanding of many-body effects, which emerge from the interaction of many particles, in such ultrafast electron dynamics has been missing due to their complexity.

The emergence of excitons is a typical case of these many-body effects, which are among the most important quasi-particles in solids from both fundamental and technological points of view. Therefore, the development of a microscopic understanding of ultrafast exciton dynamics is an important milestone towards the science of nonequilibrium electron dynamics.

Now, a research team involving a researcher at Tsukuba University has used attosecond transient reflection spectroscopy to study core-level excitons in magnesium fluoride crystals.

“Thanks to the utilization of independent calibration experiments, we were able to achieve results that matched with theoretical simulations,” says author Professor Shunsuke Sato.

An important finding was that the exciton acted like a hybrid between an electron orbiting a single atom, and an electron delocalized over a bulk material. The researchers propose methods such as adding an external electric field or mechanical strain to the crystal to precisely control this response.

 “Exploring solutions beyond classical electronics, is becoming increasingly important if we want to move beyond current technological computing limits,” Professor Sato says. “The results of our research may lead to new materials for performing computations faster and more efficiently than current systems.”

Original Paper

The work is published in Advanced Functional Materials as “Unravelling the intertwined atomic and bulk nature of localised excitons by attosecond spectroscopy” (DOI:10.1038/s41467-021-21345-7).

Correspondence

Assistant Professor SATO Shunsuke
Center for Computational Sciences, University of Tsukuba

Researchers led by the University of Tsukuba devise a new approach to show how ghost-like neutrinos helped shape the Universe

Tsukuba, Japan – Computer simulations have struggled to capture the impact of elusive particles called neutrinos on the formation and growth of the large-scale structure of the Universe. But now, a research team from Japan has developed a method that overcomes this hurdle.

In a study published this month in The Astrophysical Journal, researchers led by the University of Tsukuba present simulations that accurately depict the role of neutrinos in the evolution of the Universe.

Why are these simulations important? One key reason is that they can set constraints on a currently unknown quantity: the neutrino mass. If this quantity is set to a particular value in the simulations and the simulation results differ from observations, that value can be ruled out. However, the constraints can be trusted only if the simulations are accurate, which was not guaranteed in previous work. The team behind this latest research aimed to address this limitation.

“Earlier simulations used certain approximations that might not be valid,” says lead author of the study Lecturer Kohji Yoshikawa. “In our work, we avoided these approximations by employing a technique that accurately represents the velocity distribution function of the neutrinos and follows its time evolution.”

To do this, the research team directly solved a system of equations known as the Vlasov–Poisson equations, which describe how particles move in the Universe. They then carried out simulations for different values of the neutrino mass and systemically examined the effects of neutrinos on the large-scale structure of the Universe.

The simulation results demonstrate, for example, that neutrinos suppress the clustering of dark matter—the ‘missing’ mass in the Universe—and in turn galaxies. They also show that neutrino-rich regions are strongly correlated with massive galaxy clusters and that the effective temperature of the neutrinos varies substantially depending on the neutrino mass.

“Overall, our findings suggest that neutrinos considerably affect the large-scale structure formation, and that our simulations provide an accurate account for the important effect of neutrinos,” explains Lecturer Yoshikawa. “It is also reassuring that our new results are consistent with those from entirely different simulation approaches.”

This work represents a milestone in simulating the Universe and paves the way for further exploration of how neutrinos influence the formation and growth of the large-scale structure. For instance, the new simulation approach could be used to study the dynamics of neutrinos and unconventional types of dark matter. Ultimately, it might lead to a determination of the neutrino mass.

Original Paper

The article, “Cosmological Vlasov–Poisson Simulations of Structure Formation with Relic Neutrinos: Nonlinear Clustering and the Neutrino Mass,” was published in The Astrophysical Journal at DOI: 10.3847/1538-4357/abbd46

Correspondence

Assistant Professor YOSHIKAWA Kohji
Center for Computational Sciences, University of Tsukuba