The total electron charge density (shown in green) of a quantum dot of gallium arsenide, containing just 465 atoms. (Image: Lin-Wang Wang)
Here green shows the change in charge response when a single-electron perturbation is introduced into bulk gallium arsenide (left) and into a 465-atom quantum dot of the same material near its surface (right): except where the dots surface intervenes, the responses of the two systems are very similar. (Image: Lin-Wang Wang)
Quantum dots, tiny crystals consisting of a few hundred to a few thousand atoms, sparkle with promise for uses ranging from tagging proteins in living cells to foiling counterfeiters to enabling quantum computers. The optics and electronics of these semiconductor nanocrystals are dramatically different from the same materials in bulk. But it turns out that one of the most important electronic properties of quantum dots has been misunderstood for over a decade.
Theorists at the Department of Energy’s Lawrence Berkeley National Laboratory have shown that a quantum dot’s dielectric function (a term indicating how charge responds to an electric field) does not depend on its band gap, as researchers long believed. On the contrary, the dielectric function of a quantum dot, measured on the microscopic scale, is virtually the same as that of the bulk material -- except near the dot’s surface. "One of the interesting things about quantum dots is that their band gaps are much larger than the same material in bulk. At the same time their overall dielectric constants are much smaller," says Lin-Wang Wang of Berkeley Lab’s Computational Research Division. "Therefore it was natural to assume that the size of the band gap in a quantum dot is what determines its overall dielectric constant."
Recently French researchers led by Christophe Delerue of the Institut Supérieur d’Electronique du Nord raised doubts about this assumed relationship, however, basing their argument on approximate calculations. To test the questions posed by the French group, Wang and postdoctoral fellow Xavier Cartoixà performed, for the first time, ab initio ("from first principles") microscopic studies of the dielectric function in quantum dots. To do so they used PEtot, a quantum-mechanical electronic-structure program developed by Wang, on the Seaborg supercomputer at the Department of Energy’s National Energy Research Scientific Computing Center (NERSC), based at Berkeley Lab.
Paul Preuss | EurekAlert!
Computer model predicts how fracturing metallic glass releases energy at the atomic level
20.07.2018 | American Institute of Physics
What happens when we heat the atomic lattice of a magnet all of a sudden?
18.07.2018 | Forschungsverbund Berlin
A new manufacturing technique uses a process similar to newspaper printing to form smoother and more flexible metals for making ultrafast electronic devices.
The low-cost process, developed by Purdue University researchers, combines tools already used in industry for manufacturing metals on a large scale, but uses...
For the first time ever, scientists have determined the cosmic origin of highest-energy neutrinos. A research group led by IceCube scientist Elisa Resconi, spokesperson of the Collaborative Research Center SFB1258 at the Technical University of Munich (TUM), provides an important piece of evidence that the particles detected by the IceCube neutrino telescope at the South Pole originate from a galaxy four billion light-years away from Earth.
To rule out other origins with certainty, the team led by neutrino physicist Elisa Resconi from the Technical University of Munich and multi-wavelength...
For the first time a team of researchers have discovered two different phases of magnetic skyrmions in a single material. Physicists of the Technical Universities of Munich and Dresden and the University of Cologne can now better study and understand the properties of these magnetic structures, which are important for both basic research and applications.
Whirlpools are an everyday experience in a bath tub: When the water is drained a circular vortex is formed. Typically, such whirls are rather stable. Similar...
Physicists working with Roland Wester at the University of Innsbruck have investigated if and how chemical reactions can be influenced by targeted vibrational excitation of the reactants. They were able to demonstrate that excitation with a laser beam does not affect the efficiency of a chemical exchange reaction and that the excited molecular group acts only as a spectator in the reaction.
A frequently used reaction in organic chemistry is nucleophilic substitution. It plays, for example, an important role in in the synthesis of new chemical...
Optical spectroscopy allows investigating the energy structure and dynamic properties of complex quantum systems. Researchers from the University of Würzburg present two new approaches of coherent two-dimensional spectroscopy.
"Put an excitation into the system and observe how it evolves." According to physicist Professor Tobias Brixner, this is the credo of optical spectroscopy....
13.07.2018 | Event News
12.07.2018 | Event News
03.07.2018 | Event News
20.07.2018 | Power and Electrical Engineering
20.07.2018 | Information Technology
20.07.2018 | Materials Sciences