Exotic Patterns of interacting electrons at the metal-insulator transition
Since high school we know that matter appears in three different phases (solid, liquid, gas); yet the microscopic details about the transformation from one state to the other puzzles physicists until now. An international team of scientists led by Prof. Martin Dressel from the Institute of Physics (1) at the University of Stuttgart gained fascinating insights in electronic phase transitions by applying novel optical techniques. The results were published in the international scientific journals Applied Physics Letters and Science Advances.*
The change of correlated electrons from metallic to insulating is very similar to the freezing of water into ice, i.e. transforming from liquid to solid. Since the structures at such a phase transition are typically smaller than the wavelength of visible light, it is not possible to see them by a conventional microscope. A near-field microscope, however, utilizes an atomically thin metallic tip scanning above the material’s surface to record the local optical properties.
By combining such an apparatus with the capability to reach cryogenic temperatures, the physicists from Stuttgart could monitor for the first time the metal-insulator transition of a molecular crystal on the nanometer scale at temperatures as low as -138°C.
They discovered a peculiar pattern of alternating metallic and insulating regions, reminiscent of the black and white fur of a zebra. The stripes have a width of less than one micrometer and occur due to the interplay of tension and strain within the crystal coupled to anomalous thermal expansion. The pattern of electronic inhomogeneity and its gradual evolution with changing temperature can be nicely modeled by a mathematical simulation.
A similarly exotic pattern of electrons was also observed in a completely different material with a metal-insulator transition: vanadium dioxide shows a sudden drop of electrical resistance when heated from room temperature above 70°C. By examining the polarization of reflected light the scientists from Stuttgart revealed the formation of sub-micrometer-size metallic droplets.
They are spherical in shape and keep growing in size until they make contact at the phase transition. Cooling down again, the polarization is different from the warm-up cycle indicating that metallic regions form oblate ellipsoids, i.e. disks that become smaller in density and size until they vanish in the insulating phase; just like the melting of ice.
Although a complete theoretical understanding is pending, there is evidence for a crucial influence of the layer thickness on the shape of metallic droplets.
It seems that the Stuttgart group has discovered a completely new facet of electronic materials that exhibit fascinating patterns formed by interacting electrons.
Prof. Dr. Martin Dressel, University of Stuttgart, Institute of Physics (1), Tel.: +49 (0) 711-685 64946, Email: dressel (at) pi1.physik.uni-stuttgart.de
Andrej Pustogow, Alex S. McLeod, Yohei Saito, Dmitri N. Basov, and Martin Dressel: Internal Strain Tunes Electronic Correlations on the Nanoscale, Science Advances 4, eaau9123 vom 14. Dezember 2018
Andrea Mayer-Grenu | idw - Informationsdienst Wissenschaft
Observations of nearby supernova and associated jet cocoon provide new insights on gamma-ray bursts
18.01.2019 | George Washington University
A new twist on a mesmerizing story
17.01.2019 | ETH Zurich Department of Physics
The scientific and political community alike stress the importance of German Antarctic research
Joint Press Release from the BMBF and AWI
The Antarctic is a frigid continent south of the Antarctic Circle, where researchers are the only inhabitants. Despite the hostile conditions, here the Alfred...
World first experiments on sensor that may revolutionise everything from medical devices to unmanned vehicles
The new sensor - capable of detecting vibrations of living cells - may revolutionise everything from medical devices to unmanned vehicles.
Dead and alive at the same time? Researchers at the Max Planck Institute of Quantum Optics have implemented Erwin Schrödinger’s paradoxical gedanken experiment employing an entangled atom-light state.
In 1935 Erwin Schrödinger formulated a thought experiment designed to capture the paradoxical nature of quantum physics. The crucial element of this gedanken...
Cellulose obtained from wood has amazing material properties. Empa researchers are now equipping the biodegradable material with additional functionalities to produce implants for cartilage diseases using 3D printing.
It all starts with an ear. Empa researcher Michael Hausmann removes the object shaped like a human ear from the 3D printer and explains:
The phenomenon of so-called superlubricity is known, but so far the explanation at the atomic level has been missing: for example, how does extremely low friction occur in bearings? Researchers from the Fraunhofer Institutes IWM and IWS jointly deciphered a universal mechanism of superlubricity for certain diamond-like carbon layers in combination with organic lubricants. Based on this knowledge, it is now possible to formulate design rules for supra lubricating layer-lubricant combinations. The results are presented in an article in Nature Communications, volume 10.
One of the most important prerequisites for sustainable and environmentally friendly mobility is minimizing friction. Research and industry have been dedicated...
16.01.2019 | Event News
14.01.2019 | Event News
12.12.2018 | Event News
18.01.2019 | Materials Sciences
18.01.2019 | Life Sciences
18.01.2019 | Health and Medicine