Missing link between new topological phases of matter discovered

The Bismut doping is enhanced from 0% (left) to 2.2% (right). Measurements at BESSY II show that this leads to increased bandgaps.

The Bismut doping is enhanced from 0% (left) to 2.2% (right). Measurements at BESSY II show that this leads to increased bandgaps. © HZB

HZB-Physicists at BESSY II have investigated a class of materials that exhibit characteristics of topological insulators. During these studies they discovered a transition between two different topological phases, one of which is ferroelectric, meaning a phase in the material that exhibits spontaneous electric polarisation and can be reversed by an external electric field. This could also lead to new applications such as switching between differing conductivities.

The HZB researchers studied crystalline semiconductor films made of a lead, tin, and selenium alloy (PbSnSe) that were doped additionally with tiny amounts of the element bismuth. These semiconductors belong to the new class of materials called topological insulators, materials that conduct very well at their surfaces while behaving as insulators internally. Doping with 1-2 per cent bismuth has enabled them to observe a new topological phase transition now. The sample changes to a particular topological phase that also possesses the property of ferroelectricity. This means that an external electric field distorts the crystal lattice, whereas conversely, mechanical forces on the lattice can create electric fields.

The effect can be used to develop new functionality, which is also of interest for potential applications. Ferroelectric phase-change materials are employed in DVDs and flash memories, for example. An electrical voltage displaces atoms in the crystal, transforming the insulating material into a metallic one.

The bismuth doping in the PbSnSe films investigated at HZB served as a perturbation. The number of electrons in bismuth does not fit well in the periodic arrangement of atoms within the PbSnSe crystal. “Tiny changes in the atomic structure give rise to fascinating effects in this class of materials”, explains HZB researcher Dr. Jaime Sánchez-Barriga,  principal investigator coordinating the project.

Following detailed analyses of the measurements, only one conclusion remained: the bismuth doping causes a ferroelectric distortion in the lattice that also changes the allowable energy levels of the electrons. “This problem kept us puzzled during several beamtimes until we reproduced the scientific results on a whole new set of samples”, adds Sánchez-Barriga. “Potential applications could arise through ferroelectric phases - ones that have not been thought of before. Lossless conduction of electricity in topological materials can be switched on and off at will, by electrical pulses or by mechanical strain”, explains Prof. Oliver Rader, head  the department Materials for Green Spintronics at HZB.

 

Publication in Nature communications (2017): Topological quantum phase transition from mirror to time reversal symmetry protected topological insulator
Partha S. Mandal, Gunther Springholz, Valentine V. Volobuev, Ondrei Caha, Andrei Varykhalov, Evangelos Golias, Günther Bauer, Oliver Rader, Jaime Sánchez-Barriga

doi: 10.1038/s41467-017-01204-0

 

Note: The investigation has been conducted in close collaboration with researchers from Johannes-Kepler-Universität Linz who also grew the samples. Partha S. Mandal, who carried out the measurements on the material system as part of his dissertation was supported by the Helmholtz Virtual Institute ”New States of Matter and their Excitations”.

 

 

arö


You might also be interested in

  • Fertilisation under the X-ray beam
    Science Highlight
    19.03.2024
    Fertilisation under the X-ray beam
    After the egg has been fertilized by a sperm, the surrounding egg coat tightens, mechanically preventing the entry of additional sperm and the ensuing death of the embryo. A team from the Karolinska Institutet has now gained this new insight through measurements at the X-ray light sources BESSY II, DLS and ESRF. 
  • Where quantum computers can score
    Science Highlight
    15.03.2024
    Where quantum computers can score
    The travelling salesman problem is considered a prime example of a combinatorial optimisation problem. Now a Berlin team led by theoretical physicist Prof. Dr. Jens Eisert of Freie Universität Berlin and HZB has shown that a certain class of such problems can actually be solved better and much faster with quantum computers than with conventional methods.
  • The future of BESSY
    News
    07.03.2024
    The future of BESSY
    At the end of February 2024, a team at HZB published an article in Synchrotron Radiation News (SRN). They describe the next development goals for the light source as well as the BESSY II+ upgrade programme and the successor source BESSY III.