Directional conductivity when under strain

3 Apr 2014. A team from Department of Physics NUS exhibit the ability to conduct electricity in a certain direction when it is placed under tensile strain condition.

A research collaboration between Asst Prof Andrivo Rusydi and collaborators from Nanyang Technological University and Chinese Academy of Sciences, China have discovered that La0.7Sr0.3MnO3 (LSMO) ultra-thin films exhibit the ability to conduct electricity in a certain direction when it is placed under tensile strain conditions at low temperatures. This unique behavior to conduct electricity in a certain direction has the potential for directional superconductivity andmagnetism leading to possible new scientific discoveries and applications.

The team deposited high-quality LSMO thin films via pulsed laser deposition on different materials and due to lattice mismatch, create stress and strain conditions. For the film which is exposed to large tensile strain, the researchers observed in-plane transport anisotropy along one of the crystallographic direction. This phenomenon was not observed in other samples with stress or low strain conditions and was not reported before.  

To reveal the origin on anisotropic transports, they have used synchrotron based experimental technique of soft X-ray absorption (XAS) at Mn L-edge and O K-edge. Due to the dipole allowed transitions, the former is mainly Mn 2p --> 3d transitions. While the latter is O 1s-2p transitions. Because the O 2p hybridizes with transition metals, especially Mn 3d, one can thus determine the importance of O 2p as well as the hybridization of O 2p with Mn 3d and map the unoccupied states in a broad energy range. Surprisingly, the XAS results show that while the Mn 3d orbital occupancy is isotropic, the O 2p–Mn 3d and O 2p–La 5d/Sr 4s hybridized states are anisotropic accompanied by spectral weigh transfer suggesting that hybridizations and strong correlations play important role to the observed phenomenon.

This result will be useful in understanding the anisotropic transport behavior in other material systems and was published in Nature Communications in Nov 2013.

In a separate finding, Asst Prof Andrivo Rusydi and his collaborators successfully obtained and analyzed the conductivity of the surface and bulk contributions of a new topological conductor, Bi1.5Sb0.5Te1.8Se1.2 through the use of terahertz time-domain spectroscopy (THz-TDS) and angular-resolved photoemission spectroscopy (ARPES) (see Figure). Topological conductors are electronic materials with an insulating bulk and conducting surface. Due to free carriers in the bulk, the properties of their metallic surface are often difficult to detect and characterize.

The combination of TDS-THz and ARPES has offered a unique capability in differentiating the contribution from the bulk and surface to the conductivity of topological insulators. This work has demonstrated that the bulk insulating character can be enhanced through the compensation of donors and acceptors in Bi1.5Sb0.5Te1.8Se1.2. This result was published in Scientific Reports in Dec 2013 and would help further our understanding of the electronic states in these technologically advanced materials.

andrivo1

The supercell used for calculations is La4Sr2Mn6O18 (1x3x2) with experimental lattice constants: a = 3.952 Å, b = 3.947Å and c = 3.788 Å [Image credit: Nature Communications]