In 1928, Dirac proposed a wave equation to describe relativistic electrons1. Shortly afterwards, Klein solved a simple potential step problem for the Dirac equation and encountered an apparent paradox: the potential barrier becomes transparent when its height is larger than the electron energy. For massless particles, backscattering is completely forbidden in Klein tunnelling, leading to perfect transmission through any potential barrier2,3. The recent advent of condensed-matter systems with Dirac-like excitations, such as graphene and topological insulators, has opened up the possibility of observing Klein tunnelling experimentally4{\textendash}6. In the surface states of topological insulators, fermions are bound by spin{\textendash}momentum locking and are thus immune from backscattering, which is prohibited by time-reversal symmetry. Here we report the observation of perfect Andreev reflection in point-contact spectroscopy{\textendash}-a clear signature of Klein tunnelling and a manifestation of the underlying {\textquoteleft}relativistic{\textquoteright}physics of a proximity-induced superconducting state in a topological Kondo insulator. Our findings shed light on a previously overlooked aspect of topological superconductivity and can serve as the basis for a unique family of spintronic and superconducting devices, the interface transport phenomena of which are completely governed by their helical topological states.

}, isbn = {1476-4687}, doi = {10.1038/s41586-019-1305-1}, url = {https://doi.org/10.1038/s41586-019-1305-1}, author = {Lee, Seunghun and Stanev, Valentin and Zhang, Xiaohang and Stasak, Drew and Flowers, Jack and Higgins, Joshua S. and Dai, Sheng and Blum, Thomas and Pan, Xiaoqing and Yakovenko, Victor M. and Paglione, Johnpierre and Greene, Richard L. and Galitski, Victor and Takeuchi, Ichiro} } @article { ISI:000452687700006, title = {Anomalous normal-state resistivity in superconducting La2-xCexCuO4: Fermi liquid or strange metal?}, journal = {PHYSICAL REVIEW B}, volume = {98}, number = {22}, year = {2018}, month = {DEC 10}, pages = {224503}, abstract = {We present experimental results for the in-plane resistivity of the electron-doped cuprate superconductor La2-xCexCuO4 above its transition temperature T-c as a function of Ce doping x and temperature. For the doping x between 0.11 and 0.17, where T-c varies from 30 K (x = 0.11) to 5 K (x = 0.17), we find that the resistivity shows a T-2 behavior for all values of doping over the measurement range from 70 to 250 K. The coefficient of the T-2 resistivity term decreases with increasing x following the trend in We analyze our data theoretically and posit that n-type cuprates are better thought of as strange metals. Although the quadratic temperature dependence appears to be in naive agreement with the Fermi-liquid (FL) expectations, the facts that the measured resistivity is large and approximate T-2 scattering dominates the resistivity even up to 400 K argue against a standard normal-metal FL picture being applicable. We discuss possible origins of the strange-metal behavior.}, issn = {2469-9950}, doi = {10.1103/PhysRevB.98.224503}, author = {Sarkar, Tarapada and Greene, Richard L. and S. Das Sarma} }