Drive the Dirac electrons into Cooper pairs in SrxBi2Se3

Abstract

Topological superconductors are a very interesting and frontier topic in condensed matter physics. Despite the tremendous efforts in exploring topological superconductivity, its presence is however still under heavy debate. The Dirac electrons have been proven to exist on the surface of a topological insulator. It remains unclear whether and how the Dirac electrons fall into Cooper pairing in an intrinsic superconductor with the topological surface states. Here we show the systematic study of scanning tunnelling microscope/spectroscopy on the possible topological superconductor SrxBi2Se3. We first demonstrate that only the intercalated Sr atoms can induce superconductivity. Then we show the full superconducting gaps without any in-gap density of states as expected theoretically for a bulk topological superconductor. Finally, we find that the surface Dirac electrons will simultaneously condense into the superconducting state within the superconducting gap. This vividly demonstrates how the surface Dirac electrons are driven into Cooper pairs.

Degradation of topological surface state by nonmagnetic S doping in SrxBi2Se3

“…2 shows an enlarged view of the resistivity near the supercoducting transition temperature. It can be seen that the onset temperature of the superconducting transition () is about 2.9 K for the undoped Sr 0.066 Bi 2 Se 3 sample, which is consistent with previous reports232425262728. The width of the superconducting transition is less than 0.3 K, suggesting the high-quality of the single crystal sample.…”

“…It has also been experimentally proven that the Dirac point and the topological surface states are well-preserved in the Sr x Bi 2 Se 3 samples23242526. Furthermore, angle-dependent resistivity measurements on Sr x Bi 2 Se 3 single crystals by different groups have revealed apparent two-fold anisotropy, indicating rotational symmetry breaking in this compound2728. The nodeless and two-fold symmetric superconducting gap is consistent with the prediction of topologically nontrivial superconductivity in Sr x Bi 2 Se 3 .…”

Superconducting topological surface states in the noncentrosymmetric bulk superconductor PbTaSe 2

“…A larger superconducting gap on the TSS may also be induced as a result of parity mixing of the pairing potential enhanced by surface Dirac fermions ( 46 ). A tunneling spectrum on superconducting Cu x Bi 2 Se 3 and Sr x Bi 2 Se 3 also does not show an additional superconducting gap ( 10 , 47 , 48 ), which is proposed to be due to the doping evolution of the Fermi surface ( 46 , 49 ). Meanwhile, recent nuclear magnetic resonance ( 50 ) and specific heat ( 51 ) measurements on Cu x Bi 2 Se 3 report rotational symmetry breaking in spin rotation and superconducting gap amplitude, indicating topologically nontrivial superconductivity.…”

Superconductivity with twofold symmetry in Bi 2 Te 3 /FeTe 0.55 Se 0.45 heterostructures

“…Proximity effect is another way to induce possible topological superconductivity ( 7 ), which has been demonstrated in the Bi 2 Se 3 thin film on top of 2 H -NbSe 2 ( 8 ) and with the possible evidence of a Majorana mode within the vortex core on Bi 2 Te 3 /NbSe 2 heterostructures ( 9 ). In the doped materials M x Bi 2 Se 3 (M = Cu, Nb, and Sr), bulk superconductivity was observed ( 10 – 12 ) and full gaps have been reported on the surface by scanning tunneling microscopy/spectroscopy (STM/STS) measurements ( 13 , 14 ). Recently, Knight shift of nuclear magnetic resonance (NMR) measurements ( 15 ) and angle resolved–specific heat ( 16 ) reveal a twofold feature of superconductivity when the magnetic field rotates within the ab plane in Cu x Bi 2 Se 3 .…”