Universität zu Köln

Bagchi, Mahasweta ORCID: 0000-0002-0031-0952 (2023). Surface characterization of topological superconductor materials using scanning tunneling microscopy and spectroscopy. PhD thesis, Universität zu Köln.

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Abstract

Realizing and understanding topological superconductors that can host exotic quasiparticles such as the Majorana fermions is currently an exciting challenge in condensed-matter physics. However, not many materials have been conclusively identified as topological superconductors. Here, we examine topological superconductivity in two different systems - the family of doped Bi2Se3 superconductors and in the heterostructure of a conventional superconductor [Pb(111) thin film] grown on a topological insulator TlBiSe2. We do so by using the surface-sensitive technique of scanning tunneling microscopy (STM) and spectroscopy (STS) under ultra high vacuum (UHV) and down to 350 mK. We present the growth and characterization of three different members of the doped Bi2Se3 family. In this class of materials, bulk superconducting properties consistently show a peculiar twofold symmetry that is only compatible with a gap structure that has odd-parity and hence is considered to be topological. However, we find that superconductivity on the surface of these crystals is not robust and this constitutes a formidable challenge for our goal. Nevertheless, we have made several unexpected observations in each case. On the superconducting area on the surface of CuxBi2Se3, we measure a tunneling spectrum that suggests that the density of states around the Fermi energy is gapped but with a twofold anisotropy. This anisotropy also manifests in a 10 percent difference in the average superconducting gap measured as a function of the orientation of the in-plane magnetic field with respect to the crystal lattice. The minima in the gap structure is found to coincide with a a crystallographic mirror plane. However, contrary to expectation that one should find a mirror-symmetry protected pair of point nodes, we find a minima and not a node in the gap structure since zero integrated density of states at the Fermi level is measured in the absence of a magnetic field. For a superconducting region on CPSBS, we observe elliptical areas of enhanced quasiparticle density of states due to magnetic vortices penetrating the sample when the external magnetic field is applied normal to the sample plane. The anisotropy in the vortex profile is shown to be a consequence of the anisotropic gap structure. However, we discover that the orientation of the gap minima here is rotated by 60 degree compared to the point nodes in the bulk and that the expected gap nodes are most likely lifted in this case as well. The above results are understood by employing a symmetry-based phenomenological analysis. The latter shows that to find out the expected gap structure on the surface one must consider the effective symmetry, in particular the broken inversion symmetry at the surface as well as any other differences in crystal symmetry on the surface in comparison to the bulk. Any change in the effective symmetry on the surface will allow for a change in the superconducting gap structure. In this case the observed lifting of the gap nodes and the rotation of the gap minima are consistent with the symmetry changes. Among the three members of the doped Bi2Se3 family considered here, SrxBi2Se3 is the only one to show a near 100 percent superconducting volume fraction in the bulk. However, on the surface of these crystals no gap in the density of states is observed around the Fermi energy, as long as clean metallic probe tips are used. Only when micron-sized flakes of the sample are transferred onto the STM probe tip during prolonged scanning a superconducting gap in the density of states appears. To rationalize this observation we have claimed that superconductivity in SrxBi2Se3 crystals does not extend to the surface when the topological surface state (TSS) is intact. The existence of the TSS causes the charge distribution to be different near the surface compared to the bulk, leading to band bending and a consequential local electric field, which can kill superconductivity at the surface. Therefore, in micro-flakes, where the TSS is likely destroyed due to strain from the mechanical transfer, superconductivity can be observed on the surface. The above hypothesis applies on the surface of CuxBi2Se3 as well since no evidence for the TSS is found in the superconducting regions, hinting at the absence of any surface band bending or local electric field. Moreover, majority of the surface on CuxBi2Se3} and CPSBS turned out to be nonsuperconducting, and for such areas we propose that band bending at the surface is tied to the suppression of the superconducting order parameter. Band bending and its impact on the superconducting properties have been discussed for cuprate superconductors where the relevant parameters of the Thomas-Fermi screening length (nanometers) and carrier density are similar to that of doped Bi2Se3. In parallel to our efforts on the doped Bi2Se3 superconductors, the heterostructure of Pb(111) thin film on TlBiSe2 is also investigated for signatures of two-dimensional topological superconductivity. In particular, we searched for a localized Majorana mode inside vortex cores as well as a dispersive one-dimensional Majorana mode at the physical edge of the two-dimensional system. However, no signature of such modes is detected for this system.

Item Type:	Thesis (PhD thesis)
Creators:	Creators Email ORCID ORCID Put Code Bagchi, Mahasweta mistu101093@gmail.com orcid.org/0000-0002-0031-0952 UNSPECIFIED
URN:	urn:nbn:de:hbz:38-710809
Date:	18 September 2023
Language:	English
Faculty:	Faculty of Mathematics and Natural Sciences
Divisions:	Faculty of Mathematics and Natural Sciences > Department of Physics > Institute of Physics II
Subjects:	Physics
Uncontrolled Keywords:	Keywords Language topological superconductivity English scanning tunneling spectroscopy English
Date of oral exam:	9 June 2023
Referee:	Name Academic Title Ando, Yoichi Dr.
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/71080