Observation of two-dimensional bulk electronic states in the superconducting topological insulator heterostructureCux(PbSe)5(Bi

Abstract

We have performed angle-resolved photoemission spectroscopy (ARPES) on Cu x (PbSe) 5 (Bi 2 Se 3 ) 6 (CPSBS; x = 1.47), a superconductor derived from a topological insulator heterostructure, to elucidate the electronic states relevant to the occurrence of possible unconventional superconductivity. Upon Cu intercalation into the parent compound (PbSe) 5 (Bi 2 Se 3 ) 6 , we observed a distinct energy shift of the bulk conduction band due to electron doping. Photonenergy dependent ARPES measurements of CPSBS revealed that the observed bulk band forms a cylindrical electronlike Fermi surface at the Brillouin-zone center. The two-dimensional nature of the bulk electronic states puts strong constraints on the possible topological character of the superconducting state in CPSBS. 75.70.Tj, 79.60 In this Rapid Communication, we report high-resolution ARPES study of PSBS (m = 2) and its Cu-intercalated counterpart, CPSBS. By utilizing a relatively long escape depth of photoelectrons excited by low-energy photons, we have succeeded in observing previously unidentified, intrinsic bulk band structure of the 2-QL Bi 2 Se 3 unit lying deeper beneath the surface. Our most important finding is that the near-E F bulk band responsible for the superconductivity shows a two-dimensional (2D) character, in contrast to the 3D character of bulk Bi 2 Se 3 . We discuss the implications of the present result in relation to the possible topological superconductivity in CPSBS.High-quality single crystals of a parent compound PSBS (m = 2) were grown by a modified Bridgman method using high purity elements Pb ( between the quantized bulk CB and the lower branch of the gapped Dirac-cone state in the topmost 2-QL Bi 2 Se 3 of PSBS in Fig. 1(a). Thus, bands A and B are attributed to the quantized bulk CB and the lower branch of the gapped Dirac-cone state, respectively. It is noted that bands A and B in Fig. 2(f) are shifted downward by ∼ 0.1 eV as compared to those in Fig. 1(a). This may be due to the weakened band-bending effect (i.e. weakened outof-plane potential gradient) at the second Bi 2 Se 3 unit, which may also explain the absence of a clear Rashba splitting for band A in Figs. 2(b) and 2(f). Also, the absence of the upper branch of gapped Dirac-cone state is likely due to overlapping by CB 1st with a dominant intensity.To elucidate the Fermi-surface topology in the 3D momentum space relevant to the possible unconventional superconductivity of CPSBS, we have performed hν-dependent ARPES measurements by focusing on band A (hereafter we call CB 2nd ). As one can see in the representative second-derivative intensity plots in Fig. 3(a), we commonly see CB 2nd and CB 1st over a wide hν range of 8.4-23 eV (note that 8.4-eV photons are most bulk sensitive).A direct comparison of the EDC at theΓ point in Fig. 3 Now we discuss the implications of our results in relation to topological superconductivity.The most important finding of the present study is that the unconventional superconduc- an intriguing platform to explore Majorana fermio...

Nematic Superconductivity in Doped Bi2Se3 Topological Superconductors

“…Strictly speaking, the global crystal symmetry of this compound is orthorhombic and does not have three-fold rotational symmetry due to the neighboring PbSe layer. Nevertheless, the Bi 2 Se 3 layer of this compound, as well as its electronic structure, almost preserves the three-fold symmetry [86,87]. Therefore, the observed two-fold anisotropy in the SC state is most likely due to a nematic SC gap, rather than the conventional origin such as Fermi-velocity anisotropy.…”

Topological interface states in the natural heterostructure (PbSe) 5 ( Bi2Se3 ) 6Momida1, Bihlmayer2, Blügel3 et al. 2018Phys. Rev. BSelf Cite102110View full textAdd to dashboardCite

“…The Pb-based homologous series of (PbSe) 5 (Bi 2 Se 3 ) 3m (m = 1,2) have recently attracted attention and been studied experimentally because of their interesting and unconventional topological properties [1][2][3][4][5][6]. (PbSe) 5 (Bi 2 Se 3 ) 3m consists of an alternating layered structure of m quintuple layers (QLs) of Bi 2 Se 3 with bilayer PbSe [7,8], and it is naturally formed as a multilayer heterostructure.…”

“…Such multilayer systems possessing a topological and an ordinary insulator have also been considered to be an interesting playground for realizing novel Weyl semimetal phases [13]. Nakayama et al have observed gapped Dirac-cone dispersions within the bulk band gap for m = 2 via angle-resolved photoemission spectroscopy (ARPES), showing that topological interface states are effectively encapsulated by the PbSe block layers [2,3]. Prompted by recent interest in topological superconductors [14], an unconventional superconducting nature has been studied experimentally for (PbSe) 5 (Bi 2 Se 3 ) 3mbased materials with Ag doping [4] and Cu intercalations [3,5].…”

“…Nakayama et al have observed gapped Dirac-cone dispersions within the bulk band gap for m = 2 via angle-resolved photoemission spectroscopy (ARPES), showing that topological interface states are effectively encapsulated by the PbSe block layers [2,3]. Prompted by recent interest in topological superconductors [14], an unconventional superconducting nature has been studied experimentally for (PbSe) 5 (Bi 2 Se 3 ) 3mbased materials with Ag doping [4] and Cu intercalations [3,5]. However, a detailed understanding of this material is very limited, especially from theoretical viewpoints.…”

Unconventional superconductors with spin-orbit interaction

“…Moreover, besides the Cu doped case, upper critical field measurements indicate the nematic state in the Sr or Nb doped cases as well [94][95][96] Similarly, compound topological insulators Cu x (PdSe) 5 (Bi 2 Se 3 ) 6 can also be in a superconducting state below T c = 2.85K [98]. ARPES measurements have shown that both bulk and surface bands cross the Fermi energy [99]. In addition, the temperature dependence of the specific heat shows linear behavior near T = 0, indicating the existence of line node [98].…”