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We report on high quality local tunnel spectroscopy measurements in superconductors using in-situ fabricated superconducting tips as counterelectrode. The experiments were made at very low temperatures using a dilution refrigerator and a 3 He cryostat. Spectra obtained with superconducting tip and sample of Al show that the spectroscopic resolution of our set-up is of 15µeV . Following the observation of Josephson current in tunneling regime (with tips of Pb and of Al), we discuss the feasibility of Scanning Josephson Spectroscopy with atomic size resolution. Experiments showing new applications of these superconducting tips under applied external magnetic fields are also reported.

We report electrical conductance measurements of Bi nanocontacts created by repeated tip-surface indentation using a scanning tunneling microscope at temperatures of 4 K and 300 K. As a function of the elongation of the nanocontact we measure robust, tens of nanometers long plateaus of conductance G0 = 2e2 /h at room temperature. This observation can be accounted for by the mechanical exfoliation of a Bi(111) bilayer, a predicted QSH insulator, in the retracing process following a tipsurface contact. The formation of the bilayer is further supported by the additional observation of conductance steps below G0 before break-up at both temperatures. Our finding provides the first experimental evidence of the possibility of mechanical exfoliation of Bi bilayers, of the existence of the QSH phase in a two-dimensional crystal, and, most importantly, of the observation of the QSH phase at room temperature.Topological insulators present a gap in the bulk, but host surface states protected against backscattering by time reversal symmetry [1]. This implies that they are immune to non-magnetic disorder-induced localization, i.e., they are able to carry electrical current on the surface regardless of imperfections. 2D TIs[2], actually predicted before their three-dimensional (3D) counterparts [3], are expected to exhibit the so-called quantum spin Hall (QSH) phase, a spin filtered version of the integer quantum Hall effect [4]. While the most exotic experimental manifestation of this phase is through a nearly universal spin Hall conductivity of ≈ e/2π, a truly universal charge transport is expected to manifest, e.g., as a two-terminal conductance G 0 = 2e 2 /h.To date, two types of 2D systems have been predicted to be QSH insulators: two-dimensional crystals such as graphene [2] or Bi(111) bilayers [5] and semiconductor heterojunctions such as CdTe/HgTe[6] or, more recently, InAs/GaSb quantum wells [7]. Transport measurements in CdTe/HgTe [6] and InAs/GaSb[8] quantum wells have revealed the presence of protected edge states and provided the first experimental evidence of the QSH phase to date. The QSH state in 2D crystals, on the other hand, has not been experimentally confirmed to date. The fact that spin-orbit coupling (SOC) in graphene is so weak precludes the observation of the QSH phase in this material. Bismuth, on the contrary, presents a naturally large SOC, its mechanical and electronic properties are well characterized for bulk and surface [9], in nanowire form [10], and, recently, the existence of edge states in Bi(111) bilayers has been reported [11]. Other proposals stay, at this moment, at a more speculative level [12].Cleavage techniques are becoming common in the quest for 2D crystals [13]. With a few exceptions [14], they remain largely unexplored in the field of topological in- sulators (TI's). We report here, using scanning tunneling microscope (STM) based mechanical and electrical characterization techniques, the first evidence of the QSH phase in a two-dimensional crystal such as an exfoliated Bi(111) bi...

We present a method to produce superconducting tips to be used in Scanning Tunneling Microscopy/Spectroscopy experiments. We use these tips to investigate the evolution of the electronic density of states of NbSe 2 from 0.3K up to its critical temperature (7.2K). The use of a superconducting tip (Pb) as ounterelectrode provides an enhancement of the different features related to the DOS of NbSe 2 in the tunneling conductance curves, along all the studied thermal range. The analysis of the experimental results gives evidence of the presence of multiband superconductivity in NbSe 2 .

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