Ordered assemblies of magnetic atoms on the surface of conventional superconductors can be used to engineer topological superconducting phases and realize Majorana fermion quasiparticles (MQPs) in a condensed matter setting. Recent experiments have shown that chains of Fe atoms on Pb generically have the required electronic characteristics to form a 1D topological superconductor and have revealed spatially resolved signatures of localized MQPs at the ends of such chains. Here we report higher resolution measurements of the same atomic chain system performed using a dilution refrigerator scanning tunneling microscope (STM). With significantly better energy resolution than previous studies, we show that the zero bias peak (ZBP) in Fe chains has no detectable splitting from hybridization with other states. The measurements also reveal that the ZBP exhibits a distinctive 'double eye' spatial pattern on nanometer length scales. Theoretically we show that this is a general consequence of STM measurements of MQPs with substantial spectral weight in the superconducting substrate, a conclusion further supported by measurements of Pb overlayers deposited on top of the Fe chains. Finally, we report experiments performed with superconducting tips in search of the particle-hole symmetric MQP signature expected in such measurements.Comment: Accepted to Nature Physics; available through Advance online publicatio

Nematic quantum fluids with wavefunctions that break the underlying crystalline symmetry can form in interacting electronic systems. We examine the quantum Hall states that arise in high magnetic fields from anisotropic hole pockets on the Bi(111) surface. Spectroscopy performed with a scanning tunneling microscope shows that a combination of single-particle effects and many-body Coulomb interactions lift the six-fold Landau level (LL) degeneracy to form three valley-polarized quantum Hall states. We image the resulting anisotropic LL wavefunctions and show that they have a different orientation for each broken-symmetry state. The wavefunctions correspond to those expected from pairs of hole valleys and provide a direct spatial signature of a nematic electronic phase. Main text:Nematic electronic states represent an intriguing class of broken-symmetry phases that can spontaneously form as a result of electronic correlations (1, 2). They are characterized by reduced rotational symmetry relative to the underlying crystal lattice and have attracted considerable interest in systems such as two-dimensional electron gases (2DEGs) (3-5), strontium ruthenate (6), and high-temperature superconductors (7)(8)(9)(10)(11)(12). The sensitivity of electronic nematic phases to disorder results in short range ordering and domains, making them difficult to study using global measurements that average over microscopic configurations. The effect of perturbations, such as crystalline strain, may be used to show a propensity for nematic order, i.e. to provide evidence that vestiges of nematic behavior survive even in the presence of material imperfections (1). However, it is difficult to quantitatively correlate the experimental evidence of ordering with a microscopic description of the electronic states and the interactions responsible for nematic behavior. To put the study of nematic electronic phases on more quantitative ground, it is therefore important not only to perform local measurements, but also to find a material system for which theory can fully characterize the underlying broken-symmetry states and the electronic interactions.Multi-valley 2DEGs with anisotropic band structure have been anticipated as a model platform to explore nematic order in the quantum Hall regime (13-17). The key idea is that Coulomb interactions can spontaneously lift the valley degeneracy in materials with low disorder and thereby break rotational symmetry. In contrast to previously studied metallic nematic phases, this leads to a gapped nematic state with quantized Hall conductance. We examine such a 2DEG on the surface of single crystals of bismuth (Bi), which is one of the cleanest electronic systems, with a bulk mean free path reaching 1 mm at low temperatures (18). Interest in Bi has recently been rekindled by bulk measurements showing phase transitions and anisotropic behavior at large magnetic fields, which may be related to nematic electronic phenomena (19)(20)(21)(22). We focus here on the (111) surface of Bi, for which strong R...

High quality hexagon plate-like Na3Bi crystals with large (001) plane surfaces were grown from a molten Na flux. The freshly cleaved crystals were analyzed by low temperature scanning tunneling microscopy and angle-resolved photoemission spectroscopy, allowing for the characterization of the three-dimensional (3D) Dirac semimetal (TDS) behavior and the observation of the topological surface states. Landau levels were observed, and the energy-momentum relations exhibited a linear dispersion relationship, characteristic of the 3D TDS nature of Na3Bi. In transport measurements on Na3Bi crystals, the linear magnetoresistance and Shubnikov-de Haas quantum oscillations are observed for the first time.

The Josephson effect provides a direct method to probe the strength of the pairing interaction in superconductors. By measuring the phase fluctuating Josephson current between a superconducting tip of a scanning tunneling microscope (STM) and a BCS superconductor with isolated magnetic adatoms on its surface, we demonstrate that the spatial variation of the pairing order parameter can be characterized on the atomic scale. This system provides an example where the local pairing potential suppression is not directly reflected in the spectra measured via quasipartcile tunneling. Spectroscopy with such superconducting tips also show signatures of previously unex- A number of novel superconducting states of matter such as those appearing in disordered superconductors, heavy fermion materials, and high-T c superconductors have been predicted to have pairing order parameters that are spatially modulated on atomic length scales. These short range spatial modulations can occur due to different mechanisms such as the inhomogeneous material properties in disordered superconductors [1][2][3][4], a momentum dependent pairing interaction, such as the Fulde-Ferrell-Larkin-Ovchinnkov (FFLO) state proposed for heavy fermion materials [5][6][7], or the interplay between different forms of electronic ordering in the pair density waves proposed for high-T c cuprates [8][9][10]. Although spectroscopic mapping with a scanning tunneling microscope (STM) can provide evidence for variations in the local density of states (LDOS) through quasi-particle tunneling, such measurements probe the superconducting order parameter only indirectly. If the Josephson effect can be measured and mapped on the atomic scale, then it would allow for direct characterization of the local pairing order parameter and high-resolution studies of novel superconducting phases [11].This goal has motivated previous efforts in the use of superconducting tips in STM [12] and has led to the local observation of thermal phase fluctuating Josephson supercurrent close to the point contact regime [13][14][15]. Subsequent measurements have mapped the Josephson effect on the nanometer scale, applying this technique to vortices [16,17] and high-T c cuprates [18,19]. A major challenge in improving the resolution of these experiments has been satisfying the competing requirements of a high junction impedance necessary for imaging and a low junction impedance allowing for the strong tip-sample coupling necessary to observe the Josephson effect despite thermal fluctuations. Extending the Josephson STM measurements to millikelvin temperatures allows for mapping of the Cooper pair current at junction resistances that are compatible with atomic resolution imaging.In this letter, we use scanning Josephson spectroscopy to probe variations of the superconducting order parameter on the scale of a single atom. We map the strength of the phase fluctuating Josephson current between a superconducting Pb tip and a Pb(110) surface with a dilute concentration of magnetic impurities us...

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