In non-magnetic bulk materials, inversion symmetry protects the spin degeneracy. If the bulk crystal structure lacks a centre of inversion, however, spin–orbit interactions lift the spin degeneracy, leading to a Rashba metal whose Fermi surfaces exhibit an intricate spin texture. In superconducting Rashba metals a pairing wavefunction constructed from these complex spin structures will generally contain both singlet and triplet character. Here we examine the possible triplet components of the order parameter in noncentrosymmetric BiPd, combining for the first time in a noncentrosymmetric superconductor macroscopic characterization, atomic-scale ultra-low-temperature scanning tunnelling spectroscopy, and relativistic first-principles calculations. While the superconducting state of BiPd appears topologically trivial, consistent with Bardeen–Cooper–Schrieffer theory with an order parameter governed by a single isotropic s-wave gap, we show that the material exhibits Dirac-cone surface states with a helical spin polarization.

Spin-polarized scanning tunneling microscopy (SP-STM) has been used extensively to study magnetic properties of nanostructures. Using SP-STM to visualize magnetic order in strongly correlated materials on an atomic scale is highly desirable, but challenging. We achieved this goal in iron tellurium (Fe(1+ y)Te), the nonsuperconducting parent compound of the iron chalcogenides, by using a STM tip with a magnetic cluster at its apex. Our images of the magnetic structure reveal that the magnetic order in the monoclinic phase is a unidirectional stripe order; in the orthorhombic phase at higher excess iron concentration (y > 0.12), a transition to a phase with coexisting magnetic orders in both directions is observed. It may be possible to generalize the technique to other high-temperature superconductor families, such as the cuprates.

Atomic force microscopy (AFM) images of graphene and graphite show contrast with atomic periodicity. However, the contrast patterns vary depending on the atomic termination of the AFM tip apex and the tip-sample distance, hampering the identification of the atomic positions. Here, we report quantitative AFM imaging of epitaxial graphene using inert (carbon-monoxide-terminated) and reactive (iridium-terminated) tips. The atomic image contrast is markedly different with these tip terminations. With a reactive tip, we observe an inversion from attractive to repulsive atomic contrast with decreasing tip-sample distance, while a nonreactive tip only yields repulsive atomic contrast. We are able to identify the atoms with both tips at any tip-sample distance. This is a prerequisite for future structural and chemical analysis of adatoms, defects, and the edges of graphene nanostructures, crucial for understanding nanoscale graphene devices.

Noncontact atomic force microscopy (AFM) has recently progressed tremendously in achieving atomic resolution imaging through the use of small oscillation amplitudes and well-defined modification of the tip apex. In particular, it has been shown that picking up simple inorganic molecules (such as CO) by the AFM tip leads to a well-defined tip apex and to enhanced image resolution. Here, we use the same approach to study the three-dimensional intermolecular interaction potential between two molecules and focus on the implications of using molecule-modified AFM tips for microscopy and force spectroscopy experiments. The flexibility of the CO at the tip apex complicates the measurement of the intermolecular interaction energy between two CO molecules. Our work establishes the physical limits of measuring intermolecular interactions with scanning probes. Scanning probe methods have been the workhorse of nanotechnology in obtaining atomic-scale structural and electronic information. While earlier experiments relied mostly on scanning tunneling microscopy (STM) [1][2][3][4], noncontact atomic force microscopy (AFM) has recently revolutionized the field of atomic-scale imaging by the impressive demonstrations of atomic resolution on bulk and ultrathin insulators and atomic identification and manipulation at room-temperature on semiconductor surfaces [5][6][7][8][9][10]. Improvements in AFM instrumentation have further extended these possibilities as demonstrated by the structural characterization of single crystal surfaces and isolated organic molecules with unprecedented spatial resolution [11][12][13][14][15]. These advances are based on the use of a quartz tuning fork force sensor operating with very small tip oscillation amplitudes, down to a fraction of an Å ngström [16,17]. As a consequence, the measured frequency shift Áf (proportional to the force gradient) is dominated by the short range (chemical) forces allowing the interaction energies to be measured down to the atomic scale [10,14,[16][17][18][19].Previous experiments have shown that using a carbon monoxide terminated tip (formed by controlled vertical manipulation of CO from the sample surface onto the tip) yields a well-defined tip apex and enhanced image resolution [12,20]. Here, we use the same approach and present precise measurement of intermolecular forces between two CO molecules: one adsorbed on the Cu(111) single crystal substrate and the other on the AFM tip as schematically illustrated in Fig. 1. The frequency shift, Áf, is measured as a function of tip-sample distance (z direction) and lateral distance between tip and the adsorbed CO molecule. By performing this experiment with a metalterminated and a CO-terminated tip, a three-dimensional map of the intermolecular interaction potential with subatomic resolution is obtained. At sufficiently large intermolecular separations this approach yields the ''true'' intermolecular interaction, i.e., a combination of chemical repulsion (due to the Pauli exclusion principle) and van der Waals (vdW) attract...