Charge transport properties of metal-molecule interfaces depend strongly on the character of molecule-electrode interactions. Although through-bond coupled systems have attracted the most attention, through-space coupling is important in molecular systems when, for example, through-bond coupling is suppressed due to quantum interference effects. To date, a probe that clearly distinguishes these two types of coupling has not yet been demonstrated. Here, we investigate the origin of flicker noise in single molecule junctions and demonstrate how the character of the molecule-electrode coupling influences the flicker noise behavior of single molecule junctions. Importantly, we find that flicker noise shows a power law dependence on conductance in all junctions studied with an exponent that can distinguish through-space and through-bond coupling. Our results provide a new and powerful tool for probing and understanding coupling at the metal-molecule interface.

Abstract:The driving forces behind electronic nematicity in the iron pnictides remain hotly debated. We use atomic-resolution variable-temperature scanning tunneling spectroscopy to provide the first direct visual evidence that local electronic nematicity and unidirectional antiferroic (stripe) fluctuations persist to temperatures almost twice the nominal structural ordering temperature in the parent pnictide NaFeAs. Low-temperature spectroscopic imaging of nematically-ordered NaFeAs shows anisotropic electronic features that are not observed for isostructural, non-nematic LiFeAs. The local electronic features are shown to arise from scattering interference around crystalline defects in NaFeAs, and their spatial anisotropy is a direct consequence of the structural and stripe-magnetic order present at low temperature. We show that the anisotropic features persist up to high temperatures in the nominally tetragonal phase of the crystal. The spatial distribution and energy dependence of the anisotropy at high temperatures is explained by the persistence of large amplitude, short-range, unidirectional, antiferroic (stripe) fluctuations, indicating that strong density wave fluctuations exist and couple to near-Fermi surface electrons even far from the structural and density wave phase boundaries. Main Text:The nature of the normal state from which superconductivity emerges in unconventional superconductors remains a mystery. It is suspected that electronic interactions present in the normal state play a key role in the formation of the superconducting state 1 . In both the cuprates and the pnictide phase diagrams, magnetically ordered states exist in proximity to the superconducting state, and the pnictides additionally exhibit orbital ordering 2,3 . A crucial additional feature of the pnictides is the appearance of a "nematic'' phase in which the tetragonal rotational symmetry of the ideal pnictide lattice is spontaneously broken below a temperature T S . Recent bulk transport and scattering measurements have suggested that the nematic phase is driven by electronic, rather than lattice, degrees of freedom [4][5][6][7][8] and is observed in all electronic channels -charge 9,10 , orbital 4 , and spin 7,11 . Spin order and spin fluctuations [12][13][14][15][16] (which couple quadratically to nematicity) as well as orbital order 17,18 and orbital fluctuations 19 (which can couple linearly) have been invoked to explain the nematicity.However, the dominant interaction responsible for the nematic ordering and fluctuations remains 2 unknown and identifying it is a key experimental goal. In this paper, we use variable temperature scanning tunneling spectroscopy to provide new insights into this issue by showing that our spectroscopic signals reveal that nematicity occurs in conjuction with strong antiferroic fluctuations and that both phenomena persist up to temperatures much greater than the temperatures at which longrange order is established.The arsenide superconductors consist of one or more iron-arsenide layers with the...

Surfaces of three-dimensional topological insulators have emerged as one of the most remarkable states of condensed quantum matter 1-5 where exotic electronic phases of Dirac particles should arise 1,6-8 . Here we report a discovery of surface superconduc-

We show that a small number of intentionally introduced defects can be used as a spectroscopic tool to amplify quasiparticle interference in 2H-NbSe 2 , that we measure by scanning tunneling spectroscopic imaging. We show from the momentum and energy dependence of the quasiparticle interference that Fermi surface nesting is inconsequential to charge density wave formation in 2H-NbSe 2 . We demonstrate that by combining quasiparticle interference data with additional knowledge of the quasiparticle band structure from angle resolved photoemission measurements, one can extract the wavevector and energy dependence of the important electronic scattering processes thereby obtaining direct information both about the fermiology and the interactions.In 2H-NbSe 2 , we use this combination to show that the important near-Fermi-surface electronic physics is dominated by the coupling of the quasiparticles to soft mode phonons at a wave vector different from the CDW ordering wave vector.

The topology of Weyl semimetals requires the existence of unique surface states. Surface states have been visualized in spectroscopy measurements, but their connection to the topological character of the material remains largely unexplored. 1T'-MoTe 2 , presents a unique opportunity to study this connection. This material undergoes a phase transition at 240 K that changes the structure from orthorhombic (putative Weyl semimetal) to monoclinic (trivial metal), while largely maintaining its bulk electronic structure. Here, we show from temperature-dependent quasiparticle interference measurements that this structural transition also acts as a topological switch for surface states in 1T'-MoTe 2 . At low temperature, we observe strong quasiparticle scattering, consistent with theoretical predictions and photoemission measurements for the surface states in this material. In contrast, measurements performed at room temperature show the complete absence of the scattering wavevectors associated with the trivial surface states. These distinct quasiparticle scattering behaviors show that 1T'-MoTe 2 is ideal for separating topological and trivial electronic phenomena via temperature-dependent measurements.