The superconducting compound, LiFeAs, is studied by scanning tunneling microscopy and spectroscopy. A gap map of the unreconstructed surface indicates a high degree of homogeneity in this system. Spectra at 2 K show two nodeless superconducting gaps with ∆1 = 5.3 ± 0.1 meV and ∆2 = 2.5 ± 0.2 meV. The gaps close as the temperature is increased to the bulk Tc indicating that the surface accurately represents the bulk. A dip-hump structure is observed below Tc with an energy scale consistent with a magnetic resonance recently reported by inelastic neutron scattering.PACS numbers: 74.55.+v, 74.20.Mn, 74.20.Rp, 74.70.Xa In the new family of high temperature superconductors, the iron pnictides, a general consensus is emerging in favor of an s ± symmetry [1-3] although other paring states such as p-wave [4,5] and d-wave have also been suggested [2,3]. A challenge in unambiguously identifying the pairing state in this class of materials is that experimental investigations are occurring against a backdrop of variations in sample purity and quality. In particular many of these compounds have intrinsic limitations due to high defect density from cation doping in the bulk, or structural or electronic reconstructions which complicate surface sensitive investigations [6,7].A particularly interesting compound among the pnictides is LiFeAs which is superconducting without cation substitution [8,9]. This potentially places it in the same position that YBa 2 Cu 3 O 7−x (YBCO) holds in the cuprates [10], a stoichiometric superconductor that can be chemically and structurally perfect enough to avoid artifacts arising from disorder. LiFeAs has the additional advantage of a natural cleaving plane, exposing a non-polar surface that does not undergo reconstruction [11], making it well suited to surface sensitive spectroscopic studies such as angle resolved photoemission spectroscopy (ARPES) [12,13] and scanning tunneling microscopy and spectroscopy (STM and STS) [14][15][16], much like the cuprate Bi 2 Sr 2 CaCu 2 O 8+x (BSCCO) [17,18].In this letter, we show through STM that the surface is clean and unreconstructed. Through spatially resolved STS measurements, we find that the gap structure is extremely homogeneous, presenting an opportunity to study a clean and well-defined system. STS acquired at 2K reveal two nodeless gaps, consistent with a multiband s ± pairing state [1]. We find that the temperature dependence of the gap is BCS-like, in contrast to the fluctuation driven transition [19] and pseudogap phase present above T c in the cuprates [20]. All of these simplifying features offer a system in which to study a feature LiFeAs does have in common with the cuprates; a pronounced structure above the superconducting gap, indicating strong coupling to boson modes. Single crystals of LiFeAs were grown by a self-flux technique. Li 3 As and FeAs, pre-synthesized from Li (99.9%), Fe (99.995%) and As (99.9999%), were mixed in a composition of 1:2 and sealed under 0.3 atm Ar. The mixture was heated to 1323 K for 10 hours, then cooled ...

Defects in LiFeAs are studied by scanning tunneling microscopy and spectroscopy (STS). Topographic images of the five predominant defects allow the identification of their positions within the lattice. The most commonly observed defect is associated with an Fe site and does not break the local lattice symmetry, exhibiting a bound state near the edge of the smaller gap in this multigap superconductor. Three other common defects, including one also on an Fe site, are observed to break local lattice symmetry and are pair breaking, indicated by clear in-gap bound states, in addition to states near the smaller gap edge. STS maps reveal complex, extended real-space bound-state patterns, including one with a chiral distribution of the local density of states. The multiple bound-state resonances observed within the gaps and at the inner gap edge are consistent with theoretical predictions for the s ± gap symmetry proposed for LiFeAs and other iron pnictides.

Many-body phenomena are ubiquitous in solids, as electrons interact with one another and the many excitations arising from lattice, magnetic, and electronic degrees of freedom. These interactions can subtly influence the electronic properties of materials ranging from metals, 1 exotic materials such as graphene, 2, 3 and topological insulators, 4 or they can induce new phases of matter, as in conventional 5 and unconventional superconductors, 6-9 heavy fermion systems, 10 and other systems of correlated electrons. As no single theoretical approach describes all such phenomena, the development of versatile methods for measuring manybody effects is key for understanding these systems. To date, angle-resolved photoemission spectroscopy (ARPES) has been the method of choice for accessing this physics by directly imaging momentum resolved electronic structure. 2-4, 6, 7, 9 Scanning tunneling microscopy/spectroscopy (STM/S), renown for its real-space atomic resolution capability, can also access the electronic structure in momentum space using Fourier transform scanning tunneling spectroscopy (FT-STS Non-interacting electrons in a crystal occupy quantum states with an infinite lifetime and band dispersion (k) set by the lattice potential. Interactions with the other electrons and elementary excitations of the system scatter the electrons, resulting in an altered dispersion relation E(k) and a finite lifetime. These many-body effects are encoded in the complex self-energyThe imaginary part Σ (k, E) determines the lifetime of the state and is related to the scattering rate. The real part Σ (k, E) shifts the electronic dispersionThe tools available for studying energy and momentum resolved selfenergy are limited. 15 For example, bulk transport and optical spectroscopies provide some access to k-integrated self-energies while ARPES accesses k-resolved information for only the occupied states. It is therefore important to develop a more extensive suite of versatile techniques, especially in the context of complex systems that remain poorly understood from a theoretical perspective.STM/STS accesses momentum space electronic structure by imaging real-space maps of the modulations in differential conductance (dI/dV ), which is proportional to the local density of states (LDOS) of the sample. These modulations arise from the interference of electrons scattered elastically by defects, and contain information about the initial and final momenta that are accessible by a Fourier transform of the real space map. As the electrons are dressed by interactions, the momentum space scattering intensity map is often referred to as the quasiparticle interference (QPI) map. The dominant intensities in a QPI map occur at scattering wave vectors linking constant energy segments of the band dispersion. By tracking the energy dependence of these peaks, the electronic dispersion E(k) can be obtained. This technique has been used to map coarse dispersions in many materials 16,17 and to examine scattering selection rules. 16,17 While the influenc...

Quasiparticle interference, (QPI) by means of scanning tunneling microscopy/spectroscopy (STM/STS), angle resolved photoemission spectroscopy (ARPES), and multi-orbital tight binding calculations is used to investigate the band structure and superconducting order parameter of LiFeAs. Using this combination of techniques we identify intra-and interband scattering vectors between the hole (h) and electron (e) bands in the QPI maps. Discrepancies in the band dispersions inferred from previous ARPES and STM/STS are reconciled by recognizing a difference in the kz sensitivity for the two probes. The observation of both h-h and e-h scattering is exploited using phase-sensitive scattering selection rules for Bogoliubov quasiparticles. From this we demonstrate an s± gap structure, where a sign change occurs in the superconducting order parameter between the e and h bands.

Understanding the physical origin of threshold switching and resistance drift phenomena is necessary for making a breakthrough in the performance of low-cost nanoscale technologies related to nonvolatile phase-change memories. Even though both phenomena of threshold switching and resistance drift are often attributed to localized states in the band gap, the distribution of defect states in amorphous phase-change materials (PCMs) has not received so far, the level of attention that it merits. This work presents an experimental study of defects in amorphous PCMs using modulated photocurrent experiments and photothermal deflection spectroscopy. This study of electrically switching alloys involving germanium (Ge), antimony (Sb) and tellurium (Te) such as amorphous germanium telluride (a-GeTe), a-Ge 15 Te 85 and a-Ge 2 Sb 2 Te 5 demonstrates that those compositions showing a high electrical threshold field also show a high defect density. This result supports a mechanism of recombination and field-induced generation driving threshold switching in amorphous chalcogenides. Furthermore, this work provides strong experimental evidence for complex trap kinetics during resistance drift. This work reports annihilation of deep states and an increase in shallow defect density accompanied by band gap widening in aged a-GeTe thin films.