Local Density Of States Measurements Research Articles

Adaptive sparse sampling for quasiparticle interference imaging

Quasiparticle interference imaging (QPI) offers insight into the band structure of quantum materials from the Fourier transform of local density of states (LDOS) maps. Their acquisition with a scanning tunneling microscope is traditionally tedious due to the large number of required measurements that may take several days to complete. The recent demonstration of sparse sampling for QPI imaging showed how the effective measurement time could be fundamentally reduced by only sampling a small and random subset of the total LDOS. However, the amount of required sub-sampling to faithfully recover the QPI image remained a recurring question. Here we introduce an adaptive sparse sampling (ASS) approach in which we gradually accumulate sparsely sampled LDOS measurements until a desired quality level is achieved via compressive sensing recovery. The iteratively measured random subset of the LDOS can be interleaved with regular topographic images that are used for image registry and drift correction. These reference topographies also allow to resume interrupted measurements to further improve the QPI quality. Our ASS approach is a convenient extension to quasiparticle interference imaging that should remove further hesitation in the implementation of sparse sampling mapping schemes.• Accumulative sampling for unknown degree of sparsity• Controllably interrupt and resume QPI measurements• Scattering wave conserving background subtractions

Localized Graphitization on Diamond Surface as a Manifestation of Dopants

Doped diamond electrodes have attracted significant attention for decades owing to their excellent physical and electrochemical properties. However, direct experimental observation of dopant effects on the diamond surface has not been available until now. Here, low-temperature scanning tunneling microscopy is utilized to investigate the atomic-scale morphology and electronic structures of (100)- and (111)-oriented boron-doped diamond (BDD) electrodes. Graphitized domains of a few nanometers are shown to manifest the effects of boron dopants on the BDD surface. Confirmed by first-principles calculations, local density of states measurements reveal that the electronic structure of these features is characterized by in-gap states induced by boron-related lattice deformation. The dopant-related graphitization is uniquely observed in BDD (111), which explains its electrochemical superiority over the (100) facet. These experimental observations provide atomic-scale information about the role of dopants in modulating the conductivity of diamond, as well as, possibly, other functional doped materials.

CVD graphene/Ge interface: morphological and electronic characterization of ripples

Graphene grown directly on germanium is a possible route for the integration of graphene into nanoelectronic devices as well as it is of great interest for materials science. The morphology of the interface between graphene and germanium influences the electronic properties and has not already been completely elucidated at atomic scale. In this work, we investigated the morphology of the single-layer graphene grown on Ge substrates with different crystallographic orientations. We determined the presence of sinusoidal ripples with a single propagation direction, zig-zag, and could arise due to compressive biaxial strain at the interface generated as a result of the opposite polarity of the thermal expansion coefficient of graphene and germanium. Local density of states measurements on the ripples showed a linear dispersion relation with the Dirac point slightly shifted with respect to the Fermi energy indicating that these out-of-plane deformations were n-doped, while the graphene regions between the highs were undoped.

Probing chiral edge states in topological superconductors through spin-polarized local density of state measurements

We show that spin-polarized local density of states (LDOS) measurements can uniquely determine the chiral nature of topologically protected edge states surrounding a ferromagnetic island embedded in a conventional superconductor with spin-orbit coupling. The spin-polarized LDOS show a strong spin-polarization directly tied to the normal direction of the edge, with opposite polarizations on opposite sides of the island, and with a distinct oscillatory pattern in energy.

Scanning tunneling microscopy of superconducting topological surface states inBi2Se3

In this paper we present scanning tunneling microscopy of a large $\textrm{Bi}_2\textrm{Se}_3$ crystal with superconducting PbBi islands deposited on the surface. Local density of states measurements are consistent with induced superconductivity in the topological surface state with a coherence length of order 540 nm. At energies above the gap the density of states exhibits oscillations due to scattering caused by a nonuniform order parameter. Strikingly, the spectra taken on islands also display similar oscillations along with traces of the Dirac cone, suggesting an inverse topological proximity effect.

Soft superconducting gap in semiconductor-based Majorana nanowires

We develop a theory for the proximity effect in superconductor-semiconductor-normal metal tunneling structures, which have recently been extensively studied experimentally, leading to the observation of transport signatures consistent with the predicted zero-energy Majorana bound states. We show that our model for the semiconductor nanowire having multiple occupied subbands with different transmission probabilities through the barrier reproduces the observed "soft-gap" behavior associated with substantial subgap tunneling conductance. We study the manifestations of the soft gap phenomenon both in the tunneling conductance and in local density of states measurements and discuss the correlations between these two quantities. We emphasize that the proximity effect associated with the hybridization between low-lying states in the multiband semiconductor and the normal metal states in the lead is an intrinsic effect leading to the soft gap problem. In addition to the intrinsic contribution, there may be extrinsic effects, such as, for example, interface disorder, exacerbating the soft gap problem. Our work establishes the generic possibility of an ubiquitous presence of an intrinsic soft gap in the superconductor-semiconductor-normal metal tunneling transport conductance induced by the inverse proximity effect of the normal metal.

Local density of states study of a spin-orbit-coupling induced Mott insulatorSr2IrO4

We present scanning tunneling microscopy and spectroscopy experiments on the novel J_eff = 1/2 Mott insulator Sr2IrO4. Local density of states (LDOS) measurements show an intrinsic insulating gap of 620 meV that is asymmetric about the Fermi level and is larger than previously reported values. The size of this gap suggests that Sr2IrO4 is likely a Mott rather than Slater insulator. In addition, we found a small number of native defects which create in-gap spectral weight. Atomically resolved LDOS measurements on and off the defects shows that this energy gap is quite fragile. Together the extended nature of the 5d electrons and poor screening of defects help explain the elusive nature of this gap.

Absence of boron aggregates in superconducting silicon confirmed by atom probe tomography

Superconducting boron-doped silicon films prepared by gas immersion laser doping (GILD) technique are analyzed by atom probe tomography. The resulting three-dimensional chemical composition reveals that boron atoms are incorporated into crystalline silicon in the atomic percent concentration range, well above their solubility limit, without creating clusters or precipitates at the atomic scale. The boron spatial distribution is found to be compatible with local density of states measurements performed by scanning tunneling spectroscopy. These results combined with the observations of very low impurity level and of a sharp two-dimensional interface between doped and undoped regions show that the Si:B material obtained by GILD is a well-defined random substitutional alloy endowed with promising superconducting properties.

Observation of two-gap superconductivity in SrFe1.85Co0.15As2 single crystals by scanning tunneling microscopy and spectroscopy

Superconducting properties of SrFe1.85Co0.15As2 single crystals and their parent material, SrFe2As2, were investigated by scanning tunneling microscopy and spectroscopy (STM/S). In the parent material, we modeled surface conditions on the in situ cleaved single crystals, based on the observation of 2×1 stripe patterns and square-lattice patterns in the atomic-resolution topography images and with the help of local density of states measurements. In the STM/S studies on SrFe1.85Co0.15As2, a robust superconducting gap (2Δlarge=17.3 meV) was observed in the conductance spectra measured along a line on the SrFe1.85Co0.15As2 surface. Moreover, an additional small gap-like (2Δsmall=2.9 meV) structure was simultaneously observed. Our observation corroborates the two-gap structures in iron-based superconductors.

Fluxonic cellular automata

We propose a quantum cellular automata composed of nanostructured mesoscopic superconducting squares, where the logic states are defined by two trapped vortices in a 2×2 blind hole matrix. We present the functioning logic gate based on this fluxonic cellular automata, where the logic operations are verified through theoretical simulations within the Ginzburg-Landau formalism. The input signals are defined using the vortex interaction with current loops placed on top of the two diagonal blind holes of the input cell. The readout technology may be chosen from a large variety of modern vortex imaging methods, transport, and local density of states measurements.

Failure of scattering interference in the pseudogap state of cuprate superconductors

We calculate scattering interference patterns for various electronic states proposed for the pseudogap regime of the cuprate superconductors. The scattering interference models all produce patterns whose wavelength changes as a function of energy, in contradiction to the energy-independent wavelength seen by scanning tunneling microscopy (STM) experiments in the pseudogap state. This suggests that the patterns seen in STM local density of states measurements are not due to scattering interference, but are rather the result of some form of ordering.

Vortex core shapes measured by STM

We have studied vortex core shapes in superconducting NbSe2 by STM, as function of temperature and bias voltage. The experimentally measured tunnel current profiles are compared with the results of calculations using microscopic theory. We find that, at low temperatures (T/T c ≃ 0.25), the apparent vortex core radius strongly depends on the bias voltage, which demonstrates the energy dependence of the scale for spatial variation of the quasiparticle density of states. Good quantitative agreement between measured and calculated profiles is found by using the accepted value for the superconducting coherence length ΞS, without further adjustable parameters. This shows that the bias dependence is a useful extra tool in the interpretation of local density of states measurements.