Proximity-Induced Superconductivity in Monolayer MoS2.

Proximity effects in superconducting normal (SN) material heterostructures with metals and semiconductors have long been observed and theoretically described in terms of Cooper pair wave functions and Andreev reflections. Whereas the semiconducting N-layer materials in the proximity experiments to date have been doped and tens of nanometers thick, we present here a proximity tunneling study involving a pristine single-layer transition-metal dichalcogenide film of MoS2 placed on top of a Pb thin film. Scanning tunneling microscopy and spectroscopy experiments together with parallel theoretical analysis based on electronic structure calculations and Green's function modeling allow us to unveil a two-step process in which MoS2 first becomes metallic and then is induced into becoming a conventional s-wave Bardeen-Cooper-Schrieffer-type superconductor. The lattice mismatch between the MoS2 overlayer and the Pb substrate is found to give rise to a topographic moiré pattern. Even though the induced gap appears uniform in location, the coherence peak height of the tunneling spectra is modulated spatially into a moiré pattern that is similar to but shifted with respect to the moiré pattern observed in topography. The aforementioned modulation is shown to originate from the atomic-scale structure of the SN interface and the nature of local atomic orbitals that are involved in generating the local pairing potential. Our study indicates that the local modulation of induced superconductivity in MoS2 could be controlled via geometrical tuning, and it thus shows promise toward the integration of monolayer superconductors into next-generation functional electronic devices by exploiting proximity-effect control of quantum phases.

Spectroscopic signatures of different symmetries of the superconducting order parameter in metal-decorated graphene.

Motivated by the recent experiments indicating superconductivity in metal-decorated graphene sheets, we investigate their quasi-particle structure within the framework of an effective tight-binding Hamiltonian augmented by appropriate BCS-like pairing terms for p-type order parameter. The normal state band structure of graphene is modified not only through interaction with adsorbed metal atoms, but also due to the folding of bands at Brillouin zone boundaries resulting from a [Formula: see text] reconstruction. Several different types of pairing symmetries are analyzed utilizing Nambu-Gorkov Green's function techniques to show that [Formula: see text]-symmetric nearest-neighbor pairing yields the most enhanced superconducting gap. The character of the order parameter depends on the nature of the atomic orbitals involved in the pairing process and exhibits interesting angular and radial asymmetries. Finally, we suggest a method to distinguish between singlet and triplet type superconductivity in the presence of magnetic substitutional impurities using scanning tunneling spectroscopy.

Inter-Layer Coupling Induced Valence Band Edge Shift in Mono- to Few-Layer MoS2.

Recent progress in the synthesis of monolayer MoS2, a two-dimensional direct band-gap semiconductor, is paving new pathways toward atomically thin electronics. Despite the large amount of literature, fundamental gaps remain in understanding electronic properties at the nanoscale. Here, we report a study of highly crystalline islands of MoS2 grown via a refined chemical vapor deposition synthesis technique. Using high resolution scanning tunneling microscopy and spectroscopy (STM/STS), photoemission electron microscopy/spectroscopy (PEEM) and µ-ARPES we investigate the electronic properties of MoS2 as a function of the number of layers at the nanoscale and show in-depth how the band gap is affected by a shift of the valence band edge as a function of the layer number. Green's function based electronic structure calculations were carried out in order to shed light on the mechanism underlying the observed bandgap reduction with increasing thickness, and the role of the interfacial Sulphur atoms is clarified. Our study, which gives new insight into the variation of electronic properties of MoS2 films with thickness bears directly on junction properties of MoS2, and thus impacts electronics application of MoS2.

Coexisting Honeycomb and Kagome Characteristics in the Electronic Band Structure of Molecular Graphene.

We uncover the electronic structure of molecular graphene produced by adsorbed CO molecules on a copper (111) surface by means of first-principles calculations. Our results show that the band structure is fundamentally different from that of conventional graphene, and the unique features of the electronic states arise from coexisting honeycomb and Kagome symmetries. Furthermore, the Dirac cone does not appear at the K-point but at the Γ-point in the reciprocal space and is accompanied by a third, almost flat band. Calculations of the surface structure with Kekulé distortion show a gap opening at the Dirac point in agreement with experiments. Simple tight-binding models are used to support the first-principles results and to explain the physical characteristics behind the electronic band structures.

Nanoscale interplay of strain and doping in a high-temperature superconductor.

The highest-temperature superconductors are electronically inhomogeneous at the nanoscale, suggesting the existence of a local variable that could be harnessed to enhance the superconducting pairing. Here we report the relationship between local doping and local strain in the cuprate superconductor Bi(2)Sr(2)CaCu(2)O(8+x). We use scanning tunneling microscopy to discover that the crucial oxygen dopants are periodically distributed in correlation with local strain. Our picoscale investigation of the intraunit-cell positions of all oxygen dopants provides essential structural input for a complete microscopic theory.

Tissue welding tonsillectomy provides an enhanced recovery compared to that after monopolar electrocautery technique in adults: a prospective randomized clinical trial.

We have compared tonsillectomy (TE) with tissue welding (TW) technology using a specially designed forceps versus conventional monopolar electrocautery to evaluate whether this new technology may improve recovery after TE. This was a single-blind, randomized clinical trial with two parallel groups. Sixty healthy adult day-surgery patients were allocated into the TW-TE group (n = 31) and the monopolar electrocautery-TE group (n = 29). We recorded intraoperative events and short- and long-term recovery for 2 weeks postoperatively. The patients and study nurses evaluating patients during recovery were blinded to the operation method used. All patients in the TW-TE group completed the study as per protocol, but in the monopolar electrocautery-TE group, there was one drop-out in the hospital and another after discharge. There was no difference in the perioperative parameters and early recovery between the two groups. After discharge, recovery was significantly faster in the TW group than in the monopolar group: (1) the duration of postoperative pain was 2 days shorter, and (2) activities of normal daily living were less affected, and (3) the need for hospital contacts after discharge, and (4) the incidence of postoperative bleeding was less in the TW group than that in the monopolar group. No patients in the TW group developed secondary bleeding versus three patients in the monopolar group requiring electrocautery to control bleeding. In conclusion; our results indicate that, TW technique may provide reduced pain, faster recovery, and fewer complications compared to electrocautery TE.

Origin of the electron-hole asymmetry in the scanning tunneling spectrum of the high-temperature Bi2Sr2CaCu2O8+delta superconductor.

We have developed a material specific theoretical framework for modeling scanning tunneling spectroscopy (STS) of high-temperature superconducting materials in the normal as well as the superconducting state. Results for Bi2Sr2CaCu2O8+delta (Bi2212) show clearly that the tunneling process strongly modifies the STS spectrum from the local density of states of the dx2-y2 orbital of Cu. The dominant tunneling channel to the surface Bi involves the dx2-y2 orbitals of the four neighboring Cu atoms. In accord with experimental observations, the computed spectrum displays a remarkable asymmetry between the processes of electron injection and extraction, which arises from contributions of Cu dz2 and other orbitals to the tunneling current.

Submolecular imaging of chloronitrobenzene isomers on Cu(111).

We compare computer simulations to experimental scanning tunneling microscopy (STM) images of chloronitrobenzene molecules on a Cu(111) surface. The experiments show that adsorption induced isomerization of the molecules takes place on the surface. Furthermore, not only the submolecular features can be seen in the STM images, but different isomers can also be recognized. The Todorov-Pendry approach to tunneling produces simulated STM images which are in good accordance with the experiments. Alongside with STM simulations in a tight-binding basis, ab initio calculations are performed in order to analyze the symmetry of relevant molecular orbitals and to consider the nature of tunneling channels. Our calculations show that while the orbitals delocalized to the phenyl ring create a relatively transparent tunneling channel, they also almost isolate the orbitals of the substitute groups at energies which are relevant in STM experiments. These features of the electronic structure are the key ingredients of the accurate submolecular observations.

Imaging water on Ag(111): field induced reorientation and contrast inversion.

Water adsorbed on Ag(111) at 70 K forms circular clusters that consist of six molecules. In scanning tunneling microscopy, this cyclic hexamer is imaged as a protrusion for voltages below V(SS)=-93 meV and as a depression for voltages above V(SS). The electronic density of states, however, increases around V(SS). We explain this counterintuitive result with the aid of calculated images by a change from constructive to destructive interference between different tunneling channels due to a field induced reorientation of the molecule under the tunneling tip.

Intermolecular bond length of ice on Ag(111).

Water adsorbed in submonolayer coverage on Ag(111) at 70 K forms hydrogen-bonded networks. High resolution images in combination with calculation reveal that single protrusions represent a cyclic water hexamer with the intermolecular bond stretched to the silver lattice constant of 0.29 nm. Scanning tunneling spectroscopy indicates that the bond length within the two-dimensional hydrogen-bonded water layer is shortened. The spectra contain further information about the vibrational modes of water molecules.