Yeping Jiang

Direct observation of nodes and twofold symmetry in FeSe superconductor.

Scanning tunneling spectroscopy suggests an orbital ordering mechanism for electron pairing in an iron-based superconductor. We investigated the electron-pairing mechanism in an iron-based superconductor, iron selenide (FeSe), using scanning tunneling microscopy and spectroscopy. Tunneling conductance spectra of stoichiometric FeSe crystalline films in their superconducting state revealed evidence for a gap function with nodal lines. Electron pairing with twofold symmetry was demonstrated by direct imaging of quasiparticle excitations in the vicinity of magnetic vortex cores, Fe adatoms, and Se vacancies. The twofold pairing symmetry was further supported by the observation of striped electronic nanostructures in the slightly Se-doped samples. The anisotropy can be explained in terms of the orbital-dependent reconstruction of electronic structure in FeSe.

Topological insulator Bi2Se3 thin films grown on double-layer graphene by molecular beam epitaxy

Atomically flat thin films of topological insulator Bi2Se3 have been grown on double-layer graphene formed on 6H–SiC(0001) substrate by molecular beam epitaxy. By a combined study of reflection high energy electron diffraction and scanning tunneling microscopy, we identified the Se-rich condition and temperature criterion for layer-by-layer growth of epitaxial Bi2Se3 films. The as-grown films without doping exhibit a low defect density of 1.0±0.2×1011/cm2, and become a bulk insulator at a thickness of ten quintuple layers, as revealed by in situ angle resolved photoemission spectroscopy measurement.Atomically flat thin films of topological insulator Bi{sub 2}Se{sub 3} have been grown on double-layer graphene formed on 6H-SiC(0001) substrate by molecular beam epitaxy. By a combined study of reflection high energy electron diffraction and scanning tunneling microscopy, we identified the Se-rich condition and temperature criterion for layer-by-layer growth of epitaxial Bi{sub 2}Se{sub 3} films. The as-grown films without doping exhibit a low defect density of 1.0{+-}0.2x10{sup 11}/cm{sup 2}, and become a bulk insulator at a thickness of ten quintuple layers, as revealed by in situ angle resolved photoemission spectroscopy measurement.

Experimental investigation of the heat and mass transfer between air and liquid desiccant in a cross-flow regenerator

The regenerator is one of the essential components in a liquid desiccant air-conditioning system, whose efficiency directly influences the system performance. A performance test-bed for a cross-flow regenerator was established in the present analysis. Celdek structured packings were used in the regenerator and LiBr aqueous solution was used as the liquid desiccant. Moisture removal rate and regenerator effectiveness are adopted to describe the mass transfer performance of the regenerator. Effects of air and desiccant inlet parameters on the regenerator performance are experimentally investigated, and performance comparisons between present cross-flow regenerator and other counter-flow configurations available in the literature are also carried out. The comparison results show that the impacts of air and desiccant inlet parameters show similar tendency with those of counter-flow regenerators. A dimensionless mass transfer correlation is proposed, which gives results in good agreement with the experimental findings.

Landau quantization and the thickness limit of topological insulator thin films of Sb2Te3.

We report the experimental observation of Landau quantization of molecular beam epitaxy grown Sb{2}Te{3} thin films by a low-temperature scanning tunneling microscope. Different from all the reported systems, the Landau quantization in a Sb{2}Te{3} topological insulator is not sensitive to the intrinsic substitutional defects in the films. As a result, a nearly perfect linear energy dispersion of surface states as a 2D massless Dirac fermion system is achieved. We demonstrate that four quintuple layers are the thickness limit for a Sb{2}Te{3} thin film being a 3D topological insulator. The mechanism of the Landau-level broadening is discussed in terms of enhanced quasiparticle lifetime.

Fermi-level tuning of epitaxial Sb2Te3 thin films on graphene by regulating intrinsic defects and substrate transfer doping.

High-quality Sb2Te3 films are obtained by molecular beam epitaxy on a graphene substrate and investigated by in situ scanning tunneling microscopy and spectroscopy. Intrinsic defects responsible for the natural p-type conductivity of Sb2Te3 are identified to be the Sb vacancies and Sb(Te) antisites in agreement with first-principles calculations. By minimizing defect densities, coupled with a transfer doping by the graphene substrate, the Fermi level of Sb2Te3 thin films can be tuned over the entire range of the bulk band gap. This establishes the necessary condition to explore topological insulator behaviors near the Dirac point.

Suppression of Superconductivity by Twin Boundaries In FeSe

Low-temperature scanning tunneling microscopy and spectroscopy are employed to investigate twin boundaries in stoichiometric FeSe films grown by molecular beam epitaxy. Twin boundaries can be unambiguously identified by imaging the 90° change in the orientation of local electronic dimers from Fe site impurities on either side. Twin boundaries run at approximately 45° to the Fe-Fe bond directions, and noticeably suppress the superconducting gap, in contrast with the recent experimental and theoretical findings in other iron pnictides. Furthermore, vortices appear to accumulate on twin boundaries, consistent with the degraded superconductivity there. The variation in superconductivity is likely caused by the increased Se height in the vicinity of twin boundaries, providing the first local evidence for the importance of this height to the mechanism of superconductivity.

Direct observation and measurement of mobile charge carriers in a monolayer organic semiconductor on a dielectric substrate.

We report the electrical characterization of a single layer of an organic semiconductor grown on a dielectric surface. The dynamic response of the charge carriers in the monolayer film of pentacene was characterized through the electrostatic interactions between an electric force microscope (EFM) probe and pentacene islands of various sizes. These islands were formed in situ by segmenting a coalesced pentacene monolayer into separated regions. The size-dependent dielectric responses of the pentacene islands suggest that mobile charges exist in the organic monolayer. Local capacitance spectroscopy revealed that the charge carriers in the p-type pentacene monolayer could be depleted at high bias voltages, enabling a further determination of the charge-carrier concentration in the organic semiconductor ultrathin film.

Thermal conductivity measurement of individual CdS nanowires using microphotoluminescence spectroscopy

We present a scheme to measure the thermal conductivity of individual nanowires using temperature-dependent and microphotoluminescence spectroscopy. The CdS nanowires suspended over trenches in silicon substrate were excited with a focused laser beam in confocal configuration. The photoluminescence at the laser spot gave a measure of the local temperature rise determined by the excitation laser intensity and the thermal conductance of the nanowires. By obtaining the temperature gradient across the nanowire and the length of the suspended segment, the thermal conductivity of CdS nanowires with diameters in the range of 200–400 nm was extracted to be in the range of 4.9–6.2 W/m K.

Exchange-narrowing spin-spin interaction in the paramagnetic regime of perovskite manganites studied through the EPR measurements for (La, Y)2/3(Ca, Sr, Ba)1/3MnO3 with a wide span of Tc

The electron paramagnetic resonance (EPR) spectrum as a function of T is experimentally studied for (La,Y)2/3 (Ca,Sr,Ba)1/3 MnO3 with a wide span of TC . In the real paramagnetic regime (T Tmin ), the spectrum consists of a single line with the resonance field being independent of T . The resonance linewidth is given by Hpp = H pp ,min +b (T -Tmin ). The exchange-narrowing spin-spin interaction and the spin-lattice interaction are explained to be responsible for the terms of H pp ,min and b (T -Tmin ), respectively. Based on the exchange-narrowing mechanism, the values of H pp ,min are directly deduced from the TC -values, which are well consistent with the experimental H pp ,min -values. This confirms the possibility that the exchange-narrowing spin-spin interaction exists in the ABO3 -type perovskite manganites.

Scanning tunneling microscopy of interface properties of Bi2Se3 on FeSe.

We investigate the heteroepitaxial growth of Bi(2)Se(3) films on FeSe substrates by low-temperature scanning tunneling microscopy/spectroscopy. The growth of Bi(2)Se(3) on FeSe proceeds via van der Waals epitaxy with atomically flat morphology. A striped moiré pattern originating from the lattice mismatch between Bi(2)Se(3) and FeSe is observed. Tunneling spectra reveal the spatially inhomogeneous electronic structure of the Bi(2)Se(3) thin films, which can be ascribed to the charge transfer at the interface.