Udai Raj Singh

Construction and performance of a dilution-refrigerator based spectroscopic-imaging scanning tunneling microscope

We report on the set-up and performance of a dilution-refrigerator based spectroscopic imaging scanning tunneling microscope. It operates at temperatures below 10 mK and in magnetic fields up to 14T. The system allows for sample transfer and in situ cleavage. We present first-results demonstrating atomic resolution and the multi-gap structure of the superconducting gap of NbSe(2) at base temperature. To determine the energy resolution of our system we have measured a normal metal/vacuum/superconductor tunneling junction consisting of an aluminum tip on a gold sample. Our system allows for continuous measurements at base temperature on time scales of up to ≈170 h.

Spatial inhomogeneity of the superconducting gap and order parameter in Se0.4Te0.6

U.R.S. acknowledges support by the Alexander-von-Humboldt Foundation; furthermore, we acknowledge support by SPP1458 of the DFG through Projects No. WA2510/1-1 and No. DE1762/1-1.

A stiff scanning tunneling microscopy head for measurement at low temperatures and in high magnetic fields.

We have developed a measurement head for scanning tunneling microscopy (STM) and specifically for spectroscopic imaging STM which is optimized for high mechanical stiffness and good thermal conductivity by choice of material. The main components of the microscope head are made of sapphire. Sapphire has been chosen from several competing possibilities based on finite element modeling of the fundamental vibrational modes of the body. We demonstrate operation of the STM head in topographic imaging and tunneling spectroscopy at temperatures down to below 2 K.

Reorientation of the diagonal double-stripe spin structure at Fe1+yTe bulk and thin-film surfaces

Establishing the relation between ubiquitous antiferromagnetism in the parent compounds of unconventional superconductors and their superconducting phase is important for understanding the complex physics in these materials. Going from bulk systems to thin films additionally affects their phase diagram. For Fe1+yTe, the parent compound of Fe1+ySe1−xTex superconductors, bulk-sensitive neutron diffraction revealed an in-plane oriented diagonal double-stripe antiferromagnetic spin structure. Here we show by spin-resolved scanning tunnelling microscopy that the spin direction at the surfaces of bulk Fe1+yTe and thin films grown on the topological insulator Bi2Te3 is canted out of the high-symmetry directions of the surface unit cell resulting in a perpendicular spin component, keeping the diagonal double-stripe order. As the magnetism of the Fe d-orbitals is intertwined with the superconducting pairing in Fe-based materials, our results imply that the superconducting properties at the surface of the related superconducting compounds might be different from the bulk.

Pseudogap formation in the metallic state of La0.7Sr0.3MnO3 thin films

We report on scanning tunneling microscopy and spectroscopy (STM/S) studies of epitaxial La0.7Sr0.3MnO3 (LSMO) thin films on a lattice matched (001) La0.3Sr0.7Al0.35Ta0.35O9 substrate for both as-grown and annealed films. In contrast to the as-grown films, the films annealed at 800 °C in air show atomic terraces with spectra that develop a gaplike structure with cooling. We show that the gap structure can be attributed to the predicted pseudogap in the manganites. Unlike several previous reports, we did not find electronic inhomogeneities in LSMO by STM/S.

Tunneling study of the charge-ordering gap on the surface of La 0.350 Pr 0.275 Ca 0.375 Mn O 3 thin films

Variable temperature scanning tunneling microscopy/spectroscopy studies on (110) oriented epitaxial thin films of La

Imaging the real space structure of the spin fluctuations in an iron-based superconductor

_{0.350}

Evidence for orbital order and its relation to superconductivity in FeSe0.4Te0.6

Pr

Preparation of magnetic tips for spin-polarized STM on Fe1+yTe

_{0.275}

Towards a quantitative description of tunneling conductance of superconductors : Application to LiFeAs

Ca