Anjan K. Gupta

Microscopic electronic inhomogeneity in the high-Tc superconductor Bi2Sr2CaCu2O8+x.

The parent compounds of the copper oxide high-transition-temperature (high-Tc) superconductors are unusual insulators (so-called Mott insulators). Superconductivity arises when they are ‘doped’ away from stoichiometry. For the compound Bi2Sr2CaCu2O8+x, doping is achieved by adding extra oxygen atoms, which introduce positive charge carriers (‘holes’) into the CuO2 planes where the superconductivity is believed to originate. Aside from providing the charge carriers, the role of the oxygen dopants is not well understood, nor is it clear how the charge carriers are distributed on the planes. Many models of high-Tc superconductivity accordingly assume that the introduced carriers are distributed uniformly, leading to an electronically homogeneous system as in ordinary metals. Here we report the presence of an electronic inhomogeneity in Bi2Sr2CaCu2O8+x, on the basis of observations using scanning tunnelling microscopy and spectroscopy. The inhomogeneity is manifested as spatial variations in both the local density of states spectrum and the superconducting energy gap. These variations are correlated spatially and vary on the surprisingly short length scale of ∼14 Å. Our analysis suggests that this inhomogeneity is a consequence of proximity to a Mott insulator resulting in poor screening of the charge potentials associated with the oxygen ions left in the BiO plane after doping, and is indicative of the local nature of the superconducting state.

Compact coarse approach mechanism for scanning tunneling microscope

We report on the design and fabrication of a coarse approach mechanism, called a piezotube walker, based on a piezoelectric tube moving inside a triangular prism shaped cavity. This walker walks like a six legged insect moving its legs one by one and then the belly following. The walker works in any orientation from horizontal to vertical and its motion is found to be linear with the applied voltage above a threshold voltage. A compact scanning tunneling microscope (STM) was fabricated using this approach mechanism. The scanner tube of the STM is mounted on the inside of the walker tube, reducing the size of the STM considerably. Topographical images with atomic resolution were obtained for layered materials like graphite and NbSe2.

Scanning tunneling spectroscopy of the superconducting proximity effect in a diluted ferromagnetic alloy

We studied the proximity effect between a superconductor (Nb) and a diluted ferromagnetic alloy (CuNi) in a bilayer geometry. We measured the local density of states on top of the ferromagnetic layer, which thickness varies on each sample, with a very low temperature Scanning Tunneling Microscope. The measured spectra display a very high homogeneity. The analysis of the experimental data shows the need to take into account an additional scattering mechanism. By including in the Usadel equations the effect of the spin relaxation in the ferromagnetic alloy, we obtain a good description of the experimental data.

Evidence for topological surface states in metallic single crystals of Bi2Te3

Bi2Te3 is a member of a new class of materials known as topological insulators which are supposed to be insulating in the interior and conducting on the surface. However, experimental verification of the conductive qualities of the surface states has been hindered by parallel bulk conductions. We report low temperature magnetotransport measurements on single crystal samples of Bi2Te3. We observe metallic character in our samples and large and linear magnetoresistance from 1.5 K to 290 K with prominent Shubnikov-de Haas (SdH) oscillations whose traces persist up to 20 K. Even though our samples are metallic, we are able to obtain a Berry phase close to the value of π, which is expected for Dirac fermions of the topological surface states. This indicates that we have obtained evidence for the topological surface states in metallic single crystals of Bi2Te3. Other physical measurements obtained from the analysis of the SdH oscillations are also in close agreement with those reported for the topological surface states. The linear magnetoresistance observed in our sample, which is considered as a signature of the Dirac fermions of the surface states, lends further credence to the existence of topological surface states.

Thickness dependent lattice expansion in nanogranular Nb thin films

We report on the dependence of the lattice parameter on thickness (t) and grain size (D) in nanogranular Nb thin films deposited on Si (100) substrates by using dc magnetron sputtering. The lattice parameter is found to exponentially increase with decreasing thickness as well as grain size. We analyze the lattice expansion in terms of the excess free volume in the grain boundaries, which is found to decrease with the grain size in large thickness limit. The thickness dependence of the lattice expansion and the grain size dependence of the excess free volume are discussed in terms of the oxygen content in the grain boundaries.

Compact two-dimensional coarse-positioner for scanning probe microscopes.

We report on the design and fabrication of a compact two-dimensional xy-positioner for scanning probe microscopes. This positioner uses three piezoelectric bimorphs in flexing or length-change mode by appropriate selection of electrodes and voltage polarities. One end of these bimorphs is fixed to a rectangular metal frame while on each of the free ends two sapphire disks are fixed which can slide against the polished plates of a platform movable in the xy-plane. For moving the platform by one step, the bimorphs are deformed sequentially in one mode and they are brought back to their undeformed state simultaneously. The motion of the positioner has been tested with an optical microscope and a homemade scanning tunneling microscope.

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.

Reversibility Of Superconducting Nb Weak Links Driven By The Proximity Effect In A Quantum Interference Device

We demonstrate the role of the proximity effect in the thermal hysteresis of superconducting constrictions. From the analysis of successive thermal instabilities in the transport characteristics of micron-size superconducting quantum interference devices with a well-controlled geometry, we obtain a complete picture of the different thermal regimes. These determine whether or not the junctions are hysteretic. Below the superconductor critical temperature, the critical current switches from a classical weak-link behavior to one driven by the proximity effect. The associated small amplitude of the critical current makes it robust with respect to the heat generation by phase slips, leading to a nonhysteretic behavior.

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

Controlling hysteresis in superconducting constrictions with a resistive shunt

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