Single grain (LRE)-Ba-Cu-O superconductors fabricated by top seeded melt growth in air

We have recently reported a practical processing method for the fabrication in air of large, single grain (LRE)-Ba-Cu-O [where LRE Nd, Sm, Eu and Gd] bulk superconductors that exhibit high Tc and high Jc. The process is based initially on the development of a new type of generic seed crystal that can promote effectively the epitaxial nucleation of any (RE)-Ba-Cu-O system and, secondly, by suppressing the formation of (LRE)/Ba solid solution in a controlled manner within large LRE-Ba-Cu-O grains processed in air. In this paper we investigate the degree of homogeneity of large grain Sm-Ba-Cu-O superconductors fabricated by this novel process. The technique offers a significant degree of freedom in terms of processing parameters and reproducibility in the growth of oriented single grains in air and yields bulk samples with significantly improved superconducting and field-trapping properties compared to those processed by conventional top seeded melt growth (TSMG)

Temperature dependence of the impurity-induced resonant state in Zn-doped Bi_2Sr_2CaCu_2O8+δ_{8+\delta} by Scanning Tunneling Spectroscopy

We report on the temperature dependence of the impurity-induced resonant state in Zn-doped Bi_2Sr_2CaCu_2O$_{8+\delta}$ by scanning tunneling spectroscopy at 30 mK < T < 52 K. It is known that a Zn impurity induces a sharp resonant peak in tunnel spectrum at an energy close to the Fermi level. We observed that the resonant peak survives up to 52 K. The peak broadens with increasing temperature, which is explained by the thermal effect. This result provides information to understand the origin of the resonant peak.Comment: 4 pages, 3 figures, to appear in Phys. Rev.

STS Observations of Landau Levels at Graphite Surfaces

Scanning tunneling spectroscopy measurements were made on surfaces of two different kinds of graphite samples, Kish graphite and highly oriented pyrolytic graphite (HOPG), at very low temperatures and in high magnetic fields. We observed a series of peaks in the tunnel spectra, which grow with increasing field, both at positive and negative bias voltages. These are associated with Landau quantization of the quasi two-dimensional electrons and holes in graphite in magnetic fields perpendicular to the basal plane. Almost field independent Landau levels fixed near the Fermi energy, which are characteristic of the graphite crystalline structure, were directly observed for the first time. Calculations of the local density of states at the graphite surfaces allow us to identify Kish graphite as bulk graphite and HOPG as graphite with finite thickness effectively

Construction of a Versatile Ultra-Low Temperature Scanning Tunneling Microscope

We constructed a dilution-refrigerator (DR) based ultra-low temperature scanning tunneling microscope (ULT-STM) which works at temperatures down to 30 mK, in magnetic fields up to 6 T and in ultrahigh vacuum (UHV). Besides these extreme operation conditions, this STM has several unique features not available in other DR based ULT-STMs. One can load STM tips as well as samples with clean surfaces prepared in a UHV environment to an STM head keeping low temperature and UHV conditions. After then, the system can be cooled back to near the base temperature within 3 hours. Due to these capabilities, it has a variety of applications not only for cleavable materials but also for almost all conducting materials. The present ULT-STM has also an exceptionally high stability in the presence of magnetic field and even during field sweep. We describe details of its design, performance and applications for low temperature physics.Comment: 6 pages, 9 figures. accepted for publication in Rev. Sci. Instru

Brownian molecular motors driven by rotation-translation coupling

We investigated three models of Brownian motors which convert rotational diffusion into directed translational motion by switching on and off a potential. In the first model a spatially asymmetric potential generates directed translational motion by rectifying rotational diffusion. It behaves much like a conventional flashing ratchet. The second model utilizes both rotational diffusion and drift to generate translational motion without spatial asymmetry in the potential. This second model can be driven by a combination of a Brownian motor mechanism (diffusion driven) or by powerstroke (drift driven) depending on the chosen parameters. In the third model, elements of both the Brownian motor and powerstroke mechanisms are combined by switching between three distinct states. Relevance of the model to biological motor proteins is discussed.Comment: 11 pages, 8 figure

Scanning tunneling microscopy and spectroscopy of the electronic local density of states of graphite surfaces near monoatomic step edges

We measured the electronic local density of states (LDOS) of graphite surfaces near monoatomic step edges, which consist of either the zigzag or armchair edge, with the scanning tunneling microscopy (STM) and spectroscopy (STS) techniques. The STM data reveal that the $(\sqrt{3} \times \sqrt{3}) R 30^{\circ}$ and honeycomb superstructures coexist over a length scale of 3-4 nm from both the edges. By comparing with density-functional derived nonorthogonal tight-binding calculations, we show that the coexistence is due to a slight admixing of the two types of edges at the graphite surfaces. In the STS measurements, a clear peak in the LDOS at negative bias voltages from -100 to -20 mV was observed near the zigzag edges, while such a peak was not observed near the armchair edges. We concluded that this peak corresponds to the graphite "edge state" theoretically predicted by Fujita \textit{et al.} [J. Phys. Soc. Jpn. {\bf 65}, 1920 (1996)] with a tight-binding model for graphene ribbons. The existence of the edge state only at the zigzag type edge was also confirmed by our first-principles calculations with different edge terminations.Comment: 20 pages, 11 figure

Superconducting magnesium diboride films with Tc \approx 24K grown by pulsed laser deposition with in-situ anneal

Thin superconducting films of magnesium diboride (MgB2) with Tc \approx 24K were prepared on various oxide substrates by pulsed laser deposition (PLD) followed by an in-situ anneal. A systematic study of the influence of various in-situ annealing parameters shows an optimum temperature of about 600C in a background of 0.7 atm. of Ar/4%H2 for layers consisting of a mixture of magnesium and boron. Contrary to ex-situ approaches (e.g. reacting boron films with magnesium vapor at 900C), these films are processed below the decomposition temperature of MgB2. This may prove enabling in the formation of multilayers, junctions, and epitaxial films in future work. Issues related to the improvement of these films and to the possible in-situ growth of MgB2 at elevated temperature are discussed.Comment: 5 pages, 4 figure

High transport critical current density obtained for Powder-In-Tube-processed MgB2 tapes and wires using stainless steel and Cu-Ni tubes

MgB2 tapes and wires were fabricated by the Powder-In-Tube method. Stainless steel and Cu-Ni tubes were used as sheath materials, and no heat treatment was applied. The tapes made of stainless steel showed transport critical current density Jc of about 10,000A/cm2 at 4.2K and 5T. A high Jc of about 300,000A/cm2 was obtained by extrapolating the Jc-B curves to zero field. Multifilamentary(7-core) MgB2 wire was successfully fabricated using Cu-Ni tubes. For both tapes and wires the grain connectivity of MgB2 was as good as a high-pressure sintered bulk sample. However, the Jc of the Cu-Ni sheathed wire was lower than the stainless steel sheathed tape due to the lower packing density of MgB2.Comment: 4 pages, 3 figure

Current percolation and anisotropy in polycrystalline MgB2_2

The influence of anisotropy on the transport current in MgB$_2$ polycrystalline bulk samples and wires is discussed. A model for the critical current density is proposed, which is based on anisotropic London theory, grain boundary pinning and percolation theory. The calculated currents agree convincingly with experimental data and the fit parameters, especially the anisotropy, obtained from percolation theory agree with experiment or theoretical predictions.Comment: 5 pages, accepted for publication in Physical Review Letters (http://prl.aps.org/