We report the growth of high-quality c-axis-oriented epitaxial MgB 2 thin films by using a pulsed laser deposition technique. The thin films grown on (1    1 0 2) Al 2 O 3 substrates show a T c of 39 K. The critical current density in zero field is ∼ ∼ ∼ ∼ 6 x 10 6 A/cm 2 at 5 K and ∼ ∼ ∼ ∼ 3 x 10 5 A/cm 2 at 35 K, suggesting that this compound has great potential for electronic device applications, such as microwave devices and superconducting quantum interference devices (SQUIDs). For the films deposited on Al 2 O 3 , X-ray diffraction patterns indicate a highly c-axis-oriented crystal structure perpendicular to the substrate surface.The recent discovery of the binary metallic MgB 2 superconductor [1] having a remarkably high transition temperature (T c ) of 39 K has attracted great scientific interest [2][3][4][5][6][7][8]. With its metallic charge carrier density [2] and the strongly linked nature of the inter-grains in a polycrystalline form [9,10], this material is expected to be a very promising candidate for superconducting device [11] as well as large-scale applications. Furthermore, since the single crystal growth of MgB 2 seems very difficult, the fabrication of epitaxial thin film should be an important development for future basic research studies. However, the fabrication of thin films of this material has not been reported yet.We used a two-step method to fabricate MgB 2 thin films. First, we deposited amorphous B thin films; we sintered then at high temperature in Mg vapor, which is very similar to the growth techniques of cuprate Hg-based superconducting thin films [12,13]. We pressed commercial Boron (99.99%) powder into a disk shape with a diameter of 12.7 mm and a height of 5 mm under a pressure of 6 tons. Precursor thin films of B were deposited on Al 2 O 3 (AO) and SrTiO 3 (STO) substrates at room temperature by using pulsed laser deposition. The laser energy density was 20 -30 J/cm 2 at a laser flux of 600 mJ/pulse and a pulse frequency of 8 Hz. After a precursor thin film had been fabricated, it was put into a Ta tube together with a high purity Mg metal (99.9%) and sealed in an Ar atmosphere. The heat treatment was carried out in an evacuated quartz ampoule to prevent oxidation of the Ta tube. The typical sintering procedure was fast heating to 900 °C in 5 minutes; this temperature was held for 10 -30 minutes, and then quenched to room temperature. The typical film thickness used in this study was 0.4 µm, which was measured by a scanning electron microscope. This simple technique can be applied to other physical deposition methods, such as sputtering and electron-beam evaporation, and is highly reproducible, so mass production should be possible. The resistivity measurements were carried out using the dc four-probe method. The dc magnetic properties were measured with a Quantum Design MPMS superconducting quantum interference device magnetometer. The structures were analyzed using a x-ray diffractometer (XRD).The typical temperature dependence of the resistivity of MgB 2 grown on AO ...

The study of the anisotropic superconductor MgB2 using a combination of scanning tunneling microscopy and spectroscopy reveals two distinct energy gaps at ∆1=2.3 meV and ∆2=7.1 meV. Different spectral weights of the partial superconducting density of states (PDOS) are a reflection of different tunneling directions in this multi-band system. Our experimental observations are consistent with the existence of two-band superconductivity in the presence of interband superconducting pair interaction and quasiparticle scattering. Temperature evolution of the tunneling spectra follows the BCS scenario [1] with both gaps vanishing at the bulk Tc. Indeed, the study of tunneling junctions exhibiting only the small gap (c-axis tunneling) clearly and reproducibly show that this gap persists up to the bulk Tc. The data confirm the importance of Fermi-surface sheet dependent superconductivity in MgB2 proposed in the multigap model by Liu et al. [2] .The discovery of superconductivity in MgB 2 [3] at 39K sparked great interest in the fundamental physics and practical applications of this material. There has already been rapid progress in understanding the physical properties of this superconductor. Specific heat measurements [4,5] show that MgB 2 is an s-wave superconductor and the presence of the isotope effect [6,7] points towards phonon-mediated pairing. Tunneling and photoemission spectroscopy directly measures the superconducting energy gap and can provide further understanding of the origin of the superconductivity in this material. Earlier tunneling spectroscopy measurements show a large spread in the gap values [8][9][10] each consistent with the BCS form. More recent experiments, including STM tunneling spectroscopy [11], point-contact spectroscopy [12,13], specific heat measurements [4,5], and Raman spectroscopy [14] point towards the existence of two distinct gaps. This scenario has been predicted theoretically by Liu et al. [2]. First principle calculations show that the Fermi surface of MgB 2 consists of 2D cylindrical sheets arising from σ antibonding states of B p xy orbitals, and 3D tubular networks arising from π bonding and antibonding states of B p z orbitals. In this theoretical framework [2] two different energy gaps exist, the smaller one being an induced gap associated with the 3D bands and the larger one associated with the superconducting 2D bands. Furthermore both superconducting gaps should vanish at the bulk critical temperature T c . Due to this highly anisotropic band structure the superconducting gaps should be momentum-dependent reflecting the strength of the electron-phonon coupling of the carriers in the different bands. Up to now there has been no direct experimental evidence of the orientation dependence of the order parameter in this material. Moreover, the temperature dependence of the two gaps would give further insights into the nature of superconductivity in MgB 2 . Scanning tunneling spectroscopy is a unique technique that allows direct measure of the DOS near the Fermi energy with high...

We report a detailed comparison of experimental data and theoretical predictions for the dendritic flux instability, believed to be a generic behavior of type-II superconducting films. It is shown that a thermomagnetic model published very recently [Phys. Rev. B 73, 014512 (2006)10.1103/PhysRevB.73.014512] gives an excellent quantitative description of key features like the stability onset (first dendrite appearance) magnetic field, and how the onset field depends on both temperature and sample size. The measurements were made using magneto-optical imaging on a series of different strip-shaped samples of MgB2. Excellent agreement is also obtained by reanalyzing data previously published for Nb.

We report the observation of two-dimensional plasma filamentary arrays with more than 100 elements generated during breakdown of air at atmospheric pressure by a focused Gaussian beam from a 1.5-MW, 110-GHz gyrotron operating in 3-s pulses. Each element is a plasma filament elongated in the electric field direction and regularly spaced about one-quarter wavelength apart in the plane perpendicular to the electric field. The development of the array is explained as a result of diffraction of the beam around the filaments, leading to the sequential generation of high intensity spots, at which new filaments are created, about a quarter wavelength upstream from each existing filament. Electromagnetic wave simulations corroborate this explanation and show very good correlation to the observed pattern of filaments.