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Momentum-dependent scanning tunneling spectroscopy in MgB{sub 2}.

Description: We present study of the anisotropic superconductor MgB{sub 2} using a combination of scanning tunneling microscopy and spectroscopy. The results reveal two distinct energy gaps at {Delta}{sub 1} = 2.3 meV and {Delta}{sub 2} = 7.1 meV. Different spectral weights of the partial superconducting density of states 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 with both gaps vanishing at the bulk T{sub c}. The data confirm the importance of Fermi-surface sheet dependent superconductivity in MgB{sub 2} proposed in the multigap model by Liu et al. [1].
Date: July 31, 2002
Creator: Karapetrov, G.; Iavarone, M.; Koshelev, A. E.; Kwok, W. K.; Crabtree, G. W.; Hinks, D. G. et al.
Partner: UNT Libraries Government Documents Department

Superconducting phase diagram of single crystal MgB{sub 2}.

Description: Using magnetization, transport and single-crystal specific heat measurements we have determined the superconducting phase diagram of MgB{sub 2}. A zero-temperature in-pane coherence length of 9.4 nm is found. The superconducting anisotropy changes monotonously from a value around 2 near T{sub c} to above 4.5 near 22 K. We present strong evidence for a surface superconducting state for H{parallel}c which might account for the wide spread in reported values of the superconducting anisotropy.
Date: September 17, 2002
Creator: Welp, U.; Rydh, A.; Karapetrov, G.; Kwok, W. K.; Crabtree, G. W.; Marcenat, C. et al.
Partner: UNT Libraries Government Documents Department

The influence of structural defects on intragranular critical currents of bulk MgB[sub 2].

Description: Bulk MgB2 samples were prepared under different synthesis conditions and analyzed by scanning and transmission electron microscopy. The critical current densities were determined from the magnetization versus magnetic field curves of bulk and powder-dispersed-in-epoxy samples. Results show that through a slow cooling process, the oxygen dissolved in bulk MgB2 at high synthesis temperatures can segregate and form nanometer-sized coherent precipitates of Mg(B,O)2 in the MgB2 matrix. Magnetization measurements indicate that these precipitates act as effective flux pinning centers and therefore significantly improve the intra-grain critical current density and its field dependence.
Date: January 1, 2002
Creator: Serquis, A. (Adriana); Liao, Xiaozhou; Civale, L. (Leonardo); Zhu, Y. T. (Yuntian Theodore); Coulter, J. Y. (James Y.); Peterson, D. E. (Dean E.) et al.
Partner: UNT Libraries Government Documents Department

Phase diagram of single crystal MgB{sub 2}.

Description: Using magnetization, magneto-transport and single-crystal specific heat measurements the authors have determined the superconducting phase diagram of MgB{sub 2}. The superconducting anisotropy {gamma} changes monotonously from a value of around 2 near T{sub c} to above 4.5 at 22 K. For H{parallel}c a pronounced peak effect in the critical current occurs at the upper critical field. They present a strong evidence for a surface superconducting state for H{parallel}c which might account for the wide spread in reported values of the superconducting anisotropy {gamma}.
Date: September 30, 2002
Creator: Karapetrov, G.; Welp, U.; Rydh, A.; Iavarone, M.; Kwok, W. K.; Crabtree, G. W. et al.
Partner: UNT Libraries Government Documents Department

MICROSTRUCTURE OF SUPERCONDUCTING MGB(2).

Description: Recently, Akimitsu and co-workers [1] discovered superconductivity at 39 K in the intermetallic compound MgB{sub 2}. This discovery provides a new perspective on the mechanism for superconductivity. More specifically, it opens up possibilities for investigation of structure/properties in a new class of materials. With the exceptions of the cuprate and C{sub 60} families of compounds, MgB{sub 2} possesses the highest superconducting transition temperature T{sub c}. Its superconductivity appears to follow the BCS theory, apparently being mediated by electron-phonon coupling. The coherence length of MgB{sub 2} is reported to be longer than that of the cuprates [2]. In contrast to the cuprates, grain boundaries are strongly coupled and current density is determined by flux pinning [2,3]. Presently, samples of MgB{sub 2} commonly display inhomogeneity and porosity on the nanoscale, and are untextured. In spite of these obstacles, magnetization and transport measurements show that polycrystalline samples may carry large current densities circulating across many grains [3,4]. Very high values of critical current densities and critical fields have been recently observed in thin films [5,6]. These attributes suggest possible large scale and electronic applications. The underlying microstructure can be intriguing, both in terms of basic science and in applied areas. Subsequent to the discovery, many papers were published [1-13], most dealing with synthesis, physical properties, and theory. There have yet been few studies of microstructure and structural defects [11, 14]. A thorough understanding of practical superconducting properties can only be developed after an understanding of microstructure is gained. In this work we review transmission electron microscopy (TEM) studies of sintered MgB{sub 2} pellets [14]. Structural defects, including second phase particles, dislocations, stacking faults, and grain boundaries, are analyzed using electron diffraction, electron-energy loss spectroscopy (EELS), high resolution imaging, and structural modeling.
Date: July 12, 2001
Creator: Zhu, Y.; Li, Q.; Wu, L.; Volkov, V.; Gu, G. & Moodenbaugh, A. R.
Partner: UNT Libraries Government Documents Department

Phase Equilibria Relationships of High-Tc Superconductors

Description: As an integral part of a R&D program partially supported by the Electricity Delivery and Energy Reliability Office of DOE, we have determined phase equilibria data and phase diagrams for the three generations of superconductor materials: 1st generation, (Bi,Pb)-Sr-Ca- Cu-O systems; 2nd generation, Ba-R-Cu-O systems (R=lanthanides and yttrium); and 3rd generation, MgB2 systems. Our studies involved bulk materials, single crystals and thin films. This report gives a summary of our accomplishments, a list of publications, and 15 selected journal publications
Date: January 11, 2011
Creator: Wong-Ng, Winnie
Partner: UNT Libraries Government Documents Department

Density Functional Theory Simulations Predict New Materials for Magnesium-Ion Batteries (Fact Sheet), NREL Highlights, Science

Description: Multivalence is identified in the light element, B, through structure morphology. Boron sheets exhibit highly versatile valence, and the layered boron materials may hold the promise of a high-energy-density magnesium-ion battery. Practically, boron is superior to previously known multivalence materials, especially transition metal compounds, which are heavy, expensive, and often not benign. Based on density functional theory simulations, researchers at the National Renewable Energy Laboratory (NREL) have predicted a series of stable magnesium borides, MgB{sub x}, with a broad range of stoichiometries, 2 < x < 16, by removing magnesium atoms from MgB{sub 2}. The layered boron structures are preserved through an in-plane topological transformation between the hexagonal lattice domains and the triangular domains. The process can be reversibly switched as the charge transfer changes with Mg insertion/extraction. The mechanism of such a charge-driven transformation originates from the versatile valence state of boron in its planar form. The discovery of these new physical phenomena suggests the design of a high-capacity magnesium-boron battery with theoretical energy density 876 mAh/g and 1550 Wh/L.
Date: October 1, 2011
Partner: UNT Libraries Government Documents Department