F. Hunte

Florida State University, Tallahassee, FL, United States

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Publications (24)76.15 Total impact

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    ABSTRACT: Epitaxial thin films heterostructures of topological insulator candidate Sr3SnO (SSO) are grown on a cubic yttria-stabilized zirconia (c-YSZ)/Si (001) platform by pulsed laser deposition. X-ray and electron diffraction patterns confirm the epitaxial nature of the layers with cube-on-cube orientation relationship: (001)[100]SSO∥(001)[100]c-YSZ∥(001)[100]Si. The temperature dependent electrical resistivity shows semiconductor behavior with a transport mechanism following the variable-range-hopping model. The SSO films show room-temperature ferromagnetism with a high saturation magnetization, and a finite non-zero coercivity persisting up to room temperature. These results indicate that SSO is a potential dilute magnetic semiconductor, presumably obtained by controlled introduction of intrinsic defects.
    Applied Physics Letters 09/2013; 103(112101). · 3.79 Impact Factor
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    ABSTRACT: This paper reviews the status of high temperature superconductors for high field magnets for future devices such as a high energy LHC or a muon collider. Some of the primary challenges faced for the implementation of systems are discussed. Two conductor technologies, Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ and YBa$_2$Cu$_3$O$_{7-\delta}$, have emerged as high field conductor options, but their relative advantages and disadvantages for high field magnets are quite different. These are reviewed from an engineering perspective, including coil manufacturing, electromechanical behaviour and quench behaviour. Lastly, the important roles of "system pull" upon conductor and magnet technology development, and of interactions between the materials and magnet communities for accelerating development, are discussed.
    08/2011;
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    ABSTRACT: The quench behavior of Bi<sub>2</sub>Sr<sub>2</sub>CaCu<sub>2</sub>O x (Bi2212) wire is investigated through numerical simulations. This work is part of the U.S. Very High Field Superconducting Magnet Collaboration (VHFSMC). Numerical simulations are carried out using a one-dimensional computational model of thermal transport in Bi2212 composite wires. A quench is simulated by introducing heat in a section of the wire, and the voltage and temperature are monitored as function of time and position. The quench energy, normal zone propagation velocity, and spatial distribution of temperature are calculated for varying transport current and applied magnetic field. The relevance of these simulations in defining criteria for experimental measurements is discussed.
    IEEE Transactions on Applied Superconductivity 07/2011; · 1.20 Impact Factor
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    ABSTRACT: MgB2 thin films have superior superconducting properties compared to bulk MgB2 and demonstrate the potential for further improving the performances of MgB2 wires and tapes. Using transmission electron microscopy, we have characterized the microstructure of pure and C-doped MgB2 using various carbon sources grown by hybrid physical–chemical vapor deposition (HPCVD), and cold-grown–annealed film deposited by molecular beam epitaxy (MBE). The MgB2 HPCVD films increase in crystal quality in the order (MeCp)2Mg-sourced films, CH4-sourced films, B(CH3)3-sourced films, pure films, while the Hc2 values of these films follow the opposite order. The cold-grown–annealed MgB2 MBE film contains non-epitaxial ≤ 10 nm MgB2 grains and MgO nanoparticles. The microstructural origins of electron scattering and flux pinning in both films are discussed. We also show the structure and chemistry of the degraded phase in HPCVD films and its effects on superconducting properties.
    Superconductor Science and Technology 08/2010; 23(9):095008. · 2.76 Impact Factor
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    ABSTRACT: The full text of the corrigendum is given in the PDF file.
    Superconductor Science and Technology 01/2010; 23(4):9801-049801. · 2.76 Impact Factor
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    ABSTRACT: We report that C-doped MgB2 thin films deposited by hybrid physical–chemical vapor deposition (HPCVD) using CH4 as the carbon source have Hc2(0 K)~60 T, similar to that of HPCVD films obtained using (MeCp)2Mg for the carbon. Using transmission electron microscopy, we show that in the films doped using CH4 there is a MgB2C2 layer on top of the MgB2 film, which does not degrade the MgB2 connectivity or Jc. We also find a high density of coherent MgO nanoplatelets in the MgB2 which create strain fields which may give rise to strong π-band scattering and the very high Hc2.
    Superconductor Science and Technology 10/2009; 22(12):125001. · 2.76 Impact Factor
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    ABSTRACT: The Hc2(T) of a set of four carbon-doped MgB2 films grown on both SiC and Al2O3 substrates by HPCVD from methane CH4 at flow rates from 7 to 10 sccm were measured in fields up to 65T. Compared to early metalorganic C sources which generated high Hc2(0), these films have much lower resistivities and higher connectivities. The curvature of Hc2(T) derived from low current four point magnetoresistance shows upturn at low temperatures, which is consistent with the dominance of pi-band scattering in the theory of dirty two-gap superconductivity. Hc2^|(0) > 60T is close to the paramagnetic limit of ˜ 66T for the 10 sccm film on SiC, though still a little lower than for the previously used metalorganic (C6H7)2Mg. Differences in the Hc2(T) behavior between films grown on the two substrates are attributed to variations in strain fields produced by the substrate coupled to the film at growth.
    03/2009;
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    ABSTRACT: We present measurements of the resistivity and the upper critical field Hc2 of NdFeAs O0.7F0.3 single crystals in strong DC and pulsed magnetic fields up to 45 T and 60 T, respectively. We found that the field scale of Hc2 is comparable to ˜100 T of high Tc cuprates. Hc2(T) parallel to the c-axis exhibits a pronounced upward curvature similar to what was extracted from earlier measurements on polycrystalline samples. Thus this behavior is indeed an intrinsic feature of oxypnictides, rather than manifestation of vortex lattice melting or granularity. The orientational dependence of Hc2 shows deviations from the one-band Ginzburg-Landau scaling. The mass anisotropy decreases as T decreases, from 9.2 at 44 K to 5 at 34 K. We discuss to what extent different pairing scenarios can manifest themselves in the observed behavior of Hc2, using the two-band model of superconductivity. The results indicate the importance of paramagnetic effects on Hc2(T), which may significantly reduce Hc2(0) as compared to Hc2(0)˜200-300 T based on extrapolations of Hc2(T) near Tc down to low temperatures.
    03/2009;
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    ABSTRACT: Tokamak fusion reactors require the development of magnets capable of generating large magnetic fields under stringent structural constraints. Magnets made with high temperature superconductors (HTS) are well suited to this application, but are vulnerable to quench occurrence during operation. Temperature and strain sensors based on fiber optics are being developed as a first step to counter this contingency. Optical fibers with Bragg gratings are amenable to embedding within superconducting magnets to monitor temperature, strain, irradiation, and to detect quench occurrence. Additionally, in the case of AgX/Ag/Bi2Sr2CaCu2Ox, (Bi2212) wire magnets, fiber optics can serve as a heat treatment process monitor for wind-and-react (W&R) manufacturing. Here we show that it is possible to detect quenches using fiber Bragg grating sensors and examine the effects of Bi2212/optical fiber co-sintering on Bi2212 performance and fiber survivability.
    01/2009;
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    ABSTRACT: We present detailed measurements of the longitudinal resistivity ρxx(T,H) and the upper critical field Hc2 of NdFeAsO0.7F0.3 single crystals in strong dc and pulsed magnetic fields up to 45 and 60 T, respectively. We found that the field scale of Hc2 is comparable to Hc2∼100 T of high-Tc cuprates. Hc2(T) parallel to the c axis exhibits a pronounced upward curvature similar to what was extracted from earlier measurements on polycrystalline LaFeAs(O,F), NdFeAs(O,F), and SmFeAs(O,F) samples. Thus, this behavior of Hc2⊥(T) is indeed an intrinsic feature of oxypnictides rather than manifestation of vortex lattice melting or granularity. The orientational dependence of Hc2(θ) as a function of the angle θ between H and the c axis shows deviations from the one-band Ginzburg-Landau scaling. The mass anisotropy parameter γ(T)=(mc/mab)1/2=Hc2∥/Hc2⊥ obtained from these measurements decreases as temperature decreases from γ≃9.2 at 44 K to γ≃5 at 34 K, where ∥ and ⊥ correspond to H parallel and perpendicular to the ab planes, respectively. Spin-dependent magnetoresistance and nonlinearities in the Hall coefficient suggest contribution to the conductivity from electron-electron interactions modified by disorder reminiscent of that in diluted magnetic semiconductors. The Ohmic resistivity ρxx(T,H) measured below Tc but above the irreversibility field exhibits a clear Arrhenius thermally-activated behavior ρ=ρ0 exp[−Ea(T,H)/T] over 4–5 decades of ρxx. The activation energy Ea(T,H) has very different field dependencies for H∥ab and H⊥ab varying from 4×103 K at H=0.2 T to ∼200 K at H=35 T. We discuss to what extent different pairing scenarios suggested in the literature can manifest themselves in the observed behavior of Hc2, using the two-band model of superconductivity in oxypnictides. The results indicate the importance of paramagnetic effects on Hc2(T) in oxypnictides, which may significantly reduce Hc2(0) as compared to Hc2(0)∼200–300 T based on extrapolations of Hc2(T) near Tc down to low temperatures.
    Physical Review B 11/2008; 78(17). · 3.66 Impact Factor
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    ABSTRACT: Early studies have found quasi-reversible magnetization curves in polycrystalline bulk rare-earth iron oxypnictides that suggest either widely spread obstacles to intergranular current or very weak vortex pinning. In the present study of polycrystalline samarium and neodymium iron oxypnictide samples made by high pressure synthesis, the hysteretic magnetization is significantly enhanced. Magneto-optical imaging and study of the field dependence of the remanent magnetization as a function of particle size both show that global currents over the whole sample do exist but that the intergranular and intragranular current densities have distinctively different temperature dependences and differ in magnitude by about 1000. If the highest current density loops lie only within grains, their magnitude is ~5 × 106 A cm−2 at 5 K and self-field. Whole sample current densities, though two orders of magnitude lower at 1000–10 000 A cm−2, are some two orders of magnitude higher than in random polycrystalline cuprates. We cannot yet be certain whether this large difference in global and intragrain current density is intrinsic to the oxypnictides or due to extrinsic barriers to current flow, because the samples contain a significant second phase, some of which wets the grain boundaries and may cause a superconducting–normal–superconducting proximity effect in the whole sample critical current.
    Superconductor Science and Technology 07/2008; 21(9):095008. · 2.76 Impact Factor
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    ABSTRACT: The recent synthesis of the superconductor LaFeAsO(0.89)F(0.11) with transition temperature T(c) approximately 26 K (refs 1-4) has been quickly followed by reports of even higher transition temperatures in related compounds: 41 K in CeFeAsO(0.84)F(0.16) (ref. 5), 43 K in SmFeAsO(0.9)F(0.1) (ref. 6), and 52 K in NdFeAsO(0.89)F(0.11) and PrFeAsO(0.89)F(0.11) (refs 7, 8). These discoveries have generated much interest in the mechanisms and manifestations of unconventional superconductivity in the family of doped quaternary layered oxypnictides LnOTMPn (Ln: La, Pr, Ce, Sm; TM: Mn, Fe, Co, Ni; Pn: P, As), because many features of these materials set them apart from other known superconductors. Here we report resistance measurements of LaFeAsO(0.89)F(0.11) at high magnetic fields, up to 45 T, that show a remarkable enhancement of the upper critical field B(c2) compared to values expected from the slopes dB(c2)/dT approximately 2 T K(-1) near T(c), particularly at low temperatures where the deduced B(c2)(0) approximately 63-65 T exceeds the paramagnetic limit. We argue that oxypnictides represent a new class of high-field superconductors with B(c2) values surpassing those of Nb(3)Sn, MgB(2) and the Chevrel phases, and perhaps exceeding the 100 T magnetic field benchmark of the high-T(c) copper oxides.
    Nature 07/2008; 453(7197):903-5. · 38.60 Impact Factor
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    ABSTRACT: The new rare-earth arsenate superconductors are layered, low carrier density compounds with many similarities to the high-Tc cuprates. An important question is whether they also exhibit weak-coupling across randomly oriented grain-boundaries. In this work we show considerable evidence for such weak-coupling by study of the dependence of magnetization in bulk and powdered samples. Bulk sample magnetization curves show very little hysteresis while remanent magnetization shows almost no sample size dependence, even after powdering. We conclude that these samples exhibit substantial electromagnetic granularity on a scale approximating the grain size, though we cannot yet determine whether this is intrinsic or extrinsic.
    Applied Physics Letters 06/2008; · 3.79 Impact Factor
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    ABSTRACT: We report a new method for synthesizing magnesium diboride (MgB2) thin films which exhibit a steep increase in the slope of the upper critical field Hc2 versus decreasing temperature (i.e. |dHc2/dT|Tc|≈2.5 T K−1). An Hc2 of 38.4 T at 4.2 K and an extrapolated value of Hc2(0) above 43 T at 0 K are reported. Films of magnesium plus boron were deposited on unheated c-plane sapphire substrates by co-evaporation, and were subsequently annealed in a reducing atmosphere at temperatures below 600 °C. The use of a combination of a magnesium-rich stoichiometry (Mg/B>1/2) for the as-deposited film and a two-step annealing process was found to be critical in obtaining these high values of Hc2 and |dHc2/dT|Tc|.
    Superconductor Science and Technology 05/2008; 21(8):085009. · 2.76 Impact Factor
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    ABSTRACT: The ongoing search for new superconductors has recently yielded a new family of oxypnictides composed of alternating La_2O_{2-x}F_x and Fe_2As_2 layers [1-4] with transition temperatures T_c of 25-28 K, which can be raised to 40-43 K by replacing La with Ce [5] or Sm [6] or to 52 K by replacing La with Nd and Pr [7, 8]. Recent experiments and band structure calculations have suggested an unconventional multiband superconductivity in the layers of paramagnetic Fe ions, which would normally destroy superconductivity in the traditional mechanism of the s-wave Cooper pairing. Here we report very high-field resistance measurements up to 45T, which show a remarkable enhancement of the upper critical fields B_c2 at low temperatures, as compared to those expected from the already high slopes of dB_c2/dT ~ 2T/K near T_c . The deduced B_c2(0) ~ 63-65 T exceeds the paramagnetic limit, consistent with strong coupling and important two-band effects in LaFeAsO_0.89F_0.11. We argue that oxypnictides are emerging as a new class of high-field superconductors surpassing the B_c2 of Nb_3Sn, MgB_2, and the Chevrel phases and perhaps approaching the 100T field benchmark of the high-T_c cuprates.
    05/2008;
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    ABSTRACT: The recent discovery of a new class of superconducting oxypnictides with high transition temperatures may have profound implications for understanding unconventional high-temperature superconductivity. Like the cuprates, the oxypnictides seem to manifest an interleaving of charge donor and superconducting layers emerging upon doping of an antiferromagnetic parent semi-metal. Here we report magneto-transport measurements of three rare earth (Re = La, Nd, Sm) oxypnicide compounds with the transition temperatures near the maximum reported to date, in very high DC and pulsed magnetic fields up to 45 and 54 T, respectively. Our resistivity, Hall coefficient and critical magnetic fields data suggest that these oxypnictide superconductors bridge the gap between MgB$_2$ and YBaCu$_3$O$_{7-x}$ as far as electromagnetic and vortex properties are concerned.
    Physical review. B, Condensed matter 01/2008; 78:064511. · 3.77 Impact Factor
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    ABSTRACT: We compare magnetotransport of the three iron-arsenide-based compounds ReFeAsO (Re=La, Sm, Nd) in very high DC and pulsed magnetic fields up to 45 and 54 T, respectively. Each sample studied exhibits a superconducting transition temperature near the maximum reported to date for that particular compound. While high magnetic fields do not suppress the superconducting state appreciably, the resistivity, Hall coefficient, and critical magnetic fields, taken together, suggest that the phenomenology and superconducting parameters of the oxypnictide superconductors bridges the gap between MgB2 and YBCO.
    Physical review. B, Condensed matter 01/2008; 78(6). · 3.77 Impact Factor
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    ABSTRACT: The low Hc2 values seen in pure and well ordered MgB2 can be raised dramatically, to 35 T or more, by introducing carrier scattering by native and impurity defects. We describe three means to do this. First, He ion irradiation is used to tune Tc from 39K to less than 10K, while at Tc near 33K, Hc2 reached a maximum value. Similar behavior has been reported for neutron damage and carbon doping. Second, we introduced oxygen in the films, either in-situ or ex-situ, and again, high Hc2 values were seen and in these films, very high Jc values as well. Finally, a novel route has been investigated. We deposited MgB2 films on room temperature substrates, then annealed at temperatures just sufficient to produce crystallinity, giving Tcs in the range of 10 to 30K. Such films exhibit large dHc2/dT values near Tc, sometimes larger than 2 T/K. This work is of practical importance and gives an improved understanding of how intraband and interband carrier scattering in the ``2-gap'' superconductor MgB2 determine its Hc2, resistivity and Tc.
    01/2008;
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    Physical review. B, Condensed matter 01/2008; 78:174523. · 3.77 Impact Factor
  • ChemInform 01/2008; 39(39).