F. Hunte

North Carolina State University, Raleigh, North Carolina, United States

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Publications (37)108.88 Total impact

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    ABSTRACT: We have synthesised Bi2Se3 epitaxial thin films on c-sapphire substrates, where Se-related defects and strains are controlled precisely during pulsed laser deposition. This allows us to tune electrical and magnetotransport properties and probe the role of defects and strains as a function of processing conditions systematically. The defect microstructure has been studied in detail using high resolution X-ray diffraction and high-angle annular dark field scanning transmission electron microscopy. Magnetotransport measurements show a strong dependence on microstructure which is associated with the Se-content. With higher Se content, the film experiences large compressive strain along the [0 0 1] direction which is accompanied by the partial suppression of one family of twin domain formation. As a result, the insulating behavior becomes more pronounced at a low temperature which is understood in terms of the quantum correlation induced by electron–electron interactions. The compressive strain enhances spin–orbit coupling and topological characteristics. These results shed light on the importance of controlling the intrinsic defects during the growth of Bi2Se3 thin films, providing an effective way to suppress the bulk conductivity and establish the correlation between microstructure and strain.
    Acta Materialia 08/2015; 95. DOI:10.1016/j.actamat.2015.05.009
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    ABSTRACT: We report on the epitaxial growth and magnetic properties of Cr2O3 thin films grown on r-sapphire substrate using pulsed laser deposition. The X-ray diffraction (XRD) (2θ and Φ) and TEM characterization confirm that the films are grown epitaxially. The r-plane ( 01 1 ¯ 2 ) of Cr2O3 grows on r-plane of sapphire. The epitaxial relations can be written as [ 01 1 ¯ 2 ] Cr2O3 ‖ [ 01 1 ¯ 2 ] Al2O3 (out-of-plane) and [ 1 ¯ 1 ¯ 20 ] Cr2O3 ‖ [ 1 ¯ 1 ¯ 20 ] Al2O3 (in-plane). The as-deposited films showed ferromagnetic behavior up to 400 K but ferromagnetism almost vanishes with oxygen annealing. The Raman spectroscopy data together with strain measurements using high resolution XRD indicate that ferromagnetism in r-Cr2O3 thin films is due to the strain caused by defects, such as oxygen vacancies.
    Journal of Applied Physics 05/2015; 117(19):193907. DOI:10.1063/1.4921435
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    ABSTRACT: We report on the epitaxial growth and magnetic properties of antiferromagnetic and magnetoelectric (ME) Cr2O3 thin films deposited on cubic yttria stabilized zirconia (c-YSZ)/Si(001) using pulsed laser deposition. The X-ray diffraction (2ϴ and Φ) and TEM characterizations confirm that the films were grown epitaxially. The Cr2O3(0001) growth on YSZ(001) occurs with twin domains. There are four domains of Cr2O3 with in-plane rotation of 30° or 150° from each other about the [0001] growth direction. The epitaxial relation between the layers is given as [001]Si ‖ [001]YSZ ‖ [0001]Cr2O3 and [100]Si ǁ [100]YSZ ǁ [101¯0] Cr2O3 or [112¯0] Cr2O3. Though the bulk Cr2O3 is an antiferromagnetic with TN = 307 K, we found that the films exhibit ferromagnetic like hysteresis loops with high saturation and finite coercive field up to 400 K. The thickness dependent magnetizations together with oxygen annealing results suggest that the ferromagnetism (FM) is due to oxygen related defects whose concentration is controlled by strain present in the films. This FM, in addition to the intrinsic magneto-electric properties of Cr2O3, opens the door to relevant spintronics applications.
    Applied Physics Letters 09/2014; 105(13):132401-132401-5. DOI:10.1063/1.4896975
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    ABSTRACT: In this paper, we report on the epitaxial integration of room temperature lead-free ferroelectric BaTiO3 thin (∼1050 nm) films on Si (100) substrates by pulsed laser deposition technique through a domain matching epitaxy paradigm. We employed MgO and TiN as buffer layers to create BaTiO3/SrRuO3/MgO/TiN/Si (100) heterostructures. C-axis oriented and cube-on-cube epitaxial BaTiO3 is formed on Si (100) as evidenced by the in-plane and out-of-plane x-ray diffraction, and transmission electron microscopy. X-ray photoemission spectroscopic measurements show that Ti is in 4(+) state. Polarization hysteresis measurements together with Raman spectroscopy and temperature-dependent x-ray diffraction confirm the room temperature ferroelectric nature of BaTiO3. Furthermore, laser irradiation of BaTiO3 thin film is found to induce ferromagnetic-like behavior but affects adversely the ferroelectric characteristics. Laser irradiation induced ferromagnetic properties seem to originate from the creation of oxygen vacancies, whereas the pristine BaTiO3 shows diamagnetic behavior, as expected. This work has opened up the route for the integration of room temperature lead-free ferroelectric functional oxides on a silicon platform.
    Journal of Applied Physics 09/2014; 116(9):094103-094103-5. DOI:10.1063/1.4894508
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    ABSTRACT: Recent electrical measurements have accessed transport in the topological surface state band of thin exfoliated samples of Bi2Se3 by removing the bulk n-type doping by contact with thin films of the molecular acceptor F4-TCNQ. Here we report on the film growth and interfacial electronic characterization of F4-TCNQ grown on Bi2Se3. Atomic force microscopy shows wetting layer formation followed by 3D island growth. X-ray photoelectron spectroscopy is consistent with this picture and also shows that charge transferred to the molecular layer is localized on nitrogen atoms. Ultraviolet photoelectron spectroscopy shows a work function increase and an upward shift of the valence band edge that suggest significant reduction in carrier density at the Bi2Se3 surface.
    The Journal of Physical Chemistry C 07/2014; 118(27):14860-14865. DOI:10.1021/jp412690h
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    ABSTRACT: We report the bi-epitaxial growth of ZnO and resistance switching characteristics of Pt/ZnO/TiN-based heterojunction devices fabricated on Si(001) substrates by pulsed laser deposition. The structural properties of the heterostructures characterized by XRD (θ-2θ, φ scans) and TEM confirm that the ZnO films having hexagonal wurtzite structure (six-fold symmetry) grow bi-epitaxially on the TiN buffer layer (four-fold symmetry). The Pt(111) grows epitaxially on ZnO(0001). The epitaxial relationship between the various films is given as (111)Pt ‖ (0001)ZnO ‖ (001)TiN ‖ (001)Si and [100]TiN ‖ [100]Si, [21¯1¯0]ZnO ‖ [110]TiN or [101¯0]ZnO ‖ [110]TiN, and [101¯]Pt ‖ [21¯1¯0]ZnO. The effect of ZnO growth temperature on the electrical properties of Pt/ZnO/TiN devices is studied and correlated with the microstructure of the ZnO/TiN interface. The Pt/ZnO/TiN devices exhibited good bi-polar resistance switching characteristics at voltages as low as ±1 V.
    Journal of Applied Physics 06/2014; 115(23):234501-234501-6. DOI:10.1063/1.4883959
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    ABSTRACT: High strength dispersion strengthened (DS) Ag/Al alloys with various Al content are studied as candidates for sheathing Bi2Sr2CaCu2O8Cx (Bi2212) wire. The Ag/Al alloys are fabricated by powder metallurgy and internally oxidized in pure oxygen. The time and temperature of the internal oxidation heat treatment is varied to maximize the strength after undergoing the Bi2212 partial melt process (PMP). Vickers micro-hardness number (HVN), room temperature tensile behavior, optical and scanning electron microscopy, ion channeling contrast imaging using a focused ion beam and electrical resistivity measurements are used to characterize the alloys. An Ag/0.2wt%Mg (Ag/Mg) alloy is used for comparison. Results show that internal oxidation at 650–700 �C for 4 h produces the highest HVN for the DS Ag/Al alloy; when oxidized at 675 �C for 4 h the HVN, yield strength and tensile strength of the DS Ag/Al are 50% higher than the corresponding values of Ag/Mg. Microstructural observations show that Al2O3 precipitates play the main role in strengthening the DS Ag/Al alloy. The alloy retains its fine grain structure and strength after PMP heat treatment.
    Superconductor Science and Technology 10/2013; 26. DOI:10.1088/0953-2048/26/12/125012
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    ABSTRACT: One of the remaining challenges for the implementation of commercial Ag-alloy-sheathed Bi2Sr2CaCu2O8+x (Bi2212) wires in high-field superconducting magnets is quench protection. To develop an effective quench protection system, it is important to understand the conditions that must be avoided during a quench so that the conductor is not degraded. While these conditions are understood for NbTi and Nb3Sn, they are conductor specific and there remains a lack of data and understanding of the limiting conditions for Bi2212 wires. Here, quenches are induced in short strands and small coils of Bi2212 round wires at 4.2 K. The quench conditions are varied to identify the threshold conditions resulting in wire degradation. These conditions are quantified in terms of the maximum temperature, the maximum time rate of change of the temperature, and the maximum temperature spatial gradient along the length of the wire. It is found that the time rate of change of the temperature (thermal shock) is not a primary driver for degradation but that both the maximum temperature and its spatial gradient play a key role. It is not clear, however, whether the temperature gradient along the length of the wire, or radially from the center of the wire to the surface, dominates. It is also found that threshold values for these parameters vary between different Bi2212 wires and, thus, must be identified for the specific wire to be used in a magnet system. Implications of these results on quench protection are discussed.
    IEEE Transactions on Applied Superconductivity 10/2013; 23(5). DOI:10.1109/TASC.2013.2271255
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    ABSTRACT: The significant amount of energy stored in a large high-field superconducting magnet can be sufficient to destroy the coil in the event of an unprotected quench. For magnets based on high-temperature superconductors (HTSs), such as Bi2Sr2CaCu2Ox (Bi2212) and YBa2Cu3O7−x (YBCO), quench protection is particularly challenging due to slow normal zone propagation. A previous computational study showed that the quench behavior of HTS magnets is significantly improved if the turn-to-turn electrical insulation is thermally conducting, enhancing 3-D normal zone propagation. Here, a new doped-titania electrical insulation with high thermal conductivity is evaluated. The thermal conductivity of the insulation is measured at cryogenic temperatures, and its chemical compatibility with Bi2212 round wires is determined. Thin layers of the insulation are deposited onto the surface of Bi2212 and YBCO wires, which are then wound into small coils to study the quench behavior. Results show that the critical current and homogeneity of Bi2212 coils are improved relative to coils reacted with mullite insulation. Relative to similar coils with conventional insulation (mullite for Bi2212 and Kapton for YBCO), the turn-to-turn quench propagation is increased by a factor of 2.8 in Bi2212 coils at 4.2 K and self-field and by a factor of 2.5 in YBCO coils at 4.2 K and 5 T. These results indicate that doped-titania insulation may significantly improve Bi2212 and YBCO coils. Increased normal zone propagation velocity enhances quench detection and quench protection, and the thinness of the insulation relative to the most common alternatives increases the magnet winding pack current density and reduces the coil specific heat.
    IEEE Transactions on Applied Superconductivity 10/2013; 23(5). DOI:10.1109/TASC.2013.2269535
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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). DOI:10.1063/1.4820770
  • Liyang Ye, Frank Hunte, Justin Schwartz
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    ABSTRACT: The development of high field superconducting magnets using high temperature superconductors (HTSs) is progressing for high energy physics, nuclear magnetic resonance and energy storage applications. Yet the key issue of quench protection remains unresolved, primarily due to the slow normal zone propagation velocity (NZPV) in HTS magnets. High magnetic field may affect the quench behavior through two opposing effects: increased NZPV may result due to reduced critical temperature and current sharing temperature, but decreased NZPV may result due to reduced critical current density and thus operating current. At present it is unclear which effect dominates. Here, a series of quench experiments at high magnetic field on multilayer wind-and-react Bi2Sr2CaCu2Ox (Bi2212) coils addresses this question. The two- and three-dimensional quench behavior is investigated in a magnetic field up to 20 T at 4.2 K. With increasing magnetic field, the minimum quench energy decreases significantly. The NZPV also decreases with magnetic field, but only up to about 8 T. For magnetic fields above 8 T, the NZPV is independent of magnetic field up to at least 20 T. Thus, at low field the NZPV is dominated by the decreasing critical current density, whereas at higher magnetic field the competing effects of decreasing critical current density and decreasing temperature margin offset each other.
    Superconductor Science and Technology 03/2013; 26(5):055006. DOI:10.1088/0953-2048/26/5/055006
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    ABSTRACT: YBa2Cu3O7–x (YBCO) coated conductors are emerging as an important option for magnets for energy systems and experimental science. One of the remaining challenges for YBCO superconducting magnets is quench protection, i.e. ensuring that the YBCO is not damaged due to a fault condition. One key issue is understanding the underlying causes of degradation during a quench. Here, the microstructure of a quenched, degraded sampled is compared to that of an unquenched control sample. To facilitate microstructural analysis of the YBCO surface, the Cu stabilizer and Ag cap layer were removed by etching. Reactions between the Cu etchant and YBCO proved to be a signature of Ag/YBCO delamination. Two types of pre-existing defects were identified as initiation points of degradation. Defects on the conductor edge resulting in delaminated Ag lead to dendritic flux avalanches and high local heating, which cause further Ag delamination. This self-propagating effect results in dendritic Ag delamination, which is seen through etchant–YBCO reactions. Defects within the YBCO layer result in breaches in the protective Ag layer such that Cu etchant penetrates and reacts with the YBCO. Energy-dispersive X-ray spectroscopy analysis showed similar reactions as in the edge degradation but also showed pure Ag particles, which indicates that the local temperature was sufficient to cause localized Ag melting.
    Acta Materialia 12/2012; 60(20):6991–7000. DOI:10.1016/j.actamat.2012.09.003
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    ABSTRACT: High-strength high-elastic-modulus dispersion-strengthened (DS) silver aluminum alloys are studied for sheath-ing Bi 2 Sr 2 CaCu 2 O 8+x (Bi2212) round wire. DS is an effective method for producing a fine grain metallurgical structure that is resistant to softening during high-temperature heat treatment. Here, DS Ag/0.5-wt.% Al (AgAl) alloy sheet is produced using powder metallurgy and is compared with Ag/0.2-wt.% Mg (AgMg) alloy, which is currently the most common alloy used for Bi2212 wire. Room temperature (RT), 77-and 4.0-K tensile tests, Vickers microhardness, optical microscopy, field emission scanning electron microscopy, and electrical resistivity measurements are compared. Furthermore, Bi2212/AgMg and Bi2212/AgAl wires are produced and compared for short-sample and coil I c (4.2 K; self-field). It is found that the AgAl solid wire shows high yield stress and ultimate tensile strength in the annealed condition at both RT and 4.0 K, as well as significant ductility at 4.0 K. Electrical transport measurements show that the Bi2212/AgAl wires perform as well or better than Bi2212/AgMg wires. Furthermore, no leakage is observed after partial melt processing (PMP) of Bi2212/AgAl spirals. After PMP, the Bi2212/AgAl wire not only has yield and tensile stresses slightly higher than those of the Bi2212/AgMg wire but also exhibits > 2% elongation, which is several times higher than that of Bi2212/AgMg. Index Terms—Dispersion-strengthened (DS) alloys, high-temperature superconductor, tensile properties, transport measurements.
    IEEE Transactions on Applied Superconductivity 02/2012; 22. DOI:10.1109/TASC.2011.2179296
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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.
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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; 21(3-21):2787 - 2790. DOI:10.1109/TASC.2010.2094173
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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. DOI:10.1088/0953-2048/23/9/095008
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    ABSTRACT: The full text of the corrigendum is given in the PDF file.
    Superconductor Science and Technology 04/2010; 23(4):9801-049801. DOI:10.1088/0953-2048/23/4/049801
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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. DOI:10.1088/0953-2048/22/12/125001
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    ABSTRACT: The iron pnictide superconductors have very high upper critical field Bc2(0) of possibly over 100 T for 1111 and 50-70 T for 122. We have recently shown [1,2] that polycrystalline 1111 samples exhibit electromagnetic granular behavior, perhaps in an analogous way to that seen now to be intrinsic to the HTS cuprates. Detailed investigation is proceeding in parallel with serious efforts to make more single phase samples, since it appears that all present polycrystalline oxypnictides are multi-phase. In particular we are using magneto-optical imaging to study the local variation of current density and then performing detailed microstructural analysis by SEM, TEM and orientation analysis to understand intergranular current flow. At the present time we see that samples are multi-phase, often with a grain boundary wetting phase, but even so the global Jc attains 1000-4000 A/cm^2, some 10-40 times that seen in single phase YBCO randomly oriented polycrystalline. On the other hand, very high intra-grain critical current owing to the strong pinning reminiscent of Nb-Ti is observed in the Co doped Ba122 pnictide. We will report on our latest results on the inter- and intra-granular current properties in the high-Tc pnictides. [1] A. Yamamoto et al., Appl. Phys. Lett. 92, 252501 (2008). [2] A. Yamamoto et al., Supercond. Sci. Technol. 21, 095008 (2008).
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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.

Publication Stats

783 Citations
108.88 Total Impact Points

Institutions

  • 2011–2014
    • North Carolina State University
      • • Department of Materials Science and Engineering
      • • Department of Electrical and Computer Engineering
      Raleigh, North Carolina, United States
  • 2008–2010
    • Florida State University
      • Applied Superconductivity Center (ASC)
      Tallahassee, FL, United States