@article{akouala_kumar_punugupati_reynolds_reynolds_mily_maria_narayan_hunte_2019, title={Planar Hall effect and anisotropic magnetoresistance in semiconducting and conducting oxide thin films}, volume={125}, ISSN={0947-8396 1432-0630}, url={http://dx.doi.org/10.1007/S00339-019-2592-Y}, DOI={10.1007/s00339-019-2592-y}, number={5}, journal={Applied Physics A}, publisher={Springer Science and Business Media LLC}, author={Akouala, Christer R. and Kumar, Raj and Punugupati, Sandhyarani and Reynolds, C. Lewis and Reynolds, Judith G. and Mily, Edward J. and Maria, Jon-Paul and Narayan, Jagdish and Hunte, Frank}, year={2019}, month={Apr} }
 @article{li_zhang_qiao_yu_peterson_zafar_kumar_curtarolo_hunte_shannon_et al._2016, title={All The Catalytic Active Sites of MoS2 for Hydrogen Evolution}, volume={138}, ISSN={0002-7863 1520-5126}, url={http://dx.doi.org/10.1021/jacs.6b05940}, DOI={10.1021/jacs.6b05940}, abstractNote={MoS2 presents a promising low-cost catalyst for the hydrogen evolution reaction (HER), but the understanding about its active sites has remained limited. Here we present an unambiguous study of the catalytic activities of all possible reaction sites of MoS2, including edge sites, sulfur vacancies, and grain boundaries. We demonstrate that, in addition to the well-known catalytically active edge sites, sulfur vacancies provide another major active site for the HER, while the catalytic activity of grain boundaries is much weaker. The intrinsic turnover frequencies (Tafel slopes) of the edge sites, sulfur vacancies, and grain boundaries are estimated to be 7.5 s-1 (65-75 mV/dec), 3.2 s-1 (65-85 mV/dec), and 0.1 s-1 (120-160 mV/dec), respectively. We also demonstrate that the catalytic activity of sulfur vacancies strongly depends on the density of the vacancies and the local crystalline structure in proximity to the vacancies. Unlike edge sites, whose catalytic activity linearly depends on the length, sulfur vacancies show optimal catalytic activities when the vacancy density is in the range of 7-10%, and the number of sulfur vacancies in high crystalline quality MoS2 is higher than that in low crystalline quality MoS2, which may be related with the proximity of different local crystalline structures to the vacancies.}, number={51}, journal={Journal of the American Chemical Society}, publisher={American Chemical Society (ACS)}, author={Li, Guoqing and Zhang, Du and Qiao, Qiao and Yu, Yifei and Peterson, David and Zafar, Abdullah and Kumar, Raj and Curtarolo, Stefano and Hunte, Frank and Shannon, Steve and et al.}, year={2016}, month={Dec}, pages={16632–16638} }
 @article{punugupati_kumar_nori_hunte_narayan_2016, title={Structural, magnetic and magnetotransport properties of bi-epitaxial La0.7Sr0.3MnO3 (110) thin films integrated on Si (001)}, volume={106}, ISSN={["1873-2453"]}, DOI={10.1016/j.actamat.2015.12.054}, abstractNote={We report the growth of bi-epitaxial La0.7Sr0.3MnO3 (110) thin films on Si (001) substrate with cubic yttria stabilized zirconia (c-YSZ)/SrTiO3 (STO) buffer layers by pulsed laser deposition. The La0.7Sr0.3MnO3 and STO thin films were grown with a single [110] out-of-plane orientation and with two in-plane domain variants, which is confirmed by XRD and detailed TEM studies. The growth of STO on c-YSZ can be explained by the paradigm of domain matching epitaxy. The epitaxial relationship between STO and c-YSZ can be written as [110] (001) c-YSZ ‖ [1¯11¯] (110) STO (or) [110] (001) c-YSZ ‖ [1¯12¯] (110) STO. The La0.7Sr0.3MnO3 thin films are ferromagnetic with Curie temperature 324 K and showed metal to insulator transition at 285 K. The La0.7Sr0.3MnO3 thin films showed hysteresis loops in magnetoresistance when magnetic field is applied along both in-plane (110) and out-of-plane [110] directions. The highest magnetoresistance obtained in this study is −32% at 50 K and 50 kOe for in-plane configuration, whereas the room-temperature magnetoresistance is −4% at 10 kOe and −17% at 50 kOe. The hysteresis in the magnetoresistance and the controlled domain boundaries in bi-epitaxial La0.7Sr0.3MnO3 films integrated on Si can offer significant advantages over the polycrystalline counterparts.}, journal={ACTA MATERIALIA}, author={Punugupati, Sandhyarani and Kumar, Raj and Nori, Sudhakar and Hunte, Frank and Narayan, Jagdish}, year={2016}, month={Mar}, pages={40–47} }
 @article{lee_kumar_hunte_narayan_schwartz_2015, title={Control of intrinsic defects and magnetotransport properties of Bi2Se3/c-sapphire epitaxial heterostructures}, volume={95}, ISSN={1359-6454}, url={http://dx.doi.org/10.1016/J.ACTAMAT.2015.05.009}, DOI={10.1016/J.ACTAMAT.2015.05.009}, abstractNote={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.}, journal={Acta Materialia}, publisher={Elsevier BV}, author={Lee, Y.F. and Kumar, R. and Hunte, F. and Narayan, J. and Schwartz, J.}, year={2015}, month={Aug}, pages={57–64} }
 @article{ishmael_rogers_hunte_naderi_roach_straka_schwartz_2015, title={Current Density and Quench Behavior of MgB2/Ga Composite Wires}, volume={25}, ISSN={["1558-2515"]}, DOI={10.1109/tasc.2015.2483597}, abstractNote={Magnesium diboride (MgB 2) is a promising superconductor for many technical applications. Sufficient current densities at required magnetic fields, moderate operational temperature, low raw materials' cost, and an economical manufacturing process have enabled commercial development of MgB 2 wires. Reacted MgB 2, however, is brittle, and applications involving coils and windings with small bend radii are therefore difficult to implement. Furthermore, improvements in the critical current density are needed to expand the range of potential applications. Here, we report on the electrical behavior of novel MgB 2/Ga composite wires produced such that the proximity effect enhances connectivity, allowing the high-temperature anneal typically required for in situ and ex situ MgB 2 wires to be eliminated. Elimination of the high-temperature anneal simplifies MgB 2 manufacturing and has the potential to create a wire that is more tolerant of bending. Here, we present critical current density and quench propagation results for MgB 2/Ga composite wires sheathed in Cu.}, number={6}, journal={IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY}, author={Ishmael, Sasha A. and Rogers, Samuel and Hunte, Frank and Naderi, Golsa and Roach, Christian and Straka, Weston and Schwartz, Justin}, year={2015}, month={Dec} }
 @article{kumar_brom_redwing_hunte_2015, title={Magnetotransport phenomena in Bi2Se3 thin film topological insulators grown by hybrid physical chemical vapor deposition}, volume={117}, ISSN={["1089-7550"]}, DOI={10.1063/1.4907802}, abstractNote={Intrinsic defects in Bi2Se3 topological insulators tend to produce a high carrier concentration and current leakage through the bulk material. Bi2Se3 thin films were grown by hybrid physical chemical vapor deposition on (0001) Al2O3 substrates with high Se vapor pressure to reduce the occurrence of Se vacancies as the main type of defect. Consequently, the carrier concentration was reduced to ∼5.75 × 1018 cm−3 comparable to reported carrier concentration in Bi2Se3 thin films. Magnetotransport measurements were performed on the films and the data were analyzed for weak anti-localization using the Hikami-Larkin-Nagaoka model. The estimated α and lϕ values showed good agreement with the symplectic case of 2-D transport of topological surface states in the quantum diffusion regime. The temperature and angular dependence of magnetoresistance indicate a large contribution of the 2-D surface carriers to overall transport properties of Bi2Se3 thin film.}, number={6}, journal={JOURNAL OF APPLIED PHYSICS}, author={Kumar, Raj and Brom, Joseph E. and Redwing, Joan M. and Hunte, Frank}, year={2015}, month={Feb} }
 @article{lee_kumar_hunte_narayan_schwartz_2015, title={Microstructure and transport properties of epitaxial topological insulator Bi2Se3 thin films grown on MgO (100), Cr2O3 (0001), and Al2O3 (0001) templates}, volume={118}, ISSN={0021-8979 1089-7550}, url={http://dx.doi.org/10.1063/1.4932027}, DOI={10.1063/1.4932027}, abstractNote={We report the epitaxial integration of defect-induced room temperature ferromagnetic insulators, Cr2O3 and MgO, with topological insulators Bi2Se3 on c-sapphire substrate by pulsed laser deposition. The structural, magnetic, and magnetotransport properties of ∼15 nm Bi2Se3 thin films are investigated on each template. The lattice misfits of Cr2O3/Bi2Se3 and MgO/Bi2Se3 are ∼16% and ∼39%, respectively, where the critical thickness for pseudomorphic growth is less than one monolayer. The insulating behavior is more pronounced due to the additional scattering of the surface states of the Bi2Se3 layer by interfacing with MgO and Cr2O3. The weak antilocalization effect from the surface states is clearly suppressed, accounting for the presence of magnetic bottom layers. This work demonstrates an effective way to study the emergence of a ferromagnetic phase in topological insulators by the magnetic proximity effect in Bi2Se3, a step toward unveiling their exotic properties.}, number={12}, journal={Journal of Applied Physics}, publisher={AIP Publishing}, author={Lee, Y. F. and Kumar, R. and Hunte, F. and Narayan, J. and Schwartz, J.}, year={2015}, month={Sep}, pages={125309} }
 @article{punugupati_narayan_hunte_2015, title={Room temperature ferromagnetism in epitaxial Cr2O3 thin films grown on r-sapphire}, volume={117}, ISSN={0021-8979 1089-7550}, url={http://dx.doi.org/10.1063/1.4921435}, DOI={10.1063/1.4921435}, abstractNote={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 (011¯2) of Cr2O3 grows on r-plane of sapphire. The epitaxial relations can be written as [011¯2] Cr2O3 ‖ [011¯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.}, number={19}, journal={Journal of Applied Physics}, publisher={AIP Publishing}, author={Punugupati, Sandhyarani and Narayan, Jagdish and Hunte, Frank}, year={2015}, month={May}, pages={193907} }
 @article{singamaneni_punugupati_prater_hunte_narayan_2014, title={Ferroelectric and ferromagnetic properties in BaTiO3 thin films on Si (100)}, volume={116}, ISSN={0021-8979 1089-7550}, url={http://dx.doi.org/10.1063/1.4894508}, DOI={10.1063/1.4894508}, abstractNote={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.}, number={9}, journal={Journal of Applied Physics}, publisher={AIP Publishing}, author={Singamaneni, Srinivasa Rao and Punugupati, Sandhyarani and Prater, John T. and Hunte, Frank and Narayan, Jagdish}, year={2014}, month={Sep}, pages={094103} }
 @article{wang_hewitt_kumar_boltersdorf_guan_hunte_maggard_brom_redwing_dougherty_2014, title={Molecular Doping Control at a Topological Insulator Surface: F-4-TCNQ on Bi2Se3}, volume={118}, ISSN={["1932-7447"]}, DOI={10.1021/jp412690h}, abstractNote={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.}, number={27}, journal={JOURNAL OF PHYSICAL CHEMISTRY C}, author={Wang, J. and Hewitt, A. S. and Kumar, R. and Boltersdorf, J. and Guan, T. and Hunte, F. and Maggard, P. A. and Brom, J. E. and Redwing, J. M. and Dougherty, D. B.}, year={2014}, month={Jul}, pages={14860–14865} }
 @article{punugupati_narayan_hunte_2014, title={Strain induced ferromagnetism in epitaxial Cr2O3 thin films integrated on Si(001)}, volume={105}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.4896975}, DOI={10.1063/1.4896975}, abstractNote={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.}, number={13}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Punugupati, Sandhyarani and Narayan, Jagdish and Hunte, Frank}, year={2014}, month={Sep}, pages={132401} }
 @article{punugupati_temizer_narayan_hunte_2014, title={Structural and resistance switching properties of epitaxial Pt/ZnO/TiN/Si(001) heterostructures}, volume={115}, ISSN={0021-8979 1089-7550}, url={http://dx.doi.org/10.1063/1.4883959}, DOI={10.1063/1.4883959}, abstractNote={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.}, number={23}, journal={Journal of Applied Physics}, publisher={AIP Publishing}, author={Punugupati, Sandhyarani and Temizer, Namik K. and Narayan, Jagdish and Hunte, Frank}, year={2014}, month={Jun}, pages={234501} }
 @article{ye_hunte_schwartz_2013, title={Effects of high magnetic field on the quench behavior of Bi2Sr2CaCu2Ox coils at 4.2 K}, volume={26}, ISSN={0953-2048 1361-6668}, url={http://dx.doi.org/10.1088/0953-2048/26/5/055006}, DOI={10.1088/0953-2048/26/5/055006}, abstractNote={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.}, number={5}, journal={Superconductor Science and Technology}, publisher={IOP Publishing}, author={Ye, Liyang and Hunte, Frank and Schwartz, Justin}, year={2013}, month={Mar}, pages={055006} }
 @article{ishmael_luo_white_hunte_liu_mandzy_muth_naderi_ye_hunt_et al._2013, title={Enhanced Quench Propagation in Bi2Sr2CaCu2Ox and YBa2Cu3O7-x Coils via a Nanoscale Doped-Titania-Based Thermally Conducting Electrical Insulator}, volume={23}, ISSN={["1558-2515"]}, DOI={10.1109/tasc.2013.2269535}, abstractNote={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.}, number={5}, journal={IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY}, author={Ishmael, Sasha and Luo, Haojun and White, Marvis and Hunte, Frank and Liu, X. T. and Mandzy, Natalia and Muth, John F. and Naderi, Golsa and Ye, Liyang and Hunt, Andrew T. and et al.}, year={2013}, month={Oct} }
 @article{lee_wu_kumar_hunte_schwartz_narayan_2013, title={Epitaxial integration of dilute magnetic semiconductor Sr3SnO with Si (001)}, volume={103}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.4820770}, DOI={10.1063/1.4820770}, abstractNote={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.}, number={11}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Lee, Y. F. and Wu, F. and Kumar, R. and Hunte, F. and Schwartz, J. and Narayan, J.}, year={2013}, month={Sep}, pages={112101} }
 @article{kajbafvala_nachtrab_kumar_hunte_wong_schwartz_2013, title={High strength oxide dispersion strengthened silver aluminum alloys optimized for Bi2Sr2CaCu2O8+x round wire}, volume={26}, ISSN={["1361-6668"]}, DOI={10.1088/0953-2048/26/12/125012}, abstractNote={High strength dispersion strengthened (DS) Ag/Al alloys with various Al content are studied as candidates for sheathing Bi2Sr2CaCu2O8+x (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.}, number={12}, journal={SUPERCONDUCTOR SCIENCE & TECHNOLOGY}, author={Kajbafvala, Amir and Nachtrab, William and Kumar, Raj and Hunte, Frank and Wong, Terence and Schwartz, Justin}, year={2013}, month={Dec} }
 @article{ye_cruciani_effio_hunte_schwartz_2013, title={On the Causes of Degradation in Bi2Sr2CaCu2O8+x Round Wires and Coils by Quenching at 4.2 K}, volume={23}, ISSN={["1558-2515"]}, DOI={10.1109/tasc.2013.2271255}, abstractNote={One of the remaining challenges for the implementation of commercial Ag-alloy-sheathed $\hbox{Bi}_{2}\hbox{Sr}_{2}\hbox{CaCu}_{2}\hbox{O}_{8+{\rm x}}$ (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 $\hbox{Nb}_{3} \hbox{Sn}$, 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.}, number={5}, journal={IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY}, author={Ye, Liyang and Cruciani, Davide and Effio, Timothy and Hunte, Frank and Schwartz, Justin}, year={2013}, month={Oct} }
 @article{kajbafvala_nachtrab_lu_hunte_liu_cheggour_wong_schwartz_2012, title={Dispersion-Strengthened Silver Alumina for Sheathing Bi2Sr2CaCu2O8+x Multifilamentary Wire}, volume={22}, ISSN={["1558-2515"]}, DOI={10.1109/tasc.2011.2179296}, abstractNote={High-strength high-elastic-modulus dispersion-strengthened (DS) silver aluminum alloys are studied for sheathing Bi2Sr2CaCu2O8 + 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 Ic (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.}, number={1}, journal={IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY}, author={Kajbafvala, Amir and Nachtrab, William and Lu, Xi Feng and Hunte, Frank and Liu, Xiaotao and Cheggour, Najib and Wong, Terence and Schwartz, Justin}, year={2012}, month={Feb} }
 @article{song_hunte_schwartz_2012, title={On the role of pre-existing defects and magnetic flux avalanches in the degradation of YBa2Cu3O7-x coated conductors by quenching}, volume={60}, ISSN={["1873-2453"]}, DOI={10.1016/j.actamat.2012.09.003}, abstractNote={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.}, number={20}, journal={ACTA MATERIALIA}, author={Song, Honghai and Hunte, Frank and Schwartz, Justin}, year={2012}, month={Dec}, pages={6991–7000} }
 @article{zhu_hunte_zhuang_feng_gan_xi_larbalestier_voyles_2010, title={MgO platelets and high critical field in MgB2thin films doped with carbon from methane}, volume={23}, ISSN={0953-2048 1361-6668}, url={http://dx.doi.org/10.1088/0953-2048/23/4/049801}, DOI={10.1088/0953-2048/23/4/049801}, abstractNote={The full text of the corrigendum is given in the PDF file.}, number={4}, journal={Superconductor Science and Technology}, publisher={IOP Publishing}, author={Zhu, Y and Hunte, F and Zhuang, C G and Feng, Q R and Gan, Z Z and Xi, X X and Larbalestier, D C and Voyles, P M}, year={2010}, month={Mar}, pages={049801–049801} }
 @article{zhu_pogrebnyakov_wilke_chen_xi_redwing_zhuang_feng_gan_singh_et al._2010, title={Nanoscale disorder in pure and doped MgB2thin films}, volume={23}, ISSN={0953-2048 1361-6668}, url={http://dx.doi.org/10.1088/0953-2048/23/9/095008}, DOI={10.1088/0953-2048/23/9/095008}, abstractNote={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.}, number={9}, journal={Superconductor Science and Technology}, publisher={IOP Publishing}, author={Zhu, Y and Pogrebnyakov, A V and Wilke, R H and Chen, K and Xi, X X and Redwing, J M and Zhuang, C G and Feng, Q R and Gan, Z Z and Singh, R K and et al.}, year={2010}, month={Aug}, pages={095008} }
 @article{zhu_hunte_zhuang_feng_gan_xi_larbalestier_voyles_2009, title={MgO platelets and high critical field in MgB2thin films doped with carbon from methane}, volume={22}, ISSN={0953-2048 1361-6668}, url={http://dx.doi.org/10.1088/0953-2048/22/12/125001}, DOI={10.1088/0953-2048/22/12/125001}, abstractNote={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.}, number={12}, journal={Superconductor Science and Technology}, publisher={IOP Publishing}, author={Zhu, Y and Hunte, F and Zhuang, C G and Feng, Q R and Gan, Z Z and Xi, X X and Larbalestier, D C and Voyles, P M}, year={2009}, month={Oct}, pages={125001} }
 @article{shen_gandikota_singh_hunte_jaroszynski_larbalestier_rowell_newman_2008, title={A novel technique for synthesizing MgB2thin films with high upper critical fields}, volume={21}, ISSN={0953-2048 1361-6668}, url={http://dx.doi.org/10.1088/0953-2048/21/8/085009}, DOI={10.1088/0953-2048/21/8/085009}, abstractNote={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|.}, number={8}, journal={Superconductor Science and Technology}, publisher={IOP Publishing}, author={Shen, Y and Gandikota, R and Singh, R K and Hunte, F L and Jaroszynski, J and Larbalestier, D C and Rowell, J M and Newman, N}, year={2008}, month={May}, pages={085009} }
 @article{yamamoto_jiang_tarantini_craig_polyanskii_kametani_hunte_jaroszynski_hellstrom_larbalestier_et al._2008, title={Evidence for electromagnetic granularity in the polycrystalline iron-based superconductor LaO0.89F0.11FeAs}, volume={92}, ISSN={0003-6951 1077-3118}, url={http://dx.doi.org/10.1063/1.2952195}, DOI={10.1063/1.2952195}, abstractNote={The recently discovered oxypnictide 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.}, number={25}, journal={Applied Physics Letters}, publisher={AIP Publishing}, author={Yamamoto, A. and Jiang, J. and Tarantini, C. and Craig, N. and Polyanskii, A. A. and Kametani, F. and Hunte, F. and Jaroszynski, J. and Hellstrom, E. E. and Larbalestier, D. C. and et al.}, year={2008}, month={Jun}, pages={252501} }
 @article{yamamoto_polyanskii_jiang_kametani_tarantini_hunte_jaroszynski_hellstrom_lee_gurevich_et al._2008, title={Evidence for two distinct scales of current flow in polycrystalline Sm and Nd iron oxypnictides}, volume={21}, ISSN={0953-2048 1361-6668}, url={http://dx.doi.org/10.1088/0953-2048/21/9/095008}, DOI={10.1088/0953-2048/21/9/095008}, abstractNote={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.}, number={9}, journal={Superconductor Science and Technology}, publisher={IOP Publishing}, author={Yamamoto, A and Polyanskii, A A and Jiang, J and Kametani, F and Tarantini, C and Hunte, F and Jaroszynski, J and Hellstrom, E E and Lee, P J and Gurevich, A and et al.}, year={2008}, month={Jul}, pages={095008} }
 @article{hunte_jaroszynski_gurevich_larbalestier_jin_sefat_mcguire_sales_christen_mandrus_2008, title={Two-band superconductivity in LaFeAsO0.89F0.11 at very high magnetic fields}, volume={453}, ISSN={0028-0836 1476-4687}, url={http://dx.doi.org/10.1038/nature07058}, DOI={10.1038/nature07058}, abstractNote={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.}, number={7197}, journal={Nature}, publisher={Springer Science and Business Media LLC}, author={Hunte, F. and Jaroszynski, J. and Gurevich, A. and Larbalestier, D. C. and Jin, R. and Sefat, A. S. and McGuire, M. A. and Sales, B. C. and Christen, D. K. and Mandrus, D.}, year={2008}, month={May}, pages={903–905} }
 @article{venus_hunte_dan dahlberg_2005, title={Contribution of low-temperature degrees of freedom to the anisotropy in Co/CoO exchange coupled bilayers}, volume={286}, ISSN={0304-8853}, url={http://dx.doi.org/10.1016/j.jmmm.2004.09.038}, DOI={10.1016/j.jmmm.2004.09.038}, abstractNote={Abstract Measurements of the transverse magnetic AC-susceptibility χ ( T ) of exchange coupled polycrystalline Co/CoO bilayers have been made via the anisotropic magnetoresistance using a small AC field of 2–5 Oe and frequencies from 40 to 400 Hz. A step increase in Re χ is observed at 50–100 K, with an accompanying dissipation peak in Im χ . This feature is independent of the CoO thickness, but its dispersion with frequency suggests a thermally activated degree of freedom with an activation energy Ea∼150 K. This is consistent with a contribution to the anisotropy field due to the dynamics of the ferromagnetic rather than the antiferromagnetic grains.}, journal={Journal of Magnetism and Magnetic Materials}, publisher={Elsevier BV}, author={Venus, D. and Hunte, F. and Dan Dahlberg, E.}, year={2005}, month={Feb}, pages={191–195} }