J. S. Higgins

University of Maryland, College Park, Maryland, United States

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Publications (32)67.42 Total impact

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    ABSTRACT: The properties of electron(n)-doped cuprate superconductors show significant deviations from those of their hold(p)-doped counterparts. Experiments prior to 2000 suggested an s-wave pairing symmetry as opposed to d-wave pairing symmetry in hole-doped cuprates. Recent experiments have suggested that n-doped cuprates have a d-wave pairing symmetry. However tunneling spectroscopy of these materials have not revealed a zero bias conductance peak (ZBCP), which is a classic signature of d-wave symmetry. We present the first tunneling spectroscopy data on n-doped Pr2-xCexCuO4 (PCCO) using point contact junctions which show a systematic evolution of the ZBCP. This method of junction fabrication is important as it allows the barrier strength between the normal and the superconducting electrodes to be varied. We show that this is essential to observing the ZBCP. The n-doped cuprates have a low Tc (~25 K) and Hc2 (~10 T). The low Hc2 enables us to obtain the normal state in PCCO at low temperatures. We have used this to probe the density of states in the normal state of PCCO. We observe an anomalous gap even in the normal state.1 This normal state gap (NSG) becomes smaller on the over-doped side. We discuss the behavior of this NSG in the context of the pseudogap which has been observed in hole-doped cuprates.
    International Journal of Modern Physics B 01/2012; 16(20n22). · 0.46 Impact Factor
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    ABSTRACT: One of the outstanding issues in the electron doped cuprates is the role that oxygenation plays in the superconductivity and normal state properties. Oxygen addition can be considered both a doping and a disordering process. To disentangle these two effects, disorder can be introduced by irradiating the samples without altering doping. We report transport studies on optimal and underdoped Pr2-xCexCuO4-y films subject to proton irradiation and oxygenation. We establish a correlation between the static AFM and the metal-insulator crossover. Our separation of the disorder and doping effects also shed light on oxygen reduction effects in electron-doped cuprates. Supported by NSF grant DMR-0653535.
    03/2008;
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    W Yu, J. S. Higgins, P. Bach, R. L. Greene
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    ABSTRACT: We present magnetotransport evidence for antiferromagnetism in films of the electron-doped cuprates Pr$_{2-x}$Ce$_x$CuO$_4$. Our results show clear signature of static antiferromagnetism up to optimal doping x=0.15, with a quantum phase transition close to x=0.16, and a coexistence of static antiferromagnetism and superconductivity for 0.12$\le$x$\le$0.15.
    06/2007;
  • W. Yu, J. S. Higgins, P. Bach, R. L. Greene
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    ABSTRACT: We present magnetotransport evidence for antiferromagnetism in films of the electron-doped cuprates Pr2-xCexCuO4 . Our results show clear signature of static or quasistatic antiferromagnetism up to optimal doping x=0.15 , with a quantum phase transition close to x=0.16 , and a coexistence of antiferromagnetism and superconductivity for 0.12⩽x⩽0.15 .
    Physical review. B, Condensed matter 06/2007; 76(2):20503. · 3.77 Impact Factor
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    ABSTRACT: We performed a systematic study of the magnetic properties and the Hall effect on 5% cobalt doped (La, Sr)TiO3 thin films grown by pulsed laser deposition. The superparamagnetic nature of the system is established by several protocols of magnetic measurements. Nevertheless, the anomalous Hall effect (AHE) is observed in the system, the Hall resistivity vs magnetic field loops being found to be identical to the magnetic hysterisis loops. This once again (Phys. Rev. Lett. 92, 166601 (2004)) highlights the limitations of AHE as a tool to test the intrinsic nature of ferromagnetism in a diluted magnetic system. Possible reasons for the origin of the AHE in our system are discussed.
    03/2007;
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    ABSTRACT: We are using exchange bias at ferromagnet layer/multiferroic interfaces to study the nature of magnetism in multiferroic materials. Co 5 nm layers have been deposited by sputtering on surfaces of epitaxial BiFeO3 and TbMnO3 thin films and on LuMnO3 single crystals. Epitaxial BiFeO3 and TbMnO3 films were prepared by PLD. Magnetic properties of the Co/multiferroic bilayers are measured using SQUID, VSM, MOKE and XMCD. In BiFeO3, we find that the bilayers exhibit exchange bias even at room temperature. In the TbMnO3 system, increasing of coercive field and exchange bias was also clearly observed below the N'eel temperature. In LuMnO3, we observe positive exchange bias as well as switching of the sign of the exchange bias depending on the cooling procedure. This behavior may be related to the frustration in Mn spins. Difference in the exchange bias behavior between different multiferroic materials will be discussed. The effect of electric field on exchange bias is currently under investigation. Supported by ONR N000140110761, ONR N000140410085, NSF DMR 0094265, DMR 0231291, MRSEC DMR-00-0520471, and the W. M. Keck Foundation.
    03/2007;
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    ABSTRACT: A systematic study of the magnetic properties and the Hall effect was performed on pulsed laser deposited 5% cobalt doped (La,Sr)TiO3 thin films, especially grown at high substrate temperature. The system is found to be superparamagnetic in nature as evidenced by several protocols of magnetic measurements. Nevertheless, the anomalous Hall effect (AHE) is observed in the system, the profile of the measured Hall resistivity vs magnetic field being found to be identical to the magnetic hysteresis loops. This highlights the limitations of AHE as a tool to test the intrinsic nature of ferromagnetism in a diluted magnetic system, supporting our previous report for the Co:TiO2 case [ S. R. Shinde et al. Phys. Rev. Lett. 92 166601 (2004)]. It is believed that the magnetic clusters polarize nearby electrons and the nonzero polarization leads to a net transverse current because of the spin dependent scattering, which gives rise to the observed AHE. We found that the magnitude of the AHE signal observed in the current extrinsic diluted magnetic semiconductor (DMS) is much lower (by a few orders of magnitude) than that found in the intrinsic long range ferromagnetic ordered DMS, which raises the possibility for using this magnitude, rather than the occurrence of AHE, as a criterion for intrinsic or extrinsic diluted magnetic system.
    Physical review. B, Condensed matter 01/2007; 76(8). · 3.77 Impact Factor
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    ABSTRACT: We present a study on the effects of changing the oxygen content in the electron-doped superconducting cuprate Pr2-xCexCuO4±delta (PCCO). Epitaxial, c-axis oriented, overdoped (x = 0.17) thin films were grown using a pulsed laser deposition technique, and the oxygen content was adjusted during a post-growth annealing process. In addition to the transition temperature (Tc), measurements of the Hall effect and resistivity were performed at low temperatures (T < Tc, H > HC2) in several films of different oxygen content. We compare the disorder observed in these oxygenated samples with disorder induced by proton irradiation in an optimally annealed (x = 0.17) film. An analysis of the data demonstrates that a change in the oxygen content of PCCO has two separable effects: 1) a disorder effect, and 2) a doping effect similar to that of cerium.
    03/2006;
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    ABSTRACT: Niobium doping is shown to have a significant effect on the ferromagnetism and microstructure of dilutely cobalt-doped anatase TiO2 films. Epitaxial films of anatase TiO2 with 3% Co, with and without 1% niobium doping were grown by pulsed-laser deposition at 875 ^0C at different oxygen pressures. For growth at 10-5 Torr Niobium doping suppresses ferromagnetism, while it enhances the same in films grown at 10-4 Torr. The results of High-resolution Z-contrast Scanning Transmission Electron Microscopy and Electron Energy Loss Spectroscopy study reveal uniform surface enrichment of cobalt in the form of Ti1-x-yCoxNbyO2-deltag phase, without cobalt metal clusters. The transport and Hall effect results will also be presented and discussed.
    03/2006;
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    ABSTRACT: Among the half metallic ferromagnets, Fe3O4 is of particular interest because of its half metallicity, high curie temperature and a charge ordering transition at 120K (popularly known as Verwey transition (TV)). These materials are also expected to show 100% spin polarization. In view of these fascinating properties, we studied temperature dependent transport, magnetic, structural and interface characteristics of epitaxial schottky junctions between Fe3O4 and Nb:SrTiO3 (with different Nb concentrations). Epitaxial thin films of Fe3O4 were grown on Nb:SrTiO3 substrates by PLD technique. The films show epitaxial growth along (100)-axis direction. We also performed HR-TEM and EELS study to ensure a better quality interface. In the temperature range above TV, 300K-130K, the I-V characteristic shifts towards higher forward bias voltage upon lowering temperature. On further decreasing temperature (below TV), the trend is reversed. Junction parameters such as the Schottky barrier height (phiB) and ideality factor (eta) are extracted using thermionic emission theory at all temperatures. These parameters show interesting and systematic trends above and below TV. From the magnetic field dependence of non linear I-V characteristics data, a spin polarization of ˜80% is estimated for the magnetite electrode at TV.
    03/2006;
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    ABSTRACT: We report on resistivity and Hall measurements in thin films of the electron-doped superconducting cuprate Pr$_{2-x}$Ce$_{x}$CuO$_{4\pm\delta}$. Comparisons between x = 0.17 samples subjected to either ion-irradiation or oxygenation demonstrate that changing the oxygen content has two separable effects: 1) a doping effect similar to that of cerium, and 2) a disorder effect. These results are consistent with prior speculations that apical oxygen removal is necessary to achieve superconductivity in this compound. Comment: 5 pages, 5 figures
    Physical Review B 02/2006; · 3.66 Impact Factor
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    ABSTRACT: Thin films of (0.5%, 1%) Nb:SrTiO3 dilutely doped with (2 at. %) magnetic transition elements (Cr, Co, Fe, Mn) are examined for ferromagnetism. X-ray diffraction, Rutherford backscattering ion channeling, scanning transmission electron microscopy Z-contrast imaging, and electron energy loss spectroscopy techniques establish high crystalline quality of the films with no impurity phase(s) and highly uniform dopant distribution. Although the film conductivity improves dramatically by Nb doping, no ferromagnetism is found in any of our samples over the temperature range of 365 down to 5 K. This is contrasted to the case of ferromagnetism reported in cobalt doped (La,Sr)TiO3.
    Applied Physics Letters 01/2006; 89. · 3.79 Impact Factor
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    ABSTRACT: The temperature dependent Schottky diode characteristics of epitaxial junctions between Nb:SrTiO3 (Nb concentrations: 0.5%) and Fe3O4 are studied. Epitaxial thin films of Fe3O4 were grown on Nb:SrTiO3 substrates by pulsed laser deposition technique. The films and heterointerfaces were characterized by x-ray diffraction, Z-contrast transmission electron microscopy, magnetic susceptibility, four-probe in-plane resistivity, and the temperature dependent junction current-voltage (I-V) characteristics. The nonlinear nature of the characteristics is analyzed within the framework of thermionic emission theory. Junction parameters such as the Schottky barrier height (phiB) and ideality factor (ε) are extracted. The temperature evolution of these parameters shows interesting and systematic trends, with remarkable changes near the Verwey transition (TV=120 K). The magnetic field dependence of I-V characteristic data is also recorded and a spin polarization of ~80% is estimated for the magnetite electrode.
    Journal of Applied Physics 01/2006; 99. · 2.21 Impact Factor
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    ABSTRACT: We report a detailed magnetic and structural analysis of highly reduced Co doped rutile TiO(2-delta) films displaying an anomalous Hall effect (AHE). The temperature and field dependence of magnetization, and transmission electron microscopy, clearly establish the presence of nanosized superparamagnetic cobalt clusters of approximately 8-10 nm size in the films at the interface. The co-occurrence of superparamagnetism and AHE raises questions regarding the use of the AHE as a test of the intrinsic nature of ferromagnetism in diluted magnetic semiconductors.
    Physical Review Letters 05/2004; 92(16):166601. · 7.73 Impact Factor
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    ABSTRACT: The effect of 200 MeV Ag ion irradiation on the temperature and field dependence of critical current density (JC) of high quality MgB2 thin films is studied. Substantial increase in JC is observed over a certain field range for the film irradiated at a dose of 1012 ions/cm2. Our analysis suggests that columnar defects are not formed under irradiation conditions used in these studies, which correspond to an electronic energy loss of about 16 keV/nm. Defects clusters are likely to be responsible for the observed improvement in JC.© 2004 American Institute of Physics.
    Applied Physics Letters 03/2004; 84(13):2352-2354. · 3.79 Impact Factor
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    ABSTRACT: In recent years, ferromagnetic properties have been demonstrated above room temperature in the cobalt-doped semiconductor TiO_2. While a room temperature ferromagnetic semiconductor is important for spintronics, it is important to determine if the observed ferromagnetism is intrinsic or extrinsic (i.e. due to cobalt clusters). Here we report on Hall effect measurements in thin films of Co:TiO2 [1]. We find films with a low carrier concentration (10^18 / cm^3 -- 10^19 / cm^3) yield a linear behavior in the Hall data while those with a higher carrier concentration (10^21 / cm^3 -- 10^22 / cm^3 ) display anomalous behavior near zero magnetic field, suggestive of ferromagnetism. However, in the films that show an anomalous Hall effect, our study of the structural and magnetic properties indicates the presence of cobalt clusters. This suggests that the ferromagnetism in these films is not intrinsic.
    01/2004;
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    ABSTRACT: Thin films of (La0.6Pr0.4)0.7Ca0.3MnO3 were grown on NdGaO3(110) and LaAlO3(001) substrates by pulsed laser deposition. The substrate temperature was held at 820 °C, and the oxygen pressure during deposition and cooling was controlled from a few mTorr to 400 mTorr. X-ray diffraction data show an interesting structural evolution with oxygen deficiency, which was estimated by non-Rutherford elastic resonance scattering of He+ ions at 3.045 MeV. With increase in oxygen deficiency the insulator-metal transition temperature (Tp) shows a significant shift to lower temperature and the transition becomes much sharper and hysteretic, with a greatly enhanced temperature coefficient of resistance (TCR). Low field magnetotransport also shows peculiar characteristics. These results bring out the tendency towards increasingly first order character with oxygen deficiency, and are in clear contrast to those in oxygen deficient La0.7Ca0.3MnO3 films, wherein the TCR is seen to decrease with decreasing Tp. The observations are discussed within the framework of the phase separation picture.
    Physical Review B 01/2004; 69(23). · 3.66 Impact Factor
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    ABSTRACT: We report Hall effect measurements on thin films of cobalt-doped TiO2-delta. Films with a low carrier concentration (1018 1019/cm3) yield a linear behavior in the Hall data while those having a higher carrier concentration (1021 1022/cm3) display anomalous behavior near zero field. In the entire range of carrier concentrations, n-type conduction is observed.
    Physical Review B 01/2004; 69. · 3.66 Impact Factor
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    ABSTRACT: Ferromagnetism is observed at and above room temperature in pulsed laser deposited epitaxial films of Co-doped Ti-based oxide perovskite ( La <sub>0.5</sub> Sr <sub>0.5</sub> TiO <sub>3-δ</sub>). The system has the characteristics of an intrinsic diluted magnetic semiconductor (metal) at low concentrations (≪∼2%), but develops inhomogeneity at higher cobalt concentrations. The films range from being opaque metallic to transparent semiconducting depending on the oxygen pressure during growth and are yet ferromagnetic. © 2003 American Institute of Physics.
    Applied Physics Letters 10/2003; · 3.79 Impact Factor
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    ABSTRACT: The occurrence of room temperature ferromagnetism is demonstrated in pulsed laser deposited thin films of Sn(1-x)Co(x)O(2-delta) (x<0.3). Interestingly, films of Sn(0.95)Co(0.05)O(2-delta) grown on R-plane sapphire not only exhibit ferromagnetism with a Curie temperature close to 650 K, but also a giant magnetic moment of 7.5+/-0.5 micro(B)/Co, not yet reported in any diluted magnetic semiconductor system. The films are semiconducting and optically highly transparent.
    Physical Review Letters 08/2003; 91(7):077205. · 7.73 Impact Factor