Ted Grant

Origin of magnetism in undoped TiO2 nanotubes

Magnetic properties of undoped anatase, rutile, and amorphous titanium dioxide (TiO2) nanotubes grown by electrochemical anodization were studied by superconducting quantum interference device (SQUID) and electron spin resonance (ESR) methods in the temperature range 1.8-300 K. All anatase, rutile, and amorphous TiO2 nanotubes were found to exhibit paramagnetic behaviors in the entire temperature range when tested with magnetic center concentrations of 6×10(17), 3×10(16), and 3 × 10(15) cm(-3), respectively. The diameter of the TiO2 nanotubes varied from 40-160 nm and has no significant effect on the magnetic properties observed. SQUID data showed strong nonlinear M-H relationships for anatase at low temperatures, and Arrot plot analysis suggested ferromagnetism in the sample with a Curie temperature T(C) ~ 6 K. However, ESR studies showed no evidence for long-distance magnetic ordering. ESR studies revealed two magnetic centers with g1 = 1.928 and g2 = 2.028 that were common to all samples. The resonance peak at g1 = 1.922 was ascribed to Ti(3+) cations centers resulting from oxygen vacancies, while the peak at g2 = 2.028 was ascribed to surface absorbents. The amorphous sample ESR spectrum contained additional resonance peaks with corresponding g values at 2.228, 1.873, and 1.715 that possibly resulted from the disordered nature of these samples.

Superconductivity in non-centrosymmetric ThCoC2

Superconductivity in compounds whose crystal structure lacks inversion symmetry are known to display intriguing properties that deviate from conventional BCS superconducting behavior. Here we report magnetization, resistivity, and heat capacity measurements on polycrystalline samples of ThCoC2, which has been reported to crystallize in the non-centrosymmetric CeNiC2 prototype structure, and show clear evidence of bulk superconductivity in ThCoC2 with a critical temperature of Tc = 2.65?K. From the specific heat data we find a Sommerfeld coefficient of ? = 8.38?mJ?mol?1?K?2 and a Debye temperature of ?D = 449?K. Interestingly the Hc2 superconducting phase diagram displays positive curvature, and the specific heat at low temperature deviates from conventional exponential temperature dependence, which is suggestive of possible unconventional superconducting behavior in ThCoC2, similar to that seen in the isostructural and isoelectronic non-centrosymmetric superconductor LaNiC2.

Methodology and search for superconductivity in the La–Si–C system

In this paper we describe a methodology for the search for new superconducting materials. This consists of a parallel synthesis of a highly inhomogeneous alloy which covers large areas of the metallurgical phase diagram combined with a fast, microwave-based method which allows non-superconducting portions of the sample to be discarded. Once an inhomogeneous sample containing a minority phase superconductor is identified, we revert to well-known thorough identification methods which include standard physical and structural methods. We show how a systematic structural study helps in avoiding misidentification of new superconducting materials when there are indications from other methods of new discoveries. These ideas are applied to the La?Si?C system which exhibits promising normal state properties which are sometimes correlated with superconductivity. Although this system shows indications for the presence of a new superconducting compound, the careful analysis described here shows that the superconductivity in this system can be attributed to intermediate binary and single phases of the system.

Magnetic properties of nearly stoichiometric CeAuBi2 heavy fermion compound

Motivated by the interesting magnetic anisotropy found in the heavy fermion family CeTX2 (T = transition metal and X = pnictogen), here, we study the novel parent compound CeAu1−xBi2−y by combining magnetization, pressure dependent electrical resistivity, and heat-capacity measurements. The magnetic properties of our nearly stoichiometric single crystal sample of CeAu1−xBi2−y (x = 0.92 and y = 1.6) revealed an antiferromagnetic ordering at TN = 12 K with an easy axis along the c-direction. The field dependent magnetization data at low temperatures reveal the existence of a spin-flop transition when the field is applied along the c-axis (Hc ∼ 7.5 T and T = 5 K). The heat capacity and pressure dependent resistivity data suggest that CeAu0.92Bi1.6 exhibits a weak heavy fermion behavior with strongly localized Ce3+ 4f electrons. Furthermore, the systematic analysis using a mean field model including anisotropic nearest-neighbors interactions and the tetragonal crystalline electric field (CEF) Hamiltonian allo...

Electron Spin resonance of Gd3+ in three dimensional topological insulator Bi2Se3

Bi2Se3 has been claimed to be a three dimensional topological insulator (TI) with topologically protected metallic surface states with exotic properties. We have performed electron spin resonance (ESR) measurements on Gd3+ doped (x ≈ 0.01) Bi2Se3 single crystal grown from stoichiometric melt. For the studied crystals, our preliminary results revealed a partly resolved Gd3+ fine structure spectrum with Dysonian (metallic character) lines. At room temperature, the central line has a g ≈ 1.98, a linewidth ΔH ≈ 95 G and the spectra have a overall splitting of roughly 1300 Oe. As the temperature is decreased, the Gd3+ ESR ΔH of the central line presents a very small Korringa-like behavior b =ΔH/ΔT ≈ 0.013 Oe/K and nearly T-independent g-value. However, for T ≲ 40 K, ΔH shows a stronger narrowing effect evolving to Korringa-like behavior (b ≈ 0.15 Oe/K) for T ≲ 30 K. Concomitantly with the change in ΔH behavior, the Gd3+ central line g value starts to decrease reaching a value of 1.976 at T =4.2 K. The ESR results are discussed in terms of possible effects of protected topological surface states enlightened by complementary data from macroscopic measurements.

Corrigendum: Methodology and search for superconductivity in the La–Si–C system

IOP P UBLISHING S UPERCONDUCTOR S CIENCE AND T ECHNOLOGY Supercond. Sci. Technol. 25 (2012) 049501 (1pp) doi:10.1088/0953-2048/25/4/049501 Corrigendum: Methodology and search for superconductivity in the La–Si–C system 2011 Supercond. Sci. Technol. 24 075017 J de la Venta 1,2 , Ali C Basaran 1,2,3 , T Grant 4 , A J S Machado 4,5 , M R Suchomel 6 , R T Weber 7 , Z Fisk 4 and Ivan K Schuller 1,2 Department of Physics, University of California San Diego, La Jolla, CA 92093, USA Center for Advanced Nanoscience, University of California San Diego, La Jolla, CA 92093, USA Materials Science and Engineering, University of California San Diego, La Jolla, CA 92093, USA Department of Physics and Astronomy, University of California Irvine, Irvine, CA 92697, USA EEL—University of S˜ao Paulo, SP 12600970, Brazil Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439, USA EPR Division Bruker BioSpin Corporation, Billerica, MA 01821-3931, USA E-mail: jdelaventa@physics.ucsd.edu Received 23 January 2012 Published 14 February 2012 Online at stacks.iop.org/SUST/25/049501 The correct figure 4(c) is presented below. This correction in the abscissa axis does not affect any other results or conclusions of the paper. We gratefully acknowledge Professor I Felner for pointing out this correction. Figure 4. (c) ZFC (−−) and FC (−N−) magnetization curves at 100 Oe for sample 3. c 2012 IOP Publishing Ltd Printed in the UK & the USA