Gustavo A Alvarez

Synthesis of SmFeAsO by an easy and versatile route and its physical property characterization

We report synthesis, structure, electrical transport, and heat capacity of SmFeAsO. The title compound is synthesized by one-step encapsulation of stoichiometric FeAs, Sm, and Sm2O3 in an evacuated (10−5 Torr) quartz tube by prolong (72 h) annealing at 1100 °C. The as-synthesized compound is crystallized in tetragonal structure with P4/nmm space group having lattice parameters a=3.937 26(33) A and c=8.498 02(07) A. The resistance (R-T) measurements on the compound exhibited ground state spin-density-wave (SDW)-like metallic steps below 140 K. Heat capacity CP(T) measurements on the title compound, showed an anomaly at around 140 K, which is reminiscent of the SDW ordering of the compound. At lower temperatures the CP(T) shows a clear peak at around 4.5 K. At lower temperature below 20 K, Cp(T) is also measured under an applied field of 7 T. It is concluded that the CP(T) peak at 4.5 K is due to the antiferromagnetic ordering of Sm3+ spins. These results are in confirmation with ordering of Sm in Sm2−xCexCuO4.

Characteristics of NdBa2Cu3O7-δ/PrBa2Cu3O7-δ/NdBa2Cu3O7-δ Planar Josephson Junctions

We report on the well-characterized properties of a-axis oriented trilayer Josephson junctions made of NdBa2Cu3O7-δ and PrBa2(Cu,Co)3O7-δ films, and c-axis oriented trilayer Josephson junctions made of NdBa2Cu3O7-δ and PrBa2Cu3O7-δ films with smooth surface morphology and good crystallinity. a-Axis oriented NdBa2Cu3O7-δ/PrBa2(Cu,Co)3O7-δ/NdBa2Cu3O7-δ trilayers and c-axis oriented NdBa2Cu3O7-δ/PrBa2Cu3O7-δ/NdBa2Cu3O7-δ trilayers were prepared on (100) SrTiO3 substrates to obtain planar junctions. Their current–voltage characteristics are of the resistively-shunted-junction type at 4.2 K. They showed well-developed Shapiro steps corresponding to 5–20 GHz microwave irradiation. Moreover, the power dependences of the maximum amplitudes of the Shapiro steps were clearly obtained and consistent with the theoretical values expressed as Jn(2eV/hf). The magnetic field dependence of the Josephson current for a-axis oriented trilayer Josephson junctions showed a Fraunhofer-like pattern, suggesting the spatial uniformity of the critical current flowing over the PrBa2(Cu,Co)3O7-δ barrier layer.

Magnetic glassy behavior in ferroelectric relaxor type solid solutions: Magnetoelectric relaxor

Perovskite solid solution ceramics with compositions of 0.9Pb(Fe0.5Nb0.5)O3–0.1PbTiO3, 0.6Pb(Ni1∕3Nb2∕3)O3–0.4PbTiO3, and 0.6Pb(Co1∕3Nb2∕3)O3–0.4PbTiO3 were synthesized by the traditional solid state reaction method. Ferroelectric measurements revealed that these samples have well saturated polarization-electrical field loops. Dielectric measurements showed that abnormal dielectric peaks at their Curie temperature were frequency dependent. Both characteristics indicate that these samples are relaxor type ferroelectric materials. Field cooled and zero field cooled magnetization measurements revealed that the 0.6Pb(Ni1∕3Nb2∕3)O3–0.4PbTiO3 and 0.6Pb(Co1∕3Nb2∕3)O3–0.4PbTiO3 samples are paramagnetic down to 5K, while the 0.9Pb(Fe0.5Nb0.5)O3–0.1PbTiO3 sample shows an antiferromagnetic-like ordering starting from around 40K. Furthermore, a weak ferromagnetism is observed in the 0.9Pb(Fe0.5Nb0.5)O3–0.1PbTiO3 sample, as evidenced by the magnetic hysteresis loop measured at 10K. The ac susceptibility measurement of...

Magnetotransport and magnetic properties of weak ferromagnetic semiconductors : Ca doped LaCrO3

We report the magnetic and magnetotransport properties of Ca doped La1−xCaxCrO3 (x=0, 0.1, 0.2, and 0.3). Both the magnetic and transport properties are very sensitive to Ca substitution. The Neel temperature TN decreases substantially with increasing calcium doping concentration from 290to160K. A weak ferromagnetic state with large coercive fields of up to 3T is present for the Ca doped LaCrO3. The temperature dependence of the resistivity shows that all the Ca doped compounds are semiconducting and their resistivities decrease with increasing Ca for low doping levels. The resistivity curves show thermally activated behavior and a variable range hopping behavior at high temperatures. The magnetotransport measurements show a negative magnetoresistance. Furthermore, an anomalous peak was observed in the field dependence of magnetoresistance for the LCCO materials.

Superconductivity of various borides: The role of stretched c-parameter

The superconductivity of MgB2, AlB2, NbB2+x, and TaB2+x is intercompared. The stretched c-lattice parameter (c=3.52 A) of MgB2 in comparison to NbB2.4 (c=3.32 A) and AlB2 (c=3.25 A) decides empirically the population of their π and σ bands and as a result their transition temperature Tc values, respectively, at 39 and 9.5 K for the first two and no superconductivity for the later. The nonstoichiometry induces an increase in c parameter with Boron excess both in NbB2+x and TaB2+x. Magnetization (M-T) and resistivity measurements (ρ-T) in case of niobium boride samples show the absence of superconductivity in stoichiometric NbB2 sample (c=3.26 A) while a clear diamagnetic signal and a ρ=0 transition for boron excess NbB2+x samples. On the other hand, superconductivity is not achieved in TaB2+x case. The probable reason behind is the comparatively lesser or insufficient stretching of c parameter.

Subterahertz Josephson plasma emission in layered high-TC superconducting tunnel junctions

We investigated the emission of subterahertz frequency electromagnetic radiation from high-TC superconducting c-axis NdBa2Cu3O7−δ∕PrBa2Cu3O7−δ∕NdBa2Cu3O7−δ trilayer thin film tunneling junctions when external electric and magnetic fields are applied. The current-voltage characteristics under applied ab-plane magnetic fields H (up 8T) exhibit well defined steps, Vn, such that eVn≈hωp∕(2nπ), where the plasma frequency ωp≈0.4 THz and n=1,2,3,….. These steps may be interpreted using Josephson plasma dynamics. The applied voltage creates oscillating currents via the Josephson coupling energy EJ (EJ=hIJ∕4πe, where IJ is the Josephson current and h is Planck’s constant) and the charge energy EC (EC=e2∕2C, where C is the junction capacitance). Thus the Josephson plasma becomes excited by the tunneling current, with some of the energy being emitted as subterahertz frequency radiation. Our results provide a new insight into a solid-state quantum system with considerable potential for new solid-state terahertz emiss...

C-axis quasiparticle tunneling properties of planar NdBa2Cu3O7−δ/PrBa2Cu3O7−δ/NdBa2Cu3O7−δ quasi-homostructures☆

Abstract We report on high quality junctions fabricated from c-axis-oriented NBCO/PBCO/ NBCO multilayers. The current-voltage characteristics (CVC) exhibit supercurrent and quasi-particle tunneling that is commonly observed for superconductor-insulator-superconductor (SIS) junctions. Our results show the importance of using barrier layer materials of PBCO sandwiched between NBCO electrodes for the fabrication of electronic devices.

Improvement of the Ramp Edge Junction Properties by Interface Treatment

To modify the interface of ramp edge junction and improve current-voltage characteristics, we carried out ECR (electron cyclotron resonance) ion cleaning and annealing treatments to the surface of the ramp edge before depositing a barrier layer and counter YBCO film. The surface roughness of the ramp after the interface treatment under several conditions was investigated by Atomic Force Microscope and we fabricated the junctions using PBCO as a barrier material. The junctions, which were cleaned by ECR Ar and O2 ions and annealed under oxygen atmosphere, showed weak link characteristics, though the others showed flux flow ones. The roughness of the ramp surface for the junctions was smaller than the surface roughness after any other treatments. These results suggested that the ECR cleaning is effective to flatten the PBCO barrier on the ramp.

Preparation and characterization of a-axis oriented NdBa2Cu3O7-δ/PrBa2(Cu, Co)3O7-δ/NdBa2Cu3O7-δ planar junctions

Abstract We report on well characterized a -axis trilayers made of NdBa 2 Cu 3 O 7− δ and PrBa 2 (Cu,Co) 3 O 7− δ films with smooth surface morphology and high crystallinity. The thin films were grown on (100) SrTiO 3 substrates by rf+dc hybrid sputtering. X-ray diffraction and Rutherford backscattering spectrometry analyses provided evidence of well-equilibrated and relaxed crystal. Planar junctions were fabricated from these a -axis oriented NdBa 2 Cu 3 O 7− δ /PrBa 2 (Cu,Co) 3 O 7− δ /NdBa 2 Cu 3 O 7− δ trilayers. The current–voltage characteristics exhibited Josephson current. Well developed Shapiro steps for 7–20 GHz microwave irradiation were observed. The dependence of the maximum amplitude of the Josephson current under applied magnetic field made evident spatial variations of the critical current over the PrBa 2 (Cu,Co) 3 O 7− δ barrier layer. The current–voltage characteristics were qualitatively consistent with the resistively-shunted-junction model.

Spin wave scattering and interface magnetism in superconducting-ferromagnet-superconducting hybrid structures

In this work we report on interlayer spin-polarized transport studies carried out on YBa2Cu3O7-δ/La2/3Sr1/3MnO3/YBa2Cu3O7-δ thin film multilayers. Anisotropic YBa2Cu3O7-δ high-temperature superconductors (YBCO) and the manganite La2/3Sr1/3MnO3 (LSMO) are layered metallic systems where the c-axis interlayer current transport occurs via sequential tunneling of charge carriers. The temperature dependence of the resistance R(T) for different applied fields shows that these heterostructures are clearly influenced by both the insulator-to-metal transition of the LSMO layers at ∼320 K and the stripe fluctuations in the conductivity of the YBCO layers at the onset of electronic inhomogeneity at ∼210 K. Furthermore, the tunneling of spin-polarized charge carriers between the YBCO layers through LSMO showed sharp switching between the high and low resistance states, which we assume corresponds to antiparallel and parallel alignments of the magnetic (spin) moments of these hybrid structures. The investigation of the...