G.C. Xiong

Nano-scale GeO2 wires synthesized by physical evaporation

Abstract Germanium dioxide nanowires (GeONW) have been synthesized by a physical evaporation method. The morphology, structure and chemical composition of the wires were characterized using transmission electron microscopy (TEM) and spectroscopy of energy dispersive X-ray fluorescence (EDX). TEM observations show that the GeONWs have a very high purity and diameter ranging from 15 to 80 nm. EDX analysis reveals that the GeONWs consist of Ge and O elements. Electron diffraction patterns taken on a single GeONW are spotty patterns of single crystals and are well indexed to a GeO 2 single crystal of hexagonal structure.

Comprehensive study of the resistance switching in SrTiO3 and Nb-doped SrTiO3

We have demonstrated that the resistance switching (RS) effect can be controlled by the modification of the electrode configurations and the carrier densities in the Ag/SrTiO3 and Ag/Nb-doped SrTiO3(Nb:STO) structures. The elimination of the Schottky junction in the metal/Nb:STO completely destroys the RS effect, which suggests that the RS effect originates from the modification of Schottky-like barrier formed at the interface of metal/Nb:STO. The rectifying I-V curves revealed that the change in resistance was attributed to the trapping or detrapping carriers at the interface. The carrier density plays an important role in the determination of RS effect. The presence of the RS in SrTiO3 requires an appropriate doping level to provide conditions for trapping carriers at the interface.

Trap-assisted tunneling resistance switching effect in CeO2/La0.7(Sr0.1Ca0.9)0.3MnO3 heterostructure

We reported the resistance switching (RS) behavior in the epitaxially grown CeO2/ La0.7(Sr0.1Ca0.9)0.3MnO3 (CeO2/LSCMO) heterojunctions on SrTiO3 substrate. The CeO2/LSCMO device displayed improved switching characteristics as compared to that of metal/manganite device. The switching threshold voltage showed a strong dependence on the thickness of the CeO2 layer, where a minimum/maximum thickness was required for the appearance of the resistance switching. Both set and reset threshold voltages increase with the increase of the CeO2 layer thickness due to the trap-assisted electron tunneling effect. In the meantime, the defects or vacancies in the CeO2 films, in particular, the concentration of the defects or vacancies in the interface between CeO2 and LSCMO, have a significant impact on the switching effect. These results suggest that the electron tunneling accompanied by a trapping/detrapping process at the interface is likely responsible for the RS effect in the insulator/manganite system.

Improved initial epitaxial growth of superconducting YBa2Cu3O7 thin films on Y-ZrO2 substrates with a La1.85Sr0.15CuO4 buffer layer

Abstract Epitaxial thin films of YBa 2 Cu 3 O 7 (YBCO) have been deposited on Y–ZrO 2 (YSZ) substrates by means of the pulse laser deposition technique. It has been found that the initial epitaxy of YBCO thin films grown on YSZ can be significantly improved by using La 1.85 Sr 0.15 CuO 4 (LSCO) as a buffer layer. X-ray diffraction measurements show that the epitaxial YBCO films have single in-plane orientation with YBCO [100]∥LSCO [100] and LSCO [100]∥YSZ [110]. The real-time resistance measurements reveal that with LSCO buffer layers the initial formation of the YBCO ultra-thin films changes from the island growth to the layer-by-layer growth.

Small polaron effect on carrier recombination in perovskite manganite thin films

Abstract Photoinduced ‘transient thermoelectric effect (TTE)’ in perovskite manganite La 0.6 Ca 0.4 MnO 3 thin film has been measured under magnetic fields. The fast decay process of TTE signals is due to a recombination of photogenerated electron–hole pairs through Mn 4+ ions as capture centers, whose evaluated cross section σ obeys the power law σ ∝ T − n ( n =0.75) in the ferromagnetic phase far below T c and in the paramagnetic phase. From the observed relaxation time τ 1 , we evaluated the parameter α characterizing a small polaron effect and the effective mass m ∗ ; both are enhanced appreciably near T c . Such anomaly is attributed to the change in the thermal velocity of diffusing holes with downspin due to a small polaron effect .

Large low-field magnetoresistance observed in heterostructure multilayers

The behaviour of resistivity versus temperature in a series of (LCMO)/(GCMO) multilayers has been studied. The resistivity peak temperature of the multilayers decreases with decreasing thickness of the LCMO layers. Large low-field magnetoresistance (LFMR) of % is obtained at 78.5 K in heterostructure LCMO/GCMO multilayers when a magnetic field is swept within Oe. This result suggests that the interface strain between LCMO and GCMO layers may influence the magnetic transport properties and a suitable film structure could be used to increase the LFMR.

Magnetic manipulation by resistance switching in CeO2/PrBa2Cu3O7−δ/Pt heterostructure: The role of oxygen vacancies

Pronounced bipolar resistance switching with a good retention property has been observed in CeO2/PrBa2Cu3O7−δ/Pt heterostructure. The low resistance state and high resistance state exhibited distinguished ferromagnetic signals, as compared to the nearly non-magnetic initial state. It is found that the migration of the oxygen vacancies under electric field is mainly responsible for the electric and the magnetic changes. The modified interfacial electronic structure by the oxygen vacancy migration and the trapping/detrapping of the carriers leads to the resistance switching. The exchange interaction of the hydrogen-like orbitals formed around the singly occupied oxygen vacancies in CeO2 is accounting for the emerged and modulated ferromagnetic signals. Temperature dependence of resistance in the low resistance state follows a variable range hopping law, further confirming that the amount of oxygen vacancies in the CeO2 layer directly affects the hydrogen-like orbital radius, which determines the strength of...

Microstructure of epitaxial superconductive YBa2Cu3O7 thin films prepared at different deposition rates

A study is reported on the growth mechanism of YBa2Cu3O7 with different growth speeds by high resolution transmission microscopy (HRTEM) and analysis of the interface and thin film microstructure. Two thin films were synthesized by pulse laser deposition on [100], miscut 5°, SrTiO3 substrate at 820 °C, one with a pulse laser frequency of 1 Hz and one with 6 Hz. Cross-sections were studied by an H-9000 NAR HRTEM along the [010] direction. The growth process of the sample made at 1 Hz was as follows. First, distorted step flow growth occurred on a step-mediated substrate surface of 3–4 cells thickness. Second, about a 15 nm thickness of island shape growth becomes superimposed on the area of the step flow layer. Finally, thin film growth occurred but with growth fluctuation. The sample made at 6 Hz showed the characteristics of island growth; the growth area of island or ball shape was of small size and dense distribution, and seemed to be a confused mosaic stack. The influence of growth speed on YBCO epitaxial film microstructure was studied explicitly by HRTEM.

Growth of HgBa2CaCu2Ox thin films using seed-buffer layers of YBa2Cu3Oy

Abstract High quality superconducting HgBa2CaCu2Ox (Hg-1212) thin films have been fabricated on SrTiO3 (STO) substrates using YBa2Cu3Oy (YBCO) as a seed-buffer layer (SBL). X-Ray diffraction indicates that the Hg-1212 thin films have a c-axis epitaxial structure with (100)Hg-1212//(100)STO. The φ-scans and rocking curves demonstrate that the SBL results in a better orientation of the Hg-1212 films. Scanning electron microscopy (SEM) also shows that the film surfaces are more regular. The Hg-1212 thin films on YBCO SBL have a superconducting transition temperature Tc=115–124 K and a critical current density Jc>1×106 A/cm2 at 77 K in zero field.

Growth of HgBa2CaCu2Oy films with a buffer layer

Using YBa2 Cu3 O7 (YBCO) as a buffer layer high Tc superconducting HgBa2 CaCu2 Oy (Hg-1212) thin films with zero resistance temperature (Tc ) of 115-124 K and critical current density (Jc ) of 1 MA cm-2 at 77 K in zero magnetic field have been grown on (100)Y-ZrO2 substrates. The high Tc , Jc and good morphology indicate that YBCO is a good buffer for growing the Hg cuprate films. This novel buffer makes it possible to grow Hg cuprate films on many substrates, including some metallic tapes.