Kwang-Yong Kang

Mechanism and observation of Mott transition in VO2-based two- and three-terminal devices

An abrupt Mott metal-insulator transition (MIT) rather than the continuous Hubbard MIT near a critical on-site Coulomb energy U/U_c=1 is observed for the first time in VO_2, a strongly correlated material, by inducing holes of about 0.018% into the conduction band. As a result, a discontinuous jump of the density of states on the Fermi surface is observed and inhomogeneity inevitably occurs. The gate effect in fabricated transistors is clear evidence that the abrupt MIT is induced by the excitation of holes.When holes of about 0.018% are induced into a conduction band (breakdown of critical on-site Coulomb energy), an abrupt first-order Mott metal–insulator transition (MIT) rather than a continuous Hubbard MIT near a critical on-site Coulomb energy U/Uc=1, where U is on-site Coulomb energy between electrons, is observed on an inhomogeneous VO2 film, a strongly correlated Mott insulator. As a result, discontinuous jumps of the density of states on the Fermi surface are observed and inhomogeneity inevitably occurs. The off-current and temperature dependences of the abrupt MIT in a two-terminal device and the gate effect in a three-terminal device are clear evidence that the abrupt Mott MIT was induced by the excitation of holes. Raman spectra measured by a micro-Raman system show an MIT without the structural phase transition. Moreover, the magnitude of the observed jumps ΔJobserved at the abrupt MIT is an average over an inhomogeneous measurement region of the maximum true jump, ΔJtrue, deduced from the Brinkman–Rice picture. A brief discussion of whether VO2 is a Mott insulator or a Peierls insulator is presented.

Growth optimization and electrical characteristics of VO2 films on amorphous SiO2/Si substrates

In order to fabricate an electronic device, we have optimized growth conditions of VO2 films on amorphous SiO2/Si structure substrates by pulsed laser deposition. The phase of VO2 films observed consisted of multiphases such as VO2, V2O5, and V2O3. The correlation between phase changes and growth conditions of VO2 films was analyzed. Also, the electrical characteristics of VO2-based three terminal devices, attributed to structural and phase changes, are discussed. VO2 films under optimal growth conditions have a change in resistivity of the order of 102 near a critical temperature, Tc=340 K. Furthermore, an abrupt jump in current was observed when an electric field was applied to the three terminal device in the VO2-based SiO2/Si structure. The source–drain voltage, in which the abrupt current jump occurred, changed with application of a gate electric field.

Cubic Y–Ba–Cu–O thin films by high speed pulsed laser deposition

Abstract The crystal structure and properties of YBa2Cu3Ox (YBCO) thin films on LaSrGaO4 (100) substrate have been investigated as a function of deposition rates in the range of 0.1–12.2 nm/s using different repetition rate of pulsed laser. At a given substrate temperature of 700°C, when the film was deposited at low deposition rate of 0.1 nm/s, c-axis oriented orthorhombic YBCO thin film growth was observed. However, at high deposition rate of 12.2 nm/s, cubic YBCO thin film growth was observed. This demonstrates that the shortening of interval time between the vapor pulses is important for the cubic YBCO thin film growth. The cubic YBCO thin film growth under high deposition rates was explained by the cation kinetics.

Detected power and linewidth of Josephson self-radiation from frequency-locked and microwave-coupled arrays in a waveguide detection system

Frequency-locked and microwave-coupled arrays of high- superconducting Josephson junctions have been developed. The Josephson junctions were prepared on bicrystal MgO substrates using a pulsed laser ablation method. The maximum detected power of Josephson self-radiation, which is directly detected by a superheterodyne receiver with a central frequency of 22 GHz, from the arrays was enhanced with increasing number of Josephson junctions. The Josephson self-radiation properties of the arrays revealed a good quality of frequency locking and exactly satisfied the Josephson voltage-frequency relationship. A narrower linewidth of the Josephson self-radiation was obtained by increasing the number of Josephson junctions in the array.

EXAFS observation of two distinct Bi–O distances below Tc for a Ba0.6K0.4BiO3 single crystal

Abstract In order to find two distinct Bi–O distances as evidence for the breathing mode distortion due to an electron–phonon interaction below T c , extended X-ray absorption fine structure of Bi L 3 -edge is measured from 14 to 300 K for a high quality single-crystal Ba 0.6 K 0.4 BiO 3 . The two distinct Bi–O bond lengths are obtained from data analyzed by a two-shell fit, and the Bi–O distances are similar to that in BaBiO 3 , independent of temperature. They come from a remnant phase of BaBiO 3 . Bi–O peaks coming from the superconducting phase are not analyzed because the Bi–O peaks overlap. Further, the Debye–Waller factor (σ 2 ) for Bi–O bonds does not fit Einstein model with increasing temperature.

Observation of a linear temperature dependence of the critical current density in a Ba0.63K0.37BiO3 single crystal

Abstract For a Ba 0.63 K 0.37 BiO 3 single crystal with T c ≈31 K, H c 1 ≈750 Oe at 5 K, and dimensions 3×3×1 mm 3 , the temperature and field dependences of magnetic hysteresis loops have been measured within 5–25 K in magnetic fields up to 6 Tesla. The critical current density is J c (0) ≈1.5 × 10 5 A/cm 2 at zero field and 1×10 5 A/cm 2 at 1 kOe at 5 K. J c decreases exponentially with increasing field up to 10 kOe . A linear temperature dependence of J c is observed below 25 K, which differs from the exponential and the power-law temperature dependences in high- T c superconductors including the BKBO. The linear temperature dependence can be regarded as an intrinsic effect in superconductors.

Self-radiation power and microwave coupling of YBCO Josephson junction arrays on bicrystal MgO substrates

Abstract We have studied the microwave properties of the Josephson junction array by measurement of Josephson self-radiation power and Shapiro-steps. The serial–parallel and parallel Josephson junction arrays were prepared with and without additional inductive coupling lines. The maximum detected power of 22 GHz from the serial–parallel array is about 25 pW and its peak point revealed a good quality of frequency-locking and exactly satisfied the Josephson voltage–frequency relationship. Although the parallel array has similar physical properties of average critical current and normal resistance of Josephson junctions to serial–parallel array, the maximum detected power is only about several tens of fW. The received power of serial–parallel array is higher than that of the parallel array for the same intensity of irradiation power.

Magnetoresistance observed by decomposition of the magnetic moment in La1−xCaxMnO3 films

A ferromagnetic phase, characterized by electron carriers and a high temperature colossal magnetoresistance (HTCMR) dependent on the magnetic moment, and a semiconducting phase, characterized by hole carriers and a low temperature CMR (LTCMR), are observed in La1−xCaxMnO3 thin films by the van der Pauw method. The LTCMR is much more sensitive to the magnetic field than the HTCMR. In the ferromagnetic phase for films with anisotropic moments in two dimensions, a remnant resistivity of the order of 10−8 Ω m is observed up to 100 K and increases exponentially with both a temperature up to Tc and a magnetic field above 1 T (a positive magnetoresistivity). We found that the ferromagnetic phase below Tc is in a polaronic state with a polaronic mobile conduction, and the carrier density dips near Tc. For resistances measured by the four-probe method with line electrodes, low temperature information of the HTCMR is not revealed. The van der Pauw method is more effective for the resistance measurement of a magneti...

Fabrication of Sr2AlTaO6/YBCO interface-controlled junctions

Abstract We fabricated ramp-edge Josephson junctions with barriers formed by interface treatments instead of epitaxially grown barrier layers. A low-dielectric Sr2AlTaO6(SAT) layer was used as an ion-milling mask as well as an insulating layer for the ramp-edge junctions. An ion-milled YBA2Cu3O7−x (YBCO)-edge surface was not exposed to solvent through all fabrication procedures. The barriers were produced by structural modification at the edge of the YBCO base electrode using high energy ion-beam treatment, prior to deposition of the YBCO counterelectrode. We investigated the effects of high energy ion-beam treatment, annealing, and counterelectrode deposition temperature on the characteristics of the interface-controlled Josephson junctions. The spreads in the critical current (Ic) as well as the junction parameters such as Tc, Ic, RN were measured and discussed in relation to the interface states.