P.-H. Xiang

Tuning of the metal-insulator transition in electrolyte-gated NdNiO3 thin films

We demonstrate electrostatic control of the metal-insulator transition in the typical correlated-electron material NdNiO3 through a large effective capacitance of the electric double layer at the electrolyte/NdNiO3 interface. The metal-insulator transition temperature (TMI) of NdNiO3 is shown to decrease drastically with increasing hole concentration through the application of a negative gate voltage (VG). The shift in TMI (|ΔTMI|) is larger for thinner NdNiO3; for VG of −2.5 V, |ΔTMI| of 5-nm-thick NdNiO3 is as large as 40 K, and the resistivity change near 95 K is one order of magnitude. This study may be potentially applicable to Mott transistor devices.

Strain‐Mediated Phase Control and Electrolyte‐Gating of Electron‐Doped Manganites

A prototype Mott transistor, the electric double layer transistor with a strained CaMnO(3) thin film, is fabricated. As predicted by the strain phase diagram of electron-doped manganite films, the device with the compressively strained CaMnO(3) exhibits an immense conductivity modulation upon applying a tiny gate voltage of 2 V.

Electrolyte‐Gated SmCoO3 Thin‐Film Transistors Exhibiting Thickness‐Dependent Large Switching Ratio at Room Temperature

A Mott transistor that exhibits a large switching ratio of more than two orders at room temperature is demonstrated by using the electric double layer of an ionic liquid for gating on a strongly correlated electron system SmCoO3. From the thickness dependence of the on-state channel current, we estimate the screening length of the SmCoO3 to be ∼5 nm. The good carrier confinement within the Thomas-Fermi screening length demonstrates that the SmCoO3-channel electric double layer transistor is the first candidate for a two-dimensional Mott transistor.

Room temperature Mott metal-insulator transition and its systematic control in Sm1−xCaxNiO3 thin films

We report an epitaxial growth and electronic transport properties of Ca-doped SmNiO3 (Sm1−xCaxNiO3, 0≤x≤0.1) thin films deposited on (001)-oriented LaAlO3 substrates by the pulsed laser deposition method. Due to strong electron correlations of the Sm1−xCaxNiO3 films, the Mott metal-insulator (MI) transition appears around 370 K, which decreases to room temperature only by the 1%–2% Ca doping, and dramatically shifts to lower temperatures by increasing the Ca content. For x≥0.1, the film reveals metallic conductivity down to the lowest temperature measured. In the insulating phase of x<0.04, we observe another resistivity anomaly around 200 K corresponding to an antiferromagnetic ordering of the Ni sublattice of SmNiO3 matrix. A complete electronic phase diagram of the Sm1−xCaxNiO3 thin film is unveiled by this work.

Strain-controlled electronic properties and magnetorelaxor behaviors in electron-doped CaMnO3 thin films

We have fabricated epitaxial thin films of electron-doped manganite Ca1−xCexMnO3 (CCMO) with 0≤x≤0.08. The transport properties of CCMO films are very sensitive to substrate-controlled epitaxial strain. For the CCMO(x=0.05) film, the metallic transport characteristic is observed only on a nearly lattice-matched NdAlO3 (NAO) substrate, while tensilely and compressively stressed films are insulating. The CCMO(x=0.06) film on the NAO substrate shows a large magnetoresistance characteristic of a magnetorelaxor. This behavior can be explained in terms of the phase separation and the irreversible growth of the metallic domain in antiferromagnetic insulating matrix.

Phase diagrams of strained Ca1–xCexMnO3 films

The transport and magnetic properties were examined of thin films of the electron-doped manganite Ca1−xCexMnO3 (CCMO; 0 ≤ x ≤ 0.1) deposited on three different single-crystalline oxide substrates: YAlO3, NdAlO3, and LaSrAlO4, which subject the films to a compressive strain, a practical lattice matching, and a tensile strain, respectively. The temperature dependence of the activation energy extracted from the resistivity–temperature curves clearly indicated the phase transitions in the CCMO films. A comparison with magnetization measurements revealed that regardless of the substrates, the temperature-driven phase transition of the CCMO films changed from the G-type antiferromagnetic transition at x = 0 to the canted antiferromagnetic transition and to the charge- and/or orbital-ordered transition, as x was increased. In the canted antiferromagnetic phase, the epitaxial strains have significant impact not only on the transport properties but also on the magnetic properties such as the spontaneous magnetizat...

Colossal magnetoresistance accompanied with magnetorelaxor behavior in phase-separated Ca1−xCexMnO3 thin films and CaMnO3/Ca0.92Ce0.08MnO3 superlattices

We report on the transport properties of electron-doped manganite Ca1−xCexMnO3 (CCMO, 0≤x≤0.08) films and superlattices composed of insulating layers CaMnO3 (CMO) and Ca0.92Ce0.08MnO3 (CCMO8), deposited on nearly lattice-matched NdAlO3 substrates. The CCMO (x=0.06 and 0.07) films show colossal magnetoresistance (CMR) accompanied with magnetorelaxor behavior, which can be ascribed to the phase separation of canted G-type antiferromagnetic metal and C-type antiferromagnetic insulator. The (CMO)m/(CCMO8)n superlattices with 4≤m, n≤8 (unit cells) resemble the solid-solution CCMO (x=0.06 and 0.07) films in CMR and magnetorelaxor behavior, suggesting that the phase separation takes place in the superlattices. The CMR and magnetorelaxor behavior of the (CMO)m/(CCMO8)n superlattices strongly depend on the thicknesses of constituent CMO and CCMO8 layers. The origin of the phase separation in the superlattices is discussed in terms of the charge transfer and the phase competition at the interfaces.