L. F. Wang

Anisotropic-strain-induced antiferromagnetic-insulating state with strong phase instability in epitaxial (La0.8Pr0.2)0.67Ca0.33MnO3 films

We grow epitaxial (La0.8Pr0.2)0.67Ca0.33MnO3 films simultaneously on the lattice-closely-matched substrates, cubic (LaAlO3)0.3(Sr2AlTaO6)0.7 [LSAT(001)] and orthorhombic NdGaO3 [NGO(001) and NGO(110)]. While all as-grown films show a ferromagnetic-metallic (FM) ground state as observed for the bulk target, the annealed films show quite different magnetotransport behavior as follows: on NGO(110) they show a robust FM ground state, on LSAT(001) and NGO(001) however, they show surprisingly a coexisted antiferromagnetic insulating state with high phase instability in a wide temperature range. The phase coexistence being easily induced via the control of anisotropic epitaxial strain suggests that the phase separation in manganites could be elastically driven, and thus can be strain-engineered for devices applications.

Pseudomorphic strain induced strong anisotropic magnetoresistance over a wide temperature range in epitaxial La0.67Ca0.33MnO3/NdGaO3(001) films

Strong anisotropic magnetoresistance (AMR) was observed in La0.67Ca0.33MnO3 films grown coherently on the orthorhombic NdGaO3(001) substrates. With an increased orthorhombic lattice distortion due to the pseudomorphic strain, the films show not only a ferromagnetic-metal (FM) transition at TC of ∼265u2002K, but also the phase coexistence of FM and antiferromagnetic-insulator below ∼250u2002K. The phase competitions are very sensitive to the magnetic field, and more strikingly, to its orientations with respect to the crystal axes resulting in a large AMR in a broad temperature range, in addition to the conventional one peaked near TC. The films also show uniaxial magnetic anisotropy with the easy axis along the elongated b axis, suggesting that it is the strain induced spin-orbit-lattice coupling and the resultant phase competitions that control the AMR in epitaxial manganite films.

Electron spin relaxation due to D'yakonov-Perel' and Elliot-Yafet mechanisms in monolayer MoS 2 : Role of intravalley and intervalley processes

We investigate the electron spin relaxation due to the Dyakonov-Perel and Elliot-Yafet mechanisms, including the intra- and intervalley processes in monolayer MoS

Exciton-plasmon-photon conversion in silver nanowire: Polarization dependence

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Annealing assisted substrate coherency and high-temperature antiferromagnetic insulating transition in epitaxial La0.67Ca0.33MnO3/NdGaO3(001) films

in the absence of the external electric field. We construct the effective Hamiltonian for the conduction band using the Lowdin partition method from the anisotropic two-band Hamiltonian with the intrinsic spin-orbit coupling of the conduction band included. The spin-orbit coupling of the conduction band induces the intra- and intervalley Dyakonov-Perel relaxation for in-plane spins. In addition, the Elliot-Yafet spin relaxation also takes place for in-plane spins due to the interband spin mixing. We find that the Dyakonov-Perel mechanism dominates the in-plane spin relaxation. In the framework of this mechanism, the intravalley process is shown to play a more important role at low temperature, whereas the intervalley one becomes more important at high temperature. At the temperature in between, the leading process of the in-plane spin relaxation changes from the intervalley one to intravalley one as the electron density increases. Moreover, we find that the intravalley process is dominated by the electron-electron Coulomb scattering even with high impurity density since the dominant term in the spin-orbit coupling is isotropic, which does not lead to the spin relaxation together with the electron-impurity scattering. This is very different from previous studies in semiconductors and graphene. In addition to the in-plane spin relaxation, we also study the out-of-plane one in the presence of an in-plane magnetic field. The out-of-plane spin relaxation time is found to decrease with the increase of the in-plane magnetic field.

Interference of surface plasmon polaritons from a “point” source

Polarization dependence of the exciton-plasmon-photon conversion in silver nanowire-quantum dots structure was investigated using a scanning confocal microscope system. We found that the fluorescence enhancement of the CdSe nanocrystals was correlated with the angle between the excitation light polarization and the silver nanowire direction. The polarization of the emission was also related with the nanowire direction. It was in majority in the direction parallel with nanowire due to the nano-antenna effect.

Transmission of doughnut light through a bull’s eye structure

Bulk La0.67Ca0.33MnO3 (LCMO) and NdGaO3 (NGO) have the same Pbnm symmetry but different orthorhombic lattice distortions, yielding an anisotropic strain state in the LCMO epitaxial film grown on the NGO(001) substrate. The films are optimally doped in a ferromagnetic-metal ground state, after being ex-situ annealed in oxygen atmosphere, however, they show strikingly an antiferromagnetic-insulating (AFI) transition near 250 K, leading to a phase separation state with tunable phase instability at the temperatures below. To explain this drastic strain effect, the films with various thicknesses were ex-situ annealed under various annealing parameters. We demonstrate that the ex-situ annealing can surprisingly improve the epitaxial quality, resulting in the films with true substrate coherency and the AFI ground state. And the close linkage between the film morphology and electronic phase evolution implies that the strain-mediated octahedral deformation and rotation could be assisted by ex-situ annealing, and moreover, play a key role in controlling the properties of oxide heterostructures.

Anisotropic-strain-controlled metal-insulator transition in epitaxial NdNiO3 films grown on orthorhombic NdGaO3 substrates

The interference patterns of the surface plasmon polaritons (SPPs) on the metal surface from a “point” source are observed. Innovation to the previous works, a point SPPs source with diameter of 100 nm is generated at the freely chosen positions on Au/air interface using near field excitation method. Such a point source provides good enough coherence to generate obvious interference phenomenon. This point SPPs source may be useful in the investigation of plasmonics for its high coherence, deterministic position, and minimum requirement for the initial light source.

Anisotropic resistivities in anisotropic-strain-controlled phase-separated La0.67Ca0.33MnO3/NdGaO3(100) films

We experimentally investigate the extraordinary optical transmission of doughnut light through a bulls eye structure. Since the intensity is vanished in the center of the beam, almost all the energy reaches the circular corrugations (not on the hole), excite surface plasmons which propagate through the hole and reradiate photons. The transmitted energy is about 57 times of the input energy on the hole area. It is also interesting that the transmitted light has a similar spatial shape with the input light although the diameter of the hole is much smaller than the wavelength of light.

Controlling the sharpness of room-temperature metal-insulator transition in epitaxial Sm0.5Nd0.5NiO3 films

NdNiO3 (NNO) films were grown by pulsed laser deposition on orthorhombic (110)-, (001)-, and (100)-oriented NdGaO3 substrates. It is found that all the films are tensile-strained but show dramatically different metal-insulator transition (MIT) temperatures (TMI) (160–280u2009K), as compared with the NNO bulk (∼200u2009K). A high resemblance in the sharpness of MIT and lattice variation across the MIT was observed. The TMI is highly dependent on the magnitude of the orthorhombic distortion induced by the different substrate surface plane and tends to recover the bulk value after annealing. Our results suggest that the anisotropic epitaxial strain can effectively tune the MIT of NNO films, and the NiO6 octahedra rotation and deformation involved in accommodating the tensile strain might cause the different TMI.