Possible Origins for Paramagnetic-ferromagnetic and Insulator-metal Transitions as well as Colossal Magnetoresistance in Manganites

Systematical investigations of zero-field resistivity, magnetoresistance and magnetization were performed for a typical manganese compound La2/3Ca1/3MnO3. It is argued that the common origin for insulator-metal and paramagenetic ferromagnetic-transitions as well as colossal magnetoresistance is due to the formation of ferromagnetic clusters in the paramagnetic background. The transition to metallic state is resulted from percolation of ferromagnetic metallic clusters, while the colossal magnetoresistance is due to the application of magnetic field, which accelerates the growth of ferromagnetic metallic clusters and causes the shift of the onset temperature for the metallic percolation to higher temperature. Based on the random resistor network model, the zero-field resistivity versus temperature dependence is simulated by using experimental parameters, and experimental data well agree with those in whole temperature range, giving a strong support to our approach.

Defect-induced insulator-metal transition and negative permittivity in La_(1-x)Ba_(x)CoO_(3)perovskite structure

The development of negative permittivity materials in multifunctional applications requests expansion of their operating frequency and improvement of stability of negative permittivity.Low electron density is beneficial to reduce plasma frequency so that negative permittivity is achieved in kHz region.Negative permittivity achieved by percolating composites is restricted in practicality due to its instability nature at high temperatures.To achieve temperature-stable negative permittivity in kHz region,monophase La_(1-x)Ba_(x)CoO_(3)ceramics were prepared,and the transition from dielectric to metal was elaborated in the perspective of electrical conductivity and negative permittivity.The plasma-like negative permittivity is attained in kHz region,which is interpreted by the collective oscillation of low electron density.The temperature-stable negative permittivity is based on the fact that the plasmonic state will not be undermined at high temperatures.In addition,zero-crossing behavior of real permittivity is observed in La_(0.9)Ba_(0.1)CoO_(3)sample,which provides a promising alternative to designing epsilon-near-zero materials.This work makes the La_(1-x)Ba_(x)CoO_(3)system a source material for achieving effective negative permittivity.

Strain-mediated insulator-metal transition in topotactically hydro-reduced SrFeO_(2)

Controlling oxygen redox reactions in transition metal oxides offers an attractive route to tune their physical properties;a topotactic structural transformation from their parent phases effectively modifies the electronic state. In this work, infinitelayered SrFeO_(2) thin films were produced from brownmillerite SrFeO_(2.5) via low-temperature hydro-reduction. After the structural transition, their out-of-plane lattice constants dramatically shrank by 12%;tensilely strained samples exhibited metallic character, whereas the compressively strained ones maintained the insulating behavior of their bulk form. According to X-ray linear dichroism results, this strain-mediated electronic anisotropy may be attributed to electron redistribution within degenerated orbitals. This suggests a possible mechanism for the metallic conductivity of infinite-layered SrFeO_(2), giving a hint for understanding emergent quantum phenomena, such as the recently discovered superconductivity in nickelates, and stimulating various applications, including in ionic conductivity and oxygen catalysis.

Transport Properties and CMR Effect in La_(2/3)Ca_(1/3)MnO_3

The magnetic and electrical transport properties of the colossal magnetoresistance material La_(2/3)Ca_(1/3)MnO_3 were studied. It is found that the insulator-metal transition is well consistent with the paramagnetic-ferromagnetic transition,and shifts to higher temperature with increasing applied magnetic field. These results suggest that the transport properties are triggered by the magnetic structure transition and consequently result in a CMR near T _C.

Antenna-assisted picosecond control of nanoscale phase transition in vanadium dioxide

Nanoscale devices in which the interaction with light can be configured using external control signals hold great interest for next-generation optoelectronic circuits.Materials exhibiting a structural or electronic phase transition offer a large modulation contrast with multi-level optical switching and memory functionalities.In addition,plasmonic nanoantennas can provide an efficient enhancement mechanism for both the optically induced excitation and the readout of materials strategically positioned in their local environment.Here,we demonstrate picosecond all-optical switching of the local phase transition in plasmonic antenna-vanadium dioxide(VO_(2))hybrids,exploiting strong resonant field enhancement and selective optical pumping in plasmonic hotspots.Polarization-and wavelength-dependent pump–probe spectroscopy of multifrequency crossed antenna arrays shows that nanoscale optical switching in plasmonic hotspots does not affect neighboring antennas placed within 100 nm of the excited antennas.The antenna-assisted pumping mechanism is confirmed by numerical model calculations of the resonant,antenna-mediated local heating on a picosecond time scale.The hybrid,nanoscale excitation mechanism results in 20 times reduced switching energies and 5 times faster recovery times than a VO_(2) film without antennas,enabling fully reversible switching at over two million cycles per second and at local switching energies in the picojoule range.The hybrid solution of antennas and VO_(2) provides a conceptual framework to merge the field localization and phase-transition response,enabling precise,nanoscale optical memory functionalities.