W. Lu

Comparative High Field Magneto-transport Of Rare Earth Oxypnictides With Maximum Transition Temperatures

We compare magnetotransport of the three iron-arsenide-based compounds ReFeAsO (Re=La, Sm, Nd) in very high DC and pulsed magnetic fields up to 45 and 54 T, respectively. Each sample studied exhibits a superconducting transition temperature near the maximum reported to date for that particular compound. While high magnetic fields do not suppress the superconducting state appreciably, the resistivity, Hall coefficient, and critical magnetic fields, taken together, suggest that the phenomenology and superconducting parameters of the oxypnictide superconductors bridges the gap between MgB{sub 2} and YBCO.

Evidence for weak electronic correlations in iron pnictides

Using x-ray absorption and resonant inelastic x-ray scattering, charge dynamics at and near the Fe L edges is investigated in Fe pnictide materials, and contrasted to that measured in other Fe compounds. It is shown that the XAS and RIXS spectra for 122 and 1111 Fe pnictides are each qualitatively similar to Fe metal. Cluster diagonalization, multiplet, and density-functional calculations show that Coulomb correlations are much smaller than in the cuprates, highlighting the role of Fe metallicity and strong covalency in these materials. Best agreement with experiment is obtained using Hubbard parameters U <~;; 2eV and J ~;; 0.8eV.

Enhancement of room-temperature photoluminescence in InAs quantum dots

We report pronounced enhancement of room-temperature photoluminescence up to 80-fold induced by proton implantation and the rapid thermal annealing process in a multilayer InAs/GaAs quantum-dot structure. This effect is studied by a combination of material methods and resulted from both proton passivation and carrier capture enhancement effects. The maximum photoluminescence peak shift is about 23 meV, resulting from the intermixing of quantum dots. Linear dependence behavior as observed for both the nonradiative recombination time and carrier relaxation time on the ion-implantation dose. Maximum enhancement of the photoluminescence is observed for a proton implantation dose of 1.0×1014 cm−2 followed by rapid thermal annealing at 700 °C. These effects will be useful for quantum dot optoelectronic devices.

Perfect charge compensation in WTe2 for the extraordinary magnetoresistance: From bulk to monolayer

The electronic structure of WTe2 bulk and layers are investigated by using the first principles calculations. The perfect electron-hole (n-p) charge compensation and high carrier mobilities are found in WTe2 bulk, which may result in the large and non-saturating magnetoresistance (MR) observed very recently in the experiment [Ali et al., Nature 514, 205 (2014)]. The monolayer and bilayer of WTe2 preserve the semimetallic property, with the equal hole and electron carrier concentrations. Moreover, the very high carrier mobilities are also found in WTe2 monolayer, indicating that the WTe2 monolayer would have the same extraordinary MR effect as the bulk, which could have promising applications in nanostructured magnetic devices.

Enhanced low temperature thermoelectric performance of Ag-doped BiCuSeO

We investigated the physical properties of the silver doped layered oxyselenides BiCu1–xAgxSeO (x = 0–0.4), which crystallize in an unusual intergrowth structure with [Cu2Se2]2– and [Bi2O2]2+ layers. The total thermal conductivity is decreased because the heavier Ag doping in BiCuSeO lattice decreased the lattice thermal conductivity. The undoped BiCuSeO exhibits a semiconducting behavior, and the Ag-doped BiCuSeO performs much improved electrical conductivity. Although Ag-doping causes a decreasing Seebeck coefficient, the significant increase of the electrical conductivity compensates the moderate decrease of the Seebeck coefficient, which leads to the strongly improved power factor values. Finally, the figure of merit is improved and reaches a maximum ∼0.07 at 300 K for the sample BiCu0.7Ag0.3SeO.

Fabrication of La0.8Na0.2Mn1−xCuxO3 (x = 0, 0.05) thin films on YSZ substrates via chemical solution deposition

a-Axis oriented La0.8Na0.2MnO3 (LNMO) and La0.8Na0.2Mn0.95Cu0.05O3 (LNMCO) films with large magnetoresistance (MR) near room temperature are fabricated via chemical solution deposition on yttrium-stabilized zirconia (YSZ) (1 0 0) single crystal substrates. With slight Cu doping, it is found that the growth mode is changed from the layer-by-layer mode of the LNMO film to the island mode of the LNMCO film. The LNMO film exhibits typical transport properties of epitaxial films of manganites. However, the LNMCO film shows transport properties corresponding to polycrystalline films of manganites. The MR value of the LNMCO film at low applied magnetic field is also remarkably enhanced compared with that of the LNMO film below the insulator–metal transition temperature. MR as high as 9% is obtained at a field of 0.05 T at 5 K. The enhanced MR effect for the LNMCO film is attributed to the spin-dependent scattering of polarized electrons at the grain boundaries.

Spin-glass behavior and zero-field-cooled exchange bias in a Cr-based antiperovskite compound PdNCr3

We report the synthesis, structure, and magnetic and electrical/thermal transport properties of a Cr-based antiperovskite compound PdNCr3, which crystallizes in MgCNi3-type cubic structure (space group Pmm, No. 221). Interestingly, the spin-glass (SG) behavior, which is confirmed by the corresponding characteristic parameters (the freezing temperature T0 = 61.4(2) K, the dynamical exponent zν = 7.103(3), and the flipping time τ0 = 2.714(2) × 10−11 s), is observed in PdNCr3. Furthermore, the value of the Sommerfeld–Wilson ratio (RW ∼ 1.024(3)) for PdNCr3 is much smaller than those of cluster glass systems (RW > 100) and Kondo cluster glass systems (RW = 20–30), indicating that PdNCr3 is a canonical SG system. Density functional theory calculation shows that the origin of SG in PdNCr3 is attributed to the disordering located N vacancies, which is further confirmed by the measurement of sample PdN0.75Cr3 with more N deficiency. On the other hand, infrequently, the zero-field-cooled exchange bias (ZFC-EB) with an exchange bias field (HE) of about 350 Oe is observed after zero-field cooling from an unmagnetized state in PdNCr3. The values of HE are found to depend strongly on temperature and measuring magnetic field. For PdNCr3, the ferromagnetic unidirectional anisotropy, which is the origin of our ZFC-EB effect, is formed around the ferromagnetic–SG interface isothermally during the initial magnetization process below the blocking temperature. In addition, the training effect of ZFC-EB in PdNCr3 is observed after the zero-field cooling process and has been explained well in terms of the spin configurational relaxation model.

Observation of the large orbital entropy in Zn-doped orbital-spin-coupled system MnV2O4

We present the investigation on the magnetocaloric effect (MCE) in an orbital-spin-coupled spinel vanadate Mn0.95Zn0.05V2O4 via the dc magnetization and the specific-heat measurements. The maximum values of magnetic entropy change of Mn0.95Zn0.05V2O4 reach 9.6 and 16.5 J/kg K for field change 0–4 T around ferrimagnetic ordering temperature TC=52.5 K and orbital ordering temperature TOO=45 K. Except for a small quantity of contribution of the spin entropy, the observed giant MCE around TOO may be attributed to the orbital entropy due to the change of the orbital state of V3+.

Modulating magnetism of nitrogen-doped zigzag graphene nanoribbons

We present a study of electronic properties of zigzag graphene nanoribbons (ZGNRs) substitutionally doped with nitrogen atoms at a single edge by first principle calculations. We find that the two edge states near the Fermi level separate due to the asymmetric nitrogen-doping. The ground states of these systems become ferromagnetic because the local magnetic moments along the undoped edges remain and those along the doped edges are suppressed. By controlling the charge-doping level, the magnetic moments of the whole ribbons are modulated. Proper charge doping leads to interesting half-metallic and single-edge conducting ribbons which would be helpful for designing graphene?nanoribbon-based spintronic devices in the future.

Two-photon-absorption-induced nonlinear photoresponse in GaAs∕AlGaAs quantum-well infrared photodetectors

Using a free-electron laser(FEL) source, we have studied the two-photon-absorption (TPA) effect in GaAs∕AlGaAs quantum-well infrared photodetector (QWIP). The TPA-induced photoresponse in QWIPs has been measured under different FEL excitation power by the photoconductivity method. The effective-mass approximation theory is used for the QWIP structure to explain the photoresponse behavior. It is demonstrated that the TPA-induced photocarrier density is proportional to the square of the excitation power. Based on the experimental results, the TPA coefficients of QWIPs were obtained to be 0.0045, 0.0030, 0.0103, and 0.0061cm∕MW for the excitation lines of 10.6, 10.7, 11.9 and 13.2μm, respectively. The dependence the TPA coefficients on the excitation wavelength is explained by our theoretical model.Using a free-electron laser(FEL) source, we have studied the two-photon-absorption (TPA) effect in GaAs∕AlGaAs quantum-well infrared photodetector (QWIP). The TPA-induced photoresponse in QWIPs has been measured under different FEL excitation power by the photoconductivity method. The effective-mass approximation theory is used for the QWIP structure to explain the photoresponse behavior. It is demonstrated that the TPA-induced photocarrier density is proportional to the square of the excitation power. Based on the experimental results, the TPA coefficients of QWIPs were obtained to be 0.0045, 0.0030, 0.0103, and 0.0061cm∕MW for the excitation lines of 10.6, 10.7, 11.9 and 13.2μm, respectively. The dependence the TPA coefficients on the excitation wavelength is explained by our theoretical model.