D. C. Ling

Effect of Co, Ni, and Cu substitution on the electronic structure of hexagonal YMnO3 studied by x-ray absorption spectroscopy

X-ray absorption spectroscopy measurements have been performed to elucidate local electronic and atomic structures of orthorhombic 3d-transition metal-doped yttrium manganites (YMnO3) with chemical formulae YMn2/3Me1/3O3 (Me=Co, Ni, and Cu). The Mn L3- and K-edges x-ray absorption near-edge structure (XANES) demonstrate the direct substitution of Me2+ for Mn3+, so that the positive effective charge of Mn ions are increased. Me-doping is also found to induce substantial broadening of the Mn L3-edge feature, which suggests enhancement of the delocalization of Mn 3d eg subbands and conductivity. Local spin density approximation (LSDA)+U (Hubbard U parameter) calculations were used to understand their electronic structures.

Ultrafast dynamics of hot electrons and phonons in chemical vapor deposited graphene

The relaxation dynamics of photoexcited carriers in a chemical vapor deposited graphene transferred on quartz substrate has been investigated by using ultrafast optical-pump terahertz (THz)-probe spectroscopy. Terahertz transmission through graphene sample is reduced by optical pumping. The change of transmission decays exponentially after the optical pulse. We find the relaxation time is insensitive to the substrate temperature from 10 K to 300 K but increases sublinearly with pump fluence. We model the relaxation process involving electron-phonon coupling together with a set of rate equations to describe the transient responses of quasi-particles and optical phonons. The increases of the extracted carrier temperature and the measured relaxation time with pump fluence are associated with the fact that high pump fluence significantly increases the carrier temperature and broadens the carrier distribution. As a result, it leads to the reduction of optical phonon emission efficiency and the decrease of cool...

Role of valence-band Co 3d states on ferromagnetism in Zn1−xCoxO nanorods

This work investigates the electronic and ferromagnetic properties of Zn1−xCoxO nanorods using x-ray absorption, x-ray magnetic circular dichroism, and scanning photoelectron microscopy methods. The magnetic moment of Co ions in Zn1−xCoxO nanorods is found greatly reduced relative to that of the Co metal. The intensities of valence-band features near the valence-band maximum/Fermi level (EF) of ferromagnetic nanorods are substantially larger than those of weaker ferromagnetic nanorods, suggesting that the occupation of near-EF valence-band Co 3d states is important in determining the ferromagnetic behavior in Zn1−xCoxO nanorods.

Electronic and magnetic properties of the Ag-doped Fe3O4 films studied by x-ray absorption spectroscopy

The electronic and magnetic properties of Ag-doped Fe3O4 films were studied by x-ray absorption near-edge structure (XANES), extended x-ray absorption fine structure (EXAFS), and x-ray magnetic circular dichroism (XMCD) measurements. A comparison between the Ag K-edge EXAFS Fourier transform spectra of Ag-doped Fe3O4 and the Ag metal shows that Ag atoms aggregate into Ag granules. The O K-edge and Ag L3-edge XANES spectra consistently indicate an electron transfer from the Fe3O4 host into Ag granules. The Fe L3,2-edge XMCD spectra and hysteresis measurements reveal that Ag granules reduce the average magnetic moment of Fe ions and the saturation magnetization of Fe3O4.

Observation of weak carrier localization in green emitting InGaN/GaN multi-quantum well structure

Green emitting InGaN/GaN multi-quantum well samples were investigated using transmission electron microscopy, photoluminescence (PL), and time-resolved photoluminescence (TRPL) spectroscopy. Weak carrier localization with characteristic energy of ∼12 meV due to an inhomogeneous distribution of In in the InGaN quantum (QW) layer is observed. The temperature dependence of the PL peak energy exhibits S-shape phenomenon and is comparatively discussed within the framework of the Varshnis empirical formula. The full width at half maximum of the PL emission band shows an increasing-decreasing-increasing behavior with increasing temperature arising from the localized states caused by potential fluctuations. The radiative life time, τr, extracted from the TRPL profile shows ∼T3/2 dependence on temperature above 200 K, which confirms the absence of the effect of carrier localization at room temperature.

Correlation between magnetic properties and the electronic structures of soft magnetic ternary Fe78−xYxB22 (x = 4–9) bulk metallic glasses

Fe and Y K-edge extended x-ray absorption fine structure, Fe(Y) L(3,2)-edge (L(3)-edge) x-ray absorption near-edge structure (XANES) and valence-band photoemission spectroscopy (VB-PES) measurements have been carried out to study soft magnetic ternary Fe(78-x)Y(x)B(22) bulk metallic glasses (BMGs). The combined XANES and VB-PES results do not show broadening of the Fe 3d band to support the previous interpretation of the reduction of the magnetic moment in BMGs by Y-induced decrease of exchange splitting of Fe 3d orbitals. Instead, the density of delocalized/itinerant Fe 3d states in the vicinity of the Fermi level is found to be reduced by Y substitution, which reduces the strength of itinerant-states-mediated ferromagnetic coupling between local spins on the Fe ions and the total magnetic moment of the Fe-based BMGs.

Charged impurity-induced scatterings in chemical vapor deposited graphene

We investigate the effects of defect scatterings on the electric transport properties of chemical vapor deposited (CVD) graphene by measuring the carrier density dependence of the magneto-conductivity. To clarify the dominant scattering mechanism, we perform extensive measurements on large-area samples with different mobility to exclude the edge effect. We analyze our data with the major scattering mechanisms such as short-range static scatters, short-range screened Coulomb disorders, and weak-localization (WL). We establish that the charged impurities are the predominant scatters because there is a strong correlation between the mobility and the charge impurity density. Near the charge neutral point (CNP), the electron-hole puddles that are induced by the charged impurities enhance the inter-valley scattering, which is favorable for WL observations. Away from the CNP, the charged-impurity-induced scattering is weak because of the effective screening by the charge carriers. As a result, the local static structural defects govern the charge transport. Our findings provide compelling evidence for understanding the scattering mechanisms in graphene and pave the way for the improvement of fabrication techniques to achieve high-quality CVD graphene.

NOVEL MAGNETISM AND SUPERCONDUCTIVITY IN A Ru-BASED DOUBLE PEROVSKITE

We report the observations, using magnetic susceptibility and specific heat measurements, of the coexistence of superconductivity and magnetic ordering at low temperatures in the Cu-doping double perovskites Sr2ARu1-xCuxO6, with A=Y or Ho, and x<0.2. The observed striking features can be understood in terms of a plausible theoretical model based on the double exchange mechanism, which occurs indirectly via Ru5+ spin coupling to itinerant electrons hopping from one Ru site to another.

The electronic and magnetic properties of La0.85Zr0.15MnO3 deposited on SrTiO3 and MgO substrates

The electronic and magnetic properties of tetravalent-ion-doped La0.85Zr0.15MnO3 (LZMO) thin films that were epitaxially grown on SrTiO3 (STO) and MgO substrates were studied using temperature-dependent x-ray diffraction (XRD), x-ray absorption near-edge structure, x-ray linear dichroism, and x-ray magnetic circular dichroism at the Mn L3,2- and K-edge. XRD studies reveal that the LZMO thin films have compressive and tensile strains (along the c-axis) on the STO and MgO substrates, respectively. As the temperature is reduced from room temperature to below magnetic transition temperature, the preferentially occupied Mn majority-spin eg orbital changes from the in-plane dx2-y2 to the out-of-plane d3z2-r2 orbital for LZMO/STO, and vice versa for LZMO/MgO. Experimental results suggest that the new hopping path that is mediated by the Mn2+ ions triggers a stronger d3z2-r2 orbital ordering of Mn3+ ions and enhances the ferromagnetic coupling between the Mn spin moments of t2g electrons in LZMO/STO, whereas the ...

Effect of Cr deficiency on physical properties of triangular-lattice antiferromagnets CuCr1−xO2 (0 ≦ x ≦ 0.10)

Structural, transport, and magnetoelectric (ME) properties of delafossite oxides CuCr1−xO2 with 0 ≦ x ≦ 0.10 were extensively investigated. The Rietveld refinement shows that the Cu-O bond length decreases with increasing Cr deficiency, indicative of the presence of a mixed valence state of Cu+/Cu2+ and an enhancement of the hybridization between Cu 3d and O 2p orbitals. As a result, it leads to a decrease of room-temperature resistivity by two orders of magnitude. The deduced effective moment for the Cr-deficient samples is larger than the one only taking into account the contribution from Cr3+ with S = 3/2. This demonstrates that Cu2+ is present in the Cr-deficient samples, giving rise to excess holes at the Cu site. Below TN(Cr) ∼ 24 K, the magnetocapacitance [e(H)−e(0)]/e(0) exhibits a distinct field dependence and deviates from the square of magnetization M2. These findings suggest that the ME coupling in CuCr1−xO2 with higher x is modulated by an increase of the spin fluctuations in the CrO2 triangu...