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A Rusydi,
S Dhar,
A Roy Barman,
Ariando,
D-C Qi,
M Motapothula,
J B Yi, I Santoso,
Y P Feng,
K Yang,
Y Dai,
N L Yakovlev,
J Ding,
A T S Wee,
G Neuber,
M B H Breese,
M Ruebhausen,
H Hilgenkamp,
T Venkatesan
[show abstract]
[hide abstract]
ABSTRACT: We report room-temperature ferromagnetism (FM) in highly conducting, transparent anatase Ti(1-x)Ta(x)O(2) (x∼0.05) thin films grown by pulsed laser deposition on LaAlO(3) substrates. Rutherford backscattering spectrometry (RBS), X-ray diffraction, proton-induced X-ray emission, X-ray absorption spectroscopy (XAS) and time-of-flight secondary-ion mass spectrometry indicated negligible magnetic contaminants in the films. The presence of FM with concomitant large carrier densities was determined by a combination of superconducting quantum interference device magnetometry, electrical transport measurements, soft X-ray magnetic circular dichroism (SXMCD), XAS and optical magnetic circular dichroism, and was supported by first-principles calculations. SXMCD and XAS measurements revealed a 90 per cent contribution to FM from the Ti ions, and a 10 per cent contribution from the O ions. RBS/channelling measurements show complete Ta substitution in the Ti sites, though carrier activation was only 50 per cent at 5 per cent Ta concentration, implying compensation by cationic defects. The role of the Ti vacancy (V(Ti)) and Ti(3+) was studied via XAS and X-ray photoemission spectroscopy, respectively. It was found that, in films with strong FM, the V(Ti) signal was strong while the Ti(3+) signal was absent. We propose (in the absence of any obvious exchange mechanisms) that the localized magnetic moments, V(Ti) sites, are ferromagnetically ordered by itinerant carriers. Cationic-defect-induced magnetism is an alternative route to FM in wide-band-gap semiconducting oxides without any magnetic elements.
Philosophical Transactions of The Royal Society A Mathematical Physical and Engineering Sciences 10/2012; 370(1977):4927-43. · 2.77 Impact Factor
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A. Rusydi,
S. Dhar,
A. Roy Barman,
Ariando,
D. -C. Qi,
M. Motapothula,
J. B. Yi, I. Santoso,
Y. P. Feng,
K. Yang,
Y. Dai,
N. L. Yakovlev,
J. Ding,
A. T. S. Wee,
G. Neuber,
M. B. H. Breese,
M. Ruebhausen,
H. Hilgenkamp,
T. Venkatesan
[show abstract]
[hide abstract]
ABSTRACT: We report room-temperature ferromagnetism in highly conducting transparent
anatase Ti1-xTaxO2 (x~0.05) thin films grown by pulsed laser deposition on
LaAlO3 substrates. Rutherford backscattering spectrometry (RBS), x-ray
diffraction (XRD), proton induced x-ray emission (PIXE), x-ray absorption
spectroscopy (XAS) and time-of-flight secondary ion mass spectrometry
(TOF-SIMS) indicated negligible magnetic contaminants in the films. The
presence of ferromagnetism with concomitant large carrier densities was
determined by a combination of superconducting quantum interference device
(SQUID) magnetometry, electrical transport measurements, soft x-ray magnetic
circular dichroism (SXMCD), XAS, and optical magnetic circular dichroism (OMCD)
and was supported by first-principle calculations. SXMCD and XAS measurements
revealed a 90% contribution to ferromagnetism from the Ti ions and a 10%
contribution from the O ions. RBS/channelling measurements show complete Ta
substitution in the Ti sites though carrier activation was only 50% at 5% Ta
concentration implying compensation by cationic defects. The role of Ti vacancy
and Ti3+ was studied via XAS and x-ray photoemission spectroscopy (XPS)
respectively. It was found that in films with strong ferromagnetism, the Ti
vacancy signal was strong while Ti3+ signal was absent. We propose (in the
absence of any obvious exchange mechanisms) that the localised magnetic
moments, Ti vacancy sites, are ferromagnetically ordered by itinerant carriers.
Cationic-defect-induced magnetism is an alternative route to ferromagnetism in
wide-band-gap semiconducting oxides without any magnetic elements.
07/2012;
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F. Massee,
S. de Jong. Y. Huang,
W. K. Siu, I. Santoso,
A. Mans,
A. T. Boothroyd,
D. Prabhakaran,
R. Follath,
A. Varykhalov,
L. Patthey,
M Shi,
J. B. Goedkoop,
M. S. Golden
[show abstract]
[hide abstract]
ABSTRACT: The exact nature of the low temperature electronic phase of the manganite
materials family, and hence the origin of their colossal magnetoresistant (CMR)
effect, is still under heavy debate. By combining new photoemission and
tunneling data, we show that in La{2-2x}Sr{1+2x}Mn2O7 the polaronic degrees of
freedom win out across the CMR region of the phase diagram. This means that the
generic ground state is that of a system in which strong electron-lattice
interactions result in vanishing coherent quasi-particle spectral weight at the
Fermi level for all locations in k-space. The incoherence of the charge
carriers offers a unifying explanation for the anomalous charge-carrier
dynamics seen in transport, optics and electron spectroscopic data. The
stacking number N is the key factor for true metallic behavior, as an
intergrowth-driven breakdown of the polaronic domination to give a metal
possessing a traditional Fermi surface is seen in the bilayer system.
03/2011;
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I. Santoso,
P. K Gogoi,
H. B. Su,
H Huang,
Y Lu,
D. Qi,
W Chen,
M. A. Majidi,
Y P Feng,
A T S Wee,
K. P. Loh,
T. Venkatesan,
R. P. Saichu,
A. Goos,
A. Kotlov,
M. Ruebhausen,
A. Rusydi
[show abstract]
[hide abstract]
ABSTRACT: Using a combination of ultraviolet-vacuum ultraviolet reflectivity and
spectroscopic ellipsometry, we observe a resonant exciton at an unusually high
energy of 6.3eV in epitaxial graphene. Surprisingly, the resonant exciton
occurs at room temperature and for a very large number of graphene layers
$N$$\approx$75, thus suggesting a poor screening in graphene. The optical
conductivity ($\sigma_1$) of resonant exciton scales linearly with number of
graphene layer (up to \emph{at least} 8 layers) implying quantum character of
electrons in graphene. Furthermore, a prominent excitation at 5.4eV, which is a
mixture of interband transitions from $\pi$ to $\pi^{*}$ at the M point and a
$\pi$ plasmonic excitation, is observed. In contrast, for graphite the resonant
exciton is not observable but strong interband transitions are seen instead.
Supported by theoretical calculations, for $N \leq$ 28 the $\sigma_1$ is
dominated by the resonant exciton, while for $N >$ 28 it is a mixture between
exitonic and interband transitions. The latter is characteristic for graphite,
indicating a crossover in the electronic structure. Our study shows that
important elementary excitations in graphene occur at high binding energies and
elucidate the differences in the way electrons interact in graphene and
graphite.
01/2011;
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A. R. Barman,
M. Motapothula,
A. Annadi,
K. Gopinadhan,
Y. L. Zhao,
Z. Yong, I. Santoso,
Ariando,
M. Breese,
A. Rusydi,
S. Dhar,
T. Venkatesan
[show abstract]
[hide abstract]
ABSTRACT: Useful electronic, magnetic, and optical properties have been proposed and observed in thin films of Ti1-xMxO2 (M=Ta, Nb, V). In this work, we have studied phase formation for films of Ti1-xTaxO2 prepared by pulsed laser deposition. We show that substitutional Ta in TiO2 results in a different material system in terms of its electronic properties. Moss-Burstein shift is ruled out by comparing the electrical transport data of anatase and rutile TiO2. Vegard's law fit to the blueshift data and the high energy optical reflectivity studies confirm the formation of an alloy with a distinct band structure. (C) 2011 American Institute of Physics. [doi:10.1063/1.3553773]
Applied Physics Letters 01/2011; 98(7). · 3.84 Impact Factor
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T S Herng,
D-C Qi,
T Berlijn,
J B Yi,
K S Yang,
Y Dai,
Y P Feng, I Santoso,
C Sánchez-Hanke,
X Y Gao,
Andrew T S Wee,
W Ku,
J Ding,
A Rusydi
[show abstract]
[hide abstract]
ABSTRACT: We report direct evidence of room-temperature ferromagnetic ordering in O-deficient ZnO:Cu films by using soft x-ray magnetic circular dichroism and x-ray absorption. Our measurements have revealed unambiguously two distinct features of Cu atoms associated with (i) magnetically ordered Cu ions present only in the oxygen-deficient samples and (ii) magnetically disordered regular Cu2+ ions present in all the samples. We find that a sufficient amount of both oxygen vacancies (V(O)) and Cu impurities is essential to the observed ferromagnetism, and a non-negligible portion of Cu impurities is uninvolved in the magnetic order. Based on first-principles calculations, we propose a microscopic "indirect double-exchange" model, in which alignments of localized large moments of Cu in the vicinity of the V(O) are mediated by the large-sized vacancy orbitals.
Physical Review Letters 11/2010; 105(20):207201. · 7.37 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: After years of research into colossal magnetoresistant (CMR) manganites using bulk techniques, there has been a recent upsurge in experiments directly probing the electronic states at or near the surface of the bilayer CMR materials La2−2xSr1+2xMn2O7 using angle-resolved photoemission or scanning probe microscopy. Here, we report temperature-dependent, angle-resolved photoemission data from single crystals with a doping level of x=0.36. The first important result is that there is no sign of a pseudogap in the charge channel of this material for temperatures below the Curie temperature TC. The data show unprecedented sharp spectral features, enabling the unambiguous identification of clear, resolution-limited quasiparticle features from the bilayer split 3dx2−y2-derived Fermi surfaces both at the zone-face and zone diagonal kF locations. The data show that these low temperature Fermi surfaces describe closed shapes in k∥, centered at the (π∕a,π∕a) points in the two dimensional Brillouin zone, and are not open and arclike in nature. The second important result concerns the temperature dependence of the electronic states. The spectra display strong incoherent intensity at high binding energies and a very strong temperature dependence, both characteristics reminiscent of polaronic systems. However, the clear and strong quasiparticle peaks at low temperatures are difficult to place within a polaronic scenario. A careful analysis of the temperature-dependent changes in the Fermi surface spectra both at the zone face and zone diagonal regions in k space indicates that the coherent quasiparticle weight disappears for temperatures significantly above TC and that the k dependence of the T-induced changes in the spectra invalidates an interpretation of these data in terms of the superposition of a “universal” metallic spectrum and an insulating spectrum whose relative weight changes with temperature. In this sense, our data are not compatible with a phase separation scenario.
Phys. Rev. B. 12/2007; 76(23).
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[show abstract]
[hide abstract]
ABSTRACT: After years of research into colossal magnetoresistant (CMR) manganites using bulk techniques, there has been a recent upsurge in experiments directly probing the electronic states at or near the surface of the bilayer CMR materials La$_{2-2x}$Sr$_{1+2x}$Mn$_2$O$_7$ using angle-resolved photoemission or scanning probe microscopy. Here we report new, temperature dependent, angle resolved photoemission data from single crystals with a doping level of $x=0.36$. The first important result is that there is no sign of a pseudogap in the charge channel of this material for temperatures below the Curie temperature $T_C$. The second important result concerns the temperature dependence of the electronic states. The temperature dependent changes in the Fermi surface spectra both at the zone face and zone diagonal regions in $k$-space indicate that the coherent quasiparticle weight disappears for temperatures significantly above $T_C$, and that the $k$-dependence of the T-induced changes in the spectra invalidate an interpretation of these data in terms of the superposition of a `universal' metallic spectrum and an insulating spectrum whose relative weight changes with temperature. In this sense, our data are not compatible with a phase separation scenario.
11/2007;
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[show abstract]
[hide abstract]
ABSTRACT: In light of recent conflicting angle resolved photoemission studies on the bilayered colossal magnetoresistant (CMR) manganite La_{2-2x}Sr_{1+2x}Mn_{2}O_{7} (0.36< x< 0.40), new ARPES data are presented for x=0.36 and 0.40, showing only for the former clear quasiparticle-like features at and around the (pi, 0)-point in k-space. The electronic states are clearly renormalised, both as regards their dispersion relation and lifetime due to coupling to bosonic degrees of freedom. Importantly, both the existence of quasiparticles and their renormalisation survive well into the paramagnetic state, up to 50 K above T_Curie. This argues against strong coupling to spin modes and raises questions regarding the nature of the paramagnetic insulating phase.
12/2006;
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A Mans, I Santoso,
Y Huang,
W K Siu,
S Tavaddod,
V Arpiainen,
M Lindroos,
H Berger,
V N Strocov,
M Shi,
L Patthey,
M S Golden
[show abstract]
[hide abstract]
ABSTRACT: By combining surprising new results from a full polarization analysis of nodal angle-resolved photoemission data from pristine and modulation-free Bi(2)Sr(2)CaCu(2)O(8+delta) with structural information from LEED and ab initio one-step photoemission simulations, we prove that the shadow Fermi surface in these systems is of structural origin, being due to orthorhombic distortions from tetragonal symmetry present both in surface and bulk. Consequently, one of the longest standing open issues in the investigation of the Fermi surface of these widely studied systems finally meets its resolution.
Physical Review Letters 04/2006; 96(10):107007. · 7.37 Impact Factor
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A. Mans, I. Santoso,
Y Huang,
W. Siu,
S. Tavaddod,
V Arpiainen,
M. Lindroos,
H. Berger,
V. N. Strocov,
M Shi,
L. Patthey,
M. S. Golden
[show abstract]
[hide abstract]
ABSTRACT: By combining surprising new results from a full polarization analysis of nodal angle-resolved photoemission data from pristine and modulation-free Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ with structural information from LEED and {\it ab initio} one-step photoemission simulations, we prove that the shadow Fermi surface in these systems has structural origin, being due to orthorhombic distortions from tetragonal symmetry present in both surface and bulk. Consequently, one of the longest standing open issues in the fermiology of these widely studied systems finally meets its resolution.
09/2005;
