49 research outputs found
Metallic state in La-doped YBaCuO thin films with -type charge carriers
We report hole and electron doping in La-doped YBaCuO(YBCO) thin
films synthesized by pulsed laser deposition technique and subsequent
\emph{in-situ} postannealing in oxygen ambient and vaccum. The -type samples
show a metallic behavior below the Mott limit and a high carrier density of
10 cm at room temperature (\emph{T}) at the
optimally reduced condition. The in-plane resistivity () of the
-type samples exhibits a quadratic \emph{T} dependence in the
moderate-\emph{T} range and shows an anomaly at a relatively higher \emph{T}
probably related to pseudogap formation analogous to underdoped
NdCeCuO (NCCO). Furthermore, (T), \emph{T} and
\emph{T} with minimum resistivity (\emph{T}) were investigated in both
- and -side. The present results reveal the - asymmetry (symmetry)
within the metallic-state region in an underdoped cuprate and suggest the
potential toward ambipolar superconductivity in a single YBCO system.Comment: 4 pages, 5 figure
Cationic vacancy induced room-temperature ferromagnetism in transparent conducting anatase Ti_{1-x}Ta_xO_2 (x~0.05) thin films
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.Comment: 21 pages, 10 figures, to appear in Philosophical Transaction - Royal
Soc.
Reversible room-temperature ferromagnetism in Nb-doped SrTiO3 single crystals
The search for oxide-based room-temperature ferromagnetism has been one of
the holy grails in condensed matter physics. Room-temperature ferromagnetism
observed in Nb-doped SrTiO3 single crystals is reported in this Rapid
Communication. The ferromagnetism can be eliminated by air annealing (making
the samples predominantly diamagnetic) and can be recovered by subsequent
vacuum annealing. The temperature dependence of magnetic moment resembles the
temperature dependence of carrier density, indicating that the magnetism is
closely related to the free carriers. Our results suggest that the
ferromagnetism is induced by oxygen vacancies. In addition, hysteretic
magnetoresistance was observed for magnetic field parallel to current,
indicating that the magnetic moments are in the plane of the samples. The x-ray
photoemission spectroscopy, the static time-of-flight and the dynamic secondary
ion mass spectroscopy and proton induced x-ray emission measurements were
performed to examine magnetic impurities, showing that the observed
ferromagnetism is unlikely due to any magnetic contaminant.Comment: 6 pages, 6 figure
Direct observation of anisotropic small-hole polarons in an orthorhombic structure of BiVO₄ films
Here, we report an anisotropic small-hole polaron in an orthorhombic structure of BiVO₄ films grown by pulsed-laser deposition on yttrium-doped zirconium oxide substrate. The polaronic state and electronic structure of BiVO₄ films are revealed using a combination of polarization-dependent x-ray absorption spectroscopy at VL[subscript 3,2] edges, spectroscopic ellipsometry, x-ray photoemission spectroscopies, and high-resolution x-ray diffraction with the support of first-principles calculations. We find that in the orthorhombic phase, which is slightly different from the conventional pucherite structure, the unoccupied V 3d orbitals and charge inhomogeneities lead to an anisotropic small-hole polaron state. Our result shows the importance of the interplay of charge and lattice for the formation of a hole polaronic state, which has a significant impact in the electrical conductivity of BiVO₄, hence its potential use as a photoanode for water splitting
Electron Accumulation and Emergent Magnetism in LaMnO3/SrTiO3 Heterostructures
Emergent phenomena at polar-nonpolar oxide interfaces have been studied
intensely in pursuit of next-generation oxide electronics and spintronics. Here
we report the disentanglement of critical thicknesses for electron
reconstruction and the emergence of ferromagnetism in polar-mismatched
LaMnO3/SrTiO3 (001) heterostructures. Using a combination of element-specific
X-ray absorption spectroscopy and dichroism, and first-principles calculations,
interfacial electron accumulation and ferromagnetism have been observed within
the polar, antiferromagnetic insulator LaMnO3. Our results show that the
critical thickness for the onset of electron accumulation is as thin as 2 unit
cells (UC), significantly thinner than the observed critical thickness for
ferromagnetism of 5 UC. The absence of ferromagnetism below 5 UC is likely
induced by electron over-accumulation. In turn, by controlling the doping of
the LaMnO3, we are able to neutralize the excessive electrons from the polar
mismatch in ultrathin LaMnO3 films and thus enable ferromagnetism in films as
thin as 3 UC, extending the limits of our ability to synthesize and tailor
emergent phenomena at interfaces and demonstrating manipulation of the
electronic and magnetic structures of materials at the shortest length scales.Comment: Accepted by Phys. Rev. Let