Chemical potential shift in La(1-x)Sr(x)MnO(3): Photoemission test of the phase separation scenario

We have studied the chemical potential shift in La(1-x)Sr(x)MnO(3) as a function of doped hole concentration by core-level x-ray photoemission. The shift is monotonous, which means that there is no electronic phase separation on a macroscopic scale, whereas it is consistent with the nano-meter scale cluster formation induced by chemical disorder. Comparison of the observed shift with the shift deduced from the electronic specific heat indicates that hole doping in La(1-x)Sr(x)MnO(3) is well described by the rigid-band picture. In particular no mass enhancement toward the metal-insulator boundary was implied by the chemical potential shift, consistent with the electronic specific heat data.Comment: 7 pages, 3 figures, to be published in Europhysics Letter

Modulation Doping of a Mott Quantum Well by a Proximate Polar Discontinuity

We present evidence for hole injection into LaAlO3/LaVO3/LaAlO3 quantum wells near a polar surface of LaAlO3 (001). As the surface is brought in proximity to the LaVO3 layer, an exponential drop in resistance and a decreasing positive Seebeck coefficient is observed below a characteristic coupling length of 10-15 unit cells. We attribute this behavior to a crossover from an atomic reconstruction of the AlO2-terminated LaAlO3 surface to an electronic reconstruction of the vanadium valence. These results suggest a general approach to tunable hole-doping in oxide thin film heterostructures.Comment: 16 pages, 7 figure

Temperature-dependent photoemission spectral weight transfer and chemical potential shift in Pr1−x_{1-x}Cax_xMnO3_3 : Implications for charge density modulation

We have studied the temperature dependence of the photoemission spectra of Pr$_{1-x}$Ca$_x$MnO$_3$ (PCMO) with $x=0.25$, 0.3 and 0.5. For $x=0.3$ and 0.5, we observed a gap in the low-temperature CE-type charge-ordered (CO) phase and a pseudogap with a finite intensity at the Fermi level ($E_F$) in the high-temperature paramagnetic insulating (PI) phase. Within the CO phase, the spectral intensity near $E_F$ gradually increased with temperature. These observations are consistent with the results of Monte Carlo simulations on a model including charge ordering and ferromagnetic fluctuations [H. Aliaga {\it et al.} Phys. Rev. B {\bf 68}, 104405 (2003)]. For $x=0.25$, on the other hand, little temperature dependence was observed within the low-temperature ferromagnetic insulating (FI) phase and the intensity at $E_F$ remained low in the high-temperature PI phase. We attribute the difference in the temperature dependence near $E_F$ between the CO and FI phases to the different correlation lengths of orbital order between both phases. Furthermore, we observed a chemical potential shift with temperature due to the opening of the gap in the FI and CO phases. The doping dependent chemical potential shift was recovered at low temperatures, corresponding to the disappearance of the doping dependent change of the modulation wave vector. Spectral weight transfer with hole concentration was clearly observed at high temperatures but was suppressed at low temperatures. We attribute this observation to the fixed periodicity with hole doping in PCMO at low temperatures.Comment: 5pages, 7figure

Potential Profiling of the Nanometer-Scale Charge Depletion Layer in n-ZnO/p-NiO Junction Using Photoemission Spectroscopy

We have performed a depth-profile analysis of an all-oxide p-n junction diode n-ZnO/p-NiO using photoemission spectroscopy combined with Ar-ion sputtering. Systematic core-level shifts were observed during the gradual removal of the ZnO overlayer, and were interpreted using a simple model based on charge conservation. Spatial profile of the potential around the interface was deduced, including the charge-depletion width of 2.3 nm extending on the ZnO side and the built-in potential of 0.54 eV

Electronic charges and electric potential at LaAlO3/SrTiO3 interfaces studied by core-level photoemission spectroscopy

We studied LaAlO3/SrTiO3 interfaces for varying LaAlO3 thickness by core-level photoemission spectroscopy. In Ti 2p spectra for conducting "n-type" interfaces, Ti3+ signals appeared, which were absent for insulating "p-type" interfaces. The Ti3+ signals increased with LaAlO3 thickness, but started well below the critical thickness of 4 unit cells for metallic transport. Core-level shifts with LaAlO3 thickness were much smaller than predicted by the polar catastrophe model. We attribute these observations to surface defects/adsorbates providing charges to the interface even below the critical thickness

Soft x-ray magnetic circular dichroism study of weakly ferromagnetic Zn1−x_{1-x}Vx_xO thin film

We performed a soft x-ray magnetic circular dichroism (XMCD) study of a Zn$_{1-x}$V$_x$O thin film which showed small ferromagnetic moment. Field and temperature dependences of V 2$p$ XMCD signals indicated the coexistence of Curie-Weiss paramagnetic, antiferromagnetic, and possibly ferromagnetic V ions, quantitatively consistent with the magnetization measurements. We attribute the paramagnetic signal to V ions substituting Zn sites which are somewhat elongated along the c-axis

Phase Change Observed in Ultrathin Ba0.5Sr0.5TiO3 Films by in-situ Resonant Photoemission Spectroscopy

Epitaxial Ba0.5Sr0.5TiO3 thin films were prepared on Nb-doped SrTiO3 (100)substrates by the pulsed laser deposition technique, and were studied by measuring the Ti 2p - 3d resonant photoemission spectra in the valence-band region as a function of film thickness, both at room temperature and low temperature. Our results demonstrated an abrupt variation in the spectral structures between 2.8 nm (~7 monolayers) and 2.0 nm (~5 monolayers) Ba0.5Sr0.5TiO3 films, suggesting that there exists a critical thickness for phase change in the range of 2.0 nm to 2.8 nm. This may be ascribed mainly to the intrinsic size effects.Comment: 13 pages, 4 figure