Epitaxial growth and transport properties of Nb-doped SrTiO3_{3} thin films

Nb-doped SrTiO$_{3}$ epitaxial thin films have been prepared on (001) SrTiO$_{3}$ substrates using pulsed laser deposition. A high substrate temperature ($>1000^{\circ}{C}$) was found to be necessary to achieve 2-dimensional growth. Atomic force microscopy reveals atomically flat surfaces with 3.9 \AA $$ steps. The films show a metallic behavior, residual resistivity ratios between 10 and 100, and low residual resistivity of the order of 10$^{-4}$$\Omega$cm. At 0.3 K, a sharp superconducting transition, reaching zero resistance, is observed.Comment: 4 pages, 4 figure

A-site driven ferroelectricity in strained ferromagnetic L2NiMnO6 thin films

We report on theoretical and experimental investigation of A-site driven ferroelectricity in ferromagnetic La2NiMnO6 thin films grown on SrTiO3 substrates. Structural analysis and density functional theory calculations show that epitaxial strain stretches the rhombohedral La2NiMnO6 crystal lattice along the [111]cubic direction, triggering a displacement of the A-site La ions in the double perovskite lattice. The lattice distortion and the A-site displacements stabilize a ferroelectric polar state in ferromagnetic La2NiMnO6 crystals. The ferroelectric state only appears in the rhombohedral La2NiMnO6 phase, where MnO6 and NiO6 octahedral tilting is inhibited by the 3-fold crystal symmetry. Electron localization mapping showed that covalent bonding with oxygen and 6s orbital lone pair formation are negligible in this material.Comment: in pres

In situ photoemission study on atomically-controlled La1−x_{1-x}Srx_xMnO3_3 thin films: Composition dependence of the electronic structure

We have investigated change in the electronic structures of atomically-controlled La$_{1-x}$Sr$_x$MnO$_3$ (LSMO) thin films as a function of hole-doping level ($x$) in terms of {\it in situ} photoemission spectroscopy (PES) and x-ray absorption spectroscopy (XAS) measurements. The {\it in situ} PES measurements on a well-ordered surface of high-quality epitaxial LSMO thin films enable us to reveal their intrinsic electronic structures, especially the structure near the Fermi level ($E_F$). We have found that overall features of valence band as well as the core levels monotonically shifted toward lower binding energy as $x$ was increased, indicating the rigid-band like behavior of underlying electronic structure of LSMO thin films. The peak nearest to $E_F$ due to the $e_g$ orbital is also found to move toward $E_F$ in a rigid-band manner, while the peak intensity decreases with increasing $x$. The loss of spectral weight with $x$ in the occupied density of states was compensated by simultaneous increment of the shoulder structure in O 1$s$ XAS spectra, suggesting the existence of a pseudogap, that is depression in spectral weight at $E_F$, for all metallic compositions. These results indicate that the simple rigid-band model does not describe the electronic structure near $E_F$ of LSMO and that the spectral weight transfer from below to above $E_F$ across the gap dominates the spectral changes with $x$ in LSMO thin films.Comment: 8 pages, 8 figure

Modulation of the ferromagnetic insulating phase in Pr0.8Ca0.2MnO3 by Co substitution

Ferromagnetic insulator Pr0.8Ca0.2Mn1-yCoyO3 (0 <= y <= 0.7) thin films were epitaxially grown on (LaAlO3)0.3-(SrAl0.5Ta0.5O3)0.7 (100) substrates by pulsed laser deposition. To probe the ferromagnetic insulator state of hole-doped manganites, the Co content dependences of the structural, magnetic, and transport properties were studied. Variation of lattice constant by the substitution of Co ions is well reproduced considering that divalent and trivalent Co ions substitute for Mn ions at the perovskite B-sites. For 0 <= y <= 0.3, the Curie temperature, saturation magnetization, and magnetoresistance increase with increasing Co content, retaining the insulating properties. Detailed analyses of transport and magnetic properties indicate the contribution of both double exchange and superexchange interactions to the appearance of the ferromagnetic insulating phase.Comment: 3 pages, 3 figure

In-situ photoemission study of Pr_{1-x}Ca_xMnO_3 epitaxial thin films with suppressed charge fluctuations

We have performed an {\it in-situ} photoemission study of Pr_{1-x}Ca_xMnO_3 (PCMO) thin films grown on LaAlO_3 (001) substrates and observed the effect of epitaxial strain on the electronic structure. We found that the chemical potential shifted monotonically with doping, unlike bulk PCMO, implying the disappearance of incommensurate charge fluctuations of bulk PCMO. In the valence-band spectra, we found a doping-induced energy shift toward the Fermi level (E_F) but there was no spectral weight transfer, which was observed in bulk PCMO. The gap at E_F was clearly seen in the experimental band dispersions determined by angle-resolved photoemission spectroscopy and could not be explained by the metallic band structure of the C-type antiferromagnetic state, probably due to localization of electrons along the ferromagnetic chain direction or due to another type of spin-orbital ordering.Comment: 5 pages, 4 figure

Manifestation of Correlation Effects in the photoemission spectra of Ca1−x_{1-x}Srx_xRuO3_3

We have measured soft x-ray photoemission and O 1{\it s} x-ray absorption spectra of Ca$_{1-x}$Sr$_x$RuO$_3$ thin films prepared {\it in situ}. The coherent and incoherent parts have been identified in the bulk component of the photoemission spectra, and spectral weight transfer from the coherent to the incoherent part has been observed with decreasing $x$, namely, with increasing orthorhombic distortion. We propose that, while the Ru 4d one-electron bandwidth does not change with $x$, the distortion and hence the splitting of the $t_{2\text{g}}$ band effectively increases electron correlation strength. Although strong mass enhancement is found in the electronic specific heat data, the coherent part remains wide, suggesting enhanced band narrowing only in the vicinity of {\it E$_{F}$}

Gradual Disappearance of the Fermi Surface near the Metal-Insulator Transition in La1−x_{1-x}Srx_{x}MnO3_{3}

We report the first observation of changes in the electronic structure of La$_{1-x}$Sr$_{x}$MnO$_{3}$ (LSMO) across the filling-control metal-insulator (MI) transition by means of in situ angle-resolved photoemission spectroscopy (ARPES) of epitaxial thin films. The Fermi surface gradually disappears near the MI transition by transferring the spectral weight from the coherent band near the Fermi level ($E_{F}$) to the lower Hubbard band, whereas a pseudogap behavior also exists in the ARPES spectra in the close vicinity of $E_{F}$ for the metallic LSMO. These results indicate that the spectral weight transfer derived from strong electron-electron interaction dominates the gap formation in LSMO associated with the filling-control MI transition.Comment: 11 pages, 4 figure

Spin-filter tunnel junction with matched Fermi surfaces

Efficient injection of spin-polarized current into a semiconductor is a basic prerequisite for building semiconductor-based spintronic devices. Here, we use inelastic electron tunneling spectroscopy to show that the efficiency of spin-filter-type spin injectors is limited by spin scattering of the tunneling electrons. By matching the Fermi-surface shapes of the current injection source and target electrode material, spin injection efficiency can be significantly increased in epitaxial ferromagnetic insulator tunnel junctions. Our results demonstrate that not only structural but also Fermi-surface matching is important to suppress scattering processes in spintronic devices.Comment: 5 pages, 4 figure

Ion induced quark-gluon implosion

We investigate nuclear fragmentation in the central proton-nucleus and nucleus - nucleus collisions at the energies of LHC. We argue that within the semi-classical approximation because of fast increase with energy of cross sections of soft and hard interactions each nucleon is stripped in the average process off ``soft'' partons and fragments into a collection of leading quarks and gluons with large $p_t$. Valence quarks and gluons are streaming in the opposite directions when viewed in the c.m. of the produced system. The resulting pattern of the fragmentation of the colliding nuclei leads to an implosion of the quark and gluon constituents of the nuclei. The matter density produced at the initial stage in the nucleus fragmentation region is estimated to be $\geq$ 50 GeV/fm$^3$ at the LHC energies and probably $\geq$ 10 GeV/fm$^3$ at RHIC.Comment: 5 pages, final version, discussion of the signals of the new phase is expande

The band structure and Fermi surface of La0.6_{0.6}Sr0.4_{0.4}MnO3_{3} thin films studied by in-situ angle-resolved photoemission spectroscopy

We have performed an in situ angle-resolved photoemission spectroscopy (ARPES) on single-crystal surfaces of La$_{0.6}$Sr$_{0.4}$MnO$_{3}$ (LSMO) thin films grown on SrTiO$_{3}$ (001) substrates by laser molecular beam epitaxy, and investigated the electronic structure near the Fermi level ($E_{F}$). The experimental results were compared with the band-structure calculation based on LDA + $U$. The band structure of LSMO thin films consists of several highly dispersive O 2$p$ derived bands in the binding energy range of 2.0 - 6.0 eV and Mn 3$d$ derived bands near $E_{F}$. ARPES spectra around the $Gamma$ point show a dispersive band near $E_{F}$ indicative of an electron pocket centered at the $Gamma$ point, although it was not so clearly resolved as an electronlike pocket due to the suppression of spectral weight in the vicinity of $E_{F}$. Compared with the band-structure calculation, the observed conduction band is assigned to the Mn 3$de_{g}$ majority-spin band responsible for the half-metallic nature of LSMO. We have found that the estimated size of the Fermi surface is consistent with the prediction of the band-structure calculation, while the band width becomes significantly narrower than the calculation. Also, the intensity near $E_{F}$ is strongly reduced. The origin of these discrepancies between the experiment and the calculation is discussed.Comment: 7 pages, 5 figure