Surface composition of BaTiO3/SrTiO3(001) films grown by atomic oxygen plasma assisted molecular beam epitaxy

We have investigated the growth of BaTiO3 thin films deposited on pure and 1% Nb-doped SrTiO3(001) single crystals using atomic oxygen assisted molecular beam epitaxy (AO-MBE) and dedicated Ba and Ti Knudsen cells. Thicknesses up to 30 nm were investigated for various layer compositions. We demonstrate 2D growth and epitaxial single crystalline BaTiO3 layers up to 10 nm before additional 3D features appear; lattice parameter relaxation occurs during the first few nanometers and is completed at {\guillemotright}10 nm. The presence of a Ba oxide rich top layer that probably favors 2D growth is evidenced for well crystallized layers. We show that the Ba oxide rich top layer can be removed by chemical etching. The present work stresses the importance of stoichiometry and surface composition of BaTiO3 layers, especially in view of their integration in devices.Comment: In press in J. Appl. Phy

Structural and magnetic properties of CoPt mixed clusters

In this present work, we report a structural and magnetic study of mixed Co58Pt42 clusters. MgO, Nb and Si matrix can be used to embed clusters, avoiding any magnetic interactions between particles. Transmission Electron Microscopy (TEM) observations show that Co58Pt42 supported isolated clusters are about 2nm in diameter and crystallized in the A1 fcc chemically disordered phase. Grazing Incidence Small Angle X-ray Scattering (GISAXS) and Grazing Incidence Wide Angle X-ray Scattering (GIWAXS) reveal that buried clusters conserve these properties, interaction with matrix atoms being limited to their first atomic layers. Considering that 60% of particle atoms are located at surface, this interactions leads to a drastic change in magnetic properties which were investigated with conventional magnetometry and X-Ray Magnetic Circular Dichro\"{i}sm (XMCD). Magnetization and blocking temperature are weaker for clusters embedded in Nb than in MgO, and totally vanish in silicon as silicides are formed. Magnetic volume of clusters embedded in MgO is close to the crystallized volume determined by GIWAXS experiments. Cluster can be seen as a pure ferromagnetic CoPt crystallized core surrounded by a cluster-matrix mixed shell. The outer shell plays a predominant role in magnetic properties, especially for clusters embedded in niobium which have a blocking temperature 3 times smaller than clusters embedded in MgO

Role of disorder in half-filled high Landau levels

We study the effects of disorder on the quantum Hall stripe phases in half-filled high Landau levels using exact numerical diagonalization. We show that, in the presence of weak disorder, a compressible, striped charge density wave, becomes the true ground state. The projected electron density profile resembles that of a smectic liquid. With increasing disorder strength W, we find that there exists a critical value, W_c \sim 0.12 e^2/\epsilon l, where a transition/crossover to an isotropic phase with strong local electron density fluctuations takes place. The many-body density of states are qualitatively distinguishable in these two phases and help elucidate the nature of the transition.Comment: 4 pages, 4 figure

Determination of the cation site distribution of the spinel in multiferroic CoFe2O4 / BaTiO3 layers by X-ray photoelectron spectroscopy

International audienceThe properties of CoFe2O4/BaTiO3 artificial multiferroic multilayers strongly depend on the crystalline structure, the stoichiometry and the cation distribution between octahedral (Oh) and tetrahedral (Td) sites (inversion factor). In the present study, we have investigated epitaxial CoFe2O4 layers grown on BaTiO3, with different Co/Fe ratios. We determined the cation distribution in our samples by X-ray magnetic circular dichroism (XMCD), a well accepted method to do so, and by X-ray photoelectron spectroscopy (XPS), using a fitting method based on physical considerations. We observed that our XPS approach converged on results consistent with XMCD measurements made on the same samples. Thus, within a careful decomposition based on individual chemical environments it is shown that XPS is fully able to determine the actual inversion factor

Thermodynamic properties of the d-density wave order in cuprates

We solve a popular effective Hamiltonian of competing $d$-density wave and d-wave superconductivity orders self-consistently at the mean-field level for a wide range of doping and temperature. The theory predicts a temperature dependence of the $d$-density wave order parameter seemingly inconsistent with the neutron scattering and $\mu$SR experiments of the cuprates. We further calculate thermodynamic quantities, such as chemical potential, entropy and specific heat. Their distinct features can be used to test the existence of the $d$-density wave order in cuprates.Comment: changed to 4 pages and 4 figures. More reference added. Accepted by Phys. Rev.

Structure of self-organized Fe clusters grown on Au(111) analyzed by Grazing Incidence X-Ray Diffraction

We report a detailed investigation of the first stages of the growth of self-organized Fe clusters on the reconstructed Au(111) surface by grazing incidence X-ray diffraction. Below one monolayer coverage, the Fe clusters are in "local epitaxy" whereas the subsequent layers adopt first a strained fcc lattice and then a partly relaxed bcc(110) phase in a Kurdjumov-Sachs epitaxial relationship. The structural evolution is discussed in relation with the magnetic properties of the Fe clusters.Comment: 7 pages, 6 figures, submitted to Physical Review B September 200

Top-transmon: hybrid superconducting qubit for parity-protected quantum computation

Qubits constructed from uncoupled Majorana fermions are protected from decoherence, but to perform a quantum computation this topological protection needs to be broken. Parity-protected quantum computation breaks the protection in a minimally invasive way, by coupling directly to the fermion parity of the system --- irrespective of any quasiparticle excitations. Here we propose to use a superconducting charge qubit in a transmission line resonator (a socalled transmon) to perform parity-protected rotations and read-out of a topological (top) qubit. The advantage over an earlier proposal using a flux qubit is that the coupling can be switched on and off with exponential accuracy, promising a reduced sensitivity to charge noise.Comment: 7 pages, 5 figure

Finite Temperature Density Instability at High Landau Level Occupancy

We study here the onset of charge density wave instabilities in quantum Hall systems at finite temperature for Landau level filling $\nu>4$. Specific emphasis is placed on the role of disorder as well as an in-plane magnetic field. Beyond some critical value, disorder is observed to suppress the charge density wave melting temperature to zero. In addition, we find that a transition from perpendicular to parallel stripes (relative to the in-plane magnetic field) exists when the electron gas thickness exceeds $\approx 60$\AA. The perpendicular alignment of the stripes is in agreement with the experimental finding that the easy conduction direction is perpendicular to the in-plane field.Comment: 4 pages, 2 eps figures. We show explicitly that a transition from perpendicular to parallel stripes (relative to the in-plane magnetic field) exists when the electron gas thickness exceeds $\approx 60$\AA. The perpendicular alignment of the stripes is in agreement with the experimental finding that the easy conduction direction is perpendicular to the in-plane fiel

Density Induced Interchange of Anisotropy Axes at Half-Filled High Landau Levels

We observe density induced 90$^{\circ}$ rotations of the anisotropy axes in transport measurements at half-filled high Landau levels in the two dimensional electron system, where stripe states are proposed ($\nu$=9/2, 11/2, etc). Using a field effect transistor, we find the transition density to be $2.9\times10^{11}$cm$^{-2}$ at $\nu$=9/2. Hysteresis is observed in the vicinity of the transition. We construct a phase boundary in the filling factor-magnetic field plane in the regime $4.4<\nu<4.6$. An in-plane magnetic field applied along either anisotropy axis always stabilizes the low density orientation of the stripes.Comment: 1 revtex file, 3 eps figure