An X-Ray Induced Structural Transition in La_0.875Sr_0.125MnO_3

We report a synchrotron x-ray scattering study of the magnetoresistive manganite La_0.875Sr_0.125MnO_3. At low temperatures, this material undergoes an x-ray induced structural transition at which charge ordering of Mn^3+ and Mn^4+ ions characteristic to the low-temperature state of this compound is destroyed. The transition is persistent but the charge-ordered state can be restored by heating above the charge-ordering transition temperature and subsequently cooling. The charge-ordering diffraction peaks, which are broadened at all temperatures, broaden more upon x-ray irradiation, indicating the finite correlation length of the charge-ordered state. Together with the recent reports on x-ray induced transitions in Pr_(1-x)Ca_xMnO_3, our results demonstrate that the photoinduced structural change is a common property of the charge-ordered perovskite manganites.Comment: 5 pages, 4 embedded EPS figures; significant changes in the data analysis mad

Approach to the metal-insulator transition in La(1-x)CaxMnO3 (0<x<.2): magnetic inhomogeneity and spin wave anomaly

We describe the evolution of the static and dynamic spin correlations of La$_{1-x}$Ca$_x$MnO$_3$, for x=0.1, 0.125 and 0.2, where the system evolves from the canted magnetic state towards the insulating ferromagnetic state, approaching the metallic transition (x=0.22). In the x=0.1 sample, the observation of two spin wave branches typical of two distinct types of magnetic coupling, and of a modulation in the elastic diffuse scattering characteristic of ferromagnetic inhomogeneities, confirms the static and dynamic inhomogeneous features previously observed at x$<$0.1. The anisotropic q-dependence of the intensity of the low-energy spin wave suggests a bidimensionnal character for the static inhomogeneities. At x=0.125, which corresponds to the occurence of a ferromagnetic and insulating state, the two spin wave branches reduce to a single one, but anisotropic. At this concentration, an anomaly appears at {\bf q$_0$}=(1.25,1.25,0), that could be related to an underlying periodicity, as arising from (1.5,1.5,0) superstructures. At x=0.2, the spin-wave branch is isotropic. In addition to the anomaly observed at q$_0$, extra magnetic excitations are observed at larger q, forming an optical branch. The two dispersion curves suggest an anti-crossing behavior at some {\bf q$_0$'} value, which could be explained by a folding due to an underlying perodicity involving four cubic lattice spacings

Epitaxy and magnetotransport of Sr_2FeMoO_6 thin films

By pulsed-laser deposition epitaxial thin films of Sr_2FeMoO_6 have been pre- pared on (100) SrTiO_3 substrates. Already for a deposition temperature of 320 C epitaxial growth is achieved. Depending on deposition parameters the films show metallic or semiconducting behavior. At high (low) deposition temperature the Fe,Mo sublattice has a rock-salt (random) structure. The metallic samples have a large negative magnetoresistance which peaks at the Curie temperature. The magnetic moment was determined to 4 mu_B per formula unit (f.u.), in agreement with the expected value for an ideal ferrimagnetic arrangement. We found an ordinary Hall coefficient of -6.01x10^{-10} m^3/As at 300 K, corresponding to an electronlike charge-carrier density of 1.3 per Fe,Mo-pair. In the semiconducting films the magnetic moment is reduced to 1 mu_B/f.u. due to disorder in the Fe,Mo sublattice. In low fields an anomalous holelike contribution dominates the Hall voltage, which vanishes at low temperatures for the metallic films only.Comment: Institute of Physics, University of Mainz, Germany, 4 pages, including 5 pictures and 1 Table, submitted to Phys. Rev.

Crystal and magnetic structure of La1−x_{1-x}Cax_{x}MnO3_{3} compound $(0.11\leq x\leq 0.175

We studied the crystal and magnetic structure of the La$_{1-x}$Ca$_{x}$MnO$_{3}$ compound for $(0.11\leq x\leq 0.175)$ using stoichiometric samples. For $x<0.13$ the system's ground state is insulating canted antiferromagnetic. For $0.13\le x \le 0.175$ e below the Jahn Teller transition temperature ($T_{\rm JT}$) the crystal structure undergoes a monoclinic distortion. The crystal structure can be described with $P 2_1/c$ space group which permits two Mn sites. The unit cell strain parameter $s=2(a-c)/(a+c)$ increases for $T<T_{\rm JT}$, taking the maximum value at the Curie point, and then decreases. Below $T_{\rm M^/M^{//}}\approx 60$ K $s$ abruptly changes slope and finally approaches T=0 K with nearly zero slope. The change of $s$ at $T_{\rm M^/M^{//}}$ is connected to a characteristic feature in the magnetic measurements. As $x$ increases towards the ferromagnetic metallic boundary, although $s$ is reduced appreciably, the monoclinic structure is preserved. The monoclinic structure is discussed with relation to the orbital ordering, which can produce the ferromagnetic insulating ground state. We also studied samples that were prepared in air atmosphere. This category of samples shows ferromagnetic insulating behavior without following the particular variation of the $s$ parameter. The crystal structure of these samples is related to the so-called O$^{*}$ ($c>a>b/\sqrt{2}$) structure

Evidence of anisotropic magnetic polarons in la0.94_{0.94}Sr0.06_{0.06}MnO3_3 by neutron scattering and comparison with Ca-doped manganites

Elastic and inelastic neutron scattering experiments have been performed in a La$_{0.94}$Sr$_{0.06}$MnO$_3$ untwinned crystal, which exhibits an antiferromagnetic canted magnetic structure with ferromagnetic layers. The elastic small q scattering exhibits a modulation with an anisotropic q-dependence. It can be pictured by ferromagnetic inhomogeneities or polarons with a platelike shape, the largest size ($\approx17\AA$) and largest inter-polaron distance ($\approx$ 38$\AA$) being within the ferromagnetic layers. Comparison with observations performed on Ca-doped samples, which show the growth of the magnetic polarons with doping, suggests that this growth is faster for the Sr than for the Ca substitution. Below the gap of the spin wave branch typical of the AF layered magnetic structure, an additional spin wave branch reveals a ferromagnetic and isotropic coupling, already found in Ca-doped samples. Its q-dependent intensity, very anisotropic, closely reflects the ferromagnetic correlations found for the static clusters. All these results agree with a two-phase electronic segregation occurring on a very small scale, although some characteristics of a canted state are also observed suggesting a weakly inhomogeneous state.Comment: 11 pages, 11 figure

Transport and superconducting properties of Fe-based superconductors: SmFeAs(O1-x Fx) versus Fe1+y (Te1-x, Sex)

We present transport and superconducting properties - namely resistivity, magnetoresistivity, Hall effect, Seebeck effect, thermal conductivity, upper critical field - of two different families of Fe-based superconductors, which can be viewed in many respects as end members: SmFeAs(O1-xFx) with the largest Tc and the largest anisotropy and Fe1+y(Te1-x,Sex), with the largest Hc2, the lowest Tc and the lowest anisotropy. In the case of the SmFeAs(O1-xFx) series, we find that a single band description allows to extract an approximated estimation of band parameters such as carrier density and mobility from experimental data, although the behaviour of Seebeck effect as a function of doping demonstrates that a multiband description would be more appropriate. On the contrary, experimental data of the Fe1+y(Te1-x,Sex) series exhibit a strongly compensated behaviour, which can be described only within a multiband model. In the Fe1+y(Te1-x,Sex) series, the role of the excess Fe, tuned by Se stoichiometry, is found to be twofold: it dopes electrons in the system and it introduces localized magnetic moments, responsible for Kondo like scattering and likely pair-breaking of Cooper pairs. Hence, excess Fe plays a crucial role also in determining superconducting properties such as the Tc and the upper critical field Bc2. The huge Bc2 values of the Fe1+y(Te1-x,Sex) samples are described by a dirty limit law, opposed to the clean limit behaviour of the SmFeAs(O1-xFx) samples. Hence, magnetic scattering by excess Fe seems to drive the system in the dirty regime, but its detrimental pairbreaking role seems not to be as severe as predicted by theory. This issue has yet to be clarified, addressing the more fundamental issue of the interplay between magnetism and superconductivity

The COSPIX mission: focusing on the energetic and obscured Universe

Tracing the formation and evolution of all supermassive black holes, including the obscured ones, understanding how black holes influence their surroundings and how matter behaves under extreme conditions, are recognized as key science objectives to be addressed by the next generation of instruments. These are the main goals of the COSPIX proposal, made to ESA in December 2010 in the context of its call for selection of the M3 mission. In addition, COSPIX, will also provide key measurements on the non thermal Universe, particularly in relation to the question of the acceleration of particles, as well as on many other fundamental questions as for example the energetic particle content of clusters of galaxies. COSPIX is proposed as an observatory operating from 0.3 to more than 100 keV. The payload features a single long focal length focusing telescope offering an effective area close to ten times larger than any scheduled focusing mission at 30 keV, an angular resolution better than 20 arcseconds in hard X-rays, and polarimetric capabilities within the same focal plane instrumentation. In this paper, we describe the science objectives of the mission, its baseline design, and its performances, as proposed to ESA.Comment: 7 pages, accepted for publication in Proceedings of Science, for the 25th Texas Symposium on Relativistic Astrophysics (eds. F. Rieger &amp; C. van Eldik), PoS(Texas 2010)25

Optical study of orbital excitations in transition-metal oxides

The orbital excitations of a series of transition-metal compounds are studied by means of optical spectroscopy. Our aim was to identify signatures of collective orbital excitations by comparison with experimental and theoretical results for predominantly local crystal-field excitations. To this end, we have studied TiOCl, RTiO3 (R=La, Sm, Y), LaMnO3, Y2BaNiO5, CaCu2O3, and K4Cu4OCl10, ranging from early to late transition-metal ions, from t_2g to e_g systems, and including systems in which the exchange coupling is predominantly three-dimensional, one-dimensional or zero-dimensional. With the exception of LaMnO3, we find orbital excitations in all compounds. We discuss the competition between orbital fluctuations (for dominant exchange coupling) and crystal-field splitting (for dominant coupling to the lattice). Comparison of our experimental results with configuration-interaction cluster calculations in general yield good agreement, demonstrating that the coupling to the lattice is important for a quantitative description of the orbital excitations in these compounds. However, detailed theoretical predictions for the contribution of collective orbital modes to the optical conductivity (e.g., the line shape or the polarization dependence) are required to decide on a possible contribution of orbital fluctuations at low energies, in particular in case of the orbital excitations at about 0.25 eV in RTiO3. Further calculations are called for which take into account the exchange interactions between the orbitals and the coupling to the lattice on an equal footing.Comment: published version, discussion of TiOCl extended to low T, improved calculation of orbital excitation energies in TiOCl, figure 16 improved, references updated, 33 pages, 20 figure

The Athena X-ray Integral Field Unit: a consolidated design for the system requirement review of the preliminary definition phase

Instrumentatio

Expectation management

We consider the application of a Fleischer–Jegerlehner-like treatment of tadpoles to the calculation of neutral scalar masses (including the Higgs) in general theories beyond the Standard Model. This is especially useful when the theory contains new scalars associated with a small expectation value, but comes with its own disadvantages. We show that these can be overcome by combining with effective field theory matching. We provide the formalism in this modified approach for matching the quartic coupling of the Higgs via pole masses at one loop, and apply it to both a toy model and to the $\mu$NMSSM as prototypes where the standard treatment can break down