Hydrostatic pressure to trigger and assist magnetic transitions: baromagnetic refrigeration

The possible application of the barocaloric effect to produce solid state refrigerators is a topic of interest in the field of applied physics. In this work, we present experimental data about the influence of external pressure on the magnetic properties of a manganite with phase separation. Using the Jahn Teller effect associated with the presence of the charge ordering we were able to follow the transition to the ferromagnetic state induced by pressure. We also demonstrated that external pressure can assist the ferromagnetic state, decreasing the magnetic field necessary to generate the magnetic transition.Comment: Acepted to be published in Applied Physics Letter

Evidence for polarons in iron pnictides of the Ln-1111 and AE-122 families

Examination of the electrical resistivities of iron pnictides shows that they can be accounted by conduction by polarons. Their activation energies show a linear behaviour with the critical temperatures of the spin density waves (SDW), T*, as both vary with pressure. The slope matches the ratio SDW gap to T*, while the intercept can be related to the transition temperature of the lattice distortion, T0. An adapted Landau free energy predicts the observed order of the transitions, according to which is higher, T* or T0. Simple arguments favour combined Jahn-Teller antiferromagnetic bipolarons.Comment: 14 pages with 4 Figure

Bell's inequality tests: from photons to B-mesons

We analyse the recent claim that a violation of a Bell's inequality has been observed in the $B$--meson system [A. Go, {\em Journal of Modern Optics} {\bf 51} (2004) 991]. The results of this experiment are a convincing proof of quantum entanglement in $B$--meson pairs similar to that shown by polarization entangled photon pairs. However, we conclude that the tested inequality is not a genuine Bell's inequality and thus cannot discriminate between quantum mechanics and local realistic approaches.Comment: 5 page

Asymmetries in the Non-Mesonic Weak Decay of Polarized Lambda-Hypernuclei

The non-mesonic weak decay of polarized Lambda-hypernuclei is studied for the first time by taking into account, with a Monte Carlo intranuclear cascade code, the nucleon final state interactions. A one-meson-exchange model is employed to describe the Lambda N-> n N processes in a finite nucleus framework. The relationship between the intrinsic Lambda asymmetry parameter a_\Lambda and the asymmetry a^M_\Lambda accessible in experiments is discussed. A strong dependence of a^M_\Lambda on nucleon final state interactions and detection threshold is obtained. Our results for a^M_\Lambda are consistent with ^{11}_\Lambda B and ^{12}_\Lambda C data but disagree with observations in ^5_\Lambda He.Comment: 4 RevTeX pages, 2 figure

Pressure effects in the triangular layered cobaltites NaxCoO2

We have measured transport properties as a function of temperature and pressure up to 30GPa in the NaxCoO2 system. For the x=0.5 sample the transition temperature at 53K increases with pressure, while paradoxically the sample passes from an insulating to a metallic ground state. A similar transition is observed in the x=0.31 sample under pressure. Compression on the x=0.75 sample transforms the sample from a metallic to an insulating state. We discuss our results in terms of interactions between band structure effects and Na+ order.Comment: 18 pages, 5 figure

Inelastic X-ray scattering studies of phonon dispersions in superconductors at high pressures

Electron-phonon interaction is of central importance for the electrical and heat transport properties of metals, and is directly responsible for charge-density-waves or (conventional) superconducting instabilities. The direct observation of phonon dispersion anomalies across electronic phase transitions can provide insightful information regarding the mechanisms underlying their formation. Here, we review the current status of phonon dispersion studies in superconductors under hydrostatic and uniaxial pressure. Advances in the instrumentation of high resolution inelastic X-ray scattering beamlines and pressure generating devices allow these measurements to be performed routinely at synchrotron beamlines worldwide.Comment: 8 pages, 6 figures. Invited review submitted to Superconductor Science and Technology, Focus issue on Hydride & High-Pressure Superconductors. References in figure caption fixed. Hyperlinks adde

Temperature dependence of iron local magnetic moment in phase-separated superconducting chalcogenide

We have studied local magnetic moment and electronic phase separation in superconducting K$_{x}$Fe$_{2-y}$Se$_2$ by x-ray emission and absorption spectroscopy. Detailed temperature dependent measurements at the Fe K-edge have revealed coexisting electronic phases and their correlation with the transport properties. By cooling down, the local magnetic moment of Fe shows a sharp drop across the superconducting transition temperature (T$_c$) and the coexisting phases exchange spectral weights with the low spin state gaining intensity at the expense of the higher spin state. After annealing the sample across the iron-vacancy order temperature, the system does not recover the initial state and the spectral weight anomaly at T$_c$ as well as superconductivity disappear. The results clearly underline that the coexistence of the low spin and high spin phases and the transitions between them provide unusual magnetic fluctuations and have a fundamental role in the superconducting mechanism of electronically inhomogeneous K$_{x}$Fe$_{2-y}$Se$_2$ system.Comment: 6 pages, 5 figure