Transition énergétique

The brightness of a light source defines its applicability to nonlinear phenomena in science. The SwissFEL laser group has now overcome one of the two principal technological hurdles to produce bright pulses in the Terahertz range (0.1-5 THz).

We have studied the magnetic and superconducting properties of Ba(Fe_0.95Co_0.05)₂As₂ as a function of temperature and external magnetic field using neutron scattering and muon spin rotation. Below the superconducting transition temperature the magnetic and superconducting order parameters coexist and compete. A magnetic field can significantly enhance the magnetic scattering in the superconducting state, roughly doubling the Bragg intensity at 13.5T.

Ultrafast magnetization reversal driven by femtosecond laser pulses has been shown to be a promising way to write information. Seeking to improve the recording density has raised intriguing fundamental questions about the feasibility of combining ultrafast temporal resolution with sub-wavelength spatial resolution for magnetic recording. Here we report on the experimental demonstration of nanoscale sub-100 ps all-optical magnetization switching, providing a path to sub-wavelength magnetic recording.

X-ray magnetic circular dichroism measurements evidence a 10K shift of the magnetic Curie temperature for La_0.7Sr_0.3MnO₃ deposited on the piezoelectric substrate [Pb(Mg_1/3Nb_2/3)O₃]_0.68−[PbTiO₃]_0.32 (011) for two different remanent piezostrain states.

Although muon spin relaxation is commonly used to probe local magnetic order, spin freezing, and spin dynamics, we identify an experimental situation in which the measured response is dominated by an effect resulting from the muon-induced local distortion rather than the intrinsic behavior of the host compound.

Superconducting [(Li_1-xFe_x)OH](Fe_1-yLi_y)Se (x≈0.2, y≈0.08) was synthesized by hydrothermal methods and characterized by single-crystal and powder X-ray diffrac- tion. The structure contains alternating layers of anti-PbO type (Fe_1-yLi_y)Se and (Li_1-xFe_x)OH. Electrical resistivity and magnetic susceptibility measurements reveal superconductivity at 43K.

Quantum magnets have occupied the fertile ground between many-body theory and low-temperature experiments on real materials since the early days of quantum mechanics. However, our understanding of even deceptively simple systems of interacting spin-1/2 particles is far from complete. The quantum square-lattice Heisenberg antiferromagnet, for example, exhibits a striking anomaly of hitherto unknown origin in its magnetic excitation spectrum.

The neutron gyromagnetic ratio has been measured relative to that of the ¹⁹⁹Hg atom with an uncertainty of 0.8 ppm. We employed an apparatus where ultracold neutrons and mercury atoms are stored in the same volume and report the result γ_n/γ_Hg = 3.8424574(30).

The magnetic ground state of the J_eff = 1/2 hyperkagome lattice in Na₄Ir₃O₈ is explored via combined bulk magnetization, muon spin relaxation, and neutron scattering measurements. A short-range, frozen state comprised of quasistatic moments develops below a characteristic temperature of T_F = 6K, revealing an inhomogeneous distribution of spins occupying the entirety of the sample volume. Quasistatic, short- range spin correlations persist until at least 20 mK and differ substantially from the nominally dynamic response of a quantum spin liquid. Our data demonstrate that an inhomogeneous magnetic ground state arises in Na₄Ir₃O₈ driven either by disorder inherent to the creation of the hyperkagome lattice itself or stabilized via quantum fluctuations.

The muon decay parameter ξ″ has been determined in a measurement of the longitudinal polarization of positrons emitted from polarized and depolarized muons. The result, ξ″ = 0.981 ± 0.045_stat ± 0.003_syst, is consistent with the Standard Model prediction of unity, and provides an order of magnitude improvement in the relative precision of this parameter. This value sets new constraints on exotic couplings beyond the dominant V-A description of the leptonic weak interaction.

The 2014 Innovation Award on Synchrotron Radiation was bestowed to researchers Ana Diaz, Manuel Guizar-Sicairos, Mirko Holler, and Jörg Raabe from the Paul Scherrer Institut, Switzerland, for their contributions to method and instrumentation development, which have set new standards in high-resolution 3D hard X-ray microscopy.

SmFeO₃ has attracted considerable attention very recently due to its reported multiferroic properties above room temperature. We have performed powder and single crystal neutron diffraction as well as complementary polarization dependent soft X-ray absorption spectroscopy measurements on floating-zone grown SmFeO₃ single crystals in order to determine its magnetic structure. We found a k=0 G-type collinear antiferromagnetic structure that is not compatible with inverse Dzyaloshinskii-Moriya interaction driven ferroelectricity. While the structural data reveal a clear sign for magneto-elastic coupling at the Néel-temperature of ∼675 K, the dielectric measurements remain silent as far as ferroelectricity is concerned.

The MuCap experiment at the Paul Scherrer Institute performed a high-precision measurement of the rate of the basic electroweak process of nuclear muon capture by the proton, μ^- + p → n + ν_μ. The experimental approach was based on the use of a time projection chamber (TPC) that operated in pure hydrogen gas at a pressure of 10 bar and functioned as an active muon stopping target. The TPC detected the tracks of individual muon arrivals in three dimensions, while the trajectories of outgoing decay (Michel) electrons were measured by two surrounding wire chambers and a plastic scintillation hodoscope.

We present neutron scattering data on the structure and dynamics of melts from polyethylene oxide rings with molecular weights up to ten times the entanglement mass of the linear counterpart. The data reveal a very compact conformation displaying a structure approaching a mass fractal, as hypothesized by recent simulation work. The dynamics is characterized by a fast Rouse relaxation of subunits (loops) and a slower dynamics displaying a lattice animal-like loop displacement.

Strain is a leading candidate for controlling magnetoelectric coupling in multiferroics. Here, we use x-ray diffraction to study the coupling between magnetic order and structural distortion in epitaxial films of the orthorhombic (o-) perovskite LuMnO₃. An antiferromagnetic spin canting in the E-type magnetic structure is shown to be related to the ferroelectrically induced structural distortion and to a change in the magnetic propagation vector.

Magnetic shape-memory Heusler alloys are multiferroics stabilized by the correlations between electronic, magnetic, and structural order. To study these correlations we use time-resolved x-ray diffraction and magneto-optical Kerr effect experiments to measure the laser induced dynamics in a Heusler alloy Ni2MnGa film and reveal a set of time scales intrinsic to the system.