6. June 2017

Field-induced magnetic instability within a superconducting condensate

D. Mazzone et al., Science Advances 3, e1602055 (2017). The application of magnetic fields, chemical substitution, or hydrostatic pressure to strongly correlated electron materials can stabilize electronic phases with different organizational principles. We present evidence for a field-induced quantum phase transition, in superconducting Nd_0.05Ce_0.95CoIn₅, that separates two antiferromagnetic phases with identical magnetic symmetry. At zero field, we find a spin-density wave that is suppressed at the critical field μ₀H* = 8 T. For H > H*, a spin-density phase emerges and shares many properties with the Q phase in CeCoIn₅. These results suggest that the magnetic instability is not magnetically driven, and we propose that it is driven by a modification of superconducting condensate at H*.

28. April 2017

Bound States and Field-Polarized Haldane Modes in a Quantum Spin Ladder

S. Ward, M. Mena et al., Physical Review Letters 118, 177202 (2017). The challenge of one-dimensional systems is to understand their physics beyond the level of known elementary excitations. By high-resolution neutron spectroscopy in a quantum spin-ladder material, we probe the leading multiparticle excitation by characterizing the two-magnon bound state at zero field. By applying high magnetic fields, we create and select the singlet (longitudinal) and triplet (transverse) excitations of the fully spin-polarized ladder, which have not been observed previously and are close analogs of the modes anticipated in a polarized Haldane chain. Theoretical modeling of the dynamical response demonstrates our complete quantitative understanding of these states.

18. April 2017

20 years of SINQ

In 1997 the Swiss spallation neutron source SINQ started its user operation. PSI has celebrated the 20th anniversary of SINQ with a scientific symposium on April 18, 2017 together with many colleagues from Switzerland and abroad. At the symposium it was not only looked back at past achievements, also recent scientific highlights were presented as well as the SINQ neutron guide and instrument upgrade program that will make SINQ fit for the next 20 years. Finally, the symposium also marked the change of the NUM Division Head from Kurt N. Clausen to Christian Rüegg.

27. March 2017

Tuning the multiferroic mechanisms of TbMnO₃ by epitaxial strain

K. Shimamoto, S. Mukherjee et al., Scientific Reports 7, 44753 (2017). A current challenge in the field of magnetoelectric multiferroics is to identify systems that allow a controlled tuning of states displaying distinct magnetoelectric responses. Here we show that the multiferroic ground state of the archetypal multiferroic TbMnO₃ is dramatically modified by epitaxial strain. Neutron diffraction reveals that in highly strained films the magnetic order changes from the bulk-like incommensurate bc-cycloidal structure to commensurate magnetic order. Concomitant with the modification of the magnetic ground state, optical second-harmonic generation (SHG) and electric measurements show an enormous increase of the ferroelectric polarization, and a change in its direction from along the c- to the a-axis. Our results suggest that the drastic change of multiferroic properties results from a switch of the spin-current magnetoelectric coupling in bulk TbMnO₃ to symmetric magnetostriction in epitaxially-strained TbMnO₃. These findings experimentally demonstrate that epitaxial strain can be used to control single-phase spin-driven multiferroic states.

17. March 2017

Sub-pixel correlation length neutron imaging: Spatially resolved scattering information of microstructures on a macroscopic scale

R.P. Harti et al., Scientific Reports 7, 44588 (2017). Neutron imaging and scattering give data of significantly different nature and traditional methods leave a gap of accessible structure sizes at around 10 micrometers. Only in recent years overlap in the probed size ranges could be achieved by independent application of high resolution scattering and imaging methods, however without providing full structural information when microstructures vary on a macroscopic scale. In this study we show how quantitative neutron dark-field imaging with a novel experimental approach provides both sub-pixel resolution with respect to microscopic correlation lengths and imaging of macroscopic variations of the microstructure. Thus it provides combined information on multiple length scales. A dispersion of micrometer sized polystyrene colloids was chosen as a model system to study gravity induced crystallisation of microspheres on a macro scale, including the identification of ordered as well as unordered phases. Our results pave the way to study heterogeneous systems locally in a previously impossible manner.