[신간의 별자리x] 우리/미술, 그리고 ‘슬픔의 박물관’

日本物理学会誌及びJournal of the Physical Society of Japanの月刊について

Physical Review B

Annual review of condensed matter physics

Journal of Physics Condensed Matter: Preface

Low-energy phonons were studied in the ferromagnetic and paramagnetic phases of the Heusler alloy ${\mathrm{Ni}}_{2}\mathrm{MnGa}.$ The investigated sample shows a martensitic phase transformation with a transition temperature ${T}_{M}\ensuremath{\approx}284\mathrm{K},$ only about 80 K below the Curie temperature. Therefore, premartensitic phenomena could be studied in the ferromagnetic as well as in the paramagnetic state. The (\ensuremath{\xi} \ensuremath{\xi} 0) ${\mathrm{TA}}_{2}$-phonon branch shows a strong but incomplete softening at $\ensuremath{\xi}\ensuremath{\approx}1/3$ in the premartensitic phase when the temperature approaches ${T}_{M}.$ The temperature dependence of this softening changes at the Curie temperature which can be explained by an additional contribution of the magnetization to the Landau free energy. At the wave vector of the strongest phonon softening a central peak occurs. A second elastic peak, with the same temperature behavior, appears at $\ensuremath{\xi}\ensuremath{\approx}0.17.$ The relation of these elastic precursors to the low-temperature structure is discussed.

Premartensitic phenomena in the ferro- and paramagnetic phases of Ni 2 MnGa

At low temperatures, Tb2Ti2O7 enters a spin liquid state, despite expectations of magnetic order and/or a structural distortion. Using neutron scattering, we have discovered that in this spin liquid state an excited crystal field level is coupled to a transverse acoustic phonon, forming a hybrid excitation. Magnetic and phononlike branches with identical dispersion relations can be identified, and the hybridization vanishes in the paramagnetic state. We suggest that Tb2Ti2O7 is aptly named a "magnetoelastic spin liquid" and that the hybridization of the excitations suppresses both magnetic ordering and the structural distortion. The spin liquid phase of Tb2Ti2O7 can now be regarded as a Coulomb phase with propagating bosonic spin excitations.

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Magnetoelastic excitations in the pyrochlore spin liquid Tb2Ti2O7.

POLDI (Pulse-OverLap DIffractometer) is a multiple pulse-overlap diffractometer at PSI, which is designed mainly for strain-scanning experiments. The multiple pulse-overlap method made it possible to build a time-of-flight diffractometer at a continuous neutron source with short flight path, high resolution and high intensity. The best achievable resolutions for the relative linewidths of the Bragg reflections are between 1×10−3 and 2×10−3, depending on the lattice spacing. The layout of the instrument and the features of the chopper, the neutron mirror and the 3He detector in time-focusing geometry are presented. The overall performance of the instrument is discussed with examples of strain-field measurements.

Time-of-flight diffraction with multiple frame overlap Part II: The strain scanner POLDI at PSI

The vibrational density of states (VDOS) of bulk nanocrystalline Ni and Cu (model) samples with grain diameters between 5 and 12 nm are derived from molecular-dynamics simulations. The results show an enhancement of the density of states at low and high energies. Because of large system sizes and a decomposition of the VDOS into grain and grain-boundary components, the low-frequency region can be investigated for the first time. It is found that the anomalous increase of the VDOS is mainly caused by the high number of grain-boundary atoms and that a power-law behavior of the low-frequency grain-boundary VDOS exists, which suggests a reduced dimensionality effect.

Low-frequency vibrational properties of nanocrystalline materials.

In situ neutron diffraction was performed on Cu∕Nb nanocomposite wires composed of a multiscale Cu matrix embedding Nb nanofilaments with a diameter of 267nm and spacing of 45nm. The evolution of elastic strains and peak profiles versus applied stress evidenced the codeformation behavior with different elastic-plastic regimes: the Cu matrix exhibit size effect in the finest channels while the Nb nanowhiskers remain elastic up to the macroscopic failure, with a strong load transfer from the Cu matrix onto the Nb filaments. The measured yield stress in the finest Cu channels is in agreement with calculations based on a single dislocation regime.

/pdf/plasticity-of-multiscale-nanofilamentary-cu-nb-composite-5d6s19gcdl.pdf

Uwe Stuhr

Papers

Premartensitic phenomena in the ferro- and paramagnetic phases of Ni 2 MnGa

Magnetoelastic excitations in the pyrochlore spin liquid Tb2Ti2O7.

Time-of-flight diffraction with multiple frame overlap Part II: The strain scanner POLDI at PSI

Low-frequency vibrational properties of nanocrystalline materials.

Plasticity of multiscale nanofilamentary Cu/Nb composite wires during in situ neutron diffraction: Codeformation and size effect