16 research outputs found
Ultrasonic studies of the magnetic phase transition in MnSi
Measurements of the sound velocities in a single crystal of MnSi were
performed in the temperature range 4-150 K. Elastic constants, controlling
propagation of longitudinal waves reveal significant softening at a temperature
of about 29.6 K and small discontinuities at 28.8 K, which corresponds to
the magnetic phase transition in MnSi. In contrast the shear elastic moduli do
not show any softening at all, reacting only to the small volume deformation
caused by the magneto-volume effect. The current ultrasonic study exposes an
important fact that the magnetic phase transition in MnSi, occurring at 28.8 K,
is just a minor feature of the global transformation marked by the rounded
maxima or minima of heat capacity, thermal expansion coefficient, sound
velocities and absorption, and the temperature derivative of resistivity.Comment: 4 pages, 4 figure
Surface state atoms and their contribution to the surface tension of quantum liquids
We investigate the new type of excitations on the surface of liquid helium.
These excitations, called surfons, appear because helium atoms have discrete
energy level at the liquid surface, being attracted to the surface by the van
der Waals force and repulsed at a hard-core interatomic distance. The
concentration of the surfons increases with temperature. The surfons propagate
along the surface and form a two-dimensional gas. Basing on the simple model of
the surfon microscopic structure, we estimate the surfon activation energy and
effective mass for both helium isotopes. We also calculate the contribution of
the surfons to the temperature dependence of the surface tension. This
contribution explains the great and long-standing discrepancy between theory
and experiment on this temperature dependence in both helium isotopes. The
achieved agreement between our theory and experiment is extremely high. The
comparison with experiment allows to extract the surfon activation energy and
effective mass. The values of these surfon microscopic parameters are in a
reasonable agreement with the calculated from the proposed simple model of
surfon structure.Comment: 10 pages, 6 figure
Formation of a direct Kolmogorov-like cascade of second-sound waves in He II.
Based on measurements of nonlinear second-sound resonances in a high-quality resonator, we have observed a steady-state wave energy cascade in He II involving a flux of energy through the spectral range towards high frequencies. We show that the energy balance in the wave system is nonlocal in K space and that the frequency scales of energy pumping and dissipation are widely separated. The wave amplitude distribution follows a power law over a wide range of frequencies. Numerical computations yield results in agreement with the experimental observations. We suggest that second-sound cascades of this kind may be useful for model studies of acoustic turbulence
Interphase REE Partitioning at the Boundary between the Earth’s Transition Zone and Lower Mantle: Evidence from Experiments and Atomistic Modeling
Trace elements play a significant role in interpretation of different processes in the deep Earth. However, the systematics of interphase rare-earth element (REE) partitioning under the conditions of the uppermost lower mantle are poorly understood. We performed high-pressure experiments to study the phase relations in key solid-phase reactions CaMgSi2O6 = CaSiO3-perovskite + MgSiO3-bridgmanite and (Mg,Fe)2SiO4-ringwoodite = (Mg,Fe)SiO3-bridgmanite + (Mg,Fe)O with addition of 1 wt % of REE oxides. Atomistic modeling was used to obtain more accurate quantitative estimates of the interphase REE partitioning and displayed the ideal model for the high-pressure minerals. HREE (Er, Tm, Yb, and Lu) are mostly accumulated in bridgmanite, while LREE are predominantly redistributed into CaSiO3. On the basis of the results of experiments and atomistic modeling, REE in bridgmanite are clearly divided into two groups (from La to Gd and from Gd to Lu). Interphase REE partition coefficients in solid-state reactions were calculated at 21.5 and 24 GPa for the first time. The new data are applicable for interpretation of the trace-element composition of the lower mantle inclusions in natural diamonds from kimberlite; the experimentally determined effect of pressure on the interphase (bridgmanite/CaSiO3-perovskite) REE partition coefficients can be a potential qualitative geobarometer for mineral inclusions in super-deep diamonds