22 research outputs found
Observation of elastic anomalies driven by coexisting dynamical spin Jahn-Teller effect and dynamical molecular spin state in paramagnetic phase of the frustrated MgCrO$
Ultrasound velocity measurements of magnesium chromite spinel MgCrO
reveal elastic anomalies in the paramagnetic phase that are characterized as
due to geometrical frustration. The temperature dependence of the tetragonal
shear modulus exhibits huge Curie-type softening, which
should be the precursor to spin Jahn-Teller distortion in the antiferromagnetic
phase. The trigonal shear modulus exhibits nonmonotonic temperature
dependence with a characteristic minimum at 50 K, indicating a coupling
of the lattice to dynamical molecular spin state. These results strongly
suggest the coexistence of dynamical spin Jahn-Teller effect and dynamical
molecular spin state in the paramagnetic phase, which is compatible with the
coexistence of magnetostructural order and dynamical molecular spin state in
the antiferromagnetic phase.Comment: 6 pages, 3 figure
Elastic Instabilities within Antiferromagnetically Ordered Phase in the Orbitally-Frustrated Spinel GeCoO
Ultrasound velocity measurements of the orbitally-frustrated GeCoO
reveal unusual elastic instabilities due to the phonon-spin coupling within the
antiferromagnetic phase. Shear moduli exhibit anomalies arising from the
coupling to short-range ferromagnetic excitations. Diplike anomalies in the
magnetic-field dependence of elastic moduli reveal magnetic-field-induced
orbital order-order transitions. These results strongly suggest the presence of
geometrical orbital frustration which causes novel orbital phenomena within the
antiferromagnetic phase.Comment: 5 pages, 3 figure
Coevolutionary genetic algorithm for constraint satisfaction with a genetic repair operator for effective schemata formation
We discuss a coevolutionary genetic algorithm for constraint satisfaction. Our basic idea is to explore effective genetic information in the population, i.e., schemata, and to exploit the genetic information in order to guide the population to better solutions. Our coevolutionary genetic algorithm (CGA) consists of two GA populations; the first GA, called “H-GA”, searches for the solutions in a given environment (problem), and the second GA, called “P-GA”, searches for effective genetic information involved in the H-GA, namely, good schemata. Thus, each individual in P-GA consists of alleles in H-GA or “don't care” symbol representing a schema in the H-GA. These GA populations separately evolve in each genetic space at different abstraction levels and affect with each other by two genetic operators: “superposition” and “transcription”. We then applied our CGA to constraint satisfaction problems (CSPs) incorporating a new stochastic “repair” operator for P-GA to raise the consistency of schemata with the (local) constraint conditions in CSPs. We carried out two experiments: First, we examined the performance of CGA on various “general” CSPs that are generated randomly for a wide variety of “density” and “tightness” of constraint conditions in the CSPs that are the basic measures of characterizing CSPs. Next, we examined “structured” CSPs involving latent “cluster” structures among the variables in the CSPs. For these experiments, computer simulations confirmed us the effectiveness of our CGA</p
Spin-orbit coupling inactivity of Co ion in geometrically frustrated magnet GeCoO
We report single-crystal neutron diffraction studies on a spinel
antiferromagnet GeCoO, which exhibits magnetic order with a trigonal
propagation vector and tetragonal lattice expansion () below
K. For this inconsistency between spin and lattice in symmetry,
magnetic Bragg reflections with a tetragonal propagation vector were discovered
below . We discuss spin and orbital states of Co ion
underlying the new magnetic component.Comment: 3 pages 2 figures, submitted to ICFCM proceeding (Journal of Physics:
Conference Series, 2011
Disorder-sensitive superconductivity in the iron silicide LuFeSi studied by the Lu-site substitutions
We studied effect of non-magnetic and magnetic impurities on
superconductivity in LuFeSi by small amount substitution of the Lu
site, which investigated structural, magnetic, and electrical properties of
non-magnetic (LuSc)FeSi,
(LuY)FeSi, and magnetic
(LuDy)FeSi. The rapid depression of by
non-magnetic impurities in accordance with the increase of residual resistivity
reveals the strong pair breaking dominated by disorder. We provide compelling
evidence for the sign reversal of the superconducting order parameter in
LuFeSi.Comment: 4 pages, 5 figure
Jahn-Teller Inactivity and Magnetic Frustration in GeCoO Probed by Ultrasound Velocity Measurements
Ultrasound velocity measurements of cubic spinel GeCoO in single
crystal were performed for the investigation of shear and compression moduli.
The shear moduli in the paramagnetic state reveal an absence of Jahn-Teller
activity despite the presence of orbital degeneracy in the Co ions. Such
a Jahn-Teller inactivity indicates that the intersite orbital-orbital
interaction is much stronger than the Jahn-Teller coupling. The compression
moduli in the paramagnetic state near the Nel temperature
reveal that the most relevant exchange path for the antiferromagnetic
transition lies in the [111] direction. This exchange-path anisotropy is
consistent with the antiferromagnetic structure with the wave vector [111], suggesting the presence of bond frustration due to
competition among a direct ferromagnetic and several distant-neighbors
antiferromagnetic interactions. In the JT-inactive condition, the bond
frustration can be induced by geometrical orbital frustration of
- interaction between the Co ions which can be realized
in the pyrochlore lattice of the high spin Co with -orbital
degeneracy. In GeCoO, the tetragonal elongation below releases
the orbital frustration by quenching the orbital degeneracy.Comment: 7 pages, 7figures, to appear in Phys. Rev.
Magnetic field dependence of superconducting energy gaps in YNi2B2C: Evidence of multiband superconductivity
We present results of in field directional point contact spectroscopy (DPCS)
study in the quaternary borocarbide superconductor YNi2B2C, which is
characterized by a highly anisotropic superconducting gap function. For I||a,
the superconducting energy gap (D), decreases linearly with magnetic field and
vanishes around 3.25T which is well below the upper critical field (Hc2~6T)
measured at the same temperature (2.2K). For I||c, on the other hand, D
decreases weakly with magnetic field but the broadening parameter (G) increases
rapidly with magnetic field with the absence of any resolvable feature above
3.5T. From an analysis of the field variation of energy gaps and the zero bias
density of states we show that the unconventional gap function observed in this
material could originate from multiband superconductivity.Comment: 19 pages including figures (final version