arXiv subjects

N. Nagasawa

Publications and source records attributed to N. Nagasawa.

3 recordsLinked to original sources

arXiv2015

Low-Energy Optical Phonon Modes in the Caged Compound LaRu2Zn20

The caged compound LaRu2Zn20 exhibits a structural transition at TS =150 K, whose driving mechanism remains elusive. We have investigated atomic dynamics by the measurements of specific heat C and inelastic X-ray scattering (IXS). The lattice part of the specific heat Clat divided by T3, Clat/T3, shows a broad peak at around 15 K, which is reproduced by two Einstein modes with characteristic temperatures of 35 K and 82 K, respectively. IXS measurements along the [111] and [110] directions reveal optical phonon modes at 3 meV (35 K) and 7 meV (80 K), respectively, whose values agree with the values of Einstein temperatures. The first principles calculation has assigned the phonon modes at 3 meV as the optical modes of Zn atoms located at the middle of two La atoms. The low-energy vibration of the Zn atom perpendicular to the there-fold axis is thought to lead the structural instability of LaRu2Zn20.

cond-mat.str-el↗

arXiv2012

Simultaneous Superconducting and Antiferroquadrupolar Transitions in PrRh$_{2}$Zn$_{20}$

Superconducting and antiferroquadrupolar (AFQ) transitions in a Pr-based compound PrRh2Zn20 have been found to occur simultaneously at Tc=TQ=0.06 K. The superconducting transition manifests itself by zero resistance and large diamagnetic susceptibility. The specific heat exhibits a Schottky anomaly peaking at 14 K and magnetization curves measured at 2 K show anisotropic behaviors. The analysis of these data indicates that the crystalline electric field (CEF) ground state of the trivalent Pr ion is the non-Kramers Gamma3 doublet with the quadrupolar degrees of freedom. A sharp peak in the specific heat at 0.06 K has been attributed not to the superconducting transition but to the AFQ transition because the ordering temperature TQ decreases in B || [100] but increases in B || [110] and B || [111] with increasing B up to 6 T. This anisotropic behavior of TQ(B) can be well explained by a two-sublattice mean-field calculation, which corroborates the AFQ ordered state below TQ. The entropy release at TQ is only 10% of Rln2 expected for the Gamma3 doublet, suggesting possible interplay between the quadrupolar degrees of freedom and the superconductivity.

cond-mat.str-el↗

arXiv2003

Bosonic stimulation of cold 1s excitons into a harmonic potential minimum in Cu$_2$O

Density distribution of cold exciton clouds generated into a strain-induced potential well by two-photon excitation in Cu$_2$O is studied at 2 K. We find that an anomalous spike, which can be interpreted as accumulation of the excitons into the ground state, emerges at the potential minimum. The accumulation can be due to stimulated scattering of cold excitons, mediated by acoustic phonon emission. Possibility of the formation of the thermodynamic Bose-Einstein condensate of paraexcitons has been discussed.

cond-mat.mtrl-sci↗