1,023 research outputs found
Spin-1/2 Triangular Lattice with Orbital Degeneracy in a Metallic Oxide Ag2NiO2
A novel metallic and magnetic transition metal oxide Ag2NiO2 is studied by
means of resistivity, magnetic susceptibility, specific heat and X-ray
diffraction. The crystal structure is characterized by alternating stacking of
a Ni3+O2 layer and a (Ag2)+ layer, the former realizing a spin-1/2 triangular
lattice with eg orbital degeneracy and the latter providing itinerant
electrons. It is found that the NiO2 layer exhibits orbital ordering at Ts =
260 K and antiferromagnetic spin ordering at TN = 56 K. Moreover, a moderately
large mass enhancement is found for the itinerant electrons, suggesting a
significant contribution from the nearly localized Ni 3d state to the Ag 5s
state that forms a broad band.Comment: 9 pages, 5 figures, to be published in Rapid Communications, Phys.
Rev.
Tuning the spin dynamics of kagome systems
Despite the conceptional importance of realizing spin liquids in solid states
only few compounds are known. On the other side the effect of lattice
distortions and anisotropies on the magnetic exchange topology and the
fluctuation spectrum are an interesting problem. We compare the excitation
spectra of the two s=1/2 kagome lattice compounds volborthite and vesignieite
using Raman scattering. We demonstrate that even small modifications of the
crystal structure may have a huge effect on the phonon spectrum and low
temperature properties.Comment: 3 pages, 2 figure
Effects of Rattling Phonons on the Quasiparticle Excitation and Dynamics in the Superconducting -Pyrochlore KOsO
Microwave penetration depth and surface resistance at 27 GHz are
measured in high quality crystals of KOsO. Firm evidence for
fully-gapped superconductivity is provided from . Below the second
transition at K, the superfluid density shows a step-like
change with a suppression of effective critical temperature .
Concurrently, the extracted quasiparticle scattering time shows a steep
enhancement, indicating a strong coupling between the anomalous rattling motion
of K ions and quasiparticles. The results imply that the rattling phonons help
to enhance superconductivity, and that K sites freeze to an ordered state with
long quasiparticle mean free path below .Comment: 5 pages, 5 figures, to be published in Phys. Rev. Let
Thermal Conductivity of the Pyrochlore Superconductor KOs2O6: Strong Electron Correlations and Fully Gapped Superconductivity
To elucidate the nature of the superconducting ground state of the
geometrically frustrated pyrochlore KOs2O6 (Tc=9.6K), the thermal conductivity
was measured down to low temperatures (~Tc/100). We found that the
quasiparticle mean free path is strikingly enhanced below a transition at
Tp=7.5K, indicating enormous electron inelastic scattering in the normal state.
In a magnetic field the conduction at T ->0K is nearly constant up to ~0.4Hc2,
in contrast with the rapid growth expected for superconductors with an
anisotropic gap. This unambiguously indicates a fully gapped superconductivity,
in contrast to the previous studies. These results highlight that KOs2O6 is
unique among superconductors with strong electron correlations.Comment: 5 pages, 4 figures, accepted for publication in Phys. Rev. Let
Magnetic Phase Diagram of the Hole-doped CaNaCuOCl Cuprate Superconductor
We report on the magnetic phase diagram of a hole-doped cuprate
CaNaCuOCl, which is free from buckling of CuO
planes, determined by muon spin rotation and relaxation. It is characterized by
a quasi-static spin glass-like phase over a range of sodium concentration
(), which is held between long range antiferromagnetic
(AF) phase () and superconducting phase where the system is
non-magnetic for . The obtained phase diagram qualitatively agrees
well with that commonly found for hole-doped high-\tc cuprates, strongly
suggesting that the incomplete suppression of the AF order for is an
essential feature of the hole-doped cuprates.Comment: 5 pages, submitted to Phys. Rev. Let
A Possible Phase Transition in beta-pyrochlore Compounds
We investigate a lattice of interacting anharmonic oscillators by using a
mean field theory and exact diagonalization. We construct an effective
five-state hopping model with intersite repulsions as a model for
beta-pyrochlore AOs_2O_6(A=K, Rb or Cs). We obtain the first order phase
transition line from large to small oscillation amplitude phases as temperature
decreases. We also discuss the possibility of a phase with local electric
polarizations. Our theory can explain the origin of the mysterious first order
transition in KOs_2O_6.Comment: 4 pages, 4 figures, submitted to J. Phys. Soc. Jp
Kagom\'{e} ice state in the dipolar spin ice Dy_{2}Ti_{2}O_{7}
We have investigated the kagom\'{e} ice behavior of the dipolar spin-ice
compound Dy_{2}Ti_{2}O_{7} in magnetic field along a [111] direction using
neutron scattering and Monte Carlo simulations. The spin correlations show that
the kagom\'{e} ice behavior predicted for the nearest-neighbor (NN) interacting
model, where the field induces dimensional reduction and spins are frustrated
in each two-dimensional kagom\'{e} lattice, occurs in the dipole interacting
system. The spins freeze at low temperatures within the macroscopically
degenerate ground states of the NN model.Comment: 5 pages, 3 figures, submitted to PR
NMR Observation of Rattling Phonons in the Pyrochlore Superconductor KOs2O6
We report nuclear magnetic resonance studies on the beta-pyrochlore oxide
superconductor KOs2O6. The nuclear relaxation at the K sites is entirely caused
by fluctuations of electric field gradient, which we ascribe to highly
anharmonic low frequency oscillation (rattling) of K ions. A phenomenological
analysis shows a crossover from overdamped to underdamped behavior of the
rattling phonons with decreasing temperature and its sudden sharpening below
the superconducting transition temperature Tc. Absence of the Hebel-Slichter
peak in the relaxation rate at the O sites below Tc also indicates strong
electron-phonon coupling.Comment: 4 pages, 3 figure
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