27 research outputs found
Hopping Time Scales and the Phonon-Liquid Electron-Crystal Picture in Thermoelectric Copper Selenide
Nearest-neighbor resonating valence bonds in YbMgGaO4
Since its proposal by Anderson, resonating valence bonds (RVB) formed by a
superposition of fluctuating singlet pairs have been a paradigmatic concept in
understanding quantum spin liquids (QSL). Here, we show that excitations
related to singlet breaking on nearest-neighbor bonds describe the high-energy
part of the excitation spectrum in YbMgGaO4, the effective spin-1/2 frustrated
antiferromagnet on the triangular lattice, as originally considered by
Anderson. By a thorough single-crystal inelastic neutron scattering (INS)
study, we demonstrate that nearest-neighbor RVB excitations account for the
bulk of the spectral weight above 0.5 meV. This renders YbMgGaO4 the first
experimental system where putative RVB correlations restricted to nearest
neighbors are observed, and poses a fundamental question of how complex
interactions on the triangular lattice conspire to form this unique many-body
state.Comment: To be published in Nature Communication
Crystalline Electric Field Randomness in the Triangular Lattice Spin-Liquid YbMgGaO
We apply moderate-high-energy inelastic neutron scattering (INS) measurements
to investigate Yb crystalline electric field (CEF) levels in the
triangular spin-liquid candidate YbMgGaO. Three CEF excitations from the
ground-state Kramers doublet are centered at the energies = 39,
61, and 97\,meV in agreement with the effective \mbox{spin-1/2} -factors and
experimental heat capacity, but reveal sizable broadening. We argue that this
broadening originates from the site mixing between Mg and Ga
giving rise to a distribution of Yb--O distances and orientations and, thus, of
CEF parameters that account for the peculiar energy profile of the CEF
excitations. The CEF randomness gives rise to a distribution of the effective
spin-1/2 -factors and explains the unprecedented broadening of low-energy
magnetic excitations in the fully polarized ferromagnetic phase of YbMgGaO,
although a distribution of magnetic couplings due to the Mg/Ga disorder may be
important as well.Comment: Accepted in Phys. Rev. Let
Spin dynamics of coupled spin ladders near quantum criticality in Ba2CuTeO6
We report inelastic neutron scattering measurements of the magnetic
excitations in Ba2CuTeO6, proposed by ab initio calculations to magnetically
realize weakly coupled antiferromagnetic two-leg spin-1/2 ladders. Isolated
ladders are expected to have a singlet ground state protected by a spin gap.
Ba2CuTeO6 orders magnetically, but with a small Neel temperature relative to
the exchange strength, suggesting that the interladder couplings are relatively
small and only just able to stabilize magnetic order, placing Ba2CuTeO6 close
in parameter space to the critical point separating the gapped phase and Neel
order. Through comparison of the observed spin dynamics with linear spin wave
theory and quantum Monte Carlo calculations, we propose values for all relevant
intra- and interladder exchange parameters, which place the system on the
ordered side of the phase diagram in proximity to the critical point. We also
compare high field magnetization data with quantum Monte Carlo predictions for
the proposed model of coupled ladders.Comment: 14 pages, 12 figure
Crystalline Electric Field Randomness in the Triangular Lattice Spin-Liquid YbMgGaO
We apply moderate-high-energy inelastic neutron scattering (INS) measurements
to investigate Yb crystalline electric field (CEF) levels in the
triangular spin-liquid candidate YbMgGaO. Three CEF excitations from the
ground-state Kramers doublet are centered at the energies = 39,
61, and 97\,meV in agreement with the effective \mbox{spin-1/2} -factors and
experimental heat capacity, but reveal sizable broadening. We argue that this
broadening originates from the site mixing between Mg and Ga
giving rise to a distribution of Yb--O distances and orientations and, thus, of
CEF parameters that account for the peculiar energy profile of the CEF
excitations. The CEF randomness gives rise to a distribution of the effective
spin-1/2 -factors and explains the unprecedented broadening of low-energy
magnetic excitations in the fully polarized ferromagnetic phase of YbMgGaO,
although a distribution of magnetic couplings due to the Mg/Ga disorder may be
important as well.Comment: Accepted in Phys. Rev. Let
Avoided quasiparticle decay and enhanced excitation continuum in the spin-1/2 near-Heisenberg triangular antiferromagnet Ba3CoSb2O9
We explore the magnetic excitations of the spin-1/2 triangular
antiferromagnet Ba3CoSb2O9 in its 120 degree ordered phase using single-crystal
high-resolution inelastic neutron scattering. Sharp magnons with no decay are
observed throughout reciprocal space, with a strongly renormalized dispersion
and multiple soft modes compared to linear spin wave theory. We propose an
empirical parametrization that can quantitatively capture the complete
dispersions in the three-dimensional Brillouin zone and explicitly show that
the dispersion renormalizations have the direct consequence that one to two
magnon decays are avoided throughout reciprocal space, whereas such decays
would be allowed for the unrenormalized dispersions. At higher energies, we
observe a very strong continuum of excitations with highly-structured intensity
modulations extending up at least 4x the maximum one-magnon energy. The
one-magnon intensities decrease much faster upon increasing energy than
predicted by linear spin wave theory and the higher-energy continuum contains
much more intensity than can be accounted for by a two-magnon cross-section,
suggesting a significant transfer of spectral weight from the high-energy
magnons into the higher-energy continuum states. We attribute the strong
dispersion renormalizations and substantial transfer of spectral weight to
continuum states to the effect of quantum fluctuations and interactions beyond
the spin wave approximation, and make connections to theoretical approaches
that might capture such effects. Finally, through measurements in a strong
applied magnetic field, we find evidence for magnetic domains with opposite
senses for the spin rotation in the 120 degree ordered ground state, as
expected in the absence of Dzyaloshinskii-Moriya interactions, when the sense
of spin rotation is selected via spontaneous symmetry breaking.Comment: 20 pages, 13 figure