43 research outputs found
Spin fluctuations associated with the collapse of the pseudogap in a cuprate superconductor
Theories of the origin of superconductivity in cuprates are dependent on an
understanding of their normal state which exhibits various competing orders.
Transport and thermodynamic measurements on LaSrCuO show
signatures of a quantum critical point, including a peak in the electronic
specific heat versus doping p, near the doping p*, where the pseudogap
collapses. The fundamental nature of the fluctuations associated with this peak
is unclear. Here we use inelastic neutron scattering to show that close to
and near p*, there are low-energy collective spin excitations with
characteristic energies 5 meV. The correlation length of the spin
fluctuations does not diverge in spite of the low energy scale and we conclude
that the underlying quantum criticality is not due to antiferromagnetism but
most likely to a collapse of the pseudogap. We show that the large specific
heat near p* can be understood in terms of collective spin fluctuations. The
spin fluctuations we measure exist across the superconducting phase diagram and
may be related to the strange metal behaviour observed in overdoped cuprates
Unveiling phonons in a molecular qubit with four-dimensional inelastic neutron scattering and density functional theory
Magnetic-field dependence of low-energy magnons, anisotropic heat conduction, and spontaneous relaxation of magnetic domains in the cubic helimagnet ZnCr2Se4
Anisotropic low-temperature properties of the cubic spinel helimagnet
ZnCr2Se4 in the single-domain spin-spiral state are investigated by a
combination of neutron scattering, thermal conductivity, ultrasound velocity,
and dilatometry measurements. In an applied magnetic field, neutron
spectroscopy shows a complex and nonmonotonic evolution of the spin-wave
spectrum across the quantum-critical point that separates the spin-spiral phase
from the field-polarized ferromagnetic phase at high fields. A tiny spin gap of
the pseudo-Goldstone magnon mode, observed at wave vectors that are
structurally equivalent but orthogonal to the propagation vector of the spin
helix, vanishes at this quantum critical point, restoring the cubic symmetry in
the magnetic subsystem. The anisotropy imposed by the spin helix has only a
minor influence on the lattice structure and sound velocity but has a much
stronger effect on the heat conductivities measured parallel and perpendicular
to the magnetic propagation vector. The thermal transport is anisotropic at T <
2 K, highly sensitive to an external magnetic field, and likely results
directly from magnonic heat conduction. We also report long-time thermal
relaxation phenomena, revealed by capacitive dilatometry, which are due to
magnetic domain motion related to the destruction of the single-domain magnetic
state, initially stabilized in the sample by the application and removal of
magnetic field. Our results can be generalized to a broad class of helimagnetic
materials in which a discrete lattice symmetry is spontaneously broken by the
magnetic order.Comment: 13 pages, 8 figures + Supplemental Materia