53 research outputs found
Temperature dependence of the (π,0) anomaly in the excitation spectrum of the 2D quantum Heisenberg antiferromagnet
Dedicated to the life and work of Professor Roger Cowley. 22 pages, 8 figuresIt is well established that in the low-temperature limit, the two-dimensional quantum Heisenberg antiferromagnet on a square lattice (2DQHAFSL) exhibits an anomaly in its spectrum at short-wavelengths on the zone-boundary. In the vicinity of the (π,0) point the pole in the one-magnon response exhibits a downward dispersion, is heavily damped and attenuated, giving way to an isotropic continuum of excitations extending to high energies. The origin of the anomaly and the presence of the continuum are of current theoretical interest, with suggestions focused around the idea that the latter evidences the existence of spinons in a two-dimensional system. Here we present the results of neutron inelastic scattering experiments and Quantum Monte Carlo calculations on the metallo-organic compound Cu(DCOO)2⋅4D2O (CFTD), an excellent physical realisation of the 2DQHAFSL, designed to investigate how the anomaly at (π,0) evolves up to finite temperatures T/J∼2/3. Our data reveal that on warming the anomaly survives the loss of long-range, three-dimensional order, and is thus a robust feature of the two-dimensional system. With further increase of temperature the zone-boundary response gradually softens and broadens, washing out the (π,0) anomaly. This is confirmed by a comparison of our data with the results of finite-temperature Quantum Monte Carlo simulations where the two are found to be in good accord. At lower energies, in the vicinity of the antiferromagnetic zone centre, there was no significant softening of the magnetic excitations over the range of temperatures investigated.Accepted manuscrip
Negative-pressure-induced helimagnetism in ferromagnetic cubic perovskites Sr<sub>1-x</sub>Ba<sub>x</sub>CoO<sub>3 </sub>
Helimagnetic materials are identified as promising for novel spintronic applications. Since helical spin order is manifested as a compromise of competing magnetic exchange interactions, its emergence is limited by unique constraints imposed by the crystalline lattice and the interaction geometries as exemplified by the multiferroic perovskite manganites with large orthorhombic distortion. Here we show that a simple cubic perovskite SrCoO3 with room-temperature ferromagnetism has the potential to host helimagnetic order upon isotropic lattice expansion. Increasing the Ba content x in SrxBa1-xCoO3 continuously expands the cubic lattice, eventually suppressing the ferromagnetic order near x = 0.4 where helimagnetic correlations are observed as incommensurate diffuse magnetic scattering by neutron-diffraction measurements. The emergence of helimagnetism is semiquantitatively reproduced by first-principles calculations, leading to the conjecture that a simple cubic lattice with strong d-p hybridization can exhibit a variety of novel magnetic phases originating from competing exchange interactions
Field-controlled magnetic order in the quantum spin-ladder system (Hpip)2CuBr4
International audienceNeutron diffraction is used to investigate the field-induced, antiferromagnetically ordered state in the two-leg spin-ladder material (Hpip)2CuBr4. This “classical” phase, a consequence of weak interladder coupling, is nevertheless highly unconventional: its properties are influenced strongly by the spin Luttinger-liquid state of the ladder subunits. We determine directly the order parameter (transverse magnetization), the ordering temperature, the spin structure, and the critical exponents around the transition. We introduce a minimal microscopic model for the interladder coupling and calculate the quantum fluctuation corrections to the mean-field interaction
Spin-phonon coupled modes in the incommensurate phases of doped CuGeO
The doping effect of the folded phonon mode at 98 cm was investigated
on the Si-doped CuGeO by magneto-optical measurements in far-infrared (FIR)
region under high magnetic field. The folded phonon mode at 98 cm
appears not only in the dimerized (D) phase but also in the
dimerized-anitiferromagnetic (DAF) phase on the doped CuGeO. The splitting
was observed in the incommensurate (IC) phase and the antiferromagnetically
ordered incommensurate (IAF) phase above . The split-off branches exhibit
different field dependence from that of the pure CuGeO in the vicinity of
, and the discrepancy in the IAF phase is larger than that in the IC
phase. It is caused by the interaction between the solitons and the impurities.Comment: 7 pages, 4 figures, resubmitted to Phys. Rev.
The instrument suite of the European Spallation Source
An overview is provided of the 15 neutron beam instruments making up the initial instrument suite of the
European Spallation Source (ESS), and being made available to the neutron user community. The ESS neutron
source consists of a high-power accelerator and target station, providing a unique long-pulse time structure
of slow neutrons. The design considerations behind the time structure, moderator geometry and instrument
layout are presented.
The 15-instrument suite consists of two small-angle instruments, two reflectometers, an imaging beamline,
two single-crystal diffractometers; one for macromolecular crystallography and one for magnetism, two powder
diffractometers, and an engineering diffractometer, as well as an array of five inelastic instruments comprising
two chopper spectrometers, an inverse-geometry single-crystal excitations spectrometer, an instrument for vibrational
spectroscopy and a high-resolution backscattering spectrometer. The conceptual design, performance
and scientific drivers of each of these instruments are described.
All of the instruments are designed to provide breakthrough new scientific capability, not currently
available at existing facilities, building on the inherent strengths of the ESS long-pulse neutron source of high
flux, flexible resolution and large bandwidth. Each of them is predicted to provide world-leading performance
at an accelerator power of 2 MW. This technical capability translates into a very broad range of scientific
capabilities. The composition of the instrument suite has been chosen to maximise the breadth and depth
of the scientific impact o
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