RESUMEN
The conduction electrons in a metal experience competing interactions with each other and the atomic nuclei. This competition can lead to many types of magnetic order in metals. For example, in chromium the electrons order to form a spin-density-wave (SDW) antiferromagnetic state. A magnetic field may be used to perturb or tune materials with delicately balanced electronic interactions. Here, we show that the application of a magnetic field can induce SDW magnetic order in a quasi-2D metamagnetic metal, where none exists in the absence of the field. We use magnetic neutron scattering to show that the application of a large (B ≈ 8 T) magnetic field to the perovskite metal Sr3Ru2O7 (refs 3-7) can be used to tune the material through two magnetically ordered SDW states. The ordered states exist over relatively small ranges in field (â²0.4 T), suggesting that their origin is due to a new mechanism related to the electronic fine structure near the Fermi energy, possibly combined with the stabilizing effect of magnetic fluctuations. The magnetic field direction is shown to control the SDW domain populations, which naturally explains the strong resistivity anisotropy or 'electronic nematic' behaviour observed in this material.
RESUMEN
There is great interest in finding materials possessing quasiparticles with topological properties. Such materials may have novel excitations that exist on their boundaries which are protected against disorder. We report experimental evidence that magnons in an insulating kagome ferromagnet can have a topological band structure. Our neutron scattering measurements further reveal that one of the bands is flat due to the unique geometry of the kagome lattice. Spin wave calculations show that the measured band structure follows from a simple Heisenberg Hamiltonian with a Dzyaloshinkii-Moriya interaction. This serves as the first realization of an effectively two-dimensional topological magnon insulator--a new class of magnetic material that should display both a magnon Hall effect and protected chiral edge modes.
RESUMEN
A new family of superconductors containing layers of iron arsenide has attracted considerable interest because of their high transition temperatures (T(c)), some of which are >50 K, and because of similarities with the high-T(c) copper oxide superconductors. In both the iron arsenides and the copper oxides, superconductivity arises when an antiferromagnetically ordered phase has been suppressed by chemical doping. A universal feature of the copper oxide superconductors is the existence of a resonant magnetic excitation, localized in both energy and wavevector, within the superconducting phase. This resonance, which has also been observed in several heavy-fermion superconductors, is predicted to occur when the sign of the superconducting energy gap takes opposite values on different parts of the Fermi surface, an unusual gap symmetry which implies that the electron pairing interaction is repulsive at short range. Angle-resolved photoelectron spectroscopy shows no evidence of gap anisotropy in the iron arsenides, but such measurements are insensitive to the phase of the gap on separate parts of the Fermi surface. Here we report inelastic neutron scattering observations of a magnetic resonance below T(c) in Ba(0.6)K(0.4)Fe(2)As(2), a phase-sensitive measurement demonstrating that the superconducting energy gap has unconventional symmetry in the iron arsenide superconductors.
RESUMEN
High degeneracy in ground states leads to the generation of exotic zero-energy modes, a representative example of which is the formation of molecular spin-liquid-like fluctuations in a frustrated magnet. Here we present single-crystal inelastic neutron scattering results for the frustrated magnet MgCr(2)O(4), which show that a common set of finite-energy molecular spin excitation modes is sustained in both the liquid-like phase above magnetic ordering temperature T(N) and an ordered phase with an extremely complex magnetic structure below T(N). Based on this finding, we propose the concept of high degeneracy in excited states, which promotes local resonant elementary excitations. This concept is expected to have ramifications on our understanding of excitations in many complex systems, including not only spin but also atomic liquids, complex order systems, and amorphous systems.
RESUMEN
We report inelastic neutron scattering measurements of the resonant spin excitations in Ba(1-x)K(x)Fe(2)As(2) over a broad range of electron band filling. The fall in the superconducting transition temperature with hole doping coincides with the magnetic excitations splitting into two incommensurate peaks because of the growing mismatch in the hole and electron Fermi surface volumes, as confirmed by a tight-binding model with s(±)-symmetry pairing. The reduction in Fermi surface nesting is accompanied by a collapse of the resonance binding energy and its spectral weight, caused by the weakening of electron-electron correlations.
RESUMEN
When the transition temperature of a continuous phase transition is tuned to absolute zero, new ordered phases and physical behaviour emerge in the vicinity of the resulting quantum critical point. Sr3Ru2O7 can be tuned through quantum criticality with magnetic field at low temperature. Near its critical field Bc it displays the hallmark T-linear resistivity and a [Formula: see text] electronic heat capacity behaviour of strange metals. However, these behaviours have not been related to any critical fluctuations. Here we use inelastic neutron scattering to reveal the presence of collective spin fluctuations whose relaxation time and strength show a nearly singular variation with magnetic field as Bc is approached. The large increase in the electronic heat capacity and entropy near Bc can be understood quantitatively in terms of the scattering of conduction electrons by these spin-fluctuations. On entering the spin-density-wave ordered phase present near Bc, the fluctuations become stronger suggesting that the order is stabilised through an "order-by-disorder" mechanism.
RESUMEN
The quasi-two-dimensional antiferromagnet FePS(3) has been investigated using inelastic neutron spectroscopy with the time-of-flight spectrometer HET at the ISIS spallation neutron source. In the paramagnetic regime, two clearly resolved, high energy excitations were observed in the low scattering angle detector banks at 195(5) meV and 430(10) meV. The absence of these transitions from the high angle detector banks indicates that they are likely to be due to the crystal fields and magnetic in origin. The two transitions most probably represent electronic transitions in the Fe(2+) ion among the low lying crystal field and spin-orbit split levels raised from the ground state. It has not yet been determined why the energies are greater than those observed in a comparable Raman experiment.
RESUMEN
The MAPS direct geometry time-of-flight chopper spectrometer at the ISIS pulsed neutron and muon source has been in operation since 1999, and its novel use of a large array of position-sensitive neutron detectors paved the way for a later generations of chopper spectrometers around the world. Almost two decades of experience of user operations on MAPS, together with lessons learned from the operation of new generation instruments, led to a decision to perform three parallel upgrades to the instrument. These were to replace the primary beamline collimation with supermirror neutron guides, to install a disk chopper, and to modify the geometry of the poisoning in the water moderator viewed by MAPS. Together, these upgrades were expected to increase the neutron flux substantially, to allow more flexible use of repetition rate multiplication and to reduce some sources of background. Here, we report the details of these upgrades and compare the performance of the instrument before and after their installation as well as to Monte Carlo simulations. These illustrate that the instrument is performing in line with, and in some respects in excess of, expectations. It is anticipated that the improvement in performance will have a significant impact on the capabilities of the instrument. A few examples of scientific commissioning are presented to illustrate some of the possibilities.
RESUMEN
The spin waves in a powdered sample of a quasi-two-dimensional antiferromagnet, FePS(3), have been measured using neutron inelastic scattering. The data could be modelled and the exchange interactions determined using a two-dimensional Heisenberg Hamiltonian with single ion anisotropy. A suitable fit to the data could only be achieved by including magnetic interactions up to the third nearest neighbour, which is consistent with the findings for other members of the MPS(3) family (M=transition metal). The best fit parameters at 6 K were J(1) = 1.49 meV, J(2) = 0.04 meV, J(3) =- 0.6 meV, with an anisotropy of Δ = 3.7 meV. Measurements as a function of temperature give a coarse measure of the behaviour of the anisotropy and the nature of the phase transition.
RESUMEN
We report inelastic neutron scattering measurements and random phase approximation calculations of the dispersive crystal field excitations of UPd(3). The measured spectra at lower energies agree with those calculated using quadrupolar interaction parameters deduced from bulk and x-ray scattering measurements. The more intense excitations arising from the hexagonal sites were used to obtain exchange parameters which proved to be anisotropic.
RESUMEN
We report the inelastic neutron scattering study of spin dynamics in EuCu(2)(Si(x)Ge(1-x))(2) (x = 1, 0.9, 0.75, 0.6), performed in a wide temperature range. At x = 1 the magnetic excitation spectrum was found to be represented by the double-peak structure well below the energy range of the Eu(3+) spin-orbit (SO) excitation (7)F(0)â(7)F(1), so that at least the high-energy spectral component can be assigned to the renormalized SO transition. Change of the Eu valence towards 2 + with increased temperature and/or Ge concentration results in further renormalization (lowering the energy) and gradual suppression of both inelastic peaks in the spectrum, along with developing sizeable quasielastic signal. The origin of the spectral structure and its evolution is discussed in terms of excitonic model for the mixed valence state.
RESUMEN
We have performed inelastic neutron scattering on the near ideal spin-ladder compound La4Sr10Cu24O41 as a starting point for investigating doped ladders and their tendency toward superconductivity. A key feature was the separation of one-triplon and two-triplon scattering. Two-triplon scattering is observed quantitatively for the first time and so access is realized to the important strong magnetic quantum fluctuations. The spin gap is found to be 26.4+/-0.3 meV. The data are successfully modeled using the continuous unitary transformation method, and the exchange constants are determined by fitting to be Jleg=186 meV and Jrung=124 meV along the leg and rung, respectively; a substantial cyclic exchange of Jcyc=31 meV is confirmed.
RESUMEN
Neutron spectroscopic measurements of the magnetic excitations in PrO2 reveal (1) sharp peaks characteristic of transitions between levels of the 4f(1) configuration of Pr4+ split by the cubic crystal field, and (2) broad bands of scattering centered near 30 and 160 meV. We present a simple model based on a vibronic Hamiltonian that accounts for these contrasting features of the data. The analysis shows that 90%+/-10% of the Pr ions have a localized 4f(1) configuration and provides strong evidence for a dynamic Jahn-Teller effect in the gamma(8) electronic ground state.