Quasielastic magnetic scattering of neutrons by heavy-fermion systems

A theory of quasielastic scattering by a spin liquid with resonating valence bonds is constructed. It is demonstrated that the dependence of the scattering cross section of the spin liquid on the energy transfer has the same shape as the quasielastic peak observed experimentally in heavy-fermion systems. It is shown that a consequence of fact that the excitations of the spin-liquid state obey Fermi statistics is that the total quasielastic scattering cross section oscillates as a function of the momentum transfer. Pis’ma Zh. éksp. Teor. Fiz. 68, No. 6, 480–485 (25 September 1998)     

Time-domain interferometry using synchrotron radiation applied to diffusion in ordered alloys

Time-domain interferometry of synchrotron radiation (TDI) has recently been used as a tool for investigating diffusion in glasses. This work deals with an extension of this technique to ordered structures. In a TDI experiment performed on the B2 alloy CoGa at the APS the intensity scattered into Bragg directions showed no detectable quasielastic signal. Experimental lower limits of the elastic contribution are given. They are in accordance with the coherent scattering function derived in this paper. This result indicates that TDI can be applied to diffusion in crystalline solids, e.g. intermetallic alloys, by using diffuse scattering. Requirements and limitations of diffuse scattering experiments are discussed. Received 21 September 2000 and Received in final form 13 December 2000     

Enhanced Landau-Placzek ratio in a heavy fermion metal observed by Brillouin scattering

Brillouin spectroscopy of the heavy fermion metal UPt₃ shows strong quasielastic scattering, which has no analogue in ordinary metals. An enhanced Landau-Placzek ratio relating the quasielastic to the inelastic scattering intensity is deduced from the line shape analysis. The quasielastic line is of nonmagnetic origin and is attributed to electron density fluctuations as predicted by a recent theory of hydrodynamic fluctuations in heavy fermion systems.     

Diffusion of hydrogen in the β-phase of pd-H studied by small energy transfer neutron scattering

The diffusion of hydrogen in β-PdH_x has been studied by quasielastic neutron scattering. It is shown that the diffusion occurs through jumps between adjacent octahedral interstitial sites. The observed integrated quasielastic intensities cannot be described by a simple Debye-Waller factor. The phase transition from the β-phase to the α-phase has also been studied. No dramatic changes in the scattering patterns were observed. It is concluded that the diffusion mechanism is remarkably similar for the low concentration α-phase and the high concentration β-phase.     

Dynamics of cubic-tetragonal phase transition in KNbO<Subscript>3</Subscript> perovskite

The low-energy part of the vibration spectrum of KNbO₃ was studied by cold neutron inelastic scattering in the cubic phase. In addition to acoustic phonons, we observe strong diffuse scattering, which consists of two components. The first one is quasistatic and has a temperature-independent intensity. The second component appears as quasielastic scattering in the neutron spectrum, indicating a dynamic origin. From analysis of the inelastic data, we conclude that the quasielastic component and the acoustic phonon are mutually coupled. The susceptibility associated with the quasielastic component grows as the temperature approaches T _C .     

Neutron scattering on the strongly correlated electron CeNi Cu system: from non-magnetic behaviour to long-range magnetic order

The ground state of the strongly correlated electron CeNi_1-xCu_x compounds has been investigated by means of neutron scattering experiments. Thus, magnetic diffraction was performed for compounds showing long-range magnetic order (x > 0.2). An evolution from a collinear ferromagnetic structure for x =0.6 to a simple antiferromagnetic one for CeCu takes place through some more complex magnetic structures for intermediate compositions. The magnetic moments are continuously reduced when the Ni content increases reflecting the progressive enhancement of the Kondo screening. The large reduction found for x =0.6 compound is discussed and the existence of a spin glass like component of the magnetic moment cannot be discarded. From the quasielastic spectra, we have obtained the Kondo temperatures which are close to the magnetic ordering ones. The quasielastic line-width evolves from a linear temperature dependence to a T ^1/2 behaviour when approaching the non-magnetic limit. Then, this system provides an interesting example for the evolution of unstable 4 f shell relaxation regimes when modifying the hybridisation strength. Received 22 May 2000     

Magnetic correlations in heavy fermion CeAl3 compound

To study the effect of hybridization on inter-ion correlations the measurements of the intensity of the quasielastic scattering on momentum transfer in Ce_0.9Y _0.1Al₃, Ce_0.9La_0.1Al₃ and CeAl₃ have been performed. Some hints that quasielastic magnetic scattering more sensitive to regularity in rare-earth sublattice than to the hybridization have been obtained. We also get some indications on the presence of ferromagnetic inter-ion correlations. In a contrast to quasielastic scattering the inelastic part of spectra is very sensitive to the hybridization.     

Quasielastic electron scattering in the giant-resonance energy region

A new method is proposed for extracting the quasielastic portion of the spectrum of scattered electrons. The method is based on the so-called “bin” technique within which a multipole analysis is followed by a determination of the resonance shape and magnitude. The method essentially consists in performing a simultaneous multipole analysis of the mixture of quasielastic and resonance cross sections. A subsequent identification of resonance peaks in the cross section and its quasielastic portion is performed in the energy dependence of the transition probability for each multipolarity, since, there, the difference between the resonance and quasielastic processes is more pronounced than in the spectra of scattered electrons. The method makes it possible to reconstruct the process and to find the energy dependence of the quasielastic cross section in the primary spectra of scattered electrons. The spectra of quasielastically scattered electrons are obtained by the method in question for the ⁶⁵Cu nucleus. The positions of the quasielastic-peak maxima and their shift with respect to the case of scattering on a free nucleon are determined in the momentum-transfer range q = 0.5–1.2 fm^?1.     

Simultaneous energy analysis in a large angular range: A novel neutron spectrometer,its resolution and applications

A new type of crystal spectrometer for elastic and inelastic neutron scattering has been set up by modifying an existing flat-cone diffractormeter at the BER II-reactor in Berlin (West). The main difference to conventional triple axis spectrometers is the analyser part. It consists of large crystal plates which reflect the neutrons out of the horizontal plane into a curved multicounter. This allows simultaneous measurements in a large and continuous range of scattering angles for a constant energy transfer. The resolution function has been calculated and compared with experimental results. There is the possibility to focus acoustical phonons. We present applications together with experimental results such as quasielastic diffuse scattering in orientationally disordered crystals and inelastic scattering due to acoustical phonons. In combination with the flat-cone technique all elastic and inelastic scattering events of a single crystal can be collected in a systematic and efficient way.in collaboration with the Hahn-Meitner Institut, Berlin (West)     

Quasielastic neutron scattering by diffusive adsorbed hydrogen as a possible cause of the energy spreading of ultracold neutrons during long storage in traps

Small cooling and heating of ultracold neutrons during long storage in traps have been observed in recent experiments. It is shown that quasielastic neutron scattering due to the diffusive motion of hydrogen at the surface of adsorbed hydrogenous contaminants on the surface may be a possible cause of the spreading of the energy of ultracold neutrons during long storage in traps. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 2, 81–86 (25 January 1999) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Deep processes in nuclei and in-medium dressing of the nucleons and mesons

K⁺ scattering and quasielastic electron scattering from nuclei are expected to provide information about the nucleons and mesons in the inner regions of nuclei. The K⁺- nucleus cross sections and the quasielastic electron-nucleus response functions have been calculated taking into account the same in-medium dressing of the nucleons and the same coupling of the σ and ω mesons to the polarization of nuclear matter. We obtain a good agreement with experimental data for the two processes.     

Quasielastic scattering of light by a photoexcited electron-hole plasma induced in a GaAs layer in the presence of InAs quantum dots

The development of a method for registering quasielastic electronic light scattering spectra in the near-IR region, which makes it possible to detect light scattering by a photoexcited electron-hole plasma induced in a GaAs layer in the presence of a self-organized ensemble of InAs quantum dots, is reported. A substantial resonance intensification of such scattering, two orders of magnitude greater than the values established for the bulk material, is observed, and the main mechanism of such scattering is determined.     

Neutron quasielastic scattering from dynamic disorder in PbF2 and BaF2

Coherent neutron quasielastic scattering has been observed from powder samples of PbF₂, and BaF₂, at temperatures above their transition to fast-ion conduction. The scattering is found to be similar to that previously observed in SrCl₂ and CaF₂, suggesting that it stems from a mechanism common to these fluorites in their fast-ion phase.     

Magnetic relaxation in UCu4 + xAl8 - x

A quasielastic neutron scattering study has been performed on UCu_4+xAl_8-x, a system which reveals an alloying-induced transition from magnetic order to heavy-fermion behavior. The magnetic response can be described by a broad quasielastic Lorentzian for all concentrations. No crystal field excitations could be detected. Remarkably, the linewidth decreases with increasing hybridization strength and seems to be no measure of the Kondo lattice temperature T*. In addition, electron spin resonance (ESR) experiments were performed which revealed that, even in the heavy-fermion regime, spin fluctuations play an important role. Both experimental findings are in distinct contrast to cerium-based heavy fermion systems.     

Nuclear effects on lepton polarization in charged-current quasielastic neutrino scattering

We use a correlated local Fermi gas (LFG) model, which accounts also for long distance corrections of the RPA type and final-state interactions, to compute the polarization of the final lepton in charged-current quasielastic neutrino scattering. The present model has been successfully used in recent studies of inclusive neutrino nucleus processes and muon capture. We investigate the relevance of nuclear effects in the particular case of τ polarization in tau-neutrino induced reactions for several kinematics of relevance for neutrino oscillation experiments.     

Study of jump models for the diffusion of hydrogen in NbH0.02 by means of the quasielastic scattering of slow neutrons

The quasielastic scattering of slow neutrons in a NbH_0.02 single crystal has been measured for a wavevector transfer Q between 0.5 ≤ |Q| ≤ 2.5 Å^?1 at 293, 431 and 581 K for several sample orientations. The data have been analysed in terms of several jump models for the diffusion of hydrogen by comparing the spectral lineshape with the incoherent scattering law S_inc(Q, ω) of the models. The model of jumps to nearest-neighbor sites becomes progressively worse with increasing temperature. The discrepancy cannot be remedied by taking into account jumps to the second-neighbor sites. On the other hand the measured spectral lineshape can be well reproduced in the whole |Q| range by a double-jump model and a two-state model. For comparison, the method of fitting one Lorentzian to theory and experiment has also been used.     

The diffusive motion of silver ions in α-AgI: Results from quasielastic neutron scattering

The diffusive motion of silver ions in σ-AgI at 250°C has been studied by quasielastic cold neutron scattering. Spectra were taken in the range of wavevector transfer 0.5 < Q < 2.2Å^?1 for elastic scattering. The quasielastic line shapes contain a narrow and a broad component. They are compared to model calculations allowing for the superposition of two kinds of motion on two different time scales, a local random motion and a translational motion of the jump-diffusion type. The model closely fits the data. The local random motion takes place on a time scale of the order of 10^?12 s, with amplitudes of the order of 1 Å. It is probably caused by rapid fluctuations of the local potentials due to the diffusive motion of the other cations. The translational motion results in a mean displacement of the silver ion over a distance of the order of a lattice constant (5 Å) with a correlation time of the order of 10^?11s. This correlation time is composed of a residence time and a time-of-flight, which are both of comparable magnitude.     

Methyl rotation in acetamide: the transition from quantum mechanical tunneling to classical reorientation studied by inelastic neutron scattering

Quasielastic and inelastic incoherent neutron scattering has been used to study in detail the transition from quantum mechanical tunneling motion to classical reorientation of the methyl groups in rhombohedral acetamide CH₃CONH₂. The temperature dependence of the low temperature quasielastic and inelastic scattering due toE ^a E ^b and AE transitions of the tunneling methyl groups has been investigated with eV resolution and — together with the higher temperature quasielastic scattering-compared with theoretical predictions. Microscopic theories are capable to describe most of the experimental observations at low temperatures. A heuristic theoretical approach accounts well for the high temperature results.