Ion Density Fluctuations in Liquid GalliumThe study of ion density fluctuations in the THzrange in metals is a basic branch useful to derivegeneral information on the fluid dynamics. Inad<strong>di</strong>tion, these stu<strong>di</strong>es allow to use the ions inmetals as weak probes of the electron fluid responseat low frequency. Indeed, the effective ion-ionpotential is me<strong>di</strong>ated by the electron fluid throughthe <strong>di</strong>electric response in a frequency region which isquite small as compared to the Fermi energy, so thatthe electron response can be safely considered asstatic.Despite the long stan<strong>di</strong>ng research devoted to theanalysis of the electron gas properties, this is still aforefront problem of many body physics.Neutron and photon scattering are the most powerfultools for the investigation of the ion dynamics incondensed systems from crystals to gases.We present here an investigation of the ion dynamicsin liquid Gallium by means of neutron[1] and x-rayscattering[2], as well as, molecular dynamicssimulation[1]. Although liquid alkali metals are theprototype for the study of the electron response,polyvalent metals are also very interesting since theyprovide information on the electron gas at higherdensity than alkali metals, other than ad<strong>di</strong>tionalinformation on the role of the ion core repulsivepotential. The experimental results of Refs. 1 and 2are summarized in Fig. 1, where the experimentalresults from neutron and x-ray scattering arepresented in comparison to a large scale moleculardynamics simulation. The very good agreementbetween simulation and experiments gives a strongsupport for the ion-ion potential employed in thecomputer experiment. This potential is <strong>di</strong>rectlydeduced from the electron gas properties and it isable to reproduce, without adjustable parameters,the experimental results at two quite <strong>di</strong>fferenttemperatures. The form we employed is <strong>di</strong>rectly<strong>di</strong>ctated from the electron gas response plus arepulsive contribution equal to that used in thestandard Lennard-Jones potential. Therefore theeffective two body potential includes many bodyeffects both the ion liquid and electron liquid throughthe density of the system. In the present simulationthe potential is given by:related to the electron density. The coefficient VRand VF are deduced by comparing the simulated andexperimental structure factors at room temperature.The pair potential proves its effectiveness by beingable to reproduce the static structure factor at hightemperature (970 K) without adjustable parameters.In ad<strong>di</strong>tion the simulation is quite effective inprovi<strong>di</strong>ng a good agreement with the experimentalresults for the dynamic structure factor as measuredat room temperature and 970 K by neutronscattering and at room temperature by x-rayscattering.References[1] L. E. Bove, F. Formisano, F. Sacchetti, C. Petrillo,A. Ivanov, B. Dorner, and F. Barocchi, Phys. Rev.B71, 014207 (2005).[2] T. Scopigno, A. Filipponi, M. Krisch, G. Monaco,G. Ruocco, and F. Sette, Phys. Rev. Lett. 89, 255506(2002).AuthorsL. E. Bove,1 F. Formisano, C. Petrillo, F. Sacchetti<strong>Dipartimento</strong> <strong>di</strong> <strong>Fisica</strong>, Università <strong>di</strong> Perugia, CRS-SOFT Unità <strong>di</strong> Perugia and Centro per i MaterialiInnovativi e Nanostrutturati (CEMIN), Via A. Pascoli,I-06123 Perugia, Italy.99SOFT Scientific <strong>Report</strong> 2004-06
Scientific <strong>Report</strong> – Elastic and inelastic scattering of neutrons and X-raysFemtosecond dynamics in Ferromagnetic MetalsIn electronic Resonant Raman Scattering (RRS) thesample is resonantly excited at a core threshold anda photon is emitted bringing the sample to the finalstate. This process sets a characteristic time scaleconnected with the lifetime of the interme<strong>di</strong>ate statecore hole. If there is an evolution of the systemalong this time scale the effect is seen in the spectralfunction [1]. For this reason this approach to thestudy of fast processes is called core hole clock. Thepresent work is to our knowledge the firstimplementation of the core hole clock in the study ofmagnetism. This is done in ferromagnetic metals(Fe-Co-Ni) in the scattering channel 2p 6 3d n + hν in →2p 5 3d n+1 → 2p 6 3d n+1 3s 1 + hν out, where n is the 3doccupation number in the ground state and hν in andhν out are the incident and outgoing photon energies(in this case hν out is about 100 eV lower than hν in).We use the so called Integrated RRS (IRRS) [2]where one measures, as a function of hν in, theintegral of the scattered intensity in the scatteringchannel. The geometry is shown in the upper panelof Figure 1. The IRRS spectra show an effect notpresent in ferromagnetic insulators. A magneticcircular <strong>di</strong>chroism (MCD) is seen in IRRS spectra alsoin those regions between L 3 and L 2 and above L 2where the usual XMCD gives no signal (see the lowerpanels on Co in the figure where the red areas showthis effect on the IRRS <strong>di</strong>chroism given in green).This is due to the spin dependent screening takingplace in the interme<strong>di</strong>ate state [3] so that the corehole becomes polarised. In order to take place, thisprocess requires a non zero magnetic moment of thesystem. In the fully relaxed theoretical model of ref.[3] the <strong>di</strong>chroism (more exactly the flipping ratio)depends on the local magnetic moment of theexcited site once the core hole is fully screened. Thecrucial experimental information given here is thetrend of this <strong>di</strong>chroism (flipping ratio) along thesequence Fe-Co-Ni and in particular the fact that Ni-100.20770 790 810 770 790 810Fig.1.L 3 L 13LL 22XMCDCo metalIRRS-MCDA0Fig. 2.metal shows clearly this effect. This is importantsince in a fully relaxed situation we should see noeffects because a well screened core hole in Ni givesessentially a 3d 10 configuration with almost zeromagnetic moment. Thus along the characteristic timescale of the scattering (typically 1-2 femtoseconds)Ni has not the time to develop the spin dependentscreening of the Fermi gas.More can be seen from Figure 2 where thehistograms give the magnetic moments of therelaxed and unrelaxed sites [3] in comparison withthe flipping ratio of the IRRS-MCD given by the reddots. The data are normalised to one in the case ofFe where a relaxed model accounts for the effect asseen in Auger spectroscopy [3]. The trend showsthat Co is somewhere in between the fully screenedand unscreened situation whereas Ni is basicallyunrelaxed. Thus Ni is much slower in buil<strong>di</strong>ng up thescreening and we attribute this behaviour to thenarrow band nature of the Ni-3d holes while the Feand Co case are certainly much more itinerant. Inthis sense we see a link between the correlationproperties and the magnetic dynamics along shorttime scales. In conclusion the use of the core holeclock in soft x-ray resonant scattering points out asubstantially slower dynamics in Ni in comparisonwith Fe and Co.References[1] P.A. Brühwiler, O. Karis, and N. Mårtensson,Rev. Mod. Phys. 74, 703-740 (2002) and referencesquoted therein.[2] L. Braicovich et al. , Phys. Rev. Lett. 90, 117401(2003).[3] A. Chassé et al. , Phys. Rev B 68, 214402(2003) and references quoted therein.AuthorsL. Braicovich (a), G. Ghiringhelli (a), A. Tagliaferri(a), G. van der Laan (b), E. Annese (c), and N. B.Brookes (d)(a) INFM-CNR and Dip. <strong>di</strong> <strong>Fisica</strong>, Politecnico <strong>di</strong>Milano, Italy; (b) Magnetic Spectroscopy, DaresburyLaboratory, UK; (c) TASC, Trieste, Italy; (d)European Synchrotron Ra<strong>di</strong>ation Facility, Grenoble,France.SOFT Scientific <strong>Report</strong> 2004-06100
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