Neutron Scattering

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,~ 1 = 0 represents the totally symmetric stationary solution . The prefactor of this purely elastic term is called elastic incoherent structure factor (EISF) . It only depends on the jump geometry and allows to discern between different jump models . The presence of a purely elastic term shows that the jumping atom remains localized around the molecule - in contrast to translational processes . Quasielastic scattering is related to the degenerate eigenvalues 4 , 3 . The two phase factors represent clockwise and anticlockwise jumps . The prefactor is now the inelastic incoherent structure factor (IISF) . It bas its maximum at Qd _ 32 and thus gives access to the jump distance . The Lorentzian width yields the jump rate . More complex motions A transition to potentials of higher multiplicity, e.g. V6 , introduces different jump distances Figure 17 .6 : QNS-spectra (right) and EISF (left) of adamantan . Spektrometer : IN5, ILL . Inset, right : The molecule and its rotation axes . Solid line : 90° jumps about all C 4 axes . Dashed : 120° jumps about all C 3 axes . and jump times . Correspondingly the scattering function contains more than one Lorentzian with different IISFs [5, 6] . The unhindered motion (multiplicity=oc) allows for any orientation. This rotational diffusion and is characterized like in the case of translation by a rotational diffusion coefficient [8] . Fig . 17 .6 shows quasielastic spectra of adamantan. The large dimensions of the molecule 17- 1 3
allow a good determination of the EISF in the accessible Q range . Thus precise conclusions on the possible rotations can be drawn . Rotational jumps are thermally activated and obey the Arrhenius law (17 .23) . Po = o is called attempt frequency. Its inverse is about the time requiredby the atom at roomtemperature to pass the jump distance . For a methyl group P o - 10 13 .sec 1 . The exponential factor represents the succes rate : the larger the barrier E o , the rarer a crossing . If the shape of a potential is given one gets the potential from the activation energy E a . It is assumed that the potential does not change with temperature . In general a large Q range is required at good energy resolution to get conclusive answers . Adamantan (fig . 17 .6) is an exceptionally good example . Best suited are backscattering instruments . Time-of-flight spectrometers suffer from a small Q-range . More complex 3- dimensionaljump models involve jump matrices of higher dimensions [6] . Possible reasons for wrong conclusions may be the occurrence of multiple jumps . neutron distinghuishes only the starting and the final orientation . Double jumps about an easy axis may look as a single jump about a high barrier [7] . The scattering function is calculated on the assumption of single jumps, however . Monte Carlo simulations eau clarify discrepancies . The 17 .3 .2 Rotational tunnelling : single particle model Stochastic motions take their energy from a thermal bath . At low temperature they die out and a classical description fails . A quantummechanical theory is needed . In quantum mechanics the indistinghuishable protons of a molecule are connected by a common wave function . This introduces coherence effects . Eigenenergies of rotation are the so-called librations in the meV regime - similar te , harmonie oscillations - and the new low energy tunnelling modes in the peV regime. A "pocket states" formalisme - described in more detail below for methyl groups - gives a qualitative picture . The molecule can exist in three possible orientations 123 >, 1231 > and 1312 > . If the barrier between these orientations is large, the orientational subgroups are decoupled and molecules can perform almost harmonie oscillations only (threefold degenerate) . For lower barrier the orientational substates are coupled. In quantum mechanical language : the wave funetions overlap and the librational states split by tunnelling . Thus rotational tunnelling is not a dynamical event. Only if one could prepare a system in a Gedanken experiment in a single orientation it would move into a new orientation within a time t - ~t . Tunnelling energies h~ are of the order of peV. Monographs are [4, 8, 9] . 17- 1 4
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Forschungszentrum Jülich in der He
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Forschungszentrum Jülich GmbH Inst
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Contents . . 1 Neutron Sources H .
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1 . Neutron Sources Harald Conrad 1
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d) f, = v a, , q - ti = P a, - 0 .3
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Period Example Flux (D [1013 1950-6
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In stage 1 the primary proton knock
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get as heat, the rest is transporte
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down power and stronger absorption
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Appendix Neutron Sources - an overv
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led remotely. It is rauch more conv
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2 . Properties of the neutron, elem
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In the 60's the ferst high flux rea
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Hot neutrons in reactors are obtain
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elated values for the FRJ-2 reactor
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2 .5 Scattering amplitude and cross
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(A+k2 ) yr = u(Z:) yr V( u~r)= z "
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_du _ mn \2 A2-~2z r2~ I(k' IVI k)I
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E ô ô x z w 0 z _w U N Figure 2 .
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scattering is observed with the ave
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For a spherical electron distributi
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A more thorough introduction to neu
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3 . Elastie Seattering from Many-Bo
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Q = Q = k2 + k2 - 2kk' cos2B (3 .3)
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3 .2 Fundamental Scattering Theory
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The meaning of (3 .l4) is immediate
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1 . e. in the second approximation,
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The saure problem will bc dealt wit
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ALQ) = Y-bfe'Q'-rB(r-(n .a+m .b+p»
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du LQ) = ALQJ2 = e'Q .A' . * -i e-
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Tab . 3.1 : The scattering lengths
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We note immediately that we should
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These magnetic structures can be un
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M1LQ)=Q-MQ)xQ M(Q)= IM (r)e i Q " d
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Orbital magnetic spin angular ,,- m
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aQ . r 3 àQ . N . QR . aQ .t k _ML
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Schweika Analysis Polarization W .
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ducing a complex component and were
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ir v s ftft 3956 2 s 2 0.81[X] 80 c
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Some polarizers using Bragg reflect
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Different to the coherent nuclear s
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where QBG denotes the background .
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4 .4 Applications We now consider e
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Fig. 4 .3 .4 probably represents th
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. occur only in the non-spin flip c
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The results for the magnetization d
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a large solid angle with polarizers
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scattering to spin-flip scattering
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An experimental verification of thi
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Correlation Functions Measured by S
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In real liquids however usually an
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The term in big parentheses of equa
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4 0 -5 0 5 10 15 20 ho) [meV] Figur
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Figure 5 .5 : The pair correlation
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Figure 5 .6 : Partial differential
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Then equation (5 .21) can be conver
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The route from expression (5 .27) t
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In addition it is often useful to d
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2 . The pair correlation function h
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G(r, t) G(r, t) i(Q, t) r Q i(Q, t)
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Insertion of this result into (5 .6
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With this example we can also demon
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6. Continuum description : Grazing
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Starting point is the exact atomic
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For specular reflectivity measureme
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v v z l 2r V(z) = 2 -2erfC~ with er
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F{dV(z)/dz ) to calculate the refle
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Equation (6 .13) also shows that th
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ut is completely reflected . The cr
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0.4° . The full dynamical theory c
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1) If no magnetic induction B (whic
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Diffractometer Gernot Heger
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select a special wavelengths band (
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" The direction of the reciprocal l
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Bragg-intensities of single crystal
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monochromator, on the mosaic spread
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complex sample environments, such a
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the diffraction plane) two further
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Small-angle Scattering and Reflecto
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L= (1) ( k ) a e -(k/k T ) 2 Ak (8
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possible neutron wave lengths betwe
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figure and with neutrons of about 1
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1 .0 Danvinkurve 0 .8 0 .6 0 .4 0 .
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tails. This reflection curve leads
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For the Nickel film one gets a thic
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I The rotor of a velocity selector
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multiplier are arranged in a quadra
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9 . Crystal spectrometer : triple-a
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machine which will be followed by a
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10 .3 Beaur shaping Due to the fact
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G.k=zGz . (11) This case is fulfill
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tribution of bisecting planes (ntos
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directions which is exploited for t
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Fig . 14) Dispersion surfaces for B
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whereby E and v,, denote energy and
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[1] B . N. Brockhouse, in Inelastic
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10 . Time-of-flight spectrometers M
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Tnloderator /K V7 2/m/s A/nm Aiw/eV
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10.2 .1 Interpretation of spectra A
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----~ Probe ' (Chopper 2) Zeit I Ch
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the similar intensity distribution
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is performed without Jacobian due t
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10.2 .6 Crystal monochromators The
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larger energy transfers . However i
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10.3 Inverted TOF-spectrometer In t
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constant offset to the TOF . To be
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sample size of cm and a detection p
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Contents 10 .1 Introduction . . . .
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11 . Neutron spin-echo spectrometer
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2 0 L Tr -0 L 2 ArDet Figure 11 .1
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initial velocity-reassemble at the
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distribution (current sheets) that
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esults, where il is an irrelevant c
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10000 - 8000 - co 6000 - C ô 4000-
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e0 oX n .5 1 .0 1 .5 so 0 .0 0 .0 1
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nel" coils) the required homogeneit
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Figure 11 .9 : S(Q, t)IS(Q) of a 2
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Contents 11 .1 Introduction . . . .
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12 . Structure determination G . He
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Important: Thc measured Bragg inten
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determination of accurate atomic pa
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The knowledge of the overall occupa
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(even anharmonic) effects. This com
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Fig . 4 . Coordination ofthe D20 mo
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fenoelectric phase of orthorhombic
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(c) and (d) calculated densities at
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13 Inelastic Neutron Scattering Pho
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Figurel Schematical drawing of a ge
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equilibrium position the atom is in
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s - ' f N .mwn .I )oo K L .~um..un
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esults from the quantum mechanics o
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y absorption . 13 .1 .3 Magnetic in
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Figure 7 The plane in reciprocal sp
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5 100s1 [OSl ] IOSSI [SSS y i 4 0 r
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dynamics of ionic compounds with th
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16 14 12 F.. . 10 ô 8 q 6 cr 4 G.
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21 [ooç 1 " - 19 18 17 16 "" 298 K
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n = v - cap[i(pga . - wt)] (13 .22)
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Y c 3 c Cc E Figure 16 Magnon dispe
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. . 2 H . Bilz and W . Kress, Phono
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14 . Soft Matter : Structure Dietma
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length density of a sample against
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2 P(Q) = 1 ~exp(-Ii - j'6 Q2 ) (14
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fraction. In the region of large Q
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with the partial structure factor S
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and which lead to the scattering cr
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For the intramolecular and intermol
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500 450 -400 U 0 ;, 350 42, 300 s~.
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scaling behavior of the susceptibil
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15 Polymer Dynamics Dieter Richter
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constraints due to the mutual inter
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(15 .3) T ß ( E)=T ô exp E k,T -
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This incoherent approximation does
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10 an 0 2 0 Figure 15.4 : Relaxatio
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In order to test Eq .[15 .9] the re
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Arrhenius temperature dependence wi
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1 3kBT ~ z 2 72 2 p2 a = f2 N2 P =W
Page 339 and 340: S (Q,t) = 12 fdu exp ~ -u_ (S2Rt) v
Page 341 and 342: We now use these data, in order to
Page 343 and 344: c 0 .8 0 .6 00 .055 x 0 .1 110 .14
Page 345 and 346: small but systematic deviations fro
Page 347 and 348: S(Q,t) is predicted . Such a platea
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Page 354: 16 Magnetism Thomas Brückel
Page 357 and 358: the ground state of metallic magnet
Page 359 and 360: c~ ~ o 0 `,~i) g',, b o 0 Q! b o 0
Page 361 and 362: only "sec" this component and not t
Page 363 and 364: 200 ¬ 400 500 600 700 E 3 QI J (h0
Page 365 and 366: Fie. 16.4: Scattering geometry for
Page 367 and 368: 0 .8 ' D3 ° Stassis 3d spin - - 3d
Page 369 and 370: Here, J;j denotes the exchange cons
Page 371 and 372: Fig. 16.8 : Contour plot of the mag
Page 373 and 374: 0.0001 0.001 0 .01 0.1 2 Fig. 16 .1
Page 376: 17 Translation and Rotation M . Pra
Page 379 and 380: 17.1 Introduction Atomic and molecu
Page 381 and 382: 17 .2.2 Diffusion, microscopic appr
Page 383 and 384: 0.6 H20 2 x2tÄ z ) Figure 17 .2 :
Page 385 and 386: The scattering function of a polycr
Page 387 and 388: e (2) 1 (j 1, 0 > - 0,1 >), respect
Page 389: leads to the eigenvalue problem ~q
Page 393 and 394: m Eu W Figure 17 .7 : Eigenenergies
Page 395 and 396: -so o so Energy transfer (NeV) Ener
Page 397 and 398: Bibliography [1] T. Springer, Quasi
Page 400 and 401: 18 . Texture in Materials and Earth
Page 402 and 403: Further important structure related
Page 404 and 405: The axes of the cartesian Kp coordi
Page 406 and 407: 4.0 Experimental Texture Analysis T
Page 408 and 409: Fig. 18.14: Variation of reflection
Page 410 and 411: measurements can be performed in tr
Page 412 and 413: Apart from the global description o
Page 414 and 415: In the following, two experimental
Page 416 and 417: The neutron diffraction pole figure
Page 418 and 419: dependent pole densities, and (4) g
Page 420 and 421: Schriften des Forschungszentrums J
Page 423: Forschungszentrum Jülich in der He
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Neutron Scattering

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