MAGNETIC LINE-WIDTH OF THE IV-COMPOUND
YbCu2Si2
E. Holland-Moritz, D. Wohlleben, M. Loewenhaupt
To cite this version:
E. Holland-Moritz, D. Wohlleben, M. Loewenhaupt. MAGNETIC LINE-WIDTH OF THE IVCOMPOUND YbCu2Si2. Journal de Physique Colloques, 1978, 39 (C6), pp.C6-835-C6-836.
<10.1051/jphyscol:19786372>. <jpa-00217839>
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Submitted on 1 Jan 1978
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JOURNAL DE PHYSIQUE
Colloque C6, supplément au n" 8, Tome 39, août 1978, page
C6-835
MAGNETIC LINE-WIDTH OF THE IV-COMPOUND Y b C u 2 S i 2 +
E. Holland-Moritz, D. Wohlleben and M. Loewenhaupt
II. Phys. Institut
der Vniversitat
zu Win, W. Germany.
*Institut
ftir Festkdrperforsahung
der KFA Jtilieh, W. Germany
Résumé.- Le spectre de puissance des niveaux Zeeman 4f magnétiques sur les ions fluctuants du Yb a
été mesuré en fonction de la température par diffraction diffuse magnétique de neutrons. Une largeur de raie T/2-7 meV quasiêlastique, presque indépendante de la température à été détectée. Pour
la première fois, les mesures ont été étendues, dans cette expérience, à la spectroscopie de perte
d'énergie à kfiT«r/2.
Abstract.- The power spectrum of the magnetic 4f Zeeman-levels on the fluctuating Yb-ions was measured as a function of temperature by diffuse magnetic neutron scattering. A nearly temperature
independent quasielestic line width V/2-1 meV was detected. In this experiment, the measurements
were extended, for the first time, to the energy loss spectroscopy at k„T«r/2.
It has been established previously by measu-
energy E larger than T/2.
rements of the diffuse magnetic neutron scattering
cross-section of CePd3 /l/ and Ce Th _
that in the so-called valence
alloys 111
fluctuation phase
-50
3
-40
-30
-20
-10
0
10
r — • — • — • — • — w ~ ^
20
30
40
50
•
•
•
;
30
40
50
the 4f Zeeman-levels relax spontaneously and nearly
independent of temperature at a rate of about 10"
F( W )=—^
—
/
,. e -""'V , r / 2 , 2 , | h u ) ) ' /
s. It was not possible to establish unequivocally
r72=86meV
/
\
\
200\
the existence of this relaxation near T=0 in these
first experiments, because they were performed by
measuring the energy gain of the neutron and there-
/ /
100
\
fore suffered from severe intensity problems at
k_T«r/2. We report here the first such measurements
^JMM^>
on a Ytterbium fluctuation valence compound in
both energy gain and energy loss.
The problem with
energy gain measurements
-50
-40
-30
-20
-10
0
10
20
ENERGIE TRANSFER ImeVI
is demonstrated in figure 1. If one assumes a temperature independent Lorentzian power spectrum, the
energy dependence of the scattering cross-section
Fig. 1 : Energy dependent part of the scattering
law S(0,w).
at a given momentum transfer Q has the form of F(oo)
as shown in the upper left of figure 1. In the high
The measured spectrum will then correspond to the
temperature approximation (k_T»I72) F(u) approa-
hatched area under the heavy line. This spectrum
ches a Lorentzian with an intensity proportional to
originating from a broad quasielastic Lorentzian
k„T. In this case, energy loss and energy gain specis
troscopy can both measure the width F/2. However,
can be clearly distinguished from the usual inelastic line spectra (see also C. Balseiro and A. Lopez
/3/).
at k_T«r/2 the scattering goes to zero on the
As before /I/ along with those on the interenergy gain side, because there are no excitations
mediate valence (IV)-compound (YbCu2Si2) we have
in the sample to give energy to the neutron.
performed measurements on a diamagnetic compound
Thus in order to establish the existence of the Lo(LaCu2Si2) and a compound with a stable 4f-shell
rentzian power spectrum near T=0, energy loss spec(TbCu2Si2), in order to separate inelastic phonon
troscopy has to be performed with incoming neutron
scattering and to contrast the normal Korringa beWork was performed within the program of SFB 125
Aachen-Julich-Koln.
haviour of the line width for a stable 4f-shell
against the large temperature independent width of
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19786372
IV-compound. Figure 2 shows the energy gain spectra
at room temperature (measured by TOF method).
on the same compound.
The
hatched area is the elastic nuclear incoherent
scattering and exhibits the instrumental resolution
'
12
f
6
9
athw=O by its width. In figure 2a the scattering
intensity around -15 meV fulfilled a Q2-dependence
and is due to phonon scattering. The drawn out line
3
in figure 2b is a fit of the diffuse magnetic scat' - 2 0
tering according to
s~Q,.)
=
l g r
- c u )
2
?JB
2
2 - 5
6
8
ENERGY TRANSFER [meVl
F~(Q)-~(T).
+-PUW)
1-e
Fig. 3 : Energy loss spectra of YbCu,Si,and
LaCu,Si2 for 20=14O at 5 K; E =12.5 mev.
(I)
0
Figure 4b shows the temperature dependence of the
line width of TbCu,Si2.
Yb CuqSi2
-30 -25 -20 -15 -10
-5 0
-30 -25 -20 -15
ENERGY TRANSFER 1 meV 1
-10
-5
0
o
07
A INL
Fig. 2 : Energy gain spectra of YbCu Si, and
LaCu2Si2 for 20=20° at 300 K; Eo= 3 . 3 meV.
Here is F(Q) the local magnetic 4f-form factor,
x(T)
the static susceptibility and P(fiw) the power
0
la0
2@J
3@J0
TEMPERATURE [KI
TEMPERATURE [Kl
Fig. 4 : Quasielastic line widths of YbCu2Si2 and
TbCu2Si, plotted against temperature.
spectrum, e.g. a Lorentzian. In a time-of-flight
As previously for CePd, we now observed a
experiment the value of the momentum transfer Q
varies with energy transfer ,hu for a constant scattering angle 20. The fit includes the incoherent
scattering. The additional intensity over the fit
is due to phonon scattering. (The original fit was
performed on a difference spectrum of YbCu2Si2 and
LaCu,Si,.)
In the power spectrum P&w)
we liave as-
sumed one quasielastic magnetic line (dashed) and
one inelastic magnetic line (dotted) centered at
12 meV with a nearly temperature independent width
r/2=(15
+
very large nearly temperature independent line
width in YbCu,Si2, which at 30 K is 60 times wider
than the magnetic line of the stable valence compound TbCu,Si,.
ted by comparison of figure 3a with figure 1. This
measurement establishes more firmly than before the
fact that the large r/2 for IV-compounds is a property of the ground state.
5) meV. The width of the quasielastic li-
ne obtained from such fits is plotted as a function
of temperature down to 30 K in figure 4a (circles).
Figure 3 shows energy loss spectra of the same systems at 5 K. The energy E of the incident neutrons
0
was 12.5 meV. Fits on the basis of formula (I)
yield line widths plotted in figure 4a (triangles).
The values of r/2 are consistent with the
fluctuation energy of 6.5 meV derived from the static susceptibility 141 or the width of the zero
bias anomaly derived by tunnellin~spectroscopy /5/
The existence of the Lorentzian
spectrum at T<<r/2 is quite convincingly demonstra-
References
/I/
Holland-Moritz, E., Loewenhaupt, M., Schmatz ,
W. and Wohlleben, D., Phys. Rev. Lett. 2 (1977)
983.
/2/ Shapiro, S.M., Axe, J.D., Birgenau, R.J.,
Lawrence, J.M. and Parks, R.D., Phys. Rev.
B 16 (1977) 2225.
/3/ Balseiro, C. and Lopez, A., Phys. Rev. B
(1978) 99.
/4/ Sales, B. and Wohlleben, D., Phys. Rev. Lett.
(1975) 1240.
21
/5/ Leppin, H.P., Wohlleben, D. and Sales, B., Physica 86-888 (1977) 241.