High-Resolution Photoemission On UPdSn and UNiAl - 125264y

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High-Resolution Photoemission On UPdSn and UNiAl

Almeida, T.; Havela, L.; Sechovsky, V.; Naegele, J.R.; Bruck, E.H.

DOI

10.1016/0925-8388(92)90313-X

Publication date

1992

Published in

Journal of Alloys and Compounds

Link to publication

Citation for published version (APA):

Almeida, T., Havela, L., Sechovsky, V., Naegele, J. R., & Bruck, E. H. (1992).

High-Resolution Photoemission On UPdSn and UNiAl. Journal of Alloys and Compounds, 181(1-2),

205-210. https://doi.org/10.1016/0925-8388(92)90313-X

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JAL 8074

High resolution photoemission on UPdSn and UNiA1

L. Havela, T. A l m e i d a a n d J. R. N a e g e l e

Commission o f the European Communities, Joint Research Centre, Institute f o r T r a n s u r a n i u m Elements, D-TSO0 Karlsruhe (FRG)

V. S e c h o v s k y

Department o f Metal Physics, Charles University, Ke Karlovu S, CS-121 15 Prague 2 (Czechoslovakia)

E. B r t i c k

Van der W a a l s - Z e e m a n Laboratory, University of Amsterdam, Valckenierstraat 5S,

N1-1018 X E A m s t e r d a m (Netherlands)

A b s t r a c t

High-resolution UV photoelectron spectroscopy on heavy-fermion antiferromagnet UNiA1 displays a broad triangular 5f emission spread from E F dowIl to at least - 1 eV. The presence of 5f states at EF, in contrast to the low T value of 5 mJ mol- 1 K-2, is observed also in local-moment antiferromagnet UPdSn. The considerable degree of itinerancy is demonstrated in both cases by 4f core-level spectra.

1. I n t r o d u c t i o n V a r i a t i o n s in t h e 5 f e l e c t r o n l o c a l i z a t i o n o f a c t i n i d e i n t e r m e t a l l i c s c a n l e a d t o d i f f e r e n t t y p e s o f g r o u n d state. P h o t o e l e c t r o n s p e c t r o s c o p y is in p r i n c i p l e a p o w e r f u l m e t h o d f o r f o l l o w i n g d e t a i l s o f t h e l o c a l i z a t i o n p r o c e s s o n a m i c r o s c o p i c scale. H o w e v e r , it is n o t y e t c l e a r t h e e x t e n t to w h i c h t h e p h o t o e m i s s i o n s p e c t r a a r e a f f e c t e d b y f i n a l - s t a t e e f f e c t s in t h e c a s e o f m a t e r i a l s w i t h s t r o n g e l e c t r o n - e l e c t r o n c o r r e l a t i o n s , o r t h e e x t e n t t o w h i c h t h e y r e f l e c t t h e u n d e r l y i n g s i n g l e - p a r t i c l e e l e c t r o n i c s t r u c t u r e . T h e a i m o f t h e p r e s e n t i n v e s t i g a t i o n is t o c o n t r i b u t e to t h e u n d e r s t a n d i n g o f t h e b a s i c c h a r a c t e r i s t i c s o f t h e e l e c t r o n i c s t r u c t u r e o f u r a n i u m i n t e r m e t a l l i c s . W e h a v e c o n c e n t r a t e d o n p h o t o e m i s s i o n s p e c t r o s c o p y s t u d i e s o n t e r n a r y u r a n i u m c o m p o u n d s o f t h e t y p e UTX. S y s t e m a t i c r e s e a r c h o f t h e i r b u l k p r o p e r t i e s h a s p r o v i d e d a g o o d b a c k g r o u n d f o r s t u d i e s u s i n g m i c r o s c o p i c m e t h o d s . V a r i a t i o n in t h e T a n d X c o m p o n e n t s p r o v i d e s a w i d e s p e c t r u m o f p h y s i c a l b e h a v i o u r . F o r a n i n t r o d u c t o r y s t u d y , t w o o f t h e m , UNiA1 a n d U P d S n , w e r e s e l e c t e d a s r e p - r e s e n t a t i v e s o f d i f f e r e n t c l a s s e s o f c o m p o u n d s . UNiA1 is a n t i f e r r o m a g n e t i c b e l o w T N = 19 K, b u t t h e s l o w m a g n e t i c s a t u r a t i o n in fields a b o v e t h e m e t a m a g n e t i c t r a n s i t i o n (Be = 11.3 T) a n d t h e l o w m a g n e t i c m o m e n t s ( a b o u t 0 . 6 ~B p e r u r a n i u m a t o m ) i n d i c a t e a n i t i n e r a n t c h a r a c t e r o f t h e m a g n e t i s m . Also t h e h i g h T v a l u e o f 1 6 4 m J t o o l -1 K -2 i n d i c a t e s t h e p r e s e n c e o f i t i n e r a n t

206

5f states at EF [ 1 ]. On the contrary, the T coefficient of UPdSn is only about 5 mJ mol -~ K -2 [2-4]. This value is extraordinarily low among uranium intermetallics and can, together with the considerable size of the ordered magnetic m o m e n t s on the U atoms (2.1 ~B per uranium a t o m [5]), be explained as a consequence of localization of the 5f states. The reason for the existence of localized 5f states in UPdSn (or at least more localized than in UNiA1) can be the different geometrical a r r a n g e m e n t of the uranium atoms (only two nearest neighbours at a distance of 365 p m ) a n d / o r a reduced 5 f - d hybridization. Inspecting possible analogies in the broad spectrum of uranium intermetalli c compounds, we can find only one c o m p o u n d with such a low T value, namely UPda, where the shift of 5f states from EF is d o c u m e n t e d by electron spectroscopy [6-8].

2. E x p e r i m e n t a l d e t a i l s a n d d i s c u s s i o n

Polycrystalline samples of UNiA1 and UPdSn were prepared by arc melting of stoichiometric amounts of the constituents. The surface was cleaned by sputtering by Ar + ions during heating to 150 °C. The spectra were recorded at T = 77 K. A stoichiometric surface was obtained by in-situ scraping using a diamond file. The variation in the composition upon different preparation procedures was monitored by the evaluation of the integral intensities of the core level lines. The compositions of the surfaces subjected to standard Ar + ion sputtering (argon at 8 × 10 -5 Torr; 3 keV), which is used below for comparison with the stoichiometric surface, correspond to U1.4~Pd1.12Sno.4a and ULeNiAlo.s. The O 2p features were negligible in all spectra for at least 30 min, which demonstrates the good quality of the surface.

Valence band spectra of UPdSn were recorded for the excitation energies

hv= 16.8 eV (Ar I), 21.2 eV (He I) and 40.8 eV (He II). The instrumental resolution combined with the Fermi level temperature broadening at 77 K corresponds to a resolution A ranging from 50 to 100 meV depending on the electron analyser pass energyEo. Spectra obtained by X-ray photoelectron spectroscopy (XPS) with A1 Ka excitation ( h v = 1 4 8 6 . 6 eV) display A~-0.8 eV. The absolute accuracy of the calibration of the energy analyser voltage, which was checked by the position of the gold Fermi edge, is better t h a n 5 meV in UV photoelectron spectroscopy (UPS) and better t h a n 100 meV in XPS.

All experimental spectra were refined in a standard way by applying a correction for the transmission function of the energy analyser, and subtracting the secondary-electron background. In the case of Ne I spectra, the doublet

(hv= 16.67 and 16.85 eV) was first separated. We are aware of the fact that the secondary-electron background subtraction procedures are most reliable for higher kinetic energies (XPS, He ID, while some arbitrary estimates are necessary to cope with the strong b a c k g r o u n d variation and its high intensity for low kinetic energies of photoelectrons, which is crucial for Ne I and He I spectra.

Valence band spectra for different excitation energies obtained on a scraped surface are displayed in Figs. 1 and 2 for UNiA1 and UPdSn respectively. The basic features are compatible with what was found earlier for palladium and nickel intermetallics in general [9]. In a c o m p o u n d with the more electropositive uranium, the spectral intensity coming from Ni(Pd) d states is shifted from E F to higher binding energies leaving conduction electron states of uranium n e a r EF. Thus the maxima found at around - 2 eV for UNiA1 and at around - 3 . 5 eV for UPdSn can be mainly attributed to 3d and 4d emission respectively. This assignment is directly supported by results on ThPdA1 and LaPdA1 [9 ], where the XPS spectra were found to be dominated by a single p r o n o u n c e d m a x i m u m at around - 4 eV. A more quantitative analysis based on atomic photoionization cross-section calculations [10] shows that the cross-section of the 5f emission should increase relatively to the Ni 3d emission with increasing photoexcitation energy. Thus the development of the UNiA1 spectra from h v = 21.2 eV to h v = 40.8 eV shows that the spectral intensity, which is spread from EF towards the minimum, should be attributed mainly to the emission of 5f origin. A standard procedure of separation of the 5f emission is to subtract the spectra taken at h v = 2 1 . 2 and 40.8 eV, normalized at higher binding energies, where the 5f emission is no longer expected. The result, which is severely affected by details of the background subtraction and normalization procedures, m u s t be used with caution. Moreover, electron structure calculations (e.g. for UCoA1 [ 11 ]) show

200000 150000 100000 UNiAI T 1 4 8 6 . 6 e V \ \ , 50000 40.8 eV ~ / .( 21.2 e V ~ I 16.8 e V 0 -6 -4 -2 0 energy (eV) 8000 U P d S n 7000 - - 1486.6 eV 60OO 5000 ~ , / 4000 : '

4o.8 eV //~"

//

3000 , / //~,\

"~/

/ 2000 / / 'i' 1000 2 1 . ~ / ~ . t ~ / ! 0 ~ 1 6 . 8 e V -6 -4 -2 0 energy (eV)

Fig. 1. Valence-band photoemission spectra of UNiA1 (stoichiometric surface) recorded for different excitation energies. The shaded area is an estimate of the 5f emission obtained by the procedure described in text.

Fig. 2. Valence band photoemission spectra of UPdSn (stoichiometric surface) recorded for different excitation energies. The shaded area is an estimate of the 5f emission obtained by the procedure described in text.

208

that t h e 5 f - d hybridization causes s o m e p o r t i o n o f the d partial density o f states (DOS) to a p p e a r (or to be induced) at the m a x i m u m o f the 5f DOS and vice versa. Similar m e c h a n i s m s (hybridization o f d and 5 f states with s, p and 6d states f o r m i n g a b r o a d c o n d u c t i o n band) can be responsible f o r t h e s h a p e o f t h e s p e c t r a obtained at h r = 16.8 eV, which mimic the f e a t u r e s f o u n d for higher excitation energies even if the 3d and especially the 5f c r o s s - s e c t i o n s are v e r y small f o r such low excitation energies.

T h e relatively b r o a d triangular 5 f emission, o b s e r v e d for UNiAl in the case o f He II radiation, is a r a t h e r conventional p i c t u r e f o r m o s t o f the u r a n i u m intermetallics including heavy-fermion c o m p o u n d s [ 12 ]. A c o m p a r i s o n with t h e s p e c t r a o b t a i n e d on a s p u t t e r e d (uranium-rich) surface shows that some additional 5f intensity a p p e a r s f o r the stoichiometric surface in the e n e r g y range b e l o w - 0.3 eV. Although the influence o f nickel-related states (3d) c a n n o t be excluded, one would e x p e c t a s t r o n g e r effect in the He I s p e c t r u m in the case. Assuming a 5 f origin o f this feeble feature, we still c a n n o t decide w h e t h e r it is an u n r e s o l v e d satellite s u p e r i m p o s e d on the single-particle DOS, which is s u p p r e s s e d f o r the s p u t t e r e d surface, or a f e a t u r e of t h e 5f band. The e x c e s s o f u r a n i u m in the s p u t t e r e d surface causes a r e d u c t i o n in the m e a n U - U spacing and possible s u b s e q u e n t delocalization and loss o f c o r r e l a t i o n features. Such a satellite can originate possibly from p o o r l y s c r e e n e d 5f final states [ 12].

In UPdSn, t h e larger s e p a r a t i o n o f the 4d states from EF causes their e n e r g y overlap with electronic states a r o u n d EF to be reduced. Photoionization c r o s s - s e c t i o n calculations [ 10] show h e r e t h a t the s p e c t r a should be dominated b y the Pd 4d emission, which is considerable already for h v = 16.8 eV, and only s o m e a d m i x t u r e o f t h e 5f emission can be e x p e c t e d at higher excitation energies. This m a k e s the s e p a r a t i o n o f t h e 5 f emission even m o r e uncertain than in UNiA1, but t h e increase in t h e emission at EF with increasing excitation e n e r g y indicates again the 5 f c h a r a c t e r o f the states close to EF, which is in c o n t r a d i c t i o n with t h e low T value. The noticeable s h o u l d e r c e n t r e d at a r o u n d - 0.6 eV is quite similar t o t h e localization features ( 5 P - ' final state) o b s e r v e d in UPd2Si2 [8l. This a p p r a o c h , however, fails to explain why this s h o u l d e r b e c o m e s relatively m o r e p r o n o u n c e d when the excitation e n e r g y is d e c r e a s e d even down t o h~,= 16.8 eV, w h e r e almost no 5f emission is possible. Such a d e v e l o p m e n t could suggest a Pd 4d or U 6d origin o f this shoulder. C o m p a r i s o n with the uranium-rich surface shows that the r e d u c t i o n in t h e u r a n i u m c o n t e n t d e p r e s s e s strongly the emission at EF, but the satellite b e c o m e s relatively e n h a n c e d . Thus P d 4d states are the m o s t plausible source o f m a j o r p a r t o f spectral intensity in this shoulder.

T h e U 4 f c o r e level s p e c t r a o b t a i n e d on b o t h c o m p o u n d s are displayed in Fig. 3. F o r UNiA1, t h e y are o f the type f o u n d for u r a n i u m c o m p o u n d s with a b r o a d 5f band, i.e. b o t h t h e position and t h e s h a p e o f the 4 f spectral lines r e s e m b l e t h o s e o f a-U [13]. As c o m p a r i s o n o f the data obtained on t h e s c r a p e d and t h e s p u t t e r e d s u r f a c e shows, t h e full width at half-maximum is slightly higher in the stoichiometric case (2.5 eV in c o n t r a s t with 2 . 2 - 2 . 3 eV in t h e uranium-rich case). Although n o satellite s t r u c t u r e can be observed,

12000 ~ - 10000 ,

=: 80001

/.~

ooo 4000 j / / 2000 U N i A I f / % / ~ / -400 -390 -380 energy (eV) s c r . \ S C F . -370

Fig. 3. U 4 f c o r e level s p e c t r a ( s p i n - - o r b i t split 4 f 5~ a n d 4f 7/2 d o u b l e t ) o b t a i n e d o n s t o i c h i o m e t r i c ( s c r . ) a n d u r a n i u m - r i c h ( s p . ) s u r f a c e s o f UNiA1 a n d UPdSn.

the s p e c t r u m can be d e c o m p o s e d into two a s y m m e t r i c gaussians and b r o a d f e a t u r e s l o c a t e d at a r o u n d - 6 eV with r e s p e c t to the main lines. In UPdSn, the main 4f spectral lines are s o m e w h a t shifted to higher binding energies (by a b o u t 0.4 eV) with r e s p e c t to UNiAI and UPdSn subjected to sputtering, but still t h e position o f t h e s e main lines shows well-screened final states,

i.e. a high probability o f s c r e e n i n g b y 5 f electrons. This implies that some

itinerant c h a r a c t e r o f the 5f states m u s t be retained. The main lines are, however, a c c o m p a n i e d by a pair o f satellites at higher binding energies, s e p a r a t e d f r o m the main lines by 2.6 eV and 6.4 eV respectively. This s p e c t r u m is similar to what has b e e n f o u n d f o r e x a m p l e f o r UCus. Sarma et al. [14] analysed such a type o f s p e c t r u m using the G u n n a r s s o n - S c h 6 n h a m m e r model and c o n c l u d e d that the 5f states can be only weakly hybridized but that the correlation energies are considerable.

3. C o n c l u s i o n s

High resolution UPS and XPS shows f o r UNiA1 the 5f emission to be s p r e a d down to a b o u t - 1 eV f r o m EF. The correlation features are weaker than e x p e c t e d f o r a c o m p o u n d exhibiting magnetic ordering. The situation in UPdSn is m u c h less clear. P a r t o f the spectral intensity, which can be attributed to the Pd 4d states, f o r m s a s h o u l d e r at a r o u n d - 0 . 6 eV. The p r e s e n c e o f t h e s e states in t h e vicinity Of EF can point to the 5 f - 4 d hybridization, which implies an itinerant c h a r a c t e r o f 5f states. A considerable spectral intensity, p r o b a b l y o f the 5f origin, was f o u n d at

EF,

which is in contrast with the low T value. However, t h e e n e r g y resolution o f the s p e c t r o s c o p y is t o o p o o r to visualize details o f t h o s e states at a r o u n d EF, which are thermally accessible in the low t e m p e r a t u r e region, from which the low T is extracted. In particular t h e 4 f c o r e level s p e c t r a show that a considerable itinerancy o f the 5f states is r e t a i n e d in this c o m p o u n d .

210

A c k n o w l e d g m e n t s

T h i s w o r k i s a p a r t o f t h e r e s e a r c h p r o g r a m o f t h e S t i c h t i n g v o o r F u n d a m e n t e e l O n d e r z o e k d e r M a t e r i e , w h i c h is f i n a n c i a l l y s u p p o r t e d b y t h e N e d e r l a n d s e o r g a n i s a t i e v o o r W e t e n s c h a p p e l i j k O n d e r z o e k . P a r t o f t h e w o r k o f V.S. h a s b e e n g e n e r o u s l y s u p p o r t e d b y t h e A l e x a n d e r v o n H u m b o l d t F o u n d a t i o n .

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