Study of iodine-intercalated Bi2Sr2CaCu2Oy single crystals

PHYSICA El £EVIER

Physica C 228 (1994) 137-140

Study of iodine-intercalated

Bi2Sr2CaCu2Oysingle crystals

P. Almrras *, H. Berger, L. Perez, G. Margaritondo lnstitut de' Physique Appliqude, Ecole Polytechnique Fdd~rale, Ch-lO15 Lausanne, Switzerland Received 13 April 1994; revised manuscript received 19 May 1994

Abstract

Iodine intercalation in Bi2Sr2CaCu20,. crystals enables one to modify the critical temperature, To, of the superconducting phase, and to obtain a semiconductor phase. This doping drastically changes the size of the unit cell. A photoemission spectroscopy study enables us to see that the decrease of the critical temperature is not related only to the change in the c-axis lattice parameter, but also to an over-doping of copper planes (hole doping).

Subtle changes in crystal structure of high-To superconductors can produce important effects on the electrical behavior. Recently, the demonstrated capability to intercalate iodine [ 1 ] between BiO bilayers in Bi2Sr2CaCu2Oy stimulated much interest. One might assume that the consequent change in Tc is only due to the change in the c-axis lattice parameter. Our photoemission experiments show that, on the contrary, the change is also related to the over-doping of the CuO planes. This work is the mainstream of the effort to clarify the superconductivity mechanism in these materials, which requires understanding the role of the CuO2 sheets - in particular, their carrier doping, and the strength of the coupling between two adjacent sheets. For example, in the Bi(T1)Sr(Ba)CaCuO family, we know that T~is a function of the number of CuO2 planes, and of the anisotropy in crystal properties, e.g., direction-dependent resistivity. Furthermore, in YBaCuO/ PrBaCuO alternating thin films, when the separation between the CuO2 sheets is modified, Tc is also modified. Intercalation changes the c-axis parameter, and * Corresponding author. 0921-4534/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSD10921-4534 ( 9 4 ) 0 0 3 1 1 - 3

therefore it can be expected to change T~. This was confirmed by our results, in agreement with previous results [2-4] by other groups: the iodine intercalation increases the c-axis parameter from 30 A to 37 * and modifies the critical temperature [5-7]. The fact that iodine was intercalated rather than forming an alloy was confirmed not only by the consistency of our results with those of other authors who used different experimental probes [2-7 ], but also by the absence of corelevel photoemission shifts of elements. We extended the iodine concentration to large enough values to transform the superconductor into a semiconductor. However, by thermal annealing one can transform again the semiconductor into a superconductor, its Tc changing for different annealing temperatures. In order to elucidate the relation between iodine intercalation and critical temperature, we decided to systematically explore the electronic structure of different materials with X-ray photoemission spectroscopy (XPS). Photoemission studies were performed using a SCIENTA ESCA-300 system equipped with a rotating anode (A 1 K a radiation source), a X-ray monochromator (composed of seven quartz crystals), and a concentric hemispherical analyzer. This experimental

138

P. Ahndras et al. / l~hvstca (" 22~h' (1994) I37 14~;

Table I Sample

Iodine concentration I~Bi: tvSre IC~-1~)~4,(71.11,,(),

t'rllJcaJ telnpclalult.,

'-,i/c altm~: , axi,, t~\ i

As-grown

~

Semiconductor

~

0

,~45 K

~(}~

I 17

Semiconductor

A n n e a l e d at IO0°C

~

I 04

7{l(I K

z ?

A n n e a l e d at 2 0 0 ° C

~

098

75 {} K

~ 7 .?

A n n e a l e d at 3 0 0 ° C

v: : 0 9 6

77,(-~ K

";¢~ 2

:~,

perature of samples without iodine and, therel~re ate quite unlikely to affect the oxygen content. The samples' characterization was completed b 3 N ray diffraction studies, in particular measurements ~,I the change in lattice parameter along the c ztxi~. F i g ! shows a series of the X-ray patterns for different iodine intercalated sample~,. The ~ axis lattice paramcler increases by approximately 7 .:~ tvotn the pure t~+ the semiconductor phase. The pat-ameter decreases again when the material becomes again a superconductor after annealing, but the difference with respect to the semiconductor phase is very small for the lowest annealing temperature+ Note that annealing at 100-200 and 30{FC produces monophase superconductors with different transition temperatures. After annealing at 400°C. we see evi dence of two phases: for example, the weak peak at the left of peak O, O, 122corresponds to the peak (). O. !(_!of the pure phase. The fact that the superconducting and semiconducting phases can have close values of the c-axis latticc parameters suggests that the link between intercalation and T,. is more complicated than a mere modification

system reaches an energy resolution of 0.3 eV. All of the samples studied by XPS were high-quality single crystals. The aforementioned semiconductor phase was obtained starting from pure Bi2Sr~,CaCu20,, and performing a thermal treatment at 10 atm of iodine. A subsequent thermal annealing removes the iodine of samples which becomes again a superconductor. The critical temperature measured at zero (onset) resistivity increases as the iodine concentration decreases for different annealings; see Table 1. The width of the transition was typically 4 K. The iodine concentration was accurately estimated from microprobe (cameca C A M E B A X SX-50) measurements. W e used the same technique together with XPS to analyze possible spatial variations of the iodine concentration. None was found within the experimental accuracy; in particular, a comparison of bulk-sensitive microprobe measurements and more surface-sensitive XPS measurements excluded the possibility of significant concentration gradients between surface and bulk. We emphasize that annealing temperatures like those we used are not sufficient to modify the transition ten>

a n n e a l e d at 4 0 0 ° C

. q

2t3(deg.} C u - K ~

a n n e a l e d at 3 0 0 ° C

Tc = 80 K

a n n e a l e d at 2 0 0 ° C

T c = 75 K

a n n e a l e d at 1 0 0 ° C

Tc = 70 K

Completely 12 i n t e r c a l a t e d without annealing Semiconductor s i n g l e c r y s t a l as g r o w n T c = 8 5 K

• unidentified lmc>

Fig. 1. X-ray diffraction patterns for (from bonom to top): as-grown (superconducting) Bi2Sr:CaCu2()Gthe same material alter ~trong iodine intercalation,that produces a semiconducting phase; the same, after several different annealings at increasing temperatures

P. Ahn~ras et al. / Physica C 228 (1994) 137-140

e..

~ ul-

Cu2+

=" ;,%

i

944

i

i

i

940

936

932

Binding Energy (eV) Fig. 2. Deconvolution of Cu 2p3/z core level spectra, showing the different peaks corresponding to the Cu~+ j) and Cu~+z) states.

annealed

at

300°C

2e-

Binding Energy (eV) Fig. 3. Decrease of Cu~+ ~) component with 1intercalation.

Z° ~

I3d

e-

/

! i completely

/

/

e-

2eannealed i

i

635

630

at i

62S

300°C i

i

620

615

Binding Energy (eV) Fig. 4. 13d~/2 and 3d~/z peaks for the strongly iodine-intercalated (semiconductor) phase and for the same sample after 300°C annealing (which produces again a superconductor). of the unit cell would imply. This stimulated us to study the electronic structure of our samples in detail, with

139

XPS. From the copper core-level photoemission spectra, we can measure the oxidation degree of copper. In fact, (Fig. 2) the Cu 2p3/2 core level spectra of cuprate superconductors [ 8,9] exhibit a main peak at approximately 933 eV, primarily due to the well screened core hole final state of the 2p53d~°L configuration, in which the symbol L denotes a ligand hole, a broad satellite centered at about 943 eV due to the poorly screened final state of the 2p53d 9, and 2p53d ~° component that also contributes to the main peak. The intensity ratio 2p53dl°/(2p53d9+2p53dl°L) reflects the Cu ( + l ) / Cu ( + 2) concentration ratio. W e know from previous studies (for example on the yttrium [9] doped Bi2Sr2CaCu2Ov) that this ratio increases when the transition temperature decreases, and therefore the Cu ( + 1) concentration increases. But for our iodine-doped samples (Fig. 3), we can see that this ratio increases when the annealing temperature increases. This implies an increase of Cu ( + 2) when the critical temperature decreases, which is precisely the contrary of what we observed for Y doped samples. This means that iodine doping results in an increase of the carrier (holes) density in the copper planes. The analysis of the Cu ( + J)/Cu ~+ 2) concentration ratio is complicated by the presence in this same spectral region of the 13p~/2 signal which overlaps the Cu ~+ 1) signal. Note, however, that the 1 3pl/2 signal tends to simulate an increase in the Cu ~+ J)/Cu ~+2) ratio, so that the real hole doping increase might be even larger than suggested by the uncorrected data. As for the 1 3d spectra (Fig. 4), we see two separate spin-orbit doublets. The lower binding energy doublet was attributed by Fukuda et al. [5] to I (-1) and the other to I (+7). Fukuda et al. [5] propose that I ~ ~) could be bound to bismuth and I (+7) to oxygen in the B i - O layers. W e see that annealing reduces the iodine concentration and also dramatically decreases the intensity of the lower binding energy 1 3d doublet; the relative intensity with respect to the higher binding energy doublet changes from 21 to 3 on going from the unannealed semiconductor to 300°C annealing. In principle, this implies that the iodine in the semiconducting phase is primarily related to the lower binding energy 13d peak, and therefore, according to Ref. [5], it would imply that it is related to I ( ~). This last conclusion is uncertain, however: recently, polarization-resolved Raman measurements [ 10,11 ] on iodineintercalated single crystals provided evidence for the

I' Almeras

141)

presence

o f I~

]'

and

p o s s i b l e I~

ez

al / P h v s w a ('22~ (1994) 137 14~

~'-le+l~,

~'

Acknowledgements

T h e s e i o n s e x i s t in t h e lattice w i t h a c o n s i s t e n t alignT h i s w.'ork w a s s u p p o r t e d by t h e F o n d s N a t i o n a l

m e n t a l o n g e i t h e r the a - a x i s or the b - a x i s . W e c a n o b s e r v e an a s y m m e t r y in t h e l i n e s h a p e w h i c h is indic

S u i s s e de la r e c h e r c h e S c i e n t i f i q u e and b> the Echoic

a t i v e o f t w o t y p e s o f i o d i n e . F a u l q u e s et al. [ 1 1 ] pro-

Polytechnique Federale de Lausanne.

p o s e d that t h e p r e s e n c e o f I~

l~ is r e l a t e d to t h e p e a k

at 6 2 4 e V ; t h e r e f o r e , t h e l o w e r b i n d i n g e n e r g y c o m p o n e n t c a n b e a t t r i b u t e d to I~

i~. I f this i d e n t i f i c a t i o n

is c o r r e c t , o u r a n n e a l i n g s r e m o v e I~ I~. S i n c e w e c a n n o t rule o u t o n e o r t h e o t h e r o f t h e s e i n t e r p r e t a t i o n s . t h e i d e n t i f i c a t i o n o f t h e d i f f e r e n t 1 3d p e a k s is u n c e r t a i n at t h e p r e s e n t t i m e . In s u m m a r y , o u r r e s u l t s put a q u e s t i o n m a r k on thc i d e a that i o d i n e i n t e r c a l a t i o n c h a n g e s the critical tem~ p e r a t u r e m a i n l y by m o d i f y i n g t h e c - a x i s lattice c o n s t a n l and thereby the interaction between copper planes. We found, on the contrary, only a small difference m c-axis lattice p a r a m e t e r b e t w e e n t h e s e m i c o n d u c t o r p h a s e a n d t h e h i g h i o d i n e c o n c e n t r a t i o n s u p e r c o n d u c t o r . O n Ihe o t h e r h a n d , w e h a v e s e e n that i o d i n e i n t e r c a l a t e s as an ion w h i c h p r o d u c e s a c h a r g e t r a n s f e r in t h e c o p p e r plane, increasing the charge carrier (hole) density. This i n d i c a t e s that t h e s i z e o f t h e u n i t cell is n o t an i m p o r t a n t f a c t o r , or at least n o t t h e o n l y i m p o r t a n t f a c t o r , m the i o d i n e - i n t e r c a l a t i o n - i n d u c e d c h a n g e o f t h e critical t e m perature

-

carrier-concentration

a p p e a r to play an i m p o r t a n t role.

modifications

also

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