SOFT X-RAY EMISSION FROM NORMAL AND SUPERCONDUCTING STATES

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SOFT X-RAY EMISSION FROM NORMAL AND SUPERCONDUCTING STATES

R. Crisp

To cite this version:

R. Crisp. SOFT X-RAY EMISSION FROM NORMAL AND SUPERCONDUCTING STATES. Jour- nal de Physique Colloques, 1987, 48 (C9), pp.C9-1199-C9-1202. �10.1051/jphyscol:19879216�. �jpa- 00227337�

JOURNAL DE PHYSIQUE

Colloque C9, suppl6ment au n012, Tome 48, decembre 1987

SOFT X-RAY E M I S S I O N FROM NORMAL AND SUPERCONDUCTING STATES

R.S. CRISP

The University of Western Australia, Nedlands, 6009 Western Australia, Australia

Resume On prbsente des bandes dUmission de

Nb

Abstract Soft X-ray emission bands from Nb metal a t lOSK and <9K and from a YBa2Cu30 ceramic superconductor a t 90K are presented. No effects identifiably due to superconductivity are observed.

ODUCTORY

E-ENTAL

The intensity distributions in soft x-ray emission bands from solids reflect the density of conduction states folded with a transition probability which depends on the state symmetries. For a superconductor above and below its transition temperature, Tc, we might expect to see differences, therefore, in the band shapes. Accordingly, w e have recorded valence band spectra from a YBa2Cu30 ceramic superconductor (Tc295K) a t about 90K and from Nb (Tc-9. 17K) both above and below the transition temperature.

The samples were clamped to a copper target holder cooled with liquid nitrogen or liqud helium and the sample temperatures were measured with a copper-constantan thermocouple. Because of the difficulty of achieving a meaningful thermocouple contact, data for the ceramic are reported only for a single temperature (estimated290K) believed to be in the superconducting regime. For Nb, data were recorded from a n unambiguously "normal" target and from the same target a t a temperature almost certainly in the superconducting regime. In neither material were effects observed which could be related to a superconducting state.

The valence electrons in these solids exist in states of mixed s-, p-, d- etc symmetry and which of these states a r e "seen" in an X-ray transition depends on the symmetry of the initial core state. For the Nb-Md5 band the initial state is a d-hole and w e sample p- (and perhaps f-) final states. For the YBa2Cu30 ceramic, initial states of s-, p- o r d-symmetry may be excited in either Y. Ba, Cu or 0 cores and w e will sample final states of p-, s- and d- or p- and f-symmetry in the vicinity of the particular atom in which the core hole has been created. In addition a number of lines arising from transitions between core states are known to fall in the spectral region of observation from 13-260A (48-955eV).

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19879216

C9-1200 JOURNAL DE PHYSIQUE

A . - m

Selection rules preclude many transitions and those which were observed are shown in Figs 1 and 2 and confirm that the valence electrons are principally in p-states (presumably associated with bonding to 0 sites).

(a) Cu-La a t 13.3A arising from transitions 2p-(4s,3d) and which is intense in the metal, is not seen a t all in the ceramic. Thus a profound (but not unexpected) change in the symmetry of the 3d,4s states has taken place in going from the fcc metal to the oxide in the perovskite ceramic structure.

(b) 0-Ka a t 23.7A, arising from transitions 1s-2p, is observed with a half-width (corrected for instrumental broadening) of 3.82k0.05eV which reflects the width of the bonding electron distribution around 0 sites (folded with a Lorentzian K level width estimated 0.5

-

(c) The Y-Mq5 spectrum a t 75-100 A comprises lines arising from initial 3d vacancies filled by transitions with 4 p final states (the intense Mc lines) and a band identified a s arising from valence band final states with a width of some 8.0 +- 0.7 eV (which is more than twice as broad a s the observed bonding electron distribution around the 0 sites). In addition, lines with 3 p initial states fall in the same energy region.

120. 130. 140. 150. 160. 170.

PHOTON ENERGY (eV)

Fig 1. Y-Mq5 spectral region from YBaZCu30 ceramic superconductor.

Excitation l.OmA 3.0kV, temperature z90 K.

(dl The Ba-Nq5 spectrum a t 130-175A is similar in form to that for Y-M45 with a group of lines arising from Nq5

-

(M)

with a diffuse band, with a half-width of * 5 eV, identified a s arising from the valence states.

Fig 2. Ba-Nq5 spectral region from YBa2Cu30 ceramic superconductor.

Excitation l.OmA a t 3.0kV, temperature 4 0 K . It can be inferred:-

(i) from the intensities, that the valence o r bonding electrons a r e in p-states (unlike their s-symmetry in metallic Y or Ba), and are localised around 0 cores. Apart from the relatively intense 0-Ka, which involves a 1s initial state, the complete absence of spectra involving Zs, 3s, 4 s etc initial states is predictable due to strongly competing Auger decay, notwithstanding the assumed p-symmetry of the majority of the valence electrons. A study of CU-Kp (outside the present energy range) should be rewarding, given the demonstrated influence of both Cu valency and 0 defficiency on the superconductivity of these ceramics.

(ii) from the bands observed, that some valence electrons in p-states are to be found close to Y and Ba sites (cf s-symmetry in the pure metals).

(iii) from a comparison of the Y-Mc and Ba-Ne lines with those from the pure metals, that the Y N23 and Ba 023 levels are significantly broadened and marginally shifted (<O.leV) in the ceramic. The ratios between the line components a r e profoundly modified in the ceramic and this is evidence of a significant interaction with the lattice potential from neighbouring sites.

JOURNAL DE PHYSIQUE

B.

- b&Emai

It is confidently believed that the bands recorded with liquid helium cooling a t reduced power input (100 and 60wA a t 3.5kV) were indeed emitted a t the indicated temperature of 8-9K and are representative of Nb below Tc. As is evident from Fig 3 there is no significant change in this band in going from the normal state a t 105K t o superconducting below 9.17K.

196. 1 9 . 200. 202. 204. 206. 208.

PHOTON ENERGY (eV1

Fig 3. Nb-M45 emission band recorded above and below Tc.

Excitation 4.0mA a t 3.0kY (105K), 60pA a t 3.0kV(<9K)

Notwithstanding any understandable scepticism regarding the claim that in this experiment the temperature was in fact below 9.17K, we believe that the observation of no change in the band between the normal and superconducting state arises from the perturbation of the core hole locally breaking the Cooper pairs so that the observed transition involves just normal electrons. We intend to pursue this work further, using the much more favorable transition temperature of the ceramic material, to examine the band structure unambiguously in both states.

The author gratefully acknowledges continuing support for the soft x-ray project from the Australian Research Grants Scheme and the University of Western Australia. The ceramic superconductor samples were prepared by Dr F J Lincoln and Dr P G McCormick, members of the WA Ceramic Superconductor Research Group (WACSURG) a s part of the ongoing program a t UWA.