Bulk-sensitive Photoemission on Ti -doped Sr 2 RuO 4 Sr 2 RuO 4 is the first layered non-cuprate perovskite found to display superconductivity (T c ≈ 1K) [1]. As is the case for cuprate high-temperature superconductors, the fundamental issues to be addressed here refer to the pairing mechanism, and in particular, the role of electronic correlations. While the unusual spin-triplet pairing with p-wave symmetry suggests the importance of ferromagnetic fluctuations, in experiments, mainly evidence for antiferromagnetic fluctuations was found. Recently, it was observed that the substitution of the Ru 4 + ion by the non-magnetic d 0 -ion Ti 4 + induces local-moment magnetism, which statically orders for Sr 2 Ru 1-x Ti x O 4 with x 0.03 [2]. This behavior was confirmed by elastic neutron scattering in which the formation of a static incommensurate spin-density wave (SDW) for x = 0.09 below T SDW = 25K [3] is observed. The formation of this static SDW upon slight Ti-doping opens the possibility to study its effects on the electronic structure from a fundamental point of view and might give an important insight into the role of antiferromagnetic fluctuations in the superconducting parent compound as well. To elucidate these questions we performed photoemission (PE) measurements (see Fig. 1) at high photon energies using BL25SU to investigate both a possible change of the Fermi surface (FS ) upon doping and possible effects on the low-energy electronic structure along the high-symmetry lines of the Brillouin zone (BZ). Knowledge of the FS topology is important because it determines the manifold of possible excitations on the FS for a given momentum. If there exist two-dimensional manifolds across the FS, which can be mapped onto each other by one single vector, the respective portions of the FS are said to be nested. Fig. 1. Schematic of the photoemission process. Monochromatic photons kick out electrons from the surface of a solid. Due to energy and momentum conservation their kinetic energy and emission angle can be traced back to their energy and momentum in the crystal. h ν E kin , k θ At these locations, the FS becomes intrinsically unstable and a n SDW with a wavevector corresponding to the nesting vector may develop concomitant with an energy gap. Figure 2 displays an image of the FS of Sr 2 Ru 0.91 Ti 0.09 O 4 . Also indicated is the BZ. The arrows mark SDW vectors as experimentally observed by neutron scattering. Their starting and end points lie essentially on two mutually perpendicular pairs of FS sheets and thus give striking evidence that the SDW indeed is driven by FS nesting. They can be viewed as reflecting a hidden one dimensionality in the electronic structure. Note that the thus identified nesting vectors are also in agreement with the findings of band structure calculations. Surprisingly, the FS topology and especially the FS “volume” remain essentially unchanged in comparison with the undoped material [4], indicating that the Ti ions do not much affect the states close to the Fermi energy E F . If the electronic structure is not much affected right at E F , one would nevertheless expect a relatively large Fig. 2. FS map of Sr 2 Ru 0.91 Ti 0.09 O 4 . Indicated are the BZ (white lines), the underlying one-dimensional FS sheets (red lines), and the experimentally observed SDW vectors (arrows). The dashed white lines denote the irreducible parts of the BZ. 46 References [1] Y. Maeno et al. : N a tur e 372 ( 1 994) 532 . [ 2 ] N . Kikug a w a an d Y. Maeno : Phys . R e v . L e tt . 89 (2002) 11 700 1. [ 3 ] M. Br a d en et al. : Phys . R e v . L e tt . 88 (2002) 1 97002 . [ 4 ] A . S e kiy a m a , S . K a s a i, M. Tsu ne k a w a , Y. Ishid a , M. Si n g, A . Iriz a w a , S . Im a d a , T . M ur o , Y. S a it o h, Y. O n uki, T . Kimur a , Y. T o kur a an d S . Sug a : t o b e publish e d . [ 5 ] A . D a m a sc e lli et al. : Phys . R e v . L e tt . 85 (2000) 5 1 94 . M ich ae l Si n g a ,b* R a lph Cl ae ss en , a an d S . Sug a b ( a ) Exp e rim en t a lphysik II, U n iv e rsität Augsburg, G e rm an y (b) Gr a du a t e Sch oo l o f E n gi nee ri n g Sci en c e , Os a k a U n iv e rsity E-m a il: si n g@physik . u n i- a ugsburg . d e * Present address : Exp e rim en t a lphysik II, U n iv e rsität Augsburg, G e rm an y Intensity (arb. units) E - E F (eV) –10 0 –8 –6 –4 –2 dis o rd e r p o t en ti a l t o b e i n duc e d by th e Ti i on s . This sh o uld b e r e fl e ct e d by br oa d ene d structur e s an d th e l a ck o f disp e rsi on p a rticul a rly o f l o w-lyi n g b an ds . A typic a l s e ri e s o f ene rgy distributi on curv e s a l on g th e high-symm e try li ne Γ X is sh o w n i n F ig . 3 . I n trigui n gly, one fi n ds sh a rp disp e rsi n g f ea tur e s o v e r th e en tir e v a l en c e b an d r e gi on , i n p a rticul a r a t E F , which is a g a i n c o mp a r a bl e t o th e fi n di n gs i n th e u n d o p e d c o mp o u n d . Ye t ano th e r imp o rt an t issu e h a s t o b e a ddr e ss e d, which is a r a th e r fu n d a m en t a l one an d a lw a ys a t o pic a b o ut PE sp e ctr o sc o py . Si n c e e l e ctr on s str on gly i n t e r a ct with m a tt e r, PE i n g ene r a l is a highly surf a c e - s en sitiv e t e ch n i q u e. It is imp o rt an t t o k no w this, e sp e ci a lly i n o ur c a s e , si n c e it w a s pr e vi o usly sh o w n [ 5 ] th a t th e u n d o p e d syst e m, a lth o ugh tw o -dim en si ona l i n na tur e , e xhibits a surf a c e r e c on structi on which is a ctu a lly a ls o pr e s en t i n th e Ti-d o p e d c o mp o u n d a s r e v ea l e d by l o w- ene rgy e l e ctr on diffr a cti on ( no t sh o w n ) . This r e c on structi on imp o s e s a ne w p e ri o dicity on th e e l e ctr on syst e m a t th e surf a c e , an d h en c e , a ff e cts th e e l e ctr on ic structur e. Fo r e x a mpl e , e ff e cts such a s b an d b a ckf o ldi n g, th e o ccurr en c e o f r e plic a s o f th e F S sh ee ts o r e v en an o v e r a ll ch an g e o f th e e l e ctr on ic structur e m a y r e sult . I n d ee d, a ll such e ff e cts w e r e f o u n d i n PE sp e ctr o sc o py o f th e u n d o p e d syst e m usi n g l o w- ene rgy ph o t on s o f a ppr o xim a t e ly 30 eV [ 5 ]. H igh- ene rgy ph o t on s o f a b o ut 700 eV pr o vid e d by BL25SU tr an sf e r much m o r e ene rgy t o th e e l e ctr on s an d thus giv e th e m a sig n ific an tly high e r ch an c e t o e sc a p e fr o m d ee p e r i n th e s a mpl e i n t o th e v a cuum . I n d ee d, w e d o no t s ee an y sig n s o f th e a b o v e -m en ti one d e ff e cts an d thus c an c on clud e th a t o ur d a t a truly r e fl e cts th e i n tri n sic bulk e l e ctr on ic structur e. L oo ki n g a t th e subtl e e ff e cts o f th e SD W with a v e ry high r e s o luti on ( an d h en c e , i ne vit a bly l o w ph o t on ene rgi e s) o ur d a t a is urg en tly nee d e d t o dis en t an gl e th e surf a c e fr o m bulk c on tributi on s . I n summ a ry, b o th th e F S im a g e an d th e b an d m a ps r e c o rd e d by high- ene rgy PE surprisi n gly i n dic a t e th a t th e e ss en ti a l o v e r a ll e l e ctr on ic structur e o f Sr 2 RuO 4 is pr e s e rv e d up on slight Ti-d o pi n g . I n p a rticul a r, w e o bs e rv e sh a rp disp e rsi n g b an ds d e spit e th e l a rg e dis o rd e r du e t o th e Ti i on s . Our r e sults a r e v e ry imp o rt an t f o r d e c on v o luti n g th e surf a c e fr o m bulk c on tributi on s if one c a rri e s on with studyi n g th e subtl e e ff e cts o f th e SD W on th e e l e ctr on ic structur e a t l o w ph o t on ene rgi e s . Such i n v e stig a ti on s a r e curr en tly i n pr o gr e ss . Fig. 3. EDCs along the Γ X high-symmetry line. Note the sharp dispersing features, in particular at the Fermi energy (E F = 0). 47
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