ESR study of (Sr,La,Ca)_{14}Cu_{24}O_{41}

a r X i v :c o n d -m a t /0108512v 1 [c o n d -m a t .s t r -e l ] 30 A u g 2001

ESR study of (Sr ,La ,Ca )14Cu 24O 41

V.Kataev a ,1,?K.-Y.Choi a ,2M.Gr¨u ninger a U.Ammerahl a ,b

B.B¨u chner a ,2A.Freimuth a A.Revcolevschi b

a II.

Physikalisches Institut,Universit¨a t zu K¨o ln,50937K¨o ln,Germany

b Laboratoire

de Chimie des Solides,Universit′e Paris-Sud,91405Orsay,France

Spin ladders have recently attracted much attention in particular due to su-perconductivity with T c ≈10K observed under pressure in Sr 14?x Ca x Cu 24O 41

(SCCO)with x ≥11.5[1].SCCO contains 2-leg S =1/2Cu 2O 3ladders showing a large spin gap ?ladder ≈400K [2]and S =1/2CuO 2chains,both running along the c -axis.It is ’self-doped’with 6holes per formula unit.For x =0almost all holes reside in the chains and show quasi-2D order [3,4].In this charge ordered state spin dimers with a gap ?dimer ≈130K are formed between next-nearest-neighbor Cu spins via a localized hole [3,4].The conduc-tivity of SCCO increases with x .The prevailing viewpoint is that the chemical pressure due to Ca doping causes a substantial hole transfer from the chains to the ladders [5],i.e.,both metallic conductivity and superconductivity are con?ned to the ladders.However,recent x-ray absorption data indicate only a marginal increase of the hole content in the ladders with increasing x [6].

We measured electron spin resonance(ESR)of Cu2+ions in single crystals of(Sr,La,Ca)14Cu24O41at9.47GHz.A single resonance line of a Lorentzian shape with an anisotropic g-factor is observed.The principal values of g de-termined for the b-and c-axes are g b=2.29±0.05and g c=2.05±0.03, respectively.Since?ladder is large,the ESR signal at T<300K is attributed

to the chains.The spin susceptibilityχspin

ESR derived from the ESR intensity is

similar to the static susceptibilityχstat.In SCCOχspin

ESR andχstat for small x

are well described by the spin-dimer model.A consistent description ofχstat in SCCO for all x is obtained[7,8]in terms of a smooth crossover from spin dimers to a uniform antiferromagnetic(AF)spin chain with increasing Ca content,which assumes a small reduction of the hole content in the chains from～6to～5.This scenario is justi?ed by the following analysis of the ESR line width?H(Fig.1).For SCCO with small x(Fig.1,left panel)the curves above30K can be well approximated as?H(T)=?H0+?H?(T).?H0 is due to T-independent spin-spin interactions and inhomogeneities.?H?(T) accounts for other,T-dependent spin-relaxation mechanisms.For x=0it emerges as a strong and almost linear in T contribution to?H only above a certain temperature T?≈200K.T?decreases rapidly with Ca doping to170 K for x=2and to～80K for x=5.For still larger x it is not possible to de?ne a T?below which?H is constant.We identify T?as the charge ordering temperature,consistent with the results of other techniques[3,4,7].Activated hole motion in the chains above T?breaks spin dimers,causes spin?ips and thus broadens the ESR signal.A rapid reduction of T?with increasing x in-dicates a growing instability of the spin dimer state.Remarkably,a strong increase of?H(T)is found for all x,suggesting the presence of a substantial amount of mobile holes in the chains even at large Ca doping.A consecutive reduction of the total number of holes due to heterovalent substitution from n=6in SCCO to n=5,4,3,and1,as in Sr13La1-,La2Ca12-,La3Ca11-and La5Ca9Cu24O41,respectively,results in a much weaker increase of?H with T.The slopes d(?H)/dT are summarized in Fig.2.According to Ref.[6]all holes reside in the chains in the compounds with reduced hole doping.There-fore,the comparison of d(?H)/dT in the left and right part of Fig.2gives an upper border of approximately one hole being transferred from the chains to the ladders in fully hole doped SCCO.

The steep increase of?H at T<10K in SCCO with large x is due to the development of AF order in the chains.A detailed analysis of the data [8]shows that the increase of the Ca content continuously drives the system towards an AF instability.This is expected,if spin dimerized chains gradually transform into uniform chains which eventually order at low T due to weak interchain couplings.

With increasing x the T-independent part of the line width?H0in SCCO increases by a factor of10.In the samples with reduced hole doping it grows up to～1.5kOe in nearly undoped La5Ca9Cu24O41.A careful analysis of

the spectra suggests that this e?ect is not related to structural or magnetic inhomogeneities [11].At T <50K the resonance line experiences additional broadening when long-range AF order at T N ≈10K with an extreme magnetic anisotropy is approached [10].The width of the ESR signal in the paramagnetic regime above ～50K,where static short-range correlations vanish,is therefore determined mainly by the anisotropy of the spin-spin interactions,which in concentrated paramagnets is the major broadening mechanism.The dominant isotropic exchange in the chains with a small hole content is ferromagnetic and

occurs between nearest neighbors,H iso =J iso

S i S i +1,with J iso ≈?20K [3].

The leading anisotropy is of a symmetric type,H aniso =

S i A i ,i+1S i +1.A con-ventional estimate of A i ,i+1with ?H ～1.5kOe yields A i ,i+1～2K [11],which is as large as 10%of J iso .Such a strong anisotropy is surprising for copper oxides,which are considered to be the best experimental realizations of an isotropic Heisenberg magnet.It can be explained by the speci?c geometry of two symmetrical 90?Cu–O–Cu bonds,connecting nearest neighbor Cu sites.In this geometry the in?uence of the spin-orbit coupling on the superexchange is found to be considerably enhanced [12].The ESR data indeed show that low-dimensional cuprates with certain bonding geometries may deviate signif-icantly from the isotropic Heisenberg model.

References

[1]M.Uehara et al.,Phys.Soc.Jpn.65(1996)2764;T.Nagata et al.,Phys.Rev.

Lett.81,(1998)1090.[2]R.G.Eccleston et al.,Phys.Rev.Lett.81(1998)1702;S.Katano et al.,Phys.

Rev.Lett.82(1999)636.[3]S.A.Carter et al.,Phys.Rev.Lett.77(1996)1378.

[4]L.P.Regnault et al.,Phys.Rev.B 59(1999)1055;M.Matsuda et al.,ibid

p.1060.[5]T.Osafune et al.,Phys.Rev.Lett.78(1997)1980.[6]N.N¨u cker et al.,Phys.Rev.B 62(2000)14384.[7]U.Ammerahl,PhD thesis,Univ.of Cologne,2000.[8]V.Kataev et al.,Phys.Rev.B 64(2001),104422.[9]S.Ohsugi et al.,Phys.Rev.Lett.82(1999)4715.[10]U.Ammerahl et al.,Phys.Rev.B 62(2000)R3592.[11]V.Kataev et al.,Phys.Rev.Lett.86(2001)2882.

[12]V.Yu.Yushankhai,and R.Hayn,Europhys.Lett.47(1999)116;S.Tornow et

al.,Phys.Rev.B 60(1999)10206.

?H , O e

T, K

T, K

Fig.1.T -dependence of the ESR line width ?H of (Sr ,La ,Ca)14Cu 24O 41.Solid lines in the left panel denote the constant and linear contributions to ?H (T ),as explained in the text.T ?indicates the charge ordering temperature.

1

2

3

d (?H )/d T , O

e /K

x

Fig.2.High temperature slope of the ?H (T )dependence of the samples with full

(n =6)and reduced (n <6)hole doping.