Journal of Low Temperature Physics, Vol. 117, Nos. 314, 1999

T1-NMR Study on High-Tc OxidesTlM2CaCu2O7_d(M=Ba, Sr) :

Spin-Gap in Underdoped and Overdoped Systems

Takayuki Gotox,t, Satoru Nakajima t,E. Ohshimat, Masae Kikuchi t, Yasuhiko Syono t and Tetsuo Fukase t

*Faculty of Science and Technology. Sophia University. Tokyo 102-S534 Japant Department of Chemistry. Graduate School of Sat net. Tohokn I'ntr.

*Inst. Mater. Re-s.. Tohoku Univ. 9SO-S~>77 Japan

The Spin-lattice relaxation rate Tj l1and the Knight shift have b(( n me figuredfor Tl-based cuprates TlBa2Ca.Cu2O7+t (TB121>) and TlSr2 Yl_,r('(iJ.('ii2O7+t

(TS1212), In the underdoped sample of TS1212 with Tc=3-iK. (T^T1

showed a gap-like behavior from the temperature T^(-=120K. As for theslightly overdoped sample of TB1212 with T^=SOK. both the Knight shiftand (T\T)~l1showed a significant dfcnasr from IfJOK. suggesting the exist-ing of the spin-gap not only around </~(?r. TT). bvt also r/~0.

PACS numbers: 76.60k. 74.72.Fq.

1. Introduction

The low-energy gap in the excitation spectrum of Cu-3r/ spins in high-TC cuprates, so-called as the spin-gap1 is interpreted as the condensationof spinons in theoretical models for the electronic phase diagram of thecuprates.2'3 Those models predict the disappearance of the spin-gap fromthe normal state of the overdoped region, where the spin-gap temperaturecoincides with the superconducting TO Experimental results4 on VBCO iswell explained by them. However, recent experiments on thp Hi-system andLa-system report the existence of the pseudogap in the normal s ta tp of theoverdoped region.5"' In order to test the universality of models, accumula-tion of data on other high-Tc cuprates is expected.

We have studied by Tl-NMR on the spin-gap in TlM2('a('u2O,+7 (M =Ba. Sr) with the hole concentration from the underdoped to the overdoped

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regions. This system has the bilayer-type C'uO plane, the structure of whichis similar to YBCO. The Tl nucleus probes the C'u-3rf spin susceptibilitythrough the supertransferred hyperfine interaction via an apical oxygen. Inthis paper, we report our observation of the significant reduction of theKnight shift and (TiT)"1 in the normal state. Discussion will be on the pos-sibility of the existence of the spin-gap in the underdoped and the overdopedregions. So far, Magishi et al. has reported the gradual decrease of (T\T)~1at T>Tc for the underdoped TlSr^fCao.sLuo.rJCuiOv. if no estimations forthe gap energy has been made.8

Another interest in TB1212 and TS1212 is the difference in their Ic's.Both Ba and Sr are divalent, so that the nominal hole concentration must bethe same for stoichiometric systems. However, the state of the as-sinteredsample is quite different9-10 between TB1212 and TS1212. We reveal byNMR that this difference comes from the partial atomic exchange betweenTl and Ca-sites.

2. Experimental

Polycrystalline samples of TlBa2CaCu2O7+f and TlSr2Y1_J.Caa.C'u2O7+^(.r=0.2, 0.5) have been prepared by solid-state reaction method. The holeconcentration of TB1212 is controlled by the oxygen content.10 The as-sintered sample is in the slightly overdoped region with Tc=80K. The op-timally doped sample with Tc = 100K is obtained by the heat treatment inflowing Ar-gas. The as-sintered sample of TS1212 with .r=Q.2 and Te = 34Kbelongs to the underdoped region, and that with x=0.5 and 7o = 79K belongsto the optimally doped region.9 The Superconducting transition temperatureTC was determined as the onset of the diamagnetic shielding signal measuredby the SQUID magnetometer in the applied field 20 Oe. Samples, which wereconfirmed to be of the single phase by the powder X-ray diffraction, werepulverized and mixed with epoxy resin for the alignment of the c-axis in amagnetic field of 11 T.

Tl-NMR experiments were performed in the magnetic field around 6T.at the temperature 4.2~280K with the resonance frequency 145.27 MHz.

Fig. 1. Typical 203/205Tl-spectra. Solid lines indicate zero-shift positions.

468 T. Goto et al

Tl-NMR Study on High-TV Oxides TB1212/TS1212 469

Fig. 2. Knight shift and the relaxation rate for TBI212 and TSL212 .

Spectra were obtained by plotting the spin-echo amplitude against the ap-plied field. Figure 1 shows typical spectra for the two isotopes of 203T1 and205T1. both of which have / = 1/2. Though there is one crystallographicTl-site in the two systems. TB1212 shows spectra with extra peaks corre-sponding to another Tl-site. The signal of the extra small peak comes fromthe Tl atoms located at the Ca-site by the partial atomic exchange betweenC'a and Tl. The same phenomenon11 was reported for Tl-^BajCaCujOs+.vThe amplitude ratio of the Oa-site peak to the Tl-site peak is approximately10%. Spectra, of TS1212 were explained with a single Tl-site. indicat ing tha tthe atomic exchange does not take place in this system.

The spin-lattice relaxation rate was measured by the saturation-recoverymethod with a pulse train. In TB1212. measurements were done at the Tl-site peak. The relaxation curve followed the single exponential function. InTS1212. the curve was fitted by the function with the two relaxation rates.1 —/S • e~t'Ti — /L ' f"*'^1 to obtain the four parameters /s- 7'j\ /L and TjL.

3. Results

Figure 2 shows the temperature dependence of the Knight shift and(TiT)~l1 In higher temperature region of the normal state of TB1212.(TiT)"1 follows the Curie-Weiss law reflecting the two dimensional anti-feiTomagnetic spin fluctuation.12 With decreasing temperature, it starts todeviates from the law at rsG=170K for the optimally doped sample, andat 140K for the slightly overdoped one. The optimally doped .sample evenshows a reduction of (T\T)~l1in the normal s ta te .

The shown Knight shift of TB1212 is for Tl located at the C'a-site.Note that its sign is negative. The position of the C'a-site is equivalent asthe Y-site in YBCO, so that it well probes the static spin susceptibility ofthe CuO plane. The negative sign of the shift comes from the fact thatthe transferred hyperfine coupling constant is negative.13 The temperaturewhere —7i's starts to decrease coincides with T$G.

Next, as for TS1212. the Knight shift is temperature independent for theoptimally doped sample, and is decreasing at low temperatures for the un-clerdopecl sample. This behavior is the same as many other high-Tc cuprates.

The spin-lattice relaxation curve consists of the two components. Am-plitude ratios //,//,s'=0.2 for x=0.2. and O.1 for ,r=0.1. stayed constant inthe measured temperature range. As shown in Fig. 2. the short componentobeys the Curie-Weiss law in higher temperature region of the normal state.It shows a. reduction a.t the temperature TSG far above TO Following Ref. 1.we ma.de an Arrhenius plot of (TiT)""1 divided by the Curie-Weiss functionas shown in Fig. 3. One clearly sees a gap-like behavior below TSG- Thisresult makes a clear contrast with Ref. 8. where (T\T)~1 showed only agradual reduction with a broad maximum. The long component obeys theKorringa relation in the normal state. Both the components showed neithera kink nor a abrupt change at TO.

4. Discussion

In TB1212, (TiT)"1 deviates from the Curie-Weiss law from the temper-ature TSG much higher than TO This suggests the existence of the spin-gapeven in the overdoped region. For the Knight shift also starts to decrea.seat TSG, the possible spin-gap opens not only around the antiferromagneticq-vector (ir,ir} but also around Q~0. The change of the uniform spin sus-ceptibility due to this gap around Q~0 must be very small, because the tem-perature dependence of the Cu-Knight shift is very small (within ±0.01%)as reported previously.14

Next, the steep reduction in (TiT)"1 of TS1212 from TSG indicates theexistence of the spin-gap in nnderdoped solid-solution systems. This con-trasts with the fact that the spin-gap is easily smeared out by the impuritydoping to the CuO plane.15

The gap-opening temperature TSG decreases with increasing the holeconcentration, the behavior of which agrees qualitatively with the theoreticalmodel2'3 and experimental results4 on YBC'O. The gap energy of 240~260Kis considerably larger than that for other cuprates.1-4 Here we must notethat the analysis of the Arrhenius plot is allowed rigorously only when thesymmetry of the gap is spherical. For a further discussion on the relation be-

T. Goto et al470

Tl-NMR Study on High-TV Oxides TB1212/TS1212 471

Fig. 3. Arrhenius-plot for (T\/T) l divided by the Curie-Weiss factor.

tween the spin-gap and the superconducting gap. a more detailed theoreticalanalysis is necessary.

Finally, we comment on the difference in TC between TB1212 andTS1212. As stated above, the state of as-sintered sample is quite differ-ent between the two systems: Tll^C'aCujOy+.s is in the slightly overdopeclstate with Tc=80K, while TlSr2C'aC'u^O7+^ in the completely overdopeclstate with T^—0. The excess oxygen f< at the TIC) layer of TB1212 is re-ported to be smaller9'10 than that for TS1212. The reason for this different6 is that the potential of the T1O layer to the oxygen ion is raised by theatomic exchange between T13+ and Ca2+ in TB1212.

ACKNOWLEDGMENTSA part of experiments were performed at HFLSM. IMR. Tohoku Univ.

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