Pseudogap vs stripe fluctuations in high-Tc cuprates

PHYSlCA ELSEVIER

Physica C 341-348 (2000) 823 826 www.elsevier.nl/locate/physc

Pseudogap vs stripe fluctuations in high-Tc cuprates S. Uchida Department of Superconductivity, and Department of Advanced Material Science, University of Tokyo, Tokyo 113-8656, Japan The pseudogap in the normal state of high-T¢ cuprates is a phenomenon characteristic of the underdoped regime. It is suggested that the underdoped cuprates are also characterized by stripe fluctuations. We discuss based on the transport and optical experiments that the stripe fluctuations are competing with the pseudogap and that in La-based cuprates the stripe fluctuations dominate in the charge dynamics.

1. I N T R O D U C T I O N Recent observation of the incommensurate peaks in the inelastic neutron scattering for YBCO and BSCCO [1] suggests that the stripe fluctuations may exist in the doped CuO2 planes of any high-To cuprate materials. Then, it is one of key issues of high-Tc superconductivity whether the stripe fluctuations promote or compete with the pairing fluctuations which manifest as a pseudogap in the normal state of the underdoped cuprates. Obviously, the pseudogap predominates in the low-energy spin and charge excitations [2] in many cuprates, representatively in YBCO. In this paper we show that in La-based cuprates, such as La2_~SrxCu04 (LSCO) and La:_~_uNduSrxCu04 (LNSCO), the stripe fluctuations appear to dominate the pseudogap effect in the charge transport and optical properties. In an extreme case where the system shows a static stripe order the charge dynamics are totally different from the typical ones in YBCO and consistent with the formation of one-dimensional (1D) charge stripes. In this regard, the stipe fluctuations are competing with the pseudogap and hence with superconducting pairing fluctuations. 2. P S E U D O G A P

EFFECTS

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CHARGE DYNAMICS Although its origin is still under debate, the pseudogap effect is seen in both in-plane and caxis charge tranport in characteristic manners

(Fig. 1}. The pseudogap appears most clearly in the caxis optical conductivity [3] as a suppression lowenergy spectral weight below T* well above To. The serniconducting T dependence the c-axis resistivity (Pc) is associated with the deepening of this pseudogap with decreasing T. The suppression of the low-energy spectral weight is not seen in the in-plane optical conductivity which always shows a Drude-like peak at w = 0. However, the opening of a pseudogap is signaled by a change in the T dependence of the in-plane resistivity (Pab) below T* [4]. Pab is linear in T above T*, while it decreases more steeply below T*. The change in the T dependence ofpab suggests that the pseudogap effect on the in-plane charge dynamics is via a change in the carrier scattering time (r). Actually, the development of a pseudogap in at(w) is correlated with a rapid narrowing of the w = 0 peak width in o'ab(w). The spectral change of ~rab(W) is also argued in terms of a suppression of w-dependent scattering rate 1/r(w) below a certain w comparable with the pseudogap width in

clw) [5]. It is worthy of mentioning that the pseudogap also affects the Hall coefficient [4]. Corresponding to the T-linear resistivity Pab, R H "" 1/T is a canonical behavior of the Hall coefficient in the optimally doped materials. It is found that as Pab deviates from the T-linearity below T*, the increase of RH(-'~ I/T) with decrease of T slows down below T* and eventually decreases at lower temperatures showing a broad peak between Tc

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3. C H A R G E TRANSPOI~ STATIC STRIPE STATE

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A counter example of the pseudogapped state is a static stripe state realized in LNSCO [6]. The T dependence of Pab is typically as that shown in Fig. 2 for x = 0.12 and y -- 0.40 which exhibits a LTO-LTT phase transition at Td=75K. The static charge order develops below Td and then the spin order follows at somewhat lower temperature. A small resistivity jump is seen at T~, and at low temperatures dpab/dT becomes negative [7]. However, this does not imply that the stripe ordered state is an insulator. The in-plane optical conductivity spectrum in the infrared region is metallic without showing any gap feature [8]. Likewise, the c-axis optical conductivity does not show a clear pseudogap behavior, although coherent transport along the c-axis is more blocked than that in LSCO. The most anomalous is the T dependence of the Hall coefficient (RH). Blow Td RH shows a rapid decrease and becomes very small as T --* 0. It is shown that the suppressed RH would result from the confinement of charge carriers within one-dimensional (1D) charge stripes, as evidenced also from the 1D ARPES Fermi surface [9,10]. In this 1D state one can hardly observe a spin gap nor a clear pseudogap in the low-energy electronic excitations.

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There are growing evidences that strong stripe fluctuations are present in Nd free LSCO. The elastic neutron experiment has observed nearly static incommensurate spin modulation for x < 0.12, [11] and a 'wipeout' effect observed in NOR suggests a charge ordering in LSCO [12]. Here, we revisl't the charge dynamics in LSCO and argue that the stripe fluctuations do affect them: (1) A spin gap is hard to see in NMR and the pseudogap effect on the ARPES 'Fermi surface' is totally different from that observed for BSCCO [13]. Related to this, only a weak pseUdogap feature is observed in at(w), and correspondingly the increase of Pc with decreasing T is much slower than that for YBCO (Fig. 3). (2) The in-plane resistiv-

S. Uchida/Physica C 341-348 (2000) 823-826

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REFERENCES 3

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5. S U M M A R Y At present, it is far from clear what role is played by the stripe fluctuations in YBCO and BSCCO. Apparently, in these materials the stripe fluctuations are weaker and the low-energy electronic excitations are determined predominantly by the pseudogap effect. By contrast, the stripe fluctuations play an active role in the charge (and probably spin) dynamics whereas the pseudogap is a m~nor effect in La-cuprates. The contrasting behavior is suggestive of a competitive relationship between stripe fluctuations and pseudogap. ACKNO~E~E~ I would like to thank T. Noda, H. Eisaki, K. Kojim~ S. Tajima, and N. P. Ong for collaboration in the experiments. This work is supported by a Grant-in-Aid for Scientific Research and a COE Grant from the Mini~ry of Eduration, Science, Sports and Culture of Japan.

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