Dynamics of photogenerated charge carriers in cuprates.

New results of infrared photoexcitation experiments on undoped La2Cu04 and Nd2Cu04 are reported. We observed two broad electronic excitations within the charge-transfer energy gap and an associated phonon bleaching, possibly originating from strong charge-lattice coupling. Our results are discussed in terms of the condensation of self-localized photocarriers in the Cu02 planes at low temperatures.

theoretical models. In this Letter, we report newly found remarkable properties of charge carriers in the Cu02 planes that provide a deep insight into the nature of charge carriers and dynamic properties.
The nature of charge carriers and their interactions with the lattice in these unusual superconducting cuprates may be directly probed by standard infrared reIIectivity measurements.
However, because of the free-carrier contribution to the infrared conductivities, one can discern the residual contributions buried under the Drude-type conductivities only when all the free carriers are condensed into the superconducting state at T & T, [ll.
Therefore, it is not straightforward to verify the relevance of the background conductivities to the superconductivity.
In an attempt to address specific issues concerning the infrared properties of high-T, . cuprates, we employed the steady-state infrared photoexcitation technique to measure the infrared absorptions due to the photogenerated charge carriers (photocarriers) in the Cu02 planes of undoped La2Cu04 and Nd2Cu04, with which we can change the charge-carrier densities in situ at various temperatures.
We found two photoinduced electronic excitations, one at lower energy (LE) and the other at higher energy (HE), within the charge-transfer (CT) energy gap and a phonon bleaching associated with the Raman-active inplane breathing mode of the Cu02 plane. At T &40 K, we observed a spectral~eight transfer from the far infrared (co & 500 cm ') to LE, suggesting that LE at -0.12 eV in La2Cu04 and at -0. 16  The spectral resolution was set to 2 cm Typical photoinduced absorption spectra of each compound obtained at 4.2 K are shown in Fig. 1. There are three common features that are readily seen in each absorption spectrum: an absorption dip (bleaching) at -580 cm ' and LE peaked at -1300 cm ' ( -0.16 eV) and HE located at -5000 cm ' ( -0.62 eV) for Nd2Cu04, and the similar bleaching at -682 cm ' and LE at -1000 cm ' ( -0.12 eV) and HE at -3800 cm ' ( -0.47 eV) for LaqCu04 [4], consistent with the doping studies [5,6].
The photoinduced absorption spectra of La2Cu04 and Nd2Cu04 obtained at two temperatures are shown in Fig Figure   3 shows details of the temperature dependences. We note that the absorption of LE abruptly gains strength for T & 40 K while HE has moderately increased its oscillator strength only by -20% at 4.2 K.
The photoinduced signal is proportional to the number of photocarriers in the Cu02 planes (np) which is determined by balancing the recombination rate with the generation rate (ccI, optical pump intensity) in the steady state. Therefore, the intensity-dependence measurements can directly probe the detailed characteristics of charge carriers. As displayed in Fig. 4, the intensity dependence of LE measured at 45 K exhibits~I behavior, presumably resulting from the bimolecular recombination [7]. At 4.2 K, we observe~I behaviors for LE of both La2Cu04 and NdqCu04. We note that HE is insensitive to temperature changes, unlike LE (see Fig. 3); however, we observed the same intensity dependence for both LE and HE (not shown here) [8], implying that LE and HE share the same physical origin [2, 3,7].
Based on our results, we attempt to answer the following specific issues: Cl) Are these photoinduced spectral changes attribut able to charges in the CuOq planes? -The photoexcitation process, generating both electrons and holes, induces virtually identical infrared activities to those from chemical doping where only either holes (p-type doping) or electrons (n-type doping) are involved, suggesting that the photogenerated electrons (holes) in LaqCuOq (NdqCu04) are localized outside the CuOq planes after the charge separation. cles in an insulating medium of cKB", the absorption coefficient a can be approximated [9] as a/exa, = f(36m/ c)o~(tu)/s~in the e~&& e2 =4rro~/ tu limit. By taking f-0.008 ( -2 wt%), eKa, --2. 1   spectral weight due to free carriers in the far infrared (see the inset in Fig. 2) for T )40 K.
In principle, one can estimate the photocarrier concentration from the carrier generation rate ( -2.5&&10 ' charges/cm sec with photon Ilux -1.2X 10' photons/ cm sec corresponding to 100 mW/cin at 2.53 eV by assuming unit quantum efficiency) if the bimolecular recombination rate is given. However, the bimolecular recombination rate in cuprates is not known at this stage, and we anticipate that the rate in cuprates is dramatically diAerent from that due to trapped charges in conventional 3D systems, such as amorphous hydrogenated silicons [10], because of the unusual layered structure of cuprates consisting of CuOq sheets with La (or Nd) ions in between that may provide the trapping centers of oppositely charged particles at elsewhere out of the plane. We speculate that the unusual structure of cuprates accounts for the observed high concentration of photocarriers in the Cu02 planes with the lifetime order of -I msec.
Such long-lived photocarriers in layered cuprates have also been observed in photoconductivity measurements [11].
(2) Are these electronic excitations directly related to the superconductivity.~-The carrier recombination dynamics bears directly on the state of charge carriers. For the recombination between two uncorrelated, oppositely   [5]. Our findings appear to disagree with these suggestions because (1) we anticipate a substantial change in the energy when the charge carriers are paired at T ( T, and (2) if we assume that HE in the photoinduced absorption spectra is due to the Coulomb binding between a hole in the plane and a trapped electron out of the plane, we expect rx1 ', which is not the case.
In this experiment, we observed that the electronic excitations are tied to the IRAV modes, suggesting that the self-localization of charges breaks the local symmetry around charge carriers [2,3]. In addition, we found evidence that LE is directly involved in the charge transport.
The bleaching at -682 cm ' in La2Cu04 and at -580 cm ' of Nd2Cu04 spectra (see Fig. 1) is probably due to the Fano-like interference between LE and the IRAV mode [141 associated with the symmetric breathing mode in the Cu02 plane [15], implying the presence of strong charge-lattice coupling.
Therefore, one possible scenario is that the charges in the Cu02 planes gain stability in a self-localized state with a binding energy which develops a gap state(s) within the CT gap as evidenced by the observed electronic states and the associated IRAV modes. At T & T" these self-localized charged excitations more likely form a bound pair state in the momentum space via many-body interactions [16] as evidenced by the recombination dynamics.
In summary, we have observed two photoinduced electronic absorptions within the CT gap and a phonon bleaching associated with the Raman-active breathing mode of the Cu02 plane, resulting probably from the strong charge-lattice coupling. At T & 40 K, we observed spectral weight transfer from the far-infrared range (ta(500 cm ') to LE and changes in the intensity dependence of the photoinduced signal from I to I indicating a possible change in the state of the charge carriers into a momentum-space-paired state. Our results suggest that the electronic excitations in cuprates are charged and directly involved in the transport.