Dynamics of charge carriers in cuprates

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Metals

71 (1995) 1559-1562

Dynamics of Charge Carriers in Cuprates F. Lia, Y. H. Kima, and S.-W. Cheongb aDepartment of Physics, University of Cincinnati Cincinnati, Ohio 45221, U. S. A. bAT&T Bell Laboratories Murray Hill, New Jersey 07974, U. S. A. Detailed photoinduced infrared absorption studies of semiconducting Y lBa2Cu306+y (y&.2 ) and La2CuO4 were carried out. We observed two broad absorptions, one at - 0.13 eV and the other at - 0.62 eV in the in contrast to the previous photoexcitation results. photoinduced absorption spectrum of YlBa$tQOe+y We found that the lower energy peak positions in both Y lBa$u306+y and La2CuO4 abruptly change at T Tc of the corresponding superconducting phase. The amount of energy shift seems to be related to the pairing energy. Carrier recombination dynamics studies suggest that the the grotmd state of photocarriers in the Cu@ plane at T < Te is remarkably different from that observed at T > Te.

A central feature of all of the superconducting cuprates is that they are doped insulators, obtained by chemically adding charge carriers to a highly correlated antiferromagnetic insulating state. Although it is well-known that doping-induced excess holes (or electrons) in the CuO2 planes are responsible for the phase transition into the superconducting state from the antiferromagnetic, insulating state, the microscopic instabilities and interplay of charge carriers between superconductivity and magnetism have not been fully investigated. Standard infmred reflectivity measurements have been extensively carried out to study the dynamics of doped holes (or electrons) in the Cue plane of superconducting cuprates. The relectivity data of all of the high Te cuprates in infrared region show non-Drude behavior and it is generally accepted that there exists a substantial spectral weight in the infrared conductivities [l]. However, the relevance of this broad infrared band to the superconductivity could not be tested due the spectral complexity inherent to the system. Therefore, it is of fundamental importance to investigate the dynamics of the excess charge carriers in the Cue planes in infrared region using a different technique in order to understand the mechanism(s) of high Tc superconductivity. In this work, we have carried out infrared absorption studies of photocarrriers in insulating La2Cu04, Nd2CuO4, andYlBa2CujOe+y (y < 0.2) compounds by employing the steady-state infrared photoexcitation technique. In this experiment, the charge carriers were changed in-situ by controlling the laser pump intensity at various temperatures in an attempt to address specific issues concerning the infrared properties of high-Tc cuprates. In particular, we extensively studied the lower 0379-6779/95/$09.50 SSDI

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energy absorption peak at around - 0.13 eV to gain an insight into the nature of charge carriers and their dynamics. Steady-state infmred photoinduced absorption studies provide an excellent opportunity for investigating the underlying physics of charge carriers in the antiferromagnetic ground state. Because there is no counter ion involved in the photoexcitation process, photocarriers are expected to be free after charge separation unlike the case of dilute chemical doping. We can also unambiguously identify with these experiments the nature of the infrared electronic absorption bands and their correlations with the free carriers that are participating in the transport. More importantly, the recombination dynamics of photocarriers inferred from the infrared photoinduced activities gives direct information about the physical state of charge carriers. It has been verified that the dynamics of photogenerated charge carriers (photocarriers) in the parent compounds of high Tc superconducting cuprates is equivalent to that of doping-induced charges in the superconducting cuprates [2-41. In other words, photocarriers in the steady-state accompany the infrared absorption bands with associated infiamd active vibrational modes, consistent with infrared reflectivity studies of cuprates at various doping levels. Furthermore, recent infmred [4] and transport [5] studies of photocarriers in the parent compounds have provided deep insights into the nature of holes and their dynamics. In particular, probing the inframd activities of photocarriers in the CuO2 planes has revealed unexpectedly rich information about the dynamics of charge carriers in cuprates, information which

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F. Li et al. / Synthetic Metals 71 (1995) 1559-1562

cannot be directly obtained from the chemical doping studies of cuprates. Insulating YlBa2CUg06+y (y < 0.2) prepared from single phase, superconducting YlBa2Cu307 through N2 annealing at 650 “C. A La$uO4 pellet synthesixed with the conventional solid-state reaction was used. Excess oxygen in the La2CuO4 specimen was removed by N2 annealing at 650 “C. For the infrared photoinduced absorption measurements (500 cm-l - 8000 cm-l), the specimens were ground into -1pm size grains and mixed in KBr at a con~ntration of -2 wt%. The homogeneous mixture was then compressed into bluish black semitransparent pellets with -0.7 mm thickness. The sample was mounted on a cold fmger for infrared transmission measurements at various temperatures. The excess charge carriers were generated by optically pumping across the charge transfer gap using an Ar+-ion laser (2.41 eV) at various intensities ranging from 1.2 mW/cm2 to 140 mW/cm2. Photoinduced absorption spectra were obtained by measuring fractional changes (a) in the infrared transmission(3)ofthesemi-transparentsample(a/9(Aa)f, tiabsorption coefficients and L = absorption &p~atthepumpenergy)inresponsetotheexternallaser incident on the sample by using a Bruker 113~ infrared spectrometer for frequencies from 500 cm-l (0.05 eV) to 8000 cm-l (1 ev). The spectral resolution was set to 2 cm-l. Long time signal averaging was necessary in order to obtain a satisfactory signal-to-noise ratio and to statistically improve the baseline stability for more accuratetempea;lturedependencemeasurements. One important exerimental difficulty with Y lBazCu306+y sample is in its extreme sensitivity to the photon flux in obtaining the photoinduced absorption spectra. For example, for photon flux - 2.5 x 10’6 photons/cm%ec wrmsponding to 20 mW/cm2 at 2.53 eV the signal begins to saturate and the carriers lifetime becomes metastable unlike the case of La2CuO4 and NdZCuO4 where the satma&t sets in at - 100 mW/cm2. Such unusual sensitivity may have its physical origin in the photoinduced oxygen chemistry associated with the CuO chains. In addition, when the intensity increases above 20 mW/cm2, local heating begins to dominate the photoinduced spectra and changes the spectral features. The unusual layered structure of Y lBa2Cu306+y cuprates consisting of Cu@ sheets and CuO chains accounts for the observed high concentration of pho&arriers in the CuOZplaneswiththelifetimeontheorderof-lmsec.. Typical photoii absorption spectra of La2CuO4 and YlBa2Cu306+y obtained at 4.2 K ate shown in Figurel. Weobservedtwobroadi&aredabsorptionsone at- 1000cm-l (-0.13eV)andtheotherat-5000cm-1

(- 0.62 eV) for Y1Ba2CU306+y and at - 1000 cm-l ( 0.13 eV) and at - 3800 cm-l (- 0.47 eV) for La2CuO4 along with a phonon bleaching which is due to the Fanolike interference between the lower energy electronic absorption band and the charge-induced infrared-active vi(IRAV) mode, implying the presence of strong ciuugelatt&coupling.

<

Y1BazCu~0~( y < 0.2)_ I

Frequency

(cm-

I)

Figure 1. Photoinduced absorption spectra measured at T=4.2 K; the laser pump intensity was 30 mW/cm2 at 2.7 eV for LazCuO4 and was 10 mW/cm2 at 2.41 eV for YlBazCu306+y. Note the change of beam splitter and detectorat-4ooocm-1. As shown in the following Figure 2, two similar peaks have been observed in very lightly doped cuprate single crystals suggesting that the essential physical equivalence of the dynamics of photocarriers in the CuO2 planes with that of doping-induced charges as summarized in Table 1 together with the photoinduced peaks for La2CuO4 and Y1Ba2Cu30Gy. I

0” --0

I

1

025

0.5

I

I

I

,

/

I 0.75

1.0

1.25

E W)

Figure 2. Comparison of the optical conductivity 01 for lightly doped YlBa2Cu306+y, Nd2Cu04_y, and L~~CUO~+~single crystals obtained by Thomas et al. in Ref. 3. The dashed curves are sketched to indicate the ,band positions.

F. Li et al. / Synthetic Metals 71 (1995) 1559-1562

Notice, however, that the relative intensity ratio of the lower energy peak to the higher energy peak of the lightly doped sample is much smaller than that of the phot0indll&case.

Table 1. Peak position energies of the lower energy absorption (LE) and the higher energy absorption (HE) of the photoinduced absorption spectra of La2CuO4 and Y lBa$u306+y for temperatures below and above Te of their superconducting phase and the corresponding LP and HE of lightly doped single crystals.

otoinduced LE

V
1La2CuC4 I-O.11 eV

1

IY1Ba2Cu$kj+v

I

I

1 -0.12eV

1

-0.13eV -0.16eV

Cr>Tc) Photoinduced HE Doping-induced LE

- 0.47 eV - 0.13 eV

,Doping-induced HE

-0.53eV

- 0.62 eV -0.16e.v (y < 0.4) forallT -0.13 eV (y-1.0) forT
4

For maximally doped superconducting cuprates, the lower and higher energy peaks are not discernible at T > T, because of the Drude-like free carrier contribution to the conductivity. Only the lower energy peak is observed at -1070 cm-l (- 0.13 eV ) as the residual conductivities after all the free carriers have been condensed into the superconducting state at T < Tc. Therefore, straightforward studies of the temperature dependence of the lower energy peak in the superconducting cuprate is not feasible through standard relkctivity measurements. It has been suggested that phase separation is a general property of doping into a commensurate correlateinsulating stete produced by the interactions between the electrons and for which the associated long range order is commensurate with the lattice. The instability against phase separation in La2CuO4 was experimentally demonstrated through photoinduced absorption experiment suggesting that photocarriers are inhomogeneously distributed by forming metallic domains of the conductivity comparable to that of superconducting La1.85Sro.t$u04 [6]. Furthermore, it was veritkd that the charge carrier responsible for the lower energy absorption is directly involved in transport [4]. In this context, the lower energy absorption peak bears significant information about the charge dynamics and its relevance to the superconductivity. The photoinduced signal is proporuonal to the number of photocarriers in the Cue planes (rip). In the steady state, I+,can be determined by balancing the recombination rate with the generation rate which is proportional to the

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optical pump intensity (I) if we know both the recombination and generation rates. Unfortunately, we do not know the recombination rate for cuprates. However, we can obtain an important information about the dynamics of photocaniers in cuprates through the measurements of the intensity dependence of the photoinduced absorption spectra. The intensity dependences of the lower energy electronic absorption at - 0.12 eV measured at 10 K and at 100 K are shown in Figure 3. At 100 K, the intensity dependence is proportional to I”e5until the pump intensity reaches 10 mW/cm2 and the photoinduced signal tends to saturate beyond 10 mW/cm2. ‘Ihe Io5-dependence has been interpreted to result from the bimolecular recombination. The saturation effect may be explained in terms of photocarrier lifetime changes. If the photocarrier lifetime increases beyond our steady state time scale - 1 msec., the observed photoinduced signal no longer reflects total spectral changes, and what is observed then is the net increment in addition to the contribution from the existing photocarriers because we are measuring the net changes in the infrared transmission. This is also consistent with recent time-resolved photoconductivity measurements of cuprates. At 10 K, the hotoinduced signal becomes proportional to - P.$ consistent with the results for La2CuO4 and Nd2CuO4. Note that overall signal level is increased at lower temperamms because of the changes in the oscillator strength (hence scattering rate) at the lower energypealr.

6

10“

IO0

IO'

ld

Pump Intensity (mW/cm’)

Figure 3. Optical pump intensity dependence of the photoinduced lower energy absorption of YlBa$u&j+y attwotemperatunX. The IO.25 deqex~dewe may be understood within the picture of BCS-like condensed state in momentum space.

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F. Li et al. I Synthetic Metals 71 (1995) 1559-1562

Because there exist electron-like (n) and hole-like (p) branches in the excitation energies [7] In this picture, we obtain n - p - Io.25 for large n at a steady state. Therefore, the change in the intensity dependence in YlBazCu306+y may indicate a possible phase transition inthestateofphouXzriersintheCu~planes. Detailed temperature dependence of the lower energy peak has revealed that the peak position abruptly changes from - 0.16 eV (- 1300 cm-l) to - 0.13 eV (- 1070 cm-l) at T - 90 K for YlBaZCugO6+y and from - 0.12 eV (990 cm-l) to - 0.11 eV (- 880 cm-l) at T - 40 K for La2CuO4 as shown in Figure 4.

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We would like to thank Dr. S. B. Nam for many insightful discussions.

1. See for a review, T. Timusk and D.B. Tanner in PbyticalFqr&es ofHigh Temper&xc Superconductors I, D.M. Ginsberg (ed.), World Scientific, Singapore, 1989 and D.B. Tanner and T. Timusk, ibid.11, 1993. See also G.A. Thomas, in Proceedings of the Scottish Universities Summer School on Physics of High-Temperature Superconductors, D.P. Tunstall and W. Barford (eds.), Adam Hilger, Bristol, 1993. 2. Y.H. Kim, AJ. Heeger, L. Acedo, G. Stucky, and F. Wudl, Phys. Rev. B36 (1987) 7252; Y.H. Kim, C. Foster, AJ. Heeger, S. Cox, and G. Stucky, Phys. Rev. B38 (1988) 6478; Y.H. Kim, C.M. Foster, A.J. Heeger, S. Cox, L. Acedo, and G. Stucky, Physica Scripta T27 (1989) 19; C.M. Foster, AJ. Heeger, G. Stucky, and N. Herron, Solid State Commun. 71 (1989) 945; C. Talliani, R. Zamboni, G. Ruani, F.C. Matacotta, and K.I. Pokhodnya, Solid State Commun. 66 (1988) 487.

0

20

40

60

loo

80

Temperature ( K

_I

120

140

)

Figure 4. Temperature dependence of the lower energy peak positron The temperature dependence of the lower energy peak of La&u04 was overlooked in the previous study although thedifferenceisreadilyseeninFigure2ofRef.[4]. Also, relatively themagnitudeoftheloweresrergypeakmueases sharply at low temperatures. Therefore, together with the optical pump intensity dependence measurements, we propose that the controversial lower energy peak is directly involved in the superconductivity and that its position change might be explained in terms of the Fermi level change due to the BCS-like condensate formation. It is in&resting to note that the amount of energy shift in the lower energy absorption peak scales with the superconducting transition temperature. For example, the peak energy shift for LazCuO4 is - 110 cm-l while the shift is - 230 cm-t for YtBa$u306+y, which could be related to the energy gap. Using these energies as the pairing gap energies, we estimate the corresponding energy ratio 2A/kBT, - 3.5 for La2CuO4 with Te - 45 K and 3.6 for YIBa2Cu306+y with Te - 92 K, which might suggest that the pairing mechanism is BCS-like.

3. S. Uchida, T. Ido, H. Takagi, T. Arima, Y. Tokura, and S. Tajima, Phys. Rev. B43 (1991) 7942; G. Thomas, D.H Rapkine, S.L. Cooper, S-W. Cheong, A.S. Cooper, L.F. Schneemayer, and J.V. Waszcxak. Phys. Rev. B45 (1992)2474. 4. Y.H. Kim, S-W. Cheong, and Z. Fisk, Phys. Rev. Iett 67 (1991) 2227. 5. G. Yu, AJ. Heeger, G. Stucky, N. Herron, and E.M. McCarron, Soild State Commun. 72 (1989) 345; G. Yu, C.H. Lee, AJ. Heeger, N. Herron, and E.M. McCarron, Phys. Rev. Lett, 67 (1991) 2581. 6. Y.H. Kim, S.-W. Cheong, and Z. Fisk, Physica C200 (1992) 201. 7. F. Li, Y.H. Kim, and S.-W. Cheong, to be published (1994).