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Evolution of the electronic Raman scattering of high-Tc superconductors with doping
1
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ELSEVIER
PHYSICA © Physica C 282-287 (1997) 1017-1018
Evolution of the Electronic Raman Scattering of High-Tc Superconductors with Doping * G. Blumberg,lJ,b Moonsoo Kang,1J and M. V. Klein lJ IJScience and Technology Center for Superconductivity and Department of Physics, University of Illinois at Urbana-Champaign, Urbana, IL 61801-3080 6Institute of Chemical Physics and Biophysics, Ravala 10, Tallinn EE0001, Estonia For BhSr2CaCu20s±6 superconductors electronic Raman scattering from high- and low-energy excitations has been studied in relation to the hole doping level, temperature and energy of the incident photons. For underdoped superconductors we conclude that: short range antiferroma.gnetic correlations persist with hole doping; the holes bind in local preformed pairs of B 1g symmetry and 600 cm -1 binding energy; the pair concentration increases with temperature reduction until they gain partial coherence and condense into a colle<:tive superconducting state.
1. INTRODUCTION
The normal state properties of doped cuprates are very different from those of conventional metals and are usually viewed as manifestations of strong electron-electron correlations. Electronic Raman scattering directly probes both (a) shortrange antiferromagnetic correlations caused by the strong electron-electron interaction and (b) the superconducting order parameter through coherent excitation across the superconducting gap. In the present work we study both effects on the same set of Bi2Sr2CaCu20s±6 samples. Furthermore, we provide evidence from underdoped samples of Raman scattering from incoherent gap excitations due to preformed pairs.
2. RESULTS AND DISCUSSION Figure 1 shows the high energy part of BIg electronic Raman scattering spectra as a function of hole doping. The spectrum from the antiferromagnetic insulator (Y-doped Bi-2212 crystal) exhibits a band peaked at about 2860 cm- I that has been assigned to scattering by two-magnons. With doping the two-magnon scattering peak broadens, weakens, and shifts to lower frequency. ·We thank K. Kadowaki and C. Kendziora for providing the samples. Work is supported by NSF grants DMR 9320892 and DMR 91-20000 through the STCS. 0921-4534/97/$17.00 CO Elsevier Science B.Y. All rights reserved. PH S0921-4534(97)00588-1
However, the existence of the peak indicates that a local short-range antiferromagnetic order of a few lattice spacings persists in the superconducting cup rates. Figure 2 shows the low energy BIg Raman scattering spectra as function of the doping, temperature and excitation energy. The intensity of the nearly flat BIg Raman continuum is proportional to the incoherent quasiparticle scattering in the vicinity of wavevectors {klJn } = {(O, ±~) and (±~, and represents the frequency dependent incoherent scattering rate. The continuum extends to a few electron-volts, an energy scale comparable to the width of the electronhole excitation spectra (see Fig. 1). For higher temperatures it starts from very low frequencies, affirming strong incoherent scattering even for low lying electron-hole excitations. Cooling the underdoped samples gradually develops an electronic scattering feature at about 600 cm- 1 • This peak has defined BIg symmetry. The peak position does not show temperature or doping dependence. Simultaneously with the grow.t.h..of the 600 cm -1 peak, the spectra show reduction in the low-frequency portion of the continuum indicating a drop in the low-frequency scattering rate.
On
Based on these data, we argue that for the underdoped cup rates the single quasi-particle exci-
1018
G. Blumberg e/ al.lPhysica C 282-287 (1997) 1017-1018
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600 CD" peak
109 and 2J!/l CD" phonons ~O~109~and~~_I~=~"~P~OOoons~__~______________~ ,
3.05 excitation 0 '--_ _ _ _ _ _ _ _ _ _ _ _eV _ _---.l
o
1000
2000
3000
4000
Raman shift (em·
l
b T,. 13K (undadoped) \.. 64nA. d T,. 12K (owni
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Figure 1. The B 1g continuum and two-magnon Raman scattering spectra for BhSr2CaCu20S±6 as a function of doping for 3.05 eV excitation
tations in the vicinity of {k an } points are mostly incoherent. However, in the presence of the short range antiferromagnetic background two doped holes may form a localized bound state of B 1g symmetry. Light scattering may break the pairs into two singlets by a process similar to twomagnon scattering. We propose that this process is the origin of the 600 cm- 1 peak. Sample cooling increases the pair concentration. At a critical density the quasi-localized pairs earn partial coherence and condense into the collective superconducting, state. Simultaneously, due to the coherence effect, Raman spectra earns additional 600 cm- 1 peak intensity. For the very underdoped sample (Te < 10 K) the 600 cm- 1 peak exhibits only a weak enhancement. The sample with higher doping (Te = 83 K) exhibits an effective pair condensation in the superconducting state with formation of a 26.-1ike feature in the Raman spectra out of the 600 cm- 1 peak. In contrast, overdoped materials show a different picture. Despite the fact that normal state scattering is still mostly incoherent, the scattering intensity above Te is weaker than for underdoped materials (compare Fig. 2b and d). There is no peak near 600 cm- 1 . A strong coherent 26.peak, much stronger than the 600 cm -1 peak for the underdoped materials, develops below Te.
Figure 2. The low-energy portion of B 1g Raman scattering spectra for Bi2Sr2CaCu20s±6 as a function of doping, temperature and excitation energy.
3. CONCLUSIONS Our analysis of a broad range of Raman data for Bi-2212 cuprates at different dopings implies that for the underdoped superconductors (i) a short-range antiferromagnetic order of a few lattice spacings persists upon doping from the antiferromagnetic insulator; (ii) the single quasiparticle excitations in the vicinity of {k an }. points are mostly incoherent and neither in normal nor in the superconducting state show signatures of well-defined single quasi-particle bands; (iii) in the presence of short range antiferromagnetic order, doped holes bind in pairs of dZ;3_ y 3 symmetry and 600 cm- 1 binding energy; (iv) the pair concentration increases with temperature reduction until they gain a partial coherence (phasing) at Te and condense into the collective. ~rconduct ing state. For overdoped superconductors short range magnetic excitations become overdamped, no preformed pairs are observed above Te, and a strong coherent 26.-peak corresponding to the excitations across the superconducting gap develops below Te.
~
--
ELSEVIER
PHYSICA © Physica C 282-287 (1997) 1017-1018
Evolution of the Electronic Raman Scattering of High-Tc Superconductors with Doping * G. Blumberg,lJ,b Moonsoo Kang,1J and M. V. Klein lJ IJScience and Technology Center for Superconductivity and Department of Physics, University of Illinois at Urbana-Champaign, Urbana, IL 61801-3080 6Institute of Chemical Physics and Biophysics, Ravala 10, Tallinn EE0001, Estonia For BhSr2CaCu20s±6 superconductors electronic Raman scattering from high- and low-energy excitations has been studied in relation to the hole doping level, temperature and energy of the incident photons. For underdoped superconductors we conclude that: short range antiferroma.gnetic correlations persist with hole doping; the holes bind in local preformed pairs of B 1g symmetry and 600 cm -1 binding energy; the pair concentration increases with temperature reduction until they gain partial coherence and condense into a colle<:tive superconducting state.
1. INTRODUCTION
The normal state properties of doped cuprates are very different from those of conventional metals and are usually viewed as manifestations of strong electron-electron correlations. Electronic Raman scattering directly probes both (a) shortrange antiferromagnetic correlations caused by the strong electron-electron interaction and (b) the superconducting order parameter through coherent excitation across the superconducting gap. In the present work we study both effects on the same set of Bi2Sr2CaCu20s±6 samples. Furthermore, we provide evidence from underdoped samples of Raman scattering from incoherent gap excitations due to preformed pairs.
2. RESULTS AND DISCUSSION Figure 1 shows the high energy part of BIg electronic Raman scattering spectra as a function of hole doping. The spectrum from the antiferromagnetic insulator (Y-doped Bi-2212 crystal) exhibits a band peaked at about 2860 cm- I that has been assigned to scattering by two-magnons. With doping the two-magnon scattering peak broadens, weakens, and shifts to lower frequency. ·We thank K. Kadowaki and C. Kendziora for providing the samples. Work is supported by NSF grants DMR 9320892 and DMR 91-20000 through the STCS. 0921-4534/97/$17.00 CO Elsevier Science B.Y. All rights reserved. PH S0921-4534(97)00588-1
However, the existence of the peak indicates that a local short-range antiferromagnetic order of a few lattice spacings persists in the superconducting cup rates. Figure 2 shows the low energy BIg Raman scattering spectra as function of the doping, temperature and excitation energy. The intensity of the nearly flat BIg Raman continuum is proportional to the incoherent quasiparticle scattering in the vicinity of wavevectors {klJn } = {(O, ±~) and (±~, and represents the frequency dependent incoherent scattering rate. The continuum extends to a few electron-volts, an energy scale comparable to the width of the electronhole excitation spectra (see Fig. 1). For higher temperatures it starts from very low frequencies, affirming strong incoherent scattering even for low lying electron-hole excitations. Cooling the underdoped samples gradually develops an electronic scattering feature at about 600 cm- 1 • This peak has defined BIg symmetry. The peak position does not show temperature or doping dependence. Simultaneously with the grow.t.h..of the 600 cm -1 peak, the spectra show reduction in the low-frequency portion of the continuum indicating a drop in the low-frequency scattering rate.
On
Based on these data, we argue that for the underdoped cup rates the single quasi-particle exci-
1018
G. Blumberg e/ al.lPhysica C 282-287 (1997) 1017-1018
-. ~
=
• T, = ~~ (und.ldoped) \. 476
'=
::s .ci
300K t50K tOOK 5K
~ c:
o
t
~ V 41
III
III III
2v
2
.....s::
:; ~
~
i
600 CD" peak
109 and 2J!/l CD" phonons ~O~109~and~~_I~=~"~P~OOoons~__~______________~ ,
3.05 excitation 0 '--_ _ _ _ _ _ _ _ _ _ _ _eV _ _---.l
o
1000
2000
3000
4000
Raman shift (em·
l
b T,. 13K (undadoped) \.. 64nA. d T,. 12K (owni
-
SOOO
)
Figure 1. The B 1g continuum and two-magnon Raman scattering spectra for BhSr2CaCu20S±6 as a function of doping for 3.05 eV excitation
tations in the vicinity of {k an } points are mostly incoherent. However, in the presence of the short range antiferromagnetic background two doped holes may form a localized bound state of B 1g symmetry. Light scattering may break the pairs into two singlets by a process similar to twomagnon scattering. We propose that this process is the origin of the 600 cm- 1 peak. Sample cooling increases the pair concentration. At a critical density the quasi-localized pairs earn partial coherence and condense into the collective superconducting, state. Simultaneously, due to the coherence effect, Raman spectra earns additional 600 cm- 1 peak intensity. For the very underdoped sample (Te < 10 K) the 600 cm- 1 peak exhibits only a weak enhancement. The sample with higher doping (Te = 83 K) exhibits an effective pair condensation in the superconducting state with formation of a 26.-1ike feature in the Raman spectra out of the 600 cm- 1 peak. In contrast, overdoped materials show a different picture. Despite the fact that normal state scattering is still mostly incoherent, the scattering intensity above Te is weaker than for underdoped materials (compare Fig. 2b and d). There is no peak near 600 cm- 1 . A strong coherent 26.peak, much stronger than the 600 cm -1 peak for the underdoped materials, develops below Te.
Figure 2. The low-energy portion of B 1g Raman scattering spectra for Bi2Sr2CaCu20s±6 as a function of doping, temperature and excitation energy.
3. CONCLUSIONS Our analysis of a broad range of Raman data for Bi-2212 cuprates at different dopings implies that for the underdoped superconductors (i) a short-range antiferromagnetic order of a few lattice spacings persists upon doping from the antiferromagnetic insulator; (ii) the single quasiparticle excitations in the vicinity of {k an }. points are mostly incoherent and neither in normal nor in the superconducting state show signatures of well-defined single quasi-particle bands; (iii) in the presence of short range antiferromagnetic order, doped holes bind in pairs of dZ;3_ y 3 symmetry and 600 cm- 1 binding energy; (iv) the pair concentration increases with temperature reduction until they gain a partial coherence (phasing) at Te and condense into the collective. ~rconduct ing state. For overdoped superconductors short range magnetic excitations become overdamped, no preformed pairs are observed above Te, and a strong coherent 26.-peak corresponding to the excitations across the superconducting gap develops below Te.