Using scanning tunneling spectroscopy, we investigated the temperature dependence of the quasiparticle density of states of overdoped Bi2Sr2CuO6+δ between 275 mK and 82 K. Below Tc = 10 K, the spectra show a gap with well-defined coherence peaks at ±Δp≃12 meV, which disappear at Tc. Above Tc, the spectra display a clear pseudogap of the same magnitude, gradually filling up and vanishing at T*≃68 K. The comparison with Bi2Sr2CaCu2O8+δ demonstrates that the pseudogap and the superconducting gap scale with each other, providing strong evidence that they have a common origin

Ando, Y

Fischer, O

Kugler, M

Ono, S

Renner, C

English

UCL Discovery

VOLUME 86, NUMBER 21 PHYSICAL REVIEW LETTERS 21M AY 2001Scanning Tunneling Spectroscopy of Bi2Sr2CuO61d: New Evidence for the Common Originof the Pseudogap and SuperconductivityM. Kugler and Ø. FischerDPMC, Université de Genève, 24, Quai Ernest-Ansermet, 1211 Genève, SwitzerlandCh. RennerNEC Research Institute, 4 Independence Way, Princeton, New Jersey 08540S. Ono and Yoichi AndoCentral Research Institute of Electric Power Industry, Komae, Tokyo 201-8511, Japan(Received 3 January 2001)Using scanning tunneling spectroscopy, we investigated the temperature dependence of the quasipar-ticle density of states of overdoped Bi2Sr2CuO61d between 275 mK and 82 K. Below Tc  10 K, thespectra show a gap with well-deﬁned coherence peaks at 6Dp  12 meV, which disappear at Tc. AboveTc, the spectra display a clear pseudogap of the same magnitude, gradually ﬁlling up and vanishing atT  68 K. The comparison with Bi2Sr2CaCu2O81d demonstrates that the pseudogap and the supercon-ducting gap scale with each other, providing strong evidence that they have a common origin.DOI: 10.1103/PhysRevLett.86.4911 PACS numbers: 74.25.Jb, 73.40.Gk, 74.72.HsMany experiments provided evidence for an unusual be-havior in the normal state of high-temperature supercon-ductors (HTS) [1]: Nuclear magnetic resonance, infraredconductivity, neutron scattering, transport characteristics,speciﬁc heat, spin susceptibility, thermoelectric power, andRaman spectroscopy all showed indirect signatures whichwere interpreted as the opening of a gap in the electronicexcitation spectrum above the critical temperature Tc, theso-called pseudogap [1]. This striking observation initi-ated an intense and challenging debate about its origin,since the answer to this key question may turn out to beessential for the understanding of high-temperature super-conductivity. Two basic trends can be found in literature:Either the pseudogap has its origin in a phenomenon dif-ferent from superconductivity [2] and which may possiblybe in competition with the superconducting state, or it hasbasically the same origin as superconductivity [3] in whichcase it might reﬂect the presence of pairs above Tc.Powerful tools to investigate these issues are angular re-solved photoemission spectroscopy (ARPES) [4] and tun-neling spectroscopy [5–10], since they give a direct accessto the quasiparticle density of states (DOS). Probing theDOS of Bi2Sr2CaCu2O81d (Bi2212) single crystals as afunction of temperature clearly conﬁrmed the presence ofa gap above Tc [4–6,8]. Furthermore, ARPES demon-strated that for underdoped samples the pseudogap and thesuperconducting gap have a similar gap anisotropy in kspace [4]. Beyond these observations, scanning tunnelingspectroscopy (STS) showed that in Bi2212 the pseudogapis not restricted to the underdoped state, but that it also ex-ists in the optimally and even overdoped case [6–8]. Thesestudies strongly favor the idea that the pseudogap is relatedto superconductivity and that it might be due to precursorpairing [4–7,11]. Very recently, considerable attention hasbeen directed to superconductor-insulator-superconductor(SIS) tunneling and, in particular, to intrinsic SIS tunnelingexperiments on multiple junctions in Bi2212 mesas [12].These studies revealed a double gap structure in the tunnel-ing conductance spectra: a sharp feature which developson top of a much broader excitation gap. The authors thusclaimed to distinguish and simultaneously observe boththe superconducting gap and the pseudogap. The super-conducting gap showing a strong temperature dependenceand the pseudogap existing even below Tc led them furtherto conclude that their origins are different. The deﬁniteanswer to the origin of the pseudogap thus still appearsambiguous. Note however, that in these experiments thesuperconducting gap at T ø Tc has about the same mag-nitude as the pseudogap at various doping levels, whichis consistent with the above mentioned ARPES and STSstudies.A limiting experimental fact is that these recent re-sults were all obtained on a single compound, namely,Bi2212. In order to draw more general conclusions it iscrucial to investigate other HTS compounds where the su-perconducting parameters, such as Tc or the gap D, areradically different. One such compound is Bi2Sr2CuO61d(Bi2201). This material has a very low critical tempera-ture of Tmaxc  13 K [13], which is nearly an order ofmagnitude lower than for Bi2212 (Tmaxc  92 K; see [8]).Bi2201 therefore presents an ideal stage to test the rela-tion between the pseudogap and the superconducting gap.Here we present a scanning tunneling spectroscopy studyof Bi2201 as a function of temperature.We studied single crystals having a nonstoichiomet-ric composition of Bi2.1Sr1.9CuO61d. The samples weregrown by ﬂoating zone melting, yielding Tonsetc  10 K(DT  4 K) as measured by dc-SQUID magnetization.The temperature dependence of the Hall coefﬁcient andthe room-temperature thermopower show a characteristic0031-90070186(21)4911(4)$15.00 © 2001 The American Physical Society 4911VOLUME 86, NUMBER 21 PHYSICAL REVIEW LETTERS 21M AY 2001behavior of overdoped samples [14] and allowed a consis-tent determination of the hole concentration per Cu atom,p  0.18. The measurements presented in this Letter wereobtained using a newly developed sub-Kelvin scanningtunneling microscope which operates under ultrahigh vac-uum in variable temperatures between 275 mK and roomtemperature [15]. The tunnel junctions were made be-tween in situ cleaved Bi2201 (001) surfaces and electro-chemically etched iridium tips mounted perpendicular tothe sample surface. In this conﬁguration, the differentialtunneling conductance dIdV primarily yields an angularaverage over the ab plane DOS.In Fig. 1a we focus on the tunneling conductance mea-sured at T  275 mK, showing typical spectra obtainedat different locations on the sample surface. They werereproducibly observed on different crystals and aftersubsequent in situ cleavages [16]. The spectra presentwell-developed coherence peaks at 6Dp  12 meV [17].This is a striking result, since for a Tc of only 10 K the gapmagnitude is extremely large. It is by a factor 7 larger thanthe BCS d-wave prediction DBCS  1.8 meV [18]. Thisfurther leads to a very high ratio 2DpkBTc  28, whichis even more remarkable since our sample is overdoped.For comparison, at equivalent doping, Bi2212 yields2DpkBTc  10 [6]. This result shows that in Bi2201superconductivity is far from being BCS like, even in theoverdoped regime. Because of this extreme situation itis crucial to search for signatures of a pseudogap. If thelatter is related to the superconducting gap, we expect acorrespondingly large pseudogap in a wide temperatureinterval above Tc. On the contrary, if the origin of thepseudogap is independent of superconductivity, we expectthat in our overdoped samples the pseudogap may be ab-sent or, at least, that its magnitude is completely unrelatedto the superconducting gap. Harris et al. [17] reportedan ARPES study which showed leading edge shifts of7 meV on underdoped Bi2.3Sr1.7CuO61d (Tc , 4 K) andof 10 meV on optimally doped Bi2Sr1.65La0.35CuO61d-50 -100 0 50 100V [mV] Sample1.52.02.53.0-50 0 501.01.5dI/dV[G]Ω-1aT=275mK10 5 0X[ nm]abbFIG. 1. (a) dIdV spectra obtained at 275 mK at differentlocations on in situ cleaved Bi2201 (Tc  10 K). In the mainpanel the spectra are shifted vertically by 0.2 GV21 for clarity.In the inset they are superposed and normalized. The peaks areat 6Dp  12 meV. (b) 10 3 10 nm2 constant current image(Rt  1.5 GV, raw data) showing atomic resolution and the26 Å periodic superstructure along the b axis.(Tc  29 K) at temperatures above Tc. They interpretedthese shifts as the opening of a pseudogap, although theycould not determine at which temperature T . Tc thepseudogap closes. Note that these values are smaller thanthe gap magnitude we obtained in the superconductingstate on overdoped Bi2201. This observation furtheremphasizes the need to determine the relation betweenthe pseudogap and the superconducting gap. As we shallsee, the temperature dependence of the tunneling spectrapresents a consistent picture and gives a clear answer tothis issue.Compared to Bi2212 it is notoriously more difﬁcult togrow homogeneous Bi2201 single crystals, thus inherentlyresulting in locally varying tunneling spectra. To accessthe intrinsic DOS independently of local sample inhomo-geneities, we systematically acquired 100 spectra spacedequidistantly along a 20 nm line on the sample surface ateach selected temperature. Figure 2a shows the results atthree characteristic temperatures. The slope of the back-ground conductance is reproducible and topographic imag-ing routinely resolved atomic scale features, as shown inFig. 1b. Hence, the variations of the spectra are not dueto poor tunneling junction quality [16]. Consistently withwhat we observed on Bi2212, the periodic superstructure(Fig. 1b) does not lead to a spatial variation of the spec-tra. Figure 2b shows the average of these spectra. Thepeak positions scatter within 63 mVrms around the av-erage position and ﬁne structures of the DOS tend to besmeared out. However, the average spectra clearly de-lineate the essential features. At T1  8.8 K, below Tc,well-deﬁned coherence peaks delimit the superconductinggap. At T2  24.7 K far above Tc, a pseudogap is plainly05 0 -500.80.91.01.01.01.11.51.22.02.5norm.dI/dVdI/dV[G]Ω-10 50 -50 0 50 -50V[ m V ] Sample V[ m V ] Sample V[ m V ] SampleT =8.8K 1 T =82K 3 abcT =24.7K 2dI/dVdI/dV82KT/FIG. 2. dIdV spectra acquired at T  8.8, 24.7, and 82 K.(Rt  0.6 GV). (a) 100 superposed spectra acquired along a20 nm path. (b) Respective average spectra together with the82 K data (dashed line) and normalized to the conductance at100 mV. (c) Same spectra, but normalized to the 82 K data.4912VOLUME 86, NUMBER 21 PHYSICAL REVIEW LETTERS 21M AY 2001visible. Finally, at T3  82 K, the gap has ﬁlled up anda broad hump, which is slightly offset to negative bias,develops. Assuming that the 82 K spectrum reﬂects themetallic normal state DOS, we normalized our data to thisbackground conductance to highlight the spectral changesdue to the pseudogap and the superconducting state.To investigate the continuous evolution of the quasipar-ticle DOS, we acquired tunneling conductance spectra atvarious temperatures between 275 mK and 82 K as shownin Fig. 3. All spectra up to about 70 K show a gap. At thebulk Tc, the coherence peaks abruptly disappear, similarto Bi2212 [6]. This unambiguously shows that the largegap measured below Tc is the gap of the coherent super-conducting state of the bulk and that the conductance peaksare related to the coherent superﬂuid density [19]. Further-more, there is no sign indicating that the superconductinggap closes at Tc as predicted by the BCS theory. Instead,the superconducting gap evolves into a clear pseudogapwhich is roughly constant up to about 30 K (T ¿ Tc),where the gap structure progressively becomes larger, asalso observed in Bi2212 [6]. The pseudogap gradually ﬁllsup and ﬁnally vanishes at T  68 6 2 K, which we de-ﬁned as the temperature where the zero-bias conductance275mK1.6K2.5K3.6K4.8K6.3K8.8K9.7K T bulk cT*11.9K17.2K19.7K24.7K29.6K40.3K50.2K61.9K67.4K74.1K82.0K50 100 -50 -100 0V[ m V ] Sample82KTNormalized tunneling conductance   dI/dV   dI/dV/FIG. 3. Temperature dependence of the tunneling conductanceof Bi2201. This representative set of spectra was acquired andnormalized to the 82 K background (dashed line) as describedin Fig. 2 and shifted vertically for clarity.Temperature[K]V[mV]Sample100-100-5050001 0 2 0 3 0 4 05 0 6 0 7 0 8 0pseudogap phaseTc T*FIG. 4. Interpolated top-view representation of the spectrashown in Fig. 3. The grey scale corresponds to the normalizeddifferential tunneling conductance.reaches 95% of the normal state conductance given hereby the 82 K data.The purpose of the top-view representation (Fig. 4) is tofocusonthegeneraltrendoftheoveralltemperaturebehav-ior, rather than on detail features. The present temperaturedependence largely conﬁrms what has been observed pre-viously by STS on Bi2212 [6], with the central differencethat Bi2201 has radically different superconducting pa-rameters. The key observation in the temperature depen-dence is the extremely large pseudogap phase comparedto the superconducting one. There is a factor 7 betweenT and Tc, which is precisely the same factor we observedbetween Dp and DBCS.A consequence of these results is that the ratio of Dpand T is the same as found in other HTS materials.In Fig. 5 we plotted TTc versus 2DpkBTc for severalHTS materials which have been investigated by tunnel-ing spectroscopy. Oda et al. [11] and Nakano et al. [20]Bi2201 This workLa214 [20]Y123 [10]Bi2212 [6]Bi2212 [8]Bi2212 [11]Bi2212 [5]0 5 10 15 20 25 30 35012345678T*/TC2/ k T ∆pB C2 /k T* = 4.3 ∆p BFIG. 5. TTc versus 2DpkBTc for various cuprates com-pared to the mean-ﬁeld relation 2DpkBT  4.3 [18], whereT replaces Tc. Error bars are extracted from the givenreferences.4913VOLUME 86, NUMBER 21 PHYSICAL REVIEW LETTERS 21M AY 2001eV / Sample p ∆0.00.51.51.0dI/dV[G]Ω-1T=4.2KT=81.9K0.00.51.01.5T=98KT=4.2K Bi2212UDT =83K cBi2212ODT =74K c-8 -6 -4 -2 0 2 4 6 80.81.01.2 T=2.4KT=19.7KBi2201ODT =10K c∆p =44meV∆p =38meV∆p =12meVFIG. 6. Comparison between the pseudogap (dashed line, T .Tc) and the gap in the superconducting state (solid line, T ,Tc) for underdoped (UD) and overdoped (OD) Bi2212 [6], andoverdoped Bi2201 (present work). All curves are raw data.highlighted that Bi2212 and La214 fall close to the BCSd-wave relation 2DpkBT  4.3 [18], where Tc is re-placed by T. Strikingly, this is also true for Bi2201, al-though 2DpkBTc is by a factor 2–3 larger than for theother compounds. It thus conﬁrms that it is T, ratherthan Tc, which reﬂects the mean-ﬁeld critical temperatureof the superconductor. This consistency favors modelswhich evoke precursor pairing as the origin of the pseu-dogap phase [3]. From this point of view, the fact thatthe pseudogap temperature T is almost 1 order of magni-tude larger than Tc reveals that phase ﬂuctuations are verystrong in this single CuO2 layer cuprate, even in the over-doped regime.The central result of this Letter is related to the scal-ing between the superconducting gap and the pseudogap.In spite of the low Tc and the overdoped nature of oursamples, we ﬁnd that the ratio between the gap in the su-perconducting state and the pseudogap is the same as inBi2212. Figure 6 compares the tunneling spectra aboveand below Tc for the present study on overdoped Bi2201,as well as for over- and underdoped Bi2212 [6]. To high-light the scaling properties, we normalized the energy scaleto the respective Dp of each compound, thus demonstrat-ing that the relative size of the pseudogap with respect tothe superconducting gap is in all three cases the same. Thisconsistency is remarkable since the energy scale given byDp is a factor 3–4 smaller for Bi2201 compared to Bi2212.The present data thus bring a manifest conﬁrmation that thepseudogap scales with the superconducting gap, giving arobust evidence that the origin of the pseudogap is relatedto superconductivity.In conclusion, using scanning tunneling spectroscopy,we measured the temperature dependence of the DOS ofoverdoped Bi2201 between 275 mK and 82 K. Togetherwith earlier results on Bi2212, we obtain a systematicpicture where the pseudogap scales with the gap in thesuperconducting state, although both compounds showradically different superconducting parameters. Further-more, the surprisingly large gap value observed in Bi2201and the correspondingly large T add strong evidence thatin these materials the pseudogap has the same origin assuperconductivity.[1] T. Timusk and B. Statt, Rep. Prog. Phys. 62, 61 (1999).[2] R.S. Markiewicz, C. Kusko, and V. Kidambi, Phys. 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Scanning Tunneling Spectroscopy of Bi2Sr2CuO6+d: New Evidence for the Common Origin of the Pseudogap and Superconductivity

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