Investigation of the Tunneling Spectra in HgBr2-Intercalated Bi-2212 Single Crystals below and above Tc

0.52 MB·English

by  C. Kurter

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Investigationofthe tunneling spectra inHgBr -intercalatedBi-2212
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singlecrystalsbelow and aboveT
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n C.Kurtera, D.Mazura L.Ozyuzerb D.G.Hinksa K.E. Graya
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c aMaterialsScience Division,Argonne National Laboratory, Argonne IL 60439 USA
- bDepartment of Physics, IzmirInstitute of Technology, TR-35430 Izmir Turkey
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. Abstract
t
a
m InterlayertunnelingspectroscopymeasurementswereperformedonmesaarraysofBi-2212singlecrystals,intercalated
- withHgBr2.Tunnelingconductanceswereobtainedoverawidetemperaturerangetoexaminethespectralfeatures,
d especiallythebehaviorofthequasiparticlepeakscorrespondingtosuperconductingenergygaps(SGs).Experimental
n spectrashowedthatgap-likefeaturesarestillpresentevenforthetemperaturesfarabovethetransitiontemperature,
o
Tc. This evidenceis consistent with theidea that theSGevolves intoa pseudogap aboveTc for HgBr2-intercalated
c
Bi-2212 single crystals.
[

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v Keywords: HighTemperature Superconductors, IntrinsicJosephson Junctions, Tunneling Spectroscopy, Pseudogap.
1 PACS:
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One of the puzzles for high-T superconductors als distorts the measuredresults substantially,e.g.,
0 c
7 is the pseudogap (PG) that results in a depression possiblyleadingtoareductioninmagnitudeofSG.
0 of density of states at the Fermi level above Tc [1]. Intercalating inert HgBr2 molecules between adja-
/ Since tunneling spectroscopy can reveal the states cent Bi-O layers increases the c-axis resistivity re-
t
a neartheFermilevel,itcangiveaninsightaboutPG sultinginahundred-foldsuppressionoftheheatdis-
m
apartfrom superconducting gap(SG) in the quasi- sipationata givenvoltageper junction[3].
- particle excitation spectrum. The PGis clearly ob- Pristine Bi-2212 single crystals were grown by a
d
n servedto exist in underdoped andoptimally doped floating zone technique and intercalation was ac-
o Bi-2212singlecrystals.However,itisrelativelyhard complished by a vapor transport reaction between
c toseethesefeaturesaboveTc forheavilyoverdoped pure Bi-2212 crystals and HgBr2 in air at 230 oC
:
v Bi-2212specimens,sinceSGandPGarebarelydis- for 16 hours. After intercalation, Tc for these crys-
i tinguishable for these crystals [2]. In this study, we tals was measured to be 74 K, and it is generally
X
report the tunneling measurements of overdoped, thoughtthatintercalatedcrystalsareoverdoped[4].
ar HgBr2-intercalatedBi-2212crystalswithbulkTc ∼ Afteradditionalheattreatmentsduringmesafabri-
74 K. Although interlayer tunneling is a perfectly cation(150oC for1hourinflowingair)these crys-
convenient technique for studying naturally occur- talsmayhavefurtheroverdopedtoanevenlowerT
c
ringintrinsicJosephsonjunctions(IJJs)withincop- (see below).For evengreaterminimization ofheat-
peroxidesuperconductors,excessJouleheatingbe- ing effects, small mesa structures (10x10 µm2 by
causeofpoorthermalconductivityofthesemateri- ∼50 nm height) were patterned on the crystal sur-
face using photolithography and Ar ion milling [5].
∗ Forinterlayertunnelingmeasurements,twoconven-
Correspondingauthor. Tel.:[phone]
Email address: [email protected] (C. Kurter).
Preprintsubmitted toElsevier 6February2008
1.6 10.0
1.6
01..82 I (mA)---0001100.......0482284 8.0 (a) V(mV)p468000000 Tc VBCp(mSV) 42234.19432..55
I(mA)-000...404 -1.6 -1000 -500 V (m0V) 500 1000 424.932 dI/dV(mS)46..00 20000 50T(K1)00 150 45667789419370.5
54 85 K
69
-0.8 77 2.0
85
100
-1.2 133
145 K
0.0
1.8
-1.6
69
-1000 -500 0 500 1000 (b) 73
77
V(mV) 1.6 80
85
91
112
Fig. 1. Temperature dependence of I-V characteristics of a 1.4 125
10x10 µm2 mesa. The inset shows I-V characteristics with mS) 113435 K
quasiparticlebranches at 4.2K. V (
dI/d1.2
tionalcontactsweremadeontwoendsofthecrystal
while the third contact used a mechanically sharp- 1.0
enedgoldwireof100µmindiameter,makingagen-
tle contactontopofaparticularmesaofthe array. 0.8
In order to see clearly the hysteretic quasiparticle -1000 -500 0 500 1000
branches in I-V characteristics,the currentwas cy- V(mV)
cledbackandforthmanytimes.
Fig. 2. Temperature dependence of dI/dV-V characteristics
Figure 1 shows the temperature evolution of the
ofa10x10µm2mesa.(a)T=4.2-85K,(b)T=69-145K.The
current-voltage characteristics: for clarity, multiple
inset shows evolution of sumgappeaks withtemperature.
quasiparticlebranches(see IJJ stack dataobtained
at4.2Kininset)wereremovedtoemphasizethebe- totheBCStemperaturedependence.BelowTc,the
haviorofthesum-gapbrancheswithincreasingtem- voltage drops much more rapidly with increasing
perature. We never observe ohmic behavior above temperature in the experimentaldata,andis never
Tc (evenat145K)outtothemodestvoltagesmea- zero at Tc . There is an upward deviation beyond
sured.Theinsetshows30individualtunnelingjunc- Tc thatmaybeasignofhybridizationofsupercon-
tion resistivebranches for this mesa with a spacing ductingpeakwiththepseudogap[6].Inconclusion,
of∼24mV each. ourinterlayertunnelingmeasurementsofHgBr2 in-
The tunneling conductances, i.e. the numerical tercalated Bi-2212 single crystals endorse the exis-
derivatives of I-V curves shown in Fig. 1, are dis- tenceofPGaboveTc ∼69Kthatwehaveobserved
playedinFigs.2(a)and(b)atvarioustemperatures upto 112K.
from 4.2 K to 145 K. The quasiparticle peaks de-
creaseinheightandsmearoutwithincreasingtem-
perature. However, the data for 77 to 112 K still
show a broad gap-like quasiparticle peak structure
References
that may be taken as evidence of a pseudogap in
HgBr2dopedBi-2212samples.Thisfeaturecangive [1] L. Alffet al.,Nature422, (2003) 698.
an insight about the emergence of a pseudogapbe- [2] L. Ozyuzer et al.,Europhys. Lett. 58, (2002) 589.
low112K.Above145Ktheconductivitymonoton- [3] A.Yurgens etal.,Physica C 362,(2001) 286.
[4] A.Yurgensetal.,Int.J.Mod.Phys.B13,(1999)3758.
ically decreases with absolute voltage. The inset of
[5] C.KurterandL.Ozyuzer,J.Opt.Adv.Mat.7,(2005)
Fig. 2(a) shows the gap voltage obtained from the
415.
peakposition,Vp,vs.temperaturefromwhichaTc [6] K.Anagawa etal. Phys.Rev. B 73,(2006) 184512.
of ∼ 69 K can be inferred. From the same plot one
canjudgethecompatibilityoftheexperimentaldata
2