Table Of ContentCOND MAT
Indirect electric field doping of the CuO planes in NdBa Cu O cuprate
2 2 3 7
M. Salluzzo,1,∗ G. Ghiringhelli,2 J. C. Cezar,3 N.B. Brookes,3 G. M. De Luca,1 F. Fracassi,2 and R. Vaglio1
1CNR-INFM COHERENTIA, Dipartimento di Scienze Fisiche Universit`a di Napoli ”Federico II”,
Complesso di Monte S. Angelo, Via Cinthia, 80126 Napoli (Italy)
2CNR-INFM COHERENTIA and Dipartimento di Fisica,
Politecnico di Milano, piazza Leonardo da Vinci 32, 20133 Milano, Italy
3European Synchrotron Radiation Facility, BP220, 38043 Grenoble cedex, France
(Dated: February 2, 2008)
The mechanism of field-effect doping in the “123” high critical temperature superconductors
8 (HTS) has been investigated by x-ray absorption spectroscopy in the presence of an electric field.
0 WedemonstratethatholesarecreatedattheCuOchainsofthechargereservoirandthatfield-effect
0 dopingoftheCuO2 planesoccursbychargetransfer, from thechainstotheplanes, ofafraction of
2 theoverallinducedholes. Theelectronicpropertiesofthechargereservoirandofthedielectric/HTS
n interface determine theelectric field doping of theCuO2 planes.
a
J PACSnumbers: 74.78.-w,74.25.Jb,71.30.+h
4
1
Thephysicalphenomenaoccurringattheinterfacebe-
] tween materials having different bulk electronic proper-
l
e ties are extremely important in modern electronics [1].
- The greatsuccess of the metal-oxide semiconductor field
r
t effect transistors is due to the high technological con-
s
. trol of the Si/SiO2 interface and to the very good un-
t
a derstanding of the electric field effect mechanisms at the
m interface between a band insulator and a band semicon-
- ductor. Suchdetailedinsightismissingfortheinterfaces
d
of materials with strong electroncorrelation,such as the
n
high critical temperature superconductors (HTS).
o
c FIG. 1: (Color online)(a) Sketch of the NdBCO/STO inter-
The application of an electric field at the interface be-
[ face with the labeling of the inequivalent copper and oxygen
tweenaninsulatoranda metal/semiconductoris consid- sites used throughout the text. The interface plane is shown
1
v eredageneralmethodtochangereversiblytheelectronic as a dashed line separating theTiO2 planeof theSTO (100)
properties of thin film without introducing chemical and single crystal from the BaO plane of the NdBCO film. b)
8
7 structural disorder. In particular, the electric field-effect In situ measured four point probe channel resistance of the
8 has been used to shift the critical temperature and even threeNdBCOdevices: 4u.c. (greendiamonds)superconduct-
1 toinducephasetransitionsinconventionalandhighcrit- ing with critical temperature Tc = 20 K and carrier density
1. icaltemperatureTc superconductors[2,3,4,5,6,7]. Ex- pup.cl.=(b0la.0ck6(c0i)rchleosl)esi/nCsuulOat2inpglawniet;h3ppul.c=. 0(.r0e5d(0c)iracnleds)0.a0n4d(02)
0 periments conducted on HTS from early 1990s to nowa-
holes/CuO2 plane, respectively.
8 dayshavebeenusuallyinterpretedbysupposingthatthe
0
inducedchargesbecomecarriersintheCuO conducting
: 2
v planes, thus changing the filling of the Zhang-Rice (ZR)
Xi band [8]. The universal relationship between Tc and the a direct doping of the superconducting CuO2 planes by
the polarization charges created by the electric field has
carrier density in the CuO planes of the HTS (the so-
r 2
not been clearly demonstrated yet.
a calledHTSdome)[9],suggestedthatanexternalelectric
fieldcouldeasilyleadtocontrollablechangesoftheirelec- Here we present an x-ray absorption spectroscopy
tronicproperties. AlthoughmodulationsofT [5, 10,11] (XAS) study of the electric field doping of the well
c
and of the critical current density [6] were observed in studied SrTiO /NdBCO interface. The Cu L edge
3 3
the “123”YBa Cu O andNdBa Cu O (NdBCO)thin (2p →3d transition) XAS experiment was performed
2 3 7 2 3 7 3/2
films, the extent of the electric field effect was found to at the ID08 beam line of the European Synchrotron Ra-
be below expectations. Superconducting to insulating diation Facility. It was designed in order to allow si-
transitions were observed only in samples with a dop- multaneousspectroscopicandtransportcharacterization
ingexactlyattheseparationofthe twoelectronicphases of the samples under the application of an electric field.
[4, 7], while small changes on the sample conductivity Within the wide family of the HTS, NdBCO belongs to
wereobservedinBi Sr La CaCu O filmshavinga the “123” group, where, besides superconducting CuO
2 1.5 0.5 2 8+δ 2
singleinsulatingCuO plane dopedinarangenominally planes, an important role is played by the CuO chains
2
spanning the whole HTS dome [12]. As matter of fact, within the “charge reservoir” (CR). Transmission elec-
2
relatedtothecapacitanceandtothearealcarrierdensity
∆n through the relation Q = −e·∆n = RVgateCdV .
s s 0
With V =±900 V, about ±6.0×1013 holes/cm2 can
gate
be subtracted/added at the STO/NdBCO interface [see
also EPAPS file].
WehavestudiedNdBCOsampleswithadopingabove
andbelowthecriticalvalueseparatingthesuperconduct-
ing and insulating phases (Fig. 1b) [14]. An example of
the effect of the electric field is shown in Fig. 2. The
resistivity of the 3 u.c. NdBCO sample at 10 K non-
linearly decreases as function of ∆n , in agreement with
s
the induction of carriers in the CuO planes. It is worth
2
nothing that the field effect doping is insufficient to in-
duce superconductivity. Similarly, we could not revert
FIG. 2: (Color online) The channel resistance of the 3 u.c.
device, measured as function of thenumberof holes per unit the 4 u.c. sample from superconducting to insulating by
area induced by the electric field. In the inset Tc vs doping applying a positive gate voltage. These results are in
(ppl) characteristics of the 4 u.c. (green circle), 3 u.c. (red contradiction with simple model expectations: assuming
circle) and 2 u.c. (black circle) overlapped to the HTS dome that only one unit cell is doped and that all the carri-
(SCstandsforsuperconductingandAFforantiferromagnetic
ers are transferred to the CuO planes, about p =0.05
phases). Thehorizontalbarsaroundeachexperimentalpoint 2 pl
holes/CuO plane would be added or subtracted. In the
are the estimated field effect, in the assumption that all the 2
3 u.c. NdBCO the average hole population per CuO
inducedchargebecomecarriersintheCuO2planesofthefirst 2
unit cell at theinterface. plane is 0.05(0), consequently the electric field should
bringtozerothe conductioncarriersforV =+900V
gate
anddoublethemforV =−900V.Insuchascenario,
gate
tronmicroscopy[13]hasdemonstratedthatthefirstlayer an insulating to superconducting transition is expected,
at the SrTiO3/NdBCO interface belongs to the CR, as as shown in the inset of Fig. 2. Even assuming that all
sketchedinFig.1a. Thisisparticularlyrelevantsincethe the unit cells are doped, a phase transition should occur
effectiveelectricfieldpenetrationcalculatedbyaThomas anyhow in the samples studied, because the carrier den-
Fermi model or by any simple electrostatic estimates is sity would change by ∆p = ±0.015 holes/CuO , i.e.,
pl 2
of the order of 1 nm or less, i.e. less than one unit cell enough to induce a transition.
(u.c.). In previous studies the role of the atomic lay- To unravel the underlying microscopic mechanism of
ers directly facing the interfacewas largelyneglected. In the electric field effect, we have used the XAS technique
this paper we will show, on the contrary, that they play that is sensitive to the orbital occupation and symme-
a decisive role in the microscopic mechanism of the field try at the different copper sites of our samples. We col-
effect doping of cuprates. lected the total electron yield signalthat probes a depth
NdBCO (001) ultra-thin films have been epitaxially of few nanometers, slightly larger than the NdBCO film
grown, by high oxygen pressure diode sputtering, on thickness. By using the linear polarization of the x-rays,
TiO terminated 10x10x 0.5 mm3 SrTiO (100) (STO) spectracorrespondingtofinalstateslyingparallelorper-
2 3
surface. The samples are pseudomorphic to the STO pendicular to the CuO planes have been measured (I
2 ab
single crystals and have high structural perfection, and and I spectra in Fig. 3) [see EPAPS file].
c
very smooth surfaces. The field effect structures have The Cu L spectra, measured on the NdBCO films at
3
beencompletedbydepositiononthethinfilmofthegold 10 K, present the typical features of all cuprates, i.e.,
pads for transport measurements, and of the gate elec- a main peak at 932.0 eV and a shoulder at 933.5 eV
trode (4×2 mm2 area) on the back of the substrate, in (Fig. 3) [15]. “123” cuprates contain two inequivalent
a geometry suitable for the XAS experiment. The field copper sites, i.e., Cu(1) in the chains and Cu(2,3) in the
has been applied through the SrTiO single crystal un- CuO planes(seeFig.1a). Asshownin[16]and[17],the
3 2
derultrahighvacuumconditions. Thedeviceshavebeen main peak has a contribution from the doped Cu(1)3+in
characterizedinsituduringtheXASexperiment. Aneg- thechainsthatformaligandstate(3d9L)withtheneigh-
ative V creates holes at the interface (accumulation boring oxygen ions, which increases if holes are injected
gate
mode) and a positive V reduces the density of holes in the chains along the a−b axes. Another contribution
gate
inthe sample(depletionmode). Thecapacitanceofthis comes from the undoped Cu(2,3)2+ in the CuO planes
2
device is a function of the gate voltage,a well-knownre- (3d9state). The remaining doped Cu(2,3) sites, occu-
sult for non-linear dielectrics materials like STO. At low piedbyformallytrivalentCu(2,3)3+ ions,giverisetothe
gatevoltageitis about2.5 nFanditdecreasesto 0.4nF satellite peak at 933.5 eV. In the I spectra, this fea-
ab
at high voltages. The charge accumulated at the inter- ture is due to the Zhang-Rice (ZR) singlet [8], i.e., to
face is consequently measured by an electrometer and is the mobile carriers in the CuO planes. In the I spec-
2 c
3
FIG. 3: (Color online) Cu L3 XAS spectra, with Ekab (Iab)
and (b) Ekc (Ic), measured at low temperature (10 K). The
ZR satellite peak intensity is related to the number of holes
per CuO2 plane (see text). This feature decreases continu-
ously when going from 4 u.c. (black squares) to 2 u.c. (blue
diamonds), in agreement with transport data.
trathe satelliteisduetoCu(2,3)3+ siteshybridizedwith
theapicalO(4)oxygen,asignatureofthechargetransfer
from the chains to the planes. As shown in Fig. 3 and
in[18],thesatellitepeaksdecreaseswiththethicknessof
theNdBCOfilms,inagreementwiththetransportprop-
erties, with the reduction of the carrier density and of
the charge transfer from the chains to the CuO2 plane. FIG. 4: (Color on line). (a) Gate voltage dependence of the
The effects ofthe electric fieldonthe Iab spectraat10 IabXASspectrainthe3u.c. NdBCOdevice(theexperimen-
K are shown in Fig. 4 for the 3 u.c. sample, and similar tal configuration is shown in the inset): in the upper panel
result were found for the 2 u.c. and 4 u.c. films. Un- theIab spectra are shown for negative (black lines) and posi-
tive(redlines)gatevoltages. Thebluelineisacquiredatzero
dernegative(positive)gatevoltages,thesatelliteat933.5
field. Inthelowerpanelsthedataareshownascolormappic-
eV,acquires(looses)spectralweightbecausetheinjected
tures with different linear color-scales around the main peak
chargesfill up (empty) the ZR band, i.e., additional car- at 932 eV (left panel) and around the c3d10L satellite (right
riersappearattheCuO2 planes. However,thelargestef- panel). (b) Effect of the field shown by subtracting to each
fectsareobservedonthemainpeak,whichshowsastrong Iab XASspectrathereferencescanacquiredwithoutthefield
increase(decrease)inintensityandagradualbroadening (black closed and line joined circles). (c) Difference between
(narrowing) on the high-energy side. As shown in Fig. Iab and Ic XAS spectra at different gate voltages
4b, where the spectrum at zero field is subtracted from
the data, a peak at about 932.2 eV continuously grows
with negative gate voltages. These results are explained the chains (parallel to the ab plane). On the contrary
by an enhancement of hole density along the chains by the filling of the ZR band is accompanied by an equal
negative fields. The ratio of the holes induced in the change of Cu(2,3)-O(4) ligand states. This is a further
chains (at the Cu(1) sites) and in the CuO2 planes (at confirmationthatthedopingoftheCuO2planesisrather
the Cu(2,3) sites) as estimated from the integrated in- indirect, i.e., it occurs by a partial charge transfer from
tensities around 932.2 eV and 933.5 eV in Fig. 4b, is the heavily doped chains via the Cu(2,3)-O(4) bonding.
3.5±0.5. It does not depend on the gate voltage within The XAS results are consistent with the transport
the experimental error. data measured in situ and shown in Fig. 2. In particu-
The electric field effect is similar in the ab plane and lar we find a non-linear relationship between the change
along the c axis. However, interesting differences can be in the resistance and the weight of ZR states (propor-
noted. InFig.4cweshowtheXAS(I −I )spectraasa tional to the integrated intensity of the satellite peak)
ab c
functionofthefield: fornegative(positive)gatevoltages, from V =+1000 V to -500 V and a saturation (lower
gate
the main peak at 932 eV acquires (looses) relative in- slope) at higher negative voltages. This resembles the
plane intensity, whereas the satellite anisotropy remains non-linear relationship between the resistivity and in-
almost unaffected. This confirms that negative voltages duced carrier density shown in Fig. 2 and can be ex-
leadtoastrongenhancementoftheholepopulationalong plained by taking into account the activated tempera-
4
ture dependence, with a slope depending on the carrier
density, of the resistance in insulating cuprates.
Thedataarealsoconsistentwithpreviouslypublished
∗
Electronic address: [email protected]
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2
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c
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(2007)
ingidentifiedthephysicalmechanismsleadingtoelectric
[19] In Ref. 4 the Hall effect coefficient was used to extract
field doping of the CuO planes in the HTS, we have
2 the amount of carriers injected into the CuO2 planes. It
shown that an advanced interface engineering, i.e., the
is well known that in “123” compounds also the chains
rightchoiceoftheatomiclayersattheinterfaceandtheir contribute to the transport, consequently the Hall coef-
chemical and structuralcontrol,can eventually open the ficient depends also on the carrier present in the chains,
waytoa moreeffective field-effectdopingofhighcritical and,inthecaseofafieldeffectexperiment,onthecarrier
temperature superconductors. injected into the chains. Consequently, it is not surpris-
ingtofindanagreementbetweentheorderofmagnitude
The Authors are grateful to Jean-Marc Triscone and
oftheHalleffectcoefficientandtheestimatedfieldeffect
to Antonio Barone for useful discussion about data in-
injected carrier density.
terpretation. M. S., G. D. L. and R. V. acknowledge
[20] SeongshikOh,TrevisA.Crane,D.J.VanHarlingen,and
the support from the EU under the project Nanoxide, J. N.Eckstein, Phys. Rev.Lett. 96, 107003 (2006)
contract n. 033191.
