Search for broken time-reversal symmetry near the surface of YBCO films using Beta-NMR

0.16 MB·English

by  H. Saadaoui

Search for broken time-reversal symmetry near the surface of YBa Cu O films
2 3 7−δ
using β-NMR
H.Saadaoui,1,∗ G.D.Morris,2 Z.Salman,3,∗ Q.Song,1 K.H.Chow,4 M.D.Hossain,1 C.D.P.Levy,2 T.J.Parolin,5
M. R. Pearson,2 M. Smadella,1 D. Wang,1 L. H. Greene,6 P. J. Hentges,6 R. F. Kiefl,1,2,7 and W. A. MacFarlane5
1Department of Physics and Astronomy, University of British Columbia, Vancouver, BC, V6T 1Z1, Canada
2TRIUMF, 4004 Wesbrook Mall, Vancouver, BC, V6T 2A3, Canada
3Clarendon Laboratory, Department of Physics, Oxford University, Parks Road, Oxford, OX1 3PU, UK
4Department of Physics, University of Alberta, Edmonton, AB, T6G 2G7, Canada
1 5Chemistry Department, University of British Columbia, Vancouver, BC, V6T 1Z1, Canada
1 6Department of Physics, University of Illinois at Urbana-Champaign, Urbana, Illinois, 61801, USA
0 7Canadian Institute for Advanced Research, Toronto, ON, M5G 1Z8, Canada
2
(Dated: January 6, 2011)

n
a Weak spontaneous magnetic fields are observed near the surface of YBa2Cu3O7−δ films using
J β-detected Nuclear Magnetic Resonance. Below Tc, the magnetic field distribution in a silver film
evaporated onto the superconductor shows additional line broadening, indicating the appearance
5
of small disordered magnetic fields. The line broadening increases linearly with a weak external

magnetic field applied parallel to the surface, and is depth-independent up to 45 nm from the
]
n Ag/YBa2Cu3O7−δ interface. The magnitude of the line broadening extrapolated to zero applied
o field is less than 0.2 G, and is close to nuclear dipolar broadening in the Ag. This indicates that
c any fieldsdueto broken time-reversal symmetry are less than 0.2 G.
-
r
p PACSnumbers: 74.25.Ha,74.72.-h,76.60.-k
u
s
. The highly unconventional electronic properties of conductance peak (ZBCP) in tunneling spectra) that
t
a high-T superconductors(HTSC)giverisetonovelinter- may condense into a superfluid of different symmetry
c
m
facialphenomenathatareimportantfundamentally(e.g. than the bulk,12 e.g. s-wave, or TRSB states such as
d- inprobingsymmetryofthebulkelectronicgroundstate); dx2−y2+isanddx2−y2+idxy.13 OtheroriginsofaTRSB
as well as in applications (e.g. junction-based devices). state include, frustrated OP near grain boundaries or
n
While significantprogresshasbeen made in understand- junctions,3 the interaction of a self-induced magnetic
o
c ingthe transportpropertiesofsuchinterfaces,verylittle field caused by the OP distortion with the OP itself,14
[ is knownabout their magnetic properties,in partdue to and finite size effects in thin films.15,16

1 the lack of an appropriate local magnetic probe. A par- Experiments to detect TRSB near surfaces have
v ticularly unresolved issue is whether the superconduct- yielded controversial results. Carmi et al. measured a
0 ing order parameter (OP) breaks time-reversal symme- weak spontaneous magnetic field using SQUID magne-
3 try (TRS) near the surface.1,2 A characteristic feature tometryneartheedgesofepitaxialc-axisorientedYBCO
9 of TRS-breaking (TRSB) is spontaneous magnetization; films below T .17 Spontaneous Zeeman-like splitting of
0 c
however,Meissnerscreeningcancelsthisinthebulk,lim- the ZBCP, due to TRSB, has been seen in some tun-
.
1 iting the associated fields to within the magnetic pene- neling measurements,18 but not inothers.19 Phase sensi-
0 tration depth of defects and interfaces.3 To measure this tive measurements, which could also detect spontaneous
1 magnetization directly, one requires a sensitive depth- flux,showednoevidenceofaTRSBstate.20 Aresolution
1
dependent local magnetic probe. In this paper we use a to this disagreement requires more direct information of
:
v novel technique based on depth-controlled beta-detected interface magnetism in cuprates using a local magnetic
Xi nuclearmagneticresonance(β-NMR)tosearchforTRSB probe that can locate the origin and distribution of any
order near the surface of the high-Tc cuprate supercon- such fields on the atomic scale.
r
a ductor YBa2Cu3O7−δ (YBCO). In this study, we present direct measurements of the
In contrast to Ru-based and heavy fermion magneticfieldneartheinterfaceofsilverandYBCOfilms
superconductors,4,5 there is no evidence for TRSB using β-NMR. We measure the field distribution using a
in the bulk of HTSC cuprates, particularly YBCO,6 highly spin polarized 8Li+ beam implanted into a thin
where OP-phase-sensitive measurements have estab- silveroverlayerdepositedonYBCO.We findaninhomo-
lished spin-singlet dx2−y2-wave order.7 There are some geneousbroadeningofthefielddistributionbelowthe Tc
indications of weak magnetism,8,9 some of it related to of YBCO, with the probe ions stopping at an average
the CuO chains in YBCO,10 or to vortex cores above distance of 8 nm from the Ag/YBCO interface. By ex-
the lower critical field H .11 Surface scattering of the trapolating the line broadening to zero applied field we
c1
Cooper pairs from most surfaces perpendicular to the find the mean internal field is very close to experimental
CuO2 planes frustrates dx2−y2-wave order within a few resolution determined by the Ag nuclear dipolar fields.
coherence lengths of the interface.3 This leaves a high In this way we obtain an upper limit of 0.2 G for any
density of mobile holes (as evidenced by the zero bias spontaneous fields of electronic origin.
2
The experiment was performed using β-NMR of 8Li+
at the ISAC facility at TRIUMF in Vancouver, Canada. 0
For details, see Refs.21,22 Similar to NMR, to measure sity3000 Ag YBCO
the resonance, we apply a field along the spin polariza- T=100 K n
tion (here in the plane of the films) B0 = B0yˆ (with y−0.02 g de2000
5 ≤ B0 ≤ 150 G) and follow the polarization of 8Li+ as etr ppin1000
afunctionofthe frequencyω ofasmalltransverseradio- m−0.04 Sto 0
frequency (RF) field of amplitude B ∼ 1 G, applied m 0 10 20 30
1 Depth (nm)
along the xˆ-axis. The resonance condition is ω = γB , y
loc s 0
where for 8Li+, γ = 0.63015 kHz/G, and Bloc is the lo- A 2∆
cal field. At this ω, the polarization, initially parallel
to the yˆ-axis, is averaged by precession in the oscillat- −0.02 T=4.3 K
ing field. Inthe absenceofdynamic effects, the resulting
resonance is generally broadened by any static inhomo- B =10 G
−0.04 0
geneity in the local magnetic field. Thus, the lineshape
offers a detailed measurement of the distribution of lo- 2 4 6ω(kHz)8 10 12
calmagneticfieldsinthe sampledvolumedeterminedby
the beamspot (∼ 3 mm in diameter) and the implanta-
tion profile (see below). FIG. 1: (Color online). Typical β-NMR spectra taken by
A novel pulsed RF mode was used in this study. The implanting 2 keV 8Li+ into Ag/YBCO(110), in an external
RFfieldisappliedin90o pulsesrandomizedinfrequency field of B0 = 10 G (FC) applied along the surface of the
order, instead of the continuous wave (cw) mode com- film. Solid lines are fits to a Lorentzian of HWHM∆. Inset:
monly used.22 In randomly pulsed RF (RPRF), one ob- simulated implantation profile using TRIM.SP for 8Li+ of 2
keVin15nmofAgonYBCO.26 The8Li+stopsatanaverage
tains a high signal to noise ratio with minimal contribu-
depthof 8 nm away from theAg/YBCO interface.
tion from both variations in the incoming 8Li+ rate and
cw power broadening. Because of the limited B , the
1
RPRF mode is suitable for narrowlines up to a few kHz
in width. Fig. 1 shows the resonancespectrum at 100 K (STO) substrate measuring 8×6 mm. The (103) film
with a half width athalf maximum(HWHM) ofapprox- (T = 84 K) was grown under similar conditions. Three
c
imately ∆0 ≈ 0.15 kHz (or 0.24 G). Similar cw spectra (001) films were also studied, (i) one with Tc = 88.7
can be at least twice as broad,21 making it difficult to K grown on (001) STO under similar conditions as the
resolve a small additional broadening. (110),and(ii)two films (T ∼88.0K)grownby thermal
c
Major advantages of β-NMR in detecting TRSB are co-evaporationon8×10mmLaAlO3. The films areepi-
the abilities (i) to implant the probe 8Li+ at low energy taxialandatomicforcemicroscopywasusedtocharacter-
into thin layered structures and (ii) to control the mean ize the surface roughness which is in the range 6-12 nm.
implantationdepthonthenanometerscale. Inthisstudy, Allsampleswerecappedex-situwith15nmofAg,except
8Li+ is preferentially implanted into the thin silver over- oneofthelasttwowhichwascappedwith50nm,byDC
layer evaporated onto YBCO, instead of the supercon- sputtering 99.99%Ag at room temperature under an Ar
ductor itself. Stopping the probes in the overlayer elim- pressure of 30 mtorr at a rate of 0.5 ˚A/s while rotating
inates the possibility of the probe perturbing the super- the sample to ensure uniformity. The 8Li+ implantation
conductor. Also,the8Li+ nucleuscarriesasmallelectric energy was varied so that the probe ions are implanted
quadrupolemoment, so the spectrumin the Ag is freeof at averagedepths ranging from8 to 43 nm. The inset of
anyquadrupolesplittings whicharepresentinnoncubic Fig. 1,showsthesimulatedstoppingprofileof2keV8Li+
YBCO.21 Consequently the resonanceof8Li+ in Ag, be- ions in 15 nm of Ag on YBCO using TRIM.SP.26 Here
low1 Tesla,is single narrowline witha a T-independent the average probe-YBCO distance is ∼8 nm. At 2 keV,
linewidth attributed to nuclear dipole broadening from about 20% of 8Li+ ions stop in the YBCO, yielding no
the small nuclear moments of 107Ag and 109Ag.22 From associatedNMRsignalduetofastspin-latticerelaxation
basic magnetostatics, any inhomogeneous fields in the at low fields. To measure the NMR resonance in Ag, a
YBCO layer will decay exponentially outside the super- smallfieldisrequired,andourmeasurementsweretaken
conductor as exp(−2aπz), where a is the length scale of by field-cooling (FC) in a small field B0 or zero field-
the inhomogeneity.23–25 Thus we can only detect such cooling (ZFC). Residual magnetic fields were reduced to
fields provided our probe-YBCO stopping distance z is lessthan30mGnormaltothesurfacewhenFC,orinall
. a . Any static field inhomogeneities arising in this directions when ZFC. The samples were aligned parallel
way2πwill broadenthe intrinsic resonanceof the Ag layer. to the field with an accuracy of at least 0.5◦.
The measurements presented here were carriedout on Fig. 1 shows two resonances at 100 K and 4.3 K in
(110),(103),and(001)-orientedYBCOfilmscappedwith Ag/YBCO(110). Above T , the resonances are all iden-
c
15 or 50 nm of Ag. The (110) film (T = 86.7 K) was tical and show negligible differences in amplitude and
c
grown by RF magnetron sputtering on a (110) SrTiO linewidth, and are indistinguishable from those intrinsic
3
3

0.5 1
Ag/YBCO(110) Mean depth (nm)
0 25 50
0.4 T BA=g10 G 0.8 ∆∆− (kHz)− (kHz)0000..1155 100 AGg YBCO
0.3 c 0 z) 0.6 0 K)0 K) 10 G T=10 K
H ∆∆(1(1 000 5 10 15
0.2 k E (keV)
z) ( 0.4
H ∆
0.1
k
( Ag/YBCO(001)
∆ 0.4 Ag 0.2 T=100 K
T B =10 G 0
0.3 c 0 0 50 100
B (G)
0
0.2
FIG. 3: (Color online). The HWHM of the resonance in
Ag/YBCO(001)at10KtakenafterZFC.Thedashedlinesare
0.1 linear fits. Inset: energy and depth dependenceof the excess
0 100 200 300
T (K) HWHMat10K(∆(10K)−∆0)inthe50nmAg/YBCO(001)
sample at applied fields 10 and 100 G. The resonance at all
FIG. 2: (Color online). The T-dependence of the energies is dueto thefraction of 8Li+ landing in Ag film.
HWHM, ∆, of the resonance of 2 keV 8Li+ implanted into
Ag/YBCO(110), Ag/YBCO(001), and Ag. The data on the
Ag/YBCO(110) and Ag film was taken using FC, and ZFC
fortheAg/YBCO(001). Thewidthswere independentofFC tent with some tunneling measurements.18 In contrast,
orZFC.ThedashedlinesrepresenttheaverageAgwidth∆0, the broadening here has an onset close to Tc.
the arrows point to the Tc of the YBCO films, and the solid The line broadeningversusthe externalmagnetic field
lines are guideto the eye.
in the c-axis sample is displayed in Fig. 3. At 100 K,
∆ is field-independent as expected. At 10 K (ZFC),
∆ increases linearly with the applied field. This field-
to Ag. Below Tc, the resonance broadens, therefore is dependent broadening is attributed to inhomogeneous
reducedinamplitude. The broadeningis symmetric,un- penetration of the applied field in the form of flux vor-
like the field distribution within the bulk of a supercon- tices. Penetration of vortices would not typically occur
ductor in an ordered vortex lattice state.27 Such a sym- at these fields well-below H ,32 especially since the de-
c1
metric broadening is typical of a more disordered vortex magnetization factor for the field parallel to the film is
distribution.28–30 The HWHM, ∆, of a single Lorentzian very small. However, at the interface the flux lines may
fit to both (110) and (001) samples is plotted in Fig. 2. penetrate more easily due to suppression of the d-wave
It is nearly T-independent above Tc, consistent with the order near twin or grain boundaries.33,34 Moreover, sur-
smallnucleardipolarbroadeninginAg. Itisthesamein face roughness suppresses the Bean-Livingston surface
both samples and comparable to a control sample of Ag barrier, and vortices may nucleate at fields H ≤ H .35
c1
grownonaninsulatingSTOsubstrateundersimilarcon- SurfacevorticeshavebeenobservedinYBCOcrystalsin
ditions (open circles, Fig. 2). Below Tc, the resonance fields as small as 4 G applied parallel to the surface.36
broadens, signaling the appearance of disordered static At suchlow fields, the vortexspacing d is of the orderof
magnetic fields in the underlying YBCO. fewmicrons.37 Outsidethesuperconductor,theresulting
∆ below T is slightly larger in Ag on the (110) film field inhomogeneity leads to a depth independent broad-
c
than the (001) film. In an applied field of 10 G, the ad- ening, since z ≪ d, consistent with our results (inset of
ditional broadening, ∆−∆ , at ≈ 5 K is already very Fig. 3).
0
small, about 0.25 kHz for the (110) film, and 0.15 kHz The T-dependence ofthe linewidth in Fig. 2 is consis-
for the (001) film. This broadening is clearly caused by tentwithlinebroadeningduetovorticesbeneaththesur-
the superconducting YBCO, since it is absent above T facesinceonewouldexpectsuchvortexpenetrationclose
c
and in the Ag film without YBCO. It is, however, not toT ,andthatitwouldincreaseasthetemperaturefalls
c
accompanied by a resonance shift (see Fig. 1). The res- due to the decreasing magnetic penetration depth in the
onance frequency is constant from 300 K to 5 K in all superconductor.38,39 Field inhomogeneities are also ex-
films, independent of FC or ZFC cooling. This shows pectedfromlocalvariationsofthe shieldingcurrentden-
there is no superconducting proximity effect in the Ag sity due to surface roughness as well as twin and grain
layerwhere an induced Meissner shielding ofthe applied boundaries.40 This inhomogeneity will be enhanced by
field leads to a diamagnetic resonance shift, e.g. as seen increasing the field or decreasing the temperature below
recently in Ag/Nb heterostructures.31 The temperature- T ; due to a higher current density with pronounced lo-
c
dependent broadening below T is not linked to TRSB, cal variations. This would lead to a temperature and
c
as the latter state is expected theoretically to appear at field-dependent broadeningas observedinFigs. 2 and 3.
a second transition temperature T ≪ T ,3,12,13 consis- It is possible that flux penetration and current density
c2 c
4
inhomogeneities near the surface could be related to the iments where much stronger spontaneous fields are pre-
apparent dead layer seen in HTSC and other supercon- dicted to cause the ZBCP splitting.18
ductors using low-energy µSR.41,42 Further experiments
In conclusion, we have conducted a depth-resolved β-
on atomically flat surfaces may help elucidate the ori-
NMR study of the field distribution near the interface
gin of the magnetic field inhomogeneities reported here.
of Ag and YBCO films. In all films we find additional
However the field-dependent source of the line broaden-
broadeningoftheNMRresonancebelowT ,signalingthe
ing is not central to the current study. c
appearance of disordered internal static fields in YBCO.
The main resultofthis study is the zerofield extrapo-
We established an upper limit of 0.2 G for TRSB fields
lation of the broadening at low temperature which is an
at low temperature. This rules out any straightforward
estimateoftheTRSBfields. Thebroadeningat10Kex-
interpretation based on the TRSB state that was sug-
trapolatesto∆ ≈0.2kHzinthe(001)and(103),and
B=0 gestedbytunnelingmeasurementstobecharacterizedby
0.3 kHz in (110) (not shown). This ∆ is marginally
B=0 a much larger spontaneous magnetic field.18,19 We have
higherthanthenormalstatebroadening,∆ ≈0.15kHz.
0 shownthatsuchaputativestatemustbeconsistentwith
Thus, the net internal field in the superconducting state
asmallupperlimitonthewidthofthemagneticfielddis-
extrapolatedtozeroappliedfield,∆ −∆ islessthan
B=0 0 tributionintheadjacentAg. Wehavealsodemonstrated
0.15 kHz (or ∼ 0.2 G) in all orientations. Part of the
that β-NMR can be used as a sensitive magnetic probe
difference is due to the slight broadening of the Ag res-
of spontaneous magnetic fields near an interface.
onance upon cooling from 100 to 5 K, as seen in Fig. 2
(open circles). Thus, the additional broadening at zero We thank R. Abasalti, D. Arseneau, S. Daviel, P.
field ∆ −∆ , is anestimate of the spontaneousmag- Dosanjh, and W. K. Park for technical assistance. This
B=0 0
netic field at the Ag/YBCO interface, and has an upper workissupportedbyCIFAR,NSERC;andNRCthrough
limit of 0.2 G. This extrapolated value is close to our a contribution to TRIUMF. LHG acknowledgesthe sup-
experimental resolution determined by the Ag nuclear portof the Office ofScience, U.S. Departmentof Energy
moments. Theseadditionalfieldsatlowtemperatureare undercontractsDE-FG02-07ER46453throughtheU.of
clearly much weaker than predicted by tunneling exper- Illinois Frederick Seitz Materials Research Laboratory.
∗ Present address: Paul Scherrer Institute, Laboratory for 13 M. Fogelstro¨m, D. Rainer, and J. A. Sauls, Phys. Rev.
Muon Spin Spectroscopy,5232 Villigen PSI, Switzerland. Lett. 79, 281 (1997).
1 L. P. Gor’kov, Sov.Sci. Rev.A Phys.9, 1 (1987). 14 M. Palumbo and P. Muzikar, Phys. Rev. B 45, 12620
2 M. Sigrist and K. Ueda, Rev.Mod. Phys. 63, 239 (1991). (1992),
3 M. Sigrist, Pro. The. Phys. 99, 899 (1998). 15 A. B. Vorontsov,Phys. Rev.Lett. 102, 177001 (2009).
4 G.M.Luke,Y.Fudamoto,K.M.Kojima, M.I.Larkin,J. 16 M.H.S.Amin,S.N.Rashkeev,M.Coury,A.N.Omelyan-
Merrin,B.Nachumi,Y.J.Uemura,Y.Maeno,Z.Q.Mao, chouk, and A. M. Zagoskin, Phys. Rev. B 66, 174515
Y. Mori, H. Nakamura, and M. Sigrist, Nature 394, 558 (2002).
(1998). 17 R.Carmi,E.Polturak,G.Koren,andA.Auerbach,Nature
5 G.M.Luke,A.Keren,L.P.Le,W.D.Wu,Y.J.Uemura, 404, 853 (2000).
D. A. Bonn, L. Taillefer, and J. D. Garrett, Phys. Rev. 18 M. Covington, M. Aprili, E. Paraoanu, L. H. Greene, F.
Lett. 71, 1466 (1993). Xu, J. Zhu, and C. A. Mirkin, Phys. Rev. Lett. 79, 277
6 R. F. Kiefl, J. H. Brewer, I. Affleck, J. F. Carolan, P. (1997).
Dosanjh,W.N.Hardy,T.Hsu,R.Kadono,J.R.Kempton, 19 For a review see, G. Deutscher, Rev. Mod. Phys. 77, 109
S.R.Kreitzman,Q.Li,A.H.O’Reilly,T.M.Riseman,P. (2005).
Schleger, P. C. E. Stamp, H. Zhou, L. P. Le, G. M. Luke, 20 W.K.NeilsandD.J.VanHarlingen,Phys.Rev.Lett.88,
B. Sternlieb, Y. J. Uemura, H. R. Hart, and K. W. Lay , 047001 (2002).
Phys. Rev.Lett. 64, 2082 (1990). 21 R.F.Kiefl,W.A.MacFarlane,G.D.Morris,P.Amaudruz,
7 D. J. Van Harlingen, Rev.Mod. Phys. 67, 515 (1995). D.Arseneau,H.Azumi,R.Baartman,T.R.Beals,J.Behr,
8 J. E. Sonier, J. H. Brewer, R. F. Kiefl, R. I. Miller, G. C. Bommas, J. H. Brewer, K. H. Chow, E. Dumont, S.
D. Morris, C. E. Stronach, J. S. Gardner, S. R. Dunsiger, R. Dunsiger, S. Daviel, L. Greene, A. Hatakeyama, R. H.
D. A. Bonn, W. N. Hardy, R. Liang, and R. H. Heffner, Heffner,Y. Hirayama, B. Hitti, S.R. Kreitzman, C. D. P.
Science 292, 1692 (2001). Levy, R. I. Miller, M. Olivo, and R. Poutissou, Physica B
9 J. Xia, E. Schemm, G. Deutscher, S. A. Kivelson, D. A. 326, 189 (2003).
Bonn,W.N.Hardy,R.Liang, W.Siemons,G.Koster,M. 22 G.D.Morris, W.A.MacFarlane, K.H.Chow,Z.Salman,
M.Fejer,andA.Kapitulnik,Phys.Rev.Lett.100,127002 D. J. Arseneau, S. Daviel, A. Hatakeyama, S. R. Kreitz-
(2008). man, C. D. P. Levy, R. Poutissou, R. H. Heffner, J. E.
10 Z. Yamani, B. W. Statt, W. A. MacFarlane, R. Liang, Elenewski,L.H.Greene,andR.F.Kiefl,Phys.Rev.Lett.
D. A. Bonn, and W. N. Hardy, Phys. Rev. B 73, 212506 93, 157601 (2004).
(2006). 23 M. Xu, M. D. Hossain, H. Saadaoui, T. J. Parolin, K. H.
11 J. E. Sonier, Rep.Progr. Phys. 70, 1717 (2007). Chow, T. A.Keeler, R.F.Kiefl,G. D.Morris, Z.Salman,
12 C. R.Hu,Phys. Rev.Lett. 72, 1526 (1994). Q. Song, D. Wang, and W. A. MacFarlane, J. Mag. Res.
5
191, 47 (2008). Phys. Rev.B 35, 7129 (1987).
24 Z. Salman, K. H. Chow, R. I. Miller, A. Morello, T. J. 34 R. P. Huebener, Magnetic flux structures in superconduc-
Parolin, M. D. Hossain, T. A. Keeler, C. D. P. Levy, W. tors, p. 86, Springer(2001).
A. MacFarlane, G. D. Morris, H. Saadaoui, D. Wang, R. 35 M. Konczykowski, L. I. Burlachkov, Y. Yeshurun, and F.
Sessoli, G. G. Condorelli, and R. F. Kiefl, Nano Lett. 7, Holtzberg, Phys.Rev.B 43, 13707 (1991).
1551 (2007). 36 G. J. Dolan, F. Holtzberg, C. Feild, and T. R. Dinger,
25 H. Bluhm, Phys.Rev.B 76, 144507 (2007). Phys. Rev.Lett. 62, 2184 (1989).
26 W. Eckstein, Computer Simulation of Ion-Solid Interac- 37 S.JBendingandM.J.W.Dodgson,Condens.Matter17,
tions (Springer, Berlin, 1991). R955 (2005).
27 J. E. Sonier, J. H. Brewer, and R. F. Kiefl, Rev. Mod. 38 Ch.Niedermayer,E.M. Forgan,H.Glu¨ckler,A.Hofer, E.
Phys. 72, 769 (2000). Morenzoni, M. Pleines, T. Prokscha, T. M. Riseman, M.
28 H.Saadaoui,W.A.MacFarlane,Z.Salman,G.D.Morris, Birke,T.J.Jackson,J.Litterst,M.W.Long,H.Luetkens,
Q.Song, K.H.Chow, M. D.Hossain, C. D.P.Levy,A.I. A.Schatz,andG.Schatz,Phys.Rev.Lett.83,3932(1999).
Mansour, T. J. Parolin, M. R. Pearson, M. Smadella, D. 39 C. Panagopoulos, J. L. Tallon, and T. Xiang, Phys. Rev.
Wang, and R. F. Kiefl, Phys.Rev.B 80, 224503 (2009). B 59, R6635 (1999).
29 G.I.Menon,A.Drew,U.K.Divakar,S.L.Lee,R.Gilardi, 40 C. Jooss, A. Forkl, R. Warthmann, H.-U. Habermeier, B.
J. Mesot, F. Y. Ogrin, D. Charalambous, E. M. Forgan, Leibold, and H.Kronmu¨llerPhysica C 266, 235 (1996).
N. Momono, M. Oda, C. Dewhurst, and C. Baines, Phys. 41 R.F.Kiefl,M.D.Hossain,B.M.Wojek,S.R.Dunsiger,G.
Rev.Lett. 97, 177004 (2006). D.Morris, T. Prokscha, Z.Salman, J. Baglo, D.A.Bonn,
30 D. R.Harshman, A. T. Fiory, and R.J. Cava, Phys. Rev. R.Liang,W.N.Hardy,A.Suter,andE.Morenzoni,Phys.
Lett. 66, 3313 (1991). Rev.B 81, 180502(R) (2010).
31 E. Morenzoni et al., unpublished. 42 A. Suter, E. Morenzoni, N. Garifianov, R. Khasanov, E.
32 R. Liang, P. Dosanjh, D. A. Bonn, W. N. Hardy, and A. Kirk,H.Luetkens,T.Prokscha,andM.Horisberger,Phys.
J. Berlinsky,Phys. Rev.B 50, 4212 (1994). Rev.B 72, 024506 (2005).
33 G. Aeppli, R. J. Cava, E. J. Ansaldo, J. H. Brewer, S. R.
Kreitzman, G. M. Luke, D. R. Noakes, and R. F. Kiefl,