Puzzling evidence for surface superconductivity in the layered dichalcogenide Cu$_{10\%}$TiSe$_2$

0.89 MB·

by  F. Levy-Bertrand

Puzzling evidence for surface superconductivity in the layered dichalcogenide Cu TiSe
10% 2
F.Levy-Bertranda,b,B.Michona,b,J.Marcusa,b,C.Marcenata,c,J.Kacˇmarcˇ´ıkd,T.Kleina,b,H.Cercelliera,b
aUniv.GrenobleAlpes,InstNEEL,F-38000Grenoble,France
bCNRS,InstNEEL,F-38000Grenoble,France
cCEA,INAC-SPSMS,F-38000Grenoble,France
dCentreofLowTemperaturePhysics,InstituteofExperimentalPhysics,SlovakAcademyofSciences,04001Kosˇice,Slovakia
7
Abstract
1
0WereportonspecificheatandmagnetotransportmeasurementsperformedonsuperconductingCu TiSe singlecrystals.Weshow
10% 2
2
thatsuperconductivitypersistsintransportmeasurementsuptomagneticfieldsH wellabovetheuppercriticalfieldH deduced
R c2
nfrom the calorimetric measurements. Surprisingly this ”surface” superconductivity is present for all magnetic field orientations,
aeitherparallelorperpendiculartothelayers. ForH(cid:107)ab,thetemperaturedependenceoftheH /H ratiocanbewellreproducedby
J R c2
solvingtheGinzburg-Landauequationsinpresenceofasurfacelayerwithreducedsuperconductingproperties. Unexpectedlythis
2
temperaturedependencedoesnotdependonthefieldorientation.
1

Keywords: surfacesuperconductivity,dichalcogenide,specificheat,transport
]
nPACS:74.25.F-,74.25.Bt,74.25.Op,74.45.+c
o
c
-
r1. Introduction dichalcogenide. 1T-TiSe2 is an octahedrally coordinated
p
dichalcogenide which exhibits a charge density wave (CDW)
u
It has been shown several decades ago that surface super- accompaniedbyaperiodiclatticedistortionbelow200K.Re-
s
.conductivity can persist up to temperatures much larger than centdiscoveriesofchirality[8]andultrafastdynamicsofcharge
t
athe bulk critical temperature in the presence of a magnetic carriersinthecharge-orderedstate[9],aswellasastrongthe-
m
field parallel to the sample surface [1]. Beyond this standard oreticalactivityhaveledtoaunifiedpictureinwhichtheCDW
-effect, surface and/or interface superconductivity is at work transitionisdrivenbyacooperativeexciton-phononmechanism
d
in various systems leading to intriguing properties. For in- [10, 11, 12]. Upon doping by Cu intercalation, the CDW is
n
ostance the presence of misfit dislocations in semiconducting continuouslysuppressedandasuperconductingdomeemerges
cmonochalcogenideheterostructuresresultintotheformationof [13]. Superconductivity reaches its maximum critical temper-
[inversion layers that turn out to be superconducting with un- ature when the CDW is close to being suppressed (or at least

1 expectedly high critical temperatures [2]. Beside, thanks to istoofainttobedetected). Wepresenthereacombinedstudy
vcrystalgrowthprogress,artificiallayeredmetamaterialsorsin- ofthetransportandspecificheatpropertiesofhighquality,op-
7gle layer are nowadays accessible, generating fascinating be- timally doped single crystals. We show that the resistivity de-
8
haviors (for a review see ref.[3]). For example, the inter- viates from its normal state value for fields much larger than
2
face between the LaAlO and SrTiO insulators happens to the upper critical field H corresponding to the establishment
3 3 3 c2
0be superconducting[4] or the single FeSe epitaxial layer be- of bulk superconductivity, as deduced from specific heat mea-
.comes superconducting at a critical temperature rising up to surements. Surprisingly this surface superconductivity is only
1
about100K,exceedingbyaboutanorderofmagnitudethebulk weakly dependent on the orientation of the applied magnetic
0
7value[5]. field and remains clearly visible even for magnetic fields per-
1 Surfaceeffectsmightalsoplayafundamentalroleinuncon- pendiculartothesamplesurface.
:ventional superconductors. Indeed, those materials naturally
v
ipresent layered structures. It has been suggested that a new
X 2. Samplesandexperiments
type of superconductivity, called pair density wave [6] could
rfor instance exist in those stripped superconductors consisting Single crystals of Cu TiSe were grown by mineralization.
a x 2
of an array of superconducting regions separated by (narrow) StoichiometricamountsofCu,TiandSepowderswereheated
insulatingregions. Inthosesystems,superconductivityalsoof- up to 650oC in a quartz tube under vacuum. Sub-milimetric
tencompeteswithanotherelectronicand/ormagneticinstabil- to milimetric hexagonal platelets, with thickness on the order
ityandspatialmodulationsoftheorderparameterofthiscom- of 20 µm, were obtained (see inset of figure 1, lower panel).
peting phase may then lead to interfacial (or surface) hidden X-raydiffractionconfirmsthatthesamplescrystallizedwithin
superconductivitywithenhancedcriticaltemperature[7]. theP-3m1spacegroup. SlightunfoldingBraggpeakswereob-
In this paper we show some evidence for the presence of served,revealingstackingfaultsalongthec-axisperpendicular
some puzzling surface superconductivity in the Cu TiSe tothehexagonalplane.Scanningelectron-microscopyhasbeen
10% 2
PreprintsubmittedtoPhysicaC January13,2017
Figure 2 displays the field dependence of resistivity (lower
) 60 panel)andspecificheat(upperpanel)atT = 1.2K(asanex-
s
hm 40 c-axis ample)fortheindicatedmagneticfieldorientations. Asshown,
O contacts the shape of theC anomaly remains unchanged whatever the
m 20 sample ~20µm p
R ( ~1mm field orientation, hence allowing an unambiguous determina-
0 tion of the angular dependence of the upper critical field Hc2
1.2 (dashed vertical lines in Fig.2). Note that the clear observa-
) tion of a well defined specific heat jump is another indication
s
nit forthehighqualityofthecrystals. Asexpectedforanisotropic
u
b. superconductors, the superconducting transition is shifted to-
ar wardshigherfieldsastheappliedmagneticfieldismovedfrom
T ( 1.1 the c-axis to the ab-plane and this angular dependence can
C/ then be well fitted using the standard London model for (3D)
anisotropicsuperconductors:
3.0 3.5 4.0 4.5
Temperature (K) Hc2(θ)=(cid:34)Hsinθ (cid:35)2+(cid:34)Hcosθ (cid:35)2−1/2 (1)
c2//ab c2//c
Figure 1: Temperature dependence of the resistivity (upper panel) and spe-
cificheat(lowerpanel)inCu10%TiSe2 singlecrystals. Asshown,verysharp with Hc2//ab ∼ 1.45±0.05 T and Hc2//c ∼ 0.85±0.05 T (see
anomaliesareobservedinbothmeasurementsandthesmallshiftinthecritical dotted line in Fig.3), i.e. corresponding to a mass anisotropy
temperaturemostprobablyreflectsthesmalldispersionofthecriticaltempera- ratio of ∼ 1.7±0.1 in good agreement with previous studies
turewithinthesamplebatch.Theinsetoftheupperpanelsketchesthecontact
[15,16].
geometryandthephotointhelowerpaneldisplaysthesamplemountedonthe
specificheatprobe.
1.4
)
s
nit
performed on some of the crystals confirming that the copper u
dopingishomogenouswithin±0.4%(butsmallerthanthe10% b. 1.2
r
a
nominalvalueby∼1−2%). (
AC specific heat and four-probe resistivity measurements T γ1.0
have been performed on several crystals. The magnetic field / c
e
orientationhasbeenvariedfromthelayerab-plane,totheaxis el
C
0.8
perpendiculartothehexagonalstructure(e.g. c-axis). Intrans-
port measurements, four electrical contacts were attached on 80
the sample surface (see sketch in the inset of the upper panel )
s 60
of Fig.1) using silver paint and the current was applied in m
h
the layer plane, roughly along the a-axis. Cp measurements mO 40
have been performed using a high sensitivity AC technique in (
whichheatwassuppliedtothesamplebyalightemittingdiode R 20
andtheinducedtemperatureoscillationswererecordedwitha
0
thermocouple[14]. Special care has been taken to reduce the
0.0 0.5 1.0 1.5 2.0 2.5 3.0
electrical noise down to ∼ 5 pV (at 0.5 K) using cold trans- Field (T)
formersandverylownoisehomebuiltpre-amplifiers.
Figure2: Magneticfielddependenceoftheresistivity(lowerpanel)andspe-
cificheat(upperpanel)atT=1.2Kfortheindicatedmagneticfieldorientation.
3. Resultsanddiscussion Verticaldottedlinescorrespondstothemidpointofthespecificheatanomaly
andhencetotheuppercriticalfieldHc2correspondingtotheonsetofbulksu-
Figure1displaysthezeromagneticfieldtemperaturedepen- perconductivity.Asshown,theresistivityclearlydeviatesfromitsnormalstate
dence of the resistivity (upper panel) and specific heat (lower valueforfieldsHR muchlargerthanHc2 forallorientationsofthemagnetic
fields.
panel). Asshownaverywellresolvedspecificheatjumpisob-
servedatthecriticaltemperatureT =4.27±0.05K(theerror
c
barcorrespondstothewidthofthejump). Thistransitionisac- Onthecontrarytheshapeoftheresistivitytransitionstrongly
companiedbyasharpdropoftheresistivitytozero(seeupper depends on the field orientation. Indeed, as shown in Fig.2
panel). The slightly lower corresponding T is most probably (lowerpanel),forθ (cid:46) 60◦ asharpincreaseoftheresistivityis
c
reflectingthedispersionofthecriticaltemperatureswithinthe observedforfieldsclosetoH . Notethatthisonsetfieldcorre-
c2
different crystals of the batch. Note that the samples studied spondstotheonsetfieldofthespecificheattransitionshowing
here exhibit the highest T with the smallest transition width thatR → 0assoonasasignificantpartofthesamplebecomes
c
everreportedinthissystem. superconducting and forms percolation paths shorting out the
2
to the data in this angular range. If this angular dependence
1.8 can hence be reasonably attributed to standard surface effects
2
HC1.6 closetotheab−plane,thisisabsolutelynotthecaseforθ→0.
/ R1.4 Indeed, in this latter case, surface effects would be expected
H to vanish for fields perpendicular to the planes and H would
1.2 R
hencebeexpectedtotendtowardsH . However,asshownon
c2
2.5 the upper panel of Fig.3, H /H surprisingly remains almost
R c2
)
a constantontheentireanglerangeemphasizingtheexistenceof
sl
Te2.0 some unexpected superconductivity persisting well above the
(R bulkuppercriticalfieldeffectsforallfielddirections.
H
d 1.5 AchangeofslopeinthemagnetoresistanceofCuTiSe2crys-
n tals prior to the establishment of bulk superconductivity has
a
2 been previously reported by Morosan el al. [15]. This fea-
c
H1.0 turewasattributed,withcaution,toCuinhomogeneities,butit
isworthnotingthatthesuperconductingtransitionisverysharp
0 30 60 90 120 inzeromagneticfieldforbothreference[15]andoursamples,
Angle (degree) rather attesting for the good homogeneity of the samples. A
similar effect has also been reported in other layered systems
Figure3:AngulardependenceatT =1.2KoftheuppercriticalfieldHc2(open such as NbSe2 [19] (note that Fig.3 in this work is strikingly
circles)andofthefieldHR(closedsquares)onthebottompanel.Hc2isdeduced similar to Fig.2 of the present work) and MgB2 [20]. In this
fromspecificheatmeasurements(seeFig.2,upperpanel).HRisdeducedfrom lattersystem,thiseffecthasbeenattributedtosurfacecurrents
resistivitymeasurements(seeFig.2,lowerpanel).Thedottedlineisafittothe
flowing on the lateral side faces due to the fact that the con-
datausingthestandardLondonmodelfor3Danisotropicsystem(seeequation
tactpadswerecoveringthewholelateralsurfaceoftheplatelet.
1andtextfordetails). Thesolidlinecorrespondstotheangulardependence
ofthesurfaceconductivity(seeequation2andtextfordetails). Thesolidline This is however not the case here as point like contacts were
aimtohighlightthecharacteristiccusp-likeshapeofsurfacesuperconductivity made on the top of the platelet. Note that the observation of
formagneticfieldsclosetothelayerplane. NotethatHRissurprisinglymuch such an effect in MgB tends also to rule out any explanation
largerthanHc2 forθ → 0. Upperpanel: angulardependenceoftheHR/Hc2 2
basedonthepresenceofachargeorderintheothercompounds.
ratioshowingthatthisratioonlyweaklydependsonthefieldorientationin
strikingcontrastwithstandardsurfacesuperconductivity(solidline). Figure4displaysthetemperaturedependenceoftheH /H
R c2
ratio for fields along the c-axis and close to the ab-plane. As
raw specific heat and resistivity measurements were not taken
at the same temperature points, the H values were interpo-
electricalresistanceofthesample. Akinkisthenclearlyvisi- c2
latedusingaWHHformula[21,22](previousstudies[15,16]
bleintheRversusH curveandRonlyreachesitsnormalstate
value for H = H >> H (open squares and short solid lines demonstratedthatsuchaWHHformulaleadstoaveryreason-
inFig.2,lowerpaRnel). Ficn2ally,forθ (cid:38) 75◦ theresistivetransi- able fit to the data for both field-orientations). As pointed out
above,forfieldsclosetotheab-planes,H canbeattributedto
tionbecomesverybroadandwedidnotobserveanymoreany R
standard surface superconductivity but, as shown, the H /H
clear signature of the upper critical field in the magnetoresis- R c2
tancedata(seeFig.2forθ=90◦). ratio decreases with temperature, hence being much smaller
thantheSaintJames-deGennes1.7value. Howeverthisvalue
The angular dependence of H is displayed in Fig.3. As
R hasbeenderivedinthecaseofaninterfacebetweenthesuper-
shown,asharpcuspisclearlyvisiblecloseto90◦. Thiscuspis
conductingmaterialsandvacuum[18]andmuchsmallervalues
characteristic of standard surface conductivity and the angular
areactuallyexpectedforT →T inpresenceofatoplayerwith
dependenceofH (≡ H )canthenbewellfittedbythesurface c
R c3 depressedsuperconductingproperties[23].
superconductivityformulaproposedbyK.Yawafujietal. [17]:
Assumingthatthesystemconsistsoftwo(semi-infinite)sur-
(cid:34)Hc3(θ)sinθ(1+ 1−cosθ)(cid:35)2+ Hc3(θ)cosθ=1 (2) cfaacnes(oSl)vaendthbeullikn(eBar)iszuepdeGrcionnzdbuucrgto-rLsainndcaounteaqcuta(tTiocSn<s iTncBp)roense-
H 2tanθ H
c3(cid:107)ab c3(cid:107)c
ence of a magnetic field parallel to the interface [24] in or-
with H ≈ H ≈ H /1.7 as suggested by D. Saint- der to calculate the surface critical field H which is defined
c3(cid:107)c c2(cid:107)ab c3(cid:107)ab c3
JamesandP.G.DeGennes[1]. Notethatthenumericalvalue by the boundary conditions at the interface between the two
of the H /H ratio can actually significantly differ (both sub-layers. H /H is then given by a numerical function
c3(cid:107)ab c3(cid:107)c c3 c2
experimentally and theoretically) from the standard 1.7 Saint- F(γ,α(T))dependingonγ=σB/σS ∼(m /m )×(n /n )and
N N S B B S
James-DeGennesvalue(see[17]anddiscussionbelow). The α(T) = [ξ /ξ ]2 where σ , m, n and ξ are the normal state
S B N i i i
fit is only intended to emphasize the cusp-like angular depen- conductivity, effective masses, carrier densities and coherence
dence of H in the vicinity of the ab-planes. Note also that lengths in the bulk (i = B) and at the surface (i = S), respec-
R
thisformulahasbeenderivedforisotropicsuperconductorsand tively. Veryreasonablefittothedata(dottedlineinFig.4)can
doesnottakeintoaccounttheangulardependenceoftheupper be obtained assuming that the surface critical temperature TS
c
criticalfieldinanisotropicsystems. However,thisdependence isontheorderofTB/γandthezerotemperaturesurfacecoher-
c
can be neglected close to 90◦ leading to a very reasonable fit encelengthratioξ0/ξ0 ∼0.74−γ/130.
S B
3
ties. Thoselayerscouldaccountfortheobservationofoursur-
1.8
face(orinterface)superconductivityfor H(cid:107)ab. Surfacewould
thenmorerefertoaboundarynucleationfield(attheinterface
α between the bulk and the ”dead”-layers) being larger than the
1.6 bulkuppercriticalfield[24]. Evenifboththeangularandtem-
peraturedependenceofH couldthenbewelldescribedbythe
R
standardtheoryforfieldorientationsclosetotheab−planes,the
originofsurfaceand/orinterfaceeffectsfor H(cid:107)cremainstobe
C21.4 clarified.
H
/ R
H
4. Conclusion
1.2
Insummary, wehavedemonstratedthroughthecomparison
between specific heat and resistivity measurements the exis-
tenceofsurface(orinterface)superconductivityinCu TiSe
10% 2
1.0 single crystals. Surprisingly, this surface superconductivity is
presentforallmagneticfieldorientationseitherparallelorper-
pendiculartothelayers. ForH closetotheab-planes,thetem-
0.0 0.2 0.4 0.6 0.8 1.0
peraturedependenceoftheH /H ratioindicatesthepresence
Reduced temperature T/T R c2
C ofsurface(orinterface)layerswithdepressedsuperconducting
properties. Theinfluenceofthoselayersonthesuperconduct-
Figure4:TemperaturedependenceoftheHR/Hc2ratioforH(cid:107)candHcloseto ing properties for H(cid:107)c deserves further theoretical and experi-
theab-plane(seetextandFig.2fordefinitionofthefields). Thedottedlineis
afittothedataforH(cid:107)abassumingthepresenceofsurface(orinterface)layers mentalworks.
withdepressedsuperconductingpropertiesleadingtotheexistenceofabound-
arynucleationfield,largerthanHc2butwithHR/Hc2significantlysmallerthan
thestandard1.7SaintJames-deGennesvalue(forT →Tc). Forfieldsclose Acknowledgments
totheab-planes,thecurvescanbeshiftedbyasmallnormalizationcoefficient
α ∼ 0.9−1dependingontheexactorientationofthefieldcomparedtothe
This work is supported by the French National Research
planesbutnotethatthistemperaturedependenceisstrikingly-andsurprisingly
-similarforbothfieldorientations. Agency through Grant No. ANR-12-JS04-0003-01 SUBRIS-
SYME.JKhasbeensupportedbyAPVV-0036-11andAPVV-
0405-14.
Anestimationofγfromothertechniquesisthennecessaryin
order to be conclusive on the surface superconducting proper- References
ties.NotethatbandstructurecalculationsofTiSe slabspredict
2
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