Temperature-dependent transformation of the magnetic excitation spectrum on approaching superconductivity in Fe1-x (Ni/Cu)x Te0.5 Se0.5

0.85 MB·English

by  Zhijun Xu

Temperature-dependent transformationofthe magneticexcitationspectrum onapproaching
superconductivity inFe (Ni/Cu) Te Se
1−x x 0.5 0.5
Zhijun Xu,1,∗ Jinsheng Wen,1,2,3,∗ Yang Zhao,4,5 Masaaki Matsuda,6 Wei Ku,1 Xuerong
Liu,1 Genda Gu,1 D.-H. Lee,2,3 R. J. Birgeneau,2,3 J. M. Tranquada,1 and GuangyongXu1
1Condensed Matter Physics and Materials Science Department,
Brookhaven National Laboratory, Upton, New York 11973, USA
2Physics Department, University of California, Berkeley, CA 94720, USA
2 3MaterialsScience Division, Lawrence BerkeleyNational Laboratory, Berkeley, CA,94720, USA
1 4NISTCenterforNeutronResearch,NationalInstituteofStandardsandTechnology,Gaithersburg,Maryland20899,USA
0 5DepartmentofMaterialsScienceandEngineering,UniversityofMaryland, CollegePark,Maryland,20742, USA
2 6Quantum Condensed MatterDivision, OakRidgeNationalLaboratory, OakRidge, TN,37831, USA
(Dated:January24,2012)
n
a Spinexcitationsareoneofthetopcandidatesformediatingelectronpairinginunconventionalsuperconduc-
J
tors. Their coupling to superconductivity is evident in a large number of systems, by the observation of an

0 abruptredistributionofmagneticspectralweightatthesuperconductingtransitiontemperature,Tc,forenergies
2 comparabletothesuperconductinggap.HerewereportinelasticneutronscatteringmeasurementsonFe-based
superconductors, Fe1−x(Ni/Cu)xTe0.5Se0.5, thatemphasize anadditional signature. Theoverallshapeofthe
n] lowenergymagneticdispersionchangesfromtwoincommensurateverticalcolumnsatT ≫Tctoadistinctly
differentU-shapeddispersionatlowtemperature. Importantly,thisspectralreconstructionisapparentfortem-
o
c perature up to ∼ 3Tc. If the magnetic excitations are involved in the pairing mechanism, their surprising
- modificationontheapproachtoTcdemonstratesthatstronginteractionsareinvolved.
r
p
u
s Inweak-couplingmodelsofmagnetically-mediatedsuper- tionalLaboratory. Thelatticeconstantsarea = b = 3.81A˚,
. conductivity,magnonsessentiallyreplacephononsasthepair- andc = 6.02A˚,usingthetwo-Feunitcell. Forconvenience,
t
a ing bosons [1]. By assumption, the interaction between the we label these samples as Ni02, Ni04, and Cu10, according
m
electrons and bosons is not strong enough to modify the to the amount of Ni/Cu doping on the Fe site. The neu-
- bosonicexcitationspectrum. Inconventionalsystems,super- tron scattering experiments on the two Ni02 and Ni04 sam-
d
n conductivitydoesmodifytheself-energyofthephonons,but ples were carried out on the BT7 triple-axis-spectrometerat
o there is no significant change in the phonon dispersion [2]. the NIST Centerfor NeutronResearch, usingbeamcollima-
c In many unconventionalsuperconductors, including high-T tionsofopen-50′-S-50′-240′,afixedfinalenergyof14.7meV
c
[
cuprates[3–6],heavyFermionsuperconductors[7,8],andthe andtwopyrolyticgraphitefiltersafterthesample. TheCu10

1 recentlydiscoveredFe-basedsuperconductors[9–11],oneob- samplewasmeasuredontheHB1triple-axis-spectrometerat
v serves, oncoolingbelowT , the gappingoflow-energyspin the High Flux Isotope Reactor, Oak Ridge National Labora-
c
4 fluctuations and a shift of spectral weight to a “resonance” tory.withbeamcollimationsof48′-40′-S-60′-240′,fixedfinal
0
peak. Empirically, the magnetic spectrum found above and energyof 13.5 meV , and two pyrolytic graphitefilters after
4
4 belowTctendstobequalitativelythesame. the sample. No static order around (0.5,0,0.5)was found in
1. Here we study the low-energy spin fluctuations in single- anyofthethreesamples. Theinelasticscatteringexperiments
wereallperformedinthe(HK0)zone,sothatthescattering
0 crystalsamplesofthesuperconductorFeTe0.5Se0.5(the“1:1”
2 system,T = 14K)asweperturbthesystembymakingpar- planeisdefinedbythe[100]and[010]wave-vectors.Alldata
1 c have been normalized into absolute units of µ2eV−1/Fe by
tialsubstitutionsforFe.Substituting2%and4%ofNireduces B
: incoherentelastic scatteringintensitiesfromthesamples. X-
v T to12Kand8K,respectively,while10%ofCuresultsinan
c
i absenceofsuperconductivity,asshowninFig.1(a). Ourin- ray diffractionmeasurementsof lattice parameterswere per-
X
formedatbeamlineX22BoftheNationalSynchrotronLight
elasticneutronscatteringmeasurementsshowthatlowenergy
ar (~ω . 12 meV) magnetic excitations transform from hav- Source,BrookhavenNationalLaboratory.
ing two peaks clearly away from the antiferromagnetic(AF)
We are interested in the magnetic excitations near the AF
wave-vectorathightemperatureinthenormalstate,tohaving
wave-vectorQAF = (0.5,0.5,0). Figure1(c)-(e)showsthe
a broad maximum near the AF wave-vector at low tempera-
measuredinelasticneutronscatteringintensityasafunctionof
tureinthesuperconductingphase. Thisdrasticchangeonthe
energyobtainedatT =2.8Kand15Kforallthreesamples.
magnetic dispersion between the superconducting and non-
It has been established in previous studies [13–16] that the
superconductingphases suggests that strong correlationsbe-
unperturbedsuperconductorhasamagneticresonancepeakat
tweenelectronshavetobetakenintoaccountwhenthemag- E ∼7meV.HereweseethatE decreasesto∼6and5meV
r r
neticandelectronicpropertiesofthe“1:1”systemareconsid-
in the Ni02and N04samples, respectively,while thereis no
ered.
observableresonancein the nonsuperconductingCu10. One
SinglecrystalsofFe1−x(Ni/Cu)xTe0.5Se0.5weregrownby canalsoseeaspingapofabout3meVinNi02,butthegapis
aunidirectionalsolidificationmethod[12]atBrookhavenNa- moredifficulttoresolveforNi04.
2
Things get more interesting when we look at the wave-
vector(q)dependenceofthemagneticscattering. Ithasbeen
established in previousstudies [13, 14, 17] of superconduct-
ing FeTe1−xSex that the magnetic excitations disperse from
QAF only in the transverse direction, along [1,−1,0]. Fig-
ure2showsscansalongthisdirectionfortheNi04sampleat
a series ofenergies,illustrating thevariationofthe qdepen-
denceasthetemperaturechangesfrom2.8K(≪T )to15K
c
(&T )andthenupto100K(T ≫T ). Thevariationsaremi-
c c
noratthehigherenergies,asinFig.2(e)-(f),butbecomedra-
maticforE ∼ E ≈ 5meV,asinFig.2(a)-(c). Thechange
r
from T ≪ T to T & T is simply the standard resonance
c c
behavior.Thefeaturethatwewishtoemphasizeisthechange
fromasinglecommensuratepeakatT &T toapairofwell-
c
resolvedincommensuratepeaksatT ≫T . Thischangecan-
c
notbeconfusedwithatemperature-dependentchangeinpeak
width.
Thesamedataarepresentedagain,slightlycleanedupand
inadifferentformat,inFig.3(a)-(c). Thelower-temperature
dataexhibitaU-shapeddispersion,withthebottomoftheU
at ∼ E . Except for the change in the resonant peak, the
r
basicshapeofthedispersiondoesnotreallychangeoncross-
FIG. 2. (Color online) Wave-vector dependence of the magnetic
ing Tc. In contrast, the dispersion at 100 K is qualitatively scatteringintensityalongthetransversedirectionthroughQAF[see
different: it looks like the legs of a pair of trousers. It also Fig. 1 (b)] for the Ni04 sample at T = 2.8 K (red circles), 15 K
looks very similar to the low-temperature dispersion of the (blue squares), and 100 K (green triangles), obtained at excita-
tion energies (a) 3.5 meV, (b) 5 meV, (c) 6.5 meV, (d) 8 meV, (e)
11 meV, (f) 20 meV [which was measured in a higher zone, near
Q=(1.5,0.5,0)].Solidlinesareguidestotheeye.
non-superconductingCu10sampleshowninFig.3(d).
Thereisclearlyamajorchangeinthelow-energyportionof
thedispersionbetween15and100K,buthowdoesitchange
betweenthosetemperatures? ThisisillustratedinFig.4. Fo-
cusing in particular on the results for the Ni04 sample, in
Fig. 4(e) we see that the crossover is continuous in temper-
ature,butwithareasonablydefinedmid-pointat30±10K.
ForNi02,themidpointmaybecloserto40K.Inbothcases,
thecrossoveroccursattemperaturesoforder3T . We previ-
c
ouslyobserved[16]hintsofthistemperaturedependentmod-
ificationofthedispersioninsuperconductingFeTe Se ;
0.35 0.65
however,thehigh-temperatureincommensurabilitywasnotas
largenoraswellresolvedasfortheNi-andCu-dopedsamples
[seeFig.4(e)].
It is possible to see the incommensuratecolumnsof mag-
netic scattering even at low temperaturewhen the supercon-
ductivity is suppressed, as shown for the Cu10 sample in
FIG.1. (Coloronline)Magneticsusceptibilityandinelasticneutron Fig. 3(d). A similar low-temperaturespectrum has been ob-
scattering measurements performed on the Ni02, Ni04, Cu10, and servedpreviouslyinnon-bulk-superconducting“1:1”samples
SC50(FeTe0.5Se0.5)samples. (a)Magneticsusceptibilitymeasure- suchasFe Te Se [17]andFe Te Se [18].
1.04 0.73 0.27 1.10 0.75 0.25
ments, and (c), (d), (e) magnetic neutron scattering intensity mea-
There is an evident pattern that superconducting1:1 sam-
suredatQAFwithT =2.8K(redcircles)and15K(bluesquares).
Theerrorbarsrepresentthesquarerootofthenumberofcounts.Fit- ples have commensurate or almost commensurate mag-
tedbackground obtainedfromconstant-energyscanshasbeensub- netic excitations at the resonance energy, while non-
tractedfromall datasets. The(HK0) scatteringplaneisplottedin superconducting samples have incommensurate excitations.
(b)whilethedashedlinedenotesthedirectionfortheQ-scansshown OurresultsfortheNi-dopedsamplesshowthatitispossible
inFig.2.
fora sampletotransformfromtheincommensuratephaseat
3
FIG. 4. (Color online) Thermal evolution of the magnetic scatter-
ingat~ω = 5meV.ThedataaremeasuredthroughQAFalongthe
transversedirectionfortheNi02sampleat(a)100K,(b)40K,(c)
15K,(d)2.8K,and(e)fortheNi04sampleplottedasanintensity
FIG.3. (Coloronline)Magneticscatteringintensityplottedforthe contourmapintemperature–wave-vectorspace. Thedatahavebeen
Ni04sampleinenergy-momentumspaceat(a)2.8K,(b)15K,and smoothed. The yellow and black symbols in (e) denote the corre-
(c)100K.ResultsfortheCu10samplemeasuredat2.8Kareplot- sponding peak positions for the Ni02 sample (yellow squares) and
ted in (d). The data have been smoothed, and non-magnetic sharp forasuperconductingFe1+δTe0.35Se0.65sample[16].
spurioussignals[seeFig.2(a)]havebeenremovedforbettervisual
effects.
high temperature to the low-energy-commensuratephase on
cooling.Thecommensurabilityappearsattheenergyscaleof
theresonanceenergyatatemperatureof∼3T . Onepossible
c
explanation of this incommensurate-to-commensurate trans-
formationmayberelatedtothestrongorbitalcorrelationsin
this system. In the iron-based superconductors, it has been
proposedthattherearecompetingelectronicinstabilitiessim-
ilar to those in the cuprates [19, 20]. In addition to anti-
ferromagnetism and superconductivity, the material also has
a propensity toward xz/yz orbital ordering. Such ordering
can modify the shape of the Fermi surface (e.g., enlarging
one electron Fermi pocket while shrinking the other), lead-
ingtoenhancementofthecommensuratemagneticscattering
betweentheholeandtheelectronpockets[20]. Eveniflong- FIG.5. (Coloronline)Latticeparametersa(redcircle)andc(blue
rangeorbitalorderingdoesnotsetinuponcooling,thesystem squares)measuredontheNi04sample.
maystillhaveregionsoflocallyorderedorslowlyfluctuating
orbital correlations. It is possible that the crossover we ob-
serveat∼3Tcreflectssuchanorbitalordering“transition”in netic spectrum is unusual among unconventional supercon-
thepresenceofdisorder. Anadditionalexperimentalsupport ductors. For example, in superconducting YBa Cu O
2 3 6+x
ofthisscenarioisgivenbyx-rayscatteringmeasurementson systems [3–5], the spin resonance develops at commensu-
Fe1+ySe0.57Te0.43[21]whereitwasshownthataweaklattice ratewave-vectorsbelowTc, butthe“hour-glass”shapeddis-
distortion,reflectingthebreakingof90degreerotationsym- persion with a commensurate saddle point is still compati-
metry,setsinaround40K,similartothecrossovertempera- blewiththehightemperaturespectrum,withintheincreased
tureobservedin ourexperiment. In oursamples, we didnot q widths. In superconducting La2−xSrxCuO4 the spin res-
seesuchalatticedistortion.Nevertheless,thetemperaturede- onance occurs at lower energies where the spin fluctuations
pendenceofthelatticeparameters(Fig.5)obtainedfromthe are incommensurate [22, 23], both in the normal and super-
Ni04sampleindicateananomalousin-planeexpansionbelow conducting phases. Returning to the analogy with electron-
60K,whilecdecreasesmonotonicallywithcooling. phononcoupling, strong interactionscan lead to a modifica-
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