Large mass enhancement in RbOs2O6

0.13 MB·English

by  M. Brühwiler

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LargemassenhancementinRbOs O
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M.Bru¨hwilera, S.M.Kazakova J.Karpinskia B.Batlogga

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n aLaboratoryforSolidStatePhysics,ETHZu¨rich,8093Zu¨rich,Switzerland
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p Abstract
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s Heat capacity measurements on the recently discovered geometrically frustrated β-pyrochlore superconductor
t. RbOs2O6 (Tc = 6.4K) yield a Sommerfeld coefficient of 44mJ/molf.u./K2. This is about 4 times larger than the
a onefoundinbandstructurecalculations.Inordertospecifytheenhancementduetoelectron-electroninteractions,
m
wehavemeasuredtheelectron-phononenhancement.Byasuitableanalysis,anelectron-phononcouplingconstant
- λep = 1±0.1 is derived from the specific heat jump at Tc. This leaves a significant additional λadd = 2.1±0.3
d
forenhancementduetoothermechanisms,possiblyrelatedtothetriangularlattice.Toarriveattheseresults,an
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appropriateanalysismethodforbulkthermodynamicdatabasedonthecondensationenergywasapplied.
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Keywords: RbOs2O6,superconductivity,correlation,DOSenhancement,thermodynamicproperties
1 PACS:74.25.Bt;71.27.+a;74.70.-b
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5 Longstandinginterestinthepyrochloresstemsfrom SpecificheatmeasurementsonRbOs2O6 showthat
theirinherentgeometricalfrustrationduetothemetal the residual Sommerfeld coefficient in the supercon-
1
0 ions forming a network of corner-sharing tetrahedra. ductingstateγrandthenormal-statecoefficientγvary
6 Thus,metallicpyrochloresconstituteidealsystemsto among the various samples measured. The variation
0 study to what degree itinerant electrons are affected is consistent with the presence of a second metallic
/ by a lattice which is known to cause geometrical component, and subsequent x-ray diffraction analysis
t
a frustration for interactions among localized magnetic has confirmed the presence of OsO2 in the samples.
m moments. For similar reasons, the superconductivity We have therefore developed a suitable quantitative
- recently found in RbOs2O6 has been of considerable methodtoanalyzethermodynamicdatawhendealing
d interest.RbOs2O6isoneofonlyfourpyrochloresuper- with a superconducting sample containing a metallic
n conductorsknowntodate.Thesearetheα-pyrochlore second phase. Since the analysis is based on the con-
o Cd2Re2O7 and the β-pyrochlores AOs2O6, where A densationenergyofthesuperconductorofinterest,we
c
: = Cs, Rb, or K.[1,2,3] RbOs2O6 is a conventional callitcondensationenergyanalysis(CEA).Itinvolves
v s-wave superconductor with a critical temperature integrating theheat capacity to obtain the condensa-
Xi Tc=6.4K.[4,5,6,7]Inthisshortpaper,weprovideev- tion energy which is a reliable measure of the super-
idence for an additional electronic mass enhancement conductingfraction. From the systematic variation of
r
a beyondthecontributionfromthecouplingtophonons. thethermodynamic parameters on the superconduct-
Furthermore,thebasicthermodynamicparametersof ing fraction it is possible to extract the properties of
RbOs2O6 areextracted. thepuresample.[7]
According to the CEA, the superconducting elec-
tronicspecificheatCes isextractedfromthemeasure-
∗ Correspondingauthor.Tel:[phone]fax:+411633 pmeernctonbdyuCcteisn(Tg)m=asηsm−f1r∆acCtipon+,γ∆1TCp.H=erCe,0ηTm−isCt1h2eTsuis-
1072
thedifference in heat capacity between the supercon-
Emailaddress: [email protected]
(M.Bru¨hwiler). ducting and normal state, and γ1 is the Sommerfeld
PreprintsubmittedtoElsevierScience 6February2008
coefficient of pure RbOs2O6. γ1 turns out to be γ1 = Table1
79µJ/g/K2 (44mJ/molf.u./K2)andthisresultsinCes ThermodynamicparametersofRbOs2O6.
Parameter Value
showninFig.1.Thesampleshowninthefigurehasa
massfractionofRbOs2O6ofηm =74.9%,correspond- ξ,λeff(0K) 74˚A,252nm
ingtoavolumefractionof82.6%.Furtherthermody- κ(Tc),κ(0K) 23,34
namicparametersobtainedfromthemeasurementsby
∆Cp|Tc/(γ1Tc) 1.9
CEAarelistedinTable1.
After having determined the intrinsic Sommerfeld λep,λadd 1±0.1,2.1±0.3
coefficient γ1 of RbOs2O6, it is possible to analyze b/Tc (0.175±0.005)K−2
the specific heat anomaly at Tc. The normalized spe- ∆U1(0K) 860µJ/g(483mJ/molf.u.)
cific heat jump ∆Cp|Tc/(γ1Tc) = 1.9 is significantly Hc,Hc1,Hc2(0K) 1249,92,60000Oe
largerthanthatforaweakcouplingsuperconductor.It
corresponds to an electron-phonon coupling constant −dHc/dT,−dHc2/dT|Tc 369,12000Oe/K
λep =2R0∞α2F(ω)/ωdω≈1,[8],whereα2F(ω)isthe Q≡−H2cT(c0) ddHTc(cid:12)(cid:12)Tc 3.79±0.05
electron-phononspectraldensity.Thus,RbOs2O6 isa kBTc/(~ωln) 0.06
superconductorintheintermediate-couplingregime.
2∆(0K)/(kBTc) 3.87
1/(8π)·(γ1Tc2)/∆U1 0.15
γ1 79µJ/g/K2 (44mJ/molf.u./K2)
1.6
of a calculated Stoner enhancement of the magnetic
0
1.4 RbOs2O6 1.9 susceptibilityofroughly2,[9]wespeculatethatthead-
mJ/g/K) 011...802 ----4321 D(t) (%) Nb BWC.cS. T) = C/(1cp dtortiathIicneotrnsiauptlmsaerimnanhmtaarreinynt,ceserwicmseetiathnpetaprilmslloydowtudsheyeuntoCsamEstpoiAcinptprcoreoocRrpirsbeeelrOlatytisei2dosO.net6Aeefftrmomeoceitnnxsge-.
C (es0.6 0.0 0.2 0.4t20.6 0.8 1.0 the electron-phonon enhancement λep = 1 ± 0.1,
i.e. RbOs2O6 is an intermediate-coupling supercon-
0.4 ductor.Furthermore,RbOs2O6hasahighSommerfeld
Cen = 1 T coefficient for a pyrochlore of 44mJ/molf.u./K2 and
0.2 RbOs2O6 thus a remarkably large enhancement over the calcu-
0.0 lated band coefficient of 3.8 to 4.4. In addition to the
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
enhancement due to the coupling to phonons, there
T/Tc is thusan unusually high enhancement of thedensity
of states due to other mechanisms λadd = 2.1±0.3.
We speculate that the origin of this mass enhance-
Fig.1. SuperconductingelectronicspecificheatofRbOs2O6.
The normalized specific heat jump ∆Cp|Tc/(γ1Tc) = 1.9 is mentliesinthe3-dimensionaltriangularnatureofthe
significantlylargerthanthatforaweakcouplingsuperconduc- pyrochlorelattice.
tor and correspondsto an electron-phononcouplingconstant This study was partly supported by the Swiss Na-
λep=1±0.1.Theinsetshowsthedeviationofthenormalized tionalScienceFoundation.
criticalfieldHc/Hc(0)fromasimple1−t2 behavior.
With the calculated band Sommerfeld coefficient
γb = 17.8µJ/g/K2 of RbOs2O6 from Ref. [9], the
present result indicates a significant enhancement
References
of the electronic specific heat of (1+λep +λadd) =
(79.1µJ/g/K2)/(17.8µJ/g/K2) ≈ 4.4. This enhance-
[1] S.Yonezawaetal.,J.Phys.Soc.Jpn.73(2004)1655.
ment surpasses the one found in Sr2RuO4 of about
[2] S.Yonezawaetal.,J.Phys.Soc.Jpn.73(2004)819.
3.8 by 16% [10]. Additional to the electron-phonon
enhancementλep,thereisastrongenhancementofun- [3] S.Yonezawaetal.,J.Phys.:Condens.Matt.16(2004)L9.
knownoriginλadd ≈2.4.Weusethebandstructureγb [4] M.Bru¨hwileretal.,Phys.Rev.B70(2)(2004)020503(R).
from another calculation to estimate the uncertainty [5] R.Khasanovetal.,Phys.Rev.Lett.93(2004)157004.
inthisadditionalenhancement:Sanizetal.(Ref.[10])
[6] K.Magishietal.,PhysicalReviewB71(2)(2005)024524.
have calculated the band Sommerfeld coefficient for
[7] M.Bru¨hwileretal.,PhysicalReviewB71(2005)214517.
KOs2O6,whichis18%higherthantheonecalculated
inRef.[9].Assumingthese18%tobetheuncertainty [8] F.Marsiglioetal.,PhysicalReviewB35(1987)3219.
inγb forRbOs2O6 resultsinλadd ≈2.1±0.3.Inview [9] J.Kunesetal.,PhysicalReviewB70(17)(2004)174510.
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[10] R.Sanizetal.,PhysicalReviewB70(2004)100505(R).
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