Unconventional strong pinning in the low temperature phase of U_.9725Th_.0275Be_13

2 Pages·0.09 MB·English

by  Elisabeth Dumont

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Unconventional strongpinning inthelow temperaturephaseof
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U Th Be
.9725 .0275 13

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n Elisabeth Dumonta,1,Ana Celia Motaa,andJames L.Smithb
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c aLaboratorium fu¨rFestk¨orperpkysik, ETH H¨onggerberg,8093 Zu¨rich, Switzerland
- bLos Alamos National Laboratory, LosAlamos, NewMexico87545, USA
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. Abstract
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m Weinvestigated low field vortex dynamicsin a single crystal of U.9725Th.0275Be13.Wefound asharp transition
- in the vortex creep rate at the lower transition temperature Tc2, coincident with the second jump in the specific
d heat. In the high-temperature phase, rather strong creep rates are observed. In the low temperature phase, the
n ratesdrop toundetectabelylow levels. Thisbehaviourindicatesthat averystrongpinningmechanism ispresent
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in the low temperature phase of U.9725Th.0275Be13, which could be explained by the existence of domain walls,
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separating discreetly degenerate states of a superconductor, that can sustain fractional vortices and thus act as
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verystrong pinningcenters.

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2 Keywords: HeavyFermions; Superconductivity; Vortex creep
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0 If a small amount of uranium is substituted ginofthetransitionatTc2 hasbeendeducedfrom
0 for thorium in the heavy fermion superconductor the observation of a distinct increase in the slope
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UBe13, the superconducting transition tempera- ofthe lowercriticalfieldHc1[3]atTc2.
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t ture shows a non-monotonic dependence on tho- Recently, the low-field magnetic properties of
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m riumconcentrationx.Moreover,inasmallcritical theheavyfermionsuperconductorUPt3havebeen
- range 0.019 < x < 0.045 a second transition oc- investigated[4].Inthelow–field,low–temperature
d
cursbelowthe superconducting transition[1].The B–phase, bulk vortices remain so strongly pinned
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nature of this phase transition is still a matter that the creep rates drop from rather high finite
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c of controversy. In zero field µSR measurements, rates observed in the A–phase to undetectabely
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v a continuous increase of µ-spin-relaxation rate, low levels at Tc . The observation of zero initial
Xi whichsetsinatTc2hasbeenobserved[2].Thishas creepintheB–phaseofUPt3wastheninterpreted
been interpreted as originating from a very weak as originating from an intrinsic pinning mecha-
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a spontaneous magnetic field. Superconducting ori- nism in multicomponent superconducting phases.
Insuchphasesfractionalvorticesgettrappedindo-
mainwallsseparatingdiscreetlydegeneratestates
1 Corresponding author. Present address: Laboratorium
of the superconductor[5]. These experimental re-
fu¨rFestko¨rperpkysik,ETHHo¨nggerberg,CH8093Zu¨rich,
Switzerland. E-mail:[email protected] sultsshowthatvortexcreepmeasurementscanbe
Preprintsubmitted toPhysica B 6February2008
usedasapowerfulprobetogiveinformationabout inFig.2asafunctionoftemperature.AtT =Tc2,
the characterofasuperconductingphase. weobserveasharptransitionincreeprates.They
We present here relaxation measurements of drop by 3 orders of magnitude to zero within our
the remanentmagnetization on a single crystalof sensitivity (∂lnM/∂lnt ≈ 10−5). Also shown in
U.9725Th.0275Be13(size: 1.00 × 0.89 × 2.25mm3) Fig.2 are measurements of the lower critical field
prepared in Los Alamos National Laboratory[7]. Hc1 on the same crystal.Hc1 was obtained as the
Theexperimentalarrangementhasbeendescribed fieldwherethefirstdeviationfromtheinitialslope
earlier[4].Thesinglecrystalhasatransitiontem- ofisothermalmagnetization–curvesoccurs.
perature Tc1 = 523mK. Specific heat measure-
ments [6] show a second jump at Tc2 = 350mK. 0.03
Typicalrelaxationmeasurementsofthe remanent Tc2
magnetization are shown in fig. 1. All the values 40
of Mrem are taken with the sample in the fully
0.02
criticalstate. t
Oe) ∂ln
(1 M/
{ 8 Hc 20 n
1.0 125000 0.01∂l
250
300
)
4s 0.9 325
= 0 0.00
t 0 1.105 2.105 3.105
(
m T 2(mK2)
Mre 0.8 350
/ 360
) Fig. 2. Creep rates (closed circles, right axis) and
(t 370 lower critical field Hc1 (open diamonds, left axis) of
M rem 0.7 400 tUh.e97e2y5eT.h.0275Be13plotted vs T2. The lines are guides to
In conclusion,we have observed a sharp transi-
450
0.6 tioninvortexpinning inU.9725Th.0275Be13atTc2,
−
101 102 103 104 105 106 similartotheoneobservedinUPt3 atTc [4].This
t (s)
might hint towards a new type of strong pinning
structures and/or a new type of vortices being
Fig. 1. Isothermal relaxation measurements of rema-
nentMrem inU.9725Th.0275Be13.Temperaturesaregiven present in the low–T phases of these two multi-
inmKon the righthandsideof the graph. phasesuperconductors.
Weobservetwodifferentregimesofvortexcreep,
separated by a dramatic change in creep rates at References
T = Tc2. For temperatures above Tc2, we observe
strongvortexcreepandalogarithmictimedepen- [1]H.R. Ottet al.,Phys. Rev. B 31(1985) 1651.
dence of the decays. For temperatures below Tc2, [2]R.H. Heffner et al.,Phys. Rev. B 40(1989)806.
the initial vortex creep drops to zero. At longer [3]U.Rauchschwalbe et al.,Europhys. Lett. 3(1987)757.
times the decays start deviating from logarithmic [4]A.Amann etal.,Phys. Rev. B 57(1998) 3640.
behaviour.Thisaccelerationofvortexcreepatlong [5]M.Sigrist,K.Ueda, Rev. Mod.Phys. 63(1992) 239.
times has also been observed in UPt3[4]. Here we [6]A.Ramirez, privatecommunication.
only discuss the initial slope of the decays. The
[7]J.L.Smith, Phil.Mag. B,65(1992) 1367.
normalized creep rates (∂lnM/∂lnt) are plotted
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