Physica B 326 (2003) 378–380

Unconventional superconductivity in ðTMTSFÞ2ClO4

G.M. Lukea,*, M.T. Roversa, A. Fukayab, I.M. Gatb, M.I. Larkinb, A. Savicib,Y.J. Uemurab, K.M. Kojimac, P.M. Chaikind, I.J. Leed, M.J. Naughtone

aDepartment of Physics and Astronomy, Brockhouse Institute of Materials Research, McMaster University,

Hamilton, Ont., Canada L8S 4M1bDepartment of Physics, Columbia University, New York, NY 10027, USA

cDepartment of Physics, University of Tokyo, Tokyo, JapandDepartment of Physics, Princeton University, Princeton, NJ, USA

eDepartment of Physics, Boston College, Chestnut Hill, MA 02467, USA

Abstract

We have performed detailed measurements to characterize the superconducting ðTcE1:1 KÞ state of ðTMTSFÞ2ClO4

using both normal and deuterated samples. Using ZF-mSR measurements to search for spontaneous magnetic fields

below Tc; we see no evidence for such fields within our resolution, thus we conclude the time reversal symmetry is likelynot broken. We have also performed precession measurements in the vortex state of ðTMTSFÞ2ClO4: Thesuperconducting relaxation rate decreases strongly with increasing field, which may indicate a dimensional crossover

in the nature of the vortex lattice. We estimate that the penetration depth is roughly 535 nm:r 2002 Elsevier Science B.V. All rights reserved.

Keywords: Organic superconductors; Penetration depth; Broken time reversal symmetry

The Bechgaard salts (for a general introductionsee Ref. [1]) ðTMTSFÞ2-X are quasi-one-dimen-sional organic conductors which exhibit variousinteresting properties depending on the anionmolecule X and/or the external pressure. TheXQPF6 system at ambient pressure showsmetallic conductivity above its incommensuratespin density wave (SDW) transition at T ¼ 12 K;below which it becomes insulating. The applica-tion of external pressure changes the ground statefrom SDW to superconducting (with TcB1 KÞabove 6 kbar: The XQClO4 salt in ambientpressure corresponds to the PF6 system at

6 kbar; exhibiting superconductivity belowTcB1:2 K for samples which have been slowlycooled through the anion-ordering transition.Several experiments have indicated an uncon-

ventional (non-s-wave) superconducting state inthe XQPF6 and XQClO4 systems, including theextreme sensitivity of Tc to ppm level non-magnetic impurities and the absence of a Hebel–Slichter peak in the NMR 1=T1 relaxation rate [2]which was taken as evidence of p- or d-wavepairing [3]. More recently, measurements of theupper critical field found that Hc2 for fields parallelto the conducting plane does not saturate as T-0[4] and exceeds the so-called paramagnetic (orPauli) limit where the Zeeman splitting exceeds thesuperconducting gap for an s-wave superconduc-tor (by more than a factor of 4) [5].

*Corresponding author. Tel.: +1-905-525-9140x27639; fax:

+1-905-546-1252.

E-mail address: luke@mcmaster.ca (G.M. Luke).

0921-4526/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 1 6 3 4 - 4

In the case of non-s-wave pairing, the Cooperpairs carry either spin and/or orbital angularmomentum. Depending on the symmetry of theunderlying lattice and the details of the super-conducting state, such angular momentum canproduce non-zero static magnetic fields within thesuperconductor below Tc: In this case, the super-conducting state is said to be characterized bybroken time reversal symmetry (TRS). ZF-mSR isthe most effective method to identify TRS-break-ing superconducting states due to its superbsensitivity to weak magnetic fields. TRS-breakingfields have been detected by ZF-mSR inðU;ThÞBe13 [6], in UPt3 below the lower super-conducting transition [7], and in Sr2RuO4 [8]. Wenote that not all unconventional superconductorsexhibit broken TRS; for example, no spontaneousmagnetic fields associated with Tc have beenidentified in optimally doped high temperaturesuperconductors.We mounted several hundred needle-like crys-

tals of both normal and deuterated (in separatemeasurements) ðTMTSFÞ2ClO4 with their mostconducting a-axis perpendicular to the muon beammomentum on a high resistivity GaAs singlecrystal. The b and c axes were not aligned. GaAswas chosen for sample mounting as muons landingin GaAs form muonium, which does not con-tribute to our muon time spectrum. The samplewas cooled slowly in a dilution refrigerator atthe TRIUMFM15 beamline such that it was in theanion-ordered state.In zero applied field (ZF-mSR) we observed-

temperature independent relaxation above thesuperconducting Tc in each sample reflectingthe presence of nuclear dipole moments. Thisrelaxation, though fairly strong for nuclear di-poles, was weaker in the deuterated samples;therefore, we concentrated our search for sponta-neous fields in the superconducting state to the d-ðTMTSFÞ2ClO4 crystals. We fit the relaxation to aconvolution of Gaussian and Lorentzian fielddistributions [10], with the Gaussian part heldconstant for all runs (using a value obtained froma global fit of all runs). We show the Lorentzianpart of the relaxation in Fig. 1. We see that withinour resolution ðE0:25 GÞ there is no increase inthe relaxation below Tc; therefore we find no

evidence for broken TRS superconductivity inðTMTSFÞ2ClO4: We note that the backgroundtemperature-independent relaxation due to nucleardipole fields is considerably (by roughly a factor of4) larger than that in either UPt3 or Sr2RuO4

which reduces our sensitivity to possible fields;however, we would have detected fields of the sizeseen in those systems if they were present in d-ðTMTSFÞ2ClO4:We also performed precession (TF-mSR) mea-

surements in the mixed state of d-ðTMTSFÞ2ClO4

with the external field applied perpendicular to thea-direction to measure the magnetic penetrationdepth lðTÞ: We observed an enhancement of therelaxation below Tc due to the presence of vortices;however, the relaxation was quite weak reflectingthe long penetration depth in this material. Sincethe relaxation was so weak (less than 0:4 ms�1) wecould not employ a full lineshape analysis; instead,we used a simple Gaussian form to characterizethe relaxation (where the relaxation envelope wasgiven by expð�1=2s2t2Þ).We show the temperature dependence of the

relaxation rate ðsÞ for applied fields of 9 G and50 G in Fig. 2. We see the relaxation is tempera-ture independent above TcE1:1 K and increasesroughly linearly with temperature below Tc: Thereis a saturation of the relaxation apparent in the50 G data which may also be present in the 9 Gdata. It is difficult to make a strong statementregarding the temperature dependence of the

Fig. 1. Zero field mSR relaxation rate in d-ðTMTSFÞ2ClO4

showing no increase below TcE1:1 K:

G.M. Luke et al. / Physica B 326 (2003) 378–380 379

relaxation rate since it is relatively weak at alltemperatures. From the increase in relaxation(subtracting the low temperature relaxation ratefrom the T > TC rate in quadrature) below TC of0:25 ms�1 for B ¼ 50 G; we estimate lðT-0Þ ¼535 nm:The increase in relaxation below TC is strongly

field dependent. In fact, by 180 G (not shown) wedetected no visible increase in relaxation below TC:This strong reduction of the relaxation rate mayreflect a change in the lattice morphology fromisotropic vortices at low field to increasinglydecoupled pancake vortices at higher fields as hasbeen seen in other highly anisotropic supercon-

ductors such as Bi2Sr2CaCu2O8þd [11] andðBEDT-TTFÞ2CuðSCNÞ2: Our estimate of l isconsiderably shorter than reported previously fora non-deuterated sample [12]; those previousmeasurements were presumably performed inhigher fields where we see that the relaxation rateis strongly reduced.

Research at McMaster was supported byNSERC and the Canadian Institute for AdvancedResearch, and at Columbia by NSF (DMR-01-02752, 98-02000).

References

[1] T. Ishiguro, K. Yamaji, Organic Superconductors, in:

Springer Series in Solid-State Sciences, Vol. 88, Springer,

Berlin, 1990.

[2] M. Takigawa, H. Yasuoka, G. Saito, J. Phys. Soc. Jpn. 56

(1987) 873.

[3] Y. Hasegawa, H. Fukuyama, J. Phys. Soc. Jpn. 56 (1987)

877.

[4] I.J. Lee, M.J. Naughton, G.M. Danner, P.M. Chaikin,

Phys. Rev. Lett. 78 (1997) 3555.

[5] I.J. Lee, P.M. Chaikin, M.J. Naughton, Phys. Rev. B 62

(2000) R14669.

[6] R.H. Heffner, et al., Phys. Rev. Lett. 65 (1990) 2816.

[7] G.M. Luke, et al., Phys. Rev. Lett. 71 (1993) 1466.

[8] G.M. Luke, et al., Nature 394 (1998) 561.

[10] M.I. Larkin, et al., Physica B 289–290 (2000) 153.

[11] S.I. Lee, et al., Phys. Rev. Lett. 75 (1995) 922.

[12] D.R. Harshman, A.P. Mills Jr., Phys. Rev. B 45 (1992)

10684.

Fig. 2. TF-mSR relaxation rate in d-ðTMTSFÞ2ClO4 in applied

fields of 9 G (squares) and 50 G (triangles) showing the effects

of the vortex lattice below TcE1:1 K:

G.M. Luke et al. / Physica B 326 (2003) 378–380380