Consistence of spin fluctuations and superconductivity in UPt3

it,It, STI.~WAR I', ~, IqSK, ,LO, WII~I~IS ~Itd J,L, SMIT]'I Shwe qur rofx~ll ,r,n th~ suporcanda~ctivily ot t.JPq:, [ 1], whig:lib was ¢~,lrcatly known m he a qandi~ dat~ for I@in-l]~ctuath, ms [2], other wnrkt~ts h~w~ f{~nfir~lles.~ ~hos¢ lesalls ~ad tlltenstlf¢~d the $tlpot, ~:~mdtulclillg criti,~al field t:3,4, 5], l~l:ert~sl is high in UPt:~ because it has joined CeCuzSiz arid UBel~ il~, the slnall l'arnily of h~avy termi~lt su~e~'~:onduc~o~'s [6], wllid~ have electron mass ~;uha~,cen~ents of about 20{1, bill also because of its .diff;~rencc~s fn:,m them, particularly tl~e T n tn T ~e~'m in it:, heat capacity. Speculalk>n is ramtnant o,~ the p~,~.,~ibility of a triplet pair,~d supercon- dueling i~ta m in all of these compounds ['7. 8], For a recent revi~r w of heavy fermion systems, see r~[. [9], However, .IJPt3 is unique in the similarity of its heat capacity to':hat 6f SHe at much lower temperatures, which may make it the mast likely candidate for p-state pairing [10]. Whatever the outcome of this partict,lar speculation, the extreme properties of these corapounds will r~main a sovrce of con- Iroversy for a whi]le. For thi:; reason we report here on an exte nsion of our earlier heat capacity work [ 1 ] to heat capacity in an :applied magnetic field of 1 ] T and on magaetoresimance measurements. F~g. I sho~vs the effect of the applied magnetic field on the heat capacity of UPb. Above about 7 K... the da~a are ¢tmsistent with the point of view that sp~n fluctuations are depressed by lhe field. This is !he same as the results on UA]~ [l ]] and l'iBe, [ 12], which are the only previously known compounds that showed a T 3 in T term in their heat capacities. At k~wer temperatm'es the two data sets cross, l! is not clear how ~his might be inlerpretcd. However, two pp.{sis should be rondo, In lhe iteighhodtc~od of ? K the th~rma'L o,torgy and lhe magnoti~; 0n~rgy el: the ~le~tron~ in I1 T am c~mpnrahl~, wht0h ob,~cm.~s a ,spin,, flu~:tuatinlt interpretation, S~col~d, unlll thc~re arc candidates for a proper description (including anisotrol,yl o~[ this Fermi liquid stale that the h0al capaoity ,~hows is developing below about 10 K, we can only say that it is sol ~tne.xpe~ted to observe that afi¢td modifies that state, ~g, 2 shows the off~ct of applied magn0tic fields on the electrical resistivity of U P h at 1,2 K as measured wi~h a standard 4-1end, ac techni- que, As in the heat capacity case, them is little theoretical guidance, but the data may he com- pared to those for related compounds. For UAI.,. the power law for the field has exponents of 1..45 ( 2 T to 15T} and 1.3 (above 15T) [~3]. We measured an exponent ef 1.25 ( 2 T to l l T ) which suggests either that UPt3 has a larger pat'arnagnon contribution to the resL,~tivity than does UAI= or that there are band structt~re effects. In co,ltrast, the other heavy fermion superconductors show a negative magnetoresis- tance. For the same temperature and field UBe,.~ shows a --34% change and CeCu2Si~ shows ap- proximately a - 4 . 5 % change, [14] compared to the UPt:4 results in fig. 2 of +41%. So mag- netoresistance suggests that UPt3 is more fike the other spin fluctuators ~than like the other heavy fermion superconductors. The results shown in the figures confirm that UPt~ is unique amongst spin fluctuators and heavy ferrnion superconductors in that it is a member of both small groups. Because these 0378-4363/84/~;03.00 (~ El:~evier Science Publishers B.V. (North-Holland Physics Publishing'. Division)