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METALS, SUPERCONDUCTORS ... 2009Field Evolution of Coexisting Superconductingand Magnetic Orders in CeCoIn 5Heavy fermion compound CeCoIn 5 is one of the most intriguingexamples of a manifestation of the coexistence ofmagnetic and superconducting (SC) orders. In the SC stateof this compound application of a magnetic field (H 0 ) inducesa long range magnetic order (LRO), restricted to asmall high-field low-temperature region of the phase diagramjust below H c2 . What is more, this particular region ofthe phase diagram was initially identified as the first realizationof the long-sought Fulde, Ferrell, Larkin, and Ovchinnikov(FFLO) state, a superconducting state with a nonzeropair momentum and a spatially modulated order parameter.However, important questions regarding the truenature of this SC phase, the details of the magnetic orderand its field dependence, and the potential driving mechanismsof their coexistence remain unanswered. Therefore,CeCoIn 5 provides a strikingly rich ground to study thecomplex interplay between exotic SC and magnetic degreesof freedom. Experimentally, nuclear magnetic resonance(NMR), as a microscopic probe sensitive to both magneticand SC degrees of freedom, provides a powerful tool for theinvestigation of these puzzles.Here we report detailed low temperature (T ) 115 In NMRmeasurements on the three distinct In sites in CeCoIn 5 , forH 0 ||[100] [Koutroulakis et al., Phys. Rev. Lett. in press(arXiv:0912.3548)]. The axially symmetric In(1) is locatedin the center of the tetragonal Ce planes, while In(2 ac ) andIn(2 bc ) sites correspond to In atoms located on the lateralfaces (parallel and perpendicular to the applied field, respectively)of the unit cell. In figure 83 the H 0 evolutionof the In(2 ac ) spectra at T ≈70 mK is plotted. Loweringthe field below ≈ 11.7 T establishes magnetic LRO, whichis evident from the broadening of the In(2 ac ) line into aspectrum with two extrema/peaks and finite signal weightin between them. Such spectra are characteristic of incommensurate(IC) LRO along one spatial dimension. For9.2T H 0 10.2 T, the spectra of all In sites consist of asingle peak, i.e. no signature of the IC state is observed.However, these spectra remain significantly broader thanthe ones for H 0 9.2 T, where the linewidth of all sites canbe adequately described by the spatial distribution of magneticfields resulting from the vortex lattice of an Abrikosov(low-field) SC state (lfSC).By NMR we have thus established that at T ≈ 70mK aphase with static magnetic LRO is stabilized for fieldsabove ≈ 10.2T in the SC state. By analyzing the spectraof the different In sites, we deduce that the LRO is an incommensuratespin density wave (IC-SDW) with momentsoriented along the ĉ-axis, independent of the in-plane H 0orientation. We fit the data to simulated spectra for the IC-SDW order with magnitude of the local magnetic moment(µ 0 ) as a fitting parameter. This allows us to map the detailedfield evolution of the moment. From the analysis ofthe field dependence of the NMR shift we show that this IC-SDW coexists with a novel SC state, that is likely an FFLOphase, characterized by an enhanced spin susceptibility.Figure 83: Low temperature NMR spectra of In(2 ac ) for variousH 0 ‖â. The frequency scale is defined by subtracting ω 0 , the zeroNMR shift frequency. N denotes the normal phase, IC the LROphase, eSC the state with strong spin fluctuations, and lfSC theAbrikosov SC state. The LRO is evident in the fact that the In(2 ac )line broadens into a spectrum with two extrema. Such spectra arecharacteristic of IC LRO along one spatial dimension. Red solidlines are simulated spectra for the IC-SDW order.By consideration of the spectral lineshapes and spin decoherencetime of different In sites for 9.2T H 0 10.2Tat low-T , we conclude that this corresponds to an ‘exotic’SC (eSC) phase characterized by strong AF fluctuations.This might be a ‘true FFLO’ phase, unperturbed by LROmagnetic order. This phase is separated from the above describedhigh-field, low-T phase by the 2 nd order phase transitionpreviously identified at H ∗ ≈ 10.2 T from the NMRlineshift data. Such a two step phase transition from lfSCto FFLO (assuming it exists in eSC) and then to the IC statewas theoretically predicted when spin fluctuations are considered.The IC magnetism can arise in the FFLO state ina d-wave SC as a consequence of the formation of Andreevbound states near the zeros of the FFLO order parameter.The formation of the IC LRO is triggered by a large DOSin these bound states.M. Horvatić, C. BerthierG. Koutroulakis, V. F. Mitrović (Brown University, Providence, U.S.A.), G. Lapertot, J. Flouquet (INAC, SPSMS, CEAGrenoble, France)62