Strongly Correlated Electron Systems

Remarkably, no signature of a magnetic transition can be seen in the pressure window pc*< p...

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Quantum criticality, Antiferromagnetism and Superconductivity G. Knebel, D. Aoki, D. Braithwaite, J.-Brison, G. Lapertot, S. Raymond, B. Salce, J. Flouquet

Usually small amounts of magnetic impurities lead to a suppression of the superconducting state in a conventional phononmediated superconductor. In contrast, in several heavy fermion superconductors it is found that superconductivity appears just at the border of a magnetically ordered state and that the attractive pairing interaction is due to critical magnetic fluctuations. For CeRhIn5 we could show by performing specific heat, electrical transport, susceptibility, and neutron scattering experiments under high pressure and magnetic field that both states coexist close to the critical point where the antiferromagnetic order is suppressed. fluctuations in the spin and but also in the charge channel. The dHvA measurements show clearly that the Fermi surface changes abruptly under pressure at pc from 4f localized to itinerant as function of pressures. Spectacularly, the application of a magnetic field in the pressure window pc*
Fig. 2: H-T phase diagram of CeRhIn5 at p = 2.4 GPa from specific heat (squares) and resisitivity (circles). The insert shows the specific heat measured at H = 7.5 T. Clearly a second anomaly appears inside the super-conducting state.

Fig. 1: a) P-T phase diagram of CeRhIn5 (H=0) from specific heat (circles), susceptibility (triangles) and resistivity measurements (stars). Above the pressure pc* the antiferromagnetic order is suppressed rapidly. The dashed line gives the extrapolation of TN to zero at the critical pressure pc. b) The specific heat and resistivity at p = 1.7 GPa where SC and AF coexist. c) Temperature variation of the peak intensity measured at the wave vector Q = (1/2, 1/2, 0.4) at 1.7 GPa on IN22 at ILL/CRG.

Clear signatures of the quantum critical point pc are the enhancement of the resistivity in the normal state and the strong enhancement of the inelastic scattering term in the resistivity, the maximum of the effective mass of the charge carriers derived from the slope of the upper critical field and also determined directly in dHvA experiments (performed at Osaka University). Furthermore, the 2 electrical resistivity shows strong deviations from a T temperature dependence due to critical quantum

The microscopic nature of the AF+SC phases below pc* and the re-entrant AF+SC phase are not determined completely. Neutron scattering experiments under high pressure performed at the ILL/CRG spectrometers IN12 and IN22 have clearly shown the existence of magnetic order inside the AF+SC state at least up to 1.7 GPa (see Fig. 1c), however, the possibility of a spatial separation of AF and SC volumes can not be excluded by these experiments as well as a change in the magnetic structure from incommensurate to commensurate inside the AF+SC phase below Tc. These correlated phenomena may explain the non BCS like phase transition at Tc below pc* in contrast to the sharp anomaly of the specific heat above pc* as well as the double anomalies inside the reentrant AF+SC domain. To clarify the situation directly by neutron scattering is a future experimental challenge.

Selected publication(s): G. Knebel et al. Phys. Rev. B 74, 020501 (R) (2006), J. Phys. Soc. Jpn. 77, 114704 (2008); S. Raymond et al Phys. Rev. B 77, 172502 (2008) Grant(s) : ANR ICENET, ECCE and NEMSICOM

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Strongly Correlated Electron Systems

The discovery of heavy fermion superconductivity in the Ce based heavy fermion family CeMIn5 (M=Co, Ir, or Rh) opened a new route to investigate unconventional forms of superconductivity. While CeCoIn5 and CeIrIn5 are superconducting at ambient pressure,CeRhIn5 is antiferromagnetically ordered below TN = 3.8 K. Under application of pressure (see Fig. 1) TN (p) reaches its maximum at 1 GPa and is monotonously suppressed for higher pressure. An extrapolation TN → 0 gives pc = 2.5 GPa. However, CeRhIn5 is also a superconductor in a wide pressure region from 1 to 5 GPa, and the superconducting transition temperature Tc is maximal at pc. At the pressure pc* = 1.95 GPa the superconducting and magnetic transition temperatures fall together, TN = Tc. Remarkably, no signature of a magnetic transition can be seen in the pressure window pc*