The upper critical field H(c2)(T) of the multiband superconductor KFe2As2 has been studied via low-temperature thermal expansion and magnetostriction measurements. We present compelling evidence for Pauli-limiting effects dominating H(c2)(T) for H || a, as revealed by a crossover from second- to first-order phase transitions to the superconducting state in the magnetostriction measurements down to 50 mK. Corresponding features were absent for H || c. To our knowledge, this crossover constitutes the first confirmation of Pauli limiting of the H(c2)(T) of a multiband superconductor. The results are supported by modeling Pauli limits for single-band and multiband cases.

The low-temperature thermal expansion of CeCoIn5 single crystals measured parallel and perpendicular to magnetic fields B oriented along the c axis yields the volume thermal-expansion coefficient β. Considerable deviations of β(T ) from Fermi-liquid behavior occur already within the superconducting region of the (B, T ) phase diagram and become maximal at the upper critical field B 0 c2 . However, β(T ) and the Grüneisen parameter Γ are incompatible with a quantum critical point (QCP) at B 0 c2 , but allow for a QCP shielded by superconductivity and extending to negative pressures for B < B 0 c2 . Together with literature data we construct a tentative (p, B, T ) phase diagram of CeCoIn5 suggesting a quantum critical line in the (p, B) plane.

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Superconductors close to quantum phase transitions often exhibit a simultaneous increase of electronic correlations and superconducting transition temperatures. Typical examples are given by the recently discovered iron-based superconductors. We investigated the band-specific quasiparticle masses of AFe2As2 single crystals with A = K, Rb, and Cs and determined their pressure dependence. The evolution of electronic correlations could be tracked as a function of volume and hole doping. The results indicate that with increasing alkali-metal ion radius a quantum critical point is approached. The critical fluctuations responsible for the enhancement of the quasiparticle masses appear to suppress the superconductivity.PACS numbers: 74.70. Xa,71.18.+y,74.40.Kb,71.27.+a Unconventional superconductivity (SC) often emerges in the proximity of continuous, zero-temperature phase transitions, so-called quantum critical points (QCPs). In particular, the onset of magnetic order is generally believed to drive SC by magnetic quantum criticality. Examples encompass the cuprates, organic metals, heavyfermion systems, and the recently discovered iron-based superconductors. A particularly illustrative example is given by BaFe 2 (As 1−x P x ) 2 . Here, the application of chemical pressure, by replacing As with isovalent, smaller P ions, suppresses antiferromagnetic (AF) order resulting in an extended superconducting dome with a maximum transition temperature T c ≈ 30 K at the critical concentration x c = 0.3 [1]. The QCP at x c shielded by SC was anticipated theoretically [2] and observed through strongly enhanced quasiparticle masses and deviations from Fermi-liquid (FL) behavior. In this Letter we show that the isostructural superconductor KFe 2 As 2 can likewise be pushed towards a QCP by substituting isovalent Rb and Cs for K. In these compounds, in contrast to the examples listed above and despite general consensus, the proximity to a QCP appears to suppress SC.The alkali metal series AFe 2 As 2 (A122) with A = K, Rb, and Cs represents one of the rare examples of stoichiometric iron-arsenide superconductors. According to LDA calculations their low T c values of less than 3.5 K cannot be explained by electron-phonon coupling. Angleresolved photoemission spectroscopy and thermal conductivity measurements suggest an unconventional pairing mechanism [3][4][5][6]. Recent specific-heat measurements reveal huge Sommerfeld coefficients γ which exceed those of BaFe 2 (As 1−x P x ) 2 in apparent contradiction to the low

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