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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Electronic nematicity, a correlated state that spontaneously breaks rotational symmetry, is observed in several layered quantum materials. In contrast to their liquid-crystal counterparts, the nematic director cannot usually point in an arbitrary direction (XY nematics), but is locked by the crystal to discrete directions (Ising nematics), resulting in strongly anisotropic fluctuations above the transition. Here, we report on the observation of nearly isotropic XY-nematic fluctuations, via elastoresistance measurements, in hole-doped Ba1−xRbxFe2As2iron-based superconductors. While forx=0, the nematic director points along the in-plane diagonals of the tetragonal lattice, forx=1, it points along the horizontal and vertical axes. Remarkably, for intermediate doping, the susceptibilities of these two symmetry-irreducible nematic channels display comparable Curie–Weiss behavior, thus revealing a nearly XY-nematic state. This opens a route to assess this elusive electronic quantum liquid-crystalline state.

We present electrical resistivity and ac-susceptibility measurements of GdTe3, TbTe3 and DyTe3 performed under pressure. An upper charge-density-wave (CDW) is suppressed at a rate of dTCDW,1/dP ∼ −85 K/GPa. For TbTe3 and DyTe3, a second CDW below TCDW,2 increases with pressure until it reaches the TCDW,1(P ) line. For GdTe3, the lower CDW emerges as pressure is increased above ∼ 1 GPa. As these two CDW states are suppressed with pressure, superconductivity (SC) appears in the three compounds at lower temperatures. Ac-susceptibility experiments performed on TbTe3 provide compelling evidence for bulk SC in the low-pressure region of the phase diagram. We provide measurements of superconducting critical fields and discuss the origin of a high-pressure superconducting phase occurring above 5 GPa.

In the antiferromagnetic (AF) heavy-fermion system CePdAl the magnetic Ce ions form a network of equilateral triangles in the (001) plane, similar to the kagomé lattice, with one third of the Ce moments not participating in the long-range order. The Néel temperature TN = 2.7 K is reduced upon replacing Pd by Ni in CePd1−xNixAl, with TN → 0 for x = 0.144, where the specific heat C exhibits a C/T ∝ − log T dependence. Within the Hertz-Millis-Moriya model of quantum criticality, this behavior might indicate 2D critical antiferromagnetic fluctuations arising from the decoupling of 3D magnetic order by frustration. On the other hand, the simultaneous presence of Kondo effect and geometric frustration might entail a new route to quantum criticality.