We report the ac magnetic susceptibility ac and resistivity measurements of EuFe 2 As 2 under high pressure P. By observing nearly 100% superconducting shielding and zero resistivity at P ¼ 28 kbar, we establish that P-induced superconductivity occurs at T c $ 30 K in EuFe 2 As 2 . shows an anomalous nearly linear temperature dependence from room temperature down to T c at the same P. ac indicates that an antiferromagnetic order of Eu 2þ moments with T N $ 20 K persists in the superconducting phase. The temperature dependence of the upper critical field is also determined.KEYWORDS: iron pnictides, pressure-induced superconductivity, susceptibility, upper critical field DOI: 10.1143/JPSJ.78.083701The discovery of superconductivity (SC) at a transition temperature T c ¼ 26 K in LaFeAsO 1Àx F x by Kamihara et al.1) has triggered extensive studies of SC in layered iron pnictides and related compounds. Rotter et al. found that BaFe 2 As 2 with a simpler structure can be made superconducting by doping:2) Perhaps more importantly, it is reported that 122 compounds of the form AFe 2 As 2 (A ¼ Ca, Sr, Ba, and Eu) can be tuned to SC by the application of high pressure P. 3-10)P tuning can provide opportunities to determine the nature of the iron-pnictide high-temperature SC without being adversely affected by disorder due to doping. However, most of these reports are based only on resistivity measurements and hence cannot establish the bulk nature of P-induced SC.11) Even when magnetic measurements are reported, results are not conclusive: In ref. 5, magnetic measurements were performed on SrFe 2 As 2 and BaFe 2 As 2 , but the observed volume fraction was expressed in arbitrary units. In ref. 9, the volume fraction of the P-induced superconducting phase of CaFe 2 As 2 was estimated to be at least 50%, while in ref. 12 CaFe 2 As 2 was reported not to exhibit SC under hydrostatic P produced by the use of helium as a pressure-transmitting medium.EuFe 2 As 2 exhibits two phase transitions, at T o $ 190 K and T N $ 19 K, at ambient P.13-16) The transition at T o is a combined structural and magnetic transition, similar to those in the other 122 compounds: the crystal structure changes from tetragonal to orthorhombic and the Fe 2þ moments order antiferromagnetically. The transition at T N is due to the antiferromagnetic (AFM) ordering of the Eu 2þ moments. The AFM coupling of the Eu 2þ moments is rather weak: the field-induced paramagnetic state with a saturated moment of $7 B /Eu is easily reached by the application of $1 or 2 T in the ab-plane or along the c-axis, respectively. 17) A temperature (T)-P phase diagram has been determined from measurements: 10) while T o decreases with P and is not detected above P ¼ 23 kbar, T N is nearly P-independent up to 26 kbar (the highest P in ref. 10). The authors of ref. 10 state that P-induced SC at T c $ 30 K occurs above 20 kbar. However, their data (at P ¼ 21:6 kbar) shows only a partial drop and approximately half of the normal-state appears to remain as T ! 0. Obviously, further e...

We report resistivity measurements performed on KFe 2 As 2 single crystals down to T = 0.3 K and in magnetic fields up to 17.5 T. The in-plane resistivity vs. T curve has a convex shape down to ∼50 K and shows a T 2 dependence below ∼45 K. The ratio of the c-axis to in-plane resistivities is ∼10 at room temperature and ∼40 at 4.2 K. The superconducting

We report on a band structure calculation and de Haas-van Alphen measurements of KFe 2 As 2 . Three cylindrical Fermi surfaces are found. Effective masses of electrons range from 6 to 18m e , m e being the free electron mass. Remarkable discrepancies between the calculated and observed Fermi surface areas and the large mass enhancement (&3) highlight the importance of electronic correlations in determining the electronic structures of iron pnicitide superconductors. The discovery of superconductivity at T c ¼ 26 K in LaFeAs (O,F) 1) has given rise to intense experimental and theoretical efforts to elucidate the superconducting pairing mechanism and symmetry in iron pnictide superconductors (see ref. 2 for a recent review). Since the development of realistic theories of the mechanism requires detailed knowledge of the Fermi surface (FS), experimental determination of the FS is highly desirable.Accordingly, many angle-resolved photoemission spectroscopy (ARPES) studies have been performed.2) Their results show some level of agreement in the FS and band dispersion with conventional band structure calculations and moderate mass renormalization due to many-body effects. On the other hand, measurements of de Haas-van Alphen (dHvA) or other quantum oscillations, which are bulk probes and allow accurate determination of the FS cross sections and effective masses m à , are rather limited. dHvA measurements performed on the FeP compounds LaFePO 3,4) and SrFe 2 P 2 5) have shown that band shifts of up to $0:1 eV are necessary to bring band structure calculations into agreement with experiments and that the enhancement of effective masses over band ones is about two. Since high T c 's are found only in FeAs compounds, dHvA studies of FeAs compounds are more desired. However, because of the structural/magnetic phase transitions, measurements on the alkaline-earth 122 parent compounds AFe 2 As 2 (A ¼ Ca, Sr, and Ba) [6][7][8] have observed only small FS pockets, which makes it difficult to draw an overall picture of the electronic structures of these compounds. Very recently, dHvA measurements have been performed on BaFe 2 (As 1Àx P x ) 2 for 0:41 x 1. 9) As one goes from x ¼ 1 to 0.41, where T c $ 25 K, the electron FS's shrink and the mass enhancement factor increases from $2 to $4.KFe 2 As 2 is an end member of the high-T c binary alloy (Ba 1Àx K x )Fe 2 As 2 with the ThCr 2 Si 2 structure and has T c $ 3 K.10,11) The low-temperature resistivity exhibits a clear T 2 dependence with a large coefficient of A ¼ 0:026 m cm/K 2 , 12) and specific heat measurements have found correspondingly large Sommerfeld coefficients: exp ¼ 69 or 93 mJ/(K 2 Ámol-f.u.) (f.u. = formula unit) for poly or single crystals, respectively. 13,14) These indicate the existence of moderately large electron correlations.75 As nuclear quadrupole resonance measurements have shown that spin fluctuations (SF's) are much suppressed (compared with the optimally doped compound).13) The first ARPES measurement 15) found and hole FS's at the À point in the Bri...

In a recent Letter [1], Dong et al. have observed a T 1.5 dependence of resistivity ρ in KFe 2 As 2 at the upper critical field B c2 = 5 T parallel to the c axis and have suggested the existence of a field-induced quantum critical point (QCP) at B c2 . The value of B c2 = 5 T was determined from the onset of a resistive transition. It is much higher than a perviously reported value of B c2 = 1.25 T [2], which was determined from a midpoint. The origin of the large difference in B c2 can be attributed to broad transitions (see the inset of Fig. 1 of Ref. 1, the ρ(T ) curve at zero field shows the onset at 4.8 K and zero resistivity at 2.5 K), which indicate distribution of the transition temperature and B c2 due to inhomogeneity in the sample. Since the observed exponent 1.5 is some kind of average over the inhomogeneous sample, it is questionable to relate it to a QCP. To clarify this, let us assume that some portion of a sample becomes quantum critical at B c2 = 5 T and that ρ in that portion varies as T 1.5 . Since B c2 for the rest of the sample is below 5 T, the rest is in the Fermi liquid regime and exhibits a T 2 dependence of ρ. Then, one can not expect ρ measured across the sample varies as T 1.5 .We substantiate below the argument against the proposed QCP, mentioning data obtained for high-quality single crystals with resistivity ratios of up to ∼600, which exhibit de Haas-van Alphen (dHvA) oscillations [3].

We report measurements of the de Haas-van Alphen (dHvA) oscillation and a band structure calculation for the pnictide superconductor BaNi 2 P 2 , which is isostructural to BaFe 2 As 2 , the mother compound of the iron-pnictide high-T c superconductor (Ba 1Àx K x )Fe 2 As 2 . Six dHvA-frequency branches with frequencies up to $8 kT were observed, and they are in excellent agreement with results of the band-structure calculation. The determined Fermi surface is large, enclosing about one electron and hole per formula unit, and three-dimensional. This is in contrast to the small two-dimensional Fermi surface expected for the iron-pnictide high-T c superconductors. The mass enhancement is about two.KEYWORDS: Fermi surface, BaNi 2 P 2 , de Haas-van Alphen effect, electronic structure, pnictide superconductor DOI: 10.1143/JPSJ.78.033706The discoveries of superconductivity in LaFePO (3) by Kamihara, Watanabe, Hosono, and coworkers have initiated enthusiastic search for superconductivity in layered iron and nickel pnictides. The superconducting transition temperature T c has quickly been raised to 43 K by the application of pressure 4) and to 54 -56 K by the use of high-pressure synthesis and/or rare-earth/actinoid substitution.5-8) Different crystal structures have also been found to be compatible with this new type of high-temperature superconductivity:10) and LiFeAs (T c ¼ 18 K), 11) for example, although -FeSe is not a pnictide. The key ingredient of the high-temperature superconductivity seems to be the twodimensional square lattice of Fe 2þ ions. Since iron and nickel are archetypal magnetic elements and parent compounds, such as LaFeAsO and BaFe 2 As 2 , exhibit magnetic transitions, it is suggested that the paring mechanism is of magnetic origin.It is interesting to determine why the combination of Fe and As is unique. Among the four possible combinations, i.e., (Fe, Ni) Â (P, As), only the FeAs combination gives high T c 's. To address this issue, we have initiated a fermiology study of iron and nickel pnictides, and here, we report the first results, namely, the results of de Haasvam Alphen (dHvA) measurements and a band-structure calculation performed for BaNi 2 P 2 . BaNi 2 P 2 crystallizes in the ThCr 2 Si 2 structure as BaFe 2 As 2 and becomes superconducting below T c $ 3 K. 12,13) BaNi 2 P 2 single crystals were prepared by high-pressure synthesis from the constituent elements: the synthesis temperature and pressure were 1250 C and 1 GPa, respectively. The typical in-plane residual resistivity, residual resistivity ratio (RRR), and T c of the crystals have been reported to be 5 m cm, 18, and 2.51 K, respectively.14) The dHvA torque measurements were performed on five singlecrystal samples (samples #1, #2, #11, WM, and HM) with typical dimensions of 0:4 Â 0:2 Â 0:02 mm 3 by using piezoresistive microcantilevers. The crystal axes of samples #2 and WM were determined using X-ray diffractometry with a three-circle goniometer combined with a CCD area detector. For the other samples, the axes were identified b...