Competitive coexistence of superconductivity with antiferromagnetism in CeRhIn5

We carried out ac magnetic susceptibility measurements under pressures P on the heavy fermion antiferromagnet CeRhIn5. We report bulk superconductivity (SC) at ambient pressure with a transition temperature Tc approximately or equal to 90 mK. The degraded SC in a powdered or polished sample was restored by annealing, showing that the SC state is sensitive to inhomogeneity. In a coexistence region of the SC with antiferromagnetism (AF), we find that Tc(P)(n)TN(P)(1-n) = const where TN indicates a Néel temperature and n denotes a ratio of electronic specific heat coefficients below and above TN, indicating the competition of the SC and the AF for states at the Fermi surface.     

Spin-triplet superconductivity in UNi(2)Al(3) revealed by the (27)Al Knight shift measurement

We report (27)Al Knight shift ( (27)K) measurement on a single-crystal UNi(2)Al(3) that reveals a coexistence of superconductivity and a spin-density-wave (SDW) type of magnetic ordering ( T(SDW) = 4.5 K). The spin part of (27)K, (27)K(s), does not change down to 50 mK across the superconducting (SC) transition temperature T(c) approximately 0.9 K. In contrast with the isostructural compound UPd(2)Al(3) ( T(c) approximately 2 K), which was identified to be a spin-singlet d-wave superconductor, the behavior of (27)K strongly supports that UNi(2)Al(3) , like UPt(3) and Sr(2)RuO(4), belongs to a class of spin-triplet SC pairing state superconductors.     

Angle-resolved photoemission study of the cobalt oxide superconductor Na(x)CoO(2) x yH(2)O: observation of the Fermi surface

The cobalt oxide superconductor Na(x)CoO(2) x yH(2)O is studied by angle-resolved photoemission spectroscopy. We report the Fermi surface (FS) topology and electronic structure near the Fermi level (E(F)) in the normal state of Na(x)CoO(2) x yH(2)O. Our result indicates the presence of the hexagonal FS centered at the Gamma point, while the small pocket FSs along Gamma-K direction are absent, similar to Na(x)CoO(2). The top of the e(g)(') band, which is expected in band calculations to form the small pocket FSs, extends to within approximately 30 meV below E(F), closer to E(F) than in Na(x)CoO(2). We discuss its possible role in superconductivity, comparing with other experimental and theoretical results.     

Magnetoresistivity and filamentary superconductivity in nickel-doped BaFe_2As_2

We present magnetotransport studies on a series of BaFe_(2-x)Ni_xAs_2(0.03 ≤ x ≤ 0.10) single crystals. In the underdoped(x = 0.03) non-superconducting sample, the temperature-dependent resistivity exhibits a peak at 22 K, which is associated with the onset of filamentary superconductivity(FLSC). FLSC is suppressed by an external magnetic field in a manner similar to the suppression of bulk superconductivity in an optimally-doped(x = 0.10) compound, suggesting the same possible origin as the bulk superconductivity. Our magnetoresistivity measurements reveal that FLSC persists up to the optimal doping and disappears in the overdoped regime where the long-range antiferromagnetic order is completely suppressed, pointing to a close relation between FLSC and the magnetic order.     

Unconventional superconductivity and electron correlations in the cobalt oxyhydrate Na0.35CoO2.yH2O from nuclear quadrupole resonance

We report a careful 59Co nuclear quadrupolar resonance measurement on the recently discovered cobalt oxyhydrate Na0.35CoO2.yH(2)O superconductor from T=40 K down to 0.2 K. We find that in the normal state the spin-lattice relaxation rate 1/T(1) follows a Curie-Weiss type temperature (T) variation, 1/T(1)T=C/(T-theta), with theta=-42 K, suggesting two-dimensional antiferromagnetic spin correlations. Below T(c)=3.9 K, 1/T(1) decreases with no coherence peak and follows a T(n) dependence with n approximately 2.2 down to approximately 2.0 K but crosses over to a 1/T(1) proportional to T variation below T=1.4 K, which suggests non-s-wave superconductivity. The data in the superconducting state are most consistent with the existence of line nodes in the gap function.     

Commensurate spin dynamics in the superconducting state of an electron-doped cuprate superconductor

We report neutron scattering studies on two single crystal samples of the electron-doped (n-type) superconducting (SC) cuprate Nd2-xCexCuO4 (x=0.15) with T(c)=18 and 25 K. Unlike the hole-doped (p-type) SC cuprates, where incommensurate magnetic fluctuations commonly exist, the n-type cuprate shows commensurate magnetic fluctuations at the tetragonal (1/2 1/2 0) reciprocal points both in the SC and in the normal state. A spin gap opens up when the n-type cuprate becomes SC, as in the optimally doped p-type La2-xSrxCuO4. The gap energy, however, increases gradually up to about 4 meV as T decreases from T(c) to 2 K, which contrasts with the spin pseudogap behavior with a T-independent gap energy in the SC state of p-type cuprates.     

Doping dependent evolution of magnetism and superconductivity in Eu(1-x)K(x)Fe2As2 (x = 0-1) and temperature dependence of the lower critical field H(c1)

We have synthesized polycrystalline samples of Eu(1-x)K(x)Fe2As2 (x = 0-1) and carried out systematic characterization using x-ray diffraction, ac and dc magnetic susceptibility, and electrical resistivity measurements. A clear signature of the coexistence of a superconducting transition (T(c) = 5.5 K) with spin density wave (SDW) ordering is observed in our underdoped sample with x = 0.15. The SDW transition disappears completely for the x = 0.3 sample and superconductivity arises below 20 K. The superconducting transition temperature Tc increases with increase in the K content and a maximum Tc = 33 K is reached for x = 0.5, beyond which it decreases again. The doping dependent Tx phase diagram is extracted from the magnetic and electrical transport data. It is found that magnetic ordering of Eu moments coexists with the superconductivity up to x = 0.6. The isothermal magnetization data taken at 2 K for the doped samples suggest the 2+ valence state of the Eu ions. We also present the temperature dependence of the lower critical field H(c1) of the superconducting polycrystalline samples. The values of H(c1)(0) obtained for x = 0.3, 0.5, and 0.7 after taking the demagnetization factor into account are 202, 330, and 212 Oe, respectively. The London penetration depth λ(T) calculated from the lower critical field does not show exponential dependence at low temperature, as would be expected for a fully gapped clean s-wave superconductor. In contrast, it shows a T2 power law feature up to T = 0.3Tc, as observed in Ba(1-x)K(x)Fe2As2 and BaFe(2-x)Co(x)As2.     

Electrical transport properties of Fe Se single crystal under high magnetic field

Understanding the normal electronic state is crucial for unveiling the mechanism of unconventional superconductivity(SC). In this paper, by applying a magnetic field of up to 37T on FeSe single crystals, we could reveal the normal-state transport properties after SC was completely suppressed. The normal-state resistivity exhibited a Fermi liquid behavior at low temperatures. Large orbital magnetoresistance(MR) was observed in the nematic state with H//c, whereas MR was negligible with H//ab. The magnitude of the orbital MR showed an unusual reduction, and Kohler's rule was severely violated below 10-25 K;these were attributable to spin fluctuations. The results indicated that spin fluctuations played a paramount role in the normalstate transport properties of FeSe albeit the Fermi liquid nature was at low temperature.     

Carrier-concentration dependence of the pseudogap ground state of superconducting Bi₂Sr(₂-x)La(x)CuO(₆+δ) revealed by ⁶³,⁶⁵Cu-nuclear magnetic resonance in very high magnetic fields

We report the results of the Knight shift by ?3,??Cu-NMR measurements on single-layered copper-oxide Bi?Sr(?-x)La(x)CuO(?+δ) conducted under very high magnetic fields up to 44 T. The magnetic field suppresses superconductivity completely, and the pseudogap ground state is revealed. The ?3Cu-NMR Knight shift shows that there remains a finite density of states at the Fermi level in the zero-temperature limit, which indicates that the pseudogap ground state is a metallic state with a finite volume of Fermi surface. The residual density of states in the pseudogap ground state decreases with decreasing doping (increasing x) but remains quite large even at the vicinity of the magnetically ordered phase of x ≥ 0.8, which suggests that the density of states plunges to zero upon approaching the Mott insulating phase.     

Magnetism and superconductivity in Eu0.2Sr0.8(Fe0.86Co0.14)2As2 probed by 75As NMR

We report bulk superconductivity (SC) in Eu(0.2)Sr(0.8)(Fe(0.86)Co(0.14))(2)As(2) single crystals by means of electrical resistivity, magnetic susceptibility and specific heat measurements with T(c) is approximately equal to 20 K and an antiferromagnetic (AFM) ordering of Eu(2+) moments at T(N) is approximately equal to 2.0 K in zero field. (75)As NMR experiments have been performed in the two external field directions (H is parallel to ab) and (H is parallel to c). (75)As-NMR spectra are analysed in terms of first-order quadrupolar interaction. Spin-lattice relaxation rates (1/T(1)) follow a T(3) law in the temperature range 4.2-15 K. There is no signature of a Hebel-Slichter coherence peak just below the SC transition, indicating a non-s-wave or s(±) type of superconductivity. In the temperature range 160-18 K 1/T(1)T follows the C/(T+θ) law reflecting 2D AFM spin fluctuations.     

Antiferromagnetic spin fluctuations above the dome-shaped and full-gap superconducting states of LaFeAsO1-xFx revealed by (75)As-nuclear quadrupole resonance

We report a systematic study by (75)As nuclear-quadrupole resonance in LaFeAsO(1-x)F(x). The antiferromagnetic spin fluctuation found above the magnetic ordering temperature T(N) = 58 K for x = 0.03 persists in the regime 0.04 ≤ x ≤ 0.08, where superconductivity sets in. A dome-shaped x dependence of the superconducting transition temperature T(c) is found, with the highest T(c) = 27 K at x = 0.06, which is realized under significant antiferromagnetic spin fluctuation. With increasing x further, the antiferromagnetic spin fluctuation decreases, and so does T(c). These features resemble closely the cuprates La(2-x)Sr(x)CuO(4). In x = 0.06, the spin-lattice relaxation rate (1/T(1)) below T(c) decreases exponentially down to 0.13T(c), which unambiguously indicates that the energy gaps are fully opened. The temperature variation of 1/T(1) below T(c) is rendered nonexponential for other x by impurity scattering.     

Fermi-liquid ground state in the n-type Pr(0.91)LaCe(0.09)CuO(4-y) copper-oxide superconductor

We report nuclear magnetic resonance studies on the low-doped n-type copper-oxide Pr(0.91)LaCe(0.09)CuO(4-y) (T(c)=24 K) in the superconducting state and in the normal state uncovered by the application of a strong magnetic field. We find that when the superconductivity is removed the underlying ground state is the Fermi liquid state. This result is at variance with that inferred from previous thermal conductivity measurement and appears to contrast with that in p-type copper oxides with a similar doping level where high-T(c) superconductivity sets in within the pseudogap phase. The data in the superconducting state are consistent with the line-node gap model.     

Normal-state hourglass dispersion of the spin excitations in FeSexTe(1-x)

We use cold neutron spectroscopy to study the low-energy spin excitations of superconducting (SC) FeSe0.4Te0.6 and essentially nonsuperconducting (NSC) FeSe0.45Te0.55. In contrast with BaFe2-x(Co,Ni)xAs2, where the low-energy spin excitations are commensurate both in the SC and normal state, the normal-state spin excitations in SC FeSe0.4Te0.6 are incommensurate and show an hourglass dispersion near the resonance energy. Since similar hourglass dispersion is also found in the NSC FeSe0.45Te0.55, we argue that the observed incommensurate spin excitations in FeSe(1-x)Tex are not directly associated with superconductivity. Instead, the results can be understood within a picture of Fermi surface nesting assuming extremely low Fermi velocities and spin-orbital coupling.     

Fermi-surface reconstruction in CeRh1-xCoxIn5

The evolution of the Fermi surface of CeRh(1-x)CoxIn5 was studied as a function of Co concentration x via measurements of the de Haas-van Alphen effect. By measuring the angular dependence of quantum oscillation frequencies, we identify a Fermi-surface sheet with f-electron character which undergoes an abrupt change in topology as x is varied. Surprisingly, this reconstruction does not occur at the quantum critical concentration x(c), where antiferromagnetism is suppressed to T=0. Instead we establish that this sudden change occurs well below x(c), at the concentration x approximately 0.4, where long-range magnetic order alters its character and superconductivity appears. Across all concentrations, the cyclotron effective mass of this sheet does not diverge, suggesting that critical behavior is not exhibited equally on all parts of the Fermi surface.     

Enhancement of superconducting transition temperature due to the strong antiferromagnetic spin fluctuations in the noncentrosymmetric heavy-fermion superconductor CeIrSi3: A 29Si NMR study under pressure

We report a (29)Si NMR study on the pressure-induced superconductivity (SC) in an antiferromagnetic (AFM) heavy-fermion compound CeIrSi(3) without inversion symmetry. In the SC state at P = 2.7-2.8 GPa, the temperature (T) dependence of the nuclear-spin lattice relaxation rate 1/T(1) below T(c) exhibits a T(3) behavior without any coherence peak just below T(c), revealing the presence of line nodes in the SC gap. In the normal state, 1/T(1) follows a square root T-like behavior, suggesting that the SC emerges under the non-Fermi-liquid state dominated by AFM spin fluctuations enhanced around a quantum critical point. The reason why the maximum T(c) in CeIrSi(3) is relatively high among the Ce-based heavy-fermion superconductors may be the existence of the strong AFM spin fluctuations. We discuss the comparison with the other Ce-based heavy-fermion superconductors.     

Unique spin dynamics and unconventional superconductivity in the layered heavy fermion compound CeIrIn5: NQR evidence

We report measurements of the 115In nuclear spin-lattice relaxation rate ( 1/T1) between T = 0.09 and 100 K in the new heavy fermion (HF) compound CeIrIn5. At 0.4 < or = T< or = 100 K, 1/T1 is strongly T-dependent, which indicates that CeIrIn5 is much more itinerant than known Ce-based HFs. We find that 1/T1T, subtracting that for LaIrIn5, follows a (1 / T+straight theta)3/4 variation with straight theta = 8 K. We argue that this novel feature points to anisotropic, due to a layered crystal structure, spin fluctuations near a magnetic ordering. The bulk superconductivity sets in at 0.40 K below which the coherence peak is absent and 1/T1 follows a T3 variation, which suggests unconventional superconductivity with line-node gap.     

Specific-heat measurements of the gap structure of the organic superconductors kappa-(ET)2Cu[N(CN)2]Br and kappa-(ET)2Cu(NCS)2

We present high resolution heat capacity measurements of the organic superconductors kappa-(ET)(2)Cu[N(CN)(2)]Br and kappa-(ET)(2)Cu(NCS)(2) in fields up to 14 T. We use the high field data to determine the normal state specific heat and hence extract the behavior of the electronic specific heat C(el) in the superconducting state in zero and finite fields. We find that in both materials for T/T(c) less or similar 0.3, C(el)(H=0) approximately T2 indicating d-wave superconductivity. The data are well described by a strong coupling d-wave model from our base temperature (T/T(c) approximately 0.1) right up to T(c). Our data help to resolve the controversy regarding the order parameter symmetry in these materials.     

Quasiparticle liquid in the highly overdoped Bi(2)Sr(2)CaCu(2)O(8+delta)

Results from the study of a highly overdoped (OD) Bi(2)Sr(2)CaCu(2)O(8+delta) with a T(c) = 51 K using angle-resolved photoemission spectroscopy are presented. We observe a sharp peak in the spectra near ( pi,0) that persists well above T(c), a nodal self-energy which approaches that seen for the Mo(110) surface state, and a more k-independent line shape at the Fermi surface than the lower-doped cuprates. This allows for a realistic comparison of the lifetime values to the experimental resistivity measurements. These observations point to the validity of the quasiparticle picture for the OD even in the normal state.     

Enhancing the superconducting transition temperature of CeRh 1-x IrxIn5 due to the strong-coupling effects of antiferromagnetic spin fluctuations: an 115In nuclear quadrupole resonance study

We report on systematic evolutions of antiferromagnetic (AFM) spin fluctuations and unconventional superconductivity (SC) in heavy-fermion (HF) compounds CeRh(1-x)Ir(x)In(5) via an (115)In nuclear-quadrupole-resonance experiment. The nuclear spin-lattice relaxation rate 1/T(1) has revealed the marked development of AFM spin fluctuations as approaching an AFM ordered state. Concomitantly, the superconducting transition temperature T(c) and the energy gap Delta0 increase drastically from T(c)= 0.4K and 2Delta0/k(B)T(c)=5 in CeIrIn(5) up to T(c) =1.2K and 2Delta0/k(B)T(c) =8.3 in CeRh(0.3)Ir(0.7)In5 , respectively. The present work suggests that the AFM spin fluctuations in close proximity to the AFM quantum critical point are indeed responsible for the strong-coupling unconventional SC in HF compounds.