Strong coupling between 4f valence instability and 3d ferromagnetism in Yb(x)Fe4Sb12 studied by resonant x-ray emission spectroscopy

We have investigated the temperature and pressure dependency of the electronic structure of Yb-filled skutterudites, YbFe(4)Sb(12) and Yb(0.88)Fe(4)Sb(12), using x-ray absorption and emission spectroscopies. An anomalous increase of the Yb valence, which is beyond the conventional Anderson model picture, is found to coincide with the onset of the ferromagnetic order in the x=0.88 sample below 20 K. In contrast, the nearly stoichiometric YbFe(4)Sb(12) is paramagnetic down to 2 K and the Yb valence is independent of temperature. This evidences a close interplay between the magnetic instability of the Fe 3d electrons and valence instability of the Yb 4f electrons. Under pressure, a sudden increase in the valence is found to occur around 13 GPa for YbFe(4)Sb(12) and 17 GPa for Yb(0.88)YbFe(4)Sb(12).     

Ferrimagnetism in EuFe4Sb12 due to the interplay of f-electron moments and a nearly ferromagnetic host

We combine x-ray magnetic circular dichroism spectroscopy at Fe L2,3 edges, at Eu M4,5 edges, x-ray absorption spectroscopy (XAS) investigation of Eu valence, and local spin density calculations, to show that the filled skutterudite Eu0.95Fe4Sb12 is a ferrimagnet in which the Fe 3d moment and the Eu2+ 4f moment are magnetically ordered with dominant antiferromagnetic coupling. From Eu L3 edge XAS, we find that about 13% of the Eu have a formal valence of 3+. We ascribe the origin of ferrimagnetism at a relatively high transition temperature TC of 85 K in Eu0.95Fe4Sb12 to f-electron interaction with the nearly ferromagnetic [Fe4Sb12]2.2- host lattice.     

Evidence for unconventional strong-coupling superconductivity in PrOs4Sb12: an Sb nuclear quadrupole resonance study

We report Sb-NQR results which evidence a heavy-fermion (HF) behavior and an unconventional superconducting (SC) property in Pr(Os4Sb12 with T(c)=1.85 K. The temperature (T) dependence of nuclear-spin-lattice-relaxation rate, 1/T(1), and NQR frequency unravel a low-lying crystal-electric-field splitting below T0 approximately 10 K, associated with Pr3+(4f(2))-derived ground state. In the SC state, 1/T(1) shows neither a coherence peak just below T(c) K nor a T3-like power-law behavior observed for anisotropic HF superconductors with the line-node gap. The isotropic energy gap with its size Delta/k(B)=4.8 K seems to open up across T(c) below T(*) approximately 2.3 K. It is surprising that Pr(Os4Sb12 looks like an isotropic HF superconductor-it may indeed argue for Cooper pairing via quadrupolar fluctuations.     

Probing the superconducting gap symmetry of PrOs4Sb12: a penetration depth study

We report measurements of the magnetic penetration depth lambda in single crystals of PrOs4Sb12 down to 0.1 K, with the ac field applied along the a, b, and c directions. In all three field orientations, lambda approximately T2 and superfluid density rho(s) approximately T2 for T<0.3T(c). Data are best fit by the 3He A-phase-like gap with multidomains, each having two point nodes along a cube axis, and parameter Delta(0)(0)/k(B)T(c)=2.6, suggesting that PrOs4Sb12 is a strong-coupling superconductor with two point nodes on the Fermi surface. We also confirm the double transitions at 1.75 and 1.85 K seen in other measurements.     

Optical pseudogap from iron states in filled skutterudites AFe4Sb12 (A=Yb, Ca, Ba)

Optical investigations are presented of the filled skutterudites AFe4Sb12 with divalent cations A=Yb, Ca, Ba. For each of these compounds a very similar pseudogap structure in the optical conductivity develops in the far-infrared spectral region at temperatures below 90 K. Highly accurate local-density approximation electronic band structure calculations can consistently explain the origin of the pseudogap structure generated largely by transition metal 3d states. In particular, a 4f-conduction electron hybridization or strong correlations can be ruled out as origin for the pseudogap.     

Flux-line lattice distortion in PrOs4Sb12

We report that the flux-line lattice in the cubic superconductor Pr(Os4Sb12 is strongly distorted from an ideal hexagonal lattice at very low temperatures in a small applied field. We attribute this to the presence of gap nodes in the superconducting state on at least some Fermi-surface sheets.     

Signatures of a hybridization gap in magnetic susceptibility of Ce(1-x)La(x)Os(4)Sb(12)

The magnetic susceptibility of Ce(1-x)La(x)Os(4)Sb(12), with x ≈ 0.2, exhibits a pronounced maximum at T(m) = 80 K. This T(m) coincides roughly with the temperature below which a small gap, believed to be a hybridization gap, is observed in spectroscopic measurements in undoped CeOs(4)Sb(12). However, a similar anomaly, at a lower temperature of 50 K, is observed in LaOs(4)Sb(12). Furthermore, there is a monotonic variation of T(m) with x, for x > 0.2, suggesting the same origin of the two anomalies and undermining a simple hybridization gap interpretation of the susceptibility of Ce(1-x)La(x)Os(4)Sb(12) alloys, with x < 1. A possibility of the hybridization gap opening, induced by freezing out of local phonons, strongly coupled with electronic degrees of freedom, is discussed.     

Crystal field potential of PrOs4Sb12: consequences for superconductivity

The results of inelastic neutron scattering provide a solution for the crystal field level scheme in PrOs4Sb12, in which the ground state in the cubic crystal field potential of T(h) symmetry is a Gamma(1) singlet. The conduction electron mass enhancement is consistent with inelastic exchange scattering, and we propose that inelastic quadrupolar, or aspherical Coulomb, scattering is responsible for enhancing the superconducting transition temperature. PrOs4Sb12 appears to be the first compound in which aspherical Coulomb scattering is strong enough to overcome magnetic pair breaking and increase T(c).     

Coexistence of strongly mixed-valence and heavy-fermion character in SmOs4Sb12 studied by soft- and hard-X-ray spectroscopy

Sm-based heavy-fermion compound SmOs4Sb12 has been investigated by soft x-ray (hnu=1070-1600 eV) and hard x-ray (HX; hnu=7932 eV) spectroscopy. The HX photoemission spectroscopy clearly demonstrates that the strongly mixed-valence state and the heavy-fermion state coexist in the bulk. It is found that the Sm valence decreases below 100 K, indicating that the Kondo coherence develops with approaching the proposed Kondo temperature. Our theoretical analyses suggest that the origin of the coexistence in SmOs4Sb12 is the coincidence of two conditions, namely, (i) the energy difference between Sm divalent and trivalent states is very small and (ii) the hybridization between Sm 4f and conduction electrons is weak.     

Low-temperature specific heat of the heavy-fermion superconductor PrOs4Sb12

We report the magnetic field dependence of the specific-heat C of single crystals of the first Pr-based heavy-fermion superconductor Pr(Os4Sb12. The variation of C at low temperature and the magnetic phase diagram inferred from C, the resistivity and magnetization provide compelling evidence of a doublet ground state. Two distinct superconducting anomalies in C indicate an unconventional superconducting state, where the splitting may arise from a weak lifting of the ground state degeneracy. In combination this identifies Pr(Os4Sb12 as a strong contender for quadrupolar pairing, i.e., superconductivity that is neither electron-phonon nor magnetically mediated.     

Multiple superconducting phases in new heavy fermion superconductor PrOs4Sb12

The superconducting gap structure of recently discovered heavy fermion superconductor PrOs4Sb12 was investigated by using thermal transport measurements in magnetic field rotated relative to the crystal axes. We demonstrate that a novel change in the symmetry of the superconducting gap function occurs deep inside the superconducting state, giving a clear indication of the presence of two distinct superconducting phases with twofold and fourfold symmetries. We infer that the gap functions in both phases have a point node singularity, in contrast to the familiar line node singularity observed in almost all unconventional superconductors.     

Muon spin relaxation and isotropic pairing in superconducting PrOs4Sb12

Transverse-field muon-spin rotation measurements in the vortex-lattice of the heavy-fermion (HF) superconductor PrOs4Sb12 yield a temperature dependence of the magnetic penetration depth lambda indicative of an isotropic or nearly isotropic energy gap. This is not seen to date in any other HF superconductor and is a signature of isotropic pairing symmetry, possibly related to a novel nonmagnetic "quadrupolar Kondo" HF mechanism in PrOs4Sb12. The T=0 relaxation rate sigma(s)(0)=0.91(1) micros(-1) yields an estimated magnetic penetration depth lambda(0)=3440(20) A, which is considerably shorter than in other HF superconductors.     

多壁碳纳米管对p型Ba0.3FeCo3Sb12化合物热电性能的影响

用两步固相反应法合成了P型Ba填充方钴矿化合物Ba0.3FeCo3Sb12，并采用放电等离子烧结法(SPS)制备了Ba0.3FeCo3Sb12／多壁碳纳米管复合材料．研究了Ba0.3FeCo3Sb12／多壁碳纳米管复合材料的结构及多壁碳纳米管对其热电性能的影响规律：SEM分析表明多壁碳纳米管比较均匀地分布在Ba0.3FeCo3Sb12基体中；随着碳纳米管添加量的增加，Ba0.3FeCo3Sb12／多壁碳纳米管复合材料的电导率下降、塞贝克系数略微下降、晶格热导率大幅度降低，当碳纳米管含量为5％时其晶格热导率最小；当碳纳米管的含量为3％时，本研究得到的Ba0.3FeCo3Sb12／碳纳米管复合材料的最大ZT值达0．78．     

Pronounced enhancement of the lower critical field and critical current deep in the superconducting state of PrOs4Sb12

We have observed an unexpected enhancement of the lower critical field H(c1)(T) and the critical current I(c)(T) deep in the superconducting state below T approximately 0.6 K (T/T(c) approximately 0.3) in the filled skutterudite heavy fermion superconductor PrOs(4)Sb(12). From a comparison of the behavior of H(c1)(T) with that of the heavy fermion superconductors U(1-x)Th(x)Be(13) and UPt(3), we speculate that the enhancement of H(c1)(T) and I(c)(T) in PrOs(4)Sb(12) reflects a transition into another superconducting phase that occurs below T/T(c) approximately 0.3. An examination of the literature reveals unexplained anomalies in other physical properties of PrOs(4)Sb(12) near T/T(c) approximately 0.3 that correlate with the features we have observed in H(c1)(T) and I(c)(T).     

Superconducting PrOs4Sb12: a thermal conductivity study

The superconducting state of the heavy fermion PrOs4Sb12 is studied by heat transport measurements on a highly homogeneous single crystal exhibiting only one transition peak in the specific heat. The field and temperature dependence of the thermal conductivity confirm multiband superconductivity and point to fully open gaps on the whole Fermi surface.     

Connection between charge fluctuations and the coherent temperature in the heavy-fermion system SmOs4Sb12: a {121, 123}Sb NQR study

We report {121, 123}Sb nuclear quadrupole resonance measurements under pressure in a novel heavy fermion (HF) system SmOs4Sb12. The nuclear spin-spin relaxation rate 1/T{2} exhibits a distinct peak near the coherent temperature of the Kondo effect. The isotope effect of 121Sb and 123Sb indicates that the peak in 1/T{2} is electrical in origin. The connection between the peak in 1/T{2} and the development of coherency of the Kondo effect is robust even under pressure. It is conjectured that charge fluctuation plays an important role in forming the HF state in SmOs4Sb12.     

Magnetic behavior of Ba3Cu3Sc4O12

The chain-like system Ba(3)Cu(3)Sc(4)O(12) has potentially interesting magnetic properties due to the presence of Cu(2+) and a structure-suggested low dimensionality. We present magnetization M versus magnetic field H and temperature T, T- and H-dependent heat-capacity C(p), (45)Sc nuclear magnetic resonance (NMR), muon spin rotation (μSR), neutron diffraction measurements and electronic structure calculations for Ba(3)Cu(3)Sc(4)O(12). The onset of magnetic long-range antiferromagnetic (AF) order at T(N) ～ 16 K is consistently evidenced from the whole gamut of our data. A significant sensitivity of T(N) to the applied magnetic field H (T(N) ～ 0 K for H = 70 kOe) is also reported. Coupled with a ferromagnetic Curie-Weiss temperature (θ(CW) ～ 65 K) in the susceptibility (from a 100 to 300 K fit), it is indicative of competing ferromagnetic and antiferromagnetic interactions. These indications are corroborated by our density functional theory based electronic structure calculations, where we find the presence of significant ferromagnetic couplings between some copper ions whereas AF couplings were present between some others. Our experimental data, backed by our theoretical calculations, rule out the one-dimensional magnetic behavior suggested by the structure and the observed long-range order is due to the presence of non-negligible magnetic interactions between adjacent as well as next-nearest chains.     

Heavy fermion fluid in high magnetic fields: an infrared study of CeRu4Sb12

We report a comprehensive infrared magnetospectroscopy study of a CeRu4Sb12 compound revealing quasiparticles with a heavy effective mass m*, with a detailed analysis of optical constants in fields up to 17 T. We find that the applied magnetic field strongly affects the low-energy excitations in the system. In particular, the magnitude of m* approximately = 70 m(b) (m(b) is the quasiparticle band mass) at 10 K is suppressed by as much as 25% at 17 T. This effect is in quantitative agreement with the mean-field solution of the periodic Anderson model augmented with a Zeeman term.     

A theoretical study of tunnelling conductance in ferromagnet/ PrOs4Sb12 junctions

The tunnelling conductance spectra of ferromagnet/PrOs4Sb12 junctions are theoretically investigated by using the Blonder-Tinkham-Klapwijk theory. Three pairs of possible candidate for the pairing symmetry of superconducting energy gap of the recently discovered heavy-fermion unconventional superconductor PrOs4Sb12 are chosen for calculation. We have studied the spin-polarization effect on the conductance spectra, with respect to different strength of ferromagnetism of the ferromagnet and different strength of the interface barrier. Moreover, we have discussed the influence of nodal structures of the superconducting energy gap on the conductance spectra. Different features of the tunnelling conductance spectra were got, which may serve as useful theoretical comparisons for future experiments.     

Multiband superconductivity in the heavy fermion compound PrOs4Sb12

The thermal conductivity of the heavy fermion superconductor Pr(Os(4)Sb(12) was measured down to T(c)/40 throughout the vortex state. At lowest temperatures and for magnetic fields H approximately 0.07H(c2), already 40% of the normal state thermal conductivity is restored. This behavior (similar to that observed in MgB2) is a clear signature of multiband superconductivity in this compound.