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We present a study of the lattice dynamical properties of superconducting SrPt3P (Tc = 8.4 K) via highresolution inelastic x-ray scattering (IXS) and ab initio calculations. Density functional perturbation theory including spin-orbit coupling results in enhanced electron-phonon coupling (EPC) for the optic phonon modes originating from the Pt(I) atoms, with energies ~ 5 meV, resulting in a large EPC constant λ ~ 2. An overall softening of the IXS powder spectra occurs from room to low temperatures, consistent with the predicted strong EPC and with recent specific-heat experiments (2Δ0/ kBTc ~ 5). The low-lying phonon modes observed in the experiments are approximately 1.5 meV harder than the corresponding calculated phonon branch. Moreover, we do not find any changes in the spectra upon entering the superconducting phase. We conclude that current theoretical calculations underestimate the energy of the lowest band of phonon modes indicating that the coupling of these modes to the electronic subsystem is overestimated. [ABSTRACT FROM AUTHOR]

The CeFeAsO and CeFePO iron pnictide compounds were studied via electrical transport measurements under high pressure. In CeFeAsO polycrystals, the magnetic phases involving the Fe and Ce ions coexist for hydrostatically applied pressures up to 15 GPa, and with no signs of pressure-induced superconductivity up to 50 GPa for the less hydrostatic pressure techniques. For the CeFePO single crystals, pressure further stabilizes the Kondo screening of the Ce 4f-electron magnetic moments. [ABSTRACT FROM AUTHOR]

Since their discovery in 2008, the Fe-based superconductors have attracted a great deal of interest. Regrettably, the mechanism(s) responsible for the superconductivity has yet to be unequivocally identified. High pressure is an important variable since its application moderates the pairing interaction. Thus far, the LnFePO (Ln = La, Pr, Nd, Sm, Gd) family of superconductors has received relatively little attention. Early high-pressure studies on LaFePO found that Tc initially increased with pressure before passing through a maximum at higher pressures. The present studies on both polycrystalline and single-crystalline LaFePO, PrFePO, and NdFePO utilize the most hydrostatic pressure medium available, i.e., dense He. Surprisingly, for all samples, Tc is found to initially decrease rapidly with pressure at the rate dTc/dP ≃ -2 to -3 K/GPa. Less hydrostatic pressure media thus appear to enhance the value of Tc in these materials. These results give yet further evidence that the superconducting state in Fe-based superconductors is extraordinarly sensitive to lattice strain. [ABSTRACT FROM AUTHOR]

We have carried out a comprehensive study of the UCoi_,Fe, Ge series across the entire range of compositions 0 ≤ x ≤ 1, and report the results of x-ray diffraction, magnetization, specific heat, and electrical resistivity measurements to uncover the T-x phase diagram. Substitution of Fe into UCoGe initially results in an increase in the Curie temperature and a rapid destruction of the superconductivity. Near x = 0.22, the ferromagnetic transition is suppressed to zero temperature at an apparent ferromagnetic itinerant electron quantum critical point, where the temperature dependence of the electrical resistivity and specific heat in this region reveal non-Fermi liquid behavior. [ABSTRACT FROM AUTHOR]

Electrical resistivity, specific heat, and magnetization measurements on URu2-xFexSi2 reveal a twofold enhancement of the "hidden-order" (HO)/large-moment antiferromagnetic (LMAFM) phase boundary T0(x). The T0(Pch) curve, obtained by converting x to "chemical pressure" Pch, is strikingly similar to the T0(P) curve, where P is applied pressure, for URu2Si2 both exhibit a "kink" at 1.5 GPa and a maximum at ~7 GPa. This similarity suggests that the HO-LMAFM transition at 1.5 GPa in URu2Si2 occurs at x ≈ 0.2 (Pch ≈ 1.5 GPa) in URu2-xFexSi2. URu2-xFexSi2 provides an opportunity for studying the HO and LMAFM phases with methods that probe the electronic structure [e.g., scanning tunneling microscopy (STM), angle-resolved photoemission spectroscopy (ARPES), and point-contact spectroscopy (PCS)] but cannot be used under pressure. [ABSTRACT FROM AUTHOR]

Under ambient conditions, praseodymium metal possesses a localized 4f²-electron configuration. Near 20 GPa, the lattice volume collapses by ∼10% into the a-uranium crystal structure, and the electrical resistivity drops dramatically. This behavior is similar to that observed in cerium metal and has been taken as evidence for 4f delocalization, although the precise nature of such volume collapse transitions in rare-earth metals is still a matter of debate. Since cerium metal develops superconductivity in the collapsed phase, we undertook a search for superconductivity in praseodymium metal at high pressure. Using designer diamond anvils, we measured the electrical resistivity of high purity praseodymium metal to pressures above 1 Mbar and millikelvin temperatures. No evidence for superconductivity was found in any of the measurements. The lack of superconductivity may derive from magnetic pair-breaking effects related to incomplete screening/delocalization of the 4f-electron state. [ABSTRACT FROM AUTHOR]

The interplay between superconductivity and charge-density-wave (CDW) order plays a central role in the layered transition-metal dichalcogenides. 1T-TiSe2 forms a prime example, featuring superconducting domes on intercalation as well as under applied pressure. Here, we present high energy-resolution inelastic x-ray scattering measurements of the CDW soft phonon mode in intercalated CuxTiSe2 and pressurized 1T-TiSe2 along with detailed ab-initio calculations for the lattice dynamical properties and phonon-mediated superconductivity. We find that the intercalation-induced superconductivity can be explained by a solely phonon-mediated pairing mechanism, while this is not possible for the superconducting phase under pressure. We argue that a hybridization of phonon and exciton modes in the pairing mechanism is necessary to explain the full observed temperature-pressure-intercalation phase diagram. These results indicate that 1T-TiSe2 under pressure is close to the elusive state of the excitonic insulator. [ABSTRACT FROM AUTHOR]

In the heavily hole-doped iron-based superconductors AFe2As2 (A=K, Rb, and Cs), the electron effective mass increases rapidly with alkali-ion radius. To study the superconducting gap structure in this series, we measure the in-plane London penetration depth λab(T) in clean crystals of AFe2As2 down to low temperature T∼0.1 K. In KFe2As2, the superfluid stiffness ρsab(T)=λab²(0)/λab²(T) at low temperatures can be accounted for by the strongly band-dependent multiple gaps reported recently. Although the λab(T) in all systems exhibits similar nonexponential temperature dependence indicating nodes or small minima in the gap, we find that the quasiparticle excitations at low temperatures show a systematic suppression with increasing alkali-ion radius. A possible origin of such evolution of low-energy excitations is discussed in terms of the momentum-dependent effect of enhanced quasiparticle mass near a quantum critical point. [ABSTRACT FROM AUTHOR]

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.

X-ray diffraction, electrical resistivity, magnetic susceptibility, and specific heat measurements on Ce(1-x)Yb(x)CoIn5 (0≤x≤1) reveal that many of the characteristic features of the x=0 correlated electron state are stable for x≤0.775 and that phase separation occurs for x>0.775. The stability of the correlated electron state is apparently due to cooperative behavior of the Ce and Yb ions, involving their unstable valences. Low-temperature non-Fermi liquid behavior is observed and varies with x, even though there is no readily identifiable quantum critical point. The superconducting critical temperature T(c) decreases linearly with x towards 0 K as x→1, in contrast with other HF superconductors where T(c) scales with T(coh).

The temperature-(T-)magnetic-field (H) phase diagram for the noncentrosymmetric compound Yb(2)Fe(12)P(7), [corrected] determined from electrical resistivity (ρ), specific heat (C), and magnetization (M) measurements on single crystal specimens, is reported. This system exhibits a crossover from a magnetically ordered non-Fermi-liquid (NFL) phase at low H to another NFL phase at higher H. The crossover occurs near the value of H where the magnetic ordering temperature (T(M)) is no longer observable in C(T,H)/T and ρ(T,H), but not where T(M) extrapolates smoothly to T=0 K at a possible quantum critical point (QCP). This indicates the occurrence of a quantum phase transition between the two NFL phases. The lack of a clear relationship between the extrapolated QCP and NFL behavior suggests an unconventional route to the NFL ground states.

A series of high-pressure electrical resistivity measurements on single crystals of TbTe3 reveal a complex phase diagram involving the interplay of superconducting, antiferromagnetic and charge-density-wave order. The onset of superconductivity reaches a maximum of almost 4 K (onset) near approximately 12.4 GPa.