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We report anisotropic resistivity measurements of the heavy-fermion superconductor CeRh$_2$As$_2$ in magnetic fields up to 16 T and temperatures down to 0.35 K. The measured CeRh$_2$As$_2$ resistivity shows a signature corresponding to the suggested quadrupole density wave order state at $T_0 \sim$ 0.5 K for both measured directions. For a magnetic field applied along the tetragonal $a$ axis, $T_0$ is enhanced with magnetic field reaching $\sim$1.75 K at 16 T. Further, a magnetic field-induced transition occurs at $B_m \sim $ 8.1 T corresponding to a change to a new broken symmetry state. For a magnetic field applied along the $c$ axis, $T_0$ is suppressed below our base temperature $\sim$0.35 K by $B \sim$ 4.5 T, a field close to the previously reported field-induced transition within the superconducting state suggested to be from an even-parity to an odd-parity state. Our results indicate that the multiple superconducting phases in CeRh$_2$As$_2$ are intimately tied to the suppression of the proposed quadrupole-density-wave phase at $T_0$., Comment: 6 pages and 5 figures

Newly-discovered superconductor UTe$_2$ is a strong contender for a topological spin-triplet state wherein a multi-component order parameter arises from two nearly-degenerate superconducting states. A key issue is whether both of these states intrinsically exist at ambient pressure. Through thermal expansion and calorimetry, we show that UTe$_2$ at ambient conditions exhibits two detectable transitions only in some samples, and the size of the thermal expansion jump at each transition varies when the measurement is performed in different regions of the sample. This result indicates that the two transitions arise from two spatially separated regions that are inhomogeneously mixed throughout the volume of the sample, each with a discrete superconducting transition temperature (T$_c$). Notably, samples with higher T$_c$ only show a single transition at ambient pressure. Above 0.3 GPa, however, two transitions are invariably observed in ac calorimetry. Our results not only point to a nearly vertical line in the pressure-temperature phase diagram but also provide a unified scenario for the sample dependence of UTe$_{2}$., Comment: 6 pages, 4 figures, includes supplemental information, changed conclusion on the origin of double-transition feature observed in some UTe2 samples

We present Raman-scattering results for three materials, CeB6, TbInO3, and YbRu2Ge2, to illustrate the essential aspects of crystal-field (CF) excitations and quadrupolar fluctuations of 4f-electron systems. For CF excitations, we illustrate how the 4f orbits are split by spin-orbit coupling and CF potential by presenting spectra for inter- and intra-multiplet excitations over a large energy range. We discuss identification of the CF ground state and establishment of low-energy CF level scheme from the symmetry and energy of measured CF excitations. In addition, we demonstrate that the CF linewidth is a sensitive probe of electron correlation by virtue of self-energy effect. For quadrupolar fluctuations, we discuss both ferroquadrupolar (FQ) and antiferroquadrupolar (AFQ) cases. Long-wavelength quadrupolar fluctuations of the same symmetry as the FQ order parameter persists well above the transition temperature, from which the strength of electronic intersite quadrupolar interaction can be evaluated. The tendency towards AFQ ordering induces ferromagnetic correlation between neighboring 4f-ion sites, leading to long-wavelength magnetic fluctuations.

Multicomponent superconductivity in intermediate-valence UTe2 emerges from an antiferromagnetic quantum critical point., UTe2 is a recently discovered unconventional superconductor that has attracted much interest because of its potentially spin-triplet topological superconductivity. Our ac calorimetry, electrical resistivity, and x-ray absorption study of UTe2 under applied pressure reveals key insights on the superconducting and magnetic states surrounding pressure-induced quantum criticality at Pc1 = 1.3 GPa. First, our specific heat data at low pressures, combined with a phenomenological model, show that pressure alters the balance between two closely competing superconducting orders. Second, near 1.5 GPa, we detect two bulk transitions that trigger changes in the resistivity, which are consistent with antiferromagnetic order, rather than ferromagnetism. Third, the emergence of magnetism is accompanied by an increase in valence toward a U4+ (5f2) state, which indicates that UTe2 exhibits intermediate valence at ambient pressure. Our results suggest that antiferromagnetic fluctuations may play a more substantial role on the superconducting state of UTe2 than previously thought.

Phases of matter are usually identified through spontaneous symmetry breaking, especially regarding unconventional superconductivity and the interactions from which it originates. In that context, the superconducting state of the quasi-two-dimensional and strongly correlated perovskite Sr

Introduction Pitolisant Expanded Access Clinical Evaluation (PEACE) provided adult patients with narcolepsy access to treatment with pitolisant while it was an investigational medication in the United States. Methods Pitolisant was titrated to 35.6 mg/day (or the highest tolerable dose) over a 3-week period. Dose adjustments were permitted at the discretion of the treating physician based on patient response. Treating physicians followed their standard of care and were required to report adverse events (AEs). Demographic and baseline information for all enrolled patients, and safety results available through October 30, 2019, are reported here (presentation will include final data from the PEACE program). Results In all, 623 patients (67.9% female; 84.6% white; mean age, 40.0 years; narcolepsy type 1, 51.5%) were treated with pitolisant in the PEACE program. Nearly all patients (98.4%) had been previously treated with other narcolepsy medications (88.1% with ≥2 narcolepsy medications). Overall, 35.2% of patients discontinued from the program; 16.7% due to an AE and 12.2% for lack of effect. At Month 1, 97.3% of patients remained in the study, 88.2% at Month 3, 76.5% at Month 6, 66.9% at Month 9, and 55.0% at Month 12. In all, 256 (41.1%) patients experienced ≥1 AE; majority (52.5%) of these AEs occurred early in treatment (by Week 3). The most commonly reported AEs were headache (9.8% of AEs), nausea (6.6%), anxiety (5.6%), and insomnia (4.7%). Conclusion In the PEACE program, patient characteristics were generally reflective of the US narcolepsy patient population. The safety and tolerability profile of pitolisant was similar to that seen in the clinical development program, with no new safety signals identified. The program ceased enrollment in August 2019 after the US approval of pitolisant for the treatment of excessive daytime sleepiness in adult patients with narcolepsy. Support Harmony Biosciences, LLC.

CeCo(In_{0.990}Hg_{0.010})_{5} is a charge doped variant of the d-wave CoCoIn_{5} superconductor with coexistent antiferromagnetic and superconducting transitions occurring at T_{N}=3.4 and T_{c}=1.4 K, respectively. We use neutron diffraction and spectroscopy to show that the magnetic resonant fluctuations present in the parent superconducting phase are replaced by collinear c-axis magnetic order with three-dimensional Ising critical fluctuations. No low-energy transverse spin fluctuations are observable in this doping-induced antiferromagnetic phase and the dynamic resonant spectral weight predominately shifts to the elastic channel. Static (τ0.2 ns) collinear Ising order is proximate to superconductivity in CeCoIn_{5} and is stabilized through hole doping with Hg.

Two aspects of the ambient pressure magnetic structure of heavy fermion material CeRhIn

Electronic nematic materials are characterized by a lowered symmetry of the electronic system compared to the underlying lattice, in analogy to the directional alignment without translational order in nematic liquid crystals. Such nematic phases appear in the copper- and iron-based high-temperature superconductors, and their role in establishing superconductivity remains an open question. Nematicity may take an active part, cooperating or competing with superconductivity, or may appear accidentally in such systems. Here we present experimental evidence for a phase of fluctuating nematic character in a heavy-fermion superconductor, CeRhIn

We have carried out high-resolution angle-resolved photoemission measurements on the Cebased heavy fermion compound CePt2In7 that exhibits stronger two-dimensional character than the prototypical heavy fermion system CeCoIn5. Multiple Fermi surface sheets and a complex band structure are clearly resolved. We have also performed detailed band structure calculations on CePt2In7. The good agreement found between our measurements and the calculations suggests that the band renormalization effect is rather weak in CePt2In7. A comparison of the common features of the electronic structure of CePt2In7 and CeCoIn5 indicates that CeCoIn5 shows a much stronger band renormalization effect than CePt2In7. These results provide new information for understanding the heavy fermion behaviors and unconventional superconductivity in Ce-based heavy fermion systems., Comment: 24 pages, 10 figures

In search of novel, improved materials for magnetic data storage and spintronic devices, compounds that allow a tailoring of magnetic domain shapes and sizes are essential. Good candidates are materials with intrinsic anisotropies or competing interactions, as they are prone to host various domain phases that can be easily and precisely selected by external tuning parameters such as temperature and magnetic field. Here, we utilize vector magnetic fields to visualize directly the magnetic anisotropy in the uniaxial ferromagnet CeRu2Ga2B. We demonstrate a feasible control both globally and locally of domain shapes and sizes by the external field as well as a smooth transition from single stripe to bubble domains, which opens the door to future applications based on magnetic domain tailoring.

The Belitz-Kirkpatrick-Vojta (BKV) theory shows in excellent agreement with experiment that ferromagnetic quantum phase transitions (QPTs) in clean metals are generally first order due to the coupling of the magnetization to electronic soft modes, in contrast to the classical analogue that is an archetypical second-order phase transition. For disordered metals the BKV theory predicts that the second-order nature of the QPT is restored because the electronic soft modes change their nature from ballistic to diffusive. Our low-temperature magnetization study identifies the ferromagnetic QPT in the disordered metal UCo_{1-x}Fe_{x}Ge as the first clear example that exhibits the associated critical exponents predicted by the BKV theory.

We have investigated the magnetic ground state of the antiferromagnetic Kondo-lattice compounds CeMAl4Si2(M = Rh, Ir) using neutron powder diffraction. Although both of these compounds show two magnetic transitions T(N1) and T(N2) in the bulk properties measurements, evidence for magnetic long-range order was only found below the lower transition T(N2). Analysis of the diffraction profiles reveals a commensurate antiferromagnetic structure with a propagation vector k = (0, 0, 1/2). The magnetic moment in the ordered state of CeRhAl4Si2 and CeIrAl4Si2 were determined to be 1.14(2) and 1.41(3) μ(B) Ce(-1), respectively, and are parallel to the crystallographic c-axis in agreement with magnetic susceptibility measurements.

We investigated the electrical resistivity and heat capacity of 1% Cd-doped CeIrIn_{5} under hydrostatic pressure up to 2.7 GPa, near where long-range antiferromagnetic order is suppressed and bulk superconductivity suddenly reemerges. The pressure-induced T_{c} is close to that of pristine CeIrIn_{5} at 2.7 GPa, and no signatures of a quantum critical point under pressure support a local origin of the antiferromagnetic moments in Cd-CeIrIn_{5} at ambient pressure. Similarities between superconductors CeIrIn_{5} and CeCoIn_{5} in response to Cd substitutions suggest a common magnetic mechanism.

Electric resistivity, specific heat, magnetic susceptibility, and inelastic neutron scattering experiments were performed on a single crystal of the heavy fermion compound Ce(Ni0.935Pd0.065)2Ge2 in order to study the spin fluctuations near an antiferromagnetic (AF) quantum critical point (QCP). The resistivity and the specific heat coefficient for T ⩽ 1 K exhibit the power law behavior expected for a 3D itinerant AF QCP (ρ(T) ∼ T(3/2) and γ(T) ∼ γ0 - bT(1/2)). However, for 2 ⩽ T ⩽ 10 K, the susceptibility and specific heat vary as log T and the resistivity varies linearly with temperature. Furthermore, despite the fact that the resistivity and specific heat exhibit the non-Fermi liquid behavior expected at a QCP, the correlation length, correlation time, and staggered susceptibility of the spin fluctuations remain finite at low temperature. We suggest that these deviations from the divergent behavior expected for a QCP may result from alloy disorder.

We have used high-resolution neutron spectroscopy experiments to determine the complete spin wave spectrum of the heavy-fermion antiferromagnet CeRhIn₅. The spin wave dispersion can be quantitatively reproduced with a simple frustrated J₁-J₂ model that also naturally explains the magnetic spin-spiral ground state of CeRhIn₅ and yields a dominant in-plane nearest-neighbor magnetic exchange constant J₀=0.74(3) meV. Our results pave the way to a quantitative understanding of the rich low-temperature phase diagram of the prominent CeTIn₅ (T=Co, Rh, Ir) class of heavy-fermion materials.

Nuclear magnetic resonance (NMR) measurements on the ^{195}Pt nucleus in an aligned powder of the moderately heavy-fermion material U_{2}PtC_{2} are consistent with spin-triplet pairing in its superconducting state. Across the superconducting transition temperature and to much lower temperatures, the NMR Knight shift is temperature independent for field both parallel and perpendicular to the tetragonal c axis, expected for triplet equal-spin pairing superconductivity. The NMR spin-lattice relaxation rate 1/T_{1}, in the normal state, exhibits characteristics of ferromagnetic fluctuations, compatible with an enhanced Wilson ratio. In the superconducting state, 1/T_{1} follows a power law with temperature without a coherence peak giving additional support that U_{2}PtC_{2} is an unconventional superconductor. Bulk measurements of the ac susceptibility and resistivity indicate that the upper critical field exceeds the Pauli limiting field for spin-singlet pairing and is near the orbital limiting field, an additional indication for spin-triplet pairing.

Very high field (29)Si-NMR measurements using a fully (29)Si-enriched URu(2)Si(2) single crystal were carried out in order to microscopically investigate the "hidden order" (HO) state and adjacent magnetic phases in the high field limit. At the lowest measured temperature of 0.4 K, a clear anomaly reflecting a Fermi surface instability near 22 T inside the HO state is detected by the (29)Si shift, (29)K(c). Moreover, a strong enhancement of (29)K(c) develops near a critical field H(c) ≃ 35.6 T, and the ^{29}Si-NMR signal disappears suddenly at H(c), indicating the total suppression of the HO state. Nevertheless, a weak and shifted (29)Si-NMR signal reappears for fields higher than H(c) at 4.2 K, providing evidence for a magnetic structure within the magnetic phase caused by the Ising-type anisotropy of the uranium ordered moments.

We studied the effects of Co doping on the intrinsic anisotropic properties of aligned a-axis YBa/sub 2/Cu/sub 3-x/Co/sub x/O/sub 7-/spl delta// films pulsed laser deposited on (100) LaSrGaO/sub 4/ substrates We used X-ray-absorption fine structure analysis and resistivity data to determine the quality of Co incorporation. Higher deposition pressures provided with better Co dopant incorporation but smaller grain size. However at lower pressures Co incorporation can still be achieved by down the cooling process. For transport along the c-direction, Co dopant causes reduction interlayer coupling as evidenced by an increase resistive anisotropy (/spl rho//sub c///spl rho//sub b/) with increasing level of Co incorporation, Co doping level of x=0.22 effectively doubles the resistive anisotropy of YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// films (from -18 to -40 at 100 K).

Ultrafast optical spectroscopy is used to study the antiferromagnetic f-electron system USb(2). We observe the opening of two charge gaps at low temperatures (/~45 K), arising from renormalization of the electronic structure. Analysis of our data indicates that one gap is due to hybridization between localized f-electron and conduction electron bands, while band renormalization involving magnons leads to the emergence of the second gap. These experiments thus enable us to shed light on the complex electronic structure emerging at the Fermi surface in f-electron systems.

We have studied the local structure around Co in ${\mathrm{YBa}}_{2}$(${\mathrm{Cu}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Co}}_{\mathit{x}}$${)}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ c-axis oriented thin films with three different concentrations: x=0.07, 0.10, 0.17, and in a ${\mathrm{PrBa}}_{2}$(${\mathrm{Cu}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Co}}_{\mathit{x}}$${)}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ thin film of concentration x=0.05 using the x-ray absorption fine structure (XAFS) technique. Data were collected at the Co K edge with polarizations both parallel and perpendicular to the film surface. We find that the oxygen neighbors are well ordered and shortened in comparison with YBCO Cu-O values to \ensuremath{\sim}1.80 \AA{} and 1.88 \AA{} in the c axis and ab plane, respectively. The data also show that there are approximately two O neighbors in the main Co-O peak for the ab-plane data, which agrees with a Cu(1) substitution site but is inconsistent with most of the Co on a Cu(2) site. A comparison of further neighbor peaks for the thin film and powder data show that the amplitude of these peaks is suppressed for the thin films relative to the powder samples. This suggests that there is more disorder and/or distortions of the Co environment present in the thin films. Since x-ray diffraction shows the films to be well ordered, this is a localized distortion. We also compared the polarized XAFS results with calculations for four oxides CoO, ${\mathrm{Co}}_{2}$${\mathrm{O}}_{3}$, ${\mathrm{Co}}_{3}$${\mathrm{O}}_{4}$, and ${\mathrm{CoBaO}}_{2}$, and showed that the XAFS data are inconsistent with the presence of significant precipitates of these phases. \textcopyright{} 1996 The American Physical Society.

Many highTcsystems exhibit structural distortions or anomalies, some of which correlate withTc. We present our results for three such systems and discuss them in the context of providing any evidence for coupling of the lattice to the superconducting pairs. We then propose a modulation type of experiment that should provide unambiguous results if a significant lattice-pair coupling does exist.

We have investigated the local structure about the Cu, Ba, and Hg atoms in ${\mathrm{HgBa}}_{2}$${\mathrm{CuO}}_{4+\mathrm{\ensuremath{\delta}}}$ using the x-ray absorption fine structure (XAFS) technique. We find that most of the local structure results are consistent with the diffraction measurements except for two peaks in the XAFS spectra corresponding to the Cu-O(2) and Ba-O(2) atom pairs. These peaks are exceptionally broad, much broader than expected from the diffraction results. Our analysis, which includes neutron-diffraction results, indicates that the atomic displacements for the Cu and O(2) atoms are negatively correlated. The Ba and O(2) atoms may also be negatively correlated. Although this anticorrelation could be static or dynamic, we consider that it may be the result of the rapid hopping of charge between the apical O(2) ion and the ${\mathrm{CuO}}_{2}$ plane; this hopping may also provide a coupling mechanism for the formation of Cooper pairs.

The physical properties of the first In analog of the PuMGa(5) (M = Co, Rh) family of superconductors, PuCoIn(5), are reported. With its unit cell volume being 28% larger than that of PuCoGa(5), the characteristic spin-fluctuation energy scale of PuCoIn(5) is three to four times smaller than that of PuCoGa(5), which suggests that the Pu 5f electrons are in a more localized state relative to PuCoGa(5). This raises the possibility that the high superconducting transition temperature T(c) = 18.5 K of PuCoGa(5) stems from the proximity to a valence instability, while the superconductivity at T(c) = 2.5 K of PuCoIn(5) is mediated by antiferromagnetic spin fluctuations associated with a quantum critical point.

We report the synthesis, structure, and physical properties of single crystals of CePt(2)In(7). Single crystal x-ray diffraction analysis confirms the tetragonal I4/mmm structure of CePt(2)In(7) with unit cell parameters a = 4.5886(6) Å, c = 21.530(6) Å and V = 453.32(14) Å(3). The magnetic susceptibility, heat capacity, Hall effect and electrical resistivity measurements are all consistent with CePt(2)In(7) undergoing an antiferromagnetic order transition at T(N) = 5.5 K, which is field independent up to 9 T. Above T(N), the Sommerfeld coefficient of specific heat is γ ≈ 300 mJ mol(-1) K(-2), which is characteristic of an enhanced effective mass of itinerant charge carriers. The electrical resistivity is typical of heavy-fermion behavior and gives a residual resistivity ρ(0) ∼ 0.2 µΩ cm, indicating good crystal quality. CePt(2)In(7) also shows moderate anisotropy of the physical properties that is comparable to structurally related CeMIn(5) (M = Co, Rh, Ir) heavy-fermion superconductors.

We present small angle neutron scattering studies of the vortex lattice (VL) in CeCoIn5 with magnetic fields applied parallel (H) to the antinodal [100] and nodal [110] directions. For H is parallel to [100], a single VL orientation is observed, while a 90° reorientation transition is found for H is parallel to [110]. For both field orientations and VL configurations we find a distorted hexagonal VL with an anisotropy, Γ=2.0±0.05. The VL form factor shows strong Pauli paramagnetic effects similar to what have previously been reported for H is parallel to [001]. At high fields, above which the upper critical field (H(c2)) becomes a first-order transition, an increased disordering of the VL is observed.

We use heat-capacity measurements as a function of field rotation to identify the nodal gap structure of CeIrIn(5) at pressures to 2.05 GPa, deep inside its superconducting dome. A fourfold oscillation in the heat capacity at 0.3 K is observed for all pressures, but with its sign reversed between 1.50 and 0.90 GPa. On the basis of recent theoretical models for the field-angle-dependent specific heat, all data, including the sign reversal, imply a d(x(2)-y(2)) order parameter with nodes along [110], which constrains theoretical models of the pairing mechanism in CeIrIn(5).

(59)Co nuclear spin-lattice relaxation has been measured for the heavy-fermion superconductor CeCoIn(5) in a range of applied fields directed parallel to the c axis. An enhanced normal-state relaxation rate, observed at low temperatures and fields just above H(c2)(0), is taken as a direct measure of the dynamical susceptibility and provides microscopic evidence for an antiferromagnetic instability. The results are well described using the self-consistent renormalized theory for two-dimensional antiferromagnetic spin fluctuations, and parameters obtained in the analysis are applied to previously reported specific heat and thermal expansion data with good agreement.

Using Shubnikov-de Haas oscillations measured in URu2Si2 over a broad range in a magnetic field of 11-45 T, we find a cascade of field-induced Fermi surface changes within the hidden order phase I and further signatures of oscillations within field-induced phases III and V [previously discovered by Kim et al., [Phys. Rev. Lett. 91, 256401 (2003)]. A comparison of kinetic and Zeeman energies indicates a pocket-by-pocket polarization of the Fermi surface leading up to the destruction of the hidden order phase I at ≈35 T. The anisotropy of the Zeeman energy driving the transitions in URu2Si2 points to an itinerant hidden order parameter involving quasiparticles whose spin degrees of freedom depart significantly from those of free electrons.

The lack of superconductivity in several candidate materials that exhibit a non-spin density wave quantum critical point has raised the question of whether the associated spectra of quantum fluctuations are beneficial to forming superconducting electron pairs. Here we discuss the possibility that the prototypical heavy-fermion antiferromagnet CeRhIn5 may be the first example of unconventional superconductors where superconductivity arises from Kondo-breakdown quantum criticality.

The physical properties including magnetic susceptibility, specific heat, and electrical resistivity of two new plutonium compounds Pu2M3Si5 (M = Co, Ni) are reported. Pu2Ni3Si5 crystallizes in the orthorhombic U2Co3Si5 structure type, which can be considered a variant of the BaAl4 tetragonal structure, while Pu2Co3Si5 adopts the closely related monoclinic Lu2Co3Si5 type. Magnetic order is observed in both compounds, with Pu2Ni3Si5 ordering ferromagnetically at T(C) = 65 K then undergoing a transition into an antiferromagnetic state below T(N) = 35 K. Two successive magnetic transitions are also observed at T(mag1) = 38 K and T(mag2) = 5 K in Pu2Co3Si5. Specific heat measurements reveal that these two materials have a moderately enhanced Sommerfeld coefficient γ ∼ 100 mJ/mol Pu K(2) in the magnetic state with comparable RKKY and Kondo energy scales.

The low-temperature thermal expansion of CeCoIn(5) 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(c2)(0). However, β(T) and the Grüneisen parameter Γ are incompatible with a quantum critical point at B(c2)(0), but allow for a quantum criticality shielded by superconductivity and extending to negative pressures for BB(c2)(0). We construct a tentative (p, B, T) phase diagram of CeCoIn(5) suggesting a quantum critical line in the (p, B) plane.

We report results of ⁵⁹Co nuclear magnetic resonance measurements on a single crystal of superconducting PuCoGa₅ in its normal state. The nuclear spin-lattice relaxation rates and the Knight shifts as a function of temperature reveal an anisotropy of spin fluctuations with finite wave vector q. By comparison with the isostructural members, we conclude that antiferromagnetic XY-type anisotropy of spin fluctuations plays an important role in mediating superconductivity in these heavy fermion materials.

We have examined the relaxation of photoinduced quasiparticles in the heavy-fermion superconductor PuCoGa5. The deduced electron-phonon coupling constant is incompatible with the measured superconducting transition temperature Tc=18.5 K, which speaks against phonon-mediated superconductivity. Upon lowering the temperature, we observe an order-of-magnitude increase of the quasiparticle relaxation time in agreement with the phonon bottleneck scenario--evidence for a hybridization gap in the electronic density of states. The modification of photoinduced reflectance in the superconducting state is consistent with the heavy character of the quasiparticles that participate in Cooper pairing.

We report 75As NMR measurements in CaFe2As2, made under applied pressures up to 0.83 GPa produced by a standard clamp pressure cell. Our data reveal phase segregation of paramagnetic and antiferromagnetic (AFM) phases over a range of pressures, with the AFM phase more than 90% dominant at low temperatures. In situ rf susceptibility measurements indicate the presence of superconductivity. 75As spin-lattice relaxation experiments indicate that the 75As nuclei sample the superconductivity while in the magnetically ordered environment.