In correlated metals derived from Mott insulators, the motion of an electron is impeded by Coulomb repulsion due to other electrons. This phenomenon causes a substantial reduction in the electron's kinetic energy, leading to remarkable experimental manifestations in optical spectroscopy 1 . The hightransition-temperature (T c ) superconducting cuprates are perhaps the most studied examples of such correlated metals. The occurrence of high-T c superconductivity in the iron pnictides 2-4 puts a spotlight on the relevance of correlation effects in these materials 5 . Here, we present an infrared and optical study on single crystals of the iron pnictide superconductor LaFePO. We find clear evidence of electronic correlations in metallic LaFePO with the kinetic energy of the electrons reduced to half of that predicted by band theory of nearly free electrons. We deduce that electronic many-body effects are important in the iron pnictides despite the absence of a Mott transition.The recent discovery of superconductivity in the iron pnictides promises to be an important milestone in condensed-matter physics 2,3 . Here is a new class of materials with a layered structure and relatively high superconducting T c values 3,4 rivalling the doped cuprates. Electronic conduction is believed to occur in the ironpnictogen layers 6 , similar to the cuprates where the charge carriers are delocalized in the copper-oxygen planes. Two decades of research on the cuprates have established that a proper account of the exotic normal-state properties is a prerequisite for the understanding of the superconducting instability 7 .The correlated metallic state of the superconducting cuprates is derived through chemical doping of the parent compounds, which are strongly correlated (Mott) insulators. However, the parent compounds of the iron pnictides are metallic, albeit highly dissipative, bad metals 5 . Magnetic ordering in the parent iron arsenides at low temperatures leads to partial gapping of the Fermi surface but does not initiate an insulating state. Moreover, there is no evidence of long-range magnetic ordering in the 1111-iron phosphide LaFePO (ref. 8). Recent theoretical work on the normal state of the iron pnictides suggests that despite the apparent itinerant behaviour, Mott physics is relevant to charge dynamics and magnetic properties 5,9,10 . Thus motivated, we investigated the normal state of the 1111-iron phosphide superconductor LaFePO with infrared and optical spectroscopy, focusing on charge dynamics in the conducting planes.An optical experiment measures the dynamical response of the electron subjected to an external electromagnetic field and facilitates monitoring of many-body effects experienced by the electron in a material. These many-body effects include the interaction of the electron with other electrons, phonons as well as ordered or fluctuating spins. Figure 1a shows the real part of the ab-plane optical conductivity σ 1 (ω) of LaFePO over a broad frequency range. Sample growth and characterization procedures, and...

Understanding the origin of superconductivity in strongly correlated electron systems continues to be at the forefront of the unsolved problems of physics 1 . Among the heavy f-electron systems, CeCoIn 5 is one of the most fascinating, as it shares many of the characteristics of correlated d-electron high-T c cuprate and pnictide superconductors 2-4 , including competition between antiferromagnetism and superconductivity 5 . Although there has been evidence for unconventional pairing in this compound 6-11 , high-resolution spectroscopic measurements of the superconducting state have been lacking. Previously, we have used high-resolution scanning tunnelling microscopy (STM) techniques to visualize the emergence of heavy fermion excitations in CeCoIn 5 and demonstrate the composite nature of these excitations well above T c (ref. 12). Here we extend these techniques to much lower temperatures to investigate how superconductivity develops within a strongly correlated band of composite excitations. We find the spectrum of heavy excitations to be strongly modified just before the onset of superconductivity by a suppression of the spectral weight near the Fermi energy (E F ), reminiscent of the pseudogap state 13,14 in the cuprates. By measuring the response of superconductivity to various perturbations, through both quasiparticle interference (QPI) and local pair-breaking experiments, we demonstrate the nodal d-wave character of superconducting pairing in CeCoIn 5 .CeCoIn 5 undergoes a superconducting transition at 2.3 K. Despite evidence of unconventional pairing, consensus on the mechanism of pairing and direct experimental verification of the order parameter symmetry are still lacking [6][7][8][9]11 . Moreover, experiments have suggested that superconductivity in this compound emerges from a state of unconventional quasiparticle excitations with a pseudogap phase similar to that found in underdoped high-T c cuprates [15][16][17] . Previously, we demonstrated that scanning tunnelling spectroscopic techniques can be used to directly visualize the emergence of heavy fermion excitations in CeCoIn 5 and their quantum critical nature 12 . Through these measurements, we also demonstrated the composite nature of heavy quasiparticles and showed their band formation as the f -electrons hybridize with the spd-electrons starting at 70 K, well above T c (ref. 12). This previous breakthrough, together with our recent development of high-resolution millikelvin STM, offers a unique opportunity to measure how superconductivity emerges in a heavy electron system. Figure 1 shows STM topographs of the two commonly observed atomically ordered surfaces of CeCoIn 5 produced after the cleaving of single crystals in situ in the ultra-high vacuum environment 1 Joseph Henry Laboratories and Department of Physics, Princeton University, Princeton, New Jersey 08544, USA, 2 Condensed Matter and Magnet Science, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA. † These authors contributed equally to this work. *e-mail: yazdani@pr...

In solids containing elements with f orbitals, the interaction between f-electron spins and those of itinerant electrons leads to the development of low-energy fermionic excitations with a heavy effective mass. These excitations are fundamental to the appearance of unconventional superconductivity and non-Fermi-liquid behaviour observed in actinide- and lanthanide-based compounds. Here we use spectroscopic mapping with the scanning tunnelling microscope to detect the emergence of heavy excitations with lowering of temperature in a prototypical family of cerium-based heavy-fermion compounds. We demonstrate the sensitivity of the tunnelling process to the composite nature of these heavy quasiparticles, which arises from quantum entanglement of itinerant conduction and f electrons. Scattering and interference of the composite quasiparticles is used to resolve their energy-momentum structure and to extract their mass enhancement, which develops with decreasing temperature. The lifetime of the emergent heavy quasiparticles reveals signatures of enhanced scattering and their spectral lineshape shows evidence of energy-temperature scaling. These findings demonstrate that proximity to a quantum critical point results in critical damping of the emergent heavy excitation of our Kondo lattice system.Comment: preprint version, 26 pages, 6 figures. Supplementary: 15 pages, 14 figure

A break in periodicity occurs in the actinide series between plutonium and americium as the result of the localization of 5f electrons. The subsequent chemistry of later actinides is thought to closely parallel lanthanides in that bonding is expected to be ionic and complexation should not substantially alter the electronic structure of the metal ions. Here we demonstrate that ligation of californium(III) by a pyridine derivative results in significant deviations in the properties of the resultant complex with respect to that predicted for the free ion. We expand on this by characterizing the americium and curium analogues for comparison, and show that these pronounced effects result from a second transition in periodicity in the actinide series that occurs, in part, because of the stabilization of the divalent oxidation state. The metastability of californium(II) is responsible for many of the unusual properties of californium including the green photoluminescence.

Recent experimental and theoretical interest in the superconducting phase of the heavy fermion material URu2Si2 has led to a number of proposals in which the superconducting order parameter breaks time-reversal symmetry (TRS). In this study we measured polar Kerr effect (PKE) as a function of temperature for several high-quality single crystals of URu2Si2. We find an onset of PKE below the superconducting transition that is consistent with a TRS-breaking order parameter. This effect appears to be independent of an additional, possibly extrinsic, PKE generated above the hidden order transition at THO = 17.5 K, and contains structure below Tc suggestive of additional physics within the superconducting state.

A second-order phase transition is associated with emergence of an "order parameter" and a spontaneous symmetry breaking. For the heavy fermion superconductor URu 2 Si 2 , the symmetry of the order parameter associated with its ordered phase below 17.5 K has remained ambiguous despite 30 years of research, and hence is called "hidden order" (HO). Here we use polarization resolved Raman spectroscopy to specify the symmetry of the low energy excitations above and below the HO transition. These excitations involve transitions between interacting heavy uranium 5f orbitals, responsible for the broken symmetry in the HO phase. From the symmetry analysis of the collective mode, we determine that the HO parameter breaks local vertical and diagonal reflection symmetries at the uranium sites, resulting in crystal field states with distinct chiral properties, which order to a commensurate chirality density wave ground state. 1 arXiv:1410.6398v1 [cond-mat.str-el]

Berkelium is positioned at a crucial location in the actinide series between the inherently stable half-filled 5f(7) configuration of curium and the abrupt transition in chemical behavior created by the onset of a metastable divalent state that starts at californium. However, the mere 320-day half-life of berkelium's only available isotope, (249)Bk, has hindered in-depth studies of the element's coordination chemistry. Herein, we report the synthesis and detailed solid-state and solution-phase characterization of a berkelium coordination complex, Bk(III)tris(dipicolinate), as well as a chemically distinct Bk(III) borate material for comparison. We demonstrate that berkelium's complexation is analogous to that of californium. However, from a range of spectroscopic techniques and quantum mechanical calculations, it is clear that spin-orbit coupling contributes significantly to berkelium's multiconfigurational ground state.

Electrical resistivity, specific heat, and magnetization measurements on URu2−xFexSi2 reveal a two-fold enhancement of the "hidden order" (HO)/large moment antiferromagnetic (LMAFM) phase boundary T0(x). The T0(P ch ) curve, obtained by converting x to "chemical pressure" P ch , 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 (P ch ≈ 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. STM, ARPES, PCS) but cannot be used under pressure.