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.
Recent experimental and theoretical interest in the superconducting phase of the heavy fermion material URu$_2$Si$_2$ 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 URu$_2$Si$_2$. 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 $T_{HO}=17.5$ K, and contains structure below $T_c$ suggestive of additional physics within the superconducting state.
We have carried out a careful magnetic neutron scattering study of the heavy fermion compound URuSi to probe the possible existence of a small magnetic moment parallel to tetragonal basal plane in the hidden-order phase. This small in-plane component
of the magnetic moment on the uranium sites $S_parallel$ has been postulated by two recent models (rank-5 superspin/hastatic order) aiming to explain the hidden-order phase, in addition to the well-known out-of-plane component $S_perp ~ approx~0.01-0.04 $mu_B$/U. In order to separate $S_parallel$ and $S_perp$ we take advantage of the condition that for magnetic neutron scattering only the components of the magnetic structure that are perpendicular to the scattering vector $Q$ contribute to the magnetic scattering. We find no evidence for an in-plane magnetic moment $S_parallel$. Based on the statistics of our measurement, we establish that the upper experimental limit for the size of any possible in-plane component is $S^{rm{max}}_parallel ~ leq~1cdot 10^{-3} ~mu_B$/U.
We discuss the discrepancy between the measured optical Drude weight and band structure calculations in LaFePO and other iron-based superconductors. This discrepancy is mostly due to mass renormalization arising from electronic correlation effects.
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 electrons kinetic energy leading to remarkable experimental man
ifestations in optical spectroscopy. The high-Tc superconducting cuprates are perhaps the most studied examples of such correlated metals. The occurrence of high-Tc superconductivity in the iron pnictides puts a spotlight on the relevance of correlation effects in these materials. 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. Hallmarks of strong electronic many-body effects reported here are important because the iron pnictides expose a new pathway towards a correlated electron state that does not explicitly involve the Mott transition.
X-ray diffraction, electrical resistivity, magnetization, specific heat, and thermoelectric power measurements are presented for single crystals of the new filled skutterudite compound {CeOsAs}, which reveal phenomena that are associated with f - ele
ctron - conduction electron hybridization. Valence fluctuations or Kondo behavior dominates the physics down to $T$ $sim$ 135 K. The correlated electron behavior is manifested at low temperatures as a hybridization gap insulating state. The small energy gap $Delta$$_1$/k$_B$ $sim$ 73 K, taken from fits to electrical resistivity data, correlates with the evolution of a weakly magnetic or nonmagnetic ground state, which is evident in the magnetization data below a coherence temperature $T$$_{coh}$ $sim$ 45 K. Additionally, the low temperature electronic specific heat coefficient is small, $gamma$ $sim$ 19 mJ/mol K$^2$. Some results for the nonmagnetic analogue compound {LaOsAs} are also presented for comparison purposes.
We present the first infrared and optical study in the normal state of ab-plane oriented single crystals of the iron-oxypnictide superconductor LaFePO. We find that this material is a low carrier density metal with a moderate level of correlations an
d exhibits signatures of electron-boson coupling. The data is consistent with the presence of coherent quasiparticles in LaFePO.
Single crystals of the compound LaFePO were prepared using a flux growth technique at high temperatures. Electrical resistivity measurements reveal metallic behavior and a resistive transition to the superconducting state at a critical temperature T_
c ~ 6.6 K. Magnetization measurements also show the onset of superconductivity near 6 K. In contrast, specific heat measurements manifest no discontinuity at T_c. These results lend support to the conclusion that the superconductivity is associated with oxygen vacancies that alter the carrier concentration in a small fraction of the sample, although superconductivity characterized by an unusually small gap value can not be ruled-out. Under applied magnetic fields, T_c is suppressed anisotropically for fields perpendicular and parallel to the ab-plane, suggesting that the crystalline anisotropy strongly influences the superconducting state. Preliminary high-pressure measurements show that T_c passes through a maximum of nearly 14 K at ~ 110 kbar, demonstrating that significantly higher T_c values may be achieved in the phosphorus-based oxypnictides.
We have performed several high-pressure resistivity experiments on the recently discovered superconductors La[O_{0.89}F_{0.11}]FeAs and Ce[O_{0.88}F_{0.12}]FeAs. At ambient pressure, these materials have superconducting onset temperatures T_c of 28 K
and 44 K, respectively. While the T_c of La[O_{0.89}F_{0.11}]FeAs goes through a maximum between 10-68 kbar, in qualitative agreement with a recent report by Takahashi et al., the T_c of Ce[O_{0.88}F_{0.12}]FeAs decreases monotonically over the measured pressure range. At 265 kbar, the T_c of the cerium-based compound has been suppressed below 1.1 K.
Electrical resistivity $rho$, specific heat C, and magnetic susceptibility $chi$ measurements made on the filled skutterudite CeRu_4As_{12} reveal non-Fermi liquid (NFL) T - dependences at low T, i.e., $rho$(T) $sim$ T^{1.4} and weak power law or log
arithmic divergences in C(T)/T and $chi$(T). Measurements also show that the T - dependence of the thermoelectric power S(T) deviates from that seen in other Ce systems. The NFL behavior appears to be associated with fluctuations of the Ce valence between 3^+ and 4^+ rather than a typical Kondo lattice scenario that would be appropriate for an integral Ce valence of 3^+.
