We report the phase diagram of the doping series U_2Pt_xRh_(1-x)C_2, studied through measurements of resistivity, specific heat and magnetic susceptibility. The Neel temperature of U_2RhC_2 of ~ 22 K is suppressed with increasing Pt content, reaching
zero temperature close to x=0.7, where we observed signatures of increased quantum fluctuations. In addition, evidence is presented that the antiferromagnetic state undergoes a spin-reorientation transition upon application of an applied magnetic field. This transition shows non-monotonic behaviour as a function of x, peaking at around x=0.3. Superconductivity is observed for x>=0.9, with T_c increasing with increasing x. The reduction in T_c and increase in residual resistivity with decreasing Pt content is inconsistent with the extension of the Abrikosov-Gorkov theory to unconventional superconductivity.
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.
Quantum criticality in the normal and superconducting state of the heavy-fermion metal CeCoIn$_5$ is studied by measurements of the magnetic Gr{u}neisen ratio, $Gamma_H$, and specific heat in different field orientations and temperatures down to 50 m
K. Universal temperature over magnetic field scaling of $Gamma_H$ in the normal state indicates a hidden quantum critical point at zero field. Within the superconducting state the quasiparticle entropy at constant temperature increases upon reducing the field towards zero, providing additional evidence for zero-field quantum criticality.
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 study the ionization dynamics in intense laser-droplet interaction using three-dimensional, relativistic particle-in-cell simulations. Of particular interest is the laser intensity and frequency regime for which initially transparent, wavelength-s
ized targets are not homogeneously ionized. Instead, the charge distribution changes both in space and in time on a sub-cycle scale. One may call this the extreme nonlinear Mie-optics regime. We find that - despite the fact that the plasma created at the droplet surface is overdense - oscillating electric fields may penetrate into the droplet under a certain angle, ionize, and propagate in the just generated plasma. This effect can be attributed to the local field enhancements at the droplet surface predicted by standard Mie theory. The penetration of the fields into the droplet leads to the formation of a highly inhomogeneous charge density distribution in the droplet interior, concentrated mostly in the polarization plane. We present a self-similar, exponential fit of the fractional ionization degree which depends only on a dimensionless combination of electric field amplitude, droplet radius, and plasma frequency with only a weak dependence on the laser frequency in the overdense regime.
The heavy-fermion superconductor CeCoIn$_5$ displays an additional transition within its superconducting (SC) state, whose nature is characterized by high-precision studies of the isothermal field dependence of the entropy, derived from combined spec
ific heat and magnetocaloric effect measurements at temperatures $Tgeq 100$ mK and fields $Hleq 12$ T aligned parallel, perpendicular and $18^circ$ off the tetragonal [100] direction. For any of these conditions, we do not observe an additional entropy contribution upon tuning at constant temperature by magnetic field from the homogeneous SC into the presumed Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) SC state. By contrast, for $Hparallel [100]$ a negative isothermal entropy contribution, compatible with spin-density-wave (SDW) ordering, is found. Our data exclude the formation of a FFLO state in CeCoIn$_5$ for out-of-plane field directions, where no SDW order exists.
Since their discovery nearly a decade ago, plutonium-based superconductors have attracted considerable interest, which is now heightened by the latest discovery of superconductivity in PuCoIn5. In the framework of density functional theory (DFT) with
in the generalized gradient approximation (GGA) together with dynamical mean-field theory (DMFT), we present a comparative study of the electronic structure of PuCoIn5 with the related material, PuCoGa5. Overall, a similar GGA-based electronic structure, including the density of states, energy dispersion, and Fermi surface topology, was found for both compounds. The GGA Pu 5f band was narrower in PuCoIn5 than in PuCoGa5, resulting in an effective reduction of Kondo screening in the former system, as also shown by DMFT calculations. This phenomenon is due to the expanded lattice for PuCoIn5.
Magnetic susceptibility results for single crystals of the new cubic compounds UT$_2$Al$_{20}$ (T=Mn, V, and Mo) are reported. Magnetization, specific heat, resistivity, and neutron diffraction results for a single crystal and neutron diffraction and
inelastic spectra for a powder sample are reported for UMn$_2$Al$_{20}$. For T = V and Mo, temperature independent Pauli paramagnetism is observed. For UMn$_2$Al$_{20}$, a ferromagnetic transition is observed in the magnetic susceptibility at $T_c$ = 20 K. The specific heat anomaly at $T_c$ is very weak while no anomaly in the resistivity is seen at $T_c$. We discuss two possible origins for this behavior of UMn$_2$Al$_{20}$: moderately small moment itinerant ferromagnetism, or induced local moment ferromagnetism.
One of the major challenges of particle physics has been to gain an in-depth understanding of the role of quark flavor and measurements and theoretical interpretations of their results have advanced tremendously: apart from masses and quantum numbers
of flavor particles, there now exist detailed measurements of the characteristics of their interactions allowing stringent tests of Standard Model predictions. Among the most interesting phenomena of flavor physics is the violation of the CP symmetry that has been subtle and difficult to explore. Till early 1990s observations of CP violation were confined to neutral $K$ mesons, but since then a large number of CP-violating processes have been studied in detail in neutral $B$ mesons. In parallel, measurements of the couplings of the heavy quarks and the dynamics for their decays in large samples of $K, D$, and $B$ mesons have been greatly improved in accuracy and the results are being used as probes in the search for deviations from the Standard Model. In the near future, there will be a transition from the current to a new generation of experiments, thus a review of the status of quark flavor physics is timely. This report summarizes the results of the current generation of experiments that is about to be completed and it confronts these results with the theoretical understanding of the field.
The generation of relativistic attosecond electron bunches is observed in three-dimensional, relativistic particle-in-cell simulations of the interaction of intense laser light with droplets. The electron bunches are emitted under certain angles whic
h depend on the ratios of droplet radius to wavelength and plasma frequency to laser frequency. The mechanism behind the multi-MeV attosecond electron bunch generation is investigated using Mie theory. It is shown that the angular distribution and the high electron energies are due to a parameter-sensitive, time-dependent local field enhancement at the droplet surface.
