Within a Kondo lattice, the strong hybridization between electrons localized in real space (r-space) and those delocalized in momentum-space (k-space) generates exotic electronic states called 'heavy fermions'. In URu(2)Si(2) these effects begin at temperatures around 55 K but they are suddenly altered by an unidentified electronic phase transition at T(o) = 17.5 K. Whether this is conventional ordering of the k-space states, or a change in the hybridization of the r-space states at each U atom, is unknown. Here we use spectroscopic imaging scanning tunnelling microscopy (SI-STM) to image the evolution of URu(2)Si(2) electronic structure simultaneously in r-space and k-space. Above T(o), the 'Fano lattice' electronic structure predicted for Kondo screening of a magnetic lattice is revealed. Below T(o), a partial energy gap without any associated density-wave signatures emerges from this Fano lattice. Heavy-quasiparticle interference imaging within this gap reveals its cause as the rapid splitting below T(o) of a light k-space band into two new heavy fermion bands. Thus, the URu(2)Si(2) 'hidden order' state emerges directly from the Fano lattice electronic structure and exhibits characteristics, not of a conventional density wave, but of sudden alterations in both the hybridization at each U atom and the associated heavy fermion states.

Many correlated electron materials, such as high-temperature superconductors 1 , geometrically frustrated oxides 2 and lowdimensional magnets 3,4 , are still objects of fruitful study because of the unique properties that arise owing to poorly understood many-body effects. Heavy-fermion metals 5 -materials that have high effective electron masses due to those effects-represent a class of materials with exotic properties, ranging from unusual magnetism, unconventional superconductivity and 'hidden' order parameters 6 . The heavy-fermion superconductor URu 2 Si 2 has held the attention of physicists for the past two decades owing to the presence of a 'hidden-order' phase below 17.5 K. Neutron scattering measurements indicate that the ordered moment is 0.03μ B , much too small to account for the large heat-capacity anomaly at 17.5 K. We present recent neutron scattering experiments that unveil a new piece of this puzzle-the spin-excitation spectrum above 17.5 K exhibits well-correlated, itinerant-like spin excitations up to at least 10 meV, emanating from incommensurate wavevectors. The large entropy change associated with the presence of an energy gap in the excitations explains the reduction in the electronic specific heat through the transition.The central issue in URu 2 Si 2 concerns the identification of the order parameter that explains the reduction in the specific heat coefficient, γ = C/T, and thus the change in entropy, through the transition at 17.5 K (ref. 6). Numerous speculations about the ground state have been advanced, from quadrupolar ordering 7 , to spin-density wave formation 8 , to 'orbital currents' 9 to account for the missing entropy. Here, we present cold-neutron time-offlight spectroscopy results that shed some light on the 'hiddenorder' (HO) in URu 2 Si 2 . We have carried out experiments above and below the ordering temperature to measure how the spin excitations evolve. It is clear from our data that above T 0 the spectrum is dominated by fast, itinerant-like spin excitations emanating from incommensurate wavevectors at positions located 0.4a* from the antiferromagnetic (AF) points. From the group velocity and temperature dependence of these modes, we surmise that these are heavy-quasiparticle excitations that form below the 'coherence temperature' and play a crucial role in the formation of the heavy-fermion and HO states. The gapping of these excitations, which corresponds to a loss of accessible states, accounts for the reduction in γ through the transition at 17.5 K. Figure 1 shows the excitation spectrum of URu 2 Si 2 at 1.5 K in the H00 plane. The characteristic gaps at ∼2 meV at the AF zone centre (1, 0, 0) and ∼4 meV at the incommensurate wavevectors (0.6, 0, 0) and (1.4, 0, 0) have been known for some time 10 . The incommensurate wavevector corresponds to a displacement of ∼0.4a * from the AF zone centres (that is, where h + k + l = an odd integer, and is thus forbidden in the body-centred-cubic chemical structure). A scenario for this modesoftening at the incommensurate positi...

The structure of recently discovered identical superdeformed bands in |S| Tb and ,52 Dy and in l50 Gd and 15, Tb are discussed in terms of the strong-coupling approach. Based on the experimental evidence that the superdeformed core of ,52 Dy is extremely insensitive to the polarization effects induced by the odd particle, the bands are shown to exhibit the presence of the pseudo SU(3) symmetry at extreme conditions of large elongations and high spins.

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The y-ray spectrum in the giant dipole resonance (GDR) region associated with the reaction 40 Ar+ 70 Ge at 10 MeV/nucleon has been measured in coincidence with residues of the heavy composite systems whose excitation energy was E* -230 MeV. From the statistical-model analysis, it is deduced that the GDR strength is consistent with 100% of the energy-weighted sum rule; the energy is 16 ± 1 MeV while the width is 13±1 MeV. This value is not very different from the one measured at E* =130 MeV, thus pointing to saturation effects in the damping of the GDR.PACS numbers: 24.30.Cz, 25.70.Gh, 27.60.+J Information on the properties of nuclei at high temperature can be obtained by measuring the high-energy y rays which are emitted when they decay, in particular in the energy region of the giant dipole resonance (GDR) decay. In fact, studies of the energy, width, structure, and strength of the GDR as a function of excitation energy and spin provide direct information on the coupling of the GDR to fluctuations of the nuclear surface and on the size and strength of the average potential at finite temperature. Studies of this type have been carried out for a number of nuclei up to moderate excitation energies. The width of the GDR built on excited states in the Sn isotopes 1,2 has been found to increase nearly quadratically with the excitation energy of the compound nucleus up to E* « 130 MeV. Thermal fluctuations exploring the ensemble of nuclear shapes can account for only part of the observed increase. Indeed, the angular momentum transferred to the compound nucleus increases with bombarding energy and leads to a broadening of the GDR strength function due to deformation effects. This is supported by calculations 3 of the potential-energy surfaces of Sn nuclei as a function of nuclear temperature T and spin /, which predict that the Sn isotopes evolve from spherical shapes at low / to well deformed, predominantly oblate shapes at / > 40. Assuming that the dipole vibration couples adiabatically to the nuclear surface vibrations, the width of the GDR increases.In the present paper we report on a study of the structure of the GDR up to excitation energies is*«230 MeV in 1,0 Sn nuclei. We find that the width of the GDR at this E* does not deviate appreciably from the one measured at 130 MeV. This saturation opens up for new insights into the damping mechanism of the GDR at finite temperature.A 1-mg/cm 2 70 Ge target was bombarded by a 400-MeV 40 Ar beam from the coupled cyclotron SARA of the Institut des Sciences Nucleaires, Grenoble. The reaction products were detected in two position-sensitive parallel-plate avalance counters (PPAC's) with a sensitive area of 15x20 cm 2 . The PPAC's were located symmetrically on both sides of the beam in the forward direction and subtended an angle of ± (3°-20°) in the laboratory system. The PPAC provided information on the time of flight from the target and the energy loss of projectilelike fragments and residues from fusion and incomplete fusion. The high-energy y rays were measured in an a...

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