Role of commensurate and incommensurate low-energy excitations in the paramagnetic to hidden-order transition of URu2_2Si2_2

We report low-energy inelastic neutron scattering data of the paramagnetic (PM) to hidden-order (HO) phase transition at $T_0=17.5\,{\rm K}$ in URu$_2$Si$_2$. While confirming previous results for the HO and PM phases, our data reveal a pronounced wavevector dependence of low-energy excitations across the phase transition. To analyze the energy scans we employ a damped harmonic oscillator model containing a fit parameter $1/\Gamma$ which is expected to diverge at a second-order phase transition. Counter to expectations the excitations at $\vec{Q}_1=(1.44,0,0)$ show an abrupt step-like suppression of $1/\Gamma$ below $T_0$, whereas excitations at $\vec{Q}_0=(1,0,0)$, associated with large-moment antiferromagnetism (LMAF) under pressure, show an enhancement and a pronounced peak of $1/\Gamma$ at $T_0$. Therefore, at the critical HO temperature $T_0$, LMAF fluctuations become nearly critical as well. This is the behavior expected of a super-vector order parameter with nearly degenerate components for the HO and LMAF leading to nearly isotropic fluctuations in the combined order-parameter space.Comment: 6 pages; v3 accepted journal version; minor modifications compared to v

Midinfrared Conductivity in Orientationally Disordered Doped Fullerides

The coupling between the intramolecular vibrational modes and the doped conduction electrons in $M_3C_{60}$ is studied by a calculation of the electronic contributions to the phonon self energies. The calculations are carried out for an orientationally ordered reference solid with symmetry $Fm \bar{3} m$ and for a model with quenched orientational disorder on the fullerene sites. In both cases, the dispersion and symmetry of the renormalized modes is governed by the electronic contributions. The current current correlation functions and frequency dependent conductivity through the midinfrared are calculated for both models. In the disordered structures, the renormalized modes derived from even parity intramolecular phonons are resonant with the dipole excited single particle spectrum, and modulate the predicted midinfrared conductivity. The spectra for this coupled system are calculated for several recently proposed microscopic models for the electron phonon coupling, and a comparison is made with recent experimental data which demonstrate this effect.Comment: 32 pages + 9 postscript figures (on request), REVTeX 3.

Superconductivity in Ce- and U-based "122" heavy-fermion compounds

This review discusses the heavy-fermion superconductivity in Ce- and U-based compounds crystallizing in the body-centered tetragonal ThCr2Si2 structure. Special attention will be paid to the theoretical background of these systems which are located close to a magnetic instability.Comment: 12 pages, 9 figures. Invited topical review (special issue on "Recent Developments in Superconductivity") Metadata and references update

Jahn-Teller polarons and their superconductivity in a molecular conductor

We present a theoretical study of a possibility of superconductivity in a three dimensional molecular conductor in which the interaction between electrons in doubly degenerate molecular orbitals and an {\em intra}molecular vibration mode is large enough to lead to the formation of $E\otimes \beta$ Jahn-Teller small polarons. We argue that the effective polaron-polaron interaction can be attractive for material parameters realizable in molecular conductors. This interaction is the source of superconductivity in our model. On analyzing superconducting instability in the weak and strong coupling regimes of this attractive interaction, we find that superconducting transition temperatures up to 100 K are achievable in molecular conductors within this mechanism. We also find, for two particles per molecular site, a novel Mott insulating state in which a polaron singlet occupies one of the doubly degenerate orbitals on each site. Relevance of this study in the search for new molecular superconductors is pointed out.Comment: Submitted to Phys. Rev.

Financial diversification before modern portfolio theory: UK financial advice documents in the late nineteenth and the beginning of the twentieth century

The paper offers textual evidence from a series of financial advice documents in the late nineteenth century and the early twentieth century of how UK investors perceived of and managed risk. In the world’s largest financial centre of the time, UK investors were familiar with the concept of correlation and financial advisers’ suggestions were consistent with the recommendations of modern portfolio theory in relation to portfolio selection strategies. From the 1870s, there was an increased awareness of the benefits of financial diversification - primarily putting equal amounts into a number of different securities - with much of the emphasis being on geographical rather than sectoral diversification and some discussion of avoiding highly correlated investments. Investors in the past were not so naïve as mainstream financial discussions suggest today

Photodissociation dynamics of the HCNN radical

The photodissociation dynamics of the diazomethyl (HCNN) radical have been studied using fast radical beam photofragment translational spectroscopy. A photofragment yield spectrum was obtained for the range of 25 510-40 820 cm-1, and photodissociation was shown to occur for energies above 25 600 cm-1. The only product channel observed was the formation of CH and N2. Fragment translational energy and angular distributions were obtained at several energies in the range covered by the photofragment yield spectrum. The fragment translational energy distributions showed at least two distinct features at energies up to 4.59 eV, and were not well fit by phase space theory at any of the excitation energies studied. A revised C-N bond dissociation energy and heat of formation for HCNN, D0 (HC-NN) =1.139±0.019 eV and Δf H0 (HCNN) =5.010±0.023 eV, were determined. © 2006 American Institute of Physics.Ann Elise Faulhaber, Jason R. Gascooke, Alexandra A. Hoops, and Daniel M. Neumar

Theory of Superconducting TcT_{c} of doped fullerenes

We develop the nonadiabatic polaron theory of superconductivity of $M_{x}C_{60}$ taking into account the polaron band narrowing and realistic electron-phonon and Coulomb interactions. We argue that the crossover from the BCS weak-coupling superconductivity to the strong-coupling polaronic and bipolaronic superconductivity occurs at the BCS coupling constant $\lambda\sim 1$ independent of the adiabatic ratio, and there is nothing ``beyond'' Migdal's theorem except small polarons for any realistic electron-phonon interaction. By the use of the polaronic-type function and the ``exact'' diagonalization in the truncated Hilbert space of vibrons (``phonons'') we calculate the ground state energy and the electron spectral density of the $C_{60}^{-}$ molecule. This allows us to describe the photoemission spectrum of $C_{60}^{-}$ in a wide energy region and determine the electron-phonon interaction. The strongest coupling is found with the high-frequency pinch $A_{g2}$ mode and with the Frenkel exciton. We clarify the crucial role of high-frequency bosonic excitations in doped fullerenes which reduce the bare bandwidth and the Coulomb repulsion allowing the intermediate and low-frequency phonons to couple two small polarons in a Cooper pair. The Eliashberg-type equations are solved for low-frequency phonons. The value of the superconducting $T_{c}$, its pressure dependence and the isotope effect are found to be in a remarkable agreement with the available experimental data.Comment: 20 pages, Latex, 4 figures available upon reques

A Public Option for the Core

This paper is focused not on the Internet architecture – as defined by layering, the narrow waist of IP, and other core design principles – but on the Internet infrastructure, as embodied in the technologies and organizations that provide Internet service. In this paper we discuss both the challenges and the opportunities that make this an auspicious time to revisit how we might best structure the Internet’s infrastructure. Currently, the tasks of transit-between-domains and last-mile-delivery are jointly handled by a set of ISPs who interconnect through BGP. In this paper we propose cleanly separating these two tasks. For transit, we propose the creation of a “public option” for the Internet’s core backbone. This public option core, which complements rather than replaces the backbones used by large-scale ISPs, would (i) run an open market for backbone bandwidth so it could leverage links offered by third-parties, and (ii) structure its terms-of-service to enforce network neutrality so as to encourage competition and reduce the advantage of large incumbents

Photodissociation spectroscopy and dynamics of the CH(2)CFO radical

Alexandra A. Hoops, Jason R. Gascooke, Kathryn E. Kautzman, Ann Elise Faulhaber, and Daniel M. Neumar

Renormalization Group Approach to the Coulomb Pseudopotential for C_{60}

A numerical renormalization group technique recently developed by one of us is used to analyse the Coulomb pseudopotential (${\mu^*}$) in ${{\rm C}_{60}}$ for a variety of bare potentials. We find a large reduction in ${\mu^*}$ due to intraball screening alone, leading to an interesting non-monotonic dependence of ${\mu^*}$ on the bare interaction strength. We find that ${\mu^*}$ is positive for physically reasonable bare parameters, but small enough to make the electron-phonon coupling a viable mechanism for superconductivity in alkali-doped fullerides. We end with some open problems.Comment: 12 pages, latex, 7 figures available from [email protected]