A model for the doped copper oxide compounds

We present a relativistic spin-fermion model for the cuprates, in which both the charge and spin degrees of freedom are treated dynamically. The spin-charge coupling parameter is associated with the doping fraction. The model is able to account for the various phases of the cuprates and their properties, not only at low and intermediate doping but also for (highly) over-doped compounds. In particular, we acquire a qualitative understanding of high-T_c superconductivity through Bose-Einstein condensation of bound charge pairs. The mechanism that binds these pairs does not require a Fermi sea.Comment: 9 pages, 2 postscript figures. Version accepted for publication in Europhys. Let

Simple description of the anisotropic two-channel Kondo problem

We adapt strong-coupling methods first used in the one-channel Kondo model to develop a simple description of the spin-$1\over 2$ two-channel Kondo model with channel anisotropy. Our method exploits spin-charge decoupling to develop a compactified Hamiltonian that describes the spin excitations. The structure of the fixed-point Hamiltonian and quasiparticle impurity S-matrix are incompatible with a Fermi liquid description.Comment: 4 pages, latex (uses revtex and epsf macros) with 3 figures - all in a self unpacking uuencoded file. Revisions include changes to Fig. 1(a) and detailed discussion of the spin excitation

The fine-tuning price of the early LHC

LHC already probed and excluded half of the parameter space of the Constrained Minimal Supersymmetric Standard Model allowed by previous experiments. Only about 0.3% of the CMSSM parameter space survives. This fraction rises to about 0.9% if the bound on the Higgs mass can be circumvented.Comment: 7 pages. v3: updated with new bounds from ATLAS and CMS at 1.1/fb presented at the EPS-HEP-2011 conferenc

Algebraic Approach to Interacting Quantum Systems

We present an algebraic framework for interacting extended quantum systems to study complex phenomena characterized by the coexistence and competition of different states of matter. We start by showing how to connect different (spin-particle-gauge) {\it languages} by means of exact mappings (isomorphisms) that we name {\it dictionaries} and prove a fundamental theorem establishing when two arbitrary languages can be connected. These mappings serve to unravel symmetries which are hidden in one representation but become manifest in another. In addition, we establish a formal link between seemingly unrelated physical phenomena by changing the language of our model description. This link leads to the idea of {\it universality} or equivalence. Moreover, we introduce the novel concept of {\it emergent symmetry} as another symmetry guiding principle. By introducing the notion of {\it hierarchical languages}, we determine the quantum phase diagram of lattice models (previously unsolved) and unveil hidden order parameters to explore new states of matter. Hierarchical languages also constitute an essential tool to provide a unified description of phases which compete and coexist. Overall, our framework provides a simple and systematic methodology to predict and discover new kinds of orders. Another aspect exploited by the present formalism is the relation between condensed matter and lattice gauge theories through quantum link models. We conclude discussing applications of these dictionaries to the area of quantum information and computation with emphasis in building new models of computation and quantum programming languages.Comment: 44 pages, 14 psfigures. Advances in Physics 53, 1 (2004

Moment of Inertia and Superfluidity of a Trapped Bose Gas

The temperature dependence of the moment of inertia of a dilute Bose gas confined in a harmonic trap is determined. Deviations from the rigid value, due to the occurrence of Bose-Einstein condensation, reveal the superfluid behaviour of the system. In the noninteracting gas these deviations become important at temperatures of the order of $T_c N^{-1/12}$. The role of interactions is also discussed.Comment: 10 pages, REVTEX, 1 figure attached as postscript fil

Multi frequency evaporative cooling to BEC in a high magnetic field

We demonstrate a way to circumvent the interruption of evaporative cooling observed at high bias field for $^{87}$Rb atoms trapped in the (F=2, m=+2) ground state. Our scheme uses a 3-frequencies-RF-knife achieved by mixing two RF frequencies. This compensates part of the non linearity of the Zeeman effect, allowing us to achieve BEC where standard 1-frequency-RF-knife evaporation method did not work. We are able to get efficient evaporative cooling, provided that the residual detuning between the transition and the RF frequencies in our scheme is smaller than the power broadening of the RF transitions at the end of the evaporation ramp.Comment: 12 pages, 2 figure

Kondo tunneling through real and artificial molecules

When a cerocene molecule is chemisorbed on metallic substrate, or when an asymmetric double dot is hybridized with itinerant electrons, its singlet ground state crosses its lowly excited triplet state, leading to a competition between the Zhang-Rice mechanism of singlet-triplet splitting in a confined cluster and the Kondo effect (which accompanies the tunneling through quantum dot under a Coulomb blockade restriction). The rich physics of an underscreened S=1 Kondo impurity in the presence of low-lying triplet/singlet excitations is exposed. Estimates of the magnetic susceptibility and the electric conductance are presented.Comment: 4 two-column revtex pages including 1 eps figur

Cooper problem in the vicinity of Anderson transition

We study numerically the ground state properties of the Cooper problem in the three-dimensional Anderson model. It is shown that attractive interaction creates localized pairs in the metallic noninteracting phase. This localization is destroyed at sufficiently weak disorder. The phase diagram for the delocalization transition in the presence of disorder and interaction is determined.Comment: revtex, 4 pages, 4 figure

Finite-Temperature Transition into a Power-Law Spin Phase with an Extensive Zero-Point Entropy

We introduce an $xy$ generalization of the frustrated Ising model on a triangular lattice. The presence of continuous degrees of freedom stabilizes a {\em finite-temperature} spin state with {\em power-law} discrete spin correlations and an extensive zero-point entropy. In this phase, the unquenched degrees of freedom can be described by a fluctuating surface with logarithmic height correlations. Finite-size Monte Carlo simulations have been used to characterize the exponents of the transition and the dynamics of the low-temperature phase

Molecular modeling of hydrate-clathrates via ab initio, cell potential, and dynamic methods

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 2005.Includes bibliographical references.High level ab initio quantum mechanical calculations were used to determine the intermolecular potential energy surface between argon and water, corrected for many- body interactions, to predict monovariant and invariant phase equilibria for the argon hydrate and mixed methane-argon hydrate systems. A consistent set of reference parameters for the van der Waals and Platteeuw model, ... and ..., were developed for Structure II hydrates and are not dependent on any fitted parameters. Our previous methane-water ab initio energy surface has been recast onto a site-site potential model that predicts guest occupancy experiments with improved accuracy compared to previous studies. This methane-water potential is verified via ab initio many-body calculations and thus should be generally applicable to dense methane-water systems. New reference parameters, ... and ..., for Structure I hydrates using the van der Waals and Platteeuw model were also determined. Equilibrium predictions with an average absolute deviation of 3.4% for the mixed hydrate of argon and methane were made. These accurate predictions of the mixed hydrate system provide an independent test of the accuracy of the intermolecular potentials.(cont.) Finally, for the mixed argon-methane hydrate, conditions for structural changes from the Structure I hydrate of methane to the Structure II hydrate of argon were predicted and await experimental confirmation. We present the application of a mathematical method reported earlier' by which the van der Waals-Platteeuw statistical mechanical model with the Lennard-Jones and Devonshire approximation can be posed as an integral equation with the unknown function being the intermolecular potential between the guest molecules and the host molecules. This method allows us to solve for the potential directly for hydrates for which the Langmuir constants are computed, either from experimental data or from ab initio data. Given the assumptions made in the van der Waals-Platteeuw model with the spherical-cell approximation, there are an infinite number of solutions; however, the only solution without cusps is a unique central-well solution in which the potential is at a finite minimum at the center to the cage.(cont.) From this central-well solution, we have found the potential well depths and volumes of negative energy for sixteen single-component hydrate systems: ethane (C₂H₆), cyclopropane (C₃H₆), methane (CH₄), argon (Ar), and chlorodifluoromethane (R-22) in structure I; and ethane (C₂H₆), cyclopropane (C₃H₆), propane (C₃H₈), isobutane (C₄H₁₀), methane (CH₄), argon (Ar), trichlorofluoromethane (R-1 1), dichlorodifluoromethane (R-12), bromotrifluoromethane (R-1 3B 1), chloroform (CHC1₃), and 1,1,1,2-Tetrafluoroethane (R-134a) in structure II. This method and the calculated cell potentials were validated by predicting existing mixed hydrate phase equilibrium data without any fitting parameters and calculating mixture phase diagrams for methane, ethane, isobutane, and cyclopropane mixtures. Several structural transitions that have been determined experimentally as well as some structural transitions that have not been examined experimentally were also predicted. In the methane-cyclopropane hydrate system, a structural transition from structure I to structure II and back to structure I is predicted to occur outside of the known structure II range for the cyclopropane hydrate.(cont.) Quintuple (Lw-SI-SII-Lho-V) points have been predicted for the ethane-propane-water (277.3 K, 12.28 bar, and Xeth,waterfree = 0.676) and ethane-isobutane-water (274.7 K, 7.18 bar, and Xeth,waterfree = 0.81) systems. A two-fold mechanism for hydrate inhibition has been proposed and tested using molecular dynamic simulations for PEO, PVP, PVCap, and VIMA. This mechanism hypothesizes that (1) as potential guest molecules become coordinated by water, form nuclei, and begin to grow, nearby inhibitor molecules disrupt the organization of the forming clathrate and (2) inhibitor molecules bind to the surface of the hydrate crystal precursor and retards further growth along the bound growth plane resulting in a modified planar morphology. This mechanism is supported by the results of our molecular dynamic simulations for the four inhibitor molecules studied. PVCap and VIMA, the more effective inhibitors, shows strong interactions with the liquid water phase under hydrate forming conditions, while PVP and PEO appear relatively neutral to the surrounding water.by Brian Anderson.Ph.D