Gossamer superconductivity

A new superconducting Hamiltonian is introduced for which the exact ground state is the Anderson resonating valence bond. It differs from the t–J and Hubbard Hamiltonians in possessing a powerful attractive force. Its superconducting state is characterized by a full and intact d-wave tunnelling gap, quasiparticle photoemission intensities that are strongly suppressed, a suppressed superfluid density, and an incipient Mott–Hubbard gap.     

The use of coherent states in the variational treatment of proton-neutron interactions

Coherent states are used as trial states to determine, variationally, the structure of the eigenvectors belonging to a schematic Hamiltonian consisting of single-particle, pairing and residual proton-neutron interaction terms. It is shown that the standard proton-neutron quasiparticle random-phase approximation (pn-QRPA) is recovered, as a variational theory, by replacing quasiparticle pair creation and annihilation operators by bosons. It is also shown that an exact, algebra preserving, mapping of the Hamiltonian is needed to describe the spectrum beyond the QRPA phase transition. The role of the spurious components of the trial wave functions is discussed. Received: 19 February 2002 / Accepted: 3 May 2002     

Equivalence of random and competing nonrandom bonds for Edwards Anderson spin-glass

We map the Edwards Anderson Hamiltonian onto an effective Hamiltonian for Ising spins with nonrandom competing couplings. A high-temperature series is used to calculate the coupling constants to 20th, 16th, and 12th order for two, three, and four dimensions, respectively. We conclude the lower critical dimension to be close to three and find the correlation-length and susceptibility critical exponents to be twice as large as for thed=3 Ising model.     

to -12pt [-12pt]to 16ptRapid Note Wavefunctions for the Luttinger liquid

Standard bosonization techniques lead to phonon-like excitations in a Luttinger liquid (LL), reflecting the absence of Landau quasiparticles in these systems. Yet in addition to the above excitations some LL are known to possess solitonic states carrying fractional quantum numbers (e.g. the spin 1/2 Heisenberg chain). We have reconsidered the zero modes in the low-energy spectrum of the Gaussian boson LL Hamiltonian both for fermionic and bosonic LL: in the spinless case we find that two elementary excitations carrying fractional quantum numbers allow to generate all the charge and current excited states of the LL. We explicitly compute the wavefunctions of these two objects and show that one of them can be identified with the 1D version of the Laughlin quasiparticle introduced in the context of the Fractional Quantum Hall effect. For bosons, the other quasiparticle corresponds to a spinon excitation. The eigenfunctions of Wen's chiral LL Hamiltonian are also derived: they are quite simply the one dimensional restrictions of the 2D bulk Laughlin wavefunctions. Received 26 January 1999 and Received in final form 21 April 1999     

Validity of the Anderson and Hubbard models for the description of metallic Ce and cerium heavy fermion compounds

The importance of the inclusion of inter-site f-f hybridization in electron structure calculations for metallic Ce and cerium heavy fermion compounds was studied. We demonstrate that for heavy-fermion systems such as cerium compound CeCu₂Si₂ f-f hybridization can be neglected and Anderson model application is well justified. On another hand for cerium metal f-f hybridization is strong enough to provide the contribution to hybridization function comparable to hybridization between 4f and itinerant electrons. We argue that in the case of Ce only the most general Hamiltonian combining the Hubbard and Anderson models should be used.     

Partitioning in multiconfiguration perturbation theory

A simple and effective formulation of multi‐configuration perturbation theory is reviewed emphasizing the various possibilities for partitioning the Hamiltonian. We study different principles how traditional partitionings (like that of Møller and Plesset or Epstein and Nesbet) can be generalized to the multi‐configurational case. Level shift parameters are introduced with the purpose of fine‐tuning the partitioning. Variational conditions for optimizing level shift parameters in multi‐configuration theory are investigated, both for N‐particle levels and for quasiparticle (one‐electron) energies.     

Study of low-lying collective states using a Microscopic Anharmonic Vibrator Approach

Anharmonic features of the low-lying collective states in the ruthenium isotopes have been investigated systematically by using the Microscopic Anharmonic Vibrator Approach (MAVA). MAVA is based on a realistic microscopic G-matrix Hamiltonian, only slightly renormalized in the adopted large realistic single-particle spaces. This Hamiltonian is used to derive equations of motion for the mixing of one-and two-phonon degrees of freedom starting from collective phonons of the quasiparticle random-phase approximation. Interesting applications to analysis of double-beta-decay rates to excited collective states are foreseen. Presented by J. Suhonen at the Workshop on calculation of double-beta-decay matrix elements (MEDEX’05), Corfu, Greece, September 26–29, 2005.     

Low-lying Gamow-Teller transitions in spherical nuclei

The Pyatov Method has been used to study the low-lying Gamow-Teller transitions in the mass region of 98 ⩽ A ⩽ 130. The eigenvalues and eigenfunctions of the total Hamiltonian have been solved within the framework of proton-neutron quasiparticle random-phase approximation. The low-lying β decay log(ft) values have been calculated for the nuclei under consideration.     

The effect of exchange interaction on quasiparticle Landau levels in narrow-gap quantum well heterostructures

Using the 'screened' Hartree-Fock approximation based on the eight-band k·p Hamiltonian, we have extended our previous work (Krishtopenko et al 2011 J. Phys.: Condens. Matter 23 385601) on exchange enhancement of the g-factor in narrow-gap quantum well heterostructures by calculating the exchange renormalization of quasiparticle energies, the density of states at the Fermi level and the quasiparticle g-factor for different Landau levels overlapping. We demonstrate that exchange interaction yields more pronounced Zeeman splitting of the density of states at the Fermi level and leads to the appearance of peak-shaped features in the dependence of the Landau level energies on the magnetic field at integer filling factors. We also find that the quasiparticle g-factor does not reach the maximum value at odd filling factors in the presence of large overlapping of spin-split Landau levels. We advance an argument that the behavior of the quasiparticle g-factor in weak magnetic fields is defined by a random potential of impurities in narrow-gap heterostructures.     

AC transport through a quantum dot: from Kondo to Coulomb-blockade behaviour

We analyze the conductance ( ) of a quantum dot (QD) in an AC potential at finite temperature. The Friedel–Langreth sum rule (FLSR) is generalized to include the effect of an AC potential and finite T. We have solved the Anderson Hamiltonian by means of a self-consistent procedure which fulfills the generalized FLSR. New features are found in the density of states (DOS) and in when an AC voltage is applied. Our model describes the effect of an AC potential on the transition from Kondo regime to a Coulomb-blockade behaviour as T increases.     

Density of localized states and linear specific heat for anderson model of amorphous semiconductors

The Anderson model is used to describe the role of electron correlation in localized states in amorphous semiconductors. We adopt a quasiparticle approach and discuss both the Fermi level pinning and the linear specific heat in terms of temperature and concentration changes of the one-particle density of states.Dedicated to Professor Miroslav Trlifaj on the occasion of his sixtieth birthday.     

Microscopic structure of <Superscript>126</Superscript>Ba in IBM terms

The yrast band of the nonaxially deformed ¹²⁶Ba nucleus is described by the Hamiltonian of the interaction boson model. Its parameters are calculated on the basis of a microscopic theory within a spherical mean field, and residual interactions that include pairing and multipole factorized forces. Each state of the yrast band is considered independently of others, allowing us to study variations in the superfluid properties of the nucleus and the quasiparticle structure of collective D phonons with spin. The calculations are performed in an expanded configuration space that includes the collective D phonon states, and noncollective states in which an additional phonon of positive parity whose spin assumes values of 0 to 6 is present along with the D phonons. It is shown that the collective Hamiltonian parameters cannot be reproduced without considering the effect of the noncollective states.     

Spreading of wave packets in the Anderson model on the Bethe Lattice

The spreading of wave packets evolving under the Anderson Hamiltonian on the Bethe Lattice is studied for small disorder. The mean square distance travelled by a particle in a timet is shown to grow ast ² for larget.The author was supported in part by the NSF Grant DMS-9208029.     

The Symmetries of Fermion Fluids at Low Dimensions

We point out that the quasiparticle spectrum of the Landau Fermi liquid theory has an extra Z₂ symmetry, local in momentum space, which is not generic to the Hamiltonian with interactions. Thus the Fermi liquid is in this sense a (quantum) zero-temperature critical point.     

From String Nets to Nonabelions

We discuss Hilbert spaces spanned by the set of string nets, i.e. trivalent graphs, on a lattice. We suggest some routes by which such a Hilbert space could be the low-energy subspace of a model of quantum spins on a lattice with short-ranged interactions. We then explain conditions which a Hamiltonian acting on this string net Hilbert space must satisfy in order for the system to be in the DFib (Doubled Fibonacci) topological phase, that is, be described at low energy by an SO(3)₃ × SO(3)₃ doubled Chern-Simons theory, with the appropriate non-abelian statistics governing the braiding of the low-lying quasiparticle excitations (nonabelions). Using the string net wavefunction, we describe the properties of this phase. Our discussion is informed by mappings of string net wavefunctions to the chromatic polynomial and the Potts model.     

GW quasiparticle bandgaps of anatase TiO2 starting from DFT + U

We investigate the quasiparticle band structure of anatase TiO(2), a wide gap semiconductor widely employed in photovoltaics and photocatalysis. We obtain GW quasiparticle energies starting from density-functional theory (DFT) calculations including Hubbard U corrections. Using a simple iterative procedure we determine the value of the Hubbard parameter yielding a vanishing quasiparticle correction to the fundamental bandgap of anatase TiO(2). The bandgap (3.3 eV) calculated using this optimal Hubbard parameter is smaller than the value obtained by applying many-body perturbation theory to standard DFT eigenstates and eigenvalues (3.7 eV). We extend our analysis to the rutile polymorph of TiO(2) and reach similar conclusions. Our work highlights the role of the starting non-interacting Hamiltonian in the calculation of GW quasiparticle energies in TiO(2) and suggests an optimal Hubbard parameter for future calculations.     

On the role of image forces in chemisorption theory

The Extended Anderson Hamiltonian is used to study the effect of fluctuations of an adatom charge Q on the ionic part of the chemisorption energy. It is shown that dynamical effects essentially modify the classical expression E = ? ?Q² for the energy of interaction between a static charge Q and a metal (? is the interaction energy for a unit charge). The exact solution for the one-electron two-level model as well as a variational solution for the Extended Anderson Hamiltonian model are given. Validity conditions for a variety of approximate schemes are studied. The results are presented for the Extended Anderson Hamiltonian model parameterized so as to describe some aspects of the Li/W and Li/Mo chemisorption systems.