J. H. Samson

Magnetic, chemical and structural ordering in transition metals

The metals in the middle of the transition metal series have a tendency towards antiferromagnetism, ordered AB alloy structures and lattice distortions based on a twofold superlattice. By contrast, at the beginning and end of the series the corresponding tendency is towards ferromagnetism, chemical segregation rather than alloy formation, and stable simple structures. The authors show how all these results can be understood in a unified way in terms of a general theorem published earlier. The argument applies quite generally to any set of tight-binding bands such as the d bands, independent of the specific structure. In a general sense the superlattice formation for approximately a half-filled band is like a Peierls transition, but it results from the electronic structure as a whole with no special role attached to the Fermi surface. The case of threefold superlattices is also discussed with examples.

Superlight Small Bipolarons in the Presence of a Strong Coulomb Repulsion

We study a lattice bipolaron on a staggered triangular ladder and triangular and hexagonal lattices with both long-range electron-phonon interaction and strong Coulomb repulsion using a novel continuous-time quantum Monte-Carlo (CTQMC) algorithm extended to the Coulomb-Frohlich model with two particles. The algorithm is preceded by an exact integration over phonon degrees of freedom, and as such is extremely efficient. The bipolaron effective mass and bipolaron radius are computed. Lattice bipolarons on such lattices have a novel crablike motion, and are small but very light in a wide range of parameters, which leads to a high Bose-Einstein condensation temperature. We discuss the relevance of our results with current experiments on cuprate high-temperature superconductors and propose a route to room temperature superconductivity.

Theory of local magnetic moments in transition metals

The authors consider whether a state of disordered local moments (DLM) can exist in an itinerant magnetic metal above Tc. They develop a criterion Il(EF)>1 for the non-magnetic state to be unstable with respect to the formation of DLM. This criterion is more easily satisfied than the Stoner criterion for ferromagnetism. In(EF)>1, near the centre of a tight-binding band and less easily satisfied near the band edges. They report calculations on tight-binding d bands of 3d transition metals by means of the recursion method. These suggest that BCC Mn and Fe do possess a DLM state, whilst Cr, FCC Mn and Ni do not. They present a method of calculation of the real part of the Green function by the recursion method.

Magnetic contributions to thermal expansion of transition metals: implications for local moments above Tc

Most transition-metal magnets, although itinerant in nature, do not undergo large volume contractions on passing from the magnetic to the non-magnetic state. Such a contraction might be expected because of the very large internal magnetic pressure which has been calculated to exist in these magnets at zero temperature. By developing the theory of the spontaneous magnetovolume effect in greater generality than previously, the authors show that it is the local magnetic moment which dictates the pressure. A positive pressure also arises from the energy of disordering the moments at high temperature, and a small net expansion from the coupling of magnetic fluctuations to the anharmonic part of the lattice potential. The effects are related to the experimentally measurable specific heat and thermal coefficient of linear expansion. They analyse the data to infer root-mean-square values of the atomic magnetic moments above the Curie (Neel) temperature TC(TN) in Fe, Ni and Cr, and find a remarkably small reduction from the saturation values at T=0. Even in the Invar alloys most of the atomic moment remains above Tc.

Theory of some physical properties and competing processes in tight-binding bands

Certain physical properties, or differences of two physical quantities, oscillate in sign depending on how far the energy bandstructure is filled. The authors show how a theorem of Ducastelle and Cyrot-Lackmann (1971) concerning the number of such zeros for the case of a tight-binding band can be extended to quantities expressible in terms of Green functions. One application is to the relative ease of formation of different structures or magnetic configurations in transition metals; another application is to the Landau theory of phase transitions. The authors show the relation between the theorem and Friedel oscillations.

Superconductivity in a Hubbard-Frohlich model and in cuprates

Using the variational Monte Carlo method, we find that a relatively weak long-range electron-phonon interaction induces a d-wave superconducting state in doped Mott-Hubbard insulators and/or strongly correlated metals with a condensation energy significantly larger than can be obtained with Coulomb repulsion only. Moreover, the superconductivity is shown to exist for infinite on-site Coulomb repulsion without the need for additional mechanisms such as spin fluctuations to mediate d-wave superconductivity. We argue that our superconducting state is robust with respect to a more intricate choice of the trial-wave function and that a possible origin of high-temperature superconductivity lies in a proper combination of strong electron-electron correlations with poorly screened Frohlich electron-phonon interaction.

Effects of lattice geometry and interaction range on polaron dynamics

We study the effects of lattice type on polaron dynamics using a continuous-time quantum Monte Carlo approach. Holstein and screened Frohlich polarons are simulated on a number of different Bravais lattices. The effective mass, isotope coefficients, ground-state energy and energy spectra, phonon numbers, and density of states are calculated. In addition, the results are compared with weak- and strong-coupling perturbation theory. For the Holstein polaron, it is found that the crossover between weak- and strong-coupling results becomes sharper as the coordination number is increased. In higher dimensions, polarons are much less mobile at strong coupling, with more phonons contributing to the polaron. The total energy decreases monotonically with coupling. Spectral properties of the polaron depend on the lattice type considered, with the dimensionality contributing to the shape and the coordination number to the bandwidth. As the range of the electron-phonon interaction is increased, the coordination number becomes less important, with the dimensionality taking the leading role.

Superlight small bipolarons

Recent angle-resolved photoemission spectroscopy (ARPES) has identified that a finite-range Frohlich electron-phonon interaction (EPI) with c-axis polarized optical phonons is important in cuprate superconductors, in agreement with an earlier proposal by Alexandrov and Kornilovitch. The estimated unscreened EPI is so strong that it could easily transform doped holes into mobile lattice bipolarons in narrow-band Mott insulators such as cuprates. Applying a continuous-time quantum Monte-Carlo algorithm (CTQMC) we compute the total energy, effective mass, pair radius, number of phonons and isotope exponent of lattice bipolarons in the region of parameters where any approximation might fail taking into account the Coulomb repulsion and the finite-range EPI. The effects of modifying the interaction range and different lattice geometries are discussed with regards to analytical strong-coupling/non-adiabatic results. We demonstrate that bipolarons can be simultaneously small and light, provided suitable conditions on the electron-phonon and electron-electron interaction are satisfied. Such light small bipolarons are a necessary precursor to high-temperature Bose-Einstein condensation in solids. The light bipolaron mass is shown to be universal in systems made of triangular plaquettes, due to a novel crab-like motion. Another surprising result is that the triplet-singlet exchange energy is of the first order in the hopping integral and triplet bipolarons are heavier than singlets in certain lattice structures at variance with intuitive expectations. Finally, we identify a range of lattices where superlight small bipolarons may be formed, and give estimates for their masses in the anti-adiabatic approximation.

Superlight small bipolarons from realistic long-range Coulomb and Frohlich interactions

We report analytical and numerical results on the two-particle states of the polaronic t-Jp model derived recently with realistic Coulomb and electron-phonon (Fr¨ohlich) interactions in doped polar insulators. Eigenstates and eigenvalues are calculated for two different geometries. Our results show that the ground state is a bipolaronic singlet, made up of two polarons. The bipolaron size increases with increasing ratio of the polaron hopping integral t to the exchange interaction Jp but remains small in the whole range 0 t/Jp 1. Furthermore, the model exhibits a phase transition to a superconducting state with a critical temperature well in excess of 100K since the small bipolarons are perfectly mobile. In the range t/Jp 1, there are distinct charge and spin gaps opening in the density of states, specific heat, and magnetic susceptibility well above Tc.

Tunable refraction in a two-dimensional quantum-state metamaterial

In this paper we consider a two-dimensional quantum-state metamaterial comprising an array of qubits (two-level quantum objects). Here we propose that it should be possible to manipulate the propagation of quantum information. We show that a quantum metamaterial such as the one considered here exhibits several different modes of operation, which we have termed Aharonov-Bohm, intermediate, and quantum-Zeno. We also see interesting behavior which could be thought of as either quantum birefringence (where the material acts like a beam splitter) as well as the emergence of quantum correlations in the circuit’s measurement statistics. Quantum-state metamaterials as proposed here may be fabricated from a variety of technologies from superconducting qubits to quantum dots and would be readily testable in existing state-of-the-art laboratories.