Coexistence of solutions in dynamical mean-field theory of the Mott transition

In this paper, I discuss the finite-temperature metal-insulator transition of the paramagnetic Hubbard model within dynamical mean-field theory. I show that coexisting solutions, the hallmark of such a transition, can be obtained in a consistent way both from Quantum Monte Carlo (QMC) simulations and from the Exact Diagonalization method. I pay special attention to discretization errors within QMC. These errors explain why it is difficult to obtain the solutions by QMC close to the boundaries of the coexistence region.Comment: 3 pages, 2 figures, RevTe

Superconductivity in the Two-Band Hubbard Model in Infinite Dimensions

We study a two-band Hubbard model in the limit of infinite dimensions, using a combination of analytical methods and Monte-Carlo techniques. The normal state is found to display various metal to insulators transitions as a function of doping and interaction strength. We derive self-consistent equations for the local Green's functions in the presence of superconducting long-range order, and extend previous algorithms to this case. We present direct numerical evidence that in a specific range of parameter space, the normal state is unstable against a superconducting state characterized by a strongly frequency dependent order-parameter.Comment: 12 pages (14 figures not included, available upon request), Latex, LPTENS Preprint 93/1

The Basic Surgical Skills course in sub-Saharan Africa: an observational study of effectiveness

Background: The Basic Surgical Skills (BSS) course is a common component of postgraduate surgical training programmes in sub-Saharan Africa, but was originally designed in a UK context, and its efficacy and relevance have not been formally assessed in Africa. Methods: An observational study was carried out during a BSS course delivered to early-stage surgical trainees from Rwanda and the Democratic Republic of the Congo. Technical skill in a basic wound closure task was assessed in a formal Objective Structured Assessment of Technical Skills (OSAT) before and after course completion. Participants completed a pre-course questionnaire documenting existing surgical experience and self-perceived confidence levels in surgical skills which were to be taught during the course. Participants repeated confidence ratings and completed course evaluation following course delivery. Results: A cohort of 17 participants had completed a pre-course median of 150 Caesarean sections as primary operator. Performance on the OSAT improved from a mean of 10.5/17 pre-course to 14.2/17 post-course (mean of paired differences 3.7, p < 0.001). Improvements were seen in 15/17 components of wound closure. Pre-course, only 47% of candidates were forming hand-tied knots correctly and 38% were appropriately crossing hands with each throw, improving to 88 and 76%, respectively, following the course (p = 0.01 for both components). Confidence levels improved significantly in all technical skills taught, and the course was assessed as highly relevant by trainees. Conclusion: The Basic Surgical Skills course is effective in improving the basic surgical technique of surgical trainees from sub-Saharan Africa and their confidence in key technical skills

Mott transition at large orbital degeneracy: dynamical mean-field theory

We study analytically the Mott transition of the N-orbital Hubbard model using dynamical mean-field theory and a low-energy projection onto an effective Kondo model. It is demonstrated that the critical interaction at which the insulator appears (Uc1) and the one at which the metal becomes unstable (Uc2) have different dependence on the number of orbitals as the latter becomes large: Uc1 ~ \sqrt{N} while Uc2 ~ N. An exact analytical determination of the critical coupling Uc2/N is obtained in the large-N limit. The metallic solution close to this critical coupling has many similarities at low-energy with the results of slave boson approximations, to which a comparison is made. We also discuss how the critical temperature associated with the Mott critical endpoint depends on the number of orbitals.Comment: 13 pages. Minor changes in V

Theory of light-enhanced phonon-mediated superconductivity

We investigate the dynamics of a phonon-mediated superconductor driven out of equilibrium. The electronic hopping amplitude is ramped down in time, resulting in an increased electronic density of states. The dynamics of the coupled electron-phonon model is investigated by solving Migdal-Eliashberg equations for the double-time Keldysh Green's functions. The increase of the density of states near the Fermi level leads to an enhancement of superconductivity when the system thermalizes to the new state at the same temperature. We provide a time- and momentum-resolved view on this thermalization process, and show that it involves fast processes associated with single-particle scattering and much slower dynamics associated with the superconducting order parameter. The importance of electron-phonon coupling for the rapid enhancement and the efficient thermalization of superconductivity is demonstrated, and the results are compared to a BCS time-dependent mean-field approximation.Comment: 12 pages, 8 figure

The effect of load in a contact with boundary lubrication

The effect of the transition load on the wear in a contact with boundary lubrication was investigated. An experimental method was developed for this purpose, and parameters affecting the boundary lubrication under industrial operating conditions were identified. These parameters are the adsorbed boundary film, the contact microgeometry (surface roughness), macrogeometry, and hardness of materials used. It was found that the curve of the tops of the surface protrustion affect the transition load, and thus the boundary lubrication. The transition load also depends on the chemical nature of the contact and its geometrical and mechanical aspects

Interplane charge dynamics in a valence-bond dynamical mean-field theory of cuprate superconductors

We present calculations of the interplane charge dynamics in the normal state of cuprate superconductors within the valence-bond dynamical mean-field theory. We show that by varying the hole doping, the c-axis optical conductivity and resistivity dramatically change character, going from metallic-like at large doping to insulating-like at low-doping. We establish a clear connection between the behavior of the c-axis optical and transport properties and the destruction of coherent quasiparticles as the pseudogap opens in the antinodal region of the Brillouin zone at low doping. We show that our results are in good agreement with spectroscopic and optical experiments.Comment: 5 pages, 3 figure

Spin wave contribution to the nuclear spin-lattice relaxation in triplet superconductors

We discuss collective spin wave excitations in triplet superconductors with an easy axis anisotropy for the order parameter. Using a microscopic model for interacting electrons we estimate the frequency of such excitations in Bechgaard salts and ruthenate superconductors to be one and twenty GHz respectively. We introduce an effective bosonic model to describe spin-wave excitations and calculate their contribution to the nuclear spin lattice relaxation rate. We find that in the experimentally relevant regime of temperatures, this mechanism leads to the power law scaling of 1/T_1 with temperature. For two and three dimensional systems the scaling exponents are three and five respectively. We discuss experimental manifestations of the spin wave mechanism of the nuclear spin lattice relaxation.Comment: 4 pages, 2 figure