The Principle of Minimal Resistance in Non-Equilibrium Thermodynamics

Analytical models describing the motion of colloidal particles in given velocity fields are presented. In addition to local approaches, leading to well known master equations such as the Langevin and the Fokker-Planck equations, a global description based on path integration is reviewed. This shows that under very broad conditions, during its evolution a dissipative system tends to minimize its energy dissipation in such a way to keep constant the Hamiltonian time rate, equal to the difference between the flux-based and the force-based Rayleigh dissipation functions. At steady state, the Hamiltonian time rate is maximized, leading to a minimum resistance principle. In the unsteady case, we consider the relaxation to equilibrium of harmonic oscillators and the motion of a Brownian particle in shear flow, obtaining results that coincide with the solution of the Fokker-Planck and the Langevin equations

Search for a light dark sector particle at LHCb

A search is presented for a hidden-sector boson, $\chi$, produced in the decay $B^0 \rightarrow K^* (892)^0 \chi$, with $K^* (892)^0 \rightarrow K^+ \pi^-$ and $\chi \rightarrow \mu^+ \mu^-$ . The search is performed using a $pp$-collision data sample collected at $\sqrt{s}=7$ and 8 TeV with the LHCb detector, corresponding to integrated luminosities of 1 and 2 fb$^{-1}$ respectively. No significant signal is observed in the mass range $214 \le m_\chi \le 4350$ MeV, and upper limits are placed on the branching fraction product $\mathcal{B}(B^0 \rightarrow K^* (892)^0 \chi) \times \mathcal{B}(\chi \rightarrow \mu^+ \mu^- )$ as a function of the mass and lifetime of the $\chi$ boson. These limits place the most stringent constraints to date on many theories that predict the existence of additional low-mass dark bosons.Comment: Presentation at the DPF 2015 Meeting of the American Physical Society Division of Particles and Fields, Ann Arbor, Michigan, August 4-8, 201

Electronic structure of heavily-doped graphene: the role of foreign atom states

Using density functional theory calculations we investigate the electronic structure of graphene doped by deposition of foreign atoms. We demonstrate that, as the charge transfer to the graphene layer increases, the band structure of the pristine graphene sheet is substantially affected. This is particularly relevant when Ca atoms are deposed on graphene at CaC$_{6}$ stoichiometry. Similarly to what happens in superconducting graphite intercalated compounds, a Ca bands occurs at the Fermi level. Its hybridization with the C states generates a strong non-linearity in one of the $\pi^{*}$ bands below the Fermi level, at energies comparable to the graphene E$_{2g}$ phonon frequency. This strong non-linearity, and not manybody effects as previously proposed, explains the large and anisotropic values of the apparent electron-phonon coupling measured in angular resolved photoemission.Comment: 4 pages, 2 figures, see also M. Calandra and F. Mauri,arXiv:0707.146

Anharmonic phonon frequency shift in MgB2

We compute the anharmonic shift of the phonon frequencies in MgB2, using density functional theory. We explicitly take into account the scattering between different phonon modes at different q-points in the Brillouin zone. The shift of the E2g mode at the Gamma point is +5 % of the harmonic frequency. This result comes from the cancellation between the contributions of the four- and three-phonon scattering, respectively +10 % and -5 %. A similar shift is predicted at the A point, in agreement with inelastic X-ray scattering phonon-dispersion measurements. A smaller shift is observed at the M point.Comment: 4 pages, 1 figur

On the Arnold Conjecture and the Atiyah-Patodi-Singer Index Theorem

The Arnold conjecture yields a lower bound to the number of periodic classical trajectories in a Hamiltonian system. Here we count these trajectories with the help of a path integral, which we inspect using properties of the spectral flow of a Dirac operator in the background of a \Sp(2N) valued gauge field. We compute the spectral flow from the Atiyah-Patodi-Singer index theorem, and apply the results to evaluate the path integral using localization methods. In this manner we find a lower bound to the number of periodic classical trajectories which is consistent with the Arnold conjecture.Comment: 12 pages, references correcte

ABJM θ\theta-Bremsstrahlung at four loops and beyond: non-planar corrections

We consider the Bremsstrahlung function associated to a 1/6-BPS Wilson loop in ABJM theory, with a cusp in the couplings to scalar fields. We non-trivially extend its recent four-loop computation at weak coupling to include non-planar corrections. We have recently proposed a conjecture relating this object to supersymmetric circular Wilson loops with multiple windings, which can be computed via localization. We find agreement between this proposal and the perturbative computation of the Bremsstrahlung function, including color sub-leading corrections. This supports the conjecture and hints at its validity beyond the planar approximation.Comment: 22 page

Electrochemical doping of few layer ZrNCl from first-principles: electronic and structural properties in field-effect configuration

We develop a first-principles theoretical approach to doping in field-effect devices. The method allows for calculation of the electronic structure as well as complete structural relaxation in field-effect configuration using density-functional theory. We apply our approach to ionic-liquid-based field-effect doping of monolayer, bilayer, and trilayer ZrNCl and analyze in detail the structural changes induced by the electric field. We show that, contrary to what is assumed in previous experimental works, only one ZrNCl layer is electrochemically doped and that this induces large structural changes within the layer. Surprisingly, despite these structural and electronic changes, the density of states at the Fermi energy is independent of the doping. Our findings imply a substantial revision of the phase diagram of electrochemically doped ZrNCl and elucidate crucial differences with superconductivity in Li intercalated bulk ZrNCl.Comment: 15 pages, 14 figure

Universal enhancement of superconductivity in two dimensional semiconductors at low doping by electron-electron interaction

In two-dimensional multivalley semiconductors, at low doping, even a moderate electron-electron interaction enhances the response to any perturbation inducing a valley polarization. If the valley polarization is due to the electron-phonon coupling, the electron-electron interaction results in an enhancement of the superconducting critical temperature. By performing first principles calculations beyond density functional theory, we prove that this effect accounts for the unconventional doping-dependence of the superconducting transition-temperature (T$_c$) and of the magnetic susceptibility measured in Li$_x$ZrNCl. By finding the conditions for a maximal T$_c$ enhancement, we show how weakly-doped two-dimensional semiconductors provide a route towards high T$_c$ superconductivity.Comment: 5 Pages + Supplementary materials, 9 Pictures overall, to appear on Phys. Rev. Let