Phase Relaxation of Electrons in Disordered Conductors

Conduction electrons in disordered metals and heavily doped semiconductors at low temperatures preserve their phase coherence for a long time: phase relaxation time $\tau_\phi$ can be orders of magnitude longer than the momentum relaxation time. The large difference in these time scales gives rise to well known effects of weak localization, such as anomalous magnetoresistance. Among other interesting characteristics, study of these effects provide quantitative information on the dephasing rate $1/\tau_\phi$. This parameter is of fundamental interest: the relation between $\hbar/\tau_\phi$ and the temperature $T$ (a typical energy scale of an electron) determines how well a single electron state is defined. We will discuss the basic physical meaning of $1/\tau_\phi$ in different situations and its difference from the energy relaxation rate. At low temperatures, the phase relaxation rate is governed by collisions between electrons. We will review existing theories of dephasing by these collisions or (which is the same) by electric noise inside the sample. We also discuss recent experiments on the magnetoresistance of 1D systems: some of them show saturation of $1/\tau_\phi$ at low temperatures, the other do not. To resolve this contradiction we discuss dephasing by an external microwave field and by nonequilibrium electric noise.Comment: Order of figures and references corrected; one reference added; 15 pages, 2 figures, lecture given on 10th International Winterschool on New Developments in Solid State Physics, Mauterndorf, Salzburg, Austria; 23-27 Feb. 199

Distribution function of persistent current

We introduce a variant of the replica trick within the nonlinear sigma model that allows calculating the distribution function of the persistent current. In the diffusive regime, a Gaussian distribution is derived. This result holds in the presence of local interactions as well. Breakdown of the Gaussian statistics is predicted for the tails of the distribution function at large deviations

Diamagnetic response of Aharonov-Bohm rings: Impurity backward scatterings

We report a theoretical calculation on the persistent currents of disordered normal-metal rings. It is shown that the diamagnetic responses of the rings in the vicinity of the zero magnetic field are attributed to multiple backward scatterings off the impurities. We observe the transition from the paramagnetic response to the diamagnetic one as the strength of disorder grows using both the analytic calculation and the numerical exact diagonalization.Comment: final versio

Agency, Narrative, and Mortality

Narrative views of agency and identity arise in opposition to reductionism in both domains. While reductionists understand both identity and agency in terms of their components, narrativists respond that life and action are both constituted by narratives, and since the components of a narrative gain their meaning from the whole, life and action not only incorporate their constituent parts but also shape them. I first lay out the difficulties with treating narrative as constitutive of metaphysical identity and turn to its function in practical identity. I then explore the ways narrative shapes our agency—by tapping into our motivational structures, providing an understanding of the social background within which our agency operates, guiding our agency through an understanding of our histories and aspirations, providing the links that structure actions internally, and allowing us to change the meaning of our pasts. I suggest that putting these functions of narrative together may allow us to genuinely shape our past motivational structures through our actions. Finally, if life has the form of a narrative, it may seem as if mortality is necessary for our lives and their contents to be meaningful: a narrative, it might seem, relies on the ending for its meaning. If so, an immortal life would be meaningless. I examine the possibility that even an immortal life may draw meaning from local narratives that constitute projects within such a life, while arguing that, to the contrary, narrative may be a tool exclusively adapted to mortal lives

The Significance of Future Generations

We find meaning and value in our lives by engaging in everyday projects. But, according to a recent argument by Samuel Scheffler, this value doesn’t depend merely on what the projects are about. In many cases, it depends also on the future generations that will replace us. By imagining the imminent extinction of humanity soon after our own deaths, we can recognize both that much of our current valuing depends on a background confidence in the ongoing survival of humanity and that the survival and flourishing of those future generations matters to us. After presenting Scheffler’s argument, I will explore two twentieth century precursors—Hans Morgenthau and Simone de Beauvoir—before returning to Scheffler to see that his argument can not only show us why future generations matter, but it can also give us hope for immortality and a blueprint for embracing a changing future

Influence of trigonal warping on interference effects in bilayer graphene

Bilayer graphene (two coupled graphitic monolayers arranged according to Bernal stacking) is a two-dimensional gapless semiconductor with a peculiar electronic spectrum different from the Dirac spectrum in the monolayer material. In particular, the electronic Fermi line in each of its valleys has a strong p -> -p asymmetry due to trigonal warping, which suppresses the weak localization effect. We show that weak localization in bilayer graphene may be present only in devices with pronounced intervalley scattering, and we evaluate the corresponding magnetoresistance