Theory of a Scanning Tunneling Microscope with a Two-Protrusion Tip

We consider a scanning tunneling microscope (STM) such that tunneling occurs through two atomically sharp protrusions on its tip. When the two protrusions are separated by at least several atomic spacings, the differential conductance of this STM depends on the electronic transport in the sample between the protrusions. Furthermore two-protrusion tips commonly occur during STM tip preparation. We explore possible applications to probing dynamical impurity potentials on a metallic surface and local transport in an anisotropic superconductor.Comment: revtex, 11 pages, 6 figures upon reques

Noether symmetries, energy-momentum tensors and conformal invariance in classical field theory

In the framework of classical field theory, we first review the Noether theory of symmetries, with simple rederivations of its essential results, with special emphasis given to the Noether identities for gauge theories. Will this baggage on board, we next discuss in detail, for Poincar\'e invariant theories in flat spacetime, the differences between the Belinfante energy-momentum tensor and a family of Hilbert energy-momentum tensors. All these tensors coincide on shell but they split their duties in the following sense: Belinfante's tensor is the one to use in order to obtain the generators of Poincar\'e symmetries and it is a basic ingredient of the generators of other eventual spacetime symmetries which may happen to exist. Instead, Hilbert tensors are the means to test whether a theory contains other spacetime symmetries beyond Poincar\'e. We discuss at length the case of scale and conformal symmetry, of which we give some examples. We show, for Poincar\'e invariant Lagrangians, that the realization of scale invariance selects a unique Hilbert tensor which allows for an easy test as to whether conformal invariance is also realized. Finally we make some basic remarks on metric generally covariant theories and classical field theory in a fixed curved bakground.Comment: 31 pa

Linking the Conservation of Culture and Nature: A Case Study of Sacred Forests in Zimbabwe

This paper examines the role of traditional religious beliefs and traditional leaders in conserving remnant patches of a unique type of dry forest in the Zambezi Valley of northern Zimbabwe. We examined aerial photographs spanning more than three decades, interviewed and surveyed local residents, and met with communities to learn about the environmental history of the forests and the factors that have affected land use in the area. Our results show that forest loss is dramatically less in forests that are now considered sacred, or were in the past connected to sacred forests. This supports our hypothesis that traditional spiritual values have influenced human behavior affecting the forests, and have played a role in protecting them until now. We also found that rates of forest loss have been much higher in an area where traditional leaders are relatively disempowered within the post-independence political system compared to an area where traditional leaders have more power. These findings lead us to conclude that a strategy that links the conservation of culture and nature is likely to be more effective in conserving forests than a strategy that ignores traditional beliefs, values, and institutions

Chiral interface states in p-n graphene junctions

We present a theoretical analysis of unidirectional interface states which form near p−n junctions in a graphene monolayer subject to a homogeneous magnetic field. The semiclassical limit of these states corresponds to trajectories propagating along the p−n interface by a combined skipping-snaking motion. Studying the two-dimensional Dirac equation with a magnetic field and an electrostatic potential step, we provide and discuss the exact and essentially analytical solution of the quantum-mechanical eigenproblem for both a straight and a circularly shaped junction. The spectrum consists of localized Landau-like and unidirectional snaking-skipping interface states, where we always find at least one chiral interface state. For a straight junction and at energies near the Dirac point, when increasing the potential step height, the group velocity of this state interpolates in an oscillatory manner between the classical drift velocity in a crossed electromagnetic field and the semiclassical value expected for a purely snaking motion. Away from the Dirac point, chiral interface states instead resemble the conventional skipping (edge-type) motion found also in the corresponding Schrödinger case. We also investigate the circular geometry, where chiral interface states are predicted to induce sizable equilibrium ring currents

Scaling of the spin stiffness in random spin-1/2 chains : Crossover from pure-metallic behaviour to random singlet-localized regime

In this paper we study the localization transition induced by the disorder in random antiferromagnetic spin-1/2 chains. The results of numerical large scale computations are presented for the XX model using its free fermions representation. The scaling behavior of the spin stiffness is investigated for various disorder strengths. The disorder dependence of the localization length is studied and a comparison between numerical results and bosonization arguments is presented. A non trivial connection between localization effects and the crossover from the pure XX fixed point to the infinite randomness fixed point is pointed out.Comment: Published version, 7 pages, 6 figure

The Role of Alexithymia in Memory and Executive Functioning Across the Lifespan

Alexithymia is a personality trait characterised by difficulties identifying feelings (DIF), describing feelings (DDF), and externally oriented thinking (EOT). Alexithymia has been associated with poorer memory, at least for emotive materials, and recently, with executive and neural dysfunction. Aging is also accompanied by poorer memory and executive functioning (EF), neural dysfunction, and increasing alexithymia. Thus, the hypothesis of a general cognitive impairment in alexithymia, particularly in elders, needs investigation. Three large, independent, cross-sectional experiments (n = 296, 139 and 121, respectively) investigated memory and EF in healthy adults, ranging from young to old adulthood, with age, sex, and the three Toronto Alexithymia Scale-20 subscales (DIF, DDF, EOT) as predictors in hierarchical regressions. Across studies, alexithymia contributed to poorer memory (via EOT) and EF (via DIF), in younger and older adults. Additionally, these effects occurred in non-emotive contexts with neutral stimuli. Moreover, although memory was worse with greater age and poor EF contributed to poor memory, those who had both high EOT and poor EF had particularly poor memory. Thus, alexithymia (particularly via high DIF or high EOT) is a risk factor for age-related cognitive decline. Further research should clarify the direction and nature of these complex relationships

Quantum Transition between an Antiferromagnetic Mott Insulator and dx2−y2d_{x^2 - y^2} Superconductor in Two Dimensions

We consider a Hubbard model on a square lattice with an additional interaction, $W$, which depends upon the square of a near-neighbor hopping. At half-filling and a constant value of the Hubbard repulsion, increasing the strength of the interaction $W$ drives the system from an antiferromagnetic Mott insulator to a $d_{x^2 -y^2}$ superconductor. This conclusion is reached on the basis of zero temperature quantum Monte Carlo simulations on lattice sizes up to $16 \times 16$.Comment: 4 pages (latex) and 4 postscript figure

Re-parameterization Invariance in Fractional Flux Periodicity

We analyze a common feature of a nontrivial fractional flux periodicity in two-dimensional systems. We demonstrate that an addition of fractional flux can be absorbed into re-parameterization of quantum numbers. For an exact fractional periodicity, all the electronic states undergo the re-parameterization, whereas for an approximate periodicity valid in a large system, only the states near the Fermi level are involved in the re-parameterization.Comment: 4 pages, 1 figure, minor changes, final version to appear in J. Phys. Soc. Jp