Views from Above: Recent Forest History in the Far North of Madagascar

This study is a spatial analysis of the forests west of Mt. d’Ambre National Park in the far northern portion of Madagascar. This area has little spatial, ethnographic, or historic data pertaining to the environment. After a research trip to this area from June to August 2017, the need for a temporal study of forest change materialized. Utilizing Landsat data, NDVIs were generated to assess forest cover density from 1978 to 2018. This spatial analysis saw deforestation across the study area. Deforestation in this area is mainly caused by agriculture, large foreign companies, recent immigration, and local exploitation of timber for income. (Gezon and Freed, 1999). Supplementing past research in this area, a spatial study provides context on a larger scale. Remotely sensed data has a limited scope temporally, but the significance of the environmental events since the 1970’s forgives the short time span. Observing patterns of change in forests across primate habitat can provide a base for future research in behavior, ecology, and the interactions between humans and the immediate environment

Classical Phase Fluctuations in Incommensurate Peierls Chains

In the pseudogap regime of one-dimensional incommensurate Peierls systems, fluctuations of the phase of the order parameter prohibit the emergence of long-range order and generate a finite correlation length. For classical phase fluctuations, we present exact results for the average electronic density of states, the mean localization length, the electronic specific heat and the spin susceptibility at low temperatures. Our results for the susceptibility give a good fit to experimental data.Comment: 4 Revtex pages, 4 figures, submitted to Phys. Rev. Let

Absence of fermionic quasi-particles in the superfluid state of the attractive Fermi gas

We calculate the effect of order parameter fluctuations on the fermionic single-particle excitations in the superfluid state of neutral fermions interacting with short range attractive forces. We show that in dimensions D \leq 3 the singular effective interaction between the fermions mediated by the gapless Bogoliubov-Anderson mode prohibits the existence of well-defined quasi-particles. We explicitly calculate the single-particle spectral function in the BEC regime in D=3 and show that in this case the quasi-particle residue and the density of states are logarithmically suppressed.Comment: 4 RevTex pages, 3 figures; title changed, new Figure 1, added references. We argue that in the entire regime of the BCS-BEC crossover the quasi-particle picture breaks down in D <=3 for neutral fermions (but NOT for charged fermions

Competing orders II: the doped quantum dimer model

We study the phases of doped spin S=1/2 quantum antiferromagnets on the square lattice, as they evolve from paramagnetic Mott insulators with valence bond solid (VBS) order at zero doping, to superconductors at moderate doping. The interplay between density wave/VBS order and superconductivity is efficiently described by the quantum dimer model, which acts as an effective theory for the total spin S=0 sector. We extend the dimer model to include fermionic S=1/2 excitations, and show that its mean-field, static gauge field saddle points have projective symmetries (PSGs) similar to those of `slave' particle U(1) and SU(2) gauge theories. We account for the non-perturbative effects of gauge fluctuations by a duality mapping of the S=0 dimer model. The dual theory of vortices has a PSG identical to that found in a previous paper (L. Balents et al., cond-mat/0408329) by a duality analysis of bosons on the square lattice. The previous theory therefore also describes fluctuations across superconducting, supersolid and Mott insulating phases of the present electronic model. Finally, with the aim of describing neutron scattering experiments, we present a phenomenological model for collective S=1 excitations and their coupling to superflow and density wave fluctuations.Comment: 22 pages, 10 figures; part I is cond-mat/0408329; (v2) changed title and added clarification

Renormalization of the BCS-BEC crossover by order parameter fluctuations

We use the functional renormalization group approach with partial bosonization in the particle-particle channel to study the effect of order parameter fluctuations on the BCS-BEC crossover of superfluid fermions in three dimensions. Our approach is based on a new truncation of the vertex expansion where the renormalization group flow of bosonic two-point functions is closed by means of Dyson-Schwinger equations and the superfluid order parameter is related to the single particle gap via a Ward identity. We explicitly calculate the chemical potential, the single-particle gap, and the superfluid order parameter at the unitary point and compare our results with experiments and previous calculations.Comment: 5 pages, 3 figure

Ferromagnetic Luttinger Liquids

We study weak itinerant ferromagnetism in one-dimensional Fermi systems using perturbation theory and bosonization. We find that longitudinal spin fluctuations propagate ballistically with velocity v_m << v_F, where v_F is the Fermi velocity. This leads to a large anomalous dimension in the spin-channel and strong algebraic singularities in the single-particle spectral function and in the transverse structure factor for momentum transfers q ~ 2 Delta/v_F, where 2 Delta is the exchange splitting.Comment: 4 pages, 3 figure

Exact Numerical Calculation of the Density of States of the Fluctuating Gap Model

We develop a powerful numerical algorithm for calculating the density of states rho(omega) of the fluctuating gap model, which describes the low-energy physics of disordered Peierls and spin-Peierls chains. We obtain rho(omega) with unprecedented accuracy from the solution of a simple initial value problem for a single Riccati equation. Generating Gaussian disorder with large correlation length xi by means of a simple Markov process, we present a quantitative study of the behavior of rho (omega) in the pseudogap regime. In particular, we show that in the commensurate case and in the absence of forward scattering the pseudogap is overshadowed by a Dyson singularity below a certain energy scale omega^{ast}, which we explicitly calculate as a function of xi.Comment: 4 revtex pages, 3 figure

Exact Results for the Crossover from Gaussian to Non-Gaussian Order Parameter Fluctuations in Quasi One-Dimensional Electronic Systems

The physics of quasi one-dimensional Peierls systems is dominated by order parameter fluctuations. We present an algorithm which allows for the first time to exactly calculate physical properties of the electrons gas coupled to classical order parameter fluctuations. The whole range from the Gaussian regime dominated by amplitude fluctuations to the non-Gaussian regime dominated by phase fluctuations is accessible. Our results provide insight into the 'pseudogap' phenomenon occurring in underdoped high-temperature superconductors, quasi one-dimensional organic conductors and liquid metals.Comment: 4 pages, 4 figures, accepted for publication in Physical Review Letter

Some remarks about pseudo gap behavior of nearly antiferromagnetic metals

In the antiferromagnetically ordered phase of a metal, gaps open on parts of the Fermi surface if the Fermi volume is sufficiently large. We discuss simple qualitative and heuristic arguments under what conditions precursor effects, i.e. pseudo gaps, are expected in the paramagnetic phase of a metal close to an antiferromagnetic quantum phase transition. At least for weak interactions, we do not expect the formation of pseudo gaps in a three dimensional material. According to our arguments, the upper critical dimension d_c for the formation of pseudo gaps is d_c=2. However, at the present stage we cannot rule out a higher upper critical dimension, 2 < d_c <= 3. We also discuss briefly the role of statistical interactions in pseudo gap phases.Comment: 6 pages, accepted in PRB, relevant references added, several small change