Quantum Monte Carlo study of a magnetic-field-driven 2D superconductor-insulator transition

We numerically study the superconductor-insulator phase transition in a model disordered 2D superconductor as a function of applied magnetic field. The calculation involves quantum Monte Carlo calculations of the (2+1)D XY model in the presence of both disorder and magnetic field. The XY coupling is assumed to have the form -J\cos(\theta_i-\theta_j-A_{ij}), where A_{ij} has a mean of zero and a standard deviation \Delta A_{ij}. In a real system, such a model would be approximately realized by a 2D array of small Josephson-coupled grains with slight spatial disorder and a uniform applied magnetic field. The different values \Delta A_{ij} then corresponds to an applied field such that the average number of flux quanta per plaquette has various integer values N: larger N corresponds to larger \Delta A_{ij}. For any value of \Delta A_{ij}, there appears to be a critical coupling constant K_c(\Delta A_{ij})=\sqrt{[J/(2U)]_c}, where U is the charging energy, above which the system is a Mott insulator; there is also a corresponding critical conductivity \sigma^*(\Delta A_{ij}) at the transition. For \Delta A_{ij}=\infty, the order parameter of the transition is a renormalized coupling constant g. Using a numerical technique appropriate for disordered systems, we show that the transition at this value of \Delta A_{ij} takes place from an insulating (I) phase to a Bose glass (BG) phase, and that the dynamical critical exponent characterizing this transition is z \sim 1.3. By contrast, z=1 for this model at \Delta A_{ij}=0. We suggest that the superconductor to insulator transition is actually of this I to BG class at all nonzero \Delta A_{ij}'s, and we support this interpretation by both numerical evidence and an analytical argument based on the Harris criterion.Comment: 17 pages, 23 figures, accepted for publication in Phys. Rev.

Time-dependent embedding: surface electron emission

An embedding method for solving the time-dependent Schr\"odinger equation is developed using the Dirac-Frenkel variational principle. Embedding allows the time-evolution of the wavefunction to be calculated explicitly in a limited region of space, the region of physical interest, the embedding potential ensuring that the wavefunction satisfies the correct boundary conditions for matching on to the rest of the system. This is applied to a study of the excitation of electrons at a metal surface, represented by a one-dimensional model potential for Cu(111). Time-dependent embedding potentials are derived for replacing the bulk substrate, and the image potential and vacuum region outside the surface, so that the calculation of electron excitation by a surface perturbation can be restricted to the surface itself. The excitation of the Shockley surface state and a continuum bulk state is studied, and the time-structure of the resulting currents analysed. Non-linear effects and the time taken for the current to arrive outside the surface are discussed. The method shows a clear distinction between emission from the localized surface state, where the charge is steadily depleted, and the extended continuum state where the current emitted into the vacuum is compensated by current approaching the surface from the bulk.Comment: 15 figure

Single-Particle Density of States of a Superconductor with a Spatially Varying Gap and Phase Fluctuations

Recent experiments have shown that the superconducting energy gap in some cuprates is spatially inhomogeneous. Motivated by these experiments, and using exact diagonalization of a model d-wave Hamiltonian, combined with Monte Carlo simulations of a Ginzburg-Landau free energy functional, we have calculated the single-particle density of states LDOS$(\omega,r)$ of a model high-T$_c$ superconductor as a function of temperature. Our calculations include both quenched disorder in the pairing potential and thermal fluctuations in both phase and amplitude of the superconducting gap. Most of our calculations assume two types of superconducting regions: $\alpha$, with a small gap and large superfluid density, and $\beta$, with the opposite. If the $\beta$ regions are randomly embedded in an $\alpha$ host, the LDOS on the $\alpha$ sites still has a sharp coherence peak at $T = 0$, but the $\beta$ component does not, in agreement with experiment. An ordered arrangement of $\beta$ regions leads to oscillations in the LDOS as a function of energy. The model leads to a superconducting transition temperature $T_c$ well below the pseudogap temperature $T_{c0}$, and has a spatially varying gap at very low $T$, both consistent with experiments in underdoped Bi2212. Our calculated LDOS$(\omega,r)$ shows coherence peaks for $T T_c$, in agreement with previous work considering phase but not amplitude fluctuations in a homogeneous superconductor. Well above $T_c$, the gap in the LDOS disappears.Comment: 37 pages, 12 figures. Accepted by Phys. Rev. B. Scheduled Issue: 01 Nov 200

Two phase transitions in the fully frustrated XYXY model

The fully frustrated $XY$ model on a square lattice is studied by means of Monte Carlo simulations. A Kosterlitz-Thouless transition is found at $T_{\rm KT} \approx 0.446$, followed by an ordinary Ising transition at a slightly higher temperature, $T_c \approx 0.452$. The non-Ising exponents reported by others, are explained as a failure of finite size scaling due to the screening length associated with the nearby Kosterlitz-Thouless transition.Comment: REVTEX file, 8 pages, 5 figures in uuencoded postscrip

Phase transition in ultrathin magnetic films with long-range interactions: Monte Carlo simulation of the anisotropic Heisenberg model

Ultrathin magnetic films can be modeled as an anisotropic Heisenberg model with long-range dipolar interactions. It is believed that the phase diagram presents three phases: An ordered ferromagnetic phase I, a phase characterized by a change from out-of-plane to in-plane in the magnetization II, and a high-temperature paramagnetic phase III. It is claimed that the border lines from phase I to III and II to III are of second order and from I to II is first order. In the present work we have performed a very careful Monte Carlo simulation of the model. Our results strongly support that the line separating phases II and III is of the BKT type.Comment: 7 page

Biodesulphurized subbituminous coal by different fungi and bacteria studied by reductive pyrolysis. Part 1: Initial coal

One of the perspective methods for clean solid fuels production is biodesulphurization. In order to increase the effect of this approach it is necessary to apply the advantages of more informative analytical techniques. Atmospheric pressure temperature programming reduction (AP-TPR) coupled with different detection systems gave us ground to attain more satisfactory explanation of the effects of biodesulphurization on the treated solid products. Subbituminous high sulphur coal from ‘‘Pirin” basin (Bulgaria) was selected as a high sulphur containing sample. Different types of microorganisms were chosen and maximal desulphurization of 26% was registered. Biodesulphurization treatments were performed with three types of fungi: ‘‘Trametes Versicolor” – ATCC No. 200801, ‘‘Phanerochaeta Chrysosporium” – ME446, Pleurotus Sajor-Caju and one Mixed Culture of bacteria – ATCC No. 39327. A high degree of inorganic sulphur removal (79%) with Mixed Culture of bacteria and consecutive reduction by 13% for organic sulphur (Sorg) decrease with ‘‘Phanerochaeta Chrysosporium” and ‘‘Trametes Versicolor” were achieved. To follow the Sorg changes a set of different detection systems i.e. AP-TPR coupled ‘‘on-line” with mass spectrometry (AP-TPR/MS), on-line with potentiometry (AP-TPR/pot) and by the ‘‘off-line” AP-TPR/GC/MS analysis was used. The need of applying different atmospheres in pyrolysis experiments was proved and their effects were discussed. In order to reach more precise total sulphur balance, oxygen bomb combustion followed by ion chromatography was used

Radicalization processes and transitional phases in female and male detainees residing in Dutch terrorism wings.

Background: Radicalization, violent extremism, and terrorism are risks to societal security. Although research on terrorism-related behaviors is increasing, thorough empirical studies are rare. Methods: This study investigates radicalization processes and transitions in a matched sample of female and male terrorist suspects and convicts (N = 26) residing in Dutch penitentiary terrorism wings. Results:Results show that both men and women often experienced discrimination. A subgroup of women grew up in a stressful family environment and lacked emotional support from their family, whereas the other women did not experience such circumstances. The majority of the study sample was susceptible to connecting with radicalized friends or family members. Interestingly, factors that initially led to radicalization (e.g., a utopian image of the Islamic State) could later turn out to be factors associated with abandoning extremism. Conclusions: In this study, differences in radicalization processes and transitional phases between women and men emerged. Men more often had police contact prior to a terrorism-related offense. Making an effort to right old mistakes seemed important in the radicalization processes of men, whereas women had a stronger desire for emotional support and were more driven by experienced trauma and feelings of loneliness. This study provides input for gender-specific prevention and disengagement interventions

Community Biodiversity Management: Promoting resilience and the conservation of plant genetic resources

This book is the first to set out a clear overview of CBM as a methodology for meeting socio-environmental changes. CBM is shown to be a key strategy that promotes community resilience, and contributes to the conservation of plant genetic resources. The authors present the underlying concepts and theories of CBM as well as its methodology and practices, and introduce case studies primarily from Brazil, Ethiopia, France, India, and Nepal. Contributors include farmers, leaders of farmers’ organizations, professionals from conservation and development organizations, students and scientists

Time-dependent electron transport through a strongly correlated quantum dot: multiple-probe open boundary conditions approach

We present a time-dependent study of electron transport through a strongly correlated quantum dot. The time-dependent current is obtained with the multiple-probe battery method, while adiabatic lattice density functional theory in the Bethe ansatz local-density approximation to the Hubbard model describes the dot electronic structure. We show that for a certain range of voltages the quantum dot can be driven into a dynamical state characterized by regular current oscillations. This is a manifestation of a recently proposed dynamical picture of Coulomb blockade. Furthermore, we investigate how the various approximations to the electron-electron interaction affect the line-shapes of the Coulomb peaks and the I-V characteristics. We show that the presence of the derivative discontinuity in the approximate exchange-correlation potential leads to significantly different results compared to those obtained at the simpler Hartree level of description. In particular, a negative differential conductance (NDC) in the I-V characteristics is observed at large bias voltages and large Coulomb interaction strengths. We demonstrate that such NDC originates from the combined effect of electron-electron interaction in the dot and the finite bandwidth of the electrodes.Comment: 10 pages, 7 figure

Phase transitions in the one-dimensional frustrated quantum XY model and Josephson-junction ladders

A one-dimensional quantum version of the frustrated XY (planar rotor) model is considered which can be physically realized as a ladder of Josephson-junctions at half a flux quantum per plaquette. This system undergoes a superconductor to insulator transition at zero temperature as a function of charging energy. The critical behavior is studied using a Monte Carlo transfer matrix applied to the path-integral representation of the model and a finite-size-scaling analysis of data on small system sizes. Depending on the ratio between the interchain and intrachain couplings the system can have single or double transitions which is consistent with the prediction that its critical behavior should be described by the two-dimensional classical XY-Ising model.Comment: 13 pages, Revtex, J. Appl. Phys. (to appear), Inpe-las-00