Thomas C. Lang

Dynamical signatures of edge-state magnetism on graphene nanoribbons.

We investigate the edge-state magnetism of graphene nanoribbons using projective quantum Monte Carlo simulations and a self-consistent mean-field approximation of the Hubbard model. The static magnetic correlations are found to be short ranged. Nevertheless, the correlation length increases with the width of the ribbon such that already for ribbons of moderate widths we observe a strong trend towards mean-field-type ferromagnetic correlations at a zigzag edge. These correlations are accompanied by a dominant low-energy peak in the local spectral function and we propose that this can be used to detect edge-state magnetism by scanning tunneling microscopy. The dynamic spin structure factor at the edge of a ribbon exhibits an approximately linearly dispersing collective magnonlike mode at low energies that decays into Stoner modes beyond the energy scale where it merges into the particle-hole continuum.

Correlation Effects in Quantum Spin-Hall Insulators: A Quantum Monte Carlo Study

We consider the Kane-Mele model supplemented by a Hubbard U term. The phase diagram is mapped out using projective auxiliary field quantum Monte Carlo simulations. The quantum spin liquid of the Hubbard model is robust against weak spin-orbit interaction, and is not adiabatically connected to the spin-Hall insulating state. Beyond a critical value of U>U(c) both states are unstable toward magnetic ordering. In the quantum spin-Hall state we study the spin, charge, and single-particle dynamics of the helical Luttinger liquid by retaining the Hubbard interaction only on a ribbon edge. The Hubbard interaction greatly suppresses charge currents along the edge and promotes edge magnetism but leaves the single-particle signatures of the helical liquid intact.

Transforming growth factor-beta and ovarian carcinoma cells: regulation of proliferation and surface antigen expression

Transforming growth factor-beta (TGF-beta) is a multifunctional peptide regulating several processes in ovarian cells. The growth of ovarian carcinoma cell lines (OVCAR-3, HTB-77, 2780 and CRL-1572) was reduced by TGF-beta in a dose related manner. The antiproliferative activity was not improved by combination with other biological response modifiers. Treatment with TGF-beta augmented the expression of interferon-gamma induced class I and II antigens of the major histocompatibility complex. The presentation of another antigen namely the tumor marker CA-125 on the cell surface was markedly reduced by TGF-beta.

Antiferromagnetism in the Hubbard Model on the Bernal-Stacked Honeycomb Bilayer

Using a combination of quantum Monte Carlo simulations, functional renormalization group calculations and mean-field theory, we study the Hubbard model on the Bernal-stacked honeycomb bilayer at half-filling as a model system for bilayer graphene. The free bands consisting of two Fermi points with quadratic dispersions lead to a finite density of states at the Fermi level, which triggers an antiferromagnetic instability that spontaneously breaks sublattice and spin rotational symmetry once local Coulomb repulsions are introduced. Our results reveal an inhomogeneous participation of the spin moments in the ordered ground state, with enhanced moments at the threefold coordinated sites. Furthermore, we find the antiferromagnetic ground state to be robust with respect to enhanced interlayer couplings and extended Coulomb interactions.

Effects of Taxol on choriocarcinoma cells.

OBJECTIVE Taxol (Bristol-Myers Squibb) (paclitaxel) has been shown to be a potent inhibitor of cell growth for a variety of tumors. We were interested in the antiproliferative efficacy and biologic properties of this novel antineoplastic agent in choriocarcinoma cells. STUDY DESIGN Human choriocarcinoma cell lines JAR and BeWo were cultured as monolayers and treated with Taxol. RESULTS Proliferation of JAR and BeWo cells was inhibited by Taxol in a dose-related manner and 1 to 3 nmol/L was sufficient to achieve 50% growth reduction. This effect was accompanied by a marked induction of human chorionic gonadotropin secretion. The effect on human chorionic gonadotropin secretion was dependent on intact protein biosynthesis but not mediated by augmented messenger ribonucleic acid expression. In these choriocarcinoma cells Taxol promoted differentiation as shown by an increase in syncytiotrophoblastic-like cells. Combination of Taxol with either etoposide or methotrexate resulted in antagonistic growth inhibition. CONCLUSION Taxol is a highly effective antineoplastic agent in choriocarcinoma cells, and clinical trials in refractory disease would therefore be warranted. However, substances other than etoposide or methotrexate should be evaluated for combined treatment. In addition to growth inhibition, differentiation is also induced by Taxol, as shown by increased human chorionic gonadotropin secretion and changed morphologic features.

Quantum Phase Transitions in the Kane-Mele-Hubbard Model

We study the two-dimensional Kane-Mele-Hubbard model at half filling by means of quantum Monte Carlo simulations. We present a refined phase boundary for the quantum spin liquid. The topological insulator at finite Hubbard interaction strength is adiabatically connected to the groundstate of the Kane-Mele model. In the presence of spin-orbit coupling, magnetic order at large Hubbard U is restricted to the transverse direction. The transition from the topological band insulator to the antiferromagnetic Mott insulator is in the universality class of the three-dimensional XY model. The numerical data suggest that the spin liquid to topological insulator and spin liquid to Mott insulator transitions are both continuous.

Entanglement Spectra of Interacting Fermions in Quantum Monte Carlo Simulations

Department of Physics, Boston University, Boston, MA 02215, USA(Dated: November 25, 2013)In a recent article T. Grover [Phys. Rev. Lett. 111, 130402 (2013)] introduced a simple methodto compute Renyi entanglement entropies in the realm of the auxiliary eld quantum Monte Carloalgorithm. Here, we further develop this approach and provide a stabilization scheme to computehigher order Renyi entropies and an extension to access the entanglement spectrum. The methodis tested on systems of correlated topological insulators.

Z2topological invariants in two dimensions from quantum Monte Carlo

We employ quantum Monte Carlo techniques to calculate the

Magnetic correlations in short and narrow graphene armchair nanoribbons.

Z_2

THE CHARACTERIZATION OF TOPOLOGICAL PROPERTIES IN QUANTUM MONTE CARLO SIMULATIONS OF THE KANE–MELE–HUBBARD MODEL

topological invariant in a two-dimensional model of interacting electrons that exhibits a quantum spin Hall topological insulator phase. In particular, we consider the parity invariant for inversion-symmetric systems, which can be obtained from the bulks imaginary-time Greens function after an appropriate continuation to zero frequency. This topological invariant is used here in order to study the trivial-band to topological-insulator transitions in an interacting system with spin-orbit coupling and an explicit bond dimerization. We discuss the accessibility and behavior of this topological invariant within quantum Monte Carlo simulations.