Topological phase transition is a hot topic in condensed matter physics and computational material science. Here, we investigate the electronic structure and phonon dispersion of the two-dimensional (2D) platinum ditelluride ($PtTe_2$) using the dens
ity functional theory. It is found that the $PtTe_2$ monolayer is a trivial insulator with an indirect band gap of 0.347eV. Based on parity analysis, the biaxial tensile strain can drive the topological phase transition. As the strain reaches 19.3%, $PtTe_2$ undergoes a topological phase transition, which changes from a trivial band insulator to a topological insulator with $Z_2=1$. Unlike conventional honeycomb 2D materials with topological phase transition, which gap closes at K points, the strained $PtTe_2$ monolayer becomes gapless at M points under critical biaxial strain. The band inversion leads the switch of the parities near the Fermi level, which gives rise to the topological phase transition. The novel monolayer $PtTe_2$ has a potential application in the field of micro-electronics.
Under the Thomas-Fermi approximation, a relatively much simpler analytical solutions of the coupled Gross-Pitaevskii equations for the two-species BEC have been derived. Additionally, a model for the asymmetric states has been proposed, and the compe
tition between the symmetric and asymmetric states has been evaluated. The whole parameter-space is divided into zones, each supports a specific phase, namely, the symmetric miscible phase, the symmetric immiscible phase, or the asymmetric phase. Based on the division the phase-diagrams against any set of parameters can be plotted. Thereby, the effects of these parameters can be visualized. There are three critical values in the inter-species interaction $% V_{AB} $ and one in the ratio of particle numbers $N_{A}/N_{B}$. They govern the transitions between the phases. Two cases, (i) the repulsive $V_{AB}$ matches the repulsive $% V_{A}+V_{B}$, and (ii) the attractive $V_{AB}$ nearly cancels the effect of the repulsive $V_{A}+V_{B}$ have been particularly taken into account. The former leads to a complete separation of the two kinds of atoms , while the latter lead to a collapse. Finally, based on an equation derived in the paper, a convenient experimental approach is proposed to determine the ratio of particle numbers .
We argue that our analysis of the J-Q model, presented in Phys. Rev. B 80, 174403 (2009), and based on a field-theory description of coupled dimers, captures properly the strong quantum fluctuations tendencies, and the objections outlined by L. Isaev
, G. Ortiz, and J. Dukelsky, arXiv:1003.5205, are misplaced.
Checkerboard patterns have been proposed in order to explain STM experiments on the cuprates BSCCO and Na-CCOC. However the presence of these patterns has not been confirmed by a bulk probe such as neutron scattering. In particular, simple checkerboa
rd patterns are inconsistent with neutron scattering data, in that they have low energy incommsensurate (IC) spin peaks rotated 45 degrees from the direction of the charge IC peaks. However, it is unclear whether other checkerboard patterns can solve the problem. In this paper, we have studied more complicated checkerboard patterns (modulated checkerboards) by using spin wave theory and analyzed noncollinear checkerboards as well. We find that the high energy response of the modulated checkerboards is inconsistent with neutron scattering results, since they fail to exhibit a resonance peak at (pi,pi), which has recently been shown to be a universal feature of cuprate superconductors. We further argue that the newly proposed noncollinear checkerboard also lacks a resonance peak. We thus conclude that to date no checkerboard pattern has been proposed which satisfies both the low energy constraints and the high energy constraints imposed by the current body of experimental data in cuprate superconductors.
Recent neutron scattering measurements reveal spin and charge ordering in the half-doped nickelate, La$_{3/2}$ Sr$_{1/2}$ NiO$_4$. Many of the features of the magnetic excitations have been explained in terms of the spin waves of diagonal stripes wit
h weak single-ion anisotropy. However, an optical mode dispersing away from the (pi,pi) point was not captured by this theory. We show here that this apparent optical mode is a natural consequence of stripe twinning in a diagonal stripe pattern with a magnetic coupling structure which is two-fold symmetric, i.e. one possessing the same spatial rotational symmetry as the ground state.
We use a quantum Monte Carlo method to calculate the Neel temperature T_N of weakly coupled S=1/2 Heisenberg antiferromagnetic layers consisting of coupled ladders. This system can be tuned to different two-dimensional scaling regimes for T > T_N. In
a single-layer mean-field theory, chi_s^{2D}(T_N)=(z_2J)^{-1}, where chi_s^{2D} is the exact staggered susceptibility of an isolated layer, J the inter-layer coupling, and z_2=2 the layer coordination number. With a renormalized z_2, we find that this relationship applies not only in the renormalized-classical regime, as shown previously, but also in the quantum-critical regime and part of the quantum-disordered regime. The renormalization is nearly constant; k_2 ~ 0.65-0.70. We also study other universal scaling functions.
We discuss the magnetic excitations of well-ordered stripe and checkerboard phases, including the high energy magnetic excitations of recent interest and possible connections to the resonance peak in cuprate superconductors. Using a suitably parametr
ized Heisenberg model and spin wave theory, we study a variety of magnetically ordered configurations, including vertical and diagonal site- and bond-centered stripes and simple checkerboards. We calculate the expected neutron scattering intensities as a function of energy and momentum. At zero frequency, the satellite peaks of even square-wave stripes are suppressed by as much as a factor of 34 below the intensity of the main incommensurate peaks. We further find that at low energy, spin wave cones may not always be resolvable experimentally. Rather, the intensity as a function of position around the cone depends strongly on the coupling across the stripe domain walls. At intermediate energy, we find a saddlepoint at $(pi,pi)$ for a range of couplings, and discuss its possible connection to the resonance peak observed in neutron scattering experiments on cuprate superconductors. At high energy, various structures are possible as a function of coupling strength and configuration, including a high energy square-shaped continuum originally attributed to the quantum excitations of spin ladders. On the other hand, we find that simple checkerboard patterns are inconsistent with experimental results from neutron scattering.
