A Memristor Model with Piecewise Window Function

In this paper, we present a memristor model with piecewise window function, which is continuously differentiable and consists of three nonlinear pieces. By introducing two parameters, the shape of this window function can be flexibly adjusted to model different types of memristors. Using this model, one can easily obtain an expression of memristance depending on charge, from which the numerical value of memristance can be readily calculated for any given charge, and eliminate the error occurring in the simulation of some existing window function models

On the Intersection of Two Plane Curves

We study the following question: fix a sufficient general curve D of degree d in P^2, what is the least number of intersections between D and an irreducible curve of degree m? G. Xu proved this number i(d, m) is at least d - 2 for all m. This problem can be regarded as the algebraic part of Kobayashi conjecture on the hyperbolicity of P^2 D. We first improved Xu's bound with m fixed and then generalized his result to rational ruled surfaces.Comment: 13 pages in AMS-LATEX. To appear on MR

Critical Current Density and Resistivity of MgB2 Films

The high resistivity of many bulk and film samples of MgB2 is most readily explained by the suggestion that only a fraction of the cross-sectional area of the samples is effectively carrying current. Hence the supercurrent (Jc) in such samples will be limited by the same area factor, arising for example from porosity or from insulating oxides present at the grain boundaries. We suggest that a correlation should exist, Jc ~ 1/{Rho(300K) - Rho(50K)}, where Rho(300K) - Rho(50K) is the change in the apparent resistivity from 300 K to 50 K. We report measurements of Rho(T) and Jc for a number of films made by hybrid physical-chemical vapor deposition which demonstrate this correlation, although the "reduced effective area" argument alone is not sufficient. We suggest that this argument can also apply to many polycrystalline bulk and wire samples of MgB2.Comment: 11 pages, 3 figure

Interaction induced topological phase transition in Bernevig-Hughes-Zhang model

We study interaction induced topological phase transition in Bernevig-Hughes-Zhang model. Topological nature of the phase transition is revealed by directly calculating the Z2 index of the interacting system from the single-particle Green's function. The interacting Z2 index is also consistently checked through the edge spectra. Combined with ab initio methods, present approach is a useful tool searching for correlated topological insulating materials from the first-principle point of view.Comment: 4.5 pages, 4 figures, reference adde

Anomalous Phase Transition in Strained SrTiO3_3 Thin Films

We have studied the cubic to tetragonal phase transition in epitaxial SrTiO$_3$ films under various biaxial strain conditions using synchrotron X-ray diffraction. Measuring the superlattice peak associated with TiO$_6$ octahedra rotation in the low temperature tetragonal phase indicates the presence of a phase transition whose critical temperature is a strong function of strain, with T$_C$ as much as 50K above the corresponding bulk temperature. Surprisingly, the lattice constants evolve smoothly through the transition with no indication of a phase change. This signals an important change in the nature of the phase transition due to the epitaxy strain and substrate clamping effect. The internal degrees of freedom (TiO$_6$ rotations) have become uncoupled from the overall lattice shape.Comment: 4 pages, 3 figures, REVTeX

Derivation of Electroweak Chiral Lagrangian from One Family Technicolor Model

Based on previous studies deriving the chiral Lagrangian for pseudo scalar mesons from the first principle of QCD in the path integral formalism, we derive the electroweak chiral Lagrangian and dynamically compute all its coefficients from the one family technicolor model. The numerical results of the $p^4$ order coefficients obtained in this paper are proportional to the technicolor number $N_{\rm TC}$ and the technifermion number $N_{\rm TF}$, which agrees with the arguments in previous works, and which confirms the reliability of this dynamical computation.Comment: 6 page

Spin current through an ESR quantum dot: A real-time study

The spin transport in a strongly interacting spin-pump nano-device is studied using the time-dependent variational-matrix-product-state (VMPS) approach. The precession magnetic field generates a dissipationless spin current through the quantum dot. We compute the real time spin current away from the equilibrium condition. Both transient and stationary states are reached in the simulation. The essentially exact results are compared with those from the Hartree-Fock approximation (HFA). It is found that correlation effect on the physical quantities at quasi-steady state are captured well by the HFA for small interaction strength. However the HFA misses many features in the real time dynamics. Results reported here may shed light on the understanding of the ultra-fast processes as well as the interplay of the non-equilibrium and strongly correlated effect in the transport properties.Comment: 5 pages, 5 figure

Magnetism of Cold Fermionic Atoms on p-Band of an Optical Lattice

We carry out \textit{ab initio} study of ground state phase diagram of spin-1/2 cold fermionic atoms within two-fold degenerate $p$-band of an anisotropic optical lattice. Using the Gutzwiller variational approach, we show that a robust ferromagnetic phase exists for a vast range of band fillings and interacting strengths. The ground state crosses over from spin density wave state to spin-1 Neel state at half filling. Additional harmonic trap will induce spatial separation of varies phases. We also discuss several relevant observable consequences and detection methods. Experimental test of the results reported here may shed some light on the long-standing issue of itinerant ferromagnetism.Comment: 5 pages, 4 figure

Thermodynamics of the Mg-B system: Implications for the deposition of MgB2 thin films

We have studied thermodynamics of the Mg-B system with the modeling technique CALPHAD using a computerized optimization procedure. Temperature-composition, pressure-composition, and pressure-temperature phase diagrams under different conditions are obtained. The results provide helpful insights into appropriate processing conditions for thin films of the superconducting phase, MgB2, including the identification of the pressure/temperature region for adsorption-controlled growth. Due to the high volatility of Mg, MgB2 is thermodynamically stable only under fairly high Mg overpressures for likely growth temperatures. This constraint places severe temperature constraints on deposition techniques employing high vacuum conditions