Magnetic quantum phase transitions of the antiferromagnetic J_{1}-J_{2} Heisenberg model

We obtain the complete phase diagram of the antiferromagnetic $J_{1}$-$J_{2}$ model, $0\leq \alpha = J_2/J1 \leq 1$, within the framework of the $O(N)$ nonlinear sigma model. We find two magnetically ordered phases, one with N\' eel order, for $\alpha \leq 0.4$, and another with collinear order, for $\alpha\geq 0.6$, separated by a nonmagnetic region, for $0.4\leq \alpha \leq 0.6$, where a gapped spin liquid is found. The transition at $\alpha=0.4$ is of the second order while the one at $\alpha=0.6$ is of the first order and the spin gaps cross at $\alpha=0.5$. Our results are exact at $N\rightarrow\infty$ and agree with numerical results from different methods.Comment: 4 pages, 5 figure

Particle Creation by a Moving Boundary with Robin Boundary Condition

We consider a massless scalar field in 1+1 dimensions satisfying a Robin boundary condition (BC) at a non-relativistic moving boundary. We derive a Bogoliubov transformation between input and output bosonic field operators, which allows us to calculate the spectral distribution of created particles. The cases of Dirichlet and Neumann BC may be obtained from our result as limiting cases. These two limits yield the same spectrum, which turns out to be an upper bound for the spectra derived for Robin BC. We show that the particle emission effect can be considerably reduced (with respect to the Dirichlet/Neumann case) by selecting a particular value for the oscillation frequency of the boundary position

Roughness correction to the Casimir force : Beyond the Proximity Force Approximation

We calculate the roughness correction to the Casimir effect in the parallel plates geometry for metallic plates described by the plasma model. The calculation is perturbative in the roughness amplitude with arbitrary values for the plasma wavelength, the plate separation and the roughness correlation length. The correction is found to be always larger than the result obtained in the Proximity Force Approximation.Comment: 7 pages, 3 figures, v2 with minor change

Tailoring Graphene with Metals on Top

We study the effects of metallic doping on the electronic properties of graphene using density functional theory in the local density approximation in the presence of a local charging energy (LDA+U). The electronic properties are sensitive to whether graphene is doped with alkali or transition metals. We estimate the the charge transfer from a single layer of Potassium on top of graphene in terms of the local charging energy of the graphene sheet. The coating of graphene with a non-magnetic layer of Palladium, on the other hand, can lead to a magnetic instability in coated graphene due to the hybridization between the transition-metal and the carbon orbitals.Comment: 5 pages, 4 figure

Dynamical Casimir effect with cylindrical waveguides

I consider the quantum electromagnetic field in a coaxial cylindrical waveguide, such that the outer cylindrical surface has a time-dependent radius. The field propagates parallel to the axis, inside the annular region between the two cylindrical surfaces. When the mechanical frequency and the thickness of the annular region are small enough, only Transverse Electromagnetic (TEM) photons may be generated by the dynamical Casimir effect. The photon emission rate is calculated in this regime, and compared with the case of parallel plates in the limit of very short distances between the two cylindrical surfaces. The proximity force approximation holds for the transition matrix elements in this limit, but the emission rate scales quadratically with the mechanical frequency, as opposed to the cubic dependence for parallel plates.Comment: 6 page

The Hamilton-Jacobi Approach to Teleparallelism

We intend to analyse the constraint structure of Teleparallelism employing the Hamilton-Jacobi formalism for singular systems. This study is conducted without using an ADM 3+1 decomposition and without fixing time gauge condition. It can be verified that the field equations constitute an integrable system.Comment: 12 pages, no figur

Negative Hopping Magnetoresistance and Dimensional Crossover in Lightly Doped Cuprate Superconductors

We show that, due to the weak ferromagnetism of La$_{2-x}$Sr$_x$CuO$_4$, an external magnetic field leads to a dimensional crossover 2D $\to$ 3D for the in-plane transport. The crossover results in an increase of the hole's localization length and hence in a dramatic negative magnetoresistance in the variable range hopping regime. This mechanism quantitatively explains puzzling experimental data on the negative magnetoresistance in the N\'eel phase of La$_{2-x}$Sr$_x$CuO$_4$.Comment: 6 pages, 3 figures; published versio