Conformal QED in two-dimensional topological insulators

It has been shown recently that local four-fermion interactions on the edges of two-dimensional time-reversal-invariant topological insulators give rise to a new non-Fermi-liquid phase, called helical Luttinger liquid (HLL). In this work, we provide a first-principle derivation of this non-Fermi-liquid phase based on the gauge-theory approach. Firstly, we derive a gauge theory for the edge states by simply assuming that the interactions between the Dirac fermions at the edge are mediated by a quantum dynamical electromagnetic field. Here, the massless Dirac fermions are confined to live on the one-dimensional boundary, while the (virtual) photons of the U(1) gauge field are free to propagate in all the three spatial dimensions that represent the physical space where the topological insulator is embedded. We then determine the effective 1+1-dimensional conformal field theory (CFT) given by the conformal quantum electrodynamics (CQED). By integrating out the gauge field in the corresponding partition function, we show that the CQED gives rise to a 1+1-dimensional Thirring model. The bosonized Thirring Hamiltonian describes exactly a HLL with a parameter K and a renormalized Fermi velocity that depend on the value of the fine-structure constant $\alpha$.Comment: (5+4) pages, 2 figure

Chern-Simons theory and atypical Hall conductivity in the Varma phase

In this letter, we analyze the topological response of a fermionic model defined on the Lieb lattice in presence of an electromagnetic field. The tight-binding model is built in terms of three species of spinless fermions and supports a topological Varma phase due to the spontaneous breaking of time-reversal symmetry. In the low-energy regime, the emergent effective Hamiltonian coincides with the so-called Duffin-Kemmer-Petiau (DKP) Hamiltonian, which describes relativistic pseudospin-0 quasiparticles. By considering a minimal coupling between the DKP quasiparticles and an external Abelian gauge field, we calculate both the Landau-level spectrum and the emergent Chern-Simons theory. The corresponding Hall conductivity reveals an atypical quantum Hall effect, which can be simulated in an artificial Lieb lattice.Comment: 5 pages, 3 figures; New version with an improved discussion about our finding

Correlated versus Uncorrelated Stripe Pinning: the Roles of Nd and Zn Co-Doping

We investigate the stripe pinning produced by Nd and Zn co-dopants in cuprates via a renormalization group approach. The two dopants play fundamentally different roles in the pinning process. While Nd induces a correlated pinning potential that traps the stripes in a flat phase and suppresses fluctuations, Zn pins the stripes in a disordered manner and promotes line meandering. We obtain the zero temperature phase diagram and compare our results with neutron scattering data. A good agreement is found between theory and experiment.Comment: To appear at the proceedings of the LLD2K Conference Tsukuba, July 2000, Japan. 4 pages, 2 figure

Topological phases in a two-dimensional lattice: Magnetic field versus spin-orbit coupling

In this work, we explore the rich variety of topological states that arise in two-dimensional systems, by considering the competing effects of spin-orbit couplings and a perpendicular magnetic field on a honeycomb lattice. Unlike earlier approaches, we investigate minimal models in order to clarify the effects of the intrinsic and Rashba spin-orbit couplings, and also of the Zeeman splitting, on the quantum Hall states generated by the magnetic field. In this sense, our work provides an interesting path connecting quantum Hall and quantum spin Hall physics. First, we consider the properties of each term individually and we analyze their similarities and differences. Secondly, we investigate the subtle competitions that arise when these effects are combined. We finally explore the various possible experimental realizations of our model.Comment: 19 pages, 15 figure

Avaliação da transferência de metano no sentido gás-líquido e da capacidade de suprimento da desnitrificação biológica de esgoto sanitário com doadores endógenos de elétrons (metano e sulfeto de hidrogênio

Este trabalho apresenta uma avaliação da capacidade de desnitrificação biológica de esgoto sanitário através de fontes endógenas de elétrons (metano e sulfeto de hidrogênio) em um sistema de tratamento que associa em série processos biológicos anaeróbios e aeróbios. O metano presente no biogás produzido num reator UASB pode ser utilizado na etapa de desnitrificação através de sua transferência para a fase líquida. Essa transferência, porém, apresenta limitações que podem comprometer seu uso como única fonte de elétrons. Por esse motivo, uma das etapas desse estudo objetivou a avaliação da capacidade de transferência de metano no sentido gás-líquido. Os resultados demonstram o incremento da capacidade padrão de transferência (STR) com o aumento da taxa de aplicação de biogás. Contudo, esse incremento não se mostrou proporcional, o que implica na redução da eficiência padrão de transferência (STE). Na segunda etapa do estudo foi simulada a utilização de metano e de sulfeto de hidrogênio produzidos na etapa anaeróbia como doadores de elétrons para a desnitrificação biológica de esgoto sanitário num sistema anaeróbio aeróbio em sete cenários diferentes onde foram permutados fatores importantes ao processo. Em todos os cenários foram consideradas as relações estequiométricas da via desnitrificante. Se considerado o fornecimento de todo o metano recuperado como biogás à desnitrificação foram alcançados efluentes finais com teores nulos de N-NO3-. Essa consideração, no entanto, leva à superestimar o potencial de utilização desse doador de elétrons. A utilização do metano e sulfeto de hidrogênio dissolvidos no efluente do reator UASB e do metano recuperado como biogás e efetivamente fornecido ao sistema apresentou capacidade de remoção de 13,8 mg.L-1 de N-NO3-, 8,44 mg.L-1 de N-NO3- e 6,42 mg.L-1 de N-NO3-. Isso representa eficiências de remoção de 49,1%, 30,0% e 22,9% de todo N-NO3- do sistema, respectivamente, estando a capacidade de anulação dos teores de nitrato no efluente condicionada à utilização em conjunto desses doadores de elétrons. Mesmo que as relações C/N e S/N reais sejam maiores que as estequiométricas, e possam levar a limitações da capacidade de sustentação da desnitrificação biológica com fontes endógenas de elétrons, essas fontes podem ser utilizadas de forma a reduzir a necessidade de doadores exógenos de elétrons e, consequentemente, os custos de tratamento, potencializando a promoção da utilização dessa tecnologia no tratamento de efluentes sanitários

Stripe dynamics in presence of disorder and lattice potentials

We study the influence of disorder and lattice pinning on the dynamics of a charged stripe. Starting from a phenomenological model of a discrete quantum string, we determine the phase diagram for this system. Three regimes are identified, the free phase, the flat phase pinned by the lattice, and the disorder pinned phase. In the absence of impurities, the system can be mapped onto a 1D array of Josephson junctions. The results are compared with measurements on nickelates and cuprates and a good qualitative agreement is found between our results and the experimental data.Comment: 4 pages, 2 figure

Phase Transition and Monopoles Densities in a Nearest Neighbors Two-Dimensional Spin Ice Model

In this work, we show that, due to the alternating orientation of the spins in the ground state of the artificial square spin ice, the influence of a set of spins at a certain distance of a reference spin decreases faster than the expected result for the long range dipolar interaction, justifying the use of the nearest neighbor two dimensional square spin ice model as an effective model. Using an extension of the model presented in ref. [Scientific Reports 5, 15875 (2015)], considering the influence of the eight nearest neighbors of each spin on the lattice, we analyze the thermodynamics of the model and study the monopoles and string densities dependence as a function of the temperature.Comment: 11 pages, 8 figure