Flavio S. Nogueira

A high-temperature ferromagnetic topological insulating phase by proximity coupling

Topological insulators are insulating materials that display conducting surface states protected by time-reversal symmetry, wherein electron spins are locked to their momentum. This unique property opens up new opportunities for creating next-generation electronic, spintronic and quantum computation devices. Introducing ferromagnetic order into a topological insulator system without compromising its distinctive quantum coherent features could lead to the realization of several predicted physical phenomena. In particular, achieving robust long-range magnetic order at the surface of the topological insulator at specific locations without introducing spin-scattering centres could open up new possibilities for devices. Here we use spin-polarized neutron reflectivity experiments to demonstrate topologically enhanced interface magnetism by coupling a ferromagnetic insulator (EuS) to a topological insulator (Bi2Se3) in a bilayer system. This interfacial ferromagnetism persists up to room temperature, even though the ferromagnetic insulator is known to order ferromagnetically only at low temperatures (<17 K). The magnetism induced at the interface resulting from the large spin–orbit interaction and the spin–momentum locking of the topological insulator surface greatly enhances the magnetic ordering (Curie) temperature of this bilayer system. The ferromagnetism extends ~2 nm into the Bi2Se3 from the interface. Owing to the short-range nature of the ferromagnetic exchange interaction, the time-reversal symmetry is broken only near the surface of a topological insulator, while leaving its bulk states unaffected. The topological magneto-electric response originating in such an engineered topological insulator could allow efficient manipulation of the magnetization dynamics by an electric field, providing an energy-efficient topological control mechanism for future spin-based technologies.

Spin Josephson effect in ferromagnet/ferromagnet tunnel junctions

We consider the tunnel current between two ferromagnetic metals from a perspective similar to the one used in superconductor/superconductor tunnel junctions. We use fundamental arguments to derive a Josephson-like tunnel spin current IJspin sin (θ1 − θ2). Here the phases are associated with the planar contribution to the magnetization, c↑†c↓ ~ eiθ. The crucial step in our analysis is the fact that the z-component of the spin is canonically conjugate to the phase of the planar contribution: [θ,Sz] = i. This is the counterpart to the commutation relation [,N] = i in superconductors, where is the phase associated with the superconducting order parameter and N is the Cooper pair number operator. We briefly discuss the experimental consequences of our theoretical analysis.

Deconfinement transition in three-dimensional compact U(1) gauge theories coupled to matter fields

It is shown that permanent confinement in three-dimensional compact U(1) gauge theory can be destroyed by matter fields in a deconfinement transition. This follows from a nontrivial infrared fixed point caused by matter, and an anomalous scaling dimension of the gauge field. This leads to a logarithmic interaction between the defects of the gauge fields, which form a gas of magnetic monopoles. For logarithmic interactions, the original electric charges are unconfined. The confined phase, which is permanent in the absence of matter fields, is reached at a critical electric charge, where the interaction between magnetic charges is screened by a pair-unbinding in a Kosterlitz-Thouless-like phase transition.

Kosterlitz-Thouless - like deconfinement mechanism in the (2+1)-dimensional Abelian Higgs model

We point out that the permanent confinement in a compact (2 + 1)-dimensional U( 1) Abelian Higgs model is destroyed by matter fields in the fundamental representation. The deconfinement transition is Kosterlitz–Thouless-like. The dual theory is shown to describe a three-dimensional gas of point charges with logarithmic interactions which arises from an anomalous dimension of the gauge field caused by critical matter field fluctuations. The theory is equivalent to a sine-Gordon-like theory in (2 + 1)-dimensions with an anomalous gradient energyproportional to k 3 . The Callan– Symanzik equation is used to demonstrate that this theory has a massless and a massive phase. The renormalization group equations for the fugacity y(l) and stiffness parameter K(l)of the theory show that the renormalization of K(l) induces an anomalous scaling dimension ηy of y(l). The stiffness parameter of the theory has a universal jump at the transition determined by the dimensionality and ηy . As a byproduct of our analysis, we relate the critical coupling of the sine-Gordon-like theory to an a priori arbitrary constant that enters into the computation of critical exponents in the Abelian Higgs model at the charged infrared-stable fixed point of the theory, enabling a determination of this parameter. This facilitates the computation of the critical exponent ν at the charged fixed point in excellent agreement with one-loop renormalization group calculations for the three-dimensional XY model, thus confirming expectations based on duality transformations.  2003 Elsevier B.V. All rights reserved.

Critical properties of the topological Ginzburg-Landau model

We consider a Ginzburg-Landau model for superconductivity with a Chern-Simons term added. The flow diagram contains two charged fixed points corresponding to the tricritical and infrared stable fixed points. The topological coupling controls the fixed-point structure and eventually the region of first-order transitions disappears. We compute the critical exponents as a function of the topological coupling. We obtain that the value of the

Strong-coupling topological Josephson effect in quantum wires.

\ensuremath{\nu}

Quantum Electrodynamics in 2 + 1 Dimensions, Confinement, and the Stability of U(1) Spin Liquids

exponent does not vary very much from the

Phase structure of (2+1)-dimensional compact lattice gauge theories and the transition from Mott insulator to fractionalized insulator

\mathrm{XY}

Criticality in the (2+1)-dimensional compact Higgs model and fractionalized insulators.

value,

Deconfined quantum criticality and conformal phase transition in two-dimensional antiferromagnets

{\ensuremath{\nu}}_{\mathrm{XY}}=0.67.