Sumathi Rao

Renormalization group fixed points and the Higgs boson spectrum

Abstract We study numerically the renormalization group equations for the Higgs potential of the two-doublet model assuming perturbative unification and sufficiently large initial quartic and Higgs-Yukawa couplings such that the full nonlinearities interplay. We obtain predictions for the physical Higgs boson spectrum in the two-doublet model up to systematic differences in fermion coupling schemes. Unambiguous predictions emerge only when there exists a heavy generation in which quarks couple to both doublets. In other cases we find that the potential can become quartically unstable at low energies for arbitrary initial stable values of the coupling constants.

Low-energy manifestations of a new interactions scale: Operator analysis

We list all dimension sixSU(3)×SU(2)×U(1) invariant operators which can be used in a phenomenological analysis of deviations from the standard model due to a new interaction scale. Specific attention is focused on modifications to the masses of theW andZ bosons.

Junction of several weakly interacting quantum wires: A renormalization group study

We study the conductance of three or more semi-infinite wires which meet at a junction. The electrons in the wires are taken to interact weakly with each other through a short-range density-density interaction, and they encounter a general scattering matrix at the junction. We derive the renormalization-group equations satisfied by the S-matrix, and we identify its fixed points and their stabilities. The conductance between any pair of wires is then studied as a function of physical parameters such as temperature. We discuss the possibility of observing the effects of junctions in present day experiments, such as the four-terminal conductance of a quantum wire and crossed quantum wires.

Conductance, valley and spin polarizations, and tunneling magnetoresistance in ferromagnetic-normal-ferromagnetic junctions of silicene

We investigate charge conductance and spin and valley polarization along with the tunnelling magneto-resistance (TMR) in silicene junctions composed of normal silicene and ferromagnetic silicene. We show distinct features of the conductances for parallel and anti-parallel spin configurations and the TMR, as the ferromagnetic-normal-ferromagnetic (FNF) junction is tuned by an external electric field. We analyse the behavior of the charge conductance and valley and spin polarizations in terms of the independent conductances of the different spins at the two valleys and the band structure of ferromagnetic silicene and show how the conductances are affected by the vanishing of the propagating states at one or the other valley. In particular, unlike in graphene, the band structure at the two valleys are independently affected by the spin in the ferromagnetic regions and lead to non-zero, and in certain parameter regimes, pure valley and spin polarizations, which can be tuned by the external electric field. We also investigate the oscillatory behavior of the TMR with respect to the strength of the barrier potential (both spin-independent and spin-dependent barriers) in the normal silicene region and note that in some parameter regimes, the TMR can even go from positive to negative values, as a function of the external electric field.

Parity anomalies in gauge theories in 2 + 1 dimensions

Abstract It is shown that the introduction of massless fermions in an abelian gauge theory in 2 + 1 dimensions does not lead to any parity anomaly despite a non-commutativity of limits in the structure function of the odd part of the vacuum polarisation tensor. However, a parity anomaly does exist in non-abelian theories due to a conflict between gauge invariance under large gauge transformations and the parity symmetry.

Transport through Quasiballistic Quantum Wires: The Role of Contacts

We model one-dimensional transport through each open channel of a quantum wire by a Luttinger liquid with different interaction parameters for the leads, the contacts, and the wire, and with two barriers at the contacts. We show that this simple model explains several features of recent experiments, such as the flat conductance plateaus observed at finite temperatures and lengths, and universal conductance corrections in different channels. We discuss the possibility of seeing resonance like features at very low temperatures.

Topological mass term for gravity induced by matter

Abstract The contribution of massive fermions and topologically massive gauge fields to a Chern-Simons term for the graviton is computed to one-loop order in three dimensions. Invariance under large coordinate transformations places restrictions on the number of these massive matter fields. The parity anomaly for massless fermions is determined, and a possible parity anomaly for massless gauge fields is suggested.

Spin-polarized scanning-tunneling probe for helical Luttinger liquids.

We propose a three-terminal spin-polarized STM setup for probing the helical nature of the Luttinger liquid edge state that appears in the quantum spin Hall system. We show that the three-terminal tunneling conductance depends on the angle (θ) between the magnetization direction of the tip and the local orientation of the electron spin on the edge while the two terminal conductance is independent of this angle. We demonstrate that chiral injection of an electron into the helical Luttinger liquid (when θ is zero or π) is associated with fractionalization of the spin of the injected electron in addition to the fractionalization of its charge. We also point out a spin current amplification effect induced by the spin fractionalization.

Enhancement of Tunneling Density of States at a Junction of Three Luttinger Liquid Wires

We study the tunneling density of states (TDOS) for a junction of three Tomonaga-Luttinger liquid wires. We show that there are fixed points which allow for the enhancement of the TDOS, which is unusual for Luttinger liquids. The distance from the junction over which this enhancement occurs is of the order of x=v/(2omega), where v is the plasmon velocity and omega is the bias frequency. Beyond this distance, the TDOS crosses over to the standard bulk value independent of the fixed point describing the junction. This finite range of distances opens up the possibility of experimentally probing the enhancement in each wire individually.

Interedge interactions and fixed points at a junction of quantum Hall line junctions

We show that fixed points (characterized by matrices which specify the splitting of the currents at the junction) can be accessed in a system which contains a junction of three quantum Hall line junctions. For such a junction of fractional quantum Hall edge states, we find that it is possible for both the flower (single droplet) and islands (three droplets) configurations to be stable in an intermediate region, for a range of values of the interedge repulsive interactions. A measurement of the tunneling conductance as a function of the gate voltage controlling interedge repulsions can give a clear experimental signal of this region.