On the worldsheet theories of strings dual to free large N gauge theories

We analyze in detail some properties of the worldsheet of the closed string theories suggested by Gopakumar to be dual to free large N SU(N) gauge theories (with adjoint matter fields). We use Gopakumar's prescription to translate the computation of space-time correlation functions to worldsheet correlation functions for several classes of Feynman diagrams, by explicit computations of Strebel differentials. We compute the worldsheet operator product expansion in several cases and find that it is consistent with general worldsheet conformal field theory expectations. A peculiar property of the construction is that in several cases the resulting worldsheet correlation functions are non-vanishing only on a sub-space of the moduli space (say, for specific relations between vertex positions). Another strange property we find is that for a conformally invariant space-time theory, the mapping to the worldsheet does not preserve the special conformal symmetries, so that the full conformal group is not realized as a global symmetry on the worldsheet (even though it is, by construction, a symmetry of all integrated correlation functions).Comment: 60 pages, 17 figures, latex. v2: Added references and a minor correctio

Defining the effective temperature of a quantum driven system from current-current correlation functions

We calculate current-current correlation functions and find an expression for the zero-frequency noise of multiterminal systems driven by harmonically time-dependent voltages within the Keldysh non-equilibrium Green's functions formalism. We also propose a fluctuation-dissipation relation for current-current correlation functions to define an effective temperature. We discuss the behavior of this temperature and compare it with the local temperature determined by a thermometer and with the effective temperature defined from a single-particle fluctuation-dissipation relation. We show that for low frequencies all the definitions of the temperature coincide.Comment: 11 pages, 5 figure

NEGF Simulations of the effect of strain on scaled double gate NanoMOSFETs

The effect of biaxial strain on double gate (DG) nanoscaled Si MOSFET with channel lengths in the nanometre range is investigated using Non-Equilibrium Green’s Functions (NEGF) simulations. We have employed fully 2D NEGF simulations in order to answer the question at which body thickness the effects of strain is masked by the confinement impact. Following ITRS, we start with a 14 nm gate length DG MOSFET having a body thickness of 9 nm scaling the transistors to gate lengths of 10, 6 and 4 nm and body thicknesses of 6.1, 2.6 and 1.3 nm. The simulated I D–V G characteristics show a 6% improvement in the on-current for the 14 nm gate length transistor mainly due to the energy separation of the Δ valleys. The strain effect separates the 2 fold from the 4 fold valleys thus keeping mostly operational transverse electron effective mass in the transport direction. However, in the device with an extreme body thickness of 1.3 nm, the strain effect has no more impact on the DG performance because the strong confinement itself produces a large energy separation of valleys