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O. Tirkkonen

Publications and source records attributed to O. Tirkkonen.

10 recordsLinked to original sources
arXiv2013

Device-to-Device Data Storage for Mobile Cellular Systems

As an alternative to downloading content from a cellular access network, mobile devices could be used to store data files and distribute them through device-to-device (D2D) communication. We consider a D2D-based storage community that is comprised of mobile users. Assuming that transmitting data from a base station to a mobile user consumes more energy than transmitting data between two mobile users, we show that it can be beneficial to use redundant storage to ensure that data files stay available to the community even if some of the storing users leave the network. We derive a tractable closed-form equation stating when redundancy should be used in order to minimize the expected energy consumption of data retrieval. We find that replication is the preferred method of adding redundancy as opposed to regenerating codes. Our findings are verified by computer simulations.

cs.NI↗
arXiv1999

Excited states in the twisted XXZ spin chain

We compute the finite size spectrum for the spin 1/2 XXZ chain with twisted boundary conditions, for anisotropy in the regime $0< γ<π/2$, and arbitrary twist $θ$. The string hypothesis is employed for treating complex excitations. The Bethe Ansatz equtions are solved within a coupled non-linear integral equation approach, with one equation for each type of string. The root-of-unity quantum group invariant periodic chain reduces to the XXZ_1/2 chain with a set of twist boundary conditions ($π/γ\in Z$, $θ$ an integer multiple of $γ$). For this model, the restricted Hilbert space corresponds to an unitary conformal field theory, and we recover all primary states in the Kac table in terms of states with specific twist and strings.

hep-th↗
arXiv1996

G/G models as the strong coupling limit of topologically massive gauge theory

We show that the problem of computing the vacuum expectation values of gauge invariant operators in the strong coupling limit of topologically massive gauge theory is equivalent to the problem of computing similar operators in the $G_k/G$ model where $k$ is the integer coefficient of the Chern-Simons term. The $G_k/G$ model is a topological field theory and many correlators can be computed analytically. We also show that the effective action for the Polyakov loop operator and static modes of the gauge fields of the strongly coupled theory at finite temperature is a perturbed, non-topological $G_k/G$ model. In this model, we compute the one loop effective potential for the Polyakov loop operators and explicitly construct the low-lying excited states. In the strong coupling limit the theory is in a deconfined phase.

hep-th↗
arXiv1996

Exact Solution of the One-Dimensional Non-Abelian Coulomb Gas at Large N

The problem of computing the thermodynamic properties of a one-dimensional gas of particles which transform in the adjoint representation of the gauge group and interact through non-Abelian electric fields is formulated and solved in the large $N$ limit. The explicit solution exhibits a first order confinement-deconfinement phase transition with computable properties and describes two dimensional adjoint QCD in the limit where matter field masses are large.

hep-th↗
arXiv1996

Charge Screening and Confinement in Hot 3-D QED

We examine the possibility of a confinement-deconfinement phase transition at finite temperature in both parity invariant and topologically massive three-dimensional quantum electrodynamics. We review an argument showing that the Abelian version of the Polyakov loop operator is an order parameter for confinement, even in the presence of dynamical electrons. We show that, in the parity invariant case, where the tree-level Coulomb potential is logarithmic, there is a Berezinskii-Kosterlitz-Thouless transition at a critical temperature ($T_c=e^2/8π+{\cal O}(e^4/m)$, when the ratio of the electromagnetic coupling and the temperature to the electron mass is small). Above $T_c$ the electric charge is not confined and the system is in a Debye plasma phase, whereas below $T_c$ the electric charges are confined by a logarithmic Coulomb potential, qualitatively described by the tree-level interaction. When there is a topological mass, no matter how small, in a strict sense the theory is not confining at any temperature; the model exhibits a screening phase, analogous to that found in the Schwinger model and two-dimensional QCD with massless adjoint matter. However, if the topological mass is much smaller than the other dimensional parameters, there is a temperature for which the range of the Coulomb interaction changes from the inverse topological mass to the inverse electron mass. We speculate that this is a vestige of the BKT transition of the parity-invariant system, separating regions with screening and deconfining behavior.

hep-th↗
arXiv1995

Charge Screening in the Finite Temperature Schwinger Model

We compute the effective action and correlators of the Polyakov loop operator in the Schwinger model at finite temperature and discuss the realization of the discrete symmetries that occur there. We show that, due to nonlocal effects of massless fermions in two spacetime dimensions, the discrete symmetry which governs the screening of charges is spontaneously broken even in an effective one-dimensional model, when the volume is infinite. In this limit, the thermal state of the Schwinger model screens an arbitrary external charge; consequently the model is in the deconfined phase, with the charge of the deconfined fermions completely screened. In a finite volume we show that the Schwinger model is always confining.

hep-th↗
arXiv1994

Equivariance, BRST and Superspace

The structure of equivariant cohomology in non-abelian localization formulas and topological field theories is discussed. Equivariance is formulated in terms of a nilpotent BRST symmetry, and another nilpotent operator which restricts the BRST cohomology onto the equivariant, or basic sector. A superfield formulation is presented and connections to reducible (BFV) quantization of topological Yang-Mills theory are discussed.

hep-th↗