Temperature Dependence of the QCD Coupling

We present a one-loop calculation of a gauge invariant QCD beta function. Using both momentum and temperature renormalization group equations we investigate the running coupling in the magnetic sector as a function of temperature and momentum scale. At fixed momentum scale we find that, in contrast to $\lambda\phi^4$ or QED, high-temperature QCD is strongly coupled, even after renormalization group improvement. However, if the momentum scale is changed simultaneously with temperature in a specified manner, the coupling decreases. We also point out in what regime dimensional reduction occurs. Both the cases $N_f$ smaller and larger than $\frac{11}{2} N_c$ are discussed.Comment: 10 pages, LaTeX (5 postscript figures available), ITFA-93-11,THU-93/0

Critical Temperature and Amplitude Ratios from a Finite-Temperature Renormalization Group

We study \l\f^4 theory using an environmentally friendly finite-temperature renormalization group. We derive flow equations, using a fiducial temperature as flow parameter, develop them perturbatively in an expansion free from ultraviolet and infrared divergences, then integrate them numerically from zero to temperatures above the critical temperature. The critical temperature, at which the mass vanishes, is obtained by integrating the flow equations and is determined as a function of the zero-temperature mass and coupling. We calculate the field expectation value and minimum of the effective potential as functions of temperature and derive some universal amplitude ratios which connect the broken and symmetric phases of the theory. The latter are found to be in good agreement with those of the three-dimensional Ising model obtained from high- and low-temperature series expansions.Comment: 14 pages of LaTeX. Postscript figures available upon request form [email protected]

Elements of Epistemic Crypto Logic (Extended Abstract)

Representation of ignorance about large numbers | agent a does not know agent b's key | is not feasible in standard Kripke semantics. The paper introduces register models that allow for compact representation of such ignorance. This is used to design a sound and complete language for number guessing games. The probabilities generated by our semantics allow for and motivate Monte Carlo model checking for register models. We show that the approach can be extended to a real life setting, namely the analysis of cryptographic security protocols. We look at a well known security protocol for secret key distribution over an insecure network, and point out how this can be analyzed with our modied version of Kripke semantics

Towards Model Checking Cryptographic Protocols with Dynamic Epistemic Logic

We present a variant of Kripke models to model knowledge of large numbers, applicable to cryptographic protocols. Our Epistemic Crypto Logic is a variant of Dynamic Epistemic Logic to describe com- munication and computation in a multi-agent setting. It is interpreted on register models which eciently encode larger Kripke models. As an example we formalize the well-known Die-Hellman key exchange. The presented register models also motivate a Monte Carlo method for model checking which we compare against a standard algorithm, using the key exchange as a benchmark

The Finite-Temperature Renormalization Group Applied to λϕ⁴ Theory and QCD

In this paper we apply the finite-temperature renormalization group from the point of view of “environmentally friendly” renormalization. We study both λϕ⁴ theory and the magnetic sector of QCD. At one loop level the complete temperature range of λϕ⁴ is successfully described in terms of the parameters of the zero temperature theory. We show also how the critical temperature can be calculated in terms of the latter. For the magnetic sector of QCD, in distinction to λϕ⁴, a one-loop finite temperature renormalization group improvement is not sufficient to describe the high-temperature regime

From soft harmonic phonons to fast relaxational dynamics in CH3_{3}NH3_{3}PbBr3_{3}

The lead-halide perovskites, including CH$_{3}$NH$_{3}$PbBr$_{3}$, are components in cost effective, highly efficient photovoltaics, where the interactions of the molecular cations with the inorganic framework are suggested to influence the electronic and ferroelectric properties. CH$_{3}$NH$_{3}$PbBr$_{3}$ undergoes a series of structural transitions associated with orientational order of the CH$_{3}$NH$_{3}$ (MA) molecular cation and tilting of the PbBr$_{3}$ host framework. We apply high-resolution neutron scattering to study the soft harmonic phonons associated with these transitions, and find a strong coupling between the PbBr$_{3}$ framework and the quasistatic CH$_{3}$NH$_{3}$ dynamics at low energy transfers. At higher energy transfers, we observe a PbBr$_{6}$ octahedra soft mode driving a transition at 150 K from bound molecular excitations at low temperatures to relatively fast relaxational excitations that extend up to $\sim$ 50-100 meV. We suggest that these temporally overdamped dynamics enables possible indirect band gap processes in these materials that are related to the enhanced photovoltaic properties.Comment: (main text - 5 pages, 4 figures; supplementary information - 3 pages, 3 figures