Screening properties of the quark gluon plasma are studied from Polyakov-loop correlation in lattice QCD simulations with two flavors of improved Wilson quarks at temperatures $T/Tpc simeq 1$--4 where $Tpc$ is the pseudocritical temperature. Using the Euclidean-time reflection symmetry and the charge conjugation symmetry, we introduce various types of Polyakov-loop correlation functions and extract screening masses in magnetic and electric sectors. We find that the temperature dependence of the screening masses are well described by the weak coupling expansion. We also find that a ratio of the screening masses in the electric sector to the magnetic sector shows qualitative agreement with a prediction from the dimensionally-reduced effective field theory and the N=4 supersymmetric Yang-Mills theory at $1.3 < T/Tpc < 3$.
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