Theoretical studies on intervalley splittings in Si/SiO2 quantum dot structures |
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Authors: | S. H. Park D. Ahn |
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Affiliation: | 1.Department of Electronics Engineering,Catholic University of Daegu,Kyeongbuk,Republic of Korea;2.Institute of Quantum Information Processing and Systems,University of Seoul,Seoul,Republic of Korea |
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Abstract: | Using the coupled cluster method we investigatespin-s J 1-J′ 2 Heisenberg antiferromagnets (HAFs) on an infinite, anisotropic, two-dimensional triangular lattice for the two cases where the spin quantum number s = 1 and s = $frac{3}
{2}$frac{3}
{2}. With respect to an underlying square-lattice geometry the model has antiferromagnetic (J 1 > 0) bonds between nearest neighbours and competing (J′ 2 > 0) bonds between next-nearest neighbours across only one of the diagonals of each square plaquette, the same diagonal in each square. In a topologically equivalent triangular-lattice geometry, the model has two types of nearest-neighbour bonds: namely the J′ 2 ≡ κJ 1 bonds along parallel chains and the J 1 bonds producing an interchain coupling. The model thus interpolates between an isotropic HAF on the square lattice at one limit (κ = 0) and a set of decoupled chains at the other limit (κ → ∞), with the isotropic HAF on the triangular lattice in between at κ = 1. For both the spin-1 model and the spin-$frac{3}
{2}$frac{3}
{2} model we find a second-order type of quantum phase transition at κ c = 0.615 ± 0.010 and κ c = 0.575 ± 0.005 respectively, between a Néel antiferromagnetic state and a helically ordered state. In both cases the ground-state energy E and its first derivative dE/dκ are continuous at κ = κ c , while the order parameter for the transition (viz., the average ground-state on-site magnetization) does not go to zero there on either side of the transition. The phase transition at κ = κ c between the Néel antiferromagnetic phase and the helical phase for both the s = 1 and s = $frac{3}
{2}$frac{3}
{2} cases is analogous to that also observed in our previous work for the s = $frac{1}
{2}$frac{1}
{2} case at a value κ c = 0.80 ± 0.01. However, for the higher spin values the transition appears to be of continuous (second-order) type, exactly as in the classical case, whereas for the s = $frac{1}
{2}$frac{1}
{2} case it appears to be weakly first-order in nature (although a second-order transition could not be ruled out entirely). |
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