The phenomenological renormalisation group is applied to the 1D transverse Ising model with longitudinal imaginary field in order to investigate the Yang-Lee edge singularity (1952) of the equivalent 2D Ising model. The method is extended to calculate directly the exponent sigma of the magnetisation.
A T=0 real-space renormalization-group method is presented and applied to the quantum 1D transverse field Ising model in the presence of a longitudinal field. This is an indirect way to study the equation of state of the equivalent 2D classical Ising model at finite temperature. When the longitudinal field is purely imaginary we obtain a new critical behavior which is related to the Yang-Lee edge singularity of the corresponding classical model. We extract the edge exponent σ in agreement with previous series expansion estimates.
The quantum one-dimensional transverse Ising model with an imaginary longitudinal field is studied at T=0 using a real-space renormalization-group method. Evaluation is made of the real part of the ground-state longitudinal magnetization which is related to the Yang-Lee distribution of zeros of the partition function for the two-dimensional classical Ising model. The edge exponents are found as $\ensuremath{\sigma}=\ensuremath{-}0.25\ifmmode\pm\else\textpm\fi{}0.05$ and $\ensuremath{\eta}=\ensuremath{-}1.0\ifmmode\pm\else\textpm\fi{}0.02$, in agreement with series expansion estimates.
A quantum renormalization-group technique is applied to the one-dimensional transverse field Ising model in a complex longitudinal field. The Yang-Lee edge singularity is analysed in this model in view of its analogy with the 2d classical Ising model. Previous two-level calculations are improved by retaining four levels in the blocking procedure. A more precise edge exponent σ is found in agreement with the infinite block extrapolation of the two-level calculation, but still larger in absolute value than the latest series analysis results. The occurrence of oscillations in the real part of the magnetization appears to be an artifact of the real-space method, which introduces finite size effects.
The ground state of the quantum one-dimensional transverse-field Ising model in a longitudinal field is studied using a real-space renormalization-group method. The longitudinal magnetization in the ground state is calculated. Considering a real longitudinal field, this study provides an indirect investigation of the equation of state of the equivalent classical two-dimensional Ising model. Considering then the case of a purely imaginary longitudinal field, a new critical behavior is found which corresponds to the Yang-Lee edge singularity of the classical equivalent. In this last case the details of calculations and results are given, published earlier in a letter form.
The random Ising model in a transverse field in one dimension with Hamiltonian H=- Sigma ( Gamma i S i z +J i S i x S i+1 x ) is studied at T=O from a real-space renormalisation group block method which preserves duality transformations. The ground state magnetisation and the ground state energy are determined for random distributions P(J)(P( Gamma ))=N J J NJ-1 /J 0 N(J) for 0<J<J 0 and P(J)=0 for J>J O . A new critical behaviour corresponding to a new fixed point takes place in the presence of disorder. The crossover exponent describing the departure from the pure system behaviour is calculated. The second derivative of the ground state energy delta 2 E/ delta Gamma 2 which diverges logarithmically for the pure system is rounded in the presence of disorder but a sharp transition field still exists where the magnetisation goes to zero with an exponent beta about twice as large as the pure system exponent. Comparison is made with the analytical results of McCoy and Wu (1969) for the classical equivalent random-striped Ising 2D model.