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.. highlight:: none | ||
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Dynamical Green’s function | ||
Dynamical Green's function | ||
-------------------------- | ||
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Using :math:`{\mathcal H}\Phi`, we can calculate a dynamical Green’s | ||
function | ||
Using :math:`{\mathcal H}\Phi`, we can calculate a dynamical Green's function | ||
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.. math:: I(z) = \langle \Phi ' | \frac{1}{ {\mathcal H}- z\hat{I} } | \Phi '\rangle, | ||
.. math:: G_n^{O_l,O_r}(z) = \langle \Phi_n | \hat{O}_l (z + E_n - \hat{\cal H})^{-1} \hat{O}_r| \Phi_n \rangle | ||
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where :math:`|\Phi ' \rangle = \hat{O} | \Phi _0 \rangle` is an | ||
excited state and :math:`\hat{O}` is an excitation operator defined as a | ||
single excitation operator | ||
where :math:`\hat{O}_{l,r}` is a single exciation operator | ||
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.. math:: \sum_{i, \sigma_1} A_{i \sigma_1} c_{i \sigma_1} (c_{i\sigma_1}^{\dagger}) | ||
.. math:: \sum_{i, \sigma_1} A_{i \sigma_1} c_{i \sigma_1} \quad \textrm{or} \quad \sum_{i, \sigma_1} A_{i \sigma_1} c_{i\sigma_1}^{\dagger} | ||
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or a pair excitation operator | ||
or a pair-exciation operator | ||
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.. math:: \sum_{i, j, \sigma_1, \sigma_2} A_{i \sigma_1 j \sigma_2} c_{i \sigma_1}c_{j \sigma_2}^{\dagger} (c_{i\sigma_1}^{\dagger}c_{j\sigma_2}). | ||
.. math:: \sum_{i, j, \sigma_1, \sigma_2} A_{i \sigma_1 j \sigma_2} c_{i \sigma_1}c_{j \sigma_2}^{\dagger} \quad \textrm{or} \quad | ||
\sum_{i, j, \sigma_1, \sigma_2} A_{i \sigma_1 j \sigma_2} c_{i\sigma_1}^{\dagger}c_{j\sigma_2}. | ||
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For example, the dynamical spin susceptibilities can be calculated by | ||
defining :math:`\hat{O}` as | ||
For example, to compute the dynamical spin susceptibility, we use pair excitation operators | ||
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.. math:: \hat{O} = \hat{S}({\bf k}) = \sum_{j}\hat{S}_j^z e^{i {\bf k} \cdot \bf {r}_j} = \sum_{j}\frac{1}{2} (c_{j\uparrow}^{\dagger}c_{j\uparrow}-c_{j\downarrow}^{\dagger}c_{j\downarrow})e^{i {\bf k} \cdot \bf {r}_j}. | ||
.. math:: \hat{O}_r = \hat{S}_{\textbf{R}=\textbf{0}}^z = \frac{1}{2} (c_{\textbf{0}\uparrow}^{\dagger}c_{\textbf{0}\uparrow}-c_{\textbf{0}\downarrow}^{\dagger}c_{\textbf{0}\downarrow}) | ||
\\ | ||
\hat{O}_l = \hat{S}_{\textbf{R}}^z = \frac{1}{2} (c_{\textbf{R}\uparrow}^{\dagger}c_{\textbf{R}\uparrow}-c_{\textbf{R}\downarrow}^{\dagger}c_{\textbf{R}\downarrow}) | ||
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There are two modes implemented in :math:`{\cal H}\Phi`. One is the | ||
continued fraction expansion method by using Lanczos method | ||
[#]_ and the other is the shifted Krylov | ||
method [#]_ . See the reference | ||
for the details of each algorithm. | ||
to generate :math:`G_n^{O_l,O_r}(z)\equiv G_n^{\textbf{R}}(z)`, | ||
then perform the Fourier transformation | ||
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.. [#] \E. Dagotto, Rev. Mod. Phys. **66**, 763-840 (1994). | ||
.. [#] \S.Yamamoto, T. Sogabe, T. Hoshi, S.-L. Zhang, T. Fujiwara, Journal of the Physical Society of Japan **77**, 114713 (2008). | ||
.. math:: G_n^{\textbf{k}}(z) \equiv \sum_{\textbf{R}} \exp(i\textbf{k}\cdot\textbf{R}) G_n^{\textbf{R}}(z) | ||
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as a postprocess. | ||
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Three modes are implemented in :math:`{\cal H}\Phi`: | ||
The continued fraction expansion method by using Lanczos method [1]_, | ||
the shifted Krylov method [2]_, and | ||
the Lehmann representation with the full diagonallization | ||
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.. math:: G_n^{O_l,O_r}(z) = \sum_{m} \frac{\langle \Phi_n | \hat{O}_l | \Phi_m \rangle \langle \Phi_m |\hat{O}_r| \Phi_n \rangle}{z + E_n - E_m}. | ||
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See the reference for the details of each algorithm. | ||
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.. [1] \E. Dagotto, Rev. Mod. Phys. **66**, 763-840 (1994). | ||
.. [2] \S.Yamamoto, T. Sogabe, T. Hoshi, S.-L. Zhang, T. Fujiwara, Journal of the Physical Society of Japan **77**, 114713 (2008). |
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.. highlight:: none | ||
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.. _Sec:sec_partion_function: | ||
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Partition function and quantities at finite-temperature | ||
------------------------------------------------------- | ||
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To avoid overflow/underflow, we compute as follows: | ||
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Partition function | ||
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.. math:: | ||
Z(T) &= \sum_{i=1}^N \exp\left(-\frac{E_i}{T}\right) | ||
\nonumber \\ | ||
&= \exp\left(-\frac{E_1}{T}\right) \left[ | ||
1 + \exp\left(-\frac{E_2-E_1}{T}\right)+ \exp\left(-\frac{E_3-E_1}{T}\right) | ||
\cdots | ||
+ \exp\left(-\frac{E_N-E_1}{T}\right) | ||
\right] | ||
\nonumber \\ | ||
&= \exp\left(-\frac{E_1}{T}\right) \left[ | ||
1 + \exp\left(-\frac{E_2-E_1}{T}\right)\left[ | ||
1 + \exp\left(-\frac{E_3-E_2}{T}\right)\left[ | ||
1 + \dots | ||
\left[ | ||
1 + \exp\left(-\frac{E_N-E_{N-1}}{T}\right) | ||
\right] | ||
\right] | ||
\right] | ||
\right] | ||
Quantity at finite tempearture | ||
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.. math:: | ||
O(T) &= \frac{1}{Z(T)}\sum_i O_i \exp\left(-\frac{E_i}{T}\right) | ||
\nonumber \\ | ||
&= \exp\left(-\frac{E_1}{T}\right) \left[ | ||
O_1 + O_2 \exp\left(-\frac{E_2-E_1}{T}\right) + O_3\exp\left(-\frac{E_3-E_1}{T}\right) | ||
\cdots | ||
+ O_N\exp\left(-\frac{E_N-E_1}{T}\right) | ||
\right] | ||
\nonumber \\ | ||
&= \exp\left(-\frac{E_1}{T}\right) \left[ | ||
O_1 + \exp\left(-\frac{E_2-E_1}{T}\right)\left[ | ||
O_2 + \exp\left(-\frac{E_3-E_2}{T}\right)\left[ | ||
O_3 + \dots | ||
\left[ | ||
O_{N-1} + O_N\exp\left(-\frac{E_N-E_{N-1}}{T}\right) | ||
\right] | ||
\right] | ||
\right] | ||
\right] | ||
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DynamicalGreen_en | ||
Realtime_en | ||
Bogoliubov_en | ||
Partition_en | ||
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@@ -232,7 +232,7 @@ CG method | |
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TPQ (mTPQ/cTPQ) method | ||
~~~~~~~~~~ | ||
~~~~~~~~~~~~~~~~~~~~~~ | ||
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* ``Lanczos_max`` | ||
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