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nkavokine committed Jun 6, 2024
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Expand Up @@ -47,25 +47,23 @@ The Monte Carlo algorithms for quantum impurity problems are
based on stochastically exploring the terms in the perturbative expansion of the solution
around an exactly solvable limit. Hybridization expansion algorithms -- chief of which the
continuous-time `CTHYB` -- involve expanding around the limit of an isolated atom [@gull2011].
Three extensive libraries for the numerical treatment of quantum many-body problems are
currently available: `ALPS` [@ALPS2018], `w2dynamics` [@w2dynamics2019] and `TRIQS` [@CTHYB2016],
and each has its own implementation of `CTHYB`.
Currently, there exist implementations of `CTHYB` within three different libraries: `ALPS` [@ALPS2018], `w2dynamics` [@w2dynamics2019] and `TRIQS` [@CTHYB2016].

However, a simpler and potentially faster version of the `CTHYB` algorithm,
called `CTSEG`, can be used under the restriction of (possibly time-dependent) density-density
interactions on the impurity. `CTSEG` can be further generalized to allow for time-dependent
spin-spin interactions [@otsuki2013]. To our knowledge, no implementation of `CTSEG` has been published so far.
Our `CTSEG` solver is about twice as fast as `TRIQS-CTHYB` for a single orbital problem, and has
better scaling with the number of orbitals (400 times faster in our 5 orbital test case, see Fig. 1a).
better scaling with the number of orbitals (40 times faster in our 5 orbital test case, see Fig. 1a).
`CTSEG` has already allowed us to obtain the first numerically-exact solution of the
quantum Heisenberg spin glass [@kavokine2024].

![**a**. Running time comparison between the TRIQS implementations of CTSEG and CTHYB. The test system is a multi-orbital impurity at half-filling and inverse temperature $\beta = 20$. The Coulomb repulsion is $U = 2$ for two electrons on the same orbital and $U' = 1$ for two electrons on different orbitals. The hybridization is diagonal and identical for all orbitals: $\Delta(\omega) = 1/(\omega - 0.3)$. **b**. Spin-spin correlation function $\chi(\tau) = \langle \mathbf{S}(\tau) \cdot \mathbf{S}(0) \rangle$ of the $t-J-U$ model studied by Dumitrescu et al., obtained using CTSEG at inverse temperature $\beta = 300$ and different values of doping $p$. At long times $\chi(\tau) \sim 1/\tau^{\theta}$, with $\theta = 1$ at the QCP. Inset: exponent $\theta$ as a function of doping $p$. The QCP is located at $p \approx 0.16$.](figure_JOSS.pdf)
![**a**. Running time comparison between the TRIQS implementations of CTSEG and CTHYB. The test system is a multi-orbital impurity at half-filling and inverse temperature $\beta = 20$. The Coulomb repulsion is $U = 2$ for two electrons on the same orbital and $U' = 1$ for two electrons on different orbitals. The hybridization is diagonal and identical for all orbitals: $\Delta(\omega) = 1/(\omega - 0.3)$. **b**. Spin-spin correlation function $\chi(\tau) = \langle \mathbf{S}(\tau) \cdot \mathbf{S}(0) \rangle$ of the $t-J-U$ model studied by Dumitrescu et al., obtained using CTSEG at inverse temperature $\beta = 300$ and different values of doping $p$. At long times $\chi(\tau) \sim 1/\tau^{\theta}$, with $\theta = 1$ at the QCP. Inset: exponent $\theta$ as a function of doping $p$. The QCP is located at $p \approx 0.16$.](figure_JOSS.pdf){width=80%}

# Example of use

As a further illustration of our solver's performance, we apply it to the fully connected $t-J-U$ model
studied by [@dumitrescu2022]. At half-filling, the model forms a spin glass phase, which melts into
studied by @dumitrescu2022. At half-filling, the model forms a spin glass phase, which melts into
a metal at a doping-induced quantum critical point (QCP). Dumitrescu et al.
obtained solutions at inverse temperatures up to $\beta = 65$, limited by the fermionic sign problem
of their interaction expansion solver. The hybridization expansion carried out by `CTSEG` is
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