We present symbolic algorithms designed to investigate the perturbative expansions of the d-Hubbard model. These methods are part of recent developments in many-body perturbation theory that aim to reformulate Feynman diagrams in terms of divided differences. The direct application of this technique to the one-dimensional Hubbard model yields the coefficients of the grand potential up to the sixth order, expressed in terms of both the interacting potential (U expansions) and high-temperature expansions (β expansions). A key feature of this approach is the ability to merge the β expansions to any desired order. To verify our analytical results, we compare the derived magnetic susceptibility with the exact solution of the quantum transfer matrix method in the half-filled case.
We present symbolic algorithms designed to investigate the perturbative expansions of the d-Hubbard model. These methods are part of recent developments in many-body perturbation theory that aim to reformulate Feynman diagrams in terms of divided differences. The direct application of this technique to the one-dimensional Hubbard model yields the coefficients of the grand potential up to the sixth order, expressed in terms of both the interacting potential (U expansions) and high-temperature expansions (β expansions). A key feature of this approach is the ability to merge the β expansions to any desired order. To verify our analytical results, we compare the derived magnetic susceptibility with the exact solution of the quantum transfer matrix method in the half-filled case.
Abstract We investigated the two-dimensional Hubbard model using many-body perturbation theory, focusing on the systematic computation of thermodynamic properties. The contributions of generalized Hugenholtz (Feynman) diagrams were evaluated symbolically and reformulated using divided differences, enabling efficient and precise calculations. We derived the coefficients of the grand potential expansion up to the eighth order in both the interaction potential (U-expansion) and the inverse temperature (β-expansion). Additionally, we presented results for magnetic susceptibility and heat capacity at half-filling, providing critical insights into the thermodynamic behavior of the model in this regime.
We investigate the grand potential of the one-dimensional Hubbard model in the high-temperature limit, calculating the coefficients of the high-temperature expansion $(\ensuremath{\beta}$-expansion) of this function up to order ${\ensuremath{\beta}}^{4}$ by an alternative method. The results derived are analytical and do not involve any perturbation expansion in the hopping constant, being valid for arbitrary density of electrons in the one-dimensional model. In the half-filled case, we compare our analytical results for the specific heat and the magnetic susceptibility, in the high-temperature limit, with the ones obtained by Beni et al. [Phys. Rev. B 8, 3329 (1973)] and Takahashi's integral equations, showing that the latter result does not take into account the complete energy spectrum of the one-dimensional Hubbard model. The exact integral solution by J\"uttner et al. [Nucl. Phys. B 522, 471 (1998)] is applied to the determination of the range of validity of our expansion in $\ensuremath{\beta}$ in the half-filled case, for several different values of U.
Abstract We present a new algorithm to evaluate the grand potential at high and finite temperatures using many-body perturbation theory. This algorithm enables us to calculate the contribution of any Hugenholtz or Feynman vacuum diagrams and formulate the results as a sum of divided differences. Additionally, the proposed method is applicable to any interaction in any dimension, allowing us to calculate thermodynamic quantities efficiently at any given temperature, particularly at high temperatures.Furthermore, we apply this algorithm to the Heisenberg spin-1/2 XXZ chain. We obtain all coefficients of the high-temperature expansion of the free energy and susceptibility per site of this model up to the sixth order.
We exploit the grassmannian nature of the variables involved in the path integral expression of the grand canonical partition function for self--interacting fermionic models to show, in one-space dimension, a general relation among the terms of it expansion in the high temperature limit and a combination of co-factors of a suitable matrix with commuting entries. As an application, we apply this framework to calculate the exact coefficients, up to order β^3, of the expansion of the grand canonical partition function for the Hubbard model in d=(1+1) in the high temperature limit. The results are valid for any set of parameters that characterize the model.
This thesis develops and applies numerical tools for studying strongly correlated models at finite temperature. Strongly correlated materials exhibit rich emergent behavior, including high temperature superconductivity, exotic magnetism, and metal–insulator transitions. To study these phenomena, Feynman diagrammatics based on perturbation theory provides the foundation for both perturbative and non-perturbative numerical techniques. The thesis focuses on the diagrammatic evaluation of singleand two-particle response functions, with the truncation order acting as a control parameter. Recent advances in symbolic computation have led to the Algorithmic Matsubara Integration (AMI) technique, a central computational technique utilized in this work. AMI generates a symbolic representation of arbitrary Feynman diagrams and resolves the nested Matsubara sums via the residue theorem, yielding closedform semi-analytical expressions, with the remaining momentum integrals evaluated stochastically. The first part of the thesis adopts a pedagogical approach, introducing the generic many-body Hamiltonian, motivating the Hubbard model as a prototype for correlated systems, and reviewing Green’s function formalism in many-body theory. In the first application, we employ AMI to evaluate the pairing susceptibility to measure superconducting fluctuations in the single-band Hubbard model. We identify two dominant 𝑞 = [0,0] and 𝑞 = [π,π] 𝑑-wave pairing modes and map the correlation length and amplitude across the phase space 𝑇–𝑡'– 𝜇, defined by temperature 𝑇, nearest-neighbor hopping 𝑡' and chemical potential 𝜇. Next, we present an algorithm that assigns band indices to arbitrary Feynman topologies symbolically and iteratively using the non-zero elements of interaction tensor Uabcd. Applied to all connected topologies, it generates the full set of contributing diagrams for multiorbital problems. Combined with AMI, this framework termed MB-AMI enables systematic diagrammatic expansion for any low-energy multi-band system with arbitrary longrange interactions. We benchmark the method on molecular H₂ and on the Hubbard ii model under a Zeeman field. Using MB-AMI, we study the multilayer Hubbard model and the layer-resolved features of the pairing susceptibility. We report a monotonic but sublinear increase in the pairing response with layer stacking, assisted by pair-hopping interactions, with a well defined three-dimensional limit. Finally, motivated by the spin-dependent renormalization of screened interactions 𝑊ᵤᵤ and 𝑊ud, we propose a phenomenological extended Hubbard model where nearest-neighbor interactions 𝑉 are spin-dependent and independently tunable. We explore all four quadrants of the 𝑉ᵤᵤ–𝑉ud phase space, focusing particularly on the ferromagnetic-like regime 𝑉ud > 𝑉ᵤᵤ. In this regime, we find that competing ferromagnetic and antiferromagnetic spin fluctuations give rise to an exotic spin-density wave (SDW). We study the resulting fermi surface reconstruction and the emergence of triplet 𝑝-wave pairing from the induced SDW. In conclusion, we propose that merging MB-AMI with the Discrete Lehmann Representation (DLR) into a unified framework opens a pathway to performing high order, fully self-consistent diagrammatic calculations for accurate ab initio simulations of realistic materials.