English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Tools annual

Global: Zendy Free Plan

Global: Zendy Tools monthly

Global: Zendy Plus monthly

Three well-known perturbative approaches to deriving low-energy effectivetheories, the degenerate Brillouin-Wigner perturbation theory (projectionmethod), the canonical transformation, and the resolvent methods are compared.We use the Hubbard model as an example to show how, to fourth order in hoppingt, all methods lead to the same effective theory, namely the t-J model withring exchange and various correlated hoppings. We emphasize subtle technicaldifficulties that make such a derivation less trivial to carry out for ordershigher than second. We also show that in higher orders, different approachescan lead to seemingly different forms for the low-energy Hamiltonian. All ofthese forms are equivalent since they are connected by an additional unitarytransformation whose generator is given explicitly. The importance oftransforming the operators is emphasized and the equivalence of theirtransformed structure within the different approaches is also demonstrated.Comment: 14 pages, no figure

Higher order corrections to effective low-energy theories for strongly correlated electron systems