Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Ab initio determination of magnetic ground state of pyrochlore
  Y$_2$Mn$_2$O$_7$   There are two discrepant experimental results on the magnetic ground state of
Y$_{2}$Mn$_{2}$O$_{7}$, one study proposes a spin glass state, while another
introduces the material as a ferromagnet. In this study, we attempt to resolve
this issue by employing density functional theory and Monte Carlo simulations.
We derive different spin models by varying the Hubbard $U$ parameter in ab
initio GGA+$U$ calculations. For the most range of Hubbard $U$, We obtain that
the leading terms in the spin Hamiltonian are bi-quadratic and the nearest
neighbor Heisenberg exchange interactions. By comparing Monte Carlo simulations
of these models with the experiments, we find a ferromagnetic ground state for
Y$_{2}$Mn$_{2}$O$_{7}$ as the most compatible with experiments. We also
consider Y$_{2}$Mo$_{2}$O$_{7}$ as a prototype of the defect-free pyrochlore
system with spin-glass behavior and compare it with Y$_{2}$Mn$_{2}$O$_{7}$. The
orbital degrees of freedom are considered as a leading factor in converting a
defect-free pyrochlore such as Y$_{2}$Mn$_{2}$O$_{7}$ to a spin glass system.
By changing the $d$ orbital occupations of Mo atoms, our GGA+$U$ calculations
for Y$_{2}$Mo$_{2}$O$_{7}$ indicate many nearly degenerate states with
different $d$ orbital orientations which reveals $d$ orbital degrees of freedom
in this material. While for Y$_{2}$Mn$_{2}$O$_{7}$, we find a single ground
state with a fixed orbital orientation. Consequently, all of our ab initio
approaches confirm Y$_{2}$Mn$_{2}$O$_{7}$ as a ferromagnetic system.

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The categories of this paper are: eess
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The categories of this paper are: q-fin
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The categories of this paper are: hep-lat
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The categories of this paper are: atom-ph
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The categories of this paper are: chem-ph
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The categories of this paper are: patt-sol
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The categories of this paper are: nlin
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The categories of this paper are: quant-ph
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The categories of this paper are: q-alg
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The categories of this paper are: physics
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The categories of this paper are: q-bio
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The categories of this paper are: stat
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The categories of this paper are: supr-con
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The categories of this paper are: solv-int
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The categories of this paper are: hep-th
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The categories of this paper are: mtrl-th