Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Variational Monte Carlo Study on the Superconductivity in the
  Two-Dimensional Hubbard Model   The possibility of superconductivity (SC) in the 2D Hubbard model (2DH) was
investigated by means of the variational Monte Carlo method. The energy gain of
the d-wave SC state, obtained as the difference of the minimum energy with a
finite gap and that with zero gap, was examined with respect to dependences on
U, electron density rho and next nearest neighbor transfer t' mainly on the 10
x 10 lattice. It was found to be maximized around U = 8 (in energy unit of t).
It sharply increased for negative values of t' and had a broad peak for t' ~
-0.10. For these values of t' the energy gain was a smooth increasing function
of rho almost independent of the shell structure in the region starting from ~
0.76 up to 0.92. This clearly indicates that the result is already close to the
value in the bulk limit. For t' = 0, the energy gain depended on the electronic
shell state. Competition between the SC and the SDW states was also
investigated. When t' = 0, the ground state is SDW in the range of rho \geq ~
0.84$. The SC region slightly extends up to ~ 0.87 for t' ~ -0.10. Consequently
the present results strongly indicate that the 2DH with t' ~ -0.1 drives SC by
itself in the rho region from ~ 0.76 to ~ 0.87. The energy gain in the SC state
with suitable parameters is found to be in reasonable agreement with the
condensation energy in the SC state of YBa2Cu3O7. The corresponding t-J model
proves to give an order-of-magnitude larger energy gain, which questions its
validity.

37
The categories of this paper are: supr-con
6
The categories of this paper are: astro-ph
2
The categories of this paper are: math
33
The categories of this paper are: plasm-ph
11
The categories of this paper are: hep-lat
26
The categories of this paper are: bayes-an
25
The categories of this paper are: patt-sol
0
The categories of this paper are: hep-ph
19
The categories of this paper are: eess
14
The categories of this paper are: math-ph
17
The categories of this paper are: q-fin
9
The categories of this paper are: nlin
28
The categories of this paper are: alg-geom
15
The categories of this paper are: nucl-ex
13
The categories of this paper are: quant-ph
3
The categories of this paper are: physics