[1] Matthews M R, Anderson B P, Haljan P C, Hall D S, Wieman C
E and Cornell E A 1999 Phys. Rev. Lett. 83 2498

[2] Williams J E and Holland M J 1999 Nature 401
568

[3] Inouye S. et al 2001 Phys. Rev. Lett. 87
080402

[4] Scherer D R et al 2007 Phys. Rev. Lett. 98
110402

[5] Madison K W, Chevy F, Wohlleben W and Dalibard J 2000
Phys. Rev. Lett. 84 806

[6] Nakahara M, Isoshima T, Machida K, Ogawa S I and Ohmi T
2000 Physica B 284 17

[7] Salasnich L, Malomed B A and Toigo F 2008 Phys. Rev.
A 77 035601

[8] Schweikhard V, Coddington I, Engels P, Tung S and Cornell
E A 2004 Phys. Rev. Lett. 93 210403

[9] M\"{Ott\"{Oen M M, Pietil\"{a V and Virtanen S M M 2007
Phys. Rev. Lett. 99 250406

[10] Ginzburg V L and Pitaevskii L P 1958 Zh. Eksp. Teor.
Fiz. 34 1240

[11] Gross E P 1961 Nuovo Cimento 20 454 Pitaevskii L P 1961
Zh. Eksp. Teor. Fiz. 40 646

[12] Gross E P 1963 J. Math. Phys. 4 195

[13] Xu T 2006 Ann. of Phys. 321 2017

[14] Moffatt H K 1969 J. Fluid Mech. 35 117

[15] Duan Y S, Xu T and Fu L B 1999 Prog. Theor. Phys.
101 467

[16] Ji A C, Liu W M, Song J L and Zhou F 2008 Phys. Rev.
Lett. 101 010402

[17] Xu T 2005 Phys. Rev. E 72 036303

[18] Xu T, Su H L, Zhou G and Hu X W 2006 Mod. Phys.
Lett. A 21 1369

[19] Faddeev L D 1976 Lett. Math. Phys. 1 289

[20] Faddeev L D and Niemi A J 1997 Nature 387 58

Faddeev L D and Niemi A J 2001 Phil. Trans. A
359 1399

[21] Witten E 1989 Comm. Math. Phys. 121 351

Polyakov A M 1988 Mod. Phys. Lett. A 3 325

[22] Volovik G E and Mineev V P 1977 Zh. Eksp. Teor.
Fiz. 72 2256