Title: Dynamics of dimer and z spin component fluctuations in spin-1/2 XY chain
Author(s):

	O.Derzhko (Institute for Condensed Matter Physics of the National Academy of Sciences of Ukraine, 1 Svientsitskii Str., 79011 Lviv, Ukraine) ,
	T.Krokhmalskii (Institute for Condensed Matter Physics of the National Academy of Sciences of Ukraine, 1 Svientsitskii Str., 79011 Lviv, Ukraine) ,
	P.Hlushak (Institute for Condensed Matter Physics of the National Academy of Sciences of Ukraine, 1 Svientsitskii Str., 79011 Lviv, Ukraine) ,

One-dimensional quantum spin-1/2 XY models admit the rigorous analysis not only of their static properties (i.e. the thermodynamic quantities and the equal-time spin correlation functions) but also of their dynamic properties (i.e. the different-time spin correlation functions, the dynamic susceptibilities, the dynamic structure factors). This becomes possible after exploiting the Jordan-Wigner transformation which reduces the spin model to a model of spinless noninteracting fermions. A number of dynamic quantities (e.g. related to transverse spin operator or dimer operator fluctuations) are entirely determined by two-fermion excitations and can be examined in much detail. We consider the spin-1/2 XY chain in a transverse (|| z) magnetic field with the Hamiltonian

and calculate the dynamic structure factors

for the local spin operators {A_m,B_m}= {s^z_m, D_m} where D_m=s^x_m s^x_m+1+ s^y_m s^y_m+1 is the dimer operator. The results for the dynamic transverse structure factor S_zz(κ,ω) and for the dynamic dimer structure factor S_DD(κ,ω) are known, whereas the analysis of the dynamic structure factor S_zD(κ,ω)=(S_Dz(κ,ω))^* has not been reported so far. We compare different two-fermion dynamic quantities contrasting their generic and specific features.

Key words: quantum spin chains, dynamic structure factors
PACS: 75.10.-b