Emptiness formation probability and Painlevé V equation in the XY spin chain

Abstract

We reconsider the problem of finding $L$ consecutive down spins in the ground state of the XY chain, a quantity known as the Emptiness Formation Probability. Motivated by new developments in the asymptotics of Toeplitz determinants, we show how the crossover between the critical and off-critical behaviour of the Emptiness Formation Probability is exactly described by a $\tau$ function of a Painlevé V equation. Following a recent proposal, we also provide a power series expansion for the $\tau$ function in terms of irregular conformal blocks of a Conformal Field Theory with central charge $c=1$. Our results are tested against lattice numerical calculations, showing excellent agreement. We finally rediscuss the free fermion case where the Emptiness Formation Probability is characterized by a Gaussian decay for large $L$.

25 pages, 9 figures. References and a footnote added