Eur. Phys. J. B 61, 89-93 (2008)
https://doi.org/10.1140/epjb/e2008-00033-1

Quantum fluctuations in a scale-free network-connected Ising system

Department of Physics, Soongsil University, Seoul, 156-743, Korea

Corresponding author: ^a hyi@ssu.ac.kr

Received: 13 September 2007
Revised: 16 November 2007
Published online: 23 January 2008

Abstract

We study the effect of quantum fluctuations in an Ising spin system on a scale-free network of degree exponent γ>5 using a quantum Monte Carlo simulation technique. In our model, one can adjust the magnitude of the magnetic field perpendicular to the Ising spin direction and can therefore control the strength of quantum fluctuations for each spin. Our numerical analysis shows that quantum fluctuations reduce the transition temperature T_c of the ferromagnetic-paramagnetic phase transition. However, the phase transition belongs to the same mean-field type universality class both with and without the quantum fluctuations. We also study the role of hubs by turning on the quantum fluctuations exclusively at the nodes with the most links. When only a small number of hub spins fluctuate quantum mechanically, T_c decreases with increasing magnetic field until it saturates at high fields. This effect becomes stronger as the number of hub spins increases. In contrast, quantum fluctuations at the same number of “non-hub” spins do not affect T_c. This implies that the hubs play an important role in maintaining order in the whole network.