Spin impurities coupled to quantum critical systems

K. Chen, Y. Komijani, Even-odd effect of an spin-S impurity coupled to a quantum critical system, Phys. Rev. B 100, 094514 (2019).

• A potential trap in a superfluid--Mott insulator transition of Bose-Hubbard model can be described by a spin-1/2 XY-coupled to the Wilson-Fisher fixed point of a phi^4 theory. This the XY Bose-Kondo universality class and the spin will be partially screened at the infrared fixed point.
• Here, we were motivated by interplay between two/few of such potential traps, which can be described by two-impurity Bose-Kondo model. It turns out that at low-energies such a system can be described by a spin-S coupled to the critical system.
• For the case of S=1, we used renormalization group with dimensional and cut=-off regularization in combination with quantum Monte carlo to study various critical exponents which showed good agreements.
• Moreover, we found out that depending on whether 2S is even or odd the system can behave differently. Either full lifting of all 2S+1 degeneracies or a residual degeneracy leading to partial screening.