Quantum criticality and BEC scenario in quasi 1-D Antiferromagnets – signatures in the spin dynamics

Low dimensional quantum antiferromagnetsare excellent test-beds to study field induced exotic phases, e.g. Bose-Einstein condensation (BEC) [1], magnetization plateaux [2], and the related Quantum Critical phenomenon.They are governed by the low energy spin dynamics and the physics of ‘hard-core’ bosons. In this talk I will mainly focus on the complex but universal physics near the vicinity of Quantum Critical Points.

In the 1-D regime, Iwill present the first clear experimental demonstration of the quantum-critical behaviour of the spin dynamics in magnetic insulators - insensitive to the microscopic properties of the system and scale invariant –as revealed in the nuclear spin-lattice relaxation rate T1-1 in anisotropic spin-chain system NiCl2-4SC(NH2)2(DTN) and the spin-ladder system (C5H12N)2CuBr4 [3].

In the 3-D regime (milli Kelvin temperature range), we have confirmed microscopically, for the first time,the predicted powerlaw behavior of the phase transition to the universal class of 3-D BEC type transition in DTN. We have also established the universal nature of spin fluctuation near the 3-D quantum critical point in DTN. For both the 1-D and 3-D regime we have extracted the experimental scaling function, to serve as a reference for future theoretical descriptions.I will also present the experimental results obtained inside the 3-D ordered phasein DTN, where a BEC of magnons is predicted to occur.  

[1] T. Giamarchi, C. Rüegg, and O. Tchernyshyov, Nature Phys. 4, 198 (2008).
[2] K. Kodamaet al., Science 298, 395 (2002).
[3] S. Mukhopadhyay et al., Phys. Rev. Lett. 109, 177206 (2012).