U(1)-Symmetry breaking and violation of axial symmetry in TlCuCl3 and other insulating spin systems

Abstract

We describe the Bose-Einstein condensate of magnetic bosonic quasiparticles in insulating spin systems using a phenomenological standard functional method for T = 0. We show that results that are already known from advanced computational techniques immediately follow. The inclusion of a perturbative anisotropy term that violates the axial symmetry allows us to remarkably well explain a number of experimental features of the dimerized spin-1/2 system TlCuCl3. Based on an energetic argument we predict a general intrinsic instability of an axially symmetric magnetic condensate towards a violation of this symmetry, which leads to the spontaneous formation of an anisotropy gap in the energy spectrum above the critical field. We, therefore, expect that a true Goldstone mode in insulating spin systems, i.e., a strictly linear energy-dispersion relation down to arbitrarily small excitations energies, cannot be observed in any real material.

arXiv comment6 pages, 3 figures

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• Parametric pumping and kinetics of magnons in dipolar ferromagnets
• High-field instability of field-induced triplon Bose-Einstein condensate
• Critical exponents and intrinsic broadening of the field-induced transition in NiCl${}_2$$\cdot$4SC(NH${}_2$)${}_2$
• Thermal conductivity of IPA-CuCl_3: Evidences of ballistic magnon transport and limited applicability of the Bose-Einstein condensation model
• Bose - Einstein condensation of triplons with a weakly broken U(1) symmetry
• On Josephson effects in insulating spin systems
• Pre-saturation phase in the frustrated ferro-antiferromagnet Pb${}_2$VO(PO${}_4$)${}_2$
• Gr{ü}neisen parameter of quantum magnets with spin gap
• Spin-gapped magnets with weak anisotropies I: Constraints on the phase of the condensate wave function
• The effects of disorder in dimerized quantum magnets in mean field approximations
• Low-temperature heat transport of spin-gapped quantum magnets
• Giant dielectric nonlinearities at a magnetic Bose-Einstein condensation
• Experimental Verification of the Universality of Magnetic-Field-Induced Bose-Einstein Condensation of Magnons
• Pressure-tuned quantum criticality in the large-$D$ antiferromagnet DTN
• Characteristic temperatures of a triplon system of dimerized quantum magnets
• Field-induced ordering in a random-bond quantum spin ladder compound with weak anisotropy
• Narrowly avoided spin-nematic phase in BaCdVO(PO${}_4$)${}_2$: NMR evidence
• Experimental tests for macroscopic phase coherence in magnetic-quasiparticle condensates of insulating spin systems