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We investigate the amplitude (Higgs) mode associated with longitudinalfluctuations of the order parameter at the continuous spontaneous symmetrybreaking phase transition. In quantum magnets, due to the fast decay of theamplitude mode into low-energy Goldstone excitations, direct observation ofthis mode represents a challenging task. By focusing on a quasi-one-dimensionalgeometry, we circumvent the difficulty and investigate the amplitude mode in asystem of weakly coupled spin chains with the help of quantum Monte Carlosimulations, stochastic analytic continuation, and a chain-mean field approachcombined with a mapping to the field-theoretic sine-Gordon model. The amplitudemode is observed to emerge in the longitudinal spin susceptibility in thepresence of a weak symmetry-breaking staggered field. A conventional measure ofthe amplitude mode in higher dimensions, the singlet bond mode, is found toappear at a lower than the amplitude mode frequency. We identify these twoexcitations with the second (first) breather of the sine-Gordon theory,correspondingly. In contrast to higher-dimensional systems, the amplitude andbond order fluctuations are found to carry significant spectral weight in thequasi-1D limit.

Amplitude Mode in Quantum Magnets via Dimensional Crossover