4.6 Article

Dynamical exponent of a quantum critical itinerant ferromagnet: A Monte Carlo study

期刊

PHYSICAL REVIEW B
卷 105, 期 4, 页码 -

出版社

AMER PHYSICAL SOC
DOI: 10.1103/PhysRevB.105.L041111

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资金

  1. RGC of Hong Kong SAR of China [17303019, 17301420, AoE/P-701/20]
  2. Strategic Priority Research Program of the Chinese Academy of Sciences [XDB33000000]
  3. NSF [PHY-1748958, DMR-2045871]
  4. Office of Basic Energy Sciences, U.S. Department of Energy [DE-SC0014402]

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In this study, we investigate the effect of the coupling between two-dimensional quantum rotors and itinerant fermions near an XY ferromagnetic quantum critical point. Through large-scale quantum Monte Carlo simulations, we find that the behavior in the XY case is different from previous understanding, with a dynamical critical exponent of 2 and a fermionic self-energy proportional to the square root of frequency. This new behavior is attributed to the fact that a fermionic spin is not a conserved quantity due to spin-spin coupling to rotors. These results have significant implications for experiments.
We consider the effect of the coupling between two-dimensional (2D) quantum rotors near an XY ferromagnetic quantum critical point and spins of itinerant fermions. We analyze how this coupling affects the dynamics of rotors and the self-energy of fermions. A common belief is that near a q = 0 ferromagnetic transition, fermions induce an Omega/q Landau damping of rotors (i.e., the dynamical critical exponent is z = 3) and Landau overdamped rotors give rise to non-Fermi liquid fermionic self-energy Sigma proportional to omega(2/3). This behavior has been confirmed in previous quantum Monte Carlo (QMC) studies. Here we show that for the XY case the behavior is different. We report the results of large-scale quantum Monte Carlo simulations, which show that at small frequencies z = 2 and Sigma proportional to omega(1/2). We argue that the new behavior is associated with the fact that a fermionic spin is by itself not a conserved quantity due to spin-spin coupling to rotors, and a combination of self-energy and vertex corrections replaces 1/q in the Landau damping by a constant. We discuss the implication of these results to experiments.

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