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Complex critical exponents in quantum percolation transitions

Rafael M. Fernandes, Jörg Schmalian

Published Sep 14, 2010• 1 citation
OpenAlex

Abstract

We show that the critical behavior of quantum systems undergoing a percolation transition is dramatically affected by their topological Berry phase $2\pi\rho$. For irrational $\rho$, we demonstrate that the low-energy excitations of diluted Josephson-junctions arrays, quantum antiferromagnets, and interacting bosons are spinless fermions with fractal spectrum. As a result, critical properties not captured by the usual Ginzburg-Landau-Wilson description of phase transitions emerge, such as complex critical exponents, log-periodic oscillations and dynamically broken scale-invariance.

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We show that the critical behavior of quantum systems undergoing a percolation transition is dramatically affected by their topological Berry phase 2πρ2\pi\rho2πρ. For irrational ρ\rhoρ, we demonstrate that the low-energy excitations of diluted Josephson-junctions arrays, quantum antiferromagnets, and interacting bosons are spinless fermions with fractal spectrum. As a result, critical properties not captured by the usual Ginzburg-Landau-Wilson description of phase transitions emerge, such as complex critical exponents, log-periodic oscillations and dynamically broken scale-invariance.
We show that the critical behavior of the XY quantum-rotor model undergoing a percolation transition is dramatically affected by its topological Berry phase 2π ρ. In particular, for irrational ρ, its low-energy excitations emerge as spinless fermions with fractal spectrum. As a result, critical properties not captured by the usual Ginzburg-Landau-Wilson description of phase transitions arise, such as complex critical exponents, log-periodic oscillations and dynamically broken scale invariance.
The complex conductance $G(\ensuremath{\omega})$ of site-diluted Josephson junction arrays close to the percolation threshold was measured over a wide range of frequencies \ensuremath{\omega}. Well below Tc{T}_{c}Tc​ both the superfluid $[\ensuremath{\omega}\mathrm{Im}G]$ and dissipative [ReG][\mathrm{Re}G][ReG] components are independent of \ensuremath{\omega} below a critical frequency ${\ensuremath{\omega}}_{c}$, whereas $G(\ensuremath{\omega})\ensuremath{\propto}{\ensuremath{\omega}}^{\ensuremath{-}u}$ with $u\ensuremath{\approx}\frac{1}{2}$ for $\ensuremath{\omega}>{\ensuremath{\omega}}_{c}$. This is shown to reflect the crossover from a Euclidean regime $(\ensuremath{\omega}<{\ensuremath{\omega}}_{c})$ dominated by phononlike modes of the phase system to a fractal regime $(\ensuremath{\omega}>{\ensuremath{\omega}}_{c})$, where the relevant excitations are localized fractons. Percolative critical exponents extracted from the data are consistent with theoretical predictions.
Measurements of the IV characteristics of site-diluted Josephson-junction arrays have revealed intriguing effects of percolative disorder on the phase transition and the vortex dynamics in a two-dimensional XY system. Different from other types of phase transitions, the Kosterlitz-Thouless transition was eliminated with the introduction of percolative disorder far below the percolation threshold. Even after the Kosterlitz-Thouless order had been removed, the system remained superconducting at low temperatures by establishing a different type of order. Near the percolation threshold, evidence was found that, as a consequence of the underlying fractal structure, the critical dynamics of the phase degrees of freedom persisted down to zero temperature.
We show that the interplay of geometric criticality and quantum fluctuations leads to a novel universality class for the percolation quantum phase transition in diluted magnets. All critical exponents involving dynamical correlations are different from the classical percolation values, but in two dimensions they can nonetheless be determined exactly. We develop a complete scaling theory of this transition, and we relate it to recent experiments in La2Cu(1-p)(Zn,Mg)(p)O4. Our results are also relevant for disordered interacting boson systems.
The nature of the superfluid-to-Bose-glass (SF-BG) quantum phase transition, occurring in systems of interacting bosons immersed in a disordered environment, remains elusive. One fundamental open question is whether or not the transition obeys conventional scaling at quantum critical points (QCPs): this scaling would lock the value of the crossover exponent $\ensuremath{\phi}$---dictating the vanishing of the superfluid critical temperature upon approaching the QCP---to the value of quantum critical exponents for the ground-state transition. Yet such a relation between exponents has been called into question by several numerical as well as experimental results on the SF-BG transition. Here we revisit this issue in the case of the S=1/2S=1/2S=1/2 Heisenberg antiferromagnet on a site-diluted cubic lattice, which lends itself to efficient quantum Monte Carlo simulations. Our results show that the model exhibits a percolation transition in zero applied field, with the correlation length exponent $\ensuremath{\nu}=0.87(8)$ and $\ensuremath{\phi}=1.1(1)$ consistent with 3d3d3d percolation. When applying a sufficiently strong magnetic field, the dilution-induced transition decouples from geometric percolation, and it becomes a SF-BG transition; nonetheless, the $\ensuremath{\nu}$ and $\ensuremath{\phi}$ exponents maintain values consistent with those of the percolation transition. These results contradict the conventional scaling, which predicts $\ensuremath{\phi}\ensuremath{\geqslant}2$; and they suggest a possible relationship between the SF-BG transition and percolation of phase coherence.
We present the critical theory of a number of zero-temperature phase transitions of quantum antiferromagnets and interacting boson systems in two dimensions. The most important example is the transition of the S=1∕2S=1∕2S=1∕2 square lattice antiferromagnet between the N\'eel state (which breaks spin rotation invariance) and the paramagnetic valence bond solid (which preserves spin rotation invariance but breaks lattice symmetries). We show that these two states are separated by a second-order quantum phase transition. This conflicts with Landau-Ginzburg-Wilson theory, which predicts that such states with distinct broken symmetries are generically separated either by a first-order transition, or by a phase with co-existing orders. The critical theory of the second-order transition is not expressed in terms of the order parameters characterizing either state, but involves fractionalized degrees of freedom and an emergent, topological, global conservation law. A closely related theory describes the superfluid-insulator transition of bosons at half filling on a square lattice, in which the insulator has a bond density wave order. Similar considerations are shown to apply to transitions of antiferromagnets between the valence bond solid and the Z2{Z}_{2}Z2​ spin liquid: the critical theory has deconfined excitations interacting with an emergent U(1)\mathrm{U}(1)U(1) gauge force. We comment on the broader implications of our results for the study of quantum criticality in correlated electron systems.