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Register a new userZ2 topological order and first-order quantum phase transitions in systems with combinatorial gauge symmetry
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We study a generalization of the two-dimensional transverse-field Ising model, combining both ferromagnetic and antiferromagnetic two-body interactions, that hosts exact global and local Z2 gauge symmetries. Using exact diagonalization and stochastic series expansion quantum Monte Carlo methods, we confirm the existence of the topological phase in line with previous theoretical predictions. Our simulation results show that the transition between the confined topological phase and the deconfined paramagnetic phase is of first-order, in contrast to the conventional Z2 lattice gauge model in which the transition maps onto that of the standard Ising model and is continuous. We further generalize the model by replacing the transverse field on the gauge spins with a ferromagnetic XX interaction while keeping the local gauge symmetry intact. We find that the Z2 topological phase remains stable, while the paramagnetic phase is replaced by a ferromagnetic phase. The topological-ferromagnetic quantum phase transition is also of first-order. For both models, we discuss the low-energy spinon and vison excitations of the topological phase and their avoided level crossings associated with the first-order quantum phase transitions.
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We introduce the notion of combinatorial gauge symmetry -- a local transformation that includes single spin rotations plus permutations of spins (or swaps of their quantum states) -- that preserve the commutation and anti-commutation relations among the spins. We show that Hamiltonians with simple two-body interactions contain this symmetry if the coupling matrix is a Hadamard matrix, with the combinatorial gauge symmetry being associated to the automorphism of these matrices with respect to monomial transformations. Armed with this symmetry, we address the physical problem of how to build quantum spin liquids with physically accessible interactions. In addition to its intrinsic physical significance, the problem is also tied to that of how to build topological qubits.
A model of a mixture of spinless fermions and spin-zero hardcore bosons, with filling fractions $rho_F$ and $rho_B$, respectively, on a two-dimensional square lattice with {em composite} hopping $t$ is presented. In this model, hopping swaps the locations of a fermion and a boson at nearest-neighbor sites. When $rho_F+rho_B=1$, the fermion hopping amplitude $phi$ and boson superfluid amplitude $psi$ are calculated in the ground state within a mean-field approximation. The Fermi sector is insulating ($phi=0$) and the Bose sector is normal ($psi=0$) for $0 le rho_F < rho_c$. The model has {em coupled first-order} transitions at $rho_F = rho_c simeq 0.3$ where both $phi$ and $psi$ are discontinuous. The Fermi sector is metallic ($phi>0$) and the Bose sector is superfluid ($psi>0$) for $rho_c < rho_F < 1$. At $rho_F=1/2$, fermion density of states $rho$ has a van Hove singularity, the bulk modulus $kappa$ displays a cusp-like singularity, the system has a density wave (DW) order, and $phi$ and $psi$ are maximum. At $rho_F=rho_{kappa} simeq 0.81$, $kappa$ vanishes, becoming {em negative} for $rho_{kappa}<rho_F<1$. The role of composite hopping in the evolution of Fermi band dispersions and Fermi surfaces as a function of $rho_F$ is highlighted. The estimate for BEC critical temperature is in the subkelvin range for ultracold atom systems and several hundred kelvins for possible solid-state examples of the model.
We find that the first-order quantum phase transitions~(QPTs) are characterized by intrinsic jumps of relevant operators while the continuous ones are not. Based on such an observation, we propose a bond reversal method where a quantity $mathcal{D}$, the difference of bond strength~(DBS), is introduced to judge whether a QPT is of first order or not. This method is firstly applied to an exactly solvable spin-$1/2$ textit{XXZ} Heisenberg chain and a quantum Ising chain with longitudinal field where distinct jumps of $mathcal{D}$ appear at the first-order transition points for both cases. We then use it to study the topological QPT of a cross-coupled~($J_{times}$) spin ladder where the Haldane--rung-singlet transition switches from being continuous to exhibiting a first-order character at $J_{times, I} simeq$ 0.30(2). Finally, we study a recently proposed one-dimensional analogy of deconfined quantum critical point connecting two ordered phases in a spin-$1/2$ chain. We rule out the possibility of weakly first-order QPT because the DBS is smooth when crossing the transition point. Moreover, we affirm that such transition belongs to the Gaussian universality class with the central charge $c$ = 1.
We investigate the effects of dissipation on the quantum dynamics of many-body systems at quantum transitions, especially considering those of the first order. This issue is studied within the paradigmatic one-dimensional quantum Ising model. We analyze the out-of-equilibrium dynamics arising from quenches of the Hamiltonian parameters and dissipative mechanisms modeled by a Lindblad master equation, with either local or global spin operators acting as dissipative operators. Analogously to what happens at continuous quantum transitions, we observe a regime where the system develops a nontrivial dynamic scaling behavior, which is realized when the dissipation parameter $u$ (globally controlling the decay rate of the dissipation within the Lindblad framework) scales as the energy difference $Delta$ of the lowest levels of the Hamiltonian, i.e., $usim Delta$. However, unlike continuous quantum transitions where $Delta$ is power-law suppressed, at first-order quantum transitions $Delta$ is exponentially suppressed with increasing the system size (provided the boundary conditions do not favor any particular phase).
The hunt for exotic quantum phase transitions described by emergent fractionalized degrees of freedom coupled to gauge fields requires a precise determination of the fixed point structure from the field theoretical side, and an extreme sensitivity to weak first-order transitions from the numerical side. Addressing the latter, we revive the classic definition of the order parameter in the limit of a vanishing external field at the transition. We demonstrate that this widely understood, yet so far unused approach provides a diagnostic test for first-order versus continuous behavior that is distinctly more sensitive than current methods. We first apply it to the family of $Q$-state Potts models, where the nature of the transition is continuous for $Qleq4$ and turns (weakly) first order for $Q>4$, using an infinite system matrix product state implementation. We then employ this new approach to address the unsettled question of deconfined quantum criticality in the $S=1/2$ Neel to valence bond solid transition in two dimensions, focusing on the square lattice $J$-$Q$ model. Our quantum Monte Carlo simulations reveal that both order parameters remain finite at the transition, directly confirming a first-order scenario with wide reaching implications in condensed matter and quantum field theory.
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