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arXiv:1611.08298 · Physical review. B./Physical review. B

Emergence of string valence-bond-solid state in the frustrated J1−J2 transverse field Ising model on the square lattice

M. Sadrzadeh, Reza Haghshenas, Saeed S. Jahromi, A. Langari

Published Dec 19, 2016• 19 citations
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Abstract

We investigate the ground-state nature of the transverse field Ising model on the ${J}_{1}\text{\ensuremath{-}}{J}_{2}$ square lattice at the highly frustrated point ${J}_{2}/{J}_{1}=0.5$. At zero field, the model has an exponentially large degenerate classical ground state, which can be affected by quantum fluctuations for nonzero field toward a unique quantum ground state. We consider two types of quantum fluctuations, harmonic ones by using linear spin-wave theory (LSWT) with single-spin-flip excitations above a long-range magnetically ordered background and anharmonic fluctuations, by employing a cluster-operator approach (COA) with multispin cluster-type fluctuations above a nonmagnetic cluster-ordered background. Our findings reveal that the harmonic fluctuations of LSWT fail to lift the extensive degeneracy as well as signaling a violation of the Hellmann-Feynman theorem. However, the string-type anharmonic fluctuations of COA are able to lift the degeneracy toward a string valence-bond-solid (VBS) state, which is obtained from an effective theory consistent with the Hellmann-Feynman theorem as well. Our results are further confirmed by implementing numerical tree tensor network simulation. The emergent nonmagnetic string VBS phase is gapped and breaks lattice rotational symmetry with only twofold degeneracy, which bears a continuous quantum phase transition at $\mathrm{\ensuremath{\Gamma}}/{J}_{1}\ensuremath{\cong}0.50$ to the quantum paramagnet phase of high fields. The critical behavior is characterized by $\ensuremath{\nu}\ensuremath{\cong}1.0$ and $\ensuremath{\gamma}\ensuremath{\cong}0.33$ exponents.

DOI
10.1103/physrevb.94.214419

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We investigate the ground-state nature of the transverse field Ising model on the J1\ensuremath-J2{J}_{1}\text{\ensuremath{-}}{J}_{2}J1​\ensuremath-J2​ square lattice at the highly frustrated point J2/J1=0.5{J}_{2}/{J}_{1}=0.5J2​/J1​=0.5. At zero field, the model has an exponentially large degenerate classical ground state, which can be affected by quantum fluctuations for nonzero field toward a unique quantum ground state. We consider two types of quantum fluctuations, harmonic ones by using linear spin-wave theory (LSWT) with single-spin-flip excitations above a long-range magnetically ordered background and anharmonic fluctuations, by employing a cluster-operator approach (COA) with multispin cluster-type fluctuations above a nonmagnetic cluster-ordered background. Our findings reveal that the harmonic fluctuations of LSWT fail to lift the extensive degeneracy as well as signaling a violation of the Hellmann-Feynman theorem. However, the string-type anharmonic fluctuations of COA are able to lift the degeneracy toward a string valence-bond-solid (VBS) state, which is obtained from an effective theory consistent with the Hellmann-Feynman theorem as well. Our results are further confirmed by implementing numerical tree tensor network simulation. The emergent nonmagnetic string VBS phase is gapped and breaks lattice rotational symmetry with only twofold degeneracy, which bears a continuous quantum phase transition at \ensuremathΓ/J1\ensuremath≅0.50\mathrm{\ensuremath{\Gamma}}/{J}_{1}\ensuremath{\cong}0.50\ensuremathΓ/J1​\ensuremath≅0.50 to the quantum paramagnet phase of high fields. The critical behavior is characterized by \ensuremathν\ensuremath≅1.0\ensuremath{\nu}\ensuremath{\cong}1.0\ensuremathν\ensuremath≅1.0 and \ensuremathγ\ensuremath≅0.33\ensuremath{\gamma}\ensuremath{\cong}0.33\ensuremathγ\ensuremath≅0.33 exponents.
We study the zero-temperature phase diagram of transverse field Ising model on the J 1 − J 2 square lattice. In zero magnetic field, the model has a classical Néel phase for J 2 / J 1 < 0.5 and an antiferromagnetic collinear phase for J 2 / J 1 > 0.5. We incorporate harmonic fluctuations by using linear spin wave theory (LSWT) with single spin flip excitations above a magnetic order background and obtain the phase diagram of the model in this approximation. We find that harmonic quantum fluctuations of LSWT fail to lift the large degeneracy at J 2 / J 1 = 0.5 and exhibit some inconsistent regions on the phase diagram. However, we show that anharmonic fluctuations of cluster operator approach (COA) resolve the inconsistency of the LSWT, which reveals a string-valence bond solid ordered phase for the highly frustrated region.
We study the effect of quantum fluctuations by means of a transverse magnetic field (Γ\GammaΓ) on the antiferromagnetic J1−J2J_1-J_2J1​−J2​ Ising model on the checkerboard lattice, the two dimensional version of pyrochlore lattice. A plaquette operator formalism has been implemented to bosonise the model, in which a single boson is associated to each eigenstate of a plaquette and the inter-plaquette interactions define an effective Hamiltonian. The excitations of a plaquette would represent an-harmonic fluctuations of the model, which lead not only to lower the excitation energy compared with a single-spin flip but also to lift the extensive degeneracy in favor of a plaquette ordered state, which breaks lattice rotational symmetry for J2>J1J_2>J_1J2​>J1​ (the collinear phase). The bosonic excitation gap vanishes at the critical boundary to N\'{e}el (J2J1J_2 J_1J2​J1​) phases, respectively. At the homogeneous coupling (J1=J2J_1=J_2J1​=J2​) a (canted) resonating plaquette state (RPS) is established from an-harmonic fluctuations, which lasts for low fields, Γ/J1≲0.4\Gamma/J_1\lesssim 0.4Γ/J1​≲0.4, followed by a crossover to the fully polarized phase. We will show that our results predict an order-by-disorder as a results of transverse field at the critical point (J2=0.5J1J_2=0.5 J_1J2​=0.5J1​) of the antiferromagnetic J1−J2J_1-J_2J1​−J2​ Ising square lattice.
We investigate the ground-state phase diagram of the frustrated transverse-field Ising (TFI) model on the checkerboard lattice (CL), which consists of N\'eel, collinear, quantum paramagnet, and plaquette-valence bond solid (-VBS) phases. We implement a numerical simulation that is based on the recently developed unconstrained tree tensor network ansatz, which systematically improves the accuracy over the conventional methods as it exploits the internal gauge selections. At the highly frustrated region (J2=J1)({J}_{2}={J}_{1})(J2​=J1​), we observe a second-order phase transition from the plaquette-VBS state to the paramagnet phase at the critical magnetic-field ${\mathrm{\ensuremath{\Gamma}}}_{c}=0.28$ with the associated critical exponents $\ensuremath{\nu}=1$ and $\ensuremath{\gamma}\ensuremath{\simeq}0.4$, which are obtained within the finite-size scaling analysis on different lattice sizes $N=4\ifmmode\times\else\texttimes\fi{}4,6\ifmmode\times\else\texttimes\fi{}6,8\ifmmode\times\else\texttimes\fi{}8$. The stability of the plaquette-VBS phase at low magnetic fields is examined by a spin-spin correlation function, which verifies the presence of plaquette-VBS at J2=J1{J}_{2}={J}_{1}J2​=J1​ and rules out the existence of a N\'eel phase. In addition, our numerical results suggest that the transition from the N\'eel (for J2<J1){J}_{2}<{J}_{1})J2​<J1​) to the plaquette-VBS phase is a deconfined phase transition. Moreover, we introduce a mapping, which renders the low-energy effective theory of the TFI on the CL to be the same model on the ${J}_{1}\ensuremath{-}{J}_{2}$ square lattice (SL). We show that the plaquette-VBS phase of the highly frustrated point J2=J1{J}_{2}={J}_{1}J2​=J1​ on the CL is mapped to the emergent string-VBS phase on the SL at J2=0.5J1{J}_{2}=0.5{J}_{1}J2​=0.5J1​.
The ground-state properties of the S=1/2S=1/2S=1/2 transverse-field Ising model on the checkerboard lattice are studied using linear spin-wave theory. We consider the general case of different couplings between nearest neighbors (J1{J}_{1}J1​) and next-to-nearest neighbors (J2{J}_{2}J2​). In zero field, the system displays a large degeneracy of the ground state, which is exponential in the system size (for J1=J2{J}_{1}={J}_{2}J1​=J2​) or in the system's linear dimensions (for J2>J1{J}_{2}>{J}_{1}J2​>J1​). Quantum fluctuations induced by a transverse field are found to be unable to lift this degeneracy in favor of a classically ordered state at the harmonic level. This remarkable fact suggests that a quantum-disordered ground state can be instead promoted when nonlinear fluctuations are accounted for, in agreement with existing results for the isotropic case J1=J2{J}_{1}={J}_{2}J1​=J2​. Moreover, spin-wave theory shows sizable regions of instability, which are further candidates for quantum-disordered behavior.
Using the coupled cluster method for high orders of approximation and complementary exact diagonalization studies we investigate the ground state properties of the spin-1/2 J1−−J2{J}_{1}--{J}_{2}J1​−−J2​ frustrated Heisenberg antiferromagnet on the square lattice. We have calculated the ground state energy, the magnetic order parameter, the spin stiffness, and several generalized susceptibilities to probe magnetically disordered quantum valence-bond phases. We have found that the quantum critical points for both the N\'eel and collinear orders are ${J}_{2}^{c1}\ensuremath{\approx}(0.44\ifmmode\pm\else\textpm\fi{}0.01){J}_{1}$ and ${J}_{2}^{c2}\ensuremath{\approx}(0.59\ifmmode\pm\else\textpm\fi{}0.01){J}_{1}$, respectively, which are in good agreement with the results obtained by other approximations. In contrast to the recent study by [Sirker et al. Phys. Rev. B 73, 184420 (2006)], our data do not provide evidence for the transition from the N\'eel to the valence-bond solid state to be first order. Moreover, our results are in favor of the deconfinement scenario for that phase transition. We also discuss the nature of the magnetically disordered quantum phase.
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