# Hierarchy of the low-lying excitations for the -dimensional Potts model in the ordered phase

###### Abstract

The -dimensional Potts model was simulated with the exact diagonalization method. In the ordered phase, the elementary excitations (magnons) are attractive, forming a series of bound states in the low-energy spectrum. We investigate the low-lying spectrum through a dynamical susceptibility, which is readily tractable with the exact diagonalization method via the continued-fraction expansion. As a result, we estimate the series of (scaled) mass gaps, (: single-magnon mass), in proximity to the transition point.

###### keywords:

75.10.Jm 75.40.Mg 05.50.+q , 05.70.Jk^{†}

^{†}journal: Nuclear Physics B

## 1 Introduction

In the ordered phase for an Ising ferromagnet, the elementary excitations (magnons) are attractive, forming a series of bound states (composite particles) with the mass gaps, (: single-magnon mass). In fact, for the -dimensional Ising model Zamolodchikov98 ; Delfino04 ; Fonseca03 , there appear eight types of excitations with the (scaled) mass gaps

(1) | |||||

(2) | |||||

(3) | |||||

(4) | |||||

(5) | |||||

(6) | |||||

(7) |

under a properly scaled magnetic field so as to preserve the integrability; namely, the scaled gap ratio, , displays a universal hierarchical character. For a quasi-one-dimensional ferromagnet, CoNbO, the primary one, (golden ratio), was confirmed by means of the inelastic neutron scattering Coldea10 . For the -dimensional counterpart, such rigorous information is not available, and various approaches have been made so as to fix the hierarchical structure Caselle99 ; Lee01 ; Caselle02 ; Nishiyama08 ; Dusuel10 ; Nishiyama14 ; Rose16 ; Nishiyama16 . Meanwhile, it turned out that the spectrum for the three-state () Potts model exhibits a hierarchical character as well Chim92 ; Delfino08 ; Lepori09 ; Rutkevich15 ; Lencses15 ; Falcone07 . Related results are recalled afterwards.

According to the Monte Carlo simulations in dimensions Falcone07 ; Falcone07b , the hierarchy of the Potts model and that of the pure gauge theory look alike. Actually, for the Z case in dimensions, a duality relation Wegner71 ; Balian75 ; Fradkin78 does hold, ensuring the correspondence between them. Generically Svetitsky82 , the SU gauge theory displays a global Z symmetry (center of SU), which immediately establishes a relationship between them; for , the transition would not be critical, and the universality idea does not apply nonetheless. Meanwhile, an extensive lattice-gauge-theory simulation reveals the “weak dependence” Athenodorou15 of the SU theory, suggesting a robustness of the hierarchy, . On the one hand, the Potts model exhibits an “approximate universality” Hamer92 , even though the phase transition is of first order. Hence, it is expected that the hierarchy should display a model-independent character to some extent.

In this paper, we investigate the -dimensional Potts model Hamer90 ; Dai14 by means of the numerical diagonalization method. The method allows us to evaluate the dynamical susceptibilities, Eqs. (12) and (17), via the continued-fraction-expansion method Gagliano87 . In Fig. 1, we present a schematic drawing for a spectral function.

To be specific, we present the Hamiltonian for the -dimensional Potts model

(8) |

Here, the operator

(9) |

is placed at each square-lattice point ; namely, the base () satisfies . The summation runs over all possible nearest neighbor pairs . The operator induces the transition , and the parameter denotes the corresponding coupling constant. This model exhibits the first-order phase transition at () Hamer92 , which separates the ordered () and disordered () phases. An infinitesimal magnetic field (: linear dimension of the finite-size cluster) stabilizes Fonseca03 the ground state, and the power comes from the putative scaling theory Hamer90 for the -dimensional Potts model. Note that the first-order phase transition also obeys the remedied scaling theory Fisher82 ; Challa86 ; Campostrini15 ; Campostrini14 .

The rest of this paper is organized as follows. In the next section, we present the numerical results. The simulation algorithm is presented as well. In Sec. 3, we address the summary and discussions.

## 2 Numerical results

In this section, we present the numerical results for the Potts model (8). To begin with, we explain the simulation algorithm.

### 2.1 Numerical algorithm

We employed the exact diagonalization method to simulate the Potts model (8) for a rectangular cluster with spins. In order to treat a variety of systematically, we implemented the screw-boundary condition Novotny90 . According to Ref. Novotny90 , an alignment of spins with the first- and -th-neighbor interactions reduces to a rectangular cluster under the screw-boundary condition. Based on this idea, we express the Hamiltonian as

(10) |

Here, the diagonal matrix denotes the -th-neighbor interaction for an alignment ; that is, the diagonal element is given by with the translation operator, , and the Potts interaction, . The above formulae are mathematically closed; however, for an efficient simulation, Eqs. (9) and (10) of Ref. Nishiyama08b may be of use.

We performed the numerical diagonalization for the Hamiltonian matrix (10) by means of the Lanczos method. The single-magnon mass gap is given by

(11) |

with the ground-state () and first-excited () energy levels within the zero-momentum sector. Because the spins form a rectangular cluster, the linear dimension is given by , which sets a foundermental length scale in the subsequent scaling analyses.

### 2.2 Single-magnon mass gap

In this section, we investigate the single-magnon mass gap (11) with the scaling theory Hamer90 ; Campostrini15 . The first-order phase transition obeys the properly remedied scaling theory.

In Fig. 2, we present the scaling plot, -, for () , () , () , and () . Here, the symbol denotes Campostrini15 , and the scaling parameters, and , are taken from Refs. Hamer92 and Hamer90 ; Campostrini15 , respectively. We stress that there is no ad hoc fitting parameter involved in the scaling analysis.

The data in Fig. 2 seem to collapse into a scaling curve, indicating that the simulation data already enter the scaling regime. Encouraged by this finding, we turn to the analysis of the spectral properties.

### 2.3 Hierarchical spectral peaks via

Based on the finite-size scaling Hamer90 ; Campostrini15 demonstrated in the preceding section, we analyze the dynamical susceptibility

(12) |

with the ground-state energy (vector) () and the energy-resolution parameter . Here, the perturbation operator is set to

(13) |

with

(14) |

and the projection operator . We calculated the dynamical susceptibility (12) with the continued-fraction expansion Gagliano87 . The dynamical susceptibility (spectral function) obeys the scaling formula

(15) |

with a certain scaling function Podolsky12 ; Nishiyama16 .

In Fig. 4, we present the scaling plot, -, with fixed and for various (dotted), (solid), and (dashed); here, the scaling parameters, and , are the same as those of Fig. 2. The data appear to collapse into a scaling curve satisfactorily.

Each signal in Fig. 3 is interpreted by the diagram in Fig. 1. That is, the peaks around , and correspond to the excitations, respectively. The shoulder peak around should be the two-magnon-spectrum threshold. The signal may be either the particle or a composite one consisting of and . The ratios are estimated in the next section more in detail.

Last, we mention the choice of the perturbation operator (13). In a preliminary stage, we surveyed various types of the perturbation operators, aiming to create the particles effectively. Actually, neither the first or the second term of the Hamiltonian (8) commutes with ; otherwise, the susceptibility reduces to a mere specific heat. A key ingredient is that the exact diagonalization method permits us to treat any off-diagonal operators.

### 2.4 Universality of the scaled masses

In this section, we devote ourselves to the analysis of the scaled masses ; it is expected that each ratio takes a constant value in proximity to the transition point Hamer92 .

In Fig. 4, we present the scaling plot, -, with fixed and for various values of the scaling parameter, (dotted), (solid), and (dashes); the scaling parameters, and , are the same as those of Fig. 2. Note that these curves do not necessarily overlap, because the scaling parameter is not a constant value; see Eq. (15) for the scaling formula. Each peak position seems to be kept invariant, albeit with the scaling parameter varied. As a result, we estimate the scaled mass gaps as

(16) |

Here, each error margin was determined from the finite-size drift between and ; a dominant source of the error margin comes from the oscillatory deviation (an artifact due to the screw-boundary condition), which depends on the condition whether the system size is a quadratic number or not .

Each particle may possess a finite life time, because it is embedded within the two-particle spectrum. As a matter of fact, the data for (dotted) in Fig. 4 exhibit split peaks around , indicating that the bound state has an appreciable peak width, . Similarly, we observed for the data with and . To summarize, we estimate the intrinsic peak widths (reciprocal life time) as and . Each peak width is about one tenth of the corresponding mass gap.

This is a good position to address an overview of the related studies. First, for the classical three-dimensional Potts model, an estimate Falcone07 was reported; the notation (symmetry index) is taken from the original paper. This result may correspond to the present result, , Eq. (16), supporting an “approximate universality” Hamer92 for the Potts model. Second, as for the Z Agostini97 and SU Fiore03 gauge field theories, the results, and , respectively, were obtained. The hierarchical structures are quite reminiscent of ours, Eq. (16). As a matter of fact, for the gauge field theory, the “week dependence” of the gauge group SU was reported Athenodorou15 , indicating a robustness of the hierarchy . For the SU gauge field Falcone07b , a glueball mass, either or , was estimated. The result may correspond to , Eq. (16). Last, for the Ising model Caselle99 ; Lee01 ; Caselle02 ; Nishiyama08 ; Dusuel10 ; Nishiyama14 ; Rose16 ; Nishiyama16 , estimates, Rose16 and Caselle99 , were reported. These results resemble to ours, Eq. (16), supporting a model-independence on the hierarchy.

Last, we address a remark. Because the phase transition is discontinuous, the continuum limit cannot be taken properly. The above estimates such as the life time are specific to a lattice realization, although seemingly preferable scaling behavior was observed. However, in an approximate sense, the simulation results seem to be comparable with the related ones, as claimed by the preceeding studies Falcone07 ; Falcone07b ; Hamer92 .

### 2.5 Continuum-threshold peak via

As a comparison, we present a simulation result for (17), aiming to see to what extent the spectral weight is affected by the choice of the perturbation operator. The dynamical susceptibility is defined by

(17) |

with a perturbation operator

(18) |

Note that the operator coincides with the second term of the Hamiltonian (8). Hence, it exhibits the specific-heat-type singularity with Hamer90 ; Campostrini15 right at the transition point.

In Fig. 5, we present the scaling plot, -, with fixed and for various (dots), (solid), and (dashed); the scaling parameters, and , are the same as those of Fig. 2. In contrast with in Fig. 3, the susceptibility detects the signals and the two-magnon-spectrum-threshold peak ; instead, the bound-state hierarchy becomes hardly observable.

The result indicates that a naive external disturbance such as the specific-heat-type perturbation does not create the bound states higher than very efficiently. It is significant to set up the perturbation operator, which does not commute with any terms of the Hamiltonian. Note that the exact diagonalization method allows us to survey various types of the (off-diagonal) perturbation operators so as to observe clearly.

## 3 Summary and discussions

The hierarchy for the -dimensional Potts model (8) was investigated with the numerical diagonalization method. The method allows us to calculate the dynamical susceptibilities , Eqs. (12) and (17), via the continued-fraction expansion Gagliano87 . Through the probe , we obtained . The particles acquire intrinsic peak widths, and , respectively; these spectra are embedded within the two-magnon spectrum. According to Refs. Falcone07 ; Falcone07b , the hierarchy of the Potts model and that of the pure gauge theory are alike. For instance, as for the Z-symmetric gauge group Agostini97 , there was reported a hierarchy, , quite reminiscent of ours, Eq. (16).

As a reference, we calculated (17); here, the operator coincides with the second term of the Hamiltonian (8), and hence, it would be relevant to the experimental study. It turned out that the probe is insensitive to the hierarchy , indicating that the choice of the perturbation operator is vital to observe . In this sense, the exact diagonalization method has an advantage in that we are able to treat various perturbation operators so as to observe the hierarchy clearly.

## Acknowledgment

This work was supported by a Grant-in-Aid for Scientific Research (C) from Japan Society for the Promotion of Science (Grant No. 25400402).

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