The quantum clock model is a quantum lattice model.[1] It is a generalisation of the transverse-field Ising model . It is defined on a lattice with states on each site. The Hamiltonian of this model is

Here, the subscripts refer to lattice sites, and the sum is done over pairs of nearest neighbour sites and . The clock matrices and are generalisations of the Pauli matrices satisfying

and

where is 1 if and are the same site and zero otherwise. is a prefactor with dimensions of energy, and is another coupling coefficient that determines the relative strength of the external field compared to the nearest neighbor interaction.

The model obeys a global symmetry, which is generated by the unitary operator where the product is over every site of the lattice. In other words, commutes with the Hamiltonian.

When the quantum clock model is identical to the transverse-field Ising model. When the quantum clock model is equivalent to the quantum three-state Potts model. When , the model is again equivalent to the Ising model. When , strong evidences have been found that the phase transitions exhibited in these models should be certain generalizations [2] of Kosterlitz–Thouless transition, whose physical nature is still largely unknown.

One-dimensional model

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There are various analytical methods that can be used to study the quantum clock model specifically in one dimension.

Kramers–Wannier duality

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A nonlocal mapping of clock matrices known as the Kramers–Wannier duality transformation can be done as follows:[3]   Then, in terms of the newly defined clock matrices with tildes, which obey the same algebraic relations as the original clock matrices, the Hamiltonian is simply  . This indicates that the model with coupling parameter   is dual to the model with coupling parameter  , and establishes a duality between the ordered phase and the disordered phase.

Note that there are some subtle considerations at the boundaries of the one dimensional chain; as a result of these, the degeneracy and   symmetry properties of phases are changed under the Kramers–Wannier duality. A more careful analysis involves coupling the theory to a   gauge field; fixing the gauge reproduces the results of the Kramers Wannier transformation.

Phase transition

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For  , there is a unique phase transition from the ordered phase to the disordered phase at  . The model is said to be "self-dual" because Kramers–Wannier transformation transforms the Hamiltonian to itself. For  , there are two phase transition points at   and  . Strong evidences have been found that these phase transitions should be a class of generalizations[2] of Kosterlitz–Thouless transition. The KT transition predicts that the free energy has an essential singularity that goes like  , while perturbative study found that the essential singularity behaves as   where   goes from   to   as   increases from   to  . The physical pictures[4] of these phase transitions are still not clear.

Jordan–Wigner transformation

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Another nonlocal mapping known as the Jordan Wigner transformation can be used to express the theory in terms of parafermions.

References

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  1. ^ Radicevic, Djordje (2018). "Spin Structures and Exact Dualities in Low Dimensions". arXiv:1809.07757 [hep-th].
  2. ^ a b Bingnan Zhang (2020). "Perturbative study of the one-dimensional quantum clock model". Phys. Rev. E. 102 (4): 042110. arXiv:2006.11361. Bibcode:2020PhRvE.102d2110Z. doi:10.1103/PhysRevE.102.042110. PMID 33212691. S2CID 219966942.
  3. ^ Radicevic, Djordje (2018). "Spin Structures and Exact Dualities in Low Dimensions". arXiv:1809.07757 [hep-th].
  4. ^ Martin B. Einhorn, Robert Savit, Eliezer Rabinovici (1980). "A physical picture for the phase transitions in Zn symmetric models". Nuclear Physics B. 170 (1): 16-31. Bibcode:1980NuPhB.170...16E. doi:10.1016/0550-3213(80)90473-3. hdl:2027.42/23169.{{cite journal}}: CS1 maint: multiple names: authors list (link)