Two-dimensional critical Ising model

Last updated

The two-dimensional critical Ising model is the critical limit of the Ising model in two dimensions. It is a two-dimensional conformal field theory whose symmetry algebra is the Virasoro algebra with the central charge . Correlation functions of the spin and energy operators are described by the minimal model. While the minimal model has been exactly solved, see also, e.g., the article on Ising critical exponents, the solution does not cover other observables such as connectivities of clusters.

Contents

The minimal model

Space of states and conformal dimensions

The Kac table of the minimal model is:

This means that the space of states is generated by three primary states, which correspond to three primary fields or operators: [1]

The decomposition of the space of states into irreducible representations of the product of the left- and right-moving Virasoro algebras is

where is the irreducible highest-weight representation of the Virasoro algebra with the conformal dimension . In particular, the Ising model is diagonal and unitary.

Characters and partition function

The characters of the three representations of the Virasoro algebra that appear in the space of states are [1]

where is the Dedekind eta function, and are theta functions of the nome , for example . The modular S-matrix, i.e. the matrix such that , is [1]

where the fields are ordered as . The modular invariant partition function is

Fusion rules and operator product expansions

The fusion rules of the model are

The fusion rules are invariant under the symmetry . The three-point structure constants are

Knowing the fusion rules and three-point structure constants, it is possible to write operator product expansions, for example

where are the conformal dimensions of the primary fields, and the omitted terms are contributions of descendant fields.

Correlation functions on the sphere

Any one-, two- and three-point function of primary fields is determined by conformal symmetry up to a multiplicative constant. This constant is set to be one for one- and two-point functions by a choice of field normalizations. The only non-trivial dynamical quantities are the three-point structure constants, which were given above in the context of operator product expansions.

with .

The three non-trivial four-point functions are of the type . For a four-point function , let and be the s- and t-channel Virasoro conformal blocks, which respectively correspond to the contributions of (and its descendants) in the operator product expansion , and of (and its descendants) in the operator product expansion . Let be the cross-ratio.

In the case of , fusion rules allow only one primary field in all channels, namely the identity field. [2]

In the case of , fusion rules allow only the identity field in the s-channel, and the spin field in the t-channel. [2]

In the case of , fusion rules allow two primary fields in all channels: the identity field and the energy field. [2] In this case we write the conformal blocks in the case only: the general case is obtained by inserting the prefactor , and identifying with the cross-ratio.

In the case of , the conformal blocks are:

From the representation of the model in terms of Dirac fermions, it is possible to compute correlation functions of any number of spin or energy operators: [1]

These formulas have generalizations to correlation functions on the torus, which involve theta functions. [1]

Other observables

Disorder operator

The two-dimensional Ising model is mapped to itself by a high-low temperature duality. The image of the spin operator under this duality is a disorder operator , which has the same left and right conformal dimensions . Although the disorder operator does not belong to the minimal model, correlation functions involving the disorder operator can be computed exactly, for example [1]

whereas

Connectivities of clusters

The Ising model has a description as a random cluster model due to Fortuin and Kasteleyn. In this description, the natural observables are connectivities of clusters, i.e. probabilities that a number of points belong to the same cluster. The Ising model can then be viewed as the case of the -state Potts model, whose parameter can vary continuously, and is related to the central charge of the Virasoro algebra.

In the critical limit, connectivities of clusters have the same behaviour under conformal transformations as correlation functions of the spin operator. Nevertheless, connectivities do not coincide with spin correlation functions: for example, the three-point connectivity does not vanish, while . There are four independent four-point connectivities, and their sum coincides with . [3] Other combinations of four-point connectivities are not known analytically. In particular they are not related to correlation functions of the minimal model, [4] although they are related to the limit of spin correlators in the -state Potts model. [3]

Related Research Articles

<span class="mw-page-title-main">Noether's theorem</span> Statement relating differentiable symmetries to conserved quantities

Noether's theorem or Noether's first theorem states that every differentiable symmetry of the action of a physical system with conservative forces has a corresponding conservation law. The theorem was proven by mathematician Emmy Noether in 1915 and published in 1918. The action of a physical system is the integral over time of a Lagrangian function, from which the system's behavior can be determined by the principle of least action. This theorem only applies to continuous and smooth symmetries over physical space.

<span class="mw-page-title-main">Fokker–Planck equation</span> Partial differential equation

In statistical mechanics and information theory, the Fokker–Planck equation is a partial differential equation that describes the time evolution of the probability density function of the velocity of a particle under the influence of drag forces and random forces, as in Brownian motion. The equation can be generalized to other observables as well. The Fokker-Planck equation has multiple applications in information theory, graph theory, data science, finance, economics etc.

Linear elasticity is a mathematical model of how solid objects deform and become internally stressed due to prescribed loading conditions. It is a simplification of the more general nonlinear theory of elasticity and a branch of continuum mechanics.

In mathematics, the Hodge star operator or Hodge star is a linear map defined on the exterior algebra of a finite-dimensional oriented vector space endowed with a nondegenerate symmetric bilinear form. Applying the operator to an element of the algebra produces the Hodge dual of the element. This map was introduced by W. V. D. Hodge.

<span class="mw-page-title-main">Radon transform</span> Integral transform

In mathematics, the Radon transform is the integral transform which takes a function f defined on the plane to a function Rf defined on the (two-dimensional) space of lines in the plane, whose value at a particular line is equal to the line integral of the function over that line. The transform was introduced in 1917 by Johann Radon, who also provided a formula for the inverse transform. Radon further included formulas for the transform in three dimensions, in which the integral is taken over planes. It was later generalized to higher-dimensional Euclidean spaces and more broadly in the context of integral geometry. The complex analogue of the Radon transform is known as the Penrose transform. The Radon transform is widely applicable to tomography, the creation of an image from the projection data associated with cross-sectional scans of an object.

<span class="mw-page-title-main">Drude model</span> Model of electrical conduction

The Drude model of electrical conduction was proposed in 1900 by Paul Drude to explain the transport properties of electrons in materials. Basically, Ohm's law was well established and stated that the current J and voltage V driving the current are related to the resistance R of the material. The inverse of the resistance is known as the conductance. When we consider a metal of unit length and unit cross sectional area, the conductance is known as the conductivity, which is the inverse of resistivity. The Drude model attempts to explain the resistivity of a conductor in terms of the scattering of electrons by the relatively immobile ions in the metal that act like obstructions to the flow of electrons.

<span class="mw-page-title-main">Rabi cycle</span> Quantum mechanical phenomenon

In physics, the Rabi cycle is the cyclic behaviour of a two-level quantum system in the presence of an oscillatory driving field. A great variety of physical processes belonging to the areas of quantum computing, condensed matter, atomic and molecular physics, and nuclear and particle physics can be conveniently studied in terms of two-level quantum mechanical systems, and exhibit Rabi flopping when coupled to an optical driving field. The effect is important in quantum optics, magnetic resonance and quantum computing, and is named after Isidor Isaac Rabi.

In physics, the S-matrix or scattering matrix relates the initial state and the final state of a physical system undergoing a scattering process. It is used in quantum mechanics, scattering theory and quantum field theory (QFT).

In quantum physics, Fermi's golden rule is a formula that describes the transition rate from one energy eigenstate of a quantum system to a group of energy eigenstates in a continuum, as a result of a weak perturbation. This transition rate is effectively independent of time and is proportional to the strength of the coupling between the initial and final states of the system as well as the density of states. It is also applicable when the final state is discrete, i.e. it is not part of a continuum, if there is some decoherence in the process, like relaxation or collision of the atoms, or like noise in the perturbation, in which case the density of states is replaced by the reciprocal of the decoherence bandwidth.

<span class="mw-page-title-main">LSZ reduction formula</span> Connection between correlation functions and the S-matrix

In quantum field theory, the Lehmann–Symanzik–Zimmerman (LSZ) reduction formula is a method to calculate S-matrix elements from the time-ordered correlation functions of a quantum field theory. It is a step of the path that starts from the Lagrangian of some quantum field theory and leads to prediction of measurable quantities. It is named after the three German physicists Harry Lehmann, Kurt Symanzik and Wolfhart Zimmermann.

<span class="mw-page-title-main">Two-state quantum system</span> Simple quantum mechanical system

In quantum mechanics, a two-state system is a quantum system that can exist in any quantum superposition of two independent quantum states. The Hilbert space describing such a system is two-dimensional. Therefore, a complete basis spanning the space will consist of two independent states. Any two-state system can also be seen as a qubit.

In general relativity, the Gibbons–Hawking–York boundary term is a term that needs to be added to the Einstein–Hilbert action when the underlying spacetime manifold has a boundary.

<span class="mw-page-title-main">Maxwell's equations in curved spacetime</span> Electromagnetism in general relativity

In physics, Maxwell's equations in curved spacetime govern the dynamics of the electromagnetic field in curved spacetime or where one uses an arbitrary coordinate system. These equations can be viewed as a generalization of the vacuum Maxwell's equations which are normally formulated in the local coordinates of flat spacetime. But because general relativity dictates that the presence of electromagnetic fields induce curvature in spacetime, Maxwell's equations in flat spacetime should be viewed as a convenient approximation.

<span class="mw-page-title-main">Jaynes–Cummings model</span> Model in quantum optics

The Jaynes–Cummings model is a theoretical model in quantum optics. It describes the system of a two-level atom interacting with a quantized mode of an optical cavity, with or without the presence of light. It was originally developed to study the interaction of atoms with the quantized electromagnetic field in order to investigate the phenomena of spontaneous emission and absorption of photons in a cavity.

A vacuum Rabi oscillation is a damped oscillation of an initially excited atom coupled to an electromagnetic resonator or cavity in which the atom alternately emits photon(s) into a single-mode electromagnetic cavity and reabsorbs them. The atom interacts with a single-mode field confined to a limited volume V in an optical cavity. Spontaneous emission is a consequence of coupling between the atom and the vacuum fluctuations of the cavity field.

The purpose of this page is to provide supplementary materials for the ordinary least squares article, reducing the load of the main article with mathematics and improving its accessibility, while at the same time retaining the completeness of exposition.

In quantum field theory, and especially in quantum electrodynamics, the interacting theory leads to infinite quantities that have to be absorbed in a renormalization procedure, in order to be able to predict measurable quantities. The renormalization scheme can depend on the type of particles that are being considered. For particles that can travel asymptotically large distances, or for low energy processes, the on-shell scheme, also known as the physical scheme, is appropriate. If these conditions are not fulfilled, one can turn to other schemes, like the minimal subtraction scheme.

Lagrangian field theory is a formalism in classical field theory. It is the field-theoretic analogue of Lagrangian mechanics. Lagrangian mechanics is used to analyze the motion of a system of discrete particles each with a finite number of degrees of freedom. Lagrangian field theory applies to continua and fields, which have an infinite number of degrees of freedom.

Generalized relative entropy is a measure of dissimilarity between two quantum states. It is a "one-shot" analogue of quantum relative entropy and shares many properties of the latter quantity.

The Peierls substitution method, named after the original work by Rudolf Peierls is a widely employed approximation for describing tightly-bound electrons in the presence of a slowly varying magnetic vector potential.

References

  1. 1 2 3 4 5 6 P. Di Francesco, P. Mathieu, and D. Sénéchal, Conformal Field Theory, 1997, ISBN   0-387-94785-X
  2. 1 2 3 Cheng, Miranda C. N.; Gannon, Terry; Lockhart, Guglielmo (2020-02-25). "Modular Exercises for Four-Point Blocks -- I". arXiv: 2002.11125v1 [hep-th].
  3. 1 2 Delfino, Gesualdo; Viti, Jacopo (2011-04-21). "Potts q-color field theory and scaling random cluster model". Nuclear Physics B. 852 (1): 149–173. arXiv: 1104.4323v2 . Bibcode:2011NuPhB.852..149D. doi:10.1016/j.nuclphysb.2011.06.012. S2CID   119183802.
  4. Delfino, Gesualdo; Viti, Jacopo (2010-09-07). "On three-point connectivity in two-dimensional percolation". Journal of Physics A: Mathematical and Theoretical. 44 (3): 032001. arXiv: 1009.1314v1 . doi:10.1088/1751-8113/44/3/032001. S2CID   119246430.