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XXZ Heisenberg model

Critical points and phase transitions in magnetic systems are studied using the Quantum Heisenberg model, a statistical mechanical model created by Werner Heisenberg. The Heisenberg model replaces classical spin vectors with quantum operators to treat the spins of magnetic systems quantum mechanically, in contrast to the more straightforward Ising model.

This is a thorough description of the XXZ Heisenberg model:

What is XXZ Heisenberg model?

The quantum Heisenberg model has a particular variation known as the XXZ Heisenberg model. In one direction (the z-axis), interactions between nearby spins exhibit a certain anisotropy, which is commonly referred to as a one-dimensional spin-1/2 chain. “XXZ” is the name given to it because the coupling constants for interactions in the x and y directions are the same, but they differ from the coupling constant in the z direction.

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How does the XXZ Heisenberg model work?

The basic idea is that, for quantum mechanical reasons, two dipoles can achieve their lowest energy state when aligned due to the dominating coupling between them. The model mainly takes into account magnetic interactions between neighbouring dipoles.

Pauli spin-1/2 matrices are used to create quantum operators that represent the spins in the XXZ Heisenberg model. An external magnetic field and real-valued coupling constants that specify the strength of interaction along the x, y, and z axes are included in the model. The energy scale is set by the parameter J. In the x-y plane, ferromagnetic order is favored if J is positive, which means spins tend to align in that plane. When J is negative, it encourages antiferromagnetic alignment, in which adjacent spins align in opposition to one another.

The strength of the uniaxial anisotropy along the z-direction is measured by ∆ (Delta), a critical parameter in the XXZ model. The x-y plane interactions are in competition with this ∆ parameter. Different physical regimes are displayed by the system depending on the value of ∆:

• The axial regime occurs when |∆| > 1 (i.e., when the absolute value of ∆ is greater than 1). In this regime, ferromagnetic order along the z-axis is preferred if J∆ is positive.
• Planar regime: (|∆| < 1) when ∆’s absolute value is less than 1.

Finding the spectrum of this model’s Hamiltonian is usually the goal of studying it. This enables the partition function to be calculated and the thermodynamics of the system to be studied. By allowing researchers to concentrate on certain magnetisation sectors, the model’s conserved total magnetisation along the z-axis makes analysis easier. This means that the number of “up” or “down” spins along this direction stays constant.

Heisenberg Model Family Relationship

A larger family of quantum Heisenberg models, which are identified by their coupling constants, includes the XXZ Heisenberg model:

• Heisenberg XXX model: This is the isotropic variant, in which all three (x, y, z) directions have uniform interactions (Jx = Jy = Jz = J). Werner Heisenberg’s first simplified model was this one.
• As previously mentioned, the XXZ Heisenberg model has a different coupling in z (Jx = Jy ≠ Jz) but equal couplings in x and y.
• In the Heisenberg XYZ model, all three coupling constants (Jx, Jy, and Jz) differ from one another, making it the most general and totally anisotropic instance.

The XXZ model has several exchange couplings and is an expansion of the basic isotropic Heisenberg chain.

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Features and Qualities

• Spin-1/2 Chain: This style usually consists of a one-dimensional line of spins that can be either “up” or “down” (spin-1/2).
• Phases: The system exhibits three unique phases, which are characterized by a ferromagnetic region, a fully critical paramagnetic phase, and an antiferromagnetic phase, depending on the anisotropic coupling constant ∆.
• Ground States: a state with all spins aligned in the same direction is the lowest energy state in the ferromagnetic region (where ∆ is larger than or equal to 1). A little magnetic field can be used in certain configurations to break energy degeneracies and choose a certain ground state.
• Symmetries: The ground state and low-energy excitations of the XXZ model can be assessed by utilising its several symmetries. It has U(1) (or O(2)) symmetry when ∆ is not equal to ±1, and SU(2) symmetry at its critical points. This is a significant distinction from the Z2-only Ising model.
• Integrability: It is possible to integrate the XXZ model. It is therefore possible to solve it precisely, frequently with the aid of sophisticated mathematical methods such as the Bethe Ansatz. Large symmetry algebras, like quantum groups, are necessary for this integrability to occur.
• Critical Region: In contrast to certain models that have discrete critical points, the XXZ model has a whole critical region where it is gapless between its quantum phase transitions.
• Quench Dynamics: Research indicates that, even when the critical point is the edge of a completely critical zone, the behavior of the system during a quantum phase transition is highly dependent on the critical point itself. Reversing the direction of evolution can result in a specular time-evolution profile in the adiabatic limit (extremely slow changes), a trait that may be related to the integrability of the model.

Utilizations and Applications

Several significant applications exist for the quantum Heisenberg model, including its XXZ variant:

• The study of critical points and phase transitions in magnetic materials is a crucial aspect of magnetic systems studies.
• Theoretical Framework: It provides an important and doable theoretical illustration for using sophisticated numerical techniques like density matrix renormalisation group (DMRG).
• Link to Other Models: It can be applied to solve other models of statistical mechanics, including the six-vertex model.
• The Heisenberg model can be projected onto the half-filled Hubbard model in condensed matter physics when there are strong repulsive interactions.
• Various integrable field theories, such as the non-relativistic nonlinear Schrödinger equation and relativistic models like the S² sigma model and sine-Gordon model, are described by the model’s limits as the lattice spacing approaches zero.
• The Heisenberg model is the source of concepts such as entanglement entropy and entanglement spectrum, which are used to describe quantum phase transitions and non-equilibrium dynamics.

Advantages

• It is said to be a more accurate model for magnetic materials than the Ising model since it takes into account the quantum nature of spins.
• Accurate Solvability: Its integrability makes it possible to obtain accurate analytical solutions through the use of methods such as the Bethe Ansatz, which is advantageous for both teaching and offering significant insights.
• Theoretical Tractability: highly correlated quantum systems can be explored using this “tractable theoretical example” of employing sophisticated numerical techniques like DMRG.
• Rich Physics: It contains a variety of physical characteristics, such as quantum phase transitions and various magnetic phases, which makes it a rich system for both theoretical and numerical research.

Disadvantages

• Complexity: Although analytical solutions (such as the Bethe Ansatz equations) are precisely solvable, they can be difficult to work with due to their complexity.
• Computational Demands: Because the Hilbert space grows exponentially, direct solutions are not feasible for dynamic simulations and bigger systems (beyond relatively tiny chains). This calls for computationally limited numerical approximation techniques such as DMRG.
• Finite-Size Effects: Results may not be instantly generalisable due to numerical simulations on finite-sized systems that exhibit behaviour that differs from that expected for infinite (thermodynamic limit) systems.
• Sensitivity to Parameters: Depending on the particular regime (e.g., paramagnetic vs. antiferromagnetic) and the quench parameter selection, the behaviour and computing needs can vary substantially.
• Particular Symmetries: Although its rich symmetry is advantageous for exact solutions, knowledge from one model may not be automatically applicable to others due to variations in symmetry groups as compared to other models (such as Ising).
• Dynamic Approaches’ Drawbacks: In time-dependent simulations, methods such as the Runge-Kutta approximation may not be able to maintain unitarity over extended periods of time, which could result in instability or a loss of accuracy.

An essential tool for comprehending quantum magnetism and phase transitions, the XXZ Heisenberg model strikes a balance between theoretical tractability and physical realism, making it a fundamental component of quantum many-body physics.

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