Relation between gap and order in spin model

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SUMMARY

The discussion focuses on the relationship between spin gap and long-range order in the context of the XXZ model in an external field. It establishes that a spin gap can exist even when the system exhibits long-range order, particularly in the ferromagnetic phase. The participants also explore the order parameter for the XXZ chain model, questioning whether magnetization serves as the appropriate parameter to differentiate between the XY phase and the ferromagnetic phase, despite its value being 0 in disordered phases and 1 in fully ordered phases.

PREREQUISITES
  • Understanding of spin gap concepts in quantum mechanics.
  • Familiarity with the XXZ model in statistical mechanics.
  • Knowledge of order parameters in phase transitions.
  • Basic principles of magnetization in ferromagnetic systems.
NEXT STEPS
  • Research the implications of spin gaps in quantum phase transitions.
  • Study the XXZ model's behavior under varying external fields.
  • Examine the role of order parameters in distinguishing different magnetic phases.
  • Explore the relationship between magnetization and phase order in quantum systems.
USEFUL FOR

Physicists, particularly those specializing in condensed matter physics, quantum mechanics researchers, and students studying phase transitions and magnetic systems.

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what is the definition of spin gap?I may have ever heard a conclusion that spin gap will not exist while the system present long range order(right?),but the system of XXZ model in external field could stay in ferromagnetic phase,and have a long range order with a gapped excitation.why?
 
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One more question appended.What is the order parameter of XXZ chain model in external field to distinguish between XY phase and ferromagnetic phase? Magnetization? but order parameter has a value of 0 for disorder phase,value of 1 for complete order phase...
 

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