We investigate a quasiadiabatic thermal process for preparing finite-temperature ensembles in the thermodynamic limit. The process gradually transforms a thermal ensemble of a noninteracting system into that of an interacting system of interest over a finite operation time, with the temperature controlled by parameters associated with the entropy of the initial state. We analyze this process in both nonintegrable and integrable spin chains with translational invariance. For the nonintegrable case, numerical simulations combined with a thermodynamic argument indicate that the thermal properties of local observables are accurately reproduced with a single parameter, although the operation time increases exponentially with precision. In contrast, for the integrable transverse-field Ising model, we analytically show that an extensive number of parameters tied to local conserved quantities is generally necessary, and the performance is affected by the presence of a quantum phase transition. These results clarify the potential and limitations of the quasiadiabatic thermal process for an ensemble preparation and highlight the role of integrability in determining its efficiency.

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• Eigenstate thermalization
• Quantum simulation
• Quantum spin models
• Exact diagonalization
• Finite-size scaling
• Integrable systems