On Magnetic Models in Wavefunction Ensembles
In a wavefunction-only philosophy, thermodynamics must be recast in terms of an ensemble of wavefunctions. In this perspective we study how to construct Gibbs ensembles for magnetic quantum spin models. We show that with free boundary conditions and distinguishable “spins” there are no finite-temper...
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| Format: | Article |
| Language: | English |
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MDPI AG
2023-03-01
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| Series: | Entropy |
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| Online Access: | https://www.mdpi.com/1099-4300/25/4/564 |
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| author | Leonardo De Carlo William D. Wick |
| author_facet | Leonardo De Carlo William D. Wick |
| author_sort | Leonardo De Carlo |
| collection | DOAJ |
| description | In a wavefunction-only philosophy, thermodynamics must be recast in terms of an ensemble of wavefunctions. In this perspective we study how to construct Gibbs ensembles for magnetic quantum spin models. We show that with free boundary conditions and distinguishable “spins” there are no finite-temperature phase transitions because of high dimensionality of the phase space. Then we focus on the simplest case, namely the mean-field (Curie–Weiss) model, in order to discover whether phase transitions are even possible in this model class. This strategy at least diminishes the dimensionality of the problem. We found that, even assuming exchange symmetry in the wavefunctions, no finite-temperature phase transitions appear when the Hamiltonian is given by the usual energy expression of quantum mechanics (in this case the analytical argument is not totally satisfactory and we relied partly on a computer analysis). However, a variant model with additional “<i>wavefunction energy</i>” does have a phase transition to a magnetized state. (With respect to dynamics, which we do not consider here, wavefunction energy induces a non-linearity which nevertheless preserves norm and energy. This non-linearity becomes significant only at the macroscopic level.) The three results together suggest that magnetization in large wavefunction spin chains appears if and only if we consider indistinguishable particles and block macroscopic dispersion (i.e., macroscopic superpositions) by energy conservation. Our principle technique involves transforming the problem to one in probability theory, then applying results from large deviations, particularly the Gärtner–Ellis Theorem. Finally, we discuss Gibbs vs. Boltzmann/Einstein entropy in the choice of the quantum thermodynamic ensemble, as well as open problems. |
| first_indexed | 2024-03-11T05:03:12Z |
| format | Article |
| id | doaj.art-82a07a8d52084a0e9bdd1a45e8945058 |
| institution | Directory Open Access Journal |
| issn | 1099-4300 |
| language | English |
| last_indexed | 2024-03-11T05:03:12Z |
| publishDate | 2023-03-01 |
| publisher | MDPI AG |
| record_format | Article |
| series | Entropy |
| spelling | doaj.art-82a07a8d52084a0e9bdd1a45e89450582023-11-17T19:07:51ZengMDPI AGEntropy1099-43002023-03-0125456410.3390/e25040564On Magnetic Models in Wavefunction EnsemblesLeonardo De Carlo0William D. Wick1Scuola Normale Superiore, Piazza dei Cavalieri, 7, 56126 Pisa, ItalyIndependent Researcher, Seattle, WA 98119, USAIn a wavefunction-only philosophy, thermodynamics must be recast in terms of an ensemble of wavefunctions. In this perspective we study how to construct Gibbs ensembles for magnetic quantum spin models. We show that with free boundary conditions and distinguishable “spins” there are no finite-temperature phase transitions because of high dimensionality of the phase space. Then we focus on the simplest case, namely the mean-field (Curie–Weiss) model, in order to discover whether phase transitions are even possible in this model class. This strategy at least diminishes the dimensionality of the problem. We found that, even assuming exchange symmetry in the wavefunctions, no finite-temperature phase transitions appear when the Hamiltonian is given by the usual energy expression of quantum mechanics (in this case the analytical argument is not totally satisfactory and we relied partly on a computer analysis). However, a variant model with additional “<i>wavefunction energy</i>” does have a phase transition to a magnetized state. (With respect to dynamics, which we do not consider here, wavefunction energy induces a non-linearity which nevertheless preserves norm and energy. This non-linearity becomes significant only at the macroscopic level.) The three results together suggest that magnetization in large wavefunction spin chains appears if and only if we consider indistinguishable particles and block macroscopic dispersion (i.e., macroscopic superpositions) by energy conservation. Our principle technique involves transforming the problem to one in probability theory, then applying results from large deviations, particularly the Gärtner–Ellis Theorem. Finally, we discuss Gibbs vs. Boltzmann/Einstein entropy in the choice of the quantum thermodynamic ensemble, as well as open problems.https://www.mdpi.com/1099-4300/25/4/564quantum magnetismwavefunction ensembleslarge deviations |
| spellingShingle | Leonardo De Carlo William D. Wick On Magnetic Models in Wavefunction Ensembles Entropy quantum magnetism wavefunction ensembles large deviations |
| title | On Magnetic Models in Wavefunction Ensembles |
| title_full | On Magnetic Models in Wavefunction Ensembles |
| title_fullStr | On Magnetic Models in Wavefunction Ensembles |
| title_full_unstemmed | On Magnetic Models in Wavefunction Ensembles |
| title_short | On Magnetic Models in Wavefunction Ensembles |
| title_sort | on magnetic models in wavefunction ensembles |
| topic | quantum magnetism wavefunction ensembles large deviations |
| url | https://www.mdpi.com/1099-4300/25/4/564 |
| work_keys_str_mv | AT leonardodecarlo onmagneticmodelsinwavefunctionensembles AT williamdwick onmagneticmodelsinwavefunctionensembles |