Discerning chaos in quantum systems is an important problem as the usual route of Lyapunov exponents in classical systems is not straightforward in quantum systems. A standard route is the comparison of statistics derived from model physical systems to those from random matrix theory (RMT) ensembles, of which the most popular is the nearest-neighbor-spacing distribution, which almost always shows good agreement with chaotic quantum systems. However, even in these cases, the long-range statistics (like number variance and spectral rigidity), which are also more difficult to calculate, often show disagreements with RMT. As such, a more stringent test for chaos in quantum systems, via an analysis of intermediate-range statistics, is needed, which will additionally assess the extent of agreement with RMT universality. In this paper, we deduce the effective level-repulsion parameters and the corresponding Wigner–surmise-like results of the next-nearest-neighbor-spacing distribution (nNNSD) for integrable systems (semi-Poissonian statistics) as well as the three classical quantum chaotic Wigner–Dyson regimes, by stringent comparisons to numerical RMT models and benchmarking against our exact analytical results for Gaussian matrix models, along with a semi-analytical form for the nNNSD in the orthogonal-to-unitary symmetry crossover. To illustrate the robustness of these RMT based results, we test these predictions against the nNNSD obtained from quantum chaotic models as well as disordered lattice spin models. This reinforces the Bohigas–Giannoni–Schmit and the Berry–Tabor conjectures, extending the associated universality to longer-range statistics. In passing, we also highlight the equivalence of nNNSD in the apparently distinct orthogonal-to-unitary and diluted-symplectic-to-unitary crossovers.
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Beyond nearest-neighbor universality of spectral fluctuations in quantum chaotic and complex many-body systems
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April 2025
Research Article|
April 10 2025
Beyond nearest-neighbor universality of spectral fluctuations in quantum chaotic and complex many-body systems
Debojyoti Kundu
;
Debojyoti Kundu
(Conceptualization, Data curation, Investigation, Writing – original draft)
Department of Physics, Shiv Nadar Institution of Eminence (SNIoE)
, Gautam Buddha Nagar, Uttar Pradesh 201314, India
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Santosh Kumar
;
Santosh Kumar
c)
(Conceptualization, Investigation, Supervision, Writing – review & editing)
Department of Physics, Shiv Nadar Institution of Eminence (SNIoE)
, Gautam Buddha Nagar, Uttar Pradesh 201314, India
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Subhra Sen Gupta
Subhra Sen Gupta
d)
(Conceptualization, Investigation, Supervision, Writing – review & editing)
Department of Physics, Shiv Nadar Institution of Eminence (SNIoE)
, Gautam Buddha Nagar, Uttar Pradesh 201314, India
d)Author to whom correspondence should be addressed: [email protected]
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Debojyoti Kundu
Santosh Kumar
c)
Subhra Sen Gupta
d)
Department of Physics, Shiv Nadar Institution of Eminence (SNIoE)
, Gautam Buddha Nagar, Uttar Pradesh 201314, India
d)Author to whom correspondence should be addressed: [email protected]
b)
Present address: International Centre for Theoretical Sciences, Tata Institute of Fundamental Research, Bengaluru 560089, India.
c)
Deceased.
Chaos 35, 043122 (2025)
Article history
Received:
August 21 2024
Accepted:
March 15 2025
Citation
Debojyoti Kundu, Santosh Kumar, Subhra Sen Gupta; Beyond nearest-neighbor universality of spectral fluctuations in quantum chaotic and complex many-body systems. Chaos 1 April 2025; 35 (4): 043122. https://doi.org/10.1063/5.0234333
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