title: Quantum dynamics of excitations and decoherence in many-spin systems detected with Loschmidt echoes: its relation to their spreading through the Hilbert space.
abstract: In this work, we overview time-reversal nuclear magnetic resonance (NMR) experiments in many-spin systems evolving under the dipolar Hamiltonian. The Loschmidt echo (LE) in NMR is the signal of excitations which, after evolving with a forward Hamiltonian, is recovered by means of a backward evolution. The presence of non-diagonal terms in the non-equilibrium density matrix of the many-body state is directly monitored experimentally by encoding the multiple quantum coherences. This enables a spin counting procedure, giving information on the spreading of an excitation through the Hilbert space and the formation of clusters of correlated spins. Two samples representing different spin systems with coupled networks were used in the experiments. Protons in polycrystalline ferrocene correspond to an 'infinite' network. By contrast, the liquid crystal N-(4-methoxybenzylidene)-4-butylaniline in the nematic mesophase represents a finite proton system with a hierarchical set of couplings. A close connection was established between the LE decay and the spin counting measurements, confirming the hypothesis that the complexity of the system is driven by the coherent dynamics.
authors.affiliation: Facultad de Matemática Astronomía y Física, Universidad Nacional de Córdoba, Ciudad Universitaria, Córdoba 5000, Argentina.
authors.forename: C M
authors.initials: CM
authors.lastname: Sánchez
authors.affiliation: Facultad de Matemática Astronomía y Física, Universidad Nacional de Córdoba, Ciudad Universitaria, Córdoba 5000, Argentina Instituto de Física Enrique Gaviola (IFEG-CONICET), Ciudad Universitaria, Córdoba 5000, Argentina.
authors.forename: P R
authors.initials: PR
authors.lastname: Levstein
authors.affiliation: Facultad de Matemática Astronomía y Física, Universidad Nacional de Córdoba, Ciudad Universitaria, Córdoba 5000, Argentina Instituto de Física Enrique Gaviola (IFEG-CONICET), Ciudad Universitaria, Córdoba 5000, Argentina.
authors.forename: L
authors.initials: L
authors.lastname: Buljubasich
authors.affiliation: Facultad de Matemática Astronomía y Física, Universidad Nacional de Córdoba, Ciudad Universitaria, Córdoba 5000, Argentina Instituto de Física Enrique Gaviola (IFEG-CONICET), Ciudad Universitaria, Córdoba 5000, Argentina horacio@famaf.unc.edu.ar.
authors.forename: H M
authors.initials: HM
authors.lastname: Pastawski
authors.affiliation: Facultad de Matemática Astronomía y Física, Universidad Nacional de Córdoba, Ciudad Universitaria, Córdoba 5000, Argentina Instituto de Física Enrique Gaviola (IFEG-CONICET), Ciudad Universitaria, Córdoba 5000, Argentina.
authors.forename: A K
authors.identifier: http://orcid.org/0000-0003-0187-9654
authors.initials: AK
authors.lastname: Chattah
keywords: Loschmidt echo; decoherence; many-body dynamics; nuclear magnetic resonance; quantum dynamics