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 crystalN-(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.

En este trabajo se estudian las características dinámicas del agua de hidratación de una mezcla normal de gangliósidos utilizando la técnica de resonancia magnética nuclear. Mediciones de tiempos de relajación espín-espín permiten identificar dos rnicroambientes bien diferenciados para el agua, cuyos tiempos de correlación rotacional están en el rango 10-9 -10-8 s y 10-11 -10-10 s a temperatura ambiente. Para una relación agua/gangliósido 200:1 (mol:mol) se determina que aproximadamente 30 moléculas de agua están fuertemente ligadas a cada gangliósido y que las propiedades dinámicas del resto están alteradas respecto de las del agua libre

Utilizamos la espectroscopía de resonancia paramagnética electrónica (RPE) para estudiar estructuras formadas por moléculas anfifílicas en solución acuosa. Analizamos el efecto del colesterol en estructuras constituidas por dos variedades de glicolípidos: los monosialogangliósidos y una mezcla natural de gangliósidos totales. En solución acuosa, ambas especies puras se agregan formando micelas. A partir de los valores de microviscosidad y micropolaridad obtenidos con un marcador de espín hidrosoluble, se pudo concluir que las mezclas monosialogangliósidos colesterol forman estructuras de vesícula y que las mezclas de gangliósidos totales / colesterol, continúan formando micelas. Con el empleo de marcadores de espín que se ubican en la región hidrofóbica de los agregados se encontró que el colesterol "endereza" las cadenas hidrocarbonadas de los glicolípidos, este efecto se correlaciona con un cambio en la curvatura de la superficie de los agregados.

A suitable NMR experiment in a one-dimensional dipolar coupled spin system allows one to reduce the natural many-body dynamics into effective one-body dynamics. We verify this in a polycrystalline sample of hydroxyapatite (HAp) by monitoring the excitation of NMR many-body superposition states: the multiple-quantum coherences. The observed effective one-dimensionality of HAp relies on the quasi 1d structure of the dipolar coupled network that, as we show here, is dynamically enhanced by the quantum Zeno effect. Decoherence is also probed through a Loschmidt echo experiment, where the time reversal is implemented on the double-quantum Hamiltonian, I_i,+I_j,+ + I_i,-I_j,-. We contrast the decoherence of adamantane, a standard 3d system, with that of HAp. While the first shows an abrupt Fermi-type decay, HAp presents a smooth exponential law.

In this work we present solid-state nuclear magnetic-resonance experiments to study decoherence in the dynamics of many-spin systems. We characterize the global Loschmidt echo and the distribution of multiple-quantum coherence orders during the evolution under dipolar and double-quantum Hamiltonians. To study an infinite H-1 system and a closed cluster of nuclear spins we use polycrystalline adamantane and the liquid crystal 5CB in the nematic mesophase, respectively, as model systems. The infinite or finite nature of the system is clearly manifested through spin counting measurements. Comparison between experimental and numerical results gives insights on the decoherence mechanisms in these systems. Contrastingly, the dominating mechanism in 5CB affects all coherence orders in the same way, while in adamantane higher orders of coherence decay faster than the lower ones.

Arbutin is known to suppress melanin production in murine B16 melanoma cells and inhibit phospholipase action. This encourages the possibility to stabilize it in lipid aggregates for its administration in medical applications. Thus, it was of interest to demonstrate that monomyristoylphosphatidylcholine (14:0 lysoPC) and arbutin may form association complexes. This was studied by Electron Microscopy (EM), 31P Nuclear Magnetic Resonance (31P NMR), Electronic Paramagnetic Resonance (EPR) and Fourier Transform Infrared Spectroscopy (FTIR). EM images show the formation of particles of c.a. 6 nm in diameter. For a 1:1 lysoPC-arbutin molar ratio 31P NMR shows a spectrum with a shoulder that resembles the axially symmetric spectrum characteristic of vesicles. The addition of La3+ ions to the arbutin-lysoPC complex allows one to distinguish two phosphorous populations. These results suggest that arbutin-lysoPC forms vesicles with bilayers stabilized in an interdigitated array. FTIR spectroscopy shows that arbutin interacts with the hydrated population of the carbonyl groups and with the phosphates through the formation of hydrogen bonds. It is interpreted that hydrophobic interactions among the phenol group of arbutin and the acyl chain of lysoPC are responsible for the decrease in acyl chain mobility observed at the 5th C level by EPR. A model proposing the formation of interdigitated bilayers of arbutin-lysoPC could explain the experimental results.

Recent experiments, [G.A. Álvarez, E.P. Danieli, P.R. Levstein, H.M. Pastawski, J. Chem. Phys. 124 (2006) 194507], have reported the observation of a quantum dynamical phase transition in the dynamics of a spin swapping gate. In order to explain this result from a microscopic perspective, we introduce a Hamiltonian model of a two level system with many-body interactions with an environment whose excitation dynamics is fully solved within the Keldysh formalism. If a particle starts in one of the states of the isolated system, the return probability oscillates with the Rabi frequency ω0. For weak interactions with the environment 1/τSE<2ω0, we find a slower oscillation whose amplitude decays with a rate 1/τϕ=1/(2τSE). However, beyond a finite critical interaction with the environment, 1/τSE>2ω0, the decay rate becomes 1/τϕ∝ω02τSE. The oscillation period diverges showing a quantum dynamical phase transition to a Quantum Zeno phase consistent with the experimental observations.

Puzzling anomalies previously observed in multipulse NMR experiments in natural abundance 29Si [A.E. Dementyev, D. Li, K. MacLean, and S.E. Barrett, Phys. Rev. B 68, 153302 (2003)] such as long-lived spin echoes and even-odd asymmetries, are also found in polycrystalline C60. Further experiments controlling the phases and tilting angles of the pulse trains, as well as analytical and numerical calculations allowed us to explain the origin of these anomalies. We prove that the observation of long magnetization tails requires two conditions: i) an rf field inhomogeneity able to produce different tilting angles in different sites of the sample and ii) the absence of spin diffusion (non-effective flip-flop interactions). The last requirement is easily satisfied in diluted dipolar solids, where the frequency differences between sites, caused by disorder or other sources, are usually at least one order of magnitude larger than the dipolar couplings. Both conditions lead to the generation of stimulated echoes in Carr-Purcell (CP) and Carr-Purcell-Meiboom-Gill (CPMG) pulse trains. We show, both experimentally and theoretically, that the stimulated echoes interfere constructively or destructively with the normal (Hahn) echoes depending on the alternation or not of the pi pulse phases in the CP and the CPMG sequences. Constructive interferences occur for the CP and CPMG sequences with and without phase alternation respectively, which are the cases where long magnetization tails are observed. Sequences with two, three and four pi pulses after the pi/2 pulse allow us to disentangle the contributions of the different echoes and show how the stimulated echoes originate the even-odd asymmetry observed in both 29Si and C60 polycrystalline samples.

We have modified the polarization echo (PE) sequence through the incorporation of Lee-Goldburg cross polarization steps to quench the H1-H1H dipolar dynamics. In this way, the C13 becomes an ideal local probe to inject and detect polarization in the proton system. This improvement made possible the observation of the local polarization P00(t) and polarization echoes in the interphenyl proton of the liquid crystal N-(4-methoxybenzylidene)-4-butylaniline. The decay of P00(t) was well fitted to an exponential law with a characteristic time τC≈310 μs. The hierarchy of the intramolecular dipolar couplings determines a dynamical bottleneck that justifies the use of the Fermi Golden Rule to obtain a spectral density consistent with the structural parameters. The time evolution of P00(t) was reversed by the PE sequence generating echoes at the time expected by the scaling of the dipolar Hamiltonian. This indicates that the reversible H1-H1 dipolar interaction is the main contribution to the local polarization decrease and that the exponential decay for P00(t) does not imply irreversibility. The attenuation of the echoes follows a Gaussian law with a characteristic time τφ≈527 μs. The shape and magnitude of the characteristic time of the PE decay suggest that it is dominated by the unperturbed homonuclear dipolar Hamiltonian. This means that τφ is an intrinsic property of the dipolar coupled network and not of other degrees of freedom. In this case, one cannot unambiguously identify the mechanism that produces the decoherence of the dipolar order. This is because even weak interactions are able to break the fragile multiple coherences originated on the dipolar evolution, hindering its reversal. Other schemes to investigate these underlying mechanisms are proposed.

We formulate the many-body spin dynamics at high temperature within the non-equilibrium Keldysh formalism. For the simplest XY interaction, analytical expressions in terms of the one particle solutions are obtained for linear and ring configurations. For small rings of even spin number, the group velocities of excitations depend on the parity of the total spin projection. This should enable a dynamical filtering of spin projections with a given parity i.e., a spin projection chromatography.

We analyze theoretically and experimentally the quantum dynamics of a three-spin-1/2 system during cross polarization (CP). Our analysis takes into account a Hamiltonian behavior for a carbon C13 coupled to two protons H1 while the coupling to a spin bath is treated in the fast fluctuation approximation. This model is applied to the methylene and biphenyl groups of the smectic and nematic phases of the liquid crystal 4-n-octyl-4′-cyanobiphenyl (8CB). Experimental data from standard CP, combined with our theoretical results, allow us to separate the homonuclear H1-H1 and heteronuclear H1-C13 residual dipolar couplings. These values are in good agreement with those obtained by using a combination of CP under Lee–Goldburg conditions and standard CP data. A well differentiated relaxation behavior among the two phases seems to indicate that while the extreme narrowing approximation is appropriate for the nematic phase, the description of the smectic phase requires consideration of the slow-motion limit.

The evaluation of spin excitation dynamics in finite 1-d systems of spins 1 2 with XY exchange interaction J acquired new interest because NMR experiments at high temperature (kBT⪢J) confirmed the predicted spin wave behavior of mesoscopic echoes. In this work, we use the Jordan–Wigner transformation to obtain the exact dynamics of inhomogeneous chains and rings where the evolution is reduced to one-body dynamics. For higher dimensions, the spin excitations manifest many-body effects that can be interpreted as a simple dynamics of non-interacting fermions plus a decoherent process.

A unique experimental tool to deepen into the Boltzmann–Loschmidt controversy is provided by the NMR polarization echoes (PE). These appear when a local spin excitation, evolving with a many-body “diffusive” spin dynamics, is reversed. The attenuation of the PEs represents a progresive failure of the quantum interferences to rebuild the local excitation. Our results indicate that, in the absence of detectable environmental interactions, the characteristic time of this attenuation is determined by the reversible dynamics itself, i.e., spin–spin interaction time. This supports the Boltzmann's hypothesis of molecular “chaos”.

We extract the exact on-resonance position and local dynamical information of the H1 network from a dramatic variation of the peak amplitude of C13 signals as a function of the carrier frequency of the decoupling field for H1. Its Lorentzian dependence is explained within a simple theory.

The effect of cholesterol (Chol) on two kinds of glycolipid assemblies, one composed of monosialogangliosides (GM1a) and the other formed by a natural mixture of bovine brain gangliosides (TBG), has been analysed. The experimental approach involves spin label electron paramagnetic resonance (EPR) in aqueous lipid dispersions. The employment of a hydrosoluble spin label and a ‘quencher’ of the EPR signal that is not able to permeate lipid interfaces, allowed us to conclude that GM1a/Chol mixtures give rise to vesicles at Chol proportions for which TBG/Chol mixtures form micelles. The use of different liposoluble spin labels reveals that cholesterol produces a straightening of the hydrocarbon chains in both lipid systems. In GM1a/Chol mixtures, this feature is more pronounced and it is coupled with a decrease in polarity at the chain ends.

The NMR technique allows to create a non-equilibrium local polarization and to detect its later evolution. Besides, it is possible to change the sign of the effective dipolar Hamiltonian and therefore retrace an apparently diffusive dynamics leading to a polarization echo. Our experiments in polycrystalline samples of (C5H5)Mn(CO)(3) and (C5H5)(2)Fe showed that those echoes attenuate as function of the time elapsed until the dynamics is reverted. In the former, a strong irreversible quadrupolar interaction (non inverted), produces an exponential decay. The latter has strong many-body interactions (reversible) whose quasi-chaotic dynamics has a local instability sensitive to the presence of small residual interactions (non-inverted). Thus an irreversible Gaussian decay appears. Numerical solutions of model systems agree with this hypothesis. To control the dynamical parameters we applied to structurally similar crystals: (C5H5)(2)Fe and (C5H5)(2)Co, a pulse sequence devised ad-hoc. It limits the complexity of the dynamical state for each t(R), slowing down its contribution to the attenuation and revealing the presence of an eventual underlying source of irreversibility. For (C5H5)(2)Co an exponential decay [attributable to the magnetism of the Co(II)] emerges when the dynamics is sufficiently reduced, while for (C5H5)(2)Fe the attenuation remains Gaussian. This shows that irreversibility is controlled by the reversible dynamics.

Self-assembled structures of phosphatidylcholine (PC), gangliosides and cholesterol (CHOL) have been studied by pulsed phosphorus-31 nuclear magnetic resonance (31P-NMR). The incorporation of gangliosides to multilamellar vesicles of PC destabilizes the structure leading to the formation of micelles and small aggregates. At intermediate ganglioside/phospholipid molar ratios, besides the well-characterized signals from lamellar structures and small aggregates, a signal that we assigned to cylindrical micelles is observed. Upon addition of CHOL, an increase of the bilayer signal at the expense of that assigned to the cylindrical micellar structures, indicates that the destabilization process is reverted. This effect is consistent with the reduction of the experimental molecular area found in mixed ganglioside/CHOL monolayers as compared with the ideal non-interacting case.

The reversal of the time evolution of the local polarization in an interacting spin system involves a sign change of the effective dipolar Hamiltonian which refocuses the “spin diffusion” process generating a polarization echo. Here, the attenuation of these echo amplitudes as a function of evolution time is presented for cymantrene and ferrocene polycrystalline samples, involving one and two five spin rings per molecule, respectively. We calculate the fraction of polarization which is not refocused because only the secular part of the dipolar Hamiltonian is inverted. The results indicate that, as long as the spin dynamics is restricted to a single ring, the non-inverted part of the Hamiltonian is not able by itself to explain the whole decay of the polarization echoes. A crossover from exponential (cymantrene) to Gaussian (ferrocene) attenuation is experimentally observed. This is attributed to an increase of the relative importance of the spin dynamics, as compared with irreversible interactions, which favors dynamical irreversibility.

An ingenious pulse sequence devised by Zhang, S., Meier, B. H., and Ernst, R. R., 1992, Phys. Rev. Lett., 69, 2149 reverses the time evolution (‘spin diffusion’) of the local polarization in a dipolar coupled 1H spin system. This refocusing originates a polarization echo, whose amplitude attenuates by increasing the time t R elapsed until the dynamics are reversed. Different functional attenuations are found for a set of dipolar coupled systems: ferrocene, (C5H5)2Fe, cymantrene, (C5H5)Mn(CO)3, and cobaltocene, (C5H5)2Co. To control a relevant variable involved in this attenuation a pulse sequence has been devised to progressively reduce the dipolar dynamics. Since it reduces the evolution of the polarization echo it is referred to as the REPE sequence. Two extreme behaviours were found while characterizing the materials. In systems with a strong source of relaxation and slow dynamics the attenuation follows an exponential law (cymantrene). In systems with strong dipolar dynamics the attenuation is mainly Gaussian. By the application of the REPE sequence the characteristic time of the Gaussian decay is increased until the presence of an underlying dissipative mechanism is revealed (cobaltocene). For ferrocene, however, the attenuation remains Gaussian within the experimental timescale. These two types of behaviour suggest that the many-body quantum dynamics present an extreme intrinsic instability which, in the presence of small perturbations, leads to the onset of irreversibility. This experimental conclusion is consistent with the tendencies displayed by the numerical solutions of model systems.

Fourier Transform EPR (FT-EPR) was used to study the formation and decay of free radicals produced by photoionization of phenothiazine (PTH) solubilized in aqueous SDS and Triton X-100 micellar solutions in the absence and presence of electron acceptors. CIDEP spectra produced by PTH photoionization in micellar solution differ from those found in homogeneous solution. The effect is attributed to changes in relative importance of single-photon, singlet excited state, and biphotonic, triplet excited state, photoionization. With quinone acceptors present in the bulk aqueous phase, photoionization of PTH in SDS, results in instantaneous formation of quinone anion radicals that carry the spin polarization of the precursor hydrated electrons. If the acceptor is anchored in the micelle, electron capture cannot compete with electron escape into the aqueous phase. Instead, anion radicals are formed primarily by reductive quenching of3PTH*. This process gives rise to a spectrum that is attributed to long-lived spin-correlated radical pairs, [PTH+…Q−].