arXiv subjects

R. C. Zamar

Publications and source records attributed to R. C. Zamar.

7 recordsLinked to original sources

arXiv2025

Evolution towards quasi-equilibrium in nematic liquid crystals studied through decoherence of multi-spin multiple-quantum coherences

New evidence is presented in favor of irreversible decoherence as the mechanism which leads an initial out-of-equilibrium state to quasi-equilibrium in nematic liquid crystals. The NMR experiment combines the Jeener-Broekaert sequence with reversal of the dipolar evolution and decoding of multiple-quantum coherences to allow visualizing the evolution of the multi-spin coherence spectra during the formation of the quasi-equilibrium states. We vary the reversion strategies and the preparation of initial states and observe that the spectra amplitude attenuate with the reversion time, and notably, that the decay is frequency selective. We interpret this effect as evidence of "eigen-selection", a signature of the occurrence of irreversible adiabatic decoherence, which indicates that the spin system in liquid crystal NMR experiments conforms an actual open quantum system.

quant-ph↗

arXiv2023

Quasi-equilibrium and quantum correlation in an open spin-pair system

Quasi-equilibrium states that can be prepared in solids through Nuclear Magnetic Resonance (NMR) techniques are out-of-equilibrium states that slowly relax towards thermodynamic equilibrium with the lattice. In this work, we use the quantum discord dynamics as a witness of the quantum correlation in this kind of state. The studied system is a dipole interacting spin pair whose initial state is prepared with the NMR Jeener-Broekaert pulse sequence, starting from equilibrium at high temperature and high external magnetic field. It then evolves as an open quantum system within two different dynamic scenarios: adiabatic decoherence driven by the coupling of the pairs to a common phonon field, described within a non-markovian approach; and spin-lattice relaxation represented by a markovian master equation, and driven by thermal fluctuations. In this way, the studied model is endowed with the dynamics of a realistic solid sample. The quantum discord rapidly increases during the preparation of the initial state, escalating several orders of magnitude compared with thermal equilibrium at room temperature. Despite the vanishing of coherences during decoherence, the quantum discord oscillates around this high value and undergoes a minor attenuation, holding the same order of magnitude as the initial state. Finally, the quantum discord dissipates within a time scale shorter than but comparable to spin-lattice relaxation.

quant-ph↗

arXiv2021

Adiabatic quantum decoherence in many non-interacting subsystems induced by the coupling with a common boson bath

This work addresses quantum adiabatic decoherence of many-body spin systems coupled with a boson field in the framework of open quantum systems theory. We generalize the traditional spin-boson model by considering a system-environment interaction Hamiltonian that represents a partition of non-interacting subsystems and highlights the collective correlation that appears exclusively due to the coupling with a common environment. Remarkably, this simple, exactly solvable model encompasses relevant aspects of a many-body open quantum system and features the subtle quantum effects that arise when the size scales up to a macroscopic level. We derive an analytical expression for the time dependence of the density matrix without assuming coarse-graining. The resulting decoherence function is eigen-selective and is a complex exponential whose exponent has a real part that introduces a decay similar to that in the spin-boson model. On the contrary, the imaginary part depends on the quantum numbers and geometry of the whole partition and does not reflect the system temperature. Motivated by decoherence in solid-state NMR we apply the theoretical results to a partition of dipole-coupled spin pairs in contact with a common phonon bath, using typical parameters of hydrated salts. The proposal allows estimating the decoherence time scale in terms of the system physical constants. As a significant novelty, the decoherence function phase depends on the eigenvalue distribution throughout the sample. It plays the leading role, overshadowing the mechanism associated with the bath thermal state. Finally, we apply the formalism to describe decoherence in the "magic echo" NMR reversal experiment. We find that the system-environment correlation explains the origin of irreversibility, and both the decoherence rate value and its dependence on the dipolar frequency, are remarkably similar to the experiment.

quant-ph↗

arXiv2015

Irreversible decoherence of dipole interacting nuclear spins coupled with a phonon bath

We report a first-principle theoretical study of the adiabatic decoherence undergone by a nuclear spin system in a solid, coupled to the phonon field through the dipolar interaction. The calculations are performed for a chain of weakly interacting 1/2-spin pairs, considered as an open quantum system in contact with a bosonic heat bath. By incorporating to the whole system Hamiltonian the fluctuations of the local dipolar energy produced by low frequency phonons, and assuming that this low energy fluctuations are adiabatic, we find that the spin dynamics can be described in closed form through a spin-boson model. The obtained results show that the coupling with the phonons destroy the spin coherence, and the efficiency of the process significantly depends on the complexity of the involved spin states. By using realistic values for the various parameters of the model, we conclude that this mechanism can be particularly efficient to degrade multi-spin coherences, when the number of `active' spins involved in a given coherence is high. In this way, we show that the spin coherence in the adiabatic regime can be noticeably affected by this mechanism.

cond-mat.stat-mech↗

arXiv2013

Quantum irreversible decoherence behaviour in open quantum systems with few degrees of freedom. Application to 1H NMR reversion experiments in nematic liquid crystals

An experimental study of NMR spin decoherence in nematic liquid crystals (LC) is presented. Decoherence dynamics can be put in evidence by means of refocusing experiments of the dipolar interactions. The experimental technique used in this work is based on the MREV8 pulse sequence. The aim of the work is to detect the main features of the Irreversible Quantum Decoherence (IQD) in LC, on the basis of the theory presented by the authors recently. The focus is laid on experimentally probing the eigen-selection process in the intermediate time scale, between quantum interference of a closed system and thermalization, as a signature of the IQD of the open quantum system, as well as on quantifying the effects of non-idealities as possible sources of signal decays which could mask the intrinsic IQD. In order to contrast experiment and theory, the theory was adapted to obtain the IQD function corresponding to the MREV8 reversion experiments. Non-idealities of the experimental setting are analysed in detail within this framework and their effects on the observed signal decay are numerically estimated. It is found that, though these non-idealities could in principle affect the evolution of the spin dynamics, their influence can be mitigated and they do not present the characteristic behavior of the IQD. As unique characteristic of the IQD, the experimental results clearly show the occurrence of eigen-selectivity in the intermediate timescale, in complete agreement with the theoretical predictions. We conclude that the eigen-selection effect is the fingerprint of IQD associated with a quantum open spin system in LC. Besides, these features of the results account for the quasi-equilibrium states of the spin system, which were observed previously in these mesophases, and lead to conclude that the quasi-equilibrium is a definite stage of the spin dynamics during its evolution towards equilibrium

physics.atm-clus↗

arXiv2013

Evidence of several dipolar quasi-invariants in Liquid Crystals

In a closed quantum system of N coupled spins with magnetic quantum number I, there are about (2I + 1)^N constants of motion. However, the possibility of observing such quasi-invariant (QI) states in solid-like spin systems in Nuclear Magnetic Resonance (NMR) is not a strictly exact prediction. The aim of this work is to provide experimental evidence of several QI, in the proton NMR of small spin clusters, besides those already known Zeeman, and dipolar orders (strong and weak). We explore the spin states prepared with the Jeener-Broekaert pulse sequence by analyzing the time-domain signals yielded by this sequence as a function of the preparation times, in a variety of dipolar networks. We observe that the signals can be explained with two dipolar QIs only within a range of short preparation times. At longer times the time-domain signals have an echo-like behaviour. We study their multiple quantum coherence content on a basis orthogonal to the z-basis and see that such states involve a significant number of correlated spins. Then we show that the whole preparation time-scale can only be reconstructed by assuming the occurrence of multiple QI which we isolate experimentally.

physics.atm-clus↗

arXiv2010

Quasi-equilibrium states in thermotropic liquid crystals studied by multiple quantum NMR

We study the nature of the quasiinvariants in nematic 5CB and measure their relaxation times by encoding the multiple quantum coherences of the states following the JB pulse pair on two orthogonal bases, Z and X. The experiments were also performed in powder adamantane at 301 K which is used as a reference compound having only one dipolar quasiinvariant. We show that the evolution of the quantum states during the build up of the quasi-equilibrium state in 5CB prepared under the S condition is similar to the case of adamantane and that their quasi-equilibrium density operators have the same tensor structure. In contrast, the second constant of motion, whose explicit operator form is not known, involves a richer composition of multiple quantum coherences on the X basis of even order, in consistency with the truncation inherent in its definition. We exploited the exclusive presence coherences 4, 6, 8, besides 0 and 2 under the W condition to measure the spin-lattice relaxation time T_{W} accurately, so avoiding experimental difficulties that usually impair dipolar order relaxation measurement such as Zeeman contamination at high fields, and also superposition of the different quasiinvariants. This procedure opens the possibility of measuring the spin-lattice relaxation of a quasiinvariant independent of the Zeeman and S reservoirs, so incorporating a new relaxation parameter useful for studying the complex molecular dynamics in mesophases. In fact, we report the first measurement of T_{W} in a liquid crystal at high magnetic fields. The comparison of the obtained value with the one corresponding to a lower field (16 MHz) points out that the relaxation of the W-order strongly depends on the intensity of the external magnetic field, similarly to the case of the S reservoir, indicating that the relaxation of the W-quasiinvariant is also governed by the cooperative molecular motions.

physics.atm-clus↗