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Bulletin of the Lebedev Physics Institute

Spin state of ortho-parawater molecules in liquid

Vadim K Konyukhov

Published Apr 1, 2009• 2 citations
Publisher

Abstract

The state of the nuclear spin moments of two protons in the water molecule is considered when the latter is subjected to surrounding liquid and, for this reason, is an open quantum system. The form of the density operator of the mixed state for spin moments is postulated. Spin degrees of freedom of the molecule appear strongly protected against external influences and are not susceptible to external magnetic fields due to the zero nuclear magnetic moment.

DOI
10.3103/s106833560904006x

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The state of the nuclear spin moments of two protons in the water molecule is considered when the latter is subjected to surrounding liquid and, for this reason, is an open quantum system. The form of the density operator of the mixed state for spin moments is postulated. Spin degrees of freedom of the molecule appear strongly protected against external influences and are not susceptible to external magnetic fields due to the zero nuclear magnetic moment.
Spin dynamics and ortho-para conversion in H2_{2}2​O during the phase transition from gas to solid in external magnetic fields
The spin dynamics of water ice in the presence of external magnetic fields are investigated. The employed model is based on the approach introduced by Buntkowsky et al. [Z. Phys. Chem. 222, 1049 (2008)], which considers two nearest-neighbor water molecules and yields a four-spin system, as the abundant oxygen isotope has zero nuclear spin. The model is extended to include coupling to external magnetic fields, allowing us to analyze the interplay between magnetic dipole-dipole interactions and magnetic field coupling. Two types of configurations are examined: (i) static, homogeneous fields, corresponding to a time-independent interaction, and (ii) spatially varying sinusoidal fields in relative motion with the molecules, leading to a time-dependent interaction. All computations are performed within the density operator formalism. The ortho/para populations and the total spin projections are evaluated during the first tens of milliseconds following the gas-to-solid phase transition. For static homogeneous fields, we show that increasing field strength suppresses dipolar-induced depolarization. Assuming that all molecules are initially in the para state, we show that static homogeneous fields can drive the ortho population up to approximately 50%50\%50%, whereas suitably chosen sinusoidal-field configurations can increase it beyond 90%90\%90%. These results are relevant for schemes aiming to preserve or manipulate nuclear-spin polarization during deposition.
The possibility of appearance of spin polarized states in strongly asymmetric nuclear matter is analyzed within the framework of a Fermi liquid theory with the Skyrme effective interaction. The zero temperature dependence of the neutron and proton spin polarization parameters as functions of density is found for SLy4SLy4SLy4 and SLy5SLy5SLy5 effective forces. It is shown that at some critical density strongly asymmetric nuclear matter undergoes a phase transition to the state with the oppositely directed spins of neutrons and protons while the state with the same direction of spins does not appear. In comparison with neutron matter, even small admixture of protons strongly decreases the threshold density of spin instability. It is clarified that protons become totally polarized within a very narrow density domain while the density profile of the neutron spin polarization parameter is characterized by the appearance of long tails near the transition density.
The possibility of appearance of spin polarized states in symmetric and strongly asymmetric nuclear matter is analyzed within the framework of a Fermi liquid theory with the Skyrme effective interaction. The zero temperature dependence of the neutron and proton spin polarization parameters as functions of density is found for SkM∗^*∗, SGII (symmetric case) and SLy4, SLy5 (strongly asymmetric case) effective forces. By comparing free energy densities, it is shown that in symmetric nuclear matter ferromagnetic spin state (parallel orientation of neutron and proton spins) is more preferable than antiferromagnetic one (antiparallel orientation of spins). Strongly asymmetric nuclear matter undergoes at some critical density a phase transition to the state with the oppositely directed spins of neutrons and protons while the state with the same direction of spins does not appear. In comparison with neutron matter, even small admixture of protons strongly decreases the threshold density of spin instability. It is clarified that protons become totally polarized within a very narrow density domain while the density profile of the neutron spin polarization parameter is characterized by the appearance of long tails near the transition density.