V. P. Smilga

Analytical properties of muon polarization spectra in type-II superconductors and experimental data interpretation for mono- and polycrystalline HTSCs

Abstract The authors have solved Ginzburg-Landau equations for an intermediate magnetic field for type-II superconductors. The solution was obtained from the solution for a strong magnetic field via simple transformation. Hence, a set of results obtained at the case of the strong magnetic field is applicable to that of the intermediate field which is much easier to get experimentally and more informative. The authors analyzed properties of muon polarization spectra in mono- and polycrystalline HTSCs at the intermediate magnetic field. On the basis of analytical properties the authors suggested the way to interpret μSR experimental data.

Muonic study of type II superconductors

A theory of μSR method is developed for uniaxial anisotropic high-Tc superconductors. In two extreme cases ofHext‖c andHext⊥c analytical formulas are obtained which makes it possible to determine from the position of van Hove singularities in the Fourier-spectrum of muonium polarization, the type and parameters of the vortex lattice, such as λab (λc). In caseHext≲Hc2 we obtained the shape of the Fourier-spectrum in analytical form and the simple method of determining the Ginsburg-Landau parameter κ.Convenient expressions for numerical calculations are obtained for the arbitrary orientation of the external field, and an algorithm is provided to compute the mean fieldB, the vortex lattice parameters and the bulk field distribution in anisotropic superconductor. The Fourier spectrum of polarization based on these calculations can be used to independently check the validity of the high-Tc parameters determination for “appropriate” orientations.

Muon spin rotation in liquid noble gases

A new mechanism of slow muon depolarization in liquid noble gases is suggested. The muon spin polarization in [A-μ-A]+ complexes is calculated. The results allow one to observe [A-μ-A]+ experimentally and to measure hyperfine interaction constants.

Theory of muon spin depolarization in crystalline phases of 3He

As is obvious from the energetic point of view, positive muons must form the molecular ion ( He_2μ)+ in condensed phases of helium. A theory of positive muon spin depolarization in crystalline phase of 3He in this model is devised. The theory explains experimental results. It is shown that the abrupt temperature dependence of the muon spin depolarization rate at T < 2 K which is observed in experiments is explained by spin–phonon interaction. This interaction mechanism arises due to a modulation of the exchange interaction between host atoms of the 3He‐lattice.

The theory of muon spin depolarization in the crystalline phase of 3He

Abstract Peculiarities of a muon spin diamagnetic component behavior in condensed phases of 3He are considered. It is shown that the experimentally observed relaxation of a diamagnetic component in solid 3He could be explained theoretically if μ+ forms a molecular ion with a 3He atom. In this case the relaxation of the muon spin polarization is determined by a spin–phonon interaction at T⩽1 K . In the temperature range T⩾1 K a hyperfine interaction in a molecular ion is not effective and the relaxation is determined by a direct interaction of muon magnetic moment with fluctuating local magnetic fields in the crystal lattice.

Theory of muon spin depolarization in condensed phases of hydrogen isotopes

A theory of muon spin depolarization in the molecular ion ( H2μ)+(( D_2μ)) formed in a crystalline phase of hydrogen isotopes is presented. It is shown that the molecular ion ( H_2μ)+ has no time to thermalize during the muon lifetime, but after \tau\ll \tau_μ has time to transit to the lowest energy levels of the vibration‐rotation spectrum. The depolarization of the muon spin is determined by the interaction of the ion’s electric dipole moment with the lattice and by spin‐rotation interactions VLS in the ion. This mechanism is analogous to that of “muonium”, replacing the hyperfine interaction by VLS. The results can explain the experimental data and in particular the absence of a strong isotopic effect.

Excited states of the molecular ion (H2μ)

The rotation-vibration spectrum of (H2μ)+ is computed. Radiative lifetimes of the excited states are of order 10−4 s or more. These times can be considered infinite compared to the lifetime ofμ+. For the ion in a crystal the lifetimes are significantly decreased by interaction with polarized molecules of the lattice. Transition rates to the ground state are calculated for (H2μ)+ in a hydrogen crystal. The results make it possible to interpret the experimental data from μSR investigations of hydrogen, deuterium and hydrogen-deuterium mixtures.

Study of magnetic texture in ferromagnets by the muon method

The measurement of the muon polarization and the muon spin relaxation rate in a ferromagnet in zero external magnetic field allows the determination of such parameters as the initial magnetic texture and the magnetic inhomogeneity of the target material. The power of the μSR-method for a number of targets made of iron alloys is illustrated.

The μSR-technique as a new tool to study magnetic properties of nanocrystalline ferromagnetic metals

Abstract Possible applications of slow and low-energy muons to study important questions in ferromagnetism are considered, namely, appearance of a magnetic order in nanostructures. It is shown, that the μSR-technique gives possibilities to determine both magnetic texture and the influence of domain walls on some magnetic characteristics in nanostructures produced by different techniques. The μSR-examination of nanostructures enables the solution of important questions like vanishing ferromagnetic order when the sizes of grains are too small and, maybe, a study of a new type of phase transition.

Spin dynamics of the muon in muonium in normal metals

The question of the charge state of the proton (the positive muon) in metals is of fundamental importance for the theory of metal hydrides. The theory developed here permits determination of the charge state of μ+ in normal metals. The experimental possibilities of the observation of Mu atoms in metals at various strengths of the external magnetic field and various temperatures are analyzed.