V. N. Samovarov

Electron diffraction study of the structural transformations in free argon clusters

An electron diffraction technique is used to study the structure of clusters formed in an isentropically expanding supersonic argon jet. The formation of the hcp phase with increasing cluster size is reliably detected for the first time. Observations are made for mean cluster sizes N in the range from 1×103 to 8×104 atoms/cluster. An analysis of the shape of the diffraction peaks is carried out. It is found that in the range N⩽2×103 atoms/cluster, where the clusters are icosahedral, the profiles of the diffraction peaks are well approximated by a Lorentzian. For fcc clusters with N⩾3×103 atoms/cluster a better approximation is the standard Gaussian function. In the case N⩾1×104 atoms/cluster one observes peaks of the hcp phase in addition to the fcc peaks. The intensity of the hcp peaks increases with increasing cluster size, and for N≈8×104 atoms/cluster, the (110), (101), (103), and (202) peaks, characteristic only for the hcp phase, are clearly registered in addition to the fcc peaks. A possible m...

Manifestation of Hubbard and covalent correlations in the absorption spectra of YBa2Cu3O6+x films

The absorption spectra of single-crystal YBa2Cu3O6+x films with various doping levels in the range from x≈0.35 to x≈0.9 are measured in the energy region 0.3–3 eV. An analysis of the spectral composition of the absorption is made with allowance for intraband and interband transitions and the local dd transitions in the Cu2+ ion. It is concluded that the dd band (the transition dxy→dx2−y2 at 1.5 eV) reflects the enhancement of the covalent bonding (pd hybridization) upon metallization and that the spectral feature at ≈1.8 eV carries information about the contribution of electronic correlations, since it is sensitive to the opening of a spin gap in the insulator and to antiferromagnetic fluctuations in the metal. Although the covalent (≈1.5 eV) and correlation (≈1.8 eV) absorption peaks compete with each other, the coexistence of these bands in the metal supports the validity of a model based on the correlation polaron — a charge carrier which creates a region of covalent bonding in a Hubbard matrix of anti...

Electron diffraction study of two-component clusters Ar–Kr: Features of the nucleation, growth mechanisms, and structural states

The structure and growth processes of clusters formed in adiabatically expanding supersonic jets of gaseous mixtures of rare gases are investigated for the first time. Ar–Kr mixtures containing 0.1, 0.5, 2.5, and 20% Kr atoms are studied at temperatures and pressures of the gaseous mixtures at the nozzle entrance T0=100 and 120K and P0=0.15–2.5atm. The experiments give a number of results that shed light on the picture of cluster formation in Ar–Kr gaseous mixtures. It is established that the mechanisms of cluster formation depend substantially on the krypton impurity concentration and the temperature of the gaseous mixture at the nozzle entrance. The evolution of the character of the cluster formation, from homogeneous to heterogeneous, is traced. The intensification of rare-gas cluster growth by an impurity is revealed for the first time, and the concentration dependence of the transition from a process of formation of solid solutions to the formation of clusters containing only impurity krypton is traced.

Phase separation into pure components in mixed Ar-Xe clusters

Phase equilibrium in binary clusters of heavy inert gases has been studied using a combination of electron diffraction and optical methods. Mixed Ar-Xe clusters exhibit the phenomenon of phase separation into pure components with the formation of a sharp interface between the xenon core and argon shell, which is not observed in massive samples of this system.

Optical evidence for compatibility of antiferromagnetism and superconductivity in YBa2Cu3O6+x

The evolution of the spectral composition of the absorption in the 1.25–2.6 eV region for metallic films of YBa2Cu3O6+x with superconducting transition temperatures of Tc=51 and 74 K is measured as the films are cooled from 180 to 20 K. Particular attention is paid to the temperature effects in two absorption bands: the A band (≃1.8 eV), which reflects the appearance of holes dressed in antiferromagnetic (AFM) fluctuations, and the (A+J) band (≃2.15 eV), which reflects an additional (magnon) excitation of the short-range AFM order. It is found that the changes of these bands begin in the normal phase at T<T* in the temperature region corresponding to the opening of the pseudogap state, and the (A+J) magnon band arises in the pseudogap state even in the case when it is absent at room temperatures. At the superconducting transition the parameters of the bands stop changing, and the (A+J) magnon band is preserved in the superconducting state. The results are interpreted as evidence of a magnetic nature of th...

Experimental verification of the Hagena relation for large clusters formed in a conical nozzle

The dependence of the size of inert gas clusters formed in a conical nozzle on the gas pressure and temperature at the nozzle input was studied for free clusters with a total number of atoms above 103. An analysis of the experimental data allowed a relationship between the number of atoms in a cluster and the Hagena parameter in the case of large clusters to be checked for the first time and the value of this parameter to be refined. It is shown that the established relations are also quite well obeyed in the presence of a small amount of impurities in supersonic jets of inert gases.

Electron diffraction analysis of the FCC–HCP transition in argon clusters with a change in cluster size

A transition from FCC to mixed FCC–HCP structure as a function of the cluster size is followed for the first time in free crystals formed in supersonic argon streams. It is established that FCC reflections appear and their intensity sharply increases for linear size of the aggregations of about 110A. As cluster size increases above this value, the FCC reflections become weaker. The transition is preceded by a special structural state of the clusters where there is a substantial number of randomly arranged close-packed atomic layers. Such a transition has been observed previously in bulk samples of solid argon under compression to pressure of approximately 50GPa. In clusters where such a high pressure is not reached, the appearance of a disordered close-packed structure followed by the appearance of FCC domains is probably due to the mechanisms of aggregation growth in supersonic streams.

Structure of mixed clusters formed in supersonic jets of Ar–N2 gas mixtures

An electron diffraction study is done on substrate-free clusters formed from nanodrops in expanding supersonic jets of Ar–N2 gas mixtures. The mean characteristic linear dimension of the clusters varies from 20to200A, and the temperature of the clusters is 40K. The content of the gas mixtures spanned the entire concentration interval. These studies revealed that the mixed Ar–N2 clusters are enriched with Ar, and a method for determining the absolute concentrations of the cluster constituents is proposed. The structure of the clusters in relation to their size and the component concentrations is established for the first time. The diagram of the structural states is compared with published data for bulk samples grown from the liquid. A transformation from the fcc to the hcp structure on increasing cluster size is revealed, and also a transformation from the hcp to an hcp+fcc structure with increasing argon concentration; this is not observed in the bulk samples. In clusters the mutual transformations of th...

Size-induced structural transformations in argon clusters with a fivefold symmetry axis

An electron-diffraction study of the structure of free clusters of Ar is carried out in the interval of mean sizes N≈600–1500 atoms/cluster. The existence of an amorphous (polyicosahedral) phase in large rare gas clusters with N≈600–800 atoms/cluster is observed for the first time. It is shown that with increasing number of atoms in a cluster a transition to a multilayer icosahedral (ordered) phase occurs in a rather narrow size interval.

Antiferromagnetic correlations in superconducting YBa2Cu3O6+x samples from optical absorption data; comparison with the results of neutron and muon experiments

Narrow-band spectral features demonstrating a high sensitivity to the development of the pseudogap state are detected in metallic films of YBa2Cu3O6+x with Tc≈51 and 74 K. Attention is focused on the temperature behavior of the exciton–bimagnon band A+3J (≈2.15 eV) and the exciton–two-magnon band A+4J (≈2.28 eV); these bands arise as a result of phase separation into insulating and metallic regions. By comparing the optical results with published data on the temperature behavior of the muon depolarization rate and the integrated intensity of the (π,π) magnetic resonance it is shown for the first time that all three independent techniques give the same observed temperature dependence f(T/Tc) of measured quantities both in the normal and superconducting states. The established correlation of the optical, neutron, and muon data is analyzed from the standpoint of the formation of stripe ordering and the compatibility of antiferromagnetic order and superconductivity.