arXiv subjects
Publications and source records attributed to M. Ya. Amusia.
The cross sections of ultra-soft x-ray bremsstrahlung in at electron scattering by Ar, Kr and Xe are theoretically calculated. The results are consistent with the absolute values of the differential cross sections measured by Gnatchenko et al [Phys. Rev. A 80, 022707 (2009)] for scattering electrons with an energy of 600 eV on these atoms.
The spatial distributions of electric charges forming the C60 shell have been analyzed with the Poisson equation. It has been shown that the modification of formulas for the rectangular-potential-well as a model potential of the C60 shell by means of addition to them the Coulomb-potential-like terms does not describe the monopole polarization of the shell by the electric charge Z in the center of the shell. The phenomenological potentials simulating the C60 shell potential should belong to a family of potentials with a non-flat bottom. In the paper some model potentials have been proposed and discussed. It has been shown that by varying the semithiknesses of these potential wells we could describe the various degrees of the monopole polarization of the C60 shell that due to shifting of collectivized electrons of the shell relative to the rigid positive carcass of C60 fullerene. Constructed here, the model potentials for the C60 shell can be used to describe the processes of elastic electron scattering by endohedral A+@C60 positive ions.
We generalize here the one-level consideration in our recent paper arXiv:1901.00411 [1] to the case when an electron collides with a potential that have any number of s bound states. We investigate peculiarities in the Wigner time delay behavior for slow electron elastic s-scattering by spherically symmetric square-potential well. We have considered potential wells, the variation of parameters of which (potential depth U and its radius R) lead to arising arbitrary number of s bound states. We demonstrate that while the time delay for potential wells with no discrete s-levels always has a positive value for small electron energies, it changes sign after level arising. We found that at the moments of arising in the well not only of the first but also following s-levels as well, the time delay as a function of U experiences instant jumps from a positive value to a negative one. The amplitudes of these jumps increases with decrease of the electron wave vector k. The times delay for potential well, the variation of the radius of which R leads to the appearance of discrete levels, also change sign at these critical radii.
We analyze using Poisson equation the spatial distributions of the positive charge of carbon atomic nuclei shell and negative charge of electron clouds forming the electrostatic potential of the C60 fullerene shell as a whole. We consider also the case when an extra positive charge appears inside C60 in course of e.g. photoionization of an endohedral A@C. We demonstrate that frequently used radial square-well potential U(r) simulating the C60 shell leads to nonphysical charge densities of the shell in both cases - without and with an extra positive charge inside. We conclude that the square well U(r) modified by adding a Coulomb-potential-like term does not describe the interior polarization of the shell by the electric charge located in the center of the C60 shell. We suggest another model potential, namely that of hyperbolic cosine shape with properly adjusted parameters that is able to describe the monopole polarization of C60 shell. As a concrete illustration, we have calculated the photoionization cross-sections of H@C60 taking into account the monopole polarization of the shell in the frame of suggested model. We demonstrate that proper account of this polarization does not change the photoionization cross-section.
We investigate specific features in the Wigner time behavior for slow electron elastic scattering by shallow potential wells. We considered two types of potentials wells, the small changes in the parameters of which lead to arising bound states in the well. It appeared that the time delay for attractive potential wells with no bound levels always has a positive value for small electron energies and changes sign after level arising in the well. At the moment of arising the times delay has a jump. The value of this jump is as more as less is the difference in the potential well depth from its critical value. The values of times delay strongly depend on geometrical sizes of potential wells.
We have calculated photoionization cross-section of endohedral atoms A@CN. We took into account the polarizability of the fullerene electron shell CN that modifies the incoming photon beam and the one-electron wave functions of the caged atom A. We employ simplified versions of both static and dynamic polarization. The properly modified one-electron wave functions became a starting point of the account of the multi-electron correlations in the frame of the random phase approximation with exchange (RPAE). We treat atomic and fullerenes polarization that act upon photoelectron similarly, substituting them by static polarization potential. The photon beam polarizes the fullerene. This effect is accounted for introducing a polarization factor. As concrete objects, we have considered Ar and Xe atoms inside fullerene C60. Inclusion of polarization prominently increases the photoionization cross-section mainly close to the threshold.
We prove in the present paper that a simple modeling of a complicated highly polarizable C$_{60}$ target by a rectangular (in a radial coordinate) square well potential in combination with the static polarization potential provides a viable approximation for a low-energy elastic electron scattering off this target. The proof is based on the results of the comparison of the calculated angle-differential elastic electron scattering cross section off C$_{60}$ versus corresponding experimental data.
Within the framework of a Dirac bubble potential model for the C60 fullerene shell, we calculated the time delay in slow-electron elastic scattering by C60. It appeared that the time of transmission of an electron wave packet through the Dirac bubble potential sphere that simulates a real potential of the C60 cage exceeds by more than an order of magnitude the transmission time via a single atomic core. Resonances in the time delays are due to the temporary trapping of electron into quasi-bound states before it leaves the interaction region.
We demonstrate that in the frame of the random phase approximation with the exchange, which preserves the validity of the precise well known dipole sum rule, the partial contributions for given subshells strongly deviates from the number of electrons in these subshells. This demonstrates strong effects of intershell interaction in atoms.
Dedicated to the centenary of the Ioffe Institute, the article contains the shortest review of scientific achievements of the theorists of the institute during this time. We concentrate mainly on research in the field of elementary particle physics, astrophysics, nuclear theory and atoms. To obtain very important scientific results became possible because outstanding theoreticians worked at this Institute. The high level of research persisted in spite of several mass moves of theorists - then to Kharkov, then to Moscow, then abroad. The author can testify to the atmosphere that reigned in the FTI in person, since he works at this institute since 1958. The article deals not only with research activity, but also with the famous physics schools and their outstanding cultural programs. We present also examples of extraordinary successful activity of theorists far beyond the field of their narrow specialization.
Based on experimental data and our theoretical analysis, we provide a strategy for unambiguous establishing of gapless quantum spin liquid state (QSL) in herbertsmithite and other materials. To clarify the nature of QSL, we recommend measurements of heat transport, low-energy inelastic neutron scattering and optical conductivity under the application of external magnetic field at low temperatures. We also suggest that artificially introduced inhomogeneity into $\rm ZnCu_{3}(OH)_6Cl_2$ can stabilize QSL, and serves as a test elucidating the contribution coming from impurities. We predict the results of these measurements in the case of gapless QSL.
We report on a new state of matter manifested by strongly correlated Fermi systems including various heavy-fermion (HF) metals, two-dimensional quantum liquids such as $\rm ^3He$ films, certain quasicrystals, and systems behaving as quantum spin liquids. Generically, these systems can be viewed as HF systems or HF compounds, in that they exhibit typical behavior of HF metals. At zero temperature, such systems can experience a so-called fermion-condensation quantum phase transition (FCQPT). Combining analytical considerations with arguments based entirely on experimental grounds we argue and demonstrate that the class of HF systems is characterized by universal scaling behavior of their thermodynamic, transport, and relaxation properties. That is, the quantum physics of different HF compounds is found to be universal, emerging irrespective of the individual details of their symmetries, interactions, and microscopic structure. This observed universal behavior reveals the existence of a new state of matter manifest in HF compounds. We propose a simple, realistic model to study the appearance of flat bands in two-dimensional ensembles of ultracold fermionic atoms, interacting with coherent resonant light. It is shown that signatures of these flat bands may be found in peculiarities in their thermodynamic and spectroscopic properties. We also show that the FCQPT, in generating flat bands and altering Fermi surface topology, is an essential progenitor of the exotic behavior of the overdoped high-temperature superconductors represented by $\rm La_{2-x}Sr_xCuO_4$, whose superconductivity differs from that predicted by the classical Bardeen-Cooper-Schrieffer theory. The theoretical results presented are in good agreement with recent experimental observations, closing the colossal gap between these empirical findings and Bardeen-Cooper-Schrieffer-like theories.
We concentrate here on photon absorption as well as electron and positron scattering upon endohedrals that consist of a fullerenes shell and an inner atom A. The aim is to understand the effect of fullerene electron shell in formation of corresponding cross-section. We consider the problem substituting the action of a complex multiatomic fullerenes shell by a combination of static pseudopotential and dynamic polarization potential. The electron correlations in the atom A are taken into account in the frame of the random phase approximation with exchange (RPAE). We demonstrate that the fullerenes shell strongly affects the cross-sections, bringing in a number of peculiarities, such as confinement resonances and giant-endohedral resonances and partial wave Ramsauer-type minima. Numerical data are obtained for endohedrals A@C60 and A@C60@C240, where A are noble gas atoms He, Ar and Xe.
We investigate here ground state properties of atoms, in which substitute fermions -- electrons by bosons, e.g. $π$ - meson. We perform some calculations in the frame of modified Hartree-Fock (HF) equation. The modification takes into account the symmetry, instead of anti-symmetry of the two-identical bosons wave function. The modified HF approach thus enhances (doubles) the effect of self-action for the boson case. Therefore, we accordingly modify the HF equations by eliminating from them the self-action terms artificially. We found the binding energy of $π$ - meson negative ions of $π$ - meson atoms and the number of extra bound $π$ - meson increases with the growth of nuclear charge Z. For e.g. for Xe even four times charged negative ion exists. As an example of a simple process with a pion atom, we consider photoionization that differs essentially from that for electron atoms. Namely, it is not monotonic decreasing from the threshold but has a prominent maximum above threshold instead. We study also elastic scattering of pions by $π$ - meson atoms.
We investigate positron scattering upon endohedrals and compare it with electron-endohedral scattering. We show that the polarization of the fullerene shell considerably alters the polarization potential of an atom, stuffed inside a fullerene. This essentially affects both the positron and electron elastic scattering phases as well as corresponding cross-sections. Of great importance is also the interaction between the incoming positron and the target electrons that leads to formation the virtual positronium. We illustrate the general trend by concrete examples of positron and electron scattering upon endohedrals He@C60 and Ar@C60, and compare it to scattering upon fullerene C60. To obtain the presented results, we have employed new simplified approaches that permit to incorporate the effect of fullerenes polarizability into the He@C60 and Ar@C60 polarization potential and to take into account the virtual positronium formation. Using these approaches, we obtained numeric results that show strong variations in shape and magnitudes of scattering phases and cross-sections due to effect of endohedral polarization and virtual positronium formation.
We investigate systems of three mutually interacting particles with masses of which the inner is much bigger than the intermediate and the latter is much bigger than the outer. Then the three-body problem reduces to the two-body scattering or structure of the light one in the field of the pseudo-nucleus formed by two others. We calculate analytically the properties of considered systems, such as the scattering cross-sections, hyperfine splitting, Auger decay of exited states and Lamb shits, presenting them as expansions in powers of the ratio of light to intermediate particle masses. This ratio is the small parameter of the studied problems.
A deeper insight into electron elastic scattering off endohedral fullerenes $A$@C$_{60}$ is provided. The study accounts for polarization of both the encapsulated atom $A$ and C$_{60}$ cage by an incident electron. It is carried out in the framework of the combination of both a model and the first-principle approximations. A core principle of the model is that the C$_{60}$ cage itself is presented by an attractive spherical potential of a certain inner radius, thickness, and depth. The main idea of the first-principle approximation is that the polarization of $A$@C$_{60}$ by an incident electron is accounted with the help of the Dyson equation for the self-energy part of the Green's function of an incident electron. Calculations are performed for, and comparison is made between the individual cases: (a) when $A$@C$_{60}$ is regarded as a static system, (b) when C$_{60}$ is a static cage, but the encapsulated atom is polarizable, (c) when both C$_{60}$ and $A$ are polarized simultaneously but independently of each other, and (d), as the most general scenario, when polarizabilities of C$_{60}$ and $A$ are coupled. Spectacular similarities and discrepancies between results of each of the exploited approximations are demonstrated. Revealed features of the overall significant impact of $A$@C$_{60}$ polarization by an incident electron on its elastic scattering off $A$@C$_{60}$ are demonstrated. Choosing Ne, Xe, and Ba as "probing" atoms, the dependence of $e + A@{\rm C_{60}}$ scattering on the size and polarizability of the encapsulated atom is unraveled.
Recently Han et al. have provided an analysis of the observed behavior of $\rm ZnCu_{3}(OH)_6Cl_2$ Herbertsmithite based on a separation of the contributions to its thermodynamic properties due to impurities from those due to the kagome lattice. The authors developed an impurity model to account for the experimental data and claimed that it is compatible with the presence of a small spin gap in the kagome layers. We argue that the model they advocate is problematic, conflicting with the intrinsic properties of $\rm ZnCu_{3}(OH)_6Cl_2$ as observed and explained in recent experimental and theoretical investigations. We show that the existence of the gap in the kagome layers is not in itself of a vital importance, for it does not govern the thermodynamic and transport properties of $\rm ZnCu_3(OH)_6Cl_2$. Measurements of heat transport in magnetic fields could clarify the quantum-critical features of spin-liquid physics of $\rm ZnCu_{3}(OH)_6Cl_2$.