Unusual chemical bond and spectrum of beryllium dimer in ground X1Σg+X^1\Sigma_g^+ state

This review outlines the main results which show the dual nature of the chemical bond in diatomic beryllium molecule in the ground $X^1\Sigma_g^+$ state. It has been shown that the beryllium atoms are covalently bound at low-lying vibrational energy levels ({\nu}=0-4), while at higher ones ({\nu}=5-11) they are bound by van der Waals forces near the right turning points. High precision ab initio quantum calculations of Be$_2$ resulted in the development of the modified expanded Morse oscillator potential function which contains all twelve vibrational energy levels [A.V. Mitin, Chem. Phys. Lett. 682, 30 (2017)]. The dual nature of chemical bond in Be$_2$ is evidenced as a sharp corner on the attractive branch of the ground state potential curve. Moreover, it has been found that the Douglas-Kroll-Hess relativistic corrections also show a sharp corner when presented in dependence on the internuclear separation. The difference in energy between the extrapolated and calculated multi-reference configuration interaction energies in dependence on the internuclear separation also exhibits singular point in the same region. The other problems of ab initio quantum calculations of the beryllium dimer are also discussed. Calculated spectrum of vibrational-rotational bound states and new metastable states of the beryllium dimer in the ground state important for laser spectroscopy are presented. The vibration problem was solved for the modified expanded Morse oscillator potential function and for the potential function obtained with Slater-type orbitals [M. Lesiuk et al, Chem. Theory Comput. 15, 2470 (2019)]. The theoretical upper and lower estimates of the spectrum of vibrational-rotational bound states and the spectrum of rotational-vibrational metastable states with complex-valued energy eigenvalues and the scattering length in the beryllium dimer are presented

Toward the creation of terahertz graphene injection laser

We study the effect of population inversion associated with the electron and hole injection in graphene p-i-n structures at the room and slightly lower temperatures. It is assumed that the recombination and energy relaxation of electrons and holes is associated primarily with the interband and intraband processes assisted by optical phonons. The dependences of the electron-hole and optical phonon effective temperatures on the applied voltage, the current-voltage characteristics, and the frequency-dependent dynamic conductivity are calculated. In particular, we demonstrate that at low and moderate voltages the injection can lead to a pronounced cooling of the electron-hole plasma in the device i-section to the temperatures below the lattice temperature. However at higher voltages, the voltage dependences can be ambiguous exhibiting the S-shape. It is shown that the frequency-dependent dynamic conductivity can be negative in the terahertz range of freqiencies at certain values of the applied voltage. The electron-hole plasma cooling substantially reinforces the effect of negative dynamic conductivity and promotes the realization of terahertz lasing. On the other hand, the heating of optical phonon system can also be crucial affecting the realization of negative dynamic conductivity and terahertz lasing at the room temperatures.Comment: 10 pages, 7 figure

Magnetoresistance of a semiconducting magnetic wire with domain wall

We investigate theoretically the influence of the spin-orbit interaction of Rashba type on the magnetoresistance of a semiconducting ferromagnetic nanostructure with a laterally constrained domain wall. The domain wall is assumed sharp (on the scale of the Fermi wave length of the charge carriers). It is shown that the magnetoresistance in such a case can be considerably large, which is in a qualitative agreement with recent experimental observations. It is also shown that spin-orbit interaction may result in an increase of the magnetoresistance. The role of localization corrections is also briefly discussed.Comment: 5 pages, 2 figure

Tunneling of a Quantized Vortex: Roles of Pinning and Dissipation

We have performed a theoretical study of the effects of pinning potential and dissipation on vortex tunneling in superconductors. Analytical results are obtained in various limits relevant to experiment. In general we have found that pinning and dissipation tend to suppress the effect of the vortex velocity dependent part of the Magnus force on vortex tunneling.Comment: Latex, 12 page

Effect of self-consistent electric field on characteristics of graphene p-i-n tunneling transit-time diodes

We develop a device model for p-i-n tunneling transit-time diodes based on single- and multiple graphene layer structures operating at the reverse bias voltages. The model of the graphene tunneling transit-time diode (GTUNNETT) accounts for the features of the interband tunneling generation of electrons and holes and their ballistic transport in the device i-section, as well as the effect of the self-consistent electric field associated with the charges of propagating electrons and holes. Using the developed model, we calculate the dc current-voltage characteristics and the small-signal ac frequency-dependent admittance as functions of the GTUNNETT structural parameters, in particular, the number of graphene layers and the dielectric constant of the surrounding media. It is shown that the admittance real part can be negative in a certain frequency range. As revealed, if the i-section somewhat shorter than one micrometer, this range corresponds to the terahertz frequencies. Due to the effect of the self-consistent electric field, the behavior of the GTUNNETT admittance in the range of its negativity of its real part is rather sensitive to the relation between the number of graphene layers and dielectric constant. The obtained results demonstrate that GTUNNETTs with optimized structure can be used in efficient terahertz oscillators.Comment: 8 pages, 9 figure

EPR study of polycrystalline superconductors with YBa2Cu3O7 structure

Electron paramagnetic resonance (EPR) of Gd3+, Eu2+, and copper ions has been investigated in the high-Tc superconductor with YBa2Cu3O7-α structure. It has been established that the system is heterogeneous at 0.15≤δ≤0.5 and consists of metallic and dielectric regions. The former arises due to oxygen enrichment while the later due to oxygen deficiency. The integral of exchange interaction between Gd3+ localized moments and conduction electrons Jsf=0.016 eV has been determined from the normal state temperature dependence of Gd3+ EPR linewidth for metallic regions. Tc depression by gadolinium-localized moments for GdBa2Cu3O7-α was estimated to be ΔTc{reversed tilde equals}-2K. Anomalies in linewidth temperature dependence upon transition from the normal to the superconducting state have given information about the value and temperature behavior of the superconductor's energy gap. The model, which gives the opportunity to understand some peculiarities of the EPR signal for YBa2Cu3O7-α samples, is proposed in terms of several bottlenecked spinsubsystems: spin-liquid in CuO planes and Cu2+-O- and Cu2+-O2- fragments in CuO chains. © 1989 Plenum Publishing Corporation

Simultaneous control of magnetic topologies for reconfigurable vortex arrays

The topological spin textures in magnetic vortices in confined magnetic elements offer a platform for understanding the fundamental physics of nanoscale spin behavior and the potential of harnessing their unique spin structures for advanced magnetic technologies. For magnetic vortices to be practical, an effective reconfigurability of the two topologies of magnetic vortices, that is, the circularity and the polarity, is an essential prerequisite. The reconfiguration issue is highly relevant to the question of whether both circularity and polarity are reliably and efficiently controllable. In this work, we report the first direct observation of simultaneous control of both circularity and polarity by the sole application of an in-plane magnetic field to arrays of asymmetrically shaped permalloy disks. Our investigation demonstrates that a high degree of reliability for control of both topologies can be achieved by tailoring the geometry of the disk arrays. We also propose a new approach to control the vortex structures by manipulating the effect of the stray field on the dynamics of vortex creation. The current study is expected to facilitate complete and effective reconfiguration of magnetic vortex structures, thereby enhancing the prospects for technological applications of magnetic vortices.ope

Superstripes and complexity in high-temperature superconductors

While for many years the lattice, electronic and magnetic complexity of high-temperature superconductors (HTS) has been considered responsible for hindering the search of the mechanism of HTS now the complexity of HTS is proposed to be essential for the quantum mechanism raising the superconducting critical temperature. The complexity is shown by the lattice heterogeneous architecture: a) heterostructures at atomic limit; b) electronic heterogeneity: multiple components in the normal phase; c) superconducting heterogeneity: multiple superconducting gaps in different points of the real space and of the momentum space. The complex phase separation forms an unconventional granular superconductor in a landscape of nanoscale superconducting striped droplets which is called the "superstripes" scenario. The interplay and competition between magnetic orbital charge and lattice fluctuations seems to be essential for the quantum mechanism that suppresses thermal decoherence effects at an optimum inhomogeneity.Comment: 20 pages, 3 figures; J. Supercon. Nov. Mag. 201