Superconducting properties of a boson-exchange model of doped graphene

We study the superconducting properties of a doped one-layer graphene by using a model in which the interparticle attraction is caused by a boson (phonon-like) excitations. We study the dependencies of the superconducting gap D and the mean-field critical temperature TcMF on the carrier density, attraction strength and the characteristic (Debye) bosonic frequency. In addition, we study the temperature-carrier density phase diagram of the model by taking into account the thermal fluctuations of the order parameter. We show that the fluctuations result in a significant suppression of TcMF, such that the real (Berezinskii– Kosterlitz–Thouless) critical temperature Tc is much lower than TcMF. The region Tc < T < TcMF is characterized by a finite density of states at the Fermi level (the pseudogap phase). We show that the width of the temperature interval of the pseudogap phase strongly depends on the model parameters—carrier concentration, attraction amplitude, and boson frequency

On temperature versus doping phase diagram of high critical temperature superconductors

An attempt to describe the bell-shape dependence of the critical temperature of high-Tc superconductors on the charge carrier density is made. It is proposed to explain its linear increase in the region of small densities (underdoped regime) by the role of the order parameter phase 2D fluctuations which become less at this density growth. The critical temperature suppression in the region of large carrier densities (overdoped regime) is connected with the appearance (because of doping) of an essential damping of long-wave bosons which, within the framework of the model proposed, define the mechanism of indirect inter-fermion attraction.Зроблена спроба опису дзвоноподібної залежності критичної температури високотемпературних надпровідників від густини носіїв заряду. Пропонується пояснити її лінійне зростання в області малих густин (недолегований режим) роллю 2D флуктуацій фази параметра порядку, які стають меншими при зростанні густини. Подавлення критичної температури в області великих густин носіїв (перелегований режим) пов’язаний з появою (завдяки легуванню) суттєвого затухання довгохвильових бозонів, які в рамках запропонованої моделі визначають механізм непрямого між-ферміонного притягання

Temperature–carrier-concentration phase diagram of a two-dimensional doped d-wave superconductor

The finite-temperature properties of a two-dimensional d-wave superconductor with the Lifshitz disorder, introduced by dopants, are studied. The doping dependence of the mean-field critical temperature Tc MF and of the superconducting critical temperature Tc defined by the Berezinskii—Kosterlitz—Thouless transition are calculated at different values of coupling, dopant potential, and intermediate boson energy. It is shown that superconductivity tends to disappear with increasing doping when the dopant potential is large enough, though the metallic properties of the system are preserved

Doping-dependent superconducting properties of two-dimensional metals with different types of interparticle coupling (Review Article)

The superconducting properties of a two-dimensional metallic system with arbitrary carrier density and both local and various types of the indirect boson-exchange attractive interaction in the cases of s- and d-wave pairing are analyzed and reviewed at T = 0. In particular, the possibility of a crossover from the Bose—Einstein condensation regime to Bardeen—Cooper—Schrieffer-like superconductivity with growing carrier density and changing coupling in the case of different pairing channels is discussed. Gaussian fluctuations of the order parameter are taken into account, and the carrier density dependence of the gap magnitude is studied. The role of the form of the interparticle attractive interaction in the physical behavior of the system is also considered

Group expansions for impurities in superconductors

A new method is proposed for practical calculation of the effective interaction between impurity scatterers in superconductors, based on algebraic properties of related Nambu matrices for Green functions. In particular, we show that the density of states within the s-wave gap can have a non-zero contribution (impossible either in Born and in T-matrix approximation) from non-magnetic impurities with concentration $c \ll 1$, beginning from $\sim c^{3}$ order.Comment: 5 pages, 1 figur

On the orientational relief of the intermolecular potential and the structure of domain walls in fullerite C₆₀

A simple planar model for an orientational ordering of threefold molecules on a triangular lattice modeling a close-packed (111) plane of fullerite is considered. The system has 3-sublattice ordered ground state which includes 3 different molecular orientations. There exist 6 kinds of orientational domains, which are related with a permutation or a mirror symmetry. Interdomain walls are found to be rather narrow. The model molecules have two-well orientational potential profiles, which are slightly effected by a presence of a straight domain wall. The reason is a stronger correlation between neighbor molecules in the triangular lattice versus the square lattice previously considered. A considerable reduction (up to one order) of the orientational interwell potential barrier is found in the core regions of essentially two-dimentional potential defects, such as a three-domain boundary or a kink in the domain wall. For ultimately uncorrelated nearest neighbors the height of the interwell barrier can be reduced even by a factor of 10²

Boundary friction on molecular lubricants: rolling mode?

A theoretical model is proposed for low-temperature friction between two smooth rigid solid surfaces separated by lubricant molecules, admitting their deformations and rotations. The appearance of different modes of energy dissipation (by «rocking» or «rolling» of lubricants) at slow relative displacement of the surfaces is shown to be accompanied by stick-and-slip features and reveals a nonmonotonic (mean) friction force vs external load

Impurity and vacancy effects in graphene

A Green function analysis has been developed for quasiparticle spectrum of a 2D graphene sheet in presence of different types of substitutional disorder, including vacancies. The anomalous character of impurity effects in this system is demonstrated, compared to those in well known doped semiconductors, and explained in terms of conical singularities in the band spectrum of pure graphene. The criteria for appearance of localized states on clusters of impurity scatterers and for qualitative restructuring of band spectrum are established and a possibility for a specific metal/insulator transition at presence of vacancies is indicated

Spectral function of graphene with short-range impurity centers

Spectral function of graphene with point substitutional defects is calculated for different impurity concentrations. It is demonstrated that features in the spectral function of graphene observed in ARPES experiments can be caused by the presence of a well-defined resonance state and are clearly pronounced at the impurity concentrations, which are of the order of the critical concentration for the impurity induced spectrum rearrangement

On the theory of equilibrium magnetoelastic domain structure in easy-plane antiferromagnet

Macroscopic magnetoelastic domain structure of the defectless layered antiferromagnet of CoCl₂-type with the "easy-plane" magnetic anisotropy is studied theoretically in the framework of phenomenological approach. In assumption of mobile domain walls, the finite-size effects are shown to result in the formation of a stable domain structure that changes reversibly under the action of the external magnetic field and can be treated as equilibrium. It is found that in antiferromagnets, where (in contrast to ferromagnets) long-range forces of magnetic origin are absent, the domain structure and its collective behavior are governed by elasticity. Field dependence of a domain structure, magnetostriction and low-frequency AFMR of poly- and monodomain samples are calculated, the external magnetic field being directed perpendicular to the main symmetry axis of the crystal. The results obtained are in qualitative agreement with the available experimental data