Position Of Chemical Potential Research Articles

Half-metallicity in zigzag phosphorene nanoribbons with magnetic edges

Phosphorene nanoribbons with magnetic edges are potentially attractive for nanoelectronic and spintronic applications. Here, the attention is focused on electronic and magnetic properties of these materials. Using a self-consistent computational method it is shown that, in general, two types of configuration with spontaneous edge magnetization can be responsible for lifting the spin degeneracy, and consequently for the appearance of the half-metallic behavior. Namely, in addition to the well-known configuration with edge states ferromagnetically coupled along each edge and aligned in parallel between the two edges, there is another one (much less explored) with edge states ferromagnetically aligned on only one edge. The ground state magnetic configurations strongly depend on the chemical potential position (electron filling factor), and can be controlled by means of the external perpendicular gate voltage, and by changing the nanoribbon width. It is also shown that the magnetic edge configurations responsible for the half-metallicity can be stabilized with ferromagnetic substrates, due to the proximity effect.

Spin susceptibility of disordered gapped graphene systems

We calculate the spin susceptibility for the case of gapped graphene systems in the presence of disorder. The average single-particle density of states in gapped graphene with disorder was calculated, using the Born and the T-matrix approximations. The temperature dependence of the static spin susceptibility was analyzed. The influence of the chemical potential position and disorder is also discussed.

Дослідження бозонного спектра двовимірних оптичних ґраток зі структурою типу графену. Надплинна фаза

The energy spectrum of a system of Bose atoms in the superfluid phase in an optical lattice of the graphene type has been studied. The dispersion laws for the energy bands and the single particle spectral densities are calculated in the random phase approximation and in the framework of the hard-core boson formalism, and their changes at the transition from the normal phase to the superfluid one are described. As a result of this transformation, the number of subbands doubles. In the case of the subband energetic equivalence, the Dirac points in the spectrum survive, and their number becomes twice as much. When the subbands are energetically nonequivalent, the Dirac points are absent. The shape of spectral densities is shown to be sensitive to the changes in the temperature and the chemical potential position.

THE PSEUDOGAP IN MULTIBAND SUPERCONDUCTIVITY

The pseudogap (PG) excitation is analyzed as a natural event in multiband superconductivity. It corresponds to minimal quasiparticle excitation energy of an electron band not touched by the chemical potential. The critical points of the phase diagram are determined by vanishing conditions for normal state pseudogaps (NPG). For two bands (gapped or overlapping) these are positioned on edges of the superconducting dome. Theoretical background for a three-band system with two interband pairing channels is developed. There are three independent superconducting gaps (SCG). The PG is associated with the band component possessing a bare gap which can be quenched by doping. At low doping the PG and the SCG of another band component coexist. The critical point is not fixed in respect of the transition temperature (Tc) dome background. The depletion of the PG associated states is restored here. This effect can also be indirect by the participation of these states in determining the chemical potential position. At the critical point the PG looses its normal state contribution and continues as the SCG of the same band. Illustrative examples on the doping scale have been calculated.

Electric field effects in high-Tccuprates

Dependences of ${T}_{c}$ on hole concentration in ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7\ensuremath{-}y}$ (Y-123) and ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+x}$ (Bi-2212) cuprates have been calculated in the transverse electric field $\stackrel{\ensuremath{\rightarrow}}{E}$ in the framework of the two-band model. The phase space for the pair-transfer scattering between the overlapping bands is determined by the position of the chemical potential. The dependence of ${T}_{c}$ on the hole concentration ${n}_{h}$ has a maximum. The calculated ${T}_{c}{(n}_{h})$ agree in $\stackrel{\ensuremath{\rightarrow}}{E}=0$ with the experiment in Bi-2212 and Y-123 cuprates. The electric-field effects in cuprates are considered with the use of ${T}_{c}{(n}_{h}).$ In metal-insulator-superconductor structures, the electric field induces changes in the carrier concentration and in the position of chemical potential. The field-induced shifts of ${T}_{c}$ depend on the strength and polarity of the electric field. For maximum field effect, the superconductor has to be away from optimal doping, i.e., maximum ${T}_{c},$ ${\mathrm{dT}}_{c}{/dn}_{h}=0,$ and has to be closer to the maximum of the slope ${\mathrm{dT}}_{c}{/dn}_{h}.$ The asymmetry $|\ensuremath{\Delta}{T}_{c}(\stackrel{\ensuremath{\rightarrow}}{E})|\ensuremath{\ne}|\ensuremath{\Delta}{T}_{c}(\ensuremath{-}\stackrel{\ensuremath{\rightarrow}}{E})|$ changes with $\stackrel{\ensuremath{\rightarrow}}{E}.$ In agreement with experiment, the shifts $|\ensuremath{\Delta}{T}_{c}(\stackrel{\ensuremath{\rightarrow}}{E})|$ decrease when the film thickness increases. The field effects in both systems are similar, but in Bi-2212 they are stronger. The theory agrees qualitatively and, in some cases, quantitatively with experiment. The dependence of the ratio of surface and bulk conductivities on the film thickness $(d)$ allows us to obtain quantitative agreement between the calculated $d$ dependence of $|\ensuremath{\Delta}{T}_{c}(\stackrel{\ensuremath{\rightarrow}}{E})|$ and the experimental data in Y-123 and Bi-2212. The influence of the photodoping on ${T}_{c}$ of cuprates and photoinduced superconductivity are discussed.

Doping dependence of the density of states for CuO2 clusters in the Hubbard model.

One-particle densities of states for finite square clusters are calculated with the one- and three-band Hubbard model, within the generalized unrestricted Hartree-Fock approximation. Cluster-size and free- or periodic-boundary-conditions effects are studied. Changes induced by doping in both the density of states and the chemical potential position are presented. From the general accordance with experimental data and other theoretical results, we conclude that the unrestricted Hartree-Fock approach is valid. The prediction of the Hubbard model on the chemical potential behavior upon doping is discussed on the light of experimental findings.