arXiv subjects

S. Adam

Publications and source records attributed to S. Adam.

32 records · Page 2Linked to original sources

arXiv2008

Tuning the effective fine structure constant in graphene: opposing effects of dielectric screening on short- and long-range potential scattering

We reduce the dimensionless interaction strength in graphene by adding a water overlayer in ultra-high vacuum, thereby increasing dielectric screening. The mobility limited by long-range impurity scattering is increased over 30 percent, due to the background dielectric constant enhancement leading to reduced interaction of electrons with charged impurities. However, the carrier-density-independent conductivity due to short range impurities is decreased by almost 40 percent, due to reduced screening of the impurity potential by conduction electrons. The minimum conductivity is nearly unchanged, due to canceling contributions from the electron/hole puddle density and long-range impurity mobility. Experimental data are compared with theoretical predictions with excellent agreement.

cond-mat.mtrl-sci↗

arXiv2008

Density inhomogeneity driven percolation metal-insulator transition and dimensional crossover in graphene nanoribbons

Transport in graphene nanoribbons with an energy gap in the spectrum is considered in the presence of random charged impurity centers. At low carrier density, we predict and establish that the system exhibits a density inhomogeneity driven two dimensional metal-insulator transition that is in the percolation universality class. For very narrow graphene nanoribbons (with widths smaller than the disorder induced length-scale), we predict that there should be a dimensional crossover to the 1D percolation universality class with observable signatures in the transport gap. In addition, there should be a crossover to the Boltzmann transport regime at high carrier densities. The measured conductivity exponent and the critical density are consistent with this percolation transition scenario.

cond-mat.mes-hall↗

arXiv2008

Transport in suspended graphene

Motivated by recent experiments on suspended graphene showing carrier mobilities as high as 200,000 cm^2/Vs, we theoretically calculate transport properties assuming Coulomb impurities as the dominant scattering mechanism. We argue that the substrate-free experiments done in the diffusive regime are consistent with our theory and verify many of our earlier predictions including (i) removal of the substrate will increase mobility since most of the charged impurities are in the substrate, (ii) the minimum conductivity is not universal, but depends on impurity concentration with cleaner samples having a higher minimum conductivity. We further argue that experiments on suspended graphene put strong constraints on the two parameters involved in our theory, namely, the charged impurity concentration n_imp and d, the typical distance of a charged impurity from the graphene sheet. The recent experiments on suspended graphene indicate a residual impurity density of 1-2 \times 10^{10} cm^{-2} which are presumably stuck to the graphene interface, compared to impurity densities of ~10^{12} cm^{-2} for graphene on SiO_2 substrate. Transport experiments can therefore be used as a spectroscopic tool to identify the properties of the remaining impurities in suspended graphene.

cond-mat.mtrl-sci↗

arXiv2007

Transport in chemically doped graphene in the presence of adsorbed molecules

Motivated by a recent experiment reporting on the possible application of graphene as sensors, we calculate transport properties of 2D graphene monolayers in the presence of adsorbed molecules. We find that the adsorbed molecules, acting as compensators that partially neutralize the random charged impurity centers in the substrate, enhance the graphene mobility without much change in the carrier density. We predict that subsequent field-effect measurements should preserve this higher mobility for both electrons and holes, but with a voltage induced electron-hole asymmetry that depends on whether the adsorbed molecule was an electron or hole donor in the compensation process. We also calculate the low density magnetoresistance and find good quantitative agreement with experimental results.

cond-mat.mes-hall↗

arXiv2007

Measurement of Scattering Rate and Minimum Conductivity in Graphene

The conductivity of graphene samples with various levels of disorder is investigated for a set of specimens with mobility in the range of $1-20\times10^3$ cm$^2$/V sec. Comparing the experimental data with the theoretical transport calculations based on charged impurity scattering, we estimate that the impurity concentration in the samples varies from $2-15\times 10^{11}$ cm$^{-2}$. In the low carrier density limit, the conductivity exhibits values in the range of $2-12e^2/h$, which can be related to the residual density induced by the inhomogeneous charge distribution in the samples. The shape of the conductivity curves indicates that high mobility samples contain some short range disorder whereas low mobility samples are dominated by long range scatterers.

cond-mat.mes-hall↗

arXiv2007

Carrier transport in 2D graphene layers

Carrier transport in gated 2D graphene monolayers is theoretically considered in the presence of scattering by random charged impurity centers with density $n_i$. Excellent quantitative agreement is obtained (for carrier density $n > 10^{12} \rm{cm}^{-2}$) with existing experimental data (Ref. \onlinecite{kn:novoselov2004, kn:novoselov2005, kn:zhang2005, kn:kim2006, kn:fuhrer2006}). The conductivity scales linearly with $n/n_i$ in the theory, and shows extremely weak temperature dependence. The experimentally observed asymmetry between electron and hole conductivities is explained by the asymmetry in the charged impurity configuration in the presence of the gate voltage, while the high-density saturation of conductivity for the highest mobility samples is explained as a crossover between the long-range and the point scattering dominated regimes. We argue that the experimentally observed saturation of conductivity at low density arises from the charged impurity induced inhomogeneity in the graphene carrier density which becomes severe for $n \lesssim n_i \sim 10^{12} \rm{cm}^{-2}$.

cond-mat.mes-hall↗

arXiv2007

Rigorous derivation of the mean field Green functions of the two-band Hubbard model of superconductivity

The Green function (GF) equation of motion technique for solving the effective two-band Hubbard model of high-T_c superconductivity in cuprates [N.M. Plakida et al., Phys. Rev. B, v. 51, 16599 (1995); JETP, v. 97, 331 (2003)] rests on the Hubbard operator (HO) algebra. We show that, if we take into account the invariance to translations and spin reversal, the HO algebra results in invariance properties of several specific correlation functions. The use of these properties allows rigorous derivation and simplification of the expressions of the frequency matrix (FM) and of the generalized mean field approximation (GMFA) Green functions (GFs) of the model. For the normal singlet hopping and anomalous exchange pairing correlation functions which enter the FM and GMFA-GFs, an approximation procedure based on the identification and elimination of exponentially small quantities is described. It secures the reduction of the correlation order to GMFA-GF expressions.

cond-mat.supr-con↗

arXiv2006

Mean field solutions to singlet hopping and superconducting pairing within a two-band Hubbard model

The mean field Green function solution of the two-band singlet-hole Hubbard model for high-$T\sb{c}$ superconductivity in cuprates (Plakida, N.M. et al., Phys. Rev. B51, 16599 (1995), JETP 97, 331 (2003)) involves expressions of higher order correlation functions describing respectively the singlet hopping and the superconducting pairing. Rigorous derivation of their values is reported based on the finding that specific invariant classes of polynomial Green functions in terms of the Wannier overlap coefficients $ν\sb{ij}$ exist.

cond-mat.supr-con↗

arXiv2006

The boundary layer problem in Bayesian adaptive quadrature

The boundary layer of a finite domain [a, b] covers mesoscopic lateral neighbourhoods, inside [a, b], of the endpoints a and b. The correct diagnostic of the integrand behaviour at a and b, based on its sampling inside the boundary layer, is the first from a set of hierarchically ordered criteria allowing a priori Bayesian inference on efficient mesh generation in automatic adaptive quadrature.

physics.comp-ph↗

arXiv2003

Reliability Conditions in Quadrature Algorithms

The detection of insufficiently resolved or ill-conditioned integrand structures is critical for the reliability assessment of the quadrature rule outputs. We discuss a method of analysis of the profile of the integrand at the quadrature knots which allows inferences approaching the theoretical 100% rate of success, under error estimate sharpening. The proposed procedure is of the highest interest for the solution of parametric integrals arising in complex physical models.

math.NA↗

arXiv2002

Magnetic-field dependence of energy levels in ultrasmall metal grains

We present a theory of mesoscopic fluctuations of g tensors and avoided crossing energies in a small metal grain. The model, based on random matrix theory, contains both the orbital and spin contributions to the g tensor. The two contributions can be experimentally separated for weak spin-orbit coupling while they merge in the strong coupling limit. For intermediate coupling, substantial correlations are found between g factors of neighboring levels.

cond-mat.mes-hall↗

arXiv2002

Exchange and spin-fluctuation superconducting pairing in cuprates

We propose a microscopical theory of superconductivity in CuO$_2$ layer within the effective two-band Hubbard model in the strong correlation limit. By applying a projection technique for the matrix Green function in terms of the Hubbard operators, the Dyson equation is derived. It is proved that in the mean-field approximation d-wave superconducting pairing mediated by the conventional exchange interaction occurs. Allowing for the self-energy corrections due to kinematic interaction, a spin-fluctuation d-wave pairing is also obtained. $T\sb{c}$ dependence on the hole concentration and $\bf k$-dependence of the gap function are derived. The results show that the exchange interaction (which stems from the interband hopping) prevails over the kinematic interaction (which stems from the intraband hopping).

cond-mat.supr-con↗

arXiv1999

Signature of Fermi surface jumps in positron spectroscopy data

A subtractionless method for solving Fermi surface sheets ({\tt FSS}), from measured $n$-axis-projected momentum distribution histograms by two-dimensional angular correlation of the positron-electron annihilation radiation ({\tt 2D-ACAR}) technique, is discussed. The window least squares statistical noise smoothing filter described in Adam {\sl et al.}, NIM A, {\bf 337} (1993) 188, is first refined such that the window free radial parameters ({\tt WRP}) are optimally adapted to the data. In an ideal single crystal, the specific jumps induced in the {\tt WRP} distribution by the existing Fermi surface jumps yield straightforward information on the resolved {\tt FSS}. In a real crystal, the smearing of the derived {\tt WRP} optimal values, which originates from positron annihilations with electrons at crystal imperfections, is ruled out by median smoothing of the obtained distribution, over symmetry defined stars of bins. The analysis of a gigacount {\tt 2D-ACAR} spectrum, measured on the archetypal high-$T_c$ compound $YBa\sb{2}Cu\sb{3}O\sb{7-δ}$ at room temperature, illustrates the method. Both electronic {\tt FSS}, the ridge along $ΓX$ direction and the pillbox centered at the $S$ point of the first Brillouin zone, are resolved.

physics.data-an↗

arXiv1999

Reliable operations on oscillatory functions

Approximate $p$-point Leibniz derivation formulas as well as interpolatory Simpson quadrature sums adapted to oscillatory functions are discussed. Both theoretical considerations and numerical evidence concerning the dependence of the discretization errors on the frequency parameter of the oscillatory functions show that the accuracy gain of the present formulas over those based on the exponential fitting approach [L. Ixaru, "Computer Physics Communications", 105 (1997) 1--19] is overwhelming.

math.NA↗