Influence of metal contacts and charge inhomogeneity on transport properties of graphene near the neutrality point

There is an increasing amount of literature concerning electronic properties of graphene close to the neutrality point. Many experiments continue using the two-probe geometry or invasive contacts or do not control samples’ macroscopic homogeneity. We believe that it is helpful to point out some problems related to such measurements. By using experimental examples, we illustrate that the charge inhomogeneity induced by spurious chemical doping or metal contacts can lead to large systematic errors in assessing graphene’s transport properties and, in particular, its minimal conductivity. The problems are most severe in the case of two-probe measurements where the contact resistance is found to strongly vary as a function of gate voltage.     

Energy gaps at neutrality point in bilayer graphene in a magnetic field

Utilizing the Baym-Kadanoff formalism with the polarization function calculated in the random phase approximation, the dynamics of the ν = 0 quantum Hall state in bilayer graphene is analyzed. Two phases with nonzero energy gap, the ferromagnetic and layer asymmetric ones, are found. The phase diagram in the plane ($ \bar \Delta _0 $ \bar \Delta _0 , B), where $ \bar \Delta _0 $ \bar \Delta _0 is a top-bottom gates voltage imbalance, is described. It is shown that the energy gap scales linearly, ΔE ∼ 14B [T] K, with magnetic field.     

The charge neutrality point in covalent semiconductor surfaces

The electronic surface density of states in the entire energy spectrum is studied for a covalent semiconductor. A one-dimensional model has been firstly analyzed. The results are generalized for the (111)-surface, and it is shown that when an abrupt potential is placed between ions, the only surface band should be half occupied, in agreement with Shockley. However, this no longer holds for a more general situation.     

Magnetoresistance anomalies at the Curie point of some FeCr spinels

Magnetoresistance measurements on three sintered FeCr spinels up to H = 14.5 koe show clear peaks at the Curie points T_c. It is suggested that this effect is associated with Jahn-Teller lattice distortions and possibly with a non Néel spin configuration leading to large spin disorder at T_c.     

One-parameter scaling at the dirac point in graphene

We numerically calculate the conductivity sigma of an undoped graphene sheet (size L) in the limit of a vanishingly small lattice constant. We demonstrate one-parameter scaling for random impurity scattering and determine the scaling function beta(sigma)=dlnsigma/dlnL. Contrary to a recent prediction, the scaling flow has no fixed point (beta>0) for conductivities up to and beyond the symplectic metal-insulator transition. Instead, the data support an alternative scaling flow for which the conductivity at the Dirac point increases logarithmically with sample size in the absence of intervalley scattering--without reaching a scale-invariant limit.     

On the topological charge concentrated at a point

For SU(2) gauge fields over the 4-dimensional sphere with a finite number of points x₁, x₂, ..., and x_N removed, there are gauge transformations which modify the topological charge concentrated at x_j by adding n_j, where n₁, n₂, …, and n_N. are integers such that Σ^N_{j = 1}n_j = 0. However, the reduction modulo Z of the topological charge at a point is well defined, being given in terms of the secondary characteristic classes of Chern and Simons, except when the topological charge is indeterminate.     

Magnetoresistance measurements in Al-Ge alloys after neutron irradiation at 4.6 K

The influence of different structural defects, introduced by irradiation in a series of AlGe alloys is established by measuring the longitudinal magnetoresistance. The validity of Kohler's rule is absolute for both pure and alloyed samples that have been irradiated by neutrons at 4.6 K, while the rule breaks down after annealing at different temperatures.The origin of the breakdown of Kohler's rule appears to be the anisotropic scattering of the electrons on the clusters formed after annealing. This scattering depends on the size of the clusters which varies with the concentration of foreign atoms.     

Magnetoresistance and magnetic ordering fingerprints in hydrogenated graphene

Spin-dependent features in the conductivity of graphene, chemically modified by a random distribution of hydrogen adatoms, are explored theoretically. The spin effects are taken into account using a mean-field self-consistent Hubbard model derived from first-principles calculations. A Kubo transport methodology is used to compute the spin-dependent transport fingerprints of weakly hydrogenated graphene-based systems with realistic sizes. Conductivity responses are obtained for paramagnetic, antiferromagnetic, or ferromagnetic macroscopic states, constructed from the mean-field solutions obtained for small graphene supercells. Magnetoresistance signals up to ～7% are calculated for hydrogen densities around 0.25%. These theoretical results could serve as guidance for experimental observation of induced magnetism in graphene.     

Magnetoresistance of single-layer graphene under the conditions of short-range potential scattering

The magnetoresistance of single-layer graphene on a Si/SiO₂ substrate is measured in the temperature range of 2.5?C150 K. It is found that, at high enough temperatures and away from the Dirac point, the resistance varies as the square root of the magnetic field. This agrees with a recent theoretical calculation of the magnetoresistance for the case of charge-carrier scattering by defects characterized by a short-range potential.     

Elastic-constant measurements in solid argon at the melting point

The velocities of ultrasonic waves have been measured in solidified argon in the pressure range up to 6 kb (600 MPa). Shear wave velocities could be measured to temperatures very close to the melting point, such that both liquid and solid phases coexisted, $\text{(T?T}{}_{\mathrm{<mtext>m</mtext>}}\text{)}\text{T}{}_{\mathrm{<mtext>m</mtext>}}\text{<10}{}^{-5}$. The measurements show that neither the shear velocity tends to zero nor its derivative with temperature tends to -8 as T→T_m.     

Justification of the local space charge neutrality hypothesis in metal oxidation

The important local charge neutrality hypothesis in metal oxidation theory is shown numerically to be self-consistent with Poisson's equation and the coupled transport equations for diffusion of oppositely-charged ionic and electronic defect species deep within very thick growing oxides.     

Magnetoresistance effect in vertical NiFe/graphene/NiFe junctions

For convenient and efficient verification of the magnetoresistance effect in graphene spintronic devices, vertical magnetic junctions with monolayer graphene sandwiched between two NiFe electrodes are fabricated by a relatively simple way of transferring CVD graphene onto the bottom ferromagnetic stripes. The anisotropic magnetoresistance contribution is excluded by the experimental result of magnetoresistance (MR) ratio dependence on the magnetic field direction. The spin-dependent transport measurement reveals two distinct resistance states switching under an in-plane sweeping magnetic field. A magnetoresistance ratio of about 0.17 % is obtained at room temperature and it shows a typical monotonic downward trend with the bias current increasing. This bias dependence of MR further verifies that the spin transport signal in our device is not from the anisotropic magnetoresistance. Meanwhile, the I—V curve is found to manifest a linear behavior, which demonstrates the Ohmic contacts at the interface and the metallic transport characteristic of vertical graphene junction.     

Magnetoresistance measurements concerning the conduction band structure of cubic SrTiO3

The longitudinal and transverse magnetoresistance of cubic n-SrTiO₃ was measured with current in 〈001〉 and 〈111〉 directions at 120 K. The results support a model with a warped constant energy surface and are not consistent with a 〈100〉 many model of the lowest conduction band.     

Surface electron accumulation and the charge neutrality level in In2O3

High-resolution x-ray photoemission spectroscopy, infrared reflectivity and Hall effect measurements, combined with surface space-charge calculations, are used to show that electron accumulation occurs at the surface of undoped single-crystalline In2O3. From a combination of measurements performed on undoped and heavily Sn-doped samples, the charge neutrality level is shown to lie approximately 0.4 eV above the conduction band minimum in In2O3, explaining the electron accumulation at the surface of undoped material, the propensity for n-type conductivity, and the ease of n-type doping in In2O3, and hence its use as a transparent conducting oxide material.     

The charge neutrality level in w-AlxGa1−x N solid solutions

Energy positions of the charge electroneutrality level (CNL) and neutral vacancy levels of nitrogen are calculated for w-GaN, w-AlN, and w-Al _x Ga _1−x N versus solid-solution composition x in the virtual-crystal approximation. It is shown that within the whole range of the w-Al _x Ga _1−x N compositions, the CNL is located in the upper half of the band gap, which results in the n-type conductivity of this material upon exposure to high-energy radiation. __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 75–78, August, 2006.     

The surface potential of point charge and point dipole lattices

New expressions for the potential in the surface region of point charge and point dipole lattices reveal that a modified direct summation scheme can converge with comparable rapidity to those of the more complicated Ewald form.     

Nuclear Rms charge radii from relative electron scattering measurements at low energies

The nuclear rms charge radii measured by low energy electron scattering at Darmstadt are summarized. Improvements in the experimental equipment and method permitted a redetermination of the¹²C radius which yieldedR _m (¹²C)=2.462 ± 0.022fm. This value has been used to recalibrate the radii measured relative to¹²C.