arXiv subjects

H. T. Jonkman

Publications and source records attributed to H. T. Jonkman.

8 recordsLinked to original sources

arXiv2010

Large yield production of high mobility freely suspended graphene electronic devices on a PMGI based organic polymer

The recent observation of fractional quantum Hall effect in high mobility suspended graphene devices introduced a new direction in graphene physics, the field of electron-electron interaction dynamics. However, the technique used currently for the fabrication of such high mobility devices has several drawbacks. The most important is that the contact materials available for electronic devices are limited to only a few metals (Au, Pd, Pt, Cr and Nb) since only those are not attacked by the reactive acid (BHF) etching fabrication step. Here we show a new technique which leads to mechanically stable suspended high mobility graphene devices which is compatible with almost any type of contact material. The graphene devices prepared on a polydimethylglutarimide based organic resist show mobilities as high as 600.000 cm^2/Vs at an electron carrier density n = 5.0 10^9 cm^-2 at 77K. This technique paves the way towards complex suspended graphene based spintronic, superconducting and other types of devices.

cond-mat.mes-hall↗

arXiv2009

Linear scaling between momentum and spin scattering in graphene

Spin transport in graphene carries the potential of a long spin diffusion length at room temperature. However, extrinsic relaxation processes limit the current experimental values to 1-2 um. We present Hanle spin precession measurements in gated lateral spin valve devices in the low to high (up to 10^13 cm^-2) carrier density range of graphene. A linear scaling between the spin diffusion length and the diffusion coefficient is observed. We measure nearly identical spin- and charge diffusion coefficients indicating that electron-electron interactions are relatively weak and transport is limited by impurity potential scattering. When extrapolated to the maximum carrier mobilities of 2x10^5 cm^2/Vs, our results predict that a considerable increase in the spin diffusion length should be possible.

cond-mat.mes-hall↗

arXiv2009

Electronic spin transport in graphene field effect transistors

Spin transport experiments in graphene, a single layer of carbon atoms, indicate spin relaxation times that are significantly shorter than the theoretical predictions. We investigate experimentally whether these short spin relaxation times are due to extrinsic factors, such as spin relaxation caused by low impedance contacts, enhanced spin flip processes at the device edges or the presence of an aluminium oxide layer on top of graphene in some samples. Lateral spin valve devices using a field effect transistor geometry allowed for the investigation of the spin relaxation as a function of the charge density, going continuously from metallic hole to electron conduction (charge densities of $n\sim 10^{12}$cm$^{-2}$) via the Dirac charge neutrality point ($n \sim 0$). The results are quantitatively described by a one dimensional spin diffusion model where the spin relaxation via the contacts is taken into account. Spin valve experiments for various injector/detector separations and spin precession experiments reveal that the longitudinal (T$_1$) and the transversal (T$_2$) relaxation times are similar. The anisotropy of the spin relaxation times $τ_\parallel$ and $τ_\perp$, when the spins are injected parallel or perpendicular to the graphene plane, indicates that the effective spin orbit fields do not lie exclusively in the two dimensional graphene plane. Furthermore, the proportionality between the spin relaxation time and the momentum relaxation time indicates that the spin relaxation mechanism is of the Elliott-Yafet type. For carrier mobilities of 2-5$\times 10^3$ cm$2^$/Vs and for graphene flakes of 0.1-2 $μ$m in width, we found spin relaxation times of the order of 50-200 ps, times which appear not to be determined by the extrinsic factors mentioned above.

cond-mat.mes-hall↗

arXiv2008

Controlling the efficiency of spin injection into graphene by carrier drift

Electrical spin injection from ferromagnetic metals into graphene is hindered by the impedance mismatch between the two materials. This problem can be reduced by the introduction of a thin tunnel barrier at the interface. We present room temperature non-local spin valve measurements in cobalt/aluminum-oxide/graphene structures with an injection efficiency as high as 25%, where electrical contact is achieved through relatively transparent pinholes in the oxide. This value is further enhanced to 43% by applying a DC current bias on the injector electrodes, that causes carrier drift away from the contact. A reverse bias reduces the AC spin valve signal to zero or negative values. We introduce a model that quantitatively predicts the behavior of the spin accumulation in the graphene under such circumstances, showing a good agreement with our measurements.

cond-mat.mes-hall↗

arXiv2008

Electronic spin drift in graphene field effect transistors

We studied the drift of electron spins under an applied DC electric field in single layer graphene spin valves in a field effect transport geometry at room temperature. In the metallic conduction regime ($n \simeq 3.5 \times 10^{16}$ m$^{-2}$), for DC fields of about $\pm$70 kV/m applied between the spin injector and spin detector, the spin valve signals are increased/decreased, depending on the direction of the DC field and the carrier type, by as much as $\pm$50%. Sign reversal of the drift effect is observed when switching from hole to electron conduction. In the vicinity of the Dirac neutrality point the drift effect is strongly suppressed. The experiments are in quantitative agreement with a drift-diffusion model of spin transport.

cond-mat.mes-hall↗

arXiv2008

Anisotropic spin relaxation in graphene

Spin relaxation in graphene is investigated in electrical graphene spin valve devices in the non-local geometry. Ferromagnetic electrodes with in-plane magnetizations inject spins parallel to the graphene layer. They are subject to Hanle spin precession under a magnetic field $B$ applied perpendicular to the graphene layer. Fields above 1.5 T force the magnetization direction of the ferromagnetic contacts to align to the field, allowing injection of spins perpendicular to the graphene plane. A comparison of the spin signals at B = 0 and B = 2 T shows a 20 % decrease in spin relaxation time for spins perpendicular to the graphene layer compared to spins parallel to the layer. We analyze the results in terms of the different strengths of the spin orbit effective fields in the in-plane and out-of-plane directions.

cond-mat.mes-hall↗

arXiv2007

Kekulé Cells for Molecular Computation

The configurations of single and double bonds in polycyclic hydrocarbons are abstracted as Kekulé states of graphs. Sending a so-called soliton over an open channel between ports (external nodes) of the graph changes the Kekulé state and therewith the set of open channels in the graph. This switching behaviour is proposed as a basis for molecular computation. The proposal is highly speculative but may have tremendous impact. Kekulé states with the same boundary behaviour (port assignment) can be regarded as equivalent. This gives rise to the abstraction of Kekulé cells. The basic theory of Kekulé states and Kekulé cells is developed here, up to the classification of Kekulé cells with $\leq 4$ ports. To put the theory in context, we generalize Kekulé states to semi-Kekulé states, which form the solutions of a linear system of equations over the field of the bits 0 and 1. We briefly study so-called omniconjugated graphs, in which every port assignment of the right signature has a Kekulé state. Omniconjugated graphs may be useful as connectors between computational elements. We finally investigate some examples with potentially useful switching behaviour.

cs.OH↗

arXiv1995

Excitons in C_60 studied by Temperature Dependent Optical Second- Harmonic Generation

The electric dipole forbidden T_1g excitonic state of solid C_60 at omega=1.81 eV can be probed with a Second-Harmonic Generation (SHG) experiment. We show that the SHG line shape depends strongly on the degree of rotational order. We observe a splitting into two peaks below the rotational ordering phase transition temperature of 260 K. The origin of this splitting is discussed in terms of a possible Jahn-Teller effect, a possible Davydov splitting due to the four molecules per unit cell in the low temperature phase, and a mixing of the nearly degenerate T_1g and G_g free molecule states because of the lower symmetry in the solid. The exciton band structure is calculated with a charge transfer mediated propagation mechanism as suggested by Lof et al. and with one-electron (-hole) transfer integrals determined from band structure calculations. Comparison with our experimental SHG data leads to a reasonable agreement and shows that a mixing of T_1g and G_g states may explain the splitting at low temperature.

cond-mat↗