Ballistic pomt contacts, defined in the two-dimensional electron gas of a GaAs-AlGaAs heterostructure, have been studied in zero magnetic field The conductance changes in quantized Steps of e 2 /nh when the width, controlled by a gate on top of the heterojunction, is vaned Up to sixteen Steps are observed when the pomt contact is widened from 0 to 360 nm An explanation is proposed, which assumes quantized transverse momentum in the pomt-contact region As a result of the high mobihty attamable in the twodimensional electron gas (2DEG) in GaAs-AlGaAs heterostructures it is now becoming feasible to study ballistic transport in small devices '" 6 In metals ideal tools for such studies are constnctions havng a width W and length L much smaller than the mean free path l e These are known äs Sharvin pomt contacts 7 Because of the ballistic transport through these constnctions, the resistance is determmed by the pomt-contact geometry only Point contacts have been used extensively for the study of elastic and melastic electron scattermg With use of biased pomt contacts, electrons can be mjected mto metals at energies above the Fermi level This allows the study of the energy dependence of the scattermg mechamsms 8 With the use of a geometry containmg two pomt contacts, with Separation smaller than l e , electrons mjected by a pomt contact can be focused mto the other contact, by the application of a magnetic field This technique (transverse electron focusmg) has been applied to the detailed study of Fermi surfaces 9 In this Letter we report the first expenmental study of the resistance of ballistic pomt contacts m the 2DEG of high-mobihty GaAs-AlGaAs heterostructures The smgle-pomt contacts discussed m this paper are part of a double-pomt-contact device The results of transverse electron focusmg m these devices will be published elsewhere '° The pomt contacts are dehned by electrostatic depletion of the 2DEG underneath a gate This method, which has been used by several authors for the study of l D conduction,' 1 offers the possibility to control the width of the pomt contact by the gate voltage Control of the width is not feasible in metal pomt contacts The classical expression for the conductance of a pomt contact m two dimensions (see below) is G=(e 2 /nh)k Y W/n(1) in which kf is the Fermi wave vector and W is the width of the contact This expression is vahd if l e » W and the Fermi wavelength λρ<ίί W The first condition is satisfied in our devices, which have a maximum width W mm «= 250 nm and l e =8 5 μηι The second condition should also hold when the devices have the maximum width We expect quantum effects to become important when the width becomes comparable to λρ, which is 42 nm m our devices In this way we are able to study the transition from classical to quantum ballistic transport through the pomt contactThe pomt contacts are made on high-mobility molecular-beam-epitaxy-grown GaAs-AlGaAs heterostructures The electron density of the matenal is 3 56xl0 15 /m 2 and the mobihty 85 m 2 /V s (at 0 6 K) These values ...

The transport of spin information has been studied in various materials, such as metals 1 , semiconductors 2 and graphene 3 . In these materials, spin is transported by diffusion of conduction electrons 4 . Here we study the diffusion and relaxation of spin in a magnetic insulator, where the large bandgap prohibits the motion of electrons. Spin can still be transported, however, through the diffusion of non-equilibrium magnons, the quanta of spin wave excitations in magnetically ordered materials. Here we show experimentally that these magnons can be excited and detected fully electrically 5,6 in linear response, and can transport spin angular momentum through the magnetic insulator yttrium iron garnet (YIG) over distances as large as 40 μm. We identify two transport regimes: the diffusion limited regime for distances shorter than the magnon relaxation length, and the relaxation limited regime for larger distances. With a model similar to the diffusion-relaxation model for electron spin transport in (semi)conducting materials, we extract the magnon relaxation length = . ± . μm in a 200 nm thin YIG film at room temperature.

Transverse electron focusing in a two-dimensional electron gas is mvestigated expenmentally and theoretically for the flrst time. A split Schottky gate on top of a GaAs-Al x Ga,_^As heterostructure defines two point contacts of variable width, which are used äs mjector and collector of ballistic electrons As evidenced by their quantized conductance, these are quantum point contacts with a width comparable to the Fermi wavelength At low magnetic flelds, skipping orbits at the electrongas boundary are directly observed, thereby establishing that boundary scattenng is highly specular Large additional oscillatory structure in the focusing spectra is observed at low temperatures and for small pomt-contact size This new phenomenon is mterpreted in terms of mterference of coherently excited magnetic edge states m a two-dimensional electron gas A theory for this effect is given, and the relation with nonlocal resistance measurements in quantum ballistic transport is discussed It is pomted out, and expenmentally demonstrated, that four-termmal transport measurements m the electron-focusmg geometry constitute a determmation of either a generalized longitudmal resistance or a Hall resistance At high magnetic fields the electron-focusmg peaks are suppressed, and a transition is observed to the quantum Hall regime The anomalous quantum Hall effect m this geometry is discussed m light of a four-termmal resistance formula

We present electronic transport measurements of single-and bilayer graphene on commercially available hexagonal boron nitride. We extract mobilities as high as 125 000 cm 2 V −1 s −1 at room temperature and 275 000 cm 2 V −1 s −1 at 4.2 K. The excellent quality is supported by the early development of the ν = 1 quantum Hall plateau at a magnetic field of 5 T and temperature of 4.2 K. We also present a new and accurate transfer technique of graphene to hexagonal boron nitride crystals. This technique is simple, fast and yields atomically flat graphene on boron nitride which is almost completely free of bubbles or wrinkles. The potential of commercially available boron nitride combined with our transfer technique makes high mobility graphene devices more accessible.

Electronic spin transport in graphene field-effect transistors Popinciuc, M.; Jozsa, C.; Zomer, P. J.; Tombros, N.; Veligura, A.; Jonkman, H. T.; van Wees, B. J. Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum. Spin transport experiments in graphene, a single layer of carbon atoms ordered in a honeycomb lattice, 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 aluminum 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 ϳ 10 12 cm −2 ͒ via the Dirac charge neutrality point ͑n ϳ 0͒. 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 ʈ and Ќ , 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 spinrelaxation time and the momentum-relaxation time indicates that the spin-relaxation mechanism is of the Elliott-Yafet type. For carrier mobilities of 2 ϫ 10 3 -5ϫ 10 3 cm 2 / V s and for graphene flakes of 0.1-2 m in width, we found spin-relaxation times on the order of 50-200 ps, times which appear not to be determined by the extrinsic factors mentioned above.