Presentation on theme: "Gate Control of Spin Transport in Multilayer Graphene"— Presentation transcript:
1Gate Control of Spin Transport in Multilayer Graphene By H. Goto et al.Kun Xu
2Advantages Advantages of spin over charge: Easily manipulatable with externally applied magnetic fieldsLong coherence/relaxation timeGMR - Giant magnetoresistance (GMR) is a quantum mechanical magnetoresistance effect observed in thin film structures composed of alternating ferromagnetic and nonmagnetic layers.Already used in magnetic storage technologies.Once created it tends to stay that way for a long time – unlike charge states, which are easily destroyed by scattering or collision with defects, impurities or other charges.Seamless integration of electronic, optoelectronic and maetoelectronic multifunctionality on the single device.
3GMR Giant magnetoresistance Sandwich structure FNFSpin valve (HDD read/write heads)The 2007 Nobel Prize in physics was awarded to Albert Fert and Peter Grünberg for the discovery of GMRGMR - Giant magnetoresistance (GMR) is a quantum mechanical magnetoresistance effect observed in thin film structures composed of alternating ferromagnetic and nonmagnetic layers.Already used in magnetic storage technologies.In the absence of an external magnetic field, the direction of magnetization of adjacent ferromagnetic layers is antiparallel due to a weak anti-ferromagnetic coupling between layers. The result is high-resistance magnetic scattering.When an external magnetic field is applied, the magnetization of the adjacent ferromagnetic layers is parallel. The result is lower magnetic scattering, and lower resistance. 
4Disadvantages Existing spin devices do not amplify signals although they are successful switches orvalves).Spin amplifier constructed from well known components.The spin current polarization is detected by a spin valve and transformedinto a voltage. A conventional amplifier is used to generate a chargecurrent proportional to the detected spin current. A spin current source transformsthe charge current into the output spin current.
5Datta-Das DeviceCurrent modulated by the degrees of precession in electron spin introduced by the gate fielda structuremade from indium-aluminum-arsenideand indium-gallium-arsenideLarmor precession.The emitter emits electrons withtheir spins oriented along the directionof the electrode’s magnetization, whilethe collector (with the same electrodemagnetization) acts as a spin filter andaccepts electrons with the same spinonly. In the absence of any changes tothe spins during transport, every emittedelectron enters the collector. In thisdevice, the gate electrode produces afield that forces the electron spins toprecess, just like the precession of aspinning top under the force of gravity.The electron current is modulated bythe degree of precession in electronspin introduced by the gate field: Anelectron passes through the collector ifits spin is parallel, and does not if it isantiparallel, to the magnetization.
6Spin-based quantum Computer Qubit – intrinsic binary unitsQuantum entanglementSingle electron trapped in a quantum dotGives quantum computer to operate in parallelSince spins inherently have long coherence length/immune to the long-range electrostatic coulomb interactions between charges.Factor large integers into prime numbers
7Spin transport in graphite based devices Carbon nanotubesGrapheneMultilayer graphene (MLG)Weak spin-orbit and hyperfine interactionGate control of spin conductionWeak spin-orbit and hyperfine interaction ---- carbon: light element
12Measurement Four terminal lock-in technique 4.2K Excitation current of 1.0 uA, 119HzBack gate biasThe great promise of spintronic technology is based upon the fundamental ability of electron spins in electronicmaterials to preserve coherence for relatively long times. A typical electron “remembers” its initialspin orientation for a nanosecond. This time scale is indeed long when compared with the typical times–femtoseconds–for electron momentum relaxation. Perhaps a more revealing quantity than spin lifetime(which is usually called spin relaxation time T1 or spin decoherence time T2, depending on the context ofthe experiment) is the spin diffusion length LS which measures how far electrons diffuse in a solid withoutlosing spin coherence.Imersed in liquid Helium at 4.2k
13Spin Signal: Rs Rs=Rp-Rap Proportional to R when FN interfaces are opaqueProportional to 1/Rwhen FN interfaces are transparent