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Writing and reading spin information on mobile electronic qubits Writing and reading spin information on mobile electronic qubits Symposium on Spin Physics and Nanomagnetism =Chudnovsky-Fest, Friday, March 13, 2009 Amnon Aharony Ora Entin-Wohlman (BGU) Yasuhiro Tokura (NTT) Shingo Katsumoto (ISSP) Physics Department and Ilse Katz Nano center

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Late 70ies: amorphous magnetism 2001: collaboration on magnetic molecules

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Outline Spintronics, quantum computing Spin-Orbit interaction Spin field effect transistor Spin filter: writing information on electron spinor Quantum networks Our spin filter: simple exercise in quantum mechanics Our spin reader: measuring spinor via conductance Conclusions Writing and reading spin information on mobile electronic qubits Writing and reading spin information on mobile electronic qubits

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Alternative to electronics: spintronics

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Quantum mechanics : Particle-wave duality Schrödingers wave equation Diracs equation: spin and spinor

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Spin-orbit interactions Entin-Wohlman, Gefen, Meir, Oreg (1989, 1992) Dirac :: Aharonov-Casher

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Spin-orbit interactions Dirac :: Rashba : 2DEG, confined to a plane by an asymmetric potential along z: Strength of Rashba term can be tuned by gate voltage! A spinor y entering from the left and travelling a distance L along the x-axis will be multiplied by the 2x2 unitary matrix Rotation of spin direction around y-axis

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Spin field effect transistor

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Tunable with gate voltage Can use at low gate voltages! Das and Datta (1990): The Spin field effect transistor

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Conventional computers: information in bits, 0 or 1, +1 or -1, or Quantum computers: information in Qubits, Electron described by spinor: Complex numbers Spinor is an eigenvector of, the spin component along a, b Quantum computers

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Writing on spinor: Spin filtering : Work with mobile electrons, Generate spin-polarized current out of an unpolarized source filter polarized electrons unpolarized electrons

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Textbook method: Stern-Gerlach splitting Based on Zeeman splitting, Requires large fields, separation of beams not easy due to uncertainty Writing on spinor: Spin filtering : Work with mobile electrons, Generate spin-polarized current out of an unpolarized source

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Spin filtering : Generate spin-polarized current out of an unpolarized source Earlier work: usually calculate spin-dependent conductance, and generate partial polarization, which varies with parameters. Our aim: obtain full polarization, in a tunable direction quantum networks filter Unpolarized electrons polarized electrons

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Earlier work concentrated on spin-dependent conductance, averaged over electron energies, did not concentrate on spin filtering Our aim: use simplest quasi-1D model to generate spin filtering We use tight-binding quantum networks, Our main conclusion: can achieve full filtering provided we use both spin-orbit and Aharonov-Bohm 2-component spinor at node u 2x2 unitary matrix, representing hopping from v to u Continuum versus tight-binding networks: AA + Ora Entin-Wohlman, J. Phys. Chem. (in press); ArXiv: 0807.4088

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4 solutions, which appear in pairs, Real q: running solution. Complex q: evanescent solution. Ballistic conductance = g= number of solutions which run from left to right: g= 0, 1 or 2 For a broad range of parameters, there is only one running solution, and then the electrons are fully polarized! General solution for chain of diamonds:

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Problems: How to realize long chain? How to read information from spinor? Single diamond

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The rest of the talk described unpublished work on the transmission through a single diamond, with both Rashba spin-orbit and Aharonov-Bohm flux. Results show that there exist lines in the plane connecting the above two interactions, for which the outgoing electrons have unique spin directions, which can be tuned by the electric and magnetic fields. Thus, this is an ideal spin filter. When one sends fully polarized electrons into the single diamond, the outgoing current depends on the angle between this polarization and the special spin-direction which characterizes the filter. Appropriate tuning then allows the reading of the incoming polarization by Measuring currents..

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Conclusions: Need both Aharonov-Bohm and spin-orbit to Obtain full filtering, with unique spin. Spin is sensitive to electric and magnetic fields: small changes in parameters switch the direction of the filtered spin. Can work at fixed small magnetic field, with small changes in electric field or in electron energy.

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More to do: Add Dresselhaus spin-orbit interaction? Add Zeeman field – Aharonov-Casher? Berry phase? Dissipation: stochastic noise? phonons? Dephasing? Add e-e interactions? How can we combine beams to perform computing? Postdoc positions

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