2. The spinning computer era Spintronics Hsiu-Hau Lin National Tsing-Hua Univ

3. Quantum Information Science Wokshop Tainan, Oct 18-19, 2002 Hsiu-Hau Lin National Tsing-Hua Univ Collaborators and References Jurgen Konig Allan H. MacDonald John Schliemann Steve M. Girvin Thomas Jungwirth Phys. Rev. Lett. 84, 5628(2000) Appl. Phys. Lett. 78, 1550 (2001) Phys. Rev. Lett. 86, 5637 (2001) Lecture Notes in Physics 579, Springer (2002)

4. Quantum Information Science Wokshop Tainan, Oct 18-19, 2002 Hsiu-Hau Lin National Tsing-Hua Univ Outline Introductory spintronics Making ferromagnetic semiconductors Pushing up the critical temperature Theoretical Understanding Conclusions

5. Quantum Information Science Wokshop Tainan, Oct 18-19, 2002 Hsiu-Hau Lin National Tsing-Hua Univ Spin and Charge Spin: data storage in Magnetic materials. Charge: data processing in Semiconductors.

6. Quantum Information Science Wokshop Tainan, Oct 18-19, 2002 Hsiu-Hau Lin National Tsing-Hua Univ Spin Valve Use spin configurations to control the current -- spintronics ! Conventional transistors make use of current or voltage to control the transmitted current -- electronics.

7. Quantum Information Science Wokshop Tainan, Oct 18-19, 2002 Hsiu-Hau Lin National Tsing-Hua Univ Spin-Valve Transport metalinsulator Transport properties depend on the carriers’ spins.

8. Quantum Information Science Wokshop Tainan, Oct 18-19, 2002 Hsiu-Hau Lin National Tsing-Hua Univ Magnetic Recording Actual setup A real spin valve!

9. Quantum Information Science Wokshop Tainan, Oct 18-19, 2002 Hsiu-Hau Lin National Tsing-Hua Univ Magnetic Read Head Available commercial product of spintronics -- magnetic read-out head by IBM in 1997. Now it becomes $1,000,000,000 USD market ! Even more surprises on the way!

10. Quantum Information Science Wokshop Tainan, Oct 18-19, 2002 Hsiu-Hau Lin National Tsing-Hua Univ Nonvolatile Memory (MRAM) Weak currents flip red spin -- Data processing Strong currents flip blue spins -- Memory storage

11. Quantum Information Science Wokshop Tainan, Oct 18-19, 2002 Hsiu-Hau Lin National Tsing-Hua Univ Earlier Tries (A) Magnetic Semiconductors: Eu chalogenides, semiconducting spinels, … Hard to grow ! (B) Diluted II-VI Magnetic Semiconductors: CdTe, ZnSe, … - Easy to replaced by Mn - Hard to dope, no mobile carriers.

12. Quantum Information Science Wokshop Tainan, Oct 18-19, 2002 Hsiu-Hau Lin National Tsing-Hua Univ Success of III-V Semiconductors Ferromagnetism in (Ga 1-x, Mn x ) As: Ga 3+ Mn 2+ + hole Key to ferromagnetism ! Mn 2+ (S = 5/2) Expect n I = n e -- But ! Mobile hole (s=1/2)

13. Quantum Information Science Wokshop Tainan, Oct 18-19, 2002 Hsiu-Hau Lin National Tsing-Hua Univ Phase Diagram Low solubility, easily leads to phase separation. Molecular Beam Epitaxy (MBE) at low temperatures.

14. Quantum Information Science Wokshop Tainan, Oct 18-19, 2002 Hsiu-Hau Lin National Tsing-Hua Univ Spin Doctor Ohno Critical temperature Tc = 110K

15. Quantum Information Science Wokshop Tainan, Oct 18-19, 2002 Hsiu-Hau Lin National Tsing-Hua Univ Spin Doctor Awschalom Efficiency over 90% polarized injected current. Coherent spin transport over 100  m. Hope for Quantum Computer!

16. Quantum Information Science Wokshop Tainan, Oct 18-19, 2002 Hsiu-Hau Lin National Tsing-Hua Univ Mean-Field Prediction The critical temperature grows as the doping x increases.

17. Quantum Information Science Wokshop Tainan, Oct 18-19, 2002 Hsiu-Hau Lin National Tsing-Hua Univ Seems Optimistic? Weiss mean field theory predictions ! Wide-band gap semiconductors and oxides are our best bets!

18. Quantum Information Science Wokshop Tainan, Oct 18-19, 2002 Hsiu-Hau Lin National Tsing-Hua Univ Dream Come True? Co-doped TiO 2 : Formation of domain walls suggesting the presence of a long range magnetic moment T c is estimated as high as 400 K exceeding the room temperature.

19. Quantum Information Science Wokshop Tainan, Oct 18-19, 2002 Hsiu-Hau Lin National Tsing-Hua Univ Magnetization Curve Hysteresis is very small Small magnetoR ratio (60% under 8T at 2K) But, there is always hope!

20. Quantum Information Science Wokshop Tainan, Oct 18-19, 2002 Hsiu-Hau Lin National Tsing-Hua Univ A Simple Model Kinetic energy Exchange energy J > 0, Antiferromagnetic coupling Ignore realistic band structure, disorder, electronic correlation, and so on. Similar to CMR, Kondo,… BUT ! Not quite the same.

21. Quantum Information Science Wokshop Tainan, Oct 18-19, 2002 Hsiu-Hau Lin National Tsing-Hua Univ Ferromagnetic Ground State Impurity spins aligned by mobile electrons. Electronic bands acquire a Zeeman splitting gap. Carrier-Mediated Ferromagnetism !

22. Quantum Information Science Wokshop Tainan, Oct 18-19, 2002 Hsiu-Hau Lin National Tsing-Hua Univ Spin-Wave Spectrum Gapless spin excitations (Goldstone mode). Exchange – minimized Kinetic – twisted phase In the dilute limit,

23. Quantum Information Science Wokshop Tainan, Oct 18-19, 2002 Hsiu-Hau Lin National Tsing-Hua Univ Optical Spin Waves E =  + Kinetic Two-particle continuum. Collective mode below the continuum.

24. Quantum Information Science Wokshop Tainan, Oct 18-19, 2002 Hsiu-Hau Lin National Tsing-Hua Univ Trend of Critical Temperature Spin-wave fluctuations included self-consistently. Magnetization curve is computed. T c is not monotonically increasing with respect to doping and carrier density.

25. Quantum Information Science Wokshop Tainan, Oct 18-19, 2002 Hsiu-Hau Lin National Tsing-Hua Univ Different Regimes Weiss mean-field theory is not always correct ! Friedel oscillations become important and reduce T c. Interesting crossovers !

26. Quantum Information Science Wokshop Tainan, Oct 18-19, 2002 Hsiu-Hau Lin National Tsing-Hua Univ Conclusions Diluted magnetic semiconductors are fascinating materials. T c already exceeds the room temperature. However, lots of room for improvements. Deeper theoretical understanding of spin transport at finite temperature is in order.