The spinning computer era Spintronics Hsiu-Hau Lin National Tsing-Hua Univ.
Post on 21-Dec-2015
232 Views
Preview:
Transcript
Quantum Information Science WokshopTainan, 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)
Quantum Information Science WokshopTainan, 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
Quantum Information Science WokshopTainan, Oct 18-19, 2002
Hsiu-Hau Lin National Tsing-Hua Univ
Spin and Charge
Spin: data storage inMagnetic materials.
Charge: data processing in Semiconductors.
Quantum Information Science WokshopTainan, 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 voltageto control the transmitted current -- electronics.
Quantum Information Science WokshopTainan, Oct 18-19, 2002
Hsiu-Hau Lin National Tsing-Hua Univ
Spin-Valve Transport
metal insulator
Transport properties depend onthe carriers’ spins.
Quantum Information Science WokshopTainan, Oct 18-19, 2002
Hsiu-Hau Lin National Tsing-Hua Univ
Magnetic Recording
Actual setup
A real spin valve!
Quantum Information Science WokshopTainan, 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 $1,000,000,000USD market !
Even more surprises on the way!
Quantum Information Science WokshopTainan, 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
Quantum Information Science WokshopTainan, 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.
Quantum Information Science WokshopTainan, Oct 18-19, 2002
Hsiu-Hau Lin National Tsing-Hua Univ
Success of III-V Semiconductors
Ferromagnetism in (Ga1-x, Mnx) As:
Ga3+
Mn2+ + hole
Key to ferromagnetism !
Mn2+ (S = 5/2)
Expect nI = ne -- But ! Mobile hole (s=1/2)
Quantum Information Science WokshopTainan, 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.
Quantum Information Science WokshopTainan, Oct 18-19, 2002
Hsiu-Hau Lin National Tsing-Hua Univ
Spin Doctor Ohno
Critical temperature Tc = 110K
Quantum Information Science WokshopTainan, 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!
Quantum Information Science WokshopTainan, Oct 18-19, 2002
Hsiu-Hau Lin National Tsing-Hua Univ
Mean-Field Prediction
The critical temperaturegrows as the doping x increases.
Quantum Information Science WokshopTainan, Oct 18-19, 2002
Hsiu-Hau Lin National Tsing-Hua Univ
Seems Optimistic?
Weiss mean field theorypredictions !
Wide-band gap semiconductors and oxides are our best bets!
Quantum Information Science WokshopTainan, Oct 18-19, 2002
Hsiu-Hau Lin National Tsing-Hua Univ
Dream Come True?
Co-doped TiO2:
• Formation of domain wallssuggesting the presence of a longrange magnetic moment
• Tc is estimated as high as 400 Kexceeding the room temperature.
Quantum Information Science WokshopTainan, 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!
Quantum Information Science WokshopTainan, 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.
Quantum Information Science WokshopTainan, 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 !
Quantum Information Science WokshopTainan, 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,
Quantum Information Science WokshopTainan, Oct 18-19, 2002
Hsiu-Hau Lin National Tsing-Hua Univ
Optical Spin Waves
E = + Kinetic
• Two-particle continuum.
• Collective mode below the continuum.
Quantum Information Science WokshopTainan, 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.• Tc is not monotonically
increasing with respect to doping and carrier density.
Quantum Information Science WokshopTainan, 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 Tc .
• Interesting crossovers !
Quantum Information Science WokshopTainan, Oct 18-19, 2002
Hsiu-Hau Lin National Tsing-Hua Univ
Conclusions
Diluted magnetic semiconductors are fascinating materials.
Tc already exceeds the room temperature. However, lots of room for improvements.
Deeper theoretical understanding of spin transport at finite temperature is in order.
top related