A. Wojcik TAMU X. Wang, D.M. Mittleman, and J. Kono Rice S.A. Crooker NHMFL, Los Alamos

1. Terahertz studies of collective excitations and microscopic physics in semiconductor magneto-plasmas Alexey Belyanin Texas A&M University A. Wojcik TAMU X. Wang, D.M. Mittleman, and J. Kono Rice S.A. Crooker NHMFL, Los Alamos NSF CAREER NSF OISE

2. N-doped InSb: a classic narrow-gap semiconductor ~ 0.2 eV ~ 0.8 eV Small band gap Small electron mass ~ 0.014 m0 Strong non-parabolicity; Non-equidistant cyclotron transitions Palik & Furdyna 1970 McCombe & Wagner 1975

3. Many frequency scales in doped semiconductors fall into the THz spectral range 1 THz = 4.1 meV • Plasma frequency • Fermi energy • Electron scattering rates • Cyclotron frequency in the magnetic field of ~ 1 Tesla • Intra-donor transition frequencies • Phonon frequencies • Rich information can be extracted from THz spectroscopic studies • Exotic conditions for atoms and plasma in superstrong magnetic fields • Potential for optoelectronic devices utilizing THz coherence

4. THz time-domain spectroscopy Delay stage CPA laser 1 KHz, 800 nm Receiver Transmitter ZnTe B ZnTe WC 1/4 Current Amplifier Lock_in amplifier Sample: n-doped InSb crystal Sample #1: density = 2.1E14 cm-3 Sample #2: density = 3.5E14 cm-3 Sample #3 density = 6.1E14 cm-3 T = 1.6-300 K f = 0.1-2.5THz B = 0-10 T

5. Incident and transmitted THz pulses

6. The transmittance contour map of sample # 1 T = 1.6 K T = 40 K Frequency, THz Magnetic field, Tesla • Plasma edge • Cyclotron resonance • Intra-donor transition lines at low T and high B • Interference features

7. Transmittance at 40 K: only free-carrier effects expected

8. Free-carrier effects: interference of normal magnetoplasmon modes “Cold” plasma approximation: n2 FMS  CRI CRA CRI CRA Ei B  ~ 16

9. Transmittance at 40 K: only free-carrier effects expected CRI CRA

10. experiment Interference structure is very sensitive to the cyclotron transition energy and the density of free electrons Yields information on the electron cyclotron mass, band non-parabolicity, compensation ratio, and binding energy on donors (Tellurium) as a function of magnetic field theory

11. Temperature map at B = 0.9 T experiment Position of the peak is very sensitive to thermal band gap EgT : theory EgT = 0.215 eV

12. Electron scattering rate Temperature dependence at B = 0.9 T • Scattering mechanisms: • Ionized and neutral impurities • Acoustic deformation potential • Piezoelectric • Optical deformation potential • Polar optical phonons • intrinsic carriers Impurity scattering Polar optical phonons Electron-hole scattering

13. Electron-”ion” scattering in a strong magnetic field http://hyperphysics.phy-astr.gsu.edu Debye radius Gyroradius: Similar to magnetic white dwarfs and neutron stars!

14. Low-temperature effects: donor absorption lines and field-induced localization Nn = 2.1x1014 cm-3 40 K 1.6 K measurements

15. Donor (tellurium) transition lines Cyclotron resonance CRA 1s-2p+ transition (000)-(110) CRI 1s-2p- transition (000)-(0-10) McCombe & Wagner 1975

16. Low-temperature effects: field-induced localization T = 1.6 K, Nn = 2.1x1014 cm-3 Edwards & Sienko 1978 Quantum phase transition metal-insulator? Gradual magnetic freeze-out of carriers? B = 0: Nn ~ 6x1013 cm-3

17. Gao et al., APL 2006 Shayegan et al. PRB 1988 Freeze-out picture: Mani et al., PRB 1989,1991

18. Low-temperature effects: field-induced localization T = 1.6 K, Nn = 2.1x1014 cm-3 Not compatible with a gradual magnetic freeze-out? Trying scaling behavior of dielectric constant … “Releasing” electrons at B ~ Bc Efros & Shklovskii 1976 etc.

19. Conclusions • Coherent time-domain THz spectroscopy provides quantitative information on the band structure, electron scattering processes, and collective excitations • Intriguing low-temperature behavior of the dielectric response; nature of the magnetic field-induced localization is still unclear • Also for future studies: dispersion , deviation from ideal plasma, kinetic effects near the cyclotron resonance