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 1

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

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

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

Incident and transmitted THz pulses

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

Transmittance at 40 K: only free-carrier effects expected

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

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

experiment theory 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

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

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

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

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

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

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

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

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.

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