In-plane and perpendicular magnetic fields dependence of cyclotron resonance in two-dimensional electron systems

The cyclotron resonance of two-dimensional electron systems (2DES) of GaAs/AlGaAs heterostructures is investigated in a tilted magnetic field. In this study, the perpendicular (B) and in-plane (B_||) magnetic fields are changed systematically. More than two cyclotron resonances caused by B_|| are observed. Further, the enhancement of cyclotron mass by B_|| and its dependence on B and electron concentration of 2DES are also investigated in detail. These phenomena are well explained by the resonant subband Landau level coupling model.     

Strong resonant intersubband magnetopolaron effect in heavily modulation-doped GaAs/AlGaAs single quantum wells at high magnetic fields

Electron cyclotron resonance (CR) has been studied in magnetic fields up to 32 T in two heavily modulation-δ-doped GaAs/Al_0.3Ga_0.7As single quantum well samples. Little effect on electron CR is observed in either sample in the region of resonance with the GaAs LO phonons. However, above the LO-phonon frequency energy E_LO at B>27 T, electron CR exhibits a strong avoided-level-crossing splitting for both samples at energies close to E_LO+(E₂−E₁), where E₂, and E₁ are the energies of the bottoms of the second and the first subbands, respectively. The energy separation between the two branches is large, reaching a minimum of about 40 cm⁻¹ around 30.5 T for both samples. This splitting is due to a three-level resonance between the second LL of the first electron subband and the lowest LL of the second subband plus an LO phonon. The large splitting in the presence of high electron densities is due to the absence of occupation (Pauli-principle) effects in the final states and weak screening for this three-level process.     

两个子带占据的In0.53Ga0.47As/In0.52Al0.48As量子阱中填充因子的变化规律

研究了低温(15K)和强磁场(0—13T)条件下, InP基In_053Ga_047As/In_052Al_048As量子阱中电子占据两个子带时填充因子随磁场的变化规律.结果表明,在电子自旋分裂能远小于朗道能级展宽的情况下,如果两个子带分裂能是朗道分裂能的整数倍时,即ΔE₂₁=kω_c(其中k为整数),填充因子为偶数;当两个子带分裂能为朗道分裂能的半奇数倍时,即ΔE₂₁=(2k+1)ω_c/2,填充因子出现奇数. 关键词： 053Ga_047As/In_052Al_048As量子阱')" href="#">In_053Ga_047As/In_052Al_048As量子阱 填充因子 磁输运     

Effects of uniaxial stress on the cyclotron resonance in inversion layers on Si (100)

In electron inversion layers on Si (100) MOS capacitors uniaxial stress can cause simultaneous occupation of the two non-equivalent subbands E₀ and E₀'. Two distinct cyclotron resonance signals are observed and can be used to determine the effective masses and the relative occupation in the two subbands and their dependence on stress and electron density.     

Comparison of Shubnikov-de Haas effect and cyclotron resonance on Si(100) MOS transistors under uniaxial stress

The stress induced population of the non-equivalent subbands E₀ to E′₀ is investigated by measurements of the magnetoconductivity — the Shubnikov-de Haas oscillations — and the cyclotron resonance taken on the same MOS transistor under identical conditions. In cyclotron resonance with increasing stress, a gradual transfer of carriers from E₀ to E′₀ is observed, whereas the Shubnikov-de Haas periodicity is unaffected and always yields a valley degeneracy of two. A domain model is required to resolve this apparent discrepancy.     

Quantum cyclotron resonance of two-dimensional holes in the Ge layers of Ge-Ge1−x Six heterostructures

Quantum cyclotron resonance of two-dimensional holes in strained germanium layers of the periodic heterostructure Ge-Ge_1−x Si_x has been observed and investigated for the first time. The results are compared with the data of electrophysical measurements in strong magnetic fields. A clear dependence of the magnitude of the longitudinal effective mass of the holes on the absolute magnitude of the elastic deformation of the germanium layers (splitting energy of the hole subbands) is observed. Fiz. Tverd. Tela (St. Petersburg) 39, 2096–2100 (November 1997)     

Magneto-intersubband zener tunneling in a wide GaAs quantum well at high filling factors

The dependence of the differential resistance r _xx on the dc current density J _dc in a wide GaAs quantum well with two occupied size quantization subbands has been investigated at the temperature T = 4.2 K in the magnetic fields B < 1 T. A peak, whose position is given by the relation 2R _c eE _H = ħω_c/2, where R _c is the cyclotron radius, E _H is the Hall electric field, and ω_c is the cyclotron frequency, has been observed in the r _xx (J _dc) curves at high filling factors. The experimental results are attributed to Zener tunneling of electrons between the Landau levels of different subbands.     

Cyclotron resonance of electrons and holes in electric surface subbands of tellurium

We present first measurements of the submillimeter-cyclotron resonance of electrons and holes in electric surface subbands of tellurium. From the resonance in the inversion layer we have obtained for the magnetic field paralled to the trigonal axis, the cyclotron mass of the surface electron m_ce = (0.117 ± 0.002)m₀. The resonance of the accumulation layer splits and suggests the contribution of different nonparabolic subbands.     

High power splitting of the cyclotron resonance induced photoconductivity in n-GaAs

Conclusion In summary our results show that photoconductivity and optical absorption under cyclotron resonance conditions in high purity n-GaAs are more complex than it has been assumed previously. As cyclotron resonance can only be measured with non zero free electron concentration, and because the 2p _– shallow donor level remains below the N=0 Landau level for all magnetic field strengths, a certain population of 2p _– states must be present and thus the interference between both absorption processes is unavoidable as long as 2p _–2p ₊ electric dipole transitions are activated by ionized impuritics. This interference gets most drastically manifest at high intensities causing an apparent splitting of the photoconductivity line. Even at lower intensities however when the dip in the photoconductivity line is not observable, all optical characteristics previously attributed to cyclotron resonance are affected by the shallow donor absorption. The energy separationE _2p+–E _2p– of shallow donors in a magnetic field is exactly equal to w _CR only for isolated impurities in the effective mass approximation. The same electric stray field of ionized impurities which cause the activation of 2p _–2p ₊ absorption may shift the donor energy levels due to stark effect. For magnetic field strengthsB<1 T a field dependent deviation ofE _{2 +}–E _{1p –} from the cyclotron resonance quantum energy was observed [15] amounting up to 8 pc. In this case we expect that the peak positions of photoconductivity and absorption do not spectrally coincide and do not occur at the resonance magnetic field strength of 01 Landau level transitions. Thus effective masses determined by standard cyclotron resonance methods at low magnetic fields may be incorrect by a few percent.     

Intersubband resonance polarons in Al/δ-GaAs tunneling junctions

Tunneling spectra of the A1/δ-GaAs junctions fabricated by molecular-beam epitaxy in the regime of “intimate” contact of A1 with GaAs (100) were studied at 1.6 K in a magnetic field B parallel to the two-dimensional electron-gas layer. The concentration of 2D electrons in the δ-layer grown at a distance of 20 nm from the A1/GaAs interface was 1.1 × 10¹² cm^?2 and corresponded to a partial filling of only the lowest subband E ₀. The tunneling spectra exhibited many-particle features, viz., a zero-bias anomaly, lines of longitudinal optical (LO) phonons, and characteristic “dips” corresponding to the energies E _i of the 2D subbands. In the B fields below the critical value B _c ?11 T, the levels of 2D subbands underwent the usual diamagnetic shifts. At B≥B _c , the E ₁ (B) term pinning and the anticrossing of the E ₁(B) and E ₀(B)+2?ω_LO terms were observed, where ?ω_LO is the LO-phonon energy in GaAs. The observed effects are interpreted as manifestations of resonance intersubband polarons arising in the δ-layer upon reaching the E ₁(B _c )?E ₀(B _c )=2?ω_LO resonance.     

Spectroscopy of the electron subbands on (1 1 1)-Si

We present measurements of infrared inter-subband absorption for electron subbands on (1 1 1)-Si for densities N_s up to ～ 10¹³ cm^?2 at 4.2 K for both parallel- and perpendicular-excitation geometries. Contrary to previously published work the depolarization shift is identified as a sizeable splitting of the resonance energies E^⊥₀₁ and E^z.dfnc;₀₁. The comparison with a recent calculations is given.     

Tunable band structure effects on ballistic transport in graphene nanoribbons

Graphene nanoribbons (GNR) in mutually perpendicular electric and magnetic fields are shown to exhibit dramatic changes in their band structure and electron transport properties. A strong electric field across the ribbon induces multiple chiral Dirac points, closing the semiconducting gap in armchair GNRs. A perpendicular magnetic field induces partially formed Landau levels as well as dispersive surface-bound states. Each of the applied fields on its own preserves the even symmetry Ek=E−k of the subband dispersion. When applied together, they reverse the dispersion parity to be odd and gives Ee,k=−Eh,−k and mix the electron and hole subbands within the energy range corresponding to the change in potential across the ribbon. This leads to oscillations of the ballistic conductance within this energy range.     

Surface cyclotron resonance in InSb

We have studied cyclotron resonance of electrons in an inversion layer on the surface of p-type InSb. With increasing surface charge density n_s a number of distinct resonance peaks are found to appear. These are attributed to cyclotron resonance in different electric subbands. The various cyclotron masses are found to increase with n_s and with the frequency of the far-infrared laser source.     

Shallow acceptor levels in Ge/GeSi heterostructures with quantum wells in a magnetic field

Shallow acceptors in Ge/GeSi heterostructures with quantum wells are studied theoretically and experimentally in the presence of a magnetic field. It is shown that, in addition to the cyclotron resonance lines, magnetoabsorption spectra reveal transitions from the acceptor ground state to excited states related to Landau levels from the first and second confinement subbands, as well as the resonances caused by ionization of A⁺ centers.     

Self-consistent calculations of electric subbands in p-type GaAlAs-GaAs heterojunctions

The calculations in the Hartree-approximation indicate that only the heavy hole subband is occupied for carrier concentrations < 4×10¹²/cm² and that the spin degeneracy is lifted for finite wave vectors. Without magnetic field, two effective masses of 0.16 m_o and 0.61 m_o are deduced. A calculation of the Landau levels predicts a three-line cyclotron resonance absorption spectrum at high magnetic fields, which partially agrees with experimental data.     

Zero differential resistance in a double quantum well at high filling factors

The differential resistance r _xx in a GaAs double quantum well with two occupied size-quantization subbands have been studied at temperatures T = 1.6–4.2 K in magnetic fields B < 0.5 T. It has been found that differential resistance r _xx vanishes at the maxima of magneto-intersubband oscillations with an increase in the direct current I _dc. It has been shown that the discovered r _xx ≈ 0 state appears under the condition 2R _c E _H/ħω_c < 1/2, where R _c is the cyclotron radius of electrons at the Fermi level, E _H is the Hall electric field induced by the current I _dc, and ω_c is the cyclotron frequency.     

Effective mass of the 2-dimensional electron gas in an Al0.6Ga0.4Sb/InAs single quantum well

The 2-dimensional electron gas (2DEG) in an Al_0.6Ga_0.4Sb/InAs single quantum well is studied using cyclotron resonance and Shubnikov - de Haas (SdH) techniques. The effective mass (m^*) of the 2DEG was obtained from the peak positions of the cyclotron resonance transmission spectra. The results exhibit oscillatory behavior as a function of the magnetic field strength (B). The m^* value extracted from the temperature dependence of the SdH oscillations is in good agreement with the average value of m^* obtained from cyclotron resonance measurements. The effective mass is calculated as a function of B using an electron self-energy model based on the Hartree-Fock approximation. The calculated m^* values also show oscillatory behavior similar to that of the measured cyclotron resonance m^*. Both experiment and theory show that m^* maxima are shifted from the integral values (both odd and even) of the filling factors.     

Directional temperature of hot electrons in InSb under electric and magnetic fields

Directional differences in electron temperature are observed in n-type indium antimonide between E ∥ H and E ⊥ H geometries of electric and magnetic fields. Effects are reported in far-infrared cyclotron resonance for electrons at the donor level as well as the conduction band.     

Electron-active cyclotron resonance in p-doped InMnAs in high magnetic fields

We present a theoretical and experimental study of electron-active cyclotron resonance in p-doped InMnAs in high magnetic fields. Results are based on an 8-band Pidgeon–Brown model generalized to include finite k_z effects and s(p)–d exchange interaction between itinerant carriers and Mn d-electrons. The e-active transitions in the valence band in p-doped samples take place due to the nature of multiple valence bands (heavy and light holes). We have calculated the absorption spectra in high magnetic fields and identified optical transitions which contribute to the cyclotron resonance for both e-active and h-active polarizations. Calculations show agreement with experimental results.     

Cyclotron resonance of hot electrons and nonlinear transport phenomena in n-type indium antimonide

Impurity states and nonlinear transport phenomena in n-type indium antimonide under strong magnetic fields have been extensively studied at liquid helium temperatures through H₂O laser cyclotron resonance combined with d.c. measurements. A new type cyclotron resonance with modulation by pulsed electric field, or PEM-CR, has been utilized throughout. Origin of several weak transitions so far indefinite has been identified. Existence of the donor binding state in a magnetic field as low as 2·85 kOe for the excess donor concentration N_D ? N_A = 2 × 10¹³ cm^?3 is experimentally confirmed. Joint determination of resistivity and carrier distribution in the energy space has yielded a fair success in separating the mechanisms for the nonlinear transport behavior.