We have observed remarkable multiple-line electron spin-resonance spectra in sensitive terahertz laser pho-toresponse measurements of the two-dimensional electron gas of an asymmetric InAs quantum well in the integer quantum Hall effect regime. Near filling factor 7 with the magnetic field oriented at large angles theta from the normal to the sample surface, rather than the expected single-electron spin-resonance line, we observed several sharp pairs of features at fields across the corresponding quantum Hall plateau. At a unique angle theta c, the dominant central pair merges into a single line close to the estimated magnetic field of electron spin resonance. For theta > theta c this line splits into two sharper features whose separation in magnetic field increases with increasing theta. Surprisingly, for theta < theta c the central feature disappears. The explanation of this behavior, as well as the observation of additional pairs of sharp features with larger magnetic-field separations, is based on strong spin orbit coupling effects and the concomitant effective magnetic fields associated with pairs of oppositely directed, persistent quantum Hall edge currents combined with the behavior of edge channels near the center of the odd plateaus. Modeling the splitting of the spin resonances due to these effective spin orbit magnetic fields is in reasonable agreement with observations. These results show that it is possible to probe the widths of quantum Hall edge channels through the spectral specificity of the electron spin resonance and possibly manipulate spins with THz photons having wavelengths several orders of magnitude larger than the edge-channel widths.

Spatially indirect Type-II band alignment in magnetically-doped quantum dot (QD) structures provides unexplored opportunities to control the magnetic interaction between carrier wavefunction in the QD and magnetic impurities. Unlike the extensively studied, spatially direct, QDs with Type-I band alignment where both electrons and holes are confined in the QD, in ZnTe QDs embedded in a (Zn,Mn)Se matrix only the holes are confined in the QDs. Photoexcitation with photon energy 3.06 eV (2.54 eV) generates electron-hole pairs predominantly in the (Zn,Mn)Se matrix (ZnTe QDs). The photoluminescence (PL) at 7 K in the presence of an external magnetic field exhibits an up to three-fold increase in the saturation red shift with the 2.54 eV excitation compared to the shift observed with 3.06 eV excitation. This unexpected result is attributed to multiple hole occupancy of the QD and the resulting increased penetration of the hole wavefunction tail further into the (Zn,Mn)Se matrix. The proposed model is supported by microscopic calculations which accurately include the role of hole-hole Coulomb interactions as well as the hole-Mn spin exchange interactions.

We measure the Hall conductivity of a two-dimensional electron gas formed at a GaAs/AlGaAs heterojunction in the terahertz regime close to the cyclotron resonance frequency using highly sensitive Faraday rotation measurements. The sample is electrically gated, allowing the electron density to be changed continuously by more than a factor of 3. We observe clear plateaulike and steplike features in the Faraday rotation angle vs electron density and magnetic field (Landau-level filling factor) even at fields or frequencies very close to cyclotron resonance absorption. These features are the high frequency manifestation of quantum Hall plateaus---a signature of topologically protected edge states. We observe both odd and even filling factor plateaus and explore the temperature dependence of these plateaus. Although dynamical scaling theory begins to break down in the frequency region of our measurements, we find good agreement with theory.

Inserted narrow InAs quantum wells in InAs/InGaAs/InAlAs heterostructures have been used to achieve higher mobility for high-electron-mobility transistors (HEMTs) with ultra-low-power and low-noise amplification characteristics and for spin-based devices. Due to the large nonparabolicity of the conduction band of InAs and the penetration of the confined electronic envelope function into the adjacent layer(s), accurate calculations of effective mass and g-factor of charge carriers can be problematic. Methods of making precise determinations of the mass and other electronic parameters are thus of interest. We have applied magneto-photoresponse and -transmissions measurements at several THz laser frequencies in concert with dc magnetotransport measurements at low temperature (T = 1.6 K) to determine various electronic parameters (effective mass, carrier density, g-factor, mobility and the quantum scattering time) of the 2DEG in an InAs/In0.75Ga0.25As/In0.75Al0.25As inserted channel structure. This characterization method can also be used to probe the effect of strain, Rashba field, etc on the properties of charge carriers in such structures.

Nanostructures fabricated from narrow-gap semiconductors with strong spin-orbit interaction (SOI), such as InAs, can be used to filter momentum modes of electrons and offer the possibility to create and detect spin-polarized currents entirely by electric fields. Here, we present magnetotransport and THz magnetospectroscopy investigations of Hall-bars with back-gates made from in InGaAs/InAlAs quantum well structures with a strained 4 nm InAs-inserted channel. The two-dimensional electron gas is at 53 nm depth and has a carrier density of about 6 × 1011 cm−2 and mobility of about 2 × 105 cm2/Vs, after illumination. Electrical and THz optical transport measurements at low temperatures and in high magnetic fields reveal an effective mass of 0.038m0 and an anisotropic g-factor of up to 20, larger than for bulk InAs or InAs-based heterostructures. We demonstrate that quasi-one-dimensional channels can be formed by micro-laser lithography. The population of subbands is controlled by in-plane gates. Contrary to previous reports, symmetric and asymmetric in-plane gate voltages applied to quasi-one dimensional channels did not show indications of SOI-induced anomalies in the conductance.

We used continuous wave photoluminescence (cw-PL) and time resolved photoluminescence (TR-PL) spectroscopy to compare the properties of magnetic polarons (MP) in two related spatially indirect II-VI epitaxially grown quantum dot systems. In the ZnTe/(Zn,Mn)Se system the holes are confined in the non-magnetic ZnTe quantum dots (QDs), and the electrons reside in the magnetic (Zn,Mn)Se matrix. On the other hand, in the (Zn,Mn)Te/ZnSe system, the holes are confined in the magnetic (Zn,Mn)Te QDs, while the electrons remain in the surrounding non-magnetic ZnSe matrix. The magnetic polaron formation energies in both systems were measured from the temporal red-shift of the band-edge emission. The magnetic polaron exhibits distinct characteristics depending on the location of the Mn ions. In the ZnTe/(Zn,Mn)Se system the magnetic polaron shows conventional behavior with decreasing with increasing temperature T and increasing magnetic field B. In contrast, in the (Zn,Mn)Te/ZnSe system has unconventional dependence on temperature T and magnetic field B; is weakly dependent on T as well as on B. We discuss a possible origin for such a striking difference in the MP properties in two closely related QD systems.

tria — The electronic properties of In 0.53 Ga 0.47 As/GaAs 0.51 Sb 0.49 2D electron gas (2DEG) systems, in spite of their use in high power electronics, have not been extensively investigated. Recently, they have been suggested as potential materials for IR quantum devices such as quantum cascade lasers (QCL), and they also show a strong Rashba effect 1,2. Here accurate values of the effective masses are important. Two remotely donor (Si)-doped samples grown by MBE with a 2DEG at the single heterostructure interface were studied by FIR magneto-transmission spectroscopy with a BOMEM FTIR spectrometer. The maximum mobilities (near 70 K) are 43,000 cm 2 /Vs and 36,000 cm 2 /Vs with corresponding carrier densities of 1.07 x 10 12 cm −2 and 2.13 x 10 12 cm −2 , respectively. Cyclotron resonance measurements between 4T and 9T yielded m* = 0.0495m 0 for the more heavily doped sample. Individual transmission profiles in this case showed broadening toward high-energy, which may be due to contributions to the overall absorption profile from higher occupied subbands. The lower density sample shows an energy vs B dependence that does not extrapolate to zero at B = 0. The origin of this behavior will be discussed.

HgTe quantum wells with a gapped single Dirac cone electronic dispersion relation have been investigated by THz magneto-photoresponse (PR) and magneto-transport measurements. The QW sample has the conventional band alignment with the well thickness (6.1 nm) slightly smaller than the critical thickness for the topological phase transition. The effective gap of this structure is roughly 10 meV, and the large sheet density ([Formula: see text] m-2) of the two-dimensional electron gas (2DEG) results in a very large Fermi energy ([Formula: see text] meV). We have found several interesting effects at these high densities. In this paper we focus on an observed beating of quantum oscillations in the PR signal (at 1.83 THz) and compare it with direct measurements of oscillations in the longitudinal magneto-resistance (Rxx). The mechanism for the PR is cyclotron resonance absorption heating of the electrons (an electron bolometric effect). We attribute the beating to Rashba splitting of the spin states, which is barely observable in direct Rxx measurements under strong gate-induced electric fields.

We have measured the Landau-level spin-splitting of two-dimensional electrons in the composite InAs/InGaAs channels of two InAs/InGaAs/InAlAs heterostructures with different alloy compositions by magnetotransport and THz magneto-photoconductivity in magnetic fields up to 10 T. The structures differ importantly in the mobility of the channel, the electron density and the composition of the barriers. The magnitudes of the experimental g-factors for B along the quantization axis and their anisotropies are larger by at least a factor of 2 than the corresponding calculated single particle values. The angular dependence of many-body exchange contributions and the effects of broadening of Landau–level densities of states are necessary for understanding this behaviour. We find evidence for a marked decrease of the exchange contribution at low perpendicular magnetic fields in the higher mobility sample from coincidence measurements, but no indications of such behaviour in the lower mobility sample.

HgTe quantum wells (QWs) have shown a number of interesting phenomena, recently the first two-dimensional topological insulating state. We have studied thermoelectric photovoltages of two-dimensional electrons in a 6.1 nm wide HgTe QW induced by cyclotron resonance absorption (B = 2 to 5 T) of a THz laser beam. We have estimated thermopower coefficients by detailed analysis of the photovoltage signals developed across various contacts of a large Hall bar structure at a bath temperature of 1.6 K. The photovoltage signals are washed out at bath temperature of 18 K.

We have used THz magneto-photoresponse/transmission to measure various electronic parameters of a two-dimensional electron gas (2DEG) located primarily within an InAs inserted-channel and the surrounding InGaAs well in an asymmetric InAs/InGaAs/InAlAs inserted-well heterostructure in magnetic fields up to 10 T. We have developed an analytical approach to the photoresponse based on resonant heating of the 2DEG by cyclotron resonance (CR) absorption. The analysis incorporates a resonant T-profile mimicking the CR absorption, combined with the theoretical expression for quantized resistance oscillations of a 2DEG. Fitting of an individual set of PR data vs. magnetic field can in principle, provide g-factor, cyclotron effective mass, SdH scattering time, CR scattering time and carrier density of the 2DEs. This is a temperature differential technique, which leads to enhanced sensitivity to harmonic content of the Shubnikov-deHaas (S-dH) oscillations and thus to clearer spin-splitting of the Landau levels at lower fields than is possible in direct S-dH measurements at the same bath temperature. Results show an enhanced g-factor for electrons and large g-factor anisotropy.

We report a detailed low-temperature study of the two-dimensional (2D) electron gas in a 6.1-nm-wide HgTe quantum well with Hg0.3Cd0.7Te barriers by terahertz magnetophotoconductivity and magnetotransmission combined with magnetotransport measurements (R-xx and R-xy) in magnetic fields up to 10 T. This well width, close to that at the topological phase transition, corresponds to conventional band ordering, and we probe the "bulk" quasi-2D Landau-level (LL) spectrum of the conduction band at high energies (approximate to 135 - 160 meV) above the Dirac point. The calculated separations between adjacent LLs of the same spin based on published parameters for this structure are in fair agreement with the measured cyclotron resonance energies. However, the very large spin splittings observed (E-spin > E-cyclotron) require a significantly larger g-parameter g(e) for electrons. Tilted field coincidence experiments are consistent with the large spin splitting showing coincidences at 3/2 and twice the cyclotron energy. This large value of g(e) also leads to interesting crossings of the calculated LLs, and we find direct evidence of these crossings in the R-xx measurements at lower electron densities (Fermi energies) produced by negative gate bias.

We used time resolved photoluminescence (TRPL) spectroscopy to compare the properties of magnetic polarons in two related, spatially indirect, II-VI epitaxially grown quantum dot systems. In sample A (ZnMnTe/ZnSe), the photoexcited holes are confined in the magnetic ZnMnTe quantum dots (QDs), while the electrons remain in the surrounding non-magnetic ZnSe matrix. In sample B (ZnTe/ZnMnSe) on the other hand, the holes are confined in the non-magnetic ZnTe QDs and the electrons move in the magnetic ZnMnSe matrix. The magnetic polaron formation energies, E-MP, in these samples were measured from the temporal red-shift of the excitonic emission peak. The magnetic polarons in the two samples exhibit distinct characteristics. In sample A, the magnetic polaron is strongly bound with E-MP = 35 meV. Furthermore, E-MP has unconventionally weak dependence of on both temperature T and magnetic field B-appl. In contrast, magnetic polarons in sample B show conventional characteristics with E-MP decreasing with increasing temperature and increasing external magnetic field. We attribute the difference in magnetic polaron properties between the two types of QDs to the difference in the location of the Mn ions in the respective structures.