Showing papers on "Quantum well published in 2013"
TL;DR: In this paper, it was shown that two-dimensional tin films are QSH insulators with sizable bulk gaps of 0.3 eV, sufficiently large for practical applications at room temperature.
Abstract: The search for large-gap quantum spin Hall (QSH) insulators and effective approaches to tune QSH states is important for both fundamental and practical interests. Based on first-principles calculations we find two-dimensional tin films are QSH insulators with sizable bulk gaps of 0.3 eV, sufficiently large for practical applications at room temperature. These QSH states can be effectively tuned by chemical functionalization and by external strain. The mechanism for the QSH effect in this system is band inversion at the $\ensuremath{\Gamma}$ point, similar to the case of a HgTe quantum well. With surface doping of magnetic elements, the quantum anomalous Hall effect could also be realized.
1,113 citations
TL;DR: This work directly confirms the existence of the edge channels of the quantum spin Hall state by imaging the magnetic fields produced by current flowing in large Hall bars made from HgTe quantum wells, providing input to the question of how ballistic transport may be limited in the edge channel.
Abstract: The quantum spin Hall (QSH) state is a state of matter characterized by a non-trivial topology of its band structure, and associated conducting edge channels 1‐5 . The QSH state was predicted 6 and experimentally demonstrated 7 to be realized in HgTe quantum wells. The existence of the edge channels has been inferred from local and non-local transport measurements in sufficiently small devices 7‐9 . Here we directly confirm the existence of the edge channels by imaging the magnetic fields produced by current flowing in large Hall bars made from HgTe quantum wells. These images distinguish between current that passes through each edge and the bulk. On tuning the bulk conductivity by gating or raising the temperature, we observe a regime in which the edge channels clearly coexist with the conducting bulk, providing input to the question of how ballistic transport may be limited in the edge channels. Our results represent a versatile method for characterization of new QSH materials systems 10‐13 . Like an ordinary insulator, the QSH state has a bulk energy gap, but the QSH state supports within the gap a pair of counter-propagating spin-polarized edge modes 15 . The QSH state is predicted in HgTe/(Hg,Cd)Te quantum wells thicker than a critical thickness of 6.3nm, whereas thinner quantum wells should beordinaryinsulators 6,7 .Theedgemodesaretheoreticallyprotected against backscattering by their orthogonal spin states 1416 , and therefore should have a quantized conductance of e 2 =h, where e is
258 citations
TL;DR: In this article, the basic features of ISB transitions in III-nitride quantum wells and quantum dots are described in terms of theoretical calculations, material growth, spectroscopy, resonant transport phenomena, and device implementation.
Abstract: III-nitride nanostructures have recently emerged as promising materials for new intersubband (ISB) devices in a wide variety of applications. These ISB technologies rely on infrared optical transitions between quantum-confined electronic states in the conduction band of GaN/Al(Ga)N nanostructures, namely quantum wells or quantum dots. The large conduction band offset (about 1.8 eV for GaN/AlN) and sub-picosecond ISB relaxation of III-nitrides render them appealing materials for ultrafast photonic devices in near-infrared telecommunication networks. Furthermore, the large energy of GaN longitudinal-optical phonons (92 meV) opens prospects for high-temperature THz quantum cascade lasers and ISB devices covering the 5?10 THz band, inaccessible to As-based technologies due to phonon absorption. In this paper, we describe the basic features of ISB transitions in III-nitride quantum wells and quantum dots, in terms of theoretical calculations, material growth, spectroscopy, resonant transport phenomena, and device implementation. The latest results in the fabrication of control-by-design devices such as all-optical switches, electro-optical modulators, photodetectors, and lasers are also presented.
171 citations
TL;DR: Coherent emission from a GaAs-based quantum well microcavity diode with electrical injection with modulation doping of the wells is observed here and helps to overcome the relaxation bottleneck and to form a macroscopic and degenerate condensate.
Abstract: Inversionless ultralow threshold coherent emission, or polariton lasing, can be obtained by spontaneous radiative recombination from a degenerate polariton condensate with nonresonant excitation. Such excitation has, hitherto, been provided by an optical source. Coherent emission from a GaAs-based quantum well microcavity diode with electrical injection is observed here. This is achieved by a combination of modulation doping of the wells, to invoke polariton-electron scattering, and an applied magnetic field in the Faraday geometry to enhance the exciton-polariton saturation density. These measures help to overcome the relaxation bottleneck and to form a macroscopic and degenerate condensate as evidenced by angle-resolved luminescence, light-current characteristics, spatial coherence, and output polarization. The experiments were performed at 30 K with an applied field of 7 T.
167 citations
TL;DR: In this article, the use of layered growth in MBE affords us the fabrication of heterostructures, such as quantum wells and superlattices, which may further enable additional studies and applications, similar to those of conventional semiconductor heter-structures but with the novel properties of TI.
Abstract: Thin films of V–VI compound semiconductors (Bi2Se3, Bi2Te3 and Sb2Te3) have been synthesized recently as three-dimensional topological insulators (TIs). Although these materials have been used as thermoelectric materials for many years, for future studies and applications of the topological surface states, a major bottleneck remains the lack of high-quality bulk materials that have very few defects and the Fermi level can be moved to inside the bulk bandgap. In this paper, we review the use of molecular beam epitaxy (MBE) technique to achieve high-quality TI materials. Furthermore, the use of layered growth in MBE affords us the fabrication of heterostructures, such as quantum wells and superlattices. Thus, it may further enable additional studies and applications, similar to those of conventional semiconductor heterostructures but with the novel properties of TI. We explore the growth mechanism, providing a detail discussion on the growth parameters of thin-film synthesis by MBE. Then we discuss more complex cases, such as functional doping, heterostructures and superlattices. Potential new properties in such quantum structures are discussed. Finally, we give an outlook on this material system for both fundamental studies and applications. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
160 citations
09 Jun 2013
TL;DR: Polariton lasing under electrical pumping is observed in a multi-quantum well microcavity as mentioned in this paper, and the system's hybrid nature of part light and part matter is probed by measuring the Zeeman-splitting of the microcave mode in the regime of polariton Lasing.
Abstract: Polariton lasing under electrical pumping is observed in a multi quantum well microcavity. The system’s hybrid nature of part light and part matter is probed by measuring the Zeeman-splitting of the microcavity mode in the regime of polariton lasing.
158 citations
TL;DR: In this paper, the authors analyzed the performance of InGaN quantum-well light-emitting diodes with large-bandgap AlGaInN thin barriers with the consideration of carrier transport effect for efficiency droop suppression.
Abstract: The electrical and optical characteristics of InGaN quantum-well light-emitting diodes with large-bandgap AlGaInN thin barriers were analyzed with the consideration of carrier transport effect for efficiency droop suppression. The lattice-matched AlGaInN quaternary alloys with different compositions, thicknesses, and positions were employed as thin barrier layers (1-2 nm) surrounding the InGaN QW in LED structures. The increased effective barrier heights of AlGaInN thin barrier led to suppression of carrier leakage as compared to conventional InGaN QW LEDs with GaN barrier only. The current work provides a comprehensive simulation taking into consideration the carrier transport in self-consistent manner, and the finding indicated the use of thin layers of AlGaInN or AlInN barriers as sufficient for suppressing the droop in InGaN-based QW LEDs. The efficiency of InGaN QW LED with the insertion of lattice-matched Al0.82In0.18N thin barrier layers showed the least droop phenomenon at high current density among the investigated LEDs. The thickness study indicated that a thin layer (<; 2 nm) of large-bandgap material in the barrier region was sufficient for efficiency droop suppression.
153 citations
TL;DR: In this article, the growth, structural investigations, and lasing properties of dilute bismide Ga(AsBi)/(AlGa)As single quantum well lasers with 2.2% Bi grown by metal organic vapor phase epitaxy on GaAs (001) substrates were reported.
Abstract: The Ga(AsBi) material system opens opportunities in the field of high efficiency infrared laser diodes. We report on the growth, structural investigations, and lasing properties of dilute bismide Ga(AsBi)/(AlGa)As single quantum well lasers with 2.2% Bi grown by metal organic vapor phase epitaxy on GaAs (001) substrates. Electrically injected laser operation at room temperature is achieved with a threshold current density of 1.56 kA/cm2 at an emission wavelength of ∼947 nm. These results from broad area devices show great promise for developing efficient IR laser diodes based on this emerging materials system.
151 citations
TL;DR: In this article, the spectral and spatial emission properties of terahertz quantum cascade lasers are studied. But the authors focus on three key factors: photonic structures for extracting and confining light in a cavity, an upconversion technique based on nonlinear intracavity mixing and a frequency stabilisation technique based in femtosecond-laser combs.
Abstract: This article reviews state-of-the-art engineering of the spectral and spatial emission properties of terahertz quantum cascade lasers by focusing on three key factors: photonic structures for extracting and confining light in a cavity, an upconversion technique based on nonlinear intracavity mixing and a frequency stabilisation technique based on femtosecond-laser combs.
138 citations
TL;DR: In this paper, a 2D topological insulator with a graphene layer sandwiched between two Bi2Se3 slabs is presented, which exhibits a large intrinsic bulk band gap of 30-50 meV.
Abstract: Topological insulators (TIs) represent a new quantum state of matter characterized by robust gapless states inside the insulating bulk gap. The metallic edge states of a two-dimensional (2D) TI, known as the quantum spin Hall (QSH) effect, are immune to backscattering and carry fully spin-polarized dissipationless currents. However, existing 2D TIs realized in HgTe and InAs/GaSb suffer from small bulk gaps (<10 meV) well below room temperature, thus limiting their application in electronic and spintronic devices. Here, we report a new 2D TI comprising a graphene layer sandwiched between two Bi2Se3 slabs that exhibits a large intrinsic bulk band gap of 30-50 meV, making it viable for room-temperature applications. Distinct from previous strategies for enhancing the intrinsic spin-orbit coupling effect of the graphene lattice, the present graphene-based TI operates on a new mechanism of strong inversion between graphene Dirac bands and Bi2Se3 conduction bands. Strain engineering leads to effective control and substantial enhancement of the bulk gap. Recently reported synthesis of smooth graphene/Bi2Se3 interfaces demonstrates the feasibility of experimental realization of this new 2D TI structure, which holds great promise for nanoscale device applications.
123 citations
TL;DR: The recombination rate coefficients (RRCs) A, B, and C in MOVPE-grown single-quantum-well light emitting diodes spanning the entire blue green spectral range are determined by fitting efficiency curves and differential carrier lifetimes as mentioned in this paper.
Abstract: The recombination rate coefficients (RRCs) A, B, and C in MOVPE-grown single-quantum-well light emitting diodes spanning the entire blue-green spectral range are determined by fitting efficiency curves and differential carrier lifetimes The results show definite trends for each of the RRCs: A tendentially decreases with increasing wavelength, B definitely decreases, and C remains approximately constant Therefore, the increase of the droop with increasing wavelength (the green gap problem) is rather due to the decrease of B than an increase of C The determined values of C are shown to be similar to what has been predicted by others with first-principles computer simulations accounting for phonon-assisted Auger recombination Samples grown on sapphire and silicon substrates are compared and show significant differences only for the RRC A, presumably due to the difference in threading dislocation density
Journal Article•
TL;DR: It is shown that the intrinsic polarization of materials can be utilized to simultaneously reduce the energy gap and enhance the SOI, driving the system to a TI state.
TL;DR: Numerical calculations of the confined eigenstates suggest that the electrons and holes in their ground states are confined to extremely localized one-dimensional filaments at the corners of the hexagonal structure which extend along the length of the nanowire.
Abstract: The electronic properties of thin, nanometer scale GaAs quantum well tubes embedded inside the AlGaAs shell of a GaAs core–multishell nanowire are investigated using optical spectroscopies. Using numerical simulations to model cylindrically and hexagonally symmetric systems, we correlate these electronic properties with structural characterization by aberration-corrected scanning transmission electron microscopy of nanowire cross sections. These tubular quantum wells exhibit extremely high quantum efficiency and intense emission for extremely low submicrowatt excitation powers in both photoluminescence and photoluminescence excitation measurements. Numerical calculations of the confined eigenstates suggest that the electrons and holes in their ground states are confined to extremely localized one-dimensional filaments at the corners of the hexagonal structure which extend along the length of the nanowire.
TL;DR: In this structure, owing to Fano interference, the diffraction intensity of the grating, especially the first-order diffraction, can be significantly enhanced and this investigation may be used to develop novel photonic devices in semiconductor QW systems.
Abstract: We propose a scheme for obtaining an electromagnetically induced grating in an asymmetric semiconductor quantum well (QW) structure via Fano interference. In our structure, owing to Fano interference, the diffraction intensity of the grating, especially the first-order diffraction, can be significantly enhanced. The diffraction efficiency of the grating can be controlled efficiently by tuning the control field intensity, the interaction length, the coupling strength of tunneling, etc. This investigation may be used to develop novel photonic devices in semiconductor QW systems.
TL;DR: In this paper, the rate of nonradiative Auger recombination in InGaN/GaN quantum wells with rectangular and smooth confining potentials was calculated and a three-fold suppression of the recombination rate was demonstrated.
Abstract: We calculate the rate of nonradiative Auger recombination in InGaN/GaN quantum wells with rectangular and smooth confining potentials. The calculations show that the rate of Auger recombination in rectangular quantum wells is sufficiently high to explain the efficiency droop in nitride-based light emitting diodes (LEDs). This rate, however, can be reduced by softening of the confining potential and a three-fold suppression is demonstrated in the studied quantum wells. The suppression of the Auger recombination rate improves LED radiative efficiency and reduces the droop effect, as we show using the standard recombination (ABC) model.
TL;DR: In this paper, the optical gain and threshold characteristics of InGaN quantum wells (QWs) on ternary inGaN substrate emitting in green and yellow spectral regimes are analyzed.
Abstract: The optical gain and threshold characteristics of InGaN quantum wells (QWs) on ternary InGaN substrate emitting in green and yellow spectral regimes are analyzed. By employing the ternary substrates, the material gains were found as ~ 3-5 times higher than that of conventional method with reduced wavelength shift. The threshold carrier density is reduced by ~ 15%-50% from the use of ternary substrate method for green- and yellow-emitting lasers.
TL;DR: It is shown that the absence of coherence collapse or other CW instabilities typical of diode lasers is inherently associated with the high value of the photon to carrier lifetime ratio and the negligible linewidth enhancement factor of quantum cascade lasers.
Abstract: We study the time dependence of the optical power emitted by terahertz and mid-IR quantum cascade lasers in presence of optical reinjection and demonstrate unprecedented continuous wave (CW) emission stability for strong feedback. We show that the absence of coherence collapse or other CW instabilities typical of diode lasers is inherently associated with the high value of the photon to carrier lifetime ratio and the negligible linewidth enhancement factor of quantum cascade lasers.
TL;DR: In this paper, the influence of spin coherence on the optical bistability and multistability was investigated in a GaAs quantum well waveguide coupled by orthogonally polarized optical fields.
Abstract: In a GaAs quantum well waveguide coupled by orthogonally polarized optical fields, the influence of spin coherence on the optical bistability (OB) and multistability is investigated. It is shown that OB and multistability are very sensitive to the relative phase between applied fields.
TL;DR: In this article, the appearance and disappearance of optical bistability in an asymmetric three-coupled quantum well structure inside a unidirectional ring cavity was investigated.
Abstract: We investigate optical bistability (OB) behavior in an asymmetric three-coupled quantum well structure inside a unidirectional ring cavity. By controlling the assisting coherent driven field and the frequency detunings of the two control laser fields, we find that the appearance and disappearance of OB can easily be controlled by adjusting the positions of the dual electromagnetically induced transparency windows. Analysis in the dressed-state picture is also given. Our scheme may be used for building more efficient all-optical switches and logic-gate devices for optical computing and quantum information processing.
TL;DR: The experimental results are consistent with the theoretically predicted absorptance quantum, AQ = πα/nc for each set of interband transitions in a 2D semiconductor, where α is the fine structure constant and nc is an optical local field correction factor.
Abstract: The optical absorption properties of free-standing InAs nanomembranes of thicknesses ranging from 3 nm to 19 nm are investigated by Fourier transform infrared spectroscopy. Stepwise absorption at room temperature is observed, arising from the interband transitions between the subbands of 2D InAs nanomembranes. Interestingly, the absorptance associated with each step is measured to be ∼1.6%, independent of thickness of the membranes. The experimental results are consistent with the theoretically predicted absorptance quantum, A Q = πα/n c for each set of interband transitions in a 2D semiconductor, where α is the fine structure constant and n c is an optical local field correction factor. Absorptance quantization appears to be universal in 2D systems including III–V quantum wells and graphene.
TL;DR: This work discusses how the underlying polarity of the crystal structure drives the growth of these heterostructures with a 3-fold symmetry resulting in a nonuniform GaAs quantum well tube and AlGaAs shell, and suggests that the Al GaAs growth rate is faster along the [112] B compared to the [ 112] A directions and/or that there is a polarity driven surface reconstruction generating AlGa as growth fronts inclined to the {110} planes.
Abstract: AlGaAs/GaAs quantum well heterostructures based on core-multishell nanowires exhibit excellent optical properties which are acutely sensitive to structure and morphology. We characterize these heterostructures and observe them to have 3-fold symmetry about the nanowire axis. Using aberration-corrected annular dark field scanning transmission electron microscopy (ADF-STEM), we measure directly the polarity of the crystal structure and correlate this with the shape and facet orientation of the GaAs core, quantum wells and cap, and the width of radial Al-rich bands. We discuss how the underlying polarity of the crystal structure drives the growth of these heterostructures with a 3-fold symmetry resulting in a nonuniform GaAs quantum well tube and AlGaAs shell. These observations suggest that the AlGaAs growth rate is faster along the ⟨112⟩ B compared to the ⟨112⟩ A directions and/or that there is a polarity driven surface reconstruction generating AlGaAs growth fronts inclined to the {110} planes. In contras...
TL;DR: High-speed excitable response capabilities in optoelectronic integrated circuits operating at telecommunication wavelengths comprising a nanoscale double barrier quantum well resonant tunneling diode driving a laser diode mimicking the spiking behavior of biological neurons are demonstrated.
Abstract: We demonstrate, experimentally and theoretically, excitable nanosecond optical pulses in optoelectronic integrated circuits operating at telecommunication wavelengths (1550 nm) comprising a nanoscale double barrier quantum well resonant tunneling diode (RTD) photo-detector driving a laser diode (LD). When perturbed either electrically or optically by an input signal above a certain threshold, the optoelectronic circuit generates short electrical and optical excitable pulses mimicking the spiking behavior of biological neurons. Interestingly, the asymmetric nonlinear characteristic of the RTD-LD allows for two different regimes where one obtain either single pulses or a burst of multiple pulses. The high-speed excitable response capabilities are promising for neurally inspired information applications in photonics.
Book•
10 May 2013TL;DR: In this paper, the underlying physics, design principles and performance of quantum cascade (QC) light sources based on intersubband transitions are discussed and applications to chemical sensors for trace gas analysis with part per billion sensitivities.
Abstract: Summary form only given. Quantum cascade (QC) lasers are fundamentally new semiconductor light sources in two respects. Their wavelength is not determined by the band gap of the material and can be tailored over a wide range of the mid-infrared region (3.4 /spl mu/m to 17 /spl mu/m) using the same combination of materials by a suitable choice of the active layer thicknesses. This tutorial will emphasize the underlying physics, design principles and performance of QC lasers based on intersubband transitions. Recent results on high-resolution spectroscopy will be presented and applications to chemical sensors for trace gas analysis with part per billion sensitivities.
TL;DR: In this paper, the authors proposed a heat engine based on a hot cavity connected via quantum wells to electronic reservoirs. And they found that the device delivers a large power of about 0.18 W cm−2 for a temperature difference of 1 K, nearly doubling the power that can be extracted from a similar heat engine.
Abstract: We analyze a heat engine based on a hot cavity connected via quantum wells to electronic reservoirs. We discuss the output power as well as the efficiency both in the linear and nonlinear regime. We find that the device delivers a large power of about 0.18 W cm−2 for a temperature difference of 1 K, nearly doubling the power that can be extracted from a similar heat engine based on quantum dots. At the same time, the heat engine also has good efficiency albeit reduced from the quantum dot case. Due to the large level spacings that can be achieved in quantum wells, our proposal opens a route toward room-temperature applications of nanoscale heat engines.
TL;DR: In this article, the design and performance of multi-stack InAs/InGaAs sub-monolayer (SML) quantum dots (QD) based infrared photodetectors are reported.
Abstract: We report on the design and performance of multi-stack InAs/InGaAs sub-monolayer (SML) quantum dots (QD) based infrared photodetectors (SML-QDIP). SML-QDIPs are grown with the number of stacks varied from 2 to 6. From detailed radiometric characterization, it is determined that the sample with 4 SML stacks has the best performance. The s-to-p (s/p) polarized spectral response ratio of this device is measured to be 21.7%, which is significantly higher than conventional Stranski-Krastanov quantum dots (∼13%) and quantum wells (∼2.8%). This result makes the SML-QDIP an attractive candidate in applications that require normal incidence.
TL;DR: In this paper, the authors review different strategies to achieve a three-dimensional energy bandgap modulation in a nanowire (NW) by the introduction of self-assembled 0D, 1D and 2D quantum structures, quantum dots (QDs), quantum wires (QWRs) and quantum wells (QWs).
Abstract: We review different strategies to achieve a three-dimensional energy bandgap modulation in a nanowire (NW) by the introduction of self-assembled 0D, 1D and 2D quantum structures, quantum dots (QDs), quantum wires (QWRs) and quantum wells (QWs). Starting with the well-known axial, radial (coaxial/prismatic) or polytypic quantum wells in GaN/AlN, GaAs/AlAs or wurtzite/zinc-blende systems, respectively, we move to more sophisticated structures by lowering their dimensionality. New recent approaches developed for the self-assembly of GaN quantum wires and InAs or AlGaAs quantum dots on single nanowire templates are reported and discussed. Aberration corrected scanning transmission electron microcopy is presented as a powerful tool to determine the structure and morphology at the atomic scale allowing for the creation of 3D atomic models that can help us to understand the enhanced optical properties of these advanced quantum structures.
TL;DR: In this paper, local InGaN decomposition and resultant inhomogeneous luminescence in green laser diode (LD) epitaxial structures were investigated using micro-photoluminescence, Z-contrast scanning transmission electron microscopy, and high-resolution transmission electron microscope.
Abstract: Local InGaN quantum well (QW) decomposition and resultant inhomogeneous luminescence in green laser diode (LD) epitaxial structures are investigated using micro-photoluminescence, Z-contrast scanning transmission electron microscopy, and high-resolution transmission electron microscopy. The local InGaN QW decomposition is found to happen during p-type layer growth due to too high thermal budget and may initiate at the InGaN/GaN QW upper interface probably due to the formation of In-rich InGaN clusters there. Reducing thermal budget and optimizing InGaN/GaN QW growth suppress the local InGaN QW decomposition, and green LD structures with homogeneous luminescence and bright electroluminescence (EL) intensity are obtained.
TL;DR: In this article, the thermoelectric and physical properties of superlattices consisting of modulation doped Ge quantum wells inside Si 1−yGey barriers are presented.
Abstract: The thermoelectric and physical properties of superlattices consisting of modulation doped Ge quantum wells inside Si1−yGey barriers are presented, which demonstrate enhancements in the thermoelectric figure of merit, ZT, and power factor at room temperature over bulk Ge, Si1−yGey, and Si/Ge superlattice materials. Mobility spectrum analysis along with low temperature measurements indicate that the high power factors are dominated by the high electrical conductivity from the modulation doping. Comparison of the results with modelling using the Boltzmann transport equation with scattering parameters obtained from Monte Carlo techniques indicates that a high threading dislocation density is also limiting the performance. The analysis suggests routes to higher thermoelectric performance at room temperature from Si-based materials that can be fabricated using micro- and nano-fabrication techniques.
TL;DR: In this article, a single-peak narrow-bandwidth thermal emission with a quality factor (Q factor) of more than 100 at a wavelength of 9.1 µm was demonstrated.
Abstract: We experimentally demonstrate single-peak narrow-bandwidth thermal emission with a quality factor (Q factor) of more than 100 at a wavelength of 9.1 μm. The emission is significantly suppressed at all other wavelengths. Our emitter is based on an intersubband transition in a multiple quantum well structure combined with a single high-Q resonant mode in a two-dimensional photonic crystal slab, which allows strong light-matter interaction only at a specific wavelength. Strong thermal emission is exhibited only in a limited angular range (∼20°) from the normal direction. Our results have potential applications in bio- and environmental sensors.
TL;DR: In this article, the giant photocurrents in HgTe/HgCdTe quantum well were observed and it was shown that the photocurrent is caused by cyclotron resonance in a Dirac fermion system, which allows the effective electron velocity v≈7.2×105 m/s.
Abstract: We report on the observation of the giant photocurrents in HgTe/HgCdTe quantum well (QW) of critical thickness at which a Dirac spectrum emerges. At an exciting QW of 6.6 nm width by terahertz (THz) radiation and sweeping magnetic field we detected a resonant photocurrent. Remarkably, the position of the resonance can be tuned from negative (−0.4 T) to positive (up to 1.2 T) magnetic fields by means of optical doping. The photocurrent data, accompanied by measurements of radiation transmission as well as Shubnikov–de Haas and quantum Hall effects, prove that the photocurrent is caused by cyclotron resonance in a Dirac fermion system, which allows us to obtain the effective electron velocity v≈7.2×105 m/s. We develop a microscopic theory of the effect and show that the inherent spin-dependent asymmetry of light-matter coupling in the system of Dirac fermions causes the electric current to flow.