Te Quantum Research Articles

Suris tetrons: possible spectroscopic evidence for four-particle optical excitations of a two-dimensional electron gas.

The excitations of a two-dimensional electron gas in quantum wells with intermediate carrier density (ne∼1011 cm-2), i.e., between the exciton-trion and the Fermi-sea range, are so far poorly understood. We report on an approach to bridge this gap by a magnetophotoluminescence study of modulation-doped (Cd,Mn)Te quantum well structures. Employing their enhanced spin splitting, we analyzed the characteristic magnetic-field behavior of the individual photoluminescence features. Based on these results and earlier findings by other authors, we present a new approach for understanding the optical transitions at intermediate densities in terms of four-particle excitations, the Suris tetrons, which were up to now only predicted theoretically. All characteristic photoluminescence features are attributed to emission from these quasiparticles when attaining different final states.

Coherent Coupling of Excitons and Trions in a Photoexcited CdTe/CdMgTe Quantum Well

We present zero-, one-, and two-quantum two-dimensional coherent spectra of excitons and trions in a CdTe/(Cd,Mg)Te quantum well. The set of spectra provides a unique and comprehensive picture of the coherent nonlinear optical response. Distinct peaks in the spectra are manifestations of exciton-exciton and exciton-trion coherent coupling. Excellent agreement using density matrix calculations highlights the essential role of many-body effects on the coupling. Strong exciton-trion coherent interactions open up the possibility for novel conditional control schemes in coherent optoelectronics.

Resonant optical pumping of a Mn spin in a strain-free quantum dot

The authors fabricate strain-free shallow CdTe/(Cd,Mg)Te quantum dots containing isolated Mn atoms. The reduction in strain-induced magnetic anisotropy yields optical and dynamical properties qualitatively different from those of the strained self-assembled dots studied previously.

Plasmons due to the Interplay of Dirac and Schrödinger Fermions

We study the interplay between Dirac and Schrödinger fermions in the polarization properties of a two-dimensional electron gas (2DEG). Specifically, we analyze the low-energy sector of narrow-gap semiconductors described by a two-band Kane model. In the context of quantum spin Hall insulators, particularly, in Hg(Cd)Te quantum wells, this model is named the Bernevig-Hughes-Zhang model. Interestingly, it describes electrons with intermediate properties between Dirac and Schrödinger fermions. We calculate the dynamical dielectric function of such a model at zero temperature within random phase approximation. Surprisingly, plasmon resonances are found in the intrinsic (undoped) limit, whereas they are absent-in that limit-in graphene as well as ordinary 2DEGs. Additionally, we demonstrate that the optical conductivity offers a quantitative way to identify the topological phase of Hg(Cd)Te quantum wells from a bulk measurement.

Heating of the Mn spin system by photoexcited holes in type‐II (Zn,Mn)Se/(Be,Mn)Te quantum wells

The efficiency of the Mn‐spin system heating under pulsed laser excitation is studied in diluted magnetic semiconductor heterostructures Zn0.99 Mn0.01 Se/Be0.93 Mn0.07 Te with type‐II band alignment by means of time‐resolved photoluminescence and pump‐probe reflectivity. An essential role in the heating is played by multiple spin‐flip scatterings of a hole with localized spins of Mn2+ ions. The efficiency of the spin and energy transfer from photoexcited holes to Mn ions of the Zn0.99 Mn0.01 Se layer considerably depends on the hole lifetime in this layer. This lifetime can be limited by the hole relaxation into the Be0.93 Mn0.07 Te layers and is strongly sensitive to the excitation power and Zn0.99 Mn0.01 Se layer thickness. These dependences allow us to determine a characteristic time of about 20ps for the spin and energy transfer from photoexcited holes to the Mn spin system.

Growth and properties of HgTe quantum wells -A topic review

The properties of HgTe/Hg1−x Cdx Te superlattices (SLs) and quantum wells, (QWs), are of fundamental interest; SLs are potentially useful for infrared opto-electronic applications and QWs have aroused much interest due to recent evidence that they are topological insulators with dissipation less transport in edge states in the absence of a magnetic field. Depending on the HgTe thickness , the band structure has either the normal sequence of quantum well states or an inverted one when nm, in which the electron E1 subband is a valence subband and lies below the heavy hole H1 subband. This has been shown to lead to a very large Rashba spin–orbit (s–o) splitting of up to meV in HgTe/Hg1−x Cdx Te QWs with an inverted band structure. This as well as other transport results will be presented. These properties of the HgTe/Hg1−x Cdx Te QW, large s–o splitting and an inverted band structure, have recently been shown to result in the quantum spin Hall effect. In the quantum spin Hall effect, carriers with opposite spin move in opposite directions on a given edge in an insulator in the absence of a magnetic field. Convincing evidence for non-local edge channel transport in HgTe based QWs has been recently published. The results demonstrate that quantum transport through helical edge channels is dissipation less at zero magnetic field and agree quantitatively with the theory of the quantum spin Hall effect. Images of current in the edge channels by means of scanning gate microscopy have revealed well localized scattering sites which perturb the quantum spin Hall edge states on a sub-micrometer scale. In the micrometer sized regions between the scattering sites, current appeared to propagate unperturbed Recently, images of edge channels in HgTe QWs in the quantum spin Hall regime have been obtained by means of their magnetic fields with a scanning superconducting quantum interference device (SQUID). The edge channels were observed to dominate transport in devices with edges much longer than the scattering length, and to persist even in the presence of bulk conduction.

Benchmark Assessment of Density Functional Methods on Group II-VI MX (M = Zn, Cd; X = S, Se, Te) Quantum Dots.

In this work, we build a benchmark data set of geometrical parameters, vibrational normal modes, and low-lying excitation energies for MX quantum dots, with M = Cd, Zn, and X = S, Se, Te. The reference database has been constructed by ab initio resolution-of-identity second-order approximate coupled cluster RI-CC2/def2-TZVPP calculations on (MX)6 model molecules in the wurtzite structure. We have tested 26 exchange-correlation density functionals, ranging from local generalized gradient approximation (GGA) and hybrid GGA to meta-GGA, meta-hybrid, and long-range corrected. The best overall functional is the hybrid PBE0 that outperforms all other functionals, especially for excited state energies, which are of particular relevance for the systems studied here. Among the DFT methodologies with no Hartree-Fock exchange, the M06-L is the best one. Local GGA functionals usually provide satisfactory results for geometrical structures and vibrational frequencies but perform rather poorly for excitation energies. Regarding the CdSe cluster, we also present a test of several basis sets that include relativistic effects via effective core potentials (ECPs) or via the ZORA approximation. The best basis sets in terms of computational efficiency and accuracy are the SBKJC and def2-SV(P). The LANL2DZ basis set, commonly employed nowadays on these types of nanoclusters, performs very disappointingly. Finally, we also provide some suggestions on how to perform calculations on larger systems keeping a balance between computational load and accuracy.

Visible quantum-dot-based targeted siRNA delivery for HIF-1α gene silencing

In the present paper, we synthesized cadmium (Cd) telluride (Te) quantum dots (CdTe QDs) by a hydrothermal method using three different ligands as stabilizers: N-acetyl-L-cysteine (NAC), cysteamine hydrochloride (CA), or 2-(dimethylamino) ethanethiol hydrochloride (DMAE). The results demonstrated that CA-coated CdTe QDs had a higher zeta potential (positive) compared with the NAC- and DMAE-coated CdTe QDs. Furthermore, CA-coated CdTe QDs had greater ability to bind siRNA and greater uptake by human gastric cancer cells (SGC-7901). The release of siRNA from the surface of CdTe QDs was induced by high concentrations of reduced glutathione (GSH). mRNA and protein levels of HIF-1α in SGC-7901 cells were significantly reduced after being treated with the complex of CA-coated CdTe QDs and HIF-1α siRNA under strictly anerobic conditions. In addition, CA-coated CdTe QDs displayed excellent optical properties, good morphology, and low toxicity to normal cells.

Nonlocal resistance and its fluctuations in microstructures of band-inverted HgTe/(Hg,Cd)Te quantum wells

We investigate experimentally transport in gated microsctructures containing a band-inverted HgTe/Hg_{0.3}Cd_{0.7}Te quantum well. Measurements of nonlocal resistances using many contacts prove that in the depletion regime the current is carried by the edge channels, as expected for a two-dimensional topological insulator. However, high and non-quantized values of channel resistances show that the topological protection length (i.e. the distance on which the carriers in helical edge channels propagate without backscattering) is much shorter than the channel length, which is ~100 micrometers. The weak temperature dependence of the resistance and the presence of temperature dependent reproducible quasi-periodic resistance fluctuations can be qualitatively explained by the presence of charge puddles in the well, to which the electrons from the edge channels are tunnel-coupled.

Spin-flip Raman scattering of the neutral and charged excitons confined in a CdTe/(Cd,Mg)Te quantum well

Spin-flip Raman scattering of electrons and heavy holes is studied for resonant excitation of neutral and charged excitons in a CdTe/Cd${}_{0.63}$Mg${}_{0.37}$Te quantum well. The spin-flip scattering is characterized by its dependence on the incident and scattered light polarization as well as on the magnetic field strength and orientation. Model schemes of electric-dipole-allowed spin-flip Raman processes in the exciton complexes are compared to the experimental observations, from which we find that lowering the exciton symmetry, time of carrier spin relaxation, and mixing between electron states and, respectively, light- and heavy-hole states play an essential role in the scattering. At the exciton resonance, anisotropic exchange interaction induces heavy-hole spin-flip scattering, while acoustic phonon interaction is mainly responsible for the electron spin-flip. In resonance with the positively and negatively charged excitons, anisotropic electron-hole exchange as well as mixed electron states allow spin-flip scattering. Variations in the resonant excitation energy and lattice temperature demonstrate that localization of resident electrons and holes controls the Raman process probability and is also responsible for symmetry reduction. We show that the intensity of the electron spin-flip scattering is strongly affected by the lifetime of the exciton complex, and in tilted magnetic fields it is affected by the angular dependence of the anisotropic electron-hole exchange interaction.

Subnanosecond magnetization dynamics induced by a pulsed magnetic field in diluted magnetic semiconductor quantum wells

The magnetization dynamics induced by a pulsed magnetic field is investigated by time- and polarization-resolved photoluminescene measurements in (Cd,Mn)Te/(Cd,Mg)Te quantum wells. The magnetization dynamics of Mn${}^{2+}$ ions is found to be strongly dependent on the external static magnetic field. A dynamical response of the magnetization on a subnanosecond time scale is observed at zero static magnetic field, while it drastically slows down and approaches the spin-lattice relaxation time constant for a nonzero static field. Theoretical calculations emphasize the importance of local spin interactions that interplay with the Zeeman interaction for the observed magnetization dynamics.

Inversion asymmetry effects in modulation-doped Cd1−xMnxTe quantum wells

We report a striking in-plane anisotropy of the spin-flip Raman signals observed for dilute magnetic Cd${}_{1\ensuremath{-}x}$Mn${}_{x}$Te quantum wells containing a two-dimensional electron gas. The effect depends upon electron concentration, which can be varied within a single sample via secondary above-barrier illumination. The experimental results are described in a simple, single-electron picture by a model of the conduction band Hamiltonian that includes contributions from Dresselhaus, Rashba, and Zeeman terms.

Self-organization and photoluminescence properties of Pb0.7Sn0.3Te quantum dots embedded in a CdTe matrix

This paper describes self-organization of Pb0.7Sn0.3Te quantum dots (QDs) in a CdTe matrix and their photoluminescence (PL) properties by comparing with those of PbTe QDs in the same matrix. Owning to the large interface tension by the lattice-type mismatch of this heterosystem, postgrowth annealing brought a Pb0.7Sn0.3Te/CdTe quantum well into highly symmetric Pb0.7Sn0.3Te/CdTe QDs similar to the case of PbTe/CdTe heterosystem but with a little smaller QD size. Temperature dependent midinfrared PL spectra of these Pb0.7Sn0.3Te/CdTe and PbTe/CdTe QDs were explained by a theoretical calculation using the effective mass approximation and deformation potentials.

MBE growth and characterization of a II–VI distributed Bragg reflector and microcavity lattice-matched to MgTe

We present the realization and characterization of a 20-fold, fully lattice-matched epitaxial distributed Bragg reflector based on (Cd,Zn)Te and (Cd,Zn,Mg)Te layers. We also present a microcavity based on (Cd,Zn,Mg)Te containing a (Cd,Zn)Te quantum well. Reflectivity spectra, photoluminescence imaging in real space and in far field are presented.

Resonant spin amplification of resident electrons in CdTe/(Cd,Mg)Te quantum wells subject to tilted magnetic fields

Electron spin coherence in CdTe/(Cd,Mg)Te quantum wells is studied experimentally and theoretically in tilted external magnetic fields generated by a superconducting vector magnet. The long-lived spin coherence is measured by pump-probe Kerr rotation in the resonant spin amplification (RSA) regime. The shape of RSA signals is very sensitive to weak magnetic field components deviating from the Voigt or Faraday geometries.

Magnetospectroscopy of two-dimensional HgTe-based topological insulators around the critical thickness

Recently, a new class of materials, so-called topological insulators, has emerged. These are systems characterized by the inversion of the electronic band structure and also by a certain strength of the spin-orbit interaction. HgTe/Cd${}_{x}$Hg${}_{1\ensuremath{-}x}$Te quantum wells represent a prominent example. They can change to the topological insulator phase from the conventional insulator phase when the thickness of the quantum well is increased over the critical thickness ${d}_{c}$ = 6.3 nm. Here, we report on a far-infrared magnetospectroscopy study of a set of HgTe/Cd${}_{x}$Hg${}_{1\ensuremath{-}x}$Te quantum wells with different thicknesses from below to above the critical value ${d}_{c}$. In quantizing magnetic fields up to 16 T, both intraband and interband transitions have been clearly observed. In the widest quantum well with inverted band structure, we confirm the avoided crossing of the zero-mode Landau levels observed earlier in similar structures. In both noninverted quantum wells close to the critical thickness, we report unambiguously on the square root dependence of the transition energy on the magnetic field, as expected in the single-particle model of massless Dirac fermions. The obtained results are compared with the allowed transition energies between Landau levels in the valence and conduction bands calculated using the 8 \ifmmode\times\else\texttimes\fi{} 8 Kane model.

Transport through a quantum spin Hall quantum dot

Quantum spin Hall insulators, recently realized in HgTe/(Hg,Cd)Te quantum wells, support topologically protected, linearly dispersing edge states with spin-momentum locking. A local magnetic exchange field can open a gap for the edge states. A quantum-dot structure consisting of two such magnetic tunneling barriers is proposed, and the charge transport through this device is analyzed. The effects of the bias voltage, the gate voltage, and the charging energy in the quantum dot are studied employing Landauer and master-equation approaches. For vanishing charging energy, the differential conductance is periodic in both gate and bias voltages. For nonzero charging energy, the periodicity in the gate voltage is retained, but with increased period. A partial recurrence of the noninteracting periodicities is found for strong interactions. The possibility of controlling the edge magnetization by a locally applied gate voltage is proposed.

Optically detected magnetic resonance of Mn-related excitations in (Cd,Mn)Te quantum wells

Optically detected magnetic resonance (ODMR) was applied to reveal the exchange interaction effects between Mn2+ ions and confined holes in (Cd,Mn)Te quantum wells with 2D hole gas. Two anisotropic ODMR signals with different angular variations were found and ascribed to isolated manganese ions and to exchange-coupled complexes consisting of manganese and holes. It is shown that calculations on the basis of spin Hamiltonian for these systems are in agreement with the experimental data.

New Cd 1− x Mn x Te quantum dots for application in light-emitting diodes

Cd1−xMnxTe quantum dots (QDs) were synthesised through a one-step approach in an aqueous medium, and a red-shift in the emission peak wavelength, from 542 nm to a long wavelength of 566 nm, was observed by doping Mn2+ ions into the CdTe QDs. A red light-emitting diode (LED) device was fabricated by combining red light-emitting Cd1−xMnxTe QDs with a near-UV InGaN LED chip. CIE colour coordinates of the LED at (0.56, 0.25) demonstrated a near red light-emitting LED. The results showed that the Cd1−xMnxTe QDs are good candidates for LED applications.

Exciton–Mn exchange interactions as a function of translational wavevector in Cd1‐xMnxTe quantum wells

AbstractThe exchange interaction between excitons and manganese ions have been studied in wide quantum wells of Cd1‐x Mnx Te with Cd1‐y Mgy Te barriers. The widths L of the wells are sufficiently large for the excitons to be described by the centre of mass approximation, in which the translational wavevector is quantised according to Kz = Nπ/L. Excitonic transitions with different values of the quantisation index N can be resolved in the optical spectra. The giant Zeeman splittings induced by a magnetic field then enable the separate determination of the exchange energy for each value of the wavevector. We find that the quantity N0 (α – β), which describes the exchange splitting, is constant to within 3% over the range of wavevectors up to 4 × 106 cm–1 (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)