Te Quantum Wells Research Articles

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.

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.

Spin-polarized electric currents in diluted magnetic semiconductor heterostructures induced by terahertz and microwave radiation

We report on the study of spin-polarized electric currents in diluted magnetic semiconductor (DMS) quantum wells subjected to an in-plane external magnetic field and illuminated by microwave or terahertz radiation. The effect is studied in (Cd,Mn)Te/(Cd,Mg)Te quantum-wells (QWs) and (In,Ga)As/InAlAs:Mn QWs belonging to the well-known II-VI and III-V DMS material systems, as well as in heterovalent AlSb/InAs/(Zn,Mn)Te QWs, which represent a promising combination of II-VI and III-V semiconductors. Experimental data and developed theory demonstrate that the photocurrent originates from a spin-dependent scattering of free carriers by static defects or phonons in the Drude absorption of radiation and subsequent relaxation of carriers. We show that in DMS structures, the efficiency of the current generation is drastically enhanced compared to nonmagnetic semiconductors. The enhancement is caused by the exchange interaction of carrier spins with localized spins of magnetic ions resulting, on the one hand, in the giant Zeeman spin splitting, and, on the other hand, in the spin-dependent carrier scattering by localized Mn${}^{2+}$ ions polarized by an external magnetic field.

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)

Dissipationless phonon Hall viscosity

We study the acoustic phonon response of crystals hosting a gapped time-reversal symmetry-breaking electronic state. The phonon effective action can in general acquire a dissipationless ``Hall' viscosity, which is determined by the adiabatic Berry curvature of the electron wave function. This Hall viscosity endows the system with a characteristic frequency ${\ensuremath{\omega}}_{v}$; for acoustic phonons of frequency $\ensuremath{\omega}$, it shifts the phonon spectrum by an amount of order ${(\ensuremath{\omega}/{\ensuremath{\omega}}_{v})}^{2}$ and it mixes the longitudinal and transverse acoustic phonons with a relative amplitude ratio of $\ensuremath{\omega}/{\ensuremath{\omega}}_{v}$ and with a phase shift of $\ifmmode\pm\else\textpm\fi{}\ensuremath{\pi}/2$, to lowest order in $\ensuremath{\omega}/{\ensuremath{\omega}}_{v}$. We study several examples, including the integer quantum Hall states, the quantum anomalous Hall state in Hg${}_{1\ensuremath{-}y}$Mn${}_{y}$Te quantum wells, and a mean-field model for ${p}_{x}+i{p}_{y}$ superconductors. We discuss situations in which the acoustic phonon response is directly related to the gravitational response, for which striking predictions have been made. When the electron-phonon system is viewed as a whole, this provides an example where measurements of Goldstone modes may serve as a probe of adiabatic curvature of the wave function of the gapped sector of a system.

ODMR Study of Zn1−x Mn x Se/Zn1−y Be y Se and (Cd1−x ,Mn)Te/Cd1−y Mg y Te Diluted Magnetic Semiconductor Quantum Wells

The effects of microwave pumping with a frequency of 60 GHz on the magneto-optical properties of diluted magnetic semiconductors (DMSs) are studied in (Zn,Mn)Se/(Zn,Be)Se and (Cd,Mn)Te/(Cd,Mg)Te quantum wells. Resonant heating of the Mn2+ ions in the electron spin resonance conditions leads to an increase in the Mn-spin temperature, which exceeds the bath temperature by up to 5.2 K, as detected by the shift of exciton emission line and decrease of its integral intensity. Nonresonant heating mediated by free carriers is also observed through variation of the polarization degree of emission. Direct measurements of spin–lattice relaxation times for both materials using time-resolved optically detected magnetic resonance (ODMR) technique have been performed. The mechanisms of ODMR in nanostructures of DMSs are discussed.

Local electronic properties and magnetism of (Cd,Mn)Te quantum wells

The electronic properties of heterostructures containing (Cd,Mn)Te quantum wells are probed by soft x-ray spectroscopies. We provide experimental evidence that Mn ions are in the Mn2+ (3d5) electronic configuration, and rule out the possibility that charge transfer and crystal field effects can reduce the magnetic moment of each Mn ion to the value (0.61) extracted from the fitting of the magnetization curve with a Brillouin function. These results confirm that the observed magnetic behavior can be correctly explained by assuming an antiferromagnetic coupling between nearest-neighbor S=5/2 Mn2+ ions, rather than a paramagnetic response from an ensemble of low-spin Mn ions.

Observation of the magnetic soft mode in (Cd,Mn)Te quantum wells using spin-flip Raman scattering

We report the experimental observation of a magnetic soft mode of collective spin excitations in (Cd,Mn)Te quantum wells, containing a 2D hole gas interacting with magnetic ions. We found that with increasing hole concentration the eigenfrequency of the paramagnetic spin resonance decreases, which is predicted to occur prior to the ferromagnetic phase transition. Such a magnetic soft mode is detected in spin-flip Raman-scattering experiments as a reduction in the Mn $g$ factor. The results are described in terms of collective dynamics of strongly coupled spin systems with positive feedback. Based on this approach the evolution of such excitations with hole concentration is calculated.

Reducing influence of antiferromagnetic interactions on ferromagnetic properties of p-(Cd,Mn)Te quantum wells

In order to explain the absence of hysteresis in ferromagnetic p-type (Cd,Mn)Te quantum wells (QWs), spin dynamics was previously investigated by Monte Carlo simulations combining the Metropolis algorithm with the determination of hole eigenfunctions at each Monte Carlo sweep. Short-range antiferromagnetic superexchange interactions between Mn spins—which compete with the hole-mediated long-range ferromagnetic coupling—were found to accelerate magnetization dynamics if the layer containing Mn spins is wider than the vertical range of the hole wave function. Employing this approach it is shown here that appreciate magnitudes of remanence and coercivity can be obtained if Mn ions are introduced to the quantum well in a delta-like fashion.

Magnetization Dynamics of a (Cd,Mn)Te Quantum Well in Pulsed Magnetic Field

Magnetization dynamics in bulk (Cd,Mn)Te crystals has been studied by many authors since the eighties. It was demonstrated that in magnetic field of the order of 1 T, the magnetization relaxation is mainly caused by the spin–lattice interaction [1, 2]. Its characteristic time is of the order of microseconds for samples with Mn content equal to a few percent. Our present work is devoted to two-dimensional structures — quantum wells (QWs). In this paper we present results of magnetization relaxation studies of a (Cd,Mn)Te QW in a pulsed magnetic field.

Giant modulation of resonance Raman scattering from (Cd,Mn)Te quantum wells by secondary illumination

Large changes in the intensity of spin-flip Raman signals from (Cd,Mn)Te quantum wells arise under weak additional illumination above the barrier band-gap energy even when the secondary beam is 2 orders of magnitude weaker than the beam used to excite resonant Raman scattering. The behavior is ascribed to changes in the lifetime of the intermediate state in the Raman-scattering process caused by the light-induced alterations in the quantum well carrier density; spin-flip Raman scattering is highly sensitive to this since it is a doubly resonant process.

Spin coherence of holes and electrons in undoped CdTe/(Cd,Mg)Te quantum wells

The coherent spin dynamics of electrons and holes in an undoped CdTe/(Cd,Mg)Te quantum well structure was studied by time-resolved pump-probe Kerr rotation technique using resonant excitation of the charged exciton (trion) state. Long-living spin coherence of electrons (up to 10 ns) and holes (up to 500 ps) was measured. The unusual observation of long-living spin beats from both types of carriers in parallel is explained by coexistence of resident electrons and holes which are separated spatially in the quantum well plane. This conclusion is confirmed by experiments where type and concentration of resident carriers are tuned by above-barrier illumination.

Magnetization Dynamics Down to a Zero Field in Dilute (Cd,Mn)Te Quantum Wells

The evolution of the magnetization in (Cd,Mn)Te quantum wells after a short pulse of magnetic field was determined from the giant Zeeman shift of spectroscopic lines. The dynamics in the absence of a static magnetic field was found to be up to 3 orders of magnitude faster than that at 1 T. Hyperfine interaction and strain are mainly responsible for the fast decay. The influence of a hole gas is clearly visible: at zero field anisotropic holes stabilize the system of Mn ions, while in a magnetic field of 1 T they are known to speed up the decay by opening an additional relaxation channel.

Nanosecond Spin Dynamics in (Cd,Mn)Te Quantum Dots and Quantum Wells

Nanosecond Spin Dynamics in (Cd,Mn)Te Quantum Dots and Quantum Wells

Quantum Anomalous Hall Effect inHg1−yMnyTeQuantum Wells

The quantum Hall effect is usually observed when a two-dimensional electron gas is subjected to an external magnetic field, so that their quantum states form Landau levels. In this work we predict that a new phenomenon, the quantum anomalous Hall effect, can be realized in Hg{1-y}Mn{y}Te quantum wells, without an external magnetic field and the associated Landau levels. This effect arises purely from the spin polarization of the Mn atoms, and the quantized Hall conductance is predicted for a range of quantum well thickness and the concentration of the Mn atoms. This effect enables dissipationless charge current in spintronics devices.

Charged Excitons and Biexcitons in CdZnTe/(Cd,Zn,Mn)Te Quantum Wells in Pulse Magnetic Fields

We have studied exciton complexes such as charged excitons and biexcitons in dilute magnetic semiconductor Cd 1- x Zn x Te/Cd 1- x - y Zn x Mn y Te ( x =0.07, y =0.43) quantum wells (well widths 4 and 9 nm). Laser excitation power dependence and time-resolved photoluminescence (PL) showed unique features related to excitons, charged excitons, and biexcitons. Four-wave-mixing spectroscopy with different choices of polarized incident light exhibited beating signals arising from exciton-biexciton interaction. Circular polarized PL was measured in magnetic fields of up to 42 T using a pulse magnet. The observed Zeeman splitting and the degree of circular polarization in magnetic fields revealed that the formation of biexcitons is quenched upon applying a magnetic field, and PL from the charged excitons was preserved up to the highest magnetic field.

The Quantum Spin Hall Effect: Theory and Experiment

The search for topologically non-trivial states of matter has become an important goal for condensed matter physics. Recently, a new class of topological insulators has been proposed. These topological insulators have an insulating gap in the bulk, but have topologically protected edge states due to the time reversal symmetry. In two dimensions the helical edge states give rise to the quantum spin Hall (QSH) effect, in the absence of any external magnetic field. Here we review a recent theory which predicts that the QSH state can be realized in HgTe/CdTe semiconductor quantum wells (QWs). By varying the thickness of the QW, the band structure changes from a normal to an “inverted” type at a critical thickness d c . We present an analytical solution of the helical edge states and explicitly demonstrate their topological stability. We also review the recent experimental observation of the QSH state in HgTe/(Hg,Cd)Te QWs. We review both the fabrication of the sample and the experimental setup. For thin QWs w...