Te Quantum Research Articles

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.

Growth of Bi2Te3 quantum dots/rods in glass: a unique highly stable nanosystem with novel functionality for high performance magneto optical devices

Magneto optical materials are currently of great interest, primarily for modern applications in optical isolation, modulation and switching in telecommunication. However, single crystals are the benchmark materials still used in these devices which are rather expensive and very difficult to fabricate. In this context, we are reporting herewith a stable and novel Bi(2)Te(3) quantum dot-glass nanosystem obtained using a controlled thermo-chemical method. The Q-dots of hexagonal Bi(2)Te(3) of size 4 to 14 nm were grown along the <1 1 3> direction. Surprisingly, we obtained quantum rods of Bi(2)Te(3) of size 6 × 10 nm for the first time. The strong quantum confinement in the nanosystem is clearly shown by the optical study. The band gap of the host glass was drastically reduced (from 4.00 to 1.88 eV) due to the growth of Bi(2)Te(3) quantum dots whereas photoluminescence showed a Stokes shift ~175 meV. Faraday Rotation (FR) investigations of the Bi(2)Te(3) quantum dot-glass nanosystem show a nonlinear response in Verdet constant with a decrease in the Bi(2)Te(3) dot sizes. The Bi(2)Te(3) Q-dot-glass nanosystem with ~4 nm dots shows significant enhancement (70 times) in Verdet constant compared to the host glass and more radically better than conventional single crystal (TGG). This is the first time that such a type of unique nanosystem has been architectured and has given extremely good magneto-optical performance. We strongly feel that this novel nanosystem has tremendous applications in magneto-optical devices. It is noteworthy that expensive single crystals can be replaced with this cost effective novel glass nanosystem. Interestingly, the present quantum dot-glass nanosystem can be transformed into optical fibers very easily, which will have an exceptionally high impact on the fabrication of high performance magneto optical devices.

Interlevel hybridization phenomena in the coincidence effect under quantum Hall regime in a HgTe quantum well

As found from quantum magnetotransport under tilted magnetic fields in a (013)-oriented n-Cd0.7Hg0.3Te/HgTe/Cd0.7Hg0.3Te quantum well of ∼20nm width, the positions of magnetic level coincidences agree quite well with a traditional description of the coincidence effect like in a simple Γ6 band in spite of the Γ8 character of the conduction band in HgTe. The coincidence angles yield the effective g-factor g* = 33 while g* = 50÷60 is obtained with different magnetotransport techniques under fields oriented perpendicular to the layers. The difference is settled when assuming the g-factor anisotropy of g⊥/g||≈ 2. Level anticrossings are revealed at high values of perpendicular magnetic fields, corresponding to the quantum Hall regime. They manifest themselves in a splitting of the coincidence magnetoresistivity peaks and in a shift of the split-off peaks from the integer filling factors ν to the neighbouring half-integer filling factors. The anticrossing at ν = 3 is found to be significantly stronger than the neighbouring ones at ν = 2 and ν = 4 that is tentatively explained in terms of formation of easy-axis quantum Hall ferromagnetic states in the vicinity of the expected crossings.

Carrier dynamics and activation energy of CdTe quantum dots in a CdxZn1−xTe quantum well

We investigate the optical properties of CdTe quantum dots (QDs) in a Cd0.3Zn0.7Te quantum well (QW) grown on GaAs (100) substrates. Carrier dynamics of CdTe/ZnTe QDs and quantum dots-in-a-well (DWELL) structure is studied using time-resolved photoluminescence (PL) measurements, which show the longer exciton lifetime of the DWELL structure. The activation energy of the electrons confined in the DWELL structure, as obtained from the temperature-dependent PL spectra, was also higher than that of electrons confined in the CdTe/ZnTe QDs. This behavior is attributed to the better capture of carriers into QDs within the surrounding QW.

LASER INDUCED SEMICONDUCTOR–METAL TRANSITION IN A SEMIMAGNETIC QUANTUM WELL

The effect of laser and magnetic field on the semiconductor–metal transition in a Cd 1-x2 Mn x2 Te / Cd 1-x1 Mn x1 Te / Cd 1-x2 Mn x2 Te quantum well has been investigated using variational principle in effective mass approximation for smaller Mn composition. The effect of non parabolicity of the conduction band has been included in our calculation. The results are presented and discussed.

SHAPE EFFECT OF SEMIMAGNETIC QUANTUM DOT ON THE BOUND MAGNETIC POLARON

The effect of geometry, concentration of Mn ion and the magnetic field on the binding energy of a donor and the donor bound magnetic polaronic shift in a finite Cd 1–x1 Mn x1 Te / Cd 1–x2 Mn x2 Te Quantum Dot within the effective mass approximation is carried out employing the variational principle. The results are presented and discussed.

Positively versus negatively charged excitons: A high magnetic field study of CdTe/Cd1−xMgxTe quantum wells

A magneto-optical study of the energy and spin structure of charged excitons in a 20-nm-thick CdTe/Cd${}_{0.65}$Mg${}_{0.35}$Te quantum well is performed in strong magnetic fields up to 51 T. The type of resident carriers (holes or electrons) in the quantum well is controlled optically by above-barrier illumination, permitting a direct comparison of positively (T${}^{+}$) versus negatively (T${}^{\ensuremath{-}}$) charged excitons. The binding energies of the singlet states of these complexes behave qualitatively differently with increasing magnetic field $B$; namely, the binding energy decreases for ${\mathrm{T}}^{+}$ and increases for ${\mathrm{T}}^{\ensuremath{-}}$ with $B$. The triplet state of ${\mathrm{T}}^{+}$ is identified in strong fields with a binding energy smaller than that of the ${\mathrm{T}}^{\ensuremath{-}}$ triplet state.

Manifestation of the composition inhomogeneity of Zn1 − x Mg x Te quantum wires in Raman spectra

The resonant Raman spectra of Zn1 − xMgxTe quantum wires have been investigated. The dependences of the frequencies of longitudinal optical phonons of the ZnTe-like and MgTe-like modes (LO1 and LO2) on the photon energy have been found. The character of these dependences correlates with the variation in the frequencies of optical phonons of the Zn1 − xMgxTe alloys on the composition (x). This gives grounds to assume that the quantum wires are inhomogeneous in the composition. This assumption is also confirmed by the fact that the intensity ratio of the LO2 and LO1 modes in the Raman spectra increases with increasing excitation energy.

Probing the electronic and optical properties of silica-coated quantum dots with first-principles calculations

The electronic and optical natures of silica-coated semiconductor nanocrystals (Cd(2)Te(2)@(SiO(2))(24)) have been investigated by density functional theory (DFT) and time-dependent DFT calculations. The calculated results of Cd(2)Te(2)@(SiO(2))(24) have revealed that the structural synergy effect between the Cd(2)Te(2) quantum dots (QDs) and the silica coating shell plays a dominant role in the photoelectric properties. The binding of embedded Cd(2)Te(2) to the outer silica coating shell leads to the distortion of the silica nanocage, indicating strong coupling between the QDs and silica shell. The optical features of Cd(2)Te(2) clusters and Cd(2)Te(2)@(SiO(2))(24) complexes were evaluated using the time-dependent DFT method. It is determined that the maximal absorption peak of isolated Cd(2)Te(2) in a UV-Vis absorption spectrum appears at 584 nm, which shifts to 534 nm when the Cd(2)Te(2) QDs were encapsulated by silica, in close agreement with the experimental evidence. The excited process has a direct electronic transition character from the occupied Cd(2)Te(2) states to the outer silica nanocage excited states (core → shell electronic transitions). A deep insight into silica-coated QD systems is beneficial for understanding their optical nature and the development of core/shell QDs.

Quantum Hall states under conditions of vanishing Zeeman energy

We report on magneto-transport measurements of a two-dimensional electron gas confined in a Cd$_{0.997}$Mn$_{0.003}$Te quantum well structure under conditions of vanishing Zeeman energy. The electron Zeeman energy has been tuned via the $s-d$ exchange interaction in order to probe different quantum Hall states associated with metallic and insulating phases. We have observed that reducing Zeeman energy to zero does not necessary imply the disappearing of quantum Hall states, i.e. a closing of the spin gap. The spin gap value under vanishing Zeeman energy conditions is shown to be dependent on the filling factor. Numerical simulations support a qualitative description of the experimental data presented in terms of a crossing or an avoided-crossing of spin split Landau levels with same orbital quantum number $N$.

Robust magnetic polarons in type-II (Zn,Mn)Te/ZnSe magnetic quantum dots

We present a magneto-optical study of magnetic polarons in type-II (Zn,Mn) Te quantum dots. The polarons are formed due to the exchange coupling between the spins of the holes and those of the Mn ions, both of which are localized in the dots. In our photoluminescence studies, the magnetic polarons are detected at temperatures up to $\ensuremath{\sim}150\text{ }\text{K}$, with a formation energy of $\ensuremath{\sim}40\text{ }\text{meV}$. The emission from these dots exhibits an unusually small Zeeman shift with applied magnetic field ($\ensuremath{\sim}2\text{ }\text{meV}$ at 8 T) and at the same time a very large circular polarization. We attribute this apparently contradictory behavior by a low and weakly temperature-dependent magnetic susceptibility due to antiferromagnetic coupling of the Mn spins.

Optical control of electron spin coherence in CdTe/(Cd,Mg)Te quantum wells

Optical control of the spin coherence of quantum well electrons by short laser pulses with circular or linear polarization is studied experimentally and theoretically. For that purpose the coherent electron spin dynamics in a $n$-doped CdTe/(Cd,Mg)Te quantum well structure was measured by time-resolved pump-probe Kerr rotation, using resonant excitation of the negatively charged exciton (trion) state. The amplitude and phase shifts of the electron spin beat signal in an external magnetic field, that are induced by laser control pulses, depend on the pump-control delay and polarization of the control relative to the pump pulse. Additive and nonadditive contributions to pump-induced signal due to the control are isolated experimentally. These contributions can be well described in the framework of a two-level model for the optical excitation of the resident electron to the trion.

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.

The effect of magnetic field on acceptor impurity in a diluted magnetic quantum well system

We investigate the effects of magnetic field strength over the ground state binding energy of a hydrogenic acceptor impurity confined in a Cd 1 − x i n Mn x i n Te / Cd 1 − x o u t Mn x o u t Te quantum well. We apply the variational method using 1s-hydrogenic wavefunction, in the framework of the single band effective mass approximation. The effect of valence band degeneracy is considered through the energy dependent effective mass. The magnetic field dependence of the barrier height is discussed. The band gap tuning in an external magnetic field for different compositions of Mn ions is achieved using an empirical formula. Spin polaronic shifts are estimated using mean field theory for different magnetic field strengths and the well sizes. We present a theoretical study of diluted magnetic semiconductors treating the local p–d exchange interaction J between the itinerant carriers and the Mn electrons within a realistic band structure. These results are compared with the other existing available literature.

Optical spin control in nanocrystalline magnetic nanoswitches

We investigate the optical properties of (Cd,Mn)Te quantum dots (QDs) by looking at the excitons as a function of the Mn impurities positions and their magnetic alignments. When doped with two Mn impurities, the Mn spins, aligned initially antiparallel in the ground state, have lower energy in the parallel configuration for the optically active spin-up exciton. Hence, the photoexcitation of the QD ground state with antiparallel Mn spins induces one of them to flip and they align parallel. This suggests that (Cd,Mn)Te QDs are suitable for spin-based operations handled by light.

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.