Quantum-well Resonances Research Articles

Quantum-well resonances caused by partial confinement in MgO-based magnetic tunnel junctions

Quantum-well resonances caused by partial confinement in MgO-based magnetic tunnel junctions

Negative electron affinity opens quantum well in MgO layers on Ag(100)

Epitaxial MgO films on Ag(100) were studied by photoelectron spectroscopy. From the low-energy part of the spectra we obtain a negative electron affinity of about −0.9 eV. Even though electrons in the lowest conduction band are not confined by a potential barrier at the surface, quantum-well resonances are observed. The dispersion of the conduction band is determined in good agreement with theoretical calculations. Aspects of observing image-potential states predicted by theory on MgO films are discussed.

Magnetophonon resonance in quantum wells due to absorption and emission of confined phonon

In this paper, the effect of confined phonons described by the Huang-Zhu model, Ridley's guided mode model, and Fuchs-Kliewer slab mode model on the full-width at half-maximum (FWHM) of the optically detected magnetophonon resonance (ODMPR) peak for the inter-subband transitions (1–2) with the absorption/emission of phonon in GaAs quantum wells is investigated by using the projection operator method. The ODMPR conditions depend on the photon energy and well width are presented. Our numerical results show that the FWHM of the ODMPR peak for the inter-subband transitions with the absorption/emission of phonon increases with the growing temperature and decreases with the rise of well width for all models of phonons. Besides, the FWHM of the above ODMPR peaks for the bulk phonons has a smaller value and changes slower than it does for the confined phonons. The FWHM for the Huang-Zhu model is largest among three models of confined phonons (Huang-Zhu model, Ridley's guided mode model, and Fuchs-Kliewer slab mode model), while it for Ridley's guided mode model is smallest. This result is in good agreement with the theoretical results in Refs. [1–3]. In addition, the FWHM of the ODMPR peak for the inter-subband transitions with the emission of phonon has a smaller value than that for absorption of phonon for both the bulk and confined phonons. This result is in good agreement with the theoretical results in Ref. [4] but very different from that the theoretical results in Refs. [5,6].

Co and Phthalocyanine Overlayers on the Quantum-Well System Co(001)/Cu: Spin-Polarized Electron Reflection Experiments

The influence of a Co or phthalocyanine (Pc) molecular overlayer on the properties of quantum-well resonances (QWR) in Cu layers atop Co(001) is studied by means of spin-polarized electron reflection. For Co atoms and Pc molecules, an energy shift of the QWR-induced signal is observed with increasing coverage and is attributed to a variation of the electron reflection phase at the Cu/Co and Cu/Pc interface. For Co we find a linear energy shift in the Cu QWR energy position with increasing coverage down to the sub-monolayer regime. This shows that the phase accumulation model remains accurate within the sub-monolayer regime of a discontinuous interface. An opposite sign in the energy shift between Co and Pc overlayers could reflect an opposite impact on the Cu surface work function of overlayer adsorption.

Coupling of quantum well states and phonons in thin multilayer Pb films on Si(111)

Density functional theory calculations for the electronic and phononic band structures of Pb/Si(111) thin films with a thickness of 4 and 5 monolayers (ML) are performed. We employ a Si(111)$(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})$ unit cell to model the Pb films including the Si substrate, and identify quantum well (QW) states for film thicknesses between 3 and 6 ML. The calculations show that the quantum-confined state closest to the Si band gap acquires the character of a quantum well resonance with a major part of its wave function extending into the Si(111) substrate. This finding explains the unusually low dispersion of this state and its lacking sensitivity to phonons in the 5 ML Pb film. Moreover, several unoccupied QW states are identified in the calculations and are assigned to previously observed features in structurally simpler freestanding Pb films. The calculated phonon band structures of the Pb/Si(111)$(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})$ films display stiff surface phonon modes in the 2.3--2.5 THz range. The electron-phonon coupling strength in the quantum-confined states is addressed by means of deformation-potential theory using the calculated atomic displacements of $\mathrm{\ensuremath{\Gamma}}$-point phonons. It is found that both the acoustic shear deformation potential as well as the optical deformation potentials of unoccupied QW states are sizable. Comparing the results for 4 and 5 ML Pb films, we conclude that the optical deformation potentials are generally larger for the 4 ML film. The occupied QW resonance in the 5 ML Pb film shows weak electron-phonon coupling, in qualitative agreement with the small experimentally observed lifetime broadening of this state. Our results form the basis for addressing the role of electron-phonon scattering for the lifetime of unoccupied QWs acting as intermediate states in two-photon photoemission from Pb/Si(111) films.

Optically detected electrophonon resonance in quantum wells via two-photon absorption processes under the influence of phonon confinement

The multi-photon (nonlinear) optical absorption power by electrons in square quantum wells, taking account the electron-confined optical phonon interaction, is calculated using the state-dependent projection technique. We numerically obtain the dependence of the absorption power on the photon energy in the GaAs/AlAs semiconductor quantum well. By analysing this dependence, we show clearly the transition corresponding to each resonant peak, including the optically detected electrophonon resonance (ODEPR). Also, using a computational method, we obtain the line-width of the ODEPR as profiles of the curves. The dependence of the ODEPR line-width on the width of the quantum well shows that the ODEPR line-width for two-photon absorption is about one order of value smaller than it is for one-photon absorption.

Confined optical-phonon-assisted cyclotron resonance in quantum wells via two-photon absorption process

The effect of confined phonons on the phonon-assisted cyclotron resonance (PACR) via both one and two photon absorption processes in a quantum well is theoretically studied. We consider cases when electrons are scattered by confined optical phonons described by the Fuchs-Kliewer slab, Ridley's guided, and Huang-Zhu models. The analytical expression of the magneto-optical absorption coefficient (MOAC) is obtained by relating it to the transition probability for the absorption of photons. It predicts resonant peaks caused by transitions between Landau levels and electric subband accompanied by confined phonons emission in the absorption spectrum. The MOAC and the full-width at half-maximum (FWHM) for the intra- and inter-subband transitions are given as functions of the magnetic field, temperature, and quantum well width. In narrow quantum wells, the phonon confinement becomes more important and should be taken into account in studying FWHM.

Long-Range Phase Coherence in Double-Barrier Magnetic Tunnel Junctions with a Large Thick Metallic Quantum Well

Double-barrier heterostructures are model systems for the study of electron tunneling and discrete energy levels in a quantum well (QW). Until now resonant tunneling phenomena in metallic QWs have been observed for limited thicknesses (1-2 nm) under which electron phase coherence is conserved. In the present study we show evidence of QW resonance states in Fe QWs up to 12 nm thick and at room temperature in fully epitaxial double MgAlO_{x} barrier magnetic tunnel junctions. The electron phase coherence displayed in this QW is of unprecedented quality because of a homogenous interface phase shift due to the small lattice mismatch at the Fe-MgAlO_{x} interface. The physical understanding of the critical role of interface strain on QW phase coherence will greatly promote the development of spin-dependent quantum resonant tunneling applications.

ChemInform Abstract: MgO(001) Barrier Based Magnetic Tunnel Junctions and Their Device Applications

AbstractReview: 210 refs.

Phonon-assisted cyclotron resonance in quantum wells via the multiphoton absorption process

In this paper, we consider the theory of phonon-assisted cyclotron resonance (PACR) in quantum well semiconductors due to the scattering of electrons and acoustic, nonpolar and polar optical phonons. Using perturbation approach, we obtain the expression of absorption power in the case of multiphoton absorption. From the numerical and graphical method, we consider the dependence of absorption power on frequency, temperature, and magnetic field for a specific GaAs quantum well. Using profile method, we obtain the PACR-linewidth as profiles of the curves. The temperature and magnetic field dependence of the PACR-linewidth have been investigated. The results are compared with those in the case of monophoton absorption process.

Quantum well states in Ag thin films on MoS2(0001) surfaces

In-plane dispersions of quantum well (QW) states originating from the electron confinement of Ag sp electrons within the MoS2 band gap region are investigated by means of angle-resolved photoemission spectroscopy (ARPES). A number of QW resonances have been observed in the ARPES spectra in a binding energy range lying outside the MoS2 energy gap which is required for full confinement of the Ag sp electrons. In spite of having the expected free electron-like behavior, these QW states show a significant increase of in-plane effective mass with increasing binding energy due to the hybridization of Ag sp electrons with the MoS2 valence bands. The binding energy dependence of the bottom of the QW states (k‖ = 0) as a function of the Ag film thickness has been analyzed. The well-established phase accumulation model has been applied for calculating the phase shifts of electrons at the boundaries. Our observations show that the total phase shift behaves differently for energies above and below the MoS2 valence band maxima, due to the hybridizations being different in nature. The structure plot calculated considering the different quantum number dependent total phase shifts provides a good description of the experimental observations.

CoFe alloy as middle layer for strong spin dependent quantum well resonant tunneling in MgO double barrier magnetic tunnel junctions

We report on spin dependent quantum well (QW) resonances in the CoFe alloy middle layer of CoFe/MgO/CoFe/MgO/CoFeB double barrier magnetic tunnel junctions. The $dI/dV$ spectra reveal clear resonant peaks for the parallel magnetization configurations, which can be related to the existence of QW resonances obtained from first-principles calculations. We observe that the differential tunneling magnetoresistance (TMR) exhibits an oscillatory behavior as a function of voltage with a sign change as well as a pronounced TMR enhancement at resonant voltages at room temperature. The observation of strong QW resonances indicates that the CoFe film possesses a long majority spin mean-free path, and the substitutional disorder does not cause a significant increase of scattering. Both points are confirmed by first-principles electronic structure calculation.

MgO(001) barrier based magnetic tunnel junctions and their device applications

Spintronics has received a great attention and significant interest within the past decades, and provided considerable and remarked applications in industry and electronic information etc. In spintronics, the MgO based magnetic tunnel junction (MTJ) is an important research advancement because of its physical properties and excellent performance, such as the high TMR ratio in MgO based MTJs. We present an overview of more than a decade development in MgO based MTJs. The review contains three main sections. (1) Research of several types of MgO based MTJs, including single-crystal MgO barrier based-MTJs, double barrier MTJs, MgO based MTJs with interlayer, novel electrode material MTJs based on MgO, novel barrier based MTJs, novel barrier MTJs based on MgO, and perpendicular MTJs. (2) Some typical physical effects in MgO based MTJs, which include six observed physical effects in MgO based MTJs, namely spin transfer torque (STT) effect, Coulomb blockade magnetoresistance (CBMR) effect, oscillatory magnetoresistance, quantum-well resonance tunneling effect, electric field assisted magnetization switching effect, and spincaloric effect. (3) In the last section, a brief introduction of some important device applications of MgO based MTJs, such as GMR & TMR read heads and magneto-sensitive sensors, both field and current switching MRAM, spin nano oscillators, and spin logic devices, have been provided.

Magnetophonon Resonance in Quantum Wells with Parabolic Potential

The linear dc magnetoconductivity in the$(x,y)$ plane of a parabolic quantum well, with a magnetic field$\vec B = B\vec e_z$ applied, is evaluated for electron - opticalphonon interaction. For nonpolar optical and polar optical phonons,the magneto-conductivity oscillates as a function of the magneticfield with resonances occurring when $P\omega_c=\omega_0$, where$\omega_c$ and $\omega_0$ are cyclotron frequency and optical phononfrequency, respectively, and where $P$ is an integer. The analyticresults are numerically evaluated to show explicitly the dependenceof magneto-conductivity on the magnetic field, the confinementfrequency in $z$ direction, and the temperature of the system.

Optical spectra in the region of exciton resonances in quantum wells and quantum dots of In0.3Ga0.7As/GaAs heterostructures

The transparency, reflection and luminescence spectra of In0.3Ga0.7As structures with 8nm thickness and quantum wells limited by the barrier layer GaAs of a 9nm (upper layer) and 100nm (bottom layer) thickness had been studied in the region of photon energy 0.5–1.6eV. Lines associated with the transitions hh,lh1-e1(1s,2s,3s), hh2,lh2-e2(1s,2s,3s), hh1,lh1-e2(1s) and hh3,lh3-e3(1s) had been revealed in reflection spectra. The shapes of the reflection and transparency lines had been calculated using a single oscillator model of dispersion relations and the Kramers–Kronig integrals. The binding energy of hh,lh1-e1 excitons, the effective mass mhh∗ and mlh∗ and the damping factor for the optical transitions to QW and QD had been determined. The lifetime of charge carriers on quantum dots varies in the range of 0.04–0.1ps, while the radiative lifetime of excitons in quantum wells in the considered structure is around 2ps.

Quantum size effect driven thermal decomposition of Ag films on Fe(100) in the presence of pinhole-growth morphological defects

The growth morphology of Ag films that are deposited on an Fe(100) surface at room temperature has been investigated using low-energy electron-diffraction spot-profile analysis and low-energy electron microscopy. The superposition of periodic spot-profile modulations that are caused by interference of waves that are reflected from the surface at different exposed levels and intensity oscillations that are associated with quantum-well resonances within the film provides complementary information on film roughness and thickness. The predominant morphological defect that is present during layer-by-layer growth is determined to be pinholes that penetrate completely through the film to the Fe substrate surface. Such imperfect films are observed to decompose upon annealing to thicknesses that are stabilized by quantum-well states. This result suggests that pinholes eliminate kinetic limitations that promote the unusual bifurcation mode of thermal decomposition that was observed previously for atomically smooth films prepared at low temperature.

Quantum well states, resonances and stability of metallic overlayers

The role of quantum well states (QWSs) and quantum well resonances (QWRs) in thestability of metallic overlayers is studied. When the substrate presents a confining gap, as itis the case of Cu(111), the stability is determined by QWSs and there is experimentalevidence of the existence of magic thicknesses. For overlayers which do not have QWSs, asthose grown on Al(111), we explore the existence of thicknesses of enhanced stability dueto QWRs by comparing the strength of the oscillations in the surface energy.

Optically Detected Magnetophonon Resonances in Quantum Wells

Optically Detected Magnetophonon Resonances in Quantum Wells

Frequency limits of terahertz radiation generated by optical-phonon transit-time resonance in quantum wells and heterolayers

We develop an analytical model based on a few generalized parameters of bulk materials and two-dimensional (2D) structures which enables us to analyze the main features of terahertz (THz) radiation generation caused by optical-phonon transit-time resonance. In the framework of such a model and direct Monte Carlo simulations it is shown that for the same material (i) generation conditions are the same in quantum wells and heterolayers characterized by the same parameter of electron localization in the 2D channel and (ii) a considerable increase up to 5 times of the high-frequency cutoff for THz radiation generation is predicted in going from 3D to 2D transport.

First-Principles Theory of Quantum Well Resonance in Double Barrier Magnetic Tunnel Junctions

Quantum well (QW) resonances in Fe(001)/MgO/Fe/MgO/Fe double barrier magnetic tunnel junctions are calculated from first principles. By including the Coulomb blockade energy due to the finite size islands of the middle Fe film, we confirm that the oscillatory differential resistance observed in a recent experiment [T. Nozaki, Phys. Rev. Lett. 96, 027208 (2006)10.1103/PhysRevLett.96.027208] originates from the QW resonances from the Delta1 band of the Fe majority-spin channel. The primary source of smearing at low temperatures is shown to be the variation of the Coulomb blockade energy.