S. S. Makler

GaAs-Al x Ga 1¿x As double-barrier heterostructure phonon laser: A full quantum treatment

The aim of this work is to describe the behavior of a device capable to generate high frequency (∼ THz) acoustic phonons. This device consists in a GaAs-AlGaAs double barrier heterostructure that, when an external bias is applied, produces a high rate of longitudinal optical LO phonons. These LO phonons are confined and they decay by stimulated emission of a pair of secondary longitudinal optical (L̃O) and transversal acoustic (TA) phonons. The last ones form an intense beam of coherent acoustic phonons. To study this effect, we start from a tight binding Hamiltonian that take into account the electron-phonon (e-ph) and phonon-phonon (ph-ph) interactions. We calculate the electronic current through the double barrier and we obtain a set of five coupled kinetic equations that describes the electron and phonon populations. The results obtained here confirm the behavior of the terahertz phonon laser, estimated by rougher treatments [1].

Tuning a resonance in Fock space: Optimization of phonon emission in a resonant-tunneling device

Phonon-assisted tunneling in a double barrier resonant tunneling device can be seen as a resonance in the electron-phonon Fock space which is tuned by the applied voltage. We show that the geometrical parameters can induce a symmetry condition in this space that can strongly enhance the emission of longitudinal optical phonons. For devices with thin emitter barriers this is achieved by a wider collectors barrier.

A resonant tunneling diode based on a Ga1 xMnxAs=GaAs double barrier structure

Abstract We introduce here a device capable of producing a strongly spin-polarized current. It can be useful in spintronics as a polarizer, an analyzer and other applications. Consequently, it could be useful in quantum computing. The device consists of a resonant tunneling diode with a ferromagnetic well made of Ga1−xMnxAs. The levels at the well are spin polarized and so is the current through the system. To study the transport properties of the system we use a new formalism capable of treating, in an exact and very efficient way, an open system. The result is that the current could be polarized parallel or antiparallel to the magnetization of the well, depending on the applied bias.

Polaron effect on Raman scattering in semiconductor quantum dots

Strong coupling of a confined exciton to optical phonons in semiconductor quantum dots (QDs) leading to the formation of a polaron is considered for a model system including two lowest exciton states and several optical phonon modes. Both intra- and inter-level terms are taken into account. The Hamiltonian has been exactly diagonalized including a finite number of phonons allowed for each mode, large enough to guarantee that the result can be considered exact in the physically important region of energies. Based on this polaron spectrum, the Raman scattering probability is obtained, which is compared with the one calculated using the standard perturbation theory approach. It is shown that, when either diagonal or non-diagonal coupling is sufficiently strong, the Raman spectrum line shape and especially its resonant behaviour differ considerably from the perturbation theory predictions. The dependence of the scattering intensity on the excitation wavelength contains features similar to those expected in the optical absorption spectra of QDs.

A double-barrier heterostructure generator of terahertz phonons: many-body effects

In this paper we study the generation of coherent terahertz phonons in a double-barrier heterostructure (DBH) under the influence of an external applied bias. The system is characterized by an energy difference between the two lowest levels in the well, which resonates with the optical phonon energy, producing a high rate of emission of longitudinal optical (LO) phonons. The strong electron-phonon interaction in a polar semiconductor leads to the formation of a polaron that is relevant close to this resonance. Therefore the levels corresponding to the first excited state and the satellite of the ground state anticross in two polaronic branches. The LO phonon has a very short lifetime. It decays by stimulated emission of a pair of phonons (LO + TA) (where TA stands for transverse acoustic). As a consequence an intense beam of TA coherent phonons is produced, which could have several important applications. A rough model of this system has already been presented (Makler S S, Vasilevskiy M I, Weberszpil J, Anda E V, Tuyarot D E and Pastawski H M 1998 J. Phys.: Condens. Matter 10 5905). Several improvements to that model are presented here. Besides the more accurate treatment of the electron-phonon interaction, the phonon-phonon interaction is considered here taking into account the fact that the whole system is out of equilibrium. The results confirm that the proposed phonon laser is reliable.

Graphical estimative of diamagnetic anisotropy in some conjugated systems

Abstract A graphical method for obtaining the diamagnetic anisotropy of certain conjugated systems relative to that of benzene is proposed. The conjugation volume required is determined through the π charge density contour line of maximum conjugation. The agreement with experimental data or other theoretical treatments is satisfactory. In all cases considered, not more than one third and not less one fifth of each electron takes part in the conjugation.

Ultra-high-frequency coherent sound generation in resonant tunneling

Abstract In this paper we propose a device capable of generating ultra-high-frequency coherent sound. It is called SASER by analogy to laser. The device consists of a double barrier heterostructure (DBHS) tailored to permit phonons to resonate with the electronic system. The condition for resonance is fulfilled when the energy difference between the first and second resonant peaks of the electronic DOS in the well matches the LO-phonon frequency. Due to the low energy and short wavelength of the phonon beam this device could be useful for imaging and for the non-destructive characterization of nanostructures. It could also be used to build phonoelectronic systems (analogous to optoelectronics). We establish kinetic equations for the number of electrons in each level, the number of LO-phonons in the well and for the number of coherently generated secondary TA phonons that results from the LO-phonon decay due to anharmonicity. Coefficients of the equations that describe the tunneling process have to be obtained in a self-consistent way, considering the effects of space charge inside the well and the accumulation and depletion layers in the emitter and collector, respectively. The steady solution of the system is analyzed and the conditions for “SASER action” are discussed. The I(V) characteristics show the well known bistability at the negative differential resistence region. Also, at the beginning of the SASER emission this curve shows chaotic behaviour.

A quantum formalism for a terahertz acoustic laser

The aim of this work is to improve the study of a phonon laser (saser) proposed by us several years ago[1]. This is a device capable to generate an intense coherent beam of acoustical phonons. Our acoustic laser consists in a double barrier heterostructure tailored such the energy difference between the ground and the first excited state in the well is close to the energy of the LO phonon. The electrons are directly injected into the excited level. Therefore they decay producing a high rate of LO phonons. These phonons are confined inside the well and decay into a pair of phonons[2]: LO ® + TA. The TA phonons escape the well in the [111] direction constituting an intense coherent beam. Recently were studied (and sometimes realized experimentally) several kinds of phonon lasers. Up to our knowledge our saser is the only that has a very short wavelength (smaller than 25 A) and a very long range (greater than 1000 mm). Because of that, such beam could have applications to acoustic nanoscopy, acoustic nanolithography and phonoelectronics. In early articles[1, 3, 4, 5, 6] we get the kinetic equations for the averaged electron and phonon populations. Quantum fluctuations were not taken into account. The system Hamiltonian is H = He + Hph + He-ph + Hph-ph + He-e. To solve this Hamiltonian we expand their eigenfunctions in the basis of the eigenstates½jn1n2n3n of the single particle part of it. We obtain a set of coupled equations for the expansion coefficients that can be solved with some approximations. The results are qualitatively similar to those obtained previously.

Transport Properties of a Ga1-xMnxAs/Ga1-yAlyAs Double-Barrier

We study the transport properties of a spin filter consisting of a double-barrier resonant tunneling device in which the well is made of a semimagnetic material. Even if the device could be made of several materials, we discuss here the case of a Ga1-xMnxAs/Ga1-yAlyAs system because it can be integrated into the well known AlAs/GaAs technology. We solve the Hamiltonian H = HK+HP+HE+HM+Hh-i+Hh-h. Its terms represent the kinetic energy, the double-barrier profile, the applied bias, the magnetic interaction, the hole-impurity attraction and the hole-hole repulsion, respectively. A very simple one-dimensional Green function is introduced to solve self-consistently the Poisson equation for the profile due to the charge distribution. A real space renormalization formalism is used to calculate exactly the currents. In a previous work we have shown that the Rashba effect is weak. Therefore the results show very well defined spin-polarized currents. Our results confirm that this system is a good device for spintronics.

A selfconsistent calculation of the transport properties of a double barrier spin filter

A double barrier resonant tunneling device in which the well is made of a semi-magnetic material can work as an efficient spin filter. Today it is possible to make semiconductors that are ferromagnetic at room temperature. Therefore the device studied here has a great potential to be used as a polarizer, an analyzer and other spintronic applications. We discuss here the case of a Ga1-xMnxAs/Ga1-yAlyAs system because it can be integrated into the well known AlGaAs/GaAs technology. Our tight-binding Hamiltonian includes the kinetic energy, the double-barrier profile, the electric field, the magnetic term, the hole-impurity and the hole-hole interactions. The profile and the charge distribution are calculated self-consistently. In previous works we studied this system by solving the Hamiltonian in the reciprocal space, in order to simplify the treatment of the Poisson equation for the charge distribution. Here we introduce a simple one dimensional Green function that permits to solve all terms in the real space. Besides, a real space renormalization formalism is used to calculate exactly the electronic currents for each spin polarization. The results confirm that the proposed system is a good device for spintronics.