H Pascher

Interpretation of photoluminescence spectra in partially interdiffused PbTePb1−xSnxTe superlattices

Abstract Photoluminescence of PbTe Pb 1−x Sn x Te multiquantum wells and superlattices was observed with relatively low pump intensities using a Q-switched CO- laser. Care was taken to observe only spontaneous and not stimulated emission. From the spectra the n=1 subband separation can be obtained. The data were analyzed with an envelope function approach which is valid only for abrupt interfaces but also for a continuous variation of the composition.

Exchange interaction in semimagnetic IV-VI multi-quantum-well structures

Diluted magnetic IV-VI semiconductors like Pb1-xMnxTe or Pb1-xMnxSe exhibit a strong dependence of the effective g-factors of the free carriers on temperature and magnetic field, caused by an exchange interaction between the free carriers and the magnetic moments of the paramagnetic ions. In quantum wells or superlattices the strength of the exchange interaction depends on the probability of finding the carriers within the diluted magnetic component. With various PbSe/Pb1-xMnxSe superlattices the authors performed coherent anti-Stokes Raman scattering (CARS) experiments in the mid-infrared using CO2 lasers to obtain very precise data on the spin splittings of electrons and holes. The analysis of these CARS data in the framework of an envelope-function approximation yields a type-l alignment with electrons confined in the Pb1-xMnxSe layers and holes in the PbSe layers.

Coherent anti-Stokes Raman scattering in dilute magnetic PbTe/Pb1−xMnxTe quantum wells

Abstract In dilute magnetic narrow gap quantum well structures PbTe/Pb1−xMnxTe (x≤0.03) the effective g-factors of electrons were determined by coherent anti-Stokes Raman scattering in Voigt configuration. For this geometry, that is magnetic field parallel to the layers, the Landau levels are calculated by an envelope function approach. The observed data are in agreement with the calculated g-values. Estimates for the band offset are obtained.

Superlattices of IV-VI Compounds

Superlattices, multiquantum and single quantum- well structures of IV-VI compounds are of interest because of their electronic properties as narrow gap materials with a many valley band structure. In addition, also their structural properties are quite unique and they offer the possibility to investigate structural phase transitions in artificially layered structures. Experimental data on PbTe/Pb1-xGexTe are presented in this context. Electronic properties of superlattices are described within the frame work of the envelope function approximation and experimental data on interband magneto optical transitions in both geometries, namely B parallel and perpendicular to the layers, are presented. Coherent anti Stokes Raman scattering is used to obtain information on the electronic g-factors in PbTe/Pb1-xSnxTe (x = 0.175) MQW structures. The experimental data are analyzed with calculations of superlattice Landau states in both magnetic field configurations.

g factor spectroscopy in narrow-gap semiconductors by optical four-wave mixing

Among various non-linear Raman techniques coherent anti-Stokes Raman spectroscopy (CARS) has become most popular because of its strong, easily detectable coherent signal. For the spectroscopy of narrow-gap semiconductors which are only transparent for infrared light this advantage is of special importance. Using Maxwells theory the process can be described by a third-order non-linear susceptibility chi (3). Its strongest Raman-like resonances in a semiconductor exposed to a magnetic field are the spin-flip resonances from which the effective g factors of electrons and holes can be deduced. The interference of resonant and non-resonant contributions to the susceptibility in some cases causes complicated line shapes which have to be analysed carefully to find the correct resonance positions. A short review of the theoretical description of the process and the experimental apparatus suitable for the investigation of narrow-gap semiconductors in a magnetic field is given. Then the analysis of the line shapes is described. Results are discussed for several III-V compounds, II-VI narrow-gap materials and IV-VI epitaxial films.

Nuclear spin polarization in InSb detected by spin-flip Raman gain spectroscopy

The conduction electron spin resonance (CESR) in n-type InSb is shifted by the spin polarization of the nuclear and electron spins (Overhauser shift). The authors measured this shift by optically detecting the magnetic CESR using coherent spin-flip Raman scattering of two CO lasers in the infrared region. This method is very useful for studying the hyperfine interaction in narrow-gap semiconductors with high effective g-factors. By using a combined CESR/NMR technique, the effective g-factor in InSb was determined at different magnetic fields with the high precision available from NMR experiments. The Overhauser shift is directly proportional to the probability mod Psi (0) mod 2 of finding an electron at the nuclear site of the 115In or the 121Sb isotope, respectively. In thermal equilibrium, when the spin polarization of the nuclei is known, measurements of the Overhauser shift directly yield information on mod Psi (0) mod 2 of the conduction electrons at the respective nuclear site. Observing the shift while either building up dynamic nuclear polarization or studying the relaxation of the nuclei from high spin polarization to thermal equilibrium yields information on the nuclear spin-lattice relaxation time T1 of each of the isotopes.

Coherent anti stokes Raman scattering and magnetooptical interband- transitions in superlattices of diluted magnetic IV–VI semiconductors

Abstract In diluted magnetic IV–VI semiconductors like PbMnTe or PbMnSe there is a strong dependence of the effective g-factors of conduction and valence band on temperature and magnetic field. This modification of the spin splittings with respect to the diamagnetic host materials is caused by an exchange interaction between the free carriers and the magnetic moments of the paramagnetic ions. In quantum wells (QWs) or superlattices (SLs) the strength of the exchange interaction depends on the penetration of the wave functions of the free carriers into the diluted magnetic component. Detailed informations on the bandstructure of PbSe/PbMnSe SLs and MQWs are achieved for different concentrations of the magnetic ions and different widths of the quantum wells by interbandabsorption and coherent Raman experiments (CARS). Particularly CARS yields very precise data on the spin splittings of carriers confined in the quantum wells. In type I MQWs the interband transitions which are indirect in real space give complementary informations. The analysis of CARS data from various PbMnSe/PbSe QW structures yields a type I′ alignment with electrons confined in the PbMnSe layers and holes in the PbSe layers. The interband magnetooptical data support these conclusions.

Heterostructures of dilute magnetic IV-VI compounds

Quantum well (QW) structures composed of a diamagnetic narrow gap IV-VI semiconductor (PbTe, PbSe) and a dilute magnetic compound (PbMnTe, PbMnSe, PbEuTe) are investigated by various magneto-optical methods as coherent anti-Stokes Raman scattering (CARS), interband-absorption and photoluminescence. The CARS experiments yield precise values of the spin splittings of valence and conduction bands as a function of magnetic field and temperature. By interband transmission energy gaps and effective masses are determined. The data are analyzed in the framework of an envelope function approach taking into account the exchange interaction between free charge carriers and the magnetic moments of the paramagnetic ions in a molecular field approximation. A comparison of the spin splittings of the bands in the QWs with those observed in homogeneous materials yields information on the probabilities to find the mobile carriers in the diamagnetic and the semimagnetic layers, respectively. This information allows the determination of the type of the superlattice and of the band offsets at the interfaces.

Coherent Raman scattering and magneto-optical interband transitions in diluted magnetic IV-VI compounds

The authors performed coherent Raman spectroscopic investigations of spin-flip transitions in Pb1-xMnxTe and Pb1-xEuxSe in magnetic fields down to 500 G. In order to find a consistent set of band parameters for the Mitchell-Wallis k.p model, magneto-optical interband absorption measurements were also performed. In the Raman spectrum of Pb1-xEuxSe with x>1% a completely new resonance with high oscillator strength appears. Although the levels involved have not been identified unambiguously up to now, the experimental findings indicate the existence of a magnetic phase transition in Pb1-xEuxSe, which they observed by optical methods.

Overhauser shift and dynamic nuclear polarisation in InSb observed by coherent spin-flip Raman scattering

The authors detect the Overhauser shift of the conduction electron spin resonance (CESR) in n-type InSb via coherent spin-flip Raman scattering. In thermal equilibrium this shift is proportional to the probability mod psi (rk) mod 2 of the conduction electron at the nuclear position rk. By partially saturating the CESR, the nuclear spin polarisation is dynamically increased (DNP) and the Overhauser shift increases in proportion. The time dependence of the Overhauser shift yields directly the nuclear spin-lattice relaxation times for both In and Sb. The optical method results in very high CESR sensitivity. Narrow-gap semiconductors not accessible by classical double-resonance techniques can be investigated.