Initial Exciton Research Articles

Differential Transmission Spectroscopy (DTS) in Cu2O in the Presence of Cold Excitons

A high density of cold 1s orthoexcitons was created in Cu2O using two counterpropagating high-power femtosecond infrared (IR) laser beams tuned to half the 1s exciton resonance energy. The initial 1s exciton gas density was estimated to reach up to 1020 cm—3. The resulting differential absorption spectrum revealed a pronounced screening of the yellow np absorption lines, up to complete disappearance of the 2p exciton for the highest pump intensity. On the other hand, no change in the n = 1 phonon-assisted absorption continuum was detected, showing that the Mott condition for dissociation into a two-component plasma was not reached for the n = 1 excitons.

Photoluminescence of free and localized excitons from CdTe/CdMnTe quantum well structures in the presence of a strong external magnetic field

The exciton photoluminescence from two CdTe quantum wells with the thicknesses 4 and 8 nm in CdTe/CdMnTe structures is studied in the magnetic field up to 41 T. The transformation of the photoluminescence spectrum depends strongly on the field direction and degree of the initial exciton localization. The criterion of strong field for the diamagnetic excitons related to the Landau levels is achieved.

Nonequilibrium effects in α-particle-induced reactions in Cs and I

The excitation functions for the reactions 127I(α,n)130Cs, 127I(α,2n)129Cs, 127I(α,4n)127Cs, 133Cs(α,2n)135La, and 133Cs(α,4n)133La have been measured up to 50 MeV alpha-particle energy using the stacked-foil activation technique and Ge(Li) gamma-ray spectroscopy. The measured cross sections were compared with theoretical calculations considering equilibrium as well as the pre-equilibrium geometry-dependent hybrid models of Blann. The high-energy tails of the excitation functions show a substantial contribution from pre-equilibrium emission. A general agreement is observed between the experimental results and theoretical predictions with an initial exciton configuration n0 = 4(2n + 2p + 0h).PACS No. 25.40-h

Time-resolved magneto-optical experiments in Cd1−xMnxTe/ZnTe multiple quantum wells

Optical pump-probe experiments were performed in Cd0.9Mn0.1Te/ZnTe multiple quantum wells to study the dynamics of the exciton magnetic polaron in this material. Two transient absorptions of opposite sign were observed. The first signal is negative and indicates the decrease of optical absorption, as the excitons are approaching saturation. The second signal is positive and is interpreted as the absorption of “new excitons” after the initial excitons created by the pump pulse have decayed leaving behind a magnetic polarized Mn environment. The spin relaxation time of the Mn ions has been determined to be 601 ps. Finally, we discuss the effect of the polaron–polaron interaction, relevant in our experiment because we saturate the excitons in the quantum wells.

Article

The stacked-foil activation technique and HPGe gamma-ray spectroscopy were used for the investigation of alpha-particle-induced reactions on natural iron up to 50 MeV. Excitation functions for the production of reaction residues 57Ni, 56Ni, 58Co, 57Co, and 56Co from alpha-particle-induced reactions on 56Fe were measured with a view to improve the quality of earlier measurements. The experimental results were compared with calculations considering equilibrium as well as preequilibrium reactions according to hybrid model of Blann (ALICE/90). It was found that the initial exciton configuration n0= 4(2p + 2n + 0h ) i.e., pure particle state appears to give a good fit to the experimental data.PACS No.: 25.60

Exciton magnetic polarons in (100)- and (120)-oriented semimagnetic digital alloys (Cd,Mn)Te

We report on optical studies of exciton magnetic polarons in artificial crystals of digital alloys of diluted magnetic (semimagnetic) semiconductors built of alternating CdTe and ${\mathrm{Cd}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Te}$ layers, each only a few monolayers thick. Specially designed quantum well structures with the digital magnetic alloy in the quantum well region were grown and studied experimentally by magneto-optical methods. Precise comparative studies of the structures grown simultaneously either on (120)- or (100)-oriented substrates were performed by photoluminescence excitation spectroscopy and photoluminescence under selective excitation. While no difference in magnetic properties, as measured by the giant Zeeman splitting of excitonic states, was observed between (120)- and (100)-oriented diluted magnetic structures, the exciton magnetic polaron energy was found to be by $10--40%$ larger in (120)-oriented structures than in (100)-oriented counterparts. This increase is explained by the effect of the heavy-hole effective mass anisotropy: greater hole mass in [120] direction causes an enhancement of an initial exciton localization which, in turn, gives rise to an increase of the magnetic polaron energy. Results of model calculations are in good agreement with experimental data.

The181Ta(7Li,5n)183Osreaction: Measurement and analysis of the excitation function and isomeric cross-section ratios

Excitation function and isomeric cross-section ratios for the production of ${}^{183}{\mathrm{Os}}^{m,g}$ by ${}^{7}\mathrm{Li}$-induced reactions on ${}^{181}\mathrm{Ta}$ are obtained from the measurements of the residual activities by the conventional stacked-foils technique from threshold to 50 MeV. The excitation function and isomeric cross-section ratios for nuclear reaction ${}^{181}\mathrm{Ta}{(}^{7}\mathrm{Li}{,5n)}^{183}{\mathrm{Os}}^{m,g}$ are compared with the theoretical statistical model calculation by using the ALICE/91, STAPRE, and CASCADE codes. In the energy range of the present measurement the excitation functions are fitted fairly well by both the geometry dependent hybrid (GDH) model and the hybrid model of Blann with initial exciton number ${n}_{0}=7$ (${n}_{n}=4,$ ${n}_{p}=3,$ ${n}_{h}=0$) using the ALICE/91 code. The experimental isomeric cross-section ratios are also reproduced fairly well by the calculation using the STAPRE code. However, the CASCADE code calculations slightly underpredict the cross section but reproduce the shape. In general, the statistical model under a suitable set of global assumptions, can reproduce the excitation function as well as isomeric cross-section ratios.

Pre-equilibrium Model Analysis of Alpha Particle InducedReactions up to 80MeV

The excitation functions of the 51V[(α,n),(α,3n), (α,p3n), (α,α3n),(α,α2pn), (α,2α) and (α,2αn)]reactions were studied up to 80MeV using stacked foil activationtechnique and Ge(Li) gamma-ray spectroscopy. Secondly, a comparisonwith the pre-equilibrium hybrid model and the index model have beenmade using the initial exciton number n0 = 4(4p0h).It was observed that the hybrid model gives a better agreement thanthe index model for (α.xn) type of reactions. However,both the models failed to explain reactions involving the emission ofprotons and alpha particles.

Measurement and analysis of the excitation function and isomeric cross section ratios forα-induced reaction on Ir, Au, Re and Ta nuclei

Excitation functions and a few isomeric cross-section ratios for production of (1)192Au,193Au,194Au,195Au and192Ir nuclides inα-induced reactions on191,193Ir, (2)197Tl,197m Hg,198m,g Tl,199Tl and200Tl nuclides inα-induced reaction in197Au and (3)183Re and184m,g Re nuclides inα-induced reaction in181Ta and185Re are obtained from the measurements of the residual activities by the conventional stacked-foils technique from threshold to 50MeV. The excitation function and isomeric cross-section ratios for nuclear reaction181Ta(α,n)184m,g Re are compared with the theoretical calculation using the code Stapre which is based on exciton model for pre-equilibrium phase and Hauser-Feshbach formalism taking angular momentum and parity into account for the equilibrium phase of the nuclear reaction. All other experimental excitation functions are compared with the calculations considering equilibrium as well as pre-equilibrium reaction mechanism according to the geometry dependent hybrid (GDH) model and hybrid model of Blann using the code Alice/91. The high energy part of the excitation functions are dominated by pre-equilibrium reaction mechanism whereas the low energy parts are dominated by equilibrium evaporation with its characteristic peak. The GDH model provides a potentially better description of the physical process (i.e., a higher probability for peripheral collisions to undergo precompound decay than for central collisions) compared to hybrid model. However in the energy range of present measurement most of the excitation functions are fitted reasonably well by both GDH model and hybrid model with initial exciton numberN 0=4(N n=2,N p=2,N h=0). Barring a few reactions we have found the overall agreement between theory and experiment is reasonably good taking the limitations of the theory into account.

Excitation function measurement and reaction mechanism analysis for alpha-induced reactions on

Excitation functions have been measured for and produced by -particle-induced reactions on in the incident energy range 34-50 MeV. The excitation functions of these nuclei and those of , reported earlier, are analysed in terms of direct, pre-equilibrium and evaporation emissions. The two most commonly used models of pre-equilibrium reactions, the exciton and the hybrid models, have both been used and compared. The choice of the initial exciton number, the role of the pre-equilibrium cluster and multiple nucleon emissions have been investigated. The direct emission of clusters are found to significantly affect the excitation functions of and Au isotopes.

Excitation functions of alpha particle induced reactions on aluminium and copper

Cross-sections for the reactions with product nuclei24Na,22Na,68Ga and67Ga were investigated over the energy range of 30 to 75 MeV for alpha particle induced reactions on natural aluminium and copper, using stacked-foil activation technique. The measured excitation functions were analysed with special reference to their suitability for monitoring beam energy and intensity. The experimental results were compared with the predictions of hybrid model of Blann. The assumption of initial exciton numbern 0=4(4p0h) best satisfies the measured excitation functions in the present work.

α-particle induced reactions on yttrium and terbium

The stacked foil activation technique has been employed for the investigation of $\ensuremath{\alpha}$-particle induced reactions on the target elements yttrium and terbium up to 50 MeV. Six excitation functions for the $(\ensuremath{\alpha},xn)$ type of reactions were studied using high-resolution HPGe $\ensuremath{\gamma}$-ray spectroscopy. A comparison with Blann's geometric dependent hybrid model has been made using the initial exciton number ${n}_{0}=4(4p0h)$ and ${n}_{0}=5(5p0h)$. A broad general agreement is observed between the experimental results and theoretical predictions with an initial exciton number ${n}_{0}=4(4p0h)$.

Relaxation of hot quasi-two-dimensional excitons

A theory of the relaxation of hot quasi-two-dimensional excitons in quantum wells is presented. Based on this theory, we derive analytical expressions for the decay rates of excitons as functions of the initial exciton momentum. We take into account the penetration of electron and hole wave functions into the barrier region, an effect which becomes increasingly significant at narrow well widths. Decay rates are calculated for well widths ranging from 25 to 150 Å at low temperatures for GaAs Al 0.3Ga 0.7As single quantum wells. Our results indicate a notable increase of the decay rates with the initial momentum of excitons. Our results also show that the decay probabilities of the heavy-hole excitons are higher than that of the light-hole excitons at a fixed temperature and exciton energy. Good agreement is obtained when we compare the calculated results with available experimental data.

Analysis of the excitation functions from alpha particle induced reactions up to 120 MeV

Alpha particle induced reactions in the target element niobium were investigated in the energy range 30 to 120 MeV. Thirteen excitation functions were measured for the production of 96Tc, 95m+gTc, 95Nb, 92mNb, 90Nb, 88Zr, 88Zr, 88Zr, 88Y, 87m+gY and 88Y. Secondly a comparison with a preequilibrium index model has been made using the initial exciton number n0 = 4 (4p0h) and level density parameter a = A/8. In general the shapes of the excitation functions were fairly well reproduced with the model predictions. However, the agreement between theoretical and experimental excitation functions for the (α, zαxnyp) type of reactions is only within a factor of 5.

Measurement and analysis of excitation functions for alpha induced reactions on iodine and cesium

The excitation functions for the reactions127I(α, 2n)129Cs,127I(α, 4n)127Cs,133Cs(α, 2n)135La and133Cs(α, 4n)133La have been measured up to ≈50 MeVα-particle energy using the stacked foil activation technique. Measured excitation functions are compared with pre-equilibrium geometry dependent hybrid model calculations. It has been found that theoretical calculations using an initial exciton numbern 0=4 (2p+2n+0h) give good agreement with experimental excitation functions.

Pre-equilibrium analysis of the alpha-particle induced reactions in niobium in the energy range 30 - 80 MeV

Stacked foil activation technique and Ge(Li) gamma-ray spectroscopy have been used for the investigation of alpha particle induced reactions in the target element niobium up to 80 MeV. Eleven excitation functions were measured for the production of , , , , , , , and with a view to adding new data points in the energy region where there are no existing data, and to improve the quality of earlier measurements having divergencies beyond the experimental errors. Secondly, a comparison with the pre-equilibrium hybrid model and the index model have been made using the initial exciton number (4p0h). While the shapes of the excitation functions were fairly reproduced with both models, the hybrid model consistently resulted in a better agreement.

Analysis of the excitation function of alpha-particle-induced reactions on natural silver

Excitation functions of the reactions 107Ag[(α, n); (α, 2n); (α, αn); (α, α2n)] and,109Ag[(α, 2n); (α, 3n); (α, 4n); (α, α3n); (α, α4n)] were investigated up to 70 MeV by the stacked foil activation technique and Ge(Li) gamma-ray spectroscopy method. Since the natural silver used as the target has two odd mass stable isotopes of abundance 51.83% (107Ag) and 48.17% (109Ag), their activation in some cases gives the same residual nucleus through different reaction channels, but with very different Q values. In such cases, the individual reaction cross sections are separated with the help of the ratio of theoretical cross sections. The experimental cross sections were compared with the predictions of a pre-equilibrium hybrid model. The (α, xn) reactions are fairly well reproduced with initial exciton number n0 = 4(4p0h), whereas (α, αxn) reactions are underestimated in magnitude by a factor of five to six.

Nuclear Exciton Echo Produced by Ultrasound in Forward Scattering of Synchrotron Radiation.

The time evolution of the coherent decay of a collective nuclear excitation (nuclear exciton), induced by $14.4\mathrm{keV}$ synchrotron radiation, was studied in a system of two stainless steel foils, where one was vibrated by ultrasound (US). The US vibrations disrupted the coherent nuclear emission from the two targets. However, at times corresponding to multiples of the US period, spike increases in the decay were observed. The spikes are interpreted as echoes of the initial nuclear exciton.

STUDY OF EXCITATION FUNCTIONS IN Rh(α, xn) Ag REACTIONS

The excitation functions for the reactions 103 Rh (α, n)106 m Ag , 103 Rh (α, 2n)105 Ag , and 103 Rh (α, 3n)104 Ag have been measured between 10 and 40 MeV alpha-particle energy, using the stacked foil activation technique. Excitation functions are also calculated theoretically using the Geometry Dependent Hybrid (GDH) model. The computer code ALICE/LIVERMORE-82 has been used which takes into account the pre-equilibrium contribution along with the equilibrium decay of the compound nucleus. It has been found that initial exciton number n0=4 gives a satisfactory reproduction of experimental data.

Two-site self-trapping of bulk and surface wide band excitons

The self-trapping of bulk and surface excitons is considered. Both the initial free exciton state and the final state (diatomic quasi-molecule) are formed via resonant binding. Under self-trapping, the binding is redistributed to concentrate mainly inside the molecule, forming a strong intra-molecule bond. But some of the weak resonant bonds persist in the vicinity of the molecule, which results in a lowering of the self-trapping barrier. These phenomena manifest themselves differently in the three- and two-dimensional cases, being better pronounced in the latter case.