Search Results (52 total)

Optical and micro-structural properties of ordered/disordered/ordered InGaAsP quantum wells grown on GaAs substrates were investigated by photoluminescence spectroscopy, high-resolution transmission electron microscopy and selective area diffraction. Strong evidence of carrier localization effects was obtained from low temperature photoluminescence experiments. Photoluminescence spectra of thinner quantum wells were dominated by a broad emission band located at energies below the bandgap of the well material. The energy peak position of this emission varied considerably with the laser excitation power. Carrier localization was attributed to potential fluctuations in the barrier and well layers, as a result of two coexisting effects: Spontaneously atomic ordering and, in a minor degree, alloy inhomogeneities. We show that a reduction of the ordering degree in the bottom barrier layer resulted in a considerable decrease of localization effects in quaternary quantum well heterostructures.

We report on the first analysis of directed and elliptic flow with the new method of Lee–Yang zeros. Experimental data are presented for Ru+Ru reactions at 1.69A GeV measured with the FOPI detector at SIS/GSI. The results obtained with several methods, based on the event-plane reconstruction, on Lee–Yang zeros, and on multiparticle cumulants (up to fifth order) applied for the first time at SIS energies, are compared. They show conclusive evidence that azimuthal correlations between nucleons and composite particles at this energy are largely dominated by anisotropic flow.

We present transverse momentum and rapidity spectra of charged pions in central Ru+Ru collisions at 400A and 1528A  MeV. The data exhibit enhanced production at low transverse momenta compared to expectations from a thermal model that includes the decay of Δ(1232) resonances and thermal pions. Modifications of the Δ spectral function and the Coulomb interaction are necessary to describe the detailed shape of the transverse momentum spectra. Within the framework of the thermal model, the freeze-out radii of pions are similar at both beam energies. The isospin quantum molecular dynamics model reproduces the shapes of the transverse momentum and rapidity spectra of pions, but the predicted absolute yields are larger than in the measurements, especially at lower beam energy.

Peripheral and semiperipheral collisions have been studied in the system ⁹³Nb+⁹³Nb at 38A  MeV. The evaporative and midvelocity components of the light charged particle and intermediate mass fragment emissions have been carefully disentangled. In this way it was possible to obtain the average amount not only of charge and mass, but also of energy, pertaining to the midvelocity emission, as a function of an impact parameter estimator. This emission has a very important role in the overall balance of the reaction, as it accounts for a large fraction of the emitted mass and for more than half of the dissipated energy. As such, it may give precious clues on the microscopic mechanism of energy transport from the interaction zone toward the target and projectile remnants.

We present a complete systematics (excitation functions and system-size dependences) of global stopping and side flow for heavy ion reactions in the energy range between 0.09A and 1.93A  GeV. For the heaviest system, Au+Au, we observe a plateau of maximal stopping extending from about 0.2A to 0.8A  GeV with a fast drop on both sides. The degree of stopping, which is shown to remain significantly below the expectations of a full stopping scenario, is found to be highly correlated to the amount of side flow.

Ultrasonic attenuation and phase velocity measurements have been carried out in the ferromagnetic perovskites La_2/3Ca_1/3MnO₃ and La_2/3Sr_1/3MnO₃. Data show that the transition at the Curie temperature T_C changes from first to second order as Sr replaces Ca in the perovskite. The compound with first-order transition shows also another transition at a temperature T^*>T_C. We interpret the temperature window T_C<T<T^* as a region of coexistence of a phase-separated regime of metallic and insulating regions, in the line of recent theoretical proposals.

We present new experimental data on directed flow in collisions of Au+Au, Xe+CsI, and Ni+Ni at incident energies from 90A to 400A MeV. We study the centrality and system dependence of integral and differential directed flow for particles selected according to charge. All the features of the experimental data are compared with isospin quantum molecular dynamics (IQMD) model calculations in an attempt to extract information about the nuclear matter equation of state (EoS). We show that the combination of rapidity and transverse momentum analysis of directed flow allows to disentangle various parametrizations in the model. At 400A MeV, a soft EoS with momentum dependent interactions is best suited to explain the experimental data in Au+Au and Xe+CsI, but in the case of Ni+Ni the model underpredicts flow for any EoS. At 90A MeV incident beam energy, none of the IQMD parametrizations studied here are able to consistently explain the experimental data.

We present the centrality dependence of proton and deuteron rapidity distributions in Ru+Ru collisions at 400A MeV. Data are compared with isospin quantum molecular dynamics (IQMD) calculations under various assumptions on the nucleon-nucleon cross section in the medium. The rapidity spectra of both particles can be reproduced by IQMD with a free nucleon-nucleon cross section for the most central collisions. The ratio of baryon rapidity distributions in isospin asymmetric collision systems shows incomplete mixing and partial transparency of the projectile and target nuclei at this beam energy.

InGaAs/InP strained-layer modulation-doped quantum wells were studied by photoluminescence. The combination of the built-in strain and the quantum confinement in this system leads to a strong valence band mixing yielding direct and indirect band gap structures. We demonstrate that the optical emission line shape is strongly dependent on the valence band dispersion and it is a good method to distinguish between direct and indirect structures. The application of an external biaxial tensile strain to the samples provides an additional evidence of direct-to-indirect band gap transition in strained heterostructures.

We present experimental data on directed flow in semicentral Au+Au collisions at incident energies from 90 to 400 A MeV. For the first time for this energy domain, the data are presented in a transverse momentum differential way. We study the first order Fourier coefficient v₁ for different particle species and establish a gradual change of its patterns as function of incident energy and for different regions in rapidity.

We have observed changes in the morphology of InP films grown in epiready and processed substrates. These changes are attributed to selective InP growth in areas in the submicron range due to the presence of C-based selective micromasks. Analyzing the behavior of the height–height correlation function, we show that both the roughness exponent and the correlation length changes depending on the area in which growth is taking place. For small area surfaces the correlation length increases and the roughness exponent decreases, indicating enhanced adatom relaxation due to the presence of the borders of the finite growing areas. Nonlinear terms should be included in continuum models in order to explain the observed results.

Four different combinations of ₄₄⁹⁶Ru and ₄₀⁹⁶Zr nuclei, both as projectile and target, were investigated at the same bombarding energy of 400A MeV using a 4π detector. The degree of isospin mixing between projectile and target nucleons is mapped across a large portion of the phase space using two different isospin-tracer observables, the number of measured protons and the tritium to ³He yield ratio. The experimental results show that the global equilibrium is not reached even in the most central collisions. Quantitative measures of stopping and mixing are extracted from the data. They are found to exhibit a quite strong sensitivity to the in-medium ( n,n) cross section used in microscopic transport calculations.

The production of charged pions, protons, and deuterons has been studied in central collisions of ⁵⁸Ni on ⁵⁸Ni at incident beam energies of 1.06A, 1.45A, and 1.93A GeV. The dependence of transverse-momentum and rapidity spectra on the beam energy and on the centrality of the collison is presented. It is shown that the scaling of the mean rapidity shift of protons established for between 10A and 200A GeV at the Brookhaven AGS and the CERN SPS accelerators energies is valid down to 1A GeV. The degree of nuclear stopping is discussed; quantum molecular dynamics calculations reproduce the measured proton rapidity spectra for the most central events reasonably well, but do not show any sensitivity between the soft and the hard equation of state. A radial flow analysis, using the midrapidity transverse-momentum spectra, delivers freeze-out temperatures T and radial flow velocities β_r which increase with beam energy up to 2A GeV; in comparison to the existing data of Au on Au over a large range of energies, only β_r shows a system size dependence.

A Reply to the Comment by S. Chattopadhyay.

Primary and secondary masses of heavy reaction products have been deduced from kinematics and energy–time-of-flight measurements, respectively, for the direct and reverse collisions of ¹⁰⁰Mo with ¹²⁰Sn at 14.1A MeV. Direct experimental evidence of the correlation of energy sharing with net mass transfer and model-independent results on the evolution of the average excitation from equal-energy to equal-temperature partition are presented.

The ratio of the total charge bound in fragments with Z between 2 and 15 to the hydrogen yield, (tsum₂¹⁵M_iZi)/M_H, has been measured, and the neutron-to-proton ratio n/p has been estimated from the data of central Au + Au reactions between 100 and 400A MeV, measured with the phase I setup of the detector system FOPI and GSI, Darmstadt, in the polar-angle range between 7° and 30°. These two quantities were used to determine the entropy per nucleon S/A by comparing them with the predictions of the freesco code. The analysis allows the simulataneous extraction of the values of the baryonic entropy, temperature, and collective flow. The extracted values are in good agreement with the values obtained in earlier FOPI studies, and, for the baryonic entropy, with recent hydrodynamic calculations.

Complete distributions of the light and intermediate mass fragments ( Z  =  1–6) produced within the polar angular range 1^∘≤Θ_lab≤30^∘ in highly central collisions of 250 A MeV Au + Au are presented. The results of this measurement and a model analysis are used to study the expansion and clustering of the hot and compressed transient state formed in central collisions of such a heavy system. The influence of the initial conditions on the final observables is discussed.

Velocity correlations of intermediate mass fragments (IMF), produced in collisions of Au+Au at 100, 150, 250, and 400A MeV beam energy, are extracted from measurements with the 4π detector system (FOPI) in construction stage I at Schwerionen-Synchrotron (SIS) at the Gesellschaft für Schwerionenforschung (GSI) in Darmstadt. The IMF correlation functions of peripheral and semicentral events are found to be strongly affected by the collective sideward motion of nuclear matter. The sideflow causes an enhancement of correlations at small relative velocities. This enhancement results from the mixing of differently azimuthally oriented events; it vanishes if the events are rotated into a unique reaction plane. Selecting violent central collisions, the comparison of the data with a Coulomb dominated final-state interaction model points to a radius of the expanding and multifragmenting source of R_s≃13 fm for 100A MeV which appears shrinking by 20% when increasing the projectile energy to 400 MeV per nucleon. The deduced source radii are found to depend on the radial explosion energy used in the model. The inclusion of such a collective expansion is necessary for a reasonable description of the experimental single-particle spectra of the IMF. The unique Coulomb suppression of small relative IMF velocities, found for the given beam energy range, is attributed to rather constant averaged next-neighbor distances 〈d_IMF〉=8.6±0.2 fm of the IMF charge centers within the source at breakup time.

Using the FOPI facility at GSI Darmstadt complete data of Au on Au collisions at 150A MeV were collected for charged products (Z=1–15) at laboratory angles 1°≤Θ_lab≤30°. Central collisions were selected by applying various criteria. The kinetic energy spectra of fragments from an isolated midrapidity source are investigated in detail for center-of-mass angles 25°≤Θ_c.m.≤45°. The heavy products (Z≥3) are used to determine the collective energy which is found to be at least 10A MeV.

The characteristics of the fission step following a binary deep-inelastic interaction have been reconstructed for three-body events detected in the reaction ¹⁰⁰Mo+¹⁰⁰Mo at 18.7A MeV and ^12-Sn+¹²⁰Sn at 18.4A MeV. The observed anisotropy of the in-plane angular distributions points to the fast decay of a rotating (and strongly deformed) nuclear object formed at the end of the deep-inelastic interaction. The derived time scale of the process indicates that asymmetric divisions are faster than symmetric ones.

The multiplicities of p and α particles detected in coincidence with fragments emitted in fully relaxed collisions in the reactions of 18.5A MeV ¹³⁶Xe+⁴⁸Ti have been measured for different exit channel mass asymmetries. A kinematic source analysis of the spectra and angular distributions of the light particles has been used to separate the total multiplicities into prescission and postscission contributions. From these results, the excitation energies at scission are determined using an empirical technique based upon previous measurements of light charged particle multiplicities observed in coincidence with evaporation residues. These excitation energies are found to decrease from ∼400 MeV to 110 MeV as the fragment mass asymmetry, A_H/A_L, varies from 4.8 to 1.0. A corresponding increase of the mean lifetime of the scissioning nucleus from ∼5×10^-22 s to ∼1×10^-20 s is derived using calculated statistical model decay widths. The extent to which this variation of lifetime with mass asymmetry may be attributed to completely damped deep inelastic collisions or to dynamic delays in the decay of a compound nucleus is discussed as is the need for inclusion of dynamics in the deexcitation calculations for hot nuclei. Observed three fragment events are also discussed.

Velocity correlations of intermediate mass fragments (IMFs), produced in central collisions of Au+Au at 150 MeV beam energy, are extracted from measurements with the FOPI (phase I) detector system at SIS in GSI Darmstadt. The IMF correlation function for semicentral events is found to be affected by the directed sideward flow. When rotating the events into a unique reaction plane an enhancement of correlations, resulting from event mixing effects, vanishes. Selecting violent collisions with a high degree of azimuthal symmetry the correlation function appears nearly independent of additional event or single particle gate conditions. The comparison of the data with a Coulomb dominated final-state interaction model points to an expanding and multifragmenting soure with radius R∼14 fm.

The entropy per nucleon (S/A) has been extracted for the Au [(150–800)A MeV] + Au reaction by using the phase I setup of the 4π facility at GSI, Darmstadt. The entropy has been obtained from the comparison of various observables characterizing the dM/dZ fragment multiplicity distributions, extending up to Z∼15, with those calculated with the quantum statistical model. It is the first time that S/A values are determined by considering the full ensemble of charged products detected in the reaction. Consistent values of S/A are found from different methods. These entropy values are shown to be fairly independent of the volume of the ‘‘participant’’ region considered. They are somewhat lower than those extracted in earlier works but are in good agreement with hydrodynamic calculations and suggest a low viscosity for the hot and dense nuclear matter.

The azimuthal angle distributions of neutral pions at midrapidity from Au+Au reactions at 1 GeV/nucleon incident energy have been measured. An enhanced emission of π⁰’s perpendicular to the reaction plane is observed. The azimuthal asymmetry is dependent on the π⁰ momentum: the π⁰ spectrum perpendicular to the reaction plane is harder than in the reaction plane. The strength of the observed asymmetry appears to be more pronounced for π⁰ than for charged particles and neutrons.