arXiv subjects

E. Lehmann

Publications and source records attributed to E. Lehmann.

17 recordsLinked to original sources

arXiv2007

Probing the electron-phonon coupling in MgB2 through magnetoresistance measurements in neutron irradiated thin films

We report magnetoresistance (MR) measurements on MgB2 and the corresponding full account from ab-initio calculations; we suggest that this combination can be a useful tool to probe electron- phonon coupling. We obtain good quantitative agreement between high field measurements on neutron irradiated epitaxial thin films and calculations within Bloch-Boltzmann transport theory over a wide range of magnetic fields (0-28 T) and temperatures (40-300 K), and as a function of the field orientation. The crossovers between in-plane and out-of-plane MR, experimentally observed as a function of either disorder or temperature are well reproduced indicating that disorder and interaction with phonons strongly affect the scattering rate of s-carriers.

cond-mat.supr-con↗

arXiv2006

Systematic study of disorder induced by neutron irradiation in MgB2 thin films

The effects of neutron irradiation on normal state and superconducting properties of epitaxial magnesium diboride thin films are studied up to fluences of 1020 cm-2. All the properties of the films change systematically upon irradiation. Critical temperature is suppressed and, at the highest fluence, no superconducting transition is observed down to 1.8 K. Residual resistivity progressively increases from 1 to 190 microohmcm; c axis expands and then saturates at the highest damage level. We discuss the mechanism of damage through the comparison with other damage procedures. The normal state magnetoresistivity of selected samples measured up to high fields (28 and 45T) allows to determine unambiguously the scattering rates in each band; the crossover between the clean and dirty limit in each sample can be monitored. This set of samples, with controlled amount of disorder, is suitable to study the puzzling problem of critical field in magnesium diboride thin films. The measured critical field values are extremely high (of the order of 50T in the parallel direction at low fluences) and turns out to be rather independent on the experimental resistivity, at least at low fluences. A simple model to explain this phenomenology is presented.

cond-mat.supr-con↗

arXiv2006

Point-contact spectroscopy in neutron-irradiated Mg^{11}B_2

We report on recent results of point-contact spectroscopy measurements in Mg^{11}B_2 polycrystals irradiated at different neutron fluences up to 1.4 x 10^{20} cm^{-2}. The point contacts were made by putting a small drop of Ag paint -- acting as the counterelectrode -- on the cleaved surface of the samples. The gap amplitudes were extracted from the experimental conductance curves, showing Andreev-reflection features, through a two-band Blonder-Tinkham-Klapwijk fit and reported as a function of the Andreev critical temperature of the junctions, TcA. The resulting Delta_sigma(TcA) and Delta_pi(TcA) curves show a clear merging of the gaps when TcA ~ 9K that perfectly confirms the findings of specific-heat measurements in the same samples. Anomalous contacts with TcA > Tc (being Tc the bulk critical temperature) were often obtained, particularly in samples irradiated at very high fluences. Their fit gave a different dependence of Delta_pi on TcA. The possible origin of these contacts is discussed in terms of local current-induced annealing and/or nanoscale inhomogeneities observed by STM in the most irradiated samples.

cond-mat.supr-con↗

arXiv2006

Effects of neutron irradiation on polycrystalline Mg11B2

We studied the influence of the disorder introduced in polycrystalline MgB2 samples by neutron irradiation. To circumvent self shielding effects due to the strong interaction between thermal neutrons and 10B we employed isotopically enriched 11B which contains 40 times less 10B than natural B. The comparison of electrical and structural properties of different series of samples irradiated in different neutron sources, also using Cd shields, allowed us to conclude that, despite the low 10B content, the main damage mechanisms are caused by thermal neutrons, whereas fast neutrons play a minor role. Irradiation leads to an improvement in both upper critical field and critical current density for an exposure level in the range 1-2x1018 cm-2. With increasing fluence the superconducting properties are depressed. An in-depth analysis of the critical field and current density behaviour has been carried out to identify what scattering and pinning mechanisms come into play. Finally the correlation between some characteristic lengths and the transition widths is analysed.

cond-mat.supr-con↗

arXiv2006

Covariant Equilibrium Statistical Mechanics

A manifest covariant equilibrium statistical mechanics is constructed starting with a 8N dimensional extended phase space which is reduced to the 6N physical degrees of freedom using the Poincare-invariant constrained Hamiltonian dynamics describing the micro-dynamics of the system. The reduction of the extended phase space is initiated forcing the particles on energy shell and fixing their individual time coordinates with help of invariant time constraints. The Liouville equation and the equilibrium condition are formulated in respect to the scalar global evolution parameter which is introduced by the time fixation conditions. The applicability of the developed approach is shown for both, the perfect gas as well as the real gas. As a simple application the canonical partition integral of the monatomic perfect gas is calculated and compared with other approaches. Furthermore, thermodynamical quantities are derived. All considerations are shrinked on the classical Boltzmann gas composed of massive particles and hence quantum effects are discarded.

math-ph↗

arXiv2005

Observation of the Crossover from Two-Gap to Single-Gap Superconductivity through Specific Heat Measurements in Neutron Irradiated MgB2

We report specific heat measurements in neutron irradiated MgB2 samples, for which the critical temperature has been suppressed down to 8.5 K, but the superconducting transition remains extremely sharp, indicative of a defect structure extremely homogeneous. Our results demonstrate that the two-gap feature is evident in the temperature range above 21 K, while the single-gap superconductivity is well established as a bulk property not associated to local disorder fluctuations when Tc is decreased down to 11 K.

cond-mat.supr-con↗

arXiv2005

Magnetoresistivity in MgB2 as a probe of disorder in p- and s-bands

In this paper we present normal state magnetoresistivity data of magnesium diboride epitaxial thin films with different levels of disorder, measured at 42K in magnetic fields up to 45 Tesla. Disorder was introduced in a controlled way either by means of neutron irradiation or by carbon doping. From a quantitative analysis of the magnetoresistivity curves with the magnetic field either parallel or perpendicular to the plane of the film, we extract the ratio of the scattering times in p- and s-bands. We demonstrate that the undoped unirradiated thin film has p scattering times smaller than s ones; upon irradiation, both bands become increasingly more disordered; eventually the highly irradiated sample (neutron fluence 7.7X1017 cm-2) and the C-doped sample have comparable scattering times in the two types of bands. This description of the effect of disorder in the two kinds of bands on transport is consistent with the residual resistivity values and with the temperature dependence of the resistivity.

cond-mat.supr-con↗

arXiv2005

Enhanced flux pinning in neutron irradiated MgB2

We study the effect of neutron irradiation on the critical current density Jc of isotopically pure polycrystalline Mg11B2 samples. For fluences in the range 1017-1018 cm-2, Jc is enhanced and its dependence on magnetic field is significantly improved: we demonstrate that, in this regime, point-like pinning centers are effectively introduced in the system proportionally to the neutron fluence. Instead, for larger fluences, a strong suppression of the critical temperature accompanied by a decrease of both the upper critical field Bc2 and Jc is found.

cond-mat.supr-con↗

arXiv2004

Neutron Irradiation of Mg11B2 : From the Enhancement to the Suppression of Superconducting Properties

In this letter we present the effect of neutron irradiation up to fluences of 3.9 1019 n/cm2 on the superconducting properties of MgB2. In order to obtain a disorder structure homogeneously distributed, the experiment was carried out on bulk samples prepared with the 11B isotope. Up to fluences of 1018 n/cm2 the critical temperature is slightly diminished (36 K) and the superconducting properties are significantly improved; the upper critical field is increased from 13.5 T to 20.3 T at 12 K and the irreversibility field is doubled at 5 K. For larger neutron fluences the critical temperature is suppressed down to 12 K and the superconducting properties come out strongly degraded.

cond-mat.supr-con↗

arXiv1997

Influence of the in-medium pion dispersion relation in heavy ion collisions

We investigate the influence of medium corrections to the pion dispersion relation on the pion dynamics in intermediate energy heavy ion collisions. To do so a pion potential is extracted from the in-medium dispersion relation and used in QMD calculations and thus we take care of both, real and imaginary part of the pion optical potential. The potentials are determined from different sources, i.e. from the $Δ$--hole model and from phenomenological approaches. Depending on the strength of the potential a reduction of the anti-correlation of pion and nucleon flow in non-central collisions is observed as well as an enhancement of the high energetic yield in transverse pion spectra. A comparison to experiments, in particular to $p_t$-spectra for the reaction Ca+Ca at 1 GeV/nucleon and the pion in-plane flow in Ne+Pb collisions at 800 MeV/nucleon, generally favours a weak potential.

nucl-th↗

arXiv1996

Influence of the pion-nucleon interaction on the collective pion flow in heavy ion reactions

We investigate the influence of the real part of the in-medium pion optical potential on the pion dynamics in intermediate energy heavy ion reactions at 1 GeV/A. For different models, i.e. a phenomenological model and the $Δ$--hole model, a pionic potential is extracted from the dispersion relation and used in Quantum Molecular Dynamics calculations. In addition with the inelastic scattering processes we thus take care of both, real and imaginary part of the pion optical potential. A strong influence of the real pionic potential on the pion in-plane flow is observed. In general such a potential has the tendency to reduce the anticorrelation of pion and nucleon flow in non-central collisions.

nucl-th↗

arXiv1995

Heavy Ion Collisions and the Density Dependence of the Local Mean Field

We study the effect of the density dependence of the scalar and the vector part of the nucleonic self-energy in Relativistic Quantum Molecular Dynamics (RQMD) on observables like the transversal flow and the rapidity distribution. The stability of nuclei in RQMD is greatly improved if the density dependence is included in the self-energies compared to a calculation assuming always saturation density of nuclear matter. Different approaches are studied: The main results are calculated with self-energies extracted from a Dirac-Brückner-Hartree-Fock G-matrix of a one boson exchange model, i.e. the Bonn potential. These results are compared with those obtained by a generalization of static Skyrme force, with calculations in the simple linear Walecka model and results of the Brückner-Hartree-Fock G-matrix of the Reid soft core potential. The transversal flow is very sensitive to these different approaches. A comparison with the data is given.

nucl-th↗

arXiv1995

Relativistic Effects in Heavy-Ion Collisions at SIS Energies

The covariant and non-covariant Quantum Molecular Dynamics models are applied to investigate possible relativistic effects in heavy ion collisions at SIS energies. These relativistic effects which arise due to the full covariant treatment of the dynamics are studied at bombarding energies E$_{lab.}$ = 50, 250, 500, 750, 1000, 1250, 1500, 1750 and 2000 MeV/nucl.. A wide range of the impact parameter from b = 0 fm to b = 10 fm is also considered. In the present study, five systems $^{12}$C-$^{12}$C, $^{16}$O-$^{16}$O, $^{20}$Ne-$^{20}$Ne, $^{28}$Si-$^{28}$Si and $^{40}$Ca-$^{40}$Ca are investigated. The full covariant treatment at low energies shows quite good agreement with the corresponding non-covariant approach whereas at higher energies it shows less stopping and hence less thermal equilibrium as compared to the non-covariant approach. The collisions dynamics is less affected. The density using RQMD rises and drops faster than with QMD. The relativistic effects show some influence on the resonance matter production. Overall, the relativistic effects at SIS energies ($\leq$ 2000 MeV/nucl.) are less significant.

nucl-th↗

arXiv1995

Study of non-equilibrium effects and thermal properties of heavy ion collisions using a covariant approach

Non-equilibrium effects are studied using a full Lorentz-invariant formalism. Our analysis shows that in reactions considered here, no global or local equilibrium is reached. The heavier masses are found to be equilibrated more than the lighter systems. The local temperature is extracted using hot Thomas Fermi formalism generalized for the case of two interpenetrating pieces of nuclear matter. The temperature is found to vary linearly with bombarding energy and impact parameter whereas it is nearly independent of the mass of the colliding nuclei. This indicates that the study of temperature with medium size nuclei is also reliable. The maximum temperatures obtained in our approach are in a nice agreement with earlier calculations of other approaches. A simple parametrization of maximal temperature as a function of the bombarding energy is also given.

nucl-th↗

arXiv1995

Sensitivity of the nuclear equation of state towards relativistic effects

We analyze relativistic effects in transverse momentum using Quantum Molecular Dynamics [QMD] and its covariant extension Relativistic Quantum Molecular Dynamics [RQMD]. The strength of the relativistic effects is found to increase with the bombarding energy and with an averaged impact parameter. The variation in the intensity of the relativistic effects with variation in the mass of the colliding nuclei is not systematic. Furthermore, the hard EOS is affected drastically by the relativistic effects whereas the soft EOS is affected less. Our analysis shows that up to the bombarding energy of 1 GeV/nucl., the influence of relativistic effects is small. Whereas at higher energies, relativistic effects become naturally very important.

nucl-th↗

arXiv1995

Consequences of a covariant Description of Heavy Ion Reactions at intermediate Energies

Heavy ion collisions at intermediate energies are studied by using a new RQMD code, which is a covariant generalization of the QMD approach. We show that this new implementation is able to produce the same results in the nonrelativistic limit (i.e. 50MeV/nucl.) as the non-covariant QMD. Such a comparison is not available in the literature. At higher energies (i.e. 1.5 GeV/nucl. and 2 GeV/nucl.) RQMD and QMD give different results in respect to the time evolution of the phase space, for example for the directed transverse flow. These differences show that consequences of a covariant description of heavy ion reactions within the framework of RQMD are existing even at intermediate energies.

nucl-th↗

arXiv1994

A Resonance Model for pi N -> Y K and pi Delta -> Y K Reactions for Kaon Production in Heavy Ion Collisions

In a resonance model the reactions pi N -> Y K and pi Delta -> Y K are studied. For the reactions pi N -> Lambda K and pi Delta -> Lambda K, the resonances N(1650)(J^P=1/2^-), N(1710)(1/2^+) and N(1720)(3/2^+) are included as intermediate states. For the reactions pi N -> Sigma K, the resonances N(1710)(1/2^+), N(1720)(3/2^+) and Delta(1920)(3/2^+) are considered, while for the pi Delta -> Sigma K reactions the intermediate resonances are N(1710)(1/2^+) and N(1720)(3/2^+). Besides these resonances in the s-channel, the t-channel K^*(892) exchanges are also taken into account as a smooth background. The relevant coupling constants for the meson-baryon vertices are obtained (except for Delta(1920)) from the experimental decay branching ratios of the relevant resonances. All isospin channels of the pi N -> Y K and pi Delta -> Y K cross sections are calculated. By comparing the calculated results with the available experimental data, we find that the total cross sections of the pi N -> Y K reactions can be explained by the resonance model. The pi Delta -> Y K cross sections, for which no experimental data are available, are predicted theoretically. Parametrizations of the calculated total cross sections for all different isospin channels are given for the use of kaon productions in heavy ion collisions. The differential cross sections are also studied.

nucl-th↗