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P. Wagner

Publications and source records attributed to P. Wagner.

28 records · Page 2Linked to original sources
arXiv1999

Comparison of the Transport Mechanism in Underdoped High Temperature Superconductors and in Spin Ladders

Recently, the normal state resistivity of high temperature superconductors (in particular in La2-xSrxCuO4 single crystals) has been studied extensively in the region below Tc by suppressing the superconducting state in high magnetic fields. In the present work we report on the normal state resistance of underdoped La2-xSrxCuO4 thin films under epitaxial strain, measured far below Tc by applying pulsed fields up to 60 T. We will compare the transport measurements on these high temperature superconductors with transport data reported for the Sr2.5Ca11.5Cu24O41 spin ladder compound. This comparison leads to an interpretation of the data in terms of the recently proposed 1D quantum transport model and the charge-stripe models.

cond-mat.supr-con↗
arXiv1999

Finite size effects and the order of a phase transition in fragmenting nuclear systems

We discuss the implications of finite size effects on the determination of the order of a phase transition which may occur in infinite systems. We introduce a specific model to which we apply different tests. They are aimed to characterise the smoothed transition observed in a finite system. We show that the microcanonical ensemble may be a useful framework for the determination of the nature of such transitions.

nucl-th↗
arXiv1999

Normal State Resistivity of Underdoped YBa2Cu3Ox Thin Films and La2-xSrxCuO4 Ultra-Thin Films under Epitaxial Strain

The normal state resistivity of high temperature superconductors can be probed in the region below Tc by suppressing the superconducting state in high magnetic fields. Here we present the normal state properties of YBa2Cu3Ox thin films in the underdoped regime and the normal state resistance of La2-xSrxCuO4 thin films under epitaxial strain, measured below Tc by applying pulsed fields up to 60 T. A universal rho(T) behaviour is reported. We interpret these data in terms of the recently proposed 1D quantum transport model with the 1D paths corresponding to the charge stripes.

cond-mat.supr-con↗
arXiv1999

Influence of the cooperative Jahn-Teller effect on the transport- and magnetic properties of La_{7/8}Sr_{1/8}MnO_3 single crystals

The low-doped magnetic perovskite La_{7/8}Sr_{1/8}MnO_3 undergoes within the paramagnetic-semiconducting phase a first-order structural transition due to antiferrodistorsive ordering of Jahn-Teller deformed MnO_6 octahedra. This allows to study not only the influence of the spin configuration on the magneto-transport properties (CMR effect) but also the role of orbital order and disorder. The orbital ordering transition (at 269 K in zero magnetic field) causes a doubling of the resistivity (regardless of the CMR effect in applied magnetic fields) and a drop of the paramagnetic susceptibility. The latter might be interpreted in terms of a shrinking of spin polarons. External magnetic fields shift the ordering transition to lower temperatures according to the field-induced decrease of the carrier localization. The magnetic field - temperature phase boundary line was investigated by means of magnetoresistance (up to 12 T) and pulsed-fields magnetization measurements up to 50 T. The pronounced magnetization anomalies, associated with the phase transition, vanish for fields exceeding 20 T. This behaviour has been attributed to a field-induced crossover from antiferrodistorsive order to a nondistorsive/ferromagnetic orbital configuration.

cond-mat.str-el↗
arXiv1999

Is binary sequential decay compatible with the fragmentation of nuclei at high energy?

We use a binary sequential decay model in order to describe the fragmentation of a nucleus induced by the high energy collisions of protons with Au nuclei. Overall agreement between measured and calculated physical observables is obtained. We evaluate and analyse the decay times obtained with two different parametrisations of the decay rates and discuss the applicability of the model to high energy fragmentation.

nucl-th↗
arXiv1998

Flow angle from intermediate mass fragment measurements

Directed sideward flow of light charged particles and intermediate mass fragments was measured in different symmetric reactions at bombarding energies from 90 to 800 AMeV. The flow parameter is found to increase with the charge of the detected fragment up to Z = 3-4 and then turns into saturation for heavier fragments. Guided by simple simulations of an anisotropic expanding thermal source, we show that the value at saturation can provide a good estimate of the flow angle, $Θ_{flow}$, in the participant region. It is found that $Θ_{flow}$ depends strongly on the impact parameter. The excitation function of $Θ_{flow}$ reveals striking deviations from the ideal hydrodynamical scaling. The data exhibit a steep rise of $Θ_{\flow}$ to a maximum at around 250-400 AMeV, followed by a moderate decrease as the bombarding energy increases further.

nucl-ex↗
arXiv1998

Thermodynamics of a finite system of classical particles with short and long range interactions and nuclear fragmentation

We describe a finite inhomogeneous three dimensional system of classical particles which interact through short and (or) long range interactions by means of a simple analytic spin model. The thermodynamic properties of the system are worked out in the framework of the grand canonical ensemble. It is shown that the system experiences a phase transition at fixed average density in the thermodynamic limit. The phase diagram and the caloric curve are constructed and compared with numerical simulations. The implications of our results concerning the caloric curve are discussed in connection with the interpretation of corresponding experimental data.

nucl-th↗
arXiv1997

Abundance of Delta Resonances in 58Ni+58Ni Collisions between 1 and 2 AGeV

Charged pion spectra measured in 58Ni-58Ni collisions at 1.06, 1.45 and 1.93 AGeV are interpreted in terms of a thermal model including the decay of Delta resonances. The transverse momentum spectra of pions are well reproduced by adding the pions originating from the Delta-resonance decay to the component of thermal pions, deduced from the high transverse momentum part of the pion spectra. About 10 and 18% of the nucleons are excited to Delta states at freeze-out for beam energies of 1 and 2 AGeV, respectively.

nucl-ex↗
arXiv1997

About the determination of critical exponents related to possible phase transitions in nuclear fragmentation

We introduce a method based on the finite size scaling assumption which allows to determine numerically the critical point and critical exponents related to observables in an infinite system starting from the knowledge of the observables in finite systems. We apply the method to bond percolation in 2 dimensions and compare the results obtained when the bond probability p or the fragment multiplicity m are chosen as the relevant parameter.

nucl-th↗