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G. Roepke

Publications and source records attributed to G. Roepke.

At least 37 records · Page 2Linked to original sources
arXiv2009

Effect of medium dependent binding energies on inferring the temperatures and freeze-out density of disassembling hot nuclear matter from cluster yields

We explore the abundance of light clusters in asymmetric nuclear matter at subsaturation density. With increasing density, binding energies and wave functions are modified due to medium effects. The method of Albergo, Costa, Costanzo and Rubbino (ACCR) for determining the temperature and free nucleon density of a disassembling hot nuclear source from fragment yields is modified to include, in addition to Coulomb effects and flow, also effects of medium modifications of cluster properties, which become of importance when the nuclear matter density is above 10$^{-3}$ fm$^{-3}$. We show how the analysis of cluster yields, to infer temperature and nucleon densities, is modified if the shifts in binding energies of in medium clusters are included. While, at low densities, the temperature calculated from given yields changes only modestly if medium effects are taken into account, larger discrepancies are observed when the nucleon densities are determined from measured yields.

nucl-th↗
arXiv2008

The influence of Pauli blocking effects on the properties of dense hydrogen

We investigate the effects of Pauli blocking on the properties of hydrogen at high pressures, where recent experiments have shown a transition from insulating behavior to metal-like conductivity. Since the Pauli principle prevents multiple occupation of electron states (Pauli blocking), atomic states disintegrate subsequently at high densities (Mott effect). We calculate the energy shifts due to Pauli blocking and discuss the Mott effect solving an effective Schroedinger equation for strongly correlated systems. The ionization equilibrium is treated on the basis of a chemical approach. Results for the ionization equilibrium and the pressure in the region 4.000 K < T < 20.000 K are presented. We show that the transition to a highly conducting state is softer than found in earlier work. A first order phase transition is observed at T < 6.450 K, but a diffuse transition appears still up to 20.000 K.

physics.plasm-ph↗
arXiv2008

Dynamical Schwinger effect and high-intensity lasers. Realising nonperturbative QED

We consider the possibility of experimental verification of vacuum e^+e^- pair creation at the focus of two counter-propagating optical laser beams with intensities 10^{20}-10^{22} W/cm^2, achievable with present-day petawatt lasers, and approaching the Schwinger limit: 10^{29} W/cm^2 to be reached at ELI. Our approach is based on the collisionless kinetic equation for the evolution of the e^+ and e^- distribution functions governed by a non-Markovian source term for pair production. As possible experimental signals of vacuum pair production we consider e^+e^- annihilation into gamma-pairs and the refraction of a high-frequency probe laser beam by the produced e^+e^- plasma. We discuss the dependence of the dynamical pair production process on laser wavelength, with special emphasis on applications in the X-ray domain (X-FEL), as well as the prospects for μ^+μ^- and π^+π^- pair creation at high-intensity lasers. We investigate perspectives for using high-intensity lasers as ``boosters'' of ion beams in the few-GeV per nucleon range, which is relevant, e.g., to the exploration of the QCD phase transition in laboratory experiments.

physics.plasm-ph↗
arXiv2008

Criterion for Bose-Einstein condensation in traps and self-bound systems

The internal one-particle density matrix is discussed for Bose-Einstein condensates with finite number of particles in a harmonic trap. The outcome of the digonalization of the density matrix depends on the choice of the internal coordinates: The Pethick-Pitaevskii-type internal density matrix, whose analytical eigenvalues and eigenfunctions are evaluated, yields a fragmented condensate, while the Jacobi-type internal density matrix leads to an ideal condensate. We give a criterion for the choice of the internal coordinates: In the macroscopic limit the internal density matrix should have eigenvalues and eigenfunctions of an ideal Bose-Einstein condensate, this being a very physical condition for cases where the system is also an ideal Bose condensation in the laboratory frame. One choice fulfilling this boundary condition is given by the internal Jacobi coordinates, while the internal coordinates with respect to the center of mass do not satisfy the condition. Based on our criterion, a general definition of the internal one-particle density matrix is presented in a self-bound system, consisting of interacting bosons.

cond-mat.other↗
arXiv2008

Alpha-particle condensation in 16O via a full four-body OCM calculation

In order to explore the 4 α-particle condensate state in 16O, we solve a full four-body equation of motion based on the 4 αOCM (Orthogonality Condition Model) in a large 4 αmodel space spanned by Gaussian basis functions. A full spectrum up to the 0_6^+ state is reproduced consistently with the lowest six 0^+ states of the experimental spectrum. The 0^+_6 state is obtained at about 2 MeV above the 4 αbreakup threshold and has a dilute density structure, with a radius of about 5 fm. The state has an appreciably large αcondensate fraction of 61 %, and a large component of α+12C(0_2^+) configuration, both features being reliable evidence for this state to be of 4 αcondensate nature.

nucl-th↗
arXiv2006

Inelastic form factors to alpha particle condensate states in 12C and 16O: what can we learn ?

In order to discuss the spatial extention of the second 0+ state of 12C (Hoyle state), we analyze the inelastic form factor of electron scattering to the Hoyle state, which our 3 alpha condensate wave function reproduces very well like previous 3 alpha RGM/GCM models. The analysis is made by varying the size of the Hoyle state artificially. As a result, we find that only the maximum value of the form factor sensitively depends on its size, while the positions of maximum and minimum are almost unchanged. This size dependence is found to come from a characteristic feature of the transition density from the ground state to the Hoyle state. We further show the theoretical predictions of the inelastic form factor to the second 2+ state of 12C, which was recently observed above the Hoyle state, and of the inelastic form factor to the calculated third 0+ state of 16O, which was conjectured to correspond to the 4 alpha condensed state in previous theoretical work by the present authors.

nucl-th↗
arXiv2005

Bremsstrahlung from dense plasmas and the Landau-Pomeranchuk-Migdal effect

The suppression of the bremsstrahlung cross section due to multiple scattering of the emitting electrons is an important effect in dense media (Landau-Pomeranchuk-Migdal effect). Here, we study the emission from a dense, fully-ionized and non-relativistic hydrogen plasma. Using the dielectric approach, we relate optical properties such as emission and absorption to equilibrium force-force correlation functions, which allow for a systematic perturbative treatment with the help of thermodynamic Green functions. By considering self-energy and vertex corrections, medium modifications such as multiple scattering of the emitting electrons are taken into account. Results are presented for the absorption coefficient as a function of the frequency at various densities. It is shown that the modification of the inverse bremsstrahlung due to medium effects becomes more significant in the low frequency and high density region.

physics.plasm-ph↗
arXiv2004

Resonance states in 12C and alpha-particle condensation

The states with $J^π=0^+$, $2^+$, and $4^+$ of $^{12}$C with excitation energies less than about 15 MeV are investigated with the alpha condensate wave function with spatial deformation and by using the method of ACCC (analytic continuation in the coupling constant) which is necessary for a proper treatment of resonance states. The calculated energy and width of the recently observed $2_2^+$ state are found to be well reproduced. The obtained $2_2^+$ wave function has a large overlap with a single condensate wave function of $3α$ gas-like structure. The density distribution is shown to be almost the same as that of the $0_2^+$ state that is regarded as a $3α$ Bose-condensed state, if the energy of the $2_2^+$ state is scaled down to the same value as the one of the $0_2^+$ state. Furthermore, the kinetic energy, nuclear interaction energy, and Coulomb interaction energy of the calculated $2_2^+$ state are shown to be very similar to those of the $0_2^+$ state. We conclude that the $2_2^+$ state has a structure similar to the $0_2^+$ state of Bose-condensate character with a dilute $3α$ gas-like structure. In addition the resonance states, $0_3^+$, $0_4^+$, $4_2^+$, are also discussed.

nucl-th↗
arXiv2004

Internal vs. External Conductivity of a Dense Plasma: Many-particle theory and simulations

In the long-wavelength limit k=0, the response function has been investigated with respect to the external and internal fields which is expressed by the external and internal conductivity, respectively. Molecular dynamics (MD) simulations are performed to obtain the current-current correlation function and the dynamical collision frequency which are compared with analytical expressions. Special attention is given to the dynamical collision frequency and the description of plasma oscillations in the case of k=0. The relation between the external and internal conductivity and to the current-current correlation function is analyzed.

physics.plasm-ph↗
arXiv2002

Covariant Linear Response Theory of Relativistic QED Plasmas

We start from the QED Lagrangian to describe a charged many-particle system coupled to the radiation field. A covariant density matrix approach to kinetic theory of QED plasmas, subjected to a strong external electro-magnetic field has recently been developed [1,2]. We use the hyperplane formalism in order to perform a manifest covariant quantization and to implement initial correlations to the solution of the Liouville-von Neumann equation. A perturbative expansion in orders of the fine structure constant for the correlation functions as well as the statistical operator is applied. The non-equilibrium state of the system is given within generalized linear response theory. Expressions for the susceptibility tensor, describing the plasma response, are calculated within different approximations, like the RPA approximation or considering collisions within the Born-approximation. In particular, the process of relativistic inverse bremsstrahlung in a plasma is discussed.

quant-ph↗
arXiv2002

Reflectivity of Shock Compressed Xenon Plasma

Experimental results for the reflection coefficient of shock-compressed dense Xenon plasmas at pressures of 1.6 - 17 GPa and temperatures around 30 000 K using a laser beam with λ= 1.06 10^-6 m are compared with calculations based on different theoretical approaches to the dynamical collision frequency. It is found that a reasonable description can be given assuming a spatial electron density profile corresponding to a finite width of the shock wave front of about $2 10^-6 m.

physics.plasm-ph↗
arXiv2001

Kinetic theory of QED plasma in a strong electromagnetic field I. The covariant hyperplane formalism

We develop a covariant density matrix approach to kinetic theory of QED plasmas subjected into a strong external electromagnetic field. A canonical quantization of the system on space-like hyperplanes in Minkowski space and a covariant generalization of the Coulomb gauge is used. The condensate mode associated with the mean electromagnetic field is separated from the photon degrees of freedom by a time-dependent unitary transformation of both, the dynamical variables and the nonequilibrium statistical operator. Therefore even in the case of strong external fields a perturbative expansion in orders of the fine structure constant for the correlation functions as well as the statistical operator is applicable. A general scheme for deriving kinetic equations in the hyperplane formalism is presented.

physics.plasm-ph↗
arXiv2001

Kinetic theory of QED plasmas in a strong electromagnetic field. II. The mean-field description

Starting from a general relativistic kinetic equation, a self-consistent mean-field equation for fermions is derived within a covariant density matrix approach of QED plasmas in strong external fields. A Schrödinger picture formulation on space-like hyperplanes is applied. The evolution of the distribution function is described by the one-particle gauge-invariant 4x4 Wigner matrix, which is decomposed in spinor space. A coupled system of equations for the corresponding Wigner components is obtained. The polarization current is expressed in terms of the Wigner function. Charge conservation is obeyed. In the quasi-classical limit for the Wigner components a relativistic Vlasov equation is obtained, which is presented in an invariant, i.e. hyperplane independent, form.

quant-ph↗
arXiv2000

Medium corrections in the formation of light charged particles in heavy ion reactions

Within a microscopic statistical description of heavy ion collisions, we investigate the effect of the medium on the formation of light clusters. The dominant medium effects are self-energy corrections and Pauli blocking that produce the Mott effect for composite particles and enhanced reaction rates in the collision integrals. Microscopic description of composites in the medium follows the Dyson equation approach combined with the cluster mean-field expansion. The resulting effective few-body problem is solved within a properly modified Alt-Grassberger-Sandhas formalism. The results are incorporated in a Boltzmann-Uehling-Uhlenbeck simulation for heavy ion collisions. The number and spectra of light charged particles emerging from a heavy ion collision changes in a significant manner in effect of the medium modification of production and absorption processes.

nucl-th↗
arXiv2000

Topological invariant variables in QCD

We show that the class of functions of topologically nontrivial gauge transformations in QCD includes a zero-mode of the Gauss law constraint. The equivalent unconstrained system compatible with Feynman's integral is derived in terms of topological invariant variables, where the zero-mode is identified with the winding number collective variable and leads to the dominance of the Wu-Yang monopole. Physical consequences of Feynman's path integral in terms of the topological invariant variables are studied.

hep-th↗
arXiv2000

The alpha-particle in nuclear matter

Among the light nuclear clusters the alpha-particle is by far the strongest bound system and therefore expected to play a significant role in the dynamics of nuclei and the phases of nuclear matter. To systematically study the properties of the alpha-particle we have derived an effective four-body equation of the Alt-Grassberger-Sandhas (AGS) type that includes the dominant medium effects, i.e. self energy corrections and Pauli-blocking in a consistent way. The equation is solved utilizing the energy dependent pole expansion for the sub system amplitudes. We find that the Mott transition of an alpha-particle at rest differs from that expected from perturbation theory and occurs at approximately 1/10 of nuclear matter densities.

nucl-th↗
arXiv1999

Few-Body States in Fermi-Systems and Condensation Phenomena

Residual interactions in many particle systems lead to strong correlations. A multitude of spectacular phenomenae in many particle systems are connected to correlation effects in such systems, e.g. pairing, superconductivity, superfluidity, Bose-Einstein condensation etc. Here we focus on few-body bound states in a many-body surrounding.

nucl-th↗