arXiv subjects

G. Roepke

Publications and source records attributed to G. Roepke.

At least 55 records · Page 3Linked to original sources

arXiv1999

Deuteron Formation in Heavy Ion Collisions within the Faddeev Approach

We address the formation of correlations in a general nonequilibrium situation. As an example we calculate the formation of light composite particles in a heavy ion collision. In particular we study the formation of deuterons via three-body reactions in some detail. To calculate the relevant reaction rates we solve the Faddeev equation which is consistently modified to include the self energy corrections and Pauli blocking factors due to surrounding nucleons. We find that the time scales of the reaction as well as the number of emitted deuterons change as medium dependent rates are used in the calculation.

nucl-th↗

arXiv1999

Anomalous anharmonicity in doped cuprate superconductors

Strong nonlinear elastic effects arise due to the Lifshitz transition associated with a change in the Fermi surface topology. It is shown that an electronic contribution to the elastic characteristics of cuprates becomes important if the chemical potential is near but not too near the Van Hove critical energy $ε_c$. In the case of a saddle point singularity in the energy spectrum an enhancement factor arises for the "electronic" anharmonic constants of the 3rd and 4th order, that is proportional to $ε_F/(ε_F-ε_c)$ to the 1st and 2nd power, respectively. The behaviour of the Grueneisen coefficient in the vicinity of the Lifshitz transition is considered.

cond-mat↗

arXiv1999

Topological gauge invariant variables in QCD

We suggest that proper variables for the description of non-Abelian theories are those gauge invariant ones which keep the invariance of the winding number functional with respect to topologically nontrivial (large) gauge transformations. We present a model for these variables using the zero mode of the Gauss law constraint and investigate their physical consequences for hadron spectrum and confinement on the level of the generating functional for two-color QCD.

hep-th↗

arXiv1999

Deuteron formation in nuclear matter

We investigate deuteron formation in nuclear matter at finite temperatures within a systematic quantum statistical approach. We consider formation through three-body collisions relevant already at rather moderate densities because of the strong correlations. The three-body in-medium reaction rates driven by the break-up cross section are calculated using exact three-body equations (Alt-Grassberger-Sandhas type) that have been suitably modified to consistently include the energy shift and the Pauli blocking. Important quantities are the lifetime of deuteron fluctuations and the chemical relaxation time. We find that the respective times differ substantially while using in-medium or isolated cross sections. We expect implications for the description of heavy ion collisions in particular for the formation of light charged particles at low to intermediate energies.

nucl-th↗

arXiv1999

Isospin singlet (pn) pairing and quartetting contribution to the binding energy of nuclei

Isospin singlet (pn) pairing as well as quartetting in nuclei is expected to arise near the symmetry line $N=Z$. Empirical values can be deduced from the nuclear binding energies applying special filters. Within the local density approximation, theoretical estimates for finite nuclei are obtained from results for the condensation energy of asymmetric nuclear matter. It is shown that the isospin singlet condensation energy drops down abruptly for |N-Z|~4 for medium nuclei in the region A=40. Furthermore, alpha-like quartetting and the influence of excitations are discussed.

nucl-th↗

arXiv1999

Three-body properties in hot and dense nuclear matter

We derive three-body equations valid at finite densities and temperatures. These are based on the cluster mean field approach consistently including proper self energy corrections and the Pauli blocking. As an application we investigate the binding energies of triton and determine the Mott densities and momenta relevant for a many particle description of nuclear matter in a generalized Beth-Uhlenbeck approach. The method, however is not restricted to nuclear physics problems but may also be relevant, e.g., to treat three-particle correlations in weekly doped semiconducters or strongly coupled dense plasmas.

nucl-th↗

arXiv1999

Precritical pair fluctuations and formation of a pseudogap in low-density nuclear matter

Low-density nuclear matter at finite temperature is considered representing the strong coupling situation of a highly correlated fermion system. One-particle se lf-energies and the density of states in the vicinity of the pairing transition point are presented. Within the Green function approach model calculations are p erformed using the thermodynamic $T$ matrix in ladder approximation. As a main r esult the formation of a pseudogap in the level density near the critical temper ature of the pairing phase transition has been found. It is shown that compared to mean field, the picture of the onset of the pairing transition is essentially changed if correlations are taken into account.

nucl-th↗

arXiv1999

Deuteron formation in nuclear matter within the Faddeev approach

We consider deuteron formation in heavy ion collisions at intermediate energies. The elementary reaction rates (Nd -> NNN) in this context are calculated using rigorous Faddeev methods. To this end an in-medium Faddeev equation that consistently includes the energy shift and Pauli blocking effects has been derived and solved numerically. As a first application we have calculated the life-time of deuteron fluctuations for nuclear densities and temperatures typical for the final stage of heavy ion collisions. We find substantial differences between using the isolated and the in-medium rates.

nucl-th↗

arXiv1998

Two-particle properties in nuclear matter at finite temperatures

Correlation effects in nuclear matter at finite temperatures are studied for subnuclear densities ($ρ<ρ_0$) and medium excitation energy, where a nonrelativistic potential approach is possible. A quantum statistical approach is given, where clusters are treated under the influence of a clustered medium treated within a mean field approximation. Spectral functions, in-medium cross sections, and reaction rates are considered as well as the formation of a quantum condensate. In particluar, exploratory calculations are shown with respect to the pseudo gap in the nucleon level density close to the superfluid phase transition. Estimates of isospin singlet pairing and quartetting in nuclear binding energies are given.

nucl-th↗

arXiv1998

A quantum kinetic equation for Fermi-systems including three-body correlations

A single-time quantum transport equation, which includes effects beyond the quasiparticle approximation, is derived for Fermi-systems in the framework of non-equilibrium real-time Green's functions theory. Ternary correlations are incorporated in the kinetic description via a cluster expansion for the self-energies (e.g., the transport vertex and the width) truncated at the level of three-body scattering amplitudes. A finite temperature/density formulation of the three-body problem is given. Corresponding three-body equations reduce to the well-known Faddeev equations in the vacuum limit. In equilibrium the equation of state contains virial corrections proportional to the third quantum virial coefficient.

nucl-th↗

arXiv1998

Four-particle condensate in strongly coupled fermion systems

Four-particle correlations in fermion systems at finite temperatures are investigated with special attention to the formation of a condensate. Instead of the instability of the normal state with respect to the onset of pairing described by the Gorkov equation, a new equation is obtained which describes the onset of quartetting. Within a model calculation for symmetric nuclear matter, we find that below a critical density, the four-particle condensation (alpha-like quartetting) is favored over deuteron condensation (triplet pairing). This pairing-quartetting competition is expected to be a general feature of interacting fermion systems, such as the excition-biexciton system in excited semiconductors. Possible experimental consequences are pointed out.

cond-mat.str-el↗

arXiv1998

Excess low energy photon pairs from pion annihilation at the chiral phase transition

The photon pair production by pion annihilation in a hot and dense medium at the chiral phase transition is investigated within a chiral quark model. As a direct consequence of this transition the $σ$ meson appears as a bound state in the domain of temperatures and chemical potentials where the condition $M_σ(T,μ) \approx 2 M_π(T,μ)$ is fulfilled. This effect results in a strong enhancement of the cross section for the pion annihilation process $2 π\to 2 γ$ compared with the vacuum case. The calculation of the photon pair production rate as function of the invariant mass shows a strong enhancement and narrowing of the $σ$ meson resonance at threshold due to chiral symmetry restoration.

hep-ph↗

arXiv1997

Dynamical derivation of a quantum kinetic equation for particle production in the Schwinger mechanism

A quantum kinetic equation has been derived for the description of pair production in a time-dependent homogeneous electric field $E(t)$. As a source term, the Schwinger mechanism for particle creation is incorporated. Possible particle production due to collisions and collisional damping are neglected. The main result is a closed kinetic equation of the non-Markovian type. In the low density approximation, the source term is reduced to the leading part of the well known Schwinger formula for the probability of pair creation. We compare the formula obtained with other approaches and discuss the differences.

hep-ph↗

arXiv1997

Squeezed condensate of gluons and the mass of the eta'

The relation between the large mass of the $η'$ and the structure of the gluon vacuum via the $U_A(1)$ anomaly is discussed. A squeezed gluon vacuum is considered as an alternative to existing models. Considering Witten's formula for the $η_0$ mass we show that the contact term can give a sizable contribution and relate it to the physical gluon condensate. The values of the gluon condensate obtained through this relation are compared with the value by Shifman, Vainshtein and Zakharov and the recent update values by Narison.

hep-ph↗

arXiv1997

Relativistic quantum kinetic equation of the Vlasov type for systems with internal degrees of freedom

We present an approach to derive a relativistic kinetic equation of the Vlasov type. Our approach is especially reliable for the description of quantum field systems with many internal degrees of freedom. The method is based on the Heisenberg picture and leads to a kinetic equation which fulfills the conservation laws. We apply the approach to the standard Walecka Lagrangian and an effective chiral Lagrangian.

nucl-th↗

arXiv1997

Dielectric function of a two-component plasma including collisions

A multiple-moment approach to the dielectric function of a dense non-ideal plasma is treated beyond RPA including collisions in Born approximation. The results are compared with the perturbation expansion of the Kubo formula. Sum rules as well as Ward identities are considered. The relations to optical properties as well as to the dc electrical conductivity are pointed out.

physics.plasm-ph↗

arXiv1997

Dielectric function of nonideal plasmas and electrical dc conductivity

Within generalized linear response theory, an expression for the dielectric function is derived which is consistent with standard approaches to the electrical dc conductivity. Explicit results are given for the first moment Born approximation. Some exact relations as well as the limiting behaviour at small values of the wave number and frequency are investigated.

physics.plasm-ph↗

arXiv1997

Deuteron life-time in hot and dense nuclear matter near equilibrium

We consider deuteron formation in hot and dense nuclear matter close to equilibrium and evaluate the life-time of the deuteron fluctuations within the linear response theory. To this end we derive a generalized linear Boltzmann equation where the collision integral is related to equilibrium correlation functions. In this framework we then utilize finite temperature Green functions to evaluate the collision integrals. The elementary reaction cross section is evaluated within the Faddeev approach that is suitably modified to reflect the properties of the surrounding hot and dense matter.

nucl-th↗