Strangeness in hadronic stellar matter

We examine the presence of strangeness-bearing components, hyperons and kaons, in dense neutron star matter. Calculations are performed using relativistic mean field models, in which both the baryon-baryon and kaon-baryon interactions are mediated by meson exchange. Results of kaon condensation are found to be qualitatively similar to previous work with chiral models, if compatibility of the kaon optical potentials is required. The presence of strangeness, be it in the form of hyperons or kaons, implies a reduction in the maximum mass and a relatively large number of protons, sufficient to allow rapid cooling to take place. The need to improve upon the poorly-known couplings of the strange particles, which determine the composition and structure of neutron stars, is stressed. We also discuss generic problems with effective masses in mean field theories

Dynamics of Excited Electrons in Copper: Role of Auger Electrons

Within a theoretical model based on the Boltzmann equation, we analyze in detail the structure of the unusual peak recently observed in the relaxation time in Cu. In particular, we discuss the role of Auger electrons in the electron dynamics and its dependence on the d-hole lifetime, the optical transition matrix elements and the laser pulse duration. We find that the Auger contribution to the distribution is very sensitive to both the d-hole lifetime tau_h and the laser pulse duration tau_l and can be expressed as a monotonic function of tau_l/tau_h. We have found that for a given tau_h, the Auger contribution is significantly smaller for a short pulse duration than for a longer one. We show that the relaxation time at the peak depends linearly on the d-hole lifetime, but interestingly not on the amount of Auger electrons generated. We provide a simple expression for the relaxation time of excited electrons which shows that its shape can be understood by a phase space argument and its amplitude is governed by the d-hole lifetime. We also find that the height of the peak depends on both the ratio of the optical transition matrix elements R=|M_{d \to sp}|^2/|M_{sp \to sp}|^2 and the laser pulse duration. Assuming a reasonable value for the ratio, namely R = 2, and a d-hole lifetime of tau_h=35 fs, we obtain for the calculated height of the peak Delta tau_{th}=14 fs, in fair agreement with Delta tau_{exp} \approx 17 fs measured for polycrystalline Cu.Comment: 6 pages, 6 figure

The Equation of State of Neutron-Star Matter in Strong Magnetic Fields

We study the effects of very strong magnetic fields on the equation of state (EOS) in multicomponent, interacting matter by developing a covariant description for the inclusion of the anomalous magnetic moments of nucleons. For the description of neutron star matter, we employ a field-theoretical approach which permits the study of several models which differ in their behavior at high density. Effects of Landau quantization in ultra-strong magnetic fields ($B>10^{14}$ Gauss) lead to a reduction in the electron chemical potential and a substantial increase in the proton fraction. We find the generic result for $B>10^{18}$ Gauss that the softening of the EOS caused by Landau quantization is overwhelmed by stiffening due to the incorporation of the anomalous magnetic moments of the nucleons. In addition, the neutrons become completely spin polarized. The inclusion of ultra-strong magnetic fields leads to a dramatic increase in the proton fraction, with consequences for the direct Urca process and neutron star cooling. The magnetization of the matter never appears to become very large, as the value of $|H/B|$ never deviates from unity by more than a few percent. Our findings have implications for the structure of neutron stars in the presence of large frozen-in magnetic fields.Comment: 40 pages, 7 figures, accepted for publication in Ap

Electron-phonon relaxation and excited electron distribution in zinc oxide and anatase

We propose a first-principle method for evaluations of the time-dependent electron distribution function of excited electrons in the conduction band of semiconductors. The method takes into account the excitations of electrons by external source and the relaxation to the bottom of conduction band via electron-phonon coupling. The methods permits calculations of the non-equilibrium electron distribution function, the quasi-stationary distribution function with steady-in-time source of light, the time of setting of the quasi-stationary distribution and the time of energy loss via relaxation to the bottom of conduction band. The actual calculations have been performed for titanium dioxide in the anatase structure and zinc oxide in the wurtzite structure. We find that the quasi-stationary electron distribution function for ZnO is a fermi-like curve that rises linearly with increasing excitation energy whereas the analogous curve for anatase consists of a main peak and a shoulder. The calculations demonstrate that the relaxation of excited electrons and the setting of the quasi-stationary distribution occur within the time no more than 500 fsec for ZnO and 100 fsec for anatase. We also discuss the applicability of the effective phonon model with energy-independent electron-phonon transition probability. We find that the model only reproduces the trends in changing of the characteristic times whereas the precision of such calculations is not high. The rate of energy transfer to phonons at the quasi-stationary electron distribution also have been evaluated and the effect of this transfer on the photocatalyses has been discussed. We found that for ZnO this rate is about 5 times less than in anatase.Comment: 21 p., 9 figure

Nonequilibrium Magnetization Dynamics of Nickel

Ultrafast magnetization dynamics of nickel has been studied for different degrees of electronic excitation, using pump-probe second-harmonic generation with 150 fs/800 nm laser pulses of various fluences. Information about the electronic and magnetic response to laser irradiation is obtained from sums and differences of the SHG intensity for opposite magnetization directions. The classical M(T)-curve can be reproduced for delay times larger than the electron thermalization time of about 280 fs, even when electrons and lattice have not reached thermal equilibrium. Further we show that the transient magnetization reaches its minimum approx. 50 fs before electron thermalization is completed.Comment: 8 pages, 5 figures, revte

Dynamics of Excited Electrons in Copper and Ferromagnetic Transition Metals: Theory and Experiment

Both theoretical and experimental results for the dynamics of photoexcited electrons at surfaces of Cu and the ferromagnetic transition metals Fe, Co, and Ni are presented. A model for the dynamics of excited electrons is developed, which is based on the Boltzmann equation and includes effects of photoexcitation, electron-electron scattering, secondary electrons (cascade and Auger electrons), and transport of excited carriers out of the detection region. From this we determine the time-resolved two-photon photoemission (TR-2PPE). Thus a direct comparison of calculated relaxation times with experimental results by means of TR-2PPE becomes possible. The comparison indicates that the magnitudes of the spin-averaged relaxation time \tau and of the ratio \tau_\uparrow/\tau_\downarrow of majority and minority relaxation times for the different ferromagnetic transition metals result not only from density-of-states effects, but also from different Coulomb matrix elements M. Taking M_Fe > M_Cu > M_Ni = M_Co we get reasonable agreement with experiments.Comment: 23 pages, 11 figures, added a figure and an appendix, updated reference

Kaon effective mass and energy from a novel chiral SU(3)-symmetric Lagrangian

A new chiral SU(3) Lagrangian is proposed to describe the properties of kaons and antikaons in the nuclear medium, the ground state of dense matter and the kaon-nuclear interactions consistently. The saturation properties of nuclear matter are reproduced as well as the results of the Dirac-Br\"{u}ckner theory. Our numerical results show that the kaon effective mass might be changed only moderately in the nuclear medium due to the highly non-linear density effects. After taking into account the coupling between the omega meson and the kaon, we obtain similar results for the effective kaon and antikaon energies as calculated in the one-boson-exchange model while in our model the parameters of the kaon-nuclear interactions are constrained by the SU(3) chiral symmetry.Comment: 13 pages, Latex, 3 PostScript figures included; replaced by the revised version, to appear in Phys. Rev.

S-wave Pairing of Λ\Lambda Hyperons in Dense Matter

In this work we calculate the $^1S_0$ gap energies of $\Lambda$ hyperons in neutron star matter. The calculation is based on a solution of the BCS gap equation for an effective G-matrix parameterization of the $\Lambda-\Lambda$ interaction with a nuclear matter background, presented recently by Lanskoy and Yamamoto. We find that a gap energy of a few tenths of MeV is expected for $\Lambda$ Fermi momenta up to about 1.3 fm$^{-1}$. Implications for neutron star matter are examined, and suggest the existence of a $\Lambda$ $^1S_0$ superfluid between the threshold baryon density for $\Lambda$ formation and the baryon density where the $\Lambda$ fraction reaches $15-20$.Comment: 16 pages, Revtex, 9 figures, 33 reference

Kaon production in heavy-ion collisions and maximum mass of neutron stars

We determine an `empirical' kaon dispersion relation by analysing and fitting recent experimental data on kaon production in heavy-ion collisions. We then investigate its effects on hadronic equation of state at high densities and on neutron star properties. We find that the maximum mass of neutron stars can be lowered by about 0.4$M_\odot$, once kaon condensation as constrained by our empirical dispersion relation is introduced. We emphasize the growing interplay between hadron physics, relativistic heavy-ion physics and the physics of compact objects in astrophysics.Comment: 6 pages with 3 postscript figures, to appear in Physical Review Letter

Strange nuclear matter within Brueckner-Hartree-Fock Theory

We have developed a formalism for microscopic Brueckner-type calculations of dense nuclear matter that includes all types of baryon-baryon interactions and allows to treat any asymmetry on the fractions of the different species (n, p, $\Lambda$, $\Sigma^0$, $\Sigma^+$, $\Sigma^-$, $\Xi^-$ and $\Xi^0$). We present results for the different single-particle potentials focussing on situations that can be relevant in future microscopic studies of beta-stable neutron star matter with strangeness. We find the both the hyperon-nucleon and hyperon-hyperon interactions play a non-negligible role in determining the chemical potentials of the different species.Comment: 36 pages, LateX, includes 8 PostScript figures, (submitted to PRC