arXiv subjects

Michael Schreiber

Publications and source records attributed to Michael Schreiber.

At least 91 records · Page 5Linked to original sources

arXiv2010

Light Transmission Through Metallic-Mean Quasiperiodic Stacks with Oblique Incidence

The propagation of s- and p-polarized light through quasiperiodic multilayers, consisting of layers with different refractive indices, is studied by the transfer matrix method. In particular, we focus on the transmission coefficient of the systems in dependency on the incidence angle and on the ratio of the refractive indices. We obtain additional bands with almost complete transmission in the quasiperiodic systems at frequencies in the range of the photonic band gap of a system with a periodic alignment of the two materials for both types of light polarization. With increasing incidence angle these bands bend towards higher frequencies, where the curvature of the transmission bands in the quasiperiodic stack depends on the metallic mean of the construction rule. Additionally, in the quasiperiodic systems for p-polarized light the bands show almost complete transmission near the Brewster's angle in contrast to the results for s-polarized light. Further, we present results for the influence of the refractive indices at the midgap frequency of the periodic stack, where the quasiperiodicity was found to be most effective.

physics.optics↗

arXiv2010

Twenty Hirsch index variants and other indicators giving more or less preference to highly cited papers

The Hirsch index or h-index is widely used to quantify the impact of an individual's scientific research output, determining the highest number h of a scientist's papers that received at least h citations. Several variants of the index have been proposed in order to give more or less preference to highly cited papers. I analyse the citation records of 26 physicists discussing various suggestions, in particular A, e, f, g, h(2), h_w, h_T, \hbar, m, π, R, s, t, w, and maxprod. The total number of all and of all cited publications as well as the highest and the average number of citations are also compared. Advantages and disadvantages of these indices and indicators are discussed. Correlation coefficients are determined quantifying which indices and indicators yield similar and which yield more deviating rankings of the 26 datasets. For 6 datasets the determination of the indices and indicators is visualized.

physics.soc-ph↗

arXiv2009

Wave Packet Dynamics, Ergodicity, and Localization in Quasiperiodic Chains

In this paper, we report results for the wave packet dynamics in a class of quasiperiodic chains consisting of two types of weakly coupled clusters. The dynamics are studied by means of the return probability and the mean square displacement. The wave packets show anomalous diffusion in a stepwise process of fast expansion followed by time intervals of confined wave packet width. Applying perturbation theory, where the coupling parameter v is treated as perturbation, the properties of the eigenstates of the system are investigated and related to the structure of the chains. The results show the appearance of non-localized states only in sufficiently high orders of the perturbation expansions. Further, we compare these results to the exact solutions obtained by numerical diagonalization. This shows that eigenstates spread across the entire chain for v>0, while in the limit v->0 ergodicity is broken and eigenstates only spread across clusters of the same type, in contradistinction to trivial localization for v=0. Caused by this ergodicity breaking, the wave packet dynamics change significantly in the presence of an impurity offering the possibility to control its long-term dynamics.

cond-mat.mes-hall↗

arXiv2009

A Case Study of the Modified Hirsch Index hm Accounting for Multiple Co-authors

J. E. Hirsch (2005) introduced the h-index to quantify an individual's scientific research output by the largest number h of a scientist's papers, that received at least h citations. This so-called Hirsch index can be easily modified to take multiple co-authorship into account by counting the papers fractionally according to (the inverse of) the number of authors. I have worked out 26 empirical cases of physicists to illustrate the effect of this modification. Although the correlation between the original and the modified Hirsch index is relatively strong, the arrangement of the datasets is significantly different depending on whether they are put into order according to the values of either the original or the modified index.

physics.soc-ph↗

arXiv2008

A modification of the h-index: the hm-index accounts for multi-authored manuscripts

In order to take multiple co-authorship appropriately into account, a straightforward modification of the Hirsch index was recently proposed. Fractionalised counting of the papers yields an appropriate measure which is called the hm-index. The effect of this procedure is compared in the present work with other variants of the h-index and found to be superior to the fractionalised counting of citations and to the normalization of the h-index with the average number of authors in the h-core. Three fictitious examples for model cases and one empirical case are analysed.

physics.soc-ph↗

arXiv2008

An empirical investigation of the g-index for 26 physicists in comparison with the h-index, the A-index, and the R-index

Hirsch has introduced the h-index to quantify an individual's scientific research output by the largest number h of a scientist's papers that received at least h citations. In order to take into account the highly skewed frequency distribution of citations, Egghe proposed the g-index as an improvement of the h-index. I have worked out 26 practical cases of physicists from the Institute of Physics at Chemnitz University of Technology and compare the h and g values. It is demonstrated that the g-index discriminates better between different citation patterns. This can also be achieved by evaluating Jin's A-index which reflects the average number of citations in the h-core and interpreting it in conjunction with the h-index. h and A can be combined into the R-index to measure the h-core's citation intensity. I have also determined the A and R values for the 26 data sets. For a better comparison, I utilize interpolated indices. The correlations between the various indices as well as with the total number of papers and the highest citation counts are discussed. The largest Pearson correlation coefficient is found between g and R. Although the correlation between g and h is relatively strong, the arrangement of the data set is significantly different, depending on whether they are put into order according to the values of either h or g.

physics.soc-ph↗

arXiv2007

Magnetic susceptibility of the two-dimensional Hubbard model using a power series for the hopping constant

The magnetic susceptibility of the two-dimensional repulsive Hubbard model with nearest-neighbor hopping is investigated using the diagram technique developed for the case of strong correlations. In this technique a power series in the hopping constant is used. At half-filling the calculated zero-frequency susceptibility and the square of the site spin reproduce adequately results of Monte Carlo simulations. Also in agreement with numerical simulations no evidence of ferromagnetic correlations was found in the considered range of electron concentrations $0.8\alt\bar{n}\alt 1.2$ for the repulsion parameters $8|t|\leq U\leq 16|t|$. However, for larger $U/|t|$ and $|1-\bar{n}|\approx 0.2$ the nearest neighbor correlations become ferromagnetic. For $\bar{n}\alt 0.94$ and $\bar{n}\agt 1.06$ the imaginary part of the real-frequency susceptibility becomes incommensurate for small frequencies. The incommensurability parameter grows with departure from half-filling and decreases with increasing the frequency. This behavior of the susceptibility can explain the observed low-frequency incommensurate response observed in normal-state lanthanum cuprates.

cond-mat.str-el↗

arXiv2007

Interaction between ionic lattices and superconducting condensates

The interaction of the ionic lattice with the superconducting condensate is treated in terms of the electrostatic force in superconductors. It is shown that this force is similar but not identical to the force suggested by the volume difference of the normal and superconducting states. The BCS theory shows larger deviations than the two-fluid model.

cond-mat.supr-con↗

arXiv2007

A case study of the Hirsch index for 26 non-prominent physicists

The h index was introduced by Hirsch to quantify an individual's scientific research output. It has been widely used in different fields to show the relevance of the research work of prominent scientists. I have worked out 26 practical cases of physicists which are not so prominent. Therefore this case study should be more relevant to discuss various features of the Hirsch index which are interesting or disturbing or both for the more average situation. In particular, I investigate quantitatively some pitfalls in the evaluation and the influence of self-citations.

physics.soc-ph↗

arXiv2007

Coherent destruction of the current through molecular wires using short laser pulses

A molecular wire coupled to two electron reservoirs is investigated within a tight-binding approach including spin and Coulomb interaction. Under the assumption of weak coupling to the electron reservoirs a quantum master equation can be derived for the electron transport through the wire. Motivated by the phenomenon of coherent destruction of tunneling for monochromatic laser fields, the influence of Gaussian laser pulses on the transport through the wires is studied. For situations in which the maximum amplitude of the electric field fulfills the conditions for the destructive quantum effect the average current through the system can be suppressed even for a wire consisting of only one site. Turning on the electron correlation does not destroy the suppression of the current by the laser.

cond-mat.mes-hall↗

arXiv2007

Coherent laser control of the current through molecular junctions

The electron tunneling through a molecular junction modeled by a single site weakly coupled to two leads is studied in the presence of a time-dependent external field using a master equation approach. In the case of small bias voltages and high carrier frequencies of the external field, we observe the phenomenon of coherent destruction of tunneling, i.e. the current through the molecular junction vanishes completely for certain parameters of the external field. In previous studies the tunneling within isolated and open multi-site systems was suppressed; it is shown here that the tunneling between a single site and electronic reservoirs, i.e. the leads, can be suppressed as well. For larger bias voltages the current does not vanish any more since further tunneling channels participate in the electron conduction and we also observe photon-assisted tunneling which leads to steps in the current-voltage characteristics. The described phenomena are demonstrated not only for monochromatic fields but also for laser pulses and therefore could be used for ultrafast optical switching of the current through molecular junctions.

cond-mat.mes-hall↗

arXiv2007

Switching the current through molecular wires

The influence of Gaussian laser pulses on the transport through molecular wires is investigated within a tight-binding model for spinless electrons including correlation. Motivated by the phenomenon of coherent destruction of tunneling for monochromatic laser fields, situations are studied in which the maximum amplitude of the electric field fulfills the conditions for the destructive quantum effect. It is shown that, as for monochromatic laser pulses, the average current through the wire can be suppressed. For parameters of the model, which do not show a net current without any optical field, a Gaussian laser pulse can establish a temporary current. In addition, the effect of electron correlation on the current is investigated.

cond-mat.mes-hall↗

arXiv2007

Optimal control of molecular electronics by femtosecond laser pulses

Combining the features of molecular wires and femtosecond laser pulses gives the unique opportunity to optically switch electron currents in molecular devices with very high speed. Based on a weak-coupling approximation between wire and leads a quantum master equation for the population dynamics and the electric current through the molecular wire has been developed which allows for arbitrary time-dependent laser fields interacting with the wire. This formalism is combined with the theory of optimal control. For a tight-binding approximation of the wire we show how to compute the laser pulses to switch the current through the wire on and off. With this approach the desired pattern of the current in time can be chosen in an almost arbitrary fashion.

cond-mat.mes-hall↗

arXiv2007

The influence of self-citation corrections on Egghe's g index

The g index was introduced by Leo Egghe as an improvement of Hirsch's index h for measuring the overall citation record of a set of articles. It better takes into account the highly skewed frequency distribution of citations than the h index. I propose to sharpen this g index by excluding the self-citations. I have worked out nine practical cases in physics and compare the h and g values with and without self-citations. As expected, the g index characterizes the data set better than the h index. The influence of the self-citations appears to be more significant for the g index than for the h index.

physics.soc-ph↗

arXiv2007

Self-citation corrections for the Hirsch index

I propose to sharpen the index h, proposed by Hirsch as a useful index to characterize the scientific output of a researcher, by excluding the self-citations. Performing a self-experiment and also analyzing two anonymous data sets, it is shown that self-citations can significantly reduce the $h$ index in contrast to Hirsch's expectations. This result is confirmed by an analysis of 13 further data sets.

physics.soc-ph↗

arXiv2006

The concept of correlated density and its application

The correlated density appears in many physical systems ranging from dense interacting gases up to Fermi liquids which develop a coherent state at low temperatures, the superconductivity. One consequence of the correlated density is the Bernoulli potential in superconductors which compensates forces from dielectric currents. This Bernoulli potential allows to access material parameters. Though within the surface potential these contributions are largely canceled, the bulk measurements with NMR can access this potential. Recent experiments are explained and new ones suggested. The underlying quantum statistical theory in nonequilibrium is the nonlocal kinetic theory developed earlier.

cond-mat.supr-con↗

arXiv2006

Localization of electronic states in amorphous materials: recursive Green's function method and the metal-insulator transition at E<>0

In this paper we will investigate whether the scaling assumptions made in previous studies for the transition at energies outside the band centre can be reconfirmed in numerical calculations, and in particular whether the conductivity sigma follows a power law close to the critical energy E_c. For this purpose we will use the recursive Green's function method to calculate the four-terminal conductance of a disordered system for fixed disorder strength at temperature T=0. Applying the finite-size scaling analysis we will compute the critical exponent and determine the mobility edge, i.e. the MIT outside the band centre.

cond-mat.dis-nn↗

arXiv2005

The influence of ultra-fast laser pulses on electron transfer in molecular wires studied by a non-Markovian density matrix approach

New features of molecular wires can be observed when they are irradiated by laser fields. These effects can be achieved by periodically oscillating fields but also by short laser pulses. The theoretical foundation used for these investigations is a density matrix formalism where the full system is partitioned into a relevant part and a thermal fermionic bath. The derivation of a quantum master equation, either based on a time-convolutionless or time-convolution projection-operator approach, incorporates the interaction with time-dependent laser fields non-perturbatively and is valid at low temperatures for weak system-bath coupling. From the population dynamics the electrical current through the molecular wire is determined. This theory including further extensions is used for the determination of electron transport through molecular wires. As examples, we show computations of coherent destruction of tunneling in asymmetric periodically driven quantum systems, alternating currents and the suppression of the directed current by using a short laser pulse.

cond-mat.mes-hall↗