G. Nimtz
University of Cologne
Network
Latest external collaboration on country level. Dive into details by clicking on the dots.
Publication
Featured researches published by G. Nimtz.
Progress in Quantum Electronics | 1997
G. Nimtz; W. Heitmann
Abstract Recent experimental studies with microwave and laser pulses have revealed superluminal (faster-than-light) group, signal and energy velocities for the tunneling of electromagnetic wave packets in undersized waveguides and other photonic barriers. First we report on the historic background of tunneling and the problems of the interpretation of electronic tunneling data. The mathematical analogy of the classical tunneling, i.e. the propagation of evanescent modes, described by the Helmholtz equation, and the quantum mechanical tunneling, described by the Schro¨dinger equation, is introduced. In the next sections the experimental data on the tunneling time of electromagnetic wave packets and signals is presented. The interpretation of the experimental observations, particularly the production of superluminal tunneling velocity and its implication for the quantum mechanical electronic tunneling are discussed in the following sections. An introduction to the various theoretical approaches is included. Remarks on superluminal tunneling and on causality conclude the paper.
Journal of Crystal Growth | 1988
G. Nimtz; P. Marquardt; H. Gleiter
Abstract We report on the size-induced metal-insulator transition discovered in tiny metal crystals. Here the electric conductivity drastically decreases with particle diameter below a few μm. On the other hand, in semiconductor crystallites a size-correlated inter-band absorption blue shift was observed. The effects are a consequence of the quantum confinement in all three dimensions (“quantum dots”) and hence are of universal character. In addition to their fundamental importance, the observations now allow the design of the ultimate microelectronic device and permit the engineering of novel devices as well.
Progress in Quantum Electronics | 2003
G. Nimtz
Abstract Photonic tunneling is currently of theoretical and applied interest. In a previous review, faster-than-light (i.e. superluminal) photonic tunneling was discussed (Progr. Quantum Electron. 21 (1997) 81). Recently, superluminal photonic pulse transmission and reflection have been measured at microwave and infrared frequencies. It seems clear that superluminal photonic and electronic devices will become a reality in the near future. In the present report, we introduce new experimental and theoretical data on superluminal tunneling and reflection. Data of reflection by barriers have evidenced the nonlocal nature of tunneling. Asymmetric barriers have revealed a strange asymmetric reflection behavior in time. The principle of causality is not violated by a superluminal speed even though the time duration between cause and effect can be shortened compared with a luminal interaction exchange. An empirical relationship independent of the barrier system is found for the photonic tunneling time. This relation seems to be universal for all kind of tunneling processes in the case of single opaque barriers. We show that the superluminal velocity can be applied to speed up photonic modulation and transmission as well as to improve microelectronic devices.
Physics Letters A | 1994
W. Heitmann; G. Nimtz
Recently several approaches have been presented to prove causality for the measured superluminal photonic tunneling velocity. It is shown here that these proofs are not relevant for the frequency band limited microwave experiments in question (FM and AM signals) and that such experiments cannot be used to test Einstein causality.
Journal of Physics: Condensed Matter | 1998
P. Pissis; A Kyritsis; D. Daoukaki; G. Barut; R. Pelster; G. Nimtz
The dynamical behaviour of the glass transition of propylene glycol confined in droplets in butyl rubber (three-dimensional confinement, mean droplet diameter d = 8-11 nm) and in pores in controlled porous glasses (two-dimensional confinement, mean pore diameter d = 2.5-7.5 nm) has been studied in detail by means of broadband dielectric spectroscopy (5 Hz-2 GHz) and of thermally stimulated depolarization current measurements. Effective medium theory corrections of the data are discussed. The results indicate the existence of a relatively immobile interfacial layer close to the wall. For the volume liquid the dynamics of the glass transition becomes faster and the glass transition temperature decreases compared to the bulk liquid. The shifts increase with decreasing d, are larger in butyl rubber than in controlled porous glasses (three-dimensional versus two-dimensional confinement) and vanish for nm. These results are discussed in relation to those obtained with polymers confined in thin polymeric films (one-dimensional confinement) and in semicrystalline polymeric samples and are explained on the basis of the cooperativity concept and the model of Adam and Gibbs. The cooperativity length at is determined to be nm in both butyl rubber and controlled porous glasses. Interesting effects of confinement are observed on the shape of the dielectric response of the process associated with the glass transition.
Solid-state Electronics | 1978
Rolf R. Gerhardts; Ralf Dornhaus; G. Nimtz
Abstract The temperature dependence of the Auger-lifetime of n-Hg1−xCdxTe is investigated both theoretically and experimentally for several values of bandgap and of extrinsic carrier-concentration nex in the whole range between room and He-temperatures. For semiconducting compounds a pronounced minimum of the lifetime between 10 and 50K and an exponential increase at still lower temperatures are found. The position of the minimum and the exponent depend mainly on the value of the bandgap and on the ratio of the conduction- and valenceband effective masses. Apart from the extended temperature range, which does not allow the use of classical statistics, our calculation differs from earlier work in so far, as we take the band energies and the overlap integrals of conduction- and valenceband Bloch-functions from a k·p-calculation. For semiconducting compounds, we compare the results with those obtained from an estimate of the overlap integrals given by Antoncik and Landsberg. Whereas both results are compatible at high temperatures, characteristic differences occur at low temperatures, where we find the lifetime to be proportional to n ex −8 3 rather than to nex−2. For semimetallic compounds we calculate a weakly temperature dependent Auger-time of the order of 0.1–1 nsec.
Solid State Communications | 1988
P. Marquardt; G. Nimtz; B. Mühlschlegel
We report that both size (d) and dimensionality of the confinement imposed on an electron gas govern its conductivity. A dramatic decay of the quasi-static conductivity by more than 8 orders of magnitude has been discovered in the sub-micrometer (so-called mesoscopic) size range of metal particles when their sizes are reduced down to the nm regime. This size-induced metal-insulator transition is of universal character and can only be understood as a quantum-size effect.
Annalen der Physik | 1998
G. Nimtz
It recently has been demonstrated that signals conveyed by evanescent modes can travel faster than light. In this report some special features of signals are introduced and investigated, for instance the fundamental property that signals are frequency band limited. Evanescent modes are characterized by extraordinary properties: Their energy is negative, they are not directly measurable, and the evanescent region is not causal since the modes traverse this region instantaneously. The study demonstrates the necessity of quantum mechanics in order to understand the superluminal signal velocity of classical evanescent modes.
Physical Review E | 2002
G. Nimtz; A. Haibel; R.-M. Vetter
The time behavior of microwaves undergoing partial reflection by photonic barriers was measured in the time and in the frequency domain. It was observed that for opaque barriers the reflection delay is almost independent of the barriers length. This result corresponds to the Hartman effect in transmission.
Applied Physics Letters | 1979
G. Nimtz; B. Schlicht; R. Dornhaus
Drastic long‐term changes of transport properties and the recombination behavior of a Hg1−xCdxTe crystal are reported. Being pure n‐type material immediately after fabrication the crystal showed a p‐type conduction after a five‐year storage under room‐temperature conditions. The lifetime—formerly dominated by Auger recombination—dropped by about two orders of magnitude. A model of Hg vacancy diffusion is suggested to explain the observed data.