摘要:

AbstractWe have studied the phonon spectra resulting from laser pulse excitation of a silicon surface. We used a phonon spectrometer based on pressure tuned boron levels in silicon (Si: B‐spectrometer). A new design allowed us to use the spectrometer in vacuum for the first time. We observed phonon frequencies up to 1 THz. The pulse shapes indicated both, quasidiffusion and a long exciton lifetime. On adding helium films to the surface we have found dramatic changes of the spectrum. Full thermalisation was reached already at a thickness of 1.5 monolayers. This indicates a strong anharmonic interaction in the very first monolayers at these high frequencies and, in addition, a strong confinement of the phonons to the surface by quasidiffusio

ISSN:0003-3804    

DOI:10.1002/andp.19955070302

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractWe have studied the phonon transmission from ultraclean Si(111) surfaces into liquid helium by the phonon pulse technique. Reflection experiments and angular resolving transmission experiments were performed to measure the absolute value of the transmission coefficient and its dependence on the phonon emission angle into the solid. Clean samples etched conventionally in dilute HF show a significant anomalous transmission. An additional etch step in highly buffered HF (i.e., NH4F) with a pH‐value of 7.8 reduces the transmission coefficient drastically. Local inhomogeneities of the transmission coefficient caused by the deposition of water molecules on the surface could be visualized by the enhanced transmission. We found that the observed anomalous transmission is caused by mass defect

ISSN:0003-3804    

DOI:10.1002/andp.19955070303

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractWe report on a microwave oscillator based on Bloch oscillations of electrons in a semiconductor superlattice. Our GaAs/AlAs superlattice, at room temperature, was coupled electromagnetically by an antenna to a rectangular cavity resonator, and was operated at a current‐voltage state of negative differential conductance. We observed generation of microwave radiation at frequencies, depending on the resonator length, between 7 and 30 GHz. Electronic tuning by several percent was possible; the ratio of linewidth to frequency was of the order of 10−4. A radiation power up to 1 μW (at 10 GHz) was obtained, corresponding to a generator efficiency of the order of 10−3for the conversion of electrical power to microwave rad

ISSN:0003-3804    

DOI:10.1002/andp.19955070304

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractThe size‐dependentac‐conductivity of a small metallic particle is studied for electrons enclosed in a box with random impurities. The exact wave functions and energies are calculated numerically up to 500 electrons, then the Kubo‐formula is evaluated, and an ensemble average is performed. A strong size‐ and frequency dependence of the conductivity is found which is in good agreement with random matrix results. The size dependence of thedc‐conductivity agrees qualitatively with microwave absorption measurements on submicron

ISSN:0003-3804    

DOI:10.1002/andp.19955070305

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractThe electronic shell structure resulting from the interference of closed orbital paths is determined for mesoscopic systems like spherical clusters, discs and rings by extending the semiclassical theory of Balian and Bloch. Analytical results for the shell structure in the density of states are obtained. Thus, the dependence of the shell structure on dimension, size and geometry and potential of the mesoscopic system and on an external magnetic field can be studied systematically. Comparison of the semiclassical results and those of quantum mechanical calculations permits analysis of typical quantum mechanical effects and shows the validity of the semiclassical theory. Our results should stimulate new experiments, can be used to calculate oscillations in the binding‐energy, ionization‐potential, and can be applied to analyze oscillations in the electronic density of states of quantum dot systems like anti‐dot lat

ISSN:0003-3804    

DOI:10.1002/andp.19955070306

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractWe present simulation results for single α‐thermal chain polymers in finite volumes. For this we use a recently proposed recursive implementation of the enrichment method. In 3 dimensions it allows the simulation of extremely long chains (up toN= 300000). It is much less efficient ford= 2, but we can also there extend considerably the previously accessible range of chain lengths and densities. We verify most tested scaling laws, and we point out similarities with complex optimization proble

ISSN:0003-3804    

DOI:10.1002/andp.19955070307

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

摘要:

AbstractNew field equations of the Projective Unified Field Theory are presented which avoid potential difficulties of former versions with respect to the equivalence principle. The physical interpretation of this new version remains unchanged: constancy of the “gravitational constant”, electromagnetic polarization of the vacuum, definiteness of the energy of the stationary scalaric field, etc. Furthermore, the Klein‐Gordon field and the Dirac field are tr

ISSN:0003-3804    

DOI:10.1002/andp.19955070308

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY

ISSN:0003-3804    

DOI:10.1002/andp.19955070301

出版商:WILEY‐VCH Verlag    

年代:1995

数据来源: WILEY