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1
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Spin-Lattice Relaxation Beyond the Debye Approximation

100%
Witowski A., Bardyszewski W., Kutter C., Wyder P., Witkowska B., Mycielski A.
Acta Physica Polonica A
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1995
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vol. 87
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issue 2
533-535
EN
In this paper we present the results of magnetization relaxation in HgCdMnTe at high magnetic fields. In this mixed crystal the TA phonons have an energy lower than the spin splitting of the Mn^{2+} ground orbital singlet at about 20 T, which allows to check the effect of phonons with wave vectors from the edges of the Brillouin zone on the spin-lattice relaxation.
2
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Orientation Dependent Spin-Lattice Relaxation in CdMnSe

100%
Witowski A., Strutz T., Wyder P.
Acta Physica Polonica A
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1995
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vol. 87
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issue 2
536-538
EN
The magnetization dynamics of Cd_{1-x}Mn_{x}Se at 4.5 K as a function of magnetic field up to 22 T was measured using a nonresonant technique. For x = 0.01 the relaxation does not depend on orientation and in higher fields the relaxation rates are proportional to B^{3}. For x = 0.02 a dependence on orientation is observed. The difference does not depend on magnetic field. This suggests that either the interaction between Mn ions responsible for spin-lattice relaxation in Mn clusters is anisotropic, or the relaxing clusters are oriented in a given manner with respect to the c axis.
3
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Thermodynamics of Fcc Al Crystal from First Principles - Performance of Local Density and Generalized Gradient Approximations

100%
Scharoch P., Peisert J., Tatarczyk K.
Acta Physica Polonica A
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2007
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vol. 112
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issue 3
513-521
EN
Phonon dispersion relations in fcc Al crystal were calculated from first principles using density functional perturbation theory, as implemented in ABINIT code. The results are compared with experimental data as well as with the results of previously done ab initio calculations based on the direct method. A slightly better agreement of density functional perturbation theory phonons with experiment can be observed. The ab initio phonon energies were used to evaluate the partition function of the crystal, using the Monkhorst-Pack integration scheme. The quasiharmonic approximation was applied to relate the temperature dependent part of the free energy to volume. The lattice constant dependence of phonon energies was found to be almost linear, so the second order polynomial was considered as sufficient to approximate the dependence. A few examples of thermodynamic characteristics were evaluated: isobaric specific heat, linear thermal expansion coefficient, isothermal bulk modulus, and compared with the experimental data. The calculation was done both in the local density and the generalized gradient approximations for the exchange-correlation energy. The agreement with the experimental data appears to be very satisfactory, although better in the local density approximation than in the generalized gradient approximation.
4
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Long-wavelength Optical Phonons in Cd_{1-x}Mn_{x}Se and Cd_{1-x}Fe_{x}Se Mixed Crystals

100%
Witowski A., Hausenblas M., Wyder P.
Acta Physica Polonica A
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1991
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vol. 79
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issue 2-3
311-314
EN
The results of FIR reflectivity measurements for CdMnSe and CdFeSe are presented. These results are described theoretically using Dynamic Dielectric Function in which two phonon modes are included. The composition dependencies of mode parameters and dielectric constant are discussed.
5
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Infrared Absorption in Single Crystals of (ET)_{2}C_{60} Grown from CS_{2} Solution

100%
Spitsina N. G., Semkin V. N., Graja A.
Acta Physica Polonica A
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1995
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vol. 87
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issue 4-5
869-876
EN
Fourier transform IR absorption spectra of single crystals of the (ET)_{2}C_{60} complex grown from the CS_{2} solution as well as its components: C_{60} and ET, also grown from CS_{2} solution, are presented and discussed. It is shown that the rotation of C_{60} is strongly hindered and ET molecules are bent in the crystal structure of the complex. The arguments for the domi­nation of the van der Waals interactions are given.
6
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On Phonon Dispersion in Alkali Metals

100%
Gajjar P. N., Thakore B. Y., Patel H. K., Jani A. R.
Acta Physica Polonica A
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1995
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vol. 88
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issue 3
489-502
EN
In the present paper, the calculation of the phonon dispersion relations for alkali metals to second order in a local pseudopotential is discussed in terms of the real-space sum. Different forms of dielectric functions are employed to judge the varying effects of exchange and correlation on the phonon frequencies. The quantitative agreement of phonon frequencies for the alkali metals reflects, satisfactorily, the experimental trend.
7
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Elastic Behaviour and Lattice Vibrations in bcc V and Nb

100%
Verma A., Verma M., Rathore R.
Acta Physica Polonica A
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1996
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vol. 90
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issue 3
547-556
EN
A model is developed by extending the generalised form of exponential potential known as extended generalised exponential potential to account for: (a) a realistic realization of interactions in all separations in general and that of small separations in particular, (b) three-body and electronic effects into the interaction in an alternative and simpler form, (c) a model free from usual fitting procedure. The model is employed to compute the cohesive energy, phonon spectra and second- and third-order elastic constants for group VA bcc metals V and Nb. The computed results showing good agreement with the experimental findings lend reliability and credibility to the potential.
8
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Vibrational Modes in Si_{x}Ge_{1-x} Alloys: Temperature and Compositional Dependence

100%
Lorenc M., Humlíček J.
Acta Physica Polonica A
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1997
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vol. 92
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issue 5
899-902
EN
We report infrared absorption spectra of crystalline Si_{x}Ge_{1-x} alloys with silicon content 0 ≤ x ≤ 1 at room and liquid nitrogen temperature. We covered the spectral range from 375 to 1200 cm^{-1} that includes the "Si-Ge" and "Si-Si" single-phonon transitions, the continuum of two-phonon processes, and the localized mode of interstitial oxygen. We study the change of vibrational structure and correlation between reference (pure Si and Ge) and alloy spectra. We observed shifts to lower wave numbers by about 1 cm^{-1} of the two-phonon absorption bands per 1% increase in germanium concentration. Pronounced changes of the vibrational spectra upon lowering the temperature were detected.
9
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Electron Dispersion in Liquid Alkali Metals

80%
Gajjar P., Thakore B., Jani A.
Acta Physica Polonica A
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1998
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vol. 94
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issue 1
33-40
EN
A local pseudopotential in the second-order perturbation theory is used to study the electron dispersion relation, the Fermi energy and deviation in the Fermi energy from free electron value for the liquid alkali metals. The influence of the five different forms of the local field correction functions on the aforesaid electronic properties is examined explicitly, which reflects the varying effects of screening. The depth of the first negative hump in the electron dispersion curves of the liquid alkali metals increases in the order Na, K, Rb, Cs.
10
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Raman Spectroscopy of Cubic HgS:Fe

80%
Szuszkiewicz W., Witkowska B., Jouanne M.
Acta Physica Polonica A
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1995
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vol. 87
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issue 2
415-418
EN
From the Raman spectra performed at temperatures from 10 K to 295 K for Hg_{1-x}Fe_{x}S crystals containing up to a few percent of iron the phonon energies for selected high-symmetry points of the Brillouin zone have been determined. A very high ionicity of these mixed crystals has been found. The TO phonon frequency value for β-HgS at Γ point, equal to about 177 cm^{-1}, suggested previously in the literature, has been confirmed.
11
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The In_{4}Se_{3} Crystal as a Three-Dimensional Imitative Model of Phenomena in One-Dimensional Crystals

70%
Bercha D., Sznajder M., Bercha A., Kharkhalis L., Rushchanskii K.
Acta Physica Polonica A
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1998
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vol. 94
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issue 2
250-254
EN
For three-dimensional charge carriers described by the dispersion law with quartic terms of the wave vector, the density of states function similar as in the one-dimensional case was determined. This similarity allows the Pekar and Dejgen condenson states in the continuum approximation to exist. The calculated phonon spectrum reveals optical vibrations of a very low frequency, which favours the electron-phonon interaction and creation of the condenson states.
12
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A Model Calculation of Infrared Spectra in Cyclohexanol

70%
Nowina Konopka M., Wasiutyński T.
Acta Physica Polonica A
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1999
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vol. 96
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issue 3-4
399-407
EN
A model calculation based on semiempirical methods for the cyclohexanol molecules is presented. Energetically preferred molecular conformations and possibility of different hydrogen bond networks in the crystal are discussed. The calculated and the experimental infrared spectra are compared. The results corroborate earlier suggestions that the rich polymorphism of cyclohexanol is due to the existence of both axial and equatorial isomers in solid phases and different architecture of hydrogen bonds network.
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