Search results

1

Extension of Maier-Meier Model on Cholesteric Liquid Crystals

100%

Hoffmann J.

Acta Physica Polonica A

|

2004

|

vol. 105

|

issue 4

373-381

EN

Based on the Maier-Meier theory, a model is proposed in which the electric permittivity values of the cholesteric liquid crystals can be linked to their molecular parameters (μ - dipole moment,α - polarizability). To accomplish this project there were defined the transformation principles from the liquid crystal structure frame to the molecular frame, and a method for rendering the single molecule parameters (μ, α) into a set of averaged quantities of the macroscopic ensemble was offered. This model was used to calculate a dipole moment and polarizability for two liquid crystalline materials from electric permittivity measurements. The results are compared with the data obtained from MP3 calculations.

2

Determination of the Dielectric Anisotropy of Cholesteric Liquid Crystals

100%

Hoffmann J.

Acta Physica Polonica A

|

2003

|

vol. 103

|

issue 1

41-46

EN

The method of determination of the main components of electric permittivity tensor for the cholesteric liquid crystals was proposed. The method was used to interpret the results obtained for pure 4-[(S,S)-2,3~epoxyhexyloxy]-phenyl-4-(decyloxy)-benzoate and the binary mixture of cyanobiphenyls.

3

Ultrafast Phenomena in II-VI Semiconductors

51%

Kalt H., Wachter S., Lüerssen D., Hoffmann J.

Acta Physica Polonica A

|

1998

|

vol. 94

|

issue 2

139-146

EN

We review studies on the coherent and incoherent exciton dynamics in ZnSe-based quantum wells. First, the exciton-exciton scattering parameter is determined from femtosecond four-wave mixing as a function of background exciton density generated by a prepulse. A linear increase in the excitonic homogeneous linewidth is found as a function of the background exciton density with significant different scattering parameters for the cases of interaction with coherent or incoherent excitons. Second, the thermalization dynamics of spectrally narrow hot-exciton distributions is investigated by time-resolved phonon sideband spectroscopy. Thermalization assisted by acoustic phonons occurs on a 100 ps timescale, which is in good agreement with model calculations in time dependent perturbation theory.

4

Magnetic and Neutron Diffraction Studies of the Polycrystalline and Nanoparticle TbMnO_3

27%

Bażela W., Dul M., Dyakonov V., Gondek Ł., Hoser A., Hoffmann J., Penc B., Szytuła A., Kravchenko Z., Nosalev I., Zarzycki A.

Acta Physica Polonica A

|

2012

|

vol. 122

|

issue 2

384-390

EN

This paper reports on investigations of magnetic properties, crystal and magnetic structures on TbMnO_3 prepared in various ways, namely, as conventional polycrystalline sample and two nano particle specimens (synthesized with a sol-gel method at temperatures of 800 and 850°C). The X-ray and neutron diffraction data confirm the orthorhombic crystal structure (space group Pbnm, No. 62) without noticeable differences of the lattice parameters for poly- and nanocompounds. For the polycrystalline sample, a subsequent ordering of the Mn and Tb sublattices with decreasing temperature was observed. Namely, the Mn sublattice exhibits a modulated magnetic structure with the propagation vector k = (0, k_{x}, 0) in between 41-5 K. Below T = 21 K, a change from a collinear (A_{y} mode) into non-collinear (A_{y}G_{z} mode) structure was evidenced. Further decreasing of temperature below 10 K results in magnetic ordering of the Tb sublattice (modulated G_{x}A_{y}F_{z} mode). For nanoparticle compounds, magnetic ordering in the Mn and Tb sublattices is described by propagation vector k = (0, k_{y}, 0), with k_{y} components higher than observed for polycrystalline sample. The magnetic ordering in the Mn sublattice is described by a collinear A_{y} mode down to 1.6 K where the Tb moment becomes ordered (G_{x}A_{y} mode). The observed broadening of the Bragg peaks connected to the Tb sublattice suggests the cluster-like character of its magnetic structure.

5

Influence of the Grain Size on the Magnetic Properties of TbMnO_{3}

27%

Bażela W., Dul M., Dyakonov V., Gondek Ł., Hoser A., Hoffmann J., Penc B., Szytuła A., Kravchenko Z., Nosalev I., Zarzycki A.

Acta Physica Polonica A

|

2012

|

vol. 121

|

issue 4

785-788

EN

The magnetic properties including magnetic structure of poly and nano samples of TbMnO_{3} are determined. All the samples investigated are antiferromagnets. In these samples the Mn ad Tb moments order antiferromagnetically at different temperatures and form modulated magnetic structure described by the propagation vector k=(k_{x},0,0) with different value of k_{x} for the Mn and Tb sublattices. Comparison of the data for poly and nano samples indicates the decrease of the moment and increase of the k_{x} component of propagation vector in the nano specimens. The wide Bragg peak related to the Tb sublattice suggests that the magnetic order has the claster-like character. The magnetic moments value in both sublattices is smaller, whereas the k_{x} values are larger for nano samples.

Refine search results

5 Acta Physica Polonica A

2 2012

1 2004

1 2003

1 1998

5 Hoffmann J.

2 Bażela W.

2 Dul M.

2 Dyakonov V.

2 Gondek Ł.

2 Hoser A.

2 Kravchenko Z.

2 Nosalev I.

2 Penc B.

2 Szytuła A.

2 Zarzycki A.

1 Kalt H.

1 Lüerssen D.

1 Wachter S.

Extension of Maier-Meier Model on Cholesteric Liquid Crystals

Determination of the Dielectric Anisotropy of Cholesteric Liquid Crystals

Ultrafast Phenomena in II-VI Semiconductors

Magnetic and Neutron Diffraction Studies of the Polycrystalline and Nanoparticle TbMnO_3

Influence of the Grain Size on the Magnetic Properties of TbMnO_{3}