The structure of chiral Sm C* liquid crystals in planar samples and its change in an electric field

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The structure of chiral Sm C* liquid crystals in planar samples and its change in an electric field

M. Glogarová, J. Pavel

To cite this version:

M. Glogarová, J. Pavel. The structure of chiral Sm C* liquid crystals in planar samples and its change in an electric field. Journal de Physique, 1984, 45 (1), pp.143-149. �10.1051/jphys:01984004501014300�.

�jpa-00209730�

The structure of chiral Sm C* liquid crystals ⁱⁿ planar samples

and its change ^{in an} electric field

M. Glogarová and J. Pavel

Institute of Physics, ^{Czech. Acad.} Sci., Na Slovance 2, 18200 Prague 8, Czechoslovakia

(Rep ^le 6 juin 1983, accepte ^{le 20} septembre 1983)

Résumé. 2014 La structure de la phase lamellaire Sm C* en géométrie planaire ^a été déterminée pour différentes

épaisseurs de l’échantillon par l’observation ^au microscope polarisant ^d’un composé ^chiral (DOBAMBC) ^{et de} mélanges ^{de Sm C et de} cholestérique. ^{Dans tous les} échantillons la structure hélicoïdale près des lames de verre est déroulée par l’ancrage ^{sur ces lames et les moments} dipolaires ^{des molécules sont orientés vers le centre d’échan-}

tillon. Dans les échantillons épais, ^la compétition ^{entre un} ancrage fort ^en surface et une structure hélicoïdale dans la masse provoque ^un double réseau de disinclinaisons torses ( ± ² n) parallèles ^entre elles. Les disinclinaisons

au voisinage de la lame supérieure ^{sont décalées d’un} demi-pas (p/2) ^{de la structure} hélicoïdale par rapport ^à celles situées près ^{de la lame} inférieure.

Dans les échantillons minces (d ~ p), ^{la structure} hélicoïdale est complètement ^{déroulée mais} l’ancrage pro- voque ^une rotation des molécules le long ^de l’épaisseur ^de l’échantillon. Le processus de déroulement dans les échantillons épais ^sous champ électrique ^se passe ^en deux étapes. ^{Pour la valeur} critique ^du champ Ec1 les paires

de disinclinaisons (± ² 03C0) s’annihilent et laissent l’échantillon sans structure hélicoïdale. Lorsque ^le champ ^atteint

la deuxième valeur critique Ec2, ^la rotation le long ^de l’épaisseur de l’échantillon est supprimée. ^{Dans les} échantillons minces seulement, l’élimination de la rotation se réalise sous l’influence du champ Ec2.

Abstract.

From observation by microscope of DOBAMBC and mixtures of smectic C and cholesteric, ^{a struc-}

ture of Sm C* in planar samples ^{has been} determined as depending ^{on the} sample thickness d. In all samples

there are unwound layers ^{at the} sample ^{surfaces with} dipole-moments directed from the surface to the sample ^bulk.

In thick samples ^the regular helicoidal structure occurring ^{in the} sample ^{bulk is connected to} the unwound sur-

face layers by ^{means of ±} 2 03C0 disinclination lines, the upper and lower lines being ^shifted by ^{half a helical} step p.

In thin samples (d ~ p) ^{the helical structure} is unwound by ^surface anchoring but there is a twist along ^the sample

thickness.

The unwinding process of thick samples in electric field proceeds ^{in two} steps : unwinding of the helix via mutual annihilation of opposite disinclinations at critical field Ec1 and removal of the twist at Ec2. ^{With thin} samples only unwinding of the twist at Ec2 ^takes place.

Classification

Physics ^Abstracts

61.30G - 77.80D

Introduction.

In an infinite sample chiral smectic C (Sm C*) liquid crystal phase ^{has a helical structure where molecules}

rotate around the helical axis z by ^{an azimuthal} angle Q(z) ( ) = 2 p 1t z (p = ^helical step), keeping ^a constant tilt

P (P P p g

angle 0 from the axis ^z. In a limited sample ^{the struc-}

ture is influenced by boundary conditions ^{and is} reflected in the sample ^texture.

A characteristic texture of Sm C* liquid crystals ⁱⁿ plate-like samples ^{is known as the} system ^of equidis-

tant lines parallel ^to the smectic layers [1-5]. By ^the microscope study of the Sm C* material obtained by mixing bis-(4’-n-decyloxybenzal-2-chloro-1-4-phe-

nylene) diamine, which exhibited a Sm C phase, ^and

cholesterol cinnamate it was first found that the observed lines originated from linear defects [2]. ^It

was concluded that the defects ^are + 2 a and - 2 n twist disinclinations having ^the axis of rotation z and located near the respective upper and lower glass ^slides limiting ^{the Sm C*} sample with the smectic layers perpendicular ^{to the} sample plane [2]. ^{The distance}

of disinclinations near each of the glass plates ^was interpreted ^as the helical step p. The disinclinations

appeared in consequence of a planar anchoring ^{on the} glass slides in order to join the helical regular ^structure

in the bulk and unwound planar layers ^{near the sam-} ple ^{surfaces. When a constant tilt} angle 0 is assumed within the whole sample, ^two planar orientations

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01984004501014300

144

differring ^{in Sl} by 1t ^are possible ^{at the} glass plates.

Directors of both ^are parallel ^{to the} sample plane ^and

form an angle ^{of 2 0. These two} orientations are iden- tical with the orientations achieved by unwinding ⁱⁿ antiparallel electric fields [6].

In reference 2 a model of the structure of Sm C*

in planar samples (plate-like samples with smectic

layers perpendicular ^{to the} sample plane) ^was given

under the assumption ^{that the} alignment of molecules at both glass plates ^is planar ^{with the same director.}

The behaviour of the disinclination lines in the electric field was first studied with DOBAMBC [4, 5]

and a model of unwinding ^{of Sm C* structure in the}

electric field was proposed. ^{It is based on a compres-}

sion and a mutual annihilation of opposite ^{2 n disin-}

clinations [4, 5].

The aim of this paper is ^a detailed optical study ^of

the Sm C* structure in planar samples and its unwind-

ing ^{in an} electric field. For the study ^we used mixtures

[7] ^of

with the cholesteric phase. The mixtures exhibit Sm C*

phases ^{with a helical} step ^{from 3 to 25} pm for concen-

trations from 20 to 5 mol. % of the cholesteric sub- stance in the temperature range 75 : 110 °C [7]. ^The

mixtures with long ^helical step enable direct micro- scope observation of the unwinding process in an

electric field.

On the basis of the results obtained with the long step ^{mixtures the} unwinding process in ^an electric field of DOBAMBC was examined.

1. Experimental.

The materials, ^{heated to the} isotropic phase, ^were

confined between two glass slides covered with a

transparent conductive SnO2 layer. ^The glasses ^were thoroughly ^cleaned by spectrographic grade ^aceton.

Mylar spacers 25 and 50 pm thick were used to define the sample thickness. No treatment for obtaining ^an

easy direction ^was used. The cell was then placed ⁱⁿ

a heating chamber and connected to the source of the electric field. The chamber was placed ^on the table of a polarizing microscope. ^{An a.c.} electric field in the

frequency ^{range from 1 to 100 Hz} and the field ampli-

tude 10 kV/cm ^{were used to} achieve well-oriented

planar samples [7]. ^This procedure ^{was much more}

successful ^{with Sm} C* mixtures than with DOBAMBC,

where the a.c. field acted on the smectic layers’ ^orien-

tation only slightly.

The texture of the samples ^was examined in an

electric field of frequency ^{10-1 Hz or in a d.c. field}

which was increased step by step.

2. Results.

With the mixtures it ^was found that with the samples

of the composition yelding ^{a helical} step p comparable

to the sample thickness d (thin samples) ^{the texture}

was different from that observed for samples ^with

p d (i.e. ^thick samples). ^{For this reason we used}

two types ^of samples, which exhibited typical ^textures.

The temperature changes ^{did not} influence the texture in the Sm C* phase up ^to the vicinity ^{of the} temperature of the phase transition to the cholesteric phase [7].

2.1 THICK SAMPLES OF MIXTURES.

For a typical

thick sample ^a composition of 90 mol. % of Sm C and 10 mol. % of cholesteric substances ^was chosen. The Sm C* phase of the mixture existed from about 750 to 110 oC [7] ^{with a helical} step of about 12 _pm [7].

The sample thickness d was 50 Jlm. The typical ^texture

is shown in figures la, b where the dechiralization lines focused near the _upper (a) ^{and lower} (b) glass

Fig. ^1.

^{The texture} of 50 um thick planar sample ^{of the}

mixture with 90 mol. % of Sm C. The focused lines are the disinclination lies located near (a) upper, (b) ^lower glass

slide. Dimensions of the surface region ^are (0.25 ^x 0.33) ^mm2.

plates ^are distinguished. ^{It can be seen} that the lower and upper lines ^{are not} located one above the other,

as was supposed ^{for a} regular ^structure in references 2, 4, 5, ^but they halve the distance between the lines

pertaining ^to opposite ^surfaces.

For a small d.c. electric field with positive polarity applied ^{to the} upper electrode, ^{the lower} disinclination lines move sidewise to the right ^{or to the left} assuming

the position almost below the upper lines ^to form

pairs. The situation is shown in figure ^{2a. It can} happen

that a lower line approaches ^two neighbouring upper lines by ^{its different} parts (see Fig. 2a). The process described is reversible; if the electric field is dimi-

nished, ^{the lines return to} their initial positions.

When a negative polarity ^is applied ^on the upper electrode the process is the same, but the movement of the _upper and lower lines is mutually interchanged.

When the d.c. electric field reaches a threshold value

Ec1 = ^{5 kV/cm the upper} and lower lines in a pair

come nearer, join ^{to form a} loop ^and disappear ^as

is shown in figure 2b. The critical field E,,,, can ^differ

slightly within the sample (probably in connection with defects in smectic layer structure), ^{so in some} parts ^{of the} sample the lines persist ^{to a} slightly higher value of the applied ^field (see Fig. 2b).

Fig. ^2.

^{The texture of the same} sample ^{as in} figure ^{1 at} (a) ^E ECl’ (b) ^{E =} Ec1.

After the disappearance of the lines the sample ^is optically homogeneous but there is no optical ^extinc-

tion between crossed polarizers. ^{This fact} implies ^that

the structure is twisted along ^the perpendicular ^{to the} sample plane.

When the d.c. electric field reaches the value E,,,,

50 kV/cm, ^small irregular ^{areas, which are dark at}

crossed polarizers, appear (see Fig. 3). Within these

areas the material is not twisted along ^the sample

thickness. The area boundaries are focused near the lower glass slide if the positive polarity is connected to the upper electrode but they ^{are focused near the}

upper glass ^{slide at the} opposite polarity. ^{The areas}

grow through ^{the whole} sample.

At fields E > EC2 ^the sample ^{shows an extinction}

between crossed polarizers ^at positions which differ

by ^an angle ^{of 70} (or 20) deg. ^{for the two} polarities

of the field.

If the field is lowered below EC2’ ^first, the twisted islets _appear, which _grow through ^{the whole} sample plane. ^Below EC1 ^{1 short loops} of dechiralization lines

are created and lengthen through ^{the whole} sample.

First the lower and _upper dechiralization lines lie above each other, ^{when E ---+} 0, ^the upper and lower lines _reoccupy the alternate initial positions.

At a field EC2 ^E EC2’ i.e. in the sample ^without

dechiralization lines but twisted along ^the sample plane perpendicular, ^two types ^of significant ^defects

are present : ^Striae parallel ^{to the} former dechiraliza- tion lines, ^{which are} probably connected with hydro- dynamic instabilities (see Figs. 3, 4) and lines either closed in the form of a loop ^or ending ^on macroscopic point defects in the liquid crystal (Fig. 4).

2.2 THIN SAMPLES OF MIXTURES. - For preparation

of a typical ^thin sample ^{we have used a mixture of}

95 mol. % of Sm G and 5 mol. % ^of cholesteric mate- rials. The mixture’s helical step determined before-

Fig. ^3.

^{The 50} pm thick sample of the mixture at E = EC2

seen between crossed polarizers. ^{In the dark areas the}

unwound structure is in extinction, ^{in the} bright ^twisted

areas, striae connected with hydrodynamic unstabilities

are visible. The actual area is (0.25 ^x 0.33) ^mm2.

146

Fig. ^4.

^A linear defect in mixed sample ⁵⁰ pm thick at

ECI ^E Ec, ^{observed at crossed} polarizers. ^{The actual}

area is (0.25 ^x 0.33) ^mm2.

hand for samples 100 Jlm thick was 25 pm [7]. ^So,

for thin samples ^{we used 25} pm spacers. These samples

look optically homogeneous, usually ^no dechiraliza- tion lines are present. Only ^a strongly irregular system of lines sometimes _appears (Fig. 5). ^The samples ^show

an extinction between the polarizers ^{at the} angle ^of

20 deg. ^{This means} that the thin samples ^{are similar}

to the thick samples ^at Ec1 ^E EC2 ^i.e. they ^are

twisted by ⁷⁰ deg. along ^the sample plane perpendi-

cular.

If the electric field increases from zero to E Ir, 2

50 kV/cm ^the sample acquires ^{a tint and} changes

colour if observed at crossed polarizers. ^{At E x} EC2’

areas with extinction at crossed polarizers ^{are nu-}

cleated and _grow through ^{the whole} sample plane. ^The

area boundaries are focused near the lower (upper) glass plate ^{if the} positive polarity ^is applied ^{onto the}

upper (lower) electrode. At E > E C2 the extinction

positions ^{of the} sample between crossed polarizers

Fig. ^5.

^An irregular system of dechiralization lines which

can appear in 25 ^pm thick samples with 95 mol. % ^{of Sm C.}

Observed at crossed polarizers. The actual area is (0.5 ^x 0.66) ^mm2.

differ by ^an angle ^{of 70} deg. ^for antiparallel ^electric

fields. If the field is lowered below EC2’ ^{twisted areas}

appear and _grow, and at E ⁼ 0 the sample ^{is twisted} along ^the sample plane ^normal.

2.3 DOBAMBC. - The samples ^{25 pm and 50 gm}

thick exhibit the texture with dechiralization lines. The system of lines is too dense (p z ³ pm) ^{for the} upper and lower lines to be possible ^{to focus} separately, ⁱⁿ

order to recognize their mutual positions.

When the external field reaches a critical value of

= 3 kV/cm ^the crystal ^{unwinds to the} homogeneous

Sm C structure by ^the mutual annihilation of opposite

dechiralization lines as was described in references 4, ^5.

If the field increases sufficiently slowly ^or step by

step, ^two stages ^of unwinding ^{can be} recognized ^simi- larly ^to the mixed Sm C* described above. These are

the disappearence of the dechiralization lines and

unwinding ^{of the} twist, ^{shown as} nucleation and

growth ^{of the areas} which have an extinction between crossed polarizers (see Figs. 6a, b). The critical fields

Fig. ^6.

^The switching of DOBAMBC at E = Ec ^{as seen}

between crossed polarizers. ^{The dark areas} in the central part ^are completely unwound, ^the bright ^{areas are twisted} along ^the sample thickness. (a) ^A switching cycle ^{where the}

dechiralization lines disappeared ^{before the} untwisting.

(b) Some dechiralization lines remain a moment after the

untwisting. The actual area is (0.25 ^x 0.33) ^mm2.

for both processes ^are very close, Ec1 = Ec2 = ^{3 kV/cm.}

The nontwisted ^area boundaries are focused at the lower glass slide when positive polarity ^is applied ^to

the _upper electrode, ^{and are focused at} the upper

glass slide for the opposite polarity.

3. Interpretation ^{of the observed results.}

3.1 THE STRUCTURE OF SAMPLES WITH DECHIRALI- ZATION LINES

THICK SAMPLES.

3.1.1 Mixtures.

The occurrence of dechiralization lines in the sample ^{texture is a} result of the coexistence of unwound planar ^surface layers ^{at the} glass plates limiting ^{the Sm C*} sample and the helical structure in the sample ^bulk [2, 4]. The smectic layers ^are sup-

posed ^{to be} perpendicular ^{to the} sample plane. ^After

the vanishing ^of dechiralization lines in the electric field a twist of 70 deg. ^remains along ^the sample

thickness. The twist exists between the two planar

orientations anchored at the surface, the directors of which differ by ⁷⁰ deg. ^{These two} orientations corres-

pond ^{to the} possible planar orientations in Sm C*

samples with the tilt angle ^{0 = 35} deg. ^{The same value}

of 0 has been determined [7] for the studied mixtures

as independent ^of temperature and concentration. The permanent dipole-moments of the unwound surface

layers ^{should be} perpendicular ^{to the} director and lie in the smectic layers. ^This requirement ^{defines two} antiparallel orientations of the dipole-moments per-

pendicular ^{to the} sample plane.

From the field experiments (as explained below) ^it

follows that the dipole-moments in the surface layers point towards the bulk of the sample.

On the basis of the established facts the structure is determined as shown in figure 7a. The molecules’

orientations in the unwound upper and lower surface

layers ^are mutually ^rotated by ^the angle ^{Sl = n. The}

alternate location of - 2 n (lower) ^{and + 2 n} (upper)

disinclinations is a consequence of the opposite anchoring ^{on the} glass plates.

3.1.2 DOBAMBC. - With DOBAMBC the oppo- site anchoring ^{on the} glass plates is also confirmed

by the existence of a twisted alignment along ^the sample normal after the disappearance of disinclina- tions. The dipole-moment polarity ^{has been found to}

be the same as that for mixtures, ^the dipole-moments point ^{from the surface towards the} sample ^bulk.

Mutual positions of the upper and lower disinclination lines could not be experimentally established because the lines are very dense. Nevertheless we suppose that

they ^are in alternate positions ^{otherwise the} opposite

anchored surface layers ^{could not be} simply ^matched

to the regular ^{helical structure} in the bulk.

3.2 THE MODEL OF UNWINDING OF THICK SAMPLES IN THE ELECTRIC FIELD.

At E Er the helical structure in the bulk is deformed. As a result of the deformation a movement of disinclinations in the direction of the field ^can be supposed. Simultaneously,

Fig. ^7.

^{The structure of the} planar ^{Sm C*} sample ^for

d > p in the sample ^section perpendicular ^to the disinclina- tion lines. The molecules are represented by ^{nails the} points

of which correspond ^{to the parts} of molecules turned toward the observer. Dipole-moments ^{at surface} layers ^{are indicated} by ^arrows. (a) ^{E =} 0, (b) ^E ECI’ ^{(c) E =} E,,. ^{The full}

circles at (a) ^and (b) ^{denote +} 2 n and - 2 n disinclination sections near the upper and lower glass plates, respectively,

and n disinclination sections limiting ^the nontwisted area in

(c).

the lines which move towards the sample limit shift sidewise either left or right (Fig. 7b). The lower and upper disinclinations having opposite ^{vectors of rota-}

tion get ^{nearer and} start to attract each other. At E ⁼ ECI 1 they mutually annihilate, unwinding ^the

helix. At fields ECI ^E EC2 ^{the structure is twisted} along ^the sample plane normal. The twist is not regular being deformed under the influence of the applied

field as is shown in the right and left of figure ^7c.

The linear defects which can be observed in the twisted samples ^after unwinding of the helix are pro-

bably the isolated 2 n disinclinations which have no

opposite disinclination for mutual annihilation. These

disinclinations separate regions optically ^undistin-

148

guishable, which have opposite twists between the anchored surface alignments (Fig. 8a). The disincli- nations become pinned ^{on some defect or tend to close}

into a loop.

At E z EC2 ^{the surface layer with the} dipole-moment

directed against ^{the field starts to switch to the other} planar orientation with the opposite dipole-moment (see ^central part ^of Fig. 7c). ^The homogeneous ^untwist-

ed area is limited by ^{a n} disinclination located at the surface of the glass slide. The experiment ^{has shown}

that in all cases the n disinclination lies at the lower

(upper) surface when the electric field points ^down-

wards (upwards) (Fig. 7c). ^{From this} finding ^the pola- rity of the surface layers has been determined. The

completely ^unwound sample ^{has the structure of Sm C.}

By the described mechanisms the unwinding ^of

thick samples of DOBAMBC and mixtures can be

successfully described. With the mixtures, the coercive fields ECl ^{for the} unwinding of the helical structure and EC2 ^{for the} unwinding of the twist differ by ^an

order of magnitude. With DOBAMBC they ^are close,

so that both _processes take place almost simultaneous-

ly. ^{This is} probably ^{the reason} why they ^{have not been} recognized up ^{to now.}

3.3 THE STRUCTURE AND UNWINDING OF THIN SAM- PLES.

Thin samples where p N d ^{have been} pre-

pared only ^with long step ^{mixtures. In these} samples

the system of disinclinations is unstable and helical structure typical for Sm C* is unwound by the surface

anchoring. ^The oppositely anchored orientations at

glass plates ^{cause a} regular ^twist along ^the sample perpendicular (Fig. 8). ^{At E} EC2 ^{the structure is}

deformed and at E z EC2 the twisted structure is unwound by ^{the same mechanism as} the twisted structure of the thick samples (Fig. 7c).

4. Conclusions.

Boundary conditions have a great ^{effect on the struc-}

ture of Sm C* in finite samples. ^{If no} special ^treatment

of the glass ^slides limiting ^the planar sample ^is used,

a planar ^surface anchoring ^very probably ^{occurs. For}

both the studied materials the anchoring is undoubted-

ly such that the dipole-moments of the unwound sur-

face layers point from the surface into the sample.

Thus the orientations of molecules in the _upper and lower surface layers ^differ by ^an angle ^{Q = n, so that} they produce ^a twist with the angle n along the smectic

layers, keeping ^{a constant tilt} angle ^{from the} smectic layer perpendicular.

For chiral molecules the orientations characterized

by ^{Q = 0 or Q = n are} nonequivalent ^if occurring

in one surface layer. They differ in the direction of the

dipole-moments. ^If W 1 ^{is the} anchoring energy ^cor-

responding ^to the orientation with the dipole-moment pointing inwards, W2 that for the opposite ^orienta- tion, ^the experiment showed that W1 1 W2. ^{But the}

Fig. ^8.

^{The structure of the} sample section for d z _p.

At the right ^a cross-section of the 2 n disinclination is shown

by the full circle.

opposite orientations in lower and upper surface

layers are, ^on the contrary, equivalent, both charac- terized by ^{the same} anchoring energy Wi . ^{It can be} expected ^{that the} mutually opposite orientations in surface layers ^occur if the increase in the free _energy due to the twist between the layers ^is smaller than

W2 - W1, ^{If the} opposite ^{is true} the molecule orien- tation in both the surface layers ^is parallel ^{and thus} nonequivalent, ^so that the twist is not found. This can

take place ⁱⁿ very thin samples ^and probably ^occurs

in planar samples ^of DOBAMBC, ^about 2 pm thick,

which were found unwound and untwisted without electric field [8]. The critical thickness for the unwound untwisted samples ^can be calculated from the elastic free energy ^{as a} function of the elastic constants and of W2 - W 1 [9].

In the thick samples, probably ^{due to the} opposite

surface anchoring, ^{an alternate} arrangement ^{of lower} and upper disinclinations takes place. ^When focusing

into the sample ^centre the double number of lines is observed and the structure appears ^to have a half step.

Preliminary results of the unwinding of the Sm C*

planar sample ^{with the} opposite anchoring ^{at surfaces}

and alternate positions ^{of +} 2 n and - 2 n disin- clinations were given in reference 10 and a theoretical model of this process is studied [11].

The plate ^like samples ^{with the} planar alignment ^as

described here are used in many experiments. ^Inter- preting ^the results, ^{it is} necessary ^to bear in mind the realistic sample structure, which is decisive for the structure switching in the electric field and is probably strongly ^{reflected in its} optical ^and dielectric properties.

Properties of the thin twisted Sm C* samples ^{are worth}

a particular study.

Acknowledgments.

The authors ^are indebted to Dr. V. Janovec and Dr. L. Lejcek for valuable discussions and to Prof.

R. Blinc and Prof D. Demus for supplying ^the liquid

crystals.

References

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[5] GLOGAROVÁ, M., LEJ010DEK, L., PAVEL, J., JANOVEC, V., FOUSEK, J., Mol. Cryst. Liq. Cryst. ⁹¹ (1983) ^309.

[6] MEYER, R. B., LIÉBERT, L., STRZELECKI, L., KELLER, P., J. Physique ^{Lett. 36} (1975) ^L-69.

[7] PAVEL, J., GLOGAROVÁ, M., DEMUS, D., MÄDICKE, A., PELZL, G., Cryst. Research Tech. 18 (1983) ^915.

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[10] GLOGAROVÁ, M., PAVEL, J., LEJ010DEK, L., Proceedings

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[11] LEJ010DEK, L., private communication.