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Submitted on 1 Jan 1975
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ULTRASONIC ABSORPTION AND ORDER OF THE SMECTIC-A-NEMATIC PHASE TRANSITION
R. Bartolino, F. Scudieri, D. Sette, A. Slivinski
To cite this version:
R. Bartolino, F. Scudieri, D. Sette, A. Slivinski. ULTRASONIC ABSORPTION AND ORDER OF THE SMECTIC-A-NEMATIC PHASE TRANSITION. Journal de Physique Colloques, 1975, 36 (C1), pp.C1-121-C1-122. �10.1051/jphyscol:1975122�. �jpa-00215899�
Classification Physics Abstracts 7.130 — 7.270 — 7.480
ULTRASONIC ABSORPTION AND ORDER
OF THE SMECTIC-A-NEMATIC PHASE TRANSITION
R. BARTOLINO, F. SCUDIERI, D. SETTE and A. SLIVINSKI (*) Istituto di Fisica, Facolta' di Ingegneria, Universita' di Roma, Roma, Italy,
and Gruppo Nazionale Struttura della Materia del Consiglio Nazionale delle Ricerche, Roma, Italy
Abstract. — Ultrasonic measurements in CBOOA near the smectic A-nematic transition show the occurrence of critical absorption and therefore support the second order nature of the transition.
The order and the dynamical behaviour of smectic and on the dynamics of the processes. In the present A-nematic transition (SA-N) in liquid crystals is at communication we report some preliminary results the present a subject of theoretical and experimental which seem to support second-order nature of the investigation. McMillan's theoretical model [1], as transition in CBOOA. A pulse interferometer has been well as some experimental results in CBOOA obtained used at two frequencies (5 and 15 MHz). The smectic by him [2] and by Doane et al. [3], suggest, at least material was CBOOA (N-p-cyano-benzilidene-p-n- in some systems, the second order nature of the octyloxyaniline) furnished by Eastman-Kodak, which transition. In such cases de Gennes proposes [4] an had a SA-N transition temperature of about 83 °C.
analogy between a superconductor and a smectic A, The experiments have been performed in the tempera- obtaining interesting predictions on the temperature ture range between 74 and 90 °C, in a bath whose behaviour of deformed samples and on pretransitional temperature has been kept constant within the accuracy anomalies i. e. for the Frank elastic coefficients [5] in of + 0.05 °C. The samples used were sealed in an nematic phase. In such an analysis a complex order argon atmosphere and frequently changed in order parameter ij/ is introduced : its magnitude gives the to avoid deterioration of the material,
amplitude of the density modulation in the smectic
phase, while the phase gives the layer position. Expe- , . riments on the elastic coefficients by Durand et al. [6], Ji, -
Meyer et al. [7], Leger et al. [8] and by Slivinski cm Z
et al. [9] support de Gennes' suggestion on the diver- 6 - ^ ^
gence of the bend and twist elastic constants. ! / • Brochard [10] has more recently presented an ana- " I ,jZ*
lysis in which a close connection with the dynamic «-s—_i* I / behaviour of liquid helium at the X point has been 4 " ^ ~r^! t~ ~ * ^ - ^ ^ : jVj*
pointed out : in such a case the fluctuations of the 7^H?H^ • 5 MHl
complex order parameter should be of great impor- tance.
Interferometric measurements on the temperature -8 -4 o 4 T-TCO
dependence of the bend elastic constant near the
SA-N transition have been performed by Cladis [11] FIG. 1. — Absorption coefficient of ultrasound (in dB/cm) in CBOOA. Her conclusion is that the transition at a frequency of 5 MHz, ra. the difference T - rc. could be a small first-order transition or a k transition.
Ultrasonic absorption measurements can be used to
give further information on the nature of the transition T h e absorption coefficient vs. temperature in a range of about ± 8 °C around the transition tempe- (*) On leave from the Institute of Physics, University of r a t u r e (Te) i s Si v e n i n nSu r e l a t 5 M H z a n d i n
Gdansk, Gdansk — Poland. figure 2 at 15 MHz.
JOURNAL DE PHYSIQUE Colloque Cl, supplément au n° 3, Tome 36, Mars 1975, page Cl-121
Résumé. — Des mesures ultrasoniques dans CBOOA près de la transition smectique A-néma- tique montrent l'existence d'une absorption critique et par conséquent sont en faveur d'une transi- tion de seconde espèce.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1975122
Cl-122 R. BARTOLINO, F. SCUDIERI, D. SETTE AND A. SLIVINSKI
FIG. 2. - Absorption coefficient a of ultrasound at a frequency of 15 MHz.
We wish here to call attention to the appearence of a maximum around Tc which is more pronounced a t lower frequency. Within the limits of accuracy of the
present measurements the curve seems to be conti- nuous and roughly symmetric around Tc. The whole process occurs in the range Tc f 0.2 OC. These results seem to indicate the presence of critical processes around Tc and therefore to support the existence of a second-order transition. The critical process could be, in the Brochard picture, the interactions of sound waves with thermal fluctuations of the complex order parameter IJ, in analogy with the processes in helium at the I point [12, 13, 14, 151.
Research on sound absorption and velocity at various frequencies is at present being carried out, with better temperature definition in order to have further information both on the critical processes and on the dynamical behaviour of the two phases. We are also planning measurements on oriented monocrys- talline samples.
Assistance at the measurements of Dr G. D'Arrigo is gratefully acknowledged.
References
[l] MCMILLAN, W. L., Phys. Rev. A 4 (1971) 1238.
[2] MCMILLAN, W. L., Phys. Rev. A 7 (1973) 1419.
[3] DOANE, J. W., PARKER, R. S., CVIKL, B., JOHNSON, D. L.
and RSHEL, D. K., Phys. Rev. Lett. 28 (1972) 1964.
[4] DE GENNES, P. G., Solid State Commun. 10 (1972) 573.
[5] FRANK, F. C., Disc. Farad. Soc. 25 (1958) 19.
[6] DELAYE, M., RIBOTTA, R. and DURAND, G., Phys. Rev.
Lett. 31 (1973) 443.
[7] CHEUNG, L., MEYER, R. B. and GRULER, H., Phys. Rev.
Lett. 31 (1973).
[S] L~GER, L., Phys. Rev. Lett. 31 (1973).
[9] SLIVINSKI, A., BARTOLINO, R., SCUDIERI, F. and BERTO- LOTTI, M., to be published in J. Appl. Phys.
[l01 BROCHARD, F., J. Physique 34 (1973) 441.
[l11 CLADIS, P. E., Phys. Rev. Lett. 31 (1973) 1200.
[l21 IMAI, J. S., RUDNICK, I., Phys. Rev. Lett. 22 (1969) 694.
[l31 WILLIAMS, R. D., RUDNICK, I., Phys. Rev. Lett. 25 (1970) 276.
[l41 SETTE, D., in Varenna Summer School on critical pheno- mena (1970) p. 508.
[l51 HOHENBERG, P. C., in Varenna Summer School on critical phenomena (1970) p. 285.
