[[abstract]]Pyroelectricity has been observed in the smectic-C and smectic-H phases of l-p-decyloxybenzylidene-p′-amino-2-methylbutylcinnamate after the material is poled in a dc field. The observed pyroelectric coefficient is consistent with an estimate of its theoretical value.[[incitationindex]]SCI[[booktype]]紙本[[booktype]]電子

Yu, L. J.

Pyroelectricity has been observed in the smectic-C and smectic-H phases of l-p-decyloxybenzylidene-p′-amino-2-methylbutylcinnamate after the material is poled in a dc field. The observed pyroelectric coefficient is consistent with an estimate of its theoretical value.SCI紙本電子

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VOLUME 36, NUMBER 7 PHYSICAL REVIEW LETTERS 16 FEBRUARY 1976applies to the result for Tb.' The discussed ef-fects are also important for other strongly an-isotropic systems. A comprehensive analysis ofthe spin waves in the heavy-rare-earth metalson the basis of the systematic Bose operator ex-pansion will be published elsewhere.The hospitality of the solid state physics groupat Iowa State University and the Ames Labora-tory is gratefully acknowledged. It is a pleasureto thank B. Harmon for comments on the manu-script.*Work performed for the U. S. Energy Research andDevelopment Administration under Contract No. W-7405-eng-82.fPresent address.~A. R, . Mackintosh and H. B. Mpller, in MagneticProperties of Rare Earth Metals, edited by R. J. Elli-ott (Plenum, New York, 1970), p. 187.R. M. Nicklow, N. Wakabayashi, M. K. Wilkinson,and R. E. Reed, Phys. Rev. Lett. 27, 884 (1971).J. Jensen, J. G. Houmann, and H. Bjerrum Mgller,Phys. Rev. B 12, 808 (1975); J. Jensen and J. G. Hou-mann, Phys. Rev. B 12, 820 (1975); J. G. Houmann,J. Jensen, and P. Touborg, Phys. Rev. B 12, 832(1975).K. Niira, Phys. Rev. 117, 129 (1960).B. R. Cooper, R. J. Elliott, S. J. Nettel, and H. Suhl,Phys. Rev. 127, 57 (1962).B.R. Cooper, Phys. Rev. 169, 281 (1968).VT. Holstein and H. Primakoff, Phys. Rev. 58, 1098(1940).M. S. S. Brooks, Phys. Rev. B 1, 2257 (1970).Let us restrict the discussion to q along the hexago-nal axis in which case we may use the double-zonescheme with one atom per unit cell.P.-A. Lindgard, A. Kowalska, and P. Laut, J. Phys.Chem. Solids 28, 1357 (1967).T. A. Kaplan and D. H. Lyons, Phys. Rev. 129, 2072(196').~2P.-A. Lindg5rd and J. ,G. Houmann, in Proceedingsof the Conference on Rare Earths and Aotinides, Durham, England, l9&f, edited by E. W. Lee (Institute ofPhysics, London. , England, 1971).P.-A. Lindgard and O. Danielsen, J. Phys. C 7, 1528(1974).Q. T. Trammel, J.Appl. Phys. 81, 862S (1960); J. H.Van Vleck and R. M. Bozorth, Phys. Rev. 118, 1498(1960); B. Grover, Phys. Rev. 14o, A1944 (1965). Seealso the review by B. R. Cooper, in Magnetic I'xoper-ties of Rare Earth Metals, edited by R. J. Elliott (Ple-num, New York, 1970), p. 17.~~The details of the transformation and explicit tablesof tensor operators relevant for cubic and hexagonalsymmetry are given by P.-A. Lindgard and A. Kowal-ska, to be published.6@=0.288 was measured in a very pure Er sample byM. Habenschuss, C. Strassis, S. K. Sinha, H. W. Deck-man, and F. H. Spedding, Phys. Rev. B 10, 1020 (1974).This value was used in the present fit. However, ashallow minimum in the standard deviation occurs at Q= 0.230.iso. Rathmann, J. Als-Nielsen, P. Bak, J. Hgg, andP. Touborg, Phys. Rev. B 10, 8988 (1974).Observation of Pyroelectricity in Chiral Smectic-C and -H Liquid Crystals*L. J. Yu, H. Lee, C. S. Bak, and M. M. LabesDepartment of Chemistry, Temple University, Phi7adelphia, Pennsylvania 19722(Received 10 November 1975)Pyroelectricity has been observed in the smectic-C and smectic-H phases of l-p-de-cyloxybenzylidene-P'-amino-2-methylbutylcinnamate after the material is poled in a dcfield. The observed pyroelectric coefficient is consistent with an estimate of its theo-retical value.Recently Meyer et al. ' have presented both the-oretical arguments and some experimental evi-dence that P -1ecyloxybenzy lid ene-P '-amino- 2-methylbutylcinnamate (DBC), when prepared a,sa pure enantiomer (using l-amyl alcohol), is ferroelectric in the smectic-C and smectic-H phas-es. It occurred to us that an indication of spon-taneous polarization in these phases would be thepresence of a pyroelectric effect We have. suc-ceeded in measuring a pyroelectric current inthe smectic-C and smectic-II phases of the l-en-antiomer of DBC after aligning the phases in adc electric field, and verified that no pyroelec-tric effect is observed in the racemic form ofDBC.l- and d/-DBC were synthesized in the follow-ing manner: p-nitrocinnamic acid was convert-ed to the acid chloride via treatment with thionylchloride; I-amyl alcohol or dl-amyl alcohol wa, sthen added to form the p-nitrocinnamate ester,388VOLUME 36, NUMBER 7 PHYSICAL RKVIEW LETTERS 16 I'EBRUARY 1976which was reduc ed to the p -aminocinnamate es-ter with stannous chloride and hydrochloric acid.Finally, the Schiff base DBC was made by con-densing the p-aminocinnamate ester with n-de-cyloxybenzaldehyde. The phase transition tem-peratures were in good agreement with those pre-viously reported. 'The pyroelectric measurements were performedon samples of I-DBC and dl-DBC aligned betweentwo glass plates mhich had been coated with indi-um oxide and then with silicon monoxide to pro-mote homogeneous alignment. ' A 6.3-pm or 12.7-pm Mylar film with a 1.2x 1.2-cm' hole was usedas a spacer, The samples were heated to 125 C,8' higher than the isotropic transition tempera-ture, kept under a dc electric field of 5x 10~ Vcm ' for 1 h, and quenched to the smectic-Cphase with the field still applied. This treatmentserves two functions: Undesirable ionic speciesare removed by electrolysis, and the sample is"poled"; i.e. , the dipoles are aligned in the field.Alternatively, the sample can be poled startingfrom the smectic-A phase (115-95'C). In thesmectic-C and -II phases, the "spontaneous" cur-rents are measured after the "background" cur-rent stabilizes, which takes ™2h. A small re-sidual background current is always observed;in l-DBC a pyroelectric current is also observedwhen the sample is heated (or cooled) at a rapidheating (or cooling) rate 'The .pyroelectric cur-rents were measured in the smectic-C and -H(&63 C) phases. In the smectic-A phase, in ac-cord with theory, no pyroelectric current couldbe observed. However, the background currentin this phase was always quite high, and it wouldtherefore be difficult to distinguish a pyroelec-tric current in this phase in any event.In the experiments reported in this work, themolecular axis is parallel to the glass; i.e. , thesmectic planes are perpendicular to the glassand an electric field is applied perpendicular tothe glass plates. A macroscopic dipole momentwill only occur mhen the helicoidal smectic ar-ray is "untwisted, " i.e„mhen the pitch approach-es infinity. We found that after poling, the infin-ite-pitch smectic-C and -II phases mere partiallyretained for several hours even after the fieldwas removed; i.e. , a memory state was achieved.Microscopic observations indicated that a largeportion of the sample did not relax back to theso-called fingerprint texture; the helical arraymay be partially restored but mith a large pitch. ~For this reason, the structures of both the smee-tic-C and the smectic-Il phases, being untwisted,Smectic- C Smectic —H3.5CLEa.o—O0 2.7—2.4—LLI2.i-(3I.8—(+)CLE0I-LU+) 2I I I I I I I I I I 0TIME ( I div. =5 minutes)FIG. 1. Recorder tracings of observed currents inchirgl (-) and racemic (+) DBC. The compounds areheated from 65 to 70'C at a rate of 75'/min in thesmectic-C phase, and from 50 to 55'C at a rate of10'/min in the smeotio II phase. -should have a macroscopic dipole moment andshould therefore show a pyroelectric effect. Asa control experiment dE-DBC was treated in anidentical manner; because of the apolar charac-ter of this material, no pyroelectric effect shouldbe observable.Pyroelectric currents were measured in a man-ner previously described. ' The samples are firstheld at a fixed temperature until a stable back-ground current is observed and recorded; heat-ing rates of 75 and 10'/min for the smectic-Cand smectic-Il phases, respectively, were ap-plied and the phases heated to a 5 higher temper-ature. As can be seen in Fig. 1, this heating pro-duces a current pulse as well as a rise in thebackground current. The background currentstabilizes again as soon as the temperature sta-bilizes. When no pyroelectric current is pro-duced, as in the experiments with d/-DBC, oneobserves the rise in the background current, butno pyroelectric current pulse.The pyroelectric coefficient dP/dT can be cal-culated from the data of Fig. 1 from the followingexpression for the pyroelectric current I:I =A(dP/dT')dT/dt,where A is the electrode area and dT/dt is theheating rate. The highest value of the pyroelec-tric coefficient in the smectic-C phase is -2x10 "C deg ' cm"', and -3~10 "C deg"' cm 'in the smectic-H phase. The magnitude of the ob-389VOLUME 36, NUMBER 7 PHYSICAL REVIEW LETTERS 16 I'EBRUARY 1976served pyroelectric coefficient was often as muchas a factor of 2 less than this, the irreproduci-bility presumably related to the degree of align-ment and memory state which exists in the indi-vidual sample.An estimate of the theoretical value of dP/dTcan be made in the following manner. Polariza-tion P is defined as the macroscopic dipole mo-ment per unit volume V:P =~u/V =pu, (2)where N is the number of dipoles in the volumeV, u is the dipole moment, and p =N/V. By dif-ferentiating Etl. (2) with respect to temperatureT, one obtainsdP 1dp 1 duThe relative change in density (I/p)dp/dT is ap-proximately the volume expansion coefficient(negative sign) and should have the value of - —1x 10 ' deg '.' ' The magnitude of the second termin Eq. (3) is —10 ' deg ', ' and can therefore beneglected. ' P can be assumed' to have a value of-125 esu cm ' (=4.2x10 'C cm '). Thereforean estimate of dP/dT is - —4x 10 "C deg ' cm '.Thus the observed value of the pyroelectric co-efficient [(2 to 3)x 10 "C deg ' cm '] is tluiteclose to the theoretical value. Since neither per-fect alignment of smectic-C and -8 phases norperfect untwisting of the chiral phases can be as-sured, the agreement is rather good. Furtherwork on describing the properties of these inter-esting phases is underway.*This work was supported by the U. S. Army ResearchOffice under Grants No. DAHC04-74-G-0186 and No.DAAG29-76-G-0040,R. B. Meyer, L. Liebert, L. Strzelecki, and P. Kel-ler, J, Phys. (Paris), Lett. 36, L-69 (1975).M. Leclercq, J. Billard, and J. Jacques, Mol. Cryst.Liq. Cryst. 8, 367 (1969); A. Psarrea, C. Sandris, andG. Tsatsas, Bull. Soc. Chim. Fr. 1961, 2145.J. L. J~»~ng, Appl. Phys. Lett. 21, 173 (1972).4The lifetime of such a memory state seems to de-pend on the thickness and boundary conditions of thesample. Although Meyer et a E. (Ref. 1) did not reportmemory effects in their earlier work, observations oflong-lived untwisted states have also been made bythem (B. B. Meyer, private communication).~A. I. Baise, H. Lee, B.Oh, R. E. Salomon, andM. M. Labes, Appl. Phys. Lett. 26, 428 (1976).~L. E. Hajdo, A. C. Eringen, A. E. Lord, Jr. , andF. E. Wargocki, Lett. Appl. Eng. Sci. 3, 125 (1975).VL. E. Hajdo, A. C. Eringen, J. Giancola, and A. E.Lord, Jr., Lett. Appl. Eng. Sci. 3, 61 (1976).R. G. Kepler, in Proceedings of the Second Inter-national Conference on Electrophotography, October1973" (Society of Photographic Scientists Bnd Engi-neers, to be published).8The polarization is expected to change rapidly nearthe smectic-C-smectic-A transition temperature [see,for example, R. Blinc, Phys. Status Solidi (b) 70, K29(1975)]. Our measurements for the smectic-C phasewere therefore carried out" 20' below this transitionpoint to avoid any contribution to the current from sucha pretransitional phenomenon.COMMENTSAnomalous Angular Distribution in the Transition to the 2s, /2 State in "0T. Motobayashi, * I. Kohno, K. Katori, f M. Yoshie, f T. Ohi, t and H. KamitsuboThe Institute of Physical and Chemical Research, Wako shi, Saitama 3-51, Japan(Received 4 November 1975)The reaction 60(~4N, N) 0 has been studied at a bombarding energy of 79 MeV. Theangular distribution for the transition to the 2s&~2 state in O showed an anomaly similarto that already reported in studies of ~ C(~ N, 3N)~3C and C( B„,BBe)~3N.Recently, an anomaly has been reported in theangular distributions for population of 2s]/2 statesof "C (E„=3.09 MeV) and "N (E„=2.37 MeV) instudies of the reactions "C("N,"N)"C ' and"C("B,'Be)"N,' respectively. In these studiesit was found that exact finite-range distorted-wave Born- approximation (DWBA) calculationsassuming a direct one-step transfer reactionmechanism gave theoretical angular distributionswhich oscillated completely out of phase with the390

Physical Review Letters

Observation of Pyroelectricity in Chiral Smectic-C and -H Liquid Crystals

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Observation of Pyroelectricity in Chiral Smectic-C and -H Liquid Crystals

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