The influence of incommensurate modulation on the optical properties of ((CH₃)₄N)₂CuCl₄

Abstract

Abstract. This paper reports on the measurement of the optical properties of incommensurately modulated ((CH3)4N)2CuCÎ4 by using the High Accuracy Universal Polarimeter. With this technique it is possible to measure, simultaneously, linear birefringence, linear dichroism, circular birefringence, circular dichroism and the rotation of the optical indicatrix. Two different samples are used. The orientation of the first sample allows for the measurement of the gyration tensor element #33. In the other sample #13 can be measured. The optical effects are studied as a function of temperature, both in a region of zero linear dichroism and in a region of finite linear dichroism. The measurements reveal that the crystals have a low defect concentration. The presence of the incommensurate modulation is clearly revealed by the linear birefringence. The linear dichroism, on the other hand, appears to be unaffected by the modulation. It is interesting that if one averages the effect of the incommensurate modulation a crystal structure is obtained that is believed to be orthorhombic and centrosymmetric. Two effects are observed that are forbidden by the symmetry of this average structure. One of them is the rotation of the optical indicatrix, which contradicts an orthorhombic symmetry. The other is the onset of non-zero optical activity, approximately halfway through the incommensurate phase. In centrosymmetric media this effect is not allowed to occur. The observed temperature dependence of the optical activity £ j3 differs from the behaviour measured by other authors. It is discussed whether the observed effects can be attributed to a symmetry breaking by the incommensurate modulation.

On the Propagation of Light in Incommensurately Modulated Dielectric Crystals

“…Then the size of the effect depends upon the amplitude coefficients. Since a determines the behaviour of the gyration in the vicinity of optical axes [8], only the value of a À has a significance for the linearly birefringent crystal sections, the subject of the HAUP experiments [6,15,16]. Contrary to the conclusions of Kushnir [8] where the major contribution originates from e 0 12 c $ q 0 Dn l m al, here the macroscopic gyration is mainly attributed to the modulated gyration g 33 c $ c 0 Dnl m al 2 .…”

Microscopic Dielectric and Gyration Tensors of Incommensurately Modulated Insulators

“…This is about two orders of magnitude larger when compared to the normal wave ellipticity predicted in the study [36] where the term linear in the mesoscopic modulation wave vector is not accounted for. The value of the c parameter according to the upper limit gets into the typical sensitivity ranges characteristic of the polarimetric apparatus HAUP (see [6][7][8]) and can be, in principle, detected experimentally.…”

“…Up to date the mechanisms for the influence of the IC modulation on the effect are not reliably clarified, though a few experimental results seem to be interesting [5]. Of particular mentioning are the experimental observations of the gyration effect in a number of A 2 BX 4group crystals whose IC phases have inversion symmetry (see, e.g., the recent works [6][7][8] and references therein). Despite the numerous discrepancies in the data, those results deserve a careful theoretical analysis.…”

Influence of incommensurate modulation on the optical properties of the solid solution ((CH3)4<

“…9 It is interesting that this effect does not show plateaus in the incommensurate phase. It is possible that indicatrix rotation has a stronger dependence on the modulation amplitude than on the length of the modulation wave vector.…”

“…9 This was done with a high-accuracy universal polarimeter ͑HAUP͒ that is very well suited for this purpose. [9][10][11][12] In the case of ((CH 3 ) 4 N) 2 CuCl 4 a rotation of the optical indicatrix was detected in the incommensurate phase, which contradicts an orthorhombic symmetry. In addition, ͑small͒ optical activity was found which contradicts the presence of an inversion center.…”