Technical Report of ISSP Ser. A: Number 3140

Abstract

The x-ray diffraction experiment of (N(CH3)4)2CuCl4 under hydrostatic pressure has revealed the existence of a special confluent point, denoted by L(sup *) in the pressure-temperature (p-T) phase diagram, among the prototypical normal phase, the incommensurate phase and the commensurate phase characterized by the wave vector q = c(sup *)/3. With increasing pressure, the incommensurate phase with q less than c(sup *)/3 decreases its own stable temperature range and vanishes at L(sup *). The new incommensurate region with q greater than c(sup *)/3 reappears at L(sup *) and widens with increasing pressure. On the p-T phase diagram the second order normal-incommensurate and the first order incommensurate-commensurate phase lines meet at L(sup *). The wave vector in both incommensurate regions goes continuously to c(sup *)/3 and the jump of the satellite intensity on the first order incommensurate-to-commensurate phase transition becomes small, as the L(sup *) point is approached. These experimental facts show that the L(sup *)point is a multicritical point in incommensurate systems.

静水圧下の(N(CH3)4)2CuCl4のX線回折実験は、圧力温度(p-T)状態図でL*点と表示され、波動ベクトルq=c*/3により特徴付けられた原型の正常相、非整合相および整合相に囲まれた特殊合流点の存在を明らかにした。圧力の増加と共に、qがc*/3より小の非整合相は安定温度範囲を減少し、L*において消滅する。qがc*/3より大の新規非整合領域がL*に再出現し、圧力の増加とともに広がる。p-T状態図上で、2次正常非整合相と1次非整合・整合相線がL*において交わる。両非整合領域における波動ベクトルは連続的にc*/3に近づき、そしてL*点に接近すると、1次整合相に対する非整合相転移の際の衛線強度の飛躍は小さくなる。これらの実験的事実はL*点が非整合系での多重臨界点であることを示している。