Concomitant Structural Transition Research Articles

Thirty-Year Anniversary of κ-(BEDT-TTF)2Cu2(CN)3: Reconciling the Spin Gap in a Spin-Liquid Candidate

In 1991 the layered organic compound κ-(BEDT-TTF)2Cu2(CN)3 with a triangular lattice was synthesized for the first time. Although, originally, the focus was on the superconducting properties under pressure, this frustrated Mott insulator has been the most promising quantum-spin-liquid candidate for almost two decades, widely believed to host gapless spin excitations down to T→0. The recent observation of a spin gap rules out a gapless spin liquid with itinerant spinons and puts severe constraints on the magnetic ground state. This review evaluates magnetic, thermal transport, and structural anomalies around T⋆=6 K. The opening of a spin gap yields a rapid drop of spin susceptibility, NMR Knight shift, spin-lattice relaxation rate, and μ-SR spin fluctuation rate, but is often concealed by impurity spins. The concomitant structural transition at T⋆ manifests in thermal expansion, THz phonons and 63Cu NQR relaxation. Based on the field dependence of T⋆, a critical field of 30–60 T is estimated for the underlying spin-singlet state. Overall, the physical properties are remarkably similar to those of spin-Peierls compounds. Thus, a strong case is made that the ‘6K anomaly’ in κ-(BEDT-TTF)2Cu2(CN)3 is the transition to a valence-bond-solid state and it is suggested that such a scenario is rather the rule than the exception in materials with strong magnetic frustration.

Epitaxial growth and electrical properties of VO2 on [LaAlO3]0.3[Sr2AlTaO6]0.7 (111) substrate

The authors report the epitaxial growth and the electrical properties, especially the metal-to-insulator transition, of vanadium dioxide (VO2) thin films synthesized on [LaAlO3]0.3[Sr2AlTaO6]0.7 (LSAT) (111) (LSAT) substrates by pulsed laser deposition. X-ray diffraction studies show that the epitaxial relationship between the VO2 thin films and LSAT substrate is given as VO2(020)||LSAT(111) and VO2[001]||LSAT[112¯]. The authors observed a sharp 4 orders of magnitude change in the longitudinal resistance for the VO2 thin films around the transition temperature. The authors also measured distinct Raman spectra below and above the transition point indicating a concomitant structural transition between the insulator and metallic phases, in agreement with past investigations.

Structural changes at the semiconductor-insulator phase transition in the single-layered perovskiteLa0.5Sr1.5MnO4

The semiconductor-insulator phase transition of the single-layer manganite La0.5Sr1.5MnO4 has been studied by means of high resolution synchrotron x-ray powder diffraction and resonant x-ray scattering at the Mn K edge. We conclude that a concomitant structural transition from tetragonal I4/mmm to orthorhombic Cmcm phases drives this electronic transition. A detailed symmetry-mode analysis reveals that condensation of three soft modes -Delta_2(B2u), X1+(B2u) and X1+(A)- acting on the oxygen atoms accounts for the structural transformation. The Delta_2 mode leads to a pseudo Jahn-Teller distortion (in the orthorhombic bc-plane only) on one Mn site (Mn1) whereas the two X1+ modes produce an overall contraction of the other Mn site (Mn2) and expansion of the Mn1 one. The X1+ modes are responsible for the tetragonal superlattice (1/2,1/2,0)-type reflections in agreement with a checkerboard ordering of two different Mn sites. A strong enhancement of the scattered intensity has been observed for these superlattice reflections close to the Mn K edge, which could be ascribed to some degree of charge disproportion between the two Mn sites of about 0.15 electrons. We also found that the local geometrical anisotropy of the Mn1 atoms and its ordering originated by the condensed Delta_2 mode alone perfectly explains the resonant scattering of forbidden (1/4,1/4,0)-type reflections without invoking any orbital ordering.

Structural, electrical and magnetic properties of RE1/3Sr2/3FeO3 compounds (RE = La, Pr, Nd, Eu and Gd)

We report results of structural, magnetic and electrical measurements performed on the perovskite RE1/3Sr2/3FeO3 compounds (RE = rare earth). Samples with light RE (La, Pr or Nd) show a metal-insulator-like transition coupled to an antiferromagnetic ordering upon cooling. This transition is believed to be brought about by a charge ordering of Fe3+ and Fe5+ions in the low temperature phase accompanied by the concomitant structural transition. The replacing of light RE by heavy ones (Gd or Eu) gives rise to high-resistivity compounds with no metal-insulator transition. However, the magnetic transition remains at low temperatures. A previous Mössbauer study reports similar behaviour for RE=Eu compound. This suggests that variations in Mössbauer spectra may be of a magnetic origin in light RE compounds and not coming from a full charge disproportionation of Fe4+ as deduced previously.