NMR Study of Characteristic CDW Transition in SrAl₄

Abstract

A charge density wave (CDW) transition at T CDW = 243 K and a structural phase (SP) transition at T SP ≈ 90 K occur in SrAl 4 with the BaAl 4-type body center tetragonal structure, which is the divalent and non-4 f electron reference compound of EuAl 4. To understand the electronic states of the CDW and SP transitions, the 27 Al NMR measurements of a single crystal of SrAl 4 , which has two cyrstallographically inequivalent Al sites, referred as Al(I) and Al(II), have been carried out in magnetic fields perpendicular and parallel to the c-axis, H 0 ⊥ c and H 0 ∥ c, respectively, at temperatures between 4.2 K and 300 K. In the case of H 0 ⊥ c, the incommensurate CDW state is dominant below T CDW and the resonance line shows a characteristic double-horned shape due to the incommensurate CDW modulation. The commensurate state becomes dominant below T SP. On the other hand, in the case of H 0 ∥ c, only the commensurate state is observed in the whole temperature range below T CDW. The temperature dependences of the Knight shifts of 27 Al(I) and 27 Al(II) for H 0 ∥ c show the different behavior below T CDW. The Knight shifts of 27 Al(I) and 27 Al(II) shift largely and slightly to negative side, respectively, with deceasing temperature. The shifts to the negative side of the Knight shifts are attributable to the core polarization of the d-electrons. The contribution of d-electrons for Al(I) site is stronger than that for Al(II).

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“…As for the second scenario, as the small deviation in θ from 0 degree makes the satellite line shift toward the central transition line as described in the above equations for the satellite line positions, one may explain the characteristic asymmetric shape of the satellite lines by taking the small deviation from θ = 0 • into consideration. In fact, such scenario has been applied to explain the asymmetric lines observed in EuAl 4 where the distribution of θ is estimated to be at most 6 • or less [34]. However, for our case, this seems to be not the case.…”

Charge-Density-Wave Order and Multiple Magnetic Transitions in Divalent Europium Compound EuAl$_4$

“…This coupling between magnetism and lattice distortion could be the origin of the complex multistep transition in EuAl 4 . In addition, a possible canting of the magnetic moment from the c-axis suggested by recent NMR and Mössbauer spectroscopies 26) can bring another degree-of-freedom. The present data do not allow the corresponding magnetic structure to be revealed, as reliable quantitative analysis is difficult owing to strong wavelength-dependent neutron absorption.…”

“…Note that a similar CDW transition was found in the isostructural compound SrAl 4 without the 4 f element. [24][25][26][27] It implies that the Eu ion is not involved in the displacement, and the Al ion could be responsible for the displacement. Indeed, the electronic structure of EuAl 4 disclosed by de Haas van Alphen measurement, photoelectron spectroscopy, and band calculations shows close similarity to SrAl 4 , where 4 f electrons lie well below the Fermi level.…”