X-ray natural circular dichroism in copper metaborate
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, PARTICLES, FIELDS, GRAVITATION, AND ASTROPHYSICS
X-Ray Natural Circular Dichroism in Copper Metaborate E. N. Ovchinnikovaa, A. Rogalevb, F. Wilhelmb, K. A. Kozlovskayaa, A. P. Oreshkoa*, and V. E. Dmitrienkoc a Moscow
b
State University, Moscow, 119991 Russia European Synchrotron Radiation Facility, Grenoble, CS 40220 38043 France c Shubnikov Institute of Crystallography, Russian Academy of Sciences, Leninskii pr. 59, Moscow, 119333 Russia *e-mail: [email protected] Received November 17, 2015
Abstract—The local electronic structure of copper ions in a copper metaborate CuB2O4 crystal is studied on the ESRF synchrotron using X-ray absorption polarization-dependent spectroscopy. The X-ray natural circular dichroism near the K absorption edge of copper is measured in the direction that is perpendicular to crystal axis c. The data obtained indicate the presence of hybridized p–d electronic states of copper. Theoretical calculations are used to separate the contributions of the two crystallographically nonequivalent positions of copper atoms in the unit cell of CuB2O4 to the absorption and X-ray circular dichroism spectra of the crystal. DOI: 10.1134/S1063776116050186
crystals, including the hybridization of the electronic states of various parities that are responsible for optical activity and the magnetoelectric effect [8–12]. All types of X-ray circular dichroism and their relation to the physical properties of a medium were comprehensively considered in [11]. Since copper metaborate has boron atoms, this creates certain difficulties for using magnetic neutron diffraction analysis because of strong absorption of neutrons and is not an obstacle to X-ray methods. In this work, XNCD is used to study the local electronic properties of two copper sublattices in the low-temperature phase of copper metaborate. Experimental results are compared with quantum-mechanical calculations, which make it possible to separate the contributions of two copper positions with different symmetries.
1. INTRODUCTION Interest in the local electronic structure of copper compounds has been quickened due to the discovery of high-temperature superconductivity. Copper metaborate CuB2O4 has a tetragonal structure without an inversion center [1] and attract attention of researchers by the fact that is has a number of phase transitions at 20, 10, and 2 K. It is a weak ferromagnet in the range 10–20 K and has an incommensurate magnetic structure below 10 K [2–5]. In addition copper metaborate exhibit other interesting physical properties, such as a giant optical magnetoelectric effect, which manifests itself only in systems with a violated parity [6, 7]. Despite a large number of works, the problems of magnetic ordering in two copper sublattices (Cu(A), Cu(B)) and their local electronic structure are still unresolved [2]. The giant optical magnetoelectric effect is assumed to be caused by copper atoms at position Cu(A). This copper position is also considered to be responsible for the paramagnetic behavior above the Néel temperature. A d
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