Copper Metaborate Research Articles

Fonony, magnony i eksitony v netsentrosimmetrichnommagnitoelektrike - antiferromagnetike CuB2O4

In the last two decades copper metaborate CuB2O4 with a unique noncentrosymmetric crystal structure has become the subject of active research due to its unusual magnetic and optical properties. We consider the propagation and absorption of light in CuB2O4 based on the solution of Maxwell’s equations. We present an overview of the main results on the investigation of the phonon spectrum using infrared and Raman spectroscopy. Studies in the region of electronic transitions in Cu2+ ions in the crystal field have allowed the separation of contributions to the optical absorption from copper ions in inequivalent positions. A splitting of zero-phonon absorption lines in a magnetic field has been detected, and these results have received a theoretical explanation in terms of the exciton model. A rich structure of exciton–magnon states has been observed in the photoluminescence spectra. We have carried out a spectroscopic study of the optical second harmonic generation in the region of excitonic transitions, which has allowed the contribution of the toroidal moment and the Fano resonance to the observed signals to be revealed.

Synthesis and Optical Properties of Nickel-Doped Copper Metaborate Crystals

Synthesis and Optical Properties of Nickel-Doped Copper Metaborate Crystals

Zeeman and Davydov splitting of Frenkel excitons in the antiferromagnet CuB2O4

The optical spectra of antiferromagnetic copper metaborate ${\mathrm{CuB}}_{2}{\mathrm{O}}_{4}$ are characterized by an exceptionally rich structure of narrow absorption lines due to electronic transitions within the magnetic ${\mathrm{Cu}}^{2+}$ ions, but their unambiguous identification and behavior in magnetic fields have remained far from being fully understood. We study the polarized magnetoabsorption spectra of this tetragonal antiferromagnet with high spectral resolution across the energy range of $1.4055--1.4065$ eV in magnetic fields up to 9.5 T for temperatures from 1.6 up to the N\'eel temperature ${T}_{N}=20$ K. We observe a set of eight absorption lines at $T=1.6$ K in magnetic fields exceeding 1.4 T, which we identify as arising from Frenkel excitons related to the ground and first excited states of the ${\mathrm{Cu}}^{2+}$ ions. The number of these excitons is defined by the presence of the four ${\mathrm{Cu}}^{2+}$ ions with doubly degenerate spin state $S=1/2$ at the $4b$ positions in the crystallographic unit cell. The energies of these excitons are determined by the exchange interaction of 0.5 meV of the ${\mathrm{Cu}}^{2+}$ ions in the excited state with the surrounding ions and by the Davydov splitting of 0.12 meV. In large magnetic field the observed Zeeman splitting is controlled by the anisotropic $g$-factors of both the ground and excited states. We develop a theoretical model of Frenkel excitons in the magnetic field that accounts for specific features of the spin structure and exchange interactions in ${\mathrm{CuB}}_{2}{\mathrm{O}}_{4}$. The model is used for fitting the experimental data and evaluation of the Frenkel exciton parameters, such as the Davydov splitting, the molecular exchange energy, and the $g$-factors of the ground and excited states of the ${\mathrm{Cu}}^{2+}$ ions.

Синтез и оптические свойства кристаллов метабората меди, легированного никелем

This work presents information on the growth and spectroscopic study of single crystals of copper metaborate doped with nickel Cu1-xNixB2O4 (x = 0.05, 0.1). In the absorption spectra of both crystals, satellites related to Cu centers distorted by impurity Ni atoms were observed near the lines of zero phonon transitions. Polarization studies in the isotropic ab-plane of the tetragonal crystal Cu1-xNixB2O4 show the presence of linear magnetic dichroism in the magnetically ordered state, which was previously observed both in manganese-doped and undoped copper metaborates CuB2O4. The temperature of magnetic phase transitions into the collinear antiferromagnetic and into helicoidal structures, TN = 19.1 K and T* = 8.6 К, respectively, were determined from the temperature dependence of the dichroic signal.

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This work presents information on the growth and spectroscopic study of single crystals of copper metaborate doped with nickel Cu1-xNixB2O4 (x = 0.05, 0.1). In the absorption spectra of both crystals, satellites related to Cu centers distorted by impurity Ni atoms were observed near the lines of zero phonon transitions. Polarization studies in the isotropic ab-plane of the tetragonal crystal Cu1-xNixB2O4 show the presence of linear magnetic dichroism in the magnetically ordered state, which was previously observed both in manganese-doped and undoped copper metaborates CuB2O4. The temperature of magnetic phase transitions into the collinear antiferromagnetic and into helicoidal structures, TN = 19.1 K and T* = 8.6 К, respectively, were determined from the temperature dependence of the dichroic signal.

Diffraction of helical x-rays by optically active achiral crystals

Four crystal classes are optically active yet not chiral (non-enantiomorphic). Corresponding Bragg diffraction patterns calculated for circularly polarized (helical) x-rays tuned to an atomic resonance prove that angular anisotropy in the distribution of electrons create spots not indexed on the chemical structure (defined by a space group derived from Thomson scattering by perfect spheres of charge). Templeton-Templeton scattering, as it is usually called, of helical x-rays is quantified in terms of a chiral signature defined as the partial diffracted intensity hallmarked by x-ray helicity. Our electric dipole - electric dipole (E1-E1) chiral signature for a space group in the optically active crystal class D2d affords a complete interpretation in terms of copper quadrupoles of diffraction data collected on copper metaborate [E. N. Ovchinnikova et al., J. Synchrotron Rad. 28, 1455 (2021)]. An example of a chiral signature derived from a parity-odd resonance event (E1-E2) is included in our study. It is firmly established that tuning the energy of x-rays to an atomic resonance enhances the intensity of inherently weak Templeton-Templeton scattering and renders space-group forbidden Bragg spots element specific. Our chiral signature as a function of rotation about the reflection vector (azimuthal-angle scan) is specific to position multiplicity, Wyckoff letter and symmetry in a favourable space group.

Helicity-dependent resonant X-ray scattering in CuB2O4.

Exploitation of X-ray circular polarized beams to study forbidden Bragg reflections and new information that could be obtained in these experiments are discussed. It is shown that the intensities of such reflections can be different for the right- and left-circular polarizations (i.e. exhibiting circular dichroism) even for the dipole-dipole resonant transitions involved in the scattering process. This difference can be observed only in crystals having no center of inversion. Here, this approach is used to study helicity-dependent resonant diffraction in copper metaborate CuB2O4 single crystal, which is non-centrosymmetric but achiral. Nonetheless, a strong circular dichroism has been observed for hh0 forbidden reflections in the vicinity of the Cu K-edge. This effect is shown to originate from dipolar transitions in Cu atoms occupying the 8(d) Wyckoff position only.

Exciton and exciton-magnon photoluminescence in the antiferromagnet CuB2O4

Copper metaborate ${\mathrm{CuB}}_{2}{\mathrm{O}}_{4}$ crystallizes in a unique noncentrosymmetric structure, becomes antiferromagnetically ordered below ${T}_{\text{N1}}=20$ K, and exhibits a great diversity in magnetic, optical, and magneto-optical properties. In particular, it shows strong photoluminescence rarely observed before in other magnetically ordered copper oxides in which magnetic properties are defined by magnetic ${\mathrm{Cu}}^{2+}$ ($3{d}^{9}$, $S=1/2$) ions. Here we report on the detailed spectroscopic study of the photoluminescence originating from the ${\mathrm{Cu}}^{2+}$ ions. Our investigations are focused on understanding the energy-level scheme of the multiple excitations below the energetically lowest, crystal-field-split $d\text{\ensuremath{-}}d$ electronic transition at 1.405 eV. We identify multiple emission lines, and among them we distinguish three sets of lines, each composed of an exciton line and a satellite attributed to magnon-assisted exciton recombination. The emission intensity of the three sets changes strongly in the temperature range $1.7--40$ K, showing pronounced correlations with the magnetic phase transitions between the commensurate and incommensurate phases. Photoluminescence excitation spectra and time-resolved emission dynamics give closer insight into the energy relaxation channels populating the exciton-magnon sets.

Anomalous boron isotope effects on electronic structure and lattice dynamics of CuB2O4.

Copper metaborate had a unique crystal structure and exhibited noteworthy magnetic phase transitions at 21 and 10 K. The electronic structure and lattice dynamics of copper metaborate Cu11B2O4 single crystals were investigated and compared with the optical properties of CuB2O4, to assess the boron isotope effect. The optical absorption spectrum at room temperature revealed two charge-transfer bands at approximately 4.30 and 5.21 eV with an extrapolated direct optical band gap of 3.16 ± 0.07 eV. Compared with the data on CuB2O4, the electronic transitions were shifted to lower energies upon the replacement of a heavier boron isotope. The band gap was also determined to be lower in Cu11B2O4. Anomalies in the temperature dependence of the optical band gap were observed below 21 K. Furthermore, 38 Raman-active phonon modes were identified in the room-temperature Raman scattering spectrum of Cu11B2O4, which were also observed in CuB2O4 with a shift to lower frequencies. No broadening caused by isotopic changes was observed. As the temperature decreased, phonon frequencies shifted to higher wavenumbers and the linewidth decreased. Anomalous softening in the Raman peaks below 21 K was also revealed.

High-Resolution Spectroscopy of Low-Temperature Phase Transitions in Copper Metaborate CuB2O4

The linear magnetic dichroism in the crystallographically isotropic ab plane of a tetragonal CuB2O4 single crystal has been investigated by high-resolution Fourier spectroscopy at temperatures of 1.4–22 K. Magnetic phase transitions are found at T < 2.0 K and their nature is discussed.

Excitation of multiple phonon modes in copper metaborate CuB2O4 via nonresonant impulsive stimulated Raman scattering

Excitation of four coherent phonon modes of different symmetries has been realized in copper metaborate CuB$_2$O$_4$ via impulsive stimulated Raman scattering (ISRS). Phonons were detected by monitoring changes in the linear optical birefringence using the balanced-detection (BD) technique. We compare the results of BD-ISRS experiment to the polarized spontaneous Raman scattering spectra. We show that the agreement between the two sets of data obtained by these allied techniques in a wide phonon frequencies range of 4-14 THz can be achieved by rigorously taking into account the symmetry of the phonon modes, and the corresponding excitation and detection selection rules. It is also important to account for the difference between incoherent and coherent phonons in terms of their contributions to the Raman scattering process. This comparative analysis highlights the importance of the ratio between the frequency of a particular mode, and the pump and probe spectral widths. We demonstrate analytically that the pump and probe pulse durations of 90 and 50 fs, respectively, used in our experiments, limit the highest frequency of the excited and detected coherent phonon modes to 12 THz, and define their relative amplitudes.

On the Calculation of an X-Ray Natural Circular Dichroism Signal

The sequential derivation of an expression that describes an X-ray natural circular dichroism (XNCD) signal in the most general case is performed. The obtained expression is used as a basis to consider XNCD signals and their structural origins in copper metaborate CuB2O4 crystals at an incident radiation energy near the K absorption edge of copper and lanthanum–gallium silicate La3Ga5SiO14 crystals at an incident radiation energy near the K absorption edge of gallium. It has been demonstrated that the measurements of circularly polarized X-ray absorption spectra in combination with corresponding first-principle calculations provide an efficient method for studying the contributions of crystallographically non-equivalent atoms to the X-ray optical activity of crystals.

Copper Metaborate CuB2O4 Phase Diagrams Based on the Results of Measuring the Magnetic Moment

The magnetic phase transitions and the phase diagrams of the CuB2O4 multiferroic are studied. Phase diagrams of copper metaborate in a magnetic field directed along the [100] and [001] axes are plotted using the results of measuring the magnetic moment. Evidences for the existence of polycritical points are obtained.

Magnetic phase transitions and linear magnetic dichroism in manganese-doped copper metaborate (Cu,Mn)B2O4

The work presents a study of manganese-doped copper metaborate (Cu,Mn)B2O4 using optical spectroscopy. The temperature of the antiferromagnetic phase transition TN = 19 K has been defined according to the absorption spectra. Polarization studies (Cu,Mn)B2O4 in isotropic ab-plane show the presence of linear antiferromagnetic dichroism in the magnetically ordered state previously observed in pure copper metaborate CuB2O4. This measurement allows to find the magnetic phase transition into an elliptical structure at the temperature T* = 7.0 K.

Zeeman spectrum, magnetic neutron diffraction pattern, and Dirac multipoles for a multiferroic materialCuB2O4

Zeeman spectra, dichroic signals, and neutron Bragg diffraction patterns generated by copper ions in magnetically ordered copper metaborate $(\mathrm{Cu}{\mathrm{B}}_{2}{\mathrm{O}}_{4})$ are investigated within a minimal model of Cu atomic states. A theory platform, common to understanding optical spectra and neutron diffraction patterns, affords the immediate benefit of a unified description of the experimental probes in terms of electronic multipoles. Results for dichroic signals illustrate a nontrivial use of a general, quantum mechanical theory of photon absorption couched in terms of Dirac multipoles that are magnetic and polar. Anapoles (Dirac dipoles) are predicted to generate Bragg spots in magnetic neutron diffraction that are not indexed by the motif of conventional (axial) magnetic-dipole moments. The minimal model of Cu states is informed by magnetic symmetry, derived from an established commensurate antiferromagnetic order, with a sparse number of parameters that comply with available empirical evidence.

X-ray natural circular dichroism in copper metaborate

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.

Raman Spectroscopy Investigation of the Polar Vibrational Modes in CuB2O4

We report results of polarized Raman spectroscopy of piezoelectric copper metaborate CuB2O4 (space group I ¯42d, Z = 12) in the range from 80 to 1300 cm-1. Due to the dense population of the phonon spectra we carried out angular-dependent measurements by rotating the sample in different crystallographic planes thus performing gradual transitions from one polarization configuration to another. The different angular dependence of the scattering intensities of vibrational modes of different symmetries allowed us to obtain Raman signal from some transversal optical (TO) modes of E-symmetry and to demonstrate the singlemode behavior of the B2 modes by means of transition from TO character to longitudinal optical (LO) character of these phonons.

Lattice dynamics of piezoelectric copper metaborate CuB2O4

Tetragonal piezoelectric copper metaborate CuB${}_{2}$O${}_{4}$ (space group $I\overline{4}2d$, $Z=12$) became a subject of active research during the last decade because it demonstrates unique magnetic, optical, and magnetooptical properties. We report results of the optical studies of polarized infrared reflection and transmission spectra and Raman back-scattering spectra in the range from 20 to 2500 cm${}^{\ensuremath{-}1}$. In accordance with the symmetry analysis, infrared studies allowed us to identify all polar ${B}_{2}(z)$, and $E(x,y)$ normal vibrational modes. Frequencies of all longitudinal (LO) and transverse (TO) optical phonons were determined by fitting the observed infrared reflection spectra. Raman measurements revealed the frequencies of the majority of nonpolar ${A}_{1}(xx,yy,zz)$ and ${B}_{1}(xx,yy)$ modes and polar ${B}_{2}(z)$ and $E(x,y)$ LO modes. We show that several groups of closely spaced spectral lines observed in the vibrational spectra of CuB${}_{2}$O${}_{4}$ can be considered as a result of the Davydov splitting, which takes place in the case of several equivalent molecular units in a primitive crystal cell. Multiphonon absorption in the range of 1200--2500 cm${}^{\ensuremath{-}1}$ was analyzed and a tentative assignment of some features was given. Ab initio calculations of all zone-center polar and nonpolar modes were performed using density functional theory (DFT). The computed frequency distribution is in a good agreement with observed line positions in the infrared and Raman spectra. All these results allowed us to suggest reliable identification of all zone-center phonon modes thus providing a firm basis for further experimental and theoretical studies of this material with complex crystal, magnetic, and electronic structures and fascinating physical properties.

High-temperature heat capacity of copper metaborate CuB2O4

The heat capacity of copper metaborate CuB2O4 has been measured over a wide temperature range. A correlation between the composition of the CuO-B2O3 system and the heat capacity of the oxide compounds has been found.

Reply to comment on ‘Calculated chiral and magneto-electric dichroic signals for copper metaborate(CuB2O4) in an applied magnetic field’

From the dawn of modern electromagnetism it has been known that a magnetic field is nothanded (chiral). Arima and Saito (2009 J. Phys.: Condens. Matter 21 498001) persist withunwisdom in their repeated claim to have observed control of chirality using a magneticfield by and in itself. In our reply to their claim, we demonstrate damning errors in allchallenges in the comment levelled at our analysis of the observation reported by Saito et al (2008 Phys. Rev. Lett. 101 117402) and made on a crystal of copper metaborate.