Single Ion Transitions Research Articles

Crystal-field states and defect levels in candidate quantum spin ice Ce2Hf2O7

We report the synthesis of powder and single-crystal samples of the cerium pyrohafnate and their characterization using neutron diffraction, thermogravimetry and X-ray absorption spectroscopy. We evaluate the amount of non-magnetic Ce$^{4+}$ defects and use this result to interpret the spectrum of crystal-electric field transitions observed using inelastic neutron scattering. The analysis of these single-ion transitions indicates the dipole-octupole nature of the ground state doublet and a significant degree of spin-lattice coupling. The single-ion properties calculated from the crystal-electric field parameters obtained spectroscopically are in good agreement with bulk magnetic susceptibility data down to about 1 K. Below this temperature, the behavior of the magnetic susceptibility indicates a correlated regime without showing any sign of magnetic long-range order or freezing down to 0.08 K. We conclude that Ce$_2$Hf$_2$O$_{7}$ is another candidate to investigate exotic correlated states of quantum matter such as the octupolar quantum spin ice recently argued to exist in the isostructural compounds Ce$_2$Sn$_2$O$_7$ and Ce$_2$Zr$_2$O$_7$.

Magnetic properties of closo-carborane-based Co(II) single-ion complexes with O, S, Se, and Te bridging atoms

We present a detailed computational analysis of a newly proposed family of Co(II) single-ion complexes with large magnetic anisotropy. Starting from previously proposed structures based on oxy- and thio-bridged 1,2-dicarba-closo-dodecaborane and 1,2-dicarba-closo-hexaborane, we explore the substitution of O and S bridging atoms by Se and Te atoms. Our calculations, based on CASSCF/NEVPT2 methods, show that these substitutions also lead to some large magnetic anisotropy complexes. Particularly, one of the proposed single-ion Se-bridged Co(II) chelated by 1,2-dicarba-closo-dodecaborane has a calculated anisotropy parameter D=-144 cm−1, and a single-ion Se-bridged Co(II) chelated by 1,2-dicarba-closo-hexaborane gives a calculated D=-133 cm−1. These large D values can be attributed in part to geometric distortion from Td symmetry at the Co(II) ion caused by the ligand structure, and in part to the electric effect of the Se atoms. Our results suggest that this strategy could be a feasible alternative to increase the magnetic anisotropy of single-ion transition metal complexes.

Absorption of light by exchange coupled pairs of tetrahedrally coordinated divalent manganese in the helvite-genthelvite solid solution

Periodico di Mineralogia (2011), 80, 1 (Special Issue), 105-111 - DOI: 10.2451/2011PM0008 Special Issue in memory of Sergio Lucchesi Absorption of light by exchange coupled pairs of tetrahedrally coordinated divalent manganese in the helvite-genthelvite solid solution Ulf Halenius Department of Mineralogy, Swedish Museum of Natural History, Stockholm, Sweden ulf.halenius@nrm.se Abstract Intensified colours in the helvite-genthelvite solid solution series are demonstrated by means of single crystal optical absorption and electron microprobe techniques to be due to absorption of light by exchange coupled IV Mn 2+ - IV Mn 2+ pairs. The absorption bands caused by the pair transitions occur at energies, 21100, 22600, 23800, 26800 and 28400 cm -1 , that are comparable to those recorded for spin-forbidden single ion transitions in IV Mn 2+ , but the absorption coefficients of the bands are proportional to the square of the Mn-contents of the samples instead of simply showing a Lambert-Beer relationship to the Mn-content. The helvite structure, comprising four corner sharing MnO 3 S tetrahedra with a common sulphur anion apex, represents a favourable situation for formation of exchange coupled 3d-cation pairs. However, several common Mn-rich minerals, as, e.g., rhodonite, have comparably favourable structures that contain corner, edge or face sharing 3d-cation centred polyhedra. Consequently, may Mn 2+ -Mn 2+ pair absorption be a frequent cause for colour intensification in minerals. Key words : helvite; genthelvite; electronic spectroscopy; electron microprobe analyses.

EPR and photoluminescence properties of combustion-synthesized ZnAl2O4:Cr3+ phosphors

An efficient red emitting ZnAl2O4:Cr3+ powder phosphor material was prepared at furnace temperatures as low as 500 °C by using the combustion method. The prepared powders were analyzed by X-ray diffraction and scanning electron microscopy techniques. The optical properties were studied using photoluminescence technique. The EPR spectra exhibit an intense resonance signal at g = 3.74 which is attributed to Cr3+–Cr3+ pairs, and the weak resonance signal of at g = 1.97 is attributed to Cr3+ single ion transition. The spin population (N) has been evaluated as a function of temperature. The excitation spectrum exhibits two broad bands in the visible region which are characteristic of Cr3+ ions in octahedral symmetry and the emission spectrum exhibits zero-phonon line frequencies along with vibronic frequencies. The crystal field parameter (Dq) and Racah parameters (B and C) have been evaluated and discussed.

Diffuse reflectance spectra of U ions in ThO 2

Diffuse reflectance spectra obtained at ambient temperature for polycrystalline U x Th (1− x) O 2+ y ceramics fired at 1500 °C in 3.5% H 2/N 2 show several well-defined absorption bands of a 500 cm −1 width in the range of 4000–120 00 cm −1, extending the range of previous measurements. These bands are ascribed to interconfigurational d–d single-ion transitions of U 4+and increase in intensity with U concentration for x ⩽ 0.1. For higher x values, the intensities of the U 4+ bands peak at x ∼ 0.2 and then decrease with increasing x to small values only at x = 1. Firing in argon at 1500 °C produced enhanced absorption in the 6000–7000 cm −1 range, indicative of U 5+ formation. Further oxidation to U 5+ was produced by firing in air. The detailed shape of the U 5+ peaks depended upon the degree of oxidation and the U/Th ratio.

High speed algorithm for the calculation of magnetic and orbital excitations in rare earth based systems

An algorithm for the efficient and fast calculation of dispersive magnetic excitations in rare earth based systems has been developed. Crystal field anisotropy and anisotropy of the two ion interactions can be taken into account. It is based on the standard mean field–random phase approximation of the problem. In analogy to lattice dynamical calculations the central problem of finding the poles of the dynamical susceptibility χ( Q, ω) has been reduced to the diagonalisation of a dynamical matrix. This approach is applicable to complex magnetic structures, it is fast and easily generalised to include many single ion transitions and orbital interactions.

Fluorescence dynamics in 1D systems with phonon-assisted energy transfer

We report the results of a theoretical study of fluorescence dynamics in quasi-one-dimensional systems with phonon-assisted energy transfer between nearest neighbors. It is assumed that the phonons are three-dimensional, the one-phonon mechanism is dominant and the wavelength of the phonon involved is long in comparison with the separation between nearest neighbors. A combination of analytical and numerical techniques is used in the analysis of the decay of the narrow-line component observed in fluorescence line narrowing studies. A random distribution of single-ion transition frequencies is postulated, with no correlation between sites, and the variance of the distribution is taken to be small in comparison with ( kT/ℏ) 2. It is found that the narrow-line fluorescence decays asymptotically as t −1/3 rather than as t −1/2, as would be the case if the energy transfer were diffusive at long times. It is shown that the subdiffusive dynamics reflects a singular behavior in the limiting form of the distribution of transfer rates that is not present when the energy transfer involves two-phonon processes. Numerical results are presented for the early stages of the decay for rectangular distributions of transition frequencies with various values of the ratio of kT/ℏ σ, where σ 2 is the variance of the distribution.

Observation of single ion transitions in the ordered states of FeCl2

Electron spin resonance (ESR) experiments using a far infrared laser and a superconducting magnet have been performed on a single crystal sample of the metamagnet FeCl2. A new ESR signal has been observed at a much higher frequency than the ordinary antiferromagnetic and ferromagnetic resonance frequencies. The new ESR signal originates from a transition within the lowest triplet state of single Fe2+ ions in the presence of exchange fields.

Observation of Single-Ion Transitions in the Ordered States of FeBr2

By utilizing a far infrared laser operating in the THz range, we have observed new electron spin resonance (ESR) lines in the ordered states of a layered antiferromagnet with strong uniaxial anisotropy, FeBr 2 . We interpret these ESR lines as arising from transitions within the lowest triplet state of single Fe 2+ ions. Good agreement between theory and experiment is obtained without any adjustable parameters.

Ni3+-Ni3+ pair transitions in highly doped LiNbO3:Ni.

We report on the EPR ${\mathrm{Ni}}^{3+\mathrm{\ensuremath{-}}}$${\mathrm{Ni}}^{3+}$ pair transitions in highly doped ${\mathrm{LiNbO}}_{3}$:Ni. It is found that there are nine absorption peaks at 640, 1200, 1460, 1520, 3040, 3160, 3380, 7800, and 10 600 G in the range 0--12 000 G in highly doped ${\mathrm{LiNbO}}_{3}$:Ni (0.8 wt. %). The spectra were interpreted as both the ${\mathrm{Ni}}^{3+}$ single-ion transitions and ${\mathrm{Ni}}^{3+\mathrm{\ensuremath{-}}}$${\mathrm{Ni}}^{3+}$ pair transitions. In addition, it is found that many superhyperfine lines exist in the range 3000--3500 G which is not yet understood. Their origin is possibly an electron-phonon disturbance.

Low-field microwave absorption and Cr3+-Cr3+ pair transitions in highly doped yttrium aluminum garnet.

A single crystal of yttrium aluminum garnet (YAG) doped with ${\mathrm{Cr}}^{3+}$ (1.5 wt. %) was grown by the Czochralski technique. ESR measurements show that there are fifteen weak absorption peaks at 330, 420, 840, 1200, 1725, 2516, 3000, 3750, 4500, 4960, 5520, 5880, 6805, 7975, and 9050 G, and two intense absorption peaks at 1980 and 3464 G in the range of 0--12 000 G in highly doped YAG, ${\mathrm{Cr}}^{3+}$ (1.5 wt.%). The spectra were interpreted as both the ${\mathrm{Cr}}^{3+}$-${\mathrm{Cr}}^{3+}$ pair (with spin three) transitions and ${\mathrm{Cr}}^{3+}$ single-ion transitions. The calculation is in agreement with experiments.

Magnon sidebands and cooperative absorptions in K2ReCl6and Cs2ReCl6

Weak features between 16000 and 19000 cm-1 in the absorption spectra of Cs2ReCl6 and K2ReCl6 are assigned as cooperative absorptions involving coupled excitations of the Gamma 8(4A2g) to Gamma 6(2T1g), Gamma 8(2T1g), Gamma 8(2Eg) transitions on neighbouring complex ions. The intensity mechanism of these transitions is discussed. Below the Neel temperature of K2ReCl6, the single-ion transitions are accompanied by magnon sidebands with a magnon wavenumber of about 45 cm-1.

Fourier transform IR spectroscopy of Co 2+ excitations in KZn 1− xCo xF 3

From a temperature-dependent study of the IR absorption spectrum of crystals of cubic KZn 1− x Co x F 3 (with 0 < x < 0.15) we have identified electronic and vibronic transitions associated with isolated clusters of Co 2+ ions. Single-ion transitions within the 4T 1g( 4F) ground term were found at 950 and 1045 cm −1, respectively, together with higher-frequency vibronics, most of which could be assigned to combinations of these electronic transitions with transverse optic phonons. At higher Co 2+ ion concentrations (e.g. x = 0.12) other absorption features were observed and these have been attributed to vibronics associated with pairs of Co 2+ ions.

Spin-dependent Raman scattering from phonons and electronic excitations in the 1D Ising antiferromagnet CsCoBr3

Raman scattering from phonons, magnons and low lying electronic excitations of Co2+ in CsCoBr3 have been measured between 8 and 100K. Above TN1=28.3K, CsCoBr3 is a one-dimensional antiferromagnet and for this phase eight of the eleven possible single-ion transitions within the 4T1g(4F) ground term have been assigned. The observed energies can be accurately fitted using a 3d7 crystal-field model which includes the intrachain exchange within the molecular field approximation. Scattering from one-magnon and two-magnon bands has been identified. Below TN1 the main spin wave peak develops structure associated with the different modes of Ising chains coupled in a ferrimagnetic triangular lattice. Near TN1, the temperature dependence of the scattering intensity associated with the E1g phonon and all excitons is dominated by a spin-dependent mechanism involving the interchain pair polarisability. Analogous effects are predicted to occur for compounds having a similar structure.

Fir absorption spectrum of CoO doped with Ni 2+

We have measured the absorption spectrum of CoO doped with up to 6.0% Ni 2+ in the far infra-red at liquid helium temperatures. Two absorption lines have been found, one near 146 cm -1 and the other at 182 cm -1. Although the frequency of the latter does not vary with Ni concentration, its oscillator strength is proportional to the Ni concentration. Therefore the line at 182 cm -1 is due to an impurity-induced absorption. The frequency of the line near 146 cm -1 decreases perceptibly with increasing Ni concentration while its oscillator strength remains constant. In addition to previous Raman data, therefore, our results are evidence that this line is due to a collective AFMR mode and not to a single ion transition in CoO. A comparison of our experimental data to the results of model calculations indicates that the magnetocrystalline anisotropy of the Co 2+ ions decreases with increasing Ni concentration.

Symmetry Properties of the Planar Antiferrornagnet K2NiF4

AbstractUsing well established group theoretical methods a symmetry analysis is made of magaons and paramagnetic spin waves in antiferromagnetic K2NiF4. Excitons and single ion transitions are also considered and Selection rules for first‐order transitions are derived.

Further studies of the optical absorption spectrum of U5+in zircon

From the studies of the concentration and compensator dependence of the optical absorption spectrum of U5+ in zircon, only the two strong lines near 9030 and 6700 cm-1 appear to be single-ion transitions in which the charge compensation is remote. Zeeman studies of the two lines have been made and it is concluded that there is a near-degeneracy in the ground state of remotely compensated U5+ in zircon. The separation of the low-lying states would appear to be <1cm-1.

Neutron scattering study of the magnetic excitations in FeBr2

The magnetic excitations in FeBr2 have been studied between 2K and 9K using inelastic neutron scattering. The spin waves may be described by a simple S=1 Hamiltonian with an anisotropy field g mu BHA=0.428+or-0.016 THz, intra-plane isotropic interactions of 2J1=0.152+or-0.008 THz, 2J2=-0.051+or-0.006 THz and an inter-planar interaction 2J'1=-0.06+or-0.005 THz. The spin-wave spectrum is quite similar to FeCl2, although slightly less two-dimensional. At T>2K a second type of excitation is observed, which interacts strongly with the spin waves and is thought to correspond to the single ion transition from the Sz=0 to Sz=+1 state of the ground-state triplet.

Optical studies on crystalline hexahalorhenates—I: Low temperature absorption spectra of K 2[ReCl 6] single crystals

The optical spectrum of rhenium(IV) in single crystals of K 2[ReCl 6] has been measured in the range of 14,000 Å to 5000 Å at 4·2°K at 4·2°K and 77°K. The spectrum consists of six band systems, five of which correspond to single ion transitions within the t 2 g 3 configuration. The observed excited levels can be fitted with Δ = 29·8 kK, B = 0·42 kK and ζ 5 d = 2·3 kK. The superimposed vibrational structure involves odd modes of the [ReCl 6] 2− complex. The sixth band system centered at 5600 Å is tentatively assigned to Re 4+ pairs. The temperature dependence of the bands leads to a ground-state exchange integral J = −80 cm −1.

Spectrum of Exchange Coupled Cr3+ Pairs in YAlO3

The optical spectrum and exchange splittings of the two types of nearest neighbor Cr3+ pairs in orthorhombic YAlO3 have been studied. The emission spectrum from 7200 to 7350 Å yields the ground state exchange −JS.S+j(S.S)2 with Ja=−24.7, ja=−0.66 cm−1 and Jb=−26.2 and jb=−0.91 cm−1. From the shift of the lines under uniaxial compression along the crystal z axis we conclude these values correspond, respectively, to pairs whose axes are in the xy plane and along the z axis. The energies of several pair levels with one ion in 2E and the other in 4A2, having S=1 and 2 have been obtained. Selective enhancement of nn pair luminescence is observed when the excitation energy is at about twice the single ion 2E energy. The excitation and absorption spectra indicate these are pair levels corresponding to both ions in the 2E state and have S=0 and 1. The single ion transitions are magnetic dipole in agreement with Cr3+ being at a center of inversion. The pair lines are electric dipole, and the strongest transitions in the infrared, and the pair lines in the ultraviolet obey the selection rule ΔS=0 indicating the pair transitions obtain their intensity from the spin dependent dipole moment mechanism.