Crystal Field Excitations Research Articles

Revealing the effect of interstitial oxygen on the low-energy crystal electric field excitations of Pr3+ in 214 -nickelates

We report an inelastic neutron scattering study (INS) on the low-energy crystal electric field (CEF) excitations of ${\mathrm{Pr}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{NiO}}_{4+\ensuremath{\delta}}$ single crystals at various temperatures. The observed low-$E$ CEF level of the O-doped sample $(x=0,\phantom{\rule{4pt}{0ex}}\ensuremath{\delta}\ensuremath{\approx}0.24)$ at $\ensuremath{\sim}5.5\phantom{\rule{4pt}{0ex}}\mathrm{meV}$ appears at significantly lower energy than that of the Sr-doped sample $(x=0.5,\phantom{\rule{4pt}{0ex}}\ensuremath{\delta}=0.0)$ at $\ensuremath{\sim}8.5\phantom{\rule{4pt}{0ex}}\mathrm{meV}$. Applying the point charge (PC) model calculation, this has been interpreted as an effect of the interstitial oxygen via lowering the local symmetry and modifying the CEF environment of the central rare earth ${\mathrm{Pr}}^{3+}$ $(^{3}H_{4})$ ions.

Model for coupled 4f−3d magnetic spectra: A neutron scattering study of the Yb-Fe hybridization in Yb3Fe5O12

In this work, we explore experimentally and theoretically the spectrum of magnetic excitations of the Fe$^{3+}$ and Yb$^{3+}$ ions in ytterbium iron garnet (Yb$_3$Fe$_5$O$_{12}$). We present a complete description of the crystal-field splitting of the $4f$ states of Yb$^{3+}$, including the effect of the exchange field generated by the magnetically ordered Fe subsystem. We also consider a further effect of the Fe-Yb exchange interaction, which is to hybridise the Yb crystal field excitations with the Fe spin-wave modes at positions in the Brillouin zone where the two types of excitations cross. We present detailed measurements of these hybridised excitations, and propose a framework which can be used in the quantitative analysis of the coupled spectra in terms of the anisotropic $4f-3d$ exchange interaction.

Metamagnetism and crystal-field splitting in pseudohexagonal CeRh3Si2

CeRh$_3$Si$_2$ has been reported to exhibit metamagnetic transitions below 5~K, a giant crystal field splitting, and anisotropic magnetic properties from single crystal magnetization and heat capacity measurements. Here we report results of neutron and x-ray scattering studies of the magnetic structure and crystal-field excitations to further understand the magnetism of this compound. Inelastic neutron scattering (INS) and resonant inelastic x-ray scattering (RIXS) reveal a $J_z$\,=\,1/2 groundstate for Ce when considering the crystallographic $a$ direction as quantization axis, thus explaining the anisotropy of the static susceptibility. Furthermore, we find a total splitting of 78\,meV for the $J$\,=\,5/2 multiplet. The neutron diffraction study in zero field reveals that on cooling from the paramagnetic state, the system first orders at $T_{\text{N}_1}=4.7$\,K in a longitudinal spin density wave with ordered Ce moments along the $b$-axis (i.e. the [0 1 0] crystal direction) and an incommensurate propagation vector $\textbf{k}=(0,0.43,0$). Below the lower-temperature transition $T_{\text{N}_2}=4.48$\,K, the propagation vector locks to the commensurate value $\textbf{k}=(0,0.5,0)$, with a so-called lock-in transition. Our neutron diffraction study in applied magnetic field $H\parallel b$-axis shows a change in the commensurate propagation vector and development of a ferromagnetic component at $H=3$\,kOe, followed by a series of transitions before the fully field-induced ferromagnetic phase is reached at $H = 7$\,kOe. This explains the nature of the steps previously reported in field-dependent magnetization measurements. A very similar behaviour is also observed for the $H\parallel$ [0 1 1] crystal direction.

Crystal-field excitations and quadrupolar fluctuations of 4f-electron systems studied by polarized light scattering

We present Raman-scattering results for three materials, CeB6, TbInO3, and YbRu2Ge2, to illustrate the essential aspects of crystal-field (CF) excitations and quadrupolar fluctuations of 4f-electron systems. For CF excitations, we illustrate how the 4f orbits are split by spin-orbit coupling and CF potential by presenting spectra for inter- and intra-multiplet excitations over a large energy range. We discuss identification of the CF ground state and establishment of low-energy CF level scheme from the symmetry and energy of measured CF excitations. In addition, we demonstrate that the CF linewidth is a sensitive probe of electron correlation by virtue of self-energy effect. For quadrupolar fluctuations, we discuss both ferroquadrupolar (FQ) and antiferroquadrupolar (AFQ) cases. Long-wavelength quadrupolar fluctuations of the same symmetry as the FQ order parameter persists well above the transition temperature, from which the strength of electronic intersite quadrupolar interaction can be evaluated. The tendency towards AFQ ordering induces ferromagnetic correlation between neighboring 4f-ion sites, leading to long-wavelength magnetic fluctuations.

High-pressure study of electronic properties of a CeCuAl3 single crystal

CeCuAl3 belongs to the Ce-based family of intermetallic compounds crystallizing in a non-centrosymmetric tetragonal BaNiSn3-type structure. Multiple members of the family exhibit pressure-induced superconductivity, unconventional superconductivity, as the crystallographic lattice lacks a centre of inversion. Several indications have suggested the existence of superconductivity also in CeCuAl3. Such assumption is supported by presented measurements of anisotropic compressibility. Our measurements of the electrical resistance of a CeCuAl3 single crystal in significantly expanded pressure and temperature regions up to 10 GPa and down to 9 mK, however, indicate no transition to the superconducting state. A standard saturation of resistance to residual value and its temperature evolution attributed to Fermi-liquid behaviour is observed at low temperatures throughout the experiment. At higher temperatures, the pressure dependence of an anomaly ascribed to the Kondo effect as well as two anomalies connected to the crystal field excitations is followed and discussed with respect to heavy-fermion properties and crystal field scheme of CeCuAl3.

Mesoscale interplay between phonons and crystal electric field excitations in quantum spin liquid candidate CsYbSe2

In quantum spin liquid candidate CsYbSe2, phonon coupling to a crystal electric field mode results in a vibronic bound state, and complex, mesoscale interplay between phonon modes and CEF modes is observed.

Magnetic properties and neutron spectroscopy of lanthanoid-{tetrabromocatecholate/18-crown-6} single-molecule magnets

Lanthanoid single-molecule magnets (Ln-SMMs) exhibit slow magnetic relaxation at low temperatures. This arises from an energy barrier to magnetisation reversal associated with the crystal field (CF) splitting of the Ln(III) ion. The magnetic relaxation is impacted by the interaction of the molecule with the crystal lattice, so factors including particle size and crystal packing can play an important role. In this work, a family of compounds of general formula [Ln(18-c-6)(NO3)(Br4Cat)]·X (Ln = La, Tb, Dy; 18-c-6 = 18-crown-6; Br4Cat2− = tetrabromocatecholate) has been studied by inelastic neutron scattering (INS) and magnetometry to elucidate the effects of crystal packing on the slow magnetic relaxation of the Tb(III) and Dy(III) compounds. The deuterated analogues [Ln(18-c-6-d24)(NO3)(Br4Cat)]·CH3CN-d3 (1-LnD; Ln = La, Tb, Dy) have been synthesised, with 1-TbD and the diamagnetic analogue 1-LaD measured by INS. The dynamic magnetic properties of 1-TbD and 1-DyD have also been measured and compared for two samples with different particle sizes. To probe packing effects on the slow magnetic relaxation, two new solvatomorphs of the hydrogenous compounds [Ln(18-c-6)(NO3)(Br4Cat)]·X (2-Ln: X = CH2Cl2; 3-Ln: X = 0.5 toluene) have been obtained for Ln = Tb and Dy. The CF splitting between the ground and first excited CF pseudo-doublets has been experimentally determined for 1-TbD by INS, and strongly rare earth dependent and anharmonic lattice vibrational modes have also been observed in the INS spectra, with implications for slow magnetic relaxation. Dynamic magnetic measurements reveal significant particle-size dependence for the slow magnetic relaxation for 1-TbD, while a previously reported anomalous phonon bottleneck effect in the 1-DyD analogue does not change with particle size. Further dynamic magnetic measurements of 2-Ln and 3-Ln show that the slow magnetic relaxation in these Ln-SMMs is strongly dependent on lattice effects and crystal packing, which has implications for the future use of Ln-SMMs in devices.

Magnetic-field control of magnetoelastic coupling in the rare-earth pyrochlore Tb2Ti2O7

In the rare-earth pyrochlore ${\mathrm{Tb}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$, there are strong interactions between crystal field and phonon excitations resulting in the hybridization of crystal field excitations with both acoustic and optical phonon excitations, which may be implicated in its evasion of the expected long-range ordered states. The magnetoelastic coupling mechanisms are thought to involve large quadrupolar matrix elements between the crystal field states that allow them to couple with intersecting phonons. The character of the hybridized modes can be determined by polarized neutron scattering, as is done here for the case of a crystal field-optical phonon coupling. The coupling mechanism can be further investigated by applying a magnetic field to modify the energies of the crystal field states relative to the phonon spectrum. For a weakly dispersive optical phonon and crystal field level, this has the effect of detuning the quasidegeneracy necessary for hybridization and suppressing the magnetoelastic signal. For a strongly dispersive acoustic phonon crossing a crystal field level, the magnetic field moves the crystal field level, changing the wave vector and energy at which the modes intersect. The field-driven modification of matrix elements for dipole and quadrupole operators involved in the formation of the coupled mode results in the suppression of the magnetoelastic coupling. As the crystal field states shift to higher energy and the magnetoelastic coupling is suppressed, the spin system is driven closer to classically anticipated ordered structures.

Absence of Kondo effect in CeNiGe3 revealed by coherent phonon dynamics

Cerium-based intermetallic ${\mathrm{CeNiGe}}_{3}$ has been generally believed to be a heavy-fermion material with Kondo behavior at low temperatures. Using femtosecond-resolved coherent phonon spectroscopy, we present a temperature-dependent dynamic investigation of the bosonic quasiparticles in ${\mathrm{CeNiGe}}_{3}$. Our data do not agree with the heavy-fermion expectation and instead show that the phonon stiffening from room temperature down to 5 K can be well explained by the anharmonic effect in the absence of a Kondo mechanism. Furthermore, the coherent lattice vibration located at \ensuremath{\sim}7.9 THz exhibits a mode splitting at ${T}^{+}\ensuremath{\approx}105\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, which is close to the energy scale of the first excited crystal field splitting of the Ce $4f$ level. We argue that an orbital crossover may account for this intriguing observation.

One ion to catch them all: Targeted high-precision Boltzmann thermometry over a wide temperature range with Gd3+

Ratiometric luminescence thermometry with trivalent lanthanide ions and their 4fn energy levels is an emerging technique for non-invasive remote temperature sensing with high spatial and temporal resolution. Conventional ratiometric luminescence thermometry often relies on thermal coupling between two closely lying energy levels governed by Boltzmann’s law. Despite its simplicity, Boltzmann thermometry with two excited levels allows precise temperature sensing, but only within a limited temperature range. While low temperatures slow down the nonradiative transitions required to generate a measurable population in the higher excitation level, temperatures that are too high favour equalized populations of the two excited levels, at the expense of low relative thermal sensitivity. In this work, we extend the concept of Boltzmann thermometry to more than two excited levels and provide quantitative guidelines that link the choice of energy gaps between multiple excited states to the performance in different temperature windows. By this approach, it is possible to retain the high relative sensitivity and precision of the temperature measurement over a wide temperature range within the same system. We demonstrate this concept using YAl3(BO3)4 (YAB):Pr3+, Gd3+ with an excited 6PJ crystal field and spin-orbit split levels of Gd3+ in the UV range to avoid a thermal black body background even at the highest temperatures. This phosphor is easily excitable with inexpensive and powerful blue LEDs at 450 nm. Zero-background luminescence thermometry is realized by using blue-to-UV energy transfer upconversion with the Pr3+−Gd3+ couple upon excitation in the visible range. This method allows us to cover a temperature window between 30 and 800 K.

Optical spectroscopy study of KDy(WO4)2: Crystal-field levels of Dy3+ and the Jahn–Teller transition

Transmission spectra of a double potassium-dysprosium tungstate KDy(WO4)2 single crystal in the region of intermultiplet transitions of the Dy3+ ion and the Raman spectra of light scattering on the electronic levels of the 6H15/2 ground multiplet of Dy3+ are studied in a wide temperature range. The energy scheme for the crystal-field (CF) levels of Dy3+ in KDy(WO4)2 is created. The phase transition at the temperature 6.3 K is accompanied by the lowering of the energy of the ground Dy3+ CF state, which is evidenced by both the transmission and Raman spectroscopies. Splitting of the low-lying (∼13 cm–1) first excited CF level of the Dy3+ ion indicates the simultaneous structural phase transition. The role of Davydov interaction is discussed.

Abnormal reduction and luminescence properties of Sm2+-doped Sr5(PO4)3Cl prepared by solution combustion synthesis

Sm-doped Sr5(PO4)3Cl was prepared by a solution combustion synthesis in the air atmosphere. The pure hexagonal phases of the samples were confirmed via the XRD, Raman, and FT-IR measurements. The typical microrod shape of the products was also demonstrated by SEM, TEM, and HRTEM images. The spectral properties and lifetimes were applied to characterize the luminescence of the samples as-prepared and subsequently heated in the air atmosphere. The luminescence of as-prepared Sm-doped Sr5(PO4)3Cl contains a broad emission band and a group of narrow lines, which are ascribed to the radiative transitions of 4f5d→4f and 5D0→7FJ (J = 0, 1, 2) of Sm2+ ions, respectively. It indicates that the Sm3+ can be successfully reduced to Sm2+ ions in hexagonal Sr5(PO4)3Cl prepared by combustion synthesis. The temperature dependent luminescence spectra and decay curves of Sm2+ were measured in a very wide temperature region from 10 K to 300 K. The thermal quenching and activation energy of Sm2+ doped Sr5(PO4)3Cl were reported. The crystal field energy level diagram and excitation mechanism of Sm2+ ions-doped Sr5(PO4)3Cl were characterized by the high resolution luminescence at low temperature. The luminescence of Sm2+ in Sr5(PO4)3Cl shows a great dependence on heating temperature in the air atmosphere (50–700 °C). The heating treatments at the elevated temperature depress Sm2+ emission along with the appearance of Sm3+ ions. The temperature-induced changes in the modification of the spectral components could be applied as potential luminescence thermometry.

Spectroscopic Analysis of Vibronic Relaxation Pathways in Molecular Spin Qubit [Ho(W5O18)2]9-: Sparse Spectra Are Key.

Vibrations play a prominent role in magnetic relaxation processes of molecular spin qubits as they couple to spin states, leading to the loss of quantum information. Direct experimental determination of vibronic coupling is crucial to understand and control the spin dynamics of these nano-objects, which represent the limit of miniaturization for quantum devices. Herein, we measure the magneto-infrared properties of the molecular spin qubit system Na9[Ho(W5O18)2]·35H2O. Our results place significant constraints on the pattern of crystal field levels and the vibrational excitations allowing us to unravel vibronic decoherence pathways in this system. We observe field-induced spectral changes near 63 and 370 cm-1 that are modeled in terms of odd-symmetry vibrations mixed with f-manifold crystal field excitations. The overall extent of vibronic coupling in Na9[Ho(W5O18)2]·35H2O is limited by a modest coupling constant (on the order of 0.25) and a transparency window in the phonon density of states that acts to keep the intramolecular vibrations and MJ levels apart. These findings advance the understanding of vibronic coupling in a molecular magnet with atomic clock transitions and suggest strategies for designing molecular spin qubits with improved coherence lifetimes.

Importance of dynamic lattice effects for crystal field excitations in the quantum spin ice candidate Pr2Zr2O7

Pr$_2$Zr$_2$O$_7$ is a pyrochlore quantum spin-ice candidate. Using Raman scattering spectroscopy we probe crystal electric field excitations of Pr$^{3+}$, and demonstrate the importance of their interactions with the lattice. We identify a vibronic interaction with a phonon that leads to a splitting of a doublet crystal field excitation at around 55~meV. We also probe a splitting of the non-Kramers ground state doublet of Pr$^{3+}$ by observing a double line of the excitations to the first excited singlet state $E^0_g \rightarrow A_{1g}$. We show that the splitting has a strong temperature dependence, with the doublet structure most prominent between 50~K and 100~K, and the weight of one of the components strongly decreases on cooling. We suggest a static or dynamic deviation of Pr$^{3+}$ from the position in the ideal crystal structure can be the origin of the effect, with the deviation strongly decreasing at low temperatures.

Crystal-field excitations and vibronic modes in the triangular-lattice spin-liquid candidate TbInO3

We study the ground-state properties, the electronic excitations, and lattice dynamics in spin-liquid candidate ${\mathrm{TbInO}}_{3}$. By employing polarization-resolved Raman spectroscopy we define the intermultiplet and intramultiplet excitations, and establish the low-energy crystal-field (CF) level scheme. In particular, we demonstrate that the ground state of the ${\mathrm{Tb}}^{3+}$ ions is a non-Kramers doublet, and relate the enhanced linewidth of the CF modes to the magnetic fluctuations near the spin-liquid ground state. We identify the 38 allowed Raman-active phonon modes at low temperature. Moreover, we observe hybrid vibronic excitations involving coupled CF and low-lying phonon modes, suggesting strong spin-lattice dynamics. We develop a model for vibronic states and obtain the parameters of the bare responses and coupling strength. We further demonstrate that the obtained CF level scheme is consistent with specific-heat data.

Anomalous magnetism of uranium(IV)-oxo and -imido complexes reveals unusual doubly degenerate electronic ground states

Anomalous magnetism of uranium(IV)-oxo and -imido complexes reveals unusual doubly degenerate electronic ground states

Magnetic structure and crystal field excitations of NdOs2Al10: a neutron scattering study

We have investigated the magnetic properties of a polycrystalline sample of NdOs2Al10 using neutron diffraction and inelastic neutron scattering, magnetic susceptibility and heat capacity measurements. The magnetic susceptibility data reveal an antiferromagnetic transition at T N1 = 2.2 K while the heat capacity data which extend to lower temperatures show a further transition at T N2 = 1.1 K. The INS measurements detect four well-resolved crystal field excitations at 7.4, 12.4, 17.6 meV and 19.2 meV at 5 K. The positions and intensities of the observed CEF excitations and the temperature dependence of previously published single crystal susceptibility data are analysed based on the orthorhombic CEF model of the J = 9/2 ground state multiplet of Nd3+ ion. The neutron diffraction study reveals the magnetic structure at T = 1.6 K to be of sine wave type (κ = [0, 0.723, 0]) with an ordered state moment of μ Nd 3+ = 1.59(1) μ B aligned along the a-axis in agreement with the single ion crystal field anisotropy. This indicates that the magnetism of NdOs2Al10 is governed by the RKKY interactions in agreement with other RT 2Al10 (R = Nd, Sm, Tb, T = Fe, Ru and Os), but different to that of R = Ce based compounds where anisotropic two ions interaction as well as Kondo interaction govern the anomalous direction of the ordered state moment.

Unified description of resistivity and thermopower of Pr2Ir2O7 : Possible influence of crystal field excitation in a Kondo lattice

We present experimental evidence of incoherent Kondo scattering as the source of resistivity minima in bulk polycrystalline and nanocrystalline Pr$_2$Ir$_2$O$_7$. The temperature dependence of thermopower shows a positive maximum at high temperature followed by a negative minimum at low temperature, with the sign inversion occurring at a much higher temperature than $T_K$. Moreover, we observe little correlation between TK and intersite coupling strength given by $\theta_{CW}$. We describe the temperature dependence of thermopower and resistivity within the framework of crystal field excitation in a Kondo lattice.

Low-energy magneto-optics of Tb2Ti2O7 in a [111] magnetic field

The pyrochlore magnet Tb$_{2}$Ti$_{2}$O$_{7}$ shows a lack of magnetic order to low temperatures and is considered to be a quantum spin liquid candidate. We perform time-domain THz spectroscopy on high quality Tb$_{2}$Ti$_{2}$O$_{7}$ crystal and study the low energy excitations as a function of [111] magnetic field with high energy resolution. The low energy crystal field excitations change their energies anomalously under magnetic field. Despite several sharp field dependent changes, we show that the material's spectrum can be described not by a phase transitions, but by field dependent hybridization between the low energy crystal field levels. We highlight the strong coupling between spin and lattice degrees of freedom in Tb$_{2}$Ti$_{2}$O$_{7}$ as evidenced by the magnetic field tunable crystal field environment. Calculations based on single ion physics with field induced symmetry reduction of the crystal field environment can reproduce our data.

Effective point-charge analysis of crystal fields: Application to rare-earth pyrochlores and tripod kagome magnets R3Mg2Sb3O14

An indispensable step to understand collective magnetic phenomena in rare-earth compounds is the determination of spatially-anisotropic single-ion properties resulting from spin-orbit coupling and crystal field (CF). The CF Hamiltonian has a discrete energy spectrum -- accessible to spectroscopic probes such as neutron scattering -- controlled by a number of independent parameters reflecting the point-symmetry of the magnetic sites. Determining these parameters in low-symmetry systems is often challenging. Here, we describe a general method to analyze CF excitation spectra using adjustable effective point-charges. We benchmark our method to existing neutron-scattering measurements on pyrochlore rare-earth oxides and obtain a universal point-charge model that describes a large family of related materials. We adapt this model to the newly discovered tripod Kagome magnets ($R_{3}$Mg$_2$Sb$_{3}$O$_{14}$, $R$ = Tb, Ho, Er, Yb) for which we report broadband inelastic neutron-scattering spectra. Analysis of these data using adjustable point-charges yields the CF wave-functions for each compound. From this, we calculate thermomagnetic properties that accurately reflect our measurements on powder samples, and predict the effective gyromagnetic tensor for pseudo-spin degrees of freedom -- a crucial step to understand the exotic collective properties of these kagome magnets at low temperature. We present further applications of our method to other tripod kagome materials and triangular rare-earth compounds $R$MgGaO$_4$ ($R$ =Yb, Tm). Overall, this study establishes a widely applicable methodology to predict CF and single-ion properties of rare-earth compounds based on interpretable and adjustable models of effective point-charges.