Systematic extraction of crystal electric-field effects and quantum magnetic model parameters in triangular rare-earth magnets
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Pocs, Christopher A.↗
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X-ray scattering by a crystal in an electric field.
Strong electric field effects on atomic scattering by crystal
ELECTRIC FIELD STRENGTH↗
Mitigating CaCO 3 crystal nucleation and growth through continuous ion displacement via alternating electric fields
Mineral crystal formation poses a challenge on surfaces (e.g., heat exchangers, pipes, membranes, etc.) in contact with super-saturated fluids. Applying alternating currents (AC) to such surfaces can prevent surface crystallization under certain conditions. Here, we demonstrate that ion displacement induced by periodic charging and discharging of the electrical double layer (EDL) inhibits both heterogeneous and homogeneous nucleation (and crystal growth) of CaCO 3 . Titanium sheets (meant to simulate metallic heat exchanger surfaces) are immersed in super-saturated CaCO 3 solutions with a saturation index >11. We show that at relatively high AC frequencies, incomplete EDL formation leads to an alternating electric field that propagates far into the bulk solution, inducing rapid ion migration that overwhelms the Brownian motion of ions. Electrochemical characterization reveals EDL charging/discharging under AC conditions that greatly inhibits precipitation. Operating at 4 V pp , 0.1–10 Hz reduces turbidity by over 96% and reduces CaCO 3 coverage on the metal plates by over 92%. Based on electrokinetic and crystallization models, the ion displacement velocity (exceeding the mean Brownian velocity) and displacement length disrupts ion collision and crystal nucleation. Overall, the technique has potential for preventing mineral crystal formation in heat exchangers and many other industrially relevant systems.
42 ENGINEERING↗
Topological magnetic hysteresis in single crystals of CeAgSb 2 ferromagnet
Closed-topology magnetic domains are usually observed in thin films and in an applied magnetic field. In this work, we report the observation of rectangular cross-section tubular ferromagnetic domains in thick single crystals of CeAgSb 2 in zero applied field. Relatively low exchange energy, small net magnetic moment, and anisotropic in-plane crystal electric fields lower the domain wall energy and allow for the formation of the closed-topology patterns. The tubular domain structure irreversibly transforms into a dendritic pattern upon cycling the magnetic field. This transition between closed and open topologies results in a ‘topological magnetic hysteresis’— the actual hysteresis in magnetization, not due to the imperfections and pinning, but due to the difference in the pattern morphology. Similar physics was suggested before in pure type-I superconductors and is believed to be a generic feature of other nonlinear single (present case), or two-phase (type-I superconductor) systems where the effects similar to demagnetization (shape-dependent macroscopic variation of properties) lead to pattern formation.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Importance of dynamic lattice effects for crystal field excitations in the quantum spin ice candidate Pr 2 Zr 2 O 7
We explore dynamic interactions between the crystal lattice and magnetic degrees of freedom in a frustrated magnetic system using the example of a pyrochlore quantum spin-ice candidate Pr 2 Zr 2 O 7 . Using Raman scattering spectroscopy we demonstrate that crystal electric field excitations of Pr3+, which define the magnetic properties of Pr 2 Zr 2 O 7 , cannot be understood within a model of a static lattice. We identify vibronic interactions with a phonon which lead to a splitting of a doublet crystal field excitation at around 55 meV. We also observe an unconventional behavior of a splitting of the non-Kramers ground state doublet of Pr 3+ , revealed by observing excitations to the first excited singlet state $E$$^{0}_{g}$ → A 1 g at around 10 meV. The splitting has a strong temperature dependence, where the doublet structure is most prominent between 50 and 100 K, and the weight of one of the components strongly decreases on cooling contrary to simple thermal population tendency. Furthermore, 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.
36 MATERIALS SCIENCE↗
Explaining the extra crystal-field mode in 𝐴 Ce 𝑋 2 (𝐴 = K, Rb, Na; 𝑋 = O, S, Se, Te)
A growing list of Ce-based magnets have shown an extra and heretofore unexplained crystal electric field (CEF) mode at high energies. We describe a process whereby an optical phonon can produce a split CEF mode well above the phonon energy. We use density functional theory and point-charge model calculations to estimate the phonon distortions and coupling to model this effect in KCeO 2 , showing that it accounts for the extra CEF mode observed. Furthermore, this mechanism is generic and may explain the extra modes observed on a variety of Ce 3+ compounds.
36 MATERIALS SCIENCE↗
Ion Transport and Crystal Rotation in Plastic Crystal Electrolytes Under Applied Electric Fields
Organic ionic plastic crystal electrolytes, containing a plastic crystal and lithium salt, offer a potential balance between mechanical and electrochemical properties for solid state lithium-ion battery electrolytes. These electrolytes contain multiple mobile ionic species (three or four), resulting in complex transport mechanisms which have not yet been established. Plastic crystals are defined by long-range positional order and short-range rotational disorder. It is therefore necessary to quantify changes in the local crystal structure of the electrolyte as current flows through it. Herein, we examine the electrochemical properties of pyrrolidinium-based plastic crystal electrolytes containing lithium salt and zwitterion additives, including measurements of current fraction and limiting current. We obtain species-specific insight into electrolyte transport using pulsed-field gradient nuclear magnetic resonance spectroscopy and find that, while the zwitterion additive increases ionic conductivity, it decreases lithium diffusivity with respect to other ionic components. With operando spatiotemporally resolved wide-angle X-ray scattering we observe location-specific crystal rotations due to the passage of ionic current. In conclusion, we posit that reducing energy dissipation due to rotation is essential for using plastic crystal electrolytes in practical applications.
Yap, Kyra M. K. [University of California, Berkele↗
Close Proximity to a Quantum Phase Transition in TmZn2GaO5
TmZn2GaO5 is a newly synthesized triangular lattice magnet that exhibits a unique quantum phase characterized by strong Ising anisotropy, a pseudodoublet crystal electric field ground state, and a low-energy gapped excitation at the 𝐾 point. Unlike its well-known counterparts, TmMgGaO4 and YbMgGaO4, this material crystallizes in a distinct hexagonal structure, leading to a platform for investigating frustrated magnetism. Magnetic susceptibility, heat capacity, and inelastic neutron scattering measurements confirm the absence of long-range magnetic order down to 50 mK, placing TmZn2GaO5 in a distinct region of the transverse-field Ising model phase diagram. Theoretical calculations based on spin-wave theory and mean-field modeling reproduce key experimental observations, reinforcing the material’s placement in a quantum disordered or multipolar state near a quantum critical point that separates it from the low-energy phase of TmMgGaO4. These results highlight its potential for exploring quantum disordered states, anisotropic excitations, and exotic quantum phases in frustrated spin systems.
Ennis, Matthew [Department of Physics, Duke Univer↗
Magnetic order and excitations in Ce 3 TiBi 5 and Ce 3 ZrBi 5
The R 3 MBi 5 rare-earth intermetallics (R = rare earth, M = Ti, Zr, Sc) provide a versatile platform to explore how Kondo hybridization, Ruderman–Kittel–Kasuya–Yosida (RKKY) exchange, magnetic frustration, and broken inversion symmetry may cooperate to generate unusual magnetic behavior. We present a comprehensive neutron scattering investigation of the magnetic structure, crystal electric field (CEF), and low-energy excitations in the locally noncentrosymmetric Kondo-lattice compounds Ce 3 TiBi 5 and Ce 3 ZrBi 5 . Powder and single-crystal neutron diffraction reveals incommensurate cycloidal antiferromagnetic order in Ce 3 TiBi 5 with propagation vector k = (0, 0, 0.388) and a reduced ordered moment of m = 0.53(3)μ B . Ce 3 ZrBi 5 exhibits a qualitatively similar magnetic diffraction profile, with k ≃ (0, 0, 0.37). Inelastic neutron scattering measurements resolve two clear, well-separated CEF excitations in both compounds with nearly the same profile, confirming a well-isolated Kramers doublet ground state. At low energies, a broad, quasi-elastic magnetic response is observed at T ≃ T N , whose momentum-dependence is inconsistent with that expected from conventional collective excitations of localized moments. This discrepancy, along with a Kondo temperature estimate T K ~ 6 - 7 K—comparable to T N —indicates sizable Kondo hybridization, which accounts for the moment reduction and the spiral magnetic order that appears to involve the magnetic hard direction. Our results place these compounds in a regime where local inversion symmetry breaking, anisotropic CEF effects, and competing Kondo and RKKY interactions collectively give rise to unconventional magnetic order.
Kondo interaction↗
Spin excitations in the frustrated triangular lattice antiferromagnet NaYbO 2
In this work, we present a neutron scattering-based study of magnetic excitations and magnetic order in NaYbO 2 under the application of an external magnetic field. The crystal electric field-split J = 7=2 multiplet structure is determined, revealing a mixed |m z $\rangle$ ground state doublet and is consistent with a recent report Ding et al. Our measurements further suggest signatures of exchange effects in the crystal field spectrum, manifested by a small splitting in energy of the transition into the first excited doublet. The field-dependence of the low-energy magnetic excitations across the transition from the quantum disordered ground state into the fluctuation driven ordered regime is analyzed. Signs of a first-order phase transition into a noncollinear ordered state are revealed at the upper-field phase boundary of the ordered regime, and higher order magnon scattering, suggestive of strong magnon-magnon interactions, is resolved within the previously reported up-up-down phase. Our results reveal a complex phase diagram of field-induced order and spin excitations within NaYbO 2 and demonstrate the dominant role of quantum fluctuations cross a broad range of fields within its interlayer frustrated triangular lattice.
36 MATERIALS SCIENCE↗
Unconventional magnetic order emerging from competing energy scales in the new R Rh 3 Si 7 intermetallics ( R = Gd-Yb)
The competition between Ruderman-Kittel-Kasuya-Yosida (RKKY), crystal electric field (CEF), and Kondo energy scales has recently emerged at the heart of complex magnetism in several Ce- or Yb-based intermetallics. Hard axis magnetic order has been observed in a handful of these compounds, independent of the crystal symmetry, size of the ordered moment, or the relative scale of the Kondo and magnetic ordering temperatures. This raises the question of the role of each energy scale in driving the ground state properties. In focusing on a single class of compounds, the rhombohedral RRh 3 Si 7 , we compare the anisotropy and magnetic ground states in members of this series with only RKKY interactions (R = Gd), or RKKY and CEF effects (R = Tb-Tm), with the behavior of the R = Yb compound, where all three energy scales (RKKY, CEF, Kondo) are at play. Moreover, we extend the comparison to two other isostructural Kondo systems YbIr 3 Si 7 and YbIr 3 Ge 7 , where hard axis magnetic order is also observed. The non-Kondo compounds RRh 3 Si 7 (R = Tb-Tm) lack the complexity of magnetic order along the hard CEF axis, pointing to the dominant role of the Kondo effect in driving this magnetic order. Furthermore, the CEF-RKKY competition is still responsible for complex magnetic ground states, and it appears that the electronic and magnetic degrees of freedom are entangled in all magnetic members of this series of compounds.
36 MATERIALS SCIENCE↗
Single-Crystal Diffuse Neutron Scattering Study of the Dipole-Octupole Quantum Spin-Ice Candidate Ce 2 Zr 2 O 7 : No Apparent Octupolar Correlations Above 𝑇 = 0.05 K
The insulating magnetic pyrochlore Ce 2 Zr 2 O 7 has gained attention as a quantum spin-ice candidate with dipole-octupole character that arises from the crystal-electric-field ground-state doublet for the Ce 3+ Kramers ion. This dipole-octupole character permits both spin-ice phases based on magnetic dipoles and those based on more-exotic octupoles. This work reports low-temperature neutron diffraction measurements on single-crystal Ce 2 Zr 2 O 7 with 𝑄 coverage both at low 𝑄, where the magnetic form factor for dipoles is near maximal, and at high 𝑄, covering the region where the magnetic form factor for Ce 3+ octupoles is near maximal. This study was motivated by recent powder neutron diffraction studies of other Ce-based dipole-octupole pyrochlores, Ce 2 Sn 2 O 7 and Ce 2 Hf 2 O 7 , which each showed temperature-dependent diffuse diffraction at high 𝑄, interpreted as arising from octupolar correlations. Our measurements use an optimized single-crystal diffuse scattering instrument that allows us to screen against strong Bragg scattering from Ce 2 Zr 2 O 7 . The temperature-difference neutron diffraction reveals a low-𝑄 peak consistent with dipolar spin-ice correlations reported in previous work, and an alternation between positive and negative net intensity at higher 𝑄. These features are consistent with our numerical-linked-cluster calculations using pseudospin interaction parameters previously reported for Ce 2 Zr 2 O 7 , Ce 2 Sn 2 O 7 , and Ce 2 Hf 2 O 7 . Importantly, neither the measured data nor any of the NLC calculations show evidence for increased scattering at high 𝑄 resulting from octupolar correlations. We conclude that at the lowest attainable temperature for our measurements (𝑇 = 0.05 K), scattering from octupolar correlations in Ce 2 Zr 2 O 7 is not present in the neutron diffraction signal on the level of our observation threshold of around 0.1% of the low-𝑄 dipole scattering. We compare these results to those obtained earlier on powder Ce 2 Sn 2 O 7 and Ce 2 Hf 2 O 7 , and to low-energy inelastic neutron scattering from single-crystal Ce 2 Zr 2 O 7 .
36 MATERIALS SCIENCE↗
X-ray spectroscopic investigation of crystal fields in Ce 2 Rh 1 − x Ir x In 8 heavy fermions
The higher dimensionality in the crystal fields of the Ce 2 M In 8 ( M = Rh , Ir ) compounds and its interplay with hybridization and disorder are key ingredients to understand the complex phase diagrams by this family, which have been explored extensively by macroscopic techniques. Here, we present an investigation of the crystal-electric field schemes of Ce 2 Rh 1 − x Ir x In 8 using x-ray absorption spectroscopy. Our full multiplet calculations for the 4 f 1 configuration of Ce 3 + to describe the temperature-dependent linear dichroism in Ce 2 M In 8 are consistent with a Γ 7 1 = 1 − α 2 · | ∓ 3 2 〉 − | α | · | ± 5 2 〉 ground state containing a predominant | ± 3 / 2 〉 contribution that increases further with x . This enhancement is believed to favor superconductivity in Ce-based heavy fermion materials, observed in previous results in the Ce M In 5 family. Our recent observations shed light on the unexpected emergence of the ambient-pressure superconducting dome in the center of the composition phase diagram and its subsequent suppression on the Ir-rich side due to the early onset of fluctuations associated with the structurally more disordered state, inferred from previous neutron magnetic diffraction experiments. Published by the American Physical Society 2024
Christovam, D. S. (ORCID:0000000250698107)↗
Kondo effect in ferromagnetic quantum critical CeRh 6 Ge 4
The mechanism of a pressure-induced quantum critical point in the heavy fermion ferromagnet CeRh 6 Ge 4 has attracted interest, as ferromagnetic quantum criticality in a clean itinerant Ce compound is typically avoided. The localized versus itinerant character of the 4𝑓 electrons is a key aspect for understanding this behavior. We investigated the electronic structure of the 4𝑓 shell in CeRh 6 Ge 4 using core-level photoelectron and x-ray absorption spectroscopy, demonstrating the hybridization of Ce 4𝑓 with the conduction electrons. Linearly polarized x-ray absorption reveals a temperature-dependent linear dichroism consistent with the crystal-electric-field sequence as inferred from the static susceptibility. This dichroism cannot be described by an ionic full-multiplet model alone, but is reproduced by including the Kondo effect within a single-impurity Anderson model in the noncrossing approximation. The Kondo effect mixes higher-lying crystal-field states into a resulting multiorbital ground state with 4𝑓 occupancy, 𝑛 𝑓 ∼ 0.9. Deviations at low temperatures between the measured linear dichroism and calculated dichroism suggest an orbital-dependent Kondo effect. A scenario in which there is a multiorbital ground state and orbital-dependent Kondo hybridization should be a starting point for a model of pressure-induced criticality in CeRh 6 Ge 4 .
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Sm 2 Ru 3 Sn 5 : A Noncentrosymmetric Cubic Member of the Ln 2 M 3 X 5 Family
An optimized synthetic method is presented for Sm 2 Ru 3 Sn 5 and investigate its physical properties and electronic structure. Sm 2 Ru 3 Sn 5 is prepared by arc-melting stoichiometric ratios of the elements and is confirmed by single crystal and powder X-ray diffraction. An antiferromagnetic transition is observed at T N = 3.8 K. A modified Curie-Weiss fit to the data in the range 50–150 K yields a Curie-Weiss temperature: θ CW = −36.6 K and an effective magnetic moment: μ eff = 0.83 μ B , in agreement with a Sm 3+ oxidation state. Field-dependent magnetization up to H = 7 T at 2 K shows a maximum response of 0.06 μ B , which is significantly lower than the expected Sm 3+ saturation moment (0.71 μ B ). Resistivity measurements indicate metallic behavior, and analysis of the magnetic entropy from the heat capacity reveals a doublet ground state due to crystal electric field splitting. The electronic structure and density of states are calculated with density function theory and further supported by the local density approximation with dynamical mean-field theory. Finally, the experimental and computational results highlight localized Sm 3+ moments and suggest a possible interplay between Ruddelman–Kitel–Kasuya–Yosida and Kondo interactions, positioning Sm 2 Ru 3 Sn 5 as a promising material for studying topology and complex physical phenomena.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Giant elastocaloric effect at low temperatures in TmVO 4 and implications for cryogenic cooling
Adiabatic decompression of paraquadrupolar materials has significant potential as a cryogenic cooling technology. We focus on TmVO 4 , an archetypal material that undergoes a continuous phase transition to a ferroquadrupole-ordered state at 2.15 K. Above the phase transition, each Tm ion contributes an entropy of k B ln 2 due to the degeneracy of the crystal electric field groundstate. Owing to the large magnetoelastic coupling, which is a prerequisite for a material to undergo a phase transition via the cooperative Jahn–Teller effect, this level splitting, and hence the entropy, can be readily tuned by externally induced strain. Using a dynamic technique in which the strain is rapidly oscillated, we measure the adiabatic elastocaloric response of single-crystal TmVO 4 , and thus experimentally obtain the entropy landscape as a function of strain and temperature. The measurement confirms the suitability of this class of materials for cryogenic cooling applications and provides insight into the dynamic quadrupole strain susceptibility.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Growth-controlled twinning and magnetic anisotropy in CeSb 2
Cerium diantimonide (CeSb 2 ) is a layered heavy-fermion Kondo lattice material that hosts complex magnetism and pressure-induced superconductivity. The interpretation of its in-plane anisotropy has remained unsettled due to structural twinning, which superimposes orthogonal magnetic responses. Here, in this study, we combine controlled crystal growth with magnetization and rotational magnetometry to disentangle the effects of twinning. Nearly untwinned high-quality single crystals reveal the intrinsic in-plane anisotropy: The in-plane easy axis saturates at 𝑀 easy (4 T) ≈ 1.8 𝜇 B /Ce, while the in-plane hard axis magnetization is strongly suppressed, nearly linear, and comparable to the out-of-plane response. These results resolve long-standing discrepancies in reported magnetic measurements, in which in-plane metamagnetic transition fields and saturation magnetization varied significantly across previous studies. Growth experiments demonstrate that avoiding the proposed 𝛼 𝛽 structural transition—through Sb-rich flux and slower cooling—systematically reduces twinning. However, powder x-ray diffraction and differential thermal analysis measurements show no clear evidence of a distinct 𝛽 phase. Our results establish a consistent magnetic phase diagram and provide essential constraints for crystal-electric field models, enabling a clearer understanding of the interplay between anisotropic magnetism and unconventional superconductivity in CeSb 2 .
Weber, Jan T. [Goethe University, Frankfurt (Germa↗
Magnetic ground state and perturbations of the distorted kagome Ising metal TmAgGe
Here, we present the magnetic orders and excitations of the distorted kagome intermetallic magnet TmAgGe. Using neutron single crystal diffraction we identify the propagation vectors k = ($\frac{1}{2}$ 0 0) and k = (0 0 0) and determine the magnetic structures of the zero-field and magnetic field-induced phases for H along the $\mathcal{a}$ and [–110] crystal directions. We determine the experimental magnetic field- temperature (H, T)-phase diagram and reproduce it by Monte Carlo simulations of an effective spin exchange Hamiltonian for one distorted kagome layer. Our model includes a strong axial single-ion anisotropy and significantly smaller exchange couplings, which span up to the third-nearest neighbors within the layer. Single crystal inelastic neutron scattering (INS) measurements reveal an almost flat, only weakly dispersive mode around 7 meV that we use alongside bulk magnetization data to deduce the crystal-electric field (CEF) scheme for the Tm 3+ ions. Random phase approximation (RPA) calculations based on the determined CEF wave functions of the two lowest quasidoublets enable an estimation of the interlayer coupling that is compatible with the experimental INS spectra. No evidence for low-energy spin waves associated to the magnetic order was found, which is consistent with the strongly Ising nature of the ground state.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗