We report dielectric spectroscopy of single-crystalline NdMgAl11O19, a magnetoplumbite hexaaluminate in which localized Nd^3+ moments coexist with a polarizable AlO5 bipyramidal network. The real part of the permittivity, ε'_c(T), measured along the crystallographic c axis, increases as the temperature is lowered from 275 K to 30 K and is frequency-independent between 4 Hz and 50 kHz. At lower temperatures, a frequency-dependent decrease in permittivity is observed, followed by a further upturn below 2 K. The high-frequency ε'_c(T) is described by a Barrett formula supplemented by an effective two-level contribution, yielding a robust gap of Δ= 25.85 ± 0.32 K consistent with the lowest Nd^3+ crystal-electric-field (CEF) splitting. Below ∼ 30 K, the dielectric response becomes strongly frequency and magnetic-field dependent. Isothermal ε_c'(H) measurements reveal a reproducible low-field crossover near μ_0H_c ≃ 0.85 T, which we attribute to the competition between antiferromagnetic correlations and Zeeman splitting of the ground-state Kramers doublet. NdMgAl11O19 thus provides a Kramers reference system in which dielectric signatures of the excited-state CEF manifold can be distinguished from those of the field-tuned, correlation-dominated ground-state doublet sector in a centrosymmetric frustrated magnetoplumbite host

We report successive anomalies at low temperature in the magnetic field dependence of the thermoelectric signal in the heavy fermion compound UPd_2Al_3 inside the antiferromagnetic state up to the metamagnetic transition at H_M =18 T. Based on renormalisation perturbation theory and the partitioning of the f orbitals into localized and delocalized parts, our analysis attributes these anomalies to complex topological changes of the Fermi surface driven by Zeeman effect. The observation of a sudden change of sign both in the thermoelectric power and in the Hall coefficient at H_M in addition to the appearance of large quantum oscillations in the thermoelectric power above H_M indicate a strong Fermi surface reconstruction at the metamagnetic transition due to the unfolding of the electronic bands.

We report broadband dielectric spectra of the non-Kramers hexaaluminate PrMgAl11O19, revealing a pronounced interplay between permittivity and magnetization at cryogenic temperatures. The zero-field dielectric response follows a Barrett-type quantum-paraelectric form, while a broad dielectric anomaly near 5 K shows a complex field dependence that mirrors the multi-hump behavior of the magnetic specific heat, evidencing robust magnetoelectric coupling. The inverse permittivity epsilon('-1) (T, H) scales linearly with M-2, consistent with a biquadratic (PM2)-M-2 term in a Landau framework. Fits yield a temperature-dependent coupling constant lambda(T) that decreases with heating from (1.07 +/- 0.01) x 10(-4) mu B-2 (at 5 K) to (4.77 +/- 0.02) x 10(-5) mu B-2 (at 10 K), reflecting the thermal population of low-lying energy levels of Pr3 +. Consistently, the uniaxial thermal expansion develops an additional low-temperature hump below similar to 30 K that is progressively suppressed by magnetic field, recovering an approximately saturated response by 9 T. These results identify PrMgAl11O19 as a paradigmatic non-Kramers hexaaluminate where quantum paraelectricity and magnetoelectric interactions are intrinsically entangled, establishing hexaaluminates as a tunable platform for magnetoelectric physics in frustrated quantum materials.

We investigated the rare-earth triangular-lattice antiferromagnet NdMgAl_11O_19 using single-crystal magnetization (1.8 K ≤ T ≤ 300 K, μ_0 H ≤ 7 T) and specific-heat measurements down to 45 mK. The dc susceptibility confirms a well-isolated Kramers doublet ground state with pronounced Ising-type anisotropy, with g_c ≈ 3.7 and g_ab≈ 1.45. Curie–Weiss fits yield weak, anisotropic antiferromagnetic exchange, with θ_c = -0.54 K and θ_ab = -0.87 K. Heat-capacity measurements show no long-range magnetic order down to 40 mK, corresponding to a frustration index f ≳ 20. Instead, C_m/T exhibits a broad maximum near 0.081 K whose magnitude and field evolution are consistent with short-range correlations in an anisotropic triangular lattice. Applied magnetic fields open a Zeeman gap where the specific-heat anomaly follows Δ= g μ_B μ_0 H, and M(H,T) is well described by a Brillouin function for an effective J = 1/2 moment. The field tuning of the low-temperature entropy manifold allows self-cooling from 1.8 K to 53 mK by adiabatic demagnetisation from a 9 T field. These results identify NdMgAl_11O_19 as a nearly ideal weak-exchange triangular magnet with a field-tunable correlated ground state, where two-dimensional crossover effects may emerge from frustrated XXZ interactions.

We report dielectric spectroscopy of single-crystalline NdMgAl 11 O 19 , a magnetoplumbite hexaaluminate in which localized Nd 3 + moments coexist with a polarizable AlO 5 bipyramidal network. The real part of the permittivity ɛ c ′ ( T ) , measured along the crystallographic c axis, increases as the temperature is lowered from 275 K to 30 K and is frequency-independent between 4 Hz and 50 kHz. At lower temperatures, a frequency-dependent decrease in permittivity is observed, followed by a further upturn below 2 K. The high-frequency ɛ c ′ ( T ) is described by a Barrett formula supplemented by an effective two-level contribution, yielding a robust gap of Δ = 25.9 ( 3 ) K consistent with the lowest Nd 3 + crystal-electric-field (CEF) splitting. Below ∼ 30 K , the dielectric response becomes strongly frequency and magnetic-field dependent. Isothermal ɛ c ′ ( H ) measurements reveal a reproducible low-field crossover near μ 0 H c ≃ 0.85 T , which we attribute to the competition between antiferromagnetic correlations and Zeeman splitting of the ground-state Kramers doublet. NdMgAl 11 O 19 thus provides a Kramers reference system in which dielectric signatures of the excited-state CEF manifold can be distinguished from those of the field-tuned, correlation-dominated ground-state doublet in the centrosymmetric frustrated magnetoplumbite host.

We report the magnetic properties of the quantum triangular lattice antiferromagnet (TLAF) PrMgAl11O19 through magnetization and specific heat measurements. Strong magnetic anisotropy indicates the realization of an Ising-like magnetism in PrMgAl11O19 single crystal while no long-range magnetic ordering is realized down to 0.4 K. The splitting of the low-lying quasi-doublet into two singlets suggested by experimental data, is consistent with an effective pseudospin-1/2 scenario. The observed gapless excitations in zero field are attributed to induced quantum magnetism, they would be induced by magnetic interactions of an energy scale comparable with the splitting between the two singlets. Based on these results, we modeled the magnetic ground state of PrMgAl11O19 by a quantum Ising magnet with an intrinsic transverse field rather than a quantum spin liquid (QSL). In addition, our data show a non-monotonous response of the low-temperature specific heat to the external fields revealing a complex interplay between intrinsic and external magnetic fields.

We report dielectric and magnetoelectric studies of single-crystalline CeMgAl11O19, a Kramers triangular magnet embedded in a polarizable hexaaluminate lattice. In zero magnetic field, the permittivity epsilon'(T ) follows the Barrett law of a quantum paraelectric down to <^>25 K, below which a broad minimum develops near 3 K without evidence of static ferroelectric or magnetic order. Application of magnetic fields up to 9 T shifts this minimum to higher temperatures and broadens it, evidencing a tunable magnetoelectric response. The magnetoelectric coupling was characterized using results from magnetization measurements. The anomaly temperature T*, extracted from the local minimum of epsilon'(T ), exhibits a linear dependence on the squared magnetization M2, consistent with the biquadratic magnetoelectric coupling allowed in centrosymmetric systems. This magnetoelectric effect, mediated by spin-orbit-entangled Kramers doublets interacting with a frustrated antipolar liquid, establishes CeMgAl11O19 as a prototype for exploring quantum magnetoelectricity in frustrated systems.

The first-order transition line in the H-T phase diagram of itinerant electron metamagnets terminates at the critical end point-analogous to the critical point on the gas-liquid condensation line in the p-T phase diagram. To unravel the impact of critical magnetic fluctuations on the crystal lattice of a metamagnet at the critical end point, we performed an ultrasonic study of the itinerant electron metamagnet UTe2 across varying temperatures and magnetic fields. At temperatures exceeding 9 K, a distinct V-shaped anomaly emerges, precisely centered at the critical field of the metamagnetic transition in the isothermal field dependence of elastic constants. This anomaly arises from lattice instability, triggered by critical magnetic fluctuations via strong magnetoelastic interactions. Remarkably, this effect is maximized precisely at the critical-end-point temperature. Comparative measurements of another itinerant metamagnet, UCoAl, reveal intriguing commonalities. Despite significant differences in the paramagnetic ground state, lattice symmetry, and the expected metamagnetic transition process between UTe2 and UCoAl, both exhibit similar anomalies in elastic properties near the critical end point.

The study of magnetic frustration in classical spin systems is motivated by the prediction and discovery of classical spin liquid states. These uncommon magnetic phases are characterized by a massive degeneracy of their ground state implying a finite magnetic entropy at zero temperature. While the classical spin liquid state is originally predicted in the Ising triangular lattice antiferromagnet in 1950, this state has never been experimentally observed in any triangular magnets. The discovery of an electric analogue of classical spin liquids on a triangular lattice of uniaxial electric dipoles in EuAl12O19 is reported here. This new type of frustrated antipolar phase is characterized by a highly-degenerate state at low temperature implying an absence of long-range antiferroelectric order, despite short-range antipolar correlations. Its dynamics are governed by a thermally activated process, slowing down upon cooling toward a complete freezing at zero temperature.

We report ferromagnetic ordering at T-C = 1.3 K in the quasi-two-dimensional magnet EuAl12O19 with large spins S = 7/2. / 2. This ferromagnetic state was characterized by magnetization and specific heat measurements and the experimental results were compared with classical Monte Carlo simulations. They reveal a strong single- ion anisotropy leading to a uniaxial spontaneous magnetization along the c axis. Furthermore, the application of a magnetic field in the hard magnetization plane ab reduces the Curie temperature down to T = 0.5 . 5 K at mu H-0 approximate to 0.4 . 4 T while preserving the second-order nature of the ferromagnetic transition.

The unconventional superconductor UTe 2 exhibits numerous signatures of spin-triplet superconductivity—a rare state of matter which could enable quantum computation protected against decoherence. UTe 2 possesses a complex phase landscape comprising two magnetic field-induced superconducting phases, a metamagnetic transition to a field-polarized state, along with pair- and charge-density wave orders. However, contradictory reports between studies performed on UTe 2 specimens of varying quality have severely impeded theoretical efforts to understand the microscopic origins of the exotic superconductivity. Here, we report a comprehensive suite of high magnetic field measurements on a generation of pristine quality UTe 2 crystals. Our experiments reveal a significantly revised high magnetic field superconducting phase diagram in the ultraclean limit, showing a pronounced sensitivity of field-induced superconductivity to the presence of crystalline disorder. We employ a Ginzburg–Landau model that excellently captures this acute dependence on sample quality. Our results suggest that in close proximity to a field–induced metamagnetic transition the enhanced role of magnetic fluctuations—that are strongly suppressed by disorder—is likely responsible for tuning UTe 2 between two distinct spin-triplet superconducting phases.

UTe 2 is a spin -triplet superconductor candidate for which high quality samples with long mean free paths have recently become available, enabling quantum oscillation measurements to probe its Fermi surface and effective carrier masses. It has recently been reported that UTe 2 possesses a 3D Fermi surface component [Phys. Rev. Lett. 131 , 036501 (2023)]. The distinction between 2D and 3D Fermi surface sections in triplet superconductors can have important implications regarding the topological properties of the superconductivity. Here we report the observation of oscillatory components in the magnetoconductance of UTe 2 at high magnetic fields. We find that these oscillations are well described by quantum interference between quasiparticles traversing semiclassical trajectories spanning magnetic breakdown networks. Our observations are consistent with a quasi -2D model of this material 's Fermi surface based on prior dHvA-effect measurements. Our results strongly indicate that UTe 2 -which exhibits a multitude of complex physical phenomena -possesses a remarkably simple Fermi surface consisting exclusively of two quasi -2D cylindrical sections.

The heavy fermion paramagnet UTe$_2$ exhibits numerous characteristics of spin-triplet superconductivity. Efforts to understand the microscopic details of this exotic superconductivity have been impeded by uncertainty regarding the underlying electronic structure. Here we directly probe the Fermi surface of UTe$_2$ by measuring magnetic quantum oscillations in pristine quality crystals. We find an angular profile of quantum oscillatory frequency and amplitude that is characteristic of a quasi-2D Fermi surface, which we find is well described by two cylindrical Fermi sheets of electron- and hole-type respectively. Additionally, we find that both cylindrical Fermi sheets possess considerable undulation but negligible small-scale corrugation, which may allow for their near-nesting and therefore promote magnetic fluctuations that enhance the triplet pairing mechanism. Importantly, we find no evidence for the presence of any 3D Fermi surface sections. Our results place strong constraints on the possible symmetry of the superconducting order parameter in UTe$_2$.

The spin-triplet superconductor UTe_2 exhibits a myriad of exotic physical phenomena, including the possession of three distinct superconducting phases at ambient pressure for magnetic field μ_0 H ≤ 40 T aligned in certain orientations. However, contradictory reports between studies performed on UTe_2 specimens of varying quality have severely impeded theoretical efforts to understand the microscopic properties of this material. Here, we report high magnetic field measurements on a new generation of ultraclean UTe_2 crystals grown by a salt flux technique, which possess enhanced superconducting critical temperatures and fields compared to previous sample generations. Remarkably, for H applied close to the hard magnetic b direction, we find that the angular extent of magnetic field-reinforced superconductivity is significantly increased in these pristine quality crystals. This suggests that in close proximity to a field-induced metamagnetic transition the enhanced role of magnetic fluctuations - that are strongly suppressed by disorder - is likely responsible for tuning UTe_2 between two distinct spin-triplet superconducting phases. Our results reveal a strong sensitivity to crystalline disorder of the field-reinforced superconducting state of UTe_2.

Magnetic dilution of a well-established Kitaev candidate system is realized in the substitutional Ru_1-xRh_xCl_3 series (x = 0.02-0.6). Optimized syntheses protocols yield uniformly-doped single crystals and polycrystalline powders that are isostructural to the parental α-RuCl_3 as per X-ray diffraction. The Rh content x is accurately determined by the quantitative energy-dispersive X-ray spectroscopy technique with standards. We determine the magnetic phase diagram of Ru_1-xRh_xCl_3 for in-plane magnetic fields from magnetization and specific-heat measurements as a function of x and stacking periodicity, and identify the suppression of the magnetic order at x ≈ 0.2 towards a disordered phase, which does not show any clear signature of freezing into a spin glass. Comparing with previous studies on the substitution series Ru_1-xIr_xCl_3, we propose that chemical pressure would contribute to the suppression of magnetic order especially in Ru_1-xIr_xCl_3 and that the zigzag magnetic ground state appears to be relatively robust with respect to the dilution of the Kitaev–Γ–Heisenberg magnetic lattice. We also discovered a slight dependence of the magnetic properties on thermal cycling, which would be due to an incomplete structural transition.

We report a detailed experimental and theoretical study on the effect of hydrostatic pressure on the structural and magnetic aspects of the layered honeycomb antiferromagent $\alpha$-RuCl$_{3}$. Magnetic susceptibility measurements performed under almost ideal hydrostatic-pressure conditions yield that the phase transition to zigzag-type antiferromagnetic order at $T_N$ = 7.3 K can be rapidly suppressed to about 6.1 K. A further suppression with increasing pressure is impeded due to the occurrence of a pressure-induced structural transition at $p \geq$ 104 MPa, accompanied by a strong dimerization of Ru-Ru bonds, which gives rise to a collapse of the magnetic susceptibility. Whereas the dimerization transition is strongly first order, as reflected by large discontinuous changes in $\chi$ and pronounced hysteresis effects, the magnetic transition under varying pressure and magnetic field also reveals indications for a weakly first-order transition. We assign this observation to a strong magnetoelastic coupling in this system. Measurements of $\chi$ under varying pressure in the paramagnetic regime ($T > T_N$) and before dimerization ($p <$ 100 MPa) reveal a considerable increase of $\chi$ with pressure. These experimental observations are consistent with the results of ab-initio Density Functional Theory (DFT) calculations on the pressure-dependent structure and the corresponding pressure-dependent magnetic model. Comparative susceptibility measurements on a second crystal showing two consecutive magnetic transitions instead of one, indicating the influence of stacking faults. Using different temperature-pressure protocols the effect of these stacking faults can be temporarily overcome, transforming the magnetic state from a multiple-$T_N$ into a single-$T_N$ state.

Magnetic dilution of a well-established Kitaev candidate system is realized in the substitutional Ru1-xRhxCl3 series (x = 0.02-0.6). Optimized syntheses protocols yield uniformly doped single crystals and polycrystalline powders that are isostructural to the parental alpha-RuCl3 as per x-ray diffraction. The Rh content x is accurately determined by the quantitative energy-dispersive x-ray spectroscopy technique with standards. We determine the magnetic phase diagram of Ru1-xRhxCl3 for in-plane magnetic fields from magnetization and specific-heat measurements as a function of x and stacking periodicity and identify the suppression of the magnetic order at x approximate to 0.2 towards a disordered phase, which does not show any clear signature of freezing into a spin glass. Comparing with previous studies on the substitution series Ru1-xRhxCl3, we propose that chemical pressure would contribute to the suppression of magnetic order, especially in Ru1-xRhxCl3, and that the zigzag magnetic ground state appears to be relatively robust with respect to the dilution of the Kitaev-Gamma-Heisenberg magnetic lattice. We also discovered a slight dependence of the magnetic properties on thermal cycling, which would be due to an incomplete structural transition.

Matthias Gillig, 2, ∗ Xiaochen Hong, 3 Piyush Sakrikar, Gaël Bastien, A.U.B. Wolter, Leonie Heinze, Satoshi Nishimoto, 6 Bernd Büchner, 2, 7 and Christian Hess 3, 7 Leibniz Institute for Solid State and Materials Research, 01069 Dresden, Germany Institute for Solid State Physics, TU Dresden, 01062 Dresden, Germany Fakultät für Mathematik und Naturwissenschaften, Bergische Universität Wuppertal, 42097 Wupptertal, Germany Indian Institute of Science Education and Research (IISER) Mohali, Knowledge City, Sector 81, Mohali 140306, India Institut für Physik der Kondensierten Materie, TU Braunschweig, 38106 Braunschweig, Germany Department of Physics, Technical University Dresden, 01069 Dresden, Germany Center for Transport and Devices, TU Dresden, 01062 Dresden, Germany (Dated: October 6, 2021)

Recently, several putative quantum spin liquid (QSL) states were discovered in {\tilde S} = 1/2 S̃=1/2 rare-earth based triangular-lattice antiferromagnets (TLAF) with the delafossite structure. In order to elucidate the conditions for a QSL to arise, we report here the discovery of a long-range magnetic order in the Ce-based TLAF KCeS _2 2 below T_{\mathrm N} = 0.38 TN=0.38 K, despite the same delafossite structure. Finally, combining various experimental and computational methods, we characterize the crystal electric field scheme, the magnetic anisotropy and the magnetic ground state of KCeS _2 2 .