V. B. Filipov

Intense low-energy ferromagnetic fluctuations in the antiferromagnetic heavy-fermion metal CeB6

Heavy-fermion metals exhibit a plethora of low-temperature ordering phenomena . Among these are the so-called hidden-order phases that, in contrast to conventional magnetic order, are invisible to standard neutron diffraction experiments. One of the structurally most simple hidden-order compounds, CeB6, has been intensively studied for an elusive phase that was attributed to the antiferroquadrupolar ordering of cerium-4f moments . As the ground state of CeB6 is characterized by a more conventional antiferromagnetic (AFM) order , the low-temperature physics of this system has generally been assumed to be governed solely by AFM interactions between the dipolar and multipolar Ce moments . Here we overturn this established picture by observing an intense ferromagnetic (FM) low-energy collective mode that dominates the magnetic excitation spectrum of CeB6. Inelastic neutron-scattering data reveal that the intensity of this FM excitation significantly exceeds that of conventional spin-wave magnons emanating from the AFM wavevectors, thus placing CeB6 much closer to a FM instability than previously anticipated. This propensity for ferromagnetism may account for much of the unexplained behaviour of CeB6, and should lead to a re-examination of existing theories that have so far largely neglected the role of FM interactions.

Review of

Results of development, emission properties investigations, and applications of cathode materials based on lanthanum hexaboride (LaB₆), LaB₆-Me^IV-VIB₂, and also rhenium-tungsten (Re-W) dispenser and pressed-oxide cathodes are presented. Emission properties of a novel class of directionally crystallized lanthanum hexaboride-based eutectics have been studied. The LaB₆-Me^IV-VIB₂ composites exhibit improved emission properties and thermal cycling reliability as compared with the source lanthanum hexaboride. The most promising effective thermionic material is the eutectic composition LaB₆-VB₂. The techniques for producing pressed cathodes based on barium hafnate with tungsten and dispenser cathodes with a rhenium-tungsten matrix have been developed. It is shown that a cathode of 63% BaHfO₃-37% W (by mass) is enables a thermionic current density of 230 A/cm² at T = 2000 K. Cathodes with a rhenium-tungsten sponge with 3BaOldr0.5CaOmiddotAl₂O₃ as the emissive active substance have higher emission properties in comparison to cathodes with a tungsten sponge with normal impregnation.

\hbox{LaB}_{6}

Electron spin resonance (ESR) in strongly correlated metals is an exciting phenomenon, as strong spin fluctuations in this class of materials broaden extremely the absorption line below the detection limit. In this respect, ESR observation in CeB6 provides a unique chance to inspect Ce3+ magnetic state in the antiferroquadrupole (AFQ) phase. We apply the original high frequency (60 GHz) experimental technique to extract the temperature and angular dependences of g-factor, line width and oscillating magnetization. Experimental data show unambiguously that the modern ESR theory in the AFQ phase considering the Γ8 ground state of Ce3+ ion completely fails to predict both the g-factor magnitude and its angular dependence. Alignment of the external magnetic field along [100] axis induces a strong (more than twofold) broadening of ESR line width with respect to the other crystallographic directions and results also in the anomalous temperature dependences of the g-factor and oscillating magnetization. In this experimental geometry the latter parameter surprisingly exceeds total static magnetization by 20% at T* ~ 2.5 K. We argue that the unusual physical picture of ESR in CeB6 may be strongly affected by spin fluctuations and dynamic collective effects predominantly pronounced in [100] direction.

, Re-W Dispenser, and

CeB(6) is a model compound exhibiting antiferroquadrupolar (AFQ) order, its magnetic properties being typically interpreted within localized models. More recently, the observation of strong and sharp magnetic exciton modes forming in its antiferromagnetic (AFM) state at both ferromagnetic and AFQ wave vectors suggested a significant contribution of itinerant electrons to the spin dynamics. Here we investigate the evolution of the AFQ excitation upon the application of an external magnetic field and the substitution of Ce with non-magnetic La, both parameters known to suppress the AFM phase. We find that the exciton energy decreases proportionally to T_N upon doping. In field, its intensity is suppressed, while its energy remains constant. Its disappearance above the critical field of the AFM phase is preceded by the formation of two modes, whose energies grow linearly with magnetic field upon entering the AFQ phase. These findings suggest a crossover from itinerant to localized spin dynamics between the two phases, the coupling to heavy-fermion quasiparticles being crucial for a comprehensive description of the magnon spectrum.

\hbox{BaHfO}_{3}

The LuB12 ® LuB4 ® LuB2 phase transformations on annealing in vacuum (T=(1400÷1800) K, p < 10-2 Pa) are investigated with use of the parent single crystals. SEM and X-ray researches of the corresponding lutetium boride single crystals were carried out before and after their annealing. It is shown that the LuB12 → LuB4 phase transformation takes place in surface layer and transition region spreads inwards the single crystal bulk with time. According to the assessed Lu-B phase diagram the LuB4 phase transformation into other individual phases is impossible, and at first it is shown that under corresponding conditions the LuB4 → LuB2 spontaneous phase transformation takes place both on the surface and in the LuB4 single crystal bulk.

-W Cathode Development

For the last few decades, researchers have been intrigued by multipolar ordering phenomena and related quantum phase transitions in heavy-fermion Kondo systems. However, a criticality induced by substitution level (x), temperature (T), or magnetic field (B) is poorly understood even in the prototypical material, Ce1−xLaxB6, despite a large collection of experimental results is available. In this work, we present T–B, x–T, and x–B phase diagrams of Ce1−xLaxB6 (B || [110]). These are completed by investigating heat capacity, magnetocaloric effect (MCE), and elastic neutron scattering. A drastic increase of the Sommerfeld coefficient γ0, which is estimated from the heat capacity down to 0.05 K, is observed with increasing x. The precise T–B phase diagram including a high-entropy region is derived from the MCE analysis in which a knowledge beyond the equilibrium thermodynamics is involved. Finally, the x–B phase diagram at T = 0, which supports the existence of a quantum critical point at x > 0.75, is obtained by the same analysis. A detailed interpretation of phase diagrams strongly indicates positive correlation between the fluctuating multipoles and the effective electron mass.Phase diagrams: new features in heavy-fermion materialsThorough measurements of the phase diagrams of Ce1−xLaxB6 shed new light on the phases and properties of heavy-fermion materials. Heavy-fermion systems, in which the electrons have effective masses much higher than the mass of free electrons, exhibit different phases that can be accessed by varying the temperature (T), applied magnetic field (B) or amount of lanthanum (x). Scientists at Max Planck Institute for Chemical Physics of Solids and Technical University of Dresden have completed the T-B, x-T and x-B phase diagrams of the heavy-fermion material Ce1−xLaxB6 revealing, among other features, a new high-entropy phase and a strong correlation between multipolar fluctuations and the effective electron mass. This correlation is expected to be universal in this type of systems, thus the results should hold for other heavy-fermion materials.

Magnetic resonance anisotropy in CeB6: an entangled state of the art.

In zero magnetic field, the famous neutron spin resonance in the f-electron superconductor CeCoIn5 is similar to the recently discovered exciton peak in the non-superconducting CeB6. Magnetic field splits the resonance in CeCoIn5 into two components, indicating that it is a doublet. Here we employ inelastic neutron scattering (INS) to scrutinize the field dependence of spin fluctuations in CeB6. The exciton shows a markedly different behavior without any field splitting. Instead, we observe a second field-induced magnon whose energy increases with field. At the ferromagnetic zone center, however, we find only a single mode with a non-monotonic field dependence. At low fields, it is initially suppressed to zero together with the antiferromagnetic order parameter, but then reappears at higher fields inside the hidden-order phase, following the energy of an electron spin resonance (ESR). This is a unique example of a ferromagnetic resonance in a heavy-fermion metal seen by both ESR and INS consistently over a broad range of magnetic fields.

Magnetic field and doping dependence of low-energy spin fluctuations in the antiferroquadrupolar compound Ce

Precision measurements of charge transport characteristics (resistivity, magnetoresistance, Hall and Seebeck coefficients) and magnetic susceptibility have been carried out on high-quality single-crystals of the so-called dilute Kondo system La1-xCexB6 (χ≤0.1) in a wide temperature range 1.8–300 K. It is shown that the low temperature magnetic contribution in resistivity obeys the power law Δρ~T-α, which corresponds to the regime of weak localization of charge carriers with the critical exponent values α=0.47–0.49 for cerium concentrations χ≤ 0.1. Simultaneously an asymptotic behavior of Seebeck coefficient S~ -lnT is found together with nearly constant and negative Hall coefficient RH(T) in these dilute magnetic systems. The results of comprehensive analysis contradict to the predictions of Kondo-impurity approach for this archetypal strongly correlated electron system in dilute impurity limit. An alternative interpretation of the properties of La1-xCexB6 is developed and based on spin-polaron approach, Pauli paramagnetism and the density of states renormalization effects at low temperatures.

_{1 − x}

Spatial anisotropy generated spontaneously in the translationally invariant metallic phase, i.e. electron nematic effect, addresses a great challenge for both experimentalists and theoreticians. An interesting option for the realization of the electron nematic phase is provided by the system with orbital ordering, as long as both orbitally ordered states and electron nematic phases possess broken spatial symmetry. Here we report the detailed study of the angular dependences of the magnetoresistance in the orbitally ordered antiferroquadrupole (AFQ) phase of CeB6. Our data allowed revealing the electron nematic effect, which develops when magnetic field exceeds a critical value of 0.3–0.5T. As a result, new transition inside the AFQ phase corresponding to the change of the symmetry of magnetic scattering on spin fluctuations in CeB6 is discovered.

La

We investigate the doping-induced changes in the electronic structure of CeB