Influence of crystal-field on the thermal conductivity of PrNi5

The thermal conductivity of PrNi₅ in the temperature range 0·5 to 20 K is investigated. The analysis of crystalline electric field effect on the experimentally obtained temperature dependence of the thermal conductivity indicates that beside the main electronic component also other contributions (phonons, magnetic excitons) can take place in the heat transport in this intermetallic compound.The authors would like to thank Dr. G. Eska for lending the sample and Dr. . Jáno for his interest in the work and stimulating discussions.     

Magnetic structure of rare-earth dodecaborides

We have investigated the magnetic structure of HoB₁₂, ErB₁₂ and TmB₁₂ by neutron diffraction on isotopically enriched single-crystalline samples. Results in zero field as well as in magnetic field up to 5 T reveal modulated incommensurate magnetic structures in these compounds. The basic reflections can be indexed with q=(1/2±δ, 1/2±δ, 1/2±δ), where δ=0.035 both for HoB₁₂ and TmB₁₂ and with q=(3/2±δ, 1/2±δ, 1/2±δ), where δ=0.035, for ErB₁₂. In an applied magnetic field, new phases are observed. The complex magnetic structure of these materials seems to result from the interplay between the RKKY and dipole-dipole interaction. The role of frustration due to the fcc symmetry of dodecaborides and the crystalline electric field effect is also considered.     

Quantum oscillations and the Sermi surface of LuB12

To examine the Fermi surface of LuB₁₂, measurements of the de Haas-van Alphen (dHvA) effect were made at temperatures between 0.35 and 2 K in magnetic fields up to 12 Tesla. Oscillations in the susceptibility occurred above 5 Tesla in any field direction relative to the single crystal sample. From the Fourier transform of the data obtained, we conclude the Fermi surface of both conduction bands to have multiple extremal cross sections. For some of these orbits, the temperature dependence of the dHvA signal was investigated to determine the corresponding cyclotron mass. For a better understanding, a Full Potential Linearized Augmented Plane Wave-(FLAPW-) band structure calculation was carried out and the shapes of the Fermi surfaces were determined. In addition, we investigated the transverse magnetoresistance as a function of the field and the field direction. Its anisotropy, as well as the Shubnikov-de Haas (SdH) oscillations occurring in certain geometries, are in agreement with the results of the dHvA measurements.     

Separation of the contributions to the magnetization of Tm1 − x Yb x B12 solid solutions in steady and pulsed magnetic fields

The magnetization of substitutional Tm_{1 ? x} Yb _x B₁₂ solid solutions is studied in the composition range 0 < x ≤ 0.81. The measurements are performed at low temperatures (1.9–300 K) in steady (up to 11 T) and pulsed (up to 50 T, pulse duration of 20–100 ms) magnetic fields. An analysis of the experimental data allowed the contributions to the magnetization of the paramagnetic phase of the Tm_{1 ? x} Yb _x B₁₂ compounds to be separated. These contributions include a Pauli component, which corresponds to the response of the heavy-fermion manybody states that appears in the energy gap in the vicinity of the Fermi level (density of states (3?4) × 10²¹ cm^?3 meV^?1), and a contribution with saturation in high magnetic fields attributed to the localized magnetic moments ((0.8–3.7)μ_B per unit cell) of the nanoclusters formed by rare-earth ions with an antiferromagnetic interaction.     

Magnetic Anisotropy of the Low-Temperature Specific Heat of Ho0.01Lu0.99B12 with Dynamic Charge Stripes

JETP Letters - The specific heat of Ho0.01Lu0.99B12 single crystals with dynamic charge stripes is studied at low and ultralow temperatures (0.07–20 K) in applied magnetic fields of 0–9...     

Features of the Crystal Structure of Tm1–xYbxB12 Dodecaborides near a Quantum Critical Point and at a Metal–Insulator Transition

JETP Letters - The magnetic phase diagram of Tm1–xYbxB12 antiferromagnets is determined from the specific heat measurements performed at low and ultralow temperatures (0.07–10 K)....     

Dynamics of boron nanoclusters in RB12 (R = Yb,Lu) systems

The phonon dispersion in RB₁₂ (R = Yb, Lu) systems is studied in detail using inelastic neutron scattering over a wide range of energies (up to 55 meV). It is demonstrated that the main features of the lattice dynamics in the RB₁₂ crystals can be described in terms of a simple model of force constants with a strong hierarchy of interatomic interactions in the systems: B-B ≫ B-R ≫ R-R. An interesting feature of the low-energy portion of the phonon spectrum is the weakly dispersive optical mode. According to the model calculations, this mode is associated with the relative displacements and “breathing” vibrations of the boron nanoclusters B₁₂.

     

Specific features of the formation of the ground state in PrB6

The temperature dependences of the thermal conductivity and magnetization of a PrB₆ single crystal in magnetic fields of 0–14 T have been measured. An analysis of the data amassed has revealed that, apart from the well-known magnetic phase transitions at T ～ 7 and ～4 K, there appears a spontaneous magnetization with a relatively small magnetic moment in PrB₆ at temperatures below 20 K, which is initiated by the splitting of the ground state, apparently, due to dynamic structure distortions.     

Effects of disorder and isotopic substitution in the specific heat and Raman scattering in LuB12

Precision measurements of the specific heat and spectral intensity I(ω) of Raman scattering for Lu ^N B₁₂ single crystal samples with various boron isotopes (N = 10, 11, nat) have been performed at low and intermediate temperatures. A boson peak in the low-frequency part of the I(ω) spectrum has been observed for the first time for lutetium dodecaboride at liquid nitrogen temperatures. It has been shown that low-temperature anomalies in the specific heat, along with the features of Raman spectra, can be interpreted in terms of the transition to a cageglass state at T* = 50?70 K, which appears when Lu³⁺ ions are displaced from the centrosymmetric position in cavities of a rigid covalent boron sublattice towards the randomly located boron vacancies. The concentrations of various two-level systems that correspond to two types of vibrational clusters with correlation lengths of 12–15 and 18–22 Å, respectively, have been estimated. The vibrational density of states of LuB₁₂ has been calculated from Raman spectra in the model of soft atomic potentials. An approach has been proposed to explain the dielectrization of the properties of the YbB₁₂ compound at T < T*, as well as the features of the formation of magnetic structures in RB₁₂ antiferromagnets (R = Tb, Dy, Ho, Er, Tm) and the suppression of superconductivity in LuB₁₂.     

Anomalies of the specific heat near the quantum critical point in Tm<Subscript>0.74</Subscript>Yb<Subscript>0.26</Subscript>B<Subscript>12</Subscript>

The behavior of the specific heat near the quantum critical point x ∼ 0.3 in the Tm_{1 − x} Yb _x B₁₂ system has been studied. Detailed measurements have been performed on high-quality single-crystalline Tm_0.74Yb_0.26B₁₂ samples within a wide temperature range of 1.9–300 K in a magnetic field up to 9 T. The temperature dependence of the magnetic contribution to the specific heat has a logarithmic divergence of the form C/T ∼ lnT at T < 4 K, which can be attributed to the quantum critical behavior regime suppressed by the external magnetic field. The Schottky anomaly of the magnetic contribution to the specific heat in Tm_0.74Yb_0.26B₁₂ has been analyzed.