In the system Tm_{1 - x}Yb_{x}B_{12} the specific heat has been studied in a wide range of Yb-concentration in the vicinity of the quantum critical point x_{C} ≈ 0.3. The results were obtained on high quality single crystalline samples of Tm_{0.7}Yb_{0.3}B_{12} compound placed near quantum critical point, both for antiferromagnetic metals (x < x_{C}) as well as for paramagnetic insulators (x > x_{C}) within a wide temperature range of 1.9-300 K in magnetic field up to 9 T. The temperature dependence of the magnetic contribution to specific heat for Tm_{0.74}Yb_{0.26}B_{12} shows a logarithmic divergence of the form C_{mag}/T∿-lnT at T<4 K, which may be attributed to the quantum critical regime, and it is suppressed by strong external magnetic field. The Schottky anomaly of the magnetic contribution to specific heat in Tm_{1 - x}Yb_{x}B_{12} has been established and analyzed in detail.
The antiferromagnetic ground state has been studied by transverse magnetoresistance, heat capacity and magnetization measurements, which were carried out on high quality single crystals of Tm_{0.996}Yb_{0.004}B_{12} dodecaboride in strong magnetic fields at liquid helium temperatures. Both antiferromagnetic-paramagnetic (AF-P) and spin-orientation (AF1-AF2) phase transitions have been observed, and allowed to construct a complicated magnetic H-T phase diagram for this compound. Strong magnetoresistance anisotropy was found both in AF states (ρ(H||[110])/ρ(H||[111])~ 1.2 at H~ 20 kOe) and at the critical field of AF-P transition (H_{N}[100]/H_{N}[111]~ 1.25) in this magnetic metal with a simple fcc crystal structure.
We probed the evolution of the superconducting transition temperature T_c and the normal state parameters of Lu_xZr_{1-x}B_{12} solid solutions employing resistivity, heat capacity and magnetization measurements. In these studies of high-quality single crystals it was found that there are two types of samples with different magnetic characteristics. An unusually strong suppression of superconductivity in Lu_xZr_{1-x}B_{12} with a rate dT_c/dx=0.21 K/at.% of Lu was observed previously on the first "magnetic" set of crystals, and it was argued to be caused by the emergence of static spin polarization in the vicinity of non-magnetic lutetium ions. On the contrary, the second (current) set of "nonmagnetic" crystals demonstrates a conventional T_c(x) dependence with a rate dT_c/dx=0.12 K/at.% of Lu which is typical for BCS-type superconductors doped by nonmagnetic impurities. The reason for this difference is yet unclear. Moreover, the H-T phase diagram of the superconducting state of Lu_xZr_{1-x}B_{12} (0 ≤ x ≤ 1) solid solutions has been deduced from magnetization measurements.
The specific heat of high quality La^{N}B_{6} (N=10, 11, natural) single crystals is investigated in a wide range of temperatures 2 - 300 K. The obtained data allow to estimate correctly (i) the electronic γ· T term of specific heat (γ ≈ 2.4 mJ/(mol·K^{2})), (ii) the contribution from quasilocal vibrating mode of La^{3+} ions (Θ_{E} ≈ 150 - 152 K), (iii) the Debye-type term from rigid boron cages (Θ_{D} ≈ 1160± 40 K). Our data also suggest an additional defect-mode component (iv) which may be attributed to a contribution of 1.5% boron vacancies in LaB_{6}. The obtained results may be interpreted in terms of formation of two level systems, which appear when La^{3+} ions are displaced from their centrosymmetric positions in the cavities of rigid boron cages, apart from randomly distributed boron vacancies in the LaB_{6} matrix.
Transport and magnetic properties of polycrystalline Tm_{0.03}Yb_{0.97}B₁₂ samples were investigated at temperatures 1.8-300 K in magnetic fields up to 9 T. The activated behavior of resistivity, the Hall coefficient and thermopower is described in terms of a narrow gap ε_g ≈ 16.6 meV, which controls the charge transport in Tm_{0.03}Yb_{0.97}B₁₂ at T>40 K. The maximum of magnetic susceptibility found at 50 K is shown to be induced by a spin gap Δ ≈ 4.7 meV being close to the half of the spin fluctuation energy in YbB₁₂. Large diffusive thermopower S=AT, A=-29.1 μV/K² and the Pauli susceptibility χ₀ ≈ 7.2×10¯³ emu/mol found below 20 K seem to be associated with the many-body resonance, which corresponds to states with an enhanced effective mass m* ≈ 250m₀ (m₀ - free electron mass). The effective parameters of magnetic centers and the analysis of anomalies favor the nonequivalent states of substitute Tm ions.
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