An EPR study of the temperature dependence of the energy gap in ytterbium dodecaboride

An EPR study of ytterbium dodecaboride (YbB₁₂) showed the presence of an energy gap with a width of 2Δ=12 meV in the energy spectrum of this Kondo insulator. The temperature dependence of the energy gap was determined by interpreting the experimental data within the framework of the exciton dielectric model: Δ(T)=72 K at an absolute zero and Δ(T)=0 at ～115 K. The temperature dependence of the EPR line-width exhibits a feature at 13–15 K, which is indicative of a finite density of states inside the gap. This can be related to the presence of impurity states or bound polaron excitations in the electron spectrum of YbB₁₂.     

On the nature of the energy gap in ytterbium dodecaboride YbB12

The YbB₁₂ Kondo insulator was studied by EPR. The energy spectrum of YbB₁₂ was established to have a gap of width 2Δ≈12 meV. The experimental data are interpreted in terms of the excitonic-dielectric model. The temperature behavior of the gap was determined; it was found that Δ(T)=72 K at absolute zero and vanishes at ～115 K.     

Electron paramagnetic resonance in Kondo insulators

This review is devoted to the application of electron paramagnetic resonance (EPR) in the study of fluctuating-valence materials, which are characterized by a narrow gap in the electron energy spectrum (Kondo insulators or Kondo semiconductors). The authors’ papers on studying classical objects of this field of solid-state physics, SmB₆ and YbB₁₂, are considered as an illustration of the potentiality of the EPR method. Temperature dependences of the gap width in these materials were obtained, the static and dynamic Jahn-Teller effects on Sm³⁺ ions in SmB₆ were detected, and the formation of Yb³⁺ ion pairs and the spontaneous breaking of cubic symmetry in YbB₁₂ were observed. The results obtained indicate that preference should be given to the exciton-polaron model developed by Kikoin et al. for the ground state of Kondo insulators.     

Mössbauer study of 170Yb in some metals and metal-like compounds. Isomer shift measurements and intermediate valence states of ytterbium

We present here a Mössbauer effect study of the isomer shift and the magnetic behaviour of ¹⁷⁰Yb in compounds with metallic character where the ytterbium valence state ranges from a practically divalent and diamagnetic state (Yb in Ag, Tm and Yb) to a trivalent magnetic state (Yb in Au). Intermediate valence states, all of them probably non-magnetic, have been observed in YbAl₂, TmAl₂, YbAl₃ and Al as well as in the borides YbB₄, YbB₁₂ and TmB₁₂.In Tm metal, in addition to the quadrupole interaction, a weak magnetic hyperfine interaction is visible on ¹⁷⁰Yb below T_N.     

Magnetic dynamics in an electron-doped YbB<Subscript>12</Subscript> condo insulator

Neutron spectroscopy results are presented for electron-doped YbB₁₂-based systems. Electrondoping effects were obtained through the partial substitution of Yb for four-valence Zr, and through the introduction of a small amount of carbon in the boron sublattice of a YbB₁₂ condo insulator. The different character of the original YbB₁₂ gap-type magnetic-excitation-spectrum changes was observed.     

Low temperature properties of a new compound based on (Yb,Zr)B<Subscript>12</Subscript> substitution

The thermodynamic, kinetic, and magnetic characteristics of (Yb, Zr)B₁₂ have been investigated in detail in the temperature range from 4 to 300 K to reveal the effect of the band structure on the properties of the YbB₁₂ Kondo insulator ground state. It is found that electron doping due to 20% substitution in the Yb sublattice significantly changes the properties of the low-temperature ground state of YbB₁₂ Kondo insulator and only slightly affects the high-temperature spin-fluctuating state forming at T > 50 K.     

XPS study of rare earth dodecaborides: TmB12, YbB12 and LuB12

Single phase TmB₁₂, YbB₁₂ and LuB₁₂ were successfully obtained by the arc-melting and floating zone methods. The XPS measurements of these compounds have indicated that (1) only YbB₁₂ is a valence fluctuating compound with the valency of +2.9 at room temperature, (2) the correlation energy between Yb²⁺ and Yb³⁺ ions is about 6.0 eV, (3) Tm and Lu ions in the dodecaborides are in a trivalent state, and (4) the binding energy of the 4f level in TmB₁₂ and LuB₁₂ is 5.0 and 7.7 eV, respectively.     

Electron paramagnetic resonance of Yb<Superscript>3+</Superscript> ions in a concentrated YbRh<Subscript>2</Subscript>Si<Subscript>2</Subscript> compound with heavy fermions

The EPR signal from localized ytterbium ions was observed in an undoped YbRh₂Si₂ compound with heavy fermions in the temperature range from 1.5 to 25 K. The exponential contribution dominating the temperature dependence of EPR line width at temperatures above 15 K was shown to be caused by the random transitions from the ground to the first excited Stark sublevel of the Yb³⁺(4f¹³) ion with the activation energy Δ=115 K.     

Magnetic excitations in systems with a nonmagnetic ground state and valence fluctuations

The neutron inelastic scattering data for the spectra of f-electron excitations in valence-unstable semiconductors SmB₆, SmS, and YbB₁₂ are discussed. The conclusions on the relation of the spectral characteristics to the nature of exciton-type states formed in these systems are reported.     

Bulk and surface electron transport in topological insulator candidate YbB6–δ

We report the study of transport and magnetic properties of the YbB_6–δsingle crystals grown by inductive zone melting. A strong disparity in the low temperature resistivity, Seebeck and Hall coefficients is established for the samples with the different level of boron deficiency. The effective parameters of the charge transport in YbB_6–δ are shown to depend on the concentration of intrinsic defects, which is estimated to range from 0.09% to 0.6%. The pronounced variation of Hall mobility μ_H found for bulk holes is induced by the decrease of transport relaxation time from τ ≈ 7.7 fs for YbB_5.994 to τ ≈ 2.2 fs for YbB_5.96. An extra contribution to conductivity from electrons with μ_H≈ –1000 cm² V^–1 s^–1 and the very low concentration n /n_Yb≈ 10^–6 discovered below 20 K for all the single crystals under investigation is suggested to arise from the surface electron states appeared in the inversion layer due to the band bending. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Dimer self-organization of impurity ytterbium ions in synthetic forsterite single crystals

Paramagnetic centers formed by impurity Yb³⁺ ions in synthetic forsterite (Mg₂SiO₄) grown by the Czochralski technique are studied by X-band CW and pulsed EPR spectroscopy. These centers are single ions substituting magnesium in two different crystallographic positions denoted М1 and М2, and dimer associates formed by two Yb³⁺ ions in nearby positions М1. It is established that there is a pronounced mechanism favoring self-organization of ytterbium ions in dimer associates during the crystal growth, and the mechanism of the spin–spin coupling between ytterbium ions in the associate has predominantly a dipole–dipole character, which makes it possible to control the energy of the spin–spin interaction by changing the orientation of the external magnetic field. The structural computer simulation of cluster ytterbium centers in forsterite crystals is carried out by the method of interatomic potentials using the GULP 4.0.1 code (General Utility Lattice Program). It is established that the formation of dimer associates in the form of a chain parallel to the crystallographic axis consisting of two ytterbium ions with a magnesium vacancy between them is the most energetically favorable for ytterbium ions substituting magnesium in the position М1.     

EPR of Yb<Superscript>3+</Superscript> ions in a monoclinic KY(WO<Subscript>4</Subscript>)<Subscript>2</Subscript> single crystal

The electron paramagnetic resonance (EPR) of Yb³⁺ ions in a KY(WO₄)₂ single crystal was investigated at T=4.2 K and fixed frequency of 9.38 GHz. The resonance absorption observed on the lowest Kramers doublet represents the complex superposition of three spectra, corresponding to the ytterbium isotopes with different nuclear moments. The EPR spectrum is characterized by a strong anisotropy of the g-factors. The temperature dependence of the g-factors is shown to be caused by the strong spin-orbital and orbital-lattice coupling. The resonance lines broaden with increasing temperature due to the short spin-lattice relaxation times.     

EPR and optical properties of KYb(WO4)2 and KTb0.2Yb0.8(WO4)2 single crystals

Well oriented KYb(WO₄)₂ and KTb_0.2Yb_0.8(WO₄)₂ single crystals have been investigated for their magnetic and optical properties using the Raman and EPR techniques. The registered EPR signal is dominated by three lines ascribed to ytterbium ions: one main and two satellites. Tb ions, although non-paramagnetic, distinctly modify magnetic properties of the KYb(WO₄)₂ single crystal. Basic parameters of the spin Hamiltonian, including Zeeman and hyperfine terms (g and A matrices) as well the spatial orientation between principal and crystallographic axes systems were determined for both crystals.     

Spin dynamics in Yb- and Sm-based systems with the nonmagnetic ground state

The nature and formation conditions of the ground state and the dynamic magnetic response (the magnetic excitation spectrum) of the YbB₁₂ and Sm(Y)S systems with the nonmagnetic ground state have been studied using inelastic neutron scattering.     

Study of gamma/X‐ray interaction in Kondo insulators

We have studied the gamma/X‐ray interaction parameters such as mass attenuation coefficient, mean free path (λ), half value layer, 10th value layer, energy‐absorption buildup factors, and variation of electrical conductivity with the energy of gamma/X‐ray between 1 keV and 100 GeV for Kondo insulators such as FeSi, Ce₃Bi₄Pt₃, SmB₆, YbB₁₂, CeNiSn, CeRhSb, CeRuSn, and CeFeP. The variation of photon interaction parameters with incident energy of gamma/X‐ray is graphically represented. We have also identified the X‐ray absorption edges for the Kondo insulators. This identified X‐ray absorption edges are useful in the characterizing of Kondo insulators such as FeSi, Ce₃Bi₄Pt₃, SmB₆, YbB₁₂, CeNiSn, CeRhSb, CeRuSn, and CeFeP. Hence, these data are also useful in the material science. This type of data for Kondo insulators is not available in literature. Hence, this work is first of its kind, and it is useful in various fields of nuclear physics.     

Electron paramagnetic resonance of some lanthanide ions in yttrium and lutetium vanadate

The electron paramagnetic resonance (EPR) parameters have been determined for the trivalent ions neodymium, erbium and ytterbium, at low abundance in lutetium vanadate. Crystal field splittings produce ground states which are Kramers doublets, and measurements were made of theg-values both parallel and perpendicular to the tetragonal axis; they are compared with previous measurements in yttrium vanadate. Two of the crystal field splitting parameters of the trivalent gadolinium ion (S = 7/2) in LuVO₄ show noticeable difference from those in YVO₄.     

Asymmetric Anderson model and spin excitations in the Kondo insulator YbB<Subscript>12</Subscript>

A cluster problem is analyzed as an example demonstrating that the observed three-mode behavior of spin-triplet excitations in YbB₁₂ can be described by the asymmetric Anderson model with insulating singlet ground state. In the case of an infinite system, it is argued that the behavior of the f subsystem can be analyzed by using an effective Hamiltonian ? _J with direct antiferromagnetic f-f exchange interaction. The spin excitation spectrum is shown to have a minimum at the antiferromagnetic vector, as observed experimentally. A distinctive feature of the analysis is the use of singlet and triplet basis operators.     

Electron paramagnetic resonance in YbNiAl<Subscript>2</Subscript> single crystals with strong magnetic anisotropy

Anisotropy in the magnetic properties of YbNiAl₂ intermetallide has been studied. Electron paramagnetic resonance (EPR) signals assigned to the localized magnetic moments of trivalent ytterbium have been detected at temperatures below 20 K. Spin–lattice relaxation processes like the Orbach–Aminov process with participation of the first excited Stark sublevel of the Yb³⁺ ion with an energy of 96 K govern electron–spin dynamics and the disappearance of spectrum lines with a further increase in temperature. Strong magnetic anisotropy effects are discussed as a main reason for the appearance of electron paramagnetic resonance.     

Laser cooling and trapping of ytterbium atoms

The experiments on the laser cooling and trapping of ytterbium atoms are reported, including the two-dimensional transversal cooling, longitudinal velocity Zeeman deceleration, and a magneto-optical trap with a broadband transition at a wavelength of 399 nm. The magnetic field distributions along the axis of a Zeeman slower were measured and in a good agreement with the calculated results. Cold ytterbium atoms were produced with a number of about 10⁷ and a temperature of a few milli-Kelvin. In addition, using a 556-nm laser, the excitations of cold ytterbium atoms at ¹S₀-³P₁ transition were observed. The ytterbium atoms will be further cooled in a 556-nm magneto-optical trap and loaded into a three-dimensional optical lattice to make an ytterbium optical clock.      

Self-consistent theory of pressure on doped Kondo insulators

The effects of pressure on doped Kondo insulators are studied in the framework of slave-boson mean-field theory under the coherent potential approximation. A unified picture is presented for both the h-type Kondo insulators (SmB₆ and YbB₁₂) and the e-type Kondo insulators (Ce₃Bi₄Pt₃). The f-electron's density of states within the whole range of the concentration of nonmagnetic atoms are calculated self-consistently under various pressures. The variation of the energy gap and the Kondo temperature with pressure are also obtained. These theoretical results can be considered as a possible approach to study systematically the development of the gap behavior by doping and by mechanical pressure.