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.     

Temperature dependence of the energy gap in Bi2223 metal oxide superconductor

Spectroscopic studies of the silver-optimum-doped Bi2223 contacts show that the temperature dependence of the parameter Δ follows the BCS curve. However, the tunnel measurements performed for the same series of specimens did not reveal any temperature dependence of the energy gap Δ. The feature observed in the tunnel density of states was retained at temperatures T>T _c , manifesting the presence of the temperature-independent pseudogap E _p . The difference between the data obtained with tunnel spectroscopy and Andreev reflection spectroscopy is explained by the fact that the latter measures the true superconducting energy gap Δ_s(T), whereas the peaks of the tunneling conductivity are related to the total energy gap Δ of cuprates, which includes both the parameter Δ_s and the pseudogap $E_p :\Delta \approx \sqrt {\Delta _s^2 + E_p^2 } $ .     

Infrared spectroscopy of the intermediate-valence semiconductor YbB12

The dynamic conductivity and permittivity spectra of the intermediate-valence compound YbB₁₂ are measured in the frequency range (6–10⁴) cm^?1 (quantum energy 0.75 meV-1.24 eV) at temperatures of 5–300 K. Analysis of the spectral singularities associated with the response of free charge carriers has made it possible for the first time to determine the temperature dependences of their microscopic parameters, viz., concentration, effective mass, relaxation frequency and time, mobility, and plasma frequency. It is shown that the relaxation frequency decreases upon cooling from 300 K to the coherence temperature T ^* = 70 K for YbB₁₂, which is mainly associated with the phonon mechanism of scattering of charge carriers. For cooling below the coherence temperature T ^* = 70 K, the temperature dependence of the relaxation frequency for charge carriers of the Fermi-liquid type is found to be γ ～ γ₀ + T ², while their effective mass and relaxation time increase, respectively, to m ^*(20 K) = 34m ₀ (m ₀ is the free electron mass) and τ(20 K) = 4 × 10^?13 s, indicating the establishment of coherent scattering of carriers from localized magnetic moments of the f centers. At a temperature of T = 5 K, the conductivity spectrum contains an absorption line at a frequency of 22 cm^?1 (2.7 meV); the origin of this line can be associated with the exciton-polaron bound state. Since such a state was observed earlier in other intermediate-valence semiconductors (such as SmB₆, TmSe_1?x Te, and (Sm, Y)S), it is probably typical of this class of compounds.     

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.     

Effect of defects on ytterbium ion valency in YbB12

An EPR signal from ytterbium ions with an integer valence of +3 was detected in the YbB₁₂ intermediate-valence compound. The measurements were carried out on an YbB₁₂ single crystal in the 1.6-to 4.2-K temperature interval. The EPR spectrum is interpreted as being due to the existence of exchange-coupled ytterbium ion pairs.     

EPR observation of the spin pair correlation above Tc in lightly alkali metal-doped C60 fullerene

The existence of two temperatures: T_p-Cooper pairing and T_c-Bose–Einstein condensation in high temperature superconductors has been stipulated in a lightly potassium-doped C₆₀ by Magnetically Modulated Microwave Absorption. This doping level corresponds to the carrier density greater than the critical one: x>x₁. In case of rubidium lightly doped C₆₀, where the carrier density x was smaller than the critical one: x<x₁, anomalous EPR temperature dependence was observed. The characteristic temperature of bound electron pair formation T_p≈65 K and the energy gap 2Δ/k=30 K were estimated from the temperature dependence of the EPR signal intensity in non-superconducting state. These results suggest that the liquid fermions–liquid bosons transition can be observed as the opening of the spin gap at temperature T_p postulated in Micnas–Ranninger–Robaszkiewicz theory.     

Paramagnetic resonance linewidth of EuO near the curie temperature

Measurements of the EPR linewidth ΔH of EuO at 9 GHz are reported in the temperature range of 66–300° K, with particular attention to the region near T_c (69.6° K). Comparison with the earlier data of Eastman at 25 GHz in the critical region shows considerable suppression of ΔH at 25 GHz. The temperature-dependent behavior of ΔH in EuO at 9 GHz is similar to the observations in CrBr₃ and it is in qualitative agreement with the zero-field predictions of Huber and Maleev. The quantitative discrepancies are believed to be due to the effect of the resonance magnetic field on EPR spin dynamics near T_c and an inadequate decoupling of the four-spin correlation functions used in the theories.     

Paramagnetic centers in nonmetallic amorphous polyphthalocyanines

Electron paramagnetic resonance (EPR) spectra of nonmetallic amorphous polyphthalocyanines are investigated in the temperature range 295–500 K. The EPR spectrum of nonmetallic amorphous polyphth-alocyanine samples at room temperature prior to heating is a narrow singlet of approximately Lorentzian shape with a linewidth ΔH_pp ≈ 1.7 Oe, a splitting factor g=2.00, and an intensity I_EPR ≈ 10¹⁷ spins/g. It is found that the intensity and linewidth of the EPR spectrum increase with increasing temperature. Beginning with a characteristic temperature T₁, both parameters, ΔH_pp and I_EPR, become dependent on time (under isothermal conditions). Computer calculations of the spectra demonstrate that the EPR spectrum can be represented as a superposition of two lines with substantially differing parameters whose dependences on the temperature and micro-wave power also differ significantly. The possible reasons for the existence of electron paramagnetic resonance centers of two types with different degrees of delocalization of a charge carrier with a magnetic moment in nonmetallic amorphous polyphthalocyanines are discussed.     

Defect-induced magnetic fluctuations in YbB12

We present the results of nuclear spin–lattice relaxation rate (1/T₁) measurements in a typical Kondo insulator YbB₁₂ for ^10,11B and ¹⁷¹Yb nuclei. Above 20 K, 1/T₁ at the B sites shows thermally activated temperature dependence with a gap of about 100 K. However, it shows anomalous enhancement below 15 K, which is partially suppressed by magnetic field up to 16 T. No such anomaly was observed at the Yb sites. The ratio of 1/T₁ for ¹¹B and ¹⁰B nuclei indicates that the anomaly below 15 K is caused by dilute magnetic moments assisted by nuclear spin diffusion. The origin and the nature of the low temperature magnetic fluctuations are discussed.     

Phase transition study in α-Fe2WO6 compound by EPR

The temperature dependence of the EPR spectrum for the α-phase of iron tungstate has been investigated in the temperature range of 40–260 K. At temperatures betweenT ₁ ≈ 250 K andT ₂ ≈ 205 K where the antiferromagnetic phase transition occurs, a relatively narrow EPR line arising from the dominant iron(III) species has emerged, gaining intensity with the temperature increase. Its linewidth temperature evolution could be described by Huber equation, with T_N = 200 K, which is consistent with the peak seen in magnetic susceptibility measurements, while the correspondingg-factor shifts to higher fields reflecting the build-up of internal field emerging from increasing shortrange order in the spin system. At temperatures lower than T₂, a very broad and distorted EPR line with temperature dependentg-factor and linewidth has been observed reflecting the corresponding rise of the magnetic susceptibility below the antiferromagnetic phase transition, presumably arising from magnetic clusters embedded in the antiferromagnetic background.     

Frequency (field) dependent NMR relaxation rate in the interrupted metallic strand model

We calculate the nuclear magnetic resonance rate T^?1₁ arising from the electron-nuclear hyperfine contact interaction, within the interrupted metallic strand model. The electron levels are assumed to have an energy half width Γ and a mean spacing Δ₀ and it is assumed that all segments have the same nuclear spin temperature. In the limit Γ ? Δ₀, T^?1₁ has nearly the same behaviour for kT ? Δ₀ and kT?Δ₀. It is proportional to temperature but has a Lorentzian magnetic field dependence with halfwidth H= Γ/μ_B. At low fields it is enhanced over the value for a normal metal by the factor Δ₀/Γ.This anomalous behaviour arises from the suppression of electron spin flip processes by a magnetic field and should always occur when electronic states are localised, that is when there is a locally discrete electron energy spectrum. Therefore it may be relevant not only to certain linear chain conductors but to other cases of electron localisation.The present model provides an additional possible source of frequency dependence of T₁ in linear chain materials. In certain materials especially those containing defects, it may be more appropriate than the currently accepted mechanism which involves electron spin diffusion in one dimension.     

The effect of hydrogen in solid solution on the magnetic transitions in thulium

The influence of hydrogen on the magnetic structures in thulium was determined by measuring the temperature dependence of the electrical resistivity up to 75 K. The H acts by lowering the Neel temperature and the spin-disorder resistivity, by smearing out the manifestation of the Curie temperature, and by revealing additional magnetic transitions below T_C. This behaviour and the observation of an energy gap in the spin-wave spectrum are commented within the framework of the RKKY-model.     

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.     

The potential barrier height for a Jahn-Teller center

The EPR spectrum of the Cu²⁺ ion in a ZnSiF₆·6H₂O crystal is studied in the temperature range T=5?300K. It is shown that the EPR spectrum can be represented in the form of a superposition of three contributions with essentially different properties. The first contribution is characterized by the maximal intensity at low temperatures and is described by a spin Hamiltonian with a large anisotropy of parameters. The second contribution has the maximal intensity at high temperatures and is described by a spin Hamiltonian with a low anisotropy of parameters. The third contribution cannot be described by a spin Hamiltonian and has the form of a partly orientationally averaged EPR spectrum. The reason for the emergence of these contributions is substantiated along with the form of the temperature dependence of their intensities on the basis of variation of the populations of vibronic states upon a change in temperature. The height (E₀=4±1cm_?1) of the potential barrier separating three equivalent Jahn-Teller potential wells of the Cu²⁺ ion is determined from analysis of the temperature dependence of the integrated intensity of the EPR spectrum. The obtained value of the barrier height substantially differs from the estimate (100 cm^?1) obtained earlier [2, 3] for the Cu²⁺ ion in ZnSiF₆·6H₂O on the basis of the tunneling model. It is shown that the forms of the temperature dependences of the linewidth of the low-and high-temperature EPR spectra are essentially different. This difference indicates that the contributions of the low-and high-temperature EPR spectra are associated with quantum-mechanical transitions between these states. It is proposed that the low-and high temperature contributions to the EPR spectrum are associated with the filling of under-the-barrier and above-the-barrier vibronic states, respectively.     

Magnetic and electrical properties of the half-metallic ferromagnets Co2CrAl

This paper presents the results of measurements of the magnetic and electrical properties of the ferromagnetic alloy Co₂CrAl in two structural states: (i) after severe plastic deformation and (ii) after shortterm high-temperature annealing of the deformed specimens. The experiments have been performed at temperatures in the range from 2 to 900 K in magnetic fields H ≤ 50 kOe. The ferromagnetic Curie temperature T _C and the paramagnetic Curie temperature Θ have been determined (T _C = 305 K and Θ = 326 K), as well as the spontaneous magnetic moment μ _S and the effective magnetic moment μ_eff per molecule of the alloy (μ _S = 1.62 μ_B and μ _eff ² = 8.2 μ _B ² ). It has been shown that the magnetic crystalline anisotropy energy of the alloy is on the order of ～5 × 10⁵ erg/g. The specific features of the electrical properties are associated with the presence of an energy gap in the electronic spectrum near the Fermi level E _F and with the change in the parameters of the energy gap as a function of the temperature.     

EPR of Cr2O3 in the phase transition region

The temperature dependence of the Cr₂O₃ EPR spectra is obtained. The linewidth of the EPR is defined by the phonon's modulation of the exchange interactions at T >T_N + 4–5 K though at T_N < T < T_N + 4 K the critical broadening of the EPR in Cr₂O₃ is determined by fluctuation effects.     

Effect of correlation on electronic properties of NiO: A study from dynamical mean field theory

In order to describe a typical strongly correlated insulator NiO at electronic level, we perform a first principles calculation for temperature effect on electronic properties of NiO using a many-body method merging local density approximation (LDA) with dynamical mean field theory, so called the LDA+DMFT scheme. Band gap and density of states (DOS) are in good agreement with available experimental data and theoretical calculations, and Ni d-e_g and d-t_2g components both exhibit insulating character. Calculated hybridization functions indicate that Ni d-e_g states strongly hybrid with O p states at T = 58 K, 116 K, 145 K, 232 K and 464 K. In order to compare with experimental angle-resolved photoemission spectrum (ARPES), we also calculate momentum-resolved electronic spectrum function, which is established that obvious electronic excitation mainly arises from Ni d-t_2g states at temperature T = 232 K, and the spectrum functions between −0.5 eV and 0.0 eV are almost symmetric about certain k points. Finally, we analyze the effect of temperature on electronic properties of NiO by carrying out LDA+DMFT calculations at T = 58 K, 116 K, 145 K, 232 K and 464 K, respectively. Results show that temperature mainly influences the valence states of spectrum function and hybridization function, in particular high-lying states close to Fermi level. Electronic excitation distributions and spectrum characters in electronic spectrum function are also discussed.     

EPR study of the low temperature charge density wave state of o-TaS3

We report an EPR study of the chain conductor o-TaS₃ in the low temperature charge density wave (CDW) state. The EPR spectrum is attributed to Fe³⁺ (S=5/2) impurities. A power law for the temperature dependence of the EPR intensity, (T^−α with an exponent α∼0.8) found below ∼30 K is very close to that previously found in magnetic susceptibility measurements. The possible role of these impurities in the susceptibility data are discussed.     

Effect of interband scattering on kinetic coefficients and estimates of the parameters of the band spectrum of Sb2Te3

It has been shown that uncertainties in the interpretation of experimental data on transport phenomena in Sb₂Te₃ are resolved in the two-band model with the consistent inclusion of the interband hole scattering. The performed calculation is in quantitative agreement with the experimental data in the temperature range from 77 to 400 K for the following parameters of the band spectrum: the effective mass of the density of states of light holes m _d1 ≈ 0.6m ₀ (where m ₀ is the free electron mass), the effective mass of the density of states of heavy holes m _d2 ≈ 1.8m ₀, and the energy gap between nonequivalent extrema of the valence band ΔE _v(T) ≈ 0.15–2.5 × 10^?4 T eV.     

Nuclear magnetic resonance study of the heavy-fermion system URu2Si2

Nuclear magnetic resonance (NMR) and relaxation studies on ²⁹Si have been carried out on the heavy Fermion system URu₂Si₂. Above the Kondo temperature of about 60 K, the nuclear relaxation time T₁ is nearly temperature independent, which is consistent with the occurrence of fluctuations of localized U moments. Below about 60 K T₁ is inversely proportional to temperature suggesting that the system behaves like a Fermi liquid. A sharp increase in T₁ occurs below 17 K which is probably associated with the opening of an energy gap at the Fermi surface due to the formation of a spin density wave state. Below about 10 K, T₁ reacquires the inverse temperature dependence observed in the 17 K ∼ 60 K temperature range.