Effect of pressure on the valence state of Yb in YbPd2Si2

The intermediate valent behaviour of YbPd₂Si₂ has been studied under pressure in the temperature range from 1.2 K to 90 K by using the 84 keV Mössbauer transition in¹⁷⁰Yb. At 54 kbar and 4.2 K we obtain an increase of the electric field gradient (EFG) by a factor of 3. In addition, the EFG varies strongly with temperature, in contrast to the behaviour at ambient pressure. At 1.2 K a change of the hyperfine pattern is observed indicating a magnetic character of the Yb ion. These results provide evidence of a pressure induced change of the valence state close to 3+.Work supported by the Deutsche Forschungsgemeinschaft     

Doppelresonanzexperimente zur Untersuchung der Hyperfeinstruktur des 6s6p 3 P 1-Zustandes von171Yb und173Yb

For the odd Yb isotopes¹⁷¹Yb and¹⁷³Yb the hyperfine splitting of the 6s6p ³ P ₁ state has been measured by optical double resonance in zero magnetic field. Taking into account second order corrections due to the influence of the 6¹ P ₁ and 6³ P _0,2 states, the following results for the magnetic splitting constantA and the electric quadrupole interaction constantB have been derived: A(¹⁷¹Yb)=3958.228 (60) Mc/s, A(¹⁷³Yb)=?1094.318 (35) Mc/s, S(¹⁷³Yb)=?825.904 (85) Mc/s. The hyperfine structure anomaly of the isotopes¹⁷¹Yb and^{171 173}Yb was determined to be Δ=?0.352 (10)%.     

ESR of Yb3+ ions in a single crystal of the fluctuating-valence compound YbB12

The temperature and angular dependences of electron spin resonance (ESR) spectra of Yb³⁺ ions in a single crystal of fluctuating-valence compound YbB₁₂ were studied. The existence of Yb?Yb ion pairs was observed in a cubic-symmetry crystal. The ions forming the pairs are coupled by the isotropic exchange but interact also with the other pairs by the dipole and exchange coupling. The occurrence of a slight anisotropy in a cubic semiconductor may be the result of a spontaneous break of symmetry specific for the ground state of the Kondo dielectric. A strong temperature dependence of the amplitude of the ESR signals is found at 1.6–4.2 K and interpreted as a result of the capture of electrons by Yb³⁺ ions from electron traps with a binding energy of 18 K. ESR spectra of Yb³⁺ single ions in the Γ₆ state were observed also. The decrease of temperature from 4.2 to 1.6 K indicates a tendency to the ferromagnetic ordering of Yb?Yb pairs.     

Pressure effects on the superconducting transition of ytterbium doped Ce0.6Yb0.4FeAsO0.9F0.1

The Effect of pressure on the superconducting transition temperature of Yb doped Ce_0.6Yb_0.4FeAsO_0.9F_0.1 has been investigated for the first time using resistivity and magnetization studies. Increase in chemical pressure by substitution of smaller Yb³⁺ ions in place of Ce³⁺ ions results in a significant enhancement of T_c from 38 K (Yb free) to 47 K (40% Yb). Enhancement in T_c with external pressure has been observed for this compound up to a maximum value of T_c = 48.7 K at 1 GPa, beyond which T_c starts decreasing monotonously. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Characterization of quasiparticle states at and beyond stability in ytterbium isotopes: Spectroscopy of 175Yb, 176Yb and 177Yb

Excited states in ¹⁷⁵Yb, ¹⁷⁶Yb and ¹⁷⁷Yb were populated via the bombardment of a ¹⁷⁶Yb target with a 750 MeV ¹³⁶Xe beam. Gamma-ray decays from these states were measured with the AFRODITE multi-detector spectrometer. The rotational band previously assigned to the ground state of ¹⁷⁷Yb has been reassigned to the first-excited state of ¹⁷⁵Yb. A new rotational band based on the ground state of ¹⁷⁷Yb is presented, and the band based on the K^π = 4^- two-quasiparticle state in ¹⁷⁶Yb has been identified. Also a candidate for the rotational band based on the K^π = 8^-, T_1/2 = 11.4(3) s two-quasiparticle state in ¹⁷⁶Yb has been found. Comparisons of g_K values derived from in-band branching ratios are consistent with the ν9/2⁺[624] assignment to the ground state of ¹⁷⁷Yb, the ν²{9/2⁺[624] ⊗ 1/2^-[510]} assignment to the K^π = 4^- state and with the ν²{9/2⁺[624] ⊗ 7/2^-[514]} assignment to the K^π = 8^- metastable excited state in ¹⁷⁶Yb.     

Die Hyperfeinstruktur des 6s 6p 3 P 1-Zustandes von171Yb und173Yb

The hyperfine structure of the 6s 6p ³ P ₁ state of¹⁷¹Yb and¹⁷³Yb has been investigated by the level crossing technique and by optical double resonance. The following results for the magnetic hfs splitting constantA and for the electric quadrupole interaction constantB have been obtained:A(¹⁷¹Yb)=3959.1(14)Mc/sec;A(¹⁷³Yb)=?1094.7(6) Mc/sec;B(¹⁷³Yb)=?826.9(9) Mc/sec. The hyperfine structure anomaly of the isotopes¹⁷¹Yb and¹⁷³Yb was determined to beΔ=?0.36(3)%.     

Angular correlation studies on 172Lu(172Yb) in GaN and measurements at low temperatures

For optoelectronic devices semiconductors with large band gap doped with rare earth are used. Doping is generally performed during growth but for more structured doping the ion implantation technique is preferable. The perturbed γγ-angular correlation technique is an ideal tool to study the behavior of semiconductors after implantation. An adequate rare earth isotope for such investigations of semiconductors is ¹⁷²Yb. The temperature dependence of the hyperfine fields for ¹⁷²Lu(¹⁷²Yb) in GaN has been analysed. The total electric field gradient (EFG) at the site of this probe is a superposition of the lattice EFG due to the GaN wurtzite structure and the EFG due to the 4f shell of the rare earth probe itself. The latter is strongly temperature dependent and opposed to the lattice EFG which in contrast is nearly constant since the lattice parameters change only negligibly with temperature. At elevated temperatures the sublevels of the 4f shell, split by the crystal electric field, are equally populated. But at low temperatures the lowest level is occupied preferentially. Sign and magnitude of the EFG caused by the 4f shell can be calculated (Tomala et al, J Magn Magn Mater 89(1–2):143, 1990) and the result compared to the experimental data.     

Additive colouring of CaF2:Yb crystals: determination of Yb2+ concentration in CaF2:Yb crystals and ceramics

When growing CaF₂ crystal doped with rare-earth ions, most of these ions are present in a trivalent state. However, due to contact with graphite crucible, a small proportion of a number of ions (Eu, Sm, Yb and Tm) are reduced to a bivalent state. A similar situation takes place during fabrication of CaF₂ ceramics doped with rare-earth metals. This fact is of particular importance for laser CaF₂:Yb crystals (ceramics), a promising material for short-pulse, high-power, high-energy diode-pumped solid state lasers since the presence of bivalent Yb ions can be a source of thermal losses. To date, there has been no technique to determine Yb²⁺ concentration in as-grown crystals. The proposed technique is based on a total reduction of Yb³⁺ ions via the heating of as-grown CaF₂ crystals with known concentration of Yb in the reducing atmosphere of metal vapour and determining the cross section of absorption bands of Yb²⁺ ions. The knowledge of these parameters allows estimation of the Yb²⁺ content in CaF₂:Yb crystals or ceramics by analysing their absorption spectra. Examples of using this technique are given. The technology of CdF₂ crystals reduction (an “additive colouring”) and features of colouring of crystals doped with rare-earth ions are considered.     

Low-temperature vibrational properties of KGd(WO4)2: (Er, Yb) single crystals studied by Raman spectroscopy

Temperature-dependent polarized Raman spectra of KGd(WO₄)₂: (Er, Yb) single crystals have been analyzed over the 77-292 K temperature range. The A_g and B_g spectra obtained are discussed in terms of factor group analysis. The spectra have been found to reveal the bands related to internal and external vibrations of WO₄²⁻, WOW and WOOW molecular groups. Strong depolarization of the majority of the Raman bands has been observed in the whole temperature range. Some anomalies in the spectral parameters of selected Raman bands below 175 K have been discussed in terms of the local distortion of WO₄²⁻ ions in KGd(WO₄)₂: (Er, Yb) crystals.     

Quadrupole interaction in In0.95 Ag0.045 Ga0.005 alloy

The temperature dependence of the quadrupole interaction of¹¹¹Cd in the In_0.95 Ag_0.045 Ga_0.005 alloy is determined by the time differential perturbed angular correlation method from 77 to 422 K. The electric field gradients produced by the lattice are derived. The temperature dependence of the electric field gradient is found to follow theT ^3/2 law observed in In between 77 K and room temperature and to deviate from it at higher temperatures. The widths of the electric field gradient distribution are 0.1147(259), 0.0924(340), 0.4954(601) and 1.344(175) at 77, 298, 373 and 422 K, respectively. This illustrates that the alloy is well ordered at 77 and 298 K and that it becomes less ordered at 373 and 422 K.     

Energy transfer and fluorescence characteristics of erbium-doped ytterbium aluminum garnet

We have examined the fluorescence characteristics of the garnet-type crystal Yb₃Al₅O₁₂ : Er³⁺ (YbAlG : Er³⁺) and studied the energy transfer process between the two rare earth ions over a temperature range 78–297 K. Certain data were compared with those of YAlG : Er³⁺. In YbAlG : Er³⁺, Yb fluorescence is observed at ?1.03 μm (corresponding to the ²F_5/2 → ²F_7/2 transition); Er fluorescence occurs at ?8500 Å (⁴S_3/2 → ⁴I_13/2 transition) and ?1.6 μm (⁴I_13/2 → ⁴I_15/2 transition). In YAlG : Er³⁺, the same Er lines are observed with the addition of a band at ?1 μ (⁴I_11/2→⁴I_15/2 transition). In YbAlG : Er³⁺, the decay pattern of the Yb emission is purely exponential at all the temperatures examined; the fluorescence lifetime ranges from 36 μ s (at 78 K) to 74 μs(at 269 K). The lifetime of the Er⁴I_13/2 level in the same sample increases from 5.4 ms (at 78 K) to 6.85 ms (at 294 K). The lifetime of this Er level in YAlG : Er³⁺ is weakly temperature dependent over the same range with a value of ?12 ms. Excitation spectra were obtained for the Er 1.53 μm fluorescence in YbAlG : Er³⁺ in order to verify the presence of Yb → Er energy transfer in this sample. The presence of the Yb absorption band (?1 μm) in these spectra provides direct evidence of this energy transfer. The relative enhancement of this Yb band with respect to the Er bands in going from 78 K to 175 K is an indication of a more efficient transfer at the higher temperature. Excitation spectra obtained for the Yb 1.03 μm fluorescence in YbAlG : Er³⁺ revealed the presence of Er → Yb energy transfer as well in this sample. The existence of both Yb → Er and Er → Yb transfer is expected, due to the resonance between the ⁴I_11/2 → ⁴I_15/2 transition of Er and the ²F_5/2 → ²F_7/2 transition of Yb. The above results are explained in terms of a rate equation model in which transfer in both directions is treated in the following manner: Yb → Er transfer is considered to be much more probable than decay processes originating at the Yb ²F_5/2 level; Er → Yb transfer is treated as much more probable than decay processes originating at the Er ⁴I_11/2 level.     

Spectroscopic characterization of Yb:Sc2O3 transparent ceramics

Yb：Sc2O3 transparent ceramics are fabricated by a conventional ceramic process and sintering in H2 atmosphere. The room-temperature spectroscopic properties are investigated, and the Raman spectrum shows an obvious vibration characteristic band centred at 415 cm-1. There are three broad absorption bands around 891, 937, and 971 nm, respectively. The strongest emission peak is centred at 1.04 μm with a broad bandwidth （11 nm） and an emission cross-section of 1.8×10^-20 cm^2. The gain coefficient implies a possible laser ability in a range from 990 nm to 1425 nm. The energy-level structure shows that Yb：Sc2O3 ceramics have large Stark splitting at the ground state level due to their strong crystal field. All the results show that Yb：Sc2O3 transparent ceramics are a promising material for short pulse lasers.     

Yb:铅锌镧碲酸盐玻璃的热学、光谱和激光性质

研究了摩尔组分为70TeO2₂-(20-x)ZnO-xPbO-5La2₂O3_{3-25K22O-25Na22O（x=0，5，10，15，20）的新型多元铅 锌镧碲酸盐激光玻璃，外掺Yb22O33为玻璃摩尔组分的1％.测试了 试样的物理性质及吸收光谱、荧光光谱和荧光寿命，计算了Yb3+3+的吸收截面、受激发射截面、荧光有效线宽等 关键词： 3+掺杂碲酸盐玻璃')" href="#">Yb3+³⁺掺杂碲酸盐玻璃 光谱性质 热稳定性}     

The lattice location and hyperfine field of implanted Yb in Fe as a function of annealing temperature

A combined study has been made of the lattice location and hyperfine field of Yb implanted into Fe single crystals. The location has been determined by ion channeling and back-scattering, the hyperfine field by perturbed angular correlation (PAC) measurements on¹⁶⁹Yb (decaying to excited states in¹⁶⁹Tm). The channeling experiments indicate that initially about 60% of the Yb atoms occupy substitutional sites in the Fe lattice, while the remaining atoms are not in any specific crystallographic site. On annealing, Yb migrates from substitutional to non-substitutional sites. No Yb atoms remain substitutional after a 600°C anneal. By making PAC measurements at two temperatures for two - cascades in¹⁶⁹Tm, it is found that the PAC pattern can be described using a combined static and time-dependent magnetic interaction. The PAC results show that the average hyperfine field and relaxation parameter decrease on annealing, and that the field disappears after a 600°C anneal. The correlation between the location and the hyperfine field is discussed. A comparison of the results with previous Mössbauer results for¹⁵¹Gd implanted in Fe, together with relaxation parameter measurements on a¹⁶⁹Yb₂O₃ source, suggests that the non-substitutional Yb is internally oxidized in the Fe host.Work partly carried out while at the Clarendon Laboratory, Oxford, England and Nuclear Physics Division, AERE, Harwell, England.     

Crystal field investigations of Yb(C2H5SO4)3. 9H2O and Er: YA1G

Using the spectroscopically derived crystal field parameters for Yb(C₂H₅SO₄)₃. 9H₂O and Er³⁺: YA1G, the temperature dependence of Schottky specific heat, paramagnetic susceptibility, magnetic anisotropy and μ_eff has been calculated over a temperature range 5–400°K. The hyperfine interaction parameters for ¹⁷¹Yb³⁺, ¹⁷³Yb³⁺ and ¹⁶⁷Er³⁺ systems are also obtained and in turn used to estimate the nuclear specific heat. The nice agreement obtained for susceptibility and specific heat of Yb(C₂H₅SO₄)₃. 9H₂O at very low temperatures confirms the accuracy of CEF parameters employed and the neglect of exchange interaction. However, for Er³⁺: YA1G, the CEF parameters are adequate to explain the bulk thermal and magnetic properties but not the g-values.     

Pressure-induced magnetic phase transitions in Yb Kondo lattice systems

In this paper we give a brief overview of the effect of pressure on the magnetic and electronic properties of Yb Kondo lattices using the ¹⁷⁰Yb Mössbauer technique, electrical resistance and X-ray diffraction. The selected materials were either nonmagnetic (YbCu₂Si₂ and Yb₂Ni₂Al) or ferromagnetic (YbNiSn). We show that pressure induces a first order transition to a magnetic ground state in both YbCu₂Si₂ and Yb₂Ni₂Al. In the former compound, the transition is accompanied by a valence change towards Yb³⁺ state. The behavior of both YbCu₂Si₂ and Yb₂Ni₂Al can be understood as resulting from a pressure enhancement of the RKKY interaction which finally dominates the Kondo effect. We demonstrate that the ground state properties of YbNiSn are governed by a volume dependent competition between anisotropic exchange interactions and crystal field anisotropy rather than by a direct competition between Kondo and RKKY interactions.     

Bestimmung der Kerndipolmomente von171Yb und173Yb durch optisches Pumpen

The nuclear Zeeman levels of the odd isotopes of Ytterbium (¹⁷¹Yb,¹⁷¹ I=1/2;¹⁷³Yb,¹⁷³ I=5/2) were polarized in the atomic ground state (6s ^{2 1} S ₀) by means of optical pumping with the resonance line (6s ^{2 1} S ₀ ? 6s6p ¹ P ₁,λ=3988 å). The magnetic moments, calculated from the nuclear Hf-Zeeman transitions in a magnetic field of 228 Gauss are¹⁷¹ Μ _I=0.49188 (2)Μ _K ¹⁷³ Μ _I=0,67755 (3)Μ _K (with diamagnetic correction applied).     

Magnetic and structural studies of the alloy system REIn0.5Ag0.5

The structural and magnetic properties of the alloy system REIn_0.5Ag_0.5 [RE = Gd, Tb, Dy, Ho, Er, Tm and Yb] are reported. All these alloys (except that of Yb) crystallize in a cubic CsCl type structure at room temperature. Low temperature X-ray diffraction data does not reveal any structural phase transformation down to 8 K. On the basis of magnetic susceptibility data at a different temperature (3–300 K) and applied magnetic field (2 × 10⁵ to 8 × 10⁶ A m^-1, it has been concluded that GdIn_0.5Ag_0.5 is ferromagnetic (T_c = 118 K), TbIn_0.5Ag_0.5 and DyIn_0.5Ag_0.5 are meta magnetic (T_N = 66 and 30 K, respectively) and alloys involving Ho, Er, Tm and Yb are ferrimagnetic with Néel temperatures (T_N) equal to 24, 22, 21 and 20 K, respectively. The evaluated effective magneton number (p) is found to be slightly larger compared to theoretical values for tripositive ions of Gd, Tb and Dy and a bit smaller for Ho, Er, Tm and Yb. The results have been qualitatively explained using appropriate theories.     

Crystal growth, optical properties, and laser operation of Yb3+-doped NYW single crystal

Laser crystal Yb³⁺-doped NaY(WO₄)₂ (Yb:NYW) with excellent quality has been grown by Czochralski technique. The rocking curves from (400) plane of as-grown Yb:NYW crystal was measured and the full-width value at half-maximum was 19.92″. The effective segregation coefficients were measured by the X-ray fluorescence method. The polarized absorption spectra and the fluorescence spectra of Yb:NYW crystal were measured at room temperature. The fluorescence decay lifetime of Yb³⁺ ion in NYW crystal has been investigated. The spectroscopic parameters of Yb:NYW crystal are calculated and compared with those of Yb:YAG crystal. A continuous wave output power of 3.06 W at 1031 nm was obtained with a slope efficiency of 42% by use of diode pumping.     

Continuous wave and tunable laser operation of Yb3+ in disordered NaLa(MoO4)2

Continuous-wave Yb³⁺ laser operation is studied in single crystals of disordered NaLa(MoO₄)₂ at room temperature. The sample used was grown by the Czochralski technique and incorporates an Yb ion density of 3.1×10²⁰ cm^-3. The effect of the Yb concentration on some of the crystal properties is described as well as the spectroscopic Yb³⁺ properties at 5 K. Maximum slope efficiencies of about 40% for π and 38% for σ polarization were obtained under Ti:sapphire laser pumping near 976 nm, respectively. The maximum output power for the π polarization was 400 mW at 1039.5 nm, the threshold in this case amounted to 240 mW (absorbed pump power). The laser emission was tunable between 1016 and 1064 nm with a Lyot filter. Lasing was also realized by pumping with a fiber-coupled diode laser module. Maximum output power of 900 mW at 1035 nm was achieved in this case for the π polarization and the threshold was 280 mW. The results, in terms of output power and tunability, are superior in comparison to all previous reports on Yb-doped disordered double tungstate or molybdate crystals and represent a significant improvement in comparison to earlier experiments with low-doped Yb:NaLa(MoO₄)₂.PACS 42.55.Rz; 42.55.Xi; 42.70.Hj