Signatures of spin-glass behaviour in PrIr2B2 and heavy fermion behaviour in PrIr2B2C

The magnetic and transport properties of PrIr(2)B(2) and PrIr(2)B(2)C have been investigated by dc and ac magnetic susceptibility, specific heat, electrical resistivity and magnetoresistance measurements. PrIr(2)B(2) forms in CaRh(2)B(2)-type orthorhombic crystal structure (space group Fddd). At low fields the dc magnetic susceptibility of PrIr(2)B(2) exhibits a sharp anomaly near 46 K which is followed by an abrupt increase below 10 K with a peak at 6 K, and split-up in ZFC and FC data below 46 K. In contrast, the specific heat exhibits only a broad Schottky type hump near 9 K which indicates that there is no long range magnetic order in this compound. The thermo-remanent magnetization is found to decay very slowly with a mean relaxation time τ = 3917 s. An ac magnetic susceptibility measurement also observes two sharp anomalies; the peak positions strongly depend on the frequency and shift towards high temperature with an increase in frequency, obeying the Vogel-Fulcher law as expected for a canonical spin-glass system. The two spin-glass transitions occur at freezing temperatures T(f1) = 36 K and T(f2) = 3.5 K with shifts in the freezing temperatures per decade of frequency δT(f1) = 0.044 and δT(f2) = 0.09. An analysis of the frequency dependence of the transition temperature with critical slowing down, τ(max)/τ(0) = [(T(f)-T(SG))/T(SG)](-zν), gives τ(0) = 10(-7) s and exponent zν = 8, and the Vogel-Fulcher law gives an activation energy of 84 K for T(f1) and 27.5 K for T(f2). While zν = 8 is typical for spin-glass system, the characteristic relaxation time τ(0) = 10(-7) s is very large and comparable to that of superspin-glass systems. An addition of C in PrIr(2)B(2) leads to PrIr(2)B(2)C which forms in LuNi(2)B(2)C-type tetragonal structure (space group I4/mmm) and remains paramagnetic down to 2 K. The specific heat data show a broad Schottky type anomaly, which could be fairly reproduced with CEF analysis which suggests that the ground state is a CEF-split singlet and the first excited state singlet is situated 15 K above the ground state. The Sommerfeld coefficient γ～300 mJ mol(-1) K(-2) of PrIr(2)B(2)C is very high and reflects a heavy fermion behaviour in this compound. We believe that the heavy fermion state in PrIr(2)B(2)C has its origin in low lying crystal field excitations as has been observed in PrRh(2)B(2)C.     

Magnetic ordering in HoRu2Si2, HoRh2Si2, TbRh2Si2 and TbIr2Si2 by neutron diffraction

Neutron diffraction study of polycrystalline HoRu₂Si₂, HoRh₂Si₂, TbRh₂Si₂, and TbIr₂Si₂ was performed in the temperature range between 4.2 and 300 K. For HoRu₂Si₂ the magnetic spin alignment of a linear transverse wave mode below the Néel temperature 19 K is observed. This static moment wave is propagating along the b-axis with k=(0, 0.2, 0) and is polarized in the c-axis. The root-mean-square and maximum saturation moments per Ho atom are 9.26 and 13.09μ_B, respectively. HoRh₂Si₂, TbRh₂Si₂ an TbIr₂Si₂ are simple collinear antiferromagnets of +-+- type with Néel temperatures of (27±1), (98±2) and (72±3) K, respectively. For TbRh₂Si₂ and TbIr₂Si₂ magnetic moments are localized on RE ions only and are aligned along the tetragonal axis, while for HoRh₂Si₂ they form an angle ø = (28±3)°.     

Luminescence of defect centres in Hg2Cl2

Abstract

Luminescence in Hg₂Cl₂, crystals excited with UV light is investigated in the spectral region 0.8–2.25 eV. Measurements are performed on as-grown samples and on samples previously exposed to UV light at RT. Six emission bands are found which depend on the concentration of the photochemical entities produced by irradiation of the crystals at RT. It is concluded that infra-red (IR) luminescence of Hg₂Cl₂ originates from crystal defects. The observed emission bands are tentatively attributed to the emission of (HgCI_xBr_3?x)^? and (HgCl_xBr_4?x)^2? complexes formed with residual Br impurities. Centres responsible for IR Hg₂Cl₂ emissions are excited: (i) via excitons of Hg₂Cl₂, (ii) via excited states of isolated Hg₂Br₂ molecules, and (iii) resonantly through the excitation bands of defect centres.     

Li_2O-Al_2O_3-SiO_2-P_2O_5系统微晶玻璃受控析晶分析

应用DTA,XRD,SEM等手段研究了Li2O_Al2O3_SiO2_P2O5系统微晶玻璃的析晶过程和相组成。采用模拟计算方法考察了不同的析晶条件。引入适量的Al2O3等难熔物组分来改变析晶条件及残留玻璃相的组成和粘度。通过抑制主晶相(Li2O.2SiO2)晶粒的生长,最终获得了晶粒尺寸为0.1~0.3μm的透明微晶玻璃。     

Study of Nd2+ absorption in x-irradiated CaF2, SrF2, BaF2 crystals

The absorption spectra of x-irradiated alkaline-earth fluoride (CaF₂, SrF₂, BaF₂) crystals doped with Nd³⁺ ions have been investigated. X-irradiation results in creating the absorption bands of inter-configuration 4fⁿ–4fⁿ⁻¹–5 d¹ transitions of Nd²⁺. The charge reduction of the neodymium by irradiation is not temperature-stable and the ions reoxidation (Nd²⁺ → Nd³⁺) occurs under heating to 570 K in CaF₂, 520 K in SrF₂ and 470 K in BaF₂.     

Change of hyperfine and transferred hyperfine interaction of Eu2+ in CdF2, CaF2, SrF2, and BaF2

ENDOR measurements of the hyperfine and transferred hyperfine interaction of Eu²⁺ in CdF₂, SrF₂, and BaF₂ were performed. Compared with measurements on CaF₂, there is a radial shifting of the next fluorine ligands in CdF₂∶ +5.4%, CaF₂∶ + 3.5%, and BaF₂∶ ?3.9%. Since the SrF₂ and EuF₂ lattice constants are approximately equal, a shifting in SrF₂ was not assumed. In both europium isotopes (151 and 153) a change of hyperfine fields at the nucleus was observed. This can be explained qualitatively by the difference in radial distribution of the different europium orbitals. Also noticed was a small change of the hyperfine anomaly, which indicates contributions of the zero-point vibration.     

Neutron diffraction study of magnetic order in TbRh2Ge2 and TbRu2Ge2

TbRh₂Ge₂ orders antiferromagnetically in the AFI type structure with a magnetic moment of 9.4(3)μ_B at 4.2 K localized on the Tb³⁺ ion. TbRu₂Ge₂ exhibits a square modulated magnetic structure with moments aligned along the c-axis of the crystallographic unit cell. The magnitude of the magnetic moment localized on the Tb³⁺ ion is 9.1(1)μ_B at 4.2 K.     

HfO2中ZrO2含量对266nm HfO2/SiO2多层膜反射率的影响

用两种不同纯度的HfO2材料与同一纯度的SiO2材料组合,沉积Λ／4规整膜系(HL)^11H形成266nm的紫外反射镜,发现反射率相差0．7％左右。用X光电子能谱法分析了高反膜中表层HfO2中的成分,发现ZrO2的含量相差一个数量级左右。为确定形成这种差别的原因,用辉光放电质谱法测定了这两种HfO2材料中锆(Zr)及其钛(Ti)、铁(Fe)的含量,发现Zr是其中的最主要的杂质,两种HfO2材料中Zr含量有一个数量级的差别。说明在266nm波段,HfO2中ZrO2的含量会对HfO2／SiO2高反膜的反射率造成影响。根据HfO2单层膜的光谱曲线,推算出了这两种材料的消光系数的差别,并用Tfcalc膜系设计软件进行理论和镀制结果的模拟,得到与实验测试一致的结果。     

Peak effect studies in single crystals CeRu2 and 2H-NbS2

We have studied the peak effect (PE) phenomenon in single crystals of weakly pinned superconductors CeRu₂ and 2H-NbS₂. 2H-NbS₂ is iso-structural and iso-electronic to 2H-NbSe₂, whose similarity with CeRu₂ as regards the PE representing the order-to-disorder transformation of the flux line lattice was claimed some time ago. We report on the step change in equilibrium magnetization across the peak effect in CeRu₂. We also present the vortex phase diagram of 2H-NbS₂ obtained from the magnetization data, and compare the PE phenomenon in 2H-NbS₂ and 2H-NbSe₂.     

Nature of the A phase in CeCu2Si2

Neutron diffraction experiments have been performed on a magnetically ordered CeCu2Si2 single crystal exhibiting A-phase anomalies in specific heat and thermal expansion. Below T(N) approximately 0.8 K antiferromagnetic superstructure peaks have been detected. The propagation vector of the magnetic order appears to be determined by the topology of the Fermi surface of heavy quasiparticles as indicated by renormalized band-structure calculations. The observation of long-range incommensurate antiferromagnetic order as the nature of the A phase in CeCu2Si2 suggests that a spin-density-wave instability is the origin of the quantum critical point in CeCu2Si2.     

The absence of valence fluctuations in SmNi2Sn2

In an earlier work [1] an anomaly in the lattice volume had been observed in SmNi₂Sn₂ which indicated possible valence fluctuations of Sm ions. Using a well characterised sample of SmNi₂Sn₂, we do not find any evidence of valence fluctuations from our susceptibility, resistivity, heat capacity and the lattice parameter data. SmNi₂Sn₂ orders magnetically below 8 K and all the Sm ions are trivalent.     

Zeeman effects in the visible 2B2 ← 2A1 system of NO2

The Zeeman effect facilitates certain rotational assignments in bands of the ${}^2\text{B}{}_2\text{-}{}^2\text{A}{}_1$ electronic system of NO₂. A partial analysis of two bands of this system is reported.     

Polar optic phonons in Hg2Cl2 and Hg2Br2

Far infrared (30–430 cm^?1) reflectivity measurements of Hg₂Cl₂ and Hg₂Br₂ single crystals have been performed in polarized light. The spectra, which are in agreement with group-theoretical predictions, were analyzed by the oscillator fitting procedure and Kramers-Kronig method. The results are compared with the existing data from other measurements and the large anisotropy of polar modes is briefly discussed. The polarization vectors of all long-wavelength symmetry modes were determined group-theoretically.     

Spin fluctuation effects in substituted CeRu2Si2 and YbPd2Si2 alloys

Effects of chemical substitution in CeRu₂Si₂, a well-studied heavy fermion system and YbPd₂Si₂ have been investigated through magnetic susceptibility and x-ray diffraction in the systems CeRu _x Si₂, CeRu_2−x Os _x Si₂, CeRu₂Si_2−x Ge _x and YbPd₂Si_2−x Ge _x . Replacing silicon by germanium generates normal chemical pressure effect, namely, Ce and Yb atoms in CeRu₂Si₂ and YbPd₂Si₂ became more and less magnetic respectively. With increasing Ge concentration, CeRu₂Si_2−x Ge _x exhibits larger susceptibility at low temperature, goes to an antiferromagnetic state and finally becomes ferromagnetic. In YbPd₂Si_2−x Ge _x , increasing Ge concentration drives Yb atoms to more divalent state. Electronic effects are more pronounced in CeRu_2−x Os _x Si₂ though CeRu₂Si₂ and CeOs₂Si₂ have very nearly the same lattice parameters. It is conjectured that CeRu₂Si_2−x Ge _x may be the first Ce-based heavy fermion having a magnetic ground state. The authors felicitate Prof. D S Kothari on his eightieth birthday and dedicate this paper to him on this occasion.     

Laser-Induced Current Switching Observed in the Discharge Media of CF2Cl2-N2 and CH3Cl-N2

Switchings of discharge current induced by ArF and KrF laser pulses in the discharge media of CF2Cl2-N2 and CH3Cl-N2 were investigated using a negative point-to-plane discharge apparatus. The electron attachment rate constants of CF2Cl2 in buffer gases of N2 and Ar were measured by a parallel-plate drift-tube apparatus at various E/N, from which the dominant negative ions in the discharge media were inferred. The conductivity of the discharge medium is enhanced upon laser irradiation due to conduction electrons being produced from photoelectron-detachment of Cl-. From the dependence of the enhanced current on laser power, the photodetachment cross sections of Cl- in a given discharge condition were derived to be 2.5 × 10-17 cm2 at 193 nm and 1.0 × 10-17 cm2 at 248 nm. After the current enhancement, the current was greatly reduced as was observed in the discharge medium of CF2Cl2-N2, but not in CH3Cl-N2. The mechanism for the optically induced reduction of discharge current is discussed.     

Infrared absorption of iodomethylperoxy (syn-ICH2OO) radical generated upon photolysis of CH2I2 and O2 in solid para-H2

Irradiation of a para-hydrogen (p-H₂) matrix containing diiodomethane (CH₂I₂) and O₂ at 3.2 K with light at 280 ± 20 nm, followed by annealing of the matrix at 4.0 K, yielded infrared (IR) absorption lines at 2982.4, 1408.9, 1231.8, 1226.5/1225.6, 1085.6, 917.7, 841.6/841.1, 550.5, and 490.2 cm^?1 that are assigned to the syn-iodomethylperoxy (syn-ICH₂OO) radical. Further irradiation of the matrix at 365 nm diminished these features. Experiments with CH₂I₂ and ¹⁸O₂ yielded lines of syn-ICH₂¹⁸O¹⁸O at 1407.3, 1228.1/1227.7, 1217.7/1217.0, 1031.5, 899.9/899.4, 836.7/836.0, and 473.6 cm^?1. The assignments are based on the photolytic behaviour and comparisons of observed vibrational wavenumbers, IR intensities, and ¹⁸O-isotopic shifts with those predicted with the B3LYP/aug-cc-pVTZ-pp method. The observation is consistent with a mechanism that, upon photolysis of CH₂I₂ at 280 nm, CH₂I was formed and subsequently reacted with O₂ to yield syn-ICH₂OO. Compared with the gaseous reaction CH₂I + O₂ → CH₂OO + I at low pressure, observation of ICH₂OO in a p-H₂ matrix instead of CH₂OO in the gaseous phase indicates that the excess energy of internally excited ICH₂OO, produced upon reaction of CH₂I + O₂, was rapidly quenched in the matrix so that ICH₂OO became readily stabilised without further decomposition to form CH₂OO + I.     

On the Theory of Magnetoelectric Coupling in LiCu 2 O 2

A microscopic theory is proposed for coupling of copper spins with an external electric field in the magnetically ordered phase of LiCu2O2. The expressions are derived for the dipole moment components of exchange-coupled pairs of copper spins, as well as expressions for describing the macroscopic polarization in terms of the angles determining spatial orientations of spin rotation planes (helixes) in copper bichains. It is shown that zero electric polarization in isostructural compound NaCu2O2 can be explained by the difference in the spin structures in NaCu2O2 and LiCu2O2.     

Study of the Absorption Characteristics of Transitions in the HOT 2υ2–υ2 Band of the 34SO2 Molecule

Russian Physics Journal - The absorption coefficient of the rovibrational 2υ2–υ2 band of the 34SO2 molecule localized in the region 350–850 cm–1 is studied for the first...     

Conversion of nitrogen oxides in N2:O2:CO2 and N2:O2:CO2:NO2 mixtures subjected to a dc corona discharge

This paper concerns the influence of a direct current (dc) corona discharge on production and reduction of NO, NO₂ and N₂O in N₂:O₂:CO₂ and N₂:O₂:CO₂:NO₂ mixtures. The corona discharge was generated in a needle-to-plate reactor. The positively polarized electrode consisted of 7 needles. The grounded electrode was a stainless steel plate. The gas flow rate through the reactor was varied from 28 to 110 cm³/s. The time-averaged discharge current ranged from 0 to 6 mA. It was found that in the N₂:O₂:CO₂ mixture the corona discharge produced NO, NO₂ and N₂O. In the N₂:O₂:CO₂:NO₂ mixture the reduction of NO₂ was between 6–56%, depending on the concentration of O₂, gas flow rate and corona discharge current. The NO₂ reduction was accompanied by production of NO and N₂O. The results show that efficient reduction of nitrogen oxides by a corona discharge cannot be expected in the mixtures containing N₂ and O₂ if reducing additives are not employed.     

Valence instability of Pr impurities in ZrIr2 and ZrOs2

This paper describes a search for valence instabilities of Pr impurities in ZrMe₂ with Me=Os, Ir and Ru. Pr impurities go into solution in the 2% concentration range in ZrIr₂ and ZrOs₂ but not in ZrRu₂. Susceptibility measurements indicate a nonmagnetic groundstate of Pr in both ZrIr₂ and ZrOs₂. The effective moment in ZrIr₂ is slightly below the trivalent value in ZrIr₂ while in ZrOs₂ the susceptibility is far below the trivalent value between helium and room temperatures. The resistivity increments of the alloys are very large and show very unusual temperature dependence, even for stable rare-earth (RE) impurities in ZrIr₂, where one observes strong deviations from the Matthiessen's rule at low temperatures. The depression of the superconducting transition temperature of ZrIr₂ and ZrOs₂ by Pr impurities is about 0.5 K/at%.