Temperature dependence of the MnPS3 crystal fluorescence

Summary The fluorescence spectra of MnPS₃ single crystals have been investigated at various temperatures beginning from 300K down to 10K in the 1.5 eV to 3.0 eV range. Four structures are observed in all spectra. The strong similarity between these results and previous data concerning the MnPS₃ room temperature fluorescence suggests an intra-Mn nature for the transitions responsible for the above features. All the subband gap structures exhibit a temperature shift which can be satisfactorily interpreted by means of the crystal field theory and the so-called ?transition metal weakly interacting? model. The fine splitting in the originally degenerate {⁴ E _g,⁴ A _1g} levels is enhanced as temperature decreases. These results confirm the Mn²⁺ 3ϕ excited-state energy distribution deduced from the transition metal weakly interacting model. The authors of this paper have agreed to not receive the proofs for correction.     

Identification of the multicomponent nature of E 0 photoreflectance spectra from moderately doped GaAs substrates

The paper considers methods used to identify the multicomponent character of photoreflectance (PR) spectra in the vicinity of the E ₀ transition measured at room temperature from moderately doped GaAs substrates. It is shown that that if an E ₀ PR spectrum contains an excitonic component, its presence can be established from both a mathematical and phase analysis, as well as from measurements performed at different laser excitation densities. Fiz. Tverd. Tela (St. Petersburg) 39, 2123–2129 (December 1997)     

Theoretical studies on intervalley splittings in Si/SiO<Subscript>2</Subscript> quantum dot structures

Using the coupled cluster method we investigatespin-s J ₁-J′ ₂ Heisenberg antiferromagnets (HAFs) on an infinite, anisotropic, two-dimensional triangular lattice for the two cases where the spin quantum number s = 1 and s = $\frac{3} {2}$\frac{3} {2}. With respect to an underlying square-lattice geometry the model has antiferromagnetic (J ₁ > 0) bonds between nearest neighbours and competing (J′ ₂ > 0) bonds between next-nearest neighbours across only one of the diagonals of each square plaquette, the same diagonal in each square. In a topologically equivalent triangular-lattice geometry, the model has two types of nearest-neighbour bonds: namely the J′ ₂ ≡ κJ ₁ bonds along parallel chains and the J ₁ bonds producing an interchain coupling. The model thus interpolates between an isotropic HAF on the square lattice at one limit (κ = 0) and a set of decoupled chains at the other limit (κ → ∞), with the isotropic HAF on the triangular lattice in between at κ = 1. For both the spin-1 model and the spin-$\frac{3} {2}$\frac{3} {2} model we find a second-order type of quantum phase transition at κ _c = 0.615 ± 0.010 and κ _c = 0.575 ± 0.005 respectively, between a Néel antiferromagnetic state and a helically ordered state. In both cases the ground-state energy E and its first derivative dE/dκ are continuous at κ = κ _c , while the order parameter for the transition (viz., the average ground-state on-site magnetization) does not go to zero there on either side of the transition. The phase transition at κ = κ _c between the Néel antiferromagnetic phase and the helical phase for both the s = 1 and s = $\frac{3} {2}$\frac{3} {2} cases is analogous to that also observed in our previous work for the s = $\frac{1} {2}$\frac{1} {2} case at a value κ _c = 0.80 ± 0.01. However, for the higher spin values the transition appears to be of continuous (second-order) type, exactly as in the classical case, whereas for the s = $\frac{1} {2}$\frac{1} {2} case it appears to be weakly first-order in nature (although a second-order transition could not be ruled out entirely).     

Optical properties of new organic conductors based on the BEDT-TSeF molecule (the κ-(BETS)4Hg2.84Br8 superconductor and κ-(BETS)4Hg3Cl8 Metal) in the range 300–15 K

Polarized reflectance and optical conductivity spectra of single crystals of two new isostructural organic conductors based on the BEDT-TSeF molecule, namely, the κ-(BETS)₄Hg_2.84Br₈ superconductor (T _c =2 K) and the κ-(BETS)₄Hg₃Cl₈ metal, which undergoes a smooth transition to the dielectric state near 35 K, have been obtained in the spectral region 700–6500 cm⁻¹ at temperatures of 300–15 K. At 300 K, the spectra of both compounds are nearly identical and differ from the Drude spectrum characteristic of metals. The nature of the observed difference is discussed, and the spectra are described in terms of a cluster approach with inclusion of electron-electron correlations in the Hubbard approximation combined with the Drude model. The parameters of the theory were determined, including the electron transfer integrals between molecules in a cluster. The spectra in the conducting plane of the crystals were found to be essentially anisotropic, which should be assigned to specific features of in-plane interaction between molecules. The spectra of the superconductor and the metal become increasingly different as the temperature is lowered. The spectra of the metal obtained for T<150 K exhibit splitting of the broad electronic maximum in the mid-IR region into two bands, which is accompanied by a splitting of a vibronic feature deriving from electron interaction with intramolecular BETS vibrations of ν₃(A _g ) symmetry. No such splitting is observed in the superconductor spectra with decreasing temperature. __________ Translated from Fizika Tverdogo Tela, Vol. 46, No. 11, 2004, pp. 1921–1929. Original Russian Text Copyright ? 2004 by Vlasova, Drichko, Petrov, Semkin, Zhilyaeva, Lyubovskaya, Olejniczak, A. Kobayashi, H. Kobayashi.     

Magnetoresistivity and acoustoelectronic effects in a tilted magnetic field in <Emphasis Type="Italic">p</Emphasis>-Si/SiGe/Si structures with an anisotropic <Emphasis Type="Italic">g</Emphasis> factor

Magnetoresistivity ρ _xx and ρ _xy and the acoustoelectronic effects are measured in p-Si/SiGe/Si with an impurity concentration p = 1.99 × 10¹¹ cm⁻² in the temperature range 0.3–2.0 K and an tilted magnetic field up to 18 T. The dependence of the effective g factor on the angle of magnetic field tilt θ to the normal to the plane of a two-dimensional p-Si/SiGe/Si channel is determined. A first-order ferromagnet-paramagnet phase transition is observed in the magnetic fields corresponding to a filling factor ν = 2 at θ ≈ 59°–60°.     

Calculation of the probability of the 4 T 2→4 A 2 transition in luminescence spectra of the Mn4+ ion in gadolinium-gallium garnet

The ⁴ T ₂→⁴ A ₂ transition of the Mn⁴⁺ ion has been observed in the luminescence spectrum of Mn⁴⁺:Cd₃Ga₅O₁₂ upon intense laser pumping. It is shown that an increase in the intensity of the ⁴ T ₂→⁴ A ₂ transition with respect to the ² E→ ⁴ A ₂ transition in these crystals with increasing pumping power relates to the increased role of induced transitions. The intensity of this process is greater in the region where the ² E→⁴ A ₂ and ⁴ T ₂→⁴ A ₂ transition bands overlap most, which leads to an increase of the zero-phonon line of the latter transition, peaking at a wavelength of 694 nm. The rates of radiative and nonradiative recombination involving the ⁴ T ₂, ² E, and ⁴ A ₂ terms of the Mn⁴⁺ ion in gallium-gadolinium garnet (GGG) are calculated using data from luminescence spectra measured at different temperatures. The Mn⁴⁺:GGG crystal is proposed for use as an active element of lasers with continuous frequency tuning. __________ Translated from Optika i Spektroskopiya, Vol. 94, No. 4, 2003, pp. 590–596. Original Russian Text Copyright ? 2003 by Bulyarskiĭ, Zhukov, Prikhod’ko.     

Kinetic regularities and mechanism of polymerization of perfluoromethylvinyl ether at high pressures

The kinetics of the thermal polymerization of perfluoromethylvinyl ether (PFMVE) is studied at pressures of 3–13 kbar (300–1300 MPa) and temperatures of 80–260°C. The activation energy (E _act = (76 ± 3) kJ/mol) and activation volume (ΔV₀^⪼ = −(27 ± 2) cm³/mol) for the overall polymerization rate are determined. The inhibition method is used to estimate the activation energy of thermal initiation (E _in = (79.9 ± 3) kJ/mol). The quantity E _p − (1/2)E _t was calculated to be 36.6 ± 3 kJ/mol. The limiting polymerization temperature was evaluated: T _lim = (180 ± 3)°C. A mechanism of PFMVE polymerization is proposed on the assumption that the reaction is bimolecular.     

Microscopic study of low-lying yrast spectra and deformation systematics in neutron-rich <Superscript>98–106</Superscript>Sr isotopes

Variation-after-projection (VAP) calculations in conjunction with Hartree-Bogoliubov (HB) ansatz have been carried out for A = 98–106 strontium isotopes. In this framework, the yrast spectra with J ^Π ≥ 10⁺, B(E2) transition probabilities, quadrupole deformation parameter and occupation numbers for various shell model orbits have been obtained. The results of the calculation for yrast spectra give an indication that it is important to include the hexadecapole-hexadecapole component of the two-body interaction for obtaining various nuclear structure quantities in Sr isotopes. Besides this, it is also found that the simultaneous polarization of p _3/2 and f _5/2 proton subshells is a significant factor in making a sizeable contribution to the deformation in neutron-rich Sr isotopes.     

The estimates of periodic potentials in terms of effective masses

LetG _n=(A _n ⁻ , A _n ⁺ ),n≧1, denote the gaps,M _n ^± be the effective masses and Σn=[A _n−1 ⁺ ,A _n ^- ],A ₀ ⁺ =0, be the spectral bands of the Hill operatorT=−d ²/dx ²+V(x) inL ² (R), whereV is a 1-periodic real potential fromL ²(0,1). Let the length gapL _n=|G_n|, h_n be the height of the corresponding slit on the quasimomentum domain and Δ_n=π²(2n−1)−∣Σ_n∣>0 be the band reduction. Let ,n≧1, denote the gap length for the operator . Introduce the sequencesL={L_n}, h={h_n}, l={l_n}, Δ={Δ_n},M ^±={M _n ^± } and the norms ,m≧0. The following results are obtained: i) The estimates of‖V‖, ‖L‖, ‖h‖ ₁, ‖l‖₁, ‖δ‖ in terms of ‖M^±‖₂, ii) identities for the Dirichlet integral of quasimomentum and integral of potentials and so on, iii) the generation of i), ii) for more general potentials. The research described in this publication was made possible in part by grant from the Russian Fund of Fundamental Research and INTAS.     

Process πp → ππN at high energies and moderate momenta transferred to the nucleon and the determination of parameters of the f0(980) and f0(1300)

We present the results of simultaneous analysis of the S-wave ππ spectra in the reactions π ⁻ p → (ππ)_S n at p _lab=38 GeV/c (GAMS) and π ⁻ p → (ππ)_S n at p _lab=18 GeV/c (E852 Collaboration) at moderate momenta transferred to the nucleon, |t| ≲ 1.5 (GeV/c)². The t distributions are described by the Reggeized π and a ₁ exchanges provided by the leading and daughter trajectories, while the M _ππ spectra are determined by a set of scalar-isoscalar resonances. With M _ππ distributions averaged over different t intervals, we have found several solutions given by different t-channel-exchange mechanisms at |t| ∼ (0.5–1.5) (GeV/c)², with resonance parameters close to each other. We conclude that, despite a poor knowledge of the structure of the t exchange, the characteristics of resonances such as masses and widths can be reliably determined using the processes under discussion. As to pole positions, we have found (1031±10)−i(35±6) MeV for f ₀(980) and (1315±20)−i(150±30) MeV for f ₀(1300). From Yadernaya Fizika, Vol. 66, No. 5, 2003, pp. 960–972. Original English Text Copyright ? 2003 by Anisovich, Sarantsev. This article was submitted by the authors in English.     

Improved characteristics of the 22.5 keV (M1 + E2) nuclear transition in <Superscript>149</Superscript>Sm

The gamma ray and conversion electron spectra of the 22.5keV M1 + E2 nuclear transition depopulating the first excited level 5/2⁻ in ¹⁴⁹Sm were investigated in detail. A substantially improved value of 22500.2 ± 0.8 eV for the transition energy was determined that opens a possibility to establish an electron energy standard for the keV region based on the conversion electron lines of this transition. The E2 admixture parameter |δ(E2/M1)| = 0.0784 ± 0.0009 and the nuclear structure parameter λ = − 2 ± 10 were derived. The |δ(E2/M1)| and λ parameters were also obtained for all available experimental data and their mutual combination.     

Strong resonant intersubband magnetopolaron effect in heavily modulation-doped GaAs/AlGaAs single quantum wells at high magnetic fields

Electron cyclotron resonance (CR) has been studied in magnetic fields up to 32 T in two heavily modulation-δ-doped GaAs/Al_0.3Ga_0.7As single quantum well samples. Little effect on electron CR is observed in either sample in the region of resonance with the GaAs LO phonons. However, above the LO-phonon frequency energy E_LO at B>27 T, electron CR exhibits a strong avoided-level-crossing splitting for both samples at energies close to E_LO+(E₂−E₁), where E₂, and E₁ are the energies of the bottoms of the second and the first subbands, respectively. The energy separation between the two branches is large, reaching a minimum of about 40 cm⁻¹ around 30.5 T for both samples. This splitting is due to a three-level resonance between the second LL of the first electron subband and the lowest LL of the second subband plus an LO phonon. The large splitting in the presence of high electron densities is due to the absence of occupation (Pauli-principle) effects in the final states and weak screening for this three-level process.     

Calculation of band structure in (101)-biaxially strained Si

The structure model used for calculation was defined according to Vegard’s rule and Hooke’s law. Calculations were performed on the electronic structures of (101)-biaxially strained Si on relaxed Si_1−X Ge _X alloy with Ge fraction ranging from X = 0 to 0.4 in steps of 0.1 by CASTEP approach. It was found that [±100] and [00±1] valleys (δ₄) splitting from the [0±10] valley (Δ₂) constitute the conduction band (CB) edge, that valence band (VB) edge degeneracy is partially lifted and that the electron mass is unaltered under strain while the hole mass decreases in the [100] and [010] directions. In addition, the fitted dependences of CB splitting energy, VB splitting energy and indirect bandgap on X are all linear. Supported by the National Pre-research Foundation of China (Grant Nos. 51308040203 and 51408061105DZ0171)     

New Fluorescent Antitumour Cisplatin Analogue Complexes. Study of the Characteristics of their Binding to DNA by Flow Injection Analysis

The flow injection technique is applied to study the binding to DNA of new platinum complexes—E₁: ethylenediaminechlorocholylglycinateplatinum(II): [PtCl(CG)(en)], C₅₄H₉₂O₁₂Pt and E₂: ethylenediaminebischolylglycinateplatinum(II): [Pt(CG)₂(en)], C₂₈H₅₀ClN₃O₆Pt—derived from cisplatin in which the exchangeable ligands were replaced by bile acids, such that these anticancer drugs have less toxicity and less resistance is developed towards them. Both compounds are fluorescent and their fluorescence is enhanced when they form adducts with DNA, a property that is extremely useful for monitoring the cytotoxic activity and their mechanisms of action. The binding parameters to DNA of E₁ [apparent intrinsic binding constant K_E1: (11.2 ± 0.4) × 10³ M⁻¹ and maximum number of binding sites per nucleotide, n _E1: 0.121 ± 2 × 10⁻³) and E₂ (K_E2: 9.2 ± 0.7) × 10³ M⁻¹ and n _E2 0.098 ± 2 × 10⁻³] were determined following the Scatchard method and the type of binding was studied experimentally through the modifications introduced by each of the compounds into the ethidium bromide–DNA bond.     

Infrared reflectivity spectra of <Emphasis Type="Italic">η</Emphasis>-Na<Subscript>1.3</Subscript>V<Subscript>2</Subscript>O<Subscript>5</Subscript> in the charge disordered and ordered phase

We have measured the far-infrared reflectivity spectra of the sodium vanadium oxide η-Na_1.3V₂O₅ polycrystals in the wide temperature (80–300 K) and frequency (150–1500 cm⁻¹) range. Appearance of new phonon oscillators, phonon oscillator mode splitting and step-like shift of TO and LO frequencies at low temperatures are correlated with the charge-ordering phase transition, which takes place at about 120 K in this vanadium oxide.     

Emission of quasi-thermal ions from a solid bombarded by cluster ions

Comparative studies of the emission of quasi-thermal atomic and cluster ions from V, Nb, Ta, Au, and In targets bombarded by cluster ions Au _m ⁻(m = 1–9), as well as from Si and Bi targets bombarded by cluster ions Au _m ⁻(m = 1–9) and Bi _m ⁻(m = 1–5), with energy E ₀ ranging from 6 to 21 keV are carried out. In the case of bombardment by heavy cluster ions, the fraction of the quasi-thermal component in the energy spectra of sputtered atomic ions reaches 50 (for V, In, and Au), 70 (Nb), or more than 90% (Ta). In addition, quasi-thermal ions play a considerable part in the emission of small cluster ions Au₂⁺, In₂⁺, In₃⁺, and Bi _n ⁺(n = 2–7). The results of the generalizing investigation favor the presence of thermal spike conditions at cluster bombardment and their appreciable contribution to the emission of atomic and small cluster ions.     

Study of the Influence of the Excitation Laser Power Density on the Intermediate-Field Electric Modulation Component of the GaAs Photoreflection E 0-Signal

The influence of the excitation laser power density L on the amplitude A and characteristic time constant of the intermediate-field electric modulation component of photoreflection spectra measured in the region of the E ₀ fundamental transition of GaAs is studied. Crystalline samples with the charge carrier concentration n 10¹⁶ cm^–3 are investigated for L = 100 W/cm²–1 W/cm². A logarithmic dependence of the electric modulation signal on the excitation laser power density has been established for all examined samples. It is demonstrated that the observed change in the characteristic time constant does not have any noticeable effect on the dependence A(L).     

121Sb and 123Sb NQR and the heterophase structure in the SbSI ferroelectric

The temperature dependences of NQR line frequencies and widths of ¹²¹Sb (for the ±1/2→±3/2 transition) and of ¹²³Sb (for the ±1/2→±3/2 and ±3/2→±5/2 transitions), as well as of the principal components and the asymmetry parameter of the electric-field-gradient tensor at the ¹²³Sb nucleus have been studied in a SbSI crystal in the 115–325 K range. The dynamic and static factors governing the character of these relations are discussed. The ±1/2→±3/2 line in the ¹²¹Sb NQR spectrum splits into a doublet within a narrow (0.5 K) temperature interval near the ferroelectric phase transition (T _c=293 K), which is associated with the formation of a macroscopic heterophase structure in the crystal. Fiz. Tverd. Tela (St. Petersburg) 41, 1286–1292 (July 1999)