Fundamental reflectivity spectra of Hg1−xMnxTe mixed crystals in the 1.7 to 3.5 eV energy range

The room temperature reflectivity coefficient R(E) for the mixed crystals Hg_1?xMn_xTe (x up to 0.57) in the energy range 1.7 to 3.5 eV was investigated. Two distinct maxima E₁ and E₁ + Δ₁ connected with the transitions in the critical points on the [111] direction of the Brillouin zone for the samples with x up to 0.3 and the more diffused structure of R(E) for the samples with x > 0.3 was observed. A quadratic dependence of E₁ and E₁ + Δ₁ transition energy vs alloy composition with x up to 0.3 was found, with bowing coefficient c = 1.21 ± 0.02 and 1.06 ± 0.02 respectively. The energy variation of an additional shoulder probably connected with the e₁ transitions at L point of the Brillouin zone is also reported.     

Stability of bipolarons in the presence of a magnetic field

The stability of large Fröhlich bipolarons in the presence of a static magnetic field is investigated with the path integral formalism. We find that the application of a magnetic field (characterized by the cyclotron frequence ω _c) favors bipolaron formation: (i) the critical electronphonon coupling parameter α _c (above which the bipolaron is stable) decreases with increasing ω _c and (ii) the critical Coulomb repulsion strength U _c (below which the bipolaron is stable) increases with increasing ω _c. The binding energy and the corresponding variational parameters are calculated as a function of α, U and ω _c. Analytical results are obtained in various limiting cases. In the limit of strong electron-phonon coupling (α ? 1) we obtain for ω _c ? 1 that E _estim ? E _estim(ω _c = 0) + c(u)ω _c/α ⁴ with c(u) an explicitly calculated constant, dependent on the ratio u = U/α where U is the strength of the Coulomb repulsion. This relation applies both in 2D and in 3D, but with a different expression for c(u). For ω _c ? α ²? 1 we find in 3D E _estim ? ω _c - α ² A(u) ln²(ω _c/α ²), (also with an explicit analytical expression for A(u)) whereas in 2D E _estim ^2D ? ω _c - α√ω _cπ(u-2-√2)/2. The validity region of the Feynman-Jensen inequality for the present problem, bipolarons in a magnetic field, remains to be examined.     

Polarization properties of Li2−x Na x Ge4O9 (0.2 ≤ x ≤ 0.3) crystals

The polarization switching in sinusoidal fields and the pyroelectric properties of Li_2?x Na _x Ge₄O₉ (0.2 ≤ x ≤ 0.3) crystals are measured in the temperature range T _c ?T ≤ 40 K. The behavior of the P?E hysteresis loops with variations in temperature is investigated for crystals with phase transition temperatures T _c < 300 K and T _c > 300 K. It is shown that, for crystals with phase transition temperatures T _c < 300 K, the temperature dependence of the hysteresis loop exhibits a behavior typical of crystals with second-order phase transitions. The crystals with phase transition temperatures T _c > 300 K are characterized by double hysteresis loops in the temperature range T _c ?T ₁ ≈ 30 K. The correlation between the polarization properties and possible structural transformations of the Li_2?x Na _x Ge₄O₉ crystals due to the change in the concentration ratio of Na and Li ions is discussed.     

In Vivo Relaxation Time Measurements in the Human Placenta Using Echo Planar Imaging at 0.5 T

This paper presents the first in vivo measurements of the nuclear magnetic resonance relaxation times T₁ and T₂ at 0.5 T in the human placenta from 20 weeks gestational age until term, in both normal and compromised pregnancies. T₁ measurements were performed by using both an inversion recovery sequence and the Look-Locher echo planar imaging (EPI) sequence on a total of 41 women with normal pregnancies and 11 women with compromised pregnancies. T₂ measurements were performed by using a spin-echo EPI sequence on 36 women with normal pregnancies and 14 women with compromised pregnancies. In normal pregnancies, both the T₁ values measured with the inversion recovery sequence and the T₂ values were found to decrease with gestational age, the linear regression results gave T₁=−9.1t+1538 r²=0.23 p=0.03 T₂=−4.0t+338 r²=0.47 p=4 10⁻⁶ where t is the gestational age in weeks, and T₁ and T₂ are the relaxation times in milliseconds. T₁ values measured very rapidly with the Look-Locher EPI sequence, but, therefore, with a much lower signal-to-noise ratio, showed no significant trends.The T₁ values measured in the abnormal group were significantly lower than those measured in the normal group. Four out of eight patients with compromised pregnancies had placental T₁ values lying outside the 90% confidence limits for the normal population based about the regression line, significantly more than expected by chance (p = 0.005). Ten out of fourteen of the T₂ measurements in the abnormal group were below the regression line established for the normal group, with 4 lying below the 90% confidence interval, although these trends were only just significant (p = 0.06 and p = 0.03).     

Dynamic hysteresis of magnetic aggregates with non-integer dimension

We investigate the dynamic hysteresis of nanoscale magnetic aggregates by employing Monte Carlo simulation, based on Ising model in non-integer dimensional space. The diffusion-limited aggregation (DLA) model with adjustable sticking probability is used to generate magnetic aggregates with different fractal dimension D. It is revealed that the exponential scaling law A(H₀, ω)∼H₀^α·ω^β, where A is the hysteresis area, H₀ and ω the amplitude and frequency of external magnetic field, applies to both the low-ω and high-ω regimes, while exponents α and β decrease with increasing D in the low-ω regime and keep invariant in the high-ω regime. A mean-field approach is developed to explain the simulated results.     

Spatial scaling in fracture propagation in dilute systems

The geometry of fracture patterns in a dilute elastic network is explored using molecular dynamics simulation. The network in two dimensions is subjected to a uniform strain which drives the fracture to develop by the growth and coalescence of the vacancy clusters in the network. For strong dilution, it has been shown earlier that there exists a characteristic time t_c at which a dynamical transition occurs with a power law divergence (with the exponent z) of the average cluster size. Close to t_c, the growth of the clusters is scale-invariant in time and satisfies a dynamical scaling law. This paper shows that the cluster growth near t_c also exhibits spatial scaling in addition to the temporal scaling. As fracture develops with time, the connectivity length xi of the clusters increases and diverges at t_c as xi ∼ (t_c − t)^−ν, with ν = 0.83 ± 0.06. As a result of the scale-invariant growth, the vacancy clusters attain a fractal structure at t_c with an effective dimensionality d_f ∼ 1.85 ± 0.05. These values are independent (within the limit of statistical error) of the concentration (provided it is sufficiently high) with which the network is diluted to begin with. Moreover, the values are very different from the corresponding values in qualitatively similar phenomena suggesting a different universality class of the problem. The values of ν and d_f supports the scaling relation z = νd_f with the value of z obtained before.     

Analysis of the stimulated Raman spectrum of the ν16ν2 + ν18 Fermi dyad of benzene

Ionization-detected stimulated Raman spectroscopy was used to obtain high-resolution recordings of the two perpendicular bands of the benzene molecule centered at 1591.327 and 1609.518 cm⁻¹. The effective resolution was further enhanced by deconvolving the spectrum to a linewidth 0.003 to 0.004 cm⁻¹. Fine-tuning of the ionizing radiation made it possible to record the transitions belonging to each band separately, thus greatly simplifying the spectrum in the region of overlap. The strong ^sS and ^oO branches were, for the most part, completely resolved as were many lines in the weaker ^oP and ^sR branches and even in the central ^oQ and ^sQ branches. The observed bands belong to the E_2g fundamental ν₁₆ in nearly exact Fermi resonance with the combination ν₂ + ν₁₈. A detailed rovibrational analysis of the spectrum is reported. A perturbation detected in the ^sS_k branches of the lower-frequency band for K = 19 to 23 was identified as a quintic anharmonic resonance with the third overtone, 4ν₂₀, of the lowest lying fundamental ν₂₀, which is infrared- and Raman-inactive (species E_2u). Deperturbed spectroscopic constants for the interacting states are reported which reproduce the observed line positions with a standard deviation of 0.0013 cm⁻¹. The unperturbed origins of the ν₁₆ and ν₂ + ν₁₈ states are only 1.106 cm⁻¹ apart. The fundamental ν₁₆ was identified with the higher-frequency state with origin at 1600.976 cm⁻¹.     

Supermembranes and the signature of spacetime

We consider extended objects with s space and t time world-volume dimensions moving in a spacetime with S⩾ s space and T⩾t time dimensions. The requirements of spacetime supersymmetry and world-volume fermionic gauge invariance severely restrict the possible values of S and T. If we furthermore insist that the transverse group SO(S − s, T−t) be compact to avoid ghosts, then t=T. The results may be interpreted as a set of superconformal field theories with s+t⩽6 and N⩽8 whose superconformal groups are in one-to-one correspondence with those in Nahm's classification. Although the choice t = T = 1 is not uniquely singled out, it does seem to play a preferred role.     

Microstructure and magnetic characterization of the Sm-CoMn co-substituted SrCaM hexagonal ferrites

A series of Sm-CoMn substituted hexagonal ferrites with chemical composition of Sr_0.85-xCa_0.15Sm_xFe_12-y(Co_0.5Mn_0.5)_yO₁₉ (0.00?≤?x?≤?0.60, (0.00?≤?y?≤?0.50) were synthesized by the solid-state reaction method. Microstructure and magnetic properties of the hexaferrites have been investigated by the X-ray diffraction, field emission scanning electron microscopy and a permanent magnetic measuring system. A single magnetoplumbite phase is exhibited in the hexaferrites with the substitutiom of Sm (0.00?≤?x?≤?0.12) and CoMn (0.00?≤?y?≤?0.10) contents. For the hexaferrites containing Sm (x?≥?0.24) and CoMn (y?≥?0.20), impurity phases are observed in the structure. The FESEM micrographs exhibit that the hexaferrites with different Sm-CoMn contents have formed hexagonal structures and the grain size of the hexaferrites remains unchanged with increasing Sm-CoMn content. The remanence (B_r), H_k/H_cj ratios, and maximum energy product [(BH)_max] decrease with increasing Sm-CoMn content (0.00?≤?x?≤?0.60, (0.00?≤?y?≤?0.50). Instrinsic coercivity (H_cj) and magnetic induction coercivity (H_cb) increase with increasing Sm-CoMn content (0.00?≤?x?≤?0.12, 0.00?≤?y?≤?0.10), and then decrease with increasing Sm-CoMn content (0.12?≤?x?≤?0.36, 0.10?≤?y?≤?0.30), while for the hexaferrites with Sm (x?≥?0.36) and CoMn (y?≥?0.30), with increasing Sm-CoMn content, H_cj increases and H_cb decreases.     

Systematic study of impact parameter dependent K-vacancy probabilities in near symmetric gas- and solid-target collision systems

K-vacancy production probabilitiesP _K (b) were measured with gas and solid targets byK-x-ray particle coincidences (impact parameterb is determined by the particle detection angle) in the region of light (Z _P ?Z _T ?10) up to intermediate heavy (Z _P ?Z _T ?36) collision systems. The measuredP _K (b) reveal a very strong difference in shape between solid and gas targets independent ofZ. Only theP _K (b) measured with gas targets and those measured with solid targets at very small impact parameters show reasonable good agreement with the 2pπ _x-2pσ rotational coupling model. At largeb theP _K (b) from solid targets are strongly influenced by a multiple collision effect, where projectileL-vacancies seem to be produced in collisions beforeK-vacancy production. However, this effect cannot be understood just by a two collision process whereL-vacancy production and 2pπ _x-2pσ rotational coupling occurs in consecutive collisions.     

Observation of a transition from BCS to HTSC-like superconductivity in Ba1 − x KxBiO3 single crystals

The temperature dependences of the upper critical field B _c2(T) and surface impedance Z(T) = R(T) + iX(T) have been measured in Ba_{1 ? x} K_xBiO₃ single crystals with transition temperatures 6 ≤ T _c ≤ 32 K (0.6 > x > 0.4). A transition from the BCS to an unusual type of superconductivity has been revealed: B _c2(T) curves of the crystals with T _c > 20 K have positive curvature (as in some HTSCs), and those of the crystals with T _c < 15 K described by the usual Werthamer-Helfand-Hohenberg (WHH) formula. The R(T) and X(T) dependences of the crystals with T _c ≈ 32 K and T _c ≈ 11 K in the temperature range T ? T _c are linear (as in HTSCs) and exponential (BCS), respectively. The experimental results are discussed using the extended saddle point model by Abrikosov.     

Spatial evolutions of inversionless gain and lasing field intensity in an open V-type system with SGC

This paper studies manly spatial evolution of gain without inversion (GWI) and the Rabi frequency E (intensity ?_p) of the probe field in an open V-type three-level inversionless lasing system with spontaneously generated coherence (SGC) for both cases with and without Doppler broadening. We found that: (1) Varying sizes of SGC strength (measured by angle θ), atomic exit rate (r₀) and ratio (S) of the atomic injection rates has remarkable effect on spatial evolutions of GWI and E (?_p). This effect in the case with Doppler broadening is similar to but weaker than that in the case without Doppler broadening. (2) Regardless of that Doppler broadening is present or not, GWI and E (?_p) increase with increase of θ, r₀ and S in certain value ranges of θ, r₀ and S; in the case with SGC we can obtain GWI and E (?_p) much larger than those in the case without SGC, while by choosing values of γ₀ and S, in the open system we can obtain LWI gain and E (?_p) much larger than those in the corresponding closed system. (3) The propagation distance in which GWI exists in the case with Doppler broadening is longer than that in the case without Doppler broadening; in the case without Doppler broadening, we can obtain larger GWI than that in the case with Doppler broadening; but in the case with Doppler broadening, we can obtain larger E (?_p) than that in the case without Doppler broadening.     

Classical bouncing Universes from vector fields

For the anisotropic Universe filled with massless vector field in the General Relativity frame we obtain bouncing solution for one of scale factors. We obtain the Universe with finite maximal energy density, finite value of R,RμνRμν,RμναβRμναβ and non-zero value of a scale factor for directions transverse to a vector field. Such a bounce can be also obtained for a massive vector field with kinetic initial conditions, which gives isotropic low energy limit. We discuss the existence of a bounce for a massless vector field with additional matter fields, such as cosmological constant or dust. We also discuss bouncing solution for massless vector field domination in n+2-dimensional space-time.     

Effects of annealing on localized states in amorphous Ge films

Annealing behaviors of the activation energy for the electrical conduction E_σ, that for the thermoelectric power E_s, the optical gap E^opt_g, and the spin density in evaporated amorphous Ge are investigated. E_σ is independent of E_s and E^opt_g, and the rates of variation of E^opt_g and E_s with annealing temperature are connected by △E^opt_g = 2.5△E^opt_s. It is suggested that the position of the Fermi level does not change with annealing in contrast with amorphous Si, and the edge of the localized tail state shifts with annealing.     

Shear viscosity of the Quark–Gluon Plasma from a virial expansion

We calculate the shear viscosity η in the quark–gluon plasma (QGP) phase within a virial expansion approach with particular interest in the ratio of η to the entropy density s, i.e. η/s. The virial expansion approach allows us to include the interactions between the partons in the deconfined phase and to evaluate the corrections to a single-particle partition function. In the latter approach we start with an effective interaction with parameters fixed to reproduce thermodynamical quantities of QCD such as energy and/or entropy density. We also directly extract the effective coupling α _V for the determination of η. Our numerical results give a ratio η/s≈0.097 at the critical temperature T _c, which is very close to the theoretical bound of 1/(4π). Furthermore, for temperatures T≤1.8T _c the ratio η/s is in the range of the present experimental estimates 0.1–0.3 at RHIC. When combining our results for η/s in the deconfined phase with those from chiral perturbation theory or the resonance-gas model in the confined phase we observe a pronounced minimum of η/s close to the critical temperature T _c.     

On Two-Temperature Problem for Harmonic Crystals

We consider the dynamics of a harmonic crystal in d dimensions with n components, d,n≥1. The initial date is a random function with finite mean density of the energy which also satisfies a Rosenblatt- or Ibragimov–Linnik-type mixing condition. The random function is translation-invariant in x ₁,...,x _d?1 and converges to different translation-invariant processes as x _d →±∞, with the distributions μ _±. We study the distribution μ _t of the solution at time $t \in \mathbb{B}$ . The main result is the convergence of μ _t to a Gaussian translation-invariant measure as t→∞. The proof is based on the long time asymptotics of the Green function and on Bernstein's “room-corridor” argument. The application to the case of the Gibbs measures μ _±=g _± with two different temperatures T _± is given. Limiting mean energy current density is ?(0,...,0,C(T ₊?T _?)) with some positive constant C>0 what corresponds to Second Law.     

New features of a smectic-A-crystal-B phase transition in a homologous series of liquid crystals

The components L _j of the Lorentz tensor and the polarizability density of molecules G in the smectic-A and crystalline-B phases have been determined for homologues of the series of alkyl-p-(4-alkoxybenzylideneamino-)cinnamates. The quantity L _j (G) in both phases is a linear (quadratic) function of the orientational order parameter of molecules S, which is invariant (noninvariant) with respect to the A-B transition, which is manifested in the form of jumps δL _j and δG and enhancement of the G(S) dependence. An increase in the length of terminal molecular chains and weakening of interlayer correlation of molecules are accompanied by strengthening of the A-B transition of the first order and G(S) dependences in both phases together with an increase in δL _j and δG. Change δG and dependence G(S) in the B phase are related to change in the conformation (flattening) of aromatic molecular cores.     

1/N topological expansion for lattice QCD

A U(N_c) gauge theory with a global U(N_f) flavor symmetry is investigated in the limit both N_c and N_f large with the ratio ξ ≡ N_f/N_c fixed.     

On the fractal dimension of the Henon attractor

We re-measured the fractal dimension of the Hénon attractor by direct box-counting. We paid special attention to (a) optimal speed and use of storage, and (b) systematic corrections due to the finiteness of the number of iterations. Covering with grids of up to 9600 × 9600 boxes, we observe that the number N(?, n) of boxes visited after n iterations obeys a scaling law N(?, ∞) - N(?, n) ≈ const × ?^-αn^-β (for n → ∞) with α = 2.42 ± 0.15, β = 0.89 ± 0.03. Using this extrapolate to n → ∞, we obtain D = 1.28 ± 0.01 in disagreement with previous box-counting estimates, but in agreement with a recent indirect evaluation.     

The electrochemistry of InP in Br2/HBr solutions and its relevance to etching behaviour

Etch rate-potential curves of p-InP in HBrandBr₂/HBr solutions in the dark and under illumination were correlated with current-potential curves. It was found that InP is etched via a “chemical” mechanism both by HBrandBr₂. In aqueous HBr solutions InP is only etched at a significant rate at concentrations higher than 5 mol/ℓ. The Br₂ etchants contained 4.5M HBr; in this case HBr only serves to complex Br₂toBr^-₃. The etch rate in Br₂/HBr solutions is mass-transport controlled at InP(001) and kinetically controlled at InP with the (111) In orientation. The macroscopically obtained results are consistent with the profiles etched at resist edges in InP. The electrochemistry of Br₂ at p-InP under illumination reveals some interesting aspects with regard to the agreement between the etch rate and the Br₂ reduction.