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1.
In most homologous series of compounds made of bent-core (BC) molecules, the B2 B1 and B6 phases occur as the chain length decreases. We have studied binary mixtures of the compound 1,3-phenylene bis[4-(3-methylbenzoyloxy)]4-n-dodecylbiphenyl 4’-carboxylate (BC12) which exhibits the B2 phase with the compound 4-biphenylyl 4’-n-undecyloxybenzoate (BO11) made of rod-like (R) molecules. We find the above sequence
of occurrence of the B phases with increasing concentration of BOH. In this paper we describe the physical origin for the
formation of these phases in both pure compounds and in the mixtures. We have also found the occurrence of the biaxial smectic
A phase when the BO11 concentration is increased to 87–95.5 mol%. We also report on another binary system composed of BC12
and 4-n-octyloxy 4’- cyanobiphenyl (8OCB) made of R molecules. This system exhibits the biaxial smectic A phase down to 30°C. Using polarized
infrared spectroscopy we find that the mutual orientation of the R and BC molecules in the SmAdb liquid crystal is such that the arrow axes of the BC molecules are along the layer normal of the partial bilayer smectic
structure formed by the rods. We also describe unusual growth patterns obtained when the nematic phase transforms to the SmAdb phase in a mixture with 24 mol% of BC12. 相似文献
2.
Yu. A. Simonov 《JETP Letters》2008,87(3):121-123
The dipion spectrum for the ϒ(nS) → ϒ(n′S) transition with n < 4 has the form dw/dq ∼ (phase space) |η − x|2, with x = q
2 − 4m
π2 / (ΔM)2 − 4m
π2 < q
2 ≡ M
ππ2, and ΔM = M(nS) − M(n′S). The parameter η is calculated and the spectrum is shown to reproduce the experimental data for all three types of decays:
3 → 1, 2 → 1, and 3 → 2 with η ≈ 0.5, 0, and −3, respectively.
The text was submitted by the author in English. 相似文献
3.
An electron paramagnetic resonance (EPR) line shape simulation for nitroxide spin probes in the motional narrowing region
was carried out assuming axially symmetricg andA tensors and with different anisotropies of rotationN (=R
∥/R
⊥) whereR
∥ andR
⊥ are, respectively, elements of the diffusion tensor along and perpendicular to its principal axisz′. In addition, it was assumed that the principal axes of the diffusion tensor coincide with the molecular axes. Each of three
casesz′=x,z′=y andz′=z, which result from cyclic permutations of the molecular axesx, y andz with thez′,y′ andx′ axes of the diffusion tensor, yields its typical EPR spectrum characterized by the relative intensities of the low-, center-
and high-field lines. The parameter δ defined by and calculable from the intensities of the three lines was found to vary
linearly withN for thez′=x andz′=y cases and, as anticipated, to be practically constant at a value of 1 for thez′=z case. This suggested a method for estimatingN for a probe from its EPR spectrum. Experimental spectra over a narrow temperature range (1°C) in the vicinity of the nematic-to-isotropic
transition (about 34.6°C) ofN-(4-n-butylbenzilidene)-4-amino-2,2,6,6-tetramethylpiperidine-1-oxide at a mole fraction of 1·10−3 in 4-n-pentyl-4′-cyanobiphenyl showed a pattern of peak heights characteristic of thez′=x case with δ values that gave, neglecting effects of the mean field, higher and lowerN values in the nematic and isotropic regions, respectively. Analysis of other similar systems in the literature gave similar
results. 相似文献
4.
J. Rutkowska P. Perkowski W. Piecek Z. Raszewski J. Kędzierski 《Opto-Electronics Review》2008,16(3):262-266
Dielectric measurements were performed for two smectogens, being structurally analogous with the opposite space orientation
of the-COO ester group in the molecular core, using a HP4192A impedance analyser. Gold coated electrode cells of different
thicknesses were used. Temperature dependences of the relaxation frequency fR and the inverse of the dielectric strength Δε′┴
−1 obtained by fitting experimental values of perpendicular components, i.e., the real ε′┴ and the imaginary ε″┴ parts of the complex dielectric permittivity to the Cole-Cole equation as well as the investigation of modification of relaxation
processes under bias were determined for the SmC*, SmA*, and N* phases of studied compounds. One can conclude, on the basis of the above results, that dielectric relaxation processes observed
by us in the studied compounds are similar to those of the soft and Goldstone mode typically observed by others in the SmC*, SmA*, and N* phases. It is concluded from acomparison of their properties with other related compounds that the link between the biphenyl
moiety and-COO ester group is closely related to the stability of smectic phases. 相似文献
5.
O.V. Bychuk B. O'Shaughnessy N.J. Turro 《The European physical journal. E, Soft matter》2001,4(3):281-291
We propose and theoretically study an experiment designed to measure short-time polymer reaction kinetics in melts or dilute
solutions. The photolysis of groups centrally located along chain backbones, one group per chain, creates pairs of spatially
highly correlated macroradicals. We calculate time-dependent rate coefficients κ(t) governing their first-order recombination kinetics, which are novel on account of the far-from-equilibrium initial conditions.
In dilute solutions (good solvents) reaction kinetics are intrinsically weak, despite the highly reactive radical groups involved.
This leads to a generalised mean-field kinetics in which the rate of radical density decay - ∼S(t), where S(t) ∼t
- (1 + g/3) is the equilibrium return probability for 2 reactive groups, given initial contact. Here g≈ 0.27 is the correlation hole exponent for self-avoiding chain ends. For times beyond the longest coil relaxation time τ,
- ∼S(t) remains true, but center of gravity coil diffusion takes over with rms displacement of reactive groups x(t) ∼t
1/2 and S(t) ∼ 1/x
3(t). At the shortest times ( t
10-6s), recombination is inhibited due to spin selection rules and we find ∼tS(t). In melts, kinetics are intrinsically diffusion-controlled, leading to entirely different rate laws. During the regime limited
by spin selection rules, the density of radicals decays linearly, n(0) - n(t) ∼t. At longer times the standard result - ∼d
3(t)/d (for randomly distributed ends) is replaced by ∼d2x
3(t)/d
2 for these correlated initial conditions. The long-time behavior, t > τ, has the same scaling form in time as for dilute solutions.
Received 18 May 2000 相似文献
6.
This paper considers Hardy–Lieb–Thirring inequalities for higher order differential operators. A result for general fourth-order
operators on the half-line is developed, and the trace inequality
tr( (-D)2 - CHRd,2\frac1|x|4 - V(x) )-g £ Cgò\mathbbRd V(x)+g+ \fracd4 dx, g 3 1 - \frac d 4,\mathrm{tr}\left( (-\Delta)^2 - C^{\mathrm{HR}}_{d,2}\frac{1}{|x|^4} - V(x) \right)_-^{\gamma}\leq C_\gamma\int\limits_{\mathbb{R}^d} V(x)_+^{\gamma + \frac{d}{4}}\,\mathrm{d}x, \quad \gamma \geq 1 - \frac d 4, 相似文献
7.
This paper is concerned with the asymptotic behavior solutions of stochastic differential equations dy
t
=dω
t
−∇Γ(y
t
) dt, y
0=0 and d=2. Γ is a 2 &\times; 2 skew-symmetric matrix associated to a shear flow characterized by an infinite number of spatial scales
Γ12=−Γ21=h(x
1), with h(x
1)=∑
n
=0
∞γ
n
h
n
(x
1/R
n
), where h
n
are smooth functions of period 1, h
n
(0)=0, γ
n
and
R
n
grow exponentially fast with n. We can show that y
t
has an anomalous fast behavior (?[|y
t
|2]∼t
1+ν with ν > 0) and obtain quantitative estimates on the anomaly using and developing the tools of homogenization.
Received: 1 June 2001 / Accepted: 11 January 2002 相似文献
8.
B. B. Krichevtsov R. V. Pisarev A. A. Rzhevskii V. N. Gridnev H. -J. Weber 《JETP Letters》1998,67(8):602-606
Spectral measurements of the Voigt birefringence Δn were performed for the cubic magnetic semiconductor Cd1−x
Mn
x
Te (0≤x≤0.52) in order to investigate how the exchange interaction of Mn2+ ions with itinerant electrons depends on the electron wave vector. It was determined that Δn/x
2 is independent of x and the magnetic field direction, i.e., the effect is due to the Mn2+ ions and is isotropic. Below the band gap edge the dispersion of the birefringence Δn can be described well in all samples by the unusual dependence Δn∼(E
g
−ℏω) −3.5. This can be explained by a decrease of the exchange interaction of Mn2+ ions with itinerant electrons with increasing distance from the center of the Brillouin zone.
Pis’ma Zh. éksp. Teor. Fiz. 67, No. 8, 569–573 (25 April 1998) 相似文献
9.
O. Cozar I. Ardelean V. Simon L. David N. Vedean V. Mih 《Applied magnetic resonance》1999,16(4):473-480
Two lead-phosphate glass systems doped with both copper and vanadium ions in different ratios were studied by EPR (electron paramagnetic resonance) method. EPR spectra and parameters (g‖ = 2.44, g⊥ = 2.08 andA ‖ = 117.6 · 10−4 cm−1) obtained for x(CuO · V2O5)(l−x)[2P2O5 · PbO] glasses withx ≤ 10 mol% suggest a tetrahedral (Td) coordination of Cu2+ ions and not a tetragonally elongated octahedron as has been assumed in previous works. The ground state of the paramagnetic electron is thed xy copper orbital with a 4pz contribution of 6%. For 20 ≤x ≤ 40 mol% a broad line (ΔB = 307 G) characteristic for clustered ions appears atg = 2.18. The V4+ ions are evidenced only in the spectra of x(CuO · 2V2O5)(1 −x)[2P2O5 · PbO] glasses and the resonance parameters suggest a pentacoordinated C4v local symmetry for these ions. The hyperfine structures characteristic for Cu2+ and V4+ ions disappear for 10 ≤x ≤ 40 mol% due to the mixed exchange Cu2+−V4+ pair formation in these glasses. 相似文献
10.
A. S. Emelyanov S. I. Raevskaya F. I. Savenko I. P. Raevski M. A. Malitskaya E. I. Sitalo 《Bulletin of the Russian Academy of Sciences: Physics》2009,73(1):123-124
It is established that, as a result of application of even a small (∼0.5 kV cm−1) bias field E, the maximum of the piezoelectric coefficient d
31 in (1 − x)Pb(Mg1/3Nb2/3)O3 − xPbTiO3(x = 0.06, 0.13) crystals shifts from the Vogel-Fulcher temperature to the critical point in the E-T phase diagram of a given composition. The field dependence of the d
31(T) peak magnitude has a maximum near the E values corresponding to the critical point.
Original Russian Text ? A.S. Emelyanov, S.I. Raevskaya, F.I. Savenko, I.P. Raevski, M.A. Malitskaya, E.I. Sitalo, 2009, published
in Izvestiya Rossiiskoi Akademii Nauk. Seriya Fizicheskaya, 2009, Vol. 73, No. 1, pp. 132–133. 相似文献
11.
A. V. Tyunis V. A. Shaburov Yu. P. Smirnov A. E. Sovestnov 《Physics of the Solid State》1997,39(9):1337-1340
The electronic structure of U and Ge in the solid solutions U(Al1−x
Gex)3 is investigated by measuring x-ray line shifts. It is shown that uranium has the mixed valence U3+ [Rn](5f
3)-U4+ [Rn](5f
2) over the entire composition range (0⩽x⩽1) and that the population of the uranium 5f shell increases by ∼0.28 5f electrons/U atom from UAl3 (x=0) to UGe3 (x=1). The electronic structure of Ge is close to the electronic structure of Ge metal over the entire composition range 0<x⩽1. No variation of the population of the Ge 4p shell is detected to within the experimental error (∼0.1 4p electrons/Ge atom) as the composition varies from x=0.2 to 1. It is established that the delocalization of a U 5f electron occurs as a result of its transition to the s or d band of the same uranium atom.
Fiz. Tverd. Tela (St. Petersburg) 39, 1505–1508 (September 1997) 相似文献
12.
《The European Physical Journal C - Particles and Fields》2007,49(2):545-558
Ratios of the ψ′ over the J/ψ production cross sections in the dilepton channel for C, Ti and W targets have been measured in 920 GeV proton-nucleus
interactions with the HERA-B detector at the HERA storage ring. The ψ′ and J/ψ states were reconstructed in both the μ+μ- and the e+e- decay modes. The measurements covered the kinematic range -0.35≤xF≤0.1 with transverse momentum pT≤4.5 GeV/c. The angular dependence of the ratio has been used to measure the difference of the ψ′ and J/ψ polarization. All results for the muon and electron decay channels are in good agreement: their ratio, averaged over
all events, is Rψ′(μ)/Rψ′(e)=1.00±0.08±0.04. This result constitutes a new, direct experimental constraint on the double ratio of branching fractions,
(B′(μ)B(e))/(B(μ)B′(e)), of ψ′ and J/ψ in the two channels. The ψ′ to J/ψ production ratio is almost constant in the covered xF range and shows a slow increase with pT. 相似文献
13.
N. V. Ivashin 《Journal of Applied Spectroscopy》2010,77(1):28-37
The structure and vibrations of neutral porphine metal complexes (Me-P, Me = Co, Ni, Cu) and their d-anionic forms with an
additional electron localized in vacant dx2 -y2 - d_x^2 -_y^2 - and dz2 d_z^2 -orbitals are compared based on calculations by a DFT method. It is shown that such electron population causes a significant
increase of the electronic charge on the macrocycle rather than on the Me atom and is accompanied by a considerable redistribution
of π- and σ-electron densities (ρπ ρσ). A predominant gain of ρπ (0.49e) is found for the monoanion of Co-P (Co-P–, dz2 d_z^2 -monoanion); of ρσ (0.6e), for Ni-P–( dx2 -y2 - d_x^2 -_y^2 - monoanion). These features are reflected in both the structure of the anions and the behavior of their vibrational frequencies.
The greatest frequency shifts among IR active modes when populating the dz2 d_z^2 - and dx2 -y2 - d_x^2 -_y^2 - orbitals occur for out-of-plane vibrations (>30 cm–1) and in-plane modes (34–46 cm–1) involving MeN- and CαCm-bonds, respectively. Abnormally large frequency lowering is found for B1g-type modes (active in the resonance Raman spectrum) involving mainly CαCm-, CβCβ-, CαCβ-, and MeN-bonds. This is related to a change in the dπ-eg interaction strength during such vibrations that contributes to a decrease in the corresponding force constants. 相似文献
14.
Pramod Bhatt E. Carlegrim A. Kanciurzewska M. P. de Jong M. Fahlman 《Applied Physics A: Materials Science & Processing》2009,95(1):131-138
Thin film iron-tetracyanoethylene Fe(TCNE)
x
, x∼2, as determined by photoelectron spectroscopy, was grown in situ under ultra-high vacuum conditions using a recently developed
physical vapor deposition-based technique for fabrication of oxygen- and precursor-free organic-based molecular magnets. Photoelectron
spectroscopy results show no spurious trace elements in the films, and the iron is of Fe2+ valency. The highest occupied molecular orbital of Fe(TCNE)
x
is located at ∼1.7 eV vs. Fermi level and is derived mainly from the TCNE− singly occupied molecular orbital according to photoelectron spectroscopy and resonant photoelectron spectroscopy results.
The Fe(3d)-derived states appear at higher binding energy, ∼4.5 eV, which is in contrast to V(TCNE)2 where the highest occupied molecular orbital is mainly derived from V(3d) states. Fitting ligand field multiplet and charge
transfer multiplet calculations to the Fe L-edge near edge X-ray absorption fine structure spectrum yields a high-spin Fe2+ (3d6) configuration with a crystal field parameter 10Dq∼0.6 eV for the Fe(TCNE)
x
system. We propose that the significantly weaker Fe-TCNE ligand interaction as compared to the room temperature magnet V(TCNE)2 (10Dq∼2.3 eV) is a strongly contributing factor to the substantially lower magnetic ordering temperature (T
C
) seen for Fe(TCNE)
x
-type magnets. 相似文献
15.
The formation of inhomogeneities in CdxHg1-x
Te alloys upon post-growth cooling or upon low-temperature annealing is simulated numerically. The mechanism of the formation
of inhomogeneities is based on the diffusion instability in a system involving mercury atoms located at lattice sites, interstitial
mercury atoms, and cation vacancies. It is revealed that, upon prolonged annealing of the CdxHg1-x
Te alloys with a cadmium content x = 0.2 at a temperature of ∼200°C, the concentrations of mercury atoms at lattice sites, interstitial mercury atoms, and vacancies
are characterized by an inhomogeneous nearly periodical distribution arising from a small fluctuation when the initial equilibrium
concentration of interstitial mercury atoms exceeds a threshold value (∼3 × 1017 cm−3). The spatial and time scales of the concentration distribution are determined primarily by the equilibrium concentration
of vacancies and do not depend on the type of fluctuation involved. The spatial period of the concentration distribution increases
linearly from 0.01 to 3.00 μm as the equilibrium concentration of vacancies changes from 1019 to 1014 cm−3. At lower concentrations of vacancies, the periodic structure is formed for a considerably longer time. 相似文献
16.
Polarization-optical studies and measurements of the birefringence Δn and the angle of rotation of the optical indicatrix for the (NH4)2NbOF5 crystal have been carried out in the temperature range 100–350 K. Two anomalies of the birefringence have been revealed at
the temperatures T
01 = 258 K and T
02 ≈ 219 K. According to the twinning pattern, the crystal undergoes successive changes in symmetry: orthorhombic ↔ monoclinic
1 ↔ monoclinic 2. The twofold axis of the monoclinic phases (or the normal to the plane) is directed along [001]or. The effect of the uniaxial compression along [011]or and the electric field E ≈ 25 kV/cm along [100]or on the twin structure has been studied. The ferroelastic phase transition at T
01 is due to the appearance of the shear deformation x
4(T) and is accompanied by significant anomalies of the birefringence. Strong pretransition phenomena mask the jumps in the birefringence
Δn(T) and in the angle of rotation of the indicatrix φ(T) at T
01. 相似文献
17.
We study stable “bookshelf” smectic-A structures within a very thin plane-parallel cell of thickness L in which the mismatch between surface preferred (d
s) and intrinsic (d0) smectic layer thicknesses occurs. The Landau-Ginzburg approach based on a complex smectic order parameter is used. For a
weak enough smectic positional anchoring strength W smectic layers adopt the modified bookshelf profile. In a thick enough cell with increasing W a lattice of edge dislocations is continuously formed at the confining surfaces and then depinned from them. The structure
with dislocations is formed when the condition d
0/(
d
0/d
s - 1) ∼ 2 is fulfilled, where is the positional surface anchoring extrapolation length. If the cell is thin enough the dislocations formed at opposite
cell plates annihilate and consequently the smectic layers adopt a locked bookshelf structure. This transition is discontinuous
and takes place when d
0/(L
d
0/d
s - 1) ∼ 5 is realized. To observe these transitions in a cell of thickness L∼ 1μm the conditions W∼ 10-6
J/m
2 and
d
0/d
s - 1∼ 5 . 10-4 have to be fulfilled. All the three qualitatively different structures coexist at the triple point.
Received 21 February 2002 相似文献
18.
The excess 1/f noise in a random lattice with bond resistances r∼exp(−λx), where x is a random variable and λ≪1, is studied theoretically. It is shown that if the correlation function {δr
2}∼r
r
θ+2, then the relative spectral density of the noise in the system is expressed as C
e∼λm exp(−λ(1−p
c)), where p
c is the percolation threshold and m=νd (ν is the critical exponent of the correlation length and d is the dimensionality of the problem). It is hypothesized that the exponent m possesses a dual universality: It is independent of 1) the geometry of the lattice and 2) the θ-mechanism responsible for
the generation of the local noise. Numerical modeling in a three-dimensional lattice gives m=52.3 for θ=1 and θ=0, in agreement with the hypothesis.
Pis’ma Zh. éksp. Teor. Fiz. 63, No. 8, 614–618 (25 April 1996) 相似文献
19.
A. V. Okotrub L. G. Bulusheva A. G. Kudashov V. V. Belavin D. V. Vyalikh S. L. Molodtsov 《Applied Physics A: Materials Science & Processing》2009,94(3):437-443
Arrays of vertically aligned nitrogen-doped carbon (CN
x
) nanotubes have been synthesized by decomposition of aerosol mixture of acetonitrile and ferrocene at 850°C. Nitrogen concentration
in the outer shells of the CN
x
nanotubes was found from X-ray photoelectron spectroscopy (XPS) data to reach ∼6%. The XPS N 1s spectra and N 1s near-edge X-ray absorption fine structure (NEXAFS) spectra identified three chemical forms of nitrogen in the CN
x
nanotube arrays: pyridine-like, graphitic, and molecular nitrogen. The π
* resonance of molecular nitrogen showed clear polarization dependence that indicates predominant orientation of N2 molecules along the nanotubes axis. The estimated range of the polar angle distribution of the N2 molecules orientation in the CN
x
nanotube array amounts to 15°. 相似文献
20.
Stefano Galatolo 《Journal of statistical physics》2006,123(1):111-124
In this note we prove that for equilibrium states of axiom A systems having positive dimension the time τ
B
(x) needed for a typical point x to enter for the first time in a typical ball B with radius r behaves for small r as τ
B
(x)∼ r
−d
where d is the local dimension of the invariant measure at the center of the ball. A similar relation is proved for a full measure set of interval exchanges. Some applications to Birkoff averages of unbounded (and not L
1) functions are shown. 相似文献
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