共查询到20条相似文献,搜索用时 15 毫秒
1.
S. A. Nemov M. K. Zhitinskaya R. V. Parfen’ev D. V. Shamshur 《Physics of the Solid State》1998,40(7):1096-1097
A study is reported of the effect of low-level germanium additions (∼0.01–0.1 at. %) on the parameters of the superconducting
transition, viz. the critical temperature T
c, the second critical magnetic field H
c2, and
in PbTe doped with 2 at. % Tl, which are derived from the dependence of the electrical resistivity of a sample on temperature
(0.4–4.2 K) and magnetic field (0–1.3 T). The discontinuity revealed by experimental data is related to the onset of a Ge-induced
structural phase transition.
Fiz. Tverd. Tela (St. Petersburg) 40, 1204–1205 (July 1998) 相似文献
2.
It is shown that the slope of the upper critical field
in superconductors with d pairing drops rapidly with increasing concentration of normal impurities, while in superconductors with anisotropic s pairing
increases and reaches the well-known asymptotic level characteristic for the isotropic case. This difference makes it possible,
in principle, to employ measurements of H
c
2 in disordered superconductors as an experimental method for determining the type of pairing in high-T
c
superconductors and systems with heavy fermions.
Pis’ma Zh. éksp. Teor. Fiz. 63, No. 5, 347–352 (10 March 1996) 相似文献
3.
Buzza DM 《The European physical journal. E, Soft matter》2004,13(1):79-86
Using the complementary approaches of Flory theory and the overlap function, we study the molecular weight distribution and conformation of hyperbranched polymers formed by the melt polycondensation of A-RN0-Bf - 1 monomers in their reaction bath close to the mean field gel point pA = 1, where pA is the fraction of reacted A groups. Here
, N0 is the degree of polymerisation of the linear spacer linking the A group and the f-1 B groups and condensation occurs exclusively between the A and B groups. For
, we assume that the number density of hyperbranched polymers with degree of polymerisation N generally obeys the scaling form
and we explicitly show that this scaling assumption is correct in the mean field regime (here Nl is the largest characteristic degree of polymerisation and the function
cuts off the power law sharply for
). We find the upper critical dimension for this system is dc = 4, so that for
the mean field values for the polydispersity exponent and fractal dimension apply:
, df = 4. For d = 3, mean field theory is still correct for
where
is the Ginzburg point; for
, mean field theory applies on small mass scales N<Nc but breaks down on larger mass scales N>Nc where
is a cross-over mass. Within the Ginzburg zone (i.e., d<dc,
), we show that the hyperbranched chains on mass scales N>Nc are non-Gaussian with fractal dimension given by df = d (for d = 2,3,4). Our results are qualitatively different from those of the percolation model and indicate that the polycondensation of ABf-1, unlike polymer gelation, is not described by percolation theory. Instead many of our results are similar to those for a monodisperse melt of randomly branched polymers, a consequence of the fact that
so that polydispersity is irrelevant for excluded volume screening in hyperbranched polymer melts.Received: 15 December 2003, Published online: 2 March 2004PACS:
82.35.-x Polymers: properties; reactions; polymerization - 05.70.Jk Critical point phenomena 相似文献
4.
5.
By causality of matter one means its property not to admitsuperluminal excitations, i.e. excitations that propagate faster than the vacuum speed of lightc. In discussing the propagation of small excitations, one has to distinguish betweenphase velocities
j
/k, (1jg=number of dispersion branches),group velocities d
j
/dk, a front velocityv
f
: =
and the propagation speedv
q
:=(dp/d)1/2 of isotropic quasistatic (small) perturbations. We discuss some of their properties. In particular, the (maximal) speedv
s
of small signals is not smaller thanv
f
, and equalsv
f
whenever the dispersion branches
j
(k) behave reasonably at infinity of the complexk-plane. In essence stronger conditions guaranteev
q
<v
f
(in which casev
q
c would imply superluminal behaviour). 相似文献
6.
Pablo A. Ferrari Beat M. Niederhauser Eugene A. Pechersky 《Journal of statistical physics》2007,128(5):1159-1176
We consider the Harmonic crystal, a measure on
with Hamiltonian H(x)=∑
i,j
J
i,j
(x(i)−x(j))2+h∑
i
(x(i)−d(i))2, where x, d are configurations, x(i), d(i)∈ℝ, i,j∈ℤ
d
. The configuration d is given and considered as observations. The ‘couplings’ J
i,j
are finite range. We use a version of the harness process to explicitly construct the unique infinite volume measure at finite
temperature and to find the unique ground state configuration m corresponding to the Hamiltonian. 相似文献
7.
A. V. Seliverstov M. A. Tarasov V. S. Edel’man 《Journal of Experimental and Theoretical Physics》2017,124(4):643-656
The Andreev subgap conductance at 0.08–0.2 K in thin-film superconductor (aluminum)–insulator–normal metal (copper, hafnium, or aluminum with iron-sublayer-suppressed superconductivity) structures is studied. The measurements are performed in a magnetic field oriented either along the normal or in the plane of the structure. The dc current–voltage (I–U) characteristics of samples are described using a sum of the Andreev subgap current dominating in the absence of the field at bias voltages U < (0.2–0.4)Δc/e (where Δc is the energy gap of the superconductor) and the single-carrier tunneling current that predominates at large voltages. To within the measurement accuracy of 1–2%, the Andreev current corresponds to the formula \({I_n} + {I_s} = {K_n}\tanh \left( {{{eU} \mathord{\left/ {\vphantom {{eU} {2k{T_{eff}}}}} \right. \kern-\nulldelimiterspace} {2k{T_{eff}}}}} \right) + {K_s}{{\left( {{{eU} \mathord{\left/ {\vphantom {{eU} {{\Delta _c}}}} \right. \kern-\nulldelimiterspace} {{\Delta _c}}}} \right)} \mathord{\left/ {\vphantom {{\left( {{{eU} \mathord{\left/ {\vphantom {{eU} {{\Delta _c}}}} \right. \kern-\nulldelimiterspace} {{\Delta _c}}}} \right)} {\sqrt {1 - {{eU} \mathord{\left/ {\vphantom {{eU} {{\Delta _c}}}} \right. \kern-\nulldelimiterspace} {{\Delta _c}}}} }}} \right. \kern-\nulldelimiterspace} {\sqrt {1 - {{eU} \mathord{\left/ {\vphantom {{eU} {{\Delta _c}}}} \right. \kern-\nulldelimiterspace} {{\Delta _c}}}} }}\) following from a theory that takes into account mesoscopic phenomena with properly selected effective temperature T eff and the temperature- and fieldindependent parameters K n and K s (characterizing the diffusion of electrons in the normal metal and superconductor, respectively). The experimental value of K n agrees in order of magnitude with the theoretical prediction, while K s is several dozen times larger than the theoretical value. The values of T eff in the absence of the field for the structures with copper and hafnium are close to the sample temperature, while the value for aluminum with an iron sublayer is several times greater than this temperature. For the structure with copper at T = 0.08–0.1 K in the magnetic field B|| = 200–300 G oriented in the plane of the sample, the effective temperature T eff increases to 0.4 K, while that in the perpendicular (normal) field B ⊥ ≈ 30 G increases to 0.17 K. In large fields, the Andreev conductance cannot be reliably recognized against the background of single- carrier tunneling current. In the structures with hafnium and in those with aluminum on an iron sublayer, the influence of the magnetic field is not observed. 相似文献
8.
Hajo Kuiper 《Zeitschrift für Physik A Hadrons and Nuclei》1974,269(4):379-383
Knowledge of the crystal magnetic fieldH c is necessary for proton polarization measurements in polarized targets by the internal field method [1, 2]. Because of the importance of lanthanum magnesium nitrate (LMN) single crystals for low-energy polarized proton targets [3],H c has been calculated for LMN on the basis of new x-ray diffraction data. We find thatH c=ApP2(cosθ), wherep is the proton polarization andP 2 the second Legendre polynomial depending onθ, the angle between crystalc-axis and external magnetic field, andA is a constant characteristic for LMN. Our result \(A = - \left( {0.242 \pm \begin{array}{*{20}c} {0.028} \\ {0.090} \\ \end{array} } \right) Oe\) leads to a field only about half as large as expected according to previous assumptions. The difference is due to the markedly different proton positions, revealed by the x-ray structure analysis. 相似文献
9.
We continue our study of the collision of two solitons for the subcritical generalized KdV equations
Solitons are solutions of the type where c
0 > 0. In [21], mainly devoted to the case f (u) = u
4, we have introduced a new framework to understand the collision of two solitons , for (0.1) in the case (or equivalently, ). In this paper, we consider the case of a general nonlinearity f (u) for which , are nonlinearly stable. In particular, since f is general and c
1 can be large, the results are not perturbations of the ones for the power case in [21].
First, we prove that the two solitons survive the collision up to a shift in their trajectory and up to a small perturbation
term whose size is explicitly controlled from above: after the collision, , where is close to c
j
(j = 1, 2). Then, we exhibit new exceptional solutions similar to multi-soliton solutions: for all , there exists a solution such that
where (j = 1, 2) and converges to 0 in a neighborhood of the solitons as .
The analysis is split in two distinct parts. For the interaction region, we extend the algebraic tools developed in [21] for
the power case, by expanding f (u) as a sum of powers plus a perturbation term. To study the solutions in large time, we rely on previous tools on asymptotic
stability in [17,22] and [18], refined in [19,20].
This research was supported in part by the Agence Nationale de la Recherche (ANR ONDENONLIN). 相似文献
10.
V. I. Al’shits N. N. Bekkauer A. E. Smirnov A. A. Urusovskaya 《Journal of Experimental and Theoretical Physics》1999,88(3):523-526
A constant magnetic field is found to have a substantial effect on the macroplasticity of NaCl crystals when they are being
actively strained at a constant rate
during magnetic treatment. We have measured the dependence of the yield point σ
y
on the magnetic induction B=0–0.48 T and the strain rate
. It is shown that this magnetic effect has a threshold character and is observed only for B>B
c
, where B
c
grows with increasing
as
. The lower the strain rate
, the larger the relative decrease in the yield point σ
y
(B)/σ
y
(0) at fixed field B>B
c
. At small enough strain rates
the threshold field B
c
ceases to depend on
and goes constant. A theoretical model is proposed which is in good agreement with the observed regularities. The model is
based on the competition between thermally activated and magnetically stimulated depinning of dislocations from paramagnetic
impurity centers.
Zh. éksp. Teor. Fiz. 115, 951–958 (March 1999) 相似文献
11.
Covariant differential calculi on the quantum space
for the quantum group SL
q
(2) are classified. Our main assumptions are thatq is not a root of unity and that the differentials de
j
of the generators of
form a free right module basis for the first-order forms. Our result says, in particular, that apart from the two casesc =c(3), there exists a unique differential calculus with the above properties on the space
which corresponds to Podles' quantum sphereS
qc
/2
. 相似文献
12.
The complex impedance of the Ag2ZnP2O7 compound has been investigated in the temperature range 419–557 K and in the frequency range 200 Hz–5 MHz. The Z′ and Z′ versus frequency plots are well fitted to an equivalent circuit model. Dielectric data were analyzed using complex electrical
modulus M* for the sample at various temperatures. The modulus plot can be characterized by full width at half-height or in terms of
a non-exponential decay function
f( \textt ) = exp( - \textt/t )b \phi \left( {\text{t}} \right) = \exp {\left( { - {\text{t}}/\tau } \right)^\beta } . The frequency dependence of the conductivity is interpreted in terms of Jonscher’s law:
s( w) = s\textdc + \textAwn \sigma \left( \omega \right) = {\sigma_{\text{dc}}} + {\text{A}}{\omega^n} . The conductivity σ
dc follows the Arrhenius relation. The near value of activation energies obtained from the analysis of M″, conductivity data, and equivalent circuit confirms that the transport is through ion hopping mechanism dominated by the
motion of the Ag+ ions in the structure of the investigated material. 相似文献
13.
Takekazu Ishida Kiichi Okuda Hidehito Asaoka Yukio Kazumata Kenji Noda Humihiko Takei 《Czechoslovak Journal of Physics》1996,46(3):1217-1218
The reversible magnetic torque of untwinned YBa2Cu3O7 single crystals shows the four-fold symmetry in thea-b plane. The irreversible torque indicates evidence for a novel intrinsic pinning along thea andb axes. These facts mean that the free energy of the four-fold symmetry has a minimum when the field is applied along thea orb axis. The results are consistent with those expected from thed x 2?y 2 symmetry and rule out the possibility of thed xy symmetry. The Fermi surface anisotropy is not responsible for the observed anisotropy. This is firstbulk evidence for thek-dependent gap anisotropy on the Fermi surface. The two-fold anisotropy parameter is found as\(\gamma _{ab} = \sqrt {{{m_a } \mathord{\left/ {\vphantom {{m_a } {m_b }}} \right. \kern-\nulldelimiterspace} {m_b }}} = 1.18 \pm 0.14\). 相似文献
14.
Martin Kummer 《Communications in Mathematical Physics》1976,48(1):53-79
This paper contains a detailed study of the flow that the classical Hamiltonian
H = \tfrac12(x*20c 2 1 + y*20c 2 1 ) + \tfrac12(x*20c 2 2 + y*20c 2 2 ) + O3 H = \tfrac{1}{2}(x\begin{array}{*{20}c} 2 \\ 1 \\ \end{array} + y\begin{array}{*{20}c} 2 \\ 1 \\ \end{array} ) + \tfrac{1}{2}(x\begin{array}{*{20}c} 2 \\ 2 \\ \end{array} + y\begin{array}{*{20}c} 2 \\ 2 \\ \end{array} ) + \mathcal{O}_3 相似文献
15.
T. Nattermann M. Feigelman I. Lyuksyutov 《Zeitschrift für Physik B Condensed Matter》1991,84(3):353-359
We analyze the influence of thermal and frozen-in disorder on the flux line (FL) density
close to the lower critical fieldH
c1. Arguments which consider the steric repulsion of fluctuating FLs give
with the roughness exponent of a single FL andd the space dimensionality. We show by a phenomenological scaling approach and a renormalization group treatment, that this is correct only fordd
c
=2/–1, i.e. for
. Ford>d
c
the steric FL repulsion at scales more than some critical one is irrelevant and
. For disordered superconductorsd
c
=2 and
ford=2, 3. We also found the melting line for a FL lattice in the presence of frozen-in impurities close toH
c1. 相似文献
16.
J. T. Chayes L. Chayes Daniel S. Fisher T. Spencer 《Communications in Mathematical Physics》1989,120(3):501-523
Thed-dimensional, nearest-neighbor disordered Ising ferromagnet: $$H = - \sum {J_{ij} \sigma _i \sigma _j }$$ is studied as a function of both temperature,T, and a disorder parameter,λ, which measures the size of fluctuations of couplingsJ ij ≧0. A finite-size scaling correlation length,ζ f (T, λ), is defined in terms of the magnetic response of finite samples. This correlation length is shown to be equivalent, in the scaling sense, to the quenched average correlation lengthζ(T, λ), defined as the asymptotic decay rate of the quenched average two-point function. Furthermore, the magnetic response criterion which definesζ f is shown to have a scale-invariant property at the critical point. The above results enable us to prove that the quenched correlation length satisfies: $$C\left| {\log \xi (T)} \right|\xi (T) \geqq \left| {T - T_c } \right|^{ - {2 \mathord{\left/ {\vphantom {2 d}} \right. \kern-\nulldelimiterspace} d}}$$ which implies the boundv≧2/d for the quenched correlation length exponent. 相似文献
17.
The problem of perturbative breakdown of conformal symmetry can be avoided, if a conformally covariant quantum field j{\varphi} on d-dimensional Minkowski spacetime is viewed as the boundary limit of a quantum field f{\phi} on d + 1-dimensional Anti-deSitter spacetime (AdS). We study the boundary limit in renormalized perturbation theory with polynomial
interactions in AdS, and point out the differences as compared to renormalization directly on the boundary. In particular,
provided the limit exists, there is no conformal anomaly. We compute explicitly the one-loop “fish diagram” on AdS4 by differential renormalization, and calculate the anomalous dimension of the composite boundary field j2{\varphi^2} with bulk interaction kf4{\kappa \phi^4}. 相似文献
18.
R. Orbach 《Hyperfine Interactions》1989,49(1-4):325-333
The excitation dynamics of site diluted magnets can be described at low energies (long length scales) by magnons, and above
a crossover frequency, ωc, (short length scales) by fractons. The density of fracton states is given by
, where
is the fracton dimensionality. Dilution gives rise to a characteristic length ξ∝(p−p
c)ν, wherep
c is the critical concentration for (magnetic) percolation. The crossover frequency ωc is proportional to ξ-1[1+(θ/2)], where θ is the rate at which the diffusion constant decays with distance for diffusion on an equivalent network. A fractal
dimensionD describes the density of magnetic sites on the infinite network, and
. For percolating networks,
for all dimensions ≥2. Neutron scattering structure factor measurements by Uemura and Birgeneau compare well with calculations
using fracton concepts.
Magnons are extended at low energies, while the fracton states are geometrically localized, with a wave function envelope
proportional to exp
. Here,
is the fracton length scale at frequency ω. The exponentd
ϕ lies between 1 andd
min, the chemical length index (of the order of 1.6 in three dimensions). The localization of the magnetic excitations causes
a spread in the NMR relaxation rates. A given nuclear moment will experience only a limited set of fracton excitations, resulting
in an overall non-exponential decay of the NMR relaxation signal. When strong cross-relaxation is present, the relaxation
will be exponential, but the temperature dependence will be strongly altered from the concentrated result. 相似文献
19.
An electric molecular beam resonance spectrometer has been used to measure simultaneously the Zeeman- and Stark-effect splitting of the hyperfine structure of133Cs19F. Electric four pole lenses served as focusing and refocusing fields of the spectrometer. A homogenous magnetic field (Zeeman field) was superimposed to the electric field (Stark field) in the transition region of the apparatus. Electrically induced (Δ m J =±1)-transitions have been measured in theJ=1 rotational state, υ=0, 1 vibrational state. The obtained quantities are: The electric dipolmomentμ el of the molecule for υ=0, 1; the rotational magnetic dipolmomentμ J for υ=0, 1; the anisotropy of the magnetic shielding (σ ⊥-σ‖) by the electrons of both nuclei as well as the anisotropy of the molecular susceptibility (ξ ⊥-ξ‖), the spin rotational interaction constantsc Cs andc F, the scalar and the tensor part of the nuclear dipol-dipol interaction, the quadrupol interactioneqQ for υ=0, 1. The numerical values are:
$$\begin{gathered} \mu _{el} \left( {\upsilon = 0} \right) = 73878\left( 3 \right)deb \hfill \\ \mu _{el} \left( {\upsilon = 1} \right) - \mu _{el} \left( {\upsilon = 0} \right) = 0.07229\left( {12} \right)deb \hfill \\ \mu _J /J\left( {\upsilon = 0} \right) = - 34.966\left( {13} \right) \cdot 10^{ - 6} \mu _B \hfill \\ \mu _J /J\left( {\upsilon = 1} \right) = - 34.823\left( {26} \right) \cdot 10^{ - 6} \mu _B \hfill \\ \left( {\sigma _ \bot - \sigma _\parallel } \right)_{Cs} = - 1.71\left( {21} \right) \cdot 10^{ - 4} \hfill \\ \left( {\sigma _ \bot - \sigma _\parallel } \right)_F = - 5.016\left( {15} \right) \cdot 10^{ - 4} \hfill \\ \left( {\xi _ \bot - \xi _\parallel } \right) = 14.7\left( {60} \right) \cdot 10^{ - 30} erg/Gau\beta ^2 \hfill \\ c_{cs} /h = 0.638\left( {20} \right)kHz \hfill \\ c_F /h = 14.94\left( 6 \right)kHz \hfill \\ d_T /h = 0.94\left( 4 \right)kHz \hfill \\ \left| {d_s /h} \right|< 5kHz \hfill \\ eqQ/h\left( {\upsilon = 0} \right) = 1238.3\left( 6 \right) kHz \hfill \\ eqQ/h\left( {\upsilon = 1} \right) = 1224\left( 5 \right) kHz \hfill \\ \end{gathered} $$ 相似文献
20.
For weakly non ergodic systems, the probability density function of a time average observable
is
where
is the value of the observable when the system is in state j=1,…L. p
j
eq is the probability that a member of an ensemble of systems occupies state j in equilibrium. For a particle undergoing a fractional diffusion process in a binding force field, with thermal detailed
balance conditions, p
j
eq is Boltzmann’s canonical probability. Within the unbiased sub-diffusive continuous time random walk model, the exponent 0<α<1 is the anomalous diffusion exponent 〈x
2〉∼t
α
found for free boundary conditions. When α→1 ergodic statistical mechanics is recovered
. We briefly discuss possible physical applications in single particle experiments. 相似文献
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