共查询到20条相似文献,搜索用时 62 毫秒
1.
Jeffrey M. Harris K. Krishana N. P. Ong R. Gagnont L. Taillefer 《Journal of Low Temperature Physics》1996,105(3-4):877-886
The washboard frequency of the moving vortex lattice in untwinned YBa2 Cu3 O6.93 may be observed through mode-locking to an externally applied ac current of frequency ext. The interference between and ext results in jumps in the dc current-voltage characteristics when and ext are harmonically related1. The interference effect disappears in the vortex liquid state. The Hall conductivity xy below Tc in YBCO contains contributions2 from a positive quasiparticle (qp) term (H) and a negative vortex term (1/H). The qp term is surprisingly large well below Tc and implies a large gap anisotropy and a long qp mean free path (mfp). The thermal Hall effect3 xy is closely related to the qp xy; xy is produced by asymmetric scattering of qp by pinned vortices. The qp mfp at H = 0, extracted from xy and extended to low T by xy, increases remarkably from 90 Å at Tc to more than 0.5m at 22 K. 相似文献
2.
M. M. Talianker 《Journal of Materials Science》1982,17(11):3097-3100
The athermal transformation in Zr-2 at.% Nb alloy has been investigated by transmission electron microscopy. Analysis of the selected-area diffraction pattern has shown that the orientation relationships between the omega and the parent-phase in quenched Zr-2 at.% Nb alloy are the same as have been previously observed for the reaction in pure zirconium. Thus it was deduced that the direct transition has taken place in the alloy during cooling. The-originated -particles were visualized using the dark-field technique. The formation of the athermal omega in the-region of-stabilized Zr-Nb alloy is discussed in terms of the relative positions of the free energy equilibrium curvesT
0
,T
0
,T
0
and the correspondingM
s
,M
s
andT
s
start curves. It is concluded that the omega phase can occur over a much wider range of alloy compositions than is usually recognized on the basis of transformation data. 相似文献
3.
Starting from the nonequilibrium theory of dirty superconductors in the Ginzburg-Landau regime, spatially homogeneous states with an applied currentI=I
0+I
1 cos (t) are considered. Expressions for the linear response (I1 small) valid up to high frequencies (k
BTc) are derived and evaluated analytically for the experimentally important case of smallI
0 and 0(T). Then the nonlinear response is treated for frequencies with E1. Interesting new behavior is found for frequencies 0 1, where 0 is essentially the GL relaxation time. 相似文献
4.
Infrared absorption characteristics of-SiC particles prepared by chemical vapour deposition were studied. These particles were either solid or had a silicon core and/or were hollow. The solid particles exhibited a single absorption peak between the transverse optical frequency (TO = 794 cm–1) and the longitudinal optical frequency (LO = 976 cm–1) of-SiC. This absorption peak shifted to a lower frequency with increasing lattice parameter of-SiC and increasing free silicon content. The particles containing a silicon core and/or were hollow exhibited double absorption peaks close to TO and LO. The peak at the LO side shifted to a lower frequency and that at the TO side to a higher frequency with decreasing silicon core size and increasing hollow size. Using the calculations based on the effective medium theory assuming surface phonon mode, the relationship between the infrared absorption characteristics and microstructures of the-SiC particles are explained. 相似文献
5.
P. C. Fannin T. S. Perova A. T. Giannitis M. Nolan A. R. Moore H. S. Gamble 《Journal of Materials Science: Materials in Electronics》2001,12(4-6):347-350
We report on the measurement of the frequency-dependent complex permittivity, ()=()-i(), over the frequency range, 30 MHz to 6 GHz, of silicon wafers and of thin dielectric films formed on silicon. Measurements, as a function of temperature and time treatments, were obtained by means of an HP Network Analyzer and dielectric probe and the resulting ()and()plots for the silicon wafers are shown to have a Debye-type [1] profile, thereby indicating that the associated polarization mechanism is of the orientational variety. 相似文献
6.
Oracio Navarro 《Journal of Low Temperature Physics》1996,105(3-4):855-860
We present results of a thermodynamic analysis for the superconductors compounds BaPb0.7Bi0.3O3 and Ba0.7K0.3BiO3. The physical quantities are calculated making use of the Eliashberg theory and the electron-phonon spectra 2()F() as calculated by Shirai et al. For the superconductor BaPb0.7Bi0.3O3, several models of the 2()F() were studied looking for a better agreement with experimental data. The best fit is achieved with a simple constant scaling (C = 1.25) of the Shirai's spectra. The functional derivative of the deviation function D(t) with respect to changes in 2()F() is also calculated. 相似文献
7.
Meir Weger 《Journal of Low Temperature Physics》1994,95(1-2):131-144
It is generally believed that a high electronic density-of-states n(EF), and therefore a low Fermi velocity vF, are required to obtain a high transition temperature, since Tc ph exp(–1/) and = n(EF)V. However, V = < I2 >/M
ph
2
, and Bardeen showed that I EFkF = (1/2)k
F
2
vF. Thus one may expect that should increase with vF. While it may not be feasable to increase the one-electron velocity vF signifcantly, the velocity may increase greatly as a result of renormalization by electron-electron interactions. Such a renormalization exists in Hartree-Fock theory for an unscreened electron-gas. We found that for a medium-density electron-gas (rs 10–20) imbedded in a background with a dielectric constant () such that / > 10, there is a significant increase of vF by renormalization, even when screening is taken into consideration. The peak of v(k) at k = kF is very narrow, the half width being somewhat less than the frequency 0 at which () falls by a factor of 2. When () is due to ionic polarization, o is a typical phonon frequency. The height of the peak vF/v
F
0
is of order EF/0, and the width in units of momentum is: k/kF (0/EF)2. This velocity peak is associated with a minimum in the screening constant at Ep. Its sharpness causes the normal state properties to be highly anomalous; namely the conductivity is exceptionally high, with an anomalous temperature dependence; the conductivity anisotropy, thermoelectric power, and Hall constant are anomalous. Direct determination of v(k) in YBCO by several methods indeed suggests a large, sharp peak, the width being of order 10 me V. This peak manifests itself in the tunneling and point-contact spectroscopy I–V curves. We suggest that the reduced screening at the Fermi level, associated with this velocity peak, is responsible for the high Tc of the cuprates and several other exotic superconductors. Thus, the high Tc is a reflection of the anomalous normal-state properties. 相似文献
8.
In 1970, Ruvalds and Zawadowski (RZ) outlined a microscopic theory of the two-roton pair excitation spectrum in superfluid4He. They pointed out that because of a Base condensate-induced coupling into the density fluctuation spectrum, these pair excitations show up in inelastic neutron data for S(Q, ) as high energy multiparticle scattering above the maxon-roton quasiparticle peak. Stimulated by recent high-resolution neutron data from ILL, we have carried out an extensive study of the single-particle 1(Q, ) and the two-particle 2(Q, ) spectral densities within the RZ scenario, over a wide range of wavevectors (1 Q 3 Å–1), frequencies and temperatures. We extend the original RZ analysis (which concentrated on the two-roton spectrum) to include the multiparticle structure associated with the maxonroton and maxon-maxon spectra and present numerical results for both attractive and repulsive quasiparticle interactions. We also point out that the microscopic theory of S(Q, ) in a Bose-condensed fluid shows that it involves a weighted sum of both 1(Q, )and 2(Q, ). As a result, multiparticle structure exhibited by S(Q, ) is not easily related to theoretical results for 1(Q, ) and 2(Q, ). Previous attempts in the literature to fit neutron data for S(Q, ) to either 1(Q, ) or 2(Q, ) would not appear to have much quantitative basis, especially in the two-roton frequency region 2R. 相似文献
9.
C. C. Homes S. V. Dordevic D. A. Bonn R. Liang W. N. Hardy 《Journal of Superconductivity》2004,17(1):93-96
The optical conductivity sum rule is used to examine the evolution of the spectral weight N() in both the normal and superconducting states of optimally and underdoped YBa2Cu3O6+x
along the a axis. Differences in N() above and below T
c allow the strength of the superconducting condensate s to be determined. In the optimally-doped material, s is fully formed at energies comparable to the full superconducting gap maximum (0.1 eV), while in the underdoped material the energy scale for convergence is considerably higher (0.6 eV). This difference is discussed in terms of normal-state properties. 相似文献
10.
AC electrical properties of 410 nm think 30 at.wt% Cu-70 at.wt% GeO2 thin films are reported for the frequency range 104 to 106 Hz and temperature range 150 to 425 K. The loss tangent (tan ) and the dielectric loss (/0) are found to show striking minima around a cut-off frequency 105 Hz. In the lower frequency range (105 Hz), 1()
s
T
n
is obeyed with s (0 to 0.51) increasing as a function of temperature and n (0.10 to 0.14) showing a very weak temperature dependence. In the higher frequency region (105 Hz), 1() and /0 increase sharply leading to the quadratic behavior of 1() with s equal to 2. These processes are discussed by analyzing an equivalent circuit which shows that at lower frequencies, the effects of series resistance in leads and contacts can be neglected, while at higher frequencies such effect give rise to spurious 2 dependance for the conductance. A weakly activated AC conductivity and a frequency exponent s that increases with increasing temperature suggest that the low frequency behavior originates from carrier migration by tunneling process. 相似文献
11.
The dielectric constants and loss factors,, for pure single-crystal MgO and for Fe-and Cr-doped crystals have been measured at frequencies, , from 500 Hz to 500 kHz at room temperature. For pure MgO at 1 kHz the values of and the loss tangent, tan , (9.62 and 2.16×10–3, respectively) agree well with the data of Von Hippel; the conductivity, , varies as
n
withn=0.98±0.02. In Fe-doped crystals increases with Fe-concentration (at any given frequency); for a crystal doped with 12800 ppm Fe, was about four times the value for pure MgO. At all concentrations the variation of log with log was linear andn=0.98±0.02. A decrease in with increasing Fe-concentration was also observed. A similar, although less pronounced, behaviour was found in Cr-doped crystals. The effects are discussed in terms of hopping mechanisms. 相似文献
12.
Matthias Opel Martin Götzinger Christian Hoffmann Ralf Nemetschek Richard Philipp Francesca Venturini Rudi Hackl Andreas Erb Eric Walker 《Journal of Low Temperature Physics》1999,117(3-4):347-351
We present results of electronic Raman-scattering experiments in differently doped YBa
2
Cu
3
O
6+x
. In B
2g
symmetry, an analysis of the data in terms of a memory function approach is presented and dynamical relaxation rates (, T) and mass-enhancement factors 1 + (, T) for the carriers are obtained. Starting from temperatures T > 180K, (, T) and 1 + (,T) are extrapolated to lower temperatures and used to re-calculate Raman spectra. By comparison with our data, we find a loss of spectral weight between T
c
< T < T* at all doping levels x. T* is comparable to the pseudogap temperature found in other experiments. Below T
c
, the superconducting gap is observed. It depends on x and scales with T
c
whereas the energy scale of the pseudogap remains the same. 相似文献
13.
A theory is presented for the gain and noise in an amplifier based on a dc SQUID. In the lumped circuit approximation, the total inductance of the input circuitL
T
is coupled to the SQUID inductanceL via a mutual inductanceM
ii
=
e
(LL
T
)1/2 and is in series with a voltage source with a resistanceR
i
and a capacitanceC
i
. The results are expressed in terms of parameters for a SQUID with reduced inductance (1–
e
2
)L. The voltage gain of the amplifier at frequency /2 isM
i
V
r
/Z
T
*
(), while the total voltage noise at the output of the SQUID isV
N
r
()+M
i
2
V
r
J
N
r
() (R
i
+ 1/jC
i
)/L
TZ
T
*
(). Here,Z
T
*
()=Z
T
()–J
r
M
i
2
(R
i
+ 1/jC
i
)/L
i
, whereZ
T
() is the total impedance of the unloaded input circuit,V
r
andJ
r
are the flux-to-voltage and flux-to-circulating current transfer functions of the reduced SQUID, andV
N
r
() andJ
N
r
() are the noise voltage and noise current of the reduced SQUID. 相似文献
14.
M. Schossmann J. P. Carbotte E. Schachinger 《Journal of Low Temperature Physics》1988,70(5-6):537-545
Consideration of the functional derivative of the zero-temperature reduced upper critical magnetic fieldh
c2(0) with electron-phonon spectral density 2
F() suggests that its value can be maximized, for a given areaA under 2
F(), with the choice of a delta-function spectral density. We show thath
c2(0) is then independent ofA and that it increases steadily as the Einstein frequency E of the delta function is lowered. We argue that there is a local maximum inh
c2(0) at E=0. The value at maximum is very sensitive to impurity content and increases sharply from a clean-limit value of 1.5 atT
c
/E=1.33 to more than 3.45 in the dirty limit. 相似文献
15.
The present paper examines the elastostatic problem related to the axisymmetric rotation of a rigid circular disc bonded to a non-homogeneous half-space containing a penny-shaped crack. The shear modulus of the half-space is assumed to vary with depth according to the relation (z) = 1(z + c), c > 0 and 1, are constants. Using Hankel transforms, the solution of the problem is reduced to integral equations and finally to simultaneous Fredholm integral equations of the second kind. By solving numerically the simultaneous Fredholm integral equations, results are obtained which are used to estimate the stress intensity factor at the crack tip and the torque required to rotate the disc through an angle 0. 相似文献
16.
Summary. In this paper, a plane viscous liquid sheet is considered in contact with two passive media, from which two external perturbations of small intensity, given by local pressure distributions, act on the sheet. Of the two possible ways of action of the sources of perturbation only the symmetric one is pursued here, for which the sheet reacts in the so called varicose mode. The action of the sources is taken either as instantaneous, or as persisting infinitely long. During the persistent action the sources are considered to move with constant velocity along the sheet and the perturbation is changing harmonically with circular frequency e. In the paper, the linear response of the viscous sheet to the external perturbation is computed and analyzed. The starting point of the computation is a general formula for the exact linear response of the sheet as an inverse Fourier-Laplace transform, the derivation of which is reproduced in Appendix A of this paper. The formula shows the response to depend on several characteristic numbers, of which one is intrinsic to the unperturbed sheet, whereas the others also imply the properties of the source. The intrinsic number can be taken as in which the sheet thickness h, the density of the liquid, its kinematic viscosity , and it surface tension with respect to the two ambient media enter. It is related by to the Ohnesorge number Oh. First the concept of the long wave approximation used is discussed in the context: it implies the neglect of all the hard branches n(k),n=1,..., of spectral modes of the sheet, for which On the remaining two soft branches ±(k) of the spectrum, for which limk0±(k)=0, the asymptotic expansion of (k) fork0 is considered and the whole branches are approximated by the (extrapolation of the) lowest order of the expansion. Finally, the whole Fourier integrand of the signal is substituted by its lowest order asymptotic expression aroundk=0. Within this approximation the response of the sheet to an instantaneous point perturbation is computed in its rest frame of reference. It is a long time asymptotic form of the exact response to the instantaneous point perturbation, the kernel of the general signal evolution of the sheet. The computed asymptotic form of this kernel bears a strong formal resemblance to the evolution kernel of the heat equation. The analogy is, however, not physical: whereas for 2<4 the long wave modes of the two soft branches of the sheet spectrum are purely absorptive, like the modes of the heat equation, for 2>4 their modes are dispersive and dissipative waves, with their peculiar propagation properties. By adequate application of the superposition principle the responses to instantaneous line perturbation and to harmonically persistent perturbations by moving point and line sources are computed from the asymptotic evolution kernel. Because of the specific form of this kernel, the long time surface deflection responses to the harmonic excitations do not depend separately on the additional characteristic numbers eandW=||/ (where eand || are considered in units of 4/h2 and 2/h, respectively), but only on their ratio e/W2. The properties of these signals in dependence on and e/W2, as well as their implications for technical processes involving liquid sheets, like courtain coating, are discussed in detail.AcknowledgementThe support of this work by the German Research Foundation (DFG) is gratefully acknowledged. 相似文献
17.
Mi-Ock Mun Young Jin Kim Yu Park Jae Hoon Kim S. H. Moon H. N. Lee H. G. Kim B. Oh 《Journal of Superconductivity》2002,15(5):475-477
The reflectivity of superconducting MgB2 (T
c = 39 K) has been measured on a randomly oriented thin film at room temperature over a wide-range of frequencies, 20 < 100000 cm–1. The conductivity shows highly metallic behavior but cannot be explained with a simple Drude model alone. The electronic contribution is analyzed by a generalized Drude model. The scattering rate 1/() and the mass renormalization ratio m*()/m = 1 + () exhibit clear frequency dependence. The electron–phonon coupling strength is estimated to be 1.5 ± 0.5 while the plasma frequency p is 2.4 eV. 相似文献
18.
Our previous theory yielded for the Zeeman splitting of the imaginaryJ=1 collective mode in3He-B the result =2+0.25J
z
( is the effective Larmor frequency). In this paper we take into account the downward shift of the pair-breaking edge from 2 to 22– (2 and 1 are the longitudinal and transverse gap parameters). This leads to a complex Landé factor: the frequencies of theJ
z
=±1 components become =2+0.39J
z
, and the linewidths of these resonances become finite: =0.18. The coupling amplitudes of theJ
z
=±1 components to density are found to be proportional to gap distortion, (1–2/(/)2. Our results for the ultrasonic attenuation due to theJ
z
=±1,J=1 modes are capable of explaining the field dependence of the attenuation close to the pair-breaking edge as observed by Dobbs, Saunders, et al. The observed peak is caused by theJ
z
=–1 component: its height increases due to gap distortion as the field is increased, and the peak shifts downward in temperature and its width increases with the field due to the complex Landé factor. TheJ
z
=+1 component gives rise to a corresponding dip relative to the continuum attenuation. 相似文献
19.
From investigations of two-magnon Raman scattering (RS) under high pressures up to 430 kbar in Eu2CuO4 and YBa2Cu3O6.2 crystals, it was shown that the dependence of the superexchange integralJ on the distance between Cu and O atoms in CuO2 planesa is anomalously weak (Ja–n, n=3±0.5). The large value ofJ indicates strong initial overlapping of Cu and O wave functions in high-T
c
, materials. It was found that an increase in free carrier concentration results in a rapid increase of magnon damping and the disappearance of the two-magnon peak from RS spectra. A detailed study of electron Raman scattering has been carried out in superconducting and insulating YBa2Cu3O6–x
, single crystals. The spectral redistribution at frequencies<600 cm–1 in different polarizations indicate that the superconducting gap is strongly anisotropic. In the normal (metallic) phase the behavior of the imaginary part of the response functionR() in the polarization (xx) corresponds to the model of a marginal Fermi liquid, and in the polarization (xx), this behavior is independent of the temperature. In insulating crystals,R() is independent of temperature toT200 K in both polarizations. 相似文献
20.
Some electrical properties of hot-pressed lithium sialons, Li
x/8Si6–3x/4Al5x/8O
x
N8–x
havingx<5 and an yttrium sialon were measured between 291 and 775 K; the former consisted essentially of a single crystalline phase whereas the latter contained 98% glassy phase. For lithium sialons, the charging and discharging current followed al(t) t
–nlaw withn=0.8 at room temperature. The d.c. conductivities were about 10–13 ohm–1 cm–1 at 291 K and rose to 5×10–7 ohm–1 cm–1 at 775 K. At high temperatures electrode polarization effects were observed in d.c. measurements. The variation of the conductivity over the frequency range 200 Hz to 9.3 GHz followed the ()
n
law. The data also fitted the Universal dielectric law,()
n–1 well, and approximately fitted the Kramers-Kronig relation()/()–
=cot (n/2) withn decreasing from 0.95 at 291 K to 0.4 at 775 K. The temperature variations of conductivities did not fit linearly in Arrhenius plots. Very similar behaviour was observed for yttrium sialon except that no electrode polarization was observed. The results have been compared with those obtained previously for pure sialon; the most striking feature revealed being that d.c. for lithium sialon can be at least 103 times higher than that of pure sialon. Interpretation of the data in terms of hopping conduction suggests that very similar processes are involved in all three classes of sialon. 相似文献