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1.
The hopping motion of charged light particles in superconducting metals is discussed within the framework of a two-state model. In addition to the coupling to the superconducting electrons, the influence of phonons is investigated. It is shown that sufficiently far below the critical temperature the hopping rate is dominated by one-phonon processes if the static energy shifts between the particle ground states (so-called energy asymmetries) are finite but smaller than twice the BCS energy gap. The rôle of special two-phonon processes (named diphonon processes) resulting from non-linear coupling to the lattice is investigated. In the limit of energy asymmetries larger than twice the BCS energy gap phonon coupling does not crucially influence the jump rates.This article was processed using Springer-Verlag TEX Z. Physik B macro package 1.0 and the AMS fonts, developed by the American Mathematical Society. 相似文献
2.
E. Orignac Y. Suzumura 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,23(1):57-67
We study the interplay of Anderson localization and interaction in a two chain Hubbard ladder allowing for arbitrary ratio
of disorder strength to interchain coupling. We obtain three different types of spin gapped localized phases depending on
the strength of disorder: a pinned 4k
F Charge Density Wave (CDW) for weak disorder, a pinned 2k
F CDWπ for intermediate disorder and two independently pinned single chain 2k
F CDW for strong disorder. Confinement of electrons can be obtained as a result of strong disorder or strong attraction. We
give the full phase diagram as a function of disorder, interaction strength and interchain hopping. We also study the influence
of interchain hopping on localization length and show that localization is enhanced by a small interchain hopping but suppressed
by a large interchain hopping.
Received 6 April 2001 相似文献
3.
An exact diagonalization calculation for a small cluster in the two-dimensional t-J model has been studied to calculate two-hole
correlation. Calculations reveal dominant hole-hole correlation for holes sitting on next-nearest-neighbour (NNN) sites and
critical coupling occurs at J/t = 0.8. With the increase in negative-type NNN hopping, correlation decreases at NNN sites whereas it increases at other sites.
The thermodynamic properties such as entropy and specific heat are studied as functions of temperature with various NNN hopping
strength. Results show that with the inclusion of negative NNN hopping, the system becomes more ordered. A qualitative transition
temperature region has been estimated. It is shown that with the increase in NNN hopping strength, T
c increases. Specific heat results show non-Fermi liquid-type behaviour of the system. All our calculations establish the importance
of negative-type NNN hopping. 相似文献
4.
E. Müller-Hartmann A. Reischl 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,28(2):173-183
The derivation of effective spin models describing the low energy magnetic properties of undoped CuO2-planes is reinvestigated. Our study aims at a quantitative determination of the parameters of effective spin models from those of a multi-band model and is supposed to be relevant
to the analysis of recent improved experimental data on the spin wave spectrum of La2CuO4. Starting from a conventional three-band model we determine the exchange couplings for the nearest and next-nearest neighbor
Heisenberg exchange as well as for 4- and 6-spin exchange terms via a direct perturbation expansion up to 12th (14th for the 4-spin term) order with respect to the copper-oxygen hopping tpd. Our results demonstrate that this perturbation expansion does not converge for hopping parameters of the relevant size.
Well behaved extrapolations of the couplings are derived, however, in terms of Padé approximants. In order to check the significance
of these results from the direct perturbation expansion we employ the Zhang-Rice reformulation of the three band model in
terms of hybridizing oxygen Wannier orbitals centered at copper ion sites. In the Wannier notation the perturbation expansion
is reorganized by an exact treatment of the strong site-diagonal hybridization. The perturbation expansion with respect to
the weak intersite hybridizations is calculated up to 4th order for the Heisenberg coupling and up to 6th order for the 4-spin
coupling. It shows excellent convergence and the results are in agreement with the Padé approximants of the direct expansion.
The relevance of the 4-spin coupling as the leading correction to the nearest neighbor Heisenberg model is emphasized.
Received 8 June 2001 / Received in final form 28 May 2002 Published online 19 July 2002 相似文献
5.
P. Lunkenheimer G. Knebel A. Pimenov G. A. Emel’chenko A. Loidl 《Zeitschrift für Physik B Condensed Matter》1995,99(1):507-516
The complex conductivity of La2CuO4+δ has been investigated for frequencies 20 Hz≤ν≤4 GHz and temperatures 1.5K≤T≤450 K. Two single crystals with δ≈0 and δ≈0.02
were investigated, using dc (four-probe), reflectometric and contact-free techniques. At high temperatures the dc conductivity
is thermally activated with low values of the activation energy. For low temperatures Mott's variable range hopping dominates.
The real and imaginary parts of the ac conductivity follow a power-law dependence σ∼v
s, typical for charge transport by hopping processes. A careful analysis of the temperature dependence of the ac conductivity
and of the frequency exponents has been performed. It is not possible to explain all aspects of the ac conductivity in La2CuO4+δ by standart hopping models. However, the observed minimum in the temperature dependence of the frequency exponents strongly suggests tunneling of large polarons as dominant transport process. 相似文献
6.
It is proposed that the charge transport across the SiO
x
layer at the interface SnO2/Si proceeds by a hopping mechanism. During heat treatment of the photovoltaic cells, in air, chemical reactions occur with O2/H2O, which lead to a drastic reduction of the density of hopping sites near the Si conduction band edge. The SiO
x
hopping sites of energy near the valence band edge are less affected by these chemical reactions. Thus, photogenerated holes can still pass the barrier while the dark current flow is strongly inhibited. 相似文献
7.
Assuming that charge carriers form a Fermi liquid state, we study a model for layered high-temperature superconductors with unretarded intralayer and interlayer pairing. Guided by band structure calculations and inverse photoemission experiments, we adopt a tight binding band with nearest and next-nearest neighbors hopping within the sheets and weak interlayer hopping. The gap equations are solved numerically, without imposing a cutoff energy, characteristic to phonon mediated superconductivity. On this basis we calculate the gap parameters,T
c
, the tunneling conductance, infrared absorption and the coherence length for various band fillings =1/2–x by introducing excess holes of concentrationx. Assuming the interlayer coupling strength to be smaller than the intralayer one, our main results are as follows:T
c
is dominated by the intralayer properties, reaching a maximum atx0.3, where strong coupling features appear. In the presence of interlayer pairing, the gap becomes anisotropic perpendicular to the layers, and standard BCS-behavior is modified. In particular the BCS-square root singularity in the density of states and in the tunneling conductance is replaced by van Hove singularities characterizing the anisotropic gap. In particular, we investigate the anisotropy of the tunneling conductance for specular and diffuse tunneling for a junction parallel or perpendicular to the layers, infrared absorption, as well as the coherence length, parallel and perpendicular to the layers. 相似文献
8.
In the present work, we report the interplay of single particle and Cooper pair tunnelings on the superconducting state of
layered high-T
c cuprate superconductors. For this we have considered a model Hamiltonian incorporating the intra-planar interactions and
the contributions arising due to the coupling between the planes. The interplanar interactions include the single particle
tunneling as well as the Josephson tunneling of Cooper pairs between the two layers. The expression of the out-of-plane correlation
parameter which describes the hopping of a particle from one layer to another layer in the superconducting state is obtained
within a Bardeen-Cooper-Schriefer (BCS) formalism using the Green’s function technique. This correlation is found to be sensitive
to the various parameter of the model Hamiltonian. We have calculated the out-of-plane contribution to the superconducting
condensation energy. The calculated values of condensation energy are in agreement with those obtained from the specific heat
and the c-axis penetration depth measurements on bilayer cuprates. 相似文献
9.
V. V. Val’kov A. A. Shklyaev M. M. Korovushkin A. F. Barabanov 《Physics of the Solid State》2011,53(10):1997-2000
The normal and superconducting phases of the ensemble of spin polarons in a two-dimensional Kondo lattice have been considered
under the conditions when the hopping integral is comparable to the s-d exchange interaction energy. The polaron excitation spectrum and the superconducting transition temperature have been found
taking into account upper triplet states. The change in the concentration dependence of the critical temperature of the transition
to the superconducting phase with the relation between the hopping integral and the integral of the s-d exchange coupling has been analyzed. 相似文献
10.
Structural, morphological and transport properties of PrFe1? x Ni x O3 (x?=?0.1, 0.2, 0.3, 0.4 and 0.5) thin films grown on LaAlO3 substrate by pulsed laser deposition were studied experimentally. Structural analysis of the samples showed that they have in-plane compressive strain and single-phase epitaxial growth along with c-axis (001) orientation having orthorhombic structure with space group Pbnm. The observed strain is reduced with Ni substitution. The resistivity as a function of temperature follows the variable range hopping (VRH) model up to certain amount of Ni substitution (x?=?0.3) but fails for higher values of x. From the above model, parameters such as density of states at the Fermi level, N(E F), hopping energy, E h, and hopping distance R h, were calculated. Ni substitution leads to an increase in conductivity and this conduction is controlled by disorder-induced localization of charge carriers. With Ni substitution the gap parameter is found to decrease. The enhancement in conductivity and the failure of VRH model for higher doped compositions at high temperature is discussed. 相似文献
11.
The temperature dependences of the conductivities parallel and perpendicular to the layers in layered TlGaSe2 single crystals are investigated in the temperature range from 10 K to 293 K. It is shown that hopping conduction with a variable hopping length among localized states near the Fermi level takes place in TlGaSe2 single crystals in the low-temperature range, both along and across the layers. Hopping conduction along the layers begins to prevail over conduction in an allowed band only at very low temperatures (10–30 K), whereas hopping conduction across the layers is observed at fairly high temperatures (T?210 K) and spans a broader temperature range. The density of states near the Fermi level is determined, N F=1.3×1019eV·cm3)?1, along with the energy scatter of these states J=0.011 eV and the hopping lengths at various temperatures. The hopping length R along the layers of TlGaSe2 single crystals increases from 130 Å to 170 Å as the temperature is lowered from 30 K to 10 K. The temperature dependence of the degree of anisotropy of the conductivity of TlGaSe2 single crystals is investigated. 相似文献
12.
We propose a new mechanism to explain the magnetic structure of a recently discovered magnetoresistive double perovskite oxide
system, Sr2FeMoO6, with the help of detailed experimental and theoretical results. This model, based on a strong antiferromagnetic coupling
between the local moment and the charge carriers arising from local hopping interactions, can give rise to ferromagnetic metallic
as well as ferromagnetic insulating ground states. The relevance of this mechanism in understanding the magnetism in dilute
magnetic semiconductors such as Ga1 − x
Mn
x
As, is also discussed. 相似文献
13.
A convenient form of the Peierls-Hubbard Hamiltonian is obtained for the case when the Hubbard repulsion is the largest energy parameter. It allows to consider in the spin-wave approximation the properties of the one-hole low-lying excitations of a 2d lattice. For the parameters approximately corresponding to La2CuO4 it is shown that the hole polarons in the CuO2 planes of lightly doped samples are of large size with a solitonlike-shaped highly asymmetric wave function oriented along the diagonals of the planes or of small size depending on the value of the electron-phonon coupling. In both cases the cooperative effect of the electron-phonon and electron-magnon interactions leads to a large effective mass and to hopping transport of the excitations, with preferential jumps along the diagonals in the former case and rotationally symmetric in the latter. For hoping matrix elements which are small in comparison with a phonon quantum the competition between the interactions leads to the decrease of the total spin in the ground state with increasing electron-phonon coupling. 相似文献
14.
This paper reports conduction mechanism in a-Sb2Se3 over a wide range of temperature (238 to 338 K) and frequency (5 Hz to 100 kHz). The d.c. conductivity measured as a function
of temperature shows semiconducting behaviour with activation energy ΔE=0.42 eV. Thermally induced changes in the electrical and dielectric properties of a-Sb2Se3 have been examined. The a.c. conductivity in the material has been explained using modified CBH model. The band conduction
and single polaron hopping is dominant above room temperature. However, in the lower temperature range the bipolaron hopping
dominates. 相似文献
15.
16.
B. Lalevic N. Fuschillo B. Kuliyev W. Wang 《Applied Physics A: Materials Science & Processing》1974,5(2):127-132
Transport and dielectric properties of polycrystalline CoO films were studied as functions of the applied field, frequency
and temperature. TheI–V plots showed that the Poole-Frenkel field emission mechanism is responsible for conduction at fields>105 V/cm. The ac conductivity σ(ω), the imaginary part of the dielectric constantε
2, and tan δ plots as functions of frequency revealed three dispersion regions. The σ(ω) andε
2 frequency dependence indicates a non-adiabatic hopping of charge carriers at low frequencies and adiabatic hopping at high
frequencies. The activation energy of a dielectric oscillator is 0.15 eV.
Work supported by the Office of Naval Research. 相似文献
17.
A. Sen U. N. Maiti R. Thapa K. K. Chattopadhyay 《Applied Physics A: Materials Science & Processing》2011,104(4):1105-1111
Successful incorporation of vanadium dopant within the giant dielectric material CaCu 3Ti 4O12 (CCTO) through a conventional solid-state sintering process is achieved and its influence on the dielectric as well as electrical
properties as a function of temperature and frequency is reported here. Proper crystalline phase formation together with dopant
induced lattice constant shrinkage was confirmed through X-ray diffraction. The temperature dependence of the dielectric constant
at different constant frequencies was investigated. We infer that the correlated barrier hopping (CBH) model is dominant in
the conduction mechanism of the ceramic as per the temperature-dependent ac conductivity measurements. The electronic parameters
such as density of the states at the Fermi level, N(E
f) and hopping distance, R
ω
of the ceramic were also calculated using this model. 相似文献
18.
The thermal properties and their relationship to the charge transport properties of the La2?xSrxCu0.94Ti0.06O4 solid solution series have been investigated by means of electric resistivity and thermopower measurements. The different changes of the broad peak in S–T curves for Sr-doped samples were observed, which may result from the itinerant hole carriers. The transport mechanism of La2?xSrxCu0.94Ti0.06O4 is mainly dominated by the small-polaron hopping due to the discrepancy in the activation energy derived from the resistivity and the thermoelectric power. The small polarons are not originated from the magnetic coupling between magnetic ions and hole spins, but from the coupling between the phonon with a breathing mode and the hole carriers. 相似文献
19.
V. I. Kozub R. V. Parfen’ev D. V. Shamshur D. V. Shakura A. V. Chernyaev S. A. Nemov 《JETP Letters》2006,84(1):35-40
The superconductor-insulator transition that occurs at liquid helium temperatures in the (PbzSn1?z )0.84In0.16Te semiconductor system with varying lead concentration z = 0.5–0.9 is experimentally investigated. The transition is attributed to the change in the energy characteristics of In impurity centers due to the variation in the amount of lead. The insulator state appears with the transition from the mixed band-impurity conduction, which is characterized by resonant scattering of carriers into the quasilocal indium impurity states, to the hopping conduction between indium impurity states. The sample with z = 0.8 is found to exhibit a variable range hopping conduction described by Mott’s law. Factors that lead to the hopping conduction via impurity states are considered. 相似文献
20.
H.??en?ariková P.?Farka?ovsky 《The European Physical Journal B - Condensed Matter and Complex Systems》2010,77(3):393-399
The canonical Monte-Carlo is used to study the phase transitions from the
low-temperature ordered phase to the high-temperature disordered phase in
the two-dimensional half-filled Falicov-Kimball model with correlated
hopping. As the low-temperature ordered phase we consider the chessboard
phase, the axial striped phase and the segregated phase.
It is shown specifically for weak coupling, which is the most interesting
regime, that all three phases persist also at finite temperatures (up to
the critical temperature τ
c
) and that the phase transition at the
critical point is of the first order for the chessboard and axial striped
phase and of the second order for the segregated phase. In addition, it is
found that the critical temperature is reduced with the increasing amplitude
of correlated hopping t' in the chessboard phase and it is strongly
enhanced by t' in the axial striped and segregated phase. 相似文献