共查询到20条相似文献,搜索用时 24 毫秒
1.
A. A. Ovchinnikov M. Ya. Ovchinnikova E. A. Plekhanov 《Journal of Experimental and Theoretical Physics》1998,87(3):534-545
It is shown that d-symmetry superconductivity due to valence bond correlations is possible. Valence bond correlations are compatible with antiferromagnetic
spin order. In order to explictly construct a homogeneous state with the valence bond structure in the two-dimensional Hubbard
model for an arbitrary doping, we have used the variational method based on unitary local transformation. Attraction between
holes in the d-channel is due to modulation of hopping by the site population in course of the valence bond formation, and corresponding
parameters have been calculated variationally. An important factor for the gap width is the increase in the density of states
on the Fermi level due to antiferromagnetic splitting of the band. The gap width and its ratio to the T
c are 2Δ≃0.1t and 2Δ/kT
c≃4.5−4 for U/t≃8. The correspondence between the theoretical phase diagram and experimental data is discussed. The dependence of T
c on the doping δ=|n−1| and the Fermi surface shape are highly sensitive to the weak interaction t′ leading to diagonal hoppings. In the case of t′>0 and p-doping, the peak on the curve of T
c(δ) occurs at the doping δ
opt, when the energy of the flattest part of the lower Hubbard subband crosses the Fermi level at k∼(π,0). In underdoped samples with δ<δ
opt, the anisotropic pseudogap in the normal state corresponds to the energy difference |E(π,0)−μ| between this part of the spectrum and the Fermi level.
Zh. éksp. Teor. Fiz. 114, 985–1005 (September 1998) 相似文献
2.
S. S. Aplesnin 《Physics of the Solid State》1998,40(6):986-990
An analysis is made of the two-dimensional Heisenberg model with S=1/2, anisotropic exchange interaction between nearest neighbors, and alternating exchange in two directions, [100] and [010]
(corresponding to condensation of the (π, π) mode) and in one direction [100] (corresponding to condensation of the (π, 0) mode). The quantum Monte Carlo method is used to calculate the thermodynamic characteristics and the spin correlation
functions which are used as the basis to determine the boundary of stability of an anisotropic antiferromagnetic with respect
to alternation of exchange δ=(1−J
x,y
/J
z
)0.4 in the (π,π) model and δ=(1−J
x,y
/J
z
)0.31 in the (π,0) model. In the (π,0) model a disordered quantum state exists in the range (1−J
x,y
/J
z
)0.31<δ<(0.3–0.35). The energy (E−0.68)=0.36δ
1.80(6) and 0.21 δ
2.0(5), the energy gap between the ground and excited states H
c
(δ)=1.96δ
2.(1), 1.8(1) (δ−0.35(3))0.67(2) were determined as a function of the alternation of exchange in the (π,π)-and (π,0) models, respectively.
Fiz. Tverd. Tela (St. Petersburg) 40, 1080–1085 (June 1998) 相似文献
3.
A. A. Ovchinnikov M. Ya. Ovchinnikova E. A. Plekhanov 《Journal of Experimental and Theoretical Physics》1999,88(2):356-369
The phase diagram, nature of the normal state pseudogap, type of the Fermi surface, and behavior of the superconducting gap
in various cuprates are discussed in terms of a correlated state with valence bonds. The variational correlated state, which
is a band analogue of the Anderson (RVB) states, is constructed using local unitary transformations. Formation of valence
bonds causes attraction between holes in the d-channel and corresponding superconductivity compatible with antiferromagnetic spin order. Our calculations indicate that
there is a fairly wide range of doping with antiferromagnetic order in isolated CuO2 planes. The shape of the Fermi surface and phase transition curve are sensitive to the value and sign of the hopping interaction
t′ between diagonal neighboring sites. In underdoped samples, the dielectrization of various sections of the Fermi boundary,
depending on the sign of t′, gives rise to a pseudogap detected in photoemission spectra for various quasimomentum directions. In particular, in bismuth-and
yttrium-based ceramics (t′>0), the transition from the normal state of overdoped samples to the pseudogap state of underdoped samples corresponds to
the onset of dielectrization on the Brillouin zone boundary near k=(0,π) and transition from “large” to “small” Fermi surfaces. The hypothesis about s-wave superconductivity of La-and Nd-based ceramics has been revised: a situation is predicted when, notwithstanding the d-wave symmetry of the superconducting order parameter, the excitation energy on the Fermi surface does not vanish at all points
of the phase space owing to the dielectrization of the Fermi boundary at k
x=±
k
y. The model with orthorhombic distortions and two peaks on the curve of T
c versus doping is discussed in connection with experimental data for the yttrium-based ceramic.
Zh. éksp. Teor. Fiz. 115, 649–674 (February 1999) 相似文献
4.
E. R. Likhachev S. I. Kurganskii O. I. Dubrovskii A. M. Bugakov 《Physics of the Solid State》1997,39(3):378-380
The results of calculations of the energy band structure of thin films are used to obtain theoretical photoelectron spectra
of the valence band of the compound YBa2Cu3O7−δ
for δ=0 and δ=1 at various photon energies; the spectra are averaged over the photoelectron emission angles. The principal structural features
of the spectra are determined by the d states of copper atoms, while the variation of the shape of the spectra with increasing photon energy is attributed to a
relative decrease in the contribution from the p states of oxygen atoms. The density of d states around the Fermi level for YBa2Cu3O6 films is observed to increase relative to YBa2Cu3O7 films.
Fiz. Tverd. Tela (St. Petersburg) 39, 437–440 (March 1997) 相似文献
5.
A. A. Ovchinnikov M. Ya. Ovchinnikova 《Journal of Experimental and Theoretical Physics》1999,89(3):564-576
The properties of spin excitations superposed on a uniform ground state with antiferromagnetic (or spiral) spin structure
are studied in a 2D Hubbard model. Expressions are derived for the spin susceptibility in the random phase approximation (RPA) using split Hubbard
bands as a zeroth approximation. The calculated collective modes with dispersion ω(Q)=c|Q−(π, π)| near Q∼(π, π) reproduce well the characteristics of the spin excitations observed in undoped cuprates. For doped systems with an
antiferromagnetic structure of the ground state, calculating X″(Q,ω→0) gives the same mode with a peak at Q∼(π, π), regardless of the type of Fermi surface. It is shown that in doped systems with a spiral ground state spin structure,
X″(Q,ω→0) peaks occur with incommensurate quasimomenta Q that are coupled to the spirality vector.
Zh. éksp. Teor. Fiz. 116, 1058–1080 (September 1999) 相似文献
6.
C. Andreoiu D. Rudolph I. Ragnarsson C. Fahlander R.A.E. Austin M.P. Carpenter R.M. Clark J. Ekman R.V.F. Janssens T.L. Khoo F.G. Kondev T. Lauritsen T. Rodinger D.G. Sarantites D. Seweryniak T. Steinhardt C.E. Svensson O. Thelen J.C. Waddington 《The European Physical Journal A - Hadrons and Nuclei》2002,14(3):317-348
High-spin states in 59Cu were populated using the fusion-evaporation reaction 28Si + 40Ca at a beam energy of 122 MeV. The Gammasphere Ge-detector array in conjunction with the 4π charged-particle detector array
Microball allowed for the detection of γ-rays in coincidence with evaporated light particles. The resulting extensive high-spin
decay scheme of 59Cu is presented, which comprises more than 320 γ-ray transitions connecting about 150 excited states. Their spins and parities
have been assigned via directional correlations of γ-rays emitted from oriented states. Average quadrupole moments of rotational
bands have been determined from the analysis of residual Doppler shifts. Shell model calculations in the fp shell are invoked to study some of the low-spin states, while the experimental characteristics of the rotational bands are
analyzed in the configuration-dependent cranked Nilsson-Strutinsky (CNS) approach.
Received: 15 February 2002 / Accepted: 23 April 2002 相似文献
7.
K. Singh S. Sihotra S. S. Malik J. Goswamy D. Mehta N. Singh R. Kumar R. P. Singh S. Muralithar E. S. Paul J. A. Sheikh C. R. Praharaj 《The European Physical Journal A - Hadrons and Nuclei》2006,27(3):321-324
The collective band structures of the 125Cs nucleus have been investigated by in-beam γ-ray spectroscopic techniques following the 110Pd ( 19F, 4n) reaction at 75MeV. The previously known level scheme, with rotational bands built on πg7/2, πg9/2 and πh11/2 orbitals, has been extended and evolves into bands involving rotationally aligned ν(h11/2)2 and π(h11/2)2 quasiparticles. A strongly coupled band has been reassigned a high-K πh11/2 ⊗ νg7/2 ⊗ νh11/2 three-quasiparticle configuration and a new side band likely to be its chiral partner has been identified. Configurations
assigned to various bands are discussed in the framework of Principal/Tilted Axis Cranking (PAC/TAC) model calculations. 相似文献
8.
Kuljeet Singh Z. Naik R. Kumar J. Goswamy D. Mehta N. Singh C. R. Praharaj E. S. Paul K. P. Singh R. P. Singh S. Muralithar N. Madhavan J. J. Das S. Nath A. Jhingan P. Sugathan R. K. Bhowmik 《The European Physical Journal A - Hadrons and Nuclei》2005,25(3):345-354
High-spin states in 123Cs, populated via the 100Mo ( 28Si, p4n) fusion-evaporation reaction at E
lab = 130 MeV, have been investigated employing in-beam γ-ray spectroscopic techniques. Rotational bands, built on πg
7/2, πg
9/2 and the unique-parity πh
11/2 orbitals, have been extended and evolve into bands involving rotationally aligned ν(h
11/2)2 and π(h
11/2)2 quasiparticles. A three-quasiparticle band based on the high-K πh
11/2 ⊗ νg
7/2 ⊗ νh
11/2 configuration has also been observed. Total Routhian Surface (TRS) calculations have been used to predict the nuclear shape
parameters ( β2, β4, γ) for the various assigned configurations. The assigned configurations have been discussed in the framework of a microscopic
theory based on the deformed Hartree-Fock (HF) and angular-momentum projection techniques. 相似文献
9.
Yu. G. Chukalkin A. E. Teplykh A. N. Pirogov D. G. Kellerman 《Physics of the Solid State》2010,52(12):2545-2551
The specific features of the crystal structure and the magnetic state of stoichiometric lithium manganite in the structurally
ordered Li[Mn2]O4 and disordered Li1 − δMnδ[Mn2 − δLiδ]O4 (δ = 1/6) states have been investigated using neutron diffraction, X-ray diffraction, and magnetic methods. The structurally
disordered state of the manganite was achieved under irradiation by fast neutrons (E
eff ≥ 1 MeV) with a fluence of 2 × 1020 cm−2 at a temperature of 340 K. It has been demonstrated that, in the initial sample, the charge ordering of manganese ions of
different valences arises at room temperature, which is accompanied by orthorhombic distortions of the cubic spinel structure,
and the long-range antiferromagnetic order with the wave vector k = 2π/c(0, 0, 0.44) is observed at low temperatures. It has been established that the structural disordering leads to radical changes
in the structural and magnetic states of the LiMn2O4 manganite. The charge ordering is destroyed, and the structure retains the cubic symmetry even at a temperature of 5 K. The
antiferromagnetic type of ordering transforms into ferrimagnetic ordering with local spin deviations in the octahedral sublattice
due to the appearance of intersublattice exchange interactions. 相似文献
10.
S. S. Savinskii N. V. Khokhryakov 《Journal of Experimental and Theoretical Physics》1997,84(6):1131-1137
A simple tight-binding model of the π-electron states of carbon nanotubes is analyzed. The symmetry group of nanotubes and its relation to electronic structure
are discussed. The applicability of the simple model is analyzed on the basis of numerical calculations of the electronic
states of energy-optimized nanotubes, performed in a parametric tight-binding model that takes account of the carbon valence
electrons. Numerical data on the gap widths of optimized nanotubes and data obtained from the zone-folding model employed
in the literature are presented.
Zh. éksp. Teor. Fiz. 111, 2074–2085 (June 1997) 相似文献
11.
K. A. Kikoin M. N. Kiselev A. S. Mishchenko 《Journal of Experimental and Theoretical Physics》1997,85(2):399-414
A theory of stabilization of a spin liquid in a Kondo lattice at temperatures close to the temperature of antiferromagnetic
instability has been developed. Kondo exchange scattering of conduction electrons leads to emergence of a state of the spin
liquid of the resonating valence bonds (RVB) type at T>T
K. Owing to this stabilization, low-energy processes of Kondo scattering with energies below T
K are frozen so that the “singlet” state of the Kondo lattice is not realized; instead a strongly correlated spin liquid with
developed antiferromagnetic fluctuations occurs. A new version of the Feynman diagram technique has been developed to describe
interaction between spin fluctuations and resonant valence bonds in a self-consistent manner. Emergence of a strongly anisotropic
RVB spin liquid is discussed.
Zh. éksp. Teor. Fiz. 112, 729–759 (August 1997) 相似文献
12.
We present a calculation of the neutrino-nucleon scattering cross-section which takes into account the nuclear correlations
in the relativistic random phase approximation (RPA). Our approach is based on a quantum-hadrodynamics model with exchange
of σ, ω, π, ρ and δ mesons. In view of applications to neutrino transport in the final stages of supernova explosion and proto-neutron
star cooling, we study the evolution of the neutrino mean free path as a function of density, proton-neutron asymmetry and
temperature. Special attention was paid to the issues of renormalization of the Dirac sea, residual interactions in the tensor
channel, coupling to the delta-meson and meson mixing. In contrast with the results of other authors, we find that the neutral-current
process is not sensitive to the strength g' of the residual contact interaction. As a consequence, it is found that RPA corrections with respect to the mean-field approximation
amount to only 10% to 15% at high density.
Received: 27 June 2001 / Accepted: 14 January 2002 相似文献
13.
M. V. Krasin’kova 《Technical Physics》1998,43(11):1347-1349
The question of the dimensionality of superconductivity is considered within the framework of a model of superconductivity
via asymmetric, delocalized “crystalline” π orbitals (analogous to the corresponding molecular orbitals) extending along chains of covalently bonded copper and oxygen
ions. It is shown that superconductivity is preceded by a separation of the bonds in the CuO2 layer into covalent and ionic bonds with ordering of the covalent bonds into chains. Such an ordering facilitates the formation
of a crystalline π orbital lowering the crystal energy by the resonance energy of the π bond and is therefore favored. The superconducting current is created by non-dissipative motion of π-electron pairs along the asymmetric, “crystalline” π orbitals extending along chains of covalently bonded copper and oxygen ions, in the presence of an ionic bond between neighboring
chains extending through the easily polarizable O2− ions. This ionic bond correlates the motion of the electron pairs along all the π orbitals and stabilizes the superconducting state. Only in this sense is the apparent “onedimensionality” of superconductivity
in cuprate superconductors to be understood.
Zh. Tekh. Fiz. 68, 82–84 (November 1998) 相似文献
14.
The inclusive K
+ meson production in proton–nucleus collisions in the near threshold and subthreshold energy regimes is analyzed with respect
to the one–step (pN→K
+
YN, Y=Λ,Σ) and two–step (pN→NNπ, NN2π; πN→K
+
Y) incoherent production processes on the basis of an appropriate new folding model, which takes properly into account the
struck target nucleon removal energy and momentum distribution (nucleon spectral function), extracted from recent quasielastic
electron scattering experiments and from many–body calculations with realistic models of the NN interaction. Comparison of the model calculations of the K
+ total and double differential cross sections for the reaction p+C
12 with the existing experimental data is given, illustrating both the relative role of the primary and secondary production
channels at considered incident energies and those features of the cross sections which are sensitive to the high momentum
and high removal energy part of the nucleon spectral function that is governed by nucleon–nucleon short–range and tensor correlations.
It is found that the in–medium modifications of the available for pion and kaon production invariant energies squared due
to the respective optical potentials are needed to account for considered experimental data.
Received: 2 April 1997 / Revised version: 7 August 1997 相似文献
15.
A method is presented for approximating the effect of core electrons by a pseudopotential which is an extension of one previously presented by Dixon and Hugo. The pseudopotential is constructed in a fully ab initio manner from atomic SCF calculations. It is non-local in both radial and angular coordinates, but its matrix elements are none the less easy to evaluate. The method has been implemented within a multi-structure valence-bond framework. The approximations arising from the use of finite basis sets, both for the pseudopotential and for the valence wavefunction, inevitably lead to errors in calculated energies. However, these errors are largely atomic in origin. Thus, in addition to ab initio calculations we also use empirical atomsin-molecules corrections to minimize both basis set errors and atomic correlation errors. These method are applied to potential curves for 21 electronic states of the SO molecule. Comparison is made of the curves calculated using the ab initio multi-structure valence-bond method without and with the atoms-in-molecules corrections. The potential energy curves of three, previously unobserved, bound electronic states of SO are calculated. We estimate that these states, 1Σ-, 3Δ and 3Σ+, lie in the region of 3·2 to 3·4 eV above the ground state. 相似文献
16.
We measured the optical absorption spectra of para-benzoquinone and duroquinone in polar (methanol) and nonpolar (n-hexane) solvents. We find that the specific effect of the polar solvent, which manifests itself here as a bathochromic shift
of one of π-π* bands, is caused by the formation of hydrogen bonds between solvent molecules and the molecule under study
and, as a consequence, by a decrease in the energy gap between the corresponding occupied (π) and unoccupied (π*) molecular
orbitals. This result is obtained by TDDFT B3LYP/6-311+G(d, p)-calculations of electronic spectra, which, in the case of isolated
para-benzoquinone and duroquinone molecules, reproduce experimental optical absorption spectra of the corresponding compounds
in n-hexane and, in the case where these molecules form complexes with methanol molecules by means of hydrogen bonds, reproduce
spectra measured in methanol. 相似文献
17.
The electronic structure of a high quality superconducting Bi2Sr2CaCu2O8+δ (Bi2212) single crystal is studied by angle resolved ultra violet photoemission (ARUPS) using He I (21.2 eV). Our results
appear to show two bands crossing the Fermi level in ΓX direction of the Brillouin zone as reported by Takahashiet al. The bands at higher binding energy do not show any appreciable dispersion. The nature of the states near the Fermi level
is discussed and the observed band structure is compared with the band structure calculations. 相似文献
18.
Yuhu Zhang Furong Xu Jianjun He Zhong Liu Xiaohong Zhou Zaiguo Gan M. Oshima T. Hayakawa Y. Toh T. Shizuma J. Katakura Y. Hatsukawa M. Matsuda H. Kusakari M. Sugawara K. Furuno T. Komatsubara T. Une Shuxian Wen Zhimin Wang 《中国科学G辑(英文版)》2003,46(4):382-389
High-spin states in182Au have been produced and studied via the152Sm(35Cl,5nγ)182Au reaction. The level scheme consisting of the πh
9/2⊗νi
13/2 and πi
13/2⊗νi
13/2 bands has been established for the first time. The low spin signature inversion in both bands has been found. The observed
signature inversion phenomena can be interpreted qualitatively using the pairing and deformation self-consistent cranked Wood-Saxon
calculations. 相似文献
19.
Ts. Venkova M.-G. Porquet I. Deloncle B.J.P. Gall H. De Witte P. Petkov A. Bauchet T. Kutsarova E. Gueorgieva J. Duprat C. Gautherin F. Hoellinger R. Lucas A. Minkova N. Schulz H. Sergolle E.A. Stefanova A. Wilson 《The European Physical Journal A - Hadrons and Nuclei》1999,6(4):405-413
The 107,109Rh nuclei have been produced as fission fragments following the fusion reaction 28Si +176Yb at 145 MeV bombarding energy and studied with the Eurogam2 array. In both nuclei three new rotational bands with the odd
proton occupying the πg9/2, πp1/2 and π(g7/2/d5/2) sub-shells have been observed. In 107Rh, two other bands involving strong M1 transitions have been identified at excitation energy larger than 2 MeV. They can
be interpreted in terms of three quasiparticle excitations. In addition new structures consisting of four transitions, built
on states located at low excitation energy (680 keV in 107Rh and 642 keV in 109Rh), point out the importance of triaxial deformation in these two isotopes.
Received: 1 September 1999 相似文献
20.
N. Itagaki S. Okabe K. Ikeda I. Tanihata 《The European Physical Journal A - Hadrons and Nuclei》2002,13(1-2):43-47
The structure of Be and C isotopes are investigated based on the molecular-orbit (MO) model. The low-lying states are characterized
by several configurations of valence neutrons, which are constructed as combinations of basic orbits. In 10Be, all of the observed positive-parity bands and the negative-parity bands are described within the model. The second 0+ state of 10Be has a large α-α cluster structure, and this is characterized by a (1/2+
σ)2 configuration. An enlargement of the α-α distance due to two-valence neutrons along the α-α axis makes their wave function
smooth and reduces the kinetic energy drastically. Furthermore, the contribution of the spin-orbit interaction due to coupling
between the S
z = 0 and the S
z = 1 configurations, is important. In the ground state of 12Be, the calculated energy exhibits similar characteristics, that the remarkable α clustering and the contribution of the spin-orbit
interaction make the binding of the state with (3/2-
π)2(1/2+
σ)2 configuration properly stronger in comparison with the closed p-shell (3/2-
π)2(1/2-
π)2 configuration. This is related to the breaking of the N = 8 (closed p-shell) neutron magic number. Also, the molecule-like structure of the C isotopes is investigated using a microscopic α+α+α+n+n+ . . . model. The combination of the valence neutrons in the π- and the σ-orbit is promising to stabilize the linear-chain state
against the breathing and bending modes, and it is found that the excited states of 16C are the most promising candidates for such structure.
Received: 1 May 2001 / Accepted: 4 December 2001 相似文献