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1.
I. A. Makarov S. G. Ovchinnikov E. I. Shneider 《Journal of Experimental and Theoretical Physics》2012,114(2):329-342
The influence of interlayer hoppings on the superconducting transition temperature (T
c) in bilayer cuprates has been studied. The parameter of hopping between layers is expressed as t
⊥(k) = t
⊥(cos(k
x
) − cos(k
y
))2 and treated as a small perturbation for the states of two CuO2 planes described by the t-t′-t″-J* model. In the generalized mean field approximation for dx2 - y2{d_{{x^2} - {y^2}}} symmetry of the superconducting gap, neither the interlayer hopping or exchange interaction, nor the pair hopping between
CuO2 layers provides an additional mechanism of Cooper pair formation or an increase in T
c. In the concentration dependence of T
c, the bilayer splitting of the upper Hubbard band of quasi-holes is manifested as two peaks with temperatures slightly lower
than the maximum T
c for a single-layer cuprate. Interlayer antiferromagnetic spin correlations suppress bilayer splitting. 相似文献
2.
Recently, angle-resolved photoemission spectroscopy measurements on Bi2Sr2CaCu2O8+δ, which possesses two CuO2 layers in the same unit cell, have yielded very interesting results. For the overdoped samples, these results show a splitting
of electronic states near k=(π, 0) point of Brillioun zone. On the other hand, no splitting is observed in the underdoped samples. In view of this, the
detailed studies including the doping and temperature dependence of the spectral properties become desirable. In this paper,
we consider cuprates possessing two CuO2 layers per unit cell. Each layer in the system is described by the t-t
1-J model and the two layers are coupled via an intrabilayer hopping term (t
⊥) and an intrabilayer exchange coupling (J
⊥). A self-consistent perturbation approach is used to calculate the electronic spectral function for different values of hole
density, hole momentum and temperature. We find that the imaginary part of the self energy is strongly momentum dependent
which contradicts the suggestion that the Fermi surface of cuprates may be described by marginal Fermi liquid theory. We have
calculated the spectral function for various values of intrabilayer parameters t
⊥ and J
⊥. For larger values of intrabilayer interactions we observe the splitting in the quasi-particle peak at k=(π, 0) which is in agreement with the recent observations. The splitting is also found to be sensitive to the hole concentration
as well as the temperature of the system. We have also discussed the reasons why the splitting is absent in underdoped bilayer
cuprates at low temperature. 相似文献
3.
E. Z. Kuchinskii I. A. Nekrasov Z. V. Pchelkina M. V. Sadovskii 《Journal of Experimental and Theoretical Physics》2007,104(5):792-804
Pseudogap phenomena are observed for the normal underdoped phase of different high-T
c cuprates. Among others, the Bi2Sr2CaCu2O8 − δ (Bi2212) compound is one of the most studied experimentally. To describe the pseudogap regime in Bi2212, we use a novel generalized
ab initio LDA + DMFT + Σk hybrid scheme. This scheme is based on the strategy of one of the most powerful computational tools for real correlated materials:
the local density approximation (LDA) + dynamical mean-field theory (DMFT). Conventional LDA + DMFT equations are here supplied
with an additional (momentum-dependent) self-energy Σk in the spirit of our recently proposed DMFT + Σk approach taking into account pseudogap fluctuations. In the present model, Σk describes nonlocal correlations induced by short-range collective Heisenberg-like antiferromagnetic spin fluctuations. The
effective single-impurity problem of the DMFT is solved by the numerical renormalization group (NRG) method. Material-specific
model parameters for the effective x
2 − y
2 orbital of Cu-3d shell of the Bi2212 compound, e.g., the values of intra-and interlayer hopping integrals between different Cu sites, the
local Coulomb interaction U, and the pseudogap potential Δ were obtained within the LDA and LDA + DMFT schemes. Here, we report
on the theoretical LDA + DMFT + Σk quasiparticle band dispersion and damping, Fermi surface renormalization, momentum anisotropy of (quasi)static scattering,
densities of states, spectral densities, and angular-resolved photoemission (ARPES) spectra, taking into account pseudogap
and bilayer splitting effects for normal (slightly) underdoped Bi2212 (δ = 0.15). We show that LDA + DMFT + Σk successfully describes strong (pseudogap) scattering close to Brillouin zone boundaries. Our calculated LDA + DMFT + Σk Fermi surfaces and ARPES spectra in the presence of pseudogap fluctuations are almost insensitive to the bilayer splitting
strength. However, our LDA-calculated value of bilayer splitting is rather small to describe the experimentally observed peak-dip-hump
structure. The results obtained are in good semiquantitative agreement with various recent ARPES experiments.
The article was submitted by the authors in English. 相似文献
4.
A. Medhi S. Basu C. Kadolkar 《The European Physical Journal B - Condensed Matter and Complex Systems》2009,72(4):583-589
The phase diagram for a t-J bilayer as a function of interplanar
hopping, t⊥ and hole concentration, x is presented for a few different
values of interplanar exchange, J⊥ using variational Monte Carlo
calculations. The phase diagram shows rich features, such as a coexistence of
antiferromagnetism and superconductivity at underdoping, planar (d-wave) and
interplanar (dz-wave) superconducting correlations for small and large
J⊥, respectively at optimal and overdoping.
Another unusual feature appears in the form of a dome shaped structure in the phase
diagram where the superconducting correlations
are initially assisted as interplanar hopping is enhanced for small
t⊥, while larger t⊥ is found to be detrimental to
superconductivity. 相似文献
5.
The present paper attempts to study the Neel temperature of bilayer antiferromagnetic cuprate YBa2Cu3O6.2 within anisotropic Heisenberg model. The double time Green’s function formalism within random phase approximation (RPA) has
been used to obtain various correlation functions. The magnetization and the Neel temperature (T
N) are evaluated. It is observed that the ratio of intrabilayer to inplane exchange coupling (r=J⊥/J‖) plays an important role in the magnetic dynamics of bilayer systems. The recent experimental data of bilayer system YBa2Cu3O6.2 have been used to estimate the ratio r from the expression for Neel temperature. The estimated values of spin gap and the ratio of hopping matrix elements t⊥/t‖ are found to be in fairly good agreement with the existing experimental results. 相似文献
6.
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
7.
For a bilayer model of the high-Tc cuprates consisting of two coupled Hubbard planes we calculate within the FLEX (fluctuation exchange) approximation the quasiparticle self-energies and dynamic spin susceptibilities. We consider a Fermi surface similar to those of the YBaCuO cuprates, an on-site Coulomb repulsion U = 6 t (where t is the intraplane hopping), and interplane hoppings t’ up to t’ = 0.7 t. For sufficiently large t'(?0.3 t) the spin susceptibility gC? due to transitions between the bonding and antibonding states becomes much larger than the susceptibility gC+ for transitions within the same band. The spectral density Im gC?(q, ω) exhibits a peak centered at the antiferromagnetic wavevector q = Q = (π, π) and at the antiparamagnon frequency ω = ωsf. For decreasing temperature T and increasing t’ the peak becomes more and more pronounced. We calculate the Fermi lines of the bonding and antibonding bands for different t'. The superconducting transition temperature Tc for dx 2?y2-wave pairing decreases as t' increases. Our results are in qualitative agreement with magnetic neutron scattering, magnetic resonance, and photoemission experiments on bilayer YBaCuO cuprates. They are also consistent with the results of quantum Monte Carlo simulations. 相似文献
8.
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
9.
Takasada Shibauchi Tsuyoshi Kawakami Yuhki Terao Minoru Suzuki Lia Krusin-Elbaum 《Pramana》2006,66(1):305-312
Vortex and pseudogap states in electron-doped Sm2−x
Ce
x
CuO4−δ (x ∼ 0.14) are investigated by the interlayer transport in magnetic fields up to 45 T. To extract intrinsic properties, we fabricated
small 30 nm-high mesa structures, sufficiently thin to be free of the recently reported partial decomposition problems. On
cooling, the c-axis resistivity ρc of the mesa structures reveals a semiconductive upturn above Tc, followed by a sharp superconducting transition at 20 K. When the magnetic fieldH is applied along the c-axis, ρc(T) shows a parallel shift without significant broadening, as also observed in the hole-doped underdoped cuprates. Above the
transition we observe negative magnetoresistance (MR), which can be attributed to the field suppression of the pseudogap,
whose magnitude is as small as 38 K. Our results in thex ∼ 0.14 samples closely correspond to the interlayer transport behavior in the ‘overdoped’ regime of hole-doped Bi2Sr2 CaCu2 O8+y. 相似文献
10.
S. G. Ovchinnikov I. A. Makarov E. I. Shneyder 《Journal of Experimental and Theoretical Physics》2011,112(2):288-302
We present a theoretical study of the electronic structure of bilayer HTSC cuprates and its evolution under doping and in
a high magnetic field. Analysis is based on the t-t′-t″-J* model in the generalized Hartree-Fock approximation. Possibility of tunneling between CuO2 layers is taken into account
in the form of a nonzero integral of hopping between the orbitals of adjacent planes and is included in the scheme of the
cluster form of perturbation theory. The main effect of the coupling between two CuO2 layers in a unit cell is the bilayer splitting manifested in the presence of antibonding and bonding bands formed by a combination
of identical bands of the layers themselves. A change in the doping level induces reconstruction of the band structure and
the Fermi surface, which gives rise to a number of quantum phase transitions. A high external magnetic field leads to a fundamentally
different form of electronic structure. Quantum phase transitions in the field are observed not only under doping, but also
upon a variation of the field magnitude. Because of tunneling between the layers, quantum transitions are also split; as a
result, a more complex sequence of the Lifshitz transitions than in single-layer structures is observed. 相似文献
11.
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
12.
S. I. Vedeneev 《JETP Letters》1998,68(3):230-235
The coexistence of a superconducting gap and a normal-state gap (pseudogap) is observed in tunneling experiments with high-quality
single crystals of the single-layer cuprate Bi2Sr2CuOz. At temperatures T< T
c the normal-state gap, whose width is close to that of the super-conducting gap, does not vanish in magnetic fields H>H
c2. The large smearing of the gap structure in the tunneling spectra and the high conductance of tunnel junctions at zero bias
voltage are due to the strong angular dependence of the superconducting-and normal-state gaps. The results of the investigations
are in good agreement with recently published angle-resolved photoemission data.
Pis’ma Zh. éksp. Teor. Fiz. 68, No. 3, 217–222 (10 August 1998) 相似文献
13.
A. V. Anan’ev Yu. I. Zhdanov A. P. Gerashchenko K. N. Mikhalev S. V. Verkhovskii E. Yu. Medvedev K. A. Okulova N. I. Chebotaev V. N. Goshchitskii 《JETP Letters》1998,67(3):182-188
The spin-spin relaxation rate 63
T
2
−1
of 63Cu nuclei in CuO2 layers is measured in the normal and superconducting states of the compound YBa2Cu3O6.9 (T
c
onset
=94 K) subjected to radiation-induced disordering by a fast-neutron flux Φ to T
c
onset
=68 K (Φ=7×1018 cm−2) and T
c
onset
<4 K (Φ=12×1018 cm−2). It is found that as the structural disorder increases, the contribution of the indirect spin-spin interaction 63
T
2G
−1
, which is related to the value of the spin susceptibility at the boundary of the Brillouin zone of the copper planes χs(q={π/a; π/a}), decreases slightly at the transition to the superconducting state for the initial sample and remains unchanged for the
weakly disordered sample. This behavior of the short-wavelength contribution to the spin susceptibility attests to the stability
of the x
2−y
2 symmetry of the energy gap against structural disorder, in accordance with proposed theoretical models of Cooper pairing
for high-T
c cuprates.
Pis’ma Zh. éksp. Teor. Fiz. 67, No. 3, 172–177 (10 February 1998) 相似文献
14.
Variations in the temperature behavior of resistivity, ρ(T), in the ab plane of the anisotropic single-crystal high-T
c
superconductor BiSrCuO (2201 phase) have been observed at the insulator-metal (IM) transition. At low temperatures, as one
approaches the transition, the Mott relation for two dimensions, ln ρ ∝ T
−1/3, changes to ln ρ ∝ T
−1/2, which corresponds to hopping conduction with a Coulomb gap in the density of states. Negative temperature slopes were revealed
in the samples near the transition. Estimates suggest that superconductivity in these samples sets in from the Anderson insulator
state. The behavior of the width of the superconducting transition and of the temperature of its onset, T
con, at the IM transition has been studied from measurements of the ac magnetic susceptibility. It is shown that in the vicinity
of the IM transition the superconducting transition becomes broader, and the onset of the transition T
con shifts toward higher temperatures. This behavior is attributed to nonuniform superconductivity resulting from formation in
the crystal of superconducting droplets with different values of T
c
, which is caused by fluctuations in the local density of states due to the inherent disorder in the crystal. In these conditions,
superconductivity has a percolation character.
Fiz. Tverd. Tela (St. Petersburg) 40, 1190–1194 (July 1998) 相似文献
15.
In cuprates, in a view where pairing correlations set in at the pseudogap energy scale T* and acquire global coherence at
a lower temperature Tc, the regionT
c⪯ T ⪯ T* is a vast fluctuation regime.T
c andT* vary differently with doping and the question remains about the doping trends of the relevant magnetic field scales: the
field Hc2 bounding the superconducting response and the pseudogap closing field Hpg. In-plane thermal (Nernst) and our interlayer (tunneling) transport experiments in Bi2Sr2CaCu2O8+y report hugely different limiting magnetic fields. Here, based on pairing (and the uncertainty principle) combined with the
definitions of the Zeeman energy and the magnetic length, we show that both fields convert to the same pseudogap scaleT* upon transformation as orbital and Zeeman critical fields, respectively. The region of superconducting coherence is confined
to the ‘dome’ that coincides with the usual unique upper critical field Hc2 on the strongly overdoped side. We argue that the distinctly different orbital and the Zeeman limiting fields can co-exist
owing to charge and spin degrees of freedom separated to different parts of the strongly anisotropic Fermi surface. 相似文献
16.
A. A. Voronkov B. P. Gorshunov V. S. Nozdrin E. S. Zhukova T. Matsuoka K. Tanaka S. Miyasaka S. Tajima M. Dressel 《JETP Letters》2012,94(9):708-713
The spectra of the conductivity and dielectric constant of La1.87Sr0.13CuO4 cuprate have been directly measured in the frequency range of 0.3 to 1.2 THz (10–40 cm−1) and the temperature range of 5 to 300 K in the E | c polarization (the electric field vector of radiation is perpendicular to the copper-oxygen planes). Excitation has been observed
in the superconducting phase, and its nature has been attributed to the transverse optical excitation of the condensate of
Cooper pairs, which appears because Josephson junctions between CuO planes are modulated due to in-plane magnetic and charge
stripes. Additional quasiparticle absorption of unknown origin has been detected at frequencies below ≈15 cm−1 at liquid helium temperatures. 相似文献
17.
The rare t-quark decays t → cl
j
+
l
k
−
and t → cṽ
j
k
k
induced by scalar-leptoquark doublets are considered within the minimal model involving four-color quark-lepton symmetry
and the Higgs mechanism of quark and lepton mass splitting. The partial widths with respect to the decays being considered
and the total widths Γ(t → cl
+′l
−) = Σj,k Γ(t → cl
j
+
l
k
−
) and Γ(t → cl
+′l
−) = Σj,kΓ(t → cṽ
j
v
k
) with respect to, respectively, the charged leptonic and neutrino modes are calculated. It is shown that, at scalar-leptoquark
masses higher than the t-quark mass (m
S > m
t), the branching ratios for these modes are Br(t → cl
+′l
−) ≈ (3.5−0.4) × 10−5 and Br(t → cṽ′v) ≈ (7.1−0.8) × 10−5 at m
s
= 180–250 GeV and an appropriate value of the leptoquark-mixing angle (sin β ≈ 0.2) and can increase for m
S < m
t to Br(t → cl
+′l
−) ≈ 0.03−0.002 and Br(t → cl
+′l
−) ≈ 0.46−0.05 for the charged mode at m
S
= 150–170 GeV for sin β ≈ 1 and sin β ≈ 0.2, respectively. In the cases being considered, t-quark decays to pairs of charged leptons can be accessible to detection at LHC. In the last case, these decays could manifest
themselves (for example, in dilepton events) at the Tevatron as well.
Original Russian Text ? P.Yu. Popov, A.D. Smirnov, 2006, published in Yadernaya Fizika, 2006, Vol. 69, No. 6, pp. 1006–1016. 相似文献
18.
A system of self-consistent integral equations for the superconducting gap is formulated and solved taking account of the
instability of the normal phase of bilayer cuprates against charge-density waves. The critical parameters are calculated as
a function of the wave vector, temperature, and doping index. It is found that the region in which superconductivity coexists
with d-type charge-density waves depends strongly on the doping index. The effective energy-gap parameter, determined as the interval
between the peaks of the density of states, can have a local minimum at temperatures T<T
c
.
Pis’ma Zh. éksp. Teor. Fiz. 68, No. 7, 583–587 (10 October 1998) 相似文献
19.
A.?V.?Zaitsev G.?A.?Ovsyannikov K.?Y.?Constantinian Yu.?V.?Kislinski? A.?V.?Shadrin I.?V.?Borisenko P.?V.?Komissinskiy 《Journal of Experimental and Theoretical Physics》2010,110(2):336-344
It is shown experimentally that the superconducting current density in Nb/Au/Ca1 − x
Sr
x
CuO2/YBa2Cu3O7 − δ hybrid superconducting heterostructures with a Ca1 − x
Sr
x
CuO2 anti-ferromagnetic (AF) cuprate interlayer is anomalously high for interlayer thicknesses d
M
= 10–50 nm and the characteristic damping length for superconducting correlations is on the order of 10 nm. The experimental
results are explained on the basis of theoretical analysis of a junction of two superconductors (S′ and S) connected by a
magnetic multilayer with the AF ordering of magnetization in the layers. It is shown that with such a magnetization ordering,
anomalous proximity effect determined by the singlet component of the condensate wavefunction may take place. As a result,
the critical currents in S′/I/AF/S and S′/I/N/S structures (I denotes an insulator, and N, the normal metal) may coincide
in order of magnitude even when the thickness of the AF interlayer considerably exceeds the decay length of the condensate
wavefunction in ferromagnetic layers. 相似文献
20.
A. V. Mitin 《Bulletin of the Russian Academy of Sciences: Physics》2009,73(8):1061-1064
Intensification of segregation processes with a decrease in the charge carrier concentration in cuprates should lead, according
to the string model, to a decrease the discrete width w
ν of dominant bosonic stripes and an increase in their critical temperature T
c
up to 1200 K. A comparison of the simulated results with the experimental data gives grounds to conclude that the anomalies
of the electronic properties observed in the pseudogap regime of lightly dopes cuprates are most likely to be due to the formation
of a metastable network of superconducting nanochannels with T
c
> 200 K from the bosonic stripes limited by w
ν ≤ 2 nm. 相似文献