共查询到20条相似文献,搜索用时 31 毫秒
1.
S. Takács 《Solid State Communications》2003,128(12):455-459
We study some parameters of superconductors with δ-function type singularities in the electronic density of states (DOS), exhibiting (s+d)-wave symmetry of the order parameter. Starting with a pure s-wave pairing potential Vs, the critical temperature Tc at first slightly increases with increasing the d-wave interaction potential Vd, being determined by this interaction only for stronger Vd values. The ratio R=2|Δ(0)|/kBTc of the mean value of the zero temperature energy gap |Δ(0)| to Tc increases with increasing Vd, reaching a maximum which depends on the mixing interaction term. The maximum values of R are comparable with very high values obtained in some gap measurements. The jump in the specific heat at critical temperature is by a factor 2.4 higher for the extreme singularity of pure s-wave symmetry, as compared with the BCS theory with constant DOS. Such higher values of the jump are in agreement with the experimentally observed values, as well as with the calculations determined by extended saddle points in the electronic bands. By switching the d-wave channel, the value of the jump decreases. The results show the usefulness of calculations with δ-type singularities as a limiting case of very strong singularities in the DOS. 相似文献
2.
R. Gonczarek M. Krzyzosiak A. Gonczarek 《The European Physical Journal B - Condensed Matter and Complex Systems》2008,61(3):299-308
In this paper we find and present on diagrams in the coordinates of η=2t1/t0 (the ratio of the second and the first nearest
neighbor hopping integrals) and n (the carrier concentration) the
areas of stability for the superconducting spin-singlet s-
and d-wave and the spin-triplet p-wave order
parameters hatching out during the phase transition from the normal
to the superconducting phase. The diagrams are obtained for an
anisotropic two-dimensional superconducting system with a relatively
wide partially-filled conduction band. We study a tight-binding
model with an attractive nearest neighbor interaction with the
amplitude V1, and the on-site interaction (with the amplitude
V0) taken either as repulsive or attractive. The problem of the
coexistence of the s-, p- and d-wave order
parameters is addressed and solved for chosen values of the ratio
V0/V1. A possible island of stability of the d-wave
order parameter in the s-wave order parameter environment for
a relatively strong on-site interaction is revealed. The triple
points, around which the s-, d-, and p-wave
order parameters coexist, are localized on diagrams. It is shown
that results of the calculations performed for the two-dimensional
tight-binding band model are dissimilar with some obtained within
the BCS-type approximation. 相似文献
3.
G. E. Volovik 《JETP Letters》1997,65(2):217-223
Vortex mass in Fermi superfluids and superconductors and its influence on quantum tunneling of vortices are discussed. The
vortex mass is essentially enhanced due to the fermion zero modes in the core of the vortex: the bound states of the Bogoliubov
quasiparticles localized in the core. These bound states form the normal component, which is nonzero even in the low-temperature
limit. In the collisionless regime ω
0
τ≫1 the normal component trapped by the vortex is unbound from the normal component in a bulk superfluid/superconductor and
adds to the inertial mass of the moving vortex. In a d-wave superconductor the vortex mass has an additional factor of (B
c2/B)1/2 due to the gap nodes.
Pis’ma Zh. éksp. Teor. Fiz. 65, No. 2, 201–206 (25 January 1997)
Published in English in the original Russian journal. Edited by Steve Torstveit. 相似文献
4.
R. Gonczarek L. Jacak M. Krzyzosiak A. Gonczarek 《The European Physical Journal B - Condensed Matter and Complex Systems》2006,49(2):171-186
Based upon the tight-binding formalism a model of a
high-Tc superconductor with isotropic and anisotropic
attractive interactions is considered analytically. Symmetry
facets of the group C4v are included within a method of
successive transformations of the reciprocal space. Complete sets
of basis functions of C4v irreducible representations are
given. Plausible spin-singlet and spin-triplet superconducting
states are classified with regard to the chosen basis functions. It
is displayed that pairing interaction coefficients and the
dispersion relation, which can be characterized by the parameter
η= 2t1/t0, have a diverse and mutually competing
influence on the value of the transition temperature. It is also
shown that
in the case of a nearly half-filled conduction band
and an anisotropic pairing interaction the spin-singlet
d-wave symmetry superconducting state is realized for
small values of the parameter η, whereas in the opposite
limit, for sufficiently large values, the spin-triplet
p-wave symmetry superconducting state has to be formed.
This result cannot be obtained within the Van Hove scenario or
BCS-type approaches, where the p-wave symmetry
superconducting state absolutely dominates. The specific heat jump
and the isotope shift as functions of the parameter η are
assessed and discussed for the d-wave symmetry singlet and
the p-wave symmetry triplet states. 相似文献
5.
In d-wave superconductors, the electronic density of states (DOS) induced by a vortex exhibits a divergence at low energies: N
vortex(E) ∼1/|E|. This divergence is the result of gap nodes in the spectrum of excitation outside the vortex core. The heat capacity in
two regimes, T
2/T
c
2
≫
B/B
c
2 and T
2/T
c
2
≪
B/B
c
2, is discussed.
Pis’ma Zh. éksp. Teor. Fiz. 64, No. 9, 641–645 (10 November 1996)
Published in English in the original Russian journal. Edited by Steve Torstveit. 相似文献
6.
7.
Two-and three-body decays of γ(5S) into BB, BB*, B*B*, B
s
B
s
, B
s
B
s
*, and BB*π, B*B*π are evaluated using the theory developed earlier for dipion-bottomonium transitions. The theory contains only two parameters—vertex
masses M
br
and M
ω—known from the dipion spectra and width. Predicted values of Γtot(5S) and six partial widths Γ
k
(5S), k = BB, BB*, ... are in agreement with the experiment. The decay widths Γ5S
(πBB*) and Γ5S
(πB*B*) are also calculated and found to be on the order of 10 keV.
The text was submitted by the authors in English. 相似文献
8.
Diffusion activation energy versus the favourable energy in two-order-parameter model:A molecular dynamics study of liquid Al 下载免费PDF全文
In the present work, we find that both diffusion activation energy Ea(D) and Ea(Sex) increase linearly with pressure and have the same slope (0.022±0.001 eV/GPa) for liquid Al. The temperature and pressure dependence of excess entropy is well fitted by the expression -Sex(T,P)/kB=a(P)+b(P)T+c(P)exp(Ef/kBT), which together with the small ratio of Ef/kBT leads to the relationship of excess entropy to temperature and pressure, i.e. Sex≈-cEf/T, where c is about 12 and Ef (=Δ E-PΔV) is the favourable energy. Therefore, there exists a simple relation between Ea(Sex) and Ef, i.e. Ea(Sex)≈cEf. 相似文献
9.
Beno?t?Collins Kenneth?J.?Dykema Francisco?Torres-Ayala 《Journal of statistical physics》2010,139(5):779-799
We show that the polynomial S
m,k
(A,B), that is the sum of all words in noncommuting variables A and B having length m and exactly k letters equal to B, is not equal to a sum of commutators and Hermitian squares in the algebra R〈X,Y〉, where X
2=A and Y
2=B, for all even values of m and k with 6≤k≤m−10, and also for (m,k)=(12,6). This leaves only the case (m,k)=(16,8) open. This topic is of interest in connection with the Lieb–Seiringer formulation of the Bessis–Moussa–Villani conjecture,
which asks whether Tr (S
m,k
(A,B))≥0 holds for all positive semidefinite matrices A and B. These results eliminate the possibility of using “descent + sum-of-squares” to prove the BMV conjecture. 相似文献
10.
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
11.
We discuss the high density behavior of a system of hard spheres of diameter d on the hypercubic lattice of dimension n, in the limit n→∞, d→∞, d/n = δ. The problem is relevant for coding theory, and the best available bounds state that the maximum density of the system falls in the interval 1 ≤ ρ V
d
≤ exp (n κ(δ)), being κ(δ) > 0 and V
d
the volume of a sphere of radius d. We find a solution of the equations describing the liquid up to an exponentially large value of ρ = ρ V
d
, but we show that this solution gives a negative entropy for the liquid phase for ρ >rsimn. We then conjecture that a phase transition towards a different phase might take place, and we discuss possible scenarios for this transition.
PACS: 05.20.Jj, 64.70.Pf, 61.20.Gy 相似文献
12.
We derive a type of kinetic equation for Kelvin waves on quantized vortex filaments with random large-scale curvature, that
describes step-by-step (local) energy cascade over scales caused by 4-wave interactions. Resulting new energy spectrum E
LN(k) ∝ k
−5/3 must replace in future theory (e.g., in finding the quantum turbulence decay rate) the previously used spectrum E
KS(k) ∝ k
−7/5, which was recently shown to be inconsistent due to nonlocality of the 6-wave energy cascade. 相似文献
13.
The monoclinic (space group C2/m) superstructure of V14O6, which is formed in the atom-vacancy ordering of the tetragonal solid solution of oxygen in vanadium, is studied by the methods
of x-ray diffraction and symmetry analysis. It has been found that the channel of the order-disorder phase transition attributed
to the formation of the monoclinic suboxide V14O6 includes six superstructure vectors belonging to three non-Lifshitz stars {k
1−1}, {k
1−2}, and {k
1–3} of one type {k
1}. The distribution function of the O atoms in the V14O6 monoclinic superstructure has been calculated. It has been shown that the displacements of V atoms distort the body-centered
tetragonal metal sublattice, thus preparing the formation of the fcc sublattice and the transition from the suboxide V14O6 to the cubic vanadium monoxide with the B1 structure. 相似文献
14.
The crystallographic structure and electronic properties of HfNi were studied as a function of pressure by combining X-ray
diffraction results with the full potential linearized augmented plane wave (LAPW) calculations. No phase transition was observed
up to a pressure of 35.3 GPa, with a total volume contraction of V/V
0 = 0.85, a bulk modulus value of B
0 = 52 ± 3 GPa and B
0
′ = 1.29 ± 0.26. The calculated linear increase in the V
zz
value as a function of the pressure induced volume reduction at the hafnium site was attributed mainly to the p–p contribution,
while in the nickel site, a non negligible d–d contribution to V
zz
is also observed, and attributed to the high 3d-partial DOS near the nickel nucleus. Based on the total electronic DOS at
E
Fermi calculated for 0 K (N(E
0
Fermi)), a value of 6.85 and 5.03 (mJ/mol/k2) was calculated for the band contribution (γ
band) to the electronic specific heat coefficient (γ) at a pressure of 0 and 35.3 GPa, respectively. 相似文献
15.
The monoclinic (space group C2/m) superstructure of V14O6, which is formed in the atom-vacancy ordering of the tetragonal solid solution of oxygen in vanadium, is studied by the methods
of x-ray diffraction and symmetry analysis. It has been found that the channel of the order-disorder phase transition attributed
to the formation of the monoclinic suboxide V14O6 includes six superstructure vectors belonging to three non-Lifshitz stars {k
1−1}, {k
1−2}, and {k
1–3} of one type {k
1}. The distribution function of the O atoms in the V14O6 monoclinic superstructure has been calculated. It has been shown that the displacements of V atoms distort the body-centered
tetragonal metal sublattice, thus preparing the formation of the fcc sublattice and the transition from the suboxide V14O6 to the cubic vanadium monoxide with the B1 structure.
Original Russian Text ? A.I. Gusev, D.A. Davydov, 2007, published in Pis’ma v Zhurnal éksperimental’noĭ i Teoreticheskoĭ Fiziki,
2007, Vol. 86, No. 10, pp. 746–751. 相似文献
16.
The field dependence of the vibrational contribution to the dynamic magnetic permeability μ
V(H) is calculated for a thin (of thickness d∼λ) high-T
c superconducting wafer in a magnetic field parallel to the surface. The resulting curves are plotted on the basis of an exact
numerical analysis of the vortex structures both for the thermodynamic-equilibrium vortex lattice and in the presence of pinning
forces and the Bean-Livingston surface barrier. It is shown that the μ
V(H) curves are highly sensitive to the size factor (d/λ) and exhibit abrupt changes corresponding to a change in the number of vortex rows. The equilibrium μ
V(H) curve is found to be similar in its general behavior and absolute value (obtained with allowance for the distribution of
grain sizes and with appropriate values of λ and ϰ) to the experimental μ
V(H) curve plotted at nitrogen temperature for fine-grained YBa2Cu3Ox with grain diameters 〈D〉∼λ in an increasing magnetic field. It is established that the main cause of the experimentally observed irreversible behavior
of the μ
V(H) curves during cyclic variation of the applied magnetic field is the existence of a surface barrier to the exit of vortices
from the superconductor. The lower limit H
min(B) of stability of the mixed state in the presence of an ideal surface barrier in a thin, high-T
c superconducting wafer (d∼λ) is determined, along with the range of the vortex state (H
max-H
min) for a fixed number of vortices in micrometer-size grains of the investigated YBaCuO samples.
Fiz. Tverd. Tela (St. Petersburg) 39, 1943–1947 (November 1997) 相似文献
17.
Let λ i (Ω,V) be the i th eigenvalue of the Schrödinger operator with Dirichlet boundary conditions on a bounded domain $\Omega \subset \mathbb{R}^nLet λ
i
(Ω,V) be the i
th
eigenvalue of the Schr?dinger operator with Dirichlet boundary conditions on a bounded domain
and with the positive potential V. Following the spirit of the Payne-Pólya-Weinberger conjecture and under some convexity assumptions on the spherically rearranged potential V
*, we prove that λ2(Ω,V) ≤ λ2(S
1,V
*). Here S
1 denotes the ball, centered at the origin, that satisfies the condition λ1(Ω,V)=λ1(S
1,V
*).Further we prove under the same convexity assumptions on a spherically symmetric potential V, that λ2(B
R
, V) / λ1(B
R
, V) decreases when the radius R of the ball B
R
increases.We conclude with several results about the first two eigenvalues of the Laplace operator with respect to a measure of Gaussian or inverted Gaussian density.R.B. was supported by FONDECYT project # 102-0844.H.L. gratefully acknowledges financial support from DIPUC of the Pontifí cia Universidad Católica de Chile and from CONICYT. 相似文献
18.
On Noncommutative Multi-Solitons 总被引:2,自引:0,他引:2
Rajesh Gopakumar Matthew Headrick Marcus Spradlin 《Communications in Mathematical Physics》2003,233(2):355-381
We find the moduli space of multi-solitons in noncommutative scalar field theories at large θ, in arbitrary dimension. The
existence of a non-trivial moduli space at leading order in 1/θ is a consequence of a Bogomolnyi bound obeyed by the kinetic
energy of the θ=∞ solitons. In two spatial dimensions, the parameter space for k solitons is a K?hler de-singularization of the symmetric product (ℝ2)
k
/S
k
. We exploit the existence of this moduli space to construct solitons on quotient spaces of the plane: ℝ2/ℤ
k
, cylinder, and T
2
. However, we show that tori of area less than or equal to 2πθ do not admit stable solitons. In four dimensions the moduli
space provides an explicit K?hler resolution of (ℝ4)
k
/S
k
. In general spatial dimension 2d, we show it is isomorphic to the Hilbert scheme of k points in ℂ
d
, which for d>2 (and k>3) is not smooth and can have multiple branches.
Received: 29 May 2001 / Accepted: 16 August 2002 Published online: 7 November 2002
Communicated by R.H. Dijkgraaf 相似文献
19.
The variations of molar volume Mv and ultrasonic velocity V with temperature in N(p-n-pentyloxy benzylidene) p-toluidine are presented. The molar volume jump at the nematicisotropic transition suggests it is of first order. The adiabatic compressibility β ad, molar sound velocity or Rao number Rn , molar compressibility or Wada constant A, the thermal expansion coefficient α, and Van der Waals constant b, are computed. Molar sound velocity and molar compressibility are compared with the values of other members of the homologous series and are found to be in good agreement with Rao's and Wada's observations. The order parameter Sk, at the nematic-isotropic phase transition is 0.441 from our experimental results and the Maier-Saupe table and is found to be in good agreement with the theoretical value. 相似文献
20.
Evidence that pinning on linear or planar defects dominates the vortex dynamics in YBa2Cu3O7−x (YBCO) films is provided by complex impedance measurements at temperature 8 K<T<T
c
and magnetic field 0<B<6 T. Below the vortex lattice melting transition Bg(T) but above a threshold field Bp≈8(1-T/T
c
) T, the inductance of vortices increases as B2, much less rapidly than predicted for collective pinning of vortices by point defects. Above the vortex melting line, critical
scaling persists over the region Bg(T<B<B*(T) where the vortex correlation length ξ exceeds a characteristic length scale ξ*≡ξ(B=B*)≈450?. The value of ξ* is not sensitive to Al-doping in the Cu sites in the lattice and is close to the size of twin domains in the film. The nature
of the observed crossovers is discussed in terms of available theoretical models for a glass-liquid transition at Bg. 相似文献