30 research outputs found
Evaluation of the two-particle propagator for Hubbard model with the help of Hubbard-I approximation
The Hubbard-I approximation is generalized to allow for direct evaluation of
the equal-time anomalous two-electron propagator for Hubbard model on
two-dimensional square lattice. This propagator is compared against the quantum
Monte Carlo data obtained by Aimi and Imada [J. Phys. Soc. Jpn. {\bf 76},
113708 (2007)] in the limit of strong electron-electron interaction. The
Hubbard-I predictions are in a good qualitative agreement with the Monte Carlo
results. In particular, -wave correlations decay as ("free
electron" behaviour), if separation exceeds 2-3 lattice constants. However,
the Hubbard-I approximation underestimates coefficient by a factor of about
three. We conclude that the Hubbard-I approximation, despite its simplicity and
artefacts, captures the qualitative behaviour of the two-particle propagator
for the Hubbard model, at least for moderate values of .Comment: 17 pages, 5 figures; the text is reorganized somewhat as compared to
the preprint's previous version; an extra figure is added; some figures are
re-drawn for different parameter values; typeset with IOP styl
Josephson Coupling through a Quantum Dot
We derive, via fourth order perturbation theory, an expression for the
Josephson current through a gated interacting quantum dot. We analyze our
expression for two different models of the superconductor-dot-superconductor
(SDS) system. When the matrix elements connecting dot and leads are featureless
constants, we compute the Josephson coupling J_c as a function of the gate
voltage and Coulomb interaction. In the diffusive dot limit, we compute the
probability distribution P(J_c) of Josephson couplings. In both cases, pi
junction behavior (J_c < 0) is possible, and is not simply dependent on the
parity of the dot occupancy.Comment: 9 pages; 3 encapsulated PostScript figure
Comparative study of dye-loaded liposome accumulation in sensitive and resistant human breast cancer cells
The aim of this research is to study the dynamics and efficiency of liposome accumulation in sensitive and resistant human breast cancer cells. Methods: Methods of fluorescence microscopy, fluorescence microspectroscopy and MTT-test have been used. Results: The liposome-to-cell interaction and dye cellular uptake in sensitive, cisplatin-resistant and doxorubicin-resistant MCF-7 human breast cancer cells have been analyzed using time changes in both fluorescence resonance energy transfer signal from the donor probe DiO to the acceptor one DiI preloaded in liposomes and cell image brightness. Conclusion: Obtained results show that resistant cells accumulate dye-loaded liposomes more effectively and reveal more effective dye molecule cellular uptake
Loss of Pi-Junction Behaviour in an Interacting Impurity Josephson Junction
Using a generalization of the non-crossing approximation which incorporates
Andreev reflection, we study the properties of an infinite-U Anderson impurity
coupled to two superconducting leads. In the regime where and
are comparable, we find that the position of the sub-gap resonance in the
impurity spectral function develops a strong anomalous phase dependence-- its
energy is a minimum when the phase difference between the superconductors is
equal to . Calculating the Josephson current through the impurity, we find
that -junction behaviour is lost as the position of the bound-state moves
above the Fermi energy.Comment: 4 pages, 4 figures; labelling of Fig. 3 corrected; final published
form, only trivial change
Ground State and Spectral Properties of a Quantum Impurity in d-Wave Superconductors
The variational approach of Gunnarsson and Sch\"onhammer to the Anderson
impurity model is generalized to study d-wave superconductors in the presence
of dilute spin-1/2 impurities. We show that the local moment is screened when
the hybridization exceeds a nonzero critical value at which the ground state
changes from a spin doublet to a spin singlet. The electron spectral functions
are calculated in both phases. We find that while a Kondo resonance develops
above the Fermi level in the singlet phase, the spectral function exhibits a
low-energy spectral peak below the Fermi level in the spin doublet phase. The
origin of such a ``virtual Kondo resonance'' is the existence of low-lying
collective excitations in the spin-singlet sector. We discuss our results in
connection to recent spectroscopic experiments on Zn doped high-T
superconductors.Comment: 5 pages, 4figures, revised versio
Josephson current in unconventional superconductors through an Anderson impurity
Josephson current for a system consisting of an Anderson impurity weakly
coupled to two unconventional superconductors is studied and shown to be driven
by a surface zero energy (mid-gap) bound-state. The repulsive Coulomb
interaction in the dot can turn a junction into a 0-junction. This effect
is more pronounced in p-wave superconductors while in high-temperature
superconductors with symmetry it can exit for rather large
artificial centers at which tunneling occurs within a finite region.Comment: 4 pages 3.eps figure
Thermal conductivity via magnetic excitations in spin-chain materials
We discuss the recent progress and the current status of experimental
investigations of spin-mediated energy transport in spin-chain and spin-ladder
materials with antiferromagnetic coupling. We briefly outline the central
results of theoretical studies on the subject but focus mainly on recent
experimental results that were obtained on materials which may be regarded as
adequate physical realizations of the idealized theoretical model systems. Some
open questions and unsettled issues are also addressed.Comment: 17 pages, 4 figure
Monitoring of high energy deuteron beams in the experiments with massive targets
The influence of massive uranium target (500 kg natU) of assembly "QUINTA" on the results of high energy deuteron
beam monitoring with aluminum and copper foils was investigated. In order to increase the accuracy of deuteron
beam off-line monitoring, the measurements of the cross sections of fragmentation reaction natCu(d,x)24Na
were performed at 1.32, 2, 4 и 8 GeV deuteron energies. For the same deuteron energies the cross sections of residual
nuclei 7
Be, 42K, 52Mn, 57Co and 58Co for natCu(d,x) reaction were measuredИсследовано влияние массивной урановой мишени (500 кг natU) установки «КВИНТА» на результаты мониторирования пучков дейтронов с помощью алюминиевых и медных фольг. Для улучшения точности off-line мониторирования пучков дейтронов, бомбардирующих протяженные мишени из тяжёлых элементов, были проведены измерения сечений реакции фрагментации natCu(d,x)24Na для дейтронов с энергией 1,32; 2; 4 и 8 ГэВ. Для этих же энергий дейтронов измерены сечения выхода изотопов 7 Be, 42K, 52Mn, 57Co и 58Co в реакциях natCu(d,x).Досліджено вплив масивної уранової мішені (500 кг natU) установки «КВІНТА» на результати моніторування пучків дейтронів за допомогою алюмінієвих і мідних фольг. Для покращення точності off-line моніторування пучків дейтронів, які бомбардують протяжні мішені з важких елементів, були проведені вимірювання перетинів реакції фрагментації natCu(d,x)24Na для дейтронів з енергією 1,32; 2; 4 і 8 ГеВ. Для цих же енергій дейтронів виміряні перетини виходу ізотопів 7 Be, 42K, 52Mn, 57Co в реакціях natCu(d,x)
Pairing and Density Correlations of Stripe Electrons in a Two-Dimensional Antiferromagnet
We study a one-dimensional electron liquid embedded in a 2D antiferromagnetic
insulator, and coupled to it via a weak antiferromagnetic spin exchange
interaction. We argue that this model may qualitatively capture the physics of
a single charge stripe in the cuprates on length- and time scales shorter than
those set by its fluctuation dynamics. Using a local mean-field approach we
identify the low-energy effective theory that describes the electronic spin
sector of the stripe as that of a sine-Gordon model. We determine its phases
via a perturbative renormalization group analysis. For realistic values of the
model parameters we obtain a phase characterized by enhanced spin density and
composite charge density wave correlations, coexisting with subleading triplet
and composite singlet pairing correlations. This result is shown to be
independent of the spatial orientation of the stripe on the square lattice.
Slow transverse fluctuations of the stripes tend to suppress the density
correlations, thus promoting the pairing instabilities. The largest amplitudes
for the composite instabilities appear when the stripe forms an antiphase
domain wall in the antiferromagnet. For twisted spin alignments the amplitudes
decrease and leave room for a new type of composite pairing correlation,
breaking parity but preserving time reversal symmetry.Comment: Revtex, 28 pages incl. 5 figure
Quantum Dot in the Kondo Regime coupled to p-wave superconductors
This paper studies the physics of junctions containing superconducting
and normal leads weakly coupled to an Anderson impurity in the Kondo
regime . Special attention is devoted to the case where one of the leads
is a superconductor where mid-gap surface states play an important
role in the tunneling processes and help the formation of Kondo resonance. The
novel physics in these systems beyond that encountered in quantum dots coupled
only to to normal leads is that electron transport at finite bias in
and junctions is governed by Andreev reflections. These enable the
occurrence of dissipative current even when the bias is smaller than the
superconducting gap . Using the slave boson mean field approximation
the current, shot-noise power and Fano factor are calculated as functions of
the applied bias voltage in the sub-gap region and found to be
strongly dependent on the ratio between the Kondo temperature and
the superconducting gap . In particular, for large values of the
attenuation of current due to the existence of the superconducting gap is
compensated by the Kondo effect. This scenario is manifested also in the
behavior of the Josephson current as function of temperature.Comment: 7 pages, 5 .eps figure