1,346 research outputs found
Effect of dopant atoms on local superexchange in cuprate superconductors: a perturbative treatment
Recent scanning tunneling spectroscopy experiments have provided evidence
that dopant impurities in high- Tc superconductors can strongly modify the
electronic structure of the CuO2 planes nearby, and possibly influence the
pairing. To investigate this connection, we calculate the local magnetic
superexchange J between Cu ions in the presence of dopants within the framework
of the three-band Hubbard model, up to fifth-order in perturbation theory. We
demonstrate that the sign of the change in J depends on the relative
dopant-induced spatial variation of the atomic levels in the CuO2 plane,
contrary to results obtained within the one-band Hubbard model. We discuss some
realistic cases and their relevance for theories of the pairing mechanism in
the cupratesComment: 5 pages, 4 figures, revised versio
Experimental evidence of thermal fluctuations on the X-ray absorption near-edge structure at the aluminum K-edge
After a review of temperature-dependent experimental x-ray absorption
near-edge structure (XANES) and related theoretical developments, we present
the Al K-edge XANES spectra of corundum and beryl for temperature ranging from
300K to 930K. These experimental results provide a first evidence of the role
of thermal fluctuation in XANES at the Al K-edge especially in the pre-edge
region. The study is carried out by polarized XANES measurements of single
crystals. For any orientation of the sample with respect to the x-ray beam, the
pre-edge peak grows and shifts to lower energy with temperature. In addition
temperature induces modifications in the position and intensities of the main
XANES features. First-principles DFT calculations are performed for both
compounds. They show that the pre-edge peak originates from forbidden 1s to 3s
transitions induced by vibrations. Three existing theoretical models are used
to take vibrations into account in the absorption cross section calculations:
i) an average of the XANES spectra over the thermal displacements of the
absorbing atom around its equilibrium position, ii) a method based on the crude
Born-Oppenheimer approximation where only the initial state is averaged over
thermal displacements, iii) a convolution of the spectra obtained for the atoms
at the equilibrium positions with an approximate phonon spectral function. The
theoretical spectra so obtained permit to qualitatively understand the origin
of the spectral modifications induced by temperature. However the correct
treatment of thermal fluctuation in XANES spectroscopy requires more
sophisticated theoretical tools
Late-time evolution of charged massive Dirac fields in the Kerr-Newman background
We investigate both the intermediate late-time tail and the asymptotic tail
behavior of the charged massive Dirac fields in the background of the
Kerr-Newman black hole. We find that the intermediate late-time behavior of
charged massive Dirac fields is dominated by an inverse power-law decaying tail
without any oscillation, which is different from the oscillatory decaying tails
of the scalar field. We note that the dumping exponent depends not only on the
angular quantum numbers , the separation constant and the rotating
parameter , but also on the product of the spin weight of the Dirac
field and the charges of the black hole and the fields. We also find that the
decay rate of the asymptotically late-time tail is , and the
oscillation of the tail has the period of which is modulated by
two types of long-term phase shifts.Comment: 14 page
Observation of vortices and hidden pseudogap from scanning tunneling spectroscopic studies of electron-doped cuprate superconductor
We present the first demonstration of vortices in an electron-type cuprate
superconductor, the highest (= 43 K) electron-type cuprate
. Our spatially resolved quasiparticle tunneling spectra
reveal a hidden low-energy pseudogap inside the vortex core and unconventional
spectral evolution with temperature and magnetic field. These results cannot be
easily explained by the scenario of pure superconductivity in the ground state
of high- superconductivity.Comment: 6 pages, 4 figures. Two new graphs have been added into Figure 2.
Accepted for publication in Europhysics Letters. Corresponding author:
Nai-Chang Yeh (E-mail: [email protected]
Effects of domain walls on hole motion in the two-dimensional t-J model at finite temperature
The t-J model on the square lattice, close to the t-J_z limit, is studied by
quantum Monte Carlo techniques at finite temperature and in the underdoped
regime. A variant of the Hoshen-Koppelman algorithm was implemented to identify
the antiferromagnetic domains on each Trotter slice. The results show that the
model presents at high enough temperature finite antiferromagnetic (AF) domains
which collapse at lower temperatures into a single ordered AF state. While
there are domains, holes would tend to preferentially move along the domain
walls. In this case, there are indications of hole pairing starting at a
relatively high temperature. At lower temperatures, when the whole system
becomes essentially fully AF ordered, at least in finite clusters, holes would
likely tend to move within phase separated regions. The crossover between both
states moves down in temperature as doping increases and/or as the off-diagonal
exchange increases. The possibility of hole motion along AF domain walls at
zero temperature in the fully isotropic t-J is discussed.Comment: final version, to appear in Physical Review
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