41 research outputs found
A Two Energy Gap Preformed-Pair Scenario For the Cuprates: Implications for Angle-Resolved Photoemission Spectroscopy
We show how, within a preformed pair scenario for the cuprate pseudogap, the
nodal and antinodal responses in angle resolved photoemission spectroscopy
necessarily have very different temperature dependences. We examine the
behavior and the contrasting dependences for a range of temperatures both
below and above . Our calculations are based on a fully microscopic
-matrix approach for addressing pairing correlations in a regime where the
attraction is stronger than BCS and the coherence length is anomalously short.
Previously, the distinct nodal and anti-nodal responses have provided strong
support for the "two-gap scenario" of the cuprates in which the pseudogap
competes with superconductivity. Instead, our theory supports a picture in
which the pseudogap derives from pairing correlations, identifying the two gap
components with non-condensed and condensed pairs. It leads to reasonably good
agreement with a range of different experiments in the moderately underdoped
regime and we emphasize that here there is no explicit curve fitting. Ours is a
microscopic rather than a phenomenological theory. We briefly address the more
heavily underdoped regime in which the behavior is more complex.Comment: 12 pages; 10 figure
Pairing Fluctuation Theory of Superconducting Properties in Underdoped to Overdoped Cuprates
We propose a theoretical description of the superconducting state of under-
to overdoped cuprates, based on the short coherence length of these materials
and the associated strong pairing fluctuations. The calculated and the
zero temperature excitation gap , as a function of hole
concentration , are in semi-quantitative agreement with experiment. Although
the ratio has a strong dependence, different from the
universal BCS value, and deviates significantly from the BCS
prediction, we obtain, quite remarkably, quasi-universal behavior, for the
normalized superfluid density and the Josephson critical
current , as a function of . While experiments on
are consistent with these results, future measurements on
are needed to test this prediction.Comment: 4 pages, 3 figures, REVTeX, submitted to Phys. Rev. Let
On the Relationship Between the Pseudo- and Superconducting Gaps: Effects of Residual Pairing Correlations Below Tc
The existence of a normal state spectral gap in underdoped cuprates raises
important questions about the associated superconducting phase. For example,
how does this pseudogap evolve into its below Tc counterpart? In this paper we
characterize this unusual superconductor by investigating the nature of the
``residual'' pseudogap below Tc and, find that it leads to an important
distinction between the superconducting excitation gap and order parameter. Our
approach is based on a conserving diagrammatic BCS Bose-Einstein crossover
theory which yields the precise BCS result in weak coupling at any T<Tc and
reproduces Leggett's results in the T=0 limit. We explore the resulting
experimental implications.Comment: REVTeX, 4 pages, 1 EPS figure (included
Magnetic Field Effects in the Pseudogap Phase: A Competing Energy Gap Scenario for Precursor Superconductivity
We study the sensitivity of T_c and T^* to low fields, H, within the
pseudogap state using a BCS-based approach extended to arbitrary coupling. We
find that T^* and T_c, which are of the same superconducting origin, have very
different H dependences. This is due to the pseudogap, \Delta_{pg}, which is
present at the latter, but not former temperature. Our results for the
coherence length \xi fit well with existing experiments.We predict that very
near the insulator \xi will rapidly increase.Comment: 4 pages, 4 figures, RevTe
Theory of Radio Frequency Spectroscopy Experiments in Ultracold Fermi Gases and Their Relation to Photoemission Experiments in the Cuprates
In this paper we present an overview of radio frequency (RF) spectroscopy in
the atomic Fermi superfluids. An ultimate goal is to suggest new directions in
the cold gas research agenda from the condensed matter perspective.Our focus is
on the experimental and theoretical literature of cold gases and photoemission
spectroscopy of the cuprates particularly as it pertains to areas of overlap.
This paper contains a systematic overview of the theory of RF spectroscopy,
both momentum integrated and momentum resolved. We discuss the effects of
traps, population imbalance, final state interactions over the entire range of
temperatures and compare theory and experiment. We show that this broad range
of phenomena can be accomodated within the BCS-Leggett description of BCS-BEC
crossover and that this scheme also captures some of the central observations
in photoemission experiments in the cuprates. In this last context, we note
that the key themes which have emerged in cuprate photoemission studies involve
characterization of the fermionic self energy, of the pseudogap and of the
effects of superconducting coherence (in passing from above to below the
superfluid transition temperature, ).These issues have a counterpart in
the cold Fermi gases and it would be most useful in future to use these atomic
systems to address these and the more sweeping question of how to describe that
anomalous superfluid phase which forms in the presence of a normal state
excitation gap.Comment: 23 pages, 22 figure
Assembly of Silver Nanoparticles into Hollow Spheres Using Eu(III) Compound based on Trifluorothenoyl-Acetone
The preparation of luminescent silver hollow spheres using Eu(III) compound based on trifluorothenoyl-acetone is described. The structure and size of silver hollow spheres were determined by TEM images. The result shows the formation of hollow structure and average size of the silver hollow spheres (0.9 μm). The silver hollow spheres were further characterized by UV absorption spectrum, SNOM and SEM images, suggesting them to be formed by self-assemble of some isolated silver nanoparticles. The luminescent properties of them were also investigated and they are shown to be high emission strength; moreover, they offer the distinct advantage of a lower packing density compared with other commercial luminescent products
Pairing fluctuations and pseudogaps in the attractive Hubbard model
The two-dimensional attractive Hubbard model is studied in the weak to
intermediate coupling regime by employing a non-perturbative approach. It is
first shown that this approach is in quantitative agreement with Monte Carlo
calculations for both single-particle and two-particle quantities. Both the
density of states and the single-particle spectral weight show a pseudogap at
the Fermi energy below some characteristic temperature T*, also in good
agreement with quantum Monte Carlo calculations. The pseudogap is caused by
critical pairing fluctuations in the low-temperature renormalized classical
regime of the two-dimensional system. With increasing temperature
the spectral weight fills in the pseudogap instead of closing it and the
pseudogap appears earlier in the density of states than in the spectral
function. Small temperature changes around T* can modify the spectral weight
over frequency scales much larger than temperature. Several qualitative results
for the s-wave case should remain true for d-wave superconductors.Comment: 20 pages, 12 figure
Nanoscale Confinement and Fluorescence Effects of Bacterial Light Harvesting Complex LH2 in Mesoporous Silicas
Many key chemical and biochemical reactions, particularly in living cells, take place in confined space at the mesoscopic scale. Toward understanding of physicochemical nature of biomacromolecules confined in nanoscale space, in this work we have elucidated fluorescence effects of a light harvesting complex LH2 in nanoscale chemical environments. Mesoporous silicas (SBA-15 family) with different shapes and pore sizes were synthesized and used to create nanoscale biomimetic environments for molecular confinement of LH2. A combination of UV-vis absorption, wide-field fluorescence microscopy, and in situ ellipsometry supports that the LH2 complexes are located inside the silica nanopores. Systematic fluorescence effects were observed and depend on degree of space confinement. In particular, the temperature dependence of the steady-state fluorescence spectra was analyzed in detail using condensed matter band shape theories. Systematic electronic-vibrational coupling differences in the LH2 transitions between the free and confined states are found, most likely responsible for the fluorescence effects experimentally observed