35 research outputs found
Theory of Diamagnetism in the Pseudogap Phase: Implications from the Self energy of Angle Resolved Photoemission
In this paper we apply the emerging- consensus understanding of the fermionic
self energy deduced from angle resolved photoemisssion spectroscopy (ARPES)
experiments to deduce the implications for orbital diamagnetism in the
underdoped cuprates. Many theories using many different starting points have
arrived at a broadened BCS-like form for the normal state self energy
associated with a d-wave excitation gap, as is compatible with ARPES data.
Establishing compatibility with the f-sum rules, we show how this self energy,
along with the constraint that there is no Meissner effect in the normal phase
are sufficient to deduce the orbital susceptibility. We conclude, moreover,
that diamagnetism is large for a d-wave pseudogap. Our results should apply
rather widely to many theories of the pseudogap, independent of the microscopic
details.Comment: 15 pages, 8 figure
Observation of a pairing pseudogap in a two-dimensional Fermi gas
Pairing of fermions is ubiquitous in nature and it is responsible for a large
variety of fascinating phenomena like superconductivity, superfluidity of
He, the anomalous rotation of neutron stars, and the BEC-BCS crossover in
strongly interacting Fermi gases. When confined to two dimensions, interacting
many-body systems bear even more subtle effects, many of which lack
understanding at a fundamental level. Most striking is the, yet unexplained,
effect of high-temperature superconductivity in cuprates, which is intimately
related to the two-dimensional geometry of the crystal structure. In
particular, the questions how many-body pairing is established at high
temperature and whether it precedes superconductivity are crucial to be
answered. Here, we report on the observation of pairing in a harmonically
trapped two-dimensional atomic Fermi gas in the regime of strong coupling. We
perform momentum-resolved photoemission spectroscopy, analogous to ARPES in the
solid state, to measure the spectral function of the gas and we detect a
many-body pairing gap above the superfluid transition temperature. Our
observations mark a significant step in the emulation of layered
two-dimensional strongly correlated superconductors using ultracold atomic
gases
Unconventional particle-hole mixing in the systems with strong superconducting fluctuations
Development of the STM and ARPES spectroscopies enabled to reach the
resolution level sufficient for detecting the particle-hole entanglement in
superconducting materials. On a quantitative level one can characterize such
entanglement in terms of the, so called, Bogoliubov angle which determines to
what extent the particles and holes constitute the spatially or momentum
resolved excitation spectra. In classical superconductors, where the phase
transition is related to formation of the Cooper pairs almost simultaneously
accompanied by onset of their long-range phase coherence, the Bogoliubov angle
is slanted all the way up to the critical temperature Tc. In the high
temperature superconductors and in superfluid ultracold fermion atoms near the
Feshbach resonance the situation is different because of the preformed pairs
which exist above Tc albeit loosing coherence due to the strong quantum
fluctuations. We discuss a generic temperature dependence of the Bogoliubov
angle in such pseudogap state indicating a novel, non-BCS behavior. For
quantitative analysis we use a two-component model describing the pairs
coexisting with single fermions and study their mutual feedback effects by the
selfconsistent procedure originating from the renormalization group approach.Comment: 4 pages, 4 figure
Appearance of fluctuating stripes at the onset of the pseudogap in the high-Tc Superconductor Bi2Sr2CaCu2O8+x
Doped Mott insulators have been shown to have a strong propensity to form
patterns of holes and spins often referred to as stripes. In copper-oxides,
doping also gives rise to the pseudogap state, which transforms into a high
temperature superconductor with sufficient doping or by reducing the
temperature. A long standing question has been the interplay between pseudogap,
which is generic to all hole-doped cuprates, and stripes, whose static form
occurs in only one family of cuprates over a narrow range of the phase diagram.
Here we examine the spatial reorganization of electronic states with the onset
of the pseudogap state at T* in the high-temperature superconductor
Bi2Sr2CaCu2O8+x using spectroscopic mapping with the scanning tunneling
microscope (STM). We find that the onset of the pseudogap phase coincides with
the appearance of electronic patterns that have the predicted characteristics
of fluctuating stripes. As expected, the stripe patterns are strongest when the
hole concentration in the CuO2 planes is close to 1/8 (per Cu). While
demonstrating that the fluctuating stripes emerge with the onset of the
pseudogap state and occur over a large part of the cuprate phase diagram, our
experiments indicate that they are a consequence of pseudogap behavior rather
than its cause.Comment: preprint version, 25 pages including supplementary informatio
Nile Red fluorescence spectroscopy reports early physicochemical changes in myelin with high sensitivity
The molecular composition of myelin membranes determines their structure and function. Even minute changes to the biochemical balance can have profound consequences for axonal conduction and the synchronicity of neural networks. Hypothesizing that the earliest indication of myelin injury involves changes in the composition and/or polarity of its constituent lipids, we developed a sensitive spectroscopic technique for defining the chemical polarity of myelin lipids in fixed frozen tissue sections from rodent and human. The method uses a simple staining procedure involving the lipophilic dye Nile Red, whose fluorescence spectrum varies according to the chemical polarity of the microenvironment into which the dye embeds. Nile Red spectroscopy identified histologically intact yet biochemically altered myelin in prelesioned tissues, including mouse white matter following subdemyelinating cuprizone intoxication, as well as normal-appearing white matter in multiple sclerosis brain. Nile Red spectroscopy offers a relatively simple yet highly sensitive technique for detecting subtle myelin changes
Perfusion of His-Tagged Eukaryotic Myocilin Increases Outflow Resistance in Human Anterior Segments in the Presence of Aqueous Humor
PURPOSE. A previous study by the authors has shown that recombinant myocilin purified from a prokaryotic expression system increases outflow resistance in cultured human anterior segments. The present study was performed to determine whether full-length myocilin purified from a human trabecular meshwork cell expression system alters outflow resistance after infusion into human anterior segments. METHODS. A feline immunodeficiency virus vector encoding both full-length myocilin (amino acids 1-503 fused to C-terminal V5 and six-histidine epitopes) and puromycin resistance was used to transduce a transformed trabecular meshwork cell line (TM5). Stably expressing cells were selected with puromycin. Recombinant myocilin was purified from the media using nickel ion affinity chromatography. Control purifications were performed on media from parental TM5 cells. Anterior segments of human eyes were placed in organ culture and perfused with either Dulbecco's modified Eagle's medium (DMEM) or DMEM supplemented with 50% porcine aqueous humor. One eye received an anterior chamber exchange with recombinant myocilin (2 g/mL), whereas the fellow eye received an equal volume of control. Immunohistochemistry was performed with anti-myocilin and anti-V5 antibodies. Native polyacrylamide gel electrophoresis was used to analyze myocilin complex formation in porcine aqueous humor. RESULTS. Recombinant myocilin in porcine aqueous humor increased outflow resistance in cultured human anterior segments (91% Ϯ 68% [mean Ϯ SD] versus 18% Ϯ 31% in fellow control eye; n ϭ 9, P ϭ 0.004). Maximum outflow resistance was obtained 5 to 17 hours after infusion and remained above baseline for Ͼ3 days. Recombinant myocilin also increased outflow resistance in eyes incubated in DMEM, but only if myocilin was preincubated with porcine aqueous humor (78% Ϯ 77% when preincubated in DMEM containing porcine aqueous humor versus 13% Ϯ 15% when preincubated with DMEM alone, n ϭ 6, P ϭ 0.03). Recombinant myocilin appears to form a complex in porcine aqueous humor with a heat-labile protein(s). Immunohistochemistry revealed the presence of myocilin in the juxtacanalicular region of the trabecular meshwork. CONCLUSIONS. Myocilin purified from human trabecular meshwork cells increased outflow resistance in cultured human anterior segments, but only after incubation with porcine aqueous humor. Recombinant myocilin appears to form a complex in porcine aqueous humor that enables it to bind specifically within the trabecular meshwork. (Invest Ophthalmol Vis Sci