222 research outputs found
Scanning Tunneling Microscopy studies of single layer high-Tc cuprate Bi2Sr2-xLaxCuO6+delta
Thesis advisor: Hong DingThesis advisor: Vidya MadhavanHigh temperature superconductivity has been one of the most challenging problems in condensed matter physics since its discovery. This dissertation presents systematic studies on the single layer high temperature superconductor Bi2Sr2-xLaxCuO6+delta by scanning tunneling microscopy. The STM results have been compared to Angle-resolved photoemission spectroscopy (ARPES) data. Using STM spectroscopy and ARPES we observed two distinct gaps that coexist both in real space and in the antinodal region of momentum space, below the superconducting transition temperature. By looking at the energy scale of these two gaps along with the temperature dependence data, we fnd that the small gap is associated with superconductivity. The large gap persists above Tc, and seems linked to observed charge ordering. We also find a strong correlation between the large and small gaps suggesting that they are affected by similar physical processes. This is the first time that two coexisting and competing energy scales have been directly observed in STM spectroscopy. Combining this with ARPES data, we show that the pseudogap may be a diffferent order parameter from the superconducting phase. This provides support to the recently proposed "two gaps scenario" and should lead to more experimental discovery and theoretical discussions. In this dissertation we also discuss the spatial properties of the scanning tunneling microscopy conductance maps, as well as the charge ordering pattern at high energies. We observe interesting periodic patterns at low energies which can not be explained by a simple charge density wave picture. We also fnd the surprising bias dependence in terms of the contrast reversal. We propose a model of STM measuring effect to explain these phenomena.Thesis (PhD) — Boston College, 2009.Submitted to: Boston College. Graduate School of Arts and Sciences.Discipline: Physics
Tensor-based Intrinsic Subspace Representation Learning for Multi-view Clustering
As a hot research topic, many multi-view clustering approaches are proposed
over the past few years. Nevertheless, most existing algorithms merely take the
consensus information among different views into consideration for clustering.
Actually, it may hinder the multi-view clustering performance in real-life
applications, since different views usually contain diverse statistic
properties. To address this problem, we propose a novel Tensor-based Intrinsic
Subspace Representation Learning (TISRL) for multi-view clustering in this
paper. Concretely, the rank preserving decomposition is proposed firstly to
effectively deal with the diverse statistic information contained in different
views. Then, to achieve the intrinsic subspace representation, the
tensor-singular value decomposition based low-rank tensor constraint is also
utilized in our method. It can be seen that specific information contained in
different views is fully investigated by the rank preserving decomposition, and
the high-order correlations of multi-view data are also mined by the low-rank
tensor constraint. The objective function can be optimized by an augmented
Lagrangian multiplier based alternating direction minimization algorithm.
Experimental results on nine common used real-world multi-view datasets
illustrate the superiority of TISRL
Polarization engineering of entangled photons from a lithium niobate nonlinear metasurface
Complex polarization states of photon pairs are indispensable in various
quantum technologies. Conventional methods for preparing desired two-photon
polarization states are realized through bulky nonlinear crystals, which can
restrict the versatility and tunability of the generated quantum states due to
the fixed crystal nonlinear susceptibility. Here we present a solution using a
nonlinear metasurface incorporating multiplexed silica metagratings on a
lithium niobate film of 300 nanometer thickness. We fabricate two orthogonal
metagratings on a single substrate with an identical resonant wavelength,
thereby enabling the spectral indistinguishability of the emitted photons, and
demonstrate in experiments that the two-photon polarization states can be
shaped by the metagrating orientation. Leveraging this essential property, we
formulate a theoretical approach for generating arbitrary
polarization-entangled qutrit states by combining three metagratings on a
single metasurface, allowing the encoding of desired quantum states or
information. Our findings enable miniaturized optically controlled quantum
devices using ultrathin metasurfaces as polarization-entangled photon sources.Comment: 21 pages, 5 figures, journal pape
Spatially entangled photon-pairs from lithium niobate nonlocal metasurfaces
Metasurfaces consisting of nano-scale structures are underpinning new
physical principles for the creation and shaping of quantum states of light.
Multi-photon states that are entangled in spatial or angular domains are an
essential resource for quantum imaging and sensing applications, however their
production traditionally relies on bulky nonlinear crystals. We predict and
demonstrate experimentally the generation of spatially entangled photon pairs
through spontaneous parametric down-conversion from a metasurface incorporating
a nonlinear thin film of lithium niobate. This is achieved through nonlocal
resonances with tailored angular dispersion mediated by an integrated silica
meta-grating, enabling control of the emission pattern and associated quantum
states of photon pairs by designing the grating profile and tuning the pump
frequency. We measure the correlations of photon positions and identify their
spatial anti-bunching through violation of the classical Cauchy-Schwartz
inequality, witnessing the presence of multi-mode entanglement. Simultaneously,
the photon-pair rate is strongly enhanced by 450 times as compared to
unpatterned films due to high-quality-factor metasurface resonances, and the
coincidence to accidental ratio reaches 5000. These results pave the way to
miniaturization of various quantum devices by incorporating ultra-thin
metasurfaces functioning as room-temperature sources of quantum-entangled
photons
Exogenous brassinosteroids alleviate calcium deficiency-induced tip-burn by maintaining cell wall structural stability and higher photosynthesis in mini Chinese Cabbage
Tip-burn has seriously affected the yield, quality and commodity value of mini Chinese cabbage. Calcium (Ca2+) deficiency is the main cause of tip-burn. In order to investigate whether exogenous brassinosteroids (BRs) can alleviate tip-burn induced by calcium (Ca2+) deficiency and its mechanism, in this study, Ca2+ deficiency in nutrient solution was used to induced tip-burn, and then distilled water and BRs were sprayed on leaves to observe the tip-burn incidence of mini Chinese cabbage. The tip-burn incidence and disease index, leaf area, fluorescence parameters (Fv/Fm, NPQ, qP andφPSII) and gas exchange parameters (Tr, Pn, Gs and Ci), pigment contents, cell wall components, mesophyll cell ultrastructure and the expression of genes related to chlorophyll degradation were measured. The results showed that exogenous BRs reduced the tip-burn incidence rate and disease index of mini Chinese cabbage, and the tip-burn incidence rate reached the highest on the ninth day after treatment. Exogenous BRs increased the contents of cellulose, hemifiber, water-soluble pectin in Ca2+ deficiency treated leaves, maintaining the stability of cell wall structure. In addition, BRs increased photosynthetic rate by increasing the activities of Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and fructose 1,6-bisphosphatase (FBPase) related to Calvin cycle, maintaining relatively complete chloroplast structure and higher chlorophyll content via down-regulating the expression of BrPPH1 and BrPAO1 genes related to chlorophyll degradation. In conclusion, exogenous BRs alleviated calcium deficiency-induced tip-burn by maintaining cell wall structural stability and higher photosynthesis
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