Superconductivity from the Condensation of Topological Defects in a Quantum Spin-Hall Insulator

The discovery that spin-orbit coupling can generate a new state of matter in the form of quantum spin-Hall (QSH) insulators has brought topology to the forefront of condensed matter physics. While QSH states from spin-orbit coupling can be fully understood in terms of band theory, fascinating many-body effects are expected if the state instead results from interaction-generated symmetry breaking. In particular, topological defects of the corresponding order parameter provide a route to exotic quantum phase transitions. Here, we introduce a model in which the condensation of skyrmion defects in an interaction-generated QSH insulator produces a superconducting (SC) phase. Because vortex excitations of the latter carry a spin-$1/2$ degree of freedom numbers, the SC order may be understood as emerging from a gapless spin liquid normal state. The QSH-SC transition is an example of a deconfined quantum critical point (DQCP), for which we provide an improved model with only a single length scale that is accessible to large-scale quantum Monte Carlo simulations.Comment: 14 pages, 11 figures, 3 table

Bandwidth controlled quantum phase transition between an easy-plane quantum spin Hall state and an s-wave superconductor

The quantum spin Hall state can be understood in terms of spontaneous O(3) symmetry breaking. Topological skyrmion configurations of the O(3) order parameter vector carry a charge 2e, and as shown previously, when they condense, a superconducting state is generated. We show that this topological route to superconductivity survives easy-plane anisotropy. Upon reducing the O(3) symmetry to O(2)$\times$ Z$_2$, skyrmions give way to merons that carry a unit charge. On the basis of large-scale auxiliary field quantum Monte Carlo simulations, we show that at the particle-hole symmetric point, we can trigger a continuous and direct transition between the quantum spin Hall state and s-wave superconductor by condensing pairs of merons. This statement is valid in both strong and weak anisotropy limits. Our results can be interpreted in terms of an easy-plane deconfined quantum critical point. However, in contrast to the previous studies in quantum spin models, our realization of this quantum critical point conserves $U(1)$ charge, such that skyrmions are conserved

Thermodynamic and Dynamical Signatures of a Quantum Spin-Hall Insulator to Superconductor Transition

Thermodynamic and dynamical properties of a model of Dirac fermions with a deconfined quantum critical point (DQCP) separating an interaction-generated quantum spin-Hall insulator from an s-wave superconductor [Nature Comm.~{\bf 10}, 2658 (2019)] are studied by quantum Monte Carlo simulations. Inside the deconfined quantum critical region bound by the single-particle gap, spinons and spinless charge-2e skyrmions emerge. Since the model conserves total spin and charge, and has a single length scale, these excitations lead to a characteristic linear temperature dependence of the uniform spin and charge susceptibilities. At the DQCP, the order parameter dynamic structure factors show remarkable similarities that support emergent Lorentz symmetry. Above a critical temperature, superconductivity is destroyed by the proliferation of spin-1/2 vortices.Comment: 8 pages and 8 figure

Effective model for superconductivity in magic-angle graphene

We carry out large-scale quantum Monte Carlo simulations of a candidate field theory for the onset of superconductivity in magic-angle twisted bilayer graphene. The correlated insulating state at charge neutrality spontaneously breaks U(1) Moir\'e valley symmetry. Owing to the topological nature of the bands, skyrmion defects of the order parameter carry charge $2e$ and condense upon doping. In our calculations we encode the U(1) symmetry by an internal degree of freedom such that it is not broken upon lattice regularization. Furthermore, the skyrmion carries the same charge. The nature of the doping-induced phase transitions depends on the strength of the easy-plane anisotropy that reduces the SU(2) valley symmetry to U(1) $\times \mathbb{Z}_2$. For large anisotropy, we observe two distinct transitions separated by phase coexistence. While the insulator to superconducting transition is of mean-field character, the U(1) transition is consistent with three-dimensional XY criticality. Hence, the coupling between the gapless charge excitations of the superconducting phase and the XY order parameter is irrelevant. At small anisotropy, we observe a first-order transition characterized by phase separation.Comment: 6 pages, 5 figures, supplemental materia

High-Spatial and High-Mass Resolution Imaging of Surface Metabolites of Arabidopsis thaliana by Laser Desorption-Ionization Mass Spectrometry Using Colloidal Silver

High-spatial resolution and high-mass resolution techniques are developed and adopted for the mass spectrometric imaging of epicuticular lipids on the surface of Arabidopsis thaliana. Single cell level spatial resolution of ∼12 μm was achieved by reducing the laser beam size by using an optical fiber with 25 μm core diameter in a vacuum matrix-assisted laser desorption ionization-linear ion trap (vMALDI-LTQ) mass spectrometer and improved matrix application using an oscillating capillary nebulizer. Fine chemical images of a whole flower were visualized in this high spatial resolution showing substructure of an anther and single pollen grains at the stigma and anthers. The LTQ-Orbitrap with a MALDI ion source was adopted to achieve MS imaging in high mass resolution. Specifically, isobaric silver ion adducts of C29 alkane (m/z 515.3741) and C28 aldehyde (m/z 515.3377), indistinguishable in low-resolution LTQ, can now be clearly distinguished and their chemical images could be separately constructed. In the application to roots, the high spatial resolution allowed molecular MS imaging of secondary roots and the high mass resolution allowed direct identification of lipid metabolites on root surfaces

Deciphering the Peptide Iodination Code: Influence on Subsequent Gas-Phase Radical Generation with Photodissociation ESI-MS

Iodination of tyrosine was recently discovered as a useful method for generating radical peptides via photodissociation of carbon–iodine bonds by an ultraviolet photon in the gas phase. The subsequent fragmentation behavior of the resulting odd-electron peptides is largely controlled by the radical. Although previous experiments have focused on mono-iodination of tyrosine, peptides and proteins can also be multiply iodinated. Tyrosine and, to a lesser extent, histidine can both be iodinated or doubly iodinated. The behavior of doubly iodinated residues is explored under conditions where the sites of iodination are carefully controlled. It is found that radical peptides generated by the loss of a single iodine from doubly iodinated tyrosine behave effectively identically to singly iodinated peptides. This suggests that the remaining iodine does not interfere with radical directed dissociation pathways. In contrast, the concerted loss of two iodines from doubly iodinated peptides yields substantially different results that suggest that radical recombination can occur. However, sequential activation can be used to generate multiple usable radicals in different steps of an MSn experiment. Furthermore, it is demonstrated that in actual peptides, the rate of iodination for tyrosine versus mono-iodotyrosine cannot be predicted easily a priori. In other words, previous assumptions that mono-iodination of tyrosine is the rate-limiting step to the formation of doubly iodinated tyrosine are incorrect

Identification and characterization of a novel cross-link lesion in d(CpC) upon 365-nm irradiation in the presence of 2-methyl-1,4-naphthoquinone

We report the isolation and characterization for the first time of a cross-link lesion between two adjacent cytosines from the 2-methyl-1,4-naphthoquinone (menadione)-sensitized 365-nm irradiation of d(CpC). Electrospray ionization mass spectrometry (ESI-MS), tandem MS and (1)H NMR results indicate that the cross-link occurs between the C5 carbon atom of one cytosine and the N(4) nitrogen atom of the other cytosine. Furthermore, we synthesized d(CpC) with a (15)N being incorporated on the amino group of either of the two cytosines. We then irradiated the two (15)N-labeled dinucleoside monophosphates, isolated the cross-link products and characterized them by MS and multi-stage tandem MS. The latter results established unambiguously that the N(4) nitrogen atom of the 3′-nucleobase is involved in the covalent bond formation between the two cytosines. This, in combination with two-dimensional nuclear Overhauser effect spectroscopy (NOESY) results, demonstrates that the cross-link arises from the formation of a covalent bond between the C5 carbon atom of the 5′ cytosine and the N(4) nitrogen atom of the 3′ cytosine. We also show that the solution pH has a significant effect on the formation of the cross-link lesion, which supports that the deprotonation at the exocyclic amino group of cytosine cation radical is essential for the formation of the cross-link lesion

STABILITY RESEARCH OF FLOW-INDUCED VIBRATION OF HYBRID RIGID-FLEXIBLE PIPE CONVEYING FLUID (MT)

An articulated pipe composed of a flexible pipe and a rigid pipe is a typical hybrid rigid-flexible dynamical system involving flow-induced vibrations. Based on Hamilton′s principle, the governing equations of motion of the hybrid rigid-flexible pipe system were established. The partial differential equations of motion were discretized via Galerkin′s approach using the modal functions of a cantilevered beam. Instabilities and critical flow velocities were analyzed and calculated as a function of the stiffness of the rotational spring, mass ratio, length ratio of the rigid and flexible pipes. The results show that the critical velocities decrease as the increasing of length ratio and the system mainly suffers second mode instability. The hybrid pipe undergoes complicated transference of “second mode instability-regain stability-third mode instability” at a certain value of stiffness when the flow velocity increases. With the increasing of mass ratio of flexible pipe, a transition from second mode instability to fourth mode instability is observed