28 research outputs found
Topological Protection from Random Rashba Spin-Orbit Backscattering: Ballistic Transport in a Helical Luttinger Liquid
The combination of Rashba spin-orbit coupling and potential disorder induces
a random current operator for the edge states of a 2D topological insulator. We
prove that charge transport through such an edge is ballistic at any
temperature, with or without Luttinger liquid interactions. The solution
exploits a mapping to a spin 1/2 in a time-dependent field that preserves the
projection along one randomly undulating component (integrable dynamics). Our
result is exact and rules out random Rashba backscattering as a source of
temperature-dependent transport, absent integrability-breaking terms.Comment: 6+3 pages, 2+1 figure
Origin of -shifted three-dimensional charge density waves in kagome metal AVSb
Understanding the nature of charge density wave (CDW) and superconductivity
in kagome metal AVSb (A=Cs,Rb,K) is a recent subject of intensive
study. Due to the presence of van Hove singularities, electron-electron
interaction has been suggested to play an important role in the formation of
such broken symmetry states. Recent experiments show that the CDW order is
three-dimensional and it is staggered across different kagome layers. However,
the experimental interpretation for the precise structure of CDW varies in
terms of whether it is the star of David (SD), inverse star of David (ISD) or
the alternation of the two among neighboring layers. In this work, we show that
the origin of these distinct CDW orders can be understood in a unified picture
by considering intra- and inter-layer electron-electron interactions as well as
the coupling between electrons and lattice distortions. Utilizing an effective
9-band model with V orbitals and out-of-plane Sb orbitals, it is
demonstrated that the repulsive electron-electron interaction favors charge
bond order which induces either SD or ISD upon including lattice distortions.
As the inter-layer interaction is introduced, -shifted CDW develops with
the staggered ordering along the -axis. We also find that the phase with
alternating SD and ISD can be stabilized as the ground state under strong
inter-layer interaction
Massive Dirac fermions in moir\'e superlattices: a route towards topological flat minibands
We demonstrate a generic mechanism to realize topological flat minibands by
confining massive Dirac fermions in a periodic moir\'e potential, which can be
achieved in a heterobilayer of transition metal dichalcogenides. We show that
the topological phase can be protected by the symmetry of moir\'e potential and
survive to arbitrarily large Dirac band gap. We take the MoTe/WSe
heterobilayer as an example and find that the topological phase can be driven
by a vertical electric field. By projecting the Coulomb interaction onto the
topological fat minibands, we identify a correlated Chern insulator at half
filling and a quantum valley-spin Hall insulator at full filling which explains
the topological states observed in the MoTe/WSe in experiment. Our work
clarifies the importance of Dirac structure for the topological minibands and
unveils a general strategy to design topological moir\'e materials.Comment: 13 pages, 12 figure
Superconductivity and bosonic fluid emerging from Moir\'e flat bands
Although evidence of inter-valley attraction-mediated by phonon or
topological fluctuations is accumulating, the origin of superconductivity in
the flat-band quantum moir\'e materials remains an open question. Here, instead
of attempting to pinpoint the origin of the superconductivity, we aim at
identifying universal properties of moir\'e flat bands that shall emerge in the
presence of inter-valley attractions. We show that by matching the interaction
strength of inter-valley attraction with intra-valley repulsion, the flat-band
limit becomes exactly solvable. Away from the flat-band limit, the system can
be simulated via quantum Monte Carlo (QMC) methods without sign problem for any
fillings. Combining analytic solutions with large-scale numerical simulations,
we show that upon increasing temperature, the superconducting phase melts into
a bosonic fluid of Cooper pairs with large/diverging compressibility. In
contrast to flat-band attractive Hubbard models, where similar effects arise
only for on-site interactions, our study indicates this physics is a universal
property of moir\'e flat bands, regardless of microscopic details such as the
range of interactions and/or spin-oribt couplings. At higher temperature, the
boson fluid phase gives its way to a pseudo gap phase, where some Cooper pairs
are torn apart by thermal fluctuations, resulting in fermion density of states
inside the gap. Unlike the superconducting transition temperature, which is
very sensitive to doping and twisting angles, the gap and the temperature scale
of the boson fluid phase and the pseudo gap phase are found to be nearly
independent of doping level and/or flat-band bandwidth. The relevance of these
phases with experimental discoveries in the flat band quantum moir\'e materials
is discussed
Evolution from quantum anomalous Hall insulator to heavy-fermion semimetal in magic-angle twisted bilayer graphene
The ground states of twisted bilayer graphene (TBG) at chiral and flat-band
limit with integer fillings are known from exact solutions, while their
dynamical and thermodynamical properties are revealed by unbiased quantum Monte
Carlo (QMC) simulations. However, to elucidate experimental observations of
correlated metallic, insulating and superconducting states and their
transitions, investigations on realistic, or non-chiral cases are vital. Here
we employ momentum-space QMC method to investigate the evolution of correlated
states in magic-angle TBG away from chiral limit at charge neutrality with
polarized spin/valley, which approximates to an experimental case with filling
factor . We find that the ground state evolves from quatum anomalous
Hall insulator into an intriguing correlated semi-metallic state as AA hopping
strength reaches experimental values. Such a state resembles the recently
proposed heavy-fermion representations with localized electrons residing at AA
stacking regions and delocalized electrons itinerating via AB/BA stacking
regions. The spectral signatures of the localized and itinerant electrons in
the heavy-fermion semimetal phase are revealed, with the connection to
experimental results being discussed.Comment: 6 pages, 4 figures with supplementary material (6 pages, 11 figures
Igg, Igm and Iga Antibodies against the Novel Polyprotein in Active Tuberculosis
Background
The present study was aimed to evaluate whether IgG, IgM and IgA antibodies levels detected against a novel Mycobacterium tuberculosis polyprotein 38 F-64 F (with 38 F being the abbreviation for 38kD-ESAT6-CFP10 and 64 F for Mtb8.4-MPT64-TB16.3-Mtb8) are suitable for diagnosing active tuberculosis, and for monitoring the efficacy of chemotherapy on TB patients.
Methods
In this study, a total of 371 active TB patients without treatment were selected and categorized into S+/C+ group (n = 143), S-/C+ group (n = 106) or S-/C- group (n = 122). A series of serum samples were collected from 82 active TB patients who had undergone anti-TB chemotherapy for 0–6 months at one month interval. Humoral responses (IgG, IgM and IgA) were determined for the novel Mycobacterium tuberculosis polyprotein using indirect ELISA methods in all of serum samples.
Results
For S+/C+, S-/C+ and S-/C- active tuberculosis patients before anti-TB chemotherapy, the sensitivities of tests based on IgG were 65.7%, 46.2% and 52.5% respectively; the sensitivities based on IgM were 21.7%, 24.5% and 18.9%; and the sensitivities based on IgA were 25.2%, 17.9% and 23.8%. By combination of three isotypes, for all active tuberculosis patients, the test sensitivity increased to 70.4% with the specificity being 91.5%. After anti-TB chemotherapy, there were no significant differences between groups with different courses of anti-TB chemotherapy.
Conclusions
The novel Mycobacterium tuberculosis polyprotein 38 F-64 F represents potential antigen suitable for measuring IgG, IgM and IgA antibodies. However, the serodiagnostic test based on the 38 F-64 F polyprotein appears unsuitable for monitoring the efficacy of chemotherapy
Topology and Symmetry in Weakly and Strongly Correlated Electronic Systems
This thesis studies topological phases in various electronic crystalline systems with a focus on the interplay between symmetry and topology, including the non-interacting topological insulators, higher-order topological insulators, topological semimetals as well as the strongly correlated systems with intrinsic topological order. First, we show that when the symmetry of the system is low, the symmetry representation of energy bands may not be sufficient to determine the topological index, but the symmetry can still be utilized to define a gauge-invariant quantity, which provides a gauge-independent way to compute the topological index of topological insulators. Second, we proposed a generalization of the famous Fu-Kane-Mele Pfaffian formula from topological insulators to higher-order topological insulators, which allows many important conclusions that have been well known for topological insulators to be generalized to higher-order topological insulators. This generalization is made possible via a linear superposition of symmetry operator. Third, we proposed a diagnosis scheme for topological nodal lines with nontrivial monopole charge based on point group representations. This method predicts novel cage-like nodal structures around high-symmetry lines in the Brillouin zone beyond previous diagnosis schemes which require high-symmetry lines to be gapped. Lastly, we studied strongly correlated topological phases in twisted Moire systems and proposed to realize fractional Chern insulators in twisted bilayer transition metal dichalcogenides.PHDPhysicsUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/170015/1/heqiuli_1.pd