Topological magnetic textures in magnetic topological insulators

The surfaces of intrinsic magnetic topological insulators (TIs) host magnetic moments exchange-coupled to Dirac electrons. We study the magnetic phases arising from tuning the electron density using variational and exact diagonalization approaches. In the dilute limit, we find that magnetic skrymions are formed which bind to electrons leading to a skyrmion Wigner crystal phase while at higher densities spin spirals accompanied by chiral 1d channels of electrons are formed. The binding of electrons to textures raises the possibility of manipulating textures with electrostatic gating. We determine the phase diagram capturing the competition of intrinsic spin-spin interactions and carrier density and comment on the possible application to experiments in magnetic TIs and spintronic devices such as skyrmion-based memory.Comment: v2: highlighted the 4-parameter parameterization of skyrmions and elaborated on skyrmion Wigner crystal phase v1: 6+3 pages, 5 figure

Giant proximity exchange and flat Chern band in 2D magnet-semiconductor heterostructures

Van der Waals (vdW) heterostructures formed by two-dimensional magnets and semiconductors have provided a fertile ground for fundamental science and for spintronics. We present first-principles calculations finding a proximity exchange splitting of 14 meV equivalent to an effective Zeeman field of 120 T in the vdW magnet-semiconductor heterostructure MoS$_2$/CrBr$_3$, leading to a 2D spin-polarized half-metal with carrier densities ranging up to $10^{13}$ cm$^{-2}$. We consequently explore the effect of large exchange coupling on the electronic bandstructure when the magnetic layer hosts chiral spin textures such as skyrmions. A flat Chern band is found at a "magic" value of magnetization $\overline{m} \sim 0.2$ for Schr\"odinger electrons, and it generally occurs for Dirac electrons. The magnetic proximity induced anomalous Hall effect enables transport-based detection of chiral spin textures, and flat Chern bands provides an avenue for engineering various strongly correlated states.Comment: 5+4 pages, 7 figures; v2 accepted to Science Advance

Zero-field composite Fermi liquid in twisted semiconductor bilayers

Recent experiments have produced evidence for fractional quantum anomalous Hall (FQAH) states at zero magnetic field in the semiconductor moir\'e superlattice system $t$MoTe$_2$. Here we argue that a composite fermion description, already a unifying framework for the phenomenology of 2d electron gases at high magnetic fields, provides a similarly powerful perspective in this new context. To this end, we present exact diagonalization evidence for composite Fermi liquid states at zero magnetic field in $t$MoTe$_2$ at fillings $n=\frac{1}{2}$ and $n=\frac{3}{4}$. We dub these non-Fermi liquid metals anomalous composite Fermi liquids (ACFLs), and we argue that they play a central organizing role in the FQAH phase diagram. We proceed to develop a long wavelength theory for this ACFL state that offers concrete experimental predictions upon doping the composite Fermi sea, including a Jain sequence of FQAH states and a new type of commensurability oscillations originating from the superlattice potential intrinsic to the system.Comment: Phys. Rev. Lett. journal version. Editors' Suggestion and Featured in Physics. main: 5 pages, 3 figures. SM: 5 pages, 3 figure

Establishment of population-based surveillance for invasive pneumococcal disease in Bangalore, India

Background :Invasive pneumococcal disease (IPD) is vaccine-preventable but few data on the incidence of PD exist for Indian children. Aims: To assess the feasibility of implementing prospective, population-based surveillance for PD among children less than five years of age. Settings and Design :Hospitals and health agencies, Bangalore, India. Retrospective review and analysis of hospitalization records as well as public health and demographic data. Material and Methods : Records for 2006 hospitalizations for pneumococcal disease-associated syndromes (meningitis, pneumonia and sepsis) were identified at three pediatric referral hospitals (Indira Gandhi Institute of Child Health, Kempegowda Institute of Child Health and Vani Vilas Hospital) in Bangalore using International Classification of Diseases, 9th revision codes. Hospital microbiology laboratory records were assessed to ensure capacity for identifying S. pneumoniae. Population data were identified from national census and polio surveillance data. Results : The Bangalore city southern zone includes 33 wards occupying 51 Km 2 with 150,945 children between 0-5 years of age served by three referral pediatric hospitals. From January--December 2006, records of these three hospitals showed 2,219 hospitalizations of children less than five years of age (967 pneumonia, 768 sepsis, and 484 meningitis) with PD-associated diagnoses (southern zone area incidence: 0.15/100,000 PD-associated hospitalizations, less than five years of age). There were 178 deaths in children less than five years of age, of which 87 were attributable to sepsis, 56 to pneumonia and 35 to meningitis. Conclusion : Our analysis suggests that the PD-associated disease burden in Bangalore is high and local institutions have capacity for population-based surveillance. In a prospective study, systematic attention to potential barriers in identifying children with pneumococcal infections will improve estimation of IPD incidence in India