We show that the strongly spin-orbit coupled materials Bi 2 Te 3 and Sb 2 Te 3 and their derivati... more We show that the strongly spin-orbit coupled materials Bi 2 Te 3 and Sb 2 Te 3 and their derivatives belong to the Z 2 topological-insulator class. Using a combination of first-principles theoretical calculations and photoemission spectroscopy, we directly show that Bi 2 Te 3 is a large spin-orbit-induced indirect bulk band gap ( $ 150 meV) semiconductor whose surface is characterized by a single topological spin-Dirac cone. The electronic structure of self-doped Sb 2 Te 3 exhibits similar Z 2 topological properties. We demonstrate that the dynamics of spin-Dirac fermions can be controlled through systematic Mn doping, making these materials classes potentially suitable for topological device applications.
The growth and elementary properties of p-type Bi 2 Se 3 single crystals are reported. Based on a... more The growth and elementary properties of p-type Bi 2 Se 3 single crystals are reported. Based on a hypothesis about the defect chemistry of Bi 2 Se 3 , the p-type behavior has been induced through low level substitutions (1% or less) of Ca for Bi. Scanning tunneling microscopy is employed to image the defects and establish their charge. Tunneling and angle resolved photoemission spectra show that the Fermi level has been lowered into the valence band by about 400 meV in Bi 1.98 Ca 0.02 Se 3 relative to the n-type material. p-type single crystals with ab plane Seebeck coefficients of +180 µV/K at room temperature are reported. These crystals show a giant anomalous peak in the Seebeck coefficient at low temperatures, reaching +120 µVK -1 at 7 K, giving them a high thermoelectric power factor at low temperatures. In addition to its interesting thermoelectric properties, p-type Bi 2 Se 3 is of substantial interest for studies of technologies and phenomena proposed for topological insulators.
1 Recent theories and experiments have suggested that strong spin-orbit coupling effects in certa... more 1 Recent theories and experiments have suggested that strong spin-orbit coupling effects in certain band insulators can give rise to a new phase of quantum matter, the so-called topological insulator, which can show macroscopic entanglement effects . Such systems feature two-dimensional surface states whose electrodynamic properties are described not by the conventional Maxwell equations but rather by an attached axion field, originally proposed to describe strongly interacting particles .
Topological Phase Transition and Texture Inversion in a Tunable Topological Insulator
Science, 2011
The recently discovered three-dimensional or bulk topological insulators are expected to exhibit ... more The recently discovered three-dimensional or bulk topological insulators are expected to exhibit exotic quantum phenomena. It is believed that a trivial insulator can be twisted into a topological state by modulating the spin-orbit interaction or the crystal lattice, driving the system through a topological quantum phase transition. By directly measuring the topological quantum numbers and invariants, we report the observation of a phase transition in a tunable spin-orbit system, BiTl(S1-δSeδ)2, in which the topological state formation is visualized. In the topological state, vortex-like polarization states are observed to exhibit three-dimensional vectorial textures, which collectively feature a chirality transition as the spin momentum-locked electrons on the surface go through the zero carrier density point. Such phase transition and texture inversion can be the physical basis for observing fractional charge (±e/2) and other fractional topological phenomena.
1 When electrons are subject to a large external magnetic field, the conventional charge quantum ... more 1 When electrons are subject to a large external magnetic field, the conventional charge quantum Hall effect [1, 2] dictates that an electronic excitation gap is generated in the sample bulk, but metallic conduction is permitted at the boundary. Recent theoretical models suggest that certain bulk insulators with large spin-orbit interactions may also naturally support conducting topological boundary states in the extreme quantum limit [3, 4, 5], which opens up the possibility for studying unusual quantum Hall-like phenomena in zero external magnetic fields [6]. Bulk Bi 1−x Sb x single crystals are predicted to be prime candidates [7, 8] for one such unusual Hall phase of matter known as the topological insulator [9,. The hallmark of a topological insulator is the existence of metallic surface states that are higher dimensional analogues of the edge states that characterize a quantum spin Hall insulator [3, 4, 5, 6, 7, 8, 9,. In addition to its interesting boundary states, the bulk of Bi 1−x Sb x is predicted to exhibit three-dimensional Dirac particles , another topic of heightened current interest following the new findings of two-dimensional graphene and charge quantum Hall fractionalization observed in pure bismuth . However, despite numerous transport and magnetic measurements on the Bi 1−x Sb x family since the 1960s , no direct evidence of either topological quantum Hall-like states or bulk Dirac particles has ever been found. Here, using incident-photon-energy-modulated angle-resolved photoemission spectroscopy (IPEM-ARPES), we report the direct observation of massive Dirac particles in the bulk of Bi 0.9 Sb 0.1 , locate the Kramers' points at the sample's boundary and provide a comprehensive mapping of the topological Dirac insulator's gapless surface modes. These findings taken together suggest that the observed surface state on the boundary of the bulk insulator is a realization of the much sought exotic "topological metal" [9,. They also suggest that this material has potential application in developing next-generation quantum computing devices that may incorporate "light-like" bulk carriers and topologically protected spin-textured edge-surface currents.
Diarrheal illness in a cohort of children 0–2 years of age in rural Bangladesh: I. Incidence and risk factors
Acta Paediatrica, 2006
To describe clinical characteristics and age- and season-specific incidences of diarrheal episode... more To describe clinical characteristics and age- and season-specific incidences of diarrheal episodes, and to evaluate risk factors associated with the occurrence of diarrheal disease. A total of 252 infants from rural Bangladesh were followed through household surveillance for 2 y from birth during the years 1993-1996. Demographic and household determinants were linked to the probability of illness using logistic regression models. The overall incidence of diarrhea was 4.25 episodes per child per year. Peak rates of overall, acute, and persistent diarrhea occurred in the 6-11-mo and 12-17-mo age groups. Diarrheal rates peaked during the spring and summer. Among host-related characteristics, having a sibling in the household and having had prior diarrhea were significant risk factors for diarrhea. Among environmental characteristics, spring season remained a highly statistically significant risk factor for diarrhea. Diarrheal disease continues to be a substantial burden in young children in rural Bangladesh. Most diarrheal episodes are of short duration, and should primarily be treated with oral rehydration therapy to prevent diarrhea-related mortality. Improved knowledge of oral rehydration therapy, feeding during episodes to prevent further malnutrition, prolonged breastfeeding, and the keeping of livestock in corralled areas of the home are advocated.
Theoretical and experimental discovery of single-Dirac-cone topological-insulator class was repor... more Theoretical and experimental discovery of single-Dirac-cone topological-insulator class was reported in the same paper at arXiv: 0812.2078 (2008) [http://arxiv.org/abs/0812.2078].
Proceedings of The National Academy of Sciences, 2004
Spectroscopic studies have identified a number of proteins that appear to retain significant resi... more Spectroscopic studies have identified a number of proteins that appear to retain significant residual structure under even strongly denaturing conditions. Intrinsic viscosity, hydrodynamic radii, and small-angle x-ray scattering studies, in contrast, indicate that the dimensions of most chemically denatured proteins scale with polypeptide length by means of the power-law relationship expected for random-coil behavior. Here we further explore this discrepancy by expanding the length range of characterized denatured-state radii of gyration (R G) and by reexamining proteins that reportedly do not fit the expected dimensional scaling. We find that only 2 of 28 crosslink-free, prosthetic-group-free, chemically denatured polypeptides deviate significantly from a powerlaw relationship with polymer length. The R G of the remaining 26 polypeptides, which range from 16 to 549 residues, are well fitted (r 2 ؍ 0.988) by a power-law relationship with a best-fit exponent, 0.598 ؎ 0.028, coinciding closely with the 0.588 predicted for an excluded volume random coil. Therefore, it appears that the mean dimensions of the large majority of chemically denatured proteins are effectively indistinguishable from the mean dimensions of a random-coil ensemble.
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Papers by Zahid Hasan