501 research outputs found

    Quantum spin Hall density wave insulator of correlated fermions

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    We present the theory of a new type of topological quantum order which is driven by the spin-orbit density wave order parameter, and distinguished by Z2Z_2 topological invariant. We show that when two oppositely polarized chiral bands [resulting from the Rashba-type spin-orbit coupling αk\alpha_k, kk is crystal momentum] are significantly nested by a special wavevector Q(π,0)/(0,π){\bf Q}\sim(\pi,0)/(0,\pi), it induces a spatially modulated inversion of the chirality (αk+Q=αk\alpha_{k+Q}=\alpha_k^*) between different sublattices. The resulting quantum order parameters break translational symmetry, but preserve time-reversal symmetry. It is inherently associated with a Z2Z_2-topological invariant along each density wave propagation direction. Hence it gives a weak topological insulator in two dimensions, with even number of spin-polarized boundary states. This phase is analogous to the quantum spin-Hall state, except here the time-reversal polarization is spatially modulated, and thus it is dubbed quantum spin-Hall density wave (QSHDW) state. This order parameter can be realized or engineered in quantum wires, or quasi-2D systems, by tuning the spin-orbit couping strength and chemical potential to achieve the special nesting condition.Comment: 8 pages, 4 figure

    Outcome of cephalo-medullary nailing in intertrochanteric fracture of femur

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    Background: Intertrochanteric fracture is commonly seen in elderly and osteoporotic bone due to trivial trauma and requires hospitalization. Nonoperative treatment leads to complications like bedsore, pneumonia and deep vein thrombosis. The introduction of cephalomedullary nailing has broadened its use in complex intertrochanteric fracture with least complications.Methods: A prospective observational study was conducted in Birgunj, Nepal from 4th October 2020 to 3rd October 2021 among 50 patients of age more than 30 years with closed intertrochanteric fractures. Ethical approval was obtained from the institutional review committee and proper informed consent was taken. Modified Harris hip scoring system was used. Type of implant used was PFN long and short.Results: The mean age of 50 patients was 66.42 years, female to male ratio was 1.5:1 (30:20) and right side (72%) with fall injury (78%) was commonly observed. According to Kyles classification, 40% patients had Kyles’s type II fracture followed by type III (38%).  Most of the cases start mobilization on 2nd post operative day. Mean Harris hip score at 14 days, 6 weeks, 3 months and 6 months were 52.02, 64.50, 72.91 and 84.40 respectively. Early mobilization within 2 days of post-surgery had significant improvement in functional outcome throughout follow up as compared to more than 2 days (p=0.001, 0.001, 0.001 and 0.001 at 14 days, 6 weeks, 3 months and 6 months respectively). Radiological union was achieved in all cases within 6 months.Conclusions: The study results concluded cephalomedullary nailing is effective treatment for intertrochanteric fracture

    A Literature review on industrially accepted MPPT techniques for solar PV system

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    Solar energy is a clean renewable energy and it is available around 89,000 TW on the earth surface. To get maximum power from a solar PV system with minimum power transfer loss is one of the main design objectives of an energy transferring network. Power electronic devices perform a very important character for an efficient PV power tracking system control and either incorporates to transfer the generated power to the ac/dc grid or battery storage system. In this case the duty of the power electronics devices used in PV system is to track maximum power point under different operating conditions of environment, so that power tracking efficiency of solar PV system can be improved. This paper encapsulates based the on performance comparisions on the behavior of MPP under uniform and nonuniform operating conditions and selects the optimum duty cycle for industrially accepted MPPT techniques with their algorithm

    Probing the Fermi surface and magnetotransport properties in MoAs2_{2}

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    Transition metal dipnictides (TMDs) have recently been identified as possible candidates to host topology protected electronic band structure. These materials belong to an isostructural family and show several exotic transport properties. Especially, the large values of magnetoresistance (MR) and carrier mobility have drawn significant attention from the perspective of technological applications. In this report, we have investigated the magnetotransport and Fermi surface properties of single crystalline MoAs2_{2}, another member of this group of compounds. Field induced resistivity plateau and a large MR have been observed, which are comparable to several topological systems. Interestingly, in contrast to other isostructural materials, the carrier density in MoAs2_{2} is quite high and shows single-band dominated transport. The Fermi pockets, which have been identified from the quantum oscillation, are largest among the members of this group and have significant anisotropy with crystallographic direction. Our first-principles calculations reveal a substantial difference between the band structures of MoAs2_{2} and other TMDs. The calculated Fermi surface consists of one electron pocket and another 'open-orbit' hole pocket, which has not been observed in TMDs so far.Comment: 8 pages, 9 figure

    Large Landau level splitting with tunable one-dimensional graphene superlattice probed by magneto capacitance measurements

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    The unique zero energy Landau Level of graphene has a particle-hole symmetry in the bulk, which is lifted at the boundary leading to a splitting into two chiral edge modes. It has long been theoretically predicted that the splitting of the zero-energy Landau level inside the {\it bulk} can lead to many interesting physics, such as quantum spin Hall effect, Dirac like singular points of the chiral edge modes, and others. However, so far the obtained splitting with high-magnetic field even on a hBN substrate are not amenable to experimental detection, and functionality. Guided by theoretical calculations, here we produce a large gap zero-energy Landau level splitting (\sim 150 meV) with the usage of a one-dimensional (1D) superlattice potential. We have created tunable 1D superlattice in a hBN encapsulated graphene device using an array of metal gates with a period of \sim 100 nm. The Landau level spectrum is visualized by measuring magneto capacitance spectroscopy. We monitor the splitting of the zeroth Landau level as a function of superlattice potential. The observed splitting energy is an order higher in magnitude compared to the previous studies of splitting due to the symmetry breaking in pristine graphene. The origin of such large Landau level spitting in 1D potential is explained with a degenerate perturbation theory. We find that owing to the periodic potential, the Landau level becomes dispersive, and acquires sharp peaks at the tunable band edges. Our study will pave the way to create the tunable 1D periodic structure for multi-functionalization and device application like graphene electronic circuits from appropriately engineered periodic patterns in near future

    Effect of Tempering on Cyclic Loading of Medium Carbon Steel

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    The utility of Medium Carbon steel is well known now days. It has got such a variety of uses in distinctive commercial ventures because of its moderately low cost and great mechanical properties. The failure due to cyclic loading or dynamic loading is a crucial topic in the field of mechanical behaviour of materials since cyclic loading counts about ninety percent of the failures resulted from mechanical causes. In this work the cyclic loading behaviour of Medium Carbon Steel has been studied. The properties of steel are greatly influenced by various heat treatment techniques and tempering is the most common and important heat treatment technique. In the present work the effect of tempering on microstructure, tensile properties, hardness and most importantly on cyclic loading behaviour has been studied. The emphasis is given to the endurance limit. Attempts have been made to find out the best set of tempering conditions. The Medium Carbon Steel specimens were first austenitized at 850oC, hold for 1hr for soaking and then quenched in water bath maintained at room temperature. The quenched specimens were then tempered at three different temperatures 200oC, 400oC and 600oC the duration of tempering treatment was 60min., 90min., and 120min. for all of the above temperatures. Attempts have been made to correlate the microstructure with the mechanical properties of Medium Carbon Steel. The cyclic loading test was carried out in R.R. Moore Rotating Beam Testing Machine with a completely reversed stress pattern. Here the maximum stress and minimum stress are equal in magnitude but opposite in sense so the stress ratio i.e. the ratio of maximum stress to minimum stress becomes R= -1, the mean stress becomes zero. From this work, it may be concluded that tempering significantly improves the mechanical properties as well as the fatigue life of the material, and the best results have been seen for low temperature tempering at 200oC, for 60 min

    Pre and per operative prediction of difficult laparoscopic cholecystectomy using clinical and ultrasonographic parameters

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    Background: Cholecystectomy is the procedure of choice for symptomatic gall stones. Laparoscopic Cholecystectomy (LC) may be rendered difficult by various problems encountered during surgery. Several factors have been implicated with a difficult case, but no reliable criteria are available yet to identify patients preoperatively with a difficult LC. Preoperative prediction of a difficult LC can help the patient as well as the surgeon prepare better for the intraoperative risk and the risk of conversion to open cholecystectomy. The present study was undertaken to evaluate role of various factors responsible for conversion from laparoscopic to open cholecystectomy and also to study the intraoperative problems faced by the surgeon responsible for conversion in order to make the procedure safer for the patient as well as the surgeon.Methods: In 50 consecutive patients who underwent LC during 2013 to 2014 patient’s characteristics, clinical history, laboratory data, ultrasonography results and intraoperative details were prospectively analyzed to determine predictors of difficult LC.Results: Of 50 patients 3 (06%) required conversion to open cholecystectomy. Significant predictors of conversion were obscured anatomy of Calot’s due to adhesions, sessile gall bladder, male gender and gall bladder wall thickness >3 mm.Conclusions: With preoperative clinical and ultrasonographic parameters, proper patient selection can be made to help predict difficult LC and a likelihood of conversion to open cholecystectomy.
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