First principles prediction of novel quantum topological insulator state in two-dimensional XMg2Bi2(X=Eu/Yb).

Topological insulator (TIs), a novel quantum state of materials, has a lot of significance in the development of low-power electronic equipments as the conducting edge states display exceptional endurance against back-scattering. The absence of suitable materials with high fabrication feasibility and significant nontrivial bandgap, is now the biggest hurdle in their potential applications in devices. Here, we illustrate using first principles density functional calculations that the quintuplet layers of EuMg2Bi2and YbMg2Bi2crystals are potential two-dimensional TIs with a sizeable nontrivial gaps of 72 meV and 147 meV respectively. Dynamical stability of these quintuplet layers of EuMg2Bi2and YbMg2Bi2is confirmed by our phonon calculations. The weakly coupled layered structure of parent compounds makes it possible for simple exfoliation from a three-dimensional structure. We observed gapless edge states inside the bulk band gap in both the systems which indicate their TI nature. Further, we observed the anomalous and spin Hall conductivities to be quantized in two dimensional EuMg2Bi2and YbMg2Bi2respectively. Our findings predict two viable candidate materials as two dimensional quantum TIs which can be explored by future experimental investigations and possible applications of quantized spin and anomalous Hall conductance in spintronics.

The effect of antisite disorder on magnetic and exchange bias properties of Gd-substituted Y2CoMnO6double perovskite.

Combining experimental investigations and first-principles density functional theory (DFT) calculations, we report physical and magnetic properties of Gd-substituted Y2CoMnO6double perovskite, which are strongly influenced by antisite-disorder-driven spin configurations. On Gd doping, Co and Mn ions are present in mixed-valence (Co3+, Co2+, Mn3+and Mn4+) states. Multiple magnetic transitions have been observed: (i) paramagnetic to ferromagnetic transition is found to occur atTC= 95.5 K, (ii) antiferromagnetic transition atTN= 47 K is driven by3d-4fpolarization and antisite disorder present in the sample, (iii) change in magnetization belowT⩽20 K, primarily originating from Gd ordering, as revealed from our DFT calculations. AC susceptibility measurement confirms the absence of any spin-glass or cluster-glass phases in this material. A significantly large exchange bias effect (HEB= 1.07 kOe) is found to occur below 47 K due to interfaces of FM and AFM clusters created by antisite-disorder.

Emergence of weak pyrochlore phase and signature of field induced spin ice ground state in Dy2- x La x Zr2O7; x = 0, 0.15, 0.3.

The pyrochlore oxides Dy2Ti2O7 and Ho2Ti2O7 are well studied spin ice systems and have shown the evidences of magnetic monopole excitations. Unlike these, Dy2Zr2O7 is reported to crystallize in a distorted fluorite structure. We present here the magnetic and heat capacity studies of La substituted Dy2Zr2O7. Our findings suggest the absence of spin ice state in Dy2Zr2O7 but the emergence of the magnetic field induced spin freezing near T ≈ 10 K in ac susceptibility measurements which is similar to Dy2Ti2O7. The magnetic heat capacity of Dy2Zr2O7 shows a shift in the peak position from 1.2 K in zero field to higher temperatures in the magnetic field, with the corresponding decrease in the magnetic entropy. The low temperature magnetic entropy at 5 kOe field is R ln2 - (1/2)R ln(3/2) which is the same as for the spin ice state. Substitution of non-magnetic, isovalent La3+ for Dy3+ gradually induces the structural change from highly disordered fluorite to weakly ordered pyrochlore phase. The La3+ substituted compounds with less distorted pyrochlore phase show the spin freezing at lower field which strengthens further on the application of magnetic field. Our results suggest that the spin ice state can be stabilized in Dy2Zr2O7 either by slowing down of the spin dynamics or by strengthening the pyrochlore phase by suitable substitution in the system.

Orbital order and electron itinerancy in CoV2O4 and Mn0.5Co0.5V2O4 from first principles.

In view of the recent experimental predictions of a weak structural transition in CoV2O4 we explore the possible orbital order states in its low temperature tetragonal phases from first principles density functional theory calculations. We observe that the tetragonal phase with I41/amd symmetry is associated with an orbital order involving complex orbitals with a reasonably large orbital moment at vanadium sites while in the phase with I41/a symmetry, the real orbitals with quenched orbital moment constitute the orbital order. Further, to study the competition between orbital order and electron itinerancy we considered Mn0.5Co0.5V2O4 as one of the parent compounds, CoV2O4, lies near itinerant limit while the other, MnV2O4, lies deep inside the orbitally ordered insulating regime. Orbital order and electron transport have been investigated using first principles density functional theory and Boltzmann transport theory in CoV2O4, MnV2O4 and Mn0.5Co0.5V2O4. Our results show that as we go from MnV2O4 to CoV2O4 there is enhancement in the electron's itinerancy while the nature of orbital order remains unchanged.

An experimental and theoretical study of magnetocaloric effect in Nd0.5Dy0.5FeO3.

A significant magnetocaloric effect (MCE) has been revealed in our investigation on polycrystalline Nd0.5Dy0.5FeO3 below 30 K. Observed magnetization of the system at low temperature is 32% higher than the expected average magnetization of NdFeO3 and DyFeO3. Such an enhancement in the magnetization led to a large change in magnetic entropy (10.4 Jkg-1 K-1) at 4 K. The observed entropy change is remarkable considering the higher natural abundance of Nd compared to that of Dy and negligible MCE seen in case of NdFeO3. Theoretical calculations performed using mean-field approximation and Monte Carlo simulations on an Ising type spin model indicate that the high magnetocaloric effect is caused primarily by the ordering of rare-earth ions in C-type antiferromagnetic state in presence of molecular exchange field created by Fe ions.

Magnetocaloric effects from an interplay of magnetic sublattices in Nd2NiMnO6.

We present a combined experimental and theoretical study to understand the magnetism and magnetocaloric behavior of the double perovskite Nd2NiMnO6. The magnetic susceptibility data confirms a ferromagnetic transition with [Formula: see text] K. An additional feature at T = 25 K, indicative of antiferromagnetic correlations, is present. A positive magnetocaloric effect (MCE) near [Formula: see text] and a negative MCE around T = 25 K is inferred from the temperature dependence of the change in magnetic entropy at low magnetic fields. The negative MCE peak is suppressed on the application of a magnetic field and can be made to switch to a conventional positive MCE upon increasing magnetic field. We understand and reproduce these features in Monte Carlo simulations of a phenomenological Heisenberg model for Nd2NiMnO6. The validity of the model is tested using density functional theory calculations. We argue that this simple understanding of the experimental observations in terms of two antiferromagnetically coupled sublattices allows these results to be useful across a broader class of magnetocaloric materials.

Nature of spiral state and absence of electric polarisation in Sr-doped YBaCuFeO5 revealed by first-principle study.

Experimental results on YBaCuFeO5, in its incommensurate magnetic phase, appear to disagree on its ferroelectric response. Ambiguity exists on the nature of the spiral magnetic state too. Using first-principles density functional theory (DFT) calculations for the parent compound within LSDA + U + SO approximation, we reveal the nature of spiral state. The helical spiral is found to be more stable below the transition temperature as spins prefer to lie in ab plane. Dzyaloshinskii-Moriya (DM) interaction turns out to be negligibly small and the spin current mechanism is not valid in the helical spiral state, ruling out an electric polarisation from either. These results are in very good agreement with the recent, high quality, single-crystal data. We also investigate the magnetic transition in YBa1-xSrxCuFeO5 for the entire range (0 ≤ x ≤ 1) of doping. The exchange interactions are estimated as a function of doping and a quantum Monte Carlo (QMC) calculation on an effective spin Hamiltonian shows that the paramagnetic to commensurate phase transition temperature increases with doping till x = 0.5 and decreases beyond. These observations are consistent with experimental findings.

First-Principles Correlated Approach to the Normal State of Strontium Ruthenate.

The interplay between multiple bands, sizable multi-band electronic correlations and strong spin-orbit coupling may conspire in selecting a rather unusual unconventional pairing symmetry in layered Sr2RuO4. This mandates a detailed revisit of the normal state and, in particular, the T-dependent incoherence-coherence crossover. Using a modern first-principles correlated view, we study this issue in the actual structure of Sr2RuO4 and present a unified and quantitative description of a range of unusual physical responses in the normal state. Armed with these, we propose that a new and important element, that of dominant multi-orbital charge fluctuations in a Hund's metal, may be a primary pair glue for unconventional superconductivity. Thereby we establish a connection between the normal state responses and superconductivity in this system.

Competing electronic states in high temperature phase of NaTiO(2).

First principle density functional theory calculations on the high temperature phase of layered triangular lattice system NaTiO2 have revealed that a collective electronic state exists energetically close to the ground state but with competing transport properties: the latter is metallic with partially occupied doubly degenerate e'g orbitals, whereas the former is insulating with a1g orbital fully occupied. Significant occupation of this excited state is possible at non zero temperature either thermally or thanks to very soft (large amplitude) oxygen vibrations. Possible explanations of the experimental low conductivity based on competing orbital transport and of the specific heat jump at a structural transition based on orbital entropy are discussed.

Unraveling wetting transition through surface textures with X-rays: liquid meniscus penetration phenomena.

In this report we show that synchrotron X-ray radiography is a powerful method to study liquid-air interface penetration through opaque microtextured surface roughness, leading to wetting transition. We investigate this wetting phenomenon in the context of sessile drop evaporation, and establish that liquid interface sinking into the surface texture is indeed dictated by the balance of capillary and Laplace pressures, where the intrinsically three-dimensional nature of the meniscus must be accounted for. Air bubble entrapment in the texture underneath impacting water drops is also visualized and the mechanisms of post-impact drop evaporation are discussed.

Metallicity and ferromagnetism in nanosystem of charge ordered Nd0.5Sr0.5MnO3.

The fascinating phenomenon of destabilization of charge/orbital order in Nd0.5Sr0.5MnO3 with the reduction of grain size is critically investigated. Based on our magnetic and transport experiments followed by a theoretical analysis, we analyze various possible mechanisms and try to delineate a universal scenario behind this phenomenon. We revisit this issue carefully and discuss various evidences from experiments in nano and bulk manganites on the absence of correlation between size reduction and pressure effects on manganites. We propose a phenomenological model based on enhanced surface disorder to explain the appearance of weak ferromagnetism and metallicity in nanosize Nd0.5Sr0.5MnO3 system. All evidence seems to suggest that the transport is mediated through the surface via enhanced density of states in the nanometric grains. We provide theoretical support for this by performing an ab-initio electronic structure calculation as well as from a recent numerical simulation and argue that the mechanism is likely to be general in all nanosize charge ordered manganites.

A ground state phase diagram of a spinless, extended Falicov-Kimball model on the triangular lattice.

Correlated systems with hexagonal layered structures have come to the fore with renewed interest in cobaltates, transition metal dichalcogenides and GdI(2). While superconductivity, unusual metal and possible exotic states (prevented from long-range order by strong local fluctuations) appear to come from frustration and correlation working in tandem in such systems, they freeze at a lower temperature to crystalline states. The underlying effective Hamiltonian in some of these systems is believed to be the Falicov-Kimball model and therefore, a thorough study of the ground state of this model and its extended version on a non-bipartite lattice is important. Using a Monte Carlo search algorithm, we identify a large number of different possible ground states with charge order as well as valence and metal-insulator transitions. Such competing states, close in energy, give rise to complex charge order and other broken symmetry structures as well as the phase segregations observed in the ground state of these systems.

Proposed orbital ordering in MnV2O4 from first-principles calculations.

Based on density functional calculations, we propose a possible orbital ordering in MnV2O4 which consists of orbital chains running along crystallographic a and b directions with orbitals rotated alternatively by about 45 degrees within each chain. We show that the consideration of correlation effects as implemented in the local spin density approximation +U approach is crucial for a correct description of the space group symmetry. This implies that the correlation-driven orbital ordering has a strong influence on the structural transitions in this system. Inclusion of spin-orbit effects does not seem to influence the orbital ordering pattern. We further find that the proposed orbital arrangement favors a noncollinear magnetic ordering of V spins, as observed experimentally. Exchange couplings among V spins are also calculated and discussed.

Characterization of entamoeba histolytica antigens in circulating immune complexes in sera of patients with amoebiasis.

Isolated circulating immune complexes (CICs) from sera of patients with amoebiasis were characterized to determine Entamoeba histolytica antigens that participate in the disease process. In total, 116 serum samples were collected before starting anti-amoebic therapy, and their CICs were isolated by differential polyethylene glycol precipitation. The presence of amoeba-specific antigens in CICs was detected by antigen capture enzyme-linked immunosorbent assay (ELISA) and by immunoblot assay. Antigen capture ELISA showed significantly higher optical density (p < 0.001) in all patients with amoebiasis than in the normal healthy controls and patients of non-amoebic hepatic disorder. Immunoblot assay detected amoeba-specific CICs in all 18 patients (100%) with confirmed amoebic liver abscess, 28 (80%) of 35 patients with clinically-suspected amoebic liver abscess, and 18 (78.26%) of 23 patients with amoebic colitis. No patients with non-amoebic hepatic disorders and healthy control subjects had any detectable level of amoebic antigens in CICs. Immunoblot assay revealed E. histolytica antigens of relative molecular masses of 35, 56, 70, and 90 kDa present in CICs of 64 of 76 patients with amoebiasis. The 35-kDa polypeptide was observed in 52 patients (81.25%). The results of the study suggest that the 35-kDa polypeptide antigen can be a diagnostic marker in active amoebiasis.

Injection drug use in Calcutta: a potential focus for an explosive HIV epidemic.

Our objective was to assess HIV risk perceptions, risk behaviours and factors that may facilitate an increase in injection drug use in Calcutta. Focus group discussions and in-depth interviews were conducted with a selected group of drug users to delineate drug use patterns, languages used to express addictive and related experiences and acceptance of harm minimization messages. Results from these were used to develop a semi-structured interview instrument which was used to interview 111 drug users (76 IDUs and 35 non-IDUs) recruited from jails and detoxification centres. Secondary data of narcotic seizures for the last 5 years were collected from the Calcutta police department. Data on percentage of IDUs admitted to large detox facility in the city was also collected. Findings show that HIV/AIDS knowledge and risk perceptions were low; sharing of injection equipment was reported by 66% of the injectors; and condom use was insignificant. Non-availability, rising cost and increasing tolerance to heroin were cited as factors contributing to switch to injection. Ecological association was found between intensified police activity and an increase in: the amount of smokable heroin seized; increased injection of buprenorphine; and admission to detoxification centres. In addition to HIV, IDUs were also found to be prone to hepatitis B and C. The findings suggest an urgent need for developing and implementing community-based HIV prevention interventions targeting drug users in Calcutta.

Diurnal sensitivity of the testes to exogenous testosterone propionate in sexually regressed whitethroated munia Lonchura malabarica.

The testes in different groups of whitethroated munia (Lonchura malabarica) were studied following exogenous treatment of testosterone propionate (1 mg/100 g body wt./day) at 06.00 h or 14.00 h or 22.00 h for 15 consecutive days during the quiescent phase of the annual testicular cycle. The testicular conditions in each group of testosterone treated and appropriate control (administered only with the vehicle of hormone at corresponding time-points) group of birds were analyzed by the studies of paired testicular weight, seminiferous tubular diameter, spermatogenetic index, and cytology of the testicular germ cells. The results of the study revealed that exogenous testosterone was stimulatory to the testicular functions irrespective of the time of treatment. The gametokinetic response of the testes to exogenous testosterone also did not show any statistical difference among the birds treated at different hours of the day. The present study demonstrates for the first time that the responses of the testes to exogenous testosterone during the post-breeding phase in an annual testicular cycle of a wild bird does not vary with the time of administration.

Efficacy and safety of oral iron chelating agent deferiprone in beta-thalassemia and hemoglobin E-beta thalassemia.

OBJECTIVES: To assess efficacy and safety of oral iron chelating agent deferiprone (DFP) in patients with beta thalassemia and hemoglobin E-beta thalassemia. DESIGN: Non-randomized study. SETTING: Hematology Out-Patient Department. SUBJECTS: Forty-one patients of beta thalassemia and hemoglobin E-beta thalassemia. INTERVENTIONS: DFP was given to 20 patients, 10 patients of beta thalassemia and 10 with hemoglobin E-beta thalassemia; the rest were taken as controls. RESULTS: A significant fall in serum ferritin was observed in the study group along with rise in urinary iron excretion (p < 0.05). Adverse effects of DFP were nausea and vomiting (30%), significant arthropathy requiring stopping of the drug (30%), and reversible neutropenia in one patient. All these complications could be managed easily with medical supervision and no death or permanent disability was seen. CONCLUSIONS: DFP is an effective and fairly well tolerated oral iron chelating agent. The side effects that occur can be tackled easily if monitored properly.

Subpopulations of T lymphocytes in the peripheral blood, dermal lesions and lymph nodes of post kala-azar dermal leishmaniasis patients.

Distribution of different subpopulations of T cells in the dermal lesions, lymph nodes and peripheral blood of post kala-azar dermal lesihmaniasis (PKADL) patients was studied by using appropriate phenotypic markers for CD2+, CD4+ and CD8+ cells. Histopathological studies of skin lesions showed marginal to massive infiltration of mononuclear cells depending upon the duration of illness and type of lesions. Thus, while the hypopigmented patches were represented by small focal collections of lymphocytes with scanty parasites in the dermis, these were replaced at the nodular stage with massive granulomas consisting of lymphocytes, plasma cells and histiocytes with numerous amastigotes. The involvement of CD4+ and CD8+ cell types in these lesions also showed a gradual change from the appearance of a few cells of both the phenotypes in early hypopigmented type to massive accumulation of cells, primarily of CD8+ phenotype, in the granuloma of nodular type. However, the observed preponderance of CD8+ cells at the lesion site of chronic PKADL patients is in contrast to their peripheral blood CD4+/CD8+ cell ratio (1.9:1) which remained within the normal limits. Similar studies of lymph nodes from PKADL patients with lymphadenopathy revealed infiltration of the cortical areas by T cells which were more of CD8+ than CD4+ phenotypes. All these results document the involvement of CD8+ cells in leishmanoid lesions. Thus, it is likely that these cells, in association with appropriate subpopulations of CD4+ cells, play a profound role in the evolution of dermal pathology in PKADL.

Detection of immune complexes in tissues of persons infected with Entamoeba histolytica.

Immune complexes were detected by radioimmunoassay in colonic tissue from 71 to 122 patients (58%) excreting Entamoeba histolytica cysts and in liver tissue from 5 of 16 of these patients. Complexes were also found in colonic tissue from 9 of 56 other patients (16%) who were not excreting cysts.