Origins of multi-sublattice magnetism and superexchange interactions in double-double perovskite CaMnCrSbO6.

The multi-sublattice magnetism and electronic structure in double-double perovskite compound CaMnCrSbO6is explored using density functional theory. The bulk magnetization and neutron diffraction suggest a ferrimagnetic order (TC∼49 K) between between Mn2+and Cr3+spins. Due to the non-equivalent Mn atoms (labelled as Mn(1) and Mn(2) which have tetrahedral and planar oxygen coordinations, respectively) and the Cr atom in the centre of distorted oxygen octahedron in the unit cell, the exchange interactions are more complex than that expected from a two sublattice magnetic system. The separations between the on-site energies of thed-orbitals of Mn(1), Mn(2) and Cr obtained from Wannier function analysis are in agreement with their expected crystal field splitting. While the DOS obtained from non spin-polarized calculations show a metallic character, starting from HubbardU = 0 eV the spin-polarized electronic structure calculations yield a ferrimagnetic insulating ground state. The band gap increases withUeff(U - J), thereby showing a Mott-Hubbard nature of the system. The inclusion of anti-site disorder in the calculations show decrease in band-gap and also reduction in the total magnetic moment. Due to the ∼90∘superexchange, nearest neighbour exchange constants obtained from DFT are an order of magnitude smaller than those reported for various magnetic perovskite and double-perovskite compounds. The Mn(1)-O-Mn(2) (out of plane and in-plane), Mn(1)-O-Cr and Mn(2)-O-Cr superexchange interactions are found to be anti-ferromagnetic, while the Cr-O-O-Cr super-superexchange is found to be ferromagnetic. The Mn(2)-O-Cr superexchange is weaker than the Mn(1)-O-Cr super-exchange, thus effectively resulting in ferrimagnetism. From a simple 3-site Hubbard model, we derived expressions for the antiferromagnetic superexchange strengthJAFMand also for the weaker ferromagneticJFM. The relative strengths ofJAFMfor the various superexchange interactions are in agreement with those obtained from DFT. The expression for Cr-O-O-Cr super-superexchange strength (J~SS), which has been derived considering a 4-site Hubbard model, predicts a ferromagnetic exchange in agreement with DFT. Finally, our mean field calculations reveal that assuming a set of four magnetic sub-lattices for Mn2+spins and a single magnetic sublattice for Cr3+spins yields a much improvedTC, while a simple two magnetic sublattice model yields a much higherTC.

Spin and current transport in the robust half-metallic magnetc-CoFeGe.

Spintronics is an emerging form of electronics based on the electrons' spin degree of freedom for which materials with robust half-metallic ferromagnet character are very attractive. Here we determine the structural stability, electronic, magnetic, and mechanical properties of the half-Heusler (hH) compound CoFeGe, in particular also in its cubic form. The first-principles calculations suggest that the electronic structure is robust with 100% spin polarization at the Fermi level under hydrostatic pressure and uni-axial strain. Both the longitudinal and Hall current polarization are calculated and the longitudinal current polarization (PL) is found to be>99%and extremely robust under uniform pressure and uni-axial strain. The anomalous Hall conductivity and spin Hall conductivity of hH cubic CoFeGe (c-CoFeGe) are found to be∼-100S cm-1and∼39 ℏ/eS cm-1, respectively. Moreover, the Curie temperature of the alloy is calculated to be ∼524 K with a 3µBmagnetic moment. Lastly, the calculated mechanical properties indicate thatc-CoFeGe is ductile and mechanically stable with a bulk modulus of ≈154 GPa. Overall, this analysis reveals that cubic CoFeGe is a robust half-metallic ferromagnet and an interesting material for spintronic applications.

Thermoelectric transport properties of XAgP (X = Sr and Ba) from first principles.

For thermoelectric applications those materials are of interest that have significant power factor (PF) and low lattice thermal conductivity,κL. Here we theoretically exploreκLof two novel materials SrAgP and BaAgP using linearized Boltzmann transport equation with a single-mode relaxation time approach. We estimate the figure of meritzTby employingab-initiocalculations based on density functional theory and semiclassical Boltzmann transport theory. It is observed that at room temperature SrAgP exhibits slightly higher lattice thermal conductivity than BaAgP, which is mainly due to the large phonon group velocity. The relaxation time derived from deformation potential theory indicates a higherp-type PF for SrAgP compared to BaAgP over the entire temperature range. This provides an estimate for the figure of merit for the two materials. The low lattice thermal conductivity and higher PF make SrAgP a more promising thermoelectric material.

Impact of the Channel Thickness on the Photoresponse of Black Arsenic Mid-Infrared Photodetectors.

Recently explored black arsenic is a layered two-dimensional low-symmetry semiconducting material that, owing to its inherent narrow bandgap (∼0.31 eV) in its bulk form, is attractive for mid-infrared optoelectronics. Several studies have been conducted on its structural, charge-transport, and thermal properties for implementation in nanoelectronics. Herein, the thickness-dependent optoelectronic performance of black arsenic devices for mid-infrared wavelengths (2.0-4.0 µm) is investigated. The device was fabricated over an hBN/SiO2/Si substrate using mechanical exfoliation of black arsenic. It is observed that the optoelectronic properties of the devices vary significantly with the thickness of the black arsenic channel of the devices. A peak photoresponsivity of 244 A/W was achieved at 3.00 µm for a 60 nm-thick black arsenic channel. However, the maximum detectivity of 6.14 × 109 Jones was found for a lower thickness (∼25 nm) of black arsenic, along with an excellent (i.e., the least) noise-equivalent power of ∼89 fW/Hz1/2. Our findings reveal that the optoelectronic properties of black arsenic are excellent and can be tuned through thickness control. The promising results suggest the considerable potential of black arsenic in future opto- and nanoelectronic devices.

Structural, electronic and magnetic properties of weakly correlated metal Sr2CrTiO6: a first principles study.

Structural, electronic and magnetic behaviour of a less-explored material Sr2CrTiO6has been investigated usingabinitiocalculations with generalized gradient approximation (GGA) and GGA +Umethods, whereUis the Hubbard parameter. For each of the three possible Cr/Ti cationic arrangements in the unit cell, considered in this work, the non-magnetic band structure shows distinct traits with significant flat-band regions leading to larget2gdensity of states around the Fermi energy. The Cr4+ion in Sr2CrTiO6, which is ad2system, shows a reverse splitting of thet2gorbitals. The calculated hopping matrix contains non-zero off-diagonal elements between thedxzanddyzorbitals, while thedxyorbitals remain largely unmixed. A minimal tight binding model successfully reproduces the sixt2gbands around the Fermi energy. The Fermi surface shows a two-dimensional nesting feature for the layered arrangement of Cr and Ti atoms. Fixed spin moment studies suggest that the magnetism in this compound cannot be explained by Stoner's criterion of an itinerant band ferromagnet. In the absence of HubbardUterm, the ground state is a half-metallic ferromagnet. Calculations for the anti-ferromagnetic spin arrangement show re-arrangement of orbital character resulting in (a) narrowdxz/dyzbands and sharp peaks in the density of states at the Fermi energy and (b) highly dispersivedxybands with a broader density of states around the Fermi energy. The metallicity persists even with increasingUfor both the spin arrangements, thus suggesting that Sr2CrTiO6belongs to the class of weakly correlated metals, while the shifting of O 2pstates towards the Fermi energy withUindicates a negative charge-transfer character in Sr2CrTiO6.

An anomalous interlayer exciton in MoS2.

The few layer transition metal dichalcogenides are two dimensional materials that have an intrinsic gap of the order of ≈2 eV. The reduced screening in two dimensions implies a rich excitonic physics and, as a consequence, many potential applications in the field of opto-electronics. Here we report that a layer perpendicular electric field, by which the gap size in these materials can be efficiently controlled, generates an anomalous inter-layer exciton whose binding energy is independent of the gap size. We show this originates from the rich gap control and screening physics of TMDCs in a bilayer geometry: gating the bilayer acts on one hand to increase intra-layer screening by reducing the gap and, on the other hand, to decrease the inter-layer screening by field induced charge depletion. This constancy of binding energy is both a striking exception to the universal reduction in binding energy with gap size that all materials are believed to follow, as well as evidence of a degree of control over inter-layer excitons not found in their well studied intra-layer counterparts.

The nature of itineracy in CoV2O4: a first-principles study.

Inspired by recent experiments, we have theoretically explored the nature of itineracy in CoV2O4 under pressure and investigated, using first-principles density functional theory calculations, whether it has any magnetic and orbital ordering. Our calculations indicate that there could be two possible routes for obtaining the experimentally observed pressure induced metallicity in this system. One is via the spin­orbit interaction coupled with Coulomb correlation, which can take the system from a semiconducting state at ambient pressure to a metallic state under high pressure. The other mechanism, as indicated by our GGA + U calculations, is based on the presence of two kinds of electrons in the system: localized and itinerant. An effective Falicov­Kimball model could then possibly explain the observed insulator to metal transition. Comparison of the two scenarios with existing experimental observations leads us to believe that the second scenario offers a better explanation for the mechanism of the insulator to metal transition in CoV2O4 under pressure.

Temperature tuned defect induced magnetism in reduced graphene oxide.

The existence of ferromagnetism in the wonder material graphene has opened up the path for many future spintronics and memory applications. But simultaneously it is very important to understand the variation of these properties with temperature in regards to the device applications. Here we observed defect induced ferromagnetism in chemically reduced graphene and the effect of temperature on it. Several theoretical studies have proved that the main cause of ferromagnetism in graphene is due to various defects. The observed results established that these defects can be mended by treating the samples at elevated temperatures but sacrificing the ferromagnetism simultaneously. Hence, temperature plays a crucial role in controlling the magnetism as well as the defects in graphene. In this study we revealed that at 600 °C the self-repair mechanism helps the defects to mend but resulting in the decrement of magnetization and providing a good quality graphene with less defects.

Effect of spin-orbit coupling on magnetic and orbital order in MgV2O4.

Recent measurements on MgV(2)O(4) single crystals have reignited the debate on the role of spin-orbit (SO) coupling in dictating the orbital order in vanadium spinel systems. Density functional theory calculations were performed using the full-potential linearized augmented-plane-wave method within the local spin density approximation (LSDA), Coulomb correlated LSDA (i.e. LSDA + U), and with SO interaction (LSDA + U + SO) to study the magnetic and orbital ordering in the low temperature phase of MgV(2)O(4). It is observed that, in the experimental antiferromagnetic phase, the spin-orbit coupling affects the orbital order differently in alternate V-atom chains along the c-axis. This observation is consistent with the experimental prediction that the effect of spin-orbit coupling is intermediate between those in the cases of ZnV(2)O(4) and MnV(2)O(4).

Full potential results on the magneto-optical properties of the Heusler compounds Co(2)FeX (X = Al, Ga, Si and Ge).

We have calculated the magneto-optical (MO) properties of Co(2)FeX (X =  Al, Ga, Si and Ge) Heusler compounds using the full potential linearized augmented plane wave (FPLAPW) method as implemented in the WIEN2K code using the local spin density approximation (LSDA) and also by using the generalized gradient approximation (GGA) for the electronic exchange and correlation. In all the compounds, Kerr rotation θ(K) has a strong minimum near 2 eV, the value of |θ(K)| corresponding to this minimum being almost as large as in pure Co-Fe compounds. The calculated MO spectra help to identify the features of the experimental spectra. A comparison of the results shows that the Kerr spectrum is quite similar from both LSDA and GGA but the latter gives better agreement with experiment. Moreover, we find that inclusion of correlation effects using GGA+U removes the discrepancy in magnetic moment of Co(2)FeX (X =  Si, Ge) though it has an insignificant effect on the Kerr spectra.

First-principles calculations of electronic and optical properties of Fe(3-x)V(x)Al (x = 0-3) compounds.

We present calculations of the electronic and optical properties for Fe(3-x)V(x)Al (x = 0-3) compounds with the aim of studying the effect of increasing the concentration of vanadium. We also report calculations of the magneto-optical (MO) Kerr effect, which depends on the off-diagonal components of the optical conductivity (OC), for the magnetic compounds Fe(3)Al and V(2)FeAl. This systematic analysis of the electronic structure and optical properties of Fe(3-x)V(x)Al compounds has been carried out using the full potential linearized augmented plane wave (FPLAPW) method within the generalized gradient approximation (GGA). The results, presented at equilibrium lattice constant, show that the compounds Fe(3)Al, V(2)FeAl and V(3)Al are metallic whereas the Fermi level lies at the 'pseudogap' in Fe(2)VAl. The OC spectra exhibit one main peak around 2.5 eV for all the compounds. The calculated OC spectra for Fe(2)VAl and Fe(3)Al shows the same features as in the experimental spectra, resulting a good qualitative agreement: however, the magnitude is overestimated. The blueshifting of structures in Fe(2)VAl is discussed. Our results show that the two compounds exhibit a contrast in terms of intraband transitions which help to achieve a good agreement with experiment in Fe(3)Al and remain at low key in Fe(2)VAl. The peaks in the optical conductivity can be explained as arising out of the transitions from Al p to Fe 3d and/or V 3d in these compounds. The calculations for MO spectra of the compounds Fe(3)Al and V(2)FeAl show small Kerr rotations.

How can we make stable linear monoatomic chains? Gold-cesium binary subnanowires as an example of a charge-transfer-driven approach to alloying.

On the basis of first-principles calculations of clusters and one dimensional infinitely long subnanowires of the binary systems, we find that alkali-noble metal alloy wires show better linearity and stability than either pure alkali metal or noble metal wires. The enhanced alternating charge buildup on atoms by charge transfer helps the atoms line up straight. The cesium doped gold wires showing significant charge transfer from cesium to gold can be stabilized as linear or circular monoatomic chains.

Comparative study of optical and magneto-optical properties of GdFe(2) and GdCo(2).

We report calculations of the optical and magneto-optical properties of GdFe(2) and GdCo(2) using the full-potential linear-augmented-plane-wave method. Calculations with the Coulomb corrected local spin density approximation (LSDA+U) give a better representation of the band structure, density of states and magnetic moments compared to LSDA alone. However, both LSDA and LSDA+U approximations give fairly good agreement with experiment for the diagonal optical conductivity. The calculated results for GdCo(2) are in better agreement with the 'oxide corrected' data. Our results suggest that the data for GdFe(2) are most likely influenced by surface oxidation, due to the high reactivity of these compounds. For the much smaller off-diagonal components and Kerr rotation, LSDA results are better than the LSDA+U results, particularly in the energy range 0-3 eV. We show that the unphysical negative ellipticity values are taken care of by the use of stronger relaxation, which also improves the qualitative agreement with experimental data. Overall we have obtained a fair agreement with the experimental data for both optical and magneto-optical properties. We feel that measurements over a larger energy range are required for facilitating an exhaustive and decisive comparison and also to strengthen the bond between theory and experiments.

Magic structures and quantum conductance of silver nanowires.

We investigate the pathway of thinning process for transient [110] nanowires (NWs) of Ag. The result is in good agreement with experimental observations. An unambiguous identification of the structure of a NW requires at least two views along different directions. In the cases where two views of different NW structures are practically the same for very thin NWs which pose experimental difficulty due to small signal-to-noise ratio, our theoretical analysis helps distinguish these structures. On the basis of conductance (G) calculations vis-á-vis the structure of transient NWs, the puzzling experimental observation of fractionally quantized G values is explained by considering the existence of mixed structures for thin wires.