Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

Carbon nanotubes (CNTs) promise to advance a number of real-world technologies. Of these applications, they are particularly attractive for uses in chemical sensors for environmental and health monitoring. However, chemical sensors based on CNTs are often lacking in selectivity, and the elucidation of their sensing mechanisms remains challenging. This review is a comprehensive description of the parameters that give rise to the sensing capabilities of CNT-based sensors and the application of CNT-based devices in chemical sensing. This review begins with the discussion of the sensing mechanisms in CNT-based devices, the chemical methods of CNT functionalization, architectures of sensors, performance parameters, and theoretical models used to describe CNT sensors. It then discusses the expansive applications of CNT-based sensors to multiple areas including environmental monitoring, food and agriculture applications, biological sensors, and national security. The discussion of each analyte focuses on the strategies used to impart selectivity and the molecular interactions between the selector and the analyte. Finally, the review concludes with a brief outlook over future developments in the field of chemical sensors and their prospects for commercialization.

Carbon Nanotube Chemical Sensors

In non-centrosymmetric superconductors, where the crystal structure lacks a centre of inversion, parity is no longer a good quantum number and an electronic antisymmetric spin-orbit coupling (ASOC) is allowed to exist by symmetry. If this ASOC is sufficiently large, it has profound consequences on the superconducting state. For example, it generally leads to a superconducting pairing state which is a mixture of spin-singlet and spin-triplet components. The possibility of such novel pairing states, as well as the potential for observing a variety of unusual behaviors, led to intensive theoretical and experimental investigations. Here we review the experimental and theoretical results for superconducting systems lacking inversion symmetry. Firstly we give a conceptual overview of the key theoretical results. We then review the experimental properties of both strongly and weakly correlated bulk materials, as well as two dimensional systems. Here the focus is on evaluating the effects of ASOC on the superconducting properties and the extent to which there is evidence for singlet-triplet mixing. This is followed by a more detailed overview of theoretical aspects of non-centrosymmetric superconductivity. This includes the effects of the ASOC on the pairing symmetry and the superconducting magnetic response, magneto-electric effects, superconducting finite momentum pairing states, and the potential for non-centrosymmetric superconductors to display topological superconductivity.

https://iopscience.iop.org/article/10.1088/1361-6633/80/3/036501/pdf

Superconductivity and spin-orbit coupling in non-centrosymmetric materials: a review.

The cohesive and elastic properties of the non-oxide perovskite type superconductor MgCNi3 are calculated using the full-potential linear muffin-tin orbital method with the local density approximation as well as the generalized gradient approximation for exchange and correlation. The calculated equation of state and ground state properties (equilibrium lattice constant, bulk modulus and its pressure derivative) agree well with recent experiments. From the elastic constants the Young's modulus, shear modulus, Poisson's ratio, sound velocities and Debye temperature are obtained. By analysing the ratio between bulk modulus and shear modulus we conclude that MgCNi3 is intermediate between brittle and ductile in nature.

http://iopscience.iop.org/article/10.1088/0953-8984/19/32/326214/pdf

Elastic properties of MgCNi3 - a superconducting perovskite

A novel approach to the investigation of correlation effects in the electronic structure of magnetic crystals which takes into account a frequency dependence of the self-energy (the so-called `LDA++ approach') is developed. The fluctuation-exchange approximation is generalized to the spin-polarized multi-band case and a local version of it is proposed. As an example, we calculate the electronic quasiparticle spectrum of ferromagnetic iron. It is shown that the Fermi-liquid description of the bands near the Fermi level is reasonable, while the quasiparticle states beyond approximately the 1 eV range are strongly damped, in agreement with photoemission data. The result of the spin-polarized thermoemission experiment is explained satisfactorily. The problem of satellite structure is discussed.

https://iopscience.iop.org/article/10.1088/0953-8984/11/4/011/pdf

LDA++ approach to the electronic structure of magnets: correlation effects in iron

We propose a self-consistent approximate solution of the s - f model for describing the exchange coupling of a local moment system with a partially filled energy band. Induced electronic correlations account for the characteristic quasiparticle band effects which become manifest via striking temperature dependencies, band deformations and splittings. For weak s - f exchange interactions a `Stoner-like' spin splitting of the conduction band proportional to the f magnetization occurs. As soon as the coupling exceeds a critical value an additional spin splitting of the quasiparticle dispersion sets in, which is due to different elementary excitations. One of these appears as a repeated emission and reabsorption of a magnon by the conduction electron, resulting in an effective electron - magnon attraction. This gives rise to a polaron-like quasiparticle (a `magnetic polaron'). Other elementary processes are connected to magnon emission or absorption by the conduction electron (`scattering states'). The polarization of the conduction band due to the s - f exchange interaction J feeds back to the localized spin system leading to an indirect coupling between the spins. For weak s - f coupling the RKKY mechanism dominates , but with remarkable deviations for intermediate and strong couplings. The Curie temperature saturates with increasing J, where the saturation value is strongly dependent on the band occupation n. The oscillating behaviour of the effective exchange integral connecting the localized spins restricts ferromagnetism to special regions for n. The magnetization curve, the spin polarization of the itinerant electrons, and f - f as well as s - f spin correlation functions are worked out for a simple cubic lattice and discussed in terms of the band occupation n and the s - f exchange coupling J.

http://iopscience.iop.org/article/10.1088/0953-8984/9/6/015/pdf

Magnetism and electronic structure of a local moment ferromagnet

Results of our study on the superconducting and normal-state properties of the recently discovered superconductor ${\mathrm{MgCNi}}_{3},$ the effect of Fe and Co substitution at the Ni site and the effect of pressure are reported. It is shown that a two band model provides a consistent interpretation of the temperature dependence of the normal-state resistance and the Hall constant. Whereas band structure calculations suggest an increase in ${T}_{c}$ upon partial substitution of Ni with Fe and Co, Co substitution quenches superconductivity and Fe substitution leads to an increase followed by a decrease in ${T}_{c}.$ The observed variation of ${T}_{c}$ may be explained in terms of competition between an increase in ${T}_{c}$ due to increase in density of states and a decrease due to spin fluctuations. Based on these results, it is suggested that the spin fluctuations are weaker in Fe doped samples as compared to the Co doped ones. An initial decrease in ${T}_{c}$ (and the normal-state resistance) followed by an increase is observed on application of pressure. The decrease in ${T}_{c}$ for small applied pressures can be understood in terms of the decrease in the density of states at the Fermi level. The subsequent increase in ${T}_{c}$ with pressure is due to a lattice softening or a structural phase transition, consistent with the band structure calculations. It is conjectured that suppression of spin fluctuations by pressure may also be responsible for the observed increase in ${T}_{c}$ at higher pressures.

Normal and superconducting states of MgCNi 3 upon Fe and Co substitution and external pressure

Results of our study on the superconducting and normal state properties of the recently discovered superconductor MgCNi_3, the effect of Fe and Co substitution at the Ni site and the effect of pressure are reported. It is shown that a two band model provides a consistent interpretation of the temperature dependence of the normal state resistance and the Hall constant. Whereas band structure calculations suggest an increase in T_c upon partial substitution of Ni with Fe and Co, Co substitution quenches superconductivity and Fe substitution leads to an increase followed by a decrease in T_c. The observed variation of T_c may be explained in terms of a competition between increase in T_c due to increase in density of states and a decrease due to spin fluctuations. Based on these results, it is suggested that the spin fluctuations are weaker in Fe doped samples as compared to the Co doped ones. An initial decrease in T_c (and the normal state resistance) followed by an increase is observed on application of pressure. The decrease in T_c for small applied pressures can be understood in terms of the decrease in the density of states at the Fermi level. The subsequent increase in T_c with pressure is due to a lattice softening or a structural phase transition, consistent with the band structure calculations. It is conjectured that suppression of spin fluctuations by pressure may also be responsible for the observed increase in T_c at higher pressures.

http://arxiv.org/pdf/cond-mat/0202277.pdf

Study of Normal and Superconducting States of MgCNi_3 upon Fe and Co Substitution and External Pressure

Ab initio simulations are used to investigate the magnetic and electronic properties of freestanding Fe(1−x)M
 x
 (M = Co/Ni) nanowires. The stability of the nanowires increases with Co (Ni) addition, as seen from the increase in cohesive energy. With the addition of Co (Ni), the average magnetic moment shows a monotonic decrease, in contrast to the Slater–Pauling behavior observed in bulk Fe–Co/Ni alloys. The magnetic anisotropy energy of the nanowire is observed to change sign, from a parallel alignment of spins along the wire axis, to a perpendicular alignment with the increase of Co and Ni content. The magnetic anisotropy energy variation is seen to be correlated with the orbital moment anisotropy. The coercivity, as calculated using the Jacobs–Bean model is observed to decrease with Co (Ni) addition to the nanowire.

/pdf/compositional-variation-of-magnetic-moment-magnetic-9v6j4lrsr8.pdf

Compositional variation of magnetic moment, magnetic anisotropy energy and coercivity in Fe(1−x)M x (M = Co/Ni) nanowires: an ab initio study

Magnetism, electronic structure and half-metallic property of transition metal (V, Cr, Mn, Fe, Co) substituted Zn3P2 dilute magnetic semiconductors: An ab-initio study

S. Mathi Jaya

Papers

Magnetism and electronic structure of a local moment ferromagnet

Normal and superconducting states of MgCNi 3 upon Fe and Co substitution and external pressure

Study of Normal and Superconducting States of MgCNi_3 upon Fe and Co Substitution and External Pressure

Compositional variation of magnetic moment, magnetic anisotropy energy and coercivity in Fe(1−x)M x (M = Co/Ni) nanowires: an ab initio study

Magnetism, electronic structure and half-metallic property of transition metal (V, Cr, Mn, Fe, Co) substituted Zn3P2 dilute magnetic semiconductors: An ab-initio study