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

Multiferroics, defined for those multifunctional materials in which two or more kinds of fundamental ferroicities coexist, have become one of the hottest topics of condensed matter physics and materials science in recent years. The coexistence of several order parameters in multiferroics brings out novel physical phenomena and offers possibilities for new device functions. The revival of research activities on multiferroics is evidenced by some novel discoveries and concepts, both experimentally and theoretically. In this review, we outline some of the progressive milestones in this stimulating field, especially for those single-phase multiferroics where magnetism and ferroelectricity coexist. First, we highlight the physical concepts of multiferroicity and the current challenges to integrate the magnetism and ferroelectricity into a single-phase system. Subsequently, we summarize various strategies used to combine the two types of order. Special attention is paid to three novel mechanisms for multiferroicity generation: (1) the ferroelectricity induced by the spin orders such as spiral and E-phase antiferromagnetic spin orders, which break the spatial inversion symmetry; (2) the ferroelectricity originating from the charge-ordered states; and (3) the ferrotoroidic system. Then, we address the elementary excitations such as electromagnons, and the application potentials of multiferroics. Finally, open questions and future research opportunities are proposed.

Multiferroicity: the coupling between magnetic and polarization orders

The ground-state structural and electronic properties of ferroelectric $\mathrm{Bi}\mathrm{Fe}{\mathrm{O}}_{3}$ are calculated using density functional theory within the local spin-density approximation (LSDA) and the $\mathrm{LSDA}+U$ method. The crystal structure is computed to be rhombohedral with space group $R3c$, and the electronic structure is found to be insulating and antiferromagnetic, both in excellent agreement with available experiments. A large ferroelectric polarization of $90--100\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{C}∕{\mathrm{cm}}^{2}$ is predicted, consistent with the large atomic displacements in the ferroelectric phase and with recent experimental reports, but differing by an order of magnitude from early experiments. One possible explanation is that the latter may have suffered from large leakage currents. However, both past and contemporary measurements are shown to be consistent with the modern theory of polarization, suggesting that the range of reported polarizations may instead correspond to distinct switching paths in structural space. Modern measurements on well-characterized bulk samples are required to confirm this interpretation.

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First-principles study of spontaneous polarization in multiferroic Bi Fe O 3

Materials research has driven the development of modern nano-electronic devices. In particular, research in magnetic thin films has revolutionized the development of spintronic devices1,2 because identifying new magnetic materials is key to better device performance and design. Van der Waals crystals retain their chemical stability and structural integrity down to the monolayer and, being atomically thin, are readily tuned by various kinds of gate modulation3,4. Recent experiments have demonstrated that it is possible to obtain two-dimensional ferromagnetic order in insulating Cr2Ge2Te6 (ref. 5) and CrI3 (ref. 6) at low temperatures. Here we develop a device fabrication technique and isolate monolayers from the layered metallic magnet Fe3GeTe2 to study magnetotransport. We find that the itinerant ferromagnetism persists in Fe3GeTe2 down to the monolayer with an out-of-plane magnetocrystalline anisotropy. The ferromagnetic transition temperature, Tc, is suppressed relative to the bulk Tc of 205 kelvin in pristine Fe3GeTe2 thin flakes. An ionic gate, however, raises Tc to room temperature, much higher than the bulk Tc. The gate-tunable room-temperature ferromagnetism in two-dimensional Fe3GeTe2 opens up opportunities for potential voltage-controlled magnetoelectronics7-11 based on atomically thin van der Waals crystals.

Gate-tunable Room-temperature Ferromagnetism in Two-dimensional Fe 3 GeTe 2

Transport properties of aliovalent-ion-doped BiFeO3 (BFO) thin films have been studied in order to identify the cause of high leakage currents Doping of 2at% Ti4+ ions increased the dc resistivity by more than three orders of magnitude In contrast, doping of 2+ ions such as Ni2+ reduced the dc resistivity by two orders of magnitude Current–voltage (I–V) characteristics indicated that the main conduction mechanism for pure and Ni2+ doped BFO was space charge limited, which was associated with the free-carriers trapped by the oxygen vacancies, whereas in the Ti4+ doped BFO, field-assisted ionic conduction was dominant

Greatly reduced leakage current and conduction mechanism in aliovalent-ion-doped BiFeO3

Edge states of various two-dimensional materials such as graphene are intrinsically spin-polarized. In other materials, electric field and charge doping are required for introducing magnetism to their edges. In this work, by using first-principles calculations, we studied the effects of transverse electric field on the edge states of the armchair blue phosphorene nanoribbon (ABPNR), and found that a transverse electric field drives the edge electronic state occupied and at the same time spin-polarized. We also doped electrons to the ABPNR and found that these additional electrons occupy and spin-polarize the electronic states of both edges of the nanoribbon.

The spin-polarized edge states of blue phosphorene nanoribbons induced by electric field and electron doping.

The high tunneling electroresistance (TER) effect, generally caused by ferroelectric (FE)-modulated barrier height or width, is essential for the applications of multiferroic tunnel junctions in data storage. It is traditionally obtained by distinct electrical screening lengths of electrodes. Interface engineering can enhance the TER effect further. In this work, taking $\mathrm{Co}\text{\ensuremath{-}}{({\mathrm{TiO}}_{2}\text{\ensuremath{-}}\mathrm{BaO})}_{N}\text{\ensuremath{-}}\mathrm{Co}$ tunnel junctions as examples, we demonstrate a distinct principle than the screening lengths for designing extraordinary TER effect. We reveal that when the interfacial FE displacement is much larger than that of the FE bulk, it will bend the barrier band near the interface violently, and the interfacial polarization direction pointing to or away from the interface determines whether the energy band rises or falls. The large interfacial Ba-O displacement and its corresponding polarization direction in $\mathrm{Co}\text{\ensuremath{-}}{\mathrm{BaTiO}}_{3}\text{\ensuremath{-}}\mathrm{Co}$ tunnel junctions can be significantly modulated by the direction of FE polarization, resulting in a metallic-insulating transition of the entire thin ${\mathrm{BaTiO}}_{3}$ barrier. For thick ${\mathrm{BaTiO}}_{3}$ barrier ($N=25$, $\ensuremath{\sim}10$ nm), the effective tunnel barrier width shifts between about 2 nm and 6.5 nm as the polarization of ${\mathrm{BaTiO}}_{3}$ switches direction, which can dramatically modulate the tunneling efficiency. This effect shed light on a novel route for enhancing TER through the interface engineering.

Giant ferroelectric modulation of barrier height and width in multiferroic tunnel junctions

The existence of ferroelectricity in {$\mathrm{BiMnO}_3$} has been a long-standing question for both experimentalists and theorists. In addition to a highly distorted bulk structure, the ionic crystal planes cause a large roughness in thin films that makes it extremely difficult to nail down the physical mechanisms underlying a possible ferroelectric-ferromagnetic phase. We approach the problem by including the substrate explicitly to study the polarization. With this model, we investigate mono-, di-, and trilayer {$\mathrm{BiMnO}_3$} thin films on $\mathrm{SrTiO}_3$ substrates. We find that thin film systems have both strong ferromagnetism and strong ferroelectricity. Substrate constraints weaken the competition between displacements induced by stereochemically active Bi-$6s^2$ lone pairs and by Jahn-Teller distortions around Mn ions found in the bulk, such that the sum of off-center displacements of Bi ions in bulk {$\mathrm{BiMnO}_3$} nearly cancel. In {$\mathrm{BiMnO}_3$} thin films, in contrast, all Bi ions displace roughly in parallel, resulting in a strongly polar structure. We also find spontaneous charge disproportionation of Mn ion pairs in {$\mathrm{BiMnO}_3$} thin films.

Yun-Peng Wang

Papers

The spin-polarized edge states of blue phosphorene nanoribbons induced by electric field and electron doping.

Giant ferroelectric modulation of barrier height and width in multiferroic tunnel junctions

Ferroelectricity in $\mathbf{BiMnO_3}$ Thin Films

Thermodynamic properties of metastable wurtzite InP nanosheets

Spin dependent resonant electron tunneling through planar graphene barriers