Hua-Hua Fu

Spin Seebeck Effect and Thermal Colossal Magnetoresistance in Graphene Nanoribbon Heterojunction

Spin caloritronics devices are very important for future development of low-power-consumption technology. We propose a new spin caloritronics device based on zigzag graphene nanoribbon (ZGNR), which is a heterojunction consisting of single-hydrogen-terminated ZGNR (ZGNR-H) and double-hydrogen-terminated ZGNR (ZGNR-H2). We predict that spin-up and spin-down currents flowing in opposite directions can be induced by temperature difference instead of external electrical bias. The thermal spin-up current is considerably large and greatly improved compared with previous work in graphene. Moreover, the thermal colossal magnetoresistance is obtained in our research, which could be used to fabricate highly-efficient spin caloritronics MR devices.

Spin-dependent Seebeck Effect, Thermal Colossal Magnetoresistance and Negative Differential Thermoelectric Resistance in Zigzag Silicene Nanoribbon Heterojunciton

Spin-dependent Seebeck effect (SDSE) is one of hot topics in spin caloritronics, which examine the relationships between spin and heat transport in materials. Meanwhile, it is still a huge challenge to obtain thermally induced spin current nearly without thermal electron current. Here, we construct a hydrogen-terminated zigzag silicene nanoribbon heterojunction, and find that by applying a temperature difference between the source and the drain, spin-up and spin-down currents are generated and flow in opposite directions with nearly equal magnitudes, indicating that the thermal spin current dominates the carrier transport while the thermal electron current is much suppressed. By modulating the temperature, a pure thermal spin current can be achieved. Moreover, a thermoelectric rectifier and a negative differential thermoelectric resistance can be obtained in the thermal electron current. Through the analysis of the spin-dependent transport characteristics, a phase diagram containing various spin caloritronic phenomena is provided. In addition, a thermal magnetoresistance, which can reach infinity, is also obtained. Our results put forward an effective route to obtain a spin caloritronic material which can be applied in future low-power-consumption technology.

Nearly Perfect Spin Filter, Spin Valve and Negative Differential Resistance Effects in a Fe 4 -based Single-molecule Junction

The spin-polarized transport in a single-molecule magnet Fe4 sandwiched between two gold electrodes is studied, using nonequilibrium Greens functions in combination with the density-functional theory. We predict that the device possesses spin filter effect (SFE), spin valve effect (SVE), and negative differential resistance (NDR) behavior. Moreover, we also find that the appropriate chemical ligand, coupling the single molecule to leads, is a key factor for manipulating spin-dependent transport. The device containing the methyl ligand behaves as a nearly perfect spin filter with efficiency approaching 100%, and the transport is dominated by transmission through the Fe4 metal center. However, in the case of phenyl ligand, the spin filter effect seems to be reduced, but the spin valve effect is significantly enhanced with a large magnetoresistance ratio, reaching 1800%. This may be attributed to the blocking effect of the phenyl ligands in mediating transport. Our findings suggest that such a multifunctional molecular device, possessing SVE, NDR and high SFE simultaneously, would be an excellent candidate for spintronics of molecular devices.

The integrated spintronic functionalities of an individual high-spin state spin-crossover molecule between graphene nanoribbon electrodes

The spin-polarized transport properties of a high-spin-state spin-crossover molecular junction with zigzag-edge graphene nanoribbon electrodes have been studied using density functional theory combined with the nonequilibrium Greens-function formalism. The molecular junction presents integrated spintronic functionalities such as negative differential resistance behavior, spin filter and the spin rectifying effect, associated with the giant magnetoresistance effect by tuning the external magnetic field. Furthermore, the transport properties are almost unaffected by the electrode temperature. The microscopic mechanism of these functionalities is discussed. These results represent a step toward multifunctional molecular spintronic devices on the level of the individual spin-crossover molecule.

Spin–lattice coupling driven ferroelectric transition in one-dimensional organic quantum magnets

We propose a theoretical quantum spin model for a one-dimensional (1D) spin-Peierls (SP) system to describe its ferroelectricity driven by the spin–lattice coupling, and further investigate the ferroelectric (FE) SP transition for a known 1D organic donor–acceptor charge transfer compound, which was experimentally proved to show ferroelectricity, by means of many-body Greens function theory. It is found that the transition temperature (Tc), polarization (P) and dielectric constant are in agreement with experimental results. Meanwhile, it is shown that the two-site thermal entanglement entropy is a good indicator of a FE transition. In addition, the potential magnetoelectric behavior is taken into account. On the one hand, when the magnetic field is turned on, it makes P and Tc decrease, and drives the FE transition from the second order to the first order. Nevertheless, the FE dimerized-singlet state may collapse for high enough magnetic fields, leading to the restoration of paraelectric uniform stack of donor–acceptor. On the other hand, as the electric field is applied along the chain, the FE phase is evidenced by the electric polarization (P)–field (E) hysteresis loops. It is also found that the magnetization M decreases but the polarization P can be enhanced with increasing electric field, which makes the FE transition exhibit crossover behavior, since the electrostatic energy predominates over the spin–lattice coupling.

A theoretical model for anisotropic multiferroics

We propose a theoretical model for anisotropic multiferroics, which are one-dimensional charge transfer magnets. By means of Greens function theory, ferroelectric and magnetic properties are studied. It is found that the anisotropy not only plays an important role on the ferroelectric phase transition but also enhances the ferroelectric polarization. Under different anisotropy, the phase diagram and temperature dependence of the magnetic susceptibility and dielectric constant are also presented. It reveals that the transition temperature increases as anisotropy ascends, which is attributed to the energy gap. These results put forward a way to enhance the ferroelectric phase transition temperature.

Multiple thermal spin transport performances of graphene nanoribbon heterojuction co-doped with Nitrogen and Boron

Graphene nanoribbon is a popular material in spintronics owing to its unique electronic properties. Here, we propose a novel spin caloritronics device based on zigzag graphene nanoribbon (ZGNR), which is a heterojunction consisting of a pure single-hydrogen-terminated ZGNR and one doped with nitrogen and boron. Using the density functional theory combined with the non-equilibrium Green’s function, we investigate the thermal spin transport properties of the heterojunction under different magnetic configurations only by a temperature gradient without an external gate or bias voltage. Our results indicate that thermally-induced spin polarized currents can be tuned by switching the magnetic configurations, resulting in a perfect thermal colossal magnetoresistance effect. The heterojunctions with different magnetic configurations exhibit a variety of excellent transport characteristics, including the spin-Seebeck effect, the spin-filtering effect, the temperature switching effect, the negative differential thermal resistance effect and the spin-Seebeck diode feature, which makes the heterojunction a promising candidate for high-efficiently multifunctional spin caloritronic applications.

Field-Controlled Luttinger Liquid and Possible Crossover into Spin Liquid in Strong-Rail Ladder Systems

The thermodynamics and transport properties of strong-rail ladder systems are investigated by means of Greens function theory. It is shown that the magnetic behavior clearly manifests a typical antiferromagnetism with gapped or gapless low-lying excitations, which is in agreement with the experimental results. In addition, the temperature-field-induced phase diagram is explored, and we demonstrate a Luttinger liquid behavior in the window h(c) (marking the ending of the M=0 plateau)<h<h(s) (saturation magnetic field) within a narrow range of temperature. The spin liquid phase is uncovered for h<h(c) upon cooling down to zero temperature. It is also shown that the rung entanglement entropy is a good indicator for detecting the field-driven quantum criticality. Meanwhile, the magnetic susceptibility, the specific heat, and the thermal (spin) Drude weights are calculated to characterize the plentiful quantum phases, in which the thermal insulating and conducting behaviors can be controlled by magnetic fields.

Approach to control polarization and magnetic properties for multiferroics with Dzyaloshinskii-Moriya interaction

We propose a theoretical model for one-dimensional (charge transfer magnets with Dzyaloshinskii-Moriya (DM) interaction. By using Greens function theory, we have studied the effect of DM interaction on ferroelectric and magnetic properties, where ferroelectricity is induced through symmetric mechanism. It is shown that the uniform DM interaction reduces the polarization and makes the magnetization plateau narrow down. Moreover, the transition temperature descends as the uniform DM interaction ascends, which is attributed to the decrease of the energy gap. In addition, the staggered DM interaction, which is related to intersite distance, is also discussed. It is also found that there exists a critical point, above or below which the staggered DM interaction plays different roles on the polarization, transition temperature, and magnetic behavior. As the staggered DM interaction is larger, it enhances the polarization and transition temperature and meanwhile widens the magnetization plateau, otherwise it re...