Hua Hao

Spin-filtering transport and switching effect of MnCu single-molecule magnet

Electron transport of a single-molecule magnet (SMM) device has been investigated using the first-principles calculations. The SMM based device is constructed by a SMM MnCu [MnCuCl(5-Br-sap)2(MeOH)] bridged between semi-infinite Au(100) electrodes with thiol groups connecting the molecule and the gold electrodes. Our results exhibit crucial features of spin filtering and Kondo resonance. The spin filtering remains robust, whereas the Kondo resonance highly depends on the contact geometry. Specifically, this Kondo resonance can be switched on or off by changing the contact distance. The mechanisms of these features are formulated in details.

Orbital symmetry induced conductance switching in a graphene nanoribbon heterojunction with different edge hydrogenations

First principles calculations are performed to investigate the electron transport through a zigzag-edged graphene nanoribbon (ZGNR) heterojunction constructed by connecting a monohydrogenated ZGNR and a dihydrogenated ZGNR and its response to external magnetic fields. It is found that the heterojunction can be switched between a conducting state and an insulating state by tuning the magnetic fields. It arises from the matching or mismatching between the π or π* states of the two ribbons under different magnetic fields. This mechanism of conductance switching by tuning the orbital symmetry can be considered in the future design of graphene based electronic devices.

Electronic structures and transverse electrical field effects in folded zigzag-edged graphene nanoribbons

Folded graphene nanoribbons were observed and their unusual transport properties were reported recently. In this work, the electronic structures and the effects of external transverse electrical field in the folded zigzag-edged graphene nanoribbons (ZGNRs) are investigated by first-principles calculations. It is found that, the folding does not break the semiconductivity, the antiferromagnetically coupled edge states and edge magnetism characteristic of ZGNRs. Interestingly, when external electrical field is applied, the ribbons can be turned to metal or half-metal, depending on the direction of the field. The robustness of the half-metallicity and the edge magnetism to the electrical field is also discussed.

Tuning the electron transport properties of boron- nitride nanoribbons with electron and hole doping

By first principles calculations based on the density functional theory and nonequilibrium Green’s function technique, we have studied the electronic and transport properties of C-doped zigzag-edged boron-nitride nanoribbons (ZBNNRs). Due to the two sub-lattices in boron-nitride nanoribbons (BNNRs), C substitutions at B sites and N sites naturally provide ways for electron doping and hole doping. Different combinations of the C chain substitution schemes are utilized to tune the electron transport of nano junctions constructed with ZBNNRs. It is found that, either substitution for B or N by C, in symmetric doping, the junction always behaves as a good conductor. However, in the asymmetric doping, the performance of the junctions highly depends on the positions of the C chain. When the C atoms are doped at opposite edges on the two sides of the junction, there is no current across the junction although dopings at B site and N site can both transform a BNNR from an insulator into a metal. Interestingly, when the doping sites are moved to the middle of the ribbons, the junctions conduct very well and negative differential resistance (NDR) is observed due to the special alignment of the energy bands of the two leads.

Room temperature memory device using single-molecule magnets

To make memory devices based on an individual single-molecule magnet work far above the blocking temperature, we propose a new route, where the information is contained in the charge state of the molecule, and it works through charging and discharging the molecule by applying gate voltages. Here, a model device built on a single-molecule magnet, Fe4, is taken as an example to exhibit the validity of our proposed route. Ab initio calculations show that the two different charge states with a moderately large energy shift of 1.2 eV are responsible for the low and high conductances in this device: one corresponds to the neutral state of the molecule, and the other to its anionic state. Moreover, the transition from the neutral state to the anionic state is accompanied by a giant increase of nearly two orders of magnitude in the conductance. Additionally, the low and high conductances before and after charging the molecule are hardly dependent on the different spin configurations of the Fe4 molecule, which indicates that the performance of the Fe4 memory device is probably preserved even at room temperature.

Realizing stable fully spin polarized transport in SiC nanoribbons with dopant

Intrinsic half-metallicity recently reported in zigzag edged SiC nanoribbons is basically undetectable due to negligible energy difference between the antiferromagnetic (AFM) and ferromagnetic (FM) configurations. In this Letter, by density functional theory calculations, we demonstrate a scheme of N doping at the carbon edge to selectively close the edge state channel at this edge and achieve 100% spin filtering, no matter whether it is in an AFM state or FM state. This turns SiC nanoribbon into a promising material for obtaining stable and completely spin polarized transport and may find application in spintronic devices.

Spin-flip effect on transport properties of a Mn3 molecule

Electron transport through a single-molecule magnet [NEt4]3[Mn3Zn2(salox)3O(N3)6Cl2] is investigated by spin-polarized density functional theory combined with the Keldysh nonequilibrium Green’s function technique. Our study demonstrates that spin-filtering effect and negative differential resistance exist in the ground state of this molecule. When the magnetic state of the molecule is changed from its ground state to the spin-flip state, substantial changes are induced not only in energy levels of the molecule, but also in the coupling of molecular states with eigenstates of Ag(100) nano-electrodes, which lead to the disappearance of spin-filtering effect and negative differential resistance.

Gate-induced switching in single-molecule magnet MnIIICuII

Gate voltage effect on electronic transport through the smallest single-molecule magnet (SMM) MnCu [MnIIICuIICl(5-Br-sap)2(MeOH)] sandwiched between Au(100) electrodes is investigated by spin-polarized density functional theory calculations combined with the Keldysh nonequilibrium Green’s technique. Our study demonstrates that a certain gate voltage can induce a switching of the conductance in the equilibrium state. Under a finite bias voltage, negative differential resistance is observed in this system and can be modulated by tuning the gate voltage. More interestingly, current rectification can be achieved at a certain negative gate voltage. These effects can be understood by the responses of the benzene rings and the magnetic core to an external electrical field.

Thermal spin current in zigzag silicene nanoribbons with sp2–sp3 edges

Using first-principles calculations combined with non-equilibrium Greens function method, we study thermal spin transport of zigzag silicene nanoribbons (ZSiNRs) with unsymmetrical sp2–sp3 edges under a temperature gradient but no bias. Both in the linear and non-linear response regimes, we have opposite flow directions for different spins, which leads unambiguously to spin current. Most important is that pure spin current can be achieved and basically no tuning of the chemical potential μ is needed since the neutral point is very close to μ = 0 (the chemical potential located at the Fermi level) and this fact holds for a very large temperature range studied (110 ≤ TL ≤ 300 K). The direction of charge current induced by a temperature gradient can be easily reversed by tuning the chemical potential, while the spin current is almost unchanged in the same process, indicating that the spin current is robust and stable. In addition, both spin current and charge current present a thermoelectric diode behavior for TL ≤ 200 K in the nonlinear response regime. These findings suggest that the unsymmetrically sp2–sp3 terminated ZSiNRs are promising materials for spin caloritronic devices.

Electron transport enhanced by electrode surface reconstruction: a case study of C60-based molecular junctions

The effects of surface reconstruction on electron transport of two monolayers of C60 sandwiched between two Cu(111) bulk electrodes have been investigated by density functional theory (DFT) calculations combined with a nonequilibrium Greens function technique. Two markedly different electrode surface structures have been considered, which have been obtained in previous experimental works: one with an unreconstructed perfect surface and the other with a surface reconstruction with a 7-atom-missing hole per (4 × 4) Cu(111) cell. The results indicate that surface reconstruction induces an increase of more than 50% in the current at low bias. Molecular-orbital projected density of states (MO-PDOS) analysis reveals that the change in transport properties originates from the enhanced orbital-dependent electrode–molecule coupling and the increased charge transfer from electrodes to molecules. Our current work suggests that surface reconstruction could play a very important role in the electron transport properties; and hence surface reconstruction (or more generally realistic atomic contact details) should be taken into full consideration in the simulation and design of molecular devices, especially when it is expected to reproduce computationally the experimental observations.