Spin transport and relaxation in graphene

We review our recent work on spin injection, transport and relaxation in graphene. The spin injection and transport in single layer graphene (SLG) were investigated using nonlocal magnetoresistance (MR) measurements. Spin injection was performed using either transparent contacts (Co/SLG) or tunneling contacts (Co/MgO/SLG). With tunneling contacts, the nonlocal MR was increased by a factor of ∼1000 and the spin injection/detection efficiency was greatly enhanced from ∼1% (transparent contacts) to ∼30%. Spin relaxation was investigated on graphene spin valves using nonlocal Hanle measurements. For transparent contacts, the spin lifetime was in the range of 50-100 ps. The effects of surface chemical doping showed that for spin lifetimes in the order of 100 ps, charged impurity scattering (Au) was not the dominant mechanism for spin relaxation. While using tunneling contacts to suppress the contact-induced spin relaxation, we observed the spin lifetimes as long as 771 ps at room temperature, 1.2 ns at 4 K in SLG, and 6.2 ns at 20 K in bilayer graphene (BLG). Furthermore, contrasting spin relaxation behaviors were observed in SLG and BLG. We found that Elliot-Yafet spin relaxation dominated in SLG at low temperatures whereas Dyakonov-Perel spin relaxation dominated in BLG at low temperatures. Gate tunable spin transport was studied using the SLG property of gate tunable conductivity and incorporating different types of contacts (transparent and tunneling contacts). Consistent with theoretical predictions, the nonlocal MR was proportional to the SLG conductivity for transparent contacts and varied inversely with the SLG conductivity for tunneling contacts. Finally, bipolar spin transport in SLG was studied and an electron-hole asymmetry was observed for SLG spin valves with transparent contacts, in which nonlocal MR was roughly independent of DC bias current for electrons, but varied significantly with DC bias current for holes. These results are very important for the use of graphene for spin-based logic and information storage applications.     

Spin dynamics and quantum transport in magnetic semiconductor quantum structures

Quantum structures derived from magnetic semiconductors serve as a powerful arena within which to study the interplay between quantum electronics and thin film magnetism. In particular, the semiconductor aspects of these flexible systems allow direct access to the electronic spin degrees of freedom using both magneto-optical as well as magneto-transport probes. Here we provide an overview of recent developments in the experimental study of II–VI magnetic semiconductor quantum structures, with particular emphasis on the dynamical behavior of field-tunable electronic spin states and spin-dependent quantum transport.     

Vacancy cluster-limited electronic transport in metallic carbon nanotube

We investigate the electronic properties of metallic (7,7) carbon nanotubes (CNT) in the presence of a variety of tetra- and hexa-vacancy defects, by using the first principles density functional theory (DFT) combined with the non-equilibrium Green’s function technique. From the view point of energetic stability large vacancies tend to split into pentagon and heptagon (5-7) defects. However, this does not preclude the presence of “holes” in the carbon nanotube by the nanoelectronic lithography technique. We show that the states linked to large vacancies hybridize with the extended states of the nanotubes to modify their band structure. As a consequence, the hole-like defects in the CNT lead to more prominent electronic transport compared to the situation in the defective CNT consisting of pentagon-heptagon pair defects. Our study suggests the possibility to improve the electronic properties of a defective carbon nanotube via morphological modifications induced by irradiation techniques.     

Ballistic transport in extended Datta-Das spin field effect transistors

The model of the Datta-Das spin field effect transistor [S. Datta, B. Das, Appl. Phys. Lett. 56 (1990) 665] is extended in several respects: (1) the Rashba effect and Dresselhaus effect coexist; (2) the incoming and outgoing leads are both ferromagnetic; (3) the interfacial scattering and band mismatch are taken into account. By using the Griffith boundary conditions, the transmission coefficients and, thus, the Landauer-Büttiker conductance are obtained analytically. The transmission probability and conductance of the spin field effect transistor are studied in detail.     

La NMR and As NQR study of the As-based filled skutterudite LaOs4As12

We report experimental results of nuclear magnetic resonance (NMR) at the La site and nuclear quadrupole resonance (NQR) at the As site in the normal state of the superconducting compound LaOs₄As₁₂. Measurements have been performed on powder sample obtained from high quality single crystals. The temperature dependences of the nuclear spin-lattice relaxation rates, 1/T₁, of ⁷⁵As and ¹³⁹La nuclei were measured. No scaling between them was found indicating a local character of relaxation processes. The relaxation of ⁷⁵As nuclei can consistently be understood in terms of antiferromagnetic spin fluctuations, as deduced from the T-dependence of (1/T₁T)=C/(T−θ)^1/2.     

Quantum dots as scatterers in electronic transport: interference and correlations

Conductance through a system consisting of a wire with side-attached quantum dots is calculated. Such geometry of the device allows to study the coexistence of quantum interference, electron correlations and their influence on conductance. We underline the differences between ‘classical’ Fano resonance in which the resonant channel is of single-particle nature and ‘many-body’ Fano resonance with the resonant channel formed by Kondo effect. The influence of electron-electron interactions on the Fano resonance shape is also analyzed.     

Alternating current transport property for a two-channel interacting quantum wire

We study theoretically the alternating current (ac) transport property through a two-channel clean quantum wire of finite length in the presence of both inter-channel and intra-channel electron-electron (e-e) interactions. Using the bosonization technique and linear response theory, we have obtained analytical expressions of the ac conductance. Interestingly, the ac conductance oscillations, with two different frequencies, form a beat which governs the behavior of the transport property in the presence of inter-channel e-e interaction. This result provides us with a new way to control the transport property of narrow quantum wires by engineering the Fermi velocities in the two different channels, i.e., the electron density.     

Spin polarization and spin separation realized in the double-helical molecules

We investigate the electron transport through one double-helical molecule with four terminals, by considering one terminal to be the source and others to be the drains. It is found that notable spin polarizations simultaneously occur during the processes of intra-chain electron tunneling and inter-chain electron reflection. More importantly, in these two processes, the spin polarizations always show similar strengths and opposite directions. Based on these results, we consider that the spin polarization and spin separation can be co-realized in this system.     

Modeling the thermodynamic properties of bimetallic nanosolids

A model has been developed to account for size, shape, surface segregation, composition and dimension dependent cohesive energy of bimetallic nanosolids, and further been extended to predict the size dependent thermodynamic properties, such as melting temperature, Curie temperatures, ordering temperature and phase diagram. The cohesive energy, melting temperature, Curie temperatures and ordering temperature of bimetallic nanosolids decrease with decreasing the particle size. The depression is dramatic in the lower range of size, while it becomes smoothly in large size. For nano phase diagram, the solidus and liquidus curves drop and the two-phase zones become small, as the size of the nanosolids decreases. The two-phase zones of the nano phase are always lower than the regions indicated in the bulk Ag-Pd alloy phase diagram, and they may deteriorate into a curve at a critical size. It is also found that the thermodynamic properties of nanosolids not only depend on the compositions, the atomic diameter and the cohesive energy of each component, but also depend on the size and the shape. The model predictions are consistent with the corresponding simulation, semi-empirical model and experimental data.     

Spin torque transfer structure with new spin switching configurations

Spin torque transfer structures with new spin switching configurations are proposed, fabricated and investigated in this paper. The non-uniform current-induced magnetization switching is implemented based on both GMR and MTJ nano devices. The proposed new spin transfer structure has a hybrid free layer that consists of a layer with conductive channels (magnetic) and non-conductive matrix (non-magnetic) and traditional free layer(s). Two mechanisms, a higher local current density by nano-current-channels and a non-uniform magnetization switching (reversal domain nucleation and growth) by a magnetic nanocomposite structure, contribute in reducing the switching current density. The critical switching current density for the new spin transfer structure is reduced to one third of the typical value for the normal structure. It can be expected to have one order of magnitude or more reduction for the critical current density if the optimization of materials and fabrication processes could be done further. Meanwhile, the thermal stability of this new spin transfer structure is not degraded, which may solve the long-standing scaling problem for magnetic random access memory (MRAM). This spin transfer structure, with the proposed and demonstrated new spin switching configurations, not only provides a solid approach for the practical application of spin transfer devices but also forms a unique platform for researchers to explore the non-uniform current-induced switching process.     

Spin transport in the anisotropic easy-plane two-dimensional Heisenberg antiferromagnet

Spin transport in the easy-plane two-dimensional anisotropic Heisenberg antiferromagnet with S=1 is studied. Regular part of spin conductivity is calculated, at zero temperature, using a self-consistent harmonic approximation and the Kubo formalism. Three magnon processes provide the dominant contribution to the spin conductivity. Furthermore, the transport is ballistic and characterized by finite Drude weight.     

Enhanced spin polarization in an asymmetric magnetic graphene superlattice

The authors investigate the spin-resolved transport through an asymmetrical magnetic graphene superlattice (MGS) consisting of the periodic barriers with abnormal one in height. To quantitatively depict the asymmetrical MGS, an asymmetry factor has been introduced to measure the height change of the abnormal barrier. It is shown that the spin filter effect is strongly enhanced by the barrier asymmetry both in the Klein and the classical tunneling regimes. In the presence of abnormal barrier, the conductance with certain spin direction is suppressed with respect to different tunneling regimes, and thus high spin polarization with opposite sign can be achieved.     

Multi-carrier transport properties in p-type ZnO thin films

By the aid of temperature- and magnetic-field-dependent Hall effect measurements, we have extracted the multi-carrier transport information in N-doped and N–In codoped p- ZnO thin films grown on Si substrates through mobility spectrum analysis. It is found that owing to the compensation between free electrons and holes, the two-dimensional hole gas from ZnO/Si interface layers becomes determinant and results in the high p-type conductivity and high hole mobility in the ZnO samples. Compared with N-doping, the N–In codoping introduces many In donors and increases acceptor incorporation, as well as enhancing the free hole mobility due to the short-range dipole-like scattering.     

Linear and nonlinear critical magnetic properties and transport in Nd0.75Ba0.25MnO3 single crystal: evidence for its anomalous critical behavior near TC

The data on the resistance and magnetoresistance (MR) as well as measurements of the linear and nonlinear susceptibilities are presented for a Nd_0.75Ba_0.25MnO₃ single crystal with the Curie temperature T_C≈129 K. Although this compound remains insulating in the ferromagnetic state, its resistance has an anomaly near T_C and it reveals the colossal magnetoresistance. The data on the magnetic response are well described by the dynamic scaling theory for 3D isotropic ferromagnets in the paramagnetic critical region at τ>τ^*≈0.11, τ=(T−T_C)/T_C. Below τ^* an anomalous critical behavior is found that suggests the coexistence of two magnetic phases. This behavior is discussed in terms of a phase separation which can occur in the moderately doped manganites exhibiting an orbital ordering.     

Probing phonon emission via hot carrier transport in suspended graphitic multilayers

We study hot carrier transport under magnetic fields up to 15 T in suspended graphitic multilayers through differential conductance () spectroscopy. Distinct high-energy anomalies have been observed and shown to be related to intrinsic phonon-emission processes in graphite. The evolution of such anomalies under magnetic fields is further understood as a consequence of inter-Landau level cyclotron-phonon resonance scattering. The observed magneto-phonon effects not only shed light on the physical mechanisms responsible for high-current transport in graphitic systems, but also offer new perspectives for optimizing performance in graphitic nano-electronic devices.     

Magnon-phonon interaction in the one dimensional quantum XY model

In this paper, we study the phonon dynamics of the one dimensional quantum XY model coupled to phonons, using two different approaches: a low temperature approximation for the memory function, and the perturbation expansion in the Green function formalism. We show that at least to the lowest order in the calculation, both methods give the same result for the dynamic phonon correlation function.     

A vertical transport geometry for electrical spin injection and extraction in Si

Schottky barriers formed between ferromagnetic metal and Semiconductor are of particular interest for spin injection and detection experiments. Here, we investigate electrical spin polarized carrier injection and extraction in Si using a Co/Si/Ni vertical structure built on a 250 nm thick Si membrane. Current–voltage measurements performed on the devices at low temperatures showed evidence of the conduction being dominated by thermionic field emission, which is believed to be the key to spin injection using Schottky junctions. This, however, proved inconclusive as our devices did not show any magnetoresistance signal even at low temperatures. We attribute this partially to the high resistance-area product in our Schottky contacts at spin injection biases. We show the potential of this vertical spin-device for future experiments by numerical simulation. The results reveal that by growing a thin highly doped Ge layer at the Schottky junctions the resistance-area products could be tuned to obtain high magnetoresistance.     

Optical properties of acceptor-exciton complexes in ZnO/SiO2 quantum dots

The binding energy E_b of the acceptor-exciton complex (A⁻,X) as a function of the radius (or of the impurity position of the acceptor) and the normalized oscillator strength of (A⁻,X) in spherical ZnO quantum dots (QDs) embedded in a SiO₂ matrix are calculated using the effective-mass approximation under the diagonalzation matrix technique, including a three-dimensional confinement of the carrier in the QD and assuming a finite depth. Numerical results show that the binding energy of the acceptor-exciton complexes is particularly robust when the impurity position of the acceptor is in the center of the ZnO QDs. It has been clearly shown from our calculations that these physical parameters are very sensitive to the quantum dot size and to the impurity position. These results could be particularly helpful, since they are closely related to experiments performed on such nanoparticles. This may allow us to improve the stability and efficiency of the semiconductor quantum dot luminescence which is considered critical.     

First-principles study of the electronic transport properties of a C131 -based molecular junction

Using first-principles density functional theory and the non-equilibrium Green’s function formalism, we have studied the electronic transport properties of the dumbbell-like fullerene dimer C₁₃₁-based molecular junction. Our results show that the current-voltage curve displays an obvious negative differential resistance phenomenon in a certain bias voltage range. The negative differential resistance behavior can be understood in terms of the evolution of the transmission spectrum and the projected density of states with applied bias voltage. The present findings could be helpful for the application of the C₁₃₁ molecule in the field of single molecular devices or nanometer electronics.     

Power factor enhancement in Pr-substituted alloys prepared by high-pressure sintering

Bi₈₅Sb_15−xPr_x (x=0,1,2,3) alloys with partial substitution of Pr for Sb were synthesized by mechanical alloying followed by high-pressure sintering. The crystal structure was characterized by X-ray diffraction. The electrical conductivity and Seebeck coefficient were measured in the temperature range of 80–300 K. The results show that the electrical conductivity and Seebeck coefficient of Pr-substituted samples are both larger than those of the reference sample, Bi₈₅Sb₁₅, in the whole measurement temperature range. The power factor of Bi₈₅Sb₁₃Pr₂ reaches a maximum value of 3.83×10⁻³ W K⁻² m⁻¹ at 235 K, which is about four times larger than that of the reference sample, Bi₈₅Sb₁₅, at the same temperature.