arXiv subjects

Jing Shi

Publications and source records attributed to Jing Shi.

At least 145 records · Page 8Linked to original sources

arXiv2015

Contact Induced Spin Relaxation in Graphene Spin Valves with Al2O3 and MgO Tunnel Barriers

We investigate spin relaxation in graphene by systematically comparing the roles of spin absorption, other contact-induced effects (e.g. fringe fields, etc.), and bulk spin relaxation for graphene spin valves with MgO barriers, Al2O3 barriers, and transparent contacts. We obtain effective spin lifetimes by fitting the Hanle spin precession data with two models that include or exclude the effect of spin absorption. Results indicate that additional contact-induced spin relaxation other than spin absorption dominates the contact effect. For tunneling contacts, we find reasonable agreement between the two models with median discrepancy of ~20% for MgO and ~10% for Al2O3.

cond-mat.mes-hall↗

arXiv2015

Experimental Demonstration of XOR Operation in Graphene Magnetologic Gates at Room Temperature

We report the experimental demonstration of a magnetologic gate built on graphene at room temperature. This magnetologic gate consists of three ferromagnetic electrodes contacting a single layer graphene spin channel and relies on spin injection and spin transport in the graphene. We utilize electrical bias tuning of spin injection to balance the inputs and achieve "exclusive or" (XOR) logic operation. Furthermore, simulation of the device performance shows that substantial improvement towards spintronic applications can be achieved by optimizing device parameters such as device dimensions. This advance holds promise as a basic building block for spin-based information processing.

cond-mat.mes-hall↗

arXiv2015

Independent tuning of electronic properties and induced ferromagnetism in topological insulators with heterostructure approach

The quantum anomalous Hall effect (QAHE) has been recently demonstrated in Cr- and V-doped three-dimensional topological insulators (TIs) at temperatures below 100 mK. In those materials, the spins of unfilled d-electrons in the transition metal dopants are exchange coupled to develop a long-range ferromagnetic order, which is essential for realizing QAHE. However, the addition of random dopants does not only introduce excess charge carriers that require readjusting the Bi/Sb ratio, but also unavoidably introduces paramagnetic spins that can adversely affect the chiral edge transport in QAHE. In this work, we show a heterostructure approach to independently tune the electronic and magnetic properties of the topological surface states in (BixSb1-x)2Te3 without resorting to random doping of transition metal elements. In heterostructures consisting of a thin (BixSb1-x)2Te3 TI film and yttrium iron garnet (YIG), a high Curie temperature (~ 550 K) magnetic insulator, we find that the TI surface in contact with YIG becomes ferromagnetic via proximity coupling which is revealed by the anomalous Hall effect (AHE). The Curie temperature of the magnetized TI surface ranges from 20 to 150 K but is uncorrelated with the Bi fraction x in (BixSb1-x)2Te3. In contrast, as x is varied, the AHE resistivity scales with the longitudinal resistivity. In this approach, we decouple the electronic properties from the induced ferromagnetism in TI. The independent optimization provides a pathway for realizing QAHE at higher temperatures, which is important for novel spintronic device applications.

cond-mat.mtrl-sci↗

arXiv2015

Epitaxial growth and properties of La0.7Sr0.3MnO3 thin films with micrometer wide atomic terraces

La0.7Sr0.3MnO3 (LSMO) films with extraordinarily wide atomic terraces are epitaxially grown on SrTiO3 (100) substrates by pulsed laser deposition. Atomic force microscopy measurements on the LSMO films show that the atomic step is ~ 4 Å and the atomic terrace width is more than 2 micrometers. For a 20 monolayers (MLs) LSMO film, the magnetization is determined to be 255 +- 15 emu/cm3 at room temperature, corresponding to 1.70 + - 0.11 Bohr magneton per Mn atom. As the thickness of LSMO increases from 8 MLs to 20 MLs, the critical thickness for the temperature dependent insulator-to-metal behavior transition is shown to be 9 MLs. Furthermore, post-annealing in oxygen environment improves the electron transport and magnetic properties of the LSMO films.

cond-mat.mtrl-sci↗

arXiv2014

Heat-driven spin transport in a ferromagnetic metal

As a non-magnetic heavy metal is attached to a ferromagnet, a vertically flowing heat-driven spin current is converted to a transverse electric voltage, which is known as the longitudinal spin Seebeck effect (SSE). If the ferromagnet is a metal, this voltage is also accompanied by voltages from two other sources, i.e. the anomalous Nernst effect in both the ferromagnet and the proximity-induced ferromagnetic boundary layer. By properly identifying and carefully separating those different effects, we find that in this pure spin current circuit the additional spin current drawn by the heavy metal generates another significant voltage by the ferromagnetic metal itself which should be present in all relevant experiments.

cond-mat.mes-hall↗

arXiv2014

Proximity-induced ferromagnetism in graphene revealed by anomalous Hall effect

We demonstrate the anomalous Hall effect (AHE) in single-layer graphene exchange-coupled to an atomically flat yttrium iron garnet (YIG) ferromagnetic thin film. The anomalous Hall conductance has magnitude of ~0.09(2e2/h) at low temperatures and is measurable up to ~ 300 K. Our observations indicate not only proximity-induced ferromagnetism in graphene/YIG with large exchange interaction, but also enhanced spin-orbit coupling which is believed to be inherently weak in ideal graphene. The proximity-induced ferromagnetic order in graphene can lead to novel transport phenomena such as the quantized AHE which are potentially useful for spintronics.

cond-mat.mes-hall↗

arXiv2014

Studies of LL-type 500MHz 5-cell superconducting cavity at SINAP

A low loss (LL) type 500 MHz 5-cell superconducting niobium prototype cavity with large beam aperture has been developed successfully including the optimization, the deep drawing and electron beam welding, the surface treatment and the vertical testing. The performance of the fundamental mode was optimized and the higher order modes were damped by adopting an enlarged beam pipe for propagation. Surface preparation or treatment including mechanical polishing, buffered chemical polishing and high pressure rinsing with ultra-pure water and so on was carried out carefully to promise a perfect inner surface condition. The vertical testing results show that the accelerating voltage higher than 7.5 MV was obtained while the quality factor was better than 1E9 at 4.2 K. No obvious multipacting or field emission was found during the test. However, a quench happened while increasing the field a little higher than 7.5 MV that at present limited the cavity performance.

physics.acc-ph↗

arXiv2014

Ferroelectric-like SrTiO3 surface dipoles probed by graphene

The electrical transport properties of graphene are greatly influenced by its environment. Owing to its high dielectric constant, strontium titanate (STO) is expected to suppress the long-range charged impurity scattering and consequently to enhance the mobility. However, the absence of such enhancement has caused some controversies regarding the scattering mechanism. In graphene devices transferred from SiO2 to STO using a newly developed technique, we observe a moderate mobility enhancement near the Dirac point, which is the point of charge neutrality achieved by adjusting the Fermi level. While bulk STO is not known as a ferroelectric material, its surface was previously reported to exhibit an outward displacement of oxygen atoms and ferroelectric-like dipole moment. When placed on STO, graphene shows strong and asymmetric hysteresis in resistivity, which is consistent with the dipole picture associated with the oxygen displacement. The hysteretic response of the surface dipole moment diminishes the polarizability, therefore weakens the ability of STO to screen the Coulomb potential of the impurities.

cond-mat.mes-hall↗

arXiv2014

Direct comparison of graphene devices before and after transfer to different substrates

The entire graphene field-effect-transistor (FET) devices first fabricated on SiO2/Si are peeled from the surface and placed on a different wafer. Both longitudinal and transverse resistivity measurements of the devices before and after the transfer are measured to calculate the mobility for a direct comparison. After transferred to different SiO2/Si wafers, the mobility generally is comparable and the defect density does not show any significant increase, which indicates the degradation due to the transfer process itself is minimal. The same method can be applied to transfer graphene devices to any arbitrary substrates (e.g. SrTiO3 or STO). The transfer method developed here not only eliminates the need to locate single-layer graphene on non-SiO2/Si substrates for patterning, but also provides a convenient way to study the effects of various substrates on graphene electronic properties.

cond-mat.mes-hall↗

arXiv2013

Induced magneto-transport properties at palladium/yttrium iron garnet interface

As a thin layer of palladium (Pd) is directly deposited on an yttrium iron garnet or YIG (Y3Fe5O12) magnetic insulator film, Pd develops both low- and high-field magneto-transport effects that are absent in standalone Pd or thick Pd on YIG. While the low-field magnetoresistance peak of Pd tracks the coercive field of the YIG film, the much larger high-field magnetoresistance and the Hall effect do not show any obvious relationship with the bulk YIG magnetization. The distinct high-field magneto-transport effects in Pd are shown to be caused by interfacial local moments in Pd.

cond-mat.mtrl-sci↗

arXiv2013

77Se NMR Investigation of Fe-doped Bi2Se3

Bismuth selenide is both a thermoelectric material and topological insulator. Defects and dopants create conduction in thermoelectric applications. However, such defects may degrade the performance as a topological insulator (TI). Magnetic impurities such as iron open a band gap at the Dirac point on the surface. Since magnetically-doped TIs are important in technological applications, a good understanding of their properties is needed. In this article, 77Se nuclear magnetic resonance (NMR) spectroscopy has been used to investigate Fe-doped Bi2Se3. Spin-lattice relaxation measurements indicate that the Fe dopants provide a spin diffusion relaxation mechanism at low temperatures for the 77Se. Above 320 K, the predominant 77Se relaxation mechanism resulting from interaction with the conduction carriers is thermally induced with an activation energy of 21.5 kJ/mol (5.1 kcal/mol, 222 meV) and likely arises from inter-band excitations. Magic-angle spinning produces negligible narrowing of the 77Se resonance at 7 T, suggesting a statistical distribution of material defects and is also consistent with a dipolar interaction with the neighboring quadrupolar nucleus.

cond-mat.mtrl-sci↗

arXiv2012

Field-effect mobility enhanced by tuning the Fermi level into the band gap of Bi2Se3

By eliminating normal fabrication processes, we preserve the bulk insulating state of calcium-doped Bi2Se3 single crystals in suspended nanodevices, as indicated by the activated temperature dependence of the resistivity at low temperatures. We perform low-energy electron beam irradiation (<16 keV) and electrostatic gating to control the carrier density and therefore the Fermi level position in the nanodevices. In slightly p-doped Bi2-xCaxSe3 devices, continuous tuning of the Fermi level from the bulk valence band to the band-gap reveals dramatic enhancement (> a factor of 10) in the field-effect mobility, which suggests suppressed backscattering expected for the Dirac fermion surface states in the gap of topological insulators.

cond-mat.mes-hall↗

arXiv2012

Spin Relaxation in Single Layer Graphene with Tunable Mobility

Graphene is an attractive material for spintronics due to theoretical predictions of long spin lifetimes arising from low spin-orbit and hyperfine couplings. In experiments, however, spin lifetimes in single layer graphene (SLG) measured via Hanle effects are much shorter than expected theoretically. Thus, the origin of spin relaxation in SLG is a major issue for graphene spintronics. Despite extensive theoretical and experimental work addressing this question, there is still little clarity on the microscopic origin of spin relaxation. By using organic ligand-bound nanoparticles as charge reservoirs to tune mobility between 2700 and 12000 cm2/Vs, we successfully isolate the effect of charged impurity scattering on spin relaxation in SLG. Our results demonstrate that while charged impurities can greatly affect mobility, the spin lifetimes are not affected by charged impurity scattering.

cond-mat.mes-hall↗

arXiv2012

A Two-Dimensional CA Traffic Model with Dynamic Route Choices Between Residence and Workplace

The Biham, Middleton and Levine (BML) model is extended to describe dynamic route choices between the residence and workplace in cities. The traffic dynamic in the city with a single workplace is studied from the velocity diagram, arrival time probability distribution, destination arrival rate and convergence time. The city with double workplaces is also investigated to compared with a single workplace within the framework of four modes of urban growth. The transitional region is found in the velocity diagrams where the system undergoes a continuous transition from a moving phase to a completely jamming phase. We perform a finite-size scaling analysis of the critical density from a statistical point of view and the order parameter of this jamming transition is estimated. It is also found that statistical properties of urban traffic are greatly influenced by the urban area, workplace area and urban layout.

physics.soc-ph↗

arXiv2012

Derivative relations between electrical and thermoelectric quantum transport coefficients in graphene

We find that the empirical relation between the longitudinal and Hall resistivities (i.e. Rxx and Rxy) and its counterpart between the Seebeck and Nernst coefficients (i.e. Sxx and Sxy), both originally discovered in two-dimensional electron gases, hold remarkably well for graphene in the quantum transport regime except near the Dirac point. The validity of the relations is cross-examined by independently varying the magnetic field and the carrier density in graphene. We demonstrate that the pre-factor, α_s, does not depend on carrier density in graphene. By tuning the carrier mobility therefore the degree of disorders, we find that the pre-factor stays unchanged. Near the Dirac point, different mechanisms at low densities such as carrier localization may be responsible for the breakdown of these relations.

cond-mat.mes-hall↗

arXiv2011

Reconfigurable nanoelectronics using graphene based spintronic logic gates

This paper presents a novel design concept for spintronic nanoelectronics that emphasizes a seamless integration of spin-based memory and logic circuits. The building blocks are magneto-logic gates based on a hybrid graphene/ferromagnet material system. We use network search engines as a technology demonstration vehicle and present a spin-based circuit design with smaller area, faster speed, and lower energy consumption than the state-of-the-art CMOS counterparts. This design can also be applied in applications such as data compression, coding and image recognition. In the proposed scheme, over 100 spin-based logic operations are carried out before any need for a spin-charge conversion. Consequently, supporting CMOS electronics requires little power consumption. The spintronic-CMOS integrated system can be implemented on a single 3-D chip. These nonvolatile logic circuits hold potential for a paradigm shift in computing applications.

cond-mat.mtrl-sci↗

arXiv2011

Direct Evidence for Edge-Contaminated Vortex Phase in a Nb Single Crystal using Neutron Diffraction

We report the first direct observation of a disordered vortex matter phase existing near the edge of a bulk type-II superconductor Nb using a novel position-sensitive neutron diffraction technique. This "edge-contaminated" vortex state was implicated in previous studies using transport techniques and was postulated to have played a significant role in the behavior of vortex dynamics in a wide range of type-II superconductors. It is found that upon thermal annealing, the vortex matter in the bulk undergoes re-ordering, suggesting that the edge-contaminated bulk vortex state is metastable. The edge vortex state remains disordered after repeated thermal annealing, indicating spatial coexistence of a vortex glass with a Bragg glass. This observation resolves many outstanding issues concerning the peak effect in type-II superconductors.

cond-mat.supr-con↗

arXiv2011

Effect of charged impurities on graphene thermoelectric power near the Dirac point

In graphene devices with a varying degree of disorders as characterized by their carrier mobility and minimum conductivity, we have studied the thermoelectric power along with the electrical conductivity over a wide range of temperatures. We have found that the Mott relation fails in the vicinity of the Dirac point in high-mobility graphene. By properly taking account of the high temperature effects, we have obtained good agreement between the Boltzmann transport theory and our experimental data. In low-mobility graphene where the charged impurities induce relatively high residual carrier density, the Mott relation holds at all gate voltages.

cond-mat.mtrl-sci↗