arXiv subjects

Yongbing Xu

Publications and source records attributed to Yongbing Xu.

35 records · Page 2Linked to original sources

arXiv2019

Observation of Shubnikov-de Haas Oscillations in Large-Scale Weyl Semimetal WTe2 Films

Topological Weyl semimetal WTe2 with large-scale film form has a promising prospect for new-generation spintronic devices. However, it remains a hard task to suppress the defect states in large-scale WTe2 films due to the chemical nature. Here, we significantly improve the crystalline quality and remove the Te vacancies in WTe2 films by post annealing. We observe the distinct Shubnikov-de Haas quantum oscillations in WTe2 films. The nontrivial Berry phase can be revealed by Landau fan diagram analysis. The Hall mobility of WTe2 films can reach 1245 cm2V-1s-1 and 1423 cm2V-1s-1 for holes and electrons with the carrier density of 5 * 10^19 cm^-3 and 2 * 10^19 cm^-3, respectively. Our work provides a feasible route to obtain high-quality Weyl semimetal films for the future topological quantum device applications.

cond-mat.mtrl-sci↗

arXiv2019

Enhancement of intrinsic magnetic damping in defect-free epitaxial Fe3O4 thin films

We have investigated the magnetic damping of precessional spin dynamics in defect-controlled epitaxial grown Fe$_3$O$_4$(111)/Yttria-stabilized Zirconia (YSZ) nanoscale films by all-optical pump-probe measurements. The intrinsic damping constant of the defect-free Fe$_3$O$_4$ film is found to be strikingly larger than that of the as-grown Fe$_3$O$_4$ film with structural defects. We demonstrate that the population of the first-order perpendicular standing spin wave (PSSW) mode, which is exclusively observed in the defect-free film under sufficiently high external magnetic fields, leads to the enhancement of the magnetic damping of the uniform precession (Kittel) mode. We propose a physical picture in which the PSSW mode acts as an additional channel for the extra energy dissipation of the Kittel mode. The energy transfer from Kittel mode to PSSW mode increases as in-plane magnetization precession becomes more uniform, resulting in the unique intrinsic magnetic damping enhancement in the defect-free Fe$_3$O$_4$ film.

cond-mat.mtrl-sci↗

arXiv2018

Ultrafast photocarrier recombination dynamics in black phosphorus-molybdenum disulfide (BP/MoS2) heterostructure

Van der Waals (vdW) heterostructures constructed with two-dimensional (2D) materials have attracted great interests, due to their fascinating properties and potential for novel applications. While earlier efforts have advanced the understanding of the ultrafast cross-layer charge transfer process in 2D heterostructures, mechanisms for the interfacial photocarrier recombination remain, to a large extent, unclear. Here, we investigate a heterostructure comprised of black phosphorus (BP) and molybdenum disulfide (MoS2), with a type-II band alignment. Interestingly, it is found that the photo-generated electrons in MoS2 (transferred from BP) exhibit an ultrafast lifetime of about 5 ps, significantly shorter than those of the constituent materials. By corroborating with the relaxation of photo-excited holes in BP, it is revealed that the ultrafast time constant is as a result of efficient Langevin recombination, where the high hole mobility of BP facilitates a large recombination coefficient (approximately 2x10^-10 m^2/s). In addition, broadband transient absorption spectroscopy confirms that the hot electrons transferred to MoS2 distribute over a broad energy range following an ultrafast thermalization. The rate of the interlayer Langevin recombination is found to exhibit no energy level dependence. Our findings provide essential insights into the fundamental photo-physics in type-II 2D heterostructures, and also provide useful guidelines for customizing photocarrier lifetimes of BP for high-speed photo-sensitive devices.

cond-mat.mes-hall↗

arXiv2018

Roles of heating and helicity in ultrafast all-optical magnetization switching in TbFeCo

Using time-resolved magneto-optical Kerr effect (TR-MOKE) method, helicity-dependent all-optical magnetization switching (HD-AOS) is observed in ferrimagnetic TbFeCo films. The thermal effect and opto-magneto effects are separately justified after single circularly polarized laser pulse. The integral evolution of this ultrafast switching is characterized on different time scales and the defined magnetization reversal time of 460 fs is the fastest ever observed. Combining the heat effect and inverse Faraday effect (IFE), micromagnetic simulations based on a single macro-spin model are performed that reproduce HD-AOS following a linear reversal mechanism.

cond-mat.mtrl-sci↗

arXiv2018

Direct demonstration of the emergent magnetism resulting from the multivalence Mn in a LaMnO3 epitaxial thin film system

Atomically engineered oxide heterostructures provide a fertile ground for creating novel states. For example, a two-dimensional electron gas at the interface between two oxide insulators, giant thermoelectric Seebeck coefficient, emergent ferromagnetism from otherwise nonmagnetic components, and colossal ionic conductivity. Extensive research efforts reveal that oxygen deficiency or lattice strain play an important role in determining these unexpected properties. Herein, by studying the abrupt presence of robust ferromagnetism (up to 1.5 uB/Mn) in LaMnO3-based heterostructures, we find the multivalence states of Mn that play a decisive role in the emergence of ferromagnetism in the otherwise antiferromagnetic LaMnO3 thin films. Combining spatially resolved electron energy-loss spectroscopy, X-ray absorption spectroscopy and X-ray magnetic circular dichroism techniques, we determine unambiguously that the ferromagnetism results from a conventional Mn3+-O-Mn4+ double-exchange mechanism rather than an interfacial effect. In contrast, the magnetic dead layer of 5 unit cell in proximity to the interface is found to be accompanied with the accumulation of Mn2+ induced by electronic reconstruction. These findings provide a hitherto-unexplored multivalence state of Mn on the emergent magnetism in undoped manganite epitaxial thin films, such as LaMnO3 and BiMnO3, and shed new light on all-oxide spintronic devices.

cond-mat.mtrl-sci↗

arXiv2018

Topological Phase Transition-Induced Tri-Axial Vector Magnetoresistance in (Bi1-xInx)2Se3 Nanodevices

We report the study of a tri-axial vector magnetoresistance (MR) in nonmagnetic (Bi1-xInx)2Se3 nanodevices at the composition of x = 0.08. We show a dumbbell-shaped in-plane negative MR up to room temperature as well as a large out-of-plane positive MR. MR at three directions is about in a -3%: -1%: 225% ratio at 2 K. Through both the thickness and composition-dependent magnetotransport measurements, we show that the in-plane negative MR is due to the topological phase transition enhanced intersurface coupling near the topological critical point. Our devices suggest the great potential for room-temperature spintronic applications, for example, vector magnetic sensors.

physics.app-ph↗

arXiv2017

Effect of Notch Structure on Magnetic Domain Movement in Planar Nanowires

We present the direct observation of magnetic domain motion in permalloy nanowires with notches using a wide-field Kerr microscopy technique. The domain wall motion can be modulated by the size and shape of the notch structure in the nanowires. It is demonstrated that the coercive fields can be tuned by modulating the notches. The experimental results are consistent with the micro-magnetic simulation results. The relationship between the notch angle and the domain nuclease are also studied. This work is useful for the design and development of the notch-based spintronic devices.

cond-mat.mes-hall↗

arXiv2017

Controllable transport of a skyrmion in a ferromagnetic narrow channel with voltage-controlled magnetic anisotropy

Magnetic skyrmions have potential applications in next-generation spintronics devices with ultralow energy consumption. In this work, the current-driven skyrmion motion in a narrow ferromagnetic nanotrack with voltage-controlled magnetic anisotropy (VCMA) is studied numerically. By utilizing the VCMA effect, the transport of skyrmion can be unidirectional in the nanotrack, leading to a one-way information channel. The trajectory of the skyrmion can also be modulated by periodically located VCMA gates, which protects the skyrmion from destruction by touching the track edge. In addition, the location of the skyrmion can be controlled by adjusting the driving pulse length in the presence of the VCMA effect. Our results provide guidelines for practical realization of the skyrmion-based information channel, diode, and racetrack memory.

cond-mat.mes-hall↗

arXiv2017

Magnetic Domain Wall Engineering in a Nanoscale Permalloy Junction

Nanoscale magnetic junction provides a useful approach to act as the building block for magnetoresistive random access memories (MRAM), where one of the key issues is to control the magnetic domain configuration. Here, we study the domain structure and the magnetic switching in the Permalloy (Fe20Ni80) nanoscale magnetic junctions with different thicknesses by using micromagnetic simulations. It is found that both the 90-degree and 45-degree domain walls can be formed between the junctions and the wire arms depending on the thickness of the device. The magnetic switching fields show distinct thickness dependencies with a broad peak varying from 7 nm to 22 nm depending on the junction sizes, and the large magnetic switching fields favor the stability of the MRAM operation.

physics.comp-ph↗

arXiv2016

Unsaturated magnetoconductance of epitaxial La0.7Sr0.3MnO3 thin films in pulsed magnetic fields up to 60 T

We report on the temperature and field dependence of resistance of La0.7Sr0.3MnO3 thin films over a wide temperature range and in pulsed magnetic fields up to 60 T. The epitaxial La0.7Sr0.3MnO3 thin films were deposited by laser molecular beam epitaxy. High magnetic field magnetoresistance curves were fitted by the Brillouin function, which indicated the existence of magnetically polarized regions and the underlying hopping mechanism. The unsaturated magnetoconductance was the most striking finding observed in pulsed magnetic fields up to 60 T. These observations can deepen the fundamental understanding of the colossal magnetoresistance in manganites with strong correlation of transport properties and magnetic ordering.

cond-mat.str-el↗

arXiv2016

A light-stimulated neuromorphic device based on graphene hybrid phototransistor

Neuromorphic chip refers to an unconventional computing architecture that is modelled on biological brains. It is ideally suited for processing sensory data for intelligence computing, decision-making or context cognition. Despite rapid development, conventional artificial synapses exhibit poor connection flexibility and require separate data acquisition circuitry, resulting in limited functionalities and significant hardware redundancy. Here we report a novel light-stimulated artificial synapse based on a graphene-nanotube hybrid phototransistor that can directly convert optical stimuli into a "neural image" for further neuronal analysis. Our optically-driven synapses involve multiple steps of plasticity mechanisms and importantly exhibit flexible tuning of both short- and long-term plasticity. Furthermore, our neuromorphic phototransistor can take multiple pre-synaptic light stimuli via wavelength-division multiplexing and allows advanced optical processing through charge-trap-mediated optical coupling. The capability of complex neuromorphic functionalities in a simple silicon-compatible device paves the way for novel neuromorphic computing architectures involving photonics.

cs.ET↗

arXiv2016

Elementary specific spin and orbital moments of ultrathin CoFeB amorphous films on GaAs(100)

Nanoscale CoFeB amorphous films have been synthesized on GaAs(100) and studied with X-ray magnetic circular dichroism (XMCD) and transmission electron microscopy (TEM). We have found that the ratios of the orbital to spin magnetic moments of both the Co and Fe in the ultrathin amorphous film have been enhanced by more than 300% compared with those of the bulk crystalline Co and Fe, and in specifically, a large orbital moment of 0.56*10^-6 B from the Co atoms has been observed and at the same time the spin moment of the Co atoms remains comparable to that of the bulk hcp Co. The results indicate that the large uniaxial magnetic anisotropy (UMA) observed in the ultrathin CoFeB film on GaAs(100) is related to the enhanced spin-orbital coupling of the Co atoms in the CoFeB. This work offers experimental evidences of the correlation between the UMA and the elementary specific spin and orbital moments in the CoFeB amorphous film on the GaAs(100) substrate, which is significant for spintronics applications.

cond-mat.mtrl-sci↗

arXiv2016

Evidence of both surface and bulk Dirac bands in ZrSiS and the unconventional magnetoresistance

The unconventional magnetoresistance of ZrSiS single crystals is found unsaturated till the magnetic field of 53 T with the butterfly shaped angular dependence. Intense Shubnikov-de Haas oscillations resolve a bulk Dirac cone with a nontrivial Berry phase, where the Fermi surface area is 1.80*10^-3 Å-2 and reaches the quantum limit before 20 T. Angle resolved photoemission spectroscopy measurement reveals an electronic state with the two-dimensional nature around the X point of Brillouin zone boundary. By integrating the density functional theory calculations, ZrSiS is suggested to be a Dirac material with both surface and bulk Dirac bands.

cond-mat.mtrl-sci↗

arXiv2016

A High-Performance Mid-infrared Optical Switch Enabled by Bulk Dirac Fermions in Cd3As2

Pulsed lasers operating in the 2-5 μm band are important for a wide range of applications in sensing, spectroscopy, imaging and communications. Despite recent advances with mid-infrared gain media, the lack of a capable pulse generation mechanism, i.e. a passive optical switch, remains a significant technological challenge. Here we show that mid-infrared optical response of Dirac states in crystalline Cd3As2, a three-dimensional topological Dirac semimetal (TDS), constitutes an ideal ultrafast optical switching mechanism for the 2-5 μm range. Significantly, fundamental aspects of the photocarrier processes, such as relaxation time scales, are found to be flexibly controlled through element doping, a feature crucial for the development of convenient mid-infrared ultrafast sources. Although various exotic physical phenomena have been uncovered in three-dimensional TDS systems, our findings show for the first time that this emerging class of quantum materials can be harnessed to fill a long known gap in the field of photonics.

cond-mat.mtrl-sci↗

arXiv2015

Planar carbon nanotube-graphene hybrid films for high-performance broadband photodetectors

Graphene has emerged as a promising material for photonic applications fuelled by its superior electronic and optical properties. However, the photoresponsivity is limited by the low absorption cross section and ultrafast recombination rates of photoexcited carriers. Here we demonstrate a photoconductive gain of $\sim$ 10$^5$ electrons per photon in a carbon nanotube-graphene one dimensional-two dimensional hybrid due to efficient photocarriers generation and transport within the nanostructure. A broadband photodetector (covering 400 nm to 1550 nm) based on such hybrid films is fabricated with a high photoresponsivity of more than 100 AW$^{-1}$ and a fast response time of approximately 100 μs. The combination of ultra-broad bandwidth, high responsivities and fast operating speeds affords new opportunities for facile and scalable fabrication of all-carbon optoelectronic devices.

cond-mat.mtrl-sci↗

arXiv2015

Stable gain-switched thulium fiber laser with 140 nm tuning range

We demonstrate a gain-switched thulium fiber laser that can be continuously tuned over 140 nm, while maintaining stable nanosecond single-pulse operation. To the best of our knowledge, this system represents the broadest tuning range for a gain-switched fiber laser. The system simplicity and wideband wavelength tunability combined with the ability to control the temporal characteristics of the gain-switched pulses mean this is a versatile source highly suited to a wide range of applications in the eye-safe region of the infrared, including spectroscopy, sensing and material processing, as well as being a practical seed source for pumping nonlinear processes.

physics.optics↗

arXiv2015

Enhancing Magnetic Ordering in Cr-doped Bi2Se3 using High-TC Ferrimagnetic Insulator

We report a study of enhancing the magnetic ordering in a model magnetically doped topological insulator (TI), Bi2-xCrxSe3, via the proximity effect using a high-TC ferrimagnetic insulator Y3Fe5O12. The FMI provides the TI with a source of exchange interaction yet without removing the nontrivial surface state. By performing the elemental specific X-ray magnetic circular dichroism (XMCD) measurements, we have unequivocally observed an enhanced TC of 50 K in this magnetically doped TI/FMI heterostructure. We have also found a larger (6.6 nm at 30 K) but faster decreasing (by 80% from 30 K to 50 K) penetration depth compared to that of diluted ferromagnetic semiconductors (DMSs), which could indicate a novel mechanism for the interaction between FMIs and the nontrivial TIs surface.

cond-mat.mtrl-sci↗