arXiv subjects

Jiangxiazi Lin

Publications and source records attributed to Jiangxiazi Lin.

At least 19 recordsLinked to original sources

arXiv2021

Bridging the gap between atomically thin semiconductors and metal leads

Electrically interfacing atomically thin transition metal dichalcogenide semiconductors (TMDSCs) with metal leads is challenging because of undesired interface barriers, which have drastically constrained the electrical performance of TMDSC devices for exploring their unconventional physical properties and realizing potential electronic applications. Here we demonstrate a strategy to achieve nearly barrier-free electrical contacts with few-layer TMDSCs by engineering interfacial bonding distortion. The carrier-injection efficiency of such electrical junction is substantially increased with robust ohmic behaviors from room to cryogenic temperatures. The performance enhancements of TMDSC field-effect transistors are well reflected by the ultralow contact resistance (down to 90 Ohm um in MoS2, towards the quantum limit), the ultrahigh field-effect mobility (up to 358,000 cm2V-1s-1 in WSe2) and the prominent transport characteristics at cryogenic temperatures. This method also offers new possibilities of the local manipulation of structures and electronic properties for TMDSC device design.

cond-mat.mtrl-sci↗

arXiv2019

Interaction effects and superconductivity signatures in twisted double-bilayer WSe$_2$

Twisted bilayer graphene provides a new two-dimensional platform for studying electron interaction phenomena and flat band properties such as correlated insulator transition, superconductivity and ferromagnetism at certain magic angles. Here, we present strong evidence of correlated insulator states and superconductivity signatures in p-type twisted double-bilayer WSe$_2$. Enhanced interlayer interactions are observed when the twist angle decreases to a few degrees as reflected by the high-order satellites in the electron diffraction patterns taken from the 2H/3R-stacked domains reconstructed from a conventional Moir\'e superlattice. In contrast to twisted bilayer graphene, there is no specific magic angle for twisted WSe$_2$. The flat band properties are observed at twist angles ranging from 1 to 4 degrees. The highest superconducting transition temperature observed by transport measurement is 6 K. Our work has facilitated future study in the area of flat band related properties in twisted transition metal dichalcogenide layered structures.

cond-mat.mes-hall↗

arXiv2019

Effects of Hexagonal Boron Nitride Encapsulation on the Electronic Structure of Few-layer MoS$_2$

The hexagonal boron nitride (hBN) encapsulation has been widely used in the electronics applications of 2D materials to improve device performance by protecting 2D materials against contamination and degradation. It is often assumed that hBN layers as a dielectric would not affect the electronic structure of encapsulated 2D materials. Here we studied few-layer MoS$_2$ encapsulated in hBN flakes by using a combination of theoretical and experimental Raman spectroscopy. We found that after the encapsulation the out-of-plane A$_{1g}$ mode is upshifted, while the in-plane E$_{2g}^1$ mode is downshifted. The measured downshift of the E$_{2g}^1$ mode does not decrease with increasing the thickness of MoS$_2$, which can be attributed to tensile strains in bilayer and trilayer MoS$_2$ caused by the typical experimental process of the hBN encapsulation. We estimated the strain magnitude and found that the induced strain may cause the K-Q crossover in the conduction band of few-layer MoS$_2$, so greatly modifies its electronic properties as an n-type semiconductor. Our study suggests that the hBN encapsulation should be used with caution, as it may affect the electronic properties of encapsulated few-layer 2D materials.

cond-mat.mtrl-sci↗

arXiv2018

Intrinsic valley Hall transport in atomically thin MoS2

Electrons hopping in two-dimensional honeycomb lattices possess a valley degree of freedom in addition to charge and spin. In the absence of inversion symmetry, these systems were predicted to exhibit opposite Hall effects for electrons from different valleys. Such valley Hall effects have been achieved only by extrinsic means, such as substrate coupling, dual gating, and light illuminating. Here, we report the first observation of intrinsic valley Hall transport without any extrinsic symmetry breaking in the non-centrosymmetric monolayer and trilayer MoS2, evidenced by considerable nonlocal resistance that scales cubically with local resistance. Such a hallmark survives even at room temperature with a valley diffusion length at micron scale. By contrast, no valley Hall signal is observed in the centrosymmetric bilayer MoS2. Our work elucidates the topological quantum origin of valley Hall effects and marks a significant step towards the purely electrical control of valley degree of freedom in topological valleytronics.

cond-mat.mes-hall↗

arXiv2018

Probing Landau levels of strongly interacting massive Dirac electrons in layer-polarized MoS$_2$

Monolayer transition metal dichalcogenides are recently emerged 2D electronic systems with various novel properties, such as spin-valley locking, circular dichroism, valley Hall effects, Ising superconductivity. The reduced dimensionality and large effective masses further produce unconventional many-body interaction effects. Although recent hole transport measurements in WSe$_2$ indicate strong interactions in the valence bands, many-body interaction effects, particularly in the conduction bands, remain elusive to date. Here, for the first time, we perform transport measurements up to a magnetic field of $29$T to study the massive Dirac electron Landau levels (LL) in layer-polarized MoS$_2$ samples with mobilities of $22000$cm$^2$/(V$\cdot$s) at $1.5$K and densities of $\sim10^{12}$cm$^{-2}$. With decreasing the density, we observe LL crossing induced valley ferrimagnet-to-ferromagnet transitions, as a result of the interaction enhancement of the g-factor from $5.64$ to $21.82$. Near integer ratios of Zeeman-to-cyclotron energies, we discover LL anticrossings due to the formation of quantum Hall Ising ferromagnets, the valley polarizations of which appear to be reversible by tuning the density or an in-plane magnetic field. Our results provide compelling evidence for many-body interaction effects in the conduction bands of monolayer MoS$_2$ and establish a fertile ground for exploring strongly correlated phenomena of massive Dirac electrons.

cond-mat.mes-hall↗

arXiv2017

Isolation and Characterization of Few-layer Manganese Thiophosphite

This work reports an experimental study on an antiferromagnetic honeycomb lattice of MnPS$_3$ that couples the valley degree of freedom to a macroscopic antiferromagnetic order. The crystal structure of MnPS$_3$ is identified by high resolution scanning transmission electron microscopy. Layer dependent angle resolved polarized Raman fingerprints of the MnPS$_3$ crystal are obtained and the Raman peak at 383 cm$^{-1}$ exhibits 100% polarity. Temperature dependences of anisotropic magnetic susceptibility of MnPS$_3$ crystal are measured in superconducting quantum interference device. Magnetic parameters like effective magnetic moment, and exchange interaction are extracted from the mean field approximation mode. Ambipolar electronic transport channels in MnPS$_3$ are realized by the liquid gating technique. The conducting channel of MnPS$_3$ offers a unique platform for exploring the spin/valleytronics and magnetic orders in 2D limitation.

cond-mat.mes-hall↗

arXiv2017

Probing strong interactions in p-type few-layer WSe$_2$ by density-dependent Landau level crossing

Atomically thin transition metal dichalcogenides (TMDCs) such as MoS$_2$ and WSe$_2$ are emerging as a new platform for exploring many-body effects. Coulomb interactions are markedly enhanced in these materials because of the reduced screening and the large Wigner-Seitz radii. Although many-body excitonic effects in TMDCs have been extensively studied by optical means, not until recently did probing their strongly correlated electronic effects become possible in transport. Here, in p-type few-layer WSe$_2$ we observe highly density-dependent quantum Hall states of {\Gamma} valley holes below 12 T, whose predominant sequences alternate between odd- and even-integers. By tilting the magnetic field to induce Landau level crossings, we show that the strong Coulomb interaction enhances the Zeeman-to-cyclotron energy ratio from 2.67 to 3.55 as the density is reduced from 5.7 to 4.0$\times$10$^{12}$ cm$^{-2}$, giving rise to the even-odd alternation. Unprecedentedly, this indicates a 4.8 times enhancement of the g-factor over its band theory value at a density as high as 4.0$\times$10$^{12}$ cm$^{-2}$. Our findings unambiguously demonstrate that p-type few-layer WSe$_2$ is a superior platform for exploring strongly correlated electronic phenomena, opening a new perspective for realizing the elusive Wigner crystallization at a moderate density.

cond-mat.mes-hall↗

arXiv2017

Quantum Transport in Ambipolar Few-layer Black Phosphorus

Few-layer black phosphorus possesses unique electronic properties giving rise to distinct quantum phenomena and thus offers a fertile platform to explore the emergent correlation phenomena in low dimensions. A great progress has been demonstrated in improving the quality of hole-doped few-layer black phosphorus and its quantum transport studies, whereas the same achievements are rather modest for electron-doped few-layer black phosphorus. Here, we report the ambipolar quantum transport in few-layer black phosphorus exhibiting undoubtedly the quantum Hall effect for hole transport and showing clear signatures of the quantum Hall effect for electron transport. By bringing the spin-resolved Landau levels of the electron-doped black phosphorus to the coincidence, we measure the spin susceptibility $\chi_s=m^\ast g^\ast=1.1\pm0.03$. This value is larger than for hole-doped black phosphorus and illustrates an energetically equidistant arrangement of spin-resolved Landau levels. Evidently, the n-type black phosphorus offers a unique platform with equidistant sequence of spin-up and spin-down states for exploring the quantum spintronic.

cond-mat.mes-hall↗

arXiv2017

Gate-tunable strong-weak localization transition in few-layer black phosphorus

Atomically thin black phosphorus (BP) field-effect transistors show strong-weak localization transition which is tunable through gate voltages. Hopping transports through charge impurity induced localized states are measured at low-carrier density regime. Variable-range hopping model is applied to simulate the charge carrier scattering behavior. In the high-carrier concentration regime, a negative magnetoresistance signals the weak localization effect. The extracted phase coherence length is power-law temperature dependent ($\sim T^{-0.48\pm0.03}$) and demonstrates electron-electron interactions in few-layer BP. The competition between the Strong localization length and phase coherence length is proposed and discussed based on the observed gate tunable strong-weak localization transition in few-layer BP.

cond-mat.mes-hall↗

arXiv2017

Odd-integer quantum Hall states and giant spin susceptibility in p-type few-layer WSe2

We fabricate high-mobility p-type few-layer WSe2 field-effect transistors and surprisingly observe a series of quantum Hall (QH) states following an unconventional sequence predominated by odd-integer states under a moderate strength magnetic field. By tilting the magnetic field, we discover Landau level (LL) crossing effects at ultra-low coincident angles, revealing that the Zeeman energy is about three times as large as the cyclotron energy near the valence band top at {\Gamma} valley. This result implies the significant roles played by the exchange interactions in p-type few-layer WSe2, in which itinerant or QH ferromagnetism likely occurs. Evidently, the {\Gamma} valley of few-layer WSe2 offers a unique platform with unusually heavy hole-carriers and a substantially enhanced g-factor for exploring strongly correlated phenomena.

cond-mat.mes-hall↗

arXiv2016

Charge Density Wave Phase Transition on the Surface of Electrostatically Doped Multilayer Graphene

We demonstrate that charge density wave (CDW) phase transition occurs on the surface of electronically doped multilayer graphene when the Fermi level approaches the M points (also known as van Hove singularities where the density of states diverge) in the Brillouin zone of graphene band structure. The occurrence of such CDW phase transitions are supported by both the electrical transport measurement and optical measurements in electrostatically doped multilayer graphene. The CDW transition is accompanied with the sudden change of graphene channel resistance at T$_m$= 100K, as well as the splitting of Raman G peak (1580 cm$^{-1}$). The splitting of Raman G peak indicats the lifting of in-plane optical phonon branch degeneracy and the non-degenerate phonon branches are correlated to the lattice reconstructions of graphene -- the CDW phase transition.

cond-mat.mes-hall↗

arXiv2016

Type-controlled Nanodevices Based on Encapsulated Few-layer Black Phosphorus for Quantum Transport

We demonstrate that encapsulation of atomically thin black phosphorus (BP) by hexagonal boron nitride (h-BN) sheets is very effective for minimizing the interface impurities induced during fabrication of BP channel material for quantum transport nanodevices. Highly stable BP nanodevices with ultrahigh mobility and controllable types are realized through depositing appropriate metal electrodes after conducting a selective etching to the BP encapsulation structure. Chromium and titanium are suitable metal electrodes for BP channels to control the transition from a p-type unipolar property to ambipolar characteristic because of different work functions. Record-high mobilities of 6000 $cm^2V^{-1}s^{-1}$ and 8400 $cm^2V^{-1}s^{-1}$ are respectively obtained for electrons and holes at cryogenic temperatures. High-mobility BP devices enable the investigation of quantum oscillations with an indistinguishable Zeeman effect in laboratory magnetic field.

cond-mat.mes-hall↗

arXiv2016

Probing the Electronic States in Black Phosphorus Vertical Heterostructures

Atomically thin black phosphorus (BP) is a promising two-dimensional material for fabricating electronic and optoelectronic nano-devices with high mobility and tunable bandgap structures. However, the charge-carrier mobility in few-layer phosphorene (monolayer BP) is mainly limited by the presence of impurity and disorders. In this study, we demonstrate that vertical BP heterostructure devices offer great advantages in probing the electron states of monolayer and few-layer phosphorene at temperatures down to 2 K through capacitance spectroscopy. Electronic states in the conduction and valence bands of phosphorene are accessible over a wide range of temperature and frequency. Exponential band tails have been determined to be related to disorders. Unusual phenomena such as the large temperature-dependence of the electron state population in few-layer phosphorene have been observed and systematically studied. By combining the first-principles calculation, we identified that the thermal excitation of charge trap states and oxidation-induced defect states were the main reasons for this large temperature dependence of the electron state population and degradation of the on-off ratio in phosphorene field-effect transistors.

cond-mat.mtrl-sci↗

arXiv2015

Band bending at interfaces between topological insulator Bi2Se3 and transition metals

Interfaces between exfoliated topological insulator Bi2Se3 and several transition metals deposited by sputtering were studied by XPS, SIMS, UPS and contact I-V measurements. Chemically clean interfaces can be achieved when coating Bi2Se3 with a transition metal layer as thin as 1 nm, even without capping. Most interestingly, UPS spectra suggest depletion or inversion in the originally n-type topological insulator near the interface. Strong band bending in the topological insulator requires careful material engineering or electric biasing if one desires to make use of the spin locking in surface states in the bulk gap for potential spintronic applications

cond-mat.mes-hall↗

arXiv2015

Observation of Valley Zeeman and Quantum Hall Effects at Q Valley of Few-Layer Transition Metal Disulfides

In few-layer (FL) transition metal dichalcogenides (TMDC), the conduction bands along the Gamma-K directions shift downward energetically in the presence of interlayer interactions, forming six Q valleys related by three-fold rotational symmetry and time reversal symmetry. In even-layers the extra inversion symmetry requires all states to be Kramers degenerate, whereas in odd-layers the intrinsic inversion asymmetry dictates the Q valleys to be spin-valley coupled. In this Letter, we report the transport characterization of prominent Shubnikov-de Hass (SdH) oscillations for the Q valley electrons in FL transition metal disulfide (TMDs), as well as the first quantum Hall effect (QHE) in TMDCs. Our devices exhibit ultrahigh field-effect mobilities (~16,000 cm2V-1s-1 for FL WS2 and ~10,500 cm2V-1s-1 for FL MoS2) at cryogenic temperatures. Universally in the SdH oscillations, we observe a valley Zeeman effect in all odd-layer TMD devices and a spin Zeeman effect in all even-layer TMD devices.

cond-mat.mes-hall↗

arXiv2015

Negative Compressibility in Graphene-terminated Black Phosphorus Heterostructures

Negative compressibility generated by many-body effects in 2D electronic systems can enhance gate capacitance. We observe capacitance enhancement in a newly emerged 2D layered material, atomically thin black phosphorus (BP). The encapsulation of BP by hexagonal boron nitride sheets with few-layer graphene as a terminal ensures ultraclean heterostructure interfaces, allowing us to observe negative compressibility at low hole carrier concentrations. We explained the negative compressibility based on the Coulomb correlation among in-plane charges and their image charges in a gate electrode in the framework of Debye screening.

cond-mat.mes-hall↗

arXiv2015

Quantum Hall Effect in Ultrahigh Mobility Two-dimensional Hole Gas of Black Phosphorus

We demonstrate that a field effect transistor (FET) made of few layer black phosphorus (BP) encapsulated in hexagonal boron nitride (h-BN) in vacuum, exhibts the room temperature hole mobility of 5200 $cm^2/Vs$ being limited just by the phonon scattering. At cryogenic tempeature the FET mobility increases up to 45,000 $cm^2/Vs$, which is eight times higher compared with the mobility obtained in earlier reports. The unprecedentedly clean h-BN/BP/h-BN heterostructure exhibits Shubnikov-de Haas oscillations and quantum Hall effect with Landau level (LL) filling factors down to v=2 in conventional laboratory magnetic fields. Moreover, carrier density independent effective mass m=0.26 m_0 is measured, and Lande g-factor g=2.47 is reported. Furthermore, an indication for a distinct hole transport behavior with up and down spin orientation is found.

cond-mat.mes-hall↗

arXiv2015

Fast Transfer-free Synthesis of High-quality Monolayer Graphene on Insulating Substrates by Simple Rapid Thermal Treatment

The transfer-free synthesis of high-quality, large-area graphene on a given dielectric substrate, which is highly desirable for device applications, remains a significant challenge. In this paper, we report on a simple rapid thermal treatment method for the fast and direct growth of high-quality, large-scale monolayer graphene on a SiO2/Si substrate from solid carbon sources. The stack structure of solid carbon layer/copper film/SiO2 is adopted in the RTT process. The inserted copper film does not only act as an active catalyst for the carbon precursor but also serves as a "filter" that prevents premature carbon dissolution, and thus, contributes to monolayer graphene growth on SiO2/Si. The produced monolayer graphene exhibits high carrier mobility of up to 3000 cm2 V-1s-1 at room temperature and standard half-integer quantum oscillations.

cond-mat.mtrl-sci↗