Signatures of Fractional Quantum Anomalous Hall States in Twisted MoTe2 Bilayer. (arXiv:2304.08470v1 [cond-mat.mes-hall])

2023-04-18T04:30:40+00:00April 18th, 2023|Categories: Publications|Tags: |

The interplay between spontaneous symmetry breaking and topology can result in exotic quantum states of matter. A celebrated example is the quantum anomalous Hall (QAH) state, which exhibits an integer quantum Hall effect at zero magnetic field thanks to its intrinsic ferromagnetism. In the presence of strong electron-electron interactions, exotic fractional-QAH (FQAH) states at zero magnetic field can emerge. These states could host fractional excitations, including non-Abelian anyons – crucial building blocks for topological quantum computation. Flat Chern bands are widely considered as a desirable venue to realize the FQAH state. For this purpose, twisted transition metal dichalcogenide homobilayers in rhombohedral stacking have recently been predicted to be a promising material platform. Here, we report experimental signatures of FQAH states in 3.7-degree twisted MoTe2 bilayer. Magnetic circular dichroism measurements reveal robust ferromagnetic states at fractionally hole filled moir’e minibands. Using trion photoluminescence as a sensor, we obtain a Landau fan diagram which shows linear shifts in carrier densities corresponding to the v=-2/3 and -3/5 ferromagnetic states with applied magnetic field. These shifts match the Streda formula dispersion of FQAH states with fractionally quantized Hall conductance of -2/3$e^2/h$ and -3/5$e^2/h$, respectively. Moreover, the v=-1 state exhibits a dispersion corresponding to Chern number -1, consistent with the predicted QAH state. In comparison, several non-ferromagnetic states on the electron doping side do not disperse, i.e., are trivial correlated insulators. The observed topological states can be further electrically driven into topologically trivial states. Our findings provide clear evidence of the long-sought FQAH states, putting

Published : "arXiv Mesoscale and Nanoscale Physics".

Bulk Photovoltaic Effect in Two-Dimensional Distorted MoTe2. (arXiv:2304.08381v1 [cond-mat.mtrl-sci])

2023-04-18T02:29:37+00:00April 18th, 2023|Categories: Publications|Tags: , |

In future solar cell technologies, the thermodynamic Shockley-Queisser limit for solar-to-current conversion in traditional p-n junctions could potentially be overcome with a bulk photovoltaic effect by creating an inversion broken symmetry in piezoelectric or ferroelectric materials. Here, we unveiled mechanical distortion-induced bulk photovoltaic behavior in a two-dimensional material (2D), MoTe2, caused by phase transition and broken inversion symmetry in MoTe2. The phase transition from single-crystalline semiconducting 2H-MoTe2 to semi-metallic 1T-MoTe2 was confirmed using X-ray photoelectron spectroscopy (XPS). We used a micrometer-scale system to measure the absorption of energy, which reduced from 800 meV to 63 meV when phase transformation from hexagonal to distorted octahedral and revealed a smaller bandgap semi-metallic behavior. Experimentally, a large bulk photovoltaic response is anticipated with the maximum photovoltage VOC = 16 mV and a positive signal of the ISC = 60 uA (400 nm, 90.4 Wcm-2) in the absence of an external electric field. The maximum values of both R and EQE were found to be 98 mAW-1 and 30 %, respectively. Our findings unveil distinctive features of the photocurrent responses caused by in-plane polarity and its potential from a wide pool of established TMD-based nanomaterials, and a novel approach to reach high efficiency in converting photons-to-electricity for power harvesting optoelectronics devices.

Published in: "arXiv Material Science".

Giant spin Hall effect in AB-stacked MoTe2/WSe2 bilayers. (arXiv:2303.12881v1 [cond-mat.mes-hall])

2023-03-24T04:30:37+00:00March 24th, 2023|Categories: Publications|Tags: , |

The spin Hall effect (SHE), in which electrical current generates transverse spin current, plays an important role in spintronics for the generation and manipulation of spin-polarized electrons. The phenomenon originates from spin-orbit coupling. In general, stronger spin-orbit coupling favors larger SHEs but shorter spin relaxation times and diffusion lengths. To achieve both large SHEs and long-range spin transport in a single material has remained a challenge. Here we demonstrate a giant intrinsic SHE in AB-stacked MoTe2/WSe2 moir’e bilayers by direct magneto optical imaging. Under moderate electrical currents with density < 1 A/m, we observe spin accumulation on transverse sample edges that nearly saturates the spin density. We also demonstrate long-range spin Hall transport and efficient non-local spin accumulation limited only by the device size (about 10 um). The gate dependence shows that the giant SHE occurs only near the Chern insulating state, and at low temperatures, it emerges after the quantum anomalous Hall breakdown. Our results demonstrate moir'e engineering of Berry curvature and large SHEs for potential spintronics applications.

Published : "arXiv Mesoscale and Nanoscale Physics".

Gate-tuned ambipolar superconductivity with strong pairing interaction in intrinsic gapped monolayer 1T’-MoTe2. (arXiv:2302.14274v1 [cond-mat.supr-con])

2023-03-01T04:30:36+00:00March 1st, 2023|Categories: Publications|Tags: |

Gate tunable two-dimensional (2D) superconductors offer significant advantages when studying superconducting phase transitions. Here, we address superconductivity in exfoliated 1T’-MoTe2 monolayers with an intrinsic band gap of ~7.3 meV using electrostatic doping. Despite large differences in the dispersion of the conduction and the valence bands, superconductivity can be achieved easily for both electrons and holes. The onset of superconductivity occurs near 7-8K for both charge carrier types. This temperature is much higher than in bulk samples. Also the in-plane upper critical field is strongly enhanced and exceeds the BCS Pauli limit in both cases. Gap information is extracted using point-contact spectroscopy. The gap ratio exceeds multiple times the value expected for BCS weak-coupling. All these observations suggest a strong enhancement of the pairing interaction.

Published : "arXiv Mesoscale and Nanoscale Physics".

Electronic Janus lattice and kagome-like bands in coloring-triangular MoTe2 monolayers. (arXiv:2302.06166v1 [cond-mat.mtrl-sci])

2023-02-14T02:29:44+00:00February 14th, 2023|Categories: Publications|Tags: , |

Polymorphic structures of transition-metal dichalcogenides (TMDs) host exotic electronic states, like charge density wave and superconductivity. However, the number of these structures is limited by crystal symmetries, which poses a challenge to achieve tailored lattices and properties both theoretically and experimentally. Here, we report a coloring-triangle (CT) latticed MoTe2 monolayer, termed CT-MoTe2, constructed by controllably introducing uniform and ordered mirror-twin-boundaries into a pristine monolayer in molecular beam epitaxy. Low-temperature scanning tunneling microscopy and spectroscopy (STM/STS) together with theoretical calculations reveal that the monolayer has an electronic Janus lattice, i.e., an energy-dependent atomic-lattice and a pseudo-Te sublattice, and shares the identical geometry with the Mo5Te8 layer. Dirac-like and flat electronic bands inherently existing in the CT lattice are identified by two broad and two prominent peaks in STS spectra, respectively, and verified with density-functional-theory calculations. Two types of intrinsic domain boundaries were observed, in one of which the electronic-Janus-lattice feature maintains, implying potential applications as an energy-tunable electron-tunneling barrier in future functional devices.

Published in: "arXiv Material Science".

In situ Imaging of an Anisotropic Layer-by-Layer Phase Transition in Few-Layer MoTe2. (arXiv:2301.02694v1 [cond-mat.mtrl-sci])

2023-01-10T02:29:29+00:00January 10th, 2023|Categories: Publications|Tags: , , |

Understanding the phase transition mechanisms in two-dimensional (2D) materials is a key to precisely tailor their properties at the nanoscale. Molybdenum ditelluride (MoTe2) exhibits multiple phases at room temperature, making it a promising candidate for phase-change applications. Here, we fabricate lateral 2H-Td interfaces with laser irradiation and probe their phase transitions from micro- to atomic scales with in situ heating in the transmission electron microscope (TEM). By encapsulating the MoTe2 with graphene protection layers, we create an in situ reaction cell compatible with atomic resolution imaging. We find that the Td-to-2H phase transition initiates at phase boundaries at low temperatures (200-225 degree C) and propagates anisotropically along the b-axis in a layer-by-layer fashion. We also demonstrate a fully reversible 2H-Td-2H phase transition cycle, which generates a coherent 2H lattice containing inversion domain boundaries. Our results provide insights on fabricating 2D hetero-phase devices with atomically sharp and coherent interfaces.

Published in: "arXiv Material Science".

Anomalous circular phonon dichroism in transition metal dichalcogenides. (arXiv:2301.00917v1 [cond-mat.mes-hall])

2023-01-04T04:30:29+00:00January 4th, 2023|Categories: Publications|Tags: , |

A magnetic field can generally induce circular phonon dichroism based on the formation of Landau levels of electrons. Here, we study the magnetization-induced circular phonon dichroism in transition metal dichalcogenides, without forming Landau levels. We find that, instead of the conventional deformation potential coupling, pseudogauge-type electron-phonon coupling plays an essential role in the emergence of the phenomenon. As a concrete example, a large dichroism signal is obtained in monolayer MoTe2 on a EuO substrate, even without considering Rashba spin-orbit coupling. Due to the two-dimensional spin-valley-coupled band structure, MoTe2 shows a reciprocal and nonreciprocal absorption of circularly polarized acoustic phonons upon reversing the direction of phonon propagation and magnetization, respectively. By varying the gate voltage, a tunable circular phonon dichroism can be realized, which paves a way toward different physics and applications of two-dimensional acoustoelectronics.

Published : "arXiv Mesoscale and Nanoscale Physics".

Unconventional charge-to-spin conversions in graphene/MoTe2 van der Waals heterostructures. (arXiv:2211.09095v1 [cond-mat.mes-hall])

2022-11-17T02:29:26+00:00November 17th, 2022|Categories: Publications|Tags: , , |

Spin-charge interconversion (SCI) is a central phenomenon to the development of spintronic devices from materials with strong spin-orbit coupling (SOC). In the case of materials with high crystal symmetry, the only allowed SCI processes are those where the spin current, charge current and spin polarization directions are orthogonal to each other. Consequently, standard SCI experiments are designed to maximize the signals arising from the SCI processes with conventional mutually orthogonal geometry. However, in low-symmetry materials, certain non-orthogonal SCI processes are also allowed. Since the standard SCI experiment is limited to charge current flowing only in one direction in the SOC material, certain allowed SCI configurations remain unexplored. In this work, we performed a thorough SCI study in a graphene-based lateral spin valve combined with low-symmetry MoTe$_2$. Due to a very low contact resistance between the two materials, we could detect SCI signals using both a standard configuration, where the charge current is applied along the MoTe$_2$, and a recently introduced (3D-current) configuration, where the charge current flow can be controlled in three directions within the heterostructure. As a result, we observed three different SCI components, one orthogonal and two non-orthogonal, giving new insight into the SCI processes in low-symmetry materials. The large SCI signals obtained at room temperature, along with the versatility of the 3D-current configuration, provide feasibility and flexibility to the design of the next generation of spin-based devices.

Published in: "arXiv Material Science".

Gate-tunable heavy fermions in a moir'e Kondo lattice. (arXiv:2211.00263v1 [cond-mat.str-el])

2022-11-02T04:30:16+00:00November 2nd, 2022|Categories: Publications|Tags: , |

The Kondo lattice, describing a matrix of local magnetic moments coupled via spin-exchange interactions to itinerant conduction electrons, is a prototype of strongly correlated quantum matter. Traditionally, Kondo lattices are realized in intermetallic compounds containing lanthanide or actinide. The complex electronic structure and limited tunability of both the electron density and exchange interactions in these bulk materials pose significant challenges to study Kondo lattice physics. Here, we report the realization of a synthetic Kondo lattice in AB-stacked MoTe2/WSe2 moir’e bilayers, where the MoTe2 layer is tuned to a Mott insulating state, supporting a triangular moir’e lattice of local moments, and the WSe2 layer is doped with itinerant conduction carriers. We observe heavy fermions with a large Fermi surface below the Kondo temperature. We also observe destruction of the heavy fermions by an external magnetic field with an abrupt decrease of the Fermi surface size and quasiparticle mass. We further demonstrate widely and continuously gate-tunable Kondo temperatures through either the itinerant carrier density or Kondo interaction. Our study opens the possibility of in-situ access to the rich phase diagram of the Kondo lattice with exotic quantum criticalities in a single device based on semiconductor moir’e materials

Published : "arXiv Mesoscale and Nanoscale Physics".

Strain Induced Indirect-to-Direct Bandgap Transition, Photoluminescence Enhancement, and Linewidth Reduction in Bilayer MoTe2. (arXiv:2209.11835v1 [cond-mat.mtrl-sci])

2022-09-27T02:29:37+00:00September 27th, 2022|Categories: Publications|Tags: |

Two-dimensional (2D) layered materials provide an ideal platform for engineering electronic and optical properties through strain control because of their extremely high mechanical elasticity and sensitive dependence of material properties on mechanical strain. In this paper, a combined experimental and theoretical effort is made to investigate the effects of mechanical strain on various spectral features of bilayer MoTe2 photoluminescence (PL). We found that bilayer MoTe2 can be converted from an indirect-to direct-bandgap material through strain engineering, resulting in a photoluminescence enhancement by a factor of 2.24. Over 90% of the PL comes from photons emitted by the direct excitons at the maximum strain applied. Importantly, we show that strain effects lead to a reduction of the overall linewidth of PL by as much as 36.6%. We attribute the dramatic decrease of linewidth to a strain-induced complex interplay among various excitonic varieties such as direct bright excitons, trions, and indirect excitons. Our experimental results on direct and indirect exciton emission features are explained by theoretical exciton energies that are based on first-principle electronic band structure calculations. The consistent theory-experimental trend shows that the enhancement of PL and the reduction of linewidth are the consequences of the increasing direct exciton contribution with the increase of strain. Our results demonstrate that strain engineering can lead to a PL quality of the bilayer MoTe2 comparable to that of the monolayer counterpart. The additional benefit of a longer emission wavelength makes the bilayer MoTe2 more suitable for Silicon-photonics integration due to the reduced Silicon absorption.

Published in: "arXiv Material Science".

Valley-coherent quantum anomalous Hall state in AB-stacked MoTe2/WSe2 bilayers. (arXiv:2208.07452v1 [cond-mat.mes-hall])

2022-08-17T04:30:22+00:00August 17th, 2022|Categories: Publications|Tags: , , |

Moir’e materials provide fertile ground for the correlated and topological quantum phenomena. Among them, the quantum anomalous Hall (QAH) effect, in which the Hall resistance is quantized even under zero magnetic field, is a direct manifestation of the intrinsic topological properties of a material and an appealing attribute for low-power electronics applications. The QAH effect has been observed in both graphene and transition metal dichalcogenide (TMD) moir’e materials. It is thought to arise from the interaction-driven valley polarization of the narrow moir’e bands. Here, we show surprisingly that the newly discovered QAH state in AB-stacked MoTe2/WSe2 moir’e bilayers is not valley-polarized but valley-coherent. The layer- and helicity-resolved optical spectroscopy measurement reveals that the QAH ground state possesses spontaneous spin (valley) polarization aligned (anti-aligned) in two TMD layers. In addition, saturation of the out-of-plane spin polarization in both layers occurs only under high magnetic fields, supporting a canted spin texture. Our results call for a new mechanism for the QAH effect and highlight the potential of TMD moir’e materials with strong electronic correlations and spin-orbit interactions for exotic topological states.

Published : "arXiv Mesoscale and Nanoscale Physics".

Valley-coherent quantum anomalous Hall state in AB-stacked MoTe2/WSe2 bilayers. (arXiv:2208.07452v1 [cond-mat.mes-hall])

2022-08-17T02:29:27+00:00August 17th, 2022|Categories: Publications|Tags: , , |

Moir’e materials provide fertile ground for the correlated and topological quantum phenomena. Among them, the quantum anomalous Hall (QAH) effect, in which the Hall resistance is quantized even under zero magnetic field, is a direct manifestation of the intrinsic topological properties of a material and an appealing attribute for low-power electronics applications. The QAH effect has been observed in both graphene and transition metal dichalcogenide (TMD) moir’e materials. It is thought to arise from the interaction-driven valley polarization of the narrow moir’e bands. Here, we show surprisingly that the newly discovered QAH state in AB-stacked MoTe2/WSe2 moir’e bilayers is not valley-polarized but valley-coherent. The layer- and helicity-resolved optical spectroscopy measurement reveals that the QAH ground state possesses spontaneous spin (valley) polarization aligned (anti-aligned) in two TMD layers. In addition, saturation of the out-of-plane spin polarization in both layers occurs only under high magnetic fields, supporting a canted spin texture. Our results call for a new mechanism for the QAH effect and highlight the potential of TMD moir’e materials with strong electronic correlations and spin-orbit interactions for exotic topological states.

Published in: "arXiv Material Science".

Realization of the Haldane Chern insulator in a moir'e lattice. (arXiv:2207.02312v1 [cond-mat.mes-hall])

2022-07-07T04:30:14+00:00July 7th, 2022|Categories: Publications|Tags: , |

The Chern insulator displays a quantized Hall effect without Landau levels. In a landmark paper in 1988, Haldane showed that a Chern insulator could be realized through complex next-nearest-neighbor hopping in a honeycomb lattice. Despite its profound impact on the field of topological physics and recent implementation in cold-atom experiments, the Haldane model has remained elusive in solid-state materials. Here, we report the experimental realization of a Haldane Chern insulator in AB-stacked MoTe2/WSe2 moir’e bilayers, which form a honeycomb moir’e lattice with two sublattices residing in different layers. We show that the moir’e bilayer filled with two charge particles per unit cell is a quantum spin Hall (QSH) insulator with a tunable charge gap. Under a small out-of-plane magnetic field, it becomes a Chern insulator with Chern number c=1 from magneto-transport studies. The results are qualitatively captured by a generalized Kane-Mele tight-binding Hamiltonian. The Zeeman field splits the QSH insulator into two halves of opposite valley–one with a positive and the other a negative moir’e band gap. Our study highlights the unique potential of semiconductor moir’e materials in engineering topological lattice Hamiltonians.

Published : "arXiv Mesoscale and Nanoscale Physics".

Hydrogen-impurity induced unconventional magnetism in semiconducting molybdenum ditelluride. (arXiv:2206.03051v1 [cond-mat.mtrl-sci])

2022-06-08T02:29:43+00:00June 8th, 2022|Categories: Publications|Tags: , , |

Layered transition-metal dichalcogenides are proposed as building blocks for van der Waals (vdW) heterostructures due to their graphene-like two dimensional structure. For this purpose, a magnetic semiconductor could represent an invaluable component for various spintronics and topotronics devices. Here, we combine different local magnetic probe spectroscopies with angle-resolved photoemission and density-functional theory calculations to show that 2H-MoTe2 is on the verge of becoming magnetic. Our results present clear evidence that the magnetism can be “switched on” by a hydrogen-like impurity. We also show that this magnetic state survives up to the free surface region, demonstrating the material’s potential applicability as a magnetic component for thin-film heterostructures.

Published in: "arXiv Material Science".

Multifunctional Two-dimensional van der Waals Janus Magnet Cr-based Dichalcogenide Halides. (arXiv:2205.04053v1 [cond-mat.mtrl-sci])

2022-05-10T02:29:35+00:00May 10th, 2022|Categories: Publications|Tags: , , |

Two-dimensional van der Waals Janus materials and their heterostructures offer fertile platforms for designing fascinating functionalities. Here, by means of systematic first-principles studies on van der Waals Janus monolayer Cr-based dichalcogenide halides CrYX (Y=S, Se, Te; X=Cl, Br, I), we find that CrSX (X=Cl, Br, I) are the very desirable high TC ferromagnetic semiconductors with an out-of-plane magnetization. Excitingly, by the benefit of the large magnetic moments on ligand S2- anions, the sought-after large-gap quantum anomalous Hall effect and sizable valley splitting can be achieved through the magnetic proximity effect in van der Waals heterostructures CrSBr/Bi2Se3/CrSBr and MoTe2/CrSBr, respectively. Additionally, we show that large Dzyaloshinskii-Moriya interactions give rise to skyrmion states in CrTeX (X=Cl, Br, I) under external magnetic fields. Our work reveals that two-dimensional Janus magnet Cr-based dichalcogenide halides have appealing multifunctionalities in the applications of topological electronic and valleytronic devices.

Published in: "arXiv Material Science".

Intrinsic spin Hall torque in a moire Chern magnet. (arXiv:2205.02823v1 [cond-mat.mes-hall])

2022-05-06T04:30:24+00:00May 6th, 2022|Categories: Publications|Tags: , |

In spin torque magnetic memories, electrically actuated spin currents are used to switch a magnetic bit. Typically, these require a multilayer geometry including both a free ferromagnetic layer and a second layer providing spin injection. For example, spin may be injected by a nonmagnetic layer exhibiting a large spin Hall effect, a phenomenon known as spin-orbit torque. Here, we demonstrate a spin-orbit torque magnetic bit in a single two-dimensional system with intrinsic magnetism and strong Berry curvature. We study AB-stacked MoTe2/WSe2, which hosts a magnetic Chern insulator at a carrier density of one hole per moire superlattice site. We observe hysteretic switching of the resistivity as a function of applied current. Magnetic imaging using a superconducting quantum interference device reveals that current switches correspond to reversals of individual magnetic domains. The real space pattern of domain reversals aligns precisely with spin accumulation measured near the high-Berry curvature Hubbard band edges. This suggests that intrinsic spin- or valley-Hall torques drive the observed current-driven magnetic switching in both MoTe2/WSe2 and other moire materials. The switching current density of 10^3 Amps per square centimeter is significantly less than reported in other platforms paving the way for efficient control of magnetic order.

Published : "arXiv Mesoscale and Nanoscale Physics".

Theory of quantum anomalous Hall effect and electric-field-induced phase transition in AB-stacked MoTe2/WSe2 moire heterobilayers. (arXiv:2203.10088v1 [cond-mat.mes-hall])

2022-03-21T04:30:21+00:00March 21st, 2022|Categories: Publications|Tags: , |

In this letter, we propose a new mechanism to explain the unexpected quantum anomalous Hall (QAH) effect and the electric-field-induced Mott-to-QAH phase transition without bulk gap closure in AB-stacked MoTe2/WSe2 moire heterobilayers. We suggest that a hole-occupied band carrying a non-zero Chern number can be generated by an intrinsic band inversion between two topologically distinct Dirac bands and a Coulomb-interaction-induced gap opening in the moire band structure with broken time-reversal symmetry (TRS). The Dirac-band dispersion is induced by a pseudo-magnetic field, which is originated from the Berry phase of Bloch electrons on the moire potential. The TRS is broken by a valley-polarized interlayer-exciton condensation under an out-of-plane electric field. The valley polarization can be realized by an excitonic Bose-Hubbard (BH) model and a Berezinskii-Kosterlitz-Thouless (BKT) transition to transverse ferromagnetism. In low electric fields, the equilibrium state of the system is a Mott insulator state. At a certain electric field, the exciton condensation combining with the Chern band becomes a stabler state and the Mott-to-QAH transition occurs. Since the band inversion is intrinsic, there is no bulk gap closure at the transition.

Published : "arXiv Mesoscale and Nanoscale Physics".

Squeezed metallic droplet with tunable Kubo gap and charge injection in transition metal dichalcogenides. (arXiv:2201.11889v1 [cond-mat.mtrl-sci])

2022-01-31T02:29:19+00:00January 31st, 2022|Categories: Publications|Tags: , |

Shrinking the size of a bulk metal into nanoscale leads to the discreteness of electronic energy levels, the so-called Kubo gap. Renormalization of the electronic properties with a tunable and size-dependent Kubo gap renders fascinating photon emission and electron tunneling. In contrast with usual three-dimensional (3D) metal clusters, here we demonstrate that Kubo gap can be achieved with a two-dimensional (2D) metallic transition metal dichalcogenide (i.e., 1T’-phase MoTe2) nanocluster embedded in a semiconducting polymorph (i.e., 1H-phase MoTe2). Such a 1T’-1H MoTe2 nanodomain resembles a 3D metallic droplet squeezed in a 2D space which shows a strong polarization catastrophe while simultaneously maintains its bond integrity which is absent in traditional delta-gapped 3D clusters. The weak screening of the host 2D MoTe2 leads to photon emission of such pseudo-metallic systems and a ballistic injection of carriers in the 1T’-1H-1T’ homojunctions which may find applications in sensors and 2D reconfigurable devices.

Published in: "arXiv Material Science".

Some say, that 2D Research is the best website in the world.