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Lieb-Thirring inequalities for an effective Hamiltonian of bilayer graphene. (arXiv:1910.04296v1 [math-ph])

Publications on 2D Materials - Fri, 11/10/2019 - 12:30pm

Combining the methods of Cuenin [2019] and Borichev-Golinskii-Kupin [2009, 2018], we obtain the so-called Lieb-Thirring inequalities for non-selfadjoint perturbations of an effective Hamiltonian for bilayer graphene.

Published : "arXiv Mesoscale and Nanoscale Physics".

The post Lieb-Thirring inequalities for an effective Hamiltonian of bilayer graphene. (arXiv:1910.04296v1 [math-ph]) appeared first on 2D Research.

Ultra-thin van der Waals crystals as semiconductor quantum wells. (arXiv:1910.04215v1 [cond-mat.mes-hall])

Publications on 2D Materials - Fri, 11/10/2019 - 12:30pm

Control over the electronic spectrum at low energy is at the heart of the functioning of modern advanced electronics: high electron mobility transistors, semiconductor and Capasso terahertz lasers, and many others. Most of those devices rely on the meticulous engineering of the size quantization of electrons in quantum wells. This avenue, however, hasn’t been explored in the case of 2D materials. Here we transfer this concept onto the van der Waals heterostructures which utilize few-layers films of InSe as quantum wells. The precise control over the energy of the subbands and their uniformity guarantees extremely high quality of the electronic transport in such systems. Using novel tunnelling and light emitting devices, for the first time we reveal the full subbands structure by studying resonance features in the tunnelling current, photoabsorption and light emission. In the future, these systems will allow development of elementary blocks for atomically thin infrared and THz light sources based on intersubband optical transitions in few-layer films of van der Waals materials.

Published : "arXiv Mesoscale and Nanoscale Physics".

The post Ultra-thin van der Waals crystals as semiconductor quantum wells. (arXiv:1910.04215v1 [cond-mat.mes-hall]) appeared first on 2D Research.

Twisted bilayer graphene in a parallel magnetic field. (arXiv:1910.04185v1 [cond-mat.str-el])

Publications on 2D Materials - Fri, 11/10/2019 - 12:30pm

We study the effect of an in-plane magnetic field on the non-interacting dispersion of twisted bilayer graphene. Our analysis is rooted in the chirally symmetric continuum model, whose zero-field band structure hosts exactly flat bands and large energy gaps at the magic angles. At the first magic angle, the central bands respond to a parallel field by forming a quadratic band crossing point (QBCP) at the Moire Brillouin zone center. Over a large range of fields, the dispersion is invariant with an overall scale set by the magnetic field strength. For deviations from the magic angle and for realistic interlayer couplings, the motion and merging of the Dirac points lying near charge neutrality are discussed in the context of the symmetries, and we show that small magnetic fields are able to induce a qualitative change in the energy spectrum. We conclude with a discussion on the possible ramifications of our study to the interacting ground states of twisted bilayer graphene systems.

Published : "arXiv Mesoscale and Nanoscale Physics".

The post Twisted bilayer graphene in a parallel magnetic field. (arXiv:1910.04185v1 [cond-mat.str-el]) appeared first on 2D Research.

Topological charge pumping by sliding moir'{e} pattern. (arXiv:1910.04654v1 [cond-mat.mes-hall])

Publications on 2D Materials - Fri, 11/10/2019 - 12:30pm

We study the adiabatic topological charge pumping driven by interlayer sliding in the moir'{e} superlattices. We show that, when we slide a single layer of the twisted bilayer system relatively to the other, a moir'{e} pattern flow and a quantized transport of electrons occurs. When the Fermi energy is in a spectral gap, the number of pumped charges in the interlayer sliding process is quantized to a sliding Chern number, which obeys a Diophantine equation analogous to the quantum Hall effect. We apply the argument to the twisted bilayer graphene, and find that energy gaps above and below the nearly-flat bands has non-zero sliding Chern numbers. When the Fermi energy is in either of those gaps, the slide-driven topological pumping occurs perpendicularly to the sliding direction.

Published : "arXiv Mesoscale and Nanoscale Physics".

The post Topological charge pumping by sliding moir'{e} pattern. (arXiv:1910.04654v1 [cond-mat.mes-hall]) appeared first on 2D Research.

Large multi-directional spin-to-charge conversion in low symmetry semimetal MoTe$_2$ at room temperature. (arXiv:1910.04642v1 [cond-mat.mes-hall])

Publications on 2D Materials - Fri, 11/10/2019 - 12:30pm

Efficient and versatile spin-to-charge current conversion is crucial for the development of spintronic applications, which strongly rely on the ability to electrically generate and detect spin currents. In this context, the spin Hall effect has been widely studied in heavy metals with strong spin-orbit coupling. While the high crystal symmetry in these materials limits the conversion to the orthogonal configuration, unusual configurations are expected in low symmetry transition metal dichalcogenide semimetals, which could add flexibility to the electrical injection and detection of pure spin currents. Here, we report the observation of spin-to-charge conversion in MoTe$_2$ flakes, which are stacked in graphene lateral spin valves. We detect two distinct contributions arising from the conversion of two different spin orientations. In addition to the conventional conversion where the spin polarization is orthogonal to the charge current, we also detect a conversion where the spin polarization and the charge current are parallel. Both contributions, which could arise either from bulk spin Hall effect or surface Edelstein effect, show large efficiencies comparable to the best spin Hall metals and topological insulators. Our finding enables the simultaneous conversion of spin currents with any in-plane spin polarization in one single experimental configuration.

Published : "arXiv Mesoscale and Nanoscale Physics".

The post Large multi-directional spin-to-charge conversion in low symmetry semimetal MoTe$_2$ at room temperature. (arXiv:1910.04642v1 [cond-mat.mes-hall]) appeared first on 2D Research.

Electrically driven photon emission from individual atomic defects in monolayer WS2. (arXiv:1910.04612v1 [cond-mat.mes-hall])

Publications on 2D Materials - Fri, 11/10/2019 - 12:30pm

Optical quantum emitters are a key component of quantum devices for metrology and information processing. In particular, atomic defects in 2D materials can operate as optical quantum emitters that overcome current limitations of conventional bulk emitters, such as yielding a high single-photon generation rate and offering surface accessibility for excitation and photon extraction. Here we demonstrate electrically stimulated photon emission from individual point defects in a 2D material. Specifically, by bringing a metallic tip into close proximity to a discrete defect state in the band gap of WS2, we induce inelastic tip-to-defect electron tunneling with an excess of transition energy carried by the emitted photons. We gain atomic spatial control over the emission through the position of the tip, while the spectral characteristics are highly customizable by varying the applied tip-sample voltage. Atomically resolved emission maps of individual sulfur vacancies and chromium substituent defects are in excellent agreement with the electron density of their respective defect orbitals as imaged via conventional elastic scanning tunneling microscopy. Inelastic charge-carrier injection into localized defect states of 2D materials thus provides a powerful platform for electrically driven, broadly tunable, atomic-scale single-photon sources.

Published : "arXiv Mesoscale and Nanoscale Physics".

The post Electrically driven photon emission from individual atomic defects in monolayer WS2. (arXiv:1910.04612v1 [cond-mat.mes-hall]) appeared first on 2D Research.

Machine learning driven synthesis of few-layered WTe2. (arXiv:1910.04603v1 [cond-mat.mtrl-sci])

Publications on 2D Materials - Fri, 11/10/2019 - 10:29am

Reducing the lateral scale of two-dimensional (2D) materials to one-dimensional (1D) has attracted substantial research interest not only to achieve competitive electronic device applications but also for the exploration of fundamental physical properties. Controllable synthesis of high-quality 1D nanoribbons (NRs) is thus highly desirable and essential for the further study. Traditional exploration of the optimal synthesis conditions of novel materials is based on the trial-and-error approach, which is time consuming, costly and laborious. Recently, machine learning (ML) has demonstrated promising capability in guiding material synthesis through effectively learning from the past data and then making recommendations. Here, we report the implementation of supervised ML for the chemical vapor deposition (CVD) synthesis of high-quality 1D few-layered WTe2 nanoribbons (NRs). The synthesis parameters of the WTe2 NRs are optimized by the trained ML model. On top of that, the growth mechanism of as-synthesized 1T’ few-layered WTe2 NRs is further proposed, which may inspire the growth strategies for other 1D nanostructures. Our findings suggest that ML is a powerful and efficient approach to aid the synthesis of 1D nanostructures, opening up new opportunities for intelligent material development.

Published in: "arXiv Material Science".

The post Machine learning driven synthesis of few-layered WTe2. (arXiv:1910.04603v1 [cond-mat.mtrl-sci]) appeared first on 2D Research.

Phonon hydrodynamics and record room-temperature thermal conductivity in thin graphite. (arXiv:1910.04422v1 [cond-mat.mtrl-sci])

Publications on 2D Materials - Fri, 11/10/2019 - 10:29am

Allotropes of carbon are known as excellent conductors of heat, a feature commonly attributed to their high Debye temperature. Diamond, while dethroned by graphene, has kept remaining the best room-temperature thermal conductor among bulk solids. Here, by employing a four-probe technique to monitor the evolution of thermal conductivity as a function of temperature and thickness, we find an intimate link between high conductivity, thickness and phonon hydrodynamics in thin graphite. The in-plane thermal conductivity in 8-micron-thick graphite attains 4500 W/Km near room temperature, well above diamond and slightly larger than what was reported in isotopically purified graphene. In a wide temperature range, warming enhances thermal diffusivity, attesting that the flow of phonons is partially hydrodynamic. The observed enhancement of thermal conductivity with decreasing thickness points to a correlation between the out-of-plane momentum of phonons and the fraction of collisions which relax momentum. We argue that this can be understood given the extreme anisotropy of phonon dispersion in graphite.

Published in: "arXiv Material Science".

The post Phonon hydrodynamics and record room-temperature thermal conductivity in thin graphite. (arXiv:1910.04422v1 [cond-mat.mtrl-sci]) appeared first on 2D Research.

Robust edge photocurrent response on layered Type II Weyl semimetal WTe2. (arXiv:1910.04624v1 [cond-mat.mes-hall])

Publications on 2D Materials - Fri, 11/10/2019 - 10:29am

Photo sensing and energy harvesting based on exotic properties of quantum materials and new operation principles have great potentials to break the fundamental performance limit of conventional photodetectors and solar cells. As topological nontrivial materials, Weyl semimetals have demonstrated novel optoelectronic properties that promise potential applications in photo detection and energy harvesting arising from their gapless linear dispersion near Weyl nodes and Berry field enhanced nonlinear optical effect at the vicinity of Weyl nodes. In this work, we demonstrate robust photocurrent generation from charge separation of photoexctied electron-hole pairs at the edge of Td-WTe2, a type-II Weyl semimetal, due to crystalline-symmetry breaking along certain crystal fracture directions and possibly enhanced by robust fermi-arc type surface states. Using scanning photocurrent microscopy (SPCM) measurements, we further demonstrate that the edge current response is robust over a wide excitation photon energy. We find that this robust feature is highly generic, and shall arise universally in a wide class of quantum materials with similar crystal symmetries. In addition, possible connections between these edge photocurrents and topological properties of Weyl semimetals are explored. The robust and generic edge current response demonstrated in this work provides a new type of charge separation mechanism for photosensing and energy harvesting over broad wavelength range.

Published in: "arXiv Material Science".

The post Robust edge photocurrent response on layered Type II Weyl semimetal WTe2. (arXiv:1910.04624v1 [cond-mat.mes-hall]) appeared first on 2D Research.

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