Symmetry breaking in two-dimensional layered materials plays a significant role in their macroscopic electrical, optical, magnetic and topological properties, including but not limited to spin-polarization effects, valley-contrasting physics, nonlinear Hall effects, nematic order, ferroelectricity, Bose-Einstein condensation and unconventional superconductivity. Engineering symmetry breaking of two-dimensional layered materials not only offers extraordinary opportunities to tune their physical properties, but also provides unprecedented possibilities to introduce completely new physics and technological innovations in electronics, photonics and optoelectronics. Indeed, over the past 15 years, a wide variety of physical, structural and chemical approaches have been developed to engineer symmetry breaking of two-dimensional layered materials. In this Review, we focus on the recent progresses on engineering the breaking of inversion, rotational, time reversal and spontaneous gauge symmetries in two-dimensional layered materials, and illustrate our perspectives on how these may lead to potential new physics and applications.
Inversion symmetry in bilayer graphene allows for layered opposite Rashba spin-orbit coupling (LO-RSOC) -- the situation when the RSOC has the same magnitude but the opposite sign in two coupled spatially separated layers. We show that the LO-RSOC results in the loss of spin chirality in the momentum space, in contrast to the common uniform RSOC. This chirality loss makes it difficult to experimentally establish whether the LO-RSOC (on the scale of 10 meV) exists, because the band structure is insensitive to it. To solve this problem, we propose to identify the LO-RSOC either by gating to brea…
By combining {\it ab initio} calculations and experiments we demonstrate how the band gap of the transition metal tri-chalcogenide TiS$_3$ can be modified by inducing tensile or compressive strain. We show by numerical calculations that the electronic band gap of layered TiS$_3$ can be modified for monolayer, bilayer and bulk material by inducing either hydrostatic pressure or strain. In addition, we find that the monolayer and bilayer exhibits a transition from a direct to indirect gap when the strain is increased in the direction of easy transport. The ability to control the band gap and its…
A comprehensive rational thermal material design paradigm requires the ability to reduce and enhance the thermal conductivities of nanomaterials. In contrast to the existing ability to reduce the thermal conductivity, methods that allow to enhance heat conduction are currently limited. Enhancing the nanoscale thermal conductivity could bring radical improvements in the performance of electronics, optoelectronics, and photovoltaic systems. Here, we show that enhanced thermal conductivities can be achieved in semiconductor nanostructures by rationally engineering phonon spectral coupling between…
The magnetic interfacial Dzyaloshinskii-Moriya interaction (DMI) in multi-layered thin films can lead to exotic chiral spin states, of paramount importance for future spintronic technologies. Interfacial DMI is normally manifested as an intralayer interaction, mediated via a paramagnetic heavy metal in systems lacking inversion symmetry. Here we show how, by designing synthetic antiferromagnets with canted magnetization states, it is also possible to observe interfacial interlayer-DMI at room temperature. The interlayer-DMI breaks the symmetry of the magnetic reversal process via the emergence…
Atomic modulations of two-dimensional materials using scanning tunneling microscope (STM) tip-induced forces modifies their mechanical and electrical properties. In situ topographic and spectroscopic probing through electrical tunneling has been used for straining sheets of graphite, monolayer graphene and NbSe2. The findings also resolve a thirty-five-year-old controversy involving numerous proposed models to explain the source of anomalously high measured atomic amplitudes (of up to 24 Angstroms, expected 0.2 Angstroms) from atomic corrugation on graphite surfaces. Our findings attributes th…
Layering two-dimensional van der Waals materials provides unprecedented control over atomic placement, which could enable tailoring of vibrational spectra and heat flow at the sub-nanometer scale. Here, using spatially-resolved ultrafast thermoreflectance and spectroscopy, we uncover the design rules governing cross-plane heat transport in superlattices assembled from monolayers of graphene (G) and MoS2 (M). Using a combinatorial experimental approach, we probe nine different stacking sequences: G, GG, MG, GGG, GMG, GGMG, GMGG, GMMG, GMGMG and identify the effects of vibrational mismatch, inte…
Strain provides an effective means to tune the electrical properties while retaining the native chemical composition of the material. Unlike three-dimensional solids, two-dimensional materials withstand higher levels of elastic strain making it easier to tune various electrical properties to suit the technology needs. In this work we explore the effect of uniaxial tensile-strain on the electrical transport properties of bi- and few-layered MoS2, a promising 2D semiconductor. Raman shifts corresponding to the in-plane vibrational modes show a redshift with strain indicating a softening of the i…
Hosting unique drumhead surface states enclosed by nodal lines, topological nodal-line semimetals exhibit novel transport phenomena. Thus, the exploration of topological semimetals with different nodal-line structures has attracted much attention. In this paper, we first find a second-order nodal line semimetal (SONLS), which has coexisting hinge Fermi arcs and drumhead surface states, in a $\mathcal{PT}$-symmetry broken system. Then, without changing the intrinsic parameters, we artificially create exotic hybrid-order nodal-line semimetals hosted by different quasienergy gaps and rich nodal-l…
Type-II Weyl fermion nodes, located at the touching points between electron and hole pockets, have been recently predicted to occur in distorted octahedral ($T'$) transition metal dichalcogenide semimetals, contingent upon the condition that the layered crystal has the noncentrosymmetric orthorhombic ($T'_{or}$) stacking. Here, we report on the emergence of two shear Raman bands activated by inversion symmetry breaking in $T'$-MoTe$_2$ due to sample cooling. Polarization and crystal orientation resolved measurements further point to a phase transition from the monoclinic ($T'_{mo}$) structure …
We report broadband microwave absorption spectroscopy of the layered antiferromagnet CrCl3. We observe a rich structure of resonances arising from quasi-two-dimensional antiferromagnetic dynamics. Due to the weak interlayer magnetic coupling in this material, we are able to observe both optical and acoustic branches of antiferromagnetic resonance in the GHz frequency range and a symmetry-protected crossing between them. By breaking rotational symmetry, we further show that strong magnon-magnon coupling with large tunable gaps can be induced between the two resonant modes.
Integrated quantum photonic circuitry is an emerging topic that requires efficient coupling of quantum light sources to waveguides and optical resonators. So far, great effort has been devoted to engineering on-chip systems from three-dimensional crystals such as diamond or gallium arsenide. In this study, we demonstrate room temperature coupling of quantum emitters embedded within a layered hexagonal boron nitride to an on-chip aluminium nitride waveguide. We achieved 1.2% light coupling efficiency of the device and realise transmission of single photons through the waveguide. Our results ser…
Studying the influence of breaking time-reversal symmetry on topological insulator surface states is an important problem of current interest in condensed matter physics and could provide a route toward proof-of-concept spintronic devices that exploit spin-textured surface states. Here, we develop a new model system for studying the effect of breaking time-reversal symmetry: a hybrid heterostructure wherein a ferromagnetic semiconductor Ga1-xMnxAs, with an out-of-plane component of magnetization, is cleanly interfaced with a three-dimensional topological insulator (Bi,Sb)2(Te,Se)3 by molecular…
Tailoring spontaneous symmetry breaking in engineered van der Waals superlattices
Peter Grünberg Institut (PGI-3), Forschungszentrum Jülich（于利希研究中心）；Jülich Aachen Research Alliance, Fundamentals of Future Information Technology（亚琛于利希研究联盟）；Institut für Experimentalphysik II B, RWTH Aachen（亚琛工业大学）；Institut für Theoretische Physik und Astrophysik and Würzburg-Dresden Cluster of Excellence ctd.qmat, Universität Würzburg（维尔茨堡大学）；Department of Materials, University of Oxford（牛津大学）；John A Paulson School of Engineering and Applied Sciences, Harvard University（哈佛大学）；Institute of Physics II, Universität zu Köln（科隆大学）；Institut für Theorie der statistischen Physik, RWTH Aachen（亚琛工业大学）；Max Pl（马克斯普朗克）
We numerically investigate the impact of boron nitride (BN) domains on the transport properties of graphene nanoribbons with lengths ranging from a few to several hundreds of nanometers and lateral size up to 4 nm. By varying the size and morphology of the BN islands embedded in the graphene matrix, a wide transport tunability is obtained from perfect insulating interfaces to asymmetric electron-hole transmission profiles, providing the possibility to engineer mobility gaps to improve device performances. Even in the low-density limit of embedded BN islands, transport properties are found to b…
Manipulating Spin-Lattice Coupling in Layered Magnetic Topological Insulator Heterostructure $via$ Interface Engineering
Indian Association for the Cultivation of Science（印度科学文化研究所）；University of Maine（缅因大学）；Bose Institute（博斯研究所）；UGC-DAE Consortium for Scientific Research, Indore Centre（UGC-DAE科学研究联盟印多尔中心）
In this paper, we present a novel route to tunable spontaneous valley coherence in heterostructures of two dimensional valleytronic materials with other layered materials hosting anisotropic polaritonic modes. We first discuss the dependence of this coherence on the conductivity tensor of the anisotropic part of a general heterostructure and on its geometrical configuration. Subsequently, we propose two implementations - one, using anisotropic plasmons in phosphorene and another with hyperbolic phonon polaritons in MoO3. In both these systems we show for the first time electrostatic tunability…
The study of atomically thin two-dimensional materials is a young and rapidly growing field. In the past years, a great advance in the study of the remarkable electrical and optical properties of 2D materials fabricated by exfoliation of bulk layered materials has been achieved. Due to the extraordinary mechanical properties of these atomically thin materials, they also hold a great promise for future applications such as flexible electronics. For example, this family of materials can sustain very large deformations without breaking. Due to the combination of small dimensions, high Young's mod…
One of the fascinating properties of the new families of two-dimensional crystals is their high stretchability and the possibility to use external strain to manipulate, in a controlled manner, their optical and electronic properties. Strain engineering, understood as the field that study how the physical properties of materials can be tuned by controlling the elastic strain fields applied to it, has a perfect platform for its implementation in the atomically thin semiconducting materials. The object of this review is to give an overview of the recent progress to control the optical and electro…
Controlling magnetism in low dimensional materials is essential for designing devices that have feature sizes comparable to several critical length scales that exploit functional spin textures, allowing the realization of low-power spintronic and magneto-electric hardware. [1] Unlike conventional covalently-bonded bulk materials, van der Waals (vdW)-bonded layered magnets [2-4] offer exceptional degrees of freedom for engineering spin textures. [5] However, their structural instability has hindered microscopic studies and manipulations. Here, we demonstrate nanoscale structural control in the …
Moiré-induced symmetry breaking of charge order in van der Waals heterostructures
Donostia International Physics Center (DIPC)（圣塞巴斯蒂安国际物理中心）；Institute of Theoretical Physics, University of Hamburg（汉堡大学理论物理研究所）；Department of Physics, Indian Institute of Science Education and Research Bhopal（博帕尔印度科学教育研究系物理系）；The Hamburg Centre for Ultrafast Imaging（汉堡超快成像中心）；Centro de Física de Materiales (CSIC-UPV/EHU)（材料物理中心（西班牙国家科研委员会-巴斯克国立大学/巴斯克国立理工大学））；Max Planck Institute for the Structure and Dynamics of Matter（马克斯·普朗克物质结构与动力学研究所）；Yusuf Hamied Department of Chemistry, University of Cambridge（剑桥大学尤瑟夫·哈米德化学系）；Dept. Polymers and Advanced Materials, University of the Basque Country (UPV/EHU)（巴斯克国立大学（UPV/EHU）聚合物与先进材料系）；Ikerbasque, Basque Foundation for Science（伊克拉巴斯科，巴斯克科学基金会）
Exotic properties in single or few layers of van der Waals materials carry great promise for applications in nanoscaled electronics, optoelectronics and flexible devices. The established, distinct examples include extremely high mobility and superior thermal conductivity in graphene, a large direct band gap in monolayer MoS2 and quantum spin Hall effect in WTe2 monolayer, etc. All these exotic properties arise from the electron quantum confinement effect in the two-dimensional limit. Here we report a novel phenomenon due to one-dimensional (1D) confinement of carriers in a layered van der Waal…
Van der Waals heterostructure based on layered two-dimensional (2D) materials offers unprecedented opportunities to create materials with atomic precision by design. By combining superior properties of each component, such heterostructure also provides possible solutions to address various challenges of the electronic devices, especially those with vertical multilayered structures. Here, we report the realization of robust memristors for the first time based on van der Waals heterostructure of fully layered 2D materials (graphene/MoS2-xOx/graphene) and demonstrate a good thermal stability lack…
