When two superconductors are connected across a ferromagnet, the spin configuration of the transferred Cooper pairs can be modulated due to magnetic exchange interaction. The resulting supercurrent can reverse its sign across the Josephson junction (JJ) [1-4]. Here we demonstrate Josephson phase modulation in van der Waals heterostructures when Cooper pairs from superconducting NbSe$_2$ tunnel through atomically thin magnetic insulator (MI) Cr$_2$Ge$_2$Te$_6$. Employing a superconducting quantum interference device based on MI JJs, we probe a doubly degenerate non-trivial JJ phase ($ϕ$) originating from the magnetic barrier. This $ϕ$-phase JJ is formed by momentum conserving tunneling of Ising Cooper pairs [5] across magnetic domains in the Cr$_2$Ge$_2$Te$_6$ barrier. The doubly degenerate ground states in MI JJs provide a two-level quantum system that can be utilized as a new disipationless component for superconducting quantum devices, including phase batteries [6], memories [7,8], and quantum Ratchets [9,10].
Non-reciprocal electronic transport in a spatially homogeneous system arises from the simultaneous breaking of inversion and time-reversal symmetries. Superconducting and Josephson diodes, a key ingredient for future non-dissipative quantum devices, have recently been realized. Only a few examples of a vertical superconducting diode effect have been reported and its mechanism, especially whether intrinsic or extrinsic, remains elusive. Here we demonstrate a substantial supercurrent non-reciprocity in a van der Waals vertical Josephson junction formed with a Td-WTe2 barrier and NbSe2 electrodes…
We predict angle-dependent superfluidity for a new class of 2D materials -- transition metals trichalcogenides (TMTC). Within a mean-field approach superfluidity of indirect excitons in TMTC van der Waals heterostructures (vdWHs) is studied. We use different potentials for charged carriers interaction to analyze the influence of the screening on the studied phenomena. Our study demonstrates the angle-dependent superfluidity temperature in TMTC vdWHs: for a given density a maximum and minimum $T_c$ of superfluidity occurs along chain and $a$-directions, respectively. This work can guide experim…
Stacking two-dimensional van der Waals (vdW) materials rotated with respect to each other show versatility for the study of exotic quantum phenomena. Especially, anisotropic layered materials have great potential for such twistronics applications, providing high tunability. Here, we report anisotropic superconducting order parameters in twisted Bi2Sr2CaCu2O8+x (Bi-2212) vdW junctions with an atomically clean vdW interface, achieved using the microcleave-and-stack technique. The vdW Josephson junctions with twist angles of 0° and 90° showed the maximum Josephson coupling, which was comparable t…
Measurement of multiple Andreev reflection (MAR) in a Josephson junction made from an InAs heterostructure with epitaxial aluminum is used to quantify the highly transparent semiconductor-superconductor interface, indicating near-unity transmission. The observed temperature dependence of MAR does not follow a conventional BCS form, but instead agrees with a model in which the density of states in the quantum well acquires an effective induced gap, in our case 180 μeV, close to that of the epitaxial superconductor. Carrier density dependence of MAR is investigated using a depletion gate, reveal…
Graphene constitutes one of the key elements in many functional van der Waals heterostructures. However, it has negligible optical visibility due to its monolayer nature. Here we study the visibility of graphene in various van der Waals heterostructures and include the effects of the source spectrum, oblique incidence and the spectral sensitivity of the detector to obtain a realistic model. A visibility experiment is performed at different wavelengths, resulting in a very good agreement with our calculations. This allows us to reliably predict the conditions for better visibility of graphene i…
Proximity-induced superconductivity in a ferromagnet can induce Cooper pairs with a finite center-of-mass momentum. The resultant spatially modulated superconducting order parameter is able to stabilize Josephson junctions (JJs) with pi phase difference in superconductor-ferromagnet heterostructures and realize 'quiet' phase qubits. The emergence of two-dimensional (2D) layered superconducting and magnetic materials promises a new platform for realizing pi JJs with atomically sharp interfaces by van der Waals stacking. Here we demonstrate a thickness-driven 0-pi transition in JJs made of NbSe2…
The emergence of a topological superconducting state in van der Waals heterostructures provides a new platform for exploring novel strategies to control topological superconductors. In particular, impurities in van der Waals heterostructures, generically featuring a moire pattern, can potentially lead to the unique interplay between atomic and moire length scales, a feature absent in generic topological superconductors. Here we address the impact of non-magnetic impurities on a topological moire superconductor, both in the weak and strong regime, considering both periodic arrays and single imp…
We consider the role of deformations in graphene heterostructures with hexagonal crystals (including strain, wrinkles and dislocations) on the geometrical properties of moire patterns characteristic for a pair of two incommensurate misaligned isostructural crystals. By, employing a phenomenological theory to describe generic moire perturbation in van der Waals heterostructures of graphene and hexagonal crystals we investigate the electronic properties of such heterostructure.
Van der Waals heterostructures of graphene and hexagonal boron nitride feature a moiré superlattice for graphene's Dirac electrons. Here, we review the effects generated by this superlattice, including a specific miniband structure featuring gaps and secondary Dirac points, and a fractal spectrum of magnetic minibands known as Hofstadter's butterfly.
We report an experimental study of excitons in a double quantum well van der Waals heterostructure made of atomically thin layers of \Mo* and hexagonal boron nitride (hBN). The emission of neutral and charged excitons is controlled by gate voltage, temperature, and both the helicity and the power of optical excitation.
Indirect excitons (IXs) in van der Waals transition-metal dichalcogenide (TMD) heterostructures are characterized by a high binding energy making them stable at room temperature and giving the opportunity for exploring fundamental phenomena in excitonic systems and developing excitonic devices operational at high temperatures. We present the observation of IXs at room temperature in van der Waals TMD heterostructures based on monolayers of MoS$_2$ separated by atomically thin hexagonal boron nitride. The IXs realized in the TMD heterostructure have lifetimes orders of magnitude longer than lif…
We combine electron beam lithography and masked anodization of epitaxial aluminium to define tunnel junctions via selective oxidation, alleviating the need for wet-etch processing or direct deposition of dielectric materials. Applying this technique to define Josephson junctions in proximity induced superconducting Al-InAs heterostructures, we observe multiple Andreev reflections in transport experiments, indicative of a high quality junction. We further compare the mobility and density of Hall-bars defined via wet etching and anodization. These results may find utility in uncovering new fabri…
Van der Waals heterostructures built by vertically stacked transition metal dichalcogenides (TMDs) exhibit a rich energy landscape including interlayer and intervalley excitons. Recent experiments demonstrated an ultrafast charge transfer in TMD heterostructures. However, the nature of the charge transfer process has remained elusive. Based on a microscopic and material-realistic exciton theory, we reveal that phonon-mediated scattering via strongly hybridized intervalley excitons governs the charge transfer process that occurs on a sub-100fs timescale. We track the time-, momentum-, and energ…
We develop a theory for interlayer tunneling in van der Waals heterostructures driven under a strong electromagnetic field, using graphene/{\it h}-BN/graphene as a paradigmatic example. Our theory predicts that strong anti-resonances appear at bias voltage values equal to an integer multiple of the light frequency. These features are found to originate from photon-assisted resonant tunneling transitions between Floquet sidebands of different graphene layers, and are unique to two-band systems due to the interplay of both intraband and interband tunneling transitions. Our results point to the p…
In van der Waals heterostructures, electronic bands of two-dimensional (2D) materials, their nontrivial topology, and electron-electron interactions can be dramatically changed by a moire pattern induced by twist angles between different layers. Such process is referred to as twistronics, where the tuning of twist angle can be realized through mechanical manipulation of 2D materials. Here we demonstrate an experimental technique that can achieve in situ dynamical rotation and manipulation of 2D materials in van der Waals heterostructures. Using this technique we fabricated heterostructures whe…
Two-dimensional (2D) van der Waals heterostructures serve as a promising platform to exploit various physical phenomena in a diverse range of novel spintronic device applications. The efficient spin injection is the prerequisite for these devices. The recent discovery of magnetic 2D materials leads to the possibility of fully 2D van der Waals spintronics devices by implementing spin injection through magnetic proximity effect (MPE). Here, we report the investigation of magnetic proximity effect in 2D CrBr3/graphene van der Waals heterostructures, which is probed by Zeeman spin Hall effect thro…
Formation of moiré superlattices is common in Van der Waals heterostructures as a result of the mismatch between lattice constants and misalignment of crystallographic directions of the constituent two-dimensional crystals. We discuss theoretically electron transport in a Van der Waals tunnelling transistor in which one of the electrodes is made of two crystals forming a moiré superlattice at their interface. By investigating structures containing either the aligned graphene/hexagonal boron nitride heterostructure or twisted bilayer graphene, we show that negative differential resistance is po…
Investigating thermal transport in van der Waals heterostructure is of scientific interest and practical importance for their applications in a broad range. In this work, thermal conductivity of one-dimensional heterostructure consisting of carbon and boron nitride nanotubes is systematically investigated via molecular dynamics simulations. Thermal conductivity is found to have strong dependences on temperature, length and diameter. In addition, the axial strain and intensity of van der Waals interaction are demonstrated to be able to modulate thermal conductivity up to about 43% and 37%, resp…
We investigate excitons in stacked transition metal dichalcogenide (TMDC) layers under perpendicularly applied electric field, herein MoSe$_2$/WSe$_2$ van der Waals heterostructures. Band structures are obtained with density functional theory calculations, along with the electron and hole wave functions in conduction and valence bands, respectively. Although the type-II nature of the heterostructure leads to fully charge separated inter-layer excitons, charge carriers distribution among the layers is shown to be easily tunable by external field. Our results show that moderate values of electri…
Research on graphene and other two-dimensional atomic crystals is intense and likely to remain one of the hottest topics in condensed matter physics and materials science for many years. Looking beyond this field, isolated atomic planes can also be reassembled into designer heterostructures made layer by layer in a precisely chosen sequence. The first - already remarkably complex - such heterostructures (referred to as 'van der Waals') have recently been fabricated and investigated revealing unusual properties and new phenomena. Here we review this emerging research area and attempt to identif…
Detecting single charging events in quantum devices is an important step towards realizing practical quantum circuits for quantum information processing. In this work, we demonstrate that van derWaals heterostructure devices with gated nano-constrictions in monolayer WSe2 can be used as charge detectors for nearby quantum dots. These results open the possibility of implementing charge detection schemes based on 2D materials in complex quantum circuits.
The multi-scale computational method of combining first-principles calculation and finite element electromagnetic simulations is used to study plasmon-enhanced interlayer charge transfer (ICT) exciton of 2D lateral and van der Waals MoS2/WS2 heterostructure with 2H phase. The weak ICT excitons are observed in the 2H lateral and van der Waals MoS$_2$/WS$_2$ heterostructures. Theoretical results reveal the physical principle of plexciton resulting from strong coupling between plasmonic and ICT exciton. The weak CT exciton can be strongly enhanced by metal plasmon, can provide a new way to observ…
The isolation of a growing number of two-dimensional (2D) materials has inspired worldwide efforts to integrate distinct 2D materials into van der Waals (vdW) heterostructures. Given that any passivated, dangling bond-free surface will interact with another via vdW forces, the vdW heterostructure concept can be extended to include the integration of 2D materials with non-2D materials that adhere primarily through noncovalent interactions. In this review, we present a succinct and critical survey of emerging mixed-dimensional (2D + nD, where n is 0, 1 or 3) heterostructure devices. By comparing…
The time-dependent spin current mediated spin transfer torque behaviour has been investigated via scattering formalism within density functional theory framework supported by Green's function. Quantum magnetotransport characteristics have been revealed in a model semiconducting MoS$_2$/phosphorene van der Waals heterostructure. The dynamics of spin current channelized heterolayer transport has been studied with rotational variation in magnetization angle. It is observed that the time-dependent spin transport torque remains invariant irrespective of magnetization angle direction. The polarized …
We investigate van der Waals (vdW) heterostructures made of germanene, stanene or silicene with hexagonal Boron Nitride (h-BN). The intriguing topological properties of these buckled honeycomb materials can be maintained and further engineered in the heterostructures, where the competition between the substrate effect and external electric fields can be used to control the tunable topological phase transitions. Using such heterostructures as building blocks, various vdW topological domain walls (DW) are designed, along which there exist valley polarized quantum spin Hall edge states or valley-…
We study the anomalous Josephson effect, as well as the dependence on the direction of the critical Josephson current, in an S/N/S junction, where the normal part is realized by alternating spin-orbit coupled and ferromagnetic layers. We show that to observe these effects it is sufficient to break spin rotation and time reversal symmetry in spatially separated regions of the junction. Moreover, we discuss how to further improve these effects by engineering multilayers structures with more that one couple of alternating layers.
We report the fabrication of perpendicular ferromagnetic electrodes for tunneling devices consist of van der Waals heterostructure. We found MgO/Co/Pt films on Hexagonal BN shows perpendicular magnetic anisotropy (PMA) with the easy axis perpendicular to the substrate. Vacuum annealing enhances the perpendicular anisotropy. The easy axis along the perpendicular direction persists up to room temperature with the Pt layer thickness ranges from 1.5 to 5 nm. We employed the PMA electrodes to construct tunneling devices on graphene and monolayer WSe2, where spin injection characteristics and field …
Even if individual two-dimensional materials own various interesting and unexpected properties, the stacking of such layers leads to van der Waals solids which unite the characteristics of two dimensions with novel features originating from the interlayer interactions. In this topical review, we cover fabrication and characterization of van der Waals heterosructures with a focus on heterobilayers made of monolayers of semiconducting transition metal dichalcogenides. Experimental and theoretical techniques to investigate those heterobilayers are introduced. Most recent findings focusing on diff…
Van der Waals heterostructures formed by stacking of various two-dimensional materials are promising in electronic applications. However, the performances of most reported electronic devices based on van der Waals heterostructures are far away from existing technologies'. Here we report high performance heterostructure devices based on vertically stacked tungsten diselenide and tin diselenide. Due to the unique band alignment and the atomic thickness of the material, both charge carrier transport and energy barrier can be effectively modulated by the applied electrical field. As a result, the …
It is shown that the anomalous elasticity of membranes affects the profile and thermodynamics of a bubble in van der Waals heterostructures. Our theory generalizes the non-linear plate theory as well as membrane theory of the pressurised blister test to incorporate the power-law scale dependence of the bending rigidity and Young's modulus of a two-dimensional crystalline membrane. This scale dependence caused by long-ranged interaction of relevant thermal fluctuations (flexural phonons), is responsible for the anomalous Hooke's law observed recently in graphene. It is shown that this anomalous…
We present a phenomenological theory for coupled spin-charge dynamics in magnetic van der Waals heterostructures. The system studied consists of a layered antiferromagnet inserted into a capacitive vdW heterostructure. It has been recently demonstrated that charge doping in such layered antiferromagnets can modulate the strength, and even the sign, of exchange coupling between the layer magnetizations. This provides a mechanism for electrically generating magnetization dynamics. The central result we predict here is that the magnetization dynamics reciprocally results in inducing charge dynami…
Near-gate plasmons are a new type of plasma oscillations emerging in homogeneous two-dimensional electron systems where a gate provides partial screening of electron-electron interaction. Here we develop a theory of the near-gate plasmons in van der Waals heterostructures comprising a conducting layer separated by a thin insulator from an uncharged disk-shaped gate. We show that in these structures the near-gate plasmons form gate-size-quantized quasi-stationary discrete modes even in the collisionless limit. Belonging to continuum spectrum of two-dimensional plasmons outside of the disk-gate,…
Heterostructures formed between two different van der Waals materials enable interactions and functionalities absent in each component. In this work we show that vicinity to an atomically thin WSe$_2$ sheet dramatically impacts the energies of the symmetry-broken low Landau levels of bilayer graphene, possibly due to screening. We present a systematic study of the magnetic field and electrical displacement field dependences of the Landau level gaps at filling factor $ν$ = 1, 2, 3, and compare to BN encapsulated pristine bilayer graphene. The exchange-dominated energy splitting between the N = …
Heterostructures play significant roles in modern semiconductor devices and micro/nanosystems in a plethora of applications in electronics, optoelectronics, and transducers. While state-of-the-art heterostructures often involve stacks of crystalline epi-layers each down to a few nanometers thick, the intriguing limit would be heterto-atomic-layer structures. Here we report the first experimental demonstration of freestanding van der Waals heterostructures and their functional nanomechanical devices. By stacking single-layer (1L) MoS2 on top of suspended single-, bi-, tri- and four-layer (1L to…
Ferromagnetic van der Waals (vdW) materials are in demand for spintronic devices with all-two-dimensional-materials heterostructures. Here, we demonstrate mechanical exfoliation of magnetic-atom-intercalated transition metal dichalcogenide Cr1/3TaS2 from its bulk crystal; previously such intercalated materials were thought difficult to exfoliate. Magnetotransport in exfoliated tens-of-nanometres-thick flakes revealed ferromagnetic ordering below its Curie temperature TC ~ 110 K as well as strong in-plane magnetic anisotropy; these are identical to its bulk properties. Further, van der Waals he…
In this paper, we propose van del Waals heterostructure-based thermionic devices for the applications in cooling and power generation in the temperature range of 300 to 400 K. By using two-dimensional materials of low cross-plane thermal conductivity as the barrier materials and graphene as electrodes, our calculation demonstrates that our proposed device will have a higher efficiency as compared to other methods such as thermoelectric device and the traditional thermionic devices. By using the parameters within the current technology, we predict a cooling capability at more than 50$\%$ of the…
In this study, the electrostatically induced quantum confinement structure, quantum point contact, has been realized in p-type trilayer tungsten diselenide-based van der Waals heterostructure with modified van der Waals contact method with degenerately doped transition metal dichalcogenide crystals. Clear quantized conductance and pinch-off state through the one-dimensional confinement were observed by dual-gating of split gate electrodes and top gate. Conductance plateaus were observed at step of 0.5 $\times$ 2e$^{2}$/h at zero and high magnetic field in addition to quarter plateaus at a fini…
We numerically study the general valley polarization and anomalous Hall effect in van der Waals (vdW) heterostructures based on monolayer jacutingaite family materials Pt$_{2}$AX$_{3}$ (A = Hg, Cd, Zn; X = S, Se, Te). We perform a systematic study on the atomic, electronic, and topological properties of vdW heterostructures composed of monolayer Pt$_{2}$AX$_{3}$ and two-dimensional ferromagnetic insulators. We show that four kinds of vdW heterostructures exhibit valley-polarized quantum anomalous Hall phase, i.e., Pt$_{2}$HgS$_{3}$/NiBr$_{2}$, Pt$_{2}$HgSe$_{3}$/CoBr$_{2}$, Pt$_{2}$HgSe$_{3}$/…
The interlayer coupling, which has a strong influence on the properties of van der Waals heterostructures, strongly depends on the interlayer distance. Although considerable theoretical interest has been demonstrated, experiments exploiting a variable interlayer coupling on nanocircuits are scarce due to the experimental difficulties. Here, we demonstrate a novel method to tune the interlayer coupling using hydrostatic pressure by incorporating van der Waals heterostructure based nanocircuits in piston-cylinder hydrostatic pressure cells with a dedicated sample holder design. This technique op…
We introduce a method of local gating for van der Waals heterostructures, employing a few-layer graphene patterned bottom gate. Being a member of the 2D material family, few-layer graphene adapts perfectly to the commonly used stacking method. Its versatility regarding patterning as well as its flatness make it an ideal candidate for experiments on locally gated 2D materials. Moreover, in combination with ultra-thin hexagonal boron nitride as an insulating layer, sharp potential steps can be created and the quality of the investigated 2D material can be sustained. To underline the good feasibi…
Second-order topological insulator, which has (d-2)-dimensional topological hinge or corner states, has been observed in three-dimensional materials, but has yet not been observed in two-dimensional system. In this Letter, we theoretically propose the realization of second-order topological insulator in the van der Waals heterostructure of CoBr$_2$/Pt$_2$HgSe$_3$/CoBr$_2$. Pt$_2$HgSe$_3$ is a large gap $\mathbb{Z}_2$ topological insulator. With in-plane exchange field from neighboring CoBr$_2$, a large band gap above 70 meV opens up at the edge. The corner states, which are robust against edge…
We directly measure the electronic structure of twisted graphene/MoS2 van der Waals heterostructures, in which both graphene and MoS2 are monolayers. We use cathode lens microscopy and microprobe angle-resolved photoemission spectroscopy measurements to image the surface, determine twist angle, and map the electronic structure of these artificial heterostructures. For monolayer graphene on monolayer MoS2, the resulting band structure reveals the absence of hybridization between the graphene and MoS2 electronic states. Further, the graphene-derived electronic structure in the heterostructures r…
Excitons dominate the optics of atomically-thin transition metal dichalcogenides and 2D van der Waals heterostructures. Interlayer 2D excitons, with an electron and a hole residing in different layers, form rapidly in heterostructures either via direct charge transfer or via Coulomb interactions that exchange energy between layers. Here, we report prominent intertube excitonic effects in quasi-1D van der Waals heterostructures consisting of C/BN/MoS$_2$ core/shell/skin nanotubes. Remarkably, under pulsed infrared excitation of excitons in the semiconducting CNTs we observed a rapid (sub-picose…
Throughout the years, strongly correlated coherent states of excitons have been the subject of intense theoretical and experimental studies. This topic has recently boomed due to new emerging quantum materials such as van der Waals (vdW) bound atomically thin layers of transition metal dichalcogenides (TMDs). We analyze the collective properties of charged interlayer excitons observed recently in bilayer TMD heterostructures. We predict new strongly correlated phases - crystal and Wigner crystal - that can be selectively realized with TMD bilayers of properly chosen electron-hole effective mas…
The physics of two-dimensional (2D) materials and heterostructures based on such crystals has been developing extremely fast. With new 2D materials, truly 2D physics has started to appear (e.g. absence of long-range order, 2D excitons, commensurate-incommensurate transition, etc). Novel heterostructure devices are also starting to appear - tunneling transistors, resonant tunneling diodes, light emitting diodes, etc. Composed from individual 2D crystals, such devices utilize the properties of those crystals to create functionalities that are not accessible to us in other heterostructures. We re…
Van der Waals (vdW) heterostructures made of two-dimensional materials have been demonstrated to be versatile architectures for optoelectronic applications due to strong light-matter interactions. However, most of light-controlled phenomena and applications in the vdW heterostructures rely on positive photoconductance (PPC). Negative photoconductance (NPC) has not yet been reported in vdW heterostructures. Here we report the observation of the NPC in ReS2/h-BN/MoS2 vdW heterostructures-based floating gate phototransistor. The fabricated devices exhibit excellent performance of nonvolatile memo…
It is shown that chiral plasmons, characterized by a longitudinal magnetic moment accompanying the longitudinal charge plasmon, lead to electromagnetic near-fields that are also chiral. For twisted bilayer graphene, we estimate that the near field chirality of screened plasmons can be several orders of magnitude larger than that of the related circularly polarized light. The chirality also manifests itself in a deflection angle that is formed between the direction of the plasmon propagation and its Poynting vector. Twisted van der Waals heterostructures might thus provide a novel platform to p…
In the last six years, Indium selenide (InSe) has appeared as a new van der Waals heterostructure platform which has been extensively studied due to its unique electronic and optical properties. Such as transition metal dichalcogenides (TMDCs), the considerable bandgap and high electron mobility can provide a potential optoelectronic application. Here we present low-temperature transport measurements on a few-layer InSe van der Waals heterostructure with graphene-gated contacts. For high magnetic fields, we observe magnetoresistance minima at even filling factors related to two-fold spin degen…
We investigated experimentally the valley Zeeman splitting of excitonic peaks in the photoluminescence (PL) spectra of high-quality hBN/WS2/MoSe2/hBN heterostructures at near-zero twist angles under perpendicular magnetic fields up to 20 T. We identify two neutral exciton peaks in the PL spectra: the lower energy one exhibits a reduced g-factor relative to that of the higher energy peak, and much lower than the recently reported values for interlayer excitons in other van der Waals (vdW) heterostructures. We provide evidence that such a discernible g-factor stems from the spatial confinement o…
