Theoretical research on electronic properties in mesoscopic condensed matter systems has focused primarily on the electron charge freedom degrees, while its corresponding spin freedom degrees have not yet received the same attention. Nevertheless nowadays there has been an increment in the number of electron spin-related experiments showing unique possibilities for finding novel mechanisms of information processing and transmission, opening ample fields of opportunities in the theoretical developed of new models. In this spirit we have calculated the resonant tunneling characteristics curves in double-barrier heterostructures of GaAs-Ga1-x AlxAs under external stress and considering two charges with spin half. The resonant tunneling study has been carried out by means of the diagrammatic techniques for non equilibrium processes following the model proposed by Keldysh also a simple one-band tight-binding Hamiltonian is adopted in the theoretical framework. We have compared our results of the spin-tunneling with previous ones reported in literature.
We have theoretically investigated the properties of electronic transport in graphene heterostructures, which are consisted of two different graphene superlattices with one-dimensional periodic potentials. It is found that such heterostructures possess an unusual tunneling state occurring inside the original forbidden gaps, and the electronic conductance is greatly enhanced and Fano factor is strongly suppressed near the energy of the tunneling state. Finally we present the matching condition of the impedance of the pseudospin wave for occuring the tunneling state by using the Bloch-wave expan…
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…
Vertical graphene heterostructures have been introduced as an alternative architecture for electronic devices by using quantum tunneling. Here, we present that the current on/off ratio of vertical graphene field-effect transistors is enhanced by using an armchair graphene nanoribbon as an electrode. Moreover, we report spin-dependent tunneling current of the graphene/MoS2 heterostructures. When an atomically thin MoS2 layer sandwiched between graphene electrodes becomes magnetic, Dirac fermions with different spins feel different height of the tunnel barrier, leading to spin-dependent tunnelin…
We address the tunneling current in a graphene-hBN-graphene heterostructure as function of the twisting between the crystals. The twisting induces a modulation of the hopping amplitude between the graphene layers, that provides the extra momentum necessary to satisfy momentum and energy conservation and to activate coherent tunneling between the graphene electrodes. Conservation rules limit the tunneling to states with wavevectors lying at the conic curves defined by the intersection of two Dirac cones shifted in momentum and energy. There is a critical voltage where the intersection is a stra…
We present fermionic model based on symmetric resonant tunneling heterostructure, which demonstrates spontaneous symmetry breaking in respect to combined operations of space inversion (P) and time reversal (T). PT-symmetry breaking manifests itself in resonance coalescence (collapse of resonances). We show that resonant energies are determined by eigenvalues of auxiliary pseudo-Hermitian PT-invariant Hamiltonian.
Resonant tunneling transport in polar heterostructures is intimately connected to the polarization fields emerging from the geometric Berry-phase. In these structures, quantum confinement results not only in a discrete electronic spectrum, but also in built-in polarization charges exhibiting a broken inversion symmetry along the transport direction. Thus, electrons undergo highly asymmetric quantum interference effects with respect to the direction of current flow. By employing doping to counter the broken symmetry, we deterministically control the resonant transmission through GaN/AlN resonan…
We report on the fabrication and measurement of a graphene tunnel junction using hexagonal-boron nitride as a tunnel barrier between graphene and a metal gate. The tunneling behavior into graphene is altered by the interactions with phonons and the presence of disorder. We extract prop- erties of graphene and observe multiple phonon-enhanced tunneling thresholds. Finally, differences in the measured properties of two devices are used to shed light on mutually-contrasting previous results of scanning tunneling microscopy in graphene.
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 …
Tunneling of quasiparticles between two nearly-aligned graphene sheets produces resonant current-voltage characteristics because of the quasi-exact conservation of in-plane momentum. We claim that, in this regime, vertical transport in graphene/boron nitride/graphene heterostructures carries precious information on electron-electron interactions and the quasiparticle spectral function of the two-dimensional electron system in graphene. We present extensive microscopic calculations of the tunneling spectra with the inclusion of quasiparticle lifetime effects and elucidate the range of parameter…
We present a theory that describes the appearance of circular polarization of the photoluminescence (PL) in ferromagnet-semiconductor hybrid heterostructures due to spin-dependent tunneling of photoexcited carriers from a quantum well into a magnetic layer. The theory succeeds in explaining the experimental data on time-resolved PL for heterostructures consisting of InGaAs-based quantum well (QW) and a spatially separated Mn $δ$-layer. We show that the circular polarization of the PL originates from dynamic spin polarization of electrons due to spin-dependent leakage from the QW onto Mn donor …
We predict a new mechanism of surface plasmon amplification in graphene-insulator-graphene van der Waals heterostructures. The amplification occurs upon the stimulated interlayer electron tunneling accompanied by the emission of a coherent plasmon. The quantum-mechanical calculations of the non-local high-frequency tunnel conductivity show that a relative smallness of the tunneling exponent can be compensated by a strong resonance due to the enhanced tunneling between electron states with collinear momenta in the neighboring graphene layers. With the optimal selection of the barrier layer, the…
A theory is developed for calculating vertical tunneling current between two sheets of bilayer graphene separated by a thin, insulating layer of hexagonal boron nitride, neglecting many-body effects. Results are presented using physical parameters that enable comparison of the theory with recently reported experimental results. Observed resonant tunneling and negative differential resistance in the current-voltage characteristics are explained in terms of the electrostatically-induced band gap, gate voltage modulation, density of states near the band edge, and resonances with the upper sub-ban…
We develop a theory of electron tunneling accompanied by carrier-carrier scattering in graphene - insulator - graphene heterostructures. Due to the dynamic screening of Coulomb interaction, the scattering-aided tunneling is resonantly enhanced if the transferred energy and momentum correspond to those of surface plasmons. We reveal the possible experimental manifestations of such plasmon-assisted tunneling in current-voltage curves and plasmon emission spectra of graphene-based tunnel junctions. We find that inelastic current and plasmon emission rates have sharp peaks at voltages providing eq…
The phenomenon of resonant tunneling transport through polar double-barrier heterostructures is systematically investigated using a combined experimental and theoretical approach. On the experimental side, GaN/AlN RTDs are grown by MBE. In-situ electron diffraction is employed to monitor the number of monolayers incorporated into each tunneling barrier. Using this precise epitaxial control at the monolayer level, we demonstrate exponential modulation of the resonant tunneling current as a function of barrier thickness. Both the peak voltage and characteristic threshold bias exhibit a dependenc…
We report tunneling phenomena in double In$_{0.53}$Ga$_{0.47}$As quantum-well structures that are at odds with the conventional parallel-momentum-conserving picture of tunneling between two-dimensional systems. We found that the tunneling current was mostly determined by the correlation between the emitter and the state in one well, and not by that between those in both wells. Clear magnetic-field-dependent features were first observed before the main resonance, corresponding to tunneling channels into the Landau levels of the well near the emitter. These facts provide evidence of the violatio…
Using a quantum theory including spin-splitting effect in diluted magnetic semiconductors, we study the dependence of tunneling magnetoresistance (TMR) on barrier thickness, temperature and applied voltage in GaMnAs/GaAs/GaMnAs heterostructures. TMR ratios more than 65% are obtained at zero temperature, when one GaAs monolayer ($\approx$ 0.565 nm) is used as a tunnel barrier. It is also shown that the TMR ratio decreases rapidly with increasing the barrier thickness and applied voltage, however at high voltages and low thicknesses, the TMR first increases and then decreases. Our model calculat…
Using the transfer matrix method and the effective-mass approximation, the effect of resonant states on spin transport is studied in ZnSe/ZnMnSe/ZnSe/ZnMnSe/ZnSe structures under the influence of both electric and magnetic fields. The numerical results show that the ZnMnSe layers, which act as spin filters, polarize the electric currents. Variation of thickness of the central ZnSe layer shifts the resonant levels and exhibits an oscillatory behavior in spin current densities. It is also shown that the spin polarization of the tunneling current in geometrical asymmetry of the heterostructure wh…
We report the experimental observation of strongly enhanced tunneling between graphene bilayers through a WSe$_2$ barrier when the graphene bilayers are populated with carriers of opposite polarity and equal density. The enhanced tunneling increases sharply in strength with decreasing temperature, and the tunneling current exhibits a vertical onset as a function of interlayer voltage at a temperature of 1.5 K. The strongly enhanced tunneling at overall neutrality departs markedly from single-particle model calculations that otherwise match the measured tunneling current-voltage characteristics…
We demonstrate gate-tunable resonant tunneling and negative differential resistance in the interlayer current-voltage characteristics of rotationally aligned double bilayer graphene heterostructures separated by hexagonal boron-nitride (hBN) dielectric. An analysis of the heterostructure band alignment using individual layer densities, along with experimentally determined layer chemical potentials indicates that the resonance occurs when the energy bands of the two bilayer graphene are aligned. We discuss the tunneling resistance dependence on the interlayer hBN thickness, as well as the reson…
In this letter, we propose the resonant tunneling diode (RTD) based on a double-barrier graphene/boron nitride (BN) heterostructure as device suitable to take advantage of the elaboration of atomic sheets containing different domains of BN and C phases within a hexagonal lattice. The device operation and performance are investigated by means of a self- consistent model within the non-equilibrium Green's function formalism on a tight-binding Hamiltonian. This RTD exhibits a negative differential conductance effect which involves the resonant tunneling through both the electron and hole bound st…
We investigate interlayer tunneling in heterostructures consisting of two tungsten diselenide (WSe2) monolayers with controlled rotational alignment, and separated by hexagonal boron nitride. In samples where the two WSe2 monolayers are rotationally aligned we observe resonant tunneling, manifested by a large conductance and negative differential resistance in the vicinity of zero interlayer bias, which stem from energy- and momentum-conserving tunneling. Because the spin-orbit coupling leads to coupled spin-valley degrees of freedom, the twist between the two WSe2 monolayers allows us to prob…
We theoretically studied the in-plane tunneling spectroscopy of the hybrid structure composed of a metal and a semiconductor with Rashba spin-orbit coupling. We found that the energy spacing between two distinct features in the conductance spectrum can be used to measure the Rashba energy of the semiconductor. We also considered the effect that varying the probability of spin-conserving and spin-flip scattering at the interface has on the overall conductance. Surprisingly, an increase in interface scattering probability can actually result in increased conductance under certain conditions. Par…
In transition metal dichalcogenides, defects have been found to play an important role, affecting doping, spin-valley relaxation dynamics, and assisting in proximity effects of spin-orbit coupling. Here, we study localized states in $\mathrm{WS}_2$ and how they affect tunneling through van der Waals heterostructures of h-BN/graphene/$\mathrm{WS}_2$/metal. The obtained conductance maps as a function of bias and gate voltage reveal single-electron transistor behavior (Coulomb blockade) with a rich set of transport features including excited states and negative differential resistance regimes. Ap…
We report a bipolar field effect tunneling transistor that exploits to advantage the low density of states in graphene and its one atomic layer thickness. Our proof-of-concept devices are graphene heterostructures with atomically thin boron nitride acting as a tunnel barrier. They exhibit room temperature switching ratios ~50, a value that can be enhanced further by optimizing the device structure. These devices have potential for high frequency operation and large scale integration.
Cryogenic interface-state filling and tunneling mechanisms in strained Ge/SiGe heterostructures
University of Science and Technology of China（中国科学技术大学）；CAS Center for Excellence in Quantum Information and Quantum Physics, University of Science and Technology of China（中国科学院量子信息与量子物理学前沿卓越中心）；Hefei National Laboratory（合肥国家实验室）；Beijing Superstring Academy of Memory Technology（北京超弦记忆技术研究院）；Origin Quantum Computing Company Limited（本源量子计算科技有限责任公司）
We demonstrate gate-tunable resonant tunneling and negative differential resistance between two rotationally aligned bilayer graphene sheets separated by bilayer WSe2. We observe large interlayer current densities of 2 uA/um2 and 2.5 uA/um2, and peak-to-valley ratios approaching 4 and 6 at room temperature and 1.5 K, respectively, values that are comparable to epitaxially grown resonant tunneling heterostructures. An excellent agreement between theoretical calculations using a Lorentzian spectral function for the two-dimensional (2D) quasiparticle states, and the experimental data indicates th…
We investigate spin transport through ferromagnetic graphene vertical heterostructures where a sandwiched tunneling layer is either a normal or ferroelectric insulator. We show that the spin-polarization of the tunneling current is electronically controlled via gate voltages. We also demonstrate that the tunneling current of Dirac fermions can be prohibited when the spin configuration of ferromagnetic graphene sheets is opposite. The giant electroresistance can thus be developed by using the proposed heterostructure in this study. The effects of temperature on the spin transport and the giant …
Double barrier GaN/AlN resonant tunneling heterostructures have been grown by molecular beam epitaxy on the (0001) plane of commercially available bulk GaN substrates. Resonant tunneling diodes were fabricated; room temperature current-voltage measurements reveal the presence of a negative differential conductance region under forward bias with peak current densities of ~6.4 $kA/cm^2$ and a peak to valley current ratio of ~1.3. Reverse bias operation presents a characteristic turn-on threshold voltage intimately linked to the polarization fields present in the heterostructure. An analytic elec…
Graphene/silicon heterostructures have attracted tremendous interest as a new platform for diverse electronic and photonic devices such as barristors, solar cells, optical modulators, and chemical sensors. The studies to date largely focus on junctions between graphene and lightly-doped silicon, where a Schottky barrier is believed to dominate the carrier transport process. Here we report a systematic investigation of carrier transport across the heterojunctions formed between graphene and highly-doped silicon. By varying the silicon doping level and the measurement temperature, we show that t…
We use first-principle density functional theory (DFT) to study the transport properties of single and double barrier heterostructures realized by stacking multilayer h-BN or BC$_{2}$N, and graphene films between graphite leads. The heterostructures are lattice matched. The considered single barrier systems consist of layers of up to five h-BN or BC$_{2}$N monoatomic layers (Bernal stacking) between graphite electrodes. The transmission probability of an h-BN barrier exhibits two unusual behaviors: it is very low also in a classically allowed energy region, due to a crystal momentum mismatch b…
Recent experiments on resonant tunneling structures comprising (Ga,Mn)As quantum wells [Ohya et al., Nature Physics 7, 342 (2011)] have evoked a strong debate regarding their interpretation as resonant tunneling features and the near absences of ferromagnetic order observed in these structures. Here, we present a related theoretical study of a GaAs/(Ga,Mn)As double barrier structure based on a Green's function approach, studying the self-consistent interplay between ferromagnetic order, structural defects (disorder), and the hole tunnel current under conditions similar to those in experiment. …
Magnetic multilayer devices that exploit magnetoresistance are the backbone of magnetic sensing and data storage technologies. Here we report novel multiple-spin-filter magnetic tunnel junctions (sf-MTJs) based on van der Waals (vdW) heterostructures in which atomically thin chromium triiodide (CrI3) acts as a spin-filter tunnel barrier sandwiched between graphene contacts. We demonstrate tunneling magnetoresistance which is drastically enhanced with increasing CrI3 layer thickness, reaching a record 19,000% for magnetic multilayer structures using four-layer sf-MTJs at low temperatures. These…
Effect of spin-dependent tunneling and intervalley scattering in magnetic-semiconductor van der Waals heterostructures on exciton and trion polarization
We study the phenomenon of spontaneous symmetry breaking in dissipationless resonant tunneling heterostructures (RTS). To describe the quantum transport in this system we apply both the nonequilibrium Green function formalism based on a tight-binding model and a numerical solution of the Schroedinger equation within the envelope wavefunction formalism. An auxiliary non-Hermitian Hamiltonian is introduced. Its eigenvalues determine exactly the transparency peak positions. We present a procedure how to construct a family of non-Hermitian Hamiltonians with real eigenvalues. In general these Hamil…
Thin van der Waals (vdW) layered magnetic materials disclose the possibility to realize vdW heterostructures with new functionalities. Here we report on the realization and investigation of tunneling spin valves based on van der Waals heterostructures consisting of an atomically thin hBN layer acting as tunnel barrier and two exfoliated Fe3GeTe2 crystals acting as ferromagnetic electrodes. Low-temperature anomalous Hall effect measurements show that thin Fe3GeTe2 crystals are metallic ferromagnets with an easy axis perpendicular to the layers, and a very sharp magnetization switching at magnet…
We describe a tunneling spectroscopy technique in a double bilayer graphene heterostructure where momentum-conserving tunneling between different energy bands serves as an energy filter for the tunneling carriers, and allows a measurement of the quasi-particle state broadening at well defined energies. The broadening increases linearly with the excited state energy with respect to the Fermi level, and is weakly dependent on temperature. In-plane magnetotunneling reveals a high degree of rotational alignment between the graphene bilayers, and an absence of momentum randomizing processes.
The control of elastic and inelastic electron tunneling relies on materials with well defined interfaces. Van der Waals materials made of two-dimensional constituents form an ideal platform for such studies. Signatures of acoustic phonons and defect states have been observed in current-to-voltage ($I-V$) measurements. These features can be explained by direct electron-phonon or electron-defect interactions. Here, we use a novel tunneling process that involves excitons in transition metal dichalcogenides (TMDs). We study tunnel junctions consisting of graphene and gold electrodes separated by h…
An effect of paramagnetic impurity located in a vicinity of a quantum well (QW) on spin polarization of the carriers in the QW is analyzed theoretically. Within approach of Bardeen's tunneling Hamiltonian the problem is formulated in terms of Anderson-Fano model of configuration interaction between a localized hole state at Mn and continuum of heavy hole states in the InGaAs-based QW. The hybridization between the localized state and the QW leads to resonant enhancement of interband radiative recombination. The splitting of the configuration resonances induced by splitting of the localized sta…
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…
Magnetotunneling spectroscopy was employed to probe the confinement in vertical Si/Ge double-barrier resonant tunneling diodes with regularly distributed Ge quantum dots. Their current-voltage characteristics reveal a step-like behavior in the vicinity of zero bias, indicating resonant tunneling of heavy-holes via three-dimensionally confined unoccupied hole states in Ge quantum dots. Assuming parabolic confinement we extract the strength of the confinement potential of quantum dots.
Tunneling characteristics of twisted double bilayer graphene heterostructures
Institute for Spectroscopy, Russian Academy of Sciences（俄罗斯科学院光谱学研究所）；National Research University Higher School of Economics（高等经济国立研究大学）；Institute of Microelectronics Technology and High Purity Materials, Russian Academy of Sciences（俄罗斯科学院微电子技术与高纯材料研究所）；Programmable Functional Materials Lab, Center for Neurophysics and Neuromorphic Technologies（神经物理与类脑技术中心可编程功能材料实验室）；Moscow Center for Advanced Studies（莫斯科高级研究中心）；Department of Materials Science and Engineering, National University of Singapore（新加坡国立大学材料科学与工程系）；Institute for Functional Intelligent Materials, National University of Singapore（新加坡国立大学功能智能材料研究所）
We observe proximity-induced superconductivity in the \textit{in situ} prepared heterostructures constructed by topological insulator Bi$_{2}$Te$_{3}$ thin films and high-temperature cuprate superconductors Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$. The superconducting gap maximum is about 7.6 meV on the surface of Bi$_{2}$Te$_{3}$ thin films with a thickness of two quintuple layers, and the gap value decreases with an increase in the film thickness. Moreover, the quasiparticle interference data show a clear evidence of a twofold symmetric superconducting gap with gap minima along one pair of the pr…
This work is the first step towards understanding thermionic transport properties of graphene/phosphorene/graphene van der Waals heterostructures in contact with gold electrodes by using density functional theory based first principles calculations combined with real space Green's function formalism. We show that for monolayer phosphorene in the heterostructure, quantum tunneling dominates the transport. By adding more phosphorene layers, one can switch from tunneling dominated transport to thermionic dominated transport, resulting in transporting more heat per charge carrier, thus, enhancing …
We present a theory of quantum tunneling between 2D layers with account for Rashba and Dresselhaus spin-orbit interaction (SOI) in the layers. Energy and momentum conservation results in a single resonant peak in the tunnel conductance between two 2D layers as has been experimentally observed for two quantum wells (QW) in GaAs/AlGaAs heterostructures. The account for SOI in the layers leads to a complex pattern in the tunneling characteristic with typical features corresponding to SOI energy. For this manifestation of SOI to be observed experimentally the characteristic energy should exceed th…
This paper demonstrates the high-quality tunnel barrier characteristics and layer number controlled tunnel resistance of a transition metal dichalcogenide (TMD) measuring just a few monolayers in thickness. Investigation of vertical transport in WS2 and MoS2 TMDs in graphene/TMD/metal heterostructures revealed that WS2 exhibits tunnel barrier characteristics when its thickness is between 2 to 5 monolayers, whereas MoS2 experiences a transition from tunneling to thermionic emission transport with increasing thickness within the same range. Tunnel resistance in a graphene/WS2/metal heterostructu…
We propose the use of resonant tunneling as a route to enhance the spin-transfer torque switching characteristics of magnetic tunnel junctions. The proposed device structure is a resonant tunneling magnetic tunnel junction based on a MgO-semiconductor heterostructure sandwiched between a fixed magnet and a free magnet. Using the non-equilibrium Green's function formalism coupled self consistently with the Landau-Lifshitz-Gilbert-Slonczewski equation, we demonstrate enhanced tunnel magneto-resistance characteristics as well as lower switching voltages in comparison with traditional trilayer dev…
Recently it has been experimentally shown that a graphene nanoribbon (GNR) can be obtained by unzipping a carbon nanotube (CNT). This makes it possible to fabricate all-carbon heterostructures that have a unique interface between a CNT and a GNR. Here we demonstrate that such a heterojunction may be utilized to obtain a unique transistor operation. By performing a self-consistent non-equilibrium Green's function (NEGF) based calculation on an atomistically defined structure, we show that such a transistor may reduce energy dissipation below the classical limit while not compromising speed - th…
Magnetotransport in a laterally confined two-dimensional electron gas (2DEG) can exhibit modified scattering channels owing to a tilted Hall potential. Transitions of electrons between Landau levels with shifted guiding centers can be accomplished through a Zener tunneling mechanism, and make a significant contribution to the magnetoresistance. A remarkable oscillation effect in weak field magnetoresistance has been observed in high-mobility 2DEGs in GaAs-AlGa$_{0.3}$As$_{0.7}$ heterostructures, and can be well explained by the Zener mechanism.
We demonstrate a tunnel field effect transistor based on a lateral heterostructure patterned from an $\mathrm{LaAlO_3/SrTiO_3}$ electron gas. Charge is injected by tunneling from the $\mathrm{LaAlO_3}$/$\mathrm{SrTiO_3}$ contacts and the current through a narrow channel of insulating $\mathrm{SrTiO_3}$ is controlled via an electrostatic side gate. Drain-source I/V-curves have been measured at low and elevated temperatures. The transistor shows strong electric-field and temperature-dependent behaviour with a steep sub-threshold slope %of up to as small as $10\:\mathrm{mV/decade}$ and a transcon…
We report on experimental investigations of an electrically driven WSe2 based light-emitting van der Waals heterostructure. We observe a threshold voltage for electroluminescence significantly lower than the corresponding single particle band gap of monolayer WSe2. This observation can be interpreted by considering the Coulomb interaction and a tunneling process involving excitons, well beyond the picture of independent charge carriers. An applied magnetic field reveals pronounced magneto-oscillations in the electroluminescence of the free exciton emission intensity with a 1/B-periodicity. Thi…
