Origin of the Large Perpendicular Magnetic Anisotropy in Nanometer-thick Epitaxial Graphene/Co/Heavy Metal Heterostructures
M. Blanco-Rey, P. Perna, A. Gudin, J. M. Diez, A. Anadon Leticia de Melo Costa, Manuel Valvidares, Pierluigi Gargiani, Alejandra Guedeja-Marron, Mariona Cabero,… 展开作者
M. Blanco-Rey, P. Perna, A. Gudin, J. M. Diez, A. Anadon Leticia de Melo Costa, Manuel Valvidares, Pierluigi Gargiani, Alejandra Guedeja-Marron, Mariona Cabero, M. Varela, C. Garcia-Fernandez, M. M. Otrokov, J. Camarero, R. Miranda, A. Arnau, J. I. Cerda
A combination of theoretical modelling and experiments reveals the origin of the large perpendicular magnetic anisotropy (PMA) that appears in nanometer-thick epitaxial Co films intercalated between graphene (Gr) and a heavy metal (HM) substrate, as a function of the Co thickness. High quality epitaxial Gr/Co\n/HM(111) (HM=Pt,Ir) heterostructures are grown by intercalation below graphene, which acts as a surfactant that kinetically stabilizes the pseudomorphic growth of highly perfect Co face-centered tetragonal ($fct$) films, with a reduced number of stacking faults as the only structural defect observable by high resolution scanning transmission electron microscopy (HR-STEM). Magneto-optic Kerr effect (MOKE) measurements show that such heterostructures present PMA up to large Co critical thicknesses of about 4~nm (20~ML) and 2~nm (10~ML) for Pt and Ir substrates, respectively, while X-ray magnetic circular dichroism (XMCD) measurements show an inverse power law of the anistropy of the orbital moment with Co thickness, reflecting its interfacial nature, that changes sign at about the same critical values. First principles calculations show that, regardless of the presence of graphene, ideal Co $fct$ films on HM buffers do not sustain PMAs beyond around 6~MLs due to the in-plane contribution of the inner bulk-like Co layers. The large experimental critical thicknesses sustaining PMA can only be retrieved by the inclusion of structural defects that promote a local $hcp$ stacking such as twin boundaries or stacking faults. Remarkably, a layer resolved analysis of the orbital momentum anisotropy reproduces its interfacial nature, and reveals that the Gr/Co interface contribution is comparable to that of the Co/Pt(Ir).
The spin-orbit torques (SOTs) generated from topological insulators (TIs) have gained increasing attention in recent years. These TIs, which are typically formed by epitaxially grown chalcogenides, possess extremely high SOT efficiencies and have great potential to be employed in the next-generation spintronics devices. However, epitaxy of these chalcogenides is required to ensure the existence of topologically-protected surface state (TSS), which limits the feasibility of using these materials in industry. In this work, we show that non-epitaxial Bi$_{x}$Te$_{1-x}$/ferromagnet heterostructure…
A corner-overgrown GaAs/AlGaAs heterostructure is investigated with transmission and scanning transmission electron microscopy, demonstrating self-limiting growth of an extremely sharp corner profile of 3.5 nm width. In the AlGaAs layers we observe self-ordered diagonal stripes, precipitating exactly at the corner, which are regions of increased Al content measured by an XEDS analysis. A quantitative model for self-limited growth is adapted to the present case of faceted MBE growth, and the corner sharpness is discussed in relation to quantum confined structures. We note that MBE corner overgr…
Local electronic structures of nanometer-sized patches of epitaxial graphene and its interface layer with SiC(0001) have been studied by atomically resolved scanning tunneling microscopy and spectroscopy. Localized states belonging to the interface layer of a graphene/SiC system show to have an essential influence on the electronic structure of graphene. Giant enhancement of inelastic tunneling, reaching 50% of the total tunneling current, has been observed at the localized states on a nanometer-sized graphene monolayer surrounded by defects.
Graphene exhibits unconventional two-dimensional electronic properties resulting from the symmetry of its quasiparticles, which leads to the concepts of pseudospin and electronic chirality. Here we report that scanning tunneling microscopy can be used to probe these unique symmetry properties at the nanometer scale. They are reflected in the quantum interference pattern resulting from elastic scattering off impurities, and they can be directly read from its fast Fourier transform. Our data, complemented by theoretical calculations, demonstrate that the pseudospin and the electronic chirality i…
We demonstrate efficient intrinsic spin-to-charge current conversion in a two-dimensional electron gas using an all-epitaxial single-crystal heterostructure of LaSrMnO3/ LaAlO3 (LAO)/ SrTiO3 (STO), which can suppress spin scattering and give us an ideal environment to investigate intrinsic spin-charge conversion. With decreasing temperature to 20 K, the spin-to-charge conversion efficiency is drastically enhanced to +3.9 nm, which is the largest positive value ever reported for LAO/STO. Our band-structure calculation well reproduces this behavior and predicts further enhancement by controlling…
Using first-principles computations, we study the effect of epitaxial strains on electronic structure variations across ZnSe/ZnTe nano-heterostructures. Epitaxial strains of various types are modeled using pseudomorphic ZnSe/ZnTe heterostructures. We find that a wide range of band gaps (spanning the visible solar spectrum) and band offsets (0-1.5 eV) is accessible across the heterostructures in a controllable manner via reasonable levels of epitaxial strain. In addition to quantum confinement effects, strain in ZnSe/ZnTe heterostructures may thus be viewed as a powerful degree of freedom that …
Semiconductor heterostructures provide a powerful platform for the engineering of excitons. Here we report the excitonic properties of two-dimensional (2D) heterostructures that consist of monolayer MoS2 and WS2 stacked epitaxially or non-epitaxially in the vertical direction. We find similarly efficient interlayer relaxation and transition of excitons in both the epitaxial and nonepitaxial heterostructures. This is manifested by a two orders of magnitude decrease in the photoluminescence and the appearance of an extra absorption peak at low energy region. The MoS2/WS2 heterostructures show we…
We use time- and angle-resolved photoemission spectroscopy (tr-ARPES) to investigate ultrafast charge transfer in an epitaxial heterostructure made of monolayer WS$_2$ and graphene. This heterostructure combines the benefits of a direct gap semiconductor with strong spin-orbit coupling and strong light-matter interaction with those of a semimetal hosting massless carriers with extremely high mobility and long spin lifetimes. We find that, after photoexcitation at resonance to the A-exciton in WS$_2$, the photoexcited holes rapidly transfer into the graphene layer while the photoexcited electro…
Understanding the growth mechanisms of two-dimensional (2D) van der Waals (vdW) heterostructures is of great importance in exploring their functionalities and device applications. A custom-built system integrating physical vapor deposition and optical microscopy/Raman spectroscopy was employed to study the dynamic growth processes of 2D vdW heterostructures in situ. This allows us to identify a new growth mode with a distinctly different growth rate and morphology from those of the conventional linear growth mode. We propose a model that explains the difference in morphologies and quantifies t…
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…
In the field of spintronics, ferromagnetic/non-magnetic metallic multilayers are core building blocks for emerging technologies. Resonance experiments using stripline transducers are commonly used to characterize and engineer these stacks for applications. Up to now in these experiments, the influence of eddy currents on the excitation of the dynamics of ferromagnetic magnetization below the skin-depth limit was most often neglected. Here, using a coplanar stripline transducer, we experimentally investigated the broadband ferromagnetic resonance response of NiFe/Cu bilayers a few nanometers th…
Epitaxial heterostructures of La0.6Sr0.4MnO3 0.7Pb(Mg0.33Nb0.67)O3 0.3PbTiO3 were fabricated on LaNiO3 coated LaAlO3 (100) substrates by pulsed laser ablation. Ferromagnetic and ferroelectric hysteresis established their biferroic nature. Dielectric behviour studied under different magnetic fields over a wide range of frequency and temperatures revealed that the capacitance in these heterostructures varies with the applied magnetic field. Appearance of magnetocapacitance and its dependence on magnetic fields, magnetic layer thickness, temperature and frequency indicated a combined contribution…
Stabilizing Magnetic Bubble Domains in Epitaxial 2D Magnet/Topological Insulator Heterostructures through Interfacial Interactions
University of Southern California（南加州大学）；University of South Florida（南佛罗里达大学）；The Ohio State University（俄亥俄州立大学）；Max Planck Institute for Solid State Research（马克斯·普朗克固体研究所）；Helmholtz-Zentrum Berlin für Materialien und Energie（柏林赫尔姆霍兹材料能源中心）
Axion Insulator State in Hundred-Nanometer-Thick Magnetic Topological Insulator Sandwich Heterostructures
Department of Physics, The Pennsylvania State University（宾夕法尼亚州立大学物理系）；Department of Physics, Hong Kong University of Science and Technology（香港科技大学物理系）；Materials Research Institute, The Pennsylvania State University（宾夕法尼亚州立大学材料研究所）
We investigated perpendicular magnetic anisotropy (PMA) and related properties of epitaxial Fe (0.7 nm)/MgAl2O4(001) heterostructures prepared by electron-beam evaporation. Using an optimized structure, we obtained a large PMA energy ~1 MJ/m3 at room temperature, comparable to that in ultrathin-Fe/MgO(001) heterostructures. Both the PMA energy and saturation magnetization show weak temperature dependence, ensuring wide working temperature in application. The effective magnetic damping constant of the 0.7 nm Fe layer was ~0.02 using time-resolved magneto-optical Kerr effect. This study demonstr…
High-quality thermoelectric LaxSr1-xTiO3 (LSTO) layers (here with x = 0.2), with thicknesses ranging from 20 nm to 700 nm, have been epitaxially grown on SrTiO3(001) substrates by enhanced solid-source oxide molecular-beam epitaxy. All films are atomically flat (with rms roughness < 0.2 nm), with low mosaicity (<0.1°), and present very low electrical resistivity (<5 x 10-4 ohm.cm at room temperature), one order of magnitude lower than commercial Nb-doped SrTiO3 single-crystalline substrate. The conservation of transport properties within this thickness range has been confirmed by thermoelectri…
Epitaxial bilayered thin films consisting of La0.6Sr0.4MnO3 (LSMO) and 0.7 Pb(Mg1/3Nb2/3)O3 0.3 PbTiO3 (PMN-PT) layers of relatively different thicknesses were fabricated on LaNiO3 coated LaAlO3 (100) single crystal substrates by pulsed laser ablation technique. Ferroelectric and ferromagnetic characteristics of these heterostructures confirmed their biferroic nature. The magnetization and ferroelectric polarization of the bilayered heterostructures were enhanced with increasing PMN-PT layer thickness owing to the effect of lattice strain. Dielectric properties of these heterostructures studie…
Neutron reflectometry (NR) has emerged as a unique technique for the investigation of structure and magnetism of thin films of both biologically relevant and magnetic materials. The advantage of NR with respect to many other surface-sensitive techniques is its sub-nanometer resolution that enables structural characterizations at the molecular level. While in the case of bio-relevant samples, NR can be used to probe thin films at buried interfaces, non-destructively, even adopting a complex sample environment. Whereas the polarized version of NR is best suited for revealing the interface magnet…
The photovoltaic effect in the BiFeO3/TiO2 heterostructures can be tuned by epitaxial strain and an electric field in the visible-light region which is manifested by the enhancement of absorption activity in the heterojunction under tensile strain and an electric field based on the first-principles calculations. It is suggested that there are coupling between photon, spin carrier, charge, orbital, and lattice in the interface of the bilayer film which makes the heterojunction an intriguing candidate towards fabricating the multifunctional photoelectric devices based on spintronics. The microsc…
The giant spin Hall effect in magnetic heterostructures along with low spin memory loss and high interfacial spin mixing conductance are prerequisites to realize energy efficient spin torque based logic devices. We report giant spin Hall angle (SHA) of 28.67 (5.09) for W (Ta) interfaced epi- Co60Fe40/TiN structures. The spin-orbit torque switching current density (J_Crit) is as low as 1.82 (8.21) MA/cm2 in W (Ta)/Co60Fe40(t_CoFe)/TiN structures whose origin lies in the epitaxial interfaces. These structures also exhibit very low spin memory loss and high spin mixing conductance. These extraord…
The structures of epitaxial ultrathin Co2FeAl/MgO(001) heterostructures relating to the interface-induced perpendicular magnetic anisotropy (PMA) were investigated using scanning transmission electron microscopy, energy dispersive x-ray spectroscopy, and x-ray magnetic circular dichroism. We found that Al atoms from the Co2FeAl layer significantly interdiffuse into MgO, forming an Al-deficient Co-Fe-Al/Mg-Al-O structure near the Co2FeAl/MgO interface. This atomic replacement may play an additional role for enhancing PMA, which is consistent with the observed large perpendicular orbital magneti…
Voltage control of magnetic anisotropy (VCMA) in magnetic heterostructures is a key technology for achieving energy-efficiency electronic devices with ultralow power consumption. Here, we report the first demonstration of the VCMA effect in novel epitaxial Ru/Co2FeAl(CFA)/MgO heterostructures with interfacial perpendicular magnetic anisotropy (PMA). Perpendicularly magnetized tunnel junctions with the structure of Ru/CFA/MgO were fabricated and exhibited an effective voltage control on switching fields for the CFA free layer. A large VCMA coefficient of 108 (139) fJ/Vm for the CFA film was ach…
A topological insulator (TI) interfaced with an s-wave superconductor has been predicted to host an unusual form of superconductivity known as topological superconductivity (TSC). Molecular beam epitaxy (MBE) has been the primary approach in the scalable synthesis of the TI/superconductor heterostructures. Although the growth of epitaxial TI films on s-wave superconductors has been achieved, it remains an outstanding challenge for synthesizing atomically thin TI/superconductor heterostructures, which are critical for engineering the TSC phase. Here, we used MBE to grow Bi2Se3 films with the co…
Oscillations in the exchange coupling between ferromagnetic $La_{2/3}Ba_{1/3}MnO_3$ layers with paramagnetic $LaNiO_3$ spacer layer thickness has been observed in epitaxial heterostructures of the two oxides. This behavior is explained within the RKKY model employing an {\it ab initio} calculated band structure of $LaNiO_3$, taking into account strong electron scattering in the spacer. Antiferromagnetically coupled superlattices exhibit a positive current-in-plane magnetoresistance.
Epitaxial III-V semiconductor heterostructures are key components in modern microelectronics, electro-optics and optoelectronics. With superior semiconducting properties, halide perovskite materials are rising as promising candidates for coherent heterostructure devices. In this report, spinodal decomposition is proposed and experimentally implemented to produce epitaxial double heterostructures in halide perovskite system. Pristine epitaxial mixed halide perovskites rods and films were synthesized via Van der Waals epitaxy by chemical vapor deposition method. At room temperature, photon was a…
The challenge of parasitic bulk doping in Bi-based 3D topological insulator materials is still omnipresent, especially when preparing samples by molecular beam epitaxy (MBE). Here, we present a heterostructure approach for epitaxial BSTS growth. A thin n-type Bi$_2$Se$_3$ (BS) layer is used as an epitaxial and electrostatic seed which drastically improves the crystalline and electronic quality and reproducibility of the sample properties. In heterostructures of BS with p-type (Bi$_{1-x}$Sb$_x$)$_2$(Te$_{1-y}$Se$_y$)$_3$ (BSTS) we demonstrate intrinsic band bending effects to tune the electroni…
We experimentally observe the fast relaxation and relatively slow recombination dynamics of photogenerated electrons/holes in an epitaxial graphene-on-Si heterostructure under pumping with a 1550-nm, 80-fs pulsed fiber laser beam and probing with the corresponding terahertz (THz) beam generated by and synchronized with the pumping laser. The time-resolved electric-field intensity originating from the coherent terahertz photon emission is electro-optically sampled in total-reflection geometry. The Fourier spectrum from 1.8 to 5.2 THz agrees well the pumping photon spectrum. This result is attri…
We report on resonant tunneling magnetoresistance via localized states through a ZnSe semiconducting barrier which can reverse the sign of the effective spin polarization of tunneling electrons. Experiments performed on Fe/ZnSe/Fe planar junctions have shown that positive, negative or even its sign-reversible magnetoresistance can be obtained, depending on the bias voltage, the energy of localized states in the ZnSe barrier and spatial symmetry. The averaging of conduction over all localized states in a junction under resonant condition is strongly detrimental to the magnetoresistance.
Above-room-temperature ferromagnetism in large-area epitaxial Fe3GaTe2/graphene van der Waals heterostructures
Paul-Drude-Institut für Festkörperelektronik, Leibniz-Institut im Forschungsverbund Berlin e.V.（柏林莱布尼茨固体电子学保罗·德鲁德研究所）；Helmholtz-Zentrum Dresden-Rossendorf, Institute of Ion Beam Physics and Materials Research e.V.（德累斯顿-罗森多夫亥姆霍兹中心离子束物理与材料研究所）；QZabre AG（QZabre股份公司）；Helmholtz Zentrum Berlin for Materialien und Energie（柏林亥姆霍兹材料与能源中心）
We report on multiple fundamental qualitative improvements in the growth of improper ferroelectric hexagonal YMnO$_3$ (YMO) thin films and heterostructures by pulsed laser deposition (PLD). By a combination of pre-growth substrate annealing and low-energy-fluence PLD, we obtain a two-dimensional growth mode of the YMO films on yttria-stabilized zirconia (YSZ) with ultralow roughness and devoid of misoriented nanocrystalline inclusions. By inserting a sacrificial manganite layer capped with conducting indium-tin oxide between the substrate and the final film, the latter is grown in a fully latt…
This paper presents PANIC, a 3D discrete mesoscale dislocation dynamics model which includes a fully quantitative treatment of both dislocation climb and dislocation glide, including climb driven by both osmotic and mechanical stresses and climb enabled by both bulk and pipe diffusion, including full elastic anisotropy for materials with hexagonal symmetry. Efficient calculations can be performed for epitaxial thin films, multilayers and device structures with free surfaces, including those with irregular geometries (e.g. islands). The model also includes the capability to simulate dislocation…
Oxide based ferromagnet/semiconductor heterostructures offer substantial advantages for spin electronics. We have grown (111) oriented Fe3O4 thin films and Fe3O4/ZnO heterostructures on ZnO(0001) and Al2O3(0001) substrates by pulsed laser deposition. High quality crystalline films with mosaic spread as small as 0.03 degree, sharp interfaces, and rms surface roughness of 0.3 nm were achieved. Magnetization measurements show clear ferromagnetic behavior of the magnetite layers with a saturation magnetization of 3.2 muB/f.u. at 300 K. Our results demonstrate that the Fe3O4/ZnO system is an intrig…
A large anti-damping spin-obit torque (SOT) efficiency in magnetic heterostructures is a prerequisite to realize energy efficient spin torque based magnetic memories and logic devices. The efficiency can be characterized in terms of the spin-orbit fields generated by anti-damping torques when an electric current is passed through the non-magnetic layer. We report a giant spin-orbit field of 48.96 (27.50) mT at an applied current density of 1 MAcm-2 in beta-W interfaced Co60Fe40 (Ni81Fe19)/TiN epitaxial structures due to an anti-damping like torque, which results in a magnetization auto-oscilla…
Magneto-optic Kerr effect can probe the process of magnetization reversal in ferromagnetic thin films and thus be used as an alternative to magnetometry. Kerr effect is wavelength-dependent and the Kerr rotation can reverse sign, vanishing at particular wavelengths. We investigate epitaxial heterostructures of ferromagnetic manganite, La$_{0.7}$Sr$_{0.3}$Mn$_{0.9}$Ru$_{0.1}$O$_3$, by polar Kerr effect and magnetometry. The manganite layers are separated by or interfaced with a layer of nickelate, NdNiO$_3$. Kerr rotation hysteresis loops of trilayers, with two manganite layers of different thi…
We demonstrate a new fabrication process for hybrid semiconductor-superconductor heterostructures based on anodic oxidation (AO), allowing controlled thinning of epitaxial Al films. Structural and transport studies of oxidized epitaxial Al films grown on insulating GaAs substrates reveal spatial non-uniformity and enhanced critical temperature and magnetic fields. Oxidation of epitaxial Al on hybrid InAs heterostructures with a conducting quantum well show similarly enhanced superconducting properties transferred to the two-dimensional electron gas (2DEG) by proximity effect, with critical per…
By inserting a SrZrO$_3$ buffer layer between the film and the substrate, we demonstrate a significant reduction of the threading dislocation density with an associated improvement of the electron mobility in La:BaSnO$_3$ films. A room temperature mobility of 140 cm$^2$ V$^{-1}\text{s}^{-1}$ is achieved for 25-nm-thick films without any post-growth treatment. The density of threading dislocations is only $4.9\times 10^{9}$ cm$^{-2}$ for buffered films prepared on (110) TbScO$_3$ substrates by pulsed laser deposition.
We report on domain pattern transfer from a ferroelectric BaTiO$_{\mathrm{3}}$ substrate with a $(111)$-orientation of the surface to an epitaxial Co film grown on a Pd buffer layer. Spatially modulated interfacial strain transfer from ferroelectric/ferroelastic domains and inverse magnetostriction in the ferromagnetic film induce stripe regions with a modulation of the in-plane uniaxial magnetic anisotropy direction. Using spin-polarized low energy electron microscopy, we observe the formation of two distinct anisotropy configurations between stripe regions. Moreover, through application of a…
We demonstrate fabrication of atomically sharp interfaces between nitride antiperovskite Mn$_{3}$GaN and oxide perovskites (La$_{0.3}$Sr$_{0.7}$)(Al$_{0.65}$Ta$_{0.35}$)O$_{3}$ (LSAT) and SrTiO$_{3}$ as paradigms of nitride-antiperovskite/oxide-perovskite heterostructures. Using a combination of scanning transmission electron microscopy (STEM), atomic-resolution spectroscopic techniques, and first-principle calculations, we investigated the atomic-scale structure, composition, and boding at the interface. We show that the epitaxial growth between the antiperovskite and perovskite compounds is …
We have fabricated 5nm-high Fe(110) stripes by self-organized (SO) growth on a slightly vicinal R(110)/Al2O3(11-20) surface, with R=Mo, W. Remanence, coercivity and domain patterns were observed at room temperature (RT). This contrasts with conventional SO epitaxial systems, that are superparamagnetic or even non-magnetic at RT due to their flatness. Our process should help to overcome superparamagnetism without compromise on the lateral size if SO systems are ever to be used in applications.
Room temperature ferroelectricity is observed in lattice-matched ~18% ScAlN/GaN heterostructures grown by molecular beam epitaxy on single-crystal GaN substrates. The epitaxial films have smooth surface morphologies and high crystallinity. Pulsed current-voltage measurements confirm stable and repeatable polarization switching in such ferroelectric/semiconductor structures at several measurement conditions, and in multiple samples. The measured coercive field values are Ec~0.7 MV/cm at room temperature, with remnant polarization Pr~10 μC/cm2 for ~100 nm thick ScAlN layers. These values are sub…
The strain dependent functional properties of epitaxial transition metal oxide films can be significantly modified via substrate selection. However, large lattice mismatches preclude dislocation-free epitaxial growth on ferroelectric substrates, whose strain states are modified by applied electric fields. Here we overcome this mismatch problem by depositing an epitaxial film of ferromagnetic La0.7Sr0.3MnO3 on a single crystal substrate of well lattice matched SrTiO3 via a film of SrRuO3 that we subsequently dissolved, permitting the transfer of unstrained La0.7Sr0.3MnO3 to a ferroelectric subs…
We report on the growth of epitaxial bilayers of the La2/3Sr1/3MnO3 (LSMO) half-metallic ferromagnet and the BiFeO3 (BFO) multiferroic, on SrTiO3(001) by pulsed laser deposition. The growth mode of both layers is two-dimensional, which results in unit-cell smooth surfaces. We show that both materials keep their properties inside the heterostructures, i.e. the LSMO layer (11 nm thick) is ferromagnetic with a Curie temperature of ~330K, while the BFO films shows ferroelectricity down to very low thicknesses (5 nm). Conductive-tip atomic force microscope mappings of BFO/LSMO bilayers for differen…
Electronic devices using epitaxial graphene on Silicon Carbide require encapsulation to avoid uncontrolled doping by impurities deposited in ambient conditions. Additionally, interaction of the graphene monolayer with the substrate causes relatively high level of electron doping in this material, which is rather difficult to change by electrostatic gating alone. Here we describe one solution to these problems, allowing both encapsulation and control of the carrier concentration in a wide range. We describe a novel heterostructure based on epitaxial graphene grown on silicon carbide combined wi…
Atomically thin MoS2/graphene heterostructures are promising candidates for nanoelectronic and optoelectronic technologies. Among different graphene substrates, epitaxial graphene (EG) on SiC provides several potential advantages for such heterostructures including high electronic quality, tunable substrate coupling, wafer-scale processability, and crystalline ordering that can template commensurate growth. Exploiting these attributes, we demonstrate here the thickness-controlled van der Waals epitaxial growth of MoS2 on EG via chemical vapor deposition, giving rise to transfer-free synthesis …
The introduction of superconductivity to the Dirac surface states of a topological insulator leads to a topological superconductor, which may support topological quantum computing through Majorana zero modes. The development of a scalable material platform is key to the realization of topological quantum computing. Here we report on the growth and properties of high-quality (Bi,Sb)2Te3/graphene/gallium heterostructures. Our synthetic approach enables atomically sharp layers at both hetero-interfaces, which in turn promotes proximity-induced superconductivity that originates in the gallium film…
Heterostructures of Dirac materials such as graphene and topological insulators provide interesting platforms to explore exotic quantum states of electrons in solids. Here we study the electronic structure of graphene-Sb2Te3 heterostructure using density functional theory and tight-binding methods. We show that the epitaxial graphene on Sb2Te3 turns into quantum spin-Hall phase due to its proximity to the topological insulating Sb2Te3. It is found that the epitaxial graphene develops a giant spin-orbit gap of about ~20 meV, which is about three orders of magnitude larger than that of pristine …
Epitaxial oxide interfaces with broken translational symmetry have emerged as a central paradigm behind the novel behaviors of oxide superlattices. Here, we use scanning transmission electron microscopy to demonstrate a direct, quantitative unit-cell-by-unit-cell mapping of lattice parameters and oxygen octahedral rotations across the BiFeO3-La0.7Sr0.3MnO3 interface to elucidate how the change of crystal symmetry is accommodated. Combined with low-loss electron energy loss spectroscopy imaging, we demonstrate a mesoscopic antiferrodistortive phase transition and elucidate associated changes in…
We report on the direct observation of interface superconductivity in single-unit-cell SnSe2 films grown on graphitized SiC(0001) substrate by means of van der Waals epitaxy. Tunneling spectrum in the superconducting state reveals rather conventional character with a fully gapped order parameter. The occurrence of superconductivity is further confirmed by the presence of vortices under external magnetic field. Through interface engineering, we unravel the mechanism of superconductivity that originates from a two-dimensional electron gas formed at the interface of SnSe2 and graphene. Our findin…
The helicity of 3D topological insulator surface states has drawn significant attention in spintronics owing to spin-momentum locking where the carriers' spin is oriented perpendicular to their momentum. This property can provide an efficient method to convert charge currents into spin currents, and vice-versa, through the Rashba-Edelstein effect. However, experimental signatures of these surface states to the spin-charge conversion are extremely difficult to disentangle from bulk state contributions. Here, we combine spin- and angle-resolved photo-emission spectroscopy, and time-resolved THz …
Intense hard x-ray pulses from a free-electron laser induce irreversible structural damage in a perovskite oxide epitaxial heterostructure when pulse fluences exceed a threshold value. The intensity of x-ray diffraction from a 25-nm thick epitaxial BiFeO$_{3}$ layer on a SrTiO$_{3}$ substrate measured using a series of pulses decreases abruptly with a per-pulse fluence of 2.7 x 10$^{6}$ photons $μ$m$^{-2}$ at 9.7 keV photon energy, but remains constant for 1.3 x 10$^{6}$ photons $μ$m$^{-2}$ or less. The damage resulted in the destruction of the BiFeO$_{3}$ thin film within the focal spot area …
Gated variable resistors were manufactured by depositing epitaxial heterostructures of doped La_{2}CuO_{4} and SrTiO_{3} layers. Their conductance change as function of write current I and write time t followed a simple empirical law of the form ΔG/G = CI^A t^B. This behavior is in agreement with ionic transport that accelerates exponentially with electrical field strength.
The rapid discovery of two-dimensional (2D) van der Waals (vdW) quantum materials has led to heterostructures that integrate diverse quantum functionalities such as topological phases, magnetism, and superconductivity. In this context, the epitaxial synthesis of vdW heterostructures with well-controlled interfaces is an attractive route towards wafer-scale platforms for systematically exploring fundamental properties and fashioning proof-of-concept devices. Here, we use molecular beam epitaxy to synthesize a vdW heterostructure that interfaces two material systems of contemporary interest: a 2…
