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Atomic termination plays a key role in activating the proximity effect of antiferromagnet., This work reports the ferromagnetism of topological insulator, (Bi,Sb)2Te3 (BST), with a Curie temperature of approximately 120 K induced by magnetic proximity effect (MPE) of an antiferromagnetic CrSe. The MPE was shown to be highly dependent on the stacking order of the heterostructure, as well as the interface symmetry: Growing CrSe on top of BST results in induced ferromagnetism, while growing BST on CrSe yielded no evidence of an MPE. Cr-termination in the former case leads to double-exchange interactions between Cr3+ surface states and Cr2+ bulk states. This Cr3+-Cr2+ exchange stabilizes the ferromagnetic order localized at the interface and magnetically polarizes the BST Sb band. In contrast, Se-termination at the CrSe/BST interface yields no detectable MPE. These results directly confirm the MPE in BST films and provide critical insights into the sensitivity of the surface state.

Integration of a quantum anomalous Hall insulator with a magnetically ordered material provides an additional degree of freedom through which the resulting exotic quantum states can be controlled. Here, an experimental observation is reported of the quantum anomalous Hall effect in a magnetically-doped topological insulator grown on the antiferromagnetic insulator Cr2 O3 . The exchange coupling between the two materials is investigated using field-cooling-dependent magnetometry and polarized neutron reflectometry. Both techniques reveal strong interfacial interaction between the antiferromagnetic order of the Cr2 O3 and the magnetic topological insulator, manifested as an exchange bias when the sample is field-cooled under an out-of-plane magnetic field, and an exchange spring-like magnetic depth profile when the system is magnetized within the film plane. These results identify antiferromagnetic insulators as suitable candidates for the manipulation of magnetic and topological order in topological insulator films.

Breaking the time reversal symmetry (TRS) in a topological insulator (TI) by introducing a magnetic order gives rise to exotic quantum phenomena. One of the promising routes to inducing a magnetic order in a TI is utilizing magnetic proximity effect between a TI and a strong magnetic insulator (MI). In this article, we demonstrate a TI/MI heterostructure prepared through transferring a molecular beam epitaxy (MBE)-grown Bi2Se3 film onto a yttrium iron garnet (YIG) substrate via wet transfer. The transferred Bi2Se3 exhibits excellent quality over a large scale. Moreover, through wet transfer we are able to engineer the interface and perform a comparative study to probe the proximity coupling between Bi2Se3 and YIG under different interface conditions. A detailed investigation of both the anomalous Hall effect and quantum corrections to the conductivity in magnetotransport measurements reveals an induced magnetic order as well as TRS breaking in the transferred Bi2Se3 film on YIG. In contrast, a thin layer ...

We investigate the hydrostatic pressure dependence of interfacial superconductivity occurring at the atomically sharp interface between two non-superconducting materials: the topological insulator (TI) Bi2Te3 and the parent compound Fe1+yTe of the chalcogenide iron-based superconductors. Under pressure, a significant increase in the superconducting transition temperature Tc is observed. We interpret our data in the context of a pressure-induced enhanced coupling of the Fe1+yTe interfacial layer with the Bi2Te3 surface state, which modifies the electronic properties of the interface layer in a way that superconductivity emerges and becomes further enhanced under pressure. This demonstrates the important role of the TI in the interfacial superconducting mechanism.

The realization and application of spintronic devices would be dramatically advanced if room-temperature ferromagnetism could be integrated into semiconductor nanostructures, especially when compatible with mature silicon technology. Herein, we report the observation of such a system – an Si/MnGe superlattice with quantum dots well aligned in the vertical direction successfully grown by molecular beam epitaxy. Such a unique system could take full advantage of the type-II energy band structure of the Si/Ge heterostructure, which could trap the holes inside MnGe QDs, significantly enhancing the hole-mediated ferromagnetism. Magnetic measurements indeed found that the superlattice structure exhibited a Curie temperature of above 400 K. Furthermore, zero-field cooling and field cooling curves could confirm the absence of ferromagnetic compounds, such as Ge8Mn11 (Tc ∼ 270 K) and Ge3Mn5 (Tc ∼ 296 K) in our system. Magnetotransport measurement revealed a clear magnetoresistance transition from negative to positive and a pronounced anomalous Hall effect. Such a unique Si/MnGe superlattice sets a new stage for strengthening ferromagnetism due to the enhanced hole-mediation by quantum confinement, which can be exploited for realizing the room-temperature Ge-based spin field-effect transistors in the future.

Non-volatile memory and computing technology rely on efficient read and write of ultra-tiny information carriers that do not wear out. Magnetic skyrmions are emerging as a potential carrier since they are topologically robust nanoscale spin textures that can be manipulated with ultralow current density. To date, most of skyrmions are reported in metallic films, which suffer from additional Ohmic loss and thus high energy dissipation. Therefore, skyrmions in magnetic insulators are of technological importance for low-power information processing applications due to their low damping and the absence of Ohmic loss. Moreover, they attract fundamental interest in studying various magnon-skyrmion interactions11. Skyrmions have been observed in one insulating material Cu2OSeO3 at cryogenic temperatures, where they are stabilized by bulk Dzyaloshinskii-Moriya interaction. Here, we report the observation of magnetic skyrmions that survive above room temperature in magnetic insulator/heavy metal heterostructures, i.e., thulium iron garnet/platinum. The presence of these skyrmions results from the Dzyaloshinskii-Moriya interaction at the interface and is identified by the emergent topological Hall effect. Through tuning the magnetic anisotropy via varying temperature, we observe skyrmions in a large window of external magnetic field and enhanced stability of skyrmions in the easy-plane anisotropy regime. Our results will help create a new platform for insulating skyrmion-based room temperature low dissipation spintronic applications., Initially submitted version on 10/03/2018 with a revised arXiv webpage title

We use temperature-dependent Hall measurements to identify contributions of spin Hall, magnetic proximity, and sublattice effects to the anomalous Hall signal in heavy metal/ferrimagnetic insulator heterostructures with perpendicular magnetic anisotropy. This approach enables detection of both the magnetic proximity effect onset temperature and the magnetization compensation temperature and provides essential information regarding the interfacial exchange coupling. Onset of a magnetic proximity effect yields a local extremum in the temperature-dependent anomalous Hall signal, which occurs at higher temperature as magnetic insulator thickness increases. This magnetic proximity effect onset occurs at much higher temperature in Pt than W. The magnetization compensation point is identified by a sharp anomalous Hall sign change and divergent coercive field. We directly probe the magnetic proximity effect using x-ray magnetic circular dichroism and polarized neutron reflectometry, which reveal an antiferromagnetic coupling between W and the magnetic insulator. Finally, we summarize the exchange-coupling configurations and the anomalous Hall-effect sign of the magnetized heavy metal in various heavy metal/magnetic insulator heterostructures.

Magnetic topological insulators such as Cr-doped (Bi,Sb)2Te3 provide a platform for the realization of versatile time-reversal symmetry-breaking physics. By constructing heterostructures with N\'eel order in an antiferromagnetic CrSb and magnetic topological order in Cr-doped (Bi,Sb)2Te3, we realize emergent interfacial magnetic phenomena which can be tailored through artificial structural engineering. Through deliberate geometrical design of heterostructures and superlattices, we demonstrate the use of antiferromagnetic exchange coupling in manipulating the magnetic properties of the topological surface massive Dirac fermions. This work provides a new framework on integrating topological insulators with antiferromagnetic materials and unveils new avenues towards dissipationless topological antiferromagnetic spintronics., Comment: 25 pages, 4 figures

Studying resonance modes in exchange-coupled ferromagnetic bilayers is typically accomplished by a magnetic-field-excited ferromagnetic resonance (FMR) technique, in which case it is usually difficult to excite the optical mode, because the out-of-phase resonance cannot efficiently couple to a uniform excitatory magnetic field. This work shows that spin-torque FMR (ST-FMR) can be used to efficiently excite the optical resonance mode, because the current-induced torque naturally does not act uniformly on the two ferromagnetic layers. The results indicate that ST-FMR is an effective probe of interlayer exchange coupling in such systems, to the benefit of research on spintronic devices.

Heavy metals and topological insulators are promising materials for converting charge current into spin current for efficient manipulation of magnetization states in magnetic devices [1]–[5]. One of the most important parameters is the charge-to-spin conversion (CS) efficiency. Improving CS efficiency is critical for reducing write current of the emerging nonvolatile memory technology, spin-orbit torque MRAM (SOT-MRAM) [2], which provides comparable speed with SRAM but with a much higher memory capacity. Here, we measure CS efficiency in various topological insulators (TIs) using second-harmonic method (2eo-method) and obtain a record-high value 8.33±0.65 for insulating (BiSb)2 Te3 at room temperature. We first establish the consistency of CS efficiency obtained between spin-torque ferromagnetic resonance (ST-FMR) and 2eo-method. Then, we systematically investigate the CS efficiency in a bilayer consisting of a metallic Bi2Se3 and a CoFeB thin film using 2eo-method. By tuning the Fermi level of TI layer into bulk band gap using (BiSb)2 Te3, we improve the CS efficiency by an order of magnitude.

Magnetic skyrmions as swirling spin textures with a nontrivial topology have potential applications as magnetic memory and storage devices. Since the initial discovery of skyrmions in non-centrosymmetric B20 materials, the recent effort has focused on exploring room-temperature skyrmions in heavy metal and ferromagnetic heterostructures, a material platform compatible with existing spintronic manufacturing technology. Here, we report the surprising observation that a room-temperature skyrmion phase can be stabilized in an entirely different class of systems based on antiferromagnetic (AFM) metal and ferromagnetic (FM) metal IrMn/CoFeB heterostructures. There are a number of distinct advantages of exploring skyrmions in such heterostructures including zero-field stabilization, tunable antiferromagnetic order, and sizable spin-orbit torque (SOT) for energy-efficient current manipulation. Through direct spatial imaging of individual skyrmions, quantitative evaluation of the interfacial Dzyaloshinskii-Moriya interaction, and demonstration of current-driven skyrmion motion, our findings firmly establish the AFM/FM heterostructures as a promising material platform for exploring skyrmion physics and device applications.

The Bi2Te3/FeTe heterostructure intersects several phenomena and key classes of materials in condensed matter physics: topological insulators, superconductivity, magnetism, and the physics of interfaces. While neither the topological insulator (Bi2Te3) nor the iron chalcogenide (FeTe) are themselves superconductors, superconductivity forms in a thin 7nm interfacial layer between the two. The restricted dimensionality and the extraordinarily conductive normal state, possibly sourced by the topologically protected surface states, have led to the observation of novel phenomena such as the Likharev vortex explosion and transitions in behavior resulting from the interplay between current induced depairing and the Berezinski-Kosterlitz-Thouless regime. Bi2Te3/FeTe also displays the anomalous oscillatory magnetoresistance phenomenon, which we had previously observed in cuprates.

Recently, a logarithmic decrease of conductivity has been observed in topological insulators at low temperatures, implying a tendency of localization of surface electrons. Here, we report quantum transport experiments on the topological insulator Bi2Te3 thin films with arrayed antidot nanostructures. With increasing density of the antidots, a systematic decrease is observed in the slope of the logarithmic temperature-dependent conductivity curves, indicating the electron-electron interaction can be tuned by the antidots. Meanwhile, the weak anti-localization effect revealed in magnetoconductivity exhibits an enhanced dominance of electron-electron interaction among decoherence mechanisms. The observation can be understood from an antidot-induced reduction of the effective dielectric constant, which controls the interactions between the surface electrons. Our results clarify the indispensable role of the electron-electron interaction in the localization of surface electrons and indicate the localization of surface electrons in an interacting topological insulator., 27 pages, article+supplemental materials, published in ACS Nano

This work focuses on the formation mechanism of a superconducting Fe1+xTe/Bi2Te3 bilayer fabricated through chemical reactions between a Bi2Te3 flux and an annealed Fe layer grown on a ZnSe(111)B buffer via the molecular beam epitaxy technique. The studies using energy-dispersive X-ray spectrometry and high-resolution transmission electron microscopy performed on a number of samples fabricated with different desired schemes provide evidence that several interfacial chemical reactions taking place at the Fe/ZnSe interface through the edges of voids of the annealed Fe layer contributed to the formation of the superconducting bilayer. We have also revealed that the bonding strength of the involved Fe compounds at the bottom interface of the annealed Fe layer seems to play an essential role.

Cu doped Bi1.5Pb0.5Sr1.8La0.2Co2-xCuxOy(x = 0.0, 0.2, 0.4) samples were prepared through the solid state reaction method. The influence of different Cu doping contents on electrical resistivity, Seebeck coefficients, thermal conductivity, and the dimensionless figure of meritZTwas investigated. All the samples of Cu concentration 0.4 are single phases. The electrical resistivity of Bi1.5Pb0.5Sr1.8La0.2Co1.8Cu0.2Oydescends, and itsZTvalues are enhanced obviously. The results show that suitable element doping can modify the electric transport properties and enhance thermoelectric properties of materials.

Voltage control of magnetism in ferromagnetic semiconductor has emerged as an appealing solution to significantly reduce the power dissipation and variability beyond current CMOS technology. However, it has been proven to be very challenging to achieve a candidate with high Curie temperature (Tc), controllable ferromagnetism and easy integration with current Si technology. Here we report the effective electric-field control of both ferromagnetism and magnetoresistance in unique MnxGe1−x nanomeshes fabricated by nanosphere lithography, in which a Tc above 400 K is demonstrated as a result of size/quantum confinement. Furthermore, by adjusting Mn doping concentration, extremely giant magnetoresistance is realized from ∼8,000% at 30 K to 75% at 300 K at 4 T, which arises from a geometrically enhanced magnetoresistance effect of the unique mesh structure. Our results may provide a paradigm for fundamentally understanding the high Tc in ferromagnetic semiconductor nanostructure and realizing electric-field control of magnetoresistance for future spintronic applications.

In the interfacial superconductor Bi2Te3/Fe1+yTe, two dimensional superconductivity occurs in direct vicinity to the surface state of a topological insulator. If this state were to become involved in superconductivity, under certain conditions a topological superconducting state could be formed, which is of high interest due to the possibility of creating Majorana fermionic states. We report directional point-contact spectroscopy data on the novel Bi2Te3/Fe1+yTe interfacial superconductor for a Bi2Te3 thickness of 9 quintuple layers, bonded by van der Waals epitaxy to a Fe1+yTe film at an atomically sharp interface. Our data show highly unconventional superconductivity, which appears as complex as in the cuprate high temperature superconductors. A very large superconducting twin-gap structure is replaced by a pseudogap above ~12 K which persists up to 40 K. While the larger gap shows unconventional order parameter symmetry and is attributed to a thin FeTe layer in proximity to the interface, the smaller gap is associated with superconductivity induced via the proximity effect in the topological insulator Bi2Te3.

Polycrystalline samples of Bi1.5Pb0.5Sr2−xLaxCo2Oy (x = 0.1, 0.2, 0.3) with a layered structure were prepared by solid-state reaction method. All samples are p-type semiconductors. The thermoelectric properties, namely, the electric resistivity (ρ), Seebeck coefficient (S), and power factor (S2/ρ) of the samples are dependent on chemical composition. The values of ρ, S, and S2/ρ increase with an increase in temperature for all samples. The substitution of Pb2+ for Bi3+ and La3+ for Sr2+ improves the thermoelectric properties of the Bi2Sr2Co2Oy system owing to the simultaneous decrease of electric resistivity and increase of Seebeck coefficient. As a result, the optimal thermoelectric property has been obtained in Bi1.5Pb0.5Sr1.7La0.3Co2Oy and the power factor can reach 2.1 × 10−4 W·m−1·K−2 at 998 K.

Co2-xCuxOy (x=0.0, 0.2, 0.4) Samples were prepared by solid-state reaction method and the effect of Cu substitution on the thermoelectric properties was investigated. The presence of Cu element improved the grain size and electrical conductivity, but Seebeck coefficients were reduced by Cu substitution. It was found Cu substitution is an effective way to improve the thermoelectric performance system at high temperature.

We investigated current induced depairing in the ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}/\mathrm{FeTe}$ topological insulator-chalcogenide interface superconductor. The measured depairing current density provides information on the magnetic penetration depth and superfluid density, which in turn shed light on the nature of the normal state that underlies the interfacial superconductivity.

We have developed an incandescent Mo source to fabricate large-area single-crystalline MoSe2 thin films. The as-grown MoSe2 thin films were characterized using transmission electron microscopy, energy dispersive x-ray spectroscopy, atomic force microscopy, Raman spectroscopy, photoluminescence (PL), reflection high energy electron diffraction (RHEED) and angular resolved photoemission spectroscopy (ARPES). A new Raman characteristic peak at 1591 cm-1 was identified. Results from Raman spectroscopy, PL, RHEED and ARPES studies consistently reveal that large-area single crystalline mono-layer of MoSe2 could be achieved by this technique. This technique enjoys several advantages over conventional approaches and could be extended to the growth of other two-dimensional layered materials containing a low-vapor-pressure element.

We study the magnetic properties of W/Co40Fe40B20 (CoFeB)/MgO films using the spin-torque ferromagnetic resonance (ST-FMR) technique. This study takes the advantage of the spin Hall magnetoresistance (SMR) for generating an oscillating resistance, which is one of the necessary requirements for obtaining mixing voltage in the ST-FMR technique. We have measured both the as-grown and the annealed samples with different CoFeB layer thicknesses, which include the in-plane and out-of-plane magnetic anisotropies. The spectra for these two types of anisotropies show distinct signatures. By analyzing the ST-FMR spectra, we extract the effective anisotropy field for both types of samples. In addition, we investigate the influence of CoFeB thickness and annealing on the Gilbert damping constant. Our experiments show that by taking advantage of SMR, the ST-FMR measurement acts as an effective tool with high sensitivity for studying the magnetic properties of ultrathin magnetic films.

Superconductivity at the interface of a heterostructure confined to nanometer-sized scale offers unique opportunities to study the exotic physics of two-dimensional superconductivity. The realization of superconductivity at the interface between a topological insulator and an iron-chalcogenide compound is highly attractive for exploring several recent theoretical predictions involving these two new classes of materials. Here, we report transport measurements on a Bi2Te3/FeTe heterostructure fabricated via van der Waals epitaxy, which demonstrate superconductivity at the interface induced by the Bi2Te3 epilayer with thickness even down to one quintuple layer. The two-dimensional nature of the observed superconductivity with the highest transition temperature around 12 K was verified by the existence of a Berezinsky-Kosterlitz-Thouless transition and the diverging ratio of in-plane to out-plane upper critical field on approaching the superconducting transition temperature. With the combination of interface superconductivity and Dirac surface states of Bi2Te3, the heterostructure studied in this work provides a novel platform for realizing Majorana fermions., 24 pages, 5 figures and supplementary information

We present a study of the surface reactivity of a Pd/Bi2Te3 thin film heterostructure. The topological surface states from Bi2Te3, being delocalized and robust owing to their topological natures, were found to act as an effective electron bath that significantly enhances the surface reactivity of palladium in the presence of two oxidizing agents, oxygen and tellurium respectively, which is consistent with a theoretical calculation. The surface reactivity of the adsorbed tellurium on this heterostructure is also intensified possibly benefitted from the effective transfer of the bath electrons. A partially inserted iron ferromagnetic layer at the interface of this heterostructure was found to play two competing roles arising from the higher-lying d-band center of the Pd/Fe bilayer and the interaction between the ferromagnetism and the surface spin texture of Bi2Te3 on the surface reactivity and their characteristics also demonstrate that the electron bath effect is long-lasting against accumulated thickness of adsorbates.

CaS, with a direct bandgap of 5.38 eV, is expected to be a strong candidate as the active-layer of high performance solar-blind UV photodiodes that have important applications in both civilian and military sectors. Here, we report that a seed-layer-assisted growth approach via molecular beam epitaxy can result in high crystalline quality rocksalt CaS thin films on zincblende GaAs substrates. The Au/CaS/GaAs solar-blind photodiodes demonstrated , more than five orders in its visible rejection power, a photoresponse of 36.8 mA/w at zero bias and a corresponding quantum efficiency as high as 19% at 235 nm.

In order to reveal the microstructural features of the recently developed high Jc bulk YBCO material with Tc=91.2 K, Jc(77 K, 2 T)=2.38*104 A cm-2, large-area TEM specimens containing several grains were successfully prepared from this material by a new technique. The material has a strong (001) texture. Many grain boundaries are clean (001) small-angle twist boundaries showing large-area moire fringes in the micrographs which may be an essential cause of the improved critical current.

An array of Nb strips was fabricated on a Bi2Te3 thin film. Transport measurements show that an upturn of the resistance occurs below the Nb superconducting transition temperature. Corresponding to this resistance upturn, a differential resistance peak around zero bias and a prominent negative magnetoresistance at low magnetic fields are observed. The resistance upturn, differential resistance peak, and negative magnetoresistance can be suppressed by increasing measurement current, temperature, and applied magnetic field. We explain these phenomena in terms of an energy barrier effect induced by the Nb superconducting gap and the suppression of Andreev reflection due to the low transparency at the Nb-Bi2Te3 interface.

Molecular beam epitaxy grown MgS on GaAs(111)B substrate was resulted in wurtzite phase, as demonstrated by detailed structural characterizations. Phenomenological arguments were used to account for why wurtzite phase is preferred over zincblende phase or its most stable rocksalt phase. Results of photoresponse and reflectance measurements performed on wurtzite MgS photodiodes suggest a direct bandgap at around 5.1 eV. Their response peaks at 245 nm with quantum efficiency of 9.9% and enjoys rejection of more than three orders at 320 nm and close to five orders at longer wavelengths, proving the photodiodes highly competitive in solar-blind ultraviolet detection.

Experimental results of drop generation in a laboratory model water bath with top blowing and combined blowing are given. It was found that with top blowing alone two different mechanisms of drop generation exist, each having its own characteristic drop generation rate. The two mechanisms, identified by high speed film analysis, are shown to be a function of the blowing rate. Bottom blowing is shown to significantly increase drop generation rate and that the increase is caused by the interaction of the top blowing and the bottom blowing in the impingement zone, and not by the bottom blowing as such. Drop size distribution generated by a gas jet impinging on a liquid surface is normal. Mean drop size is increased by the introduction of bottom blowing.