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The two-dimensional (2D) semiconductor molybdenum disulfide (MoS2) has attracted widespread attention for its extraordinary electrical, optical, spin and valley related properties. Here, we report on spin polarized tunneling through chemical vapor deposited (CVD) multilayer MoS2 (~7 nm) at room temperature in a vertically fabricated spin-valve device. A tunnel magnetoresistance (TMR) of 0.5 - 2 % has been observed, corresponding to spin polarization of 5 - 10 % in the measured temperature range of 300 - 75 K. First principles calculations for ideal junctions results in a tunnel magnetoresistance up to 8 %, and a spin polarization of 26 %. The detailed measurements at different temperatures and bias voltages, and density functional theory calculations provide information about spin transport mechanisms in vertical multilayer MoS2 spin-valve devices. These findings form a platform for exploring spin functionalities in 2D semiconductors and understanding the basic phenomenon that control their performance.

We systematically investigate the effect of strain and doping on the polar metal phase in lithium osmate, LiOsO

WTe2, as a type-II Weyl semimetal, has 2D Fermi arcs on the (001) surface in the bulk and 1D helical edge states in its monolayer. These features have recently attracted wide attention in condensed matter physics. However, in the intermediate regime between the bulk and monolayer, the edge states have not been resolved owing to its closed band gap which makes the bulk states dominant. Here, we report the signatures of the edge superconductivity by superconducting quantum interference measurements in multilayer WTe2 Josephson junctions and we directly map the localized supercurrent. In thick WTe2 (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} }{}$\sim 60{\rm{\ nm}})$\end{document}, the supercurrent is uniformly distributed by bulk states with symmetric Josephson effect (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} }{}$| {I_c^ + ( B )} | {=} | {I_c^ - ( B )} |\ $\end{document}). In thin WTe2 (10 nm), however, the supercurrent becomes confined to the edge and its width reaches up to \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} }{}$1.4{\rm{\ \mu m\ }}$\end{document}and exhibits non-symmetric behavior \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{upgreek} \usepackage{mathrsfs} \setlength{\oddsidemargin}{-69pt} \begin{document} }{}$| {I_c^ + ( B )} | \ne | {I_c^ - ( B )} |$\end{document}. The ability to tune the edge domination by changing thickness and the edge superconductivity establishes WTe2 as a promising topological system with exotic quantum phases and a rich physics., We report the evidence of edge superconductivity in multilayer WTe2 Josephson junctions which remains rarely studied in type-II Weyl semimetal.

The ability to control the structure of a crystalline solid on ultrafast timescales bears enormous potential for information storage and manipulation or generating new functional states of matter [1]. In many materials where the ultrafast control of crystalline structures has been explored, optical excitation pushes materials towards their less ordered high temperature phase [2{9] as electronically driven ordered phases melt and possible concomitant structural modifications relax. Nonetheless, for a few select materials it has been shown that photoexcitation can slightly enhance the amplitude of an electronic ordering phenomenon (i.e. its electronic order parameter) [9{13]. Here we show via femtosecond hard X-ray diffraction that photodoping of the perovskite EuTiO3 transiently increases the order parameter associated with a purely structural [14] phase transition represented by the antiferrodistortive rotation of the oxygen octahedra. This can be understood from an ultrafast charge-transfer induced reduction of the Goldschmidt tolerance factor [15], which is a fundamental control parameter for the properties of perovskites, Comment: 11 pages, 3 figures

Dirac semimetals have attracted extensive attentions in recent years. It has been theoretically suggested that many-body interactions may drive exotic phase transitions, spontaneously generating a Dirac mass for the nominally massless Dirac electrons. So far, signature of interaction-driven transition has been lacking. In this work, we report high-magnetic-field transport measurements of the Dirac semimetal candidate ZrTe5. Owing to the large g factor in ZrTe5, the Zeeman splitting can be observed at magnetic field as low as 3 T. Most prominently, high pulsed magnetic field up to 60 T drives the system into the ultra-quantum limit, where we observe abrupt changes in the magnetoresistance, indicating field-induced phase transitions. This is interpreted as an interaction-induced spontaneous mass generation of the Dirac fermions, which bears resemblance to the dynamical mass generation of nucleons in high-energy physics. Our work establishes Dirac semimetals as ideal platforms for investigating emerging correlation effects in topological matters., It has been predicted that the presence of strong electronic correlations may generate new phases in materials with topologically non-trivial band structure. Here, the authors demonstrate the generation of Dirac mass in the correlated Dirac semimetal candidate ZrTe5 under high magnetic fields.

The search for proximity-induced superconductivity in topological materials has generated widespread interest in the condensed matter physics community. The superconducting states inheriting nontrivial topology at interfaces are expected to exhibit exotic phenomena such as topological superconductivity and Majorana zero modes, which hold promise for applications in quantum computation. However, a practical realization of such hybrid structures based on topological semimetals and superconductors has hitherto been limited. Here, we report the strong proximity-induced superconductivity in type-II Weyl semimetal WTe

Discovered decades ago, the quantum Hall effect remains one of the most studied phenomena in condensed matter physics and is relevant for research areas such as topological phases, strong electron correlations and quantum computing

The magnetoresistance of a material conveys various dynamic information about charge and spin carriers, inspiring both fundamental studies in physics and practical applications such as magnetic sensors, data storage, and spintronic devices. Magnetic impurities play a crucial role in the magnetoresistance as they induce exotic states of matter such as the quantum anomalous Hall effect in topological insulators and tunable ferromagnetic phases in dilute magnetic semiconductors. However, magnetically doped topological Dirac semimetals are hitherto lacking. Here, we report a systematic study of Cr-doped $\mathrm{C}{\mathrm{d}}_{3}\mathrm{A}{\mathrm{s}}_{2}$ thin films grown by molecular-beam epitaxy. With the Cr doping, $\mathrm{C}{\mathrm{d}}_{3}\mathrm{A}{\mathrm{s}}_{2}$ thin films exhibit unexpected negative transverse magnetoresistance and strong quantum oscillations, bearing a trivial Berry's phase and an enhanced effective mass. More importantly, with ionic gating the magnetoresistance of Cr-doped $\mathrm{C}{\mathrm{d}}_{3}\mathrm{A}{\mathrm{s}}_{2}$ thin films can be drastically tuned from negative to positive, demonstrating the strong correlation between electrons and the localized spins of the Cr impurities, which we interpret through the formation of magnetic polarons. Such a negative magnetoresistance under perpendicular magnetic field and its gate tunability have not been observed previously in the Dirac semimetal $\mathrm{C}{\mathrm{d}}_{3}\mathrm{A}{\mathrm{s}}_{2}$. The Cr-induced topological phase transition and the formation of magnetic polarons in $\mathrm{C}{\mathrm{d}}_{3}\mathrm{A}{\mathrm{s}}_{2}$ provide insights into the magnetic interaction in Dirac semimetals as well as their potential applications in spintronics.

We investigate the effects of strain on superconductivity with particular reference to SrTiO$_3$. Assuming that a ferroelectric mode that softens under tensile strain is responsible for the coupling, an increase in the critical temperature and range of carrier densities for superconductivity is predicted, while the peak of the superconducting dome shifts towards lower carrier densities. Using a Ginzburg-Landau approach in 2D, we find a linear dependence of the critical temperature on strain: if the couplings between the order parameter and strains in different directions differ while their sum is fixed, different behaviours under uniaxial and biaxial (uniform) strain can be understood., 5 + 3 pages; 3 + 2 figures

We present a rational design approach to customize the spin texture of surface states of a topological insulator. This approach relies on the extreme multifunctionality of organic molecules that are used to functionalize the surface of the prototypical topological insulator (TI) Bi2Se3. For the rational design we use theoretical calculations to guide the choice and chemical synthesis of appropriate molecules that customize the spin texture of Bi2Se3. The theoretical predictions are then verified in angular-resolved photoemission experiments. We show that, by tuning the strength of molecule-TI interaction, the surface of the TI can be passivated, the Dirac point can energetically be shifted at will, and Rashba-split quantum-well interface states can be created. These tailored interface properties-passivation, spin-texture tuning, and creation of hybrid interface states-lay a solid foundation for interface-assisted molecular spintronics in spin-textured materials.

We report the discovery of two ternary Zintl phases Ba

We report a magnetotransport study of Dirac semimetal $\mathrm{C}{\mathrm{d}}_{3}\mathrm{A}{\mathrm{s}}_{2}$ under high pressure. The Shubnikov--de Haas oscillations are measured at different pressures to reveal the evolution of the Fermi surface. A sudden change in the phase factor of the oscillations occurs at 1.3 GPa along with the unanticipated shrinkage of the Fermi surface, which is well below the structure transition point from the tetragonal to the monoclinic structure ($\ensuremath{\sim}2.5\phantom{\rule{0.16em}{0ex}}\mathrm{GPa}$). High-pressure x-ray diffraction also reveals an anisotropic compression of the $\mathrm{C}{\mathrm{d}}_{3}\mathrm{A}{\mathrm{s}}_{2}$ lattice around a similar pressure. Through the first-principles calculations, we find that such axial compression along the $c$ direction will shift the Dirac nodes towards the Brillouin zone center and will eventually introduce a finite energy gap. Our result unveils a possible topological phase transition in pressurized $\mathrm{C}{\mathrm{d}}_{3}\mathrm{A}{\mathrm{s}}_{2}$ without structure transition.

We study impurity scattering in the normal and d-wave superconducting states of line nodal semimetals and show that, due to additional scattering phase space available for impurities on the surface, the quasiparticle interference pattern acquires an extended character instead of a discrete collection of delta function peaks. Moreover, using the T-matrix formalism, we demonstrate that the conventional behavior of a scalar impurity in a d-wave superconductor breaks down on the surface of a line nodal semimetal in the quasi flat band limit., 11 pages,7 figures including Supplemental Material

The zero-temperature limit of a continuous phase transition is marked by a quantum critical point, which can generate exotic physics that extends to elevated temperatures. Magnetic quantum criticality is now well known, and has been explored in systems ranging from heavy fermion metals to quantum Ising materials. Ferroelectric quantum critical behaviour has also been recently established, motivating a flurry of research investigating its consequences. Here, we introduce the concept of multiferroic quantum criticality, in which both magnetic and ferroelectric quantum criticality occur in the same system. We develop the phenomenology of multiferroic quantum critical behaviour, describe the associated experimental signatures, and propose material systems and schemes to realize it., Comment: 8 pages, 4 figures

The recent discovery of hidden spin polarization emerging in bulk electronic states of specific nonmagnetic crystals is a fascinating phenomenon, though hardly explored yet. Here, we study from a theoretical perspective nonmagnetic ${\mathrm{BaNiS}}_{2}$, recently suggested to exhibit a giant Rashba-like spin-orbit splitting of the bulk bands, despite the absence of heavy elements. We employ density functional theory and Green's functions calculations to reveal the exact spin textures of both bulk and surface. We predict unambiguous signatures of spin-polarized electronic states at the surface, which reflect the bulk Rashba splitting and which could be experimentally measured with sufficient resolution: this would constitute a clear report of a bulk-Rashba-induced spin splitting at the surface of centrosymmetric crystals.

We photoexcite SrTiO3 and EuTiO3 in their purely soft-mode-driven structurally distorted phase and trace the structural order parameter via ultra-short x-rays. We observe a rapid decay for SrTiO3 and an intriguing transient enhancement for EuTiO3.

Competing orders widely exist in many material systems, such as superconductivity, magnetism, and ferroelectricity; $\mathrm{LaCrS}{\mathrm{b}}_{3}$ is a highly anisotropic magnetic material in which the spins are aligned ferromagnetically in one direction and canted antiferromagnetically in another in the Cr-Sb chains. Hole doping with $\mathrm{S}{\mathrm{r}}^{2+}$ and $\mathrm{C}{\mathrm{a}}^{2+}$ in the La site suppresses the antiferromagnetic correlations and transforms the anisotropic magnetic order into a ferromagnetic lattice in all directions. First-principles density functional theory calculations show that the canted magnetic order becomes energetically less favorable compared to the FM order upon hole doping. Doping in the La site is an effective approach to modulate the competing orders in $\mathrm{LaCrS}{\mathrm{b}}_{3}$.

We propose that illumination with light provides a useful platform for creating tunable semimetals. We show that by shining light on semimetals with a line degeneracy, one can convert them to a point node semimetal. These point nodes are adjustable and their position can be controlled by simply rotating the incident light beam. We also discuss the implications of this change in Fermi surface topology, as manifested in transport observables., Comment: Published version

Expanding the library of known inorganic materials with functional electronic or magnetic behavior is a long-standing goal in condensed matter physics and materials science. Recently, the transition metal chalcogenides including selenium and sulfur have been of interest because of their correlated-electron properties, as seen in the iron-based superconductors and the transition metal dichalcogenides. However, the chalcogenide chemical space is less explored than that of oxides, and there is an open question of whether there may be new materials heretofore undiscovered. We perform a systematic combined theoretical and experimental search over ternary phase diagrams that are empty in the Inorganic Crystal Structure Database containing cations, transition metals, and one of selenium or sulfur. In these 27 ternary systems, we use a probabilistic model to reduce the likelihood of false negative predictions, which results in a list of 24 candidate materials. We then conduct a variety of synthesis experiments to check the candidate materials for stability. While the prediction method did obtain previously unknown compositions that are predicted stable within density functional theory, none of the candidate materials formed in our experiments. We come to the conclusion that these phase diagrams are ``empty'' in the case of bulk synthesis, but it remains a possibility that alternate synthesis routes may produce some of these phases.

Since the discovery of graphene, layered materials have attracted extensive interest owing to their unique electronic and optical characteristics. Among them, Dirac semimetals, one of the most appealing categories, have been a long-sought objective in layered systems beyond graphene. Recently, layered pentatelluride ZrTe5 was found to host signatures of a Dirac semimetal. However, the low Fermi level in ZrTe5 strongly hinders a comprehensive understanding of the whole picture of electronic states through photoemission measurements, especially in the conduction band. Here, we report the observation of Dirac fermions in ZrTe5 through magneto-optics and magneto-transport. By applying a magnetic field, we observe a dependence of the inter-Landau-level resonance and Shubnikov–de Haas (SdH) oscillations with a nontrivial Berry phase, both of which are hallmarks of Dirac fermions. The angle-dependent SdH oscillations show a clear quasi-two-dimensional feature with a highly anisotropic Fermi surface and band topology, in stark contrast to the three-dimensional (3D) Dirac semimetal such as Cd3As2. This is further confirmed by the angle-dependent Berry phase measurements and the observation of bulk quantum Hall effect (QHE) plateaus. The unique band dispersion is theoretically understood: the system is at the critical point between a 3D Dirac semimetal and a topological insulator phase. With the confined interlayer dispersion and reducible dimensionality, our work establishes ZrTe5 as an ideal platform for exploring the exotic physical phenomena of Dirac fermions. The presence of fast, effectively massless electrons in layered zirconium pentatelluride (ZrTe5) crystals has been confirmed. The one-atom-thick structure of graphene enables its electrons to travel much faster than typical materials, and device designers are hunting for other layered crystals that have similar ‘Dirac fermions’. By performing magneto-optical and magneto-transport measurements, Faxian Xiu from Fudan University in China and co-workers have uncovered key signatures of electrons moving at relativistic speeds in the intriguing thermoelectric material ZrTe5. Because previous investigations of this phenomenon were hampered by ZrTe5's hard-to-access Fermi level, the team used magnetic fields to induce characteristic resonance and oscillation signals. These experiments revealed that these Dirac fermions had quasi-two-dimensional behaviour and unusual interlayer characteristics, which merit further theoretical studies and exploration of possible device applications. ZrTe5 has attracted intensive research interests because of the potential topological property. Here based on the transports and optic experiments, we observe unusual Landau quantization, non-trivial Berry phase and highly anisotropic carrier mass, which reveals quasi-two-dimensional Dirac fermions in bulk ZrTe5. The system is understood as locating at the critical point between a three-dimensional (3D) Dirac semimetal and a topological insulator. New physics or device application can be possibly realized in this quasi-2D ultra-relativistic system beyond graphene.

We study the photoresponse of two-dimensional semi-Dirac semimetals and three-dimensional Dirac semimetals to off-resonant circularly polarized light. For two-dimensional semi-Dirac semimetals we find that incident light does not open a gap in the spectrum, in contrast to the case of purely linear dispersion. For the three-dimensional case we find that applying a circularly polarized light, one can tune from a trivial insulator to three-dimensional Dirac semimetal with an inverted gap. We propose and show that application of light leads to an intriguing platform for generation, motion and merging of three-dimensional Dirac points., Published version

Two-dimensional (2D) layered transition metal dichalcogenides (TMDs) have been recently proposed as appealing candidate materials for spintronic applications owing to their distinctive atomic crystal structure and exotic physical properties arising from the large bonding anisotropy. Here we introduce the first MoS2-based spin-valves that employ monolayer MoS2 as the nonmagnetic spacer. In contrast with what is expected from the semiconducting band-structure of MoS2, the vertically sandwiched-MoS2 layers exhibit metallic behavior. This originates from their strong hybridization with the Ni and Fe atoms of the Permalloy (Py) electrode. The spin-valve effect is observed up to 240 K, with the highest magnetoresistance (MR) up to 0.73% at low temperatures. The experimental work is accompanied by the first principle electron transport calculations, which reveal an MR of ∼9% for an ideal Py/MoS2/Py junction. Our results clearly identify TMDs as a promising spacer compound in magnetic tunnel junctions and may open a new avenue for the TMDs-based spintronic applications.

Use of Permutations and Combinations in India

Magic Squares in India

We present a class of Rashba systems in hexagonal semiconducting compounds, where an electrical control over spin-orbital texture is provided by their bulk ferroelectricity. Our first-principles calculations reveal a number of such materials with large Rashba coefficients. We, furthermore, show that strain can drive a topological phase transition in such materials, resulting in a ferroelectric topological insulating state. Our findings can open avenues for interplay between Rashba effect, ferroelectricity and topological phenomena., Comment: Published version

We study with first-principles methods the interplay between bulk and surface Dirac fermions in three dimensional Dirac semimetals. By combining density functional theory with the coherent potential approximation, we reveal a topological phase transition in Na$_3$Bi$_{1-x}$Sb$_{x}$ and Cd$_3$[As$_{1-x}$P$_x$]$_2$ alloys, where the material goes from a Dirac semimetal to a trivial insulator upon changing Sb or P concentrations. Tuning the composition allows us to engineer the position of the bulk Dirac points in reciprocal space. Interestingly, the phase transition coincides with the reversal of the band ordering between the conduction and valence bands., Comment: Updated text, published version

The appearance of topologically protected states at the surface of an ordinary insulator is a rare occurrence and to date only a handful of materials are known for having this property. An intriguing question concerns the possibility of forming topologically protected interfaces between different materials. Here we propose that a topological phase can be transferred to graphene by proximity with the three-dimensional topological insulator Bi$_2$Se$_3$. By using density functional and transport theory we prove that, at the verge of the chemical bond formation, a hybrid state forms at the graphene/Bi$_2$Se$_3$ interface. The state has Dirac-cone-like dispersion at the $\Gamma$ point and a well-defined helical spin-texture, indicating its topologically protected nature. This demonstrates that proximity can transfer the topological phase from Bi$_2$Se$_3$ to graphene., Comment: 6 pages, 4 figures

We demonstrate giant magnetoresistance in $\mathrm{Fe}/\mathrm{MoS}{}_{2}/\mathrm{Fe}$ junctions by means of ab initio transport calculations. We show that junctions incorporating either a monolayer or a bilayer of $\mathrm{MoS}{}_{2}$ are metallic and that Fe acts as an efficient spin injector into $\mathrm{MoS}{}_{2}$ with an efficiency of about 45%. This is the result of the strong coupling between the Fe and S atoms at the interface. For junctions of greater thickness, a maximum magnetoresistance of $\ensuremath{\sim}$300% is obtained, which remains robust with the applied bias as long as transport is in the tunneling limit. A general recipe for improving the magnetoresistance in spin valves incorporating layered transition metal dichalcogenides is proposed.

We analyze electron transport in multiprobe quantum spin Hall (QSH) bars using the B\"{u}ttiker formalism and draw parallels with their quantum Hall (QH) counterparts. We find that in a QSH bar the measured resistance changes upon introducing side voltage probes, in contrast to the QH case. We also study four- and six-terminal geometries and derive the expressions for the resistances. For these our analysis is generalized from the single-channel to the multi-channel case and to the inclusion of backscattering originating from a constriction placed within the bar., Comment: 6 pages, 5 figures

We investigate Andreev reflection in two-dimensional heterojunctions formed by a superconductor in contact with a topological insulator ribbon either possessing or breaking time-reversal symmetry. Both classes of topological insulators exhibit perfect Andreev reflection, which is robust against disorder. This is assigned to topologically protected edge states. In the time-reversal symmetric case we show that doping one of the ribbon edges with magnetic impurities suppresses one Andreev channel, while no such suppression is seen in the broken symmetry situation. Based on this observation we suggest a tabletop transport experiment able to distinguish between the two types of topological insulators, which does not involve the direct measurement of the material band structure., Comment: Updated text, references added, published version

In this work we study the topologically protected states of the Sb(111) surface by using ab-initio transport theory. In the presence of a strong surface perturbation we obtain standing-wave states resulting from the superposition of spin-polarized surface states. By Fourier analysis, we identify the underlying two dimensional scattering processes and the spin texture. We find evidence of resonant transmission across surface barriers at quantum well states energies and evaluate their lifetimes. Our results are in excellent agreement with experimental findings. We also show that despite the presence of a step edge along a different high symmetry direction, not yet probed experimentally, the surface states exhibit unperturbed transmission around the Fermi energy for states with near to normal incidence., Comment: Updated text, reference added, published version

We demonstrate that a quantum spin Hall current, spontaneously generated at the edge of a two-dimensional topological insulator, acts as a source of spin-pumping for a magnetic impurity with uniaxial anisotropy. One can then manipulate the impurity spin direction by means of an electrical current without using either magnetic electrodes or an external magnetic field. Furthermore we show that the unique properties of the quantum spin Hall topological state have profound effects on the inelastic electron tunneling spectrum of the impurity. For low current densities inelastic spin-flip events do not contribute to the conductance. As a consequence the conductance steps, normally appearing at voltages corresponding to the spin excitations, are completely suppressed. In contrast an intense current leads to spin pumping and generates a transverse component of the impurity spin. This breaks the topological phase yielding to the conductance steps., Comment: Updated text, added figure, published version

Electrostatic doping in materials can lead to various exciting electronic properties, such as metal–insulator transition and superconductivity, by altering the Fermi level position or introducing exotic phases. Cd3As2, a three-dimensional (3D) analog of graphene with extraordinary carrier mobility, was predicted to be a 3D Dirac semimetal, a feature confirmed by recent experiments. However, most research so far has been focused on metallic bulk materials that are known to possess ultra-high mobility and giant magneto-resistance but limited carrier transport tunability. Here we report on the first observation of a gate-induced transition from band conduction to hopping conduction in single-crystalline Cd3As2 thin films via electrostatic doping by solid electrolyte gating. The extreme charge doping enables the unexpected observation of p-type conductivity in a ∼50-nm-thick Cd3As2 thin film grown by molecular beam epitaxy. More importantly, the gate-tunable Shubnikov–de Haas oscillations and the temperature-dependent resistance reveal a unique band structure and bandgap opening when the dimensionality of Cd3As2 is reduced. This is also confirmed by our first-principle calculations. The present results offer new insights toward nanoelectronic and optoelectronic applications of Dirac semimetals in general and provide new routes in the search for the intriguing quantum spin Hall effect in low-dimension Dirac semimetals, an effect that is theoretically predicted but not yet experimentally realized. The tunable quantum transport capabilities of cadmium arsenide thin films may unlock new applications for graphene-like semiconductors. Cadmium arsenide has similar electronic properties to graphene, but is easier to work with thanks to its three-dimensional crystal structure. Faxian Xiu of Fudan University in Shanghai and co-workers have now mapped out this material's band structure in confined 50-nanometre-thin film structures. By using a source-drain layout with an unconventional gate electrode — a droplet of ionic electrolyte that electrostatically dopes cadmium arsenide and changes its Fermi level — they saw remarkable conductivity switching behaviour, which is useful for ambipolar field effect transistors. Applying magnetic fields during device operation also revealed the possibility of generating quantum spin Hall effects — the team observed intriguing quantum oscillation conductivity when the Fermi level was pushed into the high-mobility conduction band. Cd3As2, which is known as a topological Dirac semimetal, has been grown on mica substrates by molecular beam epitaxy with high mobility. The temperature-dependent resistance of as-grown Cd3As2 thin films showed semiconducting behavior, indicating the band gap opening as opposed to the bulk counterpart. By solid electrolyte gating, the ambipolar effect and gate-tunable quantum oscillations were clearly demonstrated. These features make the Cd3As2 thin film system a promising platform to observe various exotic phenomena and realize new electronic applications.

We demonstrate single atom anisotropic magnetoresistance on the surface of a topological insulator, arising from the interplay between the helical spin-momentum-locked surface electronic structure and the hybridization of the magnetic adatom states. Our first-principles quantum transport calculations based on density functional theory for Mn on Bi$_2$Se$_3$ elucidate the underlying mechanism. We complement our findings with a two dimensional model valid for both single adatoms and magnetic clusters, which leads to a proposed device setup for experimental realization. Our results provide an explanation for the conflicting scattering experiments on magnetic adatoms on topological insulator surfaces, and reveal the real space spin texture around the magnetic impurity., Comment: Updated text, published version

We propose a four-terminal device designed to manipulate by all electrical means the spin of a magnetic adatom positioned at the edge of a quantum spin Hall insulator. We show that an electrical gate, able to tune the interface resistance between a quantum spin Hall insulator and the source and drain electrodes, can switch the device between two regimes: one where the system exhibits spin pumping and the other where the adatom remains in its ground state. This demonstrates an all-electrical route to control single spins by exploiting helical edge states of topological materials., 4 pages, 3 figures