Topological insulators with surface states are expected to possess perfect spin polarization. Despite the strong potential for spintronic devices, the exact value and gate tuning of spin polarization in topological insulators have not yet been clearly demonstrated. In this research, Ca is doped into the well‐established topological material Bi 2 Se 3 to enhance the spin‐orbit interaction and gate tunability. From the anisotropic magnetoresistance of the Ca‐doped Bi 2 Se 3 channel, an effective magnetic field of ≈10 T is extracted. Also, the carrier types, i.e., p ‐ and n ‐channels, as well as the spin polarization are modulated by applying an external gate voltage. This work not only suggests a measurement platform to quantitatively estimate the spin characteristics of a topological insulator but also opens a path to realize gate‐controlled pure spin current.

We have performed electron transport and angle-resolved photo-emission spectroscopy (ARPES) measurements on single crystals of transition metal dipnictide TaAs2cleaved along the (2¯01) surface which has the lowest cleavage energy. A Fourier transform of the Shubnikov-de Haas oscillations shows four different peaks whose angular dependence was studied with respect to the angle between magnetic field and the [2¯01] direction. The results indicate elliptical shape of the Fermi surface cross-sections. Additionally, a mobility spectrum analysis was carried out, which also reveals at least four types of carriers contributing to the conductance (two kinds of electrons and two kinds of holes). ARPES spectra were taken on freshly cleaved (2¯01) surface and it was found that bulk states pockets at constant energy surface are elliptical, which confirms the magnetotransport angle dependent studies. First-principles calculations support the interpretation of the experimental results. The theoretical calculations better reproduce the ARPES data if the theoretical Fermi level (FL) is increased, which is due to a small n-doping of the samples. This shifts the FL closer to the Dirac point, allowing investigating the physics of the Dirac and Weyl points, making this compound a platform for the investigation of the Dirac and Weyl points in three-dimensional materials.

Studies of the Fermi surface modification after in-situ covering NbP semimetal with heavy elements Pb and Nb ultrathin layers were performed by means of angle-resolved photoemission spectroscopy (ARPES). First, the electronic structure was investigated for pristine single crystals with two possible terminations (P and Nb) of the (0 0 1) surface. The nature of the electronic states of these two cleaving planes is different: the P-terminated surface shows spoon and bow tie-shaped surface states, whereas these shapes are not present in the Nb-terminated surface. ARPES studies show that even 1 monolayer (ML) of Pb causes topological Lifshitz transition (TLT) in P-terminated NbP where the surface Fermi arcs (SFAs) teleport to another pair of Weyl points connecting two adjacent Brillouin zones. Depositing 1 ML of Pb modifies the Fermi surface along with a shift in the Fermi energy. On the other hand, the deposition of approximately 0.8 ML of Nb modifies the electronic structure of P-terminated NbP, pushing the system on the verge of TLT but not yet fully transformed. Regardless of the dramatic surface evolution, SFAs remain connected to topologically protected Weyl points (WPs). Additionally, we studied the Nb-terminated NbP covered with 1.9 ML of Pb with only altered trivial surface states caused by an ordinary Lifshitz transition.

Using time-resolved multi-dimensional angle-resolved photoelectron spectroscopy (ARPES) we explore the angular momentum transfer of low energy polarized photons to two prototype topological insulators, Bi2Te2Se and Bi2Se3. Our comparative study is based on the analysis of circular dichroism in the photoemission yield of photoexcited Dirac states, and reveals that the spin vector of in-gap Dirac electrons in Bi2Te2Se presents a more pronounced out-of-plane component compared to that of Bi2Se3. We show that the multi-dimensional ARPES approach can be effectively used to observe the spin texture of photoexcited topological insulators, and to unambiguously disentangle experimental geometry and matrix element effects.

The possibility of inducing superconductivity in type-I Weyl semimetal through coupling its surface to a superconductor was investigated. A single crystal of NbP, grown by chemical vapor transport method, was carefully characterized by XRD, EDX, SEM, ARPES techniques and by electron transport measurements. The mobility spectrum of the carriers was determined. For the studies of interface transmission, the (001) surface of the crystal was covered by several hundred nm thick metallic layers of either Pb, or Nb, or In. DC current-voltage characteristics and AC differential conductance through the interfaces as a function of the DC bias were investigated. When the metals become superconducting, all three types of junctions show conductance increase, pointing out the Andreev reflection as a prevalent contribution to the subgap conductance. In the case of Pb-NbP and Nb-NbP junctions, the effect is satisfactorily described by modified Blonder-Tinkham-Klapwijk model. The absolute value of the conductance is much smaller than that for the bulk crystal, indicating that the transmission occurs through only a small part of the contact area. An opposite situation occurs in In-NbP junction, where the conductance at the peak reaches the bulk value indicating that almost whole contact area is transmitting and, additionally, a superconducting proximity phase is formed in the material. We interpret this as a result of indium diffusion into NbP, where the metal atoms penetrate the surface barrier and form very transparent superconductor-Weyl semimetal contact inside. However, further diffusion occurring already at room temperature leads to degradation of the effect, so it is observed only in the pristine structures. Despite of this, our observation directly demonstrates possibility of inducing superconductivity in a type-I Weyl semimetal.

Surface electronic properties of Bi2Se3 and Bi2Se3 topological insulators are known to evolve with varying surface termination. In this work, the (111) surface of Bi2Se3 has been studied with a comprehensive combination of experimental and computational (density functional theory) methods. It has been demonstrated that with proper preparation conditions the system can be forced into a new stable surface termination - a sub-monolayer of bismuth acquired through selective Se desorption - which has not been explored yet.

One of the most important challenges in the study of topological insulators is the realization of materials that are really insulating in the bulk, in order to emphasize quantum transport in the protected surface states. Irradiation with electron beams is a very promising approach toward this goal. By studying a series of samples of the prototype 3D topological insulator Bi2Te3, we show that while the topological properties of Dirac surface states are preserved after electron irradiation, their relaxation dynamics are very sensitive to the related modifications of the bulk properties. Using time- and angle-resolved photoelectron spectroscopy, we can reveal two distinct relaxation regimes after optical excitation for non-irradiated and irradiated samples. While the faster regime, corresponding to the first few picoseconds, presents a similar temporal evolution of the photoexcited population for all studied samples, the slower regime is strongly influenced by the controlled generation of defects in the bulk lattice. By adjusting the irradiation parameters in this class of materials, one can thus not only change the bulk transport properties but also tune the ultrafast response of the topological surface states.

A topological insulator gives a great concern in the field of thin film devices because it delivers a conducting state and a high spin-momentum locking at the material surface. To investigate the interfacial coupling between ferromagnet and topological insulator, the anisotropic magnetoresistance (AMR) in a Ni81Fe19/SiO2/Ca-doped Bi2Se3 structure is observed. The AMR is determined by the alignment between the magnetization direction of a ferromagnetic electrode and the bias current direction. The bias current induces a strong spin-momentum locking along the transverse direction which changes the magnetic anisotropy and switching process of the ferromagnetic layer. Furthermore, the angle dependence of magnetoresistance clearly shows that the amplitude of AMR is enhanced due to the coupling of Ni81Fe19/SiO2/Ca-doped Bi2Se3 hybrid structure. These results provide an efficient technique for manipulating magnetization reversal of the ferromagnetic material in spin-based devices.

Using femtosecond time-and angle-resolved photoemission spectroscopy, we explore the out-of-equilibrium dynamics of surface fermions in the topological system Bi2Te2Se. We show that the presence of localized states from defects at the surface is one of the key material parameters undergirding the long relaxation time of photoexcited Dirac electrons lying in the projected band gap of the bulk-insulating pristine compound. Doping this ternary compound with Sn substituting on Bi sites, while desirably increasing the resistivity at low temperatures decreases decay times of the excited homologous Dirac electrons. On the basis of these observations, we argue that long relaxation times can be ultimately controlled by a charge transfer to the surface.

Electronic and structural properties of Bi2Se3 and its extension to copper doped Bi2Se3:Cu were studied using combined ab initio simulations and transmission electron microscopy based techniques, including electron energy loss spectroscopy, energy filtered transmission electron microscopy, and energy dispersive x-ray spectroscopy. The stability of the mixed phases was investigated for substitutional and intercalation changes of basic Bi2Se3 structure. Four systems were compared: Bi-2 Se-3, structures obtaining by Cu intercalation of the van der Waals gap, by substitution of Bi by Cu in quintuple layers, and Cu2Se. The structures were identified and their electronic properties were obtained. Transmission electron microscopy measurements of Bi2Se3 and the Bi2Se3:Cu system identified the first structure as uniform and the second as composite, consisting of a nonuniform lower-Cu-content matrix and randomly distributed high-Cu-concentration precipitates. Critical comparison of the ab initio and experimental data identified the matrix as having a Bi2Se3 dominant part with randomly distributed Cu-intercalated regions having 1Cu-Bi2Se3 structure. The precipitates were determined to have 3Cu-Bi2Se3 structure.

Despite intensive investigations of Bi2Se3 in past few years, the size and nature of the bulk energy band gap of this well-known 3D topological insulator still remain unclear. Here we report on a combined magneto-transport, photoluminescence and infrared transmission study of Bi2Se3, which unambiguously shows that the energy band gap of this material is direct and reaches E g = (220 ± 5) meV at low temperatures.

Passively Q-switched fiber lasers based on low-dimensional saturable absorbers have been extensively developed due to their efficiency, robustness and great simplicity [1]. However, an environmentally stable, industrial class lasers are still lacking. Here, we present a new material for the application as a broadband saturable absorber for large-energy Q-switched ytterbium- and erbium-doped fiber lasers. Bi 2 Te 2 Se:Sn is a small band gap semiconductor and ternary topological insulator [2]. It is characterized by broadband saturable absorption [3].

Due to their broadband nonlinear optical properties, low-dimensional materials are widely used for pulse generation in fiber and solid-state lasers. Here we demonstrate novel materials, Bi2Te2Se (BTS) and Sn-doped Bi2Te2Se (BSTS), which can be used as a universal saturable absorbers for distinct spectral regimes. The material was mechanically exfoliated from a bulk single-crystal and deposited onto a side-polished fiber. We have performed characterization of the fabricated devices and employed them in polarization-maintaining ytterbium- and erbium-doped fiber lasers. This enabled us to obtain self-starting passively Q-switched regime at 1 µm and 1.56 µm. The oscillators emitted stable, linearly polarized radiation with the highest single pulse energy approaching 692 nJ. Both lasers are characterized by the best performance observed in all-polarization maintaining Q-switched fiber lasers with recently investigated new saturable absorbers, which was enabled by a very high damage threshold of the devices. This demonstrates the great potential of the investigated materials for the ultrafast photonics community.

Despite intensive investigations of Bi 2 Se 3 in past few years, the size and nature of the bulk energy band gap of this well-known 3D topological insulator still remain unclear. Here we report on a combined magneto-transport, photoluminescence and infrared transmission study of Bi 2 Se 3 , which unambiguously shows that the energy band gap of this material is direct and reaches E g = (220 ± 5) meV at low temperatures.

Bismuth selenide – the prominent topological insulator – has been recently reported as a material exhibiting an extraordinary strong Faraday rotation. This effect has been identified as due to interband excitations in bulk promoting electrons from the valence to the conduction band, in which the electron gas is partially spin-polarized due to the Zeeman effect. In this work, we test the Faraday rotation in Bi2 Se3 in high magnetic fields, when the conduction band electrons reach their full spin polarization. We find that the Faraday angle becomes in this regime almost independent of the applied magnetic field. This contrasts with the Faraday effect observed in this system at low magnetic fields, where the Faraday angle scales linearly with B and may thus be described by a conventional Verdet law.

Topological insulators are a promising class of materials for applications in the field of spintronics. New perspectives in this field can arise from interfacing metal-organic molecules with the topological insulator spin-momentum locked surface states, which can be perturbed enhancing or suppressing spintronics-relevant properties such as spin coherence. Here we show results from an angle-resolved photemission spectroscopy (ARPES) and scanning tunnelling microscopy (STM) study of the prototypical cobalt phthalocyanine (CoPc)/Bi2Se3 interface. We demonstrate that that the hybrid interface can act on the topological protection of the surface and bury the Dirac cone below the first quintuple layer.

The Faraday effect is a representative magneto-optical phenomenon, resulting from the transfer of angular momentum between interacting light and matter in which time-reversal symmetry has been broken by an externally applied magnetic field. Here we report on the Faraday rotation induced in the prominent 3D topological insulator Bi 2 Se 3 due to bulk interband excitations. The origin of this non-resonant effect, extraordinarily strong among other non-magnetic materials, is traced back to the specific Dirac-type Hamiltonian for Bi 2 Se 3 , which implies that electrons and holes in this material closely resemble relativistic particles with a non-zero rest mass.