[Show abstract][Hide abstract] ABSTRACT: The three-dimensional (3D) topological insulator (TI) is a novel state of
matter as characterized by two-dimensional (2D) metallic Dirac states on its
surface. Bi-based chalcogenides such as Bi2Se3, Bi2Te3, Sb2Te3 and their
combined/mixed compounds like Bi2Se2Te and (Bi1-xSbx)2Te3 are typical members
of 3D-TIs which have been intensively studied in forms of bulk single crystals
and thin films to verify the topological nature of the surface states. Here, we
report the realization of the Quantum Hall effect (QHE) on the surface Dirac
states in (Bi1-xSbx)2Te3 films (x = 0.84 and 0.88). With electrostatic
gate-tuning of the Fermi level in the bulk band gap under magnetic fields, the
quantum Hall states with filling factor \nu = \pm 1 are resolved with quantized
Hall resistance of Ryx = h/e2 and zero longitudinal resistance, owing to chiral
edge modes at top/bottom surface Dirac states. Furthermore, the appearance of a
\nu = 0 state (\sigma xy = 0) reflects a pseudo-spin Hall insulator state when
the Fermi level is tuned in between the energy levels of the non-degenerate top
and bottom surface Dirac points. The observation of the QHE in 3D TI films may
pave a way toward TI-based electronics.
[Show abstract][Hide abstract] ABSTRACT: Strongly correlated electron systems often exhibit very strong interactions between structural and electronic degrees of freedom that lead to complex and interesting phase diagrams. For technological applications of these materials it is important to learn how to drive transitions from one phase to another. A key question here is the ultimate speed of such phase transitions, and to understand how a phase transition evolves in the time domain. Here we apply time-resolved X-ray diffraction to directly measure the changes in long-range order during ultrafast melting of the charge and orbitally ordered phase in a perovskite manganite. We find that although the actual change in crystal symmetry associated with this transition occurs over different timescales characteristic of the many electronic and vibrational coordinates of the system, the dynamics of the phase transformation can be well described using a single time-dependent 'order parameter' that depends exclusively on the electronic excitation.
[Show abstract][Hide abstract] ABSTRACT: We studied the charge-orbital ordering in the superlattice of charge-ordered insulating Pr0.5Ca0.5MnO3 and ferromagnetic metallic La0.5Sr0.5MnO3 by resonant soft x-ray diffraction (RSXD) and hard x-ray photoemission spectroscopy (HXPES). A temperature-dependent incommensurability is found in the orbital ordering by RSXD. In addition, a large hysteresis is observed that is caused by phase competition between the insulating charge ordered and metallic ferromagnetic states. No magnetic phase transitions are observed in contrast to pure Pr0.5Ca0.5MnO3 thin films, confirming the unique character of the superlattice. Mn HXPES spectra revealed a hysteresis in the metalicity, supporting the picture of phase competition. The deviation from the commensurate orbital order can be directly related to the decrease of ordered-layer thickness that leads to dimensional crossover from three-dimensional to two-dimensional orbital ordering.
New Journal of Physics 07/2014; 16(7):073044. · 4.06 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Topological insulators are bulk electronic insulators which possess symmetry
protected gapless modes on their surfaces. Breaking the symmetries that
underlie the gapless nature of the surface modes is predicted to give rise to
exotic new states of matter. In particular, it has recently been predicted and
shown that breaking of time reversal symmetry in the form of ferromagnetism can
give rise to a gapped state characterized by a zero magnetic field quantized
Hall response and dissipationless longitudinal transport known as the Quantum
Anomalous Hall (QAH) state. A key question that has thus far remained
experimentally unexplored is the relationship of this new type of quantum Hall
state with the previously known orbitally driven quantum Hall states. Here, we
show experimentally that a ferromagnetic topological insulator exhibiting the
QAH state is well described by the global phase diagram of the quantum Hall
effect. By mapping the behavior of the conductivity tensor in the parameter
space of temperature, magnetic field, and chemical potential in the vicinity of
the QAH phase, we find evidence for quantum criticality and delocalization
behavior that can quantitatively be described by the renormalization group
properties of the quantum Hall ground state. This result demonstrates that the
QAH state observed in ferromagnetic topological insulators can be understood
within the context of the law of corresponding states which governs the quantum
Hall state. This suggests a roadmap for studying the QAH effect including
transitions to possible adjacent topologically non-trivial states and a
possible universality class for the QAH transition.
[Show abstract][Hide abstract] ABSTRACT: The low-energy electron dynamics of the quasi-two-dimensional electron-hole multiorbital system Sr2RuO4 is investigated by time-domain terahertz spectroscopy for a thin film sample. The direct observation of the real and imaginary part of the optical conductivity below 8 meV clearly demonstrates a narrowing of the coherent quasiparticle response with decreasing temperature. The optical conductivity spectra, dc conductivity, and Hall coefficient can be quantitatively well described by the multiband Drude-Lorentz model, from which band dependent scattering rates can be extracted. The resulting modest band dependent scattering rates, which are lower than 2 meV at 4 K, support a rather isotropic relaxation regime.
[Show abstract][Hide abstract] ABSTRACT: We report the study of magnetic and orbital order in epitaxial thin films grown on –. Resonant soft x-ray scattering revealed significant modifications of the magnetic order in the film as compared to the bulk. Namely (i) a different magnetic ordering wave vector, (ii) different spin directions and (iii) an additional magnetic reordering transition. We demonstrate that an analysis of the resonant scattering which is based solely on local symmetries and which does not involve a modeling of energy-dependent lineshapes allows to extract this detailed microscopic information. This approach significantly simplifies the analysis and interpretation of resonant scattering data.
New Journal of Physics 03/2014; 16(3):033006. · 4.06 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We report on the sign inversion of the anomalous Hall effect (AHE) in EuO thin films along with photoirradiation as well as a temperature scan across ̃25 K that is well below the Curie temperature (TC̃80 K). The former gives an enhancement of the mobile electron density (n) by more than 30%, but the latter gives a negligible modification of n of only 3% with a significant enhancement in mobility. It is found, in addition to the universal scaling law between longitudinal conductivity (σxx) and anomalous Hall conductivity (σAH) as |σAH|∝σxx1.6, that there is a critical value of about 102 S cm-1 in σxx that gives a boundary in the sign inversion of σAH. If n solely governs the sign of σAH, the phenomenon could be explained by a Fermi level shift across the singularity in the band structure. However, our band calculation shows that, within any realistic adjustment of band parameters, the sign inversion of AHE never occurs. Thus, we conclude that other mechanisms of AHE are necessary to account for the AHE of EuO.
[Show abstract][Hide abstract] ABSTRACT: Topological insulators are a class of semiconductor exhibiting charge-gapped insulating behaviour in the bulk, but hosting a spin-polarized massless Dirac electron state at the surface. The presence of a topologically protected helical edge channel has been verified for the vacuum-facing surface of several topological insulators by means of angle-resolved photoemission spectroscopy and scanning tunnelling microscopy. By performing tunnelling spectroscopy on heterojunction devices composed of p-type topological insulator (Bi1-xSbx)2Te3 and n-type conventional semiconductor InP, we report the observation of such states at the solid-state interface. Under an applied magnetic field, we observe a resonance in the tunnelling conductance through the heterojunction due to the formation of Landau levels of two-dimensional Dirac electrons at the interface. Moreover, resonant tunnelling spectroscopy reveals a systematic dependence of the Fermi velocity and Dirac point energy on the composition x. The successful formation of robust non-trivial edge channels at a solid-state interface is an essential step towards functional junctions based on topological insulators.
Nature Material 02/2014; 13(3):253-257. · 35.75 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The magnetic skyrmion, i.e., the nanometric swirling spin vortexlike object with the topolgical charge, is broadly observed in chiral-lattice cubic magnets, typically MnSi; where the cylindrical-shape skyrmions form the two-dimensional hexagonal crystal, in a very narrow temperature-magentic field window for bulk crystals but in a much wider one for tens of nanometer thin films under the perpendicular magnetic field applied. We have investigated the stability of two-dimensional skyrmion states emerging in epitaxial thin films of Mn1-xFexSi with various thicknesses t and iron contents x (t =10, 15, and 20 nm; x =0, 0.02, and 0.04) by changing the magnetic-field direction. Topological Hall effect arising from the skyrmions is critically suppressed in the course of tilting the applied magnetic field from the normal vector, indicating the collapse of the skyrmion state. Utilizing this observation, the stable region of the skyrmions, which depends on the film thickness relative to the helimagnetic period, can be mapped out in the temperature-magnetic field plane.
[Show abstract][Hide abstract] ABSTRACT: Torque magnetometry at low temperature and in high magnetic fields B is performed on MgZnO/ZnO heterostructures incorporating high-mobility two-dimensional electron systems. We find a sawtoothlike quantum oscillatory magnetization M (B), i.e., the de Haas-van Alphen (dHvA) effect. At the same time, nonequilibrium currents and unexpected spikelike overshoots in M are observed which allow us to identify the microscopic nature and density of the residual disorder. The acceptorlike scatterers give rise to a magnetic thaw down effect which enhances the dHvA amplitude beyond the electron-electron interaction effects being present in the MgZnO/ZnO heterostructures.
[Show abstract][Hide abstract] ABSTRACT: We have investigated the charge transport and low-energy dynamics originating from the antiferromagnetic (AF) domain wall (DW) in pyrochlore-type Nd2Ir2O7 with the all-in all-out type AF ground state by means of the magneto-resistance measurement and the terahertz time-domain spectroscopy. The resistivity is much lower in the multi-magnetic-domain state than in the nearly single-magnetic-domain one. Furthermore, the terahertz optical conductivity spectrum unravels a Drude-like response only in the multidomain state; the Drude-like absorption is strongly suppressed with increasing field and irreversibly disappears above 5 T. These observations suggest that the AF DW is highly conductive with a minimal damping constant of 2 meV, contrary to the fully-gapped (45 meV) AF insulating state of bulk. We discuss the nature of metallic states at AF DWs and their possible relevance to the gapless edge state inherent to the Weyl semimetal.
[Show abstract][Hide abstract] ABSTRACT: Gate control of percolative conduction in a phase-separated manganite system is demonstrated in a field-effect transistor geometry, resulting in ambipolar switching from a metallic state to an insulating state.
[Show abstract][Hide abstract] ABSTRACT: The dynamics of photogenerated electrons and holes in undoped anatase TiO2 were studied by femtosecond absorption spectroscopy from the visible to mid-infrared region (0.1–2.0 eV). The transient absorption spectra exhibited clear metallic responses, which were well reproduced by a simple Drude model. No mid-gap absorptions originating from photocarrier localization were observed. The reduced optical mass of the photocarriers obtained from the Drude-model analysis is comparable to theoretically expected one. These results demonstrate that both photogenerated holes and electrons act as mobile carriers in anatase TiO2. We also discuss scattering and recombination dynamics of photogenerated electrons and holes on the basis of the time dependence of absorption changes.
Journal of Applied Physics 01/2014; 115(5):053514-053514-8. · 2.21 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We examined the impact of electric field on crystal lattice of vanadium dioxide (VO2) in a field-effect transistor geometry by in-situ synchrotron x-ray diffraction measurements. Whereas the c-axis lattice parameter of VO2 decreases through the thermally induced insulator-to-metal phase transition, the gate-induced metallization was found to result in a significant increase of the c-axis length by almost 1% from that of the thermally stabilized insulating state. We also found that this gate-induced gigantic lattice deformation occurs even at the thermally stabilized metallic state, enabling dynamic control of c-axis lattice parameter by more than 1% at room temperature.
[Show abstract][Hide abstract] ABSTRACT: We report the surface conduction of a BaTiO3 thin film using electric double layer transistor (EDLT) structure. A transistor operation was observed at 220 K with an on/off ratio exceeding 105, demonstrating that ionic liquid gating is effective to induce carriers at the surface of ferroelectric materials. Temperature dependence of channel resistance exhibited a metallic behavior down to 150 K. EDLT structure is also fabricated using a commercial BaTiO3 bulk single crystal for comparison, which shows abrupt resistance increase across the orthorhombic to rhombohedral transition temperature at 183 K. This result indicates that the epitaxial strain is effective to maintain low resistance in this material with keeping the single domain structure.
[Show abstract][Hide abstract] ABSTRACT: ZnO and related alloys are an important class of materials to realize transparent electronics because of their characteristic wide band-gap and high mobility, and also because of their practical advantages, such as: available n-type materials and bulk single crystals, low-cost production, and absence of toxicity. Our studies have been conducted for more than a decade and they have enabled surface and interface engineering on an atomic scale, presenting a promising technology for developing electrical devices of various kinds. The quality of the epitaxial films was improved drastically when grown on high-temperature annealed buffer layers prepared on lattice-matched ScAlMgO4 substrates using pulsed-laser deposition. We carefully investigated the growth temperature dependence of surface morphology and electrical properties. Electron mobility was recorded as 440 cm2 V−1 s−1 at room temperature and 5500 cm2 V−1 s−1 at 1 K, leading to observation of the integer quantum Hall-effect (QHE) in abrupt ZnO/Mgx Zn1−x O interfaces. Two-dimensional electron gas (2DEG) was formed spontaneously in the interface because of the polarization mismatch between the layers. The observation of QHE enables us to access the direct determination of the interfacial electronic structure. In addition, the field-effect control of 2DEG has been demonstrated using lattice-matched interfaces as high-mobility channels.
Journal of Physics D Applied Physics 01/2014; 47(3). · 2.53 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Atomic reconstruction at the interface of MgZnO and ZnO in molecular beam epitaxy grown heterostructures is investigated. Using secondary ion mass spectroscopy, we experimentally find that Mg atomic reconstruction depends on the polarity of the interface; it is not observed in n-type interfaces (MgZnO on Zn-polar ZnO) owing to electron accumulation, while in p-type interfaces (ZnO on Zn-polar MgZnO), Mg drastically redistributes into the ZnO layer. Combined with self-consistent calculation of band profiles and carrier distributions, we reveal that the observed Mg reconstruction is not due to thermal diffusion but consequences in order to avoid hole accumulation. This tendency implies inherent significant asymmetry of energy scales of atomic and electronic reconstructions between n-type and p-type interfaces.
[Show abstract][Hide abstract] ABSTRACT: We report two-dimensionality and effective thickness of the superconductivity of a SrTiO3 single-crystal surface induced by electric double-layer gating. The carrier density was tuned from 3 × 1013 to 1.1 × 1014 cm-2 by gating, where superconductivity appears with Tc of around 0.4 K. Typical two-dimensional behavior perfectly described by the Ginzburg-Landau equation was observed in the angular and temperature dependence of the upper critical magnetic field. The effective thickness of the superconducting layer remains nearly invariant, ranging from 11 to 13 nm, with increasing charge carrier density. This invariance contradicts the expected reduction in the thickness of the accumulation layer in a triangular quantum well model. This unexpected invariance of the superconducting layer thickness is probably a unique nature for a two-dimensional electron system in the incipient ferroelectric SrTiO3.
[Show abstract][Hide abstract] ABSTRACT: We report magnetotransport properties for epitaxial thin films of
SrFeO3 and SrFe0.99Co0.01O3
with possible skyrmion-related spin textures. Resonant soft x-ray
diffraction measurements revealed formation of helical spin structures
for both samples (Q//<111>). From magnetotransport measurements we
found several distinct helimagnetic phases with multiple/single Q
vectors. A steep suppression of Hall resistivity is observed above the
critical field to reach the high-field conical state, indicating the
presence of skyrmionlike topological spin textures at lower fields
responsible for the topological Hall effect.