arXiv subjects

A. Tsukazaki

Publications and source records attributed to A. Tsukazaki.

At least 37 records · Page 2Linked to original sources

arXiv2016

Current-nonlinear Hall effect and spin-orbit torque magnetization switching in a magnetic topological insulator

Precise estimation of spin Hall angle as well as successful maximization of spin-orbit torque (SOT) form a basis of electronic control of magnetic properties with spintronic functionality. Until now, current-nonlinear Hall effect, or second harmonic Hall voltage has been utilized as one of the methods for estimating spin Hall angle, which is attributed to the magnetization oscillation by SOT. Here, we argue the second harmonic Hall voltage in magnetic/nonmagnetic topological insulator (TI) heterostructures, Cr$_x$(Bi$_{1-y}$Sb$_y$)$_{2-x}$Te$_3$/(Bi$_{1-y}$Sb$_y$)$_2$Te$_3$. From the angular, temperature and magnetic field dependence, it is unambiguously shown that the large second harmonic Hall voltage in TI heterostructures is governed not by SOT but mainly by asymmetric magnon scattering mechanism without magnetization oscillation. Thus, this method does not allow an accurate estimation of spin Hall angle when magnons largely contribute to electron scattering. Instead, the SOT contribution in a TI heterostructure is exemplified by current pulse induced non-volatile magnetization switching, which is realized with a current density of $\sim 2.5 \times 10^{10} \mathrm{A/m}^2$, showing its potential as spintronic materials.

cond-mat.mtrl-sci↗

arXiv2016

Large Unidirectional Magnetoresistance in a Magnetic Topological Insulator

We report current-direction dependent or unidirectional magnetoresistance (UMR) in magnetic/nonmagnetic topological insulator (TI) heterostructures, Cr$_x$(Bi$_{1-y}$Sb$_y$)$_{2-x}$Te$_3$/(Bi$_{1-y}$Sb$_y$)$_2$Te$_3$, that is several orders of magnitude larger than in other reported systems. From the magnetic field and temperature dependence, the UMR is identified to originate from the asymmetric scattering of electrons by magnons. In particular, the large magnitude of UMR is an outcome of spin-momentum locking and a small Fermi wavenumber at the surface of TI. In fact, the UMR is maximized around the Dirac point with the minimal Fermi wavenumber.

cond-mat.mtrl-sci↗

arXiv2016

Observation of topological Faraday and Kerr rotations in quantum anomalous Hall state by terahertz magneto-optics

Electrodynamic responses from three-dimensional (3D) topological insulators (TIs) are characterized by the universal magnetoelectric $E\cdot B$ term constituent of the Lagrangian formalism. The quantized magnetoelectric coupling, which is generally referred to as topological magnetoelectric (TME) effect, has been predicted to induce exotic phenomena including the universal low-energy magneto-optical effects. Here we report the experimental demonstration of the long-sought TME effect, which is exemplified by magneto-optical Faraday and Kerr rotations in the quantum anomalous Hall (QAH) states of magnetic TI surfaces by terahertz magneto-optics. The universal relation composed of the observed Faraday and Kerr rotation angles but not of any material parameters (e.g. dielectric constant and magnetic susceptibility) well exhibits the trajectory toward the fine structure constant $α$ $(= 2πe^2/hc \sim 1/137)$ in the quantized limit. Our result will pave a way for versatile TME effects with emergent topological functions.

cond-mat.mes-hall↗

arXiv2016

All-in-all-out magnetic domain wall conduction in pyrochlore iridate heterointerface

Pyrochlore oxides possessing "all-in-all-out" spin ordering have attracted burgeoning interest as a rich ground of emergent states. This ordering has two distinct types of magnetic domains (all-in-all-out or all-out-all-in) with broken time-reversal symmetry, and a non-trivial metallic surface state has been theoretically demonstrated to appear at their domain wall. Here, we report on observation of this metallic conduction at the single all-in-all-out/all-out-all-in magnetic domain wall formed at the heterointerface of two pyrochlore iridates. By utilizing different magnetoresponses of them with different lanthanide ions, the domain wall is controllably inserted at the heterointerface, the surface state being detected as anomalous conduction enhancement with a ferroic hysteresis. Our establishment paves the way for further investigation and manipulation of this new type of surface transport.

cond-mat.mtrl-sci↗

arXiv2016

Enhanced photogalvanic current in topological insulators via Fermi energy tuning

We achieve the enhancement of circular photogalvanic effect arising from the photo-injection of spins in topological insulator thin films by tuning the Fermi level ($E_{\rm F}$). A series of (Bi$_{1-x}$Sb$_x$)$_2$Te$_3$ thin films were tailored so that the Fermi energy ranges above 0.34 eV to below 0.29 eV of the Dirac point, i.e., from the bulk conduction band bottom to the valence band top through the bulk in-gap surface-Dirac cone. The circular photogalvanic current, indicating a flow of spin-polarized surface-Dirac electrons, shows a pronounced peak when the $E_{\rm F}$ is set near the Dirac point and is also correlated with the carrier mobility. Our observation reveals that there are substantial scatterings between the surface-Dirac and bulkstate electrons in the generation process of spin-polarized photocurrent, which can be avoided by designing the electronic structure in topological insulators.

cond-mat.mtrl-sci↗

arXiv2016

All-in-all-out magnetic domain size in pyrochlore iridate thin films as probed by local magnetotransport

Pyrochlore iridates have attracted growing attention because of a theoretical prediction of a possible topological semimetal phase originating from all-in-all-out spin ordering. Related to the topological band structure, recent findings of the magnetic domain wall conduction have stimulated investigations of magnetic domain distribution in this system. Here, we investigate the size of magnetic domains in Eu$_2$Ir$_2$O$_7$ single crystalline thin films by magnetoresistance (MR) using microscale Hall bars. Two distinct magnetic domains of the all-in-all-out spin structure are known to exhibit linear MR but with opposite signs, which enables us to estimate the ratio of the two domains in the patterned channel. The linear MR for 80 ${\times}$ 60 $μ$m$^2$ channel is nearly zero after zero-field cooling, suggesting random distribution of domains smaller than the channel size. In contrast, the wide distribution of the value of the linear MR is detected in 2 ${\times}$ 2 $μ$m$^2$ channel, reflecting the detectable domain size depending on each cooling-cycle. Compared to simulation results, we estimate the average size of a single all-in-all-out magnetic domain as 1-2 $μ$m.

cond-mat.mtrl-sci↗

arXiv2015

Magnetic modulation doping in topological insulators toward higher-temperature quantum anomalous Hall effect

Quantum anomalous Hall effect (QAHE), which generates dissipation-less edge current without external magnetic field, is observed in magnetic-ion doped topological insulators (TIs), such as Cr- and V-doped (Bi,Sb)2Te3. The QAHE emerges when the Fermi level is inside the magnetically induced gap around the original Dirac point of the TI surface state. Although the size of gap is reported to be about 50 meV, the observable temperature of QAHE has been limited below 300 mK. We attempt magnetic-Cr modulation doping into topological insulator (Bi,Sb)2Te3 films to increase the observable temperature of QAHE. By introducing the rich-Cr-doped thin (1 nm) layers at the vicinity of the both surfaces based on non-Cr-doped (Bi,Sb)2Te3 films, we have succeeded in observing the QAHE up to 2 K. The improvement in the observable temperature achieved by this modulation-doping appears to be originating from the suppression of the disorder in the surface state interacting with the rich magnetic moments. Such a superlattice designing of the stabilized QAHE may pave a way to dissipation-less electronics based on the highertemperature and zero magnetic-field quantum conduction.

cond-mat.mes-hall↗

arXiv2015

Fermi level dependent charge-to-spin current conversion by Dirac surface state of topological insulators

The spin-momentum locking at the Dirac surface state of a topological insulator (TI) offers a distinct possibility of a highly efficient charge-to-spin current (C-S) conversion compared with spin Hall effects in conventional paramagnetic metals. For the development of TI-based spin current devices, it is essential to evaluate its conversion efficiency quantitatively as a function of the Fermi level EF position. Here we exemplify a coefficient of qICS to characterize the interface C-S conversion effect by using spin torque ferromagnetic resonance (ST-FMR) for (Bi1-xSbx)2Te3 thin films whose EF is tuned across the band gap. In bulk insulating conditions, interface C-S conversion effect via Dirac surface state is evaluated as nearly constant large values of qICS, reflecting that the qICS is inversely proportional to the Fermi velocity vF that is almost constant. However, when EF traverses through the Dirac point, the qICS is remarkably suppressed possibly due to the degeneracy of surface spins or instability of helical spin structure. These results demonstrate that the fine tuning of the EF in TI based heterostructures is critical to maximizing the efficiency using the spin-momentum locking mechanism.

cond-mat.mtrl-sci↗

arXiv2015

Electric-field-induced superconductivity in electrochemically-etched ultrathin FeSe films on SrTiO3 and MgO

Among the recently discovered iron-based superconductors, ultrathin films of FeSe grown on SrTiO3 substrates have uniquely evolved into a high superconducting-transition-temperature (TC) material. The mechanisms for the high-TC superconductivity are ongoing debate mainly with the superconducting gap characterized with in-situ analysis for FeSe films grown by bottom-up molecular-beam epitaxy. Here, we demonstrate the alternative access to investigate the high-TC superconductivity in ultrathin FeSe with top-down electrochemical etching technique in three-terminal transistor configuration. In addition to the high-TC FeSe on SrTiO3, the electrochemically etched ultrathin FeSe transistor on MgO also exhibits superconductivity around 40 K, implying that the application of electric-field effectively contributes to the high-TC superconductivity in ultrathin FeSe regardless of substrate material. Moreover, the observable critical thickness for the high-TC superconductivity is expanded up to 10-unit-cells under applying electric-field and the insulator-superconductor transition is electrostatically controlled. The present demonstration implies that the electric-field effect on both conduction and valence bands plays a crucial role for inducing high-TC superconductivity in FeSe.

cond-mat.supr-con↗

arXiv2015

Odd-parity magnetoresistance in pyrochlore iridate thin films with broken time-reversal symmetry

A new class of materials termed topological insulators have been intensively investigated due to their unique Dirac surface state carrying dissipationless edge spin currents. Recently, it has been theoretically proposed that the three dimensional analogue of this type of band structure, the Weyl Semimetal phase, is materialized in pyrochlore oxides with strong spin-orbit coupling, accompanied by all-in-all-out spin ordering. Here, we report on the fabrication and magnetotransport of Eu2Ir2O7 single crystalline thin films. We reveal that one of the two degenerate all-in-all-out domain structures, which are connected by time-reversal operation, can be selectively formed by the polarity of the cooling magnetic field. Once formed, the domain is robust against an oppositely polarised magnetic field, as evidenced by an unusual odd field dependent term in the magnetoresistance and an anomalous term in the Hall resistance. Our findings pave the way for exploring the predicted novel quantum transport phenomenon at the surfaces/interfaces or magnetic domain walls of pyrochlore iridates.

cond-mat.mtrl-sci↗

arXiv2015

Quantum Hall States Stabilized in Semi-magnetic Bilayers of Topological Insulators

By breaking the time-reversal-symmetry in three-dimensional topological insulators with introduction of spontaneous magnetization or application of magnetic field, the surface states become gapped, leading to quantum anomalous Hall effect or quantum Hall effect, when the chemical potential locates inside the gap. Further breaking of inversion symmetry is possible by employing magnetic topological insulator heterostructures that host nondegenerate top and bottom surface states. Here, we demonstrate the tailored-material approach for the realization of robust quantum Hall states in the bilayer system, in which the cooperative or cancelling combination of the anomalous and ordinary Hall responses from the respective magnetic and non-magnetic layers is exemplified. The appearance of quantum Hall states at filling factor 0 and +1 can be understood by the relationship of energy band diagrams for the two independent surface states. The designable heterostructures of magnetic topological insulator may explore a new arena for intriguing topological transport and functionality.

cond-mat.mes-hall↗

arXiv2015

Magnetic field-induced insulator-semimetal transition in a pyrochlore Nd2Ir2O7

We have investigated magneto-transport properties in a single crystal of pyrochore-type Nd2Ir2O7. The metallic conduction is observed on the antiferromagnetic domain walls of the all-in all-out type Ir-5d moment ordered insulating bulk state, that can be finely controlled by external magnetic field along [111]. On the other hand, an applied field along [001] induces the bulk phase transition from insulator to semimetal as a consequence of the field-induced modification of Nd-4f and Ir-5d moment configurations. A theoretical calculation consistently describing the experimentally observed features suggests a variety of exotic topological states as functions of electron correlation and Ir-5d moment orders which can be finely tuned by choice of rare-earth ion and by magnetic field, respectively.

cond-mat.mtrl-sci↗

arXiv2015

Formation of In-plane Skyrmions in Epitaxial MnSi Thin Films as Revealed by Planar Hall Effect

We investigate skyrmion formation in both a single crystalline bulk and epitaxial thin films of MnSi by measurements of planar Hall effect. A prominent stepwise field profile of planar Hall effect is observed in the well-established skyrmion phase region in the bulk sample, which is assigned to anisotropic magnetoresistance effect with respect to the magnetic modulation direction. We also detect the characteristic planar Hall anomalies in the thin films under the in-plane magnetic field at low temperatures, which indicates the formation of skyrmion strings lying in the film plane. Uniaxial magnetic anisotropy plays an important role in stabilizing the in-plane skyrmions in the MnSi thin film.

cond-mat.str-el↗

arXiv2015

Discretized Topological Hall Effect Emerging from Skyrmions in Constricted Geometry

We investigate the skyrmion formation process in nano-structured FeGe Hall-bar devices by measurements of topological Hall effect, which extracts the winding number of a spin texture as an emergent magnetic field. Step-wise profiles of topological Hall resistivity are observed in the course of varying the applied magnetic field, which arise from instantaneous changes in the magnetic nano-structure such as creation, annihilation, and jittering motion of skyrmions. The discrete changes in topological Hall resistivity demonstrate the quantized nature of emergent magnetic flux inherent in each skyrmion, which had been indistinguishable in many-skyrmion systems on a macroscopic scale.

cond-mat.str-el↗

arXiv2014

Polarization dependent Landau level crossing in a two-dimensional electron system in MgZnO/ZnO-heterostructure

We report electrical transport measurements in a tilted magnetic field on a high-mobility two-dimensional electron system confined at the MgZnO/ZnO heterointerface. The observation of multiple crossing events of spin-resolved Landau levels (LLs) enables the mapping of the sequence of electronic states. We further measure the renormalization of electron spin susceptibility at zero field and the susceptibility dependence on the electron spin polarization. The latter manifests the deviation from the Pauli spin susceptibility. As the result, the crossing of spin-resolved LLs shifts to smaller tilt angles and the first Landau level coincidence event is absent even when the magnetic field has only a perpendicular component to the 2DES plane.

cond-mat.mes-hall↗

arXiv2014

Quantum Hall Effect on Top and Bottom Surface States of Topological Insulator (Bi1-xSbx)2Te3 Films

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 ν= \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 ν= 0 state (σxy = 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.

cond-mat.mtrl-sci↗

arXiv2014

Trajectory of Anomalous Hall Effect toward the Quantized State in a Ferromagnetic Topological Insulator

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.

cond-mat.str-el↗

arXiv2013

Challenges and opportunities of ZnO-related single crystalline heterostructures

Recent technological advancement in ZnO heterostructures has expanded the possibility of device functionalities to various kinds of applications. In order to extract novel device functionalities in the heterostructures, one needs to fabricate high quality films and interfaces with minimal impurities, defects, and disorder. With employing molecular-beam epitaxy (MBE) and single crystal ZnO substrates, the density of residual impurities and defects can be drastically reduced and the optical and electrical properties have been dramatically improved for the ZnO films and heterostructures with MgxZn1-xO. Here, we overview such recent technological advancement from various aspects of application. Towards optoelectronic devices such as a light emitter and a photodetector in an ultraviolet region, the development of p-type ZnO and the fabrication of excellent Schottky contact, respectively, have been subjected to intensive studies for years. For the former, the fine tuning of the growth conditions to make MgxZn1-xO as intrinsic as possible has opened the possibilities of making p-type MgxZn1-xO through NH3 doping method. For the latter, conducting and transparent polymer films spin-coated on MgxZn1-xO was shown to give almost ideal Schottky junctions. The wavelength-selective detection can be realized with varying the Mg content. From the viewpoint of electronic devices, two-dimensional electrons confined at the MgxZn1-xO/ZnO interfaces are promising candidate for quantum devices because of high electron mobility and strong electron-electron correlation effect. These wonderful features and tremendous opportunities in ZnO-based heterostructures make this system unique and promising in oxide electronics and will lead to new quantum functionalities in optoelectronic devices and electronic applications with lower energy consumption and high performance.

cond-mat.mtrl-sci↗