arXiv subjects

T. Sasagawa

Publications and source records attributed to T. Sasagawa.

At least 37 records · Page 2Linked to original sources

arXiv2018

Fermiology and Superconductivity of Topological Surface States in PdTe$_2$

We study the low-energy surface electronic structure of the transition-metal dichalcogenide superconductor PdTe$_2$ by spin- and angle-resolved photoemission, scanning tunneling microscopy, and density-functional theory-based supercell calculations. Comparing PdTe$_2$ with its sister compound PtSe$_2$, we demonstrate how enhanced inter-layer hopping in the Te-based material drives a band inversion within the anti-bonding p-orbital manifold well above the Fermi level. We show how this mediates spin-polarised topological surface states which form rich multi-valley Fermi surfaces with complex spin textures. Scanning tunneling spectroscopy reveals type-II superconductivity at the surface, and moreover shows no evidence for an unconventional component of its superconducting order parameter, despite the presence of topological surface states.

cond-mat.supr-con↗

arXiv2018

Discovery of switchable weak topological insulator state in quasi-one-dimensional bismuth iodide

The major breakthroughs in the understanding of topological materials over the past decade were all triggered by the discovery of the Z$_2$ topological insulator (TI). In three dimensions (3D), the TI is classified as either "strong" or "weak", and experimental confirmations of the strong topological insulator (STI) rapidly followed the theoretical predictions. In contrast, the weak topological insulator has so far eluded experimental verification, since the topological surface states emerge only on particular side surfaces which are typically undetectable in real 3D crystals. Here we provide experimental evidence for the WTI state in a bismuth iodide, $β$-Bi4I4. Significantly, the crystal has naturally cleavable top and side planes both stacked via van-der-Waals forces, which have long been desirable for the experimental realization of the WTI state. As a definitive signature of it, we find quasi-1D Dirac TSS at the side-surface (100) while the top-surface (001) is topologically dark. Furthermore, a crystal transition from the $β$- to $α$-phase drives a topological phase transition from a nontrivial WTI to the trivial insulator around room temperature. This topological phase, viewed as quantum spin Hall (QSH) insulators stacked three-dimensionally, and excellent functionality with on/off switching will lay a foundation for new technology benefiting from highly directional spin-currents with large density protected against backscattering.

cond-mat.mtrl-sci↗

arXiv2017

Full-gap superconductivity in spin-polarized surface states of topological semimetal $β$-PdBi$_2$

A bulk superconductor possessing a topological surface state at the Fermi level is a promising system to realize long-sought topological superconductivity. Although several candidate materials have been proposed, experimental demonstrations concurrently exploring spin textures and superconductivity at the surface have remained elusive. Here we perform spectroscopic-imaging scanning tunnelling microscopy on the centrosymmetric superconductor $β$-PdBi$_2$ that hosts a topological surface state. By combining first-principles electronic-structure calculations and quasiparticle interference experiments, we determine the spin textures at the surface, and show not only the topological surface state but also all other surface bands exhibit spin polarizations parallel to the surface. We find that the superconducting gap fully opens in all the spin-polarized surface states. This behaviour is consistent with a possible spin-triplet order parameter expected for such in-plane spin textures, but the observed superconducting gap amplitude is comparable to that of the bulk, suggesting that the spin-singlet component is predominant in $β$-PdBi$_2$.

cond-mat.supr-con↗

arXiv2017

Universal superconducting precursor in the cuprates

The nature of the superconducting (SC) precursor in the cuprates has been the subject of intense interest, with profound implications for both the normal and the SC states. Different experimental probes have led to vastly disparate conclusions on the temperature range of superconducting fluctuations. The main challenges have been to separate the SC response from complex normal-state behavior, and to distinguish the underlying behavior of the quintessential CuO$_{2}$ layers from compound-specific properties. Here we reveal remarkably simple and universal behavior of the SC precursor using torque magnetometry, a unique thermodynamic probe with extremely high sensitivity to SC diamagnetism. We comprehensively study four distinct cuprate compounds: single-CuO$_{2}$-layer La$_{2-x}$Sr$_{x}$CuO$_{4}$ (LSCO), Bi$_{2}$(Sr,La)$_{2}$CuO$_{6+δ}$ (Bi2201) and HgBa$_{2}$CuO$_{4+δ}$ (Hg1201), and double-layer Bi$_{2}$Sr$_{2}$Ca$_{0.95}$Y$_{0.05}$CuO$_{8+δ}$ (Bi2212). Our approach, which focuses on the nonlinear diamagnetic response, completely removes normal-state contributions and thus allows us to trace the diamagnetic signal above Tc with great precision. We find that SC diamagnetism vanishes in an unusual, yet surprisingly simple exponential manner, marked by a universal temperature scale that is independent of compound and Tc. We discuss the distinct possibility that this unusual behavior signifies the proliferation of SC clusters as a result of the intrinsic inhomogeneity known to be an inherent property of the cuprates.

cond-mat.supr-con↗

arXiv2017

Low-Energy Surface States in the Normal State of $α$-PdBi2 Superconductor

Topological superconductors as characterized by Majorana surface states has been actively searched for their significance in fundamental science and technological implication. The large spin-orbit coupling in Bi-Pd binaries has stimulated extensive investigations on the topological surface states in these superconducting compounds. Here we report a study of normal-state electronic structure in a centrosymmetric $α$-PdBi2 within density functional theory calculations. By investigating the electronic structure from the bulk to slab geometries in this system, we predict for the first time that $α$-PdBi2 can host orbital-dependent and asymmetric Rashba surface states near the Fermi energy. This study suggests that $α$-PdBi2 will be a good candidate to explore the relationship between superconductivity and topology in condensed matter physics.

cond-mat.supr-con↗

arXiv2017

Probing the superconducting gap symmetry of $α$-PdBi$_{2}$: A penetration depth study

We report measurements of the in-plane London penetration depth $λ$ in single crystals of the $α$-PdBi$_{2}$ superconductor --- the $α$-phase counterpart of the putative topological superconductor $β$-PdBi$_{2}$, down to 0.35~K using a high-resolution tunnel-diode-based technique. Both $λ$ and superfluid density $ρ_{s}$ exhibit an exponential behavior for $T\leq$ 0.35$T_{c}$, with $Δ(0)/k_{B}T_{c}\sim$2.0, $ΔC/γT_{c}$$\sim$2.0 and $λ(0)$$\sim$140~nm, showing that $α$-PdBi$_{2}$ is a moderately-coupling, fully-gapped superconductor. The values of $Δ(0)$ and $ΔC/γT_{c}$ are consistent with each other via strong-coupling corrections.

cond-mat.supr-con↗

arXiv2017

Spin-orbit scattering visualized in quasiparticle interference

In the presence of spin-orbit coupling, electron scattering off impurities depends on both spin and orbital angular momentum of electrons -- spin-orbit scattering. Although some transport properties are subject to spin-orbit scattering, experimental techniques directly accessible to this effect are limited. Here we show that a signature of spin-orbit scattering manifests itself in quasiparticle interference (QPI) imaged by spectroscopic-imaging scanning tunneling microscopy. The experimental data of a polar semiconductor BiTeI are well reproduced by numerical simulations with the $T$-matrix formalism that include not only scalar scattering normally adopted but also spin-orbit scattering stronger than scalar scattering. To accelerate the simulations, we extend the standard efficient method of QPI calculation for momentum-independent scattering to be applicable even for spin-orbit scattering. We further identify a selection rule that makes spin-orbit scattering visible in the QPI pattern. These results demonstrate that spin-orbit scattering can exert predominant influence on QPI patterns and thus suggest that QPI measurement is available to detect spin-orbit scattering.

cond-mat.mes-hall↗

arXiv2017

Narrow-band anisotropic electronic structure of ReS$_2$

We have used angle resolved photoemission spectroscopy to investigate the band structure of ReS$_2$, a transition-metal dichalcogenide semiconductor with a distorted 1T crystal structure. We find a large number of narrow valence bands, which we attribute to the combined influence of the structural distortion and spin-orbit coupling. We further image how this leads to a strong in-plane anisotropy of the electronic structure, with quasi-one-dimensional bands reflecting predominant hopping along zig-zag Re chains. We find that this does not persist up to the top of the valence band, where a more three-dimensional character is recovered with the fundamental band gap located away from the Brillouin zone centre along $k_z$. These experiments are in good agreement with our density-functional theory calculations, shedding new light on the bulk electronic structure of ReS$_2$, and how it can be expected to evolve when thinned to a single layer.

cond-mat.mtrl-sci↗

arXiv2016

Observation of Zeeman effect in topological surface state with distinct material dependence

The helical Dirac fermions on the surface of topological insulators host novel relativistic quantum phenomena in solids. Manipulating spins of topological surface state (TSS) represents an essential step towards exploring the theoretically predicted exotic states related to time reversal symmetry (TRS) breaking via magnetism or magnetic field. Understanding Zeeman effect of TSS and determining its g-factor are pivotal for such manipulations in the latter form of TRS breaking. Here, we report those direct experimental observations in Bi2Se3 and Sb2Te2Se by spectroscopic imaging scanning tunneling microscopy. The Zeeman shifting of zero mode Landau level is identified unambiguously by judiciously excluding the extrinsic influences associated with the non-linearity in the TSS band dispersion and the spatially varying potential. The g-factors of TSS in Bi2Se3 and Sb2Te2Se are determined to be 18 and -6, respectively. This remarkable material dependence opens a new route to control the spins in the TSS.

cond-mat.mes-hall↗

arXiv2015

Point nodes persisting far beyond Tc in Bi2212

In contrast to a complex feature of antinodal state, suffering from competing order(s), the "pure" pairing gap of cuprates is obtained in the nodal region, which therefore holds the key to the superconducting mechanism. One of the biggest question is whether the point nodal state as a hallmark of d-wave pairing collapses at Tc like the BCS-type superconductors, or it instead survives above Tc turning into the preformed pair state. A difficulty in this issue comes from the small magnitude of the nodal gap, which has been preventing experimentalists from solving it. Here we use a laser ARPES capable of ultrahigh energy resolution, and detect the point nodes surviving far beyond Tc in Bi2212. By tracking the temperature evolution of spectra, we reveal that the superconductivity occurs when the pair breaking rate is suppressed smaller than the single particle scattering rate on cooling, which governs the value of Tc in cuprates.

cond-mat.supr-con↗

arXiv2015

Imaging ambipolar two-dimensional carriers induced by the spontaneous electric polarization of a polar semiconductor BiTeI

Two-dimensional (2D) mobile carriers are a wellspring of quantum phenomena. Among various 2D-carrier systems, such as field effect transistors and heterostructures, polar materials hold a unique potential; the spontaneous electric polarization in the bulk could generate positive and negative 2D carriers at the surface. Although several experiments have shown ambipolar carriers at the surface of a polar semiconductor BiTeI, their origin is yet to be specified. Here we provide compelling experimental evidences that the ambipolar 2D carriers at the surface of BiTeI are induced by the spontaneous electric polarization. By imaging electron standing waves with spectroscopic imaging scanning tunneling microscopy, we find that positive or negative carriers with Rashba-type spin splitting emerge at the surface correspondingly to the polar directions in the bulk. The electron densities at the surface are constant independently of those in the bulk, corroborating that the 2D carriers are induced by the spontaneous electric polarization. We also successfully image that lateral $p$-$n$ junctions are formed along the boundaries of submicron-scale domains with opposite polar directions. Our study presents a novel means to endow non-volatile, spin-polarized, and ambipolar 2D carriers as well as, without elaborate fabrication, lateral $p$-$n$ junctions of those carriers at atomically-sharp interfaces.

cond-mat.mtrl-sci↗

arXiv2015

Topologically protected surface states in a centrosymmetric superconductor beta-PdBi2

The topological aspects of electrons in solids emerge in realistic matters as represented by topological insulators. They are expected to show a variety of new magneto-electric phenomena, and especially the ones hosting superconductivity are strongly desired as the candidate for topological superconductors (TSC). Possible TSC materials have been mostly developed by introducing carriers into topological insulators, nevertheless, those exhibiting indisputable superconductivity free from inhomogeneity are very few. Here we report on the observation of topologically-protected surface states in a centrosymmetric layered superconductor, beta-PdBi2, by utilizing spin- and angle-resolved photoemission spectroscopy. Besides the bulk bands, several surface bands, some of which crossing the Fermi level, are clearly observed with symmetrically allowed in-plane spin-polarizations. These surface states are precisely evaluated to be topological, based on the Z2 invariant analysis in analogy to 3-dimensional strong topological insulators. beta-PdBi2 may offer a TSC realized without any carrier-doping or applying pressure, i.e. a solid stage to investigate the topological aspect in the superconducting condensate.

cond-mat.mtrl-sci↗

arXiv2014

Ferroelectricity in underdoped La-based cuprates

Doping a parent antiferromagnetic Mott insulator in cuprates leads to short-range electronic correlations and eventually to high-Tc superconductivity. However, the nature of charge correlations in the lightly doped cuprates remains unclear. Understanding the intermediate electronic phase in the phase diagram (between the parent insulator and the high-Tc superconductor) is expected to elucidate the complexity both inside and outside the superconducting dome, and in particular in the underdoped region. One such phase is ferroelectricity whose origin and relation to the properties of high-Tc superconductors is subject of current research. Here we demonstrate that ferroelectricity and the associated magnetoelectric coupling are in fact common in La-214 cuprates namely, La$_{2-x}$Sr$_x$CuO$_4$, La$_2$Li$_x$Cu$_{1-x}$O$_4$ and La$_2$CuO$_{4+x}$. It is proposed that ferroelectricity results from local CuO$_6$ octahedral distortions, associated with the dopant atoms and/or clustering of the doped charge carriers, which break spatial inversion symmetry at the local scale whereas magnetoelectric coupling can be tuned through Dzyaloshinskii-Moriya interaction.

cond-mat.supr-con↗

arXiv2014

Two-stage magnetic-field-tuned superconductor-insulator transition in underdoped La$_{2-x}$Sr$_{x}$CuO$_{4}$

In the underdoped pseudogap regime of cuprate superconductors, the normal state is commonly probed by applying a magnetic field ($H$). However, the nature of the $H$-induced resistive state has been the subject of a long-term debate, and clear evidence for a zero-temperature ($T=0$) $H$-tuned superconductor-insulator transition (SIT) has proved elusive. Here we report magnetoresistance measurements in underdoped La$_{2-x}$Sr$_{x}$CuO$_{4}$, providing striking evidence for quantum critical behavior of the resistivity -- the signature of a $H$-driven SIT. The transition is not direct: it is accompanied by the emergence of an intermediate state, which is a superconductor only at $T=0$. Our finding of a two-stage $H$-driven SIT goes beyond the conventional scenario in which a single quantum critical point separates the superconductor and the insulator in the presence of a perpendicular $H$. Similar two-stage $H$-driven SIT, in which both disorder and quantum phase fluctuations play an important role, may also be expected in other copper-oxide high-temperature superconductors.

cond-mat.supr-con↗

arXiv2014

Imaging two-component nature of Dirac-Landau levels in the topological surface state of Bi2Se3

Massless Dirac electrons in condensed matter have attracted considerable attention. Unlike conventional electrons, Dirac electrons are described in the form of two-component wave functions. In the surface state of topological insulators, these two components are associated with the spin degrees of freedom, hence governing the magnetic properties. Therefore, the observation of the two-component wave function provides a useful clue for exploring the novel spin phenomena. Here we show that the two-component nature is manifested in the Landau levels (LLs) whose degeneracy is lifted by a Coulomb potential. Using spectroscopic-imaging scanning tunneling microscopy, we visualize energy and spatial structures of LLs in a topological insulator Bi2Se3. The observed potential-induced LL splitting and internal structures of Landau orbits are distinct from those in a conventional electron system and are well reproduced by a two-component model Dirac Hamiltonian. Our model further predicts non-trivial energy-dependent spin-magnetization textures in a potential variation. This provides a way to manipulate spins in the topological surface state.

cond-mat.mes-hall↗

arXiv2014

Observation of quasiparticles for phase incoherent d-wave pairing in Bi2212

In contrast to a complex feature of antinodal state, suffering from competing order(s), the "pure" pairing gap of cuprates is detected in the nodal region, which therefore holds the key to the superconducting mechanism. The pairing gap has been viewed to be rather conventional, closing at the superconducting transition temperature (Tc). However, the density of states contributed from the nodal region was claimed to have a gap-like structure even above Tc. Here we present a missing evidence for a single-particle gap near the node signifying the realization of a phase incoherent d-wave superconductivity above Tc in the optimally doped Bi2Sr2CaCu2O8+d. We find that the pair formation is formulated by momentum-independent temperature evolutions of three parameters: a BCS-type energy gap (Delta), a single particle scattering rate (Gamma_single) and a pair breaking rate (Gamma_pair). The superconductivity occurs when the Gamma_pair value is suppressed smaller than Gamma_single on cooling, and the magnitude of Tc in cuprates is governed by a condition of Gamma_single(Tc)=Gamma_pair(Tc).

cond-mat.supr-con↗

arXiv2013

Absence of Superconductivity in the "hole-doped" Fe pnictide Ba(Fe$_{1-x}$Mn$_{x}$)$_{2}$As$_{2}$: Photoemission and X-ray Absorption Spectroscopy Studies

We have studied the electronic structure of Ba(Fe$_{1-x}$Mn$_{x}$)$_{2}$As$_{2}$ ($x$=0.08), which fails to become a superconductor in spite of the formal hole doping like Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$, by photoemission spectroscopy and X-ray absorption spectroscopy (XAS). With decreasing temperature, a transition from the paramagnetic phase to the antiferromagnetic phase was clearly observed by angle-resolved photoemission spectroscopy. XAS results indicated that the substituted Mn atoms form a strongly hybridized ground state. Resonance-photoemission spectra at the Mn $L_{3}$ edge revealed that the Mn 3d partial density of states is distributed over a wide energy range of 2-13 eV below the Fermi level ($E_F$), with little contribution around $E_F$. This indicates that the dopant Mn 3$d$ states are localized in spite of the strong Mn 3d-As $4p$ hybridization and split into the occupied and unoccupied parts due to the on-site Coulomb and exchange interaction. The absence of superconductivity in Ba(Fe$_{1-x}$Mn$_{x}$)$_{2}$As$_{2}$ can thus be ascribed both to the absence of carrier doping in the FeAs plane, and to the strong stabilizaiton of the antiferromagnetic order by the Mn impurities.

cond-mat.supr-con↗

arXiv2013

Real-time manifestation of strongly coupled spin and charge order parameters in stripe-ordered nickelates via time-resolved resonant x-ray diffraction

We investigate the order parameter dynamics of the stripe-ordered nickelate, La$_{1.75}$Sr$_{0.25}$NiO$_4$, using time-resolved resonant X-ray diffraction. In spite of distinct spin and charge energy scales, the two order parameters' amplitude dynamics are found to be linked together due to strong coupling. Additionally, the vector nature of the spin sector introduces a longer re-orientation time scale which is absent in the charge sector. These findings demonstrate that the correlation linking the symmetry-broken states does not unbind during the non-equilibrium process, and the time scales are not necessarily associated with the characteristic energy scales of individual degrees of freedom.

cond-mat.str-el↗