arXiv subjects

T. Hanaguri

Publications and source records attributed to T. Hanaguri.

At least 37 records · Page 2Linked to original sources

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

Evidence for time-reversal symmetry breaking of the superconducting state near twin-boundary interfaces in FeSe

Junctions and interfaces consisting of unconventional superconductors provide an excellent experimental playground to study exotic phenomena related to the phase of the order parameter. Not only the complex structure of unconventional order parameters have an impact on the Josephson effects, but also may profoundly alter the quasi-particle excitation spectrum near a junction. Here, by using spectroscopic-imaging scanning tunneling microscopy, we visualize the spatial evolution of the local density of states (LDOS) near twin boundaries (TBs) of the nodal superconductor FeSe. The $π/2$ rotation of the crystallographic orientation across the TB twists the structure of the unconventional order parameter, which may, in principle, bring about a zero-energy LDOS peak at the TB. The LDOS at the TB observed in our study, in contrast, does not exhibit any signature of a zero-energy peak and an apparent gap amplitude remains finite all the way across the TB. The low-energy quasiparticle excitations associated with the gap nodes are affected by the TB over a distance more than an order of magnitude larger than the coherence length $ξ_{ab}$. The modification of the low-energy states is even more prominent in the region between two neighboring TBs separated by a distance $\approx7ξ_{ab}$. In this region the spectral weight near the Fermi level ($\approx\pm$0.2~meV) due to the nodal quasiparticle spectrum is almost completely removed. These behaviors suggest that the TB induces a fully-gapped state, invoking a possible twist of the order parameter structure which breaks time-reversal symmetry.

cond-mat.supr-con↗

arXiv2014

Field-induced superconducting phase of FeSe in the BCS-BEC cross-over

Fermi systems in the crossover regime between weakly coupled Bardeen-Cooper-Schrieffer (BCS) and strongly coupled Bose-Einstein-condensate (BEC) limits are among the most fascinating objects to study the behavior of an assembly of strongly interacting particles. The physics of this crossover has been of considerable interest both in the fields of condensed matter and ultracold atoms. One of the most challenging issue in this regime is the effect of large spin imbalance on a Fermi system under magnetic fields. Although several exotic physical properties have been predicted theoretically, the experimental realization of such an unusual superconducting state has not been achieved so far. Here we show that pure single crystals of superconducting FeSe offer the possibility to enter the previously unexplored realm where the three energies, Fermi energy $\varepsilon_{\rm F}$, superconducting gap $Δ$ and Zeeman energy, become comparable. Through the superfluid response, transport, thermoelectric response, and spectroscopic-imaging scanning tunneling microscopy, we demonstrate that $\varepsilon_{\rm F}$ of FeSe is extremely small, with the ratio $Δ/\varepsilon_{\rm F}\sim1 (\sim0.3)$ in the electron (hole) band. Moreover, thermal-conductivity measurements give evidence of a distinct phase line below the upper critical field, where the Zeeman energy becomes comparable to $\varepsilon_{\rm F}$ and $Δ$. The observation of this field-induced phase provides insights into previously poorly understood aspects of the highly spin-polarized Fermi liquid in the BCS-BEC crossover regime.

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↗

arXiv2012

Evidence for a cos(4φ) Modulation of the Superconducting Energy Gap of Optimally Doped FeTe_{0.6}Se_{0.4} Single Crystals Using Laser Angle-Resolved Photoemission Spectroscopy

We study the superconducting(SC)-gap anisotropy of the Γ-centered hole Fermi surface in optimally doped FeTe_{0.6}Se_{0.4} (T_c = 14.5 K), using laser-excited angle-resolved photoemission spectroscopy (ARPES). We observe sharp superconducting coherence peaks at T = 2.5 K. In contrast to earlier ARPES studies but consistent with thermodynamic results, the momentum dependence shows a \cos(4φ) modulation of the SC-gap anisotropy. The observed SC-gap anisotropy strongly indicates that the pairing interaction is not a conventional phonon-mediated isotropic one. Instead, the results suggest the importance of second-nearest-neighbor electronic interactions between the iron sites in the framework of s_\pm-wave superconductivity.

cond-mat.supr-con↗

arXiv2012

Visualizing the emergence of the pseudogap state and the evolution to superconductivity in a lightly hole-doped Mott insulator

Superconductivity emerges from the cuprate antiferromagnetic Mott state with hole doping. The resulting electronic structure is not understood, although changes in the state of oxygen atoms appear paramount. Hole doping first destroys the Mott state yielding a weak insulator where electrons localize only at low temperatures without a full energy gap. At higher doping, the 'pseudogap', a weakly conducting state with an anisotropic energy gap and intra-unit-cell breaking of 90\degree-rotational (C4v) symmetry appears. However, a direct visualization of the emergence of these phenomena with increasing hole density has never been achieved. Here we report atomic-scale imaging of electronic structure evolution from the weak-insulator through the emergence of the pseudogap to the superconducting state in Ca2-xNaxCuO2Cl2. The spectral signature of the pseudogap emerges at lowest doping from a weakly insulating but C4v-symmetric matrix exhibiting a distinct spectral shape. At slightly higher hole-density, nanoscale regions exhibiting pseudogap spectra and 180\degree-rotational (C2v) symmetry form unidirectional clusters within the C4v-symmetric matrix. Thus, hole-doping proceeds by the appearance of nanoscale clusters of localized holes within which the broken-symmetry pseudogap state is stabilized. A fundamentally two-component electronic structure11 then exists in Ca2-xNaxCuO2Cl2 until the C2v-symmetric clusters touch at higher doping, and the long-range superconductivity appears.

cond-mat.supr-con↗

arXiv2012

Scanning Tunneling Microscopy/Spectroscopy of Vortices in LiFeAs

We investigate vortices in LiFeAs using scanning tunneling microscopy/spectroscopy. Zero-field tunneling spectra show two superconducting gaps without detectable spectral weight near the Fermi energy, evidencing fully-gapped multi-band superconductivity. We image vortices in a wide field range from 0.1 T to 11 T by mapping the tunneling conductance at the Fermi energy. A quasi-hexagonal vortex lattice at low field contains domain boundaries which consist of alternating vortices with unusual coordination numbers of 5 and 7. With increasing field, the domain boundaries become ill-defined, resulting in a uniformly disordered vortex matter. Tunneling spectra taken at the vortex center are characterized by a sharp peak just below the Fermi energy, apparently violating particlehole symmetry. The image of each vortex shows energy-dependent 4-fold anisotropy which may be associated with the anisotropy of the Fermi surface. The vortex radius shrinks with decreasing temperature and becomes smaller than the coherence length estimated from the upper critical field. This is direct evidence of the Kramer-Pesch effect expected in a clean superconductor.

cond-mat.supr-con↗

arXiv2010

Reply to Comment on "Unconventional s-wave superconductivity in Fe(Se,Te)"

Mazin and Singh comment that the peaks in the Fourier-transformed spectroscopic maps of Fe(Se,Te) are not related to the quasi-particle-interference (QPI) but may be Bragg peaks associated with chalcogen lattice and possible surface reconstruction which would be triggered by surface-induced spin-density wave. We show that the peaks are too broad to be assigned as Bragg peaks and are consistent with QPI.

cond-mat.supr-con↗

arXiv2010

Momentum-Resolved Landau-Level Spectroscopy of Dirac Surface State in Bi2Se3

We investigate Dirac fermions on the surface of the topological insulator Bi2Se3 using scanning tunneling spectroscopy. Landau levels (LLs) are observed in the tunneling spectra in a magnetic field. In contrast to LLs of conventional electrons, a field independent LL appears at the Dirac point, which is a hallmark of Dirac fermions. A scaling analysis of LLs based on the Bohr-Sommerfeld quantization condition allowed us to determine the dispersion of the surface band. Near the Fermi energy, fine peaks mixed with LLs appear in the spectra, which may be responsible for the anomalous magneto-fingerprint effect [J. G. Checkelsky et al., Phys. Rev. Lett. 103, 246601 (2009)].

cond-mat.mes-hall↗

arXiv2009

Coherence factors in a high-Tc cuprate probed by quasi-particle scattering off vortices

Coherence factors are a hallmark of superconductivity as a pair-condensation phenomenon. When electrons pair, quasi-particles develop an acute sensitivity to different types of scattering potential, described by the appearance of coherence factors in the scattering amplitudes. While the effects of coherence factors are well established in isotropic superconductors, they are much harder to detect in their anisotropic counterparts, such as high-Tc cuprates. Here we demonstrate a new approach which highlights the momentum-dependent coherence factors in Ca2-xNaxCuO2Cl2. Using Fourier-transform scanning tunnelling spectroscopy to detect quasi-particle interference effects, our experiments reveal a magnetic-field dependence in quasi-particle scattering which is sensitive to the sign of the anisotropic gap. This result can be understood in terms of d-wave coherence factors and it exposes the role of vortices as quasi-particle scattering centers. We also show that a magnetic field gives rise to an enlarged gapless region around the gap nodes.

cond-mat.supr-con↗

arXiv2007

Quasi-particle interference and superconducting gap in a high-temperature superconductor Ca2-xNaxCuO2Cl2

High-transition-temperature (high-Tc) superconductivity is ubiquitous in the cuprates containing CuO2 planes but each cuprate has its own character. The study of the material dependence of the d-wave superconducting gap (SG) should provide important insights into the mechanism of high-Tc. However, because of the 'pseudogap' phenomenon, it is often unclear whether the energy gaps observed by spectroscopic techniques really represent the SG. Here, we report spectroscopic imaging scanning tunneling microscopy (SI-STM) studies of nearly-optimally-doped Ca2-xNaxCuO2Cl2 (Na-CCOC) with Tc = 25 ~ 28 K. They enable us to observe the quasi-particle interference (QPI) effect in this material, through which unambiguous new information on the SG is obtained. The analysis of QPI in Na-CCOC reveals that the SG dispersion near the gap node is almost identical to that of Bi2Sr2CaCu2Oy (Bi2212) at the same doping level, while Tc of Bi2212 is 3 times higher than that of Na-CCOC. We also find that SG in Na-CCOC is confined in narrower energy and momentum ranges than Bi2212. This explains at least in part the remarkable material dependence of Tc

cond-mat.supr-con↗

arXiv2007

Local Tunneling Spectroscopy across a Metamagnetic Critical Point in the Bi-layer Ruthenate Sr3Ru2O7

The local spectroscopic signatures of metamagnetic criticality in Sr3Ru2O7 were explored using scanning tunneling microscopy (STM). Singular features in the tunneling spectrum were found close to the Fermi level, as would be expected in a Stoner picture of itinerant electron metamagnetism. These features showed a pronounced magnetic field dependence across the metamagnetic critical point, which cannot be understood in terms of a naive Stoner theory. In addition, a pseudo-gap structure was observed over several tens of meV, accompanied by a c(2x2) superstructure in STM images. This result represents a new electronic ordering at the surface in the absence of any measurable surface reconstruction.

cond-mat.str-el↗

arXiv2007

An Intrinsic Bond-Centered Electronic Glass with Unidirectional Domains in Underdoped Cuprates

Removing electrons from the CuO2 plane of cuprates alters the electronic correlations sufficiently to produce high-temperature superconductivity. Associated with these changes are spectral weight transfers from the high energy states of the insulator to low energies. In theory, these should be detectable as an imbalance between the tunneling rate for electron injection and extraction - a tunneling asymmetry. We introduce atomic-resolution tunneling-asymmetry imaging, finding virtually identical phenomena in two lightly hole-doped cuprates: Ca1.88Na0.12CuO2Cl2 and Bi2Sr2Dy0.2Ca0.8Cu2O8+d. Intense spatial variations in tunneling asymmetry occur primarily at the planar oxygen sites; their spatial arrangement forms a Cu-O-Cu bond centered electronic pattern without long range order but with 4a0-wide unidirectional electronic domains dispersed throughout (a0: the Cu-O-Cu distance). The emerging picture is then of a partial hole-localization within an intrinsic electronic glass evolving, at higher hole-densities, into complete delocalization and highest temperature superconductivity.

cond-mat.supr-con↗

arXiv2007

Hidden Charge-Density-Wave Order in a Low-Tc Superconductor 2H-NbSe2

The Charge-density-wave(CDW) in 2H-NbSe2(TCDW ~ 33 K), which coexists with superconductivity(Tc = 7.2 K), is investigated using Angle-resolved photoemission spectroscopy as a function of temperature. Across TCDW, energy and momentum(k) distribution curves show changes associated with the primary and secondary CDW vectors. The CDW is due to hidden charge order occurring at specific k-points on Fermi surfaces in the 2-dimensional Brillouin zone(2D BZ) with spectral weight suppression, but no gap formation. The study rules out nesting in the 2D BZ and the van Hove singularity driven CDW transition in 2H-NbSe2.

cond-mat.str-el↗

arXiv2004

A `checkerboard' electronic crystal state in lightly hole-doped Ca_{2-x}Na_{x}CuO_{2}Cl_{2}

The phase diagram of hole-doped copper oxides shows four different electronic phases existing at zero temperature. Familiar among these are the Mott insulator, high-transition-temperature superconductor and metallic phases. A fourth phase, ofunknown identity, occurs at light doping along the zero-temperature bound of the `pseudogap' regime. This regime is rich in peculiar electronic phenomena, prompting numerous proposals that it contains some form of hidden electronic order. Here we present low-temperature electronic structure imaging studies of a lightly hole-doped copper oxide: Ca_{2-x}Na_{x}CuO_{2}Cl_{2}. Tunnelling spectroscopy (at energies |E|>100 meV) reveals electron extraction probabilities greatly exceeding those for injection, as anticipated for a doped Mott insulator. However, for |E|<100 meV, the spectrum exhibits a V-shaped energy gap centred on E=0. States within this gap undergo intense spatial modulations, with the spatial correlations of a four CuO_{2}-unit-cell square checkerboard, independent of energy. Intricate atomic-scale electronic structure variations also exist within the checkerboard. These data are consistent with an unanticipated crystalline electronic state, possibly the hidden electronic order, existing in the zero-temperature pseudogap regime of Ca_{2-x}Na_{x}CuO_{2}Cl_{2}.

cond-mat.supr-con↗

arXiv2004

Imaging nano-scale electronic inhomogeneity in lightly doped Mott insulator Ca_{2-x}Na_{x}CuO_{2}Cl_{2}

The spatial variation of electronic states was imaged in the lightly doped Mott insulator Ca_{2-x}Na_{x}CuO_{2}Cl_{2} using scanning tunneling microscopy / spectroscopy (STM/STS). We observed nano-scale domains with a high local density of states within an insulating background. The observed domains have a characteristic length scale of 2 nm (~4-5a, a:lattice constant) with preferred orientations along the tetragonal [100] direction. We argue that such spatially inhomogeneous electronic states are inherent to slightly doped Mott insulators and play an important role for the insulator to metal transition.

cond-mat.supr-con↗

arXiv2003

New High Field State of Flux Line Lattice in Unconventional Superconductor CeCoIn_5

Ultrasound velocity measurements of the unconventional superconductor CeCoIn_5 with extremely large Pauli paramagnetic susceptibility reveal an unusual structural transformation of the flux line lattice (FLL) in the vicinity of the upper critical field. The transition field coincides with that at which heat capacity measurements reveal a second order phase transition. The lowering of the sound velocity at the transition is consistent with the collapse of the FLL tilt modulus and a crossover to quasi two-dimensional FLL pinning. These results provide a strong evidence that the high field state is the Fulde-Ferrel-Larkin-Ovchinikov phase, in which the order parameter is spatially modulated and has planar nodes aligned perpendicular to the vortices.

cond-mat.supr-con↗

arXiv2002

Angle-resolved photoemission spectroscopy of Na-doped Ca2CuO2Cl2 single crystals: Fingerprints of a magnetic insulator in a heavily underdoped superconductor

Electronic evolution from an antiferromagnet to a high-Tc superconductor is revealed by angle-resolved photoemission experiments on tetragonal Ca(1.9)Na(0.1)CuO(2)Cl(2) single crystals, which were successfully grown for the first time under high pressures. In this underdoped superconductor, we found clear fingerprints of the parent insulator: a shadow band and a large pseudo-gap. These observations are most likely described by a "chemical potential shift", which contrasts clearly with the prevailing wisdom of the "pinned chemical potential" learned from the prototype La(2-x)Sr(x)CuO(4), demonstrating that the route to a high-Tc superconductor is not unique.

cond-mat.supr-con↗