arXiv subjects

Takeshi Kondo

Publications and source records attributed to Takeshi Kondo.

At least 91 records · Page 5Linked to original sources

arXiv2012

Collapse of the Fermi surface and fingerprints of order in the pseudogap state of cuprate superconductors

The Fermi surface in the state of cuprates is highly unusual because it appears to consist of disconnected segments called arcs. Their very existence challenges the traditional concept of a Fermi surface as closed contours of gapless excitations in momentum space. The length of the arcs in the pseudogap state was thought to linearly increase with temperature, pointing to the existence of a nodal liquid state below T*. These results were interpreted as an interplay of a d-wave pairing gap and strong scattering. Understanding the properties of the arcs is a pre-requisite to understanding the origin of the pseudogap and the physics of the cuprates. Here we use a novel approach to detect the energy gaps based on the temperature dependence of the density of states. With a significantly improved sensitivity, we demonstrate that the arcs form rapidly at T* and their length remains surprisingly constant over an extended temperature range between T* and Tarc, consistent with the presence of an ordered state below T*. These arcs span fixed points in the momentum space defining a set of wave vectors, which are the fingerprints of the ordered state that causes the pseudogap.

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arXiv2012

STM imaging of symmetry-breaking structural distortion in the Bi-based cuprate superconductors

A complicating factor in unraveling the theory of high-temperature (high-Tc) superconductivity is the presence of a "pseudogap" in the density of states, whose origin has been debated since its discovery [1]. Some believe the pseudogap is a broken symmetry state distinct from superconductivity [2-4], while others believe it arises from short-range correlations without symmetry breaking [5,6]. A number of broken symmetries have been imaged and identified with the pseudogap state [7,8], but it remains crucial to disentangle any electronic symmetry breaking from pre-existing structural symmetry of the crystal. We use scanning tunneling microscopy (STM) to observe an orthorhombic structural distortion across the cuprate superconducting Bi2Sr2Can-1CunO2n+4+x (BSCCO) family tree, which breaks two-dimensional inversion symmetry in the surface BiO layer. Although this inversion symmetry breaking structure can impact electronic measurements, we show from its insensitivity to temperature, magnetic field, and doping, that it cannot be the long-sought pseudogap state. To detect this picometer-scale variation in lattice structure, we have implemented a new algorithm which will serve as a powerful tool in the search for broken symmetry electronic states in cuprates, as well as in other materials.

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arXiv2011

What controls the phase diagram and superconductivity in Ru substituted BaFe$_2$As$_2$?

We use high resolution angle-resolved photoemission to study the electronic structure of the iron based high-temperature superconductors Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$ as a function of Ru concentration. We find that substitution of Ru for Fe is isoelectronic, i. e., it does not change the value of the chemical potential. More interestingly, there are no measured, significant changes in the shape of the Fermi surface or in the Fermi velocity over a wide range of substitution levels ($0<x<0.55$). Given that the suppression of the antiferromagnetic and structural phase has been associated with the emergence of the superconducting state, Ru substitution must achieve this via a mechanism that does not involve changes of the Fermi surface. We speculate that this mechanism relies on magnetic dilution which leads to the reduction of the effective Stoner enhancement.

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arXiv2011

Metallic surface electronic state in half-Heusler compounds RPtBi (R = Lu, Dy, Gd)

Rare-earth platinum bismuth (RPtBi) has been recently proposed to be a potential topological insulator. In this paper we present measurements of the metallic surface electronic structure in three members of this family, using angle resolved photoemission spectroscopy (ARPES). Our data shows clear spin-orbit splitting of the surface bands and the Kramers' degeneracy of spins at the Gamma and M points, which is nicely reproduced with our full-potential augmented plane wave calculation for a surface electronic state. No direct indication of topologically non-trivial behavior is detected, except for a weak Fermi crossing detected in close vicinity to the Gamma point, making the total number of Fermi crossings odd. In the surface band calculation, however, this crossing is explained by another Kramers' pair where the two splitting bands are very close to each other. The classification of this family of materials as topological insulators remains an open question.

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arXiv2010

Anomalies in the Fermi surface and band dispersion of quasi-one-dimensional CuO chains in the high-temperature superconductor YBa2Cu4O8

We have investigated the electronic states in quasi one dimensional (1D) CuO chains by microprobe Angle Resolved Photoemission Spectroscopy. We find that the quasiparticle Fermi surface consists of six disconnected segments, consistent with recent theoretical calculations that predict the formation of narrow, elongated Fermi surface pockets for coupled CuO chains. In addition, we find a strong renormalization effect with a significant kink structure in the band dispersion. The properties of this latter effect [energy scale (~40 meV), temperature dependence and behavior with Zn-doping] are identical to those of the bosonic mode observed in CuO2 planes of high temperature superconductors, indicating they have a common origin.

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arXiv2010

Importance of Fermi surface topology for high temperature superconductivity in electron-doped iron arsenic superconductors

We used angle resolved photoemission spectroscopy and thermoelectric power to study the poorly explored, highly overdoped side of the phase diagram of Ba(Fe1-xCox)2As2 high temperature superconductor. Our data demonstrate that several Lifshitz transitions - topological changes of the Fermi surface - occur for large x. Tc starts to decrease with doping when the cylindrical, central hole pocket changes to ellipsoids centering at the Z point, and goes to zero before these ellipsoids disappear around x = 0.2. Changes in thermoelectric power occur at similar x-values. Beyond this doping level the central pocket changes to electron-like and superconductivity does not exist. Our observations reveal the crucial importance of the underlying Fermiology in this class of materials. A necessary condition for superconductivity is the presence of the central hole pockets rather than perfect nesting between central and corner pockets.

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arXiv2010

Surface-driven electronic structure in LaFeAsO studied by angle resolved photoemission spectroscopy

We measured the electronic structure of an iron arsenic parent compound LaFeAsO using angle resolved photoemission spectroscopy (ARPES). By comparing with a full-potential Linear Augmented PlaneWave calculation we show that the extra large Gamma hole pocket measured via ARPES comes from electronic structure at the sample surface. Based on this we discuss the strong polarization dependence of the band structure and a temperature-dependent hole-like band around the M point. The two phenomena give additional evidences for the existence of the surface-driven electronic structure.

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arXiv2010

Disentangling Cooper-pair formation above Tc from the pseudogap state in the cuprates

The discovery of the pseudogap in the cuprates created significant excitement amongst physicists as it was believed to be a signature of pairing, in some cases well above the room temperature. In this "pre-formed pairs" scenario, the formation of pairs without quantum phase rigidity occurs below T*. These pairs condense and develop phase coherence only below Tc. In contrast, several recent experiments reported that the pseudogap and superconducting states are characterized by two different energy scales, pointing to a scenario, where the two compete. However a number of transport, magnetic, thermodynamic and tunneling spectroscopy experiments consistently detect a signature of phase-fluctuating superconductivity above leaving open the question of whether the pseudogap is caused by pair formation or not. Here we report the discovery of a spectroscopic signature of pair formation and demonstrate that in a region of the phase diagram commonly referred to as the "pseudogap", two distinct states coexist: one that persists to an intermediate temperature Tpair and a second that extends up to T*. The first state is characterized by a doping independent scaling behavior and is due to pairing above Tc, but significantly below T*. The second state is the "proper" pseudogap - characterized by a "checker board" pattern in STM images, the absence of pair formation, and is likely linked to Mott physics of pristine CuO2 planes. Tpair has a universal value around 130-150K even for materials with very different Tc, likely setting limit on highest, attainable Tc in cuprates. The observed universal scaling behavior with respect to Tpair indicates a breakdown of the classical picture of phase fluctuations in the cuprates.

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arXiv2010

Coherency of the superconducting state: the muon spin rotation and ARPES studies of (BiPb)_2(SrLa)_2CuO_{6+δ}

The superfluid density ρ_s in underdoped (T_c\simeq23K), optimally doped (T_c\simeq35K) and overdoped (T_c\simeq29K) single crystalline (BiPb)_2(SrLa)_2CuO_{6+δ} samples was studied by means of muon-spin rotation (\muSR). By combining the \muSR data with the results of ARPES measurements on similar samples [Nature 457, 296 (2009)] good self-consistent agreement is obtained between two techniques concerning the temperature and the doping evolution of ρ_s.

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arXiv2010

Critical change in the Fermi surface of iron arsenic superconductors at the onset of superconductivity

The phase diagram of a correlated material is the result of a complex interplay between several degrees of freedom, providing a map of the material's behavior. One can understand (and ultimately control) the material's ground state by associating features and regions of the phase diagram, with specific physical events or underlying quantum mechanical properties. The phase diagram of the newly discovered iron arsenic high temperature superconductors is particularly rich and interesting. In the AE(Fe1-xTx)2As2 class (AE being Ca, Sr, Ba, T being transition metals), the simultaneous structural/magnetic phase transition that occurs at elevated temperature in the undoped material, splits and is suppressed by carrier doping, the suppression being complete around optimal doping. A dome of superconductivity exists with apparent equal ease in the orthorhombic / antiferromagnetic (AFM) state as well as in the tetragonal state with no long range magnetic order. The question then is what determines the critical doping at which superconductivity emerges, if the AFM order is fully suppressed only at higher doping values. Here we report evidence from angle resolved photoemission spectroscopy (ARPES) that critical changes in the Fermi surface (FS) occur at the doping level that marks the onset of superconductivity. The presence of the AFM order leads to a reconstruction of the electronic structure, most significantly the appearance of the small hole pockets at the Fermi level. These hole pockets vanish, i. e. undergo a Lifshitz transition, at the onset of superconductivity. Superconductivity and magnetism are competing states in the iron arsenic superconductors. In the presence of the hole pockets superconductivity is fully suppressed, while in their absence the two states can coexist.

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arXiv2009

Anomalous asymmetry of the Fermi surface in the YBa2Cu4O8 high temperature superconductor revealed by Angle Resolved Photoemission Spectroscopy

We use microprobe Angle-Resolved Photoemission Spectroscopy to study the Fermi surface and band dispersion of the CuO2 planes in the high temperature superconductor, YBa2Cu4O8. We find a strong in-plane asymmetry of the electronic structure between directions along a and b axes. The saddle point of the antibonding band lies at a significantly higher energy in the a direction (pi,0) than the b direction (0,pi), whereas the bonding band displays the opposite behavior. We demonstrate that the abnormal band shape is due to a strong asymmetry of the bilayer band splitting, likely caused by a non-trivial hybridization between the planes and chains. This asymmetry has an important implication for interpreting key properties of the Y-Ba-Cu-O (YBCO) family, especially the superconducting gap, transport and results of inelastic neutron scattering.

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arXiv2009

Unusual Fermi surface nesting in parent compounds of iron arsenic high temperature superconductors revealed by Angle Resolved Photoemission Spectroscopy

We use angle resolved photoemission spectroscopy (ARPES) to study the band structure of BaFe2As2 and CaFe2As2, two of the parent compounds of the iron arsenic high temperature superconductors. We find clear evidence for band back folding and hybridization demonstrating that conduction electrons are strongly affected by the emergence of magnetic order. Our high quality data revealed that although the Fermi surface is strongly three-dimensional, it does indeed have long parallel segments along the kz direction that can lead to the emergence of magnetic order. More interestingly, we find very unusual incommensurate nesting of the Fermi surface in the a-b plane that is present only at low temperatures. We speculate that this is a signature of a failed Charge Density Wave (CDW) state that was predicted by renormalization group studies.

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arXiv2009

Three- to Two-Dimensional Transition of the Electronic Structure in CaFe2As2 - parent compound for an iron arsenic high temperature superconductor

We use angle-resolved photoemission spectroscopy (ARPES) to study the electronic properties of CaFe2As2 - parent compound of a pnictide superconductor. We find that the structural and magnetic transition is accompanied by a three- to two-dimensional (3D-2D) crossover in the electronic structure. Above the transition temperature (Ts) Fermi surfaces around Gamma and X points are cylindrical and quasi-2D. Below Ts the former becomes a 3D ellipsoid, while the latter remains quasi-2D. This finding strongly suggests that low dimensionality plays an important role in understanding the superconducting mechanism in pnictides.

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arXiv2009

Direct evidence for a competition between the pseudogap and high temperature superconductivity in the cuprates

A pairing gap and coherence are the two hallmarks of superconductivity. In a classical BCS superconductor they are established simultaneously at Tc. In the cuprates, however, an energy gap (pseudogap) extends above Tc. The origin of this gap is one of the central issues in high temperature superconductivity. Recent experimental evidence demonstrates that the pseudogap and the superconducting gap are associated with different energy scales. It is however not clear whether they coexist independently or compete. In order to understand the physics of cuprates and improve their superconducting properties it is vital to determine whether the pseudogap is friend or foe of high temperature supercondctivity. Here we report evidence from angle resolved photoemission spectroscopy (ARPES) that the pseudogap and high temperature superconductivity represent two competing orders. We find that there is a direct correlation between a loss in the low energy spectral weight due to the pseudogap and a decrease of the coherent fraction of paired electrons. Therefore, the pseudogap competes with the superconductivity by depleting the spectral weight available for pairing in the region of momentum space where the superconducting gap is largest. This leads to a very unusual state in the underdoped cuprates, where only part of the Fermi surface develops coherence.

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arXiv2008

Zero-field superfluid density in d-wave superconductor evaluated from the results of muon-spin-rotation experiments in the mixed state

We report on measurements of the in-plane magnetic penetration λ_{ab} in the optimally doped cuprate superconductor (BiPb)_2(SrLa)_2CuO_6+δ(OP Bi2201) by means of muon-spin rotation (\muSR). We show that in unconventional $d-$wave superconductors (like OP Bi2201), \muSR experiments conducted in various magnetic fields allow to evaluate the zero-field magnetic penetration depth λ_0, which relates to the zero-field superfluid density in terms of ρ_s\proptoλ_0^-2.

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arXiv2008

Imaging nanoscale Fermi surface variations in an inhomogeneous superconductor

Particle-wave duality suggests we think of electrons as waves stretched across a sample, with wavevector k proportional to their momentum. Their arrangement in "k-space," and in particular the shape of the Fermi surface, where the highest energy electrons of the system reside, determine many material properties. Here we use a novel extension of Fourier transform scanning tunneling microscopy to probe the Fermi surface of the strongly inhomogeneous Bi-based cuprate superconductors. Surprisingly, we find that rather than being globally defined, the Fermi surface changes on nanometer length scales. Just as shifting tide lines expose variations of water height, changing Fermi surfaces indicate strong local doping variations. This discovery, unprecedented in any material, paves the way for an understanding of other inhomogeneous characteristics of the cuprates, like the pseudogap magnitude, and highlights a new approach to the study of nanoscale inhomogeneity in general.

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arXiv2008

Momentum dependence of the superconducting gap in NdFeAsO1-xFx single crystals measured by angle resolved photoemission spectroscopy

We use angle resolved photoemission spectroscopy (ARPES) to study the momentum dependence of the superconducting gap in NdFeAsO1-xFx single crystals. We find that the Gamma hole pocket is fully gapped below the superconducting transition temperature. The value of the superconducting gap is 15 +- 1.5 meV and its anisotropy around the hole pocket is smaller than 20% of this value. This is consistent with an isotropic or anisotropic s-wave symmetry of the order parameter or exotic d-wave symmetry with nodes located off the Fermi surface sheets. This is a significant departure from the situation in the cuprates, pointing to possibility that the superconductivity in the iron arsenic based system arises from a different mechanism.

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arXiv2008

Evidence for competition between the superconducting and the pseudogap state in (BiPb)_2(SrLa)_2CuO_{6+δ} from muon-spin rotation experiments

The in-plane magnetic penetration depth λ_{ab} in optimally doped (BiPb)_2(SrLa)_2CuO_{6+δ} (OP Bi2201) was studied by means of muon-spin rotation. The measurements of λ_{ab}^{-2}(T) are inconsistent with a simple model of a d-wave order parameter and a uniform quasiparticle weight around the Fermi surface. The data are well described assuming the angular gap symmetry obtained in ARPES experiments [Phys. Rev. Lett {\bf 98}, 267004 (2007)], where it was shown that the superconducting gap in OP Bi2201 exists only in segments of the Fermi surface near the nodes. We find that the remaining parts of the Fermi surface, which are strongly affected by the pseudogap state, do not contribute significantly to the superconducting condensate. Our data provide evidence that high temperature superconductivity and pseudogap behavior in cuprates are competing phenomena.

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