arXiv subjects

H. Takagi

Publications and source records attributed to H. Takagi.

At least 181 records · Page 10Linked to original sources

arXiv2003

Thermal Conductivity across the Phase Diagram of Cuprates: Low-Energy Quasiparticles and Doping Dependence of the Superconducting Gap

Heat transport in the cuprate superconductors YBa$_2$Cu$_3$O$_{y}$ and La$_{2-x}$Sr$_x$CuO$_4$ was measured at low temperatures as a function of doping. A residual linear term kappa_{0}/T is observed throughout the superconducting region and it decreases steadily as the Mott insulator is approached from the overdoped regime. The low-energy quasiparticle gap extracted from kappa_{0}/T is seen to scale closely with the pseudogap. The ubiquitous presence of nodes and the tracking of the pseudogap shows that the overall gap remains of the pure d-wave form throughout the phase diagram, which excludes the possibility of a complex component (ix) appearing at a putative quantum phase transition and argues against a non-superconducting origin to the pseudogap. A comparison with superfluid density measurements reveals that the quasiparticle effective charge is weakly dependent on doping and close to unity.

cond-mat.supr-con↗

arXiv2003

Evolution of a Metal to Insulator Transition in Ca$_{2-x}$Na$_{x}$CuO$_{2}$Cl$_{2}$, as seen by ARPES

We present angle resolved photoemission (ARPES) data on Na-doped Ca$_2$CuO$_2$Cl$_2$. We demonstrate that the chemical potential shifts upon doping the system across the insulator to metal transition. The resulting low energy spectra reveal a gap structure which appears to deviate from the canonical $d_{x2-y2} ~ |cos(k_x a)-cos(k_y a)|$ form. To reconcile the measured gap structure with d-wave superconductivity one can understand the data in terms of two gaps, a very small one contributing to the nodal region and a very large one dominating the anti-nodal region. The latter is a result of the electronic structure observed in the undoped antiferromagnetic insulator. Furthermore, the low energy electronic structure of the metallic sample contains a two component structure in the nodal direction, and a change in velocity of the dispersion in the nodal direction at roughly 50 meV. We discuss these results in connection with photoemission data on other cuprate systems.

cond-mat.str-el↗

arXiv2002

The ground state of heavily-overdoped non-superconducting La$_{2-x}$Sr$_x$CuO$_4$

We report detailed thermodynamic and transport measurements for non-superconducting La$_{1.7}$Sr$_{0.3}$CuO$_4$. Collectively, these data reveal that a highly-correlated Fermi-liquid ground state exists in La$_{2-x}$Sr$_x$CuO$_4$ beyond the superconducting dome, and confirm that charge transport in the cuprates is dominated at finite temperatures by intense electron-electron scattering.

cond-mat.str-el↗

arXiv2002

Structure of end states for a Haldane Spin Chain

Inelastic neutron scattering was used to probe edge states in a quantum spin liquid. The experiment was performed on finite length antiferromagnetic spin-1 chains in Y_2BaNi_{1-x}Mg_xO_5. At finite fields, there is a Zeeman resonance below the Haldane gap. The wave vector dependence of its intensity provides direct evidence for staggered magnetization at chain ends, which decays exponentially towards the bulk (xi = 8(1) at T=0.1K). Continuum contributions to the chain end spectrum indicate inter-chain-segment interactions. We also observe a finite size blue shift of the Haldane gap.

cond-mat.str-el↗

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↗

arXiv2002

Effects of superconducting gap anisotropy on the flux flow resistivity in Y(Ni_{1-x}Pt_x)_2B_2C

The microwave complex surface impedance Z_s of Y(Ni_{1-x}Pt_x)_2B_2C was measured at 0.5 K under magnetic fields H up to 7T. In nominally pure YNi_2B_2C, which is a strongly anisotropic s-wave superconductor, the flux flow resistivity ρ_f calculated from Z_s was twice as large as that expected from the conventional normal-state vortex core model. In Pt-doped samples where the gap anisotropy is smeared out, the enhancement of ρ_f is reduced and ρ_f approaches to the conventional behavior. These results indicate that energy dissipation in the vortex core is strongly affected by the anisotropy of the superconducting gap.

cond-mat.supr-con↗

arXiv2002

Charge dynamics and "ferromagnetism" of A1-xLaxB6 (A=Ca and Sr)

Ferromagnetism has been reported recently in La-doped alkaline-earth hexaborides, A1-xLaxB6 (A=Ca, Sr, and Ba). We have performed the reflectivity, Hall resistivity, and magnetization measurements of A1-xLaxB6. The results indicate that A1-xLaxB6 can be regarded as a simple doped semimetal, with no signature of an excitonic state as suggested by several theories. It is also found that the surface of as-grown samples (10 micrometer in thickness) has a different electronic structure from a bulk one, and a fairly large number of paramagnetic moments are confined in this region. After eliminating these paramagnetic moments at the surface, we could not find any evidence of an intrinsic ferromagnetic moment in our samples, implying the possibility that the ferromagnetism of A1-xLaxB6 reported so far is neither intrinsic.

cond-mat.str-el↗

arXiv2002

Structural and Magnetic Instabilities of La$_{2-x}$Sr$_x$CaCu$_2$O$_6$

A neutron scattering study of nonsuperconducting La$_{2-x}$Sr$_x$CaCu$_2$O$_6$ (x=0 and 0.2), a bilayer copper oxide without CuO chains, has revealed an unexpected tetragonal-to-orthorhombic transition with a doping dependent transition temperature. The predominant structural modification below the transition is an in-plane shift of the apical oxygen. In the doped sample, the orthorhombic superstructure is strongly disordered, and a glassy state involving both magnetic and structural degrees of freedom develops at low temperature. The spin correlations are commensurate.

cond-mat.supr-con↗

arXiv2002

Gap Function with Point Nodes in Borocarbide Superconductor YNi_2B_2C

To determine the superconducting gap function of a borocarbide superconductor YNi_2B_2C, the c-axis thermal conductivity kappa_zz was measured in a magnetic field rotated in various directions relative to the crystal axes. The angular variation of kappa_zz in H rotated within the ab-plane shows a peculiar fourfold oscillation with narrow cusps. The amplitude of this fourfold oscillation becomes very small when H is rotated conically around the c-axis with a tilt angle of 45 degrees. Based on these results, we provide the first compelling evidence that the gap function of YNi_2B_2C has POINT NODES, which are located along the [100] and [010]-directions. This unprecedented gap structure challenges the current view on the pairing mechanism and on the unusual superconducting properties of borocarbide superconductors.

cond-mat.supr-con↗

arXiv2002

Antiferromagnetic Order Induced by an Applied Magnetic Field in a High-Temperature Superconductor

One view of the cuprate high-transition temperature (high-Tc) superconductors is that they are conventional superconductors where the pairing occurs between weakly interacting quasiparticles, which stand in one-to-one correspondence with the electrons in ordinary metals - although the theory has to be pushed to its limit. An alternative view is that the electrons organize into collective textures (e.g. charge and spin stripes) which cannot be mapped onto the electrons in ordinary metals. The phase diagram, a complex function of various parameters (temperature, doping and magnetic field), should then be approached using quantum field theories of objects such as textures and strings, rather than point-like electrons. In an external magnetic field, magnetic flux penetrates type-II superconductors via vortices, each carrying one flux quantum. The vortices form lattices of resistive material embedded in the non-resistive superconductor and can reveal the nature of the ground state - e.g. a conventional metal or an ordered, striped phase - which would have appeared had superconductivity not intervened. Knowledge of this ground state clearly provides the most appropriate starting point for a pairing theory. Here we report that for one high-Tc superconductor, the applied field which imposes the vortex lattice, also induces antiferromagnetic order. Ordinary quasiparticle pictures cannot account for the nearly field-independent antiferromagnetic transition temperature revealed by our measurements.

cond-mat.str-el↗

arXiv2002

Nonlocal Effects and Shrinkage of the Vortex Core Radius in YNi2B2C Probed by muSR

The magnetic field distribution in the vortex state of YNi2B2C has been probed by muon spin rotation (muSR). The analysis based on the London model with nonlocal corrections shows that the vortex lattice has changed from hexagonal to square with increasing magnetic field H. At low fields the vortex core radius, rho_v(H), decreases with increasing H much steeper than what is expected from the sqrt(H) behavior of the Sommerfeld constant gamma(H), strongly suggesting that the anomaly in gamma(H) primarily arises from the quasiparticle excitations outside the vortex cores.

cond-mat.supr-con↗

arXiv2001

Spin-1/2 Kagome Like Lattice in Volborthite Cu3V2O7(OH)2-H2O

A novel cuprate Volborthite, Cu3V2O7(OH)2-2H2O, containing an S-1/2 (Cu 2+ spin) kagome-like lattice is studied by magnetic susceptibility, specific heat, and 51 V NMR measurements. Signs for neither long-range order nor spin-gapped singlet ground states are detected down to 1.8 K, in spite of large antiferromagnetic couplings of ~100 K between Cu spins forming a two-dimensional kagome-like network. It is suggested that Volborthite represents a system close to a quantum critical point between classical long-range ordered and quantum disordered phases.

cond-mat.str-el↗

arXiv2001

Antiferromagnetic vortex state in a high-temperature superconductor

There is strong evidence that magnetic interactions play a crucial role in the mechanism driving high-temperature superconductivity in cuprate superconductors. To investigate this further we have done neutron scattering experiments on the simplest high-temperature superconductor La(2-x)Sr(x)CuO(4) (LSCO) in an applied magnetic field. Below the superconducting transition temperature (Tc), the field penetrates the material via an array of normal state inclusions or vortices while phase coherent superconductivity characterized by zero resistance is suppressed to the lower field-dependent irreversibility temperature. The measurements described here were performed on underdoped LSCO (x=0.10), which develops static incommensurate order below Tc in zero field. Our results show that application of a magnetic field enhances this response without changing the onset temperature. For H=5T the field-induced signal saturates to three times the zero-field signal and phase coherent superconductivity is established within the antiferromagnetic phase.

cond-mat.supr-con↗

arXiv2001

Holes in a Quantum Spin Liquid

Magnetic neutron scattering provides evidence for nucleation of antiferromagnetic droplets around impurities in a doped nickel-oxide based quantum magnet. The undoped parent compound contains a spin liquid with a cooperative singlet ground state and a gap in the magnetic excitation spectrum. Calcium doping creates excitations below the gap with an incommensurate structure factor. We show that weakly interacting antiferromagnetic droplets with a central phase shift of $π$ and a size controlled by the correlation length of the quantum liquid can account for the data. The experiment provides a first quantitative impression of the magnetic polarization cloud associated with holes in a doped transition metal oxide.

cond-mat.dis-nn↗

arXiv2000

Evolution of genetic code through isologous diversification of cellular states

Evolution of genetic code is studied as the change in the choice of enzymes that are used to synthesize amino acids from the genetic information of nucleic acids. We propose the following theory: the differentiation of physiological states of a cell allows for the different choice of enzymes, and this choice is later fixed genetically through evolution. To demonstrate this theory, a dynamical systems model consisting of the concentrations of metabolites, enzymes, amino acyl tRNA synthetase, and tRNA-amino acid complex in a cell is introduced and numerically studied. It is shown that the biochemical states of cells are differentiated by cell-cell interaction, and each differentiated type takes to use different synthetase. Through the mutation of genes, this difference in the genetic code is amplified and stabilized. Relevance of this theory to the evolution of non-universal genetic code in mitochondria is suggested. The present theory for the evolution of genetic code is based on our recent theory of isologous symbiotic speciation, which is briefly reviewed. According to the theory, phenotypes of organisms are first differentiated into distinct types through the interaction and developmental dynamics, even though they have identical genotypes, and later with the mutation in genotype, the genotype also differentiates into discrete types, while maintaining the `symbiotic' relationship between the types. Relevance of the theory to natural as well as artificial evolution is discussed.

nlin.AO↗

arXiv2000

Absence of residual quasiparticle conductivity in the underdoped cuprate YBa2Cu4O8

We report here measurements of the in-plane thermal conductivity K(T) of the underdoped cuprate YBa2Cu4O8 (Y124) below 1K. K(T) is shown to follow a simple, phononic T^3 dependence at the lowest temperatures for both current directions, with a negligible linear, quasiparticle contribution. This observation is in marked contrast with behavior reported in optimally doped cuprates, and implies that extended zero-energy (or low energy) low-energy quasiparticles are absent in YBa2Cu4O8 at low temperatures.

cond-mat.supr-con↗

arXiv2000

Microscopic Charge Inhomogeneities in Underdoped La_2-xSr_xCuO_4: Local Structural Evidence

We present local structural evidence for the existence of charge inhomogeneities at low temperature in underdoped and optimally doped La_2-xSr_xCuO_4. The inhomogeneities disappear for x > 0.2. The evidence for the charge inhomogeneities comes from an anomalous increase in the in-plane Cu-O bond length distribution in the underdoped samples as well as evidence for CuO_6 octahedral tilt inhomogeneities in the intermediate range structure. Preliminary analysis of the temperature dependence of this phenomenon indicates that the inhomogeneities set in at temperatures in the range 60 K < Tco < 130 K which depends on doping.

cond-mat.supr-con↗

arXiv2000

Neutron diffraction evidence of microscopic charge inhomogeneities in the CuO2 plane of superconducting La2-xSrxCuO4 (0 < x <0.30)

We present local structural evidence supporting the presence of charge inhomogeneities in the CuO2 planes of underdoped La2-xSrxCuO4. High-resolution atomic pair distribution functions have been obtained from neutron powder diffraction data over the range of doping 0 < x < 0.30 at 10 K. Despite the average structure getting less orthorhombic we see a broadening of the in-plane Cu-O bond distribution as a function of doping up to optimal doping. Thereafter the peak abruptly sharpens. Complementary evidence is also evident from the observation of octahedral tilt disorder in the PDF at higher atomic separation. This suggests a crossover from a charge inhomogeneous state at and below optimal doping to a homogeneous charge state above optimal doping. The strong response of the local structure to the charge-state implies a strong electron-lattice coupling in these materials.

cond-mat.supr-con↗