arXiv subjects

S. Tajima

Publications and source records attributed to S. Tajima.

At least 109 records · Page 6Linked to original sources

arXiv2002

Dependence of the superconducting transition temperature of single- and polycrystalline MgB2 on hydrostatic pressure

The dependence of Tc for MgB2 on purely hydrostatic or nearly hydrostatic pressure has been determined to 23 GPa for single-crystalline and to 32 GPa for polycrystalline samples, and found to be in good agreement. Tc decreases from 39 K at ambient pressure to 15 K at 32 GPa with an initial slope dTc/dP = -1.11(2) K/GPa. Evidence is presented that the differing values of dTc/dP reported in the literature result primarily from shear-stress effects in nonhydrostatic pressure media and not differences in the samples. Although comparison of these results with theory supports phonon-mediated superconductivity, a critical test of theory must await volume-dependent calculations based on the solution of the anisotropic Eliashberg equations.

cond-mat.supr-con↗

arXiv2002

High-pressure synthesis of superconducting Nb{1-x}B2 (x = 0-0.48) with the maximum Tc = 9.2 K

Superconductivity with Tc above 9 K was found in metal-deficient NbB2 prepared under 5 GPa, while no clear superconductivity was observed down to 3 K in stoichiometric NbB2. The superconductivity was observed above x = 0.04 in Nb1-xB2. and the lattice parameters also changed abruptly at x = 0.04. As x increased, the transition temperature Tc slightly rose and fell with the maximum value of 9.2 K at x = 0.24 for the samples sintered at 5 GPa and 1200 C. The Tc-value changed in the range from 7 K to 9 K, depending on the sintering pressure. A series of Ta1-xB2 (0 =< x =< 0.24) was also synthesized under high pressure to examine a special effect of high-pressure synthesis.

cond-mat.supr-con↗

arXiv2002

Scanning Tunneling Spectroscopic Studies of Cuprate Superconductors

Quasiparticle tunneling spectra of both hole-doped (p-type) and electron-doped (n-type) cuprates are studied using a low-temperature scanning tunneling microscope. The results reveal that neither the pairing symmetry nor the pseudogap phenomenon is universal among all cuprates, and that the response of n-type cuprates to quantum impurities is drastically different from that of the p-type cuprates. The only ubiquitous features among all cuprates appear to be the strong electronic correlation and the nearest-neighbor antiferromagnetic Cu^{2+}-Cu^{2+} coupling in the CuO_2 planes.

cond-mat.supr-con↗

arXiv2002

Crystal growth and characterization of MgB2: Relation between structure and superconducting properties

We discuss the important aspects of synthesis and crystal growth of MgB2 under high pressure (P) and temperature (T) in Mg-B-N system, including the optimisation of P-T conditions for reproducible crystal growth, the role of liquid phases in this process, the temperature dependence of crystal size and the effect of growing instabilities on single crystals morphology. Extensive experiments have been carried out on single crystals with slightly different lattice constants and defects concentration, which revealed and possible effects of Mg-deficiency and lattice strain on the superconducting properties of MgB2 (Tc, Jc, residual resistivity ratio, anisotropy etc.).

cond-mat.supr-con↗

arXiv2002

Peak effect in single crystal MgB$_2$ superconductor for ${\bf H}\parallel c$-axis

We have studied the phase diagram of MgB$_2$ superconductor using a single crystal for ${\bf H}\parallel c$-axis. For the first time we report the existence of peak effect in the screening current in MgB$_{2}$ single crystal for ${\bf H}\parallel c$-axis. In the magnetic field regime $10<H<13.5$ kOe the local fundamental diamagnetic moment displays a very narrow diamagnetic step, with a temperature width of the same size as the zero dc-magnetic field transition. For higher field this step is transformed to a peak which is related with the peak effect in the screening current. Finally, for $H<10$ kOe the diamagnetic step is transformed to a gradual transition. Our findings for the vortex matter phase diagram for the MgB$_2$ are closely related with theoretical predictions concerning the vortex matter phase diagram of a type II superconductor in the presence of weak point disorder.

cond-mat.supr-con↗

arXiv2002

New Josephson Plasma Modes in Underdoped YBa2Cu3O6.6 Induced by Parallel Magnetic Field

The c-axis reflectivity spectrum of underdoped YBa2Cu3O6.6 (YBCO) is measured below Tc=59K in parallel magnetic fields H//CuO2 up to 7T. Upon application of a parallel field, a new peak appears at finite frequency in the optical conductivity at the expense of suppression of c-axis condensate weight. We conclude that the dramatic change originates from different Josephson coupling strengths between bilayers with and without Josephson vortices. We find that the 400cm^-1 broad conductivity peak in YBCO gains the spectral weight under parallel magnetic field; this indicates that the condensate weight at ω=0 is distributed to the intra-bilayer mode as well as to the new optical Josephson mode.

cond-mat.supr-con↗

arXiv2002

de Haas-van Alphen effect in single crystal MgB_2

We report observations of quantum oscillations in single crystals of the high temperature superconductor MgB_2. Three de Haas-van Alphen frequencies are clearly resolved. Comparison with band structure calculations strongly suggests that two of these come from a single warped Fermi surface tube along the c direction, and that the third arises from cylindrical sections of an in-plane honeycomb network. The measured values of the effective mass range from 0.44-0.68 m_e. By comparing these with band masses calculated recently by three groups, we find that the electron-phonon coupling strength lambda, is a factor \~3 larger for the c-axis tube orbits than for the in-plane network orbit, in accord with recent microscopic calculations.

cond-mat.supr-con↗

arXiv2002

Anisotropic resistivity and Hall effect in MgB2 single crystals

We report resistivity and the Hall effect measurements in the normal and superconducting states of MgB2 single crystal. The resistivity has been found to be anisotropic with slightly temperature dependent resistivity ratio of about 3.5. The Hall constant, with a magnetic field parallel to the Mg and B sheets is negative in contrast to the hole-like Hall response with a field directed along the c-axis indicating presence of both types of charge carriers and, thus, multi-band electronic structure of MgB2. The Hall effect in the mixed state shows no sign change anomaly reproducing the Hall effect behavior in clean limit type-II superconductors.

cond-mat.supr-con↗

arXiv2002

Anisotropic Superconducting Properties of MgB2 Single Crystals

In-plane electrical transport properties of MgB2 single crystals grown under high pressure of 4-6 GPa and temperature of 1400-1700oC in Mg-B-N system have been measured. For all specimens we found sharp superconducting transition around 38.1-38.3K with transition width within 0.2-0.3K. Estimated resistivity value at 40K is about 1 mkOhmcm and resistivity ratio R(273K)/R(40K) of about 4.9. Results of measurements in magnetic field up to 5.5T perpendicular to Mg and B planes and up to 9T in parallel orientation show temperature dependent anisotropy of the upper critical field with anisotropy ratio increasing from 2.2 close to Tc up to about 3 below 30K. Strong deviation of the angular dependence of Hc2 from anisotropic mass model has been also found.

cond-mat.supr-con↗

arXiv2002

Anisotropic superconducting properties of MgB2 single crystals probed by in-plane electrical transport measurements

We report on study of the electronic anisotropy of the newly discovered MgB2 superconductor using the in-plane resistivity measurements in magnetic field applied perpendicular and parallel to Mg and B planes of MgB2 single crystals. The results show temperature dependent anisotropy of the upper critical field with anisotropy ratio increasing from 2.2 close to Tc up to about 3 below 30K. Our estimation of the in-plane and out-of-plane coherence length of about 68A and 23A and electronic mean-free path 240A and 60A respectively indicates MgB2 single crystal as approaching clean limit type-II superconductor.

cond-mat.supr-con↗

arXiv2002

The Superconducting Gap in MgB_2: Electronic Raman Scattering Measurements of Single Crystals

Polarisation-resolved Raman scattering measurements were performed on MgB_2 single crystals to determine the magnitude, symmetry and temperature dependence of the superconducting gap. A single sharp peak due to Cooper pair breaking appears in the electronic continuum below Tc, reaching an average maximum Raman shift of 105 +/- 1 cm^{-1} (2 Delta(0) / k_{B}Tc = 3.96 +/- 0.09) and showing up to 5 cm^{-1} anisotropy between polarised and depolarised spectra. The temperature dependence of 2 Delta follows that predicted from BCS theory, while the anisotropy decreases with decreasing temperature. It is concluded that the Raman results are consistent with a slightly anisotropic s-wave gap in a conventional BCS superconductor.

cond-mat.supr-con↗

arXiv2001

Electronic structure of MgB$_2$ from angle-resolved photoemission spectroscopy

The first angle-resolved photoemission spectroscopy results from MgB$_2$ single crystals are reported. Close to the $Γ$K and $Γ$M directions, three distinct dispersions are observed approaching the Fermi energy, as can be assigned to the theoretically predicted $σ$ (B $2p_{x,y}$) and $π$ (B $2p_z$) bands. In addition, we also observed a small parabolic-like band centered around $Γ$, and attributed it to a surface-derived state. Good agreement between our results and the band calculations suggests that the electronic structure of MgB$_2$ is of a conventional nature, thus implying that electron correlations are weak, and may be of little importance to the superconductivity in this system.

cond-mat.supr-con↗

arXiv2001

Evidence of Doping-Dependent Pairing Symmetry in Cuprate Superconductors

Scanning tunneling spectroscopy (STS) studies reveal long-range spatial homogeneity and predominantly $d_{x^2-y^2}$-pairing spectral characteristics in under- and optimally doped $\rm YBa_2Cu_3O_{7-δ}$ superconductors, whereas STS on $\rm YBa_2(Cu_{0.9934}Zn_{0.0026}Mg_{0.004})_3O_{6.9}$ exhibits {\it microscopic} spatial modulations and strong scattering near the Zn or Mg impurity sites, together with global suppression of the pairing potential. In contrast, in overdoped $\rm (Y_{0.7}Ca_{0.3})Ba_2Cu_3O_{7-δ}$, $(d_{x^2-y^2}+s)$-pairing symmetry is found, suggesting significant changes in the superconducting ground-state at a critical doping value.

cond-mat.supr-con↗

arXiv2001

Growth, structure analysis and anisotropic superconducting properties of MgB2 single crystals

Here we report the growth of sub-millimeter MgB2 single crystals of various shapes under high pressure in Mg-B-N system. Structure refinement using a single-crystal X-ray diffraction analysis gives lattice parameters a=3.0851(5) A and c=3.5201(5) A with small reliability factors (Rw =0.025, R=0.018), which enables us to analyze the Fourier and Fourier difference maps. The maps clearly show the B sp2 orbitals and covalency of the B-B bonds. The sharp superconducting transitions at Tc =38.1-38.3K were obtained in both magnetization (DTc =0.6K) and resistivity (DTc <0.3K) measurements. Resistivity measurements with magnetic fields applied parallel and perpendicular to the Mg and B sheets reveal the anisotropic nature of this compound, with upper critical field anisotropy ratio of about 2.7.

cond-mat.supr-con↗

arXiv2001

Spatial homogeneity and doping dependence of quasiparticle tunneling spectra in cuprate superconductors

Scanning tunneling spectroscopy (STS) studies reveal long-range (~100 nm) spatial homogeneity in optimally and underdoped superconducting YBa_2Cu_3O_{7-δ} (YBCO) single crystals and thin films, and macroscopic spatial modulations in overdoped (Y_{0.7}Ca_{0.3})Ba_2Cu_3O_{7-δ} (Ca-YBCO) epitaxial films. In contrast, STS on an optimally doped YBa_2(Cu_{0.9934}Zn_{0.0026}Mg_{0.004})_3O_{6.9} single crystal exhibits strong spatial modulations and suppression of superconductivity over a microscopic scale near the Zn or Mg impurity sites, and the global pairing potential is also reduced relative to that of optimally doped YBCO, suggesting strong pair-breaking effects of the non-magnetic impurities. The spectral characteristics are consistent with d_{x^2-y^2} pairing symmetry for the optimally and underdoped YBCO, and with (d_{x^2-y^2}+s) for the overdoped Ca-YBCO. The doping-dependent pairing symmetry suggests interesting changes in the superconducting ground state, and is consistent with the presence of nodal quasiparticles for all doping levels. The maximum energy gap Δ_d is non-monotonic with the doping level, while the (2Δ_d/k_BT_c) ratio increases with decreasing doping. The similarities and contrasts between the spectra of YBCO and of Bi_2Sr_2CaCu_2O_{8+x} (Bi-2212) are discussed.

cond-mat.supr-con↗

arXiv2001

Superconducting Gap and Strong In-Plane Anisotropy in Untwinned YBa2Cu3O7-d

With significantly improved sample quality and instrumental resolution, we clearly identify in the (pi,0) ARPES spectra from YBa2Cu3O6.993, in the superconducting state, the long-sought `peak-dip-hump' structure. This advance allows us to investigate the large a-b anisotropy of the in-plane electronic structure including, in particular, a 50% difference in the magnitude of the superconducting gap that scales with the energy position of the hump feature. This anisotropy, likely induced by the presence of the CuO chains, raises serious questions about attempts to quantitatively explain the YBa2Cu3O7-d data from various experiments using models based on a perfectly square lattice.

cond-mat.supr-con↗

arXiv2000

Phase fluctuations and the pseudogap in YBa2Cu3Ox

The thermodynamics of the superconducting transition is studied as a function of doping using high-resolution expansivity data of YBa2Cu3Ox single crystals and Monte-Carlo simulations of the anisotropic 3D-XY model. We directly show that Tc of underdoped YBa2Cu3Ox is strongly suppressed from its mean-field value (Tc-MF) by phase fluctuations of the superconducting order parameter. For overdoped YBa2Cu3Ox fluctuation effects are greatly reduced and Tc ~ Tc-MF . We find that Tc-MF exhibits a similar doping dependence as the pseudogap energy, naturally suggesting that the pseudogap arises from phase-incoherent Cooper pairing.

cond-mat.supr-con↗

arXiv2000

Angle-resolved photoemission study of insulating and metallic Cu-O chains in PrBa$_2$Cu$_3$O$_7$ and PrBa$_2$Cu$_4$O$_8$

We compare the angle-resolved photoemission spectra of the hole-doped Cu-O chains in PrBa$_2$Cu$_3$O$_7$ (Pr123) and in PrBa$_2$Cu$_4$O$_8$ (Pr124). While, in Pr123, a dispersive feature from the chain takes a band maximum at $k_b$ (momentum along the chain) $\sim$ $π/4$ and loses its spectral weight around the Fermi level, it reaches the Fermi level at $k_b$ $\sim$ $π/4$ in Pr124. Although the chains in Pr123 and Pr124 are approximately 1/4-filled, they show contrasting behaviors: While the chains in Pr123 have an instability to charge ordering, those in Pr124 avoid it and show an interesting spectral feature of a metallic coupled-chain system.

cond-mat.str-el↗