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Magnetism and superconductivity have been studied in a recently discovered RET 2 B 2 C borocarbide system, where RE = Y and T = Ni, Co. It was found that substitution of Co for Ni suppresses superconductivity in YNi 2 B 2 C at a rate of −0.64 K/at% Co. This suppression of T c can be interpreted as the result of a decrease in the density of states at the Fermi level, N(E F ). This suggests that the hybridization of the 4d orbits of Y and the 3d orbits of Ni plays an important role in the occurrence of superconductivity

The normal and superconducting properties in single crystal Bi 2 Sr 2 CaCu 2 O 8 (Bi2212) with T c =86 K have been investigated by systematic measurements of the 63 Cu Knight shift, 63 K( T ) and the nuclear-spin-lattice relaxation time, T 1 . In the normal state, the T -dependences of 63 K( T ) and ( T 1 T ) -1 in Bi2212 share similar magnetic behaviors to the underdoped systems such as oxygen deficient YBa 2 Cu 3 O 6+ x and YBa 2 Cu 4 O 8 , namely, 63 K( T ) decreases with lowering temperature and 63 ( T 1 T ) -1 has a peak above T c . However, from the analyses of the anisotropy of 1/ T 1 and the hyperfine field, Bi2212 is found to belong to the slightly over-doped region as in Tl 2 Ba 2 CuO 6+ x and TlSr 2 CaCu 2 O 8- x . In the superconducting state, the finding of T 1 T =const. law far below T c has revealed that the superconductivity is in a gapless regime, accompanied by the finite density of states at the Fermi level.

A TOF powder diffractometer, Sirius, installed at the Neutron Science Laboratory (KENS) of the High Energy Accelerator Research Organization (KEK), has realized both high resolution and high intensity with its large backward detector bank together with supermirror guide. Sirius also has a large 90° detector bank (0.54 str), which is used for diffraction studies under special environments, such as high pressure, high temperature, and so on. Recent works using a high-pressure cell (Prris–Edinburgh cell) and a high-temperature furnace with a gas-control device (modeled on the Miller Furnace at IPNS) are presented.

In conventional superconductors, magnetic impurities form an impurity band due to quantum interference of the impurity bound states, leading to suppression of the superconducting transition temperature. Such quantum interference effects can also be expected in d-wave superconductors. Here, we use scanning tunneling microscopy to investigate the effect of multiple non-magnetic impurities on the local electronic structure of the high-temperature superconductor Bi$_{2}$Sr$_{2}$Ca(Cu$_{1-x}$Zn$_{x}$)$_{2}$O$_{8+\delta}$. We find several fingerprints of quantum interference of the impurity bound states including: (i) a two-dimensional modulation of local density-of-states with a period of approximately 5.4 \AA\ along the $a$- and $b$-axes, which is indicative of the d-wave superconducting nature of the cuprates; (ii) abrupt spatial variations of the impurity bound state energy; (iii)an appearance of positive energy states; (iv) a split of the impurity bound state. All of these findings provide important insight into how the impurity band in d-wave superconductors is formed.

A systematic investigation on the thermodynamic properties of La-based ternary intermetallic superconductors crystallizing in a U(3)Ni(4)Si(4)-type structure is presented. The U(3)Ni(4)Si(4)-type structure consists of a characteristic intergrowth of periodic BaAl(4) (ThCr(2)Si(2))- and AlB(2)-type segments. Pristine low temperature specific heat data for recently discovered members La(3)Ni(4)Si(4) and La(3)Ni(4)Ge(4) with T(c)s of 1.0 and 0.7 K, respectively, are presented as well as La(3)Pd(4)Ge(4) with the highest T(c) of 2.5 K in the U(3)Ni(4)Si(4)-type group. Owing to the higher T(c)s of U(3)Ni(4)Si(4)-type superconductors than the related ThCr(2)Si(2)-type compounds, comparisons are drawn in our investigations of the ternary intermetallics of LaPd(2)Ge(2), LaNi(2)Si(2), and LaNi(2)Ge(2) having a ThCr(2)Si(2)-type structure. Our investigations of the thermodynamic properties show that La(3)Ni(4)Si(4) and La(3)Ni(4)Ge(4) have higher values of γ(n), N(E(F)), and Θ(D) than La(3)Pd(4)Ge(4). The same trend was found in ThCr(2)Si(2)-type compounds of LaPd(2)Ge(2), LaNi(2)Si(2), and LaNi(2)Ge(2). It turns out that the difference in T(c) between La(3)Pd(4)Ge(4), La(3)Ni(4)Si(4), and La(3)Ni(4)Ge(4), as well as the relatively higher T(c) of the U(3)Ni(4)Si(4)-type superconductors than of the related ThCr(2)Si(2)-type compounds, are largely due to the strength of electron-phonon coupling.

Magnetic hysteresis curves have been measured by using a miniature Hall probe on two single-crystal platelets of Bi 2 Sr 2 CaCu 2 O y with the magnetic field applied along the c -axis normal to the plane. With the help of detailed calculations of the critical current distribution in a ribbon-shaped sample, the observed hysteresis curves are reproduced on the assumption of the critical-state model and an exponential field dependence of J c . A tentative B - T vortex phase diagram is constructed, and its implications are discussed.

Using scanning tunneling spectroscopy, we report the correlation between spatial gap inhomogeneity and the zinc (Zn) impurity resonance in single crystals of ${\text{Bi}}_{2}{\text{Sr}}_{2}\text{Ca}{({\text{Cu}}_{1\ensuremath{-}x}{\text{Zn}}_{x})}_{2}{\text{O}}_{8+\ensuremath{\delta}}$ with different carrier (hole) concentrations $(p)$ at a fixed Zn concentration ($x\ensuremath{\sim}0.5\mathrm{%}$ per Cu atom). In all the samples, the impurity resonance lies only in the region where the gap value is less than $\ensuremath{\sim}60\text{ }\text{meV}$. Also the number of Zn resonance sites drastically decreases with decreasing $p$, in spite of the fixed $x$. These experimental results lead us to a conclusion that the Zn impurity resonance does not appear in the large-gap region although the Zn impurity evidently resides in this region.

The anisotropic pinning effect of magnetic flux lines in Bi 2 Sr 2 CaCu 2 O 8+° single crystals was investigated by torque measurements in a temperature range from 4.2 to 100 K. A sharp cusp in angular dependence of the torque was found near H · ab -plane between 15 and 82 K. The magnetic field dependence of the angle yielding the cusp ϑ c1 is clearly determined by the field component perpendicular to the CuO 2 planes H cP1 obeying H cos ϑ c 1 = H cP 1, revealing the intrinsic flux trapping between the CuO 2 planes and the start of the stepwise penetration at H cP1 . In addition, extra cusps were observed in the torque curves in a narrow temperature range from 21 to 40 K, suggesting a certain change in the vortex state. The relation between fields and angles to give extra cusps ϑ c2 also well fitted the equation H cos ϑ c 2 = H cP 2 . The temperature dependence of H cP1 and H cP2 was determined and the phase transition of the vortex state is discussed.

Magnetic hysteresis curves as well as reversible magnetization in the superconducting state of single crystalline Bi 2 Sr 2 CaCu 2 O 8+δ have been studied. Large hysteresis in magnetization at 4.2 K, which is a manifestation of strong pinning effects, rapidly diminishes around 25 K, where an anomalous peak effect is observed. This peak effect appears sharply in a certain magnetic field at a certain temperature, and is interpreted as a characteristic feature of collective pinning, which is a result of a sudden change of the elastic constant of the flux-line lattice in the vortex state. It is observed that the irreversibility line determined by magnetic hysteresis curves coincides well with the one obtained previously by the temperature dependence of magnetization.

The crystal structure of Pb 2 Sr 2 MCu 3 O 8 (M=Nd, Sm, Eu, Gd, Dy, Y 1- x Ca x , Ho and Er) has been analyzed by the Rietveld method of X-ray powder diffraction data. In order to best fit the data several structural models with the different space groups have been tried. Among them the two models with the space groups of Cmmm and P22 1 2 in which the oxygen atoms on the PbO sheet are displaced along the b -axis from the midposition between Pb atoms within the PbO sheet gave the best results. It is shown that the lattice parameters and the interatomic distances vary with the ionic radius of lanthanoides. The systematic variation of the atomic arrangement in the unit cell which consists of stacking of sheets having a different role is argued in comparison with the prototype of the Ba 2 LnCu 3 O 6+δ system. The bond lengths of the constituent atoms are also argued with special emphasis on the local environment of two copper sites. In the Pb 2 Sr 2 (Y 1- x Ca x )Cu 3 O 8 system, the interatomic distances cha...

The momentum dependence of the superconducting gap at low temperatures is precisely determined from angle resolved photoemission (ARPES) measurements on overdoped and underdoped samples of Bi2212. As the doping decreases, the maximum gap increases, but the slope of the gap near the nodes decreases. Though consistent with d-wave symmetry, the gap with underdoping cannot be fit by the simple cos (kx)- cos (ky) form. We suggest that this arises due to the increasing importance of long range interactions as one approaches the insulator. Furthermore, a comparison of our ARPES results with available penetration depth data clearly indicates that the renormalization of the linear T suppression of the superfluid density at low temperatures due to quasiparticle excitations around the d-wave nodes is large and doping dependent.

We investigate the doping dependent low energy, low temperature ($T$ = 5 K) properties of nodal quasiparticles in the d-wave superconductor Bi$_{2.1}$Sr$_{1.9}$CaCu$_2$O$_{8+\delta}$ (Bi2212). By utilizing ultrahigh resolution laser-excited angle-resolved photoemission spectroscopy, we obtain precise band dispersions near $E_{F}$, mean free paths and scattering rates ($\Gamma$) of quasiparticles. For optimally and overdoped, we obtain very sharp quasiparticle peaks of 8 meV and 6 meV full-width at half-maximum, respectively, in accord with terahertz conductivity. For all doping levels, we find the energy-dependence of $\Gamma \sim |\omega |$, while $\Gamma$($\omega =0$) shows a monotonic increase from overdoping to underdoping. The doping dependence suggests the role of electronic inhomogeneity on the nodal quasiparticle scattering at low temperature (5 K $\lsim 0.07T_{\rm c}$), pronounced in the underdoped region.

To investigate the melting transition of the vortex lattice under a force-free current parallel to the $c$ axis, we have measured the field dependence of $c$-axis resistance, critical current, and $I\text{\ensuremath{-}}V$ characteristics in vortex states of ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}\mathrm{Ca}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8+y}$ single crystals. The melting transition field ${H}_{m}$ observed in the critical current measurements was almost 20% smaller than ${H}_{m}$ determined by the resistance measurements. Furthermore, it was found that ${H}_{m}$ is reduced as the parallel current increases by measurements of the differential resistance with various bias currents. We have first demonstrated to induce the vortex-lattice melting transition by the force-free current. Although monotonical decrease of ${H}_{m}$ with increasing current is consistent with the theoretical expectations, the whole behavior of the current dependence of ${H}_{m}$ does not obey them.

To study the anisotropy dependence of the Josephson-vortex phase diagram, we measured the Josephson-vortex flow resistance as a function of magnetic field parallel to the $ab$ plane in ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}\mathrm{Ca}{\mathrm{Cu}}_{2}{\mathrm{O}}_{8+y}$ (Bi-2212) single crystals. The anisotropy of samples was controlled over a wide range by annealing. Since the Josephson-vortex flow resistance oscillates periodically as a function of the magnetic field when a dense triangular lattice is formed, the lower boundary field of the triangular lattice phase is assigned from the magnetic field where the oscillations start. We observed that the lower boundary field of the triangular lattice phase decreases systematically with increasing anisotropy parameter $\ensuremath{\gamma}$ of Bi-2212 at all measured temperatures. This behavior is consistent with theories which imply that the boundary field is inversely proportional to $\ensuremath{\gamma}$.

In the vortex core of slightly overdoped Bi2Sr2CaCu2Ox, the electron-like and hole-like states have been found to exhibit spatial modulations in anti-phase with each other along the Cu-O bonding direction. Some kind of one-dimensionality has been observed in the vortex core, and it is more clearly seen in differential conductance maps at lower biases below +-9 mV.

Josephson vortex (JV) states in Bi2Sr2CaCu2O8+y (Bi‐2212) have been studied by measuring the flow resistivity of JV’s and I‐V characteristics with a current along the c‐axis. In the flow resistivity, periodic oscillations have been found as a function of the parallel magnetic field to the superconducting layers. As this phenomenon is related to the formation of a triangular lattice of JV’s as the ground state, the 3D‐ordered state (triangular lattice phase) of JV’s could be determined. With changing the doping level of Bi‐2212, the BD‐ordered phase is shifted towards a higher magnetic field and a higher temperature, which is well explained from the theoretical analyses. Above the 3D‐ordered phase, vortex state has been studied with I‐V characteristic measurements, in which the voltage shows a distinct change in the exponent of the power‐law dependence to the current between two phases. This suggests that the 3D‐ordered phase changes to the disordered phase along the c‐axis.

The vortex phase diagram of high‐Tc superconductors in the field parallel to the c‐axis have been already extensively explored. It has been commonly observed that pinning effects are quite weak at high temperatures and high fields in high quality single crystals. Such weak pinning phases, like the vortex liquid and the Bragg glass, are good candidate to study the influence of additional artificial pinning arrays on vortex phases, because the original quenched disorders do not work well as effective pinning centers in these phases. We have introduced several types of hole arrays in Bi2Sr2CaCu2O8+y single‐crystal thin film using a focused ion beam. To preserve the quality of the crystals we have made film samples from bulk crystals by the cleaving. In samples with triangular and square pinning arrays of 1μm lattice spacing, the flow resistance of pancake vortices exhibit a matching effect at multiples of the matching fields. This matching effect occurs in the vortex liquid phase. It is difficult to explain these results by the idea of the normal vortex liquid because of the lack of in‐plane symmetries. There is a possibility that the in‐plane order is induced in the vortex liquid by the introduction of the periodic pinning arrays.

Hard x-ray photoemission spectroscopy (PES) of Cu core electronic states, with a probing depth of $\ensuremath{\sim}60\text{ }\text{ }\AA{}$, is used to show that the Zhang-Rice singlet feature is present in ${\mathrm{La}}_{2}{\mathrm{CuO}}_{4}$ but is absent in ${\mathrm{Nd}}_{2}{\mathrm{CuO}}_{4}$. Hole and electron doping in ${\mathrm{La}}_{2\ensuremath{-}x}{\mathrm{Sr}}_{x}{\mathrm{CuO}}_{4}$ (LSCO) and ${\mathrm{Nd}}_{2\ensuremath{-}x}{\mathrm{Ce}}_{x}{\mathrm{CuO}}_{4}$ (NCCO) result in new well-screened features which are missing in soft x-ray PES. Impurity Anderson model calculations establish screening from doped states as its origin, which is strongly suppressed within 15 \AA{} of the surface. Complemented with x-ray absorption spectroscopy, the small chemical-potential shift in core levels ($\ensuremath{\sim}0.2\text{ }\text{ }\mathrm{eV}$) are shown to be consistent with modifications of valence and conduction band states spanning the band gap ($\ensuremath{\sim}1\text{ }\text{ }\mathrm{eV}$) upon hole and electron doping in LSCO and NCCO.

Thin films of YNi2B2C superconductor by the magnetron sputtering technique have been synthesized. The YNi2B2C phase was found to crystallize very uniformly after annealing at 1050 °C in vacuum. The structure was investigated and the electric and magnetic properties were characterized. The films were c‐axis oriented and showed a Tc onset of ∼15 K by both magnetization and resistivity measurements. The results of the critical current measurement suggested the existence of anisotropies of pinning force and an upper critical field Bc2.

To study the Josephson-vortex system, we have measured the vortex-flow resistance as a function of magnetic field parallel to the $ab$ plane in ${\mathrm{B}\mathrm{i}}_{2}{\mathrm{S}\mathrm{r}}_{2}{\mathrm{C}\mathrm{a}\mathrm{C}\mathrm{u}}_{2}{\mathrm{O}}_{8+y}$ single crystals. Novel periodic oscillations of the vortex-flow resistance have been observed in a wide range of temperatures and magnetic fields. The period of the oscillations corresponds to the field needed to add ``one'' vortex quantum per ``two'' intrinsic Josephson junctions. The flow velocity is related to a matching effect between the lattice spacing of Josephson vortices along the layers and the width of the sample. These results suggest that Josephson vortices form a triangular lattice in the ground state where the oscillations occur.

The magnetic nanoparticles are fabricated in silica glass (SiO 2 ) using high-flux implantation of nickel negative-ions of 60 keV. Photo-absorption measurements and the cross-sectional transmission electron microscopy (XTEM) observation confirm formation of metallic Ni nanoparticles in SiO 2 , and exclude possible formation of Ni silicides (Ni 3 Si, Ni 2 Si, NiSi) and oxides (NiO) as major products. The mean diameter of the nanoparticles was in ~2.9 nm, and the depth distribution was similar to the prediction from the TRIDYN code with taking account of the sputtering. Temperature- and field- dependences of magnetization show that the nanoparticles are in the super-paramagnetic state with a blocking temperature of ~27 K.

We investigated the distribution of the impurity (Zn) resonance energy in Bi2Sr2CaCu2O8+δ by scanning tunneling microscopy and spectroscopy. The resonance energy ranged rather widely from about −6 to 3 meV. We did not observe the correlation between the resonance energy and the local energy gap expected from theories. This seems to indicate the existence of other factors that more strongly affect the resonance energy than the local energy gap.

We study the normal-state electronic excitations probed by angle-resolved photoemission spectroscopy (ARPES) in ${\mathrm{Bi}}_{1.6}{\mathrm{Pb}}_{0.4}{\mathrm{Sr}}_{2}{\mathrm{CuO}}_{6}$ (Bi2201) and ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{CaCu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ (Bi2212). Our main goal is to establish explicit criteria for determining the Fermi surface from ARPES data on strongly interacting systems where sharply defined quasiparticles do not exist and the dispersion is very weak in parts of the Brillouin zone. Additional complications arise from strong matrix element variations within the zone. We present detailed results as a function of incident photon energy, and show simple experimental tests to distinguish between an intensity drop due to matrix element effects and spectral weight loss due to a Fermi crossing. We reiterate the use of polarization selection rules in disentangling the effect of umklapps due to the BiO superlattice in Bi2212. We conclude that, despite all the complications, the Fermi surface can be determined unambiguously; it is a single large hole barrel centered about $(\ensuremath{\pi},\ensuremath{\pi})$ in both materials.

In order to study the influence of artificial pinning centers on vortex behavior in high- T c superconductors, we have measured vortex-flow resistance in Bi 2 Sr 2 CaCu 2 O 8 + y single-crystal thin films with a square array of antidots. In addition to the integer matching effect, which is expected at integer multiples of the first matching field H 1 , fractional matching effects have been found as fine dip structures at fields H f ( = fH 1 ) (f: fractional number). The observed values of f are 1/6, 1/4, 1/3, 1/2, 2/3, and 3/4. While a square lattice of vortices (or unoccupied antidots) can fit on the antidot array for the cases of 1/4 and 1/2 (3/4), different vortex configurations are required to explain for the other fractions, viz., a stripe pattern at 1/3.

FeSr 2 YCu 2 O 6+ δ with the tetragonal Ba 2 YCu 3 O 6+ δ -type structure exhibits superconductivity around 60 K, when it is properly annealed in N 2 and subsequently in O 2 . This compound also exhibits antiferromagnetic order of Fe around 20 K. We have performed neutron powder diffraction of the FeSr 2 YCu 2 O 6+ δ system at low temperature to investigate the relation between the crystal structure and the physical properties. Neutron diffraction study indicates that this compound does not have the structure phase transition due to the tilt of the FeO 6 octahedron differently from the RuSr 2 GdCu 2 O 8 magnetic superconductor with the same structure, and that the magnetic order in FeSr 2 YCu 2 O 6+ δ is not long-range order, because we can observe no extra reflection due to the crystal and magnetic superstructure.

The effect of artificially introduced periodic pinning centers fabricated by focused-ion beam milling is studied in highly anisotropic superconductor Bi 2 Sr 2 CaCu 2 O 8+ y . Step-like changes of local magnetic induction at integer multiples of the matching field are visualized by magneto-optical imaging. Temperature dependence of the matching phenomenon is understood by considering the saturation number of vortices in each pinning center.

We have locally modified a cleaved surface of single crystalline Bi 2 Sr 2 CaCu 2 O 8 + δ by local anodic oxidation using scanning tunneling microscope-assisted near-field scanning microwave microscope (STM-assisted NSMM). After the modification, we have measured the resonant frequency and the quality factor of the microwave responses near the modified region at 7.31 GHz. The resonant frequency and the quality factor increase at the modified region. It has been confirmed that the STM-assisted NSMM enables us both to modify the sample surface and to measure the microwave impedance on the modified region in nanoscale.

FeSr 2 YCu 2 O 6 + y compound (YFe1212) exhibits superconductivity only when it is annealed in a N 2 flow and subsequently in an O 2 flow. To study the relation between the superconductivity and oxygen content, 6 + y , YFe1212 with various oxygen contents ( 6 + y = 6.88 – 7.55 ) were synthesized and their crystal structures, magnetism and superconductivity were studied. After the annealing in N 2 flow, the oxygen content, 6 + y , was 6.88 and the crystal structure was orthogonal. Increasing the oxygen content, lattice constant decreased monotonically and the crystal structure changed to tetragonal at around the oxygen content 6 + y = 7.10 . Superconductivity was observed above the oxygen content 6 + y = 7.30 and the onset superconducting transition temperature increased up to 53 K with the oxygen content 6 + y = 7.55 . Increasing the oxygen content, the bond valence sum of Cu ion increased and it coincides with the result of the susceptibility measurement. In all samples, anomalies are observed at 12 K and they seems to be magnetic transition of Fe ion. The magnetic transition temperatures are independent of the oxygen content, 6 + y .

The authors report the low-temperature specific heat and the magnetization measurements of the Cu-O ferromagnet La4Ba2Cu2O10 and the related compounds. The overall feature of the specific heat in La4Ba2Cu2O10 agrees to that of the mean field approximation with 1/2 spin system and is more 3-dimensional than that of 2-dimensional Cu ferromagnet K2CuF4. The measured magnetizations on the single crystals and the estimated anisotropy suggest the Heisenberg system with Ising-like anisotropy due to the comparably large interlayer coupling. In the system of Nd substitution for La, the ferromagnetic transition is not observed because Nd ions having the f-spins may disturb the exchange interaction between interlayer Cu2+ ions.

The Fe-based copper oxide FeSr 2 YCu 2 O 6 + δ exhibits superconductivity around 50 K only when it is properly annealed in N 2 atmosphere and subsequently in O 2 atmosphere. In contract FeSr 2 NdCu 2 O 6 + δ does not exhibit superconductivity even if it is annealed along the same process as FeSr 2 YCu 2 O 6 + δ . We have synthesized the polycrystalline samples of FeSr 2 Y 1 - x Nd x Cu 2 O 6 + δ solid solution system ( 0 ⩽ x ⩽ 1 ) to investigate the Nd substitution effects. DC magnetization measurements have shown that, the samples in a range of x = 0 ∼ 0.5 exhibit superconductivity and T c was reduced with increasing the Nd content. However, we could not observe the superconductivity for x = 0.75 and 1. Rietveld refinement results revealed that due to the mutual substitution of the Sr and Nd, the presence of the extra oxygen at (0, 0, 1/2) between double CuO 2 sheets suppresses superconductivity.

To study the influence of sub-micron artificial pinning centers on vortex phases in high- T c superconductors, we have introduced holes array in Bi 2 Sr 2 CaCu 2 O 8+ y single-crystal thin films using a focused ion beam. To preserve the quality of the crystals we made film samples from bulk crystals by the cleaving, and the surface of the Bi2212 films were covered by Au protection layer. The vortex-flow resistance shows a matching effect in the samples having triangular holes array which appears in vortex liquid phase of the pristine sample. This suggests that an in-plane symmetry is induced in the vortex liquid by regular holes array. Additionally, sub-harmonics of the matching effect are newly observed near the superconducting transition temperature ( T c ). This suggests that the correlation length of pancake vortices is longer with approaching T c since the in-plane penetration depth diverges toward T c .

The FeSr 2 YCu 2 O 6+ δ compound with a tetragonal Ba 2 YCu 3 O 6+ δ -type structure exhibits superconductivity, only when it is properly annealed in N 2 and subsequently in O 2 . N 2 -annealing causes the ordering of Cu and Fe, and O 2 -annealing supplies the carrier on the CuO 2 sheets. We have synthesized a new compound FeSr 2 (Tb,Ce) 2 Cu 2 O 8+ δ with the fluorite structure instead of the Y layer between adjacent CuO 2 sheets in FeSr 2 YCu 2 O 6+ δ . Neutron diffraction study shows that the insertion of the fluorite structure is very effective on preparing the structure with ordering of Cu and Fe without the N 2 -annealing. However, enough amount of carrier is not distributed on the CuO 2 sheets to exhibit superconductivity, and resultingly we have observed very weak superconductivity in FeSr 2 (Tb,Ce) 2 Cu 2 O 8+ δ .

Li 2 Pd 3 B and Li 2 Pt 3 B show superconducting transition at the temperatures, 7.4 K and 2.6 K, respectively. These materials have the same cubic structure ( P 4 3 32) composed of distorted octahedrons, whose corners are occupied by Pd/Pt. This structure does not have mirror or inversion symmetry along any directions. It is a very interesting feature of those materials in relation to the symmetry of superconductivity. The samples were prepared by a two-stage arc-melting method. Their superconductivity is induced by Li addition. Controlling the Li stoichiometry is a key to synthesize high quality samples in the preparation process of arc-melting because Li is volatile at high temperatures. The specific heat measurements were carried out by heat-pulse relaxation and semi-adiabatic methods. Pressure effect was measured with a SQUID magnetometer using a low temperature hydrostatic micropressure cell at pressures up to around 1 GPa. The superconducting transition temperatures of both Li 2 Pd 3 B and Li 2 Pt 3 B decreased linearly with pressure. The pressure dependence, d T c /d P , were 0.091 and 0.090 K/GPa, respectively.

We have measured Josephson-vortex flow resistance in annular-type intrinsic junctions of Bi 2 Sr 2 CaCu 2 O 8+ y to study Josephson-vortex flow dynamics. Even in the annular-type junctions, the flow resistance shows periodic oscillations as a function of the fields parallel to the superconducting layers, which are superimposed with the periods determined by the effective widths of the junctions to the fields. This enables us to propose the estimation of the correlation length in the Josephson vortices.

We have performed do magnetization measurements on the over-doped Bi2Sr2CaCu208+δ single crystals with columjiar defects parallel to the c-axis by a heavy-ion irradiation in the lower magnetic field than the matching field to the defects. Pronounced peak effects have been observed at two magnetic fields in the irreversibility region; one is located at the same field of the so-called 2D-3D dimensional crossover as in the pristine sample, the other around 1/3 of the matching field. It is suggested that, by the presence of the columnar defects, correlation of the vortex lines in the direction of the columnar defects develops to induce the new phase boundary in the reversibility region with increasing a magnetic field, which remains as a boundary in the irreversibility region.

The Fermi surface is a central concept in the theory of metals. Even though the optimally doped high temperature superconductors exhibit an anomalous normal state, angle resolved photoemission spectroscopy (ARPES) has revealed a large Fermi surface despite the absence of well-defined quasiparticles and the necessity of working at finite temperatures. However, the even more unusual behavior in the underdoped high temperature superconductors, which show a pseudogap above Tc, requires us to carefully re-examine this concept. Here, we present the first results on how the Fermi surface is destroyed as a function of temperature in underdoped Bi2212 using ARPES. We find the remarkable effect that different k points become gapped at different temperatures. This leads to a break up of the Fermi surface at a temperature T* into disconnected Fermi arcs which shrink with decreasing T, eventually collapsing to the point nodes of the dx2-y2 superconducting ground state below Tc. This novel behavior, where the Fermi surface does not form a continuous contour in momentum space, is unprecedented in that it occurs in the absence of long range order. Moreover, although the d-wave superconducting gap below Tc smoothly evolves into the pseudogap above Tc, the gaps at different k points are not related to one another above Tc the same way as they are below, implying an intimate, but non-trivial relation, between the two., 4 pages, revtex, 5 eps figures, minor changes (note, a and c should be interchanged in upper right panel of old Fig. 2), accepted for Nature

Using high resolution angle-resolved photoemission spectroscopy (ARPES), we measure the superconducting gap and its momentum dependence in the high temperature superconductors Bi2Sr2CaCu2O8 (Bi22l2) and YBa2Cu3O695 (Y123). It is found thatumklapp bands due to the incommensurate superlattice in Bi2212 may obscure the intrinsicmomentum dependence of the gap. By taking these into consideration, we find a d-wave- like gap function in Bi22 12. We also report an unambiguous observation of the superconducting gap in Y123.Keywords: high temperature superconductors, superconducting gap, pairing symmetry,angle-resolved photoemission I. IntroductionOver the past few years, there has been a debate in the literature1 concerning the nature of the superconducting order parameter in the cuprate high temperature superconductors. Low temperaturepenetration depth measurements in YBa2Cu3O695 indicate the existence of line nodes in the energy gap.There is an important question whether the position of these nodes is determined by symmetry, as in thed-wave scenario, or not, as in the anisotropic s-wave scenario. A photoemission study by Shen, et al.2

High precession ac-resistivity measurements with a wide range of currents have been performed for the c-axis of single crystalline Bi2Sr2CaCu2O8+δ in magnetic fields in order to study the vortex dynamics in the vortex liquid phase. It is observed that the resistivity shows a stepwise drop at T D at the foot of the transition (ρc(T D)/ρc(T c + ) ≤ 2 × 10-4) below 80 mT, where the non-linear resistivity becomes dominant at higher current levels. The anomaly lies at considerably higher temperature than the flux-line-lattice melting (FLLM) transition temperature, T M. Since the Lorentz force is not active on the vortex line in this case, the non-linear resistivity is attributed to the non-linear Josephson coupling effect between pancake vortices. The sudden drop in ρc at T D is also understood by considering the sharp development of the long range phase correlation along the c-axis. This interpretation strongly suggests that the observed resistivity drop may represent the first order decoupling phase transition.

Dc-magnetization and ac-susceptibility measurements have been performed on new quarternary intermetallic compound YNi2B2C single crystals grown by the floating zone method. In the magnetization process, a pronounced peak effect was observed, which appears just below the upper critical field H c2 in the reversible region. It turns out that the appearance of the peak effect depends on the crystallinity and the size of the grains in the sample. This effect suggests that the peak effect in YNi2B2C may not be intrinsic origin of the nature of superconductivity, but may be rather related to the extrinsic origin such as the Pippard-mechanism[1], which has been thought to be responsible for the peak effect often observed in many conventional type II superconductors just below H c2. Nevertheless, the sharp appearance(disappearance) is reminiscent of the first order phase transition similar to the recently observed anomalous peak effect in CeRu2, UPt3 and UPd2Al3. The magnetic phase diagram of YNi2B2C is established and is argued in comparison with the recent theoretical work by Tachiki et al. [2].

We have measured the microwave surface resistance (30~60GHz) for different crystallographic orientations in the vortex state of high quality Bi2Sr2CaCu2O8+δ single crystals. A sharp magnetoabsorption resonance is observed below T c when ac electric fields and magnetic fields are applied parallel to the c-axix of Bi2Sr2CaCu2O8+δ (E ac ‖B ‖ c). We argue that the resonance arises from the collective Josephson plasma oscillations generated by interlayer Josephson currents. This resonance provides the quantitative information of the magnitude of quantum phase coherence between each CuO2 planes. From the frequency and temperature depencence of the resonance, we discuss the phase coherence in the vortex liquid and solid states.

Magnetism and superconductivity have been studied in the new family of quaternary boro-carbide compounds HoNi2B2C, which was recently successfully grown in a large single crystal form with dimensions of 6 mm in diameter and of over 100 mm in length. Antiferromagnetic order at 5.2 K was observed in the superconducting state, which appears below 6.5 K with a sharp transition, exhibiting the coexistence of antiferromagnetism with superconductivity. In order to study the interplay between superconductivity and magnetism, the magnetization process in magnetic fields parallel to the a- and c-axes have been measured. The single crystal growth mechanism of HoNi2B2C is also discussed in comparison with the one in YNi2B2C.

A metastable superconducting phase of NbB 2 with the transition temperature ( T c ) around 9 K has been found in this study, although no confirmed superconductivity in the stoichiometric NbB 2 has been reported since 1970s. This phase was induced by a rapid-quenching process and disappeared by annealing at 1100 °C. Since the superconducting transition was not sharp, the rapidly quenched NbB 2 might contain some phases with different superconducting transition temperatures ( T c s). Niobium-deficient and -rich diborides of Nb x B 2 ( x =0.67–1.11) were also prepared for the investigation of the compositional dependence of their T c s.

Magnetic phase diagram of Josephson vortices in strongly anisotropic superconductor Bi 2 Sr 2 CaCu 2 O 8+δ has been studied by measuring the Josephson-vortex flow resistance. A triangular-lattice phase, in which periodic oscillations of the flow resistance have been observed, was found to start from about 6 kOe, independent of temperature. Although the triangular lattice phase exists over 7 T below 30 K, at higher temperatures the boundary to the others (glass phase) decreases with increasing temperature, and drops suddenly at 80 K. The flow resistance also decreases close to the boundary. These facts indicate that the thermal excitation of pancake vortex–antivortex pairs or missing a long-range order in the triangular lattice along the c -axis causes the reduction of the flow resistance. Furthermore, above the triangular lattice phase there finds some change in the flow resistance, which suggests the existence of a different kind of glass phases in the Josephson-vortex system.

Recently, we have found periodic oscillations of Josephson-vortex flow resistance as a function of in-plane field in Bi 2 Sr 2 CaCu 2 O 8+ y single crystals [Phys. Rev. Lett. 89 (2002) 247002]. This phenomenon was interpreted to arise from a matching effect between the lattice spacing of Josephson vortices and the width of samples. To clarify further for this explanation, and study the effects of the sample boundaries on the Josephson-vortex flow, we have measured the in-plane anisotropy of Josephson-vortex flow resistance. When the field orientation is parallel to boundaries, the flow resistance is strongly suppressed, and the periodic oscillations are clearly observed. This suggests that the effects of sample boundaries are important for the Josephson-vortex flow, especially at the regime of low currents.