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ABSTRACT: In strongly anisotropic layered superconductors in tilted magnetic fields the
Josephson vortex lattice coexists with the lattice of pancake vortices. Due to
the interaction between them, the dissipation of the Josephson-vortex lattice
occurs to be very sensitive to the presence of the pancake vortices. If the
c-axis magnetic field is smaller then the corresponding lower critical field,
the pancake stacks are not formed but the individual pancakes may exist in the
fluctuational regime either near surface in large-size samples or in the
central region for small-size mesas. We calculate the contribution of such
fluctuating pancake vortices to the c-axis conductivity of the Josephson vortex
lattice and compare the theoretical results with measurements on small mesas
fabricated out of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ crystals. A
fingerprint of fluctuating pancakes is characteristic exponential dependence of
the c-axis conductivity observed experimentally. Our results provide strong
evidence of the existence of the fluctuating pancakes and their influence on
the Josephson-vortex-lattice dissipation.
04/2011;
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ABSTRACT: We report the systematic investigations of the oscillation of the Josephson vortex (JV) flow resistance in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ micro-fabricated junctions with various geometries and superconducting anisotropy parameters. As the applied magnetic field parallel to the $ab$-plane is increased, oscillation with a period corresponding to a $\phi_0/2$ par atomic Josephson junction changes to oscillation with a doubled period. This crossover is scaled by both the junction length and the anisotropy parameter, indicating that the bulk inductive coupling that favors the triangular JV lattice is replaced with the surface deformation energy as the dominant interaction for a JV lattice. These results suggest that the in-phase square JV lattice is pronounced at a higher magnetic field in a smaller and more anisotropic sample.
05/2009;
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ABSTRACT: The Josephson plasma resonance has been observed in a microwave frequency at 35 GHz in magnetic fields up to 6 T in the high-temperature superconductor Bi2Sr2CaCu2O8 + δ. Making use of the different dispersion relations between two Josephson plasma modes predicted by the recent theories, the longitudinal mode, which is the Nambu-Goldstone mode in a superconductor, is separated out from the transverse one experimentally. This experimental result directly proves the existence of the Nambu-Goldstone mode in a superconductor with a finite energy gap ωp = cλc/√. Such a finite energy gap implies the mass of the Nambu-Goldstone bosons in a superconductor, supporting the mass formation mechanism proposed by Anderson.
EPL (Europhysics Letters) 01/2007; 42(2):203. · 2.17 Impact Factor
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ABSTRACT: We have studied the dynamical nature of Josephson vortices in an intrinsic Josephson junction by the current‐voltage characteristics along the c‐axis of Bi2Sr2CaCu2O8+δ. We found periodic current steps with respect to the voltage which resembles the Fiske resonance observed in a single junction. We attributed this phenomenon to the resonance between JV and the Josephson plasma. © 2006 American Institute of Physics
AIP Conference Proceedings. 09/2006; 850(1):915-916.
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ABSTRACT: The vortex phases in the high‐quality Bi2Sr2CaCu2O8+δ single crystal have been studied by means of the local ac‐magnetic permeability measurements by using the miniature coils. The structural transition has been indicated in the vortex solid phase, where the linear dependence of the 1st order vortex melting phase transition Hcm(Hab), a fingerprint of the crossing lattice, sharply changes its character, and separates the strong pinning pancake‐vortex phase from the weak pinning phase. The phase transition from the crossing lattice to the tilted vortex lattice near the ab‐plane is suggested. © 2006 American Institute of Physics
AIP Conference Proceedings. 09/2006; 850(1):801-802.
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ABSTRACT: Josephson plasma excitations in the high $T_c$ superconductor Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ have been investigated in a wide microwave frequency region (9.8 -- 75 GHz), in particular, in magnetic field applied parallel to the $ab$ plane of the single crystal. In sharp contrast to the case for magnetic fields parallel to the c axis or tilted from the $ab$ plane, it was found that there are two kinds of resonance modes, which are split in energy and possess two distinctly different magnetic field dependences. One always lies higher in energy than the other and has a shallow minimum at about 0.8 kOe, then increases linearly with magnetic field. On the other hand, another mode begins to appear only in a magnetic field (from a few kOe and higher) and has a weakly decreasing tendency with increasing magnetic field. By comparing with a recent theoretical model the higher energy mode can naturally be attributed to the Josephson plasma resonance mode propagating along the primitive reciprocal lattice vector of the Josephson vortex lattice, whereas the lower frequency mode is assigned to the novel phase collective mode of the Josephson vortex lattice, which has never been observed before.
04/2005;
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ABSTRACT: Dynamical nature of the Josephson vortex (JV) system in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ (Bi2212) has been investigated in the presence of the c-axis current with magnetic field alignments very close to the $ab$-plane. As a function of magnetic fields, the c-axis JV flux flow resistance oscillates periodically in accordance with the proposed JV triangular structure. We observe that this oscillating period becomes doubled above a certain field, indicating the structure transition from triangle to square structure. This transition field becomes lower in junctions with smaller width perpendicular to the external field. We interpret that this phenomena as the effect of the edge deformation of the JV lattice due to surface current of intrinsic Josephson junctions as pointed by Koshelev.
04/2005;
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ABSTRACT: We have investigated the Josephson plasma excitations in magnetic fields parallel to the $ab$-plane in Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ single crystals in a wide microwave frequency region (9.8 -- 75 GHz). It was found that there are two kind of phase-collective modes: one increases with magnetic fields in higher fields, approaching linear asymptotic dependence with $ck$, which lies well above the inherent plasma frequency $\omega_p$, while the other does not show considerable field dependence. The higher linear mode is attributed to the Josephson plasma mode propagating along the reciprocal lattice vector of the Josephson vortex, whereas the lower one can be ascribed to the oscillation mode of Josephson vortices.
11/2002;
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ABSTRACT: We synthesized single crystalline and policrystalline MgB_2 under ambient pressures. The single crystals of MgB_2 were of good quality, where the crystal structure refinements were successfully converged with R = 0.020. The specific heat of policrystalline MgB_2 samples has been measured in a temperature range between 2 and 60 K in magnetic field up to 6 T. The measurement gave the coefficient of the linear term in the electronic specific heat, gamma = 3.51 mJ/K^2 mol, and the jump of the specific heat, 2.8 mJ/K^2 mol at 38.5 K. It is shown from the analysis of the specific heat that the electronic specific heat in the superconducting state differs largely from the conventional BCS weak coupling theory. From the results of measurements of the magnetic properties on single crystal samples, we found a sharp superconducting transition at 38 K with transition width Delta Tc = 0.8 K and the superconducting anisotropy ratio gamma increasing from about 1 near Tc to 4.0 at 25 K. Comment: 6 pages, 10 figures
07/2002;
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ABSTRACT: Vortex matter phases and phase transitions are investigated by means of Josephson plasma resonance in under-doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$ single crystals in a microwave frequency range between 19 and 70 GHz. Accompanied by the vortex lattice melting transition, a jump of the interlayer phase coherence extracted from the field dependence of the plasma frequency was observed. In the solid phase, the interlayer coherence little depends on field at a temperature region well below $T_c$ while it gradually decreases as field increases toward the melting line up to just below $T_c$. As a result, the magnitude of the jump decreases with increasing temperature and is gradually lost in the vicinity of $T_c$. This indicates that the vortex lines formed in the vortex solid phase are thermally meandering and the phase transition becomes weak especially just below $T_c$. Comment: 5pages and 4 figures. Submitted to Physica C (Proceedings of Plasma2000, Sendai)
09/2000;
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ABSTRACT: Josephson plasma resonance measurements under magnetic fields parallel to the CuO_2 layers as functions of magnetic field, temperature, and microwave frequency have been performed in Bi_2Sr_2CaCu_2O_{8+\delta} single crystals with doping range being from optimal to under-doped side. The feature of the resonance is quite unique and cannot be explained by the conventional understandings of the Josephson plasma for H \parallel c, that requires a new theory including coupling effect between Josephson vortex lattice and Josephson plasma.
07/2000;
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ABSTRACT: Electromagnetic resonant absorption phenomena in a microwave frequency range have recently been studied in the single-crystalline high-temperature superconductor Bi2Sr2CaCu2O8+δ in magnetic fields. Using a rectangular microwave cavity resonator technique with TE102 mode this absorption is unambiguously identified to be the collective longitudinal Josephson plasma excitation. Since the superconducting state is a state with a broken phase symmetry, the ordered state should be accompanied by the collective excitation (Nambu-Goldstone mode). This excitation has long been thought not to be observable, because of the formation of the large Coulomb gap (Anderson-Higgs-Kibble mechanism), above which strong damping mechanisms of excited plasma are presumably present. However, this Coulomb gap can be very small in the case of anisotropic layered system such as Bi2Sr2CaCu2O8+δ, and the plasma mode may lie in a microwave frequency region. Using characteristic dispersion relations, which enables us to separate out the longitudinal mode from the transverse one, the microwave absorption observed in Bi2Sr2CaCu2O8+δ is unambiguously attributed to the longitudinal excitations. We believe that this is the first and the direct experimental evidence of the Nambu-Goldstone mode in a superconductor and provides a direct proof that the Anderson-Higgs-Kibble (AHK) mechanism within the concept of spontaneously breaking symmetry is indeed valid in the case of superconducting phase transition. Since the finite plasma frequency observed here signifies formation of the finite mass of the plasma (phason) due to the AHK mechanism in the relativistic sense, the above-mentioned scenario in a superconductor corresponds to a direct mapping from the unified gauge-field theory of weak interaction and electromagnetic interaction shown by Weinberg and Salam.
Phys. Rev. B. 08/1997; 56(9).
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ABSTRACT: Electromagnetic resonant absorption phenomena in a microwave frequency range have recently been studied in the single-crystalline high-temperature superconductor Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}} in magnetic fields. Using a rectangular microwave cavity resonator technique with TE{sub 102} mode this absorption is unambiguously identified to be the collective longitudinal Josephson plasma excitation. Since the superconducting state is a state with a broken phase symmetry, the ordered state should be accompanied by the collective excitation (Nambu-Goldstone mode). This excitation has long been thought not to be observable, because of the formation of the large Coulomb gap (Anderson-Higgs-Kibble mechanism), above which strong damping mechanisms of excited plasma are presumably present. However, this Coulomb gap can be very small in the case of anisotropic layered system such as Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}}, and the plasma mode may lie in a microwave frequency region. Using characteristic dispersion relations, which enables us to separate out the longitudinal mode from the transverse one, the microwave absorption observed in Bi{sub 2}Sr{sub 2}CaCu{sub 2}O{sub 8+{delta}} is unambiguously attributed to the longitudinal excitations. We believe that this is the first and the direct experimental evidence of the Nambu-Goldstone mode in a superconductor and provides a direct proof that the Anderson-Higgs-Kibble (AHK) mechanism within the concept of spontaneously breaking symmetry is indeed valid in the case of superconducting phase transition. Since the finite plasma frequency observed here signifies formation of the finite mass of the plasma (phason) due to the AHK mechanism in the relativistic sense, the above-mentioned scenario in a superconductor corresponds to a direct mapping from the unified gauge-field theory of weak interaction and electromagnetic interaction shown by Weinberg and Salam. {copyright} 1997 The American Physical Society
Physical review. B, Condensed matter 08/1997; 56(9).
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ABSTRACT: Electromagnetic resonant absorption phenomena in highly anisotropic superconductor Bi2Sr2CaCu2O8 has been studied at a microwave frequency region of 35 GHz. The sharp resonance observed as a function of magnetic field can be interpreted as a plasma resonance of superconducting electrons. Making use of the difference in the dispersion relation of the two plasma modes, which gives rise to the clear sample size dependence, we are able to distinguish the longitudinal mode from the transverse one in a cavity resonator with TE102 mode. Since the longitudinal plasma mode is the Nambu-Goldstone (NG) mode in a superconductor in the context of the spontaneous symmetry breaking of the gauge field, the experimental observation of the longitudinal plasma mode clearly verifies for the first time the existence of the NG bosons in a superconductor. The finite plasma gap can be interpreted by the possession of mass of NG bosons in accordance with the Anderson's conjecture.
Physica C Superconductivity · 1.01 Impact Factor
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ABSTRACT: Polycrystalline single phase RuSr2RCu2O8 (R=Sm, Eu, Gd) samples have been synthesized by a new solid state reaction. We found superconductivity in two rare earths R=Sm and Eu with Ts(R=0)=12 and 17 K, whereas ferromagnetism occurs at Tc=146 and 139 K in these samples, respectively, in addition to the previously reported RuSr2GdCu2O8 with Ts=36 K and Tc=136 K. Huge resistivity broadening phenomena common to the high Tc superconductors were observed in magnetic fields, which suggest that strong enhancement of superconducting fluctuations due to magnetic field, suppresses the occurrence of superconducting phase coherence in these compounds.
Physica C Superconductivity · 1.01 Impact Factor
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ABSTRACT: The c-axis resistivity measurements were performed in the vicinity of the ab-plane in order to investigate the interaction between Josephson vortices and pancake vortices in Bi2Sr2CaCu2O8+δ mesoscopic single crystals. It was found that the angular dependence of the c-axis resistivity drastically changes in high magnetic field regime. The vortex lock-in transition becomes considerably broad in high magnetic fields, while the angular dependence of resistance exhibits the sharp lock-in features in low magnetic field region.
Physica C Superconductivity 469:1119-1121. · 1.01 Impact Factor
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ABSTRACT: FeSr2YCu2O6+δ exhibits superconductivity around 50 K, when it is properly annealed in N2 atmosphere and subsequently in O2 atmosphere under ambient and high pressure. Neutron powder diffraction study shows that N2-annealing causes ordering of Cu and Fe, and that O2-annealing and oxidizing under high pressure supply the charge on the CuO2 sheets. Recently, we have found a superstructure in the sample N2-annealed within a particular temperature range. Electron diffraction study indicates that it has an orthorhombic structure with the unit cell of as compared with the tetragonal FeSr2YCu2O6+δ structure with the unit cell of a×a×c. Neutron powder diffraction study indicates that it has the CoSr2YCu2O7-type structure with the FeO4 tetrahedron, due to not only ordering of Cu and Fe but also oxygen ordering on the FeOδ sheet, which is consistent with the electron diffraction study. We have also obtained the similar superstructure in FeSr2ErCu2O6+δ, but the situation is different in FeSr2NdCu2O6+δ which does not exhibit superconductivity even after O2-annealing and oxidizing. Atomic ordering in the FeSr2YCu2O6+δ system is influenced by the size of the ionic radius in the Y site, which is effective on the occurrence of superconductivity.
Physica C Superconductivity 400:43-52. · 1.01 Impact Factor
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K. Kadowaki,
H. Yamaguchi,
K. Kawamata,
T. Yamamoto,
H. Minami, I. Kakeya,
U. Welp,
L. Ozyuzer,
A. Koshelev,
C. Kurter,
K.E. Gray,
W.-K. Kwok
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ABSTRACT: We have observed intense, coherent, continuous and monochromatic electromagnetic (EM) emission at terahertz frequencies generated from a single crystalline mesa structure of the high-Tc superconductor Bi2Sr2CaCu2O8+δ intrinsic Josephson junction system. The mesa is fabricated by the Argon-ion-milling and photolithography techniques on the cleaved surface of Bi2Sr2CaCu2O8+δ single crystal. The frequency, ν, of the EM radiation observed from the sample obeys simple relations: ν = c/nλ = c/2nw and ν = 2eV/hN, where c is the light velocity in vacuum, n the refractive index of a superconductor, λ the wave length of the EM emission in vacuum, w the shorter width of the mesa, V the voltage applied to the mesa, N the number of layers of intrinsic Josephson junctions, e and h are the elementary charge and the Planck constant, respectively. These two relations strongly imply that the mechanism of the emission is, firstly, due to the geometrical resonance of EM waves to the mesa like a cavity resonance occuring in the mesa structure, and forming standing waves as cavity resonance modes, and secondly, due to the ac-Josephson effect, which works coherently in all intrinsic Josephson junctions. The peculiar temperature dependence of the power intensity emitted form samples shows a broad maximum in a temperature region between 20 and 40 K, suggesting that the nonequilibrium effect plays an essential role for the emission of EM waves in this system. The estimated total power is significantly improved in comparison with the previous report [L. Ozyuzer et al., Science 318 (2007) 1291, K. Kadowaki, et al., Physica C 437–438 (2006) 111, I.E. Batov, et al., Appl. Phys. Lett. 88 (2006) 262504], and reached as high as 5 μW from single mesa with w = 60 μm at 648 GHz, which enables us to use it for some of applications. So far, we succeeded in fabricating the mesa emitting EM waves up to 960 GHz in the fundamental mode in the w = 40 μm mesa, whereas the higher harmonics up to the 4-th order were observed, resulting in a frequency exceeding 2.5 THz. In sharp contrast to the previous reports [K. Kadowaki, et al., Physica C 437–438 (2006) 111 , M.-H. Bae, et al., Phys. Rev. Lett. 98, (2007) 027002], all the present measurements were done in zero magnetic field. Lastly, a plausible theoretical model for the mechanism of emission is discussed.
Physica C: Superconductivity.
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ABSTRACT: Dynamical nature of Josephson vortices confined within a micron sized mesoscopic intrinsic Josephson junction made of single crystalline Bi2Sr2CaCu2O8+δ has been investigated by dc-resistivity and I–V characteristics measurements of the junction. Two types of periodic oscillations of the dc-resistivity of the junction as a function of applied magnetic field H has been confirmed as observed previously, but interpreted differently, i.e., by the two purely quantum mechanical effects: one is the interference effect which gives a period of H0 = ϕ0/sL and the other is the diffraction effect which gives the same frequency of ϕ0/sL but phase shifted by ϕ0/2sL, where s is the interlayer distance, L the width of the junction perpendicular to the magnetic field and ϕ0 the quantum magnetic flux. The characteristic features of the oscillation are summarized based on the experimental findings. At higher currents, where such an oscillating behavior diminishes, we succeeded in directly observing electromagnetic radiation emitted from the intrinsic Josephson junction system towards outgoing direction of the vortices from the sample. The integrated power detected by the bolometric detector is estimated to be of the order of 100 W/cm2. The condition for radiation is discussed.
Physica C: Superconductivity.
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ABSTRACT: We synthesized single crystalline MgB2 under ambient pressure by using conventional materials and equipment. The single crystals of MgB2 were of good quality, where the crystal structure refinements were successfully converged with R=0.020. The measurements of the magnetic properties yielded a sharp superconducting transition at 38 K with transition width ΔTc=0.8 K. The upper critical field for applied field parallel to the ab plane (Hc2ab) reveals a positive curvature, while Hc2 parallel to the c axis (Hc2c) increases linearly in temperature dependence, which yields a temperature dependence of the superconducting anisotropy ratio of γ=Hc2ab/Hc2c with γ∼1 near Tc and 4.0 at 25 K.
