Temperature dependence of the magnetic penetration depth in the vortex state of the pyrochlore superconductor,Cd2Re2O7

We report transverse-field and zero-field muon spin rotation and relaxation studies of the superconducting rhenium oxide pyrochlore, Cd2Re2O7. Transverse-field measurements (H=0.007 T) show line broadening below T(c), which is characteristic of a vortex state, demonstrating conclusively the type-II nature of this superconductor. The penetration depth is seen to level off below about 400 mK (T/T(c) approximately 0.4), with a rather large value of lambda(T=0) approximately 7500 A. The temperature independent behavior below approximately 400 mK is consistent with a nodeless superconducting energy gap. Zero-field measurements indicate no static magnetic fields developing below the transition temperature.     

Spin Triplet Superconducting State due to Broken Inversion Symmetry in Li(2)Pt(3)B

We report (11)B and (195)Pt NMR measurements in noncentrosymmetric superconductor Li(2)Pt(3)B. We find that the spin susceptibility measured by the Knight shift remains unchanged across the superconducting transition temperature T(c). With decreasing temperature (T) below T(c), the spin-lattice relaxation rate 1/T(1) decreases with no coherence peak and is in proportion to T3. These results indicate that the Cooper pair is in the spin-triplet state and that there exist line nodes in the superconducting gap function. They are in sharp contrast to those in the isostructural Li(2)Pd(3)B which is a spin-singlet, s-wave superconductor, and are ascribed to the enhanced spin-orbit coupling due to the lack of spatial inversion symmetry. Our finding points to a new paradigm where exotic superconductivity arises in the absence of electron-electron correlations.     

Effect of pair breaking on mesoscopic persistent currents well above the superconducting transition temperature

We consider the mesoscopic normal persistent current (PC) in a very low-temperature superconductor with a bare transition temperature T_0(c) much smaller than the Thouless energy E(c). We show that in a rather broad range of pair-breaking strength, T_0(c) < or = Planck's/tau(s)< or =E(c), the transition temperature is renormalized to zero, but the PC is hardly affected. This may provide an explanation for the magnitude of the average PC's in the noble metals, as well as a way to determine their T_0(c)'s.     

Specific-heat evidence for two-gap superconductivity in the ternary-iron silicide Lu2Fe3Si5

We report low-temperature specific-heat studies on the single-crystalline ternary-iron silicide superconductor Lu(2)Fe(3)Si(5) with T(c)=6.1 K down to approximately T(c)/20. We confirm a reduced normalized jump in specific heat at T(c), and find that the specific heat divided by temperature C/T shows a sudden drop at approximately T(c)/5 and goes to zero with further decreasing temperature. These results indicate the presence of two distinct superconducting gaps in Lu(2)Fe(3)Si(5), similar to the typical two-gap superconductor MgB(2). We also report Hall coefficients, band structure calculations, and the anisotropy of upper critical fields for Lu(2)Fe(3)Si(5), which support the anisotropic multiband nature and reinforce the existence of two superconducting gaps in Lu(2)Fe(3)Si(5).     

Extremely high upper critical magnetic field of the noncentrosymmetric heavy fermion superconductor CeRhSi3

We report an extremely high upper critical field B(c2) in the noncentrosymmetric heavy fermion superconductor CeRhSi3 for a magnetic field B along the tetragonal c axis. B(c2)(T=0) possibly reaching 30 T is much higher than B(c2)(0)=7 T for B perpendicular c and greatly exceeds the paramagnetic limiting field. The strong anisotropy of B(c2)(0) with extremely high B(c2)(0) for B || c is qualitatively explained by the paramagnetic pair-breaking mechanism specific to the noncentrosymmetric superconductor. However, an unusual B(c2)(T) curve with a positive curvature for B || c cannot be explained satisfactorily by conventional orbital pair-breaking models.     

Superconductivity induced by longitudinal ferromagnetic fluctuations in UCoGe

From detailed angle-resolved NMR and Meissner measurements on a ferromagnetic (FM) superconductor UCoGe (T(Curie)～2.5 K and T(SC)～0.6 K), we show that superconductivity in UCoGe is tightly coupled with longitudinal FM spin fluctuations along the c axis. We found that magnetic fields along the c axis (H∥c) strongly suppress the FM fluctuations and that the superconductivity is observed in the limited magnetic-field region where the longitudinal FM spin fluctuations are active. These results, combined with model calculations, strongly suggest that the longitudinal FM spin fluctuations tuned by H∥c induce the unique spin-triplet superconductivity in UCoGe. This is the first clear example that FM fluctuations are intimately related with superconductivity.     

Superconducting fluctuations and the pseudogap in the slightly overdoped high- T(c) superconductor TlSr2CaCu2O6.8: high magnetic field NMR studies

From measurements of the 63Cu Knight shift ( K) and the nuclear spin-lattice relaxation rate ( 1/T1) under magnetic fields from zero up to 28 T in the slightly overdoped high- T(c) superconductor TlSr2CaCu2O6.8 ( T(c) = 68 K), we find that the pseudogap behavior, i.e., the reductions of 1/T1T and K above T(c) from the values expected from the normal state at high T, is strongly field dependent and follows a scaling relation. We show that this scaling is consistent with the effects of the Cooper pair density fluctuations. The present finding contrasts sharply with the pseudogap property reported previously in the underdoped regime where no field effect was seen up to 23.2 T. The implications are discussed.     

Boron isotope effect in superconducting MgB2

We report the preparation method of and boron isotope effect for MgB2, a new binary intermetallic superconductor with a remarkably high superconducting transition temperature T(c)(10B) = 40.2 K. Measurements of both temperature dependent magnetization and specific heat reveal a 1.0 K shift in T(c) between Mg11B2 and Mg10B2. Whereas such a high transition temperature might imply exotic coupling mechanisms, the boron isotope effect in MgB2 is consistent with the material being a phonon-mediated BCS superconductor.     

Mechanism of the size dependence of the superconducting transition of nanostructured Nb

A direct measurement of the superconducting energy gap by point contact spectroscopy in nanostructured Nb films shows that the gap decreases with a reduction in the average particle size. The superconducting T(c), obtained from transport and magnetic measurements, also decreases with size and scales with the energy gap. The size dependence of the superconducting properties in this intermediate coupling type II superconductor is therefore governed by changes in the electronic density of states rather than by phonon softening. Consistent with the Anderson criterion, no T(c) was observed for sizes below 8 nm.     

Disorder driven melting of the vortex line lattice

The 3D XY model with random in-plane couplings is simulated to model the phase diagram of a disordered type II superconductor as a function of temperature T and randomness strength p for fixed applied magnetic field. As p increases to a critical p(c), the first order vortex lattice melting line turns parallel to the T axis, continuing down to low temperatures, rather than ending at a critical point. Above p(c) preliminary results suggest the absence of a phase coherent vortex glass.     

Quantum and classical mode softening near the charge-density-wave-superconductor transition of CuxTiSe2

Temperature- and x-dependent Raman scattering studies of the charge-density-wave (CDW) amplitude modes in Cu(x)TiSe(2) show that the amplitude mode frequency omega(0) exhibits identical power-law scaling with the reduced temperature T/T(CDW) and the reduced Cu content x/x(c), i.e., omega(0) approximately (1-p)(0.15) for p=T/T(CDW) or x/x(c), suggesting that mode softening is independent of the control parameter used to approach the CDW transition. We provide evidence that x-dependent mode softening in Cu(x)TiSe(2) is associated with the reduction of the electron-phonon coupling constant, and that x-dependent "quantum" (T approximately 0) mode softening suggests the presence of a quantum critical point within the superconductor phase of Cu(x)TiSe(2).     

高温超导体的结构组装

 在总结铜氧化物高温超导体晶体结构特点的基础上，提出了以无限层结构Cu-O钙钛矿为核心的结构组装概念，通过和相关结构的外延组合，衍生出了高温超导体的主要晶体构型。进一步运用此观念，建立了新的三元数命名法，并对高温超导体的晶体结构进行了归类。     

Gapless magnetic and quasiparticle excitations due to the coexistence of antiferromagnetism and superconductivity in CeRhIn5: a study of 115In NQR under pressure

We report systematic measurements of ac susceptibility, nuclear-quadrupole-resonance spectrum, and nuclear-spin-lattice-relaxation time (T1) on the pressure (P)-induced heavy-fermion superconductor CeRhIn5. The temperature (T) dependence of 1/T(1) at P=1.6 GPa has revealed that antiferromagnetism (AFM) and superconductivity (SC) coexist microscopically, exhibiting the respective transition at T(N)=2.8 K and T(MF)(c)=0.9 K. It is demonstrated that SC does not yield any trace of gap opening in low-lying excitations below T(onset)(c)=2 K, but T(MF)(c)=0.9 K, followed by a T(1)T=const law. These results point to the unconventional characteristics of SC coexisting with AFM. We highlight that both of the results deserve theoretical work on the gapless nature in the low-lying excitation spectrum due to the coexistence of AFM and SC and the lack of the mean-field regime below T(onset)(c)=2 K.     

Differential sum rule for the relaxation rate in the cuprates

Motivated by recent experiments by Basov et al., we study the differential sum rule for the effective scattering rate 1/tau(omega). We show that, in a dirty BCS superconductor, the area under 1/tau(omega) does not change between the normal and the superconducting states. For magnetically mediated pairing, a similar result holds between Tor=T(c), while, in the pseudogap phase, 1/tau(omega) is just suppressed compared to 1/tau(omega) in the normal state. We argue that this violation of the differential sum rule in the pseudogap phase is due to the absence of the feedback effects from the pairing.     

Field dependence of the ground state in the exotic superconductor CeCoIn5: a nuclear magnetic resonance investigation

We report 115In nuclear magnetic resonance (NMR) measurements in CeCoIn5 at low temperature (T approximately 70 mK) as a function of the magnetic field (H0) from 2 to 13.5 T applied perpendicular to the c axis. A NMR line shift reveals that below 10 T the spin susceptibility increases as sqrt[H0]. We associate this with an increase of the density of states due to the Zeeman and Doppler-shifted quasiparticles extended outside the vortex cores in a d-wave superconductor. Above 10 T a new superconducting state is stabilized, possibly the modulated phase predicted by Fulde, Ferrell, Larkin, and Ovchinnikov. This phase is clearly identified by a strong and linear increase of the NMR shift with the field, before a jump at the first order transition to the normal state.     

Mott insulator to high Tc superconductor via pressure: resonating valence bond theory and prediction of new systems

Mott insulator superconductor transition, via pressure and no external doping, is studied in orbitally nondegenerate spin-1 / 2 systems. It is presented as another resonating valence bond route to high T(c) superconductivity. We propose a "strong coupling" hypothesis that views long range Coulomb force driven first order Mott transition as a self-doping process that also preserves superexchange on the metal side. We present a two-species t-J model where conserved N0 doubly occupied (e(-)) sites and N0 empty sites (e(+)) hop in the background of N-2N(0) singly occupied (neutral) sites in a lattice of N sites. An equivalence to the regular t-J model is made. Some old and new systems are predicted to be candidates for pressure-induced high T(c) superconductivity.     

Electric-field-effect modulation of the transition temperature, mobile carrier density, and in-plane penetration depth of NdBa2Cu3O7-delta thin films

We explore the relationship between the critical temperature T(c), the mobile areal carrier density n(2D), and the zero-temperature magnetic in-plane penetration depth lambda(ab)(0) in very thin underdoped NdBa(2)Cu(3)O(7-delta) films near the superconductor to insulator transition using the electric-field-effect technique. Having established consistency with a Kosterlitz-Thouless transition, we observe that T(KT) depends linearly on n(2D), the signature of a quantum superconductor to insulator transition in two dimensions with znu(over)=1, where z is the dynamic and nu is the critical exponent of the in-plane correlation length.     

77Se NMR study of the pairing symmetry and the spin dynamics in K(y)Fe(2-x)Se2

We present a 77Se NMR study of the newly discovered iron selenide superconductor K(y)Fe(2-x)Se2, in which T(c) = 32 K. Below T(c), the Knight shift 77K drops sharply with temperature, providing strong evidence for singlet pairing. Above T(c), Korringa-type relaxation indicates Fermi-liquid behavior. Our experimental results set strict constraints on the nature of possible theories for the mechanism of high-T(c) superconductivity in this iron selenide system.     

Subgap density of states in superconductor-normal metal bilayers in the cooper limit

We present transport and tunneling measurements of Pb-Ag bilayers with thicknesses, d(Pb) and d(Ag), that are much less than the superconducting coherence length. The transition temperature, T(c), and energy gap, Delta, in the tunneling density of states (DOS) decrease exponentially with d(Ag) at fixed d(Pb). Simultaneously, a DOS that increases linearly from the Fermi energy grows and introduces states within the gap. The integrated subgap DOS approaches 40% of the normal state value in the lowest T(c) film investigated (T(c) approximately 0.1 T(Pb)(c,bulk)). This behavior suggests that a growing fraction of quasiparticles decouple from the superconductor as T(c)-->0. The linear dependence is consistent with the quasiparticles becoming trapped on integrable trajectories in the metal layer.     

Uniform mixing of antiferromagnetism and high-temperature superconductivity in electron-doped layers of four-layered Ba(2)Ca(3)Cu(4)O(8)F(2): a new phenomenon in an electron underdoped regime

We report (63,65)Cu- and (19)F-NMR studies on a four-layered high-temperature superconductor Ba(2)Ca(3)Cu(4)O(8)F(2)((0234F(2.0)) with apical fluorine (F(-1)), an undoped 55 K superconductor with a nominal Cu(2+) valence on average. We reveal that this compound exhibits the antiferromagnetism (AFM) with a Néel temperature T(N)=100 K despite being a T(c)=55 K superconductor. Through a comparison with a related trilayered cuprate Ba(2)Ca(3)Cu(4)O(8)F(2)(0233F(2.0)), it is demonstrated that electrons are transferred from the inner plane (IP) to the outer plane (OP) in 0234F(2.0) and 0223F(2.0), confirming the self-doped high-temperature superconductivity (HTSC) having electron and hole doping in a single compound. Remarkably, uniform mixing of AFM and HTSC takes place in both the electron-doped OPs and the hole-doped IPs in 0234F(2.0).