Nodal d + id pairing and topological phases on the triangular lattice of Na(x)CoO(2).yH(2)O: evidence for an unconventional superconducting state

We show that finite angular momentum pairing chiral superconductors on the triangular lattice have point zeroes in the complex gap function. A topological quantum phase transition takes place through a nodal superconducting state at a specific carrier density x(c) where the normal state Fermi surface crosses the isolated zeros. For spin-singlet pairing, we show that the second-nearest-neighbor (d+id)-wave pairing can be the dominant pairing channel. The gapless critical state at x (c) approximately 0.25 has six Dirac points and is topologically nontrivial with a T3 spin relaxation rate below T(c). This picture provides a possible explanation for the unconventional superconducting state of Na(x)Co O(2). yH(2)O. Analyzing a pairing model with strong correlation using the Gutzwiller projection and symmetry arguments, we study these topological phases and phase transitions as a function of Na doping.     

75As NMR studies of superconducting LaFeAsO0.9F0.1

We have performed 75As nuclear magnetic resonance measurements on aligned powders of the new LaFeAsO0.9F0.1 superconductor. In the normal state, we find a strong temperature dependence of the spin shift and Korringa behavior of the spin lattice relaxation rate. In the superconducting state, we find evidence for line nodes in the superconducting gap and spin-singlet pairing. Our measurements reveal a strong anisotropy of the spin lattice relaxation rate, which suggests that superconducting vortices contribute to the relaxation rate when the field is parallel to the c axis but not for the perpendicular direction.     

自旋涨落与非常规超导配对

铜氧化物高温超导、铁基高温超导、重费米子超导和κ-型层状有机超导等超导体的超导态都与磁性有序态相邻,且超导能隙在动量空间一般存在变号.因此,这些超导体的超导机理被认为有别于常规BCS超导中的电子交换声子导致的各向同性s-波配对.在这些非常规超导中,自旋涨落被认为是导致电子形成库珀对的主要起源之一.本文主要以铜基和铁基高...     

Effect of impurities scattering potential on NMR relaxation rate in impure d-wave superconductors

The purpose of our research is to study the nuclear spin lattice relaxation rate of impure d-wave superconductors. We use the Green’s function method to derive the approximation equation of density of states including the impurity scattering potential. We can get the analytic equation of the nuclear spin lattice relaxation rate that contained the impurity scattering potential in case of weak scattering potential and strong scattering potential in the simple form as the power series of Δ(T) and T. The numerical calculations show that there is coherence peak in the weak impurity scattering potential but there is no peak in the strong impurity scattering potential.     

Using μSR to investigate the vortex lattice in high-temperature superconductors

We review some of the properties of the vortex lattice in type II superconductors and the use of muon spin rotation (μSR) to investigate high temperature superconducting oxides. As a microscopic probe of the field distribution inside the bulk of materials, μSR is shown to be a powerful tool with which to study the magnetic properties of superconductors. We also discuss how understanding the complex phenomenology of the vortex lattice in these materials is necessary in order to correctly determine fundamental parameters of the superconducting state.     

Enhancement of superconducting transition temperature due to the strong antiferromagnetic spin fluctuations in the noncentrosymmetric heavy-fermion superconductor CeIrSi3: A 29Si NMR study under pressure

We report a (29)Si NMR study on the pressure-induced superconductivity (SC) in an antiferromagnetic (AFM) heavy-fermion compound CeIrSi(3) without inversion symmetry. In the SC state at P = 2.7-2.8 GPa, the temperature (T) dependence of the nuclear-spin lattice relaxation rate 1/T(1) below T(c) exhibits a T(3) behavior without any coherence peak just below T(c), revealing the presence of line nodes in the SC gap. In the normal state, 1/T(1) follows a square root T-like behavior, suggesting that the SC emerges under the non-Fermi-liquid state dominated by AFM spin fluctuations enhanced around a quantum critical point. The reason why the maximum T(c) in CeIrSi(3) is relatively high among the Ce-based heavy-fermion superconductors may be the existence of the strong AFM spin fluctuations. We discuss the comparison with the other Ce-based heavy-fermion superconductors.     

Probing the ground state properties of iron-based superconducting pnictides and related systems by muon-spin spectroscopy

In this short review, we attempt to give a comprehensive discussion of studies performed to date by muon-spin spectroscopy (more precisely the relaxation and rotation technique, also know as μSR) on the recently discovered layered iron-based superconductors. On one side, μSR has been used to characterize the magnetic state of different families of layered iron-based systems. Similarly the subtle interplay of the magnetic state and the structural transition present in some families has been investigated. We will also discuss the information provided by this technique on the interaction between the magnetic state and the superconducting phase. Finally the μSR technique has been used to investigate the magnetic penetration depth of the superconducting ground state. The study of its absolute value, temperature and magnetic field dependence provides crucial tests for investigating possible unconventional superconducting states in such systems.     

Pressure Studies of the Unconventional Superconductors CeTIn 5 (T: Co,Ir)

In the quest for new superconductor compounds which adopt the superconducting state at increasingly higher transition temperatures T c , a non-phonon mediated coupling between the charge carriers seems to play a key role. In order to enhance our understanding of such unconventional coupling mechanisms, we studied a new family of heavy fermion (HF) superconductors CeTIn 5 (T: transition metal) whose properties point toward the realization of unconventional superconductivity (SC): the specific heat, thermal conductivity and nuclear spin-lattice relaxation rate of CeIrIn 5 and CeCoIn 5 decrease as a power law of temperature instead of exponentially for T < T c . We report on measurements of the heat capacity of CeIrIn 5 and CeCoIn 5 at hydrostatic pressures p h 1.6 GPa. In both compounds, T c increases with increasing pressure, while the mass of the quasi-particles m eff decreases, as indicated by the ratio C / T | T c . As a working hypothesis based on theories of a nearly antiferromagnetic Fermi-liquid (NAFFL), this may be interpreted as the stabilization of the superconducting state by an increase of the characteristic spin fluctuation temperature T_{\rm SF}\ (T_{\rm SF}\propto k_{\rm F}^{2}/m_{\rm eff}).     

Novel spin dynamics in a Josephson junction

We address the dynamics of a single spin embedded in the tunneling barrier between two superconductors. As a consequence of pair correlations in the superconducting state, the spin displays a rich and unusual dynamics. To properly describe the time evolution of the spin we find the generalized Wess-Zumino-Witten-Novikov term in the effective action for the spin on the Keldysh contour. The superconducting correlations lead to an effective spin action which is nonlocal in time leading to unconventional precessions. Our predictions might be directly tested for macroscopic spin clusters.     

Feedback spin resonance in superconducting CeCu2Si2 and CeCoIn5

We show that the recently observed spin resonance modes in heavy-fermion superconductors CeCoIn5 and CeCu2Si2 are magnetic excitons originating from superconducting quasiparticles. The wave vector Q of the resonance state leads to a powerful criterion for the symmetry and node positions of the unconventional gap function. The detailed analysis of the superconducting feedback on magnetic excitations reveals that the symmetry of the superconducting gap corresponds to a singlet d_{x;{2}-y;{2}} state symmetry in both compounds. In particular this resolves the long-standing ambiguity of the gap symmetry in CeCoIn5. We demonstrate that in both superconductors the resonance peak shows a significant dispersion away from Q which can be checked experimentally. Our analysis reveals the similar origin of the resonance peaks in the two heavy-fermion superconductors and in layered cuprates.     

Coexistence of magnetic fluctuations and superconductivity in the pnictide high temperature superconductor SmFeAsO1-xFx measured by muon spin rotation

Muon spin rotation experiments were performed on the pnictide high temperature superconductor SmFeAsO1-xFx with x=0.18 and 0.3. We observed an unusual enhancement of slow spin fluctuations in the vicinity of the superconducting transition which suggests that the spin fluctuations contribute to the formation of an unconventional superconducting state. An estimate of the in-plane penetration depth lambda ab(0)=190(5) nm was obtained, which confirms that the pnictide superconductors obey an Uemura-style relationship between Tc and lambda ab(0);(-2).     

Evidence for a novel state of superconductivity in noncentrosymmetric CePt3Si: a 195Pt-NMR study

We report on novel antiferromagnetic (AFM) and superconducting (SC) properties of noncentrosymmetric CePt3Si through measurements of the 195Pt nuclear spin-lattice relaxation rate 1/T(1). In the normal state, the temperature (T) dependence of 1/T(1) unraveled the existence of low-lying levels in crystal-electric-field multiplets and the formation of a heavy-fermion (HF) state. The coexistence of AFM and SC phases that emerge at T(N)=2.2 K and T(c)=0.75 K, respectively, takes place on a microscopic level. CePt3Si is the first HF superconductor that reveals a peak in 1/T(1) just below T(c) and, additionally, does not follow the T3 law that used to be reported for most unconventional HF superconductors. We remark that this unexpected SC characteristic may be related to the lack of an inversion center in its crystal structure.     

What does NMR tell us about the electronic state of high-T c superconductors?

An introduction to the models which are usually applied, when interpreting the NMR (nuclear magnetic resonance) parameter Knight shift (K _s) and spin-lattice relaxation rate 1/T ₁ in the normal and the superconducting state of high-temperature superconductors, is given. The different hyperfine interaction parameters involved, as well as the static and dynamic susceptibility χ(q,ω) will be discussed. I will point at those highlights as antiferromagnetic correlations, spin gap and vortex lattice dynamics which have emerged from the analysis of the NMR data.     

Temperature and doping dependent flat-band superconductivity on the Lieb-lattice

We consider the superconducting properties of Lieb lattice, which produces a flat-band energy spectrum in the normal state under the strong electron–electron correlation. Firstly, we show the hole-doping dependent superconducting order amplitude with various electron–electron interaction strengths in the zero-temperature limit. Secondly, we obtain the superfluid weight and Berezinskii–Kosterlitz–Thouless(BKT) transition temperature with a lightly doping level. The large ratio between the gap-opening temperature and BKT transition temperature shows similar behavior to the pseudogap state in high-T_c superconductors. The BKT transition temperature versus doping level exhibits a dome-like shape in resemblance to the superconducting dome observed in the high-T_c superconductors. However, unlike the exponential dependence of T_c on the electron–electron interaction strength in the conventional high-T_c superconductors, the BKT transition temperature for a flat band system depends linearly on the electron–electron interaction strength. We also show the doping-dependent superconductivity on a lattice with the staggered hoping parameter in the end. Our predictions are amenable to verification in the ultracold atoms experiment and promote the understanding of the anomalous behavior of the superfluid weight in the high-T_c superconductors.     

Recent progress in NMR studies on organic conductors

Recent ¹³\mathrmC\mbox-NMR studies on a family of quasi‐two‐dimensional organic conductors based on BEDT‐TTF molecules are reviewed in the light of the role of electron–electron correlation in a variety of electronic states and of the symmetry of electron pairing in the superconducting state. Comparison of the nuclear spin‐lattice relaxation rate, conducting property and molecular arrangement indicates a close relationship between the molecular arrangement and manifestation of electron correlation. The metal/nonmetal phases in the isostructural \kappa\mbox- (BEDT\mbox-TTF)₂X systems, which are in the strong dimeric regime, are understood as highly correlated metals and insulators crossing the Mott transition. For the 10‐K superconducting phase situated near the Mott transition, the absence of Hebel–Slichter coherence peak and a low‐temperature T³-dependence of the nuclear spin‐lattice relaxation rate suggest unconventional superconductivity with line nodes in gap parameter or highly anisotropic one. This revised version was published online in July 2006 with corrections to the Cover Date.     

Relaxation of polarized nuclei in superconducting rhodium

Nuclear spin lattice relaxation rates were measured in normal and superconducting (sc) rhodium with nuclear polarizations up to p = 0. 55. This was sufficient to influence the sc state of Rh, whose T(c) and B(c) are exceptionally low. Because B(c)相似文献   

Structural tuning of unconventional superconductivity in PuMGa5 (M=Co,Rh)

The superconducting properties of the recently discovered PuMGa5 (M=Co,Rh) superconductors, including the power law behavior of the specific heat, the evolution of the superconducting transition T(c) temperature with pressure, and the linear relation between T(c) and ratio of tetragonal lattice parameters c/a, are compared to those of the heavy fermion CeMIn5 (M=Co,Rh,Ir) unconventional superconductors. The striking similarity of the properties between the two families of superconductors suggests a common physics and a common (magnetically mediated) mechanism of superconductivity.     

Fulde-Ferrell-Larkin-Ovchinnikov state in layered superconductors

The phase diagram of a layered superconductor in a large parallel magnetic field is calculated. This includes a calculation of the lower critical field beyond which the superconductor is in the FFLO-state and possesses a spatially modulated order parameter and spin polarization. The order parameter, spin polarization, free-energy density, and phase boundaries of this unconventional superconducting phase are evaluated numerically in the complete B–T plane. The analysis suggests that the transition at the lower critical field is of second order, and not of first order, as previously assumed. The order parameter of the FFLO-state merges continuously into the uniform superconducting state.     

高温超导体电子结构和超导机理的角分辨光电子能谱研究

超导是一种奇异的宏观量子现象.100多年来,已发现的超导体主要分为两类:以金属或者合金为代表的常规超导体以及以铜氧化物和铁基高温超导体为代表的非常规超导体.常规超导体的超导机理能被BCS超导理论完美解释,但高温超导体的超导机理至今仍未达成共识,已经成为凝聚态物理领域中长期争论且充满挑战的重大科学问题.从实验上揭示非常规超导材料的微观电子结构,是理解其奇异正常态和超导电性机理、建立新理论的前提和基础.角分辨光电子能谱技术,由于可以实现对材料中电子的能量、动量和自旋的直接测量,在高温超导研究中发挥了重要的作用.本文综述了我们利用角分辨光电子能谱技术在铜氧化物和铁基高温超导体电子结构和超导机理研究中取得的一些进展,主要包括母体的电子结构、正常态的非费米液体行为、超导态的能带和超导能隙结构以及多体相互作用等.这些结果为理解铜氧化物和铁基高温超导体的物性及超导机理提供了重要的信息.     

Phase diagram of the electron-doped La2-xCexCuO4 cuprate superconductor from Andreev bound states at grain boundary junctions

We use quasiparticle tunneling across La2-xCexCuO4 grain boundary junctions to probe the superconducting state and its disappearance with increasing temperature and magnetic field. A zero bias conductance peak due to zero energy surface Andreev bound states is a clear signature of the phase coherence of the superconducting state. Hence, such a peak must disappear at or below the upper critical field Bc2(T). For La2-xCexCuO4 this approach sets a lower bound for Bc2(0) approximately 25 T which is substantially higher than reported previously. The method of probing the superconducting state via Andreev bound states should also be applicable to other cuprate superconductors.