A new numerical method for quasi-particle spectroscopy around a pair of vortices in triplet superconductors

Triplet superconductors such as Sr₂RuO₄ and Na_xCoO₂·yH₂O are now found to be p-wave (p_x±ip_y) or f-wave ((p_x±ip_y)coscp_z) superconductors. In conventional singlet superconductors, vortices are quantized because phase of order parameter must rotate by 2π around a vortex. But triplet superconductors have a degree of freedom of spin, which is described by d-vector. The d-vector and phase can rotate by π around a vortex, separately. Therefore appearance of HQVs is predicted. Theoretically, it is found that a pair of HQVs is more stable than a singly quantized vortex, for several parameter regions.In this study, in order to investigate quasi-particle bound states around two vortices in s-wave superconductors, we have developed a new numerical method to solve the BdG equation for two vortices state, using Mathieu functions. We confirmed the validity of this method for two vortices state and applied it in case of a pair of vortices. And we solved it.     

Electronic band structure and Fermi surface for new layered superconductor LaO0.5F0.5BiS2 in comparison with parent phase LaOBiS2 from first principles

By means of ab initio calculations, we have probed the peculiarities of the electronic band structure and Fermi surface for the recently discovered layered superconductor LaO_0.5F_0.5BiS₂ in comparison with the parent phase LaOBiS₂. The electronic factors promoting the transition of LaOBiS₂ upon fluorine doping to superconducting state: inter-layer charge transfer, the evolution of the Fermi surface, and the dependence of the near-Fermi densities of states on x for LaO_{1 ? x} F _x BiS₂ are evaluated and discussed in comparison with the available experiments.     

Direct growth of CdSe nanorods on ITO substrates by co-anchoring of ZnO nanoparticles and ethylenediamine

This work presents results from a study carried out on the Al/Cu₃BiS₃/Buffer/ZnO stacked layer, using high-resolution transmission electron microscopy (HRTEM). This system is used to fabricate solar cells with Al/Cu₃BiS₃/In₂S₃/ZnO and Al/Cu₃BiS₃/ZnS/ZnO structures. The conforming layers function as electrical contact, absorber layer, buffer layer, and optical window, respectively. The detailed results of Cu₃BiS₃ thin film investigation by HRTEM are presented. The Cu₃BiS₃ thin films are non-homogeneous and are strongly dependent on deposition conditions with grain size between 6.5 and 20?nm showing a nano-crystalline character. We found that the buffer layer of In₂S₃ grows in a polycrystalline structure, whereas the layer of ZnS reveals an amorphous structure. The performed study of these solar cells gives us significant information about their crystalline structure and allows us to visualize each of the constituting layers as well as of the Al/Cu₃BiS₃, Cu₃BiS₃/buffer, and buffer/ZnO interfaces. This study was correlated with electrical properties.     

Structural and physical properties of SrMn2As2

SrMn₂As₂ single crystals were grown by the Sn flux method. Structural features of these crystals were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The XRD results show that the single crystal has a rhombohedral structure and grows along the c-axis direction. The microstructure and layered structural features of this material have been examined by SEM and high-resolution TEM observations. The measurements of in-plane resistivity as a function of temperature demonstrate that SrMn₂As₂ undergoes a phase transition of semiconductor-insulator at a low temperature; the active energies are estimated to be Δ=0.64 and 0.29 eV for two distinct regions. Magnetic measurements show a clear antiferromagnetic (AFM) transition at about T_N=125 K. Therefore, the SrMn₂As₂ material is an AFM insulator at low temperatures and could be a potential parent compound for superconductors.     

B NMR relaxation in superconductors: YRuB2 and LuRuB2

To investigate the electronic states in YRuB₂ and LuRuB₂, we have carried out ¹¹B NMR measurements. In the normal state, the spin-lattice relaxation rates 1/T₁'s in these compounds are proportional to the temperature T. 1/T₁'s show a small coherence peak just below the superconducting transition temperature T_c and decrease exponentially well below T_c. YRuB₂ and LuRuB₂ are found to be BCS superconductors with the energy gap 2Δ(0)=3.52 k_BT_c.     

Doping effect on the carrier scattering in iron-pnictide superconductors studied by charge transport

In order to reveal the role of “carrier doping” in the iron-based superconductors, we investigated the transport properties of the oxygen-deficient iron-arsenides LnFeAsO_1−y (Ln=La, Ce, Pr and Nd) over a wide doping range. We found that the effect of “doping” in this system is mainly on the carrier scattering rather than carrier density, quite distinct from that in high-T_c cuprates. In the case of La system with lower T_c, the low temperature resistivity is dominated by T² term and fairly large magnetoresistance is observed. On the other hand, in the Nd system with higher T_c, carriers are subject to stronger scattering showing nearly T-linear resistivity and small magnetoresistance. Such strong scattering appears intimately correlated with high-T_c superconductivity in the iron-based system.     

Stripes, electron fractionalization, and ARPES

Although structurally the high temperature superconductors are quasi-two-dimensional, there is both theoretical and experimental evidence of a substantial range of temperatures in which ‘stripe’ correlations make the electronic structure locally quasi-one-dimensional. We consider an array of Josephson coupled, spin gapped one dimensional electron gases as a model of the high temperature superconductors. For temperatures above T_c, this system exhibits electron fractionalization, yielding a single particle spectral response which is sharp as a function of momentum, but broad as a function of energy. For temperatures below the spin gap but above T_c, there are enhanced one-dimensional superconducting fluctuations and pseudogap phenomena. Pair tunneling induces a crossover to three-dimensional physics as T_c is approached. Below T_c, solitons are confined in multiplets with quantum numbers which are simply related to the electron, and a coherent piece of the single particle spectral function appears. The weight of this coherent piece vanishes in the neighborhood of T_c in proportion to a positive power of the interchain superfluid density. This behavior is highly reminiscent of recent ARPES measurements on the high temperature superconductors.     

Pre-formed Cooper pairs and Bose-Einstein condensation in cuprate superconductors

A two-dimensional (2D) assembly of noninteracting, temperature-dependent, pre-formed Cooper pairs in chemical/thermal equilibrium with unpaired fermions is examined in a binary boson-fermion statistical model as the Bose-Einstein condensation (BEC) singularity temperature T_c is approached from above. Compared with BCS theory (which is not a BEC theory) substantially higher T_cs are obtained without any adjustable parameters, that fall roughly within the range of empirical T_cs for quasi-2D cuprate superconductors.     

Increase of Tc in the 1:2:3 superconductors YBCO and (CaxLa1−x)(LauBa1−u)2Cu3Oy after slow cooling or low temperature annealing

The well known phenomenon of the increase of T_c of YBCO after slow cooling or low temperature annealing without change of the oxygen content, was found also for the YBCO like tetragonal superconductors of (Ca_xLa_1−x)(La_uBa_1−u)₂Cu₃O_y (this compound has been previously denoted as CLBLCO, CLBCO or CaLaBaCuO). It has been observed at 150 and 100 °C for oxygen underdoped, optimally- and overdoped ceramics. The products retain their tetragonal unit cells. The possible reasons of this phenomenon are discussed.     

D-wave superconductivity in doped Mott insulators

The effect of proximity to a Mott insulating phase on the charge transport properties of a superconductor is determined. An action describing the low energy physics is formulated and different scenarios for the approach to the Mott phase are distinguished by different variation with doping of the parameters in the action. A crucial issue is found to be the doping dependence of the quasiparticle charge which is defined here and which controls the temperature and field dependence of the electromagnetic response functions. Presently available data on high-T_c superconductors are analyzed. The data, while neither complete nor entirely consistent, suggest that neither the quasiparticle velocity nor the quasiparticle charge vanish as the Mott phase is approached, in contradiction to the predictions of several widely studied theories of lightly doped Mott insulators. Implications of the results for the structure of vortices in high-T_c superconductors are determined.     

Pnicogen-bridged antiferromagnetic superexchange interactions in iron pnictides

The first-principles electronic structure calculations made substantial contribution to study of high T_c iron-pnictide superconductors. By the calculations, LaFeAsO was first predicted to be an antiferromagnetic semimetal, and the novel bi-collinear antiferromagnetic order was predicted for α-FeTe. Moreover, based on the calculations the underlying mechanism was proposed to be Arsenic-bridged antiferromagnetic superexchange interaction between the next-nearest neighbor Fe moments. In this article, this physical picture is further presented and discussed in association with the elaborate first-principles calculations on LaFePO. The further discussion of origin of the magnetism in iron-pnictides and in connection with superconductivity is presented as well.     

Structure and magnetic properties of Ca1−x−yMgxCu2+yO3 influenced by isoelectronic substitution x

Phases of the composition Ca_1−x−yMg_xCu_2+yO₃ have been prepared for the first time. The compounds are isostructural with the known end-members CaCu₂O₃ and MgCu₂O₃ showing a two-leg spin-ladder-like connection of copper and oxygen atoms within the Cu₂O₃-layer. Opposite the spin ladders this layer is folded, which results in a long-range antiferromagnetic ordering of these phases. The Néel temperature can be adapted by variation of x in Ca_1−x−yMg_xCu_2+yO₃ between 24 and 80 K. Several structural features, which influence the magnetic ordering, are discussed.     

Comprehensive study of high-Tc interface superconductivity

Using ALL-MBE technique, we have synthesized different heterostructures consisting of an insulator La₂CuO₄ (I) and a metal La_1.56Sr_0.44CuO₄ (M) layer neither of which is superconducting by itself. The M-I bilayers were superconducting with a critical temperature T_c≈30-36 K. This highly robust phenomenon is confined within 1-2 nm from the interface and is primarily caused by the redistribution of doped holes across the interface. In this paper, we present a comprehensive study of the interface superconductivity by a range of experimental techniques including transport measurements of superconducting properties.     

Comparison of the local structures of Ca0.82La0.18FeAs2 and Ba0.64K0.36Fe2As2 pnictide superconductors using atomic pair distribution function analysis

A comparative local structure study of pnictide superconductors Ca_0.82La_0.18FeAs₂ (112-type, T_c∼ 40 K) and Ba_0.64K_0.36Fe₂As₂ (122-type, T_c∼ 37 K), using room temperature x-ray total scattering measurements is reported. The Fe–As superconducting active layer is found to be globally similar in both the systems consisting of edge-sharing FeAs_4/4 tetrahedra as in all the iron-pnictide superconductors discovered so far. Although optimally superconducting, the active layer in these compounds is found to sustain a large local inhomogeneity. These results thus imply that a nanoscopic manipulation of the Fe–As active layer, rather than its isotropic structural tuning, is the key parameter to control the superconducting properties of the iron-based systems.     

Solvothermal preparation and characterization of nanocrystalline Bi2Te3 powder with different morphology

Bi₂Te₃-based alloys are currently best-known, technological important thermoelectric materials near room temperature. In this paper, nanocrystalline Bi₂Te₃ with different morphologies was synthesized by a solvothermal process based on the reaction between BiCl₃, Te, and KBH₄ in N,N-dimethylformamide at 100-180 °C. KBH₄ was used as a reducing agent. The products were characterized by X-ray diffraction and transmission electron microscopy (TEM). The particle morphologies and size are dependent on the reaction temperature and time. A possible formation mechanism is proposed.     

Phase diagram for stripes in YBa2Cu3O6+x superconductors

Neutron scattering has been used to measure the charge and spin structure in the YBa₂Cu₃O_6+x superconductors. Incommensurate static charge ordering is found at low doping levels while only charge fluctuations are found at higher doping. The spin structure is complex with both a commensurate resonance and incommensurate structure observed at low temperatures. The scattering results are used to construct a phase diagram for stripes in the YBa₂Cu₃O_6+x system.     

Electronic structure reconstruction of CaFe2As2 in the spin density wave state

The electronic structure of CaFe₂As₂, a parent compound of iron-based superconductors, is studied with high-resolution angle-resolved photoemission spectroscopy. The electronic structure of CaFe₂As₂ in the paramagnetic state is consistent with that of density-functional theory calculations. We show that the electronic structure of this compound is significantly reconstructed when entering the spin density wave state. We could resolve two hole-like pockets and four electron-like pockets around the (0, 0) point, and one electron-like pocket surrounded with a pair of electron- and hole-like pockets around the (π, π) point in the spin density wave state. Therefore, the complicated Fermi surface topology and electronic structure near Fermi surface of CaFe₂As₂ illustrate that there exists unconventional electronic reconstruction in the spin density wave state, which cannot be explained by the band folding and Fermi surface nesting pictures.     

Superconducting gap of overdoped Tl2Ba2CuO6+δ observed by Raman scattering

We report Raman scattering spectra for single crystals of overdoped Tl₂Ba₂CuO_6+δ (Tl-2201) at low temperatures. It was observed that the pair-breaking peaks in A_1g and B_1g spectra radically shift to lower energy with carrier doping. We interpret it as s-wave component mixing into d-wave, although the crystal structure is tetragonal. Since similar phenomena were observed also in YBa₂Cu₃O_y and Bi₂Sr₂CaCu₂O_z, we conclude that s-wave mixing is a common property for overdoped high-T_c superconductors.     

Many-body interactions in hole-doped high-Tc cuprates studied by high-resolution ARPES

We have performed high-resolution angle-resolved photoemission spectroscopy on hole-doped high-T_c cuprate superconductors (HTSCs) to study many-body interaction and the universality of low-energy excitation gap. In Bi₂Sr₂CaCu₂O₈ (Bi2212), we observed a kink in the dispersion in the off-nodal region in the superconducting state, which remarkably weakens on impurity substitution. We also find that the appearance of the kink in the off-nodal region is a common feature of Bi2212 and La_1.85Sr_0.15CuO₄ (LSCO), while the energy scale is remarkably different between two compounds (70 and 20 meV). We discuss universality of the kink in dispersion in the hole-doped HTSCs in terms of the coupling of electrons with spin fluctuations.     

Si doped Cu0.5Tl0.5Ba2Ca2Cu3O10−δ superconductors

The Si doped (Cu_0.5Tl_0.5)Ba₂Ca₂Cu_3−ySi_yO_10−δ (y=0, 0.25, 0.5, 0.75, 1.0, 1.25) superconductor samples have been synthesized to investigate the effect of mobile carriers on the critical temperature and magnitude of diamagnetism of this family of cuprates. The Si doped samples have tetragonal structure as observed from the X-ray diffraction spectra. The c-axis length of the unit cell of (Cu_0.5Tl_0.5)Ba₂Ca₂Cu_3−ySi_yO_10−δ was increased after the increase of Si concentration, whereas the critical temperature and the magnitude of diamagnetism have been decreased. The decrease in T_c(0) and magnitude of diamagnetism is possibly due to the deficiency of carriers in CuO₂/SiO₂ planes caused by the +4 state of Si atoms. However, the post-annealing of these samples in oxygen atmosphere has increased the critical temperature and the magnitude of diamagnetism. The FTIR absorption measurements of (Cu_0.5Tl_0.5)Ba₂Ca₂Cu_3−ySi_yO_10−δ samples have shown a softening of the apical oxygen mode of the type Cu(1)O_ACu(2) and TlO_ACu(2) with increased concentration of Si in the unit cell; the softening of this mode is directly linked with the increase in the c-axis lattice parameter of the (Cu_0.5Tl_0.5)Ba₂Ca₂Cu_3−ySi_yO_10−δ superconductor.