Fermi arc formation by chiral spin textures in high-temperature superconductors

In angle-resolved photoemission spectroscopy pseudogap phenomenon in high-temperature superconductors is observed as Fermi arcs, or truncated Fermi surface. Here I argue that the hole induced chiral spin texture scenario naturally leads to Fermi arcs by including hole hopping processes. Disappearance of part of the Fermi surface is associated with the effect of the coherence factor. Suppressed spectral weight of the holes turns out to be an electron-like component which has weight near (π,0) only and has some charge instability.     

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.     

Pinning efficiency of splayed columnar defects in Bi-2212 single crystal: Evidence of a cage pinning effect

A three-directional configuration of columnar defects has been induced in a Bi₂Sr₂CaCu₂O₈ single crystal by irradiation with heavy ions of high energy. Persistent current densities have been extracted, using the Bean model, from hysteresis loops recorded in the orientation H||c. We have shown that improvements in pinning properties are larger in this three-directional splayed configuration than in the one obtained with columnar defects parallel to the c-axis. This effect exists only for H larger than H_Φ, where H_Φ is the matching field, and disappears as temperature is increased and vortices become less stiff. This is the first time that such a beneficial effect is reported for a compound of such a high electronic anisotropy.     

Searching for key parameter for determining Tc in Fe-based superconductors: Study of P/As substitution in RFe(P,As)(O,F) [R=La and Nd]

We have investigated the relation between the temperature-dependence of resistivity and superconducting transition temperature T_c in RFeP_1−xAs_xO_0.90F_0.10 (x=0-1.0) (R=La and Nd). In contrast to the linear change of the crystal structure with increasing x, the temperature dependence of resistivity and T_c show non-monotonous x-dependence. When the As concentration x is increased, the temperature-dependence of resistivity changes from T² to T-linear, and T_c distinctly increases in all the La compounds and the Nd ones with x<0.60. The results indicate that the substitution of As for P induces the spin fluctuation and resultantly enhances T_c. On the other hand, we could not find any relation between the temperature-dependence of resistivity and T_c in the Nd samples with x>0.60. This may suggest the existence of other parameters for determining T_c besides the antiferromagnetic correlation in this system.     

Equilibrium of 1:2:3 CLBLCO superconductors with oxygen: effect of cooling upon the oxygen content and the homogeneity of its distribution

Liquid nitrogen, liquid oxygen and liquid argon were tested as coolers for quenching performed after equilibrium of (Ca_xLa_1−x)(Ba_1.75−xLa_0.25+x)Cu₃O_y (x=0.1 or 0.4) with oxygen has been attained. This compound has been previously denoted as CLBLCO, CLBCO or CaLaBaCuO. Absorption of O₂ during quenching in liquid oxygen was found and measured. Such samples are oxygen inhomogeneous. The transition to superconductivity is wide and begins 20 K higher than for a homogeneous sample having the same oxygen content. Liquid nitrogen, which is usually used as an external cooling agent containing 2-3% of oxygen, also leads to notable oxygen absorption. Only quenching in oxygen free liquid argon or in oxygen free liquid nitrogen does not cause oxygen absorption and may be used for the preparation of homogeneous samples of CLBLCO after equilibration at any temperature in the range from 300 to 950 °C.     

What drives pseudogap physics in high-Tc cuprates? A view from the (resistance) bridge

Taking into account the evolution of the in-plane resistivity with temperature and doping, a candidate proposal for the (hole-doped) cuprate phase diagram is constructed. Many features of the phase diagram are viewed as a consequence of an anisotropic interaction that intensifies with decreasing doping from the heavily overdoped side. At a critical doping p_c, that coincides with the development of the normal state pseudogap, the interaction becomes sufficiently strong that all electronic states near the zone boundary are effectively incoherent even at T = 0.     

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.     

Charge conduction process and photoelectrical properties of bulk heterojunction device based on sulphonated nickel phthalocyanine and rose Bengal

The dependence of photovoltaic performance of the bulk heterojunction photovoltaic device based on the blend of sulphonated nickel phthalocyanine (NiPcS) and rose Bengal (RB) on their composition, thermal annealing and oxygen exposure has been investigated. It is found that both electron and hole mobility in RB phase and NiPcS phase, respectively has been increased on thermal annealing. The power conversion efficiency of the device increases upon thermal annealing, attributed the balance charge transport. The power conversion efficiency of the device experiences a drastic increase upon oxygen exposure, which attributed to the photo-induced doping, increase in exciton diffusion length in NiPcS phase and increased volume of exciton dissociation interfacial sites. From the impedance spectroscopy, we conclude that the change in bulk resistance and dielectric constant of the active material due to the illumination has a direct relevance to the photocurrent generated by the device.     

The non-isotropic character of electric and dielectric properties of ammonium zinc chloride crystal

The dielectric constant, ε, and the d.c. conductivity, σ, were measured along the a-, b- and c-axes of (NH₄)₂ZnCl₄ (AZC) crystal in the 300-450 K temperature range. Crystals of AZC grown from aqueous solutions containing excess of ZnCl₂ were used. The value of the dielectric permittivity of AZC is extremely small compared to other ferroelectric crystals. Pronounced broad or step-like peaks at the phase transition temperatures were detected along the a- and b-axes, while ε along the c-axis is temperature independent up to the end of the measuring range. Reciprocal of the dielectric permittivity in the range of the commensurate to incommensurate phase transition obeys a relation similar to the Curie-Weiss law that is valid for second order ferroelectric/paraelectric phase transitions. The constants of the proposed relationship applied to the cooling run are given. The J-E characteristics along the three crystallographic axes were measured in the normal, incommensurate, commensurate and antiferroelectric phases. Hence, the type of conduction mechanism has been estimated. Parameters of Poole-Frenkel and Richardson-Schottky types of conduction mechanism have been determined. The effect of applied electric field on the conductivity measurement was also tested. Conductivity anomalies with different character were observed at the phase transition temperatures. The lnσ−1000/T dependence revealed thermal activation energy of conduction along the a-, b- and c-axes with different values in different phases of AZC.     

Berezinsky-Kosterlitz-Thouless transition in phase fluctuating superconductors with inhomogeneous Gaussian distributed couplings

Within the framework of phase fluctuation picture for the pseudogap state of cuprate superconductors, we study the effects of both spatial inhomogeneity of coupling strength and thermal phase fluctuations on the superconducting transition temperature. Such a Berezinsky-Kosterlitz-Thouless (BKT) transition is characterized by a two-dimensional (2D) classical XY model, in which the bond coupling is assumed to be roughly proportional to the superconducting bond order parameter. In recent STM experiments with lattice-tracking spectroscopy technique, a Gaussian-like spatially distributed pairing strength is observed. Our Monte Carlo simulations using Wolff cluster update on such 2D classical XY model, in which the bond coupling obeys a similar spatial Gaussian distribution, indicate that the enhancement of the variance of Gaussian distribution may suppress the BKT transition temperature. In addition, we calculate the related physical quantities, including the spin stiffness, free energy, specific heat, magnetization and magnetic susceptibility, by changing the inhomogeneity variance.     

The influence of out-of-plane disorder on the formation of pseudogap and Fermi arc in Bi2Sr2−xRxCuOy (R=La and Eu)

We found that the length of the Fermi arc decreases with increasing out-of-plane disorder by performing angle resolved photoemission spectroscopy (ARPES) measurements in the superconducting state of optimally doped R=La and Eu samples of Bi₂Sr_2−xR_xCuO_y. Since out-of-plane disorder stabilizes the antinodal pseudogap as was shown in our previous study of the normal state, the present results indicate that this antinodal pseudogap persists into the superconducting state and decreases the Fermi arc length. We think that the shrinkage of the Fermi arc reduces the superfluid density, which explains the large suppression of the superconducting transition temperature when out-of-plane disorder is increased.     

Spin and charge order in Mott insulators and d-wave superconductors

We argue that aspects of the anomalous, low temperature, spin and charge dynamics of the high temperature superconductors can be understood by studying the corresponding physics of undoped Mott insulators. Such insulators display a quantum transition from a magnetically ordered Néel state to a confining paramagnet with a spin gap; the latter state has bond-centered charge order, a low energy S=1 spin exciton, confinement of S=1/2 spinons, and a free S=1/2 moment near non-magnetic impurities. We discuss how these characteristics, and the quantum phase transitions, evolve upon doping the insulator into a d-wave superconductor. This theoretical framework was used to make a number of predictions for STM measurements and for the phase diagram of the doped Mott insulator in an applied magnetic field.     

Relaxation processes in mesoscopic superconductors

We investigate the possibility of a novel kind of optical pump probe spectroscopy where the two laser pulses are focused on different areas of the sample. The response to the destruction of the superconducting state in a large part of a mesoscopic ring is studied numerically. We use the time dependent Ginzburg-Landau equations with periodic boundary conditions and external magnetic field. We evaluate the relaxation rates of the superconducting order parameter as well as the voltage induced by the charge imbalance. Computer simulations confirm that the perturbation of superconductivity on one part of the ring induces a voltage which decelerates the superconducting electrons on the other part of the ring. This deceleration results in the decrease of the superconducting current and the superfluid density. The relaxation times are of the order of the picosecond, the induced voltage of few millivolts and the variation of the superconducting gap of 10% which we believe to be suitable for time resolved femtosecond optical spectroscopy.     

Electrical conduction and magnetoelectric effect of (x) BaTiO3 + (1−x) Ni0.92Co0.03Cu0.05Fe2O4 composites in ferroelectric rich region

Particulate composites with composition (x)BaTiO₃+(1−x)Ni_0.92Co_0.03Cu_0.05Fe₂O₄ in which x varies as 1, 0.85, 0.70, 0.55 and 0 (in mol%) were prepared by the conventional double sintering ceramic technique. The presence of two phases viz. ferromagnetic (Ni_0.92Co_0.03Cu_0.05Fe₂O₄) and ferroelectric (BaTiO₃) was confirmed by X-ray diffraction analysis. The dc resistivity and thermo-emf measurements were carried out with variation of temperature. The ac conductivity (σ_ac) measurements investigated in the frequency range 100 Hz to 1 MHz conclude that the conduction in these composites is due to small polarons. The variation of dielectric constant and loss tangent with frequency (20 Hz to 1 MHz) was studied. The static magnetoelectric conversion factor, i.e. dc (dE/dH)_H was measured as a function of intensity of applied magnetic field. The changes were observed in electrical properties as well as in magnetoelectric voltage coefficient as the molar ratio of the constituent phases was varied. A maximum value of magnetoelectric conversion factor of 536.06 μV/cm Oe was observed for the composite with 70% BaTiO₃+30% Ni_0.92Co_0.03Cu_0.05Fe₂O₄ at a dc magnetic field of 2.3 K Oe. The maximum magnetoelectric conversion output has been explained in terms of ferrite-ferroelectric content, applied static magnetic field and resistivity.     

New features in collective dynamics of intrinsic Josephson junctions

We discuss the breakpoint phenomenon, which reflects a resonance between Josephson and plasma oscillations in the system of coupled Josephson junctions. The resonance leads to the creation of the longitudinal plasma waves (LPW) at the breakpoint and to the oscillation dependence of the breakpoint current as a function of the dissipation and coupling parameters. A group behavior of the current-voltage characteristics (IVC) of the stacks with different number of junctions is predicted. Particularly, in the materials with dissipation parameter β=0.35 and coupling parameter α=3, we expect four groups of the IVC, which demonstrate a different character of changing of the breakpoint current with the number of junctions in the stack. Using the k-αβ-method, we find the values of the wave numbers k of the LPW and explain this group behavior of the IVC.     

Three-dimensional simulation of quasi-particle structures in nano-scaled superconductors

We have developed numerical simulation method for quasi-particle structures in the three-dimensional nano-sized superconductors, using the three-dimensional finite element method and the Bogoliubov-de Gennes equation. Using this method, we analyzed the superconducting state in the nano-sized cubic superconductors. We found the spatial oscillations of order parameter because of the confinement of superconducting electrons, and also we found the quasi-particle bound states at the corners of the cubic superconductors because of suppression of superconductivity at the corners.     

The effects of elements doping on transport and thermoelectric properties of Sr3Ti2O7

The Ruddlesden–Popper (RP) phase compounds (Sr_0.95R_0.05)₃Ti₂O₇ (R=Er, Y, Dy, Gd, Eu, Sm, Nd and La) were prepared, and their transport and thermoelectric properties were investigated. The results indicate that high-T electrical resistivity ρ (300 K<T<1000 K) increases monotonically with temperature and basically has a relation ρ∝T^M, with M varying from 0.91 to 1.92 at temperatures T>~650 K, suggesting acoustic phonon scattering is dominant. At low temperatures (5 K<T<300 K), ρ for (Sr_0.95R_0.05)₃Ti₂O₇ (R=Nd and La) decreases monotonously with decreasing temperature, whereas ρ for (Sr_0.95R_0.05)₃Ti₂O₇ (R=Er, Y, Dy, Gd, Eu and Sm) decreases first, and then increases instead as T decreases to a critical temperature T_c. Moreover, electrical conductivity σ∝T^1/2 holds at lower temperatures, indicating that the electron–electron interaction caused by the presence of disorder dominates the transport process at the low temperatures. Besides, experiments show that at T<~400 K the lattice thermal conductivity of the doped compounds basically decreases with increase of the atomic mass of dopants. Generally, the figure of merit (ZT) at 1000 K increases first, and then decreases with the increase of the dopants' ionic radius, and the largest ZT is achieved in (Sr_0.95Gd_0.05)₃Ti₂O₇ mainly owing to its lower lattice thermal conductivity.     

Upper critical field of the 122-type iron pnictide superconductors

The upper critical fields (H_c2) of the single crystals (Sr,Na)Fe₂As₂ and Ba_0.55K_0.45Fe₂As₂ were determined by means of measuring the electrical resistivity, ρ_xx(μ₀H), using the facilities of pulsed magnetic field at Los Alamos. In general, these compounds possess a very large upper critical field (H_c2(0)) with a weak anisotropic effect. The detailed curvature of H_c2(T_c) may depend on the magnetic field orientation and the sample compositions. We argue that such a difference mainly results from the multi-band effect, which might be modified via doping.     

Metal-insulator transition and variable-range hopping conductivity of n-CdSb in magnetic field

Resistivity, ρ, of a II-V group semiconductor n-CdSb doped with In is investigated in pulsed magnetic fields up to and at temperatures . The low-temperature resistivity ρ(T) increasing with T in the range of B<4 T is found to have an upturn around B∼4 T and strong activated behavior at further increase of B. These observations give evidence for magnetic-field-induced metal-insulator transition (MIT). In the insulating side of the MIT, Mott variable-range hopping (VRH) conductivity with two types of asymptotic behavior, ln ρ (T, B)∼T^−3/4B² and ln ρ (T, B)∼(B/T)^1/3, is established in low and high magnetic fields, respectively. The VRH conductivity is analyzed using a model of the near-edge electron energy spectrum established by investigations of the Hall effect. The VRH conductivity is shown to take place over the band tail states of one out of two impurity bands, which for T=0 and B=0 lie above the conduction band edge.     

Microstructure and magneto-dielectric properties of ferrimagnetic composite GdIGX:YIG1−X at radio and microwave frequencies

The gadolinium iron garnet (GdIG, Gd₃Fe₅O₁₂) and yttrium iron garnet (YIG, Y₃Fe₅O₁₂), and their GdIG_X:YIG_1−X composites, were obtained by a new solid-state procedure. The microstructure of the samples was studied by X-ray powder diffraction and Rietveld refinement, Vickers microhardness and scanning electron microscopy. The density and microhardness behavior were studied as a function of the lattice parameters for the composite samples. In the present work, the magnetic and dielectric properties of the GdIG_X:YIG_1−X composite at radio and microwave frequencies were also studied. The magnetic permeability and dielectric permittivity and loss properties for the composite GdIG_X:YIG_1−X showed good potential for use in components, when one is looking for moderate ε_r′ and μ_r′ materials with low loss in the radio-frequency and microwave range.