Kondo Signatures of a Quantum Magnetic Impurity in Topological Superconductors.

We study the Kondo physics of a quantum magnetic impurity in two-dimensional topological superconductors (TSCs), either intrinsic or induced on the surface of a bulk topological insulator, using a numerical renormalization group technique. We show that, despite sharing the p+ip pairing symmetry, intrinsic and extrinsic TSCs host different physical processes that produce distinct Kondo signatures. Extrinsic TSCs harbor an unusual screening mechanism involving both electron and orbital degrees of freedom that produces rich and prominent Kondo phenomena, especially an intriguing pseudospin Kondo singlet state in the superconducting gap and a spatially anisotropic spin correlation. In sharp contrast, intrinsic TSCs support a robust impurity spin doublet ground state and an isotropic spin correlation. These findings advance fundamental knowledge of novel Kondo phenomena in TSCs and suggest experimental avenues for their detection and distinction.

Interaction-induced localization of fermionic mobile impurities in a Larkin-Ovchinnikov superfluid.

We theoretically investigate the interplay between the fermionic mobile impurity atoms and a Larkin-Ovchinnikov (LO) superfluid in a two dimensional optical lattice. We find that the impurity atoms get localized and can form pairs when the interaction between the impurity atoms and the LO superfluid is strong enough. These features are due to the phenomena of self-localization whose underlying mechanism is revealed by an effective model. The impurity atoms with finite concentrations can drive the transition from a two-dimensional, checkerboardlike LO state to a quasi-one-dimensional, stripelike one. Experimental preparations to observe these features are also discussed.

Superfluid density in the s±-wave state of clean iron-based superconductors.

Based on a phenomenological model and the Kubo formula, we investigate the superfluid density ρ(s)(T) and then the penetration depth λ(T) of the iron-based superconductors in the coexistence region of the spin-density wave and superconductivity, and also in the overdoped region. Our calculations show a dramatic increase of λ(0) with the decrease of the doping concentration x below x = 0.1. This result is consistent with the experimental observations. At low temperatures, ρ(s)(T) shows an exponential-law behavior, while at higher temperatures, the linear-in-T behavior is dominant before it trends to vanish. It is in qualitative agreement with the direct measurement of superfluid density in films of Fe-pnictide superconductor at x = 0.08. The evolution of Δλ(T) can be roughly fitted by a power-law function with the exponent depending on the doping concentration. We show that the Uemura relation holds for the iron-based superconductors only at very low doping levels.

Model of vortex states in hole-doped iron-pnictide superconductors.

Based on a phenomenological model with competing spin-density-wave (SDW) and extended s-wave superconductivity, the vortex states in Ba(1-x)K(x)Fe2As2 are investigated by solving Bogoliubov-de Gennes equations. Our result for the optimally doped compound without induced SDW is in qualitative agreement with recent scanning tunneling microscopy experiment. We also propose that the main effect of the SDW on the vortex states is to reduce the intensity of the in-gap peak in the local density of states and transfer the spectral weight to form additional peaks outside the gap.

Supercurrent transferring through c-axis cuprate Josephson junctions with thick normal-metal bridge.

With a simple but exactly solvable model, we investigate the supercurrent transferring through the c-axis cuprate superconductor-normal metal-superconductor junctions with the clean normal metal much thicker than its coherence length. It is shown that the supercurrent as a function of thickness of the normal metal decreases much slower than the exponential decaying expected by the proximity effect. The present result may account for the giant proximity effect observed in the c-axis cuprate SNS junctions.

Surface states of gapped electron systems and semi-metals.

With a generic lattice model for electrons occupying a semi-infinite crystal with a hard surface, we study the eigenstates of the system with a bulk band gap (or the gap with nodal points). The exact solution to the wave functions of scattering states is obtained. From the scattering states, we derive the criterion for the existence of surface states. The wave functions and the energy of the surface states are then determined. We obtain a connection between the wave functions of the bulk states and the surface states. For electrons in a system with time-reversal symmetry, with this connection, we rigorously prove the correspondence between the change of Kramers degeneracy of the surface states and the bulk time-reversal Z 2 invariant. The theory is applicable to systems of (topological) insulators, superconductors, and semi-metals. Examples for solving the edge states of electrons with/without the spin-orbit interactions in graphene with a hard zigzag edge and that in a two-dimensional d-wave superconductor with a ([Formula: see text]) edge are given in appendices.

Photochemical and Nonphotochemical Fluorescence Quenching Processes in the Diatom Phaeodactylum tricornutum.

Nonphotochemical fluorescence quenching was found to exist in the dark-adapted state in the diatom Phaeodactylum tricornutum. Pretreatment of cells with the uncoupler carbonylcyanide m-chlorophenylhydrazone (CCCP) or with nigericin resulted in increases in dark-adapted minimum and maximum fluorescence yields. This suggests that a pH gradient exists across the thylakoid membrane in the dark, which serves to quench fluorescence levels nonphotochemically. The physiological processes involved in establishing this proton gradient were sensitive to anaerobiosis and antimycin A. Based on these results, it is likely that this energization of the thylakoid membrane is due in part to chlororespiration, which involves oxygen-dependent electron flow through the plastoquinone pool. Chlororespiration has been shown previously to occur in diatoms. In addition, we observed that cells treated with 3-(3,4-dichlorophenyl)-1,1-dimethylurea exhibited very strong nonphotochemical quenching when illuminated with actinic light. The rate and extent of this quenching were light-intensity dependent. This quenching was reversed upon addition of CCCP or nigericin and was thus due primarily to the establishment of a pH gradient across the thylakoid membrane. Preincubation of cells with CCCP or nigericin or antimycin A completely abolished this quenching. Cyclic electron transport processes around photosystem I may be involved in establishing this proton gradient across the thylakoid membrane under conditions where linear electron transport is inhibited. At steady state under normal physiological conditions, the qualitative changes in photochemical and nonphotochemical fluorescence quenching at increasing photon flux densities were similar to those in higher plants. However, important quantitative differences existed at limiting and saturating intensities. Dissimilarities in the factors that regulate fluorescence quenching mechanisms in these organisms may account for these differences.

The Effects of Excess Irradiance on Photosynthesis in the Marine Diatom Phaeodactylum tricornutum.

The response of Phaeodactylum tricornutum to excess light was remarkably similar to that observed in higher plants and green algae and was characterized by complex changes in minimal fluorescence yields of fully dark-adapted samples and declines in maximum variable fluorescence levels and oxygen evolution rates. In our study the parallel decreases in the effective rate constant for photosystem II (PSII) photochemistry, the variable fluorescence yield of a dark-adapted sample, and light-limited O2 evolution rates after short (0-10 min) exposures to photoinhibitory conditions could not be attributed to damage or down-regulation of PSII reaction centers. Instead, these changes were consistent with the presence of nonphotochemical quenching of PSII excitation energy in the antennae. This quenching was analogous to that component of nonphotochemical quenching studied in higher plants that is associated with photoinhibition of photosynthesis and/or processes protecting against photoinhibition in that it did not relax readily in the dark and persisted in the absence of a bulk transthylakoid proton gradient. The quenching was most likely associated with photoprotective processes in the PSII antenna that reduced the extent of photoinhibitory damage, particularly after longer exposures. Our results suggest that a large population of damaged, slowly recovering PSII centers did not form in Phaeodactylum even after 60 min of exposure to excess actinic light.

The photosynthetic apparatus of Prochlorococcus: Insights through comparative genomics.

Within the vast oceanic gyres, a significant fraction of the total chlorophyll belongs to the light-harvesting antenna systems of a single genus, Prochlorococcus. This organism, discovered only about 10 years ago, is an extremely small, Chl b-containing cyanobacterium that sometimes constitutes up to 50% of the photosynthetic biomass in the oceans. Various Prochlorococcus strains are known to have significantly different conditions for optimal growth and survival. Strains which dominate the surface waters, for example, have an irradiance optimum for photosynthesis of 200 mumol photons m(-2) s(-1), whereas those that dominate the deeper waters photosynthesize optimally at 30-50 mumol photons m(-2) s(-1). These high and low light adapted 'ecotypes' are very closely related - less than 3% divergent in their 16S rRNA sequences - inviting speculation as to what features of their photosynthetic mechanisms might account for the differences in photosynthetic performance. Here, we compare information obtained from the complete genome sequences of two Prochlorococcus strains, with special emphasis on genes for the photosynthetic apparatus. These two strains, Prochlorococcus MED4 and MIT 9313, are representatives of high- and low-light adapted ecotypes, characterized by their low or high Chl b/a ratio, respectively. Both genomes appear to be significantly smaller (1700 and 2400 kbp) than those of other cyanobacteria, and the low-light-adapted strain has significantly more genes than its high light counterpart. In keeping with their comparative light-dependent physiologies, MED4 has many more genes encoding putative high-light-inducible proteins (HLIP) and photolyases to repair UV-induced DNA damage, whereas MIT 9313 possesses more genes associated with the photosynthetic apparatus. These include two pcb genes encoding Chl-binding proteins and a second copy of the gene psbA, encoding the Photosystem II reaction center protein D1. In addition, MIT 9313 contains a gene cluster to produce chromophorylated phycoerythrin. The latter represents an intermediate form between the phycobiliproteins of non-Chl b containing cyanobacteria and an extremely modified beta phycoerythrin as the sole derivative of phycobiliproteins still present in MED4. Intriguing features found in both Prochlorococcus strains include a gene cluster for Rubisco and carboxysomal proteins that is likely of non-cyanobacterial origin and two genes for a putative varepsilon and beta lycopene cyclase, respectively, explaining how Prochlorococcus may synthesize the alpha branch of carotenoids that are common in green organisms but not in other cyanobacteria.

Interaction-induced localization of mobile impurities in ultracold systems.

The impurities, introduced intentionally or accidentally into certain materials, can significantly modify their characteristics or reveal their intrinsic physical properties, and thus play an important role in solid-state physics. Different from those static impurities in a solid, the impurities realized in cold atomic systems are naturally mobile. Here we propose an effective theory for treating some unique behaviors exhibited by ultracold mobile impurities. Our theory reveals the interaction-induced transition between the extended and localized impurity states, and also explains the essential features obtained from several previous models in a unified way. Based on our theory, we predict many intriguing phenomena in ultracold systems associated with the extended and localized impurities, including the formation of the impurity-molecules and impurity-lattices. We hope this investigation can open up a new avenue for the future studies on ultracold mobile impurities.

Hall conductivity of a spin-triplet superconductor.

We calculate the Hall conductivity for a spin-triplet superconductor, using a generalized pairing symmetry dependent on an arbitrary phase phi. A promising candidate for such an order parameter is Sr2RuO4, whose superconducting order parameter symmetry is still subject to investigation. The value of this phase can be determined through Kerr rotation and dc Hall conductivity measurements. Our calculations impose significant constraints on phi.

Weak localization of Dirac fermions in graphene.

In the presence of the charged impurities, we study the weak localization effect by evaluating the quantum interference correction to the conductivity of Dirac fermions in graphene. With the inelastic scattering rate due to electron-electron interactions obtained from our previous work, we investigate the dependence of the quantum interference correction on the carrier concentration, the temperature, the magnetic field, and the size of the sample. It is found that weak localization is present in large size samples at finite carrier doping. Its strength becomes weakened or quenched when the sample size is less than a few microns at low temperatures as studied in the experiments. In the region close to zero doping, the system may become delocalized. The minimum conductivity at low temperature for experimental sample sizes is found to be close to the data.

Fluctuation-exchange study of antiferromagnetism in disordered electron-doped cuprate superconductors.

On the basis of the Hubbard model, we extend the fluctuation-exchange (FLEX) approach to investigating the properties of the antiferromagnetic (AF) phase in electron-doped cuprate superconductors. Furthermore, by incorporating the effect of scatterings due to the disordered dopant atoms into the FLEX formalism, our numerical results show that the antiferromagnetic transition temperature, the onset temperature of pseudogap due to spin fluctuations, the spectral density of the single particle near the Fermi surface, and the staggered magnetization in the AF phase as a function of electron doping can consistently account for the experimental measurements.

Theory of the phonon Hall effect in paramagnetic dielectrics.

Based upon Raman spin-lattice interaction, we propose a theoretical model for the phonon Hall effect in paramagnetic dielectrics, which was discovered recently in an experiment [C. Strohm, G. L. J. A. Rikken, and P. Wyder, Phys. Rev. Lett. 95, 155901 (2005).]. The phonon Hall effect is revealed to be a phonon analogue to the anomalous Hall effect in electron systems. The thermal Hall conductivity is calculated by using the Kubo formula. Our theory reproduces the essential experimental features of the phonon Hall effect, including the sign, magnitude, and linear magnetic field dependence of the thermal Hall conductivity.

Local tunneling spectroscopy as a signature of the Fulde-Ferrell-Larkin-Ovchinnikov state in s- and d-wave superconductors.

The Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) states for two-dimensional s- and d-wave superconductors (s- and d-SCs) are self-consistently studied under an in-plane magnetic field. While the stripe solution of the order parameter is found to have lower free energy in s-SCs, a square lattice solution appears to be energetically more favorable in the case of d-SCs. At certain symmetric sites, we find that the features in the local density of states (LDOS) can be ascribed to two types of bound states. We also show that the LDOS maps for d-SCs exhibit bias-energy-dependent checkerboard patterns. These characteristics can serve as signatures of the FFLO states.

Spin-Hall effect in two-dimensional electron systems with Rashba spin-orbit coupling and disorder.

Using the four-terminal Landauer-Bu ttiker formula and Green's function approach, we calculate numerically the spin-Hall conductance in a two-dimensional junction system with the Rashba spin-orbit (SO) coupling and disorder. We find that the spin-Hall conductance can be much greater or smaller than the universal value e/8pi, depending on the magnitude of the SO coupling, the electron Fermi energy, and the disorder strength. The spin-Hall conductance does not vanish with increasing sample size for a wide range of disorder strength. Our numerical calculation reveals that a nonzero SO coupling can induce electron delocalization for disorder strength smaller than a critical value, and the nonvanishing spin-Hall effect appears mainly in the metallic regime.

Nondissipative spin Hall effect via quantized edge transport.

The spin Hall effect in a two-dimensional electron system on honeycomb lattice with both intrinsic and Rashba spin-orbit couplings is studied numerically. Integer quantized spin Hall conductance is obtained at the zero Rashba coupling limit when electron Fermi energy lies in the energy gap created by the intrinsic spin-orbit coupling, in agreement with recent theoretical prediction. While nonzero Rashba coupling destroys electron spin conservation, the spin Hall conductance is found to remain near the quantized value, being insensitive to disorder scattering, until the energy gap collapses with increasing the Rashba coupling. We further show that the charge transport through counterpropagating spin-polarized edge channels is well quantized, which is associated with a topological invariant of the system.

Limitations of the pulse-modulated technique for measuring the fluorescence characteristics of algae.

Precise measurements of the minimal fluorescence yield (F(o)) and maximal fluorescence yield (F(m)) of a dark-adapted sample are prerequisites for the quantification of other fluorescence parameters. The pulse amplitude-modulated chlorophyll fluorometer (PAM 101 Chlorophyll Fluorometer, Heinz Walz, Effeltrich, Germany) and saturating pulse technique have frequently been used in measuring F(o) and F(m) and in resolving the contributions of photochemical and nonphotochemical quenching to the total fluorescence yield. The extent to which instrument-dependent factors may affect the accurate measurement of F(o) and F(m) is addressed. It is shown that the increase in pulse amplitude-modulated measuring beam intensity at 1.6 and 100 kHz was nonlinear at higher light intensity settings. The implications of this for measurements of F(o) (1.6 kHz) and F(m) (100 kHz) are discussed. It is also demonstrated that underestimation of F(m) may result due to saturation of the PAM 101 photodiode by scattered infrared light associated with intense light pulses. In addition, it is shown how sample-dependent factors may affect measurements of F(o) and F(m) in samples with low chlorophyll concentrations, in particular, dilute algal suspensions of Phaeodactylum tricornutum and Chlamydomonas reinhardtii. A technique is presented for the accurate measurement of F(o) in algal suspensions (<8 mug chlorophyll a mL(-1)). The importance of examining the saturating pulse transient and F(m) level as a function of the damping setting, pulse width, and pulse intensity, and in the presence of 3-(3,4-dichlorophenyl)-1, 1-dimethylurea is discussed.

States of local moment induced by nonmagnetic impurities in cuprate superconductors.

By using a model Hamiltonian with d-wave superconductivity and competing antiferromagnetic (AF) orders, the local staggered magnetization distribution due to nonmagnetic impurities in cuprate superconductors is investigated. We show that the net moment induced by a single impurity corresponds to a local spin with S(z)=0 or 1/2 depending on the strength of the AF interaction U and the impurity scattering strength epsilon. Phase diagram of epsilon versus U for the moment formation is presented. We discuss the connection of this result with the Kondo problem. When two impurities are placed at the nearest neighboring sites, the net moment is always zero, unusually robust to parameter changes. For two neighboring strong impurities, separated by a Cu-ion site, the induced net moment has S(z)=0, 1/2, or 1.

Quasiparticle states at a d-wave vortex core in high- T(c) superconductors: induction of local spin density wave order.

The local density of states (LDOS) at the vortex lattice cores in a high- T(c) superconductor is studied by using a self-consistent mean-field theory including interactions for both antiferromagnetism (AF) and d-wave superconductivity (DSC). In a zero-field optimally doped sample the AF order is completely suppressed while DSC prevails. In the mixed state, we show that the local AF-like spin density wave order appears near the vortex core and acts as an effective local magnetic field on electrons via Zeeman coupling. As a result, the LDOS at the core exhibits a double-peak structure near the Fermi level that is in good agreement with recent scanning tunneling microscopy observations.