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We investigate quantum turbulence in a two-dimensional trapped supersolid and demonstrate that both the wave and vortex turbulence involve triple rather than dual cascades, as in a superfluid. Because of the presence of a second gapless mode associated with translation symmetry breaking, a new $k^{-13/3}$ scaling law is predicted to occur in the wave turbulence. Simultaneous fast vortex-antivortex creation and annihilation in the interior of the oscillating supersolid results in a $k^{-1}$ scaling law in the vortex turbulence. Numerical simulations based on the Gross-Pitaevskii equation confirmed the predictions., Comment: 5 pages, 3 figures

We show that the five possible ordered states in a quantum spin-1/2 system with long-range exchange interactions: Neel, ladder, Peierls, coincidence, and domain states, can be realized in a binary Rydberg-dressed BEC system in the supersolid phase. In such a system, blockade phenomenon is shown to also occur for pairs of different excited-state atoms, which results in similar intra- and inter-species long-range interactions between ground-state atoms. It suggests that a pseudo spin-1/2 system can be possibly formed in the ground state of ultracold rudibium.

Motivated by a recent experiment of Song \emph{et al.} [Science {\bf 332}, 1410 (2011)], we theoretically study the spin dynamics, charge dynamics, and point-contact Andreev-reflection spectroscopy (PCARS) of two-band iron-based superconductors of a possible extended $s_\pm$-wave pairing symmetry. We consider the case of a dominant $s_\pm$ gap blended by a secondary extended $s$ component in which gap nodes can develop in the Fermi pockets near zone corner and/or boundary. Due to the strong nesting effect associated with nodal regions, dynamical spin and charge susceptibilities can exhibit strong peaks at momenta near $(\pm\pi/2,0)$, $(\pm\pi,\pm\pi/2)$, as well as $(\pm\pi,0)$ in the unfolded Brillouin zone. For PCARS, considering an anisotropic band effect induced by an applied voltage, [100] differential conductance can exhibit a $V$-shape behavior manifesting a gap node occurring in such direction. It is highly suggested that the above features can be experimentally investigated to help sorting out the pairing symmetry of iron-based superconductors.

We investigate the condensate wave function and elementary excitations of strongly interacting bosonic polar molecules in a harmonic trap, treating the scattering amplitude beyond the standard first Born approximation (FBA). By using an appropriate trial wave function in the variational method, effects of the leading order correction beyond the FBA have been investigated and shown to be significantly enhanced when the system is close to the phase boundary of collapse. How such leading order effect of going beyond the FBA can be observed in a realistic experiment is also discussed., Comment: 7 pages, 4 figures

We investigate the effect of anisotropic and long-range dipole-dipole interaction (DDI) on the center motions of nonoverlapping Bose-Einstein condensates (BEC) in bilayer and multilayer stacks. In the bilayer, it is shown analytically that while DDI plays no role in the in-phase modes of center motions of condensates, out-of-phase mode frequency ($\omega_o$) depends crucially on the strength of DDI ($a_d$). At the small-$a_d$ limit, $\omega_o^2(a_d)-\omega_o^2(0)\propto a_d$. In the multilayer stack, transverse modes associated with center motions of coupled condensates are found to be optical phonon like. At the long-wavelength limit, phonon velocity is proportional to $\sqrt a_d$., Comment: 7 pages, 5 figures

The behaviors of a rapid rotating Bose-Einstein condensate under extreme elongation in a 2D anisotropic harmonic plus quartic trap are investigated. Due to the quartic trap, the system remains stable at high rotating velocity, $\Omega\geq \omega_\perp$ ($\omega_\perp$ is the radial harmonic trap frequency), and vortex lattices form even in the absence of the repulsive s-wave interaction ($g$). When $g$ is present, the interplay between $g$ and the quartic trap potential can lead to rich vortex lattice transition states as a function of $\Omega$, to which vortex lattices vanish eventually at some higher $\Omega$., Comment: 5 pages, 5 figures

Inspired by a recent experiment of localization-delocalization transition (LDT) of indirect excitons in lateral electrostatic lattices [M. Remeika \textit{et al.}, Phys. Rev. Lett. \textbf{102}, 186803 (2009)], we investigate the interplay between periodic potential and nonlinear interactions of indirect excitons in coupled quantum wells. It is shown that the model involving both attractive two-body and repulsive three-body interactions can lead to a natural account for the LDT of excitons across the lattice when reducing lattice amplitude or increasing particle density. In addition, the observations that the smooth component of the photoluminescent energy increases with increasing exciton density and exciton interaction energy is close to the lattice amplitude at the transition are also qualitatively explained. Our model provides an alternative way for understanding the underlying physics of the exciton dynamics in lattice potential wells., Comment: 5 pages, 3 figures

Equilibrium vortex formation in rotating binary Bose gases with a rotating frequency higher than the harmonic trapping frequency is investigated theoretically. We consider the system being evaporatively cooled to form condensates and a combined numerical scheme is applied to ensure the binary system being in an authentic equilibrium state. To keep the system stable against the large centrifugal force of ultrafast rotation, a quartic trapping potential is added to the existing harmonic part. Using the Thomas-Fermi approximation, a critical rotating frequency \Omega_c is derived, which characterizes the structure with or without a central density hole. Vortex structures are studied in detail with rotation frequency both above and below ?\Omega_c and with respect to the miscible, symmetrically separated, and asymmetrically separated phases in their nonrotating ground-state counterparts., Comment: 7 pages, 7 figures

Spontaneously crystalline ground states, called quantum crystals, of a trapped Rydberg-dressed Bose-Einstein condensate are numerically investigated. As a result described by a mean-field order parameter, such states simultaneously possess crystalline and superfluid properties. A hexagonal droplet lattice is observed in a quasi-two-dimensional system when dressing interaction is sufficiently strong. Onset of these states is characterized by a drastic drop of the non-classical rotational inertia proposed by Leggett [Phys. Rev. Lett. 25, 1543 (1970)]. In addition, an AB stacking bilayer lattice can also be attained. Due to an anisotropic interaction possibly induced by an external electric field, transition from a hexagonal to a nearly square droplet lattice is also observed., Comment: 4 pages, 4 figures

A phenomenological theory of tunneling spectroscopy for Fe-pnictide superconductors is developed by taking into consideration of asymmetric interface scattering between particle and holes. It is shown that in consistent with anti-phase $s^{\pm}$-wave pairing, appreciable zero-energy surface bound states exist on the [100] surface of Fe-pnictide superconductors. However, in contrast to the [110] bound states in $d$-wave cuprate superconductors, these bound states arise as a result of non-conservation of momentum perpendicular to the interface for tunneling electrons and the $s^{\pm}$ pairing, and hence they can only exist in a small window ($\sim \pm 6^{\circ}$) in the orientation of edges near [100] direction. Our results explain why zero-bias conductance peak is often observed in tunneling spectroscopy and when it disappears, two coherent peaks show up. These results provide unambiguous signals to test the possible $s^{\pm}$-wave pairing in Fe-pnictide superconductors., Comment: 10 pages, 4 figures

Based on a two-band model, we study the electronic Raman scattering intensity in both normal and superconducting states of iron-pnictide superconductors. For the normal state, due to the match or mismatch of the symmetries between band hybridization and Raman vertex, it is predicted that overall $B_{1g}$ Raman intensity should be much weaker than that of the $B_{2g}$ channel. Moreover, in the non-resonant regime, there should exhibit a interband excitation peak at frequency $\omega\simeq 7.3 t_1 (6.8t_1)$ in the $B_{1g}$ ($B_{2g}$) channel. For the superconducting state, it is shown that $\beta$-band contributes most to the $B_{2g}$ Raman intensity as a result of multiple effects of Raman vertex, gap symmetry, and Fermi surface topology. Both extended $s$- and $d_{xy}$-wave pairings in the unfolded BZ can give a good description to the reported $B_{2g}$ Raman data [Muschler {\em et al.}, Phys. Rev. B. {\bf 80}, 180510 (2009).], while $d_{x^2-y^2}$-wave pairing in the unfolded BZ seems to be ruled out., Comment: 7 pages, 4 figures

Motivated by a recent experiment of spatial and temperature dependent average exciton energy distribution in coupled quantum wells [S. Yang \textit{et al.}, Phys. Rev. B \textbf{75}, 033311 (2007)], we investigate the nature of the interactions in indirect excitons. Based on the uncertainty principle, along with a temperature and energy dependent distribution which includes both population and recombination effects, we show that the interplay between an attractive two-body interaction and a repulsive three-body interaction can lead to a natural and good account for the nonmonotonic temperature dependence of the average exciton energy. Moreover, exciton energy maxima are shown to locate at the brightest regions, in agreement with the recent experiments. Our results provide an alternative way for understanding the underlying physics of the exciton dynamics in coupled quantum wells., Comment: 8 pages, 5 figures

We study the gap solitons and nonlinear Bloch waves of interacting bosons in one-dimensional optical lattices, taking into account the interaction from the weak to the strong limits. It is shown that composition relation between the gap solitons and nonlinear Bloch waves exists for the whole span of the interaction strength. The linear stability analysis indicates that the gap solitons are stable when their energies are near the bottom of the linear Bloch band gap. By increasing the interaction strength, the stable gap solitons can turn into unstable. It is argued that the stable gap solitons can easily be formed in a weakly interacting system with energies near the bottoms of the lower-level linear Bloch band gaps., Comment: 8 papges, 7 figures

Fresnel single aperture diffraction (FSAD) is proposed as a phase-sensitive probe for pairing symmetry and Fermi surface of a superconductor. We consider electrons injected, through a small aperture, into a thin superconducting (SC) layer. It is shown that in case of SC gap symmetry $\Delta(-k_x,\mathbf{k}_\parallel)=\Delta(k_x,\mathbf{k}_\parallel)$ with $k_x$ and $\mathbf{k}_\parallel$ respectively the normal and parallel component of electron Fermi wavevector, quasiparticle FSAD pattern developed at the image plane is zeroth-order minimum if $k_x x=n\pi$ ($n$ is an integer and $x$ is SC layer thickness). In contrast, if $\Delta(-k_x,\mathbf{k}_\parallel)=-\Delta(k_x, \mathbf{k}_\parallel)$, the corresponding FSAD pattern is zeroth-order maximum. Observable consequences are discussed for iron-based superconductors of complex multi-band pairings., Comment: 5 pages, 3 figures

Recently we proposed a theory of point-contact spectroscopy and argued that the splitting of zero-bias conductance peak (ZBCP) in electron-doped cuprate superconductor point-contact spectroscopy is due to the coexistence of antiferromagnetic (AF) and d-wave superconducting orders [Phys. Rev. B {\bf 76}, 220504(R) (2007)]. Here we extend the theory to study the tunneling in the ferromagnetic metal/electron-doped cuprate superconductor (FM/EDSC) junctions. In addition to the AF order, the effects of spin polarization, Fermi-wave vector mismatch (FWM) between the FM and EDSC regions, and effective barrier are investigated. It is shown that there exits midgap surface state (MSS) contribution to the conductance to which Andreev reflections are largely modified due to the interplay between the exchange field of ferromagnetic metal and the AF order in EDSC. Low-energy anomalous conductance enhancement can occur which could further test the existence of AF order in EDSC. Finally, we propose a more accurate formula in determining the spin polarization value in combination with the point-contact conductance data., Comment: 9 pages, 8 figures

In the hole-doped $d_{x^{2}-y^{2}}$-wave cuprate superconductor, due to the midgap surface state (MSS), a zero bias conductance peak (ZBCP) is widely observed in [110] interface point contact spectroscopy (PCS). However, ZBCP of this geometry is rarely observed in the electron-doped cuprates, even though their pairing symmetry is still likely the $d_{x^{2}-y^{2}}$-wave. We argue that this is due to the coexistence of antiferromagnetic (AF) and the superconducting (SC) orders. Generalizing the Blonder-Tinkham-Klapwijk (BTK) formula to include an AF coupling, it is shown explicitly that the MSS is destroyed by the AF order. The calculated PCS is in good agreement with the experiments., Comment: 5 pages, 2 figures. Replaced with published version

An antiferromagnetic (AF) spin fluctuation induced pairing model is proposed for the electron-doped cuprate superconductors. It suggests that, similar to the hole-doped side, the superconducting gap function is monotonic d_{x^2-y^2}-wave and explains why the observed gap function has a nonmonotonic d_{x^2-y^2}-wave behavior when an AF order is taken into account. Dynamical spin susceptibility is calculated and shown to be in good agreement with the experiment. This gives a strong support to the proposed model., Comment: 5 RevTeX4 pages, 2 EPS figures

Gaussian distribution is commonly used as a good approximation to study the trapped one-component Bose-condensed atoms with relatively small nonlinear effect. It is not adequate in dealing with the one-component system of large nonlinear effect, nor the two-component system where phase separation exists. We propose a modified Gaussian distribution which is more effective when dealing with the one-component system with relatively large nonlinear terms as well as the two-component system. The modified Gaussian is also used to study the breathing modes of the two-component system, which shows a drastic change in the mode dispersion at the occurrence of the phase separation. The results obtained are in agreement with other numerical results., Comment: 7 pages, 7 figures, accepted for publication in Phys. Rev. A

Using a nonlinear Schr\"odinger equation including short-range two-body attraction and three-body repulsion, we investigate the spatial distribution of indirect excitons in semiconductor coupled quantum wells. The results obtained can interpret the experimental phenomenon that annular exciton cloud first contracts then expands when the number of confined excitons is increased in impurity potential well, as observed by Lai \emph{et al.} [Lai $et al.$, Science \textbf{303}, 503 (2004)]. In particular, the model reconciles the patterns of exciton rings reported by Butov \emph{et al.} [Butov $et al.$, Nature \textbf{418}, 751 (2002)]. At higher densities, the model predicts much richer patterns, which could be tested by future experiments., Comment: 5 Revtex4 pages, 3 figures

Oscillations of atomic Bose-Einstein condensates in a 1D optical lattice with a two-point basis is investigated. In the low-frequency regime, four branches of modes are resolved, that correspond to the transverse in-phase and out-of-phase breathing modes, and the longitudinal acoustic and optical phonon modes of the condensates. Dispersions of these modes depend intimately on the values of two intersite Josephson tunneling strengths, $J_1$ and $J_2$, and the on-site repulsion $U$ between the atoms. Observation of these mode dispersions is thus a direct way to access them., Comment: 5 pages,2 figures

We have analyzed the $B_{1g}$ and $B_{2g}$ Raman spectra of electron-doped cuprate superconductors Nd$_{2-x}$Ce$_{x}$CuO$_4$ and Pr$_{2-x}$Ce$_{x}$CuO$_4$ using a weakly coupled two-band model. One of these two bands is centered around $(\pm \pi/2, \pm \pi/2)$ and couples more strongly with the $B_{2g}$ mode, while the other is centered around $(\pm \pi, 0)$ and $(0, \pm \pi)$ and couples more strongly with the $B_{1g}$ mode. This model explains in a natural way why the $B_{2g}$ Raman peak occurs at a higher frequency than the $B_{1g}$ one at optimal doping, and how these two peaks change with doping in agreement with experiments. Our result suggests that the superconducting pairing in electron-doped high-$T_c$ cuprates has the $d_{x^2-y^2}$-wave symmetry and results from the same mechanism as for hole doped materials., Comment: 5 RevTeX4 pages, 3 EPS figures, replaced with published version (two tables added)

Based on a Fermi liquid picture, the temperature effect on the impurity-induced spatial modulation of local density of states (LDOS) is investigated for the d-wave superconductor Bi$_2$Sr$_2$CaCu$_2$O$_8$, in the context of scanning tunneling microscopy (STM). It is found that stripe-like structure exists even in the normal state due to a local-nesting mechanism, which is different from the octet scattering mechanism proposed by McElroy $et al$. {[Nature {\bf 422}, 592 (2003)]} in the d-wave superconducting ($d$SC) state. The normal-state spectra, when Fourier-transformed into the reciprocal space, can reveal the information of the entire Fermi surface at a single measuring bias, in contrast to the point-wise tracing proposed by McElroy $et al$. This may serve as another way to check the reality of Landau quasiparticles in the normal state. We have also re-visited the spectra in the $d$SC state and pointed out that, due to the Umklapp symmetry of the lattice, there should exist additional peaks in the reciprocal space, but experimentally yet to be found., Comment: 5 pages, 5 figures

Comment on ``Relating atomic-scale electronic phenomena to wave-like quasiparticle states in superconducting Bi$_2$Sr$_2$CaCu$_2$O$_{8+\delta}$'', Comment: 1 pages, 1 figures

Motivated by recent angle-resolved photoemission spectroscopy (ARPES) measurement that superconducting YBa$_2$Cu$_3$O$_{7-\delta}$ (YBCO) exhibits a $d_{x^2-y^2} + s$-symmetry gap, we show possible quasi-one-dimensional modulations of local density of states in YBCO. These aniostropic gap and defect induced stripe structures are most conspicuous at higher biases and arise due to the nesting effect associated with a Fermi liquid. Observation of these spectra by scanning tunneling microscopy (STM) would unify the picture among STM, ARPES, and inelastic neutron scattering for YBCO., Comment: 4 pages, 4 figures

We calculate the ultrasonic absorption and the thermal conductivity in the superconducting state of UPt$_{3}$ as functions of temperature and direction of propagation and polarization. Two leading candidates for the superconducting order parameter are considered: the $E_{1g}$ and $E_{2u}$ representations. Both can fit the data except for the ultrasonic absorption in the $A$ phase. To do that, it is necessary to suppose that the system has only a single domain, and that must be chosen as the most favorable one. However, the $E_{2u}$ theory requires fine-tuning of parameters to fit the low temperature thermal conductivity. Thus, transport data favor the $E_{1g}$ theory. Measurements of the thermal conductivity as a function of pressure at low temperature could help to further distinguish the two theories., Comment: 7 pages, 4 figures

Recent ARPES measurement observed a large $ab$-axis gap anisotropy, $\Delta(0,\pi)/\Delta(\pi,0)=1.5$, in clean YBa$_2$Cu$_3$O$_{7-\delta}$. This indicates that some sub-dominant component may exist in the $d_{x^2-y^2}$-wave dominant gap. We propose that the interlayer pairing tunneling contribution can be determined through the investigation of the order parameter anisotropy. Their potentially observable features in transport and spin dynamics are also studied., Comment: 4 pages, 3 figures

Recent experiments make it appear more likely that the order parameter of the unconventional superconductor Sr$_2$RuO$_4$ has a spin-triplet $f$-wave symmetry. We study ultrasonic absorption and thermal conductivity of superconducting Sr$_2$RuO$_4$ and fit to the recent data for various $f$-wave candidates. It is shown that only $f_{x^2-y^2}$-wave symmetry can account qualitatively for the transport data., Comment: 4 pages, 2 figures, references added and updated

We derive the Ginzburg-Landau-Wilson theory for the superconducting phase transition in two dimensions and in the magnetic field. Without disorder the theory describes a fluctuation induced first-order quantum phase transition into the Abrikosov lattice. We propose a phenomenological criterion for determining the transition field and discuss the qualitative effects of disorder. Comparison with recent experiments on MoGe films is discussed., Comment: 7 pages, 2 figures

A detailed study of the Inelastic Neutron Scattering (INS) spectra of the high-$T_c$ cuprates based on the Fermi liquid (FL) picture is given. We focus on the issue of the transformation between the commensurate and incommensurate (IC) excitation driven by frequency or $temperature$. For La$_{2-x}$Sr$_x$CuO$_4$ (LSCO), the condition of small $\Delta(0)/v_F a$ (where $a$ is the lattice constant, and henceforth will be set to 1) can simultaneously reproduces the always existing IC peaks in the superconducting (SC) and normal state, and the always fixed location at temperature or frequency change. For YBa$_2$Cu$_3$O$_{6+x}$ (YBCO), a moderate $\Delta(0)/v_F a$ and proximity of the van Hove singularity (vHS) at ${\bar M}=(0,\pi)$ to the Fermi level can reproduce the frequency- and temperature-driven shifting IC peaks in the SC state, and the vanishing of the IC peak in the normal state. The commensurate peak is found to be more appropriately described as a random phase approximation (RPA) effect. We address the conditional peak shifting behavior to a refined consideration on the nesting effect which is previously overlook. As a result, both the data on LSCO and the recent data on YBCO (on YBa$_2$Cu$_3$O$_{6.7}$ by Arai $et$ $al.$ and YBa$_2$Cu$_3$O$_{6.85}$ by Bourges $et$ $al.$) can be reasonably reconciled within a FL picture. We also point out that the one-dimensional-like data by Mook $et$ $al.$ on a detwinned and more underdoped sample YBa$_2$Cu$_3$O$_{6.6}$ could be due to a gap anisotropy effect discussed by Rendell and Carbotte, and we proceed to suggest a way of clarifying it., Comment: 13 pages, 13 figures, revised version

We present an interpretation of the recent neutron data on the commensurate and incommensurate spin fluctuations found in YBa_2d$Cu_3O_{6+y} based on a special configuration of the electronic dispersion and intervention from the d_{x^2-y^2}-wave superconducting phase. The observed switch over between the commensurate and incommensurate fluctuation spectra at the change of frequency or temperature is naturally accounted within this scenario., Comment: 4 pages and 4 figs

Motivated by recent experimental measurements on the Fermi surface(FS) destruction in underdoped high-$T_{c}$ cuprates, we examine its effect on the transport properties based on the Boltzmann equation approach. The effect is modeled by simply taking the density of states for electrons in the gapped regions to be zero. Within the nearly antiferromagnetic Fermi liquid model, we calculate the temperature dependences of the dc resistivity, the inverse Hall angle and the Hall coefficient. It is shown that the effect of the FS destruction on transport properties is sensitive to the existance and the range of the flat band near $(0,\pm\pi)$ in the dispersion of electrons, and the anistropy of the relaxation rate along the Fermi surface. We find that the experimental data are better described by the cold spot model, i.e., the transports are determined mainly by the contribution of the electrons near the Brillouin-zone diagonals., Comment: 8 pages, 7 figures

We investigate the universal electronic transport for a mixed $d_{x^2-y^2}$+s-wave superconductor in the presence of an anisotropic elliptical Fermi surface. Similar to the universal low-temperature transport predicted in a $d_{x^2-y^2}$-wave superconductor with a circular Fermi surface, anisotropic universal features are found in the low-temperature microwave conductivity, and thermal conductivity in the anisotropic system. The effects of anisotropy on the penetration depth, impurity induced $T_c$ suppression, and the zero-frequency density of states are also considered. While a small amount of anisotropy can lead to a strong suppression of the effective scattering rate and hence the density of states at zero frequency, experimental data suggests that large effects are restored by a negative $s$-component gap admixture., Comment: 8 pages

There exists significant in-plane anisotropy between $a$ and $b$ axis for various properties in YBCO. However recent thermal conductivity measurement by Chiao et al. which confirms previous microwave conductivity measurement by Zhang et al., shows no obvious anisotropy in the context of universal transport. We give a possible explanation of why the anisotropy is seen in most properties but not seen in the universal transport., Comment: 4 pages, 4 figures

It is known that for an unconventional superconductor with nodes in the gap, the in-plane microwave or dc conductivity saturates at low temperatures to a universal value independent of the impurity concentration. We demonstrate that a similar feature can be accessed using channel-dependent Raman scattering. It is found that, for a $d_{x^2-y^2}$-wave superconductor, the slope of low-temperature Raman intensity at zero frequency is universal in the $A_{1g}$ and $B_{2g}$ channels, but not in the $B_{1g}$ channel. Moreover, as opposed to the microwave conductivity, universal Raman slopes are sensitive not only to the existence of a node, but also to different pairing states and should allow one to distinguish between such pairing states., Comment: 5 pages

The magnetic susceptibility of high-T_c superconductors is investigated in the normal state using a coupled bilayer model. While this model describes in a natural way the normal-state pseudogaps seen in c-axis optical conductivity on underdoped samples, it predicts a weakly increasing susceptibility with decreasing temperature and cannot explain the magnetic pseudogaps exhibited in NMR measurements. Our result, together with some experimental evidence suggest that the mechanism governing the c-axis optical pseudogap is different from that for the $a-b$ plane magnetic pseudogap., Comment: 5 pages, 2 figures

We consider the effect of interband transitions on the c-axis conductivity, DC resistivity, and thermal conductivity in a plane-chain bilayer model of a cuprate. The relation between the c-axis resistivity and thermal conductivity is governed by the Wiedemann-Franz law. When the perpendicular hopping matrix element between chain and plane ($t_\perp$) is small, the c-axis DC resistivity shows a characteristic upturn as the temperature is lowered and the infrared conductivity develops a pseudogap. As $t_\perp$ is increased, intraband transitions start to dominate and a more conventional response is obtained. Analytical results for a simple plane-plane bilayer are also given, including the frequency sum rule of the optical conductivity., Comment: 9 pages, 7 figures, submitted to Phys. Rev. B

A simplified model of c axis transport in the high T_c superconductors is presented. Expressions are found for the c axis optical conductivity, the d.c. resistivity, and the c axis penetration depth. Within the framework of this model, the pseudogap in the optical conductivity arises naturally as a result of the layered band structure of the high T_c materials. We discuss the occurence of the pseudogap in terms of three parameters: a band gap Delta_{ps}, a temperature dependent scattering rate Gamma(T), and the strength of the interlayer coupling t_{perp}. We are also able to find analytic expressions for the d.c. conductivity and the low temperature penetration depth in terms of these three parameters. This work is an attempt to present a simple, unified picture of c axis properties in the high T_c cuprates., Comment: 3 pages, 2 figures, Proceedings of Spectroscopies of Novel Superconductors '97, To be published in J. Phys. Chem. Solids

We investigate the Raman spectra in the geometry where both incident and scattered photon polarizations are parallel to the $\hat{z}$-direction, for a plane-chain bilayer coupled via a single-particle tunneling $t_\perp$. The Raman vertex is derived in the tight-binding limit and in the absence of Coulomb screening, the Raman intensity can be separated into intraband ($\propto t_\perp^4$) and interband ($\propto t_\perp^2$) transitions. In the small-$t_\perp$ limit, the interband part dominates and a pseudogap will appear as it does in the conductivity. Coulomb interactions bring in a two-particle coupling and result in the breakdown of intra- and interband separation. Nevertheless, when $t_\perp$ is small, the Coulomb screening ($\propto t_\perp^4$) has little effect on the intensity to which the unscreened interband transitions contribute most. In general, the total Raman spectra are strongly dependent on the magnitude of $t_\perp$., Comment: 23 pages, 6 figures, submitted to Phys. Rev. B

Generalizing recent work, the Raman scattering intensity from a semi-infinite superconducting superlattice is calculated taking into account the surface contribution to the density response functions. Our work makes use of the formalism of Jain and Allen developed for normal superlattices. The surface contributions are shown to strongly modify the bulk contribution to the Raman-spectrum line shape below $2\Delta$, and also may give rise to additional surface plasmon modes above $2\Delta$. The interplay between the bulk and surface contribution is strongly dependent on the momentum transfer $q_\parallel$ parallel to layers. However, we argue that the scattering cross-section for the out-of-phase phase modes (which arise from interlayer Cooper pair tunneling) will not be affected and thus should be the only structure exhibited in the Raman spectrum below $2\Delta$ for relatively large $q_\parallel\sim 0.1\Delta/v_F$. The intensity is small but perhaps observable., Comment: 14 pages, RevTex, 6 figures