arXiv subjects

K. Levin

Publications and source records attributed to K. Levin.

At least 127 records · Page 7Linked to original sources

arXiv2000

Commensurate and Incommensurate Structure of the Neutron Cross Section in LaSrCuO and YBaCuO

We study the evolution of the d-wave neutron cross-section with variable frequency ωand fixed T (below and above Tc) in two different cuprate families. The evolution from incommensurate to commensurate to incommensurate peaks is rather generic within an RPA-like scheme. This behavior seems to be in reasonable accord with experiments, and may help distinguish between this and the "stripe" scenario.

cond-mat.str-el↗

arXiv2000

Frequency Evolution of Neutron Peaks Below Tc: Commensurate and Incommensurate Structure in LaSrCuO and YBaCuO

We study the evolution of the neutron cross-section with variable frequency $ω$ and fixed $T$ below $T_c$ in two different cuprate families. Our calculations, which predominantly probe the role of d-wave pairing, lead to generic features, independent of Fermi surface shapes. Among our findings, reasonably consistent with experiment, are (i) for $ω$ near the gap energy $Δ$, both optimal {LaSrCuO} and slightly underdoped YBCO exhibit (comparably) incommensurate peaks (ii) peak sharpening below $T_c$ is seen in {LaSrCuO}, (iii) quite generically, a frequency evolution from incommensurate to commensurate and then back to incommensurate structure is found with increasing $ω$. Due to their narrow $ω$ regime of stability, commensurate peaks in {LaSrCuO} should be extremely difficult to observe.

cond-mat.supr-con↗

arXiv1999

Nodal Quasiparticles versus Phase Fluctuations in High Tc Superconductors: An Intermediate Scenario

We explore a BCS Bose Einstein crossover scenario for $0 \leq T \leq T_c$ and its implications for the superfluid density and specific heat. The low lying excitations consist of nodal (fermionic) quasi-particles as well as excited (bosonic) pair states. Semi-quantitative comparison with cuprate data is reasonable, with no compelling indications for Landau parameter effects.

cond-mat.supr-con↗

arXiv1999

Pair Excitations, Collective Modes and Gauge Invariance in the BCS -- Bose-Einstein Crossover Scenario

In this paper we study the BCS Bose Einstein condensation (BEC) crossover scenario within the superconducting state, using a T-matrix approach which yields the ground state proposed by Leggett. Here we extend this ground state analysis to finite temperatures T and interpret the resulting physics. We find two types of bosonic-like excitations of the system: long lived, incoherent pair excitations and collective modes of the superconducting order parameter, which have different dynamics. Using a gauge invariant formalism, this paper addresses their contrasting behavior as a function of T and superconducting coupling constant g. At a more physical level, our paper emphasizes how, at finite T, BCS-BEC approaches introduce an important parameter Δ^2_{pg} = Δ^2 - Δ_{sc}^2 into the description of superconductivity. This parameter is governed by the pair excitations and is associated with particle-hole asymmetry effects which are important for sufficiently large g. In the fermionic regime, Δ_{pg}^2 represents the difference between the square of the excitation gap Δ^2 and that of the superconducting order parameter Δ_{sc}^2. The parameter Δ_{pg}^2, which is necessarily zero in the BCS (mean field) limit increases monotonically with the strength of the attractive interaction g. It follows that there is a significant physical distinction between this BCS-BEC crossover approach (in which g is the essential variable which determines Δ_{pg}) and the widely discussed (Coulomb-modulated) phase fluctuation scenario in which the plasma frequency is the tuning parameter.

cond-mat.supr-con↗

arXiv1999

Superconducting transitions from the pseudogap state: d-wave symmetry, lattice, and low-dimensional effects

We investigate the behavior of the superconducting transition temperature within a previously developed BCS-Bose Einstein crossover picture. This picture, based on a decoupling scheme of Kadanoff and Martin, further extended by Patton, can be used to derive a simple form for the superconducting transition temperature in the presence of a pseudogap. We extend previous work which addressed the case of s-wave pairing in jellium, to explore the solutions for T_c as a function of variable coupling in more physically relevant situations. We thereby ascertain the effects of reduced dimensionality, periodic lattices and a d-wave pairing interaction. Implications for the cuprate superconductors are discussed.

cond-mat.supr-con↗

arXiv1998

Incoherent Pair Tunneling as a Probe of the Cuprate Pseudogap

We argue that incoherent pair tunneling in a cuprate superconductor junction with an optimally doped superconducting and an underdoped normal lead can be used to detect the presence of pairing correlations in the pseudogap phase of the underdoped lead. We estimate that the junction characteristics most suitable for studying the pair tunneling current are close to recently manufactured cuprate tunneling devices.

cond-mat.str-el↗

arXiv1998

Pairing Fluctuation Theory of Superconducting Properties in Underdoped to Overdoped Cuprates

We propose a theoretical description of the superconducting state of under- to overdoped cuprates, based on the short coherence length of these materials and the associated strong pairing fluctuations. The calculated $T_c$ and the zero temperature excitation gap $Δ(0)$, as a function of hole concentration $x$, are in semi-quantitative agreement with experiment. Although the ratio $T_c/Δ(0)$ has a strong $x$ dependence, different from the universal BCS value, and $Δ(T)$ deviates significantly from the BCS prediction, we obtain, quite remarkably, quasi-universal behavior, for the normalized superfluid density $ρ_s(T)/ρ_s(0)$ and the Josephson critical current $I_c(T)/I_c(0)$, as a function of $T/T_c$. While experiments on $ρ_s(T)$ are consistent with these results, future measurements on $I_c(T)$ are needed to test this prediction.

cond-mat.supr-con↗

arXiv1998

On the Theory of Magnets with Competing Double Exchange and Superexchange Interactions

In the CMR materials, ferromagnetic double exchange (DE) presumably coexists with a direct nearest-neighbour antiferromagnetic interaction. We construct a single-site mean field theory that explicitly takes into account the different nature of carrier-mediated ferromagnetism vs. Heisenberg-like superexchange. We find, in contrast to previous results in the literature, that the competition between these two exchange interactions leads to ferro- or antiferromagnetic order with incomplete saturation of the magnetization (or sublattice magnetization), rather than spin canting. The associated experimental implications are discussed.

cond-mat↗

arXiv1998

On the Relationship Between the Pseudo- and Superconducting Gaps: Effects of Residual Pairing Correlations Below Tc

The existence of a normal state spectral gap in underdoped cuprates raises important questions about the associated superconducting phase. For example, how does this pseudogap evolve into its below Tc counterpart? In this paper we characterize this unusual superconductor by investigating the nature of the ``residual'' pseudogap below Tc and, find that it leads to an important distinction between the superconducting excitation gap and order parameter. Our approach is based on a conserving diagrammatic BCS Bose-Einstein crossover theory which yields the precise BCS result in weak coupling at any T<Tc and reproduces Leggett's results in the T=0 limit. We explore the resulting experimental implications.

cond-mat.supr-con↗

arXiv1998

Pairing Correlations and the Pseudo-Gap State: Application of the Pairing Approximation Theory

We investigate the pseudogap onset temperature $T^*$, the superconducting transition temperature $T_c$ and the general nature of the pseudogap phase using a diagrammatic BCS-Bose Einstein crossover theory. This decoupling scheme is based on the pairing approximation of Kadanoff and Martin, further extended by Patton (KMP). Our consideration of the KMP pairing approximation is driven by the objective to obtain BCS like behavior at weak coupling, (which does not necessarily follow for other diagrammatic schemes). The breakdown of the Fermi liquid state at $T^*$ is investigated within the lowest order theory and is associated with intermediate values of the coupling. The superconducting instability $T_c$ is evaluated by introducing mode coupling effects, in which the long lived pairs are affected by the single particle pseudogap states and vice versa. Our $T_c$ equations, which turn out to be rather simple as a result of the KMP scheme, reveal a rich structure as a function of $g$ in which the pseudogap is found to compete with superconductivity. Our results are compared with alternate theories in the literature.

cond-mat.supr-con↗

arXiv1997

A Theory of Magnets with Competing Double Exchange and Superexchange Interactions

We study the competition between ferromagnetic double exchange (DE) and nearest-neighbour antiferromagnetic exchange in CMR materials. Towards this end, a single site mean field theory is proposed which emphasizes the hopping-mediated nature of the DE contribution. We find that the competition between these two exchange interactions leads to ferro- or antiferromagnetic order with incomplete saturation of the (sub)lattice magnetization. This conclusion is in contrast to previous results in the literature which find a canted spin arrangement under similar circumstances. We attribute this difference to the highly anisotropic exchange interactions used elsewhere. The associated experimental implications are discussed.

cond-mat↗

arXiv1997

Superconductivity from a pseudogapped normal state: a mode coupling approach to precursor superconductivity

We derive a phase diagram for the pseudogap onset temperature $T^*$ (associated with the breakdown of the Fermi liquid state, due to strong pairing correlations) and the superconducting instability, $T_c$, as a function of variable pairing strength. Our diagrammatic approach to the BCS - Bose-Einstein cross-over problem self consistently treats the coupling between the single particle and pair propagators, and leads to a continuous evolution of these propagators into the standard $T T_c$.

cond-mat.supr-con↗

arXiv1997

Pseudogap effects induced by resonant pair scattering

We demonstrate how resonant pair scattering of correlated electrons above T_c can give rise to pseudogap behavior. This resonance in the scattering T-matrix appears for superconducting interactions of intermediate strength, within the framework of a simple fermionic model. It is associated with a splitting of the single peak in the spectral function into a pair of peaks separated by an energy gap. Our physical picture is contrasted with that derived from other T-matrix schemes, with superconducting fluctuation effects, and with preformed pair (boson-fermion) models. Implications for photoemission and tunneling experiments in the cuprates are discussed.

cond-mat.supr-con↗

arXiv1996

Coulomb Correlations and Pseudo-gap Effects in a Pre-formed Pair Model for the Cuprates

We extend previous work on pre-formed pair models of superconductivity to incorporate Coulomb correlation effects. For neutral systems, these models have provided a useful scheme which interpolates between BCS and Bose Einstein condensation with increasing coupling and thereby describes some aspects of pseudo-gap phenomena. However, charge fluctuations (via the plasmon, $ω_p$) significantly modify the collective modes and therefore the interpolation behavior. We discuss the resulting behavior of the pseudo-gap and thermodynamic quantities such as $T_c$, $χ$ and $C_v$ as a function of $ω_p$.

cond-mat↗

arXiv1996

What does d-wave symmetry tell us about the pairing mechanism?

In this paper we argue that d-wave symmetry is a general consequence of superconductivity driven by repulsive interactions. Van Hove (or flat band) effects, deriving from the two dimensionality of the $CuO_2$ plane are important in stabilizing this state. By extending the original Kohn-Luttinger picture to a 2 D lattice, we find that the screened Coulomb term has important wave vector structure which leads to $d_{x^2-y^2}$ superconductivity

cond-mat↗

arXiv1995

Theory of the c-Axis Penetration Depth in the Cuprates

Recent measurements of the London penetration depth tensor in the cuprates find a weak temperature dependence along the c-direction which is seemingly inconsistent with evidence for d-wave pairing deduced from in-plane measurements. We demonstrate in this paper that these disparate results are not in contradiction, but can be explained within a theory based on incoherent quasiparticle hopping between the CuO2 layers. By relating the calculated temperature dependence of the penetration depth λ_c(T) to the c-axis resistivity, we show how the measured ratio λ_c^2(0) / λ_c^2(T) can provide insight into the behavior of c-axis transport below Tc and the related issue of ``confinement.''

cond-mat↗

arXiv1995

An Alternative Route to D-wave Superconductivity: the Charge Channel

We demonstrate that a $d_{x^2-y^2}$ superconducting pairing is naturally associated with the screened Coulomb interaction $V_{eff}({\bf q},ω)$ within the $CuO_2$ plane of high $T_c$ superconductors. This pairing instability arises (via the dielectric constant) from wave-vector structure in $V_{eff}({\bf q},ω)$ at ${\bf q}=(π,π)$, which is associated with 2D Van Hove and local field effects. Our results, which are independent of the bandstructure, suggest that direct Coulomb intereactions are significant (when compared to the spin channel) and will, at the least, enhance any other underlying mechanism for $d_{x^2-y^2}$ wave pairing.

cond-mat↗

arXiv1995

Superconducting Order Parameter Symmetry in Multi-layer Cuprates

We discuss the allowed order parameter symmetries in multi-layer cuprates and their physical consequences using highly non-specific forms of the inter- and intra-plane interactions. Within this framework, the bi-layer case is discussed in detail with particular attention paid to the role of small orthorhombic distortions as would derive from the chains in YBCO or superlattice effects in BSCCO. In the orthorhombic bi-layer case the (s,-s) state is of special interest, since for a wide range of parameters this state exhibits pi phase shifts in corner Josephson junction experiments. In addition, its transition temperature is found to be insensitive to non-magnetic inter-plane disorder, as would be present at the rare earth site in YBCO, for example. Of particular interest, also, are the role of van Hove singularities which are seen to stabilize states with d_{x^2 - y^2}-like symmetry, (as well as nodeless s-states) and to elongate the gap functions along the four van Hove points, thereby leading to a substantial region of gaplessness. We find that d_{x^2 - y^2}-like states are general solutions for repulsive interactions; they possess the fewest number of nodes and therefore the highest transition temperatures. In this way, they should not be specifically associated with a spin fluctuation driven pairing mechanism.

cond-mat↗