Survival of the d-wave superconducting state near the edge of antiferromagnetism in the cuprate phase diagram

Optical Absorption in Tilted Geometries as an Indirect Measurement of Longitudinal Plasma Waves in Layered Cuprates

Electromagnetic waves propagating in a layered superconductor with arbitrary momentum, with respect to the main crystallographic directions, exhibit an unavoidable mixing between longitudinal and transverse degrees of freedom. Here we show that this basic physical mechanism explains the emergence of a well-defined absorption peak in the in-plane optical conductivity when light propagates at small tilting angles relative to the stacking direction in layered cuprates. More specifically, we show that this peak, often interpreted as a spurious leakage of the c-axis Josephson plasmon, is instead a signature of the true longitudinal plasma mode occurring at larger momenta. By combining a classical approach based on Maxwell's equations with a full quantum derivation of the plasma modes based on modeling the superconducting phase degrees of freedom, we provide an analytical expression for the absorption peak as a function of the tilting angle and light polarization. We suggest that an all-optical measurement in tilted geometry can be used as an alternative way to access plasma-wave dispersion, usually measured by means of large-momenta scattering techniques like resonant inelastic X-ray scattering (RIXS) or electron energy loss spectroscopy (EELS).

Non-linear Terahertz driving of plasma waves in layered cuprates

The hallmark of superconductivity is the rigidity of the quantum-mechanical phase of electrons, responsible for superfluid behavior and Meissner effect. The strength of the phase stiffness is set by the Josephson coupling, which is strongly anisotropic in layered cuprates. So far, THz light pulses have been used to achieve non-linear control of the out-of-plane Josephson plasma mode, whose frequency lies in the THz range. However, the high-energy in-plane plasma mode has been considered insensitive to THz pumping. Here, we show that THz driving of both low-frequency and high-frequency plasma waves is possible via a general two-plasmon excitation mechanism. The anisotropy of the Josephson couplings leads to markedly different thermal effects for the out-of-plane and in-plane response, linking in both cases the emergence of non-linear photonics across T_c to the superfluid stiffness. Our results show that THz light pulses represent a preferential knob to selectively drive phase excitations in unconventional superconductors.

Josephson coupling determines the superconducting phase stiffness and sets the energy scale of plasma waves. Here, the authors show that THz light can induce two-plasmon excitations of both out-of-plane and in-plane phase modes, leading however to markedly different resonant and thermal effects due to the strong anisotropy of the Josephson couplings.

Interlayer penetration depth in the pseudogap phase of cuprate superconductors

The opening of a pseudogap in the electronic structure of the underdoped high Tc cuprates has a profound effect on superconducting properties. Here we consider the c-axis penetration depth. A phenomenological model of the pseudogap due to Yang, Rice, and Zhang (YRZ) is used. It is based on the idea of a resonating valence bond spin liquid. A simplifying limit, the arc model, is also considered as it provides useful analytic formulas. The zero temperature value of the superfluid density n(s)(T = 0) is greatly reduced with increasing values of the pseudogap (Δpg). This value reflects the reconstruction of the Fermi surface from the large contour of Fermi liquid theory to ever smaller Luttinger pockets as Δpg becomes larger. Also, as temperature is increased the ratio n(s)(T)/n(s)(0) as a function of the reduced temperature t = T/T(c) decreases more rapidly than in the corresponding Fermi liquid (Δpg = 0) as states which have both superconducting and pseudogap become more significantly sampled.

The Origin of Tc Enhancement in Heterostructure Cuprate Superconductors

Recent experiments on heterostructures composed of two or more films of cuprate superconductors of different oxygen doping levels have shown a remarkable Tc enhancement (up to 50%) relative to single compound films. We provide a simple explanation of the enhancement which arises naturally from a collection of experimental works. We show that the enhancement could be caused by a structural change in the lattice, namely an increase in the distance of the apical oxygen from the copper-oxygen plane. This increase modifies the effective off-site interaction in the plane which in turn enhances the d-wave superconductivity order parameter. To illustrate this point we study the extended Hubbard model using the fluctuation exchange approximation.

Superfluid density in a highly underdoped YBa_{2}Cu_{3}O_{6+y} superconductor

The superfluid density rho_{s}(T) identical with1/lambda;{2}(T) has been measured at 2.64 GHz in highly underdoped YBa_{2}Cu_{3}O_{6+y}, at 37 dopings with T_{c} between 3 and 17 K. Within limits set by the transition width DeltaT_{c} approximately 0.4 K, rho_{s}(T) shows no evidence of critical fluctuations as T-->T_{c}, with a mean-field-like transition and no indication of vortex unbinding. Instead, we propose that rho_{s} displays the behavior expected for a quantum phase transition in the (3+1)-dimensional XY universality class, with rho_{s0} proportional, variant(p-p_{c}), T_{c} proportional, variant(p-p_{c});{1/2}, and rho_{s}(T) proportional, variant(T_{c}-T);{1} as T-->T_{c}.

Nodeless d-wave superconducting pairing due to residual antiferromagnetism in underdoped Pr2-xCexCuO4-delta

We investigate the doping dependence of the penetration depth versus temperature in electron-doped Pr(2-x)Ce(x)CuO(4-delta) using a model which assumes the uniform coexistence of (mean-field) antiferromagnetism and superconductivity. Despite the presence of a d(x2-y2) pairing gap in the underlying spectrum, we find nodeless behavior of the low-T penetration depth in the underdoped case, in accord with experimental results. As doping increases, a linear-in-T behavior of the penetration depth, characteristic of d-wave pairing, emerges as the lower magnetic band crosses the Fermi level and creates a nodal Fermi surface pocket.

Kosterlitz-Thouless behavior in layered superconductors: the role of the vortex core energy

In layered superconductors (SC) with small interlayer Josephson coupling vortex-antivortex phase fluctuations characteristic of quasi two-dimensional (2D) Kosterlitz-Thouless behavior are expected to be observable at some energy scale T(d). While in the 2D case T(d) is uniquely identified by the KT temperature T(KT) where the universal value of the superfluid density is reached, we show that in a generic anisotropic 3D system T(d) is controlled by the vortex-core energy, and can be significantly larger than the 2D scale T(KT). These results are discussed in relation to recent experiments in cuprates, which represent a typical experimental realization of layered anisotropic SC.

Electric-field-effect modulation of the transition temperature, mobile carrier density, and in-plane penetration depth of NdBa2Cu3O7-delta thin films

We explore the relationship between the critical temperature T(c), the mobile areal carrier density n(2D), and the zero-temperature magnetic in-plane penetration depth lambda(ab)(0) in very thin underdoped NdBa(2)Cu(3)O(7-delta) films near the superconductor to insulator transition using the electric-field-effect technique. Having established consistency with a Kosterlitz-Thouless transition, we observe that T(KT) depends linearly on n(2D), the signature of a quantum superconductor to insulator transition in two dimensions with znu(over)=1, where z is the dynamic and nu is the critical exponent of the in-plane correlation length.

Onset of a Boson mode at the superconducting critical point of underdoped YBa2Cu3Oy

The thermal conductivity kappa of underdoped YBa2Cu3Oy was measured in the T-->0 limit as a function of hole concentration p across the superconducting critical point at pSC identical with 5.0%. The evolution of bosonic and fermionic contributions to kappa was tracked as the doping level evolved continuously in each of our samples. For p< or =pSC, we observe a T3 component in kappa which we attribute to the boson excitations of a phase with long-range spin or charge order. Fermionic transport, observed as a T-linear term in kappa which persists unaltered through pSC, violates the Wiedemann-Franz law, since the electrical resistivity varies as log(1/T) and grows with decreasing p.

Correlation between superfluid density and T(C) of underdoped YBa2Cu3O6+x near the superconductor-insulator transition

We report measurements of the ab-plane superfluid density n(s) (magnetic penetration depth lambda) of heavily underdoped films of YBa2Cu3O6+x, with T(C)'s from 6 to 50 K. We find the characteristic length for vortex unbinding transition equal to the film thickness, suggesting strongly coupled CuO2 layers. At the lowest dopings, T(C) is as much as 5 times larger than the upper limit set by the 2D Kosterlitz-Thouless-Berezinskii transition temperature calculated for individual CuO2 bilayers. Our main finding is that T(C) is not proportional to n(s)(0); instead, we find T(C) proportional to ns(1/2.3+/-0.4). This conflicts with a popular point of view that quasi-2D thermal phase fluctuations determine the transition temperature.

Effective theory of high-temperature superconductors

The field theory of a fluctuating d-wave superconductor is constructed and proposed as an effective description of superconducting cuprates at low energies. The theory is used to resolve a puzzle posed by recent experiments on superfluid density in severely underdoped YBa2(Cu3)O(6+x). In particular, the overall temperature dependence of the superfluid density at low dopings is argued to be described well by the strongly anisotropic weakly interacting Bose gas, and thus approximately linear in temperature with an almost doping-independent slope.

Coincidence of checkerboard charge order and antinodal state decoherence in strongly underdoped superconducting Bi2Sr2CaCu2O8 + delta)

The doping dependence of nanoscale electronic structure in superconducting Bi(2)Sr(2)CaCu(2)O(8 + delta) is studied by scanning tunneling microscopy. At all dopings, the low energy density-of-states modulations are analyzed according to a simple model of quasiparticle interference and found to be consistent with Fermi-arc superconductivity. The superconducting coherence peaks, ubiquitous in near-optimal tunneling spectra, are destroyed with strong underdoping and a new spectral type appears. Exclusively in regions exhibiting this new spectrum, we find local "checkerboard" charge ordering of high energy states, with a wave vector of Q = (+/- 2pi/4.5a(0),0); (0, +/- 2pi/4.5a(0)) +/- 15%. Surprisingly, this spatial ordering of high energy states coexists harmoniously with the low energy Bogoliubov quasiparticle states.