Metal-to-insulator crossover in YBa2Cu3Oy probed by low-temperature quasiparticle heat transport

It was recently demonstrated that in La2-xSrxCuO4 the magnetic-field (H) dependence of the low-temperature thermal conductivity kappa up to 16 T reflects whether the normal state under high magnetic field is a metal or an insulator. We measure the H dependence of kappa in YBa(2)Cu(3)O(y) (YBCO) at subkelvin temperatures for a wide doping range, and find that at low doping the kappa(H) behavior signifies the change in the ground state in this system as well. Surprisingly, the critical doping is found to be located deeply inside the underdoped region, about the hole doping of 0.07 hole/Cu; this critical doping is apparently related to the stripe correlations as revealed by the in-plane resistivity anisotropy.     

Excitation gap in the normal and superconducting state of underdoped Bi2Sr2Ca1-xDyxCu2O8+\delta thin film and single crystals

We summarize photoemission data from underdoped Bi₂Sr₂Ca_1-xDy_xCu₂O_8+\delta that revealed anomalous properties: i) We observed an excitation gap in the normal state of underdoped samples, and this normal state gap closes in overdoped samples; ii) The normal state gap has similar magnitude and momentum dependence as the superconducting gap, which is consistent with a d_x ² _-y ² order parameter; iii) The normal state gap persists to a temperature range much higher than T_c; iv) The superconducting gap in the underdoped regime does not scale with T_c. These results are consistent with theoretical models that suggest the underdoped regime can be characterized by two temperatures, a mean‐field temperature below which there is pairing, and a lower superconducting transition temperature at which the pairs become phase coherent. This revised version was published online in August 2006 with corrections to the Cover Date.     

Doping dependent evolution of infrared spectra in underdoped YBa2Cu3Oy in terms of two-component optical conductivity

In access to optical spectroscopy of heavily underdoped detwinned YBa₂Cu₃O_y (YBCO) crystals, we re-examine the doping dependent evolution of infrared spectra in the CuO₂ plane of underdoped YBCO in terms of two-component optical conductivity. The extended Drude model analysis is applied to the two-component conductivity, and the results are compared with experimental data in the pseudogap state. We demonstrate that a model consisting of a Drude and Lorentz oscillator components reproduces characteristics of infrared spectra in underdoped YBCO.     

Collective modes in the loop ordered phase of cuprate superconductors

We show that the two branches of collective modes discovered recently in underdoped cuprates with a large spectral weight are a necessary consequence of the loop-current state. Such a state has been shown in earlier experiments to be consistent with the symmetry of the order parameter competing with superconductivity in four families of cuprates. We also predict a third branch of excitations which cannot be discovered by neutron scattering but may be discovered by other techniques. Using parameters to fit the observed modes, we show that quantum fluctuations change the direction of the effective moments in the ground state to lie at an angle to the c axis as observed in experiments.     

Magnetic field induced insulator–metal crossover in underdoped cuprates

Within the charge-spin separation fermion-spin theory, the influence of a strong external magnetic field on the normal state transport of the underdoped cuprates is discussed. It is shown that when superconductivity in the underdoped cuprates is suppressed in the presence of a strong external magnetic field, the system reveals a low temperature normal state insulator–metal crossover. It is also shown that this striking insulator–metal crossover behavior is intriguingly related to the antiferromagnetic correlation.     

Field-induced thermal metal-to-insulator transition in underdoped La(2-x)Sr(x)CuO(4+delta)

The transport of heat and charge in cuprates was measured in single crystals of La(2-x)Sr(x)CuO(4+delta) (LSCO) across the doping phase diagram at low temperatures. In underdoped LSCO, the thermal conductivity is found to decrease with increasing magnetic field in the T-->0 limit, in striking contrast to the increase observed in all superconductors, including cuprates at higher doping. In heavily underdoped LSCO, where superconductivity can be entirely suppressed with an applied magnetic field, we show that a novel thermal metal-to-insulator transition takes place upon going from the superconducting state to the field-induced normal state.     

Anomalous proximity effect in underdoped YBa2Cu3O6+x josephson junctions

Josephson junctions were photogenerated in underdoped thin films of the YBa2Cu3O6+x family using a near-field scanning optical microscope. The observation of the Josephson effect for separations as large as 100 nm between two wires indicates the existence of an anomalously large proximity effect and shows that the underdoped insulating material in the gap of the junction is readily perturbed into the superconducting state. The critical current of the junctions was found to be consistent with the conventional Josephson relationship. This result appears to constrain the applicability of SO(5) theory to explain the phase diagram of high critical temperature superconductors.     

Tunneling into the normal state of Pr2-xCexCuO4

The temperature dependence of the tunneling conductance was measured for various doping levels of Pr(2-x)CexCuO4 using planar junctions. A normal state gap is seen at all doping levels studied, x=0.11 to x=0.19. We find it to vanish above a certain temperature T*. T* is greater than T(c) for the underdoped region and it follows T(c) on the overdoped side. This behavior suggests finite pairing amplitude above T(c) on the underdoped side.     

Time-reversal symmetry breaking by a (d+id) density-wave state in underdoped cuprate superconductors

It was proposed that the id(x(2)-y(2)) density-wave state (DDW) may be responsible for the pseudogap behavior in the underdoped cuprates. Here we show that the admixture of a small d(xy) component to the DDW state breaks the symmetry between the counterpropagating orbital currents of the DDW state and, thus, violates the macroscopic time-reversal symmetry. This symmetry breaking results in a nonzero polar Kerr effect, which has recently been observed in the pseudogap phase.     

Resonant impurity states in the d-density-wave phase

We study the electronic structure near impurities in the d-density-wave (DDW) state, a possible candidate phase for the pseudogap region of the high-temperature superconductors. We show that the density of states near a nonmagnetic impurity in the DDW state is qualitatively different from that in a superconductor with dx(2)(-y(2)) symmetry. Thus, the electronic structure near impurities can provide insight into the nature of the two phases recently observed by scanning tunneling microscopy experiments in the superconducting state of underdoped Bi-2212 compounds.     

Magneto-optical evidence for a gapped fermi surface in underdoped YBa2Cu3O6+x

The infrared (900-1100 cm(-1)) Faraday rotation and circular dichroism are measured in the normal state of underdoped High T(c) superconductors and used to study the magnetotransport. YBa2Cu3O6+x thin films are investigated in the temperature range 10-300 K in magnetic fields up to 8 T and as a function of oxygen concentration. A dramatic increase of the Hall frequency is observed for underdoped samples, which is not consistent with the approach to a Mott transition but is consistent with a partial gapping of the Fermi surface as predicted in density wave models.     

Breakdown of universal transport in correlated d-wave superconductors

The prediction and observation of low-temperature universal thermal conductivity in cuprates has served as a keystone of theoretical approaches to the superconducting state, but recent measurements on underdoped samples show strong violations of this apparently fundamental property of d-wave nodal quasiparticles. Here, we show that the breakdown of universality may be understood as the consequence of disorder-induced magnetic droplets arising from enhanced antiferromagnetic correlations in the underdoped state, even as these same correlations protect the nodal density of states.     

Spin-fluctuation gating in the c-axis non-Drude conductivity of the high Tc cuprates

An ideal antiferromagnetic (AF) ordering of the spins of the CuO layers of an underdoped cuprate prevents the low energy tunneling of the charge carriers between the layers. In order to obtain a non-vanishing c-axis conductivity (σ_c), we invoke ground state fluctuations of the spin system. These provide a frequency-dependent gating effect by changing the direction of the AF order parameter within one layer relative to that in a neighboring layer, thereby permitting some tunneling. The calculated σ_c compares favorably with experimental data in a) being small and b) having a weak frequency dependence of distinctly non-Drude form.     

Superconductor-to-insulator transition and transport properties of underdoped YBa2Cu3O(y) crystals

The carrier-concentration-driven superconductor-to-insulator (SI) transition as well as transport properties in underdoped YBa2Cu3O(y) twinned crystals is studied. The SI transition takes place at y approximately 6.3, carrier concentration n(SI)H approximately 3x10(20) cm(-3), anisotropy rho(c)/rho(ab) approximately 10(3), and the threshold resistivity rho(SI)ab approximately 0.8 mOmega cm which corresponds to a critical sheet resistance h/4e2 approximately 6.5 kOmega per CuO2 bilayer. The evolution of a carrier, nH infiniti y - 6.2, is clearly observed in the underdoped region. The resistivity and Hall coefficient abruptly acquire strong temperature dependence at y approximately 6.5 indicating a radical change in the electronic state.     

The c—Axis Infrared Conductivity of a dx^2—y^2— Wave Superconductor with the Hopping Assisted by the Spin Fluctuations

A theory of the c-axis infrared conductivity of a dx^2-y^2- wave superconductor due to the competition between the interlayer direct hopping and the hopping assisted by the spin fluctuations has been developed.The prediction of our theory captures the main feature of the experiment.Thus we argue that the anomalous behavior of the c-axis infrared conductivity of the underdoped cuprates in superconducting state may be properly understood within the theory.     

Coherent d-wave superconducting gap in underdoped La2-xSrxCuO4 by angle-resolved photoemission spectroscopy

We present angle-resolved photoemission spectroscopy data on moderately underdoped La1.855Sr0.145CuO4 at temperatures below and above the superconducting transition temperature. Unlike previous studies of this material, we observe sharp spectral peaks along the entire underlying Fermi surface in the superconducting state. These peaks trace out an energy gap that follows a simple d-wave form, with a maximum superconducting gap of 14 meV. Our results are consistent with a single gap picture for the cuprates. Furthermore our data on the even more underdoped sample La1.895Sr0.105CuO4 also show sharp spectral peaks, even at the antinode, with a maximum superconducting gap of 26 meV.     

Influence of the Pseudogap on the Inelastic Neutron Scattering Spectra of the Underdoped Lanthanum Cuprate

The influence of pseudogap on the inelastic neutron scattering spectra of the underdoped lanthanum cuprate is studied on the basis of the model which incorporates both the superconducting state and pseudogap state.It is found that the striking effects of the influence of the pseudogap on the incommensurability of the spin excitation spectrum are that in the superconducting state the pseudogap makes the intensity of the incommensurate peak increase,in the normal state the pseudogap not only makes the intensity of the incommensurate peak increase,but also sharpens the incommensurate peak and increases incommensurability.     

Kinetic energy driven pairing in cuprate superconductors

Pairing occurs in conventional superconductors through a reduction of the electronic potential energy accompanied by an increase in kinetic energy. In the underdoped cuprates, optical experiments show that pairing is driven by a reduction of the electronic kinetic energy. Using the dynamical cluster approximation we study superconductivity in the two-dimensional Hubbard model. We find that pairing is indeed driven by the kinetic energy and that superconductivity evolves from an unconventional state with partial spin-charge separation, to a superconducting state with quasiparticle excitations.     

Enhancement of the superconducting transition temperature of La2-xSrxCuO4 bilayers: role of pairing and phase stiffness

The superconducting transition temperature T(c) of bilayers comprising underdoped La(2-x)Sr(x)CuO(4) films capped by a thin heavily overdoped metallic La(1.65)Sr(0.35)CuO(4) layer, is found to increase with respect to T(c) of the bare underdoped films. The highest T(c) is achieved for x=0.12, close to the "anomalous" 1/8 doping level, and exceeds that of the optimally doped bare film. Our data suggest that the enhanced superconductivity is confined to the interface between the layers. We attribute the effect to a combination of the high pairing scale in the underdoped layer with an enhanced phase stiffness induced by the overdoped film.     

Evidence for two separate energy gaps in underdoped high-temperature cuprate superconductors from broadband infrared ellipsometry

We present broadband infrared ellipsometry measurements of the c-axis conductivity of underdoped RBa_{2}Cu_{3}O_{7-delta} (R=Y, Nd, and La) single crystals. Our data show that separate energy scales are underlying the redistributions of spectral weight due to the normal state pseudogap and the superconducting gap. Furthermore, they provide evidence that these gaps do not share the same electronic states and do not merge on the overdoped side. Accordingly, our data are suggestive of a two gap scenario with a pseudogap that is likely extrinsic with respect to superconductivity.