high-Tc Cuprate Superconductors Research Articles

Local transport characteristics of break junction in Sr2RuO4 microbridge

We have measured tunnel conductance of spin-triplet superconductor Sr2RuO4 (SRO) break junction which was made by micro fabrication technique with a focused ion beam. This is a new type of tunnel junctions made of SRO, which is different from those made of SRO and other materials. Since the tunnel conductance is sensitive to the internal phase of superconductivity, it enables us to examine the chiral p-wave pairing state, which is the most probable candidate of SRO. The tunnel conductance spectrum of the junction showed a broad zero-bias conductance peak whose shape is different from that of high-Tc cuprate superconductors. The shape of the spectrum is in quite good agreement with the calculated spectrum of a chiral p-wave/insulator/normal metal junction.

Observation of strong intrinsic pinning inMgB2 films

By using three types of MgB2 superconductors, such as c-axis-oriented single-crystal films,c-axis-oriented columnar-structure films and films withoutc-axisorientation perpendicular to the substrate surface, we have investigated the intrinsic pinning effectin MgB2 superconductors. The strong field performance ofJc was observed by turning the orientation of grains from thec axis to thea axis. Noc-axis-orientedMgB2 films showed a noticeableincrease of Jc at highfields compared with c-axis-oriented films, whether they had columnar structures or not. Our results clearly show thatMgB2 has strong intrinsic pinning caused by the large anisotropy ofthe superconducting energy gap in the boron layers like high-Tc cuprate superconductors with a layered structure.

Quantum oscillations in the high- T c cuprates

We review recent progress in the study of quantum oscillations as a tool for uniquely probing low-energy electronic excitations in high-T(c) cuprate superconductors. Quantum oscillations in the underdoped cuprates reveal that a close correspondence with Landau Fermi-liquid behaviour persists in the accessed regions of the phase diagram, where small pockets are observed. Quantum oscillation results are viewed in the context of momentum-resolved probes such as photoemission, and evidence examined from complementary experiments for potential explanations for the transformation from a large Fermi surface into small sections. Indications from quantum oscillation measurements of a low-energy Fermi surface instability at low dopings under the superconducting dome at the metal-insulator transition are reviewed, and potential implications for enhanced superconducting temperatures are discussed.

Single-particle dispersion of the 2D p–d model

The two-dimensional p–d model has been solved within the Composite Operator Method (COM) in an original four-pole approximation where the basic four-component composite field contains both the standard electronic fields p and d, the latter split into two Hubbard operators, and a composite operator describing the coupling between the p electrons and the spin excitations of d electrons. The use of such a rich basis plus the enforcement of Algebra Constraints, typical of COM, in order to compute the unknowns of the theory and set the correct Hilbert space, allowed us to obtain an excellent agreement with the numerical results available in the literature. Supported by this evidence, we have now started a systematic comparison with the plethora of experimental results available in the literature for cuprate superconductors. In particular, in this short manuscript, we discuss the single-particle dispersion of the model, as obtained by means of the COM, and compare it to the ARPES experimentally observed one for Lanthanum family of monolayer high-Tc cuprate superconductors.

Filling and temperature dependence of the spin susceptibility of the two-dimensional Hubbard model in the superconducting d-wave phase

The two-dimensional Hubbard model in the superconducting d-wave phase has been solved within the Composite Operator Method (COM) in the two-pole approximation [1]. The unknowns of the theory have been computed exploiting the operatorial relations, dictated by the Pauli principle, existing between the composite operators belonging to the basis. Such a procedure has also allowed to correctly fix the Hilbert space of the problem avoiding to average on unphysical states and permitting to obtain a very good qualitative agreement with the experimental results available in the literature as regards the phase diagram in the T–n plane. Given such an encouraging result, in this short manuscript, we have analyzed the filling and temperature dependence of the static and uniform spin susceptibility, as obtained by means of the COM within a one-loop-like approximation, and we have compared them to the experimentally observed ones for single-layer high-Tc cuprate superconductors.

Doped high-Tccuprate superconductors elucidated in the light of zeros and poles of the electronic Green’s function

We study electronic structure of hole- and electron-doped Mott insulators in the two-dimensional Hubbard model to reach a unified picture for the normal state of cuprate high-Tc superconductors. By using a cluster extension of the dynamical mean-field theory, we demonstrate that structure of coexisting zeros and poles of the single-particle Green's function holds the key to understand Mott physics in the underdoped region. We show evidence for the emergence of non-Fermi-liquid phase caused by the topological quantum phase transition of Fermi surface by analyzing low-energy charge dynamics. The spectra calculated in a wide range of energy and momentum reproduce various anomalous properties observed in experiments for the high-Tc cuprates. Our results reveal that the pseudogap in hole-doped cuprates has a d-wave-like structure only below the Fermi level, while it retains non-d-wave structure with a fully opened gap above the Fermi energy even in the nodal direction due to a zero surface extending over the entire Brillouin zone. In addition to the non-d-wave pseudogap, the present comprehensive identifications of the spectral asymmetry as to the Fermi energy, the Fermi arc, and the back-bending behavior of the dispersion, waterfall, and low-energy kink, in agreement with the experimental anomalies of the cuprates, do not support that these originate from (the precursors of) symmetry breakings such as the preformed pairing and the d-density wave fluctuations, but support that they are direct consequences of the proximity to the Mott insulator. Several possible experiments are further proposed to prove or disprove our zero mechanism.

High-Quality Large-Sized Single Crystals of Pb-Doped Bi2Sr2CuO6+δ High-Tc Superconductors Grown with Traveling Solvent Floating Zone Method

High quality Pb-doped Bi2Sr2CuO6+δ (Pb-Bi2201) single crystals are grown by the traveling solvent floating zone technique, with dimensions as large as ∼ 50 mm× ∼ 5.0 mm× ∼ 2 mm. The Pb-Bi2201 single crystals with different doping levels are obtained by the annealing process which covers a wide doping range of the overdoped region. We describe in detail the growth and annealing procedures and the characterization and physical property measurements of the Pb-Bi2201 crystals. The availability of these crystals provides a good opportunity to experimentally investigate high-Tc cuprate superconductors, particularly in the overdoped region.

Erratum: Tuning the in-plane electron behavior in high-Tccuprate superconductors via apical atoms: A first-principles Wannier-states analysis [Phys. Rev. B79, 214512 (2009)

Erratum: Tuning the in-plane electron behavior in high-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:msub><mml:mi>T</mml:mi><mml:mi>c</mml:mi></mml:msub></mml:math>cuprate superconductors via apical atoms: A first-principles Wannier-states analysis [Phys. Rev. B<b>79</b>, 214512 (2009)

Particle–hole symmetry breaking in the pseudogap state of Bi2201

In conventional superconductors, a gap exists in the energy absorption spectrum only below the transition temperature (Tc), corresponding to the energy price to pay for breaking a Cooper pair of electrons. In high-Tc cuprate superconductors above Tc, an energy gap called the pseudogap exists, and is controversially attributed either to pre-formed superconducting pairs, which would exhibit particle-hole symmetry, or to competing phases which would typically break it. Scanning tunnelling microscopy (STM) studies suggest that the pseudogap stems from lattice translational symmetry breaking and is associated with a different characteristic spectrum for adding or removing electrons (particle-hole asymmetry). However, no signature of either spatial or energy symmetry breaking of the pseudogap has previously been observed by angle-resolved photoemission spectroscopy (ARPES). Here we report ARPES data from Bi2201 which reveals both particle-hole symmetry breaking and dramatic spectral broadening indicative of spatial symmetry breaking without long range order, upon crossing through T* into the pseudogap state. This symmetry breaking is found in the dominant region of the momentum space for the pseudogap, around the so-called anti-node near the Brillouin zone boundary. Our finding supports the STM conclusion that the pseudogap state is a broken-symmetry state that is distinct from homogeneous superconductivity.

Incommensurate spin correlations induced by magnetic Fe ions substituted into overdopedBi1.75Pb0.35Sr1.90CuO6+z

Spin correlations in the overdoped region of Bi1.75Pb0.35Sr1.90CuO6+z have been explored with Fe-doped single crystals characterized by neutron scattering, muon-spin-rotation (muSR) spectroscopy, and magnetic susceptibility measurements. Static incommensurate spin correlations induced by the Fe spins are revealed by elastic neutron scattering. The resultant incommensurability delta is unexpectedly large (~0.2 r.l.u.), as compared with delta ~ 1/8 in overdoped superconductor La2-xSrxCuO4. Intriguingly, the large delta in this overdoped region is close to the hole concentration p. This result is reminiscent of the delta ~ p trend observed in underdoped La2-xSrxCuO4; however, it is inconsistent with the saturation of delta in the latter compound in the overdoped regime. While our findings in Fe-doped Bi1.75Pb0.35Sr1.90CuO6+z support the commonality of incommensurate spin correlations in high-Tc cuprate superconductors, they also suggest that the magnetic response might be dominated by a distinct mechanism in the overdoped region.

Superconductivity in just four pairs of (BETS)2GaCl4 molecules

How small can a sample of superconducting material be and still display superconductivity? This question is relevant to our fundamental understanding of superconductivity, and also to applications in nanoscale electronics, because Joule heating of interconnecting wires is a major problem in nanoscale devices. It has been shown that ultrathin layers of metal can display superconductivity, but any limits on the size of superconducting systems remain a mystery. (BETS)2GaCl4, where BETS is bis(ethylenedithio)tetraselenafulvalene, is an organic superconductor, and in bulk it has a superconducting transition temperature Tc of approximately 8 K and a two-dimensional layered structure that is reminiscent of the high-Tc cuprate superconductors. Here, we use scanning tunnelling spectroscopy to show that a single layer of (BETS)2GaCl4 molecules on an Ag(111) surface displays a superconducting gap that increases exponentially with the length of the molecular chain. Moreover, we show that a superconducting gap can still be detected for just four pairs of (BETS)2GaCl4 molecules. Real-space spectroscopic images directly visualize the chains of BETS molecules as the origin of the superconductivity.

Proximity to understanding the cuprates

Modern materials engineering is increasingly dominated by the nanoscale, where surface area trumps volume, so it stands to reason that interfaces between materials are at the forefront for the discovery of new properties. The last few years have seen the discovery of new interface states such as a metal [1] or superconductor [2] between two insulators, magnetism between two nonmagnetic materials [3], and a topologically protected spin current between a semiconductor and vacuum [4]. One type of interface that has been studied for 50 years is still yielding new surprises: the superconductor-metal interface. Writing in Physical Review Letters[5], Colin Parker, Aakash Pushp, Abhay Pasupathy, Kenjiro Gomes, Jinsheng Wen, Zhijun Xu, Shimpei Ono, Genda Gu, and Ali Yazdani, in a collaboration between Princeton University and Brookhaven National Laboratory, both in the US, and the Central Research Institute of Electric Power Industry in Japan, use scanning tunneling microscopy to shed new light on this familiar old interface, resolving it for the first time with nanoscale precision in a cuprate high-Tc superconductor. This precise mapping may bring new insight to a most contentious debate in cuprate superconductivity.

Broken rotational symmetry in the pseudogap phase of a high-Tc superconductor

The nature of the pseudogap phase is a central problem in the effort to understand the high-transition-temperature (high-T(c)) copper oxide superconductors. A fundamental question is what symmetries are broken when the pseudogap phase sets in, which occurs when the temperature decreases below a value T*. There is evidence from measurements of both polarized neutron diffraction and the polar Kerr effect that time-reversal symmetry is broken, but at temperatures that differ significantly from one another. Broken rotational symmetry was detected from both resistivity measurements and inelastic neutron scattering at low doping, and from scanning tunnelling spectroscopy at low temperature, but showed no clear relation to T*. Here we report the observation of a large in-plane anisotropy of the Nernst effect in YBa(2)Cu(3)O(y) that sets in precisely at T* throughout the doping phase diagram. We show that the CuO chains of the orthorhombic lattice are not responsible for this anisotropy, which is therefore an intrinsic property of the CuO(2) planes. We conclude that the pseudogap phase is an electronic state that strongly breaks four-fold rotational symmetry. This narrows the range of possible states considerably, pointing to stripe or nematic order.

Checkerboard-like bound states around a vortex in high-Tc cuprate superconductors

We have studied the bound states around a single vortex in high-Tc cuprate superconductors, using the Bogoliubov-de Gennes equation. Experimentally, the bound states around the vortex in high-Tc cuprate superconductors are very different from those in the conventional superconductors. There are no peaks in the local density of states (LDOS) around the Fermi energy and the bound states shows checkerboard-like oscillations. These feature can not be explained in the frameworks of the theory of d-wave superconductivity(dSC). Therefore, we study the effect of pseudogap states, which coexist with d-wave superconductivity. We have adopted the d-wave spin-density-wave(dSDW) theory for the pseudogap state. After deriving the Bogoliubov-de Gennes equation for dSDW+dSC, we have solved it numerically, and obtained the LDOS, which shows bound states with checkerboard-like oscillations. This result shows that the dSDW states is a good candidate for the pseudogap state.

Single composite crystal structure analysis of incommensurate spin-ladder compound Sr2.5Ca11.5Cu24O41

Single-crystal X-ray structure analysis of mutually incommensurate (Sr0.36Ca1.64Cu2O3)0.706CuO2, “Sr2.5Ca11.5Cu24O41” has been performed on the basis of superspace group approach. The lattice constants of (Sr0.36Ca1.64Cu2O3)0.706CuO2 are a=1.1298(1), b=1.2582(1), c1=0.2750(1)nm for the CuO2 part and a=1.1298(1), b=1.2582(1), c2=0.3897(1)nm for the (Sr0.36Ca1.64Cu2O3) one. From the average structure analysis, it has been expected that the formation of CuO5 with pyramidal coordination by O atoms occurs in the Cu2O3 ladder. The minimum distance between Cu atom in the Cu2O3 ladder and O atom in the CuO2 chain has clearly shown that O in the CuO2 acts as apical oxygen to form the CuO5 coordination. It has been concluded that (Sr0.36Ca1.64Cu2O3)0.706CuO2 bears a marked structural resemblance to the cuprate High Tc superconductors.

Angle-resolved photoemission study of the tri-layer high-[formula omitted] superconductor [formula omitted]: Effects of inter-layer hopping

We have performed angle-resolved photoemission spectroscopy study of the electronic structure of the tri-layer high-Tc cuprate superconductor Bi2Sr2Ca2Cu3O10+δ and observed separate band dispersions and Fermi surfaces corresponding to the outer and inner CuO2 planes. From the band dispersions, the c-axis hopping parameter t⊥ between the outer and inner CuO2 planes is estimated to be ∼56meV.

Crystal growth and characterization of T∗ cuprate superconductor [formula omitted

One of the simplest structural and “low”-Tc cuprates, T∗-type cuprates, SmLa1-xSrxCuO4-δ(SLSCO,Tc(max)∼32K) and Nd1.6-ySr0.4CeyCuO4(NSCCO,Tc(max)∼25K), can be a key material to considerate the mechanism for high-Tc cuprate superconductor. Contrary to SLSCO whose as-grown sample does not show superconductivity at all, due to heavy deficiency of apical oxygen, NSCCO shows superconductivity even in as-grown crystal. We grew some compositions of wide range of Ce concentration and carefully investigated the change of Tc and show that NSCCO becomes sufficient bulk superconductivity, even without an extreme oxygen annealing.

Coexistence of Fermi arcs and Fermi pockets in a high-Tc copper oxide superconductor

In the pseudogap state of the high-transition-temperature (high-T(c)) copper oxide superconductors, angle-resolved photoemission (ARPES) measurements have seen Fermi arcs-that is, open-ended gapless sections in the large Fermi surface-rather than a closed loop expected of an ordinary metal. This is all the more puzzling because Fermi pockets (small closed Fermi surface features) have been suggested by recent quantum oscillation measurements. The Fermi arcs cannot be understood in terms of existing theories, although there is a solution in the form of conventional Fermi surface pockets associated with competing order, but with a back side that is for detailed reasons invisible to photoemission probes. Here we report ARPES measurements of Bi(2)Sr(2-x)La(x)CuO(6+delta) (La-Bi2201) that reveal Fermi pockets. The charge carriers in the pockets are holes, and the pockets show an unusual dependence on doping: they exist in underdoped but not overdoped samples. A surprise is that these Fermi pockets appear to coexist with the Fermi arcs. This coexistence has not been expected theoretically.

Temperature dependence of the superconductivity-induced collective mode in the c-axis infrared spectra of bilayer cuprate superconductors

The low-temperature spectra of the c-axis infrared conductivity of bilayer high-Tc cuprate superconductors (HTCS) exhibit two superconductivity-induced modes [Li Yu et al., Phys. Rev. Lett. 100 (2008) 177004; and references therein]. Both can be understood in terms of a microscopic theory developed recently [J. Chaloupka, C. Bernhard, D. Munzar, Phys. Rev. B 79 (2009) 184513]. Here we summarize the elements of the theory and report on the temperature dependence (TD) of the low-energy mode and of the total optical spectral weight (SW). The calculated TD of the mode is consistent with experiment but the trends of the SW are not.

Transition from band insulator to Bose-Einstein-condensate superfluid and Mott state of cold Fermi gases with multiband effects in optical lattices

We study two models realized by two-component Fermi gases loaded in optical lattices. We clarify that multi-band effects inevitably caused by the optical lattices generate a rich structure, when the systems crossover from the region of weakly bound molecular bosons to the region of strongly bound atomic bosons. Here the crossover can be controlled by attractive fermion interaction. One of the present models is a case with attractive fermion interaction, where an insulator-superfluid transition takes place. The transition is characterized as the transition between a band insulator and a Bose-Einstein condensate (BEC) superfluid state. Differing from the conventional BCS superfluid transition, this transition shows unconventional properties. In contrast to the one particle excitation gap scaled by the superfluid order parameter in the conventional BCS transition, because of the multi-band effects, a large gap of one-particle density of states is retained all through the transition although the superfluid order grows continuously from zero. A reentrant transition with lowering temperature is another unconventionality. The other model is the case with coexisting attractive and repulsive interactions. Within a mean field treatment, we find a new insulating state, an orbital ordered insulator. This insulator is one candidate for the Mott insulator of molecular bosons and is the first example that the orbital internal degrees of freedom of molecular bosons appears explicitly. Besides the emergence of a new phase, a coexisting phase also appears where superfluidity and an orbital order coexist just by doping holes or particles. The insulating and superfluid particles show differentiation in momentum space as in the high-Tc cuprate superconductors.