Naoki Momono

Correlation between the Doping Dependences of Superconducting Gap Magnitude 2Δ 0 and Pseudogap Temperature T* in High-T c Cuprates

We report that in La 2- x Sr x CuO 4 (La214) and Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212), the resistivity ρ and magnetic susceptibility χ decrease more sharply below T * , corresponding to the onset temperature of a spin gap (pseudogap) at least for Bi2212. In each system, T * scales with the superconducting gap magnitude 2Δ 0 ( T * ∼2Δ 0 /4.3 k B ), which increases as the doping level is lowered. We also report that 2Δ 0 is almost proportional to T max (> T * ), where T max is the temperature exhibiting a broad peak of χ and k B T max has been considered to give the measure of effective exchange energy J eff ; the α factors in 2Δ 0 ∼α k B T max ∼α J eff are nearly one and two for La214 and Bi2212, respectively.

Direct observation of the coexistence of the pseudogap and superconducting quasiparticles in Bi2Sr2CaCu2O8 + y by time-resolved optical spectroscopy.

We report the ultrafast optical response of quasiparticles (QPs) in both the pseudogap (PG) and superconducting (SC) states of an underdoped Bi2Sr2CaCu2O8 + y (Bi2212) single crystal measured with the time-resolved pump-probe technique. At a probe energy variant plancks over omegapr = 1.55 eV, it is found that the reflectivity change DeltaR/R changes its sign at exactly Tc, which allows the direct separation of the charge dynamics of PG and SC QPs. Further systematic investigations indicate that the transient signals associated with PG and SC QPs depend on the probe beam energy and polarization. By tuning them below Tc, two distinct components can be detected simultaneously, providing evidence for the coexistence of PG and SC QPs.

Rotated Stripe Order and its Competition with Superconductivity in La1.88Sr0.12CuO4

We report the observation of a bulk charge modulation in La1.88Sr0.12CuO4 (LSCO) with a characteristic in-plane wave-vector of (0.236, ±�), with �=0.011 r.l.u. The transverse shift of the ordering wave-vector indicates the presence of rotated charge-stripe ordering, demonstrating that the charge ordering is not pinned to the Cu-O bond direction. On cooling through the superconducting transition, we find an abrupt change in the growth of the charge correlations and a suppression of the charge order parameter indicating competition between the two orderings. Orthorhombic LSCO thus helps bridge the apparent disparities between the behavior previously observed in the tetragonal “214” cuprates and the orthorhombic yttrium and bismuth-based cuprates and thus lends strong support to the idea that there is a common motif to charge order in all cuprate families. Charges doped into the copper oxide planes of the insulating parent compounds frequently organize into rich electronic textures. The most well-known example is the charge-spin stripe state, first seen in La1.48Nd0.4Sr0.12CuO4 [1], and subsequently in a number of other “214” cuprates, most notably La2−xBaxCuO4 (LBCO) [2–8]. More recently, charge modulations have also been observed in other cuprate families including YBa2Cu3O6+y (“123”) [9–16], and (Bi2−xPbx)(Sr2−yLay)Can−1CunO2n+4+� [17–21]. This

STM/STS Study on 4a×4a Electronic Charge Order of Superconducting Bi2Sr2CaCu2O8+δ

We performed low-bias STM measurements on underdoped Bi2212 crystals, and confirmed that a two-dimensional (2D) superstructure with a periodicity of four lattice constants (4 a ) is formed within t...

Electron scattering, charge order, and pseudogap physics in La1.6–xNd0.4SrxCuO4: An angle-resolved photoemission spectroscopy study

We report an angle-resolved photoemission study of the charge stripe ordered La1.6-xNd0.4SrxCuO4 (Nd-LSCO) system. A comparative and quantitative line-shape analysis is presented as the system evolves from the overdoped regime into the charge ordered phase. On the overdoped side (x = 0.20), a normal-state antinodal spectral gap opens upon cooling below 80 K. In this process, spectral weight is preserved but redistributed to larger energies. A correlation between this spectral gap and electron scattering is found. A different line shape is observed in the antinodal region of charge ordered Nd-LSCO x = 1/8. Significant low-energy spectral weight appears to be lost. These observations are discussed in terms of spectral-weight redistribution and gapping originating from charge stripe ordering.

Superconducting Condensation Energy and Pseudogap Formation in La2-xSrxCuO4: New Energy scale for Superconductivity

The electronic specific heat C el was measured for La 2- x Sr x CuO 4 to examine the superconducting condensation energy at T =0, U 0 . We reconfirmed that U 0 is markedly reduced even in slightly underdoped samples whose T c s are still more than 30 K. The reduction of U 0 results from the suppression of the effective superconducting gap, in addition to that of the density of states at the Fermi level E F , caused by the pseudogap formation in the normal state. The doping-level ( p ) dependence of U 0 as well as T c can be explained over a wide p range by introducing an effective energy gap Δ eff =β p Δ 0 (β=4.5) instead of Δ 0 , where Δ 0 is the maximum gap at T ≪ T c .

Pseudogap phase of cuprate superconductors confined by Fermi surface topology

The properties of cuprate high-temperature superconductors are largely shaped by competing phases whose nature is often a mystery. Chiefly among them is the pseudogap phase, which sets in at a doping p* that is material-dependent. What determines p* is currently an open question. Here we show that the pseudogap cannot open on an electron-like Fermi surface, and can only exist below the doping pFS at which the large Fermi surface goes from hole-like to electron-like, so that p*u2009≤u2009pFS. We derive this result from high-magnetic-field transport measurements in La1.6−xNd0.4SrxCuO4 under pressure, which reveal a large and unexpected shift of p* with pressure, driven by a corresponding shift in pFS. This necessary condition for pseudogap formation, imposed by details of the Fermi surface, is a strong constraint for theories of the pseudogap phase. Our finding that p* can be tuned with a modest pressure opens a new route for experimental studies of the pseudogap.High-temperature superconductors exhibit pseudogap behaviour that remains of unknown origin, despite many years of intensive study. Here the authors study the onset of the pseudogap under pressure, providing evidence that it requires a hole-like Fermi surface and constraining future theoretical developments.

The Fermi surface and band folding in La2−xSrxCuO4, probed by angle-resolved photoemission

A systematic angle-resolved photoemission study of the electronic structure of La2−xSrxCuO4 in a wide doping range is presented in this paper. In addition to the main energy band, we observed a weaker additional band, the (π, π) folded band, which shows unusual doping dependence. The appearance of the folded band suggests that a Fermi surface reconstruction is doping dependent and could already occur at zero magnetic field.

What is the energy scale in determining the Tc of cuprate superconductivity

For the understanding of the mechanism of a superconducting transition, it is of great interest to clarify what the energy scale in determining the critical temperature Tc is. According to the BCS mean-field theory, the energy gap at T<<Tc, 2Δo, is proportional to Tc; 2Δo = 4.3kBTc for d-wave superconductors. In the case of high-Tc cuprates, however, 2Δo increases monotonically with the lowering of the hole-doping level p even in the underdoped region, where Tc is largely suppressed, and does not scale with Tc, except in a highly overdoped region. Interestingly, in a wide p range spreading over the under- and overdoped regions, 2Δo is almost proportional to crossover temperature T*, around which a spin gap and/or a (small) pseudogap of almost the same energy scale as 2Δo start to develop progressively; 2Δo/kBT* is nearly independent of p and comparable to the BCS value in the entire p range examined. On the other hand, 2Δo/kBTc is inversely proportional to p, 2Δo/kBTc~1/p, except in the highly overdoped region, where Tc is very close to the BCS expectation. This indicates that the energy scale in determining Tc is of order pΔo in high-Tc cuprates. In this article, we will discuss some scenarios for the transition from pseudogap to superconducting states, where the energy scale in determining Tc is expected to be of order pΔo.

On the relations among the pseudogap, electronic charge order and Fermi-arc superconductivity in Bi2Sr2CaCu2O8+δ

On the basis of STM/STS, break-junction tunneling and electronic Raman scattering experiments on Bi2Sr2CaCu2O8+δ reported so far, we suggest that the static, electronic charge order is associated with inhomogeneous electronic states on antinodal parts of the Fermi surface that are outside the Fermi-arc around the node and responsible for the pseudogap, and coexists with the homogeneous superconductivity caused by the pairing of coherent quasiparticles on the Fermi-arc, the so-called Fermi-arc superconductivity, in the real space, although the two electronic orders or the corresponding energy gaps compete with each other in the k-space.