Pseudogap Features Research Articles

Possible origin of the reduced ordered magnetic moment in iron pnictides: A dynamical mean-field theory study

We investigate the phase diagram of a two-band frustrated Hubbard model in the framework of dynamical mean field theory. While a first-order phase transition occurs from a paramagnetic (PM) metal to an antiferromagnetic (AF) insulator when both bands are equally frustrated, an intermediate AF metallic phase appears in each band at different $U_c$ values if only one of the two bands is frustrated, resulting in continuous orbital-selective phase transitions from PM metal to AF metal and AF metal to AF insulator. We argue that such intermediate phases are possibly related to the puzzling AF metallic state with small magnetization observed in undoped iron-pnictide superconductors as well as to the pseudogap features observed in optical experiments.

Effects due to backscattering and pseudogap features in graphene nanoribbons with single vacancies

We present a systematic study of electron backscattering phenomena during conduction for graphene nanoribbons with single-vacancy scatterers and dimensions within the capabilities of modern lithographic techniques. Our analysis builds upon an \textit{ab initio} parameterized semiempirical model that breaks electron-hole symmetry and nonequilibrium Green's function methods for the calculation of the conductance distribution $g$. The underlying mechanism is based on wavefunction localizations and perturbations that in the case of the first $\pi-\pi{}^*$ plateau can give rise to impurity-like pseudogaps with both donor and acceptor characteristics. Confinement and geometry are crucial for the manifestation of such effects. Self-consistent quantum transport calculations characterize vacancies as local charging centers that can induce electrostatic inhomogeneities on the ribbon topology.

Pseudogap phase boundary in overdoped [formula omitted] studied by measuring out-of-plane resistivity under the magnetic fields

In order to elucidate the relationship between the pseudogap and high-Tc superconductivity, the characteristic pseudogap temperature T∗ in Bi2Sr2CaCu2O8+δ system has been systematically evaluated as a function of doping, especially focusing on its overdoped region, by measuring the out-of-plane resistivity ρc. Overdoped samples have been prepared by annealing TSFZ-grown Bi2Sr2CaCu2O8+δ single crystals under the high oxygen pressures (990kgf/cm2). At a zero field, the ρc showed a metallic behavior down to Tc(=62K), while under the magnetic fields of over 3T, ρc showed typical upturn behavior from around 65K upon decreasing temperature. This result suggests that the pseudogap and superconductivity are different phenomena.

Effects of Zn on magnetic properties and pseudogap of optimally doped La 2−xSr xCuO 4

The effects of Zn substitution on the uniform ( q = 0) magnetic susceptibility, χ( T), of optimally doped ( x = 0.15) La 2− x Sr x Cu 1− y Zn y O 4 sintered samples were investigated over a wide range of Zn contents ( y). Non-magnetic Zn was found to enhance χ( T) systematically and depress T c very effectively. We have extracted the characteristic pseudogap energy scale, ε g , from the analysis of χ( T) data. Unlike T c , ε g was found to be fairly insensitive to the level of Zn substitution. This supports the scenario where the pseudogap phenomenon has non-superconducting origin. We have also analyzed the Zn-induced Curie-like enhancement of the χ( T) data using different models and discussed the various possible implications.

Cluster dynamical mean-field study of the Hubbard model on a 3D frustrated hyperkagome lattice

We study the Hubbard model on a geometrically-frustrated hyperkagome lattice by a cluster extension of the dynamical mean field theory. We calculate the temperature (T) dependences of the specific heat (C) and the spin-lattice relaxation time (T1) in correlated metallic region. C/T shows a peak at T = Tp1 and rapidly decreases as T → 0. On the other hand, 1/T1T has a peak at a higher temperature Tp2 than Tp1, and largely decreases below Tp2, followed by the Korringa law 1/T1 ∝ T as T → 0. Both peak temperatures are suppressed and the peaks become sharper as electron correlation is increased. These behaviors originate from strong renormalization of the energy scales in the peculiar electronic structure in this frustrated system; a pseudo-gap like feature, the van-Hove singularity, and the flat band. The results are discussed in comparison with the experimental data in the hyperkagome material, Na4Ir3O8.

Competition between the pseudogap and superconductivity in the high-Tc copper oxides

In a classical Bardeen-Cooper-Schrieffer superconductor, pairing and coherence of electrons are established simultaneously below the critical transition temperature (T(c)), giving rise to a gap in the electronic energy spectrum. In the high-T(c) copper oxide superconductors, however, a pseudogap extends above T(c). The relationship between the pseudogap and superconductivity is one of the central issues in this field. Spectral gaps arising from pairing precursors are qualitatively similar to those caused by competing electronic states, rendering a standard approach to their analysis inconclusive. The issue can be settled, however, by studying the correlation between the weights associated with the pseudogap and superconductivity spectral features. Here we report a study of two spectral weights using angle-resolved photoemission spectroscopy. The weight of the superconducting coherent peak increases away from the node following the trend of the superconducting gap, but starts to decrease in the antinodal region. This striking non-monotonicity reveals the presence of a competing state. We demonstrate a direct correlation, for different values of momenta and doping, between the loss in the low-energy spectral weight arising from the opening of the pseudogap and a decrease in the spectral weight associated with superconductivity. We therefore conclude that the pseudogap competes with the superconductivity by depleting the spectral weight available for pairing.

Extraction of the pseudogap energy scale from the static magnetic susceptibility of single and doubleCuO2 planehigh-Tc cuprates

The effect of added holes per CuO2 plane, p, on the temperature-dependent uniform(q = 0, whereq is the wavevector)magnetic susceptibility, χ(T), of double CuO2 plane YBa2Cu3O7−δ and single CuO2 plane La2−xSrxCuO4 compounds was investigated over a wide range of oxygen deficiencies(δ) and Sr contents(x). Systematicvariations in χ(T) werefound with changing p. The characteristic pseudogap energy scale,εg(p), was extracted from theanalysis of the χ(T,p) data.From the evolution of εg(p), we have found quite abrupt signs of the pseudogap in the quasi-particlespectral density appearing first at around a planar hole content ofp = 0.190 ± 0.005.εg(p) rapidly grows (almost linearly) with decreasingp. In the heavilyunderdoped region, εg(p) extrapolates to ∼Jex (the antiferromagnetic exchange energy of the parent compound) as . These characteristic features of the pseudogap energy scale are almost identical forYBa2Cu3O7−δ and La2−xSrxCuO4 even though the superconducting transition temperature,Tc, and anisotropy are widely different for these two compounds. We have scrutinizeddifferent existing scenarios for the pseudogap in the light of our findings.

Pseudogap-Like Phenomena in Cuprates as a Manifestation of Charge-Density Waves

Differential conductance G as a function of the bias voltage V was measured for break-junctions of superconducting Bi2Sr2CaCu2O8+δ and YBa2Cu3O7−δ. The dependences G(V ) for both materials clearly demonstrate the so-called dip–hump structures outside the gap region. A theory, which suggests the charge-density-wave origin of the dip–hump structures and explains its specific form by intrinsic inhomogeneity of cuprate materials, was developed. The well-known pseudogap features in the tunnel spectra of high-Tc oxides found both below and above the superconducting critical temperature are also described by the theory, which testifies that both the pseudogap and the dip–hump structures have the same origin. Competing theories and various G(V ) peculiarities found for a number of superconducting oxides are briefly discussed.

PSEUDOGAP IN CUPRATE SUPERCONDUCTORS: EXPLORING SCALING AND QUANTUM CRITICAL END POINT

We explore extremely overdoped regime in the phase diagram of cuprate superconductors. Interlayer tunneling transport measurements under high magnetic fields are employed to detect the field dependence of low-energy density of states (DOS) in hole-doped (BSCCO and TBCO), and electron-doped (SCCO) cuprates. We find a strong doping dependence of the pseudogap closing field whose Zeeman energy universally scales with the pseudogap onset temperature, indicating a preeminent role of spin-singlet correlations in forming the pseudogap regardless of the doping type. Excess dissipation due to the pseudogap is found to follow a 'universal' correlation with the pseudogap closing field. The pseudogap features appear absent in the extremely overdoped TBCO, pointing to a possible quantum critical point very near the edge of the superconducting 'dome'.

NMR probe of pseudogap characteristics inCaAl2−xSi2+x

We report the results of a $^{27}\mathrm{Al}$ nuclear magnetic resonance (NMR) study of $\mathrm{Ca}{\mathrm{Al}}_{2\ensuremath{-}x}{\mathrm{Si}}_{2+x}$, near the stoichiometric composition with $x=0$. The low-temperature NMR relaxation rates for stoichiometric $(x=0)$ and nonstoichiometric ($x=\ensuremath{-}0.1$ and 0.1) compounds follow a Korringa law, associated with a finite density of carriers at the Fermi level. High-temperature relaxation rates for $x\ensuremath{\geqslant}0$ go over to a semiconductorlike activated form, providing information about the electronic structure near the Fermi energy. The results are consistent with pseudogap features identified by recent band-structure calculations. An analysis of the pseudogap change vs composition further points out that the band-filling picture is proper for the understanding of the NMR observations in $\mathrm{Ca}{\mathrm{Al}}_{2\ensuremath{-}x}{\mathrm{Si}}_{2+x}$.

Spin Polarized Current Injection Through ${\rm HgBr}_{2}$ Intercalated Bi2212 Intrinsic Josephson Junctions

To investigate the effect of polarized current on tunneling characteristics of intrinsic Josephson junctions (IJJs), spin- polarized and spin-degenerate current have been injected through the c-axis of HgBr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> intercalated Bi <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2.1</sub> Sr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1.5</sub> Ca <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1.4</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8+delta</sub> (Bi2212) single crystals on which 10 times 10 mum <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> mesas have been fabricated. These two spin conditions are achieved by depositing either Au (15 nm)/Co (80 nm)/Au (156 nm) multilayers or single Au film on HgBr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> intercalated Bi2212 with T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> = 74 K followed by photolithography and Ar ion beam etching. The I-V characteristics have been measured with and without a magnetic field parallel to c-axis at 4.2 K. A fine, soft Au wire is used to make a gentle mechanical contact on the top of a particular mesa in the array. Tunneling conductance characteristics were obtained and the magnetic field dependence of sumgap voltage peaks was investigated. These peaks do not change in position with increasing magnetic field for both contact configurations. In addition, the temperature dependence of tunneling characteristics of the IJJs are obtained and existence of pseudogap feature is observed above T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> for HgBr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> intercalated Bi2212.

Photoemission spectroscopy study of si-ZnMgHo quasicrystals

The results of photoemission spectroscopy of the simple-icosahedral single-grain ZnMgHo quasicrystals are presented. Synchrotron radiation photoemission measurements were performed on in situ cleaved samples at 10−10mbar pressure and low, 160K, temperature. The valence band photoemission spectra measured reveal a simple metal type valence band with a distinct Fermi edge and a sharp pseudogap feature, which originates from the van Hove singularities in the density of states. The photoemission spectra of the Mg 2p core level show the coordination shifts of Mg 2p spin–orbit doublet due to inequivalent Mg sites in the quasicrystalline lattice.

Effect of the pseudogap on the uniform magnetic susceptibility of Y 1− xCa xBa 2Cu 3O 7− δ

The effects of planar hole content, p, on the temperature-dependent uniform ( q = 0) magnetic susceptibility, χ( T), of polycrystalline Y 1− x Ca x Ba 2Cu 3O 7− δ compounds with 0.0 ⩽ x ⩽ 0.20 were investigated over a wide range of oxygen deficiency ( δ). Systematic variations in χ( T) were found with changing p. The characteristic pseudogap temperature scale, E g( p)/ k B, was extracted from the analysis of the χ( T, p) data. We have found clear evidence that the pseudogap in the quasi-particle spectral weight appears abruptly at a planar hole content of p ∼ 0.19, and that it grows rapidly with decreasing p.

Resonating bipolaron driven pseudogap phase

The spin as well as charge pseudogap features in the high Tc cuprate can be traced back to the dynamical properties on small clusters (consisting of Cu ions together with their ligand O ions environments) which make up the overall structure of these materials. These ligand complexes being deformable, causes the itinerant electrons on the Cu ions to enter into resonating bipolaron states via binary collision processes, similar to the Feshbach resonance in ultra-cold fermionic gases. We illustrate such resonant pairing on an isolated cluster and discuss how, upon coupling together such clusters, the local physics evolves into global macroscopic features, such as lattice mediated driven superconducting, respectively insulating, states of such resonating bipolarons.

Low-energy electronic structure of thehigh-Tc cuprates La2−xSrxCuO4 studied by angle-resolved photoemission spectroscopy

We have performed a systematic angle-resolved photoemission spectroscopy (ARPES) study of thehigh-Tc cuprates La2−xSrxCuO4, ranging from the underdoped insulator to the superconductor to the overdoped metal. Wehave revealed a systematic doping evolution of the band dispersions and (underlying)Fermi surfaces, pseudogap and quasi-particle features under the influence of strongelectron–electron interaction and electron–phonon interaction. The unusual transport andthermodynamic properties are explained by taking into account the pseudogap opening andthe Fermi arc formation, due to which the carrier number decreases as the doped holeconcentration decreases.

Tunneling spectroscopy of single-crystal clathrate Ba 8Ga 16Sn 30

Tunneling measurements have been carried out on the Ba 8Ga 16Sn 30. The break-junction was employed to obtain reliable tunneling spectra. The pseudo-gap like feature is obtained as 2 Δ≈175 meV, at 4.2 K, which agrees with thermoelectric power data at low temperature. We have also obtained a small gap 2 Δ≈30–40 meV.

Electronic structure of charge-density-wave state in quasi-2D KMo6O17 purple bronze characterized by angle resolved photoemission spectroscopy

Abstract We report on an angle-resolved-photoemission spectroscopy (ARPES) investigation of layered quasi-two dimensional (2D) Molybdenum purple bronze KMo6O17 in order to study and characterizes the transition to a charge-density-wave (CDW) state. We have performed photoemission temperature dependent measurements cooling down from room temperature (RT) to 32 K, well below the Peierls transition for this material, with CDW transition temperature Tc =110 K. The spectra have been taken at a selected kF point of the Fermi surface (FS) that satisfies the nesting condition of the FS, looking for the characteristic pseudo-gap opening in this kind of materials. The pseudogap has been estimated and it result to be in agreement with our previous works. The shift to lower binding energy of crossing Fermi level ARPES feature have been also confirmed and studied as a function of temperature, showing a rough like BCS behaviour. Finally we have also focused on ARPES measurements along Γ M ¯ high symmetry direction for both room and low temperature states finding some insight for ‘shadow’ or back folded bands indicating the new periodicity of real lattice after the CDW lattice distortion.

In-plane charge dynamics in underdoped YBCO

We report on the in-plane optical spectra in underdoped YBa 2Cu 3O 7 (YBCO) using a twin-free single crystal. At low temperatures, there exist two components in the optical conductivity which are considered as a Drude term by quasiparticles in a nodal direction and the pseudogap excitation in an antinodal direction. A large residual conductivity together with pseudogap feature is consistent with the picture of inhomogeneous in-plane electronic state observed by scanning tunneling microscopy (STM). We estimate a superfluid density n s from optical spectra and discuss a universal relation between T c and n s. The value estimated from our results is considerably smaller than that of the μSR measurement. It is considered due to the inhomogeneity of in-plane, the difference of applying field, and a property on pseudogap.

Phase Fluctuations in Strongly Coupledd-Wave Superconductors

We present a numerically exact solution for the BCS Hamiltonian at any temperature, including the degrees of freedom associated with classical phase, as well as amplitude fluctuations via a Monte Carlo integration. This allows for an investigation over the whole range of couplings: from weak attraction, as in the well-known BCS limit, to the mainly unexplored strong-coupling regime of pronounced phase fluctuations. In the latter, two characteristic temperatures T(*) and T(c), associated with short- and long-range ordering, respectively, can be identified in a mean-field-motivated Hamiltonian. T(*) at the same time corresponds to the opening of a gap in the excitation spectrum. In addition to introducing a novel procedure to study strongly coupled d-wave superconductors, our results indicate that classical phase fluctuations are not sufficient to explain the pseudogap features of high-temperature superconductors.

Doping phase diagram ofY1−xCaxBa2(Cu1−yZny)3O7−δfrom transport measurements: Tracking the pseudogap belowTc

The effects of planar hole concentration, p, on the resistivity, r(T), of sintered Y1-xCaxBa2(Cu1-yZny)3O7-d samples were investigated over a wide range of Ca, Zn, and oxygen contents. Zn was used to suppress superconductivity and this enabled us to extract the characteristic pseudogap temperature, T*(p), from r(T,p) data below Tco(p) [ = Tc (y = 0)]. We have also located the characteristic temperature, Tscf, marking the onset of significant superconducting fluctuations above Tc, from the analysis of r(T,H,p) and r(T,p) data. This enabled us to identify T*(p) near the optimum doping level where the values of T*(p) and Tscf(p) are very close and hard to distinguish. We again found that T*(p) depends only on the hole concentration p, and not on the level of disorder associated with Zn or Ca substitutions. We conclude that (i) T*(p) (and therefore, the pseudogap) persists below Tco(p) on the overdoped side and does not merge with the Tco(p) line and (ii) T*(p), and thus the pseudogap energy, extrapolates to zero at the doping p = 0.19 +/- 0.01. PACS numbers: 74.25.Dw, 74.25. 74.62.Dh, 74.72.-h Key words: Pseudoap, Superconducting fluctuations