Pseudogap Features Research Articles

First-principles investigation of Cu/Ti-TM/Si (TM=W, Ru) interfaces: Role of Ti-TM binary alloys as diffusion barrier layers

This study employs first-principles calculations to investigate the role of Ti-TM (TM=W, Ru) binary alloys as diffusion barrier layers in Cu/Ti-TM/Si interfaces, which are critical for enhancing the performance and reliability of microelectronic devices. The calculated results reveal that TiRu and Ti4W12 alloys exhibit exceptional stability and low contact resistance, outperforming traditional Cu/Si interfaces. The introduction of these alloys into the interface structure results in a reduced Cu s-d coupling effect, which is pivotal for inhibiting Cu diffusion. Additionally, the presence of pseudogap features and enhanced electron distribution at the interface indicate strong atomic interactions, contributing to the formation of covalent bonds and further improving barrier properties. The findings suggest that TiRu and Ti4W12 alloys are promising candidates for diffusion barrier layers, offering a balance of low contact resistance, thermal stability, and effective Cu diffusion inhibition. Therefore, from the results, it is concluded that the formation of the new interface structure will improve performance in various aspects, and thus the strategy can be applied in various directions, such as new energy materials, electronic materials, and so on. In addition, the combination of the previous work and the present work suggests that Ti-based alloys have the potential to be used as diffusion barrier materials.

Superconductivity and pseudogap features of an isolated FeAs layer in KCa2Fe4As4F2 unraveled by STM/STS

Superconductivity and pseudogap features of an isolated FeAs layer in KCa2Fe4As4F2 unraveled by STM/STS

Observation of periodic structures and pseudogaps in pristine compound α-TiNCl by STM/STS and break junction methods

Scanning tunneling microscopy and spectroscopy (STM/STS) and break-junction tunnel spectroscopy (B.TTS) measurements are performed on the pristine layered α-TiNCl semiconductor (pri-TiNCl), being the precursor of superconducting nitride chlorides. The STM topography of pri-TiNCl shows basic crystal structures with the lattice periods of a0≈ 0.38 nm and b0≈ 0.31 nm, ensuring that a clean a–b surface of micro-single crystals is obtained. From the STS measurements, the averaged conductance versus voltage dependence dI/dV(V) reveals kink structures at relatively high bias voltages of |F| ∼ −50 mV and −100 mV. The spatial (r-) conductance distributions dI/dV (V, r) (dI/dV maps) proportional to those of quasiparticle local densities of states (LDOSes) exhibit the bias-independent streak structures with the period of (4.8 ± 0.2) b0, which are predominantly observed within the range of |V| < 95 mV. The temperature dependence of the dI/dV B.TTS spectra shows the gap structure with the energy scale of 4Σ = 180 meV vanishing at Ts ∼ 120 K. Thus, the ratio of the gap Σ to the transition temperature Ts is 2Σ/kBTs ∼ 10. Here, kB is the Boltzmann constant. Such a ratio is typical of the pseudogap features in cuprate superconductors and dielectric gap characteristics in layered chalcogenides with charge-density waves.

Topological pseudogap in highly polarizable layered systems with 2D hole-like dispersion

Pseudogap in hole-doped cuprate superconductors is acknowledged as a possible key to understanding their ground normal state, however, existing pseudogap models have essential inconsistencies with experiments. In our approach pseudogap emerges due to impact of autolocalized carriers on stationary states of delocalized ones and topology of the 2D hole-like dispersion. Autolocalized carriers create potential which transforms Bloch quasiparticles into distributed wave packets (DWPs) with different momentums in areas with different potential. Topology of hole-like constant energy curves in 2D-conducting cuprates forbids DWPs with average momentums near antinode. Manifestation of permitted DWPs in ARPES spectra demonstrates all known pseudogap features including midpoint shift, giant broadening and Fermi momentum misalignment. Calculated doping dependence of the pseudogap width and onset temperature agrees with experiments. The obtained ground normal state of the hole-doped cuprates, in which all the doped holes are autolocalized until high doping is reached and near-antinodal electron DWPs are absent, explains Fermi surface reconstruction from small electron pocket to large hole-like Fermi surface observed in quantum oscillation measurements. Our results open a possibility of creating systems with artificial pseudogap and switchable density of states on the base of highly polarizable layered structures with 2D conductivity and hole-like dispersion.

Orbital selectivity in the normal state of KFe2Se2 superconductor

Using density functional dynamical mean-field theory, we show how correlation effects lead to pseudogap and Kondo-quasiparticle features in the electronic structure of pure and doped KFe2Se2 superconductor. Therein, correlation- and doping-induced orbital differentiation are linked to the emergence of an incoherent-coherent crossover in the normal state of KFe2Se2 superconductor. This crossover explains the puzzling temperature- and doping-dependent evolution of resistivity and Hall coefficient, seen in experiments of alkali-metal intercalated iron-selenide superconductors. Our microscopic description emphasises the role of incoherent and coherent electronic excitations towards unconventional transport responses of strange, bad metals.

Variation of charge dynamics upon antiferromagnetic transitions in the Dirac semimetal EuMnBi2

We have investigated the temperature- and field-variation of electronic state for the Dirac semimetal of ${\mathrm{EuMnBi}}_{2}$ by means of optical spectroscopy and theoretical calculation. The optical conductivity spectra show a clear Drude peak in the paramagnetic phase, which gradually diminishes in the Mn-$3d$ antiferromagnetic phase with decreasing temperature. Meanwhile, the absorption peaks due to the interband transition grow at low temperatures, resulting in a pseudogap feature with an energy scale of 0.07 eV. The analysis of Drude weight shows that the Drude response is nearly governed by the Dirac electrons at low temperatures. On the contrary, both the antiferromagnetic transition and spin reorientation of Eu-$4f$ moment do not significantly change the spectra except the moderate variation of Drude weight. As a comparison, we have also investigated the charge dynamics for ${\mathrm{EuZnBi}}_{2}$, which is an analog without the Mn-$3d$ antiferromagnetic ordering. In ${\mathrm{EuZnBi}}_{2}$, the optical conductivity spectra do not show the pseudogap structure, but show an intense Drude peak at all temperatures. Combined with the results of ab initio calculation, in ${\mathrm{EuMnBi}}_{2}$, it is likely that the reconstruction of electronic state driven by the Mn-$3d$ antiferromagnetic ordering causes the Dirac semimetallic state with tiny hole pockets, wherein electronic states other than the Dirac band are nearly gapped-out from the Fermi level.

In-plane resistivity of hole doped cuprates: role of pseudogap and quantum criticality*

The anomalous behaviour of dc charge dynamics in high-Tc cuprates cannot be explained by the prevalent Boltzman transport theory. In high-Tc cuprates the in-plane and out-of-plane dc resistivities exhibit high anisotropy and unconventional temperature dependence. In this study we have modeled the temperature (T) and hole content (p) dependent in-plane resistivity, ρa (T, p), of pure and Ca doped Y123 (Y(Ca)Ba2Cu3O7- δ ). We have adapted and extended the formalism developed by Naqib et al. (Physica C 471 1598 (2011)) by taking account of two generic features present in all hole doped cuprates, namely - (i) the presence of the pseudogap in the quasiparticle spectrum and (ii) high-T linear behavior, to elucidate non-Fermi liquid ρa (T, p) of Y(Ca)Ba2Cu3O7- δ over a wide range of temperature and hole contents. The characteristic pseudogap energy scale, εg (p), extracted from the analysis of the resistivity data was found to agree well with those found in a variety of earlier studies. Other extracted parameters from the analysis of ρp (T, p) data showed methodical variations with the variation of hole content. Important features of the analysis are briefly discussed.

Stacking-driven gap formation in layered 1 T - TaS2

We present a state-of-the-art density functional theory (DFT) study which models crucial features of the partially disordered orbital order stacking in the prototypical layered transition metal dichalcogenide 1T-TaS2 . Our results not only show that DFT models with realistic assumptions about the orbital order perpendicular to the layers yield band structures which agree remarkably well with experiments. They also demonstrate that DFT correctly predicts the formation of an excitation pseudo-gap which is commonly attributed to Mott-Hubbard type electron-electron correlations. These results highlight the importance of interlayer interactions in layered transition metal dichalcogenides and serve as an intriguing example of how disorder within an electronic crystal can give rise to pseudo-gap features.

Pair Breaking, Pseudogap, and Superconducting Tc of Hole-Doped Cuprates: Interrelations and Implications

Irrespective of the class they belong to, all the hole doped high-Tc cuprate superconductors show an anti-correlation between the superconducting transition temperature and the characteristic pseudogap energy in the underdoped region. The doping dependent pseudogap in the quasiparticle spectral density is believed to remove low-energy electronic states and thereby reduce the superconducting condensate. Impurities within the CuO2 plane, on the other hand, break Cooper pairs in the unitarity limit and diminish superfluid density. Both pseudogap in pure cuprates and impurity scattering in disordered cuprates reduces Tc very effectively. In this study we have compared and contrasted the mechanisms of Tc degradation due to pseudogap and impurity scattering in hole doped cuprates. We have suggested a framework where both these factors can be treated on somewhat equal footing. Beside impurity and pseudogap dependent superconducting transition temperature, the proposed scenario has been employed to investigate the disorder and hole content dependent isotope exponent in high-Tc cuprates in this work.

Re-entrant charge order in overdoped (Bi,Pb)2.12Sr1.88CuO6+δ outside the pseudogap regime.

In the underdoped regime, the cuprate high-temperature superconductors exhibit a host of unusual collective phenomena, including unconventional spin and charge density modulations, Fermi surface reconstructions, and a pseudogap in various physical observables. Conversely, overdoped cuprates are generally regarded as conventional Fermi liquids possessing no collective electronic order. In partial contradiction to this widely held picture, we report resonant X-ray scattering measurements revealing incommensurate charge order reflections for overdoped (Bi,Pb)2.12Sr1.88CuO6+δ (Bi2201), with correlation lengths of 40-60 lattice units, that persist up to temperatures of at least 250 K. The value of the charge order wavevector decreases with doping, in line with the extrapolation of the trend previously observed in underdoped Bi2201. In overdoped materials, however, charge order coexists with a single, unreconstructed Fermi surface without nesting or pseudogap features. The discovery of re-entrant charge order in Bi2201 thus calls for investigations in other cuprate families and for a reconsideration of theories that posit an essential relationship between these phenomena.

Thermal transitions, pseudogap behavior, and BCS-BEC crossover in Fermi-Fermi mixtures

We study the mass imbalanced Fermi-Fermi mixture within the framework of a two-dimensional lattice fermion model. Based on the thermodynamic and species dependent quasiparticle behavior we map out the finite temperature phase diagram of this system and show that unlike the balanced Fermi superfluid there are now two different pseudogap regimes as PG-I and PG-II. While within the PG-I regime both the fermionic species are pseudogapped, PG-II corresponds to the regime where pseudogap feature survives only in the light species. We believe that the single particle spectral features that we discuss in this paper are observable through the species resolved radio frequency spectroscopy and momentum resolved photo emission spectroscopy measurements on systems such as, 6$_{Li}$-40$_{K}$ mixture. We further investigate the interplay between the population and mass imbalances and report that at a fixed population imbalance the BCS-BEC crossover in a Fermi-Fermi mixture would require a critical interaction (U$_{c}$), for the realization of the uniform superfluid state. The effect of imbalance in mass on the exotic Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) superfluid phase has been probed in detail in terms of the thermodynamic and quasiparticle behavior of this phase. It has been observed that in spite of the s-wave symmetry of the pairing field a nodal superfluid gap is realized in the LO regime. Our results on the various thermal scales and regimes are expected to serve as benchmarks for the experimental observations on 6$_{Li}$-40$_{K}$ mixture.

Spontaneous breaking of rotational symmetry in copper oxide superconductors.

The origin of high-temperature superconductivity in copper oxides and the nature of the 'normal' state above the critical temperature are widely debated. In underdoped copper oxides, this normal state hosts a pseudogap and other anomalous features; and in the overdoped materials, the standard Bardeen-Cooper-Schrieffer description fails, challenging the idea that the normal state is a simple Fermi liquid. To investigate these questions, we have studied the behaviour of single-crystal La2-xSrxCuO4 films through which an electrical current is being passed. Here we report that a spontaneous voltage develops across the sample, transverse (orthogonal) to the electrical current. The dependence of this voltage on probe current, temperature, in-plane device orientation and doping shows that this behaviour is intrinsic, substantial, robust and present over a broad range of temperature and doping. If the current direction is rotated in-plane by an angle ϕ, the transverse voltage oscillates as sin(2ϕ), breaking the four-fold rotational symmetry of the crystal. The amplitude of the oscillations is strongly peaked near the critical temperature for superconductivity and decreases with increasing doping. We find that these phenomena are manifestations of unexpected in-plane anisotropy in the electronic transport. The films are very thin and epitaxially constrained to be tetragonal (that is, with four-fold symmetry), so one expects a constant resistivity and zero transverse voltage, for every ϕ. The origin of this anisotropy is purely electronic-the so-called electronic nematicity. Unusually, the nematic director is not aligned with the crystal axes, unless a substantial orthorhombic distortion is imposed. The fact that this anisotropy occurs in a material that exhibits high-temperature superconductivity may not be a coincidence.

Effects of spin-orbit coupling on the structural, electronic and magnetic properties of [formula omitted]-BaIrO3

3C-BaIrO3 which crystallizes in the tetragonal structure has Ir in +4 valence state. For such systems with near-perfect octahedrally coordinated Ir ions, spin-orbit coupling (SOC) in conjunction with moderate Coulomb correlations are expected to drive an insulating state by virtue of Jeff splitting of the Ir 5d manifold. However, experiments find3C-BaIrO3 tobe a Pauli paramagnet with conducting ground state. We present a comprehensive investigation of its electronic structure by means of first principles density functional theory based calculations. The calculations show that SOC introduces a pseudo-gap like feature in the anti-bonding region, reminiscent of an incomplete splitting of the Jeff states due to the strong Ir t2g - O 2p hybridization. Furthermore, it is anticipated from the electronic structure that p−type doping may introduce a metal-insulator transition in 3C-BaIrO3, in contrast to iso-electronic SrIrO3. Besides, we also investigate the effects of Coulomb correlations and magnetic properties of 3C-BaIrO3.

Valence band electronic structure of Pd based ternary chalcogenide superconductors

We present a comparative study of the valence band electronic structure of Pd based ternary chalcogenide superconductors Nb2Pd0.95S5, Ta2Pd0.97S6 and Ta2Pd0.97Te6 using experimental photoemission spectroscopy and density functional based theoretical calculations. We observe a qualitatively similarity between valence band (VB) spectra of Nb2Pd0.95S5 and Ta2Pd0.97S6. Further, we find a pseudogap feature in Nb2Pd0.95S5 at low temperature, unlike other two compounds. We have correlated the structural geometry with the differences in VB spectra of these compounds. The different atomic packing in these compounds could vary the strength of inter-orbital hybridization among various atoms which leads to difference in their electronic structure as clearly observed in our DOS calculations.

Annealing condition dependence of the superconducting property and the pseudo-gap in the protect-annealed electron-doped cuprates

Annealing as-grown electron-doped cuprates under a low oxygen-partial-pressure condition is a necessary step to achieve superconductivity. It has been recently found that the so-called protect annealing results in much better superconducting properties in terms of the superconducting transition temperature and volume fraction. In this article, we report on angle-resolved photoemission spectroscopy studies of a protect-annealed electron-doped cuprate <TEX>$Pr_{0.9}La_{1.0}Ce_{0.1}CuO_4$</TEX> on annealing condition dependent superconducting and pseudo-gap properties. Remarkably, we found that the one showing a better superconducting property possesses almost no pseudo-gap while others have strong pseudo-gap feature due to an anti-ferromagnetic order.

Momentum structure of the self-energy and its parametrization for the two-dimensional Hubbard model

We compute the self-energy for the half-filled Hubbard model on a square lattice using lattice quantum Monte Carlo simulations and the dynamical vertex approximation. The self-energy is strongly momentum dependent, but it can be parametrized via the non-interacting energy-momentum dispersion $\varepsilon_{\mathbf{k}}$, except for pseudogap features right at the Fermi edge. That is, it can be written as $\Sigma(\varepsilon_{\mathbf{k}},\omega)$, with two energy-like parameters ($\varepsilon$, $\omega$) instead of three ($k_x$, $k_y$ and $\omega$). The self-energy has two rather broad and weakly dispersing high energy features and a sharp $\omega= \varepsilon_{\mathbf{k}}$ feature at high temperatures, which turns to $\omega= -\varepsilon_{\mathbf{k}}$ at low temperatures. Altogether this yields a Z- and reversed-Z-like structure, respectively, for the imaginary part of $\Sigma(\varepsilon_{\mathbf{k}},\omega)$. We attribute the change of the low energy structure to antiferromagnetic spin fluctuations.

Ultrafast dynamics of quasiparticles and coherent acoustic phonons in slightly underdoped (BaK)Fe2As2

We have utilized ultrafast optical spectroscopy to study carrier dynamics in slightly underdoped (BaK)Fe2As2 crystals without magnetic transition. The photoelastic signals due to coherent acoustic phonons have been quantitatively investigated. According to our temperature-dependent results, we found that the relaxation component of superconducting quasiparticles persisted from the superconducting state up to at least 70 K in the normal state. Our findings suggest that the pseudogaplike feature in the normal state is possibly the precursor of superconductivity. We also highlight that the pseudogap feature of K-doped BaFe2As2 is different from that of other iron-based superconductors, including Co-doped or P-doped BaFe2As2.

Normal state incoherent pseudogap in FeSe superconductor

The normal state of Iron chalcogenide superconductors show a range of unconventional features. Bad-metallic resistivity and proximity to insulating state manifest themselves in spectral and transport responses. In particular, obervation of low-energy pseudogap feature in the normal state raises the issue of the nature of processes underpinning its emergence as well as its relation to unconventional superconductivity. Here, using the LDA+DMFT method, we show how correlation-induced orbital-selective pseudogap-like physics underpin these incoherent features in stoichimetric and electron-doped FeSe superconductor. We discuss the pseudogap regime microscopically, along with implications for the superconductive instability.

Point contact tunneling spectroscopy of the density of states in Tb–Mg–Zn quasicrystals

According to theoretical predictions the quasicrystalline (QC) electronic density of states (DOS) must have a rich and fine spiky structure which actually has resulted elusive. The problem with its absence may be related to poor structural characteristics of the studied specimens, and/or to the non-existence of this spike characteristic. Recent calculations have shown that the fine structure indeed exists, but only for two dimensional approximants phases. The aim of the present study is to show our recent experimental studies with point contact tunnel junction spectroscopy performed in samples of very high quality. The studies were performed in icosahedral QC alloys with composition Tb9Mg35Zn56. We found the presence of a pseudogap feature at the Fermi level, small as compared to the pseudogap of other icosahedral materials. This study made in different spots on the QC shows quite different spectroscopic features, where the observed DOS was a fine non-spiky structure, distinct to theoretical predictions. In some regions of the specimens the spectroscopic features could be related to Kondo characteristics due to Tb magnetic atoms acting as impurities. Additionally, we observed that the spectroscopic features vanished under magnetic field.

Optical properties ofTlNi2Se2: Observation of pseudogap formation

The quasi-two-dimensional nickel chalcogenides $TlNi_2Se_2$ is a newly discovered superconductor. We have performed optical spectroscopy study on $TlNi_2Se_2$ single crystals over a broad frequency range at various temperatures. The overall optical reflectance spectra are similar to those observed in its isostructure $BaNi_2As_2$. Both the suppression in $R(\omega)$ and the peaklike feature in $\sigma_1(\omega)$ suggest the progressive formation of a pseudogap feature in the midinfrared range with decreasing temperatures, which might be originated from the dynamic local fluctuation of charge-density-wave (CDW) instability. We propose that the CDW instability in $TlNi_2Se_2$ is driven by the saddle points mechanism, due to the existence of van Hove singularity very close to the Fermi energy.