Pseudogap Research Articles

The Dependence of China’s Monetary Policy Rules on Interest Rate Regimes: Empirical Analysis Based on a Pseudo Output Gap

Although a large number of scholars have studied the policy preferences and monetary policy rules of China’s central bank, most have found no evidence that China’s central bank has adjusted the nominal interest rates against the output gap. By constructing the pseudo output gap defined by the deviation of the real output growth rate and the target growth rate, this paper finds that China’s central bank prefers to adjust the nominal interest rates against the pseudo output gap. The monetary policy preferences and rules of China’s central bank in different interest rate regimes are investigated based on the threshold Taylor rule model. It is found that, in the high-interest-rate regime, the central bank adjusts the nominal interest against the inflation gap and the pseudo output gap, while in the low-interest-rate regime, there is no evidence that the central bank adjusts the nominal interest rates against the pseudo output gap. The lower bound of interest rate reduction and the weakening of interest rate policy effects caused by the liquidity trap of the interest rate are the possible reasons for China’s central bank not to adjust the nominal interest rates against the pseudo output gap.

Fasciculoventricular accessory pathway unmasked by a pseudo gap phenomenon

A 28 year old female with manifest preexcitation underwent electrophysiology study for intermittent palpitation. During progressively premature atrial extrastimuli, bypass tract was blocked before relative refractory period of AV node, making it unfeasible to observe the change of H-V interval and QRS morphology during decremental nodal conduction. However, dual AV node physiology, presented as a marked increase of A-H interval, occurred when a short-coupled extrastimulus was delivered, followed by a preexcited QRS with an H-V interval identical to that in sinus rhythm. This was an example of gap phenomenon involving accessory pathway and AV node. Diagnosis of fasciculoventricular bypass tract was made and ablation was therefore not performed.

Intimate link between charge density wave, pseudogap and superconducting energy scales in cuprates

The cuprate high temperature superconductors develop spontaneous charge density wave (CDW) order below a temperature $T_{CDW}$ and over a wide range of hole doping (p). An outstanding challenge in the field is to understand whether this modulated phase is related to the more exhaustively studied pseudogap and superconducting phases. To address this issue it is important to extract the energy scale $\Delta_{CDW}$ associated with the charge modulations, and to compare it with the pseudogap (PG) $\Delta_{PG}$ and the superconducting gap $\Delta_{SC}$. However, while $T_{CDW}$ is well-characterized from earlier works little has been known about $\Delta_{CDW}$ until now. Here, we report the extraction of $\Delta_{CDW}$ for several cuprates using electronic Raman spectroscopy. Crucially, we find that, upon approaching the parent Mott state by lowering $p$, $\Delta_{CDW}$ increases in a manner similar to the doping dependence of $\Delta_{PG}$ and $\Delta_{SC}$. This shows that CDW is an unconventional order, and that the above three phases are controlled by the same electronic correlations. In addition, we find that $\Delta_{CDW} \approx \Delta_{SC}$ over a substantial doping range, which is suggestive of an approximate emergent symmetry connecting the charge modulated phase with superconductivity.

Toward the Mott state with magnetic cluster formation in heavily Cu-doped NaFe1−xCuxAs

Recent neutron scattering measurements indicate that NaFe$_{1-x}$Cu$_{x}$As forms an antiferromagnetic stripe phase near $x\approx 0.5$ in a Mott insulating state. This copper concentration is well in excess of that required for superconductivity, $x < 0.04$. We have investigated the development of magnetism in this compound using $^{23}$Na nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation measurements performed on single crystals ($x$ = 0.13, 0.18, 0.24, and 0.39). We find multiple inequivalent Na sites, each of which is associated with a different number of nearest neighbor Fe sites occupied by a Cu dopant. We show that the distribution of Cu substituted for Fe is random in-plane for low concentrations ($x = 0.13$ and 0.18), but deviates from this with increasing Cu doping. As is characteristic of many pnictide compounds, there is a spin pseudo gap that increases in magnitude with dopant concentration. This is correlated with a corresponding increase in orbital NMR frequency shift indicating a change in valence from Cu$^{2+}$ to a Cu$^{1+}$ state as $x$ exceeds 0.18, concomitant with the change of Fe$^{2+}$ to Fe$^{3+}$ resulting in the formation of magnetic clusters. However, for $x\leq 0.39$ there is no evidence of long-range static magnetic order.

Fermi surface reconstruction in underdoped cuprates: Origin of electron pockets

A new phenomenological model is proposed to describe the evolution of the Fermi surface (FS) in a wide range of dopings. It reproduces the key features of the cuprates in the underdoped phase above the superconducting temperature $T_c$. It is shown that the explicit accounting of strong electron correlation in the framework of the $t-J$ model taken in complementary to the translational symmetry breaking induced by the charge density wave (CDW) gives rise to the Fermi surface reconstruction (FSR) into small electron pockets. While the strong Coulomb repulsion leads to an emergence of the arc-like Fermi surface in the pseudogap (PG) phase, the Bragg reflection on the boundaries of the reduced Brillouin zone (BZ) opens up a possibility to close the quasiparticle orbits. Direct calculation of the FS properties allows us to unveil the scenario of the experimentally observed transition to the CDW phase that sets in at the doping level $\delta\approx0.08$ and is accompanied by a divergence of the carriers effective mass and the sign reversals of the Hall and Seebeck coefficients.

Study on the surface activity of t-YSZ nanomaterials by first-principles calculation

The internal mechanism underlying the high surface activity of t-YSZ nanomaterials composed of low Miller index facets is not completely understood at present. Using first-principles calculations, the surface energies, as well as geometrical and electronic properties of t-YSZ morphology are simulated and analyzed by conventional bulk surface and new microfacet models. The results show that the surface energies follow the trend Esurface [(0 1 0 × 0 1 0)] > Esurface [(1 1 1 × 0 1 0)] > Esurface [(1 0 1 × 0 1 0)] > Esurface [(1 1 1 × 1 0 1)] > Esurface [(1 0 1 × 1 0 1)] > (1 1 1) bulk surface > (1 0 1) bulk surface > (0 1 0) bulk surface, where the surface energies of the microfacet models are several times greater than those of the bulk surfaces. The surface activity of the microfacet is therefore much more vigorous than that of the bulk surface. The very high chemical activity of the microfacets is derived from their large Fermi energy, pseudo energy gap, and change in Mulliken population. Although the low Miller index surfaces exhibit weak activity, their interface, such as [(0 1 0 × 0 1 0)], have high chemical activity, because of which they are easily and quickly corroded by CaO-MgO-Al2O3-SiO2 (CMAS), as confirmed by experimental reports. Hence, our findings can be considered a first and vital step toward understanding the unusual properties of nano-YSZ.

Anomalous Magnetic and Electrical Properties of Fe2V2−&lt;italic&gt;x&lt;/italic&gt;Al&lt;italic&gt;x&lt;/italic&gt; (&lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;$x = {0-1}$ &lt;/tex-math&gt; &lt;/inline-formula&gt;) Alloys

Intriguing correlated electronic structure is revealed when vanadium is substituted in place of iron in ferromagnetic DO 3 -Fe 3 Al intermetallic alloy and transforms it into a Fe 2 VAl Heusler alloy with L2 1 crystal structure. This particular composition exhibits nonmagnetic semi-metallic character [1], [2]. Both DFT calculations and experiments revealed the existence of pseudogap or gap ranging up to 0.5 eV. The origin of the gap and the role of hybridization in these alloys is still debated [3]. These studies reveal that the vanadium reduces the moment, yet the role of aluminium in altering the electronic properties is still need to be investigated. Therefore, in the present investigation, we have studied the structural and magnetic properties of Fe 2 V 2-x Al x alloys with x varying from 0 to1 to unravel the role of Al in Fe 2 VAl which is known to be exhibiting the features similar to that of equiatomic FeV. Alloy ingots were synthesized by arc melting the elemental constituents and subsequent annealing at 1273 K for 3 days. From X-Ray Diffraction studies it is observed that x=0 composition (FeV) stabilized in $\sigma -$phase, while a mixed phase (${\sigma }$ + cubic) is obtained for x=0.2. It is observed that as the Al-composition varies from 0.4 to 1 it exhibits a cubic phase. DC magnetic measurements were performed for all samples, which revealed that the magnetic character decreases with Al concentration. The extreme compositions (well ordered) of this series namely, FeV (with $\sigma -$phase) and Fe 2 VAl (L2 1 -phase) are found to be paramagnetic down to lowest temperature. The temperature dependence of magnetization curves shown in Fig.1 for composition x=0.2 suggest ferromagnetic character with Curie temperature (T c ) close to 240 K and for x=0.4 with T c ~130K, while the compositions x=0.8, 1 exhibit nonmagnetic behavior down to 5K (Fig.1). The magnetic isotherms of these compositions exhibit anomalous magnetic character i.e., exhibit higher coercivity at 300K compared to 5K (except for Fe 2 VAl), with lower saturation magnetization. The existence of hysteresis behavior above T c indicates the possibility of the presence of ferromagnetic clusters in intermediate compositions. Analysis of magnetic data suggests that disorder plays a significant role in addition to the composition. To see whether the magnetic anomalies affect the electrical transport, we carried out electrical resistivity measurements in the low temperature range, shown in Fig.2. The electrical resistivity shows a positive temperature coefficient of resistivity (TCR) for x=0 to x=0.8 and the resistivity at temperatures below 250K decreases with Al concentration which indicates the presence of Al improves the metallic nature of these samples. But the sample with highest Al concentration i.e Fe 2 VAl shows negative TCR which also exhibits high values of resistivity compared to other compositions of the series. This kind of behavior can be attributed to band structure formed due to structural disorder in samples, x=0.2, 0.4, 0.8, and pseudo band gap formed in Fe 2 VAl near Fermi level [4], [5]. From above studies, it is quite evident that the material going from metallic state to semi-metallic state with a small variation in Al concentration. The resistivity increased by two orders of magnitude from Fe 2 V 1.2 Al 0.8 Fe 2 VAl, whereas both the samples were paramagnetic down to 5K. An inflection point in resistivity curve near the magnetic transition temperature T c is observed, which suggests a correlation between transport and magnetic properties. These magnetic and electrical transport anomalies show even non-magnetic aluminum plays a significant role in electronic properties and will be discussed in detail.

First-principles calculations of electronic structures and ferromagnetism of Fe3Si(001)//MgO(001) films

The first-principles calculations based on density functional theory (DFT) were carried out in investigating electronic structures and ferromagnetism of Fe3Si films epitaxial on MgO(001). Firstly, the various geometric structures of Fe3Si(001)//MgO(001) constructed near lattice constant c = 3.995 Å were optimized to gain the most steady equilibrium state at c = 3.980 Å. Then, the calculated cohesive energy and negative heat of formation indicate that Fe3Si(001)//MgO(001) formed in this manner obtain high structural stability. The calculated results of spin-polarized energy band structures and density of states show that Fe3Si(001)//MgO(001) exhibit the metallic feature whose bonding orbitals are constituted by covalent bond and metallic bond. Two peaks located in both the sides of the Fermi level and the total density of states (TDOS) in this energy range are all due to the Fe 3d states, which implies that the pseudo energy gap exists in the Fermi level and covalent electron orbit hybridization takes place. Ferromagnetism of Fe3Si(001)//MgO(001) are determined by the 3d states of Fe atoms. There are two occupied sites for Fe atoms with different local magnetic moments, which is 1.34 [Formula: see text]/atom for Fe[A, C] atoms and a value of 2.68 [Formula: see text]/atom for Fe[B] atoms, likewise indicating Fe3Si films epitaxial on MgO(001) are ferromagnetic.

Mesoporous MnO2 structured by ultrathin nanosheet as electrocatalyst for oxygen reduction reaction in air-cathode microbial fuel cell

In order to couple merits of two dimension and porous materials, by using unique solvent free method, the mesoporous layered α-MnO2 structured by 2 nm nanosheet was successfully synthesized, which possesses ultra-high surface area (339 m2 g−1), meso pore size of 6 nm and large amount of oxygen vacancies. When serving as a cathodic oxygen reduction reaction catalyst in microbial fuel cell, it exhibits superior electrocatalytic performance comparable to the commercial Pt/C and achieves the maximum power density of 1671 ± 26 mW m−2 with a exchange current density of 21.18 A cm−2. Meanwhile, density functional theory study reveals that the obviously shrinking of forbidden band and the pseudo band gap between two sharp peak of d DOS, which results in an excellent electrical conductivity, efficient electron transport and low overpotential of electrode. Therefore, it can be expected that the as-prepared MnO2 catalyst, with extraordinary electrocatalytic performance through a combination of the ultrathin nanosheet architecture and oxygen vacancies, is promising and produces high power density in microbial fuel cell.

Theoretical Prediction of Structural, Electronic, and Magnetic Properties of Rh2Y In (Y = Nd, Sm) Full-Heusler Alloys

We have studied the structural, electronic, and magnetic properties of Rh2Y In (Y = Nd, Sm) alloys based on rare earth elements, using the full-potential linearized augmented plane-wave method (FP-LAPW) implemented in the Wien2K computer code with the parameterization of the generalized gradient approximation (GGA) and the local density approximation (LDA). Our results reveal a metallic behavior for rare earth-based full-Heusler Rh2Y In (Y = Nd, Sm) alloys. Indeed, the structure of the state density illustrates a polarization for the up orientation (metal character) and a pseudo gap at the down orientation.

Magnetotransport properties and pseudogap phase diagram of superconducting EuBa2Cu3Oy thin films: the influence of Eu substitution

This research explores and debates magnetotransport properties and the pseudogap phase diagram of high-temperature superconducting EuBa2Cu3Oy (EBCO) films through ρxx(T) and ρxy(T) measurements. The mixed state transport properties of samples with higher doped levels show a power-law field-dependence of activation energy U, U ∝ H−α with α = 0.52 ± 0.05, while the slightly doped samples reveal a logarithmic field-dependence of U. In addition, for the first time, all samples in the underdoped region display an unusual power-law behavior of U ∝ pγ with γ = −5.90 ± 0.28, which is qualitatively explained. A sign reversal of transverse resistivity in the mixed state is also present in the nearly optimum-doped samples, which have strong pinning energies. In the normal state, the cotangent of the Hall angle shows a wide range quadratic temperature dependence for all the samples, which can be described by Anderson’s theory. The values of spin excitation bandwidth W are coincident between two results obtained from the hidden Fermi-liquid theory and from the Hall angle for the slightly overdoped sample. Moreover, the phase diagram of EBCO, containing the superconducting transition temperatures, the T* and T** temperatures in the pseudogap (PG) phase, and the antiferromagnetic transition temperatures θN, in the whole underdoped region has been completed and compared with those for YBa2Cu3Oy (YBCO). The findings demonstrate that EBCO has lower T* and T** values, versus those of YBCO in the same doping region. The observed smaller magnitude of PG for EBCO is explained by considering the oxygen vibrational amplitudes in Cu−O−Cu bonds, which agrees with the inference based on the fluctuating bond model. The influence of Eu substitution is examined and discussed.

Voltammetric Determination of Serine Using L‐Cysteine Modified Platinum Electrode and Brief Exploration from the Theoretical Perspective

AbstractA new method for the detection of serine was described using L‐cysteine chemically modified electrode, which was prepared by cyclic voltammetry. The factors, such as pH, scan rate, and soaking time for the experimental process were discussed. Under the optimal conditions, the linear range for serine concentration was 3×10−7∼8×10−5 mol L−1, the detection limit was 2.34×10−7 mol L−1 (S/N=3), and the relative standard deviation was 2.14 % (n=10) for 3×10−7 mol L−1 serine solution. The method was applied to the detection of serine in apple cider vinegar sample with a recovery of 97.5 %–101.9 %. To further discuss the change of the property of platinum surface on atomic level, theoretical calculations were carried out. The change of adsorption energy indicated that the possible interaction of Pt and cysteine was O−C−O bridge adsorption. The analysis about the density of states (DOS) of the system and pseudo band gap showed an increase in the Pt surface activity.

Tunneling probe of fluctuating superconductivity in disordered thin films

Disordered thin films close to the superconducting-insulating phase transition (SIT) hold the key to understanding quantum phase transition in strongly correlated materials. The SIT is governed by superconducting quantum fluctuations, which can be revealed for example by tunneling measurements. These experiments detect a spectral gap, accompanied by suppressed coherence peaks that do not fit the BCS prediction. To explain these observations, we consider the effect of finite-range superconducting fluctuations on the density of states, focusing on the insulating side of the SIT. We perform a controlled diagrammatic resummation and derive analytic expressions for the tunneling differential conductance. We find that short-range superconducting fluctuations suppress the coherence peaks, even in the presence of long-range correlations. Our approach offers a quantitative description of existing measurements on disordered thin films and accounts for tunneling spectra with suppressed coherence peaks observed, for example, in the pseudo gap regime of high-temperature superconductors.

STM/STS study on electronic superstructures in the superconducting state of high-Tc cuprate Bi2Sr2CaCu2O8+δ

We report STM/STS measurements at 8 K in underdoped Bi2Sr2CaCu2O8+δ crystals (Tc = 76 K and hole-doping level p ∼ 0.12) whose energy spectra around the Fermi level are characterized by a two-gap structure consisting of spatially inhomogeneous pseudogap (PG) and comparatively homogeneous superconducting gap (SCG). Two electronic superstructures, checkerboard modulation (CBM) and Cu-O-Cu bond-centered modulation (BCM), are observed with mapping spectral weights at low energies within the SCG and the ratio of spectral weights at ±ΔPG (PG energy), respectively. On the basis of the present findings, we suggest that the lower-energy scale CBM is an intrinsic property of Cu-O planes and can coexist with the BCM whose characteristic energy is ∼ΔPG in identical regions in real space.

Effect of Fe–V nonstoichiometry on electrical and thermoelectric properties of Fe2VAl films

We study the effect of Fe–V nonstoichiometry on electrical and thermoelectric properties of Fe2VAl films. We find that temperature dependence of electrical resistivity and carrier type for Fe2−xV1+xAl films are similar to those for bulk samples reported previously. In addition, the electrical and thermoelectric properties can be modulated by varying x. These results indicate that the electronic band structure having a pseudo gap at around the Fermi level is demonstrated even in thin-film Fe2VAl samples. This study will lead to further improvement in thermoelectric properties of the thin-film Fe2VAl.

Quasistatic internal magnetic field detected in the pseudogap phase of Bi2+xSr2−xCaCu2O8+δ by muon spin relaxation

We report muon spin relaxation ($\mu$SR) measurements of optimally-doped and overdoped Bi$_{2+x}$Sr$_{2-x}$CaCu$_2$O$_{8+\delta}$ (Bi2212) single crystals that reveal the presence of a weak temperature-dependent quasi-static internal magnetic field of electronic origin in the superconducting (SC) and pseudogap (PG) phases. In both samples the internal magnetic field persists up to 160~K, but muon diffusion prevents following the evolution of the field to higher temperatures. We consider the evidence from our measurments in support of PG order parameter candidates, namely, electronic loop currents and magnetoelectric quadrupoles.

Electronic, optical and thermoelectric properties of SnGa2GeX6 (X = S, Se) compounds

Electronic, optical and thermo-electric properties of two quaternary chalcogenides SnGa2GeX6 (X = S, Se) have been studied by using density functional theory. Modified Becke-Johnson exchange potential (mBJ-GGA) was used to calculate the band gaps of these compounds. Both compounds show pseudo direct band gap of 1.74 and 1.24 eV for SnGa2GeS6 and SnGa2GeSe6, respectively. The band gap decreases when replacing the X cations from S to Se. The optical properties such as dielectric function, energy loss function, extension coefficient, refractive index, and reflectivity of these compounds are discussed in details. The thermo-electric properties are studied using Boltzmann statistics through BoltzTrap code. High absorption peaks and figure of merits (ZT) for both compounds reveal that they are good candidates for the optoelectronics and thermo-electric devices.

Pairing in two dimensions and possible consequences for superconductivity: extension of Cooper's approach

Relation between the binding energy of a pair of electrons in Cooper's configuration and the magnitude of the attractive interaction between them is investigated theoretically for a two-dimensional electron gas when the electron pair has (a) zero and (b) finite center of mass momentum (CMM) first (i) in the background of a passive Fermi sea and (ii) with inclusion of the effect of Pauli exclusion principle in the presence of an active Fermi sea, at zero temperature. In the Cooper's approach (Cooper 1956 Phys. Rev. 104 1189) two pairing electrons are considered above the Fermi sea. Due to Pauli exclusion principle, these two electrons are forbidden to go into the states whose energies are less than the Fermi energy. Consequently, the role of the electrons which form the Fermi sea is passive in formation of electron pair (Fujita and Godoy 2001 Theory of High Temperature Superconductivity (Kluwer Academic Publishers)). However, in the case of an active Fermi sea, implying possible presence of multiple pairs of electrons coming out of the Fermi sea, the two electrons will have less available region in the momentum space for pair formation due to the Pauli blocking effect. In both the situations, threshold values of attractive interaction are found to exist to bind the electrons having finite CMM even with respect to the Fermi energy while with arbitrarily small value of attractive interaction they can be bound only in the case of zero CMM and that too in the presence of a passive Fermi sea. However, the presence of an active Fermi sea strongly opposes the pair formation having lower CMM. Furthermore, raising the range of the attractive interaction below and above the Fermi surface makes the pair formation more difficult due to enhanced Pauli blocking in this case. In the case of a passive Fermi sea however, with the increase in the range of the attractive interaction it becomes easier for the electron pairs to be in both Cooper pair state (CP) and true bound state (TB). These results are totally distinct from those obtained by Randeria et al (1989 Phys. Rev. Lett. 62 981, 1990 Phys. Rev. B 41 327) that a fermionic bound state in true vacuum is a necessary and sufficient condition for the Cooper pairing instability in the presence of a passive Fermi sea, independent of attractive interaction strength implying that for a potential which is attractive everywhere in two dimension, a two-body bound state exists for an arbitrarily weak attraction. Our calculations show, in the mean field approximation, that co-existence of TB state and CP state with various CMMs can occur only beyond a critical attractive interaction strength. This could lead to the formation of incoherent pairs even under strong coupling situation, which may hint onto the pseudo gap formation leading to some of the exotic features in the anomalous normal state seen in some of the exotic superconductors.

Explanation of Non-linear In-Plane Resistivity and Hall Coefficient in the Normal State of Cuprates: Polaronic Approach

We present a theoretical study of the in-plane resistivity ρ a b (T) and Hall coefficient R H (T) within the polaronic model and precursor pairing scenario by considering a two-component charge carrier picture in the normal state of high-temperature superconducting cuprates (HTSC). Here, we use a Boltzmann-equation approach and extended BCS-like model to compute ρ a b (T) and R H (T) in the τ-approximation. The opening of the pseudogap (PG) in the normal state of the cuprates should affect their transport properties. We have found that the transition to the PG regime and the effective conductivity of charge carriers in the normal state are responsible for the pronounced non-linear temperature dependence of ρ a b and R H . With the two-component model analysis, we conclude that the opening of the BCS-like PG, while the non-linear temperature dependence of ρ a b and R H could be understood as a consequence of pairing fluctuations in the PG state of cuprate superconductors. The calculated results for ρ a b (T) and R H (T) were compared with the experimental data obtained for various hole-doped cuprates. For all the considered cases, a good quantitative agreement was found between theory and experimental data. We also show that the energy scales of the binding energies of charge carriers are identified by PG crossover temperature on the cuprate phase diagram.

Effect of Multiple Orders on Superconducting Transition Temperature of Hole Doped Cuprates

Hole doped high-Tc cuprate superconductors are strongly correlated electronic systems. In these materials, various electronic orders are often found, but whether they support or compete with superconducting order is not unambiguous. Superconductivity normally manifests itself by a superconducting gap in the electronic density of states (EDOS). In cuprates, a gap appears even in the normal state called the pseudogap (PG). For certain doping range, spin density wave and charge density wave coexist with superconductivity by inducing corresponding additional gaps in the EDOS. In this study, we have tried to obtain expression for superconducting transition temperature, Tc by solving the BCS (Bardeen-Cooper-Schrieffer) energy gap equation in the presence of depleted EDOS of various origins and types. We have been successful to solve the weak-coupling BCS integral equation analytically in some special cases and also in the general case by using numerical integration. We have found that depending on conditions these non-pairing gaps/orders can enhance as well as reduce Tc.