Pseudogap Research Articles

Enhancing atomic ordering, magnetic and transport properties of Mn2VGa Heusler alloy thin films toward negatively spin-polarized charge injection

Magnetic materials with negative spin polarization have attracted attention for their potential to increase the design freedom of spintronic devices. This study investigated the effects of off-stoichiometry on the atomic ordering, microstructure, and magneto-transport properties in Mn2+xV1−xGa (x = −0.2, 0, +0.2, +0.4) Heusler alloy films, which are predicted to have large negative spin polarization derived from a pseudo band gap in the majority spin channel. The Mn2+xV1−xGa films epitaxially grown on MgO(001) substrates exhibits variations of B2 and L21 order with the Mn concentration. A high-quality L21 ordered film was achieved in the Mn-rich composition (x = +0.2) with B2 and L21 order parameters of 0.97 and 0.86, respectively, and a saturation magnetization of 1.4 μB/f.u., which agrees with the Slater-Pauling rule. Scanning transmission electron microscopy observations showed that B2 and L21 phases coexist in Mn-poor and stoichiometric films, while the L21 phase is dominant in the Mn-rich film with small amounts of Mn–V and Mn–Ga disorders, as revealed by laboratory and anomalous X-ray diffraction. Combined first-principles calculations and anisotropic magnetoresistance analysis confirms that the addition of excess Mn preserves the high spin polarization by suppressing the formation of detrimental antisites of V atoms occupying Mn sites. Therefore, the Mn-rich composition is promising for negatively spin-polarized charge injection in Mn2VGa-based spintronic applications.

Free energy of a two-liquid system of charge carriers in strongly coupled electron and phonon fields and common nature of three phases in hole-doped cuprates

Hole-doped cuprates exhibit partially coexisting pseudogap (PG), charge ordering (CO) and superconductivity; we show that there exists a class of systems in which they have a single nature as it has recently been supposed. Since the charge-ordered phase exhibits large frozen deformation of the lattice, we develop a method for calculating the phase diagram of a system with strong long-range (Fröhlich) electron–phonon interaction. Using a variational approach, we calculate the free energy of a two-liquid system of carriers with cuprate-like dispersion comprising a liquid of autolocalized carriers (large polarons and bipolarons) and Fermi liquid of delocalized carriers. Comparing it with the free energy of pure Fermi liquid and calculating (with standard methods of Bose liquid theory) a temperature of the superfluid transition in the large-bipolaron liquid we identify regions in the phase diagram with the presence of PG (caused by the impact of the (bi)polarons potential on delocalized quasiparticles), CO and superconductivity. They are located in the same places in the diagram as in hole-doped cuprates, and, as in the latter, the shape of the calculated phase diagram is resistant to wide-range changes in the characteristics of the system. As in cuprates, the calculated temperature of the superconducting transition increases with the number of conducting planes in the unit cell, the superfluid density decreases with doping at overdoping, the bipolaron density (and bipolaronic plasmon energy) saturates at optimal doping. Thus, the similarity of the considered system with hole-doped cuprates is not limited to the phase diagram. The results obtained allow us to discuss ways of increasing the temperature of the superfluid transition in the large-bipolaron liquid and open up the possibility of studying the current-carrying state and properties of the bipolaron condensate.

Pseudogap in Sr2−xLaxIrO4: Gor'kov-Teitel'baum thermal activation model

Recently, Hall effect measurements are done on Lanthanum doped Strontium Iridate Sr2−xLaxIrO4 which is 5d analogue of cuprates [1]. Hall effect measurements show that the effective carrier density nH exhibits a crossover from nH∼x to nH∼1+x near x≃0.16. This is very similar to that found in cuprates around p≃0.19. It is proposed that a pseudogap (PG) state in Sr2−xLaxIrO4 exists and is ending at x≃0.16[1]. However, PG boundary (in doping-temperature phase diagram) remains unknown. In this work, we apply a very successfull the Gor'kov-Teitel'baum Thermal Activation (GTTA) model to Sr2−xLaxIrO4 and obtain its PG phase boundary and draw an updated phase diagram. Our results agree with previously known signatures of PG phase in this system [2–4]. Using results from GTTA model we also obtain the evolution of “Fermi arcs” in this system as a function temperature for doping concentration x≃0.08 which are in qualitative agreement with the reported results.

Ultrabroadband High Photoresponsivity at Room Temperature Based on Quasi-1D Pseudogap System (TaSe4 )2 I.

Narrow bandgap materials have garnered significant attention within the field of broadband photodetection. However, the performance is impeded by diminished absorption near the bandgap, resulting in a rapid decline in photoresponsivity within the mid-wave infrared (MWIR) and long-wave infrared (LWIR) regions. Furthermore, they mostly worked in cryogenic temperature. Here, without the assistance of any complex structure and special environment, it is realized high responsivity covering ultra-broadband wavelength range (Ultraviolet (UV) to LWIR) in a single quasi-1D pseudogap (PG) system (TaSe4 )2 I nanoribbon, especially high responsivity (From 23.9 to 8.31AW-1 ) within MWIR and LWIR region at room temperature (RT). Through direct probing the carrier relaxation process with broadband time-resolved transient absorption spectrum measurement, the underlying mechanism of majorly photoconductive effect is revealed, which causes an increased spectral weight extended to PG region. This work paves the way for realizing high-performance uncooled MWIR and LWIR detection by using quasi-1D PG materials.

Compressibility behavior of CoxB: Experiment and computation

Structural stability studies on Co2B and Co3B are performed using Mao-Bell type diamond anvil cell. The tetragonal structure of Co2B and the orthorhombic structure of Co3B are stable up to 41 and 43 GPa, respectively. The bulk modulus obtained from high-pressure XRD experiments is 282 ± 21 GPa and 254 ± 10 GPa for Co2B and Co3B, respectively, which reasonably agrees with the computed value. The first principle calculations are carried out to understand the elastic and mechanical properties of CoxB (x: 1, 2, and 3). Adding B into Co enhances the elastic and mechanical properties marginally up to Co2B. Whereas in CoB, a significant increase in the elastic and mechanical properties was observed. The electronic density of states of CoB reveals the presence of a pseudo gap at Fermi energy leading to higher bulk modulus among the cobalt borides. Bader charge analysis shows a transfer of charge from Co to B. The valence charge variation indicates the strengthening of B–B bonds at the expense of Co–Co bonds leading to enhanced elastic properties.

Statistics of the cuprate pairon states on a square lattice

In this paper the fundamental parameters of high-Tc superconductivity are shown to be connected to the statistics of pairons (hole pairs in their antiferromagnetic (AF) environment) on a square lattice. In particular, we study the density fluctuations and the distribution of the area surrounding each pairon on the scale of the AF correlation length , for the complete range of hole concentration. We show that the key parameters of the phase diagram, the Tc dome, and the pseudogap (PG) temperature , emerge from the statistical properties of the pairon disordered state. In this approach, the superconducting and the PG states appear as inseparable phenomena. The condensation energy, which fixes the critical temperature, is directly proportional to the correlation energy between pairons and not to the energy gap, contrary to conventional superconductors. When the correlation energy between pairons is suppressed by fluctuations, either thermally, by disorder, or in the vortex core, the PG state of disordered pairons is obtained. We attribute the unique features of cuprate superconductivity to this order–disorder transition in real space, which clearly differs from the Bardeen–Cooper–Schrieffer mechanism. Our predictions are in quantitative agreement with low-temperature tunneling and photoemission spectroscopy experiments.

Thermoelectric power in high-temperature superconductors: Theory and experiment (Review article)

A comprehensive review of theoretical models devoted to thermoelectric power (thermoEMF) in high-temperature superconductors (HTSCs), as well as relevant experimental works, has been carried out. It is shown that the modified two-band model with an additional temperature term provides the best fit for thermoEMF S(T) for YBa2Cu3O7–δ (model 4). However, all the models discussed, including model 4, do not match the experimental data S(T) at T < T*, where T* is the pseudogap (PG) opening temperature, which is much higher than the superconducting transition temperature Tc. This is due to the experimental fact that, at T < T*, both the resistivity ρ(T) and the density of charge carrier nf, the density of states at the Fermi level decrease, and Seebeck coefficient S(T) increases sharply, which indicates a rearrangement of the Fermi surface. Thus, the rearrangement of the Fermi surface with the appearance of PG is well established, at least in YBCO. In addition, three different types of behavior of S/T as a function of log T around the quantum critical point were observed at a normalized carrier density p = p* ≈ 0.16. Thus, three different modes of quantum criticality in YBCO were revealed at p ≈ p*, below which some order sets in HTSC, which causes the rearrangement of the Fermi surface.

Spin-momentum locking and quantum anomalous Hall effect in d+id-density wave ordered pseudo gap phase of Bi2212

We model Bi 2 Sr 2 CaCu 2 O 8 + δ (Bi2212) bilayer system by a time reversal symmetry (TRS) and inversion symmetry (IS) broken Bloch Hamiltonian H. The pseudo-gap phase of the bilayer is assumed to be chiral d-density wave ordered. One of the focal points of the paper is the theoretical study of the spin-momentum locking (SML), due to the presence of a strong spin–orbit coupling in Bi2212, reported earlier in this phase in a spin-and angle-resolved photoemission spectroscopic measurement. The non-trivial spin texture in k -space, which is an outcome of the broken IS for the bilayer system, is found tuneable by electric field (and also by intercalation). We also show that this system exhibits anomalous quantum Hall effect due to the broken TRS and the presence of a strong spin–orbit coupling. • The emergent quantum anomalous Hall state due to the time reversal asymmetry. • The tuneable non-trivial spin-texture by electric field and intercalation. • The CDDW order is the raison d’etre of the freezing of anti-nodal quasi-particles. • The spin current density is a skew matrix only at the anti-nodal points. • The Majorana zero mode exists for the Zeeman field ⩾ a critical value.

The Influence of Dimensionality on the Charge‐Density‐Wave Transition and its Application on Mid‐Infrared Photodetection

Abstract2D charge density wave (CDW) materials receive much attention for high responsivity and broadband photodetection in recent years due to their collective electron transport and narrow bandgap. However, the high dark current density problem hinders their real application. Here, a sharp CDW transition in quasi‐1D (TaSe4)2I is reported and applied for broadband photodetection. Especially at mid‐infrared region, the device shows both high photo responsivity of 1.18 ×103 A W−1 and large light on/off ratio of 80, which is superior to 2D CDW TaS2 and most reported low‐dimensional materials. Such high performance relies on two aspects. One is the much lower dark current density resulting from the pseudo gap associated with 1D Luttinger liquid state, which is supported by finite size scaling of nonlinear I–V at variable temperatures and occurrence of 1D structural phase transition consolidated by in situ Raman spectroscopy. The other is the high photocurrent associated with the “Fröhlich superconductivity” state, manifested by an ultrasensitive switching, which can be only accessible in 1D CDW materials, in agreement with the authors’ density functional theory calculation. This work thus reveals the pivotal role of dimensionality in CDW phase transition and paves a way for implementing highly sensitive broadband photodetector.

Structural complexity in the apparently simple crystal structure of Be2Ru.

The structural features of the hexagonal layered crystal structure of Be2Ru (a = 5.7508(3) Å, c = 3.0044(2) Å, space group P -62m) were investigated by single crystal X-ray diffraction and transmission electron microscopy (TEM). The residual electron density and high-resolution TEM images show that the real structure can be described as an intergrowth of the main hexagonal matrix of the Fe2P type with minor orthorhombic inclusions of its stacking variants. Such atomic arrangement is stabilized by the charge transfer from Be to Ru and by a system of polar three- and four-atomic bonds involving both components. The calculated electronic density of states (DOS) of Be2Ru revealed, contrarily to typical intermetallic compounds, a pseudo gap (dip) in the vicinity of the Fermi level. The temperature dependence of the electrical resistivity of Be2Ru shows metal behavior in agreement with the non-zero DOS at the Fermi level.

Interrelations among zero-field critical current density, irreversibility field and pseudogap in hole doped high-Tc cuprates

Critical current density and the irreversibility magnetic fields are two parameters which control most of the applications of high-temperature superconductors. In this brief communications we have investigated the intrinsic effects that determine the magnitudes of these two parameters. The roles of hole concentration in the CuO2 plane (p) and oxygen deficiency (δ) in the CuO1-δ chains on the zero-field low-temperature critical current density, Jc0, were studied for a series of Y1-xCaxBa2Cu3O7-δ (x = 0.00, 0.05, 0.10, and 0.20) thin films. Jc0 above the optimum hole concentration were found to be high. The magnitude of Jc0 were largely determined by the hole concentration, maximizing at p ∼ 0.185 ± 0.005, regardless of the values of x and δ. This implied that oxygen deficiency/disorder plays a secondary role and the intrinsic Jc0 is governed largely by the number of doped holes in the CuO2 planes of Y1-xCaxBa2Cu3O7-δ. Further support in favor of this finding was garnered from the analysis of the in-plane resistive transitions of thin films of YBa2Cu3O7-δ (YBCO) with magnetic fields (H) applied along the crystallographic c-direction. The characteristic magnetic field (H0), linked to the vortex activation energy and the irreversibility field, exhibits similar p-dependence as shown by Jc0(p). We have explained these observations in terms of the doping dependent pseudogap (PG) energy/temperature scale in the low-energy electronic energy density of states observed in high-Tc cuprates. Both the intrinsic critical current density and the irreversibility field depend directly on the superconducting condensation energy, which in turn is primarily determined by the magnitude of the hole concentration dependent PG in the quasiparticle spectral density.

Evolution of the pseudogap and excess conductivity of YBa2Cu3O7–δ single crystals in the course of long-term aging

The temperature dependences of both fluctuation conductivity (FLC) σ′(T) and pseudogap (PG) Δ*(T) derived from measurements of resistivity ρ(T) of an optimally doped YBa2Cu3O7−δ single crystal subjected to long-term storage have been studied. The as-grown sample S1 exhibits characteristics typical of optimally doped YBa2Cu3O7−δ single crystals containing twins and twin boundaries. Analysis of both FLC and PG showed an unexpected improvement in all characteristics of the sample after 6 years of storage (sample S2), indicating that the effect of twin boundaries is somehow limited. After 17 years of storage, all characteristics of the sample changed dramatically, which indicates a strong influence of internal defects formed during the aging process. For the first time, the temperature dependences of both FLC and PG were obtained after 17 years of storage.

Pseudogap formation due to charge-transfer transition and Kondo effect

We investigate the doping evolution of the electronic state of the three-band t-J-U model considering the normal state of the hole-doped high- superconducting cuprate. In our model, when some number of holes are doped into the undoped state, the d electron exhibits the charge-transfer (CT)-type Mott-Hubbard transition along with a chemical potential jump. A reduced CT gap is formed from the p band and the coherent component of the d band, and it shrinks due to charge fluctuations as more holes are doped as in the pseudogap (PG) phenomenon. This trend is reinforced as the d-p band hybridization is increased, and a Fermi liquid state is retrieved as in the Kondo effect. These suggest that the PG in the hole-doped cuprate emerges due to the CT transition and the Kondo effect.

The halide ion replacement effects on the physical properties of Cs2BX6 variant perovskites

Variant perovskites of the halide family have gained great attention in the recent times due to their prominent characteristics favorable for various applications. The variant perovskite Cs2BX6 (B= Hf, Ti, Zr and X = Cl, Br, I) have been the subject of some reports for experimental growth and measurement to computational studies. However, so far no work is available for the comparative study and detail optical properties. Here in this work, the structural, electronic and optical properties of Cs2BX6 determined using FP-LAPW + lo method and compared with the existing literature. The obtained results show improvement to good comparison with the earlier reports. The optimized band structure reveals that the band gap is decreased gradually switching from Cl to Br and I. The band structures show that most of variant perovskite under consideration are pseudo direct band gap with wide energy gaps. The optical properties like dielectric constant, absorption coefficient and optical conductivity peaks are also found up to incident photon energy of 20 eV. The materials show rising peaks in the ultraviolet region showing their great potential for high frequency applications.

Local pairs in high-temperature superconductors: The concept of pseudogap

The origin of the high-temperature superconductivity of cuprates remains a subject of debate after almost four decades of investigation. One of the main questions — what is the physics behind the mechanism of superconducting pairing, which makes it possible to obtain real Cooper pairs at temperatures much higher than 100 K, remains open. It is believed that the answer can be obtained by the studying the properties of cuprates in the normal state above Tc, where the pseudogap (PG) opens at T* >> Tc. The number of papers devoted to the study of PG is extraordinarily large, but its physics also remains in question. In cuprates, the question arises whether the pseudogap phase arises exclusively due to antiferromagnetic fluctuations, spin or charge density waves, or whether it can be explained by the formation below T* of specific paired fermions, the so-called local pairs. We review studies on both approaches to PG that should clarify this issue. In addition to theoretical considerations, we analyze and discuss various experimental results of fluctuation conductivity and PG measurements, as well as magnetic measurements, in an attempt to prove the decisive role of local pairs in the formation of the PG state. Accordingly, various types of supposed magnetic interactions can be considered as possible mechanisms of superconducting pairing in high-temperature superconductors.

Topological spin texture in the pseudogap phase of a high-Tc superconductor.

An outstanding challenge in condensed-matter-physics research over the past three decades has been to understand the pseudogap (PG) phenomenon of the high-transition-temperature (high-Tc) copper oxides. A variety of experiments have indicated a symmetry-broken state below the characteristic temperature T* (refs. 1-8). Among them, although the optical study5 indicated the mesoscopic domains to be small, all these experiments lack nanometre-scale spatial resolution, and the microscopic order parameter has so far remained elusive. Here we report, to our knowledge, the first direct observation of topological spin texture in an underdoped cuprate, YBa2Cu3O6.5, in the PG state, using Lorentz transmission electron microscopy (LTEM). The spin texture features vortex-like magnetization density in the CuO2 sheets, with a relatively large length scale of about 100 nm. We identify the phase-diagram region in which the topological spin texture exists and demonstrate the ortho-II oxygen order and suitable sample thickness to be crucial for its observation by our technique. We also discuss an intriguing interplay observed among the topological spin texture, PG state, charge order and superconductivity.

YBCO polycrystal co-added with BaTiO3 and WO3 nanoparticles: Fluctuation induced conductivity and pseudogap studies

We report the effect of the addition of BaTiO3 and WO3 nanoparticles on the pseudogap and excess conductivity of YBCO polycrystal. The excess conductivity analysis showed that the experimental data of pure and sample added with low amount of WO3 and BaTiO3 were perfectly described by the 2D Maki-Thompson (MT) and 3D Aslamazov-Larkin (AL) models. A sharp decrease in excess conductivity was obtained for samples added with high amount of nanoparticles where the analysis indicated the presence of the AL contribution and Lawrence-Doniach (LD) models. The pseudogap (PG) results showed that the value of Tpair (the temperature at which local pairs are transformed from strongly bound bosons into the fluctuating Cooper pairs) increased from around 117 K for the pure sample to around 187 K for sample co-added with low amount of nanoparticles. The pseudogap Δ*(TG) reduced from 230.9 K (for pure sample) to 223.4 K (for added sample).

Effects of annealing on the fluctuation conductivity and pseudogap in slightly doped HoBa2Cu3O7–δ single crystals

The effect of annealing at room temperature on the fluctuation conductivity (FLC) σ′(T) and pseudogap (PG) Δ*(7) in the basal ab plane of ReBa2Cu3O7–δ (Re = Ho) single crystals with a lack of oxygen has been studied. It is shown that at all stages of annealing, the FLC near Tc can be described by the Aslamazov-Larkin and Maki-Thompson fluctuation theories, demonstrating a 3D-2D crossover with increasing temperature. The crossover temperature T0 was used to determine the coherence length along the c axis, ξc(0) = (2.82 ± 0.2) Å. At the intermediate stage of annealing, an anomalous increase in 2D FLC was revealed, which is associated with the influence of uncompensated magnetic moments in HoBa2Cu3O7–5 (HoBCO): μeff, Ho = 9.7μΒ. For the quenched sample S1, the temperature dependence of the PG has a shape typical of single crystals with a large number of defects. However, Δ*(T) has two small additional maxima at high temperature, which is a feature of HoBCO single crystals with pronounced twins and indicates the two-phase nature of the sample. Upon annealing, the shape of Δ*(T) noticeably changes, very likely due to an increase in the magnetic interaction (sample S2). More important is the change in the slope of the data at high temperatures, which has become about 3.5 times steeper. The ordering of the oxygen distribution due to the diffusion process during annealing somewhat compensates for the influence of magnetic interaction. But the slope does not change (sample S3). Interestingly, the slope turns out to be the same as for FeAs-based superconductors, suggesting the possibility of the existence of spin density waves in HoBCO in the PG state. The comparison of the pseudogap parameter Δ∗(T)/Δmax∗ near Tc with the Peters-Bauer theory revealed a slight increase in the density of local pairs < n↑n↓>, which should explain the observed increase in Tc by 9 K during annealing.

On the $$\alpha $$-lazy version of Markov chains in estimation and testing problems

Given access to a single long trajectory generated by an unknown irreducible Markov chain M, we simulate an \(\alpha \)-lazy version of M which is ergodic. This enables us to generalize recent results on estimation and identity testing, that were stated for ergodic Markov chains, in a way that allows fully empirical inference. In particular, our approach shows that the pseudo spectral gap introduced by Paulin (Electron J Probab 20:32, 2015) and defined for ergodic Markov chains may be given a meaning already in the case of irreducible but possibly periodic Markov chains.

Non-Hermitian skin effect and lasing of absorbing open-boundary modes in photonic crystals

We explore absorbing open-boundary modes in non-Hermitian photonic systems. The modes have a continuum spectrum in the infinite system-size limit and can exhibit the non-Hermitian skin effect. In contrast to the conventional non-Hermitian skin modes under the fixed-end open-boundary condition, the modes concerned exhibit a strongly size-dependent spectrum that gradually converges to the non-Bloch-band dispersion. The modes correspond to the poles of the $S$ matrix, and are closely related to the lasing. We demonstrate these properties in a two-dimensional non-Hermitian photonic crystal with gain having a point-(pseudo)gap topology.