In the high-transition-temperature (high-Tc) superconductors the pseudogap phase becomes predominant when the density of doped holes is reduced1. Within this phase it has been unclear which electronic symmetries (if any) are broken, what the identity of any associated order parameter might be, and which microscopic electronic degrees of freedom are active. Here we report the determination of a quantitative order parameter representing intra-unit-cell nematicity: the breaking of rotational symmetry by the electronic structure within CuO2 unit cell. We analyze spectroscopic-imaging scanning tunneling microscope images of the intra-unit-cell states in underdoped Bi2Sr2CaCu2O8+δ and, using two independent evaluation techniques, find evidence for electronic nematicity of the states close to the pseudogap energy. Moreover, we demonstrate directly that these phenomena arise from electronic differences at the two oxygen sites within each unit cell. If the characteristics of the pseudogap seen here and by other techniques all have the same microscopic origin, this phase involves weak magnetic states at the O sites that break 90o -rotational symmetry within every CuO2 unit cell.
Understanding the nature of the mysterious pseudogap phenomenon is one of the most important issues associated with cuprate high-$T_c$ superconductors. Here, we report $^{17}$O nuclear magnetic resonance (NMR) studies on two planar oxygen sites in stoichiometric cuprate YBa$_2$Cu$_4$O$_8$ to investigate the symmetry breaking inside the pseudogap phase. We observe that the Knight shifts of the two oxygen sites are identical at high temperatures but different below $T_{\rm nem} \sim$ 185 K, which is close to the pseudogap temperature $T^{\ast}$. Our result provides a microscopic evidence for int…
During the last decade, translational and rotational symmetry-breaking phases -- density wave order and electronic nematicity -- have been established as generic and distinct features of many correlated electron systems, including pnictide and cuprate superconductors. However, in cuprates, the relationship between these electronic symmetry-breaking phases and the enigmatic pseudogap phase remains unclear. Here, we employ resonant x-ray scattering in a cuprate high-temperature superconductor La$_{1.6-x}$Nd$_{0.4}$Sr$_{x}$CuO$_{4}$ (Nd-LSCO) to navigate the cuprate phase diagram, probing the rel…
The pseudogap phenomenon in cuprates is the most mysterious puzzle in the research of high-temperature superconductivity. In particular, whether the pseudogap is associated with a crossover or phase transition has been a long-standing controversial issue. The tetragonal cuprate HgBa$_2$CuO$_{4+δ}$, with only one CuO$_2$ layer per primitive cell, is an ideal system to tackle this puzzle. Here, we measure the anisotropy of magnetic susceptibility within the CuO$_2$ plane with exceptionally high-precision magnetic torque experiments. Our key finding is that a distinct two-fold in-plane anisotropy…
The nature of the pseudogap and its relationship with superconductivity are one of the central issues of cuprate superconductors. Recently, a possible scenario has been proposed that the pseudogap state is a distinct phase characterized by spontaneous rotational symmetry breaking called "nematicity" based on transport and magnetic susceptibility measurements, where the symmetry breaking was observed below the pseudogap temperature $T^*$. Here, we report a temperature-dependent ARPES study of nematicity in slightly overdoped Bi${}_{1.7}$Pb${}_{0.5}$Sr${}_{1.9}$CaCu${}_2$O${}_{8+δ}$ triggered by…
In the present paper II, we will gain an understanding of the nematicity, insulating ground state (IGS), nematicity to stripe phase transition, Fermi pockets evolution, and resistivity temperature upturn, as to be metal - insulator (fermion-boson)- crossover (MIC) phenomena for the pseudogap (PG) region of cuprates. While in the paper I [1], we obtained an understanding of the observed heat conductivity downturn, anomalous Lorentz ratio, insulator resistivity boundary, nonlinear entropy as manifestations of the same MIC. The recently observed nematicity and hidden magnetic order are related to…
Nematicity has emerged as a key feature of cuprate superconductors, but its link to other fundamental properties such as superconductivity, charge order and the pseudogap remains unclear. Here we use measurements of transport anisotropy in YBa$_2$Cu$_3$O$_y$ to distinguish two types of nematicity. The first is associated with short-range charge-density-wave modulations in a doping region near $p = 0.12$. It is detected in the Nernst coefficient, but not in the resistivity. The second type prevails at lower doping, where there are spin modulations but no charge modulations. In this case, the on…
The anomalous momentum and doping dependence of the electron spectral function and electron dispersion for copper oxide materials in the underdoped regime are studied within the t-J model. It is shown that the electron spectrum is changed with dopings, and the electron dispersion exhibits a flat band around ($π$,0) point in the Brillouin zone, which leads to the normal-state pseudogap formation. The theoretical results are consistent with the experiments and numerical simulations.
One of the leading issues in high-$\rm T_c$ copper oxide superconductors is the origin of the pseudogap phase in the underdoped regime of their phase diagram. Using polarized neutron diffraction, a novel magnetic order has been identified as an hidden order parameter of the pseudogap as the transition temperature corresponds to what is expected for the pseudogap. The observed magnetic order preserves translational symmetry as predicted for orbital moments in the circulating current theory. Being now reported in three different cuprates familles, it appears as a universal phenomenon whatever th…
Within the t-J model, the charge dynamics of copper oxide materials in the underdoped regime is studied based on the fermion-spin theory. It is shown that both in-plane charge dynamics and c-axis charge dynamics are mainly governed by the scattering from the in-plane fluctuation, which would be suppressed when the holon pseudogap opens at low temperatures, leading to the temperature linear to the nonlinear range in the in-plane resistivity and crossovers to the semiconducting-like range in the c-axis resistivity.
We study the coexisting smectic modulations and intra-unit-cell nematicity in the pseudogap states of underdoped Bi2Sr2CaCu2O8+δ. By visualizing their spatial components separately, we identified 2πtopological defects throughout the phase-fluctuating smectic states. Imaging the locations of large numbers of these topological defects simultaneously with the fluctuations in the intra-unit-cell nematicity revealed strong empirical evidence for a coupling between them. From these observations, we propose a Ginzburg-Landau functional describing this coupling and demonstrate how it can explain the c…
Electronic nematic phases have been proposed to occur in various correlated electron systems and were recently claimed to have been detected in scanning tunneling microscopy (STM) conductance maps of the pseudogap states of the cuprate high-temperature superconductor Bi2Sr2CaCu2O8+x (Bi-2212). We investigate the influence of anisotropic STM tip structures on such measurements and establish, with a model calculation, the presence of a tunneling interference effect within an STM junction that induces energy-dependent symmetry-breaking features in the conductance maps. We experimentally confirm t…
We apply strong magnetic fields of H=28.5 \sim 43 T to suppress superconductivity (SC) in the cuprates Bi_{2}Sr_{2-x}La_xCuO_{6+δ} (x=0.65, 0.40, 0.25, 0.15 and 0), and investigate the low temperature (T) normal state by ^{63}Cu nuclear spin-lattice relaxation rate (1/T_1) measurements. We find that the pseudogap (PG) phase persists deep inside the overdoped region but terminates at x \sim 0.05 that corresponds to the hole doping concentration of approximately 0.21. Beyond this critical point, the normal state is a Fermi liquid characterized by the T_1T=const relation. A comparison of the supe…
Overshadowing the superconducting dome in hole-doped cuprates, the pseudogap state is still one of the mysteries that no consensus can be achieved. It has been suggested that the rotational symmetry is broken in this state and may result in a nematic phase transition, whose temperature seems to coincide with the onset temperature of the pseudogap state $T^*$ around optimal doping level, raising the question whether the pseudogap results from the establishment of the nematic order. Here we report results of resistivity measurements under uniaxial pressure on several hole-doped cuprates, where t…
We report measurements of the anisotropy of the spin echo decay for the inner layer Cu site of the triple layer cuprate, Hg_0.8Re_0.2Ba_2Ca_2Cu_3O_8 (T_c=126 K) in the pseudogap T regime below T_pg ~ 170 K and the corresponding analysis for their interpretation. As the field alignment is varied, the shape of the decay curve changes from Gaussian (H_0 \parallel c) to single exponential (H_0 \perp c). The latter characterizes the decay caused by the fluctuations of adjacent Cu nuclear spins caused by their interactions with electron spins. The angular dependence of the second moment (T_{2M}^{-2}…
Surface probes such as scanning tunneling microscopy (STM) have detected complex patterns at the nanoscale, indicative of electronic inhomogeneity, in a variety of high temperature superconductors. In cuprates, the pattern formation is associated with the pseudogap phase, a precursor to the high temperature superconducting state. Symmetry breaking (i.e. from C4 to C2) in the form of electronic nematicity has recently been implicated as a unifying theme of the pseudogap phase,[1] however the fundamental physics governing the nanoscale pattern formation has not yet been identified. Here we use u…
We report the hole doping dependencies of the pseudogap phase energy scale, $2Δ_{\rm PG}$, the anti-nodal (nodal) superconducting energy scales $2Δ^{AN}_{\rm SC}$ ($2Δ^{N}_{\rm SC}$) and the charge density wave energy scale, $2Δ_{\rm CDW}$. They have been extracted from the electronic Raman responses of distinct copper oxide families. For all the cuprates studied, we reveal universal doping dependencies which suggest that $2Δ_{\rm PG}$, $2Δ^{AN}_{\rm SC}$ and $2Δ_{\rm CDW}$ are governed by common microscopic interactions and that these interactions become relevant well above the superconductin…
In the quest to understand high-temperature superconductivity in copper oxides, a vigorous debate has been focused on the pseudogap - a partial gap that opens over portions of the Fermi surface in the 'normal' state above the bulk critical temperature ($T_{c}$). The pseudogap has been attributed to precursor superconductivity, to the existence of preformed pairs, or to competing orders such as charge-density waves. A direct determination of the charge of carriers as a function of temperature and bias could help resolve among these alternatives. Here, we report measurements of the shot noise of…
We present a SU(2) gauge theory of fluctuating magnetic order in the two-dimensional Hubbard model. The theory is based on a fractionalization of electrons in fermionic chargons and bosonic spinons. The chargons undergo Néel or spiral magnetic order below a density dependent transition temperature $T^*$. Fluctuations of the spin orientation are described by a non-linear sigma model obtained from a gradient expansion of the spinon action. The spin stiffnesses are computed from a renormalization group improved random phase approximation. Our approximations are applicable for a weak or moderate H…
In copper-oxide and iron-based high temperature (high-$T_{\rm c}$) superconductors, many physical properties exhibit in-plane anisotropy, which is believed to be caused by a rotational symmetry-breaking nematic order, whose origin and its relationship to superconductivity remain elusive. In many iron-pnictides, a tetragonal-to-orthorhombic structural transition temperature $T_{\rm s}$ coincides with the magnetic transition temperature $T_{\rm N}$, making the orbital and spin degrees of freedom highly entangled. NaFeAs is a system where $T_{\rm s}$ = 54 K is well separated from $T_{\rm N}$ = 42…
The pseudogap (PG) state and its related intra-unit-cell symmetry breaking remain the focus in the research of cuprate superconductors. Although the nematicity has been studied in Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$, especially underdoped samples, its behavior in other cuprates and different doping regions is still unclear. Here we apply a scanning tunneling microscope to explore an overdoped (Bi, Pb)$_2$Sr$_2$CuO$_{6+δ}$ with a large Fermi surface (FS). The establishment of a nematic order and its real-space distribution is visualized as the energy scale approaches the PG.
The pseudogap regime of the cuprate high-temperature superconductors is characterized by a variety of competing orders, the nature of which are still widely debated. Recent experiments have provided evidence for electron nematic order, in which the electron fluid breaks rotational symmetry while preserving translational invariance. Raman spectroscopy, with its ability to symmetry resolve low energy excitations, is a unique tool that can be used to assess nematic fluctuations and nematic ordering tendencies. Here, we compare results from determinant quantum Monte Carlo simulations of the Hubbar…
Within a two component approach to high Tc copper oxides including polaronic couplings, we identify the pseudogap phase as the onset of polaron ordering. This ordering persists in the superconducting phase. A huge isotope effect on the pseudogap onset temperature is predicted and in agreement with experimental data. The anomalous temperature dependence of the mean square copper oxygen ion displacement observed above, at and below Tc stems from an s-wave superconducting component of the order parameter, whereas a pure d-wave order parameter alone can be excluded.
Strongly interacting electrons can exhibit novel collective phases, among which the electronic nematic phases are perhaps the most surprising as they spontaneously break rotational symmetry of the underlying crystal lattice. The electron nematicity has been recently observed in the iron-pnictide and cuprate high-temperature superconductors. Whether such a tendency of electrons to self-organise unidirectionally has a common feature in these superconductors is, however, a highly controversial issue. In the cuprates, the nematicity has been suggested as a possible source of the pseudogap phase, w…
This article is devoted to a discussion of stripe and electron-nematic order and their connection to electronic properties in the pseudogap regime of copper-oxide superconductors. We review basic properties of these symmetry-breaking ordering phenomena as well as proposals which connect them to quantum-oscillation measurements. Experimental data indicate that these orders are unlikely to be the cause of the pseudogap phenomenon, implying that they occur on top of the pseudogap state which itself is of different origin. Specifically, we discuss the idea that the non-superconducting pseudogap gr…
We report atomic scale characterization of the pseudogap state in a high-T$_c$ superconductor, Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$. The electronic states at low energies within the pseudogap exhibit spatial modulations having an energy-independent incommensurate periodicity. These patterns, which are oriented along the copper-oxygen bond directions, appear to be a consequence of an electronic ordering phenomenon--the observation of which correlates with the pseudogap in the density of electronic states. Our results provide a stringent test for various ordering scenarios in the cuprates, which have b…
The pseudogap state is widely regarded as a precursor to high-temperature superconductivity yet remains poorly understood. Using high field (30 T) NMR, we report two electronically distinct oxygen sites within the copper-oxygen (Cu-O) planes of an underdoped cuprate in the pseudogap state. At temperatures well above the bulk superconducting transition, roughly 25% of the oxygen sites evidence superconducting pair formation, the remaining evidence antiferromagnetic ordering on the nearest neighbor Cu sites. Any superconducting fluctuations are slower than the microsecond timescales of the NMR p…
The role of the anti-bonding state in the electron correlations in Copper Oxide HTSC is analyzed. Then the t-J Hamiltonian is used to establish the formation of the charge stripes in underdoped oxides. It is proposed that these stripes make up the boundaries between the two degenerate anti-ferromagnetic (AFM) states, and that they are a key factor in switching between these states. We also provide a theoretical expression for the charge driven AFM magnons that have been observed by Neutron scattering experiments. Finally, the double correlation theory is applied to the stripe phase of holes to…
The generic band structure of high-T$_C$ copper oxides is simulated by the nearly free-electron model (NFE) in two dimensions (2-D) with parameters from band calculations. Interaction between phonons and spin waves will cause potential modulations and pseudogaps, and the strength of the modulations, the wave lengths and the doping, are all related. A Fermi-surface "arc" is found for dynamic spin/phonon waves. The confinement of superconductivity between two limiting dopings can be a result of competition with the pseudogap at low doping and weak coupling at high doping.
Here I extend my last work about the origin of the pseudo-gaps in underdoped cuprates (arXiv: cond-mat. 1011.3206), to include the mechanism of superconductivity. This is done by adapting the formalism of the double correlations in systems with nested Fermi surfaces to the semi one dimensional system of strings of holes. It is proposed that magnetic interaction is crucial for the establishment of the pseudogap and the high temperature superconductivity. It is shown that superconductivity disturbs the completeness of the strings of holes, and creates fluctuations in their shapes. This, in turn,…
We report nuclear magnetic resonance studies on the low-doped n-type copper-oxide Pr_{0.91}LaCe_{0.09}CuO_{4-y} (T_c=24 K) in the superconducting state and in the normal state uncovered by the application of a strong magnetic field. We find that when the superconductivity is removed, the underlying ground state is the Fermi liquid state. This result is at variance with that inferred from previous thermal conductivity measurement and contrast with that in p-type copper-oxides with a similar doping level where high-T_c superconductivity sets in within the pseudogap phase. The data in the superco…
Although it is generally accepted that superconductivity (SC) is unconventional in the high- transition temperature copper oxides (high-Tc cuprates), the relative importance of phenomena such as spin and charge (stripe) order, SC fluctuations, proximity to a Mott insulator, a pseudogap phase, and quantum criticality are still a matter of great debate1. In electron-doped cuprates, the absence of an anomalous pseudogap phase in the underdoped region of the phase diagram2 and weaker electron correlations3,4, suggest that Mott physics and other unidentified competing orders are less relevant and t…
The Copper-Oxygen planes in cuprates have been at the center of the search for a theory of high-temperature superconductivity. We conduct an extensive study of the ground state of the three-band Hubbard (Emery) model in the underdoped regime. We focus on the magnetic and charge orders, and present results from generalized Hartree-Fock (GHF) calculations. The ground-state properties at the thermodynamic limit are challenging to pin down because of sensitivity to computational details including the shapes and sizes of the supercells. We employ large-scale computations with various technical impr…
The last few years have seen significant experimental progress in characterizing the copper-based hole-doped high temperature superconductors in the regime of low hole density, p. Quantum oscillations, NMR, X-ray, and STM experiments have shed much light on the nature of the ordering at low temperatures. We review evidence that the order parameter in the non-Lanthanum-based cuprates is a d-form factor density-wave. This novel order acts as an unexpected window into the electronic structure of the pseudogap phase at higher temperatures in zero field: we argue in favor of a `fractionalized Fermi…
In the pseudogap state of the high-Tc copper-oxide (cuprate) superconductors, angle-resolved photoemission (ARPES) measurements have seen an Fermi arc, i.e., an open-ended gapless section in the large Fermi surface, rather than a closed loop expected of an ordinary metal. This is all the more puzzling because Fermi pockets (small closed Fermi surface features) have been suggested from recent quantum oscillation measurements. The Fermi arcs have worried the high-Tc community for many years because they cannot be understood in terms of existing theories. Theorists came up with a way out in the f…
A central issue in the quest to understand the superconductivity in cuprates is the nature and origin of the pseudogap state, which harbours anomalous electronic states such as Fermi arc, charge density wave (CDW), and $d$-wave superconductivity. A fundamentally important, but long-standing controversial problem has been whether the pseudogap state is a distinct thermodynamic phase characterized by broken symmetries below the onset temperature $T^*$. Electronic nematicity, a fourfold ($C_4$) rotational symmetry breaking, has emerged as a key feature inside the pseudogap regime, but the presenc…
A compact expression has been obtained for the superexchange coupling of magnetic ions via intermediate anions with regard to polaron effects at both magnetic ions and intermediate anions. This expression is used to analyze the main features of the behavior of isotope shifts for temperatures of three types in layered cuprates: the Neel temperatures ($T_N$), critical temperatures of transitions to a superconducting state ($T_C$), and characteristic temperatures of the pseudogap in the normal state ($T^*$).
We report the results of the Knight shift by 63,65Cu-nuclear-magnetic resonance (NMR) measurements on single-layered copper-oxide Bi2Sr2-xLaxCuO6+delta conducted under very high magnetic fields up to 44 T. The magnetic field suppresses superconductivity completely and the pseudogap ground state is revealed. The 63Cu-NMR Knight shift shows that there remains a finite density of states (DOS) at the Fermi level in the zero-temperature limit, which indicates that the pseudogap ground state is a metallic state with a finite volume of Fermi surface. The residual DOS in the pseudogap ground state dec…
We report high-resolution photoemission spectroscopy of newly-discovered iron-based layered superconductor La(O$_{0.93}$F$_{0.07}$)FeAs (Tc = 24 K). We found that the superconducting gap shows a marked deviation from the isotropic s-wave symmetry. The estimated gap size at 5 K is 3.6 meV in the s- or axial p-wave case, while it is 4.1 meV in the polar p- or d-wave case. We also found a pseudogap of 15-20 meV above Tc, which is gradually filled-in with increasing temperature and closes at temperature far above Tc similarly to copper-oxide high-temperature superconductors.
High-temperature superconductivity in iron-arsenic materials (pnictides) near an antiferromagnetic phase raises the possibility of spin-fluctuation-mediated pairing. However, the interplay between antiferromagnetic fluctuations and superconductivity remains unclear in the underdoped regime, which is closer to the antiferromagnetic phase. Here we report that the superconducting gap of the underdoped pnictides scales linearly with the transition temperature, and that a distinct pseudogap coexisting with the SC gap develops on underdoping. This pseudogap occurs on Fermi surface sheets connected b…
The most intriguing observation of cuprate experiments is most likely the metal-insulator-crossover (MIC), seen in the underdome region of the temperature-doping phase diagram of copper-oxides under a strong magnetic field, when the superconductivity is suppressed. This MIC, which results in such phenomena as heat conductivity downturn, anomalous Lorentz ratio, nonlinear entropy, insulating ground state, nematicity- and stripe-phases and Fermi pockets, reveals the nonconventional dielectric property of the pseudogap-normal phase. Since conventional superconductivity appears from a conducting n…
We derive analytic expressions for the critical temperatures of the superconducting (SC) and pseudogap (PG) transitions of the high-Tc cuprates as a function of doping. These are in excellent agreement with the experimental data both for single-layered materials such as LSCO, Bi2201 and Hg1201 and multi-layered ones, such as Bi2212, Bi2223, Hg1212 and Hg1223. Optimal doping occurs when the chemical potential vanishes, thus leading to an universal expression for the optimal SC transition temperatures. This allows for the obtainment of a quantitative description of the growth of such temperature…
Close to optimal doping, the copper oxide superconductors show 'strange metal' behavior, suggestive of strong fluctuations associated with a quantum critical point. Such a critical point requires a line of classical phase transitions terminating at zero temperature near optimal doping inside the superconducting 'dome'. The underdoped region of the temperature-doping phase diagram from which superconductivity emerges is referred to as the 'pseudogap' because evidence exists for partial gapping of the conduction electrons, but so far there is no compelling thermodynamic evidence as to whether th…
The pseudogap is one of the most pervasive phenomena of high temperature superconductors. It is attributed either to incoherent Cooper pairing setting in above the superconducting transition temperature Tc, or to a hidden order parameter competing with superconductivity. Here we use inelastic neutron scattering from underdoped YBa(2)Cu(3)O(6.6) to show that the dispersion relations of spin excitations in the superconducting and pseudogap states are qualitatively different. Specifically, the extensively studied "hour glass" shape of the magnetic dispersions in the superconducting state is no lo…
We present a short review of our basic understanding of the physics of copper - oxide superconductors and formulate the list of "solved" and "unsolved" problems. The main problem remains theoretical description of the properties of the normal state, requiring clarification of the nature of the so called pseudogap state. We review simplified models of the pseudogap state, based on the scenario of strong electron scattering by (pseudogap) fluctuations of "dielectric" (AFM, CDW) short - range order and the concept of "hot" spots (patches) on the Fermi surface. Pseudogap fluctuations are described…
The phase diagram of superconducting copper oxides is calculated as a function of doping based on a theory of dynamic stripe induced superconductivity. The two major conclusions from the theory and the numerical analysis are that T* (the pseudogap onset) and Tc (superconductivity onset) are correlated through the pseudogap, which induces a gap in the single particle energies that persists into the superconducting state. By decreasing the doping the pseudogap Delta* increases and T* increases, but when Delta* exceeds a certain critical value the superconducting transition temperature Tc is supp…
The phase diagram of the cuprate superconductors continues to pose formidable scientific challenges. While these materials are typically viewed as doped Mott insulators, it is well known that they are Fermi liquids at high hole-dopant concentrations. It was recently demonstrated that at moderate doping, in the pseudogap region of the phase diagram, the charge carriers are also best described as Fermi liquid. Nevertheless, the relationship between the two Fermi-liquid regions and the nature of the strange-metal state at intermediate doping have remained unsolved. Here we show in the case of the…
In a multiorbital model of the cuprate high-temperature superconductors soft antiferromagnetic (AF) modes are assumed to reconstruct the Fermi surface to form nodal pockets. The subsequent charge ordering transition leads to a phase with a spatially modulated transfer of charge between neighboring oxygen p_x and p_y orbitals and also weak modulations of the charge density on the copper d_{x^2-y^2} orbitals. As a prime result of the AF Fermi surface reconstruction, the wavevectors of the charge modulations are oriented along the crystalline axes with a periodicity that agrees quantitatively wit…
Unusual quantum phenomena usually emerge upon doping Mott insulators. Using a combinatorial molecular beam epitaxy system integrated with cryogenic scanning tunneling microscopy, we investigate the electronic structure of a modulation-doped Mott insulator Sn/Si(111)-($\sqrt{3}\times \sqrt{3}$)-R30$^\textrm{o}$. In underdoped regions, we observe a universal pseudogap opening around the Fermi level, which changes little with the applied magnetic field and the occurrence of Sn vacancies. The pseudogap gets smeared out at elevated temperatures and alters in size with the spatial confinement of the…
Mechanism of unconventional superconductivity is still unknown even if more than 25 years have been passed since the discovery of high-Tc cuprate superconductors by J.G. Bednorz and K. A. Muller. Here, we explore the cuprate phase diagram by electronic Raman spectroscopy and shed light on the superconducting state in hole doped cuprates. Namely, how superconductivity and the critical temperature Tc are impacted by the pseudogap.
Here I present a new model for the itinerancy of the strings of holes in the Cuprates HTSC. The model assumes various scenarios with respect to the order of the holes hopping and evaluates the weighting parameters for the different scenarios. The new model still results in the aggregation of holes into strings, but yields a spectral distribution for the itinerancy rates of the strings. From this distribution I infer a spectral distribution for the magnetic interaction between the strings, which suggests also a spectral distribution for the pseudogap parameter, and some relevant experimental fu…
Among all experimental observations of cuprate physics, the metal-insulator-crossover (MIC), seen in the pseudogap (PG) region of the temperature-doping phase diagram of copper-oxides under a strong magnetic field, when the superconductivity is suppressed, is most likely the most intriguing one. Since it was expected that the PG-normal state for these materials, as for conventional superconductors, is conducting. This MIC, revealed in such phenomena as heat conductivity downturn, anomalous Lorentz ratio, insulator resistivity boundary, nonlinear entropy, resistivity temperature upturn, insulat…
Band calculations for the hole doped La$_2$CuO$_4$ system show that artificial periodicities of Ba dopants can give the material different properties than from a uniform distribution of dopants. A periodicity within the planes make static pseudogaps which could be tuned to raise the density-of-states (DOS) at $E_F$ and the superconducting $T_C$. A periodic doping dependence perpendicular to the CuO planes can increase the matrix element for spin fluctuations.
To explain the high-temperature superconductivity of hole-doped copper-oxide high-temperature superconductors (HDCO-HTSCs), Anderson proposed a theory: (A) the pseudogap state is a resonating valence-bond (RVB) state below T* and (B) the RVB state translates itself into high-temperature superconducting state below Tc. In this paper we abandon Anderson theory B but still retain Anderson theory A and add three new hypotheses. Jointed three hypotheses with Anderson theory A, we construct an effective Hamiltonian of HDCO-HTSCs and explain why Tc-line is a dome in phase diagram and why HDCO-HTSCs h…
The intra-unit-cell nematic phase is studied within the three-band Emery model of the cuprates with the use of the approach based on the diagrammatic expansion of the Gutzwiller wave function (DE-GWF). According to our analysis the spontaneous $C_4$ symmetry breaking of the electronic wave function, leading to the nematic behavior, can appear due to electron correlations induced mainly by the onsite Coulomb repulsion, even in the absence of the corresponding intersite oxygen-oxygen repulsion term. The latter has been considered as the triggering factor of the nematic state formation in a numbe…
The temperature dependences of g factors of underdoped YBa2Cu3O7-x (Tc approximates 86.5K) indicate that the pseudogap onset temperature T*( approximates 190 K) coincides with the phase boundary established by resistivity, Nernst effect, neutron diffraction and ultrasound measurements in YBCO phase diagram. The obtained empirical relationship between g factors and pseudogap is appropriate for the entire charactering temperature range of Ortho-III YBCO. The EPR signals are ascribed to the localized Cu2+(1) sites based on the quantitative calculations of the g factors by involving the local stru…
The presence of charge order in high-transition-temperature copper oxides (high-Tc cuprates) was identified a decade ago. Now it is a universally observed order like the antiferromagnetic and the superconducting orders of the cuprates. The charge order shows up in various forms depending on materials, and it overlaps other orders in the phase diagram. Because of this diversity and complexity it has been far from clear whether or not the charge order has a direct relevance to superconductivity and to the mysterious pseudogap phenomenon. However, the research development in the last few years ha…
The mechanism of spin-phonon coupling (SPC) and possible consequencies for the properties of high-$T_C$ copper oxides are presented. The results are based on ab-initio LMTO band calculations and a nearly free-electron (NFE) model of the band near $E_F$. Many observed properties are compatible with SPC, as for the relation between doping and $\vec{q}$ for spin excitations and their energy dependence. The main pseudogap is caused by SPC and waves along [1,0,0], but it is suggested that secondary waves, generated along [1,1,0], contribute to a 'waterfall' structure. Conditions for optimal $T_C$, …
