We theoretically investigate the T-B phase diagram of cuprates building on the SU(2) pseudogap order recently obtained from the spin-fermion model. At low temperatures, our analysis reveals an almost temperature-independent critical magnetic field B_{QDW} at which d-wave superconductivity switches to the quadrupole density wave (QDW) phase. For temperatures beyond a certain value of the order of the zero-field superconducting transition temperature T_c, the critical field B_{QDW}(T) sharply grows. We compare our results derived within the effective SU(2) non-linear sigma-model with the phase diagram recently obtained by sound velocity measurements by LeBoeuf et al. [Nat. Phys. 9, 79 (2013)].
We derive a phase diagram for the pseudogap onset temperature $T^*$ (associated with the breakdown of the Fermi liquid state, due to strong pairing correlations) and the superconducting instability, $T_c$, as a function of variable pairing strength. Our diagrammatic approach to the BCS - Bose-Einstein cross-over problem self consistently treats the coupling between the single particle and pair propagators, and leads to a continuous evolution of these propagators into the standard $T<T_c$ counterparts. A rich structure is found in $T_c$ which reflects the way in which the superconducting instab…
Weakly correlated electrons on a square lattice are studied by angle-resolved functional renormalization group. Upon renormalization the interaction starts to depend on momenta and has pole-like solutions near a doping-dependent characteristic critical energy scale. Near half-filling this scale is the pseudogap temperature T*. In the overdoped regime the critical scale is the mean-field like critical temperature for d-wave superconductivity.
A new model of high-T_c superconductivity (HTSC) has been developed by us which we find is able to explain almost all the HTSC properties, like high T_c, origin and nature of NSPG (normal state pseudogap), anomalous superconducting state (SS) energy gap (SSEG) properties, absence of NMR spin relaxation rate coherence peak, existence of NSPG below T_c, nature of high-T_c magnetic superconductivity, external magnetic field (H) dependence of the NSPG formation temperature T*, etc. The coexistence of NSPG and SS BCS energy gap (EG), below T_C, has been predicted by the model before its experimenta…
We present a theoretical framework for a quantitative understanding of the full doping dependence of the optical spectra of the cuprates. In accord with experimental observations, the computed spectra show how the high-energy Mott features continue to persist in the overdoped regime even after the mid-infrared (MIR) peak originating from the pseudogap has collapsed in a quantum critical point. In this way, we reconcile the opposing tendencies of the MIR and Mott peaks to shift in opposite directions in the optical spectra with increasing doping. The competition between the pseudogap and the Mo…
The t-J model in the large N limit (N denotes the number of spin components) yields a pseudogap phase in the underdoped region which is related to a d-wave charge density wave (d-CDW). We present results for the doping dependence of the superconducting and d-CDW order parameters as well as for collective excitations in the presence of these two order parameters. We argue that the electronic Raman spectrum with B$_{1g}$ symmetry probes the amplitude fluctuations of the d-CDW at zero momentum.
We study the sensitivity of T_c and T^* to low fields, H, within the pseudogap state using a BCS-based approach extended to arbitrary coupling. We find that T^* and T_c, which are of the same superconducting origin, have very different H dependences. This is due to the pseudogap, Δ_{pg}, which is present at the latter, but not former temperature. Our results for the coherence length ξfit well with existing experiments.We predict that very near the insulator ξwill rapidly increase.
The interplay between superconductivity and the pseudogap is an important aspect of cuprate physics. However, the nature of the pseudogap remains controversial, in part because different experiments have suggested different gap functions. Here we present a photon-energy-dependence angle-resolved photoemission spectroscopy (ARPES) study on Bi$_{1.5}$Pb$_{0.55}$Sr$_{1.6}$La$_{0.4}$CuO$_{6+δ}$. We find that antinodal ARPES spectra at low photon energies are dominated by background signals which can lead to a misevaluation of the spectral gap size. Once background is properly accounted for, indepe…
An intricate interplay between superconductivity, pseudogap and Mott transition, either bandwidth driven or doping driven, occurs in materials. Layered organic conductors and cuprates offer two prime examples. We provide a unified perspective of this interplay in the two-dimensional Hubbard model within cellular dynamical mean-field theory on a $2\times 2$ plaquette and using the continuous-time quantum Monte Carlo method as impurity solver. Both at half filling and at finite doping, the metallic normal state close to the Mott insulator is unstable to d-wave superconductivity. Superconductivit…
In the course of seeking the microscopic mechanism of superconductivity in cuprate high temperature superconductors, the pseudogap phase\textemdash the very abnormal 'normal' state on the hole-doped side\textemdash has proven to be as big of a quandary as superconductivity itself. Angle-resolved photoemission spectroscopy (ARPES) is a powerful tool for assessing the momentum-dependent phenomenology of the pseudogap, and recent technological developments have permitted a more detailed understanding. This report reviews recent progress in understanding the relationship between superconductivity …
The origin of the pseudogap behavior, found in many high-$T_c$ superconductors, remains one of the greatest puzzles in condensed matter physics. One possible mechanism is fermionic incoherence, which near a quantum critical point allows pair formation but suppresses superconductivity. Employing quantum Monte Carlo simulations of a model of itinerant fermions coupled to ferromagnetic spin fluctuations, represented by a quantum rotor, we report numerical evidence of pseudogap behavior, emerging from pairing fluctuations in a quantum-critical non-Fermi liquid. Specifically, we observe enhanced pa…
The emergence of superconductivity in unconventional superconductors usually accompanies the normal-state phases of pseudogap, strange metal and Fermi liquid. It indicates these phases are strongly related to the superconducting state and should be dominated by yet unclear mechanism of superconductivity. Here, we propose a microscopic theory that, according to the coupling between localized and extended states, the pairing electrons which leap on and off a localized state result in superconducting instability. Two distinguishable pairing gaps are observed in these states, one dominates the sup…
We study a two-dimensional model of an isolated narrow topological band at partial filling with local attractive interactions. Numerically exact quantum Monte Carlo calculations show that the ground state is a superconductor with a critical temperature that scales nearly linearly with the interaction strength. We also find a broad pseudogap regime at temperatures above the superconducting phase that exhibits strong pairing fluctuations and a tendency towards electronic phase separation; introducing a small nearest neighbor attraction suppresses superconductivity entirely and results in phase s…
By measuring the low temperature specific heat, the low energy quasi-particle excitation has been derived and analyzed in systematically doped La$_{2-x}$Sr$_{x}$CuO$_{4}$ single crystals. The Volovik's relation predicted for a d-wave superconductor has been well demonstrated in wide doping regime, showing a robust evidence for the d-wave pairing symmetry. Furthermore the nodal gap slope $v_Δ$ of the superconducting gap is derived and is found to follow the same doping dependence of the pseudogap obtained from ARPES and tunnelling measurement. This strongly suggests a close relationship between…
In the high-temperature ($T_{c}$) cuprate superconductors, increasing evidence suggests that the pseudogap, existing below the pseudogap temperature $T$*, has a distinct broken electronic symmetry from that of superconductivity. Particularly, recent scattering experiments on the underdoped cuprates have suggested that a charge ordering competes with superconductivity. However, no direct link of this physics and the important low-energy excitations has been identified. Here we report an antagonistic singularity at $T_{c}$ in the spectral weight of Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+δ}$ as a compel…
We analyze superconducting state (both s and d - wave) in a simple exactly solvable model of pseudogap state, induced by short - range order fluctuations (e.g. antiferromagnetic), which is based upon model Fermi - surface with "hot patches". It is shown that superconducting energy gap averaged over these fluctuations is non zero even for the temperatures larger than mean - field T_c of superconducting transition in a sample as a whole. For temperatures T>T_c superconductivity apparently exists within separate regions ("drops"). We study the spectral density and the density of states and demons…
We report a doping dependant Electronic Raman Scattering (ERS) study of HgBa$_2$CuO$_{4+δ}$ (Hg-1201) single crystals. We investigate the dynamics of the antinodal and nodal quasiparticles. We show that the dynamical response of the antinodal quasiparticles is strongly reduced towards the underdoped regime in both the normal and superconducting states. When probing the nodal quasiparticles, we are able to distinguish between the energy scale of the pseudogap and that of the superconducting gap. A simple model relating the suppression of the dynamical response of the antinodal quasiparticles to…
Near a Mott transition, strong electron correlations may enhance Cooper pairing. This is demonstrated in the Dynamical Mean Field Theory solution of a twofold-orbital degenerate Hubbard model with inverted Hund's rules on-site exchange, which favors local spin-singlet configurations. Close to the Mott insulator, which is a local version of a valence bond insulator, a pseudogap non-Fermi-liquid metal, a superconductor, and a normal metal appear, in striking similarity with the physics of the cuprates. The strongly correlated superconducting state has a larger Drude weight than the corresponding…
Superconductivity emerges from the cuprate antiferromagnetic Mott state with hole doping. The resulting electronic structure is not understood, although changes in the state of oxygen atoms appear paramount. Hole doping first destroys the Mott state yielding a weak insulator where electrons localize only at low temperatures without a full energy gap. At higher doping, the 'pseudogap', a weakly conducting state with an anisotropic energy gap and intra-unit-cell breaking of 90\degree-rotational (C4v) symmetry appears. However, a direct visualization of the emergence of these phenomena with incre…
In this paper, we try to understand the pseudogap phenomenon observed in the cuprate superconductor through a model study. Specifically, we explore the so-called low-temperature pseudogap state by turning off the superconducting off diagonal long range order in an ansatz state for the $t$-$J$ model [New Journal of Physics {\bf 13}, 103039 (2011)]. Besides strong non-Gaussian superconducting fluctuations, the resulting state also exhibits a systematic pseudogap behavior in both spin and charge degrees of freedom, manifested in the uniform spin susceptibility, specific heat, non-Drude resistivit…
We investigate interplay between magnetic fluctuations and superconductivity in the effective five-band Hubbard model for iron-oxypnictide superconductors on the basis of the fluctuation-exchange approximation. As for the normal-state properties, we find the pseudogap behavior in the NMR relaxation rate and the spectral weight in the electron-doped region, while we cannot find such behavior in the hole-doped region. The pseudogap behavior originates from the band structure effect, that is, existence of high density of states just below the Fermi level. Solving the superconducting Eliashberg eq…
We analyze the anomalies of superconducting state (s and d-wave pairing) in a simple model of pseudogap state, induced by fluctuations of short - range order (e.g. antiferromagnetic), based on the model Fermi surface with "hot patches". We derive a system of recursion relations for Gorkov's equations which take into account all diagrams of perturbation theory for electron interaction with fluctuations of short-range order. Then we find superconducting transition temperature and gap behavior for different values of the pseudogap width and correlation lengths of short-range order fluctuations. I…
We analyze the anomalies of superconducting state (both s- and d-wave pairing) in the model of the pseudogap state induced by Heisenberg spin fluctuations of antiferromagnetic short-range order, and based on the scenario of strong scattering near the "hot spots" on the Fermi surface. We present microscopic derivation of Ginzburg-Landau expansion, taking into account all Feynman graphs of perturbation theory for electron interaction with fluctuations of short-range order and in the "ladder" approximation for electron scattering by normal (nonmagnetic) impurities. We determine the dependence of …
In a recent letter, Chatterjee et al. (Nature Phys. 4, 108 (2008)) reported tunnelling data obtained below and above the critical temperature (Tc = 15 K) by STM in overdoped Bi2-yPbySr2CuO6+x (Bi2201) and some interpretations of the data1. The main experimental findings of their work are observations of native-impurity resonances which coexist spatially with the superconducting (SC) gap below Tc and survive unchanged on warming through Tc. No doubts, the data in the letter are an important piece of information for assembling the high-Tc SC jigsaw puzzle; however, there are inaccuracies in the …
Recently developed numerical methods have enabled the explicit construction of the superconducting state of the Hubbard model of strongly correlated electrons in parameter regimes where the model also exhibits a pseudogap and a Mott insulating phase. $d_{x^2-y^2}$ symmetry superconductivity is found to occur in proximity to the Mott insulator, but separated from it by a pseudogapped nonsuperconducting phase. The superconducting transition temperature and order parameter amplitude are found to be maximal at the onset of the normal-state pseudogap. The emergence of superconductivity from the nor…
A full Fermi surface exists in underdoped high-temperature superconductors if the temperature T lies above the pseudogap temperature T*. Below T* only arcs of Fermi surface survive, scaling with T/T* as T -> 0, with T* displaying strong doping dependence. There is no accepted explanation for this behavior. We show that generalizing the BCS theory of normal superconductivity to include d-wave pairs and antiferromagnetism leads to the origin and doping dependence of the T* scale, and a quantitative description of Fermi arcs. These results place strong constraints on viable theories of high-tempe…
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 use electronic Raman scattering to study the model single-layer cuprate superconductor HgBa2CuO4+d. In an overdoped sample, we observe a pronounced amplitude enhancement of a high-energy peak related to two-magnon excitations in insulating cuprates upon cooling below the critical temperature Tc. This effect is accompanied by the appearance of the superconducting gap and a pairing peak above the gap in the Raman spectrum, and it can be understood as a consequence of feedback of the Cooper pairing interaction on the high-energy magnetic fluctuations. All of these effects occur already above T…
In this paper we study the effects of the strong coupling superconductivity on the normal state electronic structure. We point out that the pseudogap phenomena in High-T_c cuprates are naturally explained as a precursor of the strong coupling superconductivity. We expand the reciprocal of the T-matrix with respect to the momentum and the frequency, and we discuss the properties of the expansion parameters. We estimate these parameters and carry out the calculations for the single particle self-energy using the T-matrix and the self-consistent T-matrix approximations, respectively. We show that…
The study of interaction between different degrees of freedom in solids is of fundamental importance to understand the functionalities of materials. One striking example of such interaction is the intertwined coupling or competition between superconductivity (SC), charge density wave (CDW), pseudogap state (PG), and other exotic phases in cuprate superconductors. Recent emergence of nonlinear Terahertz (THz) third harmonic generation (THG) spectroscopy provides a powerful tool for exploring the collective (Higgs) modes of superconductivity order parameters, and its interaction with intertwined…
Ultrafast broadband transient reflectivity experiments are performed to study the interplay between the non-equilibrium dynamics of the pseudogap and the superconducting phases in Bi$_{2}$Sr$_{2}$Ca$_{0.92}$Y$_{0.08}$Cu$_{2}$O$_{8+δ}$. Once superconductivity is established the relaxation of the pseudogap proceeds $\sim$ 2 times faster than in the normal state, and the corresponding transient reflectivity variation changes sign after $\sim$ 0.5 ps. The results can be described by a set of coupled differential equations for the pseudogap and for the superconducting order parameter. The sign and …
This Chapter of the Jülich Summer School 2022 provides for a first reading on diagrammatic extensions of dynamical-mean-field theory and their application to spin fluctuations, pseudogap physics and supercondcutivtiy. The contents is as follows: 1 Introduction 2 Dynamical vertex approximation 3 Ladder dynamical vertex approximation 4 Hubbard model, cuprates and nickelates 5 Spin fluctuations 6 Pseudogap 7 Superconductivity 8 Conclusion and outlook
The effect of Zn substitution on the superconducting transition temperature, Tc, was investigated for the La2-xSrxCu1-yZnyO4 (Zn-LSCO) compounds over a wide range of hole concentration, p (= x), and Zn content (y) in the CuO2 plane. Zn induced rate of suppression of Tc, dTc/dy, was found to be strongly p-dependent and showed a systematic variation with hole concentration, except in the vicinity of p ~ 0.12, i.e., near the so-called 1/8th anomaly where the charge/spin stripe correlations are at its strongest in La2-xSrxCuO4. Near p ~ 0.12, the static striped charge ordering is widely believed t…
We show conclusively that a pseudogap state can arise at $T > T_c$, for reasonable pairing interaction strength, from order parameter fluctuations in a two dimensional minimal model of $d$-wave superconductivity. The occurrence of the pseudogap requires neither strong correlation nor the presence of competing order. We study a model with attractive nearest neighbor interaction and establish our result using a combination of cluster based Monte Carlo for the order parameter field and a twisted-boundary scheme to compute the momentum-resolved spectral function. Apart from a dip in the density of…
A term first coined by Mott back in 1968 a `pseudogap' is the depletion of the electronic density of states at the Fermi level, and pseudogaps have been observed in many systems. However, since the discovery of the high temperature superconductors (HTSC) in 1986, the central role attributed to the pseudogap in these systems has meant that by many researchers now associate the term pseudogap exclusively with the HTSC phenomenon. Recently, the problem has got a lot of new attention with the rediscovery of two distinct energy scales (`two-gap scenario') and charge density waves patterns in the cu…
This is a short review of the theoretical work on the two-dimensional Hubbard model performed in Sherbrooke in the last few years. It is written on the occasion of the twentieth anniversary of the discovery of high-temperature superconductivity. We discuss several approaches, how they were benchmarked and how they agree sufficiently with each other that we can trust that the results are accurate solutions of the Hubbard model. Then comparisons are made with experiment. We show that the Hubbard model does exhibit d-wave superconductivity and antiferromagnetism essentially where they are observe…
In the last few years evidence has been accumulating that there are a multiplicity of energy scales which characterize superconductivity in the underdoped cuprates. In contrast to the situation in BCS superconductors, the phase coherence temperature Tc is different from the energy gap onset temperature T*. In addition, thermodynamic and tunneling spectroscopies have led to the inference that the order parameter $Δ_{sc}$ is to be distinguished from the excitation gap $Δ$; in this way, pseudogap effects persist below Tc. It has been argued by many in the community that the presence of these dist…
The electronic structure of Bi2Sr2-xRxCuOy(R=La, Eu) near the (pi,0) point of the first Brillouin zone was studied by means of angle-resolved photoemission spectroscopy (ARPES). The temperature T* above which the pseudogap structure in the ARPES spectrum disappears was found to have an R dependence that is opposite to that ofthe superconducting transition temperature Tc. This indicates that the pseudogap state is competing with high-Tc superconductivity, and the large Tc suppression observed with increasing the out-of-plane disorder is due to the stabilization of the pseudogap state.
Spin fluctuations (SF) and SF-mediated superconductivity (SC) in quasi-two-dimensional metals around the antiferrromagnetic (AF) quantum critical point (QCP) are investigated by using the self-consistent renormalization theory for SF and the strong coupling theory for SC. We introduce a parameter y0 as a measure for the distance from the AFQCP which is approximately proportional to (x-xc), x being the electron (e) or hole (h) doping concentration to the half-filled band and xc being the value at the AFQCP. We present phase diagrams in the T-y0 plane including contour maps of the AF correlation…
We analyze the t-J model on a square lattice using bosonic spinons and fermionic holons for low density x of holes. Spinons are paired into singlets, which condense below a temperature T*. The condensate evolves out of the Mott phase - preserving its symmetry. For T > T* holons and spinons are confined (by gauge forces), so that there is no coherent charge propagation. Metallic conduction and d-wave superconductivity result from separate, sublattice-preserving, holon hopping processes which originate below T* from a coupling with the condensate. A simple effective Hamiltonian describing these …
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…
In the pseudogap phase of a high-temperature cuprate superconductor, conflicting evidence from different experiments points to a competing state or a precursor-to-superconductivity state. One single experiment now determines that both states exist.
Alkali-fulleride superconductors with a maximum critical temperature Tc of 40 K exhibit similar electronic phase diagram with unconventional high-Tc superconductors where the superconductivity resides proximate to a magnetic Mott-insulating state. However, distinct from cuprate compounds, which superconduct through two-dimensional (2D) CuO2 planes, alkali fullerides are attributed to the three-dimensional (3D) members of high-Tc family. Here, we employ scanning tunneling microscopy to show that trilayer K3C60 displays fully gapped strong coupling s-wave superconductivity that coexists spatiall…
We demonstrate that many features ascribed to strong correlation effects in various spectroscopies of the cuprates are captured by a calculation of the self-energy incorporating effects of spin and charge fluctuations. The self energy is calculated over the full doping range from half filling to the overdoped system. In the normal state, the spectral function reveals four subbands: two widely split incoherent bands representing the remnant of the two Hubbard bands, and two additional coherent, spin- and charge-dressed in-gap bands split by a spin-density wave, which collapses in the overdoped …
We study the t-J-V model of a doped Mott insulator in connection to high-T_c superconductors. The nearest neighbor Coulomb interaction (V) is treated quantum mechanically on equal footing as the antiferromagnetic exchange interaction (J). Motivated by the SU(2) symmetry at half-filling, we construct a large-N theory which allows a systematic study of the interplay between staggered flux order and superconductivity upon doping. We solve the model in the large-N limit and obtain the ground state properties and the phase diagram as a function of doping. We discuss the competition and the coexiste…
In this thesis, I present a non-perturbative approach to the single-band attractive Hubard model which is an extension of previous work by Vilk and Tremblay on the repulsive model. Exact results are derived in the general context of functional derivative approaches to many-body theories. The first step of the approximation is based on a local field type ansatz. All physical quantities can be expressed as a function of double-occupancy (in addition to temperature and filling). Double-occupancy is determined without adjustable parameter by imposing the Pauli principle and a crucial sum-rule, mak…
We demonstrate that the observed dependences of $T_c$ and $T^*$ on small magnetic fields can be readily understood in a precursor superconductivity approach to the pseudogap phase. In this approach, the presence of a pseudogap at $T_c$ (but not at $T^*$) and the associated suppression of the density of states lead to very different sensitivities to pair-breaking perturbations for the two temperatures. Our semi-quantitative results address the puzzling experimental observation that the coherence length $ξ$ is weakly dependent on hole concentration $x$ throughout most of the phase diagram. We pr…
Pressure-induced decoupling of the pseudogap and superconductivity in a bilayer cuprate
Institute of Physics, Chinese Academy of Sciences（中国科学院物理研究所）；University of Chinese Academy of Sciences（中国科学院大学）；Nanyang Technological University（南洋理工大学）；Songshan Lake Materials Laboratory（松山湖材料实验室）
We demonstrate that the formation of a chiral d-density wave (CDDW) state generates a Topological Meissner effect (TME) in the absence of any kind of superconductivity. The TME is identical to the usual superconducting Meissner effect but it appears only for magnetic fields perpendicular to the plane while it is absent for in plane fields. The observed enhanced diamagnetic signals in the non-superconducting pseudogap regime of the cuprates may find an alternative interpretation in terms of a TME, originating from a chiral d-density wave pseudogap.
We report on the phase diagram of antiferromagnetism (AFM) and superconductivity (SC) in three-layered Ba_2Ca_2Cu_3O_6(F,O)_2 by means of Cu-NMR measurements. It is demonstrated that AFM and SC uniformly coexist in three-layered compounds as well as in four- and five-layered ones. The critical hole density p_c for the long range AFM order is determined as p_c ~ 0.075, which is larger than p_c ~ 0.02 and 0.055 in single- and bi-layered compounds, and smaller than p_c ~ 0.08-0.09 and 0.10-0.11 in four- and five-layered compounds, respectively. This variation of p_c is attributed to the magnetic …
We point out that a system which supports chiral superconductivity should also support a chiral pseudogap phase: a finite temperature phase wherein superconductivity is lost but time reversal symmetry is still broken. This chiral pseudogap phase can be viewed as a state with phase incoherent Cooper pairs of a definite angular momentum. This physical picture suggests that the chiral pseudogap phase should have definite magnetization, should exhibit a (non-quantized) charge Hall effect, and should possess protected edge states that lead to a quantized thermal Hall response. We explain how these …
Layered doped Mott insulators, such as the cuprates, show unusual temperature dependence of the resistivity. Intriguingly, the resistivity perpendicular to the CuO$_2$ planes, $ρ_c(T)$, shows both metallic ($dρ_c/dT > 0$) and semi-conducting ($dρ_c/dT<0$) behavior. We shed light on this puzzle by calculating $ρ_c$ for the two-dimensional Hubbard model within plaquette cellular dynamical mean-field theory and strong-coupling continuous-time quantum Monte Carlo as the impurity solver. The temperature, $T$, and doping, $δ$, dependencies of $ρ_c$ are controlled by the first-order transition betwee…
Cluster dynamical mean field methods are used to calculate the superconductivity-induced changes in the interplane conductivity and Raman scattering cross section of the two dimensional Hubbard model. When superconductivity emerges from the pseudogap, the superconducting response is found to be diminished in amplitude, broadened and, in the case of the interplane conductivity, shifted to higher frequency. The results are in agreement with data on high temperature copper-oxide superconductors indicating that the Hubbard model contains the essential low energy physics of the pseudogap and its in…
One of the most prominent problems in high temperature superconductivity is the nature of the pseudogap phase in underdoped regimes; particularly important is the role of phase fluctuations. The Gutzwiller-projected BCS Hamiltonian is a useful model for high temperature superconductivity due to an exact mapping to the Heisenberg model at half filling and generally a very close connection to the t-J model at moderate doping. We develop the dynamical mean field theory for the d-wave BCS Hamiltonian with on-site repulsive interaction, $U$, physically imposing the partial Gutzwiller projection. Fo…
In this study, we conducted various magnetotransport measurements on Fe$_{1+y}$Te$_{1-x}$Se$_{x}$ single crystals from which excess iron was sufficiently removed. Our results revealed that crossover from the incoherent to the coherent electronic state and opening of the pseudogap occur at high temperatures ($\approx$ 150 K for $x$ = 0.2). This is accompanied by a more substantial pseudogap and the emergence of a phase with a multi-band nature at lower temperatures (below $\approx$ 50 K for $x$ = 0.2) before superconductivity sets in. A comparison of these results with those of the as-grown (no…
High-temperature superconductivity emerges in the CuO$_2$ plane upon doping a Mott insulator. To ascertain the influence of Mott physics plus short-range correlations, we solve a three-band copper-oxide model in the charge-transfer regime using cellular dynamical mean-field theory with continuous-time quantum Monte Carlo as an impurity solver. We report the normal and superconducting phase diagram of this model as a function of doping, interaction strength and temperature. Upon hole doping of the charge-transfer insulator, the phase boundary between pseudogap and correlated metal consists of a…
We analyze the anomalies of superconducting state within a simple exactly solvable model of the pseudogap state, induced by fluctuations of ``dielectric'' short range order, for the model of the Fermi surface with ``hot'' patches. The analysis is performed for the arbitrary values of the correlation length xi_{corr} of this short range order. It is shown that superconducting energy gap averaged over these fluctuations is non zero in a wide temperature range above T_c - the temperature of homogeneous superconducting transition. This follows from the absence of self averaging of the gap over the…
