Collapse of the normal-state pseudogap at a Lifshitz transition in the Bi(2)Sr(2)CaCu(2)O(8+δ) cuprate superconductor

Evidence for a Square-Square Vortex Lattice Transition in a High-T_{c} Cuprate Superconductor

Using sound velocity and attenuation measurements in high magnetic fields, we identify a new transition in the vortex lattice state of La_{2-x}Sr_{x}CuO_{4}. The transition, observed in magnetic fields exceeding 35 T and temperatures far below zero field T_{c}, is detected in the compression modulus of the vortex lattice, at a doping level of x=p=0.17. Our theoretical analysis based on Eilenberger's theory of the vortex lattice shows that the transition corresponds to the long-sought 45° rotation of the square vortex lattice, predicted to occur in d-wave superconductors near a van Hove singularity.

Disappearance of superconductivity due to vanishing coupling in the overdoped Bi[Formula: see text]Sr[Formula: see text]CaCu[Formula: see text]O[Formula: see text]

In cuprate superconductors, superconductivity is accompanied by a plethora of orders and phenomena that complicate our understanding of superconductivity in these materials. Prominent in the underdoped regime, these orders weaken or vanish with overdoping. Here, we approach the superconducting phase from the more conventional overdoped side. We present angle-resolved photoemission spectroscopy studies of Bi[Formula: see text]Sr[Formula: see text]CaCu[Formula: see text]O[Formula: see text], cleaved and annealed in ozone to increase the doping all the way to the non-superconducting phase. We show that the mass renormalization in the antinodal region of the Fermi surface that possibly reflects the pairing, weakens with doping and completely disappears precisely where superconductivity disappears. This is the evidence that in the overdoped regime, superconductivity is determined primarily by the coupling strength. A doping dependence and an abrupt disappearance above the transition temperature eliminate phononic mechanism of the observed renormalization and identify the onset of spin-fluctuations as its likely origin.

Crossover to strange metal phase: quantum criticality in one unit cell Bi2Sr2CaCu2O[Formula: see text]

Transport measurements can be used to determine the phase diagram of high temperature superconductors by detecting variations in temperature dependence of resistance in different regions of the phase diagram. While for bulk measurements several samples with varying chemical doping are used, we continuously vary carrier density in our ultra-thin two-dimensional Bi₂Sr₂CaCu₂O[Formula: see text] device by electrostatic means and the space charge doping method. Here we concentrate on a low-disorder, high quality single unit cell thick sample. We establish the crossover to strange metal from the pseudogap and Fermi liquid phases in the normal state, close to the superconducting dome. By extrapolation we demarcate a critical doping region which is thought to correspond to a quantum phase transition at very low temperature.

Incoherent strange metal sharply bounded by a critical doping in Bi2212

In normal metals, macroscopic properties are understood using the concept of quasiparticles. In the cuprate high-temperature superconductors, the metallic state above the highest transition temperature is anomalous and is known as the "strange metal." We studied this state using angle-resolved photoemission spectroscopy. With increasing doping across a temperature-independent critical value p _c ~ 0.19, we observed that near the Brillouin zone boundary, the strange metal, characterized by an incoherent spectral function, abruptly reconstructs into a more conventional metal with quasiparticles. Above the temperature of superconducting fluctuations, we found that the pseudogap also discontinuously collapses at the very same value of p _c These observations suggest that the incoherent strange metal is a distinct state and a prerequisite for the pseudogap; such findings are incompatible with existing pseudogap quantum critical point scenarios.

Nematic fluctuations in the cuprate superconductor Bi2Sr2CaCu2O8+δ

Establishing the presence and the nature of a quantum critical point in their phase diagram is a central enigma of the high-temperature superconducting cuprates. It could explain their pseudogap and strange metal phases, and ultimately their high superconducting temperatures. Yet, while solid evidences exist in several unconventional superconductors of ubiquitous critical fluctuations associated to a quantum critical point, in the cuprates they remain undetected until now. Here using symmetry-resolved electronic Raman scattering in the cuprate \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta}$$\end{document}Bi2Sr2CaCu2O8+δ, we report the observation of enhanced electronic nematic fluctuations near the endpoint of the pseudogap phase. While our data hint at the possible presence of an incipient nematic quantum critical point, the doping dependence of the nematic fluctuations deviates significantly from a canonical quantum critical scenario. The observed nematic instability rather appears to be tied to the presence of a van Hove singularity in the band structure.

Solid evidence of quantum fluctuations associated to a quantum critical point in cuprate superconductors remains elusive. Here, Auvray et al. report Raman scattering evidence of enhanced electronic nematic fluctuations near the endpoint of the pseudogap phase in Bi\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${}_{2}$$\end{document}2Sr\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${}_{2}$$\end{document}2CaCu\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${}_{2}$$\end{document}2O\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${}_{8+\delta }$$\end{document}8+δ.

Phase diagram of Bi2Sr2CaCu2O8+δ revisited

In cuprate superconductors, the doping of carriers into the parent Mott insulator induces superconductivity and various other phases whose characteristic temperatures are typically plotted versus the doping level p. In most materials, p cannot be determined from the chemical composition, but it is derived from the superconducting transition temperature, Tc, using the assumption that the Tc dependence on doping is universal. Here, we present angle-resolved photoemission studies of Bi2Sr2CaCu2O8+δ, cleaved and annealed in vacuum or in ozone to reduce or increase the doping from the initial value corresponding to Tc = 91 K. We show that p can be determined from the underlying Fermi surfaces and that in-situ annealing allows mapping of a wide doping regime, covering the superconducting dome and the non-superconducting phase on the overdoped side. Our results show a surprisingly smooth dependence of the inferred Fermi surface with doping. In the highly overdoped regime, the superconducting gap approaches the value of 2Δ0 = (4 ± 1)kBTc.

Three-Dimensional Fermi Surface of Overdoped La-Based Cuprates

We present a soft x-ray angle-resolved photoemission spectroscopy study of overdoped high-temperature superconductors. In-plane and out-of-plane components of the Fermi surface are mapped by varying the photoemission angle and the incident photon energy. No k_{z} dispersion is observed along the nodal direction, whereas a significant antinodal k_{z} dispersion is identified for La-based cuprates. Based on a tight-binding parametrization, we discuss the implications for the density of states near the van Hove singularity. Our results suggest that the large electronic specific heat found in overdoped La_{2-x}Sr_{x}CuO_{4} cannot be assigned to the van Hove singularity alone. We therefore propose quantum criticality induced by a collapsing pseudogap phase as a plausible explanation for observed enhancement of electronic specific heat.

Photoemission perspective on pseudogap, superconducting fluctuations, and charge order in cuprates: a review of recent progress

In the course of seeking the microscopic mechanism of superconductivity in cuprate high temperature superconductors, the pseudogap phase- the very abnormal 'normal' state on the hole-doped side- 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 and the pseudogap, the Fermi arc phenomena, and the relationship between charge order and pseudogap from the perspective of ARPES measurements.

Higgs Mode in the d-Wave Superconductor Bi_{2}Sr_{2}CaCu_{2}O_{8+x} Driven by an Intense Terahertz Pulse

We investigate the terahertz (THz)-pulse-driven nonlinear response in the d-wave cuprate superconductor Bi_{2}Sr_{2}CaCu_{2}O_{8+x} (Bi2212) using a THz pump near-infrared probe scheme in the time domain. We observe an oscillatory behavior of the optical reflectivity that follows the THz electric field squared and is markedly enhanced below T_{c}. The corresponding third-order nonlinear effect exhibits both A_{1g} and B_{1g} symmetry components, which are decomposed from polarization-resolved measurements. A comparison with a BCS calculation of the nonlinear susceptibility indicates that the A_{1g} component is associated with the Higgs mode of the d-wave order parameter.

Correlation-Driven Lifshitz Transition at the Emergence of the Pseudogap Phase in the Two-Dimensional Hubbard Model

We study the relationship between the pseudogap and Fermi-surface topology in the two-dimensional Hubbard model by means of the cellular dynamical mean-field theory. We find two possible mean-field metallic solutions on a broad range of interactions, doping, and frustration: a conventional renormalized metal and an unconventional pseudogap metal. At half filling, the conventional metal is more stable and displays an interaction-driven Mott metal-insulator transition. However, for large interactions and small doping, a region that is relevant for cuprates, the pseudogap phase becomes the ground state. By increasing doping, we show that a first-order transition from the pseudogap to the conventional metal is tied to a change of the Fermi surface from hole- to electronlike, unveiling a correlation-driven mechanism for a Lifshitz transition. This explains the puzzling link between the pseudogap phase and Fermi surface topology that has been pointed out in recent experiments.

Comprehensive phase diagram of two-dimensional space charge doped Bi2Sr2CaCu2O8+x

The phase diagram of hole-doped high critical temperature superconductors as a function of doping and temperature has been intensively studied with chemical variation of doping. Chemical doping can provoke structural changes and disorder, masking intrinsic effects. Alternatively, a field-effect transistor geometry with an electrostatically doped, ultra-thin sample can be used. However, to probe the phase diagram, carrier density modulation beyond 10¹⁴ cm⁻² and transport measurements performed over a large temperature range are needed. Here we use the space charge doping method to measure transport characteristics from 330 K to low temperature. We extract parameters and characteristic temperatures over a large doping range and establish a comprehensive phase diagram for one-unit-cell-thick BSCCO-2212 as a function of doping, temperature and disorder.

The determination of the phase diagram of cuprate superconductors involves chemical doping which introduces disorder and could mask intrinsic effects. Sterpetti et al. establish this phase diagram with transport measurements in ultra-thin samples by modulating the carrier density with an alternative electrostatic method.

Pseudogap phase of cuprate superconductors confined by Fermi surface topology

The properties of cuprate high-temperature superconductors are largely shaped by competing phases whose nature is often a mystery. Chiefly among them is the pseudogap phase, which sets in at a doping p* that is material-dependent. What determines p* is currently an open question. Here we show that the pseudogap cannot open on an electron-like Fermi surface, and can only exist below the doping p _FS at which the large Fermi surface goes from hole-like to electron-like, so that p* ≤ p _FS. We derive this result from high-magnetic-field transport measurements in La_1.6-x Nd_0.4Sr _x CuO₄ under pressure, which reveal a large and unexpected shift of p* with pressure, driven by a corresponding shift in p _FS. This necessary condition for pseudogap formation, imposed by details of the Fermi surface, is a strong constraint for theories of the pseudogap phase. Our finding that p* can be tuned with a modest pressure opens a new route for experimental studies of the pseudogap.

Entropic Origin of Pseudogap Physics and a Mott-Slater Transition in Cuprates

We propose a new approach to understand the origin of the pseudogap in the cuprates, in terms of bosonic entropy. The near-simultaneous softening of a large number of different q-bosons yields an extended range of short-range order, wherein the growth of magnetic correlations with decreasing temperature T is anomalously slow. These entropic effects cause the spectral weight associated with the Van Hove singularity (VHS) to shift rapidly and nearly linearly toward half filling at higher T, consistent with a picture of the VHS driving the pseudogap transition at a temperature ~T*. As a byproduct, we develop an order-parameter classification scheme that predicts supertransitions between families of order parameters. As one example, we find that by tuning the hopping parameters, it is possible to drive the cuprates across a transition between Mott and Slater physics, where a spin-frustrated state emerges at the crossover.

Pressure-dependent semiconductor to semimetal and Lifshitz transitions in 2H-MoTe2: Raman and first-principles studies

High pressure Raman spectroscopy of bulk 2H-MoTe₂ up to  ∼29 GPa is shown to reveal two phase transitions (at  ∼6 and 16.5 GPa), which are analyzed using first-principles density functional theoretical calculations. The transition at 6 GPa is marked by changes in the pressure coefficients of A _1g and [Formula: see text] Raman mode frequencies as well as in their relative intensity. Our calculations show that this is an isostructural semiconductor to a semimetal transition. The transition at  ∼16.5 GPa is identified with the changes in linewidths of the Raman modes as well as in the pressure coefficients of their frequencies. Our theoretical analysis clearly shows that the structure remains the same up to 30 GPa. However, the topology of the Fermi-surface evolves as a function of pressure, and abrupt appearance of electron and hole pockets at [Formula: see text] GPa marks a Lifshitz transition.

Magnetic Lifshitz transition and its consequences in multi-band iron-based superconductors

In this paper we address Lifshitz transition induced by applied external magnetic field in a case of iron-based superconductors, in which a difference between the Fermi level and the edges of the bands is relatively small. We introduce and investigate a two-band model with intra-band pairing in the relevant parameters regime to address a generic behaviour of a system with hole-like and electron-like bands in external magnetic field. Our results show that two Lifshitz transitions can develop in analysed systems and the first one occurs in the superconducting phase and takes place at approximately constant magnetic field. The chosen sets of the model parameters can describe characteristic band structure of iron-based superconductors and thus the obtained results can explain the experimental observations in FeSe and Co-doped BaFe₂As₂ compounds.

Unconventional High-Energy-State Contribution to the Cooper Pairing in the Underdoped Copper-Oxide Superconductor HgBa_{2}Ca_{2}Cu_{3}O_{8+δ}

We study the temperature-dependent electronic B_{1g} Raman response of a slightly underdoped single crystal HgBa_{2}Ca_{2}Cu_{3}O_{8+δ} with a superconducting critical temperature T_{c}=122  K. Our main finding is that the superconducting pair-breaking peak is associated with a dip on its higher-energy side, disappearing together at T_{c}. This result reveals a key aspect of the unconventional pairing mechanism: spectral weight lost in the dip is transferred to the pair-breaking peak at lower energies. This conclusion is supported by cellular dynamical mean-field theory on the Hubbard model, which is able to reproduce all the main features of the B_{1g} Raman response and explain the peak-dip behavior in terms of a nontrivial relationship between the superconducting gap and the pseudogap.