Session T61: Copper Oxide and Related Systems-II

Hidden strange metallic state in underdoped electron-doped cuprates

The low-temperature linear-in-T resistivity of “strange metals,” such as the metallic state of the cuprate high-temperature superconductors, has long been thought to be associated with a quantum critical point. However, recent transport studies of the cuprates have found this behavior persists over a wide range of overdoping. In this work [1], we report magnetoresistance and Hall effect results for electron-doped cuprate La_{2− x}Ce_xCuO₄ for temperatures from 0.7 to 45 K and magnetic fields up to 65 T. For x= 0.12 and 0.13, just below the Fermi surface reconstruction (FSR) at x= 0.14, the normal state in-plane resistivity exhibits a well-known upturn at low temperature. Our new results show that this resistivity upturn is eliminated at a high magnetic field and the resistivity becomes linear-in-temperature. The magnitude of the linear coefficient scales with T_c and doping, as found previously [2,3] for dopings above the FSR. This finding suggests that the strange metal is not confined to a single “critical point” in the phase diagram, but rather is a robust universal feature of the metallic ground state of the cuprates.

1. T. Sarkar, et al., Phys. Rev. B 103, 224501 (2021).

2. K. Jin,  et al.,Nature(London) 476, 73 (2011).

3. T. Sarkar, et al., Sci. Adv. 5, eaav6753 (2019).   

Incoherent transport and the evolution of power-law scaling of the magnetoresistance in cuprate superconductors.

The normal state of overdoped cuprate superconductors within the strange metal regime (p* < p < p_sc) is characterized by a T-linear in-plane resistivity that persists to lowest temperatures and grows with the strength of superconductivity as optimal doping is approached from the overdoped end of the superconducting dome at p_sc until p* (the doping at which the pseudogap temperature reaches 0 K). Analysis of the high-field, low-temperature Hall coefficient reveals a simultaneous reduction in carrier density n_H from 1+p → p [1] indicating that coherent quasiparticles are being lost. We report high-field in-plane magnetoresistance (MR) studies of Tl2201 and Bi2201 in which signatures of incoherent transport are observed. In particular, an unexpectedly large linear-in-field MR has been observed at high H and low T that is insensitive to disorder, magnetic-field orientation and obeys quadrature (H/T) scaling [2]. The growth of the magnitude of the linear-in-H MR coincides with the growth of the low-T T-linear resistivity [3]. At p < p* ∼ 0.19, the growth of the linear-in-field MR at high fields persists, yet a dramatic change in the low-field behavior occurs. The quadrature (H/T) power-law scaling that is obeyed for p > p* abruptly cedes to H/T² scaling as the pseudogap regime is entered [3]. The anticorrelation of the loss of coherent quasiparticles probed via the Hall effect and the growth of the incoherent MR indicates the presence of two charge sectors. Such an anticorrelation naturally prompts the question: from which of these charge sectors does superconductivity emerge? We also present evidence to support the hypothesis that superconductivity emerges from those carriers that exhibit signatures of incoherent transport [4].   

X-ray and neutron diffuse scattering measurements of the local structure of cuprate superconductors

Nanoscale inhomogeneity is a well-known feature of the cuprate high-temperature superconductors, and recent theoretical and experimental developments have generated renewed interest in the effects of inhomogeneity and local structural distortions on the electronic physics of these materials. We used X-ray and neutron diffuse scattering measurements to study the local structure of several cuprate superconductors, with a focus on the model compound HgBa₂CuO_4+δ which has a small unit cell, simple-tetragonal symmetry at all temperatures, and a high superconducting T_c at optimal doping. Dedicated inelastic neutron scattering measurements were also used to confirm the distinction between static distortions and low-energy fluctuations. We developed structural models guided by 3D-∆PDF analysis of the scattering data in order to understand the local structure of HgBa₂CuO_4+δ. Based on the X-ray and neutron scattering data combined with the modelling results, we discuss the temperature- and doping-dependence of the local structure of HgBa₂CuO_4+δ, the effects it may have on the electronic physics, and the implications for other cuprate materials.   

Robust superconductivity across the antiferromagnetic Fermi surface reconstruction in Nd1.85Ce0.15CuO4

The electron-doped cuprates are an important piece of the puzzle to understand high-transition-temperature cuprate superconductors, but progress remains challenging due to conflicting thermodynamic, spectroscopic, and phase-sensitive measurements on the nature of the superconducting state. Recent advances in sample preparation have elucidated the role of annealing as an independent tuning parameter, allowing the exploration of new phase space. Here we report angle-resolved photoemission spectroscopy data on Nd_1.85Ce_0.15CuO₄ samples annealed with conventional and protected methods, resulting in different antiferromagnetic (AF) correlations and Fermi surface (FS) topologies but retain the same superconducting temperature. Through resolving the elusive Bogoliubov quasiparticle peaks, we quantitatively identify strong-coupling superconducting gap (Δ_SC) maxima at the hot spots where the unreconstructed FS crosses the AF zone boundary. With varying spectral signatures across the AF-induced FS reconstruction, the differently annealed samples exhibit comparable Δ_SC magnitudes and profiles at off-nodal momenta, but show a dramatic gap-to-gapless transformation at the node. This result suggests the superconducting properties in the electron-doped cuprates are insensitive to remarkable changes of the nodal electronic structure near the Fermi energy. Our findings pave the way to reconciling previous measurements and provide important clues for understanding high temperature superconductivity.   

Mechanism for the Nanoscale Condensate Disorder of Cuprates

Nanoscale electronic disorder has long remained a mystery in carrier-doped CuO₂ superconductors. We combine scanned electron-pair (Josephson) and single-electron tunneling microscopy to visualize simultaneously the superconductive condensate electron-pair density n_P(r) and the spectrum N(r) of single-electron quasiparticles in Bi₂Sr₂CaCu₂O₈. Motivated by (arXiv:2108.03655) our objective is to study and eventually explain the nanoscale disorder in n_P(r), the singe-particle energy-gap Δ(r) and the superconductive coherence-peak height C(r).   

Critical Nematic Correlations Throughout the Doping Range inBi2−zPbzSr2−yLayCuO6+x

Rapidly expanding experimental capabilities have led to a growing wealth of data on multiple length scales, revealing rich electronic textures at the nanoscale and mesoscale in many correlated oxides. We have defined new conceptual frameworks for interpreting and understanding the multiscale electronic textures observed at the surface of these materials by employing theoretical tools from fractal mathematics and disordered statistical mechanics. This allows us to use the rich spatial information available from scanning probes in order to diagnose criticality from the spatial structure alone, without the need of a sweep of temperature or external field.  These new methods have enabled the discovery of universal, fractal electronic textures across a variety of quantum materials.  [Nat. Commun. 10, 4568 (2019);  PRL 116, 036401 (2016); Nat. Commun. 3, 915 (2012)]  Applying these cluster techniques to scanning tunneling microscopy on Bi_2−zPb_zSr_2−yLa_yCuO_6+x, we show that the locally 1D charge modulations are actually a bulk effect, since they have the kind of power law orientational correlations that only 3D models can produce.  We observe the power law behavior in underdoped, optimally doped, and slightly overdoped Bi_2−zPb_zSr_2−yLa_yCuO_6+x.    

Response of electronic nematicity and charge order under uniaxial strain in cuprate superconductors

There is a growing realization that the essential physics of many quantum materials, such as high-temperature superconductors, involves a complex interplay between many different electronic phases (charge density wave order, superconductivity, magnetism, orbital order, nematicity) that have very similar energy and temperature scales. Understanding how these phases compete, co-exist, and sometimes co-operate is of general interest to a wide family of materials. 

Here we elucidate how charge density wave order (CDW) in a canonical La-based cuprate superconductor La_2-xM_xCuO₄ (M = Sr, Nd) is influenced by electronic nematicity (a breaking of the rotational symmetry of the electronic structure) and structural distortions of the lattice using resonant soft x-ray scattering on samples under applied uniaxial strain.   

Evolution of electronic nematicity through the pseudogap phase in cuprate superconductors

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. Recently, we have employed resonant x-ray scattering in La-based cuprate high-temperature superconductors La_2-xM_xCuO₄ (M = Ba, Sr, Nd, Eu) to navigate the cuprate phase diagram, probing the relationship between electronic nematicity of the Cu 3d orbitals, charge order, and the pseudogap phase. The results established a clear link between electronic nematicity, the pseudogap, and its associated quantum critical point in overdoped cuprates. Here we will present the measurements extended to a wider doping range and to related compounds, La_2-x-yEu_ySr_xCuO4 and La_2-xBa_xCuO₄, to fully exploring the evolution of nematicity in the pseudogap phase.   

Departures from Linear-in-T resistivity as a function of doping in cuprate superconductors

A linear-in-T or “strange-metal” resistivity is one of the hallmark features of cuprate superconductors near optimal doping, with this behavior unexpected from the conventional Fermi liquid theory of the electron gas.  In contrast, at very high doping levels, it is generally agreed that the Fermi liquid (quadratic) scattering is recovered, though the way in which this recovery occurs is not well understood or characterized.  Here, we carefully study this evolution in a series of cuprate superconductors, with principle focus on the Bi₂Sr₂CaCu₂O_8+δ family of bilayer superconductors. We fit the resistivity with various parameterizations, check for scaling of parameters, and do a careful comparison to scattering rates observed from ARPES.  Our results show a clear break in parameters at a critical doping point near p=0.22 – a similar doping level obtained from a range of other experiments. The success of our parameterizations suggests a more general model for describing the energy and temperature dependences of the electronic interactions in cuprate superconductors.   

Non-linear Hall effect in PCCO electron-doped cuprate

The Hall effect in electron-doped cuprates shows non-trivial behaviors as a function of doping and temperature. For example, the Hall coefficient of Pr_2-xCe_xCuO₄ at x = 0.17 shows two sign changes between 0 and 300K. This can be explained in part using a two-carrier model. We present a quantitative analysis of the non-linear Hall effect for PCCO thin films as a function of doping. We measure the longitudinal and Hall resistivity of high quality PCCO thin films deposited by Pulsed Laser Deposition patterned using photolithography techniques. We focus on the non-linear magnetic field dependence of the Hall resistivity easily evidenced at temperatures where R_H ~ 0. Considering higher orders of field in the equation . we observe in fact that the Hall coefficient shows this non-linear behavior at all temperatures and that it evolves also with doping. An analysis of this non-linear field dependence within a simple two-band model is presented. Results show that one needs to go beyond this simplistic model to improve our understanding of the band structure of these materials and the impact of the Fermi surface transformation as a function of doping.   

Thermal Hall Conductivity of Electron Doped Cuprates

Measurements of the thermal Hall conductivity in hole-doped cuprates showed that phonons acquire chirality in a magnetic field, both in the pseudogap phase [1,2] and in the Mott insulator state [3]. The microscopic mechanism at play is still unclear. A number of theoretical proposals are being considered, including skew scattering of phonons by various defects [4,5,6], the coupling of phonons to spins [7] and a state of loop-current order with the appropriate symmetries [8]. But more experimental information is required. 

Here we present our study of the thermal Hall conductivity in the electron-doped cuprates Nd_2-xCe_xCuO₄ and Pr_2-xCe_xCuO₄, for dopings across the phase diagram, from x = 0, in the insulating antiferromagnetic phase, up to x = 0.17, in the metallic phase above optimal doping. We observe a large negative thermal Hall conductivity at all dopings, in both materials. Measurements with a heat current perpendicular to the CuO₂ planes confirm that phonons are responsible for this thermal Hall signal, as in hole-doped cuprates. We discuss the possibility that short-range spin correlations may be involved in the mechanism that confers chirality to the phonons.     

Universal spin texture in high-temperature cuprate superconductors

Spin-orbit coupling emerging out of a material's global inversion symmetry breaking has been long known to act as a pair breaking mechanism to the superconducting order parameter. However, when spin-orbit coupling emerges from local inversion symmetry breaking, an unexpected coexistence with superconductivity might occur. Little is known about the connection between spin-orbit coupling and local lattice asymmetry, and how the combination of the two can impact electronic properties and the microscopic classification of the order parameter. By using high-resolution spin- and angle-resolved photoemission spectroscopy we performed a full doping dependent study of the spin-orbit coupling previously found in cuprates to examine this interplay. The implication of this connection in steering the ground state properties of cuprates superconductors is discussed.   

Nematic superconducting phase fluctuations in a cuprate superconductor

We present angle-resolved transverse magnetoresistance measurements of single crystal thin films of the cuprate superconductor La_2-xSr_xCuO₄, 0.02 ≤ x ≤ 0.25. The strong nematic signal in the normal state transverse resistance close to T_c originally found in [Wu, J., Bollinger, A.T., He, X. and Božović, I., 2017. Nature, 547(7664), pp.432-435] is identified as being due to superconducting fluctuations. By modelling the conductivity due to normal electrons and thermally occupied Cooper pairs (i.e. Aslamazov-Larkin paraconductivity) we find that the pair stiffness is exceptionally anisotropic, quasi one-dimensional, at low doping. We discuss the observations in the context of theories where anisotropic superconductivity is expected such as stripe or pair-density wave order.    

Stripe order enhanced superconductivity in the Hubbard model

Unidirectional (``stripe'') charge-density-wave order has now been established as a ubiquitous feature in the phase diagram of the cuprate high temperature (HT) superconductors, where it generally competes with superconductivity (SC).  None-the-less, on theoretical grounds it has  been conjectured that stripe order (or other forms of ``optimal'' inhomogeneities) may play an essential positive role in the mechanism of HTSC.  Here we report density matrix renormalization group studies of the Hubbard model on long 4 and 6 leg cylinders where the hopping matrix elements transverse to the long direction are periodically modulated - mimicing the effect of putative period-2 stripe order.  We find even modest amplitude modulations can enhance the long-distance SC correlations by many orders of magnitude, and drive the system into a phase with a substantial spin gap and SC quasi-long-range-order with a Luttinger exponent, K_sc ~ 1.