Session H08: Superconductivity: Copper Oxide - Stripes, CDW and local probes

Doping-dependent precursor charge density wave correlations in La2-xBaxCuO4

Recently using resonant inelastic x-ray scattering (RIXS), we discovered, in La2-xBaxCuO4 x=0.125, precursor charge density wave (CDW) correlations that exist above the CDW transition temperature and observed that its wavevector varies with temperature and becomes unlocked from the spin wavevector [1]. This observation may help to reconcile the distinct doping-dependence of CDW wavevector in different cuprates families [1,2]. Here we show our new RIXS study that reveals the doping-dependent evolution of precursor CDW in La2-xBaxCuO4.
[1] H. Miao et al., PNAS 114, 12430-12435 (2017).
[2] H. Miao et al., Phys. Rev. X 8, 011008 (2018).   

Temperature Dependence of CDW phenomena in optimally-doped Bi2212 via RIXS

Charge density wave (CDW) in double-layered Bi-2212 copper oxide superconductor has long been studied by STM; yet the questions of how the CDW’s origin and how it interacts with other degrees of freedom as a function of temperature still remain largely unexplored. Here we utilize high-resolution resonant inelastic x-ray scattering (RIXS) at the Cu L-edge to investigate the CDW in optimally-doped Bi2212. A CDW scattering in the quasi-elastic region can be clearly observed, unambiguously demonstrating that CDW is a bulk property and exist in the optimally-doped Bi2212. Its temperature dependence and interaction with the phonon degrees of freedom will also be discussed.   

Orbital symmetries of charge density wave order in YBa2Cu3O6+x

Charge density wave (CDW) order has been shown to compete and co-exist with superconduc- tivity in underdoped cuprate superconductors. Moreover, the CDW order has been proposed to exhibit a novel d-symmetry form factor—a quadrupolar modulation of the charge on oxygen sites surrounding each Cu site — evidence for which has emerged from STM measurements in BSCCO, NCCOC and from resonant x-ray scattering measurements of YBa₂Cu₃O_6+x (YBCO). Here, we revisit resonant x-ray scattering measurements at the Cu L edge of the CDW form factor in YBCO, utilizing a variation in the measurement geometry to provide enhanced sensitivity to the CDW orbital symmetry. Contrary to previous reports, the (0 0.31 L) peak measured at the Cu L edge is dominated by a simple s form factor, revealing no evidence for a dominant d form factor CDW. However, by measuring the CDW orbital symmetry at both the (0.31 0 L) and (0 0.31 L) peaks, we identify a pronounced difference in the orbital symmetry of the CDW order propagating along the a and b axes, with the CDW propagating along the a-axis exhibiting some form of orbital order in addition to charge order. Model calculations indicate that a candidate for the unusual orbital symmetry of the a-axis CDW to break ac and ab plane mirror symmetries.   

Ground states of striped cuprates at high magnetic fields (H)

The interplay of superconductivity and charge orders found in all hole-doped cuprates remains an open question in the limit of high H. To address this issue, we have investigated the H-tuned superconducting-normal phase transitions in ultra-high-quality single crystals of La_1.7Eu_0.2Sr_0.1CuO₄ and La_1.48Nd_0.4Sr_0.12CuO₄, which have a striped charge order, under extreme conditions (temperatures T down to 0.016 K and H up to 45 T). A variety of linear and non-linear transport measurements have been performed, and the results reveal a full sequence of states in the T=0 limit as a function of H: a superconductor, a wide regime of 2D superconducting phase fluctuations, and a high-H normal state. Within the phase-fluctuations regime, an unanticipated insulatinglike region emerges that is consistent with a novel type of ordering of localized Cooper pairs within the charge stripes in CuO₂ planes. This regime appears to compete with the H=0 superconductivity. We will present our results and discuss the role of disorder, as well as the similarities to other 2D systems and materials with competing orders that undergo superconducting phase transitions.   

Stripes and superconductivity in the two-dimensional Hubbard model

The stripes have been shown the ground state in the doped two-dimensional Hubbard model for a particular interaction. In this work we discuss the superconductivity in the stripes. We employ two of the most powerful methods, density matrix renormalization group and constrained-path auxiliary-field Monte Carlo to study the superconductivity in the stripes on finite-size cylinders and in the thermodynamic limit. By studying the pair-pair correlation and the local pairing order, we show that no long-range superconductivity exists in the ground-state stripes. In addition we discuss the mixture of stripes with different stripe fillings, and the effect on the superconductivity.   

Temperature and field dependence of the intrinsic tunneling structure in overdoped Bi2Sr2CaCu2O8

Tunneling in stacked ‘intrinsic’ junctions in the high-T_c superconductor doped Bi₂Sr₂CaCu₂O₈ is a bulk probe of the quasiparticle density of states, which can be used to complement surface spectroscopic techniques such as ARPES and scanning tunneling microscopy. However, in order for this technique to give useful data above the superconducting sum-gap voltage, stacks with area of less than 1 (μm)² and containing approximately 10 or fewer intrinsic junctions must be used in order to ensure negligible self-heating.

Here we present intrinsic tunneling data for mesa structures fabricated on three over- and optimally-doped Bi_2.15Sr_1.85CaCu₂O_8+δ crystals, with 0.16–0.19 holes per CuO₂ unit, for a wide range of temperature (T) and applied magnetic field (H). The differential conductance above the gap edge shows a clear dip structure which is highly suggestive of strong coupling to a narrow boson mode. Data below the gap edge gives clear evidence for strong T-dependent pair breaking. These findings could help theorists make a detailed Eliashberg analysis and thereby contribute towards understanding the pairing mechanism.   

Bosonic Mode in the Tunneling Spectra of Fe-based and Cuprate Superconductors

The above-gap, SIN tunneling spectral dip feature in the bilayer cuprate Bi₂Sr₂CaCu₂O_8+δ (Bi2212) is highly reproducible and unambiguously linked to the resonance spin excitation. The search for similar bosonic mode features in the tunneling spectra of Fe-based and other cuprate superconductors is presented. The single-layer cuprate Hg1201Tc (94K) exhibit sharp spectra and a well resolved bosonic mode feature similar to Bi2212. STS data on FeSe (Tc = 8.9K) reveal a bosonic mode feature quite similar to Bi2212, also linked to the neutron resonance mode. It is demonstrated that SIS break junctions or intrinsic c-axis junctions reveal a strongly enhanced dip feature allowing the observation of a bosonic mode in strongly disordered Bi2201 with Tc values ≤ 5K. Over a wide range of Tc values the mode energy Ω remains inside the superconducting gap 2Δ as found in high Tc cuprates. The conclusion is that irrespective of Tc, composition and degree of disorder, the bosonic mode Ω retains its character as a spin exciton and appears to be a universal signature of unconventional superconductivity.   

Can Short-Range CDW Order Cause the Fermi Surface Reconstruction in the Cuprates?

In the cuprates, short-range charge density wave (CDW) order is generally thought to account for the reconstructed Fermi surface apparent in quantum oscillation (QO) experiments. However, since the measured CDW correlation length may sometimes be shorter than the cyclotron radius, it is unclear whether sharp QOs reflecting the reconstructed Fermi surface are theoretically expected. In this work, we derive a simple quantization rule appropriate for disordered CDWs and show that it involves a random Berry phase. We demonstrate that under most circumstances, the predicted QO amplitude is not easily reconciled with observations in the cuprates.   

Visualizing the electronic structure of infinite-layer cuprate by scanning tunnelling spectroscopy

Infinite layer Sr_1-xLa_xCuO_2+δ is a prototype to study the high-T_c superconductivity in cuprates. Using oxide-MBE technique, we fabricate Sr_1-xLa_xCuO_2+δ films with different doping levels. Mottness has been observed in lightly electron-doped Sr_1-xLa_xCuO₂ by in-situ STM. We also find a phase transition to hole-doped Sr_1-xLa_xCuO_2+δ when x exceeds 0.1. Apical oxygens are responsible for this electron- to hole-cuprate transition, which is supported by transmission electron microscope measurements. By comparing the electronic structures in electron- and hole-doped Sr_1-xLa_xCuO_2+δ, we give some clues to understand the e-h asymmetry in cuprates.   

Real-Space Characterization of Charge Ordering in Epitaxial La2-xSrxCuO4 films

Injection of charge carriers into the parent Mott insulator gives rise to a plethora of electronic orders in cuprates. Investigation of the behaving and relationship of these exotic orders with high-T_c is of pivotal importance in solving the cuprate conundrum. For this, we present a detailed study of the doping dependent charge orderings in the canonical La_2-xSr_xCuO₄ cuprate thin films. By in-situ STM, we unveil a 4a₀-modulated stripe phase in the vicinity of 0.125 doping, whose commensurability is lost with decreasing Sr concentration. When heavier doped, a grid phase appears whose size is averaged to 4 nm. In the lightly doped regime, on the other hand, a Wigner crystal phase with a distorted-hexagonal patterned electronic structure is captured. Our results contribute to the construction of electronic landscape in the cuprate doping phase diagram, and is of help in illuminating its interplay with superconductivity.   

Charge trapping and super-Poissonian noise centers in a cuprate superconductor

We present new insight into the mystery of highly anisotropic transport in the cuprates and how they can trap additional charges. Above the superconducting transition these materials are perfectly metallic along the crystal planes (ab-plane), but are insulating in the c-axis, with ratios between in-plane and perpendicular resistance exceeding 10⁴. This anisotropy has been identified as one of the key mysteries in the cuprates and has been connected to the mechanism of high-temperature superconductivity. Here, we employ our newly developed scanning noise spectroscopy technique, for which we combined a scanning tunneling microscope (STM) with a novel MHz frequency amplifier to bring noise-spectroscopy measurements to the atomic scale [1]. We discover surprising deviations from the expected Poissonian noise of uncorrelated electrons. A behavior that can only happen in highly polarizable insulators and represents strong evidence for trapping of charge in the charge reservoir layers of the cuprates. We will show how these new observations connect to the mystery of anisotropic transport in high-temperature superconductors and shed new light onto this issue [2].

[1] KM Bastiaans et al. Rev.Sci.Instrum. 89, 093709 (2018)
[2] KM Bastiaans, D Cho et al. Nature Physics AOP   

Towards the development of YBa2Cu3O7-based S-I-S tunneling Josephson junction device

Josephson junctions are used to fabricate superconductor quantum interference devices (SQUIDs), qubits, and terahertz frequency devices. YBa₂Cu₃O₇ (YBCO)-based S-I-S tunneling Josephson junctions offer many advantages over the conventional low temperature superconductor-based Josephson junctions including low cost, cryogenic system simplicity, and high I_cR_n product value (with I_c being the junction critical current and R_n the normal resistance). However, the development of a well-controlled and reproducible YBCO based S-I-S tunneling Josephson junction is very challenging since the thickness of the electrical insulation layer (I) on the junction has to be only about a nanometer and it must provide a very high potential barrier in the junction.

Here, we present our work towards the development of the YBCO based S-I-S tunneling Josephson junction device by discussing growth, structure, and electrical transport properties of (110)-YBCO, various metals (M) doped PrBa₂(Cu_0.8M_0.2)₃O₇ (PBCMO)_, and heterostructures of YBCO and PBCMO_. In particular, we discuss thin film deposition of (110)-oriented YBCO, PBCMO, and heterostructures of YBCO and PBCMO and their electrical transport properties including the electrical resistivity, conduction mechanism, and proximity effects.
  

Quantum Sensing of Layered Superconductor at the Nanoscale

We introduce single spin magnetometry based on nitrogen-vacancy (NV) centers in diamond as a new platform to probe the microwave response of a two-dimensional layered high Tc superconductor Bi₂Sr₂CaCu₂O_8+x (BSCCO) flake at the nanoscale. We observed a temperature-dependent spatial profile of the microwave field around the BSCCO flake, which can be attributed to the variation of the conductivity in BSCCO. Our technique provides a novel method to characterize the superconducting phase transitions without making electrical contacts and opens up new possibilities for nanoscale imaging of the charge dynamics in layered materials.   

Emergent superconductivity upon disordering a charge density wave ground state

We explore the interplay of a charge density wave (CDW) order and s-wave superconductivity in a disordered system. Recent experiments on 1T-TiSe₂, where the pristine sample has a commensurate CDW order and the superconductivity appears upon copper intercalation, motivates our study. Starting with an extended Hubbard model, with parameters which yield a CDW ground state within Hartree-Fock-Bogoliubov formalism in pure systems, we show that the addition of disorder quickly wipes out the global charge order by disrupting periodic modulation of density at some (low) strength of disorder. Along with this, the subdominant superconducting order emerges in regions that spatially anticorrelate with islands of strong local CDW order. The short-range density modulations, however, continue to persist and show discernible effects up to a larger disorder strength. The local CDW puddles reduce in size with increasing disorder and they finally lose their relevance in effecting the properties of the system. Implication of our model calculations on the experimental findings of transition-metal-dichalcogenides will also be discussed.