Session Z61: Superconductivity: Copper Oxide and Related Systems-III

Growth, characterization, and optimization of (110)-oriented YBa2Cu3O7 (S) / PrBa2 (Cu0.8Ga0.2)3O7 (I) / YBa2Cu3O (S) trilayer heterostructure.

Epitaxial growth and characterization of cuprate heterostructures are of the utmost importance for developing many superconductor electronic devices such as Josephson junctions, three-terminal devices, and circuit applications such as interconnects, ground planes, and multichip modules. The heterostructures made with high critical temperature (T_c) superconductor YBa₂Cu₃O_7-x (YBCO) have applications for fundamental science research, such as studies of mechanisms for high-T_c superconductivity, 2D superconductivity, and measurement of the correlation energy. These heterostructures typically have the S/D/S, S/N/S, S/I/S geometries (S=superconductor, D=dielectric, N=normal metal, I=insulator), with the middle layer as the isolation layer, normal metal, or the insulator. Using the pulsed laser-based thin film deposition technique (PLD), we fabricated (110)-oriented YBa₂Cu₃O₇ (YBCO) / PrBa₂(Cu_0.8Ga_0.2)₃O₇ (PBCGO) bi-layer and YBCO/PBCGO/YBCO tri-layer heterostructures with (110)-oriented PBCGO insulator (I) as thin as ~1nm. Afterward, we performed x-ray diffraction, atomic force microscopy, electrical transport, Raman, and Auger electron spectroscopy measurements on these heterostructures. Here, we present our experimental results from various x-ray measurements, including grazing incident x-ray diffraction, rocking curve measurement, x-ray reflectivity measurement, pole figures, and reciprocal space mapping. We also present electrical transport, Raman scattering, and Auger electron spectroscopy results on the heterostructures.

    

Resonance condensation in the high-Tc cuprates

he cold atomic Fermi gas has become the essential paradigm for understanding the crossover between the weak coupling Bardeen-Schrieffer-Cooper (BCS) and strong coupling Bose-Einstein Condensation (BEC) regimes of paired fermion condensates. On tuning the pairing interactions through the crossover, the superfluid condensate becomes maximally robust between the BCS and BEC regimes at what is referred to as the unitary point, giving rise to a maximally peaked phase transition anomaly in the specific heat. We find here, by tracing the jump Δγ in the electronic specific heat coefficient γ at the transition temperature T_c as a function of the gap ratio 2Δ/k_BT_c, that this same thermodynamic signature occurs in the high-T_c superconducting cuprates.  On taking Δ as the pairing gap in a cold atomic Fermi gas and as the antinodal pseudogap energy in the cuprates, we find Δγ to be asymmetrically peaked at the same value of 2Δ/k_BT_c ≅ 6.5. We show that the observation of this peak in the cuprates, and its coincidence with a normal state maximum in γ, provides a previously overlooked  thermodynamic signature of condensation at a unitary point.    

Non-mean field breakdown of heterogeneous superconductivity in overdoped (Pb,Bi)2Sr2CuO6+δ

The overdoped side of the cuprate phase diagram is generally considered to be the most conventional: a BCS-like superconductor emerging from a Fermi liquid normal state. Surprisingly, it was recently found that the superfluid drops to zero with increased overdoping, breaking with the previous consensus. The origin of this anomalous reduction is unknown_. Our scanning tunneling spectroscopy measurements in the overdoped regime of the (Pb,Bi)₂Sr₂CuO_6+δ high-temperature superconductor show that it is due to the emergence of a heterogeneous state with nanoscale superconducting puddles in a metallic matrix, confirming theoretical proposals within the Bardeen-Cooper-Schrieffer (BCS) mean-field theory. Our measurements further reveal that this puddling is driven by gap filling, not gap closing. Unexpectedly, we find that a basic rule from the mean-field description is dramatically violated: mean-field theory predicts that pair-breaking should always go hand in hand with a diminishment of the gap magnitude, but what we observe instead is that a larger gap coincides with stronger pair breaking. This suggests that the mechanism governing superconductivity is qualitatively different from the conventional mean-field theory.    

Mechanism of Superconductivity in Cuprates: Oxygen Hole Content, Charge Transfer Gap and Superexchange

Using cluster generalizations of dynamical mean-field theory for the three-band Hubbard model, we explain three apparently unrelated experiments that suggest how to optimize T_c in cuprates: i) NMR experiments that show that T_c is optimized by maximizing oxygen hole content [1] ii) Scanning Tunneling spectroscopy that shows that T_c is optimized by decreasing the charge transfer gap [2] and iii) neutron experiments that show that T_c is optimized by increasing superexchange [3].  The unified explanation of these three experiments that we offer also explains the mechanism for superconductivity in cuprates. The results suggest new avenues to discover compounds that superconduct at even higher temperature [4].

 

References

[1] Rybicki D, et al. Nature Communications 7:11413 (2016)

[2] Ruan W, et al., Science Bulletin 61(23):1826–1832 (2016)

[3] Wang L, et al. arXiv:2011.05029

[4] Nicolas Kowalski, Sidhartha Shankar Dash, Patrick Sémon, David Sénéchal, and André-Marie Tremblay, PNAS (Proceedings of the National Academy of Sciences) 118 (40) e2106476118 (2021).   

Search for charge density wave order in (Y1-xPrx) Ba2Cu307

High-T_c superconductivity in cuprates occurs in proximity to competing low-temperature density-wave-order states. Superconductivity appears when spin order of the parent compound is destroyed and for underdoped compounds it competes with charge density wave (CDW) order. Resonant elastic soft x-ray scattering (RSXS) combines x-ray scattering with x-ray spectroscopy, making it possible to probe a variety of exotic spin, charge, orbital and structural ordering phenomena. We report on the growth and characterization of single crystals of Y_1-xPr_xBa₂Cu₃0₇ cuprate superconductors with 0 < x < 1.0 by means of XRD, magnetic susceptibility, and electrical resistivity measurements, as well as RSXS measurements performed to probe the relationship between CDW order, the pseudogap, and high T_c superconductivity in this unexplored system.   

Discovery of an electronic crystal in very underdoped RBCO

The quantum electronic state in underdoped copper oxide high temperature superconductors tends to break the translational symmetry of the parent lattice. The resulting emergent, collective phases of matter compete with superconductivity and manifest in two main forms – charge/spin stripes and incommensurate charge-density-waves (CDWs) – but in all cases emerge out of a strongly correlated metallic state. 

In the present study, we used a combination of resonant X-ray scattering, electronic transport, and muon spin rotation measurements to study these electronic orders in the RBa₂Cu₃O₇ family (RBCO; R = rare earth), using rare earth substitution to reach the very underdoped regime. There, we discovered a multiorbital density wave originating from Cu and Pr orbitals, signaling the presence of a spatially modulated electronic state deep in the ‘Mott’ limit.

Our observations of an electronic crystal state in a magnetic insulating cuprate reveal that this electronic state is a new and potentially ubiquitous form of electronic symmetry breaking in the CuO_­2 planes. Ultimately, these findings strongly hint at a strong coupling origin of charge order in the RBCO family and underscore the prominent role of Coulomb-frustrated electronic phase separation across all cuprates.   

Oxygen Isotope Effect Study of (Y, Pr)Ba2Cu3O7-δ Thin Films

For cuprate superconductors, oxygen isotope effect (OIE) has been used to probe the extent of electron-phonon interactions in the pairing mechanism [1] and in the pseudogap formation [2]. In this talk, we present a new OIE study of Pr-doped YBa₂Cu₃O_7-δ (YPBCO), which is the only member of the RBCO (R = rare earth) family where the superconductivity can be suppressed by doping without changing the oxygen content. To maximize doping and isotope homogeneity, epitaxial YPBCO thin films grown by pulsed laser deposition are used. The substitution of ¹⁶O by ¹⁸O is done by gas-phase diffusion and confirmed using Raman spectroscopy. Electrical transport measurements are then used to determine both the superconducting critical temperature T_c and the pseudogap onset temperature T*, to look for correlations between the isotope shifts of T_c and of T*. Our results are discussed in light of past and recent studies of the pseudogap [2, 3], and in connection with the charge order that is ubiquitouly seen in underdoped cuprates [4].

[1] G. Zhao et. al., PRB 51 16487(R) (1995)

[2] D. Temperano etl., PRL 84 1990 (1999)

[3] L. Zhao et. al., Nat. Phys. 13 250 (2017)

[4] R. Comin et. al., Annu. Rev. Con. Mat. Phys. 7 369 (2016)   

B1g phonon anomaly driven by Fermi surface instability at intermediate temperature in YBa2Cu3O7-δ

We performed temperature- and doping-dependent high-resolution Raman spectroscopy experiments on YBa₂Cu₃O_7−δ to study B_1g phonons. The temperature dependence of the real part of the phonon self-energy shows a distinct kink at T=T_B1g above Tc due to softening, in addition to the one due to the onset of the superconductivity. T_B1g is clearly different from the pseudogap temperature with a maximum in the underdoped region and resembles charge density wave onset temperature, TCDW. We attribute the B_1g phonon softening to an energy gap on the Fermi surface induced by a charge density wave order, which is consistent with the results of a recent electronic Raman scattering study.   

On the Electron Pairing Mechanism of Copper-Oxide High Temperature Superconductivity.

Hole doping the CuO₂ plane generates the transition from a charge-transfer superexchange mediated antiferromagnetic insulator, to a high temperature superconducting state whose electron-pairing is exceptional. A leading proposal for the mechanism of this intense electron-pairing is that hole doping destroys magnetic order, but preserves superexchange interactions with a charge-transfer energy scale ε. In Bi₂Sr₂CaCu₂O_8+x, we use both single-particle and electron-pair (Josephson) STM to visualize ε and the electron-pair density n_p. We determine their responses to the modulations in the distance δ between planar Cu and apical O atoms. The responses we determine of ε and n_p to δ, and crucially of n_p to ε conform closely to strong-coupling theories in which charge-transfer superexchange is the electron-pairing mechanism (arXiv:2108.03655).   

Fractional spin excitations in the infinite-layer cuprate CaCuO2

The spin 1/2 square-lattice antiferromagnet is one of the most studied systems in condensed matter theory because of its purely quantum mechanical nature. After the seminal paper by Anderson, much effort has been put in the quest for exotic ground states such as the Resonating Valence Bond (RVB), which have been predicted to arise in many Heisenberg Hamiltonians with frustrating or multi-spin couplings. These states are interesting because they exhibit fractionalized collective excitations, each carrying spin 1/2, that are the 2-dimensional version of spinons in 1D spin chains. Despite many efforts, these fractional magnetic excitations have so far eluded conclusive observation in the AF square lattice. In this context, we have used Cu L₃ Resonant Inelastic X-ray Scattering (RIXS) to study the spin excitations in the infinite-layer CaCuO₂, which is special for the very large nearest neighbour AF coupling J (∼180 meV), and an exceptionally large ring exchange J_c ∼ J. Close to the magnetic zone boundary, we observe a decay of the magnon into a broad asymmetric continuum at high energies, accounting for more than 80% of the total spectral weight, and composed of ΔS=1 excitations. Our observation are in line for what is expected from a continuum of spinon pairs.   

Electrochemical Tuning of YBa2Cu3O7-x Transport Properties in Superconducting Transistor-like Structure

The ground state of strongly-correlated oxide superconductors is strongly dependent on the oxygen stoichiometry, which allows tuning the superconducting phase through a controlled motion of oxygen ions. Depositing a more electronegative layer on top of the superconducting material, in a transistor-like geometry, allows for a reversible electrochemical modulation of the oxygen content via redox chemical reactions, paving the way for novel field-effect switching superconducting devices. In this work, we explore such effects in planar devices based on the high-temperature cuprate superconductor YBa₂Cu₃O_7-x, in which oxygen diffusion is controlled via an aluminium gate. We will show that, upon gating, the gate and source-drain conductance as well as the superconducting critical temperature are modulated between non-volatile states in either a reversible/irreversible fashion depending on the Al and YBa₂Cu₃O_7-x thickness.   

Ab initio study of pressure-dependence of overdoped YBa2Cu3O7

To further elucidate our previous ab-initio results on competing stripe and magnetic phases in overdoped YBa₂Cu₃O₇ [1] we will study the role of pressure in doping the sample. We find that pressure strongly modifies the magnetic order, but it has more effects than simply changing the density.

[1] Proc. Natl. Acad. Sci. 117, 68 (2020)   

Quasiparticle evolution on a high-Tc superconductor surface decorated with sub-monolayer magnetic atoms

Unconventional superconductivity has been a fertile research field for both its basic science and application values. Among the most exciting endeavors is the investigation of electronic states inside and around superconducting vortices, where elusive new quasiparticles (Majorana bound states) or competing orders (superconducting pair density waves) thrive. So far, direct investigation of pinned vortices in momentum space is scarce, mainly due to the inherent difficulty of performing perform photoemission experiments under a macroscopic magnetic field. Here, by combining molecular beam epitaxy (MBE) and angle-resolved photoemission spectroscopy (ARPES), we study the evolution of electronic structures of the high-Tc cuprate Bi2Sr2CaCu2O8 when a monolayer of iron is deposited on the surface. We discuss the nature of the surface chemical environment and charge transfer, and report on the low energy quasiparticle evolution in energy-momentum space during this process. We discuss the general scope of using surface magnetic ion decoration as a versatile method to induce time-reversal symmetry breaking for in-situ photoemission experiments, and its specific impact on the pairing symmetry in high-Tc cuprates.