Session F22: Superconductivity:Mostly_CuO_2

An investigation of the effect of self-assembled 3-dimensional BaZrO3 nanostructures on the critical current density of second-generation superconducting coated conductors.

This study investigates the GdBa₂Cu₃O_7–x (Gd123) films with several types of self-assembled BaZrO₃ (BZO) nanostructure on the ion-beam-assisted deposited MgO (I-BAD) buffered with LaSrMnO₃ substrate by the pulsed laser deposition (PLD). The sample with additionally In-situ annealing between the BZO-doped-Gd123/BZO multilayer, the BZO-doped Gd123, and the BZO bilayer, reveals a three-dimensional (3D) BZO nanostructure network in the Gd123 matrix system, Which tremendously boosted the flux pinning force (F_p) and critical current density (J_C)   under high magnetic fields along the ab-plane and c-axis. The studies on the resistance variation near transition temperature and the flux dynamic show that the flux pinning in the coated conductor with a 3D BZO nanostructure network is more 3D-like. This 3D BZO nanostructure network helps to coated conductor’s performance in J_C (extracted from transport and magnetic data) and angular dependence at the high magnetic field is superior to the current reported results, it could be further enhanced by optimizing BZO nanostructure conditions.

    

A universally strong renormalization of the thermodynamic effective mass by electron-electron interactions in the high-Tc cuprates

The strength of the pairing interactions and the degree of effective mass renormalization in a superconductor are inextricably linked. In the high transition temperature T_c superconducting cuprates, it is debated whether the thermodynamic effective mass is moderately renormalized as in a conventional Bardeen-Schrieffer-Cooper (BCS) superconductor, or more strongly renormalized as is the case in liquid ³He. While existing estimates of the  effective mass renormalization in the cuprates are based largely on photoemission measurements, questions remain as to whether this represents the elementary excitations of the many body state. To obtain the thermodynamic effective mass that is most relevant to thermodynamic properties of the superconducting state, one must refer to measurements of physical quantities that are a thermodynamic function of state such the magnetic quantum oscillation amplitude and the specific heat. The discovery of magnetic quantum oscillations in as many a seven distinct cuprate families makes such a thermodynamic endeavor much more viable today than it was when high temperature superconductivity was first discovered. Here, using the orbitally-averaged Fermi velocity as a means for combining quantum oscillation and calorimetry data (both at low and high temperatures) with new hole doping-dependent and magnetic field orientation-dependent measurements of the single layer mercury cuprate Hg1201, we arrive at a clear answer to the question of the overall thermodynamic effective mass renormalization in the cuprates.   

Observation of closed Fermi-pockets in the cuprate pseudogap phase above the superconducting critical temperature

The anomolous nature of pseudogap electronic structure of the cuprates is typified by the observation of truncated Fermi-arcs with photoemission spectroscopy. We report on angular-dependent magnetoresistance oscillations which reveal the existence of closed small Fermi-pockets in the pseudogap phase above the superconducting critical temperature T_c=74 K in the underdoped cuprate HgBa₂CuO_4+δ. The use of high temperatures avoids the widely examined subsequent Fermi-surface reconstruction at low-temperatures and high-fields that obscures the true pseudogap phase. Salient features of the data are reproduced by a Fermi-surface comprising nodal elliptical pockets. Notably, long-ranged broken translational symmetry, such as antiferromagnetism and charge-density wave, is known to be absent in the pseudogap phase of HgBa₂CuO_4+δ  above T_c, thus suggesting a different mechanism for producing a pseudogap electronic structure.    

First-Principles Electron-Phonon Interactions and Electron Spectral Functions in Lanthanum Cuprates

Cuprate high-temperature superconductors possess many unusual properties that are not completely understood. A striking one is their electron spectral function, which exhibits “pseudogaps” in angle-resolved photoemission spectroscopy (ARPES), violating conventional band theory. It is believed that both electronic correlations and electron-phonon (e-ph) interactions play a critical role in the cuprates. Even though much research has focused on hole-doped lanthanum cuprates, studies of the parent phase La₂CuO₄ (LCO) are rather limited. Because LCO is a Mott insulator, probing its electronic structure is difficult experimentally and also challenging theoretically due to the correlated d orbitals. 

In this work, we employ Hubbard-corrected density functional theory (DFT+U) and its linear-response extension to compute e-ph interactions in LCO with a correct treatment of electronic correlations [1]. Using a Hubbard-U value determined from first principles, we obtain band gap and magnetic moment in excellent agreement with experiments. Starting from this accurate ground state, we compute the temperature dependent electron spectral functions, employing a cumulant approach able to capture strong e-ph coupling, and find pronounced broadening and satellite peaks in the spectral functions. These results are a clear evidence for strong e-ph interactions and polaron effects in LCO. The talk will analyze these findings and contrast them with those for hole-doped LCO.   

Electronic structure of copper-oxide monolayer

Copper-based high temperature superconductors have a common layered structure CuO₂ plane where superconductivity takes place. Therefore, it draws attention that whether superconductivity occurs in a single layer of CuO₂ plane and how the superconductivity in this 2D structure differs from that in bulk cuprate system. To study the superconductivity of the CuO₂ monolayer, we construct a heterostructure system for surface-accessible CuO₂ monolayer. With pulsed laser deposition and angle-resolved photoemission spectroscopy, we synthesized single layer of La_2-xSr_xCuO₄(LSCO) and measured its electronic structure. Monolayer LSCO has metallic electronic structure and bulk-like fermi surface. The hole doping ratio of the monolayer LSCO depends on underlying buffer layer since the monolayer is strongly affected by cation intermixing at the interface. Our work would provide a platform for researching 2D cuprate superconductivity.   

Introducing High-Frequency Phonons to Cuprates with Light Elements

Cuprate metal oxides host unconventional high-temperature superconductivity, whose microscopic mechanism is still under debate and investigation. Despite this lack of clarity, a great deal of research has been done on modifying the superconductivity which has provided information and constraints on the superconducting mechanisms.  Separately, within the BCS framework and its refinement by Eliashberg, high-frequency phonons, which can be introduced by light elements such as hydrogen and lithium, can give rise to relatively high-temperature superconductivity but go together with a sign-preserving s-wave gap that is not compatible with the observed d-wave superconductivity. Interestingly, recent studies have shown that electron-phonon coupling with vertex corrections can lead to sign-changing superconducting gap symmetry [Schrodi, PRB, 2021]. Inspired in part by these ideas, we aim to see how incorporation of light elements in cuprates may change the phonon modes and their effects on electronic structure. In our first principles theoretical density functional theory work, we choose Bi₂Sr₂CaCu₂O₈ (BSCCO, Bi-2212) as a prototype and study its structural, electronic, and phononic response to lithium and hydrogen doping.   

Traces of electron-phonon coupling in one-dimensional cuprates

The appearance of certain spectral features in one-dimensional (1D) cuprate materials has been attributed to a strong, extended attractive coupling between electrons. Here, using time-dependent density matrix renormalization group methods on a Hubbard-extended Holstein model, we show that extended electron-phonon (e-ph) coupling presents an obvious choice to produce such an attractive interaction that reproduces the observed spectral features and doping dependence seen in angle-resolved photoemission experiments: diminished 3k_F spectral weight, prominent spectral intensity of a holon-folding branch, and the correct holon bandwidth. While extended e-ph coupling does not qualitatively alter the ground state of the 1D system compared to the Hubbard model, it quantitatively enhances the long-range superconducting correlations and suppresses spin correlations. Such an extended e-ph interaction may be an important missing ingredient in describing the physics of the structurally similar two-dimensional high-temperature superconducting layered cuprates, which may tip the balance between intertwined orders in favor of uniform d-wave superconductivity.   

Development of three-dimensional electronic structure via enhanced chain coupling in Pr-doped YBa2Cu3O7

High-T_c cuprate superconductors undergo drastic property change upon Pr doping, including the resurrection of antiferromagnetism, the decimation of superconductivity, and more recently, the development of a 3D charge density wave (CDW) order. Previous results suggested a strong electronic structure modification on CuO₂ planes brought by their strong hybridization with 4f electrons from Y-site Pr. A more 3D low-energy electronic structure with considerable f orbital contents on portions of the Fermi surface have been predicted. Such hybridization provides a potential tuning knob of the electronic states in CuO₂ planes and, therefore, serves as a powerful handle to study intertwined orders. However, the hybridization itself and its consequences have been poorly examined experimentally. Here we performed angle-resolved photoemission spectroscopy measurements on Pr-doped YBa₂Cu₃O₇ at different Pr doping levels. Contrary to the conventional picture, traces of f electrons were absent in the low-energy electronic structure. A 3D electronic structure develops through enhanced chain coupling facilitated by Pr doping. Such change may promote the 3D CDW order recently observed. Our work exposed the importance of out-of-plane interaction in superconducting cuprates and shed light on the microscopic origin of the mysterious 3D CDW order. These results provided insights in understanding high-T_c superconductivity and paved a way for future precise-tuning studies of cuprates.

    

Probing superconducting order in overdoped CaxY1−xBa2Cu3O7 by neutron diffraction measurements of the vortex lattice.

Bragg diffracted intensities and q values for crystalline structures with long repeat distances may be obtained by small angle neutron scattering (SANS) investigations. We have developed a method and an adapted Christen formula [1] for use on TOF instruments to obtain the form factor from the integrated intensity detected in vortex lattices in superconductors [2]. We illustrate this with data obtained from high magnetic field measurements on the high-temperature superconductors YBa₂Cu₃O₇ [3] and Ca_0.15Y_0.85Ba₂Cu₃O₇ [4] at HZB (Berlin).  As a result of this, we have shown that in YBa₂Cu₃O₇ and Ca_0.15Y_0.85Ba₂Cu₃O₇ [3,4], there are deviations from the London model at the highest fields measured. Also comparing with Ca_0.04Y_0.96Ba₂Cu₃O₇ results [4], here we present that for the Ca_xY_1-xBa₂Cu₃O₇ compounds, the series of vortex lattice structure transitions have shifted down in field relative to those reported for the undoped compound. We attribute this mainly to the weakening of the 1-D superconductivity in the Cu-O chains by the disorder introduced by doping. The high-field structure of the vortex lattice is similar to recent measurements on the parent compound in fields of 25 T, which indicates that the fundamental d-wave nature of the superconducting gap is unchanged by calcium doping.

[1] D. K. Christen et al., Phys. Rev. B, 15, 4506, (1977).

[2] E. Campillo, et al., J. Appl. Cryst. 55, 1314-1323 (2022).

[3] E. Campillo, et al., Phys. Rev. B, 105, 184508 (2022).

[4] A. Cameron & E. Campillo, et al., arXiv:2208.06706.

    

Magnetic field-induced abrupt enhancement in critical current density in LSMO/YBCO/LSMO/STO heterostructures

In this study, we report the influence of the complex coupling at the high-Tc-superconductor/oxide-ferromagnetic interface of LSMO/YBCO/LSMO trilayer heterostructures in terms of the superconducting critical current (I_c). We grew epitaxial tri-layer thin films by using the PLD technique, where LSMO bottom and top layer thicknesses were 10 nm and 15 nm, respectively, while the thickness of the middle YBCO layer was 15nm and 30 nm for the LY₁₅L and the LY₃₀L heterostructure respectively. In order to study the interface effect, the initial interface conditions (the work function difference, the orbital hybridization, and the magnetic coupling between the induced Cu and Mn moments) were preset by a cooling process with various field-cooled fields (FCF). In the FCF process, the sample was cooled down from 120K to 40K under a magnetic field parallel to the film surface and along the direction of the applied in-plane current. The I_c as a function of the FCF shows that I_c initially increases at lower FCF and attains maximum I_c at the intermediate range and then it starts to decrease with further increase in FCF. We observed the critical current I_c was greatly enhanced by ~18% at 0.5T for the LY₁₅L and ~90% at 0.6T for the LY₃₀L heterostructures. Moreover, we also observed the I_c increases with the increase in the applied magnetic field (AF) for the LY₁₅L heterostructure. This behavior is opposite to a normal superconducting system where I_c decreases with the increase in the applied magnetic field, and strongly implies the possible existence of triplet superconducting pairs at the interface and inside the FM layer. We proposed phenomenological and combination models which are based on singlet and triplet superconductivity mechanisms to explain the enhancement in critical current at the interface of LSMO/YBCO/LSMO heterostructures.

 

    

The lack of Zero-biased conductance peaks at the vortex core of cuprates

Recently evidence of zero-bias conductance peaks (ZBCP) in Bi-based cuprate is reported by using spectroscopic imaging scanning tunneling microscopy (SISTM) under the extremely low field for heavily doped samples [1]. This result rekindles a long-standing puzzle about the lack of ZBCP in Bi-based cuprates in most previous STM results.  The aim of this study is to investigate the ZBCP by using renormalized mean field theory (RMFT) under a magnetic field. As in the previous RMFT study [2-5], pair density wave solution from RMFT has been applied with the Wannier function to obtain the continuum Green's function or conductance.  In this work, the electronic state in the vortex core will be shown with dopant dependence and field dependence. The physics behind the absence of ZBCP will also be discussed.

 

 

[1]Wang-MacDonald d-Wave Vortex Cores Observed in Heavily Overdoped  Phys. Rev. B, 52, 6, R3876, 2021 (Gadic et al.)

[2]. Genesis of charge orders in high temperature superconductors, Scientific Reports 6, 18675 (2016), (Wei-Lin Tu and T. K. Lee).

[3]. Evolution of pairing orders between pseudogap and superconducting phases of Cuprate Superconductors, Scientific Reports 9, 1719 (2019) (Wei-Lin Tu and T. K. Lee).

[4]. Incommensurate charge ordered states in the t-t’-J model, New J. Phys. 19, 013028 (2017), (Peayush Choubey, Wei-Lin Tu, T.K. Lee and P. J. Hirschfeld).

    

Hall anomaly and vortex charge in Bi2Sr2CaCu2Ox (Bi2212)

The sign reversal of the Hall coefficient in the cuprates superconductors, the "Hall anomaly", is still an open question in the field of high-Tc superconductors.

The Hall sign change was attributed to different effects, including various vortex dynamics, competing phases to superconductivity, and vortex charge among others.

We present a systematic study of the Hall conductance in Bi2Sr2CaCu2Ox (Bi2212) thin films over a large

range of doping. We find that in a large part of the phase diagram the Hall coefficient changes sign as a function

of temperature in the flux-flow regime. By comparing data from many samples, we show that the sign reversal

is tied to the superconducting transition and is not a result of a competing order. The measurement in the flux-flow regime allows us to access the superconductivity-suppressed state which is present in the vortex cores, otherwise accessible only by the use of very high magnetic fields. We then compare our data

to the predictions of the Bardeen-Stephan model and show that in all samples there is an additional negative

contribution to the Hall conductivity. We extract from the negative excess Hall a vortex-charge that is found to

be strongly doping dependent.   

Superfluid density in overdoped cuprates: Thin films versus bulk samples

Recent study of overdoped La$_{2−x}$Sr$_x$CuO$_4$ cuprate superconductor thin films has revealed

several unexpected findings, most notably the violation of the BCS description which was believed to adequately

describe overdoped cuprates. In particular, it was found that the superfluid density in La$_{2−x}$Sr$_x$CuO$_4$

films decreases on the overdoped side as a linear function of critical temperature T$_c$, which was taken as evidence

for the violation of Homes’ law. We show explicitly that the law is indeed violated, and as the main reason

for violation we find that the superfluid density in La$_{2−x}$Sr$_x$CuO$_4$ films is suppressed more strongly than in bulk

samples. Based on the existing literature data, we show that the superfluid density in bulk cuprate samples does

not decrease with doping, but instead tends to saturate on the overdoped side. The result is also supported by our

recent measurement of a heavily overdoped bulk La$_{2−x}$Sr$_x$CuO$_4$ sample. Moreover, this saturation of superfluid

density might not be limited to cuprates, as we find evidence for similar behavior in two pnictide superconductor

families. We argue that quantum phase fluctuations play an important role in suppressing the superfluid density

in thin films.   

Doping dependence of a quasi-1D cuprate investigated by resonant inelastic X-ray scattering

The experimental investigation of one-dimensional spin chains is an important angle to benchmark theoretical frameworks tackling the physics of strongly correlated electron systems, such as the Hubbard model. Yet, relevant experimental studies are scarce due to the lack of dope-able 1D materials. Recently, thin films of the quasi-1D cuprate Ba_2-xCuO_3+d (BCO) were synthesized and successfully hole doped using molecular beam epitaxy (MBE). Here we present resonant inelastic x-ray scattering (RIXS) results on these quasi-1D spin chains. We identify dominant electronic and magnetic contributions to the RIXS cross section and the dependence of these spectral signatures as a function of doping. From these measurements, we find robust signatures of dispersive two-spinon and orbital excitations for doping concentrations exceeding 20%. Furthermore, we perform a comparison with numerical results invoking an extended 1D Hubbard model. The potential implications of our findings for higher dimensional cuprate systems will also be discussed.