Session L30: Copper Oxide Superconductors: Spectroscopy

Resolving the Octahedral Tilt Order in La2-xSrxCuO4

The pattern of tilts of CuO₆ octahedra in 214 cuprates can have a large impact on the electronic order. For example, a phase in which the octahedra rotate about the Cu-O bonds is known to pin stripe order in La_2-xBa_xCuO₄. In La_2-xSr_xCuO₄, it has been believed that the octahedral rotations are only about an axis at 45° to the Cu-O bonds. In a recent neutron scattering study of an x = 0.07 crystal, we found evidence that there is also a tilt component about the Cu-O bonds [1]. To further explore the situation, we have performed a crystallographic study of the parent compound, La₂CuO₄, on the 4-circle diffractometer HB-3A at HFIR (ORNL). While confirming that the crystal has monoclinic symmetry, as previously reported, we have verified a new set of superlattice peaks (hkl) with h and k odd, l even. These reflections are consistent with a tilt component about the Cu-O bonds and require a revision of the space group.
[1] H. Jacobsen et al., Phys. Rev. B 92, 174525 (2015).
  

Quasi-Static Internal Magnetic Field Detected in the Pseudogap Phase of Bi2+xSr2−xCaCu2O8+δ by μSR

We report muon spin relaxation (μSR) measurements of optimally-doped and overdoped Bi_2+xSr_2−xCaCu₂O_8+δ (Bi2212) single crystals that reveal the presence of a weak temperature dependent quasi-static internal magnetic field of electronic origin in the superconducting (SC) and pseudogap (PG) phases. In both samples the internal magnetic field persists up to 160 K, but muon diffusion prevents following the evolution of the field to higher temperatures. We consider the evidence from our measurments in support of PG order parameter candidates, namely, electronic loop currents and magnetoelectric quadrupoles.   

Oxygen-isotope effects in Bi2Sr2CaCu2O8+y from deeply underdoped to heavily overdoped regimes

Oxygen-isotope effects have been studied in double-layer superconducting Bi₂Sr₂CaCu₂O_8+y in a wide doping range, which crosses a region from the deeply underdoped to the heavily overdoped regimes. The doping levels of the samples are controlled by varying the oxygen content, which can be achieved through annealing them in different oxygen partial pressures up to 600 bars. The oxygen-isotope effect on the superconducting transition temperature T_c is the smallest for optimal doping and increases systematically as T_c drops in both the underdoped and overdoped regions. The present result provides an essential constraint on the microscopic pairing mechanism of high-temperature superconductivity.   

Field Induced Density Wave State in Cuprates

Cuprate superconductors are the prototypical example of a complex condensate where superconductivity may co-exist and intertwine with charge and spin orders. Many questions regarding the cuprate phase diagram remain: “What symmetries are broken?, “Which order parameters are in the driving seat?” and “What is the nature of the high magnetic field state?”. To address these questions I will present spectroscopic imaging STM measurements of Bi₂Sr₂CaCu₂O_8+δ as a function of magnetic field.   

Optical studies of symmetry breaking in cuprate superconductors

We probe the spectral character of broken symmetry states in high –temperature cuprate superconductors (HTS) using polarization sensitive Faraday rotation measurements. We measure the complex Faraday angleat zero magnetic field on a series of HTS thin films with various levels of doping as a function of energy (0.1-3 eV), temperature (10-300K) and sample orientation. We observe sinusoidal variation of Faraday rotation angle with respect to the sample orientation (linear dichroism) which indicates the presence of broken linear symmetry. The amplitude of the linear dichroism signal is strongest in the underdoped films and increases with decreasing temperature.   

Terahertz Dynamics in the Stripe-ordered Cuprate La2-xBaxCuO4 (x = 0.115) Following Near-infrared Excitation

Numerous cuprate-based superconductors exhibit a complex interplay between spin order, charge order and superconductivity. We utilize THz time domain spectroscopy to monitor the c-axis Josephson Plasma Resonance (JPR) as a direct probe of the superconducting condensate following excitation with 1.55 eV pulses. La_2-xBa_xCuO₄ (x=0.115) exhibits charge order below ~50K, with an onset of spin order at ~40K and a superconducting transition temperature of T_C = 13K [1]. Recent work on this compound has suggested a brief enhancement of superconductivity following photoexcitation [2] which has motivated further investigation. We show that starting from the superconducting state, photoexcitation with 1.5eV near infrared pulses results in a novel electrodynamic response which persists for >300ps. In particular, c-axis polarized excitation with fluences <1mJ/cm² induces a new plasma edge around ~0.8THz along with a commensurate increase in the spectral weight below 0.5THz. For photoexcitation at temperatures above TC, a qualitatively similar state emerges that decays on the order of several picoseconds. [1] C. C. Homes, et al, Phys. Rev. B 85, 134510 (2012) [2] D. Nicoletti, et al, Phys. Rev. B 90, 100503(R) (2014)   

Magnetic Field Effect and Phonon Anomalies in 214 La-based Cuprates.

ABSTRACT.
The competition between superconductivity and electronic periodicities is a central theme in condensed matter physics, since it has shed light into the complexity of the phase diagram of cuprates. Understanding this relationship with other Fermi surface instabilities is essential to develop a satisfactory theory of superconductivity.
214 La-based cuprates [1], YBa₂Cu₃O_6+δ (YBCO) and related compounds exhibit a momentum and field dependence of short ranged, weakly correlated charge periodicities (Charge Density Wave, CDW) for doping levels 0.08≤p≤0.15 [2]. Here, I will present Resonant X Ray Scattering measurements at the Cu L- and O K-edges that indicate a strong stability of the stripe order in the Low Temperature Tetragonal (LTT) phase. In addition, Inelastic X Ray Scattering shows a softening and broadening of the low energy acoustic phonon at Q_CO below the superconducting temperature T_C. The data indicate a strong influence of the lattice symmetry and e-ph coupling in the stabilization of the charge stripe order and reveals nesting properties of the Fermi surface [3].
References
[1] J. M. Tranquada et al. Nature 375, 561 (1995).
[2] G. Ghiringhelli et al. Science 337, 821 (2012).
[3] S. Blanco-Canosa et al. in preparation.   

Modulated Susceptibility in Thin Film LBCO

In addition to being the first instance of high temperature superconductivity in the cuprate perovskite superconductors, lanthanum barium copper oxide (La_2-xBa_xCuO₄) has garnered recent interest for nematicity and interesting behavior at 1/8^th doping. In this experiment, we used Scanning Superconducting Quantum Interference Device (SQUID) Microscopy to spatially resolve the magnetic fields and susceptibility in a number of thin film LBCO samples with doping in the vicinity of the 1/8^th anomaly. We measured the critical temperatures and superfluid densities of the samples at various positions. We observed strong stripe-like modulations in the susceptibility (“striae”) of multiple samples with the surprisingly long periods of 2-4um. The periods of these striae are 1000 times longer than the periods of stripes observed in similar samples using such techniques as neutron diffraction. We speculate on the sources of these effects.   

Signatures of pair-density wave order via phase sensitive measurement of La2−xBaxCuO4 and La2-x-yEuySrxCuO4 SQUIDs and Josephson junctions

La_2−xBa_xCuO₄ (LBCO) exhibits a sharp drop in the transition temperature near x=1/8 doping, which coincides with charge order, spin order and the suppression of interlayer coupling between its CuO₂ planes. In this regime, charge, spin and superconducting orders are predicted to be intertwined into a pair-density wave (PDW) state, an ordered stripe phase characterized by sign changes in the superconducting order parameter between adjacent stripes. We present Josephson interferometry measurements of asymmetric SQUIDs incorporating crystals of LBCO at x=1/8 doping, from which we can extract the current-phase relation (CPR). We see features which are suggestive of a cancellation of the first-order Josephson tunneling and the onset of higher harmonics in the CPR, a signature of a pair-density wave state. We compare this data to measurements of LBCO devices at other dopings, as well as devices fabricated onto the striped material La_2-x-yEu_ySr_xCuO₄ (LESCO), another candidate for the formation of a PDW-state.   

Resonant Soft X-ray Scattering studies with Transition Edge Sensors

Resonant Soft X-ray has been one of the key techniques to study charge orders in high Tc cuperates. To solve the issue of unwanted enhancement of inelastic florescence background at resonance, we have developed an energy-resolving superconducting Transition-Edge Sensor microcalorimeters. These superconducting sensors obtain exquisite energy resolution by exploiting the superconducting-to-normal transition to photon energy at cryogenic temperatures (∼ 70 mK) where thermal noise is minimal. This TES has demonstrated ∼ 1.5 eV resolution around Cu L₃ edge (932.7 eV). We present first results of using this detector to study elastic diffuse scattering and charge orders from a single crystal of stripe-ordered La_2-xBa_xCuO₄ (x=0.125). Use of this detector for studying excitations and models of rejecting fluorescence background will be further discussed.   

Revised NMR Shift Phenomenology for Cuprate Superconductors

We have compiled an unbiased overview of the planar Cu NMR shifts of the superconducting cuprates based on all available NMR literature data from the last three decades. Resulting plots reveal rather astonishing features. First of all, they show that the hitherto employed models of the cuprate hyperfine scenario must be wrong. Without invoking a particular model, we point to a number of salient features and trends in the data. For example, a large isotropic shift component must be present in the strongly overdoped cuprates and disappears at lower doping levels; while the low-temperature shift vanishes for all materials if the magnetic field lies in the plane, the hallmark of spin singlet pairing, a substantial shift remains at the lowest temperatures if the magnetic field is perpendicular to the planes; revised orbital shifts become apparent, which seem to solve the old rift between experimentally deduced orbital shifts and first-principle calculations. We also discuss the data in the context of recent insights from NMR, like the material-specific charge distribution that controls the maximum T_c and superfluid density, or the material-specific nuclear relaxation rate 1/T₁.   

Towards a new interpretation of NMR and cuprate electronic properties

Recently, for a number of different materials we established that a single spin component cannot explain the NMR shifts, pointing also to a different magnetic hyperfine scenario. Driven by these finding we compiled literature shift data for planar Cu, from which it becomes obvious, now, that the hitherto adopted interpretation of the NMR shifts is wrong, e.g., a large isotropic shift reigns on the strongly overdoped, Fermi liquid side of the phase diagram and starts disappearing as doping and/or temperature are lowered. Also recently, we showed that the charges in the CuO2 plane can be quantified with NMR, which, e.g., led to the discovery that the sharing of holes between Cu and O (not the doping) is responsible for various cuprate properties, e.g., their maximum Tc. In yet another set of experiments we solved a long-standing NMR conundrum that considered Y-1237 and Y-1248 to be very ‘homogeneous’ materials, while most other cuprates show large electric field variations in the plane. We find that these homogeneous systems are in fact highly charge ordered systems. This charge ordering that responds to pressure, temperature, and magnetic field is likely to be ubiquitous to the CuO2 plane as literature shows.   

Percolative Superconducting Pre-pairing in Cuprates

One of the important open questions in the physics of cuprate superconductors is the nature of superconducting pre-pairing above T_c. We present fundamental new insight into the pre-pairing regime, using a combination of techniques and several cuprate systems. We employ an unconventional probe – nonlinear conductivity – which is zero in the normal state, to find that the signals above T_c show an universal temperature dependence, irrespective of hole doping and cuprate family. Similar behavior is observed in nonlinear torque magnetization measurements, which unequivocally separate superconducting diamagnetism from normal-state paramagnetism, and in linear conductivity experiments. The results are incompatible with Ginzburg-Landau theory, but also rule out an extended temperature range of superconducting fluctuations. To explain our data, we employ a simple percolative model based on inhomogeneity of superconducting gaps. The calculated linear and nonlinear conductivity show excellent agreement with experiment, and the magnetization is explained qualitatively. The pre-pairing is thus controlled by intrinsic, universal nanoscale gap inhomogeneity.   

Imaging Gap Nodal Structure of Unconventional Superconductors through the Anisotropic Nonlinear Meissner Effect

We present a new measurement method which can be used to image gap nodal structure of superconductors whose pairing symmetry is unknown. This method utilizes photoresponse from a microwave resonance of the superconducting sample perturbed by a scanned laser spot. For an epitaxial thin film or single crystal sample, the anisotropy of this photoresponse is directly related to that of the gap function via the non-linear Meissner coefficient, so the gap nodal directions can be inferred from the photoresponse image. Also, this method is able to simultaneously measure the change in magnetic penetration depth, whose low temperature behavior gives an important clue for gap structure. By combining results for gap symmetry from photoresponse, and low temperature behavior of the penetration depth, the presented method can make a more conclusive judgement on the gap nodal structure and hence pairing symmetry. The data taken from an example unconventional superconductors will be presented and discussed.   

Van Hove exciton model of the cuprate pseudogap

There is a dichotomy of the Van Hove singularity (VHS) in cuprates, with competing instabilities at wave numbers q = (0,0) (diverging density-of-states [DOS]) and Q=(pi,pi). The latter dominates for small doping x and/or small second-neighbor hopping t', but in most cuprates the DOS anomaly and associated anti-nodal nesting dominate at higher x. The Q-VHS displays an intimate connection with pseudogap physics [1]. The strong sensitivity of the Q-VHS to Pauli blocking indicates a close connection with excitonic physics, suggesting that the Mott-Slater transition that we find is related to the BEC-BCS transition of an underlying excitonic insulator [2].


1 R.S. Markiewicz, I.G. Buda, P. Mistark, C. Lane, and A. Bansil, Scientific Reports 7, 44008 (2017).

2. R.S. Markiewicz and A. Bansil, arXiv:1708.2270