Tracing the dynamics of superconducting order via transient terahertz third-harmonic generation.

Ultrafast optical control of quantum systems is an emerging field of physics. In particular, the possibility of light-driven superconductivity has attracted much of attention. To identify nonequilibrium superconductivity, it is necessary to measure fingerprints of superconductivity on ultrafast timescales. Recently, nonlinear THz third-harmonic generation (THG) was shown to directly probe the collective degrees of freedoms of the superconducting condensate, including the Higgs mode. Here, we extend this idea to light-driven nonequilibrium states in superconducting La2-xSrxCuO4, establishing an optical pump-THz-THG drive protocol to access the transient superconducting order-parameter quench and recovering on few-picosecond timescales. We show in particular the ability of two-dimensional TH spectroscopy to disentangle the effects of optically excited quasiparticles from the pure order-parameter dynamics, which are unavoidably mixed in the pump-driven linear THz response. Benchmarking the gap dynamics to existing experiments shows the ability of driven THG spectroscopy to overcome these limitations in ordinary pump-probe protocols.

Spectral signatures of a unique charge density wave in Ta2NiSe7.

Charge Density Waves (CDW) are commonly associated with the presence of near-Fermi level states which are separated from others, or "nested", by a wavector of q. Here we use Angle-Resolved Photo Emission Spectroscopy (ARPES) on the CDW material Ta2NiSe7 and identify a total absence of any plausible nesting of states at the primary CDW wavevector q. Nevertheless we observe spectral intensity on replicas of the hole-like valence bands, shifted by a wavevector of q, which appears with the CDW transition. In contrast, we find that there is a possible nesting at 2q, and associate the characters of these bands with the reported atomic modulations at 2q. Our comprehensive electronic structure perspective shows that the CDW-like transition of Ta2NiSe7 is unique, with the primary wavevector q being unrelated to any low-energy states, but suggests that the reported modulation at 2q, which would plausibly connect low-energy states, might be more important for the overall energetics of the problem.

Fano interference between collective modes in cuprate high-Tc superconductors.

Cuprate high-Tc superconductors are known for their intertwined interactions and the coexistence of competing orders. Uncovering experimental signatures of these interactions is often the first step in understanding their complex relations. A typical spectroscopic signature of the interaction between a discrete mode and a continuum of excitations is the Fano resonance/interference, characterized by the asymmetric light-scattering amplitude of the discrete mode as a function of the electromagnetic driving frequency. In this study, we report a new type of Fano resonance manifested by the nonlinear terahertz response of cuprate high-Tc superconductors, where we resolve both the amplitude and phase signatures of the Fano resonance. Our extensive hole-doping and magnetic field dependent investigation suggests that the Fano resonance may arise from an interplay between the superconducting fluctuations and the charge density wave fluctuations, prompting future studies to look more closely into their dynamical interactions.

Emergent Magnetic Fan Structures in Manganite Homojunction Arrays.

Devices with tunable magnetic noncollinearity are important components of superconducting electronics and spintronics, but they typically require epitaxial integration of several complex materials. The spin-polarized neutron reflectometry measurements on La1-x Srx MnO3 homojunction arrays with modulated Sr concentration reported herein have led to the discovery of magnetic fan structures with highly noncollinear alignment of Mn spins and an emergent periodicity twice as large as the array's unit cell. The neutron data show that these magnetic superstructures can be fully long-range ordered, despite the gradual modulation of the doping level created by charge transfer and chemical intermixing. The degree of noncollinearity can be effectively adjusted by low magnetic fields. Notwithstanding their chemical and structural simplicity, oxide homojunctions thus show considerable promise as a platform for tunable complex magnetism and as a powerful design element of spintronic devices.

Morphologic Characteristics of the Sigmoid Notch of the Distal Radius for Patients With Peripheral Triangular Fibrocartilage Complex Tear.

The distal radioulnar joint (DRUJ) is stabilized by the bony anatomy of the contact surfaces. The authors analyzed the morphologic characteristics and radiologic parameters at the sigmoid notch of patients with a peripheral triangular fibrocartilage complex (TFCC) tear compared with asymptomatic patients. Preoperative axial computed tomography scans were reviewed for 76 wrists with peripheral TFCC injuries, including foveal avulsion, and 76 wrists of age- and sex-matched control subjects. The authors used axial computed tomography scans of the DRUJ to classify the patients into 4 groups according to the type of sigmoid notch, namely, flat face, ski-slope, C-type, and S-type. They also measured the tilting angle, depth, width of the sigmoid notch, and radioulnar ratio (RUR). Statistical analyses were performed with the chi-square test or paired t test (P<.05). The mean proportions of flat face, ski-slope, C-type, and S-type sigmoid notches among patients with peripheral TFCC injuries were 42%, 22%, 29%, and 7%, respectively, whereas those for the control group were 33%, 1%, 65%, and 1%, respectively (P<.05). The tilting angle was lower (TFCC injury, 84.5°; control, 86.2°; P<.05) and the RUR was significantly higher (TFCC injury, 0.67; control, 0.56) in the TFCC group, particularly for men (P<.05). Depth (TFCC injury, 1.0 mm; control, 1.3 mm; P>.05) and width (TFCC injury, 14.8 mm; control, 14.5 mm; P>.05) were similar between the groups. Patients with ski-slope or dorsally tilted sigmoid notches may be at greater risk for peripheral TFCC injuries. [Orthopedics. 2021;44(6):e729-e734.].

Atomic-Scale Tuning of the Charge Distribution by Strain Engineering in Oxide Heterostructures.

Strain engineering of complex oxide heterostructures has provided routes to explore the influence of the local perturbations to the physical properties of the material. Due to the challenge of disentangling intrinsic and extrinsic effects at oxide interfaces, the combined effects of epitaxial strain and charge transfer mechanisms have been rarely studied. Here, we reveal the local charge distribution in manganite slabs by means of high-resolution electron microscopy and spectroscopy via investigating how the strain locally alters the electronic and magnetic properties of La0.5Sr0.5MnO3-La2CuO4 heterostructures. The charge rearrangement results in two different magnetic phases: an interfacial ferromagnetically reduced layer and an enhanced ferromagnetic metallic region away from the interfaces. Further, the magnitude of the charge redistribution can be controlled via epitaxial strain, which further influences the macroscopic physical properties in a way opposed to strain effects reported on single-phase films. Our work highlights the important role played by epitaxial strain in determining the spatial distribution of microscopic charge and spin interactions in manganites and provides a different perspective for engineering interface properties.

Optical conductivity and superconductivity in highly overdoped La2-x Ca x CuO4 thin films.

We have used atomic layer-by-layer oxide molecular beam epitaxy to grow epitaxial thin films of [Formula: see text] with x up to 0.5, greatly exceeding the solubility limit of Ca in bulk systems ([Formula: see text]). A comparison of the optical conductivity measured by spectroscopic ellipsometry to prior predictions from dynamical mean-field theory demonstrates that the hole concentration p is approximately equal to x. We find superconductivity with [Formula: see text] of 15 to 20 K up to the highest doping levels and attribute the unusual stability of superconductivity in [Formula: see text] to the nearly identical radii of La and Ca ions, which minimizes the impact of structural disorder. We conclude that careful disorder management can greatly extend the "superconducting dome" in the phase diagram of the cuprates.

Proximity effect in [Nb(1.5 nm)/Fe(x)]10/Nb(50 nm) superconductor/ferromagnet heterostructures.

We have investigated the structural, magnetic and superconduction properties of [Nb(1.5 nm)/Fe(x)]10 superlattices deposited on a thick Nb(50 nm) layer. Our investigation showed that the Nb(50 nm) layer grows epitaxially at 800 °C on the Al2O3(1-102) substrate. Samples grown at this condition possess a high residual resistivity ratio of 15-20. By using neutron reflectometry we show that Fe/Nb superlattices with x < 4 nm form a depth-modulated FeNb alloy with concentration of iron varying between 60% and 90%. This alloy has weak ferromagnetic properties. The proximity of this weak ferromagnetic layer to a thick superconductor leads to an intermediate phase that is characterized by a suppressed but still finite resistance of structure in a temperature interval of about 1 K below the superconducting transition of thick Nb. By increasing the thickness of the Fe layer to x = 4 nm the intermediate phase disappears. We attribute the intermediate state to proximity induced non-homogeneous superconductivity in the structure.

Phase-resolved Higgs response in superconducting cuprates.

In high-energy physics, the Higgs field couples to gauge bosons and fermions and gives mass to their elementary excitations. Experimentally, such couplings can be inferred from the decay product of the Higgs boson, i.e., the scalar (amplitude) excitation of the Higgs field. In superconductors, Cooper pairs bear a close analogy to the Higgs field. Interaction between the Cooper pairs and other degrees of freedom provides dissipation channels for the amplitude mode, which may reveal important information about the microscopic pairing mechanism. To this end, we investigate the Higgs (amplitude) mode of several cuprate thin films using phase-resolved terahertz third harmonic generation (THG). In addition to the heavily damped Higgs mode itself, we observe a universal jump in the phase of the driven Higgs oscillation as well as a non-vanishing THG above Tc. These findings indicate coupling of the Higgs mode to other collective modes and potentially a nonzero pairing amplitude above Tc.

Suppression of Three-Dimensional Charge Density Wave Ordering via Thickness Control.

Barium bismuth oxide (BaBiO_{3}) is the end member of two families of high-T_{c} superconductors, i.e., BaPb_{1-x}Bi_{x}O_{3} and Ba_{1-x}K_{x}BiO_{3}. The undoped parent compound is an insulator, exhibiting a charge density wave that is strongly linked to a static breathing distortion in the oxygen sublattice of the perovskite structure. We report a comprehensive spectroscopic and x-ray diffraction study of BaBiO_{3} thin films, showing that the minimum film thickness required to stabilize the breathing distortion and charge density wave is ≈11 unit cells, and that both phenomena are suppressed in thinner films. Our results constitute the first experimental observation of charge density wave suppression in bismuthate compounds without intentionally introducing dopants.