Tomographic mapping of the hidden dimension in quasi-particle interference

Quasiparticle interference (QPI) imaging is well established to study the low-energy electronic structure in strongly correlated electron materials with unrivalled energy resolution. Yet, being a surface-sensitive technique, the interpretation of QPI only works well for anisotropic materials, where the dispersion in the direction perpendicular to the surface can be neglected and the quasiparticle interference is dominated by a quasi-2D electronic structure. Here, we explore QPI imaging of galena, a material with an electronic structure that does not exhibit pronounced anisotropy. We find that the quasiparticle interference signal is dominated by scattering vectors which are parallel to the surface plane however originate from bias-dependent cuts of the 3D electronic structure. We develop a formalism for the theoretical description of the QPI signal and demonstrate how this quasiparticle tomography can be used to obtain information about the 3D electronic structure and orbital character of the bands.

Conduction Band Engineering of Half-Heusler Thermoelectrics Using Orbital Chemistry

Semiconducting half-Heusler (HH, XYZ) phases are promising thermoelectric materials owing to their versatile electronic properties. Because the valence band of half-Heusler phases benefit from the valence band extrema at several high-symmetry points in the Brillouin zone (BZ), it is possible to engineer better p-type HH materials through band convergence. However, the thermoelectric studies of n-type HH phases have been lagging behind since the conduction band minimum is always at the same high-symmetry point (X) in the BZ, giving the impression that there is little opportunity for band engineering. Here we study the n-type orbital diagram of 69 HHs, and show that there are two competing conduction bands with very different effective masses actually at the same X point in the BZ, which can be engineered to be converged. The two conduction bands are dominated by the d orbitals of X and Y atoms, respectively. The energy offset between the two bands depends on the difference in electron configuration and electronegativity of the X and Y atoms. Based on the orbital phase diagram, we provide the strategy to engineer the conduction band convergence by mixing the HH compounds with the reverse band offsets. We demonstrate the strategy by alloying VCoSn and TaCoSn. The V0.5Ta0.5CoSn mixture presents the high conduction band convergence and corresponding significantly larger density-of-states effective mass than either VCoSn or TaCoSn. Our work indicates that analyzing the orbital character of band edges provides new insight into engineering thermoelectric performance of HH compounds.

The Role of Orbital Nesting in the Superconductivity of Iron-Based Superconductors

We analyze the magnetic excitations and the spin-mediated superconductivity in iron-based superconductors within a low energy model that operates in the band basis, but fully incorporates the orbital character of the spin excitations. We show how the orbital selectivity, encoded in our low energy description, simplifies substantially the analysis and allows for analytical treatments, while retaining all the main features of both spin excitations and gap functions computed using multiorbital models. Importantly, our analysis unveils the orbital matching between the hole and electron pockets as the key parameter to determine the momentum dependence and the hierarchy of the superconducting gaps, instead of the Fermi surface matching, as in the common nesting scenario.

Light-induced enhancement of superconductivity in iron-based superconductor FeSe0.5Te0.5

Illumination of light on matter normally causes heating and destroys the ordered ground states. Despite this common understanding, recent advances in ultrafast light sources have enabled the non-thermal control of quantum phases. Here, we report the light-induced enhancement of superconductivity in a thin film of an iron chalcogenide FeSe0.5Te0.5, which exhibits multiple quantum condensates associated with the multi-orbital character. Upon the photoexcitation, we observed a transient increase of the superfluid density as indicated by the optical conductivity in the frequency range of superconducting gaps. The light-induced enhancement of superconductivity is further corroborated by the photoinduced enhancement of terahertz third harmonic generation, which is accounted for by the Higgs mode response. The ultrafast dynamics of two superfluid components revealed by frequency- and time-resolved terahertz measurements indicate the interplay between the condensates through the interband Cooper pairings while suggesting the potential tunability of the pairing interaction by light in the ultrafast timescale.

Clock Transition Due to a Record 1240 G Hyperfine Interaction in a Lu(II) Molecular Spin Qubit

Spins in molecules are particularly attractive targets for next-generation quantum technologies, enabling chemically programmable qubits and potential for scale-up via self-assembly. Here, we demonstrate chemical control of the degree of s-orbital mixing into the spin-bearing d-orbital associated with a series of spin-½ La(II) and Lu(II) molecules. Increased s-orbital character reduces spin-orbit coupling and enhances the electron-nuclear Fermi contact interaction. Both outcomes are beneficial for quantum applications: the former reduces spin-lattice relaxation, while the latter gives rise to a record molecular hyperfine interaction for Lu(II) that, in turn, generates a massive 9 GHz hyperfine clock transition and an order of magnitude increase in phase memory time. These findings suggest new strategies for development of molecular quantum technologies, akin to trapped ion systems.

Clock Transition Due to a Record 1240 G Hyperfine Interaction in a Lu(II) Molecular Spin Qubit

Spins in molecules are particularly attractive targets for next-generation quantum technologies, enabling chemically programmable qubits and potential for scale-up via self-assembly. Here, we demonstrate chemical control of the degree of s-orbital mixing into the spin-bearing d-orbital associated with a series of spin-½ La(II) and Lu(II) molecules. Increased s-orbital character reduces spin-orbit coupling and enhances the electron-nuclear Fermi contact interaction. Both outcomes are beneficial for quantum applications: the former reduces spin-lattice relaxation, while the latter gives rise to a record molecular hyperfine interaction for Lu(II) that, in turn, generates a massive 9 GHz hyperfine clock transition and an order of magnitude increase in phase memory time. These findings suggest new strategies for development of molecular quantum technologies, akin to trapped ion systems.

Author Correction: Long-ranged Cu-based order with $$d_{z^2}$$ orbital character at a YBa2Cu3O7/manganite interface

A Correction to this paper has been published: https://doi.org/10.1038/s41535-021-00323-8

Long-ranged Cu-based order with $$d_{z^2}$$ orbital character at a YBa2Cu3O7/ manganite interface

The interplay of nearly degenerate orders in quantum materials can lead to a myriad of emergent phases. A prominent case is that of the high-Tc cuprates for which the relationship between superconductivity and a short-ranged, incommensurate charge density wave in the CuO2 planes involving the $$d_{x^2 - y^2}$$ d x 2 − y 2 orbitals (Cu-CDW) is a subject of great current interest. Strong modifications of the strength and coherence of this Cu-CDW have been achieved by applying large magnetic fields, uniaxial pressure, or via the interfacial coupling in cuprate/manganite multilayers. However, such modifications do not alter the dominant orbital character. Here we investigate cuprate/manganite multilayers with resonant inelastic X-ray scattering (RIXS) and show that a new kind of Cu-based density wave order can be induced that has not been previously observed in the cuprates. This order has an unusually small in-plane wave vector in the range of Q|| < 0.1 reciprocal lattice units (r.l.u.), a large correlation length of about 40 nm, and a predominant $$d_{z^2}$$ d z 2 orbital character, instead of the typical $$d_{x^2 - y^2}$$ d x 2 − y 2 one. Its appearance is determined by the hole doping of the manganite which is a key parameter controlling the interfacial charge transfer and orbital reconstruction. We anticipate that the observation of a previously unknown type of density wave order at the YBCO interface will allow for fresh perspectives on the enigmatic relation between superconductivity and charge order (CO) in the cuprates.

Multiorbital charge-density wave excitations and concomitant phonon anomalies in Bi2Sr2LaCuO6+δ

Charge-density waves (CDWs) are ubiquitous in underdoped cuprate superconductors. As a modulation of the valence electron density, CDWs in hole-doped cuprates possess both Cu-3dand O-2porbital character owing to the strong hybridization of these orbitals near the Fermi level. Here, we investigate underdoped Bi2Sr1.4La0.6CuO6+δusing resonant inelastic X-ray scattering (RIXS) and find that a short-range CDW exists at both Cu and O sublattices in the copper-oxide (CuO2) planes with a comparable periodicity and correlation length. Furthermore, we uncover bond-stretching and bond-buckling phonon anomalies concomitant to the CDWs. Comparing to slightly overdoped Bi2Sr1.8La0.2CuO6+δ, where neither CDWs nor phonon anomalies appear, we highlight that a sharp intensity anomaly is induced in the proximity of the CDW wavevector (QCDW) for the bond-buckling phonon, in concert with the diffused intensity enhancement of the bond-stretching phonon at wavevectors much greater than QCDW. Our results provide a comprehensive picture of the quasistatic CDWs, their dispersive excitations, and associated electron-phonon anomalies, which are key for understanding the competing electronic instabilities in cuprates.