The nature of superconductivity in the dilute semiconductor SrTiO has remained an open question for more than 50 y. The extremely low carrier densities ([Formula: see text]-[Formula: see text] cm) at which superconductivity occurs suggest an unconventional origin of superconductivity outside of the adiabatic limit on which the Bardeen-Cooper-Schrieffer (BCS) and Migdal-Eliashberg (ME) theories are based. We take advantage of a newly developed method for engineering band alignments at oxide interfaces and access the electronic structure of Nb-doped SrTiO, using high-resolution tunneling spectroscopy. We observe strong coupling to the highest-energy longitudinal optic (LO) phonon branch and estimate the doping evolution of the dimensionless electron-phonon interaction strength ([Formula: see text]). Upon cooling below the superconducting transition temperature ([Formula: see text]), we observe a single superconducting gap corresponding to the weak-coupling limit of BCS theory, indicating an order of magnitude smaller coupling ([Formula: see text]). These results suggest that despite the strong normal state interaction with electrons, the highest LO phonon does not provide a dominant contribution to pairing. They further demonstrate that SrTiO is an ideal system to probe superconductivity over a wide range of carrier density, adiabatic parameter, and electron-phonon coupling strength.

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Resonant inelastic x-ray scattering (RIXS) has become an important tool for studying elementary excitations in correlated materials. Here, we present a systematic theoretical investigation of the Cu L-edge RIXS spectra of undoped and doped cuprate two-leg spin-ladders in both the non-spinconserving (NSC) and spin-conserving (SC) channels. The spectra are rich and host many exotic excitations. In the NSC-channel of the undoped case, we identify one-triplon and bound triplet two-triplon excitations in the strong-rung coupling limit, as well as confined spinons in the weakrung coupling limit. In the doped case, we observe a quasiparticle excitation formed from a bound charge and spin-1 2 in the strong-rung coupling limit. In the SC-channel, we also identify several new features, including bound singlet two-triplon excitations and confined spinons in the undoped ladders in the strong-and weak-rung coupling limits, respectively. Conversely, in the doped case, the SC channel primarily probes both gapless and gapped charge excitations. Finally, we revisit the available data for the ladder compound Sr14Cu24O41 in the context of our results. arXiv:1904.00530v1 [cond-mat.str-el] 1 Apr 2019

We report the observation of multiple phonon satellite features in ultra thin superlattices of form nSrIrO3/mSrTiO3 using resonant inelastic x-ray scattering. As the values of n and m vary the energy loss spectra show a systematic evolution in the relative intensity of the phonon satellites. Using a closed-form solution for the RIXS cross section, we extract the variation in the electronphonon coupling strength as a function of n and m. Combined with the negligible carrier doping into the SrTiO3 layers, these results indicate that tuning of the electron-phonon coupling can be effectively decoupled from doping. This work showcases both a feasible method to extract the electron-phonon coupling in superlattices and unveils a potential route for tuning this coupling which is often associated with superconductivity in SrTiO3-based systems.Despite the discovery of several new classes of superconductors, a comprehensive understanding of superconductivity continues to evade the community, preventing attempts to systamtically control its behavior. The discovery of superconductivity at the interface of two insulating compounds, SrTiO 3 (STO) and LaAlO 3 , is particularly promising for expanding our understanding of superconductivity due to the myriad of control parameters introduced by the heterostructure morphology [1][2][3][4]. Furthermore, superconductivity in monolayer FeSe was recently found to be remarkably enhanced by an order of magnitude when interfaced with STO [5-7]. These findings point to heterostructuring as a promising route towards the rational engineering of the superconducting ground state.While debate remains, the coupling of the conduction electrons to the longitudinal optical (LO 4 ) phonon branch is routinely regarded as an essential ingredient in STO-based superconductors [8][9][10][11][12][13]. Recent angleresolved photoemission spectroscopy (ARPES) experiments observed the systematic tuning of the electron- * dmeyers@bnl.gov † tberlijn@gmail.com ‡

The observation of a charge density wave in the underdoped cuprate high T c superconductors (Cu-CDW) raised a debate about its relationship with superconductivity. In bulk YBa 2 Cu 3 O 7−δ the Cu-CDW is incipient and mainly pinned by defects. Nevertheless, a large magnetic field can induce a true long-range Cu-CDW order as it suppresses superconductivity. An enhanced Cu-CDW order was also observed in YBa 2 Cu 3 O 7 /La 2/3 Ca 1/3 MnO 3 multilayers. Here, we show that the magnitude of the Cu-CDW in YBa 2 Cu 3 O 7−δ / Nd 0.65 (Ca 1y Sr y) 0.35 MnO 3 multilayers can be varied by adjusting the strength of the manganite charge and orbital order via the Sr content (tolerance factor). Furthermore, we resolve the reconstruction of the crystal field levels of the interfacial Cu ions that are also affected by the manganite charge and orbital order. This tuneable interfacial coupling and Cu-CDW in YBa 2 Cu 3 O 7−δ can be used for studying the relationship between the Cu-CDW and superconductivity and, possibly, for inducing new intertwined quantum states.

In ultrathin films of FeSe grown on SrTiO3 (FeSe/STO), the superconducting transition temperature Tc is increased by almost an order of magnitude, raising questions on the pairing mechanism. As in other superconductors, antiferromagnetic spin fluctuations have been proposed to mediate SC making it essential to study the evolution of the spin dynamics of FeSe from the bulk to the ultrathin limit. Here, we investigate the spin excitations in bulk and monolayer FeSe/STO using resonant inelastic x-ray scattering (RIXS) and quantum Monte Carlo (QMC) calculations. Despite the absence of long-range magnetic order, bulk FeSe displays dispersive magnetic excitations reminiscent of other Fe-pnictides. Conversely, the spin excitations in FeSe/STO are gapped, dispersionless, and significantly hardened relative to its bulk counterpart. By comparing our RIXS results with simulations of a bilayer Hubbard model, we connect the evolution of the spin excitations to the Fermiology of the two systems revealing a remarkable reconfiguration of spin excitations in FeSe/STO, essential to understand the role of spin fluctuations in the pairing mechanism.

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.

Monte Carlo (MC) simulations are essential computational approaches with widespread use throughout all areas of science. We present a method for accelerating lattice MC simulations using fully-connected and convolutional artificial neural networks that are trained to perform local and global moves in configuration space, respectively. Both networks take local spacetime MC configurations as input features and can, therefore, be trained using samples generated by conventional MC runs on smaller lattices before being utilized for simulations on larger systems. This new approach is benchmarked for the case of determinant quantum Monte Carlo (DQMC) studies of the twodimensional Holstein model. We find that both artificial neural networks are capable of learning an unspecified effective model that accurately reproduces the MC configuration weights of the original Hamiltonian and achieve an order of magnitude speedup over the conventional DQMC algorithm. Our approach is broadly applicable to many classical and quantum lattice MC algorithms.