26 Search Results

Many metallic quantum materials display anomalous transport phenomena that defy a Fermi liquid description. In this report we use numerical methods to calculate thermal and charge transport in the doped Hubbard model and observe a crossover separating high- and low-temperature behaviors. Distinct from the behavior at high temperatures, the Lorenz number L becomes weakly doping dependent and less sensitive to parameters at low temperatures. At the lowest numerically accessible temperatures, L roughly approaches the Wiedemann-Franz constant L, even in a doped Mott insulator that lacks well-defined quasiparticles. Decomposing the energy current operator indicates a compensation between kinetic and potential contributions,more » which may help to clarify the interpretation of transport experiments beyond Boltzmann theory in strongly correlated metals.« less

As primarily an electronic observable, the room-temperature thermopower S in cuprates provides possibilities for a quantitative assessment of the Hubbard model. Using determinant quantum Monte Carlo, we demonstrate agreement between Hubbard model calculations and experimentally measured room-temperature S across multiple cuprate families, both qualitatively in terms of the doping dependence and quantitatively in terms of magnitude. We observe an upturn in S with decreasing temperatures, which possesses a slope comparable to that observed experimentally in cuprates. From our calculations, the doping at which S changes sign occurs in close proximity to a vanishing temperature dependence of the chemical potential atmore » fixed density. Our results emphasize the importance of interaction effects in the systematic assessment of the thermopower S in cuprates.« less

Signatures of superconductivity at elevated temperatures above T_c in high-temperature superconductors have been observed near 1/8 hole doping for photoexcitation with infrared or optical light polarized either in the CuO₂ plane or along the c axis. While the use of in-plane polarization has been effective for incident energies aligned to specific phonons, c-axis laser excitation in a broad range between 5 μm and 400 nm was found to affect the superconducting dynamics in striped La_1.885Ba_0.115CuO₄, with a maximum enhancement in the 1/ω dependence to the conductivity observed at 800 nm. This broad energy range, specifically 800 nm, is not resonantmore » with any phonon modes, yet induced electronic excitations appear to be connected to superconductivity at energy scales well above the typical gap energies in the cuprates. A critical question is, What can be responsible for such an effect at 800 nm? Using time-dependent exact diagonalization, we demonstrate that the holes in the CuO₂ plane can be photoexcited into the charge reservoir layers at resonant wavelengths within a multiband Hubbard model. This orbitally selective photoinduced charge transfer effectively changes the in-plane doping level, which can lead to an enhancement of T_c near the 1/8 anomaly.« less

Using determinant Quantum Monte Carlo, we compare three methods of evaluating the DC Hall coefficient $$R_H$$ of the Hubbard model: the direct measurement of the off-diagonal current-current correlator $$χ_{xy}$$ in a system coupled to a finite magnetic field (FF), $$χ^{FF}_{xy}$$; the three-current linear response to an infinitesimal field as measured in the zero-field (ZF) Hubbard Hamiltonian, $$χ^{ZF}_{xy}$$; and the leading order recurrent expansion $$R^{(0)}_{H}$$ in terms of thermodynamic susceptibilities. The two quantities $$χ^{FF}_{xy}$$ and $$χ^{ZF}_{xy}$$ can be compared directly in imaginary time. Proxies for RH constructed from the three-current correlator $$χ^{ZF}_{xy}$$ can be determined under different simplifying assumptions and comparedmore » with $$R^{(0)}_{H}$$. We find these different quantities to be consistent with one another, validating previous conclusions about the close correspondence between Fermi surface topology and the sign of $$R_H$$, even for strongly correlated systems. These various quantities also provide a useful set of numerical tools for testing theoretical predictions about the full behavior of the Hall conductivity for strong correlations.« less

A defining signature of strongly correlated electronic systems is a rich phase diagram, which consists of multiple broken symmetries, such as magnetism, superconductivity, and charge order. In the recently discovered nickelate superconductors a large antiferromagnetic exchange energy has been reported, which implies the existence of strong electronic correlations. However, signatures of a broken-symmetry state other than superconductivity have not yet been observed. Here, we observe charge ordering in infinite-layer nickelates La_1-xSr_xNiO₂ using resonant x-ray scattering. The parent compound orders along the Ni-O bond direction with an incommensurate wave vector, distinct from the stripe order observed in other nickelates that propagatesmore » along a direction 45º to the Ni-O bond. The resonance profile we measure indicates that ordering originates from the nickelate layers and induces a parasitic charge modulation of lanthanum electrons. After doping, the charge order diminishes and its wave vector shifts toward commensurate, indicating that strong electronic correlations are likely to be responsible for the ordered state. Our results suggest that the existence of charge order and its potential interplay with antiferromagnetic fluctuations and superconductivity are important themes in nickel-based superconductors.« less

The recent discovery of superconductivity in oxygen-reduced monovalent nickelates has raised a new platform for the study of unconventional superconductivity, with similarities to and differences from the cuprate high-temperature superconductors. In this paper, we investigate the family of infinite-layer nickelates RNiO₂ with rare-earth R spanning across the lanthanide series, introducing a new and nontrivial “knob” with which to tune nickelate superconductivity. When traversing from La to Lu, the out-of-plane lattice constant decreases dramatically with an accompanying increase of Ni d_x²–y² bandwidth; however, surprisingly, the role of oxygen charge transfer diminishes. In contrast, the magnetic exchange grows across the lanthanides, whichmore » may be favorable to superconductivity. Moreover, compensation effects from the itinerant 5d electrons present a closer analogy to Kondo lattices, indicating a stronger interplay between charge transfer, bandwidth renormalization, compensation, and magnetic exchange. We also obtain the microscopic Hamiltonian using the Wannier downfolding technique, which will provide the starting point for further many-body theoretical studies.« less

We report the discovery of infinite-layer nickelate superconductors has spurred enormous interest. While the Ni¹⁺ cations possess nominally the same 3d⁹ configuration as Cu²⁺ in cuprates, the electronic structure variances remain elusive. Here, we present a soft X-ray photoemission spectroscopy study on parent and doped infinite-layer Pr-nickelate thin films with a doped perovskite reference. By identifying the Ni character with resonant photoemission and comparison with density functional theory + U (on-site Coulomb repulsion energy) calculations, we estimate U ~5 eV, smaller than the charge transfer energy Δ ~8 eV, confirming the Mott-Hubbard electronic structure in contrast to charge-transfer cuprates. Nearmore » the Fermi level (E_F), we observe a signature of occupied rare-earth states in the parent compound, which is consistent with a self-doping picture. Our results demonstrate a correlation between the superconducting transition temperature and the oxygen 2p hybridization near E_F when comparing hole-doped nickelates and cuprates.« less

Establishing a minimal microscopic model for cuprates is a key step towards the elucidation of a high-T_c mechanism. By a quantitative comparison with a recent in situ angle-resolved photoemission spectroscopy measurement in doped 1D cuprate chains, our simulation identifies a crucial contribution from long-range electron-phonon coupling beyond standard Hubbard models. Using reasonable ranges of coupling strengths and phonon energies, we obtain a strong attractive interaction between neighboring electrons, whose strength is comparable to experimental observations. Additionally, nonlocal couplings play a significant role in the mediation of neighboring interactions. Considering the structural and chemical similarity between 1D and 2D cuprate materials,more » this minimal model with long-range electron-phonon coupling will provide important new insights on cuprate high-T_c superconductivity and related quantum phases.« less

The pseudogap regime of the cuprate high-temperature superconductors is characterized by a variety of competing orders, the nature of which are still widely debated. Recent experiments have provided evidence for electron nematic order, in which the electron fluid breaks rotational symmetry while preserving translational invariance. Raman spectroscopy, with its ability to symmetry resolve low energy excitations, is a unique tool that can be used to assess nematic fluctuations and nematic ordering tendencies. Here we compare results from determinant quantum Monte Carlo simulations of the Hubbard model to experimental results from Raman spectroscopy in La_2-xSr_xCuO₄, which show a prominent increase inmore » the B_1g response around 10% hole doping as the temperature decreases, indicative of a rise in nematic fluctuations at low energy. Finally, our results support a picture of nematic fluctuations with B_1g symmetry occurring in underdoped cuprates, which may arise from melted stripes at elevated temperatures.« less

In the cuprates, one-dimensional (1D) chain compounds provide a distinctive opportunity to understand the microscopic physics, owing to the availability of reliable theories. However, progress has been limited by the challenge of controllably doping these materials. We report the synthesis and spectroscopic analysis of the 1D cuprate Ba_2-xSr_xCuO_3+δ over a wide range of hole doping. Our angle-resolved photoemission experiments reveal the doping evolution of the holon and spinon branches. We identify a prominent folding branch whose intensity fails to match predictions of the simple Hubbard model. An additional strong near-neighbor attraction, which may arise from coupling to phonons, quantitatively explainsmore » experiments for all accessible doping levels. Furthermore, considering structural and quantum chemistry similarities among cuprates, this attraction may play a similarly important role in high-temperature cuprate superconductors.« less