Abstract
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 at fixed density. Our results emphasize the importance of interaction effects in the systematic assessment of the thermopower
S
in cuprates.
Abstract
A microscopic understanding of the strongly correlated physics of the cuprates must account for the translational and rotational symmetry breaking that is present across all cuprate ...families, commonly in the form of stripes. Here we investigate emergence of stripes in the Hubbard model, a minimal model believed to be relevant to the cuprate superconductors, using determinant quantum Monte Carlo (DQMC) simulations at finite temperatures and density matrix renormalization group (DMRG) ground state calculations. By varying temperature, doping, and model parameters, we characterize the extent of stripes throughout the phase diagram of the Hubbard model. Our results show that including the often neglected next-nearest-neighbor hopping leads to the absence of spin incommensurability upon electron-doping and nearly half-filled stripes upon hole-doping. The similarities of these findings to experimental results on both electron and hole-doped cuprate families support a unified description across a large portion of the cuprate phase diagram.
Fermi surface (FS) topology is a fundamental property of metals and superconductors. In electron-doped cuprate Nd2−x
CeₓCuO₄ (NCCO), an unexpected FS reconstruction has been observed in optimal- and ...overdoped regime (x = 0.15–0.17) by quantum oscillation measurements (QOM). This is all the more puzzling because neutron scattering suggests that the antiferromagnetic (AFM) long-range order, which is believed to reconstruct the FS, vanishes before x = 0.14. To reconcile the conflict, a widely discussed external magnetic-field–induced AFM long-range order in QOM explains the FS reconstruction as an extrinsic property. Here, we report angle-resolved photoemission (ARPES) evidence of FS reconstruction in optimal- and overdoped NCCO. The observed FSs are in quantitative agreement with QOM, suggesting an intrinsic FS reconstruction without field. This reconstructed FS, despite its importance as a basis to understand electron-doped cuprates, cannot be explained under the traditional scheme. Furthermore, the energy gap of the reconstruction decreases rapidly near x = 0.17 like an order parameter, echoing the quantum critical doping in transport. The totality of the data points to a mysterious order between x = 0.14 and 0.17, whose appearance favors the FS reconstruction and disappearance defines the quantum critical doping. A recent topological proposal provides an ansatz for its origin.
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BFBNIB, NMLJ, NUK, PNG, SAZU, UL, UM, UPUK
Abstract The formation of charge density waves is a long-standing open problem, particularly in dimensions higher than one. Various observations in the vanadium antimonides discovered recently ...further underpin this notion. Here, we study the Kagome metal CsV 3 Sb 5 using polarized inelastic light scattering and density functional theory calculations. We observe a significant gap anisotropy with $$2{\Delta }_{\max }/{k}_{{{\rm{B}}}{T}_{{{\rm{CDW}}}\, \approx \, 20$$ 2 Δ max / k B T CDW ≈ 20 , far beyond the prediction of mean-field theory. The analysis of the A 1g and E 2g phonons, including those emerging below T CDW , indicates strong phonon-phonon coupling, presumably mediated by a strong electron-phonon interaction. Similarly, the asymmetric Fano-type lineshape of the A 1g amplitude mode suggests strong electron-phonon coupling below T CDW . The large electronic gap, the enhanced anharmonic phonon-phonon coupling, and the Fano shape of the amplitude mode combined are more supportive of a strong-coupling phonon-driven charge density wave transition than of a Fermi surface instability or an exotic mechanism in CsV 3 Sb 5 .
We examined the electronic structure in LaMO3 perovskite oxides (M = Cr, Mn, Fe, Co, Ni) by combining information from X-ray emission, absorption, and photoelectron spectroscopy. Through ...first-principles density functional theory simulations, we identified complementary hybridization features present in the transition metal and oxygen X-ray emission spectra. We then developed a method for the self-consistent alignment of the emission data onto a common energy scale using these features, providing a valuable supplementary technique to photoelectron spectroscopy for studying the partial density of states in perovskites. The combined information from X-ray emission and absorption was used to explore trends in electronic structure characteristics under the Zaanen–Sawatzky–Allen frameworknamely the extent of metal–oxygen hybridization, band gap, and charge-transfer gap. We further established a method that allows for the experimental determination of the occupied and unoccupied band positions relative to the oxide Fermi level, as well as on an absolute energy scale.
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IJS, KILJ, NUK, PNG, UL, UM
Nonequilibrium pump-probe time-domain spectroscopies can become an important tool to disentangle degrees of freedom whose coupling leads to broad structures in the frequency domain. Here, using the ...time-resolved solution of a model photoexcited electron-phonon system, we show that the relaxational dynamics are directly governed by the equilibrium self-energy so that the phonon frequency sets a window for “slow” versus “fast” recovery. The overall temporal structure of this relaxation spectroscopy allows for a reliable and quantitative extraction of the electron-phonon coupling strength without requiring an effective temperature model or making strong assumptions about the underlying bare electronic band dispersion.
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CMK, CTK, FMFMET, IJS, NUK, PNG, UL, UM, UPUK
The bilayer Hubbard model with electron-hole doping is an ideal platform to study excitonic orders due to suppressed recombination via spatial separation of electrons and holes. However, suffering ...from the sign problem, previous quantum Monte Carlo studies could not arrive at an unequivocal conclusion regarding the presence of phases with clear signatures of excitonic condensation in bilayer Hubbard models. Here, we develop a determinant quantum Monte Carlo algorithm for the bilayer Hubbard model that is sign-problem-free for equal and opposite doping in the two layers and study excitonic order and charge and spin density modulations as a function of chemical potential difference between the two layers, on-site Coulomb repulsion, and interlayer interaction. In the intermediate coupling regime and in proximity to the SU(4)-symmetric point, we find a biexcitonic condensate phase at finite electron-hole doping, as well as a competing (π,π) charge density wave state. We extract the Berezinskii-Kosterlitz-Thouless transition temperature from superfluid density and a finite-size scaling analysis of the correlation functions and explain our results in terms of an effective biexcitonic hard-core boson model.
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CMK, CTK, FMFMET, IJS, NUK, PNG, UL, UM
Abstract
The field of two-dimensional (2D) ferromagnetism has been proliferating over the past few years, with ongoing interests in basic science and potential applications in spintronic technology. ...However, a high-resolution spectroscopic study of the 2D ferromagnet is still lacking due to the small size and air sensitivity of the exfoliated nanoflakes. Here, we report a thickness-dependent ferromagnetism in epitaxially grown Cr
2
Te
3
thin films and investigate the evolution of the underlying electronic structure by synergistic angle-resolved photoemission spectroscopy, scanning tunneling microscopy, x-ray absorption spectroscopy, and first-principle calculations. A conspicuous ferromagnetic transition from Stoner to Heisenberg-type is directly observed in the atomically thin limit, indicating that dimensionality is a powerful tuning knob to manipulate the novel properties of 2D magnetism. Monolayer Cr
2
Te
3
retains robust ferromagnetism, but with a suppressed Curie temperature, due to the drastic drop in the density of states near the Fermi level. Our results establish atomically thin Cr
2
Te
3
as an excellent platform to explore the dual nature of localized and itinerant ferromagnetism in 2D magnets.
Photon-based spectroscopies have had a significant impact on both fundamental science and applications by providing an efficient approach to investigate the microscopic physics of materials. Together ...with the development of synchrotron X-ray techniques, theoretical understanding of the spectroscopies themselves and the underlying physics that they reveal has progressed through advances in numerical methods and scientific computing. In this Review, we provide an overview of theories for angle-resolved photoemission spectroscopy and resonant inelastic X-ray scattering applied to quantum materials. First, we discuss methods for studying equilibrium spectroscopies, including first-principles approaches, numerical many-body methods and a few analytical advances. Second, we assess the recent development of ultrafast techniques for out-of-equilibrium spectroscopies, from characterizing equilibrium properties to generating transient or metastable states, mainly from a theoretical point of view. Finally, we identify the main challenges and provide an outlook for the future direction of the field.
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