SIMES Publications

Abstract

Understanding spin excitations and their connection to unconventional superconductivity have remained central issues since the discovery of cuprates. Direct measurement of the dynamical spin structure factor in the parent compounds can provide key information on important interactions relevant in the doped regime, and variations in the magnon dispersion have been linked closely to differences in crystal structure between families of cuprate compounds. Here, we elucidate the relationship between spin excitations and various controlling factors thought to be significant in high-$T_c$ materials by systematically evaluating the dynamical spin structure factor for the three-orbital Hubbard model, revealing differences in the spin dispersion along the Brillouin zone axis and the diagonal. Generally, we find that the absolute energy scale and momentum dependence of the excitations primarily are sensitive to the effective charge-transfer energy, while changes in the on-site Coulomb interactions have little effect on the details of the dispersion. In particular, our result highlights the splitting between spin excitations along the axial and diagonal directions in the Brillouin zone. This splitting decreases with increasing charge-transfer energy and correlates with changes in the apical oxygen position, and general structural variations, for different cuprate families.

Abstract

We develop the theory for nonresonant Raman scattering of strongly correlated electrons in time-resolved pump- probe experiments. The electrons are initially pumped with an intense pulse of light and then a probe pulse measures the Raman response function after an adjustable time delay. We describe how the width of the probe pulse and the strength of the electron correlations affect the Raman cross section. The theory is developed for the case of B1g symmetry, where incident and scattered light are polarized perpendicular to each other. We illustrate the exact solution with the Falicov-Kimball model, which is solved via nonequilibrium dynamical mean-field theory.

Abstract

Batteries including lithium-ion, lead–acid, redox-flow and liquid-metal batteries show promise for grid-scale storage, but they are still far from meeting the grid’s storage needs such as low cost, long cycle life, reliable safety and reasonable energy density for cost and footprint reduction. Here, we report a rechargeable manganese–hydrogen battery, where the cathode is cycled between soluble Mn2+ and solid MnO2 with a two-electron reaction, and the anode is cycled between H2gas and H2O through well-known catalytic reactions of hydrogen evolution and oxidation. This battery chemistry exhibits a discharge voltage of ~1.3 V, a rate capability of 100 mA cm−2 (36 s of discharge) and a lifetime of more than 10,000 cycles without decay. We achieve a gravimetric energy density of ~139 Wh kg−1 (volumetric energy density of ~210 Wh l−1), with the theoretical gravimetric energy density of ~174 Wh kg−1 (volumetric energy density of ~263 Wh l−1) in a 4 M MnSO4electrolyte. The manganese–hydrogen battery involves low-cost abundant materials and has the potential to be scaled up for large-scale energy storage.

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.

Abstract

The pairing symmetry of interacting Dirac fermions on the $\pi$-flux lattice is studied with the determinant quantum Monte Carlo and numerical linked-cluster expansion methods. The $s^{\ast }$- (i.e., extended $s$-) and $d$-wave pairing symmetries, which are distinct in the conventional square lattice, are degenerate under the Landau gauge. We demonstrate that the dominant pairing channel at strong interactions is an unconventional $d{s}^{\ast }$-wave phase consisting of alternating stripes of $s^{\ast }$- and $d$-wave phases. A complementary mean-field analysis shows that while the $s^{\ast }$- and $d$-wave symmetries individually have nodes in the energy spectrum, the $d{s}^{\ast }$ channel is fully gapped. The results represent a new realization of pairing in Dirac systems, connected to the problem of chiral $d$-wave pairing on the honeycomb lattice, which might be more readily accessed by cold-atom experiments.

Abstract

We investigated the doping dependence of magnetic excitations in the lightly doped cuprate ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x{\mathrm{CuO}}_4$ via combined studies of resonant inelastic x-ray scattering (RIXS) at the Cu $L_3$ edge and theoretical calculations. With increasing doping, the magnon dispersion is found to be essentially unchanged, but the spectral width broadens and the spectral weight varies differently at different momenta. Near the Brillouin zone center, we directly observe bimagnon excitations that possess the same energy scale and doping dependence as previously observed by Raman spectroscopy. They disperse weakly in energy-momentum space, and they are consistent with a bimagnon dispersion that is renormalized by the magnon-magnon interaction at the zone center.

Abstract

We explore adiabatic pumping in the presence of a periodic drive, finding a new phase in which the topologically quantized pumped quantity is energy rather than charge. The topological invariant is given by the winding number of the micromotion with respect to time within each cycle, momentum, and adiabatic tuning parameter. We show numerically that this pump is highly robust against both disorder and interactions, breaking down at large values of either in a manner identical to the Thouless charge pump. Finally, we suggest experimental protocols for measuring this phenomenon.

Abstract

Ultrathin transition metal dichalcogenides (TMDCs) have recently been extensively investigated to understand their electronic and optical properties. Here we study ultrathin Mo0.91W0.09Te2, a semiconducting alloy of MoTe2, using Raman, photoluminescence (PL), and optical absorption spectroscopy. Mo0.91W0.09Te2 transitions from an indirect to a direct optical band gap in the limit of monolayer thickness, exhibiting an optical gap of 1.10 eV, very close to its MoTe2 counterpart. We apply tensile strain, for the first time, to monolayer MoTe2 and Mo0.91W0.09Te2 to tune the band structure of these materials; we observe that their optical band gaps decrease by 70 meV at 2.3% uniaxial strain. The spectral widths of the PL peaks decrease with increasing strain, which we attribute to weaker exciton–phonon intervalley scattering. Strained MoTe2 and Mo0.91W0.09Te2extend the range of band gaps of TMDC monolayers further into the near-infrared, an important attribute for potential applications in optoelectronics.

Abstract

Doped semiconductors are the most important building elements for modern electronic devices¹. In silicon-based integrated circuits, facile and controllable fabrication and integration of these materials can be realized without introducing a high-resistance interface^2,3. Besides, the emergence of two-dimensional (2D) materials enables the realization of atomically thin integrated circuits^4,5,6,7,8,9. However, the 2D nature of these materials precludes the use of traditional ion implantation techniques for carrier doping and further hinders device development¹⁰. Here, we demonstrate a solvent-based intercalation method to achieve p-type, n-type and degenerately doped semiconductors in the same parent material at the atomically thin limit. In contrast to naturally grown n-type S-vacancy SnS₂, Cu intercalated bilayer SnS₂ obtained by this technique displays a hole field-effect mobility of ~40 cm² V⁻¹s⁻¹, and the obtained Co-SnS₂ exhibits a metal-like behaviour with sheet resistance comparable to that of few-layer graphene⁵. Combining this intercalation technique with lithography, an atomically seamless p–n–metal junction could be further realized with precise size and spatial control, which makes in-plane heterostructures practically applicable for integrated devices and other 2D materials. Therefore, the presented intercalation method can open a new avenue connecting the previously disparate worlds of integrated circuits and atomically thin materials.

Abstract