Engineering flat bands in twisted-bilayer graphene away from the magic angle with chiral optical cavities

Twisted bilayer graphene (TBG) is a recently discovered two-dimensional superlattice structure which exhibits strongly-correlated quantum many-body physics, including strange metallic behavior and unconventional superconductivity. Most of TBG exotic properties are connected to the emergence of a pair of isolated and topological flat electronic bands at the so-called magic angle, $\theta \approx 1.05^{\circ}$, which are nevertheless very fragile. In this work, we show that, by employing chiral optical cavities, the topological flat bands can be stabilized away from the magic angle in an interval of approximately $0.8^{\circ}<\theta<1.3^{\circ}$. As highlighted by a simplified theoretical model, time reversal symmetry breaking, induced by the chiral nature of the cavity, plays a fundamental role in flattening the isolated bands and gapping out the rest of the spectrum. The efficiency of the cavity is discussed as a function of the twisting angle, the light-matter coupling and the optical cavity characteristic frequency. Our results demonstrate the possibility of engineering flat bands in TBG using optical devices, extending the onset of strongly-correlated topological electronic phases in Moir\'e superlattices to a wider range in the twisting angle.Comment: v1: comments welcome

A quantitative theoretical model of the boson peak based on stringlet excitations

The boson peak (BP), a low-energy excess in the vibrational density of states over the phonon Debye contribution, is usually identified as one of the distinguishing features between ordered crystals and amorphous solid materials. Despite decades of efforts, its microscopic origin still remains a mystery and a consensus on its theoretical derivation has not yet been achieved. Recently, it has been proposed, and corroborated with simulations, that the BP might stem from intrinsic localized modes which involve string-like excitations ("stringlets") having a one-dimensional (1D) nature. In this work, we build on a theoretical framework originally proposed by Lund that describes the localized modes as 1D vibrating strings, but we specify the stringlet size distribution to be exponential, as observed in independent simulation studies. We show that a generalization of this framework provides an analytically prediction for the BP frequency $\omega_{BP}$ in the temperature regime well below the glass transition temperature in both 2D and 3D amorphous systems. The final result involves no free parameters and is in quantitative agreement with prior simulation observations. Additionally, this stringlet theory of the BP naturally reproduces the softening of the BP frequency upon heating and offers an analytical explanation for the experimentally observed scaling with the shear modulus in the glass state and changes in this scaling in cooled liquids. Finally, the theoretical analysis highlights the existence of a strong damping for the stringlet modes at finite temperature which leads to a large low-frequency contribution to the 3D vibrational density of states, as observed in both experiments and simulations

Correlation between optical phonon softening and superconducting TcT_c in YBa2_2Cu3_3Ox_x

We provide an extended mathematical description of the strong correlation between the experimentally observed softening of Raman modes associated with in-plane oxygen motions and the corresponding superconducting critical temperature $T_c$, as a function of oxygen doping $x$, in YBa$_2$Cu$_3$O$_x$. The model provides a direct link between physical trends of soft optical $A_g$ (in-plane) oxygen modes, the level of oxygen doping $x$, and the superconducting $T_c$. Different regimes observed in the trend of $T_c$ vs doping can be mechanistically explained in terms of corresponding regimes of optical phonon softening in the Raman spectra. These results provide further evidence related to the physical origin of high-temperature superconductivity in rare-earth cuprate oxides and to the significant role of electron-phonon coupling therein

Glassy heat capacity from overdamped phasons and a hypothetical phason-induced superconductivity in incommensurate structures

Phasons are collective low-energy modes that appear in disparate condensed matter systems such as quasicrystals, incommensurate structures, fluctuating charge density waves, and Moir\'e superlattices. They share several similarities with acoustic phonon modes, but they are not protected by any exact translational symmetry. As a consequence, they are subject to a wavevector independent damping, and they develop a finite pinning frequency, which destroy their acoustic linearly propagating dispersion. Under a few and simple well-motivated assumptions, we compute the phason density of states, and we derive the phason heat capacity as a function of the temperature. Finally, imagining a hypothetical s-wave pairing channel with electrons, we compute the critical temperature $T_c$ of the corresponding superconducting state as a function of phason damping using the Eliashberg formalism. We find that for large phason damping, the heat capacity is linear in temperature, showing a distinctive glass-like behavior. Additionally, we observe that the phason damping can strongly enhance the effective Eliashberg coupling, and we reveal a sharp non-monotonic dependence of the superconducting temperature $T_c$ on the phason damping, with a maximum located at the underdamped to overdamped crossover scale. Our simple computations confirm the potential role of overdamped modes in explaining the glassy properties of incommensurate structures, but also in possibly inducing strongly-coupled superconductivity therein, and enhancing the corresponding $T_c$.Comment: v2: appendices added, references adde

Sharp Kohn-like phonon anomalies due to charge order can strongly enhance the superconducting TcT_c

Phonon softening is a ubiquitous phenomenon in condensed matter systems which is usually associated with charge density wave (CDW) instabilities and anharmonicity. The interplay between phonon softening, CDW and superconductivity is a topic of intense debate. In this work, the effects of anomalous soft phonon instabilities on superconductivity are studied based on a recently developed theoretical framework that accounts for phonon damping and softening within the Migdal-Eliashberg theory. Model calculations show that the phonon softening in the form of a sharp dip in the phonon dispersion relation, either acoustic or optical (including the case of Kohn-type anomalies typically associated with CDW), can cause a manifold increase of the electron-phonon coupling constant $\lambda$. This, under certain conditions, which are consistent with the concept of optimal frequency introduced by Bergmann and Rainer, can produce a large increase of the superconducting transition temperature $T_c$. In summary, our results suggest the possibility of reaching high-temperature superconductivity by exploiting soft phonon anomalies restricted in momentum space.Comment: v2: references added, discussion improve

When Bifunctional Catalyst Encounters Dual MLC Modes: DFT Study on the Mechanistic Preference in Ru-PNNH Pincer Complex Catalyzed Dehydrogenative Coupling Reaction

Metal ligand cooperation (MLC) plays an important role in the development of homogeneous catalysts. Two major MLC modes have generally been proposed, known as the M-L bond mode and the (de)­aromatization mode. To reveal the role of the dual potential functional sites on the MLC process, we present a detailed mechanistic study on a novel-designed Ru-PNNH complex possessing dual potential MLC functional sites for the M-L bond mode and the (de)­aromatization mode, respectively. Our results indicate that the Ru-PNNH complex prefers the M-L mode exclusively during different stages of the catalytic cycle. The unusual double deprotonation process and the mechanistic preference are rationalized. The N-arm deprotonation is attributed to the small steric hindrance of the amido N-arm and the conjugation stabilization effect of the amido group. The origin of the unexpected exclusive mechanistic preference on the M-L bond mechanism is due to the conjugation effect of the amido group, which stabilizes the dearomatized complex and diminishes the driving force of the (de)­aromatization mode. This study highlights the pivotal role of the ligand’s electronic effect on the MLC mechanism and should provide valuable information for the development of highly efficient bifunctional catalysts

The Effect of HSAB on Stereoselectivity: Copper- and Gold-Catalyzed 1,3-Phosphatyloxy and 1,3-Halogen Migration Relay to 1,3-Dienes

The origin of stereodivergence between copper- and gold-catalyzed cascade 1,3-phosphatyloxy and 1,3-halogen migration from α-halo-propargylic phosphates to 1,3-dienes is rationalized with density functional theory (DFT) studies. Our studies reveal the significant role of the relative hardness/softness of the metal centers in determining the reaction mechanism and the stereoselectivity. The relative harder Cu­(I/III) center prefers an associative pathway with the aid of a phosphate group, leading to the (<i>Z</i>)-1,3-dienes. In contrast, the relative softer Au­(I/III) center tends to undergo a dissociative pathway without coordination to a phosphate group, resulting in the (<i>E</i>)-1,3-dienes, where the <i>E</i> type of transition state is favored due to the steric effect. Our findings indicate the intriguing role of hard–soft/acid–base (HSAB) theory in tuning the stereoselectivity of metal-catalyzed transformations with functionalized substrates

Two-Dimensional Charge-Separated Metal–Organic Framework for Hysteretic and Modulated Sorption

A charge-separated metal–organic framework (MOF) has been successfully synthesized from an imidazolium tricarboxylate ligand, <i>N</i>-(3,5-dicarboxylphenyl)-<i>N</i>′-(4-carboxylbenzyl)­imidazolium chloride (DCPCBImH₃Cl), and a zinc­(II) dimeric secondary building unit, namely, <b>DCPCBim-MOF-Zn</b>, which shows an unprecedented 3,6-connected two-dimensional net topology with the point (Schläfli) symbol (4².6)₂(4⁴.6⁹.8²). The framework contains one-dimensional highly polar channels, and density functional theory calculations show that positive charges are located on the imidazolium/phenyl rings and negative charges on the carboxylate moieties. The charge-separated nature of the pore surface has a profound effect in their adsorption behavior, resulting in remarkable hysteretic sorption of various gases and vapors. For CO₂, the hysteretic sorption was observed to occur even up to 298 K. Additionally, trace chloride anions present in the pore channels are able to modulate the gas-sorption behavior

Hydrogenation of Carbon Dioxide Using Half-Sandwich Cobalt, Rhodium, and Iridium Complexes: DFT Study on the Mechanism and Metal Effect

The hydrogenation of carbon dioxide catalyzed by half-sandwich transition metal complexes (M = Co, Rh, and Ir) was studied systematically through density functional theory calculations. All metal complexes are found to process a similar mechanism, which involves two main steps, the heterolytic cleavage of H₂ and the hydride transfer. The heterolytic cleavage of H₂ is the rate-determining step. The comparison of three catalytic systems suggests that the Ir catalyst has the lowest activation free energy (13.4 kcal/mol). In contrast, Rh (14.2 kcal/mol) and Co (18.3 kcal/mol) catalysts have to overcome relatively higher free energy barriers. The different catalytic efficiency of Co, Rh, and Ir is attributed to the back-donation ability of different metal centers, which significantly affects the H₂ heterolytic cleavage. The highest activity of an iridium catalyst is attributed to its strong back-donation ability, which is described quantitatively by the second order perturbation theory analysis. Our study indicates that the functional group of the catalyst plays versatile roles on the catalytic cycle to facilitate the reaction. It acts as a base (deprotonated) to assist the heterolytic cleavage of H₂. On the other hand, during the hydride transfer, it can also serve as Brønsted acid (protonated) to lower the LUMO of CO₂. This ligand assisted pathway is more favorable than the direct attack of hydride to CO₂. These finds highlight that the unique features of the metal center and the functional ligands are crucial for the catalyst design in the hydrogenation of carbon dioxide

General H₂ Activation Modes for Lewis Acid–Transition Metal Bifunctional Catalysts

A general mechanism for H₂ activation by Lewis acid–transition metal (LA-TM) bifunctional catalysts has been presented via density functional theory (DFT) studies on a representative nickel borane system, (^PhDPB^Ph)­Ni. There are four typical H₂ activation modes for LA-TM bifunctional catalysts: (1) the cis homolytic mode, (2) the trans homolytic mode, (3) the synergetic heterolytic mode, and (4) the dissociative heterolytic mode. The feature of each activation mode has been characterized by key transition state structures and natural bond orbital analysis. Among these four typical modes, (^PhDPB^Ph)Ni catalyst most prefers the synergetic heterolytic mode (Δ<i>G</i>^‡ = 29.7 kcal/mol); however the cis homolytic mode cannot be totally disregarded (Δ<i>G</i>^‡ = 33.7 kcal/mol). In contrast, the trans homolytic mode and dissociative heterolytic mode are less feasible (Δ<i>G</i>^‡ = ∼42 kcal/mol). The general mechanistic picture presented here is fundamentally important for the development and rational design of LA-TM catalysts in the future