Kagome off-scale

Kagome materials have emerged as a promising platform to study novel quantum phases of matter that arise from the interplay between lattice geometry, band topology, and electronic correlations. They host a variety of electronic and structural instabilities, with charge density waves being a prominent example.

In this talk, I will describe how first principles calculations can be used to explore the rich physics associated with charge density waves. Using bilayer kagome metal ScV6Sn6 as an illustrative example, I will explore competing charge density waves, the role of anharmonic phonon-phonon interactions in the charge density wave transition, and the control of charge density waves by external parameters such as pressure and doping.

Two dimensional di-chalcogenide materials with Ising spin orbit coupling often show enhanced charge density wave transition temperature and signatures of unconventional superconductivity unlike their three dimensional bulk counterparts. Some of these experiments in monolayer NbSe2 include enhancement of superconducting transition temperature with increasing disorder concentration, observation of two fold symmetric gap in the presence of in plane magnetic field, and observation of Leggett modes in tunnelling experiments.
In this talk, we will first present our calculation of 3Q CDW ordering and role of disorder in normal state of monolayer NbSe2. We will then present our results for the superconducting state assuming a spin fluctuation mediated pairing scenario and explore the dominant gap functions within this theory. Finally, we will discuss the effects of an in-plane Zeeman field on the homogenous superconducting state.
Indian Institute Of Technology Ma

Quantum properties, like superposition, entanglement, and fractionalization, enable fascinating functionalities for quantum technologies. However, it remains unclear whether these properties are exclusive to quantum concepts or can be generalized to the classical vector space. This talk addresses this question. We introduce a generic symmetry group construction built from a vector field in a plaquette of classical spins, demonstrating how classical spins superpose in the irreducible representations (irreps). The corresponding probability amplitudes serve as order parameters and local spins as fragmented excitations. Moreover, the formalism offers a many-body vector field representation for diverse ground states, encompassing spin liquids and fragmented phases described as degenerate ensembles of irreps. Finally, we apply the theory in a square Kagome lattice, showing how the spin liquid state is unstable to either vortex orderings or fragmented phases with any finite Dzyaloshinskii-Mor...

The phase diagram of the kagome metal family AV3Sb5 (A = K, Rb, Cs) features both super- conductivity and charge density wave (CDW) instabilities, which have generated tremendous attention. We've studied the temperature evolution and demise of the CDW state using low resolution X-ray diffraction x-ray scattering study of CsV3Sb5 over a broad range of temperatures from 300 K to ∼ 2 K, below the onset of its super-conductivity at Tc ∼ 2.9 K. Such measurements are optimized for diffuse X-ray scattering. Order parameter measurements of the 2 × 2 × 2 CDW structure show an unusual and extended linear temperature dependence onsetting at T ∗ ∼ 160 K, much higher than the susceptibility anomaly associated with CDW order at TCDW = 94 K. This implies strong CDW fluctuations exist to ∼ 1.7 × TCDW. The CDW order parameter is also observed to be constant from T = 16 K to 2 K, implying that the CDW and superconducting order co-exist below Tc, and, at ambient pressure, any possible competition between...

In this talk, I will report our recent progress on Kagome superconductor AV3Sb5 (A=K, Rb, and Cs). As the first-step toward understanding of their largely unconventional natures, we conducted systematic analyses based on density functional theory calculation. Tight-binding parameters computed by maximally localized Wannier functions technique demonstrates that the out-of-plane Sb out -p orbital is a key element for the description of Van Hove singularity structures known in this material near Fermi level. Correlation strengths are also found to be largely determined by Sb out -p states. Based on constrained random phase approximation, we found that on- site and inter-site interaction parameter are both significantly affected by Sb out -p screenings. The chemical effect on the van Hove singularity structure and the possible suggestion of detecting time reversal symmetry breaking phase will be presented and discussed.

We derive a minimal low-energy model for Bernal bilayer graphene and related rhombohedral graphene multilayers at low electronic densities by constructing Wannier orbitals defined in real-space supercells of the original primitive cell. Starting from an ab-initio electronic structure theory comprising the atomic carbon $p_z$-orbitals, momentum locality of the Fermi surface pockets around $K,K'$ is circumvented by backfolding the $\pi$-bands to the concomitant mini-Brillouin zone of the supercell, reminiscent of their (twisted) moiré counterparts. The supercell Wannier functions reproduce the spectral weight and Berry curvature of the microscopic model and offer an intuitive real-space picture of the emergent physics at low electronic densities being shaped by flavor-polarized wave packets with mesoscopic extent. By projecting an orbital-resolved, dual-gated Coulomb interaction to the effective Wannier basis, we find that the low-energy physics of Bernal bilayer graphene is governed by ...

van Hove singularities (vHS) -- momenta for which the group velocity of a Bloch state vanishes, and the density of states diverges -- have a dramatic impact on interaction effects when located near the Fermi level, resulting in a rich competition between superconductivity and charge order. While the presence of vHS near the Fermi level is typically unusual, it appears to be a ubiquitous feature of many recently discovered kagome metals. In this talk I will relate the novel properties of many of these materials to the nature of their vHS. Firstly I will discuss AV3Sb5, in which ARPES identifies twofold vHS near the Fermi level with opposite concavity. The opposite concavity of the vHS results in a weak-coupling instabilitly towards excitonic order, hybridising the two bands. Landau theory predicts the coexistence of charge density wave and excitonic order, offering a possible explanation of many of the unconventional responses seen in AV3Sb5. Second, I will discuss ScV6Sn6. Recent STM e...

I will discuss recent theoretical investigations of disorder response and the spin susceptibility of unconventional superconductivity on the kagome lattice. Despite the existence of a sign-changing gap structure, which sums to zero over the Fermi surface, such unconventional pairing states remain robust to disorder and exhibit a Hebel-Slichter peak in the temperature-dependent spin-relaxation rate. It originates from destructive interference effects peculiar to the kagome lattice. For the same reason, unconventional pairing states on the kagome lattice do not exhibit a neutron resonance peak. These results build on previous theoretical studies of the surprising robustness of unconventional pairing states to disorder on the kagome lattice. Taken together these results imply that unconventional superconductivity on the kagome lattice is deceptive in the sense that its properties may appear similar to conventional non-sign-changing superconductivity. These results may be of relevance to t...

In this talk, I will discuss new progress in understanding electronic loop current states in correlated electron systems. A brief review of this type states will be given for cuprates and Kagome lattice superconductors. We will develop correlated electron models where the loop current states are ground states and discuss the physics behind it.

In this talk, I will discuss the charge correlations in two different classes of kagome metals, each with electron fillings near saddle points in their band structures. In the first compound, CsV3Sb5, a dominant breathing mode of the kagome network drives the formation of a metastable charge density state. Charge correlations in this state undergo an unusual evolution upon tuning the carrier filling, suggesting the presence of a nearby nematic instability. In the second compound, ScV6Sn6, charge order is driven by an out-of-plane instability of the Sc-Sn chains that thread through the kagome planes. This drives a form of frustrated charge order likely responsible for the pseudogap and anomalous electronic properties reported in this material. The differing routes to charge order across multiple families of kagome metals will be discussed.

In low dimensional superconductors, disorder and many body interactions leads to a veritable zoo of intriguing phases and phase transitions. A exemplar of the same is for instance the highly debated anomalous metallic phase which exists circumventing the celebrated Anderson localisation. On the other hand, disorder itself triggers a host of very intriguing and exotic effects such as the Quantum Griffiths Phase (QGP) that stems from the associated Infinite Randomness Fixed Point (IRFP). In this talk we provide incontrovertible proof of emergence of a QGP in the 2D electron gas formed in the LaScO_3 /SrTiO_2 heterostructure. The signatures of the QGP are embedded in the magneto-resistance exhibited by the sample. In particular, we show that in the Griffiths phase (that obtains at higher temperatures), the effective dynamical exponent diverges as a function of the magnetic field. Further, at lower temperatures we argue that the divergence signalling the QGP is cut-off by interplay of diso...

In this talk, I will discuss recent results for charge density wave (CDW) and pair density wave (PDW) instabilities on the kagome lattice near a van Hove singularity. I then discuss the relevance of these results to experimental observations in kagome metals such as CsV_3Sb_5.