Even as the understanding of the mechanism behind correlated insulating states in magic-angle twisted bilayer graphene converges towards various kinds of spontaneous symmetry breaking, the metallic normal state above the insulating transition tempera
ture remains mysterious, with its excessively high entropy and linear-in-temperature resistivity. In this work, we focus on the effects of fluctuations of the order-parameters describing correlated insulating states at integer fillings of the low-energy flat bands on charge transport. Motivated by the observation of heterogeneity in the order-parameter landscape at zero magnetic field in certain samples, we conjecture the existence of frustrating extended range interactions in an effective Ising model of the order-parameters on a triangular lattice. The competition between short-distance ferromagnetic interactions and frustrating extended range antiferromagnetic interactions leads to an emergent length scale that forms stripe-like mesoscale domains above the ordering transition. The gapless fluctuations of these heterogeneous configurations are found to be responsible for the linear-in-temperature resistivity as well as the enhanced low temperature entropy. Our insights link experimentally observed linear-in-temperature resistivity and enhanced entropy to the strength of frustration, or equivalently, to the emergence of mesoscopic length scales characterizing order-parameter domains.
Moire systems provide a rich platform for studies of strong correlation physics. Recent experiments on hetero-bilayer transition metal dichalcogenide (TMD) Moire systems are exciting in that they manifest a relatively simple model system of an extend
ed Hubbard model on a triangular lattice. Inspired by the prospect of the hetero-TMD Moire systems potential as a solid-state-based quantum simulator, we explore the extended Hubbard model on the triangular lattice using the density matrix renormalization group (DMRG). Specifically, we explore the two-dimensional phase space of the kinetic energy relative to the interaction strength $t/U$ and the further-range interaction strength $V_1/U$. We find competition between Fermi fluid, chiral spin liquid, spin density wave, and charge density wave. In particular, our finding of the optimal further-range interaction for the chiral correlation presents a tantalizing possibility.
Motivated by the question of whether disorder is a prerequisite for localization to occur in quantum many-body systems, we study a frustrated one-dimensional spin chain, which supports localized many-body eigenstates in the absence of disorder. When
the system is prepared in an initial state with one domain wall, it exhibits characteristic signatures of quasi-many-body localization (quasi- MBL), including initial state memory retention, an exponentially increasing lifetime with enlarging the size of the system, a logarithmic growth of entanglement entropy, and a logarithmic light cone of an out-of-time-ordered correlator. We further show that the localized many-body eigenstates can be manipulated as pseudospin-1/2s and thus could potentially serve as qubits. Our findings suggest a new route of using frustration to access quasi-MBL and preserve quantum coherence.
A mysterious incoherent metallic (IM) normal state with $T$-linear resistivity is ubiquitous among strongly correlated superconductors. Recent progress with microscopic models exhibiting IM transport has presented the opportunity for us to study new
models that exhibit direct transitions into a superconducting state out of IM states within the framework of connected Sachdev-Ye-Kitaev (SYK) quantum dots. Here local SYK interactions within a dot produce IM transport in the normal state, while local attractive interactions drive superconductivity. Through explicit calculations, we find two features of superconductivity arising from an IM normal state: First, despite the absence of quasiparticles in the normal state, the superconducting state still exhibits coherent superfluid transport. Second, the non-quasiparticle nature of the IM Greens functions produces a large enhancement in the ratio of the zero-temperature superconducting gap $Delta$ and transition temperature $T_{sc}$, $2Delta/T_{sc}$, with respect to its BCS value of $3.53$.
Interest in modulated paired states, long sought since the first proposals by Fulde and Ferrell and by Larkin and Ovchinnikov, has grown recently in the context of strongly coupled superconductors under the name of pair density wave. However, there i
s little theoretical understanding of how such a state might arise out of strong coupling physics in simple models. Although density matrix renormalization group has been a powerful tool for exploring strong coupling modulation phenomena of spin and charge stripe in the Hubbard model and the t-J model, there has been no numerical evidence of PDW within these models using DMRG. Here we note that a system with inversion breaking, C3v point group symmetry may host a PDW-like state. Motivated by the fact that spin-valley locked band structure of hole-doped group VI transition metal dichalcogenides materializes such a setting, we use DMRG to study the superconducting tendencies in spin-valley locked systems with strong short-ranged repulsion. Remarkably we find robust evidence for a PDW and the first of such evidence within DMRG studies of a simple fermionic model.
Recent experiments demonstrating large spin-transfer torques in topological insulator (TI)-ferromagnetic metal (FM) bilayers have generated a great deal of excitement due to their potential applications in spintronics. The source of the observed spin
-transfer torque, however, remains unclear. This is because the large charge transfer from the FM to TI layer would prevent the Dirac cone at the interface from being anywhere near the Fermi level to contribute to the observed spin-transfer torque. Moreover, there is yet little understanding of the impact on the Dirac cone at the interface from the metallic bands overlapping in energy and momentum, where strong hybridization could take place. Here, we build a simple microscopic model and perform first-principles-based simulations for such a TI-FM heterostructure, considering the strong hybridization and charge transfer effects. We find that the original Dirac cone is destroyed by the hybridization as expected. Instead, we find a new interface state which we dub descendent state to form near the Fermi level due to the strong hybridization with the FM states at the same momentum. Such a `descendent state carries a sizable weight of the original Dirac interface state, and thus inherits the localization at the interface and the same Rashba-type spin-momentum locking. We propose that the `descendent state may be an important source of the experimentally observed large spin-transfer torque in the TI-FM heterostructure.
One of the key motivations for the development of atomically resolved spectroscopic imaging STM (SI-STM) has been to probe the electronic structure of cuprate high temperature superconductors. In both the d-wave superconducting (dSC) and the pseudoga
p (PG) phases of underdoped cuprates, two distinct classes of electronic states are observed using SI-STM. The first class consists of the dispersive Bogoliubov quasiparticles of a homogeneous d-wave superconductor. These are detected below a lower energy scale |E|={Delta}0 and only upon a momentum space (k-space) arc which terminates near the lines connecting k=pm({pi}/a0,0) to k=pm(0, {pi}/a0). In both the dSC and PG phases, the only broken symmetries detected in the |E|leq {Delta}0 states are those of a d-wave superconductor. The second class of states occurs at energies near the pseudogap energy scale |E| {Delta}1 which is associated conventionally with the antinodal states near k=pm({pi}/a0,0) and k=pm(0, {pi}/a0). We find that these states break the 90o-rotational (C4) symmetry of electronic structure within CuO2 unit cells, at least down to 180o rotational (C2) symmetry (nematic) but in a spatially disordered fashion. This intra-unit-cell C4 symmetry breaking coexists at |E| {Delta}1 with incommensurate conductance modulations locally breaking both rotational and translational symmetries (smectic). The properties of these two classes of |E| {Delta}1 states are indistinguishable in the dSC and PG phases. To explain this segregation of k-space into the two regimes distinguished by the symmetries of their electronic states and their energy scales |E| {Delta}1 and |E|leq{Delta}0, and to understand how this impacts the electronic phase diagram and the mechanism of high-Tc superconductivity, represents one of a key challenges for cuprate studies.
In the stripe-ordered state of a strongly-correlated two-dimensional electronic system, under a set of special circumstances, the superconducting condensate, like the magnetic order, can occur at a non-zero wave-vector corresponding to a spatial peri
od double that of the charge order. In this case, the Josephson coupling between near neighbor planes, especially in a crystal with the special structure of La_{2-x}Ba_xCuO_4, vanishes identically. We propose that this is the underlying cause of the dynamical decoupling of the layers recently observed in transport measurements at x=1/8.
The elementary excitations of fractional quantum Hall (FQH) fluids are vortices with fractional statistics. Yet, this fundamental prediction has remained an open experimental challenge. Here we show that the cross current noise in a three-terminal tu
nneling experiment of a two dimensional electron gas in the FQH regime can be used to detect directly the statistical angle of the excitations of these topological quantum fluids. We show that the noise also reveals signatures of exclusion statistics and of fractional charge. The vortices of Laughlin states should exhibit a ``bunching effect, while for higher states in the Jain sequences they should exhibit an ``anti-bunching effect.
