Condensed Matter

A single layer graphene hides many body effects in the dense viscous 
fluid of p-pi electrons, Bilayer graphene, with AA and AB registry, on 
the other hand, exposes some of them. A twisted bilayer springs more 
surprises. We discuss recently seen superconductivity in twisted bilayer 
graphene. Resonating valence bond (RVB) physics contained in the dense 
electron fluid in graphene is invoked [1]. RVB fails to produce 
superconductivity in neutral graphene, as carrier are absent at the 
Fermi level. In a twisted bilayer, interlayer tunneling adds equal 
number of electron and hole carriers in Moire superlattice of dominant 
AA registry. These carriers use RVB pairing and develop charge -2e and 
+2e Cooper pair correlations, in spite of Coulomb repulsions. Resulting 
Moire lattice of Cooper pair puddles form a Josephson lattice. Coulomb 
blockade makes it a Bosonic Mott insulator. Gate voltage dopes the Bose 
Mott insulator and interesting consequences follow.

[1] G. Baskaran, arXiv:1804.00627

Systems of strongly interacting particles can give rise to topological phases beyond non-interacting limit.  Although unique features of strongly interacting topological phases, such as fractionalization of quantum degrees of freedom, have important applications in quantum information processing, these topological phases are still far from experimental realizations. In this talk, by presenting constructions of two strongly interacting topological phases, I will argue the key mechanism of their realizations is to add interactions near topological phase transitions.  I will first introduce a model of interacting Majorana fermions that describes a superconducting phase with Fibonacci topological order. Then I will show that a correlated fluid of electrons and holes, dubbed fractional excitonic insulator phase, can exhibit a fractional quantum Hall effect at zero magnetic field. I will present physical evidence and conjecture that this phase can be realized in a higher angular momentum excitonic paired system in the presence of interactions.   

Finding a quantum phase transition between two even well-studied phases of matter can be a hard problem. Free probability theory suggests a new method which can give an answer if the Hamiltonians of two phases satisfy certain conditions. In the latter case, the spectral gap behavior can be calculated without solving the full Hamiltonian but using probabilistic estimation instead. As an example, I will consider generic artificial topological systems created by a periodic drive, including Floquet Majorana modes, and show how FPT can be used to predict and characterize disorder-driven phase transitions.

See also: Phys. Rev. Lett. 121, 126803

Interacting electrons in high magnetic fields exhibit rich physical phenomena including the gapped fractional quantum Hall effects and the gapless states. The composite Fermi liquids (CFLs) are gapless states that can occur at even denominator Landau level fillings. Due to the celebrated work of Halperin, Lee and Read (94), the CFLs were understood as Fermi liquids of composite fermions, which are bound states of electron and electromagnetic flux quanta. However, at 1/2 filling, it is not obvious why the HLR description is consistent with the particle hole symmetry. Motivated by this, recently Son (15) proposed an alternative description for CFLs at 1/2, according to which the composite fermions are instead emergent Dirac fermions. Importantly, Son’s theory predicts a Pi Berry curvature singularity at the composite Fermi sea center. In the first part of this talk [2,3], I will present our numerical work about detecting this Z2 Berry phase at 1/2 filling. In the second part [1], I will present how and why Dirac fermions can emerge at all the other filling fractions (1/2m and 1-1/2m when m is integer) even without the particle hole symmetry.

[1] arXiv 1808.07529. JW.

[2] arXiv 1711.07864. Geraedts, JW, Rezayi, Haldane.

[3] arXiv 1710.09729. JW, Geraedts, Rezayi, Haldane.

We study the chiral anomaly in disordered Weyl semimetals, where the broken translational symmetry prevents the direct application of Nielsen and Ninomiya’s mechanism and disorder is strong enough that quantum effects are important. In the weak disorder regime, there exists rare regions of the random potential where the disorder strength is locally strong, which gives rise to quasi-localized resonances and their effect on the chiral anomaly is unknown. We numerically show that these resonant states do not affect the chiral anomaly only in the case of a single Weyl node. At energies away from the Weyl point, or with strong disorder where one is deep in the diffusive regime, the chiral Landau level itself is not well defined and the semiclassical treatment is not justified. In this limit, we analytically use the supersymmetry method and find that the Chern-Simons (CS) term in the effective action which is not present in non-topological systems gives rise to a non- zero average level velocity which implies chiral charge pumping. We numerically establish that the non-zero average level velocity serves as an indicator of the chiral anomaly in the diffusive limit. 

It is widely known that topological orders have long-range entangled gapped ground states from which nontrivial properties can be extracted. We introduce a new theoretical framework named the information convex, the (convex) set of reduced density matrices of a subsystem in its lowest energy, to characterize topological orders. (1) As a concrete example, we present the calculated topology dependent structure of information convex in the quantum double models and show it reveals properties of bulk anyons and deconfined topological excitations along a gapped boundary, and the condensation rules relating them. (2) As a step towards answering "why the structure of information convex looks that way and whether it has predictive power?" we look into some quantum informational constraints. The topological contribution to von Neumann entropy from each topological excitation type and certain fusion constraints are shown to emerge due to strong subadditivity, assuming fusion multiplicity is encoded in certain topological invariant manner.

It is well-known that sufficiently strong interactions can destabilize some SPT phases of free fermions, while others remain stable even in the presence of interactions. It is also known that certain interacting phases cannot be realized by free fermions. In this talk, we will study both of these phenomena in low dimensions and determine the map from free to interacting SPT phases for an arbitrary unitary symmetry. We will also describe how to compute invariants characterizing interacting phases for free band Hamiltonians with symmetry (in any dimension) using only representation theory.

A recent experiment in the Rydberg atom chain observed unusual oscillatory quench dynamics with a charge density wave initial state, and theoretical works identified a set of many-body ``scar states'' in the Hamiltonian as potentially responsible for the atypical dynamics. In the same nonintegrable Hamiltonian, we discover several eigenstates at infinite temperature that can be represented exactly as matrix product states with finite bond dimension, for both periodic boundary conditions (two exact E = 0 states) and open boundary conditions (two E = 0 states and one each E =± √2). This discovery explicitly demonstrates violation of strong eigenstate thermalization hypothesis in this model. These states show signatures of translational symmetry breaking with period-2 bond-centered pattern, despite being in 1D at infinite temperature. We show that the nearby many-body scar states with energies E ~± 1.33 andE ~ ± 2.66 can be well approximated as ``quasiparticle excitations" on top of our exact E = 0 states, and propose a quasiparticle explanation of the strong oscillations observed in experiments.

Experiments on organic crystals whose structure is well-described by the two-dimensional triangular lattice have found a lack of magnetic ordering down to the lowest accessible temperatures, indicative of a quantum spin liquid phase; however, the precise nature of this phase remains an open question.  In this talk, I present strong evidence that the triangular lattice Hubbard model at half filling, a physically motivated model of these organic crystals, realizes a chiral spin liquid phase.  In particular, I show that the model has a nonmagnetic insulating phase between a metallic phase for weak interactions and a magnetically ordered phase for strong interactions, and that the intermediate phase exhibits the expected properties of a chiral spin liquid: spontaneous breaking of time-reversal symmetry, topological ground state degeneracy, a quantized spin Hall effect, and characteristic level counting in the entanglement spectrum.  These results were obtained using the infinite-system density matrix renormalization group (iDMRG) method in a mixed real- and momentum-space basis; in the talk, I will also discuss the benefits of this mixed-space approach to DMRG in general, including its applicability to systems such as twisted bilayer graphene for which a large unit cell makes real-space DMRG impractical. 

Supersymmetry (SUSY) has not been verified so far as a fundamental symmetry in particle physics. Emergent phenomena in condensed matter physics bring the possibility of realizing SUSY as an IR symmetry. We show that 2+1D N=2 Nf=2 supersymmetric quantum electrodynamics (SQED3) with dynamical gauge bosons and fermionic gauginos emerges naturally at the tricritical point of nematic pair-density-wave (PDW) quantum phase transition on the surface of a correlated topological insulator hosting three Dirac cones, such as the topological Kondo insulator SmB6. It provides a first example of emergent supersymmetric gauge theory in condensed matter systems. We also investigate the possibility of emergent 3+1D SUSY theory in lattice models. By constructing an explicit fermionic lattice model featuring two 3D Weyl nodes, we find a continuous PDW quantum phase transition as a function of attractive Hubbard interaction. We further show that N=1 3+1D SUSY emerges at the PDW transition, which we believe is the first realization of emergent 3+1D space-time SUSY in microscopic lattice models. Supersymmetry allows us to determine certain critical exponents and the optical conductivity at the strongly coupled fixed point exactly, which may be measured in future experiments.

Operator scrambling is a crucial ingredient of quantum chaos. Specifically, in the quantum chaotic system, a simple operator can become increasingly complicated under unitary time evolution. This can be diagnosed by various measures such as square of the commutator (out-of-time-ordered correlator), operator entanglement entropy etc. In this talk, we discuss operator dynamics in three representative models: a 2-local spin model with all-to-all interaction, a chaotic spin chain with long-range interactions,  and the quantum linear map. In the first two examples, we explore the operator dynamics by using the quantum Brownian circuit approach and transform the operator spreading into a classical stochastic problem. Although the speeds of scrambling are quite different, a simple operator can eventually approach a "highly entangled" operator with operator entanglement entropy taking a volume law value (close to the Page value). Meanwhile, the spectrum of the operator reduced density matrix develops a universal spectral correlation which can be characterized by the Wishart random matrix ensemble.  In contrast, in the third example (the quantum linear map), although the square of commutator can increase exponentially with time, a simple operator does not scramble but performs chaotic motion in the operator basis space determined by the classical linear map. We show that once we modify the quantum linear map such that operator can mix in the operator basis, the operator entanglement entropy can grow and eventually saturate to its Page value, thus making it a truly quantum chaotic model.

This year there appear several amazing experiments  in the graphene moire superlattices.  In this talk I will focus on the ABC trilayer graphene/h-BN system. Mott-like  insulators at 1/4 and 1/2 of the valence band have already been reported by Feng Wang’s group at Berkeley. The sample is dual gated on top and bottom with voltage V_t and V_b.  V_t+V_b controls the density of electrons. Interestingly we find that the displacement field D=V_t-V_b can control both the topology and the bandwidth of the valence band. For one sign of D (for example D>0), there are two narrow Chern bands with opposite Chern numbers C=3,-3 for the two valleys. For D<0, the bands of the two valleys are trivial and have localized Wannier orbitals on a triangular lattice.  As a result, the physics is governed by a spin-valley Hubbard model on a triangular lattice. This talk focuses on the D<0 side and  consists of two parts:  (1) I will provide the details of this spin-valley Hubbard model and discuss some subtleties special to the moire systems.   (2) In the second part I want to show some of our theoretical attempts on this Hubbard model. First I will show that this system is a perfect platform for studying metal-insulating transition. I will provide a theory of continuous Mott transition between a Fermi liquid and a spinon Fermi surface Mott insulator. Second I will discuss some possible metallic phases upon doping away from the Mott insulator.  Unlike the familiar spin 1/2 case, the spin-valley Hubbard model may not be in a conventional Fermi liquid phase even in the over-doped region. I will provide some candidates of possible unconventional metals based on a six-flavor slave boson theory for the hole doped side.

References:

Feng Wang et.al. arxiv: 1803.01985

Ya-Hui Zhang, Dan Mao, Yuan Cao, Pablo Jarillo-Herrero and T. Senthil.  arXiv:1805.08232

Ya-Hui Zhang and T. Senthil, arxiv: 1809.05110

Ya-Hui Zhang and T. Senthil, ongoing work