Session EE06: V: Strongly Correlated Systems, Including Quantum Fluids and Solids I

Author not Attending

The promising quasi-2d QSL candidate alpha-RuCl3 may realize a field-induced QSL with its nature yet known. We explore the possibility of a simplest gapped QSL description with emergent Z2 gauge fields (Z2 QSL), and study the spectroscopic and transport signals from emergent bosonic quasiparticles known as visons. We construct a model of Z2 QSL that aligns qualitatively with numerical results, and permutes topologically distinct visons upon one translation. The model fits the phenomenology of alpha-RuCl3, including low-field magnetic orders, spectroscopy and thermal hall signals at intermediate fields etc.

The results are built on a complete classification of translation-enriched Z2 orders. Translation actions in (3+1)d U(1)  quantum spin liquids are also classified; in particular  we show that the full duality of the free photon theory could be realized by translations.   

Existence of multiple phases in double perovskite La2CoTiO6 under varying pressure

Double perovskites sparked a lot of attention in recent years due to their unique features including exotic magnetism, quantum spin-liquid, Mott insulating state, etc. The electronic states obtained from their d-orbitals determine the intriguing electrical and magnetic properties of transition metal oxides. External tuning parameter, such as hydrostatic pressure can drastically change physical properties. Here we study the first-principles density-functional theory spin-polarized electronic structure calculations for a rather simple, but with interesting physical properties, a double perovskite, namely, La₂CoTiO₆ where Ti is in 4⁺ state, i.e., nonmagnetic. The compound shows an antiferromagnetic ground state with T_N = 14.6 K [1]. At ambient pressure, the distorted structure (∠Co-O-Ti between 151–153°) shows an insulating behavior with a band gap of 0.999 eV consistent with the experiment. However, when the distortion is removed, namely, the undistorted structure (angle between Co-O-Ti is 180°) shows a very interesting ferromagnetic half-metallic (HM) behavior. With the application of hydrostatic pressure on the distorted structure, applied maximum 130 GPa, the system transforms to a metallic state (Mott transition) from an insulating state via a HM phase, which could be interesting to study the spin-controlled electronics or spintronics.

Reference

Rise and fall of insulating state in oxide perovskites

Paramagnetic (PM) ABO3 perovskites appear either as metals or as insulators or as systems able to interconvert between such phases. Here, we demonstrate the role of structural and magnetic symmetry breaking in PM phases identifying: (i) False PM metals, predicted incorrectly to be metals because of ignoring symmetry breaking energy lowering whereas in reality they are  PM insulators (e.g., PM-SrMnO3); (ii) Real PM insulators due to effective symmetry breaking leading to stabilization and gapping (e.g., JT distorted PM-LaMnO3); (iii) Extrinsic metals formed by application of external knobs such as temperature or pressure on true insulators, suppressing symmetry breaking thus restoring (i) as an actual (not false) metal (e.g., PM-LaTiO3). (iv) Intrinsic true metal not benefiting from symmetry breaking, i.e., when symmetry breaking does not open the band gap (e.g., PM-SrVO3). We will show that the rise of the insulating state emerges when (i) is replaced by (ii), whereas the fall of the insulating state and emergence of narrow-gap state emerges when (ii) transforms to (iii) under the application of external knobs, or when or the would-be Mott insulator (ii) lacks the ability to symmetry break and is this naturally a metal (iv).

    

Generalized multifractality at metal-insulator transitions and in metallic phases of two-dimensional disordered systems

We study generalized multifractality characterizing fluctuations and correlations of eigenstates in disordered systems of various symmetry classes. Both metallic phases and Anderson-localization transitions are considered. By using the nonlinear sigma-model approach, we construct pure-scaling eigenfunction observables. The construction is verified by numerical simulations of appropriate microscopic models, which also yield numerical values of the corresponding exponents. In the metallic phases, the numerically obtained exponents satisfy Weyl symmetry relations as well as generalized parabolicity (proportionality to eigenvalues of the quadratic Casimir operator). At the same time, the generalized parabolicity is strongly violated at critical points of metal-insulator transitions, signaling violation of local conformal invariance. Moreover, in classes D and DIII, even the Weyl symmetry breaks down at critical points of metal-insulator transitions. This last feature is related to a peculiarity of the sigma-model manifolds in these symmetry classes: they consist of two disjoint components. Domain walls associated with these additional degrees of freedom are crucial for ensuring Anderson localization and, at the same time, lead to the violation of the Weyl symmetry.

Further we study the impact of the additional U(1) group in the manifolds of the chiral classes on the eigenoperator construction and Weyl symmetry.   

Structural Observation of Voltage Induced Transitions in Neuromorphic Devices

Voltage induced phase transitions in VO2 and LSMO neuromorphic devices are one pathway to next generation computing. Barrier formation in LMSO has been limited to MOKE measurements, while filament formation in VO2 devices have been observed using optical reflectivity measurements. We present a pathway to observing the structural effects of a voltage induced phase transition in these devices using dark field x-ray microscopy. In addition, preliminary observations of the barrier formation in LSMO are presented and the difficulties with performing such measurements are discussed.   

Visualization of structural domains in NdNiO3/NdGaO3 heterostructures

Ionic liquid gating is a versatile method for inducing phase transitions in a wide range of oxide materials, e.g., in the epitaxial NdNiO₃ films [1]. However, the microscopic mechanism of the induced phase transition, such as, the evolution of domain structures remains elusive and difficult to be probed under operando conditions. Here, we combine X-ray reflection interfacial microscopy (XRIM) and X-ray photon correlation spectroscopy (XPCS) to reveal the changes in the domain configuration and structures in NdNiO₃ films with TFSI-Dmim as the ionic liquid gating top layer. We compare the domain morphology of NdNiO₃ on different orientations of the NdGaO3 substrates both in the pristine state, during and after the ionic gating process. This comparison yields valuable information concerning the impact of substrate strain on the domain size, distribution and their mechanical rigidity upon repeating electric cycling. Our investigation sheds light on the pathway towards rational improvement and design for the ionic gating, and help accelerate the application of these devices in future microelectronics.

    

The competing roles of structural and magnetic symmetry breaking in describing gapping and then metallization in VO2

Structural and magnetic symmetry breakings could lead to insulating phases in open d-shell transition metal oxides that could otherwise stay metallic. Here, we find from density functional theory supercell (as well as molecular dynamics) calculations how structural symmetry breaking of V-V dimerization and magnetic spin symmetry breaking shape the insulator-to-metal transition in VO₂. The V-V dimerization tends to remove the magnetic moments of the dimerized V⁴⁺ ions whose occupied d states form a spin-singlet, and the magnetism tends to weaken the V-V dimerization. In the high-temperature metallic rutile phase of VO₂, the V-V dimers are uncoupled leading to unavoidable local magnetic moments on the V⁴⁺ ions with residual spin splitting, according to DFT supercell calculations. This type of competitive interaction between the structural and magnetic symmetry breakings could deepen the understanding of the insulator-to-metal transition mechanism in open d-shell transition metal oxides and be utilized to realize a type of structural-magnetic logic devices.   

Phase transition from superconductivity to phase separation

We study the evolution of one-body propagator from superconducting state to a phase separated state of a spin-less fermion model with nearest neighboring attraction V. Our sign-free determinant quantum Monte Carlo calculations give two phases of distinct characteristics. Under weak attraction, the system is in a superconducting state displaying a superconducting gap in the weakly dressed dispersion. Under strong attraction, the system is in a phase separated state in which most particles clustered together, resulting in a strong binding energy 4V in the spectral function. As the attraction decreases near a critical value, the cluster begins to dissolve and charge 2e bosons start to emerge. We further investigate the phase transition between these two distinct quantum states of matter.   

Flat-band-based multifractality in the all-bands-flat diamond chain.

Flat band systems have drawn significant interest in recent times as they exhibit the twin features of compact localization and large-scale degeneracy. The localization properties and associated repressed transport have been discussed in the context of engineered lattices in one, two and three dimensions. Conventional wisdom suggests that introducing disorder in such systems would lead to the destruction of compact localization of the eigenstates and the associated degeneracy. This poster examines the interplay of dispersionless bands, compact localized eigenstates and quasiperiodic Aubry-Andr´e (AA) disorder in a one-dimensional all-band-flat (ABF) diamond chain. We show that surprisingly when the AA potential is applied in a specific symmetric manner to an ABF diamond chain, compact localization remains robust, although the degeneracy is broken. An even more exciting finding is that when the disorder is applied in an antisymmetric manner, all the eigenstates exhibit a multifractal nature below a critical strength of the applied potential with the existence of a central band that remains multifractal at all strengths of the potential. A further novel aspect of our work is the finding that chiral symmetry is robust even in the presence of disorder, which we show with explicit construction of the chiral symmetry operator.   

Transition to an excitonic-insulator from a two-dimensional conventional-insulator

We present a general formulation to investigate the ground-state and elementary excitations of an excitonic insulator (EI) in real materials. In addition, we discuss the out-of-equilibrium state induced (albeit transiently) by high-intensity light illumination of a conventional 2D insulator. We then, present various band-structure models which allow us to study the transition from a conventional insulator to an EI in two-dimensional (2D) materials as a function of the dielectric constant, the conventional insulator gap (and chemical potential), the bandwidths of the conduction and valence bands and the Bravais lattice unit-cell size. One of the goals of this investigation is to determine which range of these experimentally determined parameters to consider in order to find the best candidate materials to realize the excitonic insulator. The numerical solution to the EI gap equation for various band-structures shows a significant and interesting momentum-dependence of the EI gap function and of the zero-temperature electron and hole momentum-distribution across the Brillouin zone. Last, we discuss that these features can be detected by tunneling microscopy.