Interaction Effects and Non-Integer Pseudo-Landau Levels in Engineered Periodically Strained Graphene.

Strain superlattices (SL) in 2D materials like graphene provide an ideal test bed for generating flat bands and exploring the effects of strong correlations. Here we report STM/STS measurements on an engineered SL generated by placing graphene on a periodic array of silica nanospheres. A pseudomagnetic field as high as 55 T is observed along with the formation of pseudo-Landau levels (pLLs), not only at the expected integer values but also at fractional values.

Strong-coupling critical behavior in three-dimensional lattice Abelian gauge models with charged N-component scalar fields and SO(N) symmetry.

We consider a three-dimensional lattice Abelian Higgs gauge model for a charged N-component scalar field ϕ, which is invariant under SO(N) global transformations for generic values of the parameters. We focus on the strong-coupling regime, in which the kinetic Hamiltonian term for the gauge field is a small perturbation, which is irrelevant for the critical behavior. The Hamiltonian depends on a parameter v, which determines the global symmetry of the model and the symmetry of the low-temperature phases.

Deconfinement transitions in three-dimensional compact lattice Abelian Higgs models with multiple-charge scalar fields.

We investigate the nature of the deconfinement transitions in three-dimensional lattice Abelian Higgs models, in which a complex scalar field of integer charge Q≥2 is minimally coupled with a compact U(1) gauge field. Their phase diagram presents two phases separated by a transition line where static charges q, with q View Article and Find Full Text PDF

Sphaleron Rate of N_{f}=2+1 QCD.

We compute the sphaleron rate of N_{f}=2+1 QCD at the physical point for a range of temperatures 200  MeV≲T≲600  MeV. We adopt a strategy recently applied in the quenched case, based on the extraction of the rate via a modified version of the Backus-Gilbert method from finite-lattice-spacing and finite-smoothing-radius Euclidean topological charge density correlators. The physical sphaleron rate is finally computed by performing a continuum limit at fixed physical smoothing radius, followed by a zero-smoothing extrapolation.

Coulomb-Higgs phase transition of three-dimensional lattice Abelian Higgs gauge models with noncompact gauge variables and gauge fixing.

We study the critical behavior of three-dimensional (3D) lattice Abelian Higgs (AH) gauge models with noncompact gauge variables and multicomponent complex scalar fields, along the transition line between the Coulomb and Higgs phases. Previous works that focused on gauge-invariant correlations provided evidence that, for a sufficiently large number of scalar components, these transitions are continuous and associated with the stable charged fixed point of the renormalization-group flow of the 3D AH field theory (scalar electrodynamics), in which charged scalar matter is minimally coupled with an electromagnetic field. Here we extend these studies by considering gauge-dependent correlations of the gauge and matter fields, in the presence of two different gauge fixings, the Lorenz and the axial gauge fixing.

Quantum critical behaviors and decoherence of weakly coupled quantum Ising models within an isolated global system.

We discuss the quantum dynamics of an isolated composite system consisting of weakly interacting many-body subsystems. We focus on one of the subsystems, S, and study the dependence of its quantum correlations and decoherence rate on the state of the weakly-coupled complementary part E, which represents the environment. As a theoretical laboratory, we consider a composite system made of two stacked quantum Ising chains, locally and homogeneously weakly coupled.

Color-flavor reflection in the continuum limit of two-dimensional lattice gauge theories with scalar fields.

We address the interplay between local and global symmetries in determining the continuum limit of two-dimensional lattice scalar theories characterized by SO(N_{c}) gauge symmetry and non-Abelian O(N_{f}) global invariance. We argue that, when a quartic interaction is present, the continuum limit of these models corresponds in some cases to the gauged nonlinear σ model field theory associated with the real Grassmannian manifold SO(N_{f})/(SO(N_{c})×SO(N_{f}-N_{c})), which is characterized by the invariance under the color-flavor reflection N_{c}↔N_{f}-N_{c}. Monte Carlo simulations and finite-size scaling analyses, performed for N_{f}=7 and several values of N_{c}, confirm the emergence of the color-flavor reflection symmetry in the scaling limit and support the identification of the continuum limit.

Critical crossover phenomena driven by symmetry-breaking defects at quantum transitions.

We study the effects of symmetry-breaking defects at continuous quantum transitions (CQTs) of homogeneous systems, which may arise from localized external fields coupled to the order-parameter operator. The problem is addressed within renormalization-group (RG) and finite-size scaling frameworks. We consider the paradigmatic one-dimensional quantum Ising models at their CQT, in the presence of defects which break the global Z_{2} symmetry.

Phase diagram and Higgs phases of three-dimensional lattice SU(N_{c}) gauge theories with multiparameter scalar potentials.

We consider three-dimensional lattice SU(N_{c}) gauge theories with degenerate multicomponent (N_{f}>1) complex scalar fields that transform under the fundamental representation of the gauge SU(N_{c}) group and of the global U(N_{f}) invariance group, interacting with the most general quartic potential compatible with the global (flavor) and gauge (color) symmetries. We investigate the phase diagrams, identifying the low-temperature Higgs phases and their global and gauge symmetries, and the critical behaviors along the different transition lines. In particular, we address the role of the quartic scalar potential, which determines the Higgs phases and the corresponding symmetry-breaking patterns.

Breaking of Gauge Symmetry in Lattice Gauge Theories.

We study perturbations that break gauge symmetries in lattice gauge theories. As a paradigmatic model, we consider the three-dimensional Abelian-Higgs (AH) model with an N-component scalar field and a noncompact gauge field, which is invariant under U(1) gauge and SU(N) transformations. We consider gauge-symmetry breaking perturbations that are quadratic in the gauge field, such as a photon mass term and determine their effect on the critical behavior of the gauge-invariant model, focusing mainly on the continuous transitions associated with the charged fixed point of the AH field theory.

Lattice gauge theories in the presence of a linear gauge-symmetry breaking.

We study the effects of gauge-symmetry breaking (GSB) perturbations in three-dimensional lattice gauge theories with scalar fields. We study this issue at transitions in which gauge correlations are not critical and the gauge symmetry only selects the gauge-invariant scalar degrees of freedom that become critical. A paradigmatic model in which this behavior is realized is the lattice CP^{1} model or, more generally, the lattice Abelian-Higgs model with two-component complex scalar fields and compact gauge fields.

Higher-charge three-dimensional compact lattice Abelian-Higgs models.

We consider three-dimensional higher-charge multicomponent lattice Abelian-Higgs (AH) models, in which a compact U(1) gauge field is coupled to an N-component complex scalar field with integer charge q, so that they have local U(1) and global SU(N) symmetries. We discuss the dependence of the phase diagram, and the nature of the phase transitions, on the charge q of the scalar field and the number N≥2 of components. We argue that the phase diagram of higher-charge models presents three different phases, related to the condensation of gauge-invariant bilinear scalar fields breaking the global SU(N) symmetry, and to the confinement and deconfinement of external charge-one particles.

Optical conductivity of triple point fermions.

As a low-energy effective theory on non-symmorphic lattices, we consider a generic triple point fermion Hamiltonian, which is parameterized by an angular parameter. We find strongdependence in both Drude and interband optical absorption of these systems. The deviation of thecoefficient of the Drude weight from Dirac/Weyl fermions can be used as a quick way to optically distinguish the triple point degeneracies from the Dirac/Weyl degeneracies.

Quantum Advantage in the Charging Process of Sachdev-Ye-Kitaev Batteries.

The exactly solvable Sachdev-Ye-Kitaev (SYK) model has recently received considerable attention in both condensed matter and high energy physics because it describes quantum matter without quasiparticles, while being at the same time the holographic dual of a quantum black hole. In this Letter, we examine SYK-based charging protocols of quantum batteries with N quantum cells. Extensive numerical calculations based on exact diagonalization for N up to 16 strongly suggest that the optimal charging power of our SYK quantum batteries displays a superextensive scaling with N that stems from genuine quantum mechanical effects.

Graphene Plasmonics: Fully Atomistic Approach for Realistic Structures.

We demonstrate that the plasmonic properties of realistic graphene and graphene-based materials can effectively and accurately be modeled by a novel, fully atomistic, yet classical, approach, named ωFQ. Such a model is able to reproduce all plasmonic features of these materials and their dependence on shape, dimension, and fundamental physical parameters (Fermi energy, relaxation time, and two-dimensional electron density). Remarkably, ωFQ is able to accurately reproduce experimental data for realistic structures of hundreds of nanometers (∼370k atoms), which cannot be afforded by any method.

Scaling properties of the dynamics at first-order quantum transitions when boundary conditions favor one of the two phases.

We address the out-of-equilibrium dynamics of a many-body system when one of its Hamiltonian parameters is driven across a first-order quantum transition (FOQT). In particular, we consider systems subject to boundary conditions favoring one of the two phases separated by the FOQT. These issues are investigated within the paradigmatic one-dimensional quantum Ising model, at the FOQTs driven by the longitudinal magnetic field h, with boundary conditions that favor the same magnetized phase (EFBC) or opposite magnetized phases (OFBC).

Three-dimensional monopole-free CP^{N-1} models.

We investigate the phase diagram and the nature of the phase transitions of three-dimensional monopole-free CP^{N-1} models, characterized by a global U(N) symmetry, a U(1) gauge symmetry, and the absence of monopoles. We present numerical analyses based on Monte Carlo simulations for N=2, 4, 10, 15, and 25. We observe a finite-temperature transition in all cases, related to the condensation of a local gauge-invariant order parameter.

Three-dimensional phase transitions in multiflavor lattice scalar SO(N_{c}) gauge theories.

We investigate the phase diagram and finite-temperature transitions of three-dimensional scalar SO(N_{c}) gauge theories with N_{f}≥2 scalar flavors. These models are constructed starting from a maximally O(N)-symmetric multicomponent scalar model (N=N_{c}N_{f}), whose symmetry is partially gauged to obtain an SO(N_{c}) gauge theory, with O(N_{f}) or U(N_{f}) global symmetry for N_{c}≥3 or N_{c}=2, respectively. These systems undergo finite-temperature transitions, where the global symmetry is broken.

Publisher Correction: Control of electron-electron interaction in graphene by proximity screening.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Control of electron-electron interaction in graphene by proximity screenings.

Electron-electron interactions play a critical role in many condensed matter phenomena, and it is tempting to find a way to control them by changing the interactions' strength. One possible approach is to place a studied system in proximity of a metal, which induces additional screening and hence suppresses electron interactions. Here, using devices with atomically-thin gate dielectrics and atomically-flat metallic gates, we measure the electron-electron scattering length in graphene and report qualitative deviations from the standard behavior.

Phase Diagram, Symmetry Breaking, and Critical Behavior of Three-Dimensional Lattice Multiflavor Scalar Chromodynamics.

We study the nature of the phase diagram of three-dimensional lattice models in the presence of non-Abelian gauge symmetries. In particular, we consider a paradigmatic model for the Higgs mechanism, lattice scalar chromodynamics with N_{f} flavors, characterized by a non-Abelian SU(N_{c}) gauge symmetry. For N_{f}≥2 (multiflavor case), it presents two phases separated by a transition line where a gauge-invariant order parameter condenses, being associated with the breaking of the residual global symmetry after gauging.

Multicomponent compact Abelian-Higgs lattice models.

We investigate the phase diagram and critical behavior of three-dimensional multicomponent Abelian-Higgs models, in which an N-component complex field z_{x}^{a} of unit length and charge is coupled to compact quantum electrodynamics in the usual Wilson lattice formulation. We determine the phase diagram and study the nature of the transition line for N=2 and N=4. Two phases are identified, specified by the behavior of the gauge-invariant local composite operator Q_{x}^{ab}=z[over ¯]_{x}^{a}z_{x}^{b}-δ^{ab}/N, which plays the role of order parameter.

Three-dimensional ferromagnetic CP^{N-1} models.

We investigate the critical behavior of three-dimensional ferromagnetic CP^{N-1} models, which are characterized by a global U(N) and a local U(1) symmetry. We perform numerical simulations of a lattice model for N=2, 3, and 4. For N=2 we find a critical transition in the Heisenberg O(3) universality class, while for N=3 and 4 the system undergoes a first-order transition.

Josephson Field-Effect Transistors Based on All-Metallic Al/Cu/Al Proximity Nanojunctions.

We demonstrate proximity-based mesoscopic superconductor-normal metal-superconductor (SNS) field-effect controlled Josephson transistors (SNS-FETs) and show their full characterization from the critical temperature down to 50 mK in the presence of both electric and magnetic fields. The ability of a static electric field-applied by means of a lateral gate electrode-to suppress the critical current in a proximity-induced superconductor is proven for both positive and negative gate voltage values. reached typically about one-third of its initial value, saturating at high gate voltages.