Showing papers on "Symmetry (physics) published in 2014"

Generalized Global Symmetries

Abstract: A $q$-form global symmetry is a global symmetry for which the charged operators are of space-time dimension $q$; e.g. Wilson lines, surface defects, etc., and the charged excitations have $q$ spatial dimensions; e.g. strings, membranes, etc. Many of the properties of ordinary global symmetries ($q$=0) apply here. They lead to Ward identities and hence to selection rules on amplitudes. Such global symmetries can be coupled to classical background fields and they can be gauged by summing over these classical fields. These generalized global symmetries can be spontaneously broken (either completely or to a subgroup). They can also have 't Hooft anomalies, which prevent us from gauging them, but lead to 't Hooft anomaly matching conditions. Such anomalies can also lead to anomaly inflow on various defects and exotic Symmetry Protected Topological phases. Our analysis of these symmetries gives a new unified perspective of many known phenomena and uncovers new results.

Edge Nonlinear Optics on a MoS2 Atomic Monolayer

Abstract: The translational symmetry breaking of a crystal at its surface may form two-dimensional (2D) electronic states. We observed one-dimensional nonlinear optical edge states of a single atomic membrane of molybdenum disulfide (MoS2), a transition metal dichalcogenide. The electronic structure changes at the edges of the 2D crystal result in strong resonant nonlinear optical susceptibilities, allowing direct optical imaging of the atomic edges and boundaries of a 2D material. Using the symmetry of the nonlinear optical responses, we developed a nonlinear optical imaging technique that allows rapid and all-optical determination of the crystal orientations of the 2D material at a large scale. Our technique provides a route toward understanding and making use of the emerging 2D materials and devices.

Evidence for a New Soft Graviton Theorem

Abstract: The single-soft-graviton limit of any quantum gravity scattering amplitude is given at leading order by the universal Weinberg pole formula. Gauge invariance of the formula follows from global energy-momentum conservation. In this paper evidence is given for a conjectured universal formula for the finite subleading term in the expansion about the soft limit, whose gauge invariance follows from global angular momentum conservation. The conjecture is non-trivially verified for all tree-level graviton scattering amplitudes using a BCFW recursion relation. One hopes to understand this infinity of new soft relations as a Ward identity for a new superrotation Virasoro symmetry of the quantum gravity S-matrix.

Black hole hair in generalized scalar-tensor gravity.

Abstract: The most general action for a scalar field coupled to gravity that leads to second-order field equations for both the metric and the scalar---Horndeski's theory---is considered, with the extra assumption that the scalar satisfies shift symmetry. We show that in such theories, the scalar field is forced to have a nontrivial configuration in black hole spacetimes, unless one carefully tunes away a linear coupling with the Gauss-Bonnet invariant. Hence, black holes for generic theories in this class will have hair. This contradicts a recent no-hair theorem which seems to have overlooked the presence of this coupling.

Weakly interacting massive particle-nucleus elastic scattering response

Abstract: A model independent formulation of WIMP-nucleon scattering was recently developed in Galileaninvariant effective field theory and embedded in the nucleus, determining the most general WIMP-nucleus elastic response. This formulation shows that the standard description of WIMP elastic scattering in terms spin-dependent and spin-independent responses frequently fails to identify the dominant operators governing the scattering, omitting four of the six responses allowed by basic symmetry considerations. Consequently comparisons made between experiments that are based on a spin-independent/spindependent analysis can be misleading for many candidate interactions, mischaracterizing the magnitude and multipolarity (e.g., scalar or vector) of the scattering. The new responses are associated with velocitydependent WIMP couplings and correspond to familiar electroweak nuclear operators such as the orbital angular momentum ~l(i) and the spin-orbit interaction ~σ(i) ·~l(i). Such operators have distinct selection rules and coherence properties, and thus open up new opportunities for using low-energy measurements to constrain ultraviolet theories of dark matter. The community’s reliance on simplified descriptions of WIMP-nucleus interactions reflects the absence of analysis tools that integrate general theories of dark matter with standard treatments of nuclear response functions. To bridge this gap, we have constructed a public-domain Mathematica package for WIMP analyses based on our effective theory formulation. Script inputs are 1) the coefficients of the effective theory, through which one can characterize the low-energy consequences of arbitrary ultraviolet theories of WIMP interactions; and 2) one-body density matrices for commonly used targets, the most compact description of the relevant nuclear physics. The generality of the effective theory expansion guarantees that the script will remain relevant as new ultraviolet theories are explored; the use of density matrices to factor the nuclear physics from the particle physics will allow nuclear structure theorists to update the script as new calculations become available, independent of specific particle-physics contexts. The Mathematica package outputs the resulting response functions (and associated form factors) and also the differential event rate, once a galactic WIMP velocity profile is specified, and thus in its present form provides a complete framework for experimental analysis. The Mathematica script requires no a priori knowledge of the details of the non-relativistic effective field theory or nuclear physics, though the core concepts are reviewed here and in [1].

Semiclassical Virasoro symmetry of the quantum gravity S-matrix

Abstract: It is shown that the tree-level $$ \mathcal{S} $$ -matrix for quantum gravity in four-dimensional Minkowski space has a Virasoro symmetry which acts on the conformal sphere at null infinity.

Scalar phi^4 field theory for active-particle phase separation

Abstract: Active matter can separate into dense and dilute phases in the absence of inter-particle attraction, making it distinct from passive matter. Wittkowski et al. develop a dynamical theory to elucidate the physics at play, which includes a specific term that violates microscopic time-reversal symmetry.

Building Water Models: A Different Approach.

Abstract: Simplified classical water models are currently an indispensable component in practical atomistic simulations. Yet, despite several decades of intense research, these models are still far from perfect. Presented here is an alternative approach to constructing widely used point charge water models. In contrast to the conventional approach, we do not impose any geometry constraints on the model other than the symmetry. Instead, we optimize the distribution of point charges to best describe the “electrostatics” of the water molecule. The resulting “optimal” 3-charge, 4-point rigid water model (OPC) reproduces a comprehensive set of bulk properties significantly more accurately than commonly used rigid models: average error relative to experiment is 0.76%. Close agreement with experiment holds over a wide range of temperatures. The improvements in the proposed model extend beyond bulk properties: compared to common rigid models, predicted hydration free energies of small molecules using OPC are uniformly clos...

Theory of plasma confinement in non-axisymmetric magnetic fields

Abstract: The theory of plasma confinement by non-axisymmetric magnetic fields is reviewed. Such fields are used to confine fusion plasmas in stellarators, where in contrast to tokamaks and reversed-field pinches the magnetic field generally does not possess any continuous symmetry. The discussion is focussed on magnetohydrodynamic equilibrium conditions, collisionless particle orbits, and the kinetic theory of equilbrium and transport. Each of these topics is fundamentally affected by the absence of symmetry in the magnetic field: the field lines need not trace out nested flux surfaces, the particle orbits may not be confined, and the cross-field transport can be very large. Nevertheless, by tailoring the magnetic field appropriately, well-behaved equilibria with good confinement can be constructed, potentially offering an attractive route to magnetic fusion. In this article, the mathematical apparatus to describe stellarator plasmas is developed from first principles and basic elements underlying confinement optimization are introduced.

PT-symmetry in optics

Abstract: We review quantum mechanical and optical pseudo- Hermitian systems with an emphasis on PT-symmetric systems important for optics and electrodynamics. One of the most

Analytical perspective for bound states in the continuum in photonic crystal slabs.

Abstract: We investigate the formation of photonic bound states in the continuum (BICs) in photonic crystal slabs from an analytical perspective. Unlike the stationary at-Γ BICs which originate from the geometric symmetry, the tunable off-Γ BICs are due to the weighted destructive via the continuum interference in the vicinity of accidental symmetry when the majority of the radiation is precanceled. The symmetric compatible nature of the off-Γ BICs leads to a trapping of light that can be tuned through continuously varying the wave vector. With the analytical approach, we explain a reported experiment and predict the existence of a new BIC at an unrevealed symmetry.

Monopole operators and Hilbert series of Coulomb branches of 3d \mathcal{N} = 4 gauge theories

Abstract: This paper addresses a long standing problem - to identify the chiral ring and moduli space (i.e. as an algebraic variety) on the Coulomb branch of an $ \mathcal{N} $ = 4 superconformal field theory in 2+1 dimensions. Previous techniques involved a computation of the metric on the moduli space and/or mirror symmetry. These methods are limited to sufficiently small moduli spaces, with enough symmetry, or to Higgs branches of sufficiently small gauge theories. We introduce a simple formula for the Hilbert series of the Coulomb branch, which applies to any good or ugly three-dimensional $ \mathcal{N} $ = 4 gauge theory. The formula counts monopole operators which are dressed by classical operators, the Casimir invariants of the residual gauge group that is left unbroken by the magnetic flux. We apply our formula to several classes of gauge theories. Along the way we make various tests of mirror symmetry, successfully comparing the Hilbert series of the Coulomb branch with the Hilbert series of the Higgs branch of the mirror theory.

Low's subleading soft theorem as a symmetry of QED.

Abstract: It was shown by F. Low in the 1950s that the subleading terms of soft photon Smatrix elements obey a universal linear relation. In this paper we give a new interpretation to this old relation, for the case of massless QED, as an infinitesimal symmetry of the S-matrix. The symmetry is shown to be locally generated by a vector field on the conformal sphere at null infinity. Explicit expressions are constructed for the associated charges as integrals over null infinity and shown to generate the symmetry. These charges are local generalizations of electric and magnetic dipole charges.

A way forward in the study of the symmetry energy: experiment, theory, and observation

Abstract: The symmetry energy describes how the energy of nuclear matter rises as one goes away from equal numbers of neutrons and protons. This is very important to describe neutron rich matter in astrophysics. This article reviews our knowledge of the symmetry energy from theoretical calculations, nuclear structure measurements, heavy-ion collisions, and astronomical observations. We then present a roadmap to make progress in areas of relevance to the symmetry energy that promotes collaboration between the astrophysics and the nuclear physics communities.

Many-body localization and symmetry-protected topological order

Abstract: Recent work shows that highly excited many-body localized eigenstates can exhibit broken symmetries and topological order, including in dimensions where such order would be forbidden in equilibrium. In this paper we extend this analysis to discrete symmetry-protected order via the explicit examples of the Haldane phase of one-dimensional spin chains and the topological Ising paramagnet in two dimensions. We comment on the challenge of extending these results to cases where the protecting symmetry is continuous.

New Schwarzschild-like solutions in f(T) gravity through Noether symmetries

Abstract: Spherically symmetric solutions for $f(T)$ gravity models are derived by the so-called Noether symmetry approach. First, we present a full set of Noether symmetries for some minisuperspace models. Then, we compute analytical solutions and find that spherically symmetric solutions in $f(T)$ gravity can be recast in terms of Schwarzschild-like solutions modified by a distortion function depending on a characteristic radius. The obtained solutions are more general than those obtained by the usual solution methods.

Total absorption by degenerate critical coupling

Abstract: We consider a mirror-symmetric resonator with two ports. We show that, when excited from a single port, complete absorption can be achieved through critical coupling to degenerate resonances with opposite symmetry. Moreover, any time two resonances with opposite symmetry are degenerate in frequency and absorption is always significantly enhanced. In contrast, when two resonances with the same symmetry are nearly degenerate, there is no absorption enhancement. We numerically demonstrate these effects using a graphene monolayer on top of a photonic crystal slab, illuminated from a single side in the near-infrared.

Holographic Metals and Insulators with Helical Symmetry

Abstract: Homogeneous, zero temperature scaling solutions with Bianchi VII spa- tial geometry are constructed in Einstein-Maxwell-Dilaton theory. They correspond to quantum critical saddle points with helical symmetry at nite density. Assum- ing AdS5 UV asymptotics, the small frequency/(temperature) dependence of the AC/(DC) electric conductivity along the director of the helix are computed. A large class of insulating and conducting anisotropic phases is found, as well as isotropic, metallic phases. Conduction can be dominated by dissipation due to weak breaking of translation symmetry or by a quantum critical current.

Three-Dimensional Spirals of Atomic Layered MoS2

Abstract: Atomically thin two-dimensional (2D) layered materials, including graphene, boron nitride, and transition metal dichalcogenides (TMDs), can exhibit novel phenomena distinct from their bulk counterparts and hold great promise for novel electronic and optoelectronic applications. Controlled growth of such 2D materials with different thickness, composition, and symmetry are of central importance to realize their potential. In particular, the ability to control the symmetry of TMD layers is highly desirable because breaking the inversion symmetry can lead to intriguing valley physics, nonlinear optical properties, and piezoelectric responses. Here we report the first chemical vapor deposition (CVD) growth of spirals of layered MoS2 with atomically thin helical periodicity, which exhibits a chiral structure and breaks the three-dimensional (3D) inversion symmetry explicitly. The spirals composed of tens of connected MoS2 layers with decreasing areas: each basal plane has a triangular shape and shrinks graduall...

Symmetry-protected topological orders for interacting fermions: Fermionic topological nonlinear σ models and a special group supercohomology theory

Abstract: Symmetry-protected topological (SPT) phases are gapped quantum phases with a symmetry, which can be smoothly connected to the trivial product states only if we break the symmetry. For a given symmetry, we can have many different SPT phases. But how to describe/construct those different SPT phases that can not be distinguished by their symmetry? It has been shown that different bosonic SPT phases in any dimensions and for any symmetry groups can be described/constructed using group cohomology theory of the symmetry group. In this paper, we introduce a group super-cohomology theory which is a generalization of the standard group cohomology theory. Using the group super-cohomology theory, we can describe/construct different interacting fermionic SPT phases, in any dimensions and for symmetry groups where the fermions form 1D representations of the symmetry group. Just like the boson case, our systematic construction is based on constructing discrete fermionic topological non-linear σ-models from the group super-cohomology theory. Our discrete fermionic topological non-linear σ-model, when defined on a space-time with boundary, can be viewed as a “non-local” boundary effective Lagrangian, which is a fermionic and discrete generalization of the bosonic continuous Wess-Zumino-Witten term. Thus we believe that the boundary excitations of a non-trivial SPT phase are gapless if the symmetry is not broken. As an application of this general result, we construct three non-trivial SPT phases in 3D, for interacting fermionic superconductors with coplanar spin order (which have T 2 = 1 time-reversal Z 2 and fermion-number-parity Z 2 symmetries described by a full symmetry group Z T 2 × Z 2 ). We also construct three interacting fermionic SPT phases in 2D with a full symmetry group Z2×Z 2 . Those 2D fermionic SPT phases all have central-charge c = 1 gapless edge excitations, if the symmetry is not broken.

A source of antihydrogen for in-flight hyperfine spectroscopy

Abstract: Antihydrogen, a positron bound to an antiproton, is the simplest antiatom. Its counterpart-hydrogen--is one of the most precisely investigated and best understood systems in physics research. High-resolution comparisons of both systems provide sensitive tests of CPT symmetry, which is the most fundamental symmetry in the Standard Model of elementary particle physics. Any measured difference would point to CPT violation and thus to new physics. Here we report the development of an antihydrogen source using a cusp trap for in-flight spectroscopy. A total of 80 antihydrogen atoms are unambiguously detected 2.7 m downstream of the production region, where perturbing residual magnetic fields are small. This is a major step towards precision spectroscopy of the ground-state hyperfine splitting of antihydrogen using Rabi-like beam spectroscopy.

Group theory analysis of phonons in two-dimensional transition metal dichalcogenides

Abstract: Transition metal dichalcogenides (TMDCs) have emerged as a new two-dimensional material's field since the monolayer and few-layer limits show different properties when compared to each other and to their respective bulk materials. For example, in some cases when the bulk material is exfoliated down to a monolayer, an indirect-to-direct band gap in the visible range is observed. The number of layers $N$ ($N$ even or odd) drives changes in space-group symmetry that are reflected in the optical properties. The understanding of the space-group symmetry as a function of the number of layers is therefore important for the correct interpretation of the experimental data. Here we present a thorough group theory study of the symmetry aspects relevant to optical and spectroscopic analysis, for the most common polytypes of TMDCs, i.e., $2Ha$, $2Hc$ and $1T$, as a function of the number of layers. Real space symmetries, the group of the wave vectors, the relevance of inversion symmetry, irreducible representations of the vibrational modes, optical activity, and Raman tensors are discussed.

Evidence for TriangularD3hSymmetry inC12

Abstract: The discovery of a new excited state in carbon-12 hints at this nucleus's triangular symmetry.

A way forward in the study of the symmetry energy: experiment, theory, and observation

Abstract: The symmetry energy describes how the energy of nuclear matter rises as one goes away from equal numbers of neutrons and protons. This is very important to describe neutron rich matter in astrophysics. This article reviews our knowledge of the symmetry energy from theoretical calculations, nuclear structure measurements, heavy ion collisions, and astronomical observations. We then present a roadmap to make progress in areas of relevance to the symmetry energy that promotes collaboration between the astrophysics and the nuclear physics communities.

Coexisting massive and massless Dirac fermions in symmetry-broken bilayer graphene

Abstract: Charge carriers in bilayer graphene are widely believed to be massive Dirac fermions that have a bandgap tunable by a transverse electric field. However, a full transport gap, despite its importance for device applications, has not been clearly observed in gated bilayer graphene, a long-standing puzzle. Moreover, the low-energy electronic structure of bilayer graphene is widely held to be unstable towards symmetry breaking either by structural distortions, such as twist, strain, or electronic interactions that can lead to various ground states. Which effect dominates the physics at low energies is hotly debated. Here we show both by direct band-structure measurements and by calculations that a native imperfection of bilayer graphene, a distribution of twists whose size is as small as ~0.1°, is sufficient to generate a completely new electronic spectrum consisting of massive and massless Dirac fermions. The massless spectrum is robust against strong electric fields, and has a unusual topology in momentum space consisting of closed arcs having an exotic chiral pseudospin texture, which can be tuned by varying the charge density. The discovery of this unusual Dirac spectrum not only complements the framework of massive Dirac fermions, widely relevant to charge transport in bilayer graphene, but also supports the possibility of valley Hall transport.

Effect of interactions on two-dimensional fermionic symmetry-protected topological phases with Z 2 symmetry

Abstract: We study the effect of interactions on 2D fermionic symmetry-protected topological (SPT) phases using the recently proposed braiding statistics approach. We focus on a simple class of examples: superconductors with a Z2 Ising symmetry. Although these systems are classified by Z in the noninteracting limit, our results suggest that the classification collapses to Z8 in the presence of interactions -- consistent with previous work that analyzed the stability of the edge. Specifically, we show that there are at least 8 different types of Ising superconductors that cannot be adiabatically connected to one another, even in the presence of strong interactions. In addition, we prove that each of the 7 nontrivial superconductors have protected edge modes.

On fractional Choquard equations

Abstract: We investigate a class of nonlinear Schrodinger equations with a generalized Choquard nonlinearity and fractional diffusion. We obtain regularity, existence, nonexistence, symmetry as well as decays properties.