arXiv subjects

Max A. Metlitski

Publications and source records attributed to Max A. Metlitski.

50 records · Page 3Linked to original sources

arXiv2008

Edge and impurity response in two-dimensional quantum antiferromagnets

Motivated by recent Monte-Carlo simulations of Hoglund and Sandvik (arXiv:0808.0408), we study edge response in square lattice quantum antiferromagnets. We use the O(3) non-linear sigma-model to compute the decay asymptotics of the staggered magnetization, energy density and local magnetic susceptibility away from the edge. We find that the total edge susceptibility is negative and diverges logarithmically as the temperature vanishes. We confirm the predictions of the continuum theory by performing a 1/S expansion of the microscopic Heisenberg model with the edge. We propose a qualitative explanation of the edge dimerization seen in Monte-Carlo simulations by a theory of valence-bond-solid correlations in the Neel state. We also discuss the extension of the latter theory to the response of a single non-magnetic impurity, and its connection to the theory of the deconfined critical point.

cond-mat.str-el↗

arXiv2008

Destruction of Neel order in the cuprates by electron-doping

Motivated by the evidence in PCCO and NCCO of a magnetic quantum critical point at which Neel order is destroyed, we study the evolution with doping of the T=0 quantum phases of the electron doped cuprates. At low doping, there is a metallic Neel state with small electron Fermi pockets, and this yields a fully gapped d_{x^2-y^2} superconductor with co-existing Neel order at low temperatures. We analyze the routes by which the spin-rotation symmetry can be restored in these metallic and superconducting states. In the metal, the loss of Neel order leads to a topologically ordered `doublon metal' across a deconfined critical point with global O(4) symmetry. In the superconductor, in addition to the conventional spin density wave transition, we find a variety of unconventional possibilities, including transitions to a nematic superconductor and to valence bond supersolids. Measurements of the spin correlation length and of the anomalous dimension of the Neel order by neutron scattering or NMR should discriminate these unconventional transitions from spin density wave theory.

cond-mat.str-el↗

arXiv2008

Valence bond solid order near impurities in two-dimensional quantum antiferromagnets

Recent scanning tunnelling microscopy (STM) experiments on underdoped cuprates have displayed modulations in the local electronic density of states which are centered on a Cu-O-Cu bond (Kohsaka et. al., cond-mat/0703309). As a paradigm of the pinning of such bond-centered ordering in strongly correlated systems, we present the theory of valence bond solid (VBS) correlations near a single impurity in a square lattice antiferromagnet. The antiferromagnet is assumed to be in the vicinity of a quantum transition from a magnetically ordered Neel state to a spin-gap state with long-range VBS order. We identify two distinct classes of impurities: i) local modulation in the exchange constants, and ii) a missing or additional spin, for which the impurity perturbation is represented by an uncompensated Berry phase. The `boundary' critical theory for these classes is developed: in the second class we find a `VBS pinwheel' around the impurity, accompanied by a suppression in the VBS susceptibility. Implications for numerical studies of quantum antiferromagnets and for STM experiments on the cuprates are noted.

cond-mat.str-el↗

arXiv2007

Wilson Loops in Non-Compact U(1) Gauge Theories at Criticality

We study the properties of Wilson loops in three dimensional non-compact U(1) gauge theories with global abelian symmetries. We use duality in the continuum and on the lattice, to argue that close to the critical point between the Higgs and Coulomb phases, all correlators of the Wilson loops are periodic functions of the Wilson loop charge, Q. The period depends on the global symmetry of the theory, which determines the magnetic flux carried by the dual particles. For single flavour scalar electrodynamics, the emergent period is Q = 1. In the general case of N complex scalars with a U(1)^{N-1} global symmetry, the period is Q = N. We also give some arguments why this phenomenon does not generalize to theories with a full non-abelian SU(N) symmetry, where no periodicity in Q is expected. Implications for lattice simulations, as well as for physical systems, such as easy plane antiferromagnets and disordered superfluids, are noted.

hep-th↗

arXiv2007

Impurity spin textures across conventional and deconfined quantum critical points of two-dimensional antiferromagnets

We describe the spin distribution in the vicinity of a non-magnetic impurity in a two-dimensional antiferromagnet undergoing a transition from a magnetically ordered Neel state to a paramagnet with a spin gap. The quantum critical ground state in a finite system has total spin S=1/2 (if the system without the impurity had an even number of S=1/2 spins), and recent numerical studies in a double layer antiferromagnet (K. H.Hoglund et al., cond-mat/0611418) have shown that the spin has a universal spatial form delocalized across the entire sample. We present the field theory describing the uniform and staggered magnetizations in this spin texture for two classes of antiferromagnets: (i) the transition from a Neel state to a paramagnet with local spin singlets, in models with an even number of S=1/2 spins per unit cell, which are described by a O(3) Landau-Ginzburg-Wilson field theory; and (ii) the transition from a Neel state to a valence bond solid, in antiferromagnets with a single S=1/2 spin per unit cell, which are described by a deconfined field theory of spinons.

cond-mat.str-el↗

arXiv2006

Is Schwinger Model at Finite Density a Crystal?

It has been believed since the paper by Fischler, Kogut and Susskind (FKS) that in QED_2 at finite charge density the chiral condensate exhibits a spatially inhomogeneous, oscillating behaviour. In this paper we demonstrate that this inhomogeneity is due to unphysical explicit breaking of the translational invariance by a uniform background charge density. Moreover, we investigate in the context of a simple statistical model what happens if the neutralizing background is composed instead of heavy, but dynamical, particles. We find that in contrast to the standard picture of FKS, the chiral condensate will not exhibit coherent oscillations on large distance scales, unless the heavy neutralizing particles themselves form a crystal and the density is high.

hep-th↗

arXiv2005

$θ$-dependence of QCD at Finite Isospin Density

We probe the $θ$-dependence of QCD at finite isospin chemical potential $μ_I$ using the effective chiral Lagrangian approach. The phase diagram in the $θ$, $μ_I$ plane is constructed and described in detail in terms of chiral and pion condensates. The physics at $θ~ π$ is investigated in both the normal and superfluid phase. Finally, the behaviour of the gluon condensate at finite $μ_I$ is computed.

hep-ph↗

arXiv2005

$θ-$ Parameter in 2 Color QCD at Finite Baryon and Isospin Density

We use 2-color QCD as a model to study the effects of simultaneous presence of the so-called $θ$ parameter, chemical potentials for baryon number, $μ_B$ and for isospin charge, $μ_I$. We pay special attention to $θ$, $μ_B$, $μ_I$ dependence of different vacuum condensates, including chiral and diquark condensates, as well as the gluon condensate, $ $, and the topological susceptibility. We find that two phase transitions of the second order will occur when $θ$ relaxes from $θ=2π$ to $θ=0$, if $μ$ is of order of the pion mass. We demonstrate that the transition to the superfluid phase at $θ= π$ occurs at a much lower chemical potential than at $θ= 0$. We also show that the strong $θ$ dependence present near $θ= π$ in vacuum (Dashen's phenomenon), becomes smoothed out in the superfluid phase. Finally, we comment on the relevance of this study for the real world with N_c=3.

hep-ph↗

arXiv2005

Anomalous Axion Interactions and Topological Currents in Dense Matter

Recently an effective Lagrangian for the interactions of photons, Nambu-Goldstone bosons and superfluid phonons in dense quark matter has been derived using anomaly matching arguments. In this paper we illuminate the nature of certain anomalous terms in this Lagrangian by an explicit microscopic calculation. We also generalize the corresponding construction to introduce the axion field. We derive an anomalous axion effective Lagrangian describing the interactions of axions with photons and superfluid phonons in the dense matter background. This effective Lagrangian, among other things, implies that an axion current will be induced in the presence of magnetic field. We speculate that this current may be responsible for the explanation of neutron star kicks.

hep-ph↗

arXiv2005

Currents on Superconducting Strings at Finite Chemical Potential and Temperature

We consider the model giving rise to Witten's superconducting cosmic strings at finite fermion chemical potential and temperature. We demonstrate how various symmetries of the hamiltonian can be used to exactly compute the fermion electric current in the string background. We show that the current along the string is not sensitive to the profiles of the string fields, and at fixed chemical potential and temperature depends only on the string winding number, the total gauge flux through the vortex and, possibly, the fermion mass at infinity.

hep-ph↗

arXiv2004

Neutron Stars as Type-I Superconductors

In a recent paper by Link, it was pointed out that the standard picture of the neutron star core composed of a mixture of a neutron superfluid and a proton type-II superconductor is inconsistent with observations of a long period precession in isolated pulsars. In the following we will show that an appropriate treatment of the interacting two-component superfluid (made of neutron and proton Cooper pairs), when the structure of proton vortices is strongly modified, may dramatically change the standard picture, resulting in a type-I superconductor. In this case the magnetic field is expelled from the superconducting regions of the neutron star leading to the formation of the intermediate state when alternating domains of superconducting matter and normal matter coexist.

astro-ph↗

arXiv2004

Vortices and type-I superconductivity in neutron stars

In a recent paper by Link, it was pointed out that the the standard picture of the neutron star core, composed of a mixture of a neutron superfluid and a proton type-II superconductor, is inconsistent with observations of long period precession in isolated pulsars. In the following we will show that intervortex force between the magnetic flux tubes may be attractive resulting in a type-I (rather than type-II) superconductor. In this case the magnetic field cannot exist in the form of magnetic flux tubes, supporting Link's observation. This behavior of the system is due to the strong interaction between the proton-neutron Cooper pairs, which was previously ignored. We also calculate the critical magnetic fields H_c and H_c2 for type-I/II superconductors. These results also support our claim of type-I superconductivity in the cores of neutron stars.

hep-ph↗

arXiv2003

Drum vortons in high density QCD

Recently it was shown that high density QCD supports of number of topological defects. In particular, there are U(1)_Y strings that arise due to K^0 condensation that occurs when the strange quark mass is relatively large. The unique feature of these strings is that they possess a nonzero K^+ condensate that is trapped on the core. In the following we will show that these strings (with nontrivial core structure) can form closed loops with conserved charge and currents trapped on the string worldsheet. The presence of conserved charges allows these topological defects, called vortons, to carry angular momentum, which makes them classically stable objects. We also give arguments demonstrating that vortons carry angular momentum very efficiently (in terms of energy per unit angular momentum) such that they might be the important degrees of freedom in the cores of neutron stars.

hep-ph↗

arXiv2003

Vortex Rings in two Component Bose-Einstein Condensates

We study the structure of the vortex core in two-component Bose-Einstein condensates. We demonstrate that the order parameter may not vanish and the symmetry may not be restored in the core of the vortex. In this case such vortices can form vortex rings known as vortons in particle physics literature. In contrast with well-studied superfluid $^4He$, where similar vortex rings can be stable due to Magnus force only if they move, the vortex rings in two-component BECs can be stable even if they are at rest. This beautiful effect was first discussed by Witten in the cosmic string context, where it was shown that the stabilization occurs due to condensation of the second component of the field in the vortex core. This second condensate trapped in the core may carry a current along the vortex ring counteracting the effect of string tension that causes the loop to shrink. We speculate that such vortons may have been already observed in the laboratory. We also speculate that the experimental study of topological structures in BECs can provide a unique opportunity to study cosmology and astrophysics by doing laboratory experiments.

cond-mat↗