arXiv subjects

Zohar Nussinov

Publications and source records attributed to Zohar Nussinov.

121 records · Page 7Linked to original sources

arXiv2004

Orbital ordering in transition-metal compounds: I. The 120-degree model

We study the classical version of the 120-degree model. This is an attractive nearest-neighbor system in three dimensions with XY (rotor) spins and interaction such that only a particular projection of the spins gets coupled in each coordinate direction. Although the Hamiltonian has only discrete symmetries, it turns out that every constant field is a ground state. Employing a combination of spin-wave and contour arguments we establish the existence of long-range order at low temperatures. This suggests a mechanism for a type of ordering in certain models of transition-metal compounds where the very existence of long-range order has heretofore been a matter of some controversy.

cond-mat.stat-mech↗

arXiv2004

Orbital order in classical models of transition-metal compounds

We study the classical 120-degree and related orbital models. These are the classical limits of quantum models which describe the interactions among orbitals of transition-metal compounds. We demonstrate that at low temperatures these models exhibit a long-range order which arises via an "order by disorder" mechanism. This strongly indicates that there is orbital ordering in the quantum version of these models, notwithstanding recent rigorous results on the absence of spin order in these systems.

cond-mat.stat-mech↗

arXiv2004

Commensurate and Incommensurate O(n) Spin Systems: Novel Even-Odd Effects, A Generalized Mermin-Wagner-Coleman Theorem, and Ground States

We examine n component spin systems with arbitrary two spin interactions (of unspecified range) within a general framework to highlight some new subtleties present in incommensurate systems. We determine the ground states of all translationally invariant O(n>1) systems and prove that barring commensurability effects they are always spiral- no other ground states are possible. We study the effect of thermal fluctuations on the ground states to discover a novel odd-even n effect. Soft spin analysis suggests that algebraic long range order is possible in certain frustrated incommensurate even n systems while their odd n counterparts exhibit an exponential decay of correlations. We illustrate that many frustrated incommensurate continuous spin systems display smectic like thermodynamics. We report on a generalized Mermin-Wagner-Coleman theorem for all two dimensional systems (of arbitrary range) with analytic kernels in momentum space. A new relation between generalization Mermin-Wagner-Coleman bounds and dynamics is further reported. We suggest a link between a generalized Mermin-Wagner-Coleman theorem to divergent decoherence (or bandwidth) time scale in the quantum context. A generalization of the Peierls bound for commensurate systems with long range interactions is also discussed. We conclude with a discussion of O(n) spin dynamics in the general case.

cond-mat.stat-mech↗

arXiv2004

Novel Spin Dynamics in a Josephson Junction

We address the dynamics of a single spin embedded in the tunneling barrier between two superconductors. As a consequence of pair correlations in the superconducting state, the spin displays a rich and unusual dynamics. To properly describe the time evolution of the spin we find the generalized Wess-Zumino-Witten-Novikov term in the effective action for the spin on the Keldysh contour. The superconducting correlations lead to an effective spin action which is non-local in time leading to unconventional precessions. Our predictions might be directly tested for macroscopic spin clusters.

cond-mat.mes-hall↗

arXiv2003

Avoided Phase Transitions and Glassy Dynamics In Geometrically Frustrated Systems and Non-Abelian Theories

We demonstrate that the application of any external uniform non-abelian gauge background, no matter how small, leads to a greatly enhanced degeneracy. This degeneracy is so large that even a non-abelian background field of infinitesimal strength leads to a shocking change in the thermodynamics. The critical temperature might be discontinuously depressed and an "avoided critical point" will emerge. Similar models were previously employed to describe the micropscopics of metallic glasses and correctly predicted the locations of the structure factor peaks. We will compare one of the fitted parameters in these plots (the magnitude of the "avoided critical temperature" for each of the supercooled liquids) and demonstrate that, on average, these might be well predicted by our microscopic picture. We find a generalized version of the Mermin-Wagner inequality which connects the magnitude of the order parameter (the absence of entropic effects) to the characteristic relaxation times present in any system. We derive Vogel-Fulcher dynamics for a multitude of glass models (including non-Abelian gauge backgrounds), and try to fortify earlier ideas suggesting avoided critical dynamics.

cond-mat↗

arXiv2002

A Novel Approach Applied to the Largest Clique Problem

A novel approach to complex problems has been previously applied to graph classification and the graph equivalence problem. Here we apply it to the NP complete problem of finding the largest perfect clique within a graph $G$.

cond-mat↗

arXiv2002

A Novel Approach to Complex Problems

A novel approach to complex problems has been previously applied to graph classification and the graph equivalence problem. Here we consider its applications to a wide set of NP complete problems, namely, those of finding a subgraph g inside a graph G.

cond-mat.dis-nn↗

arXiv2002

Planar and Stripe Orders of Doped Mott Insulators In Dual Spin Models

We focus on very general, very large U, doped Mott Insulators with arbitrary hopping and interactions. We provide simple testimony to the competition between magnetic and superconducting orders in these systems. By mapping hard core bosons, spinless, and spinful fermions onto XXZ models, we aim to make very simple precise statements. We try to address optimal and expected filling fractions of holes within the plane and on stripes in a variety of hole and hole pair geometries. We examine the role of attractions/repulsion amongst hole pairs and single holes, and provide trivial expected numerical values for filling fractions in various scenarios. We demnostrate that plaquette states seem to naturally provide the correct stripe filling fractions.

cond-mat.str-el↗

arXiv2002

A Possible Via For Proving The Approach Of The Hubbard Ground States Toward Stripes And Other Doping States

In this simple note, we suggest a way to rigorously establishing constraints on the form of the ground states of the Hubbard and t-J models and extended longer (yet finite) range variants for various dopings, once exact numerical results are established for these Hamiltonians on finite size clusters with two different (both open and periodic) boundary conditions. We demonstrate that strong bounds will be established if non-uniform minima subject periodic boundary conditions are found. An offshoot of our proposal might enable rigorously establishing the strong tendency toward especially stable stripe formation at the magical commensurate doping of 1/8 on the infinite lattice, as well as at other dopings.

cond-mat.str-el↗

arXiv2002

Exact Solutions to Special High Dimensional O(n) Models, Dimensional Reductions, gauge redundancy, and special Frustrated Spin and Orbital models

This work addresses models (e.g. potential models of directed orbital systems- the manganates) in which an effective reduction dimensionality occurs as a result of a new symmetry which is intermediate between that of global and local gauge symmetry. This path towards dimensional reduction is examined in simple O(n) spin models and lattice gauge theories. A high temperature expansion is employed to map special anisotropic high dimensional models into lower dimensional variants. We show that it is possible to have an effective reduction in the dimension without the need of compactifying some dimensions. These models are frustrated and display a symmetry intermediate between local and global gauge symmetries. Some solutions are presented. Our dimensional reductions are a generlization of the trivial dimensional reduction that occur in pure two dimensional gauge theories. It will be further seen that the absence of a ``phase interference'' effect plays an important role in high dimensional problems. By identifying another (``permutational'') symmetry present in the large n limit, we will further show how to generally map global high dimensional spin systems onto a one dimensional chain and discuss implications.

cond-mat.stat-mech↗

arXiv2002

Glassy behavior in systems with Kac-type step-function interaction

We study a system with a weak, long-range repulsive Kac-type step-function interaction within the framework of a replicated effective $ϕ^4$-theory. The occurrence of extensive configurational entropy, or an exponentially large number of metastable minima in the free energy (characteristic of a glassy state), is demonstrated. The underlying mechanism of mesoscopic patterning and defect organizations is discussed.

cond-mat.soft↗

arXiv2000

Stripes And Nodal Fermions As Two Sides Of The Same Coin

One of the central problems in High T_{c} superconductivity is to reconcile the abundant evidence for stripe-like physics at `short' distances with the equally convincing evidence for BCS like physics at large distance scales (the `nodal fermions'). Our central hypothesis is that the duality notion applies: the superconductor should be viewed as a condensate of topological excitations associated with the fully ordered stripe phase. As we will argue, the latter are not only a form of `straighforward' spin and charge order but also involve a form of `hidden' or `topological' long range order which is also responsible for the phenomenon of spin-charge separation in 1+1D. The topological excitation associated with the destruction of this hidden order is of the most unusual kind. We suggest that the associated disorder field theory has a geometrical, gravity like structure concurrent with topological phases with no precedent elsewhere.

cond-mat.str-el↗

arXiv1998

Avoided Critical Behavior in O(n) Systems

Long-range frustrating interactions, even if their strength is infinitesimal, can give rise to a dramatic proliferations of ground or near-ground states. As a consequence, the ordering temperature can exhibit a discontinuous drop as a function of the frustration. A simple model of the doped Mott insulator, where the short-range tendency of the holes to phase separate competes with long-range Coulomb effects, exhibits this "avoided critical" behavior. This model may serve as a paradigm for many other systems.

cond-mat.stat-mech↗