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Recent documents in Physics: Faculty Publicationsen-usWed, 09 Jun 2021 14:14:38 PDT3600Evolution of magnetic field induced ordering in the layered quantum Heisenberg triangular-lattice antiferromagnet Ba<sub>3</sub> CoSb<sub>2</sub> O<sub>9</sub>
https://scholarworks.smith.edu/phy_facpubs/75
https://scholarworks.smith.edu/phy_facpubs/75Mon, 24 May 2021 08:58:55 PDT
Quantum fluctuations in the effective spin- 1/2 layered triangular-lattice quantum Heisenberg antiferromagnet Ba_{3}CoSb_{2}O_{9} lift the classical degeneracy of the antiferromagnetic ground state in magnetic field, producing a series of novel spin structures for magnetic fields applied within the crystallographic ab plane, including a celebrated collinear “up-up-down” spin ordering with magnetization equal to 1/3 of the saturation magnetization over an extended field range. Theoretically unresolved, however, are the effects of interlayer antiferromagnetic coupling and transverse magnetic fields on the ground states of this system. Additional magnetic field induced phase transitions are theoretically expected and in some cases have been experimentally observed, but details regarding their number, location, and physical character appear inconsistent with the predictions of existing models. Conversely, an absence of experimental measurements as a function of magnetic-field orientation has left other key predictions of these models untested. To address these issues, we have used specific heat, neutron diffraction, thermal conductivity, and magnetic torque measurements to map out the phase diagram as a function of magnetic field intensity and orientation relative to the crystallographic ab plane. For H||ab, we have discovered an additional magnetic field induced phase transition at low temperature and an unexpected tetracritical point in the high-field phase diagram, which coupled with the apparent second-order nature of the phase transitions eliminates several theoretically proposed spin structures for the high-field phases. Our calorimetric measurements as a function of magnetic field orientation are in general agreement with theory for field-orientation angles close to plane parallel (H||a) but diverge at angles near plane perpendicular; a predicted convergence of two phase boundaries at finite angle and a corresponding change in the order of the field induced phase transition are not observed experimentally. Our results emphasize the role of interlayer coupling in selecting and stabilizing field induced phases, provide guidance on the nature of the magnetic order in each phase, and reveal the need for new physics to account for the nature of magnetic ordering in this archetypal two-dimensional spin- 1/2 triangular-lattice quantum Heisenberg antiferromagnet.
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Nathanael Alexander Fortune et al.Quantum Confinement Transition in a D-Wave Superconductor
https://scholarworks.smith.edu/phy_facpubs/74
https://scholarworks.smith.edu/phy_facpubs/74Fri, 23 Apr 2021 07:46:24 PDT
We study the nature of the zero-temperature phase transition between a d-wave superconductor and a Mott insulator in two dimensions. In this "quantum confinement transition", spin and charge are confined to form the electron in the Mott insulator. Within a dual formulation, direct transitions from d-wave superconductors at half-filling to insulators with spin-Peierls (as well as other) order emerge naturally. The possibility of striped superconductors is also discussed within the dual formulation. The transition is described by nodal fermions and bosonic vortices, interacting via a long-ranged statistical interaction modeled by two coupled Chern-Simons gauge fields, and the critical properties of this model are discussed.
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Courtney Lannert et al.Electron Spectral Function in Two-Dimensional Fractionalized Phases
https://scholarworks.smith.edu/phy_facpubs/73
https://scholarworks.smith.edu/phy_facpubs/73Fri, 23 Apr 2021 07:46:23 PDT
We study the electron spectral function of various zero-temperature spin-charge separated phases in two dimensions. In these phases, the electron is not a fundamental excitation of the system, but rather “decays” into a spin-1/2 chargeless fermion (the spinon) and a spinless charge e boson (the chargon). Using low-energy effective theories for the spinons (d-wave pairing plus possible Néel order) and the chargons (condensed or quantum-disordered bosons), we explore three phases of possible relevance to the cuprate superconductors: (1) (formula presented) a fractionalized antiferromagnet where the spinons are paired into a state with long-ranged Néel order and the chargons are 1/2-filled and (Mott) insulating; (2) the nodal liquid, a fractionalized insulator where the spinons are d-wave paired and the chargons are uncondensed; and (3) the d-wave superconductor, where the chargons are condensed and the spinons retain a d-wave gap. Working within the (formula presented) gauge theory of such fractionalized phases, our results should be valid at scales below the energy gap of the vison—the basic vortex excitation in the theory. However, on a phenomenological level, our results should apply to any spin-charge separated system where the excitations have these low-energy effective forms. Comparison with angle-resolved photoemission spectroscopy data in the undoped, pseudogapped, and superconducting regions is made.
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Courtney Lannert et al.Inelastic Neutron Scattering Signal from Deconfined Spinons in a Fractionalized Antiferromagnet
https://scholarworks.smith.edu/phy_facpubs/72
https://scholarworks.smith.edu/phy_facpubs/72Fri, 23 Apr 2021 07:46:21 PDT
We calculate the contribution of deconfined spinons to inelastic neutron scattering (INS) in the fractionalized antiferromagnet (AF*), introduced elsewhere. We find that the presence of free spin-1/2 charge-less excitations leads to a continuum INS signal above the Néel gap. This signal is found above and in addition to the usual spin-1 magnon signal, which to lowest order is the same as in the more conventional confined antiferromagnet. We calculate the relative weights of these two signals and find that the spinons contribute to the longitudinal response, where the magnon signal is absent to lowest order. Possible higher-order effects of interactions between magnons and spinons in the AF* phase are also discussed.
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Courtney Lannert et al.Critical Dynamics of Superconductors in the Charged Regime
https://scholarworks.smith.edu/phy_facpubs/71
https://scholarworks.smith.edu/phy_facpubs/71Fri, 23 Apr 2021 07:46:19 PDT
The charged regime of the superconductor-metal transition was analyzed by applying a finite temperature critical dynamics. A transverse gage field coupling was applied to the superconducting order parameter. A new dynamic universality class characeterized by a finite fixed point ratio between the transport coefficients associated with the order parameter and gage fields was found by assuming relaxational dynamics for both the order parameter and gage fields within a renormalization group scheme. It was found that various features of the dynamic universality class of the charged superconductor appeared in measurable quantities.
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Courtney Lannert et al.Structure and Stability of Mott-Insulator Shells of Bosons Trapped in an Optical Lattice
https://scholarworks.smith.edu/phy_facpubs/70
https://scholarworks.smith.edu/phy_facpubs/70Fri, 23 Apr 2021 07:46:18 PDT
We consider the feasibility of creating a phase of neutral bosonic atoms in which multiple Mott-insulating states coexist in a shell structure and propose an experiment to spatially resolve such a structure. This spatially inhomogeneous phase of bosons, arising from the interplay between the confining potential and the short-ranged repulsion, has been previously predicted. While the Mott-insulator phase has been observed in an atomic gas, the spatial structure of this phase in the presence of an inhomogeneous potential has not yet been directly probed. In this paper, we give a simple recipe for creating a structure with any desired number of shells, and explore the stability of the structure under typical experimental conditions. The stability analysis gives some constraints on how successfully these states can be employed for quantum information experiments. The experimental probe we propose for observing this phase exploits transitions between two species of bosons, induced by applying a frequency-swept, oscillatory magnetic field. We present the expected experimental signatures of this probe, and show that they reflect the underlying Mott configuration for large lattice potential depth.
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B. Demarco et al.Dynamics of Condensate Shells: Collective Modes and Expansion
https://scholarworks.smith.edu/phy_facpubs/69
https://scholarworks.smith.edu/phy_facpubs/69Fri, 23 Apr 2021 07:46:17 PDT
We explore the physics of three-dimensional shell-shaped condensates, relevant to cold atoms in "bubble traps." We study the ground state of the condensate wave function, spherically symmetric collective modes, and expansion properties of such a shell using a combination of analytical and numerical techniques. We find two breathing-type modes with frequencies that are distinct from that of the filled spherical condensate. Upon trap release and subsequent expansion, we find that the system displays self-interference fringes. We estimate characteristic time scales, degree of mass accumulation, three-body loss, and kinetic energy release during expansion for a typical system of Rb87.
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Courtney Lannert et al.Coexistence of Superfluid and Mott Phases of Lattice Bosons
https://scholarworks.smith.edu/phy_facpubs/68
https://scholarworks.smith.edu/phy_facpubs/68Fri, 23 Apr 2021 07:46:16 PDT
Recent experiments on strongly interacting bosons in optical lattices have revealed the coexistence of spatially separated Mott-insulating and number-fluctuating phases. We explore the condensate properties of the number fluctuating phase trapped between the Mott-insulating regions and derive the associated collective-mode structure. We discuss the crossover of the interlayer properties between two- and three-dimensional behavior as a function of the lattice parameters and estimate the critical temperatures for the transition from the normal to superfluid state.
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R. A. Barankov et al.Supercurrent Survival under a Rosen-Zener Quench of Hard-Core Bosons
https://scholarworks.smith.edu/phy_facpubs/67
https://scholarworks.smith.edu/phy_facpubs/67Fri, 23 Apr 2021 07:46:15 PDT
We study the survival of supercurrents in a system of impenetrable bosons on a lattice, subject to a quantum quench from its critical superfluid phase to an insulating phase. We show that the evolution of the current when the quench follows a Rosen-Zener profile is exactly solvable. This allows us to analyze a quench of arbitrary rate, from a sudden destruction of the superfluid to a slow opening of a gap. The decay and oscillations of the current are analytically derived and studied numerically along with the momentum distribution after the quench. In the case of small supercurrent boosts ν, we find that the current surviving at long times is proportional to ν3.
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I. Klich et al.Josephson Physics Mediated by the Mott Insulating Phase
https://scholarworks.smith.edu/phy_facpubs/66
https://scholarworks.smith.edu/phy_facpubs/66Fri, 23 Apr 2021 07:46:14 PDT
We investigate the static and dynamic properties of bosonic lattice systems in which condensed and Mott insulating phases coexist due to the presence of a spatially varying potential. We formulate a description of these inhomogeneous systems and calculate the bulk energy at and near equilibrium. We derive the explicit form of the Josephson coupling between disjoint superfluid regions separated by Mott insulating regions. We obtain detailed estimates for the case of alternating superfluid and Mott insulating spherical shells in a radially symmetric parabolically confined cold atom system.
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Smitha Vishveshwara et al.Probing Condensate Order in Deep Optical Lattices
https://scholarworks.smith.edu/phy_facpubs/65
https://scholarworks.smith.edu/phy_facpubs/65Fri, 23 Apr 2021 07:46:13 PDT
We study interacting bosons in optical lattices in the weak-tunneling regime in systems that exhibit the coexistence of Mott-insulating and condensed phases. We discuss the nature of the condensed ground state in this regime and the validity of the mean-field treatment thereof. We suggest two experimental signatures of condensate order in the system. (1) We analyze the hyperfine configuration of the system and propose a set of experimental parameters for observing radio-frequency spectra that would demonstrate the existence of the condensed phase between Mott-insulating phases. We derive the structure of the signal from the condensate in a typical trapped system, taking into account Goldstone excitations, and discuss its evolution as a function of temperature. (2) We study matter-wave interference patterns displayed by the system upon release from all confining potentials. We show that as the density profiles evolve very differently for the Mott-insulating phase and the condensed phase, they can be distinguished from one another when the two phases coexist.
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Kuei Sun et al.Small-Network Approximations for Geometrically Frustrated Ising Systems
https://scholarworks.smith.edu/phy_facpubs/64
https://scholarworks.smith.edu/phy_facpubs/64Fri, 23 Apr 2021 07:46:11 PDT
The study of frustrated spin systems often requires time-consuming numerical simulations. As the simplest approach, the classical Ising model is often used to investigate the thermodynamic behavior of such systems. Exploiting the small correlation lengths in frustrated Ising systems, we develop a method for obtaining first approximations to the energetic properties of frustrated two-dimensional Ising systems using small networks of less than 30 spins. These small networks allow much faster numerical simulations, and more importantly, analytical evaluations of their properties are numerically tractable. We choose Ising systems on the triangular lattice, the kagome lattice, and the triangular kagome lattice as prototype systems and find small systems that can serve as good approximations to these prototype systems. Through comparisons between the properties of extended models and small systems, we develop a set of criteria for constructing small networks to approximate general infinite two-dimensional frustrated Ising systems. This method of using small networks provides a different and efficient way to obtain a first approximation to the properties of frustrated spin systems.
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Bilin Zhuang et al.Shell potentials for microgravity Bose–Einstein condensates
https://scholarworks.smith.edu/phy_facpubs/63
https://scholarworks.smith.edu/phy_facpubs/63Fri, 23 Apr 2021 07:46:09 PDT
Extending the understanding of Bose–Einstein condensate (BEC) physics to new geometries and topologies has a long and varied history in ultracold atomic physics. One such new geometry is that of a bubble, where a condensate would be confined to the surface of an ellipsoidal shell. Study of this geometry would give insight into new collective modes, self-interference effects, topology-dependent vortex behavior, dimensionality crossovers from thick to thin shells, and the properties of condensates pushed into the ultradilute limit. Here we propose to implement a realistic experimental framework for generating shell-geometry BEC using radiofrequency dressing of magnetically trapped samples. Such a tantalizing state of matter is inaccessible terrestrially due to the distorting effect of gravity on experimentally feasible shell potentials. The debut of an orbital BEC machine (NASA Cold Atom Laboratory, aboard the International Space Station) has enabled the operation of quantum-gas experiments in a regime of perpetual freefall, and thus has permitted the planning of microgravity shell-geometry BEC experiments. We discuss specific experimental configurations, applicable inhomogeneities and other experimental challenges, and outline potential experiments.
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N. Lundblad et al.Vortex-Antivortex Physics in Shell-Shaped Bose-Einstein Condensates
https://scholarworks.smith.edu/phy_facpubs/62
https://scholarworks.smith.edu/phy_facpubs/62Fri, 23 Apr 2021 07:46:07 PDT
Shell-shaped hollow Bose-Einstein condensates (BECs) exhibit behavior distinct from their filled counterparts and have recently attracted attention due to their potential realization in microgravity settings. Here we study distinct features of these hollow structures stemming from vortex physics and the presence of rotation. We focus on a vortex-antivortex pair as the simplest configuration allowed by the constraints on superfluid flow imposed by the closed-surface topology. In the two-dimensional limit of an infinitesimally thin shell BEC, we characterize the long-range attraction between the vortex-antivortex pair and find the critical rotation speed that stabilizes the pair against energetically relaxing towards self-annihilation. In the three-dimensional case, we contrast the bounds on vortex stability with those in the two-dimensional limit and the filled sphere BEC, and evaluate the critical rotation speed as a function of shell thickness. We thus demonstrate that analyzing vortex stabilization provides a nondestructive means of characterizing a hollow sphere BEC and distinguishing it from its filled counterpart.
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Karmela Padavić et al.Evolution of Magnetic-Field-Induced Ordering in the Layered Structure Quantum Heisenberg Triangular-Lattice Antiferromagnet Ba<sub>3</sub>CoSb<sub>2</sub>O<sub>9</sub>
https://scholarworks.smith.edu/phy_facpubs/61
https://scholarworks.smith.edu/phy_facpubs/61Tue, 05 Jan 2021 05:19:40 PST
Quantum fluctuations in the effective spin-1/2 layered structure triangular-lattice quantum Heisenberg antiferromagnet Ba_{3}CoSb_{2}O_{9} lift the classical degeneracy of the antiferromagnetic ground state in magnetic field, producing a series of novel spin structures for magnetic fields applied within the crystallographic ab plane, including a celebrated collinear ‘up-up-down’ spin ordering with magnetization equal to 1/3 of the saturation magnetization over an extended field range. Theoretically unresolved, however, are the effects of interlayer antferromagnetic coupling and transverse magnetic fields on the ground states of this system. Additional magnetic-field-induced phase transitions are theoretically expected and in some cases have been experimentally observed, but details regarding their number, location, and physical character appear inconsistent with the predictions of existing models. Conversely, an absence of experimental measurements as a function of magnetic-field orientation has left other key predictions of these models untested. To address these issues, we have used specific heat, neutron diffraction, thermal conductivity, and magnetic torque measurements to map out the phase diagram as a function of magnetic field intensity and orientation relative to the crystallographic ab plane. For H||ab, we have discovered an additional, previously unreported magnetic-field-induced phase transition at low temperature and an unexpected tetracritical point in the high field phase diagram, which — coupled with the apparent second-order nature of the phase transitions — eliminates several theoretically proposed spin structures for the high field phases. Our calorimetric measurements as a function of magnetic field orientation are in general agreement with theory for field-orientation angles close to plane parallel (H||a) but diverge at angles near plane perpendicular; a predicted convergence of two phase boundaries at finite angle and a corresponding change in the order of the field induced phase transition is not observed experimentally. Our results emphasize the role of interlayer coupling in selecting and stabilizing field-induced phases, provide new guidance into the nature of the magnetic order in each phase, and reveal the need for new physics to account for the nature of magnetic ordering in this archetypal 2D spin-1/2 triangular lattice quantum Heisenberg antiferromagnet.
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Nathanael Alexander Fortune et al.Caustics in Tachyon Matter and Other Born-Infeld Scalars
https://scholarworks.smith.edu/phy_facpubs/60
https://scholarworks.smith.edu/phy_facpubs/60Wed, 02 Sep 2020 07:11:43 PDT
We consider scalar Born-Infeld type theories with arbitrary potentials V(T) of a scalar field T. We find that for models with runaway potentials V(T) the generic inhomogeneous solutions after a short transient stage can be very well approximated by the solutions of a Hamilton-Jacobi equation that describes free streaming wave front propagation. The analytic solution for this wave propagation shows the formation of caustics with multi-valued regions beyond them. We verified that these caustics appear in numerical solutions of the original scalar BI non-linear equations. Our results include the scalar BI model with an exponential potential, which was recently proposed as an effective action for the string theory tachyon in the approximation where high-order spacetime derivatives of T are truncated. Since the actual string tachyon dynamics contain derivatives of all orders, the tachyon BI model with an exponential potential becomes inadequate when the caustics develop because high order spatial derivatives of T become divergent. BI type tachyon theory with a potential decreasing at large T could have interesting cosmological applications because the tachyon field rolling towards its ground state at infinity acts as pressureless dark matter. We find that inhomogeneous cosmological tachyon fluctuations rapidly grow and develop multiple caustics. Any considerations of the role of the tachyon field in cosmology will have to involve finding a way to predict the behavior of the field at and beyond these caustics.
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Gary Felder et al.Cosmology with Negative Potentials
https://scholarworks.smith.edu/phy_facpubs/59
https://scholarworks.smith.edu/phy_facpubs/59Wed, 02 Sep 2020 07:11:34 PDT
We investigate cosmological evolution in models where the effective potential V(f) may become negative for some values of the field f. Phase portraits of such theories in the space of variables (f,f˙ ,H) have several qualitatively new features as compared with phase portraits in the theories with V(f).0. Cosmological evolution in models with potentials with a ‘‘stable’’ minimum at V(f),0 is similar in some respects to the evolution in models with potentials unbounded from below. Instead of reaching an AdS regime dominated by the negative vacuum energy, the universe reaches a turning point where its energy density vanishes, and then it contracts to a singularity with properties that are practically independent of V(f). We apply our methods to investigation of the recently proposed cyclic universe scenario. We show that in addition to the singularity problem there are other problems that need to be resolved in order to realize a cyclic regime in this scenario. We propose several modifications of this scenario and conclude that the best way to improve it is to add a usual stage of inflation after the singularity and use that inflationary stage to generate perturbations in the standard way.
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Gary Felder et al.Warped Geometry of Brane Worlds
https://scholarworks.smith.edu/phy_facpubs/58
https://scholarworks.smith.edu/phy_facpubs/58Wed, 02 Sep 2020 07:11:27 PDT
We study the dynamical equations for extra-dimensional dependence of a warp factor and a bulk scalar in 5D brane world scenarios with induced brane metric of constant curvature. These equations are similar to those for the time dependence of the scale factor and a scalar field in 4D cosmology, but with the sign of the scalar field potential reversed. Based on this analogy, we introduce novel methods for studying the warped geometry. We construct the full phase portraits of the warp factor/scalar system for several examples of the bulk potential. This allows us to view the global properties of the warped geometry. For flat branes, the phase portrait is two dimensional. Moving along typical phase trajectories, the warp factor is initially increasing and finally decreasing. All trajectories have timelike gradient-dominated singularities at one or both of their ends, which are reachable in a finite distance and must be screened by the branes. For curved branes, the phase portrait is three dimensional. However, as the warp factor increases the phase trajectories tend towards the two-dimensional surface corresponding to flat branes. We discuss this property as a mechanism that may stretch the curved brane to be almost flat, with a small cosmological constant. Finally, we describe the embedding of branes in the 5D bulk using the phase space geometric methods developed here. In this language the boundary conditions at the branes can be described as a 1D curve in the phase space. We discuss the naturalness of tuning the brane potential to stabilize the brane world system.
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Gary Felder et al.Tachyonic Instability and Dynamics of Spontaneous Symmetry Breaking
https://scholarworks.smith.edu/phy_facpubs/57
https://scholarworks.smith.edu/phy_facpubs/57Wed, 02 Sep 2020 07:11:20 PDT
Spontaneous symmetry breaking usually occurs due to the tachyonic (spinodal) instability of a scalar field near the top of its effective potential at φ=0. Naively, one might expect the field φ to fall from the top of the effective potential and then experience a long stage of oscillations with amplitude O(v) near the minimum of the effective potential at φ=v until it gives its energy to particles produced during these oscillations. However, it was recently found that the tachyonic instability rapidly converts most of the potential energy V(0) into the energy of colliding classical waves of the scalar field. This conversion, which was called “tachyonic preheating,” is so efficient that symmetry breaking typically completes within a single oscillation of the field distribution as it rolls towards the minimum of its effective potential [G. Felder et al., Phys. Rev. Lett. 87, 011601 (2001)]. In this paper we give a detailed description of tachyonic preheating and show that the dynamics of this process crucially depends on the shape of the effective potential near its maximum. In the simplest models where V(φ)∼−m2φ2/2 near the maximum, the process occurs solely due to the tachyonic instability, whereas in the theories −λφn with n>2 one encounters a combination of the effects of tunneling, tachyonic instability and bubble wall collisions.
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Gary Felder et al.Dynamics of Symmetry Breaking and Tachyonic Preheating
https://scholarworks.smith.edu/phy_facpubs/56
https://scholarworks.smith.edu/phy_facpubs/56Wed, 02 Sep 2020 07:11:11 PDT
We reconsider the old problem of the dynamics of spontaneous symmetry breaking (SSB) using 3D lattice simulations. We develop a theory of tachyonic preheating, which occurs due to the spinodal instability of the scalar field. Tachyonic preheating is so efficient that SSB typically completes within a single oscillation as the field rolls towards the minimum of its effective potential. We show that, contrary to previous expectations, preheating in hybrid inflation is typically tachyonic. Our results may also be relevant for the theory of the formation of topological defects and of disoriented chiral condensates in heavy ion collisions.
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Gary Felder et al.