Mott transition of excitons in coupled quantum wells

In this work we study the phase diagram of indirect excitons in coupled quantum wells and show that the system undergoes a phase transition to an unbound electron-hole plasma. This transition is manifested as an abrupt change in the photoluminescence linewidth and peak energy at some critical power density and temperature. By measuring the exciton diamagnetism, we show that the transition is associated with an abrupt increase in the exciton radius. We find that the transition is stimulated by the presence of direct excitons in one of the wells and show that they serve as a catalyst of the transition.     

Detecting topological order through a continuous quantum phase transition

We study a continuous quantum phase transition that breaks a Z2 symmetry. We show that the transition is described by a new critical point which does not belong to the Ising universality class, despite the presence of well-defined symmetry-breaking order parameter. The new critical point arises since the transition not only breaks the Z2 symmetry, it also changes the topological or quantum order in the two phases across the transition. We show that the new critical point can be identified in experiments by measuring critical exponents. So measuring critical exponents and identifying new critical points is a way to detect new topological phases and a way to measure topological or quantum orders in those phases.     

A random walk representation of the Dirac propagator

We define a discrete random walk with a matrix-valued transition function and show that the scaling limit of the two-point function of the walk is given by the Dirac propagator. We study the scaling limit of similar walks with curvature-dependent transition functions, which are analogous to the Ornstein-Uhlenbeck process, and show that the Dirac propagator can be recovered by a limiting procedure.     

Non-equilibrium phase transition from AdS/CFT

Using AdS/CFT correspondence we study a non-equilibrium phase transition in the presence of a constant external magnetic field. The transition occurs when the sign of the differential conductivity reverses. Utilizing numerical method we show that the type of the transition depends on the value of magnetic field as well as the temperature of the gauge theory. Moreover we show that this transition does not depend on supersymmetry preserved by the system at zero temperature and the dimension of the subspace on which the fundamental matter field lives.     

Intrinsic inhomogeneities in manganite thin films investigated with scanning tunneling spectroscopy

Thin films of La0.7Sr0.3MnO3 on MgO show a metal insulator transition and colossal magnetoresistance. The shape of this transition can be explained by intrinsic spatial inhomogeneities, which give rise to a domain structure of conducting and insulating domains at the submicrometer scale. These domains then undergo a percolation transition. The tunneling conductance and tunneling gap measured by scanning tunneling spectroscopy were used to distinguish and visualize these domains.     

Controllable transition between optical bistability and multistability in graphene/dielectric/graphene structure

We theoretically investigated the controllable transition between optical bistability and multistability in graphene/dielectric/graphene structure for TE and TM polarizations. We show such a transition strongly depends on the Fermi energy of graphene, and thus, it can be controlled by adjusting the gate voltage applied on the graphene. In addition, we also show that the transition can be tuned by changing the thickness and permittivity of the dielectric layer, or by varying the wavelength and the incident angle of the input light. Furthermore, three S-type curves appear in our studied structure, which result in quadristability. Our results may find potential applications in optoelectronic devices.     

Bootstrap Percolation and Kinetically Constrained Models on Hyperbolic Lattices

We study bootstrap percolation (BP) on hyperbolic lattices obtained by regular tilings of the hyperbolic plane. Our work is motivated by the connection between the BP transition and the dynamical transition of kinetically constrained models, which are in turn relevant for the study of glass and jamming transitions. We show that for generic tilings there exists a BP transition at a nontrivial critical density, 0<ρ _c <1. Thus, despite the presence of loops on all length scales in hyperbolic lattices, the behavior is very different from that on Euclidean lattices where the critical density is either zero or one. Furthermore, we show that the transition has a mixed character since it is discontinuous but characterized by a diverging correlation length, similarly to what happens on Bethe lattices and random graphs of constant connectivity.     

Frustrated polyelectrolyte bundles and T= 0 Josephson-junction arrays

We establish a one-to-one mapping between a model for hexagonal polyelectrolyte bundles and a model for two-dimensional, frustrated Josephson-junction arrays. We find that the T = 0 insulator-to-superconductor transition of the quantum system corresponds to a continuous liquid-to-solid transition of the condensed charge in the finite-temperature classical system. We find that the role of the vector potential in the quantum system is played by elastic strain in the classical system. Exploiting this correspondence we show that the transition is accompanied by a spontaneous breaking of a discrete symmetry associated with the chiral patterning of the array and that at the transition the polyelectrolyte bundle adopts a universal response to shear.     

Phase separation close to the density-driven Mott transition in the Hubbard-Holstein model

The density-driven Mott transition is studied by means of dynamical mean-field theory in the Hubbard-Holstein model, where the Hubbard term leading to the Mott transition is supplemented by an electron-phonon (e-ph) term. We show that an intermediate e-ph coupling leads to a first-order transition at T=0, which is accompanied by a phase separation between a metal and an insulator. The compressibility in the metallic phase is substantially enhanced. At quite larger values of the coupling, a polaronic phase emerges coexisting with a nonpolaronic metal.     

Realization of a Ramsey-Bordé matter wave interferometer on the molecule

We show first results and systematic investigations of a matter wave interferometer with the K₂ molecule, using a transition between the electronic ground state X and the state b . This spin forbidden transition is observable due to spin-orbit coupling between the states b and A . The experimental results are compared with numerical simulations, which show the power of the interferometer to observe small phase shifts by weak interactions. Received 13 March 2000     

Nature of the high-pressure transition in Fe2O3 hematite

We present a new method to separate the crystallographic and electronic phase transitions in hematite using x-ray emission spectroscopy and x-ray diffraction. Our observations, based on the behavior of a metastable high-pressure phase in the stability domain of the low-pressure phase, show that the electronic transition is preempted by the crystallographic transition. The former occurs only afterwards in the high-pressure phase, possibly as a result of a Mott transition. The idea that the electronic transition drives the transition in hematite is therefore invalidated. Such methods should help elucidate the mechanics and the driving forces behind a number of first-order high-pressure phase transitions.     

THERMALLY ACTIVATED HOPPING IN Dy1-xPrxBa2Cu3O7-δ SYSTEM

The temperature dependence of resistivity in Dy_1-xPr_xBa₂Cu₃O_7-δ system with x>0.6 was measured. The experimental results show that Pr substitution leads to the localization of mobile holes and such a localization is enhanced with increasing Pr concentration. The gradually enhancing of localization induces Anderson transitions one by one in this system, including the transition from the conduction by excitation of holes to the one by thermal activation hopping between localized states, the so called Anderson transition type-I, and the transition from nearest neighbor hopping (NNH) to variable range hopping (VRH), the Anderson transition type-II, and the Anderson transition type-lI from 3D to 2D.     

Interaction-driven real-space condensation

We study real-space condensation in a broad class of stochastic mass transport models. We show that the steady state of such models has a pair-factorized form which generalizes the standard factorized steady states. The condensation in this class of models is driven by interactions which give rise to a spatially extended condensate that differs fundamentally from the previously studied examples. We present numerical results as well as a theoretical analysis of the condensation transition and show that the criterion for condensation is related to the binding-unbinding transition of solid-on-solid interfaces.     

Dipole-induced, first-order phase transitions of nano-rod monolayers

The isotropic-nematic transition of nano-rod monolayers with fore-aft symmetry is second order, in stark contrast to the first-order phase transition explained by Onsager [L. Onsager, Ann. (N.Y.) Acad. Sci. 51 (1949) 627] for rods in three dimensions. Here we show that the coupling of a dipole potential to excluded volume is sufficient to re-instate a first-order phase transition of rods confined to two dimensions.     

Convection in chemical fronts with quadratic and cubic autocatalysis

Convection in chemical fronts enhances the speed and determines the curvature of the front. Convection is due to density gradients across the front. Fronts propagating in narrow vertical tubes do not exhibit convection, while convection develops in tubes of larger diameter. The transition to convection is determined not only by the tube diameter, but also by the type of chemical reaction. We determine the transition to convection for chemical fronts with quadratic and cubic autocatalysis. We show that quadratic fronts are more stable to convection than cubic fronts. We compare these results to a thin front approximation based on an eikonal relation. In contrast to the thin front approximation, reaction-diffusion models show a transition to convection that depends on the ratio between the kinematic viscosity and the molecular diffusivity. (c) 2002 American Institute of Physics.     

Phase transition and critical properties of spin-orbital interacting systems

Phase transition and critical properties of Ising-like spin-orbital interacting systems in 2-dimensional triangular lattice are investigated. We first show that the ground state of the system is a composite spin-orbital ferro-ordered phase. Though Landau effective field theory predicts the second-order phase transition of the composite spin-orbital order, however, the critical exponents obtained by the renormalization group approach demonstrate that the spin-orbital order-disorder transition is far from the second-order, rather, it is more close to the first-order. The unusual critical behavior near the transition point is attributed to the fractionalization of the composite order parameter.     

Novel two-dimensional transition metal chalcogenides created by epitaxial growth

Two-dimensional(2D) materials have been a very important field in condensed matter physics, materials science, chemistry, and electronics. In a variety of 2D materials, transition metal chalcogenides are of particular interest due to their unique structures and rich properties. In this review, we introduce a series of 2D transition metal chalcogenides prepared by epitaxial growth. We show that not only 2D transition metal dichalcogenides can be grown, but also the transition metal chalcogenides that do not have bulk counterparts, and even patterned transition metal chalcogenides can be fabricated. We discuss the formation mechanisms of the novel structures, their interesting properties, and potential applications of these 2D transition metal chalcogenides. Finally, we give a summary and some perspectives on future studies.     

Formal model of internal measurement: Alternate changing between recursive definition and domain equation

We sketch a paradox generally resulting from recursivity, and propose a novel model to express evolutionary processes that requires identification of an interaction with internal measurement. In this model, a paradox is not resolved and the notion of relativity of any resolution is implicit. In a dynamical system a certain transition rule is used recursively along time. If one takes the foundation (or context) of recursivity into consideration, one obtains a fixed point or one confronts a paradox. In order to resolve this paradox, we adopt Scott's technical way to identify the form of a fixed point with a domain equation and to obtain a reflective domain, however we simultaneously show that any resolution is destined to be relative. In utilizing this notion, we construct a model of dynamical process by embedding a measurement process in one time step. Any time transition involves the process of doubting the foundation of a transition rule leading to a fixed point. Solving it and obtaining a reflexive domain is used as a new transition rule. Also, this process perpetually proceeds along time, and then the system perpetually proceeds while any solution is destined to be relative. We illustrate this type of model by using a dynamically changing contraction mapping as the interface of state and transition rule. Finally, we show that one can formalize emergent properties by using this model and discuss the relationship between endo-physics and internal measurement.     

Phase transitions and soft elasticity of smectic elastomers

Smectic-C elastomers can be prepared by cross-linking, e.g., liquid crystal polymers, in the smectic-A phase followed by a cooling through the smectic-A to smectic-C phase transition. This transition from D(infinityh) to C(2h) symmetry spontaneously breaks rotational symmetry in the smectic plane as does the transition from a smectic-A to a biaxial smectic phase with D(2h) symmetry. We study these transitions and the emergent elasticity of the smectic-C and biaxial phases in three related models and show that these phases exhibit soft elasticity analogous to that of nematic elastomers.     

Metal-to-semiconductor transition in squashed armchair carbon nanotubes

We investigate electronic transport properties of the squashed armchair carbon nanotubes, using tight-binding molecular dynamics and the Green's function method. We demonstrate a metal-to-semiconductor transition while squashing the nanotubes and a general mechanism for such a transition. It is the distinction of the two sublattices in the nanotube that opens an energy gap near the Fermi energy. We show that the transition has to be achieved by a combined effect of breaking of mirror symmetry and bond formation between the flattened faces in the squashed nanotubes.