Mean-field Phase Boundary Research Articles

Scattering of multicomponent polymer blends

The random phase approximation for polymer blends was developed by H. Benoît and described small angle scattering functions as well as mean field phase boundaries. It is a pure mean field theory that loses validity close to the real phase boundaries due to strong fluctuations. However, it gives a very clear roadmap about phase diagrams and scattering functions. A simplification of the random phase approximation is discussed that comes into effect when several polymers are mixed that involve a rather low number of chemically different repeat units. Then, the correlation functions of the same repeat unit pairs can be added up in a specific way such that the overall complexity for the calculations is reduced. The scattering functions and mean field phase boundaries are discussed within this concept.Graphical abstract

Renormalization-group analysis ofp-orbital Bose-Einstein condensates in a square optical lattice

We investigate the quantum fluctuation effects in the vicinity of the critical point of a $p$-orbital bosonic system in a square optical lattice using the Wilsonian renormalization group, where the $p$-orbital bosons condense at nonzero momenta and display rich phases including both time-reversal symmetry invariant and broken Bose-Einstein condensation states. The one-loop renormalization-group analysis generates corrections to the mean-field phase boundaries. We also show the quantum fluctuations in the $p$-orbital system tend to induce the ordered phase but not destroy it via the Coleman-Weinberg mechanism, which is qualitatively different from the ordinary quantum fluctuation corrections to the mean-field phase boundaries in $s$-orbital systems. Finally, we discuss the observation of these phenomena in the realistic experiment.

Quantum entangled ground states of two spinor Bose-Einstein condensates

We revisit in detail the non-mean-field ground-state phase diagram for a binary mixture of spin-1 Bose-Einstein condensates including quantum fluctuations. The non-commuting terms in the spin-dependent Hamiltonian under single spatial mode approximation make it difficult to obtain exact eigenstates. Utilizing the spin z-component conservation and the total spin angular momentum conservation, we numerically derive the information of the building blocks and evaluate von Neumann entropy to quantify the ground states. The mean-field phase boundaries are found to remain largely intact, yet the ground states show fragmented and entangled behaviors within large parameter spaces of interspecies spin-exchange and singlet-pairing interactions.

Mott phases and superfluid-insulator transition of dipolar spin-three bosons in an optical lattice: Implications forCr52atoms

We study the Mott phases and superfluid-insulator transition of spin-three bosons in an optical lattice with an anisotropic two dimensional optical trap. We chart out the phase diagrams for Mott states with $n=1$ and $n=2$ atoms per lattice site. It is shown that the long-range dipolar interaction stabilizes a state where the chains of the ferromagnetically aligned spins run along the longer trap direction while the spin ordering is staggered between nearby chains, leading to an antiferromagnetic ordering along the shorter trap direction. We also obtain the mean-field phase boundary for the superfluid-insulator transition in these systems and study the nature of spin ordering in the superfluid state near the transition. We show that, inside the superfluid phase and near the superfluid-insulator phase boundary, the system undergoes a first order antiferromagnetic-ferromagnetic spin ordering transition. We discuss implications of our results for $^{52}$Cr atoms and suggest possible experiments to detect different phases in such systems.

Competing field pulse induced dynamic transition in Ising Models

We discuss dynamic magnetization-reversal phenomena in the Ising model under a finite-duration external magnetic field, which competes with the existing order for . The nature of the phase boundary is estimated from the mean-field (MF) equation of motion. The susceptibility and relaxation time diverge at the MF phase boundary. A Monte Carlo study also shows divergence of the relaxation time and order-parameter fluctuations at the phase boundary. The fourth-order cumulant shows two distinct behaviors. For low temperatures and pulse durations, the value of the cumulant at the crossing point for different system sizes is far less than that for the static transition in the same dimension. This suggests a new universality class for the dynamic transition. For higher temperatures and pulse durations, the transition falls in a MF-like weak-singularity universality class.

Simulation for the first-order vortex-lattice melting transition in high-Tc superconductors

Monte Carlo simulations are performed to investigate vortex states in high-Tc superconductors. Latent heat and δ-function like divergence in the specific heat are observed as the vortex-line lattice is melt into a liquid of pancake vortex. As the anisotropy is reduced, another transition between the liquid of vortex line and that of pancake vortex is found associated with large thermal fluctuations and a weak latent heat. In relation to the vortex line liquid in systems with columnar defects, the results for smaller anisotropy are discussed. A crossover to a pancake vortex gas phase at an even higher temperature is observed as the trajectory of the mean-field phase boundary Hc2.

Superconductor-insulator phase transition in the boson Hubbard model.

We investigate the superconductor-insulator phase transition in the two-dimensional boson Hubbard system with short-range interactions. Fluctuations of both the phase and the amplitude of the superfluid order parameter are included in the determination of the phase diagram at zero and finite temperatures. The mean-field phase boundary is compared to quantum Monte Carlo results. We also calculate the frequency-dependent conductivity in the vicinity of the phase transition and find it universal at the multicritical point.

Phase boundary of a cubic superconducting circuit in a magnetic field of arbitrary magnitude and direction.

An exact analytic expression for the mean-field phase boundary ${\mathit{T}}_{\mathit{c}}$(H) of a cubic superconducting circuit in an arbitrary external-magnetic-field vector H is derived. The phase boundary of this circuit is shown to depend in a complex and sensitive way on both the magnitude and the direction of the magnetic field. Some practical applications of these properties are also suggested.