Phase diagram of the Kohn-Luttinger superconducting state for bilayer graphene

The effect of Coulomb interaction between Dirac fermions on the formation of the Kohn-Luttinger superconducting state in bilayer doped graphene is studied disregarding of the effect of the van der Waals potential of the substrate and impurities. The phase diagram determining the boundaries of superconductive domains with different types of symmetry of the order parameter is built using the extended Hubbard model in the Born weak-coupling approximation with allowance for the intratomic, interatomic, and interlayer Coulomb interactions between electrons. It is shown that the Kohn-Luttinger polarization contributions up to the second order of perturbation theory in the Coulomb interaction inclusively and an account for the long-range intraplane Coulomb interactions significantly affect the competition between the superconducting phases with the f-, p + ip-, and d + id-wave symmetries of the order parameter. It is demonstrated that the account for the interlayer Coulomb interaction enhances the critical temperature of the transition to the superconducting phase.     

Phase diagram and skyrmion energy for bilayer heterostructures at integer filling factors

The phase diagram of a bilayer heterostructure at integer filling factors was established using the hidden symmetry method. Three phases, namely, ferromagnetic, canted antiferromagnetic (CAP), and spin-singlet, were found. We confirm early results of Das Sarma et al. Each phase violates the SU(4) hidden symmetry and is stabilized by anisotropy interactions. A charged excitation in the bilayer, i.e., a skyrmion, was found and its anisotropic energy gap was calculated. The gap has a prominent minimum in the CAP.     

Phase diagram of the frustrated spin ladder with the next nearest intrachain couplings

By the density matrix renormalization group method, the phase diagram of the frustrated spin-1/2 ladder with the next nearest intrachain and ferromagnetic interchain couplings is obtained. According to our results, the two-rung entanglement is shown to be more sensitive than order parameters for determining phase boundary in such a frustrated spin ladder model. Furthermore, the additional next nearest intrachain coupling is found to enhance the intermediate columnar dimerized phase evidently.     

Phase transitions of bilayer spin-1/2 Baxter-Wu model with repulsive interlayer interactions

Using a Monte Carlo simulation and the single histogram reweighting technique, we study the critical behaviors and phase transitions of the Baxter-Wu model on a two-layer triangular lattice with repulsive interlayer interactions. Via the finite-size analysis, we obtain the transition temperatures and critical exponents at different interlayer coupling strengths. The reduced energy cumulants and the histograms reveal pseudo-first-order phase transitions, and the critical exponents derivate from the standard Baxter-Wu model if the coupling is weak. As the increase of the coupling, the behavior of the pseudo-first order transition disappears and the critical exponents are in accordance with the ones known for the 2D 4-state Potts model.     

Phase diagram for Nflation

Recently, it was showed that there is a large N phase transition in Nflation, in which when the number of fields is large enough, the slow roll inflation phase will disappear. In this brief report, we illustrate the phase diagram for Nflation, and discuss the entropy bound and some relevant results. It is found that near the critical point the number of fields saturates dS entropy.     

Superconductivity of bilayer phosphorene under interlayer compression

According to first-principles calculations, it is our prediction that bilayer phosphorene(BLP) will become a quasitwo-dimensional superconductor under a certain degree of interlayer compression. A decreasing interlayer distance may realize the transition in the BLP from a semiconducting phase to a metallic phase. On the other hand, a severe vertical compression may make the BLP lattice become dynamically unstable. It is found that in the stable metallic phase of the BLP, interlayer phonon modes dominate the electron-phonon coupling λ. The obtained λ can be greater than 1 and the superconducting temperature T_c can be higher than 10 K.     

Phase diagram for a lattice model for confining bags

Summary The two 1-dimensional spin systems proposed by Aharonov and Schwartz as models of confinement isomorphic with the MIT bag model are thoroughly analysed by using an improved mean-field scheme (which has the customary mean-field theory as its zero-th-order approximation), recently suggested to deal with fluctuation in a perturbative cluster-cumulant aproach. The ground state and its phase diagram are discussed in detail.

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Riassunto I due modelli di spin unidimensionali proposti da Aharonov e Schwartz come modelli di confinamento isomorfi al ?MIT bag model? sono analizzati compiutamente mediante uno schema di campo medio migliorato (che ammette l’ordinaria teoria di campo medio come approssimazione di ordine zero), elaborato recentemente per trattare le fluttuazioni con un approccio perturbativo che utilizza il calcolo dei cumulanti su grappoli. Lo stato fondamentale ed il corrispondente diagramma di fase sono discussi in dettaglio.

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Реэюме Проводится аналиэ двух одномерных спиновых систем, предложенных Аароновым и Щварцем в качестве моделей удержания, иэоморфного в случае MIT-модели ?мешка?. Для аналиэа исполыэуется усовершенствованная схема среднего поля (в которой обычная теория среднего поля чредставляет приближение нулевого порядка). Эта схема была недавно предложена для рассмотрения флуктуаций в пертурбационном кластерном подходе. Подробно обсуждаются основное состояние и соответствуюшая фаэовая диаграмма.

     

Evidence for spontaneous interlayer phase coherence in a bilayer quantum Hall exciton condensate

Recent experimental work on the quantized Hall state at total filling factor ν_T=1 in bilayer 2D electron systems has revealed a number of striking phenomena, including a giant and sharply resonant enhancement of the interlayer tunneling conductance at zero bias. The tunneling enhancement is a compelling indicator of spontaneous interlayer phase coherence among the electrons in the system. Such phase coherence is perhaps the single most important attribute of the excitonic Bose condensate which describes this remarkable quantum Hall state.     

Magnetic phase diagram of a ferromagnet-antiferromagnet bilayer with a rough interface

The thickness-roughness phase diagram of a thin ferromagnetic film on an antiferromagnetic substrate is studied in the case where the roughness of the interface between the layers causes frustration of the exchange interaction between them. It is shown that the inclusion of single-ion anisotropy makes the phase diagram significantly more complicated in comparison with that calculated within the exchange approximation. The evolution of a new type of domain walls caused by frustrations is traced with an increase in the film thickness and the width of the atomic steps on the film-substrate interface.     

Phase diagram of a traffic roundabout

We propose a simple cellular automaton model to study the traffic dynamics in a roundabout. Both numerical and analytical results are presented. We are able to obtain exact solutions in the full parameter space. Exact phase diagrams are derived. When the traffic from two directions mixed, there are only five distinct phases. Some of the combinations from naive intuition are strictly forbidden. We also compare the results to a signaled intersection.     

Phase diagram for electron-hole droplets

The phase diagram for electron-hole droplets is obtained using a phenomenological spin one lattice gas model treated in mean field approximation. Very good agreement with experimental data can be obtained.     

Phase diagram of a random tiling quasicrystal

We study the phase diagram of a two-dimensional random tiling model for quasicrystals. At proper concentrations the model has 8-fold rotational symmetry. Landau theory correctly gives most of the qualitative features of the phase diagram, which is in turn studied in detail numerically using a transfer matrix approach. We find that the system can enter the quasicrystal phase from many other crystalline and incommensurate phases through first-order or continuous transitions. Exact solutions are given in all phases except for the quasicrystal phase, and for the phase boundaries between them. We calculate numerically the phason elastic constants and entropy density, and confirm that the entropy density reaches its maximum at the point where phason strains are zero and the system possesses 8-fold rotational symmetry. In addition to the obvious application to quasicrystals, this study generalizes certain surface roughening models to two-dimensional surfaces in four dimensions.     

Quantum phase diagram of the t-J(z) chain model

We present the quantum phase diagram of the one-dimensional t- J(z) model for arbitrary spin (integer or half-integer) and sign of the spin-spin interaction J(z), using an exact mapping to a spinless fermion model that can be solved exactly using the Bethe ansatz. We discuss its superconducting phase as a function of hole doping nu. Motivated by the new paradigm of high temperature superconductivity, the stripe phase, we also consider the effect the antiferromagnetic background has on the t- J(z) chain intended to mimic the stripe segments.