Polaritons, mixed light-matter quasiparticles, undergo a transition to a condensed, macroscopically coherent state at low temperatures or high densities. Recent experiments show that coupling light to organic molecules inside a microcavity allows condensation at room temperature. The molecules act as saturable absorbers with transitions dressed by molecular vibrational modes. Motivated by this, we calculate the phase diagram and spectrum of a modified Tavis-Cummings model, describing vibrationally dressed two-level systems, coupled to a cavity mode. Coupling to vibrational modes can induce re-entrance, i.e. a normal-condensed-normal sequence with decreasing temperature and can drive the transition first-order.
We derive an effective Hamiltonian for the one-dimensional Hubbard-Holstein model, valid in a regime of both strong electron-electron (e-e) and electron-phonon (e-ph) interactions and in the nonadiabatic limit (t/ω0 ≤ 1), by using a non-perturbative approach. We obtain the phase diagram at quarter-filling by employing a modified Lanczos method and studying various density-density correlations. The spin-spin AF (antiferromagnetic) interactions and nearest-neighbor repulsion, resulting from the e-e and the e-ph interactions respectively, are the dominant terms (compared to hopping) and compete to determine the various correlated phases. As e-e interaction (U/t) is increased, the system transits from an AF cluster to a correlated singlet phase through a discontinuous transition at all strong e-ph couplings 2 ≤ g ≤ 3 considered. At higher values of U/t and moderately strong e-ph interactions (2 ≤ g ≤ 2.6), the singlets break up to form an AF order and then to a paramagnetic order all in a single sublattice; whereas at larger values of g (> 2.6), the system jumps directly to the spin disordered charge-density-wave (CDW) phase.
We uncover four new spin-charge ordered ground states in the strong coupling limit of the Kondo lattice model on triangular geometry. Two of the states at one-third electronic filling ($n=1/3$) consist of decorated ferromagnetic chains coupled antiferromagnetically with the neighboring chains. The third magnetic ground state is noncollinear, consisting of antiferromagnetic chains separated by a pair of canted ferromagnetic chains. An even more unusual magnetic ground state, a variant of the $120^{\circ}$ Yafet-Kittel phase, is discovered at $n=2/3$. These magnetic orders are stabilized by opening a gap in the electronic spectrum: a "band effect". All the phases support modulations in the electronic charge density due to the presence of magnetically inequivalent sites. In particular, the charge ordering pattern found at $n=2/3$ is observed in various triangular lattice systems, such as, 2H-AgNiO$_2$, 3R-AgNiO$_2$ and Na$_x$CoO$_2$.Comment: 5 pages, 4 figure
We study topological crystalline insulators doped with magnetic impurities, in which ferromagnetism at the surface lowers the electronic energy by spontaneous breaking of a crystalline symmetry. The number of energetically equivalent groundstates is sensitive to the crystalline symmetry of the surface, as well as the precise density of electrons at the surface. We show that for a SnTe model in the topological state, magnetic states can have two-fold, six-fold symmetry, or eight-fold degenerate minima. We compute spin stiffnesses within the model to demonstrate the stability of ferromagnetic states, and consider their ramifications for thermal disordering. Possible experimental consequences of the surface magnetism are discussed.PACS numbers: 73.20. At,75.70.Rf,75.30.Gw Introduction -Topological crystalline insulators (TCI's) are a class of materials in which the energy bands can host non-trivial topology protected by a crystalline symmetry [1]. These systems support surface states [2] which remain gapless provided the crystal symmetry is unbroken, and are believed to present themselves in (Sn,Pb)Te and related alloys [3][4][5][6][7][8]. Interesting effects may arise when the symmetry protecting a topological band structure is broken. In topological insulators protected by time-reversal symmetry (TRS), magnetic impurities on a surface break this symmetry and form collective states [9][10][11][12][13], which may be understood in terms of a gap opening in the surface spectrum [14].In contrast, TCI's are not protected by TRS, so the loss of this symmetry does not by itself energetically favor ordering of magnetic moments [15,16]. However, a uniform magnetization can undermine one or more relevant crystalline symmetries [17,18]. Indeed, the most common such symmetry is reflection across a mirror plane, of which there can be several. We show below that spontaneous surface magnetization opens a maximal gap when oriented along axes dictated by the bulk symmetries of the system. For a generic surface with a single mirror plane, there are two surface Dirac points at different momenta and energies [19], and in such cases at low temperature this results in a metallic, Ising-like ferromagnet, with the easy axis determined by the chemical potential µ. Importantly, the number of degenerate low-energy directions is enhanced for surfaces with further symmetries. Rotational symmetries in particular yield multiple mirror planes, and connect distinct surface Dirac cones to one another, yielding a multiplicity of easy axis directions. For sufficiently high symmetry, all the surface Dirac points may be related by symmetry operations, resulting in a fully gapped surface spectrum and a large number of groundstate orientations.To illustrate this physics, we present detailed calculations for the (111) surface of (Sn,Pb)Te [8,[20][21][22], using a known model Hamiltonian [3,23]. The (111) surface states are characterized in this system by four surface Dirac points, one at theΓ point and one at each of three Fig. 1(a).] When the system...
We show that a nearest-neighbor singlet phase results (from an effective Hamiltonian) for the one-dimensional Hubbard-Holstein model in the regime of strong electron-electron and electronphonon interactions and under non-adiabatic conditions (t/ω0 ≤ 1). By mapping the system of nearest-neighbor singlets at a filling Np/N onto a hard-core-boson (HCB) t-V model at a filling Np/(N − Np), we demonstrate explicitly that superfluidity and charge-density-wave (CDW) occur mutually exclusively with the diagonal long range order manifesting itself only at one-third filling. Furthermore, we also show that the Bose-Einstein condensate (BEC) occupation number n0 for the singlet phase, similar to the n0 for a HCB tight binding model, scales as √ N ; however, the coefficient of √ N in the n0 for the interacting singlet phase is numerically demonstrated to be smaller.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.