We investigate the existence of symmetry-protected topological phases in onedimensional alkaline-earth cold fermionic atoms with general half-integer nuclear spin I at half filling. In this respect, some orbital degrees of freedom are required. They can be introduced by considering either the metastable excited state of alkaline-earth atoms or the p-band of the optical lattice. Using complementary techniques, we show that SU(2) Haldane topological phases are stabilised from these orbital degrees of freedom. On top of these phases, we find the emergence of topological phases with enlarged SU(2I + 1) symmetry which depend only on the nuclear spin degrees of freedom. The main physical properties of the latter phases are further studied using a matrix-product state approach. On the one hand, we find that these phases are symmetryprotected topological phases, with respect to inversion symmetry, when I = 1/2, 5/2, 9/2, . . ., which is directly relevant to ytterbium and strontium cold fermions. On the other hand, for the other values of I(=half-odd integer), these topological phases are stabilised only in the presence of exact SU(2I + 1)-symmetry.
We investigate possible realizations of exotic SU(N ) symmetry-protected topological (SPT) phases with alkaline-earth cold fermionic atoms loaded into one-dimensional optical lattices. A thorough study of twoorbital generalizations of the standard SU(N ) Fermi-Hubbard model, directly relevant to recent experiments, is performed. Using state-of-the-art analytical and numerical techniques, we map out the zero-temperature phase diagrams at half-filling and identify several Mott-insulating phases. While some of them are rather conventional (nondegenerate, charge-density wave, or spin-Peierls-like), we also identify, for even N , two distinct types of SPT phases: an orbital Haldane phase, analogous to a spin-N/2 Haldane phase, and a topological SU(N ) phase, which we fully characterize by its entanglement properties. We also propose sets of nonlocal order parameters that characterize the SU(N ) topological phases found here.
We analyze the phase transition of the frustrated J1-J2 Ising model with antiferromagnetic nearestand strong next-nearest neighbor interactions on the square lattice. Using extensive Monte Carlo simulations we show that the nature of the phase transition for 1/2 < J2/J1 1 is not of the weakly universal type -as commonly believed -but we conclude from the clearly doubly peaked structure of the energy histograms that the transition is of weak first order. Motivated by these results, we analyze the phase transitions via field-theoretic methods; i.e., we calculate the central charge of the underlying field theory via transfer-matrix techniques and present, furthermore, a field-theoretic discussion on the phase-transition behavior of the model. Starting from the conformally invariant fixed point of two decoupled critical Ising models (J1 = 0), we calculate the effect of the nearest neighbor coupling term perturbatively using operator product expansions. As an effective action we obtain the Ashkin-Teller model.
We investigate classical Heisenberg spins on the Shastry-Sutherland lattice and under an external magnetic field. A detailed study is carried out both analytically and numerically by means of classical Monte-Carlo simulations. Magnetization pseudo-plateaux are observed around 1/3 of the saturation magnetization for a range of values of the magnetic couplings. We show that the existence of the pseudo-plateau is due to an entropic selection of a particular collinear state. A phase diagram that shows the domains of existence of those pseudo-plateaux in the (h, T ) plane is obtained.
Thin films of disordered hexaborides CaB 6 and SrB 6 deposited by pulsed-laser deposition on MgO (100) or Al 2 O 3 (001) substrates are ferromagnetic. A typical room-temperature moment per unit area of substrate is 350 B nm −2 , with the largest values being found for CaB 6 on Al 2 O 3 . Lattice defects are the likely origin of the exotic, high-temperature magnetism. The moment, which is present in films as thin as 12 nm, appears to reside in an interface layer whose polarization is approximately 0.4 Tesla.
Four-spin exchange interaction has been raising intriguing questions regarding the exotic phase transitions it induces in two-dimensional quantum spin systems. In this context, we investigate the effects of a cyclic four-spin exchange in the quasi-1D limit by considering a general N-leg spin ladder. We show by means of a low-energy approach that, depending on its sign, this ring exchange interaction can engender either a staggered or a uniform dimerization from the conventional phases of spin ladders. The resulting quantum phase transition is found to be described by the SU(2)_N conformal field theory. This result, as well as the fractional value of the central charge at the transition, is further confirmed by a large-scale numerical study performed by means of Exact Diagonalization and Density Matrix Renormalization Group approaches for N \le 4
Motivated by the on-going investigation of SrCu2(BO3)2 under pressure, we study a variant of the twodimensional Shastry-Sutherland (SS) spin-1/2 model with two types of dimers. Combined with the frustration of the SS model, this modification induces, in a large parameter range, a dimensional reduction at low energies, with nearly decoupled effective S = 1 Haldane chains forming along one of the diagonals of the lattice. We also present evidence that the intermediate plaquette solid phase of the undistorted SS model remains stable in a finite region of the phase diagram. Introduction.-An exciting route for exploring new phases in frustrated magnets consists in applying hydrostatic pressure so as to considerably modify the dominant magnetic interactions and thus alter the relevant low-energy degrees of freedom. The quasi-2D compound SrCu 2 (BO 3 ) 2 [1] offers such a possibility since it is in the close vicinity of a quantum phase transition from an exact tensor product of singlets toward a plaquette solid [2,3]. Indeed, Nuclear Magnetic Resonance (NMR) [4,5], Inelastic Neutron Scattering (INS) [6], as well as earlier susceptibility measurements [7], show strong evidence that SrCu 2 (BO 3 ) 2 undergoes at least one phase transition under high hydrostatic pressure, the nature of which is still not fully understood. What is clear however is that the four-fold (C 4 ) symmetry around the void plaquettes is quickly lost under pressure, so that nearest-neighbor dimers pointing in different directions become inequivalent (see Fig.1). This naturally leads to an extension of the Shastry-Sutherland (SS) model with two different diagonal bonds J 1 and J 2 described by the spin S = 1/2 Heisenberg Hamiltonian
A few exactly solvable interacting quantum many-body problems with impurities were previously reported to exhibit unusual features such as non-localization and absence of backscattering. In this work we consider the use of these integrable impurities as boundary conditions in the framework of linear transport problems. We first show that such impurities enhance the density of states at the Fermi surface, thus increasing the effective system size. The study of the real time-dynamics of a wave packet sent through a series of them inserted in both noninteracting and interacting leads then indicates that these impurities are transparent and do not add artefacts to the measurement of transport properties. We finally apply these new boundary conditions to study the conductance of an interacting scatterer using the embedding method.
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