Quantum criticality of semi-Dirac fermions in 2 + 1 dimensions

Two-dimensional semi-Dirac fermions are quasiparticles that disperse linearly in one direction and quadratically in the other. We investigate instabilities of semi-Dirac fermions toward charge and spin density wave and superconducting orders, driven by short-range interactions. We analyze the critical behavior of the Yukawa theories for the different order parameters using Wilson momentum shell renormalization group. We generalize to a large number Nf of fermion flavors to achieve analytic control in 2+1 dimensions and calculate critical exponents at one-loop order, systematically including 1/Nf corrections. The latter depend on the specific form of the bosonic infrared propagator in 2+1 dimensions, which needs to be included to regularize divergencies. The 1/Nf corrections are surprisingly small, suggesting that the expansion is well controlled in the physical dimension. The order parameter correlations inherit the electronic anisotropy of the semi-Dirac fermions, leading to correlation lengths that diverge along the spatial directions with distinct exponents, even at the mean-field level. We conjecture that the proximity to the critical point may stabilize novel modulated order phases

Charge-Density-Wave and Superconductor Competition in Stripe Phases of High Temperature Superconductors

We discuss the problem of competition between a superconducting (SC) ordered state with a charge density wave (CDW) state in stripe phases of high $T_c$ superconductors. We consider an effective model for each stripe motivated by studies of spin-gapped electronic ladder systems. We analyze the problem of dimensional crossover arising from inter-stripe SC and CDW couplings using non-Abelian bosonization and renormalization group (RG) arguments to derive an effective $O(4)$-symmetric nonlinear $\sigma$-model in $D=2+1$ for the case of when both inter-stripe couplings are of equal magnitude as well as equally RG relevant. By studying the effects of various symmetry lowering perturbations, we determine the structure of the phase diagram and show that, in general, it has a broad regime in which both orders coexist. The quantum and thermal critical behavior is discussed in detail, and the phase coexistence region is found to end at associated $T=0$ as well as $T>0$ tetracritical points. The possible role of hedgehog topological excitations of the theory is considered and argued to be RG irrelevant at the spatially anisotropic higher dimensional low-energy fixed point theory. Our results are also relevant to the case of competing N\'eel and valence bond solid (VBS) orders in quantum magnets on 2D isotropic square as well as rectangular lattices interacting via nearest neighbor Heisenberg exchange interactions.Comment: 9 pages, 3 figures (one with 3 subfigures

Boundary effects on the local density of states of one-dimensional Mott insulators and charge density wave states

We determine the local density of states (LDOS) for spin-gapped one-dimensional charge density wave (CDW) states and Mott insulators in the presence of a hard-wall boundary. We calculate the boundary contribution to the single-particle Green function in the low-energy limit using field theory techniques and analyze it in terms of its Fourier transform in both time and space. The boundary LDOS in the CDW case exhibits a singularity at momentum 2kF, which is indicative of the pinning of the CDW order at the impurity. We further observe several dispersing features at frequencies above the spin gap, which provide a characteristic signature of spin-charge separation. This demonstrates that the boundary LDOS can be used to infer properties of the underlying bulk system. In presence of a boundary magnetic field mid-gap states localized at the boundary emerge. We investigate the signature of such bound states in the LDOS. We discuss implications of our results on STM experiments on quasi-1D systems such as two-leg ladder materials like Sr14Cu24O41. By exchanging the roles of charge and spin sectors, all our results directly carry over to the case of one-dimensional Mott insulators.Comment: 28 page

Dynamical response functions in the quantum Ising chain with a boundary

We determine dynamical response functions $<{\cal O}^\dagger(t,x_1){\cal O}(0,x_2)>$ in the scaling limit of the quantum Ising chain on the half line in the presence of a boundary magnetic field. Using a spectral representation in terms of infinite volume form factors and a boundary state, we derive an expansion for the correlator that is found to be rapidly convergent as long as |\frac{x_1+x_2}{\xi}|\agt 0.2 where $\xi$ is the correlation length. At sufficiently late times we observe oscillatory behaviour of the correlations arbitrarily far away from the boundary. We investigate the effects of the boundary bound state that is present for a range of boundary magnetic fields.Comment: 32 page

Topological orbital ladders

We unveil a topological phase of interacting fermions on a two-leg ladder of unequal parity orbitals, derived from the experimentally realized double-well lattices by dimension reduction. $Z_2$ topological invariant originates simply from the staggered phases of $sp$-orbital quantum tunneling, requiring none of the previously known mechanisms such as spin-orbit coupling or artificial gauge field. Another unique feature is that upon crossing over to two dimensions with coupled ladders, the edge modes from each ladder form a parity-protected flat band at zero energy, opening the route to strongly correlated states controlled by interactions. Experimental signatures are found in density correlations and phase transitions to trivial band and Mott insulators.Comment: 12 pages, 5 figures, Revised title, abstract, and the discussion on Majorana numbe

Central Opioidergic System Interplay with Histamine on Food Intake in Neonatal Chicks: Role of µ-Opioid and H1/H3 Receptors

ABSTRACT The present study was designed to examine the role of Opioidergic and Histaminergic systems on feeding behavior in 3-hour food deprived neonatal meat- type chicks. In experiment 1, chicks received intracerebroventricular (ICV) injection of (A) control solution, (B) α-FMH (alpha fluoromethyl histidine; 250 nmol), (C) DAMGO (µ-opioid receptor agonist, 125 pmol) and (D) α-FMH + DAMGO. Experiments 2-4 were similar to experiment 1, except chicken ICV injected with Chlorpheniramine (histamine H1 receptors antagonist; 300 nmol), famotidine (histamine H2 receptors antagonist; 82 nmol) and Thioperamide (histamine H3 receptors antagonist; 300 nmol) instead of the α-FMH. In experiments 5-8, birds ICV injected with the same procedure as experiments 1-4, except they were injected with DPDPE (δ-opioid receptor agonist, 40 nmol) instead of DAMGO. Experiments 9-12 were similar to the experiments 1-4, except neonatal broilers ICV were injected with U-50488H (κ-opioid receptor agonist, 30 nmol) instead of DAMGO. Then the cumulative food intake was measured until 120 min post injection. According to the results, ICV injection of DAMGO, significantly decreased food intake (p0.05). Also, the hyperphagic effect of DPDPE and U-50488 had no affect by α-FMH, Chlorpheniramine, famotidine and Thioperamide (p>0.05). These results suggested that an interconnection between central opioidergic and histaminergic systems on feeding behavior is mediated via µ-opioid and H1/H3 receptors in neonatal broilers